Condis Crystals of Small Molecule VI. The Differences Between Smectic and Condis Phases, Evaluated by a Solid State ¹³C NMR Study of N,N′-bis(4-n-octyloxybenzal)-1,4-phenylenediamine (OOBPD)

Abstract

Carbon-13 NMR spectra have been obtained for N,N′-bis(4-n-octyloxybenzal)-1,4-phenylenediamine (OOBPD) in the condis and smectic phases. Sharp, well-resolved spectra were obtained without magic angle sample spinning in the smectic phases under the conditions of cross-polarization or single carbon pulse followed by high-power proton decoupling. The smectic I-to-smectic C transition involves a discontinuous change in ¹³C chemical shifts due to a large degree to orientational disordering. Similar behavior was found between the spectra of smectic C and the isotopic state. Order parameters for various smectic phases are calculated based on the observed differences in ¹³C chemical shift between the isotropic state and the smectic phase. The spectra were measured at different sample orientation with respect to the external magnetic field, and show that reorienting the smectic molecules in a magnetic field is facile. Larger difference can be found between the spectra obtained by cross polarization and those by single carbon pulse for a smectic liquid crystal that is closer in temperature to the condis phase. The condis-smectic transition is accompanied by the disappearance of the asymmetric chemical shift anisotropy (CSA) and the appearance of an axially symmetric CSA pattern due to the uniaxial molecular orientational motion and ordering.

Key Words:

• N,N′-bis(4-n-octyloxybenzal)-1,4-phenylenediamine (OOBPD)
• condis state
• liquid crystal
• smectic phase
• conformational motion
• orientational motion
• solid state NMR
• chemical shift anisotropy pattern
• order parameter