A review of the research advances made in the design and development of highly reactive and functional titanium oxide photocatalysts, which can utilize not only UV but also visible or solar light, and a clarification of the active sites as well as the detection of the reaction intermediates at the molecular level have been presented here. The potential for the effective utilization and conversion of solar energy into useful and safe chemical energy by the modification of the electronic properties of such TiO2 photocatalysts is great when one considers their myriad applications as well as their non-polluting qualities. Such a modification process by methods such as ion-implantation can be applied not only for semiconducting bulk TiO2 photocatalysts but also for TiO2 thin film photocatalysts and titanium oxide photocatalysts highly dispersed within zeolite frameworks. Moreover, the photocatalytic reactivity of semiconducting TiO2 nano-powders was found to be dramatically enhanced by the loading of small amounts of Pt. This worked to enhance the reduction reaction, resulting in the charge separation of the electrons and holes generated by light irradiation. In addition, highly dispersed titanium oxide species prepared within zeolite frameworks as well as SiO2 or Al2O3 matrices showed much higher and unique photocatalytic performances as compared to semiconducting bulk TiO2 photocatalysts. Significantly, a new alternative method to directly prepare such visible light-responsive TiO2 thin film photocatalysts on various substrates has been successfully developed by applying a RF magnetron sputtering deposition method.
Thermodynamics and kinetics for formation of an intramolecular dimer radical cation of a series of α,ω-di(2-naphthyl)alkanes in solutions were quantitatively investigated by near-IR transient absorption spectroscopy, which allows one to observe dimer radical cations directly. The standard enthalpy (−ΔH°) for the formation of the intramolecular dimer radical cation increased as the chain length between the two naphthyl moieties increased, and was smaller than that of the intermolecular dimer radical cation of 2-ethylnaphthalene. This shows that −ΔH° depends on the strain required for the chain to form the ring-closure configuration. Destabilization due to repulsion between the two naphthyl moieties in the intramolecular dimer radical cation was evaluated to be ca. 20 kJ mol−1. This value is smaller than that of the naphthalene excimer, indicating that the separation distance and/or the overlap between the two naphthyl moieties are not as restricted as those of the excimer. The activation energy for the formation of the intramolecular dimer radical cation was comparable to the energy for local motions of a few methylene units linking the two naphthalene moieties.
Selenium-substituted TTM-TTP (2,5-bis[4,5-bis(methylthio)-1,3-dithiol-2-ylidene]-1,3,4,6-tetrathiapentalene) derivatives (1a–1d), in which several 1,3-dithiole rings of the bis-fused TTF framework are replaced by 1,3-diselenole rings, have been prepared. In particular, we have accomplished the first selenium substitution of the inner TTP part (1a). The 1:1 composition iodine salt, (1a)I3, is an insulator constructed of the donor trimers. The 1:1 GaCl4 salt, (1a)GaCl4, has a uniform column isostructural to the sulfur analog, (TTM-TTP)FeBr1.8Cl2.2; it exhibits metallic conduction down to about 60 K, the lowest metal–insulator (M–I) transition temperature in 1:1 salts. The donors 1c and 1d give the iodine salts isostructural to the sulfur analog, (TTM-TTP)(I3)5/3. By the selenium substitution, the M–I transition of (1d)(I3)5/3 is entirely suppressed.
In organic solvents, 2-methylnaphth[2,3-d]oxazole (MNO) exhibits a single emission, whereas it exhibits dual emissions (the 350-nm band fluorescence and the longer-wavelength fluorescence) in water (pH 6.1). In acidic aqueous solution, the 350-nm band fluorescence alone is observed with a disappearance of the longer-wavelength fluorescence. In pH 6.1 buffers, α-CD has been found to form a 2:1 inclusion complex with MNO. Upon the addition of α-CD to MNO solution, the longer-wavelength fluorescence is significantly reduced in intensity, relative to the 350-nm band fluorescence. The incorporation of an MNO molecule into the α-CD cavities obstructs the generation of the excited state responsible for the longer-wavelength fluorescence. The longer-wavelength fluorescence is most likely due to the intramolecular charge transfer (ICT) state of MNO, which is induced by water molecules of high polarity. MNO forms a 1:1 inclusion complex with β-CD. In the presence of β-CD, the intensity ratio of the longer-wavelength fluorescence to the 350-nm band fluorescence is decreased. When 1-propanol is added to MNO solution containing β-CD, a ternary inclusion complex is formed among β-CD, MNO, and 1-propanol. In β-CD solution containing 1-propanol, the intensity ratio of the longer-wavelength fluorescence to the 350-nm band fluorescence is slightly reduced. These findings support the conclusion that the longer-wavelength fluorescence of MNO is due to the ICT state, whose generation is promoted by the water environment.
