ONIOM studies on thermally activated delayed fluorescence of copper(I) dimers in crystal

Herein we have employed M06 and TD-M06 methods with the ONIOM approach to study the thermally activated delayed fluorescence (TADF) phenomenon of three halide-bridged Cu(I) dimers bearing amino-phosphane ligands in crystal (Cl-, Br-, and I-coordinated ones). On the basis of spectroscopic properties, ground- and excited-state geometric and electronic structures, and photophysical properties, we have found that the S₁ and T₁ states have a very small energy gap $▵{\text{E}}_{S_1-T_1}$, less than 4.0 kcal/mol, which makes the forward and reverse intersystem crossing ISC and rISC processes between S₁ to T₁ very efficient and a population equilibrium can be established. The rISC rates are visibly sensitive to temperature and increasing temperature clearly enhances the rISC rates thus rationalizing experimentally observed TADF phenomenon. Moreover, we have also found that the low-frequency vibrational modes related to the torsional motion of the coordinated ligands have large Huang-Rhys factors, which increase from Cl via Br to I-coordinated Cu(I) dimers. In this varying process, the ISC rates increase visibly whereas the rISC rates change slightly. Our work provides useful microscopic knowledge to understand the TADF emission of the Cu(I) dimers, which could be helpful for the rational design of TADF materials with excellent performance.