Effect of intermolecular interaction on excited-state properties of thermally activated delayed fluorescence molecules in solid phase: A QM/MM study

Recently, thermally activated delayed fluorescence (TADF) molecules have attracted great attention since nearly 100% exciton usage efficiency was obtained in TADF molecules. Most TADF molecules used in organic light emitting diodes are in aggregation state, so it is necessary to make out the intermolecular interaction on their photophysical properties. In this work, the excited-state properties of the molecule Al-Cz in solid phase are theoretically studied by the combined quantum mechanics and molecular mechanics (QM/MM) method. Our results show that geometry changes between the ground state (S-0) and the first singlet excited state (S-1) are limited due to the intermolecular pi-pi and CH-pi interactions. The energy gap between S-1 and the first triplet excited state is broadened and the transition properties of excited states are changed. Moreover, the Huang-Rhys factors and the reorganization energy between S-0 and S-1 are decreased in solid phase, because the vibration modes and rotations are hindered by intermolecular interaction. The non-radiative rate has a large decrease in solid phase which improves the light-emitting performance of the molecule. Our calculation provides a reasonable explanation for experimental measurements and highlights the effect of intermolecular interaction on excited-states properties of TADF molecules. (C) 2018 Elsevier B.V. All rights reserved.