Ruthenium(II)–Polyimine–Coumarin Light‐Harvesting Molecular Arrays: Design Rationale and Application for Triplet–Triplet‐Annihilation‐Based Upconversion

Ru^II–bis‐pyridine complexes typically absorb below 450 nm in the UV spectrum and their molar extinction coefficients are only moderate (ε<16 000 M ^?1 cm^?1). Thus, Ru^II–polyimine complexes that show intense visible‐light absorptions are of great interest. However, no effective light‐harvesting ruthenium(II)/organic chromophore arrays have been reported. Herein, we report the first visible‐light‐harvesting Ru^II–coumarin arrays, which absorb at 475 nm (ε up to 63 300 M ^?1 cm^?1, 4‐fold higher than typical Ru^II–polyimine complexes). The donor excited state in these arrays is efficiently converted into an acceptor excited state (i.e., efficient energy‐transfer) without losses in the phosphorescence quantum yield of the acceptor. Based on steady‐state and time‐resolved spectroscopy and DFT calculations, we proposed a general rule for the design of Ru^II–polypyridine–chromophore light‐harvesting arrays, which states that the ¹IL energy level of the ligand must be close to the respective energy level of the metal‐to‐ligand charge‐transfer (M LCT) states. Lower energy levels of ¹IL/³IL than the corresponding ¹M LCT/³M LCT states frustrate the cascade energy‐transfer process and, as a result, the harvested light energy cannot be efficiently transferred to the acceptor. We have also demonstrated that the light‐harvesting effect can be used to improve the upconversion quantum yield to 15.2 % (with 9,10‐diphenylanthracene as a triplet‐acceptor/annihilator), compared to the parent complex without the coumarin subunit, which showed an upconversion quantum yield of only 0.95 %.     

A Methylthio‐Functionalized‐MOF Photocatalyst with High Performance for Visible‐Light‐Driven H2 Evolution

Recently, the emergence of photoactive metal–organic frameworks (MOFs) has given great prospects for their applications as photocatalytic materials in visible‐light‐driven hydrogen evolution. Herein, a highly photoactive visible‐light‐driven material for H₂ evolution was prepared by introducing methylthio terephthalate into a MOF lattice via solvent‐assisted ligand‐exchange method. Accordingly, a first methylthio‐functionalized porous MOF decorated with Pt co‐catalyst for efficient photocatalytic H₂ evolution was achieved, which exhibited a high quantum yield (8.90 %) at 420 nm by use sacrificial triethanolamine. This hybrid material exhibited perfect H₂ production rate as high as 3814.0 μmol g^?1 h^?1, which even is one order of magnitude higher than that of the state‐of‐the‐art Pt/MOF photocatalyst derived from aminoterephthalate.     

Bis‐substituted thiophene‐containing oxime sulfonates photoacid generators for cationic polymerization under UV–visible LED irradiation

Three novel types of thiophene‐containing oxime sulfonates with a big π‐conjugated system were reported as non‐ionic photoacid generators. The irradiation of the newly synthesized photoacid generators using near UV–visible light‐emitting diodes (LEDs) (365–475 nm) results in the cleavage of two weak N O bonds in single molecules, which lead to the generation of different sulfonic acids in good quantum and chemical yields. The mechanism for the N O bond cleavage for acid generation was supported by the UV–visible spectra and real‐time ¹H NMR spectra. They are developed as high‐performance photoinitiators without any additives for the cationic polymerization of epoxide and vinyl ether upon exposure to near‐UV and visible LEDs (365–475 nm) at low concentration. In the field of photopolymerization, especially visible light polymerization, it has great potential for application. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 776–782     

D‐π‐a‐type oxime sulfonate photoacid generators for cationic polymerization under UV–visible LED irradiation

