Multi-Stimuli Responsive Donor–Acceptor Tetraphenylethylene Substituted Benzothiadiazoles

In order to understand how the donor
(D)/acceptor (A) substituents
and their substitution pattern affect the solution and solid-state
optical properties, a series of symmetrical and unsymmetrical TPE
substituted BTDs 3–8 were designed
and synthesized by the Suzuki and Stille coupling reactions. Their
solvatochromic, aggregation induced emission (AIE), mechanochromic,
and vapochromic properties were studied and compared. The single-crystal
X-ray structures of BTDs 5 and 6 are reported.
The BTDs 3–8 are highly fluorescent
with the tunable emissions. The solvent dependent emission was observed
in BTDs 3–8 and their Lippert–Mataga
plots show a linear correlation of the Stokes shift with solvent polarity.
The emission study in different tetrahydrofuran (THF):water percentages
show enhanced emission in aggregates. The BTDs exhibit a reversible
multi-stimuli response toward mechanical force, solvent, and heat.
The detailed study using single-crystal X-ray, photophysical properties,
powder X-ray diffraction, scanning electron microscopy, and theoretical
calculation reveals that the planarization induced enhancement in
conjugation and conversion of crystalline to amorphous state are responsible
for mechanochromism and vapochromism. Our work has elucidated that
this general approach can be utilized to develop a promising class
of multi-stimuli responsive materials.