Excimer and monomer emission of pyrene crystals associated with the phase transition

Emission spectra and decay times of the fluorescence under the excitation of various energies were measured in pyrene crystals. At liquid nitrogen temperature, a new excimer emission was found under 390 nm excitation, and its peak shifts a little to the high energy in comparison with that of the familiar excimer. The decay time of the former emission abruptly changes around 125°K, which seems to be due to the phase transition in pyrene crystals. Also, when the crystals were rapidly cooled below 125°K, the monomer emission in the crystals was observed, and its decay time was about 600 nsec.