Effect Of Molecular Length Research Articles

Molecular rectification assisted by spin-polarized hybrid interfacial states

Based on first-principles method, we construct molecular rectifiers by designing asymmetric hybrid interfacial states. The devices contain a ferromagnetic interface with sp3-d hybrid interfacial states and a nonmagnetic interface without hybrid interfacial states. The transport calculation demonstrates that obvious charge-current rectification and spin-current rectification are realized. The analysis of spin-resolved transmission reveals that the asymmetric shift of hybrid interfacial states plays an important role in determining the rectification ratio as well as the spin polarization orientation of the spin current, and hence the form of spin-current rectification. The effect of molecular length is further discussed. A length-induced increase of rectification ratio is observed for the charge-current rectification, while the largest spin-current rectification ratio appears at a specific length. This work demonstrates the potential of hybrid interfacial states in designing excellent molecular spinterface devices.

Effect of terminal metamers and homologous with imine and ester groups on mesomorphism and their DFT study

ABSTRACT Three liquid crystalline series with different terminal chains have been prepared, where two of the series are metamer of each other, and all three series are homologues of each other in context to the terminal chain. All the homologues prepared were found to be enantiotropically mesogenic and were screened under a polarising optical microscope (POM). Differential scanning calorimetry (DSC) technique was used to obtain thermograms of phase transition, enthalpy and entropy change. Characterisation of all homologues was carried out by means of standard spectroscopic methods; infrared, proton magnetic resonance and ultraviolet spectroscopy. Series I n exhibited polymorphism as of smectic C, smectic A and nematic mesophases, while series II n and III n exhibit only smectic A and nematic mesophases. The geometry of all homologues was obtained using DFT calculation, and the effect of the twist angle of the aromatic ring on mesomorphism was examined. The effect of branching and length of the broken alkoxy terminal chain on the type of the mesophase was studied by comparing the terminal chain homologues. The prepared series were also compared with four structurally similar compounds to understand the effect of the relative position of groups and the effect of molecular length on thermal stability and mesophase length.

Effect of proportion on rectification in organic co-oligomer spin rectifiers

The rectification behaviours in organic magnetic/nonmagnetic co-oligomer spin rectifiers are investigated theoretically. It is found that both the charge current and the spin current through the device are rectified at the same time. By adjusting the proportion between the magnetic and nonmagnetic components, the threshold voltage and the rectification ratio of the rectifier are modulated. A large rectification ratio is obtained when the two components are equal in length. The intrinsic mechanism is analysed in terms of the asymmetric localization of molecular orbitals under biases. The effect of molecular length on the rectification is also discussed. These results will be helpful in the future design of organic spin diodes.

Metal-Molecule-Silicon Junctions Produced by Flip Chip Lamination of Dithiols: Effect of Molecular Length and Backbone

The integration of organic molecules with silicon is increasingly being studied for potential as hybrid electronic devices. Creating dense and highly ordered organic monolayers on silicon with reliable metal contacts still remains a challenge. A novel technique, flip chip lamination (FCL), has been developed to create uniform metal-molecule-semiconductor junctions. FCL uses nanotransfer printing to covalently attach self-assembled monolayers to a hydrogen-passivated Si(111) surface. Several dithiol molecules were studied to explore the role of molecular length and chemical structure on the physical and electronic properties of the molecules. The effects of the FCL process on the chemical and physical properties of the imbedded molecular layer were interrogated with polarized-backside reflectance absorption infrared spectroscopy. Electrical measurements were also performed to characterize device structure and to offer better insight into the mechanisms at play in the electronic transport.

Density functional theory for molecular orientation of hard rod fluids in hard slits

A density functional theory (DFT) is used to investigate molecular orientation of hard rod fluids in a hard slit. The DFT approach combines a modified fundamental measure theory (MFMT) for excluded-volume effect with the first order thermodynamics perturbation theory for chain connectivity. In the DFT approach, the intra-molecular bonding orientation function is introduced. We consider the effects of molecular length (i.e. aspect ratio of rod) and packing fraction on the orientations of hard rod fluids and flexible chains. For the flexible chains, the chain length has no significant effect while the packing fraction shows slight effect on the molecular orientation distribution. In contrast, for the hard rod fluids, the chain length determines the molecular orientation distribution, while the packing fraction has no significant effect on the molecular orientation distribution. By making a comparison between molecular orientations of the flexible chain and the hard rod fluid, we find that the molecular stiffness distinctly affects the molecular orientation. In addition, partitioning coefficient indicates that the longer rodlike molecule is more difficult to enter the confined phase, especially at low bulk packing fractions.

Effect of molecular structure on the phase behaviour of some liquid crystalline compounds and their binary mixtures VI[1]. The effect of molecular length

The apparent solution dipole moments of compounds based on 4,4-di-substituted phenyl benzoate (ROC6H4COOC6H4X), I, where X is a cyano group and R a terminal linear alkyl chain ranging from C12 to C20, were determined in cyclohexane at 30 C. The compounds were also thermally characterized by polarized light microscopy and differential scanning calorimetry. Phase diagrams were constructed for various binary mixtures prepared from any two homologues of series I as well as for every one of them with the nitro analogue II, where X is a nitro group and R C16H33. The study was undertaken in order to investigate the effect of the alkyl chain length on the mesophase behaviour. In order to assess the influence of structural variation in the central mesogenic group on the mesophase stability of pure and mixed compounds, the investigation was extended to cover binary mixtures of any two of the three compounds: analogue II (X=OC16H33), and the symmetric dialkoxy substituted phenyl esters (C16H33OC6H4COO)2 A, where A is the 1,4-phenylene group (IV) or 4,4-biphenylene (V).

