| 1. |
- Andersson, Johan, et al.
(författare)
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New paradigm of transition metal polypyridine complex photochemistry
- 2004
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Ingår i: Faraday Discussions. - : Royal Society of Chemistry (RSC). - 1364-5498. ; 127, s. 295-305
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Tidskriftsartikel (refereegranskat)abstract
- Using polarization sensitive ultrafast transient absorption spectroscopy we have studied energy transfer and excited state relaxation pathways in a tetranuclear transition metal [( osmium)( ruthenium)(3)] polypyridine complex. Contrary to the generally accepted picture of transition metal complex photochemistry we find that ultrafast energy transfer ( less than or equal to 60 fs) occurs from the excited singlet MLCT state of the peripheral Ru-chromophores to the central Os-core, in efficient competition with intersystem crossing. Energy transfer between relaxed triplet MLCT states is more than an order of magnitude slower ( 600 fs).
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| 2. |
- Andersson, Johan, et al.
(författare)
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Ultrafast singlet energy transfer competes with intersystem crossing in a multi-center transition metal polypyridine complex
- 2004
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 386:4-6, s. 336-341
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal polypyridine complexes are finding widespread applications within many areas of chemistry. For their light-induced processes, the generally accepted picture is that all function emanates from triplet states because the singlet states initially prepared by light absorption are depopulated via intersystem crossing on the 100-fs time scale, before they are significantly involved in chemical reactions. Here we show that this is not always true. With ultrafast spectroscopy applied to a (ruthenium)(3)-osmium complex we show that transition metal polypyridine complexes can be designed where energy transfer between excited singlet states located on different metal centers efficiently competes with intersystem crossing, thus decreasing population of the lower-lying triplet states and concomitant energy loss. (C) 2004 Elsevier B.V. All rights reserved.
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| 3. |
- Benko, Gabor, et al.
(författare)
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Interligand electron transfer determines triplet excited state electron injection in RuN3-sensitized TiO2 films
- 2004
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 108:9, s. 2862-2867
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Tidskriftsartikel (refereegranskat)abstract
- Electron injection from the transition metal complex Ru(dcbpy)(2)(NCS)(2) (dcbpy = 2,2'-bipyridine-4,4'-dicarboxylate) into a titanium dioxide nanoparticle film occurs along two pathways. The dominating part of the electron injection proceeds from the initially excited singlet state of the sensitizer into the conduction band of the semiconductor on the sub-hundred-femtosecond time scale. The slower part of the injection occurs from the thermalized triplet excited state on the picosecond time scale in a nonexponential fashion, as was shown in a previous study (Benko, G.; et al. J. Am. Chem. Soc. 2002, 124, 489). Here we show that the slower channel of injection is the result of the excited state being localized on a ligand of the sensitizer that is not attached to the semiconductor; hence, the electron cannot be injected directly from such an excited state into the semiconductor. Before being injected, it has to be transferred from the non-surface-attached ligand to the attached one. The results show that the interligand electron-transfer time is on the picosecond time scale, depends on the relative energies of the two ligands, and controls the electron injection from the excited triplet state of the sensitizer. The findings provide information relevant to the design of molecular-based assemblies and devices.
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| 4. |
- Benko, Gabor, et al.
(författare)
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Nonergodic dye-to-nanocrystalline semiconductor film electron transfer
- 2002
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Ingår i: 7th International Conference on Nanometer-Scale Science and Technology and 21st European Conference on Surface Science.
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Konferensbidrag (refereegranskat)abstract
- The dominating part of electron injection from the transition metal complex Ru(dcbpy)2(NCS)2 into a titanium dioxide nanocrystalline film proceeds extremely rapidly from the initially populated, vibronically nonthermalized, singlet excited state of the molecule. The results are especially relevant to the understanding and design of molecular-based photovoltaic devices and artificial photosynthetic assemblies
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| 5. |
- Benko, Gabor, et al.
(författare)
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Nonergodic dye-to-semiconductor electron transfer
- 2002
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Ingår i: Thirteenth International Conference on Ultrafast Phenomena. Technical Digest (TOPS Vol.72). - 1557527032 ; , s. 434-435
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Konferensbidrag (refereegranskat)abstract
- The dominating part of electron injection from the transition metal complex Ru(dcbpy)2(NCS)2 into a titanium dioxide nanocrystalline film proceeds extremely rapidly from the initially populated, vibronically nonthermalized, singlet excited state of the molecule. Ultrafast transient absorption measurements with laser pulses of the order of ~30 fs have been performed
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| 6. |
- Benko, Gabor, et al.
(författare)
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Particle size and crystallinity dependent electron injection in Fluorescein 27-sensitized TiO2 films.
- 2003
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 107:6, s. 1370-1375
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Tidskriftsartikel (refereegranskat)abstract
- Influence of processing parameters, such as autoclaving and firing temperature, on the optical properties of nanocrystalline anatase TiO2 film and on the process of electron injection from the dye fluorescein 27 to the as-prepared films is studied. Transmission electron microscopy and steady-state and time-resolved femtosecond spectroscopy measurements indicate that the larger the TiO2 particle and the better its overall crystallinity, the faster the process of electron injection. Unraveling factors that control the properties of the sub-20-nm sized semiconductor particles, and by this the electron injection to them, is important for understanding the process of interfacial electron transfer from the dye to the semiconductor, as well as future optimization of the function of the photoelectrochemical cell based on dye-sensitized TiO2 films.
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| 7. |
- Benko, Gabor, et al.
(författare)
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Photoinduced Electron Injection from Ru(dcbPY)2(NCS)2 to SnO2 and TiO2 Nanocrystalline Films.
