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Superfast Dynamics of Bipyridinium Ions at Interfaces and Polar Solutions.

The super-fast dynamics of solutes in polar solvents and at solid-liquid interface is investigated by femtosecond time-resolved Raman scattering (RRS) and computer simulation of molecular dynamics (MD). Femtosecond RRS is used to investigate four bipyridinium radicals in aqueous solution: methylviologen monocation MV+, benzylviologen monocation, 4 )4'-bipyridinium-N,N'-di( propylsulfonate) monoanion and NtN'-ethylene-2,2'bipyridinium monocation. Time-resolution of the dynamics of the four radicals is earned out by a pump and probe technique using the time-dependent transient intensity of Stokes and anti-Stokes RRS of C-C stretching mode. It is found that the lifetime of the electronic excited state Bau 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. The photo-induced interfacial electron transfer from colloidal CdS particles to an adsorbed methylviologen is studied by femtosecond RRS. It is found by RRS spectra that part of the electron transfer and the accompanying aromatic-to-quinoid structure change of methylviologen occurs within the laser pulse width 350 fs. Time-resolving the photo-induced MV+ RRS band by a pump-probe scheme shows that the photo-induced MV+ dynamics is a double­ exponential consisting of two components: 270fs and 6.8ps. The 270fs fast component is assigned to electron transfer from shallow traps and accounts for