LOW MELTING THERMOTROPIC LIQUID CRYSTALS-p-p' DISUBSTITUTED PHENYLBENZOATES-II :(a) p(p'-n-alkoxybenzoyloxy) acetophenones(b) p(p'-n-alkoxybenzoyloxy) benzaldehydes

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LOW MELTING THERMOTROPIC LIQUID CRYSTALS-p-p’ DISUBSTITUTED

PHENYLBENZOATES-II :(a) p(p’-n-alkoxybenzoyloxy) acetophenones(b) p(p’-n-alkoxybenzoyloxy)

benzaldehydes

J. Dave, G. Kurian

To cite this version:

J. Dave, G. Kurian. LOW MELTING THERMOTROPIC LIQUID CRYSTALS-p-p’ DIS- UBSTITUTED PHENYLBENZOATES-II :(a) p(p’-n-alkoxybenzoyloxy) acetophenones(b) p(p’-n- alkoxybenzoyloxy) benzaldehydes. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-403-C1-407.

�10.1051/jphyscol:1975166�. �jpa-00216245�

JOURNAL DE PHYSIQUE ColZoque C1, supplkment au no 3, Tome 36, Mdrs 1975, page C1-403

Classification Physics Abstracts

7.130

LOW MELTING THERMOTROPIC LIQUID CRYSTALS-p-p' DISUBSTITUTED PHENYLBENZOATES-I1 :

(a) p(p'-n-alkoxybenzoyloxy) acetophenones (b) p(p1-n-alkoxybenzsyloxy) benzaldehydes

J. S. DAVE and G. KURIAN

Chemistry Department, M. S. University of Baroda, Baroda 390 002, India

BBsumB. - Deux series homologues d'esters mesomorphes ont bte synthetises par reaction des chlorures d'acides p-n-alkoxybenzdques ayec la p-hydroxyac6tophknone et la p-hydroxybenzal- dkhyde. Ces esters ont un point de fusion peu elev6 et ont des phases nematiques et smectiques ; la phase nkmatique a une tendance ⁱⁱ l'homeotropie. Les deux series ont une tempQature de transition nkmatique-isotrope qui augmente avec le nombre de C , avec l'effet pair-impair habituel. Pour les alkoxy-benzoiloxy-a&toph6nones, la temperature de transition smectique-isotrope atteint un maximum puis diminue. Dans cette serie, la courbe de transition smectique-nematique coupe Ia courbe nematique-isotrope pour les chaines impaires aprks le derive en C7. A partir du dQiv6 en Ce jusqu'g celui en Clo, la tempkrature de transition smectique-isotrope montre l'effet pair-impair. Au contraire, la s6rie des benzaldehydes ne montre pas cet effet. La courbe de transition smectique- ntmatique coupe la courbe nematique-isotrope au dMv6 en C I ~ . Les compods mCsomorphes avec un groupe formyl ont etC peu etudiQ. Les proprietes thermiques de ces deux series sont comparkes B celles d'autres series semblables.

Abstract. - Two homologous series of mesomorphic esters are synthesised by reacting p-n-alko- xybenzoyl chlorides with p-hydroxy acetophenone and p-hydroxy benzaldehyde. These esters are low melting and polymesomorphic exhibiting nematic and smectic mesophases ; the nematic mesophase is homeotropic in nature. Both the series exhibit a tendency for rising nematic-isotropic transitions as the series are ascended and show the usual odd-even effect. In p(pl-n-alkoxybenzoyl- oxy) acetophenone series the smectic-isotropic transition curve rises to a nlaximum and then falls off. In this series the smectic-nematic curve merges with the odd member curve of the nematic- isotropic transition beyond the heptyl derivative. The smectic-isotropic transition curve for the octyl to decyl derivative shows an odd-even effect. The benzaldehyde series does not exhibit this behaviour. In this series the smectic-nematic transition curve merges with the nematic-isotropic transition curve at the dodecyl derivative. The mesomorphic compounds with a formyl group are rarely known. The thermal properties of both the series are compared with those of the related series.

1. Introduction. - Low melting thermotropic liquid crystal synthesis has currently aroused considerable interest because of their potential use in display devices.

Such compounds should satisfy certain basic require- ments ; they should possess a suitable mesomorphic range, should be chemically, photochemically and electrochemically stable, colourless and safe to handle.

Although a number of such compounds are recently available only a few satisfy all these requirements.

