Yasuhiro Kobori - Academia.edu (original) (raw)

Papers by Yasuhiro Kobori

Communications Chemistry

In the biological magnetic compass, blue-light photoreceptor protein of cryptochrome is thought t... more In the biological magnetic compass, blue-light photoreceptor protein of cryptochrome is thought to conduct the sensing of the Earth’s magnetic field by photoinduced sequential long-range charge-separation (CS) through a cascade of tryptophan residues, WA(H), WB(H) and WC(H). Mechanism of generating the weak-field sensitive radical pair (RP) is poorly understood because geometries, electronic couplings and their modulations by molecular motion have not been investigated in the secondary CS states generated prior to the terminal RP states. In this study, water dynamics control of the electronic coupling is revealed to be a key concept for sensing the direction of weak magnetic field. Geometry and exchange coupling (singlet–triplet energy gap: 2J) of photoinduced secondary CS states composed of flavin adenine dinucleotide radical anion (FAD−•) and radical cation WB(H)+• in the cryptochrome DASH from Xenopus laevis were clarified by time-resolved electron paramagnetic resonance. We foun...

Chemistry, an Asian journal, Jan 15, 2016

The photoinduced persistent intramolecular charge-transfer state of 4-carbazolyl-3-(trifluorometh... more The photoinduced persistent intramolecular charge-transfer state of 4-carbazolyl-3-(trifluoromethyl)benzoic acid was confirmed. It showed a higher catalytic activity in terms of yield and selectivity in the photochemical reduction of alkyl halides compared to the parent carbazole. Even unactivated primary alkyl bromides could be reduced by this photocatalyst. The high catalytic activity is rationalized by considering the slower backward single electron transfer owing to the spatial separation of the donor and acceptor subunits.

The Journal of Physical Chemistry Letters, 2015

The Journal of Physical Chemistry C, 2015

The Journal of Chemical Physics, 2003

The electronic and solvation structures of N,N-dimethylaniline in acetonitrile are examined by me... more The electronic and solvation structures of N,N-dimethylaniline in acetonitrile are examined by means of the ab initio reference interaction site model self-consistent-field theory coupled with the method to evaluate nonequilibrium solvation free energy developed by Chong et al. [J. Phys. Chem. 99, 10 526 (1995)]. The key quantities characterizing the solvation process—the free-energy profile governing the solvent fluctuations and solvent reorganization—are evaluated from first principles. A new scheme, which enables us to partition solvent reorganization into atomic contributions in the solute molecule, is proposed and used to analyze the process at the atomic level. We found that the linear response approximation holds well and the overall observable λs is not much affected by solute geometry, while the individual atomic contribution is significantly changed, especially by the wagging motion of the amino group.

Applied Magnetic Resonance, 2010

We present a theoretical approach to investigate the electron spin polarization (ESP) of the exci... more We present a theoretical approach to investigate the electron spin polarization (ESP) of the excited triplet state that has been detected by using the time-resolved EPR (TREPR) method in the Photosystem II reaction center (PS II RC) of the plants. We show, by using the stochastic-Liouville equation, that the ESP pattern created in the accessory chlorophyll (Chl accD1 ) which reside near the P D1 chlorophyll of the active branch is explained by one-step, concerted double electron transfer model initiating from the singlet-triplet conversion of the light-induced charge-separated (CS) state composed of P D1 radical cation and pheophytin radical anion. We also considered the sequential electron spin polarization transfer (ESPT) model via 1) the triplet charge-recombination (CR) and 2) the triplet-triplet energy transfer processes. It has been clearly shown that the ESP created in the 3 Chl accD1 * is dependent on the rate constant (k TT ) of the triplet-triplet energy transfer from the 2 intermediate triplet state created by the CR. Also we show that relative orientation of the principal axes of the spin-dipolar interaction in the intermediate triplet state ( 3 P D1 *, as an example) may play a role on the ESP pattern when the k TT is smaller than the angular frequency of the Zeeman energy.

