QM/MM Studies on Thermally Activated Delayed Fluorescence of a Dicopper Complex in the Solid State

Abstract

Dicopper complexes with thermally activated delayed fluorescence (TADF) phenomena are important in enriching the arsenal of organic light-emitting diodes materials. However, the TADF mechanism is still elusive, especially in the solid state. Herein, we chose a TADF dicopper complex and investigated its geometric and electronic structures and absorption and emission spectra using DFT, TD-DFT, and QM/MM methods. On the basis of these results, we further estimate the fluorescence emission rate from the S 1 state, phosphorescence emission rate from the T 1 state, and forward and reverse intersystem crossing (ISC and rISC) rates between S 1 and T 1 . The present work shows that both the S 1 and the T 1 states have mixed metal-to-ligand and interligand charge-transfer character. Good spatial separation between the HOMO and the LUMO makes the S 1 −T 1 energy gap small, ca. 2.8 kcal/mol. This small energy gap leads to efficient ISC and rISC processes, whose rates are much larger than the fluorescence and phosphorescence emission rates at 300 K, therefore enabling TADF. In contrast, at 77 K, the rISC process is blocked because its rate is much smaller than the phosphorescence emission. Thus, TADF disappears at 77 K. Further analysis shows that high-frequency deformation and lowfrequency torsional vibrational modes make a large contribution to the Huang−Rhys factors, but Duschinsky rotation effects are essentially negligible for both the ISC and the rISC processes.

Theoretical studies on the excited-state properties of thermally activated delayed fluorescence molecules with aggregation induced emission

“…Thus, K nr is found to be sensitive to the temperature, which is consistent with the previous research. 26 These results indicate that the excited state properties are associated with thermal motions. Furthermore, molecular conformation induces changes in excited state properties is investigated, and corresponding emission wavelengths, energies and decay rates are calculated and listed in Tables S3 and S4 (ESI †).…”

“…Both the abovementioned methods and applications can be found in the studies by Shuai, Peng, and Cui and our work. [23][24][25][26][27][28][29][30][31][32]…”

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

“…The time interval for integration in eq is 1.0 × 10 –4 ps with a total 3 ps integral time to ensure these results converged. All ISC and rISC rates including Huang–Rhys factors and reorganization energies were calculated by our own developed code. , More details can be found in previous works. ,, …”

“…Recently, George group reported a PmDI-based CzPhPmDI emitter and observed TADF in PMMA films; however, its luminescence mechanism is still unclear, and it is questionable whether highly excited states are involved in the TADF. In addition, previous theoretical calculation works are usually carried out in crystal environments, ,,− and thin film environments are scarcely reported. Inspired by this, we use the QM/MM method to model CzPhPmDI in PMMA film environments.…”

Theoretical studies on boron dimesityl-based thermally activated delayed fluorescence organic emitters: excited-state properties and mechanisms

“…Numerous mechanism investigations support experimental luminescence phenomena and predict the properties of TADF molecules. [31][32][33][34][35][36][37][38][39][40][41][42][43] For example, the Shuai group investigated the TADF mechanism of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene and found that the rotation of the Cz ligand between two cyano groups leads to the interconversion between S 1 and T 1 . The intramolecular steric hindrance hinders the nonradiative transition of S 1 -S 0 , which in turn enhances the fluorescence.…”