Varying solvent polarity to tune the enantioselective quenching of a calixarene host

Calixarene L1 has been designed to behave as a fluorescent molecular sensor capable of distinguishing between chiral amines on the basis of their size and chirality. Fluorescence quenching studies of calixarene L1 in methanol demonstrated no enantiomeric selectivity for a short chain amino alcohol, phenylglycinol, while excellent selectivity was observed for a longer chain amino alcohol, phenylalaninol (PA). The effect of solvent on the fluorescent properties of this calixarene in the presence of PA has been studied, and demonstrates that varying solvent polarity allows the wavelength of enantiomer selectivity to be tuned from 227 nm to 440 nm. While enantiomeric selectivity is observed in methanol at 227 nm, no discrimination is achieved in acetonitrile. Chiral discrimination is statistically possible with L1 and PA in chloroform at 227 nm, but it is not comparable with the extent of discrimination achieved in methanol. In chloroform a new emission band at a longer wavelength (440 nm) is formed with R-PA in solution with L1, an effect that is not observed with the S-enantiomer. This new band in chloroform at 440 nm allows very effective chiral discrimination and has been attributed to the presence of two different conformations of calixarene L1, which is reinforced by 1H-NMR studies and molecular modelling studies.