Attachment of an Electron-Withdrawing Fluorophore to a Cryptand for Modulation of Fluorescence Signaling

The laterally nonsymmetric aza cryptand synthesized by condensing tris(2-aminoethyl)amine (tren) with tris{2-[(3-(oxomethyl)phenyl)oxy]ethyl}amine readily forms mononuclear inclusion complexes with both transition- and main-group-metal ions. The fluorophore 7-nitrobenz-2-oxa-1,3-diazole is attached to one of the secondary amines, to
give an integrated fluorophore−receptor configuration. The fluorophoric system does not show any appreciable
emission when excited due to an efficient photoinduced intramolecular electron transfer (PET) from the nitrogen
lone pair. When a metal ion enters the cavity, the PET is blocked, causing recovery of fluorescence; Cd(II) gives
the highest quantum yield. The fluorophore, with π-accepting ability, drastically alters the binding property of the
cryptand. With perchlorate or tetrafluoroborate salts of Cd(II), the metal ion enters the cavity, causing recovery of
fluorescence. However, in the presence of coordinating ions such as Cl-, N3-, and SCN-, the metal ion comes
out of the cavity, causing PET to take place once again, and the fluorescence is lost. Thus, translocation of Cd(II)
between the inside and outside of the cryptand cavity can lead to a reversible fluorescence on/off situation.