Effect of fluorocarbon chains on the mesomorphic properties of chiral imines and their complexes with copper(II)

Article abstract of DOI:10.1039/b905104j

The synthesis and mesomorphism of new chiral imine compounds incorporating a fluorocarbon chain which have calamitic molecular shape and their complexes with copper(II) are reported. The liquid crystalline properties were investigated using polarised optical microscopy, differential scanning calorimetry and X-ray diffraction. Imine compounds with a semifluorinated chain and a short chiral chain show chiral smectic mesophases with dramatically enhanced mesophase stabilities compared to the related hydrocarbon derivatives. In addition, the first examples of mesogenic chiral salicylaldiminate copper(II) complexes carrying semifluorinated alkyl chains at their peripheries have been synthesized and their liquid crystalline properties were investigated.

Full text of DOI:10.1039/b905104j

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Effect of fluorocarbon chains on the mesomorphic properties of chiral imines
and their complexes with copper(II)†
a
a
Hale Ocak, Belkız Bilgin-Eran, Carsten Tschierske,b Ute Baumeisterc and Gerhard Pelzlc
*
Received 12th March 2009, Accepted 26th June 2009
First published as an Advance Article on the web 5th August 2009
DOI: 10.1039/b905104j
The synthesis and mesomorphism of new chiral imine compounds incorporating a fluorocarbon chain
which have calamitic molecular shape and their complexes with copper(II) are reported. The liquid
crystalline properties were investigated using polarised optical microscopy, differential scanning
calorimetry and X-ray diffraction. Imine compounds with a semifluorinated chain and a short chiral
chain show chiral smectic mesophases with dramatically enhanced mesophase stabilities compared to
the related hydrocarbon derivatives. In addition, the first examples of mesogenic chiral
salicylaldiminate copper(II) complexes carrying semifluorinated alkyl chains at their peripheries have
been synthesized and their liquid crystalline properties were investigated.
The work described herein deals with the synthesis and char-
1. Introduction
acterization of liquid crystalline properties of two types of new
imines, 6 and 7, combining a semifluorinated chain and a chiral
alkyl chain. As these imine compounds possess the ability to
coordinate to metals the first examples of mesogenic salicyl-
aldimine derived copper(II) complexes16 combining chain fluo-
rination and chirality are reported.
The introduction of fluorine and the choice of its position within
liquid crystalline compounds allow the preparation of materials
which have considerable technological interest for display and
non-display applications.1–3 The combination of small size and
high polarity of fluoro substituents permits the modification of
the melting point, transition temperatures, mesophase type and
a wide range of physical properties of liquid crystal materials.3–11
Fluorine atoms can replace H atoms at the rigid core or at the
flexible chains. Usually, perfluorination of the terminal chains of
rod-like molecules suppresses the formation of nematic phases
and induces smectic ones. The main reason for this behaviour is
the incompatibility of these chains with aromatic and aliphatic
building blocks of the mesogenic molecules (nano-segregation,
fluorophobic effect); the increased stiffness of perfluoroalkyl
chains and steric effects due to the larger size of F compared to
H are also important. Fluorinated chains can significantly
influence the mode of organization and the specific properties of
smectic phases, as for example they can lead to a strongly tilted
organization with alternating tilt direction (orthoconic LC if the
tilt angle is 45ꢀ)12 or give rise to randomly tilted deVries type
materials.13 Chiral rod-like liquid crystals are of interest as they
can have ferroelectric or antiferroelectric properties which are
also of interest for applications.14 Our group has reported a
variety of fluorinated mesogens and studied the structure-property
relationships in different molecular types;7–9 for example, fluo-
rinated imine compounds and their metal complexes were
investigated in depth.15
2. Results and discussion
2.1. Synthesis
The synthesis of the compounds is presented in Scheme 1. The
chiral moiety was introduced by reaction of (S)-2-methyl-
butyltosylate17 (prepared from (S)-(ꢁ)-2-methyl-1-butanol,
Fluka, 95.0%, [a]2D0 ꢁ6.3 ꢂ 0.5ꢀ, c ¼ 10 in EtOH), with p-nitro-
phenol in DMF to give (S)-1-(2-methylbutoxy)-4-nitrobenzene
(2) as described previously;17b 2 was then reduced to the
corresponding aniline 3.
