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Kimio Akiyama

Publications and source records attributed to Kimio Akiyama.

14 recordsLinked to original sources
PubMed2005

Divergent oxidative rearrangements in solution and in a zeolite: distal vs proximal bond cleavage of methylenecyclopropanes.

Irradiation of 9,10-dicyanoanthracene (DCA) or p-chloranil in the presence of E-1-benzylidene-2-phenylcyclopropane (E-5) in CH(2)Cl(2) causes E-5 to undergo methylenecyclopropane rearrangement. An adduct, Z-7, between DCA and 5 firmly supports the involvement of a bifunctional trimethylenemethane radical cation. In contrast, incorporation of E-5 into HZSM-5 produces trans,trans-1,4-diphenyl-1,3-butadiene radical cation sequestered in the HZSM-5 interior, tt-8(.+)@HZSM-5, identified by ESR and diffuse reflectance spectroscopy. In addition, low yields of tt-8, its cis,trans-isomer (ct-8), and 1-phenyl-1,2-dihydronaphthalene (9) were isolated from the supernatant solution. The sharp contrast between the photoinduced electron-transfer reaction with photosensitizers in solution and the spontaneous reaction with redox-active acidic zeolite offers the prospect of further zeolite-induced regiodivergent reactions in a range of additional substrates.

Crystallography, X-Ray↗
PubMed2005

ESR studies on DNA cleavage induced by enediyne C-1027 chromophore.

C-1027 belongs to the family of chromoprotein antitumor antibiotics, which contain a carrier apoprotein and a highly unstable enediyne chromophore. The enediyne spontaneously aromatizes to generate p-benzyne biradical, and subsequently abstracts hydrogens from the DNA sugar backbone, resulting in cleavage of the double strand. Using spin-trapping methods, we obtained direct proof of radical intermediates during an DNA cleavage, and found intriguing difference in behavior between the trapping agents 2-methyl-2-nitrosopropane (MNP) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO): MNP added to the sugar radicals of the DNA, whereas DMPO directly trapped a phenyl radical or p-benzyne biradical derived from the C-1027 chromophore.

Aminoglycosides↗
PubMed2005

Primary charge-recombination in an artificial photosynthetic reaction center.

Photoinduced primary charge-separation and charge-recombination are characterized by a combination of time-resolved optical and EPR measurements of a fullerene-porphyrin-linked triad that undergoes fast, stepwise charge-separation processes. The electronic coupling for the energy-wasting charge recombination is evaluated from the singlet-triplet electronic energy gap in the short-lived, primary charge-separated state. The electronic coupling is found to be smaller by approximately 40% than that for the primary charge-separation. This inhibition of the electronic interaction for the charge-recombination to excited triplet state largely results from a symmetry-broken electronic structure modulated by configuration interaction between 3(b1u,b3g) and 3(au, b3g) electronic states of the free-base porphyrin.

Electrons↗
PubMed2005

Carrier generation process on photoconductive polymer films as studied by magnetic field effects on the charge-transfer fluorescence and photocurrent.

We have studied the magnetic field effects (MFEs) on the charge-transfer fluorescence and transient photocurrent of a 1,2,4,5-tetracyanobenzene-doped poly(N-vinylcarbazole) film, which reflect the recombination and escape yields of the carriers, respectively. The recombination yield dependence of the external magnetic field (B) clearly shows two types of the MFEs, growth with increasing B due to the hyperfine mechanism (HFM) and a negative dip due to the level-crossing mechanism (LCM). On the other hand, the escape yield indicates complementary MFEs with a sharp decrease in yield with increasing B and then a positive dip. Simultaneous observation of the HFM- and LCM-MFEs proves the stepwise hole-hopping mechanism rather the long-range hole-jumping one. The quantitative analysis of the recombination and escape MFEs is performed using the stochastic Liouville equations (SLE) for a one-dimensional lattice model in which the stepwise hole hops take place between the nearest neighbor carbazole units with spin conservation. The SLE analysis provides the recombination and hole transfer rate constants of 7.0 x 10(7) and 4.5 x 10(8) s(-1), respectively. The boundary site number for the ion pairs in the one-dimensional model is estimated by the best fit to the experimental results. The interionic distance of the boundary ion pair in the one-dimensional model including eight sites agrees with the thermalization distance in the Onsager model. Hence, it is concluded that the elementary processes in the Onsager model applied to molecular amorphous solids are the stepwise hole hops rather than a long-range hole jump.

Fluorescence↗
PubMed2005

Photoinduced omega-bond dissociation in the higher excited singlet (S2) and lowest triplet (T1) states of a benzophenone derivative in solution.

