Nitric oxide monooxygenation (NOM) reaction of cobalt-nitrosyl {Co(NO)}8 to CoII-nitrito {CoII(NO2−)}: base induced hydrogen gas (H2) evolution

Here, we report the nitric oxide monooxygenation (NOM) reactions of a Co^III-nitrosyl complex (1, {Co-NO}⁸) in the presence of mono-oxygen reactive species, i.e., a base (OH⁻, tetrabutylammonium hydroxide (TBAOH) or NaOH/15-crown-5), an oxide (O²⁻ or Na₂O/15-crown-5) and water (H₂O). The reaction of 1 with OH⁻ produces a Co^II-nitrito complex {3, (Co^II-NO₂⁻)} and hydrogen gas (H₂), via the formation of a putative N-bound Co-nitrous acid intermediate (2, {Co-NOOH}⁺). The homolytic cleavage of the O–H bond of proposed [Co-NOOH]⁺ releases H₂via a presumed Co^III-H intermediate. In another reaction, 1 generates Co^II-NO₂⁻ when reacted with O²⁻via an expected Co^I-nitro (4) intermediate. However, complex 1 is found to be unreactive towards H₂O. Mechanistic investigations using ¹⁵N-labeled-¹⁵NO and ²H-labeled-NaO²H (NaOD) evidently revealed that the N-atom in Co^II-NO₂⁻ and the H-atom in H₂ gas are derived from the nitrosyl ligand and OH⁻ moiety, respectively.

Base-induced hydrogen (H₂) gas evolution in the nitric oxide monoxygenation reaction.

As a radical species, nitric oxide (NO) has attracted great interest from the scientific community due to its major role in various physiological processes such as neurotransmission, vascular regulation, platelet disaggregation and immune responses to multiple infections.¹ Nitric oxide synthase (NOS),² and nitrite reductase (NiR)³ enzymes are involved in the biosynthesis of NO. NOSs produce NO by the oxidation of the guanidine nitrogen in l-arginine.⁴ However, in mammals and bacteria, NO₂⁻ is reduced to NO by NiRs in the presence of protons, i.e., NO₂⁻ + e⁻ + 2H⁺ → NO + H₂O.⁵ Biological dysfunctions may cause overproduction of NO, and being radical it leads to the generation of reactive nitrogen species (RNS), i.e., peroxynitrite (PN, OONO⁻)⁶ and nitrogen dioxide (˙NO₂),⁷ upon reaction with reactive oxygen species (ROS) such as superoxide (O₂˙⁻),⁸ peroxide (H₂O₂),⁹ and dioxygen (O₂).¹⁰ Hence, it is essential to maintain an optimal level of NO. In this regard, nitric oxide dioxygenases (NODs)¹¹ are available in bio-systems to convert excess NO to biologically benign nitrate (NO₃⁻).¹²NO₂⁻ + Fe^II + H⁺ ↔ NO + Fe^III + OH⁻1[M–NO]ⁿ + 2OH⁻ → [M–NO₂]⁽ⁿ⁻²⁾ + H₂O2NOD enzymes generate NO₃⁻ from NO;^11b,12−13 however, the formation of NO₂⁻ from NO is still under investigation. Clarkson and Bosolo reported NO₂⁻ formation in the reaction of Co^III-NO and O₂.¹⁴ Nam and co-workers showed the generation of Co^II-NO₂⁻ from Co^III-NO upon reaction with O₂˙⁻.¹⁵ Recently, Mondal and co-workers reported NO₂⁻ formation in the reaction of Co^II-NO with O₂.¹⁶ Apart from cobalt, the formation of Cu^II-NO₂⁻ was also observed in the reaction of Cu^I-NO and O₂.¹⁷ For metal-dioxygen adducts, i.e., Cr^III-O₂˙⁻ and Mn^IV-O₂²⁻, NOD reactions led to the generation of Cr^III-NO₂⁻ (ref. 18) and Mn^V O + NO₂⁻,¹⁹ respectively. However, the NOD reaction of Fe^III-O₂˙⁻ and Fe^III-O₂²⁻ with NO and NO⁺, respectively, generated Fe^III-NO₃⁻via Fe^IV O and ˙NO₂.²⁰ Ford suggested that the reaction of ferric-heme nitrosyl with hydroxide leads to the formation of NO₂⁻ and H⁺.¹² Lehnert and co-workers reported heme-based Fe-nitrosyl complexes²¹ showing different chemistries due to the Fe^II-NO⁺ type electronic structures. On the other hand, Bryan proposed that the one-electron reduction of NO₂⁻ to NO in ferrous heme protein is reversible (eqn (1)).²² Also, it is proposed that excess NO in biological systems is converted to NO₂⁻ and produces one equivalent of H⁺ upon reaction with ˙OH.²³ Previously reported reactivity of M–NOs of Fe²⁴ with OH⁻ suggested the formation of NO₂⁻ and one equivalent of H⁺, where H⁺ further reacts with one equivalent of OH⁻ and produces H₂O (eqn (2)).²⁵Here in this report, we explore the mechanistic aspects of nitric oxide monooxygenation (NOM) reactions of the Co^III-nitrosyl complex, [(12TMC)Co^III(NO⁻)]²⁺/{Co(NO)}⁸ (1),^15,26 bearing the 12TMC ligand (12TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) with mono-oxygen reactive species (O²⁻, OH⁻ and H₂O) (Scheme 1). Complex 1 reacts with the base (OH⁻, tetrabutylammonium hydroxide (TBAOH)/or NaOH in the presence of 15-crown-5 as the OH⁻ source) and generates the corresponding Co^II-nitrito complex, [(12TMC)Co^II(NO₂⁻)]⁺ (3), with the evolution of hydrogen gas (H₂) via the formation of a plausible N-bound Co-nitrous acid intermediate ([Co-NOOH]⁺, 2) in CH₃CN at 273 K (Scheme 1, reaction (I)). Also, when 1 reacts with the oxide (O²⁻ or Na₂O in the presence of 15-crown-5), it generates the Co^II-nitrito complex (3) via a probable Co^I-nitro, [(12TMC)Co^I(NO₂⁻)] (4), intermediate (Scheme 1, reaction (II)); however, 1 does not react with water (Scheme 1, reaction (III)). Mechanistic investigations using ¹⁵N-labeled-¹⁵NO, D-labeled-NaOD and ¹⁸O-labelled-¹⁸OH⁻ demonstrated, unambiguously, that the N and O-atoms in the NO₂⁻ ligand of 3 resulted from NO and OH⁻ moieties; however, the H-atoms of H₂ are derived from OH⁻. To the extent of our knowledge, the present work reports the very first systematic study of Co^III-nitrosyl complex reactions with H₂O, OH⁻ and O²⁻. This new finding presents an alternative route for NO₂⁻ generation in biosystems, and also illustrates a new pathway of H₂ evolution, in addition to the reported literature.^12,27Open in a separate windowScheme 1Nitric oxide monooxygenation (NOM) reactions of cobalt-nitrosyl complex (1) in the presence of a base (OH⁻), sodium oxide (Na₂O) and water (H₂O).To further explore the chemistry of [(12TMC)Co^III(NO⁻)]²⁺ (1),^15,26 and the mechanistic insights of NOM reactions, we have reacted it with a base (OH⁻), an oxide (O²⁻), and water (H₂O). When complex 1 was reacted with TBAOH in CH₃CN, the color of complex 1 changed to light pink from dark pink. In this reaction, the characteristic absorption band of 1 (370 nm) disappears within 2 minutes (Fig. 1a; ESI, Experimental section (ES) and Fig. S1a^†), producing a Co^II-nitrito complex, [(12TMC)Co^II(NO₂⁻)]⁺ (3), with H₂ (Scheme 1, reaction (Ib)), in contrast to the previous reports on base induced NOM reactions (eqn (2)).^12,25,28 The spectral titration data confirmed that the ratio-metric equivalent of OH⁻ to 1 was 1 : 1 (ESI, Fig. S1b^†). 3 was determined to be [(12TMC)Co^II(NO₂⁻)](BF₄) based on various spectroscopic and structural characterization experiments (vide infra).^15,26bOpen in a separate windowFig. 1(a) UV-vis spectral changes of 1 (0.50 mM, black line) upon addition of OH⁻ (1 equiv.) in CH₃CN under Ar at 273 K. Black line (1) changed to red line (3) upon addition of OH⁻. Inset: IR spectra of 3-¹⁴NO₂⁻ (blue line) and 3-¹⁵NO₂⁻ (red line) in KBr. (b) ESI-MS spectra of 3. The peak at 333.2 is assigned to [(12TMC)Co^II(NO₂)]⁺ (calcd m/z 333.1). Inset: isotopic distribution pattern for 3-¹⁴NO₂⁻ (red line) and 3-¹⁵NO₂⁻ (blue line).The FT-IR spectrum of 3 showed a characteristic peak for nitrite stretching at 1271 cm⁻¹ (Co^II-14NO₂⁻) and shifted to 1245 cm⁻¹ (Co^II-15NO₂⁻) when 3 was prepared by reacting ¹⁵N-labeled NO (Co^III-15NO) with OH⁻ (Inset, Fig. 1a and Fig. S2^†). The shifting of NO₂⁻ stretching (Δ = 30 cm⁻¹) indicates that the N-atom in the NO₂⁻ ligand is derived from Co^III-15NO. The ESI-MS spectrum of 3 showed a prominent peak at m/z 333.2, [(12TMC)Co^II(¹⁴NO₂⁻)]⁺ (calcd m/z 333.2), which shifted to 334.2, [(12TMC)Co^II(¹⁵NO₂⁻)]⁺ (calcd m/z 334.2), when the reaction was performed with Co^III-15NO (Inset, Fig. 1b; ESI, Fig. S3a^†); indicating clearly that NO₂⁻ in 3 was derived from the NO moiety of 1. In addition, we have reacted 1 with Na¹⁸OH (ES and ESI^†), in order to follow the source of the second O-atom in 3-NO₂⁻. The ESI-MS spectrum of the reaction mixture, obtained by reacting 1 with Na¹⁸OH, showed a prominent peak at m/z 335.2, [(12TMC)Co^II(¹⁸ONO⁻)]⁺ (calcd m/z 335.2), (SI, Fig. S3b^†) indicating clearly that NO₂⁻ in 3 was derived from ¹⁸OH⁻. The ¹H NMR spectrum of 3 did not show any signal for aliphatic protons of the 12TMC ligand, suggesting a bivalent cobalt center (Fig. S4^†).^26b Furthermore, we have determined the magnetic moment of 3, using Evans'' method, and it was found to be 4.62 BM, suggesting a high spin Co(ii) metal center with three unpaired electrons (ESI^† and ES).²⁹ The exact conformation of 3 was provided by single-crystal X-ray crystallographic analysis (Fig. 2b, ESI, ES, Fig. S5, and Tables T1 and T2^†) and similar to that of previously reported Co^II-NO₂⁻/M^II-NO₂⁻.^15,26b Also, we have quantified the amount of nitrite (90 ± 5%), formed in the above reaction, using the Griess reagent (ESI, ES, and Fig. S6^†).Open in a separate windowFig. 2Displacement ellipsoid plot (20% probability) of 3 at 100 K. Disordered C-atoms of the TMC ring, anion and H-atoms have been removed for clarity.As is known from the literature, a metal-nitrous acid intermediate may form either by the reaction of a metal-nitrosyl with a base²⁷ or by the metal-nitrite reaction with an acid (nitrite reduction chemistry);^26b however, the products of both the reactions are different. Here, for the first time, we have explored the reaction of Co^III-nitrosyl (1) with a base. In this reaction, it is clear that the formation of Co^II-nitrito would be accomplished by the release of H₂ gas via the generation of a transient N-bound [Co-(NOOH)]⁺ intermediate (Scheme 2, reaction (II)). The formation of Co^II-NO₂⁻ (3) from the [Co-(NOOH)]⁺ intermediate is likely to proceed by either (i) homolytic cleavage of the O–H bond and release of H₂via the proposed Co^III-H transient species (Co^III-H = Co^II + 1/2H₂)³⁰ (Scheme 2, reaction (III)), as reported in previous literature where the reduced cobalt, in a number of different ligand environments, is a good H⁺ reduction catalyst and generates H₂ gas via a Co^III-H intermediate³¹ or (ii) heterolytic cleavage of the O–H bond and the formation of Co^I-NO₂⁻ + H⁺.²⁷ In the present study, we observed the formation of 3 and H₂via the plausible homolytic cleavage of the NOO–H moiety of 2 as shown in Scheme 2, in contrast to the previous reports on base-induced reactions on metal-nitrosyls (eqn (3)).²⁷ Taking together both possibilities, (i) is the most reasonable pathway for the NOM reaction of complex 1 in the presence of a base (as shown in Scheme 2, reaction (III)). And the reaction is believed to go through a Co^III-H intermediate as reported previously in Co^I-induced H⁺ reduction in different ligand frameworks and based on literature precedence, we believe that complex 1 acts in a similar manner.³¹Open in a separate windowScheme 2NOM reaction of complex 1 in the presence of OH⁻, showing the generation of Co^II-nitrito (3) and H₂via a Co(iii)-hydrido intermediate.In contrast to an O-bound Co^II-ONOH intermediate, where N–O bond homolysis of the ON-OH moiety generates H₂O₂ (Scheme 2, reaction (IV)),^26b the N-bound [Co-(NOOH)]⁺ intermediate decomposes to form NO₂⁻ and a Co(iii)-H transient species, arising from β-hydrogen transfer from the NOO–H moiety to the cobalt-center (Scheme 2, reaction (II)).^30a,c,32 The Co(iii)-hydrido species may generate H₂ gas either (a) by its transformation to the Co(ii)-nitrito complex (2) and H₂ gas as observed in the case of Co^III-H intermediate chemistry^30a,c,e−g as proposed in the chemistry of the Co^I complex with H⁺ reduction³¹ and other metal-hydrido intermediates³² and also explained in O₂ formation in PN chemistry^17,33 or (b) by the reacting with another [Co-(NOOH)]⁺ intermediate (Scheme 2, reaction (III)).Furthermore, we have confirmed the H₂ formation in the NOM reaction of 1 with OH⁻ by headspace gas mass spectrometry (Fig. 3a). Also, carrying out the reaction of 1 with NaOD leads to the formation of the [Co-(NOOD)]⁺ intermediate, which then transforms to a Co^III-D transient species. Further, as described above, the Co^III-D species releases D₂ gas, detected by headspace gas mass spectrometry (Fig. 3b), which evidently established that H₂ gas formed in the reaction of 1 with OH⁻. In this regard, we have proposed that in the first step of this reaction, the nucleophilic addition of OH⁻ to {Co-NO}⁸ generates a transient N-bound [Co-(NOOH)]⁺ intermediate that is generated by an internal electron transfer to Co^III (Scheme 2, reaction (I)). By following the mechanism proposed in the case of Co^III-H,^30a−c O₂,¹⁵ and H₂O₂(ref. 26b) formation, we have proposed the sequences of the NOM reaction of 1, which leads to the generation of Co^II-nitrito and H₂ (Scheme 2, reaction (I)–(III) and Scheme 3). In the second step, O–H bond homolytic cleavage generates a Co^III-H transient species + NO₂⁻via a β-hydrogen elimination reaction of the [Co-(NOOH)]⁺ intermediate.³² The Co^III-H intermediate may undergo the following reactions to generate H₂ gas and Co^II-nitrito either (a) by the natural decomposition of the Co^III-H transient species to generate H₂,^30a,c,e−g or (b) by the H-atom abstraction from another [Co-(NOOH)]⁺ intermediate (Scheme 3). Also, to validate our assumption that the reaction goes through a plausible N-bound [Co-(NOOH)]⁺ intermediate followed by its transformation to the Co^III-H species (vide supra), we have performed the reaction of 1 with NaOH/NaOD (in 1 : 1 ratio). In this reaction, we have observed the formation of a mixture of H₂, D₂, and HD gases, which indicates clearly that the reaction goes through the formation of Co^III-H and Co^III-D transient species via the aforementioned mechanism (Fig. 3c). This is the only example where tracking of the H atoms has confirmed the H₂ generation from an N-bound NOO–H moiety as proposed for H₂ formation from Co^III-H.³⁰Open in a separate windowFig. 3Mass spectra of formation of (a) H₂ in the reaction of 1 (5.0 mM) with NaOH (5.0 mM), (b) D₂ in the reaction of 1 (5.0 mM) with NaOD (5.0 mM), (c) D₂, HD, and H₂ in the reaction of 1 (5.0 mM) with NaOD/NaOH (1 : 1), and (d) H₂ in the reaction of 1 (5.0 mM) with NaOH in the presence of 2,4 DTBP (50 mM).Open in a separate windowScheme 3NOM reaction of complex 1 in the presence of OH⁻, showing the different steps of the reaction.While, we do not have direct spectral evidence to support the formation of the transient N-bound [Co-(NOOH)]⁺ intermediate and its decomposition to the Co^III-H transient species via β-hydrogen transfer from the NOOH moiety to the cobalt center, support for its formation comes from our finding that the reactive hydrogen species can be trapped by using 2,4-di-tert-butyl-phenol (2,4-DTBP).³⁴ In this reaction, we observed the formation of 2,4-DTBP-dimer (2,4-DTBP-D, ∼67%) as a single product (ESI, ES, and Fig. S7^†). This result can readily be explained by the H-atom abstraction reaction of 2,4-DTBP either by [Co-(NOOH)]⁺ or Co^III-H, hence generating a phenoxyl-radical and 3 with H₂ (Fig. 3d and Scheme 2, reaction (a)). Also, we have detected H₂ gas formation in this reaction (ESI,^† ES, and Fig. 3d). In the next step, two phenoxyl radicals dimerized to give 2,4-DTBP-dimer (Scheme 2c, reaction (II)). Thus, the observation of 2,4-DTBP-dimer in good yield supports the proposed reaction mechanism (Scheme 2, reaction (a) and (b)). Further, the formation of 2,4 DTBP as a single product also rules out the formation of the hydroxyl radical as observed in the case of an O-bound nitrous acid intermediate.^26bFurthermore, we have explored the NOM reactivity of 1 with Na₂O/15-crown-5 (as the O²⁻ source) and observed the formation of the Co^II-nitrito complex (3) via a plausible Co^I-nitro (4) intermediate (Scheme 1, reaction (IIa); also see the ESI^† and ES); however, 1 was found to be inert towards H₂O (Scheme 1, reaction (III); also see the ESI, ES and Fig. S8^†). The product obtained in the reaction of 1 with O²⁻ was characterized by various spectroscopic measurements.^15,26b The UV-vis absorption band of 1 (λ_max = 370 nm) disappears upon the addition of 1 equiv. of Na₂O and a new band (λ_max = 535 nm) forms, which corresponds to 3 (ESI, Fig. S9^†). The FT-IR spectrum of the isolated product of the above reaction shows a characteristic peak for Co^II-bound nitrite at 1271 cm⁻¹, which shifts to 1245 cm⁻¹ when exchanged with ¹⁵N-labeled-NO (¹⁵N¹⁶O) (ESI, ES, and Fig. S10^†), clearly indicating the generation of nitrite from the NO ligand of complex 1.^26b The ESI-MS spectrum recorded for the isolated product (vide supra) shows a prominent ion peak at m/z 333.1, and its mass and isotope distribution pattern matches with [(12-TMC)Co^II(NO₂)]⁺ (calc. m/z 333.1) (ESI, Fig. S11^†). Also, we quantified the amount of 3 (85 ± 5%) by quantifying the amount of nitrite (85 ± 5%) using the Griess reagent test (ESI, ES, and Fig. S6^†).In summary, we have demonstrated the reaction of Co^III-nitrosyl, [(12-TMC)Co^III(NO⁻)]²⁺/{CoNO}⁸ (1), with mono-oxygen reactive species (O²⁻, OH⁻ and H₂O) (Scheme 1). For the first time, we have established the clear formation of a Co^II-nitrito complex, [(12TMC)Co^II(NO₂⁻)]⁺ (3), and H₂ in the reaction of 1 with one equivalent of OH⁻via a transient N-bound [Co-(NOOH)]⁺ (2) intermediate. This [Co-(NOOH)]⁺ intermediate undergoes the O–H bond homolytic cleavage and generates a Co^III-H transient species with NO₂⁻, via a β-hydrogen elimination reaction of the [Co-(NOOH)]⁺ intermediate, which upon decomposition produces H₂ gas. This is in contrast to our previous report, where acid-induced nitrite reduction of 3 generated 1 and H₂O₂via an O-bound Co^II-ONOH intermediate.^26b Complex 1 was found to be inert towards H₂O; however, we have observed the formation of 3 when reacted with O²⁻. It is important to note that H₂ formation involves a distinctive pathway of O–H bond homolytic cleavage in the [Co-(NOOH)]⁺ intermediate, followed by the generation of the proposed Co^III-H transient species (Co^II + 1/2H₂)³⁰ prior to H₂ evolution as described in Co^I chemistry with H⁺ in many different ligand frameworks.³¹ The present study is the first-ever report where the base induced NOM reaction of Co^III-nitrosyl (1) leads to Co^II-nitrito (3) with H₂ evolution via an N-bound [Co-(NOOH)]⁺ intermediate, in contrast to the chemistry of O-bound Co^II-ONOH^26b, hence adding an entirely new mechanistic insight of base induced H₂ gas evolution and an additional pathway for NOM reactions.     

