On the parity in helical twisting power of Ru(III) 1,3-diketonates of C2 symmetry in nematic liquid crystals

Article abstract of DOI:10.1021/ja042549u

It is demonstrated that the sign of helical twisting power (HTP) of an enantiomeric Ru(III) complex of type [Ru(acac)₂L] can be switched by choosing L from either L_per or L_para, which is elongated either perpendicular or p

Full text of DOI:10.1021/ja042549u

Published on Web 05/20/2005
On the Parity in Helical Twisting Power of Ru(III)
1,3-Diketonates of C2 Symmetry in Nematic Liquid Crystals
Jun Yoshida,† Hisako Sato,†,‡ Akihiko Yamagishi,†,‡ and Naomi Hoshino*,§
Contribution from the Department of Earth and Planetary Science, Graduate School of Science,
The UniVersity of Tokyo, Tokyo 113-0033, CREST, Japan Science and Technology Agency, 4-1-8
Honcho, Kawaguchi, Saitama 332-0012, DiVision of Chemistry, Graduate School of Science,
Hokkaido UniVersity, Sapporo 060-0810, Japan
Received December 11, 2004; Revised Manuscript Received April 15, 2005; E-mail: hoshino@sci.hokudai.ac.jp
Abstract: It is demonstrated that the sign of helical twisting power (HTP) of an enantiomeric Ru(III) complex
of type [Ru(acac)2L] can be switched by choosing L from either Lper or Lpara, which is elongated either
perpendicular or parallel to the C2 symmetry axis, and four states become available in combination with
∆Λ-chirality of the metal center. Complexes 1-n, in which 4,4′-dialkoxylated dibenzoylmethanate ligands
are used as Lper, and 2 having Lpara ) 3-(4′-decyloxyphenyl)pentane-2,4-dionate ligand were prepared for
this purpose. They were optically resolved into the enantiomers by means of a clay column chromatography,
and their performance as chiral dopants was evaluated in nematic liquid crystals including a room-
temperature system, N-methoxybenzylidene-4-n-butylaniline (MBBA), which allowed facile measurements
of the helical pitch lengths and CD spectra in the induced chiral nematic states. The induced CD signals
have provided a clear evidence for the helical inversion between the two structure types, 1 and 2, of the
same chirality. The twisting power of these six-coordinate metal complexes and their structure versus twist
sense correlations are interpreted by the shape model. Intrinsically high HTP of ∆-[Ru(acac)2Lper] has also
allowed for observation of the pitch band due to the selective reflection in the visible wavelength range at
the doping level of 2 mol % in MBBA.
Chart 1. Complexes Studied (Illustrated for ∆-Enantiomers) and
Introduction
the Principal Axis Systems of Ordering Tensor Deduced from the
Nematic liquid crystals are anisotropic fluids that report
sensitively on the presence of a chiral substance by way of
forming helical superstructures. The induction of a chiral
nematic (N*) phase is thought to be thresholdless, and, when
the pitch is in the UV-vis wavelength range, a bright coloration
of samples due to the selective reflection can be utilized as a
convenient analytical tool for stereoselective syntheses.1 It is
important to work out a theoretical framework to describe the
phenomenon for device applications in prospect, and investiga-
tions into the helical twisting powers (HTP) of chiral dopants
of various molecular types should not only merit such endeavors
but also deepen our understanding of the properties of chiral
liquid crystals.2
We wish to report herein that the sign of HTP of metal
complex dopants of type [Ru(acac)2L] (acac ) acetylacetonate,
and L ) 1,3-diketonate containing phenylene groups) can be
switched by a rational choice of L between Lper and Lpara, which
are “elongated” perpendicular and parallel to the C2 symmetry
axis, respectively. Four states become available in combination
with ∆Λ-chirality of the metal center. Chart 1 illustrates
Structures
∆-enantiomers of complexes 1-n (with Lper) and 2 (with Lpara
studied in this work.
)
Results and Discussion
Synthesis and Optical Resolution. Two homologues of 1-n
(n ) 6 and 12) and 2 were prepared following the literature
procedures3 and optically resolved into pure enantiomers by
(3) For the Lper-type diketones in 1, see: (a) Serrette, A. G.; Lai, C. K.; Swager,
T. M. Chem. Mater. 1994, 6, 2252-2268. (b) Zheng, H.; Lai, C. K.; Swager,
T. M. Chem. Mater. 1995, 7, 2067-2077. Synthesis of the Lpara ligand in
2 was after: (c) Cativiela, C.; Serrano, J. L.; Zurbano, M. M. J. Org. Chem.
