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Takayuki Hirai

Publications and source records attributed to Takayuki Hirai.

At least 19 recordsLinked to original sources
PubMed2006

A triethylenetetramine bearing anthracene and benzophenone as a fluorescent molecular logic gate with either-or switchable dual logic functions.

Fluorescence behaviors of a triethylenetetramine bearing anthracene (AN) and benzophenone (BP) fragments at the respective ends, L1, have been studied in water, where effects of pH (H+) and metal cations on the emission properties have been studied in detail. L1 behaves as a fluorescent molecular logic gate driven by H+ (Input1) and metal cations (Input2) as input chemicals. The most notable feature of L1 is that this molecule expresses the "either-or" switchable dual logic functions. Operation of L1 with Cu2+ as Input2 expresses the INHIBIT logic function, where a strong AN fluorescence appears only at pH 4 (with H+) without Cu2+ [Input1(1)-Input2(0)]. In contrast, operations of L1 with all other metal cations as Input2 express the TRANSFER logic function, where the presence of H+ allows strong AN fluorescence regardless of whether the metal cation exists or not [Input1(1)-Input2(0); Input1(1)-Input2(1)]. These emission switching behaviors of L1 are driven by the difference in the coordination stability between L1 and metal cations and the photoinduced intramolecular electron and energy transfer processes: (i) a pH-induced electron transfer from unprotonated nitrogen atoms of the polyamine chain to the photoexcited AN [ELT(N-->AN*)]; (ii) a pH- and metal coordination-induced electron transfer from the photoexcited AN to the ground-state BP [ELT(AN*-->BP)]; and (iii) a Cu2+ coordination-induced energy transfer from the photoexcited AN to Cu2+ [ENT(AN*-->Cu2+)].

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PubMed2006

Ti-containing mesoporous organosilica as a photocatalyst for selective olefin epoxidation.

We previously found that Ti-containing mesoporous silica (T-S) with isolated and tetrahedrally coordinated Ti-oxide species, when photoactivated in acetonitrile with molecular oxygen (O(2)), catalyzes highly selective epoxidation of olefins (Chem. Commun. 2005, 5977). The system showed the highest epoxide selectivity among the photocatalytic systems proposed so far, but showed insufficient olefin conversion. In the present work, we have employed Ti-containing mesoporous organosilicas (T-OS), synthesized by a surfactant-templating method with an organosilane precursor, as the photocatalyst and have studied the effects on the olefin conversion and the epoxide selectivity. The T-OS catalysts demonstrate the same high epoxide selectivity as does T-S, but scarcely improve the olefin conversion. Photoluminescence measurement reveals that the T-OS catalysts with high surface hydrophobicity enhance the access of hydrophobic olefins to the photoexcited Ti-oxide species as expected, but destabilize the excited species themselves. ESR analysis demonstrates that the T-OS catalysts also destabilize the active oxygen radical (O(3)(*-)), a crucial oxidant for olefin epoxidation, formed on the excited Ti-oxide species. These destabilizations counteract the enhanced olefin access to the excited species, resulting in almost no improvement in olefin conversion. Through detailed analyses, we have summarized the changes in photocatalytic properties of the Ti-oxide species, associated with the organic modification of the catalyst.

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PubMed2006

pH- and H2O-driven triple-mode pyrene fluorescence.

[reaction: see text] A simple-structured molecule L1, a diethylenetriamine bearing two end pyrene fragments, demonstrates triple-mode fluorescence consisting of monomer and short- and long-lived excimer emissions in water, which are precisely controlled by pH and an addition of a less-polar organic solvent.

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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

Vanadium-containing mesoporous silica of high photocatalytic activity and stability even in water.

Vanadium-containing mesoporous silica molecular sieve (V proportional HMS) with tetrahedrally coordinated V-oxide species (VVO4) has been prepared by a modified surfactant-templating method, consisting of an addition of surfactant to a mixture of water, alcohol, and Si and V precursors followed by calcination. The V proportional HMS demonstrates high photocatalytic activity even in the presence of water, while other V proportional HMS's prepared by conventional templating methods and V/HMS prepared by an impregnation method show almost no activity owing to hydrolysis of the VVO4 species. ESR and photoluminescence measurements reveal that the modified templating method creates VVO4 species confined within a silica layer, while other methods create VVO4 species exposed on silica surface. The former VVO4 species are highly stabilized by the confinement within the silica, thus suppressing the hydrolysis. Another notable property of the confined VVO4 species is the higher photocatalytic activity even without water, despite their confined structure. This is explained by higher electrophilicity and longer lifetime of the excited-state VVO4 species (VIVO4*) derived from their distorted structure. The obtained findings suggest potential use of the modified surfactant-templating method for synthesis of stable and recyclable V-containing mesoporous silica with high photocatalytic activity.

