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Bridge Mediated Electron Transfer in Conjugated and Cross-Conjugated Donor-Acceptor Compounds
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry. (Leif Hammarström)
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Detailed understanding of electron transfer reactions is important in many aspects of chemistry, biology and solar energy conversion.

The main aim of this thesis is to provide further insight into electron transfer through highly conjugated bridge structures. Towards this end, three series of donor-acceptor dyads have been studied, all using an oligo(1,4-phenylene-ethynylene) moiety as the bridge. A common theme in these series is that they explore the effects of having either an ethynylene or phenylene as the attachment group between the bridge and the donor or acceptor. Photophysical characterization of these dyads was carried out by means of time resolved laser spectroscopy. The results show that having an ethynylene as attachment group results in higher rates for bridge mediated electron and energy transfer compared to similar systems, where a phenylene was used.

It was also found that most of the investigated systems show a fast back electron transfer. A notable exception is a zinc(II) phthalocyanine- gold(III) porphyrin dyad, where very fast photoinduced electron transfer (kPET = 1.0×1012 s-1) was followed by relatively slow back electron transfer (kBET = 1.0×109 s-1). A complementary DFT investigation indicated that the charge shifted state involves a reduction of the gold ion, rather than the porphyrin ring. This results in lower electronic coupling between the reduced gold porphyrin and the bridge and thus slower back electron transfer.

A series of zinc porphyrin platinum acetylide dyads was used to explore the effects on electronic coupling of different attachments points on the porphyrin ring. For the investigated system it was found that linking at the meso-position results in an eight-fold increase in electron transfer rate compared to the β-position.

In addition, a series of mixed valence compounds was used to investigate electronic coupling mediated by cross-hyperconjugated or cross-π-conjugated bridges. The results indicate coupling elements of 100-400 cm-1, with the cross-π-conjugated bridge having the largest coupling. A complementary TD-DFT study indicates that both through bond and through space coupling can be active in these systems. The relative contribution of these two mechanisms to the electronic coupling is highly conformer dependent.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. , 71 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 974
Keyword [en]
Electron transfer, Ultrafast, Long-range, Intervalence charge transfer, cross conjugation, hyperconjugation
National Category
Physical Chemistry
Research subject
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-180442ISBN: 978-91-554-8474-3 (print)OAI: oai:DiVA.org:uu-180442DiVA: diva2:552524
Public defence
2012-10-19, Polhemssalen, Ångström lab, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2012-10-01 Created: 2012-09-07 Last updated: 2013-01-23Bibliographically approved
List of papers
1. Strongly coupled zinc phthalocyanine-tin porphyrin dyad performing ultra-fast single step charge separation over a 34 Å distance
Open this publication in new window or tab >>Strongly coupled zinc phthalocyanine-tin porphyrin dyad performing ultra-fast single step charge separation over a 34 Å distance
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2007 (English)In: Chemical Communications, ISSN 1359-7345, E-ISSN 1364-548X, no 44, 4629-4631 p.Article in journal (Refereed) Published
Abstract [en]

A zinc(ii) phthalocyanine-tin(iv) porphyrin dyad with a strong electronic coupling was synthesized and upon light excitation shown to exhibit ultra-fast, long-range electron transfer in a single step.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-12069 (URN)10.1039/b711642j (DOI)000250787400013 ()17989814 (PubMedID)
Available from: 2007-11-21 Created: 2007-11-21 Last updated: 2017-12-11Bibliographically approved
2. Photoinduced Electron Transfer in Zn(II)porphyrin-Bridge-Pt(II)acetylide Complexes: Variation in Rate with Anchoring Group and Position of the Bridge
Open this publication in new window or tab >>Photoinduced Electron Transfer in Zn(II)porphyrin-Bridge-Pt(II)acetylide Complexes: Variation in Rate with Anchoring Group and Position of the Bridge
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2010 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 49, no 21, 9823-9832 p.Article in journal (Refereed) Published
Abstract [en]

The synthesis and photophysical characterization of two sets of zinc porphyrin platinum acetylide complexes are reported. The two sets of molecules differ in the way the bridging phenyl-ethynyl unit is attached to the porphyrin ring. One set is attached via an ethynyl unit on the beta position, while the other set is attached via a phenyl unit on the meso position of the porphyrin. These were compared with previously studied complexes where attachment was made via an ethynyl unit on the meso position. Femtosecond transient absorption measurements showed in all systems a rapid quenching of the porphyrin singlet state. Electron transfer is suggested as the quenching mechanism, followed by an even faster recombination to form both the porphyrin ground and triplet excited states. This is supported by the variation in quenching rate and porphyrin triplet yield with solvent polarity, and the observation of an intermediate state in the meso-phenyl linked systems. The different linking motifs between the dyads resulted in significant variations in electron transfer rates.

