Effects of high pressure on the photoluminescence transitions of excitons in α-PTCDA (perylene tetracarboxylic dianhydride) crystals

We report investigations of the photoluminescence (PL) spectrum of α-PTCDA single crystals at 11 K and 300 K as a function of applied pressures to 54 kbar carried out using a diamond-anvil cell with annealed methanol-ethanol medium. The effects of pressure are observed on several excitonic PL features principally associated with: an excimer transition for the 300 K spectra, or an indirect Frenkel exciton and charge transfer exciton transitions (between stacked molecules in different unit cells) for the 11 K spectra. Under applied pressure, all of these features redshift at rates that are in general accord with the π-π* broadening caused by increasing overlap between PTCDA molecules, but with variations in pressure dependence, especially in the quadratic shifts, that derive from the specific character and Stokes-shift mechanisms of the exciton transitions. The observed pressure behavior conforms to a recent model of the exciton transitions in α-PTCDA based on a theoretical analysis of 1 atm time-resolved PL experiments.