Synthesis of novel truxene derivatives as chemosensors for nitroaromatics and metal ions

Abstract

Two fluorescent compounds with truxene as a fluorophore are synthesized and developed as turn-off fluorescent sensors for nitroaromatic compounds (T1) and copper(II) ion (T2). Six hydrophobic n-butyl groups in T1 and six hydrophilic diglycol chains in T2 prevent pi-pi stacking of truxene core and enhance their solubilities in organic or aqueous media. The synthesis of each compound is based primarily on the Sonogashira coupling reactions, which produced the desired compounds T1 and T2 in the overall yields of 14 and 18 %, respectively. The photophysical and sensing properties of T1 depend on the solvents. In chloroform, T1 is selectively quenched by 2-nitrophenol. However, the quenching is the fluorescence signal of selectively observed with picric acid in aqueous tetrahydrofuran. The detection limits are determined as 1.54 and 0.15 ppm for 2-nitrophenol and picric acid, respectively. The considerable overlapping of the absorption spectra of T1 and that of the analytes, along with the correlation of the fraction of photons absorbed by T1 and quenching efficiencies, the sensing mechanism significantly involves inner-filter effect, where the analytes compete with T1 during the absorption process. However, the lower quenching efficiency at higher temperature suggested that energy transfer via static quenching may also involve.

Compound T2 can dissolve well in tetrahydrofuran containing up to 70% of water and shows effective and selective fluorescence quenching towards copper(II) ion without any interference from other metal ions. The detection limit of this sensing system is 0.06 ppm. The mass analysis using a mixture between T2 and copper(II) ion corresponds to the mass of a 1:1 complex between T2 and copper(II), as verified by a Job’s plot. Thus, the quenching mechanism should involves the formation of such non fluorescent ground state complex. In addition, an anion sensing system is developed from the complex of T2 and copper(II) ion, which shows fluorescence recovery upon the addition of hydrogen phosphate and several biological phosphate compounds. The signal restoration involves the coordination of phosphate group to copper(II) preventing the complexation of copper(II) ion with T2.