Two new fluorescent Zn2+ sensors exhibiting different sensing mode with subtle structural changes

Two novel receptors HL-1 and HL-2 without and with hydroxyl groups were designed and synthesised. Both receptors showed highly selective coordination towards Zn2+ and exhibited diverse sensing behaviour due to the structural variations. HL-1 showed monotonous ‘turn-on’ response towards Zn2+ while HL-2 displayed highly Zn2+ sensitive ‘turn on’ and ‘ratiometric’ properties. Detailed Job plot experiment, single crystal data, 1H NMR, ESI-MS, UV–vis and density functional theory calculation studies were conducted to understand the binding modes of HL-1 and HL-2 with Zn2+. These results revealed the binding stoichiometric ratio between receptors and Zn2+ were 1:1 with low detection limits and high binding constants. Two novel receptors HL-1 and HL-2 without and with hydroxyl groups were designed and synthesised via TCR reaction. Both of them showed selective coordination to Zn2+ ions but exhibited incongruent sensing mode due to structural variations. HL-1 showed monotonous ‘turn-on’ whereas HL-2 displayed highly sensitive ‘turn on’ and ‘ratiometric’ properties towards Zn2+.