Cyano End Groups Research Articles

Thermodynamic Driving Forces for Substrate Atom Extraction by Adsorption of Strong Electron Acceptor Molecules.

A quantitative structural investigation is reported, aimed at resolving the issue of whether substrate adatoms are incorporated into the monolayers formed by strong molecular electron acceptors deposited onto metallic electrodes. A combination of normal-incidence X-ray standing waves, low-energy electron diffraction, scanning tunnelling microscopy, and X-ray photoelectron spectroscopy measurements demonstrate that the systems TCNQ and F4TCNQ on Ag(100) lie at the boundary between these two possibilities and thus represent ideal model systems with which to study this effect. A room-temperature commensurate phase of adsorbed TCNQ is found not to involve Ag adatoms, but to adopt an inverted bowl configuration, long predicted but not previously identified experimentally. By contrast, a similar phase of adsorbed F4TCNQ does lead to Ag adatom incorporation in the overlayer, the cyano end groups of the molecule being twisted relative to the planar quinoid ring. Density functional theory (DFT) calculations show that this behavior is consistent with the adsorption energetics. Annealing of the commensurate TCNQ overlayer phase leads to an incommensurate phase that does appear to incorporate Ag adatoms. Our results indicate that the inclusion (or exclusion) of metal atoms into the organic monolayers is the result of both thermodynamic and kinetic factors.

Stereoisomeric selection upon adsorption: A structural and optical study of curcuminoid derivatives on ultrathin films of KCl on Au(111) and on bulk KCl(001)

We present a combined scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM), and differential reflectance spectroscopy (DRS) study aiming at the characterization of the relationship between structural and optical properties of a supramolecular assembly of a curcuminoid derivative adsorbed on two kinds of substrates: bilayers of KCl on Au(111) [2 ML KCl/Au(111)] and KCl(001) bulk. The molecule features a difluoroboron (${\mathrm{BF}}_{2}$) complex as well as cyano (CN) end groups, which induces an internal dipole moment of a few Debyes. The comparative study of the structural properties of the molecules adsorbed on 2 ML KCl/Au(111), which allows for STM characterization, and KCl bulk, which allows for nc-AFM and DRS characterization, shows that the molecular adsorption is similar on both substrates. STM and nc-AFM give evidence that the on-surface condensation of the molecules into supramolecular assemblies is steered by two main interactions: the molecule-substrate interaction between the CN end groups of the molecule and the cationic ${\mathrm{K}}^{+}$ species, as well as an in-plane intermolecular interaction involving ${\mathrm{BF}}_{2}$ and phenyl groups. Based on density functional theory calculations, we propose an epitaxial molecular structure described by a two-molecule unit cell forming an $8\ifmmode\times\else\texttimes\fi{}2$ supercell on the KCl surface that grows in form of ribbons along the $\ensuremath{\langle}100\ensuremath{\rangle}$ or $\ensuremath{\langle}010\ensuremath{\rangle}$ directions of the substrate, which are polar. Among several possible stereoisomers for the molecule, it is found that the balance between vertical and in-plane interactions selects the one whose size matches the distance between ${\mathrm{K}}^{+}$ species to condense on the substrate and not the most stable one stemming from the gas phase. Finally, optical spectroscopy in solvent and DRS measurements performed on the supramolecular assembly exhibit similar features identified as three absorption peaks tracing a $\ensuremath{\pi}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{*}$ electronic transition followed by a vibronic progression, plus a broader band featuring a set of higher-energy transitions. Due to both the weak electrostatic interactions and a lack of $\ensuremath{\pi}\text{\ensuremath{-}}\ensuremath{\pi}$ interactions in the condensed molecular phase, DR spectra exhibit no major excitonic effect, but a nonrigid redshift of the molecular absorption band compared to the situation in solvent. This observation is found to be consistent with the isomer selection due to the transition from the gas phase to the on-surface condensed phase.

Azido alkanes as convenient reporters for mobility within lipid membranes

A test compound featuring azido and cyano end groups linked by an alkyl spacer, az-(CH2)11-CN (az11CN), was designed as a probe of mobility of synthetic lipid membrane interiors and mobility of solvents accessed via 2DIR spectral diffusion measurements. The results indicate that the az11CN compound is oriented in lipid membranes such that the azido group is well inside the membrane, while the cyano group resides in the region of moderate polarity around the carbonyl groups of the lipid. The lineshape of the azido moiety transition at 2100 cm−1 shows significant broadening in nonpolar solvents such as hexane. In silico analysis demonstrated that the structural softness of the azido group, intrinsic Fermi resonance at the azido site, and broad wagging, twisting, and rocking vibrational CH2 bands of the alkane chain result in a large number of Fermi resonances with the fundamental transition at 2100 cm−1, thus providing a smooth and featureless linewidth.