Weak ion association equilibria between inorganic anions, ranging in charge from −1 to −4, and symmetrical tetraalkylammonium ions, R4N+ (R = Me, Et, Pr, Bu, n-Am), were investigated in formate buffer solutions containing a variable composition of ethanol (0–62.9% (m/m)). The individual effective mobilities of the inorganic anions in a series of tetraalkylammonium chloride solutions were measured using an ordinary silica capillary maintained at 25.0 ± 0.1 °C under a negative potential (−10 kV) and direct UV detection (λ = 220 nm). Under the stated experimental conditions, the association constants, Kass, showed a modest increase as the amount of ethanol in the buffer was increased. The concepts of medium effect and Gibbs free energy of transfer were applied to weak ion equilibria in order to investigate changes in association constant as the ethanol content in the buffer solutions was increased. The above treatment on selected associations revealed that the increased instability of the inorganic anion in mixed ethanolic solutions is responsible for the observed increase in Kass.
The Cu(II) complexes of oligopeptides, Cu(H−iL); i = 1, 2, or 3, reacted with N-acetyl-L-histidine (AHis) to form ternary complexes, Cu(H−jL)(AHis); j = 1 or 2. The Cu(H−1L) species formed Cu(H−1L)(AHis), which was either dimerized to [Cu(H−1L)]2(AHis−) or converted stepwise to Cu(AHis)n; n = 3, 4. The reaction of the Cu(H−2L) or Cu(H−3L) species with AHis led to the formation of Cu(H−2L)(AHis), and the formation constants of the ternary complexes were small, on the magnitude of 10−1–101 M−1.
The magnetic properties of quaternary oxides Ba3PrM2O9 and Ba3CeM2O9 (M = Ir, Ru) have been investigated. They crystallize in the 6H-perovskite structure with the space group P63/mmc, in which the cation sites with the face-sharing octahedra are occupied by Ir (or Ru) ions, and those within the corner-sharing octahedra are occupied by Pr (or Ce) ions. Magnetic susceptibility measurements for Ba3PrIr2O9 show that an antiferromagnetic interaction between two Ir ions in the Ir2O9 dimer results in a large temperature-independent paramagnetic susceptibility over a wide temperature range. Another interesting result is in the small effective magnetic moments for Ba3PrIr2O9 (0.31(1) μB) and for Ba3PrRu2O9 (0.75(1) μB). The electron paramagnetic resonance (EPR) spectrum of Pr4+ in these compounds was measured by diluting them in the corresponding isomorphous compounds Ba3CeIr2O9 and Ba3CeRu2O9 and lowering the temperature to 4.2 K. A very large hyperfine interaction with the 141Pr nucleus was observed. The results were analyzed based on the weak field approximation. The measured g values are much smaller than |−10/7|, showing that the crystal field effect on the behavior of a 4f electron is large. The low g values correspond to the small effective magnetic moments of the Pr compounds.
A 19F NMR study on myoglobin reconstituted with a ring-fluorinated C2 symmetric heme, 13,17-bis(2-carboxylatoethyl)-2,8,12,18-tetramethyl-3,7-difluoroporphyrinatoiron(II), demonstrated that the porphyrin π system is affected by the bent orientation of the Fe–O–O unit relative to the heme in oxy myoglobin and Fe dπ → CO π* back-donation in carbonmonoxy myoglobin.
