The Blue Emission From Cis‐stilbenes*

Under irradiation in glassy solution at 77°K cis‐stilbene shows a broad emission spectrum with a maximum at about 4180 Å. A similar emission is observed from 1,2‐diphenyl‐cyclopentene. The emission is shown to be fluorescence and is shown not to arise from either phenanthrene or the: intermediate (presumed to be a dihydrophenanthrene) involved in the photooxidation of cis‐stilbene to phenanthrene. The observations imply that, at least in a solid matrix, there is a barrier to rotation in the lowest excited singlet. Formation of the cyclic isomer, believed to be a dihydrophenanthrene, occurs on irradiation at room temperature but not at 77°K. Isomerization must provide a path for decay of excited singlets that becomes unavailable at low temperature. The possible general importance of such “chemical” decay mechanisms is discussed.