- 2003
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 125:5, s. 1118-1119
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Tidskriftsartikel (refereegranskat)abstract
- Photoinduced electron injection from the sensitizer Ru(dcbpy)2(NCS)2 (RuN3) into SnO2 and TiO2 nanocrystalline films occurs by two distinct channels on the femto- and picosecond time scales. The faster electron injection into the conduction band of the different semiconductors originates from the initially excited singlet state of RuN3, and occurs in competition with intersystem crossing. The rate of singlet electron injection is faster to TiO2 (1/55 fs-1) than to SnO2 (1/145 fs-1), in agreement with higher density of conduction band acceptor states in the former semiconductor. As a result of competition between the ultrafast processes, for TiO2 singlet, whereas for SnO2 triplet electron injection is dominant. Electron injection from the triplet state is nonexponential and can be fitted with time constants ranging from ~1 ps (2.5 ps for SnO2) to ~50 ps for both semiconductors. The major part of triplet injection is independent of the semiconductor and is most likely controlled by intramolecular dynamics in RuN3. The overall time scale and the yield of electron injection to the two semiconductors are very similar, suggesting that processes other than electron injection are responsible for the difference in efficiencies of solar cells made of these materials.
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| 8. |
- Benko, Gabor, et al.
(författare)
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Photoinduced Ultrafast Dye-to-Semiconductor Electron Injection from Nonthermalized and Thermalized Donor States.
- 2002
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 124:3, s. 489-493
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Tidskriftsartikel (refereegranskat)abstract
- Electron injection from the transition metal complex Ru(dcbpy)(2)(NCS)(2) (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) into a titanium dioxide nanocrystalline film occurs on the femto- and picosecond time scales. Here we show that the dominating part of the electron transfer proceeds extremely rapidly from the initially populated, vibronically nonthermalized, singlet excited state, prior to electronic and nuclear relaxation of the molecule. The results are especially relevant to the understanding and design of molecular-based photovoltaic devices and artificial photosynthetic assemblies.
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| 9. |
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| 10. |
- Billsten, Helena, et al.
(författare)
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Dynamics of energy transfer from lycopene to bacteriochlorophyll in genetically-modified LH2 complexes of Rhodobacter sphaeroides
- 2002
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 41:12, s. 4127-4136
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Tidskriftsartikel (refereegranskat)abstract
- LH2 complexes from Rb. sphaeroides were modified genetically so that lycopene, with I I saturated double bonds, replaced the native carotenoids which contain 10 saturated double bonds. Tuning the S, level of the carotenoid in LH2 in this way affected the dynamics of energy transfer within LH2, which were investigated using both steady-state and time-resolved techniques. The S I energy of lycopene in n-hexane was determined to be similar to12 500 +/- 150 cm(-1), by direct measurement of the S-1-S-2 transient absorption spectrum using a femtosecond IR-probing technique, thus placing an upper limit on the S, energy of lycopene in the LH2 complex. Fluorescence emission and excitation spectra demonstrated that energy can be transferred from lycopene to the bacteriochlorophyll molecules within this LH2 complex. The energy-transfer dynamics within the mutant complex were compared to wild-type LH2 from Rb. sphaeroides containing the carotenoid spheroidene and from Rs. molischian1l7n, in which lycopene is the native carotenoid. The results show that the overall efficiency for Crt --> B850 energy transfer is similar to80% in lyco-LH2 and similar to95% in WT-LH2 of Rb. sphaeroides. The difference in overall Crt --> BChl transfer efficiency of lyco-LH2 and WT-LH2 mainly relates to the low efficiency of the Crt S-1 --> BChl pathway for complexes containing lycopene, which was 20% in lyco-LH2. These results show that in an LH2 complex where the Crt Si energy is sufficiently high to provide efficient spectral overlap with both B800 and B850 Q(y) states, energy transfer via the Crt S, state occurs to both pigments. However, the introduction of lycopene into the Rb. sphaeroides LH2 complex lowers the S-1 level of the carotenoid sufficiently to prevent efficient transfer of energy to the B 800 Q, state, leaving only the Crt S-1 --> B 850 channel, strongly suggesting that Crt S-1 --> BChl energy transfer is controlled by the relative Crt S-1 and BChl Q(y) energies.
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| 11. |
- Billsten, Helena, et al.
(författare)
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Dynamics of vibrational relaxation in the S-1 state of carotenoids having 11 conjugated C=C bonds
- 2002
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Ingår i: Chemical Physics Letters. - 0009-2614. ; 355:5-6, s. 465-470
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Tidskriftsartikel (refereegranskat)abstract
- Transient absorption spectra and kinetics in the 470-650 nm region were recorded for lycopene, P-carotene and zeaxanthin, all carotenoids with 11 conjugated double bonds, in two solvents with different polarity. Analysis of the red wing of the carotenoid SI-S, transition revealed presence of a pronounced shoulder at early delay times. The kinetics recorded at this low-energy shoulder of the SI-S, transition yields an additional decay component of 500-800 fs in addition to the main S, decay. This dynamics is ascribed to a vibrational relaxation in the S, state of the carotenoids. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 12. |
- Chekalin, S, et al.
(författare)
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Femtosecond pump–probe investigation of primary photoinduced processes in C60–Sn nanostructures
- 2003
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Ingår i: Synthetic Metals. - 0379-6779. ; 139:3, s. 799-802
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Tidskriftsartikel (refereegranskat)abstract
- Investigation of photoinduced processes in different C60–Sn nanostructures excited by 150 fs laser pulses at 400 nm (109 W/cm2) was performed with femtosecond supercontinuum probing of difference absorption and reflection in the spectral range of 400–1600 nm. The dynamics of relaxation are quite different for various nanostructures depending on the deposition mode and the ratio of tin and fullerene content. Non-monotonic relaxation, observed in the samples with Sn nanocrystallites, is explained by electron exchange between C60 and the metal.
