Highly Emissive Hierarchical Uniform Dialkylfluorene-Based Dimer Microcrystals for Ultraviolet Organic Laser

The hierarchically uniform condensed structure of light-emitting
conjugated molecules is an effective platform to obtain an excellent
electronic landscape with little physical defect and further improve
the photophysical property. Herein, a highly emissive dialkylfluorene-based
dimer crystal structure is constructed with crystalline-enhanced emission
for ultraviolet organic microlasers. Instable planar conformation
of a fluorene dimer can be stabilized in a rigid crystalline framework,
which is a fundamental factor to fabricate the hierarchically uniform
structure (including planar conformational molecules and periodical
molecular packing in the crystalline structure). Therefore, a high
photoluminescence quantum efficiency of 50 and 75% is obtained for
neat spin-coated films and single crystals, respectively, attributing
to the little physical defect in this hierarchical oriented structure.
This robust crystalline-enhanced emission enables their microcrystals
to construct ultraviolet organic lasers (395 nm) with a low threshold
of 7.65 × 10–3 nJ cm–2 pulse–1, which is 4 times lower than that of the controlled
trimer one (TDMeF). These observations reasonably confirm that the
hierarchically ordered architecture of conjugated materials shows
great promise in optoelectronic applications.