Chem Phys Lett, 1993

Time-resolved ESR spectra of the α-hydroxybenzyl radical were measured in benzene and 2-propanol ... more Time-resolved ESR spectra of the α-hydroxybenzyl radical were measured in benzene and 2-propanol solutions by the photo-dissociation of benzoin. The hyperfine structure (hfs) of α-hydroxybenzyl depends on the solvents. In a benzene solution containing triethylamine, two species with different hyperfine structure appeared simultaneously. As the ratio of intensity for the two species depends on the concentration of triethylamine, one of them is assigned to the bare α-hydroxybenzyl and the other to the 1:1 complex of α-hydroxybenzyl and triethylamine. The equilibrium constant of complex formation was estimated to be about 450 M -1 from the analysis of CIDEP intensities.

Http Dx Doi Org 10 1080 00268970110113579, Nov 23, 2009

CIDEP signals of semireduced thionine radicals produced by reacting thionine triplets with anilin... more CIDEP signals of semireduced thionine radicals produced by reacting thionine triplets with aniline and halogenated anilines were measured by time resolved CW and pulsed FT EPR. For aniline as quencher, the polarization was emissive while for 4-Br-and 3-I-aniline a time dependent change in polarization from emissive to enhanced absorption was observed. For 4-I-aniline the signals were in enhanced absorption for all delay times. The time and concentration dependence of the signals was analysed in terms of a sequential double triplet mechanism: polarization of the thionine triplet due to selective population of the molecular triplet substates (classical`p-type' triplet mechanism) and modi®cation of this polarization by substate selective, heavy atom induced depopulation of triplet exciplexes (triplet contact radical pairs) formed as intermediates in the triplet quenching by electron transfer (`d-type' triplet mechanism). A quantitative theoretical treatment that combines the time-integrated solution of the stochastic Liouville equations for precursor triplet and triplet exciplex with the kinetic rate equation of the bimolecular quenching process is presented. The equations derived allow the extraction of two polarization enhancement factors, V d for the pure d-type and V pd for the combined p-and d-type triplet mechanism from the concentration dependence of the time dependent CIDEP signals. The CIDEP curves and the previously observed magnetic ®eld and heavy atom e ects on the free radical yield can be quantitatively simulated with a consistent set of kinetic parameters.

Journal of the American Chemical Society Jacs, Feb 1, 2006

Photoinduced, proton-coupled electron transfer (ET) between 9,10-anthraquinone-2,6-disulfonate (A... more Photoinduced, proton-coupled electron transfer (ET) between 9,10-anthraquinone-2,6-disulfonate (ADQS) and an amino acid residue of tryptophan in human serum albumin (HSA) was observed using time-resolved electron paramagnetic resonance (TREPR). The ET reaction reduces the protein binding affinity of the ligand. TREPR chemically induced dynamic electron polarization (CIDEP) spectra establish that photoinduced ET takes place from the tryptophan residue (W214) to the excited triplet state of AQDS2- while bound in subdomain IIA, a protein cleft of HSA. The TREPR CIDEP signals also reveal that the anion radical of the ligand escapes toward the bulk water region by a one-dimensional translation diffusion process within the protein's pocket area. This pilot study of HSA demonstrates how TREPR CIDEP can provide significant means to investigate dynamic characteristics of protein-surface reactions.

The journal of physical chemistry. B, Jan 19, 2016

To elucidate how local molecular conformations play a role on electronic couplings for the long-r... more To elucidate how local molecular conformations play a role on electronic couplings for the long-range photoinduced charge-separated (CS) states in protein systems, we have analyzed time-resolved electron paramagnetic resonance (TREPR) spectra by polarized laser irradiations of 9,10-anthraquinone-1-sulfonate (AQ1S(-)) bound to human serum albumin (HSA). Analyses of the magnetophotoselection effects on the EPR spectra and a docking simulation clarified the molecular geometry and the electronic coupling of the long-range CS states of AQ1S(•2-)-tryptophan214 radical cation (W214(•+)) separated by 1.2 nm. The ligand of AQ1S(-) has been demonstrated to be bound to the drug site I in HSA. Molecular conformations of the binding region were estimated by the docking simulations, indicating that an arginine218 (R218(+)) residue bound to AQ1S(•2-) mediates the long-range electron-transfer. The energetics of triad states of AQ1S(•2-)-R218(+)-W214(•+) and AQ1S(-)-R218(•)-W214(•+) have been comput...

