The study of elastic organic single crystals (EOSCs) has emerged as a cutting-edge research of crystal engineering. Although a few EOSCs have been reported recently, those suitable for optical/optoelectronic applications have not been realized. Here, we report an elastic crystal of a Schiff base, (E)-1-(4-(dimethylamino)phenyl)iminomethyl-2-hydroxyl-naphthalene. The crystal is highly bendable under external stress and able to regain immediately its original straight shape when the stress is released. It displays bright orange-red emission with a high fluorescence quantum yield of 0.43. Intriguingly, it can serve as a low-loss optical waveguide even at the highly bent state. Our result highlights the feature and utility of "elasticity" of organic crystals.

Flexible organic single crystals capable of plastic or elastic deformations have a variety of potential applications. Although the integration of plasticity and elasticity in a crystal is theoretically possible and it may cause rich and complex deformations which are highly demanded for potential applications, the integration is hard to realize in practice. Here, we show that through utilizing different modes of external forces for influencing molecular packing in different crystallographic directions, plastic helical twisting and elastic bending can both be achieved for a crystal, and they can even be realized simultaneously. Detailed crystallographic analyses and contrast experiments disclose the mechanisms behind these two kinds of distinct deformations and their mutual compatibility. Based on the plastically twistable nature of the crystal, a new application field of flexible organic single crystals, namely polarization rotators, is successfully opened up.

The very simple organic molecules have been employed to construct highly efficient single-benzene solid emitters (quantum yields: 0.72-0.84) with crystal lasing properties based on aggregation-induced emission (AIE) generated through an excited-state intramolecular proton transfer (ESIPT) reaction.

With the increasing popularity and burgeoning progress of space technology, the development of ultralowtemperature flexible functional materials is a great challenge. Herein, we report a highly emissive organic crystal combining ultralow-temperature elasticity and self-waveguide properties (when a crystal is excited, it emits light from itself, which travels through the crystal to the other end) based on a simple singlebenzene emitter. This crystal displayed excellent elastic bending ability in liquid nitrogen (LN). Preliminary experiments on optical waveguiding in the bent crystal demonstrated that the light generated by the crystal itself could be confined and propagated within the crystal body between 170 and À196 8C. These results not only suggest a guideline for designing functional organic crystals with ultralow-temperature elasticity but also expand the application region of flexible materials to extreme environments, such as space technology.

Organic single crystals with elastic bending capability and potential applications in flexible devices and sensors have been elucidated. Exploring the temperature compatibility of elasticity is essential for defining application boundaries of elastic materials. However, related studies have rarely been reported for elastic organic crystals. Now, an organic crystal displays elasticity even in liquid nitrogen (77 K). The elasticity can be maintained below ca. 150 °C. At higher temperatures, the heat setting property enables us to make various shapes of crystalline fibers based on this single kind of crystal. Through detailed crystallographic analyses and contrast experiments, the mechanisms behind the unusual low‐temperature elasticity and high‐temperature heat setting are disclosed.

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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.

Structurally simple pyrazole derivatives that exhibit excited-state intramolecular proton transfer (ESIPT) were synthesized. While these compounds displayed deep violet fluorescence in solution, in the crystalline state they showed white emission from the enol and keto forms.