Multicolor polymeric carbon dots: synthesis,separation and polyamide-supported molecular fluorescence

Multicolor carbon dots (CDs) have been developed recently and demonstrate great potential in bio-imaging, sensing, and LEDs. However, the fluorescence mechanism of their tunable colors is still under debate, and efficient separation methods are still challenging. Herein, we synthesized multicolor polymeric CDs through solvothermal treatment of citric acid and urea in formamide. Automated reversed-phase column separation was used to achieve fractions with distinct colors, including blue, cyan, green, yellow, orange and red. This work explores the physicochemical properties and fluorescence origins of the red, green, and blue fractions in depth with combined experimental and computational methods. Three dominant fluorescence mechanism hypotheses were evaluated by comparing time-dependent density functional theory and molecular dynamics calculation results to measured characteristics. We find that blue fluorescence likely comes from embedded small molecules trapped in carbonaceous cages, while pyrene analogs are the most likely origin for emission at other wavelengths, especially in the red. Also important, upon interaction with live cells, different CD color fractions are trafficked to different sub-cellular locations. Super-resolution imaging shows that the blue CDs were found in a variety of organelles, such as mitochondria and lysosomes, while the red CDs were primarily localized in lysosomes. These findings significantly advance our understanding of the photoluminescence mechanism of multicolor CDs and help to guide future design and applications of these promising nanomaterials.

Understanding the origin and sensitivity of carbon dot emission will improve their utility in various applications.

Since the accidental discovery of luminescent carbon fragments in 2004,¹ carbon dots (CDs) have attracted great research interest due to the diverse synthetic methods, tunable luminescence, and applicability in a broad range of fields, including bio-imaging,^2–4 sensing,^5,6 and light emitting diodes (LEDs).^7,8 Typically, CDs are fluorescent carbon nanostructures of sizes less than 10 nm, composed of carbon, oxygen, and nitrogen.^9–12 CDs can be produced through bottom-up methods, which involve small molecular precursors like citric acid, malic acid, urea, ethylenediamine, and so on.^13–15 In a high temperature reaction, polymerization and dehydration occur among various functional groups, and the resulting products are usually a mixture of small molecule residues, oligomers, and long chain polymers.¹⁶ The unclear fluorescence mechanisms and poorly understood internal structure of CDs limit the ability to understand, tune, and fully exploit their fluorescence properties.Fortunately, in recent years, breakthrough syntheses of multicolor CDs have been achieved.^17–19 Several different multicolor CDs have been synthesized with aromatic compounds such as phenylenediamine.^4,20–22 However, it should be noted that precursors such as aniline and phenol may have toxic effects on human health and the environment,^23,24 and thus should be avoided where possible. Syntheses of colorful CDs from non-aromatic compounds such as citric acid and urea often employ solvothermal methods. Utilizing different solvents such as formamide and dimethylformamide have been shown to play a significant role in tuning CD emission.^25,26 In addition, chromatographic post-treatment of as-made CDs plays a critical role in obtaining different colored fractions, using techniques such as anion-exchange column chromatography,²⁶ normal phase silica chromatography,²⁷ and reversed phase silica chromatography.¹⁵ Compared with high performance liquid chromatography (HPLC), the aforementioned column chromatography techniques help to separate CDs on a larger scale. These separations are based on charge²⁶ or polarity,²¹ and are efficient in isolating the desired fractions with distinct colors so that detailed structural characterization can be performed.To gain insight into the fluorescence mechanism of these multicolor CDs, researchers have considered three hypotheses: quantum size effects,²⁸ the inclusion of molecular fluorophores,²⁹ and surface state-induced emission.³⁰ For example, Rogach and coworkers developed solid-state CDs with tunable fluorescence via the seeded growth method. They attributed the tunable emission to the size of π-conjugated domains.³¹ Yang and coworkers synthesized CDs by hydrothermal treatment of citric acid and ethylenediamine. They identified a small molecule fluorophore, IPCA (1,2,3,5-tetrahydro-5-oxo-imidazo[1,2-α]pyridiine-7-carboxylic acid) from CD column separation fractions, which contributed to the blue fluorescence.¹³ Xiong and coworkers synthesized CDs from urea and p-phenylenediamine that emitted a range of colors and separated them with silica column chromatography. They found the degree of carbon oxidation increased as the emission redshifted and thus, they endorsed the surface state hypothesis.²¹ In addition to the above mechanisms, computational methods such as density functional theory (DFT) have also been applied to analyze the fluorescence origins of CDs. The charge transfer between functional groups on the polymeric unit of CDs made from citric acid and ethylenediamine was found to facilitate blue emission.¹⁶The goal of present work is to understand the fluorescence origin of multicolor CDs. The model multicolor CDs were obtained by reacting citric acid and urea in formamide via a microwave-assisted hydrothermal treatment. An automated chromatographic apparatus was employed to separate as-made CD mixtures into distinct color fractions. The individual separation process took around 20 minutes, and the obtained CD fractions exhibit discrete illumination-induced emission throughout the visible region of the spectrum. Interestingly, the sizes of separated CD fractions are not statistically different from one another, suggesting that the quantum size effects are not the source of differential emission. Solvatochromism experiments showed that the blue and green fractions have similar fluorescence behavior as a function of solvent polarity, but the red fraction behaved differently. Using computational simulations, three models of the fluorescence origin were constructed and evaluated, showing that the formation of small blue fluorescent molecules is likely and pyrene analogs could be the origins for various emission colors. Moreover, two representative CD fractions, the blue- and red-emitting fractions, were chosen for subsequent cell imaging experiments. The localization pattern for the CD fractions differed: blue-emitting CDs were observed in a wide range of organelles, while red-emitting CDs were primarily enclosed in lysosomes. Understanding the origin and the sensitivity of CD emission will improve their utility in bioimaging applications.     

Correction: Crystal structure and metallization mechanism of the π-radical metal TED

Radical electrons tend to localize on individual molecules, resulting in an insulating (Mott–Hubbard) bandgap in the solid state. Herein, we report the crystal structure and intrinsic electronic properties of the first single crystal of a π-radical metal, tetrathiafulvalene-extended dicarboxylate (TED). The electrical conductivity is up to 30 000 S cm⁻¹ at 2 K and 2300 S cm⁻¹ at room temperature. Temperature dependence of resistivity obeys a T³ power-law above T > 100 K, indicating a new type of metal. X-ray crystallographic analysis clarifies the planar TED molecule, with a symmetric intramolecular hydrogen bond, is stacked along longitudinal (the a-axis) and transverse (the b-axis) directions. The π-orbitals are distributed to avoid strong local interactions. First-principles electronic calculations reveal the origin of the metallization giving rise to a wide bandwidth exceeding 1 eV near the Fermi level. TED demonstrates the effect of two-dimensional stacking of π-orbitals on electron delocalization, where a high carrier mobility of 31.6 cm² V⁻¹ s⁻¹ (113 K) is achieved.

The molecular arrangement that enables metallic conduction in a single-component pure organic crystal is revealed by single-crystal X-ray diffraction.

Organic molecular solids are typically insulating due to their paired electrons in spatially localized s- and p-orbitals. The concept of charge-transfer (CT) between donor and acceptor¹ enabled the development of conducting molecular complexes (salts) including semiconducting perylene-bromine,² metallic tetrathiafulvalene (TTF)-tetracyano-p-quinodimethane (TCNQ),³ and polyacethylene doped with halogen molecules.⁴ A different strategy was proposed in the 1970s based on organic radicals with an open-shell electronic structure.⁵ π-Radicals such as neutral-,⁶ fully-conjugated⁷ and zwitterionic (betainic)⁸ molecules, with an unpaired electron in their singly occupied molecular orbital (SOMO), offered potential candidates. However, all these π-radicals were insulators or semiconductors with a finite bandgap, which is due to the SOMO being localized on an individual molecule. In the Mott–Hubbard model,⁹ the case of the π-radical solids can be described by the on-site Coulomb repulsion U being larger than the electronic bandwidth W (U/W > 1), in contrast to U/W < 1 in molecular metals like CT metal systems (Fig. 1a).Open in a separate windowFig. 1(a) Schematic representation of the electronic band structure of Mott–Hubbard insulators (left) and possible molecular metals (right), respectively. Solids formed from typical π-radicals possess large on-site Coulomb repulsion U compared with the electronic bandwidth W, resulting a finite bandgap (left). This requires a new mechanism to expand W and overcome U to achieve a metallic state at ambient pressure in π-radical crystals. (b) Molecular structure of the zwitterionic radical, tetrathiafulvalene-extended dicarboxylate (TED) with a symmetric intramolecular hydrogen bond.A straightforward approach to realize high conductivity in π-radical systems is to enhance the intermolecular interaction by applying high pressure.¹⁰ Bisdithiazolyl radical crystal achieved W ∼ 1 eV near the Fermi level and the room temperature conductivity σ_RT = 2 S cm⁻¹ under 11 GPa pressure.^10c An alternative route is to decrease the interatomic spacing by incorporating a metal ion. Introduction of a semimetal Se and intermolecular hydrogen bonding in a donor-type radical succeeded to improve a conductivity to σ_RT = 19 S cm⁻¹ but still required high pressure over 1 GPa for breaking its insulating character.^10d An organometallic compound with a transition metal, [Ni(tmdt)₂], by contrast, is known to form a three-dimensional (3D) Fermi surface with W = 0.48 eV and metallic conduction with σ_RT = 400 S cm⁻¹ at ambient pressure.¹¹ A breakthrough concept for expanding W at ambient pressure is desired for achieving metallization in pure organic π-radicals.Tetrathiafulvalene-extended dicarboxylate (TED) is an organic air-stable zwitterionic radical (Fig. 1b),¹² which was designed based on carrier generation induced by a stably-introduced protonic defect (–H⁺) in hydrogen-bonding molecules without adopting CT between multiple molecules.¹³ A polycrystalline film of TED exhibited metallic conduction at ambient pressure, but the mechanism has not been clarified yet due to lack of single crystal information.¹² Herein, we report the first crystal structure and intrinsic electronic properties of the recently grown single crystal TED. Structural analysis and quantum chemical simulations based on the single crystal reveal the origin of its metallic behavior.     

Ferro-self-assembly: magnetic and electrochemical adaptation of a multiresponsive zwitterionic metalloamphiphile showing a shape-hysteresis effect

Metallosurfactants are molecular compounds which combine the unique features of amphiphiles, like their capability of self-organization, with the peculiar properties of metal complexes like magnetism and a rich redox chemistry. Considering the high relevance of surfactants in industry and science, amphiphiles that change their properties on applying an external trigger are highly desirable. A special feature of the surfactant reported here, 1-(Z)-heptenyl-1′-dimethylammonium-methyl-(3-sulfopropyl)ferrocene (6), is that the redox-active ferrocene constituent is in a gemini-position. Oxidation to 6⁺ induces a drastic change of the surfactant''s properties accompanied by the emergence of paramagnetism. The effects of an external magnetic field on vesicles formed by 6⁺ and the associated dynamics were monitored in situ using a custom-made optical birefringence and dual dynamic light scattering setup. This allowed us to observe the optical anisotropy as well as the anisotropy of the diffusion coefficient and revealed the field-induced formation of oriented string-of-pearls-like aggregates and their delayed disappearance after the field is switched off.

The self-organization properties of a stimuli responsive amphiphile can be altered by subjecting the paramagnetic oxidized form to a magnetic field of 0.8 T and monitored in real time by coupling optical birefringence with dynamic light scattering.

Amphiphiles (or surfactants) combine hydrophilic (the so-called headgroups) and lipophilic entities (the so-called tails) as integral parts of their molecular structures. This particular construction principle provides them with the ability to display concentration-dependent self-organization in nonpolar and polar solvents.¹ Amphiphiles with advanced functions that go far beyond the traditional ones as emulsifiers, stabilizing agents for interfaces, or detergents were meanwhile realized by skillful manipulation of any of its constituents.^2–4 Recent examples are micellar LEDs,^5,6 catalysts,^7–9 or batteries.¹⁰ Such applications are important hallmarks on the way to even more sophisticated amphiphiles such as the ones found in nature, e.g. in the pockets of enzymes.^11–18 An important milestone is the advent of (multi-) stimuli-responsive amphiphiles, whose encoded functionalities respond to (different) external triggers. Such systems are capable of adaptive self-assembly, which can be controlled using an external input such as the pH, temperature, ionic strength, or redox state.^19–26Paramagnetic amphiphiles, recently reviewed by Eastoe and coworkers, constitute a fascinating family of stimuli-responsive surfactants.²⁷ Particular attention has been paid to magnetic ionic liquids based on amphiphilic transition metal complexes, as their properties are often superior to those of conventional magnetic fluids (ferrofluids).^28–31 Self-assembly results in high effective concentrations of the paramagnetic metal centers, and this in turn allows us to control their physico-chemical properties and the morphologies of their superstructures through an external magnetic field. Such a scheme has the added advantage that the external stimulus is non-invasive. In many current realizations of such systems, however, the magneto-active (transition) metal ion is only present as a constituent of the counterion of a cationic surfactant, but is not an integral constituent of the surfactant itself.^21,30,31Some of us have previously reported redox-switchable as well as paramagnetic stimuli-responsive amphiphiles of relevance to the current work.^32,33 We thought that ferrocene would be an ideal building block in order to combine both these kinds of stimuli within one single amphiphile.^34–37 On oxidation, the diamagnetic, hydrophobic ferrocene nucleus is transformed into a paramagnetic S = 1/2 ferrocenium ion with a distinct hydrophilic character.^38–41 Oxidation does hence not only generate a magnetic moment, but also transfers the ferrocene nucleus from the lipo- to the hydrophilic part of the amphiphile, thereby changing its entire structure. A 1,1′-disubstitution pattern of the ferrocene scaffold, which is synthetically well accessible,^34,42–44 seemed particularly suited for such an endeavor.Studies on paramagnetic amphiphiles are often thwarted by the non-trivial analytics involved in their characterization. Detailed investigations often rely on small-angle neutron scattering (SANS), which is time-consuming and costly and suffers from poor availability.^{27,30,31,45–47} Moreover, SANS is only of limited value for following kinetically fast processes which would be desirable for the live monitoring of structural changes occurring in solution. Optical birefringence is a well-established method to monitor the dynamic response of materials to external fields.^48–50 Although of high intrinsic value, optical birefringence measurements in magnetic fields were only rarely applied for the study of paramagnetic amphiphiles.²⁹We here report the zwitterionic, ferrocene-based amphiphile FcNMe₂SO₃Heptene 6 (see Fig. 1, Fc = ferrocenyl) with a sultone headgroup. Compound 6 is unique in that its self-assembly properties can be controlled by three different external stimuli, namely the (i) addition of an electrolyte, (ii) addition of an oxidant/reductant, and (iii) exposure to an external magnetic field. We also demonstrate that optical birefringence in combination with dynamic light scattering (DLS) measurements in two orthogonal directions provides detailed insights into the functional response of aggregated magnetic nanoparticles formed by 6⁺ to an external magnetic field in real time. Specifically, we have observed the formation of string-of-pearls-like aggregates of 6⁺ in a magnetic field (0.8 T), the field-induced anisotropy of the diffusion of aggregated nanoparticles, and a hysteresis effect for their disappearance after the magnetic field is switched off. Thus, the anisotropy of larger aggregates persists for more than 5 min, while the structural alignment of smaller ones vanishes at a significantly faster rate.Open in a separate windowFig. 1Synthesis of FcNMe₂SO₃Heptene (6). (a) Synthesis of 6; (b) molecular structure of 6 crystallized from acetonitrile. C; dark grey, N; turquoise, Fe; orange, S; yellow, O; red, H atoms are omitted for clarity.     

