Fluorescence and photodimerization of crystalline 9-cyanoanthracene. Spectroscopic evidence for a distinct nucleation step

Abstract

The intensity of emission in freshly grown crystals of 9-cyanoanthracene (9CNA) at 300 K is low; u.v. irradiation produces a large increase, up to 20 times, without affecting the fluorescence lifetime or λ_max. A similar effect on the 300 K emission is also produced, more slowly, by irradiation at 77 K. This is attributed to the photoreaction of trans-9CNA pairs initially present at defects and the creation of cis-type traps adjacent to the newly formed photodimer.

The lifetimes, τ, of excimeric emission from 9CNA crystals have been measured at various temperatures from 77 to 300 K. Below 170 K, τ shows only a slight dependence on temperature; above 170 K the effect is much greater, corresponding to an activation energy of 3 kJ mol^–1. This confirms earlier proposals that there is a change of emissive site on heating.