Theoretical study on the mechanism of aggregation-induced emission in red thermally activated delayed fluorescence molecules: trans/ cis-arrangement effect

Fluorescent emitters with both high exciton utilization and luminescence efficiency show promising applications in organic light-emitting diodes (OLEDs), especially those with simultaneously aggregation-induced emission (AIE) and thermally activated delayed fluorescence (TADF). In this work, we performed multi-scale simulations to investigate the photophysical properties of the reported red AIDF compounds T-DA-1, C-DA-1, T-DA-2 and C -DA-2, molecules with the same donor-acceptor group but with very different emission wavelengths and pho-toluminescence quantum yields (phi PL) due to different connection points. The packing modes of molecules in film are obtained by molecular dynamics (MD) simulations, the crystal structure is given from the experiment, and then the photophysical properties with the consideration of the solid-state effect are studied by using the combined quantum mechanics and molecular mechanics (QM/MM) method. Natural transition orbitals (NTOs), adiabatic singlet-triplet state energy levels, reorganization energies and HR factors are analysed in detail. In addition, radiative and non-radiative transitions as well as inter-system crossing (ISC) and reverse inter-system crossing (RISC) processes are also investigated. Our results indicate that there is an important role of different substitution positions on the photo physical properties of TADF molecules. The trans -arrangement substitutions restrict the geometry variations, hinder the non-radiative energy consumption process, and promote the radiative process of TADF molecules. In addition, the trans -arrangement in the aggregated state has a smaller LEST and a larger SOC, which could favorite the RISC process. Last, the films and crystals of the trans -arrangement molecules have stronger 7C-7C stacking, it limit the movement of molecules, which may inhibit the nonradiative deactivation caused by intramolecular torsional and vibrational excited-energy loss. Our work reasonably elucidates the experimental results and highlights the effect of different substitution positions on TADF properties. This could provide a theoretical perspective for designing efficient AIDF molecules.