Structure-function relationships in liquid-crystalline halogen-bonded complexes

Article abstract of DOI:10.1002/chem.201000717

New liquid-crystalline materials were prepared by self-assembly driven by halogen bonding between a range of 4-alkoxystilbazoles, 4-alkyl-, and 4-alkoxy-substituted pyridines as halogen-bonding acceptors, and substituted derivatives of4-iodotetrafluorophenyl as halogen-bonding donors. Despite the fact that the starting materials are not mesomorphic, the dimeric, halogen-bonded complexes obtained exhibited nemetic and SmA phases, depending on the length of the alkylchains present on the components. The modularity of this approach also led to new chiral mesogens starting from non-mesomorphic chiral compounds.

Full text of DOI:10.1002/chem.201000717

FULL PAPER
DOI: 10.1002/chem.201000717
Structure–Function Relationships in Liquid-Crystalline Halogen-Bonded
Complexes
Duncan W. Bruce,*[a] Pierangelo Metrangolo,*[b] Franck Meyer,[b] Tullio Pilati,[c]
Carsten Prꢀsang,[a] Giuseppe Resnati,*[b] Giancarlo Terraneo,[b]
Stephen G. Wainwright,[a] and Adrian C. Whitwood[a]
Abstract: New liquid-crystalline materials were prepared by self-assembly driven
by halogen bonding between a range of 4-alkoxystilbazoles, 4-alkyl-, and 4-alkoxy-
substituted pyridines as halogen-bonding acceptors, and substituted derivatives of
4-iodotetrafluorophenyl as halogen-bonding donors. Despite the fact that the start-
ing materials are not mesomorphic, the dimeric, halogen-bonded complexes ob-
tained exhibited nematic and SmA phases, depending on the length of the alkyl
chains present on the components. The modularity of this approach also led to
new chiral mesogens starting from non-mesomorphic chiral compounds.
Keywords: fluorinated mesogens ·
halogen bonding · liquid crystals ·
self-assembly
chemistry
·
supramolecular
Introduction
genic species using, for example, hydrogen-bonding, charge-
transfer, and quadrupolar interactions.[2]
In addition to their ubiquity in flat-panel display devices,
liquid crystals are increasingly attracting attention as materi-
als with actual and postulated application in fields as diverse
as biotechnology, nanoscience, and several other classes of
advanced optical materials.[1] Such advances are based pri-
marily on covalent materials, but the attractions of con-
structing new species from non-covalent interactions have
led many in the field to explore the generation of new meso-
As a counterpart of hydrogen bonding, halogen bonding
is emerging in the field of crystal engineering and supra-
molecular chemistry in general.[3] It is commonly established
that halogen bonding concerns any non-covalent interaction
involving a halogen atom as an acceptor of electronic densi-
ty.[3e,4]
This topic has ensured the construction of supramolecular
architectures in fields of research as diverse as solid state
synthesis,[5] nonlinear optics,[6] polymer coatings,[7] separation
technologies,[8] and the formation of highly interpenetrated
networks.[9]
Recently, halogen bonding has been shown to be useful in
the realisation of new liquid-crystalline materials assembled
from non-mesomorphic components.[10] Subsequent investi-
gations pursued the design and synthesis of halogen-bonded
liquid crystals in polymeric systems[11] and in trimeric com-
plexes,[12] one example of which led to spontaneous resolu-
tion into chiral forms despite the absence of chirality in the
complexes themselves.[13] However, all of this work reports
the mesomorphism of a quite diverse selection of halogen-
bonded mesogens, which means that there is little chance
for systematic structural comparison. More than that, rather
few of the low molar mass systems reported have enantio-
tropic mesophases and none possesses a chiral function.
In many different topics, fluorinated compounds present
an enhancement of physical properties in contrast with their
hydrocarbon homologues.[14] As expected, fluorinated sub-
[a] Prof. Dr. D. W. Bruce, Dr. C. Prꢀsang, S. G. Wainwright,
Dr. A. C. Whitwood
Department of Chemistry, University of York
Heslington, York YO10 5DD (UK)
Fax : (+44)1904-432516
E-mail: db519@york.ac.uk
[b] Prof. Dr. P. Metrangolo, Dr. F. Meyer, Prof. Dr. G. Resnati,
Dr. G. Terraneo
NFMLab and CNST-IIT@POLIMI c/o Department of Chemistry
Materials, and Chemical Engineering “Giulio Natta”
Politecnico di Milano 7, via Mancinelli; 20131 Milan (Italy)
Fax : (+39)02-2399-3080
E-mail: pierangelo.metrangolo@polimi.it
giuseppe.resnati@polimi.it
[c] Dr. T. Pilati
CNR—Institute of Molecular Science and Technology
University of Milan 19, via Golgi; 20133 Milan (Italy)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000717.
Chem. Eur. J. 2010, 16, 9511 – 9524
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9511

stituents have been incorporated successfully into liquid-
crystal compounds because of the combination of high po-
Lewis bases (Scheme 2) used were 4-octylpyridine (7) and 4-
octyloxypyridine (8),[20] both well-known compounds. On
the other hand, several, related families of iodotetrafluoro-
benzene derivatives were employed in this study as new hal-
ogen-bonding donors, the synthesis of which is now de-
scribed (Scheme 3).
larity and small size, and because the high strength of the
[15]
ꢀ
C F bond confers excellent chemical stability. However,
significant modifications are encountered frequently in re-
spect of transitions temperatures, melting points, mesophase
morphologies, and the many essential physical properties of
liquid crystals, such as visco-elastic, dielectric, and optical re-
sponses. Moreover, fluorination has also been demonstrated
to be a driving force in molecular recognition, and micro-
phase separation between perfluorocarbon and hydrocarbon
chains has led to control over the liquid-crystalline phases a
mesogen exhibits. Thus, the covalent incorporation of per-
fluorocarbon chains into mesogens will induce a microphase
separation as a result of the fluorophobic effect.[16] In gener-
al, this feature gives rise to the effective absence of nematic
phases and the enhancement of the smectic character of a
mesomorphic material.[17]
The partially fluorinated stilbene compounds 2 were ob-
tained by a Wittig reaction between 4-alkoxybenzaldehyde
and the phosphonium salt of 4-H-tetrafluorobenzyl bromide.
Alkylation of 4-hydroxybenzaldehyde with the appropriate
iodoalkane was carried out in acetone under reflux in the
presence of K2CO3. For the chiral compounds, (S)- and (R)-
citronellyl were chosen as the chains for the alkylation of 4-
hydroxybenzaldehyde. Finally, the iodination reactions of
the 4-H-tetrafluorophenyl rings were carried out by using
BuLi/I2 in dry THF, affording compounds 2 in good yield.
The key intermediate in the preparation of 3-n was 4-io-
dotetrafluorophenol, which was obtained by nucleophilic
substitution of iodopentafluorobenzene by using OHꢀ. The
final ester was prepared using standard dicyclohexyl carbo-
diimide/4-dimethylaminopyridine (DCC/DMAP) conditions.
