Color-Tunable and ESIPT-Inspired Solid Fluorophores Based on Benzothiazole Derivatives: Aggregation-Induced Emission, Strong Solvatochromic Effect, and White Light Emission

Organic
solid materials with color-tunable emissions have been
extensively applied in various fields. However, a rational design
and facile synthesis of an ideal fluorophore are still challenging
due to the undesirable aggregation-caused quenching effect in concentrated
solution and solid form. Herein, we have developed a series of 2-(2′-hydroxyphenyl)­benzothiazole
(HBT)-derived color-tunable solid emitters by switching
functional groups at the ortho-position of a hydroxyl group via formylation
and an aldol condensation reaction. By tuning the electron-withdrawing
ability and the π-conjugated framework introduced by the functional
groups, fluorophores emit light covering the full-color range from
blue to near-infrared regions with high quantum yields in their solid
form and show a significant solvatochromic effect in polar solvents.
The aggregation-induced emission (AIE) or aggregation-induced emission
enhancement (AIEE) and excited-state intramolecular proton transfer
(ESIPT) involving fluorescence mechanism, along with their inter/intramolecular
interactions in crystals, are elucidated to depict the key factors
for tunable emissions and high emitting efficiency. Furthermore, high-quality
white-light-emitting materials are obtained in various solvents and
polydimethylsiloxane (PDMS) films with combined fluorophores. Overall,
these studies report a promising strategy for the construction of
organic solid materials with color-tunable emission and shed light
on methods for obtaining desirable emission efficiency.