Homogeneous Photocatalytic System Research Articles

Photocatalytic CO2 Reduction Using Homogeneous Carbon Dots with a Molecular Cobalt Catalyst.

A simple and precious-metal free photosystem for the reduction of aqueous CO2 to syngas (CO and H2) is reported consisting of carbon dots (CDs) as the sole light harvester together with a molecular cobalt bis(terpyridine) CO2 reduction co-catalyst. This homogeneous photocatalytic system operates in the presence of a sacrificial electron donor (triethanolamine) in DMSO/H2O solution at ambient temperature. The photocatalytic system exhibits an activity of 7.7 ± 0.2 mmolsyngas gCDs -1 (3.6 ± 0.2 mmolCO gCDs -1 and 4.1 ± 0.1 mmolH2 gCDs -1) after 24hours of full solar spectrum irradiation (AM 1.5G). Spectroscopic and electrochemical characterization supports that this photocatalytic performance is attributed to a favorable association between CDs and the molecular cobalt catalyst, which results in improved interfacial photoelectron transfer and catalytic mechanism. This work provides a scalable and inexpensive platform for the development of CO2 photoreduction systems using CDs.

Rational design and performance prediction of organic photosensitizer based on TATA+ dye for hydrogen production by photocatalytic decomposition of water.

In comparison to metal complexes, organic photosensitive dyes employed in photocatalytic hydrogen production exhibit promising developmental prospects. Utilizing the organic dye molecule TA+0 as the foundational structure, a series of innovative organic dyes, denoted as TA1-1 to TA2-6, were systematically designed. Employing first-principles calculations, we methodically explored the modifying effects of diverse electron-donating groups on the R1 and R2 positions to assess their application potential. Our findings reveal that, relative to the experimentally synthesized TATA+03, the TA2-6 molecule boasts a spatial structure conducive to intramolecular electron transfer, showcasing the most negative reduction potential (Ered = -2.11eV) and the maximum reaction driving force (△G0 2 = -1.26eV). This configuration enhances its compatibility with the reduction catalyst, thereby facilitating efficient hydrogen evolution. The TA2-6 dye demonstrates outstanding photophysical properties and a robust solar energy capture capacity. Its maximum molar extinction coefficient (ε) stands at 2.616 × 104M-1·cm-1, representing a remarkable 292.8% improvement over TATA+03. In conclusion, this research underscores the promising potential of the TA2-6 dye as an innovative organic photosensitizer, positioning it as an efficacious component in homogeneous photocatalytic systems.

Ordered Integration and Heterogenization of Catalysts and Photosensitizers in Metal-/Covalent-Organic Frameworks for Boosting CO2 Photoreduction.

ConspectusSolar-driven CO2 reduction into value-added chemicals, such as CO, HCOOH, CH4, and C2+ products, has been regarded as a potential way to alleviate environmental pollution and the energy crisis. In the past decades, numerous pioneered homogeneous catalytic systems composed of soluble photosensitizers (PSs) and catalytic active sites (CASs) have been explored for CO2 photoreduction. Nevertheless, inefficient electron migration based on random collision between CASs and PSs in homogeneous catalytic systems usually causes mediocre performance. Moreover, the relatively poor separation/recycling capability of the homogeneous systems has inevitably reduced their reusability and practicality. The rational combination of PSs and CASs have been proven to play critical roles in the development of highly efficient heterogeneous catalysts to improve their performance, such as anchoring them onto the solid matrixes or connecting them through bridging ligands. However, developing effective assembly strategies to achieve the ordered orientation and uniform heterogenization of PSs and CASs remains a great challenge, mainly due to the lack of crystallinity heterogeneous transformation and structural tailoring ability of traditional solid catalysts. Moreover, due to the lack of assembly and synthesis strategies, many efficient homogeneous photocatalytic systems are still unable to achieve high crystallinity heterogeneous transformation.Metal-organic frameworks (MOFs) and covalent-organic frameworks (COFs) have recently attracted broad interest toward CO2 photocatalysis because of their diverse precursors, well-defined and tailorable structures, abundant exposed CASs and high surface areas, etc. Especially, the highly ordered orientation and uniform combination of PSs and CASs in MOFs and COFs are beneficial for improved light harvesting and charge separation, greatly helping to address the aforementioned challenges. Moreover, the well-defined crystalline structures of MOFs and COFs facilitate the establishment of the structure-activity relationship. Therefore, it is increasingly important to summarize the integration of PSs and catalysts to provide deep insight into MOF/COF-based photocatalysts.In this Account, we summarize the ordered integration of PSs and CASs in MOFs and COFs for CO2 photoconversion and describe the structure-activity relationships to guide the design of effective catalysts. Given the unique structural features of MOFs and COFs, we have emphasized the integration of PSs and CASs to optimize their photocatalytic performance, including the confinement of catalytic active nanoparticles (NPs) into photosensitizing frameworks, co-coordination of PSs and CASs, and ligand-to-metal charge-transfer and anchoring CASs on the secondary building units of the photosensitizing frameworks. The catalytic activity, selectivity, sacrificial agent, and stability of these systems were then discussed. More importantly, MOFs and COFs provide powerful platforms to understand the key steps for boosting CO2 photoreduction and exploring the catalytic mechanism, involving light harvesting, electron-hole separation/migration, and surface redox reactions. Finally, the perspective and challenge of CO2 photoreduction in MOF/COF platforms are further proposed and discussed. It is expected that this Account would provide deep insight into the integration of PSs and catalysts in COFs and MOFs with well-defined structures and afford significant inspiration toward enhanced performance in heterogeneous catalysis.