The Journal of Physical Chemistry, 1994

The concentration dependence of free radicals (TEMPO) of enhanced absorptive chemically induced d... more The concentration dependence of free radicals (TEMPO) of enhanced absorptive chemically induced dynamic electron polarization (CIDEP) spectra was studied quantitatively to clarify the mechanism of CIDEP generation in excited singlet stateradical systems. It was experimentally demonstrated that the CIDEP intensity on the free radical was in proportion to the concentration of the spin-polarized free radical. The rate constant of the CIDEP generation was determined to be (6.3 f 0.2) X lo9 M-l s-l from the Stern-Volmer plots of CIDEP intensity in the coronene-TEMPO system in benzene. The absorptive CIDEP spectra were attributed to the free radicals which enhanced the SI-Tl intersystem crossing and were interpreted by introducing the stochastic-Liouville equation into the radical-triplet pair mechanism with doublet precursor.

Journal of the American Chemical Society, Jan 15, 2016

To shed a light on fundamental molecular functions of photoinduced charge conductions by organic ... more To shed a light on fundamental molecular functions of photoinduced charge conductions by organic photovoltaic materials, it is important to directly observe molecular geometries of the intermediate charges just after the photoinduced electron-transfer reactions. However, highly inhomogeneous molecular environments at the bulk heteojunction interfaces in the photoactive layers have prevented us from understanding the mechanism of the charge conductions. We have herein investigated orbital geometries, electronic couplings and hole-dissociation dynamics of photoinduced charge-separated (CS) states in a series of poly(3-hexylthiophene)-fullerene linked dyads bridged by rigid oligo-p-phenylene spacers by using time resolved EPR spectroscopy. It has been revealed that one-dimensional intramolecular hole-dissociations exothermically take place from localized holes in initial CS states, following bridge-mediated, photoinduced charge-separations via triplet exciton diffusions in the conjugat...

Mol Phys, 2002

The spin dynamics of the duroquinone anion radical (DQ · ¡ ) generated by photoinduced electron t... more The spin dynamics of the duroquinone anion radical (DQ · ¡ ) generated by photoinduced electron transfer reactions from triplet eosin Y ( 3 EY 2¡ ) to DQ have been studied by using transient absorption and pulsed EPR spectroscopy. Unusual net-absorptive electron spin polarization plus net-emissive polarization were observed, suggesting the production of the triplet exciplex or contact radical pair as the reaction intermediate. The kinetic parameters and intrinsic enhancement factors of the electron spin polarization were determined in various alcoholic solvents. The net-absorptive electron spin polarization was also observed in ethanol± water mixed solvents. The solvent e ects on the radical yield are analysed on the basis of a stochastic Liouville equation established for the magnetic ®eld e ects on the radical yield. The zero-®eld splitting constants of the triplet exciplex are estimated from the solvent viscosity dependence of the enhancement factors due to spin±orbit coupling induced depopulation of the reaction intermediate.

Journal of the American Chemical Society, Aug 17, 2005

Melanin, a ubiquitous, heterogeneous biological polymer composed of many different monomers, cont... more Melanin, a ubiquitous, heterogeneous biological polymer composed of many different monomers, contains a population of stationary, intrinsic semiquinone-like radicals. Additional extrinsic semiquinone-like radicals are reversibly photogenerated with visible or UV irradiation. The free radical chemistry of melanin is complex and not well characterized, especially the photochemistry of melanin in the presence of oxygen. To determine directly how melanin reacts in the presence of oxygen, time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used to examine melanin free radical chemistry in human retinal pigment epithelium (RPE) cells under aerobic and anaerobic conditions. A TREPR difference spectrum was used to explore the nature of melanin chemistry in the presence of oxygen. The position and symmetrical line shape of the TREPR three-dimensional difference spectrum shows that when reactive oxygen species (ROS) are scavenged, only one of the two or more chemically different melanin free radical species participates in ROS scavenging. This protective melanin radical species exists in both the extrinsic and intrinsic populations of melanin free radicals, allowing melanin to protect the RPE from toxic species in both the light and dark.

Angewandte Chemie (International ed. in English), Jan 26, 2015

The unprecedented dependence of final charge separation efficiency as a function of donor-accepto... more The unprecedented dependence of final charge separation efficiency as a function of donor-acceptor interaction in covalently-linked molecules with a rectilinear rigid oligo-p-xylene bridge has been observed. Optimization of the donor-acceptor electronic coupling remarkably inhibits the undesirable rapid decay of the singlet charge-separated state to the ground state, yielding the final long-lived, triplet charge-separated state with circa 100 % efficiency. This finding is extremely useful for the rational design of artificial photosynthesis and organic photovoltaic cells toward efficient solar energy conversion.

