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

Two new Ru(II)Pt(II) dimers, [Ru(bpy)(2)(mu-L2)PtCl(2)](2+) (5) and [Ru(bpy)(2)(mu-L3)PtCl(2)](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)PtCl(2)](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; -CH(3) for 6). Electrochemical studies revealed that the first reduction potential of 5 (E(1/2) = -1.