Molecular Engineering Strategy for Flexible Organic Crystal Materials Integrating Low Temperature Elasticity and Optical Waveguide Properties Based on Bromo‐Hydroxy Chalcone Derivatives

Flexible organic crystal materials with optical waveguide property have attracted much attention for various applications. Meanwhile, the rising demand for deep space and polar explorations have brought about a growing interest in materials with low temperature flexibility. However, the development of organic crystal materials integrating optical waveguide and low temperature elasticity remains a significant challenge. Here, three flexible organic crystals with bromo‐hydroxy chalcone backbone are developed via molecular engineering strategy. The 4BHIE crystal with 4‐bromo‐N‐ethyl substituent exhibits superior 2D elasticity under mechanical external forces with ≈180° bending and 1.30 mm of curvature. The low optical loss coefficient of only 0.309 dB mm−1 also demonstrates potential applications in flexible optoelectronic waveguides. Interestingly, the introduction of a longer alkyl chain onto N atom of indole moiety (4BHIB) exhibits more remarkable flexibility with 0.35 mm of curvature due to its richer and more complex network of intermolecular interactions compared with that of 4BHIE. Furthermore, 5BHIE crystal with 5‐bromo‐N‐ethyl substituent shows not only elasticity at room temperature but also low‐temperature elasticity in liquid nitrogen with reversible temperature response owing to the strengthening intermolecular interactions at low temperature. 5BHIE crystal displays potential optical waveguide application in low temperature environments. This work successfully develops three flexible organic crystals with Br‐hydroxy chalcone backbone via molecular engineering strategy. The crystals exhibit excellent elasticity with optimal elastic strain of 13.03% and excellent optical waveguide with optical loss coefficient as low as 0.309 dB mm−1. The crystal with 5‐bromo‐N‐ethyl substituent maintains excellent flexibility and optical waveguide properties even at liquid nitrogen temperature.