The more highly fluorinated esters, 4-n, required the use of
4-alkoxytetrafluorobenzoic acid, which was obtained from
alkoxy-2,3,5,6-tetrafluorobenzene (itself obtained by O-alky-
lation of 2,3,5,6-tetrafluorophenol) by lithiation (BuLi) and
then bubbling CO2 through the solution. Reversed esters 5-n
were prepared by esterification of 4-iodo-2,3,5,6-tetrafluoro-
benzoic acid with a 4-alkoxyphenol. The iodobenzoic acid
was prepared from pentafluorobenzoic acid by removing the
para-fluorine using Zn in aqueous ammonia and then iodi-
nating the 2,3,4,5-tetrafluorobenzoic acid so produced using
BuLi/I2. Finally, compounds 6-n were prepared by using Wil-
liamson ether procedures to O-alkylate the phenol before
introducing the iodine using BuLi and I2 (see Supporting In-
formation for the synthetic details).
In the present study, therefore, we have undertaken an in-
vestigation in which we have prepared a number of supra-
molecular halogen-bonded mesogens and have varied the
structures systematically to provide new insights into these
materials. The basis for these new materials is a 4-substitut-
ed pyridine, in most cases an alkoxystilbazole (halogen-
bonding acceptor) and a 4-sub-
stituted iodotetrafluorobenzene
(halogen-bonding donor) as
shown in Scheme 1, and we
have varied the total number of
aromatic rings and their dispo-
Scheme 1. General
dimeric
sition, as well as the length and
nature of the terminal chains.
The formation and mesomor-
phism of these dimeric systems
is now described in detail.
halogen-bonded liquid-crystal-
line materials.
Crystal and molecular Structure of 1-10·2-6 and 1-6·6-6: The
halogen-bonded complex 1-10·2-6 (Scheme 4) was crystal-
lised by mixing equimolar amounts of stilbazole 1-10 and
fluorinated stilbene 2-6 in THF, leading to the formation of
the dimeric species as illustrated. Slow evaporation of the
solvent at room temperature after two days yielded good
quality needle-like single crystals of 1-10·2-6 suitable for X-
ray diffraction. The halogen-bonded dimers found in the
crystal structure of 1-10·2-6 are illustrated in Figure 1 and
crystallographic data are reported in Table 1.
Results and Discussion
Synthesis: Alkoxystilbazoles[18] (1-m in which m is the
number of carbon atoms in the terminal alkoxy chain) have
been shown to be versatile starting units for the design of
functional materials and more particularly liquid-crystalline
compounds,[19] and so they formed the basis for the majority
of halogen-bonding acceptors used in this study. The other
The supramolecular dimers in 1-10·2-6 are linked by N···I
halogen bonds with a distance of 2.798(2) ꢂ, which corre-
sponds to a reduction of approximately 21% of the sum of
van der Waalsꢃ radii for N and I (3.53 ꢂ) (Figure 1).[21] The
[22]
ꢀ
N···I C angle is almost linear (178.81(7)8). Moreover, all
four phenyl rings of the dimer are coplanar and the alkyl
chains adopt a perfect all-trans linear arrangement and are
coplanar with the phenyl rings. Through a centre of symme-
try, two different dimers are held together in an antiparallel
fashion in the plane of the aromatic rings by intermolecular
H···F contacts (2.49 ꢂ), involving the hydrogen atoms of the
Scheme 2. The families of nitrogen bases (1-m, 7, and 8) used in this
study.
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Liquid Crystals
FULL PAPER
about 2.57 ꢂ) between antipar-
allel molecules of 2-6.[23] These
ribbons are packed together
only by residual forces.
The
complex
1-6·6-6
(Scheme 4) was similarly crys-
tallised from THF giving rise to
very thin, colourless plate-like
crystals of poor quality, which
were, however, submitted to
single-crystal X-ray diffraction
and a structure solved. The two
independent, halogen-bonded
dimers found in the crystal
structure of 1-6·6-6 are illustrat-
ed in Figure 2 and crystallo-
graphic data are reported in
Table 1.
The structures of the two
dimers in 1-6·6-6 are quite dif-
ferent from each other and
from the one seen in 1-10·2-6.
In fact, the mean-square planes
of the stilbazole rings in 1-6·6-6
are rotated with respect to
those of the halogen-bonded
modules 6-6 by 31.1 and 13.18,
while in 1-10·2-6 the same angle
is 8.68. Moreover, both in 1-6
and in 6-6 the alkoxy chains are
not coplanar with the aromatic
rings, as seen in 1-10·2-6. In par-
ticular, the dihedral angles for
1-6 are 44.58 and 46.18, while
the same angle for 1-10 is 1.98;
also the conformations of the
alkoxy chains are different,
being gttt and tgtt in 1-6 and tttt
in 1-10. In 6-6 the dihedral
Scheme 3. Preparation of the iodo compounds used in this study.
Scheme 4. General structures of 1-10·2-6 and 1-6·6-6.
Figure 1. The halogen-bonded dimeric complex 1-10·2-6 forming planar
ribbons. Colours are as follows: C, grey; H, white; F, green; I, purple; N,
blue; O, red. Halogen bonds contacts are shown by dotted black lines. El-
lipsoids drawn at the 20% probability level and non-covalent interactions
are selecetd to be shorter than the sum of the van der Wallsꢃ radii less
0.1 ꢂ.
pyridyl ring of 2-6. This leads to the formation of supra-
molecular tetramers that are now packed in an infinite
ribbon thanks to other intermolecular H···F contacts (of
Chem. Eur. J. 2010, 16, 9511 – 9524
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P. Metrangolo, D. W. Bruce, G. Resnati et al.
Table 1. Crystal data and structure refinement for the halogen-bonded
dimeric complexes 1-10·2-6 and 1-6·6-6.
well and so for some materials it was not possible to obtain
crystalline materials in this way. In these cases (which will
be discussed below), the complexes were prepared by
weighing out equimolar quantities of the two components
very carefully and mixing them together in the melt. Some
of us have used such a procedure before in preparing hydro-
gen-bonded liquid-crystalline materials and found it to be a
satisfactory procedure—indeed on many occasions it was ab-
solutely necessary, for example, if one components was
rather insoluble.[24] In the present work, the efficacy of this
approach was tested by preparing complexes in the melt for
pairings for which crystallisation was also successful. The
differences will be outlined below. Two extensive series of
related materials were first prepared to examine structure/
mesomorphism relationships in detail and then some repre-
sentative examples were prepared using a slightly wider
1-10·2-6
1-6·6-6
formula
C23H31NO·C20 H19F4IO
815.74
C19H23NO·C12H13F4IO
657.51
Mr [gmolꢀ1
]
crystal size [mm3]
crystal system
space group
a [ꢂ]
b [ꢂ]
c [ꢂ]
0.12ꢄ0.15ꢄ0.48
triclinic
0.02ꢄ0.14ꢄ0.22
triclinic
¯
¯
P1 (no. 2)
P1 (no. 2)
6.0015(12)
15.906(3)
21.269(5)
78.082(10)
84.652(10)
88.812(10)
1977.9(7)
2
9.5019(11)
12.7799(14)
24.939(3)
85.107(2)
79.141(2)
86.225(2)
2959.5(6)
4
a [8]
b [8]
g [8]
V [ꢂ3]
Z
1calcd [gcmꢀ3
]
1.370
1.476
F
(000)
840
1336
T [K]
295(2)
110(2)
range of iodides or the alkylACTHNUTRGENUGN(oxy)pyridines. It should be
qmax [8]
26.41
25.04
index ranges
ꢀ7ꢁhꢁ7
ꢀ19ꢁkꢁ19
ꢀ26ꢁlꢁ26
8135
28132 [Io ꢂ 2s(Io)]
8135 (Rint =0.0254)
R1 =0.0265
wR2 =0.0595
1.034
ꢀ11ꢁhꢁ11
ꢀ15ꢁkꢁ15
ꢀ29ꢁlꢁ29
6597
23757 [Io ꢂ 2s(Io)]
10409 (Rint =0.0891)
R1 =0.0727
wR2 =0.1773
0.962
noted that none of the components was liquid crystalline
and so all mesomorphic properties arise from halogen-
bonded dimers (the possibility that quadrupolar arene/per-
fluoroarene interactions were involved was discounted pre-
viously on the basis of experimental evidence).[10]
reflns measured
observed reflns
independent reflns
final R indices[a]
(obs. data)
Complexes with achiral stilbenes 2-n: Systematic crystallisa-
tion of 1-m with 2-n (n=m=4, 6, 8, 10, 12) produced
twenty-five dimers (Scheme 5; Table 2), the thermal behav-
S[b]
Max/av D/s
0.001/0.000
ꢀ0.23/0.42
0.000/0.000
ꢀ2.44/2.55
D1min/max [eꢂꢀ3
]
[a] R1 =SjjFo jꢀjFc jj/SjFo j, wR2 =[Sw(F2oꢀFc2)2/SwFo4]1/2. [b] Goodness-
of-fit S=[Sw(F2oꢀF2c)2/(nꢀp)]1/2, in which n is the number of reflections
and p the number of parameters.
