Mechanism of quenching by oxygen of the excited states of ruthenium(II) complexes in aqueous media.: Solvent isotope effect and photosensitized generation of singlet oxygen, O2(1Δg), by [Ru(diimine)(CN)4]2- complex ions

In this study we report on the photophysical properties of some [RuL(CN)(4)](2)- complex ions where L = 2,2 '-bipyridine (bpy), 5,5 '-dimethyl-2,2 '-bipyridine (dmb), 1,10-phenanthroline (phen), 1-ethyl-2-(2-pyridyl) benzimidazole (pbe), 2,2 ':6 ',2 '''-terpyridine (tpy) and [RuL3](2+) where L = bpy or phen. Measurements were carried out in H2O and D2O. The effect of the deuterium isotope effect on the lifetime of these complexes is discussed. It has also been found that the presence of cyano groups has a pronounced effect on the lifetime of the excited metal-to-ligand charge transfer ((MLCT)-M-3) of these complexes. Quenching of the (MLCT)-M-3 states by oxygen is reported in H2O and D2O. The rate constants, k(q), for quenching of the (MLCT)-M-3 states of these ruthenium complex ions by molecular oxygen are in the range (2.55 to 7.01) x 10(9) M-1 s(-1) in H2O and (3.38 to 5.69) x 10(9) M-1 s(-1) in D2O. The efficiency of singlet oxygen, O-2((1)Delta(g)), production as a result of the (MLCT)-M-3 quenching by oxygen, f(Delta)(T), is reported in D2O and found to be in the range 0.29-0.52. The rate constants, k(q)(Delta), for quenching of singlet oxygen by ground state sensitizers in D2O is also reported and found to be in the range (0.15 to 3.46) x 10(7) M-1 s(-1). The rate constants and the efficiency of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a non-charge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of (1)(T-1(3)Sigma) and (3)(T-1(3)Sigma) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O-2((1)Delta(g)), and O-2((3)Sigma(-)(g)). The balance between CT and nCT deactivation channels which is described by the relative contribution p(CT) of CT induced deactivation is discussed. The kinetic model proposed for the quenching of pi-pi* triplet states by oxygen can also be applied to the quenching of 3MLCT states by oxygen.