Noboru Mataga

The bell-shaped energy gap dependence of the charge recombination reaction of geminate radical ion pairs produced by fluorescence quenching reaction in acetonitrile solution

Abstract In view of the theoretical prediction that the observation of the inverted region in the photoinduced charge separation will be difficult, as it is actually the case in the fluorescence quenching reaction, but the charge recombination of the produced geminate ion pair will show a clear-cut bell-shaped energy gap dependence, we have made systematic studies on the charge recombination processes of geminate ion pairs by directly observing their dynamics with ultrafast laser spectroscopy. We have obtained not only the results for the inverted region, but also the results for the top region as well as normal region, confirming the bell-shaped energy gap dependence of the charge recombination. On the basis of this result, some discussions on the nature of the inter- and intra-molecular ion pairs with respect to the charge recombination processes have been given in the case of some typical exciplexes and porphyrin-quinone systems.

Dynamics of excited flavoproteins: picosecond laser photolysis studies

Abstract— Molecular mechanism of fluorescence quenching of flavins in flavodoxin from Desulfovibrio vulgaris, strain Miyazaki, and riboflavin binding protein from egg white has been investigated by means of picosecond laser photolysis technique. In the case of flavodoxin, a transient absorption band characteristic of the non‐fluorescent exciplex formed by electron transfer from indole to excited flavins in model systems has been observed around 600 nm at the delay time of 33 ps from exciting ps pulse pulse width, 25 ps). In the case of riboflavin binding protein, the transient absorption spectra were different from those of flavin‐indole exciplex and rather similar to the spectra of the model system of flavin‐phenol. These results suggest that tryptophan residue exists near the isoalloxazine nucleus in flavodoxin, and in riboflavin binding protein, tyrosine residue exists near the flavin. Direct measurements of the ultrafast process of the electron transfer in flavoproteins as developed here could provide useful information for elucidating protein dynamics, associated with redox reaction, in the picosecond time region.

Mechanisms of the strongly exothermic charge separation reaction in the excited singlet state. Picosecond laser photolysis studies on aromatic hydrocarbon-tetracyanoethylene and aromatic hydrocarbon-pyromellitic dianhydride systems in polar solutions

Abstract In relation to the fact that there are no clear-cut experimental results indicating the “inverted region” in the strongly exothermic charge separation (CS) in the fluorescence quenching reaction, the possibilities of (a) formation of a nonfluorescent charge transfer complex in the course of quenching and (b) participation of excited electronic states of the ion pair in the course of CS at the encounter of fluorescer and quencher are examined for aromatic hydrocarbon-tetracyanoethylene and aromatic hydrocarbon-pyromellitic dianhydride systems in acetonitrile with the picosecond laser photolysis method. Both (a) and (b) are shown improbable as mechanisms for the lack of an inverted region in the photoinduced CS reaction. Discussions on these results are given on the basis of a new theoretical treatment.

Fluorescence quenching dynamics of tryptophan in proteins. Effect of internal rotation under potential barrier

In many proteins fluorescence from single tryptophan exhibits a nonexponential decay function. To elucidate the origin of this nonexponential decay, we have examined the fluorescence decay function and time-resolved fluorescence anisotropy of a fluorophore covalently bound to a macromolecule by solving a rotational analogue of the Smoluchowski equation. An angular-dependent quenching constant and potential energy for the fluorophore undergoing internal rotation were introduced into the equation of motion for fluorophore. Results of numerical calculations using the equations thus obtained predict that both the fluorescence decay function and time-resolved anisotropy are dependent on rotational diffusion coefficients of fluorophore and potential energy for the internal rotation. The method was applied to the observed fluorescence decay curve of the single tryptophan in apocytochrome c from horse heart. The calculated decay curves fit the observed ones well.