The Journal of Physical Chemistry a, 2004

ABSTRACT

The Journal of Physical Chemistry Letters, 2015

Recent progress is overviewed on experimental elucidations of fundamental molecular functions of ... more Recent progress is overviewed on experimental elucidations of fundamental molecular functions of the light-energy conversions by the photoactive layers of the organic photovoltalic (OPV) cells by means of the time-resolved electron paramagnetic resonance spectroscopy. Positions and orientations of the unpaired electrons and electronic coupling matrix elements are clarified in photoinduced, primary charge-separated (CS) states. Connections between the molecular geometries and the electronic couplings have been characterized for the initial CS states to elucidate how the structure, orbital delocalization, and molecular libration play roles on exothermic carrier dissociation via a vibrationally relaxed charge-transfer complex with prevention of the energy-wasting charge recombination. Superior functions to biological molecules are presented for the efficient photocurrent generations induced by orbital delocalization and by shallow trap depths at polymer-stacking domains. The above structural and electronic characteristics of the primary electron-hole pairs are essential to evaluations, designs, and developments of the efficient solar cells using organic molecules.

The Journal of Physical Chemistry, 1994

The concentration dependence of free radicals (TEMPO) of enhanced absorptive chemically induced d... more The concentration dependence of free radicals (TEMPO) of enhanced absorptive chemically induced dynamic electron polarization (CIDEP) spectra was studied quantitatively to clarify the mechanism of CIDEP generation in excited singlet stateradical systems. It was experimentally demonstrated that the CIDEP intensity on the free radical was in proportion to the concentration of the spin-polarized free radical. The rate constant of the CIDEP generation was determined to be (6.3 f 0.2) X lo9 M-l s-l from the Stern-Volmer plots of CIDEP intensity in the coronene-TEMPO system in benzene. The absorptive CIDEP spectra were attributed to the free radicals which enhanced the SI-Tl intersystem crossing and were interpreted by introducing the stochastic-Liouville equation into the radical-triplet pair mechanism with doublet precursor.

Communications Chemistry

In the biological magnetic compass, blue-light photoreceptor protein of cryptochrome is thought t... more In the biological magnetic compass, blue-light photoreceptor protein of cryptochrome is thought to conduct the sensing of the Earth’s magnetic field by photoinduced sequential long-range charge-separation (CS) through a cascade of tryptophan residues, WA(H), WB(H) and WC(H). Mechanism of generating the weak-field sensitive radical pair (RP) is poorly understood because geometries, electronic couplings and their modulations by molecular motion have not been investigated in the secondary CS states generated prior to the terminal RP states. In this study, water dynamics control of the electronic coupling is revealed to be a key concept for sensing the direction of weak magnetic field. Geometry and exchange coupling (singlet–triplet energy gap: 2J) of photoinduced secondary CS states composed of flavin adenine dinucleotide radical anion (FAD−•) and radical cation WB(H)+• in the cryptochrome DASH from Xenopus laevis were clarified by time-resolved electron paramagnetic resonance. We foun...

Chemistry, an Asian journal, Jan 15, 2016

The photoinduced persistent intramolecular charge-transfer state of 4-carbazolyl-3-(trifluorometh... more The photoinduced persistent intramolecular charge-transfer state of 4-carbazolyl-3-(trifluoromethyl)benzoic acid was confirmed. It showed a higher catalytic activity in terms of yield and selectivity in the photochemical reduction of alkyl halides compared to the parent carbazole. Even unactivated primary alkyl bromides could be reduced by this photocatalyst. The high catalytic activity is rationalized by considering the slower backward single electron transfer owing to the spatial separation of the donor and acceptor subunits.

