sp2 Carbon-Conjugated Covalent Organic Frameworks (sp2c-COFs): Synthesis and Application in Photocatalytic Water Splitting

Preparation of irreversible sp2 carbon-conjugated covalent organic frameworks (sp2c-COFs) with specific porosity, easy structural functionalization, high chemical stability, and unique π-electron conjugation structure (especially the combination of π-π stacking interactions and conjugation system), can remove the barrier of electron transfer and provide a unique advantage for photocatalytic water splitting. Herein, based on three kinds of reactions (Aldol condensation reaction, Knoevenagel condensation reaction, and Horner-Wadsworth-Emmons reaction) and guided by the precise modulation of ligand structure and topology, this review summarizes the synthesis of sp2c-COFs and their applications in photoelectrocatalytic water splitting (hydrogen evolution and oxygen evolution reactions). Furthermore, challenges and possible research directions for sp2c-COFs in photocatalytic water splitting are also provided.

Metal-free Amorphous Cross-Linked Porous Organic Polymers for Photocatalytic Hydrogen Evolution and Carbon Dioxide Reduction

The use of solar energy to produce valuable chemicals has gained significant attention in recent years. In this area, photocatalytic hydrogen evolution and carbon dioxide reduction have become key focal points. Recently, amorphous cross-linked porous organic polymers (CPOPs) have emerged as promising photocatalysts due to their tunable band gaps, broad light absorption range, and high porosity. In this article, we highlight recent advancements in cutting-edge metal-free amorphous CPOPs for photocatalytic hydrogen evolution and carbon dioxide reduction. We examine the design principles, synthetic strategies, and structure-property relationships of various cross-linked porous polymers to achieve high photocatalytic performance. Additionally, we propose future research directions and offer insights to further advance CPOP-based photocatalysts for efficient solar-driven hydrogen evolution and carbon dioxide reduction.

Asymmetric Push-Pull Type Co(II) Porphyrin for Enhanced Electrocatalytic CO2 Reduction Activity

Molecular electrocatalysts for electrochemical carbon dioxide (CO₂) reduction has received more attention both by scientists and engineers, owing to their well-defined structure and tunable electronic property. Metal complexes via coordination with many π-conjugated ligands exhibit the unique electrocatalytic CO₂ reduction performance. The symmetric electronic structure of this metal complex may play an important role in the CO₂ reduction. In this work, two novel dimethoxy substituted asymmetric and cross-symmetric Co(II) porphyrin (PorCo) have been prepared as the model electrocatalyst for CO₂ reduction. Owing to the electron donor effect of methoxy group, the intramolecular charge transfer of these push-pull type molecules facilitates the electron mobility. As electrocatalysts at -0.7 V vs. reversible hydrogen electrode (RHE), asymmetric methoxy-substituted Co(II) porphyrin shows the higher CO₂-to-CO Faradaic efficiency (FE_CO) of ~95 % and turnover frequency (TOF) of 2880 h^-1 than those of control materials, due to its push-pull type electronic structure. The density functional theory (DFT) calculation further confirms that methoxy group could ready to decrease to energy level for formation *COOH, leading to high CO₂ reduction performance. This work opens a novel path to the design of molecular catalysts for boosting electrocatalytic CO₂ reduction.

Fully unsaturated all-carbon bifluorenylidene-based polymeric frameworks: synthesis and efficient photocatalysis

Conjugated porous polymers with fully unsaturated all-carbon frameworks possess strong visible light-absorbing abilities, enabling efficient photodegradation of dye pollutants.

Pyrene- and 1,3,5-triazine-based D–A two-dimensional polymers for sunlight-driven hydrogen evolution: the influence of the linking pattern

Two conjugated organic polymers constructed from electron-rich pyrenes and electron-deficient 2,4,6-triphenyl-1,3,5-triazine demonstrated good photocatalytic hydrogen evolution activity.

Carbazole- and thiophene-containing conjugated microporous polymers with different planarity for enhanced photocatalytic hydrogen evolution

We report the synthesis of two carbazole-thiophene-based conjugated microporous polymers (Cz-3Th and Cz-4Th CMPs) with different degrees of planarity for photocatalytic hydrogen evolution from water. Depending upon the building linker's planarity, we found that the porous structure, hydrogen-evolution rate, and photocatalytic stability of the resultant CMPs varied.

