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Seiji Tobita

Publications and source records attributed to Seiji Tobita.

16 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

Environment-sensitive fluorophore emitting in protic environments.

The unusual fluorescence properties of 8-methoxy-4-methyl-2H-benzo[g]chromen-2-one (1) are described. The fluorophore 1 is almost nonfluorescent in aprotic solvent (e.g., fluorescence quantum yield Phi(f) < 0.0003 in n-hexane), whereas it strongly fluoresces at long wavelengths (>450 nm) in protic solvent (e.g., Phi(f) = 0.21 in methanol). The fluorophore 1 also shows good applicability in developing a new fluorogenic (fluorescent "off-on") sensor. [structure: see text]

Chemical Phenomena↗
PubMed2006

Effects of localized triplet exciton on reactivity of photoinduced omega-bond dissociation in naphthyl phenyl ketones having pi,pi* lowest triplet (T1) states studied by laser flash photolysis.

Photochemical properties of photoinduced omega-bond dissociation in naphthyl phenyl ketones having a phenylthiyl moiety as a leaving group, p-(alpha-naphthoyl)benzyl phenyl sulfide (NBPS) and 4-benzoyl-1-naphthylmethyl phenyl sulfide (BNMPS), in solution were investigated by laser flash photolysis techniques. Both ketones were shown to undergo photoinduced omega-bond cleavage of the C-S bond to release the phenyl thiyl radical (PTR) at room temperature. Irrespective of excitation wavelengths of NBPS, a quantum yield (Phi(rad)) of the PTR formation was obtained to be 0.1, whereas that for BNMPS was found to depend on the excitation wavelength, i.e., absorption bands from the ground state (S0) to the excited singlet states, S3, S2, and S1 of BNMPS; Phi(rad)(S3) = 0.77 and Phi(rad)(S2) = Phi(rad)(S1) = 1.0. By using triplet sensitization of p-phenylbenzophenone (PBP), efficiencies (alpha(rad)) of the radical formation in the lowest triplet state (T1(pi,pi*)) of NBPS and BNMPS were determined to be 0 and 1.0, respectively. The agreement between Phi(rad)(S1) and alpha(rad) values for BNMPS indicates that the C-S bond dissociation occurs in the T1 state via the S1 state via a fast intersystem crossing from the S1 to the T1 state. The wavelength dependence of the radical yields upon direct excitation of BNMPS was interpreted in terms of the C-S bond cleavage in the S3 state competing with internal conversion from the S3 to the S2 state. The smaller value of Phi(rad)(S3) than those of Phi(rad)(S1) and Phi(rad)(S2) was proposed to originate from the geminate recombination of singlet radical pairs produced by the bond dissociation via the S3 state. Photoinduced omega-cleavage of NBPS was concluded to take place only in the S1(n,pi*) state. Difference in reactivity of omega-cleavage between the triplet states of NBPS and BNMPS was interpreted in terms of localized triplet exciton in the naphthoyl moieties.

Journal Article↗
PubMed2006

Extreme fluorescence sensitivity of some aniline derivatives to aqueous and nonaqueous environments: mechanistic study and its implication as a fluorescent probe.

Effects of solvent water on the photophysical properties of a series of meta- and para-substituted anilines have been investigated by means of time-resolved fluorescence, transient absorption, and photoacoustic measurements. Some aniline derivatives exhibit extremely short fluorescence lifetime (tau(f)) and small quantum yield (Phi(f)) in water (e.g., tau(f) = 45 ps and Phi(f) = 0.0019 for m-cyanoaniline (m-ANCN) in H(2)O), which is in marked contrast with their much larger values in nonaqueous solvents (tau(f) = 7.3 ns and Phi(f) = 0.14 for m-ANCN in acetonitrile). Photoacoustic and transient absorption measurements show that the remarkable fluorescence quenching of m-ANCN in water is attributed almost exclusively to fast internal conversion. The lifetime measurements of m-ANCN in H(2)O/acetonitrile binary solvent mixtures reveal that the quenching is related to variation of hydrogen-bonding interactions between the amino group and water molecules and the conformational change of the amino group upon electronic excitation. Similar fluorescence quenching due to solvent water is also found for N-alkylated m-ANCNs. The drastic differences in the fluorescence intensity and lifetime of m-ANCNs under hydrophobic and hydrophilic environments and also the large solvent polarity dependence of the fluorescence band position suggest the possibility that they can be utilized as fluorescent probes for investigating the microenvironment of biological systems. In suspensions of human serum albumin (HSA) in water, remarkable enhancement of the fluorescence intensity and lifetime is observed for m-ANCN and its N-alkylated derivatives, demonstrating that m-ANCNs can be a candidate for novel fluorescent probe with small molecular size.

