Schiffbase ligands can be considered key to designing fluorescent chemosensors due to their chro- mogenic properties. Here, we study this view and demonstrate that there is a remarkable fluorescence enhancement upon Al 3 + addition to a new macroacyclic Schiffbase ligand (H2L) compared with 15 dif- ferent metal ions, while the limit of detection (LOD) value is in good agreement with the values need for biological applications ( 2 . 1 ×1 0 −9 M). Besides, we show that it can be a satisfactory linear relationship between the gradually added concentrations of Al 3 + and the fluorescence intensity of H2L. On the other hand, some transition shifts toward lower wavelengths were observed using the spectroscopic titration of H2L with Al 3 + ions. From the theoretical point of view, NBO analysis revealed that the band gaps var- ied from 1.715 eV for H2L to 2.167 eV for [AlL] + , confirming the blue shift upon complexation. The high difference in band gaps approved the occurrence of a significant electrochemical signal. Furthermore, the nature of bonds in H2L and [AlL] + was investigated by the QTAIM method specifying the shared prop- erty of bonds in H2L and the closed-shell metal-ligand interactions in [AlL] + . Ellipticity
