Overall water splitting using a semiconductor photocatalyst with sunlight has long been viewed as a potential means of large-scale H2 production from renewable resources. In general, the reaction can be accomplished when a photocatalyst is modified with a suitable cocatalyst that efficiently promotes water reduction. It is therefore essential to develop both photocatalysts and cocatalysts in harmony. Certain metal (oxy)nitrides are potential candidates as water-splitting photocatalysts because of their suitable band edge positions for water reduction/oxidation, small band gaps (<3 eV), and stability under irradiation. However, efficient water splitting using visible-light-responsive oxynitrides has still remained a challenge. This account describes our recent developments over the last 10 years of new oxynitrides as well as cocatalysts for overall water splitting under visible light.
Overall water splitting using a heterogeneous photocatalyst has attracted attention as a potential means of large-scale H2 production from renewable resources. In this article, oxynitride photocatalysts and nanoparticle cocatalysts designed for visible-light water splitting by our group are highlighted.
The development of a Pd-catalyzed highly enantio- and diastereoselective [3+2] cycloaddition of 5-vinyloxazolidinones and activated trisubstituted alkenes is described in detail. This protocol for the single-step construction of densely functionalized pyrrolidines with three contiguous stereocenters including vicinal quaternary stereocenters depends on the remarkable ability of phosphine ligands possessing a chiral ammonium ion to promote intermolecular cycloaddition reactions with a precise control of absolute stereochemistry. A series of control experiments show that a chiral ammonium–phosphine hybrid ligand enabled the individual, yet simultaneous stereocontrol of each chiral center in the annulation reaction. The reaction mechanism is also discussed with particular focus on the stereodetermining processes.
A Pd-catalyzed highly enantio- and diastereoselective [3+2] cycloaddition of 5-vinyloxazolidinones and trisubstituted alkenes is developed by using chiral ammonium–phosphine hybrid ligands. This protocol enables the single-step construction of densely functionalized pyrrolidines with three contiguous stereocenters including vicinal quaternary stereocenters.
Density functional theory (DFT) calculations were performed to analyse the magnetic properties of dinuclear Ni–Ni and Cu–Cu bis(phenolate)-bridged complexes with two antiferromagnetically-coupled metals. The estimated coupling constants J are −384.8 and −375.3 cm−1 for the two Cu complexes and −85.7 cm−1 for the Ni complex, which are consistent with the experimental values. Analysis of overlap integral can explain the difference in the coupling J between Ni and Cu complexes: Two dx2 − y2 orbitals of the copper complexes are located on the same plane as that of a phenolate ligand, and therefore produce the strong J coupling, whereas those of the nickel complex are tilted from the plane of phenolate ligand because of asymmetric environment around Ni atom, and produce moderate J coupling. We also examined the dependences of magnetic interaction in terms of several important geometrical parameters. The M–O–M bond angle is the key parameter for antiferro–ferromagnetic transition, which is consistent with previous studies for the other complexes. Also, increasing the M–O–O–M dihedral angle, decreasing the M–O–M bond angle and large hinge distortions on these complexes effectively enhance the ferromagnetic exchange, which is a desired condition for a better molecular magnet.
The ability of dichlorozirconium(IV) complex 4 incorporating an [OSSO]-type bis(phenolate) ligand with dMAO (dried methylaluminoxane) as an activator for the copolymerization of ethylene with iPr3Si-protected 5-hexen-1-ol, 5-hexen-1-oxytriisopropylsilane (5H1OSi), is described. This catalyst system efficiently promoted ethylene copolymerization with different feed ratios of 5H1OSi at 25 °C under an atmospheric pressure producing ethylene/5H1OSi copolymers with molecular weights Mw of 3800–10800 g mol−1. In the 1H NMR spectroscopic analysis, the contents of 5H1OSi in the copolymers were found to be in the range of 7.5–21.4 mol % depending upon the feed concentration of 5H1OSi. The sequence distributions and calculated reactive ratios (γEγP = 1.48–3.48) obtained from the 13C NMR spectra revealed that the catalytic system produced highly random ethylene/5H1OSi copolymers.
The solid-state photochromism of two diarylethenes, 1,2-bis(2,4-dimethyl-5-phenyl-3-thienyl)perfluorocyclopentene (1a) and 1,2-bis(2-methylbenzothiophen-3-yl)perfluorocyclopentene (2a), under shear stress has been investigated. It was found that 1a and 2a exhibited photochromic behavior, transforming from their open-ring isomers to closed-ring isomers during irradiation with visible light while under shear stress. This phenomenon was similar to that observed for cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (CMTE). These phenomena in 1a, 2a, and CMTE are intriguing because, ordinarily, photochromism in diarylethenes requires irradiation with ultraviolet (UV) light. The application of shear stress allowed non-photochromic crystal 2a to undergo photochromism. Visible absorption spectra for the three diarylethenes indicated that shear stress induced a red shift in the absorption, spanning from the UV region to the visible region. As a result, the diarylethenes in the solid state were able to absorb visible light around 400 nm for 1a and 2a, and 500 nm for CMTE, with photochromism confirmed by the appearance of a new absorption. Herein, we explore the comparative effects of shear stress and hydrostatic pressure.