In order to investigate how the sheet-like network along the side-by-side direction constructed by the strong inter-column S···N and O···H atom–atom contacts influences the stability of the metallic state of novel one-dimensional metallic anion radical salts of 2,6-bis(dicyanomethylene)-2,6-dihydro-4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one (CPDT-TCNQ), the 3,5-difluoro (F2CPDT-TCNQ) and 3,5-dimethyl (Me2CPDT-TCNQ) derivatives of CPDT-TCNQ, not possessing hydrogen atoms at the 3,5-positions to construct the inter-column O···H contacts, have been synthesized. The anion radical salts, Me4X(Me2CPDT-TCNQ)2 (X = P and As), showed a metallic temperature dependence of the resistivity from room temperature down to around 240 K and 200 K, respectively, whereas Me4X(Me2CPDT-TCNQ)2 (X = N and Sb) were semiconducting. The metal–insulator transition temperatures (TMI) of Me4X(Me2CPDT-TCNQ)2 (X = P and As) are significantly higher than those of MX4(CPDT-TCNQ)2 (X = P and As). Moreover, the extent of destabilization of the metallic state is much more significant in Me4N(Me2CPDT-TCNQ)2, since MeN4(CPDT-TCNQ)2 is metallic down to 130 K. Although electric conduction can not occur along the side-by-side direction in either salt of CPDT-TCNQ and Me2CPDT-TCNQ, these facts suggest that the two-dimensional sheet-like molecular network along the a-axis may contribute to stabilize the one-dimensional metallic states of the anion radical salts of CPDT-TCNQ.
The origins of stereo- and face-selective Diels–Alder reactions of 5-substituted 1,3-cyclopentadienes (CP) with some dienophiles were elucidated by means of semiempirical PM5, DFT(B3LYP), and ab initio RHF methods. The activation energies by the improved PM5 and B3LYP/6-31+G(d) methods were in good agreement with the experimental results. We partitioned the activation energy (ΔEact) into diene deformation (ΔEdf-dien) and dienophile deformation (ΔEdf-dphil), and the diene–dienophile interaction (Eint) energies, in addition to the new intrinsic reaction coordinate (IRC) energy (ΔEirc) partitioning. Such analysis revealed the major factors in the determining endo/exo and syn/anti selectivities of the reactions of 5-substituted CP. The syn-selectivity of the 5-F-CP is explained by the lower ΔEact (2–3 kcal/mol) caused by relatively smaller ΔEdf-dien, which leads to slighter repulsion between the reactants. This low activation energy is inferred to be caused by the better second orbital interaction or intramolecular orbital interactions between the diene π and fluoro-substituent n orbital moieties, as well as by better intermolecular π–π orbital interactions between the diene-π and dienenophile-π moieties. On the other hand, the anti-selectivity of the methyl substituted CP mainly appears from bigger Edf-dien in the syn-isomer. A novel economical method of IRC energy partitioning to the reactants, TS intermediates and products is also introduced in order to verify the energy balance of other calculation methods and other reactions.
A novel method for the synthesis of 3-substituted 4-hydroxy-1-methyl- and 1-phenyl-2-quinolones is presented. The compounds are produced in a one-step reaction in very good yields (51–76%). The major advantage of the methodology is the short time for the synthesis in contrast to previous methodologies requiring several steps and harsh conditions for the synthesis of quinolone derivatives.
Irradiation (λ > 300 nm) of di(9-anthryl)diazomethane (1) in degassed benzene gave the trimer (3) of dianthrylcarbene (2) as the main product along with a small amount of a tetramer (4). The structures of those oligomers were characterized as caged compounds produced as a result of three- and four-fold coupling at the 10 and 10′ positions of 2. Photolysis of 1 in the presence of oxygen with short wavelength UV light resulted in the formation of di(9-anthryl) ketone (5) and anthraquinone (6), while similar irradiation with a longer wavelength light gave 5 and 9-anthryl 9-anthracenecarboxylate (9), which was found to give 6 upon irradiation with a short wavelength light in the presence of oxygen. On the other hand, the photolysis of 4 gave 3 as the main product, while 3 was almost completely photostable under similar conditions. However, irradiation of both 3 and 4 in the presence of oxygen gave 6. In order to elucidate the reactions observed, we carried out spectroscopic studies (matrix EPR and UV/vis spectroscopies as well as laser flash photolysis) with 1, 3, and 4. These studies revealed that 3 generated a trimer diradical (10) formed as a result of C–C bond cleavage upon irradiation and that 10 reformed 3 almost exclusively, while similar photolysis of 1 and 4 generated the same two transient species, 10 and a dimer diradical. It is proposed that triplet carbene 2 equilibrates with the dimer diradical 2-2 and that the main decay pathway of this mixture is to generate the trimer diradical 10 while dimer diradical 2-2 undergoes coupling to produce the tetramer 4. The structure of the triplet carbene in equilibrium with diradicals is also discussed.
