Metal-free synthesis of pyridyl conjugated microporous polymers with tunable bandgaps for efficient visible-light-driven hydrogen evolution

Conjugated microporous polymers (CMPs) with tunable bandgaps have attracted increasing attention for photocatalytic hydrogen evolution. However, the synthesis of CMPs usually needs expensive metal-based catalysts. Herein, we report a metal-free synthetic route to fabricate pyridyl conjugated microporous polymers (PCMPs) via a condensed polymerization between aldehyde and aryl ketone monomers. The PCMPs show widely tunable specific surface areas (347–418 m²/g), which were controlled via changing the used monomers. The PCMPs synthesized using monomers of dialdehyde and diacetylbenzene (diacetylpyridine) in the presence of pyridine exhibited the highest visible-light driven hydrogen evolution rate (9.56 µmol/h). These novel designed PCMPs provide wide adaptability to current materials designed for high-performance photocatalysts in different applications.     

Symmetry-breaking synthesis of Janus Au/CeO2 nanostructures for visible-light nitrogen photofixation

Precise manipulation of the reactive site spatial distribution in plasmonic metal/semiconductor photocatalysts is crucial to their photocatalytic performance, but the construction of Janus nanostructures through symmetry-breaking synthesis remains a significant challenge. Here we demonstrate a synthetic strategy for the selective growth of a CeO₂ semi-shell on Au nanospheres (NSs) to fabricate Janus Au NS/CeO₂ nanostructures with the assistance of a SiO₂ hard template and autoredox reaction between Ag⁺ ions and a ceria precursor. The obtained Janus nanostructures possess a spatially separated architecture and exhibit excellent photocatalytic performance toward N₂ photofixation under visible-light illumination. In this scenario, N₂ molecules are reduced by hot electrons on the CeO₂ semi-shell, while hole scavengers are consumed by hot holes on the exposed Au NS surface, greatly promoting the charge carrier separation. Moreover, the exposed Au NS surface in the Janus structures offers an additional opportunity for the fabrication of ternary Janus noble metal/Au NS/CeO₂ nanostructures. This work highlights the genuine superiority of the spatially separated nanoarchitectures in the photocatalytic reaction, offering instructive guidance for the design and construction of novel plasmonic photocatalysts.

We demonstrate a synthetic strategy to selectively grow a CeO₂ semi-shell on Au nanospheres through the symmetry-breaking synthesis. The asymmetric nanostructures facilitate the charge carrier separation during the visible-light N₂ photofixation.     

High-performance polymer field-effect transistors: from the perspective of multi-level microstructures

The multi-level microstructure of conjugated polymers is the most critical parameter determining the charge transport property in field-effect transistors (FETs). However, controlling the hierarchical microstructures and the structural evolution remains a significant challenge. In this perspective, we discuss the key aspects of multi-level microstructures of conjugated polymers towards high-performance FETs. We highlight the recent progress in the molecular structures, solution-state aggregation, and polymer crystal structures, representing the multi-level microstructures of conjugated polymers. By tuning polymer hierarchical microstructures, we attempt to provide several guidelines for developing high-performance polymer FETs and polymer electronics.

The multi-level microstructures of conjugated polymers, including solution-state aggregation and crystal structures, are reviewed due to their influence on charge transport in polymer field-effect transistors.     

The origin of unidirectional charge separation in photosynthetic reaction centers: nonadiabatic quantum dynamics of exciton and charge in pigment–protein complexes

Exciton charge separation in photosynthetic reaction centers from purple bacteria (PbRC) and photosystem II (PSII) occurs exclusively along one of the two pseudo-symmetric branches (active branch) of pigment–protein complexes. The microscopic origin of unidirectional charge separation in photosynthesis remains controversial. Here we elucidate the essential factors leading to unidirectional charge separation in PbRC and PSII, using nonadiabatic quantum dynamics calculations in conjunction with time-dependent density functional theory (TDDFT) with the quantum mechanics/molecular mechanics/polarizable continuum model (QM/MM/PCM) method. This approach accounts for energetics, electronic coupling, and vibronic coupling of the pigment excited states under electrostatic interactions and polarization of whole protein environments. The calculated time constants of charge separation along the active branches of PbRC and PSII are similar to those observed in time-resolved spectroscopic experiments. In PbRC, Tyr-M210 near the accessary bacteriochlorophyll reduces the energy of the intermediate state and drastically accelerates charge separation overcoming the electron–hole interaction. Remarkably, even though both the active and inactive branches in PSII can accept excitons from light-harvesting complexes, charge separation in the inactive branch is prevented by a weak electronic coupling due to symmetry-breaking of the chlorophyll configurations. The exciton in the inactive branch in PSII can be transferred to the active branch via direct and indirect pathways. Subsequently, the ultrafast electron transfer to pheophytin in the active branch prevents exciton back transfer to the inactive branch, thereby achieving unidirectional charge separation.

