Abstract
All-organic thermally activated delayed fluorescence (TADF) materials have emerged
as potential candidates for optoelectronic devices and biomedical applications. However,
the development of organic TADF probes with strong emission in the longer wavelength
region (> 600 nm) remains a challenge. Strong π-conjugated rigid acceptor cores substituted
with multiple donor units can be a viable design strategy to obtain red TADF probes.
Herein, 3,6-di-t-butyl carbazole substituted to a dibenzopyridoquinoxaline acceptor core resulted
in a T-shaped donor–acceptor–donor compound, PQACz-T, exhibiting red TADF in polymer-embedded
thin-films. Further, PQACz-T self-assembled to molecular nanoaggregates of diverse
size and shape in THF–water mixture showing bright red emission along with delayed
fluorescence even in an aqueous environment. The self-assembly and the excited-state
properties of PQACz-T were compared with the nonalkylated analogue, PQCz-T. The delayed
fluorescence in nanoaggregates was attributed to the high rate of reverse intersystem
crossing. Moreover, an aqueous dispersion of the smaller-sized, homogeneous distribution
of fluorescent nanoparticles was fabricated upon encapsulating PQACz-T in a triblock
copolymer, F-127. Cytocompatible polymer-encapsulated PQACz-T nanoparticles with large
Stokes shift and excellent photostability were demonstrated for the specific imaging
of lipid droplets in HeLa cells.
Key words
TADF - dibenzopyridoquinoxaline - nanoaggregates - fluorescence imaging - lipid droplets
