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 1Σg+ and 1Δg and the triplet ground state 3Σg- of molecular oxygen are formed with varying rate constants kT1Σ, kT1Δ, and kT3Σ, respectively during the quenching by O2 of triplet states T1 of sufficient energy ET. The present paper reports these rate constants for a series of 10 biphenyl sensitizers of very different oxidation potential, Eox, but almost constant and rather large ET. Strong and graduated charge transfer (CT) effects on kT1Σ, kT1Δ, and kT3Σ are observed. These data are analyzed considering data of kT1Σ, kT1Δ, and kT3Σ determined previously for sensitizers of very different ET and Eox. The results clearly demonstrate that the quenching of triplet states by O2 proceeds via two different channels, each capable of producing O2(1Σg+), O2(1Δg), and O2(3Σg-). One quenching channel originates from excited 1,3(T1·3Σ) complexes with no CT character (nCT); the other originates from 1,3(T1·3Σ) exciplexes with partial charge transfer character (pCT). A common energy gap law determines the rate constants of O2(1Σg+), O2(1Δg), and O2(3Σg-) 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 O2(1Σg+), O2(1Δg), and O2(3Σg-) 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(T1·3Σ) and 3(T1·3Σ) is only observed in the nCT but not in the pCT channel.