Novel ferrocene-based chiral Schiff's base derivative with a twist-grain boundary phase (TGBA) and a blue phase

Article abstract of DOI:10.1039/a709140k

Synthesis of a monosubstituted ferrocene-based chiral Schiff's base derivative with unusual mesomorphic behavior is reported namely the first metallomesogen which exhibits TGBA and blue phases apart from S_C*, S_A and N*phase transformations.

Full text of DOI:10.1039/a709140k

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Novel ferrocene-based chiral Schiff’s base derivative with a twist-grain
boundary phase (TGBA) and a blue phase
Tarimala Seshadri*† and Hans-Ju¨rgen Haupt
Department of Inorganic and Analytical Chemistry, University of Paderborn, Warburger Straße 100, 33098 Paderborn, Germany
Synthesis of a monosubstituted ferrocene-based chiral
Schiff’s base derivative with unusual mesomorphic behavior
is reported namely the first metallomesogen which exhibits
TGBA and blue phases apart from SC*, SA and N* phase
transformations.
structures of low molar mass chiral liquid crystals to attain such
phases must be a very challenging and attractive subject for
study from both the academic and the applications point of
view. Especially, introduction of ferrocene units as components
of chiral liquid-crystalline materials may bring about high
thermal stability and other additional properties among which
its ability to undergo reversible one-electron transfer processes
is one of the most important.9
However, studies on the synthesis of chiral smectic C
mesophases comprising monosubstituted ferrocenyl group are
scanty except the one reported by Imrie and Loubser.10 The
limited studies in this direction are attributed to the fact that
monosubstituted ferrocenes, because of their unfavourable
molecular shape (L-shape) and also due to repulsive steric
effects of the ferrocene unit which hinders the ability of the
molecules to pack in layers thus favouring mostly the formation
of nemetic phases.11–13
We therefore attempted to prepare several ferrocene based
chiral mesophases and surprisingly synthesized a compound
that exhibits enantiotropic SC*, SA, a twist grain boundary phase
(TGBA), a cholesteric as well as a blue phase. This is the first
chiral metallomesogen which exhibits a TGBA and a blue
phase.
The synthesis of the chiral Schiff’s base derivative 8 is
described in Scheme 1. To start with, we used the readily
available (S)-(2)-2-methylbutan-1-ol for this purpose. In 8 the
chiral centre with a small terminal chain is located adjacent to
the rigid molecular core in order to couple strongly with the
Chirality in ordered fluids has become one of the most
fascinating and innovative fields of current research in liquid
crystals. The quest to develop new chiral mesomorphic
compounds led to the serendipitous discovery of new types of
physical behavior namely the twist grain boundary (TGB)1 and
the blue phases2 apart from the ferri- and antiferro-electric2,3
liquid crystalline phases. The TGBA phase first discovered by
Goodby et al.,4 in 1989 was initially predicted by Gennes5 in
1972. The appearance of the TGB phase6 has induced enormous
interest due to the formal analogy of its Landau free energy to
that of the Shubnikov phase occurring in the type II super-
conductors.5,7 Renn and Lubensky7 in 1988 described this phase
as a frustrated intermediate between N* and SA phases. Since
then, several new organic compounds showing such a phase
were synthesized by different workers and their phase se-
quences were identified.6
Both the TGB and the blue phases are two kinds of frustrated
phases which are found exclusively in chiral organic com-
pounds. None of the chiral metallomesogens reported until now
exhibited such phase formation, although several of their Pd, Cu
and V complexes bearing one or more stereocenters were
synthesized.8 Nevertheless, incorporation of metals into the
O
O
(CH2)11
O
C
C
(CH2)10Br
(CH2)11Br
p-HOC6H4CO2Me
O
C
OMe
ButNH2•BH3
AlCl3
Cl
(CH2)10Br
AlCl3
Fe
Fe
Fe
Fe
1
2
3
KOH HCl
O
O
O
p-HOC6H4NO2
DCC
(CH2)11
O
C
(CH2)11
O
C
OH
NO2
Fe
Fe
5
4
Pd(10%)C
H2
O
O
(CH2)11
O
C
OHC
CH
CH
O
+
NH2
H
C
Fe
O
CH2 C* CH2CH3
CH3
6
7
O
O
(CH2)11
O
C
N
CH
CH
CH
Fe
O
O
H
C
CH2
C CH CH
2 3
*
8
CH3
Scheme 1
Chem. Commun., 1998
735

