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Time-Resolved Electron Transfer in Porphyrin-Coordinated Ruthenium Dimers: From Mixed-Valence Dynamics to Hot Electron Transfer
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry. (Leif Hammarström)
University of Carlifonia.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry. (Leif Hammarström)
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry. (Leif Hammarström)
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2015 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 9, 4479-4487 p.Article in journal (Refereed) Published
Abstract [en]

Presented here is the first effort to study the formation and dynamics of the triruthenium cluster (Ru3O) pyrazine-bridged dimer mixed-valence state. Femtosecond transient absorption spectroscopy was implemented to follow photoinduced electron-transfer reactions in a series of asymmetric porphyrin-coordinated dyads, which form strongly coupled mixed-valence species upon single reduction. Excitation of the porphyrin subunit resulted in electron transfer to the Ru3O dimer with a time constant τ ≈ 0.6 ps. The intramolecular electron transfer was confirmed by excitation of the Ru3O MLCT, which resulted in the formation of a vibrationally unrelaxed porphyrin ground state. Under both excitation experiments, the back electron transfer was extremely fast (τCR < 0.1 ps), preventing complete time-resolved exploration of the mixed-valence state. These complexes enabled the observation of excited product states following electron-transfer processes, resulting from porphyrin S1 and S2 excitation. Although the charge recombination itself could not be observed, the yield of unrelaxed ground states supports the conclusion that delocalization takes place at least partially on a sub-100 fs time scale.

Place, publisher, year, edition, pages
2015. Vol. 119, no 9, 4479-4487 p.
Keyword [en]
electron transfer, ET, mixed valence, intervalence charge transfer, ivct, porphyrin, ZnTPP, unrelaxed ground states, hot ground states
National Category
Chemical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-235460DOI: 10.1021/jp510782cISI: 000350840700005OAI: oai:DiVA.org:uu-235460DiVA: diva2:760425
Available from: 2014-11-03 Created: 2014-11-03 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Ultrafast, Non-Equilibrium Electron Transfer Reactions of Molecular Complexes in Solution
Open this publication in new window or tab >>Ultrafast, Non-Equilibrium Electron Transfer Reactions of Molecular Complexes in Solution
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Photoinduced electron transfer is a fundamentally interesting process; it occurs everywhere in the natural world. Studies on electron transfer shed light on questions about the interaction between molecules and how the dynamics of these can be utilized to steer the electron transfer processes to achieve a desired goal. The goal may be to get electrons to the electrode of a solar cell, or to make the electrons form an energy rich fuel such as hydrogen, and it may also be an input or output for molecular switches. The importance of electron transfer reactions will be highlighted in this thesis, however, the main motivation is to gain a better understanding of the fundamental processes that affect the rate and direction of the electron transfer.

A study of photoinduced electron transfer (ET) in a series of metallophorphyrin/bipyridinium complexes in aqueous solution provided fresh insight concerning the intimate relationship between vibrational relaxation and electron transfer. The forward electron transfer from porphyrin to bipyridinium as well as the following back electron transfer to the ground state could be observed by femtosecond transient absorption spectroscopy. Both the reactant and the product states of the ET processes were vibrationally unrelaxed, in contrary to what is assumed for most expressions of the ET rates. This could be understood from the observation of unrelaxed ground states. The excess energy given by the initial excitation of the porphyrin does not relax completely during the two steps of electron transfer. This is an unusual observation, not reported in the literature prior the studies presented in this thesis. This study also gave the first clear evidence of electronically excited radical pairs formed as products of intramolecular electron transfer. Signs of electronically excited radical pairs were seen in transient spectra, and were further verified by the observation that the rates followed a Marcus normal region behavior for all excitation wavelengths, despite the relatively large excess energy of the second excited state.

This thesis also concerns electron transfer in solar cell dyes and mixed valence complexes. In the ruthenium polypyridyl complex Ru(dcb)2(NCS)2, where dcb = 4,4’-dicarboxy-2,2’-bipyridine, inter-ligand electron transfer (ILET) in the 3MLCT state was followed by means of femtosecond transient absorption anisotropy that was probed in the mid-IR region. Unexpectedly, ILET was not observed because electron density was localized on the same bpy during the time-window allowed by the rotational lifetime.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 90 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1205
Keyword
electron transfer, laser, spectroscopy, transient absorption, anisortopy, inter ligand electron transfer, dye sensitized solar cell, DSSC, vibrational relaxation, ultrafast dynamics, fs spectroscopy
National Category
Chemical Sciences
Research subject
Chemistry with specialization in Chemical Physics
Identifiers
urn:nbn:se:uu:diva-235461 (URN)978-91-554-9107-9 (ISBN)
Public defence
2014-12-19, Häggsalen, Ångströmslaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2014-11-26 Created: 2014-11-03 Last updated: 2015-02-03

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Petersson, JonasHammarström, Leif

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