  We report detailed temperature dependent photoluminescence (PL) spectra of
pentacene (PEN), perfluoropentacene (PFP), and PEN:PFP mixed thin films grown
on SiO2. PEN and PFP are particularly suitable for this study, since they are
structurally compatible for good intermixing and form a model donor/acceptor
system. The PL spectra of PEN are discussed in the context of existing
literature and compared to the new findings for PFP. We analyze the optical
transitions observed in the spectra of PEN and PFP using time-dependent density
functional theory (TD-DFT) calculations. Importantly, for the mixed PEN:PFP
film we observe an optical transition in PL at 1.4 eV providing evidence for
coupling effects in the blend. We discuss a possible charge-transfer (CT) and
provide a tentative scheme of the optical transitions in the blended films.