Scheme 5. General structure for complexes 1-m·2-n.
Table 2. Observed phases in 5ꢄ5 square matrix: parentheses indicate a
monotropic phase.
2-n/1-m
1-4
1-6
1-8
1-10
1-12
Figure 2. The two asymmetric dimers in the crystal structure of the halo-
gen-bonded complex 1-6·6-6. Colours are as follow: C, grey; H, white; F,
green; I, purple; N, blue; O, red. Halogen bonds are dotted blue lines.
2-4
2-6
2-8
2-10
2-12
N, (SmA)
N
N
N
N
N
N
N
N
N
N
N
N
N, (SmA)
N, (SmA)
N, (SmA)
N, SmA
N
N
N
N
N, (SmA)
N, (SmA)
N, (SmA)
N, SmA
angles are 89.7 and 52.48, while in 1-10·2-6 the alkoxy chains
are almost coplanar with the aromatic rings of 2-6, the angle
being 2.48. This difference is probably due to the steric hin-
drance present in 6-6 between the hydrogen atoms of the
iour of which was evaluated by using polarised optical mi-
croscopy and differential scanning calorimetry (DSC); the
results are collected in Table 3. Melting points are typically
in the range 99.3–119.18C and clearing points are in the
range 108.5–130.48C, giving an average range of 98C.
The thermal data were first analysed with respect to a
fixed chain length for the stilbene. Thus, the typical feature
of compounds 1-6·2-4 to 1-12·2-4 was an enantiotropic nem-
atic phase with a range between 9.1 to 13.38C, the phase
being identified clearly by its schlieren texture (Figure 3,
left). Figure 4 shows these transition temperatures and those
for 1-m·2-6 and reveals a monotonic destabilisation of the
ꢀ
ꢀ
ꢀ
O CH2 group and the F atoms in the ortho-position of
the tetrafluorophenyl ring.
Mesomorphic properties of the complexes: Having obtained
the stilbazoles (1-m), pyridines (7 and 8) and the various
iodotetrafluobenzenes (2-n to 6-n), several series of halo-
gen-bonded complexes were prepared to evaluate their
liquid-crystal properties. In almost every case, the complexes
were prepared by co-crystallisation from THF and the meso-
morphism reported is that of the crystalline material so ob-
tained. However, not every donor–acceptor pairing behaved
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Liquid Crystals
FULL PAPER
Table 3. Thermal data for complexes 1-m·2-n.
Transition[a]
T [8C]
DH [kJmolꢀ1
]
1-4·2-4
Cr–N
N–I
(SmA–N)
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
(SmA–N)
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
(N–SmA)
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
(SmA-N)
Cr–N
N–I
(SmA-N)
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
(SmA–N)
Cr–N
SmA–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
Cr–N
N–I
119.4
130.3
(116.4)
114.5
123.5
109.5
122.6
104.2
117.5
104.3
114.2
(93.5)
115.8
125.5
114.2
121.5
107.3
116.8
105.9
116.1
105.4
111.7
(108.3)
109.4
120.8
107.1
116.3
104.4
114.6
104.4
112.5
(87.2)
102.9
111.3
(99.8)
110.4
117.5
106.5
117.3
103.9
113.6
103.2
112.3
(95.4)
103.0
105.0
110.2
109.5
111.8
108.0
110.0
107.3
109.0
105.1
108.5
(100.0)
106.0
108.6
111.1
19.8
2.1
(0.3)
50.9
7.6
27.1
2.9
15.2
1.4
65.9
4.6
–
55.6
6.6
36.8
3.5
8.8
0.8
52.5
4.91
43.4
3.8
–
75.1
8.7
36.9
5.4
40.8
2.9
22.9
1.3
(0.4)
21.8
3.0
(0.3)
29.6
3.3
46.1
4.8
34.0
4.4
46.6
5.3
(0.2)
56.3
0.5
7.7
31.8
1.9
41.4
4.4
39.2
5.3
41.3
5.1
(0.1)
66.3
0.7
1-6·2-4
1-8·2-4
1-10·2-4
1-12·2-4
Figure 3. Left: nematic phase of 1-4·2-4; middle: smectic A phase of 1-
4·2-4; right: mainly homeotropic SmA phase of 1-10·2-8.
1-4·2-6
1-6·2-6
1-8·2-6
1-10·2-6
1-12·2-6
1-4·2-8
1-6·2-8
1-8·2-8
1-10·2-8
Figure 4. Chart of the thermal behaviour of complexes 1-m·2-4 and 1-
m·2-6 (light grey is the crystal phase; dark grey is nematic).
1-12·2-8
nematic phase with increasing m, which is common behav-
iour in calamitic mesogens with two terminal chains. A simi-
lar picture emerges when considering the variation of the
stilbene chain length for other stilbazoles.
It is similarly possible to consider the data for a fixed
chain length for the stilbazole, and Figure SI-1 (see Support-
ing Information) shows a similar monotonic decrease in TN–I
as a function of the stilbene chain length for 1-4·2-4 to 1-4·2-
12 and 1-6·2-4 to 1-6·2-12.
1-4·2-10
1-6·2-10
1-8·2-10
1-10·2-10
However, what Figure 4 and Figure SI-1 do not show is
that for the longest stilbazole chain lengths (and for 1-4·2-4;
Figure 3 middle), there is also a monotropic SmA phase,
and for stilbenes 2-8, 2-10 and 2-12 the monotropic SmA
phase is also seen for 1-10 until at 1-12·2-12 it appears enan-
tiotropically; the evolution of this behaviour is illustrated in
Figure 4. All of this is consistent with the conventional view
that increasing the terminal chain length stabilises the for-
mation of layered phases, and the fact that a monotropic
SmA phase is seen in complex 1-4·2-4, suggests the latent
existence of the phase in all of the complexes studied.
The complexes reported here are of some interest as they
represent the first series of halogen-bonded mesogens in
which the predominant mesomorphism is enantiotropic.
This point will be discussed below.
