Biomimetic Donor–Acceptor Motifs in Conjugated Polymers for Promoting Exciton Splitting and Charge Separation

Copper-Bridged Tetrakis(4-ethynylphenyl)ethene Aggregates with Photo-Regulated 1 O2 and O2 .- Generation for Selective Photocatalytic Aerobic Oxidation

Active species regulation is a key scientific issue that essentially determines the selectivity and activity of a photocatalyst. Herein, Cu^I -bridged tetrakis(4-ethynylphenyl)ethene aggregates (T₄ EPE-Cu) with photo-regulated ¹ O₂ and O₂ ^.- generation were demonstrated for selective photocatalytic aerobic oxidation. In this system, transient photovoltage combined with the density functional theory calculations confirmed that Cu-alkynyl was the main oxygen activation site. The adsorbed O₂ tends to produce O₂ ^.- because of the potential well effect of Cu-alkynyl under high-energy light excitation. But under low-energy light, O₂ tends to produce ¹ O₂ via resonance energy transfer with Cu-alkynyl. For α-terpinene oxidation, the ratios of ¹ O₂ products to O₂ ^.- products can be controlled from 1.3 (380 nm) to 10.7 (600 nm). Furthermore, T₄ EPE-Cu exhibited ultrahigh photocatalytic performance for Glaser coupling and benzylamine oxidation, with a conversion and selectivity of over 99 %.

Carbon Nitride Photocatalysts with Integrated Oxidation and Reduction Atomic Active Centers for Improved CO2 Conversion

Single-atom active-site catalysts have attracted significant attention in the field of photocatalytic CO₂ conversion. However, designing active sites for CO₂ reduction and H₂ O oxidation simultaneously on a photocatalyst and combining the corresponding half-reaction in a photocatalytic system is still difficult. Here, we synthesized a bimetallic single-atom active-site photocatalyst with two compatible active centers of Mn and Co on carbon nitride (Mn₁ Co₁ /CN). Our experimental results and density functional theory calculations showed that the active center of Mn promotes H₂ O oxidation by accumulating photogenerated holes. In addition, the active center of Co promotes CO₂ activation by increasing the bond length and bond angle of CO₂ molecules. Benefiting from the synergistic effect of the atomic active centers, the synthesized Mn₁ Co₁ /CN exhibited a CO production rate of 47 μmol g^-1  h^-1 , which is significantly higher than that of the corresponding single-metal active-site photocatalyst.

Structural Engineering of Covalent Organic Frameworks Comprising Two Electron Acceptors Improves Photocatalytic Performance

Covalent organic frameworks (COFs) have recently attracted much attention as potential photocatalysts for hydrogen production. The effective separation of photogenerated charges is a key objective to improve the photocatalytic activity of COFs. Here, four COFs were synthesized through the Schiff-base reaction to investigate whether the presence (simultaneous or not) of triazine and ketone as acceptors in COFs improved electron-hole separation efficiency. Evidence indicated that charge separation was more efficient when triazine and ketone were simultaneously present in the COF. The COF comprising two acceptors displayed the highest photocatalytic hydrogen production rate (31.43 μmol h^-1 ; 41.2 and 3.4 times as large as those of the COFs containing only triazine or ketone, respectively). Moreover, the effect of the distance between the two acceptors on the electron-hole separation was investigated by changing the length of a bridging biphenyl ring. It turned out that the transport distance of a single phenyl group was more favorable for the catalytic reaction. This work affords insight and support for the design of efficient COF photocatalysts.

Synergy of Iron Doping and Cyano Groups for Enhanced Photocatalytic Hydrogen Production over C3 N4

Improving the insufficient carrier separation dynamics is still of significance in carbon nitride (C₃ N₄ ) research. Extensive research has been devoted to improving the carrier separation efficiency through a single strategy, while ignoring the synergistic enhancement effect produced by coupling two or more conventional strategies. Herein, we reported the fabrication of cyano group-containing Fe-doped C₃ N₄ porous materials via direct co-calcination of iron acetylacetonate and melamine for synergistically improving the photocatalytic performance. Iron acetylacetonate can promote the generation of cyano groups and form Fe-doping in C₃ N₄ , thereby increasing the visible-light absorption and reactive sites. Further, the internal donor-acceptor system formed by cyano groups and Fe-doped sites promoted charge carrier separation and inhibited the radiation recombination of e^- -h⁺ pairs. The optimized photocatalytic activity of Fe-CN-2 sample was 4.5 times of bulk C₃ N₄ (BCN).

