Sensitization of singlet oxygen via encounter complexes and via exciplexes of ππ* Triplet excited sensitizers and oxygen

Abstract

Both excited singlet states, 1Σg+ and 1Δg, and the triplet ground state, 3Σg-, of molecular oxygen are competitively formed during the quenching by O2 of triplet (T1) excited sensitizers of sufficient energy. The corresponding overall rate constants kT1Σ, kT1Δ and kT3Σ as well as the T1 state energies ET and the oxidation potentials Eox have been determined for a series of six fluorene derivatives. Graduated and in part strong charge transfer (CT) effects on kT1Σ, kT1Δ, and kT3Σ are observed. These and literature data strongly indicate that quenching occurs in two different channels each capable of producing O2(1Σg+), O2(1Δg), and O2(3Σg-). One proceeds via internal conversion (IC) of excited1,3(T1·3Σ) complexes with no CT character (nCT), which cannot be distinguished from encounter complexes, the other via IC of1,3(T1·3Σ) exciplexes with partial CT character (pCT). The contributions of nCT and pCT deactivation channels to the overall formation of O2(1Σg+), O2(1Δg), and O2(3Σg-) depend on ET and Eox. The rate constants of the nCT channel are controlled by the excess energies of the respective IC processes by an energy gap law. The rate constants of the pCT channel depend on the change of free energy ΔGCET for complete electron transfer from T1 excited sensitizer to O2. Equations are presented which show the functional form of the dependence of the oxygen quenching rate constants on ET and Eox. Particular emphasis is laid on the question of whether these relations could generally be valid for ππ* triplet sensitizers. © 2002 The Royal Society of Chemistry and Owner Societies.