Jump to main content
Jump to site search

Issue 25, 2010
Previous Article Next Article

Frontier orbital engineering of photo-hydrogen-evolving molecular devices: a clear relationship between the H2-evolving activity and the energy level of the LUMO

Author affiliations

Abstract

Two new Ru(II)Pt(II) dimers, [Ru(bpy)2(μ-L2)PtCl2]2+ (5) and [Ru(bpy)2(μ-L3)PtCl2]2+ (6), were synthesized and characterized, and their electrochemical and spectroscopic properties together with their photo-hydrogen-evolving activities were evaluated (bpy = 2,2′-bypridine; L2 = 4′-[1,10]phenanthrolin-5-ylcarbamoyl)-[2,2′]bipyridinyl-4-carboxylic acid ethyl ester; L3 = 4′-methyl-[2,2′]bipyridinyl-4-carboxylic acid [1,10]phenanthrolin-5-ylamide). The structures of 5 and 6 are basically identical with that of the first active model of a photo-hydrogen-evolving molecular device developed in our group, [Ru(bpy)2(μ-L1)PtCl2]2+ (4) (L1 = 4′-([1,10]phenanthrolin-5-ylcarbamoyl)-[2,2′]bipyridinyl-4-carboxylic acid), except for the difference in the substituent group at the 4-position of the bpy moiety bound to Pt(II) (–COOH for 4; –COOEt for 5; –CH3 for 6). Electrochemical studies revealed that the first reduction potential of 5 (E1/2 = −1.23 V) is nearly consistent with that of 4 (E1/2 = −1.20 V) but is more positive than that of 6 (E1/2 = −1.39 V), where the first reduction is associated with the reduction of the bpy moiety bound to Pt(II), consistent with a general tendency that the first reduction of bpy shows an anodic shift upon introduction of electron-withdrawing group. Density functional theory (DFT) calculations for 5 and 6 also show that the lowest unoccupied molecular orbital (LUMO) corresponds to the π* orbital of the bpy moiety bound to Pt(II) for all the Ru(II)Pt(II) dimers, and the energy level of the LUMO of 6 is destabilized compared with those of 4 and 5, consistent with the results of the electrochemical studies. The photochemical hydrogen evolution from water driven by 4–6 in the presence a sacrificial electron donor (EDTA) was investigated. 5 was found to be active as an H2-evolving catalyst, while 6 shows no activity at all. However, 6 was found to drive photochemical H2 evolution in the presence of both EDTA and methyl viologen (N,N′-dimethyl-4,4′-bipyridinium, MV2+), indicating that the 3MLCT excited state of the Ru(bpy)2(phen)2+ moiety is once oxidatively quenched by MV2+ to give MV+˙ and then hydrogen evolution from water by MV+˙ proceeds as a dark reaction. Emission decays and transient absorption spectra also show that the intramolecular electron transfer (IET) is accelerated in the active Ru(II)Pt(II) dimers 4 and 5, while such acceleration is not realized for the inactive Ru(II)Pt(II) dimer 6. The driving forces (ΔG°ET) for the IET processes are estimated to be −0.16 eV for 4, −0.09 eV for 5 and 0.03 eV for 6, indicating that the IET process in 6 is uphill. It is concluded that efficient IET is required to drive the photochemical H2 evolution from water with these Ru(II)Pt(II)-based molecular devices.

Graphical abstract: Frontier orbital engineering of photo-hydrogen-evolving molecular devices: a clear relationship between the H2-evolving activity and the energy level of the LUMO

Back to tab navigation

Supplementary files

Publication details

The article was received on 05 Mar 2010, accepted on 26 Apr 2010 and first published on 26 May 2010


Article type: Paper
DOI: 10.1039/C0DT00077A
Citation: Dalton Trans., 2010,39, 5868-5876
  •   Request permissions

    Frontier orbital engineering of photo-hydrogen-evolving molecular devices: a clear relationship between the H2-evolving activity and the energy level of the LUMO

    S. Masaoka, Y. Mukawa and K. Sakai, Dalton Trans., 2010, 39, 5868
    DOI: 10.1039/C0DT00077A

Search articles by author

Spotlight

Advertisements