Photochromism of the complex between 4′-(2-hydroxyethoxy)-7-hydroxyflavylium and β-cyclodextrin, studied by 1H NMR, UV–Vis, continuous irradiation and circular dichroism

• The chalcone derived from a flavylium compound forms an inclusion complex with β-cyclodextrin.
• The host–guest complex was characterized by rotating frame nuclear Overhauser spectroscopy (ROESY) and circular dichroism.
• A mathematical model was deduced to account for the thermodynamic and kinetics of the photochromic host–guest system.

The network of chemical reactions of the compound 4′-(2-hydroxyethoxy)-7-hydroxyflavylium and its photochromic properties were studied in the absence and presence of β-cyclodextrin. The system was characterized by 1H NMR, UV–Vis, continuous irradiation and circular dichroism. In the presence of β-cyclodextrin 4.8 mM the pK′a is reduced to 2.2. The association constants with β-cyclodextrin, follow the order trans-chalcone > quinoidal base > flavylium cation, respectively 1.7 × 103 M−1; 5.6 × 102 M−1; ≈0. The rate of inter-conversion of the network species increases in the presence of β-cyclodextrin circa 2.6 times. The appearance of an induced circular dichroism signal of trans-chalcone inside the β-cyclodextrin is described for the first time. It gives information about the position of the guest inside the host and allows the calculation of the respective association constant.

Incorporation of the chalcone into β-cyclodextrin leads to an induced circular dichroism signal providing direct evidence for the inclusion complex.Figure optionsDownload as PowerPoint slide