Journal Article↗
PubMed2006

Photophysical and photochemical processes of 9,10-dihydro-9-silaphenanthrene derivatives: photochemical formation and electronic structure of 9-silaphenanthrenes.

Photophysical and photochemical processes of 9-methyl- and 9-phenyl-9,10-dihydro-9-silaphenanthrene derivatives have been studied at room temperature and 77 K in comparison with the carbon analogue, 9,10-dihydrophenanthrene. These 9,10-dihydro-9-silaphenanthrene derivatives show smaller fluorescence quantum yield and remarkably larger Stokes shifts than those of the carbon analogue. In contrast, their phosphorescence quantum yields are two times larger than those of the carbon analogue, although the absolute value is not so large (approximately 0.1). Reaction products and intermediates produced by the 266 nm light photolysis have been studied, and it has been confirmed that 9-methyl- and 9-phenyl-9-silaphenanthrenes have been photochemically formed in methylcyclohexane at 77 K, in addition to the formation of radical cations of 9,10-dihydro-9-silaphenanthrene derivatives and the carbon-centered radical: 9-hydro-9-silaphenanthrenyl radical.

Free Radicals↗
PubMed2005

Synthesis and photochemical properties of stilbenophanes tethered by silyl chains. Control of (2pi + 2pi) photocycloaddition, cis-trans photoisomerization, and photocyclization.

[structures: see text] Novel macrocyclic and medium-size stilbenophanes tethered by silyl chains were synthesized, and their photochemical and photophysical properties were examined. Direct photoirradiation of macrocyclic stilbenophanes gave intramolecular photocycloadducts stereoselectively, and the efficiency increased with decreasing distance between the two stilbene units. The triplet-sensitized photoreaction of stilbenophanes caused cis-trans photoisomerization. Photoreactions of cis-fixed stilbenophanes under an oxygen atmosphere selectively gave phenanthrenophanes. Fluorescence quantum yields increased with the introduction of silyl substituents, and hence those of silyl-tethered stilbenophanes were larger than that of unsubstituted trans-stilbene. Intramolecular excimer emission was observed when the distances between two stilbene units in the stilbenophanes were sufficiently small.

Journal Article↗
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

Synthesis and complexing properties of [2.n](2,6)pyridinocrownophanes.

[2.n](2,6)Pyridinocrownophanes (3a-e) were efficiently prepared by intramolecular [2 + 2] photocycloaddition of vinylpyridine derivatives under irradiation using a 400-W high-pressure mercury lamp through a Pyrex filter. They were of cis-configuration with respect to the cyclobutane ring, which was proven by the specific methine proton NMR resonances at delta 3.98-4.08. From ESI-MS analysis 3a-e were found to form 1:1 complexes with Ag+ cation. In a liquid-liquid extraction, 3 showed the highest affinity toward Ag+ cation among several heavy metal nitrates. In this series, 3c possessing four ethereal oxygen atoms was found to show the highest Ag+ affinity, according to the liquid-liquid extraction and determination of stability constant with the cation. The photoreactivity of precursor vinylpyridines is discussed by the predictor gamma(r(c)) proposed by Caldwell.

Crown Ethers↗
PubMed2005

Intramolecular and intermolecular hydrogen-bonding effects on photophysical properties of 2'-aminoacetophenone and its derivatives in solution.

