Bis-azamacrocyclic Anthracene as a Fluorescent Chemosensor for Cations in Aqueous Solution

A bis-azamacrocyclic anthracene (L2), which has two a 12-membered cyclic tetraamine (cyclen) connected through a 9,10-dimethylanthracene spacer, has been synthesized as a new fluorescent chemosensor for detection of pH and metal cations in aqueous solution (cyclen = 1,4,7,10-tetraazacyclododecane, L2 = 9,10-bis(1,4,7,10-tetraazacyclododecane-1-ylmethyl)anthracene). The fluorescence response of L2 has been studied in comparison to that of the previously reported monoazamacrocyclic anthracene (L1 = 1-(9-anthrylmethyl)-1,4,7,10-tetraazacyclododecane). Plots of the fluorescence intensity of L2 against pH demonstrate a sigmoidal curve with pK_a 7.4, which is lower than that of L1 (8.3). Potentiometric titration reveals that the increase in the L2 fluorescence requires protonation of both cyclen rings, thus resulting in the lower pK_a value. L2 demonstrates impressive fluorescence response against metal cations. At basic pH, upon addition of Zn²⁺ or Cd²⁺, L1 leads to an increase in the fluorescence intensity with a 1:1 metal−intensity response. L2, however, shows a 2:1 response to Zn²⁺, while showing a 1:1 response to Cd²⁺. At neutral pH, L1 fluorescence decreases upon addition of Zn²⁺ or Cd²⁺ because of a formation of metal−anthracene π complex. L2, however, still demonstrates a Zn²⁺-induced increase in intensity with a 2:1 response, while no change in intensity is observed upon Cd²⁺ addition. The obtained findings suggest potential utilities of L2 as a new type fluorescent chemosensor for the detection of cations in aqueous solution.