Pathway-Resolved Hierarchical Self-Assembly of Biomimetic Double-Walled Nanotubes

Self-assembly, the spontaneous organization of molecular components into ordered structures without external intervention, offers a powerful route to complex nanomaterials, yet the molecular-level pathways, particularly under non-equilibrium conditions, remain poorly understood. We track in real time the formation of double-walled nanotubes (DWNTs) from the amphiphilic cyanine dye C8S3, a synthetic analogue of light-harvesting chlorosomes in green sulfur bacteria. Combining microfluidic control with two-dimensional electronic spectroscopy, cryogenic transmission electron microscopy, and polarization-resolved single-object microscopy, we identify transient intermediates at both structural and excitonic levels. The results show that the outer nanotube structures first, while the inner nanotube follows with a delay, ultimately giving rise to electronically coupled coaxial architectures. While principal excitonic and morphological features emerge within minutes of self-assembly, axial elongation and orientational refinement proceed over tens of hours in a nucleation–elongation fashion. Notably, suppressing local concentration gradients through more efficient mixing abolishes DWNT formation, establishing spatial heterogeneity as a key design parameter for hierarchical self-assembly. These findings provide a framework for controlling biomimetic light-harvesting nanomaterials, with implications for directing structural outcomes in advanced materials engineering.