Photocatalytic Ketyl Radical Initiated Cketyl -Csp2 /Csp3 Coupling on ZnIn2 S4.

Visible-light-driven C-C bond formation utilizing ketyl radical (Cketyl ) species has attracted increasing attention recently, as it provides a direct route for the synthesis of complex molecules. However, the most-developed homogeneous photocatalytic systems for the generation and utilization of ketyl radicals usually entail noble metal-based (e. g., Ru and Ir) photosensitizers, which suffer from not only high cost but also potential degradation and hence pose challenges in product separation and purification. In contrast, readily accessible, inexpensive, and recyclable semiconductors represent a class of attractive and alternative photocatalysts but remain much less explored for photocatalytic ketyl radical initiated C-C bond formation. This work demonstrates that a wide range of industrially important chemicals, including substituted chromanes and tertiary alcohols, can be produced on ZnIn2 S4 under visible light irradiation through intramolecular cyclization (Cketyl -Csp2 ) and intermolecular cross-coupling (Cketyl -Csp3 ) reactions, respectively, using ketyl radicals. A suite of experimental studies aided by computational investigation were carried out to shed light on the mechanistic insights of these two types of ketyl radical initiated C-C coupling reactions on ZnIn2 S4 .

In situ Detection of Cobaloxime Intermediates During Photocatalysis Using Hollow-Core Photonic Crystal Fiber Microreactors.

Hollow-core photonic crystal fibers (HC-PCFs) provide a novel approach for in situ UV/Vis spectroscopy with enhanced detection sensitivity. Here, we demonstrate that longer optical path lengths than afforded by conventional cuvette-based UV/Vis spectroscopy can be used to detect and identify the CoI and CoII states in hydrogen-evolving cobaloxime catalysts, with spectral identification aided by comparison with DFT-simulated spectra. Our findings show that there are two types of signals observed for these molecular catalysts; a transient signal and a steady-state signal, with the former being assigned to the CoI state and the latter being assigned to the CoII state. These observations lend support to a unimolecular pathway, rather than a bimolecular pathway, for hydrogen evolution. This study highlights the utility of fiber-based microreactors for understanding these and a much wider range of homogeneous photocatalytic systems in the future.

In situ Detection of Cobaloxime Intermediates During Photocatalysis Using Hollow‐Core Photonic Crystal Fiber Microreactors

AbstractHollow‐core photonic crystal fibers (HC‐PCFs) provide a novel approach for in situ UV/Vis spectroscopy with enhanced detection sensitivity. Here, we demonstrate that longer optical path lengths than afforded by conventional cuvette‐based UV/Vis spectroscopy can be used to detect and identify the CoI and CoII states in hydrogen‐evolving cobaloxime catalysts, with spectral identification aided by comparison with DFT‐simulated spectra. Our findings show that there are two types of signals observed for these molecular catalysts; a transient signal and a steady‐state signal, with the former being assigned to the CoI state and the latter being assigned to the CoII state. These observations lend support to a unimolecular pathway, rather than a bimolecular pathway, for hydrogen evolution. This study highlights the utility of fiber‐based microreactors for understanding these and a much wider range of homogeneous photocatalytic systems in the future.