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| 13. |
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| 14. |
- Dahlbom, Mats, et al.
(författare)
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Collective excitation dynamics and polaron formation in molecular aggregates
- 2002
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Ingår i: Chemical Physics Letters. - 0009-2614. ; 364:5-6, s. 556-561
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Tidskriftsartikel (refereegranskat)abstract
- Real-space collective excitation dynamics in molecular aggregates is studied using a model where the electronic system is described via exciton theory with surface hopping. The nuclear dynamics are included using the Langevin equation where temperature and zero-point motions are entered via the fluctuation-dissipation theorem. Dynamic processes like exciton relaxation, localization, polaron formation and diffusion of self-trapped excitons, which commonly require different theories, are simultaneously described with our approach. Numerical simulations of small linear aggregates are performed. Contrary to the common view we show that exciton relaxation can temporarily increase exciton delocalization. The results are discussed based on the photosynthetic light-harvesting pigment-protein complexes. (C) 2002 Elsevier Science B.V. All rights reserved.
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| 15. |
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| 16. |
- Danielsson, Jonas, 1974-
(författare)
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Computational chemistry studies of UV induced processes in human skin
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents and uses the techniques of computational chemistry to explore two different processes induced in human skin by ultraviolet light. The first is the transformation of urocanic acid into a immunosuppressing agent, and the other is the enzymatic action of the 8-oxoguanine glycosylase enzyme. The photochemistry of urocanic acid is investigated by time-dependent density functional theory. Vertical absorption spectra of the molecule in different forms and environments is assigned and candidate states for the photochemistry at different wavelengths are identified. Molecular dynamics simulations of urocanic acid in gas phase and aqueous solution reveals considerable flexibility under experimental conditions, particularly for for the cis isomer where competition between intra- and inter-molecular interactions increases flexibility. A model to explain the observed gas phase photochemistry of urocanic acid is developed and it is shown that a reinterpretation in terms of a mixture between isomers significantly enhances the agreement between theory and experiment , and resolves several peculiarities in the spectrum. A model for the photochemistry in the aqueous phase of urocanic acid is then developed, in which two excited states governs the efficiency of photoisomerization. The point of entrance into a conical intersection seam is shown to explain the wavelength dependence of photoisomerization quantum yield. Finally some mechanistic aspects of the DNA repair enzyme 8-oxoguanine glycosylase is investigated with density functional theory. It is found that the critical amino acid of the active site can provide catalytic power in several different manners, and that a recent proposal involving a SN1 type of mechanism seems the most efficient one.
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| 17. |
- Ghanem, Raed, et al.
(författare)
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Light-driven tyrosine radical formation in a ruthenium-tyrosine complex attached to nanoparticle TiO2
- 2002
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 41:24, s. 6258-6266
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a possibility of multistep electron transfer in a supramolecular complex adsorbed on the surface of nanocrystalline TiO2. The complex mimics the function of the tyrosinez and chlorophyll unit P-680 in natural photosystem II (PSII). A ruthenium(II) tris(bipyridyl) complex covalently linked to a L-tyrosine ethyl ester through an amide bond was attached to the surface of nanocrystalline TiO2 via carboxylic acid groups linked to the bpy ligands. Synthesis and characterization of this complex are described. Excitation (450 nm) of the complex promotes an electron to a metal-to-ligand charge-transfer (MLCT) excited state, from which the electron is injected into TiO2. The photogeneration of Ru(III) is followed by an intramolecular electron transfer from tyrosine to Ru(III), regenerating the photosensitizer Ru(II) and forming the tyrosyl radical. The tyrosyl radical is formed in less than 5 us with a yield of 15%. This rather low yield is a result of a fast back electron transfer reaction from the nanocrystalline TiO2 to the photogenerated Ru(III).
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| 18. |
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| 19. |
- Grage, Mette, et al.
(författare)
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Conformational Disorder and Energy Migration in MEH-PPV with Partially Broken Conjugation.
- 2003
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606. ; 118:16, s. 7644-7650
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Tidskriftsartikel (refereegranskat)abstract
- In order to obtain a better understanding of the role of conformational disorder in the photophysics of conjugated polymers the ultrafast transient absorption anisotropy of partially deconjugated MEH-PPV has been measured. These data have been compared to the corresponding kinetics of Monte Carlo–simulated polymer chains, and estimates of the energy hopping time and energy migration distances for the polymers have been obtained. We find that the energy migration in the investigated MEH-PPV is approximately 3 times faster than in previously studied polythiophenes. We attribute this to a more disordered chain conformation in MEH-PPV. ©2003 American Institute of Physics.
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| 20. |
- Grage, Mette, et al.
(författare)
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Ultrafast Excitation Transfer and Trapping in a Thin Polymer Film.
- 2003
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 67:20, s. 1-205207
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Tidskriftsartikel (refereegranskat)abstract
- Transient absorption anisotropy of a polythiophene polymer in a thin film was studied on a femtosecond time scale. The anisotropy has a non-exponential decay on the sub-picosecond time scale, with a fastest component characterized by an ∼40 fs time constant. To simulate the anisotropy decay an incoherent energy migration model has been used. Comparison between the simulated and experimental kinetics enabled us to estimate the nearest-neighbor pair wise hopping time (τh=1±0.1 ps), the fraction of the interchain aggregates (∼10%) and the structural disorder of the polymer. The initial ∼30 fs anisotropy decay does not originate from incoherent hopping energy transfer but from some other relaxation among electronic excited states within a spectroscopic unit.
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| 21. |
- Gulbinas, V, et al.
