A Photophysical Investigation of d6 Metal Polypyridyl Complexes
A number of polypyridyl complexes based on d6 metals have been investigated using a range of spectroscopic and computational techniques. Interest in these systems stems from potential applications in molecule based devices, such as dye sensitised solar cells, as well as the potential to gain insight into fundamental photophysical properties of metal complexes and the development of techniques to probe these. Several techniques have been used and are described. The ground states of complexes have been characterised using FT-IR and FT-Raman spectroscopic techniques, which allow for verification of observable quantities obtained from density functional theory calculations. Electronic absorption and resonance Raman allow the initially excited state to be investigated, while transient absorption and transient and time resolved resonance Raman are sensitive toward excited states with the longest lifetimes. Picosecond time resolved infrared is used to bridge this gap, to provide a comprehensive description of processes post initial photoexcitation. Chapter 3 involves discussion of N3, a commonly used dye for dye sensitised solar cells, and derivatives thereof. Derivatisation was carried out to enhance the solar absorption properties of the dyes; however, it was found that the acceptor molecular orbitals do not extend over the attached units in the absorbing state. Time resolved infrared spectroscopy was carried out to probe the excited state, which showed, for an anthrylethenyl-substituted complex, an unusual charge-separated excited state based on a single ligand. In Chapters 4 and 7, investigations on complexes based on the ligand 2,2’;6’,2”-terpyridine, including multimetallic assemblies, are described. The terpyridine allows for the constructions of linear structures; however, corresponding metal complexes possess short lifetimes. Replacing two carbon of this ligand with nitrogen gives 2,6-di-2-pyridyl-1,3,5-triazine, which imbues the corresponding ruthenium complexes with extended lifetimes. Resonance Raman spectroscopy was able to show predominant population of the triazine-containing ligand when the complex is excited in the visible region. The ability of multimetallic assemblies comprised of similar ligands to quench excited state populations was probed. To that end, transient absorption and time resolved resonance Raman spectroscopic techniques have been used to show that initially populated excited state electrons return to the ground state by interacting with attached secondary metal subunits. In Chapters 5 and 6, the effects of substitution of electron donating units on well studied complexes of the ligands 2,2’-bipyridine and dipyrido[3,2-a:2’,3’-c]phenazine are investigated. Rhenium complexes of these ligands show unusual excited states that are thought to originate from the electron donating properties of the substituents. The bipyridine complexes, substituted with ethenyl-linked diphenylaniline, possess an intraligand charge transfer reaction that exists without contribution from the metal; these are well described using resonance Raman and computational methods. Complexes of the substituted dipyridophenazine ligand possess a complex system of excited states that are shown to interconvert on the picosecond and nanosecond timescales. Time resolved infrared spectroscopy was used to assign these states.
Advisor: Gordon, Keith C
Degree Name: Doctor of Philosophy
Degree Discipline: Chemistry
Publisher: University of Otago
Keywords: Raman; rhenium; ruthenium; bipyridine; terpyridine; dppz; polypyridyl; triplet state calculation; DFT; TD-DFT; N3; transient absorption; transient emission; time resolved resonance Raman; time resolved infrared; infrared; excited state dynamics; dye sensitised solar cell; metal to ligand charge transfer; intraligand charge transfer; ligand centered charge transfer; HATN; oxadiazole; triarylamine; triphenylamine; multimetallic assembly
Research Type: Thesis