Rosenberg and Champa [I], Fishel and Pate1 121, Dietrich and Steiger [3], Pate1 [4], van der Veen and Grobben [5] and Gardlund et al. [6] have reported low melting Schiff bases. Kelker et al. [7], van der Veen [8]

and Weygand [9] hate reported low melting azo and azoxy compounds. Gray et al. [lo] have studied substi- tuted biphenyl mesogens. Billard et al. [11] have reported disubstituted fluorenes and 9;lO-dihydro- phenanthrenes and substituted toIans. Young et al. [12]

studied low melting substituted stilbenes. Schiff bases are prone to hydrolysis and oxidation. Azo and azoxy compounds eventhough more stable than Schiff bases are coloured. Steinstraesser [13], Van Meter and Klan- derman [14] and Dave and Vora [15] have reported low melting substituted phenyl benzoates. Here we present the mesomorphic properties of two homologous series ofp, p'-disubstituted phenylbenzoates, having an acetyl and formyl group respectively at one end of the mole- cule. It is interesting to note that mesomorphic compounds with formyl group are rarely known.

2. Results. - 2.1 SERIES (A). - p(p'-n-alkoxyben- zoyloxy) acetophenones. A homologous series of fourteen esters is synthesised by reacting p-n-alkoxy- benzoyl chlorides with p-hydroxy acetophenone. The melting points and transition temperatures are given in Table I.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975166

J. S. DAVE AND G. KURIAN

Compound

-

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Alkyl group

- CH3 C2H5 C3H7 C4H9 CsHil C6H13 C7H15 C8H17 C ~ H I ~ CioHz~

C12H25 C14Hz9 C16H33 C18H37

Transition temperature (OC) smectic nematic isotropic

- - - - (67.0) (*) 157.5 - (81.0) (*) 144.0 - (76.0) (*) 107.0 (86.5) 105.0 (76.0) (84.0) 90.0 (85.0) 88.5 90.0 (89.0) (90.5) 101.0

84.0 - 95.0

87.0 - 95.5

89.0 - 98.0

87.5 - 102.0

90.0 - 102.5

88.5 - 101.0

85.0 - 95.0

(*) Values obtained by extrapolation.

Values in parenthesis indicate monotropy.

The first three members of the series are non-meso- morphic ; mesomorphic properties appear at the fourth member which is monotropic nematic. The smectic phase commences from the pentyl derivative. Pentyl, hexyl and heptyl derivatives are polymesomorphic i. e.

smectic and nematic. These transitions are monotropic in nat&re except the hexyl derivative in which the

NUMBER OF C A R B O N A T O M S IN ALKYL CHAIN-

FIG. 1. - p(p'-n-alkoxybenzoyloxy) acetophenones R0.C6H4.C02.C6H4.C0.CH3

- - - . Solid-mesomorphic or isotropic.

n-a Smectic-nematic.

0-0 Smectic-isotropic.

0-0 Nematic-isotropic.

nematic-isotropic transition is enantiotropic. Octyl to octadecyl derivatives exhibit purely enantiotropic smec- tic mesomorphism. Figure 1 shows the relationship of transition temperatures to carbon numbers in the alkyl chain.

The nematic-isotropic transition points (butyl to heptyl derivatives) fall on two curves which show an ascending tendency as well as the usual odd-even effect.

2.2 SERIES (B). - p@'-n-alkoxybenzoyloxy) ben- zaldehydes. This homologous series comprises fourteen esters which are obtained by reacting p-n-alkoxy- benzoyl chlorides with p-hydroxy benzaldehyde. The melting points and transition temperatures are summa- rised in Table 11.

Compound

-

15 16 17 18 19 20 21 22 23 24 25 26 27 28

Alkyl group - CHs CzHs C3H7 C4H9 CsHii C S H I ~ C7H15 C8Hi7 c9H19 CioHzl CizHz5 C14H29 C16H33 C18H37

Transition temperature (OC) smectic nematic isotropic

- - - - (25.0) (*) 92.0 - (50.0) (*) 128.0

-

(40.0) (*) 82.5 - (57.0) (*) 91.5

- 48.0 51.5

- 60.0 64.0

- 55.0 62.0

- 54.5 70.0

- 54.0 69.0

(62.5) 64.0 73.5

66.5 - 76.5

75.0 - 79.0

78.0 - 82.5

73.0

83.0

(*) Values obtained by extrapolation.

Values in parenthesis indicate monotropy.

The first four members of the series are non-meso- morphic. Pentyl to decyl derivatives are enantiotropic nematic. The smectic phase appears at the decyl deri- vatives as a monotropic phase. The higher members, dodecyl to octadecyl derivatives exhibit only enantio- tropic smectic mesophase. Figure 2 shows the plot of the transition temperatures versus the number of carbon atoms in the alkyl chain.

