Laser-induced fluorescence of 1,3,5-trimethoxylbenzene radical cation in solution at room temperature

Laser excitation at 532 nm of 1,3,5-trimethoxybenzene radical cation (1(.+)), which is generated by photoinduced electron transfer in aerated acetonitrile containing 1.4-dicyanonaphthalene and biphenyl (BP) as a sensitizer-cosensitizer system by the use of XeCl laser (308 nm), gave fluorescence around 620 nm, The assignment to that from the excited state of 1(.+) (1(.+*)) is based on the agreement of the rates for the formation of 1(.+) obtained by laser-induced fluorescence and absorption spectroscopies. The fluorescence quantum yield for 1(.+) is ca. 2 x 10(-3). There was no observable fluorescence from other di- and trimethoxybenzenes. It was revealed by molecular orbital and configuration interaction calculations that the fluorescence corresponds to the D-2-D-0 transition of 1(.+). The high quantum yield of 1(.+) would be attributable to the large energy separation between the emissive D-2 and a non-emissive D-3 states of 1(.+) preventing their mixing which leads to the non-radiative decay.