A simple theoretical model for dual phosphorescence

Abstract

A simple theoretical model is presented to explain the observed anomalous dual phosphorescences of certain aromatic carbonyl compounds in some rigid media. The phenomenon of dual phosphorescence for large molecules violates the well-known Kasha rule stating that the 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 rather slow T 2 → T 1 radiationless process which is unable to quench the T 2 emission completely. Two cases of T 1 = 3nπ *, T 2 = 3ππ * and T 1 = 3ππ *, T 2 = 3nπ * are discussed at both the low-temperature and the high-temperature limits.