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Ultrafast Electron Transfer Dynamics in a Series of Porphyrin/Viologen Complexes: Involvement of Electronically Excited Radical Pair Products
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)
2015 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 119, no 24, 7531-7540 p.Article in journal (Refereed) Published
Abstract [en]

Ultrafast electron transfer was studied for a series of metalloporphyrin/bipyridinium complexes in aqueous solution, using laser excitation in the Soret or Q-bands of the porphyrin. Electron transfer occurred before electronic and vibrational relaxation of the initial excited state. This allowed for a thorough investigation of the dependence of electron transfer rate constants on the driving force and the nature of the product state. The driving force dependence showed that electron transfer from the S-2 state occurred to an electronically excited radical pair state, and the present results provide the most direct evidence to date for the formation of such states in photoinduced electron transfer reactions. We also found that subsequent recombination of the radical pair produced vibrationally excited ground states; the excess energy of the radical pair generated from the initial state is not completely dissipated during the lifetime of the radical pair. The porphyrin/bipyridinium complexes where recombination lies deeper in the Marcus inverted region show less formation of unrelaxed ground states, contrary to what is expected from equilibrium electron transfer theories. Instead, the rate of the electron transfer, which competes with vibrational relaxation, was the main parameter controlling the relative yield of unrelaxed ground states within this series of complexes.

Place, publisher, year, edition, pages
2015. Vol. 119, no 24, 7531-7540 p.
Keyword [en]
electron transfer, electronically excited states, unrelaxed ground states, hot ground states, porphyrin, S2, ZnTPPS, MgTPPS, viologen, vibrational relaxation
National Category
Chemical Sciences
Research subject
Chemistry with specialization in Chemical Physics
Identifiers
URN: urn:nbn:se:uu:diva-235459DOI: 10.1021/jp5113119ISI: 000356754800043PubMedID: 25766332OAI: oai:DiVA.org:uu-235459DiVA: diva2:760410
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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