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Ultrafast, Non-Equilibrium Electron Transfer Reactions of Molecular Complexes in Solution
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry. (Leif Hammarström)
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 [en]
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: urn:nbn:se:uu:diva-235461ISBN: 978-91-554-9107-9 (print)OAI: oai:DiVA.org:uu-235461DiVA: diva2:762342
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
List of papers
1. Variation of Excitation Energy Influences the Product Distribution of a Two-Step Electron Transfer: S-2 vs S-1 Electron Transfer in a Zn(II)porphyrin-Viologen Complex
Open this publication in new window or tab >>Variation of Excitation Energy Influences the Product Distribution of a Two-Step Electron Transfer: S-2 vs S-1 Electron Transfer in a Zn(II)porphyrin-Viologen Complex
2009 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 131, no 23, 7940-7941 p.Article in journal (Refereed) Published
Abstract [en]

We have, for the first time for molecular systems in solution, shown a case where the variation of excitation energy influences the product distribution of a two-step electron transfer. The photoexcitation Was to the porphyrin-localized S-2 state or either of the S-1(v = 1) or S-1(v = 0) states of an aqueous Zn(II)-meso-tetrasulphonatophenyl-porphyrin methylviologen (ZnTPPS4-/MV2+) complex. Both forward and back electron transfer occur on a subpicosecond time scale (tau(FET) = 0.2, tau(BET) 0.7 ps). The excess energy Of the higher excitations partially survives both electron transfer steps. This is seen by different distributions of unrelaxed ground states, which are generated by the back electron transfer and has unique UV-vis spectroscopic signatures. State selective electron transfer opens interesting possibilities for reaction control, and the results represent initial steps in that direction

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-128393 (URN)10.1021/ja900729j (DOI)000267623100002 ()
Available from: 2010-07-23 Created: 2010-07-20 Last updated: 2017-12-12Bibliographically approved
2. Ultrafast Electron Transfer Dynamics of a Zn(II)porphyrin-Viologen Complex Revisited S-2 vs S-1 Reactions and Survival of Excess Excitation Energy
Open this publication in new window or tab >>Ultrafast Electron Transfer Dynamics of a Zn(II)porphyrin-Viologen Complex Revisited S-2 vs S-1 Reactions and Survival of Excess Excitation Energy
2010 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 114, no 45, 14329-14338 p.Article in journal (Refereed) Published
Abstract [en]

The photoinduced electron transfer reactions in a self-assembled 1 1 complex of zinc(II)tetrasulphonatophe nylporphyrin (ZnTPPS4-) and methylviologen (MV2+) in aqueous solution were investigated with transient absorption spectroscopy ZnTPPS4- was excited either in the Soret or one of the two Q-bands corresponding to excitation into the S-2 and S-1 states respectively The resulting electron transfer to MV2+ occurred surprisingly with the same time constant of tau(FET) = 180 fs from both electronic states The subsequent back electron transfer was rapid and the kinetics was independent of the initially excited state (tau(BET) = 700 fs) However ground state reactants in a set of vibrationally excited states were observed The amount of vibrationally excited ground states detected increased with increasing energy of the initial excited state showing that excess excitation energy survived a two-step electron transfer reaction in solution Differences in the ZnTPSS3-/MV+. spectra suggest that the forward election transfer from the S-2 state at least partially produces an electronically excited charge transfer state which effectively suppresses the influence of the inverted regime Other possible reasons for the similar election transfer rates for the different excited states are also discussed

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-139266 (URN)10.1021/jp9116867 (DOI)000284018000026 ()
Available from: 2010-12-22 Created: 2010-12-22 Last updated: 2017-12-11Bibliographically approved
3. Ultrafast Electron Transfer Dynamics in a Series of Porphyrin/Viologen Complexes: Involvement of Electronically Excited Radical Pair Products
Open this publication in new window or tab >>Ultrafast Electron Transfer Dynamics in a Series of Porphyrin/Viologen Complexes: Involvement of Electronically Excited Radical Pair Products
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.

Keyword
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:nbn:se:uu:diva-235459 (URN)10.1021/jp5113119 (DOI)000356754800043 ()25766332 (PubMedID)
Available from: 2014-11-03 Created: 2014-11-03 Last updated: 2017-12-05Bibliographically approved
4. Time-Resolved Electron Transfer in Porphyrin-Coordinated Ruthenium Dimers: From Mixed-Valence Dynamics to Hot Electron Transfer
Open this publication in new window or tab >>Time-Resolved Electron Transfer in Porphyrin-Coordinated Ruthenium Dimers: From Mixed-Valence Dynamics to Hot Electron Transfer
Show others...
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.

