A noninteractive model for dual phosphorescence in aromatic carbonyls

Abstract

Abstract A non-interactive density of states model is presented to explain the observed anomalous dual phosphorescences of certain aromatic carbonyl compounds. Although the experiments are in rigid media the model is in terms of the free molecule. The phenomenon of dual phosphorescence in a large molecule violates the well-known Kasha rule: emission can occur only from the lowest excited electronic state of a given multiplicity. For a small energy gap between the second triplet state (T 2) and the first triplet state (T 1), the sparse density of T 1 vibronic levels isoenergetic with the T 2 vibrationless level leads to a slow T 2⇝ T 1 radiationless process which is unable to quench the T 2 emission completely. Two cases: T 1 = 3nπ *, T 2 = 3ππ *; and T 1 = 3ππ *, T 2 = 3nπ * are discussed at both low and high temperature limits. An important result is that raising the temperature in the former case is predicted to increase the intensity of the T 1 emission at the expense of the T 2 emission, and conversely for the latter case. According to the model dual phosphorescence should also be observable in certain azines.