Photochemical properties of photoinduced omega-bond dissociation in p-benzoylbenzyl phenyl sulfide (BBPS) in solution were investigated by time-resolved EPR and laser flash photolysis techniques. BBPS was shown to undergo photoinduced omega-bond cleavage to yield the p-benzoylbenzyl radical (BBR) and phenyl thiyl radical (PTR) at room temperature. The quantum yield (phi(rad)) for the radical formation was found to depend on the excitation wavelength, i.e., on the excitation to the excited singlet states, S2 and S1 of BBPS; phi(rad)(S2) = 0.65 and phi(rad)(S1) = 1.0. Based on the CIDEP data, these radicals were found to be produced via the triplet state independent of excitation wavelength. By using triplet sensitization of xanthone, the efficiency (alpha(rad)) of the C-S bond fission in the lowest triplet state (T1) of BBPS was determined to be unity. The agreement between phi(rad)(S1) and alpha(rad) values indicates that the C-S bond dissociation occurs in the T1 state via the S1 state due to a fast intersystem crossing from the S1 to the T1 state. In contrast, the wavelength dependence of the radical yields was interpreted in terms of the C-S bond cleavage in the S2 state competing with internal conversion from the S2 to the S1 state. The smaller value of phi(rad)(S2) than that of phi(rad)(S1) was proposed to originate from the geminate recombination of singlet radical pairs produced by the bond dissociation via the S2 state. Considering the electronic character of the excited and dissociative states in BBPS showed a schematic energy diagram for the omega-bond dissociation of BBPS.

Benzophenones↗
PubMed2005

Spin dynamic study on the electric field dependence of carrier generation.

Using an acceptor-doped poly(N-vinylcarbazole) film, the magnetic field effect (MFE) on the generation efficiency of photoinduced charge was measured under various electric fields in order to clarify how the applied electric field affects the elementary processes in the photocarrier generation in photoconductive polymeric molecular solids. The external magnetic field influenced the electron spin dynamics among the geminate electron-hole pairs within a scale of a few nanometers and decreased the photocarrier generation efficiency. The observed MFE due to a hyperfine mechanism was almost independent of the electric field. By employing the stochastic Liouville equations based on a one-dimensional lattice model, we performed some model calculations for the dissociation, hopping, and recombination rate dependence of MFE on the generation efficiency. From a comparison between the observed and calculated MFE, it was concluded that the electric field affects the dissociation more than the hopping and the recombination. This coincides with the concepts in the Onsager model that is used to analyze the electric field dependence of carrier generation efficiency so far. The one-dimensional lattice model is a proper model for the carrier generation in polymeric molecular solids, which is qualitatively consistent with the Onsager model except for the long-range hole jump.

Journal Article↗
PubMed2004

Rearrangement of a 2-methylenecyclobutanone derivative triggered by photoinduced electron transfer: an unprecedented oxa analogue of the tetramethyleneethane radical cation.

On irradiation of p-chloranil (CA), 2,2-bis(p-methoxyphenyl)-3,3-dimethyl-4-methylenecyclobutanone (1) gives 2,2-bis(p-methoxyphenyl)-4-isopropylidenecyclobutanone (2), whereas 2 affords a CA adduct (4), suggesting that a novel rearrangement of 1 to give 2 occurs irreversibly via intermediate 3*+, a radical cation variant of an unprecedented oxa analogue of tetramethyleneethane.

Journal Article↗
PubMed2003

Symmetry switching of the fluorescent excited state in alpha,omega-diphenylpolyynes.

By using MO calculations based on DFT, absorption, and fluorescence spectroscopy, we have comprehensively studied the low-lying excited singlet states of alpha,omega-diphenylpolyynes (DPY) having 1-6 triple bonds. The a(g) vibrational modes of the C(triple bond)C stretching and of the phenyl ring motion were observed in the fluorescence spectra of diphenylacetylene and 1,4-diphenylbutadiyne. On the other hand, in the fluorescence spectra of the long DPY with the triple-bond number (N) more than two, the phenyl ring motion with a(g) symmetry disappeared and the b(1g) modes of the phenyl ring twisting (approximately 400 cm(-1)) and of the C-H bending (approximately 900 cm(-1)) were detected. The observed fluorescent states of DPY with N < or = 2 and N > or = 3 are assigned to the 1(1)B(1u) (pi(x)pi(x*)) and 1(1)A(u) (pi(x)pi(y*) and/or pi(y)pi(x*)) states, respectively, based on the vibronic structures, the relatively short lifetimes, and the solvatochromic shifts of the fluorescence spectra. Not only the allowed transition of 1(1)B(1u) <-- S(0) but also the forbidden transition of 1(1)A(u) <-- S(0) was detected in the fluorescence excitation spectra of the long DPY with N > or = 3. The low-lying excited state with A(u) symmetry is characteristic in polyyne, which does not exist in polyene. The oscillator strength (f) of the first absorption band in DPY decreases with an increase in N, which is the opposite behavior of the all-trans-alpha,omega-diphenylpolyenes. The N-dependence of the f value is understood by the configuration interaction between the 1(1)B(1u) and 2(1)B(1u) (pi(y)pi(y*)) states, which is consistent with the reduction of the nonlinear optical response of polyyne.

