Structural Chemistry Enables Fluorescence of Amino Acids in the Crystalline Solid State

Abstract

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.