Quenching of excited doublet states of organic radicals by stable radicals

Abstract

Rate constants for the quenching of excited diphenylmethyl radicals by stable free radicals were measured. The stable radicals were galvinoxyl and three nitroxide radicals, di-tert-butyl nitroxide (DTBN), 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO), and 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy (HTEMPO). Half-wave potentials for the four stable radicals were measured. The overall quenching rates were in the range of kq = (2-8) × 109 M-1 s-1. The possible mechanisms for this quenching are discussed. The electron-transfer mechanism is implicated for the quenching of the excited diphenylmethyl radical by both the nitroxide radicals and galvinoxyl. These electron-transfer reactions may be limited by steric effects and/or nonadiabatic electron transfer or by simultaneous, spin/energy restrictions on ion-pair formation. Interactions that are mediated by electron-exchange interactions could not be entirely excluded, and some dipole-dipole energy transfer could be involved in the quenching by the stable radicals, especially galvinoxyl. However, radical-radical addition does not appear to be contributing to the quenching by nitroxide radicals. The rate constants for the reaction of the ground state of the diphenylmethyl radical with HTEMPO were measured as (2.0-2.4) × 107 M-1 s-1 in various solvents. © 1989 American Chemical Society.