Mechanism study of TADF and phosphorescence in dinuclear copper (I) molecular crystal using QM/MM combined with an optimally tuned range-separated hybrid functional

Abstract

Abstract Investigations of the detailed photophysical processes are of great significance for future material improvements and novel designing strategies. Herein, the interconversion and decay rates of the first excited singlet state (S1) and triplet state (T1) for the Cu2I2(P^N)3 complex are calculated using the thermal vibration correlation function (TVCF) theory, combined with the optimally tuned range-separated hybrid functional (OT-ωB97XD) method at different temperature. A methodology with the building different ONIOM models, was carried over into simulation of crystal environment. All calculated results perfectly match the experimentally available data, demonstrating the validity of our applied theoretical approach. It has been found that the reverse intersystem crossing (RISC) rate kRISC from T1 to S1 is 3.11 × 1010 s−1 at 300 K, about 7 order of magnitude larger than the phosphorescence rate kr(T) = 3.71 × 103 s−1, and far more than ISC rate kISC(T1-S0) of 6.38 s−1. The S1 state can be an efficient thermal population from the T1 state by the RISC pathway, leading to an occurrence of thermally activated delayed fluorescence (TADF), and the estimated delayed time of τ(TADF) = 10 μs. On the other hand, the T1 state also exhibits stronger admixtures with higher lying singlet states due to stronger SOC, having a larger ZFS of 17 cm−1, thus, a relatively fast phosphorescence decay rate of kr(T) = 8.542 × 103 s−1 estimated by Einstein emission formula is observed. Further calculation results show that the emission intensities are stemming by 91% from the S1 state as TADF and by 9% as phosphorescence from the T1 state at 300 K, which indicates the ambient temperature emission represents the combined luminescence of TADF and phosphorescence. Our work would be useful for improving and designing the luminescent material combined high-efficiency TADF with phosphorescence.