Masanobu Shirai

Extraordinary carrier multiplication gated by a picosecond electric field pulse

The study of carrier multiplication has become an essential part of many-body physics and materials science as this multiplication directly affects nonlinear transport phenomena, and has a key role in designing efficient solar cells and electroluminescent emitters and highly sensitive photon detectors. Here we show that a 1-MVcm−1 electric field of a terahertz pulse, unlike a DC bias, can generate a substantial number of electron–hole pairs, forming excitons that emit near-infrared luminescence. The bright luminescence associated with carrier multiplication suggests that carriers coherently driven by a strong electric field can efficiently gain enough kinetic energy to induce a series of impact ionizations that can increase the number of carriers by about three orders of magnitude on the picosecond time scale.

Localizing nature of photo-excited states in SrTiO3

Abstract The absorption and luminescence measurements of SrTiO 3 are made between room temperature and 10 K. From the absorption spectra, it is shown that this material has an indirect band gap energy of 3.16 eV at 10 K. No exciton absorption is observed. The luminescence spectra indicate the existence of STE. A time-resolved luminescence measurements revealed that localizing of electrons and holes plays an important role in the kinetics of photo-excited states.

Post-crystal engineering of zinc-substituted myoglobin to construct a long-lived photoinduced charge-separation system

Photoinduced electron transfer (ET) in native photosynthesis reactions is efficiently achieved by the accumulation of different types of redox cofactors within protein assemblies immobilized in cell membranes. The precise arrangement of each cofactor in the molecular spaces enables them to retain the long-lived charge-separated state, which promotes multistep reactions in biological systems. To elucidate the mechanism of the biological ETreactions and to develop light energy conversion systems, artificial ET proteins have been constructed using de novo proteins, chemical modification of native cofactors, photocatalytic reaction centers engineered into protein assemblies, and design of synthetic metal complexes immobilized in protein–protein ET systems. The reported systems have provided insights into control of ET rates in terms of the distance between donors and acceptors, hydrogen-bonding interactions, reorganization energy of cofactors, and other factors. Control of the dense accumulation of the different redox cofactors observed in natural photosystems required to achieve long-lived charge-separated state has caused difficulties in efforts to duplicate this process using artificial protein systems in solution. Thus, the design of novel protein frameworks that allow construction of a dense array of various cofactors is a worthwhile goal. Protein crystals can be regarded as excellent candidates for the development of artificial ET reaction systems because the crystal lattices are expected to allow different types of cofactors to be arranged in three-dimensional frameworks that mimic the native ET systems. ET reactions in single protein crystals have been investigated for the dependence of long-range ET on the structures and orientations of redox centers within proteins. Gray et al. constructed photochemically-initiated protein–protein ET reactions in protein crystals containing zinc-substituted cytochrome c peroxidase or ruthenium-modified azurin. Moreover, protein crystals provide nanosized spaces for the fixation of metal ions, metal complexes, and the diffusion of organic molecules. For instance, accumulation of metal ions and metal complexes in a protein crystal lattice spaces was accomplished simply by soaking of the crystals in a solution containing their precursors. Anisotropic diffusion of small molecules in hen egg-white lysozyme (HEWL) crystals has been investigated by experimental and simulation approaches. 22] The results suggest that these features are governed by steric repulsion and electrostatic interaction induced by amino acid residues located on the internal surface of the crystal lattices. Thus, if we can precisely arrange donor and acceptor molecules and mediators in protein crystals, it is expected that the novel three-dimensional framework will allow us to achieve a longlived charge-separated state. Herein, we construct an artificial long-lived photoinduced charge-separation system using a protein crystal with different redox cofactors fixed in defined locations. We demonstrate the photoinduced multistep ET in a sperm whale myoglobin (Mb) single crystal. Methyl viologen (MV)mediated ET occurs in the crystal between zinc porphyrin (ZnP; electron donor) and an oxo-centered triruthenium cluster (Ru3O; electron acceptor; Scheme 1). The Mb crystals with space group P6 form several channel structures (diameter 2–4 nm), which provide enough space for accumulation of nanosized functional molecules as previously reported. The Mb crystal spaces are available for site-specific fixation of Ru3O and anisotropic diffusion of MV. Moreover, fixation of zinc porphyrin units with a light-harvesting function in the Mb crystal is achieved by crystallization of zinc porphyrin substituted myoglobin (ZnMb). Our engineered Mb crystals, in which ZnP and Ru3O clusters are fixed at specific sites and which allow MV molecules to diffuse, contribute to providing the extremely long half-life of the final charge-separated state (ZnPC-Ru3O ), which is 2800 times longer than that of a previously reported model system in organic solution. This [*] Dr. T. Koshiyama, Dr. M. Shirai, Prof. Dr. K. Tanaka, Prof. Dr. S. Kitagawa, Prof. Dr. T. Ueno Institute for Integrated Cell-Material Sciences (iCeMS) Kyoto University iCeMS Lab Funai Center, Kyoto University Katsura Nishikyo-ku, Kyoto 615-8510 (Japan) Fax: (+ 81)75-383-2812 E-mail: kitagawa@icems.kyoto-u.ac.jp taka@icems.kyoto-u.ac.jp

