Keitaro Yoshihara

Transient luminescence studies of electron injection in dye sensitised nanocrystalline TiO2 films

Abstract We employ fluorescence upconversion spectroscopy to monitor the dynamics of electron injection in tetracarboxyphenyl zinc porphyin (ZnTCPP) sensitised nanocrystalline TiO 2 films. A good agreement is found between these measurements and previous studies of electron injection employing transient absorption spectroscopy. In both case, nonexponential kinetics are observed, with electron injection occurring on timescales ranging from

ENERGY TRANSFER IN A PORPHYRIN CHELATE ASSEMBLY

Abstract Energy transfer among isoenergetic pigments, three oxinyl-substituted free-base porphyrins in an octahedral chelate Ga(III) complex, has been studied by the femtosecond fluorescence up-conversion technique. The anisotropy decay of the complex has a fast component of ∼10 ps which is assigned to the energy transfer between the nearest porphyrin units in the complex. The anisotropy decay of the monomer (oxinyl-substituted porphyrin) is much slower and reflects monomer rotational dynamics in solution with decay times of 50 and ∼200 ps in dichloromethane.

Primary processes in plant photosynthesis: photosystem I reaction center

The photosystem I (PSI) pigment-protein complex of plants converts light energy into a transmembrane charge separation, which ultimately leads to the reduction of carbon dioxide. Recent studies on the dynamics of primary energy transfer, charge separation, and following electron transfer of the reaction center (RC) of the PSI prepared from spinach are reviewed. The main results of femtosecond transient absorption and fluorescence spectroscopies as applied to the P700-enchied PSI RC are summarized. This specially prepared material contains only 12–14 chlorophylls per P700, which is a special pair of chlorophyll a and has a significant role in primary charge separation. The P700-enriched particles are useful to study dynamics of cofactors, since about 100 light-harvesting chlorophylls are associated with wild PSI RC and prevent one from observing the elementary steps of the charge separation. In PSI RC energy and electron transfer were found to be strongly coupled and an ultrafast up-hill energy equilibration and charge separation were observed upon preferential excitation of P700. The secondary electron-transfer dynamics from the reduced primary electron acceptor chlorophyll a to quinone are described. With creating free energy differences (ΔG0) for the reaction by reconstituting various artificial quinones and quinoids, the rate of electron transfer was measured. Analysis of rates versus ΔG0 according to the quantum theory of electron transfer gave the reorganization energy, electronic coupling energy and other factors. It was shown that the natural quinones are optimized in the photosynthetic protein complexes. The above results were compared with those of photosynthetic purple bacteria, of which the structure and functions have been studied most.

Excitation Wavelength Dependence of the Excitation Transfer in Photosystem I Reaction Center With Reduced Number of Antenna Chlorophylls

The most recent X-ray analysis of the crystal structure of Photosystem I (PS I) has shown six chlorophyll a molecules (Chl) in the electron transfer system [1]. There seem to be two pathways of the electron transfer: P700→A→A0 and P700→A’→A0’. It is not yet clarified which pathway is used nor how A and/or A’ are involved in the electron transfer. This situation is analogous to those of purple bacterial reaction centers (RC). One unique feature of PS I RC is that there are two “connecting” Chls which structurally link core antenna Chls to the electron transfer system [1]. Main excitation transfer pathway might be like: core antenna→ “connecting” Chl→A0→A→P700 and core antenna→ “connecting” Chl’→A0’→A’→P700. It is thus important to characterize the spectral properties and the excitation transfer processes associated with the “connecting” Chls, A, A’, A0 and A0’. This is, however, an extremely difficult task, because there are ≈ 100 core antenna Chls which mask the excitation transfer steps.

Relaxation processes of photoexcited carriers in GaAs/AlAs multiple quantum well structures grown by molecular beam epitaxy at low temperatures

The relaxation processes of photoexcited carriers in GaAs/AlAs multiple quantum well structures grown at low temperatures by molecular beam epitaxy were studied by a tunable single-beam femtosecond pump–probe method. Concentrations of singularly ionized antisite arsenic ions, AsGa+, in the quantum wells, which were considered as traps of photoexcited carriers, were estimated from flux conditions and substrate temperatures in the growth. Transient transmittivity of the structures were measured by varying the pump–probe photon energy. The trapping rate of photoexcited carriers, which corresponded to the reciprocal of the carrier lifetime, was derived from the relaxation profile at the pump–probe photon energy close to the exciton resonant excitation energy for each structure. The trapping rate was found to increase linearly with AsGa+ in a lower concentration range and superlinearly in a higher concentration range. Photoluminescence and absorption spectra were observed at room temperature and their correlat...

Ultrafast excited state dynamics of polybenzonitrile investigated by using femtosecond time-resolved fluorescence up-conversion

Abstract The photoluminescence dynamics of a new polymer, (–CN–) conjugated polybenzonitrile, was investigated with a femtosecond time-resolved fluorescence up-conversion technique. The experimental results revealed a bi-exponential relaxation dynamics for excited state and it was explained with the photo-generated excitons’ migration model.

Ultrafast charge separation in the plant photosystem I reaction center

The primary charge separation in photosystem I reaction center of plant at room temperature is found to occur intrinsically on a subpicosecond time scale, which is faster than that of purple bacterial reaction center.

Chapter 99 - Femtosecond time-resolved studies on spectral sensitization of AgBr nanocrystals

Spectral sensitization and supersensitization of silver bromide nanocrystals are studied by the femtosecond fluorescence up-conversion technique. Fluorescence from J-aggregates of cyanine dyes adsorbed on AgBr nanocrystals (40-900 nm) with different shapes (cubic or octahedral) is measured. Fast non-exponential fluorescence decays are observed with a fast component ranging from 400 fs to 2.5 ps depending on the type and size of the crystals. The rates of electron injection from J-aggregate dye to the conduction band of AgBr at various conditions are determined. The increase of the electron injection rate with the increase of the grain size is observed and explained by the space charge theory. Upon addition of a super-sensitizer (SS), which is different cyanine dye co-adsorbed on the surface of silver bromide grains, the fluorescence decay becomes several times faster. The results are analyzed in the framework of the “hole-trapping” supersensitization model. The effective hole trapping (electron transfer from SS to the excited J-aggregate) rate constant is found to be independent of the grain size and equal to ∼0.60 ps -1 . By fluorescence anisotropy measurement, the ultrafast energy transfer of the J-aggregates on the octahedral crystal (200 nm) is studied. The anisotropy decays are biphasic with two time constants of ∼100 fs and a few picoseconds, the short time constant being assigned to energy transfer between epitaxially arranged J-aggregates on the {111} surface.

Spectral Sensitization, Supersensitization and Ultrafast Exciton Migration on AgBr Semiconductor Surface

The fast electron photo-injection rate from the adsorbed dye to AgBr was found to be dependent on the type of AgBr surface, size of grains, and pAg of emulsion. Ultrafast exciton migration is discussed in terms of dimensionality of J-aggregates.

Photodissociation of CH 2 I 2 and the Subsequent Electron Transfer in Its Cluster Formed in Solution

We have studied photodissociation and the subsequent reactions of methylene iodide (CH2I2) upon excitation at 268 nm in acetonitrile and hexane solutions by the femtosecond transient absorption spectroscopy. In addition to dissociation and geminate recombination in the femtosecond region, formation of CH2I2 + is observed in the early picosecond region. The reaction is proposed to occur by electron transfer between photofragment I and CH2I2. This indicates that the solute molecules aggregate even in a dilute solution and they are responsible for the cation formation.