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 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.