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

Efficient integration of covalent triazine frameworks (CTFs) for augmented photocatalytic efficacy: A review of synthesis, strategies, and applications

Heterogeneous photocatalysis emerges as an exceptionally appealing technological avenue for the direct capture, conversion, and storage of renewable solar energy, facilitating the generation of sustainable and ecologically benign solar fuels and a spectrum of other pertinent applications. Heterogeneous nanocomposites, incorporating Covalent Triazine Frameworks (CTFs), exhibit a wide-ranging spectrum of light absorption, well-suited electronic band structures, rapid charge carrier mobility, ample resource availability, commendable chemical robustness, and straightforward synthetic routes. These attributes collectively position them as highly promising photocatalysts with applicability in diverse fields, including but not limited to the production of photocatalytic solar fuels and the decomposition of environmental contaminants. As the field of photocatalysis through the hybridization of CTFs undergoes rapid expansion, there is a pressing and substantive need for a systematic retrospective analysis and forward-looking evaluation to elucidate pathways for enhancing performance. This comprehensive review commences by directing attention to diverse synthetic methodologies for the creation of composite materials. And then it delves into a thorough exploration of strategies geared towards augmenting performance, encompassing the introduction of electron donor-acceptor (D-A) units, heteroatom doping, defect Engineering, architecture of Heterojunction and optimization of morphology. Following this, it systematically elucidates applications primarily centered around the efficient generation of photocatalytic hydrogen, reduction of carbon dioxide through photocatalysis, and the degradation of organic pollutants. Ultimately, the discourse turns towards unresolved challenges and the prospects for further advancement, offering valuable guidance for the potent harnessing of CTFs in high-efficiency photocatalytic processes.

Functionalized Linear Conjugated Polymer/TiO2 Heterojunctions for Significantly Enhancing Photocatalytic H2 Evolution

Conjugated polymers (CPs) have attracted much attention in recent years due to their structural abundance and tunable energy bands. Compared with CP-based materials, the inorganic semiconductor TiO2 has the advantages of low cost, non-toxicity and high photocatalytic hydrogen production (PHP) performance. However, studies on polymeric-inorganic heterojunctions, composed of D-A type CPs and TiO2, for boosting the PHP efficiency are still rare. Herein, an elucidation that the photocatalytic hydrogen evolution activity can actually be improved by forming polymeric-inorganic heterojunctions TFl@TiO2, TS@TiO2 and TSO2@TiO2, facilely synthesized through efficient in situ direct C-H arylation polymerization, is given. The compatible energy levels between virgin TiO2 and polymeric semiconductors enable the resulting functionalized CP@TiO2 heterojunctions to exhibit a considerable photocatalytic hydrogen evolution rate (HER). Especially, the HER of TSO2@TiO2 heterojunction reaches up to 11,220 μmol g-1 h-1, approximately 5.47 and 1260 times higher than that of pristine TSO2 and TiO2 photocatalysts. The intrinsic merits of a donor-acceptor conjugated polymer and the interfacial interaction between CP and TiO2 account for the excellent PHP activity, facilitating the separation of photo-generated excitons. Considering the outstanding PHP behavior, our work discloses that the coupling of inorganic semiconductors and suitable D-A conjugated CPs would play significant roles in the photocatalysis community.

Performance and Solution Structures of Side-Chain-Bridged Oligo (Ethylene Glycol) Polymer Photocatalysts for Enhanced Hydrogen Evolution under Natural Light Illumination

Converting solar energy into hydrogen energy using conjugated polymers (CP) is a promising solution to the energy crisis. Improving water solubility plays one of the critical factors in enhancing the hydrogen evolution rate (HER) of CP photocatalysts. In this study, a novel concept of incorporating hydrophilic side chains to connect the backbones of CPs to improve their HER is proposed. This concept is realized through the polymerization of carbazole units bridged with octane, ethylene glycol, and penta-(ethylene glycol) to form three new side-chain-braided (SCB) CPs: PCz2S-OCt, PCz2S-EG, and PCz2S-PEG. Verified through transient absorption spectra, the enhanced capability of PCz2S-PEG for ultrafast electron transfer and reduced recombination effects has been demonstrated. Small- and wide-angle X-ray scattering (SAXS/WAXS) analyses reveal that these three SCB-CPs form cross-linking networks with different mass fractal dimensions (f) in aqueous solution. With the lowest f value of 2.64 and improved water/polymer interfaces, PCz2S-PEG demonstrates the best HER, reaching up to 126.9 µmol h-1 in pure water-based photocatalytic solution. Moreover, PCz2S-PEG exhibits comparable performance in seawater-based photocatalytic solution under natural sunlight. In situ SAXS analysis further reveals nucleation-dominated generation of hydrogen nanoclusters with a size of ≈1.5 nm in the HER of PCz2S-PEG under light illumination.

