Impact of Polymer Matrix on Polymer Mechanochromism from Excited State Intramolecular Proton Transfer†

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Abstract
Comprehensive Summary: Mechanochromic polymers based on non‐covalent changes have attracted much attention recently. Herein, we report the impact of inter/intramolecular hydrogen bonds on polymer mechanochromism from the excited state intramolecular proton transfer (ESIPT) process. PhMz‐NH2‐OH and PhMz=2A are designed and obtained by simple and high‐yield synthesis, and are connected into polyurethane and poly(methyl acrylate‐co‐2‐ethylhexyl acrylate), respectively. In the initial state, the PhMz‐NH2‐OH@PU sample shows blue fluorescence from the excited enol form (E*) excitons, owing to intermolecular hydrogen bonds that interrupt the ESIPT reactions but the PhMz=2A@PMA‐2‐EA sample expresses cyan fluorescence belonging to the excited keto form (K*) emission, implying that the intramolecular hydrogen bonds matter. Furthermore, under stretching, external force can tune the emission of the PhMz=2A@PMA‐2‐EA sample from K* to E* state. Though external force can putatively still promote a bond rotation, ESIPT reactions remain equivalently interrupted in both the relaxed and stressed states in a hydrogen‐bond donating environment. DFT calculation confirms the force‐induced increase in dihedral angle for the transition of ESIPT‐on/off. Thus, PhMz‐NH2‐OH@PU and PhMz=2A@PMA‐2‐EA showed disparate initial ESIPT states and further different responses/sensitivity to force. This study reports a novel and efficient strategy for enriching mechanochromic investigation and extending the applications of ESIPT reactions.
Funding Information
  • Natural Science Foundation of Beijing Municipality (2182054)
  • Fundamental Research Funds for the Central Universities at Beijing University of Chemical Technology (XK180301)
  • National Natural Science Foundation of China (21704002, 22175015, 22375013)

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