π-Spacer Engineering: Driving Near-infrared Aggregation Induced-emission and Mechanofluorochromism in Carbazole-biscyanostilbenes

In this study, we report the design and synthesis of two novel carbazole-based bis-cyanostilbenes incorporating phenyl and thiophene π-spacer units to investigate their distinct impacts on photophysical properties. Notably, the thiophene-based derivative exhibits remarkable far-red/near-infrared (NIR) solid-state emission, with an emission peak at 732 nm, which shifts to 750 nm upon mechanical grinding, demonstrating pronounced mechanochromic fluorescence in the NIR region. Although its quantum yield is moderate, the ability to modulate its emission through mechanical stimuli opens exciting opportunities for stimuli-responsive NIR applications. Conversely, the phenyl-based analogue shows excellent solid-state emission at 596 nm, achieving significantly higher quantum yields, due to suppressing π-π interactions. Both compounds also exhibit strong AIE, with the thiophene system emitting at 730 nm and the phenyl-based analogue at 580 nm in the aggregate state. In addition to their optical properties, both derivatives demonstrate remarkable thermal stability and reversible MFC. These intriguing behaviors highlight the critical role of π-spacer engineering in fine-tuning solid-state emission, enhancing stimuli-responsive capabilities, and ensuring robust thermal performance. Overall, our findings provide valuable insights into the design of next-generation NIR-emissive materials, with promising potential for advancing applications in optoelectronics, bioimaging, and smart sensing technologies.