Effect of ESIPT-Induced Photoisomerization of Keto-Enamine Linkages on the Photocatalytic Hydrogen Evolution Performance of Covalent Organic Frameworks

Photoexcitation of keto-enamine allows intramolecular proton transfer from C-NH to C=O, leading to tautomerization, while the photogenerated isomers are excluded from the study of photocatalytic applications. Herein, we demonstrate the photoisomerization of keto-enamine linkages on covalent organic frameworks (COFs) induced by excited-state intramolecular proton transfer (ESIPT). Partial enolization generates partially enolized photoisomers with a mixture of keto (C=O) and enol (OH) forms, conferring extended π-conjugation with an increase in electron density. The spatially separated D-A configuration is thus rebuilt with the enol-imine-linked branch as a donor and the keto-enamine-linked branch as an acceptor, and in turn, the photoinduced charges transfer between the two adjacent branches with a long lifetime. We further prove that the partially enolized photoisomer is a key transition instead of the keto-enamine form as an excited-state model to understand the photocatalytic behaviors. Therefore, ESIPT-induced photoisomerization must be considered for rationally designing keto-enamine-linked COFs with enhanced photocatalytic activity. Also, our study points toward the importance of controlling excited-state structures for long-lived separated charges, which is of particular interest for optoelectronic applications.

Super-Oxidizing Covalent Triazine Framework Electrocatalyst for Two-Electron Water Oxidation to H2 O2

Electrochemical two-electron water oxidation (2e WOR) is gaining surging research traction for sustainable hydrogen peroxide production. However, the strong oxidizing environment and thermodynamically competitive side-reaction (4e WOR) posit as thresholds for the 2e WOR. We herein report a custom-crafted covalent triazine network possessing strong oxidizing properties as a proof-of-concept metal-free functional organic network electrocatalyst for catalyzing 2e WOR. As the first-of-its-kind, the material shows a maximum of 89.9 % Faradaic Efficiency and 1428 μmol/h/cm2 H2 O2 production rate at 3.0 V bias potential (vs reversible hydrogen electrode, RHE), which are either better or comparable to the state-of-the-art electrocatalysts. We have experimentally confirmed a stepwise 2e WOR mechanism which was further computationally endorsed by density functional theory studies.

Emerging conjugated polymers for heterogeneous photocatalytic chemical transformation

In recent decades, the efficient utilization of solar energy through heterogeneous photocatalytic chemical transformation has attracted much attention. As emerging metal-free, pure organic and heterogeneous photocatalysts, π-conjugated polymers (CPs) have been used in visible-light-driven chemical transformations due to their stability, high specific surface area, metal-free nature, and high structural designability. In this review, we summarize the synthesis protocols and design strategies for efficient CP-based photocatalysts based on the photocatalytic mechanisms. Then we highlight the key progress in light-driven chemical transformation using CPs developed by our group. Finally, we present the outlook and possible challenges for future progress of the field.

Efficient photocatalytic hydrogen evolution: Linkage units engineering in triazine-based conjugated porous polymers

Conjugated porous polymers (CPPs) have been widely reported as promising photocatalysts. However, the realization of powerful photocatalytic hydrogen production performance still benefits from the rational design of molecular frameworks and the appropriate choice of building monomers. Herein, we synthesized two novel conjugated porous polymers (CPPs) by copolymerizing pyrene and 1,3,5-triazine building blocks. It is found that minor structural changes in the peripheral groups of the triazine units can greatly affect the photocatalytic activity of the polymers. Compared with the phenyl-linkage unit, the thiophene-linkage unit can give CPP a wider absorption range of visible light, a narrower band gap, a higher transmission and separation efficiency of photo-generated carriers (electrons/holes), and a better interface contact with the photocatalytic reaction solution. The catalyst containing thiophene-triazine (ThPy-CPP) has an efficient photocatalytic hydrogen evolution rate of 21.65 and 16.69 mmol g^-1h^-1 under full-arc spectrum and visible light without the addition of a Pt co-catalyst, respectively, much better than the one containing phenyl-triazine (PhPy-CPP, only 5.73 and 3.48 mmol g^-1h^-1). This study provides a promising direction to design and construct highly efficient, cost-effective CPP-based photocatalysts, for exploring the application of noble metal-free catalysts in photocatalytic hydrogen evolution.

Fabrication of hybrid covalent triazine framework-zinc ferrite spinel to uplift visible light-driven photocatalytic organic pollutant degradation

The tunability of porous covalent triazine frameworks (CTFs) can mitigate poor photostability and rapid hole-electron recombination. Herein, an excellent improvement of visible light-driven photocatalytic pollutant degradation was achieved using a hybrid semiconductor of covalent triazine framework-zinc ferrite spinel catalysts (CTF-ZnFe₂O₄). The as-prepared CTF-ZnFe₂O₄ composites were fabricated using a facile one-pot ionothermal method. The hybrid photocatalysts were identified using X-ray diffraction (XRD), scanning electron microscopy/energy-dispersive X-ray (SEM-EDX), X-ray photoelectron spectrometer (XPS), Brunauer-Emmett-Teller (BET), Fourier transform infrared (FTIR), and UV-visible diffuse reflection spectroscopy (UV-vis DRS) characterizations. The analysis reveals that hybridization successfully ensued and altered the crystallinity structure, morphology, surface area, and bandgap energy of hybrid material. It was found that CTF-ZnFe₂O₄ 90:10 is very effective for the degradation of MB in a UV-vis light photocatalytic process with the efficiency of 95.4% and k_obs of 0.421 min^-1 for degradation of 50 mg/L MB with 0.5 g/L dosages for 120 min. Additionally, the scavenger study, effect of additional oxidants, and stability were performed for the practical application of a hybrid photocatalyst. CTF-ZnFe₂O₄ 90:10 shows outstanding pollutant degradation in sunlight irradiation and high stability with only a 5.2% reduction after a five-times sequential recycling process. Moreover, the photocatalytic mechanism of as-prepared CTF-ZnFe₂O₄ was mainly influenced by [Formula: see text] radical compared to [Formula: see text] and [Formula: see text] radicals. Overall, The as-prepared CTF-ZnFe₂O₄ shows significant potential to be utilized for photocatalytic wastewater treatment.

