Excited state dynamics for hybridized local and charge transfer state fluorescent emitters with aggregation-induced emission in the solid phase: a QM/MM study

Highly efficient organic light-emitting diodes (OLEDs) based on fluorescent emitters with a hybridized local and charge transfer (HLCT) state have attracted significant attention. Recently, a near-infrared fluorescent compound, 2,3-bis(4'-(diphenylamino)-[1,1'-biphenyl]-4-yl) fumaronitrile (TPATCN), with an HLCT state has been synthetized, and the features of OLEDs based on this compound have been explored. In this study, excited state dynamics of TPATCN in the solid phase has been theoretically studied through a combined quantum mechanics and molecular mechanics (QM/MM) method. By analyzing the changes in geometry, the Huang-Rhys factor, and reorganization energy, non-radiative consumption ways through the torsional motions of diphenylamino and central fumaronitrile in low frequency regions (< 200 cm(-1)) are effectively hindered by the restricted intramolecular rotation (RIR) effect in the solid phase. The fluorescence efficiency of the OLED has been quantitatively calculated. The results show that the fluorescence efficiency is greatly enhanced from 0.16% in the gas phase to 52.1% in the solid phase; this demonstrates the aggregation-induced emission (AIE) mechanism for the OLED. Furthermore, by combining the dynamics of the excited states and the adiabatic energy structures calculated in the solid phase, the so-called hot-exciton process from higher triplet states to a singlet state has been illustrated. Our investigation elucidates the experimental measurement and helps understand the AIE mechanism for HLCT compounds, which is beneficial for developing highly efficient emitters.