Fluorescence Quenching by Sequential Hydrogen, Electron, and Proton Transfer in the Proximity of a Conical Intersection

Radical-pair disproportionation through sequential electron-proton transfer (Polanyi's harpoon) is one efficient mechanism leading to quenching fluorescence of n,π*-excited states by hydrogen donors. This mechanism is demonstrated by ab initio MS-CASPT2 calculations and experimental investigations (photoproduct studies and EPR spin-trapping experiments) on the quenching of singlet-excited azoalkanes by chlorinated hydrocarbons.