Crystallization-Induced Red Phosphorescence and Grinding-Induced Blue-Shifted Emission of a Benzobis(1,2,5-thiadiazole)–Thiophene Conjugate

Mechanochromic
luminogens are of significant importance in both
academic and technical aspects. Thus far, most mechanochromic compounds
exhibit bathochromically shifted emission upon grinding; the examples
of those that exhibit blue-shifted emission still remain limited.
Herein, a donor–acceptor–donor (D–A–D)-structured
conjugate, namely 4,7-di­(2-thienyl)-2,1,3-benzothiadiazole (DTBT),
comprising benzobis­(1,2,5-thiadiazole) and thiophene units, has been
carefully synthesized and investigated. DTBT exhibits typical intramolecular
charge transfer (ICT) characteristics, crystallization-induced phosphorescence
(CIP), and remarkable mechanochromism. Although it merely emits fluorescence
in solutions with distinct ICT features, its crystals demonstrate
bright-red room-temperature phosphorescence (616 nm) with efficiency
up to 25.0% and generate yellow excimer fluorescence (578 nm) upon
mechanical grinding, accompanying decreased lifetimes from 10.9 μs
to 3.5 ns and a blue-shifted emission of 38 nm. These results highly
indicate the feasibility to fabricate novel CIP luminogens with blue-shifted
mechanochromism.