Tetrahedral Cu(I) Complexes for Thermally Activated Delayed Fluorescence: A Density Functional Benchmark Study with QM/MM Models

Tetrahedral Cu(I) complexes represent a major class of organometallic thermally activated delayed fluorescence (TADF) emitters. However, due to the d¹⁰ electronic structure and low-lying metal-to-ligand charge transfer (MLCT) states, these systems exhibit (pseudo) Jahn-Teller distortions in the excited state, resulting in tetrahedral to square planar geometry flattening. From a computational point of view, this poses a major challenge since theoretical studies are often conducted with isolated single molecule models. Such models are incapable of describing the suppressing effect of the surrounding solid-state environment on geometry relaxation and often result in overly relaxed excited-state geometries and inaccurate transition energies. Crystal models based on quantum mechanics/molecular mechanics (QM/MM) approaches have emerged as viable candidates for modeling the solid-state environment. Here, we report a study, conducted on 56 experimentally known tetrahedral Cu(I) TADF emitters, comparing the isolated and QM/MM models together with five commonly used density functionals. Our results show that while differences in ground-state geometries and excitation energies are small, significant deviations are observed in the excited-state geometries and fluorescence energies. Because of the added rigidity, the QM/MM models show less (pseudo) Jahn-Teller effect induced geometry flattening, which consequently results in blue-shifted fluorescence energies compared with isolated models.