Smectic and columnar liquid crystalline phases through charge-transfer interactions

Article abstract of DOI:10.1080/15421400490435369

New heterocyclic electron donors based on a 1,3,5-triazine nucleus are presented. Three phenyl rings are grafted to the triazine core either via secondary amino groups or by a direct C,C-linkage and a specific number of decyloxy chains is attached to the

Full text of DOI:10.1080/15421400490435369

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Smectic and Columnar Liquid
Crystalline Phases through
Charge-Transfer Interactions
Daniela Goldmann a , A. Nordsieck a , Dietmar
Janietz b , Thomas Frese c , Claudia Schmidt c
Joachim Wendorff c
&
a Fachbereich Chemie und Institut für Dünnschicht
technologie und Mikrosensorik , Universität
Potsdam , Teltow, Germany
b Fraunhofer-Institut für Angewandte
Polymerforschung , Golm, Germany
c Institut für Physikalische Chemie
und Wissenschaftliches Zentrum für
Materialwissenschaften , Universität Marburg ,
Marburg, Germany
Published online: 18 Oct 2010.
To cite this article: Daniela Goldmann , A. Nordsieck , Dietmar Janietz , Thomas
Frese , Claudia Schmidt & Joachim Wendorff (2004) Smectic and Columnar Liquid
Crystalline Phases through Charge-Transfer Interactions, Molecular Crystals and Liquid
Crystals, 411:1, 337-344, DOI: 10.1080/15421400490435369
To link to this article: http://dx.doi.org/10.1080/15421400490435369
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Mol. Cryst. Liq. Cryst., Vol. 411, pp. 337=[1379]–344=[1386], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490435369
SMECTIC AND COLUMNAR LIQUID CRYSTALLINE
PHASES THROUGH CHARGE-TRANSFER
INTERACTIONS
Daniela Goldmann and A. Nordsieck
Universitat Potsdam, Fachbereich Chemie und Institut fur
¨
Dunnschicht technologie und Mikrosensorik,
D-14513 Teltow, Germany
¨
¨
Dietmar Janietz
¨
Fraunhofer-Institut fur Angewandte Polymerforschung,
D-14476 Golm, Germany
Thomas Frese, Claudia Schmidt, and Joachim H. Wendorff
¨
¨
Universitat Marburg, Institut fur Physikalische Chemie und
¨
Wissenschaftliches Zentrum fur Materialwissenschaften,
D-35032 Marburg, Germany
New heterocyclic electron donors based on a 1,3,5-triazine nucleus are pre-
sented. Three phenyl rings are grafted to the triazine core either via secondary
amino groups or by a direct C,C-linkage and a specific number of decyloxy
chains is attached to the molecular periphery. The compounds are non-liquid
crystalline in their pure states. Lamellar or columnar mesophases are induced
by attractive interactions with electron acceptors.
Keywords: 1,3,5-triazine; mesophase inductions; smectic and columnar phases
INTRODUCTION
It has become apparent that thermotropic mesomorphism is not restricted
to form-anisotropic molecules. During the last few years various non-
anisometric compounds have been reported to exhibit liquid crystalline
The financial support of the Deutsche Forschungsgemeinschaft and of the Land
Brandenburg are kindly acknowledged.
Address correspondence to Dietmar Janietz, Fraunhofer Institut fu¨r Angewandte Polymer-
forschung, D-14476 Golm, Germany
337=[1379]

