Abstract
Self-assembled helical polymers hold great promise as new functional materials, where helical handedness controls useful properties such as circularly polarized light emission or electron spin. The technique of subcomponent self-assembly can generate helical polymers from readily prepared monomers. Here we present three distinct strategies for chiral induction in double-helical metallopolymers prepared via subcomponent self-assembly: (1) employing an enantiopure monomer, (2) polymerization in a chiral solvent, (3) using an enantiopure initiating group. Kinetic and thermodynamic models were developed to describe the polymer growth mechanisms and quantify the strength of chiral induction, respectively. We found the degree of chiral induction to vary as a function of polymer length. Ordered, rod-like aggregates more than 70 nm long were also observed in the solid state. Our findings provide a basis to choose the most suitable method of chiral induction based on length, regiochemical, and stereochemical requirements, allowing stereochemical control to be established in easily accessible ways.
Highlights
Encoding stereochemical information in the form of stereocenters appended to the monomer is an effective means of dictating helical handedness.[33−35] this method requires the preparation of enantiomerically pure building blocks, the ease of which is specific to the choice of monomer
A more flexible approach would employ an additive[36,37] that dictates the helical handedness of a polymer comprising either achiral or racemic monomer units.[38−43] This approach has borne fruit[44] for the helically folded polymers known as foldamers.[44−46] These synthetic oligomers are formed of amide-linked achiral monomer units terminated with end-groups containing chiral moieties, which dictate the preferred foldamer helicity.[38,47−49] Enantioenriched supramolecular polymers have been formed using an enantiopure initiating species, whereby the chiral information imprinted at initiation propagates along the length of the polymer.[39,41,50]
Polymerization proceeded down the M and P pathways to afford a racemic mixture of polymers, reflected in a featureless circular dichroism (CD) spectrum (Figure 2)
Summary
Chiral molecules interact differentially with circularly polarized light and electrons in different spin states, giving rise to useful properties[1,2] and functions.[3−7] Helical materials are employed throughout biological systems, serving as scaffolds[8−10] for mechanical support and for the precise spatial arrangement of dipoles in ion channels.[11,12] Recently, interest has grown in using helical structures for applications[13−17] that include circularly polarized light emission,[3,4,6,18−21] information storage,[22] and in spintronics as electron-spin filters.[1,5] Helical polymers[23−28] are useful in these contexts[26] due to their modular structures and scalable methods of preparation.[29,30] Achieving control over the helical handedness of these materials is necessary for their use.[5,31,32]. We infer that this plateau corresponds to a gabs wherein minimal contributions occurred from the standard polymerization mechanism; the likelihood of a polymer nucleating in the absence of a chiral inducer is minimized under these conditions Under such conditions of maximal stereoinduction, the difference between the observed gabs and the maximum gabs must arise from the intrinsic directing strength of the endcapping group, which renders M-CP1 and P-CP1 energetically inequivalent (Figure 6); our statistical mechanics model reported a free-energy penalty ( f1) for (S)-C that disfavored growth of the M helical screw-sense by 0.74 kBT. The higher effective concentration of F with respect to A promoted termination at the expense of polymer growth under conditions of slow addition This set of circumstances resulted in more F residues terminating short oligomers, while the residual A and CuI polymerized independently (via the standard polymerization mechanism, Figure 1), thereby reducing the helical handedness bias. These applications are currently under investigation, in addition to exploring means of isolating bulk quantities of enantiopure polymer
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