Metal-free carbazole-thiophene photosensitizers designed for a dye-sensitized H2-evolving photocatalyst in Z-scheme water splitting.

Dye-sensitized photocatalysts with molecular dyes and widegap semiconductors have attracted attention because of their design flexibility, for example, tunable light absorption for visible-light water splitting. Although organic dyes are promising candidates as metal-free photosensitizers in dye-sensitized photocatalysts, their efficiency in H2 production has far been unsatisfactory compared to that of metal-complex photosensitizers, such as Ru(II) tris-diimine-type complexes. Here, we demonstrate the substantial improvement of carbazole-thiophene-based dyes used for dye-sensitized photocatalysts through systematic molecular design of the number of thiophene rings, substituents in the thiophene moiety, and the anchoring group. The optimized carbazole-thiophene dye-sensitized layered niobate exhibited a quantum efficiency of 0.3% at 460 nm for H2 evolution using a redox-reversible I- electron donor, which is six-times higher than that of the best coumarin-based metal-free dye reported to date. The dye-sensitized photocatalyst also facilitated overall water splitting when combined with a WO3-based O2-evolving photocatalyst and an I3-/I- redox shuttle mediator. The present metal-free dye provided a high dye-based turnover frequency for water splitting, comparable to that of the state-of-the-art Ru(II) tris-diimine-type photosensitizer, by simple adsorption onto a layered niobate. Thus, this study highlights the potential of metal-free organic dyes with appropriate molecular designs for the development of efficient water splitting.

Synthesis of Microporous Aluminosilicate by Direct Thermal Activation of Phenyl-Substituted Single-Source Aluminosilicate Molecular Precursors.

The construction of aluminosilicates from versatile molecular precursors (MPs) represents a promising alternative strategy to conventional processes based on monomeric molecular or polymeric Al and Si sources. However, the use of MPs often suffers from drawbacks such as the decomposition of the core structures in the presence of solvents, acids, or bases. In this work, we demonstrate a simple thermal synthesis of porous aluminosilicates from single-source spiro-7-type MPs that consist of a tetrahedral Al atom and six Si atoms functionalized with 12 phenyl (Ph) groups, (C+)[Al{Ph2Si(OSiPh2O)2}2]- (C+[AlSi6]-; C+ = pyridinium cation (PyH+), Na+, K+, Rb+, or Cs+), without using a solvent or activator. Microporous aluminosilicates synthesized via the thermal treatment of C+[AlSi6]- under a 79% N2 + 21% O2 atmosphere exhibited extremely low carbon contents (0.10-1.28%), together with Si/Al ratios of 3.9-6.7 ± 0.2 and surface areas of 103.1-246.3 m2/g. The solid-state 27Al and 29Si MAS NMR spectra suggest that the obtained aluminosilicates with alkali cations retain a tetrahedral Al site derived from the spiro-7-type core structure. After a proton-exchange reaction, the aluminosilicates showed almost 1.5 times higher reactivity in the catalytic ring-opening of styrene oxide than the aluminosilicate before proton exchange due to the catalytically active OH site being predominantly bridged by tetrahedral Al and Si atoms. These results suggest that the present MP strategy is a promising method for the introduction of key structures into active inorganic materials.

Conduction Band Control of Oxyhalides with a Triple-Fluorite Layer for Visible Light Photocatalysis.

The discovery of building blocks offers new opportunities to develop and control properties of extended solids. Compounds with fluorite-type Bi2O2 blocks host various properties including lead-free ferroelectrics and photocatalysts. In this study, we show that triple-layered Bi2MO4 blocks (M = Bi, La, Y) in Bi2MO4Cl allow, unlike double-layered Bi2O2 blocks, to extensively control the conduction band. Depending on M, the Bi2MO4 block is truncated by Bi-O bond breaking, resulting in a series of n-zigzag chain structures (n = 1, 2, ∞ for M = Bi, La, Y, respectively). Thus, formed chain structures are responsible for the variation in the conduction band minimum (-0.36 to -0.94 V vs SHE), which is correlated to the presence or absence of mirror symmetry at Bi. Bi2YO4Cl shows higher photoconductivity than the most efficient Bi2O2-based photocatalyst with promising visible-light photocatalytic activity for water splitting. This study expands the possibilities of thickening (2D to 3D) and cutting (2D to 1D) fluorite-based blocks toward desired photocatalysis and other functions.

