Translocation of copper within the cavity of cryptands: reversible fluorescence signaling

摘要

The movement of a copper ion inside the cavity of cryptands having two distinct metal binding sites at each end is linked to a fluorescence ON-OFF process in a reversible manner, providing a new way of fluorescence switching.rnTransition metal ions used for signal transduction in fluorescence signaling systems are attractive due to the availability of various redox states for the metal ion. We have been using aza cryptands as receptors in the configuration "fluorophore-spacer-receptor" for emission enhancement by transition/heavy metal ions. Herein, we describe two new cryptand receptors with NS_3 and N_4 donor sets at each end of the cavity, segregated by aromatic spacers. Our design is based on the fact that Cu(Ⅰ) has a preference to bind soft S donors, whereas Cu(Ⅱ) prefers N donors. Thus, a copper ion, depending upon its oxidation state, can translocate between the ends of the cryptand receptor, leading to fluorescence switch-ability (Scheme 1). This appears to be the first example of the translocation of a metal ion inside a cryptand cavity. Translocation and fluorescence modulation by metal ions in cyclic and acyclic systems has been achieved through a change of pH~(2d) and redox state of the metal ion. Besides this, the use of a triple-stranded helical complex as molecular redox switch, a calixarenc as a molecular syringe for metal ions and metal tunneling are some of the other important results in this field.