Mechanism of Photosensitized Generation of Singlet Oxygen during Oxygen Quenching of Triplet States and the General Dependence of the Rate Constants and Efficiencies of O2(1Σg+), O2(1Δg), and O2(3Σg-) Formation on Sensitizer Triplet State Energy and Oxidation Potential

Abstract

Rate constants of photosensitized generation of O2(1Σg+), O2(1Δg), and O2(3Σg-) have been determined for a series of ππ* triplet sensitizers with strongly varying oxidation potential (Eox), triplet energy (ET), and molecular structure, in CCl4. We demonstrate that one common dependence on Eox and ET successfully describes these rate constants for the molecules studied here and also for all previously investigated ππ* sensitizers, independently of molecular structure or any other parameter. Photosensitized singlet oxygen generation during O2 quenching of ππ* triplet states can be generally described by a mechanism involving the successive formation of excited noncharge transfer (nCT) encounter complexes and partial charge transfer (pCT) exciplexes of singlet and triplet multiplicity 1,3(T13Σ), following interaction of O2(3Σg-) with the triplet excited sensitizer. Both 1,3(T13Σ) nCT and pCT complexes decay by internal conversion (ic) to yield O2(1Σg+), O2(1Δg), and O2(3Σg-) and the sensitizer ground state. ic is the rate-limiting step in the nCT channel, whereas exciplex formation is rate determining in the pCT channel. Rotation of the O2 molecule within the solvent cage of 1,3(T13Σ) nCT complexes is fast enough to allow for a completely established intersystem crossing (isc) equilibrium, whereas significant noncovalent binding interactions slow rotation and inhibit isc between 1(T13Σ) and 3(T13Σ) pCT complexes. Upon the basis of this mechanism, we propose a semiempirical relationship that can be generally used to estimate rate constants and efficiencies of photosensitized singlet oxygen generation during O2 quenching of ππ* triplet states in CCl4. The data set includes 127 rate constants for derivatives of naphthalene, biphenyl, fluorene, several ketones, fullerenes, porphyrins and metalloporphyrins, and other homocyclic and heterocyclic aromatics of variable molecular structure and size. It is suggested that the general relationship presented here can be used for the optimization of the singlet oxygen photosensitization ability of many molecules, including those used in biological and medical applications, such as the photodynamic therapy of cancer.