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 dicyanoanthracene, 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 radiolysis-laser flash photolysis combined method. The radical anions generated by pulse radiolysis do not change upon irradiation with a laser flash at 532 nm. The radical anions in the excited state decay into the ground state within the laser flash (5 ns). Lifetimes of approximately 4 ns are estimated for three radical anions in the excited state assuming a diffusion-controlled rate constant for the electron-transfer quenching. The shorter lifetimes of 1.0-1.4 ns for methyl and chloro substituents on anthraquinone are discussed in terms of internal conversion from the excited to the ground state of the radical anions accelerated by rotation of the substituents. The energy gap between the excited and ground states of the radical anions is a significant factor for the rate of the internal conversion. The quencher radical anion-neutral molecule pair is suggested as an intermediate in the electron-transfer quenching of the radical anions during the excited state by the quencher and is discussed with respect to separation and back electron transfer in the pair.