1-12·2-10
1-4·2-12
1-6·2-12
1-8·2-12
1-10·2-12
(SmA–N)
Cr–N
SmA–N
N–I
1-12·2-12
4.9
[a] Where a monotropic transition is written in heating sequence, it
means that it was possible to obtain transition the temperature (and en-
thalpy where given) on reheating, giving the true thermodynamic temper-
ature.
Complexes with chiral stilbazoles 1-(R) and 1-(S) and chiral
stilbenes 2-(R) and 2-(S): Having demonstrated mesomor-
phism in achiral derivatives of the stilbazole/iodostilbene
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P. Metrangolo, D. W. Bruce, G. Resnati et al.
Table 4. Thermal data for the chiral halogen-bonded complexes.
complexes, we extended the study to the preparation of
chiral analogues by functionalising both components with
chiral chains derived from citronellol. The R and S deriva-
tives of each component were prepared and for each enan-
tiomeric component, three complexes were prepared with
three chain lengths of the complementary component (m,
n=4, 8, 12) and complexes were prepared in which both
halves were chiral to give a family of sixteen new mesogens.
The complexes were prepared by co-crystallisation of the
two components from chloroform.
Transition
T [8C]
DH [kJmolꢀ1
]
1-(S)·2-4
Cr–N*
N*–I
Cr–N*
N*–I
Cr–N*
N*–I
Cr–N*
N*–I
Cr–N*
N*–I
Cr–N*
N*–I
Cr–N*
N*–I
Cr–N*
N*–I
Cr–N*
N*–I
Cr–N*
N*–I
Cr-SmA*
SmA*–I
Cr-SmA*
SmA*–I
Cr–N*
N*–I
Cr–N*
N*–I
Cr–N*
N*–I
Cr–N*
N*–I
73.8
92.2
74.2
92.5
75.5
84.5
74.8
84.1
86.9
91.7
87.3
91.0
74.0
95.4
73.2
94.4
75.3
93.3
74.6
94.1
81.5
94.5
81.9
95.2
65.0
69.5
37.9
63.8
38.4
64.2
64.6
69.4
43.4
5.5
17.5
2.4
36.2
3.1
74.4
11.8
49.9
4.9
44.3
4.3
5.3
0.9
35.4
5.7
22.1
2.0
66.6
15.9
14.9
6.7
19.8
8.3
51.9
7.1
28.4
3.2
27.5
3.0
10.3
1.0
1-(R)·2-4
1-(S)·2-8
1-(R)·2-8
1-(S)·2-12
1-(R)·2-12
1-4·2-(S)
Complexes formed by the self-assembly of the stilbazole
1-(S) with the fluorinated stilbenes 2-4, 2-8 and 2-12 showed
an enantiotropic N* mesophase (Figure 5) that exhibited a
1-4·2-(R)
1-8·2-(S)
1-8·2-(R)
1-12·2-(S)
1-12·2-(R)
1-(S)·2-(S)
1-(S)·2-(R)
1-(R)·2-(S)
1-(R)·2-(R)
Figure 5. Optical texture of the phase transition of 1-S·2-4 detected at the
same temperature on cooling. Fan-like texture (left), oily streak texture
after shearing (right).
typical focal conic fan texture, which is the main feature of
N* chiral liquid crystals with short pitch length. This texture
changed after shearing to give a planar texture with oily
streaks. The liquid-crystalline range was 18.48C for 1-(S)·2-
4, a little higher than for the equivalent achiral complex
bearing the same number of carbon atoms on the alkyl
chain. However, this does not reflect the fact that both melt-
ing and clearing points are destabilised with respect to the
achiral analogues owing to the presence of the doubly
branched chain and unsaturated link, both of which are de-
stabilising. The enantiomeric materials, 1-(R)·2-n mirrored
the transition temperatures of the S enantiomer almost pre-
cisely. The transition temperatures for all complexes are re-
ported in Table 4.
Complexes between stilbazoles (1-m) and alkoxytetra
AHCTUNGTRENNUNG
ACHTUNGTRENNUNGfluACHTUNGTRENNUNGoACHTUNGTRENNUNGro-
(Scheme 6) were prepared by crystallisation from a solution
Scheme 6. General structure for complexes 1-m·6-n.
We next prepared the analogous complexes (1-m·2-(S)
and 1-m·2-(R)) in which the chiral function was located on
the iodostilbene. In this case we again found little difference
between the transition temperatures of the two enantiomers
and in fact the melting points of the m=4 and 8 derivatives
vary little between the four series examined. However, the
difference lies with 1-12·2-(S) and 1-12·2-(R), which instead
of showing a N* phase in fact formed a SmA* phase. This
formed as a focal-conic texture and was distinguished from
the N* phase by its behaviour on shearing. In this way, com-
plex 1-12·2-(S) (or its R enantiomer) more closely resembles
the achiral analogues that showed a SmA phase when the
stilbazole chain length was m=12. Interestingly then, this
implies that the stilbazole may have a slightly greater effect
on the mesomorphism in these complexes.
of the two components in THF, but for some combinations
(see Table 5) the crystallisation failed. In these cases, the
complexes were prepared by weighing equimolar quantities
of the two components and melting them together carefully.
In order to assess the appropriateness of this approach, we
re-prepared by this method some complexes that we could
obtain by crystallisation and found that the mesophase seen
was identical, although transition temperatures were be-
tween 1 and 38C lower. These results suggest that the
method is adequate to enable a proper comparison to be
made, although it does suggest that crystallisation is prefera-
ble where it works. The thermal data for these complexes
are collected in Table 5.
9516
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Liquid Crystals
FULL PAPER
However, in previous papers[12b,c] we have noted the tenden-
cy for some complexes to exhibit partial dissociation when
passing to the isotropic phase and we suspect that this is the
case here, too.
Table 5. Thermal data for complexes between alkoxystilbazoles (1-m)
and alkoxytetrafluoroiodobenzenes (6-n).
Transition
T [8C]
DH [kJmolꢀ1
]
1-4·6-4
Cr–Iso
73.3
(67.1)
64.5
66.7
60.9
65.0
53.9
65.3
61.0
(60.8)
66.7
72.1
55.7
70.4
66.6
70.6
58.1
68.3
60.4
65.2
71.4
77.0
57.4
74.0
52.4
75.0
58.5
72.4
57.4
72.8
65.6
79.3
38.9
78.3
60.5
77.5
51.4
73.4
59.3
73.3
58.7
79.5
57.0
76.9
53.6
76.4
53.2
75.9
71.8
77.3
22.7
(5.7)
19.5
8.7
24.1
9.8
27.9
8.5
37.7
(7.4)
26.0
9.7
(SmA–Iso)
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–Iso
In general, the transition temperatures for these com-
plexes are rather low given their structure, which contains
three aromatic rings. It would be expected that clearing
points for such materials would be much higher and, for ex-
ample, hydrogen-bonded complexes between alkoystilba-
zoles and 4-alkoxybenzoic acids typically give mesophases
between 100 and 1508C .[19] In turn, these are again lower
than might be expected for covalent analogues due to the
extra flexibility conferred by the hydrogen bond. The transi-
tion temperatures can again be plotted in two ways: one
(shown in Figure 6) that shows how they depend on the stil-
1-4·6-6
1-4·6-8
1-4·6-10
1-4·6-12
1-6·6-4
(SmA–Iso)
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
Cr–SmA
SmA–Iso
1-6·6-6
18.5
11.2
28.8
12.1
37.4
10.6
47.6
10.1
6.9
13.6
19.5
12.2
22.2
14.0
43.8
11.9
36.4
14.4
23.9
14.3
14.7
9.8
37.9
26.8
24.7
15.0
35.0
13.8
27.3
16.6
24.0
15.6
26.1
18.7
26.4
17.3
51.4
19.8
1-6·6-8
1-6·6-10
1-6·6-12
1-8·6-4
1-8·6-6
1-8·6-8
1-8·6-10
1-8·6-12
1-10·6-4[a]
1-10·6-6[a]
1-10·6-8[a]
1-10·6-10[a]
1-10·6-12[a]
1-12·6-4[a]
1-12·6-6[a]
1-12·6-8[a]
1-12·6-10[a]
1-12·6-12[a]
Figure 6. Plot of transition temperatures for complexes 1-m·6-n as a func-
tion of stilbazole chain length (dark grey is crystal phase, light grey is
SmA).