Heterogeneous Photocatalysis as a Potent Tool for Organic Synthesis: Cross-Dehydrogenative C-C Coupling of N-Heterocycles with Ethers Employing TiO2/N-Hydroxyphthalimide System under Visible Light.

Despite the obvious advantages of heterogeneous photocatalysts (availability, stability, recyclability, the ease of separation from products and safety) their application in organic synthesis faces serious challenges: generally low efficiency and selectivity compared to homogeneous photocatalytic systems. The development of strategies for improving the catalytic properties of semiconductor materials is the key to their introduction into organic synthesis. In the present work, a hybrid photocatalytic system involving both heterogeneous catalyst (TiO2) and homogeneous organocatalyst (N-hydroxyphthalimide, NHPI) was proposed for the cross-dehydrogenative C-C coupling of electron-deficient N-heterocycles with ethers employing t-BuOOH as the terminal oxidant. It should be noted that each of the catalysts is completely ineffective when used separately under visible light in this transformation. The occurrence of visible light absorption upon the interaction of NHPI with the TiO2 surface and the generation of reactive phthalimide-N-oxyl (PINO) radicals upon irradiation with visible light are considered to be the main factors determining the high catalytic efficiency. The proposed method is suitable for the coupling of π-deficient pyridine, quinoline, pyrazine, and quinoxaline heteroarenes with various non-activated ethers.

Uranyl nitrate as a recyclable homogeneous photocatalyst for selective cross-coupling of N-substituted amines and indoles

A homogeneous photocatalytic recyclable system for the selective radical-radical cross-coupling of N-substituted amines and indoles has been established. This system could conduct in water or acetonitrile, featuring the reuse of uranyl nitrate as the recyclable photocatalyst via a simple extraction. With this mild strategy in hand, good to excellent yields of cross-coupling products could be achieved even under the irradiation of sunlight, including 26 natural product derivatives and 16 natural product inspired re-engineered compounds. A radical-radical cross-coupling mechanism was newly proposed based on experimental evidence and reported literature. This strategy has been also applied to a gram scale synthesis to demonstrate its practical utility.

Phenoxazine‐Sensitized CO2‐to‐CO Reduction with an Iron Porphyrin Catalyst: A Redox Properties‐Catalytic Performance Study

AbstractWe have evaluated six phenoxazine derivatives as visible light photosensitizers for the photochemical reduction of CO2 to CO with an iron porphyrin catalyst in organic media. The phenoxazine core was functionalized with electron‐donating or ‐withdrawing groups to modify the photophysical properties. Both singlet and triplet excited state potentials of the sensitizers spanned several hundred mV range and the ground state oxidation potentials spanned 250 mV. We observed that no correlation can be established between the production of CO and the excited state potential of the phenoxazine, which determines the driving force for electron transfer to the catalyst. On the contrary, a clear correlation can be made between the oxidation potentials of the phenoxazine and the production of CO. This observation indicates that the process was limited by photosensitizer regeneration and highlights the fact that electron transfers not directly related to the catalyst activation could play a key role in homogeneous photocatalytic systems.

Porphyrins and phthalocyanines as biomimetic tools for photocatalytic H2 production and CO2 reduction.

The increasing energy demand and environmental issues caused by the over-exploitation of fossil fuels render the need for renewable, clean, and environmentally benign energy sources unquestionably urgent. The zero-emission energy carrier, H2 is an ideal alternative to carbon-based fuels especially when it is generated photocatalytically from water. Additionally, the photocatalytic conversion of CO2 into chemical fuels can reduce the CO2 emissions and have a positive environmental and economic impact. Inspired by natural photosynthesis, plenty of artificial photocatalytic schemes based on porphyrinoids have been investigated. This review covers the recent advances in photocatalytic H2 production and CO2 reduction systems containing porphyrin or phthalocyanine derivatives. The unique properties of porphyrinoids enable their utilization both as chromophores and as catalysts. The homogeneous photocatalytic systems are initially described, presenting the various approaches for the improvement of photosensitizing activity and the enhancement of catalytic performance at the molecular level. On the other hand, for the development of the heterogeneous systems, numerous methods were employed such as self-assembled supramolecular porphyrinoid nanostructures, construction of organic frameworks, combination with 2D materials and adsorption onto semiconductors. The dye sensitization on semiconductors opened the way for molecular-based dye-sensitized photoelectrochemical cells (DSPECs) devices based on porphyrins and phthalocyanines. The research in photocatalytic systems as discussed herein remains challenging since there are still many limitations making them unfeasible to be used at a large scale application before finding a large-scale application.