Essential factors leading to unidirectional charge separation in photosynthetic reaction centers are clarified via nonadiabatic quantum dynamics calculations.     

Visible-light-induced intramolecular charge transfer in the radical spirocyclisation of indole-tethered ynones

Indole-tethered ynones form an intramolecular electron donor–acceptor complex that can undergo visible-light-induced charge transfer to promote thiyl radical generation from thiols. This initiates a novel radical chain sequence, based on dearomatising spirocyclisation with concomitant C–S bond formation. Sulfur-containing spirocycles are formed in high yields using this simple and mild synthetic protocol, in which neither transition metal catalysts nor photocatalysts are required. The proposed mechanism is supported by various mechanistic studies, and the unusual radical initiation mode represents only the second report of the use of an intramolecular electron donor–acceptor complex in synthesis.

Indole-tethered ynones form an intramolecular electron donor–acceptor complex that can undergo visible-light-induced charge transfer to promote thiyl radical generation from thiols.     

Catalytic (3 + 2) annulation of donor–acceptor aminocyclopropane monoesters and indoles

The efficient catalytic activation of donor–acceptor aminocyclopropanes lacking the commonly used diester acceptor is reported here in a (3 + 2) dearomative annulation with indoles. Bench-stable tosyl-protected aminocyclopropyl esters were converted into cycloadducts in 46–95% yields and up to 95 : 5 diastereomeric ratio using catalytic amounts of triethylsilyl triflimide. Tricyclic indoline frameworks containing four stereogenic centers including all-carbon quaternary centers were obtained.

A catalytic dearomatization of indoles with D–A aminocyclopropane monoesters for the synthesis of highly substituted indolines.     

Cyanine-based near infra-red organic photoredox catalysis

Direct metal-free near infra-red photoredox catalysis is applied to organic oxidation, photosensitization and reduction, involving cyanines as photocatalysts. This photocatalyst is competitive with conventional reactions catalyzed under visible light. Kinetic and quenching experiments are also reported. Interestingly, these systems are compatible with water media, opening perspective for various applications.

Direct metal-free near infra-red photoredox catalysis is applied to oxidation, reduction and photosensitization, involving cyanines as photocatalysts. Mechanistic insights through kinetic and quenching experiments are also reported.     

C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles by tuning Pd catalytic modes: Pd(i)–Pd(ii) catalysis vs. Pd(ii) catalysis

Efficient C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles have been developed. The former route enables C4-arylation in a highly efficient and mild manner and the latter route provides an alternative straightforward protocol for synthesis of C2/C4 disubstituted indoles. The mechanism studies imply that the different reaction pathways were tuned by the distinct acid additives, which led to either the Pd(i)–Pd(ii) pathway or Pd(ii) catalysis.

C4-arylation via Pd(i)–Pd(ii) catalysis and domino C4-arylation/3,2-carbonyl migration of indoles via Pd(ii) catalysis tuning by acids have been developed.     

Sequential C–O decarboxylative vinylation/C–H arylation of cyclic oxalates via a nickel-catalyzed multicomponent radical cascade

A selective, sequential C–O decarboxylative vinylation/C–H arylation of cyclic alcohol derivatives enabled by visible-light photoredox/nickel dual catalysis is described. This protocol utilizes a multicomponent radical cascade process, i.e. decarboxylative vinylation/1,5-HAT/aryl cross-coupling, to achieve efficient, site-selective dual-functionalization of saturated cyclic hydrocarbons in one single operation. This synergistic protocol provides straightforward access to sp³-enriched scaffolds and an alternative retrosynthetic disconnection to diversely functionalized saturated ring systems from the simple starting materials.

A selective, sequential C–O decarboxylative vinylation/C–H arylation of cyclic alcohol derivatives enabled by visible-light photoredox/nickel dual catalysis has been described.     

