NMR investigation of the oxyethylene unit ordering in some related dialkoxy laterally substituted nematogens

Abstract

Laterally dialkylated nematogens containing a short terminal polyoxyethylene chain present a wide nematic range starting from room temperature. The ordering of the oxyethylene (OE) unit has been studied by 13C NMR in the liquid crystalline phase. The values of the order parameters were derived from the transient oscillations observed during variable contact-time cross-polarization experiments. Along the oxyethylene chain, the order parameters decrease monotonically. These order parameter values do not present the usual odd-even effect and are noticeably smaller than those found for terminal alkoxy chains. Entering the nematic phase, the 13C thermal evolution of the field-induced chemical shift in the OE unit is far smaller than the one usually observed for terminal alkyl or alkoxy chains, regardless of their non-negligible order parameters. In addition,no linear correlation between the order parameter and the field-induced chemical shift is observed for the carbons in the OE unit. This is an indication that the proportion between gauche - and trans -conformations in the OE unit is temperature dependent.