Tunable Synchronicity of Molecular Valence Tautomerism with Macroscopic Solid-Liquid Transition by Molecular Lattice Engineering.

The combination of a cobalt-dioxolene core that exhibits valence tautomerism (VT) with pyridine-3,5-dicarboxylic acid functionalized with chains bearing two, four, or six oxyethylene units led to new complexes ConEGEspy (n = 2, 4, and 6). These complexes commonly form violet crystals of the low-spin (ls)-[CoIII (nEGEspy)2 (3,6-DTBSQ)(3,6-DTBCat)] (ls-[CoIII ], 3,6-DTBSQ = 3,6-di-tert-butyl semiquinonato, 3,6-DTBCat = 3,6-di-tert-butyl catecholato). Interestingly, violet crystals of Co2EGEspy in the ls-[CoIII ] transitioned into a green liquid, accompanied by an almost complete VT shift (94 %) to the high-spin (hs)-[CoII (nEGEspy)2 (3,6-DTBSQ)2 ] (hs-[CoII ]) upon melting. In contrast, violet crystals of Co4EGEspy and Co6EGEspy in the ls-[CoIII ] exhibited partial VT (33 %) and only a 9.3 % VT shift after melting, respectively. These data demonstrate the tunability of the synchronicity of the molecular VT and macroscopic solid-liquid transitions by optimizing the tethered chains, thus establishing a new strategy for coupling bistable molecules with the macroscopic world.

Functional Group-Directed Photochemical Reactions of Aromatic Alcohols, Amines, and Thiols Triggered by Excited-State Hydrogen Detachment: Additive-free Oligomerization, Disulfidation, and C(sp2)-H Carboxylation with CO2.

Exploring new types of photochemical reactions is of great interest in the field of synthetic chemistry. Although excited-state hydrogen detachment (ESHD) represents a promising prospective template for additive-free photochemical reactions, applications of ESHD in a synthetic context remains scarce. Herein, we demonstrate the expansion of this photochemical reaction toward oligomerization, disulfidation, and regioselective C(sp2)-H carboxylation of aromatic alcohols, thiols, and amines. In the absence of any radical initiators in tetrahydrofuran upon irradiation with UV light (λ = 280 or 300 nm) under an atmosphere of N2 or CO2, thiols and catechol afforded disulfides and oligomers, respectively, as main products. Especially, the photochemical disulfidation proceeded highly selectively with the NMR and quantum yields of up to 69 and 0.46%, respectively. In stark contrast, the photolysis of phenylenediamines and aminophenols results in photocarboxylation in the presence of CO2 (1 atm). p-Aminophenol was quantitatively carboxylated by photolysis for 17 h with a quantum yield of 0.45%. Furthermore, the photocarboxylation of phenylenediamines and aminophenols proceeds in a highly selective fashion on the aromatic C(sp2)-H bond next to a functional group, which is directed by the site-selective ESHD of the functional groups for the formation of aminyl and hydroxyl radicals.

Identification of Prime Factors to Maximize the Photocatalytic Hydrogen Evolution of Covalent Organic Frameworks.

Visible-light-driven hydrogen (H2) production from water is a promising strategy to convert and store solar energy as chemical energy. Covalent organic frameworks (COFs) are front runners among different classes of organic photocatalysts. The photocatalytic activity of COFs depends on numerous factors such as the electronic band gap, crystallinity, surface area, exciton migration, stability of transient species, charge separation and transport, etc. However, it is challenging to fine tune all of these factors simultaneously to enhance the photocatalytic activity. Hence, in this report, an effort has been made to understand the interplay of these factors and identify the key factors for efficient photocatalytic H2 production through a structure-property-activity relationship. Careful molecular engineering allowed us to optimize all of the above plausible factors impacting the overall catalytic activities of a series of isoreticular COFs. The present study determines three prime factors: light absorption, charge carrier generation, and its transport, which influence the photocatalytic H2 production of COFs to a much greater extent than the other factors.

Molecular Insights into the Ligand-Based Six-Proton- and Six-Electron-Transfer Processes Between Tris-ortho-Phenylenediamines and Tris-ortho-Benzoquinodiimines.

