IR dichroism of individual vibrational bands of liquid crystals in various phase states

Abstract

Decytoxybenzoic acid was chosen for the investigation. The appearance of two smectic phases, of types "C" and "D" [4], and a nematic phase is observed for this acid, having a long alkyl chain. In the nematic liquid crystal (NLC)~ the molecules' long axes lie parallel to each other without formation of a layered structure. However, the domains in the nematics are arranged rather chaotically, and, even in the presence of orientational effects (for example, rubbing of KBr plates), some disorder is possible, something which will affect the dichroism value in this liquid-crystalli ne state. The phase transition from the nematic to the 'C" smectic state can occur during simple translation of the centers of gravity of the molecules with formation of alayered structure. But if the phase transition is to other types of smectic states, rotation of the molecules about the center of gravity is also possible. Thus, in a smectic liquid crystal (SLC) of type "C," the molecules are arranged in layers, and the inclination of the molecules in the layer with respect to the plane of the layer makes an angle close to 35 ~ [5]. During phase transition to a smectic state of type "D," this angle must change. Figure 1 shows the IR spectra of decyloxybenzoic acid in polarized light for various phase states. An identical sign of the dichroism of individual vibrational bands was observed in the phase states of the nematic liquid crystals, "C" smectic liquid crystals, and solid crystals, although the absolute value of the dichroism changed during nematic liquid-crystal-" C" smectic liquid-crystal-. ..- solid-crystal-ph as e transitions. In the "D" smectic liquid-crystalli ne state, the sign of the dichroism changed to the sign which was opposite to that of other phase states. This may indicate that the direction of the dipole moment of vibrational transitions of individual groups of atoms in the molecule changed by some angle in comparison with their location in the nematic liquid-crystalli ne, "C" smectic liquid-crystalli ne, and solid-crystallin e states. Reorientation of the molecules in the "D" smectic liquid crystals occurred with respect to their location in the "C" smectic liquid crystals. Table 1 gives the values of the dichroism and its sign for individual vibrational bands of decyloxybenzoic acid and the assignment of these bands according to the data of [6] and [7]. From Table 1, it is evident that the dichroism sign changed for the bands belonging both to the alkyl chain and to vibrations of the benzene ring. However, the d[chroism value of the bands belonging to vibrations of the benzene-ring skeleton was insignificant and differed little from their dichroism value in the "C" smectic liquidcrystalline and nematic liquid-crystalli ne states, although the sign of the dichroism changed to the opposite sign. This indicates that the location of the dipole moment of these groups of atoms changed by an angle close to 9 0 ~ in comparison with its location in the "C" smectic liquid-crystalli ne and nematic liquid-crystalli ne states. The maximum change in the dichroism value in the "C" smectic liquid crystals was observed in the band of benzene-ring bending vibrations. Apparently, during transition to this liquid-crystalli ne state from the nematic liquid-crystalli ne state, not only rotation of the benzene ring occurred, but also its rotation by some angle about the molecule's long axis so that the change in the dipole moment of this vibration lay in thedirec