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Authors: Moll, Johannes
Advisor: Heinze, Katja
Title: Ligand Controlled Electronic Structure of Iron and Ruthenium Complexes
Online publication date: 24-Feb-2022
Year of first publication: 2022
Language: english
Abstract: Ruthenium(II) is well applied as photosensitizer. Due to its low abundance and consequently high cost it is worthwhile to replace ruthenium(II) by the more abundant homologue iron(II). The intrinsically lower ligand field splitting makes it challenging to design iron(II) complexes with sufficiently long excited state lifetime for photochemical applications. This work describes the development of pseudo-octahedral iron(II) and ruthenium(II) complexes combining the concept of high local symmetry with a push-pull ligand design to stabilize metal-to-ligand charge transfer (MLCT) excited states and destabilize quenching metal centered states. Using the novel 6,2’’-carboxylpyridyl-2,2’-methylaminepyridyl-pyridine (cpmp) ligand combining both, the s-donating N-CH3 bridge and one p-accepting C=O bridge in a single ligand, the homoleptic push-pull complexes [Ru(cpmp)2]2+ and [Fe(cpmp)2]2+ were synthesized. Additionally the heteroleptic [Ru(cpmp)(ddpd)]2+ and [Fe(cpmp)(ddpd)]2+ complexes bearing the literature known N,N’-dimethyl-N,N’-dipyridine-2-yl-pyridine-2,6- diamine (ddpd) ligand with two s-donating N-CH3 bridges were synthesized. The ruthenium(II) complexes [Ru(cpmp)2]2+ and ([Ru(cpmp)(ddpd)]2+ show phosphorescence at 709 nm and 755 nm, respectively, with quantum yields of 1.3 % and 0.04 %, respectively. The lower quantum yield of the heteroleptic complex can be rationalized with the more flexible N-CH3 bridges enabling a lower energy barrier for non-radiative relaxation. Nevertheless, both complexes act as photosensitizer in a green-light-sensitized thiol-ene click reaction. The homologue iron(II) complexes show low-energy MLCT absorption bands around 610 nm. However, the MLCT lifetime below 100 fs is too short for phosphorescence or photochemistry. The measured excited state lifetime of around 500 ps can be assigned to the 5T state. Excited state dynamics calculations confirm relaxation occurs along the typical 1MLCT→3MLCT→3MC→5MC excited state relaxation cascade. As the class of [Fe(N^N^C)(N^N^N)]+ type cyclometalated iron(II) complexes were predicted by quantum chemical calculations to be good candidates for increased excited state lifetime, the [Fe(pbpy)(tpy)]+ (Hpbpy=6-phenyl-2,2’-bipyridine and tpy=2,2’:6’,2’’-terpyridine) complex was synthesized by Jakob Steube. The photophysical properties are significantly improved in comparison to [Fe(tpy)3]2+. The 3MLCT lifetime is increased to 800 fs compared to 145 fs for [Fe(tpy)3]2+. It was estimated by timeresolved UV/Vis spectroscopy, performed by Ayla Kruse (geb. Päpcke). Spectroelectrochemical investigations by Johannes Moll were used for the assignment. However, this lifetime is too short for phosphorescence or photochemistry. The 2,6-diguanidylpyridine (dgpy) ligand is a strong s-donor. Expecting this ligand to stabilize charge transfer excited states, the performance of dgpy in iron complexes was investigated. Both, the homoleptic iron(II) and the homoleptic iron(III) complex are non-emissive. While the iron(III) complex is proved to have a low-spin ground state, the iron(II) complex shows spin crossover behavior in its meridional isomer, implying a rather low ligand field strength. Furthermore, the cis-facial isomer is present in solution, featuring a high-spin ground state.
DDC: 540 Chemie
540 Chemistry and allied sciences
Institution: Johannes Gutenberg-Universität Mainz
Department: FB 09 Chemie, Pharmazie u. Geowissensch.
Place: Mainz
URN: urn:nbn:de:hebis:77-openscience-55c1bc55-3f3c-4980-8085-147d6065b1d63
Version: Original work
Publication type: Dissertation
License: In Copyright
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Extent: 301 Seiten, Illustrationen, Diagramme
Appears in collections:JGU-Publikationen

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