Photosensitized generation of singlet oxygen from vinyl linked benzo-crown-ether-bipyridyl ruthenium(II) complexes

Photophysical properties of 10 substituted methoxybenzene or vinyl linked b enzo-crown-ether-2,2'-bipyridyl ruthenium(II) complexes are reported. The l ifetimes of the excited triplet metal to ligand charge transfer states, (ML CT)-M-3, of the complex ions are in the range 0.85 +/- 0.12 Cls with two ex ceptions where the lifetime drops to about half this value. The rate consta nts, k(q), for quenching in acetonitrile of the (MLCT)-M-3 states of these ruthenium complex ions by molecular oxygen, O-2(3 Sigma(g)(-)), and the var iations in the efficiency, f(Delta)(T), with which excited singlet oxygen, O-2*((1)Delta(g)), is thereby produced are reported. The quenching rate con stants are in the range 2.2-4.2 x 10(9) dm(3) mol(-1) s(-1), and efficienci es of singlet oxygen production are in the range 0.21-0.74. Those complexes with the highest values of k(q), tend to be those with lowest f(Delta)(T), values; that is, k(q) and f(Delta)(T) show a reasonable inverse correlatio n. The product k(q)f(Delta)(T) gives the rate constant for oxygen quenching with energy transfer to oxygen, k(q)(1) and k(q) - k(q)(1) gives the rate constant, k(q)(3) for oxygen quenching by any path which does not lead to e nergy transfer. The values of k(q)(1), k(q)(3) and k(q) are compared with t hose in the literature which are mainly available for organic sensitizers o f singlet oxygen. The similarities and differences between these two classe s of compounds are discussed taking into account the fact that ruthenium co mplex ions are Likely to show enhanced intersystem crossing, due to the hea vy atom effect and the likely dependence of k(q)(1) and k(q)(3), and thereb y the efficiencies of singlet oxygen production, on the energies of the exc ited 3MLCT states and on steric factors.