Conformational Kinetics in Chiral Poly(diphenylacetylene)s: A Dynamic P/M Memory Effect

Abstract

Dynamic P/M (plus/minus) helical memory in chiral dissymmetric poly(diphenylacetylene)s (PDPA) is shown using a PDPA that bears the benzamide of (L)‐alanine methyl ester as pendant. For a single chiral polymer, it is possible to obtain either P or M helical structures in a specific solvent without the presence of any chiral external stimuli. To do that, it is necessary to combine the conformational control at the pendant group with a high steric hindrance at the backbone. In this case, by thermal annealing in low‐polar solvents, an anti‐conformer is stabilized at the pendant which commands a P helix in the PDPA. Next, solvent removal followed by addition of a polar solvent such as dimethyl sulfoxide (DMSO), results in the kinetic conformationally trapped P helix. However, in this medium, the preferred handedness and the thermodynamic macromolecular helix for poly‐(L)‐1 is M. This process also occurs in the opposite way. Electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) studies show that the dynamic memory effect is present both in ground and excited states.

Stimuli-responsive synthetic helical polymers

“…Freire performed also solvent-dependent studies on PDPAs functionalized with chiral amino acids as pendants, 147,392,393 obtaining syn / anti conformational equilibria similar to those obtained for PPAs, although the activation energy to promote helix inversion is very high providing the polymers a memory effect. Also, the same group studied how the aromatic substitution in chiral PPAs ( i.e.…”

“…The authors used the ability of this polymer to form cis amide bonds in THF to create a stereocomplex, where poly-(R)-166 interacts with its enantiomeric and complementary helix to form fibrelike aggregates that evolve into a gel at high concentrations. 391 Freire performed also solvent-dependent studies on PDPAs functionalized with chiral amino acids as pendants, 147,392,393 obtaining syn/anti conformational equilibria similar to those obtained for PPAs, although the activation energy to promote helix inversion is very high providing the polymers a memory effect. Also, the same group studied how the aromatic substitution in chiral PPAs (i.e., ortho, meta, para) and/or the connector used to link pendant and backbone (i.e., anilide, benzamide) affects the dynamic behaviour of PPAs and their solvent response (Fig.…”

“…11b). 147 This polymer can adopt either P or M helical sense in different solvents at high temperatures (DMSO, M helix; CHCl 3 , P helix). This fact is due to a different conformational composition in the pendant.…”

Thermoresponsive Helical Dendronized Poly(phenylacetylene)s: Remarkable Stabilization of Their Helicity via Photo-Dimerization of the Dendritic Pendants

“…PPAs bearing ( R )-α-methoxyphenylacetic acid pendants adopted the racemic helical conformation in solution, and the helicity could be selectively enhanced through coordination with mono- or divalent metal cations. ,, Similarly, poly­( N -propargylamide) carrying alanine residues could transform from random coils to predominantly one-handed helices in aqueous solutions through complexing with Cu 2+ . Dynamically racemic monosubstituted polyacetylene derivatives interacted with nonracemic molecules, such as chiral amine, to induce excess one-handed helical conformation, and the induced helicity was memorized after the replacement of the nonracemic with achiral molecules. , Subsequently, for poly­(diphenylacetylene)­s and poly­(biphenylylacetylene)­s with large helix-inversion barriers, the induced helicity could have long-lasting memory even without the assistance of achiral molecules. − Furthermore, chiral memories also could be maintained by noncovalent cross-linking in the gels. , Although noncovalent strategies are flexible to modulate helical conformation of polymers, the stabilization effect is often limited.…”

P/M Macromolecular Switch Based on Conformational Control Exerted by an Achiral Side Chain within an Axially Chiral Locked Pendant

“…Dynamic helical polymers are macromolecular switches − where the P/M screw sense control is achieved by resorting to different helix induction mechanisms that arise from the conformational manipulation of the pendant of a monomer repeating unit because of interactions with different stimuli. Thus, information from the chiral center to the main chain of the polymer can be transmitted directly, across space, through helical induction effects such as tele-induction, − chiral overpass, , or substituent overpass, or indirectly, in a two-step process, where information from a chiral group placed at a remote position on the pendant is first transmitted to an achiral spacer and then harvested by the polyene backbone (chiral harvesting). − In copolymers, Zentel et al designed an isocyanate copolymer that shows reversible helix-inversion induced by isomerization of an azobenzene group. , …”