Probing molecular orientations and conformations at the free surfaces of liquid crystals

Sum-frequency vibrational spectroscopy was used to probe molecular orientations and conformations of the free surfaces of cyanobiphenyl (nCB, with n ranging from 2 to 11) liquid crystalline media. In both isotropic and liquid crystalline phases, the alkyl chains were found to be projecting out from the liquid crystalline surface. Spectral analysis of the C--H stretch modes from different input/output polarization combinations indicated that the chromophores of the molecules are oriented more or less along the surface normal with a broad distribution. As the alkyl chain length increases, there are more trans-gauche defects appearing along the chains. The close similarity of the spectra for different nCB however suggests that the average surface molecular orientations of all nCB are very much the same.