Molecular connectivity-driven tuning of aggregation-induced emission and mechanoresponsive properties in fused TPE-carbazole materials.

Precise control of solid-state fluorescence through molecular design is central to the development of advanced functional materials for sensing, imaging, and optical information storage. In particular, aggregation-induced emission (AIE) and mechanochromic luminogens have emerged as promising platforms for stimuli-responsive photonic applications. Herein, four constitutional isomers of carbonyl-bridged tetraphenylethylene (TPE)-carbazole derivatives were synthesised to systematically investigate how molecular connectivity governs their optoelectronic, aggregation, and mechanoresponsive properties. Distinct structure-property relationships were established, demonstrating that subtle variations in connectivity produce significant differences in electronic structure, molecular packing, aggregation-induced emission, and mechanochromic behaviour. Compounds 1, 3, and 4 exhibit AIE characteristics, whereas compound 2 displays aggregation-caused quenching-like behaviour arising from differences in accessible conformational and solid-state packing states. Among the four isomers, compounds 1 and 2 exhibit mechanochromism, with compound 1 showing highly reversible fluorescence switching, while compound 2 undergoes an irreversible pressure-induced transition associated with a metastable amorphous phase. Fluorescence microscopy revealed an exceptionally low pressure-sensing threshold of 0.2 MPa for compound 1 deposited on paper substrates, enabling sensitive detection of gentle mechanical stimuli and reversible invisible-ink write-erase cycles. These findings establish constitutional isomerism as an effective strategy for controlling aggregation, solid-state organisation, pressure sensitivity, and mechanochromic reversibility, providing molecular design principles for next-generation low-force pressure sensors and stimulus-responsive organic materials.