High-pressure effects on fluorescence of anthracene crystals

Pressure-induced changes on the fluorescence spectra of anthracene crystals were examined up to 5 GPa. Changes in the spectra are demonstrated to be sensitive both to the sample morphology and to the compression conditions (e.g., hydrostaticity). Under hydrostatic pressure, there was no excimer fluorescence from anthracene single crystals. This result is in sharp contrast to previously published data. Because the normal fluorescence was not obscured by the excimer emission, pressure-induced energy shifts for vibronic bands could be determined. These shifts cannot be described adequately by available theoretical descriptions. Strong excimer fluorescence from anthracene crystals was observed only under nonhydrostatic conditions. This fluorescence is attributed to the emission from structural defects formed at the onset of plastic deformation in the crystals. The fluorescence spectra of polycrystalline samples exhibit a new weak band, identified as defect-related excimer emission. The intensity of this emission can be enhanced by pressure, mainly because of the increasing concentration of defects.