Superfast dynamics of bipyridinium ions at interfaces and polar solutions.

摘要

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 N,N′-ethylene-2,2′ -bipyridinium monocation. Time-resolution of the dynamics of the four radicals is carried 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 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.; 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 the part of MV+ produced within the pulse width. However the 6.8 ps slow component is assigned to electron transfer from relatively deep traps.; The solvation dynamics upon solute ionization in bulk Stockmayer fluids and at surface is studied using MD simulation. For 20–40 selected thermodynamics states, the non-equilibrium solvation, starting from a neutral polar solute in bulk or adsorbed to a surface is studied by investigating the (complementary) solvent response function, solvent numbers in the first solvent shell, spatial solvent distribution, and pair distribution function. The dependence of the solvation dynamics on solute charge, solvent dipole moment, and surface parameters is studied. A mechanism with two kinds of transient states is proposed to explain the 3-mode dynamics. The computer simulation compliments the solvation dynamics upon electron transfer to a solute.