Fluorescent Chemo-Sensor for Metal Cations Based on Thiacalix[4]arenes Modified with Dansyl Moieties at the Lower Rim

The selective de-tert-butylation of p-tert-butyl thiacalix[4]arene (p-tert-butyl TCA) with AlCl3 at 80°C has been investigated. Mono-, di-, and tri-p-tert-butyl thiacalix[4]arene (mono-, di- and tri-TCA, respectively) were obtained in yields of 6.5, 20 and 21%, respectively, when p-tert-butyl TCA was treated with 7.1 equiv. of AlCl3 for 30 min. De-tert-butyl TCA (TCA) was exclusively obtained when 10.5 equiv. of AlCl3 was used. These derivatives are converted to fluorescent active derivatives by modification with dansyl moiety at the lower rim, which are used as metal sensors in aqueous solution. The extent of the fluorescent variations with the metal cations was used to display the sensing factors, ΔI/I0. These hosts show much higher sensitivity for transition metal cations such as Co2+, Ni2+, and Cd2+ than those of alkali metal cations such as Li+, Na+, and K+. The effect of p-tert-butyl groups on metal sensing ability of these hosts was studied. It was found that p-tert-butyl groups decrease the sensing ability of these hosts. It is the first example to show that water soluble TCA derivatives can detect metal cations directly in aqueous solution by variation of fluorescence intensity upon addition of a metal cation.