J . Phys. Chem. 1985,89, 3939-3941
3939
Cholesterization of a Biaxial Nematic Lyomesophase Studied by X-ray Diffraction and Optical Microscopy A. M. Figueiredo Neto,*+Y. Galerne, and L. Liebert Laboratoire de Physique des Solides,? Bdt. 510, Universitd Paris-Sud, 91405 Orsay, France, and Laboratoire pour I'Utilisation du Rayonnement Electromagndtique (LURE), Universitd Paris-Sud, 91 405 Orsay Cddex, France (Received: February 5, 1985; In Final Form: April 18, 1985)
A new cholesteric phase is observed by optical microscopy and X-ray diffraction in the lyotropic system of NadS/decanol/H20/brucine sulfate, between the usual cholesteric discotic (ChD) and cholesteric calamitic (Chc) phases. This new phase (ChBx) is the nematic biaxial phase cholesterized by the chiral compound. It may be oriented by a magnetic field. Only one cholesteric pitch (identical with the ChD pitch) is observed in the ChBx phase.
Introduction Nematic lyotropic liquid crystals' are mixtures of amphiphilic compounds and water in appropriate concentration-temperature conditions. The amphiphilic molecules form anisotropic aggregates2 which may be magnetically oriented. Three different phases have been observed3 in these systems, two uniaxial and one biaxial (NBX). Depending o n whether the direztor (n') orients parallel or perpendicular to the magnetic field these uniaxial phases have been classified2 as calamitic (Nc) and discotic (ND) respectively. Cholesteric lyotropic liquid crystals are obtained4' by the addition of a chirat compound to a nematic lyomesophase. Two different cholesteric phases have been ~ b s e r v e d : ~calamitic ,~ cholesteric (Chc) and discotic cholesteric (Ch,). In the presence of a sufficiently high magnetic field, a _ChDphase orients with the cholesteric planes perpendicular to H,and the cholesteric structure isuntwisted in the C b phase. The attempt of cholesterization of a nematic biaxial phase (Ch,,) has not been achieved to date. In this paper, we report what we believe to be the first observation of the ChBx phase. X-ray diffraction patterns of this phase are also discussed.
(w,
Experimental Section The Sample. The cholesteric lyomesophases are prepared according to conventional procedures3 with the following composition in weight percent: sodium decyl sulfate (NadS) (from Merck, purity >99%), 38.95; 1-decanol (DeOH) (from Fluke, purity >99%), 7.49; H20, 53.38; and brucine sulfate heptahydrate (BS) (from Fluka, purum), 0.18. M , is the molar concentration ratio4 defined as
[BSI
[BS]
+ [DeOH] + [NadS] x 100
in our case Ma = 0.09. A small quantity of ferrofluid ( 0.3, Chsx and Chc coexistence regions are observed, Parts a and b of Figure 2 illustrate this case with and without If, respectively. The periodic structures with the great periodicity of about 150 p m (which is of the same order of magnitude as the sample thickness, 200 rrm) are hydrodynamic instabilities, as discussed above, and produced in the Chc domains (11) de Vries.
H.Acta crVsto//o~r.1951. 4. 219.
J
a
b
Rgme 3. Small-angleX-ray diffraction pttrms of the cholestericpbasca in a 1.5-mm thick capillary sst up in the vertical d i e i o n in the plane of the figure. Sample magnetically oriented with H panllel to the helicoidal axis ( I axis), perpendicular to the X-ray beam (2 axis): (a) cholateric dimtic (ChD); (b) cholesteric biaxial (Ch.,).
by the wall orientational effect after the removal of r%' X-ray Dijj?acfion. Typical diffraction patterns of the ChDand ChBxphases with fi perpendicular to the X-ray beam arc presented in Figure 3, parts a and b, respectively.'z Both patterns present two intmse spots (with a wcak ssond orda with a spacing ratio I:2) along the 3 axis at s j l (38.3 f 0.8) A joined by two weaker arcs along the 1 axis at sl-l (47 f I ) A and (53 1) A
*
(12) The diffnnion pttem in thc d t - t i o n
arc ring as diMured in rcf 5.
A' pnllcl to thc beam
J . Phys. Chem. 1985,89, 3941-3946
for the ChD and ChBx, respectively. The existence of the second-order band already observed in the nematic seems to indicate that at a microscopic scale, the nematic pseudolamellar order" is maintained in the cholesteric phases. Also, the continuous increase of sI-lfor increasing temperatures is analogous to previous experiments in nematic phases.13 These results indicate that in the ChBx phase the long axes of the biaxial micelles are oriented parallel to the helicoidal axis (the two other orthogonal axes twist from one nematic plane to another).
Concluding Remarks The o5ervation of only one cholesteric pitch in ChBx (with or without H) in our experimental conditions is remarkable. "A priori" three different pitches could be expected in this phase, corresponding to the twist along the three independent twofold symmetry axes of the local biaxial system. In this case without H,a defect lattice typical of a blue phaselo would have been observed. But such a network of line defects costs energy. This may explain that, even very close to the ChBx transitions, the system prefers to twist only one axis (directed along the long axis of the micelles) and to untwist the two other directions. Recently, (13) Figueiredo Neto, A.M.; Galerne, Y.; Levelut, A. M.; Liebert, L. J. Phys. Lett. 1985,46,L-499.
