Redox-Modulated Fluorescent Halogen Bonding and Hydrogen Bonding Anion Sensing

Anion sensing via either optical or electrochemical readouts has separately received enormous attention, however a judicious combination of the advantages of both modalities remains almost entirely 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 the 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 both anion binding strength and fluorescence switching magnitude, displaying significant fluorescence turn-OFF upon anion binding. In contrast, redox-inactive controls display a large 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.