Red Fluorescence from a Cocrystal of Rhodamine 6G and Terephthalic Acid

Abstract Molecular cocrystallization provides an effective strategy for regulating the structure and properties of organic solids. Here, we report the growth of red-emissive Terephthalic Acid-Rhodamine 6G (TPA-R6G) molecular cocrystals by a vapor-phase transport process. The resulting crystals exhibit well-defined plate-like morphologies and intense red fluorescence. Single-crystal X-ray diffraction reveals a 1:1 cocrystalline structure, in which TPA and modified R6G molecules are alternately stacked along the crystallographic a-axis to form layered architectures. These layers are stabilized by multiple intermolecular hydrogen-bonding interactions involving the carboxyl groups of TPA. Spectroscopic characterization reveals a pronounced solid-state emission centred at 720 nm, extending from 600 to 870 nm, together with a fluorescence lifetime of 1.03 ns. In addition, nanoindentation measurements demonstrate a modulus of 8.539 ± 0.137 GPa and a hardness of 0.666 ± 0.009 GPa. Overall, these results establish a direct relationship between molecular packing and the optical and mechanical properties of TPA-R6G cocrystals, highlighting molecular cocrystallization as an effective strategy for constructing multifunctional organic crystalline materials.