Tuning the Excited-State Dynamics of Acetophenone Using Metal Ions in Solution.

The effects of dissolved metal salts on the excited-state dynamics of acetophenone in solution have been explored by using ultrafast transient absorption spectroscopy at two UV excitation wavelengths. In the absence of metal ions, the S1(nπ*) transition of acetophenone is excited at 320 nm, with intersystem crossing (ISC) occurring with a time constant τISC = 5.95 ± 0.47 ps in acetonitrile solution. Excitation at 280 nm accesses the S2(ππ*) state, which internally converts (<0.2 ps) to S1 before undergoing ISC with τISC = 4.36 ± 0.14 ps. Coordination to Mg2+ ions makes the S2 state accessible to excitation at 320 nm, with the rate of S2 → S1 internal conversion reducing 3-fold but the ISC rate increasing. These changes to the excited-state energies and dynamics of this model photosensitizer indicate that dissolved metal salts could modify the photochemistry of synthetically useful homogeneous photocatalytic systems.

Merging Photocatalysis with Electrochemistry: The Dawn of a new Alliance in Organic Synthesis.

The merging of a homogeneous photocatalytic system with an electrochemical cell, having exchanged electrons as the only common point, has been recently demonstrated. This combination opens unexplored pathways in synthesis and allowed net-oxidative photocatalytic processes to be realized in the absence of a chemical oxidant, including: 1) the C-H alkylation of heteroarenes and 2) the coupling of azoles with arenes in the presence of an electrogenerated photocatalyst.

Electrochemical Generation and Spectroscopic Characterization of the Key Rhodium(III) Hydride Intermediates of Rhodium Poly(bipyridyl) H2-Evolving Catalysts.

We previously reported that the [RhIII(dmbpy)2Cl2]+ (dmbpy = 4,4'-dimethyl-2,2'-bipyridine) complex is an efficient H2-evolving catalyst in water when used in a molecular homogeneous photocatalytic system for hydrogen production with [RuII(bpy)3]2+ (bpy = 2,2'-bipyridine) as photosensitizer and ascorbic acid as sacrificial electron donor. The catalysis is believed to proceed via a two-electron reduction of the Rh(III) catalyst into the square-planar [RhI(dmbpy)2]+, which reacts with protons to form a Rh(III) hydride intermediate that can, in turn, release H2 following different pathways. To improve the current knowledge of these key intermediate species for H2 production, we performed herein a detailed electrochemical investigation of the [RhIII(dmbpy)2Cl2]+ and [RhIII(dtBubpy)2Cl2]+ (dtBubpy = 4,4'-di- tert-butyl-2,2'-bipyridine) complexes in CH3CN, which is a more appropriate medium than water to obtain reliable electrochemical data. The low-valent [RhI(Rbpy)2]+ and, more importantly, the hydride [RhIII(Rbpy)2(H)Cl]+ species (R = dm or dtBu) were successfully electrogenerated by bulk electrolysis and unambiguously spectroscopically characterized. The quantitative formation of the hydrides was achieved in the presence of weak proton sources (HCOOH or CF3CO3H), owing to the fast reaction of the electrogenerated [RhI(Rbpy)2]+ species with protons. Interestingly, the hydrides are more difficult to reduce than the initial Rh(III) bis-chloro complexes by ∼310-340 mV. Besides, 0.5 equiv of H2 is generated through their electrochemical reduction, showing that Rh(III) hydrides are the initial catalytic molecular species for hydrogen evolution. Density functional theory calculations were also performed for the dmbpy derivative. The optimized structures and the theoretical absorption spectra were calculated for the initial bis-chloro complex and for the various rhodium intermediates involved in the H2 evolution process.