Single-Atom Catalysts on Covalent Triazine Frameworks: at the Crossroad between Homogeneous and Heterogeneous Catalysis

Heterogeneous single-site and single-atom catalysts potentially enable combining the high catalytic activity and selectivity of molecular catalysts with the easy continuous operation and recycling of solid catalysts. In recent years, covalent triazine frameworks (CTFs) found increasing attention as support materials for particulate and isolated metal species. Bearing a high fraction of nitrogen sites, they allow coordinating molecular metal species and stabilizing particulate metal species, respectively. Dependent on synthesis method and pretreatment of CTFs, materials resembling well-defined highly crosslinked polymers or materials comparable to structurally ill-defined nitrogen-containing carbons result. Accordingly, CTFs serve as model systems elucidating the interaction of single-site, single-atom and particulate metal species with such supports. Factors influencing the transition between molecular and particulate systems are discussed to allow deriving tailored catalyst systems.

Visible-Light-Promoted Switchable Selective Oxidations of Styrene Over Covalent Triazine Frameworks in Water

Using photocatalytic oxidation to convert basic chemicals into high value compounds in environmentally benign reaction media is a current focus in catalytic research. The challenge lies in gaining controllability over product formation selectivity. We design covalent triazine frameworks as heterogeneous, metal-free, and recyclable photocatalysts for visible-light-driven switchable selective oxidation of styrene in pure water. Selectivity in product formation was achieved by activation or deactivation of the specific photogenerated oxygen species. Using the same photocatalyst, by deactivation of photogenerated H₂ O₂ , benzaldehyde was obtained with over 99 % conversion and over 99 % selectivity as a single product. The highly challenging and sensitive epoxidation of styrene was carried out by creating peroxymonocarbonate as an initial epoxidation agent in the presence of bicarbonate, which led to formation of styrene oxide with a selectivity up to 76 % with near quantitative conversion. This study demonstrates a preliminary yet interesting example for simple control over switchable product formation selectivity for challenging oxidation reactions of organic compounds in pure water.

Regulating the charge carrier transport rate via bridging ternary heterojunctions to enable CdS nanorods' solar-driven hydrogen evolution

Solar-driven hydrogen generation using single-semiconductor photocatalysts for hydrogen evolution seems to be challenging due to their poor solar to fuel conversion efficiency because of their fast charge carrier recombination. The ternary heterostructure was prepared by an advanced approach to suppress the recombination of photogenerated charge carriers and has contributed a new platform for designing highly efficient photocatalytic systems. Herein, we fabricated a ternary heterojunction with ultrathin WS₂-SnS₂ nanosheets and CdS nanorods, and the photocatalytic activity was studied. The optimized CdS/SnS₂-WS₂ (6 wt%) nanostructures were found to be highly stable and exhibited the highest hydrogen evolution rate of 232.45 mmol g^-1 h^-1, which was almost 93-fold higher than that of the pristine CdS nanorods. Also, Density Functional Theory (DFT) calculations confirmed that the favorable band alignment for charge transport and superior catalytic activity of the newly fabricated ternary nanostructures make them a potential candidate for solar-driven hydrogen production.

Impact of Interfaces, and Nanostructure on the Performance of Conjugated Polymer Photocatalysts for Hydrogen Production from Water

The direct conversion of sunlight into hydrogen through water splitting, and by converting carbon dioxide into useful chemical building blocks and fuels, has been an active area of research since early reports in the 1970s. Most of the semiconductors that drive these photocatalytic processes have been inorganic semiconductors, but since the first report of carbon nitride organic semiconductors have also been considered. Conjugated materials have been relatively extensively studied as photocatalysts for solar fuels generation over the last 5 years due to the synthetic control over composition and properties. The understanding of materials' properties, its impact on performance and underlying factors is still in its infancy. Here, we focus on the impact of interfaces, and nanostructure on fundamental processes which significantly contribute to performance in these organic photocatalysts. In particular, we focus on presenting explicit examples in understanding the interface of polymer photocatalysts with water and how it affects performance. Wetting has been shown to be a clear factor and we present strategies for increased wettability in conjugated polymer photocatalysts through modifications of the material. Furthermore, the limited exciton diffusion length in organic polymers has also been identified to affect the performance of these materials. Addressing this, we also discuss how increased internal and external surface areas increase the activity of organic polymer photocatalysts for hydrogen production from water.

