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1.	Aarnio, Harri, et al. (författare) Recombination studies in a polyfluorene copolymer for photovoltaic applications 2005 Ingår i: Synthetic Metals. - : Elsevier BV. - 0379-6779. ; 155:2, s. 299-302 Tidskriftsartikel (refereegranskat)abstract We present detailed continuous wave (cw) and transient photoinduced absorption (PA) measurements in thin films of a novel alternating polyfluorene copolymer, poly[2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4',7'-di-2-thienyl-2',1',3-benzo-thiadiazole)] (DiO-PFDTBT), and its blends with the sol. fullerene deriv. [6,6]-phenyl-C61-butyric acid Me ester (PCBM) in wt. ratios of 1:0, 4:1 and 1:4. We measure the frequency, intensity and temp. dependence of the PA signal in the frequency domain, and compare with the results obtained from the transient PA decay measurements in the time domain. In all blends, the PA spectrum shows a broad high energy PA band ranging from .apprx.1 eV to 2 eV as well as a low energy band peaking at .apprx.0.35 eV. We attribute the low energy band to the P1 transition of polarons and part of the high energy band to the correlated P2 transition of polarons. Both frequency and time domain measurements show that the high energy band has two decay components, a faster component in the microsecond time regime and a slower component in the millisecond time regime. The slow component is strongly dispersive, whereas the fast component is practically non-dispersive. [on SciFinder (R)]
2.	Abramavicius, Vytautas, et al. (författare) Carrier motion in as-spun and annealed P3HT:PCBM blends revealed by ultrafast optical electric field probing and Monte Carlo simulations 2014 Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 16:6, s. 2686-2692 Tidskriftsartikel (refereegranskat)abstract Charge transport dynamics in solar cell devices based on as-spun and annealed P3HT:PCBM films are compared using ultrafast time-resolved optical probing of the electric field by means of field-induced second harmonic generation. The results show that charge carriers drift about twice as far during the first 3 ns after photogeneration in a device where the active layer has been thermally annealed. The carrier dynamics were modelled using Monte-Carlo simulations and good agreement between experimental and simulated drift dynamics was obtained using identical model parameters for both cells, but with different average PCBM and polymer domain sizes. The calculations suggest that small domain sizes in as-spun samples limit the carrier separation distance disabling their escape from geminate recombination.
3.	Andersson, Johan, et al. (författare) New paradigm of transition metal polypyridine complex photochemistry 2004 Ingår i: Faraday Discussions. - : Royal Society of Chemistry (RSC). - 1364-5498. ; 127, s. 295-305 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).
4.	Andersson, Johan, et al. (författare) Ultrafast singlet energy transfer competes with intersystem crossing in a multi-center transition metal polypyridine complex 2004 Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 386:4-6, s. 336-341 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.
5.	Andersson, Mattias, et al. (författare) Unified Study of Recombination in Polymer:Fullerene Solar Cells Using Transient Absorption and Charge-Extraction Measurements 2013 Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society. - 1948-7185. ; 4:12, s. 2069-2072 Tidskriftsartikel (refereegranskat)abstract Recombination in the well-performing bulk heterojunction solar cell blend between the conjugated polymer TQ-1 and the substituted fullerene PCBM has been investigated with pump-probe transient absorption and charge extraction of photo-generated carriers (photo-CELIV). Both methods are shown to generate identical and overlapping data under appropriate experimental conditions. The dominant type of recombination is bimolecular with a rate constant of 7 x 10(-12) cm(-3) s(-1). This recombination rate is shown to be fully consistent with solar cell performance. Deviations from an ideal bimolecular recombination process, in this material system only observable at high pump fluences, are explained with a time-dependent charge-carrier mobility, and the implications of such a behavior for device development are discussed.
6.	Aumanen, Jukka, et al. (författare) Internal Dynamics and Energy Transfer in Dansylated POPAM Dendrimers and Their Eosin Complexes 2010 Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 114:4, s. 1548-1558 Tidskriftsartikel (refereegranskat)abstract Internal dynamics of dansylated poly(propyleneamine) dendrimers (POPAM, G1-G4) in solution and excitation energy transfer from dansyls to eosin in POPAM-eosin complexes have been studied by time-resolved fluorescence spectroscopy and molecular dynamics (MD) simulations. Combining the results from fluorescence anisotropy and the MD simulation studies suggests three time domains for the internal dynamics of the G3 and G4 generations, about 60 ps for motions of the outer-sphere dansyls, 500-1000 ps for restricted motions of back-folded dansyls, and 1500-2600 ps for the overall rotation. For the smaller generations, the contribution from the restricted motions was not entirely evident. Eosin binding hinders fast rotation of the dansyl fragments in the largest G4 dendrimer, but the motion of back-folded dansyls is more hindered in the pure dendrimer. Both fluorescence anisotropy and MD results for the G4 dendrimer support the "soft" dendrimer picture with almost free mobility and substantial back-folding of the dansyls of the dendrimers in solution. Analysis of time-dependent spectral shifts of fluorescence reveals 20-30 ps excited-state solvation relaxation around a single dansyl of a dendrimer. Dendrimer-independent excitation energy transfer from 4 to 8 ps from dansyls to eosins in POPAM-eosin complexes G2-G4 was observed.
7.	Benko, Gabor, et al. (författare) Interligand electron transfer determines triplet excited state electron injection in RuN3-sensitized TiO2 films 2004 Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 108:9, s. 2862-2867 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.
