Small Organic Molecule Based on Benzothiadiazole for Electrocatalytic Hydrogen Production

Hydrophilic Photocrosslinkers as a Universal Solution to Endow Water Affinity to a Polymer Photocatalyst for an Enhanced Hydrogen Evolution Rate

A universal approach for enhancing water affinity in polymer photocatalysts by covalently attaching hydrophilic photocrosslinkers to polymer chains is presented. A series of bisdiazirine photocrosslinkers, each comprising bisdiazirine photophores linked by various aliphatic (CL-R) or ethylene glycol-based bridge chains (CL-TEG), is designed to prevent crosslinked polymer photocatalysts from degradation through a safe and efficient photocrosslinking reaction at a wavelength of 365 nm. When employing the hydrophilic CL-TEG as a photocrosslinker with polymer photocatalysts (F8BT), the hydrogen evolution reaction (HER) rate is considerably enhanced by 2.5-fold compared to that obtained using non-crosslinked F8BT photocatalysts, whereas CL-R-based photocatalysts yield HER rates comparable to those of non-crosslinked counterparts. Photophysical analyses including time-resolved photoluminescence and transient absorption measurements reveal that adding CL-TEG accelerates exciton separation, forming long-lived charge carriers. Additionally, the in-depth study using molecular dynamics simulations elucidates the dual role of CL-TEG: it enhances water penetration into the polymer matrix and stabilizes charge carriers after exciton generation against undesirable recombination. Therefore, the strategy highlights endowing a high-permittivity environment within polymer photocatalyst in a controlled manner is crucial for enhancing photocatalytic redox reactivity. Furthermore, this study shows that this hydrophilic crosslinker approach has a broad applicability in general polymer semiconductors and their nanoparticulate photocatalysts.

Rational Design of Organic Electrocatalysts for Hydrogen and Oxygen Electrocatalytic Applications

Efficient electrocatalysts are pivotal for advancing green energy conversion technologies. Organic electrocatalysts, as cost-effective alternatives to noble-metal benchmarks, have garnered attention. However, the understanding of the relationships between their properties and electrocatalytic activities remains ambiguous. Plenty of research articles regarding low-cost organic electrocatalysts started to gain momentum in 2010 and have been flourishing recently though, a review article for both entry-level and experienced researchers in this field is still lacking. This review underscores the urgent need to elucidate the structure-activity relationship and design suitable electrode structures, leveraging the unique features of organic electrocatalysts like controllability and compatibility for real-world applications. Organic electrocatalysts are classified into four groups: small molecules, oligomers, polymers, and frameworks, with specific structural and physicochemical properties serving as activity indicators. To unlock the full potential of organic electrocatalysts, five strategies are discussed: integrated structures, surface property modulation, membrane technologies, electrolyte affinity regulation, and addition of anticorrosion species, all aimed at enhancing charge efficiency, mass transfer, and long-term stability during electrocatalytic reactions. The review offers a comprehensive overview of the current state of organic electrocatalysts and their practical applications, bridging the understanding gap and paving the way for future developments of more efficient green energy conversion technologies.

Derivatization of 2,1,3-Benzothiadiazole via Regioselective C-H Functionalization and Aryne Reactivity

Despite growing interest in 2,1,3-benzothiadiazole (BTD) as an integral component of many functional molecules, methods for the functionalization of its benzenoid ring have remained limited, and many even simply decorated BTDs have required de novo synthesis. We show that regioselective Ir-catalyzed C-H borylation allows access to versatile 5-boryl or 4,6-diboryl BTD building blocks, which undergo functionalization at the C4, C5, C6, and C7 positions. The optimization and regioselectivity of C-H borylation are discussed. A broad reaction scope is presented, encompassing ipso substitution at the C-B bond, the first examples of ortho-directed C-H functionalization of BTD, ring closing reactions to generate fused ring systems, as well as the generation and capture reactions of novel BTD-based heteroarynes. The regioselectivity of the latter is discussed with reference to the Aryne Distortion Model.

