Excimer Disaggregation Enhanced Emission: A Fluorescence “Turn-On” Approach to Oxoanion Recognition

Abstract

A new approach to anion sensing that involves excimer disaggregation induced emission (EDIE) is reported. It involves the anion-mediated disaggregation of the excimer formed from a cationic macrocycle. This leads to an increase in the observed fluorescence intensity. The macrocycle in question, cyclo[1] N², N⁶-dimethyl- N², N⁶-bis(6-(1 H-imidazolium-1-yl)pyridin-2-yl)pyridine-2,6-diamine[1]1,4-dimethylbenzene (1²⁺; prepared as its PF₆^- salt), is obtained in ca. 70% yield via a simple cyclization. X-ray diffraction analyses of single crystals revealed that, as prepared, this macrocycle exists in a supramolecular polymeric form in the solid state. Macrocycle 1²⁺ is weakly fluorescent in acetonitrile. The emission intensity is concentration dependent, with the maximum intensity being observed at [1²⁺] ≈ 0.020 mM. This finding is ascribed to formation of an excimer, followed possibly by higher order aggregates as the concentration of 1²⁺ is increased. Addition of tetrabutylammonium pyrophosphate (HP₂O₇^3-) to 1²⁺ (0.020 mM in acetonitrile) produces a ca. 200-fold enhancement in the emission intensity (λ_ex = 334 nm; λ_em = 390-650 nm). These findings are rationalized in terms of the HP₂O₇^3- serving to break up essentially non-fluorescent excited-state dimers of 1²⁺ through formation of a highly fluorescent anion-bound monomeric complex, 1²⁺·HP₂O₇^3-. A turn-on in the fluorescence intensity is also seen for H₂PO₄^- and, to a lesser extent, HCO₃^-. Little (HSO₄^-, NO₃^-) or essentially no (N₃^-, SCN^-, F^-, Cl^-, Br^- and I^-) response is seen for other anions. Solid-state structural analysis of single crystals obtained after treating 1²⁺ with HP₂O₇^3- in the presence of water revealed a salt form wherein a H₂P₂O₇^2- anion sits above the cone-like macrocycle.