Journal Article↗
PubMed2003

Spectroscopic and calorimetric studies on the mechanism of methylenecyclopropane rearrangements triggered by photoinduced electron transfer.

2-(dideuteriomethylene)-1,1-bis(4-methoxyphenyl)cyclopropane (d(2)-1) undergoes degenerate rearrangement in both singlet- and triplet-sensitized electron-transfer photoreactions. Nanosecond time-resolved absorption spectroscopy on laser flash photolysis of the unlabeled 1 with 9,10-dicyanoanthracene, 1,2,4,5-tetracyanobenzene, or N-methylquinolinium tetrafluoroborate as an electron-accepting photosensitizer gives rise to two transients with lambda(max) at 500 and 350 nm assigned to the dianisyl-substituted largely twisted trimethylenemethane cation radical (6.+) and the corresponding diradical (6..), respectively. These intermediates are also detected, respectively, by steady state and nanosecond time-resolved EPR with chloranil or anthraquinone as a sensitizer. The degenerate rearrangement of d(2)-1 thus proceeds via these two different types of intermediates in a cation radical cleavage-diradical cyclization mechanism. Energetics based on nanosecond time-resolved photoacoustic calorimetry support this mechanism. A comparison of the reactivities and the spectroscopic results of 1, 1,1-bis(4-methoxyphenyl)-2-methylenespiro[2.2]pentane (2), and 1-cyclopropylidene-2,2-bis(4-methoxyphenyl)cyclopropane (3) suggest that the reversible methylenecyclopropane rearrangement between 2 and 3 proceeds via a similar mechanism.

Journal Article↗
PubMed2003

Bis(tri-tert-butylsilyl)silylene: triplet ground state silylene.

Upon irradiation with lambda = 254 nm light over the temperature range of 9-80 K, methylcyclohexane glass matrixes of 1a and 1b gave a characteristic broad EPR signal at 845 mT (X-band, 9.4 GHz) due to bis(tri-tert-butylsilyl)silylene, (tBu3Si)2Si: (2). The signal intensity as a function of temperature (9-80 K) gave a linear relation, and the spin multiplicity of 2 in the ground state was established to be a triplet. Product analysis, from which disilacyclobutane derivative 3 and dihydrosilane (tBu3Si)2SiH2 (4) were formed, also supports the conclusion about the multiplicity of 2.

Journal Article↗
PubMed2003

Long-range jump versus stepwise hops: magnetic field effects on the charge-transfer fluorescence from photoconductive polymer films.

Magnetic field effects on the charge-transfer (CT) fluorescence of a 1,2,4,5-tetracyanobenzene-doped poly(N-vinylcarbazole) thin film were investigated to clarify the primary process in photoconductive organic amorphous solid. The CT fluorescence increased with increasing magnetic field until 10 mT, and then it showed the dip around 40-50 mT. The hyperfine coupling mechanism observed in the low field and the level-crossing mechanism observed around 46 mT clearly indicate that the spin-conservative stepwise hole hops take place in the films. The boundary distance determined from the simulation based on the stepwise hopping model almost agreed with the interionic separation estimated within the Onsager analysis.

Journal Article↗
PubMed2002

Electronic structure of the lowest excited triplet state of 5,12-naphthacenequinone.

Continuous-wave time-resolved EPR (cw-TREPR) and pulsed electron nuclear double resonance (ENDOR) studies have been carried out to clarify the electronic structure of the lowest excited triplet (Tl) state of 5,12-naphthacenequinone (5,12-NpQ) as well as 1,4-anthraquinone (1,4-AQ) and 6,13-pentacenequinone (6,13-PeQ). The Tl energy level and the D value of the zero-field splitting (ZFS) parameters only slightly decreased with the increasing pi-conjugated system. The Tl states of these linear para-acenequinones were assigned to the pi pi* character. In triplet 5,12-naphthacenequinone, more than 80% of the unpaired electron spins are localized on the naphthalene aromatic sub-system.

Anthraquinones↗