Photochemical closing and opening of the guest-binding cavity of cyclodextrins

CYCLODEXTRINS and modified derivatives can bind, and sometimes modify the properties of, guest molecules in their torus-shaped cavities^1,2. They have also been used as the building blocks of molecular materials and devices³. The propensity to bind and retain a guest is not easily predictable or controllable, however. There is currently much interest in the switching on and off of chemical⁴ and biological5 activity, particularly by photochemical means⁶, as such functions will be required of molecular-scale devices. Here we report the controlled binding and release of guest molecules in cyclodextrins modified with substituents that can reversibly form bridging units across the cavity openings. Irradiation of percinna-moylated α- or β-cyclodextrin in N-methylpyrrolidin-2-one (NMP) leads to the formation of intramolecular cyclobutane bridges which trap a bound NMP molecule. Irradiation at a different wavelength breaks open the cyclobutane rings and releases the guest.