In this paper we explore the effects of medium dynamics on activationl
ess and inverted-region electron transfer (ET), when medium-induced dy
namics is slow on the time scale of the electronic processes. ET dynam
ics, with electron-nuclear coupling to the medium modes, was character
ized in terms of incoherent population decay of vibronic states in the
initial donor-acceptor manifold, which is characterized by nonadiabat
ic, energy (E)-dependent, microscopic ET rates, k(E). These k(E)'s are
determined by average Franck-Condon densities (AFDs), which were eval
uated by quantum and classical formalisms, with model calculations bei
ng performed for multimode harmonic systems with displaced potential s
urfaces. In spite of the intrinsic limitations of the classical AFDs,
which do not account for mode specificity and nuclear tunneling effect
s, the classical Franck-Condon factors provide a good description of t
he E dependence of the microscopic ET rates. For activationless ET we
show that k(E) is-proportional-to (E+nepsilon)-1/2, where ne is the ze
ro point energy, implying a weak energy dependence of k(E). Accordingl
y, the averaged experimental activationless ET rates exhibit a weak va
riation between the limits of slow medium-induced relaxation and that
of fast medium-induced dynamics. Subsequently, the theory of k(E) was
extended to include the effects of ET-induced excitations of high-freq
uency intramolecular vibrational modes, providing a unified descriptio
n of the weak E dependence of k(E) in the activationless and inverted
regions. We predict that for activationless and inverted-region ET the
experimental ET rates are only weakly dependent on the characteristic
s of medium relaxation dynamics, and can be appreciably higher than th
e solvent-controlled values (i.e., the reciprocal values of the medium
relaxation time induced by a constant charge distribution). Our analy
sis provides an adequate explanation for recent experimental observati
ons of ultrafast (k = (1 ps)-1-(100 fs)-1) activationless and inverted
-region ET, which apparently violate the predictions of solvent-contro
lled ET theory.