Enhanced mechano-responsive luminescence in polyurethanes by supramolecular mechanophores based on synergy between quadruple H-bonding and π–π stacking

Abstract

Mechano-responsive luminescent (MRL) polymers possessing low thresholds, high contrast and good reversibility are urgently needed in fields that detect and visualize weak forces occurring in polymeric materials. Herein, a supramolecular fluorescent mechanophore UPy-PDI(–OH)-UPy employing perylene diimide (PDI) that is capable of forming intermolecular excimers as luminescent moieties and 2-urea-4[1H]-pyrimidinone (UPy) with quadruple H-bonding as the spatial constraint group is presented. MRL polyurethanes (PUs) were constructed by the integration of mechanophores into PU molecules. The MRL performance was determined by the mechanophore structures and the mechano-transduction process, which can be efficiently improved by the synergy between quadruple H-bonding and π–π stacking, and chemical bonding of the mechanophores as dynamic supramolecular chain extenders into the PU backbones. Compared with the PUs with mechanophores only based on π–π stacking or physical doping, polymers with UPy-PDI(–OH)-UPy showed enhanced sensitivities by 7.4-fold and 3.5-fold, respectively. A perceptible change from orange-yellow to yellow-green under 0–500% strain, with reversibility maintained over more than 20 cycles, was observed. The current work provides a feasible strategy for improving the MRL performance of PU polymers.