1995, 60, 3074-3083. (d) Okuro, K.; Furuune, M.; Miura, M.; Nomura,
M. J. Org. Chem. 1993, 58, 7606-7607. (e) Thiruvikraman, S. V.; Suzuki,
H. Bull. Chem. Soc. Jpn. 1985, 58, 1597-1598. Complexes were prepared
following: (f) Kasahara, Y.; Hoshino, Y.; Shimizu, K.; Satoˆ, G. P. Chem.
Lett. 1990, 381-384.
† The University of Tokyo.
‡ Japan Science and Technology Agency.
§ Hokkaido University.
(1) Eelkema, R.; van Delden, R. A.; Feringa, B. L. Angew. Chem., Int. Ed.
2004, 43, 5013-5016.
(2) Chirality in Liquid Crystals; Kitzerow, H.-S., Bahr, C., Eds.; Springer-
Verlag: New York, 2001.
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J. AM. CHEM. SOC. 2005, 127, 8453-8456
8453

A R T I C L E S
Yoshida et al.
Table 1. Values for the HTP (âM)a Evaluated for the Enantiomeric
Samples of Ru(III) Complexes and the ICD Spectral Data (θ/deg)b
for Doped N* Phases
1
â
/
M µm-
θ/deg
dopant
∆
Λ
hostc
T/
°C
∆
Λ
λ/nm
p/µm
1-6
-146
130 MBBA
120 MBBA
72 EBBA
58 ZLI-1132
39d PAA
30 -0.83
30 -0.78
60 -0.69
35 -1.00
120 -1.34
1.05 420
0.59 420
0.88 420
1.07 330 17
1.33 480 7.2
5.1
8.3
7.5
1-12 -127
-66
-55
-63d
2
24 -25 MBBA
16 -13 EBBA
24 -22 ZLI-1132
17 -13d PAA
30
60
0.13 -0.16 420 14
0.55 -0.41 420 13
0.22 -0.28 330 24
0.25 -0.37 480 15
35
120
Figure 1. Selected plots of the inverse helical pitch (p-1) versus mole
fraction of the dopant (x, in mol %) for doped N* phases obtained with
either ∆- or Λ-enantiomer of 1-12 and 2. Symbols for the base liquid crystals
are b for MBBA (30 °C), 9 for EBBA (60 °C), and 2 for ZLI-1132 (35
°C).
a The signs + and - indicate that P- and M-helical superstructures are
induced, respectively. b Ellipticity readings taken at band edges of the ICD
spectra, which are exemplified in Figure 2. Concentrations of the listed
samples with ∆- and Λ-eantiomers are not exactly matched, and the mean
helical pitch length is given for each pair, for comparison to the specified
wavelength (λ/nm). c Abbreviations and experimental nematic ranges: N-(4-
methoxybenzylidene)-4-n-butylaniline (MBBA, 21-47 °C), N-(4-ethoxy-
benzylidene)-4-n-butylaniline (EBBA, 36-73 °C), and 4,4′-azoxydianisole
(PAA, 118-135 °C). ZLI-1132 is a mixture of 4-(4-alkylcyclohexyl)ben-
zonitrile and 4-(4-alkylcyclohexyl)-4′-cyanobiphenyl derivatives (Merck
Japan, TNI ) 72.3 °C, average mw ) 263.2). d Rough estimates from finite
concentrations within a range of x ) 0.14-0.18 mol %.
∂p-1
∂x xf0
â )
(1)
(
)
M
The most notable feature at a glance here is that the HTPs
differ in sign between ∆-1 and ∆-2, while both of them are
∆-enantiomers. For example, complex ∆-1 has proven to be a
remarkably powerful dopant (âM ) -1.4 × 102 and -1.2 ×
102 µm-1 for n ) 6 and 12, respectively, in MBBA), and the
performance of complex ∆-2 is also pretty good (+24 µm-1).
