Naoto Tamai

Femtosecond transient absorption spectroscopy of a spirooxazine photochromic reaction

Abstract The dynamics of Cue5f8O bond cleavage followed by the structural change to merocyanine in the photochromic reaction of a spirooxazine derivative has been studied by femtosecond transient absorption spectroscopy. Within 2 ps after excitation, the very broad spectra assigned to the absorption of the transition state, following the S n ←S 1 absorption spectrum of spirooxazine, were observed. The rate constants of Cue5f8O bond cleavage and the relaxation from the transition state to a metastable merocyanine were estimated to be approximately (700 fs) −1 and (470 fs) −1 , respectively.

Singlet excitation energy transfer in conformationally restricted zinc-free-base hybrid diporphyrins

Abstract A series of conformationally restricted zinc-free-base hybrid diporphyrins has been synthesized. Efficient intramolecular singlet-singlet energy transfer from the zinc prophyrin to the free-base porphyrin in these molecules has been studied by picosecond time-resolved fluorescence spectroscopy. The dipole-dipole Forster mechanism is proposed to be dominant in the energy transfer process on the basis of the geometry dependence of its rate. However, for shorter separations, the Dexter exchange mechanism may open an additional channel for the energy transfer.

Structure and function of the photoreceptor stentorins in Stentor coeruleus. I. Partial characterization of the photoreceptor organelle and stentorins

The unicellular ciliary protozoan, Stentor coeruleus, exhibits photophobic and phototactic responses to visible light stimuli. The pigment granule contains the photoreceptor chromoproteins (stentorins). Stentorin localized in the pigment granules of the cell serves as the primary photoreceptor for the photophobic and phototactic responses in this organism. An initial characterization of the pigment granules has been described in terms of size, absorbance spectra and ATPase activity. Two forms of the stentorin pigments have been isolated from the pigment granules. Stentorin I has an apparent molecular weight of 68,600 and 52,000 by SDS-PAGE (at 10 and 13% gel, respectively) or 102,000 by steric exclusion HPLC, whereas stentorin II is a larger molecular assembly probably composed of several proteins (mol. wt. greater than 500,000). Stentorin I is composed of at least two heterologous subunits corresponding to apparent mol. wts. of 46,000 (fluorescent, Coomassie blue negative) and 52,000 (fluorescent, Coomassie blue positive) on SDS-PAGE (13% gel). However, these values were found to be strongly dependent on the degree of crosslinking in the acrylamide gel. Stentorin II appears to be the primary photoreceptor whose absorption and fluorescence properties are consistent with the action spectra for the photoresponses of the ciliate to visible light.

Intersystem crossing of benzophenone by femtosecond transient grating spectroscopy

Abstract Transient grating spectroscopy using a femtosecond white-light continuum was developed and applied to the spectral analysis of intersystem crossing of benzophenone in solution. The diffraction spectra corresponding to Sn←S1 and Tn←T1 transient absorption were observed for the first time. The rate constants of intersystem crossing in acetonitrile and CCl4 were estimated to be (9.6 ± 0.9 ps)−1 and (11.6 ± 0.9 ps)−1, respectively.

Structure and function of the photoreceptor stentorins in Stentor coeruleus. II. primary photoprocess and picosecond time-resolved fluorescence

Stentorin serves as the photoreceptor for the photophobic and negative phototactic responses in Stentor coeruleus. Two forms of the stentorin have been isolated and purified. The strongly fluorescent form, stentorin I at pH 7.8, exhibited nearly exponential fluorescence decay monitored at 620 nm, having two comparable lifetime decay components of 2.53 ns (47%) and 5.95 ns (53%). Stentorin I showed no significant time-resolved fluorescence emission spectra in the picosecond-nanosecond time scales. The weakly fluorescent form, stentorin II, exhibited an ultrafast fluorescence decay component (10 ps) at an emission wavelength of 630 nm and pH 7.8. The amplitudes of the multi-component fluorescence in stentorin II were found to be emission wavelength-dependent. Furthermore, the fluorescence emission spectrum was time-resolvable in the picosecond time scales. Effects of pH and pD on the fluorescence decay kinetics and time-resolved spectra of stentorins I and II have also been investigated. Results are suggestive of proton dissociation as a primary photoprocess from the excited state of stentorin II.

