1,1′-Disubstituted ferrocene containing hexacatenar thermotropic liquid crystals

Article abstract of DOI:10.1039/b008904o

The preparation and characterization of ferrocene containing hexacatenar metallomesogens 4a - h based on alkoxy terminal groups with hexyloxy (4a), octyloxy (4b), decyloxy (4c), dodecyloxy (4d), tetradecyloxy (4e), hexadecyloxy (4f), octadecyloxy (4g) and eicosyloxy (4h) chains are described. The introduction of alkyl chains of different lengths induces a rich variety of self-assembled liquid crystalline structures. In the crystalline state, the ferrocene containing hexacatenar metallomesogens 4a - h organize into a microphase-separated monolayer lamellar structure. In contrast, a dramatic phase change after crystalline melting of the metallomesogens is observed with variation of the chain length. The ferrocene containing hexacatenar metallomesogens 4b and c display a bicontinuous cubic mesophase with Ia3d symmetry, while the ferrocene containing hexacatenar metallomesogens 4d - g exhibit a hexagonal columnar mesophase. Further increasing the length of the alkyl chain as in the case of 4h suppresses liquid crystallinity and induces only a crystalline phase. This unique behavior in the ferrocene containing metallomesogenic molecules can be understood to originate from the anisotropic aggregation of ferrocene containing aromatic segments and consequent entropic penalties associated with chain stretching.

Full text of DOI:10.1039/b008904o

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1,1’-Disubstituted ferrocene containing hexacatenar thermotropic
liquid crystals{
Jin-Sung Seo, Yong-Sik Yoo and Moon-Gun Choi*
Department of Chemistry and Molecular Structure Laboratory, Yonsei University, 134
Shinchon-dong, Sodaemoon-ku, Seoul 120-749, Korea. E-mail: choim@yonsei.ac.kr
Received 7th November 2000, Accepted 20th February 2001
First published as an Advance Article on the web 23rd March 2001
The preparation and characterization of ferrocene containing hexacatenar metallomesogens 4a–h based on
alkoxy terminal groups with hexyloxy (4a), octyloxy (4b), decyloxy (4c), dodecyloxy (4d), tetradecyloxy (4e),
hexadecyloxy (4f), octadecyloxy (4g) and eicosyloxy (4h) chains are described. The introduction of alkyl chains
of different lengths induces a rich variety of self-assembled liquid crystalline structures. In the crystalline state,
the ferrocene containing hexacatenar metallomesogens 4a–h organize into a microphase-separated monolayer
lamellar structure. In contrast, a dramatic phase change after crystalline melting of the metallomesogens is
observed with variation of the chain length. The ferrocene containing hexacatenar metallomesogens 4b and c
display a bicontinuous cubic mesophase with Ia3d symmetry, while the ferrocene containing hexacatenar
metallomesogens 4d–g exhibit a hexagonal columnar mesophase. Further increasing the length of the alkyl
chain as in the case of 4h suppresses liquid crystallinity and induces only a crystalline phase. This unique
behavior in the ferrocene containing metallomesogenic molecules can be understood to originate from the
anisotropic aggregation of ferrocene containing aromatic segments and consequent entropic penalties associated
with chain stretching.
and has a more or less bent molecular structure, this offers
Introduction
multiple possibilities for fine tuning of the mesomorphic
properties through connecting rod-like mesogenic units.
The goal of this article is to describe the synthesis and
characterization of hexa-substituted rod-like metallomesogens
derived from ferrocene-1,1’-dicarbonyl dichloride. We also
describe the mesomorphic phase behavior of the resulting
metallomesogens, characterized by a combination of DSC,
optical polarized microscopy and powder X-ray scattering
experiments.
Self-organizing materials which include liquid crystalline
molecules, block copolymers, hydrogen bonded complexes,
and many biological polymers have been widely studied and
observed to have great potential for various functional
materials.1 Manipulation of supramolecular nanostructure in
self-organizing materials is of critical importance to achieving
the desired functions and properties in solid state and liquid
crystalline state molecular materials.1–3 Thus, diverse mole-
cular architectures are being explored as a means to manipulate
the supramolecular structure which has dramatic effects on the
physical properties of molecules. A typical example of self-
assembling systems is provided by metallomesogenic molecules
consisting of organic ligands and transition metals.4 Metallo-
mesogenic molecules containing transition metals which share
certain general characteristics of both organic molecules and
the metals provide typical examples for liquid crystals with high
intermolecular interactions. Linear and square planar coordi-
nations of metal atoms into conventional organic mesogens
have been used to increase the aspect ratio of the aromatic core
and therefore to enhance the thermal stability of both crystal
and liquid crystal phases.
