Charge transfer induced quenching of triplet sensitizers by ground state oxygen and of singlet oxygen by ground state sensitizers: A common deactivation channel

The photosensitized production of singlet molecular oxygen O-2((1)Delta (g) ) during oxygen quenching of excited triplet states, T-1, and the deactivat ion of O-2((1)Delta (g)) by the sensitizer have been investigated for a set of biphenyl derivatives, in carbon tetrachloride, dichloromethane and acet onitrile. The rate constants of the quenching of T-1 by ground state oxygen , O-2((3)Sigma (-)(g)), leading to O-2((3)Sigma (-)(g)) and of O-2((1)Delta (g)) quenching by ground state sensitizers, S-0, have been subjected to a common analysis. It is shown that the rate constants for both processes are described by one Marcus-type plot in each solvent. This indicates that bot h processes proceed via exciplexes (T(1)(3)Sigma) and (S(0)(1)Delta) of the same partial charge transfer (pCT) from sensitizer to O-2. These findings are in accordance with results obtained previously with a series of naphtha lene derivatives. Further comparative analysis leads to a relation for the estimation of the average charge transfer character of the (T(1)(3)Sigma) a nd (S(0)(1)Delta) pCT complexes, which is consistent with the existing conc ept of the reorganization energy. The average charge transfer character var ies only slightly between the series of biphenyl and naphthalene derivative s, and is shown to increase in each case with solvent polarity. The overall reorganization energies could be separated into constant intramolecular an d solvent-dependent outer reorganization contributions. The small but signi ficant differences in the reorganization energy indicate that the structure of pCT complexes varies between these two series.