Anion-enhanced excited state charge separation in a spiro-locked N-heterocycle-fused push-pull zinc porphyrin

A new type of push–pull charge transfer complex, viz., a spiro-locked N-heterocycle-fused zinc porphyrin, ZnP-SQ, is shown to undergo excited state charge separation, which is enhanced by axial F⁻ binding to the Zn center. In this push–pull design, the spiro-quinone group acts as a ‘lock’ promoting charge transfer interactions by constraining mutual coplanarity of the meso-phenol-substituted electron-rich Zn(ii) porphyrin and an electron deficient N-heterocycle, as revealed by electrochemical and computational studies. Spectroelectrochemical studies have been used to identify the spectra of charge separated states, and charge separation upon photoexcitation of ZnP has been unequivocally established by using transient absorption spectroscopic techniques covering wide spatial and temporal regions. Further, global target analysis of the transient data using GloTarAn software is used to obtain the lifetimes of different photochemical events and reveal that fluoride anion complexation stabilizes the charge separated state to an appreciable extent.

A new type of push–pull charge transfer complex, viz., a spiro-locked N-heterocycle-fused zinc porphyrin, ZnP-SQ, is shown to undergo excited state charge separation, which is enhanced by axial F⁻ binding to the Zn center.     

Charge stabilization via electron exchange: excited charge separation in symmetric,central triphenylamine derived,dimethylaminophenyl–tetracyanobutadiene donor–acceptor conjugates

Photoinduced charge separation in donor–acceptor conjugates plays a pivotal role in technology breakthroughs, especially in the areas of efficient conversion of solar energy into electrical energy and fuels. Extending the lifetime of the charge separated species is a necessity for their practical utilization, and this is often achieved by following the mechanism of natural photosynthesis where the process of electron/hole migration occurs distantly separating the radical ion pairs. Here, we hypothesize and demonstrate a new mechanism to stabilize the charge separated states via the process of electron exchange among the different acceptor entities in multimodular donor–acceptor conjugates. For this, star-shaped, central triphenylamine derived, dimethylamine–tetracyanobutadiene conjugates have been newly designed and characterized. Electron exchange was witnessed upon electroreduction in conjugates having multiple numbers of electron acceptors. Using ultrafast spectroscopy, the occurrence of excited state charge separation, and the effect of electron exchange in prolonging the lifetime of charge separated states in the conjugates having multiple acceptors have been successfully demonstrated. This work constitutes the first example of stabilizing charge-separated states via the process of electron exchange.

The significance of electron exchange in stabilizing the charge-separated state is revealed in multi-modular donor–acceptor conjugates.     

Catalytic enantioselective synthesis of macrodiolides and their application in chiral recognition

New types of C₂-symmetric chiral macrodiolides are readily obtained via chiral N,N′-dioxide-scandium(iii) complex-promoted asymmetric tandem Friedel–Crafts alkylation/intermolecular macrolactonization of ortho-quinone methides with C3-substituted indoles. This protocol provides an array of enantioenriched macrodiolides with 16, 18 or 20-membered rings in moderate to good yields with high diastereoselectivities and excellent enantioselectivities through adjusting the length of the tether at the C3 position of indoles. Density functional theory calculations indicate that the formation of macrocycles is more favorable than that of 9-membered-ring lactones in terms of kinetics and thermodynamics. The potential utility of these intriguing chiral macrodiolide molecules is demonstrated in the enantiomeric recognition of aminols and chemical recognition of metal ions.

An asymmetric tandem Friedel–Crafts alkylation/intermolecular macrolactonization of ortho-quinone methides with C3-substituted indoles was achieved by using a chiral N,N′-dioxide-scandium(iii) complex.     

Single-atom nickel terminating sp2 and sp3 nitride in polymeric carbon nitride for visible-light photocatalytic overall water splitting

Polymeric carbon nitride (PCN) has been widely used as a metal-free photocatalyst for solar hydrogen generation from water. However, rapid charge carrier recombination and sluggish water catalysis kinetics have greatly limited its photocatalytic performance for overall water splitting. Herein, a single-atom Ni terminating agent was introduced to coordinate with the heptazine units of PCN to create new hybrid orbitals. Both theoretical calculation and experimental evidence revealed that the new hybrid orbitals synergistically broadened visible light absorption via a metal-to-ligand charge transfer (MLCT) process, and accelerated the separation and transfer of photoexcited electrons and holes. The obtained single-atom Ni terminated PCN (PCNNi), without an additional cocatalyst loading, realized efficient photocatalytic overall water splitting into easily-separated gas-product H₂ and liquid-product H₂O₂ under visible light, with evolution rates reaching 26.6 and 24.0 μmol g⁻¹ h⁻¹, respectively. It was indicated that single-atom Ni and the neighboring C atom served as water oxidation and reduction active sites, respectively, for overall water splitting via a two-electron reaction pathway.

