Femtosecond Resonance Raman Investigation of Four Univalent Bipyridinium Radicals

Femtosecond time-resolved resonance Raman scattering (RRS) spectroscopy is used to investigate four bipyridinium radicals: methylviologen monocation MV +, benzylviologen monocation BV +, 4,4‘-bipyridinium- N, N ‘-di(propylsulfonate) monoanion BPDPS -, and N, N ‘-ethylene-2,2‘-bipyridinium monocation EB + . Time resolution of the dynamics of the four radicals using the 379.6 nm laser of 350 fs pulse duration is carried out by a pump and probe technique using the time-dependent transient intensity of Stokes and anti-Stokes RRS of the C−C stretching mode. It is found that the lifetime of the electronic excited state B 3u is less than 350 fs and the vibrational relaxation rates in the electronic ground state are some 2−5 ps. The possible vibrational relaxation mechanisms including the radical structure and charge effect on the vibrational relaxation are discussed. Assignments of some in-plane stretching vibrational modes for BV + and BPDPS - are tentatively made on the basis of the similarity of their molecular skeletons to those of MV 2+, MV +, and BV 2+ . The shifts in vibrational frequency in the N−R mode are discussed according to the mass and charge effect of the substituted group R, whereas the frequency shifts in the C−C inter-ring stretching vibration upon reduction of BV 2+ to BV + are interpreted as the increase of inter-ring C−C bond order.