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Fig. 2 Polarized optical micrograph of the blue phase platelet texture on
cooling from the isotropic liquid to 156.1 °C; magnification 3 100.
Fig. 1 Polarized optical micrograph of the TGBA phase on heating to
143.8 °C; magnification 3 100
the one side and a relatively small terminal group appended to
the chiral centre on the other side. This ferrocene derivative and
derivatives thereof should be interesting candidates for prepar-
ing ferroelectric smectic C* materials useful in fast switching
electro-optic devices.
dipolar regions of the core. The phase assignments and
transition temperatures (°C) of 8 shown below were determined
by polarized optical microscopy using clean, untreated glass
microscopic slides and cover slips as well as by differential
scanning calorimetry (DSC) measurements.
Notes and References
C 116 SC* 136 SA 143.8 TGBA 144.9 N* 158 BP 159.2 I
I 157.2 BP 154.6 N* 143.1 TGBA 141.2 SA 132.6 SC* 77.9 C
† E-mail: ts@chemie.uni-paderborn.de
‡ Detailed synthetic procedures for the intermediate ferrocenyl derivatives
1–6 will be reported separately. The final Schiff’s base compound 8 is
obtained by refluxing equimolar amounts of 6 and 7 in ethanol containing
a few drops of acetic acid for 1 h and filtered hot through a sintered glass.
The precipitate obtained after cooling was dissolved in dichloromethane and
reprecipitated by the addition of heptane. Yield: 82%. Anal. Calc. for
At the SA–SC* transition, a dramatic textural change was
observed whereby the dark homeotropic texture of the SA phase
gives rise to a schlieren texture characteristic of the SC phase.
The thermogram indicates that owing to the small enthalpy
associated with these transitions, some of the peaks could not be
detected. The calculated enthalpies (kJ mol21) are as follows.
SC*–SA, 0.28; SA–TGBA–N*, 0.55 and BP–I, 0.81.
C49H57FeNO5: C, 73.95; H, 7.22; N, 1.76. Found: C, 74.32; H, 7.27, N.
1.89%. FTIR (KBr) n/cm21: 2919, 2848, 1714, 1606, 1513, 1311, 1253,
1188, 1162, 1068. 1H NMR (CDCl3), d 0.90–0.99 (m, 6 H, CH3), 1.20–1.70
(m, 18 H, 8 3 CH2, 1 CH2), 1.85 (m, 3 H, 1 CH2, 1 CH), 2.35 (m, 2 H, CH2),
4.03–4.09 (m, 4 H, C5H4 + 4 H, 2 3 OCH2), 6.57 (d, 1 H, NCH), 7.75 (d,
2 H, Ar), 7.93–7.96 (d, 1 H, NCH), 7.14–7.17 (d, 2 H, Ar), 8.50 (s, 1 H,
NNCH)
When heating slowly (0.2° min21), the filament structure of
the TGBA phase shown in Fig. 1, grows slowly in the
homeotropic regions of the smectic A phase and ends up in a fire
ball which coalesces into a cholesteric phase with a fan-shaped
texture. Also oily streaks are seen upon subjecting the
preparation to mechanical stress.
1 J. W. Goodby, M. A. Waugh, S. M. Stein, E. Chin, R. Pindak and
J. S. Patel, Nature, 1989, 337, 449.
2 J. W. Goodby, I. Nishiyama, J. Slaney, C. J. Booth and K. J. Toyne, Liq.
Cryst., 1993, 14, 37.
3 A. Fukuda, Y. Takanishi, T. Isozaki, K. Ishikawa and H. Takezoe,
J. Mater. Chem., 1994, 4, 997.
4 J. W. Goodby, M. A. Waugh, S. M. Stein, E. Chin, R. Pindak and
J. S. Patel, J. Am. Chem. Soc., 1989, 111, 8119.
5 P. G. De Gennes, Solid State Commun., 1972, 10, 753.
6 H. S. Kitzerow, A. J. Slaney and J. W. Goodby, Ferroelectrics, 1996,
179, 61; M. H. Li, V. Laux, H. T. Nguyen, G. Sigaud, P. Barois and
N. Isaert, Liq. Cryst., 1997, 23, 389 and refs. therein.
7 S. R. Renn and T. C. Lubensky, Phys. Rev. A, 1988, 38, 2132.
8 J. Buey, L. Diez, P. Espinet, H. S. Kitzerow and J. A. Miguel, Chem.
Mater., 1996, 8, 2375 and refs. therein.
9 R. Deschenaux, M. Schweissguth and A. M. Levelut, Chem. Commun.,
1996, 1275.
10 C. Imrie and C. Loubser, J. Chem. Soc., Chem. Commun., 1994,
2159.
Both during heating and cooling cycles (0.2° min21), the
formation of the blue phase is clearly seen as a platelet texture
which is shown in Fig. 2.
The influence of orientation of ester linkages which directs
the electron delocalization and the coplanarity of the two
aromatic rings of benzylideneaniline (the angle between the two
planes of these rings deduced from X-ray14 and UV data15 was
found to be 66°) probably enhances the polarisability of the
rigid core due to extended conjugation. This in turn enhances
the preponderance of smectic phases and also creates favourable
packing conditions. Furthermore, the presence of the flexible
spacer between the phenyl ring and the ferrocenyl unit in 8 is
primarily responsible for the formation of highly ordered
phases, in the absence of which only a cholesteric phase was
observed.
Compound 8 is very stable and no decomposition was seen
during repeated heating and cooling cycles. Furthermore, a free-
standing film can be prepared by heating the compound into its
smectic state (130 °C) without any decomposition.
In conclusion, we report here a monosubstituted ferrocene-
based metallomesogen with a chiral Schiff’s base which shows
for the first time a relatively long SC* domain, a TGBA phase
which mediates between SA and N* and a blue phase just before
the clearing point, in spite of a bulky pendant ferrocene unit on
11 R. Deschenaux and J. W. Goodby, in Ferrocenes, ed. A. Togni and T.
Hayasi, VCH, Weinheim, 1995, ch. 9.
12 C. Loubser and C. Imrie, J. Chem. Soc., Perkin Trans. 2, 1997, 399.
13 T. Seshadri and H.-J. Haupt, J. Mater. Chem., in press.
14 H. B. Bu¨rgi and J. D. Dunitz, Helv. Chim. Acta, 1970, 53, 1747.
15 J. Van Der Veen and A. H. Grobben, Mol. Cryst. Liq. Cryst., 1971, 15,
239.
Received in Cambridge, UK, 22nd December 1997; 7/09140K
736
Chem. Commun., 1998