HEXASUBSTITUTED TRIPHENYLENES : A NEW MESOMORPHIC ORDER

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HEXASUBSTITUTED TRIPHENYLENES : A NEW MESOMORPHIC ORDER

C. Destrade, M. Mondon, J. Malthete

To cite this version:

C. Destrade, M. Mondon, J. Malthete. HEXASUBSTITUTED TRIPHENYLENES : A NEW MESOMORPHIC ORDER. Journal de Physique Colloques, 1979, 40 (C3), pp.C3-17-C3-21.

�10.1051/jphyscol:1979305�. �jpa-00218695�

HEXASUBSTITUTED TRIPHENYLENES : A NEW MESOMORPHIC ORDER

Abstract. — We have prepared two series of hexasubstituted triphenylene derivatives : hexa- ethers from CH

to C

H

and hexaesters from C

H

to C

H

. Most of them exhibit a thermo- tropic mesophase with a large temperature range. We have found the existence of an additional transition within the mesophase domain of some hexaesters with long alkyl chains. The structure of this new type of liquid crystal is discussed with respect to ours results concerning miscibilities and heat of transition.

1. Introduction. — The existence of mesophases built up with bidimensional central rigid part (by opposition of unidimensional ones : rod-like, or tridimensional : plastic crystals) has been proposed by Professor P. G. De Gennes for years. The first example of such phases obtained with disc-like molecules has been recently described : some benzene hexa-n-alkanoates [1] and some hexa-n-alkoxy 2, 3, 6, 7, 10, 11 triphenylenes [2] exhibit a thermotropic mesomorphism. The nature, the molecular orga- nization and the physical properties of these new liquid-crystals may open a very large field of research.

2. Synthesis methods. — In a recent paper [2], we have proposed two different ways for the preparation of hexaalkoxy-triphenylenes, these two methods in spite of providing us the new espected disc-like mesogens are unsatisfactory from a chemical point of view. We start in the two cases with pyrocatechol substituted by the required alkyl chains. The trime- risation is performed wether by the action of n-butyl Lithium in dry THF on the iodo bromide derivative,

(*) Laboratoire de Chimie Organique des Hormones, College de France, 75231 Paris Cedex, France.

or following the Musgrave and Webster method [3]

by the action of p-chloranyl in sulfuric acid

The first one leads to very poor yields. The second one, in spite of relatively good yields for short alkyl chains (CH

to C

H

), require long and tidious purifications. On the other hand the necessity to obtain other derivatives of the triphenylene with various substituents, leads us to look for a new method of preparation simpler and general.

The method we proposed [4] consists first in pre- paring the hexahydroxy 2, 3, 6, 7, 10, 11 triphenylene which is obtained from the corresponding methoxy derivative [3]. Then the reaction of the required alkyl bromide or acid chloride gives respectively the hexaethers or esters derivatives

3

JOURNAL DE PHYSIQUE Colloque C3, supplément au n° 4, Tome 40, Avril 1979, page C3-17

C. DESTRADE, M. C. MONDON and J. MALTHETE (*)

Centre de Recherche Paul-Pascal, Domaine Universitaire, 33405 Talence, France

Résumé. — On a synthétisé deux séries de dérivés hexasubstitués du triphénylène : hexaéthers de CH

à C

H

et hexaesters de C

H

à C

H

. La plupart de ces composés présentent une mésophase thermotrope à large gamme de température. Nous avons mis en évidence l'existence d'un polymorphisme mésomorphe pour les hexaesters à chaînes longues. La structure de ce nou- veau type de mésophase est discutée à partir de nos résultats concernant les miscibilités et les chaleurs de transition.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979305

C. DESTRADE, M. C. MONDON AND J. MALTHETE

This very simple and direct method has allowed the preparation with very good yields of both ethers and esters with long alkyl chains.

The purity of the different prepared compounds has been checked by elementary analysis and Mass Spectroscopy.

