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‐R 2 NC 6 H 4 NNC 6 H 4 ‐CHCH[ 6 a , R 2 N n Bu 2 N; 6 b , R 2 N( n Bu)(C 4 H 8 OTHP)N; 6 c , R 2 N( n Bu)(C 4 H 8 OH)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’ (R 2 NMe 2 N) 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 π– π *, n –π * 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.