Amidofluorene-appended lower rim 1,3-diconjugate of calix[4]arene: synthesis, characterization and highly selective sensor for Cu(2.)

Abstract

Functionalization of calix[4]arene with amidofluorene moieties at the lower rim led to formation of the 1,3-diconjugate of calix[4]arene L as a novel fluorescent chemosensor for Cu(2+). The receptor molecule L exhibited a pronounced selectivity towards Cu(2+) over other mono and divalent ions. The formation of the complex between L and Cu(2+) was evaluated by absorption, fluorescence and (1)H NMR spectroscopy. The sensor L showed a remarkable color change from colorless to purple and a fluorescence quenching only upon interaction with Cu(2+). The 1:1 stoichiometry of the obtained complex has been determined by Job's plot. The association constant determined by fluorescence titration was found to be 1.8 × 10(6) M(-1). The sensor showed a linear response toward Cu(2+) in the concentration range from 1 to 10 µM with a detection limit of 9.6 × 10(-8) M.

Keywords: calix[4]arene; chemosensor; copper ions; fluorene; fluorescence.

Scheme 1

Fluorene appended 1,3-diconjugate of calix[4]arene.

Scheme 2

Synthesis route of fluorene-appended amido-linked 1,3-diconjugate of calix[4]arene L.

Figure 1

Absorption spectra of L (1.0 × 10⁻⁵ M) and its complexes with different metals (10 equiv) in MeCN.

Figure 2

Color changes of receptor L upon addition of 10 equiv of various metal ions as their perchlorate salts.

Figure 3

Influence of the addition of increasing amounts (0–100 equiv) of Cu²⁺ on the absorption spectra of L (1.0 × 10⁻⁵ M) in MeCN.

Figure 4

Fluorescence spectra of L (1.0 × 10⁻⁵ M) in MeCN upon addition of different metal ions (10 equiv) with an excitation at 280 nm.

Figure 5

(a) Fluorescence spectra (1.0 × 10⁻⁵ M) of L recorded upon the addition of copper ion (0–5 equiv) in MeCN; excitation at 280 nm and emission at 365 nm. (b) Calibration curve of fluorescence intensity of L with Cu²⁺ ion concentrations.

Figure 6

Fluorescence intensity of L (1.0 × 10⁻⁵ M) upon the addition of 10 equiv Cu²⁺ in the presence of 10 equiv of other metal ions (as perchlorate salts) in MeCN.

Figure 7

¹ H NMR (400 MHz, CD₃CN) spectra of L upon addition of (a) 0.0 equiv, (b) 0.2 equiv, (c) 0.4 equiv, (d) 0.6 equiv, (e) 0.8 equiv, (f) 1.0 equiv, (g) 1.2 equiv and (h) 1.5 equiv of Cu²⁺.

Scheme 3

A proposed binding mode between L and Cu²⁺.