Femtosecond resonance Raman investigation of four univalent bipyridinium radicals

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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 B3u 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 MV2+, MV+, and BV2+. 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 BV2+ to BV+ are interpreted as the increase of inter-ring C-C bond order. © 1996 American Chemical Society.

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Journal of Physical Chemistry

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