Scientists build polymers with two distinct molecular faces

Researchers have created a new class of polymers exhibiting chirality-induced spin selectivity exceeding 90% in thin films, with opposite signs for the two enantiomers. These feature rigid, double-stranded backbones bearing two deliberately differentiated molecular faces, achieved through a chirality-assisted synthesis using a specifically designed C2-chiral monomer. The resulting films display circular dichroism intensities over 100-fold stronger than those of their monomeric or non-ladder analogues, demonstrating a platform for emergent self-assemblies and functions in organic polymeric materials.

Chirality-Assisted Synthesis Enables Bifacial Ladder Polymers

Solid-state films of newly synthesized bifacial ladder polymers exhibit circular dichroism intensities exceeding 100-fold greater than those of comparable monomeric or non-ladder materials, signaling a substantial advancement in chiroptical behavior. The key to achieving this unique architecture lay in the design of a C2-chiral 2,8-dibromoindenofluorene monomer bearing two different substituents in syn stereochemistry, such that only one ladderization geometry is accessible. This precise control over monomer arrangement was essential for preventing the uncontrolled orientation of repeat units in previous attempts at ladder polymer synthesis.

The researchers utilized Suzuki-Miyaura polymerization of enantiopure monomers with a diboronate comonomer, enabling regioselective ladderization and preserving facial differentiation throughout the polymer structure. This ensured a uniform facial orientation along the polymer chain, a critical feature of the “bifacial” architecture.

Beyond enhanced chiroptical properties, these bifacial ladder polymers also exhibit remarkable chirality-induced spin selectivity, achieving spin-polarization values exceeding 90%. Importantly, the sign of the polarization is opposite for each enantiomer, a level of chirality-induced spin selectivity surpassing that of the precursor polymers.

The researchers report that the term “bifacial” specifically refers to a ladder polymer architecture where two different substituents are persistently oriented on opposite faces of the rigid ladder backbone, generating permanent facial asymmetry. The ability to control facial asymmetry along the polymer chain opens possibilities for face-selective intermolecular interactions and the formation of higher-order supramolecular organization.

The rigid ladder structure, fixed by covalent bonds, may facilitate the contribution of otherwise weak interactions, thereby promoting the formation of higher-order supramolecular organization and the emergence of associated functional properties. The team’s geometrically guided synthetic strategy, based on considerations of monomer symmetry and connectivity, represents a development in the creation of polymeric materials with tailored properties and functionalities.

Bifacial Ladder Polymers: A Novel Structural Concept

The creation of polymers with deliberately asymmetrical molecular faces represents a departure from conventional polymer design, and researchers have now demonstrated a method for building these with increased control over their structure. Unlike typical polymers where the backbone can rotate freely, these new materials feature rigid, double-stranded backbones that maintain a fixed orientation of their two distinct faces. This specific monomer design limits polymerization to a single, predictable geometry, ensuring consistent facial differentiation throughout the resulting polymer chain.

Polymerization under Suzuki-Miyaura conditions yielded precursor polymers that then underwent regioselective ladderization, preserving the facial differentiation throughout the process. This precise control over facial orientation is critical, as it enables the potential for face-selective intermolecular interactions and the development of materials with unique properties. The resulting homochiral bifacial ladder polymers display emergent chiroptical behavior in the solid state, forming one-handed supramolecular helices.

Suzuki-Miyaura Polymerization of Precursor Polymers

The polymerization of the enantiopure monomers with a diboronate comonomer under Suzuki-Miyaura conditions affords precursor polymers, which undergo regioselective ladderization to yield well-defined bifacial ladder polymers with fully preserved facial differentiation. This controlled process ensures that the two differentiated molecular faces of each monomer unit maintain a consistent orientation along the entire polymer chain.

This amplification of the circular dichroism signal is a direct consequence of the ordered arrangement of the chiral faces within the polymer structure. The team’s geometrically guided synthetic strategy, based on considerations of monomer symmetry and connectivity, represents a step towards creating advanced polymeric materials with tailored properties and functionalities.

Chirality-Induced Spin Selectivity Exceeds 90%

The creation of polymers exhibiting over 90% chirality-induced spin selectivity represents an advance in the field of spintronics and organic materials science, potentially enabling more efficient and compact devices for data storage and processing. Researchers achieved this level of control by constructing polymers with deliberately asymmetrical molecular structures and carefully controlling their arrangement during synthesis. This enhanced control over spin polarization stems from a novel synthetic strategy centered on a C2-chiral 2,8-dibromoindenofluorene monomer. The design incorporates two distinct substituents in syn stereochemistry, effectively limiting the possible configurations during polymerization and ensuring a predictable ladderization geometry.

More significantly, these films exhibit robust chirality-induced spin selectivity, with spin-polarization values exceeding 90% and, crucially, opposite signs for each enantiomer. This level of selectivity far surpasses that of the precursor polymers, suggesting a strong coupling between the polymer’s chirality and the spin of electrons passing through it. This ability to manipulate spin using chiral organic materials opens new avenues for developing advanced spintronic devices with enhanced performance and reduced energy consumption, potentially impacting fields ranging from data storage to quantum computing.

Facial Differentiation and Persistent Asymmetry in Polymers

Conventional polymer design often prioritizes chemical composition over precise three-dimensional architecture, but a new class of materials challenges this approach by deliberately engineering asymmetry into the polymer backbone itself. Researchers have successfully synthesized rigid, double-stranded structures with two distinctly different molecular faces, a feat previously hindered by the difficulty of controlling facial orientation during polymerization. This meticulous molecular arrangement limits the possible configurations during polymerization, effectively ensuring a single, predictable geometry.

To achieve this, they employed an enantiopure monomer, meaning a single, non-superimposable mirror image form, and a Suzuki-Miyaura polymerization technique. This combination ensured that the resulting polymer maintained a consistent facial differentiation throughout its structure, creating a truly “bifacial” architecture. Solid-state films of these bifacial ladder polymers exhibit remarkably high chirality-induced spin selectivity, exceeding 90% with opposite signs for the two enantiomers.

The researchers state that this is not simply a matter of increased signal strength, but a fundamental shift in how the material interacts with spin. The success of this chirality-assisted synthesis demonstrates a powerful strategy for building complex polymeric materials with tailored properties. This highlights the critical role of both chiral design and controlled polymerization in achieving persistent asymmetry. The team’s work provides a platform for the face-selective arrangement of substituents and functional groups, potentially enabling the creation of materials with unique intermolecular interactions and emergent functionalities.

Ladder Polymers Expand Organic Polymeric Materials’ Functions

Researchers detailed a chirality-assisted synthesis enabling the creation of rigid, double-stranded structures where each face can be independently functionalized, opening avenues for advanced materials with tailored properties. This approach overcomes limitations of previous ladder polymers, which lacked control over the orientation of repeat units. The ability to predictably arrange functional groups on opposing faces of the polymer backbone unlocks possibilities for creating materials with anisotropic properties and unique interfacial behaviors.