Stimuli-responsive polymers VI. Azobenzene modified copolyaramides with chiral helical geometries: Influence of rigid, helix-disrupting segments on physiochemical and chiroptical behavior

Abstract

Polymers comprised of alternating trans-4,4′-azobenzene and helix directing (R)-2,2′-binaphthylene backbone linkages adopt single handed helical conformations in solution that are reversibly distorted by light and heat regulated trans ↔ cis isomerization processes. The replacement of (R)-binaphthylene backbone groups with helix-disrupting 4,4′-diphenylfluorene segments affords permanent changes in some of the properties exhibited by this family of optically active materials. Polymer solubilities in organic solvents like THF and acetone are significantly diminished while thermal stabilities and photophysical behavior remain largely unchanged. Optical rotation magnitudes measured at the sodium D-line drop in a linear manner with the loss of the atropisomeric binaphthylene linkages. However, all of the polymer variants exhibit stimuli-responsive chiroptical behavior when evaluated in DMAC or THF solution environments. Specific rotation magnitudes are reversibly diminished by the near-UV light induced trans → cis isomerization reaction. Optical rotatory power is restored by reverse cis → trans isomerization processes triggered by heat or by visible light illumination. That stimuli-responsive chiroptical properties are exhibited by all polymer variants regardless of their monomer compositions indicates that global helical order along the polymer backbone is not necessarily required for this effect. Rather, isomerization induced perturbations to the local dihedral angles residing in the atropisomeric binaphthy-lene main chain segments likely serve as the genesis for photo- and thermo-regulated behavior in these materials.