A cofacial metal–organic framework based photocathode for carbon dioxide reduction

Innovative and robust photosensitisation materials play a cardinal role in advancing the combined effort towards efficient solar energy harvesting. Here, we demonstrate the photocathode functionality of a Metal–Organic Framework (MOF) featuring cofacial pairs of photo- and electro-active 1,4,5,8-naphthalenediimide (NDI) ligands, which was successfully applied to markedly reduce the overpotential required for CO₂ reduction to CO by a well-known rhenium molecular electrocatalyst. Reduction of [Cd(DPNDI)(TDC)]_n (DPNDI = N,N′-di(4-pyridyl)-1,4,5,8-naphthalenediimide, H₂TDC = thiophene-2,5-dicarboxylic acid) to its mixed-valence state induces through-space Intervalence Charge Transfer (IVCT) within cofacial DPNDI units. Irradiation of the mixed-valence MOF in the visible region generates a DPNDI photoexcited radical monoanion state, which is stabilised as a persistent species by the inherent IVCT interactions and has been rationalised using Density Functional Theory (DFT). This photoexcited radical monoanion state was able to undergo charge transfer (CT) reduction of the rhenium molecular electrocatalyst to effect CO generation at a lower overpotential than that required by the discrete electrocatalyst itself. The exploitation of cofacial MOFs opens new directions for the design philosophy behind light harvesting materials.

The photocathode functionality of a Metal–Organic Framework (MOF) featuring cofacial photo- and electro-active ligands provides a new approach to CO₂ reduction via charge transfer with a rhenium electrocatalyst.

The development of photocathode materials for CO₂ reduction and hydrogen evolution catalyses has traditionally focussed on photosensitising transition metal complexes or nanostructured solid state semiconductors.^1,2 At the nascent frontier between robust solid state semiconductors and synthetically protean metal complexes are photo-/electro-active Metal–Organic Frameworks (MOFs) that consolidate the flexibility of homogeneous systems into the robust heterogeneous phase.³ Contrasting with reported MOF examples, natural photosynthesis remains one of the most efficient light harvesting systems.⁴ One common reaction centre adopted in photosynthesis features a redox-active cofacial dimer of chlorophyll pigment molecules.⁵ This cofacial moiety stabilises the photoexcited charge separated state through intra-dimer Intervalence Charge Transfer (IVCT) interactions, enabling the trapping and conversion of light to chemical energy. Recently, we characterised IVCT interactions upon reduction to the mixed-valence state in the MOF [Zn₂(TDC)₂(DPPTzTz)₂]_n (DPPTzTz = 2,5-bis(4-(4-pyridyl)phenyl)thiazolo[5,4-d]thiazole and H₂TDC = thiophene-2,5-dicarboxylic acid) featuring cofacial dimers of the thiazolothiazole redox-active core, and probed its structure–activity dependence computationally and experimentally.^6–9 Subsequently, we sought design a new MOF featuring cofacial pairs of the photo- and redox-active N,N′-di(4-pyridyl)-1,4,5,8-naphthalenediimide (DPNDI) ligand, as a conceptually neoteric photosensitiser for incorporation into systems relevant towards artificial photosynthesis.The naphthalene diimide (NDI) core was selected for its photoactive radical monoanion state.¹⁰ For a number of discrete systems, Wasielewski and coworkers have computationally and experimentally demonstrated the ability to photoexcite the easily accessible NDI radical monoanion using visible light, facilitating its transient photoelectrochemical reduction of Re based catalytic CO₂ reduction sites.^2,11–14 Recently, Goswami et al. synthesised a Zr NDI-based MOF, applying this as a radical state heterogeneous photosensitiser to decompose dichloromethane.¹⁵Here, we describe the synthesis of a new photo- and redox-active MOF [Cd(DPNDI)(TDC)]_n, denoted csiMOF-6 (cofacial stacked IVCT), featuring cofacial dimers of the DPNDI ligand. Cofacial DPNDI MOFs have been reported previously by Takashima et al.¹⁶ and Sikdar et al.,¹⁷ where guest dependent charge transfer (CT) and neutral state photoexcitation behaviours were examined. Dinolfo et al. also incorporated DPNDI into a rhenium based cofacial complex, where its mixed-valence IVCT behaviour was probed using electrochemical and spectroelectrochemical (SEC) techniques.¹⁸ We envisaged that the cofacial NDI units in csiMOF-6 would stabilise its photoexcited radical monoanion state by IVCT interactions, akin to cofacial moieties in natural photosynthsesis processes. This strengthens the persistence of the NDI photoexcited radical monoanion state, thereby improving its efficacy at photoelectrochemical reduction of catalytically active sites. Effectiveness of the cofacial design principle behind csiMOF-6 photocathodes was verified using a combined experimental and computational approach. The successful photocathode performance of csiMOF-6 under broad band visible light irradiation encompassed its photoelectrochemical reduction of the [Re(bipy-tBu)(CO)₃Cl] (bipy-tBu = 4,4′-di-tert-butyl-2,2′-bipyridine, developed by Smieja et al.¹⁹) CO₂ reduction electrocatalyst, resulting in CO generation at reduced overpotential requirements.     

The sporothriolides. A new biosynthetic family of fungal secondary metabolites

The biosynthetic gene cluster of the antifungal metabolite sporothriolide 1 was identified from three producing ascomycetes: Hypomontagnella monticulosa MUCL 54604, H. spongiphila CLL 205 and H. submonticulosa DAOMC 242471. A transformation protocol was established, and genes encoding a fatty acid synthase subunit and a citrate synthase were simultaneously knocked out which led to loss of sporothriolide and sporochartine production. In vitro reactions showed that the sporochartines are derived from non-enzymatic Diels–Alder cycloaddition of 1 and trienylfuranol A 7 during the fermentation and extraction process. Heterologous expression of the spo genes in Aspergillus oryzae then led to the production of intermediates and shunts and delineation of a new fungal biosynthetic pathway originating in fatty acid biosynthesis. Finally, a hydrolase was revealed by in vitro studies likely contributing towards self-resistance of the producer organism.

A new family of fungal biosynthetic pathways is elucidated based on the use of fatty acid and citrate-like intermediates.

Gamma-lactone and alkyl citrate compounds derived from oxaloacetate are widespread natural products in fungi and often possess potent biological activities. Examples include sporothriolide 1,^1,2 piliformic acid 2,³ tyromycin 3⁴ and the cyclic maleidrides including byssochlamic acid 4^5,6 among others (Fig. 1). In some cases, for example those of 4 and squalestatin S1 5,⁷ detailed molecular studies have revealed that a dedicated polyketide synthase (PKS) produces a carbon skeleton that is then condensed with oxaloacetate by a citrate synthase (CS) to give an early alkyl citrate intermediate that is further oxidatively processed. In other cases, such as 1 and the sporochartines 6, the biosynthetic pathways are not yet clear.Open in a separate windowFig. 1Structures of γ-lactone and alkyl citrate metabolites from fungi. Bold bonds show oxaloacetate-derived carbons where known.Sporochartines 6a–6d^8,9 from the fungus Hypoxylon monticulosum CLL 205 (now referred to as Hypomontagnella spongiphila)¹⁰ possesses potent cytotoxicity (IC₅₀: 7.2 to 21.5 μM) vs. human cancer cell lines and are proposed to be Diels Alder (DA) adducts of the furofurandione sporothriolide 1, itself a potent antifungal agent (EC₅₀: 11.6 ± 0.8 μM),¹¹ and trienylfuranol A 7,¹² originally obtained from an endophytic fungus Hypoxylon submonticulosum DAOMC 242471 (now referred to as Hypomontagnella submonticulosa).¹³ Since the biosynthesis of sporothriolide 1 and related compounds is unknown, and biological DA reactions in fungi are currently of high interest,¹⁴ we decided to examine the biosynthesis of the sporochartines 6 in the Hypomontagnella spp. strains MUCL 54604 and CLL 205 (ref. 10 and 13) in detail.     

Nanoscale electrochemistry in a copper/aqueous/oil three-phase system: surface structure–activity-corrosion potential relationships

Practically important metal electrodes are usually polycrystalline, comprising surface grains of many different crystallographic orientations, as well as grain boundaries. In this study, scanning electrochemical cell microscopy (SECCM) is applied in tandem with co-located electron backscattered diffraction (EBSD) to give a holistic view of the relationship between the surface structure and the electrochemical activity and corrosion susceptibility of polycrystalline Cu. An unusual aqueous nanodroplet/oil (dodecane)/metal three-phase configuration is employed, which opens up new prospects for fundamental studies of multiphase electrochemical systems, and mimics the environment of corrosion in certain industrial and automotive applications. In this configuration, the nanodroplet formed at the end of the SECCM probe (nanopipette) is surrounded by dodecane, which acts as a reservoir for oil-soluble species (e.g., O₂) and can give rise to enhanced flux(es) across the immiscible liquid–liquid interface, as shown by finite element method (FEM) simulations. This unique three-phase configuration is used to fingerprint nanoscale corrosion in a nanodroplet cell, and to analyse the interrelationship between the Cu oxidation, Cu²⁺ deposition and oxygen reduction reaction (ORR) processes, together with nanoscale open circuit (corrosion) potential, in a grain-by-grain manner. Complex patterns of surface reactivity highlight the important role of grains of high-index orientation and microscopic surface defects (e.g., microscratches) in modulating the corrosion-properties of polycrystalline Cu. This work provides a roadmap for in-depth surface structure–function studies in (electro)materials science and highlights how small variations in surface structure (e.g., crystallographic orientation) can give rise to large differences in nanoscale reactivity.

Probing Cu corrosion in an aqueous nanodroplet/oil/metal three-phase environment revealed unique patterns of surface reactivity. The electrochemistry of high-index facets cannot be predicted simply from the low-index {001}, {011} and {111} responses.

Corrosion has long been studied, as a significant concern and a costly issue (ca. 3% of the GDP of industrialised countries) for the modern world.^1,2 For metals, in particular, electrochemical techniques, allied to complementary analytical and microscopy methods, play a central role in unveiling corrosion and corrosion protection mechanisms.^3–7 However, a limitation of many experimental approaches is that the electrochemical perturbation (and measurement) is applied globally at a macroscopic electrode immersed in a bulk solution,⁸ but most corrosion processes are initiated and perpetuated at (sub)microscopic surface sites (e.g., grain boundaries, inclusions, microscratches etc.).^9–14 Mismatch between the scale of key corrosion phenomena and conventional electrochemical methods makes it difficult to unambiguously identify the key anodic/cathodic sites driving corrosion. This issue is compounded for the case of atmospheric corrosion,¹⁵ or corrosion in certain automotive/industrial environments (vide infra),^16,17 which take place due to the action of small droplets on the surface in a confined system. Corrosion science needs electrochemical techniques that operate at the (sub)microscale, and allow activity and surface structure to be correlated commensurately at this scale.Among the limited library of electrochemical techniques that can routinely operate at the (sub)microscale,^18,19 scanning electrochemical cell microscopy (SECCM) is attracting significant attention.^20–22 SECCM maps electrochemistry locally and directly via a nanoscale electrochemical meniscus cell (formed at the end of a fluidic probe) that makes measurements over an array of points (typically thousands of discrete areas) on an electrode (or other) surface. For polycrystalline surfaces, SECCM measurements are powerfully combined with co-located electron backscattered diffraction (EBSD), to elucidate nanoscale structure–activity, as exemplified by studies of various electrochemical processes at a range of polycrystalline materials, including Pt,^23–26 Au,²⁷ Pd,²⁸ low carbon steel,^29–31 Zn³² and boron-doped diamond.³³In addition to its high spatiotemporal resolution, the meniscus cell configuration of SECCM facilitates rapid reactant/product exchange with the surrounding environment, mimicking a gas diffusion electrode, with an enhanced flux of gases into the meniscus cell (i.e., at the three-phase boundary).^24,27,34 When operated in air, SECCM emulates the configuration of atmospheric corrosion, with gas exchange (e.g., oxygen, O₂) taking place across the liquid/gas interface of the meniscus in contact with a surface of interest. As recently reported, and expanded upon herein, SECCM can also be operated under oil immersion,^32,35 which not only aids in confinement of the meniscus cell during prolonged measurements,³⁵ but also opens up the possibility of studying the effect of oil-soluble species (e.g., corrosion inhibitors, organic contaminants, redox mediators etc.) on local reactions at the solid/liquid/liquid interface with high spatial-resolution. This configuration is regaining interest for fundamental studies,^36,37 as well as being of great practical importance (e.g., phase-transfer reactions in industrial chemical processes, biology etc.).³⁸A key attribute of SECCM is that a number of conventional dynamic electrochemistry techniques (e.g., potentiometry, amperometry and voltammetry) can be translated readily to the confines of the meniscus cell.^20,22,39 Herein, the versatility of chronopotentiometry for local corrosion and electrochemical measurements is demonstrated. First, it is possible to make meniscus contact at zero applied current, corresponding to open circuit potential (OCP), which is measured. This corresponds to the corrosion (mixed) potential, where the rate of anodic dissolution of the metal (forming metal ions) and the rate of reduction of oxygen are balanced. Surface ion release under this condition is then analysed by subsequent “electrochemical titration” of a portion of the released metal ions, by applying a cathodic current and recording the resulting chronopotentiometric curve.⁴⁰ This allows the evaluation of intrinsic corrosion susceptibility, in situ, with high spatial-resolution, for the entire range of crystallographic orientations of a polycrystalline metal (i.e., revealed through co-located EBSD analysis). Chronopotentiometry measurements with and without O₂ present, and the use of an anodic pulse to induce the anodic dissolution (as well as the cathodic measurements mentioned) allow all of the key electrochemical processes underpinning localised corrosion to be studied. The patterns of surface reactivity establish the intimate link between corrosion susceptibility, electrochemical kinetics and surface structure at the nanoscale.     

Serine is the molecular source of the NH(CH2)2 bridgehead moiety of the in vitro assembled [FeFe] hydrogenase H-cluster

The active site of [FeFe] hydrogenase, the H-cluster, consists of a canonical [4Fe–4S]_H subcluster linked to a unique binuclear [2Fe]_H subcluster containing three CO, two CN⁻ and a bridging azadithiolate (adt, NH(CH₂S⁻)₂) ligand. While it is known that all five diatomic ligands are derived from tyrosine, there has been little knowledge as to the formation and installation of the adt ligand. Here, by using a combination of a cell-free in vitro maturation approach with pulse electronic paramagnetic resonance spectroscopy, we discover that serine donates the nitrogen atom and the CH₂ group to the assembly of the adt ligand. More specifically, both CH₂ groups in adt are sourced from the C3 methylene of serine.

The CH₂NHCH₂ bridgehead moiety of the [FeFe] hydrogenase H-cluster is derived from serine as revealed by isotope labeling and EPR spectroscopy.

