Naturally and Elastically Bent Organic Polymorphs for Multifunctional Optical Applications

Recently, mechanically bendable organic single crystals have been widely studied as emerging flexible materials. However, only a very small percentage of organic crystals have been found to be elastic or plastic. In this study, crystal engineering is employed as a powerful strategy to improve the probability of constructing flexible organic crystals. Based on an organic compound, two polymorphs Cry-R and Cry-O with bright red and orange emissions, respectively, are obtained. Cry-R, being brittle inherently, can form a naturally bent crystal with an optical waveguide as efficient as the straight crystal. The other polymorph Cry-O can be elastically bent, almost into a loop, and displays an optical waveguide and amplified spontaneous emission in both the straight and bent state, demonstrating its multifunctional applications in flexible optical devices. In addition, the Y-shaped crystals of Cry-O obtained by natural growth are found to transduce single emitted light through the two branches and thus generate dual output signals simultaneously, which further highlights the utility of “crystal flexibility”. The results not only suggest a guideline to modify the mechanical compliance by crystal engineering but also provide a model of flexible organic crystals for multifunctional optical applications.