| 1. |
- Eng, Mattias P, 1977, et al.
(författare)
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Concentration-Dependent Hole Mobility and Recombination Coefficient in Bulk Heterojunctions Determined from Transient Absorption Spectroscopy
- 2010
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Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 1:20, s. 3096-3100
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Tidskriftsartikel (refereegranskat)abstract
- A simple analytical function based on the multiple trapping model, is used to describe the biomolecular recombination of charge carriers in a bulk heterojunction (BHJ) film in the presence of an exponential energetic tail of localized hole "trap" states. The function is used to fit charge carrier decay data from an unannealed P3HT/PCBM film measured by transient absorption. The analysis assumes that only free holes participate in recombination and transport. This implies an effective recombination rate coefficient which varies with the ratio of free to trapped holes. The fit parameters yield a bimolecular recombination constant for free holes with free electrons (k(o) = 3.4 x 10(-12) cm(3) s(-1)) and information about the distribution of trap states (trap distribution parameter beta = 0.29) Assuming the Langevin recombination limit, the analysis yields a concentration dependent effective hole mobility saturating at mu(o) approximate to 7 x 10(-2) cm(2) V-1 s(-1). This approach should be useful to compare BHJs in a consistent and meaningful manner.
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| 2. |
- Kuimova, M. K., et al.
(författare)
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Determination of the triplet state energies of a series of conjugated porphyrin oligomers
- 2007
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Ingår i: Photochemical and Photobiological Sciences. - : Springer Science and Business Media LLC. - 1474-9092 .- 1474-905X. ; 6:6, s. 675-682
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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.
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| 3. |
- Albinsson, Bo, 1963, et al.
(författare)
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Electron and energy transfer in donor-acceptor systems with conjugated molecular bridges
- 2007
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 9:44, s. 5847-5864
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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.
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| 4. |
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| 5. |
- Andreasson, Joakim, 1973, et al.
(författare)
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A dihydroindolizine-porphyrin dyad as molecule-based all-photonic AND and NAND gates
- 2011
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Ingår i: Dyes and Pigments. - : Elsevier BV. - 0143-7208 .- 1873-3743. ; 89:3, s. 284-289
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Tidskriftsartikel (refereegranskat)abstract
- A molecular dyad consisting of a photochromic dihydroindolizine unit covalently linked to a porphyrin performs, when illuminated through a third-harmonic-generating crystal, the functions of both an AND and a NAND Boolean logic gate with shared all-optical inputs. The NAND gate is of particular interest as it is a so-called universal gate, and hence all other digital systems can be implemented by combinations of NAND gates. The functions of the AND and the NAND gates rely on changes in absorption and emission of the dyad in the visible spectral region upon isomerization of the photochromic unit. The change in absorption which forms the basis for the AND gate function is ascribed to the colorization/decolorization of the photochrome itself in response to the optical inputs. The variation in emission intensity which constitutes the NAND gate function is a result of the changes in redox properties of the photochrome that follow upon isomerization, such that only one of the two isomers is competent to quench the porphyrin emission by electron transfer.
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| 6. |
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| 7. |
- Eng, Mattias P, 1977, et al.
(författare)
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Impact of concentration self-quenching on the charge generation yield of fullerene based donor-bridge-acceptor compounds in the solid state
- 2011
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Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 13:9, s. 3721-3729
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Tidskriftsartikel (refereegranskat)abstract
- A fullerene based Donor-Bridge-Acceptor (DBA) compound, incorporating a pi-extended tetrathiafulvalene electron donor, is investigated with respect to its photophysics in solution versus solid state. Solid films of neat DBA are compared with blend films where the DBA compound is diluted in the inert, low dielectric, polymer poly(styrene). It is found that the moderate intermolecular electronic coupling and donor-acceptor separation (22 angstrom) in this case leads to the generation of more dissociated, intermolecular charges than a mixture of the donor and acceptor reference compounds. However, the increased intermolecular interactions in the solid state lead to the excited state of the fullerene suffering from concentration self-quenching. This is found to severely affect the charge generation yield in solid films. The impact of competing intra and intermolecular interactions in the solid state upon the film photophysics is analysed in terms of a kinetic model which includes both the effects of concentration self-quenching and the impact of film composition upon the dielectric stabilisation of charge separated states. We conclude that both concentration self-quenching and dielectric stabilisation are critical in determining the photophysics of the blend films, and discuss strategies based upon our observations to enhance the charge photogeneration properties of organic films and photovoltaic devices based upon DBA compounds.