Photosystem II-based biomimetic assembly for enhanced photosynthesis

Photosystem II (PSII) is a fascinating photosynthesis-involved enzyme, participating in sunlight-harvest, water splitting, oxygen release, and proton/electron generation and transfer. Scientists have been inspired to couple PSII with synthetic hierarchical structures via biomimetic assembly, facilitating attainment of natural photosynthesis processes, such as photocatalytic water splitting, electron transfer and ATP synthesis, in vivo. In the past decade, there has been significant progress in PSII-based biomimetic systems, such as artificial chloroplasts and photoelectrochemical cells. The biomimetic assembly approach helps PSII gather functions and properties from synthetic materials, resulting in a complex with partly natural and partly synthetic components. PSII-based biomimetic assembly offers opportunities to forward semi-biohybrid research and synchronously inspire optimization of artificial light-harvest micro/nanodevices. This review summarizes recent studies on how PSII combines with artificial structures via molecular assembly and highlights PSII-based semi-natural biosystems which arise from synthetic parts and natural components. Moreover, we discuss the challenges and remaining problems for PSII-based systems and the outlook for their development and applications. We believe this topic provides inspiration for rational designs to develop biomimetic PSII-based semi-natural devices and further reveal the secrets of energy conversion within natural photosynthesis from the molecular level.

Asymmetric Acceptor-Donor-Acceptor Polymers with Fast Charge Carrier Transfer for Solar Hydrogen Production

Construction of local donor-acceptor architecture is one of the valid means for facilitating the intramolecular charge transfer in organic semiconductors. To further accelerate the interface charge transfer, a ternary acceptor-donor-acceptor (A₁ -D-A₂ ) molecular junction is established via gradient nitrogen substituting into the polymer skeleton. Accordingly, the exciton splitting and interface charge transfer could be promptly liberated because of the strong attracting ability of the two different electron acceptors. Both DFT calculations and photoluminescence spectra elucidate the swift charge transfer at the donor-acceptor interface. Consequently, the optimum polymer, N₃ -CP, undergoes a remarkable photocatalytic property in terms of hydrogen production with AQY_405 nm =26.6 % by the rational design of asymmetric molecular junctions on organic semiconductors.

A photoelectrochemical aptasensor for the determination of bisphenol A based on the Cu (I) modified graphitic carbon nitride

Bisphenol A (BPA) has been penetrating every corner of our daily life via the entities of children's toys, food containers and electronic equipment. The ubiquitous exposure of BPA urges the implementation of supervising its emission in environment. This work designs a method of photoelectrochemical (PEC) aptasensing for the determination of BPA based on the Cu(I) modified carbon nitride (Cu/g-C₃N₄). The Cu/g-C₃N₄ was prepared by solvothermal reaction with the ionic liquid bis(1-hexadecyl-3-methylimidazolium) tetrachlorocuprate (II) as Cu source. Cu/g-C₃N₄ displays excellent PEC performances due to the introduction of Cu(I). The visible light absorption capacity and conductivity of g-C₃N₄ can be enhanced by introducing Cu(I). With the help of BPA-binding aptamer immobilized on the surface of Cu/g-C₃N₄, the Cu/g-C₃N₄ PEC aptasensor has adopted for the determination of BPA. The PEC aptasensor exhibits a well-fitted linear correlation between the response photocurrent signal and the logarithm of the concentration of BPA. The PEC aptasensor shows a distinguished capability of BPA detection with a wide detection range of 5.00 × 10^-11 to 5.00 × 10^-5 g L^-1 and low detection limit of 1.60 × 10^-11 g L^-1 (at S/N = 3). This work provides a profound insight for detecting BPA in environmental water.

Molecular Engineering of Fully Conjugated sp2 Carbon-Linked Polymers for High-Efficiency Photocatalytic Hydrogen Evolution

The diverse nature of organic precursors offers a versatile platform for precisely tailoring the electronic properties of semiconducting polymers. In this study, three fully conjugated sp² carbon-linked polymers have been designed and synthesized for photocatalytic hydrogen evolution under visible-light illumination, by copolymerizing different C₃ -symmetric aromatic aldehydes as knots with the 1,4-phenylene diacetonitrile (PDAN) linker through a C=C condensation reaction. The hydrogen evolution (HER) is achieved at a maximum rate of 30.2 mmol g^-1  h^-1 over a polymer based on 2,4,6-triphenyl-1,3,5-triazine units linked by cyano-substituted phenylene, with an apparent quantum yield (AQY) of 7.20 % at 420 nm. Increasing the degree of conjugation and planarity not only extends visible-light absorption, but also stabilizes the fully conjugated sp² -carbon-linked donor-acceptor (D-A) polymer. Incorporating additional electron-withdrawing triazine units into the D-A polymer to form multiple electron donors and acceptors can greatly promote exciton separation and charge transfer, thus significantly enhancing the photocatalytic activity.