Experimental
Materials
Pyridine was distilled from Na–benzophenone, CH2Cl2 and
˚
triethylamine from CaH2. The solvents were stored over 4 A
molecular sieve under N2. Ferrocene-1,1’-dicarbonyl dichloride
was prepared following the literature procedure.14 All reactions
were carried out under nitrogen. Propyl 4-hydroxybenzoate
(99%) propyl gallate (97%), 1,3-diisopropylcarbodiimide
(DIPC, 99%), 4-(dimethylamino)pyridine (99%), 4,4’-biphenyl-
diol (97%), 1-bromohexane (98%), 1-bromooctane (99%), 1-
bromodecane (98%), 1-bromododecane (97%), 1-bromotetra-
decane (97%), 1-bromohexadecane (97%), 1-bromooctadecane
(96%), 1-bromoeicosane (98%), 1-bromodocosane (96%) (all
from Aldrich), ferrocene (99%) (from Strem chemicals) and the
other conventional reagents were used as recieved.
A large and predominant class of metallomesogens is
ferrocene containing metallomesogens, ferrocene being a
sandwich compound consisting of two cyclopentadienyl rings
and one iron atom as center.5–13 Ferrocene containing
metallomesogens offer high thermal stability and low oxida-
tion–reduction potential. Since the ferrocene moiety is bulky
Techniques
{Electronic supplementary information (ESI) available: 1H NMR data
of compounds 2b–h and 3b–h, a DSC diagram exhibited during the first
heating scan by ferrocene containing hexacatenar metallomesogens,
and the X-ray pattern of 4a in a crystalline phase. See http://
www.rsc.org/suppdata/jm/b0/b008904o/
1H-NMR spectra were recorded from CDCl3 solution on a
Bruker DPX-250 spectrometer. TMS was used as internal
standard. A Perkin Elmer DSC-7 differential scanning
calorimeter, equipped with a 1020 thermal analysis controller
1332
J. Mater. Chem., 2001, 11, 1332–1338
DOI: 10.1039/b008904o
This journal is # The Royal Society of Chemistry 2001

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was used to determine the thermal transitions which were
reported as maxima of their endothermic or exothermic peaks,
respectively. In all cases, heating and cooling rate were
10 ‡C min21 unless otherwise specified. A Nikon Optiphot 2-
pol optical polarized microscope (magnification:1006)
equipped with a Mettler FP 82 hot stage and a Mettler FP90
central processor was used to observe the thermal transitions
and to analyze the anisotropic textures. Microanalyses were
performed with a Perkin Elmer 240 elemental analyzer at the
Korea Research Institute of Chemical Technology. X-Ray
scattering measurements were performed in transition mode
with synchrotron radiation at the 3C2 X-ray beam line at
Pohang Accelerator Laboratory, Korea. In order to investigate
structural changes on heating, the samples were held in an
aluminium sample holder which was sealed with a window of
8 mm thick Kapton films on both sides. The samples were
heated with two cartridge heaters and the temperature of the
samples was monitored by a thermocouple placed close to the
sample. Background scattering correction was made using the
scatterings from the Kapton. The correction for the slit length
smearing was applied by means of Strobl’s algorithm.
and 5 ml of pyridine under nitrogen. 1,3-Diisopropylcarbodi-
imide (DIPC, 2.1 ml, 10.7 mmol) was added dropwise to the
mixture. The mixture was stirred at room temperature under
nitrogen overnight and neutralized in 1 M HCl. The resulting
solution was poured into water and extracted with methylene
chloride. The methylene chloride layer was washed with water,
dried over anhydrous magnesium sulfate and filtered. The
solvent was removed under reduced pressure and the crude
product was purified by flash column chromatography (silica
gel, 40–60 mm, using methylene chloride as the eluent) to yield
1
1.9 g (yield: 46%) of a white solid. H-NMR (CDCl3, TMS, d,
ppm): 0.85–0.88 (m, 9H, CH3), 1.33–1.56 (m, 18H,
CH3(CH2)3), 1.74–1.86 (m, 6H, OCH2CH2), 4.03–4.09 (m,
6H, OCH2), 4.93 (s, 1H, OH), 6.90 (d, 2Ar-H, m to OH,
J~8.6 Hz), 7.22 (d, 2Ar-H, m to O2C, J~8.6 Hz), 7.42 (s, 2Ar-
H, o to CO2), 7.47 (d, 2Ar-H, o to OH, J~8.6 Hz), 7.58 (d,
2Ar-H, o to O2C, J~8.6 Hz).
