Biomedical subjects

Sachiko Tojo

Publications and source records attributed to Sachiko Tojo.

At least 19 recordsLinked to original sources

PubMed2006

S-S bond mesolysis in alpha,alpha'-dinaphthyl disulfide radical anion generated during gamma-radiolysis and pulse radiolysis in organic solution.

A dissociation mechanism of the S-S bond in the alpha,alpha'-dinaphthyl disulfide radical anion (NpSSNp*-) in organic solution was investigated on the basis of transient absorption measurements and DFT calculations. NpSSNp*- generated during gamma-radiolysis of NpSSNp in MTHF at 77 K showed the absorption band at 430 nm, which shifted to 560 nm with an increase of the ambient temperature up to room temperature. With the aid of DFT calculations at the B3LYP/6-31G(d) level, the shift of the absorption band was interpreted in terms of molecular conformational changes of NpSSNp*- due to the elongation of the S-S bond. It was observed that NpSSNp*- dissociates into naphthylthiyl radical and thionaphtholate anion in organic solution with a first-order rate constant in the magnitude of 10(6) s-1. From Arrhenius plots of the decay rate constants of NpSSNp*- in a temperature range of 160-293 K, an activation energy for the S-S bond cleavage in NpSSNp*- in solution was determined along with a frequency factor. Based on the state energies of NpSSNp*- calculated at the B3LYP/6-31G(d) level, a Morse-like energy potential for the S-S bond cleavage of NpSSNp*- is depicted as a function of the S-S bond distance.

Anions↗

PubMed2006

Emission from charge recombination during the pulse radiolysis of 9-cyano-10-phenylethynylanthracenes with donor and acceptor substituents.

Emission from 9-cyano-10-phenylanthracene and 9-cyano-10-phenylethynylanthracenes having donor and acceptor substituents (RA = PA, PEA, OEA, NEA, and DEA) was studied with the time-resolved fluorescence measurement during the pulse radiolysis of RAs in benzene (Bz). PA and DEA showed only monomer emission, while other RAs (PEA, OEA, and NEA) showed both monomer and excimer emissions with much lower intensities. On the basis of the steady-state and transient absorption and emission measurements, the formation of RA in the singlet excited state ((1)RA*) can be attributed to the charge recombination between RA radical cation and anion (RA*+ and RA*-, respectively) which are initially generated from the radiolytic reaction in Bz. It is expected that for PA with a twisted geometry, the charge recombination between PA*+ and PA*- occurs to give (1)PA* during the pulse radiolysis in Bz. For PEA and OEA, pi-stacking interaction is possible for the formation of an encounter complex during the charge recombination between RA*+ and RA*-. For NEA, it is expected that NEA*+ and NEA*- collide neck-to-neck to generate the excimer due to the twisted geometry. For DEA, a considerably twisted structure is assumed to give (1)DEA* with strong ICT character but not (1)(DEA)2* because of the bulky donor substituent.

Journal Article↗

PubMed2006

Reactivity of triarylphosphine peroxyl radical cations generated through the reaction of triarylphosphine radical cations with oxygen.

One-electron oxidation of triarylphosphines (Ar3P, Ar = phenyl and substituted phenyl) in benzonitrile (PhCN) has been studied using pulse radiolysis technique. One-electron oxidation of Ar3P occurred to yield the radical cation (Ar3P*+) which showed an intense absorption with a peak at 360-370 nm together with a broad band at 500-600 nm. The addition of molecular oxygen (O2) to the phosphorus atom of Ar3P*+ took place at the second-order rate constant of 10(7)-10(9) dm(3) mol(-1) s(-1) to yield the peroxyl triarylphosphinyl radical cation (Ar3P+OO*). It is found that the electron-releasing substituents on the para position of the phenyl ring of Ar3P influence the rate constants of the reaction of Ar3P*+ with O2 and that o-methyl substituents on the phenyl ring influence the reactivity of Ar3P+OO*.

Journal Article↗

PubMed2006

One-electron oxidation pathways during beta-cyclodextrin-modified TiO2 photocatalytic reactions.