Enhanced Built-in Electric Field Promotes Photocatalytic Hydrogen Performance of Polymers Derived from the Introduction of B←N Coordination Bond

High concentrations of active carriers on the surface of a semiconductor through energy/electron transfer are the core process in the photocatalytic hydrogen production from water. However, it remains a challenge to significantly improve photocatalytic performance by modifying simple molecular modulation. Herein, a new strategy is proposed to enhance the photocatalytic hydrogen evolution performance using boron and nitrogen elements to construct B←N coordination bonds. Experimental results show that polynaphthopyridine borane (PNBN) possessing B←N coordination bonds shows a hydrogen evolution rate of 217.4 µmol h-1 , which is significantly higher than that of the comparison materials 0 µmol h-1 for polyphenylnaphthalene (PNCC) and 0.66 µmol h-1 for polypyridylnaphthalene (PNNC), mainly attributed to the formation of a strong built-in electric field that promotes the separation of photo-generated electrons/holes. This work opens up new prospects for the design of highly efficient polymeric photocatalysts at the molecular level.

Effect of Nitrogen Atom Introduction on the Photocatalytic Hydrogen Evolution Activity of Covalent Triazine Frameworks: Experimental and Theoretical Study

The construction of high-performance photocatalyst has always been explored. Covalent organic frameworks (COFs), especially keto-amine-linked COFs, have many advantages, such as adjustable bandgaps, π-π stacking structure, excellent response ability to visible light, high specific surface area, high mobility of carrier carriers, good physical and chemical stability, and so on, showing strong potential applications in photocatalytic solar energy conversion and hydrogen production. Two analogous covalent triazine frameworks (CTFs), T3H-CTF and T3N-CTF, have been synthesized via Schiff-base condensation reactions between 2,4,6-trihydroxybenzene-1,3,5-tricarbalehyde (MOP) and the corresponding triazine-based aromatic amines under solvothermal condition. For T3N-CTF, the peripheral aromatic linker to the central triazine unit was the pyridine unit, instead of the benzene unit in the T3H-CTF unit. T3N-CTF had a hydrogen production rate (HPR) of 6485.05 μmol g^-1  h^-1 , much higher than that of T3H-CTF (2028.06 μmol g^-1  h^-1 ). Accordingly, T3N-CTF had a much higher apparent quantum yield (AQY) of 12.2 % than that of T3H-CTF (4.12 %) at 405 nm. The experimental and theoretical results showed that the extended light absorption range, enlarged surface area, and enhanced separation and transportation efficiencies of charge carriers of T3N-CTF compared with T3H-CTF were uniformly induced by the introduction of peripheral nitrogen atoms into the skeleton of former CTF, which eventually boosted the visible-light induced hydrogen evolution reaction (HER). The work suggests a new method for enhancing the intrinsic HER activity by modulating the electronic features of the conjugated COFs by the introduction of pyridinic N atoms.

Conjugated Polymer Polypyrrole Nanostructures: Synthesis and Photocatalytic Applications

Conjugated polymers (CPs) have been recently widely investigated for their properties and their applications in different fields including photocatalysis. Among the family of CPs, polypyrrole (PPy) has been the most extensively studied owing to its good environmental stability, high electrical conductivity, superior redox properties and easy synthesis. Besides, nanostructured polypyrrole-based nanomaterials are a type of active organic materials for photocatalysis, which is one of their emerging applications. Nanostructuration of polypyrrole can reduce the electron-hole recombination because of short charge transfer distances and reactant adsorption, and product desorption can be enhanced owing to the high surface area offered by nanostructures. This review summarizes synthesis of different nanostructures based on π-conjugated polymer polypyrrole and the latest developments for photocatalytic applications, including degradation of organic pollutants and hydrogen generation.

Porous Organic Polymers: Promising Testbed for Heterogeneous Reactive Oxygen Species Mediated Photocatalysis and Nonredox CO2 Fixation

Catalysts play a pivotal role in achieving the global need for food and energy. In this context, porous organic polymers (POPs) with high surface area, robust architecture, tunable pore size, and chemical functionalities have emerged as promising testbeds for heterogeneous catalysis. Amorphous POPs having functionalized interconnected hierarchical porous structures activate a diverse range of substrates through covalent/non-covalent interactions or act as a host matrix to encapsulate catalytically active metal centers. On the other hand, conjugated POPs have been explored for photoinduced chemical transformations. In this personal account, we have delineated the evolution of various POPs and the specific role of pores and pore functionalities in heterogeneous catalysis. Subsequently, we retrospect our journey over the last ten years towards designing and fabricating amorphous POPs for heterogeneous catalysis, specifically photocatalytic reactive oxygen species (ROS)-mediated organic transformations and nonredox chemical fixation of CO₂ . We have also outlined some of the future avenues of POPs and POP-based hybrid materials for diverse catalytic applications.