Compounds 2b–2h were prepared by a similar method to
that described for compound 2a. Their 1H NMR data are
available as Electronic Supplementary Information.{
4-Benzyloxybenzoic acid. 4-Hydroxybenzoic acid n-propyl
ester (1 g, 5.5 mmol), K2CO3 (2.8 g, 20.6 mmol) were dissolved
in 100 ml of dry THF under nitrogen. Benzyl bromide (3.5 g,
20.6 mmol) was then added to the mixture. The mixture was
heated at reflux for 18 h and then cooled to room temperature.
The solvent was removed under reduced pressure and excess
KOH aqueous solution added. The mixture was dissolved in
100 ml of ethanol. The mixture solution was stirred at room
temperature for 12 h and neutralized with HCl. The resulting
solution was poured into water and extracted with methylene
chloride. The methylene chloride layer was washed with water,
dried over anhydrous magnesium sulfate, and filtered. The
solvent was removed on a rotary evaporator to yield 1.1 g
Synthesis
A general outline of the synthetic procedures is shown in
Scheme 1.
3,4,5-Trialkoxybenzoic acid (1a–1h). 1a–1h were synthesized
according to the procedure reported in a previous publica-
tion.15
4’-Hydroxybiphenyl 3,4,5-tris(hexyloxy)phenylbenzoate (2a).
Compound 1a (3 g, 7.1 mmol), 4,4’-biphenyldiol (6.6 g,
35.5 mmol) and 4-dimethylaminopyridine (DMAP, 1.3 g,
10.7 mmol) were dissolved in 30 ml of methylene chloride
Scheme 1 Synthesis of the ferrocene containing hexacatenar metallomesogens.
J. Mater. Chem., 2001, 11, 1332–1338
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1
(84%) of a white solid. H-NMR (CDCl3, TMS, d, ppm): 5.14
(s, 2H, CH2O), 7.04 (d, 2Ar-H, m to COOH, J~8.8 Hz), 7.34–
7.45 (m, 5Ar-H, Ph-CH2O), 8.09 (d, 2Ar-H, o to COOH,
J~8.8 Hz).
4Ar-H, o to O2C-Cp, J~8.7 Hz), 7.42 (s, 4Ar-H, tris(octyloxy)-
phenyl), 7.61–7.65 (m, 8Ar-H, 2 o (biphenyl) to CO2), 8.27 (d,
4Ar-H, m to OH, J~8.7 Hz); Anal. Calcd for C110H146O10Fe:
C, 73.58; H, 7.61. Found C, 73.54; H, 7.61%.
Bis{4-[4’-(3@,4@,5@-tris(decyloxy)phenylcarbonyloxy)biphenyl-
4’-(4@-Hydroxyphenylcarbonyloxy)biphenyl 3,4,5-tris(hexyl-
oxy)benzoate (3a). Compound 2a (1 g, 1.7 mmol), 4-benzyl-
oxybenzoic acid (0.58 g, 2.5 mmol) and 4-dimethylamino-
pyridine (0.30 ml, 2.5 mmol) were dissolved in 30 ml of dry
methylene chloride under nitrogen, then 1,3-diisopropyl-
carbodiimide (0.50 ml, 2.5 mmol) was added dropwise to the
mixture. The mixture was stirred at room temperature under
nitrogen overnight. The resulting solution was poured into
water and extracted with methylene chloride. The methylene
chloride solution was washed with water, dried over anhydrous
magnesium sulfate, and filtered. The solvent was removed
under reduced pressure and the crude compound was isolated
by flash column chromatograpy (silica gel, 40–60 mm, using
methylene chloride as the eluent) to yield a white solid. The
solid was dissolved in methylene chloride (80 ml) and methanol
(20 ml) and then 5% palladium-on-charcoal catalyst (30 mg)
was added. The mixture was stirred under a hydrogen
atmosphere until no further hydrogen was taken up. Solvents
were removed under reduced pressure and the residue was
dissolved in methylene chloride. The catalyst was removed by
filtration through Celite and the solvent was removed by
evaporation under reduced pressure. After recrystallization
from ethanol a white solid was obtained (0.95 g, yield: 76%).
1H-NMR (CDCl3, TMS, d, ppm): 0.91–0.94 (m, 9H, CH3),
1.25–1.56 (m, 18H, CH3(CH2)3), 1.72–1.90 (m, 6H,
OCH2CH2), 4.04–4.11 (m, 6H, OCH2), 6.1 (s, 1H, OH), 6.82
(d, 2Ar-H, o to OH, J~8.6 Hz), 7.29 (m, 4Ar-H, 2 m to O2C),
7.44 (s, 2Ar-H, tris(hexyloxy)phenyl), 7.60–7.64 (m, 4Ar-H, 2o
(biphenyl) to CO2), 8.07 (d, 2Ar-H, m to OH, J~8.6 Hz).
