Mechanofluorochromic Elastomers Based on Reversible Helical Transition of Poly(phenylacetylene)s

Mechanofluorochromic (MFC) materials possessing both high sensitivity and excellent reversibility are crucial for achieving real-time mechanical sensing and biomedical probing. Nonetheless, it currently faces the challenge of possessing both critical properties at excellent levels. Herein, we have proposed a reversible polymer mechanochromic mechanophore utilizing a novel mechanism that enables "turn-off" fluorescence, triggered by a force-induced helical conformational transition in poly(phenylacetylene) (PPA) from the fluorescent compact cis-cisoid (cc) to the non-fluorescent loose cis-transoid (ct) conformation. Upon release, the restoring force, coupled with the reformation of intramolecular hydrogen bonds, drives the helical backbone to revert to cc conformation, thus enabling reversible mechanofluorochromism. A series of helical PPAs featuring vinyl groups underwent covalent crosslinking with polydimethylsiloxane (PDMS), yielding a range of MFC elastomers. We developed an in situ stretching-fluorescence testing device to achieve real-time monitoring of force-dependent photoluminescence (PL) spectra. By adjusting the crosslinking density, Vi-CP-0.01@PDMS-0.003 exhibited the highest MFC sensitivity. Significantly, the novel MFC elastomer showcases outstanding reversibility, maintaining its performance across 10 cycles. This study demonstrates a novel MFC mechanism that produces elastomers with high sensitivity and excellent reversibility, paving the way for further MFC materials development based on innovative mechanism.