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LIQUID CRYSTALLINE ISOTHIOCYANATES
J. van der Veen
To cite this version:
LIQUID CRYSTALLINE ISOTfflOCYANATES
J. VAN DER VEENPhilips Research Laboratoires, Eindhoven, The Netherlands
Abstract. — The synthesis of a new class of liquid crystals, the isothiocyanates, is reported. For some phenylbenzoates and a biphenylisothiocyanate the transition temperatures and transition enthalpies are given. Both smectic and nematic phases are observed. The influence of the —NCS group on the dielectric behaviour is also investigated.
1. Introduction. — Recently we have shown [1], for two series of liquid crystals
and
that the clearing temperature, Te and the anisotropy
in the polarisability of the CAr—X bond, Aax, can be
related to each other via Tc = C Aax, C being constant.
Such a relation can be expected from the molecular-statistical theory of Maier and Saupe for the nematic phase if we assume that the molar volume at Tc is
almost constant within the series and that the polari-sabilities of the various groups are additive. Of the investigated substituents X the •—CN group gives the largest value of Aax (4.4 A3) and consequently the
highest Tc.
These results have prompted us to search for other substituents with an even larger value of Aa^. We found that for the isothiocyanate group ( — N = C = S ) a value for Aax of 12.3 A3 is calculated from the molar
Kerr constant (in solution) of phenylisothiocyanate [2]. In this calculation the assumption is made that the CAr—N—C bond angle is equal or close to 180°. This
assumption follows from the interpretation of the permanent dipole moments of ^-substituted phenyliso-thiocyanates [3]. The permanent dipole moment of phenylisothiocyanate is found to be 2.9 D. From these data is seems possible that the synthesis of iso-thiocyanates could lead to hitherto unknown liquid
crystalline compounds with a high Tc and relatively
large positive dielectric anisotropy, due to the high Ac^ and the large dipole moment, respectively.
/j-Cyano substituted phenylbenzoates [4] and diphe-nyls [5] are known to possess a high positive dielectric anisotropy, a good chemical stability and low melting points. Therefore we decided to concentrate on the preparation of the corresponding isothiocyanates.
Results and discussion. — The results will be pre-sented in the following sequence : the transition temperatures, the transition enthalpies, the dielectric behaviour and X-ray diffraction studies.
1. The transition temperatures are given in table I. For comparison the date of the corresponding cyano substituted compounds are given.
In spite of the larger value of Aa^, the isothiocyanates have in general clearing temperatures not very different from those of the corresponding cyano substituted compounds. This could be due to a molar volume effect because the —NCS group is larger than the —CN group. Furthermore it is remarkable that whereas the cyano substituted compounds show only nematic phases the isothiocyanates all show smectic phases. The SA phases were classified by their simple focal-conic
texture and their miscibility with other SA phases. In a
miscibility study of compound 5 with an SA phase, we
observed transition bars at the phase transition, described by Sackmann [8] as typical for the SA SE
transition. The SE phase of 5 is observed in the mosaic
texture [8].
2. The transition enthalpies measured with diffe-rential scanning calorimetry (Perkin Elmer DSC IB) are presented in table II.
JOURNAL DE PHYSIQUE Colloque C3, supplément au n° 6, Tome 37, Juin 1976, page C3-13
Résumé. — La synthèse d'une nouvelle classe des cristaux liquides, les isothiocyanates, est rap-portée. Pour quelques benzoates de phényl et un isothiocyanate de biphényl, les températures de transition et les enthalpies de transition sont données. Des phases smectiques ainsi que des phases nématiques sont observées. L'influence du groupe — NCS sur le comportement diélectrique est investi aussi.
J. VAN DER VEEN
Transition temperatures (OC) (monotropic transitions are shown in parentheses)
(a) Second crystal modification, (b) SE I.
Transition enthalpies (kJ/mole)
(a) Not observed due to crystallization.
(b) SE I.
The transition enthalpy for the S, I transition of compound 5 is rather large, which can be understood from the high ordering [9].
3. The dielectric behaviour of compound 2 was investigated in the nematic phase. At 670 this com- pound shows a dielectric anisotropy of
+
10.1 (ell = 17.7 ; E, = 7.6). The corresponding cyanosubstituted compound has a dielectric anisotropy of
+
16.1 (el, = 27.8 ; E, = 11.7) at 71.80 [7]. This difference can be attributed to the difference in dipole moments of the -NCS group (2.9 D) and the -CN group (4.0 D) [lo], leading to a much smaller value of E , , for the isothiocyanate.4. To study the different smectic phases in more detail an X-ray diffraction pattern was recorded for the compounds 2 and 5. From the sharp reflection at small angles the interplanar distances were found to be 22.3 and 20.0
A,
in good agreement with the estimated molecular lengths of 22.2 and 20.3A
(from a molecu- lar scale model). Compound 2 gave in addition oneX = - C N mP NI ref. - 92 104 [61 87 (77) 171 66 (41) [61 44 (49) [61
diffuse reflection at large angles as expected for an S, phase. For compound 5 three sharp reflections were observed at large angles, similar as found for other S, phases [9].