Four D‐π‐A‐type nonionic oxime sulfonate photoacid generators (PAGs) have been designed and synthesized for use in light‐emitting diode (LED) excitable cationic photoinitiators, in which N,N‐diphenylamino was used as electron donor with trifluoroacetophenone‐based oxime sulfonates (trifluoromethanoesulfonate and p‐toluenesulfonate) as electron acceptor and substituted fluorene and biphenyl groups as the π‐conjugated systems. PAG‐Ben‐Tol (with biphenyl and p‐toluenesulfonate) and PAG‐Flu‐Tol (with fluorene and p‐toluenesulfonate) showed high quantum yields of photoacid generation (0.33–0.50) and very good thermal stability (over 250 °C). The absorbance spectra of these PAGs were consistent with the emission spectra of commercially gained UV–visible LED light sources. The potential of these PAGs for cationic photoinitiators was tested in two cationic monomer systems. These PAGs needed low light intensity and low concentration for photopolymerization with high conversions of monomer, for example, over 80%, gained at 3.0 mW cm⁻² from 365 to 470 nm LEDs. The photochemical mechanisms of these PAGs are comprehensively investigated and discussed in detail. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1146–1154     

Effects of Substituents on Metastable‐State Photoacids: Design,Synthesis, and Evaluation of their Photochemical Properties

Recently, metastable‐state photoacids have been widely used to control proton transfer in numerous chemical and biological processes as well as applications with visible light. Generally, substituents have a great influence on the photochemical properties of molecules, which will further affect their applications. Yet, the effects of substituents on metastable‐state photoacids have not been studied systematically. In this work, 16 metastable‐state photoacid derivatives were designed and synthesized on the basis of substituents having a large range of σ–π electron–donor–acceptor capabilities. The effects of substituents on the color display [or maximum absorption band(s)], solubility, pK_a values, dark/photoacidity, photosensitivity, and relaxation kinetic(s) were investigated in detail. This study will be helpful for the targeted design and synthesis of promising photoacids and the application of their photocontrolled proton‐release processes in functional materials/devices.     

Hydrogen‐Bonded Organic Frameworks (HOFs): A New Class of Porous Crystalline Proton‐Conducting Materials

Two porous hydrogen‐bonded organic frameworks (HOFs) based on arene sulfonates and guanidinium ions are reported. As a result of the presence of ionic backbones appended with protonic source, the compounds exhibit ultra‐high proton conduction values (σ) 0.75× 10^?2 S cm^?1 and 1.8×10^?2 S cm^?1 under humidified conditions. Also, they have very low activation energy values and the highest proton conductivity at ambient conditions (low humidity and at moderate temperature) among porous crystalline materials, such as metal–organic frameworks (MOFs) and covalent organic frameworks (COFs). These values are not only comparable to the conventionally used proton exchange membranes, such as Nafion used in fuel cell technologies, but is also the highest value reported in organic‐based porous architectures. Notably, this report inaugurates the usage of crystalline hydrogen‐bonded porous organic frameworks as solid‐state proton conducting materials.     

Nature‐Inspired Environmental “Phosphorylation” Boosts Photocatalytic H2 Production over Carbon Nitride Nanosheets under Visible‐Light Irradiation

Inspired by the crucial roles of phosphates in natural photosynthesis, we explored an environmental “phosphorylation” strategy for boosting photocatalytic H₂ production over g‐C₃N₄ nanosheets under visible light. As expected, a substantial improvement was observed in the rate of H₂ evolution to 947 μmol h^?1, and the apparent quantum yield was as high as 26.1 % at 420 nm. The synergy of enhanced proton reduction and improved hole oxidation is proposed to account for the markedly increased activity. Our findings may provide a promising and facile approach to highly efficient photocatalysis for solar‐energy conversion.     

