Trans-cis Photoisomerization of 1-Methyl-4-(4′-hydroxystyryl)pyridinium in inclusion complexes of β-cyclodextrin and its derivatives

The effects of β-cyclodextrin (β-CyD), heptakis(2,6-di-O-methyl)-β-cyclodextrin (DMβCyD) and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin (TMβCyD) on trans-cis photoisomerization of 1-ethyl-4-(4′-hydroxystyryl)pyridinium (POH) have been studied in aqueous solutions. The ratio of [cis]/[trans] for POH in the photostationary state at pH 8.54 was remarkably reduced by the presence of βCyD or DMβCyD. The reduction of the [cis]/[trans] ratio in the photostationary state was explained in terms of the shift of the equilibrium of POH₊^trans ⇌ PO_trans + H^- toward PO_trans formation. The binding constants of βCyD and DMβCyD for PO_trans were 2.00- and 1.36-fold larger than those for POH₊^trans, respectively. The binding constants of TMβCyD for both species are much smaller than those of βCyD and DMβCyD. This result indicates that PO_trans, which has a betain structure, forms stable complexes with βCyD and DMβCyD with its hydrophobic parts inside and the charged parts outside the CyD cavities.