Cooperative effects and long range interactions: How form shapes the substance

In this thesis we analyze how the architecture of a complex system made of a huge number of constituents can affect its functionality. In particular, in our study we focused on the light-harvesting systems found in GSB and PB antennae, which are among the most efficient aggregates of BChl molecules capable of performing photosynthesis with high internal efficiency even under low light conditions. In the literature there is evidence that their behavior is strictly related to the geometry of the BChl molecules in the antenna complexes, in particular the position and the TDM orientation. This relationship remains an open question in the field of quantum biology. Therefore, motivated by previous findings, this thesis aims to provide a possible explanation for this phenomenon. In this work we demonstrate that the high internal efficiency found in GSB and PB complexes arises from the emergence of cooperative behaviors. In particular, we investigated the coherent emission of radiation from PB and GSB antenna complexes. Our findings show that such systems can support a few red-shifted eigenstates characterized by a fast radiative decay rate (superradiant), which is enhanced with respect to that of a single emitter. This phenomenon, also called ’single-photon superadiance’, has been investigated by using a non-hermitian Hamiltonian approach and it has been found not only in smaller portions of the antennae, but also in the entire light-harvesting complexes comprising up to 10^5 emitters. Furthermore, we found that the superradiant decay rate is robust even in presence of static and thermal noise comparable to room temperature energy. Another crucial aspect explored in this thesis is how cooperativity can influence the energy transfer process, from absorption to charge separation in the RCs. We address this issue with two different approaches. For the GSB light-harvesting apparatus we model the entire process of excitation energy transfer by using incoherent rate equations for the populations of the system, accounting for the weak interaction of our aggregates with sunlight and a thermal bath, while for the PB antennae we develope a theoretical model based on the network-theory. Our results showed that the emergence of superradiance ensures a fast incoherent energy transfer from a donor to an acceptor unit before radiative and non radiative losses occur. For GSB we have found an internal efficiency close to ∼ 80%, proving that almost all the absorbed excitation is funneled to the RCs for charge separation, while the results for PB antennae have shown a charge separation quantum yield close to ∼ 90%, comparable to the literature. Finding new strategies for light-harvesting and clean energy production is a recent goal of quantum biology. Nowadays the study is not limited to understanding natural processes, but also extends to the development of quantum devices able to mimic the interesting and efficient behavior of such systems. Within this framework, this thesis also offers a brief explanation of a possible application of the unique properties exhibited by natural light-harvesting antennae. Within this context, the main goal of the APACE project is to turn the incoherent solar radiation into a coherent laser beam by using light-harvesting systems from natural and artificial antennae for absorption and funneling of the excitation to a lasing medium.