The global demand for energy and the concerns over climate issues renders the development of alternative renewable energy sources such as hydrogen (H2 ) important. A high-spin (hs) FeII complex with o-phenylenediamine (opda) ligands, [FeII (opda)3 ]2+ (hs-[6R]2+ ), was reported showing photochemical H2 evolution. In addition, a low-spin (ls) [FeII (bqdi)3 ]2+ (bqdi: o-benzoquinodiimine) (ls-[0R]2+ ) formation by O2 oxidation of hs-[6R]2+ , accompanied by ligand-based six-proton and six-electron transfer, revealed the potential of the complex with redox-active ligands as a novel multiple-proton and -electron storage material, albeit that the mechanism has not yet been understood. This paper reports that the oxidized ls-[0R][PF6 ]2 can be reduced by hydrazine giving ls-[FeII (opda)(bqdi)2 ][PF6 ]2 (ls-[2R][PF6 ]2 ) and ls-[FeII (opda)2 (bqdi)][PF6 ]2 (ls-[4R][PF6 ]2 ) with localized ligand-based proton-coupled mixed-valence (LPMV) states. The first isolation and characterization of the key intermediates with LPMV states offer unprecedented molecular insights into the design of photoresponsive molecule-based hydrogen-storage materials.

Photocatalytic CO2 reduction to formic acid using a Ru(II)-Re(I) supramolecular complex in an aqueous solution.

In an aqueous solution, photophysical, photochemical, and photocatalytic abilities of a Ru(II)-Re(I) binuclear complex (RuReCl), of which Ru(II) photosensitizer and Re(I) catalyst units were connected with a bridging ligand, have been investigated in details. RuReCl could photocatalyze CO2 reduction using ascorbate as an electron donor, even in an aqueous solution. The main product of the photocatalytic reaction was formic acid in the aqueous solution; this is very different in product distribution from that in a dimethylformamide (DMF) and triethanolamine (TEOA) mixed solution in which the main product was CO. A (13)CO2 labeling experiment clearly showed that formic acid was produced from CO2. The turnover number and selectivity of the formic acid production were 25 and 83%, respectively. The quantum yield of the formic acid formation was 0.2%, which was much lower, compared to that in the DMF-TEOA mixed solution. Detail studies of the photochemical electron-transfer process showed back-electron transfer from the one-electron-reduced species (OERS) of the photosensitizer unit to an oxidized ascorbate efficiently proceeded, and this should be one of the main reasons why the photocatalytic efficiency was lower in the aqueous solution. In the aqueous solution, ligand substitution of the Ru(II) photosensitizer unit proceeded during the photocatalytic reaction, which was a main deactivation process of the photocatalytic reaction. The product of the ligand substitution was a Ru(II) bisdiimine complex or complexes with ascorbate as a ligand or ligands.

Light-induced electron transfer/phase migration of a redox mediator for photocatalytic C-C coupling in a biphasic solution.

Photocatalytic molecular conversions that lead to value-added chemicals are of considerable interest. To achieve highly efficient photocatalytic reactions, it is equally important as it is challenging to construct systems that enable effective charge separation. Here, we demonstrate that the rational construction of a biphasic solution system with a ferrocenium/ferrocene (Fc+/Fc) redox couple enables efficient photocatalysis by spatial charge separation using the liquid-liquid interface. In a single-phase system, exposure of a 1,2-dichloroethane (DCE) solution containing a Ru(II)- or Ir(III)-based photosensitizer, Fc, and benzyl bromide (Bn-Br) to visible-light irradiation failed to generate any product. However, the photolysis in a H2O/DCE biphasic solution, where the compounds are initially distributed in the DCE phase, facilitated the reductive coupling of Bn-Br to dibenzyl (Bn2) using Fc as an electron donor. The key result of this study is that Fc+, generated by photooxidation of Fc in the DCE phase, migrates to the aqueous phase due to the drastic change in its partition coefficient compared to that of Fc. This liquid-liquid phase migration of the mediator is essential for facilitating the reduction of Bn-Br in the DCE phase as it suppresses backward charge recombination. The co-existence of anions can further modify the driving force of phase migration of Fc+ depending on their hydrophilicity; the best photocatalytic activity was obtained with a turnover frequency of 79.5 h-1 and a quantum efficiency of 0.2% for the formation of Bn2 by adding NBu4+Br- to the biphasic solution. This study showcases a potential approach for rectifying electron transfer with suppressed charge recombination to achieve efficient photocatalysis.

Excited-state hydrogen detachment from a tris-(o-phenylenediamine) iron(ii) complex in THF at room temperature.