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| 8. |
- Eng, Mattias P, 1977, et al.
(författare)
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Non-exponential distance dependence of bridge-mediated electronic coupling
- 2006
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Ingår i: Angewandte Chemie - International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 45:34, s. 5626-5629
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Tidskriftsartikel (refereegranskat)abstract
- Donor chromophores attached to oligo-(phenyleneethynylene) molecular bridges of varying length have attenuation factors for electron tunneling β that vary systematically with the donor-bridge (DB) energy gap (see picture). Investigations show that for bridges with ethynylene and vinylene repeating subunits, the electronic coupling increases with increasing donor-acceptor (DA) distance. A simple model accounts for this unexpected observation. (Figure Presented). © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.
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| 9. |
- Eng, Mattias P, 1977, et al.
(författare)
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Temperature dependence of electronic coupling through oligo-p-phenyleneethynylene bridges
- 2008
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Ingår i: Chemistry - A European Journal. - : Wiley. - 1521-3765 .- 0947-6539. ; 14:9, s. 2819-2826
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Tidskriftsartikel (refereegranskat)abstract
- A series of donor-bridge-acceptor (D-B-A) systems with varying donor–acceptor distances has been studied with respect to the temperature dependence of the triplet excitation energy transfer (TEET) rates. The donor and acceptor, zinc(II) and free-base porphyrin, respectively, were separated by oligo-p-phenyleneethynylene (OPE) bridges, where the number of phenyleneethynylene groups was varied between two and five, giving rise to edge-to-edge separations ranging between 12.7 and 33.4 Å. The study was performed in 2-MTHF between room temperature and 80 K. It was found that the distance dependence was exponential, in line with the McConnell model, and the attenuation factor, β, was temperature dependent. The experimentally determined temperature dependence of β was evaluated by using a previously derived model for the conformational dependence of the electronic coupling based on results from extensive quantum chemical, DFT and time-dependent DFT (TD-DFT), calculations. Two regimes in the temperature interval could be identified: one high-temperature, low-viscosity regime, and one low-temperature, high-viscosity regime. In the first regime, the temperature dependence of β was, according to the model, well described by a Boltzmann conformational distribution. In the latter, the molecular motions that govern the electronic coupling are slowed down to the same order of magnitude as the TEET rates. This, in effect, leads to a distortion of the conformational distribution. In the high-temperature regime the model could reproduce the temperature dependence of β, and the extracted rotational barrier between two neighboring phenyl units of the bridge structure, Ei=1.1 kJ mol−1, was in line with previous experimental and theoretical studies. After inclusion of parameters that take the viscosity of the medium into account, successful modeling of the experimentally observed temperature dependence of the distance dependence was achieved over the whole temperature interval.
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| 10. |
- Eng, Mattias P, 1977, et al.
(författare)
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The dependence of the electronic coupling on energy gap and bridge conformation - Towards prediction of the distance dependence of electron transfer reactions
- 2009
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Ingår i: Chemical Physics. - : Elsevier BV. - 0301-0104. ; 357:1-3, s. 132-139
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Tidskriftsartikel (refereegranskat)abstract
- The attenuation factor, β, for the distance dependence of electron exchange reactions is a sensitive function of the donor–bridge energy gap and bridge conformation. In this work the electronic coupling for electron and triplet excitation energy transfer has been investigated for five commonly used repeating bridge structures. The investigated bridge structures are OF (oligo fluorene), OP (oligo phenylene), OPE (oligo p-phenyleneethynylene), OPV (oligo phenylenevinylene), and OTP (oligo thiophene). Firstly, the impact of the donor–bridge energy gap was investigated by performing calculations with a variety of donors appended onto bridges that were kept in a planar conformation. This resulted in, to our knowledge, the first presented sets of bridge specific parameters to be inserted into the commonly used McConnell model. Secondly, since at experimental conditions large conformational flexibility is expected, a previously developed model that takes conformational disorder of the bridge into account has been applied to the investigated systems [M.P. Eng, T. Ljungdahl, J. Mårtensson, B. Albinsson, J. Phys. Chem. B 110 (2006) 6483]. This model is based on Boltzmann averaging and has been shown to describe the temperature dependence of the attenuation factor through OPE-bridges. Together, the parameters describing the donor–bridge energy gap dependence, for planar bridge structures, and the Boltzmann averaging procedure, describing the impact of rotational disorder, have the potential to a priori predict attenuation factors for electron and excitation energy transfer reactions through bridged donor–acceptor systems.
