Fluorinated metallomesogens - Lamellar versus columnar phase formation

Article abstract of DOI:10.1039/b819165d

Two series of tetradental cis-enaminoketone Ni(ii) and Cu(ii) complexes with their mesogenic core substituted by different combination of hydrocarbon or fluorocarbon chains have been synthesized. Depending on the type and position of the substituted group

Full text of DOI:10.1039/b819165d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Fluorinated metallomesogens – lamellar versus columnar phase formation†
*
ꢀ
ꢀ
Agata G1e˛bowska, Pawe1 Przybylski, Marta Winek, Paulina Krzyczkowska, Adam Krowczynski,
ꢀ
Jadwiga Szyd1owska, Damian Pociecha and Ewa Gorecka
Received 28th October 2008, Accepted 27th November 2008
First published as an Advance Article on the web 22nd January 2009
DOI: 10.1039/b819165d
Two series of tetradental cis-enaminoketone Ni(II) and Cu(II) complexes with their mesogenic core
substituted by different combination of hydrocarbon or fluorocarbon chains have been synthesized.
Depending on the type and position of the substituted groups, compounds reveal either calamitic or
columnar mesophases. Replacing the alkyl chains by perfluorinated chains stabilizes the columnar
phase. A unique phase sequence of isotropic–columnar hexagonal–re-entrant isotropic–smectic
(I–Colh–Ire–SmA) has been observed on cooling for some of the obtained compounds.
interactions between fluorocarbon and hydrocarbon chains
Introduction
cause the microsegregation of molecular fragments within the
mesogenic structure, and in some cases a new type of liquid-
crystalline phases are expected to occur.26–30
It is generally accepted that the type of liquid-crystalline phase
depends primarily on a molecular shape, which is determined by
the anisotropy of the molecular rigid part – the mesogenic core.1
Molecules with rod-like or disc-like cores form lamellar or
columnar phases, respectively.2,3 However, some compounds
have been synthesized, for which this rule is not fully applied. It
has been established that for some compounds even a minor
changes within the mesogenic core can lead to major differences
in molecular organization and therefore in liquid-crystalline
behaviour.4 There have been reported homologous series of
compounds in which both types of phases, lamellar and
columnar, occur upon changing the number and the position of
substituents5–8 or upon changing the length of peripheral alkyl
chains.9 Also, temperature can influence the type of molecular
packing.10–14 Lamellar-to-columnar phase transition induced by
temperature has been observed for molecules with overall rod-
like,15 disk-like16 and polycatenar17,18 shapes. The transition
between mesophases might be direct,10 or in some cases the
phases are separated by intermediate structures: bicontinuous
cubic phase (Cub)11,19 or fluid, optically isotropic phase (Ire).13,14
Here, we present the synthesis of another type of mesogenic
system exhibiting the lamellar-to-columnar cross-over behav-
iour, which was achieved by incorporation into molecular
structure perfluorinated chains, which affects the flexibility of the
molecule.20–22 Previously, it had been reported that the replace-
ment of hydrocarbon by fluorocarbon chains enhances the
thermal stability of mesophases.23 Careful studies on fluo-
roalkylated liquid crystals have revealed that fluorocarbon
substituents stabilize the smectic, columnar and cubic phases
compared to their hydrocarbon analogues by increasing their
clearing points.24 This effect is due to the incompatibility of
fluoroalkyl chains with alkyl groups caused by the fluorophilic
In this paper, we describe mesogenic properties of Ni(II) and
Cu(II) complexes with a disc-shaped core substituted by fluoro-
carbon and/or hydrocarbon chains. The triangular core is made
of two cis-enaminoketone moieties (Scheme 1). Compounds of
series I have at least one fluorocarbon chain in their molecular
structure, while the compounds of series II have only hydro-
carbon chains. In series I, the perfluorinated chain R1 is attached
to the tetrasubstituted phenyl ring and the substituents R2 and
R3 at both disubstituted phenyl rings are either octyloxy or
perfluorooctyl groups. For the series II, R1 substituent is a nonyl
chain, whereas R2 and R3 substituents are octyloxy or nonyl
groups.
