Dipyrene-Terminated Oligosilanes Enable Ratiometric Fluorescence Response in Polymers toward Mechano- and Thermo-Stimuli

Developing fluorescent motifs capable of reversibly ratiometric indicating mechano- and thermo-stimuli are appealing in monitoring deformation or temperature which polymers are subjected to. Here, a series of excimer-type chromophore Sin-Py (n = 1‒3) consisting of two pyrenes linked with oligosilanes of one to three silicon atoms is developed as the fluorescent motif incorporated in polymer. The fluorescence of Sin-Py is steered with the linker length where Si2-Py and Si3-Py with disilane and trisilane linkers display prominent excimer emission accompanied with pyrene monomer emission. Covalent incorporation of Si2-Py and Si3-Py in polyurethane gives fluorescent polymers PU-Si2-Py and PU-Si3-Py, respectively, where intramolecular pyrene excimers and corresponding combined emission of excimer and monomer are obtained. Polymer films of PU-Si2-Py and PU-Si3-Py display instant and reversible ratiometric fluorescence change during uniaxial tensile test. The mechanochromic response arises from the reversible suppression of excimer formation during mechanically induced separation of the pyrene moieties and relaxation. Furthermore, PU-Si2-Py and PU-Si3-Py show thermochromic response toward temperature and the inflection point from the ratiometric emission as a function of temperature gives indication of glass transition temperature (Tg) of the polymers. The design of excimer-based mechanophore with oligosilane provides a generally implementable way to develop of mechano- and thermo- dual responsive polymers.