Role of the Benzothiadiazole Unit in Organic Polymers on Photocatalytic Hydrogen Production

Organic polymers based on the donor-acceptor structure are a promising class of efficient photocatalysts for solar fuel production. Among these polymers, poly(9,9-dioctylfluorene-alt-1,2,3-benzothiadiazole) (PFBT) consisting of fluorene donor and benzothiadiazole acceptor units has shown good photocatalytic activity when it is prepared into polymer dots (Pdots) in water. In this work, we investigate the effect of the chemical environment on the activity of photocatalysis from PFBT Pdots for hydrogen production. This is carried out by comparing the samples with various concentrations of palladium under different pH conditions and with different sacrificial electron donors (SDs). Moreover, a model compound 1,2,3-benzothiadiazole di-9,9-dioctylfluorene (BTDF) is synthesized to investigate the mechanism for protonation of benzothiadiazole and its kinetics in the presence of an organic acid-salicylic acid by cyclic voltammetry. We experimentally show that benzothiadiazole in BTDF can rapidly react with protons with a fitted value of 0.1-5 × 1010 M-1 s-1 which should play a crucial role in the photocatalytic reaction with a polymer photocatalyst containing benzothiadiazole such as PFBT Pdots for hydrogen production in acidic conditions. This work gives insights into why organic polymers with benzothiadiazole work efficiently for photocatalytic hydrogen production.

Small Organic Molecular Electrocatalysts for Fuels Production

In recent years, heterocyclic organic compounds have been explored as molecular electrocatalysts in relevant reactions for energy conversion and storage. Merging mimetics of biological systems that perform hydride transfer with rational synthetic chemical design has opened many opportunities for organic molecules to be tuned at the atomic level conferring them interesting reactivities. These molecular electrocatalysts represent an alternative to traditional metallic materials and metal complexes employed for water oxidation, hydrogen production, and carbon dioxide reduction. This minireview describes recent reports concerning design, catalytic activity and the mechanism of synthetic molecular electrocatalysts towards solar fuels production.

Directing Group Strategy for the Isolation of Organoselenium(VI) Benzoselenonates: Metal-Free Catalysts for Hydrogen Evolution Reaction

The exploration of fourth-period organoelements, particularly organoseleniums in their highest VI oxidation state, is limited owing to their stability and synthesis. Herein, the isolation of a new class of quinolinyl-embedded, hexavalent selenium(VI) benzoselenonates has been discussed and further evaluated for a metal-free electrocatalytic hydrogen evolution reaction (HER). The Se(VI) benzoselenonates exhibited high Faradaic efficiency (F.E.) of metal-free H2 gas production up to 86% with a very good turnover number (TON) up to 43 and moderate overpotential (η) of 500 mV; in the presence of mild acetic acid source in a less deprotonating DMF solvent. Taken together with various (NMR, UV-vis, and EPR) spectroscopic and DFT computation studies, a plausible HER pathway is proposed, which suggests that the electrochemical reduction of quinolinyl ring is the initiation step and Se(VI) acts as the reaction site by involving a hydridic type of intermediate for the electrochemical H2 gas generation.

Effects of donor and acceptor substituents on the photophysics of 4-ethynyl-2,1,3-benzothiadiazole derivatives

