The light harvesting systems of photosynthetic organisms harness solar energy by efficient light capture and subsequent transport of the light’s energy to a chemical reaction center. Man-made optical devices could benefit by mimicking these naturally occurring light harvesting processes. Supramolecular organic nanostructures, composed of the amphiphilic carbocyanine dye 3,3’-bis- (2-sulfopropyl)-5,5’,6,6’-tetrachloro-1,1’- dioctylbenzimida-carbocyanine (C8S3), self assemble in aqueous solution to form tubular, double-walled J-aggregates. These J-aggregates have drawn comparisons to light harvesting systems, owing to their optical and structural similarities to the cylindrical chlorosomes (antenna) from green sulfur bacteria. This research utilizes optical spectroscopy and microscopy to study the supramolecular origins of the exciton transitions and fundamental nature of exciton energy transport in C8S3 artificial light harvesting systems. Two J-aggregate morphologies are investigated: well-separated, double-walled nanotubes and bundles of agglomerated nanotubes. Linear dichroism spectroscopy of flow-aligned nanotubes is used to generate the first quantitative, polarized model for the complicated C8S3 nanotube excitonic absorption spectrum that is consistent with theoretical predictions. The C8S3 J-aggregate photophysical properties are further explored, as the Stokes shift, quantum yield, and spectral line broadening are measured as a function of temperature from 77 – 298 K. The temperature-dependent emission ratios of the C8S3 J-aggregate two-band fluorescence spectra reveal that nanotube emission is well described with Boltzmann partitioning between states, while the bundles’ is not. Finally, understanding energy transport in these materials is critical for the proposed use of artificial light harvesting systems in optoelectronic devices. The spatial extent of energy transfer in individual C8S3 J- aggregate structures is directly determined using fluorescence imaging. We find that aggregate structural hierarchy greatly influences exciton transport distances: impressive average exciton migration distances of ~ 150 nm are measured along the nanotubes, while these distances increase to over 500 nm in the bundle superstructures. / text

In the past decade, DNA origami self-assembly has been widely applied for creating customised nanostructures with base-pair precision. In this technique, the unique chemical addressability of DNA can be harnessed to create programmable architectures, using components ranging from dye or protein molecules to metallic nanoparticles. In this thesis, we apply DNA nanotechnology for developing novel light-harvesting and optical voltage sensing nano-devices. We use the programmable positioning of dye molecules on a DNA origami plate as a mimic of a light-harvesting antenna complex required for photosynthesis. Such a structure allows us to systematically analyse optimal design concepts using different dye arrangements. Complementary to this, we use the resistive-pulse sensing technique in a range of electrolytes to characterise the mechanical responses of DNA origami structures to the electric field applied. Based on this knowledge, we assemble voltage responsive DNA origami structures labelled with a FRET pair. These undergo controlled structural changes upon application of an electric field that can be detected through a change in FRET efficiency. Such a DNA-based device could ultimately be used as a sensitive voltage sensor for live-cell imaging of transmembrane potentials.