bazole chain length and the other (see Supporting Informa-
tion, Figure SI-2) that shows how they depend on the alk-
ACHUTNGRENoNUG xyACHUTNGTRENtNNGU etrafluoroiodobenzene chain length. What is interesting
is that Figure 6 shows a clear dependence on the stilbazole
chain length, while Figure SI-2 (Supporting Information)
shows a rather small dependence on the iodobenzene chain
length, with the clearing point decreasing slowly with in-
creasing iodide chain length.
[a] Complexes prepared by melting equimolar amounts of the compo-
nents.
Complexes between stilbazoles (1-m) and the two-ring
esters 3-n, 4-n and 5-n: Examination of the thermal data for
these complexes (Scheme 7; Table 6) shows that the majority
exhibit a monotropic mesophase, which is, for most com-
plexes, nematic. However, again we note that the transition
temperatures are significantly lower than one might expect
for a complex that is so anisotropic, being composed of four
aromatic rings. For example, the four-ring mesogen:
C8H17O-Ph-CO2-Ph-CH=N-Ph-O2C-Ph-OC8H17 shows N–Iso
at 2988C.[25] Another point worthy of note here is that it is
possible to predict which of the esters should form the stron-
All of the complexes showed only a SmA phase, which
was readily identified by the appearance of a focal conic fan
texture and/or a homeotropic texture with oily streaks on
shearing and for all but two complexes (1-4·6-4 and 1-4·6-
12), the mesophases were enantiotropic. The complexes
showed a range of melting enthalpies, but clearing enthal-
pies varied from about 6 up to almost 27 kJmolꢀ1. These
higher values are rather surprising and are somewhat great-
er than would be expected for the clearing of a SmA phase.
Chem. Eur. J. 2010, 16, 9511 – 9524
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9517

P. Metrangolo, D. W. Bruce, G. Resnati et al.
Scheme 7. General structures for complexes 1-m·3-n, 1-m·4-n and 1-m·5-
n.
Table 6. Thermal data for complexes (all prepared in the melt) between
alkoxystilbazoles (1-m) and esters 3-n, 4-n and 5-n.
Transition
T [8C]
DH [kJmolꢀ1
]
1-6·3-6
Cr–Iso
(N–Iso)
Cr–Iso
(N–Iso)
Cr–Iso
Cr–Iso
Cr–N
135.5
(102.5)
128.5
(102.4)
129
127.8
85.5
101.9
74.8
63.6
–
68.8
–
69.3
74.3
20.1
6.5
–
1-6·3-10
Figure 7. Plot of clearing points as a function of fluoroester and stilbazole
chain length. Data in light grey refer to N–Iso, while those in dark grey
refer to SmA–Iso.
1-10·3-6
1-10·3-10
1-4·4-6
N–Iso
Cr–N
pare these complexes (Scheme 8), we had identified a three-
ring motif analogous to complexes 1-m·6-n, except that it
was the two-ring unit that carried the iodine. Most com-
plexes showed monotropic nematic phases (Table 7), but
what was remarkable was the extremely low transition tem-
peratures for the complexes of the esters, so much so that
the clearing points of 7·4-6 and 7·4-10 were sub-ambient,
while that of 7·5-6 was found at ꢀ98C. However, these com-
1-6·4-6
N–Iso
Cr–N
N–Iso
Cr–SmA
SmA–N
N–Iso
Cr–Iso
(N–Iso)
Cr–Iso
(N–Iso)
Cr–Iso
(N–Iso)
Cr–Iso
(N–Iso)
Cr–Iso
(SmA–Iso)
Cr–Iso
(SmA–Iso)
86.7
87.9
92.6
79.1
82.6
87.0
88.5
(87.4)
98.7
(95.4)
113.8
(106.4)
104.8
(96.5)
108.0
(93.0)
101.1
(88.9)
–
1-6·4-10
1-10·4-6
40.0
8.7
–
–
–
52.2
–
34.6
–
26.4
–
23.5
–
1-10·4-10
1-12·4-6
1-6·5-6
1-6·5-10
1-10·5-6
1-10·5-10
19.6
–
47.3
–
gest halogen bonds with the stilbazoles, if it is assumed that
the most d+ iodine will interact most strongly with the pyri-
dine nitrogen. On this basis, we suggest that the order would
be: 5-n>4-n>3-n. If the clearing points of the different
complexes are now considered (Figure 7), it is readily appar-
ent that there is no systematic dependence of the clearing
point on the type of ester, which suggests strongly that in
this series, rupture of the halogen bond is not a significant
factor in driving the clearing transition.
Complexes between octylpyridine(7) and the two-ring esters
3-n, 4-n and 5-n and iodostilbenes 2-n: In selecting to pre-
Scheme 8. General structures for complexes 7·2-5-n.
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Liquid Crystals
FULL PAPER
Table 7. Thermal data for complexes between octylpyridine (7) and
esters 3-n, 4-n and 5-n.
tween these pyridines and alkoxybenzoic acids showed
liquid-crystal phases at and around ambient temperature.[26]
Therefore we prepared four complexes in which each of 7
and 8 was complexed to 6-4 and 6-10 (Scheme 10). None of
these complexes was mesomorphic and all showed melting
points below 08C.
Transition
T [8C]
DH [kJmolꢀ1
]
7·2-6
Cr–N
N–Iso
57.2
62.3
–
–
7·2-10
7·3-6
Cr–N
N–Iso
Cr–Iso
(N–Iso)
Cr–N
56.8
61.5
39.1
(23.8)
35.0
36.7
42.1
(13.0)
34.1
(17.7)
25.6
–
–
56.2
(4.7)
30.1
5.9
32.9
(2.4)
44.1
(3.7)
13.5
–
7·3-10
7·4-6
N–Iso
Cr–Iso
(N–Iso)
Cr–Iso
(N–Iso)
Cr–Iso
(N–Iso)
Cr–Iso
(N–Iso)
7·4-10
7·5-6
Scheme 10. General structures for complexes 7/8·6-n.