The present work explores the photophysical, electrochemical, and fluorescence polarization properties of a group of π-conjugated phenylethynyl-2,1,3-benzothiadiazole derivatives (BTDs) bearing different electron-donating (ED) or electron-withdrawing (EW) substituents at the para position of the phenylethynyl moiety. The BTDs were synthesized through the Sonogashira cross-coupling reaction between 4-bromo-2,1,3-benzothiadiazole and the respective para-substituted phenylethynyl derivatives. The BTDs with the EW-substituents show relatively weak solvatochromic behavior, while the BTDs with the strong ED-substituents like methoxy and N,N-dimethylamino-based substituents (BTDPhOMe and BTDPhNMe2) exhibit a pronounced solvatochromic behavior. The change in dipole moments in the excited states of the derivatives was calculated using Lippert-Mataga plots. The conclusions drawn on the spectral behavior of the molecules could be rationalized by TD-DFT calculations involving electron density difference (EDD) maps that correlate with the ICT characteristics of the molecules. The experimental and theoretical calculations reveal that the BTDs with the strong ED-substituents (strong push-pull type BTDs) have a strong ICT character in the excited state. These strong push-pull type BTDs show high fluorescence quantum yield (ΦF) in apolar solvents and low ΦF in polar solvents. In contrast, the BTDs with the weak ED-substituents (weak push-pull type BTDs) and EW-substituents (pull-pull type BTDs) have a weaker ICT character with low ΦF in apolar and high ΦF in polar solvent media. There is good a agreement among the HOMO-LUMO band gaps obtained from absorption spectroscopy and electrochemical studies and theoretical calculations. The fluorescence anisotropy measurement in the glycerol medium shows that the studied BTDs generally exhibit higher sensitivity towards microviscosity than the traditional DPH fluorescence anisotropy probe.

The current status of hydrogen energy: an overview

Hydrogen is the most environmentally friendly and cleanest fuel that has the potential to supply most of the world's energy in the future, replacing the present fossil fuel-based energy infrastructure. Hydrogen is expected to solve the problem of energy shortages in the near future, especially in complex geographical areas (hills, arid plateaus, etc.) and harsh climates (desert, ice, etc.). Thus, in this report, we present a current status of achievable hydrogen fuel based on various scopes, including production methods, storage and transportation techniques, the global market, and the future outlook. Its objectives include analyzing the effectiveness of various hydrogen generation processes and their effects on the economy, society, and environment. These techniques are contrasted in terms of their effects on the environment, manufacturing costs, energy use, and energy efficiency. In addition, hydrogen energy market trends over the next decade are also discussed. According to numerous encouraging recent advancements in the field, this review offers an overview of hydrogen as the ideal renewable energy for the future society, its production methods, the most recent storage technologies, and transportation strategies, which suggest a potential breakthrough towards a hydrogen economy. All these changes show that this is really a profound revolution in the development process of human society and has been assessed as having the same significance as the previous industrial revolution.

Exploiting the NADP+/NADPH-like Hydride-Transfer Redox Cycle with Bis-Imidazolium-Embedded Heterohelicene for Electrocatalytic Hydrogen Evolution Reaction

Generation of clean energy in a viable manner demands efficient and sustainable catalysts. One prospective method of clean energy generation is the electrochemical hydrogen evolution reaction (HER). Over the years, various transition metal-based complexes/polymeric organic materials were utilized in HER. However, the use of a redox-active small organic molecule as a catalyst for HER has not been explored well. The requirements of a strongly acidic solution, very high overpotential, and stability under acidic conditions pose several challenges for applying organic electrocatalysts for HER. Considering these challenges, herein, we demonstrated an NADP+-like organic system (NADP+ = nicotinamide adenine dinucleotide phosphate), a bis-imidazolium-fused heterohelicene, which acts as a catalyst for HER with mild acid (acetic acid) as a proton source at moderate overpotential. The unique structural backbone of this dicationic heterohelicene allowed to exploit the NADP+/NADPH-type (NADPH = reduced nicotinamide adenine dinucleotide phosphate) hydride transfer-based redox cycle efficiently under the applied conditions, where the NADPH-like hydride intermediate transfers the hydride to the proton of the mild acid to generate H2. The Faradaic efficiency and turnover number for the present HER were achieved up to 85 ± 5% and 50 ± 3, respectively. In addition, the maximum turnover frequency, TOFmax, value of 410 s-1 was observed, which is around 400 times that obtained for the existing reported NADP+-like organic compounds used as catalysts for HER. Thorough mechanistic studies were conducted experimentally and computationally to establish a plausible catalytic cycle. This advancement could help in designing efficient organic electrocatalysts for HER from a mild proton source.