Λ-Enantiomers were also studied and, of course, resulted in
the same âM values within experimental errors but with opposite
signs. The + and - signs of âM indicate P- and M-helical twists
of the host nematic director, respectively, which was verified
by measuring the ICD spectra due to the helical arrangement
of conjugated imine chromophores of MBBA molecules.8 Figure
2 exemplifies such intense ICD spectra, which are positive for
Λ-1-12 and ∆-2 and negative for ∆-1-12 and Λ-2. In other
words, the chirality versus twist sense correlation for complex
1 is ∆-M and Λ-P, and for complex 2 ∆-P and Λ-M. The
intensities for the latter are relatively weak despite their higher
concentrations, reflecting a smaller magnitude of HTP. Es-
sentially the same correlations were observed in other nematics,
for which longitudinal chromophoric components are recogniz-
able (Table 1).
means of a clay column chromatography.4 This method involves
a column material modified with ∆-[Ru(phen)3]2+ and has
proven to be useful for neutral metal complexes. Once the
complexes had been purified chemically by a usual silica-based
column (while racemic), the optical resolution was accomplished
with further separation of impurities. Two major peaks off this
chiral column were well-separated from each other to the
“baseline” level in the very first run of elution with methanol.
Identification of the stereoisomers has been established in the
case of [Ru(acac)3]. An intense positive band at 275 nm,
followed by a negative and a positive band, in the CD spectrum
was ascribed basically to the ligand π-π* exciton band of the
∆-form,5a which was corroborated later by the single crystal
determination of this absolute configuration.5b Accordingly,
more retained fractions in all cases of 1-n and 2 exhibiting the
same CD spectral pattern were assigned to the ∆-enantiomers,
and the less retained to their antipodes, in this study (Supporting
Information).6
Structure-HTP Relations. The molecular HTP, âM defined
by eq 1, has been evaluated by the Cano method,7 plotting the
inverse pitch (p-1) against mole fraction (x) of the dopant in
four kinds of nematic liquid crystals including a room-
temperature system, N-methoxybenzylidene-4-n-butylaniline
(MBBA). Induced CD (ICD) spectra were also recorded for each
of the induced N* phases at appropriate concentrations. Table
1 summarizes the âM values thus evaluated and the ICD spectral
data relevant to their signs. The pitch data collected by the
polarizing optical microscopy are also compiled in Figure 1 for
representative cases.
Theoretical Interpretations. Thus, the complexes prepared
in this work have proven not only to contain a structural element
necessary for powerful twisters particularly for MBBA, but also
to provide a set of parity relations. The opposite handedness of
induced helices arises naturally from the ∆Λ-isomerism, but
the effect has its origin also in the distinction between Lper and
Lpara. To interpret the observed behaviors in the induction of
N* phases, a shape model proposed by Nordio, Ferrarini and
co-workers is quoted.9 Equation 29d describes the “chirality
parameter” Q, to which the HTP is proportional (â Q), and
this is given in turn by the elements of “surface chirality tensor”
Q and of the orientational ordering tensor S. Both of the tensors
(4) Kashiwara, S.; Takahashi, M.; Nakata, M.; Taniguchi, M.; Yamagishi, A.
J. Mater. Chem. 1998, 8, 2253-2257.
(5) (a) Kobayashi, H.; Matsuzawa, H.; Kaizu, Y.; Ichida, A. Inorg. Chem. 1987,
26, 4318-4323. (b) Matsuzawa, H.; Ohashi, Y.; Kaizu, Y.; Kobayashi, H.
Inorg. Chem. 1988, 27, 2981-2985.
(8) (a) Pirkle, W. H.; Rinaldi, P. L. J. Org. Chem. 1980, 45, 1379-1382. (b)
Rinaldi, P. L.; Naidu, M. S. R.; Conaway, W. E. J. Org. Chem. 1982, 47,
3987-3991. (c) Rinaldi, P. L.; Wilk, M. J. Org. Chem. 1983, 48, 2141-
2146.
(9) (a) Ferrarini, A.; Moro, G. J.; Nordio, P. L. Liq. Cryst. 1995, 19, 397-
399. (b) Ferrarini, A.; Moro, G. J.; Nordio, P. L. Mol. Phys. 1996, 87,
485-499. (c) Ferrarini, A.; Moro, G. J.; Nordio, P. L. Phys. ReV. E 1996,
53, 681-688. (d) di Matteo, A.; Todd, S. M.; Gottarelli, G.; Solladie´, G.;
Williams, V. E.; Lemieux, R. P.; Ferrarini, A.; Spada, G. P. J. Am. Chem.
