Frontier orbital engineering of photo-hydrogen-evolving molecular devices: a clear relationship between the H-2-evolving activity and the energy level of the LUMO

Two new Ru(II)Pt(II) dimers, [Ru(bpy)(2)(mu-L2)PtCl2](2+) (5) and [Ru(bpy)(2)(mu-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)(mu-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 (E-1/2 = -1.23 V) is nearly consistent with that of 4 (E-1/2 = -1.20 V) but is more positive than that of 6 (E-1/2 = -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 pi* 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 H-2-evolving catalyst, while 6 shows no activity at all. However, 6 was found to drive photochemical H-2 evolution in the presence of both EDTA and methyl viologen (N,N'-dimethyl-4,4'-bipyridinium, MV2+), indicating that the (MLCT)-M-3 excited state of the Ru(bpy)(2)(phen)(2+) moiety is once oxidatively quenched by MV2+ to give MV+center dot and then hydrogen evolution from water by MV+center dot 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 (Delta G degrees(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.