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| 2. |
- Winters, Mikael, 1976, et al.
(författare)
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Probing the efficiency of electron transfer through porphyrin-based molecular wires
- 2007
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 129:14, s. 4291-4297
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Tidskriftsartikel (refereegranskat)abstract
- Electron transfer over long distances is important for many future applications in molecular electronics and solar energy harvesting. In these contexts, it is of great interest to find molecular systems that are able to efficiently mediate electrons in a controlled manner over nanometer distances, that is, structures that function as molecular wires. Here we investigate a series of butadiyne-linked porphyrin oligomers with ferrocene and fullerene (C-60) terminals separated by one, two, or four porphyrin units (P-n, n = 1, 2, or 4). When the porphyrin oligomer bridges are photoexcited, long-range charge separated states are formed through a series of electron-transfer steps and the rates of photoinduced charge separation and charge recombination in these systems were elucidated using time-resolved absorption and emission measurements. The rates of long-range charge recombination, through these conjugated porphyrin oligomers, are remarkably fast (k(CR2) = 15 - 1.3 x 10(8) s(-1)) and exhibit very weak distance dependence, particularly comparing the systems with n = 2 and n = 4. The observation that the porphyrin tetramer mediates fast long-range charge transfer, over 65 angstrom, is significant for the application of these structures as molecular wires.
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| 3. |
- Diosa, J.E. 0, et al.
(författare)
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Dielectric relaxation in NH4HSO4 above room temperature
- 2006
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Ingår i: SOLID STATE IONICS. - : Elsevier BV. - 0167-2738. ; 177:13-14, s. 1107-1110
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Tidskriftsartikel (refereegranskat)abstract
- The dispersion curves of the dielectric response of NH4HSO4 show that the corrected imaginary part of permittivity, epsilon", and its real part epsilon' versus frequency reveal a dielectric relaxation around 9.1 x 10(5) Hz at 31 degrees C, which shifts to higher frequencies (similar to 10(6) Hz) as the temperatures increases. The relaxation frequency shows an activated relaxation process over the temperature range 31-83 degrees C with activation energy E-a = 0.14 eV, which is close to that derived from the dc conductivity. We suggest that the observed dielectric relaxation could be produced by the H+ jump and SO4- reorientation that cause distortion and change the local lattice polarizability inducing dipoles like HSO4-.
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| 4. |
- Diosa, J.E. 0, et al.
(författare)
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Dielectric relaxation studies in Cs2SO4
- 2006
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Ingår i: FERROELECTRICS. - 0015-0193. ; 333, s. 253-258
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Tidskriftsartikel (refereegranskat)abstract
- Dielectric measurements of Cs2SO4 show a distinct relaxation at low frequencies at several isotherms ( T < 651 degrees C). For example, the relaxation frequency is around 5.1 kHz at 376 degrees C and increases to approximately 1 MHz at 651 degrees C. The relaxation has an activation energy of 0.97 eV, which is in close agreement whit that of Cs+ transport. We suggest that the observed dielectric relaxation could be produced by the Cs+ jump and SO4- reorientation that cause distortion and change the local lattice polarizability inducing dipoles like CsSO4-.
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| 5. |
- Diosa, J.E. 0, et al.
(författare)
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Impedance spectroscopy studies of K2SO4 in the intermediate temperature regime
- 2005
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Ingår i: Solid State Communications. - : Elsevier BV. - 0038-1098. ; 136:11-12, s. 601-604
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Tidskriftsartikel (refereegranskat)abstract
- Impedance spectroscopy measurements of K2SO4 have been carried out from 5 Hz to 13 MHz over the temperature range 403-782 T. The plots of corrected imaginary part of permittivity, epsilon", and its real part epsilon' vs. frequency clearly show a new dielectric relaxation around epsilon") = 1.4 X 10(4) Hz at 403 degrees C, which shifts to higher frequencies (similar to 1 MHz) as the temperatures increases. The fma (calculated from the peak position of epsilon") vs. reciprocal T shows an activated relaxation process over the temperature range 403-600 T with activation energy E-a = 1.02 eV, which is close to that derived from the dc-conductivity, E-a = 1.15 eV. We suggest that the observed dielectric relaxation could be produced by the K+ jump and SO4- reorientation that cause distortion and change the local lattice polarizability inducing dipoles like KSO4-.
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| 6. |
- Diosa, J.E. 0, et al.
