Enhanced mechanofluorochromic properties of 1,4-dihydropyridine-based fluorescent molecules caused by the introduction of halogen atoms

Abstract

Four cyano(ethoxycarbonyl)methylene-1,4-dihydropyridine (CMD) derivatives were designed and synthesized to study the influence of halogen atoms on their mechanofluorochromic (MFC) properties. These CMD derivatives are demonstrated to have twisted molecular conformations and display aggregation-induced emission properties. In the crystalline state, the compounds adopt loose J-aggregates and exhibit yellow-green or green solid-state fluorescence. Upon grinding, the original samples of CMD-H, CMD-F, CMD-Cl, and CMD-Br show 21, 60, 61, and 52 nm red shifts in their fluorescence spectra, respectively, revealing MFC characteristics. The MFC properties are demonstrated to be due to the crystalline-to-amorphous transition and the red shifts in the fluorescence spectra are attributed to the increased molecular conjugations by the planarization of molecular conformations. X-ray crystallographic analyses reveal that halogen-substituted CMD derivatives have more twisted conformations and looser stacking arrangements than CMD-H, which are responsible for their higher contrast MFC properties. This work indicates that the introduction of halogen atoms to a specific fluorescent molecule might be used as a simple and feasible route for developing high contrast MFC-active materials.