19F NMR-based lithium sensors with fluoronaphthalene crown ethers

Abstract

We herein report 19F NMR-based lithium ion sensing methodology using fluorinated naphthalene compounds, which are further validated by UV–vis spectroscopy. The compounds consist of a monofluorinated naphthalene moiety with crown ether (1-aza-12-crown-4 or 1-aza-15-crown-5), which were abbreviated as FNC12 or FNC15, respectively. The two compounds were synthesized through Buchwald-Hartwig amination. Absorption spectra upon lithium binding to FNC12 exhibit the hypsochromic shifts, which can be attributed to the decreased conjugation length. 19F NMR spectra measured with FNC12 and lithium show downfield chemical shifts accompanied with peak broadening, resulting from rapid equilibrium of metal-ligand interactions. In 1H NMR spectra, similar chemical shifts and peak broadening were also observed. Limit of detection (LOD) and stability constant (Ks) were 17 μM and 50 M−1, respectively, determined from 19F NMR spectroscopy. To further examine the effect of a crown-ether ring size, we used FNC15 and performed identical titration experiments. Although it was previously known that 15-crown-5 can be bound to sodium in a selective way, we found that lithium can also bind to FNC15, stronger than sodium in our condition. Ks in FNC15–Li and FNC15–Na were around at 4000 M−1 and 2000 M−1, respectively. The effect of water on sensing was examined. This work provides a new lithium sensing method with 19F NMR spectroscopy and UV–vis spectroscopy.