Solar energy plays a central role in life, providing a direct or indirect energy source for most organisms on the Earth. Photosynthesis makes use of sunlight to convert carbon dioxide into useful biomass. In view of the potential technological applications for alternative energy sources development, several interdisciplinary works have been inspired to uncover the physical mechanisms ruling this photosynthetic process. Despite huge efforts, bio-inspired artificial systems remain still less efficient and less stable than their natural counterparts. In order to realize efficient artificial photosystems, it is crucial to determine the involved electronic energy levels and relevant energy pathways in natural light harvesters. Unravelling the coupling between molecular electronic and vibrational degrees of freedom would provide the chance to unveil the physical mechanism responsible for efficient coherent energy transfer in these prototypical compounds. This project employs combined time-resolved vibrationally and electronically sensitive experimental approaches to elucidate the molecular mechanisms of light-harvesting (LH) in natural pigment-protein photosynthetic complexes and optimize them for artificial LH systems. The aim of this project is to perform high time resolution ultrafast optical spectroscopy on highly controlled samples of both natural (from plants and mosses) and artificial (organic photovoltaic materials) LH systems. The experimental results of natural LH complexes will be compared with the ones obtained for mutant and artificial matrices, where site-directed mutagenesis will be exploited for modifying the excitonic level structure and the coupling to vibrations. This will allow us to elucidate the molecular mechanisms and the role played by quantum coherences in LH processes, and their relation to the systems structural and electronic properties, paving the way to artificial materials design through bio-inspired strategies.
The successful completion of our project will have both a scientific and a technological impact.
Scientific impact:
Photosynthesis is a fundamental biological process which provides the primary source of energy for almost all terrestrial life. Photosynthetic complexes have optimized the mechanisms of photon energy collection over the last three billion years; hence they provide ideal models for transferring design principles to artificial light-harvesting devices. A detailed understanding of the mechanisms of photosynthesis is thus of the greatest scientific importance. Our project aims at answering some key questions to this regard: do quantum coherences play a significant role in natural light harvesting systems, by providing a selective advantage to organisms through the optimization of light collection and photoprotection from excess light? can they be exploited also in artificial systems to optimize their performance? Our project is in an excellent position to address these questions, because it brings together a unique combination of partners providing high quality bio-systems, advanced optical spectroscopy and state-of-the-art numerical modeling.
Technological impact:
Rising standards of living in a growing world population will cause global energy consumption to increase dramatically over the next half-century. Almost 80% of our current energy needs are met by burning fossil fuels such as oil, natural gas and coal, which are estimated to be enough for only 50 to 100 years. In addition, carbon dioxide emission from fossil fuel combustion is the biggest source of the anthropogenic greenhouse effect that could alter the global climate system. Mankind is, therefore, facing a major challenge to invent, develop and deploy carbon-neutral, sustainable and renewable fuels. The most abundant renewable energy source available in our planet is solar radiation: suffice it to say that the daily dose of sunshine on the earth's surface is enough to satisfy our energy needs for 45 years. Capturing solar light for energy production and storage, efficiently converting it into chemical or electrical energy, is one of the grand challenges of the next decades.
Our project can contribute to this challenge in two ways:
i) By understanding the mechanisms of natural photosynthesis and finding ways to optimize it. Photosynthetic systems in fact have been designed not for maximum efficiency in energy conversion, but rather for survival under highly varying illumination conditions. To this purpose, photoprotection mechanisms are in place which set in at relatively moderate illumination intensities, turning on dissipation of the absorbed light energy and thus significantly reducing the photosynthetic yield. There is currently a great interest in the possibility of modifying the structure of light-harvesting complexes to improve photosynthesis, with the aim of increasing crop yields for food and biofuels. The detailed knowledge of the light-harvesting and photoprotection processes gained in the course of our project will provide key information in order to re-design the light harvesting function within photosynthetic proteins, to allow for a substantial increase in biofuel production efficiency.
ii) By deriving new, bioinspired design criteria and device architectures for organic photovoltaics, that go well beyond the traditional approach of designing the electronic structure of donor and acceptor materials. Organic solar cells consist of nanostructured blends of conjugated polymers, acting as light absorbers, and fullerenes, acting as electron acceptors. They provide unique advantages, such as low cost, light weight, flexibility and easy processability, that make them attractive with respect to the inorganic counterparts. The observation of vibronic quantum coherence in organic photovoltaic materials, which is one of the goals of our project, might suggest using such coherence to steer charge motion in intrinsically disordered organic thin films, thus optimizing the efficiency of charge separation.
This project, if successful, will represent the birth of a unique Italian network able to interact with the leading research groups, already established in many countries around the world, so as to address the grand challenge of an efficient capture and storage of solar energy. The network combines the different and highly complementary, multidisciplinary expertise that are required to tackle such a complex and important problem, as well as a robust network of industrial connections which will be very helpful to maximize the transfer of technologically relevant results.