The photocatalytic one-electron oxidation reactions of aromatic sulfides using the carboxymethyl-beta-cyclodextrin (CM-beta-CD)-modified TiO(2) nanoparticles (TiO(2)/CM-beta-CD) were investigated by using nano- and femtosecond transient absorption spectroscopies. The one-electron oxidation processes of the substrate (S) by the valence band hole (h(VB) (+)) at the TiO(2) surface and the trapped hole at the adsorption site of the CM-beta-CD (h(CD) (+)) were examined. The transient absorption spectra and time traces observed for the charge carriers and the radical cation of S (S(.+)) revealed that the one-electron oxidation reaction of S during the nano- and femtosecond laser flash photolyses of TiO(2)/CM-beta-CD is significantly enhanced relative to bare TiO(2). The kinetics of the decay and the dimerization processes between S(.+)s are discussed on the basis of the results obtained by the pulse radiolysis technique.

Journal Article↗

PubMed2006

Photodissociation of naphthalene dimer radical cation during the two-color two-laser flash photolysis and pulse radiolysis-laser flash photolysis.

Photodissociation of naphthalene (Np) dimer radical cation (Np2*+) to give naphthalene radical cation (Np*+) and Np and the subsequent regeneration of Np2*+ by the dimerization of Np*+ and Np were directly observed during the two-color two-laser flash photolysis in solution at room temperature. When Np2*+ was excited at the charge-resonance (CR) band with the 1064-nm laser, the bleaching and recovery of the transient absorption at 570 and 1000 nm, assigned to the local excitation (LE) and CR bands of Np2*+, respectively, were observed together with the growth and decay of the transient absorption at 685 nm, assigned to Np*+. The dissociation of Np2*+ proceeds via a one-photon process within the 5-ns laser flash to give Np*+ and Np in the quantum yield of 3.2 x 10(-3) and in the chemical yield of 100%. The recovery time profiles of Np2*+ at 570 and 1000 nm were equivalent to the decay time profile of Np*+ at 685 nm, suggesting that the dimerization of Np*+ and Np occurs to regenerate Np2*+ in 100% yield. Similar experimental results of the photodissociation and regeneration of Np2*+ were observed during the pulse radiolysis-laser flash photolysis of Np in 1,2-dichloroethane. The photodissociation mechanism can be explained based on the crossing between two potential surfaces of the excited-state Np2*+ and ground-state Np*+.

Journal Article↗

PubMed2006

Photoinduced charge separation in titania nanotubes.

The photocatalytic one-electron oxidation reaction of an aromatic compound during UV light irradiation of titania nanotubes and nanoparticles was investigated using time-resolved diffuse reflectance spectroscopy. Remarkably long-lived radical cations of the aromatic compound and trapped electrons were observed for the nanotubes when compared to those for nanoparticles. The influences of the morphology on the one-electron oxidation process of an aromatic compound adsorbed on the surface were discussed in terms of the charge recombination dynamics between the radical cation and electrons in TiO2.

Catalysis↗

PubMed2006

Temperature-driven oxygenation rate control by polymeric photosensitizer.

A polymeric photosensitizer, poly(NIPAM-co-BP), consisting of N-isopropylacrylamide (NIPAM) and benzophenone (BP) units, demonstrates a temperature-controlled oxygenation activity in water. The system promotes a heat-induced oxygenation enhancement at <17 degrees C and suppression at >22 degrees C. This unprecedented photo-oxygenation activity is triggered by a heat-induced phase transition of the polymer from coil to micelle, and then to globule state, cleverly controlling the stability and diffusion of singlet oxygen and the location of substrate.

Acrylamides↗

PubMed2006

Visible light-induced degradation of ethylene glycol on nitrogen-doped TiO2 powders.

