  We have investigated photoinduced intramolecular electron transfer dynamics
following metal-to-ligand charge-transfer (MLCT) excitation of
[Fe(CN)$_4$(2,2'-bipyridine)]$^{2-}$ (1),
[Fe(CN)$_4$(2,3-bis(2-pyridyl)pyrazine)]$^{2-}$ (2) and
[Fe(CN)$_4$(2,2'-bipyrimidine)]$^{2-}$ (3) complexes in various solvents with
static and time-resolved UV-visible absorption spectroscopy and Fe 2p3d
resonant inelastic X-ray scattering. We observe $^3$MLCT lifetimes from 180 fs
to 67 ps over a wide range of MLCT energies in different solvents by utilizing
the strong solvatochromism of the complexes. Intramolecular electron transfer
lifetimes governing $^3$MLCT relaxation increase monotonically and
(super)exponentially as the $^3$MLCT energy is decreased in 1 and 2 by changing
the solvent. This behavior can be described with non-adiabatic classical Marcus
electron transfer dynamics along the indirect $^3$MLCT->$^3$MC pathway, where
the $^3$MC is the lowest energy metal-centered (MC) excited state. In contrast,
the $^3$MLCT lifetime in 3 changes non-monotonically and exhibits a maximum.
This qualitatively different behaviour results from direct electron transfer
from the $^3$MLCT to the electronic ground state (GS). This pathway involves
nuclear tunnelling for the high-frequency polypyridyl skeleton mode
($\hbar\omega$ = 1530 cm$^{-1}$), which is more displaced for 3 than for either
1 or 2, therefore making the direct pathway significantly more efficient in 3.
To our knowledge, this is the first observation of an efficient $^3$MLCT->GS
relaxation pathway in an Fe polypyridyl complex. Our study suggests that
further extending the MLCT state lifetime requires (1) lowering the $^3$MLCT
state energy with respect to the $^3$MC state and (2) suppressing the
intramolecular distortion of the electron-accepting ligand in the $^3$MLCT
excited state to suppress the rate of direct $^3$MLCT->GS electron transfer.
