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Mesogenic Properties of Amphiphilic Liquid Crystals with an Unusual Head-Group Topology.

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Mesogenic Properties of Amphiphilic
Liquid Crystals with an Unusual
Head-Group Topology**
F r a n k Hildebrandt, Jorg A n d r e a s Schroter, Carsten
Tschierske.* Reinhard Festag, Ralf Kleppinger,
a n d Joachim Heinz Wendorff
Liquid crystalline phases may be displayed by anisotropic as
well as by amphiphilic molecules. Anisotropic (calamitic or discotic) molecules are arranged with their long axes parallel to
each other to give thermotropic smectic or columnar liquidcrystalline phases. In contrast, the driving force of the self-organization of amphiphilic molecules is their ability to segregate
incompatible (for example, hydrophilic and hydrophobic) parts
of the individual molecules. This gives rise to large aggregates
which form the basis of lamellar. columnar, or cubic thermotropic and lyotropic mesophases. A combination of these
two structural principles can provide new liquid-crystalline materials. consisting of rigid units connected with one, two, or even
more hydrophilic groups at the termini of their alkyl chains.[']
The segregation of the different parts of the molecules is facilitated by the parallel arrangement of the rigid cores and visa
versa. In amphiphilic polyhydroxy compounds this gives rise to
significant mesophase stabilization.r2.3 1 The question arose:
What happens if a hydrophilic group is not terminally
(Fig. 1, A ) , but laterally (C) attached to a rigid core? The two
different organizing forces of liquid-crystalline phases would be
perpendicularly directed to each other in these compounds, and
the parallel arrangement of the calamitic units would disturb the
segregation of hydrophilic and hydrophobic parts of the molecules and visa versa. We have therefore synthesized novel
amphiphilic compounds with hydrophilic groups laterally attached to a rigid 4,4"-didecyloxy-p-terphenylcore.
. i l x Tibli.
is the first
compound containing this molecular structure. Polarization microscopy established that it is a liquid-crystalline material. On
cooling from the isotropic melt at 114°C the transition to a
liquid-crystalline phase can be seen by the formation of batonnets that rapidly coalesce to a focal conic fan texture with pseudoisotropic areas indicating a mesophase of the smectic A type,
which crystallizes at 36 ' C . No additional phase transition was
detected in this temperature range. Reheating gives a melting
point of 83 T.
Insrirut Cur Orpinischc Chemie der Universivit
Wcinbergwcg 16. D-06015 Halle (Germany)
TtIcliix l i l t code + (34S)SSl 1182
I<. testeg. Dr. R . Kleppinger, Prof. Dr. J. H. Wendorff
tachbereich Physikalische Chemie und Wissenschaftliches Zentrum
I'ur hl~iierial\l.issenschaftender Universitit Marburg (Germany)
x
x =
1
-CH,
(51) 68
F r n
Fig. 2 . The influence of lateral substituents on the mesophase behavior of 7'-substituted 4.4'-didecyloxy-terphenyl derivatives. The transition trinperatures are displayed as columns. black areas indicate the crystalline state and hatched or blank
areas correspond to liquid-crystalline phases (S, and N. r e s p e c t i d y ) . I f the liquidcrystalline phase appears below the melting point, these are metastable (monotropic) mesophases; they were detected by cooling from the irotropic melt. The numbers
above the bars indicate that phase-transition temperatures as determined by polarization microscopy. Values in brackets refer to monotropic plxise transitions. Abbreviations. N = nematic mesophase. S, = smectic A Phasc. I n the case of I a
monotropic sinecfic C phase occurs at 70 C.
In addition the mesophase of compound 3 was investigated
with X-ray scattering. The X-ray pattern shows several sharp
reflections in the small-angle region, which can be attributed to
a layered structure: the peaks can be indexed to the third order.
