C-13 Nuclear Magnetic Resonance Investigations of Molecular Mesogens with a Terminal Phenyl Ring Linked via a Spacer: Odd-Even Effect

A setof mesogens considered as model molecules to the technologicallyimportant twist-bend (N-TB) nematogens are investigated.They consist of a three-ring core connected to a phenyl ring via aflexible spacer and displayed enantiotropic nematic and smectic Cmesophases. In such systems, odd or even number of atoms present inthe spacer could influence the orientation of the terminal phenylring and thus have a bearing on designing the N-TB phase,considered as intermediate between the nematic and the cholestericphases. One-dimensional (1D) and two-dimensional (2D) C-13 NMR spectra have been recorded in the liquid crystalline phasesand the alignment-induced chemical shifts (AIS) and the C-13-H-1 dipolar couplings obtained. The order parametersof the phenyl rings reveal features relatable to the odd/even numberof atoms of the flexible spacer and the type of linkage. The AIS plotsof the phenyl rings of the even spacer-based mesogen showed the usualbehavior for all of the phenyl rings with a decrease in AIS with increasingtemperature. However, for the odd-spacer mesogens, unusual behaviorsare noted for the terminal phenyl ring. Thus, two of the mesogensshowed an increase of AIS in the smectic C phase that continued tillthe middle of the nematic phase temperature range, followed by a decrease.The other two odd-spacer mesogens also showed different behaviors.These observations indicate that the terminal phenyl ring is orientedat an angle with respect to the long molecular axis for the odd-spacermesogens that changes as a function of temperature. The angles havebeen found to depend on the nature of the atom/group connecting thespacer to the terminal ring and the spacer length. Thus, the presentstudy provides critical information on the design of the odd dimersthat are recognized to generate fascinating N-TB mesophases.