Optical and X-ray studies of a four component cyano liquid crystal mixture: Comparison of properties with that of a similar mixture

Optical and X-ray studies have been conducted on a four component liquid crystal mixture (code name 2013) with cyano–end groups in all the four components and a lateral substitution of fluorine atom in one of them. The variations in the optical birefringence with temperature, have been investigated and the thermal dependence of polarisability anisotropy and orientational order parameter have been evaluated. X-ray studies have been conducted to determine the effective molecular characteristics such as apparent molecular length and inter molecular distance and their variations with temperature. From nature of molecular alignment as envisaged from X-ray photographs the order parameter variation with temperature has been determined and compared with macroscopic order parameter variation as determined from birefringence studies. The results have been compared with these obtained for another structurally similar four-component liquid crystal mixture (code name 2014) with isothiocyanato end group in place of cyano.

Rheology of nematic liquid crystals with highly polar molecules.

We report experimental studies on the rheology of a few nematic liquid crystals with highly polar molecules (CCH-7, PCH-7, CB-7). The selected molecules have the same alkyl chain (-C(7)H(15)) and cyano (-CN) end group. In the core part of the molecule, CCH-7 has two cyclohexane rings, PCH-7 has one cyclohexane and one aromatic ring, and CB-7 has two aromatic rings. Two viscosities were measured as a function of temperature, namely, η(2) (director parallel to the shear direction) and η(1) (director perpendicular to the shear direction). The orientation of the director was studied using small angle light scattering techniques. η(2) was measured in presheared sample, whereas the electrorheological technique was used to measure η(1). We show that both viscosities of the liquid crystals depend on the number of aromatic rings and Kirkwood correlation factor. The temperature dependent viscosities can be understood based on the intramolecular π-electron conjugation and intermolecular association of highly polar molecules.

Current–voltage characteristics of single-molecule diarylethene junctions measured with adjustable gold electrodes in solution

We report on an experimental analysis of the charge transport through sulfur-free photochromic molecular junctions. The conductance of individual molecules contacted with gold electrodes and the current–voltage characteristics of these junctions are measured in a mechanically controlled break-junction system at room temperature and in liquid environment. We compare the transport properties of a series of molecules, labeled TSC, MN, and 4Py, with the same switching core but varying side-arms and end-groups designed for providing the mechanical and electrical contact to the gold electrodes. We perform a detailed analysis of the transport properties of TSC in its open and closed states. We find rather broad distributions of conductance values in both states. The analysis, based on the assumption that the current is carried by a single dominating molecular orbital, reveals distinct differences between both states. We discuss the appearance of diode-like behavior for the particular species 4Py that features end-groups, which preferentially couple to the metal electrode by physisorption. We show that the energetic position of the molecular orbital varies as a function of the transmission. Finally, we show for the species MN that the use of two cyano end-groups on each side considerably enhances the coupling strength compared to the typical behavior of a single cyano group.

Mixed self-assembled monolayers of azobenzene photoswitches with trifluoromethyl and cyano end groups

Mixed self-assembled monolayers (SAMs) of alkanethiolates carrying azobenzene chromophores with either a trifluoromethyl or a cyano substituent have been studied. High-resolution x-ray photoelectron spectroscopy proves that the ratio of adsorbed molecules can be arbitrarily adjusted via the molar fractions in solution. As a function of these molar fractions core level shifts are observed which are attributed to local work-function changes. By simulating the electric dipole field distribution, the continuous core level shifts are ascribed to a homogeneous mixture of molecules with different end groups adsorbed on adjacent lattice sites. Near-edge x-ray absorption fine structure measurements reveal formation of well-ordered SAMs. Despite the difference in dipole moment of the end groups, the molecular tilt and twist angles are identical for both single-component SAMs and a 1:1 mixed SAM.

Orbital-Symmetry-Dependent Electron Transfer through Molecules Assembled on Metal Substrates.

Femtosecond charge-transfer dynamics in self-assembled monolayers of cyano-terminated ethane-thiolate on gold substrates was investigated with the core hole clock method. By exploiting symmetry selection rules rather than energetic selection, electrons from the nitrogen K-shell are state-selectively excited into the two symmetry-split π* orbitals of the cyano end group with X-ray photons of well-defined polarization. The charge-transfer times from these temporarily occupied orbitals to the metal substrate differ significantly. Theoretical calculations show that these two π* orbitals extend differently onto the alkane backbone and the anchoring sulfur atom, thus causing the observed dependence of the electron-transfer dynamics on the symmetry of the orbital.