The structural control and biodegradability of enzymatically synthesized polyphenols have been investigated. The peroxidase-catalyzed oxidative polymerization of various 4-substituted phenols was performed in aqueous organic solvents, in which the ratio of phenylene and oxyphenylene units of the polymer product could be precisely controlled by the nature of the solvents and the monomers. The unit ratio strongly depended on their hydrophobic parameters. The control of the structure was also examined by changing the feed ratio of the enzymatic copolymerization of the phenol derivatives. The biodegradability was evaluated by the BOD method. Poly(4-t-butylphenol) showed relatively good biodegradability, whereas little degradation of the polyphenol from unsubstituted phenol was observed.
The mechanism and intermediates of photoinduced hydrogen atom transfer and photoisomerization in 1-(1-hydroxy-2-naphthyl)-3-(1-naphthyl)-2-propen-1-one (1) and 1-(1-methoxy-2-naphthyl)-3-(1-naphthyl)-2-propen-1-one (2) have been investigated using nanosecond time-resolved infrared spectroscopy. For compound 1, a bleach signal was observed at 1575 cm−1 just after the laser excitation and recovered within 2 ms. The transient absorption at 1500 cm−1 decayed with two components of 1 μs and 2 ms, which are due to the triplet state and the ground state tautomer, respectively. On the basis of these results, the potential energy surface of hydrogen atom transfer in compound 1 was deciphered. Upon laser excitation of compound 2, the transient absorption signals at 1528 cm−1 and 1392 cm−1 assigned to the triplet state were observed in addition to the bleach signals at 1664 cm−1 and 1600 cm−1. The bleach signal at 1600 cm−1 remained as a permanent bleaching after the fast recovery with the 1.2 μs time constant. This result indicates that compound 2 underwent trans-to-cis isomerization upon laser excitation. By utilizing the data obtained in stationary and transient IR spectroscopy, we have directly determined the quantum yield of trans-to-cis isomerization to be 0.23 ± 0.01 from the difference in the IR signals.
Several 2- and 3-(phenylazo)pyridine derivatives were prepared. Most of their trans isomers have a large absorption band (∼2 × 104 M−1 cm−1 (1 M = 1 mol dm−3)) around 320 nm and a small absorption band (∼400 M−1 cm−1) around 450 nm. All of these compounds photoisomerize upon irradiation of the shorter (trans-to-cis) and longer (cis-to-trans) wavelength absorption bands. trans Isomers of 3-(phenylazo)pyridines axially coordinate more strongly to Zn–porphyrins than cis isomers, while the opposite is the case for 2-(phenylazo)pyridines, due to steric reasons. These phenylazopyridines quench the fluorescence of Zn–porphyrin upon coordination. These properties were exploited for the light-triggered fluorescence modulation of Zn–porphyrins. Light was irradiated to a mixed solution of phenylazopyridine and Zn–porphyrin to induce photoisomerization of the phenylazopyridine, which underwent association with, or dissociation from, the Zn–porphyrin, resulting in a decrease or increase of Zn–porphyrin fluorescence. For example, the fluorescence intensity of Zn–tetraphenylporphyrin reversibly changed by up to 50%, when 4-methoxy-2-(phenylazo)pyridine was employed.
The electrochemical reduction of N-acyliminium ions, which were generated by the “cation pool” method, led to the formation of carbon free radicals. Carbon radicals thus generated underwent homo-coupling reactions and the reactions with activated olefins, such as methyl acrylate. In the latter case, a mechanism involving the addition of a carbon radical to the carbon–carbon double bond, followed by one-electron reduction to give carbanions, has been proposed. The present study opens a new possibility for radical-mediated carbon–carbon bond formation based on the reduction of carbocations.
