Bis(phenylthienyl)ethene-tethered beta-cyclodextrin dimers as photoswitchable hosts.

Two beta-cyclodextrin dimers tethered by photoswitchable bis(phenylthienyl)ethene moieties were synthesized as potentially tunable receptor molecules. The cyclodextrin cavities of these dimers were linked via their secondary sides, with the photochromic bis(phenylthienyl)ethene unit either directly connected to the secondary rim (7) or via propyl spacers (10). By irradiation with light the dimers were reversibly switched between a relatively flexible (open) form and a rigid (closed) form. The photostationary states for both dimers consisted of 92% of the open and 8% of the closed form, enabling the nearly complete conversion between the two forms. The binding properties of the open and closed forms of dimers 7 and 10 were assessed by complexation studies with meso-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) using isothermal titration calorimetry. For the rigidly tethered dimer 7, a factor 8 difference in binding affinity between the open and closed form of the dimer was found. This difference in binding affinity reflects the difference in enthalpy of binding for the two dimers, indicating that the beta-cyclodextrin cavities of the closed dimer 7b are spaced too far apart from each other by the rigid closed bis(phenylthienyl)ethene tether to cooperatively bind TSPP. The difference in binding affinity was sufficient to enable the phototriggered release of TSPP from dimer. The thermodynamic parameters obtained for dimer 10 suggested that the closed tether substantially contributes to the binding of TSPP. The open and closed form of dimer 10 bound TSPP with similar association constants, although the enthalpy of binding for the complexation of TSPP by the closed form of dimer 10 was more favorable than that found for the open form of the dimer.

Photocontrolled release and uptake of a porphyrin guest by dithienylethene-tethered beta-cyclodextrin host dimers.

Two photoswitchable dithienylethene-tethered beta-cyclodextrin dimers were synthesized to function as host molecules with an externally controllable binding affinity. The cyclodextrin cavities of these dimers are linked through their secondary sides by a photochromic dithienylethene unit that is connected to the secondary rim either directly (4) or through propyl spacers (9). Irradiation with light switches these dimers between a relatively flexible (open) and a rigid (closed) form. The binding properties of the dimers depend on the configuration of the dithienylethene spacer, as is shown by microcalorimetry performed with tetrakis-sulfonatophenyl porphyrin (TSPP) as a guest molecule. The differences in binding properties are most pronounced for the more rigid dimer 4, which binds TSPP 35 times more strongly in the open form (4 a) than in the closed form (4 b). The values found for the enthalpy of binding (deltaH degrees ) indicate that this difference in binding is due to the loss of cooperativity between the two beta-cyclodextrin cavities in the closed form. Molecular modeling shows that 4 b is not able to bind TSPP effectively in both cyclodextrin cavities. The open and closed forms of the more flexible dimer 9 show no substantial difference in their binding of TSPP. Thermodynamic values indicative of strong binding of TSPP by two beta-cyclodextrin cavities were measured for both forms of the dimer, and molecular modeling confirms that both are flexible enough to tightly bind TSPP. The binding differences between the forms of dimer 4 allow the photocontrolled release and uptake of TSPP, which renders control of the ratio of complexed to free TSPP in solution possible.

A dithienylethene-tethered beta-cyclodextrin dimer as a photoswitchable host.

The binding of TSPP by a dithienylethene-tethered beta-cyclodextrin dimer can be altered reversibly by irradiation with light.