A novel chemosensor based on rhodamine and azobenzene moieties for selective detection of Al³⁺ ions

The high absorption coefficient, fluorescence quantum yield, photo stability and relatively long wavelengths of rhodamine dye have expanded its applicability as a promising fluorescent probe. In the present investigation, two dyes, namely rhodamine and azobenzene, have been conjugated for the detection of Al³⁺ in aqueous ethanol. The turn on fluorescence response of chemosensor L towards Al³⁺ is attributed to the inhibited PET (photo-induced electron transfer) and CHEF (chelation enhanced fluorescence) process along with the spirolactam (non-fluorescent) to ring-opened amide (fluorescent) process. The presence of the PET and CHEF process was corroborated by time-resolved photoluminescence study and the spirolactam ring opening was confirmed by ¹³C NMR and infrared spectroscopy. In the presence of Al³⁺, the opened spirolactam ring forms a 1:1 binding complex with the metal, which is supported by its high binding constant (K_a = 7.033 × 10³ M^-1). The limit of detection (LOD) and limit of quantification (LOQ) values are found to be 1.1 × 10^-7 M and 3.6 × 10^-7 M, respectively. The reversible recognition of Al³⁺ was also proved in the presence of Na₂EDTA by both UV-Vis and fluorometric titration. The sensing behaviour of the chemosensor towards Al³⁺ was supported by DFT/TDDFT calculations.