Photoisomerizable Bipyridine Ligands and Macroligands: Absorption, Photoisomerization Properties and Theoretical Study

Two 2,2'-bipyridines, substituted at the 4,4'-positions by p-dialkylaminophenylazostyryl moieties p-R2NC6H4N=NC6H4-CH=CH [6a, R2N=nBu2N; 6b, R2N=(nBu)(C4H8OTHP)N; 6c, R2N= (nBu)(C4H8OH)N], were successfully synthesized by using Wadworth- Emmons reactions. The X-ray structure of 6a has been determined. Esterification of 6c with 2-bromoisobutyroylbromide afforded 6d. This ligand was used as an initiator for the living radical polymerization of methylmethacrylate (MMA) and gave rise to macroligand 6e. Thin films of good optical quality were obtained by the spin-coating technique. Photoisomerization experiments were carried out on 6a in solution and on 6e 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 6a' (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 mol1 less stable than the E-E isomer, is nonplanar. The theoretical studies also reveal that several transitions of p- p*, n-p* and charge-transfer (CT) types, are involved in the longwavelength transition of 6a (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.