338=[1380]
D. Goldmann et al.
behaviour. Microphase segregation between incompatible molecular sub-
units, for example of polar and non-polar regions leads in this case to the
formation of lamellar, cylindrical or spheroidal aggregates which organize
to smectic, columnar and cubic mesophases [1]. We reported in this context
on 2,4,6-triarylamino-1,3,5-triazines substituted with six long peripheral
alkoxy chains which exhibit columnar liquid crystalline phases [2] and on
hydrogen bonding of the melamines with aromatic carboxylic acids giving
rise to the control of the two-dimensional lattice symmetries [3,4].
Mesomorphic structure formation can also arise for non-covalently
bonded aggregates formed, for example, by charge transfer (CT) interac-
tions. Thus, doping of electron rich flat aromatic compounds with electron
acceptors usually may cause the stabilization as well as the induction of col-
umnar mesophases [5–7]. However, more recently we found six-fold alkoxy
substituted triarylmelamines to be the first examples of columnar phase
forming electron donors which display an induction of smectic liquid crys-
talline structures by mixing with nitrofluorenone based acceptors [8].
We report in this contribution the three-fold alkoxy substituted 2,4,6-
triarylamino-1,3,5-triazine 4 and the triaryltriazine 8 with six peripheral
alkoxy tails but with a direct linkage of the phenyl groups to the triazine
nucleus (Scheme 1). We will focus on the synthesis of compounds 4 and
8, their thermal behaviour in the pure state as well as the structure
formation in mixtures with electron deficient counterparts.
EXPERIMENTAL
Mixtures of the triarylmelamine 4 and the triaryltriazine 8 with either TNF
9 or the cyano modified acceptor 10 (Scheme 2) were prepared by dissolv-
ing the components separately in dichloromethane and combining the solu-
tions. The thermal investigations were performed after evaporating the
solvents and drying the residues in vacuum.
Texture observations were made with an Olympus polarizing microscope
fitted with a Linkam TMH=S 600 hot stage in conjunction with a Linkam TP
92 control unit. Photo micrographs were obtained with an Olympus OM-4
Ti system camera. Calorimetric investigations were performed with a
Netzsch DSC 200. Wide angle X-ray scattering analysis was carried out with
a Siemens 5000 diffractometer.
RESULTS AND DISCUSSION
Synthesis
The synthesis of 1,3,5-triazine 4 substituted with three 4-decyloxyphenyl-
amino groups was carried out starting from 4-hydroxyacetanilide.

Smectic and Columnar Liquid Crystalline Phases
339=[1381]
SCHEME 1 Synthetic routes to the 1,3,5-triazines 4 and 8.
SCHEME 2 Chemical structure of the nitrofluorenone based electron acceptors 9
and 10.

340=[1382]
D. Goldmann et al.
Etherification with decylbromide provided the 4-decyloxyacetanilide 2. N-
acetate cleavage of compound 2 with hydrochloric acid and ethanol as the
solvent resulted in the aniline 3. Finally, the alkoxy substituted triarylme-
lamine 4 was obtained by the reaction of compound 3 with cyanuric chlor-
ide in the presence of potassium carbonate.
To prepare the 2,4,6-triaryl-1,3,5-triazine 8, the 1,2-bis-decyloxybenzene
5 was brominated yielding the dialkoxy substituted bromobenzene
compound 6. Compound 6 was converted into the corresponding boronic
acid 7 by reaction of the intermediate lithium organyl with trimethyl
borate. By applying conditions we previously reported for the synthesis
of 4,6-diphenyl-1,3,5-triazines [9], palladium-catalysed cross-coupling
reaction between the phenylboronic acid 7 and cyanuric chloride yielded
the triaryltriazine 8.
Thermal Properties
Each compound 4 and 8 exhibits only a direct transition from the crystal-
line state to the isotropic liquid (Table 1). It seems that for both triazines 4
and 8 mesomorphic structure formation is suppressed by a disturbed
balance of the molecular polar – non-polar relationship.
In case of the melamine 4 the three phenyl substituents are linked to the
central heterocyclic triazine ring via secondary amino groups giving rise to
a significant polarity of the central molecular region. However, the tri-
phenylmelamine core is substituted with only three non-polar aliphatic
chains. Thus, the lipophilic character of the molecular periphery is reduced
compared with structurally related columnar phase forming triarylme-
lamines bearing six lipophilic alkyl chains [2].
TABLE 1 Phase Transition Temperatures (ꢀC) of the Pure 1,3,5-triazines 4 and 8
and of Equimolar Compositions with the Electron Acceptors 9 and 10 (DSC 2nd
Heating at 10 K=min); Transition Enthalpies (kJ=mol) in Parenthesis; Monotropic
Phase Transitions in Square Brackets
Sample
Transition temperatures
4
Cr
Cr
Cr
Cr
Cr
Cr
111.5 (42.12)
116.5 (133.2)
139.1 (20.1)
100.6 (54.2)
[78.2 (ꢁ47.2)]
104.6 (24.6)
I
I
I
I
I
I
8
4=9
8=9
SmA
154.9 (3.28)
Col
Col
[86.5 (ꢁ3.6)]
8=10
130.7 (3.3)
Cr: crystalline; SmA: smectic A; Col: columnar; I: isotropic.

