Investigation of glass transition and polymorphism of smectogenic, partially fluorinated chiral compound by X-ray diffraction and infra-red spectroscopy

The X-ray diffraction and infra-red spectroscopy measurements are performed as a function of temperature upon heating of the glassy smectic CA* phase of liquid crystalline (S)-4 '-(1-methylheptylcarbonyl)biphenyl-4-yl 4-[7(2,2,3,3,4,4,4-heptafluorobutoxy)heptyl-1-oxy]-3-fluorobenzoate (3F7FPhH6) for the search of relationship between the structural changes and intra-molecular vibrations. The smectic layer spacing and correlation length, determined from the X-ray diffraction patterns, show different temperature dependences in the glassy state and above the glass transition temperature. The vibrational spectra do not show distinct changes upon the glass transition. Density-functional theory calculations for an isolated molecule in two conformations are applied for assignment of the IR absorption bands. The positive correlation between the average intermolecular distances and positions of two absorption bands, originating from the C=O stretching, is noticed and interpreted as a decreasing amount of the weak C-H...O=C hydrogen bonds upon heating in the smectic CA*, smectic A*, and isotropic liquid phases. The results are compared with the previously reported X-ray diffraction and infra-red data for the isostructural (S)-4 '-(1-methylheptylcarbonyl)biphenyl-4-yl 4-[7-(2,2,3,3,4,4,4-heptafluorobutoxy) heptyl-1-oxy]-2-fluorobenzoate (3F7HPhF6).