Observation of triplet excimer emission in 2-bromobenzophenone

Phosphorescence spectra of 2-bromobenzophenone (2Br-BP) crystals were measured from 70 K to room temperature. A capital crossover in the emission mechanism takes place within this temperature range, leaving at room temperature only a broad two-hump feature. Additional phosphorescence experiments, carried out to understand the nature of this spectrum, include measurements from 2Br-BP solutions in ethanol as well as time-resolved and varying-excitation-intensity measurements from crystals. Based on results of these experiments and on our own single-crystal X-ray structure data, we conclude that the above two-band spectrum recorded near room temperature is due to the emission of single-photon bi-molecular triplet excimer formed by the carbonyl groups of two neighbor 2-bromobenzophenone molecules.