Highly efficient circularly polarized near-infrared phosphorescence in both solution and aggregate

Abstract

Circularly polarized phosphorescence (CPP) is a spin-forbidden radiative process, and its underlying mechanism is not comprehensively understood, mainly due to the limited examples of efficient triplet emission from small chiral organic molecules with well-defined structures. Here we investigate a pair of chiral enantiomers, R- and S-BBTI, that feature highly distorted spiral ring-locked heteroaromatics with heavy iodine atoms. These chiral molecules are found to exhibit large dissymmetry factors up to 0.013 and emit near-infrared CPP with an efficiency of 4.2% and a lifetime of 119 μs in dimethyl sulfoxide solution excited by 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 photoactivatable in dimethyl sulfoxide. Therefore, R/S-BBTI can be applied for hypoxia imaging in cells and tumours, expanding the application scope of CPP materials.