Effects of intramolecular and intermolecular interactions on excited state properties of two isomeric Cu complexes with AIE and TADF mechanisms in solid phase: A QM/MM study

Molecular photophysical properties are not only related to the basic structures but also affected by their packing modes. In this work, excited state properties of two isomeric Cu complexes (Complex 1 and Complex 2) with aggregation induced emission (AIE) and thermally activated delayed fluorescence (TADF) features are theoretically studied. The surrounding environments for molecule in solution and solid phase are considered by polarizable continuum model (PCM) and the combined quantum mechanics and molecular mechanics (QM/MM) method respectively. In addition, the intermolecular interaction and intramolecular interaction are visualized by the reduced density gradient (RDG) function. Our investigations show that the geometrical changes between ground state (S-0) and lowest singlet excited state (S-1) are hindered in solid phase by the restricted intramolecular rotation (RIR) effect, which brings smaller Huang-Rhys factors. Moreover, the decreased reorganization energy from solution to solid phase, is mainly contributed by the reduction of dihedral angle in low frequency (< 500 cm(-1)) regions. Thus, non-radiative energy consumption process of S1 is hindered and AIE mechanism is revealed for two complexes. Furthermore, stronger intramolecular and intermolecular interactions in solid phase are found for Complex 1 than these for Complex 2, this brings decreased non-radiative decay rate and increased reverse intersystem crossing rate, remarkable prompt and delayed fluorescence efficiencies are detected for Complex 1. Thus, the effects of intramolecular and intermolecular interactions on photophysical properties of two AIE-Cu-TADF molecules are revealed and the experimental measurements are reasonably elucidated. Our work could promote molecular engineering for designing new efficient AIE-TADF materials in practical organic light emitting diodes (OLEDs).