Resonance Raman studies of the p-benzosemiquinone and p, p′-biphenylsemiquinone free-radical anions

Abstract

The p-benzosemiquinone and p, p′-biphenylsemiquinone free-radical anions have been produced by both chemical and photochemical methods. Resonance Raman spectra recorded from these free radicals are compared with those of the neutral parent compounds. The observed parent-to-radical Raman wavenumber shifts have been interpreted in terms of the structural changes that occur upon radical formation. Electronic absorption spectra of both radical species have been recorded and analysed. These have been used in conjunction with the resonance Raman data and excitation profiles to elucidate the excited-state geometries of the radicals. The resonance Raman spectrum of the p, p′-biphenyl-semiquinone radical-anion dimer also is reported, as is its electronic absorption spectrum.