Acidic enol electrooxidation-coupled hydrogen production with ampere-level current density

Hydrogen production coupled with biomass upgrading is vital for future sustainable energy developments. However, most biomass electrooxidation reactions suffer from high working voltage and low current density, which substantially hinder large-scale industrial applications. Herein, we report an acidic hydrogen production system that combined anodic ascorbic acid electrooxidation with cathodic hydrogen evolution. Unlike C-H and O-H bonds cleavage with slow kinetics in conventional organic oxidation, the highly active enol structure in ascorbic acid allows for an ultralow overpotential of only 12 mV@10 mA/cm² using Fe single-atom catalysts, and reaches 1 A/cm² at only 0.75 V (versus reversible hydrogen electrode) with approximately 100% Faraday efficiency for hydrogen production. Furthermore, the fabricated two-electrode membrane-free electrolyser delivers an industrial current density of 2 A/cm²@1.1 V at 60 °C (2.63 kWh/Nm³ H₂), which requires half of the electricity consumption in conventional water electrolysis (~5 kWh/Nm³ H₂). This work provides a new avenue for achieving industrial-scale hydrogen production from biomass.

Substituent Effect on Ligand-Centered Electrocatalytic Hydrogen Evolution of Phosphorus Corroles

There have been few reports on the substituent effect of main-group-element corrole complexes as ligand-centered homogeneous electrocatalysts for the hydrogen evolution reaction (HER). The key to comprehend the catalytic mechanism and develop efficient catalysts is the elucidation of the effects of electronic structure on the performance of energy-related small molecules. In this work, the "push-pull" electronic effect of the substituents on electrocatalytic HER of phosphorus corroles was investigated by using 5,10,15-tris(phenyl) corrole phosphorus (1P), 10-pentafluorophenyl-5,15-bis(phenyl) corrole phosphorus (2P), 10-phenyl-5,15-bis(pentafluorophenyl) corrole phosphorus (3P), 5,10,15-tris(pentafluorophenyl) corrole phosphorus (4P) complexes bearing hydroxyl axial ligands and different numbers of fluorine atoms on the meso-aryl substituents. The results revealed that the catalytic HER activity of phosphorus corroles decreased with the increasing of fluorine atom numbers, it follows in the order 1P>2P>3P>4P. Density functional theory (DFT) calculations show that the corrole 1P has the lowest free energy barrier in catalytic HER.

Bis-Imidazolium-Embedded Heterohelicene: A Regenerable NADP+ Cofactor Analogue for Electrocatalytic CO2 Reduction

Biomimetic NAD(P)H-type organic hydride donors have recently been advocated as potential candidates to act as metal-free catalysts for fuel-forming reactions such as the reduction of CO₂ to formic acid and methanol, similar to the natural photosynthesis process of fixing CO₂ into carbohydrates. Although these artificial synthetic organic hydrides are extensively used in organic reduction chemistry in a stoichiometric manner, translating them into catalysts has been challenging due to problems associated with the regeneration of these hydride species under applied reaction conditions. A recent discovery of the possibility of their regeneration under electrochemical conditions via a proton-coupled electron-transfer pathway triggered intense research to accomplish their catalytic use in electrochemical CO₂ reduction reactions (eCO₂RR). However, success is yet to be realized to term them as "true" catalysts, as the typical turnover numbers (TONs) of the eCO₂RR processes on inert electrodes for the production of formic acid and/or methanol reported so far are still in the order of 10^-3-10^-2; thus, sub-stoichiometric only! Herein, we report a novel class of structurally engineered heterohelicene-based organic hydride donor with a proof-of-principle demonstration of catalytic electrochemical CO₂ reduction reaction showing a significantly improved activity with more than stoichiometric turnover featuring a 100-1000-fold enhancement of the existing TON values. Mechanistic investigations suggested the critical role of the two cationic imidazolium motifs along with the extensive π-conjugation present in the backbone of the heterohelicene molecules in accessing and stabilizing various radical species involved in the generation and transfer of hydride, via multielectron-transfer steps in the electrochemical process.

