Ligand Controlled Electronic Structure of Iron and Ruthenium Complexes
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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.