(författare)
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Impedance spectroscopy studies of KHSO4 above room temperature
- 2005
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Ingår i: Solid State Ionics. - : Elsevier BV. - 0167-2738. ; 176:39-40, s. 2913-2916
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Tidskriftsartikel (refereegranskat)abstract
- The dielectric permittivity of polycrystalline KESO4 was measured from 5 Hz to 13 MHz over the temperature range 51-187 degrees C. The corrected imaginary part of permittivity, epsilon", and its real part epsilon' vs. frequency clearly show a new dielectric relaxation around f(max) = 1.5* 10(5) Hz at 51 degrees C which shifts to higher frequencies (similar to 1 MHz) as the temperatures increases. If the measurements are done when increasing the temperature, f(max) decreases sharply (more than two orders of magnitude) around 164 degrees C; however, above this transition temperature, it continues shifting to higher frequencies as the temperature increases. However, if the measurements are done when decreasing, the temperature, f(max) increases abruptly around 146 degrees C, thus showing a hysteretic behaviour. We suggest that the observed dielectric relaxation could be produced by proton jumps and SO4- reorientation, causing distortion and changing the local lattice polarizability to induce dipoles like HSO4-. (c) 2005 Elsevier B.V. All rights reserved.
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| 7. |
- Sandin, Peter, 1977, et al.
(författare)
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Fluorescent Properties of DNA Base Analogue tC upon Incorporation into DNA - Negligible Influence of Neighboring Bases on Fluorescence Quantum Yield
- 2005
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Ingår i: Nucleic Acids Research. - : Oxford University Press (OUP). - 0305-1048 .- 1362-4962. ; 33:16, s. 5019-5025
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Tidskriftsartikel (refereegranskat)abstract
- The quantum yield of the fluorescent tricyclic cytosine analogue, 1,3-diaza-2-oxophenothiazine, tC, is high and virtually unaffected by incorporation into both single- and double-stranded DNA irrespective of neighbouring bases (0.17-0.24 and 0.16-0.21, respectively) and the corresponding fluorescence decay curves are all mono-exponential, properties that are unmatched by any base analogue so far. The fluorescence lifetimes increase when going from tC free in solution (3.2 ns) to single- and double-stranded DNA (on average 5.7 and 6.3 ns, respectively). The mono-exponential decays further support previous NMR results where it was found that tC has a well-defined position and geometry within the DNA helix. Furthermore, we find that the oxidation potential of tC is 0.4 V lower than for deoxyguanosine, the natural base with the lowest oxidation potential. This suggests that tC may be of interest in charge transfer studies in DNA as an electron hole acceptor. We also present a novel synthetic route to the phosphoramidite form of tC. The results presented here together with previous work show that tC is a very good C-analogue that induces minimal perturbation to the native structure of DNA. This makes tC unique as a fluorescent base analogue and is thus highly interesting in a range of applications for studying e.g. structure, dynamics and kinetics in nucleic acid systems.
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| 8. |
- Tumpane, John, 1983, et al.
(författare)
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Addressable high-information-density DNA nanostructures
- 2007
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 440:1-3, s. 125-129
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Tidskriftsartikel (refereegranskat)abstract
- We present a strategy for self-assembly of the smallest yet reported DNA nanostructures that are also addressable in terms of their DNA-base code. Using linear as well as novel branched three-way DNA., oligonucleotide building-blocks we demonstrate the formation of a nano-network's fundamental cell, a DNA pseudo-hexagon of side 4 nm. The network's inherent addressability will allow functionalization with sub-nanometer precision yielding unprecedented richness in information density, important in the context of Moore's Law and nano-chip technology. (c) 2007 Published by Elsevier B.V.
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| 9. |
- Winters, Mikael, 1976, et al.
(författare)
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Control of electron transfer in a conjugated porphyrin dimer by selective excitation of planar and perpendicular conformers
- 2007
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Ingår i: Chemistry - A European Journal. - : Wiley. - 1521-3765 .- 0947-6539. ; 13:26, s. 7385-7394
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Tidskriftsartikel (refereegranskat)abstract
- A donor-acceptor system is presented in which the electron-transfer rates can be sensitively controlled by means of excitation wavelength and temperature. The electron donor is a butadiyne-linked zinc porphyrin dimer that is connected to a C-60 electron acceptor. The broad distribution of conformations allowed by the butadiyne linker makes it possible to selectively excite perpendicular or planar donor conformers and thereby prepare separate initial states with driving forces for electron transfer that differ by almost 0.2 eV This, as well as significant differences in electronic coupling, leads to distinctly different rate constants for electron transfer, which in consequence can be controlled by changing excitation wavelength. By extending the system with a secondary donor (ferrocene), a second, long-range charge-separated state can be formed. This system has been used to test the influence of conformational heterogeneity on electron transfer mediated by the porphyrin dimer in the ground state. It was found that if the dimer is forced to a planar conformation by means of a bidentate ligand, the charge recombination rate increased by an order of magnitude relative to the unconstrained system. This illustrates how control of conformation of a molecular wire can affect its behaviour.
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