Cobalt(II)–Salen Complexes for Photocatalytic Hydrogen Production in Noble Metal-Free Molecular Systems

Developing cost-effective hydrogen-production systems is always appealing in order to satisfy the future energy request. Here, two Co(II)–salen complexes 1 and 2 were firstly introduced into homogeneous photocatalytic hydrogen-evolution systems. Based on these two cobalt complexes, we developed noble-metal-free hydrogen production systems using xanthene dyes [such as Disodium salts of Eosin Y (EY2−), Rose Bengal (RB2−) and Fluorescein (Fl2−)] as photosensitizers and trimethylamine (TEA) as sacrificial donor. Complex 1 presented the best catalytic performance for hydrogen production with a TON of 319 based on catalyst within 9 h irradiation, which is superior to other cobalt complexes in noble-metal-free systems. UV–Vis spectra studies demonstrate that EY2− suffered quick decomposition, especially under the presence of complex 1, which should be responsible for hydrogen-production deactivation. The thermodynamically favorable photo-induced electron transfer from 3*EY2− to complex 1 was supported by investigations involving fluorescence quenching and cyclic voltammetry studies. However, the immediate color change (within seconds) as the hydrogen production systems exposed under the irradiation indicates that EY2− firstly suffered dehalogenation which functions as the real active dyes.

Poly(benzothiadiazoles) and Their Derivatives as Heterogeneous Photocatalysts for Visible-Light-Driven Chemical Transformations

Visible-light-driven chemical transformations via photocatalysis have witnessed an explosive growth in the past decade, accompanying the enormous development of the photocatalyst design. Among the intensely investigated systems, molecular photocatalysts as transition-metal complexes and conjugated organic dyes have been established as efficient homogeneous photocatalytic systems for organic photoredox reactions. Nonetheless, heterogeneous photocatalysts possess considerable advantages such as being highly stable, easily separable, and reusable. Especially, the organic, macromolecular heterogeneous photocatalysts have emerged as a nontoxic and potentially more environmentally friendly alternative to the traditional catalytic systems. Among them, poly(benzothiadiazoles) and their derivatives have demonstrated the ability to catalyze various organic photoredox reactions under visible-light irradiation. In this Review, the recent development of metal-free and heterogeneous photocatalytic systems based on poly(benzothiadiazoles) and their derivatives is summarized. An overview of organic photoredox reactions mediated via radicals obtained from the photogenerated electron and hole of the photocatalysts is given, and the underlying mechanisms of photochemical transformations are illuminated. The structural design principles of poly(benzothiadiazoles) for targeted photoredox reactions are also discussed.

Challenging the [Ru(bpy)3]2+ Photosensitizer with a Triazatriangulenium Robust Organic Dye for Visible-Light-Driven Hydrogen Production in Water

Photosensitizers used in homogeneous photocatalytic systems for artificial photosynthesis, such as hydrogen production, are typically based on expensive transition metal complexes such as d6 ruthenium(II) or iridium(III). In this work, we demonstrate efficient H2 production in acidic water by using an organic dye derived from the triazatriangulenium (TATA+) family as a visible-light-absorbing photosensitizer (PS). By associating the hydrosoluble tris(ethoxyethanol)triazatriangulenium with an efficient H2-evolving cobalt catalyst and ascorbic acid as sacrificial electron donor (SD), remarkable photocatalytic performances were reached in aqueous solution at pH 4.5, under visible-light irradiation, with up to 8950 catalytic cycles versus catalyst. The performances of this dye largely exceed those of the benchmark Ru tris-bipyridine in the same experimental conditions when low concentrations of catalyst are used. This higher efficiency has been clearly ascribed to the remarkable robustness of the reduced form...

Efficient electrocatalytic and photocatalytic hydrogen evolution using a linear trimeric thiolato complex of nickel

