The production of hydrogen from sunlight and water is finding an increasingly important role in the production of clean fuels from sustainable and abundant energy sources. In this process, commonly referred to as artificial photosynthesis, the role of the dye sensitizer is critical for optimizing the harvesting of visible light and triggering the reduction reaction at the catalytic active site. In recent decades organometallic sensitizers have mainly been studied, often requiring the use of scarce and, in some cases, toxic elements. This microreview describes the state of the art in the use of metal-free organic sensitizers, highlighting advantages over their organometallic counterparts. The main design and synthetic strategies, specific properties, and device performances are presented. Thanks to recent advances and lower manufacturing costs, organic sensitizers seem set to be of increasing importance for next-generation clean fuels

Dibranched donor-(π-acceptor)2 dyes, where phenothiazine is the donor core, cyanoacrylic acid is the acceptor/anchoring group, and π is represented by mono- and poly-cyclic simple and fused thiophene derivatives, were tested as photosensitizers in the photocatalytic production of H2 , in combination with a Pt/TiO2 catalyst. The optical and electrochemical properties of the dyes were investigated, showing that careful design of the thiophene-based π spacer afforded enhanced optical properties. In the H2 production over 20 h, the new thiophene-based sensitizers revealed improved stability after longer irradiation times and enhanced performances, in terms of H2 production rates and light-to-fuel efficiencies, after an initial activation period, which were for the first time associated with enhanced stability under photocatalytic production of H2 and the absence of critical dye degradation.

Multi‐branched multi‐anchoring metal‐free dyes as photosensitizers for dye‐sensitized solar cells (DSSCs) are reviewed. The article outlines design strategies, main synthetic routes and optical and photovoltaic properties of two families of multi‐branched sensitizers: (a) D–(π–A)n (D = donor, π = π‐spacer, A = acceptor/anchoring functionality) structures containing arylamine, carbazole, phenothiazine or phenoxazine derivatives as D groups, and (b) multi‐donor multi‐anchoring architectures from interconnected mono‐branched D–π–A arms, together with X‐ and Y‐shaped dyes. Although this class has been reported only in the last five years, a variety of molecular architectures, donors, and π‐spacers have been used and combined in multi‐branched molecules. The multi‐branched geometry induces distinctive features including enhanced qualitative and quantitative optical properties, increased currents and quantum efficiencies, and control of molecular aggregation.

A thiophene-based donor-acceptor phenothiazine dye has been functionalized with a peripheral glucose unit (PTZ-GLU) to bust its affinity to water and enhance dye-sensitized photogeneration of hydrogen. Compared to the corresponding alkyl derivative (PTZ-ALK), as well as the common hydrophilic triethylene glycol substitution (PTZ-TEG), the sugar derivative shows a lower contact angle; PTZ-GLU performed twice more efficient than PTZ-TEG in the photogeneration of hydrogen in terms of evolved gas and turnover number.