Lithium acetate (AcOLi)-catalyzed aldol reactions between trimethylsilyl enolates and aldehydes proceed smoothly in anhydrous DMF or pyridine to afford the corresponding aldols in good to high yields under weakly-basic conditions (Tables 1–5). This catalytic aldol reaction is performed smoothly also by using other metal carboxylates that are easily prepared in situ by treating carboxylic acids with lithium carbonate (Li2CO3) (Table 2, Scheme 5). In order to show the effect of mild and readily-available AcOLi catalyst, the aldol reaction in water-containing DMF was studied in detail. AcOLi and various metal carboxylates behaved as effective Lewis base catalysts in aldol reactions between trimethylsilyl enolate and aldehydes in DMF–H2O (50:1) (Tables 6, 7). One of the most characteristic points of the above reaction that took place in water-containing DMF is that the aldehydes having a free amide and a hydroxy or even a carboxyl group reacted smoothly and afforded the desired aldols 29–31 in moderate to high yields (Table 8, Entries 12–15). Trimethylsilyl enolates derived from carboxylic esters behaved similarly as excellent nucleophiles in the above reaction. This is the first example of Lewis base-catalyzed aldol reactions to afford the desired aldol adducts even when silyl enolates derived from carboxylic esters were used in a water-containing organic solvent.
Oxidation–reduction condensation using in situ formed alkoxydiphenylphosphines, 2,6-dimethy-1,4-benzoquinone, and carboxylic acids provides a useful method for the preparation of inverted tertiary alkyl carboxylates from the corresponding chiral tertiary alcohols under mild and neutral conditions. Similarly, it has afforded alkyl carboxylates successfully in good-to-high yields by the combined use of alkoxydiphenylphosphines having primary, secondary, or tertiary alkoxy groups, carboxylic acids, and simple 1,4-benzoquinone. When chiral secondary or tertiary alcohols are used, the corresponding inverted secondary or tertiary alkyl carboxylates are also obtained in good-to-high yields. In addition, a convenient method for the preparation of phenyl carboxylates in high yields has been established by utilizing oxidation–reduction condensation in toluene at 110 °C using phenoxydiphenylphosphines in situ-formed from phenols and chlorodiphenylphosphine, 2,6-dimethyl-1,4-benzoquinone, and carboxylic acids.
(Diiodomethyl)trimethylsilane (Me3SiCHI2, 1) is produced by treatment of iodoform with manganese in the presence of Me3SiCl. Aldehydes are converted to (E)-1-trimethylsilyl-1-alkenes in a stereoselective manner with a geminal dichromium reagent generated from 1, manganese, Me3SiCl, and a catalytic amount of CrCl3[thf]3 in THF. Similarly, (E)-1-alkenylboronic esters are prepared stereoselectively in good to excellent yields by treatment of aldehydes with a geminal dichromium reagent derived from Cl2CHB(OR)2 [(OR)2 = OCMe2CMe2O] and LiI instead of 1.
5-Pyrimidyl alkanol with up to 97% ee was formed using chiral ephedrine immobilized on silica gel in the enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde in conjunction with asymmetric autocatalysis.
Novel acidic or basic aryl-alkyl disulfides having phenol, benzoic acid, benzenesulfonic acid, aniline, and benzylamine groups were synthesized as protective agents for gold (Au) nanoparticles. Monodisperse Au nanoparticles passivated by these ligands were prepared using a chemical reduction technique, and self-assembled into well-ordered hexagonal close-packed (hcp) 2D superlattices on the substrate. We also present, for the first time, the formation of an aggregated multilayer of Au nanoparticles at a water–organic solvent interface by mixing separately synthesized acidic and basic Au nanoparticles. This film was definitely different from those obtained by the LB method and the conventional self-assembly method through solvent evaporation.
Siliceous and aluminum-containing self-standing mesoporous silica films with the thickness of ca. 50 μm were synthesized by the solvent evaporation method from tetramethoxysilane, aluminum chloride hexahydrate, and octadecyltrimethylammonium chloride. The films possessed highly ordered mesostructures and large surface areas (over ca. 700 m2 g−1). The mesostructures of the products were controlled by the chemical compositions (Si/surfactant); siliceous mesostructured materials with hexagonal (P6m) and cubic (Pm3n) phases were obtained when the molar Si/surfactant ratios were 9 and 8, respectively. The aluminum-containing films with the molar Si/Al ratios of 50, 20, and 10 were also synthesized.
Polysilanes with an optically active terminal (+) or (−)-menthoxy group, prepared by the anionic polymerization of masked disilene, adopt a preferential helical-sense conformation in solution at low temperature and also in the solid state at room temperature. The dependence on the molecular weight of the helical induction is also discussed. In a good/poor mixed solvent system, the induction of a helical conformation in the polysilanes was observed when forming aggregates.