Cooperative Self-Assembly of Helical Exciton-Coupled Biosurfactant-Functionalized Porphyrin Chromophores

Biobased, self-organizing molecules are of considerable interest as functional materials due to their structural versatility, sophisticated nanoarchitectures, and sustainable biosynthesis. Here, we study the self-assembly of a systematic series of bioconjugate sophorolipid-functionalized zinc porphyrin complexes with potential applications in optoelectronic devices. Our results provide insight into the molecular features that control the polymerization pathway, in particular the influence of carbohydrate chirality and noncovalent hydrogen-bonding on biosurfactant-driven self-organization of sophisticated light-absorbing supramolecular polymers. The self-assembly process is driven by a combination of hydrogen-bond, steric, and π-π interactions. Compounds under investigation were synthesized to examine the influence of peripheral chiral carbohydrate hydrogen bonding on chromophore aggregation and physicochemical properties through selective acetylation of the sophorose moiety. In dilute methanol/water solution, we found that excitonically coupled helical structures form by strong carbohydrate hydrogen-bonding interactions, in contrast to weakly coupled J-type aggregate formation with acetyl-group substitution of sugar hydroxyl moieties. Temperature-dependent UV/vis absorption and circular dichroism revealed that supramolecular polymerization proceeds through a cooperative mechanism of self-assembly for compounds bearing free OH groups capable of forming hydrogen-bond interactions. In contrast, per-acetylation of the sophorolipid's sugar group leads to micellar aggregation that is governed by a sterically driven isodesmic (noncooperative) assembly mechanism.