Asymmetrical Fluorene[2,3‐b]benzo[d]thiophene Derivatives: Synthesis,Solid‐State Structures,and Application in Solution‐Processable Organic Light‐Emitting Diodes

A series of novel asymmetrical fused compounds containing the backbone of fluorene[2,3‐b]benzo[d]thiophene (FBT) were effectively synthesized and fully characterized. Single‐crystal X‐ray studies demonstrated that the length of the substituent side chains greatly affects the solid‐state packing of the obtained fused compounds. DFT, photophysical, and electrochemical studies all showed that the FBTs have large band gaps, low‐lying HOMO energy levels, and therefore good stability toward oxidation. Moreover, the substituents strongly influence the fluorescence properties of the resulting FBT derivatives. The di‐n‐hexyl compound exhibits intense fluorescence in solution with the highest quantum yield of up to 91 %. Solution‐processed green phosphorescent organic light‐emitting diodes with the di‐n‐butyl derivative as the host material exhibited a maximum brightness of 14 185 cd m^?2 and a luminescence efficiency of 12 cd A^?1.     

2,5‐Difluorenyl‐Substituted Siloles for the Fabrication of High‐Performance Yellow Organic Light‐Emitting Diodes

2,3,4,5‐Tetraarylsiloles are a class of important luminogenic materials with efficient solid‐state emission and excellent electron‐transport capacity. However, those exhibiting outstanding electroluminescence properties are still rare. In this work, bulky 9,9‐dimethylfluorenyl, 9,9‐diphenylfluorenyl, and 9,9′‐spirobifluorenyl substituents were introduced into the 2,5‐positions of silole rings. The resulting 2,5‐difluorenyl‐substituted siloles are thermally stable and have low‐lying LUMO energy levels. Crystallographic analysis revealed that intramolecular π–π interactions are prone to form between 9,9′‐spirobifluorene units and phenyl rings at the 3,4‐positions of the silole ring. In the solution state, these new siloles show weak blue and green emission bands, arising from the fluorenyl groups and silole rings with a certain extension of π conjugation, respectively. With increasing substituent volume, intramolecular rotation is decreased, and thus the emissions of the present siloles gradually improved and they showed higher fluorescence quantum yields (Φ_F=2.5–5.4 %) than 2,3,4,5‐tetraphenylsiloles. They are highly emissive in solid films, with dominant green to yellow emissions and good solid‐state Φ_F values (75–88 %). Efficient organic light‐emitting diodes were fabricated by adopting them as host emitters and gave high luminance, current efficiency, and power efficiency of up to 44 100 cd m^?2, 18.3 cd A^?1, and 15.7 lm W^?1, respectively. Notably, a maximum external quantum efficiency of 5.5 % was achieved in an optimized device.     

Triarylboron‐Linked Conjugated Microporous Polymers: Sensing and Removal of Fluoride Ions

A luminescent conjugated microporous polymer (BCMP‐3) has been synthesized in high yield by a carbon–carbon coupling reaction using triarylboron as a building unit. BCMP‐3 was fully characterized by using powder X‐ray diffraction analysis, Fourier transform infrared spectroscopy, ¹³C solid‐state NMR spectroscopy, field emission scanning electron microscopy, thermogravimetric analysis, and nitrogen and carbon dioxide adsorption. The new three‐dimensional conjugated framework possess a high Brunauer–Emmett–Teller (BET) specific surface area up to 950 m² g^?1 with a pore volume of 0.768 cm³ g^?1, good stability, and abundant boron sites in the skeleton. Under excited‐light irradiation, BCMP‐3 exhibits strong fluorescent emission at 488 nm with a high absolute quantum yield of 18 % in the solid state. Polymer BCMP‐3 acts as a colorimetric and fluorescent chemosensor with high sensitivity and selectivity for F^? over other common anions. In addition, the polymer also works as an adsorbent for F^? removal and shows good adsorption capacities of up to 24 mg g^?1 at equilibrium F^? concentrations of 16 mg L^?1 and a temperature of 298 K. The adsorption kinetics and isotherm were analyzed by fitting experimental data with pseudo‐second‐order kinetics and Langmuir equations. Furthermore, we highlight that BCMP‐3 is an adsorbent for fluoride removal that can be efficiently reused many times without loss of adsorption efficiency.     

