Organic Micro-/Nanocrystals of SFX-Based Attractor–Repulsor Molecules with the Feature of Crystal-Induced Luminescence Enhancement

Spirofluorenexanthanes
(SFXs), as burgeoning second-generation
spiro compounds beyond spirobifluorenes (SBFs), can serve as versatile
scaffolds for low-cost organic semiconductors in various fluorescent,
phosphorescent and delayed fluorescent emitters, hole/electron transport
materials, charge-trapping elements, and photovoltaic donor/acceptors.
However, the detailed effects of structural factors on the diverse
aggregate-dependent behaviors are still required for high-performance
material designs to illustrate the mechanisms and advantages that
are related to not only the nature of a single molecular state but
also the aggregation states. Herein, we focus on attractor–repulsor
molecules of four SFX-based hole-transport materials including SFX-2-Cz,
SFX-2,7-DCz, SFX-2-DPA, and SFX-2,7-DDPA (where Cz is carbazole and
DPA is diphenylamine) to examine the nanocrystallization effect (distinguished
from amorphous states) on their excitonic photophysics. Both synthesized
via the C–N coupling reaction, SFX-2-DPA can form one-dimensional
microrods, whereas SFX-2,7-DDPA can be transformed into two-dimensional
nanosheets. Furthermore, the photoluminescence quantum yield (PLQY)
increases were observed to be ∼530% for SFX-2,7-DDPA (∼410%
for SFX-2,7-DCz) and ∼58% for SFX-2-DPA (∼210% for SFX-2-Cz)
in micro-/nanocrystal films compared with that of the amorphous films.
In addition, the 2D nanosheets of SFX-2,7-DDPA exhibited high fluorescence
quantum yields (∼57%) as a whispering-gallery-mode (WGM) microcavity,
reaching a laser threshold of 154.5 mW/cm2. In summary,
these results demonstrate that SFXs are the key building blocks for
the rational attractor–repulsor design of organic nanocrystalline
semiconductors with crystal-induced luminescence or stability enhancement
(CLoSE) features.