| 1. |
- Andreasson, Joakim, 1973, et al.
(author)
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Photoinduced hole transfer from the triplet state in a porphyrin-based donor-bridge-acceptor system
- 2003
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In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 107:42, s. 8825-8833
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Journal article (peer-reviewed)abstract
- The triplet excited-state deactivation of a gold porphyrin (AuP) in porphyrin-based donor-bridge-acceptor (D-B-A) systems has been studied. The results from room temperature and 80 K measurements are presented. The primary objectives have been to investigate whether electrons/electron holes or excitation energy could be transferred from (AuP)-Au-3 to the appended zinc porphyrin (ZnP) in the dimers. As the bridging chromophores in our D-B-A systems separate the ZnP and AuP moieties by 19 A edge-to-edge, we do not expect a significant contribution to either electron or energy transfer from a direct (through space) exchange mechanism. This gives us the opportunity to scrutinize how the bridging chromophores influence the transfer reactions. The results show that quenching of (AuP)-Au-3 occurs with high efficiency in the dimers that are connected by fully conjugated bridging chromophores, whereas no quenching is observed when the conjugation of the bridge is broken. We also observed that the decay of (AuP)-Au-3 is complex at temperatures below 110 K. In addition to the two previously published lifetimes on the order of some 10-100 mus, we have found a third lifetime on the nanosecond time scale.
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| 2. |
- Börjesson, Karl, 1982, et al.
(author)
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A membrane anchored DNA-based energy/electron transfer assembly
- 2008
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In: Nucleic acids symposium series (2004). - 1746-8272. ; :52, s. 691-691
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Journal article (peer-reviewed)abstract
- In this work the trapping and conversion of visible light energy into chemical energy is examined using a supramolecular assembly. This consists of a light absorbing antenna and a porphyrin redox centre both covalently attached to a DNA strand, which in turn is bound to a lipid membrane. The excitation energy is finally trapped as a benzoquinone radical anion that could potentially be used in subsequent chemical reactions.
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| 3. |
- Börjesson, Karl, 1982, et al.
(author)
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Functionalized Nanostructures: Redox-Active Porphyrin Anchors for Supramolecular DNA Assemblies
- 2010
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In: ACS Nano. - : American Chemical Society (ACS). - 1936-086X .- 1936-0851. ; 4:9, s. 5037-5046
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Journal article (peer-reviewed)abstract
- We have synthesized and studied a supramolecular system comprising a 39-mer DNA with porphyrin-modified thymidine nucleosides anchored to the surface of large unilamellar vesicles (liposomes). Liposome porphyrin binding characteristics, such as orientation, strength, homogeneity, and binding site size, was determined, suggesting that the porphyrin is well suited as a photophysical and redox-active lipid anchor, in comparison to the inert cholesterol anchor commonly used today. Furthermore, the binding characteristics and hybridization capabilities were studied as a function of anchor size and number of anchoring points, properties that are of importance for our future plans to use the addressability of these redox-active nodes in larger DNA-based nanoconstructs. Electron transfer from photoexcited porphyrin to a lipophilic benzoquinone residing in the lipid membrane was characterized by steady-state and time-resolved fluorescence and verified by femtosecond transient absorption.
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| 4. |
- Börjesson, Karl, 1982, et al.
(author)
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Membrane-Anchored DNA Assembly for Energy and Electron Transfer
- 2009
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 131:8, s. 2831-2839
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Journal article (peer-reviewed)abstract
- In this work we examine the trapping and conversion of visible light energy into chemical energy using a supramolecular assembly. The assembly consists of a light-absorbing antenna and a porphyrin redox center, which are covalently attached to two complementary 14-mer DNA strands, hybridized to form a double helix and anchored to a lipid membrane. The excitation energy Is finally trapped In the lipid phase of the membrane as a benzoquinone radical anion that could potentially be used In subsequent chemical reactions. In addition, In this model complex, the hydrophobic porphyrin moiety acts as an anchor into the liposome positioning the DNA construct on the lipid membrane surface. The results show the suitability of our system as a prototype for DNA-based light-harvesting devices, In which energy transfer from the aqueous phase to the interior of the lipid membrane Is followed by charge separation. © 2009 American Chemical Society.
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| 5. |
- Eng, Mattias P, 1977, et al.
