Polymorphism-Dependent Enhanced Emission in Molecular Aggregates: J-Aggregate versus X-Aggregate.

The photoluminescence properties of organic fluorescent materials in the solid state are closely related to their aggregation structures. Herein, we present an organic fluorescent molecule 9,10-bis(2,2-di(4-fluorophenyl)vinyl)anthracene (BDFVA) with two crystal polymorphisms to provide an ideal molecular model for understanding the nature of luminescence from the J- and X-aggregates. Detailed structural and photophysical studies reveal that both J- and X-aggregate crystals exhibit enhanced emission in comparison with the corresponding dilute solution because of crystallization-induced emission enhancement (CIEE). Additionally, the X-aggregate (B-phase) crystal with crossed molecular packing exhibits a higher fluorescence efficiency (Φ = 0.60), giving a 1.4-fold promotion of Φ compared to the J-aggregate crystal (G-phase) with staggered molecular packing because the split excited states are all optically allowed in the X-aggregate. Optical waveguide experiments show that the two crystals exhibit excellent amplified spontaneous emission (ASE), demonstrating a promising potential application in organic solid-state lasers.