On the Effect of Electron Relay Redox Potential on Electron Transfer Reactions in a Water Photoreduction Model System

The influence of electron relay redox potential ( E 1/2 ) on the electron‐transfer reactions occurring in the photo‐induced hydrogen generation from water has been investigated using the Ru(bipy) 2+ 3 /MV 2+ /EDTA/colloidal Pt model system. Quenching rate constants k q for electron transfer quenching of Ru(bipy) 2+ * 3 by a series of 2,2′‐ and 4,4′‐bipyridinium ions and by a series of phenanthroline ions having different redox potential E 1/2 have been determined by laser flash spectroscopy in deaerated aqueous solutions at pH 5. The rate constants k b of the back electron transfer following the quenching reaction have been obtained for five bipyridinium ions under the same experimental conditions. Our results show that: (1) for the same E 1/2 , the quencher molecular structure of the two homogeneous series does not affect k q significantly; (2) for the two series, k q increases with increasing E 1/2 up to a value which is close to the diffusion‐controlled limit. The same correlation between k q and E 1/2 is obtained for the two series. Such a correlation is satisfactorily interpreted in the frame of existing theories describing electron‐transfer reactions, particularly with the Rehm‐Weller treatment, provided that an intrinsic barrier Δ G *(0) = 5.2 kcal. M −1 is used; (3) k b is close to the diffusion‐controlled limit for E 1/2 comprised between ‐ 0.33 and ‐ 0.65 V (vs NHE); (4) a correlation between H 2 production rates and E 1/2 is suggested, and a maximum H 2 production is observed for E 1/2 ≃ −0.45 V (vs. NHE) corresponding to compounds 1 and 14 .