An amorphous halo occurs in the wide-angle region. The compound 3 thus displays a smectic layer structure without longrange order in the layers (S, phase). A layer thickness of
3.13 nm (at 92 'C) results. Using CPK models and assuming an
all-truns conformation of the alkyl chains, the molecular length
was estimated to be 3.8 nm. The individual molecules are thus
arranged parallel to each other in the layers, and the diol groups
are placed between the terphenyl groups. The thickness of the
smectic layers is significantly smaller than the length of the
individual molecules. This can be attributed to a partial intercalation of the terminal alkyl chains. Because the lateral groups
are comparatively large, the terphenyl groups are forced apart.
The additional free space between the alkyl chains allows a deep
interdigitation, which results in a layer distance significantly
lower than the molecular length.
A comparison of the diol compound 3 with the 4,4"-didecyloxy-2'-methylterphenyl (1. Fig. 2)[51indicates an influence of
the bulky lateral diol group that stabilizes the mesophase. It is
indeed very surprising that although the diol3 has a significantly larger lateral substituent, it displays the greater mesophase
stability. This mesophase stabilization is especially evident from
the comparison of the diol3 with compound 2 carrying a lateral
substituent of comparable size but without hydroxyl groups; it
is contradictory to all previous experience. Until now it was well
established that larger substituents in the lateral position of the
rigid core of calamitic liquid crystals give rise to a more or less
significant destabilization of the liquid-crystalline phases.[']
We therefore have to assume, that hydrogen bonding between
the diol groups is responsible for the unexpected mesophase
stabilization of compound 3.
Although the mesophase stabilization by hydrogen bonding
is well documented for amphiphilic diols with terminal diol
groups.I2] the mesophase stabilization by lateral diol groups is
rather surprising. The formation of a smectic layer structure can
only be explained if one assumes a parallel arrangement of the
terphenyl units with the diol groups placed between the rigid
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cores within the layers. This arrangement contrasts with the
usually observed segregation of hydrophilic and hydrophobic
parts of amphiphilic molecules and should disturb the hydrogen
bonding.
The IR spectra of compound 3 display a broad absorption in
the frequency range of the 0 - H stretching vibrations, indicating the presence of large hydrogen bonding networks. Temperature-dependent measurements of the pure compound show an
unusually large shift of its frequency with increasing temperature (Fig. 3). This temperature
dependence
can be attributed to a
T,oc
successiv collapse of in205
termolecular hydrogen
bonding with increasing
170
temperature.
Inter3360 cm-l
loo
molecular H-bonding is
dominant in the crystalline state (J,,, =
90
3360cm-') and also
in the liquid-crystalline
state. The aggregates
4 000
3 000
are significantly smaller
fi/crn-'
in the liquid state,
Fig. 3. Sections of the temnerature-depenand an absorption at
dent IR spectra of compound 3 in the range
3580 cm- 1 indicates the
2500- 4000 cin- I , measured with a BIORADpresence of a substantial
FTS-40 spectrophotoineter. in a heated thernumber of single molema1 cell SPECEC P;N 21 500.
cules with intramolecular hydrogen bonding
between the OH groups within the diol unit. The arrangement of
the molecules in layers seems to allow the most efficient intermolecular hydrogen bonding and any disturbence of the longitudinal order causes their breakdown.
In order to estimate the influence of the lateral hydrogen bonding on the molecular order within the smectic mesophase, the
nonamphiphilic compound 1 was also investigated by X-ray scattering. In the S, phase the layer thickness is 3.47 nm (at 85 ' C ) .
This is a little bit surprising, because this distance is larger than
that of compound 3. However, the lateral side group of compound 1 is only a methyl group and thus much smaller than the
bulky 2-oxa-4.5-dihydroxypentylgroup of compound 3. The possibilities for interdigitation are reduced. and the thickness is consequently much closer to the molecular length. Another remarkable finding is the smaller number of small-angle peaks (three
distinct peaks for compound 3 and only two at compound 1).
This indicates a better longitudinal order of the layers built up by
the diol derivative 3. The better ordering is evidently due to the
formation of hydrogen bonds between the diol groups. The position of these groups and therefore of the hydrogen bonds between
the rigid cores leads to an enhancement of the longitudinal ordering of those cores.
