Four series of multiarmed chiral aryl ethers carrying two, three, five, or eight side-chains on a variety of aromatic core molecules (2-5) were prepared. The structure and conformation of 2 and 3 (in the solid state) were determined by the X-ray crystallographic analyses. While a pair of alternated (anti) conformers (i.e, up-down and down-up) were found in the crystal of 2, three side-arms in 3 were aligned in the same direction to give a C(3)-symmetric syn-conformation. Examinations by dispersion-corrected density functional (DFT-D) calculations revealed that two out of six anti- and two out of four syn-conformers of 2 are energetically most important. Two calculated structures of anti-conformers are in good agreement with those found in the solid state by X-ray analysis. Similarly, relevant conformations of syn-3, fully alternated 4, and C(5)-symmetric 5 were optimized at the DFT-D-B-LYP/TZVP level. The structure and conformation of the side-arms in 2-5 in solution were further studied by temperature dependent (1)H NMR and UV-vis spectroscopy. In addition, comparative experimental and theoretical CD spectral studies were carried out in order to elucidate the contribution of the thermodynamically less-stable minor isomers in solution. The CD spectral changes observed for 2 and 3 at varying temperatures were quite different, while the parent chiral arene 1, as well as 4 and 5, only showed an increased intensity of the negative Cotton effect for the (1)L(b) band. The latter behavior is readily accounted for in terms of the conformational freezing of the chiral groups at low temperatures. The unusual CD spectral behavior observed for 2 and 3 was rationalized by the conformational alteration of the side-arms. Because of attractive van der Waals interactions between the aromatic units of the arms in nonpolar solvents, the syn-conformations become gradually more important for 2 at low temperatures, which eventually results in a weak positive Cotton effect for the (1)L(b) band. This was also supported by the SCS-MP2/TZVPP single-point energy calculations for the relevant conformers of 2. For 3, the contribution of the C(3)-symmetrical conformer becomes more important than the less-symmetrical isomers at low temperatures. The conformations of 2 and 3 in their excited states as well as in the oxidized states were also examined.
Owing to the absence of a band gap, the direct applicability of graphene for transistors and logic devices is still limited. Recent studies have shown that the van der Waals heterostructure of graphene and other two-dimensional (2D) materials can be a great candidate to overcome these issues. The layered structure of these materials exhibits no direct chemical bonding between the different layers. Thus the vdW junction between these materials is, in principle, free from lattice mismatch issues and interface reactions. With these advantages, these vdW heterostructures has been demonstrated vertical field effect transistor (FET) operations as well as large current ON-OFF ratio [1]. However, realization of a large ON-OFF ratio simultaneously with a large ON current density has been still challenging. Here we fabricated exfoliated-graphene/MoS 2 /Ti vertical heterostructure including vdW interface built from freshly cleaved surface of graphene and MoS 2 by using mechanical exfoliation and dry transfer method (Fig. 1(a)). The vertical transport of the device measured under the modulation of back gate voltage V G exhibits large modulation of its conductance (Fig. 1(b)). We obtained large current ON-OFF ratio of 10 5 as well as large ON current density of 10 4 A/cm 2 [2]. Such large current modulation is attributed to the modulation of the Schottky barrier at graphene/MoS 2 interface as schematically depicted in the right panel of Fig. 1(b). At the V G = +50 V, the Fermi level of the graphene is highest; therefore the Schottky barrier height at graphene/MoS 2 junction is lowest. The reduction of Schottky barrier height makes device to be low resistance ON state. On the contrary, at the V G = -50 V, the Fermi level of the graphene is lowest; therefore the Schottky barrier height is highest. Here current flow in the device is greatly suppressed and works as OFF state. The importance of Schottky-barrier modulation on the performance of the transistor could be confirmed by the plot between the current ON-OFF ratio I ON /I OFF vs. modulation of barrier height Δ φ = φ max - φ min ; these data are obtained from the series of devices with different thickness of MoS 2 (Fig. 1(c)); results reveal correlation between two values [3]. These results suggest that even larger current modulation can be possible if one could have larger modulation of Schottky barrier height. For this purpose, we compared MoS 2 , MoSe 2 , and α-MoTe 2 as transition metal dichalcogenide (TMD) materials in graphene/TMD/Ti heterostructure; these materials have different electron affinity and thus provide systematic change of band alignment to graphene (Fig. 1(d)). We show that the transport in the graphene/TMD junction is sensitive to the band alignment [4]. The V G dependence of the current revealed systematic change from MoS 2 to MoTe 2 (Fig. 1(e)). We demonstrated large current ON-OFF ratio and ON current density on MoSe 2 -based heterostructure; the value is comparable to that of MoS 2 -based device. These comparisons suggest that further improvement may be possible by optimizing device structure. We believe that our fabricated exfoliated-graphene/TMD/metal vertical heterostructure reveal superior performance to other existing graphene-based vertical transistors and present an important advance toward electronics applications. References [1] T. Georgiou et al. , Nature Nanotechnol. 8 (2013) 100. [2] R. Moriya et al. , Appl. Phys. Lett. 105 (2014) 083119. [3] Y. Sata, R. Moriya et al. , Jpn. J. Appl. Phys. 54 (2015) 04DJ04. [4] T. Yamaguchi, R. Moriya, et al. , Appl. Phys. Lett. 105 (2014) 223109. Figure 1
Polyethylene-graft-poly(n-butyl acrylate) (PE-g-PBA) and polyethylene-graft-poly(methyl methacrylate) (PE-g-PMMA) were prepared through the combination of metallocene-catalyzed ethylene/10-undecen-1-ol copolymerization and conversion of the copolymer into a macroinitiator for atom transfer radical polymerization (ATRP). Well-defined acrylate and methacrylate graft copolymers were successfully prepared under dilute ATRP conditions. This was confirmed by analyses of the detached side chains. The molecular weights of both PBA and PMMA side chains agreed with predicted values, and molecular weight distributions were narrow. The PE-g-PMMA was evaluated as a compatibilizer for a blend of PE and PMMA and demonstrated the potential of such nonpolar/polar graft copolymers as a compatibilizing agent.