Compounds 3b–3h were prepared by a similar method to
that described for compound 3a. Their 1H NMR data are
available as Electronic Supplementary Information.{
oxycarbonyl]phenyl}ferrocene-1,1’-dicarboxylate
(4c). Yield:
75%; 1H-NMR (CDCl3, TMS, d, ppm): 0.85–0.90 (m, 18H,
CH3), 1.26–1.55 (m, 84H, CH3(CH2)7), 1.74–1.86 (m, 12H,
OCH2CH2), 4.02–4.09 (m, 12H, OCH2), 4.68 (t, 4Cp,
b(ferrocene) to CO2, J~1.9 Hz), 5.11 (t, 4Cp, a(ferrocene) to
CO2, J~1.9 Hz), 7.28–7.31 (m, 8Ar-H, 2 m to O2C), 7.39 (d,
4Ar-H, o to O2C-Cp, J~8.7 Hz), 7.42 (s, 4Ar-H, tris(decyloxy)-
phenyl), 7.61–7.67 (m, 8Ar-H, 2 o (biphenyl) to CO2), 8.27 (d,
4Ar-H, m to OH, J~8.6 Hz); Anal. Calcd for C122H170O10Fe:
C, 74.59; H, 8.17. Found C, 74.47; H, 8.54%.
Bis{4-[4’-(3@,4@,5@-tris(dodecyloxy)phenylcarbonyloxy)biphe-
nyloxycarbonyl]phenyl}ferrocene-1,1’-dicarboxylate
(4d). Yield: 68%; H-NMR (CDCl3, TMS, d, ppm): 0.85–0.90
1
(m, 18H, CH3), 1.26–1.50 (m, 108H, CH3(CH2)9), 1.74–1.86
(m, 12H, OCH2CH2), 4.02–4.09 (m, 12H, OCH2), 4.68 (t, 4Cp,
b(ferrocene) to CO2, J~1.9), 5.13 (t, 4Cp, a(ferrocene) to CO2,
J~1.9 Hz), 7.28–7.31 (m, 8Ar-H, 2 m to O2C), 7.35 (d, 4Ar-H,
o to O2C-Cp, J~8.7 Hz), 7.39 (s, 4Ar-H, tris(dodecyloxy)phe-
nyl), 7.61–7.68 (m, 8Ar-H, 2 o (biphenyl) to CO2), 8.27 (d, 4Ar-
H, m to OH, J~8.7 Hz); Anal. Calcd for C134H194O10Fe: C,
75.45; H, 8.66. Found: C, 75.49; H, 9.10%.
Bis{4-[4’-(3@,4@,5@-tris(tetradecyloxy)phenylcarbonyloxy)bi-
phenyloxycarbonyl]phenyl}ferrocene-1,1’-dicarboxylate
(4e). Yield: 72%; H-NMR (CDCl3, TMS, d, ppm): 0.85–0.90
1
(m, 18H, CH3), 1.18–1.49 (m, 132H, CH3(CH2)11), 1.71–1.86
(m, 12H, OCH2CH2), 4.02–4.09 (m, 12H, OCH2), 4.69 (t, 4Cp,
b(ferrocene) to CO2, J~1.9 Hz), 5.13 (t, 4Cp, a(ferrocene) to
CO2, J~1.9 Hz), 7.27–7.34 (m, 8Ar-H, 2 m to O2C), 7.39 (d,
4Ar-H, o to O2C-Cp, J~8.7 Hz), 7.42 (s, 4Ar-H, tris(tetrade-
cyloxy)phenyl), 7.62–7.68 (m, 8Ar-H, 2 o (biphenyl) to CO2),
8.27 (d, 4Ar-H, m to OH, J~8.7 Hz); Anal. Calcd for
C146H218O10Fe: C, 76.18; H, 9.07. Found C, 76.15; H, 9.15%.
Bis{4-[4’-(3@,4@,5@-tris(hexyloxy)phenylcarbonyloxy)biphenyl-
oxycarbonyl]phenyl}ferrocene-1,1’-dicarboxylate (4a). Ferro-
cene-1,1’-dicarbonyl dichloride (0.4 g, 1.3 mmol) and triethy-
lamine (2 ml) in 30 ml of methylene chloride were added to
compound 3a (1.6 g, 2.3 mmol) in 10 ml of methylene chloride.
