Microscopic organization of molecules in smectic A and chiral (racemic) smectic C phases: Dynamic molecular deformation effect on the S_Ato S_C∗ (S_C) transition

Abstract

We investigated the correlation between orientational order and microscopic organization of the molecules in smectic A and chiral (racemic) smectic C phases by means of solid-state C-13 NMR, powder X-ray diffraction, and electrooptical measurements. The compounds under investigation are 4-((S)-2-methyloctanoyl)phenyl 4'-nonylbiphenyl-4-carhoxylate ((5')-MONBIC) and its corresponding racemic compound ((S, R)-MONBIC). Static C-13 NMR indicates that: (1) the orientational angle of the tail with respect to the magnetic field decreases slightly both in the SA and S; phases as decreasing temperature, and ( 2 ) the angle of the core with respect to the field decreases in the SA phase but increases in the S$ phase as decreasing temperature. Analysis of C-13 TI reveals that the dynamic molecular deformation for the core part can occur near the transition. We discuss the dynamic molecular deformation in comparison with the reorientation of the director at the SA to S; transition. Based on the experimental results, we propose the structural model in which describes the microscopic organization of the molecules in the mesophases.

Twist Grain Boundary Liquid‐Crystalline Phases under the Effect of the Magnetic Field: A Complete2H and13C NMR Study

“…The 1 H 908 pulse width was 4.2 ms and the temperature gradient across the sample was estimated to be within 0.5 8C. The 13 C peak assignments in the aligned sample were based on previous 13 C peak assignments of liquid crystals [36,39,44,45] having a similar aromatic core.…”

Recent NMR Studies of Thermotropic Liquid Crystals

“…These are the fluctuating proton-carbon dipolar interaction for a carbon atom directly bonded with a proton, the fluctuating 13 C CSA, and the coupling between these two interactions giving rise to cross relaxation terms [214]. Different strategies to separate individual contributions to 13 C relaxation have been proposed [215,216]. However, it is not straightforward to carry out these in reality.…”

“…However, it is not straightforward to carry out these in reality. To list a few recent 13 C relaxation works, these include 10B1M7 [217], 4-(1-methylheptyloxycarbonyl)phenyl-4 0 -octyloxybiphenyl-4-carboxylate (MHPOBC) [218][219][220], 4-(2-methyloctanoyl)phenyl 4 0 -nonylbiphenyl-4-carboxylate (MONBIC) [216], 4-(1-methylhexyloxycarbonyl) phenyl 4 0 -octyloxybiphenyl-4-carboxylate (MH(6)POBC) [221]. In general, the 13 C T À1 1 rates of the aromatic carbons increase with decreasing temperature in a manner similar to the observed spectral densities for the aromatic deuterated sites (see for instant, ZLL 7/* [189]).…”

Molecular Dynamics in Smectic and Crystalline Phases Studied by13C-NMR Spin Lattice Relaxation Time Observation

“…As for its dynamical aspect, however, details of the hindered molecular rotation associated with the emergence of spontaneous polarization are still open for question, although several experimental approaches have been carried so far [1][2][3][4][5][6][7][8][9][10][11][12][13]. Although nuclear magnetic resonance (NMR) spectroscopy is a powerful method for investigating molecular dynamics in liquid-crystalline compounds, neither the slowing down nor the hindrance of molecular rotation has been confirmed even in recent studies [14][15][16][17][18][19][20][21][22][23][24][25].…”