Hydrogenases catalyze the reversible reactions of H₂ oxidation and proton reduction, and are involved in many microbial metabolic pathways.¹ [FeFe] hydrogenases in particular are hyper-efficient, with turnover rates up to 10⁴/s.² This has led to intense focus on [FeFe] hydrogenases for sustainable production of H₂ and the design of fuel cells.³ The active site of [FeFe] hydrogenases is a six-iron cofactor called the H-cluster (Scheme 1), which consists of a canonical cuboid [4Fe–4S]_H subcluster linked through a bridging cysteine (Cys) residue to a binuclear [2Fe]_H subcluster in which the two iron ions are coordinated by three CO, two CN⁻ and an azadithiolate (adt, NH(CH₂S⁻)₂) bridging ligand. The [2Fe]_H subcluster has been proposed to be the site for H₂ binding and hydride formation,^4,5 which serves as a natural blueprint for designing small molecule catalysts for hydrogen evolution reactions.⁶ The unique structure and catalytic activity has thus raised much interest in the biosynthesis of the H-cluster, which poses a great challenge in cofactor assembly that involves toxic ligands, oxygen sensitivity and an organic adt ligand that has little inherent stability.Open in a separate windowScheme 1Bioassembly of the H-cluster highlighting the source of each moiety.While the [4Fe–4S]_H subcluster in the H-cluster can be formed by the housekeeping gene products that are used to assemble such standard Fe–S clusters, the in vivo bioassembly of the unique [2Fe]_H subcluster requires three special Fe–S “maturase” proteins: HydE, HydF, and HydG.^7,8 Although the functions of HydE and HydF have not been fully elucidated,^9–12 recent studies indicate that HydG is a bifunctional 4Fe–4S radical S-adenosyl-l-methionine (SAM) enzyme which lyses tyrosine to generate CO and CN⁻ and forms a [(Cys)Fe(CO)₂(CN)] organometallic precursor to the H-cluster on a dangler Fe(Cys) site in HydG.^13–16 More recently, by using a synthetic [(Cys)Fe(CO)₂(CN)] carrier we have shown that the two sulfur atoms in the adt ligand are derived from the precursor-bound Cys, but that the CH₂NHCH₂ component is not.¹⁷ Taken together, the biosynthetic origins of the [Fe₂S₂(CO)₃(CN)₂] part of the [2Fe]_H subcluster are depicted in Scheme 1: all five diatomic ligands are tailored from tyrosine by HydG;¹⁸ the two sulfur atoms and the two Fe atoms are from the dangler Fe(Cys) site in HydG (which can be reconstituted with Fe²⁺ and free Cys in solution¹⁹). Remarkably, these components are all delivered to the binuclear cluster assembly in the form of the [(Cys)Fe(CO)₂(CN)] product of HydG. Given these recent advances, the only missing part of the puzzle is the crucial NH(CH₂)₂ moiety: what are its molecular precursors? It has been hypothesized that HydE, which is also a 4Fe–4S radical SAM enzyme, may be involved in the formation of adt, though its physiological substrate and reaction mechanism remains under investigation.^9,10 As for any enzymatic reaction, knowing the actual substrate(s) for the reaction is crucial for unraveling the ultimate mechanism. Therefore, determining of molecular sourcing of the CH₂NHCH₂ component of the adt bridge, currently unknown, is the focus of this work.Assembly of the H-cluster in the lab can be achieved by semi-synthetic and biochemical approaches other than directly co-expressing hydA, hydE, hydF and hydG genes in cells. One very useful method alleviates the need for HydG, HydE, and in some cases, HydF, by using a synthetic [Fe₂(adt)(CO)₄(CN)₂] complex as a direct donor to the [2Fe]_H subcluster assembly.^20–22 Another “cell free synthesis” approach uses HydE/F/G in an in vitro H-cluster maturation reaction developed by the Swartz group.^23,24 The specific in vitro maturation reaction used in our current investigation contains a mixture of E. coli cell lysate containing separately overexpressed HydE, HydF, HydG (all from Shewanella oneidensis), apo-HydA1 (from Chlamydomonas reinhardtii) that harbors the [4Fe-4S]_H subcluster, and a cocktail of low molecular weight cofactors and precursors.²³ This biochemical approach gives us the opportunity to use the same set of enzymes that build the H-cluster in cells, but also enables us to determine the molecular source of each of the components in the H-cluster by using isotope-labeled cofactors/precursors, a procedure that would be very difficult to carry out and fully control in vivo. For example, by supplementing 1-¹³C-Tyr or 2-¹³C-Tyr into the in vitro maturation reaction, the CO or CN⁻ ligands to the diiron subcluster of the maturated HydA1 are respectively labeled with ¹³C.^25,26 The presence of these ¹³C labels can then in turn be detected and analyzed by using advanced electron paramagnetic resonance (EPR) spectroscopy to measure the hyperfine couplings between the magnetic ¹³C nuclei and the unpaired electron spin distributed over the H-cluster in its redox-poised paramagnetic states. In this work, we now search for the source(s) of the CH₂NHCH₂ moiety by using a similar strategy of in vitro maturation coupled to high resolution EPR to screen the assembly products formed with various isotopically labeled small molecule candidates. The presence of nitrogen element in the CH₂NHCH₂ fragment suggests an amino acid origin as one possibility. A systematic screening by pulse EPR of the in vitro maturation products generated with ¹³C, ¹⁵N, and ²H-labeled amino acids reveals that serine (Ser) serves as a molecular source for the NH(CH₂)₂ moiety of the H-cluster.     

Two helices from one chiral centre – self organization of disc shaped chiral nanoparticles

Gold nanoparticles (AuNPs) have been prepared and surface-functionalized with a mixture of 1-hexanethiol co-ligands and chiral discogen ligands separated from a disulfide function via a flexible spacer. Polarized optical microscopy together with differential scanning calorimetry showed that the organic corona of the nanocomposite forms a stable chiral discotic nematic phase with a wide thermal range. Synchrotron X-ray diffraction showed that gold NPs form a superlattice with p2 plane symmetry. Analysis indicated that the organic corona takes up the shape of a flexible macrodisk. Synchrotron radiation-based circular dichroism signals of thin films are significantly enhanced on the isotropic-LC transition, in line with the formation of a chiral nematic phase of the organic corona. At lower temperatures the appearance of CD signals at longer wavelengths is associated with the chiral organisation of the NPs and is indicative of the formation of a second helical structure. The decreased volume required and the chiral environment of the disc ligands drives the nanoparticles into columns that arrange helically, parallel to the shortest axis of the two dimensional lattice.

Gold nanoparticles (NPs) were surface functionalized with hexanethiol groups and chiral nematic discogen ligands. A superlattice, liquid crystal behaviour and helix formation of the discs and a second helical organisation of the NPs were detected.

The exploration of chiral nanomaterials is a rapidly evolving fascinating field with uses ranging from catalysis, chiral molecular sensing, stereoselective separations to super-resolution imaging.^1–3 Bottom-up approach design strategies for nanoparticles (NPs) or nanorods (NRs) as composites in liquid crystal (LC) matrices have been developed, making use of progress in synthesis strategies and characterization techniques.^4–6 For optical applications symmetry breaking by introducing chirality into the systems at the nanoscale is critical.⁷ So far, efforts to obtain chiroptical NP-LC systems have relied mainly on NPs doped in LC hosts: either chiral nematic templates disperse NPs into chiral assemblies⁸ or the NPs functionalized with chiral ligands are included in achiral nematic LCs;⁹ both strategies aim to transfer and amplify chiral effects.^10,11 Beyond the scientific challenge of generating nanocomposites where the interplay of bottom-up structuring enables plasmonic interactions of the NPs, leading to novel, so called metamaterials properties, there are new uses ranging from novel display devices,¹² to more advanced optical communications using spin information between LEDs¹³ and in quantum teleportation for optical computing.¹⁴Precise size control of NPs in LC matrices in combined systems has been found critical to avoid phase separation or precipitation of NPs.¹⁵ For intrinsic liquid crystalline NP-LC systems, targeting of the LC phase range to be close to ambient is of utmost importance, as delamination of ligands occurs at elevated temperatures, even for very stable thiol functions. Degradation tends to occur above 100 °C.¹⁶ Inclusion of chiral groups in the organic corona can lead to chiral LC phase behavior, but there is little evidence that the NPs experience a chiral distortion in their packing; experimental evidence indicates that the nanocomposites minimize energy by packing in non-chiral superlattices and chirality is confined to the corona.¹⁷Here we show that a combination of small NPs with a carefully designed chiral organic coating results in chiral assembly behaviour of both the NPs and the organic corona. Our approach is strictly modular. The surface of the gold NPs is covered and passivated by hexylthiol groups and chiral lipoic acid derivates bearing via a spacer the mesogenic groups. We note that lipoic acid, a naturally occurring chiral antioxidant explored elsewhere for the treatment of several diseases, is bound with a dithiolate bridge via two atoms to the Au surface (see Chart 1).¹⁸ This places the chiral moiety rigidly in position onto the NPs, making thus a chiral surface.¹⁹ A pentaalkynylbenzene (PA) derivative was selected as mesogenic group, as this structural motif is known to promote reliably discotic nematic phase (N_D) behaviour.²⁰ The much larger volume of the PA group, when compared to typical rod shaped mesogens, whilst also delivering LC behaviour close to ambient, is anticipated to impact more strongly on the spatial organization of neighbouring NPs when compared to calamitic functionalized NP systems.¹⁷ The formation of helically ordered superstructures is studied by a combination of dedicated thin film optical polarizing microscopy, X-ray diffraction and synchrotron radiation-based circular dichroism (SRCD). A structural model is proposed based on a chiral nematic phase made up of AuNPs coated with chiral shells forming flexible macrodisks which in turn assemble in a superlattice.^21–23Open in a separate windowChart 1Sketch of the functionalized gold nanoparticle system AuDLC*.The design, synthesis and chemical characterization of the chiral discogen is described in the ESI.^† The precursor disk-like mesogen which forms exclusively a N_D phase above 100 °C consists of six aromatic rings flanked by flexible pentyloxy chains and an undecyloxy chain bearing a terminal hydroxy group (Fig. S1^†). The attachment of the flexible (R)-(+)-1,2-dithiolane-3-pentanoic tail was predicted to promote the desired phase. The new target discogen was obtained in a high yield (93.1%) reaction. For the synthesis and purification of LC-functionalized AuNPs, denoted as AuDLC*, a synthetic pathway was developed involving first the preparing and purifying of the alkylthiol coated NPs to which LC groups are linked, using an exchange reaction (for details see the Scheme 1 in ESI^†).^24,25The thermal behaviour of the chiral discogen was examined by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The target discogen shows clearly a chiral nematic phase with the characteristic bundles of oily streaks (Fig. S2^†). On heating (Fig. S3^†), phase formation was observed at 49.0 °C from the crystalline state, with the material turning into an isotropic liquid at 95.7 °C (ΔH = 0.20 J g⁻¹ or 0.28 kJ mol⁻¹). Formation of the mesophase from the isotropic state on cooling started from 94.5 °C with an enthalpy change of 0.12 J g⁻¹ (0.17 kJ mol⁻¹), followed by a slow crystallisation occurring at 5.1 °C. In brief, the chiral phase behaviour of the target ligand is exclusively chiral nematic, enantiotropic and with a wide thermal range of up to 89.4 °C.The ¹H-NMR spectra of the investigated AuDLC* nanocomposite and the comparison with the spectra of the monomer ligand (Fig. S4^†) indicate that the chiral discogen with the disulfide group are chemically attached onto the surface of AuNPs with high efficiency. A typical feature for such NPs are the broadened ¹H-NMR spectra caused by the reduced mobility of alkyl chains of the ligands when anchored to NPs and additionally the sharp peaks present in the free ligand have disappeared, typical for such systems.²⁶ The absence of mobile free discogen using thin layer chromatography (silica, with dichloromethane as mobile phase) confirms this result. The amount of chiral discogen bounded onto the gold surface relative to the number of 1-hexanethiol groups was determined from the ¹H-NMR spectra of AuDLC*. The ratio of hydrocarbons to discogens was found to be 2 : 3. Transmission Electron Microscopy (TEM) investigations confirm that this sequential synthesis affords nanoparticles of low polydispersity (Fig. 1a); the average particle size is ∼2.5 ± 0.5 nm. Thermo-gravimetric analysis (TGA) was performed (Fig. S6^†) and gives the weight fraction of gold in AuDLC* to be 47.8% (wt/wt). The mesogen density per Au NP was calculated (ESI^† Part 7) by combining the ¹H-NMR results (Fig. S4^†) with the model developed by Gelbart et al.²⁷ On the basis of these data, we obtain that there are on average 211 Au atoms per particle, and a total number of 53 ligands per particle, of which 32 are chiral discogens and 21 are hexylthiol groups (Table S1^†). This is in accord with a ligand density of up to ∼6.3 ligands per nm².²⁸ Additionally, the characteristic peaks in the UV-vis spectra of AuDLC* arise from the combination of gold (∼500 nm) and LC ligands (absorption maxima at 238, 261 and 351 nm with two shoulder peaks at 378 and 421 nm) (Fig. S7^†).Open in a separate windowFig. 1(a) TEM image of 2.5 nm AuDLC* nanocomposite (inset: the size distribution of AuDLC*). POM micrographs of AuDLC* (b) 86.5 °C (90° crossed polarizer) (×100 μm). (c) 37.8 °C (90° crossed polarizer) (×100 μm). (d) DSC of AuDLC* at heating and cooling rate of 10.0 °C min⁻¹ (e) SAXS diffractograms of AuDLC* on heating from 30 °C to 140 °C. (f) SRCD spectra of sheared AuDLC*. Recorded in 5 °C steps on cooling from 100 °C to 30 °C. The enlarged plots in dotted regions A–C are shown in Fig. S12b–d.^† The CD intensities at a selected wavelengths in each region (A, B and C) are plotted as a function of temperature in Fig. S13^† and these plots clearly show the difference in the formation process between the two helical structures.The mesophase behaviour of the AuDLC* nanocomposite was first characterized by POM observation. As shown in Fig. 1b and c, upon cooling from the isotropic liquid, the colour in the POM texture changes from light green to yellow and we associate this with the helical pitch in AuDLC* increasing with decreasing temperature. In some regions of the POM slides, a chiral nematic phase with the typical Grandjean texture (Fig. S8a and b^†) is found. The LC textures in AuDLC* do recover after gently pressing the microscope coverslip indicating that the LC-decorated NPs form a stable phase and birefringent textures remain stable at ambient (Fig. S9^†). DSC investigations (Fig. 1d) confirm that the AuDLC* nanocomposite shows a thermodynamically stable (enantiotropic) mesophase. In DSC experiments on cooling a small peak associated with the isotropic to phase transition (92.2 °C) can be detected, and at a low temperature (3.8 °C), the glass transition occurs. The transition peak for the transition is wider than in the pure chiral discogen, typical for such systems.⁴ This is associated with the reduced mobility of the LC groups attached to the NPs. The thermal transitions determined by DSC for the pure chiral discogen and AuDLC* are collected in Table 1, showing the low values typical for transitions.²⁰ In contrast to the free chiral ligand, the NPs coated with the chiral ligand and hexylthiol groups show a small decrease in transition temperatures. This could be attributed to both the plastifying effects of the hexylthiol co-ligands and packing constraints of the LC groups due to the attachment to the NPs. This view is supported by the DSC data for the transition at 0.043 J g⁻¹ (0.12 kJ mol⁻¹, considering the evaluated 32 chiral discogen ligands per particle).Transition temperatures (°C) of free chiral discogen and AuDLC*, as determined by DSC (second cooling at rate of 10.0 °C min⁻¹)^a

Hyperpolarisation of weakly binding N-heterocycles using signal amplification by reversible exchange

Signal Amplification by Reversible Exchange (SABRE) is a catalytic method for improving the detection of molecules by magnetic resonance spectroscopy. It achieves this by simultaneously binding the target substrate (sub) and para-hydrogen to a metal centre. To date, sterically large substrates are relatively inaccessible to SABRE due to their weak binding leading to catalyst destabilisation. We overcome this problem here through a simple co-ligand strategy that allows the hyperpolarisation of a range of weakly binding and sterically encumbered N-heterocycles. The resulting ¹H NMR signal size is increased by up to 1400 times relative to their more usual Boltzmann controlled levels at 400 MHz. Hence, a significant reduction in scan time is achieved. The SABRE catalyst in these systems takes the form [IrX(H)₂(NHC)(sulfoxide)(sub)] where X = Cl, Br or I. These complexes are shown to undergo very rapid ligand exchange and lower temperatures dramatically improve the efficiency of these SABRE catalysts.

The scope of the hyperpolarisation method Signal Amplification by Reversible Exchange (SABRE) is dramatically expanded through the use of co-ligands to substrates that weakly interact with the active cataylst.

