Photosynthesis, the process by which plants and some bacteria convert sunlight into usable chemical energy, is a remarkable process that feeds nearly all of life on earth. At the smallest scales, photosynthesis is driven by molecular machinery in the form of various light-harvesting protein complexes (LHCs) contained in bacterial cells and in the chloroplasts of plant cells. At the earliest stages in photosynthesis, a photon of sunlight is absorbed by a small light-absorbing molecule (a "chromophore") and that energy is transported through a network of chromophores contained in these proteins towards specialized reaction centers where chemistry begins. This transfer process is remarkably efficient, and can result in nearly all absorbed photons reaching the reaction centers successfully. Understanding what factors of the design of these protein complexes give rise to this efficiency is thus of great interest for developing novel light-harvesting technologies. In this work, we present a series of studies focused on understanding the factors important to achieving efficient light harvesting, by spectro- and spatiotemporally resolving the few picoseconds-to-nanoseconds after photoexcitation in artificial and natural light harvesting complexes.

Chapter 1 presents a brief introduction to photosynthetic light-harvesting complexes in a variety of organisms. These complexes present a number of challenges to systematic study that kept open questions about their structure-function relationships even after over a century of study. We summarize some of these challenges and lay out our approaches to overcoming them.

Chapter 2 gives an introduction to transient absorption (TA) and transient absorption anisotropy (TAA) spectroscopy, from basic principles to practical implementation. TAA is a powerful technique for measuring energy transfer by measuring the depolarization of the transient absorption signal caused by energy transfer. We detail the implementation of a high signal-to-noise ratio TAA apparatus built in order to apply this technique to artificial light harvesting complexes.

Chapter 3 explores the role of disorder in intra-LHC energy transfer in an artificial LHC based on a circular permutant of the tobacco mosaic virus coat protein (cpTMV). This model system afford far greater control to perform systematic investigation than natural LHCs. In this study we measure the intra-complex energy transfer in these model LHCs using TAA spectroscopy and model that transport via kinetic Monte Carlo simulations. We find that fast site-to-site hopping as high as 1.6 ps^-1 is occurring in these complexes. With these simulations, we identify static disorder in orientation, site energy, and degree of coupling as key remaining factors to control to achieve long-range energy transfer in these systems. We thereby establish this system as a highly promising, bottom-up model for studying long-range energy transfer in light-harvesting protein complexes.

Chapter 4 introduces stroboscopic interferometric scattering microscopy (stroboSCAT), a label-free, time-resolved microscopy technique that can directly image energy carriers from excitons to heat in a broad range of materials. We provide an overview of the technique and detail improvements made to the technique to increase time resolution in order to study the first few picoseconds of natural light harvesting complexes. To achieve this objective we coupled an ultrafast laser source into the microscope, increasing the time resolution of the apparatus by over two orders of magnitude, to below 1 ps. We present a study of short-lived photogenerated charge carriers' migration in silicon, previously barely detectable with our lower time resolution, where we observe density-dependent diffusivity as a result of carrier-carrier scattering.

Chapter 5 presents ongoing spatially-resolved measurements of exciton migration and exciton-exciton annihilation in de-enveloped thylakoid membranes from green plants via stroboSCAT. We find that exciton-exciton annihilation dominates the observed spatial response and present a model to simultaneously fit exciton diffusion and annihilation, leveraging the spatial resolution to capture both. Finally, we provide a number of future directions and propose improvements to the apparatus to facilitate future experiments on these samples.

Taken together, this dissertation presents a set of novel approaches to studying energy transfer in LHCs that reveals the role of disorder and many-body interactions in photosynthetic light-harvesting. Both by studying novel model systems via a more well-established spectroscopic technique and by studying well-established natural photosynthetic samples via a novel microscopic technique, we unveil new findings about the important role that disorder plays in these fascinating systems.