Single-atom Ni terminating agent is introduced to coordinate with sp² or sp³ N atoms in the heptazine units of PCN, realizing visible-light photocatalytic overall water splitting to H₂O₂ and H₂ without additional cocatalyst.     

Quantifying internal charge transfer and mixed ion-electron transfer in conjugated radical polymers

Macromolecular radicals are receiving growing interest as functional materials in energy storage devices and in electronics. With the need for enhanced conductivity, researchers have turned to macromolecular radicals bearing conjugated backbones, but results thus far have yielded conjugated radical polymers that are inferior in comparison to their non-conjugated partners. The emerging explanation is that the radical unit and the conjugated backbone (both being redox active) transfer electrons between each other, essentially “quenching” conductivity or capacity. Here, the internal charge transfer process is quantified using a polythiophene loaded with 0, 25, or 100% nitroxide radicals (2,2,6,6-tetramethyl-1-piperidinyloxy [TEMPO]). Importantly, deconvolution of the cyclic voltammograms shows mixed faradaic and non-faradaic contributions that contribute to the internal charge transfer process. Further, mixed ion-electron transfer is determined for the 100% TEMPO-loaded conjugated radical polymer, from which it is estimated that one triflate anion and one propylene carbone molecule are exchanged for every electron. Although these findings indicate the reason behind their poor conductivity and capacity, they point to how these materials might be used as voltage regulators in the future.

Conjugated radical polymers can exhibit internal electron transfer depending on the radical loading.     

A case of chain propagation: α-aminoalkyl radicals as initiators for aryl radical chemistry

The generation of aryl radicals from the corresponding halides by redox chemistry is generally considered a difficult task due to their highly negative reduction potentials. Here we demonstrate that α-aminoalkyl radicals can be used as both initiators and chain-carriers for the radical coupling of aryl halides with pyrrole derivatives, a transformation often employed to evaluate new highly reducing photocatalysts. This mode of reactivity obviates for the use of strong reducing species and was also competent in the formation of sp² C–P bonds. Mechanistic studies have delineated some of the key features operating that trigger aryl radical generation and also propagate the chain process.

Aminoalkyl radicals can be used as both initiators and chain-carriers for the conversion of aryl halides into the corresponding radicals. This approach by-passes the requirement for strongly reducing photocatalysts.     

Long-range light-modulated charge transport across the molecular heterostructure doped protein biopolymers

Biological electron transfer (ET) across proteins is ubiquitous, such as the notable photosynthesis example, where light-induced charge separation takes place within the reaction center, followed by sequential ET via intramolecular cofactors within the protein. Far from biology, carbon dots (C-Dots) with their unique optoelectronic properties can be considered as game-changers for next-generation advanced technologies. Here, we use C-Dots for making heterostructure (HS) configurations by conjugating them to a natural ET mediator, the hemin molecule, thus making an electron donor–acceptor system. We show by transient absorption and emission spectroscopy that the rapid intramolecular charge separation happens following light excitation, which can be ascribed to an ultrafast electron and hole transfer (HT) from the C-Dot donor to the hemin acceptor. Upon integrating the HS into a protein matrix, we show that this HT within the HS configuration is 3.3 times faster compared to the same process in solution, indicating the active role of the protein in supporting the rapid light-induced long-range intermolecular charge separation. We further use impedance, electrochemical, and transient photocurrent measurements to show that the light-induced transient charge separation results in an enhanced ET and HT efficiency across the protein biopolymer. The charge conduction across our protein biopolymers, reaching nearly 0.01 S cm⁻¹, along with the simplicity and low-cost of their formation promotes their use in a variety of optoelectronic devices, such as artificial photosynthesis, photo-responsive protonic–electronic transistors, and photodetectors.

This work reports on a chimeric protein matrix with C-Dot–hemin heterostructures as cofactors. We show how the protein environment facilitates an ultrafast charge separation, resulting in long-range electron conduction across the protein matrix.     

