The role of intermolecular interaction in regulating the thermally activated delayed fluorescence and charge transfer properties: a theoretical perspective
Thermally activated delayed fluorescence (TADF) materials show promising application in organic light emitting diodes (OLEDs). However, the molecular packing modes and intermolecular interactions play a significant role in defining the nature of the excited states, their energies and dynamics, further influence the performance of TADF-OLEDs. A theoretical perspective to study the photophysical and charge transfer properties with explicit consideration of intermolecular interactions in solid phase is highly desired. Herein, the excited state dynamics of two novel TADF molecules BPPZ-PXZ and mDPBPZ-PXZ in toluene and solid phase are studied by polarizable continuum model (PCM) and the combined quantum mechanics and molecular mechanics (QM/MM) method respectively. The intermolecular interactions are visually evaluated by the independent gradient model (IGM) and quantitatively calculated through the molecular force field energy decomposition method. Based on the thermal vibration correlation function (TVCF) method, the intersystem crossing (ISC) and reverse intersystem crossing (RISC) processes are studied. Moreover, the charge carrier mobilities of two compounds are investigated by Marcus equation and Monte Carlo simulations. Results indicate that the different packing modes and intermolecular interactions bring remarkable influence on S1-T1 gap, spin orbital coupling effect and transition properties. The stronger intermolecular interaction in mDPBPZ-PXZ crystal brings promoted RISC process and superior TADF feature than that of BPPZ-PXZ. Furthermore, the balanced and bipolar charge transfer features are verified for studied compounds, a wise molecular design strategy with modifications in BPPZ and mDPBPZ units is proposed to regulate the photophysical and charge transfer properties. Through this work, we would like to shed light on the relationship among molecular structures, intermolecular interactions and TADF as well as charge transfer properties, which may help take advantage of the unique molecular design to develop high performance emitting molecules.