Ultrafast dynamics of the low-lying 3MLCT states of [Ru(bpy)2(dppp2)]2+.

The solvent dependence of the excited state dynamics of [Ru(bpy)(2)(dppp2)](2+) (bpy = 2,2'-bipyridine, dppp2 = pyrido[2',3':5,6]pyrazino[2,3-f][1,10]phenanthroline) were investigated using ultrafast transient absorption spectroscopy. In contrast to the "DNA light-switch" complex [Ru(bpy)(2)(dppz)](2+) (dppz = dipyrido[3,2-a:2',3'-c]phenzine), the structurally related [Ru(bpy)(2)(dppp2)](2+) exhibits discrete formation of the lowest-lying (3)MLCT(dis) (MLCT = metal-to-ligand charge transfer) state from the higher-energy (3)MLCT(prox) state in the picosecond time scale. In (3)MLCT(dis) and (3)MLCT(prox), the transferred electron is localized on the portion of the dppp2 ligand distal and proximal to the metal, respectively. The greater dipole moment of (3)MLCT(dis) compared to (3)MCLT(prox), together with the ability of the dppp2 ligand to hydrogen bond, results in a strong dependence of the kinetics of the interconversion process on solvent polarity, decreasing from 67 to 26 ps in CH(2)Cl(2) and CH(3)CN, respectively. Similarly, the lifetime of the emission of the lowest energy state of [Ru(bpy)(2)(dppp2)](2+) also decreases from 273 to 35 ns in the same solvents. In CH(3)CH(2)OH, both the rate of interconversion and the decay of (3)MLCT(dis) are significantly faster, 6.7 ps and 1.7 ns, respectively. Such dependence of the excited state properties is not observed for [Ru(bpy)(2)(dppz)](2+). The results are consistent with the relative stabilization of (3)MLCT(dis) of the dppp2 complex in CH(3)CN compared to CH(2)Cl(2) due to solvent polarity, which may be further stabilized by hydrogen bonding in CH(3)CH(2)OH.