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.