Dual-channel ratiometric recognition of Al3+ and F- ions through an ESIPT-ESICT signalling mechanism.

An optical probe 1 has been synthesized comprising naphthalimide unit conjugated with Schiff base, exhibiting excited state intramolecular proton transfer and intramolecular charge transfer as a potential sensor for Al3+ and F- ions using standard spectroscopic techniques. The probe 1 exhibited local and charge-transfer excitation at 340 nm and 460 nm, respectively. On excitation at 460 nm, probe 1 displayed two emission bands at 510 nm and 610 nm, accompanied by Stokes' shift of 50 nm and 150 nm, respectively. The solvatochromic effect and theoretical calculation depicted that the representative emissions resulted from the ESICT/ESIPT phenomenon. Upon addition of Al3+ ions, the charge transfer excitation at 460 nm was enhanced ratiometrically to local excitation at 340 nm and showed a color change from orange to yellow. Similarily, probe 1.Al3+ displayed emission enhancement at 540 nm in H2O/CH3CN (1:9; v/v) and showed a color change from yellow to blue-green emission. Following the detection of Al3+ ions, hydrolysis of probe 1 to its reacting precursors was observed. The detection of Al3+ ions was also demonstrated in surfactant-containing water. The limit of detection (LOD) of probe 1 (H2O/CH3CN (1:9; v/v)) towards Al3+ ions was measured to be 3.2 × 10-8 M. The probe 1 displayed a ratiometric absorption response towards F- ions with a new peak at 570 nm and showed a color change from orange to purple. The probe 1.F- displayed a decrease in emission at 635 nm. The LOD of probe 1 (CH3CN) towards F- ions was measured to be 7.5 × 10-7 M.