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

Abstract

Covalently-attached fluorophores may impart enhanced chemosensing capabilities to calixarene frameworks. Synthesis and characterization of six novel dansyl-appended calix[4]arenes, namely, H/Dan₄, NO₂/Dan₄, H/(OH)₂Dan₂, H/(Ester)₂(Dan)₂, t-Bu/(OH)₂Dan₂, and t-Bu/(Ester)₂Dan₂, containing two or four dansyl moieties are reported. Among these, fluorescence intensity of NO₂/Dan₄ is observed to decrease significantly in the presence Hg²⁺ in the solution. Based on the decrease in fluorescence, a limit of detection for Hg²⁺ of 20 ppb is obtained. NO₂/Dan₄ as a chemosensing agent for Hg²⁺ shows excellent selectivity and adequate reversibility. Complexation of NO₂/Dan₄ with Hg²⁺ is investigated using fluorescence spectroscopy and is observed to be 2:1. The formation constant of (NO₂/Dan₄)₂Hg²⁺ is estimated to be 5.2(±0.8) × 10¹⁰M⁻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/Dan₄ is in the 1,3-alternate conformation. Stokes shift versussolvent orientational polarizability for NO₂/Dan₄ 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/Dan₄ 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.