Photosensitized generation of singlet oxygen from ruthenium(II)-substituted benzoaza-crown-bipyridine complexes

Photophysical properties in dilute acetonitrile solution are reported for a number of vinyl-linked benzoaza-15-crown-5-bipyridine ruthenium(ii) comple xes and for three multinuclear Ru(ii) bipyridine complexes. Absorption and emission spectra are found to depend on the number of conjugated benzoaza-1 5-crown-5-bipyridine ligands present in the complex. The bi-, tri- and tetr anuclear Ru(ii) complexes show absorption and emission maxima very close to those of the parent mono-complex, Ru(ii) tris-bipyridine. For those comple xes with similar phosphorescence maxima, in the range 607-615 nm, the lifet imes of the lowest excited triplet metal to ligand charge-transfer ((MLCT)- M-3) states in de-aerated acetonitrile are also very similar, i.e., in the range 0.71 to 0.88 mu s. However, for two of the studied compounds, where t he phosphorescence maxima shift to 692 and 699 nm, the phosphorescence life times increase to 2.2 and 3.0 mu s, respectively. Rate constants, k(q), for quenching by molecular oxygen of the lowest excited (MLCT)-M-3 states are in the range (2.4-4.6)x10(9) d mol(-1) s(-1). Efficiencies of singlet oxyge n production, f(Delta)(T), sensitized by these ruthenium complexes are in t he range of 0.26-0.69, lower values being associated with those compounds s howing low potentials for oxidation of conjugated ligands. The product of k (q) and f(Delta)(T) gives the net rate constant for quenching due to energy transfer to produce singlet oxygen k(q)(1), and k(q)-k(q)(1) equals k(q)(3 ), the net rate constant for quenching due to energy dissipation of the exc ited (MLCT)-M-3 states without energy transfer. Quenching rate constants, k (q) and k(q)(3), were found to give an inverse correlation with the energy of the excited state being quenched. However, the dependence of k(q)(1) val ues on the energy of the excited states being quenched by oxygen was more c omplicated, with complexes falling into two groups showing either high or l ow efficiencies for quenching with energy transfer.