Photosensitized generation of singlet oxygen from ruthenium(ii) and osmium(ii) bipyridyl complexes

Abstract

Photophysical properties for a number ruthenium(II) and osmium(II) bipyridyl complexes are reported in dilute acetonitrile solution. The lifetimes of the excited metal to ligand charge transfer states (MLCT) of the osmium complexes are shorter than for the ruthenium complexes. Rate constants, k_q, for quenching of the lowest excited metal to ligand charge transfer states by molecular oxygen are found to be in the range (1.1–7.7) × 10⁹ dm³ mol⁻¹ s⁻¹. Efficiencies of singlet oxygen production, f_Δ^T, following oxygen quenching of the lowest excited states of these ruthenium and osmium complexes are in the range of 0.10–0.72, lower values being associated with those compounds having lower oxidation potentials. The rate constants for quenching of the excited MLCT states, k_q, are found to be generally higher for osmium complexes than for ruthenium complexes. Overall quenching rate constants, k_q were found to give an inverse correlation with the energy of the excited state being quenched, and also to correlate with the oxidation potentials of the complexes. However, when the contribution of quenching due exclusively to energy transfer to produce singlet oxygen, k_q¹, is considered, its dependence on the energy of the excited states is more complex. Rate constants for quenching due to energy dissipation of the excited MLCT states without energy transfer, k_q³, were found to show a clear correlation with the oxidation potential of the complexes. Factors affecting both the mechanism of oxygen quenching of the excited states and the efficiency of singlet oxygen generation following this quenching are discussed. These factors include the oxidation potential, the energy of the lowest excited state of the complexes and spin–orbit coupling constant of the central metal.