Influence of charge transfer interactions on the sensitization of singlet oxygen:: Formation of O2(1Σg+), O2(1Δg), and O2(3Σg-) during oxygen quenching of triplet excited biphenyl derivatives

Both excited singlet states (1)Sigma (+)(g) and (1)Deltag and the triplet ground state (3)Sigma (-)(g) of molecular oxygen are formed with varying rate constants k(T)(1 Sigma), k(T)(1 Delta), and k(T)(3 Sigma), respectively during the quenching by O-2 of triplet states T-1 of sufficient energy E-T. The present paper reports these rate constants for a series of 10 biphenyl sensitizers of very different oxidation potential, E.,, but almost constant and rather large ET. Strong and graduated charge transfer (CT) effects on k(T)(1 Sigma), k(T)(1 Delta), and k(T)(3 Sigma) are observed. These data are analyzed considering data of k(T)(1 Sigma), k(T)(1 Delta), and k(T)(3 Sigma) determined previously for sensitizers of very different E-T and E-ox. The results clearly demonstrate that the quenching of triplet states by O-2 proceeds via two different channels, each capable of producing O-2((1)Sigma (+)(g)), O-2((1)Delta (g)), and O-2((3)Sigma (-)(g)). One quenching channel originates from excited (1,3)(T(1)(.3)Sigma (-)(g)) complexes with no CT character (nCT); the other originates from 1,3 (T(1)(.3)Sigma) exciplexes with partial charge transfer character (pCT). A common energy gap law determines the rate constants of O-2((1)Sigma (-)(g)), O-2((1)Delta (+)(g)), O-2((3)Sigma (-)(g)) formation in the nCT channel. However, the respective rate constants vary on a logarithmic scale linearly with the free energy of complete electron transfer in the pCT channel. The statistical weights of the pCT processes leading to O-2((1)Sigma (+)(g)), O-2( (1)Delta (g)), and O-2((3)Sigma (-)(g)) formation are 0.67, 0,33, and 3, leading to efficiencies of overall singlet and ground state oxygen formation of 0.25 and 0.75 of pCT complexes in accordance with the spin-statistical weight ratio 1:3. A fast intersystem crossing equilibrium between (1)(T(1)(.3)Sigma) and (3)(T(1)(.3)Sigma) is only observed in the nCT but not in the pCT channel.