Theoretical Insights into Halogen Substitution Effects on Room Temperature Phosphorescence in Twisted Halogenated Tetraphenylene Derivatives.

Abstract

The introduction of heavy atoms, halogen atoms and heteroatoms into organic room-temperature phosphorescence (RTP) molecules can effectively enhance the spin-orbit coupling (SOC) effect. However, this strategy often simultaneously accelerates both radiative and nonradiative decay rates, significantly reducing the RTP efficiency and lifetime. As a wise molecular design strategy, the synergistic effect between twisted molecular conformations and halogen substitution can overcome the limitations. Therefore, this study aims to provide a theoretical elucidation of the regulatory mechanism underlying this synergistic effect on RTP performance. Building upon this foundation, we employ twisted tetraphenylene (TeP) and its halogenated derivatives (TeP-F, TeP-Cl, and TeP-Br) as model to theoretically investigate their excited-state properties and luminescence mechanisms based on first-principles calculations. Results indicate that, compared with the molecule in solvent, the solid state effect in crystal significantly inhibits the molecular geometry changes, leading to reduced nonradiative decay rates, both RTP efficiency and lifetime are increased. In addition, the increase in the atomic number of halogen substitutions increases the degree of molecular distortion that changes the distribution of electrons within the molecule, increasing the proportion of n-π* transitions, which in turn leads to an enhanced SOC effect and an increased intersystem crossing (ISC) rate. However, heavy halogen atoms excessively enhance the SOC effect and increase the reorganization energy, thereby accelerating the nonradiative decay rate. Consequently, both the lifetime and efficiency do not monotonically increase with the atomic number of halogen substituents. Among these systems, TeP-F molecule is verified to possess high efficiency and long lifetime primarily due to its moderate SOC strength and low nonradiative decay rate. Furthermore, to explore the influence of the number of halogen substitutions, we theoretically construct two new molecular aggregates (difluoro-substituted TeP-2F and tetrafluoro-substituted TeP-4F) using atomic replacement in the TeP-F crystal framework for ONIOM models. The results indicate that the SOC effect is enhanced compared with that of TeP-F, while the RTP performance is not improved due to the remarkably increased nonradiative decay rate. This study theoretically elucidates the influence of the type and number of halogen substitutions on the RTP performance of twisted halogenated TeP derivatives, providing important theoretical insights for the design of efficient, long-persistent pure organic RTP materials.