Short Molecular Axis Research Articles

Molecular dynamics studies in isotropic phase of n-TPEB's by dielecolc relaxation method

The result of dielectric relaxation measurements (1 kHz ÷ 1 GHz) for series of mesogenic molecules C nH 2n+1-φ-φ-CH 2CH 2-φ-NCS, n-TPEB, (n = 7 ÷ 10) in the isotropic phase, have been presented. Two Debye-type bands have been interpreted as a dielectric absorption due to the rotation around the long and short molecular axis. An optimization of the geometry of non-rigid TPEB molecules have been made using a quantum mechanics semi-empirical method PM3.

FT-IR study of liquid crystal molecules on alignment layers

Abstract Alignment control at the molecular level is crucial for realizing high-performance LCDs. In particular, the structure of the interface between the liquid crystal and the alignment layer must be clarified. By utilizing RAS (reflection-absorption spectroscopy), a highly sensitive FT-IR spectroscopic technique, we have obtained the following information about the orientation of liquid crystal molecules; (1) the cyano group in a liquid crystal molecule behaves like an electron donor with respect to an SiO layer, but like an electron acceptor with respect to a polyimide layer; (2) with the use of polarized IR spectroscopy, it was discovered that the liquid crystal 8CB is aligned with a slant away from the aligning direction, regardless of the type of alignment layer. The angle between the molecular short axis within the plane of the core and the surface normal was found to be smaller on polyimide layers than on SiO layers.

Position-dependent methylation effects on the triplet states of trans-2,2′-bipyridine, metal-free cis-2,2′-bipyridine and zinc(II) complex with 2,2′-bipyridine. A time-resolved EPR study

The anisotropy of the triplet sublevel populating rates was determined for the trans and metal-free cis conformers of 2,2′-bipyridine (bpy), methyl-2,2′-bipyridines (Mebpys), symmetrically substituted dimethyl-2,2′-bipyridines (Me 2bpys) and their Zn 2+ complexes in rigid solutions at 77 K by a time-resolved EPR technique. Following the conformational changes of bpy, Mebpys and Me 2bpys, the sublevel preferentially populated by intersystem crossing (ISC) changes from T z in the trans conformers to T y in the cis conformers (the y and z axes are close to the long and short in-plane molecular axis, respectively). The out-of-plane T x sublevels are preferentially populated in the Zn 2+ complexes with 6-Mebpy, 6,6′-Me 2bpy and 6,6′-dibutyl-2,2′-bipyridine. The observed anisotropy in the ISC of these Zn 2+ complexes is anomalous for a ligand-localized 3ππ * state.

The Polarized Absorption Spectrum of 1,3-Diphenyl-1,2-propadiene: Interaction between Mutually Perpendicular Equivalent π-Electronic Systems

Abstract The polarized UV absorption spectrum of 1,3-diphenyl-1,2-propadiene (DPA) was measured in a stretched polymer film at 101 K, and the polarization directions of the electronic absorption bands were determined. Pariser–Parr–Pople (PPP) calculations extended to three dimensions were performed, and the position, intensity, and polarization of each absorption band of DPA were obtained from the calculations. The observed 289.8 and 265.2 nm bands polarized along the longer molecular axis were assigned to the calculated S1←S0 and S3←S0 transitions, respectively, and the 237.1 nm band along the shorter molecular axis was assigned to the S4←S0 transition. The results of configuration analysis indicate that the second transitions (at 260.5 nm) of two styrenes, whose π-electronic systems are perpendicular to each other, contribute equally to the third and fourth transitions observed at 265.2 and 237.1 nm of DPA. In addition, it was clarified that the first absorption band of DPA corresponding to Parity’s forbidden transition was intensified by an increase of the ionization energy of the sp hybridized carbon atom in the allenic moiety.

Molecular distributions in interphases: statistical mechanical theory combined with molecular dynamics simulation of a model lipid bilayer

