Investigation of Conversion and Decay Processes in Thermally Activated Delayed Fluorescence Copper(I) Molecular Crystal: Theoretical Estimations from an ONIOM Approach Combined with the Tuned Range-Separated Density Functional Theory.

Accurate research of the photophysical processes is of great significance for the rational design of excellent thermally activated delayed fluorescence (TADF) materials. Herein, the interconversion and decay rates of the first excited singlet state (S1) and triplet states (T1) in the Cu(pop)(pz2BH2) complex are computed using the thermal vibration correlation function (TVCF) theory at different temperature. For consideration of the solid-state environment, a methodology that is based on the ONIOM model, combined with the optimally tuned range-separated hybrid functional (CAM-B3LYP*) method, was applied. Our calculated results are in excellent agreement with the experimentally available data. It has been found that the energy dissipation of the nonradiative processes from the S1 to ground state is promoted by low frequency vibrational modes in the solution phase, resulting in the high knr(S) = 1.68 × 108 s-1 at 300 K. However, for the crystal phase, they are easily hindered through intermolecular interactions, knr(S) is predicted to be decreased by about 5 orders of magnitude upon aggregation (2.98 × 103 s-1). With temperature increase, the reverse intersystem crossing (RISC) rate kRISC from T1 to S1 is drastically increased to 6.12 × 104 s-1 at 300 K, while the change of other rates is still small, which can compete with the radiative decay rate of kr(T) = 4.75 × 104 s-1 and nonradiative intersystem crossing rate of kISC(T1-S0) = 6.63 × 102 s-1 at the T1 state. This implies that the S1 state can be an efficient thermal population from the T1 state, leading to an occurrence of delayed fluorescence, and the complexes exhibit high emission quantum yields, 58.7%. But, at low temperature T < 100 K, the RISC rate is sharply change, kRISC ≪ kr(T) or kISC, which cannot induce an occurrence of delayed fluorescence. Our investigation would be helpful for designing novel, high-efficiency TADF materials.