Nonaromatic luminogens have recently emerged as highly attractive materials for biological imaging and sensing applications, due to their good hydrophilicity and biocompatibility. Here, we report that natural nonaromatic and aromatic amino acids, including l-histidine, l-glutamine, l-isoleucine, l-asparagine, l-valine, l-threonine, and l-methionine, exhibit crystallization-induced emission. The crystalline state of these amino acids shows a wide range of fluorescence emission, in striking contrast to barely any emission in the solution phase. We determined the atomic structure of these amino acids in crystalline state using X-ray crystallography. A structural analysis implies that the compact interactions through the hydrogen-bonding network of the crystallized amino acids potentially restrict intramolecular rotations and vibrations and thus enhance the radiative transitions in the crystalline state. Because these noncovalent interactions can be easily modulated by varying the chemical environment, this phenomenon of crystallization-induced emission may represent a general strategy to induce fluorescence from weakly emissive or nonemissive nonaromatic molecules.

中文翻译：

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结构化学使晶体固态的氨基酸荧光得以实现

非芳香族发光剂由于其良好的亲水性和生物相容性，最近已成为用于生物成像和传感应用的极具吸引力的材料。在这里，我们报告了天然的非芳香族和芳香族氨基酸，包括l-组氨酸，l-谷氨酰胺，l-异亮氨酸，l-天冬酰胺，l-缬氨酸，l-苏氨酸和l-甲硫氨酸，表现出结晶诱导的发射。这些氨基酸的晶态显示出宽范围的荧光发射，与溶液相中几乎没有任何发射形成鲜明对比。我们使用X射线晶体学确定了处于结晶状态的这些氨基酸的原子结构。结构分析表明，通过结晶氨基酸的氢键网络进行的紧密相互作用可能会限制分子内的旋转和振动，从而增强晶态下的辐射跃迁。因为可以通过改变化学环境来轻松调节这些非共价相互作用，所以这种结晶诱导的发射现象可能代表了一种从弱发射或非发射非芳族分子中诱导荧光的一般策略。