Electronic structure and properties of materials for artificial photosynthesis

Abstract

The oxygen evolution reaction during the photosynthesis process performed in plants, algae and cyanobacteria is possibly one of the most important reactions on the planet that sustain most life on our planet. Understanding the structure and function of the "engine of life", the oxygen-evolving complex (OEC) in the active site of Photosystem II (PSII), has been one of the great and persistent challenges of modern science. Over the past decades, immense progress has been achieved in understanding the structure and mechanism of photosynthetic reactions. This progress is in large part due to the refinement of preparative protocols, X-Ray Diffractometry (XRD), site-directed mutagenesis, Electron Paramagnetic Resonance (EPR) spectroscopy, the coming of age of X-ray Free Electron Laser (XFEL) diffractometry and computational approaches in the investigation of PS II. Nevertheless, key mechanistic and electronic details of water oxidation still remain highly contentious. Elucidation of these details is complicated by the fact that the active site of PSII exists in four natural metastable oxidation states, as well as putative unnatural forms that are plausibly induced during experimental investigation. The leading motivation of the scientific community studying PSII is ultimately the development of new catalysts and even bio-inspired solar cells, that will produce clean and sustainable energy for the world. Over the last hundred years, approximately 80% of worldwide energy consumption has been based on fossil fuels, including coal, oil, and natural gas. However, humankind now has to face the consequences arising from this dependence on fossil fuels. Worldwide energy consumption is expected to increase by over 50% by the mid-2000s (see Fig. 0.1). Because fossil fuels are finite and regional around the world, it is greatly challenging to ensure that this demand can be met, in the face of possible political tensions and other potential problems with energy supplies. Due to the usage of fossil fuels, large quantities of emissions, e.g., CO2, SO2, and oxide particles, are the predominant reasons for global warming and severe pollution. Recent reports from the Intergovernmental Panel on Climate Change emphasized the necessity of decreasing CO2 emissions on a global scale to the zero level before the next century. These arguments make the development of sustainable and carbon-neutral energy technologies one of the most urgent challenges facing humankind all over the world. Wind, ocean currents, tides, and waves are all potential sources of energy, but by far the most abundant renewable energy source on the planet is solar energy: solar illumination on Earth every hour is greater than the worldwide energy consumption for a whole year [35]. Therefore, the conversion and utilization of solar energy is a promising solution for energy problems. An intriguing potential solution to the expected shortfall in energy supplies is artificial photosynthesis [108], whereby light energy can be stored in chemical bonds and, hence, be made available as fuels [18, 200, 19, 364]. Synthetic molecular and heterogeneous manganese analogues still struggle to mimic the function and performance of the OEC. This is partly because these distinctive features are not intrinsic to the Mn4CaO5 core of the OEC but depend on its environment and result from elaborate gating and regulation mechanisms for coordinating the coupling of proton-electron transfer and the access, delivery, binding, positioning, activation, and coupling of substrate waters to form dioxygen. The high level of geometric and electronic control, both spatial and temporal, extends along the whole catalytic cycle and involves simultaneously the Mn4CaO5 cluster, its first coordination sphere, and the protein matrix that controls the flow of electrons, protons, substrates, and products. From the side of theoretical methods great progresses have been made in recent years. Due to the success of the density functional theory (DFT), not only in the field of solid state physics, but also on liquids and molecular compounds, it is possible to obtain the electronic structure of few hundreds atoms with an acceptable computational effort. Using the information provided by the experiments as starting point, it is possible to employ DFT to refine the geometries in relationship with the electron ground-state or different electronic states, to calculate the electron and spin density for a given system and to estimate spectroscopic properties. The coupling of DFT with molecular dynamics also allows us to perform ab-initio molecular dynamics of large systems at finite temperature to fully consider entropic contributions and low-energy conformational changes. Computational techniques can also provide considerable support in the analysis and interpretation of the complex IR spectra of such biological systems. In this thesis, the molecular and electronic structures of the multinuclear manganese containing bioinorganic system together with oxygen-evolving complex of PS II are investigated using DFT-based methods for the theoretical modeling of vibrational spectra in the gas phase by normal mode analysis and molecular dynamics simulations. Research on biological water oxidation traverses scientific fields and concentrates the efforts of a multitude of experimental and theoretical approaches. Different methods of investigation naturally lead to distinct views on the OEC. These are often complementary but at times are contradictory, and it is not always obvious whether the contradictions already exist in the data or arise from their suggested interpretations. Nevertheless, the overarching goals are common to all experimental and theoretical studies. These are not limited to the geometric and electronic structure of the cluster in each state of the cycle but encompass the role of the protein matrix, the channels, and secondary components of the second sphere of the cluster, such as the chloride ions. Chapter I of the thesis considers in detail the progress that have been done so far in structural and spectroscopic studies of OEC and its synthetic mimics given together with the general introduction on photosynthetic reactions occurring in the leaf. Theoretical background of the computational methods used in present work is given in detail in Chapter II. In this thesis, we explored the potentialities and the reliability of different state-of-the-art computational techniques for the investigation of the structural and vibrational properties of complex macromolecular materials of biochemical importance. The use of FTIR spectroscopy to probe the structure and function of the OEC complex in PS II has a long history. The synthesis of a very close structural mimic of the catalytic center has opened up the opportunity to perform a comprehensive and parallel study of both the natural and artificial compounds and of their vibrational modes. Chapter III is dedicated to the detailed assignment of the bands in the midand low-frequencies region by static and dynamic vibrational spectra calculations of the unique biomimetic complex. The detailed parallel analysis between the Natural and Synthetic complexes also provided a comprehensive characterization of the vibrational fingerprints in such class of cubane-like Mn-based compounds and is reported in Chapter IV. In Chapter V of the thesis we discussed the electronic and structural properties of the novel Mn4O4 synthetic compound mimicking the EPR spectroscopic nature of OEC in S2 state.