3941
a phase of this type has been predicted and described theoretically.I4 In this case, the biaxial cholesteric and the uniaxial cholesteric (which in fact is also locally biaxial15) have the same symmetry. As first noticed by Brand and Pleiner,16 such a cholesteric to cholesteric transition similar to the liquid-gas transition cannot be of second order. Since the transitions between the three cholesteric phases look perfectly continuous under all our observations, we may conclude that, either the transitions are weakly first order for small concentrations of BS and our techniques are not sensitive to detect it, or they are not really phase transitions as the liquid-gas transition above the critical point. Acknowledgment. We thank Drs. A. M. Levelut, H. Pleiner, and H. R. Brand for helpful discussions. We gratefully acknowledge the donors of Petroleum Research Fund, administered by the American Chemical Society, for financial support of this work. Registry No. NadS, 151-21-3; decanol, 112-30-1;brucine sulfate, 4845-99-2. (14) Pleiner H.;Brand, H. R., submitted for publication. (15) Yaniv, 2.;Vaz, N. A. P.; Chidichimo, G.; Doane, J. W. Phys. Rev. Lett. 1981,47,46. (16)Brand, H.R.;Pleiner, H., submitted for publication.
Investlgatlon of the Quasi-Liquid Crystal Structure Felix P. Shvartsman, Ivan R. Cabrera, Alexander L. Weis,+ Ellen J. Wachtel,+ and Valeri A. Krongauz* Department of Structural Chemistry, The Weizmann Institute of Science, Rehovot 76100, Israel (Received: April I , 1985)
Quasi-liquid crystals, a metastable mesophase of thermwhromic spiropyrans containing mesogenic groups, were investigated by different methods. Miscibility studies, X-ray diffraction,and UV-visible and FT-IR spectroscopy indicate that the material behaves basically as a nematic mesophase. Comparison of data of the present work with that obtained earlier by optical and ESR investigation of the behavior of fluorescent and paramagnetic probes results in the formulation of a structural model for quasi-liquid crystals.
Introduction Spiropyrans are among the most extensively investigated organic photo- and thermochromes.' The color changes are associated with the following reversible reactiom2
SPIROPYRAN
MEROCYANINE
Earlier we observed different types of molecular assemblies, formed on conversion of spiropyrans into merocyanine The assemblies are based on the capability of the merocyanine dyes or spiropyran-merocyanine complexes to form giant molecular stacks.g Recently we have reported a new type of structural organization for spiropyran-merocyanine systems, quasi-liquid crystals (QLCs), obtained from spiropyrans containing mesogenic groups.lOJ1 Y= Crystals of these spiropyrans (X = RPhCOOPhCH-N-; CH,-) melt into isotropic liquid. However, metastable amorphous films of these compounds prepared by fast evaporation of the solvent from their solution give a birefringent texture upon heating. 'Department of Organic Chemistry. 'Department of Polymer Research.
Appearance of texture, characteristic of a mesophase, coincides with a sharp increase in the merocyanine concentration, resulting in a color change of the films from yellow to green, due to thermochromic spiropyran merocyanine conversion. Further increase in temperature leads to the disappearance of texture but it appears again upon cooling. The temperature range of the birefringent texture is rather wide (for example, spiropyran with R = CH30- has a texture between 50-130 "C) and lies much
-
(1)Bertelson, R. C. 'Photochromism"; Brown, G. H., Ed.; Wiley: New York, 1971;Chapter 3. (2) Fischer, E., Hirshberg, Y. J . Chem. Soc. 1952,4522. (3) Krongauz, V. A.;Fishman, S. N.; Goldburt, E. S. J. Phys. Chem. 1978, 82,2469. (4) Meredith, G. R.; Williams, D. J.; Fishman, S. M.; Goldburt, E. S.; Krongauz, V. A. J. Phys. Chem. 1983,87, 1697. ( 5 ) Krongauz, V. A.; Goldburt, E. S. Macromolecules 1981, 14, 1382. (6)Goldburt, E.S.; Shvartsman, F. P.; Krongauz, V. A. Macromolecules 1984, 17, 1876. (7) Goldburt, E. S.;Shvartsman, F.P.; Fishman, S. N.; Krongauz, V. A. Macromolecules 1984,17, 1225. (8) Cabrera, I. R.; Shvartsman, F. P.; Veinberg, 0. N.; Krongauz, V. A. Science 1984,226, 341. (9) Sturmer, D. M.; Heseltine, D. W. In "The Theory of the Photographic Process," Jammes, H.,Ed.; MacMillan: New York, 1977;Chapters 7 and 8. (10) Shvartsman, F. P.; Krongauz, V. A. Nature (London) 1984,309,608. (11) Shvartsman, F.P.;Krongauz, V. A. J. Phys. Chem. 1984,88,6448.
0022-3654/85/2089-3941$01.50/00 1985 American Chemical Society
