Direct Measurement of Energy Migration in Supramolecular Carbocyanine Dye Nanotubes

Exciton transport lengths in double-walled and bundled cylindrical 3,3′-bis- (2-sulfopropyl)-5,5′,6,6′-tetrachloro-1,1′-dioctylbenzimida-carbocyanine (C8S3) J-aggregates were measured using direct imaging of fluorescence from individual aggregates deposited on solid substrates. Regions identified in confocal images were excited with a focused laser spot, and the resulting fluorescence emission was imaged onto an electron multiplying charged coupled device camera. A two-dimensional Gaussian fitting scheme was used to quantitatively compare the excitation beam profile to the broadened aggregate emission profiles. The double-walled tubes exhibit average exciton transport lengths of 140 nm, while exciton transport in the bundled nanotubes was found to be remarkably long, with distances reaching many hundreds of nanometers. A steady-state one-dimensional diffusion model for the broadening of the emission profiles yields diffusion coefficients of 120 nm2 ps–1 for the nanotubes and 7000 nm2 ps–1 for the aggregate bundles. The level of structural hierarchy dramatically affects the exciton transport capabilities in these artificial light-harvesting systems, and energy migration is not limited to a single dimension in J-aggregate bundles.