Synthesis and photophysical and cation-binding properties of mono- and tetranaphthylcalix[4]arenes as highly sensitive and selective fluorescent sensors for sodium

The syntheses and properties of two calixarene-based fluorescent molecular sensors are reported. These comprise tert-butylcalix[4]arene either with on e appended fluorophore and three ester groups (Calix-AMN1), or with four ap pended fluorophores (Calix-AMN4). The fluorophore is 6-acyl-2-methoxy-napht halene (AMN), which contains an electron-donating substituent (methoxy grou p) conjugated to an electron-withdrawing substituent (carbonyl group); this allows photoinduced charge transfer (PCT) to occur upon excitation. The in vestigated fluoroionophores thus belong to the family of PCT fluorescent mo lecular sensors. In addition to the expected red shifts of the absorption a nd emission spectra upon cation binding, a drastic enhancement of the fluor escence quantum yield-in an "off-on" fashion comparable to that seen in pho toinduced electron transfer (PET) molecular sensors-was observed. For Calix -AMN1, it increases from 10(-3) for the free ligand to 0.68 for the complex with Ca2+. This exceptional behaviour can be interpreted in terms of the r elative locations of the n pi* and pi pi* levels, which depend on the charg e density of the bound cation. For Calix-AMN4, in addition to the photophys ical effects observed for Calix-AMN1, interactions between the chromophores by complexation with some cations have been found in the ground state (hyp ochromic effect) and in the excited state (excimer formation). Steady-state fluorescence anisotropy measurements for the system Na+ subset of Calix-AM N4, show a depolarization effect due to energy transfer (homotransfer) betw een the fluorophores. Regarding the complexing properties, a high selectivi ty for Na+ over K+, Li+, Ca2+ and Mg2+ was observed in ethanol and ethanol- water mixtures. The selectivity (Na+/other cations) expressed as the ratio of the stability constants was found to be more than 400.