3. Experimental results. - 3.1 TEMPERATURE

The transitions were studied by differential scanning calorimetry using a Dupont 990 thermal analyser. Optical textures were observed with a polarizing microscop equipped with a heating and cooling stage (Metler FP5). The temperatures, enthalpies and entropies of the prepared compounds are given in table I (hexaethers) and I1 (hexaesters).

Figure 1 and 2 represent the corresponding variation of temperature and heat of transition versus the length of the alkyl chain.

3.2 OPTICAL

Ethers and esters tex- tures are quite different (an example in each series is given in figures 3 and 4). Broken fan textures (Fig. 4) are often observed with short alkyl chains esters and are very similar with those founded by Chandrasekhar et al. [I] in the case of benzene-n- alkanoates. We must also point out that, in both series, textures are very similar when increasing the length of the chains. On cooling the mesophase gene- rally makes its appearance as flower-like domains.

With ethers one can observe first the appearance of little homeotrop hexagons which, on further cooling, deformes to give very typical indentations (Fig. 3). Let us notice at last that textures very similar to smectics, can be observed in the low and high temperature mesophase of long alkyl chain esters (Figs. 5 and 6).

Enthalpies, entropies and transitions temperatures of the prepared hexaalcoxy-triphenylenes

R TC-M AHC-M ASC-M TM-I A H ASM-I

( C - I )

- - - - - - -

R

CH3 317

CzH5 247

C3H7 177 4.03 8.96

C4H9 88.6 5.52 15.2 145.6 4.50 10.07

CsHll (b) 69 7.80 22.8 122 2.04 5.16

C t 5 H ~ ,

68 8.68 25.5 97 0.86 2.32 RO

C7H15

68.6 14.4 42.1 93 1.20 3.28 0

66.8 19.92 58.6 85.6 1.04 2.90 OR

C8H1,

CgHl, 58 57 17.0 17.0 51.5 51.3 69 77.6 0.64 0.46 R O 1.35 1.83 : . . 0 R ClOH,,

C13H25

(49) p)

(") T

temperature (Celcius), M

mesophase, C

crystal, I

isotropic, AH = molar enthalpie

(K.cal), AS

molar entropie (cal/K).

(b) See also reference [5].

(e) Monotropic transition.

HEXASUBSTITUTED TRIPHENYLENES

A NEW MESOMORPHIC ORDER

TABLE I1

Enthalpies, entropies and transition temperatures of the prepared triphenylene hexa-n-alkanoates (")

O* / C - R

o=c'

: ^I

: :

O , C / ~ 0 II

04c,o

0

O k C / 0 R,c I =

R I

(a)

T

temperature (Celcius), C

crystal, M

mesophase, M,

low temperature mesophase, M,

high temperature meso- phase, I

isotropic, AH

molar enthalpie (K.cal), AS

molar entropie (cal/K).

(b)

Crystal-crystal transition.

(c)

From C,H,,

C13H17 different crystal-crystal transitions are observed we give only the higher transition temperature and sum of the enthalpies.

.

-

- .

5 : * * *

' I V ' I ' I r r r

1 1 . 1 1 1 1 1 1 1

CRMSTAL -ISOTROPIC

*[

V

. ^A

-

-

-

' I ' I ' I . v ' I

-

.

* .

-

I * - I I I I I I I I

3 4 5 6 7 8 9 10 11 12 13,

3 . 3 ISOBAR PHASES DIAGRAMS. - We have deter-

mine several isobar phases diagram by the well-known ^{FIG. 2.}

Variation of temperature and heat of transition versus temperature for the prepared triphenylene hexa-n-alkanoates.

contact method. Some of these diagrams are shown in figure 7.

In good agreement with opticals observations we two studied compounds present not too different note total miscibility in each series (but only if the alkyl chain length, see for instance figures 7a and 7b).

AH

15

10

T'c

200

150

100

50 9

OH

?5

10

5

T

1.50

100

50

FIG. 1.

Variation of temperature and heat of transition versus temperature for the prepared hexaalcoxy-triphenylenes.

~.sar.mo~e-'

l

- -

-

.

v ' ' I T v .

1 1 1 ' 1 1 1 1 I

.

//

-

-

-

-

v

- v

'I 'I

e m * . .