8.	Benko, Gabor, et al. (författare) Nonergodic dye-to-nanocrystalline semiconductor film electron transfer 2002 Ingår i: 7th International Conference on Nanometer-Scale Science and Technology and 21st European Conference on Surface Science. 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
9.	Benko, Gabor, et al. (författare) Nonergodic dye-to-semiconductor electron transfer 2002 Ingår i: Thirteenth International Conference on Ultrafast Phenomena. Technical Digest (TOPS Vol.72). - 1557527032 ; , s. 434-435 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
10.	Benko, Gabor, et al. (författare) Particle size and crystallinity dependent electron injection in Fluorescein 27-sensitized TiO2 films. 2003 Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 107:6, s. 1370-1375 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.
11.	Benko, Gabor, et al. (författare) Photoinduced Electron Injection from Ru(dcbPY)2(NCS)2 to SnO2 and TiO2 Nanocrystalline Films. 2003 Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 125:5, s. 1118-1119 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.
12.	Benko, Gabor, et al. (författare) Photoinduced Ultrafast Dye-to-Semiconductor Electron Injection from Nonthermalized and Thermalized Donor States. 2002 Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 124:3, s. 489-493 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.
13.	Benkö, Gabor, et al. (författare) Particle Size and Crystallinity Dependent Electron Injection in Flourescein 27-Sensitized TiO2 Films 2003 Ingår i: Journal of Physical Chemistry B. - : Americal Chemical Society. ; 107, s. 1370-1375 Tidskriftsartikel (refereegranskat)
14.	Biasin, Elisa, et al. (författare) Femtosecond X-Ray Scattering Study of Ultrafast Photoinduced Structural Dynamics in Solvated[Co(terpy)2]2$ 2016 Ingår i: Physical Review Letters. - : American Physical Society (APS). - 1079-7114 .- 0031-9007. ; 117:1 Tidskriftsartikel (refereegranskat)abstract We study the structural dynamics of photoexcited [Co(terpy)2]2+ in an aqueous solution with ultrafast x-ray diffuse scattering experiments conducted at the Linac Coherent Light Source. Through direct comparisons with density functional theory calculations, our analysis shows that the photoexcitation event leads to elongation of the Co-N bonds, followed by coherent Co-N bond length oscillations arising from the impulsive excitation of a vibrational mode dominated by the symmetrical stretch of all six Co-N bonds. This mode has a period of 0.33 ps and decays on a subpicosecond time scale. We find that the equilibrium bond-elongated structure of the high spin state is established on a single-picosecond time scale and that this state has a lifetime of ∼7 ps.
15.	Billsten, Helena, et al. (författare) Dynamics of energy transfer from lycopene to bacteriochlorophyll in genetically-modified LH2 complexes of Rhodobacter sphaeroides 2002 Ingår i: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 41:12, s. 4127-4136 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.
16.	Billsten, Helena, et al. (författare) Dynamics of vibrational relaxation in the S-1 state of carotenoids having 11 conjugated C=C bonds 2002 Ingår i: Chemical Physics Letters. - 0009-2614. ; 355:5-6, s. 465-470 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.
17.	Billsten, Helena, et al. (författare) Excited-state processes in the carotenoid zeaxanthin after excess energy excitation 2005 Ingår i: The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory. - : American Chemical Society (ACS). - 1520-5215. ; 109:31, s. 6852-6859 Tidskriftsartikel (refereegranskat)abstract Aiming for better understanding of the large complexity of excited-state processes in carotenoids, we have studied the excitation wavelength dependence of the relaxation dynamics in the carotenoid zeaxanthin. Excitation into the lowest vibrational band of the S-2 state at 485 nm, into the 0-3 vibrational band of the S2 state at 400 nm, and into the B-2(u)+ state at 266 nm resulted in different relaxation patterns. While excitation at 485 nm produces the known four-state scheme (S-2 -> hot S-1 -> S-1 -> S-0), excess energy excitation led to additional dynamics occurring with a time constant of 2.8 ps (400 nm excitation) and 4.9 ps (266 nm excitation), respectively. This process is ascribed to a conformational relaxation of conformers generated by the excess energy excitation. The zeaxanthin S* state was observed regardless of the excitation wavelength, but its population increased after 400 and 266 nm excitation, suggesting that conformers generated by the excess energy excitation are important for directing the population toward the S* state. The S-2-S-1 internal conversion time was shortened from 135 to 70 fs when going from 485 to 400 nm excitation, as a result of competition between the S-2-S-1 internal conversion from the vibrationally hot S2 state and S2 vibrational relaxation. The S, lifetime of zeaxanthin was within experimental error the same for all excitation wavelengths, yielding similar to 9 ps. No long-lived species have been observed after excitation by femtosecond pulses regardless of the excitation wavelength, but excitation by nanosecond pulses at 266 nm generated both zeaxanthin triplet state and cation radical.