(author)
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Triplet excitation energy transfer in porphyrin-based donor-bridge-acceptor systems with conjugated bridges of varying length: An experimental and DFT study
- 2006
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In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 110:13, s. 6483-6491
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Journal article (peer-reviewed)abstract
- A series of donor-bridge-acceptor (D-B-A) systems with varying donor-acceptor distances have been studied with respect to their triplet energy transfer properties. The donor and acceptor moieties, zinc(II), and free-base porphyrin, respectively, were separated by 2-5 oligo-p-phenyleneethynylene units (OPE) giving rise to edge-to-edge separations ranging between 12.7 and 33.4 angstrom. The study was performed in 2-MTHF at 150 K and it was established that triplet excitation energy transfer occurs with high efficiency in all of the studied D-B-A systems. The distance dependence was exponential with an attenuation factor, beta, equal to 0.45 +/- 0.015 angstrom(-1). The experimental study was also supported by quantum mechanical DFT and TD-DFT calculations on a series of closely related model systems. A thorough analysis of the OPE-bridge conformational dynamics led to an equation that quantitatively models the distance dependence of the electronic coupling found in the experimental study.
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| 6. |
- Eng, Mattias P, 1977, et al.
(author)
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Triplet photophysics of gold(III) porphyrins
- 2005
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In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 109:9, s. 1776-1784
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Journal article (peer-reviewed)abstract
- Gold porphyrins are often used as electron-accepting chromophores in donor-acceptor complexes for the study of photoinduced electron transfer, and they can also be involved in triplet-triplet energy-transfer interactions with other chromophores. Since the lowest excited singlet state is very short-lived (240 fs), the triplet state is usually the starting point for the transfer reactions, and it is therefore crucial to understand its photophysics. The triplet state of various gold porphyrins has been reported to have a lifetime of around 1.5 ns at room temperature and to have a biexponential decay both in emission and in transient absorption with decay times of around 10 and 100,us at 80 K. In this paper, the triplet photophysics of two gold porphyrins (Au-III 5,15-bis(3,5-di-tert-butylphenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetrame thylporphyrin and Au-III 5,10,15,20-tetra(3,5-di-tert-butylphenyl)porphyrin) are studied by steady-state and time-resolved absorption and emission spectroscopy over a wide temperature range (4-300 K). The study reveals the existence of a dark state with an approximate lifetime of 50 ns, which was not previously observed. This state acts as an intermediate between the short-lived singlet and the triplet state manifold. In addition, we present DFT calculations, in which the core electrons of the central metal were replaced by a pseudopotential to account for the relativistic effects, which suggest that the lowest excited singlet state is an optically forbidden ligand-to-metal charge-transfer (LMCT) state. This LMCT state is an obvious candidate for the experimentally observed dark state, and it is shown to dictate the photophysical properties of gold porphyrins by acting as a gate for triplet state formation versus direct return to the ground state.
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| 7. |
- Ljungdahl, Thomas, 1974, et al.
(author)
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Geometrically homogeneous series of covalently linked zinc/free-base porphyrin dimers of varying length; design, synthesis and characterization
- 2006
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In: European Journal of Organic Chemistry. - : Wiley. - 1099-0690 .- 1434-193X. ; :14, s. 3087-3096
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Journal article (peer-reviewed)abstract
- Singlet excitation energy transfer, SEET, can be mediated by a bridge, connecting an energy donor and acceptor, via a superexchange mechanism. The mediation efficiency depends on the energy difference between the first excited states of the donor and the bridge, DEDB, as well as the donor-acceptor distance, RDA. We have previously constructed a series of donor-bridge-acceptor, D-B-A, systems that allowed us to study how SEET depends on DEDB. To expand this study into a second dimension, the distance dependence, a new series of D-B-A systems were constructed. This series was based on the same zinc/free-base porphyrin couple as the donor-acceptor pair in the previous series. Their relative orientation was also retained. In contrast to our first series, the bridges in the latter were of varying length. The bridges were oligo(phenyleneethynylene)phenylene (OPE) structures and the length was systematically changed by increasing the no. of phenyleneethynylene units from 1 to 4. To obtain high quality samples, the D-B-A systems were assembled by a building block approach where the zinc and free-base porphyrins were introduced sep. using Heck alkynylations. The performance of the OPE structure as a mediator and scaffold is discussed in terms of singlet excited state energies and flexibility. For the first time, when combining the topical D-B-A systems with our previous subset, a homogeneous series of D-B-A systems has been synthesized that allows for studies of both the distance dependence and the energy difference dependence of SEET.
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| 8. |
- Ljungdahl, Thomas, 1974, et al.