Smectic and Columnar Liquid Crystalline Phases
341=[1383]
The reverse situation is valid for the phenyltriazine 8. Compared with
compound 4 the peripheral lipophilicity is enhanced by introducing six long
chain alkoxy groups. However, the secondary exocyclic amino groups were
omitted and replaced by direct C,C-linkages between the triazine hetero-
cycle and the phenyl substituents. The polarity of the core region is thus
reduced.
We expected that the tendency of the triazines 4 and 8 towards liquid
crystalline structure formation should be enhanced through interactions
with flat electron acceptors such as 2,4,7-trinitrofluoren-9-one (TNF) 9
and 2,4,7-trinitrofluoren-9-ylidene malodinitrile 10 (Scheme 2).
Doping of the triarylamino-1,3,5-triazine 4 with TNF 9 leads to the
induction of enantiotropic mesomorphism at an equimolar ratio of the
components. Upon cooling from the isotropic state focal-conic fan textures
develop (Fig. 1), indicative of the formation of a SmA phase. The X-ray dia-
gram of the equimolar mixture 4=9 displays three equidistant reflections in
˚
˚
˚
the small angle region (d100 ¼ 28.8 A; d200 ¼ 14.7 A; d300 ¼ 10.6 A) and an
amorphous halo at larger scattering angles. The small angle peaks reveal a
periodic layer structure along one direction whereas the diffuse scattering
in the wide angle region indicates the presence of only a short range order
within the layers. The X-ray investigations, thus, confirm the findings from
polarizing microscopy that the melamine 4 mixed with the acceptor 9
exhibits a smectic A-type mesophase.
The 1,3,5-triazine 8 forms a monotropic liquid crystalline phase in
1:1 mixtures with TNF 9 on cooling from the isotropic liquid (Table 1).
FIGURE 1 Optical texture of the mesophase of the ꢁequimolar mixture 4=9 between
crossed polarizers; 1st cooling (1 K minꢀ1) at 153.8 C. (See COLOR PLATE VIII)

342=[1384]
D. Goldmann et al.
Enantiotropic mesomorphism is found for the equimolar system 8=10. On
cooling from the isotropic phase the growth of domains of digitated stars
with hexagonal symmetry is observed by polarizing microscopy for the mix-
tures 8=9 and 8=10 (Fig. 2). These optical textures are typical for a hex-
agonal columnar mesophase.
The deep brownish colour that appears by mixing the triazine and the
acceptor components clearly indicates a charge-transfer. However, both
attractive interactions provided by electron donor-acceptor interactions
perpendicular to the planes of the aromatic cores [10,11] and attractive
electrostatic quadrupole interactions [12,13] may contribute to the
observed mesophase inductions.
In case of the melamine 4 the phenyl rings can rotate around the C-N
single bonds and the core region of the compound can adopt different
specific conformations. Taking the fact into account that the formation of
smectic liquid crystalline phases is predominantly caused by a rod-like
anisometric shape of the molecules it seems likely that the smectic meso-
phase originates from an elongated linear conformation of the triarylmela-
mine 4 which becomes favoured during interaction with TNF.
One such conformation with an almost flat geometry of the triphenyla-
minotriazine core is shown in Figure 3a along with a molecular model of
TNF 3, arranged closely face-to-face with the phenyl rings of the hetero-
cyclic donor 4. Two of the alkoxyphenyl groups of compound 4 are
arranged parallel to each other whereas the third one is turned to
the opposite direction. Distinct layered polar regions are formed which
FIGURE 2 Microphotograph of the columnar phase of the mixed system 8=10
between crossed polarizers; 2nd cooling (5 K minꢀ1) at 118.3ꢁC. (See COLOR
PLATE IX)