Experimental
Synthetic procedure
The synthetic procedure for Ni(II) and Cu(II) complexes of two
designed series of compounds is sketched in Schemes 2–4. The
aminophenyl derivatives containing various substituents (f and l)
were obtained in six-step reaction procedures starting from
commercially available substances: N-(4-hydroxyphenyl)-
acetamide and N-(3-hydroxyphenyl)acetamide (Scheme 2 and 3).
effect with
a strong affinity of neighbouring fluorinated
chains.25,26 Moreover, it is also well known that fluorophobic
_
University of Warsaw, Department of Chemistry, Al. Zwirki i Wigury 101,
02-089 Warsaw, Poland. E-mail: aglebowska@chem.uw.edu.pl
† Electronic supplementary information (ESI) available: Detailed
descriptions of the syntheses as well as analytical data of all
compounds studied. See DOI: 10.1039/b819165d
Scheme 1 General structure of complexes studied
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Scheme 2 Synthetic route to 4-octyloxy-5-perfluorooctyl-1,2-phenyl-
enediamine
Scheme 4 Synthetic route to cis-enaminoketone complexes
Results and discussion
Mesophase behaviour
The mesophase sequences, phase transition temperatures and
enthalpy changes for studied complexes are summarized in Table
1. All compounds of series II without fluorinated chains show
direct transition from crystalline-to-isotropic liquid phase, for
some of them monotropic lamellar phase (SmC) was observed,
when the samples were supercooled. Introducing at least two
fluorinated alkyl chains into molecular structure (series I)
increases significantly the clearing temperatures of materials by
more than 100 degrees. The above observation agrees with the
common idea in the chemistry of fluoroalkylated liquid crystals
that the stability of their mesophases is significantly enhanced
compared to their hydrocarbonated counterparts.23 However,
for studied materials the increase of mesophase stability is also
clearly accompanied by the change of the phase type, from
lamellar to columnar. In the case of mono-fluoroalkylated I-1$Ni
and I-1$Cu complexes, no increase in clearing temperatures
compared to their analogues of series II was observed, neither
was the change in type of the formed mesophases – they exhibit
lamellar phase.
Scheme 3 Synthetic route to 4-octyloxy-5-nonyl-1,2-phenylenediamine
The condensation at room temperature between appropriate
formyl ketone sodium salts (m, n or o) obtained by the Claisen
formylation reaction and 1,2-phenylenediamine component (f or
l) proceeds selectively at only one amino group, resulting in
crystalline monosubstituted (p, q, r or s) products (Scheme 4).
The remaining unsubstituted amino group reacts further at
higher temperature with proper formyl ketone derivative to give
free ligands, which were used without purification in the next
reaction step. To obtain designed structures of final Ni(II) and
Cu(II) complexes metal acetates were used.16 Detailed descrip-
tions of the syntheses as well as analytical data of all compounds
studied are presented in the ESI†.
All of the LC phases were fully characterized by microscopic
observation of optical textures and X-ray diffraction studies. In
the small-angle region of X-ray patterns of smectic phases only
commensurate signals related to layer thickness were observed,
while the hexagonal columnar phase was distinguished by the set
1396 | J. Mater. Chem., 2009, 19, 1395–1398
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Table 1 Liquid-crystalline phases, phase transition temperatures (in ꢁC) and enthalpy changes (in J Â gÀ1) for complexes studied
Compound
R1
R2
R3
Nickel(II) complexes
Copper(II) complexes
I-1
I-2
C8F17
C8F17
OC8H17
OC8H17
OC8H17
C8F17
Cr 126.8 (34.9) [SmA 125.3 (3.7)] I
Cr 138.1 (11.7) [Ire 86.6 (1.1)] Colh
277a I
Cr 141.6 (38.4) [SmA 114.6 (3.2)] I
Cr 121.4 (20.0) [SmA 64.0 (1.7) Ire
101.9 (0.8)] Colh 290a I
Cr 105a Colh 267a I
I-3
I-4
II-1
II-2
II-3
II-4
C8F17
C8F17
C9H19
C9H19
C9H19
C9H19
C8F17
C8F17
OC8H17
OC8H17
C9H19
C9H19
OC8H17
C8F17
OC8H17
C9H19
OC8H17
C9H19
Cr 140.7 (27.0) Colh 215a I
Cr 139a Colh 270a I
Cr 120a Colh 270a I
Cr 127a I
Cr 95.0 (26.5) [SmC 90.0 (8.0)] I
Cr 116.0 (34.5) [SmC 96.4 (8.5)] I
Cr 93.9 (23.6) [SmC 74.8 (8.4)] I
Cr 105a I
Cr 87.7 (26.5) [SmC 69.1 (6.2)] I
Cr 103.4 (32.7) [SmC 73.4 (7.5)] I
Cr 92a I
a
Data from microscopy.
