Novel dansyl-appended calix[4]arene frameworks: fluorescence properties and mercury sensing

Covalently-attached. fluorophores may impart enhanced chemosensing capabilities to calixarene frameworks. Synthesis and characterization of six novel dansyl-appended calix[4]arenes, namely, H/Dan(4), NO2/Dan(4), H/(OH)(2)Dan(2), H/(Ester)(2)(Dan)(2), t-Bu/(OH)(2)Dan(2), and t-Bu/(Ester)(2)Dan(2), containing two or four dansyl moieties are reported. Among these,. fluorescence intensity of NO2/Dan(4) is observed to decrease significantly in the presence Hg2+ in the solution. Based on the decrease influorescence, a limit of detection for Hg2+ of 20 ppb is obtained. NO2/Dan(4) as a chemosensing agent for Hg2+ shows excellent selectivity and adequate reversibility. Complexation of NO2/Dan(4) with Hg2+ is investigated using. fluorescence spectroscopy and is observed to be 2:1. The formation constant of (NO2/Dan(4))(2)Hg2+ is estimated to be 5.2(+/- 0.8) x 10(10) M-2 at ambient conditions. These observations are traced to the fact that while all other dansyl-appended calix[4] arenes show cone conformation in the solution, NO2/Dan(4) is in the 1,3-alternate conformation. Stokes shift versus solvent orientational polarizability for NO2/Dan(4) also indicates the difference in the ground-to excited-state dipole moment of this compound to be the maximum among all six, rendering it most sensitive to its environment. Fluorescence emission of NO2/Dan(4) in nonpolar chloroform, polar-aprotic acetonitrile, and polar-protic ethanol is observed to be different than that of the rest of the dansyl-appended compounds as well.