We must conclude that the liquid-crystalline phases of calamitic
mesogens can be stabilized by lateral substituents that strongly
interact with each other by hydrogen bonding. In order to verify
this hypothesis. the chemical structure of these new amphiphilic
liquid crystals has been varied by changing the lateral hydrophilic
group. The thermotropic liquid crystalline properties of the
amides 5-8''' are compared in Figure 4 (for spectroscopic data
of 7 and 8, see Table 1). It indicates that the mesophase stability
of the S, phase indeed rises with increasing number of hydroxyl
groups in the growing lateral chain. The "glucamide" 8 (1-[2.5bis(4-decyloxyphenyl)benzoylamido]- 1-deoxy-D-sorbitol) represents the first example of an entirely novel type of liquidcrystalline carbohydrates.[81 The microscopic texture of the
T /"C
X
x =
4
150
-OCHJ
I
5
-
100
111
-NHC*H,
I
OH OH
115
I
129
112
147
~
Fig.4. Thc inlluence OF the nuinher of hydroxyl groups on the therniotropic
mesophase behavior of ?'-substituted 4,4"-didec).cloxyterphenyl derivatives. For an
explanation see Fig. 2.
Table 1. Spectroscopic data of 3. 7, and 8 [a]
3 1HNMR(200MH7),CDCI,).ij=7.64(d.J=1.7Hz.l Hj.7.49-~7.57(m.4H),
7.24-7.33 (2d. 4 H ) . 6.91 -6.99 (2d. 4 H ) . 4.49 (5, 2 H ) . 3.99 (t. J = 6.5 HL. 4 H ) .
3 81 3 84 (m, I H ) , 3.55-3.71 (m. 2 H ) . 3.42-3.56 (in. 2 H ) . 1.80 (m. 4 H ) , 1.27
1-64 (m, 28H). 0.8X (1. J = 6.0H7. 6 H ) : M S . 111:: 646 ( [ M ] + .100%)
7:'HNMR(500MH7,CDC13) d =7.8h(d.J=2.0Hr.l H).7.63(dd.J=2.0Hz.
J = N . 0 Hz, I H). 7.54 (d, J = 8.7 Hz. 2 H ) . 7.36 (d. .I = 8.0 Hz, 1 H). 7.33 (d.
J=8.7Hz.2H),h.93~6.97(2d.4H).5.72(t.J=6.0Hz.IH).3.99(t.J=6.7Hz.
2 H ) . 3 . 9 X ( t . J = 6 . 7 H ~ . Z H j . 3 . 5 7 ( d d d ,I H ) . 3 . 4 5 ( d d . J = 4 . 5 H z . . l = 1 1 . 6 H z .
I H ) . 3.40 (dd. J = 5.2 Hr. J = l l . 6 Hz. 1 H ) . 3.32-3.38 (m. I H ) . 3.21 3.27 (m.
IH). 1.79(m.4H). 1.27~~1.50(m.28H).O.87(t.J=6.5Hz,6H):MS:n~::659
( [ M I + .100%)
8: ' H N M R (500 MHz, [DJDMSO): 6 = 8.13 (t. J = 5.4 Hz, 1 H). 7.56-7.71 (m.
4 H ) . 7.38 (d. J = 8.1 Hz, 1 H ) . 7.33 (d. J = 8.6 Hz, ZHj, 7.02 (d. J = 8.5 Hz. 2 H j .
6.93 (d, J = 8.5 Hz. 2 H ) . 4.72 (d. J = 4.8 Hz,1 H ) , 4.44 (d, .I = 5.4 Hz. 1 H ) , 4.34
(d. J = 5.X Hz, I H). 4.29 (t. J = 5.6 Hz. 1 H), 4 22 (d, J = 6.6 Hr. 1 H ) . 3.96-4.01
( 2 1.4 H) .3.31- 3.69 (m. 7 H),3.16 (m, 1 H), 1.72 (m. 4 H) , 1.26- 1 42 (m. 28 H ) , O . X 5
( t . J = 6.8 Hz. 6 H ) : MS: id: 749 ([,MI+. 670/u).