(författare)
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Charge carrier photogeneration in conjugated polymer
- 2002
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Ingår i: Ultrafast phenomena in semiconductors 2001 / Material Science Forum. - 0255-5476. - 0878498907 - 9780878498901 ; 384-385, s. 279-286
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Konferensbidrag (refereegranskat)abstract
- The electric field-assisted charge carrier photogeneration in a Ladder-Type Methyl substituted Poly(Para-Phenylene) was investigated by studying the electromodulated ultrafast transient differential absorption spectra and kinetics. The field stimulated appearance of the polaron absorption and quenching of the singlet exciton absorption proves that dissociation of excitons into geminately bound electron-hole pairs occurs within the entire exciton lifetime and proceeds from the relaxed state. An influence of the optically created excitons and charge pairs on the Stark-shift of the absorption band was observed and interpreted as caused by the modification of the dielectric constant of the medium. The initial separation distance of the charge carriers and its evolution in time were determined from the Stark-shift dynamics. The initial separation distance was obtained being slightly larger than the distance between adjacent polymer chains.
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| 22. |
- Gulbinas, V, et al.
(författare)
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Dynamics of charge pair generation in ladder-type poly(para-phenylene) at different excitation photon energies
- 2004
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Ingår i: Physical Review B (Condensed Matter and Materials Physics). - 1098-0121. ; 70:3, s. 1-035215
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Tidskriftsartikel (refereegranskat)abstract
- Generation of charge pairs in a ladder-type poly(para-phenylene) was investigated by means of electric field modulated femtosecond pump-probe absorption spectroscopy upon excitation at 3.1 and 4.66 eV. Exciting at 3.1 eV in the optical S-0-->S-1 transition, the number of geminately bound electron-hole pairs rises smoothly within the entire exciton life time. Upon excitation at 4.66 eV an instantaneous generation of geminate pairs superimposed on the more gradual increase is observed. Both processes are field-assisted, albeit differently. The former is attributed to ultrafast dissociation of a hot Franck-Condon state, while the latter is assigned to dissociation of vibrationally relaxed singlet excitons.
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| 23. |
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| 24. |
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| 25. |
- He, Jianjun, et al.
(författare)
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Modified phthalocyanines for efficient near-IR sensitization of nanostructured TiO2 electrode
- 2002
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 124:17, s. 4922-4932
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Tidskriftsartikel (refereegranskat)abstract
- A zinc phthalocyanine with tyrosine substituents (ZnPcTyr), modified for efficient far-red/near-IR performance in dye-sensitized nanostructured TiO2 solar cells, and its reference, glycine-substituted zinc phthalocyanine, (ZnPcGly), were synthesized and characterized. The compounds were studied spectroscopically, electrochemically, and photoelectrochemically. Incorporating tyrosine groups into phthalocyanine makes the dye ethanol-soluble and reduces surface aggregation as a result of steric effects. The performance of a solar cell based on ZnPcTyr is much better than that based on ZnPcGly. Addition of 3alpha,7alpha-dihydroxy-5beta-cholic acid (cheno) and 4-tert-butylpyridine (TBP) to the dye solution when preparing a dye-sensitized TiO2 electrode diminishes significantly the surface aggregation and, therefore, improves the performance of solar cells based on these phthalocyanines. The highest monochromatic incident photo-to-current conversion efficiency (IPCE) of similar to24% at 690 nm and an overall conversion efficiency (eta) of 0.54% were achieved for a cell based on a ZnPcTyr-sensitized TiO2 electrode. Addition of TBP in the electrolyte decreases the IPCE and eta considerably, although it increases the open-circuit photovoltage. Time-resolved transient absorption measurements of interfacial electron-transfer kinetics in a ZnPoTyr-sensitized nanostructured 702 thin film show that electron injection from the excited state of the dye into the conduction band of TiO2 is completed in similar to500 fs and that more than half of the injected electrons recombines with the oxidized dye molecules in similar to300 ps. In addition to surface aggregation, the very fast electron recombination is most likely responsible for the low performance of the solar cell based on ZnPcTyr.
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| 26. |
- He, Zhi, et al.
(författare)
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Influence of the protein binding site on the excited states of bacteriochlorophyll: DFT calculations of B800 in LH2
- 2002
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:44, s. 11606-11612
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Tidskriftsartikel (refereegranskat)abstract
- Effects of hydrogen bonding and the axial ligand interaction on the B800 band in two LH2 complexes Rhodopseudomonas (Rps.) acidophila and Rhodospirillum (Rs.) molischianum have been theo retically investigated by using density functional theory. The local electrostatic environment of the B800 bacteriochlorophyll is simulated as an atomic charge field consisting of the pigments in the protomer unit. Despite the fact that the B800 binding sites in two structures are very different, their absorption spectra are almost identical. Our calculations indicate that the charged axial ligand in Rs. molischianum and the hydrogen bonding in Rps. acidophila lead to similar red shifts, possibly explaining the above controversy. We also found (i) additional B800 bacteriochlorophyll transitions located between the Q and Soret regions for both LH2 complexes and (ii) the ligand to the B800 charge-transfer excited states in the long-wavelength region for the B800-alphaAsp(6) complex in,the Rs. molischianum LH2 system.
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| 27. |
- Herek, Jennifer, et al.
(författare)
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Ultrafast carotenoid band shifts: Experiment and theory
- 2004
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 108:29, s. 10398-10403
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Tidskriftsartikel (refereegranskat)abstract
- The ultrafast carotenoid band shift upon excitation of nearby bacteriochlorophyll molecules was studied in three different light harvesting complexes from purple bacteria. The results were analyzed in terms of changes in local electric field of the carotenoids. Time dependent density functional theory calculations based on known and model structures led to good agreement with experimental results, strongly suggesting that the mutual orientation of the pigment molecules rather than the type of the carotenoid molecules determines the extent of the ultrafast band shift. We further estimate that the protein induced local field nearby carotenoid molecule is about 4 or 6 MV/cm, depending on the orientation of the change of the electrical dipole in the carotenoid upon optical transition.
