| 1. |
- Albinsson, Bo, 1963, et al.
(författare)
-
Electron and energy transfer in donor-acceptor systems with conjugated molecular bridges
- 2007
-
Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 9:44, s. 5847-5864
-
Tidskriftsartikel (refereegranskat)abstract
- Electron and energy transfer reactions in covalently connected donor-bridge-acceptor assemblies are strongly dependent, not only on the donor-acceptor distance, but also on the electronic structure of the bridge. In this article we describe some well characterised systems where the bridges are π-conjugated chromophores, and where, specifically, the interplay between bridge length and energy plays an important role for the donor-acceptor electronic coupling. For any application that relies on the transport of electrons, for example molecule based solar cells or molecular scale electronics, it will be imperative to predict the electron transfer capabilities of different molecular structures. The potential difficulties with making such predictions and the lack of suitable models are also discussed. © the Owner Societies.
|
|
| 2. |
- Kuimova, M. K., et al.
(författare)
-
Determination of the triplet state energies of a series of conjugated porphyrin oligomers
- 2007
-
Ingår i: Photochemical and Photobiological Sciences. - : Springer Science and Business Media LLC. - 1474-9092 .- 1474-905X. ; 6:6, s. 675-682
-
Tidskriftsartikel (refereegranskat)abstract
- We report a systematic study of the photophysical parameters relevant to photodynamic therapy (PDT) by a new type of sensitizers, conjugated porphyrin oligomers. Due to the strong nonlinear properties of oligomers containing 2, 4 and 8 porphyrin units, these molecules are attractive candidates for PDT via multiphoton excitation. The triplet state energy levels for all molecules have been determined by the triplet quenching method, phosphorescence measurements and DFT calculations. We find that the triplet energies of all the oligomers are sufficient to generate singlet oxygen, > 94 kJ mol(-1). However, low singlet oxygen quantum yields are observed for the tetramer and the octamer, as compared to the conjugated dimer and monomeric porphyrin, reflecting the decrease in triplet yield. Thus the conjugated porphyrin dimer is the most promising core structure for PDT applications via multiphoton excitation.
|
|
| 3. |
- Westerlund, Fredrik, 1978, et al.
(författare)
-
Binding geometry and photophysical properties of DNA-threading binuclear ruthenium complexes
- 2007
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 111:1, s. 310-317
-
Tidskriftsartikel (refereegranskat)abstract
- The DNA binding conformation and the photophysical properties of the semiflexible binuclear ruthenium complex [mu-bidppz(phen)(4)Ru-2](4+) (2) were studied with optical spectroscopy and compared to the rigid, planar homologue in syn conformation [mu-dtpf(phen)(4)Ru-2](4+) (3) and the parent "light-switch" complex [Ru(phen)(2)dppz](2+) (1). Comparison of calculated and observed absorption bands of the bridging ligand, bidppz, confirm earlier suggestions that 2 is significantly nonplanar, both free in solution and when intercalated into poly(dAdT)(2), but the conclusion that the intercalated conformation is an anti rotamer is not substantiated by comparison of linear and circular dichroism spectra of 2 and 3. The behavior of the emission quantum yield as a function of temperature is similar for the two binuclear complexes 2 and 3 in different protic solvents, and a quantitative analysis suggests that, in solution, the solvent is more strongly hydrogen bonded to the excited state of 2 than to 1. However, the observation that for 2 the radiative rate constant increases to a value similar to 1 upon intercalation into DNA suggests that the difference between 1 and 2 in accepting hydrogen bonds is less pronounced when intercalated.
|
|
| 4. |
|
|
| 5. |
- Winters, Mikael, 1976, et al.
(författare)
-
Control of electron transfer in a conjugated porphyrin dimer by selective excitation of planar and perpendicular conformers
- 2007
-
Ingår i: Chemistry - A European Journal. - : Wiley. - 1521-3765 .- 0947-6539. ; 13:26, s. 7385-7394
-
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.