Fabrication of a Covalent Organic Framework-Based Heterojunction via Coupling with ZnAgInS Nanosphere with High Photocatalytic Activity

Covalent organic frameworks (COFs) exhibit visible-light activity for the degradation of organic pollutants. However, the recombination rates of their photoinduced electron-hole pairs are relatively high, limiting their practical application. In this work, we fabricated a 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa-1) (TpPa-1) COF-based heterojunction through coupling the TpPa-1 COF with a ZnAgInS nanosphere via a facile oil bath heating method. The results show that the prepared heterojunction exhibits outstanding catalytic activity for the degradation of high concentrations the antibiotic tetracycline (TC) and the dye rhodamine B (RhB), which is driven by simulated sunlight. Its degradation rates for RhB and TC were 30× and 18× higher than that of the pure TpPa-1 COF, respectively. The greatly enhanced photocatalytic performances can be ascribed to the formed heterojunction with good band-gap match, which promotes the migration and separation of light-induced electrons and holes and increases both light absorbance and the specific surface area. This study introduces an effective and feasible strategy for improving the photocatalytic performances of COFs via subtly integrating TpPa-1 COFs with a ZnAgInS nanosphere into an organic-inorganic hybrid. The results of the photocatalytic experiments indicate that the fabricated hybrid has a potential application in the highly efficient removal of organic pollutants.

Graphitic Aza-Fused π-Conjugated Networks: Construction, Engineering, and Task-Specific Applications

2D π-conjugated networks linked by aza-fused units represent a pivotal category of graphitic materials with stacked nanosheet architectures. Extensive efforts have been directed at their fabrication and application since the discovery of covalent triazine frameworks (CTFs). Besides the triazine cores, tricycloquinazoline and hexaazatriphenylene linkages are further introduced to tailor the structures and properties. Diverse related materials have been developed rapidly, and a thorough outlook is necessitated to unveil the structure-property-application relationships across multiple subcategories, which is pivotal to guide the design and fabrication toward enhanced task-specific performance. Herein, the structure types and development of related materials including CTFs, covalent quinazoline networks, and hexaazatriphenylene networks, are introduced. Advanced synthetic strategies coupled with characterization techniques provide powerful tools to engineer the properties and tune the associated behaviors in corresponding applications. Case studies in the areas of gas adsorption, membrane-based separation, thermo-/electro-/photocatalysis, and energy storage are then addressed, focusing on the correlation between structure/property engineering and optimization of the corresponding performance, particularly the preferred features and strategies in each specific field. In the last section, the underlying challenges and opportunities in construction and application of this emerging and promising material category are discussed.

Modification of Covalent Triazine-Based Frameworks for Photocatalytic Hydrogen Generation

The conversion of solar energy and water to hydrogen via semiconductor photocatalysts is one of the efficient strategies to mitigate the energy and environmental crisis. Conjugated polymeric photocatalysts have advantages over their inorganic counterparts. Their molecular structures, band structures, and electronic properties are easily tunable through molecular engineering to extend their spectral response ranges, improve their quantum efficiencies, and enhance their hydrogen evolution rates. In particular, covalent triazine-based frameworks (CTFs) present a strong potential for solar-driven hydrogen generation due to their large continuous π-conjugated structure, high thermal and chemical stability, and efficient charge transfer and separation capability. Herein, synthesis strategies, functional optimization, and applications in the photocatalytic hydrogen evolution of CTFs since the first investigation are reviewed. Finally, the challenges of hydrogen generation for CTFs are summarized, and the direction of material modifications is proposed.

Acetylene/Vinylene-Bridged π-Conjugated Covalent Triazine Polymers for Photocatalytic Aerobic Oxidation Reactions under Visible Light Irradiation

Solar-driven photocatalytic chemical transformation provides a sustainable strategy to produce valuable feedstock, but designing photocatalysts with high efficiency remains challenging. Herein, two acetylene- or vinylene-bridged π-conjugated covalent triazine polymers, A-CTP-DPA and V-CTP-DPE, were successfully fabricated toward metal-free photocatalytic oxidation under visible light irradiation. Compared to the one without acetylene or vinylene bridge, both resulting polymers exhibited superior activity in photocatalytic selective oxidation of sulfides and oxidative coupling of amines; in particular, A-CTP-DPA delivered an optimal photocatalytic performance. The superior activity was attributed to the broadened spectral response range, effective separation, rapid transportation of photogenerated charge carriers, and abundant active sites for photogenerated electrons due to the existence of the acetylene bridge in the framework. This work highlights the potential of acetylene and vinylene bridges in tuning catalytic efficiency of organic semiconductors, providing a guideline for the design of efficient photocatalysts.

Technological Innovations in Photochemistry for Organic Synthesis: Flow Chemistry, High-Throughput Experimentation, Scale-up, and Photoelectrochemistry

Photoinduced chemical transformations have received in recent years a tremendous amount of attention, providing a plethora of opportunities to synthetic organic chemists. However, performing a photochemical transformation can be quite a challenge because of various issues related to the delivery of photons. These challenges have barred the widespread adoption of photochemical steps in the chemical industry. However, in the past decade, several technological innovations have led to more reproducible, selective, and scalable photoinduced reactions. Herein, we provide a comprehensive overview of these exciting technological advances, including flow chemistry, high-throughput experimentation, reactor design and scale-up, and the combination of photo- and electro-chemistry.

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.