Effects of intra- and intermolecular hydrogen-bonds on the photophysical properties of 2'-aminoacetophenone derivatives (X-C6H4-COCH3) having a substituted amino group (X) with different hydrogen-bonding ability to the carbonyl oxygen (X: NH2(AAP), NHCH3(MAAP), N(CH3)2(DMAAP), NHCOCH3(AAAP), NHCOCF3(TFAAP)) are investigated by means of steady-state and time-resolved fluorescence spectroscopy and time-resolved thermal lensing. Based on the photophysical parameters obtained in aprotic solvents with different polarity and protic solvents with different hydrogen-bonding ability, the characteristic photophysical behavior of the 2'-aminoacetophenone derivatives is discussed in terms of hydrogen-bonding and n,pi*-pi,pi* vibronic coupling. The dominant deactivation process of AAP and MAAP in nonpolar aprotic solvents is the extremely fast internal conversion (k(ic)= 1.0 x 10(11) s(-1) for AAP and 3.9 x 10(10) s(-1) for MAAP in n-hexane). The internal conversion rates of both compounds decrease markedly with increasing solvent polarity, suggesting that vibronic interactions between close-lying S1(pi,pi*) and S2(n,pi*) states lead to the large increase in the non-radiative decay rate of the lowest excited singlet state. It is also suggested that for MAAP, which has a stronger hydrogen-bond as compared to AAP, an intramolecular hydrogen-bonding induced deactivation is involved in the dissipation of the S1 state. For DMAAP, which cannot possess an intramolecular hydrogen-bond, the primary relaxation mechanism of the S1 state in nonpolar aprotic solvents is the intersystem crossing to the triplet state, whereas in protic solvents very efficient internal conversion due to intermolecular hydrogen-bonding is induced. In contrast, the fluorescence spectra of AAAP and TFAAP, which have an amino group with a much stronger hydrogen-bonding ability, give strongly Stokes-shifted fluorescence, indicating that these compounds undergo excited-state intramolecular proton transfer reaction upon electronic excitation.

Journal Article↗
PubMed2005

A picosecond time-resolved study on prototropic reactions of electronically excited 1,5- and 1,8-diaminonaphthalenes in aqueous solution.

The proton transfer to solvent in the excited state of protonated diaminonaphthalenes, 1,5-diaminonaphthalene (1,5-DAN) and 1,8-diaminonaphthalene (1,8-DAN), in aqueous solution, has been investigated by picosecond time-resolved fluorescence measurements. The deprotonation rate constants of the dications of 1,8-DAN and 1,5-DAN in the excited state to produce the corresponding monocations are determined to be 1.3 x 10(10) and 5.6 x 10(9) s(-1), respectively, from dynamic analyses of their fluorescence time profiles. The much larger proton-dissociation rates compared with that of 1-aminonaphthalene (0.6 x 10(9) s(-1)) can be attributed to an electron-withdrawing effect due to the ammonium group at the 5- or 8-position in the naphthalene ring. The remarkably large proton-dissociation rate in 1,8-DAN can be ascribed to its larger reaction exergonicity which results from the electrostatic repulsion between the two ammonium groups in the reactant (the dication state) and the stabilization of the monocation state due to hydrogen bonding interactions between the NH3+ and NH2 moieties. The difference in their acidities in the excited state is discussed in terms of the reaction free energy and the proton affinities are evaluated from ab initio MO calculations.

Journal Article↗
PubMed2005

Photochemical reactions of triplet p-phenylbenzyl derivatives studied by using laser flash triplet-sensitization techniques.

Photochemical properties in the triplet states of p-phenylbenzyl derivatives (PBX; X = H, Cl, Br, OH or SH) were investigated by using laser photolysis techniques of triplet-acetone sensitization. Beta-bond dissociation of triplet phenylbenzyl mercaptan (X = SH) was shown for the first time while formation of triplet states were seen for phenyl toluene and phenylbenzyl alcohol by triplet energy transfer. Beta-bond cleavage was absent for halide compounds (X = Cl and Br) upon sensitization because of the short triplet lifetimes due to a heavy atom effect. The reactivity for beta-bond cleavage was interpreted in terms of difference in the bond enthalpy, D(C-X), relative to the triplet energies.

Journal Article↗
PubMed2004

Triplet-sensitized photolysis of the photoisomer of a 2,11-diaza[3,3](9,10)anthracenoparacyclophane: an adiabatic cycloreversion and a [2pia + 2pia + 2sigmas] rearrangement in a triplet state of the biplanophane system.