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| 11. |
- Eng, Mattias P, 1977
(författare)
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Triplet Excitation Energy Transfer in Donor-Bridge-Acceptor Systems: Experimental and Theoretical Investigations
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is a contribution to the understanding of bridge mediation of electron exchange processes in Donor-Bridge-Acceptor (D-B-A) systems. The field of research is relevant for the development and optimization of dye-sensitized solar cells, artificial photosynthesis, and the possible future development of molecular electronic devices. First an investigation of porphyrins, a group of molecules commonly used as donors and acceptors in D-B-A systems, is presented. Then an experimental and theoretical investigation of the influence of several system parameters on triplet excitation energy transfer (TEET) ensues. Finally, the work aims for future applications with the characterization of self-assembled monolayers (SAMs) of OPE-structures, commonly used as bridges in D-B-A systems, on gold.The main focus of this thesis is to investigate what governs the bridge mediation of TEET in D-B-A systems. The investigation starts with an experimental study of TEET in a series of D-B-A systems, where a zinc(II) porphyrin acts as the donor and the corresponding free-base porphyrin acts as the acceptor. The donor and acceptor chromophores are separated by oligo-p-phenyleneethynylene bridge units where the number of phenyleneethynylene groups varies between 2 and 5. The experiments are supported by a large number of theoretical calculations, leading to a model for the conformational dependence of the electronic coupling that reproduces the experimentally determined temperature dependence of the bridge mediation.In a parallel, purely theoretical, investigation, the bridge mediation of the electronic coupling for TEET is found to depend on the energy gap between relevant states, EDB, and the donor-acceptor separation, RDA, in accordance with the McConnell model. The applicability of this model to systems with conjugated bridges is discussed and examples of systems where the model does not apply are identified. For these systems an alternative model, based on electron tunneling through a square barrier, first derived by Gamow, is found to qualitatively describe the data. A more elaborate version of this model is found to accurately describe both the dependence of donor-acceptor separation and donor-bridge energy gap on the electronic coupling for experimental data from literature. The advantage of this model is that it will potentially allow for the use of molecular properties of the individual building blocks, available from experiments and/or calculations, to predict the properties of D-B-A systems.
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| 12. |
- Eng, Mattias P, 1977, et al.
(författare)
-
Triplet excitation energy transfer in porphyrin-based donor-bridge-acceptor systems with conjugated bridges of varying length: An experimental and DFT study
- 2006
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 110:13, s. 6483-6491
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Tidskriftsartikel (refereegranskat)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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| 13. |
- Eng, Mattias P, 1977, et al.
(författare)
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Triplet photophysics of gold(III) porphyrins
- 2005
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 109:9, s. 1776-1784
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Tidskriftsartikel (refereegranskat)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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| 14. |
- Fortage, Jerome, et al.