A mean-field statistical mechanical theory has been developed to describe molecular distributions in interphases. The excluded volume interaction has been modeled in terms of a reversible work that is required to create a cavity of the solute size against a pressure tensor exerted by the surrounding interphase molecules. The free energy change associated with this compression process includes the configuration entropy as well as the change in conformational energy of the surrounding chain molecules. The lateral pressure profile in a model lipid bilayer (30.5 A2/chain molecule) has been calculated as a function of depth in the bilayer interior by molecular dynamics simulation. The lateral pressure has a plateau value of 309 +/- 48 bar in the highly ordered region and decreases abruptly in the center of the bilayer. Model calculations have shown that for solute molecules with ellipsoidal symmetry, the orientational order increases with the ratio of the long to short molecular axes at a given solute volume and increases with solute volume at a given axial ratio, in accordance with recent experimental data. Increased lateral pressure (p perpendicular) results in higher local order and exclusion of solute from the interphase, in parallel with the effect of surface density on the partitioning and local order. The logarithm of the interphase/water partition coefficient for spherical solutes decreases linearly with solute volume. This is also an excellent approximation for elongated solutes because of the relatively weak dependence of solute partitioning on molecular shape. The slope is equal to (2p perpendicular - p parallel)/3KBT, where p parallel is the normal pressure component, and different from that predicted by the mean-field lattice theory. Finally, the lattice theory has been extended herein to incorporate an additional constraint on chain packing in the interphase and to account for the effect of solute size on partitioning.

A microscopic model considering the effect of molecular biaxiality on ferroelectric ordering of liquid crystalline smectic C∗ phases

Abstract On the basis of a mean-field approach, a microscopic model is presented to explain some ferroelectric properties of induced smectic C∗ phases of chiral dipolar guest molecules in an achiral smectic C host phase. The molecular biaxiality of the chiral dopants has been taken into account, resulting in a polar and quadrupolar ordering of the molecular short axes in the rotational distribution function of the guest molecules. This model explains the dependence of the magnitude and sign of the spontaneous polarization on the molecular structure of a series of cyclo-hexanone derivatives used as chiral dopants, as well as the effect of a local field at higher dopant concentrations.

Dielectric Studies on Liquid Crystals under High Pressure: V. Static Permittivity and Low Frequency Relaxation Process in 4‐n‐Heptyl‐4'‐Cyanobiphenyl (7CB)

AbstractDielectric studies have been performed on 4‐n‐heptyl‐4'‐cyanobiphenyl, 7CB, between 305 and 341 K and at elevated pressures up to 150MPa. From measurements of the dielectric relaxation (up to 13 MHz) we calculate the activation enthalpy (decreasing with increasing pressure) and the activation volume (decreasing with increasing temperature) in the nematic state. The low‐frequency process connected with molecular rotations about the short molecular axis was analysed in terms of the Martin‐Meier‐Saupe theory, yielding q ≈︁ 5 kJ/mol for the nematic potential. The dielectric behaviour is similar to previous results for 5CB and 6CB.

Dielectric Properties in Ferroelectric and Antiferroelectric Liquid Crystals with Isotropic-Chiral Smectic C Phase Sequence

Dielectric properties in ferroelectric and antiferroelectric liquid crystals which show isotropic (Iso.)→chiral smectic C (SmC*) and Iso.→chiral smectic CA (SmCA*) phase transitions have been studied. In the ferroelectric liquid crystal with the Iso.-SmC* phase sequence, two dielectric relaxations are observed under dc bias voltage. One appears around 700 kHz in frequency and is attributed to the soft mode. The other is related to the contribution of the Goldstone mode whose relaxation frequency is increased by the application of bias voltage. The antiferroelectric liquid crystal with the Iso.-SmCA* phase sequence also has two relaxations in the SmCA* phase. One originates from the molecular rotation around the short molecular axis and the other may correspond to the soft mode.

Dielectric Properties of Mesomorphic n-TPEB's in Isotropic Phase

Dielectric relaxation studies have been performed for the isotropic phase of a series of mesogenic molecules of l-[3-n-alkyl-biphenyl]-2-[4-isothiocyanatophenyl]ethane, CnH2n+1-Φ-Φ-CH2CH2-Φ-NCS (n-TPEB), for n = 7, 8 and 9. Two Debye-type dielectric absorption bands have been interpreted as corresponding to the rotation around the short and long molecular axis.

Crystal Structures of Metallic and Insulating Molecular Complexes between Naphthaceno[5,6-cd: 11,12-c′d′]bis[1,2]diselenole and 4,8-Bis(dicyanomethylene)-4H,8H-benzo[1,2-c: 4,5-c′]bis[1,2,5]thiadiazole: (TSeN)(BTDA-TCNQ) and (TSeN)(BTDA-TCNQ)(C6H5Cl)