Melting of n-butanol and n-pentanol monolayers adsorbed on graphite: Effect of molecular length on melting

The structures and melting of n-butanol and n-pentanol monolayers adsorbed on graphite have been studied by x-ray diffraction. The structures of both monolayers change between partial and complete monolayers. The complete monolayer of butanol, as well as the partial monolayer of pentanol, take herringbone structures similar to those of butane and hexane monolayers and have zigzag chains of hydrogen bonds. Two-dimensional smectic liquid crystals are formed in melting processes of the complete monolayers of both molecules while the partial monolayers melt directly into liquids. In the mesophases their solid structures are essentially maintained. The results are compared with those of the monolayers of shorter linear-chain alcohol molecules and the effect of molecular length on the melting behavior is discussed.

Molecular dynamics studies of the melting of butane and hexane monolayers adsorbed on the basal-plane surface of graphite

The effect of molecular steric properties on the melting of quasi-two-dimensional solids is investigated by comparing results of molecular dynamics simulations of the melting of butane and hexane monolayers adsorbed on the basal-plane surface of graphite. These molecules differ only in their length, being members of the n-alkane series [CH3(CH2)n−2CH3] where n=4 for butane and n=6 for hexane. The simulations employ a skeletal model, which does not include the hydrogen atoms explicitly, to represent the intermolecular and molecule–substrate interactions. Nearest-neighbor intramolecular bonds are fixed in length, but the molecular flexibility is preserved by allowing the bend and dihedral torsion angles to vary. The simulations show a qualitatively different melting behavior for the butane and hexane monolayers consistent with neutron and x-ray scattering experiments. The melting of the low-temperature herringbone (HB) phase of the butane monolayer is abrupt and characterized by a simultaneous breakdown of translational order and the orientational order of the molecules about the surface normal. In contrast, the hexane monolayer exhibits polymorphism in that the solid HB phase transforms to a rectangular-centered structure with a short coherence length in coexistence with a fluid phase. A significant result of the simulations is that they demonstrate the importance of molecular flexibility on the nature of the melting transition. The formation of gauche molecules is essential for the melting process in the hexane monolayer but unimportant for butane. The effect of molecular length on the qualitative nature of the melting process is discussed for both monolayers.

Effects of Molecular Length on Nematic Mixtures. III. Anisotropic Properties of 4-Alkylphenyl 4-Alkoxybenzoate Mixtures

Abstract Properties of multicomponent nematic mixtures of 4-alkylphenyl 4-alkoxybenzoates are studied as a function of temperature and their average molecular length (L). The L's vary between 21.47 and 27.67 A for mixtures with clearpoints in the 43 to 60°C range. The refractive indices, density, and the e1 component of dielectric permitivity all decrease approximately linearly as L increases. There are much larger changes in flow viscosity (η), conductivity anisotropy ([sgrave]|/[sgrave]|), and dielectric anisotropy (Δe). At 25°C as L increases: The is relatively high and increases sharply (from 47 to 78 cP), the [sgrave]|/[sgrave]| decreases and drops below unity (from 1.35 to 0.58), and the Δe changes from positive to negative (from +0.27 to -0.26). The temperature dependence of [sgrave]|/[sgrave]|, with tetrabutyl-ammonium tetraphenylboride as a dopant, indicates cybotactic nematic characteristics are present for the longer L mixtures in which there are an average of about nine or more alkyl carbons f...

Effects of Molecular Length on Nematic Mixtures

The properties of nematic liquid-crystal mixtures of 4-alkoxyphenyl trans-4-alkylcyclohexane-carboxylates are studied as a function of temperature and their average molecular length (L). Mixtures are prepared with clearpoints near 72° C (±3° C) and with [Lbar] varying between 21.20 and 26.15 A. At 25° C, their flow viscosity increases exponentially (from 16.3 to 50.6 cP) as [Lbar] increases. The conductivity anisotropy (σ∥/στ) also varies tremendously, decreasing from 1.62 to 0.49 as L increases, when compared at 25° C with tetrabutylammonium tetraphenylboride as dopant. The low values of σ∥/στ and its temperature dependence indicate that cybotatic nematic characteristics occur when the average total number of alkyl carbons from both end groups is 8.5 or more. Short range smectic effects are dominant in the longer mixtures. All of the mixtures have a negative dielectric anisotropy, which linearly becomes less negative with increasing L. At 25° C, dynamic scattering (DS) is observed only in the sh...

Effects of Molecular Length on Nematic Mixtures

Abstract The properties of nematic liquid-crystal mixtures of 4-alkoxyphenyl trans-4-alkylcyclohexane-carboxylates are studied as a function of temperature and their average molecular length (L). Mixtures are prepared with clearpoints near 72° C (±3° C) and with [Lbar] varying between 21.20 and 26.15 A. At 25° C, their flow viscosity increases exponentially (from 16.3 to 50.6 cP) as [Lbar] increases. The conductivity anisotropy (σ∥/στ) also varies tremendously, decreasing from 1.62 to 0.49 as L increases, when compared at 25° C with tetrabutylammonium tetraphenylboride as dopant. The low values of σ∥/στ and its temperature dependence indicate that cybotatic nematic characteristics occur when the average total number of alkyl carbons from both end groups is 8.5 or more. Short range smectic effects are dominant in the longer mixtures. All of the mixtures have a negative dielectric anisotropy, which linearly becomes less negative with increasing L. At 25° C, dynamic scattering (DS) is observed only in the sh...