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Abstract. The influence of electron relay redox potential (EI/') on the electrontransfer reactions occurring in the photo-induced hydrogen generation from water has been investigated using the Ru(bipy)~+ IMV'+ IEDTA/colloidal Pt model system. Quenching rate constants k q for electron transfer quenching of Ru(bipy)~+" by a series of 2,2'-and 4,4'-bipyridinium ions and by a series of phenanthroline ions having different redox potential EI/2 have been determined by laser flash spectroscopy in deaerated aqueous solutions at pH 5. The rate constants k h 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: (I) for the same EI/" 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/' up to a value which is close to the diffusion-controlled limit. The same correlation between k q and E'I2 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 .:la"(O) = 5.2 kcal. M-I is used; (3) k h is close to the diffusion-controlled limit for E 112 comprised between -0.33 and -0.65 V (vs NHE); (4) a correlation between H, production rates and EI/, is suggested, and a maximum H, production is observed for EI/' = -0.45 V (vs. NHE) corresponding to compounds 1 and 14.

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