Donor-Acceptor Cyanocarbazole-Based Supramolecular Photocatalysts for Visible-Light-Driven H2 Production

Highly efficient, stable, and metal-free supramolecular photocatalysts for H₂ production are uncommon and of significant interest. In this study, a donor-acceptor (D-A) cyanocarbazole-based supramolecule (2CzPN) is assembled by intermolecular C≡N⋅⋅⋅H-Ar H-bonding interactions to form an efficient, stable, and metal-free photocatalyst for H₂ evolution. The catalyst affords an H₂ production rate of 91.8 μmol h^-1 (20 mg of samples, λ>420 nm) and a corresponding apparent quantum efficiency of 7.5 % at 420 nm. The photon-generated carrier separation of the 2CzPN supramolecule, which is higher than that of the analogous polymer, can be attributed to the strong D-A characteristics and high crystallinity. This study offers the first experimental evidence of visible-light H₂ evolution among D-A cyanocarbazole-based supramolecules and it enriches the variety of supramolecular photocatalysts. This stable and effective metal-free 2CzPN supramolecular photocatalyst has obvious advantages among very few supramolecular photocatalysts for H₂ production.

Enhancing Visible-Light Hydrogen Evolution Performance of Crystalline Carbon Nitride by Defect Engineering

Crystalline carbon nitride (CCN)-based semiconductors have recently attracted widespread attention in solar energy conversion. However, further modifying the photocatalytic ability of CCN always results in a trade-off between high crystallinity and good photocatalytic performance. Herein, a facile defect engineering strategy was demonstrated to modify the CCN photocatalysts. Results confirmed that the obtained D-CCN maintained the high crystallinity; additionally, the hydrogen production rate of D-CCN was approximately 8 times higher than that of CCN. Particularly, it could produce H₂ even if the incident light wavelength extended to 610 nm. The significantly improved photocatalytic activity could be ascribed to the introduction of defects into the CCN polymer network to form the midgap states, which significantly broadened the visible-light absorption range and accelerated the charge separation for photoredox catalysis.

Quenching-Induced Structural Distortion of Graphitic Carbon Nitride Nanostructures: Enhanced Photocatalytic Activity and Electrochemical Hydrogen Production

Engineered nanomaterials are emerging in the field of environmental chemistry. This study involves the analysis of the structural, electronic, crystallinity, and morphological changes in graphitic carbon nitride (g-C₃N₄), an engineered nanomaterial, under rapid cooling conditions. X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, Brunauer-Emmett-Teller, Fourier transform infrared, Raman, band gap, and Mott-Schottky analyses strongly proved that the liquid N₂-quenched sample of g-C₃N₄ has structural distortion. The photocatalytic efficiency of engineered g-C₃N₄ nanostructures was analyzed through the degradation of reactive red 120 (RR120), methylene blue (MB), rhodamine B, and bromophenol as a representative dye. The photocatalytic dye degradation efficiency was analyzed by UV-vis spectroscopy and total organic carbon (TOC) analysis. The photocatalytic efficiency of g-C₃N₄ under different quenching conditions included quenching at room temperature in ice and liquid N₂. The degradation efficiencies are found to be 4.2, 14.7, and 82.33% for room-temperature, ice, and liquid N₂ conditions, respectively. The pseudo-first-order reaction rate of N₂-quenched g-C₃N₄ is 9 times greater than the ice-quenched g-C₃N₄. Further, the TOC analysis showed that 55% (MB) and 59% (RR120) of photocatalytic mineralization were achieved within a time duration of 120 min by the liquid N₂-quenched g-C₃N₄ nanostructure. In addition, the quenched g-C₃N₄ electrocatalytic behavior was examined via the hydrogen (H₂) evolution reaction in acidic medium. The liquid N₂-quenched g-C₃N₄ catalyst showed a lower overpotential with high H₂ evolution when compared with the other two g-C₃N₄-quenched samples. The results obtained provide an insight and extend the scope for the application of engineered g-C₃N₄ nanostructures in the degradation of organic pollutants as well as for H₂ evolution.

Intramolecular Charge Transfer and Extended Conjugate Effects in Donor-π-Acceptor-Type Mesoporous Carbon Nitride for Photocatalytic Hydrogen Evolution

Inspired by donor-acceptor (D-A) polymers in organic solar cell and the extended conjugation effect, a conceptual design of D-π-A-type mesoporous carbon nitride with benzene or thiophene as a π-spacer is proposed as an efficient photocatalyst for hydrogen evolution. The photocatalyst was successfully synthesized by a one-pot thermopolymerization based on nucleophilic substitution and a Schiff-base chemical reaction. On the molecular level, the insertion of an in-plane benzene as a π-spacer by forming covalent bonds C=N (acceptor) and C-N (donor) interrupts the continuity of tri-s-triazine units and maintains the intrinsic π-π conjugated electronic system. Synchronously, the enlarged electron delocalization and the intramolecular charge transfer induced by polarization provide force-directed migration of electrons, leading to boosted optical absorption capability and enhanced photogenerated carrier separation. With the synergistic effects of the mesoporous structure and excellent optical and electronic properties, a fivefold increase in the H₂ evolution rate compared with that of pristine g-C₃ N₄ was achieved with robust performance. In addition, other simple aromatic heterocyclic compounds (e.g., pyridine, thiophene and furan)-based D-π-A structures with a higher hydrogen evolution rate (up to sevenfold increase) were also explored to broaden the application for the design of novel photocatalysts.