Time-resolved resonance Raman spectroscopic investigation of the trans-stilbene cation radical kinetics in photolytically induced electron-transfer reactions

The formation of the trans-stilbene cation radical in photoreactions involving trans-stilbene and the electron-acceptor molecules fumaronitrile, 9-cyanoanthracene, and 9,10-dicyanoanthracene in acetonitrile solution at room temperature is established by the observation of identical transient resonance Raman (TR³) spectra. The excitation profile of the resonance Raman bands closely follows the electronic spectrum of the trans-stilbene radical cation. The decay kinetics of the Raman band intensities measured under various experimental conditions give evidence for two reactions, which are competitive with the homogeneous recombination of the trans-stilbene cation radical with the corresponding radical anion: (i) recombination with a fumaronitrile dimer anion radical and (ii) recombination with superoxide ion to yield oxidation products. trans-Stilbene is one of the first molecules for which the resonance Raman spectra of its short-lived cation and anion radical in the same solvent are now known. In contrast to the electronic absorption spectra, the resonance Raman spectra of the two radical ions are significantly different.