(author)
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Solvent and base dependence of copper-free palladium-catalyzed cross-couplings between terminal alkynes and arylic iodides: Development of efficient conditions for the construction of gold(III)/free-base porphyrin dimers
- 2006
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In: Journal of Organic Chemistry. - : American Chemical Society (ACS). - 0022-3263 .- 1520-6904. ; 71:4, s. 1677-1687
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Journal article (peer-reviewed)abstract
- In this paper, our attempts to optimize the Heck alkynylation (copper-free Sonogashira) reaction are presented. An efficient copper-free coupling protocol was needed for the synthesis of gold/zinc porphyrin dimers because previous methods had failed. Previous studies have usually focused on ligands, whereas this work focuses on the choice of solvent and base. The catalytic system throughout the investigation was formed from the stable precursor [Pd-2(dba)(3)center dot CHCl3] together with the ligand triphenylarsine, an easy-to-handle, air-stable ligand. A model study was conducted to examine the dependence of the Heck alkynylation on the solvent and base. The most successful modification proved to be the addition of methanol, as a cosolvent, in combination with a nucleophilic tertiary base. The success of the methanol additive is hypothesized to be caused by the presence of a rate-determining deprotonation step featuring a charge-separated transition state. Finally, the very high yielding and successful synthesis of a series of porphyrin systems using these new conditions is presented. For the first time, gold porphyrin substrates could efficiently be coupled in Heck alkynylation reactions.
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| 9. |
- Ljungdahl, Thomas, 1974, et al.
(author)
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Two competing mechanisms for the copper-free Sonogashira cross-coupling reaction
- 2008
-
In: Organometallics. - : American Chemical Society (ACS). - 0276-7333 .- 1520-6041. ; 27:11, s. 2490-2498
-
Journal article (peer-reviewed)abstract
- The mechanism of the copper-free Sonogashira cross-coupling was investigated using a model reaction with differently para-substituted phenylacetylenes and 4-iodobenzotrifluoride as coupling partners and a Pd 2 (dba) 3 -CHCl 3 -AsPh 3 catalyst system in methanol. A carbopalladation mechanism was ruled out through a series of experiments in which the equivalent of a carbopalladation reaction intermediate was synthesized by an alternate route, and its conversion to product was monitored. A Hammett correlation study revealed a possible mechanistic changeover when going from electron-rich to electron-poor alkynes in the model reaction. It is advocated that the reaction mechanism changes from a pathway involving a fast proton transfer from a slowly forming cationic Pd complex to a pathway involving a slow proton transfer from a neutral Pd complex on going from electron-rich to electron-poor alkynes. The amine base is believed to act as a base in both pathways and as a nucleophile promoting the formation of the cationic complex in the reactions involving electron-rich alkynes. This was substantiated by the observation of a primary isotope effect (K Alkyne-H /K Aikyne-D ≈ 2) for the electron-poor alkyne and a pronounced base dependence for the electron-rich one. © 2008 American Chemical Society.
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| 10. |
- Pettersson, Karin, 1975, et al.
(author)
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Interplay between Barrier Width and Height in Electron Tunneling: Photoinduced Electron Transfer in Porphyrin-Based Donor-Bridge-Acceptor Systems
- 2006
-
In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 110:1, s. 319-326
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Journal article (peer-reviewed)abstract
- The rate of electron tunneling in mol. donor-bridge-acceptor (D-B-A) systems is detd. both by the tunneling barrier width and height, i.e., both by the distance between the donor and acceptor as well as by the energy gap between the donor and bridge moieties. These factors are therefore important to control when designing functional electron transfer systems, such as constructs for photovoltaics, artificial photosynthesis, and mol. scale electronics. The authors have studied a set of D-B-A systems in which the distance and the energy difference between the donor and bridge states (DEDB) are systematically varied. Zinc(II) and gold(III) porphyrins were chosen as electron donor and acceptor because of their suitable driving force for photoinduced electron transfer (-0.9 eV in butyronitrile) and well-characterized photophysics. The authors have previously shown, in accordance with the superexchange mechanism for electron transfer, that the electron transfer rate is proportional to the inverse of DEDB in zinc/gold porphyrin D-B-A systems with bridges of const. edge to edge distance (19.6 .ANG.) and varying DEDB (3900-17,600 cm-1). Here, the authors use the same donor and acceptor but the bridge is shortened or extended giving a set of oligo-p-phenyleneethynylene bridges (OPE) with four different edge to edge distances ranging from 12.7 to 33.4 .ANG.. These two sets of D-B-A systems-ZnP-RB-AuP+ and ZnP-nB-AuP+-have one bridge in common, and hence, for the first time both the distance and DEDB dependence of electron transfer can be studied simultaneously in a systematic way. [on SciFinder (R)]
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