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Issue 4, 2014
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Mechanistic studies of photoinduced intramolecular and intermolecular electron transfer processes in RuPt-centred photo-hydrogen-evolving molecular devices

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Abstract

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)PtCl2; 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 (ΔGPET = −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.

Graphical abstract: Mechanistic studies of photoinduced intramolecular and intermolecular electron transfer processes in RuPt-centred photo-hydrogen-evolving molecular devices

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Publication details

The article was received on 27 Aug 2013, accepted on 15 Nov 2013 and first published on 15 Nov 2013


Article type: Paper
DOI: 10.1039/C3CP54630F
Citation: Phys. Chem. Chem. Phys., 2014,16, 1607-1616
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    Mechanistic studies of photoinduced intramolecular and intermolecular electron transfer processes in RuPt-centred photo-hydrogen-evolving molecular devices

    C. V. Suneesh, B. Balan, H. Ozawa, Y. Nakamura, T. Katayama, M. Muramatsu, Y. Nagasawa, H. Miyasaka and K. Sakai, Phys. Chem. Chem. Phys., 2014, 16, 1607
    DOI: 10.1039/C3CP54630F

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