Linear Dichroism Microscopy Resolves Competing Structural Models of a Synthetic Light-Harvesting Complex.

The initial stages of photosynthesis in light-harvesting antennae, driven by excitonic energy transport, have inspired the design of artificial light-harvesting complexes. Double-walled nanotubes (DWNTs) formed from the cyanine dye C8S3 provide a robust, self-assembled system that mimics natural chlorosomes in both structure and optical properties. Two competing molecular packing models─bricklayer (BL) and herringbone (HB)─have been proposed to explain the structural and optical characteristics of these DWNTs. This study resolves the debate by combining theoretical analysis with advanced polarization-resolved wide-field photoluminescence microscopy. Quantum-classical simulations reveal reduced linear dichroism (LDr) as a decisive parameter for distinguishing between the models. Experimental measurements of single DWNTs yielded LDr values as high as 0.93, strongly favoring the BL model. The BL model's unique excitonic patterns, dominated by negative couplings among individual chromophores, generate superradiant exciton states with transition dipoles preferentially aligned along the nanotube axis. In contrast, the HB model's mixed positive and negative couplings produce destructive interference, leading to a weaker alignment of transition dipoles. Our approach deepens the understanding of the structure-property relationships in self-assembled systems and demonstrates the potential of slip-stacking engineering to fine-tune excitonic properties for artificial light-harvesting applications.