ꢁ
Table 2 Calculated unit cell parameter a [in A] for Colh phase and r -
density [in g/cm3] calculated from X-ray data, assuming one molecule per
ꢁ
crystallographic unit cell and 3.4 A of the columnar slide dimension along
the column
Nickel(II) complexes
Copper(II) complexes
Compound
T/ꢁC
a
r
T/ꢁC
a
r
I-2
I-3
I-4
155
180
165
25.8
25.9
28.7
1.286
1.276
1.238
150
150
180
25.8
25.8
29.4
1.290
1.290
1.183
Fig. 2 2-D X-ray patterns for complex I-2 in Colh phase at 150 ꢁC. The
ꢁ
Bragg reflection at 22.3 A is due to the hexagonal arrangement of the
ꢁ
ꢁ
columns. The diffused signals at 3.4 A and 5.6 A are related to the average
distance between discs within the column and between CF2 groups in the
molten fluorinated chains, respectively.
of reflections with relative positions q1 : q2 as 1 : 1/O3 and 1 : 1/2.
The lattice parameters for columnar hexagonal phase of
compounds of series I calculated from the X-ray data are
collected in Table 2. The obtained intercolumnar distance (a) was
smaller than the calculated molecular dimension for the most
extended molecular conformation. It suggests that molecules
from neighbouring columns interdigitate their terminal chains at
a distance of about four carbon atoms. Assuming one molecule
per crystallographic unit cell, the density of the Colh close to 1 g
cmÀ3 can be obtained, confirming that the columnar cross-
sections are made of a single molecule. In the SmA phase of
Remarkably, it was found that complexes I-2$Ni and I-2$Cu
form the re-entrant isotropic liquid phase below the columnar
hexagonal phase. The isotropic phase was identified from non-
birefringent texture re-appearing on cooling the Colh phase.
Moreover, it was observed that the X-ray signal related to
columnar organization that was sharp and Bragg-type in the
columnar phase weakens and becomes diffused as the isotropic
phase re-enters. (Fig. 3). This reflects that the order of molecules
becomes short range. In both isotropic phases the average
ꢁ
compound I-1, the layer distance is 29.3 A and close to molecular
ꢁ
distance between molecules (24.6 A for I-2$Cu complex) and
length (Fig. 1).
correlation length (2–3 molecular distances) is nearly the same. It
is worthwhile to mention that no changes in the wide-angle
region of the X-ray pattern were observed. Moreover, for
compound I-2$Cu below re-isotropic phase the lamellar (SmA)
phase was detected. The layer spacing for the SmA phase was
For materials of series I, in the wide-angle region of the X-ray
pattern, apart from diffused signal corresponding to the average
ꢁ
distance between discs in the column (at ꢀ3.5 A), the broad
signal related to the average distance of –CF2– groups of molten
ꢁ
terminal chains was detected at ꢀ5.6 A (Fig. 2).
ꢁ
ꢁ
slightly larger (28.9 A) than the intercolumnar distance (25.9 A).
Both phase transitions Colh–Ire and Ire–SmA were also detected
by calorimetric method and found to be exothermic on cooling.
Fig. 3 2-D small-angle X-ray patterns for partially aligned sample
(surface free drop) of complex I-2$Cu in I, Colh and Ire and SmA phases.
The Bragg reflection with six-fold symmetry observed in the Colh phase
becomes diffused and without azimuthal modulation in both, I and Ire,
phases. In the SmA phase, Bragg reflections from layers are visible.
Fig. 1 Proposed structure of smectic (a) and hexagonal columnar (b)
phase made of complexes with fluorinated alkyl chains (dark green).
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of molecular organization, lamellar or columnar, for the studied
group of materials can be engineered by the type of the terminal
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columnar phase increases with the number of introduced fluo-
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The work was supported by COST 52/2006 Programm. The
X-ray diffraction measurements were conducted at the Structural
Research Laboratory, established with financial support from
European Regional Development Fund – project no: WKP_
1/1.4.3./1/2004/72/72/165/2005/U.
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