|
|
| 6. |
|
|
| 7. |
- Winters, Mikael, 1976
(författare)
-
Influence of Bridge Structures on the Electron Transfer in Donor-Acceptor Systems
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this Thesis, porphyrin-based donor-acceptor systems have been used to study photoinduced electron transfer processes. Principally, two different sets of systems have been investigated. The first is a series in which a ferrocene-fullerene donor-acceptor couple is bridged by conjugated zinc porphyrin oligomers (Fc-Pn-C60, n = 1, 2, 4), and the second is a system containing a zinc porphyrin donor connected by a secondary electron acceptor to a gold(III) porphyrin primary acceptor.Strong inter-porphyrin conjugation in the oligomer series was achieved by using a butadiyne-link. This link allows for a broad distribution of dihedral conformations, which has important effects on the photophysical properties of the oligomers. The ground-state absorption spectrum is in effect an average of all dihedral conformations. The photophysics of the dimer was studied extensively, and it was found that the perpendicular and planar conformations can be addressed selectively. It was furthermore found that the ground-state barrier for rotation is approximately 0.7 kcal/mol (BLYP/6-31G(d)), whereas there is a strong bias towards the planar conformation in the excited state. Conformation is critical for the electronic coupling between the porphyrin macro-cycles and has significant effect on the electron transfer properties of these systems. By using the dimer as electron donor, considerable control over the electron transfer rate can be exercised by selective excitation of planar or perpendicular conformations.The main objective for investigating the series of conjugated porphyrin oligomers was to establish how suitable they are for use in future molecular electronic applications. This was done by photo excitation of the Pn-bridge, which results in the charge separated state Fc+-Pn-C60 by a sequence of electron transfer steps. The subsequent charge recombination was then monitored. It was found that the oligomers mediate charge very efficiently at distances up to 65 Å for n = 4 with remarkably weak distance dependence. The conformational flexibility of these systems has important implications for the mediation of electronic coupling, as this is expected to be a strong function of porphyrin-porphyrin dihedral angle. It was found that the charge recombination rate increased by an order of magnitude when Fc-P2-C60 was constrained to a planar conformation by the addition of a bi dentate ligand. Further, as a part of this study, the triplet energies of the oligomers were determined.In the second set of donor-acceptor systems, two electron transfer processes are viable: a direct, long-range electron transfer, and a sequential electron transfer. High solvent polarity stabilises an intermediary state, where the electron is localised on the anthracene-based bridge unit, which thus serves as a secondary electron acceptor. It is shown in this work that the two mechanisms operate in parallel, and moreover, that the direct electron transfer is faster than the sequential mechanism by approximately a factor of two, at room temperature. As the temperature dependence of the sequential mechanism is stronger than that of the direct mechanism, temperature served as an important means to control the competition. In butyronitrile, the sequential mechanism was essentially switched off relative to the direct electron transfer at low temperatures.
|
|
| 8. |
- Winters, Mikael, 1976, et al.
(författare)
-
Photophysics of a butadiyne-linked porphyrin dimer: Influence of conformational flexibility in the ground and first singlet excited state
- 2007
-
Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 111:19, s. 7192-7199
-
Tidskriftsartikel (refereegranskat)abstract
- The photophysics of a butadiyne-linked porphyrin dimer has been investigated by spectroscopy and quantum mechanical calculations. Primarily, the influence of conformation on the ground and first singlet excited states was studied, and two spectroscopically distinct limiting cases were identified. Experiments show that the twisted and planar conformers are separate spectroscopic species that can be selectively excited and have unique absorption and emission spectra. Calculated ground-state spectra compare well with experimental spectra of the two species. A spectrum of the planar conformer was obtained by the addition of a dipyridyl pyrrole ligand, which forms a 1:1 complex with the dimer and thus forces it to stay planar. The absorption spectrum of the twisted conformer could be deduced from the excitation spectrum of its emission. The interpretation of the ground-state spectrum of the free noncomplexed dimer is that it represents an average of a broad distribution of conformations. Calculations support this conclusion by indicating that the barrier for rotation is relatively small in the ground state (0.7 kcal/mol). Studies of the temperature dependence of the fluorescence spectrum of the dimer indicate a mother-daughter relationship between the twisted and planar conformations in the excited state, where the former has approximately 3.9 kcal/mol higher energy. Furthermore, time-correlated single-photon counting experiments also suggest that the twisted population adopts a planar configuration in the first singlet excited state with a rate constant of k(rot) = 8.8 x 10(9) s(-1) in 2-MTHF at room temperature. The temperature dependence of the fluorescence lifetimes indicated that an activation energy barrier of approximately 2 kcal/mol, in part related to solvent viscosity, is associated with this rate constant.
|
|
| 9. |
- Winters, Mikael, 1976, et al.
(författare)
-
Probing the efficiency of electron transfer through porphyrin-based molecular wires
- 2007
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 129:14, s. 4291-4297
-
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.
|
|
