Multicolor Polymer Mechanochromism from an Ultrasensitive Concentration‐Dependent Excited State Intramolecular Proton Transfer Mechanophore

Abstract An ultrasensitive concentration‐dependent ESIPT mechanochromic mechanophore (MM) NaMz (4‐amino‐2‐(1 H ‐naphtho2,3‐ d imidazol‐2‐yl)phenol), which exhibits real‐time and reversible multicolor mechanochromism in polyurethane is reported. NaMz has one single link point and can be obtained facilely by one‐step reaction. As the concentration of NaMz increased from 0.01 to 0.05 mol%, the intensity ratio IE/IK of enol emission to keto emission changed significantly and concentration‐dependent multicolor fluorescence (yellow, white, blue) can be realized. Then, a simple strategy, force‐tuning the aggregation state of NaMz molecules, to adjust the ESIPT process to achieve ultrasensitive colorful mechanochromism is proposed. Intriguingly, force‐tuned white light emitting can be realized for 0.035 mol%NaMz@PU. Because the extended conjugation structure, NaMz@PU shows a red‐shifted keto emission at 541 nm with a larger Stoke shift compared with PhMz‐NH2@PU, which is also the key to multicolor concentration‐dependent polymer mechanochromism of NaMz@PU. Control experiments of NaMz‐OH@PU and NaMz‐3OH@PU show that the aliphatic hydroxyl group on the naphthimidazole moiety impedes the planarization of molecular structure and thus hinders aggregation, further verifying that the multicolor concentration‐dependent mechanochromism of NaMz@PU is indeed caused by force induced disaggregation. The excellent color‐changing effect, real‐time reversible force‐optical coupling and precise regulation of this study will make a new contribution to polymer mechanochemistry.