Abstract The charge-transfer complex between naphthaceno[5,6-cd:11,12-c′d′]bis[1,2]diselenole (TSeN) and 4,8-bis(dicyanomethylene)-4H,8H-benzo[1,2-c:4,5-c′]bis[1,2,5]thiadiazole (BTDA-TCNQ) is found to show at least three different types of crystal modifications; insulating Type-I has a (1 : 1 : 1) ratio containing solvent molecules (chlorobenzene), metallic Type-II has a (1 : 1) stoichiometry, and semiconductive Type-III is conjectured to have a (1 : 2) or (2 : 1) stoichiometry. The crystal structures of Type-I and Type-II were determined by a single-crystal X-ray diffraction method. Type-I comprises mixed-stack molecular columns, and Type-II segregated-stack molecular columns. In both types the neighboring columns are linked along short molecular axes by two Se···N contacts that are shorter than the corresponding van der Waals contact. In the Type-II crystal, there are other short distances between TSeN and BTDA-TCNQ in the direction of the long molecular axes. From the C=C bond length in the C=C(CN)2 group of BTDA-TCNQ, the degrees of charge transfer of these molecular complexes were estimated as 0.2 ± 0.3 in Type-I and 0.9 ± 0.2 in Type-II.

Laser dye spectroscopy of some pyrromethene-BF_2 complexes

To improve the laser action properties of the pyrromethene-BF(2) (P-BF(2)) complexes, we studied substitution effects at different positions of the dye molecule on the electronic spectra of several derivatives. Specifically, we used laser photoselection spectroscopy to measure the triplet-triplet (T-T) absorption and polarization spectra as well as the fluorescence and visible absorption singlet-singlet (S-S) spectra of the following compounds: 1,2,3,5,6,7,8-heptamethyl; 8-acetoxymethyl-1,3,5,7-tetramethyl-2,6-diethyl; 1,3,5,7-tetramethyl-8-p-methoxyphenyl; 3,5-dimethyl-1,7-diphenyl; and 1,3,5,7,8-pentamethyl- 2,6-diphenyl P-BF(2) complexes. The 1,3,5,7-tetramethyl P-BF(2) complex itself exhibits weak T-T absorption, which stretches from the green to the near-IR spectral region. This band consists of two overlapping (differently polarized) T-T transitions. Short molecular axis (i.e., 8 and 1,7 positions) substitution causes the positively polarized T-T transition to gain considerably in intensity. Significantly, the negatively, long-axis-polarized T-T transition was unaffected by the long-axis 2,6- position disubstitution. Therefore, only the 2,6-position disubstitution is expected to produce superior new P-BF(2) complex laser dyes. Substitution effects from strongly interacting groups with chromophores on S-S as well as T-T absorption spectra, together with cw laser photoselection spectroscopy, are briefly reviewed.

NMR study of molecular dynamics in a mixture of two polar liquid crystals (CBOOA and DOBCA)

Abstract A mixture containing 70 per cent 4-cyanobenzylidene-4′-n-octylaniline (CBOOA) and 30 per cent 4-n-dodeciloxybenzylidene-4′-cyanoaniline (DOBCA) has been studied by optical microscopy, differential scanning calorimetry, X-ray diffraction and, primarily, by NMR spectroscopy. The smectic phase of the mixture is a partial bilayer smectic A phase (SAd), in which about one half of the molecules are associated in pairs through their polar cyano end groups. The molecular dynamics in the smectic Ad phase of the mixture was studied by proton spin-lattice relaxation. Self-diffusion and rotations/reorientations were found to be essential relaxation mechanisms while the contribution of the order director fluctuations seems to be very small in all frequency regions. The correlation times associated with the molecular rotational motion around the short molecular axis, τS, are of the order 10−10 s and the ratio τS/τL ≈6.2. The translational diffusion coefficients D∥ are of the order 10−11m2s−1. The possible cont...

Comparative dielectric investigations of two liquid-crystalline side chain polymers

Abstract Dielectric relaxation measurements of two isomeric siloxane polymers with opposite direction of the carboxyl group in the liquid crystal part of the side chain were carried out in the frequency and temperature range from 10 Hz to 10 MHz and 150 K to 400 K. For every sample two intensive absorption ranges (∊″ >0·1) and two dielectric absorptions with lower intensities (∊″ <0·05) were detected. The first two are interpreted as reorientation processes of the side group as a whole around the short and long molecular axes, respectively. The less intensive absorptions can be observed below the glass temperature. They are discussed with respect to the different structures as the reorientation of the carboxyl electric dipoles in the middle part of the side group and of the terminal methoxy group, respectively.

