Tuning Thiophene-Based Phenothiazines for Stable Photocatalytic Hydrogen Production.

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.

Microsecond X-ray Absorption Spectroscopy Identification of Co(I) Intermediates in Cobaloxime-Catalyzed Hydrogen Evolution.

Rational development of efficient photocatalytic 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 the 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/Vis 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.