The reaction mixture was refluxed for 8 h, and the solvent was
removed by evaporation under reduced pressure, and the crude
product was purified by column chromatography (silica gel,
40–60 mm, using methylene chloride as the eluent) to yield
1.38 g (yield: 71%) of an orange solid. 1H-NMR (CDCl3, TMS,
d, ppm): 0.87–0.90 (m, 18H, CH3), 1.26–1.53 (m, 36H,
CH3(CH2)3), 1.72–1.87 (m, 12H, OCH2CH2), 4.05–4.07 (m,
12H, OCH2), 4.67 (t, 4Cp, b(ferrocene) to CO2, J~1.9 Hz),
5.13 (t, 4Cp, a(ferrocene) to CO2, J~1.9 Hz), 7.28–7.31 (m,
8Ar-H, 2 m to O2C), 7.39 (d, 4Ar-H, o to O2C-Cp, J~8.7 Hz),
7.42 (s, 4Ar-H, tris(hexyloxy)phenyl), 7.61–7.64 (m, 8Ar-H, 2 o
(biphenyl) to CO2), 8.27 (d, 4Ar-H, m to OH, J~8.7 Hz); Anal.
Calcd for C98H130O10Fe: C, 72.18; H, 6.96. Found: C, 72.35; H,
6.92%.
Bis{4-[4’-(3@,4@,5@-tris(hexadecyloxy)phenylcarbonyloxy)bi-
phenyloxycarbonyl]phenyl}ferrocene-1,1’-dicarboxylate
(4f). Yield: 60%; H-NMR (CDCl3, TMS, d, ppm): 0.85–0.90
1
(m, 18H, CH3), 1.18–1.49 (m, 156H, CH3(CH2)13), 1.74–1.86
(m, 12H, OCH2CH2), 4.02–4.06 (m, 12H, OCH2), 4.69 (t, 4Cp,
b(ferrocene) to CO2, J~1.9 Hz), 5.13 (t, 4Cp, a(ferrocene) to
CO2, J~1.9 Hz), 7.27–7.34 (m, 8Ar-H, 2 m to O2C), 7.39 (d,
4Ar-H, o to O2C-Cp, J~8.7 Hz), 7.42 (s, 4Ar-H, tris(hexa-
decyloxy)phenyl), 7.62–7.68 (m, 8Ar-H, 2 o (biphenyl) to CO2),
8.27 (d, 4Ar-H, m to OH, J~8.7 Hz); Anal. Calcd for
C
158H242O10Fe: C, 76.82; H, 9.42. Found C, 76.79; H, 9.56%.
Bis{4-[4’-(3@,4@,5@-tris(octadecyloxy)phenylcarbonyloxy)bi-
phenyloxycarbonyl]phenyl}ferrocene-1,1’-dicarboxylate
(4g). Yield: 73%; H-NMR (CDCl3, TMS, d, ppm): 0.85–0.90
1
(m, 18H, CH3), 1.18–1.49 (m, 180H, CH3(CH2)15), 1.74–1.86
(m, 12H, OCH2CH2), 4.02–4.09 (m, 12H, OCH2), 4.69 (t, 4Cp,
b(ferrocene) to CO2, J~1.9 Hz), 5.13 (t, 4Cp, a(ferrocene) to
CO2, J~1.9 Hz), 7.28–7.34 (m, 8Ar-H, 2 m to O2C), 7.39 (d,
4Ar-H, o to O2C-Cp, J~8.7 Hz), 7.42 (s, 4Ar-H, tris(octade-
cyloxy)phenyl), 7.61–7.65 (m, 8Ar-H, 2 o (biphenyl) to CO2),
8.27 (d, 4Ar-H, m to OH, J~8.7 Hz); Anal. Calcd for
Compounds 4b–4h were prepared by a similar method to
that described for compound 4a.
Bis{4-[4’-(3@,4@,5@-tris(octyloxy)phenylcarbonyloxy)biphenyl-
oxycarbonyl]phenyl}ferrocene-1,1’-dicarboxylate (4b). Yield:
70%; 1H-NMR (CDCl3, TMS, d, ppm): 0.87–0.90 (m, 18H,
CH3), 1.26–1.55 (m, 60H, CH3(CH2)5), 1.72–1.87 (m, 12H,
OCH2CH2), 4.02–4.09 (m, 12H, OCH2), 4.67 (t, 4Cp,
b(ferrocene) to CO2, J~1.9 Hz), 5.12 (t, 4Cp, a(ferrocene) to
CO2, J~1.9 Hz), 7.28–7.31 (m, 8Ar-H, 2 m to O2C), 7.39 (d,
C
170H266O10Fe: C, 77.37; H, 9.74. Found C, 77.34; H, 9.72%.
