Photosensitized Generation of Singlet Oxygen from (Substituted Bipyridine)ruthenium(II) Complexes

Photophysical properties in dilute MeCN solution are reported for seven Ru II complexes containing two 2,2′‐bipyridine (bpy) ligands and different third ligands, six of which contain a variety of 4,4′‐carboxamide‐disubstituted 2,2′‐bipyridines, for one complex containing no 2,2′‐bipyridine, but 2 of these different ligands, for three multinuclear Ru II complexes containing 2 or 4 [Ru(bpy) 2 ] moieties and also coordinated via 4,4′‐carboxamide‐disubstituted 2,2′‐bipyridine ligands, and for the complex [(Ru(bpy) 2 (L)] 2+ where L is N , N ′‐([2,2′‐bipyridine]‐4,4′‐diyl)bis[3‐methoxypropanamide]. Absorption maxima are red‐shifted with respect to [Ru(bpy) 3 ] 2+ , as are phosphorescence maxima which vary from 622 to 656 nm. The lifetimes of the lowest excited triplet metal‐to‐ligand charge transfer states 3 MLCT in de‐aerated MeCN are equal to or longer than for [Ru(bpy) 3 ] 2+ and vary considerably, i.e. , from 0.86 to 1.71 μs. Rate constants k q for quenching by O 2 of the 3 MLCT states were measured and found to be well below diffusion‐controlled, ranging from 1.2 to 2.0⋅10 9 dm 3 mol −1 s −1 . The efficiencies f of singlet‐oxygen formation during oxygen quenching of these 3 MLCT states are relatively high, namely 0.53 – 0.89. The product of k q and f gives the net rate constant k for quenching due to energy transfer to produce singlet oxygen, and k q − k equals k , the net rate constant for quenching due to energy dissipation of the excited 3 MLCT states without energy transfer. The quenching rate constants were both found to correlate with ΔG CT , the free‐energy change for charge transfer from the excited Ru complex to oxygen, and the relative and absolute values of these rate constants are discussed.