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| 28. |
- Jespersen, Kim, et al.
(författare)
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The electronic states of polyfluorene copolymers with alternating donor-acceptor units
- 2004
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606. ; 121:24, s. 12613-12617
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Tidskriftsartikel (refereegranskat)abstract
- We calculate the electronic states of the low bandgap polyfluorene-based copolymer DiO-PFDTBT, which consists of alternating 9,9-dioctyl-9H-fluorene and 4,7-di-thiophen-2-ylbenzo[1,2,5]thiadiazole (TBT) units, and compare with the steady-state absorption, emission, and excitation spectrum. Using the semiempirical quantum-chemical (ZINDO) method we can assign the characteristic bands of the "camel-back" absorption spectrum to one charge transfer state at lower energy localized on the TBT unit, and one delocalized excitonic state at higher energy corresponding to the -conjugated electron system. Additional "dark" charge transfer states in the gap between these bands have been revealed. Calculations are also made on the red light emitting polyfluorene-based copolymer poly(fluorene-co-benzothiadiazole) (F8BT), which contains benzo[1,2,5]thiadiazole instead of TBT. The nature of the electronic states in F8BT and DiO-PFDTBT are found to be qualitatively the same. ©2004 American Institute of Physics.
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| 29. |
- Kallioinen, Jani, et al.
(författare)
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Electron transfer from the singlet and triplet excited states of Ru(dcbpy)(2)(NCS)(2) into nanocrystalline TiO2 thin films
- 2002
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:17, s. 4396-4404
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Tidskriftsartikel (refereegranskat)abstract
- Time-resolved absorption spectroscopy was used to study the femtosecond and picosecond time scale electron injection from the excited singlet and triplet states of Ru(dcbpY)(2)(NCS)(2) (RuN3) into titanium dioxide (TiO2) nanocrystalline particle film in acetonitrile. The fastest resolved time constant of similar to30 fs was shown to reflect a sum of two parallel ultrafast processes, nonergodic electron transfer (ET) from the initially excited singlet state of RuN3 to the conduction band of TiO2 and intersystem crossing (ISC). The branching ratio of 1.5 between the two competing processes gives rate constants of 1/50 fs(-1) for ET and 1/75 fs(-1) for ISC. Following the ultrafast processes, a minor part of the electron injection (40%) occurs from the thermalized triplet state of RuN3 on the picosecond time scale. The kinetics of this slower phase of electron injection is nonexponential and can be fitted with time constants ranging from similar to1 to similar to60 ps.
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| 30. |
- Kallioinen, J, et al.
(författare)
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Photoinduced ultrafast dynamics of Ru(dcbpy)(2)(NCS)(2)-sensitized nanocrystalline TiO2 films: The influence of sample preparation and experimental conditions
- 2004
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 108:20, s. 6365-6373
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Tidskriftsartikel (refereegranskat)abstract
- In most of the previous ultrafast electron injection studies of Ru(dcbpy)(2)(NCS)(2)-sensitized nanocrystalline TiO2 films, experimental conditions and sample preparation have been different from study to study and no studies of how the differences affect the observed dynamics have been reported. In the present paper, we have investigated the influence of such modifications. Pump photon density, environment of the sensitized film (solvent and air), and parameters of the film preparation (crystallinity and quality of the film) were varied in a systematic way and the obtained dynamics were compared to that of a well-defined reference sample: Ru(dcbpy)(2)(NCS)(2)-TiO2 in acetonitrile. In some cases, the induced changes in the dynamics were uncorrelated to the electron injection process. High pump photon density (not in the linear response region) and exposure of the sensitized film to air altered the picosecond-time- scale kinetics considerably, and the changes were attributed mostly to degradation of the dye. In other cases, changes in the measured kinetics were related to the electron injection processes: reducing the firing temperature of the nanocrystalline film or making the film via electron beam evaporation (EBE) resulted in a decrease of the overall crystallinity of the film, and the electron injection slowed. In the sensitized EBE films, in addition to an increased contribution of triplet excited-state electron injection, a new electron transfer (ET) process with a time constant of 200 fs was observed.
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| 31. |
- Kuhn, O, et al.
(författare)
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Fluorescence depolarization dynamics in the B850 complex of purple bacteria
- 2002
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Ingår i: Chemical Physics. - 0301-0104. ; 275:1-3, s. 15-30
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Tidskriftsartikel (refereegranskat)abstract
- The fluorescence anisotropic decay is modeled for the B850 bacteriochlorophyll a complex of the purple bacterium Rhodopseudomonas acidophila. Structural information is combined with experimental data to derive a Hamilton operator which models the S-0-S-1 excitation energy transfer between the pigments as well as the energy dissipation into the protein environment. The time-resolved fluorescence signal is determined from the solutions of the equations of motion for the one-exciton density matrix. Nonsecular terms in the Redfield relaxation tensor are shown to have a dramatic influence on the calculated time scales for depolarization.
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| 32. |
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| 33. |
- Liu, G. M., et al.
