Hg2+ recognition by triptycene-derived heteracalixarenes: selectivity tuned by bridging heteroatoms and macrocyclic cavity

The binding properties of triptycene-derived oxacalixarenes containing 1,8-naphthyridine subunits 1a and 1b toward metal ions were investigated in detail by spectroscopic methods. A couple of new N,O-bridged diazadioxacalixarenes 2a, 2b were synthesized and their binding properties to metal ions were also evaluated. The results showed that the oxacalixarene 1a, a cis-isomer with a boat-like 1,3-alternate conformation and a symmetrical cavity structure, exhibited a highly selective fluorescence response toward Hg(2+) ions, while 1b, 2a and 2b did not show selective response towards any specific metal ions. Such selectivity can thus be controlled by the bridging heteroatoms and cavity structures of the macrocycles. Moreover, it was found that the fluorescence of 1a was considerably quenched upon the addition of Hg(2+), and a 1 : 2 stoichiometry host-guest complex was proposed on the basis of the Job plot and (1)H NMR titrations.