Bis{4-[4’-(3@,4@,5@-tris(eicosyloxy)phenylcarbonyloxy)biphe-
nyloxycarbonyl]phenyl}ferrocene-1,1’-dicarboxylate
(4h). Yield: 66%; H-NMR (CDCl3, TMS, d, ppm): 0.85–0.90
(m, 18H, CH3), 1.18–1.49 (m, 204H, CH3(CH2)17), 1.74–1.86
(m, 12H, OCH2CH2), 4.02–4.09 (m, 12H, OCH2), 4.69 (t, 4Cp,
1
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J. Mater. Chem., 2001, 11, 1332–1338

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b(ferrocene) to CO2, J~1.9 Hz), 5.13 (t, 4Cp, a(ferrocene) to
CO2, J~1.9 Hz), 7.28–7.34 (m, 8Ar-H, 2 m to O2C), 7.39 (d,
4Ar-H, o to O2C-Cp, J~8.7 Hz), 7.42 (s, 4Ar-H, tris(eicosyloxy)-
phenyl), 7.61–7.65 (m, 8Ar-H, 2 o (biphenyl) to CO2), 8.27 (d,
4Ar-H, m to OH, J~8.7 Hz); Anal. Calcd for C182H290O10Fe:
C, 77.86; H, 10.01. Found C, 77.79; H, 10.18%.
Table 1 Thermal transitions of the ferrocene containing hexacatenar
metallomesogensa
Phase transition/‡C
(corresponding enthalpy changes/kJ mol21 a
)
Compound
Heating
Cooling
4a
4b
k 200.2 (69.8) i
k 162.9 (69.4) bcc
172.9 (7.2) i
i 159.6 (78.6) k
i 158.0 (2.2) bcc
143.8 (67.0) k
Results and discussion
4c
4d
4e
4f
k 126.5 (128.7) bcc
174.6 (4.4) i
k 117.1 (98.9) col
172.3 (1.6) i
k 112.6 (82.9) col
170.7 (1.8) i
k 120.4 (110.0) col
171.9 (1.4) i
i 162.2 (3.2) bcc
Synthesis and thermal characterization
The synthesis of ferrocene containing hexacatenar metallome-
sogens was performed according to the procedure described in
Scheme 1. The metallomesogenic precursor molecules 3a–h
were synthesized by esterification of 3,4,5-trialkoxy substituted
benzoic acid with biphenyldiol and another esterification with
4-benzyloxybenzoic acid. The resulting precursors 3a–h were
treated with ferrocene-1,1’-dicarbonyl dichloride to yield
ferrocene containing hexacatenar metallomesogens 4a–h
based on alkoxy terminal groups from hexyloxy to dodecyloxy.
All analytical data of the metallomesogenic molecules are
consistent with expected structures.
i 165.1 (1.4) col
i 164.7 (1.7) col
i 169.0 (2.1) col
62.7 (69.0) k
i 169.5 (2.0) col
92.8 (122.8) k
i 97.0 (135.5) k
58.7 (34.2) k
4g
4h
k 120.9 (120.4) col
172.9 (1.6) i
k 118.5 (122.0) i
ak~crystalline, bcc~bicontinuous cubic, col~hexagonal columnar,
i~isotropic.
The thermotropic phase behavior of the ferrocene containing
metallomesogens was investigated by
a combination of
exhibits a crystalline melting at 117.1 ‡C, followed by a
mesophase which, in turn, undergoes transformation into
isotropic liquid at 172.3 ‡C. Optical microscopic observations
of this compound are consistent with this behavior. Transition
from an isotropic liquid can be seen by the formation of
dendritic domains which merge into a pseudo-focal conic
texture which is characteristic of a hexagonal columnar
mesophase exhibited by other hexagonal columnar liquid
crystals.17,18 The ferrocene complexes 4e, 4f and 4g also show
phase behavior similar to that of the complex 4d which exhibits
a crystalline melting followed by a hexagonal columnar
mesophase. Fig. 2 shows a representative texture exhibited by
the hexagonal columnar liquid crystalline phase of 4d. Further
increasing the length of flexible coil as in the case of 4h
suppresses the formation of liquid crystallinity. This is most
probably due to the high entropic contribution of the long
terminal chains on the melt state.