The photocatalytic degradation processes of ethylene glycol (EG) during the UV or visible light irradiation of pure anatase and nitrogen (N)-doped TiO2 powders (TiO(2-x)N(x), x = 0, 0.002, 0.003, and 0.007) were investigated using time-resolved diffuse reflectance (TDR) and solid-state NMR spectroscopies. The TDR spectra and time traces observed for the charge carriers indicated that the scavenging of photogenerated holes (h+) by EG occurred during the 355-nm laser photolysis of the N-doped TiO2 powders, while no direct oxidation reaction of EG by h+ occurred during the 460-nm laser photolysis, although the charge carriers were sufficiently generated upon excitation. The solid-state magic-angle spinning (MAS) NMR measurements revealed that EG is preferentially chemisorbed on the surface of the N-doped TiO2 powders, in contrast to the pure TiO2, and degrades under visible light irradiation.

Journal Article↗

PubMed2006

Emission from charge recombination during the pulse radiolysis of arylethynylpyrenes.

Emission from several 1-(arylethynyl)pyrenes with a substituent on the aryl group (REPy, R = phenyl (PEPy), 4-dimethylaminophenyl (NPEPy), 4-isopropoxyphenyl (OPEPy), 2-quinonyl (QEPy), and 9-(10-cyanoanthracenyl) (AEPy)) was studied with time-resolved fluorescence measurements during pulse radiolysis in benzene. NPEPy and AEPy showed only monomer emission, while PEPy, OPEPy, and QEPy showed both monomer and excimer emissions during pulse radiolysis. In addition, REPy's also showed long-lived emissions with very weak intensities in the absence of oxygen, which were assigned to the "P-type" delayed fluorescence derived from the triplet-triplet annihilation. The formation of REPy's in the singlet excited state (1REPy*) can be interpreted as the charge recombination between the REPy radical cation and anion (REPy*+ and REPy*-, respectively), which are initially generated from the radiolytic reaction in benzene. Both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PEPy are localized on the 1-pyrenyl (Py) moiety, while the HOMO of REPy's with an electron donating or withdrawing substituent on the benzene ring (R(D)EPy such as NPEPy and OPEPy or R(A)EPy such as QEPy and AEPy) is mainly localized on the donor moieties (R(D) or Py) and the LUMO on the acceptor ones (Py or R(A), respectively). Therefore, it is suggested that the one-electron oxidation and reduction of REPy's can occur from the donor and acceptor moieties, respectively. This scheme reasonably explains the relationship between the annihilation enthalpy changes (-Delta H' degrees) for the charge recombination of REPy*+ and REPy*- and the singlet excitation energies (E'(S1) of the REPy's. The results are compared with those in electrogenerated chemiluminescence.

Journal Article↗

PubMed2006

Effects of substituents on aryl groups during the reaction of triarylphosphine radical cation and oxygen.

In a previous report (S. Yasui, S. Tojo and T. Majima, J. Org. Chem., 2005, 70, 1276), we presented the results from the laser flash photolysis (LFP) and product analysis of the 9,10-dicyanoanthracene (DCA)-photosensitized oxidation of triarylphosphine (Ar(3)P) in acetonitrile under air, which showed that the photoreaction results in the oxidation of Ar(3)P to give the corresponding phosphine oxide (Ar(3)P=O) in a nearly quantitative yield, and that the reaction is initiated by the electron transfer (ET) from Ar(3)P to DCA in the singlet excited state ((1)DCA*), producing the triarylphosphine radical cation Ar(3)P (+). This radical cation decays through radical coupling with O(2) to afford the peroxy radical cation Ar(3)P(+)-O-O*, which we proposed to be the intermediate leading to the product Ar(3)P=O. We now examined this photoreaction in more detail using ten kinds of Ar(3)P with various electronic and steric characteristics. The decay rate of Ar(3)P*(+) measured by the LFP was only slightly affected by the substituents on the aryl groups of Ar(3)P. During the photolysis of trimesitylphosphine (Mes(3)P), the peroxy radical cation intermediate (Mes(3)P(+)-O-O*) had a lifetime long enough to be spectrophotometrically detected. The quantum yields of Ar(3)P=O increased with either electron-withdrawing or -releasing substituents on the aryl groups, suggesting that a radical center is developed on the phosphorus atom during the step when the quantum yield is determined. In addition, the o-methyl substituents in Ar(3)P decreased the quantum yield. These results clearly indicated that Ar(3)P(+)-O-O* undergoes radical attack upon the parent phosphine Ar(3)P that eventually produces the final product, Ar(3)P=O.

