Ferrocene-containing liquid-crystalline dendrimers: A novel family of mesomorphic macromolecules

Article abstract of DOI:10.1039/a702850d

The synthesis and liquid-crystalline properties of a first-generation dendritic core substituted with six mesomorphic ferrocene units are reported; the investigated compound exhibited a broad enantiotropic smectic A phase (polarized optical microscopy, DSC, and XRD studies) and good thermal stability (thermogravimetry).

Full text of DOI:10.1039/a702850d

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Ferrocene-containing liquid-crystalline dendrimers: a novel family of
mesomorphic macromolecules
Robert Deschenaux,*a Elisabeth Serranoa and Anne-Marie Levelutb
a
Institut de Chimie, Universite´ de Neuchaˆtel, Av. de Bellevaux 51, 2000 Neuchaˆtel, Switzerland
Universite´ Paris-Sud, Laboratoire de Physique des Solides, Baˆtiment 510, 91405 Orsay Cedex, France
b
The synthesis and liquid-crystalline properties of a first-
generation dendritic core substituted with six mesomorphic
ferrocene units are reported; the investigated compound
CO2R
Fe
HO
O2C
exhibited a broad enantiotropic smectic A phase (polarized
optical microscopy, DSC, and XRD studies) and good
thermal stability (thermogravimetry).
2
i
3
CO2R
Fe
Increasing interest is currently accorded to the design and
synthesis of dendrimers.1 The possibility of creating new
materials, characterized by a well defined and aesthetic
macromolecular structure, displaying unique properties (such as
catalysis,2 encapsulation,3 chiroptical activity4 and redox
activity5) strongly motivated these studies.
O2C
XO
O
(CH2)12
O
CO2
4a X = Bn
4b X = OH
ii
iii
5
CO2R
CO2R
Liquid-crystalline dendrimers have been recently reported.
An AB2 rod-like mesogenic monomer was used to synthesize
dendrimers up to the fourth generation.6 Nematic and smectic
phases were observed for these materials. First-7 and second-
generation8 polysiloxane dendrimers were functionalized by
mesogenic moieties, and showed smectic A or smectic C
phases. Metal complexes were prepared from various metal ions
and first- or second-generation ethyleneimine dendrimers and a
hexagonal columnar phase was identified for a CuII complex.9
These novel structures form a most interesting class of
anisotropic materials from a fundamental and practical view-
point: the structural features of such compounds should lead to
important basic information for a better understanding of the
structure–mesomorphic properties relationship for liquid-crys-
talline macromolecules and, ultimately, could be exploited for
elaborating new liquid-crystalline devices. To our knowledge,
organometallic-type liquid-crystalline dendrimers have not
been reported.
Fe
Fe
O2C
O2C
O
O
(CH2)12
(CH2)12
O
CO2
CO2
CO2
CO2
XO
O
iv
6a X = Bn
6b X = OH
7
v
O
O
O
O
To further explore the capability of mesogenic ferrocene
derivatives to act as mesomorphic building blocks for the
construction of liquid-crystalline macromolecules,10 we ex-
tended our interest to ferrocene-containing thermotropic den-
drimers. This communication describes the synthesis and
mesomorphic properties of the ferrocene-containing thermo-
tropic dendrimer 1, which represents the first organometallic-
type liquid-crystalline dendrimer.
O
O
CO2R
RO2C
Fe
O
O
Fe
O
O
O(CH2)12O
CO2
O2C
O
O
O2C
O2C
O(CH2)12O
O
O
O
O
O
O
A first-generation dendrimer was selected to explore the
thermal and liquid-crystalline behavior of such a new structure.
A ferrocene-based cholesterol framework was used as a
mesomorphic promoter owing to the high liquid-crystalline
tendency of cholesterol derivatives: recently, we reported the
first fullerene[60]-containing thermotropic liquid crystal ob-
tained by derivatizing the C60 core with a twin-cholesterol
derivative.11
1
The synthesis of 1 is outlined in Scheme 1. Condensation of
ferrocene derivative 2† with 4-(4A-benzyloxyphenoxy)-
dodecyloxybenzoic acid chloride 3† gave the protected ferro-
cene-containing unramified sub-unit 4a. Deprotection of the
latter species under standard conditions gave the phenol
derivative 4b, which was subsequently reacted with 5-benzyl-
oxyisophthaloyl dichloride 5† to furnish the protected ferro-
cene-containing ramified branch 6a. Removal of the benzyl
protecting group under standard conditions led to the phenol
intermediate 6b. Finally, treatment of 6b with 1,3,5-ben-
(CH2)10
O
CO2
R =
Scheme
1 Reagents and conditions: i, 4-(4A-Benzyloxyphenoxy)-
dodecyloxybenzoic acid chloride 3, Et3N, CH2Cl2, reflux, overnight, 96%;
ii, H2, Pd/C, CH2Cl2–EtOH, room temp., overnight, 96%; iii, 5-Benzylox-
yisophthaloyl dichloride 5, Et3N, CH2Cl2, reflux, overnight, 73%; iv, H2,
Pd/C, CH2Cl2–EtOH, room temp., 2 d, 63%; v, 1,3,5-benzenetricarbonyl
trichloride 7, Et3N, CH2Cl2, reflux, 24 h, 86%.
Chem. Commun., 1997
1577

