H. Stegemeyer

Treatment of cholesteric liquid crystalline mixtures by means of the goossens theory

Abstract The helical twisting power p−1 of binary cholesteric mixtures has been measured as a function of mole fraction x. Whenever the molecular structure of the components is different the function p−1(x) deviates considerably from linearity. In some cases unexpected helical inversions had been observed. The properties of the different mixtures can be understood by means of the molecular statistical theory of Goossens extended by consideration of the concentration dependence of the dispersion energy terms.

The fluorescence of the cis and trans forms of naphthyl-ethylenes

Fluorescence spectra and quantum yields of the two 1-phenyl-2-naphthyl ethylenes and the three 1,2-dinaphthyl ethylenes were studied between +25° and -180°, both of the trans and cis isomers. Contrary to recent reports by other authors we find a close similarity between the fluorescence behaviour of the two isomers of stilbene and the above five compounds. The fluorescence spectra of the cis isomers are unstructured and red-shifted in comparison with the trans isomers. The quantum yield of the trans isomers is 0.8–0.9, that of the cis isomers <0.01 at temperatures above -100°, but increases sharply on cooling.

Pressure dependence of the helical structure of cholesteric mesophases

Abstract The pitch of two cholesteric mixtures of cholesteryloleylcarbonate/cholesterylchloride is sensitively influenced by ambient static external pressure. The measurement of the wavelength of maximum light reflection of these mesophases up to 2000 bars yields a remarkable red-shift. This shift can be understaood on the basis of the improved theory of Keating when taking into account the probably large increase of the mesophase viscosity with pressure. Perhaps, however, the effect must be explained in terms of phase transitions.

Induction of optical activity in a nematic mesophase by l-menthol. Optical properties of liquid crystal mixtures☆

Abstract The helical twisting power of l -menthol in a nematic solvent (MBBA) has been measured by means of induced optical activity and circular dichroism. The optical rotatory dispersion curve is extremely broadened corresponding to a widened region of total reflection. The induced twist sharply increases with concentration resulting in a maximum value prior to the breakdown of liquid crystal structure. These results are in excellent agreement with the predictions of the molecular theory of Goossens.

Optical activity and stereochemistry of 4a,4b-dihydropentahelicene

Abstract In the photocyclization reaction of cis -di-β-naphthyl ethylene with circulary polarized an optically active dihydro aromatic (DHP) has been detected as an intermediate. The chirality of the DHP evidences the trans configuration of the tertiary hydrogens. It follows that the ring closure reaction takes place in the excited singlet state of the olefin according to the Woodward—Hofmann rules.

On the order of cholesteric to smectic-A phase transition in cholesteric mixtures☆

Abstract In a series of mixtures of cholesteryl oleylcarbonate and cholesteryl chloride the temperature dependence of the molar volume V has been determined by dilatometric measurements near the cholesteric to smectic-A phase transition. It could be shown from the V ( T ) curves that the phase transition changes from first to second order at a definite mole fraction ⩽ 0.63 cholesteryl oleylcarbonate. This behaviour can be understood in terms of the theories of McMillan and Kobayashi.