Chemical and physical properties of orthoconic liquid crystals: ²H NMR spectroscopy and molecular dynamics...

In the field of chiral smectic liquid crystals, orthoconic antiferroelectric liquid crystals (OAFLCs) have attracted the interest of the scientific community due to the very high tilt angle, close to 45°, and the consequent optical properties. In the present study, the first ²H NMR investigation is reported on two samples, namely 3F5HPhF9 and 3F7HPhF8, showing the phase sequence isotropic-SmC*-SmC_A* and the phase sequence isotropic-SmA-SmC*-SmC_A*, respectively, when cooling from the isotropic to the crystalline phases. To this aim, the liquid crystals were doped with a small amount of deuterated probe biphenyl-4,4'-diol-d₄. The trend of ²H NMR spectra versus temperature indicates the presence of very high values of the tilt of the deuterated probe in the antiferroelectric phase for both samples. The trend of the local order parameters and that of the tilt angle were compared with the results obtained for the same samples by means of different experimental techniques, namely X-ray diffraction and electrooptical measurements. Moreover, a computational study was performed on the sample labelled 3F5HPhF9 using fully atomistic classical molecular dynamics simulation of the orthoconic phase. The results obtained from the MD simulations show a very large molecular tilt of the molecules (about 42°) when packed in layers and this value is in very good agreement with experimental results. The present research aims to give additional clues about the molecular origin of the very peculiar high tilt angle of orthoconic liquid crystals in the antiferroelectric phase.