(ꢀ9.1)
7·5-10
61.4
35.3
–
Discussion
(13.0)
In the majority of cases reported in this paper, the com-
plexes studied were prepared by co-crystallising the two
components, which tended to lead to materials with sharp
melting points; this remains the preferred method of prepa-
ration. However, in some cases such a co-crystallisation
failed and so samples were prepared by melting together
carefully weighed amounts of the two components. In con-
trol situations, comparison of transition temperatures with
samples prepared by crystallisation gave very similar results
indicating that melting the samples together is good enough
to determine and compare mesomorphism. However, the
fact that sharp melting was not observed shows that complex
formation was not complete. This differs from our experi-
ence with hydrogen-bonded liquid crystals and is perhaps in-
dicative of a possibly greater lability in these halogen-
bonded systems.
plexes do not make for a good comparison with the series 1-
m·6-n, as the former contain an ester function, which will
tend to act to destabilise the mesophases. Thus it is com-
plexes 7·2-6 and 7·2-10 that must be used. In both cases, an
enantiotropic nematic phase was found and there was re-
markably little difference between the transition tempera-
tures of the two.
The phase behaviour of these two complexes can be com-
pared with that of 1-8·6-6, 1-8·6-10, 1-6·6-8 and 1-10·6-8, for
which the clearing point varies from 70.6 to 77.58C. Al-
though the TNIso values for these complexes are a little
higher, they are no so different from those of 7·2-6 and 7·2-
10, showing that the major factor determining the meso-
phase stability is the overall molecular composition.
One of the major observations is that the majority of the
materials showed enantiotropic mesophases, something that
was not a feature of several previous reports.[10,12a,b] The ob-
servation of a mesophase as monotropic or enantiotropic
represents a balance between the melting point of the com-
plex and the stability of its liquid-crystal phase and, com-
pared to previous materials reported by us, it is the destabi-
lisation of the crystal phase that accounts for the predomi-
nance of enantiotropic phases. It is possible to rationalise
this behaviour. Thus, we have reported previously the phase
behaviour of 2:1 complexes of alkoxystilbazoles with 1,4-
diiodotetrafluorobenzene and found typically that the com-
plexes melted (Cr–Iso) between 115–1308C, with monotrop-
ic clearing points around 110–1138C.[12b] In the present case,
melting points for complexes 1-m·2-n are mostly around
104–1158C, whereas clearing points are between 110–1208C.
In comparing the two series, the 2:1 complexes of the diio-
dotetrafluorobenzene are more anisotropic, containing five
rings and so it is perhaps not surprising that they melt at
temperatures slightly higher than those found for 1-n·2-n. In
principle, this anisotropy ought also to be reflected in higher
clearing points, but the fact that it is not points to an impor-
tant aspect of the mesomorphism of moieties formed by
non-covalent, monodentate interactions.
While not wishing to extend this series too much further,
we did prepare two examples of analogous complexes using
4-octyloxypyridine (Scheme 9), and these materials also
Scheme 9. General structures for complexes 8·3-n.
showed monotropic phases, but at appreciably higher transi-
tion temperatures. That these transition temperatures are
higher is consistent with the stabilising effect of the ether
oxygen.
However, having realised materials with rather low clear-
ing points, we prepared one, final set of complexes between
octyl- (or octyloxy-) pyridine and the alkoxyiodobenzenes 6-
n. Thus, as part of his work on hydrogen-bonded systems,
Kato and co-workers had shown that complexes formed be-
Chem. Eur. J. 2010, 16, 9511 – 9524
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9519

P. Metrangolo, D. W. Bruce, G. Resnati et al.
For example, in previous work with hydrogen-bonded
mesogens (Scheme 11), an analogy was drawn between a
pyridine hydrogen-bonded to a benzoic acid and a simple
bonds will show a greater destabilisation. Thus, the fact that
so many of the complexes studied show enantiotropic meso-
morphism when compared to the previous 2:1 systems can
be attributed to a significantly smaller mesophase destabili-
sation (only one halogen bond) accompanied by a destabili-
sation of the crystal phase owing to the reduced anisotropy
of the complex.
Yet despite the fact that the presence of the halogen bond
destabilises the clearing point, the complexes are otherwise
rather insensitive to its presence. This is shown clearly by
Figure 8 in which the clearing points for 1-m·2-n are plotted
Scheme 11. General structures of hydrogen-bonded mesogens,[25] ana-
logues of halogen-bonded mesogens.
ester.[27] Such an analogy allowed the effects of the flexibility
of the hydrogen bond to be assessed and it was found to be
substantial, reducing clearing points by comfortably in
excess of 100 K. If a halogen bond is then considered as
structurally similar to a s-bond (a space-filling representa-
tion of 1-10·2-6 shows that the size of the iodine is not an
additional factor), then a search of the literature shows no
directly analogous compounds for which liquid-crystal prop-
erties are reported. However, by comparing the clearing
points of 1-8·2-8 (114.68C) and the 2:1 complex of octyloxy-
stilbazole with 1,4-diiodotetrafluorobenzene (113.58C)[12b]
with those of the four-ring (9)[25] and five-ring (10)[28] com-
pounds shown below (Scheme 12), some progress can be
made.
Figure 8. Clearing point as a function of total carbon chain length for
complexes 1-m·2-n.
as a function of the total chain length, indicating that the
complexes do not discriminate between the positioning of
the chains. This is supported by the observations with the
chiral analogues, for which clearing points were also insensi-
tive to the moiety bearing the chiral and the achiral chain.
Furthermore, this monotonic increase with decreasing total
chain length shown in Figure 8 argues that where there is a
single halogen bond, mesophase stability does not appear to
be limited by halogen bond strength at least to around
1308C.
Finally, we note that in some series of materials, there is a
pronounced dependence of the mesomorphic properties on
the stilbazole, which is not mirrored as a function of the
iodine acceptor. The reasons for this are not clear, but it is
an observation that deserves mention given the data in
Figure 8. It is to be expected that this point will addressed
in due course through further study.
Scheme 12. Structures of liquid-crystalline materials with four (9) and
five aromatic rings (10).
Thus, the clearing point of 9 is 2988C, while that of 10 is
at least 4228C. Using these to compare with the halogen-
bonded mesogens shows that one halogen bond can destabi-
lise the clearing point by close to 2008C, while two halogen
bonds lead to a reduction of around 3008C. It is possible
that this last comparison represents an overestimate, as we
argued that it was conceivable that, in contrast to the situa-
tion in hydrogen-bonded mesogens, the clearing points in
the 2:1 complexes (with both 1,4- and 1,3-diiodotetrafluoro-
benzene) were limited by the stability of the halogen bond,
as evidenced by the relatively insensitivity of the clearing
point to chain length. However, complexes 1-n·2-n do show
sensitivity to chain length, suggesting that halogen-bond
rupture does not drive the clearing behaviour and so the de-
stabilisation of around 2008C can be regarded as valid.
Nonetheless, it is reasonable to assume that two halogen
Conclusion
In reporting some ninety new complexes in this work, we
present for the first time a substantial and systematic study
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Chem. Eur. J. 2010, 16, 9511 – 9524

Liquid Crystals
FULL PAPER
(5:1), to give the products as solid powder (yield 73% for 2-SII-butyl;
62% for 2-SII-hexyl; 85% for 2-SII-octyl; 95% for 2-SII-decyl; 81% for
2-SII-dodecyl; 51% for 2-SII-R-citronellyl and 59% for 2-SII-S-citronell-
yl).
of halogen-bonded liquid crystals. Thus, the present study
shows that halogen bonding may be used reliably for the
construction of new, supramolecular mesogens and that at
least in cases in which there is a single halogen bond pres-
ent, mesophase stability is not limited by halogen-bond
strength. Furthermore, general arguments that are used to
account for the mesophase stability of covalent mesogens
may also be used for halogen-bonded systems once allow-
ance is made for the flexibility of the halogen bond. Of
course, there remain many mesogenic systems that as yet
have no analogy in halogen-bonded equivalents and only
time will show if they are similarly well behaved. Nonethe-
less, the results of this systematic study present an optimistic
prognosis.