Soc. 2001, 123, 7842-7851.
(6) Although it might seem intuitive that the Λ-isomers would be more strongly
adsorbed to the ∆-modified clay, it has been shown previously that the
situation is reverse for certain tris- and mixed 1,3-diketonates of Ru(III)
(ref 4).
(7) (a) Matsumura, K.; Iwayanagi, S. Oyo Butsuri (J. Appl. Phys., Jpn.) 1974,
43, 126-131. (b) Hoshino, N.; Matsuoka, Y.; Okamoto, K.; Yamagishi,
A. J. Am. Chem. Soc. 2003, 125, 1718-1719.
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8454 J. AM. CHEM. SOC. VOL. 127, NO. 23, 2005

Helical Twisting Power of Ru(III) 1,3-Diketonates
A R T I C L E S
3
3
3
3
Qxx ) -
r0 , Qyy ) 0, Qzz ) +
r0
(4)
x8
x8
It is safely concluded that Qxx < 0 and Qzz (∼ -Qxx) > 0 for
∆-1, and this yields Q < 0 (hence â < 0) from eq 2 in a uniaxial
ordering potential (Szz is positive and the largest element).12
The signs of Qxx and Qzz flip upon transforming the axis
system from (x, y, z) to (z, -y, x) for ∆-[Ru(acac)2Lpara], and
therefore a positive Q should result (hence â > 0). This is
exactly as reported above for ∆-2. In fact, this complex carries
bis-chelate group at its terminus, and this must be a disadvantage
in achieving a large Szz. Just one 1,4-phenylene group in Lpara
ligand may not be sufficient either, and this very group may
also be in hindered rotational state, possibly making a nonneg-
ligible contribution to Qyy. At any rate, not only the sign but
also the magnitude of HTP can thus be interpreted consistently.12
Table 1 shows that the shortening of two tails in 1 (from n
) 12 to 6) has only a small enhancing effect on the HTP, which
seems to justify the approach based on the rigid part of a dopant.
A more quantitative analysis of the magnitude of HTP requires
knowledge of the proportionality constant relating Q to â, which
include such temperature-dependent properties as “orienting
strength”9d ê, twist elastic constant K22, and molar volume for
the nematic medium Vm, within the above theoretical framework.
Although we evaluated the âM values over only a small range
of reduced temperatures (T/TN*I ) 0.95-0.96, except for the
ZLI formulation), the nature in the interactions between the
dopant and liquid crystal molecules can vary from one host to
another. In fact, it came to our surprise that N-(4-ethoxyben-
zylidene)-4-n-butylaniline (EBBA), which is a higher homologue
of MBBA by only one methylene unit, experiences the HTP of
1-12 nearly half that of MBBA. It is suggested by a literature
report13a that the solvent elastic constant K22 for EBBA amounts
to roughly twice that for MBBA at temperatures of the
experiments, and this could account for the large gap observed,
since â is inversely proportional to K22. Effects of other
parameters cannot be excluded, however, and weakening in the
solute-solvent interactions (lowering ê/Vm) and the solute
orientational disordering (lowering Szz) in EBBA at a higher
temperature may also contribute to the decrease in â. The ZLI
formulation has also reported on generally lower HTP for the
dopants studied here, but this we previously observed for a
similar complex of Ru(II).7b Higher temperature ranges of some
Figure 2. ICD spectra for MBBA* materials doped with ∆- and
Λ-enantiomers of 1-12 (0.1 mol %, left) and 2 (0.3 mol %, right). Each
spectrum was recorded at 30 °C in a glass cell of 25-µm gap at normal
incidence.
are traceless,10 and the principal axis system of the latter is
usually chosen for eq 2. Most plausible axis systems are included
in Chart 1 for our complexes ∆-1-n and ∆-2. Of primary
importance is that the direction along which the ligand Lper in
1 or Lpara in 2 has been elongated by design should be designated
as the z axis, as it should have the highest propensity of
alignment.
2
Q ) -
(QxxSxx + QyySyy + QzzSzz)
(2)
x3
In short, the HTP depends not only on how chiral the dopant’s
surface is seen along each axis x, y, or z, but also on how well
these axes are aligned to the local nematic director. The surface
elements are defined by eq 3:9d
3
Qij )
[s (sˆ × br) + (sˆ × br) s ]dbr
(3)
∫
i
j
i j
S
x8
where br is a vector giving the position of a point on the
molecular surface, sˆ is the surface normal outward at the same
point, and the integral covers the whole molecular surface.