18.	Billsten, Helena, et al. (författare) Self-assembled aggregates of the carotenoid zeaxanthin: time-resolved study of excited states 2005 Ingår i: The Journal of Physical Chemistry Part A: Molecules, Spectroscopy, Kinetics, Environment and General Theory. - : American Chemical Society (ACS). - 1520-5215. ; 109:8, s. 1521-1529 Tidskriftsartikel (refereegranskat)abstract In this study, we present a way of controlling the formation of the two types of zeaxanthin aggregates in hydrated ethanol: J-zeaxanthin (head-to-tail aggregate, characteristic absorption band at 530 nm) and H-zeaxanthin (card-pack aggregate, characteristic absorption band at 400 run). To control whether J- or H-zeaxanthin is formed, three parameters are important: (1) pH, that is, the ability to form a hydrogen bond; (2) the initial concentration of zeaxanthin, that is, the distance between zeaxanthin molecules; and (3) the ratio of ethanol/water. To create H-aggregates, the ability to form hydrogen bonds is crucial, while J-aggregates are preferentially formed when hydrogen-bond formation is prevented. Further, the formation of J-aggregates requires a high initial zeaxanthin concentration and a high ethanol/water ratio, while H-aggregates are formed under the opposite conditions. Time-resolved experiments revealed that excitation of the 530-nm band of J-zeaxanthin produces a different relaxation pattern than excitation at 485 and 400 run, showing that the 530-nm band is not a vibrational band of the S-2 state but a separate excited state formed by J-type aggregation. The excited-state dynamics of zeaxanthin aggregates are affected by annihilation that occurs in both J- and H-aggregates. In H-aggregates, the dominant annihilation component is on the subpicosecond time scale, while the main annihilation component for the J-a-gregate is 5 ps. The S-1 lifetimes of aggregates are longer than in solution, yielding 20 and 30 ps for H- and J-zeaxanthin, respectively. In addition, H-type aggregation promotes a new relaxation channel that forms the zeaxanthin triplet state.
19.	Biswas, Abhijit, et al. (författare) Control of the size and shape of TiO2 nanoparticles in restricted media 2013 Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 24:19 Tidskriftsartikel (refereegranskat)abstract Template-capped TiO2 nanostructures have been synthesized. In certain template conditions, TiO2 hexagons are found to form. These hexagonal structures can be effectively sensitized by fluorescein dye without any change in the protonation state of the dye. Bare TiO2 nanoparticles are not so useful for sensitization with dyes like fluorescein as they alter the dye protonation state. The novelty of this work is twofold-the hitherto elusive hexagonal phase of TiO2 nanoparticles has been stabilized and the synthesis of TiO2 in the rutile phase has been achieved under mild conditions.
20.	Biswas, Sudipta, et al. (författare) Facile synthesis of luminescent TiO2 nanorods using an anionic surfactant: Their photosensitization and photocatalytic efficiency 2014 Ingår i: Materials Chemistry and Physics. - : Elsevier BV. - 0254-0584. ; 147:3, s. 761-771 Tidskriftsartikel (refereegranskat)abstract Mixed phase (rutile/anatase)TiO2 nanorods have been synthesized using an anionic surfactant template. Nanorod synthesis has been achieved using mild reaction conditions and without using any rod-shaped template. The shape and crystallinity of the TiO2 nanomaterials can be modulated by careful control of the surfactant concentration. The novelty of the present work is that the anatase/rutile mixed phase nanorods were formed without using the well-known layer by layer assembly method or the hydrothermal method. These mixed phase TiO2 nanorods are highly luminescent, can be easily sensitized by fluorescent dyes, show significant dye adsorption ability and function as efficient photocatalysts. (C) 2014 Elsevier B.V. All rights reserved.
21.	Bressler, C., et al. (författare) Solvation dynamics monitored by combined X-ray spectroscopies and scattering: photoinduced spin transition in aqueous [Fe(bpy)(3)](2+) 2014 Ingår i: Faraday Discussions. - : Royal Society of Chemistry (RSC). - 1364-5498. ; 171, s. 169-178 Tidskriftsartikel (refereegranskat)abstract We have studied the photoinduced low spin (LS) to high spin (HS) conversion of aqueous Fe(bpy)(3) with pulse-limited time resolution. In a combined setup permitting simultaneous X-ray diffuse scattering (XDS) and spectroscopic measurements at a MHz repetition rate we have unraveled the interplay between intramolecular dynamics and the intermolecular caging solvent response with 100 ps time resolution. On this time scale the ultrafast spin transition including intramolecular geometric structure changes as well as the concomitant bulk solvent heating process due to energy dissipation from the excited HS molecule are long completed. The heating is nevertheless observed to further increase due to the excess energy between HS and LS states released on a subnanosecond time scale. The analysis of the spectroscopic data allows precise determination of the excited population which efficiently reduces the number of free parameters in the XDS analysis, and both combined permit extraction of information about the structural dynamics of the first solvation shell.
22.	Canton, S. E., et al. (författare) Watching the dynamics of electrons and atoms at work in solar energy conversion 2015 Ingår i: Faraday discussions. - : Royal Society of Chemistry. - 1359-6640 .- 1364-5498. ; 185, s. 51-68 Tidskriftsartikel (refereegranskat)abstract The photochemical reactions performed by transition metal complexes have been proposed as viable routes towards solar energy conversion and storage into other forms that can be conveniently used in our everyday applications. In order to develop efficient materials, it is necessary to identify, characterize and optimize the elementary steps of the entire process on the atomic scale. To this end, we have studied the photoinduced electronic and structural dynamics in two heterobimetallic ruthenium-cobalt dyads, which belong to the large family of donor-bridge-acceptor systems. Using a combination of ultrafast optical and X-ray absorption spectroscopies, we can clock the light-driven electron transfer processes with element and spin sensitivity. In addition, the changes in local structure around the two metal centers are monitored. These experiments show that the nature of the connecting bridge is decisive for controlling the forward and the backward electron transfer rates, a result supported by quantum chemistry calculations. More generally, this work illustrates how ultrafast optical and X-ray
23.	Canton, Sophie, et al. (författare) Probing the Anisotropic Distortion of Photoexcited Spin Crossover Complexes with Picosecond X-ray Absorption Spectroscopy 2014 Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 118:8, s. 4536-4545 Tidskriftsartikel (refereegranskat)abstract For numerous spin crossover complexes, the anisotropic distortion of the first coordination shell around the transition metal center governs the dynamics of the high-spin/low-spin interconversion. However, this structural parameter remains elusive for samples that cannot be investigated with crystallography. The present work demonstrates how picosecond X-ray absorption spectroscopy is able to capture this specific deformation in the photoinduced high-spin state of solvated [Fe(terpy)(2)](2+), a complex which belongs to the prominent family of spin crossover building blocks with nonequivalent metal-ligand bonds. The correlated changes in Fe-N-Axial, Fe-N-Distal, and bite angle N-Distal-Fe-N-Axial extracted from the measurements are in very good agreement with those predicted by DFT calculations in D-2d symmetry. The outlined methodology is generally applicable to the characterization of ultrafast nuclear rearrangements around metal centers in photoactive molecular complexes and nanomaterials, including those that do not display long-range order.