(författare)
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XPS and UPS characterization of the TiO2/ZnPcGly heterointerface : Aligmment of energy levels
- 2002
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 106:23, s. 5814-5819
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Tidskriftsartikel (refereegranskat)abstract
- The electronic structure and the highest occupied molecular orbitals (HOMO)/the lowest unoccupied molecular orbitals (LUMO) alignment at the molecular semiconductor heterointerface of nanostructured TiO2/ZnPcGly dye sensitizer were characterized by X-ray and ultraviolet photoemission spectroscopy (XPS and UPS). The HOMO level of the dye ZnPcGly was determined to be located at 1.62 eV below the Fermi edge, and the corresponding LUMO level was estimated to be 0.10 eV above the conduction band of TiO2 based on the HOMO/LUMO gap (1.82 eV) of ZnPcGly determined by optical absorption measurements. This energy level matching between the orbitals of the dye and the bands of TiO2 can enable efficient electron transfer from photoexcited ZnPcGly to TiO2, which is very important in photoinduced charge-transfer reactions and for applications in dye-sensitized solar cells.
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| 34. |
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| 35. |
- Pan, Jie, et al.
(författare)
-
Photoinduced electron transfer between a carotenoid and TiO2 nanoparticle
- 2002
-
Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 124:46, s. 13949-13957
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Tidskriftsartikel (refereegranskat)abstract
- The dynamics of photoinduced electron injection and recombination between all-trans-8'-apo-beta-caroten-8'-oic acid (ACOA) and a TiO2 Colloidal nanoparticle have been studied by means of transient absorption spectroscopy. We observed an ultrafast (similar to360 fs) electron injection from the initially excited S-2 state of ACOA into the TiO2 conduction band with a quantum yield of similar to40%. As a result, the ACOA(.+) radical cation was formed, as demonstrated by its intense absorption band centered at 840 nm. Because of the competing S-2-S-1 internal conversion, similar to60% of the S-2-state population relaxes to the S-1 state. Although the S-1 state is thermodynamically favorable to donate electrons to the TiO2, no evidence was found for electron injection from the ACOA S, state, most likely as a result of a complicated electronic nature of the S, state, which decays with a similar to18 ps time constant to the ground state. The charge recombination between the injected electrons and the ACOA(.+) was found to be a highly nonexponential process extending from picoseconds to microseconds. Besides the usual pathway of charge recombination forming the ACOA ground state, about half of the ACOA(.+) recombines via the ACOA triplet state, which was monitored by its absorption band at 530 nm. This second channel of recombination proceeds on the nanosecond time scale, and the formed triplet state decays to the ground state with a lifetime of similar to7.3 mus. By examination of the process of photoinduced electron transfer in a carotenoid-semiconductor system, the results provide an insight into the photophysical properties of carotenoids, as well as evidence that the interfacial electron injection occurs from the initially populated excited state prior to electronic and nuclear relaxation of the carotenoid molecule.
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| 36. |
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| 37. |
- Pan, Jingxi, et al.
(författare)
-
Stepwise charge separation from a ruthenium-tyrosine complex to a nanocrystalline TiO2 film
- 2004
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 108:34, s. 12904-12910
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Tidskriftsartikel (refereegranskat)abstract
- A supramolecular complex composed of Ru(II) tris-bipyridine, tyrosine, and dipicolylamine was synthesized and characterized. This complex was attached to TiO2 nanocrystalline films via ester groups at the Ru(II) chromophore, and photoinduced multistep electron transfer was investigated by laser flash photolysis and electron paramagnetic resonance techniques. Following ultrafast electron injection from the metal-ligand charge transfer excited states of Ru(II) to the conduction band of TiO2, fast intramolecular electron transfer from the tyrosine moiety to the photogenerated Ru(III) was observed, characterized by a rate constant of similar to2 x 10(6) s(-1). By comparison of recovery kinetics at the isosbestic point with that of the reference compound lacking the tyrosine, it was found that the intramolecular electron-transfer efficiency is 90%. A hydrogen-bond-promoted electron-transfer mechanism is proposed.
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| 38. |
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| 39. |
- Papagiannakis, E, et al.
(författare)
-
A Near-Infrared Transient Absorption Study of the Excited-State Dynamics of the Carotenoid Spirilloxanthin in Solution and in the LH1 Complex of Rhodospirillum rubrum.
- 2003
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Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 107:40, s. 11216-11223
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Tidskriftsartikel (refereegranskat)abstract
- The spectroscopic properties of spirilloxanthin in an nhexane solution and bound to the core light-harvesting LH1) complex of Rhodospirillum rubrum were studied by near infrared ultrafast transient absorption spectroscopy. Global analysis of the kinetic traces measured after excitation of spirilloxanthin to the S2 (1Bu +) state enabled us to estimate the species-associated difference spectra that correspond to the excited-state absorption signals originating from the S1 (2Ag-) and S2 states. Analysis of the absorption originating from the S2 state has provided further insight into the characterization of the spirilloxanthin excited states, while by analyzing the profile of the S1-S2 transition, we place the energy of the S1 state of all-trans-spirilloxanthin at 11 500 cm-1, both in solution and in the LH1 complex. This low value excludes excitation energy transfer from the S1 state of spirilloxanthin to bacteriochlorophyll in the LH1 complex of Rs. rubrum and explains the observed low energy transfer efficiency from spirilloxanthin to bacteriochlorophyll in that complex. Our results indicate that the S* state of spirilloxanthin, which was recently found both in solution and in the LH1 complex (Gradinaru, C. C., et al. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 2364), does not exhibit detectable spectral features in the near-infrared region.
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| 40. |
- Polivka, Tomas, et al.
(författare)
-
Carotenoid S(1) state in a recombinant light-harvesting complex of Photosystem II.