~
[a] Correct C,H analyses m d ' H NMR spectra
iiie
available for all compounds.
mesophase between 112 ;C and 147 'C is typical for a smectic A
phase. The X-ray pattern exhibits a sharp Bragg reflection and
the corresponding reflections of second and third order (Fig. 5 ) .
The d value amounts approximately 3.3 nm, which again corresponds to a smectic layer structure in which the terphenyl units
are arranged parallel to each other.
201"
-
Fig. 5 X-ray scattering ofcoinpound 8 in the mesophase at 1 1h'C. I = intensity in
'irbirary units.
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Though no detailed miscibility studies have been carried out.
preliminary contact preparations suggest that the smectic A
phases of compounds 1 and 3 and those of the compounds 3 and
8 are completely miscible.
Intermolecular hydrogen bonding is also found in carboxylic
acids that are known to form dimers. In Figure 6 the carboxylic
acid 10 is compared with its acid chloride 9 and the methyl
carboxyliite 4. Again the carboxylic acid (the compound that
111 H. Ringsdorf, B. Schlarb. J. Venzmer. 4ngm C'lwni. 1988. 100. 117.- 162;
Angrit.. C / i m . Int. Ed. Engl. 1988, 27. 113 15X.
[2] D. Joachimi, C. Tschierske. H. Miiller. J. H . Wendorff. L Schiieider. R. Kleppinger. 4wgei1,. Chrm. 1993, 1/15, 1205 1207: Anjirii. ~ ' / i ~ w z/ n. / . Ed. Engl.
1993.32, 1165 1367.
[3] F. Hentrich. C . Tschierske. S. Diele, C. Sauet-. J. Marcr ( ' / i w ~1994.
.
4. 15471558.
141 Compound 3 was obtained by phasc-transfcr-cat;ilyLe~l ethcrilication o f 2'bromoinethyl-4.4-d1dccyloxyterphen~I 151 with 4-liqdroxytiicthq1-2,I-dimethyl-1.3-dioxolane ( 5 0 % NaOH, cat. Bu,NHSO,. 100 C. 2 h) followed by
acid-ca talyred hydrolysis (pyridiniuin tolueiie-4-sull~~n~1tc
(Py TosOH.
MeOH)) of the isopropylidene protective group (yield 3 0 " 0 ) . For the aynlhcsis
of 2 n-propatiol was used.
(51 J. Aiidersch. S. Diele. P. Gdring, L A . Schrbter, c'. Tschienke. 1. C/irn?.Sw.
Chiwii. Ciininiun. 1995, 107-108.
[6] W. Weissflog, D. Demus. Crj.\l. RCL Teckriol. 1984, I Y . i S 64.
ynthesised by Pd-catalyzed cross-coupiing of nieihyl2.5-dibromobcnioate with two equiv. 4-decyloxyphenyl boi-onic acid [S] (yield
93"/0). Saponification gave the carboxylic acid 10. which wab transformed into
the acid chloride 9 by treatment with oxalyl chloride ( R A d a m . L. H. Ulich,
J. Am. Chrm Soc. 1920, 42,599). Compounds 6 8 wcrc obtained by aminolqsis of crude compound 9 with excess 2-aniinoethanol. ?.3-dihydroxypropylamine. and I,-glucamine. respectively (yields 76-91 % )
[8] Rcviews: B. Pfannemiiller, Sturch 1988. 40. 476-480. G A. Jeffrey. . k c .
Cheni. Rev. 1986, I Y . 168: G. A. Jeffrey, L . M Wingert. L i q C,:l,\i 1992. 12.
179- 202.
[Y] W Weissflog, D . Demus. S. Diele, P. Nitschke, W. Wedlei-. Liq CS.1 \I. 1989.5,
11 1 123: A. C. Griffin, S. F Thames. M S. Ronner. M r r / C n .sf. Liq. Crr.vr.
1977. 34. I35 139.