Hyperpolarised magnetic resonance is receiving increasing attention from both the analytical science and medical communities due to its ability to create signals that are many orders of magnitude higher than those normally detected under Boltzmann control.^1–6 The time and cost benefits associated with this improvement have propelled this area of research forward over the past few decades. Two of the most prominent techniques used to create hyperpolarisation are dissolution Dynamic Nuclear Polarisation (d-DNP) and Para-Hydrogen Induced Polarisation (PHIP),^7,8 which derive their non-Boltzmann spin energy level populations from interactions with unpaired electrons and para-hydrogen (p-H₂, the singlet spin isomer of hydrogen), respectively. Both of these methods have been reviewed in detail.^3–5,9,10Signal Amplification by Reversible Exchange (SABRE) is a PHIP method that does not involve the chemical incorporation of p-H₂ into the target substrate.^11,12 Instead, under SABRE, spin order transfer proceeds catalytically through the temporary formation of a scalar coupling network between p-H₂ derived hydride ligands and the substrate''s nuclei whilst they are located in a transient metal complex. The most common catalysts are of the type [Ir(H)₂(NHC)(sub)₃]Cl (where NHC = N-heterocyclic carbene and sub = the substrate of interest, Fig. 1a),^13,14 although other variants are known.^15–17 For SABRE to be accomplished, the target substrate must be able to reversibly ligate to the metal centre and this limits the methods applicability; although several routes to overcome this have been reported.^18–20 Recently, the use of bidentate ancillary ligands such as NHC-phenolates¹⁶ and phosphine-oxazoles²¹ has been shown to expand the applicability of SABRE for a variety of different ligands and solvents (Fig. 1b). For example, use of the PHOX ligand (PHOX = (2-diphenylphosphanyl)phenyl-4,5-dihydrooxazole) gives ¹H NMR signal gains of up to 132-fold for 2-picoline; a substrate previously shown to be unpolarised under classic SABRE conditions.²²Open in a separate windowFig. 1Development of the SABRE method for hyperpolarisation of a range of substrates.The use of co-ligands to stabilise the active SABRE catalyst has proven successful for substrates that weakly associate to the catalyst (Fig. 1c). Of particular note is the hyperpolarisation of sodium [1,2]-¹³C₂-pyruvate²³ and sodium ¹³C-acetate²⁴ which could be used as in vivo metabolic probes. The importance of co-ligands in breaking the chemical symmetry of the SABRE catalyst is also well established and co-ligands such as acetonitrile,²⁵ sulfoxides,^23,26 1-methyl-1,2,3-triazole²⁷ and substrate isotopologues²⁸ have been employed.We report here on the use of co-ligands to allow the NMR hyperpolarisation of weakly binding N-heterocyclic derived substrates with functionality in the ortho-position that have proven to be routinely inaccessible to the SABRE technique (Fig. 1d). ¹H signal gains of up to 1442 ± 84-fold were obtained for some of these substituted pyridines at 9.4 T and the expansion of this approach to ¹³C and ¹⁵N detection and other N-heterocyclic motifs is also exemplified.     

Multiscale modeling of molecular structure and optical properties of complex supramolecular aggregates

Supramolecular aggregates of synthetic dye molecules offer great perspectives to prepare biomimetic functional materials for light-harvesting and energy transport. The design is complicated by the fact that structure–property relationships are hard to establish, because the molecular packing results from a delicate balance of interactions and the excitonic properties that dictate the optics and excited state dynamics, in turn sensitively depend on this packing. Here we show how an iterative multiscale approach combining molecular dynamics and quantum mechanical exciton modeling can be used to obtain accurate insight into the packing of thousands of cyanine dye molecules in a complex double-walled tubular aggregate in close interaction with its solvent environment. Our approach allows us to answer open questions not only on the structure of these prototypical aggregates, but also about their molecular-scale structural and energetic heterogeneity, as well as on the microscopic origin of their photophysical properties. This opens the route to accurate predictions of energy transport and other functional properties.

Multiscale modeling resolves the molecular structure of a synthetic light-harvesting complex, unraveling the microscopic origin of its photophysical properties.

Supramolecular structures may self-assemble from a variety of building blocks, resulting in a wide range of advanced materials with attractive biomimetic, sensing, catalytic, optoelectronic and photonic functionalities.^1–10 The close-packed nanoscale organization of the individual molecules within a supramolecular system, held together via noncovalent interactions, gives rise to the aggregate''s (collective) properties. Assemblies consisting of dye molecules often exhibit unique collective optical properties and are of interest for opto-electronic applications as well as artificial light-harvesting complexes that mimic natural antenna systems of photosynthetic bacteria and plants.^11–13 For example, chlorosomal antenna complexes of photosynthetic green sulfur bacteria are self-assembled into multilayer tubular structures having bacteriochlorophyll pigments as building blocks.^14–16 The structure of these antenna complexes and the underlying molecular arrangement ensures that the process of light-harvesting and excitation energy transport is very efficient, even under extremely low light conditions.^17,18 The quest to recreate such efficiency under laboratory conditions has sparked numerous studies of synthetic self-assembled systems mimicking natural chlorosomes, e.g. using porphyrins,¹⁹ zinc chlorin,²⁰ and cyanine dyes.²¹ Of particular interest are the tubular aggregates of 3,3′-bis(2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimidacarbocyanine (C8S3).^22–25 Cryo-TEM reveals a hierarchy of supramolecular architectures, including double-walled nanotubes; under certain conditions, bundles of nanotubes arise.²⁶ Thus, this system allows for the occurrence of electronic excitation energy transport at various levels: within one wall, between walls of one tube, and between different tubes, similar to the situation in natural systems.^27,28To understand how such supramolecular systems work, as well as propose design rules for new materials, it is essential to determine the relationship between molecular structure and optical properties. Current experimental techniques, however, are unable to resolve the structure at the molecular level. This, in combination with the sensitivity of spectral properties to the details of the molecular packing, leads to a crucial role for theoretical modeling.²⁹ For example, molecular dynamics (MD) simulations have been used to predict the molecular packing within a variety of supramolecular assemblies.^30–34 However, synthetic amphiphiles with aromatic groups, such as cyanine dyes—often used to prepare aggregates with optical functionality—tend to fall into kinetic traps during spontaneous self-assembly simulations and the packing of the aromatic chromophores remains highly disordered on the accessible time scale, leading to predicted (optical) spectra that are not consistent with experimental data.³⁵ This problem can be overcome by building assemblies based upon proposed architectures and assessing their stability in relatively short MD simulations.^36–38 The drawback of this approach is the requirement of a thorough understanding of what to use as a starting point and how to validate the structure. In any case, proper validation requires the modeling of the optical spectra of the obtained structure, and finally, comparing it to the experiment. The demanding character of such methods explains why an important role is played by phenomenological modeling, in which a molecular packing is guessed and the optics is obtained from parametrizing an exciton model that describes the collective excited states of the assembly with interactions dictated by the guessed packing. By comparing the calculated spectra to experimental ones, the structure and exciton model may be fine-tuned. While this method has been successful in describing spectra,^23,39 it is limited in its predictive power and also lacks access to essential microscopic parameters, such as tuning of the optical excitation energies imposed by the environment, disorder in these energies and structural heterogeneity.In this work, we use an advanced multiscale approach to determine structure–optical property relationships for the C8S3 double-walled nanotubes, guided by comparison to experiments. The optical spectrum of these aggregates, in which multiple exciton peaks may be discerned, suggests a rather complex underlying molecular packing. This fact, combined with their sheer size going up to many thousands of molecules, makes these systems exceptionally challenging to resolve and leaves important questions concerning structure–function relationships unanswered or under debate, for instance the origin of the splitting between the two lowest-energy spectral bands.^23,38 Here, we answer these questions by iteratively combining MD simulations to capture the details of molecular packing and structural disorder, an exciton Hamiltonian approach to calculate optical signatures, and explicit microelectrostatic calculations to estimate energetic disorder and solvent shifts. Previous attempts to reveal the structure of cyanine-based nanotubes were limited to small-scale system sizes,^37,38 modeling optical features phenomenologically rather than using atomistic information³⁸ or featuring simpler, single-walled systems.³⁷ In addition to answering important questions for the C8S3 double-walled nanotubes, our study opens the way to explain and predict at an unprecedented level of detail the functional properties of other highly complex molecular materials.     

Bidirectional triplet exciton transfer between silicon nanocrystals and perylene

Hybrid materials comprised of inorganic quantum dots functionalized with small-molecule organic chromophores have emerged as promising materials for reshaping light''s energy content. Quantum dots in these structures can serve as light harvesting antennas that absorb photons and pass their energy to molecules bound to their surface in the form of spin-triplet excitons. Energy passed in this manner can fuel upconversion schemes that use triplet fusion to convert infrared light into visible emission. Likewise, triplet excitons passed in the opposite direction, from molecules to quantum dots, can enable solar cells that use singlet fission to circumvent the Shockley–Queisser limit. Silicon QDs represent a key target for these hybrid materials due to silicon''s biocompatibility and preeminence within the solar energy market. However, while triplet transfer from silicon QDs to molecules has been observed, no reports to date have shown evidence of energy moving in the reverse direction. Here, we address this gap by creating silicon QDs functionalized with perylene chromophores that exhibit bidirectional triplet exciton transfer. Using transient absorption, we find triplet transfer from silicon to perylene takes place over 4.2 μs while energy transfer in the reverse direction occurs two orders of magnitude faster, on a 22 ns timescale. To demonstrate this system''s utility, we use it to create a photon upconversion system that generates blue emission at 475 nm using photons with wavelengths as long as 730 nm. Our work shows formation of covalent linkages between silicon and organic molecules can provide sufficient electronic coupling to allow efficient bidirectional triplet exchange, enabling new technologies for photon conversion.

We demonstrate that silicon quantum dots can exchange spin triplet excitons with molecules covalently attached to their surface. Such hybrid materials can enable systems that upconvert incoherent far-red light into the visible spectral range.

Hybrid materials comprised of inorganic quantum dots (QDs) interfaced with small-molecule organic chromophores have emerged as a promising platform for materials that convert near-infrared radiation into the visible spectral range.^1–3 In these structures, QDs act as light-harvesting antennas, absorbing long-wavelength photons and passing their energy to organic molecules bound to their surface in the form of spin-triplet excitons. These excitons can then be transferred into a surrounding medium, typically a solution or thin film, where pairs of them can fuse to form a bright spin-singlet state that can emit a short-wavelength photon.^4–8 Due to the long lifetime of molecular triplet excitons, which can range from several microseconds to milliseconds, these materials can operate at low photon flux, enabling their integration into light-harvesting systems that operate under solar flux^9,10 and limiting heat dissipation during their use in biological applications, such as phototherapy,^11,12 live-cell imaging,^13,14 and optogenetics.¹⁵ These hybrid materials can also be used to study interfacial energy transfer processes fundamental to the operation of solar cells that use triplet fusion''s inverse process, singlet fission, to enhance their performance.^9,16–21 The simplest design for a cell of this type is one that interfaces a singlet fission material directly in line with a back-contacted semiconductor solar cell.^22–24 In these structures, the singlet fission material acts as a light sensitizer that captures high-energy photons and uses their energy to generate pairs of triplet excitons that can be passed to the semiconductor to produce photocurrent. As molecules can be readily attached to QDs via a variety of chemical tethers, these materials allow detailed study of how the structure of the organic:inorganic interface impacts the ability of triplet excitons to move from one material to the other.For both triplet fusion-based light upconversion and singlet fission-based light harvesting, silicon represents a key material of interest. While several upconversion systems have been derived using QDs containing toxic elements, such as Cd^5,7,25 or Pb,^6,8,26,27 Si QDs are nontoxic, making them attractive for biological applications.²⁸ Silicon also dominates the solar energy market, accounting for ∼90% of solar power production,^29,30 making Si:organic interfaces that readily transmit triplet excitons a key design target for singlet fission-based solar cells.^18,19,22 Previously, we have shown triplet exciton transfer from Si QDs to surface-bound anthracene molecules can power a photon upconversion system that operates with 7% efficiency.³¹ However, the inverse energy transfer process that is key for singlet fission devices, triplet exciton transfer from surface-bound molecules to Si, was not observed in our prior work.In this report, we address triplet exciton transfer from molecules to Si by demonstrating a hybrid Si QD:perylene system wherein photoexcitation of the Si QD establishes a spin-triplet exciton population that exists in a dynamic equilibrium between the QD and perylene molecules bound to its surface. While such exciton cycling has been reported for other QD:molecule systems,^32–34 our work represents the first observation of this behavior in Si QD based systems. Using nanosecond transient absorption spectroscopy, we find triplet exciton transfer from Si to perylene takes place on a 4.2 μs timescale while energy transfer in the reverse direction occurs more than two orders of magnitude faster, on a 22 ns timescale. We attribute this difference in energy transfer rates to differences in the exciton density of states between perylene molecules and Si QDs. To demonstrate the utility of triplet excitons produced by this system for photon conversion applications, we have constructed a photon upconversion system by interfacing perylene-functionalized Si QDs with a complementary perylene-based triplet fusion annihilator. We find this system performs well, upconverting radiation with a wavelength as long as 730 nm into blue light centered near 475 nm. Under 532 nm illumination, the system upconverts light with an efficiency of 1.5% under incident light fluxes as low as 80 mW cm⁻². This performance is comparable to that recently demonstrated using the same perylene annihilator coupled with a Pd-porphyrin light absorber.³⁵ Our work demonstrates that the introduction of short, chemical linkers between molecules and Si can enable triplet exciton exchange between these materials for the design of new systems for both photon upconversion and light harvesting.     

Crystals springing into action: metal–organic framework CUK-1 as a pressure-driven molecular spring

Mercury porosimetry and in situ high pressure single crystal X-ray diffraction revealed the wine-rack CUK-1 MOF as a unique crystalline material capable of a fully reversible mechanical pressure-triggered structural contraction. The near-absence of hysteresis upon cycling exhibited by this robust MOF, akin to an ideal molecular spring, is associated with a constant work energy storage capacity of 40 J g⁻¹. Molecular simulations were further deployed to uncover the free-energy landscape behind this unprecedented pressure-responsive phenomenon in the area of compliant hybrid porous materials. This discovery is of utmost importance from the perspective of instant energy storage and delivery.

Mercury porosimetry and in situ high pressure single crystal X-ray diffraction revealed the wine-rack CUK-1 MOF as a unique crystalline material capable of a fully reversible mechanical pressure-triggered structural contraction.

