Toshiaki Kakitani

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.

Calculation of the excitation transfer matrix elements between the S2 or S1 state of carotenoid and the S2 or S1 state of bacteriochlorophyll

Formalism of the excitation transfer matrix element applicable for any multiconfigurational wave functions is made. On the basis of the resultant formulas, the excitation transfer matrix elements between the S2 or S1 state of a carotenoid, neurosporene, and the S2 or S1 state of bacteriochlorophyll a are calculated at various stacked configurations of the two molecules. The results show that the excitation transfer from the carotenoid S1 state to the bacteriochlorophyll S1 state via the Coulomb mechanism including multipole–multipole interactions takes place very efficiently in a speed more rapid than that via the electron‐exchange mechanism. The results also show that the excitation transfer from carotenoid to bacteriochlorophyll occurs directly from the carotenoid S2 state, as well as from the carotenoid S1 state. Furthermore, it is shown that the excitation transfer matrix element due to the electron‐exchange interaction has an oscillatory dependence on the displacement of one molecule from the other w...

Photoinduced electron transfer in polar solutions. I. New aspects of the role of the solvent mode in electron-transfer processes in charge-separation reactions

Abstract A new aspects of the role of the solvent mode in the photoinduced electron-transfer process of electron donor and acceptor system in polar solvents has been exploited. Taking into account the important fact that the vibrational frequency of the solvent mode in the initial neutral state of the reactants is considerably smaller than that in the final ionic state, we have derived a new formula for the energy-gap dependence of the electron-transfer rate. In this formulation, the activation energy is greatly reduced and the electron-transfer rate is almost independent of the energy gap over a wide down-hill energy region. This qualitative feature explains the experimental results for the relation between the bimolecular quenching rate constant k w and the standard free-energy change Δ G ° associated with electron transfer in the “anomalous region”.

Determination of the bimolecular rate constant of photoinduced charge separation in the energy gap region where the reaction is diffusion controlled — analysis of the transient effect in the course of fluorescence quenching reaction

Abstract The photoinduced intermolecular charge separation (CS) rate constant ( k CS ) at the encounter in acetonitrile solution has been evaluated for various donor—acceptor pairs by analyzing the transient effect in the fluorescence decay curves, in the energy gap region −Δ G 0 =0.37–2.21 eV, where the reaction is diffusion controlled. Although the obtained k CS values are larger than the diffusion rate constant, they do not show the typical bell-shaped −Δ G 0 dependence as observed in the charge recombination reaction of the geminate ion pairs produced by fluorescence quenching reaction, but show a rather flat shape.

A possible new mechanism of temperature dependence of electron transfer in photosynthetic systems

A new theory for the electron transfer by the non-adiabatic process is formulated taking into account the origin shift and the frequency change of the vibration. The resultant formulas are quite similar to those of Jortner (Jortner, J. (1976) J. Chem. Phys. 64, 4860-4867) except that the free energy gap delta G is used instead of the energy gap delta E. By applying this theory to the photosynthetic electron transfer, the role of the remarkable temperature dependence of the electron transfer from cytochrome to P+ in Chromatium vinosum and the experimental data were reproduced very well using a small value of the coupling strength in contrast with the previous theory. This implies that proteins play a role to exclude many of the solvent molecules from the region of the electron transfer reaction between the donor and acceptor molecules. The negative activation process in the back electron transfer from QA- to P+, the very slow back electron transfer from I- to P+ and the solvent isotope effect on the cytochrome oxidation are also successfully explained by this new theory. It is shown that even a qualitative conclusion as to the molecular parameters obtained from the temperature dependence of the electron transfer is different between the present theory and that of Jortner.

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.

Examination of the viability of the Collins-Kimball model and numerical calculation of the time-dependent energy gap law of photoinduced charge separation in polar solution

Abstract Viability of the analysis using the Collins-Kimball model for the rate constant of photoinduced charge separation reactions in polar solutions is investigated. In the Collins-Kimball model, it is assumed that the reaction occurs only at a specified distance in the encounter between donor and acceptor molecules under mutual diffusional motion while in real systems the reaction can occur over a range of distances. By comparing the fluorescence decay curve obtained by the Collins-Kimball model with the decay curve calculated numerically using the model incorporating a suitable distance dependence of the reaction rate, we found that the Collins-Kimball model is viable with some reservation. Using the numerically calculated distance distribution function of donor and acceptor molecules, we calculated time-dependent energy gap laws of photoinduced charge separation reactions. The calculated energy gap law for t = 0 fits well the experimental data obtained by a transient effect analysis of the fluorescence decay curve (S. Nishikawa, T. Asahi, T. Okada, N. Mataga and T. Kakitani, Chem. Phys. Letters 185 (1991) 237). The calculated energy gap law for t → ∞ fits very well the experimental data obtained by the stationary state fluorescence quenching measurements (D. Rehm and A. Weller, Israel J. Chem. 8 (1970) 259).

Electron transfer via a midway molecule as seen in primary processes in photosynthesis; a new process describable as superexchange or sequential in mutually opposite limits

Abstract Electron transfer (ET) mediated by a midway molecule should be described as a process new in itself, not in terms of direct ET between two molecules as a two-step sequential or a unistep superexchange mechanism. They describe only mutually opposite limits (not working in parallel as often assumed) that the reorganization time of phonons at the midway state is respectively much shorter or longer than the lifetime of an electron therein, when this state has energies comparable to the donor and acceptor states. We can obtain the rate constant of the ET by perturbational expansion in electronic coupling.

Molecular Mechanism for the Initial Process of Visual Excitation. I. Model of Photoisomerization in Rhodopsin and Its Theoretical Basis by a Quantum Mechanical Calculation of Adiabatic Potential

A model for explaining the photoisomerization of the chromophore in rhodopsin molecule with a high quantum yield is proposed on the basis of the strain effect. Necessary conditions assumed in the model are shown to be satisfied by the calculation using a self-consistent HMO theory which is recently developed by one of the authors. It is also shown that when a double-bond is twisted by about 40°, a steep change occurs in the molecular geometry, and that calculated adiabatic potentials, absorption wavelengths and transition dipole moments show some anomalies, corresponding to the change. Calculated energy barriers for isomerization around single-bonds of Schiff-bases of retinal in the ground state are very small.

THEORETICAL STUDY OF COLOR CONTROL MECHANISM IN RETINAL PROTEINS. I. ROLE OF THE TRYPTOPHAN RESIDUE, TYROSINE RESIDUE and WATER MOLECULE

Abstract –We calculated the opsin shift due to the electrostatic interaction between tryptophan or tyrosine residues and the chromophore by the perturbation method for various mutual configurations. The obtained opsin shift maps for these configurations demonstrated that when the above residues reside around the ionone ring side, the positive opsin shift (bathochromic shift) is obtained, and when they reside around the Schiff‐base side, the negative opsin shift (hypsochromic shift) is obtained. These properties hold true, irrespective of the orientation of those residues, indicating that higher order multipoles of the group play a central role. The maximum value of the opsin shift by these groups amounts to several hundred wavenumbers. These results indicate that the location of some of those amino acid residues at proper positions around the chromophore can cause a considerable opsin shift. We also calculated opsin shift maps for the various mutual configurations between a water molecule and the chromophore for comparison.