General procedure for the formation of 1-[2-(4-alkyloxyphenyl)vinyl]-
2,3,5,6-tetrafluoro-4-iodobenzene (2-n): The fluorinated derivative (2-SII;
1.54 mmol) was stirred in suspension in THF (5 mL) at ꢀ788C. nBuLi
(1.4m, 2.31 mmol) was added slowly and the mixture was warmed up to
room temperature. After 20 min, I2 (2.31 mmol) was added and the solu-
tion was mixed 25 min. Then, the reaction was hydrolysed with Na2S2O3
sat., the aqueous layer was extracted three times with CH2Cl2 and the or-
ganic phase was dried over anhydrous Na2SO4. After evaporation of the
solvent, the compound 2-n was recovered in pure form without further
purification (yield 95% for 2-4; 90% for 2-6; 85% for 2-8; 95% for 2-10;
81% for 2-12; 79% for 2-(R) and 90% for 2-(S)).
General procedure for the formation of 4-iodo-2,3,5,6-tetrafluorophenol
(3-SI) (Scheme 14): Pentafluoroiodobenzene (34 mmol) with KOH
(102 mmol) was heated under reflux in tert-butyl alcohol (30 mL) for
6.5 h (oil bath heated to 908C). Aqueous hydrochloric acid (5%, 20 mL)
Experimental Section
Materials and methods: Commercial HPLC-grade solvents were used
without further purification. Starting materials were purchased from
Sigma–Aldrich, Acros Organics, and Apollo Scientific. 1H, 13C and
19F NMR spectra were recorded at ambient temperature with Bruker
250, 270, 400 and 500 MHz spectrometers. Unless otherwise stated,
CDCl3 was used as both solvent and internal standard in 1H and
13C NMR spectra. For 19F NMR spectra, CDCl3 was used as solvent and
CFCl3 as internal standard. All chemical shift values are given in ppm.
The mass spectra were recorded on a GC-MS AGILENT GC-MSD5975.
Differential scanning calorimetry (DSC) analysises were performed on a
Mettler Toledo DSC823e and DSC822e instruments, aluminium light
20 mL sample pans and the Mettler: STARe software for calculation.
Melting points were also determined Reichert instrument by observing
the melting and crystallising process through an optical microscope.
Scheme 14. Synthetic scheme for compounds 3-n.
was added and the aqueous tert-butyl alcohol was distilled off under
vacuum. The residue was acidified to ꢃpH 2 with aqueous hydrochloric
acid (5%). The filtrate was extracted with ether and dried over anhy-
drous Na2SO4. The solvent was removed by rotary evaporation and the
white solid collected (60% yield).[29]
Optical microscopy was performed using an Olympus BX50 microscope
and Olympus BX51 at ꢄ100 and ꢄ200 magnification with a Linkam Sci-
entific LTS 350 heating stage and VWR international 18ꢄ18 mm borosi-
licate glass microscope cover slips of a thickness no. 1. Optical rotations
were measured on a Jasco DIP-181 polarimeter.
General procedure for the formation of 4-(4-alkoxyphenyl)-2,3,5,6-tetra-
fluoroiodobenzoate (3-n): Compound 3-SI (6.8 mmol), 4-alkyloxybenzoic
acid (6.8 mmol), 1,3-dicyclohexylcarbodiimide (7.5 mmol) and 4-(N,N-di-
methylamino)pyridine (0.3 mmol) were placed in a round-bottomed flask
with dry dichloromethane (50 mL) and stirred for 48 h at room tempera-
ture. The white precipitate was filtered off and the filtrate was washed
with aqueous acetic acid (5%, 100 mL). The organic layer was separated;
the solvent removed by rotary evaporation and the pale yellow solid was
collected. The crude product was re-dissolved in pentane and ethyl ace-
tate (30:1) and passed through a silica flash chromatography column. The
solvent was then removed by rotary evaporation and the white powder
was collected (yield 59% for 3-6 and 8% for 3-10).
All synthetic details, NMR spectra, melting points and mass spectra for
the synthesised compounds are reported in the Supporting Information.
General procedure for the formation of 3-[2-(4-alkyloxyphenyl)vinyl]-
1,2,4,5-tetrafluorobenzene (2-SII) (Scheme 13): Phosphonium salt
(2.57 mmol) and NaH (3.34 mmol) were stirred in DMF (5 mL) for
30 min. Then, the aldehyde (2-SI;[18] 4.95 mmol) was added to the mixture
and the solution was heated to 408C. After 24 h, the reaction was poured
on H2O and a white solid was filtered. The residue was purified by flash
chromatography on silica gel (240–400 mesh), eluent hexane/CH2Cl2
General procedure for the formation of 4-(4-alkoxy-2,3,5,6-tetrafluoro-
benzoyloxy)-2,3,5,6-tetrafluoroiodobenzenes (4-SII) (Scheme 15): 2,3,5,6-
Tetrafluorophenol (24 mmol), anhydrous potassium carbonate (29 mmol)
and acetonitrile (40 mL) were placed in a round-bottomed flask fitted
with a reflux condenser, nitrogen inlet and magnetic stirrer. The reaction
mixture was heated to reflux (oil bath heated to 858C) and stirred. Once
reflux was reached, 1-bromoalkane (23 mmol) was added dropwise over
10 min. The mixture was heated under reflux for 12 h before cooling.
Once the mixture had cooled to room temperature, it was treated with
water (100 mL) and extracted with petroleum ether (50 mL). The organic
layer was then washed with NaOH (10%, 2ꢄ50 mL each) and finally
with water until neutrality was reached. The organic solution was dried
over anhydrous Na2SO4 and the solvent removed using rotary evapora-
tion leaving a pale yellow oil, no further purification was carried out
(yield 97% for 4-SI-hexyl and 79% for 4-SI-decyl). n-Butyl lithium (2.5m
in hexane, 10 mmol) was cooled in an acetone/dry ice bath (ꢀ708C) in
dried glassware. Dry THF (20 mL) and diisopropylamine (10 mmol) were
added and allowed to cool. Compound 4-SI (10 mmol) in dry THF
(10 mL) was added drop-wise over 5 min. The reaction was left to stir (at
ꢀ708C) for 30 min. CO2 was then bubbled through the mixture and it
Scheme 13. Synthetic scheme for compounds 2-n.
Chem. Eur. J. 2010, 16, 9511 – 9524
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9521

P. Metrangolo, D. W. Bruce, G. Resnati et al.
water until neutral. The intermediate
(3 mmol) was dissolved in ethanol
(20 mL) and Pd/C was added and
stirred vigorously under hydrogen for
24 h. The catalyst was filtered off,
washed with dichloromethane and sol-
vent removed by rotary evaporation
and the products collected (yield 24%
for 5-SII-hexyl and 60% for 5-SII-
decyl over two steps).
Scheme 15. Synthetic scheme for compounds 4-n.