Synergistic Contribution of Oligo(ethylene glycol) and Fluorine Substitution of Conjugated Polymer Photocatalysts toward Solar Driven Sacrificial Hydrogen Evolution

To separately explore the importance of hydrophilicity and backbone planarity of polymer photocatalyst, a series of benzothiadiazole-based donor-acceptor alternating copolymers incorporating alkoxy, linear oligo(ethylene glycol) (OEG) side chain, and backbone fluorine substituents is presented. The OEG side chains in the polymer backbone increase the surface energy of the polymer nanoparticles, thereby improving the interaction with water and facilitating electron transfer to water. Moreover, the OEG-attached copolymers exhibit enhanced intermolecular packing compared to polymers with alkoxy side chains, which is possibly attributed to the self-assembly properties of the side chains. Fluorine substituents on the polymer backbone produce highly ordered lamellar stacks with distinct π-π stacking features; subsequently, the long-lived polarons toward hydrogen evolution are observed by transient absorption spectroscopy. In addition, a new nanoparticle synthesis strategy using a methanol/water mixed solvent is first adopted, thereby avoiding the screening effect of surfactants between the nanoparticles and water. Finally, hydrogen evolution rate of 26 000 µmol g^-1  h^-1 is obtained for the copolymer incorporated with both OEG side chains and fluorine substituents under visible-light irradiation (λ > 420 nm). This study demonstrates how the glycol side chain strategy can be further optimized for polymer photocatalysts by controlling the backbone planarity.

Suppressing the Excitonic Effect in Covalent Organic Frameworks for Metal-Free Hydrogen Generation

Photocatalytic hydrogen generation is a promising solution for renewable energy production and plays a role in achieving carbon neutrality. Covalent organic frameworks (COFs) with highly designable backbones and inherent pores have emerged as novel photocatalysts, yet the strong excitonic effect in COFs can impede the promotion of energy conversion efficiency. Here, we propose a facile approach to suppress the excitonic effect in COFs, which is by narrowing the band gap and increasing the dielectric screening via a rational backbone design and chemical modifications. Based on the GW-BSE method, we uncover a linear relationship between the electronic dielectric constant and the inverse square of the optical band gap of COFs of the Lieb lattice. We further demonstrate that both reduced exciton binding energy and enhanced sunlight absorption can be simultaneously realized in COFs with a narrow band gap. Specifically, we show that one of our designed COFs whose exciton binding energy is nearly half that of g-C₃N₄ is capable of metal-free hydrogen production under near-infrared light irradiation. Our results showcase an effective method to suppress the excitonic effect in COFs and also pave the way for their applications in photocatalytic, photovoltaic, and other related solar energy conversions.

(E)-7-(4-(Diphenylamino)styryl)benzo[c][1,2,5]thiadiazole-4-carbonitrile

Donor-acceptor-donor (D-A-A) structures with 2,1,3-benzothiadiazole as an internal acceptor and the 4(7)-cyanogroup as anchor acceptor have been investigated for various photovoltaic applications such as dye-sensitized solar cells (DSSCs) and organic light emitting diodes (OLEDs). They are usually obtained by cyanation of 4(7)-bromo-2,1,3-benzothiadiazoles. In this communication, the reaction of (E)-4-(2-(7-bromobenzo[c][1,2,5]thiadiazol-4-yl)vinyl)-N,N-diphenylaniline with cyanating agents was studied and it was shown that the best yield of (E)-7-(4-(diphenylamino)styryl)benzo[c][1,2,5]thiadiazole-4-carbonitrile was achieved by heating with zinc cyanide in NMP at 120 °C in the presence of tetrakis(triphenylphosphine)palladium (0). The structure of newly synthesized compound was established by means of an elemental analysis, high resolution mass-spectrometry, 1H, 13C-NMR, IR, UV spectroscopy and mass-spectrometry.

Preparation, characterization, evaluation and mechanistic study of organic polymer nano-photocatalysts for solar fuel production

This review provides the guidelines and knowledge gained so far on current strategies used to prepare, optimize and investigate polymer nanoparticles for fuel production, highlighting the future directions of polymer nano-photocatalyst development.