- 2002
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Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 41:2, s. 439-450
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Tidskriftsartikel (refereegranskat)abstract
- The carotenoid species lutein, violaxanthin, and zeaxanthin are crucial in the xanthophyll-dependent nonphotochemical quenching occurring in photosynthetic systems of higher plants, since they are involved in dissipation of excess energy and thus protect the photosynthetic machinery from irreversible inhibition. Nonetheless, important properties of the xanthophyll cycle carotenoids, such as the energy of their S(1) electronic states, are difficult to study and were only recently determined in organic solvents [Polívka, T. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 4914. Frank, H. A. (2000) Biochemistry 39, 2831]. In the present study, we have determined the S(1) energies of three carotenoid species, violaxanthin, lutein, and zeaxanthin, in their LHCII (peripheral light-harvesting complex of photosystem II) protein environment by constructing recombinant Lhcb1 (Lhc = light-harvesting complex) proteins containing single carotenoid species. Within experimental error the S(1) energy is the same for all three carotenoids in the monomeric LHCII, 13,900 +/- 300 cm(-1) (720 +/- 15 nm), thus well below the Q(y)() transitions of chlorophylls. In addition, we have found that, although the S(1) lifetimes of violaxanthin, lutein, and zeaxanthin differ substantially in solution, when incorporated into the LHCII protein, their S(1) states have in fact the same lifetime of about 11 ps. Despite the similar spectroscopic properties of the carotenoids bound to the LHCII, we observed a maximal fluorescence quenching when zeaxanthin was present in the LHCII complex. On the basis of these observations, we suggest that, rather than different photochemical properties of individual carotenoid species, changes in the protein conformation induced by binding of carotenoids with distinct molecular structures are involved in the quenching phenomena associated with Lhc proteins.
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| 41. |
- Polívka, Tomáš, et al.
(författare)
-
Direct observation of the S1 level of the carotenoid spheroidene using near-infrared femtosecond spectroscopy
- 2001
-
Ingår i: Ultrafast Phenomena XII. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 9783642625121 - 9783642565465 ; 66, s. 668-670
-
Bokkapitel (refereegranskat)abstract
- In this work, we have determined the energy of the S1 state of the carotenoid spheroidene. The energy of this state is 13,400 ± 90 cm-1 at both 293 K and 186 K, showing that there is no temperature-induced shift of the S1 level. A discrepancy of about 800 cm-1 between the S1 energy determined here and that obtained from previous fluorescence and resonance Raman measurements is explained in terms of the different conformational species co-existing in the S1 excited state.
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| 42. |
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| 43. |
- Polívka, Tornáš, et al.
(författare)
-
Near-infrared time-resolved study of the S-1 state dynamics of the carotenoid spheroidene
- 2001
-
Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 105:5, s. 1072-1080
-
Tidskriftsartikel (refereegranskat)abstract
- Using a novel experimental approach based on near-infrared femtosecond absorption spectroscopy, we have determined the energy of the S-1 state of the carotenoid spheroidene. The energy of this state is 13 400 +/- 90 cm(-1) at both 293 and 186 K, showing that there is no temperature-induced shift of the S-1 level. A discrepancy of about 800 cm(-1) between the S-1 energy determined here and that obtained from previous fluorescence and resonance Raman measurements is explained in terms of the different conformational species coexisting in the S-1 excited state. Measurements of kinetics in the near-infrared region revealed that at least three relaxation processes take place after excitation of spheroidene into its S-2 state. Ultrafast S-2 --> S-1 internal conversion occurs within the first 300 fs, followed by vibrational cooling in the SI state, which occurs on a time scale of similar to 700 fs. The S-1 lifetime is 8 ps at 293 K, in good agreement with previous measurements of the S-1 --> S-N transition. A somewhat longer S-1 lifetime of 9.5 ps is observed at 186 K.
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| 44. |
- Polivka, Tomas, et al.
(författare)
-
The carotenoid S-1 state in LH2 complexes from purple bacteria Rhodobacter sphaeroides and Rhodopseudomonas acidophila: S-1 energies, dynamics, and carotenoid radical formation
- 2002
-
Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 106:42, s. 11016-11025
-
Tidskriftsartikel (refereegranskat)abstract
- Using near-infrared femtosecond absorption spectroscopy, we have determined the S-1 energies of the carotenoids spheroidene and rhodopin glucoside in LH2 complexes of purple bacteria. The S-1 energies in the LH2 complexes yield values of 13400 +/- 100 cm(-1) for spheroidene and 12550 +/- 150 cm(-1) for rhodopin glucoside, which are very close to the S-1 energies obtained for both carotenoids in solution. The 850 cm(-1) difference between the S-1 energies of these two carotenoids significantly affects the energy transfer pathways within the LH2 complexes. The S-1 energy of spheroidene in the LH2 complex of Rhodobacter (Rb.) sphaeroides is high enough to allow efficient energy transfer from the S, state to bacteriochlorophylls, resulting in a substantial shortening of the spheroidene S-1 lifetime in the LH2 complex (1.7 ps) compared with the lifetime in solution (8.5 ps). Rhodopin glucoside, which occurs in Rhodopseudomonas (Rps.) acidophila, has an S-1 energy in the LH2 complex too low for efficient S-1-mediated energy transfer and therefore the S-2 state becomes the main energy donor in LH2 complexes containing this carotenoid. In addition, a distinct carotenoid spectral band not observed in solution, was detected at around 960 nm in the LH2 complex of Rb. sphaeroides. This band is assigned to a spheroidene radical cation, which is formed in similar to200 fs and decays within 8 ps. The yield of the spheroidene radical formation is estimated to be in the range of 5-8%.
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| 45. |
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| 46. |
- Polivka, Tomas, et al.
