Theoretical perspective for molecular folding degree and heavy atom effect on room temperature phosphorescence properties

Since room temperature phosphorescence (RTP) molecules typically exhibit small spin–orbit coupling (SOC) effect and a rapid non-radiative decay process, achieving efficient RTP emission is challenging. Therefore, it is imperative to enhance the SOC constant to facilitate efficient RTP and related structure–property relationship needs to be clarified. Herein, based on first-principle calculations, this paper elucidates the impact of molecular folding structure on SOC and RTP efficiency, excited state properties and exciton conversion processes are detailed studied. Results indicate that both the heavy atom effect and folding degree affect the SOC constants and the latter factor plays a dominate role than the former for studied systems. In addition, the intermolecular interactions can inhibit the geometric changes, decrease the reorganisation energies and increase transition dipole moment, thus the radiative decay rates are increased and non-radiative decay rates are decreased, efficient RTP emissions are realised for TA and PX molecules, no emission feature is determined for DX, previous experimental measurements are reasonably illustrated. These findings reveal the inner relationship among molecular folding structure, environmental effects and RTP performance. This study offers a theoretical perspective for elucidating the mechanism underlying fold-induced RTP enhancement, which provides an innovative molecular design strategy for developing efficient RTP emitters.