Hinge-Like Mechanochromic Mechanophores Based on [2.2]Paracyclophane

A hinge-like supramolecular mechanophore based on a [2.2]paracyclophane core and two excimer-forming 1,6-bis(phenylethynyl)pyrene luminophores is presented. Each luminophore shares one phenyl group with the [2.2]paracyclophane, resulting in a rigid and strained structure that forces the two luminophores into close proximity. As a consequence, the photoluminescence of the mechanophore in THF solution and in solid films of a polyurethane containing the new motif is dominated by excimer emission. Stretching the polymer films causes an easily discernible change from bright yellow excimer to blue-green monomer-dominated emission. The ratio of excimer to monomer emission intensities traces the nonlinear stress-strain curves of the polymer well and is a good indicator for the macroscopically applied force. The reversibility of the mechanoresponse, theoretical analyses, and reference experiments with a similar mechanophore in which the emitters and the [2.2]paracyclophane core are connected by flexible linkers support the conclusion that the mechanoactivation is caused by distorting the molecule into a bent, more open conformation and not the scission of covalent bonds. The operating principle was further confirmed by investigating a second hinge-like mechanophore based on a [2.2]paracyclophane core and 1,4-bis(phenylethynyl)benzene emitters.