The semiperfluorinated aldehydes 4 and 5 were obtained by
reaction of 2,4-dihydroxybenzaldehyde and 4-hydroxy-
benzaldehyde, respectively, with the appropriate semifluorinated
alkyl bromides,11 using KHCO3 or K2CO3 as base and DMF as
solvent.
The synthesis of the imine ligands 6 and 7 as well as the
copper(II) complexes 8 is also outlined in Scheme 1. The
condensation of the aldehydes 4, 5 and (S)-4-(2-methylbutoxy)-
aniline in toluene with p-toluenesulfonic acid as catalyst gave the
Schiff bases 6 and 7. The copper complexes 8 were obtained by
the reaction of the salicylaldimine ligands with copper(II) acetate
dihydrate in ethanol (see the ESI†). Compounds 6a and 7a were
first reported by Serrano et al.18 and have been included for
comparison.
aDepartment of Chemistry, Yildiz Technical University, Davutpaxsa
Yerlexsim Birimi, TR-34210 Esenler, Istanbul, Turkey
bInstitute of Chemistry, Organic Chemistry, Martin-Luther University
Halle-Wittenberg, Kurt Mothes-Str. 2, D-06120 Halle, Germany
cInstitute of Chemistry, Physical Chemistry, Martin-Luther University
The imines 6 and 7 were characterized by the common spec-
troscopic methods (1H-, 13C-NMR, UV-VIS, IR and MS) and by
elemental analysis. The copper(II) complexes 8 were character-
ized by UV-VIS, IR and MS. The spectroscopic data of the
materials are consistent with their proposed structures (see the
ESI†).
€
Halle-Wittenberg, Muhlpforte 1, D-06118 Halle, Germany
† Electronic supplementary information (ESI) available: Experimental
conditions for X-ray scattering and additional X-ray data;
electrooptical investigations of 6c; synthesis and analytical data. See
DOI: 10.1039/b905104j
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349.0 nm, whereas the absorption bands of the complexes 8 is
shifted to 381.0 nm. The imine compounds 7 without 2-hydroxyl
group show the absorption band around 335 nm. The IR spectra
of the salicylaldimine ligands 6 and their complexes 8 show the
C]N stretching vibration at around 1615–1618 cmꢁ1 and 1610–
1612 cmꢁ1, respectively. The frequency of the C]N stretching
vibration of the starting salicylaldimines decreases by about
5–7 cmꢁ1 due to complex formation as also reported previously in
similar studies.19 In addition, the C]N stretching band of the
imines is 7 shifted to 1604–1607 cmꢁ1
.
As the stereogenic center of (S)-(ꢁ)-2-methyl-1-butanol is not
touched during the synthesis it can be assumed that the optical
purity of the products corresponds to the starting material. An
optical rotation of [a]52089 +11.8ꢀ (c ¼ 0.4 in CHCl3) was measured
for compound 6c as a representative example.
2.2. Liquid crystalline properties
The mesomorphic properties of the obtained two series of imines
6 and 7 and the copper(II) complexes 8 were investigated using
polarised light optical microscopy (PM), differential scanning
calorimetry (DSC) and some of them by X-ray diffraction. All
of the salicylaldimines, their copper(II) complexes and
the imines show thermotropic mesomorphic behaviour and
exhibit smectic mesophases. The transition temperatures,
corresponding enthalpy values and mesophase types observed
for the compounds 6, 7 and 8 are summarized in Tables 1 and 2,
respectively.
Mesomorphic properties of salicylaldimine compounds 6. The
imine 6a, containing two peripheral alkyl chains (one alkyl chain
and one short and branched chiral chain), exhibits a short range
of a smectic A phase with fan-shaped texture and at lower
temperature the fingerprint texture of a chiral smectic C* phase
was observed. This is distinct from the phase sequence described
previously,18 where enantiotropic SmC* and chiral nematic
phases were reported (see Table 1).
Scheme 1 Synthesis of the salicylaldimines 6a–c, the imines 7a–c and the
salicylaldiminate Cu(II) complexes 8a–c.