2. Experimental. - The following reaction scheme is used for the synthesis of the isothiocyanato phenyl benzoates :
Thep-isothiocyanatophenol is prepared from, commer- cially available, p-aminophenol and thiophosgene in water, following the method of Dyson [ll]. The reaction product is extracted with ether and purified by destillation in vacuo (b. p. 141-142O/1 mm). The benzoyl chlorides are prepared with thionyl chloride from the corresponding benzoic acids, which are commercially available. The condensation of the phenol and the acid chlorides in equimolar quantities is carried out 'in pyridine at reflux temperature, or at room temperature for 24 hours [12]. The reaction product is isolated after dilution with water and recrys- tallized from hexane or petr. ether 60-80. IR and NMR
LIQUID CRYSTALLINE ISOTHIOCYANATES The diphenyl derivative is synthesized in the following way
0
11
AlCl,(I)
+
C,H,C-Cl ---+ C,H~--C-N-C-CF,II II
The starting product draminobiphenyl (carcino- genic) is commercially available. The reaction of trifluoroacetic anhydride with 4-aminobiphenyl is carried out, following the procedure described by Stauffer [13]. The Friedel-Crafts acylation is carried out in CS, [14] at room temperature for 60 hours. The reduction with hydrazine hydrate 80
%
is performed using the procedure of Steinstraesser [15]. The iso- thiocyanation is carried out in a water-chloroform mixture, according to the literature [16] method. Theintermediates 1, 2 and 3 and p-pentyl-p-isothiocyana- tobiphenyl were characterized by their IR
-
and NMR spectra. The intermediates 1 , 2 and 3 show the following m.p.'s after recrystallization 201-202O, 207-2090 and 71-720.Acknowledgment. - The author wishes to thank Dr. W. H; de Jeu for help with the classification of the smectic phases and Mr. J. A. de Poorter for the record- ing of the X-ray diffraction patterns.
References
[l] VAN DER VEEN, J., J. Physique Colloq. 36 (1975) C 1-375. [2] CHENG, C. L., LE F ~ V R E , R. J. W. and RITCHIE, G. L. D.,
J. Chem. Soc. (33) (1971) 435.
[3] ANTOS, K., MARTVON, A. and KRISTIAN, P., CON. Czech. Chem. Commun. 31 (1966) 3737.
[4] BOLLER, A., SCHERRER, SCHADT, M. and WILD, P., Proc.
I. E. E. E. 60 (1972) 1002.
[5] GRAY, G. W., HARRISON, K. J. and NASH, J. A., Electron. Lett. 9 (1973) 130.
[6] TITOV, V. V., KOVSHEV, E. I., PAVLUCHENKO, V. T., LAZA- REVA, V. T. and GREBENKIN, M. F., J. Physique Colloq. 36 (1975) C 1-387.
[7] KLINGBIEL, R. T., GENOVA, D. J., CRISWELL, T. R. and
VAN METER, J. P., J. Am. Chem. Soc. 96 (1974) 7651. [8] DEMUS, D., KOLZ, K. H. and SACKMANN, H., 2. Phj~s. Chem.
Leip. 252 (1973) 93.
[9] DOUCET, J., LEVELUT, A. M., LAMBERT, M., LIEBERT, L. and STRZELECKI, L., J. Physique Colloq. 36 (1975) C 1- 13.
[lo] MINKIN, V. I., OSIPOV, 0. A. and ZHDANOV, Yu. A., Dipole Moments in Organic Chemistry (Plenum Press New York) 1970, p. 91.
[Ill DYSON, G. M., Organic Synthesis, Collective Volume 1 ( J . Wiley, New York) 1967, p. 165.~
[12] SCHROEDER, 5. P. and BRISTOL, D. W., J. Qrg. Chem. 38 (1973) 3160.
[I31 STAUFFER, C. E., J. Am. Chem. Soc. 94 (1972) 7887. [14] MISRA, V. S. and KHARE, M. P., J. Indian Chem. Soc. 30
(1953) 43.
[15] STEINSTRAESSER, R., 2. Natuvforsch. 27b (1972) 774. [16] KRTSTIAN, P., KOVAC, S. and ANTOS, K., Coll. Czech. Chem.
