Spectral properties of trap states and the orientational phase transition in Naphthalene-Tetracyanobenzene Charge-Transfer crystal doped with Pyrene

Temperature changes of the emission spectra of deep traps, formed by pyrene-tetracyanobenzene chargetransfer complexes in the host lattice of naphthalene-tetracyanobenzene crystal, were investigated with regard to the phase transition which the host lattice undergoes at 72 +/- 1 K. Selective excitation of traps revealed distinct differences in the spectral and temporal characteristics of the trap phosphorescence in the low- and high-temperature phases of the crystal. The value of the activation energy, determined from the observed changes of the phosphorescence, indicates thermally activated reorientational motion of pyrene molecules during the phase transition. This conclusion corroborates the results of recent ENDOR experiments.