Journal Article↗

PubMed2006

Transannular distance dependence of stabilization energy of the intramolecular dimer radical cation of cyclophanes.

The intramolecular dimer radical cation and charge-transfer complex of various cyclophanes were investigated by using pulse radiolysis measurements. The charge resonance band due to the dimer radical cation of cyclophanes appeared in the near-IR region, which showed a blue-shift as the distance between the two benzene rings of cyclophane decreased. The stabilization energy of the dimer radical cation, which was estimated from the peak position of the charge resonance band, was explained by the exchange interaction, while the substituent effect was small. The absorption peak of the charge-transfer complex with chlorine atom also showed the shift in accordance with the oxidation potential of cyclophanes.

Cations↗

PubMed2006

One-electron redox processes during polyoxometalate-mediated photocatalytic reactions of TiO(2) studied by two-color two-laser flash photolysis.

The one-electron redox processes of several compounds during polyoxometalate (POM)-mediated photocatalytic reactions of TiO(2) were investigated using the two-color two-laser flash-photolysis technique. The efficiency of the one-electron oxidation of aromatic sulfides by the trapped hole (h(tr) (+)) or the surface-bound OH radical (OH(s) (.)) is found to be significantly enhanced due to electron transfer from the conduction band (CB) of TiO(2) to the POM. The efficiency of the electron transfer from the CB of TiO(2) to the POM decreases in the order H(2)W(12)O(40) (6-) < SiW(12)O(40) (4-) < PW(12)O(40) (3-), that is, it depends on the reduction potential (E(red)) of the POMs. Electron injection from PW(12)O(40) (4-) in the excited state (PW(12)O(40) (4-*)) to the CB of TiO(2) was clearly observed using the two-color two-laser flash-photolysis technique. Storage of electrons in the TiO(2)/PW(12)O(40) (3-)/methyl viologen (MV(2+)) ternary system was also achieved upon two-color two-laser irradiation.

Catalysis↗

PubMed2006

Probing the surface adsorption and photocatalytic degradation of catechols on TiO2 by solid-state NMR spectroscopy.

The local structure of the TiO2 surface modified with electron-donating bidentate ligands, such as catechols, has been investigated by solid-state NMR spectroscopy. The adsorption and degradation processes of catechols at the TiO2 surface were observed. The photocatalytic degradation mechanism of catechols at the TiO2 surface was interpreted in terms of the interfacial charge recombination reaction with conduction band electrons.

Adsorption↗

PubMed2005

Ultrafast photoinduced intramolecular charge separation and recombination processes in the oligothiophene-substituted benzene dyads with an amide spacer.

Photoinduced intramolecular charge separation (CS) and recombination (CR) processes of the tetrathiophene-substituted benzene dyads with an amide spacer (4T-PhR, R = 4-H (1), 4-CN (2), 3,4-(CN)2 (3), 4-NO2 (4), 3,5-(NO2)2 (5)) in solvents of different polarities were investigated using various fast spectroscopies. It was revealed that the CS rates depend on the ability of the acceptor and solvent polarity. Ultrafast CS with the rate of 5 x 10(12) s(-1) was revealed for 5 in PhCN and MeCN. The ultrafast CS can be attributed to the large electronic coupling matrix element between the donor and the acceptor despite the relative long donor-acceptor distance. The existence of the state with large electron density on the spacer between 14T*-PhR and LUMO should facilitate the CS process in the present dyad system. It was also revealed that the CR rates in these dyads were rather fast because of the enhanced superexchange interaction through the amide spacer.

Journal Article↗

PubMed2005

Synthesis and properties of terthiophene-modified oligodeoxynucleotides.