Structure and molecular dynamics of the mesophases exhibited by an organosiloxane tetrapode with strong polar terminal groups

The polymorphism of a new organosiloxane tetrapode compound with cyano terminal polar groups was characterized by means of polarizing optical microscopy and x-ray diffraction. The compound exhibits smectic- A and smectic- C phases with a partial bilayer arrangement due to a certain degree of head-to-head association of the mesogenic units through their cyano end groups. On the basis of x-ray diffraction results, evidencing the microsegregation of polyphilic molecules, packing models for the smectic- A and smectic- C phases are proposed. A high degree of smectic positional order and a relatively low value of the tilt angle in the smectic- C phase are indicated. Molecular dynamics of the studied compound was investigated by means of proton NMR relaxometry. The frequency dispersions of the spin-lattice relaxation time (T1) show that the relaxation is induced by three rotational modes of individual dendrimer arms with frequencies between 10;{6} and 10;{9}Hz . In the smectic phases, the effect of individual rotations is overwhelmed by a well expressed contribution of layer undulations at Larmor frequencies below approximately 10MHz . The appearance of this relaxation mechanism over the frequency range of three decades is so far unique in the case of thermotropic liquid crystals. The analysis of the layer undulations contribution supports the microsegregation model of the smectic phases by revealing a slowing-down of translational diffusion and the lack of interactions among the sublayers formed by the mesogenic groups.

Two-dimensionally periodic phases in mixtures of compounds made of rodlike and bent-core molecules

We report two distinct types of two-dimensionally periodic phases in different concentration regions of a binary system composed of the bent-core (BC) compound 1,3-phenylene bis[4-(3-methylbenzoyloxy)] 4(') - n-dodecylbiphenyl 4(') -carboxylate (BC12) and the rodlike (R) compound 4-n-octyloxy 4(') -cyanobiphenyl (8OCB). BC12 exhibits only the lamellar B2 phase. 8OCB which has a highly polar cyano end group exhibits the smectic-A(d) phase with a lamellar structure of partial bilayers made of antiparallel pairs of the polar molecules. The molecular arrangements in the two-dimensionally periodic phases have been deduced based on optical microscopy, polarized infrared and x-ray diffraction studies. The arrangement in the two-dimensionally periodic phase in lower concentration ranges of BC12 (24 to 35 mol% ) is dictated by the partial bilayer structure formed by the rods. The BC molecules arrange themselves with their arrow axes parallel to the long axes of the R molecules and the structure conforms to the c2mm space group symmetry. In higher concentration ranges of BC12 (35 to 60 mol% ) the structure is dominated by the BC molecules, to form the familiar B1 phase with the p2mg space group symmetry. The R molecules lie at the interfaces of these domains stabilizing the structure.

Role of Molecular Dipoles in Liquid Crystals

A vast majority of compounds exhibiting liquid crystalline phases have polar molecules. The nematic director is apolar, even when the molecules have cyano or nitro end-groups, as neighbouring molecules tend to have an antiparallel short-range order. This leads to a partial bilayer structure in the smectic A phase of rod-like molecules as also many interesting phase transitions. The latter can be understood on the basis of a change over to a polar short-range order as the density of the medium is increased. If the molecules have bent-cores and cyano end-groups, a partial bilayer biaxial smectic A phase is also found, implying the formation of quartets with an apolar structure. Lateral dipolar components lead to a tilting of molecules in smectic phases. This can be understood on the basis of the off-axis locations of such dipoles. A mean field theory of smectic C liquid crystals based on this mechanism gives results which reflect experimental trends. In appropriate cases, the possibility of occurrence of a cone-phase consisting of double-tilt cylinders is also pointed out.

