Effects of a long-short axis skeleton on the excited-state properties of ultraviolet hot exciton molecules: luminescence mechanism and molecular design

Wise design strategies for efficient ultraviolet (UV) hot exciton molecules are highly desired. In this work, inspired by the long-short axis skeleton strategy, a theoretical study on the substituent effect of the long-axis on the photophysical properties of UV hot exciton molecules is performed. A multiscale simulation is performed to study the photophysical properties of the reported compound 2BuCz-CNCz and theoretically designed promising compounds 2Cz-CNCz, 2TPA-CNCz2TPA-CNCz, 2Na-CNCz and 2An-CNCz, which all possess unique features of UV emission and hot exciton properties. The packing modes of the five molecules in a film are obtained by molecular dynamics (MD) simulations, and then the photophysical properties with the consideration of the SSE (solid-state effect) are studied by using the combined quantum mechanics and molecular mechanics (QM/MM) method. Finally, the exciton evolution process is revealed by the rate equations. The results show that different substituents in the long axis have relatively little effect on the larger twist angle in the short axis. The tert-butyl and triphenylamine groups increase the vibration of the molecule, enhance the non-radiative rate of the molecule and intensify the energy dissipation, but the vibration of tert-butyl can be greatly restrained in the solid state. Furthermore, our designed 2Na-CNCz compound possesses the maximum reverse intersystem crossing rate and radiative decay rate. Therefore, when studying the effect of the long-axis substituents on the properties of hot excitons, 2Na-CNCz could be a profitable candidate molecule. This work should enrich the theoretical calculation methods to investigate the luminescence properties of organic molecules in OLEDs.