Single crystal X-ray structure determination and temperature-dependent structural studies of the smectogenic compound 7OS5.

Structural characteristics of solid and liquid crystalline phases of 7OS5 (4-n-pentylphenyl-4'-n-heptyloxythiobenzoate), the achiral smectogenic mesogen with the shortest terminal carbon chain in the nOS5 homologous series, are studied by complementary methods. Simultaneously perfomed X-ray diffraction and differential scanning calorimetry occur to be a powerful tool to study metastable phases. The single crystal structure of a high-temperature phase, supercooled from the room temperature down to -183°C [orthorhombic crystal system; space group Pca21; a = 54.285 (5) Å, b = 5.5843 (3) Å, c = 14.841 (1) Å, Z = 8] is determined. Lamellar ordering of elongated molecules is stabilized by hydrogen bonds . Temperature dependence of unit-cell parameters in two crystal phases as well as structural parameters of liquid crystalline phases (smectic layer spacing, tilt angle, average distance between the long axes of molecules and correlation lengths) are determined by X-ray diffraction. The obtained results are compared with the data available for other compounds in the nOS5 homologous series.

Phase Behavior and Dynamics of Binary and Multicomponent Thioester Liquid Crystal Mixtures.

The results of studies conducted by means of complementary methods, differential scanning calorimetry, transmitted light intensity measurements, polarized optical microscopy, the electro-optical method, as well as dielectric relaxation spectroscopy of two new liquid crystal mixtures are presented. The first mixture is an equimolecular binary mixture consisting of two ferroelectric chiral liquid crystals from the homologous series, abbreviated as (S)-MHOBSn. The second mixture is a multicomponent mixture and consists of four mesogens from the homologous series of nOS5 as the base of the mixture, abbreviated as 610712 and chiral MHOBS8. The binary MHOBS4 + MHOBS7 mixture has an enantiotropic phase sequence, as follows: Cr, SmG*, SmI*, SmC*, N*. The second mixture has the same phase sequence but a much wider temperature range of ferroelectric SmC* phase. The switching time changes from 50 to 90 µs in the SmC* phase of the binary MHOBS4 + MHOBS7 mixture, whereas in 610712 + MHOBS8, the change is more than an order of magnitude higher, ranging from 500 to 3500 µs. Numerical analysis of the dielectric spectra results point toward the complex dynamics of the MHOBS4 + MHOBS7 and 610712 + MHOBS8 mixtures. The relaxation processes in the crystalline, SmC*, SmI*, and SmG* subphases have been observed and described. The relaxation processes have been detected up to 12 °C. There is a very low intensity Goldstone mode in SmC* and a low-frequency noncancellation mode in the SmI* and SmG* phases.

Polymorphism of resorcinol explored by complementary vibrational spectroscopy (FT-RS, THz-TDS, INS) and first-principles solid-state computations (plane-wave DFT).

The polymorphism of resorcinol has been complementary studied by combining Raman, time-domain terahertz, and inelastic neutron scattering spectroscopy with modern solid-state density functional theory (DFT) calculations. The spectral differences, emerging from the temperature-induced structural phase transition, have been successfully interpreted with an emphasis on the low-wavenumber range. The given interpretation is based on the plane-wave DFT computations, providing an excellent overall reproduction of both wavenumbers and intensities and revealing the source of the observed spectral differences. The performance of the generalized gradient approximation (GGA) functionals in prediction of the structural parameters and the vibrational spectra of the normal-pressure polymorphs of resorcinol has been extensively examined. The results show that the standard Perdew, Burke, and Ernzerhof (PBE) approach along with its "hard" revised form tends to be superior if compared to the "soft" GGA approximation.