Orientational order parameters of a de Vries–type ferroelectric liquid crystal obtained by polarized Raman spectroscopy and x-ray diffraction

The orientational order parameters 〈P2〉 and 〈P4〉 of the ferroelectric, de Vries–type liquid crystal 9HL have been determined in the SmA* and SmC* phases by means of polarized Raman spectroscopy, and in the SmA* phase using x-ray diffraction. Quantum density functional theory predicts Raman spectra for 9HL that are in good agreement with the observations and indicates that the strong Raman band probed in the experiment corresponds to the uniaxial, coupled vibration of the three phenyl rings along the molecular long axis. The magnitudes of the orientational order parameters obtained in the Raman and x-ray experiments differ dramatically from each other, a discrepancy that is resolved by considering that the two techniques probe the orientational distributions of different molecular axes. We have developed a systematic procedure in which we calculate the angle between these axes and rescale the orientational order parameters obtained from x-ray scattering with results that are then in good agreement with the Raman data. At least in the case of 9HL, the results obtained by both techniques support a “sugar loaf” orientational distribution in the SmA* phase with no qualitative difference to conventional smectics A. The role of individual molecular fragments in promoting de Vries–type behavior is considered

Aromaticity in cyanuric acid

This study analyzes the aromatic nature of cyanuric acid (hexahydrotriazine) and some of its derivatives, in terms of aromatic stabilization energy (ASE) and electronic behavior. The simplest molecule (C3N3O3H3) is the most aromatic item out of the entire set, but some of the others also display aromatic character. The structure of all the rings is analyzed considering their molecular orbitals as well as studying the inductive effect

Comparative Study of Orientational Order of Some Liquid Crystals from Various Homologous Series

The long-range orientational order of liquid crystals from homologous series of 4-n-alkyl-4'-cyanobiphenyl, 4-n-alkoxy-4'-cyanobiphenyl, trans-4-n-alkyl(4'-cyano-phenyl)-hexane, and 4-(trans-4'-n -alkylcyclohexyl)-isothiocyanato-benzene was studied by means of classical methods of optical spectroscopy: absorption of the polarized light, fluorescence depolarization, and Raman scattering depolarization. The absorption, emission, and Raman scattering spectra of linearly polarized light were recorded as a function of temperature in the whole range of the mesophase. On the basis of these spectra the order parameters ⟨P$\text{}_{2}$⟩ and ⟨P$\text{}_{4}$⟩ as well as the orientational distribution function were determined. The results obtained for members of various series with the same number of carbon atom in the alkyl chain were compared. It was found that the orientational order of liquid crystal molecules depends on the structure of the rigid core, on the kind of the terminal group as well as on the alkyl chain length. The odd-even effect for the order parameters, explained as the alteration of interaction between alkyl chains, was observed

Self-assembly of a [2 x 2] hydrogen bonded grid

Formation of 24 cooperative hydrogen bonds drives the spontaneous assembly of a rigid bifunctional trimelamine and bis(barbituric acid) to give selectively the [2 × 2] hydrogen-bonded grid, in preference to the corresponding [1 × 1] or polymeric assemblies

Comparative Study of Orientational Order of Some Liquid Crystals from Various Homologous Series

The long-range orientational order of liquid crystals from homologous series of 4-n-alkyl-4'-cyanobiphenyl, 4-n-alkoxy-4'-cyanobiphenyl, trans-4-n-alkyl(4'-cyano-phenyl)-hexane, and 4-(trans-4'-n -alkylcyclohexyl)-isothiocyanato-benzene was studied by means of classical methods of optical spectroscopy: absorption of the polarized light, fluorescence depolarization, and Raman scattering depolarization. The absorption, emission, and Raman scattering spectra of linearly polarized light were recorded as a function of temperature in the whole range of the mesophase. On the basis of these spectra the order parameters ⟨P$\text{}_{2}$⟩ and ⟨P$\text{}_{4}$⟩ as well as the orientational distribution function were determined. The results obtained for members of various series with the same number of carbon atom in the alkyl chain were compared. It was found that the orientational order of liquid crystal molecules depends on the structure of the rigid core, on the kind of the terminal group as well as on the alkyl chain length. The odd-even effect for the order parameters, explained as the alteration of interaction between alkyl chains, was observed

Thermodynamic stabilities of linear and crinkled tapes and cyclic rosettes in melamine-cyanurate assemblies: a model description

In this paper we describe model calculations for the self-assembly of N,N-disubstituted melamines 1 and N-substituted cyanuric acid or 5,5-disubstituted barbituric acid derivatives 2 into linear or crinkled tapes and cyclic rosettes via cooperative hydrogen bond formation. The model description considers all possible stereoisomeric tape structures consisting of two to eight different components (270 different species in total) and one cyclic hexameric rosette structure. Furthermore, eight steric parameters (R12-R28) are included that represent the different types of steric interactions within the assemblies. Most importantly, the model calculations clearly show that the tape/rosette ratio is very sensitive to changes in parameters that directly affect the internal energy of the rosette structure. In this respect, three parameters have been characterized, i.e., the basic equilibrium constant K0 for the bimolecular association of a melamine and cyanurate, the equilibrium constant Kr/K0 for the cyclization of a linear hexamer, and the parameter R12-a(Z)b, representing attractive or repulsive interactions between adjacent melamine and cyanurate moieties. For example, an increase in K0 from 100 to 10 000 M-1 ([A]0 = [B]0 = 10 mM, Kr = 0.01 M) or in Kr from 0.001 to 0.1 M ([A]0 = [B]0 = 10 mM, K0 = 1000 M-1) raises the concentration of the rosette from <5 to ~90% or from ~10 to ~85%, respectively. Similarly, a change in R12-a(Z)b from 1.0 (no repulsive or attractive interactions) to 1.5 (slight attractive interaction) raises the rosette fraction of the mixture from 25% to 45%. In sharp contrast to this, the model calculations show that parameters that only affect the internal energy of the tapes (R13-R28) hardly change the tape/rosette ratio. For example, by changing R13-a(EE)a from 1.0 (no repulsive or attractive interactions) to 0.001 (maximum repulsion), the rosette fraction in the mixture changes by no more than 8%. Including all possible sterics that occur only in tapes (i.e., R13-R28), the maximum change in rosette fraction is no more than 16%. These predictions can be rationalized by considering that any change in the stability of the tapes only affects the rosette concentration by means of shifting the equilibrium between free 1 and 2 and the rosette. Since there are 270 different tapelike structures in equilibrium, this mixture represents the best buffer solution in the world. These model calculations seem to conflict with the concept of peripheral crowding as put forward by Whitesides et al., which states that bulky substituents on the periphery of the melamine (and cyanurate) components can be used to shift the tape/rosette equilibrium completely toward the rosette structure. Computer simulations (CHARMm 24.0) show that linear tapes with bulky substituents are severely distorted from planarity, while the corresponding rosette remains planar. Therefore, tapelike structures with bulky substituents are expected to have a much higher solubility than the corresponding rosettes, which can explain the observed crystal data