Reducing the world''s fossil fuel dependence is the focus of many global initiatives,¹ aiming to mitigate the effects of climate change through tapping into sustainable energy resources such as solar and wind power. However, increasing reliance on these renewable energy sources has introduced difficulties due to the offset between power availability and demand peaks. Complementary technologies are necessary to alleviate intermittent supply, such as peaking power plants, demand-side energy management, or large scale energy storage.² The latter is particularly desirable as it can decouple electricity production and consumption, however the lack of a “one size fits all” approach has led the scientific community to envisage unconventional energy storage strategies.One such avenue emerging in recent years is the storage of mechanical energy via the compression of a suitable stimuli-responsive system, either through the intrusion of a non-wetting fluid into hydrophobic porous frameworks,³ or by means of application of an external pressure on flexible materials.⁴The former approach, first pioneered using water intrusion in zeolites and silicas,¹¹ has recently been extended to small pore zeolitic imidazolate frameworks.¹² Unfortunately, besides requiring highly hydrophobic systems, water intrusion achieves a relatively low stored energy density,³ of around 3–25 J g⁻¹. The second strategy takes advantage of the compliant nature of bulk materials. Energy is stored through structural deformations, manifesting as continuous or sudden volume changes under external pressure. The energy stored in flexible materials over a compression/decompression cycle can be an order of magnitude higher compared to the values achieved using fluid intrusion in rigid porous systems.¹³ In theory, three types of pressure-induced structural behaviour can be envisioned for such a responsive system. If the structure contraction is non-reversible, all energy is dissipated and the system is categorized as a nano-shock absorber (Fig. 1b). For structural changes that are reversible upon decompression two families of system can be distinguished, i.e. a nano-damper (Fig. 1c) or an ideal nano-spring (Fig. 1d) when the p–V curves show hysteresis or fully overlap, respectively.¹⁴Open in a separate windowFig. 1(A) Schema of mechanical energy storage in compliant crystalline materials, implying a unit cell volume change between open (op) and contracted (cp) structures, and prototypical pressure-volume curves of stimuli-responsive materials under mechanical pressure for (B) nano-shock absorbers, exemplified by MIL-53(Al),⁵ MIL-53(Ga)-FA⁶ and ZIF-4(Zn),⁷ (C) nano-dampers e.g. MIL-53(Cr),⁸ MIL-47(V)⁹ and MIL-53(Al)-FA¹⁰ and (D) nano-springs, insofar exhibited exclusively by CUK-1 presented herein.Metal–organic frameworks (MOFs), a class of porous, crystalline materials comprised of metal vertices interconnected by organic linkers, are known to exhibit responsiveness to a variety of stimuli,^15,16 including external pressure.¹⁷ Recently, several frameworks of this family of hybrid materials have been shown to act as energy storing nano-dampers or energy dissipative nano-shock absorbers, as is the case for the highly flexible MIL-53(M)^5,8,10 and MIL-47(V)⁹ series and more recently ZIF-4(Zn)⁷ (see Fig. 1b and c for their related structural behaviours). In such flexible crystalline materials compression is associated with a displacive phase transition between distinct structures of differing unit cell volumes, denoted as open (op) and contracted (cp) forms^15,16 and illustrated in Fig. 1a, occurring reversibly or irreversibly for a nano-damper or nano-shock absorber, respectively. The considerable stored energy associated with this transition, in the range of 30–200 J g⁻¹ (up to 4 kJ g⁻¹ for shock absorbers¹⁸) is highly attractive from the perspective of mechanical energy storage. However, the hysteretic compression/decompression curve characterising known nano-damper MOFs leads to a partial loss of work energy, lowering the potential storage efficiency, as well as creating issues through heat dissipation. Insofar, the search for a ideal spring-like crystalline material, capable of reversible pressure-induced structural switching without any hysteresis (Fig. 1d) has been fruitless, precluding their applicability for efficient, high density energy storage applications. Herein, a subtle combination of Hg-porosimetry, high-pressure single crystal X-ray diffraction (SC-XRD) and molecular simulations reveals the 1D-channel CUK-1 (M, M = Co, Mg)¹⁹ MOF as the first compliant hybrid porous material with a spring-back mechanical breathing behaviour.Such unique mechanically-triggered structural response implies a continuous pore contraction/expansion between op and cp forms in a narrow pressure range of 280–290 MPa, accompanied by a unit cell volume change of 20.9%. This optimal scenario paves the way towards fast energy storage/delivery system of about 40 J g⁻¹. The channel-like CUK-1(M) composed of chains of μ₃-OH/O edge and vertex sharing metal octahedra (M = Co,¹⁹ Mg²⁰) coordinated by bidentate 2,4-pyridinedicarboxylic ligands, recently emerged as an attractive porous material owing to its promising sorption performance combined with environmentally-friendly hydrothermal synthesis and high thermal and chemical stability.^20–22 Its wine-rack topology and its relatively rigid behaviour upon guest adsorption are reminiscent to that of MIL-47(V) a MOF which interestingly underwent a hysteretic, reversible structural contraction upon exerting an external pressure of 125 MPa,⁹ associated with a stored/delivered energy of 33 J g⁻¹. Inspired by our previous findings on MIL-47(V), we deliberately explored the pressure-induced structural behaviour of CUK-1 in its isostructural Co and Mg forms. MOF synthesis was performed according to the protocol detailed in ESI.^† Phase purity was confirmed by powder XRD (Fig. S3, S4 and Table S1^†) while their textural features, including BET area and pore volume, were found to match previously reported data.^19,20Mercury intrusion curves were recorded on the powder samples up to a maximum of 413 MPa as shown for CUK-1(Co) in Fig. 2, its Mg variant being reported in Fig. S6, ESI,^† together with full experimental details. A substantial amount of Hg intrudes at low pressure (<10 MPa), due to compaction of the crystals and filling of inter-particle porosity. This is followed by a sudden volume change at 281 MPa where a sharp step is observed (see inset of Fig. 2). By analogy with the conclusions previously drawn for the series of MIL-53(M)/MIL-47(V) frameworks,^5,8–10 this intruded Hg volume increase is associated with a structural contraction of CUK-1(Co), as its channel size (approx. 6.6 Å) is an order of magnitude below the pore dimension where non-wetting mercury can intrude in this pressure range (at 52 Å). The extrusion curve shows a near-perfect overlap, indicating that the framework behaves as an ideal spring, with no hysteresis between the intrusion/extrusion branches.Open in a separate windowFig. 2Sequential mercury intrusion–extrusion curves on CUK-1(Co) powder, in blue line and red, respectively. Line is a guide for eye. Volume below 1 MPa corresponds to powder compaction and intercrystallite void filling. Dotted horizontal lines demarcate contraction lower and upper bounds. Inset highlights the intrusion step in a linear scale with the op/cp contraction marked with an arrow.Moreover, this behaviour is highly repeatable, as confirmed by four consecutive pressure cycles (in Fig. S5^†). Interestingly, the same behaviour also holds true for CUK-1(Mg) (Fig. S6^†), with a similar intrusion pressure of 288 MPa. Since the two metal ions show relatively similar ionic radius (Co²⁺: 1.50(7) Å and Mg²⁺: 1.41(7) Å),²⁴ the averaged metal–oxygen distance is nearly identical in their corresponding coordination: spheres: (Co–O: 2.107(20) Å and Mg–O: 2.073(20) Å). Such analogous metal-linker bond strength is most likely at the origin of the very similar pressure-induced response of the two materials. The high transition pressure of CUK-1(Co) underpins the inability of guest adsorption to induce a breathing effect as observed previously.^20,21 Indeed, the adsorption stresses encountered throughout guest insertion are simply insufficient to overcome the energetic penalty of transition.^14,25 The 0.143 mL g⁻¹ volume change associated with the observed step in the CUK-1(Co) intrusion curve corresponds to a 20.9% change in unit cell volume, lower than in the similar phenyl-based MIL-47(V) of 43%.⁹ However, the stored energy calculated through W = P × ΔV is 40 J g⁻¹, 20% larger than the value reported for MIL-47(V)⁹ of 33 J g⁻¹. Here, the higher pressure of CUK-1(Co) switching, 281 MPa vs. 125 MPa for MIL-47(V) balances out the ΔV term. Moreover, owing to its relatively dense framework, the volumetric energy density of CUK-1(Co) remains attractive when compared to water intrusion systems (Table S4^†).Considering an initial unit cell volume for the CUK-1(Co) op form of 2467 ų from PXRD (see ESI^†), the resulting cp form is estimated to exhibit a unit cell volume of 1950 ų, based on the Hg intruded volume increase at 281 MPa. In order to directly observe the contracted form and identify the mechanism underpinning these intriguing dynamics, high pressure SC-XRD experiments were carried out in a membrane diamond anvil cell (mDAC). Individual CUK-1(Co) crystals were placed in a gasket between the polished diamonds of the mDAC, and immersed in a hydrostatic pressure transmitting medium of silicone oil AP-100, with a gold flake used to monitor inner mDAC pressure (full single crystal synthesis conditions and SC-XRD methodology available in the ESI^†).At ambient pressure, the indexed unit cell volume of the initial op form of CUK-1(Co) is nearly identical (2492 ų) to that of the previously reported²⁰ dehydrated monoclinic phase (2466.72 ų). Upon increasing DAC pressure to around 0.3 GPa, a volume contraction to the cp phase begins, which is in line with Hg porosimetry experiments. Reflections obtained from integrated 2D diffraction images were used to solve the pressure-induced structure through a dual space recycling algorithm in an expanded P1 setting, then further refined on F² using the SHELX suite²⁶ (complete data treatment methodology available in the ESI^†). The structure maintains the same C2/c space group throughout the transition between the two forms, and as such the spring-like dynamics of the framework can be described as a continuous contraction in a narrow pressure range. Above 0.5 GPa, the cp form is attained, with further pressure application leading to a linear decrease of its unit cell volume by 4% up to 1.8 GPa (Fig. 3).Open in a separate windowFig. 3Evolution of the CUK-1(Co) unit cell volume determined through indexation of Bragg reflections as a function of applied pressure as recorded in a DAC. Unit cell parameters corresponding to each pressure point can be found in Table S5, ESI.^†The unit cell dimensions of the solved cp form at 0.5 GPa are provided in Table 1, alongside as-indexed pristine op form parameters with Fig. 4a illustrating the two structures. The anisotropic transition is similar in nature to that of MIL-53(M)/MIL-47(V), characterised by a compaction in the b-direction (from approx. 13 Å to 9 Å) and an elongation along the a-axis (from 18 to nearly 20 Å). The change in the c-parameter is minimal, with only a slight increase, as it lies in the plane of the highly rigid octahedrally coordinated metal chains. A lowering of the angle (from 103 to 99°) is also observed, as the 1D parallel pores are straightened via the linker-induced torsion. A table comparing specific atomic distances, angles and torsions in the two forms is available in Table S7, ESI.^† The unit cell volume of the identified cp phase at 0.5 GPa of 1972 ų is only slightly higher than the value estimated from porosimetry measurements (as 1950 ų). We attribute this offset to the different interactions of the crystal surface with the respective pressure transmitting medium (mercury vs. silicone oil), as observed previously.⁵Crystallographic data of the pristine (op) and high pressure (cp) phases as determined from the CUK-1(Co) SC-XRD

Pressure-and temperature induced phase transitions,piezochromism, NLC behaviour and pressure controlled Jahn–Teller switching in a Cu-based framework

In situ single-crystal diffraction and spectroscopic techniques have been used to study a previously unreported Cu-framework bis[1-(4-pyridyl)butane-1,3-dione]copper(ii) (CuPyr-I). CuPyr-I was found to exhibit high-pressure and low-temperature phase transitions, piezochromism, negative linear compressibility, and a pressure induced Jahn–Teller switch, where the switching pressure was hydrostatic media dependent.

In situ high-pressure single-crystal diffraction and spectroscopic techniques have been used to study a previously unreported Cu-framework bis[1-(4-pyridyl)butane-1,3-dione]copper(ii) (CuPyr-I).

High-pressure crystallographic experiments over the last 25 years or so have proved to be a unique tool in probing the mechanical properties of the organic solid state,¹ metal-complexes, and 2D/3D coordination compounds.² In particular, high-pressure techniques have been used to study an array of mechanical and chemical properties of crystals, such as changes in electrical and thermal conductivity,³ pressure-induced melting,⁴ solubility,⁵ amorphisation,⁶ post-synthetic modification,⁷ and chemical reactions such as polymerisation,⁸ cycloaddition⁹ and nanoparticle formation.¹⁰ Previous high-pressure experiments on porous metal-organic framework (MOF) materials have shown that on loading a diamond anvil cell (DAC) with a single-crystal or polycrystalline powder, the hydrostatic medium that surrounds the sample (to ensure hydrostatic conditions) can be forced inside the pores on increasing pressure, causing the pore and sample volume to increase with applied pressure.¹¹ This technique has also been used to determine the position of CH₄ and CO₂ molecules inside the small pores of a Sc-based MOF at room temperature using a laboratory X-ray diffractometer, and has proved useful in experimentally determining the maximum size of guest molecules that can penetrate into a pore.¹²On direct compression of more dense frameworks, negative linear compressibility (NLC) has also been observed, which results in an expansion of one or more of the unit cell dimensions with an overall contraction in volume. Such changes in the compressibility behaviour of metal-containing framework materials is usually as a result of common structural motifs which rotate or bend in order to accommodate increases in length along particular crystallographic directions.¹³ Changes in coordination environment can also be induced at pressure, as metal–ligand bonds are more susceptible to compression than covalent bonds.^2a In previous high pressure studies on metal complexes or coordination compounds, in which the metal ion has an asymmetric octahedral environment caused by Jahn–Teller (JT) distortions for example (such as those observed in Cu²⁺ and Mn³⁺ complexes), the application of pressure can result in compression of the JT axis, and can even be switched to lie along another bonding direction within the octahedron.¹⁴ Such distortions often result in piezochromism, often observed within a single crystal.¹⁵Here, we present a high-pressure crystallographic study on a novel and unreported Cu-framework bis[1-(4-pyridyl)butane-1,3-dione]copper(ii) (hereafter referred to as CuPyr-I). On application of pressure, CuPyr-I is highly unusual in that it demonstrates several of these phenomena within the same framework, including a single-crystal to single-crystal phase transition, a switching of the JT axis that depends on the hydrostatic medium used to compress the crystal, piezochromism and NLC behaviour. To date, we are unaware of any other material which exhibits all of these phenomena, with the first ever reported hydrostatic media ‘tuneable’ JT-switching.Under ambient temperature and pressure CuPyr-I crystallises in the rhombohedral space group R$\bar{3}$ (a/b = 26.5936(31) Å, c = 7.7475(9) Å). Each Cu-centre is coordinated to four 1-(4-pyridyl)butane-1,3-dione linkers, two of these ligands are bound through the dione O-atoms, with the final two bonding through the N-atom of the pyridine ring to form a 3D polymer. The crystal structure of CuPyr-I is composed of an interpenetration of these 3D polymers to form one-dimensional porous channels (∼2 Å in diameter) that run along the c-axis direction (Fig. 1).Open in a separate windowFig. 1Ball and stick model showing the coordination environment around the Cu²⁺ ion in CuPyr-I, and 3D-pore structure as viewed along the c-axis direction. The yellow sphere represents the available pore-space. Colour scheme is red: oxygen, blue: nitrogen, black: carbon, white: hydrogen and cyan: copper. The Cu²⁺ octahedron is illustrated in green.On increasing pressure from 0.07 GPa to 1.56 GPa using Fluorinert FC-70 (a mixture of large perfluorinated hydrocarbons) as a hydrostatic medium, compression of the framework occurs, resulting in a 9.89% reduction in volume, while the a/b-axes and c-axis are reduced by 4.46% and 1.25% respectively (Fig. 2 (blue triangles) and Table S1^†).Open in a separate windowFig. 2 a/b and c-axes as a function of pressure in a hydrostatic medium of FC-70 (blue triangles) and MeOH (red/black circles). The vertical line indicates the transition from CuPyr-I (red circles) to CuPyr-II (black circles) above 2.15 GPa. Errors in cell-lengths are smaller than the symbols plotted.On increasing pressure to 1.84 GPa, the framework became amorphous, though this is unsurprising as the hydrostatic limit for FC-70 is ∼2 GPa, and compression of frameworks in non-hydrostatic conditions usually results in amorphisation.¹⁶ On increasing pressure to 1.56 GPa, the three-symmetry independent Cu–O/N bond lengths to the ligand were monitored (Fig. 3 and Table S5^†). Under ambient pressure conditions, the two Cu–N1 pyridine bonds are longer than the four Cu–O1/O2 dione bonds, typical for an elongated JT distorted Cu²⁺ complex. However, on increasing pressure the direction of the JT axis gradually changed from Cu–N1 to the Cu–O1 bond (the dione oxygen in the 3-position), becoming equidistant at ∼0.57 GPa. By 1.56 GPa, the lengths of the Cu–N1 and Cu–O1 bonds had steadily reduced and increased by 12.3% and 8.9%, respectively. Throughout this the Cu–O2 bond remained essentially unchanged.Open in a separate windowFig. 3Cu–O1 (orange), Cu–N1 (blue) and Cu–O2 (green) bond lengths on increasing pressure in both FC-70 (triangles and dashed lines) and MeOH (circles and solid lines).Pressure induced JT switching has been observed in other systems, including a Mn₁₂ single-molecule magnet cluster that re-orientates the JT axis on one of the Mn centres at 2.5 GPa.^14a A similar transition was also observed in [CuF₂(H₂O)₂(pyz)] (pyz = pyrazine) and Rb₂CuCl₄(H₂O)₂,¹⁵ where the JT axis was reoriented from the Cu–N bond to the perpendicular Cu–O bond, though this occurs during a crystallographic phase transition at 1.8 GPa.¹⁸ Here, in CuPyr-I, no phase transition takes place, and unusually the JT switching appears to occur progressively on increasing pressure with no phase transition.^14bUsing methanol (MeOH) as the hydrostatic medium, CuPyr-I was compressed in two separate experiments, from 0.52 GPa to 5.28 GPa using synchrotron radiation, and from 0.34 GPa to 2.95 GPa using a laboratory X-ray diffractometer. On increasing pressure to 2.15 GPa, the a/b and c-axes compressed by 6.22% and 0.39% respectively (Fig. 2, Tables S2 and S3^†). On increasing pressure from 2.15 GPa to 2.78 GPa, CuPyr-I underwent a single-crystal to single-crystal isosymmetric phase transition to a previously unobserved phase (hereafter referred to as CuPyr-II).The transition to CuPyr-II resulted in a doubling of the a/b-axes, whilst the c-axis remained essentially unchanged. On increasing the pressure further, the a/b-axes continued to be compressed, whilst the c-axis increased in length, exhibiting negative linear compressibility (NLC) until the sample became amorphous at 5.28 GPa. The diffraction data were of poor quality after the phase transition, and only the connectivity of the CuPyr-II phase could be determined at 3.34 GPa. Above 3.34 GPa, only unit cell dimensions could be extracted. The occurrence of positive linear compressibility (PLC) followed by NLC is unusual in a framework material, and we could find only a few examples in the literature where this occurs.¹⁹During the NLC, the c-axis expanded by 1.46%, to give a compressibility of K_NLC = −5.3 (0.8) TPa⁻¹ (Δp = 2.23–4.90 GPa). K_NLC is calculated using the relationship K = −1/l(∂l/∂p)_T, where l is the length of the axis and (∂l/∂p)_T is the length change in pressure at constant temperature.²⁰ The value of K_NLC here is rather small compared to the massive NLC behaviour observed in the low pressure phase of Ag₃[Co(CN)₆]⁹ (K_NLC = −76(9) TPa⁻¹, Δp = 0–0.19 GPa) or the flexible MOF MIL-53(Al) (K_NLC = −28 TPa⁻¹, Δp = 0–3 GPa) for example,^17b and is much more comparable to the dense Zn formate MOF [NH₄][Zn(HCOO)₃] (−1.8(8) TPa⁻¹ (Δp = 0–0.94 GPa)).²¹ Because of the quality of the data, the exact nature, or reason for the NLC in CuPyr-II is unknown, although we aim to investigate this in the future.On increasing pressure using MeOH, the JT axis was again supressed on compression, with the Cu–N1 bond reducing in length by 0.288 Å (12%) between 0.34 and 2.15 GPa, while the Cu–O1 bond length increased by 0.216 Å (11%). The pressure at which Cu–N1 and Cu–O1 became equidistant was 1.28 GPa, measuring 2.140(5) Å and 2.131(6) Å respectively (Fig. 3 and Table S6^†). Across the entire pressure range, little to no compression or expansion was observed in the Cu–O2 bond in the 1-position of the dione in CuPyr-I, the same trend observed when compressed in FC-70. The JT switching pressure in MeOH however was 0.71 GPa higher than observed by direct compression in FC-70 (0.57 GPa). This, to our knowledge, is the first time that pressure induced JT switching has been observed to be hydrostatic media dependent.Changes to the Cu–N and Cu–O bond lengths were supported by high-pressure Raman spectroscopy of CuPyr-I, using MeOH as the hydrostatic medium (Fig. S10^†). Gradual growth of a shoulder on a band at ∼700 cm⁻¹ during compression is tentatively assigned to the Cu²⁺ coordination environment shifting from elongated to compressed JT distorted geometry. The shouldered peak becomes split above 2 GPa, after which the isosymmetric phase transition occurs.The gradual JT switch is thought to be principally responsible for reversible piezochromism in single crystals of CuPyr-I, which change in colour from green to dark red under applied pressure (Fig. 4b, S1 and S2^†). UV-visible spectroscopy confirms a bariometric blue-shift in the absorption peak at ∼700 nm assigned to d–d electronic transitions, and a red-shift of the tentatively assigned ligand-to-metal charge-transfer (LMCT) edge around 450 nm during the elongated to compressed switch (Fig. 4a and S8^†), accounting for this colour change. The red-shift is observed during compression in both Fluorinert® FC-70 and MeOH hydrostatic media, with a slightly suppressed shift measured in the latter due to filling of the framework pores (Table S2^†). Geometric switching at the metal centre leads to electronic stabilisation of the Cu²⁺ ion, as electrons transfer from higher energy d_x²−y² (Cu–O) orbitals to the lower energy d_z² (Cu–N) state (Fig. S7^†), evidenced by the blue-shift of the d–d intraconfigurational band as the d_z² (Cu–N) is progressively mixing with d_x²−y² (Cu–O) increasing its energy with respect to the lower energy d_xy and d_xz,yz levels becoming the highest energy level at the nearly compressed rhombic geometry. On the other hand, the redshift in the hesitantly assigned O²⁻ to Cu²⁺ LMCT band below 450 nm is ascribed to increase of the Cu–O bond distance and a likely bandwidth broadening with pressure both yielding a pressure redshift of the absorption band gap edge (Fig. 4a).Open in a separate windowFig. 4(a) UV-visible spectroscopy of CuPyr-I during compression in Fluorinert® FC-70 showing a gradual BLUE-shift in the d–d intraconfigurational band (∼700 nm) and a gradual red-shift of the absorption band assigned to LMCT (∼450 nm) with increasing pressure. (b) Gradual pressure-induced Jahn–Teller switch of the Cu²⁺ octahedral coordination environment in CuPyr-1 from tetragonal elongated (left, green) to rhombic compressed (right, red), causing piezochromism. Atom colouring follows previous figures.Compression of the coordination bonds was not the only distortion to take place in CuPyr-I, with the Cu-octahedra also twisting with respect to the 1-(4-pyridyl)butane-1,3-dione linkers on increasing pressure. Twisting of the Cu-octahedra in CuPyr-I with respect to the dione section of the linker could be quantified by measuring both the ∠N1Cu1O2C4 and the ∠N1Cu1O1C2 torsion angles from the X-ray data, which in MeOH gradually decrease and increase by 12.2° and 7.3°, respectively, to 2.15 GPa (Table S8^†). In FC-70, ∠N1Cu1O2C4 and ∠N1Cu1O1C2 decrease and increase by 5.4° and 2.8°, respectively, to 1.56 GPa. On increasing pressure to 1.57 GPa in a hydrostatic medium of MeOH, a difference of ∼5° for both angles was observed compared to FC-70 at 1.56 GPa. Twisting about the octahedra allows compression of the channels to take place in a ‘screw’ like fashion and has been observed in other porous materials with channel structures.²² The overall effect is to reduce the pore volume, and decrease the size of the channels (Tables S2 and S3^†). Using MeOH as a hydrostatic medium therefore appears to reduce this effect by decreasing the compressibility of the framework.It was not possible to determine the pressure dependence in other longer-chain alcohols, including ethanol (EtOH) and isopropanol (IPA), due to cracking of the crystal upon loading into the diamond anvil cell (Fig. S1^†). We believe this is a result of these longer chain alcohols acting as reducing agents, as indicated by the loss in colour of the crystals.To ascertain the origin of the hydrostatic media-induced change in the JT switching pressure and unit cell compressibility, the pore size and content were monitored as a function of pressure. A dried crystal of CuPyr-I was collected at ambient pressure and temperature in order to compare to the high-pressure data and is included in the ESI.^† The pore volume and electron density were estimated and modelled respectively using the SQUEEZE algorithm within PLATON (Tables S1–S3^†).²³ CuPyr-I under ambient pressure conditions has three symmetry equivalent channels per unit cell with a total volume of ∼1152 ų containing diethyl ether (2.5 wt%) trapped in the pores during the synthesis of the framework, confirmed by TGA analysis (Fig. S6^†).On surrounding the crystal with FC-70, direct compression of the framework occurred. The pore content remained almost constant during compression up to 0.88 GPa, inferring no change in the pore contents. On increasing pressure further to 1.56 GPa, an increase in the calculated electron density was observed (23%), though the data here were of depreciating quality and less reliable. During compression of CuPyr-I in MeOH to 0.52 GPa, the pore volume and electron density in the channels increased by 4.5% and 54%, respectively, reflecting ingress of MeOH into the pores. The electron density in the channels continued to increase to a maximum of 0.466e⁻ A⁻³ by 0.96 GPa, although the pore volume began to decrease at this pressure. The uptake of MeOH into the pores therefore results in the marked decrease in compressibility, as noted above.Previous high-pressure experiments on porous MOFs have resulted in similar behaviour on application of pressure, with the uptake of the media significantly decreasing the compressibility of the framework.²⁴ However, using different hydrostatic media to control the JT switch in any material is, to the best of our knowledge, previously unreported. On increasing pressure above 0.96 GPa, the electron density in the pores decreases, and coincides with a steady reduction in volume of the unit cell. Both an initial increase and then subsequent decrease in uptake of hydrostatic media is common in high-pressure studies of MOFs, and has been seen several times, for example in HKUST-1 (ref. 24c) and MOF-5.^24a The ingress of MeOH into the pores on initially increasing pressure to 0.52 GPa is also reflected in a twisting of the octahedra, in-particular the ∠N1Cu1O2C4 angle decreases by 5.8° in MeOH, whereas on compression in FC-70, little to no change is observed in the ∠N1Cu1O2C4 angle to 1.56 GPa. These angles represent a twisting of the dione backbone, which we speculate must interact with the MeOH molecules which penetrate into the framework.Upon compression in n-pentane, the lightest alkane that is a liquid at ambient temperature, we see different behaviour to that in MeOH or FC-70. Poor data quality permitted only the extraction of unit cell parameters but from this it can be seen that CuPyr-I has undergone the transition to CuPyr-II by 0.77 GPa. This is a significantly lower pressure than is required to induce the phase transition in MeOH (ca. 2.15 GPa). We speculate that this difference in pressure is caused by the n-pentane entering the channels at a lower pressure than MeOH due to the hydrophobic nature of the channels. This can be overcome by MeOH but not until substantially higher pressures, as seen in other MOFs that contain hydrophobic pores.²⁵On undergoing the transition to CuPyr-II at 2.78 GPa the unit cell volume quadruples, resulting in three symmetry independent channels (12 per unit cell), with the % pore volume continuing to decrease (Table S4^†). Additionally, the reflections become much broader, significantly depreciating the data quality. Nevertheless, changes in metal–ligand bond lengths and general packing features can be extracted. In particular, the transition to CuPyr-II results in two independent Cu-centres, with six independent Cu–N/O bond distances per Cu. Each exhibits a continuation of the trend seen in CuPyr-I, with the Cu–O bonds (equivalent to the Cu–O1 bond in CuPyr-I) remaining longer than the JT suppressed Cu–N bonds. However, the transition to CuPyr-II results in both an increase and decrease in three of the four Cu–N and Cu–O bonds respectively, compared to CuPyr-I at 2.15 GPa (Table S6^†). The net result is a framework which contains a Cu-centre where the coordination bonds are more equidistant, while the JT axis becomes much more prominent in the other Cu-centre, with the Cu–O dione bond continuing to increase in length. The data for CuPyr-II depreciates rapidly after the phase transition, and more work would be required to study the effect of the anisotropic compression of the JT axis in CuPyr-II on increasing pressure further.It is difficult to determine the mechanism behind the NLC behaviour observed upon compression of CuPyr-II because the phase transition results in a significant reduction in data quality. Further work will be carried out computationally in order to elucidate the structural mechanism that gives rise to the PLC followed by NLC. However, we propose this effect is inherent to this framework and the ingress of MeOH molecules into the channels allows the retention of crystallinity to allow this behaviour to be observed crystallographically.In order to determine whether the JT switch could be induced by decreasing temperature and remove any effect the ingress of hydrostatic media has into the pores on the JT switch, variable temperature X-ray diffraction measurements were undertaken on a powder and single-crystal sample. On cooling below 175 K and 150 K in a powder and single-crystal sample respectively, a phase transition was observed, however, this was to a completely different triclinic phase, hereafter referred to as CuPyr-III. The transition here appears to occur when the disordered diethyl ether becomes ordered in the pores, confirmed by determination of the structure by single-crystal X-ray diffraction, where the diethylether could be modelled inside the pore-channel (see ESI Sections 7 & 8 for details^†).In conclusion, we have presented a compression study on the newly synthesised Cu-based porous framework bis[1-(4-pyridyl)butane-1,3-dione]copper(ii), referred to as CuPyr, compressed in FC-70 to 1.56 GPa and MeOH to 4.90 GPa. In both FC-70 and MeOH hydrostatic media, the JT axis, which extends along the Cu–N pyridyl bond, steadily compresses and then switches to lie along one of the Cu–O dione bonds. Compression in MeOH results in ingress of the medium into the framework pores, which increases the JT switching pressure to 1.47 GPa, compared with 0.64 GPa during compression in Fluorinert® FC-70. Interaction of stored MeOH with the host framework prompts twisting of the ligand backbone, which is not observed in the absence of adsorbed guest. Suppression of the JT axis is accompanied by a piezochromic colour change in the single crystals from green to dark red, as confirmed by crystallographic and spectroscopic measurements. Increasing the applied pressure to at least 2.15 GPa causes the framework to undergo an isosymmetric phase transition to a previously unobserved phase, characterised by a doubling of the a/b axes. Between 2.15 GPa and 4.90 GPa, NLC behaviour is observed.This is to the best of our knowledge the first time a phase transition, NLC, piezochromic and pressure induced JT switching behaviour have been observed within the same material. We have also reported for the first time a pressure induced JT axis switch which is hydrostatic media dependent. In further analysis of this system, we intend to study the magnetic properties under ambient and high pressure.     

