Synthesis of azobenzene containing urea groups as dicarboxylate anion receptor and sensor

Abstract

Eight azobenzene derivatives containing urea and thiourea units were synthesized by coupling diaminoazobenzenes with isocyanate or thioisocyanate. Complexation studies of ligands 2b and 5a with dicarboxylate anions such as oxalate, malonate, succinate, glutarate, adipate, pimelate, suberate and azelate were carried out by [superscript 1]H-NMR titrations. Ligands 2b and 5a were able to form 1:1 complexes with all dicarboxylate anions. Ligands 2b and 5a form the most stable complexes with suberate anion. Complexation studies of ligands 2d and 5d with dicarboxylate anions by UV-visible spectrophotometry show dramatic change in UV spectra. Additionally, ligands 2d and 5d can bind dicarboxylate anions and give a color change that can be detected by naked eyes. Ligands 2b, 2d, 5a and 5d underwent an observable cis-trans isomerization by irradiation with the UV light. Complexation studies of 2b and 5a with anions after irradiation were carried out by [superscript 1]H-NMR titrations. Association constants of ligands 2b and 5a (the cis form) toward suberate were about 10-30 times higher than ligands 2b and 5a before irradiation (the trans form). It was attributed to the matching of the chain length of dicarboxylate with the cavity size of the cis-form of ligands 2b and 5a. After irradiation, ligands 2d and 5d in DMSO solution underwent trans to cis isomerization and gave a color change that was able to be detected by naked eyes upon addition of dicarboxylate anions.