Asymmetric living crystallization-driven self-assembly (CDSA) has recently emerged as a robust strategy toward the precision creation of π-conjugated chiroptical nanostructures by taking advantage of the intrinsic optoelectronic properties and crystallinity of π-conjugated blocks and the aggregation-induced chirality amplification effect. However, the field of asymmetric living CDSA remains in its infancy with a limited understanding of the relationship between the structure of π-conjugated building blocks and asymmetric living CDSA behavior. In this contribution, we prepared a series of block copolymers consisting of core-forming oligo(p-phenylene ethynylene) pentamers and heptamers with linear pentyloxy, branched racemic 2-methylbutyloxy, and chiral (S)-2-methylbutyloxy side chains (denoted L-OPEn, rac-OPEn, and (S)-OPEn, n = 5 and 7, respectively) and corona-forming poly(N-isopropylacrylamide) (PNIPAMn, n = 36 and 40). We then investigated their CDSA behavior in detail. It was found that the increase of the OPE chain length and the decrease of the PNIPAM chain length could promote the crystallization of block copolymers. More importantly, the structure of the side chain significantly affected asymmetric CDSA behavior. In comparison with the linear side chains, branched racemic and chiral side chains can not only significantly enhance π–π stacking strength but also induce regularly twisted stacking of OPE units to give helical nanofibers with a preferred handedness. (S)-OPE5/7 and rac-OPE5/7 units of block copolymers adopted a single-layer face-to-face twisted stacking mode to form helical nanofibers with comparable circular dichroism (CD) signals in methanol. On the contrary, L-OPE5/7 units of counterparts followed a face-to-face/side-by-side packing mode to form CD-silent nanofibers and nanoribbons. By one-step heating/cooling and self-seeding approaches, both (S)-OPE7-b-PNIPAM36 and rac-OPE7-b-PNIPAM36 gave uniform helical nanofibers of controlled lengths, showing typical living/controlled characteristics in micellar elongation. In stark contrast, the L-OPE7-containing counterpart only formed CD-silent and ill-defined nanofibers and nanoribbons. The results manifested that it was the subtle interplay of π–π stacking of the OPE backbone and the conformation effect of branched racemic and chiral side chains that rendered the regularly twisted stacking of rac-OPE5/7 and (S)-OPE5/7 units to give helical nanofibers with a preferred handedness. This work provides additional insights into the correlation between the structure of π-conjugated segments, especially the side chains, and the asymmetric CDSA behavior. More appealingly, this work illustrates a more economical and efficient “racemic”-side-chain-directed asymmetric living CDSA approach toward precision creation of chiroptical nanostructures from diverse π-conjugated entities.
Chen et al. (Thu,) studied this question.