Photochromic molecular recognition of β-cyclodextrin bearing spiropyran moiety for organic guests

The photochromic behaviour and guest binding properties of β-cyclodextrin modified with 1′,3′,3′-trimethyl-6-nitro-8-methylspiro[2H-1-benzopyran-2,2′-indoline](1) has been investigated. Compound 1 exhibits a unique photochromic response in comparison to those of the 1′-(3-oxopropyl)-3′,3′-dimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-indoline] modified β- and γ-cyclodextrins (2 and 3, respectively). Compound 1 shows reverse photochromism in aqueous or ethylene glycol solutions, however, in less polar solvent such as dimethyl sulfoxide, 1 exhibits a mixed type of normal and reverse photochromism. The closed form of 1 in aqueous solution is converted to the opened form by keeping it in the dark. The halflives of the closed form in the dark are affected by the presence of a guest. The highest value is obtained when 1-borneol was used. The magnitude of the half-lives and the binding constant for a guest examined are comparable, and is dependent on the guest molecular size.