OPTOELECTRONICALLY ACTIVE DINUCLEAR RHENIUM(I) AND MANGANESE(I) COMPLEXES: FROM DESIGN TO APPLICATIONS

Energy is definitely the most important resource for mankind, and sunlight is without any doubt the ultimate energy source.[1] Unfortunately, solar energy is not useful for mankind unless converted into the final usable forms: heat, electricity, and fuels. Conversion of solar energy into heat is straightforward, but conversion of solar energy into electricity or fuel poses several problems, strongly limiting the conversion efficiency.[2] Since we cannot modify the solar spectrum, we need to find materials capable of exploiting sunlight through the threshold mechanism with the highest possible efficiency. Taking into account the average spectral distribution of solar energy, the most favorable threshold is about 885 nm (1.4 eV), which, in principle, allows 33% of energy conversion efficiency.[3] Summing up, the amount of energy we can actually get from the average solar power striking the surface of the earth depends on our capacity of developing the conversion and storage devices we need with the materials we have on our planet. In the last five years, the photovoltaic systems worldwide have undergone substantial development in terms of manufacturing distribution (largely shifted from Europe to Asia), global deployment, and even new photoactive materials.[4] Over 90% of today commercial solar cells are still based on the very same material and basic concepts developed in the 50’s at the Bell Laboratories: light-induced charge separation at a p–n junction between two wafers of p- and n-doped silicon in either single-crystal or polycrystalline form (sc-Si and poly-Si, respectively). The global share of Si PV has increased from 80% in 2009 to over 90% in 2014, because the main competitors, the so called “2nd generation solar cells”, thin film technologies like cadmium telluride (CdTe), copper-gallium-indium selenide (CIGS), and amorphous silicon (a-Si) have grown at a much lower rate.[4] Indeed, Silicon is the second most abundant and uniformly distributed element on the earth’s crust and there is no risk of shortage in any foreseeable future. By contrast, In, Ga, Se, Te, and Cd exhibit a way smaller crustal abundance and, accordingly, they are collected only as byproducts of minerals containing mostly other elements (Cu, Zn, and Al).[5] The third wave of PV technologies entering the market should be based on DSSC and OPV. Expectations for their market debut have been high for years,[6-8] but so far they have materialized only to a very small extent. At present, the market share of these two technologies is still virtually zero, despite a few flagship demonstration projects, which support technical feasibility.[9-10] Compared to the already established technologies, DSSC and OPV can offer easier building integration, in windows and facades, good performances also in non-standard illumination and temperature conditions, and lower requirements in terms of quantity and quality of raw materials. They can be manufactured at smaller economic and energetic cost and their energy payback times are estimated to be shorter than conventional thin-film technologies. The photoactive materials, including dyes, polymeric and small molecular semiconductors, play a key role in influencing physical processes involved in energy conversion, which in turn determine the electrical characteristics of the solar cell. Several types of organic and inorganic dyes are now available, as well as solid-state devices including the redox mediator, as a result of a massive research effort throughout 25 years. Despite the marked increase in the understanding of the DSSC and OPV solar cells, there remain numerous challenges related to cell/module performance and stability that need to be addressed before this technology can be deployed on a large scale. In this Ph.D. thesis we have focused our attention on dinuclear Re and Mn complexes able to act as active materials in optoelectronic applications, such as dye-sensitized solar cells, organic photovoltaics devices (Re) and electrocatalytic reduction of CO2 or H2 generation (Mn). Indeed, considering the state-of-the-art of the knowledge about the dinuclear rhenium complexes containing 1,2-diazine ligands developed in