The thermal behavior of the metallomesogens based on
ferrocene determined from heating and cooling DSC curves as
well as thermal optical polarized microscopy is presented in
Fig. 3. As shown in Fig. 3, the transition temperatures
associated with the crystalline melting decrease and the
mesophase temperature ranges become broader with increasing
length of terminal chains. A notable feature in this figure is the
existence of a cubic phase in the compounds containing short
terminal carbon chains. Similar to lyotropic and conventional
techniques consisting of differential scanning calorimetry
(DSC), thermal optical polarized microscopy and X-ray
scattering experiments. Fig. 1 presents the DSC heating and
cooling curves of the metallomesogens. The transition
temperatures and the corresponding enthalpy changes of all
complexes obtained from DSC are summarized in Table 1. In
contrast to the precursor molecules which exhibit only a
crystalline phase, all the metallomesogens except 4a and 4h
exhibit a thermotropic liquid crystalline phase. As can be
observed from Fig. 1 and Table 1, 4a based on hexyloxy
terminal chains exhibits only
a crystalline melting at
200.2 ‡C. In contrast, 4b based on octyloxy terminal chains
shows a crystalline melting at 162.9 ‡C, followed by a
mesophase which, in turn, undergoes transformation into an
isotropic liquid at 172.9 ‡C. 4c shows a similar phase behavior
to that of 4b which exhibits a crystalline melting followed by a
mesophase. On optical microscopy, however, no birefringence
between cross polarizers after melting of 4b and 4c could be
observed, strongly suggesting the existence of an optically
isotropic cubic mesophase.16
The metallomesogen 4d based on dodecyloxy terminal chains
Fig. 2 Representative optical micrography (1006) of texture exhibited
by the hexagonal columnar mesophase of 4f at 168 ‡C on a cooling
scan.
Fig. 1 DSC diagram exhibited during (a) the second heating scan and
(b) the first cooling scan by ferrocene containing hexacatenar
metallomesogens.
J. Mater. Chem., 2001, 11, 1332–1338
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Fig. 3 Dependence of the phase-transition temperature of 4a–h. Data
are obtained from the DSC heating scan. The open symbols for n~11
are obtained from the equimolar mixture of 4c and 4d (k, crystalline;
bcc, bicontinuous cubic; col, columnar; i, isotropic).
block copolymer systems,19,20 the topology in this region
should be a bicontinuous cubic phase since such a structure can
attain a smaller interfacial curvature than those of the
columnar phase. Therefore, we assume that the complexes
based on octyloxy and decyloxy terminal chains exhibit a
bicontinuous cubic phase. Interestingly, the equimolar mixture
of 4c and 4d exhibits both a cubic phase at lower temperatures
and a columnar phase at higher temperatures.
Fig. 4 SAXS spectra obtained in (a) the bicontinuous cubic mesophase
at 150 ‡C for 4c and (b) the hexagonal columnar mesophase at 150 ‡C
for 4d [q~4psin(h/l)].
X-Ray diffraction studies
To investigate the self-assembled crystalline and liquid crystal-
line states, X-ray scattering experiments have been performed
with the selected complexes at various temperatures. The first
order spacings and the lattice constants determined by small
angle X-ray diffractions are summarized in Table 2. In the
crystalline state, the X-ray diffraction pattern of the complex
4a displays three reflections in the spacing ratio of 1, 1/2, 1/3 in
the small angle region, while a number of sharp reflections are
observed in the wide angle region, indicative of a lamellar
structure consisting of a microphase separated aromatic rod
domains and the aliphatic chain domains with a lattice
˚
parameter for the cubic phase of 4c is 136.7 A. At a wide angle
only a diffuse halo remains for all the rod–coil molecules as
evidence of the lack of any positional long-range order other
than the three dimensional cubic packing of supramolecular
units (Fig. 5).
On the basis of the X-ray diffraction data described above
and its position in the phase diagram, located between lamellar
crystalline and hexagonal columnar liquid crystalline struc-
tures, the cubic phase can be best described as a bicontinuous
cubic phase with Ia3d symmetry occurring frequently in
lyotropic systems.20 Thus a similar model proposed for the
lyotropic bicontinuous cubic phase may be used for its
description. Although a bicontinuous cubic phase with Ia3d
symmetry is commonly observed in both lyotropic amphiphilic
systems and conventional diblock copolymers, it has been
observed in relatively few thermotropic systems such as
amphiphilic glucitol derivatives,21 polycatenar silver com-
plexes,22 and glycolipid derivatives.23 In particular, formation
of thermotropic bicontinuous cubic phases in ferrocene
containing metallomesogenic systems is very rare.
˚
constant of 34.9 A. The layer spacings are close to the
corresponding estimated repeating unit length of the metallo-
mesogenic molecules, indicative of a monolayer lamellar
structure in which rod units are fully interdigitated.