24.	Canton, Sophie, et al. (författare) Toward Highlighting the Ultrafast Electron Transfer Dynamics at the Optically Dark Sites of Photocatalysts 2013 Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 4:11, s. 1972-1976 Tidskriftsartikel (refereegranskat)abstract Building a detailed understanding of the structure function relationship is a crucial step in the optimization of molecular photocatalysts employed in water splitting schemes. The optically dark nature of their active sites usually prevents a complete mapping of the photoinduced dynamics. In this work, transient X-ray absorption spectroscopy highlights the electronic and geometric changes that affect such a center in a bimetallic model complex. Upon selective excitation of the ruthenium chromophore, the cobalt moiety is reduced through intramolecular electron transfer and undergoes a spin flip accompanied by an average bond elongation of 0.20 +/- 0.03 angstrom. The analysis is supported by simulations based on density functional theory structures (B3LYP*/TZVP) and FEFF 9.0 multiple scattering calculations. More generally, these results exemplify the large potential of the technique for tracking elusive intermediates that impart unique functionalities in photochemical devices.
25.	Canton, Sophie, et al. (författare) Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses. 2015 Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 6 Tidskriftsartikel (refereegranskat)abstract Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor-acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances. Experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined.
26.	Chábera, Pavel, et al. (författare) A low-spin Fe(iii) complex with 100-ps ligand-to-metal charge transfer photoluminescence 2017 Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 543:7647, s. 695-699 Tidskriftsartikel (refereegranskat)abstract Transition-metal complexes are used as photosensitizers1, in light-emitting diodes, for biosensing and in photocatalysis2. A key feature in these applications is excitation from the ground state to a charge-transfer state3,4; the long charge-transfer-state lifetimes typical for complexes of ruthenium5 and other precious metals are often essential to ensure high performance. There is much interest in replacing these scarce elements with Earth-abundant metals, with iron6 and copper7 being particularly attractive owing to their low cost and non-toxicity. But despite the exploration of innovative molecular designs6,8,9,10, it remains a formidable scientific challenge11 to access Earth-abundant transition-metal complexes with long-lived charge-transfer excited states. No known iron complexes are considered12 photoluminescent at room temperature, and their rapid excited-state deactivation precludes their use as photosensitizers13,14,15. Here we present the iron complex [Fe(btz)3]3+ (where btz is 3,3′-dimethyl-1,1′-bis(p-tolyl)-4,4′-bis(1,2,3-triazol-5-ylidene)), and show that the superior σ-donor and π-acceptor electron properties of the ligand stabilize the excited state sufficiently to realize a long charge-transfer lifetime of 100 picoseconds (ps) and room-temperature photoluminescence. This species is a low-spin Fe(iii) d5 complex, and emission occurs from a long-lived doublet ligand-to-metal charge-transfer (2LMCT) state that is rarely seen for transition-metal complexes4,16,17. The absence of intersystem crossing, which often gives rise to large excited-state energy losses in transition-metal complexes, enables the observation of spin-allowed emission directly to the ground state and could be exploited as an increased driving force in photochemical reactions on surfaces. These findings suggest that appropriate design strategies can deliver new iron-based materials for use as light emitters and photosensitizers.
27.	Chábera, Pavel, et al. (författare) Band-selective dynamics in charge-transfer excited iron carbene complexes 2019 Ingår i: Faraday Discussions. - : Royal Society of Chemistry (RSC). - 1359-6640 .- 1364-5498. ; 216:2019, s. 191-210 Tidskriftsartikel (refereegranskat)abstract Ultrafast dynamics of photoinduced charge transfer processes in light-harvesting systems based on Earth-abundant transition metal complexes are of current interest for the development of molecular devices for solar energy conversion applications. A combination of ultrafast spectroscopy and first principles quantum chemical calculations of a recently synthesized iron carbene complex is used to elucidate the ultrafast excited state evolution processes in these systems with particular emphasis on investigating the underlying reasons why these complexes show promise in terms of significantly extended lifetimes of charge transfer excited states. Together, our results challenge the traditional excited state landscape for iron-based light harvesting transition metal complexes through radically different ground and excited state properties in alternative oxidation states. This includes intriguing indications of rich band-selective excited state dynamics on ultrafast timescales that are interpreted in terms of excitation energy dependence for excitations into a manifold of charge-transfer states. Some implications of the observed excited state properties and photoinduced dynamics for the utilization of iron carbene complexes for solar energy conversion applications are finally discussed.