The Journal of Physical Chemistry Letters, 2015

The Journal of Physical Chemistry C, 2015

The Journal of Chemical Physics, 2003

The electronic and solvation structures of N,N-dimethylaniline in acetonitrile are examined by me... more The electronic and solvation structures of N,N-dimethylaniline in acetonitrile are examined by means of the ab initio reference interaction site model self-consistent-field theory coupled with the method to evaluate nonequilibrium solvation free energy developed by Chong et al. [J. Phys. Chem. 99, 10 526 (1995)]. The key quantities characterizing the solvation process—the free-energy profile governing the solvent fluctuations and solvent reorganization—are evaluated from first principles. A new scheme, which enables us to partition solvent reorganization into atomic contributions in the solute molecule, is proposed and used to analyze the process at the atomic level. We found that the linear response approximation holds well and the overall observable λs is not much affected by solute geometry, while the individual atomic contribution is significantly changed, especially by the wagging motion of the amino group.

Applied Magnetic Resonance, 2010

We present a theoretical approach to investigate the electron spin polarization (ESP) of the exci... more We present a theoretical approach to investigate the electron spin polarization (ESP) of the excited triplet state that has been detected by using the time-resolved EPR (TREPR) method in the Photosystem II reaction center (PS II RC) of the plants. We show, by using the stochastic-Liouville equation, that the ESP pattern created in the accessory chlorophyll (Chl accD1 ) which reside near the P D1 chlorophyll of the active branch is explained by one-step, concerted double electron transfer model initiating from the singlet-triplet conversion of the light-induced charge-separated (CS) state composed of P D1 radical cation and pheophytin radical anion. We also considered the sequential electron spin polarization transfer (ESPT) model via 1) the triplet charge-recombination (CR) and 2) the triplet-triplet energy transfer processes. It has been clearly shown that the ESP created in the 3 Chl accD1 * is dependent on the rate constant (k TT ) of the triplet-triplet energy transfer from the 2 intermediate triplet state created by the CR. Also we show that relative orientation of the principal axes of the spin-dipolar interaction in the intermediate triplet state ( 3 P D1 *, as an example) may play a role on the ESP pattern when the k TT is smaller than the angular frequency of the Zeeman energy.

Chem Phys Lett, 1993

Time-resolved ESR spectra of the α-hydroxybenzyl radical were measured in benzene and 2-propanol ... more Time-resolved ESR spectra of the α-hydroxybenzyl radical were measured in benzene and 2-propanol solutions by the photo-dissociation of benzoin. The hyperfine structure (hfs) of α-hydroxybenzyl depends on the solvents. In a benzene solution containing triethylamine, two species with different hyperfine structure appeared simultaneously. As the ratio of intensity for the two species depends on the concentration of triethylamine, one of them is assigned to the bare α-hydroxybenzyl and the other to the 1:1 complex of α-hydroxybenzyl and triethylamine. The equilibrium constant of complex formation was estimated to be about 450 M -1 from the analysis of CIDEP intensities.

Http Dx Doi Org 10 1080 00268970110113579, Nov 23, 2009

CIDEP signals of semireduced thionine radicals produced by reacting thionine triplets with anilin... more CIDEP signals of semireduced thionine radicals produced by reacting thionine triplets with aniline and halogenated anilines were measured by time resolved CW and pulsed FT EPR. For aniline as quencher, the polarization was emissive while for 4-Br-and 3-I-aniline a time dependent change in polarization from emissive to enhanced absorption was observed. For 4-I-aniline the signals were in enhanced absorption for all delay times. The time and concentration dependence of the signals was analysed in terms of a sequential double triplet mechanism: polarization of the thionine triplet due to selective population of the molecular triplet substates (classical`p-type' triplet mechanism) and modi®cation of this polarization by substate selective, heavy atom induced depopulation of triplet exciplexes (triplet contact radical pairs) formed as intermediates in the triplet quenching by electron transfer (`d-type' triplet mechanism). A quantitative theoretical treatment that combines the time-integrated solution of the stochastic Liouville equations for precursor triplet and triplet exciplex with the kinetic rate equation of the bimolecular quenching process is presented. The equations derived allow the extraction of two polarization enhancement factors, V d for the pure d-type and V pd for the combined p-and d-type triplet mechanism from the concentration dependence of the time dependent CIDEP signals. The CIDEP curves and the previously observed magnetic ®eld and heavy atom e ects on the free radical yield can be quantitatively simulated with a consistent set of kinetic parameters.

