Abstract:
From studies of the emission spectra of 9-cyanoanthracene (9 CNA) solutions, glasses, polycrystalline films and single crystals at 298 and 77 K a centrosymmetric excimeric species with emission maximum at ≈ 490 nm has been observed. The presence of “incipient-dimer” pairs giving rise to such emission has been confirmed by investigations of the photolytic and thermal cleavage of the photodimer in 9-cyanoanthracene single crystals and polycrystalline matrices. Luminescence decay time measurements substantiate our views. The emission spectrum of 9 CNA single crystals is asymmetric, with a maximum at 490 nm at room temperature, but upon cooling to 77 K the peak becomes symmetrical and occurs at lower energy. The energy difference of ≈ 2000 cm–1 is appreciably higher than the value predicted on the basis of the “frequency effect” and often used to explain the temperature shift of an excimeric emission in both “pair” and “stack” structures. At the higher temperatures in 9 CNA single crystals at least two excimeric forms exist in thermal equilibrium but at the lower temperatures emission occurs from the mirror-symmetric form.
