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 fluorescenc e from an excited state of the radical anion of 1,4-benzoquinone (BQ(- )), and the first report of a fluorescence quantum yield from any radi cal anion. BQ(-) was observed in both a 77 K 2-methyltetrahydrofuran m atrix and a room temperature isooctane solution. The low fluorescence quantum yield, phi(f) = 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 sou rce of the weak fluorescence, is formally forbidden. This conclusion i s supported by both semiempirical and ab initio molecular orbital calc ulations. In addition, we determined the excited state lifetime of BQ( -) at 77 K to be 63 ns, with an excited state absorption spectrum peak ing at 415 nm.