Tuned Al³⁺ selectivity and π-extended properties of di-2-picolylamine-substituted quinoline-based tolan

Di-2-picolylamine (DPA)-substituted quinoline-type tolan was synthesized herein. The electron withdrawing CN substituted tolan (1) exhibited clear Al³⁺ selectivity, unlike simple DPA-substituted quinoline (2). Additionally, owing to the π-extension to quinoline (2), a redshifted fluorescence band and the shoulder of tolan (1) were observed at 491 and 537 nm, respectively, whereas the quinoline (2) fluorescence band was observed at approximately 450 nm. Consequently, Al³⁺ bound tolan was more redshifted in comparison with other metal ions bound by 2. Fluorescence titration of Al³⁺ to 1 (20 μM) in HEPES buffer (10 mM, pH = 7.4) containing 50% MeOH exhibited a limit of detection of 36 nM.