Induced circular dichroism spectra of α-, β- and γ-cyclodextrin complexes with sodium 4′-hydroxy-3′-isopropylazobenzene-4-sulfonate and sodium 4′-hydroxy-3′,5′-diisopropylazobenzene-4-sulfonate

Abstract

The induced circular dichroism (ICD) spectra of α-, β- and γ-cyclodextrin complexes with sodium 4′-hydroxy-3′-isopropylazobenzene-4-sulfonate, 3-Prⁱ(4-OH)C₆H₃–NN–C₆H₄SO₃^–Na⁺5 and sodium 4′-hydroxy-3′,5′-diisopropylazobenzene-4-sulfonate, 3,5-(Prⁱ)₂(4-OH)C₆H₂–NN–C₆H₄SO₃^–Na⁺8 in aqueous solution have been measured. ICD is observed on the absorption bands of the achiral azo guest molecules 5 and 8 which are included in the dissymmetric field of the chiral cyclodextrin (CD) cavity. The ICD spectral patterns change drastically depending on the shape and size of the alkyl substituents (Rⁱ and R^j) of the phenol moiety [RⁱR^j(4-OH)C₆H₂–] of the azo guest molecules, RⁱR^j(4-OH)C₆H₂–NN–C₆H₄SO₃^–Na⁺. The π→π* electronic transitions which are polarized along the long axis of the slim guest molecule 5 show almost a single positive ICD in all of the α-, β- and γ-CD inclusion complexes. This result indicates that the guest 5 is included from the long-axis side into the CD cavity. However, the ICD spectra of the β- and γ-CD complexes of the bulky guest molecule 8 show a split-type positive and negative ICD signal in the π→π* transition band. This splitting pattern may arise from the dimer formation of 8 in the form of the 2:2 (guest:CD)β- and γ-CD inclusion complexes.