While the theory has been applied with much success to the
structure-performance relations for organic chiral dopants,9,11
it involves extensive calculation of each of the tensor elements.
In contrast, the rigid and orthogonal coordination geometry of
metal chelates allows one to estimate principal elements Qij’s
with little computational efforts and with reasonable accuracy.
Our reasoning is explained for the ∆-enantiomers first. As a
crude but useful approximation, the acac ligand can be regarded
as a rectangular blade of side length r0 and the backbone ligand
Lper constrained in the xz plane. The elements can then be
analytically estimated as follows for ∆-[Ru(acac)2Lper].
(12) Input in eq 2 of a test parameter set, Qxx ) -Qzz ) -77 Å3 (r0 ) 5 Å),
for the ∆-(acac)2 moiety of 1 together with plausible order parameters for
uniaxial rodlike behavior, Sxx ) -0.3 and Szz ) 0.6, leads to â ) -1.2 ×
102 µm-1, reproducing the HTP of ∆-1-12 (the proportionality constant
between â and Q, which is appropriate for MBBA at 300 K, has been
taken from ref 9c). If Qxx ) -Qzz ) 77 Å3 is assumed instead for ∆-2,
with the constant unchanged, an order parameter set such as Sxx ) -0.1
and Szz ) 0.2 yields â ) 39 µm-1. This means a far less ordered state,
while rodlike behavior is still assumed. Further disordering towards Sxx
0 (while Szz ) 0.2) would yield â diminishing toward 26 µm-1
)
.
(13) (a) Tolmachev, A.; Ferdoryako, A.; Lisetski, L. Mol. Cryst. Liq. Cryst.
1990, 191, 395-399. From the plots reported therein of temperature
dependences of the elastic constants for a homologous series of N-(4-
alkoxybenzylidene)-4-n-butylanilines, the “K22” readings for EBBA and
MBBA at TNI - T ) 13 °C compare at about 2:1 ratio, although the axis
labeling for these plots is not clear. (b) Karat, P. P.; Madhusudana, N. V.
Mol. Cryst. Liq. Cryst. 1977, 40, 239-245. The odd-even effect is also
seen in the K22 values of 4′-n-alkyl-4-cyanobiphenyls (nCB). The values
interpolated at TNI - T ) 15 °C compare roughly at 8:5 (in pN) for 6CB
to 7CB, for instance. (c) Bualek, S.; Patumtevapibal, S.; Siripitayananon,
J. Chem. Phys. Lett. 1981, 79, 389-391. The odd-even alternation in the
HTP values of certain chiral dopants has been observed in the presence (at
20 mol %) of 4,4′-dialkoxyazoxyzenzenes in a nematic host, although the
effect upon elongation from methoxy to ethoxy terminal groups is much
weaker (2-5% diminution) in this case.
(10) The definition of Q being traceless, and consequently eqs 2 and 3, involves
an approximation in which the isotropic contribution of a dopant is left
out of consideration. In reality, effects of chirality are not necessarily
canceled by the isotropic distribution, and Q is better regarded as anisotropic
content of the surface chirality. For a full tensorial approach, see: Kuball,
H.-G.; Ho¨fer, T. In Chirality in Liquid Crystals; Kitzerow, H.-S., Bahr,
C., Eds.; Springer-Verlag: New York, 2001; Chapter 3, pp 67-100.
(11) (a) Ferrarini, A.; Gottarelli, G.; Nordio, P. L.; Spada, G. P. J. Chem. Soc.,
Perkin Trans. 2 1999, 411-417. (b) Pieraccini, S.; Donnoli, M. I.; Ferrarini,
A.; Gottarelli, G.; Licini, G.; Rosini, C.; Superchi, S.; Spada, G. P. J. Org.
Chem. 2003, 68, 519-526.
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A R T I C L E S
Yoshida et al.
Figure 4. “Difference” transmittance (∆I) spectra for MBBA* materials
doped with ∆-1-12 (1.9 and 2.2 mol % for yellow and green spectra,
respectively), showing dips due to the selective reflection in the N* state.
The original spectra were measured at 20 °C in glass cells of 25-µm gap at
normal incidence.
