  The non-Markovianity of the electron transfer in an oligothiophene-fullerene
heterojunction described by a spin-boson model is analyzed using the time
dependent decoherence canonical rates and the volume of accessible states in
the Bloch sphere. The dynamical map of the reduced electronic system is
computed by the hierarchical equations of motion methodology (HEOM) providing
an exact dynamics. Transitory witness of non-Markovianity is linked to the bath
dynamics analyzed from the HEOM auxiliary matrices. The signature of the
collective bath mode detected from HEOM in each electronic state is compared
with predictions of the effective mode extracted from the spectral density. We
show that including this main reaction coordinate in a one-dimensional vibronic
system coupled to a residual bath satisfactorily describes the electron
transfer by a simple Markovian Redfield equation. Non-Markovianity is computed
for three inter fragment distances and compared with a priori criterion based
on the system and bath characteristic timescales.
