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

A bis-azamacrocyclic anthracene (<b>L2</b>), 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, <b>L2</b> = 9,10-bis(1,4,7,10-tetraazacyclododecane-1-ylmethyl)anthracene). The fluorescence response of <b>L2</b> has been studied in comparison to that of the previously reported monoazamacrocyclic anthracene (<b>L1 = </b>1-(9-anthrylmethyl)-1,4,7,10-tetraazacyclododecane). Plots of the fluorescence intensity of <b>L2</b> against pH demonstrate a sigmoidal curve with p<i>K</i>_a 7.4, which is lower than that of <b>L1</b> (8.3). Potentiometric titration reveals that the increase in the <b>L2</b> fluorescence requires protonation of both cyclen rings, thus resulting in the lower p<i>K</i>_a value. <b>L2</b> demonstrates impressive fluorescence response against metal cations. At basic pH, upon addition of Zn²⁺ or Cd²⁺, <b>L1</b> leads to an increase in the fluorescence intensity with a 1:1 metal−intensity response. <b>L2</b>, however, shows a 2:1 response to Zn²⁺, while showing a 1:1 response to Cd²⁺. At neutral pH, <b>L1</b> fluorescence decreases upon addition of Zn²⁺ or Cd²⁺ because of a formation of metal−anthracene π complex. <b>L2</b>, 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 <b>L2</b> as a new type fluorescent chemosensor for the detection of cations in aqueous solution.