Modification of Porous Protein Crystals in Development of Biohybrid Materials

Protein assemblies have attracted increasing attention for construction of biohybrid materials. Protein crystals can also be regarded as solid protein assemblies. The present work demonstrates that protein crystals can be employed as porous biomaterials by site-specific modifications of the crystals of recombinant sperm whale myoglobin mutants. The myoglobin crystals of space group P6 provide hexagonal pores consisting of the building blocks of six Mb molecules, which form a pore with a diameter of 40 A. On the basis of the lattice structure of the Mb crystals, we have selected appropriate residues located on the surface of the pores for replacement with cysteine. This enables modification of the pore surface via coupling with maleimide derivatives. We have succeeded in crystallizing the modified Mb mutants, retaining the P6 lattice, and consistently aligning nanosized functional molecules such as fluorescein, eosin, and Ru(bpy)(3) into the hexagonal pores of the Mb crystals. Our strategy for site-specific modification of protein crystal pores is applicable to various protein crystals with porous structures. We believe that modified porous protein crystals will provide attractive candidates for novel solid materials in nanotechnology applications.

Time-Resolved Spectroscopic Study on the Type I Self-Trapped Excitons in Alkali Halide Crystals: II. Excitation Spectra and Relaxation Processes

Excitation spectra for the fluorescent and phosphorescent components of type I bands in seven alkali halides, NaCl, NaBr, KBr, RbBr, NaI, KI and RbI, are measured in both energy ranges of the free exciton absorption and the band-to-band transition. Using SR pulses as the excitation light source, the two components are resolved by the method of simultaneous photon-counting through two independent time-windows. Excitation spectra for the phosphorescent bands of type II or III are also measured for comparison. From these spectra, total luminescence yield and the fraction of the type I band are determined as a function of excitation energy. On the basis of these results, the relaxation processes of free electron-hole pairs and free excitons, especially on the difference between them, are discussed.

Self-trapped states and related luminescence in PbCl2 crystals

We have comprehensively investigated localized states of photoinduced electron-hole pairs with the electron-spin-resonance technique and photoluminescence (PL) in a wide temperature range of 5-200 K. At low temperatures below 70 K, holes localize on Pb 2 + ions and form self-trapping hole centers of Pb 3 + . The holes transfer to other trapping centers above 70 K. On the other hand, electrons localize on two Pb 2 + ions at higher than 50 K and form self-trapping electron centers of Pb 3 + 2 . From the thermal stability of the localized states and PL, we clarify that the blue-green PL band at 2.50 eV is closely related to the self-trapped holes.

Self-trapped electrons and holes in PbBr2 crystals

We have directly observed self-trapped electrons and holes in PbBr 2 crystals with the electron-spin-resonance (ESR) technique. The self-trapped states are induced below 8 K by two-photon interband excitation with pulsed 120-fs-width laser light at 3.10 eV. Spin-Hamiltonian analyses of the ESR signals have revealed that the self-trapping electron centers are the dimer molecules of Ph 3 + 2 along the crystallographic a axis and the self-trapping hole centers are those of Br 2 - with two possible configurations in the unit cell of the crystal. Thermal stability of the self-trapped electrons and holes suggests that both of them are related to the blue-green luminescence band at 2.55 eV coming from recombination of spatially separated electron-hole pairs.

Correlation between the Spin State and Structure of Self-Trapped Excitons in Alkali Halides

In order to clarify the origin of the preferential relaxation of singlet self-trapped excitons (STEs) into the on-center configuration in alkali halides, we have studied variation of exchange splitting energy along the relaxation path from the on-center to the off-center configurations using a simple phenomenological model. Estimated exchange energy is several tens of meV both at the on-center and off-center configurations, and takes its maximum in between. It provides a larger potential barrier intervening between the on- and off-center minima for the singlet STE than that for the triplet STE. This will inhibit the singlet STE to be relaxed into the off-center configuration.

SYMMETRY LOWERING IN THE PHOTOINDUCED PHASE IN SPIN-CROSSOVER COMPLEXES

We investigated the resonant Raman scattering of the spin-crossover complex, [Fe(2-pic)3]Cl3EtOH, with varying the temperature and calarified for the first time that the photoinduced phase is completely different state from the thermally-induced phase. In the photoinduced phase we observed splits of Raman lines and a number of additional lines which are not observed in the high- and te low-temperature phase. These splits and appearances strongly indicate that a symmetry lowering should take place in the photoinduced phase. We imagined that the symmetry lowering is induced by the Jahn-Teller effect in the photo-excited state of the low-temperature phase.

Luminescence Channels of Manganese-Doped MgGa2O4

Green and red emissions were observed by pumping Mn-doped MgGa 2 O 4 crystals, respectively, at the band edge below 300 nm and between 300 and 500 nm. These responses are in contrast to those in the case of Mn-doped MgAl 2 O 4 . We propose a model of these luminescence channels by taking account of the smaller band gap of MgGa 2 O 4 than that of MgAl 2 O 4 , and the ESR spectrum showing Mn 2+ located at Ga-sites.