The optical properties of N'N'Bis (2,6-xylyl)perylene-3,4:9,10-bis(dicarboximide) (DPP-PTCDI) thin films of various thicknesses have been studied. While the absorption spectra are redshifted compared to the monomer absorption spectra in solution, the photoluminescence spectra show an additional inhomogeneously broadened unstructured band, which increases in intensity with increasing film thickness. Photoluminescence measurements reveal that this broadband emission originates from defect states that exist in the ground state. The bulky functional group of the DPP-PTCDI molecule inhibits excimer formation, otherwise typical for perylene derivatives. Time-resolved photoluminescence (TRPL) measurements show that the molecular decay is directly related to the film thickness due to an increasing density of defects with increasing film thickness.