Construction of Helical Structures with Multiple Fused Anthracenes: Structures and Properties of Long Expanded Helicenes

Polycyclic aromatic compounds consisting of four or five fused anthracene units were synthesized by PtCl₂-catalyzed cycloisomerization as novel long expanded helicenes. These compounds have helical structures with significant stacking of the terminal anthracene moieties at 0.33 nm interlayer distance. In the UV-vis and fluorescence spectra, the absorption and emission bands were red-shifted as the number of fused anthracene units was increased. The characteristic broad and long-lived emission bands of the long analogues are explained by the excimer-like stabilization of the excited state. These photophysical data as well as their cyclic voltammetric data are discussed on the basis of the π-conjugation and interlayer π⋅⋅⋅π interactions in the molecular structures and the molecular orbitals. The barrier and mechanism of helical inversion are also reported.     

Depolymerization of poly(2,6-dimethyl-1,4-phenylene oxide) under oxidative conditions

Depolymerization of an engineering plastic, poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), was accomplished by using 2,6-dimethylphenol (DMP) under oxidative conditions. The addition of an excess amount of DMP to a solution of PPO in the presence of a CuCl/pyridine catalyst yielded oligomeric products. When PPO (M(n)=1.0x10(4), M(w)/M(n)=1.2) was allowed to react with a sufficient amount of DMP, the molecular weight of the product decreased to M(n)=4.9x10(2) (M(w)/M(n)=1.5). By a prolonged reaction with the oxidant, the oligomeric product was repolymerized to produce PPO essentially identical to the starting material, making the oligomer useful as a reusable resource. During the depolymerization reaction, an intermediate phenoxyl radical was observed by ESR spectroscopy. Kinetic analysis showed that the rate of the oxidation of PPO was about 10 times higher than that of DMP. These results show that a monomeric phenoxyl radical attacks the polymeric phenoxyl to induce the redistribution via a quinone ketal intermediate, leading to the substantial decrease in the molecular weight of PPO, which is much faster than the chain growth.     

Preparation of hollow titania and strontium titanate spheres using sol–gel derived silica gel particles as templates

A facile approach, based on polyelectrolyte-mediated electrostatic adsorption of a water-soluble titanium complex on colloidal templates and hydrothermal treatment, is presented for the formation of hollow titania (TiO₂) and strontium titanate (SrTiO₃) spheres. Monodispersed silica gel particles were prepared by the sol?Cgel method and adopted as core templates. Deposition of a water-soluble titanium complex, titanium (IV) bis(ammoniumlactato)dihydroxide (TALH), on the silica gel particles was carried out via the layer-by-layer assembly technique. Hollow spheres were successfully formed from the core?Cshell particles. The silica gel particles used as core templates dissolved during hydrothermal treatment because of the particles?? undeveloped siloxane network. In addition, the hydrothermal treatment induced crystallization of the hollow shells. Therefore, the hydrothermal treatment played two roles; removal of the silica templates and crystallization of the hollow shells. When deionized water was used, hollow TiO₂ spheres were obtained. Hollow SrTiO₃ spheres could also be formed when an aqueous solution of Sr(OH)₂ was used. The approach presented here could be exploited as a novel and sustainable approach for the fabrication of a range of different inorganic hollow spheres.     

Robust Inclusion Complexes of Crown Ether Fused Tetrathiafulvalenes with Li+@C60 to Afford Efficient Photodriven Charge Separation

Inclusion complexes of benzo‐ and dithiabenzo‐crown ether functionalized monopyrrolotetrathiafulvalene (MPTTF) molecules were formed with Li⁺@C₆₀ ( 1? Li⁺@C₆₀ and 2? Li⁺@C₆₀). The strong complexation has been quantified by high binding constants that exceed 10⁶ M ^?1 obtained by UV/Vis titrations in benzonitrile (PhCN) at room temperature. On the basis of DFT studies at the B3LYP/6‐311G(d,p) level, the orbital interactions between the crown ether moieties and the π surface of the fullerene together with the endohedral Li⁺ have a crucial role in robust complex formation. Interestingly, complexation of Li⁺@C₆₀ with crown ethers accelerates the intersystem crossing upon photoexcitation of the complex, thereby yielding ³(Li⁺@C₆₀)*, when no charge separation by means of ¹Li⁺@C₆₀* occurs. Photoinduced charge separation by means of ³Li⁺@C₆₀* with lifetimes of 135 and 120 μs for 1? Li⁺@C₆₀ and 2? Li⁺@C₆₀, respectively, and quantum yields of 0.82 in PhCN have been observed by utilizing time‐resolved transient absorption spectroscopy and then confirmed by electron paramagnetic resonance measurements at 4 K. The difference in crown ether structures affects the binding constant and the rates of photoinduced electron‐transfer events in the corresponding complex.     

Interaction of poly(styrene sulfonate) with polycations carrying charges in the chain backbone

The interaction between sodium poly(styrene sulfonate) (NaSS) and side-chain charged polycation polymer (pendent type) or main-chain charged polycation polymer (integral type) has been studied. It was found that the polyion complex (the reaction product of these polyelectrolytes) of pendent–pendent type has an equimolar composition at any mixing ratio of two component polymers. However, a polyion complex of integral–pendent type can form a water-soluble complex with a ratio of [polycation]/[polyanion] = 1/3, in addition to a complex with a equimolar composition. The mechanism of formation of this specific complex is discussed.     

Selective and efficient transformation of N-(4-substituted benzoyl)-α-dehydroarylalanine alkyl esters into 4,5-dihydrooxazole derivatives via photoinduced electron transfer

The irradiation of N-(4-substituted benzoyl)-α-dehydroarylalanine alkyl esters (1) in methanol containing triethylamine (TEA) was found to quantitatively give cis- and trans-4,5-dihydrooxazole derivatives (2), which were described as being formed via electron transfer from TEA to the excited-state (E)-1 followed by kinetically-controlled cyclization of the (E)-1-derived anion radical. A product composition analysis showed that the cis-2/trans-2 composition ratio is greatly varied depending on the stereoelectronic properties of the substituents, the polarity of protic solvents and the concentration of TEA.     

The effect of iron spin transition on electrical conductivity of (Mg,Fe)O magnesiowüstite

We measured the electrical conductivity of Mg_0.81Fe_0.19O magnesiowüstite, one of the important minerals comprising Earth’s lower mantle, at high pressures up to 135 GPa and 300 K in a diamond-anvil cell (DAC). The results demonstrate that the electrical conductivity increases with increasing pressure to about 60 GPa and exhibits anomalous behavior at higher pressures; it conversely decreases to around 80 GPa and again increases very mildly with pressure. These observed changes may be explained by the high-spin to low-spin transition of iron in magnesiowüstite that was previously reported to occur in a similar pressure range. A very small pressure effect on the electrical conductivity above 80 GPa suggests that a dominant conduction mechanism changes by this electronic spin transition. The electrical conductivity below 2000-km depth in the mantle may be much smaller than previously thought, since the spin transition takes place also in (Mg,Fe)SiO₃ perovskite.