1
In the H-NMR spectra the characteristic resonance of the
imine protons of the salicylaldimines and the imines without
ortho-hydroxyl group appeared at around 8.49 ppm and 8.38
ppm, respectively. These values are well within the range
expected for imines. In the electronic spectra, the absorption
band of the salicylaldimine ligands 6 is observed at around
The chiral salicylaldimines 6b and 6c, containing fluorocarbon
terminal chains, show chiral smectic mesophases. On cooling
compounds 6b and 6c from the homeotropically aligned SmA
phase the optically isotropic regions become birefringent and
Table 1 Phase transition temperatures and transition enthalpies of semifluorinated imine compounds 6 and 7a
Compound
X
R
T/ꢀC [DH kJ/mol]
6ab
6b
6c
OH
OH
OH
H
H
H
–C10H21
Cr 50 [25.0] SmC* 84 [0.9] SmA 86 [1.0] Iso
Cr 78 [24.7] SmC* 106 [0.1] SmA 146 [6.1] Iso
Cr 81 [15.0] SmC* 125 [0.2] SmA 175 [6.9] Iso
Cr 90 [46.4] (SmC* 76 SmA 78 [4.9]) Iso
Cr 101 [34.4] (SmC* 99 [0.3]) SmA 127 [4.5] Iso
Cr 94 [28.3] (SmX* 70 [1.3]) SmC* 118 [0.2] SmA 142 [4.2] Iso
–(CH2)6C4F9
–(CH2)4C6F13
–C10H21
–(CH2)6C4F9
–(CH2)4C6F13
7ac
7b
7c
a
Perkin-Elmer DSC-7; heating rates 10 K minꢁ1; enthalpy values are given behind the phase transition temperatures in italics in square parentheses;
abbreviations: Cr ¼ crystalline, SmA ¼ nontilted smectic phase, SmC* ¼ chiral smectic C phase, SmX ¼ low temperature mesophase with
b
unknown structure and Iso ¼ isotropic liquid phase; monotropic transitions are indicated by parentheses. In ref. 18 for compound 6a: Cr 50.7
[7.3] SmC* 86.2 [0.5] Ch* 88.2 [0.5] Iso. c In ref. 18 for compound 7a: Cr 83.8 [10.6] (SmC* 79.5 SmA 81.2 [1.2]) Iso.
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Table 2 Phase transition temperatures and transition enthalpies of
semifluorinated copper(II) complexes 8a
a Schlieren texture with exclusively four-brush disclinations is
formed. Inthe regionsof thefan texturethesefans becomebroken.
These textural changes indicate a transition from the SmA phase
where the molecules are organized onaverage perpendicular to the
layers to a tilted SmC* phase. The typical textures of the smectic
mesophases of compound 6c are shown in Fig. 1.
X-Ray investigations confirm layer structures for the meso-
phases of compound 6c. The pattern of the smectic A phase
shows two Bragg reflections in the small-angle region with
d-value ratios of 1 : ½, corresponding to a layer distance of
d ¼ 3.3 nm (see Fig. 2b for temperature dependence). This
d-value corresponds to the maximal possible molecular length of
L ¼ 3.3 nm as determined with CPK models,20 assuming the most
stretched conformation. As it is unlikely that the molecules adopt
a strictly linear conformation in the LC state, it is assumed that
the structure is composed of antiparallel pairs of molecules (see
Fig. S5 in the ESI†) as in case of the related compound 9c15
containing a hexyl chain at the N-substituted phenyl ring, the
aliphatic and aromatic segments being completely intercalated
with separate RF sublayers and in these RF layers there is only
a partial intercalation of the RF segments.
Compounds
R
T/ꢀC [DH kJ/mol]
8a
8b
8c
–C10H21
–(CH2)6C4F9
–(CH2)4C6F13
Cr 131 [42.3] (SmA 120 [6.1]) Iso
Cr 122 [36.7] SmA 205 [10.1] Iso
Cr 144 [37.8] SmA 223 [3.9] Iso
a
For conditions and abbreviations, see Table 1.
The 2D pattern of an aligned sample (Fig. 3a) displays the
maxima of the diffuse outer scattering in a direction perpendic-
ular to that of the layer reflections as expected for the SmA
phase. The position of the outer diffuse scattering with respect to
that of the layer reflections changes only slightly below the
Fig. 1 Polarised light optical photomicrographs of the mesophases of 6c
as observed on cooling: (a) SmA phase at 146 ꢀC; (b) SmC* phase at
110 ꢀC and (c) SmC* phase at 83 ꢀC (distinct region with Schlieren
texture, magnification ꢃ200).
Fig. 2 Compound 6c: a) DSC scan (first heating and first cooling); b)
temperature dependence of the layer spacing d on heating (see Table S1 in
the ESI,† dashed line indicates the SmC*-SmA phase transition region).
