Spectroscopic and computational study of vanillin-dipyrrin ligand that capable of colorimetric sensor for zinc and copper ions

•Vanillin-dipyrrin ligand is a promising colorimetric sensor for detecting zinc and copper ions.•In methanol, the ligand showed a red-magenta color owing to the formation of hemiquinone species.•Spectroscopic and computational studies revealed that phenolic species were formed during complexation, resulting in a yellowish color.•Quantitatively, the ligand showed detection limits of up to 60 and 30 ppb for zinc and copper ions, respectively. 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. Vanillin-dipyrrin ligand is a promising colorimetric sensor for detecting zinc and copper ions. In methanol, the ligand showed a red-magenta color owing to the formation of hemiquinone species. Spectroscopic and computational studies revealed that phenolic species were formed during complexation, resulting in a yellowish color. Quantitatively, the ligand showed detection limits of up to 60 and 30 ppb for zinc and copper ions, respectively. [Display omitted]