Fluorescence Color Modulation by Intramolecular and Intermolecular π−π Interactions in a Helical Zinc(II) Complex

When a fluorescent compound shows unique optical properties, an elucidation of the mechanism may lead to an important development of novel sensing strategies. A helical 3,3‘-di- tert -butylsalen−zinc(II) complex, Zn 2 L 1 2 , has a red-shifted fluorescence as compared to that of Zn L 2 2 , a half-structured mononuclear complex of Zn 2 L 1 2 ; in addition, Zn 2 L 1 2 exhibits a fluorescence color change from green to light blue under external stimulations. We investigated the origins of these phenomena by spectroscopy, fluorescence lifetime measurement, fluorescence microscopy, X-ray powder diffraction, and X-ray single-crystal analysis. From the experimental data, we concluded that intramolecular and intermolecular π−π interactions are critical elements that determine the shifts of the fluorescence to a longer wavelength.