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PubMed2006

Visible light-induced partial oxidation of olefins on Cr-containing silica with molecular oxygen.

Photocatalytic oxidation of olefins on Cr-containing silica with molecular oxygen by visible light irradiation (lambda > 400 nm) has been investigated. Cr-SiO(2) catalyst prepared by a conventional sol-gel method, containing highly dispersed chromate species, catalyzes efficient olefin oxidation with very high selectivity for partially oxidized products (>90%), whereas semiconductor TiO(2) promotes complete decomposition (CO(2) production). The Cr-SiO(2) catalyst shows much higher activity than Cr/SiO(2) prepared by an impregnation method or Cr proportional variant MCM-41 prepared by a templating method. ESR analysis reveals that photoirradiation of the chromate species with a tetrahedral coordination (T(d)(6+)) on Cr/SiO(2) and Cr proportional variant MCM-41 catalysts leads to the formation of excited state T(d)(5+) species (T(d)(5+*)), while irradiation to T(d)(6+) on Cr-SiO(2) produces T(d)(4+*) species. This can be explained by a homogeneous T(d)(6+) arrangement with Si species on the Cr-SiO(2) catalyst. On the strongly reduced T(d)(4+*), olefins are strongly attracted by an electron and/or proton donation, resulting in high oxidation activity. The Cr-SiO(2) catalyst is applicable to partial oxidation of various aliphatic and aromatic olefins with very high selectivity, and does not promote undesirable dimerization. The obtained findings suggest a potential use of Cr-SiO(2) as an efficient and recyclable heterogeneous photocatalyst for partial oxidation of olefins.

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PubMed2005

Bis-azamacrocyclic anthracene as a fluorescent chemosensor for cations in aqueous solution.

A bis-azamacrocyclic anthracene (L2), which has two a 12-membered cyclic tetraamine (cyclen) connected through a 9,10-dimethylanthracene spacer, has been synthesized as a new fluorescent chemosensor for detection of pH and metal cations in aqueous solution (cyclen = 1,4,7,10-tetraazacyclododecane, L2 = 9,10-bis(1,4,7,10-tetraazacyclododecane-1-ylmethyl)anthracene). The fluorescence response of L2 has been studied in comparison to that of the previously reported monoazamacrocyclic anthracene (L1 = 1-(9-anthrylmethyl)-1,4,7,10-tetraazacyclododecane). Plots of the fluorescence intensity of L2 against pH demonstrate a sigmoidal curve with pKa 7.4, which is lower than that of L1 (8.3). Potentiometric titration reveals that the increase in the L2 fluorescence requires protonation of both cyclen rings, thus resulting in the lower pKa value. L2 demonstrates impressive fluorescence response against metal cations. At basic pH, upon addition of Zn2+ or Cd2+, L1 leads to an increase in the fluorescence intensity with a 1:1 metal-intensity response. L2, however, shows a 2:1 response to Zn2+, while showing a 1:1 response to Cd2+. At neutral pH, L1 fluorescence decreases upon addition of Zn2+ or Cd2+ because of a formation of metal-anthracene pi complex. L2, however, still demonstrates a Zn2+-induced increase in intensity with a 2:1 response, while no change in intensity is observed upon Cd2+ addition. The obtained findings suggest potential utilities of L2 as a new type fluorescent chemosensor for the detection of cations in aqueous solution.

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PubMed2005

A fluorescent molecular logic gate with multiply-configurable dual outputs.

A simple molecule, L, diethylenetriamine bearing anthracene fragments at both ends, behaves as a fluorescent molecular logic gate with "multiply-configurable dual outputs", capable of demonstrating five different logic functions operated by proton (H+) and transition metal cations (Mn+) as inputs.

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PubMed2005

Adsorption-driven photocatalytic activity of mesoporous titanium dioxide.

Titanium dioxide with a mesoporous structure, when photoactivated in water, demonstrates an unprecedented photocatalytic activity, driven strongly by an adsorption degree of molecules onto the catalyst surface, which promotes a preferential conversion of a well-adsorbed molecule. This catalyzes a selective transformation of a well-adsorbed molecule into a less-adsorbed molecule, so-labeled "stick-and-leave" transformation, which promotes a direct hydroxylation of benzene to phenol, one of the most difficult synthetic reactions, with very high selectivity (>80%) and using water as a source of oxidant.

