ELECTRON-TRANSFER VIA BRIDGES

In this paper we explore the energetic control of sequential and super exchange electron transfer (ET) mechanisms on the basis of quantum-mec hanical simulations and calculations for long-range ET in DBA systems, where the donor (D) and the acceptor (A) are separated by a bridge (B ). We studied ET dynamics in a Franck-Condon (FC) system characterized by three multi-dimensional displaced harmonic potential surfaces, whe re an initial single vibronic doorway state \alpha>) (with energy E-al pha) in the DBA (=D) electronic state is coupled to the mediating {\be ta]} vibronic quasicontinuum of the D(+)B(-)A (=B) electronic state, w hich in rum is coupled to the final {\gamma>} vibronic quasicontinuum of the D(+)BA(-)(=A) electronic state. The level structure was describ ed by the vibrational frequencies (for a four-mode harmonic system) an d the energy gaps Delta G(DB) and Delta G(DA) between the origins of t he corresponding electronic states (with n(alpha) = 1 - 50, n(beta) = 1000 - 2000, and n(gamma) = 1000 - 2000 states in the {\alpha>}, {\bet a>}, and {\gamma>} manifolds, respectively), while the couplings were characterized by the spectral densities and by the pair correlations ( specified in terms of correlation parameters eta(alpha alpha') and eta (beta beta')) between states belonging to the same manifold. The corre lation parameters eta(alpha alpha')(alpha, alpha' = 1 - 40) for the do orway-quasicontinuum coupling and eta(beta beta') (beta, beta' = 150 - 190) for the interquasicontinuum coupling are considerably lower than unity (\eta(alpha alpha')\ less than or equal to 0.4 and \eta(beta be ta')\ less than or equal to 0.3), obeying propensity rules with the hi ghest values of \eta(alpha alpha')\ and \eta(beta beta') which corresp ond to a single vibrational quantum difference, while for multimode ch anges between alpha and alpha' or between beta and beta' very low valu es of \eta(alpha alpha')\ or \eta(beta beta')\ are exhibited, Radiatio nless transitions theory was applied for quantum-mechanical simulation s based on the dynamcis of wave packets of molecular eigenstates for r esonance (Delta G(DB) < E-alpha) and for off-resonance (Delta G(DB) > E-alpha) coupling. Resonance \alpha> - {\beta>} - {\gamma>} coupling r esults in two-step sequential ET kinetics for all doorway states \alph a>, manifesting phase erosion due to weakly correlated intercontinuum coupling, without the need of intermediate state phonon induced therma lization. Off-resonance \alpha> - {\beta>} coupling in conjunction wit h {\beta>} - {\gamma>} resonance interactions results in unistep super exchange ET kinetics. The simulated sequential ET rates and the supere xchange rate are in good agreement with the calculated quantum-mechani cal rates obtained using the electronic couplings and FC densities. Th e energy-gap (Delta G(DB)) dependence of the simulated and the calcula ted ET rates from a single doorway state reveal a ''transition'' from sequential to superexchange ET with increasing Delta G(DB). For a fini te-temperature system, characterized by a fixed Delta G(DB) (> 0) smal l energy gap, the thermally averaged rate from a canonical ensemble of doorway states will result in the superposition of both superexchange and sequential mechanisms. (C) 1997 American Institute of Physics.