Synthesis of a novel structural triblock copolymer of poly( γ -benzyl- l-glutamic acid)- b-poly(ethylene oxide)- b-poly( ε-caprolactone)

A novel structural triblock copolymer of poly( γ-benzyl- l-glutamic acid)- b-poly(ethylene oxide)- b-poly( ε-caprolactone) (PBLG-PEO-PCL) was synthesized by a new approach in the following three steps: (1) sequential anionic ring opening polymerization (ROP) of ethylene oxide and ε-caprolactone with an acetonitrile/potassium naphthalene initiator system to obtain a diblock copolymer CN-PEO-PCL with a cyano end-group; (2) conversion of the CN end-group into NH 2 end-group by hydrogenation to obtain NH 2-PEO-PCL; (3) ROP of γ-benzyl- l-glutamate- N-carboxyanhydrides (Bz- l-GluNCA) with NH 2-PEO-PCL as macroinitiator to obtain the target triblock copolymer. The structures from CN-PEO precursor to the triblock copolymers were confirmed by FT-IR and 1H NMR spectroscopy, and their molecular weights were measured by gel permeation chromatography. The monomer of Bz- l-GluNCA can react almost quantitatively with the amino end-groups of NH 2-PEO-PCL macroinitiator by ROP.

Dielectric investigations and molecular association in non-mesogenic and mesogenic solutions

The aggregation properties of some mesogenic and non-mesogenic molecules in solutions, which have a structural relationship to the nematogens, were investigated using static dielectric method. The role of the cyano-end group, biphenyl core, and flexible tail in molecular association, were also investigated for cyanobiphenyl and some closely related structures of its derivatives. Analysis of the static dielectric data for solution of the liquid crystals indicated a high molecular association, comparable to that observed in the nematic or isotopic phases. There is also a complicated interplay between dipolar and polarizable group in the aggregation of the compounds.

Bistable Memory Switching of (Liquid Crystalline Copolymer/ Low Molecular Weight Liquid Crystal) Composite Systems

Bistable electro-optical switching characteristics have been investigated for composite systems in a smectic phase induced by mixing a side chain type nematic liquid crystalline polymer (nematic LCP) or its copolymer with weak polar methoxy terminal groups in the side chains and low molecular weight nematic liquid crystals (nematic LCs) with strong polar cyano end groups. The liquid crystalline copolymer (LCcoP) with weak polar methoxy terminal groups in the side chains attaching to a polysiloxane main chain was used in order to improve the response speed of the bistable light switching for the binary composite system. Although the LCcoP with 52.5 mol% substituted mesogenic side chains did not exhibit any mesophase characteristics at any temperature, this LCcoP induced a smectic state by mixing nematic LCs. A reversible and bistable electro-optical switching with a short response time (∽100 ms) was realized for the induced smectic binary composite system upon the application of an appropriate electric field at room temperature.

MALDI/TOF/MS and SEC Study of Astromol Dendrimers Having Cyano End Groups

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI/TOF/ MS), size exclusion chromatography (SEC), and dilute solution viscometry were used to analyze the structure of three generations of Astromol (trademark of DSM) dendrimers having cyano end groups. The molecular weights, the defect contents, and the structures of the imperfect dendrimers were determined by MALDI/TOF/MS analysis with fractions collected using a SEC fraction collector. The peak broadening observed for these materials in SEC was studied by adding different co-solvents to the SEC mobile phase. The hydrodynamic radius of the dendrimers was measured both by dilute solution viscometry and by SEC universal calibration. Our radii data are comparable to previously published results for these dendrimers obtained by neutron scattering and support the opinion that dendrimers have compact spherical structures.

Spacer Length Dependence of Mesomorphic Characteristics and Bistable Electro-Optical Effects for Smectic (Liquid Crystalline Copolymer/Liquid Crystals) Composite Systems

Mesomorphic characteristics and bistable electro-optical switching effects of smectic composite systems composed of side chain liquid crystalline copolymer (LCcoP) with a weak polar methoxy terminal group in a side chain and low molecular weight nematic liquid crystals (nematic LCs) with each strong polar cyano end group have been investigated. The spacer length of the mesogenic side chain in LCcoP has strongly affected the mesomorphic characteristics and the electro-optical effects of the (LCcoP/LCs) composite systems. The response speed of the binary composite systems became faster with a decrease of the spacer length. The binary composite systems showed reversible and bistable electro-optical switching characteristics with response time of ∼100 ms, that is, a light scattering state and a transparent one upon the application of ac electric fields with low frequency (up to - Hz) and high one (10–105 Hz), respectively. However, a light scattering state was also observed due to the sign reversal of dielectric anisotropy upon the application of an ac electric field above ∼105 Hz.