Adsorption↗
PubMed2005

Immobilization of CdS nanoparticles formed in reverse micelles onto aluminosilicate supports and their photocatalytic properties.

CdS nanoparticles, prepared in reverse micellar system, were immobilized onto thiol-modified aluminosilicate particles (ASSH) by a simple operation: addition of ASSH in the micellar solution and mild stirring. The resulting CdS nanoparticles-aluminosilicate composites (ASCdS) were used as photocatalysts for H2 generation from 2-propanol aqueous solution. The chemical properties of the aluminosilicate, such as affinity for water and other reactants, were found to affect the photocatalytic property of the CdS nanoparticles immobilized. Zeolite particles, having affinity for water and 2-propanol, gave a good ASCdS photocatalyst with respect to H2 generation.

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PubMed2005

Titanosilicate molecular sieve for size-screening photocatalytic conversion.

Titanosilicate molecular sieves, when activated by ultraviolet light irradiation in water in the presence of molecular oxygen, catalyze a conversion of molecules having a size close to the pore of the catalysts but are inactive for molecules having much larger or smaller size. This unprecedented size-screening photocatalytic activity is triggered by a combination of H2O-induced shortened lifetime of active species (charge-transfer excited state of tetrahedrally coordinated titanium oxide) and restricted diffusion of a molecule inside the pore. This catalytic property demonstrates a potential utility of the catalyst for selective transformation of molecules that is associated with a size reduction of molecules, so-labeled "molecular shave" transformation.

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PubMed2005

Photosensitized oxygenation of sulfides within an amphiphilic dendrimer containing a benzophenone core.

Photosensitized oxygenation of sulfides within amphiphilic dendrimers, Gn [n(generation) = 1-3], consisting of a benzophenone (BZP) sensitizing core, apolar interior based on n-undecane spacer, and polar dendron exterior based on 2,2-bis(hydroxymethyl)propionic acid, has been investigated in O(2)-saturated methanol. Sulfoxide formation occurring via reaction of O(2) with triplet excited-state sulfide ((3)sulfide), which is formed by a triplet energy transfer (TET) from photoformed (3)BZP to sulfide, was accelerated by the dendric sensitizers, where G2 showed the highest yields of alkylaryl and dialkyl sulfoxides. Laser photolysis studies revealed that enhanced access of sulfide to the (3)BZP core inside the apolar microenvironment accelerates the TET to sulfide, whereas prompt migration of polar sulfoxide to the polar outer shell of the dendrimer suppresses a competitive TET to sulfoxide, thus resulting in effective (3)sulfide formation. Another notable feature of the dendric sensitizer appears in oxygenation of diaryl sulfide, which is promoted by a persulfoxide intermediate formed by photooxygenation of dialkyl sulfide; photoirradiation of a mixture of diethyl sulfide (1a) and diphenyl sulfide (4a) with G2 gave 17-fold higher diphenyl sulfoxide (4b) yield than that obtained with unmodified BZP. The apolar microenvironment within the dendric sensitizer encapsulates a large quantity of 4a, which is oxidized effectively by the persulfoxide of 1a, thus resulting in high 4b yield. The BZP core within the dendric sensitizer is stable even by photoexcitation in protic solvent, suggesting potential utilities of this dendric system for effective and selective photosensitized oxygenation of sulfides.

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PubMed2005

Preparation of ZnO nanoparticles in a reverse micellar system and their photoluminescence properties.

ZnO nanoparticles with spherical morphology and narrow size distribution were obtained by calcination of Zn(OH)2 nanoparticles, which were prepared in a polyethylene glycol mono-4-nonylphenyl ether (NP-5)/cyclohexane reverse micellar system and incorporated into polyurea (PUA) via an in situ polymerization of hexamethylene diisocyanate (HDI). The resulting ZnO nanoparticles demonstrated a near-UV emission and a green emission, the intensity ratio of which depended on calcination conditions. For the nanoparticles studied, the calcination atmosphere influenced remarkably the photoluminescence properties such as intensity ratio of the near-UV emission to green emission, rather than the size, morphology, and crystallinity of the ZnO nanoparticles. The green emission decreased by calcination in O2 flow but increased by calcination in N2 flow, as compared with the case calcined in air flow. This finding suggests that the green emission is enhanced with the increase of the number of oxygen vacancies of the ZnO nanoparticles and thus the photoluminescence properties of the nanoparticles were successfully controlled by the calcination condition, without changing the size and morphology.

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