28.	Chábera, Pavel, et al. (författare) FeII Hexa N-Heterocyclic Carbene Complex with a 528 ps Metal-To-Ligand Charge-Transfer Excited-State Lifetime 2018 Ingår i: Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 9:3, s. 459-463 Tidskriftsartikel (refereegranskat)abstract The iron carbene complex [FeII(btz)3](PF6)2 (where btz = 3,3′-dimethyl-1,1′-bis(p-Tolyl)-4,4′-bis(1,2,3-Triazol-5-ylidene)) has been synthesized, isolated, and characterized as a low-spin ferrous complex. It exhibits strong metal-To-ligand charge transfer (MLCT) absorption bands throughout the visible spectrum, and excitation of these bands gives rise to a 3MLCT state with a 528 ps excited-state lifetime in CH3CN solution that is more than one order of magnitude longer compared with the MLCT lifetime of any previously reported FeII complex. The low potential of the [Fe(btz)3]3+/[Fe(btz)3]2+ redox couple makes the 3MLCT state of [FeII(btz)3]2+ a potent photoreductant that can be generated by light absorption throughout the visible spectrum. Taken together with our recent results on the [FeIII(btz)3]3+ form of this complex, these results show that the FeII and FeIII oxidation states of the same Fe(btz)3 complex feature long-lived MLCT and LMCT states, respectively, demonstrating the versatility of iron N-heterocyclic carbene complexes as promising light-harvesters for a broad range of oxidizing and reducing conditions.
29.	Chábera, Pavel, et al. (författare) Photofunctionality of iron(III) N-heterocyclic carbenes and related d5 transition metal complexes 2021 Ingår i: Coordination Chemistry Reviews. - : Elsevier BV. - 0010-8545. ; 426 Forskningsöversikt (refereegranskat)abstract Despite a few reports of photoluminescent and strongly photo-oxidizing transition metal complexes with a d5 electronic configuration, the photophysics and photochemistry of this class of transition metal complexes have largely remained unexplored. Recent investigations of earth-abundant iron(III) N-heterocyclic carbene (NHC) complexes have demonstrated promising photophysical and photochemical properties associated with low-spin (doublet) ligand-to-metal charge transfer (2LMCT) excitations, including nanosecond photoluminescence (PL) and capabilities to drive both photo-oxidation and photo-reduction reactions. These encouraging results are at first sight surprising in light of the general scarcity of known photofunctional complexes of any transition metal complexes with a d5 electronic configuration, including 1st, 2nd and 3rd row transition metal complexes of Mn(II), Tc(II), Re(II), Fe(III), Ru(III) and Os(III). Here, we review the photophysical and photochemical properties of the new Fe(III) NHC complexes together with related d5 transition metal complexes as a basis for a broader understanding of the unorthodox photophysical and photochemical properties associated with this open-shell electronic configuration. This includes considerations of the role of charge and spin effects on the ground state electronic structure, as well as discussions of charge transfer (CT) and metal centered (MC) excited state properties. The special properties of 2LMCT excited states are emphasized as a key feature to understand the photophysics of many photofunctional d5 transition metal complexes. Further aspects of excited state dynamics with d5 light-harvesting complexes, including both intra- and inter-molecular charge transfer processes, are also discussed. Finally, some fundamental challenges and emerging opportunities for further development of photofunctional Fe(III) NHC and related LMCT/d5 complexes for light-harvesting, light-emitting, and photo(electro)chemical applications are outlined. This includes some general considerations of how the specific photochemical properties of the LMCT/d5 complexes provides an exciting opportunity to develop a unique niche within the diversity of photofunctional molecular systems alongside other types of organic and inorganic chromophores commonly used in the field of molecular photochemistry.
30.	Chekalin, S, et al. (författare) Femtosecond pump–probe investigation of primary photoinduced processes in C60–Sn nanostructures 2003 Ingår i: Synthetic Metals. - 0379-6779. ; 139:3, s. 799-802 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.
31.	Chekalin, SV, et al. (författare) Femtosecond spectroscopy of primary photoexcitation relaxation in Sri- and Ti-doped C60 films 2002 Ingår i: Izvestiya Akadem II Nauk Seriya Fizicheskaya. - 0367-6765. ; 66:3, s. 305-308 Tidskriftsartikel (refereegranskat)
32.	Corani, Alice, et al. (författare) Bottom-Up Approach to Eumelanin Photoprotection: Emission Dynamics in Parallel Sets of Water-Soluble 5,6-Dihydroxyindole-Based Model Systems. 2012 Ingår i: The Journal of Physical Chemistry Part B. - : American Chemical Society (ACS). - 1520-5207 .- 1520-6106. ; 116:44, s. 13151-13158 Tidskriftsartikel (refereegranskat)abstract The molecular mechanisms by which the black eumelanin biopolymers exert their photoprotective action on human skin and eyes are still poorly understood, owing to critical insolubility and structural heterogeneity issues hindering direct investigation of excitation and emission behavior. Recently, we set up strategies to obtain water-soluble 5,6-dihydroxyindole (DHI)-based polymers as useful models for disentangling intrinsic photophysical properties of eumelanin components from aggregation and scattering effects. Herein, we report the absorption properties and ultrafast emission dynamics of two separate sets of DHI-based monomer-dimer-polymer systems which were made water-soluble by means of poly(vinyl alcohol) or by galactosyl-thio substitution. Data showed that dimerization and polymerization of DHI result in long-lived excited states with profoundly altered properties relative to the monomer and that glycosylation of DHI imparts monomer-like behavior to oligomers and polymers, due to steric effects hindering planar conformations and efficient interunit electron communication. The potential of S-glycation as an effective tool to probe and control emission characteristics of eumelanin-like polymers is disclosed.
