Chiral emergence in multistep hierarchical assembly of achiral conjugated polymers

Abstract

Intimately connected to the rule of life, chirality remains a long-time fascination in biology, chemistry, physics and materials science. Chiral structures, e.g., nucleic acid and cholesteric phase developed from chiral molecules are common in nature and synthetic soft materials. While it was recently discovered that achiral but bent-core mesogens can also form chiral helices, the assembly of chiral microstructures from achiral polymers has rarely been explored. Here, we reveal chiral emergence from achiral conjugated polymers, in which hierarchical helical structures are developed through a multistep assembly pathway. Upon increasing concentration beyond a threshold volume fraction, dispersed polymer nanofibers form lyotropic liquid crystalline (LC) mesophases with complex, chiral morphologies. Combining imaging, X-ray and spectroscopy techniques with molecular simulations, we demonstrate that this structural evolution arises from torsional polymer molecules which induce multiscale helical assembly, progressing from nano- to micron scale helical structures as the solution concentration increases. This study unveils a previously unknown complex state of matter for conjugated polymers that can pave way to a field of chiral (opto)electronics. We anticipate that hierarchical chiral helical structures can profoundly impact how conjugated polymers interact with light, transport charges, and transduce signals from biomolecular interactions and even give rise to properties unimagined before.

Not All Aggregates Are Made the Same: Distinct Structures of Solution Aggregates Drastically Modulate Assembly Pathways, Morphology, and Electronic Properties of Conjugated Polymers

“…Our recent work has demonstrated that, when dissolved in a mutual solvent such as chloroform and chlorobenzene, PII‐2T polymer chains/fiber aggregates adopt chiral helical structure shown in Figure 2 and Figure S10 (Supporting Information). [ 33 ] Unlike the amorphous ones in CN, the semicrystalline aggregates in DCB give a vibronic progression in the UV–vis low‐energy absorption band, which we attribute to aggregation‐induced backbone planarization (Figure 1c). The vibronic progression in DCB, however, exhibits a lower A 0–0 / A 0–1 ratio than that in decane, possibly owing to a relatively tortional/helical chain conformation in the aggregates.…”

“…As shown in Figure S12 (Supporting Information), the 0–0 and 0–1 peak locations blueshift, and their absorption peak ratio 0–0/0–1 decreases with concentration, indicating a decrease in the J ‐coupling character and possibly an increase in backbone torsion from isotropic to various mesophases. [ 33 ] These mesophases discovered in DCB solutions are comprised of aligned polymer fiber aggregates instead of single polymers according to SEM shown in Figure 3b. Therefore, we deduce that balanced backbone and side‐chain interactions conferred by mutual solvents are important for forming such liquid crystals.…”

“…As shown in Figure , when dissolved in DCB and other mutual solvents (Figure S11, Supporting Information), PII‐2T nanofiber aggregates form a series of lyotropic LC mesophases upon progressively increasing solution concentration. [ 33 ] When the solution surpasses a threshold concentration at ≈60 mg mL −1 , the liquid crystals first nucleate in confined droplets, commonly referred to as nematic tactoids. Further increasing solution concentration to 130 mg mL −1 causes tactoids to merge into a continuous domain, which appears birefringent under cross polarized optical microscopy (CPOM).…”

“…This phase is defined as a twist‐bent mesophase. [ 33 ] At an even higher concentration of 200 mg mL −1 , another chiral mesophase appears and exhibits a twinned morphology wherein a few micrometer periodicity is observed in its stiped texture. As shown in Figure S12 (Supporting Information), the 0–0 and 0–1 peak locations blueshift, and their absorption peak ratio 0–0/0–1 decreases with concentration, indicating a decrease in the J ‐coupling character and possibly an increase in backbone torsion from isotropic to various mesophases.…”

“…As another example, our recent work has discovered that PII‐2T aggregates into helical nanofibers; the helical structure is unfavorable for intrachain charge transport as compared to more planar counterpart. [ 32,33 ]…”