The triplet-sensitized photoreactions of the title biplanophane system 6, the photoisomer of a 2,11-diaza[3,3](9,10)anthracenoparacyclophane derivative 5, were investigated by stationary and laser-flash photolyses using xanthone (XT) and benzophenone (BP) as triplet sensitizers. When photoisomer 6 underwent XT-sensitized irradiation, a triplet cyclophane 5 and a novel polycyclic product 7 were obtained via an adiabatic cycloreversion and a formal [2pia + 2pia + 2sigmas] rearrangement, respectively. The maximum quantum yield for the formation of cyclophane 5 (0.69) and the upper-limit efficiency for the formation of polycycle 7 (0.31) were determined by laser photolysis techniques. For BP-sensitized photolysis of photoisomer 6, oxetane 8, in addition to triplet cyclophane 5 and polycycle 7, was formed by a Paterno-Buchi reaction. The quenching rate constant (k(q)) of triplet BP by photoisomer 6 (3.4 x 10(8) dm(3) mol(-)(1) s(-)(1)) was found to be 1 order of magnitude smaller than that for XT (5.0 x 10(9) dm(3) mol(-)(1) s(-)(1)). On the basis of the relationship between k(q) and the triplet donor-acceptor energy gap, the triplet energy level of photoisomer 6 was estimated to be approximately 71 kcal mol(-)(1). The photochemical and the photophysical processes involved in the sensitized photolyses are summarized in an energetic reaction diagram and discussed in detail.

Journal Article↗
PubMed2003

Fluorescence on-off switching mechanism of benzofurazans.

Many fluorescent reagents with a benzofurazan (2,1,3-benzoxadiazole) skeleton have been developed and widely used in bio-analyses. In this study, we try to elucidate the fluorescence on-off switching mechanism of three fluorogenic reagents and their derivatives. Ten 4,7-disubstituted benzofurazans were used for this purpose and the measurements of their fluorescence, phosphorescence, photolysis, and time-resolved thermal lensing signal in acetonitrile were obtained in order to understand the relaxation processes of these compounds. These results indicate that the competition of fluorescence with a fast intersystem crossing or fast photoreaction plays a key role in the fluorescence on-off switching. Semi-empirical molecular orbital calculations show that the existence of the triplet n pi* state is responsible for the fast intersystem crossing while the proximity of the reactive second single pi pi* state to the first singlet pi pi* state contributes to the fast photoreaction in the excited states.

Journal Article↗
PubMed2002

Photochemical synthesis, conformational analysis, and transformation of [60]fullerene-o-quinodimethane adducts bearing a hydroxy group.

The photochemical reactions of [60]fullerene with various aromatic aldehydes or ketones 1a-n carrying an alkyl group at the ortho position were examined. Some of them afforded stable o-quinodimethane adducts 2 with a hydroxy group attached to the cyclohexene ring. The adducts 2 were found to adopt one or both of two conformers A and E, which possess pseudoaxial and pseudoequatorial hydroxy groups, respectively. The conformer ratios depended remarkably on the substituents attached to the aromatic nucleus and the cyclohexene ring. The dynamic behavior of 2 was also investigated by the VT-NMR technique.

Journal Article↗
PubMed2002

Photochemical properties of the triplet pi,pi* state, anion and ketyl radicals of 5,12-naphthacenequinone in solution studied by laser flash photolysis: electron transfer and phenolic H-atom transfer.

Photochemical properties of the lowest triplet pi,pi* state of 5,12-naphthacenequinone (5,12-NQ) have been investigated in solution by means of nanosecond laser flash photolysis at 295 K. The transient absorption spectrum of triplet 5,12-NQ was measured in CCl4 and the molar absorption coefficient was determined. In CCl4, triplet 5,12-NQ is shown to be quenched by the ground-state 5,12-NQ. In acetonitrile, triplet 5,12-NQ is quenched by 1,2,4,5-tetramethoxybenzene (TMB) via electron transfer to produce the TMB cation and 5,12-NQ anion radicals. The absorption spectrum and the molar absorption coefficient of the 5,12-NQ anion radical were determined based on those of the TMB cation radical. Although triplet 5,12-NQ does not abstract H-atom of ethanol or cyclohexane, H-atom transfer occurs from phenol and p-phenylphenol to triplet 5,12-NQ, producing the 5,12-NQ ketyl radical. The absorption spectrum and the molar absorption coefficient of the 5,12-NQ ketyl radical were determined based on those of the p-phenylphenoxyl radical. It is shown that the quenching rate constants by phenol for triplet 5,12-NQ and other triplet paraquinones previously reported obey a Rehm-Weller relationship. The mechanism for the phenolic H-atom transfer to triplet paraquinones is interpreted in terms of coupled electron and proton transfer.

Anions↗