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.