General procedure for the formation
of
4-(4-alkyloxybenzoyloxy)-2,3,5,6-
tetrafluoroiodobenzenes (5-n): Com-
pounds 5-SI (1.5 mmol) and 5-SII (1.5 mmol), 1,3-dicyclohexylcarbodii-
mide (1.7 mmol) and 4-(N,N-dimethylamino)pyridine (0.07 mmol) were
used in the same method described in the preparation of 3-n and the
white powder was collected (yield 79% for 5-6 and 71% for 5-10).
was slowly allowed to return to room temperature. The solvent was re-
moved by vacuum and the crude product treated with water (30 mL) and
HCl (1m) to bring the pH to ꢃ2. The product was extracted with diethyl
ether, which was then removed by rotary evaporation to leave a white
solid (yield 51% for 4-SII-hexyl and 45% for 4-SII-decyl) .[30]
General procedure for the formation of 4-alkoxy-2,3,5,6-tetrafluoroben-
zenes (6-SI) (Scheme 17): 2,3,5,6-Tetrafluorophenol (24 mmol), anhy-
drous potassium carbonate (29 mmol) and acetonitrile (40 mL) were
General procedure for the formation of 4-(4-alkyloxy-2,3,5,6-tetrafluoro-
benzoyloxy)-2,3,5,6-tetrafluoroiodobenzenes (4-n): Compound (4-SII)
(5.1 mmol), 1,3-dicyclohexylcarbodiimide (5.6 mmol) and 4-dimethylami-
nopyridine (0.2 mmol) were used in the same method described in the
preparation of 3-n compounds and the white powder was collected (yield
51% for 4-6 and 45% for 4-10).
General procedure for the formation of 4-(4-alkoxybenzoyloxy)-2,3,5,6-
tetrafluoroiodobenzenes (5-SI) (Scheme 16): Pentafluorobenzoic acid
(48 mmol), zinc powder (0.19 mol) and 20% aqueous ammonia solution
Scheme 17. Synthetic scheme for compounds 6-n.
placed in a round-bottomed flask fitted with a reflux condenser, nitrogen
inlet and magnetic stirrer. The reaction mixture was heated to reflux
(858C) and stirred. Once reflux was reached, 1-bromoalkane (23 mmol)
was added dropwise over 10 min. The mixture was heated under reflux
for 12 h before cooling. Once the mixture had cooled to room tempera-
ture, it was transferred to
a separating funnel, treated with water
(100 mL) and extracted with petroleum ether (50 mL). The organic layer
was then washed with NaOH (10%, 50 mL each) and finally with water
until neutrality was reached. The organic solution was dried over anhy-
drous Na2SO4 and the solvent removed using rotary evaporation leaving
a pale yellow oil, no further purification was carried out. (yield 95% for
6-SI-butyl, 97% for 6-SI-hexyl, 79% for 6-SI-octyl, 79% for 6-SI-decyl
and 93% for 6-SI-dodecyl).
General procedure for the formation of 4-alkoxy-2,3,5,6-tetra
ACHUTGTNRENNUGfluACHTUNGTRENNUNGoACHTUNGTRENNUNGroACHTUNGTRENNUNGiodo-
AHCTUNGTRENNUNG
were cooled to ꢀ708C in dried glassware. nBuLi (2.5m in hexane,
29 mmol) was added drop wise over 15 min and left to stir for a further
15 min at ꢀ708C. Iodine (29 mmol) dissolved in anhydrous THF (20 mL)
was then slowly added to the reaction mixture until the colour remained
brown. This was then allowed to slowly warm up to room temperature,
while stirring. H2SO4(aq) (20 mL) and water (30 mL) were then added and
the aqueous phase was extracted with diethyl ether (50 mL). The organic
layer was then washed with water (50 mL), once with Na2S2O3(aq) (50 mL)
and then again with water before being dried over MgSO4. The solvent
was removed using rotary evaporation and the residue passed through
silica with petroleum ether, which was then removed by rotary evapora-
tion to leave colourless oil (yield 40% for 6-4, 55% for 6-6, 52% for 6-8,
53% for 6-10 and 18% for 6-12).
Scheme 16. Synthetic scheme for compounds 5-n.
(90 mL) were stirred at room temperature for 24 h. The zinc was re-
moved and the remaining solution acidified with HCl. The product was
extracted with diethyl ether which was later removed by rotary evapora-
tion and the intermediate collected. 2,3,5,6-Tetrafluorobenzoic acid
(27 mmol) was treated with nBuLi (2.5m in hexane, 53 mmol) and iodine
added (26 mmol) using the same procedure reported for the preparation
of 6-n (yield 59% for 5-SI) .[31]
General procedure for the formation of 4-alkyloxyphenol (5-SII): Small
pieces of metallic sodium (10 mmol) were carefully dissolved in ethanol
(20 mL). To this 4-benzyloxyphenol (9 mmol) and 1-bromoalkane
(10 mmol) were added and the mixture was heated under reflux for 24 h
(858C). The mixture was cooled and the NaBr formed was removed by
filtration and washed with dichloromethane. The solvent was then re-
moved by rotary evaporation. The crude product was re-dissolved in
chloroform and washed with water (50 mL), HCl (lm) and again with
Synthesis of 7: nBuLi (2.5m in hexane, 70 mmol) was added drop wise to
diisopropylamine (70 mmol) in dry THF (20 mL) in dried glassware
under nitrogen, placed in a dry ice/ethanol bath (ꢀ708C) and allowed to
stir for 30 min. This was then allowed
to warm to ꢀ308C and the 4-methyl-
pyridine (70 mmol) was slowly added
and stirred for 1 h. 1-Bromoheptane
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Chem. Eur. J. 2010, 16, 9511 – 9524

Liquid Crystals
FULL PAPER
(70 mmol) was then added slowly to the reaction mixture and then stirred
for a further hour. The mixture was allowed to return to room tempera-
ture and stirred for 48 h before quenching with water. The product was
extracted with petroleum ether and dried over MgSO4 before removing
the solvent by rotary evaporation. The crude product was then distilled
(at 1598C) and the pure compound collected (32% yield).[32]
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Analysis by X-ray diffraction: The 1-10·2-6 single-crystal X-ray structure
was collected at 295(3) K with a Bruker KAPPA APEX II with CCDC
area detector diffractometer using graphite-monochromated MoKa radia-
tion (l=0.71069 ꢂ); w and f scans; data collection and data reduction
were performed with the SMART and SAINT program packages. The 1-
6·6-6 single crystal X-ray structure was determined at 110(3) K using a
Bruker SMART-APEX diffractometer equipped with CCDC area detec-
tor using graphite-monochromated MoKa radiation (l=0.71069 ꢂ); w
and f scans; data collection and data reduction were performed with the
SMART and SAINT program packages. The structures were solved by
SIR2002[33] and refined by SHELXL-97[34] programs for 1-10·2-6 and
using SHELXS-97[34] and refined with SHELXL-97[34] for 1-6·6-6. The re-
finement was carried on by full-matrix least-squares on F2. Hydrogen
atoms were placed using standard geometric models and with their ther-
mal parameters riding on those of their parent atoms.
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CCDC-769517 (1-10·2-6) and 769518 (1-6·6-6) contain the supplementary
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Acknowledgements
We would like to thank Fondazione Cariplo (Project: New-Generation
Fluorinated Materials as Smart Reporter Agents in 19F MRI) and
EPSRC for financial support and the Royal Society for a Joint Project
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Received: March 22, 2010
Published online: July 21, 2010
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Chem. Eur. J. 2010, 16, 9511 – 9524