Functionalizing β-Cyano Oligo(p-phenylene vinylene) Chromophores for Use in Mechanochromic Materials

Mechanochromic materials are materials that change color upon mechanical deformation. This concept can be applied in areas of quality control, since the knowledge of damaged parts will easily be detected due to a color change in the material that has undergone deformation. Typically these types of materials are enabled by employing functional small molecules. Often, these are conjugated organic molecules or chromophores, which fluoresce different colors determined by the stimulus (or lack thereof) acting upon the material. In this work, we explore a new avenue to elicit color changes in polymeric materials based on the process of restriction of intramolecular rotations (RIR) phenomena in β-cyano oligo(p-phenylene vinylene) (cyano-OPVs) molecules. To develop the molecular structure-property relationships required for engineering stimuli-responsive polymers based on the cyano-OPV motif, this study systematically varied functional group placement and electronic characteristics on the cyano-OPV framework. A series of eight molecules was synthesized in which hydrogen, methyl, dimethyl, cyano, fluoro, and methoxy functionalities were placed onto the terminal phenyl rings. Structural characterization was accomplished using nuclear magnetic resonance (NMR) spectroscopy and the thermal characteristics of the molecules were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The absorption spectra were recorded using UV-VIS spectroscopy and molar absorptivities of the chromophores were calculated based on the Beer-Lambert relationship. Finally, the RIR for these molecules was investigated with the use of photoluminescence measurements performed on both the pure solids as well as solutions of the cyano-OPVs.