Bright persistent luminescence from pure organic molecules through a moderate intermolecular heavy atom effect
Generally, strong phosphorescence always implies a short luminescence lifetime. So, pure organic molecules with strong persistent phosphorescence at room temperature are rarely explored. Here, a new strategy is used to explore strong persistent phosphorescence of carbazole derivatives through an intermolecular moderate heavy atom effect. Flexible alkyl chains were inserted between the heavy atom and carbazole moieties so as to avoid a strong internal heavy atom affect. The results indicate that the photoluminescence properties of crystals strongly depend on molecular stacking in crystal states. Moreover, seven molecules among eight carbazole derivatives exhibit persistent room-temperature phosphorescence in their crystals and the ΦP values of three compounds exceed 9.5%. Notably, when a 6-(4-bromophenoxy)hexyl group is linked to carbazole, the ambient phosphorescent quantum yield reaches 39.5% and the lifetime is as long as 200 ms. Moreover, it emitted white light with a luminescent efficiency of 72.6% owing to the coexistence of strong fluorescence and phosphorescence.