The photoinduced electron transfer properties of two photo-hydrogen- evolving molecular devices (PHEMDs) [(bpy)2Ru(ii)(phen-NHCO-bpy-R) Pt(ii)Cl2]2+ (i.e., condensation products of [Ru(bpy) 2(5-amino-phen)]2+ and (4-carboxy-4′-R-bpy)PtCl 2; bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline; RuPt-COOH for R = COOH and RuPt-CN for R = CN) were investigated. RuPt-CN demonstrates higher photocatalytic performance relative to RuPt-COOH arising from a larger driving force for the intramolecular photoinduced electron transfer (PET) associated with a stronger electron-withdrawing effect of R (ΔG PET = -0.43 eV for RuPt-CN and -0.16 eV for RuPt-COOH). This is the first study on PET events using ultrafast spectroscopy. Dramatic enhancement is achieved in the rate of PET in RuPt-CN (1.78 × 1010 s -1) relative to RuPt-COOH (3.1 × 109 s -1). For each system, the presence of three different conformers giving rise to three different PET rates is evidenced, which are also discussed with the DFT results. Formation of a charge-separated (CS) state [(bpy) 2Ru(iii)(phen-NHCO-bpy--R)Pt(ii)Cl2] 2+ in the sub-picosecond time regime and recombination in the picosecond time regime are characterized spectrophotometrically. The CS-state formation was found to compete with reductive quenching of the triplet excited state by EDTA whose dianionic form ion-pairs with dicationic RuPt-COOH. Thus, a key intermediate [(bpy)2Ru(ii)(phen-NHCO-bpy--R)Pt(ii) Cl2]+ (i.e., the one-electron-reduced species) prior to the H2 formation was found to be formed either via reduction of the CS state by EDTA or via formation of [(bpy)2Ru(ii)(phen --NHCO-bpy-R)Pt(ii)Cl2]+ by reductive quenching of the triplet excited state. More importantly, it is also shown that some of the conformers in solution possess a CS lifetime sufficiently long to drive hydrogen evolution from water.
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