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zenetricarbonyl trichloride 7 gave the targeted ferrocene-
containing dendrimer 1 as an orange solid. The latter species
was purified by column chromatography (silica gel, eluent:
CH2Cl2–acetone–heptane 10:0.5:2 v/v/v) and crystallization
(CH2Cl2–heptane). Its structure and purity were confirmed by
1H NMR spectroscopy and elemental analysis.‡
The thermal and liquid-crystalline properties of 1 were
investigated by a combination of differential scanning calo-
rimetry (DSC), polarized optical microscopy (POM), and X-ray
diffraction studies.§
A glass transition temperature (Tg) was detected by DSC
(second heating run) at 47 °C (onset transition) followed by an
endotherm at 150 °C (peak transition, DH = 69.6 kJ mol21).
POM examinations clearly revealed the formation of a liquid-
crystalline phase between Tg and the endothermic peak. The
latter corresponded to the clearing point. Slow cooling of the
sample from the isotropic liquid into the mesophase gave rise to
a focal conic texture and homeotropic areas. From these
observations, the mesophase was identified as an enantiotropic
smectic A phase.
Footnotes and References
† The syntheses of 2 [from ferrocene-1,1A-dicarboxylic acid chloride,12
cholest-5-en-3b-yl-4-(10-hydroxydecyloxy)benzoate11 and hydroquinone
monobenzyl ether], 3 (from 12-bromododecanol, 4-hydroxymethyl benzo-
ate, and hydroquinone monobenzyl ether), and 5 (from dimethyl 5-hydrox-
yisophthalate and hydroquinone monobenzyl ether) and thermal and liquid-
crystalline properties (smectic A and/or chiral nematic phases) of all
ferrocene intermediates will be described in a full report.
‡ Selected data for 1: 1H NMR (400 MHz, CDCl3): d 9.33 (s, 3 H, arom.),
8.96 (s, 3 H, arom.), 8.37 (s, 6 H, arom.), 8.14 (d, 12 H, arom.), 7.97 (d, 12
H, arom.), 7.28 (s, 24 H, arom.), 7.15 (d, 12 H, arom.), 6.96–6.86 (m, 36 H,
arom.), 5.42 (d, 6 H, CNCH, cholesteryl), 4.97 (t, 12 H, Cp), 4.91 (t, 12 H,
Cp), 4.90–4.70 (sh, 6 H, CHO, cholesteryl), 4.51 (m, 24 H, Cp), 4.20 [t, 12
H, CO2CH2(CH2)9], 4.11–3.97 [33t, 36 H, (CH2)9CH2O and OCH2(CH-
)
2 10CH2O], 2.45 (d, 12 H, cholesteryl), 2.04–0.65 (series of m, 462 H, 246
cholesteryl protons and 216 aliphatic protons). Anal. Calc. for
555H687Fe6O84 (9037.56): C, 73.76; H, 7.66. Found: C, 73.48; H,
C
7.69%.
§ For instrumentation, see: R. Deschenaux, I. Kosztics and B. Nicolet,
J. Mater. Chem., 1995, 5, 2291; M. Veber, P. Sotta, P. Davidson, A.-M.
Levelut, C. Jallabert and H. Strzelecka, J. Phys. Fr., 1990, 51, 1283.
The nature of the liquid-crystalline phase was clearly and
unambiguously established from X-ray data. Firstly, diffraction
patterns typical of disordered smectic phases, characterized by
a sharp diffraction peak at the low-angle region (a second-order
diffraction peak was observed in some cases) and a diffuse peak
at the wide-angle region, were obtained. The layered structure
was retained below Tg. The d-layer spacing was found to be 40.7
Å at room temperature and 41.3 Å at 86 °C (the sample was
heated to the isotropic liquid, cooled to room temperature, and
re-heated to the desired temperature). Secondly, a sample was
oriented by mechanical shearing on mica close to the clearing
point, and was analysed at room temperature. A diffraction
pattern typical of a smectic phase, consisting of aligned
equidistant sharp spots at small angles and of two diffuse arcs at
wide angles, was obtained. The sharp spots are aligned along the
shearing direction (meridian) and the points of highest intensity
in the diffuse arcs are located in the equatorial plane. Therefore
the director is parallel to the shearing direction and perpendicu-
lar to the layer planes. The mesophase is actually smectic A.
Finally, compound 1 showed good thermal stability as
confirmed by thermogravimetry (10 °C min21, under N2),
which indicated that no decomposition occurred up to ca.
250 °C. Mass losses of 1, 5 and 10% were detected at 258, 281
and 322 °C, respectively.
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The above results demonstrate that ferrocene-containing
liquid-crystalline dendrimers represent a valuable family of
mesomorphic macromolecules, showing a pronounced liquid-
crystalline tendency and good thermal stability. Furthermore,
ferrocenyl functionalized dendrimers have emerged as an
interesting class of electro-active macromolecules.5a,b The pres-
ence of the ferrocene unit in mesomorphic dendrimers is
particularly attractive with potential for elaborating dendrimer-
based switchable molecular units: we have recently shown that
electron transfer can be used to generate mesomorphism in the
ferrocene–ferrocenium redox system.13
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Chem., Int. Ed. Engl., 1996, 35, 1858.
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R. D. acknowledges the Swiss National Science Foundation
(grant 20-45588.95) for financial support.
Received in Bath, UK, 25th April 1997; 7/02850D
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Chem. Commun., 1997