CsAlB3O6F: a beryllium-free deep-ultraviolet nonlinear optical material with enhanced thermal stability

The design of new beryllium-free deep-ultraviolet nonlinear optical materials is important but challenging. Here, we describe a new strategy to search for such materials based on rational selection of fundamental structural units. By combining asymmetric AlO₃F tetrahedra and π-conjugated B₃O₆ rings, a new aluminum borate fluoride, CsAlB₃O₆F was obtained. It exhibits excellent linear and nonlinear optical properties including a high optical transmittance with a cut-off edge shorter than 190 nm, large second harmonic generation intensities (2.0× KH₂PO₄, KDP), and suitable birefringence for phase-matching under 200 nm. It also has good thermal stability and can be synthesized easily in an open system.

A new potential deep-ultraviolet nonlinear optical material CsAlB₃O₆F was designed by a rational selection of fundamental structural units. This material does not require toxic raw materials and can be grown in an open system.

The exploration of new deep-ultraviolet (DUV) nonlinear optical (NLO) materials is intriguing and of great importance because these materials are crucial for the development of all-solid-state DUV lasers.^1–3 For NLO materials, the main obstacles to DUV application are 3-fold:^4,5 (i) a wide DUV transparency window (wavelength cut-off edge < 200 nm), (ii) high NLO coefficients (>1× commercial KH₂PO₄, KDP), and (iii) sufficient birefringence to satisfy phase matching conditions in the DUV region. Until now, only KBe₂BO₃F₂ (KBBF) can certainly break through these barriers and generate lasers with wavelengths shorter than 200 nm by direct second harmonic generation (SHG).⁶ However, KBBF is limited in practical use because of its adverse layered crystal growth habit and use of the highly toxic beryllium component. To find a KBBF replacement, many new NLO crystals have been developed continuously, but have so far been unable to achieve desired NLO properties.^7–10Recently, it was shown that fluorooxoborates and fluorophosphates with mixed O/F anionic groups might be recognized as new sources for discovering DUV NLO materials. For example, Pan''s group proposed that the [BO_xF_4−x] (x = 1, 2, 3) tetrahedra are good units to balance the multiple criteria of DUV NLO materials.¹¹ Accordingly, monofluorophosphates with non-π-conjugated asymmetric [PO₃F] units were also paid attention, exhibiting superior optical properties.^12,13 Thereafter, numerous fluorooxoborates and fluorophosphates with an unprecedented crystal structure and high performance as potential DUV NLO materials have been reported.^14–17 Nevertheless, because these materials are relatively unstable at high temperature in air, one need to develop a suitable crystal growth method under sealed conditions and/or search for a suitable flux (solvent) system at relatively low temperature.^14,16 Alternatively, it is possible to achieve a beryllium-free, higher stability DUV NLO crystal by combining other anionic groups, such as [AlO₄], [PO₄], [SiO₄], and [ZnO₄].^18–21Enlightened by the successful synthesis of NLO-active fluorooxoborates and fluorophosphates, we proposed that aluminum borate fluorides with [AlO_mF_n] (m + n = 4, 5, 6, AlOF for short) units might be another choice for exploring new NLO materials. Four specific aspects were considered: (I) aluminum borates are preferred because of the chemical and coordination environment similarity between Al³⁺ and Be²⁺ cations.²² (II) similar to fluorooxoborates, the substitution of O²⁻ with larger electronegativity F⁻ can increase the bandgap and optical anisotropy due to the ionicity of the Al–F bond. (III) The rich coordination environment of AlOF groups provides more structural possibilities than those of other mixed-anionic groups (e.g. [BeO₃F], [BO₃F], [BO₂F₂], see Fig. S1^†). Different from the B or Be atom, the Al atom has empty d orbitals, and it can form sp³, sp³d, and sp³d² hybrid orbitals when bonding with O/F atoms. Consequently, diverse AlOF groups without anion-site disorder, such as [AlO₃F] tetrahedra, [AlO₃F₂] or [AlO₄F] trigonal bipyramids, and [AlO₅F], [AlO₄F₂], [AlO₂F₄], or [AlOF₅] octahedra, have been achieved (Fig. S1^†). Replacing oxygen atoms with fluorine in the Al–O polyhedra not only increases the degree of freedom (e.g. cis/trans conformation), but also causes a (local) symmetry breaking and results in increased microscopic susceptibility and optical anisotropy, which are beneficial to build a noncentrosymmetric (NCS) material. (IV) Aluminum borate fluorides without B–F bonds (e.g. BaAlBO₃F₂,²³ Rb₃Al₃B₃O₁₀F,²⁴ and K₃Ba₃Li₂Al₄B₆O₂₀F²⁰) show good thermal stability, and large crystals could be obtained in air.By applying the strategy described above, we tried to design a new DUV NLO material according to the blueprints shown in Fig. 1a. From KBBF to fluorooxoborates (e.g. NH₄B₄O₆F,²⁵ ABF), the nontoxic [BO₃F] units were selected to replace the [BeO₃F] tetrahedra of KBBF while the NLO properties were retained. Meanwhile, benzene-like [B₃O₆] rings (the same as that in β-BaB₂O₄, BBO) with a better conjugated π-orbital system were utilized to replace [BO₃] triangles (the case in CsB₄O₆F,²⁶ CBF), which could contribute to the increase of SHG responses compared to KBBF. In this work, more stable [AlO₃F] units combined with the [B₃O₆] rings were chosen as fundamental building units (FBUs) and a new potential DUV NLO material CsAlB₃O₆F (CABF) has been successfully synthesized. The material can be obtained easily in an open system and it exhibits a short UV cutoff edge below 190 nm with a powder SHG response of 2.0× KDP under 1064 nm incident radiation. The first-principles calculations reveal that CABF possesses moderate SHG coefficients and sufficient birefringence for DUV phase-matching. With the introduction of new AlO₃F units, CABF continues to maintain the excellent structural features of both KBBF and β-BBO, and thus exhibits a great potential to be a Be-free DUV material for nonlinear light–matter interactions.Open in a separate windowFig. 1(a) The structural evolution from KBe₂BO₃F₂ to CsAlB₃O₆F. (b) The 2D [AlB₃O₆F]_∞ layer of CABF. (c) Crystal structure of CABF.Single crystals of CABF were obtained by the conventional flux method in an open system (see the Experimental details in the ESI^†). CABF crystallizes in the NCS orthorhombic polar space group Pna2₁ (Tables S1–S3^†). Its crystal structure is built up from [AlB₃O₆F]_∞ 2D layers and the Cs⁺ cations reside in the interlayer space (Fig. 1b and c). The FBUs of CABF are the [AlB₃O₈F] groups composed of one [B₃O₆] ring and one [AlO₃F] tetrahedron. Three crystallographically independent boron atoms are all three-coordinated to oxygen atoms forming the BO₃ triangles, which further form a B₃O₆ ring by sharing their terminal O atoms. The B–O bond distances and the O–B–O angles are in the range of 1.342(10) to 1.395(11) Å and 117.2(7) to 121.9(8)°, respectively. The Al³⁺ cation is coordinated to three oxygen atoms (Al–O bond lengths: 1.719(6)–1.734(6) Å) and one fluorine atom (Al–F bond length equals 1.675(5) Å) to form a distorted AlO₃F tetrahedron. In the bc-plane, the [AlO₃F] tetrahedra bond with discrete [B₃O₆] rings to create [AlB₃O₆F]_∞ layers that contain 18-membered rings (Fig. 1b). In one layer, the apical F atoms of [AlO₃F] tetrahedra point upward and downward regularly. The Cs⁺ cations are twelve-coordinated, forming the [CsO₁₁F] polyhedra with a Cs–F distance of 3.292(7) Å and Cs–O distances ranging from 3.256(5) to 3.585(6) Å (Fig. S2^†). Because the Cs⁺ cations reside in the tunnels created by the 18-membered rings and adjacent [AlB₃O₆F]_∞ layers, the interlayer distance in CABF is 4.03 Å. Compared to the distance of two adjacent layers in KBBF (6.25 Å), CABF should exhibit stronger interlayer interactions. Theoretical calculation based on the density functional theory (DFT) also confirms that CABF shows a larger interlayer binding energy than KBBF (0.178 vs. 0.0314 eV Å⁻² per layer). These results indicate that CABF might possess a better crystal growth habit without layering. The bond valence sum calculation²⁷ (Table S2^†) and IR spectrum (Fig. S3^†) indicate that all atoms of CABF have the expected oxidation states and coordination environments.Polycrystalline CABF was synthesized by a stoichiometric solid-state reaction of CsF, Al₂O₃ and H₃BO₃ at 500 °C, and the phase purity was confirmed by powder X-ray diffraction (PXRD) Rietveld refinement (Fig. S4^†). To check the stability of CABF, thermal measurements and further PXRD analysis were performed on its polycrystalline samples. Based on the re-crystallization experiment, the CABF sample after melting was almost the same as the original one; however, owing to the loss of volatile fluorines, a small amount of Cs₂Al₂(B₃O₆)₂O²⁸ was obtained (Fig. S6a^†). Thermogravimetric (TG) analysis shows that CABF has nearly no weight loss until 900 °C, and the differential scanning calorimetry (DSC) data show two sharp endothermic peaks around 505 and 702 °C on the heating curve (Fig. 2a). In addition, variable temperature PXRD measurements were performed (Fig. S6b^†), demonstrating a possible phase transition of CABF near 500 °C (consistent with the first peak on the DSC curve). Therefore, an excessive amount of fluorides should be used as the flux for crystal growth of CABF. Different from many fluorooxoborates (e.g. CaB₅O₇F₃ and SrB₅O₇F₃ (ref. 29–31)), CABF can survive at relatively high temperature in an open system after replacing BO₃F with AlO₃F units, which is an important advantage for large crystal growth.Open in a separate windowFig. 2(a) Thermal behaviour of CABF. (b) The diffuse reflectance spectrum of CABF. (c) PSHG measurements at 1064 nm. (d) Calculated type I phase-matching condition of CABF. Dashed lines: refractive-indices of fundamental light. Solid lines: refractive-indices of second-harmonic light. The λ_SH of CABF is estimated by satisfying n_Z(ω) = n_X(2ω). The SHG-weighted electron density maps of the occupied (e) and unoccupied (f) states in the VE process.CABF exhibits a wide transparency window from 300 to 1100 nm, with a short UV cut-off edge below 190 nm (Fig. 2b). Even at 190 nm, the reflectance is nearly 40%. DFT calculations of the band gap using the HSE06 hybrid functional with high accuracy result in 7.49 eV (corresponding to 166 nm), further indicating that CABF could be an excellent candidate for optical materials operating in the DUV region. As described earlier,^26,28 the large band gap of CABF could be attributed to the elimination of dangling bonds in [B₃O₆] units and the introduction of fluorine.Powder SHG (PSHG) measurement using the Kurtz–Perry method³² reveals that CABF has SHG intensities of 2.0× KDP (particle size range: 200–250 μm) under 1064 nm fundamental wave laser radiation. It also suggests that CABF is phase matchable as the SHG intensities continue to rise until it attains the maximum (Fig. 2c). Under 532 nm fundamental wave laser radiation, CABF also shows phase matching behavior and has an SHG response about 1/6 times that of BBO in the particle size range of 200–250 μm (Fig. S7^†). The SHG response of CABF is larger than that of KBBF (∼1.2× KDP) and suitable for DUV NLO applications. Applying Kleinmann symmetry in point group mm2, there are three nonzero and independent SHG coefficients, and our calculated SHG coefficients for CABF are d₁₅ = −0.027 pm V⁻¹, d₂₄ = 0.941 pm V⁻¹, and d₃₃ = −0.955 pm V⁻¹, which are consistent with the PSHG results. The SHG ability of CABF is comparable to those of other DUV NLO materials, including CBF (1.9× KDP),²⁶ ABF (3× KDP),²⁵ MB₅O₇F₃ (M = Ca and Sr, 2.3–2.5× KDP),^29–31 (NH₄)₂PO₃F (1.0× KDP),¹² and NaNH₄PO₃F·H₂O (1.1× KDP).¹⁶ Compared with the CBF archetype, the increased SHG response of CABF can be understood by considering the geometry factor of NLO-active [B₃O₆] units. Structurally, CABF is pretty much like CBF; however, the substitution of [BO₃F] by [AlO₃F] results in structural modulation of the [B₃O₆] groups. As shown in Fig. S8,^† the rotation angle (φ) and deviation angle (θ) of the B₃O₆ groups decrease from 33 and 6.8° in CBF, to 25 and 4.4° in CABF, respectively. The smaller φ and θ angles in CABF result in more “coplanar and aligned” [B₃O₆] units, that is, a favorable arrangement for generating the SHG response according to the anionic group approach.⁶The phase-matching ability is a key factor that should never be ignored for any practical DUV NLO application, which relies on both birefringence (Δn) and its dispersion of the crystal.³³ The calculated Δn of CABF is 0.091 at 1064 nm, large enough to satisfy DUV phase-matching (Fig. S10^†). Based on the dispersion of the refractive indices, the shortest SHG phase-matching wavelength (λ_SH) of CABF can reach 182 nm (Fig. 2d), which is comparable to that of KBBF and other recent reported NLO materials (Table S4^†). To the best of our knowledge, the phase-matching wavelength of CABF is the shortest among the SHG-active aluminum borates and aluminum borate fluorides (Table S4^†).To correlate the observed optical properties and electronic structure of CABF, band structure calculations were performed. The partial density of states (PDOS) projected on the constitutional atoms (see Fig. S9^†) demonstrates that B-2p, Al-3p, O-2p, and F-2p states occupy the frontier orbitals (the top of the valence band and the bottom of the conduction band). The high overlap of these orbitals indicates that both the [B₃O₆] and [AlO₃F] groups determine the SHG response and optical birefringence in CABF, while the Cs⁺ cations show a very small contribution. This observation was also supported by the SHG-weighted electron density analysis.³⁴ The directly perceived images (Fig. 2e, f, and S11^†) indicate that the 2p orbitals of O/F atoms in the occupied states and the anti π-orbitals of the [BO₃] groups in the unoccupied states dominate the SHG contribution. In particular, the O or F atoms in [AlO₃F] groups reveal considerable SHG density values, which further suggests that the [AlO₃F] group could be a promising NLO-active unit.     

