Distinctly Diverse PLQY and Inverse Solid-State Luminescent Properties in Structure-Similar Diphenyl Sulfone TADF Molecules: A Role of C-C

Thermally activated delayed fluorescence (TADF) materials can greatly increase internal quantum efficiencies (IQEs) of organic light-emitting diodes (OLEDs) beyond the spin statistical bottleneck of 25%. However, the mechanisms behind TADF and solid-state luminescence are not clear. Recently, two diphenyl sulfone TADF molecules with quite similar structures, A1-D1 and A2-D1, have been observed to demonstrate distinctly diverse photoluminescence quantum yield (PLQY) and inverse solid-state luminescent properties. In order to understand the origin of this phenomenon, the TADF mechanism of both molecules is studied using density function theory. The results show that strong spin-orbit coupling, low minimum energy cross point, dense triplet energy level distribution, and a mediate locally excited triplet state ((LE)-L-3) can facilitate the rapidly reverse intersystem crossing (RISC) process in A1-D1. Moreover, the solid states of both molecules are characterized via the combined quantum mechanics and molecular mechanics (QM/MM) method. It is attributed to the compact intermolecular stacking; the nonradiative consumption of A2-D1 is suppressed substantially by the restricted intermolecular rotation (RIR) effect. Therefore, A2-D1 with low PLQY can exhibit an aggregation-induced emission (AIE) phenomenon in neat films. This work may benefit the rational design of nondoped OLEDs.