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

Abstract

Photophysical properties in dilute acetonitrile solution are reported for a number of vinyl-linked benzoaza-15-crown-5-bipyridine ruthenium(II) complexes and for three multinuclear Ru(II) bipyridine complexes. Absorption and emission spectra are found to depend on the number of conjugated benzoaza-15-crown-5-bipyridine ligands present in the complex. The bi-, tri- and tetranuclear Ru(II) complexes show absorption and emission maxima very close to those of the parent mono-complex, Ru(II) tris-bipyridine. For those complexes with similar phosphorescence maxima, in the range 607–615 nm, the lifetimes of the lowest excited triplet metal to ligand charge-transfer (³MLCT) states in de-aerated acetonitrile are also very similar, i.e., in the range 0.71 to 0.88 μs. However, for two of the studied compounds, where the phosphorescence maxima shift to 692 and 699 nm, the phosphorescence lifetimes increase to 2.2 and 3.0 μs, respectively. Rate constants, k_q, for quenching by molecular oxygen of the lowest excited ³MLCT states are in the range (2.4–4.6)×10⁹ dm³ mol⁻¹ s⁻¹. Efficiencies of singlet oxygen production, f_Δ^T, sensitized by these ruthenium complexes are in the range of 0.26–0.69, lower values being associated with those compounds showing low potentials for oxidation of conjugated ligands. The product of k_q and f_Δ^T gives the net rate constant for quenching due to energy transfer to produce singlet oxygen k_q¹, and k_q−k_q¹ equals k_q³, the net rate constant for quenching due to energy dissipation of the excited ³MLCT states without energy transfer. Quenching rate constants, k_q and k_q³, were found to give an inverse correlation with the energy of the excited state being quenched. However, the dependence of k_q¹ values on the energy of the excited states being quenched by oxygen was more complicated, with complexes falling into two groups showing either high or low efficiencies for quenching with energy transfer.