Reversing electron transfer in a covalent triazine framework for efficient photocatalytic hydrogen evolution

Covalent triazine-based frameworks (CTFs) have emerged as some of the most important materials for photocatalytic water splitting. However, development of CTF-based photocatalytic systems with non-platinum cocatalysts for highly efficient hydrogen evolution still remains a challenge. Herein, we demonstrated, for the first time, a one-step phosphidation strategy for simultaneously achieving phosphorus atom bonding with the benzene rings of CTFs and the anchoring of well-defined dicobalt phosphide (Co₂P) nanocrystals (∼7 nm). The hydrogen evolution activities of CTFs were significantly enhanced under simulated solar-light (7.6 mmol h⁻¹ g⁻¹), more than 20 times higher than that of the CTF/Co₂P composite. Both comparative experiments and in situ X-ray photoelectron spectroscopy reveal that the strong interfacial P–C bonding and the anchoring of the Co₂P cocatalyst reverse the charge transfer direction from triazine to benzene rings, promote charge separation, and accelerate hydrogen evolution. Thus, the rational anchoring of transition-metal phosphides on conjugated polymers should be a promising approach for developing highly efficient photocatalysts for hydrogen evolution.

Reversing the electron transfer in a covalent triazine-based framework by Co₂P anchoring achieved highly efficient photocatalytic hydrogen evolution from water splitting.

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.

Time-Resolved Raman Spectroscopy of Polaron Formation in a Polymer Photocatalyst

Polymer photocatalysts are a synthetically diverse class of materials that can be used for the production of solar fuels such as H₂, but the underlying mechanisms by which they operate are poorly understood. Time-resolved vibrational spectroscopy provides a powerful structure-specific probe of photogenerated species. Here we report the use of time-resolved resonance Raman (TR³) spectroscopy to study the formation of polaron pairs and electron polarons in one of the most active linear polymer photocatalysts for H₂ production, poly(dibenzo[b,d]thiophene sulfone), P10. We identify that polaron-pair formation prior to thermalization of the initially generated excited states is an important pathway for the generation of long-lived photoelectrons.

A Solvent-Polarity-Induced Interface Self-Assembly Strategy towards Mesoporous Triazine-Based Carbon Materials

Triazine-based materials with porous structure have recently received numerous attentions as a fascinating new class because of their superior potential for various applications. However, it is still a formidable challenge to obtain triazine-based materials with precise adjustable meso-scaled pore sizes and controllable pore structures by reported synthesis approaches. Herein, we develop a solvent polarity induced interface self-assembly strategy to construct mesoporous triazine-based carbon materials. In this method, we employ a mixed solvent system within a suitable range of polarity (0.223≤Lippert-Mataga parameter (Δf) ≤0.295) to induce valid self-assembly of skeleton precursor and surfactant. The as-prepared mesoporous triazine-based carbon materials possess uniform tunable pore sizes (8.2-14.0 nm), high surface areas and ultrahigh nitrogen content (up to 18 %). Owing to these intriguing advantages, the fabricated mesoporous triazine-based carbon materials as functionalized porous solid absorbents exhibit predominant CO₂ adsorption performance and exceptional selectivity for the capture of CO₂ over N₂ .

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.

stk: An extendable Python framework for automated molecular and supramolecular structure assembly and discovery

Computational software workflows are emerging as all-in-one solutions to speed up the discovery of new materials. Many computational approaches require the generation of realistic structural models for property prediction and candidate screening. However, molecular and supramolecular materials represent classes of materials with many potential applications for which there is no go-to database of existing structures or general protocol for generating structures. Here, we report a new version of the supramolecular toolkit, stk, an open-source, extendable, and modular Python framework for general structure generation of (supra)molecular structures. Our construction approach works on arbitrary building blocks and topologies and minimizes the input required from the user, making stk user-friendly and applicable to many material classes. This version of stk includes metal-containing structures and rotaxanes as well as general implementation and interface improvements. Additionally, this version includes built-in tools for exploring chemical space with an evolutionary algorithm and tools for database generation and visualization. The latest version of stk is freely available at github.com/lukasturcani/stk.

Covalent triazine frameworks constructed via benzyl halide monomers showing high photocatalytic activity in biomass reforming

Here we report the synthesis of covalent triazine frameworks (CTFs) using benzyl halide monomers which are more cost-effective and with higher availability than previous ones. The resulting CTFs were successfully applied for efficient photocatalytic reforming of glucose for the first time, with a high hydrogen evolution rate up to 330 μmol g^-1 h^-1 under pH = 12. This work presented a new way to synthesize CTFs and further exhibited their potential applications in photocatalytic biomass reforming.

Covalent Organic Frameworks: Synthesis, Properties and Applications-An Overview

Covalent Organic Frameworks (COFs) are an exciting new class of microporous polymers with unprecedented properties in organic material chemistry. They are generally built from rigid, geometrically defined organic building blocks resulting in robust, covalently bonded crystalline networks that extend in two or three dimensions. By strategically combining monomers with specific structures and properties, synthesized COF materials can be fine-tuned and controlled at the atomic level, with unparalleled precision on intrapore chemical environment; moreover, the unusually high pore accessibility allows for easy post-synthetic pore wall modification after the COF is synthesized. Overall, COFs combine high, permanent porosity and surface area with high thermal and chemical stability, crystallinity and customizability, making them ideal candidates for a myriad of promising new solutions in a vast number of scientific fields, with widely varying applications such as gas adsorption and storage, pollutant removal, degradation and separation, advanced filtration, heterogeneous catalysis, chemical sensing, biomedical applications, energy storage and production and a vast array of optoelectronic solutions. This review attempts to give a brief insight on COF history, the overall strategies and techniques for rational COF synthesis and post-synthetic functionalization, as well as a glance at the exponentially growing field of COF research, summarizing their main properties and introducing the numerous technological and industrial state of the art applications, with noteworthy examples found in the literature.

