LIFETIMES OF RADICAL-ANIONS OF DICYANOANTHRACENE, PHENAZINE, AND ANTHRAQUINONE IN THE EXCITED-STATE FROM THE SELECTIVE ELECTRON-TRANSFER QUENCHING

The selective eletron-transfer quenching of the radical anions of dicy anoanthracene, phenazine, and anthraquinones in the excited state by a quencher such as fumaronitrile or dicyanobenzene is investigated in N ,N-dimethylformamide solution at room temperature using the pulse radi olysis-laser flash photolysis combined method. The radical anions gene rated by pulse radiolysis do not change upon irradiation with a laser flash at 532 nm. The radical anions in the excited state decay into th e ground state within the laser flash (5 ns). Lifetimes of approximate ly 4 ns are estimated for three radical anions in the excited state as suming a diffusion-controlled rate constant for the electron-transfer quenching. The shorter lifetimes of 1.0-1.4 ns for methyl and chloro s ubstituents on anthraquinone are discussed in terms of internal conver sion from the excited to the ground state of the radical anions accele rated by rotation of the substituents. The energy gap between the exci ted and ground states of the radical anions is a significant factor fo r the rate of the internal conversion. The quencher radical anion-neut ral molecule pair is suggested as an intermediate in the electron-tran sfer quenching of the radical anions during the excited state by the q uencher and is discussed with respect to separation and back electron transfer in the pair.