Ratiometric Naphthalimide Fluorescent Sensor For Cadmium

In this article, a highly selective ratiometric fluorescent sensor 1a was synthesized for Cd²⁺, which can work under physiological pH range. Based on 4,5-disubstitued-1,8-Naphthalimide as the fluorophore, this sensor uses di-2-picolylamine and another 2-picolyl group as the receptor. Because the metal cation will influence the electron-donating ability of the NH moiety, the shifts of the wavelength can be expected. Sensor 1a first demonstrates good selectivity. Even under high concentration of physiologically abundant anion, such as Na⁺, K⁺, Mg²⁺ and Ca²⁺, its fluorescent signals will not be affected. In particular, sensor la shows excellent signal discrimination between Cd²⁺ and Zn²⁺. Because Cd²⁺ and Zn²⁺ are in the same group of the Periodic Table, and have similar properties, fluorescent sensors usually perform same signals towards them, including the shifts of wavelength and the changes of intensity. However, upon the interaction with Cd²⁺ and Zn²⁺, sensor 1a shows reverse wavelength shifts—Cd²⁺ reduces the electron donating ability of the NH moiety and causes blue shift of the emission wavelength (from 531 nm to 487 nm), whereas Zn²⁺ causes the depronation of the NH moiety and the red shift of the emission (from 531 nm to 558 nm). Therefore, the convenient and conspicuous discrimination of the two cations is realized. To our best knowledge, it is the first fluorescent sensor that can undergo different ICT processes upon interacting with two different analytes.To further understand this unique photophysical phenomena, structure derivation and NMR studies are carried out to reveal the relationship between structure and selectivity. Sensor 1b and 1c are synthesized using 3-picolylamine and 2-ethylenepyridineamine as the substitution group of 2-picolylamine respectively. The results show that both sensor 1b and sensor 1c are silent to Cd²⁺ and Zn²⁺, which are consistent with the NMR studies. Based on these facts, it can be concluded that the 2-picolyl group plays a unique role in the recognition process of senor 1a toward Cd²⁺ and Zn²⁺. Unlike the DPA group, which grasps the metal ion through N-metal ion interaction, the 2-picolyl group may play an important role in constructing and maintaining the conformation of the receptor.In the last part of the article, a preliminary study is carried out for the synthesis of the 3,4-diamino-1,8-naphthalimide, hoping that this similar structure would also perform some unique properties. In addition, an attempt is made to improve the synthetic routine of the receptor.