Intrinsic Calix[6]arene Fluorescence for Quantitative Characterization of Group I and Group II Metal Cation Complexation

Calixarenes lacking additionally introduced fluorophores have long been considered insufficiently fluorescent for fluorimetric sensing. We have recently shown that simple calix[4]arene derivatives possess weak intrinsic fluorescence which significantly changes upon cation binding, raising a question whether the same holds for their larger and more flexible analogs. The aim of this work was to investigate the intrinsic fluorescence of the tertiary-amide calix[6]arene derivative L and to establish whether the changes accompanying complexation can be used for quantitative characterization of alkali and alkaline earth metal cation complexation. Stability constants of the 1:1 and 2:1 (cation:ligand) complexes in acetonitrile were determined by spectrofluorimetric titrations and validated by UV absorption spectrophotometry and isothermal titration calorimetry. Compared with the latter techniques, fluorimetry could be performed at much lower receptor concentrations, in some cases in the micromolar range. Complex stoichiometries and structures were further examined by NMR spectroscopy, mass spectrometry, and molecular dynamics simulations. Although L contains no additional fluorophore, it exhibits relatively weak intrinsic fluorescence which is strongly affected by cation binding. Among the alkali metal ions, pronounced fluorescence enhancement was observed for K+ and to a slightly lesser extent Rb+, whereas Li+ and Cs+ caused fluorescence quenching. Sodium ions induced both effects, depending on concentration. In contrast, all alkaline earth metal cations reduced the fluorescence intensity. The emission changes were interpreted in terms of variations in the absorption spectra and conformational rigidification upon complexation, as well as possible suppression of the photoinduced electron transfer and involvement of charge-transfer character of the excited state. Fluorimetry based on the intrinsic fluorescence of L proved superior to absorption spectrophotometry and microcalorimetry for the quantitative characterization of complexation reactions, offering a valuable advantage for poorly soluble systems and highly stable complexes.