Fluorescence of the 1,4-Benzoquinone Radical Anion

Information about excited states of radical ions can enable their use as powerful oxidizing and reducing agents capable of driving chemical reactions. This article presents the first measurements of fluorescence from an excited state of the radical anion of 1,4-benzoquinone (BQ-), and the first report of a fluorescence quantum yield from any radical anion. BQ- was observed in both a 77 K 2-methyltetrahydrofuran matrix and a room temperature isooctane solution. The low fluorescence quantum yield, φf = 0.003, and the presence of a 0.5 eV red shift of the emission band edge (593 nm) from the absorption band edge (475 nm) imply that the lowest energy transition in BQ-, which is the source of the weak fluorescence, is formally forbidden. This conclusion is supported by both semiempirical and ab initio molecular orbital calculations. In addition, we determined the excited state lifetime of BQ- at 77 K to be 63 ns, with an excited state absorption spectrum peaking at 415 nm.