The effect of ΔΛ chirality on molecular organization in two-dimensional films of a Ru(II) complex with a mesogenic ligand

Article abstract of DOI:10.1039/b110161g

A novel amphiphilic Ru(II) complex, [Ru(acac)₂L] (acac = acetylacetonato, L = 5,5′-bis(4-octylphenyloxycarbonyl)-2,2′-bipyridyl), in which L undergoes SmC, SmA and N liquid crystal phases, exhibits a remarkable chirality effect on its monolayer state: that is, a racemic mixture gives a monolayer consisting of spike-like aggregates of 1.2 nm (in height) × 50 nm (in diameter), whereas the Δ-enantiomer gives a uniform monolayer.

Full text of DOI:10.1039/b110161g

The effect of DL chirality on molecular organization in two-dimensional
films of a Ru(II) complex with a mesogenic ligand
Kentaro Okamoto,a Yuki Matsuoka,a Noboru Wakabayashi,a Akihiko Yamagishia and Naomi
Hoshino*b
a Division of Biological Sciences, Graduate School of Science, Hokkaido University, Sapporo 060-0810,
Japan. E-mail: okamoto@sci.hokudai.ac.jp
b Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan.
E-mail: hoshino@sci.hokudai.ac.jp
Received (in Cambridge, UK) 8th November 2001, Accepted 12th December 2001
First published as an Advance Article on the web 24th January 2002
A novel amphiphilic Ru(II) complex, [Ru(acac)2L] (acac =
acetylacetonato, L = 5,5A-bis(4-octylphenyloxycarbonyl)-
2,2A-bipyridyl), in which L undergoes SmC, SmA and N
liquid crystal phases, exhibits a remarkable chirality effect
on its monolayer state: that is, a racemic mixture gives a
monolayer consisting of spike-like aggregates of 1.2 nm (in
height) 3 50 nm (in diameter), whereas the D-enantiomer
gives a uniform monolayer.
24 h under a nitrogen atmosphere. The product was purified on
a silica gel column eluting with 3+1 THF+hexane. The
compound thus obtained was identified with NMR, ICP and
elemental analyses.‡
Column chromatography was applied for optical resolution.
A glass tube (1.0 cm i.d.) was packed with 2.0 g of an ion-
exchange adduct of a clay mineral (synthetic laponite) and D-
[Ru(phen)3]2+. This packing material is capable of resolving
various kinds of tris- or bis-acetylacetonato complexes.4 The
racemic mixture of [Ru(acac)2L] (ca. 1 3 1025 mol) dissolved
in 1.0 mL of a 1+1 chloroform+methanol mixture was mounted
and eluted with 1+4 chloroform+methanol under moderate
pressure. The chromatogram showed a broad single band with a
shoulder. The initial and final fractions exhibited the positive
and negative peaks at 400 nm in the circular dichroism spectra.
By repeating recrystallization from methanol solvent at 24 °C,
the optical purity of the material improved until it attained the
limiting values of De = +2.7 and 22.9 at 400 nm for the less
and more retained isomers, respectively. Comparing these
values to those of D- and L-[Ru(acac)3] (De = +1.0 and 21.0
at 380 nm, respectively), we assume that the material has been
separated nearly fully into pure D- and L-isomers of [Ru-
(acac)2L].
The racemic mixture did not melt until 233 °C, around which
the material rapidly blackened. Samples rich in the D-
enantiomer did not show any sign of mesomorphism either,
though they exhibited seemingly lower melting temperatures.
We then studied the monolayer states of [Ru(acac)2L] with a
purpose of obtaining information on the factors of achieving
liquid crystal phases for the present type of metallomesogen. A
chloroform solution of the racemic mixture or the D-isomer was
spread onto pure water at 20 °C.
It is well known that chirality has a profound effect on
mesomorphism.1 The primary role of chiral centers introduced
into the liquid crystal phases is to lower the symmetry of the
phase, but the intricate nature of the interactions in fluid phases
of chiral molecules remains largely unspecified. One approach
for resolving such a problem is to study the monolayer behavior
of liquid crystal molecules at an air–water interface.2 Various
spectroscopic techniques can be applied to reveal the molecular
organization in floating and deposited monolayers. Use of the
D,L-isomerism of 6-coordinate tris-chelate complexes would
also be enlightening in studies of the effect of chirality owing to
their unique steric and electronic characteristics. In this
communication, we report on the monolayer behaviour of an
amphiphilic metal complex in which one of the ligands is
mesomorphic. As a result, a remarkable difference has been
found in the molecular aggregation of the deposited films
between a racemic mixture and the D-enantiomer.
We selected a mesogenic bpy derivative from the literature3
and eliminated the oxo groups (dipolar components) from the
attachment of alkyl tails. The mesomorphic phase sequence of
the modified bpy ligand L [5,5A-bis(4-octylphenyloxycarbonyl)-
2,2A-bipyridyl, Fig. 1] was determined by polarizing optical
microscopy† and differential scanning calorimetry (first heating
run at 10 °C min21):
The solid and dotted curves in Fig. 2 show the surface
pressure versus molecular area isotherms for the racemic
mixture and the D-isomer, respectively. In both cases, the
surface pressure rose from zero at the molecular area of 0.35
nm2 molecule21. This lift-off area is close to the cross-sectional
area of the complex when the molecular rigid cores lie flat on
the water surface. Thus the metal complex forms a monolayer at
the air–water interface. The surface pressure rises more steeply
for the racemic mixture than for the D-isomer. It implies that the
racemic mixture forms a more rigid monolayer than the
enantiomer, while the molecular packing is less dense. The
isotherm for the D-isomer exhibits an inflection point around
0.15 nm2 molecule21, suggesting that some structural change
occurs at this molecular area. Passing through this point, the
surface pressure increases as steeply as for the racemic mixture.
We assume that the molecules in both of the monolayers orient
initially flat on the water surface around 0.35 nm2 molecule21
and that they change to vertical orientation upon compression.
The drawings within Fig. 2 show the possible molecular
packing of the racemic mixture and the D-enantiomer in the
K2 91 K1 110 SmJ 136 SmC 230 SmA 245 N 255 I.
The phase assignment is in accordance with the literature data
for the octyloxy analogue.3†
A Ru(II) complex, [Ru(acac)2L] (acac = acetylacetonato,
Fig. 1), was synthesized by reducing [Ru(acac)3] with zinc
powder in THF, and after adding L the solution was refluxed for
highly condensed states below 0.20 and 0.12 nm2 molecule21
respectively.
,
The floating films were transferred onto mica at a surface
Fig. 1 The molecular structure and model of D-[Ru(acac)2L] [acac =
acetylacetonato, L = 5,5A-bis(4-octylphenyloxycarbonyl)-2,2A-bipyridyl].
pressure of 15 mN m21 and the surface structures studied with
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This journal is © The Royal Society of Chemistry 2002

