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)]Cl-2 as catalyst, and triethylamine as sacrificial electron donor, are highly active for visible-light-driven (lambda > 450 nm) hydrogen production from water. The turnover frequency is up to 54 TON/min versus RB2- with a RB2-/[Co(bpy)(3)]Cl-2 molar ratio of 1:10 in CH3CN/H2O under optimal conditions in the first half hour of irradiation (lambda > 450 rim), and the turnover number is up to 2076 versus RB2-. Comparative studies show the following: (1) The photocatalytic H-2-evolving activity of the cationic cobalt complex [Co(bpy)(3)]Cl-2, 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)]Cl-2/[Co(bpy)(3)]Cl-2. (2) The UV-vis absorptions of xanthene dyes are red shifted to different extents upon addition of [Co(bpy)(3)]Cl-2 to the aqueous or CH3CN/H2O solutions of these dyes, while no change was observed in UV-vis absorptions of photosensitizer with addition of the cobaloximes to the aqueous solution of RB2- or addition of [Co(bpy)(3)]Cl-2 to the aqueous solution of [Ru(bpy)(3)]Cl-2. (3) The fluorescence of RB2- is significantly quenched by [Co(bpy)(3)]Cl-2, but not by the cobaloximes. These special performances of [Co(bpy)(3)]Cl-2 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)]Cl-2, and triethylamine is discussed in detail on the basis of fluorescence. fand transient absorption spectroscopic studies.