Synthesis and properties of oligodeoxynucleotides (ODNs) containing terthiophene (Thp) were described. One-electron oxidation of Thp-modified ODN resulted in the formation of Thp radical cation (Thp(.+)), which remained stable in the experimental time window up to 200mus, showing that charge may be carried along DNA by Thp as Thp(.+).

Base Sequence↗

PubMed2005

Exclusion of aromatic radical cations from cyclodextrin nanocavity studied by pulse radiolysis.

The influence of hydroxypropyl-beta-cyclodextrin (HP-beta-CD) on the one-electron oxidation reaction of aromatic sulfides (S) with Br2*- and the decay process of the S radical cation (S*+) was investigated by pulse radiolysis. The dissociation kinetics of S*+ from the CD cavity was examined in terms of the apparent equilibrium constants (Kapp) for the formation and decay processes of S*+. Inhibition of the one-electron oxidation reaction of S by Br2*- was clearly observed in the presence of HP-beta-CD. On the basis of a comparison between the determined Kapp values, it was found that the binding ability of S*+ with HP-beta-CD is much lower than that of S, because of the hydrophobic nature of the cavity. The formation process of the dimer radical cation of 4-(methylthio)phenylmethanol ((MTPM)2*+), which is generated between MTPM(*+) and neutral MTPM in solution, was also inhibited by the addition of HP-beta-CD.

Cations↗

PubMed2005

Efficient emission from charge recombination during the pulse radiolysis of electrochemical luminescent donor-acceptor molecules with an ethynyl linkage.

Efficient monomer and excimer emission from various donor-acceptor substituted phenylethynes (PE), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PE in the singlet excited state (1PE*) and the excimer (1PE2*) can be interpreted by the charge recombination between the PE radical cation (PE.+) and the PE radical anion (PE.-) which are generated initially from the radiolytic reaction in benzene. It is suggested that the positive and negative charges are localized on the donor and acceptor moieties in the radical cation and anion, respectively. This mechanism is reasonably explained by the relationship between the annihilation enthalpy changes (-DeltaH' degrees ) and singlet excitation energies of donor-substituted phenyl(9-acridinyl)ethynes (1(a-e)). In addition to the monomer emission, the compounds bearing weak donors (1(a-d)) show the excimer emission due to a very small twist angle between the donor and acceptor moieties. For the phenyl(9-cyano-10-anthracenyl)ethynes (2(c) and 2(f)), although they also show the monomer and excimer emissions, it cannot be explained by the relationship between -DeltaH' degrees values and their singlet excitation energies, suggesting the formation of the ICT state and H-type excimer in which two 9-cyano-10-anthracenyl moieties are stacked face-to-face with donor bearing a benzene ring projecting perpendicularly away from each other through the charge recombination between 2.+) and 2.-) and/or triplet-triplet annihilation.

Journal Article↗

PubMed2005

Efficient emission from charge recombination during the pulse radiolysis of electrochemical luminescent substituted quinolines with donor-acceptor character.

Efficient emission from various donor-acceptor quinolines with an ethynyl linkage (PnQ), which are known as efficient electrogenerated chemiluminescent molecules, was observed with time-resolved fluorescence measurement during the pulse radiolysis in benzene. On the basis of the transient absorption and emission measurements, and steady-state measurements, the formation of PnQ in the singlet excited state can be interpreted by charge recombination between the PnQ radical cation and the PnQ radical anion which are generated initially from the radiolytic reaction in benzene. The strong electronic coupling between the donor and acceptor through conjugation is responsible for the efficient emission during the pulse radiolysis of PnQ in benzene. It is suggested that the positive and negative charges are localized on the donor and acceptor moieties in the radical cation and anion, respectively. This mechanism is reasonably explained by the relationship between the annihilation enthalpy changes and singlet excitation energies of PnQ. The formation of the intramolecular charge transfer state is assumed for PnQ in the singlet excited state with a strong electron donating substituent. The emission from PnQ is suggested to originate from PnQ in the singlet excited state formed from the charge recombination between the PnQ radical cation and the PnQ radical anion during the pulse radiolysis. This is strong evidence for the efficient electrogenerated chemiluminescence of PnQ.

Journal Article↗