(författare)
-
Ultrafast formation of a carotenoid radical in LH2 antenna complexes of purple bacteria
- 2004
-
Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 108:39, s. 15398-15407
-
Tidskriftsartikel (refereegranskat)abstract
- The SI-mediated carotenoid to BChl a energy-transfer pathway and carotenoid radical formation have been studied using femtosecond time-resolved transient absorption spectroscopy. A series of LH2 complexes (LH2 = light-harvesting complex 2) from Rhodobacter sphaeroides incorporating carotenoids neurosporene, spheroidene, and spheroidenone were used to explore the effect of conjugation length. The S, lifetimes of carotenoids in the LH2 complex are significantly shorter than in solution, resulting in efficiencies of the S-1-mediated energy transfer of 94%, 82%, and 76%, respectively. In addition to the S-1 decay, a low-amplitude component of similar to9 ps was revealed for all carotenoids in LH2 and assigned to the S* state. Besides energy transfer, excitation of the carotenoid S-2 state in LH2 complexes also leads to the formation of a carotenoid radical, and this pathway is also conjugation length dependent. The carotenoid radical is formed as a result of electron transfer between the carotenoid and B800 BChl a molecules. The yield of the radical formation is highest for neurosporene (10-15%), while a lower efficiency was observed for spheroidene (5-8%). For spheroidenone, no distinct radical signal was found, The precursor of the carotenoid radical is a charge-transfer state that is populated directly from the S-2 state. This charge-transfer state, which represents an excited state of the carotenoid-B800 complex, decays in 300 fs (neurosporene) or 400 fs (spheroidene) to form ground-state carotenoid cation and BChl anion. The charge-separated state has a lifetime of 2 ps (neurosporene) and 8 ps (spheroidene).
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| 47. |
- Rasmusson, M, et al.
(författare)
-
Photodissociation of bromobenzene in solution
- 2003
-
Ingår i: Chemical Physics Letters. - 0009-2614 .- 1873-4448. ; 367:5-6, s. 759-766
-
Tidskriftsartikel (refereegranskat)abstract
- The photodissociation of bromobenzene in solution was investigated with ultrafast transient absorption spectroscopy following excitation at 266 nm. Ab initio calculations of lower singlet and triplet states were performed in order to guide the interpretations. The main feature of the kinetics measured between 300 and 930 nm in acetonitrile is a 9 +/- 1ps decay, which we mainly assign to predissociation. Similar decays were observed in hexane, dichloromethane and tetrachloromethane at 400 and 800 nm. Other features in acetonitrile, such as complicated short-time dynamics between 420 and 620 nm and a long-lived component, might indicate the involvement of lower triplet states.
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| 48. |
- Rasmusson, M, et al.
(författare)
-
Photodissociation of CH2ICH2I, CF2ICF2I, and CF2BrCF2I in solution
- 2002
-
Ingår i: The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory. - : American Chemical Society (ACS). - 1520-5215. ; 106:31, s. 7090-7098
-
Tidskriftsartikel (refereegranskat)abstract
- The photodissociation dynamics of CH2ICH2I, CF2ICF2I, and CF2BrCF2I have been studied in solution after excitation at 266 nm. Formation of I-2 is apparent within tens of picoseconds in solutions of CH2ICH2I and CF2ICF2I in acetonitrile. but not from CF2BrCF2I, More I-2 is formed from CH2ICH2I rather than from CF2-ICF2I, as expected if the 12 formation is a result of secondary dissociation of the haloethyl radicals followed by geminate combination. For CH2ICH2I, the 12 formation is ultrafast (<2 ps) in several different solvents. The quantum yields Of 12 formation (after a few microseconds) were found to be 1.0+/-0.1 for CH2ICH2I and 0.75-0.95 for CF2ICF2I in hexane, dichloromethane, and tetrachloromethane. The data on a nanosecond to microsecond time scale indicate that all haloethyl radicals decay within tens of nanoseconds. We suggest that the decay of haloethyl radicals can occur partially via abstraction.
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| 49. |
- Ruseckas, Arvydas, et al.
(författare)
-
Conformations and Photophysics of a Stilbene Dimer.
- 2003
-
Ingår i: The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory. - : American Chemical Society (ACS). - 1520-5215. ; 107:40, s. 8029-8034
-
Tidskriftsartikel (refereegranskat)abstract
- Photophysical studies of pseudo-p-distyryl[2.2]paracyclophane, a model molecule of a stilbene dimer arranged in a brick-wall geometry, in tetrahydrofurane solution indicate the existence of at least two conformers in the ground state. The conformer A with the smallest optical gap of 3 eV has a short radiative lifetime of 3.3 ns, while the conformer B with an optical gap about 0.3 eV larger is much more abundant in solution and its radiative lifetime is about 10 times longer. Ab initio calculations show that the ground-state energy has a shallow minimum with respect to the torsional angle between the styryl side group and the paracyclophane core around the zero position, with the lowest energy conformation at a torsional angle of -21. On this basis, A and B are assigned to flat and twisted conformations, respectively. Conformer B shows a partial decay of excited-state absorption and fluorescence with a time constant ranging from 5 to 30 ps, depending on excitation photon energy. We attribute this to isomerization of the stilbene moiety, which initially keeps conjugation in the ground state. The isomerization product is long-lived and tends to accumulate in solution.
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| 50. |
- Ruseckas, A., et al.
(författare)
-
Luminescence from inter-chain aggregates in polythiophene films
- 2001
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Ingår i: Synthetic Metals. - 0379-6779 .- 1879-3290. ; 119:1-3, s. 603-604
-
Tidskriftsartikel (refereegranskat)abstract
- We report time-resolved photoluminescence of two polythiophene derivatives with different chain packing. Emission of inter-chain aggregates is distinguished from that of intra-chain singlet excitons. The aggregate luminescence is shifted towards lower energies relative to that of intra-chain exciton by 0.17 eV, and the Huang-Rhys factor of the coupling to the intra-chain C double bondC vibration is S = 1.5. The excitonic coupling in aggregates is of intermediate strength.
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