Theoretical perspective on the luminescence mechanism of a hybridized local and charge transfer state emitter with aggregation induced emission: a QM/MM study

Abstract

Apart from thermally activated delayed fluorescence (TADF) systems, new-generation fluorescent materials with aggregation induced emission (AIE) and hybridized local and charge transfer (HLCT) state features have shown promising applications in organic light-emitting diodes (OLEDs). However, the inner luminescence mechanisms and the relationship between basic molecular structure and photophysical properties are rarely explored. Herein, the excited state properties of a newly reported blue emitter (named as TPEPO) with AIE and HLCT features, as proposed by Xue, are theoretically studied through a combined quantum mechanics and molecular mechanics (QM/MM) method coupled with the thermal vibration correlation function (TVCF). Based on the independent gradient model (IGM), intermolecular interactions are visualized and they are quantitatively calculated by the symmetry-adapted perturbation theory (SAPT) method. Restricted geometry changes are found in the solid phase and decreased Huang–Rhys (HR) factors and reorganization energies are detected especially for these in low frequency regions. Non-radiative energy consumption processes are hindered and vibronic coupling effects are weakened by the rigid environment, where an AIE mechanism is revealed. Furthermore, through analyzing the energy landscape and the calculated intersystem crossing (ISC) rate, the transfer process from high triplet states to a singlet state is determined and a hot-exciton mechanism is detected. Thus, the solid-state effects on molecular photophysical properties are highlighted and a structure–property relationship is theoretically detected.