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Abstract Circularly polarized phosphorescence (CPP) is a spin-forbidden radiative process with chiroptical activity. The CPP mechanism is far from comprehensively understood, mainly due to the limited examples of efficient triplet emission from small chiral organic molecules with well-defined structures. Herein, a pair of chiral enantiomers R/S-BBTI is reported, featuring the highly distorted spiral ring-locked heteroaromatics with heavy iodine atoms. These chiral molecules emit NIR phosphorescence and exhibit considerable high dissymmetry factors up to 0.013 with an efficiency of 4.2% and a lifetime of 119 µs in dimethyl sulfoxide (DMSO) solution after ultraviolet irradiation. Their crystals show efficient CPP with 7.0% quantum efficiency and a lifetime of 166 µs. Extensive experimental chiroptical investigations combined with theoretical calculations reveal an efficient spin-flip process that modulates the electron and magnetic transition dipole moments to enhance CPP performance. Moreover, the phosphorescence of R/S-BBTI is oxygen-sensitive and photoactivated in DMSO. Therefore, R/S-BBTI can be applied for hypoxia imaging in cells and tumors, expanding the scope of CPP applications.
Tang et al. (Mon,) studied this question.