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a number of sharp reflections in the wide-angle region (see Fig. 3c
and Table S1 and Fig. S3 in the ESI†) it is assumed to be
composed of layers with a spacing of 2.72 nm exhibiting long-
range molecular order in the layers but also a comparatively high
degree of disorder indicated by the diffuse part of the wide-angle
scattering. On heating, this phase transforms to a conventional
crystalline phase showing Bragg reflections in the whole scat-
tering range (see Fig. 3d and Fig. S3 in the ESI†).
A comparison of the phase transition data for the semi-
fluorinated chiral salicylaldimines 6a–c with the related
compounds 9a–c15 containing a hexyl chain at the N-substituted
phenyl rings (see Fig. 4) shows that the kind of side chain plays
a determining role in the formation of the liquid crystalline
phases. The clearing points of salicylaldimine 6a and semi-
fluorinated analogues 6b and 6c decrease compared to the
compounds with hexyl chains 9a, 9b and 9c, whereas the melting
points increase. Hence, the salicylaldimines 6 show mesophase
Fig. 3 2D X-ray diffraction patterns of a surface-aligned sample of 6c: a)
SmA phase at T ¼ 150 ꢀC on heating; b) SmC* phase at T ¼ 85 ꢀC on
heating, c) monotropic crystalline layer phase at T ¼ 25 ꢀC as obtained
upon rapid cooling from the isotropic liquid, d) crystalline phase at T ¼
70 ꢀC on heating for comparison. The lower part of the patterns is shaded
by the heating stage.
ꢀ
intervals about 14–18 C narrower than the related compounds
with hexyl chains 9. Nevertheless, the SmC* ranges are often
broader in the series of compounds 6. In this series there is also
a pronounced effect on the SmC* phase stability which rises with
increasing degree of fluorination.
Mesomorphic properties of imine compounds 7. Compound 7a,
which was described in the literature,18 shows monotropic SmA
and SmC* mesophases with fingerprint and broken fan textures,
respectively. Both chiral imine compounds with semifluorinated
alkyl chains 7b and 7c exhibit an enantiotropic smectic A phase.
On cooling compound 7b, with only 9 fluorine atoms in the
semifluorinated side chain, a transition to a monotropic SmC*
phase takes place at 99 ꢀC, as seen by the formation of a Schlieren
texture in the homeotropic regions of the SmA phase. For
compound 7c with an increased degree of fluorination (13 fluo-
rine atoms in the semifluorinated side chain), the transition
temperatures were further increased and the smectic C* phase
was stabilized and becomes enantiotropic and in addition
a monotropic mesophase (SmX*) occurs below this smectic C*
phase. The transition from SmC* to SmX* is associated with
a colour change in the broken fan-shaped texture (see Fig. 5b, c).
X-Ray investigations on compound 7c confirm a layer structure
transition to the SmC* phase (see Fig. 3b), though the layer
distance decreases significantly (see Fig. 2b) as typical for the
onset of tilt which increases with decreasing temperature. As
a consequence, the tilt angles derived from the positions for the
maxima of the outer diffuse scattering are significantly smaller
than those calculated from the layer spacings in the SmC* phase
and those near the phase transition in the SmA phase by cos(scalc
)
¼ d(SmC*)/d(SmA) (see Table S2 and Fig. S4 in the ESI†), which
might be explained as follows. The maximum ofꢀthe diffuse wide
angle scattering is between d ¼ 0.54 nm at 170 C and d ¼ 0.50
ꢀ
nm at 85 C, which means that it is mainly determined by the
mean distance between the RF chains (highest electron density)
and hence the tilt of the aromatic and aliphatic parts (weaker
scatterer) could in fact be stronger than actually recognized in the
diffraction patterns. Electrooptical investigations (see Fig. S1 in
the ESI†) indicate a polar switching at T < 125 ꢀC which is also in
line with the proposed SmA-to-SmC* transition.
ꢀ
ꢀ
with d ¼ 3.24 nm at 130 C (SmA) and d ¼ 2.96 nm at 90 C
(SmC*) which are very similar to the layer spacings observed for
the related salicylaldimine 6c with the same molecular length.
