Dynamic Axial‐to‐Helical Communication Mechanism in Poly[(allenylethynylenephenylene)acetylene]s under External Stimuli

Abstract

Helix inversion in chiral dynamic helical polymers is usually achieved by conformational changes at the pendant groups induced through external stimuli. Herein, a different mechanism of helix inversion in poly(phenylacetylene)s (PPAs) is presented, based on the activation/deactivation of supramolecular interactions. We prepared poly[(allenylethynylenephenylene)acetylene]s (PAEPAs) in which the pendant groups are conformationally locked chiral allenes. Therefore, their substituents are placed in specific spatial orientations. As a result, the screw sense of a PAEPA is fixed by the allenyl substituent with the optimal size/distance relationship to the backbone. This helical sense command can be surpassed by supramolecular interactions between another substituent on the allene and appropriate external stimuli, such as amines. So, a helix inversion occurs through a novel axial‐to‐helical communication mechanism, opening a new scenario for taming the helices of chiral dynamic helical polymers.

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

“…In a recent work, we demonstrated that poly-[(allenylethynylenephenylene)acetylene]s (PAEPAs) are helical polymers whose screw sense excess is determined by the axial conformational stability of the chiral allene. 37 Accordingly, PAEPAs comprise the two first requisites of the desired macromolecular gear, i.e., a unique spatial distribution of the different substituents in the two perpendicular planes of the allene. So, to complete our design, two tert-butyl groups were included in two perpendicular planes of the allene (side chains a and b) and a (dimethyl)methyl-p-tolyl-sulfonamide group in the last vacant position of the allene (side chain c), which can easily form hydrogen bonds with Lewis base solvents or anions 42 (Figure 2a).…”

“…In a recent work, we demonstrated that poly­[(allenylethynylenephenylene)­acetylene]­s (PAEPAs) are helical polymers whose screw sense excess is determined by the axial conformational stability of the chiral allene . Accordingly, PAEPAs comprise the two first requisites of the desired macromolecular gear, i.e., a unique spatial distribution of the different substituents in the two perpendicular planes of the allene.…”

“…So, to complete our design, two tert -butyl groups were included in two perpendicular planes of the allene (side chains a and b ) and a (dimethyl)­methyl- p -tolyl-sulfonamide group in the last vacant position of the allene (side chain c ), which can easily form hydrogen bonds with Lewis base solvents or anions (Figure a). Therefore, monomers mono-( P )- 1 and mono-( M )- 1 were synthetized from previously reported allenes ,, by derivatizing the propargylic alcohol with a sulfonamide group using FeCl 3 as a catalyst (Section 2 of the Supporting Information). ECD studies of both monomeric enantiomers in different solvents show, as expected, that the P or M axial chirality of the allene moiety remains unaltered due to restricted rotation along the two consecutive double bonds of the allene (Figure b).…”

“…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. , …”

Stimuli-responsive synthetic helical polymers

“…To show this effect, poly- 10 was chosen as illustrative example. 107 This polymer bears an allene pendant (Fig. 4b) in which its two consecutive double bonds place their four substituents in two perpendicular planes.…”