Induced circular dichroism spectra of α-, β-, and γ-cyclodextrin complexes with π-conjugate compounds. Part 2. Chiral dimer formation and polarization directions of the π–π* transitions in some hydroxyazo guests having a naphthalene nucleus †

Abstract

The absorption and circular dichroism spectra of some o- and p-hydroxyazo compounds having a naphthalene nucleus in the presence of α-, β-, and γ-cyclodextrin (CDx) have been measured in the wavelength region of 200–600 nm. The induced circular dichroism (ICD) is observed on the absorption bands of the π–π* transitions of the azo chromophore and the naphthalene moiety. The signs in the observed ICD spectra are discussed in terms of the orientation of the transition moment of the π–π* states to the axis of the CDx cavity and the polarization analysis by using the ZINDO approximation. Judging from the single positive sign of the ICD signal, the polarization directions of the first π–π* transitions of the azo chromophore are almost parallel to the CDx axis. Some β- and γ-CDx complexes of the title azo compounds show a split-type [(+) to (–) or (–) to (+)] ICD band at the π–π* transitions of the azo group, suggesting that a chiral dimer is trapped within the CDx cavity. The ICD bands of the naphthalene moiety may also be useful to detect the formation of the chiral dimer.