An engineered biosynthetic–synthetic platform for production of halogenated indolmycin antibiotics

Indolmycin is an antibiotic from Streptomyces griseus ATCC 12648 with activity against Helicobacter pylori, Plasmodium falciparum, and methicillin-resistant Staphylococcus aureus. Here we describe the use of the indolmycin biosynthetic genes in E. coli to make indolmycenic acid, a chiral intermediate in indolmycin biosynthesis, which can then be converted to indolmycin through a three-step synthesis. To expand indolmycin structural diversity, we introduce a promiscuous tryptophanyl-tRNA synthetase gene (trpS) into our E. coli production system and feed halogenated indoles to generate the corresponding indolmycenic acids, ultimately allowing us to access indolmycin derivatives through synthesis. Bioactivity testing against methicillin-resistant Staphylococcus aureus showed modest antibiotic activity for 5-, 6-, and 7-fluoro-indolmycin.

A semi-synthetic system for producing indolmycin, an antibiotic, was developed and used to make indole-substituted, halogenated derivatives of indolmycin, some with modest bioactivity against methicillin-resistant Staphylococcus aureus.

Antibiotic-resistant bacteria pose a great threat to human health,^1–4 and the rates of new antibiotic discoveries and clinical approvals have been in a steep decline since the 1980s.¹ Without the discovery and development of new antibiotics, drug-resistant pathogens, such as methicillin-resistant Staphylococcus aureus (MRSA), will become increasingly prevalent.^2–4 One strategy to increase antibiotic development has been to “rediscover” known, but underdeveloped, antibiotics.¹ One such example is indolmycin, which was originally discovered in 1960 from Streptomyces griseus ATCC 12648⁵ but was not originally developed for clinical use because of its narrow spectrum of activity^6–10 and its interference with tryptophan catabolism in the liver.^9,10 However, reignited interest in this old antibiotic led to the discovery of its activity against Helicobacter pylori,⁶Plasmodium falciparum,¹¹ and MRSA.¹² For MRSA, indolmycin was found to be active against mupirocin- and fuscidic acid-resistant MRSA strains, with strains resistant to indolmycin emerging infrequently and with reduced fitness compared to sensitive strains.¹² In addition, indolmycin has been shown to have minimal activity against common members of the human microbiota, suggesting that its narrow spectrum of activity is an asset.⁶ The first indole-substituted derivatives, 5-hydroxy and 5-methoxyindolmycin, were made by precursor-directed feeding of the indolmycin producer, Streptomyces griseus ATCC 12648, and they showed modest improvements in bioactivity against S. aureus and Escherichia coli.¹³ Two practical synthetic routes to indolmycin and some indole-substituted derivatives have been reported more recently,^14,15 which enabled access to a small variety of indole-substituted derivatives. Additionally, a previous patent has described synthetic methods to produce a variety of derivatives; however, these methods do not appear to offer stereochemical control, and some require tailoring steps specific to each analog.¹⁶ Therefore, further development of indolmycin would benefit from a simpler diversification method that could be applied to produce a wider variety of analogs with stereochemical control.Inspired by early biosynthetic studies,^17,18 our group previously identified the indolmycin gene cluster and elucidated the biosynthetic pathway, demonstrating that indolmycin (1) is assembled from tryptophan, arginine and S-adenosylmethionine (SAM) in a three-part process (Fig. 1).¹⁹ In the first part, l-arginine is oxidized by Ind4 in an oxygen- and PLP-dependent reaction to 4,5-dehydro-2-iminoarginine, which is then enantioselectively reduced by imine reductase Ind5 and its chaperone Ind6 to 4,5-dehydro-d-arginine. In the second part, tryptophan (2) is deaminated by PLP-dependent transaminases, giving indole pyruvate (3). Compound 3 is then methylated by SAM-dependent C-methyltransferase Ind1 to 3-methyl-indolepyruvate (4) which is reduced by NADH-dependent ketone reductase Ind2 to form indolmycenic acid (5). Then, in the third part, 4,5-dehydro-d-arginine and 5 are coupled in an ATP-dependent fashion by Ind3 and Ind6, resulting in an oxazolinone-cyclized molecule, N-desmethyl-indolmycin, which is finally N-methylated by Ind7, a SAM-dependent N-methyltransferase, to form 1.Open in a separate windowFig. 1Indolmycin biosynthesis from Streptomyces griseus ATCC 12648. (a) Indolmycin biosynthetic gene cluster. (b) Indolmycin biosynthetic pathway from Streptomyces griseus ATCC 12648. (c) Semi-synthetic scheme towards indolmycin and derivatives using indolmycin biosynthetic genes. The dashed arrow indicates a predicted side-product based on LC-MS analysis.Armed with the elucidated biosynthetic pathway for 1, we set out to create an in vivo system to make 1 in E. coli. We first cloned all necessary genes into four plasmids and co-expressed the genes in E. coli (Fig. S1^†), a strain which we named E. coli I1234670P5 (Table S1^†). We found that the genes needed to produce indolmycin in E. coli were ind1, ind2, ind3, ind4, ind6, ind7, ind0 and pel5, a homologous gene of ind5 from Paenibacillus elgii B69 showing better production of active protein in E. coli.^20,21 We also relied on the activity of endogenous E. coli aminotransferases to catalyze the initial tryptophan deamination step. However, only a small amount of 1 was produced (∼170 μg L⁻¹ of bacterial culture) and the yield could not be improved despite our best efforts (Fig. 2a). However, we found that this construct produced substantial amounts of 5 ([M + H]⁺ = 220 m/z) at 40–50 mg L⁻¹ of culture, along with a shunt product, C-desmethyl-indolmycenic acid (6; [M + H]⁺ = 206 m/z).Open in a separate windowFig. 2Biosynthetic production of 5 and semi-synthetic production of 1. (a) Extracted ion chromatograms show production of 5 with minimal production of 1 from E. coli I1234670P5. (b) Synthetic scheme to 1 from 5, adapted from literature methods.^14,15 (c) Total ion chromatogram of compound 1 isolated after semi-synthesis and final purification by semi-preparative HPLC. Compounds are indicated with coloured boxes and numbered.Compound 5 itself has been a focus of total synthetic efforts toward 1, as it is the key chiral precursor.^{14,15,22–26} Since production of 5 was much higher than that of 1 from E. coli I1234670P5, we pursued a semi-synthetic method of obtaining 1 using our biosynthetic platform to access 5, combined with a three-step chemical transformation (Fig. 2b). We attempted to remove extraneous genes from our biosynthetic platform by only including ind0, ind1, and ind2, but the changes resulted in reduced amounts of 5 (Fig. S2^†). At this time, it is unclear which of the other genes may be contributing to the production of compound 5. Therefore, we employed the full eight-gene construct toward synthesis of 5. We then adapted the three-step synthesis to make indolmycin,^14,15 in which purified 5 was esterified to make the ethyl ester (7; [M + H]⁺ = 248 m/z; Fig. S3a^†), cyclized to give N-desmethyl-indolmycin (8; [M + H]⁺ = 244 m/z; Fig. S3b and S4a^†), and methylated at the exocyclic nitrogen to give 1 ([M + H]⁺ = 258 m/z; Fig. 2, S4b and Table S4^†).Then, we wanted to make derivatives of 1 from indole derivatives, which are more widely accessible than derivatives of 2. The tryptophan synthase (TrpS) from Salmonella enterica has been previously shown to couple a wide variety of indole derivatives to l-serine to generate derivatives of 2.²⁷ We were able to replace pel5 with trpS in our biosynthetic platform without a reduction in the amount of 5 produced (Fig. S1b^†), and we named the resulting strain E. coli I1234670TS. When we fed 5-fluoroindole to E. coli I1234670TS, we observed increased production of fluorinated metabolites, 5-fluoro-indolmycenic acid (5F-5; [M + H]⁺ = 238 m/z) and 5-fluoro-C-desmethyl-indolmycenic acid (5F-6; [M + H]⁺ = 224 m/z) (Fig. 3a). We then optimized the feeding conditions, finding that 5F-5 amounts were optimal when we fed E. coli I1234670TS with 0.5 mM 5-fluoroindole per day over two days (Fig. S5^†).Open in a separate windowFig. 3Addition of the trpS gene to the biosynthetic platform allows incorporation of substituted indoles into 5. (a) LC-MS analysis of strains with and without trpS (E. coli I1234670TS and E. coli I1234670P5, respectively) when fed 5-fluoroindole. Chemical structures of compounds with the corresponding extracted ion chromatogram are shown to the right of the traces. (b) Indole derivatives tested. Dark blue shows indoles that were incorporated into an analog of 5 (>48% of underivatized 5 by LC-MS analysis; Table S5^†) and further purified; medium blue indicates indoles that were incorporated into an analog of 5 at lower levels (6–22% of underivatized 5 by LC-MS analysis; Table S5^†) but were not further verified by purification; and light blue indicates indoles that did not show detectable incorporation into 5.To determine the scope of indole derivatives accepted by our biosynthetic platform, we fed a variety of indoles to E. coli I1234670TS and monitored the production of 5 and its derivatives by LC-MS. Out of the indoles tested, we found that fluorinated and chlorinated indoles substituted at the 5-, 6- and 7-positions were the best accepted by the biosynthetic platform (Fig. 3b, S6 and S7^†). We predict that lower acceptance of indoles substituted at the 4-position may be due to steric hindrance, as 4-fluoroindole was moderately accepted, while 4-chloroindole was not observed at all. Although we observed LC-MS peaks consistent with conversion of some of the azaindoles and hydroxyindoles into 5 derivatives, further work is required to confirm, optimize and scale up the purification of these compounds (Fig. S7^†). Cultures producing derivatives of 5, substituted at 5-, 6- and 7-positions, were further scaled up for purification of the 5-derivatives and downstream synthesis of 1-derivatives (Fig. S8–S10 and Table S4^†). Each purified derivative of 5 and 1 was characterized by HR-MS and NMR (Table S3 and ESI Methods^†). Overall, cultures fed with the fluorinated indoles produced a higher amount of 5-derivatives than the cultures fed with the chlorinated indoles. 1 and its derivatives were tested against MRSA (Fig. S11^†). While the fluorinated derivatives showed bioactivity, the chlorinated derivatives of 1 did not show bioactivity at the maximum amount tested in the disk diffusion assay (30 μg). We determined MIC₅₀ values for each fluorinated compound (Table 1). The MIC₅₀ values demonstrate that 1 is a more potent inhibitor of MRSA than its derivatives, while 6F-1 showed the most potent inhibition of MRSA compared to any of the derivatives, followed by 7F-1 and 5F-1. The lack of bioactivity of the chlorinated compounds may be due to the bulky chlorinated substituent hindering the compounds'' abilities to bind to the tryptophanyl-tRNA synthetase (TrpRS) target, which is supported by docking studies of the analogs into a bacterial TrpRS structure (Fig. S12^†).MIC₅₀ values determined for 1 and its derivatives against MRSA. Values represent the average of three replicates ± the standard deviation. For 1, 5F-1, 6F-1 and 7F-1, concentrations ranging from 50 μg mL⁻¹ to 0.128 ng mL⁻¹ were tested, and for 5Cl-1, 6Cl-1 and 7Cl-1, concentrations ranging from 200 μg mL⁻¹ to 0.512 ng mL⁻¹ were tested. The reported MIC₅₀ value for indolmycin is 0.5 μg mL⁻¹ (range: 0.125–2 μg mL⁻¹) against MRSA¹²