Stretching-enhanced emission behavior of polyurethane composites containing pyrene derivatives

A series of mechanoresponsive fluorescence composite films were prepared by doping pyrene (Py) and its derivatives including 1-hydroxypyrene (HPY), 1-pyrenemethanol (PYM) and 1-pyrenylboronic acid (PBA) into thermoplastic polyurethane (PU) via physical blending, respectively. The fluorescence property of the composite films was found to be influenced by the polar group of pyrene derivatives and the content of hard segment of PU. The doped PYM showed a stronger trend to aggregate in PU matrixes, relative to other dyes. Contrary to PU/Py and most of previously reported mechanoresponsive materials, the emission intensity of aggregated PYM molecules in both PU25/PYM and PU50/PYM films exhibited a stretching-enhanced-emission feature. When PU25/PYM was stretched up to 500%, the intensity ratio of its dual emission peaks at 490 and 400 nm increased from 2.3 to 5.3. This interesting mechanoresponsive behavior was related to the formation of strong hydrogen bonding between PYM and hard segment of PU matrixes proved by FT-IR spectrograph. Enhancing instead of reducing the emission intensity of dyes through stretching would be remarkable in the field of mechanoresponsive materials.