Solar Energy Harvesting with Fe(II)-polypyridines: Strategies for Tuning the Light Absorption and Interfacial Electron Transfe

Metal polypyridines are popular molecular systems used as sensitizers in dyesensitized solar cells (DSSCs) for solar energy capture. Among these, Ru(II) polypyridines are the most common, due to their stability, tunable redox properties, and power conversion efficiencies of over 11%. Unfortunately, Ru is expensive and rare, which renders these sensitizers unsuitable for large-scale applications. Therefore, it is desirable to replace Ru(II) polypyridines with dyes that contain cheaper and more abundant metals, such as Fe.;The Earth-abundant sensitizers based on Fe(II) polypyridines are the focus of this dissertation. The major hurdle towards utilization of Fe(II) sensitizers is the short lifetime of their photoactive metal-to-ligand charge transfer (MLCT) states that undergo ultrafast intersystem crossing (ISC) into the low-lying metal-centered (MC) states. There are at least two ways of improving the efficiencies of Fe(II)-sensitized solar cells. The first route seeks to increase the rate of interfacial electron transfer (IET) from MLCT states into the conduction band (CB) of TiO2. Another route endeavors to slow down the ISC, which competes with IET, via destabilizing the MC states by designing molecules with strong ligand fields.;The computational studies presented in this dissertation focus on [Fe(tpy) 2]2+ (tpy = 2,2OE:6OE,2OEOE-terpyridine) and related complexes, however it is anticipated that the findings will be broadly applicable to any Fe(II) sensitizer. Chapter 1 describes the working of a DSSC, and provides a broad overview of this dissertation. Initially, the focus will be on strategies that can be used to improve the ligand field strength of Fe sensitizers. One way to improve the ligand field strength is by incorporating stronger sigma donor ligands into the Fe complex; i.e replacing the pyridine moieties by aryl groups, making Fe-C bonds, referred to as cyclometalation (Chapter 2). These studies found that cyclometalation is a feasible route to increasing the IET efficiency of Fe(II) photosensitizers by destabilizing the low-lying highspin MC states, but care is needed in the way the sensitizers are attached to the surface of TiO2.;A well-known long-lived Fe(II) photosensitizer that benefits from strong o donor ligands, in this case heterocyclic carbenes (CNC), is discussed in Chapter 3. We have employed molecular dynamics and quantum dynamics simulations to show that the high, 92 %, quantum efficiency of this sensitizer is due to the increased lifetime of the photoactive 3MLCT states or slower ISC rates, as opposed to faster IET rates.;Another possible pathway to increasing the ligand field strength of a transition metal complex is by imposing a nearly perfect octahedral environment around the metal center through inclusion of pyridine-bridging groups into the tpy ligand, as discussed in Chapter 4. The impact of the chemical identity of the bridging group on the resulting ligand geometry and the ligand field strength of Fe(II) sensitizers was also explored. These studies found that the two properties with the strongest impact on the ligand field strength of Fe(II)- polypyridines are Fe-ligand bond lengths and the strength of th bonding interactions. Both of these properties can be tuned via the choice of a pyridine bridging group.;In Chapter 5, a series of complexes where the tpy ligand is substituted at the 4, 4', and 4'' positions by electron donor and acceptor groups was studied. It was found that the presence of heterocyclic, th-conjugated donor groups leads to a marked improvement of the absorption properties, since the highest occupied molecular orbital (HOMO) is no longer solely metal-localized but is delocalized over both the metal center and the ligand. Additional improvement in the absorption properties can be achieved by further extending the pi- conjugation of the donor groups. This is due to the phenomenon of "HOMO inversion", where the HOMO changes from being metal-localized, to fully ligand-localized. Finally, Chapter 6 summarizes the findings of this dissertation. The wide scope of various electronic and structural features examined throughout this work can be used to formulate rational strategies for the design of new metal-polypyridine sensitizers for DSSCs that incorporate iron or other first row transition metals.