I101 M. C. Carey. D. M. Small. Arch. Inrern. Mcd. 1972, 110, 506, 7319-7320:
T. M . Stein, S. H. Gellman, J. .4n7 Cliern. S i x 1992. 11.1. 3943 3950: D G .
Barrett. S. H. Gcllman, ?hid 1993, 115, 9343-9344.
'
I
l
l
l
l
l
l
l
l
l
l
l
68 73
4 X
=
COOCH,
-1
(40j 5 8
Fig. 6. ('oinp;ii-imi of the phase-transition temperatures T' 'C of 2,5-bia(4-decyloxyphenyl)btii7uic acid (LO) and the corresponding acid chloride 9 and methyl
bcn7o:ite 4 (determined by po1;irization microscopy).
~
~
can form hydrogen bonding) displays the higher mesophase
stability. The recently described laterally connected Siamese
twin m e s o g e n ~ [ ~and
l also the covalent. laterally connected
trimesogens151may be looked upon as examples for covalent,
laterally fixed molecules. These compounds also display higher
mesophase stabilities compared with structurally related
calaniitic single mesogens.
If one coinpares the compounds 1-10 it is obvious that lateral fixing of calamitic molecules by hydrogen bonding also increases the structural order, which stabilizes the layered smectic
A phase with respect to the nematic phase (N) of the nonamphiphilic compounds. These new compounds also differ in this
respect from laterally alkyl substituted calamitic compounds
which usually favor the nematic phase."]
In summary, we have shown that there is an ambivalent influence of hydrophilic lateral substituents. Their steric requirements tend to separate the rigid cores from each other. which
decreases the molecular order and should thus give rise to a
significant mesophase destabilization. However, attractive
forces between these substituents-such as hydrogen bonding-strongly fix the individual molecules to each other. If these attractive forces exceed the repulsive forces, the longitudinal order
of the molecules is increased, and smectic liquid-crystalline
phases are stabilized. In this way, lateral fixing of calamitic
mesogens by hydrogen bonding is a new approach to smectic
materials with broad mesomorphic ranges.
Furthermore. these new amphiphilic diol derivatives resemble
an amphiphilic architecture which closely resembles that of the
naturally occurring cholic acid salts and some peptides;["] both
consist of rigid stuctures with surfaces of different polarity.
Thc biomimetic potential of these facial amphiphiles makes it
exciting to study their ability to form other self-organizing
supramolecular systems, such as lyotrophic mesophases, micelles. and thin films. These investigations are under way.
Bromides of Rare Earth Metal Boride Carbides
-A System of Building Blocks**
Hansjiirgen Mattausch and Arndt Simon*
Onlya few metal-rich rare earth metal halides MX,, with n I 2
are known namely MZX3[1-31and LaLL4IOther compounds
described as binary phases and accessible in low yields, however,
finally turned out to be "stabilized" by interstitial atoms.r51Due
to the electropositive nature of the rare earth metals these
interstitial atoms have anionic character.
Addition of H, C, N, 0 in experiments to reduce rare earth
trihalides led to numerous new ternary coinpounds, whose
structures could be systematized in terms of a condensation of
characteristic building units[,]: M,O tetrahedra, M6N, double
tetrahedra, M,H tetrahedra, and M,H, octahedra as well as
M,C and M,C, octahedra were repeatedly found. These units
can be combined as was demonstrated recently with compounds
containing the interstitial atoms C/H.[" C/O.['. and C!"'",
'I
simultaneously. Here we report new phases with B/C which
particularly well illustrate this building block principle.
Gd,Br,C,B, La,Br,C,B, and Ce,Br,C,B, are the first members of a presumably large class of compounds. They can be
[*] Prof. Dr. A. Simon. Dr. H. Mattauach
Max-Planck-Inatitut fur Festkorperforschung
Heisenbergstrdsse I, D-70569 Stuttgart (German>)
Telefax: Int code + (713)689-1642
[**I
The help of C. Hochrathner, R. Eger. R Pdttgen, and N Weishaupt in d r a w
ing, preparation, X-ray diffraction and electrical measuruiiient?. respectively. I S
gratefully acknowledged.
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