| Popis: |
Sunlight powers most of life on Earth. Through billions of years of steady and inexorable evolution, living organisms have adapted progressively more complex and ingenious mechanisms to harvest energy from the Sun and use it to their advantage, branching out in a plethora of different strategies. Sunlight also represents the most abundant, reliable and easily accessible source of energy on our planet, with the promising potential of satisfying all the energy needs of humanity for millennia to come. Especially nowadays, at the brink of a global energy crisis, it is of paramount importance to explore various alternatives to our current energy consumption habits. In the search for new strategies, a natural place where to shift our focus is offered by photosynthesis. The interest in the early steps of photosynthesis, immediately following light absorption by pigment molecules, experienced an unprecendented growth in the last decade, after the observation of oscillatory signals in their optical response, initially interpreted as signatures of quantum coherent dynamics. Those observations, together with the remarkably high efficiency of the energy-transfer and charge-separation processes, led to formulate the hypothesis that photosynthetic organisms—and living beings in general—might take advantage of non-trivial quantum effects to perform some physiologically relevant task more efficeintly than allowed by classical physics alone. While intra-molecular vibrations and their coupling to delocalized electronic excitations (excitons) have been under intense scrutiny as possible sources of the observed long-lived oscillations, fewer works have examined the mechanisms regulating excitation energy transfer on longer temporal and spatial scales. The observation of exciton transfer across distances of few micrometers in self-assembled light-harvesting arrays, initially thought to be a signature of macroscopic quantum coherence, has largely remained without a satisfactory explanation. In this thesis we reconcile these experimental observations with well-established properties of light-harvesting complexes. We offer a microscopic basis for long-range exciton transfer, harnessing quantum coherence within modular units of tightly bound pigments. This allows us to formulate design guidelines for efficient macroscopic energy transfer in the presence of realistic sources of noise and dissipation. Thanks to recent progress in chemical synthesis of conjugated assemblies of light-absorbing molecules, it is now possible to engineer bio-inspired architectures that reproduce the essential features of their biological counterparts, without the undesired complications presented by their complexity. On one hand, this allows us to explore bio-inspired architectures beyond the capabilities of biological systems. We do this by theoretically examining the energy transfer dynamics along a linear aggregate of coherently wired modular units, extracting design guidelines for efficient energy transfer in the quantum coherent regime. On the other hand, it provides us with controllable platforms to study specific aspects of biological light harvesting. With this respect, we present a model that clarifies the photophysics of a newly synthesized artificial light-harvester, specifically designed to reproduce the absorption and energy-transfer properties of the bacterial LH1-RC core complex. The ultimate goal of both natural and artificial light-harvesting is to store the harvested energy for later use in the form of an electrochemical potential. To this end, we present some preliminary results using a minimal model of the photosynthetic reaction center as an autonomous heat engine. Despite its simplicity, the model is able to qualitatively reproduce power-voltage characteristics of typical photocells. We conclude by discussing the non-trivial role of incoherent vibrations in optimizing the output power of such a simple thermoelectric device. |