Probing Dynamics of Water Mass Transfer in Organic Porous Photocatalyst Water-Splitting Materials by Neutron Spectroscopy

The quest for efficient and economically accessible cleaner methods to develop sustainable carbon-free energy sources induced a keen interest in the production of hydrogen fuel. This can be achieved via the water-splitting process and by exploiting solar energy. However, the use of adequate photocatalysts is required to reach this goal. Covalent triazine-based frameworks (CTFs) are potential target photocatalysts for water splitting. Both electronic and structural characteristics of CTFs, particularly energy levels, optical band gaps, and porosities are directly relevant to water splitting and can be engineered through chemical design. Porosity can, in principle, be beneficial to water splitting by providing a larger surface area for the catalytic reactions to take place. However, porosity can also affect both charge transport within the photocatalyst and mass transfer of both reactants and products, thus impacting the overall kinetics of the reaction. Here, we focus on the link between chemical design and water (reactant) mass transfer, which plays a key role in the water uptake process and the subsequent hydrogen generation in practice. We use neutron spectroscopy to study the mass transfer of water in two porous CTFs, CTF-CN and CTF-2, that differ in the polarity of their struts. Quasi-elastic neutron scattering is used to quantify the amount of bound water and the translational diffusion of water. Inelastic neutron scattering measurements complement the quasi-elastic neutron scattering study and provide insights into the softness of the CTF structures and the changes in librational degrees of freedom of water in the porous CTFs. We show that two different types of interaction between water and CTFs take place in CTF-CN and CTF-2. CTF-CN exhibits a smaller surface area and lower water uptake due to its softer structure than CTF-2. However, the polar cyano group interacts locally with water leading to a large amount of bound water and a strong rearrangement of the water hydration monolayer, while water diffusion in CTF-2 is principally impacted by microporosity. The current study leads to new insights into the structure-dynamics-property relationship of CTF photocatalysts that pave the road for a better understanding of the guest-host interaction on the basis of water-splitting applications.

Impact of Chemical Structure on the Dynamics of Mass Transfer of Water in Conjugated Microporous Polymers: A Neutron Spectroscopy Study

Hydrogen fuel can contribute as a masterpiece in conceiving a robust carbon-free economic puzzle if cleaner methods to produce hydrogen become technically efficient and economically viable. Organic photocatalytic materials such as conjugated microporous materials (CMPs) are potential attractive candidates for water splitting as their energy levels and optical band gap as well as porosity are tunable through chemical synthesis. The performances of CMPs depend also on the mass transfer of reactants, intermediates, and products. Here, we study the mass transfer of water (H2O and D2O) and of triethylamine, which is used as a hole scavenger for hydrogen evolution, by means of neutron spectroscopy. We find that the stiffness of the nodes of the CMPs is correlated with an increase in trapped water, reflected by motions too slow to be quantified by quasi-elastic neutron scattering (QENS). Our study highlights that the addition of the polar sulfone group results in additional interactions between water and the CMP, as evidenced by inelastic neutron scattering (INS), leading to changes in the translational diffusion of water, as determined from the QENS measurements. No changes in triethylamine motions could be observed within the CMPs from the present investigations.

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.

Visible-light-driven photocatalyst based upon metal-free covalent triazine-based frameworks for enhanced hydrogen production

An environment-friendly photocatalyst was constructed by loading reduced graphene oxide (rGO) onto a covalent triazine framework CTF-1 in this work for efficient utilization of solar energy to produce H₂.

Nano-assemblies of a soluble conjugated organic polymer and an inorganic semiconductor for sacrificial photocatalytic hydrogen production from water

Nanostructured materials have interesting optical and electronic properties that are often drastically different from those of their bulk counterparts. While bulk organic/inorganic semiconductor composites have attracted much attention in the past decade, the preparation of organic/inorganic semiconductor nanocomposites (OISNs) still remains challenging. This work presents an assembly method for the co-encapsulation of titanium dioxide dots (TDs) with a cyano-substituted soluble conjugated polymer (CSCP) into a particular nanoparticle. The as-prepared CSCP/TD semiconductor nanocomposites (CSCP/TD NCs) exhibit different particle surfaces and morphologies depending on the mass ratio of the CSCP to TDs. We then tested them as photocatalysts for sacrificial hydrogen production from water. We found that nanocomposites outperformed nanoparticles of the individual components and physical mixtures thereof. The most active CSCP/TD NC had a catalytic H2 production rate that was 4.25 times higher than that of pure polymer nanoparticles prepared under the same conditions. We ascribe this to energy transfer between the semiconductors, where direct phase contact is essential, highlighting a potential avenue for using soluble, visible light-absorbing conjugated organic polymers to build Z-schemes for overall water splitting in the future.

