Nematic Liquid Crystalline Solvent Research Articles

Defect-mediated colloidal interactions in a nematic-phase discotic solvent.

Interactions between colloidal inclusions dispersed in a nematic discotic liquid-crystalline solvent were investigated for different solute–solvent coupling conditions. The solvent was treated at the level of Gay–Berne discogens. Colloidal inclusions were coupled to the solvent with a generalized sphere-ellipsoid interaction potential. Energy strengths were varied to promote either homeotropic or planar mesogenic anchoring. Colloid–colloid interactions were modeled using a soft, excluded-volume contribution. Single-colloid and colloid-pair samples were evolved with Molecular Dynamics simulations. Equilibrium trajectories were used to characterize structural and dynamical properties of topological defects arising in the mesomorphic phase due to colloidal inclusions. Boojums were observed with planar anchoring, whereas Saturn rings were obtained with homeotropic anchoring. The manner in which these topological defects drive colloidal interactions was assessed through a free energy analysis, taking into account the relative orientation between a colloidal dyad and the nematic-field director. The dynamical behavior of defects was qualitatively surveyed from equilibrium trajectories borne from computer simulations.

Orientation and Morphology Control of the Liquid Crystalline Block Copolymer Thin Film by Liquid Crystalline Solvent.

The critical challenge to engineer the morphological structures in the strongly phase-segregated block copolymer thin film is to overcome the preferential wetting of the blocks at the interface and direct the self-assembly process. Herein, we utilize surface activity and selective solvation of a nematic (N) liquid crystalline (LC) solvent, 5CB, to facilely alter the LC anchoring and the orientation of the nanophase separated structures of the smectic-nematic (S-N) LC block copolymer thin film. For the neat S-N diblock copolymer thin film, the nanostructures are parallel aligned. In contrast, with continuous introduction of 5CB into the system, the orientations of the characteristic nanostructures and the morphologies of the LC thin film can be consequently changed, yielding the perpendicularly oriented lamellar or cylindrical structures with the feature size below 10 nm. The homeotropic alignment of the 5CB nematics near the air interface plays a critical role to induce this unique behavior in the S-N/5CB systems, which offers an opportunity to fine-tune the interfacial structures and the morphological patterning in the block copolymer thin film.

Helicity Control of Supramolecular Gel Fibers Consisting of an Achiral NiII Complex in a Chiral Nematic Solvent.

The supramolecular chirality of aggregates consisting of achiral trans-bis(salicylaldiminato)NiII complex 1 bearing long alkyl chains can be generated and controlled precisely in a chiral nematic liquid-crystalline (LC) solvent, whereas the complex naturally forms achiral gel fibers in achiral nematic LC solvents and crystals in nonmesogenic solvents. The direction and intensity of the helicity of the gel fibers of 1 in the LC gel state can be adjusted by means of the nature of the helical twisting and the concentration of the chiral dopants. Helicity control was precisely detected in the circular dichroism (CD) spectra of LC gels and by direct SEM observation of the dried gel fibers. XRD analysis revealed that the flexibility of the herringbone-based lamellar alignment of this complex is the key to the LC-specific gelation and helicity control of the gel fibers.

Testing a new analytical approach for determination of vibrational transition moment directions in low symmetry planar molecules: 1-D- and 2-D-naphthalene

A new analytical approach for improving the precision in determination of vibrational transition moment directions of low symmetry molecules (lacking orthogonal axes) is discussed in this paper. The target molecules are partially uniaxially oriented in nematic liquid crystalline solvent and are studied by IR absorption spectroscopy using polarized light. The fundamental problem addressed is that IR linear dichroism measurements of low symmetry molecules alone cannot provide sufficient information on molecular orientation and transition moment directions. It is shown that computational prediction of these quantities can supply relevant complementary data, helping to reveal the hidden information content and achieve a more meaningful and more precise interpretation of the measured dichroic ratios.The combined experimental and theoretical/computational method proposed by us recently for determination of the average orientation of molecules with Cs symmetry has now been replaced by a more precise analytical approach. The new method introduced and discussed in full detail here uses a mathematically evaluated angle between two vibrational transition moment vectors as a reference. The discussion also deals with error analysis and estimation of uncertainties of the orientational parameters. The proposed procedure has been tested in an analysis of the infrared linear dichroism (IR–LD) spectra of 1-D- and 2-D-naphthalene complemented with DFT calculations using the scaled quantum mechanical force field (SQM FF) method.

