Prolonging the phosphorescence lifetime of pure organic phosphorescent materials by a methyl-substitution strategy is described. We present a chemical strategy for improving the phosphorescence lifetime of triplet excitons under ambient conditions by incorporating methyl groups into the chemical structures. This is observed by a long-lived phosphorescence lifetime of up to 0.83 s detected for methylated 9-(4-(mesitylsulfonyl)phenyl)-9H-carbazole (>3M), compared to 0.36 s for 9-(4-(phenylsulfonyl)phenyl)-9H-carbazole (>0M) without any methyl groups. Additionally, enhanced phosphorescence efficiency can be obtained at an appropriate methylation degree, because of the smaller ΔEST (singlet and triplet energy gap) and ΔETT* (normal phosphorescence and long-lived phosphorescence energy gap). A comprehensive investigation on the packing mode in the crystalline state reveals that the methyl groups occupy the free volume and result in a suppression of non-radiative decay, accounting for the enhanced phosphorescence lifetime.
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