Crystal phase dependent photoluminescence of 6,13-pentacenequinone

Films of 6,13-pentacenequinone (PQ, C22H12O2) of nominal thicknesses 5 and 30 nm grown in high vacuum onto SiO2 have been UV (325 nm) excited and their photoluminescence (PL) response has been investigated with the support of parallel scanning electron microscopy (FE-SEM) and x-ray diffraction (XRD) experiments. The photoluminescence spectra typically show a multiplet structure that can be ultimately assigned to the sum of the emission spectra from two different crystal phases, namely the "bulk" and the "thin film" phase, emitting respectively in the 500-600 nm and 600-750 nm spectral ranges. The assignment is done via the systematic parallel SEM and XRD investigation of the samples. Data are also discussed in comparison with PL spectra of PQ reported in the literature. The blue shift of the "bulk" phase PL spectrum is assigned to its smaller (about 9% less than in the "thin film" phase) molecular packing density in the ab-plane, and accordingly, to a decreased ?-? orbital overlap.