The nematic-isotropic transition points lie on two curves, the upper curve representing the even members as usual. In this case also the nematic-isotropic tran- sition curve shows an ascending trend.

It is interesting to note that most of the members in

both the series adopt a homeotropic texture very easily

and near the transition (smectic-nematic, smectic-

isotropic or nematic-isotropic), the field becomes bire-

fringent ; the transitions could thus be determined

without much difficulty. The obscure transition tempe-

ratures, in both the series, for the non-mesomorphic

members are obtained by the extrapolation of the

nematic-isotropic transition temperature curves. These

extrapolated values lie well below the crystallization

points of these compounds and that is probably the

reason why these members are non-mesomorphic.

LOW MELTING LIQUID CRYSTALS-PHENYLBENZOATES I1 (21-405

I 2 3 4 5 6 7 8 9 I 0 1 1 I 2 I3 14 15 16 17 I8

NUMBER

FIG. 2. - p(pr-IZ-alkoxyben~oyloxy) benzaldehydes RO . C ~ H ~ . C O Z . C ~ H ~ . C H O

. _ _ _ ^. Solid-mesomorphic or isotropic.

Smectic-nematic.

0---@ Smectic-isotropic.

a---a Nematic-isotropic.

3. Discussion. - These two homologous series deviate from the normal mesomorphic homologous series in the relation of transition temperatures to substituent chain length. They exhibit a tendency for rising nematic-isotropic transition in ascending series.

This type of behaviour is generally found in series having low transition temperatures.

The homologous series with such ascending ten- dency are of three types. The first type are molecules where lateral substituents prevent a close packing in the mesomorphic phases, such as N(p-i-alkoxybenzyli- dene) - 1' - aminonaphthalene - 4' - azobenzenes with branches in the alkyl chain [9] and substituted N(p-n- a1koxybenzylidene)-p'-aminobiphenyls [16]. This has been ascribed by Gray [I 61 as due to the decreasing ratio of laternal to terminal interactions because of the thickening of the molecules due to steric effect.

The second type of compounds which behave in this manner include NCp-n-alkoxybenzy1idene)-p-alkyl anilines. Dietrich and Steiger [3] attribute this pheno- menon in such compounds to low intermolecular cohesiveness.

The third type, which shows this behaviour possesses a terminal carbonyl group conjugated with an aromatic system, for example, N(p-n-alkoxybenzy1idene)-p-

aminoacetophenones and propiophenones 1171. Castel- lano and McCaffrey 1171 have suggested that the acetyl group produces strong terminal attractions and this would result in a low ratio of lateral to terminal cohe- sions. Other series with similar strong permanent dipoles e. g., butoxy and acetoxy homologues [3] show a decreasing trend in their transition-chain length plots.

The series (A), p(p'-n-alkoxybenzoyloxy) acetophe- nones, possesses a terminal acetyl group conjugated with an aromatic system, and the series (B) p(pl-n- alkoxybenzoyloxy) benzaldehyde has the formyl group in conjugation with the aromatic system. Perhaps the dipole of the carbonyl group adopts a more coaxial orientation due to its conjugation with the aromatic system and thus the ascending tendency of the nematic- isotropic transitions in these two homologous series is justified.

Another distinguishing feature noticeable in figure I is the alternation for the smectic-isotropic transition points for octyl to decyl derivatives. The smectic- nematic curve merges with the nematic-isotropic curve and beyond the heptyl derivative the series is purely smectic. It is a general observation that in a purely smectic series the smectic-isotropic transitions exhibit the odd-even effect. Recently a few series are also reported to exhibit the alternation in the smectic- nematic curve [3, 41. In the present series the smectic- isotropic transition curve rises to a maximum at tetra- decyl derivative and then falls off. The smectic isotropic transition temperature will be determined by the weak lateral attractions. These attractions will increase as the chain grows longer and the smectic phase will increase in thermal stability but only upto a point ; for, as the alkyl chain lengthens still further the terminal cohesions grow weaker and the probability of inter- penetration of the smectic strata at a lower tempera- ture increases. For the higher homologues, therefore, the weakening terminal cohesions determine the ther- mal stabilities of the smectic mesophase, and explain the subsequent fa11 in the transition curve as the series is ascended. However in series (B) the smectic-isotropic curve rise smoothly but does not fall off. It may be because of the low thermal stability as well as the late commencement of the smectic phase in this series.

