Induced circular dichroism spectra of alpha-, beta-, and gamma-cyclodextrin complexes with pi-conjugate compounds. Part 2. Chiral dimer formation andpolarization directions of the pi-pi* transitions in some hydroxyazo guests having a naphthalene nucleus

The absorption and circular dichroism spectra of some o- and p-hydroxyazo c ompounds having a naphthalene nucleus in the presence of alpha-, beta-, and gamma-cyclodextrin (CDx) have been measured in the wavelength region of 20 0-600 nm. The induced circular dichroism (ICD) is observed on the absorptio n bands of the pi-pi* transitions of the azo chromophore and the naphthalen e moiety. The signs in the observed ICD spectra are discussed in terms of t he orientation of the transition moment of the pi-pi* states to the axis of the CDx cavity and the polarization analysis by using the ZINDO approximat ion. Judging from the single positive sign df the ICD signal, the polarizat ion directions of the first pi-pi* transitions of the azo chromophore are a lmost parallel to the CDx axis. Some beta- and gamma-CDx complexes of the t itle azo compounds show a split-type [(+) to (-) or (-) to (+)] ICD band at the pi-pi* 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.