Structural Isomerization Effect on the Triplet Energy Consumption Process of Organic Room-Temperature Phosphorescence Molecules: A QM/MM Study

Organic room-temperature phosphorescence (RTP) materials with long lifetimes and high efficiency have attracted great attention in recent studies. Structural isomerism with ester substituents at different positions could intrinsically influence the luminescence efficiency and operational lifetime of RTP molecules. A theoretical study to reveal the intrinsic structure-property relationship is highly desired. Herein, based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT), the geometric and electronic properties of three isomers (o-MCBA, m-MCBA, and p-MCBA proposed by Tang) are investigated. Furthermore, the Huang-Rhys factor and reorganization energy are analyzed, and exciton dynamic processes, such as the intersystem crossing (ISC) process and three decay channels for the energy consumption process of the first triplet excited state (T-1) based on the thermal vibration correlation function (TVCF) method, are discussed in detail. The results show that intermolecular interactions can restrict the rotation motions of the dihedral angle and the vibration motions of the bond angle for o-MCBA and m-MCBA. In addition, decreased Huang-Rhys factor and reorganization energy are found and a hindered nonradiative consumption process is determined. For p-MCBA in the solid phase, the rotation motions are partly restricted by the solid-state effect and the vibration motions of the bond length are effectively promoted by intermolecular H-bond interactions. In addition, the spin-orbit coupling (SOC) effect is enhanced by the solid-state effect, which is helpful to facilitate the ISC process. Through this study, we pursue opportunities to detect the relationship between basic molecular structures and RTP properties, which could take advantage of the unique molecular design to develop high-performance emitting molecules.