title: Photoisomerizable bipyridine ligands and macroligands: absorption, photoisomerization properties and theoretical study.
abstract: Two 2,2'-bipyridines, substituted at the 4,4'-positions by p-dialkylaminophenylazostyryl moieties p-R2N-C6H4-N=N-C6H4-CH=CH-[6 a, R2N=nBu2N; 6 b, R2N=(nBu)(C4H8OTHP)N; 6 c, R2N=(nBu)(C4H8OH)N], were successfully synthesized by using Wadworth-Emmons reactions. The X-ray structure of 6 a has been determined. Esterification of 6 c with 2-bromoisobutyroylbromide afforded 6 d. This ligand was used as an initiator for the living radical polymerization of methylmethacrylate (MMA) and gave rise to macroligand 6 e. Thin films of good optical quality were obtained by the spin-coating technique. Photoisomerization experiments were carried out on 6 a in solution and on 6 e in both solution and film, and the kinetics of photochemical (E/Z) and thermal (Z/E) isomerization were investigated. They were found to show Z-E back isomerization typical of aminoazobenzene-type rather than of push-pull-type molecules. Density functional theoretical (TD-DFT) calculations were performed on model compound 6 a' (R2N=Me2N) to understand the structural and electronic transitions of the corresponding E-E, E-Z and Z-Z isomers. It was found that the E-E isomer is almost planar as observed experimentally by X-ray diffraction, whereas the Z-Z isomer, which is 35.4 kcal mol(-1) less stable than the E-E isomer, is nonplanar. The theoretical studies also reveal that several transitions of pi-pi*, n-pi* and charge-transfer (CT) types, are involved in the long-wavelength transition of 6 a (E-E). The same observations can be made for the (Z-Z) isomer, and the TD-DFT simulated spectrum fits quite nicely to the experimental, reproducing and explaining the apparition of a blue-shifted charge-transfer band at 390 nm.
authors.affiliation: Organométalliques et Catalyse: Chimie et Electrochimie Moléculaires, UMR 6509 CNRS-Université de Rennes 1, Institut de Chimie de Rennes, Campus de Beaulieu, 35042 Rennes Cedex, France.
authors.forename: Lydie
authors.initials: L
authors.lastname: Viau
authors.forename: Itamar
authors.initials: I
authors.lastname: Malkowsky
authors.forename: Karine
authors.initials: K
authors.lastname: Costuas
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Boulin
authors.forename: Loïc
authors.initials: L
authors.lastname: Toupet
authors.forename: Eléna
authors.initials: E
authors.lastname: Ishow
authors.forename: Keitaro
authors.initials: K
authors.lastname: Nakatani
authors.forename: Olivier
authors.initials: O
authors.lastname: Maury
authors.forename: Hubert
authors.initials: H
authors.lastname: Le Bozec