Keyword
electron transfer, ET, mixed valence, intervalence charge transfer, ivct, porphyrin, ZnTPP, unrelaxed ground states, hot ground states
National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-235460 (URN)10.1021/jp510782c (DOI)000350840700005 ()
Available from: 2014-11-03 Created: 2014-11-03 Last updated: 2017-12-05Bibliographically approved
5. Interligand Electron Transfer Dynamics in the MLCT Excited State of Ru(4,4’-dicarboxylate-2,2’-bipyridine)2cis(NCS)2 Studied by Transient IR Absorption Anisotropy
Open this publication in new window or tab >>Interligand Electron Transfer Dynamics in the MLCT Excited State of Ru(4,4’-dicarboxylate-2,2’-bipyridine)2cis(NCS)2 Studied by Transient IR Absorption Anisotropy
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We have studied the interligand electron transfer (ILET) in the N712 dye (deprotonated N3) by means of IR transient IR absorption anisotropy. By probing vibrational transitions in the IR we can provide stronger evidence than previous studies that probed the electronic transitions in the visible. The results show that the observation of ILET is limited by the rotational time of the complex, and this gives a lower rate of 800 ns for ILET. Previously reported results have been both very fast and very slow, with timeconstants 20 ps and 3 ns respectively and the results presented here supports the slower rate. The initial anisotropy trace show an increase with a 5 ps time-constant, and the initial value corresponds to what is expected for a delocalized state. We note however that the time-scale of delocalization is unexpectedly slow.

Keyword
electron transfer, inter ligand electron transfer, dye sensisized solar cells, N3, N712
National Category
Chemical Sciences
Research subject
Chemistry with specialization in Chemical Physics
Identifiers
urn:nbn:se:uu:diva-235458 (URN)
Available from: 2014-11-03 Created: 2014-11-03 Last updated: 2015-02-03
6. Multichromophoric Sensitizers Based on Squaraine for NiO Based Dye-Sensitized Solar Cells
Open this publication in new window or tab >>Multichromophoric Sensitizers Based on Squaraine for NiO Based Dye-Sensitized Solar Cells
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2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 1, 103-113 p.Article in journal (Refereed) Published
Abstract [en]

Three sensitizers were synthesized and utilized as panchromatic dyes for nanocrystalline NiO films: an iodo-squaraine (SQ), a squaraine-perylene monoimide (SQ-PMI) dyad, and a squaraine-perylene monoimide-naphthalene diimide (SQ-PMI-NDI) triad. Photophysical and photovoltaic studies showed that hole injection into the NiO valence band from the SQ excited state is ultrafast, but also that subsequent recombination is very rapid, preventing SQ from being an efficient sensitizer for photovoltaic purposes. The introduction of a second light harvesting unit (PMI) and a terminal electron acceptor (NDI) significantly improved the photovoltaic performances of the system. Irrespective of which light harvesting unit was photoexcited in NiO/SQ-PMI and NiO/SQ-PMI-NDI, intramolecular charge separation leading to SQ+ and PMI or NDI is the main excited state deactivation process. Intramolecular charge separation occurred in spite of the favorable conditions for energy transfer to the SQ unit. Subsequent hole injection from SQ+ into NiO was in competition with intramolecular recombination, which may have significantly decreased the overall photovoltaic performances. The control of this side-reaction is therefore crucial for the successful design of multichromophoric systems for dye-sensitized solar cells (DSSCs). The quantum yield of NiO(+)/SQ-PMI-NDI after 50 ns is higher than that of NiO(+)/SQ-PMI, and much higher than that of NiO(+)/SQ; intramolecular recombination was slowed down by the localization of the electron further away from the SQ+ hole and consequently from NiO+. The three sensitizers were tested in NiO based DSSC devices using either the conventional triiodide/iodide electrolyte or a cobaltIII/II(4,4′-di-tert-butyl-2,2′-bipyridine)3 electrolyte. The photoconversion efficiencies steadily increased in the following order: SQ < SQ-PMI < PMI-NDI SQ-PMI-NDI. The multichromophoric sensitizers had broader absorption spectra, more long-lived charge separation, and better photovoltaic performance than single unit chromophores. This indicates that bichromophoric systems, ones in which the antenna serves both as electron acceptor and photon harvester, are realistic sensitizers to boost photovoltaic performances. These findings are important for engineering new panchromatic and more efficient sensitizers for p-type DSSCs.

National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-218951 (URN)10.1021/jp408900x (DOI)000329678200014 ()
Available from: 2014-02-27 Created: 2014-02-20 Last updated: 2017-12-05Bibliographically approved

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