Polarity-induced dual room-temperature phosphorescence involving the T-2 states of pure organic phosphors

Emissions from higher excited states are of theoretical and experimental interest but rare. A series of pure organic phosphors, 2,5-dihexyloxy-4-bromobenzaldehyde (Br6A) derivatives, show intriguing dual-phosphorescence characteristics. Their phosphorescence emissions are blue-shifted by 20 nm at room temperature compared to those at 78 K. These shifts are accompanied by increasing contributions from a fast-decaying component. The mechanism responsible for the emission shifts observed for Br6A was studied in a 2,5-dihexyloxy-4-bromobenzene (Br6) solid matrix and solvents of various polarities. While the decay curve for Br6A in the Br6 matrix showed a single slow process, pure Br6A exhibited an additional faster decay. We demonstrated that the faster decay of pure Br6A arises from the T-2 state, while Br6A in the Br6 matrix emits solely from T-1. Compared to the T-1 state (pi, pi*), the n, pi* character of the T-2 state ensures more efficient spin-orbit coupling with the ground state, which facilitates faster decay. The larger polarity of Br6A compared to that of Br6 leads to a lower T-2 level and a smaller T-2-T-1 energy gap in pure Br6A, but not when in the Br6 solid matrix, which enables a T-1 to T-2 thermal population shift in pure Br6A at room temperature. Consistently, the emissions from the Br6A solutions gradually red-shifted with increasing solvent polarity at 78 K. Moreover, the decay curves of Br6A in solvents of low polarity were single-exponential unlike the double-exponential ones observed in polar solvents. These results provide insight into the effect of the environment on the dual phosphorescence emissions of pure organic phosphors.