Institut für Organische Chemie, Technische Universität Berlin, W. Germany
Nematic phases of liquid crystals are converted to induced cholesteric mesophases by addition of a small amount of a chiral compound. This behavior can be studied by EPR spectroscopy provided that a paramagnetic spin probe, e.g., a nitroxide radical is used as a reporter. Whereas single-crystal-type spectra are observed in nematic phases, the helical structure of cholesteric phases gives rise to powder-type spectra [1,2]. However, in X-band EPR experiments the sample is necessarily exposed to an external magnetic field of 0.33 T which causes a distortion or even a complete unwinding of the cholesteric helix. Comparison with computer-simulated spectra allows the determination of the pitch.
Recently chiral compounds were discovered which exhibit a strong temperature dependence of the helical twisting power. Moreover, with several derivatives of 1,1'-binaphthyl even the phenomenon of a temperature-dependent inversion of the handedness of the helix was observed. This behavior is reflected in the EPR spectra of a suitable spin probe. Thus, at low temperatures a powder-type spectrum characteristic of cholesteric phases is obtained. With increasing temperature, the appearance of the spectrum changes and finally a single-crystal-type spectrum corresponding to a nematic phase is observed. However, a cholesteric phase forms again when the temperature is raised further, and correspondingly a powder-type spectrum is observed. Addition of a second chiral compound with known handedness changes the inversion temperature and allows the determination of the handedness of the mesophase. In ENDOR experiments, orientational selection can be achieved by means of a proper field setting.
 G. R. Luckhurst and H. J. Smith, Mol. Cryst. Liq. Cryst. 20, 319 (1973).
 G. Gottarelli, G. F. Pedulli and C. Zannoni, Chem. Phys. 64, 143 (1982).