Red-Emissive Organic Crystals of a Single-Benzene Molecule: Elastically Bendable and Flexible Optical Waveguide

Organic crystals are easily cracked into pieces or powders under applied stress because of their intrinsic brittle nature. This undesired mechanical property directly limits their application in flexible optical and optoelectronic devices. Herein, we developed a compact single-benzene molecule dimethyl 2,5-bis((2-hydroxyethyl)amino)terephthalate, which was easily crystallized to form two polymorphs, A and B. Featuring a single-benzene pi-system, both polymorphs A and B display red fluorescence in crystals. More importantly, crystals of polymorph A are flexible and can be elastically bent under mechanical force. Given these advantages, a flexible optical waveguide has been realized in the crystal of polymorph A with a bent shape, highlighting its potential application in flexible devices. In addition, the thermal transformation of crystals from polymorph A to polymorph B, which was accompanied by the change of optical property as well as mechanical elasticity, has been observed.