Abstract
A new topological design of fluorescent probes for sensing copper ion is disclosed. The calix[4]arene-oxacyclophane (Calix-OCP) receptor, either wired-in-series in arylene-alt-ethynylene conjugated polymers or standing alone as a sole molecular probe, display a remarkable affinity and selectivity for Cu(II). The unique recognition properties of Calix-OCP system toward copper cation stem from its pre-organised cyclic array of O-ligands at the calixarene narrow rim, which is kept in a conformational rigid arrangement by a tethered oxacyclophane sub-unit. The magnitude of the binding constants (Ka = 5.30 − 8.52 × 104 M−1) and the free energy changes for the inclusion complexation (−ΔG = 27.0 − 28.1 kJmol−1), retrieved from fluorimetric titration experiments, revealed a high sensitivity of Calix-OCP architectures for Cu(II) species. Formation of supramolecular inclusion complexes was evidenced from UV-Vis spectroscopy. The new Calix-OCP-conjugated polymers (polymers 4 and 5), synthesized in good yields by Sonogashira–Hagihara methodologies, exhibit high fluorescence quantum yields (ΦF = 0.59 − 0.65). Density functional theory (DFT) calculations were used to support the experimental findings. The fluorescence on–off behaviour of the sensing systems is tentatively explained by a photoinduced electron transfer mechanism.
Highlights
Molecular fluorescent systems for signalling supramolecular interactions have been in use for more than forty years [1,2]
(4 and 5) tethered were synthesized by a Pd-catalysed methodology
The three-dimensionality of the host cavity, providing a conformationally rigid cyclic array of O-ligands at the narrow rim of the calixarene fostered by the tethered oxacyclophane sub-unit, was found to be essential for the development of a strong binding between the host and the copper cation
Summary
Molecular fluorescent systems for signalling supramolecular interactions have been in use for more than forty years [1,2]. When aimed for ion detection, the system usually comprises either a fluorogenic unit covalently linked through some sort of a spacer to an ion receptor or an integrated fluorophore-ion recognition site assembly [1,2,3]. The photophysical changes that occur upon cation binding, which are the basis for signalling the binding event, may have different origins. The electronic and stereochemical characteristics of the recognition unit and the fluorophore, as well as the type of metal ion, dictate the actual mechanism. Examples covering all the above-mentioned processes are known [4]
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