Realizing high hydrogen evolution activity under visible light using narrow band gap organic photocatalysts

The design and synthesis of conjugated semiconducting polymers for photocatalytic hydrogen evolution have engendered intense recent interest. However, most reported organic polymer photocatalysts show a relatively broad band gap with weak light absorption ability in the visible light region, which commonly leads to a low photocatalytic activity under visible light. Herein, we synthesize three novel dithieno[3,2-b:2',3'-d]thiophene-S,S-dioxide (DTDO) containing conjugated polymer photocatalysts by a facile C-H arylation coupling polymerization reaction. The resulting polymers show a broad visible light absorption range up to 700 nm and a narrow band gap down to 1.81 eV due to the introduction of the DTDO unit. Benefiting from the donor-acceptor polymer structure and the high content of the DTDO unit, the three-dimensional polymer PyDTDO-3 without the addition of a Pt co-catalyst shows an attractive photocatalytic hydrogen evolution rate of 16.32 mmol h-1 g-1 under visible light irradiation, which is much higher than that of most reported organic polymer photocatalysts under visible light.

Covalent Triazine Framework Nanoparticles via Size-Controllable Confinement Synthesis for Enhanced Visible-Light Photoredox Catalysis

For metal-free, organic conjugated polymer-based photocatalysts, synthesis of defined nanostructures is still highly challenging. Here, we report the formation of covalent triazine framework (CTF) nanoparticles via a size-controllable confined polymerization strategy. The uniform CTF nanoparticles exhibited significantly enhanced activity in the photocatalytic formation of dibenzofurans compared to the irregular bulk material. The optoelectronic properties of the nanometer-sized CTFs could be easily tuned by copolymerizing small amounts of benzothiadiazole into the conjugated molecular network. This optimization of electronic properties led to a further increase in observed photocatalytic efficiency, resulting in total an 18-fold enhancement compared to the bulk material. Full recyclability of the heterogeneous photocatalysts as well as catalytic activity in dehalogenation, hydroxylation and benzoimidazole formation reactions demonstrated the utility of the designed materials.

Water Oxidation with Cobalt-Loaded Linear Conjugated Polymer Photocatalysts

The first examples of linear conjugated organic polymer photocatalysts that produce oxygen from water after loading with cobalt and in the presence of an electron scavenger are reported. The oxygen evolution rates, which are higher than for related organic materials, can be rationalized by a combination of the thermodynamic driving force for water oxidation, the light absorption of the polymer, and the aqueous dispersibility of the relatively hydrophilic polymer particles. We also used transient absorption spectroscopy to study the best performing system and we found that fast oxidative quenching of the exciton occurs (picoseconds) in the presence of an electron scavenger, minimizing recombination.

Illustrating the Role of Quaternary-N of BINOL Covalent Triazine-Based Frameworks in Oxygen Reduction and Hydrogen Evolution Reactions

Defective nitrogen-doped carbon materials have shown a promising application as metal-free electrocatalysts in the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER). However, there are still some challenges in the tuning of metal-free electrocatalysts and in understanding the roles of various nitrogen species in their electrocatalytic performance. Herein, we design a covalent triazine framework (CTF)-based material as an effective metal-free bifunctional electrocatalyst. We chose BINOL-CN (2,2'-dihydroxy-[1,1'-binaphthalene]-6,6'-dicarbonitrile) as both a carbon and a nitrogen source for the fabrication of N-containing CTF-based materials. Four BINOL-CTFs with varying N-functionalities (pyridinic-N/triazine-N, pyrrolic-N, quaternary-N, and pyridine-N-oxide) were successfully obtained. These materials were evaluated in the ORR and the HER in basic and acidic conditions, respectively. The best material has an onset potential of 0.793 V and a half-wave potential of 0.737 V, and it follows first-order kinetics in a 4e^- pathway in the ORR reaction. The same material shows an impressive HER activity with an overpotential of 0.31 V to achieve 10 mA/cm² and a small Tafel slope of 41 mV/dec, which is comparable to 31 mV/dec for Pt/C, making it a potential bifunctional electrocatalyst. We showed that the ORR and HER reactivity of CTF-based materials depends exclusively on the amount of quaternary-N species and on the available surface area and pore volume. This work highlights the engineering of CTF materials with varying amounts of N species as high-performance bifunctional electrocatalysts.

Advances in Conjugated Microporous Polymers

Conjugated microporous polymers (CMPs) are a unique class of materials that combine extended π-conjugation with a permanently microporous skeleton. Since their discovery in 2007, CMPs have become established as an important subclass of porous materials. A wide range of synthetic building blocks and network-forming reactions offers an enormous variety of CMPs with different properties and structures. This has allowed CMPs to be developed for gas adsorption and separations, chemical adsorption and encapsulation, heterogeneous catalysis, photoredox catalysis, light emittance, sensing, energy storage, biological applications, and solar fuels production. Here we review the progress of CMP research since its beginnings and offer an outlook for where these materials might be headed in the future. We also compare the prospect for CMPs against the growing range of conjugated crystalline covalent organic frameworks (COFs).

Structure–activity relationships in well-defined conjugated oligomer photocatalysts for hydrogen production from water

Oligomer chain length and backbone twisting were found to have a strong effect on optoelectronic properties but a trimer of dibenzo[b,d]thiophene sulfone was found to have high photocatalytic activity approaching that of its polymer analogue.