33.	Corani, Alice, et al. (författare) Excited-State Proton-Transfer Processes of DHICA Resolved: From Sub-Picoseconds to Nanoseconds 2013 Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 4:9, s. 1383-1388 Tidskriftsartikel (refereegranskat)abstract Excited-state proton transfer has been hypothesized as a mechanism for UV energy dissipation in eumelanin skin pigments. By using time resolved fluorescence spectroscopy, we show that the previously proposed, but unresolved, excited-state intramolecular proton transfer (ESIPT) of the eumelanin building block 5,6-dihydroxyindole-2-carboxylic acid (DHICA) occurs with a time constant of 300 fs in aqueous solution but completely stops in methanol. The previously disputed excited-state proton transfer involving the 5- or 6-OH groups of the DHICA anion is now found to occur from the 6-OH group to aqueous solvent with a rate constant of 4.0 x 10(8) s(-1).
34.	Corani, Alice, et al. (författare) Superior photoprotective motifs and mechanisms in eumelanins uncovered. 2014 Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 136:33, s. 11626-11635 Tidskriftsartikel (refereegranskat)abstract Human pigmentation is a complex phenomenon commonly believed to serve a photoprotective function through the generation and strategic localization of black insoluble eumelanin biopolymers in sun exposed areas of the body. Despite compelling biomedical relevance to skin cancer and melanoma, eumelanin photoprotection is still an enigma: What makes this pigment so efficient in dissipating the excess energy brought by harmful UV-light as heat? Why has Nature selected 5,6-dihydroxyindole-2-carboxylic acid (DHICA) as the major building block of the pigment instead of the decarboxylated derivative (DHI)? By using pico- and femtosecond fluorescence spectroscopy we demonstrate herein that the excited state deactivation in DHICA oligomers is 3 orders of magnitude faster compared to DHI oligomers. This drastic effect is attributed to their specific structural patterns enabling multiple pathways of intra- and interunit proton transfer. The discovery that DHICA-based scaffolds specifically confer uniquely robust photoprotective properties to natural eumelanins settles a fundamental gap in the biology of human pigmentation and opens the doorway to attractive advances and applications.
35.	Corani, Alice, et al. (författare) Ultrafast Dynamics of Hole Injection and Recombination in Organometal Halide Perovskite Using Nickel Oxide as p-Type Contact Electrode. 2016 Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 7, s. 1096-1101 Tidskriftsartikel (refereegranskat)abstract There is a mounting effort to use nickel oxide (NiO) as p-type selective electrode for organometal halide perovskite-based solar cells. Recently, an overall power conversion efficiency using this hole acceptor has reached 18%. However, ultrafast spectroscopic investigations on the mechanism of charge injection as well as recombination dynamics have yet to be studied and understood. Using time-resolved terahertz spectroscopy, we show that hole transfer is complete on the subpicosecond time scale, driven by the favorable band alignment between the valence bands of perovskite and NiO nanoparticles (NiO(np)). Recombination time between holes injected into NiO(np) and mobile electrons in the perovskite material is shown to be hundreds of picoseconds to a few nanoseconds. Because of the low conductivity of NiO(np), holes are pinned at the interface, and it is electrons that determine the recombination rate. This recombination competes with charge collection and therefore must be minimized. Doping NiO to promote higher mobility of holes is desirable in order to prevent back recombination.
36.	Dahlbom, Mats, et al. (författare) Collective excitation dynamics and polaron formation in molecular aggregates 2002 Ingår i: Chemical Physics Letters. - 0009-2614. ; 364:5-6, s. 556-561 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.
37.	Dahlbom, Mats, et al. (författare) Collective excitation dynamics in molecular aggregates: Exciton relaxation, self-trapping and polaron formation. 2002 Ingår i: Royal Society of Chemistry. Special Publications. - 0260-6291. ; 283:Suppl., s. 118-135 Tidskriftsartikel (refereegranskat)
38.	Danielsson, Jonas, 1974- (författare) Computational chemistry studies of UV induced processes in human skin 2004 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.
39.	De, Swati, et al. (författare) Exciton Dynamics in Alternating Polyfluorene/Fullerene Blends 2008 Ingår i: Journal of Chemical Physics. - College Park, MD, United States : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 350:1-3, s. 14-22 Tidskriftsartikel (refereegranskat)abstract Exciton dynamics in alternating copolymer/fullerene solar cell blends have been investigated using femtosecond transient absorption spectroscopy. The acceptor concentrations have been varied over a wide range. Experimental data, kinetic modeling and simulations, all indicate that the efficiency of exciton conversion to charges is 100% even at acceptor concentrations as low as 20 wt%. The reported dependence of solar cell efficiency on fullerene concentration may thus arise from other factors. However, there exists an acceptor concentration threshold (5 wt%) below which a substantial fraction of the excitations remain unquenched. The results, we believe are very relevant to optimization of performance efficiency by clever manipulation of morphology. We have also observed exciton–exciton energy transfer in these blends at low acceptor concentrations.
