Nanostructure Control of a Regioregular Poly(3-alkylthiophene)Using an Oligopeptide Side Chain

With the aim of probing the competitive roles of π–π stacking and hydrogen bonding in modulating the aggregation state of conjugated polymers, a functionalized thiophene monomer containing a clickable azide group was copolymerized with 2,5-dibromo-3-hexylthiophene via Grignard metathesis (GRIM) to form either a block or a random copolymer, which was then covalently linked via click chemistry to a tetrapeptide (Gly-Val-Gly-Val) that is prone to forming β-sheets. A series of polythiophene–peptide hybrids with similar backbone lengths but different oligopeptide side-chain contents were obtained by adjusting the ratio of the clickable monomer during the polymerization process. The self-assembly behavior of the oligopeptide–polythiophene hybrids was studied in detail. Very interestingly, it is found that the self-assembly behavior strongly depends on the amount of the tetrapeptide side chain in the hybrids, while the influence of the regioregularity (random or block copolymer) is less profound. Specifically, both the random and block copolymers with low oligopeptide contents (33 mol % and lower) self-assemble into nanofibers. On the other hand, the block and random copolymers with high oligopeptide contents (higher than 40 mol %) and the homopolymer with a 100 mol % oligopeptide side chain form spheres with a diameter of around 0.05–1.0 μm. Fourier-transformed infrared spectroscopy (FTIR) analysis demonstrates that more β-sheets are formed in the copolymers containing fewer oligopeptide side chains, more random coils are formed in the copolymers with higher oligopeptide side-chain contents, and only random coils are formed in the homopolymer. Ultraviolet-visible light spectroscopy analysis shows that only when the oligopeptide side-chain content is low could a strong intermolecular π–π stacking be observed; when the oligopeptide side-chain content increased, such an intermolecular electronic communication disappeared. This study offers new insights on how the complex roles of the hydrogen bonding and π–π stacking control the self-assembly evolution of polythiophenes decorated with oligopeptide side chains.