Table I11 summarises the average thermal stabilities of the following series :

p(pl-n-alkoxybenzoyloxy) acetophenone . . . A

p(pl-n-alkoxybenzoyloxy) benzaldehydes . . . . . . B

p-n-alkoxybenzylidene-p'-aminoacetophenone [17] C

p-n-alkoxybenzylidene-p'-ethylanilines [4] . . . . D

The moiecules of series (A) are thermally more stable

than those of series (B) ; both the smectic and nematic

thermal stabilities are increased. This increase in the

thermal stabilities can be attributed .to the increased

polarizability of the compounds due to the presence of

the acetyl group in the molecules of series (A). How-

ever, the nematic thermal stability is increased a little

more. Series (A) is thermally less stable than series ( C ) ;

C1-406 J. S. DAVE AND G . KURIAN

Average transition

temperature (OC) A B C D

- - - -

Nematic-Isotropic 88.2 59.2 112.7 64.8

(C,-C,) (C,-C,)

Smectic-Isotropic 100.1 80.2 120.0 77.3

(c12-c18) (cl 2-c14)

Commencement of

the smectic phase c g CIO c3 c 4

both the thermal stabilities are almost equally affected.

The two series differ only in the central linkage ; series (C) has the central azomethine (-CH=N-) group whereas series (A) has the central carboxy (-COO-) group. The oxygen atom of this central carboxy group in the molecule of series (A) will be bumping into the non-bonded sides of the adjacent hydrogens of the aromatic ring thereby causing considerable strain on the molecule. This will cause some twist around the C - 0 bond and force the benzene ring out of the plane of the molecule, thus reducing the co-planarity of the molecules and making them thick [la]. The hydrogen atom of the central azomethine group will also behave in the same way and bring about the twist in the mole- cules of series (C) but the twist in this case will be less than that in the case of the molecules of series (A) ; hence the molecules of series (A) will be more non- coplanar than those of series (C). Naturally both the thermal stabilities of series (A) will be decreased. The thermal stabilities of series (B) are also less than those of series (C) ; in this case also the same argument will hold good as the two series differ in the central group in the same way as series (A) and (C). But here the thermal stabilities of series (B) are affected to a greater extent than those of series (A). This is because series (A) has an acetyl group at the end of its mole- cules whereas the series (B) has the formyl group at the end of its molecules. Further, the effect is more on the nematic mesophase ; this also can be attributed to the presence of the acetyl group in the molecules of series (C) in place of the formyl group in the molecules

of series (B). Compared to series (A) and (B), series (D) has also a central azomethine group. Naturally the thermal stabilities of series (D) should be more than those of series (A) and (B). But actually the thermal stabilities of series (A) are higher than those of series (D). This can be attributed to the presence of the acetyl group in series (A) compared with presence of the ethyl group in series (D). The thermal stabilities of series (B) are almost equal to those of series (D). This can be attributed to the presence of the carboxy and the formyl groups in series (B) compared with the azomethine and ethyl groups in series (D).

In series (C) and (D) the smectic phase commences at the propyl and butyl derivatives respectively whereas it commences at the pentyl derivative in series (A) and at the decyl derivative in series (B). Series (C) and (D) have the central azomethine groups compared with the carboxy groups in series (A) and (B) ; the molecules of series (C) and (D) as explained before, will be less non- coplanar compared with the molecules of series (A) and (B). This may be the reason for the early comrnen- cement of the smectic phase in series (C) and (D). In the case of series (A) and (B).the two molecules are similar except in the end groups at one end of the molecule. The acetyl group at the end of series (A) may be responsible for the early commencement of the smectic mesophase of series (A) at the pentyl derivative 4. Experimental. - Determination of transition temperatures : Preliminary measurements were made by the optical method of Dave and Dewar [19]. The

Compound -

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

% required

C H

- - 71.10 5.19 71.82 5.67 72.46 6.08 73.06 6.45 73.60 6.79 74.09 7.12 74.55 7.39 74.97 7.66 75.36 7.91 75.72 8.13 76.38 8.55 76.95 8.91 77.46 9.23 77.91 9.51 70.30 4.72 71.10 5.22 71.82 5.67 72.46 6.08 73.06 6.45 73.60 6.79 74.09 7.11 74.55 7.39 74.97 7.66 75.36 7.91 76.06 8.34 76.65 8.73 77.21 9.07 77.69 9.37

% found

C H

- -

71.03 4.87

71.62 5.52

72.35 5.67

73.21 6.33

73.26 6.47

74.56 6.75

74.15 6.90

74.63 7.26

75.23 7.55

75.61 7.89

76.30 8.14

77.29 8.49

77.25 8.76

77.59 9.65

70.18 4.47

71.20 4.83

71.61 5.56

72.71 5.68

73.50 6.28

73.57 6.54

74.47 6.83

74.92 7.03

74.85 7.56

75.26 7.79

75.91 7.95

76.32 8.26

76.81 8.60

77.75 9.33

LOW MELTING LIQUID CRYSTALS-PHENYLBENZOATES I1 C1-407 precise measurements were made with the Leitz Ortho-

lux Polarising Microscope equiped with a Leitz heating stage.

