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

Abstract

Both excited singlet states, ¹Σ_g⁺ and ¹Δ_g, and the triplet ground state, ³Σ_g⁻, of molecular oxygen are competitively formed during the quenching by O₂ of triplet (T₁) excited sensitizers of sufficient energy. The corresponding overall rate constants k_T^1Σ, k_T^1Δ and k_T^3Σ as well as the T₁ state energies E_T and the oxidation potentials E_ox have been determined for a series of six fluorene derivatives. Graduated and in part strong charge transfer (CT) effects on k_T^1Σ, k_T^1Δ, and k_T^3Σ are observed. These and literature data strongly indicate that quenching occurs in two different channels each capable of producing O₂(¹Σ_g⁺), O₂(¹Δ_g), and O₂(³Σ_g⁻). One proceeds via internal conversion (IC) of excited ^1,3(T₁·³Σ) complexes with no CT character (nCT), which cannot be distinguished from encounter complexes, the other via IC of ^1,3(T₁·³Σ) exciplexes with partial CT character (pCT). The contributions of nCT and pCT deactivation channels to the overall formation of O₂(¹Σ_g⁺), O₂(¹Δ_g), and O₂(³Σ_g⁻) depend on E_T and E_ox. 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 ΔG_CET 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 E_T and E_ox. Particular emphasis is laid on the question of whether these relations could generally be valid for ππ* triplet sensitizers.