Two-dimensional conjugated polymers with weakened interlayer interaction for highly sensitive visible responsiveness to compression stresses

Detection of mechanical stresses requires microscopic molecular motion triggered by macroscopically applied force. Polydiacetylene (PDA), a stimuli-responsive color-changing material, generally has no responsiveness to compression stresses. We found that a layered PDA with weakened interlayer interactions exhibits direct visible color changes in response to compression stresses. An amphiphilic diacetylene (DA) monomer, 1-(10,12-pentacosadiynyl) pyridinium bromide (PCPy⁺Br⁻), formed a lamellar structure with weakened interlayer and intermolecular interactions originating from the bulky cations and anions in the interlayer space. The resultant PDA exhibited blue-to-red color changes in response to compression stresses (P) in the range of 2.5-125 MPa. In previous works, visible detection of P < 50 MPa without excitation light has not been achieved using only mechanoresponsive materials. Moreover, the compression-stress distribution was visualized using the PDA-coated paper substrate.