Participation of fractional charge transfer on the efficiency of singlet oxygen production: Heteroleptic ruthenium (II) bipyridine derivatives

[Display omitted] •Luminescence quenching of the excited 3MLCT by molecular oxygen.•Photosensitized generation of singlet oxygen by heteroleptic ruthenium (II) complexes.•Energy transfer versus charge transfer in the quenching process.•Effect of the partial charge transfer on the efficiency of singlet oxygen production. The photophysical properties of some synthesized heteroleptic polypyridyl ruthenium(II) complexes of the form [Ru(bpy)2L](PF6)2 where L = 2,2′-bipyridine, 4,4′-dimethoxy-2,2′-bipyridine, 1,10-phenanthroline, 4,4′-dimethyl-2,2′-bipyridine, 4,7-diphenyl-1,10-phenanthroline and 3,4,7,8-tetramethyl-1,10-phenanthroline, are investigated in acetonitrile. The excited state lifetime of the metal to ligand charge transfer (3MLCT) states of these complex ions are found to be ligand dependent and were found to be in the range 0.56 μs to 1.5 μs. Oxygen quenching of the excited states rate constants, kq, were found to be in the range 1.50–2.24 × 109 M−1s−1. Singlet oxygen quantum yield values, ΦΔ, are found to vary from 0.40 to 0.60 and the efficiency, fΔT, with which excited singlet oxygen, O2 (1Δg), is thereby produced was found to be in the range 0.55–0.85. Treatment of the data with charge transfer and non-charge transfer pathways using Schmidt’s model shows that partial charge transfer parameter was in the range of 0.18 to 0.58. It has also been found that fΔT decreases and oxygen quenching rate constants, kq, increases as pCT increases.