A mechanistic study on unexpected and solvent-based pH-tuneable role of benzylic pendant side-arm on N₂O₂-donor naphthodiaza-crown macrocyclic ligand as a sensitive fluorogenic chemosensor for Al³⁺ in aqueous solution: x-ray, NMR, photophysical, and computational studies

Herein, we describe for the first time synthesis, characterization, chemosenor behavior, and mechanism of metal ion-ligand interaction of two novel side-armed naphthodiaza-crown macrocycles (L1 and L2) by employing of IR, 1H and 13C NMR, UV–vis, fluorescence spectroscopy, CHN microanalysis and single crystal X-ray diffraction. Preliminary fluorescence studies in ethanol as solvent revealed that the metal ion recognition by L1 was strongly different from L2 suggesting the importance of the number of benzylic side-arms on the extent of the interaction with Mn+. For instance employing fluorescence titration in ethanol showed binding constant values 105 M−1 and 2.6×106 M−1 for L1/Al3+ and L2/Al3+, respectively. Such metal ion selectivity by L2, however, was sharply improved in aqueous media (1:1 v/v ethanol/water) in comparison with L1. The selective complex formation of L2 with Al3+ supported very low limit of detections, i.e. 5×10−7 and 7×10−6 M in ethanol and aqueous medium, respectively. Single crystal X-ray diffraction analysis (in solid) as well as 1H NMR spectroscopy in acidic DMSO-d6 solvent revealed that unparalleled direction of the electron lone pairs on the donor atoms of L2 could not let Al3+ cation to be coordinated simultaneously by both nitrogen-donor groups on the macrocycle ring. The coordination of Al3+ with both of those nitrogen donors on L2, however, was evident from the 1H NMR spectra of L2/Al3+ where the solvent was DMSO-d6.