Conformational Analysis of 2,2′-Bithiophene Revisited: The Maximum Entropy Method Applied to Large Sets of H−H and 13C−H Partially Averaged Dipolar Couplings

Two recently determined sets of partially averaged H-H and (13)C-H NMR dipolar couplings of 2,2'-bithiophene as a solute in nematic liquid-crystalline solvents have been analyzed by means of the maximum entropy method. This investigation originated to ascertain the real information content of the new (13)C-H data. To this end, a procedure has been employed which consists in adding the experimental data one after the other and monitoring, during each of these steps, the behavior of the minimum of the associated work function as well as that of the root-mean-square deviation. It turns out that nearly all (13)C-H data do not provide new pieces of information with respect to a subset of seven H-H dipolar couplings. Inclusion of the very few which do, however, alters significantly the conformational distribution function calculated taking into account the sole H-H dipolar couplings. The final form of this function is characterized by a prominent peak corresponding to the trans conformation and two smaller peaks corresponding to the cis conformation and of the conformation in which the two rings are mutually orthogonal. The form of the conformational distribution function is the result of a considerable orientational-conformational coupling. The implications of the present results on the form of the torsional potential-energy function of the isolated molecule are discussed.

3H NMR of the tritiated isotopologues of methane in nematic liquid-crystal solvents

The NMR spectra of the tritiated isotopologues of methane dissolved in several nematic liquid-crystalline solvents are measured. The spectral parameters obtained agree extremely well with those predicted from earlier NMR studies of the deuterated isotopologues, providing excellent confirmation of the theory for vibration–reorientation interaction developed earlier.

Optical and Electro-Optical Properties of Poly(2-hydroxyethylmethacrylate)/5CB Systems

This paper reports on the phase behaviour of photochemically polymerized 2-hydroxyethyl methacrylate in a nematic liquid crystalline solvent. Samples were prepared via polymerization by ultraviolet (UV) radiation of initial solutions made of the reactive monomer 2-hydroxyethylmethacrylate (HEMA) and a photoinitiator (Darocur 1173). The swelling and deswelling properties were studied by polarized optical microscopy in a wide range of temperature for the low molecular weight liquid crystal 4-cyano-4′-pentylbiphenyl (5CB). Immersion of polymerized HEMA in an excess of 5CB did not exhibit any significant change of sample size and showed only limited dependence on temperature, indicating strong polymer/liquid crystal interactions. Polymer/liquid crystal films were prepared by polymerization-induced phase separation of HEMA/Darocur 1173/5CB blends using UV radiation. The electro-optical behaviour of the films obtained was investigated as function of the initial composition and applied voltage. Strong memory effects have been observed in the case of samples prepared from 60 weight% 5CB/40 weight% HEMA.

A comparison of proton‐detected 13C local field experiments with deuterium NMR at natural abundance for studying liquid crystals

A comparison is made between the information which can be derived on structure, conformation and orientational order of the molecules in a nematic liquid crystal 4‐pentyl‐4′‐cyanobiphenyl (5CB) from the NMR spectra of the deuterium atoms at natural abundance (NAD NMR), and the two‐dimensional proton‐detected 13C local field experiment (PDLF). The nine residual quadrupolar splittings, Δν k , obtained experimentally have been compared with quadrupolar tensors and a geometry and conformational potentials calculated by the DFT method B3LYP/6‐311G**. The PDLF experiment yielded 42 scaled 13C–1H residual dipolar couplings, kD CiHj . The scaling factor, k, is determined experimentally by comparing unscaled and scaled residual dipolar couplings in a sample of fluorobenzene dissolved in a nematic liquid crystalline solvent. The corrected residual dipolar couplings, D CiHj , are used to investigate the structure and rotational potentials about each bond in the molecule.

The indirect through-space F–F coupling in peri-difluoronaphthalene: is it anisotropic?

The (1)H, (19)F and (13)C spectra have been obtained of a sample of peri-difluoronaphthalene dissolved in the nematic liquid crystalline solvent ZLI 1695. The (13)C satellite spectra from the six, single-(13)C isotopomers at natural abundance in both the (1)H and (19)F spectra were identified and analysed to yield a set of residual total, anisotropic spin-spin couplings, T(ij). This was achieved by first obtaining residual (13)C-(19)F and (13)C-(1)H couplings from a proton-encoded, (13)C detected, local field 2D spectrum. The 45 values of T(HH), T(HF) and T(CH) were used to obtain the structure of the molecule, and then to estimate whether there is a significant contribution from the component along the magnetic field, J, of the anisotropic, electron-mediated, spin-spin coupling tensors for (13)C-(19)F and (19)F-(19)F pairs. It is found that there is strong evidence for a significant contribution of J to T(FF) but not for the (13)C-(19)F pairs.