40.	De, Swati, et al. (författare) Geminate charge recombination in alternating polyfluorene copolymer/fullerene blends 2007 Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 129:27, s. 8466-8472 Tidskriftsartikel (refereegranskat)abstract By measuring excited state and charge dynamics in blends of an alternating polyfluorene copolymer and fullerene derivative over nine orders in time and two orders in light intensity, we have monitored the light-induced processes from ultrafast charge photogeneration to much slower decay of charges by recombination. We find that at low light intensities relevant to solar cell operation relatively fast (∼30 ns) geminate recombination is the dominating charge decay process, while nongeminate recombination has a negligible contribution. The conclusion of our work is that under solar illumination conditions geminate recombination of charges may be directly competing with efficient charge collection in polymer/fullerene solar cells. © 2007 American Chemical Society.
41.	Dietzek, Benjamin, et al. (författare) Appearance of coherent artifact signals in femtosecond transient absorption spectroscopy in dependence on detector design 2007 Ingår i: Laser Physics Letters. - : IOP Publishing. - 1612-2011 .- 1612-202X. ; 4:1, s. 38-43 Tidskriftsartikel (refereegranskat)abstract The appearance of coherent artifact signals in transient absorption spectroscopy employing a spectrally integrated detection system is studied. The influence of the detection design on the shape of the observed signal is detailed and the experimentally very important case, in which the shape of the coherent artifact is strongly influenced by the presence of the sample itself, is considered - leading to the situation that the artifact signal cannot be accounted for by simple comparison of the kinetics obtained for the solvent only. Finally an estimate of the relative contribution of the artifact to the overall transient absorption change is presented facilitating the interpretation of short time transients in the presence of artifact contributions and allowing to estimate the excited state absorption cross-section for a known pump-intensity dependence of the artifact signal.
42.	Dietzek, Benjamin, et al. (författare) Experimental observation of different-order components of a vibrational wave packet in a bulk dielectric using high-order Raman scattering 2007 Ingår i: Physical Review Letters. - 1079-7114. ; 98:18 Tidskriftsartikel (refereegranskat)abstract We use high-order Raman scattering in a bulk dielectric to characterize coherent dynamics with precision typical for gas phase experiments. The experimental pump-probe approach allows for the simultaneous observation and separation in space and time of the individual contributions of different-order Raman processes to a coherent wave packet without relying on phase-matching conditions and within the same experimental geometry. We propose a novel technique to discriminate between stimulated excitation of vibronic levels in the impulsive and intermediate excitation regimes, futhermore allowing us to distinguish between different pathways contributing to the same fifth-order Raman processes.
43.	Dietzek, Benjamin, et al. (författare) Protochlorophyllide a: A Comprehensive Photophysical Picture 2009 Ingår i: ChemPhysChem. - : Wiley. - 1439-7641 .- 1439-4235. ; 10:1, s. 144-150 Tidskriftsartikel (refereegranskat)abstract The photochemistry of protochlorophyllide a, a precursor in the biosynthesis of chlorophyll and substrate of the light regulated enzyme protochlorophyllide oxidoreductase, is investigated by pump-probe spectroscopy. Upon excitation into the lowest lying Q-band the light induced changes ore recorded over a wide range of probe wavelengths in the visible and near-IR region between 500 and 1000 nm. Following excitation, an initial ultrafast 450 Is process is observed related to the motion out of the Franck-Condon region on the excited state surface; thus directly unraveling previous suggestions based on time-resolved fluorescence measurements (ChemPhysChem 2006, 7, 1727-1733). Furthermore, the data reveals a previously concealed photointermediate, whose formation on a nanosecond timescale matches the overall fluorescence decay and is assigned to a triplet state. The implications of this finding with respect to the photochemistry of NADPH.-protochlorophyllide oxidoreductase (POR) ore discussed.
44.	Dietzek, Benjamin, et al. (författare) The excited-state chemistry of protochlorophyllide a: A time-resolved fluorescence study 2006 Ingår i: ChemPhysChem. - : Wiley. - 1439-7641 .- 1439-4235. ; 7:8, s. 1727-1733 Tidskriftsartikel (refereegranskat)abstract The excited-state processes of protochlorophyllide a, the precursor of chlorophyll a in chlorophyll biosynthesis, are studied using picosecond time-resolved fluorescence spectroscopy. Following excitation into the Soret band, two distinct fluorescence components, with emission maxima at 640 and 647 nm, ore observed. The 640 nm emitting component appears within the time resolution of the experiment and then decays with a time constant of 27 ps. In contrast, the 647 nm emitting component is built up with a 3.5 ps rise time and undergoes a subsequent decay with a time constant of 3.5 ns. The 3.5 ps rise kinetics are attributed to relaxations in the electronically excited state preceding the nanosecond fluorescence, which is ascribed to emission out of the thermally equilibrated S, state. The 27 ps fluorescence, which appears within the experimental response of the streak camera, is suggested to originate from a second minimum on the excited-state potentiol-energy surface. The population of the secondary excited state is suggested to reflect a very fast motion out of the Franck-Condon region along a reaction coordinate different from the one connecting the Franck-Condon region with the S-1 potential-energy minimum. The 27 ps-component is an emissive intermediate on the reactive excited-state pathway, as its decay yields the intermediate photoproduct, which has been identified previously (J. Phys. Chem. B 2006, 110, 4399-4406). No emission of the photoproduct is observed. The results of the time-resolved fluorescence study allow a detailed spectral characterization of the emission of the excited states in protochlorophyllide a, and the refinement of the kinetic model deduced from ultrafast absorption measurements.
