Chain-Length-Dependent Self-Assembly Behaviors of Discrete Conjugated Oligo(3-hexylthiophene)
Youngkwon Kim, Hyeonjung Park, Allison Abdilla, Hongseok Yun, Junghun Han, Gila E. Stein, Craig J. Hawker, Bumjoon J. Kim
Abstract
Discrete oligomers (i.e., highly monodisperse) can provide a deep understanding of chain-length-dependent properties of polymers and their self-assembly behaviors. Herein, discrete oligo(3-hexylthiophene)s (D-o3HTs) with a dispersity (Đ) of 1.0 and degree of polymerization (DP) between 6 and 18 were obtained through a simple synthetic procedure of 3-hexlythiophene trimer-based polymerizations and automated column chromatography purification. As the DP of D-o3HTs increases, longer conjugation lengths cause red shifts in their optical properties and yield tunable crystalline properties. Interestingly, D-o3HTs with DP ≤ 9 have a dominant face-on Form II structure in thin films, while a fiber morphology is also not observed in thin films. In contrast, D-o3HTs with DP ≥ 12 assemble into a dominant edge-on Form I structure in thin films and show highly ordered fiber morphologies. In addition, Bragg rod patterns are observed in thin films by transmission electron microscopy and grazing incidence X-ray scattering with these patterns being distinctive when compared to those for conventional regioregular poly(3-hexylthiophene) with Đ = 1.1. Finally, the formation of 2-dimensional flowerlike nanostructures with overall micrometer dimensions is obtained from D-o3HTs via solvent-mediated self-assembly. These results offer an understanding of self-assembly behaviors of discrete conjugated polymers, leading to exquisite control over their crystallinity and nanoscale morphology.