Ordering of Poly(p-phenylene ethynylene)s in Liquid Crystals

Poly(phenylene ethynylene)s were designed and synthesized that are completely soluble in nematic liquid crystalline solvents at molecular weights (GPC, relative to PS) of more than 100 000. These materials experience a chain extended rod conformation under these conditions with the polymer chains highly aligned with the liquid crystal director and an enhanced conjugation length determined by optical spectroscopy. Structure−property relationships are investigated that reveal the role of steric congestion about the polymer mainchain in determining the order parameter of the polymers. It was also found that the order polymer parameters increased with increasing molecular weights.

Electronic absorption spectra of cresyl violet acetate in anisotropic and isotropic solvents

The isotropic and anisotropic solvation characteristics of cresyl violet acetate (CVA) were investigated in isotropic liquid solutions and in polar nematic matrices as a function of the solvent type and concentration. The interaction of the ionic dye with the anisotropic surrounding and with that of the isotropic solvents was investigated and compared. The experimental result suggests that the nematic liquid crystalline solvents might create stronger solvation than the isotropic solvents. The spectral shifts were correlated by the solvent permittivity and Kamlet-Taft parameters. The polarized absorption spectra of cresyl violet acetate were measured between 400 and 800 nm and the dichroic ratio R and degree of anisotropy S of this dye in the liquid crystalline host determined.

An investigation of the structure and bond rotational potential of some fluorinated ethanes by NMR spectroscopy of solutions in nematic liquid crystalline solvents

NMR spectra of 1,2-dibromo-1,1-difluoroethane and 1-bromo-2-iodo-tetrafluoroethane dissolved in nematic liquid crystalline solvents have been analysed to yield the magnitudes and signs of the scalar couplings, J ij , and total anisotropic couplings, T ij , between all the 1H, 19F, and 13C nuclei, except for those between two 13C nuclei. The values obtained for T ij in principle contain a contribution from J ij aniso , the component along the static applied magnetic field of the anisotropic part of the electron-mediated spin–spin coupling. Neglecting this contribution allows partially averaged dipolar couplings, D ij , to be extracted from the T ij , and these were used to determine the structure, orientational order, and the conformational distribution generated by rotation about the C–C bond. The values obtained are compared with the results of calculations by ab initio and density functional methods. The differences found are no greater than those obtained for similar compounds which do not contain fluorine, so that there is no definitive evidence for significant contributions from J CF aniso or J FF aniso in the two compounds studied.

Tetraethyl stannane: structure, conformations, and orientational order when dissolved in a nematic liquid crystalline solvent

Quantum chemical calculations are used to investigate the structure and the distribution of conformers of tetraethyl stannane, and these are compared with those of tetraethyl methane. The calculations predict that the most abundant conformers in tetraethyl stannane are those with symmetry C1, in contrast to tetraethyl methane, in which the D2d and S4 conformers are dominant. The structural and conformational information on tetraethyl stannane are then used, together with the additive potential model, to explain the finite quadrupolar splittings observed in the deuterium NMR spectrum of tetraethyl stannane dissolved in a nematic liquid crystalline solvent. This analysis emphasizes that to understand the orientational order of a flexible molecule in a liquid crystalline phase it is essential to consider the symmetry of individual conformers rather than that of an average structure.

Obtaining the structure and bond rotational potential of a substituted ethane by NMR spectroscopy of solutions in nematic liquid-crystalline solvents

Partially averaged dipolar couplings (also referred to as residual dipolar couplings) D(ij) can be obtained from the analysis of the NMR spectra of molecules dissolved in liquid-crystalline solvents. Their values for a nonrigid molecule depend upon the bond lengths and angles, the rotational potentials, and the orientational order of the molecules. The molecule studied, 1-chloro-2-bromoethane, is one of the simplest example of a substituted alkane in which the rotational potential has three minimum-energy positions, trans and gauche+/-conformations, and the present investigation explores the problems inherent in deriving the form of the potential and the molecular geometry from the set of partially averaged couplings between the protons, and between protons and (13)C nuclei. The geometrical parameters and the rotational potential obtained are compared with the results from a density-functional theory method.

