Villy Sundström

Carotenoid to chlorophyll energy transfer in the peridinin-chlorophyll-a-protein complex involves an intramolecular charge transfer state

Carotenoids are, along with chlorophylls, crucial pigments involved in light-harvesting processes in photosynthetic organisms. Details of carotenoid to chlorophyll energy transfer mechanisms and their dependence on structural variability of carotenoids are as yet poorly understood. Here, we employ femtosecond transient absorption spectroscopy to reveal energy transfer pathways in the peridinin–chlorophyll-a–protein (PCP) complex containing the highly substituted carotenoid peridinin, which includes an intramolecular charge transfer (ICT) state in its excited state manifold. Extending the transient absorption spectra toward near-infrared region (600–1800 nm) allowed us to separate contributions from different low-lying excited states of peridinin. The results demonstrate a special light-harvesting strategy in the PCP complex that uses the ICT state of peridinin to enhance energy transfer efficiency.

Dynamics of vibrational relaxation in the S-1 state of carotenoids having 11 conjugated C=C bonds

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

Conformational Disorder and Energy Migration in MEH-PPV with Partially Broken Conjugation.

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.

Photoinduced Ultrafast Dynamics of Ru(dcbpy)2(NCS)2-Sensitized Nanocrystalline TiO2 Films: The Influence of Sample Preparation and Experimental Conditions

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.

Fluorescence depolarization dynamics in the B850 complex of purple bacteria

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.

Synthesis and properties of an iron hydrogenase active site model linked to a ruthenium tris-bipyridine photosensitizer

This thesis describes the synthesis and properties of ruthenium complexes relevant to artificial photosynthesis. The work includes preparation of RuIIpolypyridine complexes as well as multi component systems where RuII(bpy)3 or RuII(tpy)2 type complexes are used as photosesnsitizers.In the first part, the synthesis and characterisation of bipyridyl(pyridyl)methane type ligands and the corresponding ruthenium(II) bistridentate polypyridyl complexes is described. The bipyridyl-pyridyl methane type ligands were designed to increase the excited state lifetime of ruthenium(II) bisterpyridine-type complexes by altering the ligand field as compared to normal terpyridine ligands.In the second part photoinduced electron transfer and formation of charge separated states in donor-photosensitizer dyads or donor-photosensitizer-acceptor triads is studied. The first covalently linked donor-photosensitizer-acceptor triad with tyrosine as electron donor was prepared, and long lived light induced charge separation was observed. RuIIterpyridine complexes linked to carotenoid or tyrosine were also prepared, for studies of light induced charge separation on a TiO2 surface. Tryptofan was covalently linked to Ru(bpy)3 and proton coupled electron transfer from tryptophan to photogenerated ruthenium(III) was demonstrated. A pH-dependent study of the electron transfer rate gave insight into the mechanism of proton coupled electron transfer in amino acids.Finally, the last part of the thesis presents the preparation and properties of the first complex containing a photosensitizer covalently linked to a Fe-hydrogenase active site model.

In vitro self-assembly of the light harvesting pigment-protein LH2 revealed by ultrafast spectroscopy and electron microscopy

Controlled ensemble formation of protein-surfactant systems provides a fundamental concept for the realization of nanoscale devices with self-organizing capability. In this context, spectroscopic monitoring of pigment-containing proteins yields detailed structural information. Here we have studied the association behavior of the bacterial light-harvesting protein LH2 from Rhodobacter spheroides in an n,n-dimethyldodecylamine-n-oxide/water environment. Time-resolved studies of the excitation annihilation yielded information about aggregate sizes and packing of the protein complexes therein. The results are compared to transmission electron microscopy images of instantaneously frozen samples. Our data indicate the manifestation of different phases, which are discussed with respect to the thermodynamic equilibrium in ternary protein-surfactant-water systems. Accordingly, by varying the concentration the formation of different types of aggregates can be controlled. Conditions for the appearance of isolated LH2 complexes are defined.

Photodissociation of bromobenzene in solution

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±1 ps 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.

Ruthenium phthalocyanines with axial carboxylate ligands. Synthesis and function in solar cells based on nanocrystalline TiO2

The synthesis and characterization of phthalocyaninato-ruthenium (PcRu) complexes with potential functional axial ligands are described. The solubility of these PcRu complexes was much improved compared to their parent phthalocyanines without Ru, enabling purification by normal flash column chromatography and also NMR measurements in common solvents (e. g. DMSO-d(6) and CDCl3). Adsorption of these phthalocyanine dyes onto the surface of a semiconductor through the carboxyl group(s) in the axial ligands prevents to some extent formation of H-aggregates, which is typical for phthalocyanines. It also prevents stacking of the dye molecules on the surface. The photovoltaic behavior of sandwich solar cells based on nanostructured TiO2 films sensitized by these PcRu complexes was studied under irradiation with visible light. For a solar cell based on bis(4-carboxypyridine)-phthalocyaninato ruthenium(II) (1) sensitized nanoporous-nanocrystalline TiO2, a monochromatic incident photon-to-current conversion efficiency (IPCE) of 21% was obtained at 640 nm. The overall conversion efficiency (eta) was 0.61%, which is one of the best results for a solar cell based on a phthalocyanine dye. For a cell based on (4-carboxypyridine)-(4-(2-ethoxy)ethyloxycarbonylpyridine)-2,3,9,10,16,1 7,23,24-octa(n-pentyloxy)-phthalocyaninato ruthenium(II) (5) sensitized TiO2, a IPCE of 6.6% at 640 nm and eta of 0.58% were obtained. (Less)

Collective excitation dynamics and polaron formation in molecular aggregates

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.