Tuned Cd Selectivity: Showcase of Electronic and Regio-Effect of π-Extended Di-2-Picolylamine-Substituted Quinoline-Based Tolans.

π-Extended di-2-picolylamine (DPA)-substituted 8-hydroxyquinoline (8-HQ) tolans () were synthesized for testing electronic and regio-effects. The electron-poor CN-tolan () showed clear selectivity for Cd (>>Zn) over other metal ions via turn-on fluorescence, while the electron-rich MeO-tolan () displayed no clear metal selectivity. Furthermore, considering that there was no significant energy difference between the Cd complexes of and , the intended regio-effect (7- vs. 5-substituted effect) did not induce steric hindrance. Thus, the regio-effect is mainly electronic. Considering the above, and constitute a complete showcase in which electronic and regio-effects modulate the metal selectivity. The fluorescence titration of (10 mM) with Cd showed that the limit of detection (LOD) of the Cd-selective was 158 nM in PBS (phosphate-buffered saline) (10 mM, pH 7.2) containing 50% MeOH.