.

- ^v

, 1 1 1 1 1 1 1 , 1 1 1

2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 n

C3-20 C. DESTRADE, M. C. MONDON AND J. MALTHETE

FIG. 3. - Optical textures observed with the C,Hl10 derivative at 120 OC on cooling (2 OC/min.).

FIG. 4. - Optical textures observed with the C,H,,COO derivative at 120

on cooling (2 OC/min.).

50

C9%0 % ^{Hf7 COO}

T4C.

100

-

FIG. 5.

Optical texture of the Cl1Hz3COO derivative at 90 OC

on cooling (2 ~C/min.)

low temperature mesophase M,.

-

d )

Cfo

Coo ^{Cg H, COO}

I

FIG.

to 7e.

Isobar phases diagrams of different binary FIG. 6.

Optical texture of the CllH,3CO0 derivative at 116 OC

mixtures of ethers and esters

the compounds are figured by

high temperature mesophase M,. their substituants.

HEXASUBSTITUTED TRIPHENYLENES

A NEW MESOMORPHIC ORDER C3-21

4. Discussion.

The thennotropic mesophases we get with hexasubstituted triphenylenes present ' a larger mesomorphic range than those observed in the case of benzene-n-alkanoates [I], furthermore the number of mesogenic compounds is more impor- tant in these series. This kind of mesomorphism seems connected with the central disc-like diameter group.

An other interesting fact resides in the evolution of the heat of transition with the alkyl chains length : these values are at the same time increasing at the crystal-mesophase transition and decreasing at the mesophase-isotropic transition. They are similar to those usualy measured for classical liquid crystals especially in the case of long alkyl derivatives.

At last we have shown the existence of a meso- morphic polymorphism in some esters derivatives (with R 2 C1,H,,) and these new transitions are practically second order ones ( A H

25 cal .molep l).

5. Structure. - Some X Ray diffraction measure- ments [I] and optical textures agree well with an hexagonal close-packed arrangement of discs in which the discs are stacked forming liquid columns.

But an important question has to be answered : what is the localisation of the alkyl chains with respect to the central disc like group ?

Taking into account Votgell work [6] on some benzene derivatives similar to Chandrasekhar ones and ours results (heat of transition crystal-mesophase for instance see table I and 11) a model with fully extended chains in the plane of the central disc is very improbable in the mesophase. We suppose that, for short alkyl chains, the partial melting of chains leads to diabolo like molecules (Fig. 8).

Non miscibility of ethers and esters (even with similar diameter such as C,H,,COO and C9HI9O substituents) and of short chains with long chains derivatives, polymorphism and at last large crystal-

FIG. 8.

Structural model for the mesophase (short chains)

diabolo-like.

FIG. 9.

Structural model for the mesophase (long chains)

tubular arrangement.

mesophase heat of transition could support another hypothesis in which the alkyl chains are roughly perpendicular to the central disc and form octopus molecuIes [q and a tubular arrangement of them (Fig. 9).

Acknowledgments.

The authors are indebted to

~ r o f e ~ s o r J. Jacques, Professor Gasparoux and J. Prost for their help and useful1 suggestions and comments.

They thank also G. Sigaud and M. F. Achard for their help in phase diagrams interpretations and calorimetric measurements. At last they are very grateful to the chemical group for technical assistance.

References

[l] CHANDRASEKHAR, S., SADASHIVA, B. K. and SURESH, K. A., [4] MALTHETE, J. and DESTRADE,

TO be published.

Pramana 9 (1977) 47 1.

BILLARD, J., DUBOIS, J. C., NGUYEN, H. T. and ZANN, A., [2] NGUYEN, H. T., DUBOIS, J. C., MALTHETE, J. and DESTRADE, C., Nouv. J. Chimie, to be published.

C . R. Hebd. Skan. Acad. Sci. C 286 (1978) 463.

F. and WEBER, E., Angew. Chem. Internat. (Edit.) 13 [3] MUSGRAVE, 0. C. and WEBSTER, C. J., J. Chem. Soc. C (1971) (1974) 814.

1397.