Transition metal (II)/(III), Eu(III), and Tb(III) ions induced molecular photonic OR gates using trianthryl cryptands of varying cavity dimension

Abstract

Three heteroditopic cryptands with different cavity dimensions have been synthesized in high yields at 278 K without employing any templating metal ion. The three secondary amino nitrogens in each cryptand could be derivatized with anthryl groups to have a fluorophore-spacer-receptor configuration. The fluorophores in these systems do not show any fluorescence due to an efficient photoinduced intramolecular electron transfer (PET) from nitrogen lone pairs. However, the fluorescence can be recovered to different extents in the presence of different metal ions and protons as well. On complexation by a transition metal ion or on protonation in a solvent like dry THF, each exhibits large fluorescence enhancement as the nitrogen lone pairs responsible for PET are engaged in bonding. Inner-transition-metal ions like Eu(III) or Tb(III) show remarkable discrimination and give high fluorescence enhancement only in one case where the cavity size is smaller than that of other two. Each system exhibits large fluorescence enhancement with Pb(II) among the heavy metal ions studied. The present study shows that transition metal ions and Pb(II), which are known for quenching, can indeed cause fluorescence enhancement in cryptand-based systems. It is also reported for the first time that inner-transition-metal ions can also cause fluorescence. The enhancement in each case is interpreted in terms of a communication gap between the metal ion and fluorophore. Such cryptand-based fluorophores can be useful as potential molecular photonic devices and metal ion sensors as well.