National Category
Chemical Sciences
Identifiers
urn:nbn:se:uu:diva-133736 (URN)10.1021/ic100605t (DOI)000283279800017 ()
Available from: 2010-11-17 Created: 2010-11-15 Last updated: 2017-12-12Bibliographically approved
3. Long-Range Electron transfer in Zinc-Phthalocyanine-oligo(phenylene-ethynylene)-based donor-bridge-acceptor dyads
Open this publication in new window or tab >>Long-Range Electron transfer in Zinc-Phthalocyanine-oligo(phenylene-ethynylene)-based donor-bridge-acceptor dyads
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2012 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 51, no 21, 11500-11512 p.Article in journal (Refereed) Published
Abstract [en]

In the context of long-range electron transfer for solar energy conversion, we present the synthesis, photophysical, and computational characterization of two new zinc(II) phthalocyanine oligophenylene-ethynylene based donor-bride-acceptor dyads: ZnPc-OPE-AuP+ and ZnPc-OPE-C60. A gold(III) porphyrin and a fullerene has been used as electron accepting moieties, and the results have been compared to a previously reported dyad with a tin(IV) dichloride porphyrin as the electron acceptor (Fortage et al. Chem. Commun.2007, 4629). The results for ZnPc-OPE-AuP+ indicate a remarkably strong electronic coupling over a distance of more than 3 nm. The electronic coupling is manifested in both the absorption spectrum and an ultrafast rate for photoinduced electron transfer (kPET = 1.0 × 1012 s–1). The charge-shifted state in ZnPc-OPE-AuP+ recombines with a relatively low rate (kBET = 1.0 × 109 s–1). In contrast, the rate for charge transfer in the other dyad, ZnPc-OPE-C60, is relatively slow (kPET = 1.1 × 109 s–1), while the recombination is very fast (kBET ≈ 5 × 1010 s–1). TD-DFT calculations support the hypothesis that the long-lived charge-shifted state of ZnPc-OPE-AuP+ is due to relaxation of the reduced gold porphyrin from a porphyrin ring based reduction to a gold centered reduction. This is in contrast to the faster recombination in the tin(IV) porphyrin based system (kBET = 1.2 × 1010 s–1), where the excess electron is instead delocalized over the porphyrin ring.

Keyword
Electron transfer, Phthalocyanine, long-range
National Category
Physical Chemistry
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-180432 (URN)10.1021/ic3013552 (DOI)000313220200038 ()
Available from: 2012-09-07 Created: 2012-09-06 Last updated: 2017-12-07Bibliographically approved
4. Photoinduced Electron and Hole Transfer in Asymmetrically Bridged Bis-porphyrin Dyads
Open this publication in new window or tab >>Photoinduced Electron and Hole Transfer in Asymmetrically Bridged Bis-porphyrin Dyads
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(English)Manuscript (preprint) (Other academic)
National Category
Physical Chemistry
Research subject
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-180438 (URN)
Available from: 2012-09-06 Created: 2012-09-06 Last updated: 2012-10-01
5. Charge Transfer Through Cross-π-conjugated and Cross-hyperconjugated Bridges: An Intervalence Charge Transfer Study
Open this publication in new window or tab >>Charge Transfer Through Cross-π-conjugated and Cross-hyperconjugated Bridges: An Intervalence Charge Transfer Study
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(English)Manuscript (preprint) (Other academic)
Keyword
Intervalence charge transfer, Cross-conjugation, Cross-hyperconjugation
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-180437 (URN)
Available from: 2012-09-07 Created: 2012-09-06 Last updated: 2012-10-01Bibliographically approved

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