UV‐Light‐Induced Hydrogen Transfer in Guanosine–Guanosine Aggregates

Aggregates of a lipophilic guanine (G) derivative have been studied in n‐hexane by femtosecond‐to‐microsecond UV‐visible broadband transient absorption, stationary infrared and UV‐visible spectroscopy and by quantum chemical calculations. We report the first time‐resolved spectroscopic detection of hydrogen transfer in GG aggregates, which leads to (G?H) ^. radicals by means of G⁺G^? charge transfer followed by proton transfer. These radicals show a characteristic electronic spectrum in the range 300–550 nm. The calculated superimposed spectrum of the species that result from NH???N proton transfer agrees best with the experimental spectrum.     

A New p‐Metal‐n Structure AgBr‐Ag‐BiOBr with Superior Visible‐Light‐Responsive Catalytic Performance

A novel visible‐light‐driven AgBr‐Ag‐BiOBr photocatalyst was synthesized by a facile hydrothermal method. Taking advantage of both p‐n heterojunctions and localized surface plasmon resonance, the p‐metal‐n structure exhibited a superior performance concerning degradation of methyl orange under visible‐light irradiation (λ>420 nm). A possible photodegradation mechanism in the presence of AgBr‐Ag‐BiOBr composites was proposed, and the radical species involved in the degradation reaction were investigated. HO₂^?/^?O₂^? played the same important role as ^?OH in the AgBr‐Ag‐BiOBr photocatalytic system, and both the electron and hole were fully used for degradation of organic pollutants. A dual role of metallic Ag in the photocatalysis was proposed, one being surface plasmon resonance and the other being an electron‐hole bridge. Due to the distinctive p‐metal‐n structure, the visible‐light absorption, the separation of photogenerated carriers and the photocatalysis efficiency were greatly enhanced.     

Efficient NIR‐Light Emission from Solid‐State Complexes of Boron Difluoride with 2′‐Hydroxychalcone Derivatives

This article describes a series of nine complexes of boron difluoride with 2′‐hydroxychacone derivatives. These dyes were synthesized very simply and exhibited intense NIR emission in the solid state. Complexation with boron was shown to impart very strong donor–acceptor character into the excited state of these dyes, which further shifted their emission towards the NIR region (up to 855 nm for dye 5 b , which contained the strongly donating triphenylamine group). Strikingly, these optical features were obtained for crystalline solids, which are characterized by high molecular order and tight packing, two features that are conventionally believed to be detrimental to luminescence in organic crystals. Remarkably, the emission of light from the π‐stacked molecules did not occur at the expense of the emission quantum yield. Indeed, in the case of pyrene‐containing dye 4 , for example, a fluorescence quantum yield of about 15 % with a fluorescence emission maximum at 755 nm were obtained in the solid state. Moreover, dye 3 a and acetonaphthone‐based compounds 1 b , 2 b , and 3 b showed no evidence of degradation as solutions in CH₂Cl₂ that contained EtOH. In particular, solutions of brightly fluorescent compound 3 a (brightness: ε×Φ_f=45 000 M ^?1 cm^?1) could be stored for long periods without any detectable changes in its optical properties. All together, these new dyes possess a set of very interesting properties that make them promising solid‐state NIR fluorophores for applications in materials science.     

Highly Efficient Photocatalytic H2 Evolution from Water using Visible Light and Structure‐Controlled Graphitic Carbon Nitride

The major challenge of photocatalytic water splitting, the prototypical reaction for the direct production of hydrogen by using solar energy, is to develop low‐cost yet highly efficient and stable semiconductor photocatalysts. Herein, an effective strategy for synthesizing extremely active graphitic carbon nitride (g‐C₃N₄) from a low‐cost precursor, urea, is reported. The g‐C₃N₄ exhibits an extraordinary hydrogen‐evolution rate (ca. 20 000 μmol h^?1 g^?1 under full arc), which leads to a high turnover number (TON) of over 641 after 6 h. The reaction proceeds for more than 30 h without activity loss and results in an internal quantum yield of 26.5 % under visible light, which is nearly an order of magnitude higher than that observed for any other existing g‐C₃N₄ photocatalysts. Furthermore, it was found by experimental analysis and DFT calculations that as the degree of polymerization increases and the proton concentration decreases, the hydrogen‐evolution rate is significantly enhanced.     

