Stimuli-responsive polymers. III. Poly(aryl ether ketone amide)s with reversibletrans- ?cis-4,4?-azobenzene and fixed 2,2?-binaphthyl kinking elements in the main chain

Stimuli-Responsive Polymers. 5. Azobenzene Modified Polyaramides Containing Atropisomeric Binaphthyl Linkages: Tuning Chiroptical Behavior with Light and Heat

“…Starting from the 65:35 cis- to- trans photostationary state reached in 1(R) , a series of thermally mediated cis → trans relaxation steps within the polymer's main chain will simultaneously restore both the number of trans- azobenzene linkages and the helical conformations that they induce, thus reversing the chiroptical response shown in plot T 0 of Figure . That the CD plot obtained for 1(R) after 214 h in the dark was nearly identical to the spectrum for the preirradiated polymer sample is consistent with a cis- azobenzene half-life near 60 h determined by optical absorbance spectroscopy under these experimental conditions …”

“…In an effort to construct high-performance materials endowed with stimuli-responsive chiroptical behavior, we have recently prepared a series of azobenzene modified polymers and related model compounds that are fitted with axially asymmetric R - or S -binaphthyl groups (Scheme ). As was reported earlier, these film forming materials are amorphous, highly tractable, and thermally stable at temperatures exceeding 400 °C. , A number of these constructs adopt single-handed helical geometries in solution that are strongly perturbed by the trans − cis isomerization reaction. Herein, we demonstrate that light and heat can be effectively used to tune the chiroptical properties exhibited by this new polymer family.…”

“…This value is in good accord with an activation energy determined for the cis f trans isomerization reaction by optical absorbance spectroscopy. 8,9 The azobenzene isomerization process predicted to restore helical backbone structure and the optical rotatory power exhibited by the polymer sample thus appear to be well correlated. This finding is fully consistent with our explanation posited above for the origins of chiroptical behavior in these stimuli-responsive materials.…”

“…Activation energies for thermal "back"-reactions that restored the population of trans-azobenzene groups in the polymer chains were reported earlier. 8,9 These values fell near 21-22 kcal mol -1 when evaluated in THF or DMAC solvent media. Importantly, cis-isomer half-life values at room temperature were greater than 1 day, enabling the acquisition of chiroptical data over short time periods that were uncomplicated by rapidly shifting cistrans isomer ratios.…”

Stimuli-Responsive Polymers VI. Azobenzene Modified Copolyaramides with Chiral Helical Geometries: Influence of Rigid, Helix-Disrupting Segments on Physiochemical and Chiroptical Behavior

“…Importantly, cis-isomer half-life values determined at room temperature for all of the polymers exceeded 100 h, allowing for the acquisition of chiroptical data over short time periods that were uncomplicated by rapidly shifting cis-trans isomer populations within the polymer backbones. As observed for other azobenzene modified copolyaramides synthesized in our laboratory, 4,6,8,13 the interconversion of trans-and cisisomeric states within these constructs was completely reversible over numerous isomerization cycles without any apparent hysteresis or photodegradation effects.…”