Alignment structure of dimeric liquid crystal molecules having bent molecular structure

Antiferroelectric liquid crystals (AFLCs) composed of low-mass mesogens do not show the nematic (N) phase. Recently, it has been reported that an AFLC containig dimeric molecules may show the N phase. However, it is not obvious why the antiferroelectric SmC (SmCA) and N phases may coexist in the phase sequence of the dimeric LCS. In this study, the molecular alignment structure has been investigated by X-ray diffractometry (XRD) and polarized Fourier transform infrared spectroscopy (FT-IR). As a result, It is found that dimeric LCs, mPY11OCB (m = 6 and 8), have the same layer spacing between the SmA and SmCA phases, and therefore, the molecular shape is bent even in the SmA phase. The difference between both phases is due to the thermal motion of bent molecules around the molecular axis. Furthermore, it is obvious that in the SmCA phase, the alignment of dimeric LC molecules can be localized in two directions in a cell with rubbed alignment films. Furthermore, we tried to estimate the bent angle of the dimeric molecule by analyzing the relation between the absorbance of IR and the thermal motion of molecules, and then, found that the bent angles of 6PY11OCB and 8PY11OCB are 40 and 38 deg, respectively.