Design and synthesis of a highly sensitive off-on fluorescent chemosensor for zinc ions utilizing internal charge transfer

Fluorescence imaging is a powerful tool for the visualization of biological molecules in living cells, tissue slices, and whole bodies, and is important for elucidating biological phenomena. Furthermore, zinc (Zn²⁺) is the second most abundant heavy metal ion in the human body after iron, and detection of chelatable Zn²⁺ in biological studies has attracted much attention. Herein, we present a novel, highly sensitive off-on fluorescent chemosensor for Zn²⁺ by using the internal charge transfer (ICT) mechanism. The rationale of our approach to highly sensitive sensor molecules is as follows. If fluorescence can be completely quenched in the absence of Zn²⁺, chemosensors would offer a better signal-to-noise ratio. However, it is difficult to quench the fluorescence completely before Zn ²⁺ binding, and most sensor molecules still show very weak fluorescence in the absence of Zn²⁺. But even though the sensor shows a weak fluorescence in the absence of Zn²⁺, this fluorescence can be further suppressed by selecting an excitation wavelength that is barely absorbed by the Zn²⁺-free sensor molecule. Focusing on careful control of ICT within the 4-amino-1,8-naphthalimide dye platform, we designed and synthesized a new chemosensor (1) that shows a pronounced fluorescence enhancement with a blueshift in the absorption spectrum upon addition of Zn ²⁺. The usefulness of 1 for monitoring Zn²⁺ changes was confirmed in living HeLa cells. There have been several reports on 4-amino-1,8-naphthalimide-based fluorescent sensor molecules. However, 1 is the first Zn²⁺-sensitive off-on fluorescent sensor molecule that employs the ICT mechanism; most off-on sensor molecules for Zn²⁺ employ the photoinduced electron transfer (PeT) mechanism.