Increasing interest has been paid to the development of earth-abundant metal complexes as promising surrogates of noble-metal platinum and biological catalysts hydrogenases for catalyzing the hydrogen evolution reaction. In this study, we report on a molecular H2-evolving catalyst based on a linear trimeric thiolato complex of nickel Ni3(LN2S2)2 (LN2S2 = N,N′-dimethyl-N-N′-bis(2-mecaptoethyl)-ethylenediaminato). Electrochemical studies showed that the trinuclear nickel complex Ni3(LN2S2)2 can electrocatalyze hydrogen evolution from weakly acidic solutions with remarkable turnover frequencies (3495 s−1 at −1.98 V and 715 s−1 at −1.58 V vs SCE). An efficient noble-metal-free homogeneous photocatalytic system for hydrogen generation from water working under visible light irradiation was further constructed by using the target nickel complex as photocatalyst, fluorescein (Fl) as photosensitizer (PS), and triethylamine (TEA) as sacrificial electron donor. Our studies showed that Ni3(LN2S2)2 can be used in purely aqueous solution and gave a turnover number (TON, vs catalyst) for H2 evolution of 790, corresponding to a TOF 60 h−1. The results show that multinuclear nickel(II) complexes are a promising new direction for molecular catalysts for the electro- and photoreduction of protons.

Highly efficient photocatalytic hydrogen evolution from nickel quinolinethiolate complexes under visible light irradiation

Earth-abundant metal complexes have emerged as promising surrogates of platinum for catalyzing the hydrogen evolution reaction (HER). In this study, we report the design and synthesis of two novel nickel quinolinethiolate complexes, namely [Ni(Hqt)2(4, 4′-Z-2, 2′-bpy)] (Hqt = 8-quinolinethiol, Z = H [1] or CH3 [2], bpy = bipyridine). An efficient three-component photocatalytic homogeneous system for hydrogen generation working under visible light irradiation was constructed by using the target complexes as catalysts, triethylamine (TEA) as sacrificial electron donor and xanthene dyes as photosensitizer. We obtain turnover numbers (TON, vs. catalyst) for H2 evolution of 5923/7634 under the optimal conditions with 5.0 × 10−6 M complex 1/2 respectively, 1.0 × 10−3 M fluorescein and 5% (v/v) TEA at pH 12.3 in EtOH/H2O (1:1, v/v) mixture after 8 h irradiation (λ > 420 nm). We discuss the mechanism of H2 evolution in the homogeneous photocatalytic system based on fluorescence spectrum and cyclic voltammetry data.

Synthesis of a new iron–sulfur cluster compound and its photocatalytic H2 evolution activity through visible light irradiation

A new iron–sulfur cluster compound, namely [(μ‐BNT)Fe2(CO)6] (A; BNT = (R)‐1,1′‐binaphthalene‐2,2′‐dithiol), was synthesized by self‐assembly of BNT with [Fe3(CO)12] and characterized using 1H NMR, 13C NMR, infrared spectra and elemental analysis. The H2 evolution activity of A was evaluated in a constructed homogeneous photocatalytic system by combining A as catalyst, xanthene dyes as photosensitizer and triethylamine as sacrificial reagent, to give efficient H2 generation under visible‐light irradiation (λ &gt; 420 nm). The maximum H2 evolution of 404 turnovers (versus catalyst) was recorded under optimal conditions in CH3CN–H2O (1:1, v/v) after 4 h irradiation. The mechanism of H2 evolution is briefly discussed using fluorescence spectra and electrochemical analysis. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Photo-catalytic H2 evolution, structural effect and electron transfer mechanism based on four novel [2Fe2S] model complexes by photochemical splitting water

Four novel [2Fe2S] model complexes (1–4) had been synthesized and characterized by IR, 1H NMR, elemental analysis and single crystal X-ray crystallography (for 2). The key parameters affecting H2 evolution of title compounds (1–4) were optimized by constructing homogeneous photo-catalytic system consisting of title compounds (1–4) as catalyst, erythrosin B sodium salt (EBS2−) as photo-sensitizer (PS) and triethylamine (TEA) as sacrificial reagent in CH3CN/H2O solution under the irradiation of visible light. The maximum H2 evolution was separately 211.4±6.5μmol (52.9±1.6TON vs. 4), 199.4±13.5μmol (49.9±3.4TON vs. 1), 124.8±7.6μmol (31.2±1.9TON vs. 3) and 34.9±6.9μmol (8.7±1.7TON vs. 2) under the optimal conditions with catalyst of 2×10−4mol·L−1, EBS2− of 4×10−4mol·L−1, TEA of 10% (v/v) and pH 12 in CH3CN/H2O (1/1, v/v) solution. Furthermore, the structural effect and mechanism of electron transfer in the present system was carefully discussed by fluorescence spectra and cyclic voltammetry (CV) measurements.