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, [Zn2L21], has a red-shifted fluorescence as compared to that of [ZnL22], a half-structured mononuclear complex of [Zn2L21]; in addition, [Zn2L21] 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 pi-pi interactions are critical elements that determine the shifts of the fluorescence to a longer wavelength.