Smectic and Columnar Liquid Crystalline Phases
343=[1385]
FIGURE 3 a) Fork-like conformation of the melamine 4 (Cerius 2; force field
Dreiding 2); b) Structure model for the chemically induced smectic A mesophase
of compound 4.
are separated by the non-polar alkyl chains. The layer spacing determined
by X-ray is slightly less than the maximum molecular length for linear con-
formers of compound 4. Furthermore, one can imagine, on the average, an
antiparallel alignment of the melamine 4, arranged like a fork, since this
gives rise to an optimum of space filling within one layer (Fig. 3b).
Compared with the triarylamino-1,3,5-triazine 4 the conformational
mobility of the core region is significantly reduced for the triaryltriazine
8. In particular, the C,C-linkage between the N-heterocycle and the phenyl
moieties prevents conformational changes of the central core region from a
discoid shape to a more rod-like molecular geometry. Interactions with the
electron acceptors perpendicular to the aromatic rings probably result in a
flattening of the central triphenyltriazine part of compound 8. The associ-
ation with the polar nitrofluorenone acceptors gives rise to a stiffening of
the molecular core and facilitates microphase separation. Thus, micro-
segregation along with a flat disc-like shape that becomes favoured for
the triazine may explain the hexagonal columnar mesophases of the
equimolar mixtures 8=9 and 8=10 quite well.
CONCLUSIONS
The 1,3,5-triazines discussed in this contribution combine polar and
non-polar segments with the conformational mobility of the central
(hetero)aromatic part of the molecules. Depending on the connection of
the three phenyl groups with the central 1,3,5-triazine nucleus either

344=[1386]
D. Goldmann et al.
rod-like or disc-shaped molecular conformations become favoured by inter-
action with nitrofluorenone based acceptors. Associations with the electron
acceptors, furthermore, improve the gradient of the molecular polarity.
This combination of chemically induced anisometry of the phenyl substi-
tuted heterocyclic units and the segregation effects to distinct polar and
non-polar molecular regions facilitates the induction of lamellar and colum-
nar supermolecular structures.
REFERENCES
[1] Tschierske, C. (1998). J. Mater. Chem., 8, 1485.
[2] Goldmann, D., Janietz, D., Festag, R., Schmidt, C., & Wendorff, J. H. (1996). Liq. Cryst.,
21, 619.
[3] Janietz, D. (1998). J. Mater. Chem., 8, 265.
[4] Goldmann, D., Dietel, R., Janietz, D., Schmidt, C., & Wendorff, J. H. (1998). Liq. Cryst.,
24, 407.
[5] Bengs, H., Ebert, M., Karthaus, O., Kohne, B., Praefcke, K., Ringsdorf, H., Wendorff, J. H.,
& Wu¨stefeld, R. (1990). Adv. Mater., 2, 141.
[6] Ebert, M., Frick, G., Baehr, C., Wendorff, J. H., Wu¨stefeld, R., & Ringsdorf, H. (1992). Liq.
Cryst., 11, 293.
[7] Praefcke, K. & Holbrey, J. D. (1996). J. Incl. Phenom. Mol. Recog. Chem., 24, 19.
[8] Goldmann, D., Janietz, D., Schmidt, C., & Wendorff, J. H. (2000). Angew. Chem., 112,
1922.
[9] Janietz, D. & Bauer, M. (1993). Synthesis, 33.
[10] Diederich, F., Philp, D., & Seiler, P., (1994). J. Chem. Soc., Chem. Commun., 205.
[11] Markovitsi, D., Bengs, H., & Ringsdorf, H. (1992). J. Chem. Soc. Faraday Trans., 88,
1275.
[12] Bates, M. A. & Luckhurst, G. R. (1998). Liq. Cryst., 24, 229.
[13] Lunkwitz, R., Neumann, B., & Tschierske, C. (1998). Liq. Cryst., 25, 403.