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

Abstract

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 helixdisrupting 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 binaphthylene main chain segments likely serve as the genesis for photo-and thermo-regulated behavior in these materials.KEY WORDS Stimuli-Responsive Chiroptical Behavior / trans-cis Azobenzene Isomerization / Helical Polymers / Circular Dichroism / Optical Rotation / Exciton Coupling / Cardo Polymers / Macromolecules endowed with single-handed helical geometries can be prepared by a variety of synthetic methods. Monomers based on chiral 1,1 -binaphthylenes, 1,1 -biphenylenes and other axially dissymmetric ring systems provide convenient access to condensation polymers with chiral helical motifs. The polycondensation of (R)-(+)-1,1 -binaphthyl-2,2 -diamine with terephthaloyl chloride was first reported by Schulz and Jung over thirty years ago. 1 The resulting polyaramide exhibited significant optical rotatory power that was attributed to the presence of a single helical screw sense along the polymer's conformationally restricted backbone. In the decades that have followed, a variety of other optically active macromolecules have been assembled from atropisomeric, helix-directing monomers using this same generalized approach. 2 Earlier, this laboratory reported the preparation of a number of helical trans-azobenzene modified polyaramides from both (R)-and (S )-1,1 -binaphthyl-2,2 -diamine. 3, 4 Some of these materials were specifically designed to undergo reversible helix order ↔ disorder transitions in response to multiple trans ↔ cis azobenzene isomerization reactions triggered within the polymer's main chain. [5]...

Stimuli-responsive polymers. 11. Highly adaptive poly(urea-amide)s that display solvent, light and heat modulated chiroptical behavior

“…That polycondensation reactions involving 2,2 0 -disubstituted binaphthylene monomers generally lead to polymers having only modest molecular weights was originally reported by Takeishi et al [25]. For these systems, the growth of a polymer chain can be suppressed when atropisomeric binaphthylene units undergoing addition adopt cisoid conformations that place their 2-and 2 0 -substituents into close spatial proximities [9,10,26]. As we have Scheme 1.…”

9,9-Diarylfluorene-Based Poly(alkyl aryl ether)s: Synthesis and Property

“…Meanwhile, such polymers also display excellent filler-dispersing ability [10][11][12] in addition to their high thermal stability, [13][14][15][16] high solubility, [17][18][19] and so on. [20][21][22][23][24][25] We have recently reported the special -interaction of the 9,9-diarylfluorene moiety in the matrix polymers toward -face of the carbon fillers.…”

“…[1][2][3][4][5][6][7][8][9] Such optical property is caused by the presence of many aromatic rings in the 9,9-diarylfluorene moiety which occupy the different planes to disturb the interchromophore packing, effectively decreasing the optical anisotropy. Meanwhile, such polymers also display excellent filler-dispersing ability [10][11][12] in addition to their high thermal stability, [13][14][15][16] high solubility, [17][18][19] and so on. [20][21][22][23][24][25] We have recently reported the special -interaction of the 9,9-diarylfluorene moiety in the matrix polymers toward -face of the carbon fillers.…”