Visible‐Light Photocatalytic Hydrogen Production Activity of ZnIn2S4 Microspheres Using Carbon Quantum Dots and Platinum as Dual Co‐catalysts

ZnIn₂S₄ microspheres (ZIS MSs) were for the first time decorated with carbon quantum dots (CQDs) and platinum nanoparticles (NPs) as dual co‐catalysts of for photocatalytic H₂ production. The ZIS MSs co‐loaded with CQDs and Pt exhibited a high photocatalytic H₂ production rate of 1032.2 μmol h^?1 g^?1 with an apparent quantum efficiency of 2.2 % (420 nm) in triethanolamine aqueous solution under visible‐light irradiation, which was much higher than the respective photocatalytic rates of pure ZIS, Pt loaded ZIS, and CQDs‐decorated ZIS. Such a great enhancement was attributed to the integrative effect of good crystallization, enhanced light absorption, high electrical conductivity of CQDs, and the vectorial electron transfer from ZIS to CQDs and Pt NPs (ZIS→CQDs→Pt).     

Designing Sub‐2 nm Organosilica Nanohybrids for Far‐Field Super‐Resolution Imaging

Stimulated emission depletion (STED) microscopy enables ultrastructural imaging of biological samples with high spatiotemporal resolution. STED nanoprobes based on fluorescent organosilica nanohybrids featuring sub‐2 nm size and near‐unity quantum yield are presented. The spin–orbit coupling (SOC) of heavy‐atom‐rich organic fluorophores is mitigated through a silane‐molecule‐mediated condensation/dehalogenation process, resulting in bright fluorescent organosilica nanohybrids with multiple emitters in one hybrid nanodot. When harnessed as STED nanoprobes, these fluorescent nanohybrids show intense photoluminescence, high biocompatibility, and long‐term photostability. Taking advantage of the low‐power excitation (0.5 μW), prolonged singlet‐state lifetime, and negligible depletion‐induced re‐excitation, these STED nanohybrids present high depletion efficiency (>96 %), extremely low saturation intensity (0.54 mW, ca. 0.188 MW cm^?2), and ultra‐high lateral resolution (ca. λ_em/28).     

Light‐Harvesting Photocatalysis for Water Oxidation Using Mesoporous Organosilica

An organic‐based photocatalysis system for water oxidation, with visible‐light harvesting antennae, was constructed using periodic mesoporous organosilica (PMO). PMO containing acridone groups in the framework (Acd‐PMO), a visible‐light harvesting antenna, was supported with [Ru^II(bpy)₃²⁺] complex (bpy=2,2′‐bipyridyl) coupled with iridium oxide (IrO_x) particles in the mesochannels as photosensitizer and catalyst, respectively. Acd‐PMO absorbed visible light and funneled the light energy into the Ru complex in the mesochannels through excitation energy transfer. The excited state of Ru complex is oxidatively quenched by a sacrificial oxidant (Na₂S₂O₈) to form Ru³⁺ species. The Ru³⁺ species extracts an electron from IrO_x to oxidize water for oxygen production. The reaction quantum yield was 0.34 %, which was improved to 0.68 or 1.2 % by the modifications of PMO. A unique sequence of reactions mimicking natural photosystem II, 1) light‐harvesting, 2) charge separation, and 3) oxygen generation, were realized for the first time by using the light‐harvesting PMO.     

