Electron Transfer Quenching of Dye Triplets by No~ and N~. a Spin-orbit Coupling Effect on the Radical Yield

Electron transfer between electronically excited states and electron donors or acceptors in the ground state has been established as an important mechanism for the quenching of excited states. The paper of Rehm and Weller [1], demonstrating for several aromatic excited singlets and numerous aromatic quenchers in acetonitrlle a clear-cut correlation between the quenching rate constant kq and the free enthalpy change AGO of the electron transfer between excited molecule and quencher, represented a landmark in the understanding of this mechanism. In the meantime Rehm-Weller-type correlations have been shown to be valid also for reactions of excited triplet states [2-4] and excited transition-metal complexes [5,6]. Such correlations are now widely applied in the interpretation of excited-state quenching data to assess the role of electron transfer in the quenching process. With organic molecules as quenchers in polar solvents the electron-transfer mechanism is generally corroborated by the detection of radical ions whenever the Rehm-Weller correlation (log kq/AGOt) is observed. The situation is less clear when using in-