1,2,3-Triazole-Containing Molecular Pockets Derived from Cholic Acid: The Influence of Structure on Host-Guest Coordination Properties

Two cholic acid-containing trimers with 1,2,3-triazole groups close to the connecting point ("top") and at the end of the cholic acid arms ("bottom") were synthesized. These molecules arc able to form hydrophobic pockets and solubilize pyrene and other hydrophobic molecules in polar media due to the facial amphiphilicity of cholic acid. Heavy metal ions such as Cu(II) can also be coordinated by the 1,2,3-triazole groups, as shown by EPR spectroscopy. Due to the vicinity of metal ions and pyrene in the pockets, the fluorescence of pyrene is quenched. However, the position of the 1,2,3-triazole groups has a crucial influence on the metal ion complexation. The trimer with 1,2,3-triazole groups at the top is found to bind metal ions more effectively and gives rise to a significantly enhanced fluorescence quenching efficiency. Here, the metal ions act as one tridentate chelating agent, while the triazoles on the bottom rather behave as single entities without any cooperativity in binding to the metal. In the latter case, the quenching effect is reduced considerably despite the fact that both trimers are able to bind Cu(II). This indicates that the specific and strong binding of Cu(II) at the top leads to closer spatial proximity between metal ion and pyrene when compared to the Cu(II) bound on the bottom.