Colorimetric ‘naked eye’ sensing of anions using a thiosemicarbazide receptor: a case study of recognition through hydrogen bonding versus deprotonation

The thiosemicarbazide receptors 1 and 2, containing three N–H donors (including a thiourea motif), have been synthesized and studied for their anion binding abilities. The recognition of 1 and 2 with various anions [AcO−, F−, H2PO4−, SO42− and OH− for 1; OAc− and F− for 2] as their tetrabutylammonium salts was studied spectroscopically in CH3CN and EtOH. Significant changes were observed in the UV–vis absorption spectra of 1 which was red-shifted in both solvents when titrated with these anions. A dramatic colour change from yellow to red was also clearly observed. In contrast, the absorption changes for 2 were smaller with a colour change from colourless to yellow. These changes were determined to be due to deprotonation of the central N–H moiety (belonging to the thiourea) and not due to hydrogen bonding of these anions to the receptors. This was confirmed using single crystal X-ray crystallography, as crystals of 1 grown with TBAAc were found to correspond to the deprotonated structure 3.