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 (ΦF = 0.60), giving a 1.4-fold promotion
of ΦF 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.