  Multi-molecular excited states accompanied by an intra- and inter-molecular
geometric relaxation are commonly encountered in optical and electrooptical
studies and applications of organic semiconductors as, for example excimers or
charge transfer states. Understanding the dynamics of these states is crucial
to improve organic devices such as light emitting diodes and solar cells. Their
full microscopic description, however, demands for sophisticated tools such as
ab-initio quantum chemical calculations which come at the expenses of high
computational costs and are prone to errors by assumptions as well as iterative
algorithmic procedures. Hence, the analysis of spectroscopic data is often
conducted on a phenomenological level only. Here, we present a toolkit to
analyze temperature dependent luminescence data and gain first insights into
the relevant microscopic parameters of the molecular system at hand. By means
of a Franck-Condon based approach considering a single effective
inter-molecular vibrational mode and different potentials for the ground and
excited state we are able to explain the luminescence spectra of such
multi-molecular states. We demonstrate that by applying certain reasonable
simplifications the luminescence of charge transfer states as well as excimers
can be satisfactorily reproduced for temperatures ranging from cryogenics to
above room temperature. We present a semi-classical and a quantum-mechanical
description of our model and, for both cases, demonstrate its applicability by
analyzing the temperature depended luminescence of the amorphous donor-acceptor
heterojunction tetraphenyldibenzoperiflanthene:C$_{60}$ as well as
polycrystalline zinc-phthalocyanine to reproduce the luminescence spectra and
extract relevant system parameters such as the excimer binding energy.

Materials Science