Synthesis of new tripodal receptors -: a 'PET' based 'off-on' recognition of Ag+

A new set of tripodal receptors based upon an aromatic platform have been synthesized in high yields. The compounds have been characterized by spectroscopic techniques and by single crystal X-ray crystallography. These receptors are found to have good extraction ability and high transport rate for Ag(I). The receptor with imine linkages exhibits weak fluorescence emission bands lambda(max)=413 and 540 nm, upon excitation lambda(max)=365 nm. The fluorescence spectrum of the receptor shows enhancement in the intensity of the signal at 413 nm on binding with the Ag+ cation. No such significant changes are observed with other metal ions. An absorption at similar to 365 nm is typical of an intraligand (pi-pi*) transition involving the imine chromophore, which produces a weak emission band at 413 nm due to quenching caused by PET from a neighboring -OH group. Participation of OH group in coordination to the metal ion reduces PET and an enhancement of fluorescence intensity is observed, signaling recognition of the metal ion. (C) 2008 Elsevier Ltd. All rights reserved.