Whether the combination of AIE and TADF functional groups produces AIE-type TADF? –A theoretical study on the synergistic effect of TPE and carbazole donor group/thianthrene-tetraoxide acceptor group

Abstract

Two sets of triphenylethylene/tetraphenylethylene (TPE) derivatives containing carbazole and thianthrene-9,9,10,10-tetraoxide were calculated with density functional theory/time-dependent density functional theory to explore the synergistic effect of the popular aggregation-induced emission (AIE) groups (TPE) and the thermally activated delayed fluorescence (TADF) functional groups (carbazole and thianthrene-tetraoxide). Through comparing the molecular geometric and electronic structures and the properties of the excited states, the significant influences of transition configurations and natures as well as spin-orbit coupling of the excited states on the intersystem crossing and reverse intersystem crossing (RISC) processes have been clarified. In addition, the aggregation process of molecules in water was simulated, and the transformation of molecular conformation and final proportion of each conformation were explored, whose influence on RISC rate, a determinant of TADF, was further analyzed. These investigations explain why only one set of the molecules present TADF when both sets of them have small singlet-triplet energy gap (ΔEST). The comparison of the reorganization energy between the molecule in isolated state and in solid state demonstrates that the AIE phenomenon is mainly due to the restricted low-frequency motion of the benzene ring in TPE group in aggregate surroundings, which indicates TPE plays its role as an AIE functional group. Therefore, it is necessary to comprehensively investigate the various indicators that affect the TADF performance when combine TPE with TADF groups for AIE-type TADF materials, and a small ΔEST value is not a sufficient condition.