Thermodynamics and molecular ordering of carbonate-type dimer liquid crystals with emphasis on the geometrical characteristics of the linking group

Mainchain liquid crystals having a regularly alternating rigid-flexible repeating structure often exhibit distinct odd-even oscillation in their thermodynamic quantities with respect to the number of methylene units in the spacer. The effect has been widely observed from dimer (DLC) to polymer liquid crystals (PLC), but not in the monomer liquid crystals (MLC) which comprise a mesogenic unit carrying one or two tails. The degree of oscillation varies depending on the chemical structure of the linking group used to join the spacer with mesogenic units. In our previous work, we have suggested that such characteristics are mainly due to the geometrical arrangement of the linkage. In this work, we have prepared a series of carbonate-type MLCs NC phi phi OCOO(CH2)(n-1)CH3, where phi is a phenyl group, as well as DLCs NC phi phi OOCO(CH2)(n)O-COO phi phi CN. The thermodynamic behaviors at the nematic-isotropic NI transition have been compared with those previously reported for the ether-type DLCs. For the dimer series, the molecular orientation and the spacer conformation were investigated by using H-2 NMR technique. The origin of the difference in the odd-even behaviors has been traced back to the geometrical characteristics of the linking groups, which affect the relative orientation of the terminal mesogenic cores, thus leading to modification of the conformational distribution in the nematic state. The analysis has yielded the nematic conformation which satisfactorily reproduces the observed values of the dipolar and quadrupolar couplings. The bond conformation vs. bond order plots are shown for the n odd and n even systems together with the corresponding results derived previously for the ether-type DLCs. Although the numbers of bonds of the flexible spacer are not exactly comparable with each other, the conformational ordering along the spacer tends to be pronounced in both DLC systems.