γ-Cyclodextrin (CD) derivatives with a naphthalene moiety anchored to one or two of the glucose units of the CD were synthesized in order to investigate the effects of flexible and rigid capping upon complexation, as well as Förster resonance energy transfer (FRET) and photochirogenic behavior of anthracenecarboxylate (AC) moieties. UV-vis, circular dichroism and fluorescence spectral studies revealed that two AC molecules are simultaneously included in the modified γ-CD cavity to form a right-handed screw and also that the naphthalene cap efficiently transfers the singlet energy to AC included in the CD cavity via the FRET mechanism. Compared to native γ-CD, the modified γ-CDs showed much higher first association constants (K(1)) but relatively lower second association constants (K(2)) for AC, leading to two-fold larger overall affinities (K(1)K(2)). Photocyclodimerization of AC with these modified γ-CDs produced more head-to-head (HH) dimers in much better enantiomeric excesses (ee) for anti-HH dimer compared to native γ-CD. Interestingly, FRET excitation further enhanced the chemical and optical yields of anti-HH dimer up to 36% and 35% ee, for which the highly efficient FRET sensitization within the CD cavity, minimizing the "contamination" from the achiral "outside" photoreaction, is responsible. FRET sensitization also enabled us to achieve the catalytic photocyclodimerization of AC with a sub-equivalent amount of chiral supramolecular host.
New titanium complexes 5−8 with two indolide−imine chelate ligands [7-(RNCH)C8H5N]2TiCl2 (R: 5, phenyl; 6, 2,6-difluorophenyl; 7, 2,4,6-trifluorophenyl; 8, pentafluorophenyl) were synthesized and investigated as ethylene polymerization catalysts. On activation with methylalumoxane (MAO), all of the complexes were active ethylene polymerization catalysts at 25 °C to produce linear polyethylenes. The catalytic activity (TOF) increased sharply with the number of fluorine atom in the ligand. In addition, complexes 5−8 potentially exhibit the characteristics of a living ethylene polymerization. Complexes 6 and 7 promoted room temperature living ethylene polymerization to produce polyethylenes having extremely narrow molecular weight distributions (6, Mw/Mn 1.09−1.14; 7, Mw/Mn 1.05−1.23). On the other hand, at −10 °C complex 8 afforded monodisperse polyethylenes (Mw/Mn 1.12−1.25), with exceptionally high activities for a living ethylene polymerization (TOF: maximum 1155 min-1 atm-1). Using complex 7/MAO catalyst system, a polyethylene-b-poly(ethylene-co-propylene) block copolymer was successfully synthesized.
Abstract Multiple introduction of phosphonate groups around the quinone oxygen greatly deforms the p-benzoquinone ring through the steric and electronic repulsion. The repulsion produces not a boat but a chair conformation with a bent angle as large as 17.8°.
The electronic circular dichroism (CD) spectra were recorded for three diastereomeric eclipsed-staggered pairs of charge-transfer cyclophanes with different substituents, i.e., (4R(p);12R(p))- and (4S(p);12R(p))-12,15-dimethoxy[2.2]paracyclophane-4,7-dicarboxylic acid derivatives (1a-c and 2a-c, where a, b, and c denote methyl ester, carboxylic acid, and carboxylate, respectively). The effects of altering the donor-acceptor interaction between the π systems on the chiroptical properties were experimentally investigated. The anisotropy (g) factors of eclipsed species 2 were not significantly affected by the sort of substituent, while staggered 1 behaved in a significantly different way in CD spectra depending on the charge, affording contrasting CD profiles for neutral a/b versus anionic c. This study provides not only the novel insights into the planar chirality of substituted [2.2]paracyclophanes but also a basis for the potential application of such dramatic CD spectral difference between the acid-base pair to the chiroptical pH-sensors.
Abstract Aus dem Diol (I) werden Makrocyclen (III) hergestellt, die sauer oder bei katalytischer Hydrierung zu Diolen (IV) gespalten und dann weiter zu den spirocyclischen Makroethern (V) kondensiert werden.
On direct photoisomerization of cyclo-octene, a trans : cis photostationary ratio of 49 : 51 was observed, which is anomalously high for a strained cycloalkene, whereas the trans : cis ratios on toluene photosensitization in the liquid and vapour phase were 5 : 95 and 13 : 87, respectively.