On cyanogen photopolymerization

Cyanogen photopolymer was prepared by irradiating its solution in polar solvents such as tetrahydrofuran (THF), methanol and ethanol and non polar aliphatic solvent with UV light of a high pressure mercury lamp (main emission: λ>270 nm) and by sunlight irradiation ( λ>300 nm). The resulting photopolymers were black tarry products. When hexane was used as solvent, the photoproduct was a viscous oil. Elemental analysis showed incorporation of solvent molecules in the photopolymer. Photopolymer produced in THF can be represented by the minimal formula [(C 2N 2)(CH 2CH 2O)] X , while that produced in methanol by the formula [(C 3N 3)(OH)] X . All products were studied by Fourier transform–infrared (FT–IR) and electronic spectroscopy and a model paracyanogen compound was prepared from 2,4,6-tricyano-1,3,5-triazine. Based on the data generated it has been shown that cyanogen photopolymer has a polypyrazino–pyrazine structure with 1,2,4-triazine and cyano end group. It can be considered a pentamer of cyanogen. Triazine-based paracyanogen is an isomer of cyanogen photopolymers as confirmed by elemental analysis but FT–IR and especially electronic spectroscopy permit its differentiation from cyanogen photopolymers. A short discussion on cyanogen photopolymers in some bodies of the solar system has been included.

Optical anisotropy of comb-like macromolecules of lateral and semilateral structure

Maxwell effect and Kerr effect techniques were used to study the intramoleculare axial ordering of anisotropic side groups of an isolated macromolecule in solution. Three comb-like polymers with a methacrylic main chain were investigated. The side chains contain aliphatic spacers separating the mesogenic fragments R from the backbone. R contains two or three phenylene rings with ester linking groups and cyano end groups. Fragments R are attached to the side chains, by the end or middle-point, through the para- or ortho-phenylene ring. The main-chain conformation in solution was identified to assess the hydrodynamic data using the Gaussian coil approximation. The optical anisotropy of the macromolecule segment was determined and related to the monomer unit structure. The specific Kerr constant obtained in diluted benzene solution of polymer was compared with those of low molecular weight analogues of the polymer mesogenic groups. The mobility and intramolecular ordering of the mesogenic groups in the isolated macromolecules are discussed. These properties are compared with those of comb-like polymers with another type of attachment of mesogenic groups, and with another (siloxane) chemical structure of the backbone chain. The differences in restrictions of side-chain mobility due to the backbone linkage in different types of comb-like polymers are also discussed.

Optical anisotropy of comb-like macromolecules of lateral and semilateral structure

Maxwell effect and Kerr effect techniques were used to study the intramoleculare axial ordering of anisotropic side groups of an isolated macromolecule in solution. Three comb-like polymers with a methacrylic main chain were investigated. The side chains contain aliphatic spacers separating the mesogenic fragments R from the backbone. R contains two or three phenylene rings with ester linking groups and cyano end groups. Fragments R are attached to the side chains, by the end or middle-point, through the para- or ortho-phenylene ring. The main-chain conformation in solution was identified to assess the hydrodynamic data using the Gaussian coil approximation. The optical anisotropy of the macromolecule segment was determined and related to the monomer unit structure. The specific Kerr constant obtained in diluted benzene solution of polymer was compared with those of low molecular weight analogues of the polymer mesogenic groups. The mobility and intramolecular ordering of the mesogenic groups in the isolated macromolecules are discussed. These properties are compared with those of comb-like polymers with another type of attachment of mesogenic groups, and with another (siloxane) chemical structure of the backbone chain. The differences in restrictions of side-chain mobility due to the backbone linkage in different types of comb-like polymers are also discussed.

Optical anisotropy of comb-like macromolecules of lateral and semilateral structure

Maxwell effect and Kerr effect techniques were used to study the intramoleculare axial ordering of anisotropic side groups of an isolated macromolecule in solution. Three comb-like polymers with a methacrylic main chain were investigated. The side chains contain aliphatic spacers separating the mesogenic fragments R from the backbone. R contains two or three phenylene rings with ester linking groups and cyano end groups. Fragments R are attached to the side chains, by the end or middle-point, through the para- or ortho-phenylene ring. The main-chain conformation in solution was identified to assess the hydrodynamic data using the Gaussian coil approximation. The optical anisotropy of the macromolecule segment was determined and related to the monomer unit structure. The specific Kerr constant obtained in diluted benzene solution of polymer was compared with those of low molecular weight analogues of the polymer mesogenic groups. The mobility and intramolecular ordering of the mesogenic groups in the isolated macromolecules are discussed. These properties are compared with those of comb-like polymers with another type of attachment of mesogenic groups, and with another (siloxane) chemical structure of the backbone chain. The differences in restrictions of side-chain mobility due to the backbone linkage in different types of comb-like polymers are also discussed.