Infrared and Raman polarization spectroscopy of molecules aligned in nematic liquid crystals: results from cooperation with Bojidar Jordanov

Molecules, dissolved in an anisotropic solvent, tend to adopt the preferred orientation of the host. Probing the aligned molecules with polarized radiation reveals the orientation of their individual vibrational transition moments not only within the sample but also with respect to the molecular axes. The results - whether obtained with infrared (linear dichroism) or Raman spectroscopy - yield insight in the conformation and the symmetry of the molecules. There is a good choice of different nematic liquid crystals, which offer the desired uniaxial anisotropy within a comfortable temperature range, sufficient solubility for organic molecules and a favourably weak absorption spectrum, which does not impede the precise measurement of most bands of the dissolved (guest) molecules. The experimental procedure and some examples are represented. When chiral molecules are embedded in a similar way, they twist the nematic order into a cholesteric one, i.e. a screw-like, helical structure whose period (pitch) often equals infrared wavelengths. Enantiomers produce oppositely twisted structures and this effect is used to discriminate between them. For its easy observation with conventional spectrometers several simple experimental set-ups have been developed.