Theoretical study on the origin of the dual phosphorescence emission from organic aggregates at room temperature

Purely organic materials with dual room temperature phosphorescence (RTP) phenomenon were reported recently, but the underlying mechanism was still ambiguous. Herein, we revealed the source of dual RTP emission, taking CzDPS crystal as prototype, by using hybrid quantum mechanics and molecular mechanics (QM/MM) coupled with the thermal vibration correlation function rate theory. Theoretical calculations verified that the emission lifetimes are prolonged from 70 ms in the higher triplet state T₂ to 216 ms in the lowest triplet state T₁, which is well consistent with the increase of RTP lifetimes from 74 ms for the peak at 465 nm to 627 ms for the band at 565 nm. This is because the radiative and nonradiative decay rates are larger for T₂ → S₀ than that of T₁ → S₀, which was mainly contributed by the synergistic effect of the increase of spin-orbit coupling and excitation energy, as well as the decrease of reorganization energy. Moreover, the simulated RTP spectra agree well with the experimental ones, including the emission position and profiles. Therefore, the upper-lying triplet excited states are responsible for the dual RTP in CzDPS crystal. This work could contribute to further understanding on the multiple luminescence of organic aggregates.