Solvent relaxation effect on transient hole-burning spectra of organic dyes

Abstract Picosecond transient absorption spectra of organic dyes in solution are measured at room temperature. When a dye molecule is dissolved in viscous solvents, a transient spectrum initially shows a slightly sharp hole around the exciting energy, accompanied by the time-dependent broadening of its shape. This phenomenon corresponds to the solvent relaxation effect reported frequently in time-resolved fluorescence spectroscopy.

Picosecond dynamics of intramolecular electron and energy transfer in porphyrin dimer model compounds

Abstract In order to elucidate the dynamics and mechanisms of photoinduced electron transfer and back electron transfer deactivation as well as excitation transfer in some hybrid dimers of porphyrins connected by covalent chains, picosecond time-resolved transient absorption spectral measurements as well as picosecond time-resolved fluorescence measurements were made on these systems. A large difference between the rates of the photoinduced electron transfer and back electron transfer has been observed even though the exothermicities of the two processes are the same. A theoretical interpretation has been advanced to account for this difference.

Analysis of internal motion of single tryptophan in Streptomyces subtilisin inhibitor from its picosecond time-resolved fluorescence

A mode of internal motion of single tryptophan, Trp 86, of Streptomyces subtilisin inhibitor, was analyzed from its time-resolved fluorescence. The intensity and anisotropy decays of Trp 86 were measured in the picosecond range. These decays were analyzed with theoretical expressions derived assuming that the indole ring of tryptophan as an asymmetric rotor rotates around covalent bonds connecting indole with the peptide chain and an effective quencher of fluorescence of Trp 86 is the nearby SS bond of Cys 35-Cys 50. First, the intensity decays at 6 degrees, 20 degrees, and 40 degrees C were analyzed, and then the both decays of the intensity and anisotropy at 20 degrees C were simultaneously simulated with common parameters. Constants concerning geometrical structures of the protein used for the analysis were obtained from x-ray crystallographic data. Best fit between the observed and calculated decay curves was obtained by a nonlinear least squares method by adjusting a quenching constant averaged over the rotational angles, koq height of the potential energy, p, and three of six diffusion coefficients, Dxx, Dyy, Dzz, Dxy, Dyz, and Dzx, as variable parameters. The obtained results revealed that the internal motion of the indole ring became faster, the quenching rate of the fluorescence of Trp 86 was enhanced and the height of potential energy became lower at higher temperatures, and suggested that Trp 86 was wobbling around the long axis of the indole ring in the protein.

Picosecond time-resolved absorption study of all-trans- and 9-cis-retinal: configurational relaxation in the triplet state

Abstract Picosecond time-resolved absorption spectra of all-trans- and 9-cis-retinal in n -hexane solution have been measured by using an Nd 3+ :YAG laser photolysis system. A drastic change of the T n ← T 1 ] absorption spectrum of the 9-cis isomer has been observed in the picosecond to nanosecond time region, which should reflect the configurational relaxation in the triplet state.

Picosecond laser photolysis studies on dimer model systems in relation to photosynthetic charge separation processes

Abstract In relation to the photoinduced charge separation in the special pair of photosynthetic reaction center, solvation induced intramolecular electron transfer in the excited state of composite system with identical halves have been investigated with picosecond spectroscopy in the case of 1,2-dienthrylethanes. The energy gap dependences of charge separation and charge recombination in these systems are demonstrated and compared with those of hybrid dimer model of metal porphyrins, porphyrin-quinone systems and various aromatic donor-acceptor systems. The significance of these results in the understanding of efficient charge separation in biological reaction center is discussed.

On the properties of geminate electron-hole pairs in liquid cyclohexane produced by picosecond 266 nm simultaneous two-photon excitation

Abstract The nature of geminate ion pairs in liquid cyclohexane produced in the course of the relaxation from the highly excited state attained by the simultaneous two-photon excitation (9.32 eV) with picosecond 266 nm laser light to the fluorescent state has been investigated by measuring the transient absorbance due to the fluorescent state in the presence of electron scavengers. It has been demonstrated that the distribution of the geminate electron produced by the present method can be reproduced by a δ-function with its maximum at ≈ 30–40 A.