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(μ-L2)PtCl ₂]²⁺ (5) and [Ru(bpy)₂(μ-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(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 (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 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.