The X-ray diffraction pattern of 4c in its liquid crystalline
state displays several sharp reflections in the small angle region
as shown in Fig. 4. The relative positions of these reflections are
pffiffi pffiffi pffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffi pffiffiffiffiffi
1/ 6, 1/ 8, 1/ 14, 1/ 16, 1/ 22, 1/ 24 which can be indexed
as the 211, 220, 321, 400, 332 and 442 reflections of a body
centered cubic phase with Ia3d symmetry.12 From the observed
d-spacing of the 211 reflection, the best fit value for the lattice
The X-ray diffraction pattern of the birefringent mesophase
of 4d displays three sharp reflections with the ratio of 1 : 1/
pffiffi pffiffi
3 : 1/ 4 in the small angle region, characteristic of the two-
dimensional hexagonal structure (hexagonally packed supra-
Table 2 Characterization of the ferrocence containing hexacatenar
metallomesogens by small angle X-ray scattering
17
molecular cylinders) with a lattice constant of 64.3 A. The
˚
Liquid crystalline phase
Crystalline
phase
observed d-spacings and the lattice constants are given in
Table 2 and a representative small angle X-ray diffraction
pattern of 4d is shown in Fig. 4. At the wide angles only a
diffuse halo remains as evidence of a lack of any positional long
range order other than the two dimensional hexagonal packing
of supramolecular columns (Fig. 5). These results together with
optical microscopic observations indicate that this complex
displays a disordered hexagonal columnar mesophase.
Cubic
Columnar
Lamellar
˚
Lattice Lattice
d211/A constant a/A d100/A constant a/A
˚
˚
˚
˚
Compound d001/A
4a
4c
4d
4h
34.9
69.9
55.8
136.7
55.7
64.3
On the basis of the X-ray results, schematic representations
can be constructed as shown in Fig. 6. A notable feature of our
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structures with a larger interfacial area, instead of a layered
smectic structure.
The results described in this paper demonstrate that
systematic variation in the length of coil in the ferrocene
containing metallomesogenic system can change the supramo-
lecular architecture from lamellar, bicontinuous cubic to
hexagonal columnar phases. The main organizing forces
responsible for this change are believed to be the anisotropic
orientation of ferrocene containing rod segments and conse-
quent entropic penalties associated with chain stretching.
Conclusion
We have prepared hexacatenar metallomesogens based on
ferrocene. The resulting metallomesogens were characterized
by DSC, thermal optical microscopy and X-ray scattering
measurements. In the crystalline state, the metallomesogens
have been observed to organize into a microphase separated
monolayer lamellar structure. In the liquid crystalline state, the
metallomesogens with octyloxy and decyloxy terminal groups
4b and 4c self-assemble into an optically isotropic cubic
mesophase. This phase has been identified by X-ray scattering
methods to be a bicontinuous cubic phase with Ia3d symmetry.
Increasing the length of the terminal chains induces a
hexagonal columnar mesophase as exhibited by 4d, 4e, 4f
and 4g. Further increasing the length of the terminal chains as
in the case of 4h suppresses liquid crystallinity and gives rise to
only a crystalline phase. This unique phase behavior in the
ferrocene containing metallomesogens can be understood to
originate from the anisotropic aggregation of rod segments and
consequent entropic penalties associated with chain stretching.
These results demonstrate that a rich variety of self-assembled
supramolecular structures can be induced in the rod-like
metallomesogenic systems through the introduction of differ-
ent lengths of terminal chains.
Fig. 5 WAXS patterns of (a) the 4c and (b) the 4d at a liquid crystalline
phase [q~4psin(h/l)].
system consists in the ability of the metallomesogens based on
ferrocene building blocks to self-assemble into ordered
structures with curved interfaces in their liquid crystalline
states. Formation of bicontinuous cubic and hexagonal/
columnar assemblies in the metallomesogens is in contrast to
the general behavior of conventional liquid crystalline
molecules based on rigid rod-like mesogens.24 The molecular
organization of the metallomesogens into the bicontinuous
cubic or the columnar structures is also remarkably different
from that usually observed in those of block polymers based on
immiscible coil blocks since the ferrocene containing aromatic
rod segments consisting of a core domain favor anisotropic
aggregation with their long axes even in the melt state, rather
than isotropic aggregation.
Acknowledgements
This work was supported by the Korea Science and Engineer-
ing Foundation (2000) and CRM-KOSEF (2000).
Formation of the ordered structures with interfacial
curvature from the rod-like metallomesogens can be rationa-
lized by considering the entropic penalties associated with the
chain stretching and anisotropic arrangement of aromatic rod
segments. Trisubstituted flexible alkoxy chains induce curva-
ture at the aromatic/aliphatic interface, arising from the
connectivity of the stiff rod and flexible chains, constraint of
constant density, and minimization of chain stretching. At the
interface separating the rod and chain domains in the layered
structure, the relatively smaller area per junction favored by
rod–block results in chain stretching of the aliphatic segments,
which is energetically unfavorable. Therefore, the molecules
self-assemble into bicontinuous cubic or hexagonal columnar
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