Selective colorimetric sensing of fluoride ion via H-bonding in 80% aqueous solution by transition metal chelates

Abstract

A series of four coordinated transition metal chelates of composition [M(R1)2], where M=Cu(II), Co(II), Ni(II) and Zn(II) and R1=quinone-imidazole ensemble, have been prepared and characterized by using standard analytical and spectral techniques. These metal chelates selectively (over many anions) and sensitively (detection limit down to nM) sense fluoride ion colorimetrically with an instantaneous striking colour change from yellow to red. They do so in H2O:DMF (80:20% v/v) medium, while free R1 imparts similar colour change only in DMF. The mechanism of sensing involves formation of H-bond between imidazole NH group and fluoride ion. The chelates binds strongly (Ka 107M−1) with fluoride ion to form a 1:2 (receptor:fluoride) complex. The coordination of transition metals to R1 not only enhances the H-bond donor ability of the NH group but also accommodates 80% of water in the sensing medium, which is a novel finding. Electrochemical and theoretical calculations have also been used to substantiate the results obtained in the spectral studies. The spectral data were applied to construct logic gate at molecular level.