Luminescent Mechanochromic Sensors Based on Poly(vinylidene fluoride) and Excimer-Forming p-Phenylene Vinylene Dyes

Cyano-substituted excimer-forming oligo(phenylene vinylene) dyes (cyano-OPVs) with terminal alkyl tails of different length were blended with two fluorinated host polymers with similar chemical composition but differing crystallinity. These blends were used to fabricate luminogenic mechanochromic thin films, which change their emission color upon deformation. The alkyl tails affect the solubility of the chromophores in the polymer matrix and lead to different aggregation properties; this is of importance because the mechanochromic fluorescence color change of the blends is related to the self-assembly of the excimer-forming dye in the unperturbed polymer matrix, and the dispersion of the dye aggregates upon deformation. Besides the length of the solubilizing tails, the dye concentration has an important influence on the aggregate size, which is crucial to creating a mechanochromic response, since the dye aggregates must be small enough to be dispersed during the deformation process. In-situ opto-mechanical measurements have shown that the mechanochromic effect occurs primarily during plastic deformation and that the mechanically induced dispersion of the dye aggregates becomes more pronounced as the crystallinity of the matrix polymer increases.