Flexible Organic Crystalline Fibers and Loops with Strong Second Harmonic Generation

Flexible organic crystals represent a novel class of smart materials that open many opportunities for optical applications. While it has been established that elastic or plastic deformation of slender molecular crystals can be commonly induced by external intervention, crystals that grow in bent or curled shapes naturally are rarely reported. This study introduces an extraordinarily flexible organic crystalline fibrous material, (Z)-3-(2,3-dichloropyridin-4-yl)-2-(3,5-dimethylphenyl)acrylonitrile (DPA), that crystallizes both as straight and curled crystals. Crystals of DPA are easily obtained from solution either as long fibers or as crystals that are curled to various extent, and sometimes even closed into a loop. The straight crystalline fibers can be bent mechanically by applying force or photochemically by exposure to ultraviolet light. The straight and curled crystals are both polar and capable of highly efficient second harmonic generation (SHG) with respective intensities of 2.03 ± 0.15 and 1.52 ± 0.12 times (equivalent strain ≈ 1%) that of urea. Curling during crystal growth provides direct access to curved SHG-active flexible organic optical waveguiding elements, such as crystalline optical ring resonators, thereby circumventing the necessity for manual crystal bending, which is usually not readily scalable. This work highlights the unconventional properties and capabilities that fibrous molecular crystalline materials bring to the global materials space and their potential applications as shape-conforming, nonlinear organic materials.