An Optical Waveguiding Organic Crystal with Phase-Dependent Elasticity and Thermoplasticity over Wide Temperature Ranges

Abstract

Elastic or plastic bendable organic crystals have attracted increasing attention in the field of crystal engineering. For the application of flexible materials, the applicable temperature range can not be ignored. However, studies on the flexible organic crystals reported so far have not involved the effect of temperature on the mechanical properties of these materials. Here, organic crystals of 9,10-bis(pheny-lethynyl)anthracene with phase-dependent mechanical properties over wide temperature ranges are reported. Phase A displays elastic bending ability under external stress in the temperature range of −196 to 150 °C. Phase B obtained from A through a heating-induced single-crystal-to-single-crystal (SCTSC) transition exhibits thermoplastic behavior (bendable at 120–240 °C): once heated above 120 °C, its shape was arbitrarily changeable. The newly formed shape could be preserved after cooling below 120 °C and the crystal could be reshaped when reheated over 120 °C. This unique property allows the organic crystals of phase B to be thermally processed into the desired shapes. Furthermore, the potential application in flexible optical devices was preliminarily evaluated by measuring the optical waveguide of the bent crystals. This study not only expands the crystal flexibility to thermoplastic bending, but also provides a strategy for tuning the mechanical property of organic single crystals by temperature-controlled phase transition.