5. Preparation of compounds.

1) p-n-alkoxybenzoic acids. These were prepared by the method of Gray and Jones [20].

2) p-n-alkoxybenzoyl chlorides. These were pre- pared from the corresponding acids [21].

ride (0.015 mol) in dry (A. R.) pyridine. The mixture is warmed for an hour and then allowed to stand over-night. It is then acidified with dilute hydro- chloric acid. The precipitates are filtered and then washed with very dilute sodium hydroxide solution followed by water. The solid esters are recrystallized from methanol or ethyl acetate to fine white needles.

Yield about 70 %.

The analytical data are given in Table IV.

') P - ~ Y ~ ~ ~ ~ Y acetophenone was prepared from A&nowledgments. - The authors take this oppor- phenyI acetate [22]. tunity to express their sincere thanks to Prof. Suresh 4) Esters. ~ e t h n a for his keen interest in the work. One of us A solution of p-hydroxy acetophenone orp-hydroxy (G. K.) is thankful to the Gujarat Government Scien- benzaldehyde (A. R.) (0.01 mol) in dry (A. R.) pyri- tific and Industrial Research Committee for the grant dine (10 ml) is mixed with p-n-alkoxybenzoyl chlo- of a research assistantship.

References

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Cryst. ll(1970) 191.

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[3] DIETRICH, H. J. and STEIGER, E. L., MoI. Cryst. Liqu. Cryst.

16 (1972) 263.

[4] PATEL, P. R., J. Indian Chem. Soc. 50 (1973) 514.

[5] VAN DER VEEN, J. and GROBBEN, A. H., MoI. Cryst. Liqu.

Cryst. 15 (1971) 239.

[6] GARDLUND, Z. G., CURTIS, R. J. and SMITH, G. W., J. Chem.

Soc. Chem. Comm. (1973) 202.

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[8] VAN DER VEEN, J., M01. CQJSt. L i q ~ . Cryst. 17 (1972) 291.

[9] WEYGAND, C., Hand- und Jahrbuch der Chem. Physik., Vol. 2 ; CHAP, C., Akad. Verl. (Leipzig) 1941, Chem. Abstv.

37 (1943) 1078.

1101 GRAY, G. W., HARRISON, K. J. and NASH, J. A., Paper presented at the Liquid Crystal Conference, Bangalore, India, 1973, Abstract No. 25.

I l l ] CANCEILL, J., BILLARD, J. and JACQUES, J., Paper presented at the Liquid Crystal Conference, Bangalore, India, 1973, Abstract No. 26;

DVOLAITSKY, M., GABARD, J., LECLERCQ, M., MALTHETE, J., BILLARD, J. and JACQUES, J., Paper presented at the Fourth International Liquid Crystal Conference, Kent State University, Kent, Ohio, U. S. A., 1972, Mol.

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[12] YOUNG, W. R., AVIRAM, A. and Cox, R. J., J. Amer. Chem.

SOC. 94 (1972) 3976.

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[14] VANMETER, J. P. and KLANDERMAN, B. H., J. Amer. Chem.

SOC. 95 (1973) 626.

[I51 DAVE, J. S. and VORA, R. A., Paper presented at the Liquid Crystal Conference, Bangalore, India, 1973.

[16] GRAY, G. W., Molecular Structure and the properties of Liquid Crystals (Academic Press, London) 1962, p. 230.

[17] CASTELLANO, J. A. and MCCAFFREY, M. T., Liquid Crystals and Ordered Fluids (Plenum Press, New York) 1970, p. 293 ;

HALLER, I. and COX, R. J., ibid. p. 393 ;

ARORA, S. L., TAYLOR, T. R. and FERGASON, J. L., ibid. 321 ; Research report, p. 54, Liquid Crystal Institute, Kent State University, Kent, Ohio, U. S. A., N. A. S. A.

Research Contract No. NGR-36-007025, Washington D. C.

[IS] ARORA, S. L., FERGASON, J. L. and TAYLOR, T. R., J. Org.

Chem. 35 (1970) 4055.

1191 DAVE, J. S. and DEWAR, M. J. S., J. Chem. Soc. (1954) 4616.

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1211 DAVE, J. S. and VORA, R. A., Liquid Crystals and Ordered Fluids (Plenum Press, New York) 1970, 477.

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(Longmans Green and Co.L td., London) 1968, p. 676.