Kenkichi Sakamoto

CORRELATION BETWEEN THE PRETILT ANGLE OF LIQUID CRYSTAL AND THE INCLINATION ANGLE OF THE POLYIMIDE BACKBONE STRUCTURE

We have determined the inclination angle of the backbone structure of polyimide with alkyl side-chains in rubbed films. Four different polyimides with the same backbone structure but different lengths of alkyl side-chains were used in this study. The inclination angle of the backbone structure increases with the number of carbon atoms in the alkyl side-chains. We found a linear relation between the inclination angle of the backbone structure and the pretilt angle of liquid crystal (LC) that is in contact with the polyimide films. We conclude that the inclination of the polyimide backbone structure determines the pretilt angle of LC.

Anionic polymerization of masked disilenes. Regioselective synthesis of monomers, structures of polymers and mechanism of initiation for polysilylenes of the type (R1R2SiSiMe2)n

Abstract Several masked disilene monomers, i.e. 1-phenyl-7,8-disilabicyclo[2.2.2]octa-2,5-dienes, were prepared regioselectively by the reaction of unsymmetrically substituted dichlorodisilanes with biphenyl radical anions. These monomers gave polysilylenes with highly ordered structures. Important information on the tacticity of polysilylenes was obtained by detailed analysis of n.m.r. spectra. The mechanism of monomer formation as well as the polymerization mechanism are discussed.

Aromatic character of nanographene model compounds.

Superaromatic stabilization energy (SSE) defined to estimate the degree of macrocyclic aromaticity can be used as a local aromaticity index for individual benzene rings in very large polycyclic aromatic hydrocarbons (PAHs) and finite-length graphene nanoribbons. Aromaticity patterns estimated using SSEs indicate that the locations of both highly aromatic and reactive rings in such carbon materials are determined primarily by the edge structures. Aromatic sextets are first placed on the jutting benzene rings on armchair edges, if any, and then on inner nonadjacent benzene rings. In all types of nanographene model compounds, the degree of local aromaticity varies markedly near the edges.

Superaromatic Stabilization Energy as a Novel Local Aromaticity Index for Polycyclic Aromatic Hydrocarbons

Superaromatic stabilization energy (SSE), previously proposed by us, can be used as a novel local aromaticity index for benzene rings in polycyclic aromatic hydrocarbons (PAHs). SSE can be interpreted as the first local aromaticity index explicitly related to all relevant circuits in a polycyclic π-system, an origin of local aromaticity, being free of local aromaticity arising from adjacent six-site circuits. Therefore, this quantity is best suited for characterizing the electronic structure of large pericondensed PAHs and graphene nanoflakes.

Formation of a stannylstannylene via intramolecular carbene addition of a transient stannaacetylene (RSn[triple bond, length as m-dash]CR').

Photolysis of diazomethylstannylene [, Me(3)SiC([double bond, length as m-dash]N(2))(Ar)Sn:, Ar = C(6)H(3)-2,6-Tip(2), Tip = C(6)H(2)-2,4,6-(i-Pr)(3)] afforded unusual stannylstannylene via intramolecular carbene addition to an aromatic pi bond of stannaacetylene ; the structures of compounds and were fully characterized by X-ray crystallography.

Theoretical prediction of vertical transition energies of diaminosilylenes and aminosubstituted disilenes

Vertical electronic transition energies of diaminosilylenes and their dimers (disilenes and nitrogen‐bridged) were investigated by ab initio and density functional calculations. A good linear correlation was found between the observed UV transition energies of various silylenes and disilenes and those of model compounds calculated using the CIS and TD–DFT methods. On the basis of these computations the experimental UV absorption maximum observed for the dimer of (i‐Pr2N)2Si: (λmax 439 nm at 77 K), could be assigned to an SiSi bonded dimer with an unusually long SiSi distance of 2.472 Å, and the isomeric amino‐bridged cyclic dimer could be discarded.

Functional transformation of poly(dialkylaminotrimethyldisilene) prepared by anionic polymerization of the masked disilenes. The preparation of a true polysilastyrene

Abstract The SiN bond of the amino-substituted polysilane, poly[1,1,2-trimethyl-2-(dibutylamino)disilene] yields chloro-substituted polysilanes. Substitution with some nucleophiles, such as a Grignard reagent, organolithium reagents, hydrides, and alcohols, yields a new class of polysilanes. This synthetic route provides a true polysilastyrene with a head-to-tail structure.

Incident Angle Dependence of the Infrared Absorbance of Thin Rubbed Polyimide Films on CaF2 and Si Substrates

We have measured the incident angle dependence of the infrared (IR) absorption of thin rubbed polyimide films on CaF2 and Si substrates. The molecular orientation of the films was determined by fitting the experimental result with a theoretical calculation. The incident angle dependence for the rubbed film on CaF2 was very different from that for the rubbed film on Si. On the other hand, the actual molecular orientation determined by the fitting procedure is similar in both samples. Thus this result clearly shows that the incident angle dependence is strongly dependent on the substrate. This effect arises from the interference between the forward and backward propagating light in the film. This experiment demonstrates that the interference effect must be taken into account in determining the molecular orientation of thin films on solid substrates. We have also calculated the incident angle dependence for rubbed films with different inclination angles of polyimide chains on CaF2 and Si substrates. We found that CaF2 is a more suitable substrate than Si for determining the molecular orientation of rubbed films with a relatively high inclination angle of polyimide chains.

Orientation of the backbone structure of polyimide with alkyl side-chains : Determination by infrared absorption spectroscopy

Abstract By infrared absorption spectroscopy we have determined the orientation of the backbone structure of polyimide with alkyl side-chains in the rubbed film. The backbone orientation was determined from the dichroic ratio at normal incidence and the incident angle dependence of absorption by the IR active vibrations polarized along the backbone structure. We found that the backbone structure is oriented along the rubbing direction and tilted up on average by 23° from the surface. The pretilt angle of bulk liquid crystal (LC) in contact with this rubbed film is 9.6°. The average inclination angle of the backbone structure and the pretilt angle are much higher than those for the polyimide without a side-chain reported in our previous papers. This result indicates that the high inclination angle of the backbone structure of polyimide with alkyl side-chains plays an important role in causing a high pretilt angle in bulk LC.

Correlation field splitting of intramolecular vibrations of crystalline tetracene in terahertz region

The temperature dependence of terahertz (THz) absorption was observed for molecular crystals of tetracene using the Fourier transform infrared (FT-IR) spectroscopy. Over the frequency range of 1 to 6 THz (33 to 200 cm-1), THz absorptions corresponding to phonon and intramolecular vibrational modes were confirmed. Furthermore, the intramolecular vibrational modes exhibited correlation field splitting that originated from crystallization. A temperature-induced phase transition was successfully detected for the intramolecular vibrational modes of tetracene, even in powder form. Our results showed that these THz absorptions are highly sensitive to the molecular orientations within the crystal.