The Structure and Conformations of 2‐Thiophenecarboxaldehyde Obtained from Partially Averaged Dipolar Couplings

The proton NMR spectra of samples of 2-thiophenecarboxaldehyde dissolved in a nematic liquid crystalline solvent, including those from all five singly labelled 13C isotopomers, have been obtained. These have been analysed to yield sets of partially averaged dipolar couplings which have been used to determine the structure and the relative amounts of the cis and trans forms, which are the two minimum-energy structures generated by rotation about the ring-aldehyde bond. A procedure for applying vibrational corrections to the dipolar couplings in the presence of large amplitude motions is discussed.

Conjugation Enhancement of Intramolecular Exciton Migration in Poly(p-phenylene ethynylene)s

Efficient energy migration in conjugated polymers is critical to their performance in photovoltaic, display, and sensor devices. The ability to precisely control the polymer conformation is a key issue for the experimental investigations and deeper understanding of the nature of this process. We make use of specially designed iptycene-containing poly(p-phenylene ethynylene)s that display chain-extended conformations when dissolved in nematic liquid crystalline solvents. In these solutions, the polymers show a substantial enhancement in the intrachain exciton migration rate, which is attributed to their increased conjugation length and better alignment. The organizational enhancement of the energy transfer efficiency, as determined by site-selective emission from lower energy traps at the polymer termini, is accompanied by a significant increase of the fluorescence quantum yield. The liquid crystalline phase is a necessary requirement for these phenomena to occur, and when the temperature was increased above the nematic-isotropic transition, we observed a dramatic reduction of the energy transfer efficiency and fluorescence quantum yield. The ability to improve the exciton migration efficiency through precise control of the polymer structure with liquid crystalline solutions demonstrates the importance of a polymer's conformation for energy transfer, and provides a way to improve the energy transporting performance of conjugated polymers.

Vibrational spectra of partially oriented molecules having two conformers in nematic and isotropic solutions: furfural and 2-chlorobenzaldehyde

Vibrational analysis of the two conformers of furfural and 2-chlorobenzaldehyde has been carried out on the basis of their IR and Raman spectra measured in isotropic and anisotropic (nematic liquid crystalline) solvent. The average orientation of the individual conformers in the nematic solvent has been determined by means of a recently developed approach for low symmetry planar molecules using DFT calculations of the vibrational transitions moments. The complex shape of the carbonyl band additionally split into several components is interpreted as an effect of Fermi resonance.

The orientational order and conformational distributions of the two enantiomers in a racemic mixture of a chiral, flexible molecule dissolved in a chiral nematic liquid crystalline solvent

The orientational order and conformational distributions of the two enantiomers of (±)-α-ethylhexanoic acid-d15, a flexible chiral molecule, dissolved in a chiral nematic liquid crystalline solvent made of PBLG in an organic co-solvent are obtained by analysis of NMR data. The anisotropic, NMR parameters are obtained from the separated analysis of the two enantiomers using carbon–deuterium, deuterium–deuterium 2D correlation experiments and proton-coupled carbon-13 1D experiments. The analysis of conformational distributions, and the conformationally dependent orientational order parameters are derived using the additive potential, AP, model.

Absorption anisotropy and molecular association of some ionic dyes in liquid crystalline solution

Dichroic ratios R and order parameters S of solutions of methylene blue, rhodamine 6G and rhodamine B in the nematic liquid crystalline solvent were investigated by the measuring of the intensity of the absorption bands in the visible region of parallel aligned samples. The molecular association of the dyes in a nematic solvent and also in an amphiphilic solution was studied using optical spectroscopy and in a guest–host system. The nature of the interacting pairs in these dyes was discussed using the exciton theory.

Absorption anisotropy and molecular association of some ionic dyes in liquid crystalline solution

Dichroic ratios R and order parameters S of solutions of methylene blue, rhodamine 6G and rhodamine B in the nematic liquid crystalline solvent were investigated by the measuring of the intensity of the absorption bands in the visible region of parallel aligned samples. The molecular association of the dyes in a nematic solvent and also in an amphiphilic solution was studied using optical spectroscopy and in a guest–host system. The nature of the interacting pairs in these dyes was discussed using the exciton theory.