Fluorescent sensors from pyrene excimer formation and colorimetric sensors from nitobenzene for anion detection

Abstract

In the first part, the polyethylene glycol (PEG) containing pyrene thioureas at both ends, L1 – L3, and methoxy benzene pyrene thiourea, L4, were synthesized. Upon binding with F-, L1 – L3 exhibited strong excimer emission bands and weak monomer emission bands, while L4 displayed the same intensity of both bands. L1 possessed the highest binding constant because a longer PEG chain may reduce steric hindrance on the receptor molecule. However, little or no change was observed in fluorescence spectra of L1 upon adding OH-, AcO-, BzO-, H₂PO₄-, Cl-, Br- and I-. Fluorescence intensity ratios of L1∙F- complex were dependent on the concentration of L1, implying that the dimerization of L1 proceeded via intermolecular excimer interactions. Moreover, L1 was able to give the excimer emission band even when the concentration of F- was as low as 2.3 ppm in CHCl3 solution. The second part focused on the comparison of anion sensing properties between flexible and semi-rigid receptors of thiourea-nitrobenzene derivatives, R1 and R2, by using UV-vis and 1H-NMR spectroscopy techniques. All the receptors were sensitive to F-, AcO-, BzO- and H₂PO₄-. The selectivity trends of R1 and R2 with anions were F- > BzO- > AcO- > H₂PO₄- in 1: 2 ratios. In the presence of Na+ and K+, receptors R1 and R2 were found to bind the mentioned anions to a different extent. Therefore, these anions can be distinctively detected by UV-vis spectrophotometry.