Fig. 2 Surface pressure versus molecular area isotherms for the racemic
mixture (solid line) and the D-isomer (dotted line). The sub-phase was pure
water at 20 °C.
Fig. 3 The 2 mm 3 2 mm AFM image of the deposited film of the racemic
mixture (A) and the D-isomer (B). The films were transferred onto mica at
an atomic force microscope (AFM) in a tapping mode. Fig. 3(A)
is the 2 mm 3 2 mm AFM image of the deposited film of the
racemic mixture. The film consists of an aggregate of spikes
whose height and diameter are estimated to be 1.2 and 50 nm,
respectively, and one spike contains ca. 103 molecules. The
height is nearly equal to the length of the octyl moieties. Thus
the vertical molecular arrays may either become tilted or change
their structure during deposition. The strong lateral adhesion
between metal centers in the DL pairs as suggested in the
packing model (Fig. 2) would probably be related to such
transformations. Fig. 3(B) is the 2 mm 3 2 mm AFM image of
the deposited film of the D-isomer. The surface is very flat over
the observed region. There were only a small number of dotted
regions. Since the thickness of the film is difficult to estimate,
the orientation of molecules in the film is uncertain. Never-
theless, it can be concluded that the enantiomers are oriented
vertically, considering a very small molecular area at deposition
(0.12 nm2 molecule21).
As a concluding remark, close contact of D- and L-isomers
of metal complexes may well lead to three-dimensional
ordering, which should be controlled if one wishes to obtain
liquid crystals of either two- (columnar)5 or one-dimensional
(smectic) order. The enantiomers of the present type appear to
form rather compressible monolayers, and their packing
characteristics are to be fully scrutinized. In this light, our test
for their performance as a chiral dopant has proven highly
promising.6
a surface pressure of 15 mN m21
.
Notes and references
† The SmC?SmA transition was of second order and, intriguingly, the
SmA?N transition was also of weakly first order. The reported transition
temperatures were therefore determined by polarizing optical microscopy
alone. The optical textures for the N and SmA phases commonly involved
homeotropic areas, and the temperature ranges for each of these phases was
estimated from the duration of coexisting marble or fan-shaped textures.
The SmA/C transition was in turn recognized by a change of the
homeotropic area to birefringent pattern of a blurred Schliren type. The rest
of the transitions were associated with the enthalpy changes of 11.7
(K2?K1), 8.0 (K1?SmJ), 31.0 (SmJ?SmC), and 1.6 kJ mol21 (N?I); all
of these values are in accordance with the literature.3
‡ Calculated for C50H62N2O8Ru: C, 65.27; H, 6.79; N, 3.04. Found: C,
65.43; H, 6.97; N, 3.18%.
1 Chirality in Liquid Crystals, ed. C. Bahr and H. Kitzerow, Springer-
Verlag, New York, 2000.
2 T. Martynski, R. Hertmonwski and D. Bauman, Liq. Cryst., 2001, 28,
437.
3 K. E. Rowe and D. W. Bruce, Liq. Cryst., 1996, 20, 183.
4 S. Kashiwara, M. Taniguchi and A. Yamagishi, J. Chem. Soc., Dalton
Trans., 1996, 11, 2301.
5 S. T. Trazaska, H. Hsu and T. M. Swager, J. Am. Chem. Soc., 1999, 121,
4518.
6 K. Okamoto, A. Yamagishi and N. Hoshino, manuscript in preparation.
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