Investigation of electric dichroism of cetylpyridinium bromide by spectroelectrochemistry with a long optical path length thin-layer cell

In this paper, the electric dichroism of cetylpyridinium bromide (CPB) has been found and studied by spectroelectrochemistry with a long optical path length thin-layer cell (LOPTLC) for the first time. The CPB molecule with a long carbon chain and a polar pyridinium ring is anisotropic in molecular configuration or in polarizability. In the electric field of a thin-layer cell, the CPB molecule reorientates along the direction of the electric field and exhibits electric dichroism, which results in the increase of absorbance of CPB in the UV-vis range. By use of in situ measurement of spectroelectrochemistry, the order parameters of long molecular axis (S = 0.845) and short molecular axis (D = 0.155) and the angle between the long axis direction of the CPB molecule and the direction normal to the electrode surface (theta = 18-degrees 44') have been determined. These data were used to describe the state of arrangement of the molecules in the solution. The reorientation of CPB molecules is the result of the interaction between the anisotropic molecules and electric field. The effects of the concentration of CPB and of the applied electric field on the electric dichroism have been investigated.

Dielectric Relaxation and Short Range Order of a Liquid Crystalline Benzoate

AbstractDielectric investigations on 4‐nitrobenzyl 2,5‐bis‐(4‐n‐hexyloxybenzoyloxy)‐benzoate A (10Hz – 100MHz) and a mixture of A and rodlike benzoates B (10Hz – 100kHz) reveal for A two main relaxation mechanisms in the liquid crystalline phases. The low frequency one connected with the reorientation around the short molecular axis shows in mixtures an anormal increase of the intensity with temperature. The unexpected behaviour of the static dielectric constants of A as well as the systematic change of the dielectric data in the mixture with B are interpreted by a systematic change of the antiparallel correlation of A in the short range.

Dynamic Investigation in a Nematic Mixture with High Glass‐Transition Temperature

AbstractWörterDielectric measurements have been performed on oriented samples of a nematic mixture with a glass‐transition temperature of about 285 K in a frequency range between 0.1 and 106 Hz and on unoriented, quenched samples of this mixture from 156 K up to 300 K. Rotations around the short and long molecular axis gave a Vogel‐Fulcher‐Tammann‐behaviour; the seperate rotation of polar end‐groups caused the appearance of a high‐frequency‐Arrhenius‐like mechanism which was observable at very low temperatures.

ChemInform Abstract: Polarized Absorption Spectra of Some Aromatic Radical Anions in Stretched Polyethylene Films

Six different aromatic radical anions were generated in stretched polyethylene films by 60Co γ-ray irradiation at 77 K, and the polarized absorption spectra were observed at 77 K; the long and short molecular axis polarized components were determined by invoking the stepwise reduction procedure. The parent compounds are biphenyl, 4,4′-bipyridyl, phenathrene, benzo[c]-cinnoline, 1,2,3,4-dibenzanthracene and 1,2,3,4-dibenzophenazine. The restricted Hartree–Fock SCF m.o. CI calculations are in good agreement with the observed results. The spectral variations brought about by nitrogen atoms are interpreted in terms of a simple aza-perturbation treatment.

A linear dichroism and cyclic voltammetric study of substituted naphthoquinones and an ESR investigation of the corresponding anion and cation radicals

We have carried out an investigation on benzo[b]naphtho[2,3-e][1,4]dioxin-6,11-dione (1) and benzo[b]naphtho[2,3-e][1,4]dithi-in-6,11-dione (2) by absorption and linear dichroism spectroscopy and cyclic voltammetry. The corresponding anion and cation radicals have been produced and studied by ESR. From these studies it has been concluded that the main contribution to the low-energy absorption for both compounds is an intramolecular charge-transfer transition which is polarized along the short molecular axis.

Low Frequency Dielectric Relaxation Behaviour at the N-SA and SA-SC Transitions of 8S5

It is shown that the dielectric relaxation time, connected with the reorientation about the short molecular axis, changes more or less continuously at the transitions: N-SA and SA-SC of 4-n-pentylp...

Studies on the Molecular Orientation of Flavin Langmuir-Blodgett Films by FT-IR Spectroscopy

The molecular orientation of flavin Langmuir-Blodgett films was studied by FT-IR transmission, reflection absorption and polarized transmission spectroscopy. It was concluded from these measurements that the flavin (isoalloxazine) ring takes a biaxial orientation; the long axis of the isoalloxazine ring almost takes an in-plane orientation to the substrate plane and is almost parallel to the dipping direction of the substrate at the time of preparation of the film. It was also concluded that the short molecular axis is oriented obliquely to the film plane.