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

An ultrasensitive concentration-dependent ESIPT mechanochromic mechanophore (MM) NaMz (4-amino-2-(1H-naphtho[2,3-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.