A minimalist [2]rotaxane with orthogonal Li⁺ and F^- binding sites and differential fluorescence response

A minimalist [2]rotaxane, comprising a benzimidazole-dimesitylborane axle mechanically interlocked with a crown ether macrocycle, was designed and synthesized as a dual-responsive heteroditopic receptor. Single-crystal X-ray diffraction unambiguously confirmed the interlocked architecture, revealing hydrogen-bonding interactions between the imidazolyl N-H donor and the crown ether oxygen atoms. ¹H NMR binding studies demonstrated that the rotaxane binds Li⁺ ions at a cooperative binding pocket formed by the crown ether and the imidazole unit, while F^- ions bind selectively at the Lewis acidic boron center via formation of a stable fluoroborate species under a slow-exchange regime. Competition experiments established that Li⁺ binding thermodynamically prevails over F^- binding, leading to quantitative displacement of fluoride upon addition of a lithium salt. Density functional theory (DFT) and molecular dynamics (MD) calculations were performed to further examine the 1 : 1 binding complexes for both ionic guests. Fluorescence spectroscopy revealed markedly distinct signal responses: Li⁺ binding induced a 4.6-fold fluorescence enhancement at 456 nm attributable to rigidification of the molecular framework, whereas F^- binding resulted in appreciable quenching observed upon addition of a stoichiometric amount of F^- ion. Importantly, subsequent addition of Li⁺ to the pre-formed [1 + F^-] adduct restored the fluorescence. The mutual exclusivity of the two binding events and the differential fluorescence outputs render this rotaxane a functional prototype for molecular logic operations, demonstrating that sophisticated dual responsiveness can be realized within a structurally concise mechanically interlocked framework.