Long‐Lived Room‐Temperature Near‐IR Phosphorescence of BODIPY in a Visible‐Light‐Harvesting N^C^N PtII–Acetylide Complex with a Directly Metalated BODIPY Chromophore

Room‐temperature long‐lived near‐IR phosphorescence of boron‐dipyrromethene (BODIPY) was observed (λ_em=770 nm, Φ_P=3.5 %, τ_P=128.4 μs). Our molecular‐design strategy is to attach Pt^II coordination centers directly onto the BODIPY π‐core using acetylide bonds, rather than on the periphery of the BODIPY core, thus maximizing the heavy‐atom effect of Pt^II. In this case, the intersystem crossing (ISC) is facilitated and the radiative decay of the T₁ excited state of BODIPY is observed, that is, the phosphorescence of BODIPY. The complex shows strong absorption in the visible range (ε=53800 M ^?1 cm^?1 at 574 nm), which is rare for Pt^II–acetylide complexes. The complex is dual emissive with ³M LCT emission at 660 nm and the ³IL emission at 770 nm. The T₁ excited state of the complex is mainly localized on the BODIPY moiety (i.e. ³IL state, as determined by steady‐state and time‐resolved spectroscopy, 77 K emission spectra, and spin‐density analysis). The strong visible‐light‐harvesting ability and long‐lived T₁ excite state of the complex were used for triplet‐triplet annihilation based upconversion and an upconversion quantum yield of 5.2 % was observed. The overall upconversion capability (η=ε×Φ_UC) of this complex is remarkable considering its strong absorption. The model complex, without the BODIPY moiety, gives no upconversion under the same experimental conditions. Our work paves the way for access to transition‐metal complexes that show strong absorption of visible light and long‐lived ³IL excited states, which are important for applications in photovoltaics, photocatalysis, and upconversions, etc.     

A Theranostic Agent Combining a Two‐Photon‐Absorbing Photosensitizer for Photodynamic Therapy and a Gadolinium(III) Complex for MRI Detection

The convergent synthesis and characterization of a potential theranostic agent, [DPP‐ZnP‐GdDOTA]^?, which combines a diketopyrrolopyrrole‐porphyrin component DPP‐ZnP as a two‐photon photosensitizer for photodynamic therapy (PDT) with a gadolinium(III) DOTA complex as a magnetic resonance imaging probe, is presented. [DPP‐ZnP‐GdDOTA]^? has a remarkably high longitudinal water proton relaxivity (19.94 mm ^?1 s^?1 at 20 MHz and 25 °C) for a monohydrated molecular system of this size. The Nuclear Magnetic Relaxation Dispersion (NMRD) profile is characteristic of slow rotation, related to the extended and rigid aromatic units integrated in the molecule and to self‐aggregation occurring in aqueous solution. The two‐photon properties were examined and large two‐photon absorption cross‐sections around 1000 GM were determined between 910 and 940 nm in DCM with 1 % pyridine and in DMSO. Furthermore, the new conjugate was able to generate singlet oxygen, with quantum yield of 0.42 and 0.68 in DCM with 1 % pyridine and DMSO, respectively. Cellular studies were also performed. The [DPP‐ZnP‐GdDOTA]^? conjugate demonstrated low dark toxicity and was able to induce high one‐photon and moderate two‐photon phototoxicity on cancer cells.     

Guest‐Assisted Proton Conduction in the Sulfonic Mesoporous MIL‐101 MOF

Post‐synthesis modification of MIL‐101(Cr)‐NO₂ was explored in order to decorate the organic backbone by propyl‐sulfonic groups, with the aim to incorporate mobile and acidic protons for solid‐state proton electrolyte applications. The resulting solid switched from insulating towards proton superconductive behavior under humidity, while the conductivity recorded at 363 K and 95 % relative humidity reached 4.8×10^?3 S cm^?1. Propitiously, the impregnation of the material by strong acidic molecules (H₂SO₄) further boosted the proton conductivity performances up to the remarkable σ value of 1.3×10^?1 S cm^?1 at 363 K/95 % RH, which reaches the performances of the best proton conductive MOF reported so far.