Hence, the models of the organization in the SmA and SmC*
Compound 6c forms a monotropic (metastable) crystalline
phase on rapidly cooling from the isotropic liquid to ambient
temperature. Judging from the 2D X-ray pattern at 25 ꢀC
showing layer reflections of the first to the third order and
Fig. 4 Comparison of the mesomorphic behaviour of compounds 6 with that of the hexyl substituted compounds 9.15
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Fig. 5 Polarised light optical photomicrographs of the mesophases of 7c as observed on cooling: (a) smectic A phase at 136 ꢀC; (b) smectic C* phase at
110 ꢀC; (c) SmX* phase at 65 ꢀC (magnification ꢃ200).
phases of compound 6c can also be applied here. At lower
temperature the samples crystallize, so that the SmX* phase
could not be investigated in more detail. The clearing points and
mesophase ranges of the imine compounds 7a to 7c increase with
increasing number of fluorine atoms within the terminal alkyl
chain leading to a stabilisation of smectic mesophases.
fluorolkylated metallomesogens 8b and 8c exhibit enantiotropic
smectic A phases whereas the mesophase of the hydrocarbon
analog 8a is only monotropic. Increasing the number of fluorine
atoms in the fluoroalkyl chain leads to a stabilisation of smectic
mesophases, hence the semifluorinated metallomesogens 8b and
8c exhibit relatively high clearing points accompanied by partial
decomposition. The layer distance in the SmA phase of
compound 8b, for example, is d ¼ 3.1 nm (see Table S1 in the
ESI†), indicating that the model of organization suggested for
ligands 6c and 7c can also be applied to the organization of the
Cu(II) complexes in the SmA phases. The loss of tilt (no transi-
tion to SmC*) could probably be due to the fact that complex-
ation with Cu(II) increases the space required by the rigid core
structure and therefore space filling in these regions is achieved
without tilting of the aromatics.
A comparison of the mesomorphic properties of the salicyl-
aldimines 6a–c and the related imines 7a–c (see Fig. 6) shows that
there is a greater mesophase stability for the salicylaldimines
because of the presence of a lateral hydroxy group which can
affect the rigidity and polarisability of the mesogenic aromatic
core due to intramolecular hydrogen bonding.17a,21 The additional
OH group also increases the polarity of the core region and
enhances the incompatibility with the non-polar chains which
contributes to a compensation of the steric distortion provided by
the lateral OH group. Intermolecular hydrogen bonding and
dipolar interactions could also contribute to this effect.
Comparing the complexes 8 with their ligands 6 indicates an
increase of the transition temperatures by metal coordination by
about 34 to 61 K. This might largely be due to the increased
polarizability of the Cu complexes and the preorganization of the
rod-like cores by dimerization via metal coordination.
Mesomorphic properties of copper(II) complexes 8. In order to
examine the influence of fluorination on the bidentate salicyl-
aldimato copper(II) complexes, a series of compounds 8 bearing
a chiral and a semiperfluorinated chain were synthesized. The
phase transition temperatures and enthalpies of these complexes
8 are given in Table 2. All of the mononuclear complexes 8
exhibit smectic A phases with typical fan-shaped texture
(see Fig. S2 in ESI†), which can be aligned homeotropically. The
Fig. 7 gives a graphical overview of the behaviour of the
branched copper(II) complexes 8 and compounds 10 with
n-hexyl chains.15 The melting and clearing points and also the
mesophase range of the non-fluorinated copper complex 8a are
similar to those of 10a, but for the complex 8a the smectic phase
is only monotropic, whereas it is enantiotropic for 10a.
Fig. 6 Comparison of the thermal behaviour of compounds 6 and 7.
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Fig. 7 Comparison of the thermal behaviour of the copper(II) complexes 8 and 10 (SmY ¼ low temperature smectic phase with unknown structure).15
The mesophase ranges of the semifluorinated complexes 8b
and 8c are slightly broader than those of the hexyl substituted
complexes 10b and 10c, whereas the melting points are nearly the
same. Overall, in the series of the chiral compounds 8 with
relatively short and branched alkyl chains there is a stronger
influence of the degree of fluorination on the mesophase stability
compared to the related compounds 10 with longer linear alkyl
chains.
by Yildiz Technical University Scientific Research Projects
Coordination Department. Project Number: 24-01-02-03.
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Acknowledgements
B. Bilgin-Eran is grateful to the Alexander von Humboldt
€
Foundation, for a research fellowship at Universitat Halle-
Wittenberg, Halle, Germany. This research has been supported
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15 B. Bilgin-Eran, C. Yorur, C. Tschierske, M. Prehm and
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21 D. Lopez de Murillas, R. Pinol, M. B. Ros, J. L. Serrano,
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This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 6995–7001 | 7001