Authors:	Kreitner, Christoph
Title:	Bis(tridentate) polypyridine Ruthenium(II) complexes with push-pull character : synthesis, understanding and application
Online publication date:	15-Aug-2016
Year of first publication:	2016
Language:	english
Abstract:	The profound understanding of the electronic properties of a class of transition metal complexes is of interest to chemists and material scientists as it allows the tuning of their properties and the development of suitable applications. In the work presented herein, the synthesis and characterization of novel push-pull substituted bis(tridentate) ruthenium complexes is presented. In the first section, a dinuclear amide-bridged bis(terpyridine) ruthenium complex with a high electronic symmetry despite the intrinsic structural asymmetry is studied. No metal-metal-interaction is detected in the mixed valent state with the odd electron being entirely localized at one of the two ruthenium centers. This is because the spin carrying orbitals and the mediating orbitals of the bridging ligand are energetically fairly separated. In the photo-excited state, on the other hand, the bridging ligand is formally reduced by one electron. This enables electronic coupling between the two metal centers, so that two valence tautomers are detected simultaneously at room temperature by their dual emission. Further, cyclometalated Ru(N^N^N)(N^C^N)]+ complexes with substituents on either the cyclometalating ligand or both ligands have been synthesized and investigated. Their absorption properties were studied using a combination of spectroscopic and theoretical methods showing that the low-energy absorption bands are composed of Ru(N^N^N) and Ru(N^C^N) transitions to a similar extent in all cases. All complexes are very weakly emissive at room temperature from a 3MLCT (metal-to-ligand charge transfer) state, that is efficiently depopulated via a well-known metal-centered excited state and a previously unrecognized ligand-to-ligand charge transfer state. This was evidenced by temperature-dependent quantum yield measurements and supplementary density functional theory calculations. Additionally, [Ru(N^N)2(N^C)]+ and [Ru(N^N^N)(N^N^C)]+ complexes were studied on a theoretical basis highlighting common features and differences in the excited state depopulation mechanics of the different classes of complexes. In analogy to the dinuclear bis(terpyridine) ruthenium complex, a structurally related complex with an amide-linked biscyclometalating bridging ligand was synthesized and studied. The altered bridge’s frontier orbitals result in electronic coupling between the metal centers in the mixed-valent state as evidenced from an intense intervalence charge transfer band in near infrared region of the electromagnetic spectrum. In the photo-excited state, dual emission is observed at room temperature from two electronically uncoupled 3MLCT states localized at peripheral terpyridine ligands. The distance between the emissive states is too large to allow for a thermally equilibrating energy transfer to occur. Only upon freezing the solution, the non-radiative decay processes are retarded sufficiently to allow for equilibration. Considering these findings, cyclometalated polypyridine ruthenium complexes bearing triarylamine substituents were devised for use as sensitizers in dye-sensitized solar cells. These complexes are substantially valence-delocalized in the mixed-valent state allowing for mesomeric charge delocalization away from the semiconductor surface after charge injection. However, this delocalization results in a measurable resonance stabilization of the mixed-valent state that hampers dye regeneration by the electrolyte in several of the employed dye/electrolyte combinations. As a consequence, the efficiency of solar cells employing benchmark sensitizer N719 are unmatched by the developed cyclometalated polypyridine ruthenium dyes when combined with iodide/triiodie as electrolyte. Using cationic polypyridine cobalt electrolytes, N719 and the cyclometalated dyes exhibit similar performances, but the overall efficiencies are lower than with iodide/triiodide.
DDC:	540 Chemie 540 Chemistry and allied sciences
Institution:	Johannes Gutenberg-Universität Mainz
Department:	FB 09 Chemie, Pharmazie u. Geowissensch.
Place:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-4607
URN:	urn:nbn:de:hebis:77-diss-1000006142
Version:	Original work
Publication type:	Dissertation
License:	In Copyright
Information on rights of use:	https://rightsstatements.org/vocab/InC/1.0/
Extent:	316 Seiten
Appears in collections:	JGU-Publikationen