Synthesis of calix[4]arenes bearing thiosemicarbazone moieties with naphthalene groups: Highly selective turn off/on fluorescent sensor for Cu(II) recognition

Calix[4]arenes-based fluorescent chemosensors containing thiosemicarbazone as a binding site and naphthalene as a fluorogenic unit in a 1,3-alternate (L1) and cone (L2) conformation, were synthesized and examined for their binding abilities towards anions and cations using different spectroscopic techniques. Both receptors L1 and L2, exhibited an off/on fluorescence behaviour due to complex formation with Cu(II). They showed selective recognition of Cu(II) in presence of other cations and anions detecting up to 4.25 µg/L for L1 and 1.58 µg/L for L2, both in acetonitrile. The complexation behaviour was studied by 1H NMR and FT-IR spectral analysis, indicating that copper interacts with sulphur and with imine nitrogen of the thiosemicarbazone. No major differences in the selectivity and sensitivity were found between calix[4]arene receptors in 1,3-alternate and cone conformation. The 1H NMR spectra indicates that receptor L2 changed its conformation from cone to pinched cone, stiffening the conformation due to interaction of the thiosemicarbazone moiety with copper.