(författare)
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Charge transfer state and large first hyperpolarizability in a highly electronically coupled zinc and gold porphyrin dyad
- 2009
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Ingår i: Chemistry - A European Journal. - 1521-3765 .- 0947-6539. ; 15:36, s. 9058-9067
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Tidskriftsartikel (refereegranskat)abstract
- We report the synthesis and the characterizations of a novel dyad composed of a zinc porphyrin (ZnP) linked to a gold porphyrin (AuP) through an ethynyl spacer. The UV/Vis absorption spectrum and the electrochemical properties clearly reveal that this dyad exhibits a strong electronic coupling in the ground state as evidenced by shifted redox potentials and the appearance of an intense charge-transfer band localized at lambda = 739 nm in dichloromethane. A spectroelectrochemical study of the dyad along with the parent homometallic system (i.e., ZnP-ZnP and AuP-AuP) was undertaken to determine the spectra of the reduced and oxidized porphyrin units. Femtosecond transient absorption spectroscopic analysis showed that the photoexcitation of the heterometallic dyad leads to an ultrafast formation of a charge-separated state (+ZnP-AuP center dot) that displays a particularly long lifetime (tau=4 ns in toluene) for such a short separation distance. The molecular orbitals of the dyad were determined by DFT quantum-chemical calculations. This theoretical study confirms that the observed intense band at lambda = 739 nm corresponds to an interporphyrin charge-transfer transition from the HOMO orbital localized on the zinc porphyrin to LUMO orbitals localized on the gold porphyrin. Finally, a Hyper-Rayleigh scattering study shows that the dyad possesses a large first molecular hyperpolarizability coefficient (beta = 2100 x 10(-30) esu at lambda = 1064 nm), thus highlighting the valuable nonlinear optical properties of this new type of push-pull porphyrin system.
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| 15. |
- Johansson, Johan, 1980, et al.
(författare)
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Bridging Ligand Length Controls AT Selectivity and Enantioselectivity of Binuclear Ruthenium Threading Intercalators
- 2013
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Ingår i: Chemistry-a European Journal. - : Wiley. - 0947-6539 .- 1521-3765. ; 19:20, s. 6246-6256
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Tidskriftsartikel (refereegranskat)abstract
- The slow dissociation of DNA threading intercalators makes them interesting as model compounds in the search for new DNA targeting drugs, as there appears to be a correlation between slow dissociation and biological activity. Thus, it would be of great value to understand the mechanisms controlling threading intercalation, and for this purpose we have investigated how the length of the bridging ligand of binuclear ruthenium threading intercalators affects their DNA binding properties. We have synthesised a new binuclear ruthenium threading intercalator with slower dissociation kinetics from ct-DNA than has ever been observed for any ruthenium complex with any type of DNA, a property that we attribute to the increased distance between the ruthenium centres of the new complex. By comparison with previously studied ruthenium complexes, we further conclude that elongation of the bridging ligand reduces the sensitivity of the threading interaction to DNA flexibility, resulting in a decreased AT selectivity for the new complex. We also find that the length of the bridging ligand affects the enantioselectivity with increasing preference for the enantiomer as the bridging ligand becomes longer.
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| 16. |
- Nilsson, Daniel, et al.
(författare)
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Characterization of Self-Assembled Monolayers of Oligo(phenyleneethynylene) Derivatives of Varying Shapes on Gold : Effect of Laterally Extended ∏-Systems
- 2007
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Ingår i: Langmuir. - : American Chemical Society. - 0743-7463 .- 1520-5827. ; 23:11, s. 6170-6181
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Tidskriftsartikel (refereegranskat)abstract
- Fully conjugated organic molecules, such as the oligo(phenyleneethynylene) (OPE) systems, are of growing interest within the field of molecular electronics, as is the self-assembly of well-defined molecular thin films with predefined functions. The structure and function of such films are intimately related and governed by the structures of their molecular constituents, through the intermolecular interactions and the interactions between the molecules and the substrate, onto which the film is assembled. Here we report on the synthesis of a series of three OPE derivatives, with the general structure phenylethynylene−aryl−ethynylenephenylene−headgroup, and the structural investigation of the self-assembled monolayers (SAMs) formed from them on Au(111) surfaces. The SAMs were characterized by infrared reflection−absorption spectroscopy, spectroscopic ellipsometry, high-resolution X-ray photoemission spectroscopy, and near-edge X-ray absorption fine structure spectroscopy. The effective thickness of the SAMs was observed to decrease as the π-system of the aryl moiety of the OPE adsorbate was extended perpendicular to its molecular long axis. Changing the aryl moiety from benzene to naphthalene to anthracene resulted in lower molecular surface densities and larger molecular inclination. The average tilt angles for the benzene, naphthalene, and anthracene SAMs were found to be about 30°, 40°, and 42° from the surface normal, respectively. For the largest adsorbate, the anthracene derivative, there is spectroscopic evidence suggesting the existence of nonequivalent binding sites. The differences observed between the SAMs are rationalized in terms of the shape of the adsorbates and the strength of the π−π interactions between them
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| 17. |
- Shoaee, Safa, et al.
