Theoretical Study on the Light-Emitting Mechanism of Multifunctional Thermally Activated Delayed Fluorescence Molecules

Multifunctional thermally activated delayed fluorescence (TADF) molecules have recently attracted great attention. The light-emitting mechanisms of the TADF molecule PTZ-AQ with aggregation-induced emission (AIE) and mechanochromic luminescence (MCL) in three different aggregation states are studied using the combined quantum mechanics and molecular mechanics (QM/MM) method as well as the thermal vibration correlation function method. It is found that different aggregation patterns would induce different intermolecular interaction and molecular geometries. The different energy diagram of excited states also induce variation of emission colors and (reverse) intersystem crossing processes. By comparing with the light-emitting properties in solvent, the AIE property is confirmed, which is induced by enhanced radiative rate and decreased nonradiative rates in aggregation. The bicolor switch mechanism for the molecule in the solid state should have a close relationship with the dihedral angle change between the donor and the acceptor. The calculation results provide valuable information for understanding the light-emitting mechanism of the PTZ-AQ based polymorphs with AIE and MCL properties, which would favor the design of a new type multifunctional TADF molecules.