Pathways towards Boosting Solar-Driven Hydrogen Evolution of Conjugated Polymers

Photocatalytic H₂ evolution under solar illumination has been considered to be a promising technology for green energy resources. Developing highly efficient photocatalysts for photocatalytic water splitting is long-term desired but still challenging. Conjugated polymers (CPs) have attracted ongoing attention and have been considered to be promising alternatives for solar-driven H₂ production due to the excellent merits of the large π-conjugated system, versatile structures, tunable photoelectric properties, and well-defined chemical composites. The excellent merits have offered numerous methods for boosting photocatalytic hydrogen evolution (PHE) of initial CP-based photocatalysts, whose apparent quantum yield is dramatically increased from <1 to >20% in recent five years. According to the photocatalytic mechanism, this review herein systematically summarizes three major strategies for boosting photocatalytic H₂ production of CPs: 1) enhancing visible light absorption, 2) suppressing recombination of electron-hole pairs, and 3) boosting surface catalytic reaction, mainly involving eleven methods, that is, copolymerization, modifying cross-linker, constructing a donor-acceptor structure, functionalization, fabricating organic heterojunction, loading cocatalyst, and surface modification. Finally, the perspectives towards the future development of PHE are proposed.

Interfacial Synthesis of a Monolayered Fluorescent Two-Dimensional Polymer through Dynamic Imine Chemistry

A fluorescent monolayered two-dimensional polymer (2DP) containing both tetraphenylethylene (TPE) and imine linkages is synthesized at air-water interface using the Langmuir-Blodgett method. We designed TPE-based monomers with long distances between the TPE and the imine linkages to avoid the charge transfer and therefore keep the fluorescence. A monolayered 2DP provided with more than 104 μm2 in domain size and around 0.8 nm thickness was obtained through a successive Schiff base reaction at air-water interface. The nanostructures and fluorescent property of 2DP films were characterized by optical microscopy, SEM, TEM, AFM and fluorescence spectrum. Most importantly, the tip-enhanced Raman spectroscopy (TERS) was utilized here to confirm the success of the polycondensation of monolayered 2DP.

Bisulfone-Functionalized Organic Polymer Photocatalysts for High-Performance Hydrogen Evolution

Conjugated polymers show great potential in the application of photocatalysis, particularly for the photoreduction reaction of water to generate hydrogen. Molecular structure design is a key part for building a high-performance organic photocatalyst. Herein, two bisulfone-containing conjugated polymer photocatalysts were constructed with 1D or 3D polymer structures, and the effect of polymer geometry on photocatalytic activity was studied. It was found that the linear polymer PySEO-1 exhibited increased photocatalytic performance compared with the 3D polymer network PySEO-2 because the enhanced coplanarity of the polymeric chain in PySEO-1 promoted the photogenerated charge-carrier transmission along the 1D main chain. As a result, an attractive hydrogen generation rate of 9477 μmol h^-1  g^-1 was obtained with PySEO-1 under broadband light irradiation. PySEO-1 also exhibited a high external quantum efficiency of 4.1 % at an incident light wavelength of 400 nm, demonstrating that the bisulfone-containing polymers are attractive as organic photocatalysts for hydrogen production.

Bisulfone-Functionalized Organic Polymer Photocatalysts for High-Performance Hydrogen Evolution

Conjugated polymers show great potential in the application of photocatalysis, particularly for the photoreduction reaction of water to generate hydrogen. Molecular structure design is a key part for building a high-performance organic photocatalyst. Herein, two bisulfone-containing conjugated polymer photocatalysts were constructed with 1D or 3D polymer structures, and the effect of polymer geometry on photocatalytic activity was studied. It was found that the linear polymer PySEO-1 exhibited increased photocatalytic performance compared with the 3D polymer network PySEO-2 because the enhanced coplanarity of the polymeric chain in PySEO-1 promoted the photogenerated charge-carrier transmission along the 1D main chain. As a result, an attractive hydrogen generation rate of 9477 μmol h^-1  g^-1 was obtained with PySEO-1 under broadband light irradiation. PySEO-1 also exhibited a high external quantum efficiency of 4.1 % at an incident light wavelength of 400 nm, demonstrating that the bisulfone-containing polymers are attractive as organic photocatalysts for hydrogen production.

Structurally Diverse Covalent Triazine-Based Framework Materials for Photocatalytic Hydrogen Evolution from Water

A structurally diverse family of 39 covalent triazine-based framework materials (CTFs) are synthesized by Suzuki-Miyaura polycondensation and tested as hydrogen evolution photocatalysts using a high-throughput workflow. The two best-performing CTFs are based on benzonitrile and dibenzo[b,d]thiophene sulfone linkers, respectively, with catalytic activities that are among the highest for this material class. The activities of the different CTFs are rationalized in terms of four variables: the predicted electron affinity, the predicted ionization potential, the optical gap, and the dispersibility of the CTFs particles in solution, as measured by optical transmittance. The electron affinity and dispersibility in solution are found to be the best predictors of photocatalytic hydrogen evolution activity.