Promoting Effect of Electrostatic Interaction between a Cobalt Catalyst and a Xanthene Dye on Visible-Light-Driven Electron Transfer and Hydrogen Production

The readily obtained noble-metal-free molecular catalyst systems, with xanthene dyes (Rose Bengal, RB2–; Eosin Y, EY2–; and Eosin B, EB2–) as photosensitizers, [Co(bpy)3]Cl2 as catalyst, and triethylamine as sacrificial electron donor, are highly active for visible-light-driven (λ > 450 nm) hydrogen production from water. The turnover frequency is up to 54 TON/min versus RB2– with a RB2–/[Co(bpy)3]Cl2 molar ratio of 1:10 in CH3CN/H2O under optimal conditions in the first half hour of irradiation (λ > 450 nm), and the turnover number is up to 2076 versus RB2–. Comparative studies show the following: (1) The photocatalytic H2-evolving activity of the cationic cobalt complex [Co(bpy)3]Cl2 is apparently higher than the neutral cobaloxime complexes with xanthene dyes as potosensitizers, and also much higher than the analogous system of [Ru(bpy)3]Cl2/[Co(bpy)3]Cl2. (2) The UV–vis absorptions of xanthene dyes are red-shifted to different extents upon addition of [Co(bpy)3]Cl2 to the aqueous or CH3CN/H2O solutions of these dyes, while no change was observed in the UV–vis absorptions of photosensitizer with addition of the cobaloximes to the aqueous solution of RB2– or addition of [Co(bpy)3]Cl2 to the aqueous solution of [Ru(bpy)3]Cl2. (3) The fluorescence of RB2– is significantly quenched by [Co(bpy)3]Cl2, but not by the cobaloximes. These special performances of [Co(bpy)3]Cl2 are attributed to the electrostatically attractive interaction between the anionic organic dyes and the cationic cobalt catalyst. The probable mechanism for photoinduced hydrogen production catalyzed by the system of RB2–, [Co(bpy)3]Cl2, and triethylamine is discussed in detail on the basis of fluorescence and transient absorption spectroscopic studies.