Reaction mechanism and kinetics of gas-phase reaction of methylthiyl radical (CH3S (2A′)) with ozone (1A1) on the lowest doublet electronic state has been theoretically investigated. The composite method of G3MP2 was employed to obtain the optimized geometries of the stationary points and their energies. The most probable path for the title reaction is a barrier-less association channel to form a chemically activated intermediate of CH3SOOO*. The major products at lower temperatures are CH3SO and O2 while species like, CH3, SO, SO2, CH2S, CH2SOH, CH2SO, HO2, and OH are among the products of the probable secondary reactions. Direct hydrogen abstraction reaction of O3 with CH3S to form CH2S and HO3 is not an important channel at lower temperatures. Multichannel RRKM calculations along with the steady-state assumption for the energized intermediate and canonical variational transition-state theory have been used to predict the rate constants for the formation of the most important products. Rate constants for dissociation and isomerization of vibrationally “hot” CH3SO to form CH3 + SO and CH2SOH, respectively, are calculated by means of RRKM method, which are compared with those calculated for the vibrationally “cold” CH3SO. To comply with the previous experimental observations, the major product of the title reaction (CH3SO) should be produced as vibrationally “hot”.
The structural, electronic, and nonlinear optical (NLO) properties of pristine dodecadehydrotribenzoannulene (DBA) functionalizing with the alkali metal atom (Li and Na), NH2 and NO2 functional groups and its location are inspected through density functional theory (DFT) calculations. The NBO analysis indicates that functionalizing DBA caused increasing charge transfer (CT) and introduced a new acceptor–donor model for NLO materials due to the structure with of π-electron of DBA. The results demonstrate that the functional groups notably narrow the energy gap of DBA, and the electronic character of DBA is sensitive to the functional groups. Furthermore the considered functional groups enhance the static first and second hyperpolarizability of DBA. Interestingly, replacement of H atom by Na and NO2 in DBA leads to large static first hyperpolarizability (β0) values of 3977.40 and 3722.56 au and very great static second hyperpolarizability (γtot) 980380.40 and 752886.20 au, respectively, which is the largest among the considered functionalizing structures. Also transition energies (ΔE) decrease due to functionalizing, and the best location for functionalizing is position 2.
Insoluble hierarchical polybithiophene/poly(ethylenedioxythiophene) thin films with a double-layered structure were prepared on an indium tin oxide transparent electrode by sequential electrochemical polymerization. The formation of the hierarchical structure was supported by the transmission absorption spectra and Raman scattering spectra of the double-layered polythiophene thin films. In the presence of residual oxygen as an electron acceptor, light irradiation on the polythiophene-modified electrodes generated stable cathodic photocurrents. A broad peak at approximately 440 nm corresponding to the absorption of neutral polythiophene was observed in the incident photon to current conversion efficiency profile of the polybithiophene/poly(ethylenedioxythiophene)-modified electrode. The photocurrent density of the polybithiophene/poly(ethylenedioxythiophene)-modified electrode was higher than that of the poly(ethylenedioxythiophene)-modified electrode.
The currently accepted mechanism for nucleophilic attack at silicon in tetraalkoxysilanes, e.g. Si(OEt)4 is suggested to involve formation of penta- and then hexacoordinated intermediates as supported by the apparent exclusive formation of R3SiOR′ and R4Si from nucleophilic attack by RLi and RMgX. Our recent discovery of a direct route from biogenic silica to tetraalkoxyspirosiloxanes prompted us to revisit this reaction as a potential route to diverse silicon-containing species with single Si–C bonds as early studies demonstrate that spirosiloxanes form quite stable pentacoordinated alkoxysilane compounds. As anticipated, Si(2-methyl-2,4-pentanediolato)2 (SP) reacts with RLi (R = Ph, anthracene, phenylacetylene, etc.) at −78 °C to form pentacoordinated Si, e.g. LiPhSP equilibrates with the starting reagents even at 3:1 ratios of PhLi:SP with no evidence for formation of hexacoordinated species by mass spectral, NMR and quenching studies. Thus, quenching with MeI or Me3SiCl allows isolation of monosubstituted products from RLi:SP; RSi(OR′)3 including some ring-opened oligomers. Comparative studies of reactions of PhLi with Si(OEt)4 allows isolation of mono- and disubstituted products again even at 1:1 ratios of PhLi:Si(OEt)4. However, on standing at −78 °C for long periods of time or on warming to 0 °C, the primary product for both reactions is Ph4Si even with 0.5 equivalents of PhLi. At reaction temperatures ≥0 °C the primary product is again Ph4Si. These results suggest that hexacoordinated intermediates are not part of the substitution mechanism and may suggest that the higher-substituted compounds arise from disproportionation processes. We also briefly describe the conversion of anthracenylSP and 9,9-dimethylfluoreneSP to silsesquioxanes.
The relationship between molecular orientation and charge-transporting properties in vapor-deposited films of two doubly oxygen-bridged triphenylamine dimers, which differ in their connection mode, was examined. A comparison of the fluorescence spectra obtained from these films, as well as from solution and the crystalline state revealed that the dimers retain two different states in amorphous films, i.e., a monomer-like random orientation and a crystalline-like π-stacking orientation. Regarding the crystalline-like π-stacking orientation in the films, one dimer forms a one-dimensional π-stacking structure, whereas the other dimer forms a two-dimensional π-stacking structure, thus reflecting the difference in packing structure in the crystals. Time-of-flight measurements showed a strong dependence of the charge mobilities in the film of the dimer with a one-dimensional π-stacking structure on the electric field. The film of the dimer with a two-dimensional π-stacking structure exhibited ambipolar charge-transport properties with high electron and hole mobilities. When these dimers were used as hole injection layers in organic light-emitting diode devices, the external quantum efficiencies improved by a factor of 1.2 relative to standard devices.