Anion Sensing through Redox‐Modulated Fluorescent Halogen Bonding and Hydrogen Bonding Hosts**

Abstract

Anion sensing via either optical or electrochemical readouts has separately received enormous attention, however a judicious combination of the advantages of both modalities remains unexplored. Toward this goal we herein disclose a series of novel redox‐active, fluorescent halogen bonding (XB) and hydrogen bonding (HB) BODIPY‐based anion sensors, wherein introduction of a ferrocene motif induces remarkable changes in fluorescence response. Extensive fluorescence anion titration, lifetime and electrochemical studies reveal anion‐binding induced emission modulation through intramolecular photo‐induced electron transfer (PET), the magnitude of which is both dependent on the nature of the XB/HB donor and anion. Impressively, the XB sensor outperformed its HB congener in terms of anion binding strength and fluorescence switching magnitude, displaying significant fluorescence turn‐OFF upon anion binding. In contrast, redox‐inactive controls display a turn‐ON response, highlighting the pronounced impact of introduction of the redox‐active ferrocene on optical sensing performance. Additionally, the redox‐active ferrocene motif also serves as an electrochemical reporter group, enabling voltammetric anion sensing in competitive solvents. The combined advantages of both sensing modalities were further exploited in a novel, proof‐of‐principle, fluorescence spectro‐electrochemical anion sensing approach, enabling simultaneous and sensitive read out of optical and electrochemical responses in multiple oxidation states and at very low receptor concentration.