We previously reported that a tris-(o-phenylenediamine) iron(ii) complex promotes photochemical H2 generation and C-H carboxylation of o-phenylenediamine without any additives under N2 and CO2 atmospheres, respectively, in tetrahydrofuran at room temperature. Herein, the key mechanistic process, namely, excited-state hydrogen detachment from the o-phenylendiamine moiety, is demonstrated under an N2 atmosphere.

Fe/Ru Oxide as a Versatile and Effective Cocatalyst for Boosting Z-Scheme Water-Splitting: Suppressing Undesirable Backward Electron Transfer.

The atificial Z-scheme is a promising and rational strategy for solar-to-chemical energy conversion such as water-splitting. In the Z-scheme, backward redox processes are an essential drawback that should be overcome to increase its efficiency. Here, we demonstrate that the simple co-loading of Fe/Ru oxide, (Fe,Ru)Ox, onto various photocatalysts effectively improves the efficiency of water oxidation by suppressing the undesirable backward oxidation of the redox reagent Fe2+. The (Fe,Ru)Ox co-loading on Bi4TaO8Cl afforded the highest water-splitting activity (apparent quantum efficiency of 1.6% at 420 nm) among the Z-scheme systems employing mixed-anion compounds as O2-evolving photocatalysts. The results of photoelectrochemical and electrochemical measurements along with time-resolved spectroscopy clarified the key roles of Fe/Ru oxide; the Ru oxide component functions as a "collector" of photogenerated carriers and active sites for surface redox reactions, while the Fe oxide component acts as a "blocker" against unfavorable Fe2+ oxidation. The versatile availability of Fe/Ru oxide has been demonstrated for other visible-light-responsive photocatalysts.

Flux Synthesis of Layered Oxyhalide Bi4NbO8Cl Photocatalyst for Efficient Z-Scheme Water Splitting Under Visible Light.

An oxyhalide photocatalyst Bi4NbO8Cl has recently been proven to stably oxidize water under visible light, enabling the Z-scheme water splitting when coupled with another photocatalyst for water reduction. We herein report the synthesis of Bi4NbO8Cl particles via a flux method, testing various molten salts to improve its crystallinity and hence photocatalytic activity. The eutectic mixture of CsCl/NaCl with a low melting point allowed the formation of single-phase Bi4NbO8Cl at as low as 650 °C. Thus, synthesized Bi4NbO8Cl particles exhibited a well-grown and plate-like shape while maintaining surface area considerably higher than those grown with others fluxes. They showed three times higher O2 evolution rate under visible light than the samples prepared via a solid-state reaction. Time-resolved microwave conductivity measurements revealed greater signals (approximately 4.8 times) owing to the free electrons in the conduction band, indicating much improved efficiency of carrier generation and/or its mobility. The loading of RuO2 or Pt cocatalyst on Bi4NbO8Cl further enhanced the activity for O2 evolution because of efficient capturing of free electrons, facilitating the surface chemical reactions. In combination with a H2-evolving photocatalyst Ru/SrTiO3:Rh along with an Fe3+/Fe2+ redox mediator, the RuO2/Bi4NbO8Cl is an excellent O2-evolving photocatalyst, exhibiting highly effective water splitting into H2 and O2 via the Z-scheme.

Visible-light-driven CO2 reduction on a hybrid photocatalyst consisting of a Ru(ii) binuclear complex and a Ag-loaded TaON in aqueous solutions.

A hybrid photocatalyst consisting of a Ru(ii) binuclear complex and a Ag-loaded TaON reduced CO2 by visible light even in aqueous solution. The distribution of the reduction products was strongly affected by the pH of the reaction solution. HCOOH was selectively produced in neutral conditions, whereas the formation of HCOOH competed with H2 evolution in acidic conditions. Detailed mechanistic studies revealed that the photocatalytic CO2 reduction proceeded via 'Z-schematic' electron transfer with step-by-step photoexcitation of TaON and the photosensitizer unit in the Ru(ii) binuclear complex. The maximum turnover number for HCOOH formation was 750 based on the Ru(ii) binuclear complex under visible-light irradiation, and the optimum external quantum efficiency of the HCOOH formation was 0.48% using 400 nm monochromic light with ethylenediaminetetraacetic acid disodium salt as a sacrificial reductant. Even in aqueous solution, the hybrid could also convert visible-light energy into chemical energy (ΔG0 = +83 kJ mol-1) by the reduction of CO2 to HCOOH with methanol oxidation.