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Modulation of the lowest metal-to-ligand charge-transfer state in [Ru(bpy)(2)(N-N)](2+) systems by changing the N-N from hydrazone to azine: Photophysical Consequences
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Photochemistry and Molecular Science, Chemical Physics.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Photochemistry and Molecular Science, Chemical Physics.
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2006 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 45, no 23, 9580-9586 p.Article in journal (Refereed) Published
Abstract [en]

Two Ru( II) complexes, [ Ru( bpy) L-2]( ClO4) 2 ( 1) and [ Ru( bpy)(2)L']( BF4) 2 ( 2), where bpy is 2,2'-bipyridine, L is diacetyl dihydrazone, and L' 1: 2 is the condensate of L and acetone, are synthesized. From X-ray crystal structures, both are found to contain distorted octahedral RuN62+ cores. NMR spectra show that the cations in 1 and 2 possess a C-2 axis in solution. They display the expected metal-to-ligand charge transfer ( (MLCT)-M-1) band in the 400 - 500 nm region. Complex 1 is nonemissive at room temperature in solution as well as at 80 K. In contrast, complex 2 gives rise to an appreciable emission upon excitation at 440 nm. The room-temperature emission is centered at 730 nm ( lambda(max)(em)) with a quantum yield ( em) of 0.002 and a lifetime ( tau(em)) of 42 ns in an air-equilibrated methanol - ethanol solution. At 80 K, Phi(em) = 0.007 and tau(em)= 178 ns, with a lambda(max)(em) of 690 nm, which is close to the 0 - 0 transition, indicating an (MLCT)-M-3 excited-state energy of 1.80 eV. The radiative rate constant ( 5 x 10(4) s(-1)) at room temperature and 80 K is almost temperature independent. From spectroelectrochemistry, it is found that bpy is easiest to reduce in 2 and that L is easiest in 1. The implications of this are that in 2 the lowest (MLCT)-M-3 state is localized on a bpy ligand and in 1 it is localized on L. Transient absorption results also support these assignments. As a consequence, even though 2 shows a fairly strong and long-lived emission from a Ru( II) -> bpy CT state, the Ru( II) -> L CT state in 1 shows no detectable emission even at 80 K.

Place, publisher, year, edition, pages
2006. Vol. 45, no 23, 9580-9586 p.
National Category
Inorganic Chemistry
URN: urn:nbn:se:uu:diva-95089DOI: 10.1021/ic061116kISI: 000241778900057OAI: oai:DiVA.org:uu-95089DiVA: diva2:169165
Available from: 2006-11-09 Created: 2006-11-09 Last updated: 2011-05-13Bibliographically approved
In thesis
1. Tuning of the Excited State Properties of Ruthenium(II)-Polypyridyl Complexes
Open this publication in new window or tab >>Tuning of the Excited State Properties of Ruthenium(II)-Polypyridyl Complexes
2006 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Processes where a molecule absorbs visible light and then converts the solar energy into chemical energy are important in many biological systems, such as photosynthesis and also in many technical applications e.g. photovoltaics. This thesis describes a part of a multidisciplinary project, aiming at a functional mimic of the natural photosynthesis, with the overall goal of production of a renewable fuel from sun and water. More specific, the thesis is focused on design and photophysical characterization of new photosensitizers, i.e. light absorbers that should be capable of transferring electrons to an acceptor and be suitable building blocks for supramolecular rod-like donor-photosensitizer-acceptor arrays.

The excited state lifetime, the excited state energy and the geometry are important properties for a photosensitizer. The work presented here describes a new strategy to obtain longer excited state lifetimes of the geometrically favorable Ru(II)-bistridentate type complexes, without a concomitant substantial decrease in excited state energy. The basic idea is that a more octahedral coordination around the Ru will lead to longer excited state lifetimes. In the first generation of new photosensitizers a 50-fold increase of the excited state lifetime was observed, going from 0.25 ns for the model complex to 15 ns for the best photosensitizer. The second generation goes another step forward, to an excited state lifetime of 810 ns. Furthermore, the third generation of new photosensitizers show excited state lifetimes in the 0.45 - 5.5 microsecond region at room temperature, a significant improvement. In addition, the third generation of photosensitizers are suitable for further symmetric attachment of electron donor and acceptor motifs, and it is shown that the favorable properties are maintained upon the attachment of anchoring groups. The reactivity of the excited state towards light-induced reactions is proved and the photostability is sufficient so the new design strategy has proven successful.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2006. 79 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 237
Physical chemistry, Artificial photosynthesis, Ruthenium(II), Bistridentate complexes, Excited state lifetime, Linear donor-photosenstizer-acceptor arrays, Temperature dependence, Excited state decay, Fysikalisk kemi
urn:nbn:se:uu:diva-7230 (URN)91-554-6707-5 (ISBN)
Public defence
2006-12-01, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:30
Available from: 2006-11-09 Created: 2006-11-09 Last updated: 2011-06-10Bibliographically approved

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