45.	El Nahhas, Amal, et al. (författare) Photochemistry of Pheomelanin Building Blocks and Model Chromophores: Excited-State Intra- and Intermolecular Proton Transfer 2014 Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 5:12, s. 2094-2100 Tidskriftsartikel (refereegranskat)abstract Pheomelanins, the epidermal pigments of red-haired people responsible for their enhanced UV susceptibility, contain 1,4-benzothiazines and 1,3-benzothiazole as main structural components. Despite the major role played in pheomelanin phototoxicity, the photoreactivity of these species has so far remained unexplored. Static and time-resolved fluorescence spectroscopy was used to identify excited-state reactions of the two main pheomelanin benzothiazole building blocks, namely, the 6-(2-amino-2-carboxyethyl)-4-hydroxy-1,3-benzothiazole (BT) and the 2-carboxy derivative (BTCA) together with model chromophores lacking some of the ionizable functions. The results show that in aqueous buffer solution the OH at 4-position and the benzothiazole nitrogen atom control the photochemistry of both BT and BTCA via excited-state proton transfer to solvent (ESPT) and excited-state intramolecular proton transfer (ESIPT), while the amino acidic groups of the alanyl chain have a minor influence on the photochemistry. The ESPT and ESIPT produce several different excited-state ionic species with lifetimes ranging from similar to 100 ps to similar to 3 ns.
46.	Ericson, Fredric, et al. (författare) Electronic structure and excited state properties of iron carbene photosensitizers - A combined X-ray absorption and quantum chemical investigation 2017 Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 683, s. 559-566 Tidskriftsartikel (refereegranskat)abstract The electronic structure and excited state properties of a series of iron carbene photosensitizers are elucidated through a combination of X-ray absorption measurements and density functional theory calculations. The X-ray absorption spectra are discussed with regard to the unusual bonding environment in these carbene complexes, highlighting the difference between ferrous and ferric carbene complexes. The valence electronic structure of the core excited FeIII-3d5 complex is predicted by calculating the properties of a CoIII-3d6 carbene complex using the Z+1 approximation. Insight is gained into the potential of sigma-donating ligands as strategy to tune properties for light harvesting applications.
47.	Fredin, Lisa A., et al. (författare) Molecular and Interfacial Calculations of Iron(II) Light Harvesters 2016 Ingår i: ChemSusChem. - : Wiley. - 1864-5631. ; 9:7, s. 667-675 Tidskriftsartikel (refereegranskat)abstract Iron-carbene complexes show considerable promise as earth-abundant light-harvesters, and adsorption onto nanostructured TiO2 is a crucial step for developing solar energy applications. Intrinsic electron injection capabilities of such promising FeII N-heterocyclic complexes (Fe-NHC) to TiO2 are calculated here, and found to correlate well with recent experimental findings of highly efficient interfacial injection. First, we examine the special bonding characteristics of Fe-NHC light harvesters. The excited-state surfaces are examined using density functional theory (DFT) and time-dependent DFT (TD-DFT) to explore relaxed excited-state properties. Finally, by relaxing an Fe-NHC adsorbed on a TiO2 nanocluster, we show favorable injection properties in terms of interfacial energy level alignment and electronic coupling suitable for efficient electron injection of excited electrons from the Fe complex into the TiO2 conduction band on ∼100 fs time scales.
48.	Fredin, Lisa, et al. (författare) Exceptional Excited-State Lifetime of an Iron(II)-N-Heterocyclic Carbene Complex Explained 2014 Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 5:12, s. 2066-2071 Tidskriftsartikel (refereegranskat)abstract Earth-abundant transition-metal complexes are desirable for sensitizers in dye-sensitized solar cells or photocatalysts. Iron is an obvious choice, but the energy level structure of its typical polypyridyl complexes, featuring low-lying metal-centered states, has made such complexes useless as energy converters. Recently, we synthesized a novel iron-N-heterocyclic carbene complex exhibiting a remarkable 100-fold increase of the lifetime compared to previously known iron(II) complexes. Here, we rationalize the measured excited-state dynamics with DFT and TD-DFT calculations. The calculations show that the exceptionally long excited-state lifetime (similar to 9 ps) is achieved for this Fe complex through a significant destabilization of both triplet and quintet metal-centered scavenger states compared to other Feu complexes. In addition, a shallow (MLCT)-M-3 potential energy surface with a low-energy transition path from the (MLCT)-M-3 to (MC)-M-3 and facile crossing from the (MC)-M-3 state to the ground state are identified as key features for the excited-state deactivation.
49.	Fullagar, Wilfred, et al. (författare) A broadband laser plasma X-ray source for application in ultrafast chemical structure dynamics 2007 Ingår i: Review of Scientific Instruments. - : AIP Publishing. - 1089-7623 .- 0034-6748. ; 78:11, s. 1-115105 Tidskriftsartikel (refereegranskat)abstract A plasma source free from characteristic emission lines is described, based on laser irradiation of a water jet in a helium atmosphere. Various key aspects of the laser interaction are presented along with practical characterization of the observed isotropic ~4–10 keV x-ray emissions, measurements of which indicate subpicosecond duration. Observations are consistent with a vacuum heating plasma mechanism at the helium-water interface and indicate strong potential for in-house ultrafast chemical structure dynamics application when coupled to contemporary detector developments.
50.	Fullagar, Wilfred K., et al. (författare) Beating Darwin-Bragg losses in lab-based ultrafast X-ray experiments 2017 Ingår i: Structural Dynamics. - : AIP Publishing. - 2329-7778. ; 4:4 Tidskriftsartikel (refereegranskat)abstract The use of low temperature thermal detectors for avoiding Darwin-Bragg losses in lab-based ultrafast experiments has begun. An outline of the background of this new development is offered, showing the relevant history and initiative taken by this work.

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