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

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 (K_a = 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 (Φ_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.

Keywords: calix[4]arene; copper; density functional theory; fluorescence; inclusion complex; sensor; supramolecular.

Chart 1

Chemical structures of Calix-OCP-2-CBZ (a) [28] and Calix-OCP-PPE (b) [26].

Scheme 1

Cross-coupling of calix[4]arene diiodo derivatives 1 and 2 with 2,7-diethynyl-9-propyl-9H-carbazole (3) using PdCl₂(PPh₃)₂/CuI catalytic system in toluene/NEt₃ at 35 °C for 24–48 h.

Figure 1

Absorption (green line), excitation (red line, monitored at emission maxima) and emission (blue line) spectra (λ_exc = 380 nm, CHCl₃) of polymer 4 (a) and polymer 5 (b).

Figure 2

(a) Emission spectra of polymer 5 (5.0 × 10⁻⁶ M in CH₃CN) upon addition of increasing amounts (0.25 − 5.8 equivalent) of Cu(ClO₄)₂ (λ_exc = 380 nm). Inset: photo of polymer 5 fluorescence under UV irradiation (366 nm) before (1) and after (2) Cu(ClO₄)₂ addition (5 equivalent); (b) Binding isotherm for the fluorimetric titration of 5 with Cu(II) with fitted curve and confidence intervals (see text).

Figure 3

Job plot for complex formation between host 5 and Cu(II) in CH₃CN (at constant 1.0 × 10⁻⁵ M total concentration) as obtained from changes in fluorescence (λ_exc = 380 nm).

Chart 2

Chemical structure of bis-Calix-TriPr-2-CBZ [23].

Figure 4

Absorption spectra of (a) polymer 5 (8.2 × 10⁻⁵ M in CH₃CN:CHCl₃, 1:1) and (b) Calix-OCP-2-CBZ (5.0 × 10⁻⁶ M in CH₃CN) upon addition of increasing amounts of Cu(ClO₄)₂.

Figure 5

Energy and geometry optimised structures of Calix-OCP (a) and Calix-OCP-Cu(II) (b) models (side views). β-HOMO (c) and α-HOMO (d) molecular orbitals mapped on the structure of the complex (side views). DFT calculations run at the B3LYP/6-31G (d) level of theory in vacuum [44]. Hydrogens omitted for clarity. Colour codes for elements: red = oxygen, grey = carbon, green = copper.

Figure 6

(a) Emission spectra of polymer 4 (5.0 × 10⁻⁶ M in CH₃CN) upon addition of increasing amounts (up to 14.5 eq.) of Cu(ClO₄)₂ (λ_exc = 380 nm); (b) Binding isotherm for the fluorimetric titration of 4 with Cu(II) along with the fitted curve and confidence intervals.