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

Two novel receptors <b>HL-1</b> and <b>HL-2</b> 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. <b>HL-1</b> showed monotonous ‘turn-on’ response towards Zn²⁺ while <b>HL-2</b> 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 <b>HL-1</b> and <b>HL-2</b> 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 <b>HL-1</b> and <b>HL-2</b> 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. <b>HL-1</b> showed monotonous ‘turn-on’ whereas <b>HL-2</b> displayed highly sensitive ‘turn on’ and ‘ratiometric’ properties towards Zn²⁺.