This paper reports the synthesis of a Schiff base ligand (L) by reaction of 5,5’-methylene-bis-salicylaldehyde with amidol (2,4-diaminophenol). The applicability of this ligand as a new fluorescent Cu(II) sensor in aqueous medium was assessed. A metal to ligand ratio of 2:1 was identified and the structural properties and molecular orbitals of the complex formed between L and Cu2+ ions were explored using quantum chemical computation. The overall approach of the authors in conducting the experimental as well as the supporting computational part represents a complete and thorough work on sensing properties. I have some comments for improving the quality of the results.
- There is some error in representation of the starting aldehyde. The aldehyde should be CHO instead of COH.
- The reaction was not straight forward in terms of isolating the desired product. The authors did not mention the reaction time. On carrying out the reaction for 2 hours, very little precipitate was obtained. After overnight stay, crystalline precipitate was obtained in 53% yield.
- Due to insolubility of the ligand in water, the use of 30% DMSO-water mixture is workable but it also presents some doubt in the Cu2+ selectivity as coordination with DMSO has a major role to play in this.
- The fluorescence properties of L and its binding with K+, Na+, Al3+, Ni2+, Co2+, Fe3+, Zn2+, Pb2+ in presence of Cu2+ is well characterized and reproducible.
- The quantum chemical calculations do provide substantial support for the fluorescent sensing of Cu2+, but the role of DMSO in the fluorescence needs further investigation. I would suggest carrying out a proton NMR spectral assignment experiment in DMSO-D6 and D2O to investigate the binding of Cu2+ with DMSO.