Figure 3. CD spectrum for an MBBA* material doped with ∆-1-12 (2.1
mol %, nominal), recorded at 22 °C in a glass cell of 2-µm gap at normal
incidence.
hosts have allowed observations on the thermal effect(s). The
pitch lengths of the EBBA* specimens traced from 60 to 30 °C
showed little temperature dependence (slight concavity by 2%
at most), evidencing for the inert nature of the helix induction
of 1-12. Use of PAA at elevated temperatures resulted in rather
unstable textural signatures, though we managed to run the Cano
experiments. It appeared after standing at 120 °C overnight that
racemization had occurred to some extent. Despite these
variations, it is stressed again that the sign of HTP could be
controlled regardless of its magnitude, and the chirality versus
twist sense correlation, which is reversed between 1-12 and 2,
has been confirmed in four different systems.14
concentration higher than the one displayed in Figure 3. The
spectral lines have been corrected for absorption effects by
subtracting the transmittance baseline recorded in the isotropic
liquid state for each sample, and the remaining dips are
characteristic of the N* state, as the intensity of transmitting
light passing along the helical axis should be halved for the
pitch band. It is shown here that the band shifts from around
690 to 610 nm with increasing the doping level from 1.9 to 2.2
mol %, and thus the nature of the observed coloring phenom-
enon is actually the selective reflection associated with the
optical structure that is helical. The present findings will
encourage further studies on CPL related applications.
Selective Reflections. It is worth mentioning that the complex
1-12 has gained increased solubility possibly owing to the full
optical resolution as well as to the peripheral alkyl appendage,
and that a general immiscibility of metal complexes with liquid
crystal solvents may be amended. As we managed to dissolve
∆-1-12 up to 2 mol % in MBBA, the material stayed in the N*
phase barely at room temperature and exhibited greenish
coloration when a planar preparation was viewed upright. Figure
3 shows its CD spectrum, recorded at normal incidence under
cooling. It contains a huge positive band around 660 nm in
addition to the negative ICD below ca. 400 nm. The former
signal indicates selective reflection of the left-handed circularly
polarized light (SR-LCPL) and is consistent with the relationship
for the pitch band,15 λ0 ) navp ) nav/(âMx), in which nav and λ0
are the average of refractive indices (∼1.6 for MBBA)16 and
the corresponding wavelength of reflection, respectively. It is
rarely possible to obtain such a spectrum on the same UV-vis
scale with doped N* samples, while literature examples exist
for cholesteryl materials.17
In summary, the metal complex dopants of a composition
[M(blade)2(backbone)] have acquired another degree of freedom
to manipulate their HTP in addition to their ∆Λ-chirality. This
is an important asset in view of the fact that the symmetry axis
in chiral mesophases (helical axis generated) is perpendicular
to the director for nematics, but parallel to the layer normal for
smectics.18 Coordination space in metal complexes seems to
offer a variety of new opportunities for designing chiral dopants
with versatile and practical uses in liquid crystals.
Acknowledgment. This work was supported by a Grant-in-
Aid for Scientific Research on Priority Areas (417) and in part
by Grant No. 15655016 from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) of Japan. We thank
Prof. Nobuyuki Matsushita of The Universtity of Tokyo for his
assistance in some of the optical measurements and Prof. Hiroshi
Orihara of Hokkaido University and Dr. Yoichi Takanishi of
Tokyo Institute of Technology for informative discussion.
We measured the visible spectra of MBBA* specimens of
different concentrations in ∆-1-12, and Figure 4 displays two
examples, one in concentration lower than and the other in
Supporting Information Available: The Experimental Sec-
tion and the associated analytical and solution CD spectral data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(14) We thank the reviewer for directing our attention to nematics other than
MBBA.
JA042549U
(15) de Gennes, P. G.; Prost, J. The Physics of Liquid Crystals, 2nd ed.;
Oxford: New York, 1993.
(16) Mitra, M.; Majumdar, B.; Paul, R.; Paul, S. Mol. Cryst. Liq. Cryst. 1990,
180B, 187.
(18) For doping of smectic hosts, see: Hegmann, T.; Meadows, M. R.; Wand,
M. D.; Lemieux, R. P. J. Mater. Chem. 2004, 14, 185-190 and references
therein.
(17) Saeva, F. D.; Wysocki, J. J. J. Am. Chem. Soc. 1971, 93, 5928-5929.
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8456 J. AM. CHEM. SOC. VOL. 127, NO. 23, 2005