Photoinduced electron transfer reactions in quinone-linked zinc porphyrin arrays

Abstract Intramolecular photoinduced electron transfer reactions of conformationally restricted quinone-linked zinc porphyrin monomer, dimer, and trimers have been studied by picosecond time-resolved absorption spectroscopy. Both charge separation and charge recombination are very rapid in the monomeric model, while charge recombination is partly impeded in the dimeric model. In the trimeric models, the singlet excited state of the distal coplanar diporphyrin is efficiently quenched by the attached quinone and long-lived charged separated states are formed most probably by electron transfer from the diporphyrin part to the monomeric porphyrin part competing with rapid charge recombination reaction.

EXCITATION ENERGY FLOW IN A PHOTOSYNTHETIC BACTERIUM LACKING B850. FAST ENERGY TRANSFER FROM B806 TO B870 IN ERYTHROBACTER SP. STRAIN OCH 114

Abstract Excitation energy flow in the aerobic photosynthetic bacterium Erythrobacter sp. strain OCh 114 was investigated at physiological temperature by the time-resolved fluorescence spectroscopy in the picosecond time range. This bacterium has two forms of bacteriochlorophyll (Bchl) on separate antenna complexes absorbing around 806 nm (B806) and 870 nm (B870); however, B850 is not detected in this species. Upon selective excitation of B806, the fluorescence from B870 was dominant even in the initial time, indicating that energy transfer from B806 to B870 complexes occurred within the time resolution of the apparatus (6 ps). This fast transfer was confirmed by the fact that no rise term was resolved by the kinetic analysis of the fluorescence from B870. The fast energy transfer suggests tight association between the different classes of antenna complexes. The antenna pigment with energy level lower than that of reaction center was also found in this species (B888) as in some purple photosynthetic bacteria. Its content was estimated to be at most 3 Bchl per reaction center, which is significantly less than that in Rhodobacter sphaeroides .

Direct trapping excitation energy transfer from rhodamine B to crystal violet in Langmuir-Blodgett monolayer and stacked multilayer films

Abstract Picosecond time-resolved fluorescence decays of N,N′-dioctadecyl rhodamine B (DORB) have been measured in order to examine both the morphology of chromophores and the excitation energy transfer in restricted geometries of Langmuir-Blodgett (LB) monolayer and stacked multilayer films which contain both DORB and octadecyl crystal violet (ODCV). Fluorescence decays of DORB deposited in the LB films are affected by ODCV, as a result of direct trapping excitation energy transfer from DORB to ODCV. The decay analysis shows that ODCV in the LB monolayer film distributes near uniformly at low concentrations below 6 mol% and that ODCV distributes with a fractal-like structure with the Hausdorff fractal dimension of 1.4 at high concentrations above 6 mol%. Thus, the morphology of ODCV in the LB monolayer films shows a kind of phase transition at around 6 mol%. The efficiency of the energy transfer from DORB to ODCV in LB monolayer films is much lower than that in solution or on vesicle surfaces. Fluorescence decay which is characterized by the two-dimensional system for the LB monolayer films is seen to approach the one characterized by the three-dimensional system, as the number of stacking layers increases.

Solvation dynamics of a coumarin dye at liquid—solid interface layer. Picosecond total internal reflection fluorescence spectroscopic study

Abstract Fluorescence rise and decay curves of coumarin 460 in 1-butanol/sapphire interface layer were measured by time-resolved total internal reflection fluorescence spectroscopy. The fast rise and decay components due to the solvent relaxation became slow in the interface layer, and the anomaly was observed up to the penetration depth of 100 nm. The result was interpreted in terms of the longitudinal relaxation of 1-butanol affected by hydrogen bonding interactions between the hydrophilic sapphire surface and 1-butanol.

Femtosecond transient absorption spectroscopy of a single perylene microcrystal under a microscope

Abstract The photophysical properties of single α- and β-perylene microcrystals were investigated using femtosecond transient absorption spectroscopy in conjunction with a microscope at 293 K. Formation of dimer cations in respective microcrystals, and the mechanism of exciton—exciton annihilation were analyzed in terms of transient absorption spectra and their decay dynamics as well as crystal structures.