Fluorescent Chromophore Construction via Through-Space Conjugation of Caprolactam and Itaconic Acid: Mechanistic...

Fluorescence characteristics are generally attributed to conjugated molecular structures. Substances exhibiting chromatic or fluorescent effects hold significant application value in functional coatings, biochemical detection, anti-counterfeiting technologies, and pharmaceutical tracking due to their distinctive optical identification properties. Although numerous fluorescent materials have been developed, research on fluorescence mechanisms in low-molecular-weight substances remains insufficient. This study serendipitously discovered that a simple thermal mixing reaction between caprolactam (CPL) and itaconic acid (ITA) can produce a fluorescent-colored metastable colloidal system. The colloid exhibits exceptional stability at ambient temperature, maintaining noncrystalline status or forming novel butterfly/leaf-like crystal structures upon cooling, with reversible colloidal characteristics through thermal cycling. Through comprehensive characterization using fourier transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), high-performance liquid chromatography (HPLC), and polarized optical microscopy, we elucidated the chromatic and fluorescent mechanisms: intermolecular hydrogen bonding networks facilitate the construction of spatial conjugation systems containing unsaturated groups, achieving photoluminescence via π-electron orbital transitions. The reliability of fluorescence color generation mechanism was confirmed by 3D printing side. This material exhibits excellent solubility in common solvents and shows promising potential for developing fluorescent composites through polymer matrix incorporation.