Rational development of efficient photo-catalytic systems for hydrogen production requires understanding the catalytic mechanism and detailed information about the structure of intermediates in the catalytic cycle. We demonstrate how time-resolved X-ray absorption spectroscopy in the microsecond time range can be used to identify such intermediates and to determine their local geometric structure. This method was used to obtain the solution structure of the Co(I) intermediate of cobaloxime, which is a non-noble metal catalyst for solar hydrogen production from water. Distances between cobalt and nearest ligands including two solvent molecules and displacement of the cobalt atom out of plane formed by the planar ligands have been determined. Combining in situ X-ray absorption and UV-visible data, we demonstrate how slight modification of the catalyst structure can lead to the formation of a catalytically inactive Co(I) state under similar conditions. Possible deactivation mechanisms are discussed. KeywordsTransient X-ray absorption; photocatalysis; XANES; X-ray absorption spectroscopy; solar fuel Light-driven catalytic systems for hydrogen evolution from water are crucial components of our energy future.[1,2] Efficient photo-catalytic hydrogen evolution systems contain platinum nanoparticles or other noble metals that are expensive and of limited availability. This has triggered the development of catalysts based on earth-abundant 3d elements, such as cobalt, iron and nickel. [3][4][5][6][7][8] Cobaloximes are perspective and popular hydrogen evolving molecular catalysts [7][8][9] and they have been implemented in many homogeneous multicomponent photocatalytic systems. [7,[10][11][12][13][14][15][16] (Fig. 1, top panel). In the absence of a sacrificial electron donor, charge recombination returns the system to its initial state. The solution structure of the Co(I) intermediate has been predicted using DFT, [17,20,22] but was never experimentally probed. A crystal structure of the Co(I) derivative [Co(dpgBF 2 ) 2 (CH 3 CN)] -has been reported, [18] but the structure of the intermediate in solution can be significantly different from those in the solid phase. Here, we combine in situ time-resolved X-ray absorption near edge structure (XANES) spectroscopy and UV-visible spectroscopy to investigate the early stages of hydrogen evolution mediated by Co1 and Co2 catalysts. We report the first experimental determination of the structure of the Co(I) intermediate formed from Co2 in solution and highlight a major difference in terms of reactivity and stability between Co2 and Co1 under photo-catalytic conditions.XANES spectra contain element-specific information about the structure of metal complexes.[23-25] Time-resolved X-ray absorption spectroscopy in the laser pump-X-ray probe mode was initially established for experiments in the picosecond -nanosecond time range [26,27]. We recently extended the technique to the microsecond range (pumpsequential-probes[28] and pump-flow-probe[29] methods, see SI). These new...

A D‐π‐A organic dye carrying a pyridine‐N‐oxide 2‐carboxylic acid anchoring group (BC1) was synthesized together with two analogs lacking the N‐oxide (BC2) or the carboxylic acid moiety (BC3). The distribution and energy of their molecular orbitals was determined, and modelling of their spectroscopic properties was performed through a TD‐DFT computational study. The photo‐ and electrochemical properties of the dyes were assessed together with their desorption kinetics from nanocrystalline TiO2. In solution, the absorption spectra of dyes BC1 and BC3 were red‐shifted compared with BC2, with the maximum absorption wavelength influenced by the dye protonation level. The 2‐substituted carbonitrile dye BC3 was not adsorbed on the titania surface. On the other hand, the pseudo‐first order desorption rate constants of BC1 and BC2 suggest that BC1 was removed from TiO2 more slowly than BC2 a reference cyanoacrylate dye, demonstrating that simultaneous use of the N‐oxide and the carboxylic acid anchoring functions enhanced the stability of the dye/semiconductor assembly. When used as a photosensitizer for dye‐sensitized solar cells, the photovoltaic performance of BC1 was better than BC2, which corresponds to approx. 66 % of that recorded with the reference dye.

The CuAAC reaction was used for the development of a click approach to a series of triazole-substituted bipyridinyl derivatives. 4,4¢-Diethinyl 2,2¢-bipyridine and 4,4¢-diazido 2,2¢-bipyridine were synthesized and tested in the cycloaddition reactions. While 4,4¢-diazido 2,2¢-bipyridine revealed unreactive in CuAAC reactions, its corresponding N,N¢-dioxide afforded the expected cycloaddition product.

The cover picture shows the emerging role of metal‐free organic molecules as photosensitizers in the production of hydrogen from water in the presence of sunlight irradiation. Organic dyes offer several advantages compared to other photosensitizers, such as virtually infinite design variety, improved photon‐absorption properties, proper molecular‐orbital energy levels, low‐cost synthesis, and enhanced photocatalytic efficiencies. 3D structures of some representative dyes are shown in the picture. Details are presented in the Microreview by A. Abbotto et al. on http://onlinelibrary.wiley.com/doi/10.1002/ejoc.201600653/abstract.