Excimer and photochemically induced fluorescence of 9-cyanoanthracene crystals

The polarized emission spectra of melt and vapor grown single crystals of carefully purified 9-cyanoanthracene (9-CNA) are reported. This materials has a quasi one-dimensional structure with molecules stacked along the c-axis. In crystals previously unirradiated at room temperature, the emission is excimer-like with a double peak (550 nm, E⊥c) and (525 nm, E|c) at 2 K, and a single peak at 300 K (588 nm, E, |c). Exposure to UV radiation at room temperature induces fluorescence associated with the production of photodimers. At 2 K this fluorescence is highly structured and coexists with excimer emission. It also extends to higher energies than the origin of the exciton band of the first singlet, supporting a model in which this component of the fluorescence arises from 9-CNA molecules separated from bulk by a matrix of photodimers. The photoproduct thus acts somewhat like a Shpol'skii matrix for these unreacted molecules. © 1976.