Designing Efficient and Ultralong Pure Organic Room-Temperature Phosphorescent Materials by Structural Isomerism

Pure organic materials with ultralong room-temperature phosphorescence (RTP) are attractive alternatives to inorganic phosphors. However, they generally show inefficient intersystem crossing (ISC) owing to weak spin-orbit coupling (SOC). A design principle based on the realization of small energy gap between the lowest singlet and triplet states (Delta E-ST) and pure pi pi* configuration of the lowest triplet state (T-1) via structural isomerism was used to obtain efficient and ultralong RTP materials. The meta isomer of carbazole-substituted methyl benzoate exhibits an ultralong lifetime of 795.0 ms with a quantum yield of 2.1%. Study of the structure-property relationship shows that the varied steric and conjugation effects imposed by ester substituent at different positions are responsible for the small Delta E-ST and pure pi pi* configuration of T-1.