Spectroscopic and Computational Study of Vanillin-Dipyrrin Ligand that Capable of Colorimetric Sensor for Zinc and Copper Ions

Abstract

Phenolic-dipyrrin is a promising colorimetric chemosensor due to its tautomeric formation (phenolic-hemiquinone forms). In methanol solution, the vanillin-dipyrrin compound exists as a hemiquinone species, whereas the complexes exist in a phenolic form. This molecule showed a red-magenta color in methanol, in contrast to yellowish allyl-protected-dipyrrin. In the presence of zinc or copper ions, the vanillin-dipyrrin solution turned into a pale-yellow complex, similar to that of protected-dipyrrin. A computational study of the ligands and complexes showed a close relevance, where the energy transition between the highest molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the ligand in the hemiquinone form had a lower energy transition, and the two complexes showed slightly different transition energies. Apart from naked-eye color changes, vanillin-dipyrrin showed limits of detection of 9.2 × 10–7 M and 4.8×10−7 M for zinc and copper ions, respectively.