Products Cited in Publications Worldwide Effect of Ligand Modification on Properties of Iron-Tetracarbene Photosensitisers: Design, Synthesis and Photovoltaic Applications

Effect of Ligand Modification on Properties of Iron-Tetracarbene Photosensitisers: Design, Synthesis and Photovoltaic Applications

Persson, Samuel

DOI:

Abstract

The use of N-heterocyclic carbenes in conjunction with iron has been studied over the last decade for the purpose of achieving well-functioning iron-based photosensitisers. This is based on the strong σ-donating ability of the carbene, and the effect this has on the metal-based orbitals of iron, for achieving more stable excited states. This work has been done to harness this earth-abundant metal’s potential for widespread application as a photosensitiser. Within this thesis are presented a number of new iron N-heterocyclic carbene complexes, and investigations into their use as photosensitisers. All complexes are based on a core bis-tridentate ligand structure, where each ligand has a pyridine unit, flanked by two N-heterocyclic carbene moieties. First is the study of heteroleptic iron complexes, and their use in photovoltaic application. The study served to give insight into the synthetic demands of the heteroleptic iron complexes, with a difference in nucleophilicity of the two involved ligands seemingly being detrimental to the yield. The new complexes showed an improved efficiency of up to 1.3%, compared to their homoleptic parent complex’s efficiency of 0.7%. Further optimisation of the various other parts of the solar cell architecture resulted in even further improvement of the photovoltaic efficiencies. These investigations simultaneously revealed hysteresis effects to be present in these devices. Additionally, the study of the excited state dynamics of the photosensitisers at the solar cell surface seemingly revealed charge recombination with an excited state of the dye. This is followed by the synthesis and investigation of a series of new iron N-heterocyclic carbene complexes bearing phenyl-ethynyl substituents. These substituents were further modified to study the effect of electron withdrawing and electron donating groups. These new substituents served to double the excited state lifetime, from 9 ps to 18 ps, compared to the unsubstituted parent complex. Furthermore, the complexes showed no major population of the metal centred states during the excitation. However, a difference in substituents on the new phenyl-ethynyl moieties had almost no effect on the excited state dynamics of the iron centre. In addition is outlined the as of yet unrealised efforts to synthesise analogs of these designs, for use in solar cell applications. Lastly, is presented the post-complexation formation, and subsequent photophysical study, of an iron Nheterocyclic carbene complex incorporating dihydroimidazolylidene carbenes rather than simple imidazolylidenes used for other complexes. This led to a metal centred excited state lifetime of 75 ps, compared to the excited state lifetime of 9 ps for the non-hydrogenated parent complex. This strategy was not by itself adequate to improve the charge transfer state lifetime of the complex.

Keywords

complexation ; dye-senstised solar cell ; earth-abundant ; electron transfer ; iron ; iron complex ; ligand synthesis ; N-heterocyclic carbene ; photosensitiser ; photovoltaics

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