Theoretical perspective for internal quantum efficiency of thermally activated delayed fluorescence emitter in solid phase: A QM/MM study

Excited state dynamics and charge transfer property of an orange-red thermally activated delayed fluorescence (TADF) emitter are theoretically investigated by a quantum mechanics/molecular mechanics (QM/MM) method and kinetic Monte Carlo simulation. The factors that influence internal quantum efficiency of the organic light-emitting diode (OLED) based on an asymmetric donor-acceptor (D-A) type molecule 10-(7-fluoro-2,3-diphenylquinoxalin-6-yl)-10H-phenoxazine (FDQPXZ) are analyzed. The results show that the intramolecular rotation of donor unit is restricted because of the enhanced intermolecular interaction in solid phase, which hinders the non-radiative consumption of the excited state energy. The decreased reorganization energy in solid phase is mainly contributed by dihedral angle in low-frequency (<500 cm(-1)) region. Moreover, the non-radiative decay rate from the first singlet excited state (S1) to the ground state (S0) in solid phase is shown to be smaller than that in gas phase. In order to explore the charge transfer process in the film of FDQPXZ, Marcus theory is used to study the hole and electron transfer rates, and the charge mobility is thus obtained by Monte Carlo simulation. The theoretical calculation indicates that the FDQPXZ film is a p-type organic molecular material under the hopping mechanism. Intermolecular interaction for theoretical simulation of the working principle of OLEDs is highlighted. (C) 2017 Published by Elsevier B.V.