Journal of the American Chemical Society Jacs, Feb 1, 2006

Photoinduced, proton-coupled electron transfer (ET) between 9,10-anthraquinone-2,6-disulfonate (A... more Photoinduced, proton-coupled electron transfer (ET) between 9,10-anthraquinone-2,6-disulfonate (ADQS) and an amino acid residue of tryptophan in human serum albumin (HSA) was observed using time-resolved electron paramagnetic resonance (TREPR). The ET reaction reduces the protein binding affinity of the ligand. TREPR chemically induced dynamic electron polarization (CIDEP) spectra establish that photoinduced ET takes place from the tryptophan residue (W214) to the excited triplet state of AQDS2- while bound in subdomain IIA, a protein cleft of HSA. The TREPR CIDEP signals also reveal that the anion radical of the ligand escapes toward the bulk water region by a one-dimensional translation diffusion process within the protein's pocket area. This pilot study of HSA demonstrates how TREPR CIDEP can provide significant means to investigate dynamic characteristics of protein-surface reactions.

The journal of physical chemistry. B, Jan 19, 2016

To elucidate how local molecular conformations play a role on electronic couplings for the long-r... more To elucidate how local molecular conformations play a role on electronic couplings for the long-range photoinduced charge-separated (CS) states in protein systems, we have analyzed time-resolved electron paramagnetic resonance (TREPR) spectra by polarized laser irradiations of 9,10-anthraquinone-1-sulfonate (AQ1S(-)) bound to human serum albumin (HSA). Analyses of the magnetophotoselection effects on the EPR spectra and a docking simulation clarified the molecular geometry and the electronic coupling of the long-range CS states of AQ1S(•2-)-tryptophan214 radical cation (W214(•+)) separated by 1.2 nm. The ligand of AQ1S(-) has been demonstrated to be bound to the drug site I in HSA. Molecular conformations of the binding region were estimated by the docking simulations, indicating that an arginine218 (R218(+)) residue bound to AQ1S(•2-) mediates the long-range electron-transfer. The energetics of triad states of AQ1S(•2-)-R218(+)-W214(•+) and AQ1S(-)-R218(•)-W214(•+) have been comput...

The Journal of Physical Chemistry, 1994

The concentration dependence of free radicals (TEMPO) of enhanced absorptive chemically induced d... more The concentration dependence of free radicals (TEMPO) of enhanced absorptive chemically induced dynamic electron polarization (CIDEP) spectra was studied quantitatively to clarify the mechanism of CIDEP generation in excited singlet stateradical systems. It was experimentally demonstrated that the CIDEP intensity on the free radical was in proportion to the concentration of the spin-polarized free radical. The rate constant of the CIDEP generation was determined to be (6.3 f 0.2) X lo9 M-l s-l from the Stern-Volmer plots of CIDEP intensity in the coronene-TEMPO system in benzene. The absorptive CIDEP spectra were attributed to the free radicals which enhanced the SI-Tl intersystem crossing and were interpreted by introducing the stochastic-Liouville equation into the radical-triplet pair mechanism with doublet precursor.

Journal of the American Chemical Society, Jan 15, 2016

To shed a light on fundamental molecular functions of photoinduced charge conductions by organic ... more To shed a light on fundamental molecular functions of photoinduced charge conductions by organic photovoltaic materials, it is important to directly observe molecular geometries of the intermediate charges just after the photoinduced electron-transfer reactions. However, highly inhomogeneous molecular environments at the bulk heteojunction interfaces in the photoactive layers have prevented us from understanding the mechanism of the charge conductions. We have herein investigated orbital geometries, electronic couplings and hole-dissociation dynamics of photoinduced charge-separated (CS) states in a series of poly(3-hexylthiophene)-fullerene linked dyads bridged by rigid oligo-p-phenylene spacers by using time resolved EPR spectroscopy. It has been revealed that one-dimensional intramolecular hole-dissociations exothermically take place from localized holes in initial CS states, following bridge-mediated, photoinduced charge-separations via triplet exciton diffusions in the conjugat...

Mol Phys, 2002

The spin dynamics of the duroquinone anion radical (DQ · ¡ ) generated by photoinduced electron t... more The spin dynamics of the duroquinone anion radical (DQ · ¡ ) generated by photoinduced electron transfer reactions from triplet eosin Y ( 3 EY 2¡ ) to DQ have been studied by using transient absorption and pulsed EPR spectroscopy. Unusual net-absorptive electron spin polarization plus net-emissive polarization were observed, suggesting the production of the triplet exciplex or contact radical pair as the reaction intermediate. The kinetic parameters and intrinsic enhancement factors of the electron spin polarization were determined in various alcoholic solvents. The net-absorptive electron spin polarization was also observed in ethanol± water mixed solvents. The solvent e ects on the radical yield are analysed on the basis of a stochastic Liouville equation established for the magnetic ®eld e ects on the radical yield. The zero-®eld splitting constants of the triplet exciplex are estimated from the solvent viscosity dependence of the enhancement factors due to spin±orbit coupling induced depopulation of the reaction intermediate.

