Photoswitchable assembly of long-lived azobenzenes in water using visible light.

HYPOTHESIS: Switchable assemblies relevant for bio-applications may be accessed from water-soluble tetra-ortho-substituted azobenzenes that reversibly self-assemble and form complexes with ß-cyclodextrin under visible light. EXPERIMENTS: Two azobenzenes bearing either four fluorines or two chlorines and two fluorines in the ortho positions were synthesised with short poly(ethylene oxide) tails for water solubility. Photophysical properties were determined by UV-vis and 1H NMR spectroscopies, complexation with ß-cyclodextrin was assessed by 1H NMR spectroscopy, and self-assembly in water was investigated by static and dynamic light scattering. FINDINGS: Both molecules underwent trans-cis isomerization at 530 nm and cis-trans isomerization at 415 nm, with the cis forms exhibiting thermal half-lives > 300 days at room temperature. Both molecules formed inclusion complexes with ß-cyclodextrin in water, with cis-4F-AZO-PEO binding 3-fold stronger than trans, and 2Cl2F-AZO-PEO binding significantly weaker. Self-assembly of pure 2Cl2F-AZO-PEO in water showed an open association process regardless of configuration, while 4F-AZO-PEO showed an open association process for cis (Nagg increasing from 30 to 1000) but a closed association process for trans (Nagg stable at âˆ¼ 170). Aqueous solutions of 2Cl2F-AZO-PEO showed cloud points close to 45 °C, while the 4F-AZO-PEO isomers presented well-separated cloud points allowing reversible and all-visible transition between clear and turbid states at room temperature.

Quantitative Kinetic Modeling in Photoresponsive Supramolecular Chemistry: The Case of Water-Soluble Azobenzene/Cyclodextrin Complexes.

Hydrophilic host-guest complexes, consisting of water-soluble azobenzene and α-, ß-, or γ-cyclodextrins, have been proposed as a model to study supramolecular photoresponsive systems in aqueous environments through a full spectrometric approach combined with a simulation and data fitting methodology. Various essential and complementary spectroscopic techniques have been used: circular dichroism to determine whether the complex is formed or not, NMR for the stoichiometry elucidation, and UV-visible spectrophotometry to obtain the association equilibrium constant of each complex and the quantum yield for each photochemical process. A step-by-step fitting procedure is presented, which enables the determination of all thermodynamic and photokinetic parameters. A sequential methodology is applied to dissipate all uncertainties on the variability of the results and to develop a relevant and reliable protocol applicable to other types of complexes. The proposed procedure has thus been shown to be very robust and largely applicable to other photoresponsive host-guest systems.