Temperature-dependent Raman study of the smectic to nematic phase transition and vibrational analysis using density functional theory of the liquid crystalline system 4-decyloxy benzoic acid.

Room-temperature Raman spectra of the thermotropic liquid crystalline system, 4-decyloxy benzoic acid (4DBA) have been recorded and the experimentally observed bands are assigned by density functional theory (DFT) for the first time. The C-O and C-C stretching and C-H in-plane bending modes of the phenyl ring and C=O stretching modes of the -COOH group are the marker bands for the smectic (S) --> nematic (N) and nematic (N) --> isotropic (I) transitions for this system. The temperature-dependent Raman spectra for these bands in the heating cycle clearly characterize the S-->N and the N-->I transition over a range <1 degree C, which is much better than the earlier range of 23 degrees C for S-->N and 26 degrees C for the N-->I transition. The approximately 773, approximately 807, approximately 881, and approximately 1146 cm(-1) bands disappear, whereas a band at approximately 830 cm(-1) appears at the S-->N transition. The relative intensity of the approximately 1257 and approximately 1280 cm(-1) bands distinguishes the three phases, namely smectic, nematic, and isotropic, in 4DBA. The variation of line width and peak wavenumber of the approximately 1128 and approximately 1168 cm(-1) bands also clearly shows the two transitions. The molecular reorientation at the transition and the effect of local fields present in the liquid crystalline mesophases are also briefly discussed on the basis of changes in intensity, linewidth and peak wavenumber with temperature.