Intrinsic fluorescence of calixarenes: the origin and applications

Calixarene derivatives have been widely recognized as promising fluorescent sensors when suitably functionalized with fluorophoric groups, as it has been believed that these compounds are not enough intrinsically fluorescent. In this work, we investigated the intrinsic fluorescence of three calix[4]arenes lacking additional fluorescent moieties and explored the possibility of quantitative fluorimetric examination of their cation-binding processes taking place in different solvents (water, methanol, and acetonitrile).

The obtained results clearly showed that, although the fluorescence of free calixarenes in solutions was relatively low, it substantially increased, and was blue shifted to some extent, upon alkali and alkaline earth metal cation binding. For comparison, fluorescence of a monomeric compound which constitutes tertiary-amide calixarene derivative and the corresponding complexation-induced spectral changes were studied as well. The excited-state lifetimes of the free ligands and their cation complexes were measured and discussed. The detailed quantum chemical calculations provided an insight into the origin of the observed luminescence.

Overall, the results of this comprehensive and integrated investigation indicate that, contrary to current opinion, spectrofluorimetry can be successfully employed as an effective and sensitive technique for the quantitative monitoring of complexation reactions involving calixarene derivatives and related compounds, regardless of whether additional fluorophores are present in their structures.