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.
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Mizukami et al. (2004) studied this question.