Benzylic C(sp3)–C(sp2) cross-coupling of indoles enabled by oxidative radical generation and nickel catalysis

A mechanistically unique functionalization strategy for a benzylic C(sp³)–H bond has been developed based on the facile oxidation event of indole substrates. This novel pathway was initiated by efficient radical generation at the benzylic position of the substrate, with subsequent transition metal catalysis to complete the overall transformation. Ultimately, an aryl or an acyl group could be effectively delivered from an aryl (pseudo)halide or an acid anhydride coupling partner, respectively. The developed method utilizes mild conditions and exhibits a wide substrate scope for both substituted indoles and C(sp²)-based reaction counterparts. Mechanistic studies have shown that competitive hydrogen atom transfer (HAT) processes, which are frequently encountered in conventional methods, are not involved in the product formation process of the developed strategy.

A mechanistically distinct Ni-catalysed benzylic functionalization of indoles is developed by the facile oxidation of arenes. The method exhibits a wide substrate scope and pronounced chemoselectivity that cannot be accessed via known protocols.     

Small peptide diversification through photoredox-catalyzed oxidative C-terminal modification

A photoredox-catalyzed oxidative decarboxylative coupling of small peptides is reported, giving access to a variety of N,O-acetals. They were used as intermediates for the addition of phenols and indoles, leading to novel peptide scaffolds and bioconjugates. Amino acids with nucleophilic side chains, such as serine, threonine, tyrosine and tryptophan, could also be used as partners to access tri- and tetrapeptide derivatives with non-natural cross-linking.

A photoredox approach for the generation of N-acyliminiums derived from peptides enabling diversification via Friedel–Crafts reactions.     

Rhodium catalysed C-3/5 methylation of pyridines using temporary dearomatisation

Pyridines are ubiquitous aromatic rings used in organic chemistry and are crucial elements of the drug discovery process. Herein we describe a new catalytic method that directly introduces a methyl group onto the aromatic ring; this new reaction is related to hydrogen borrowing, and is notable for its use of the feedstock chemicals methanol and formaldehyde as the key reagents. Conceptually, the C-3/5 methylation of pyridines was accomplished by exploiting the interface between aromatic and non-aromatic compounds, and this allows an oscillating reactivity pattern to emerge whereby normally electrophilic aromatic compounds become nucleophilic in the reaction after activation by reduction. Thus, a set of C-4 functionalised pyridines can be mono or doubly methylated at the C-3/5 positions.

Electron poor pyridines can be activated by reduction and then methylated at C3/5 using formaldehyde.     

Intrinsic photogeneration of long-lived charges in a donor-orthogonal acceptor conjugated polymer

Efficient charge photogeneration in conjugated polymers typically requires the presence of a second component to act as electron acceptor. Here, we report a novel low band-gap conjugated polymer with a donor/orthogonal acceptor motif: poly-2,6-(4,4-dihexadecyl-4H-cyclopenta [2,1-b:3,4-b′]dithiophene)-alt-2,6-spiro [cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene]-2′,7′-dicarbonitrile, referred to as PCPDT-sFCN. The role of the orthogonal acceptor is to spatially isolate the LUMO from the HOMO, allowing for negligible exchange energy between electrons in these orbitals and minimising the energy gap between singlet and triplet charge transfer states. We employ ultrafast and microsecond transient absorption spectroscopy to demonstrate that, even in the absence of a separate electron acceptor, PCPDT-sFCN shows efficient charge photogeneration in both pristine solution and film. This efficient charge generation is a result of an isoenergetic singlet/triplet charge transfer state equilibrium acting as a reservoir for charge carrier formation. Furthermore, clear evidence of enhanced triplet populations, which form in less than 1 ps, is observed. Using group theory, we show that this ultrafast triplet formation is due to highly efficient, quantum mechanically allowed intersystem crossing between the bright, initially photoexcited local singlet state and the triplet charge transfer state. Remarkably, the free charges that form via the charge transfer state are extraordinarily long-lived with millisecond lifetimes, possibly due to the stabilisation imparted by the spatial separation of PCPDT-sFCN''s donor and orthogonal acceptor motifs. The efficient generation of long-lived charge carriers in a pristine polymer paves the way for single-material applications such as organic photovoltaics and photodetectors.

The spatial separation of PCPDT-sFCN''s donor and orthogonal acceptor motifs allows efficient photogeneration of extraordinarily long-lived charge carriers in the pristine polymer, providing an important step towards single-material optoelectronics.