our research group, starting from the pioneering studies on the halide derivatives and their application as dopants in OLED devices and as dyes for bio-imaging, we tried here to extend and tune the properties of these complexes, potentially widening their possible application in various sub-fields of optoelectronics. Therefore, starting from complexes with general formula [M2(μ-X)(μ-Y)(CO)6(μ-R-diazine)] (M=Re, Mn), being X and Y two anionic bridging ligands, we have carried out tailored syntheses with joint experimental and theoretical studies, in order to gain a deeper insight into the electronic processes involved in these classes of compounds. The spectroscopic and/or catalytic properties of the new complexes have been modulated by varying the substituents on the diazine ligand, as well as the nature of the ancillary ligands, thus modulating the LUMO and the HOMO energy level, respectively. Some of these materials were also successfully tested as dyes in DSSC devices. This thesis is basically divided in four main sections 1) New class of Re complexes with lower energy-gap and/or long lived excited state as triplet photosensitizer for triplet-triplet annihilation (TTA) based upconversion 2) New class of hydrido Re complexes and their applications in DSSC solar cells 3) New low-band gap metallo-copolymers based on Re complexes as donors in bulk-heterojunction solar cell 4) New polynuclear Mn complexes containing diazine ligands It is clear that this thesis is the result of a highly multidisciplinary, and therefore collaborative, research work. We have collaborated with various research groups both in Italy and Europe: The electrochemical characterizations have been carried out in collaboration with Prof. Patrizia Mussini (Dipartimento di Chimica, Università degli Studi di Milano, Italy). The theoretical calculation and the molecules’ design, together with the solid state analysis of the complexes, have been performed by Dr. Pierluigi Mercandelli of the same department. The test concerning the TTA upconversion (chapter 4) has been performed in collaboration with Prof. Paola Ceroni (Dipartimento di Chimica, Alma Mater Studiorum - Università di Bologna, Italy). A preliminary photophysical characterization was previously carried out by Dr. Matteo Mauro and Prof. Luisa De Cola (Institut de Science et d'Ingénierie Supramoléculaires (ISIS), Strasbourg, France). Two different research groups have been involved in the fabrication of the DSSC devices (chapter 5): preliminary tests were performed by Dr. Francesca De Rossi and Prof. Thomas M. Brown (Center for Hybrid and Organic Solar Energy – CHOSE, Rome, Italy), while the optimization of the cells and a second series of tests has been carried out in collaboration with Dr. Kazuteru Nonomura and Prof. Anders Hagfeldt (Laboratory of Photomolecular Science (LSPM), École Polytechnique Fédérale de Lausanne (EPFL), Switzerland). Dr. Stefania Zappia and Dr. Silvia Destri have been involved in the synthesis and the characterization of the metallo-copolymers (chapter 6) (Istituto per lo Studio delle Macromolecole, Consiglio Nazionale delle Ricerche (ISMAC-CNR), Milan, Italy). [1]N. Armaroli, V. Balzani, Chem. Eur. J. 2016, 22, 32–57 [2] N. Armaroli, V. Balzani, Energy for a Sustainable World—From the Oil Age to a Sun Powered Future, Wiley-VCH, Weinheim (Germany), 2011 [3] G. Porter, Criteria for Solar Energy Conversion, in Light, Chemical Change and Life. A Source Book in Photochemistry (Eds.: J. D. Coyle, R. R. Hill, D. R. Roberts), Open University Press, Milton Keynes (UK), 1982, 338 [4] International Energy Agency, Technology Roadmap—Solar Photovoltaic Energy, 2014 https://www.iea.org [5] L. T. Peiró, G. Villalba Mendez, R. U. Ayres, Environ. Sci. Technol. 2013, 47, 2939
 [6] M. Jacoby, Chem. Eng. News 2010, 88 (34), 12 [7] NanoMarkets Report, Dye-Sensitized Cell Markets 2012—Nano-531, 2012, http://ntechresearch.com/market reports/dye sensitized cell markets 2012 [8] International Energy Agency, Technology Roadmap-Solar Photovoltaic Energy 2010, https://www.iea.org [9] École Polytechnique Fédérale de Lausanne, EPFL’s Campus Has the World’s First Solar Window, can be found under https://actu.epfl.ch [10] F. C. Krebs, N. Espinosa, M. Hosel, R. R. Sondergaard, M. Jorgensen, Adv. Mater. 2014, 26, 29