Journal of the American Chemical Society, Aug 17, 2005

Melanin, a ubiquitous, heterogeneous biological polymer composed of many different monomers, cont... more Melanin, a ubiquitous, heterogeneous biological polymer composed of many different monomers, contains a population of stationary, intrinsic semiquinone-like radicals. Additional extrinsic semiquinone-like radicals are reversibly photogenerated with visible or UV irradiation. The free radical chemistry of melanin is complex and not well characterized, especially the photochemistry of melanin in the presence of oxygen. To determine directly how melanin reacts in the presence of oxygen, time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used to examine melanin free radical chemistry in human retinal pigment epithelium (RPE) cells under aerobic and anaerobic conditions. A TREPR difference spectrum was used to explore the nature of melanin chemistry in the presence of oxygen. The position and symmetrical line shape of the TREPR three-dimensional difference spectrum shows that when reactive oxygen species (ROS) are scavenged, only one of the two or more chemically different melanin free radical species participates in ROS scavenging. This protective melanin radical species exists in both the extrinsic and intrinsic populations of melanin free radicals, allowing melanin to protect the RPE from toxic species in both the light and dark.

Angewandte Chemie (International ed. in English), Jan 26, 2015

The unprecedented dependence of final charge separation efficiency as a function of donor-accepto... more The unprecedented dependence of final charge separation efficiency as a function of donor-acceptor interaction in covalently-linked molecules with a rectilinear rigid oligo-p-xylene bridge has been observed. Optimization of the donor-acceptor electronic coupling remarkably inhibits the undesirable rapid decay of the singlet charge-separated state to the ground state, yielding the final long-lived, triplet charge-separated state with circa 100 % efficiency. This finding is extremely useful for the rational design of artificial photosynthesis and organic photovoltaic cells toward efficient solar energy conversion.

The Journal of Physical Chemistry a, 2004

ABSTRACT

The Journal of Physical Chemistry Letters, 2015

Recent progress is overviewed on experimental elucidations of fundamental molecular functions of ... more Recent progress is overviewed on experimental elucidations of fundamental molecular functions of the light-energy conversions by the photoactive layers of the organic photovoltalic (OPV) cells by means of the time-resolved electron paramagnetic resonance spectroscopy. Positions and orientations of the unpaired electrons and electronic coupling matrix elements are clarified in photoinduced, primary charge-separated (CS) states. Connections between the molecular geometries and the electronic couplings have been characterized for the initial CS states to elucidate how the structure, orbital delocalization, and molecular libration play roles on exothermic carrier dissociation via a vibrationally relaxed charge-transfer complex with prevention of the energy-wasting charge recombination. Superior functions to biological molecules are presented for the efficient photocurrent generations induced by orbital delocalization and by shallow trap depths at polymer-stacking domains. The above structural and electronic characteristics of the primary electron-hole pairs are essential to evaluations, designs, and developments of the efficient solar cells using organic molecules.

The Journal of Physical Chemistry, 1994

The concentration dependence of free radicals (TEMPO) of enhanced absorptive chemically induced d... more The concentration dependence of free radicals (TEMPO) of enhanced absorptive chemically induced dynamic electron polarization (CIDEP) spectra was studied quantitatively to clarify the mechanism of CIDEP generation in excited singlet stateradical systems. It was experimentally demonstrated that the CIDEP intensity on the free radical was in proportion to the concentration of the spin-polarized free radical. The rate constant of the CIDEP generation was determined to be (6.3 f 0.2) X lo9 M-l s-l from the Stern-Volmer plots of CIDEP intensity in the coronene-TEMPO system in benzene. The absorptive CIDEP spectra were attributed to the free radicals which enhanced the SI-Tl intersystem crossing and were interpreted by introducing the stochastic-Liouville equation into the radical-triplet pair mechanism with doublet precursor.