  Electron transfer (ET) in molecular donor-acceptor dye systems is crucial for
charge transport in organic semiconductors. Classically, ET rates should
decrease with increasing donor-acceptor distance while the microscopic
mechanism is more complex and shows intricate dependencies on the excitation
conditions. In this paper, we introduce highly soluble N,N'-dialkyl perylene
dihydrazides (PDH) - perylene dyes with a dialkylamino -NR$_2$ donor
functionality directly bonded to both of their imide nitrogen atoms. We compare
the PDH electron-transfer dynamics with a group of classical
N,N'-bisalkylperylene diimides (PDI) equipped with a -NR$_2$ donor linked to
the PDI acceptor core via a varying number of alkylene -(CH$_2$)- spacer
groups, thus at distinctively different distance. Special physicochemical
design features of our study objects include: i) amine moieties as donor group
to minimize spin-orbit coupling; ii) substitution solely at both imide
positions to avoid major impact on HOMO and LUMO levels and distortions of the
PDI backbone; iii) control of donor-acceptor separation by non-conjugated
alkylene groups to exclude any additional effects due to delocalized $\pi$
electron systems. All materials show non-single-exponential photoluminescence
decay dynamics. A rate equation analysis supported by electrochemical and
absolute photoluminescence efficiency measurements yields evidence for
efficient intersystem-crossing without heavy elements and reveals that the
charge-transfer efficiency across the intramolecular interface strongly depends
on the surplus excitation energy.
