Thermally Activated Delayed Fluorescence of a Pyromellitic Diimide Derivative in the Film Environment Investigated by Combined QM/MM and MS-CASPT2 Methods.

Arylene diimide compounds exhibit thermally activated delayed fluorescence (TADF), but its mechanism remains elusive. Herein we studied the TADF mechanism of a carbazole-substituted pyromellitic diimide derivative (CzPhPmDI) in poly(methyl methacrylate) (PMMA) film by using DFT, TD-DFT, and MS-CASPT2 methods within the QM/MM framework. We found that the TADF mechanism involves three electronic states (i.e., S 0 , S 1 , and T 1 ), but the T 2 state is not involved because its energy is higher than the S 1 state by 6.9 kcal/mol. By contrast, the T 1 state is only 3.2 kcal/mol lower than the S 1 state and such small energy difference benefits the reverse intersystem crossing (rISC) process from T 1 to S 1 thereto TADF. This point is seconded by relevant radiative and nonradiative rates calculated. At room temperature, the ISC rate from S 1 to T 1 is calculated to be 6.1 × 10 6 s -1 , which is larger than the fluorescence emission rate, 2.2 × 10 5 s -1 ; thus, the dominant S 1 population converts to the T 1 state. However, in the T 1 state, the rISC process (1.8 × 10 4 s -1 ) becomes the most important channel because of the negligible phosphorescence emission rate (3.5 × 10 -2 s -1 ). So, the T 1 population is still converted back to the S 1 state to fluoresce enabling TADF. Unfortunately, the rISC process is blocked in low temperature. Besides, we found that relevant Huang-Rhys factors have dominant contribution from low-frequency vibrational motion related to the torsional motion of functional groups. These gained insights could provide useful information for the design of organic TADF materials with excellent luminescence efficiency.