Encapsulation of Cinnamic Acid by Cucurbit[7]uril for Enhancing Photoisomerization

Cis- orZ-configuration is required for the plant growth-promoting activity of cinnamic acid (CA), whereas theE-form is inactive. Herein, we describe the encapsulation ofE-CA by cucurbit[7]uril (CB7) and show that photoisomerization reactions can be more efficiently controlled in aqueous solutions by utilizing this supramolecular approach. Measurements of UV-visible absorption and proton NMR spectra at different pH values confirm thatE-CA and its methyl ester, methyl-E-cinnamate (MC), form stronger 1:1 host-guest complexes with CB7 compared to cucurbit[8]uril (CB8) or three cyclodextrins (alpha-, beta-, and gamma-CD). Irradiation of (300 nm) UV light to an aqueous solution of the CB7-boundEisomers inducesEtoZphotoisomerization and the dissociation of the complex. When the same solution is irradiated by (254 nm) UV light,ZtoEconformational changes of the unboundZisomers are observed and are accompanied by restoring the host-guest complex formation.