(författare)
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Influence of nanoscale phase separation on geminate versus bimolecular recombination in P3HT:fullerene blend films
- 2010
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Ingår i: Energy and Environmental Sciences. - 1754-5692 .- 1754-5706. ; 7:3, s. 971-976
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we compare the charge recombination dynamics observed in films comprising poly(3-hexylthiophene) blended with three fullerene derivatives: PCBM and two alternative pyrazolinofullerenes. Transient absorption data indicate that replacement of PCBM with either of the pyrazolinofullerene derivatives results in a transition from bimolecular to monomolecular (geminate) recombination dynamics. We show that this transition cannot be explained by a difference in interfacial energetics. However, this transition does correlate with nanomorphology data which indicate that both pyrazolinofullerenes yield a much finer phase segregation, with correspondingly smaller domain sizes, than observed with PCBM. Our results therefore provide clear evidence of the role of nanomorphology in determining the nature of recombination dynamics in such donor/acceptor blends.
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| 18. |
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| 19. |
- Westerlund, Fredrik, 1978, et al.
(författare)
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Binding geometry and photophysical properties of DNA-threading binuclear ruthenium complexes
- 2007
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 111:1, s. 310-317
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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.
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| 20. |
- Westerlund, Fredrik, 1978, et al.
(författare)
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Enantioselective luminescence quenching of DNA light-switch [Ru(phen) 2dppz]2+ by electron transfer to structural homologue [Ru(phendione)2dppz]2
- 2005
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 109:36, s. 17327-17332
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Tidskriftsartikel (refereegranskat)abstract
- The quenching of the luminescence of [Ru(phen)2dppz] 2+ by structural homologue [Ru(phendione)2dppz] 2+, when both complexes are bound to DNA, has been studied for all four combinations of Δ and Λ enantiomers. Flow linear dichroism spectroscopy (LD) indicates similar binding geometries for all the four compounds, with the dppz ligand fully intercalated between the DNA base pairs. A difference in the LD spectrum observed for the lowest-energy MLCT transition suggests that a transition, potentially related to the final localization of the excited electron to the dppz ligand in [Ru(phen)2dppz]2+, is overlaid by an orthogonally polarized transition in [Ru(phendione) 2dppz]2+. This would be consistent with a low-lying LUMO of the phendione moiety of [Ru(phendione)2dppz]2+ that can accept the excited electron from [Ru(phen)2dppz]2+, thereby quenching the emission of the latter. The lifetime of excited A-[Ru(phen)2dppz]2+ is decreased moderately, from 664 to 427 ns, when bound simultaneously with the phendione complex to DNA. The 108 ns lifetime of opposite enantiomer, A-[Ru(phen)2dppz]2+, is only shortened to 94 ns. These results are consistent with an average rate constant for electron transfer of approximately 1·106 s -1 between the phenanthroline- and phendione-ruthenium complexes. At binding ratios close to saturation of DNA, the total emission of the two enantiomers is lowered equally much, but for the A enantiomer, this is not paralleled by a decrease in luminescence lifetime. A binding isotherm simulation based on a generalized McGhee - von Hippel approach shows that the Δ enantiomer binds approximately 3 times stronger to DNA both for [Ru(phendione)2dppz]2+ and [Ru(phen)2dppz] 2+. This explains the similar decrease in total emission, without the parallel decrease in lifetime for the A enantiomer. The simulation also does not indicate any significant binding cooperativity, in contrast to the case when Δ-[Rh(phi)2bipy]3+ is used as quencher. The very slow electron transfer from [Ru-(phen)2dppz]2+ to [Ru(phendione)2dppz]2+, compared to the case when [Rh(phi)2phen]3+ is the acceptor, can be explained by a much smaller driving free-energy difference.
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| 21. |
- Winters, Mikael, 1976, et al.
(författare)
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Photophysics of a butadiyne-linked porphyrin dimer: Influence of conformational flexibility in the ground and first singlet excited state
- 2007
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 111:19, s. 7192-7199
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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.
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