Highly Sensitive and Selective Fluorescent Probes for Cu(II) Detection Based on Calix[4]arene-Oxacyclophane Architectures

• Main Authors: Alexandra I. Costa, Patrícia D. Barata, Carina B. Fialho, José V. Prata
• Format: Article
• Language: English
• Published: MDPI AG 2020-05-01
• Series: Molecules
• Subjects: calix[4]arene; copper; supramolecular; inclusion complex; fluorescence; sensor
• Online Access: https://www.mdpi.com/1420-3049/25/10/2456

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-<i>alt</i>-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 <i>O</i>-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 (<i>K</i>_a = 5.30 − 8.52 × 10⁴ M⁻¹) and the free energy changes for the inclusion complexation (−Δ<i>G</i> = 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 <b>4</b> and <b>5</b>), synthesized in good yields by Sonogashira–Hagihara methodologies, exhibit high fluorescence quantum yields (<i>Φ</i>_F = 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.