Molecular mechanism of anomalous increase in the helical pitch of cholesteric liquid crystals induced by achiral dopants

Article abstract of DOI:10.1021/jp034916e

A dopant-induced anomalous increase in the helical pitch of dicholesteryl 10,12-docosadiynedioate 1 is explained by generation of smectic clusters in the cholesteric phase where the dopants act as promoters of smectic domains. The dopants, having a mesogenic core and the proper length of alkyl chains on both their molecular sides, such as 4,4′-dialkylazobenzenes, 4,4′-dialkylazoxybenzenes and 4,4′-dialkylbiphenyls, considerably increase the pitch. However, dopants having benzene or a binaphthyl structure in the molecular center, and alkyl chain at one side of the molecules and Z-isomers of azobenzene and azoxybenzene derivatives slightly decrease the pitch by acting as impurities that decrease the transition temperature of the host. At X-ray diffraction study of the cholesterics revealed that the pitch increases with an increase in size and amount of the smectic domains induced by the appropriate dopants.

Full text of DOI:10.1021/jp034916e

12054
J. Phys. Chem. B 2003, 107, 12054-12061
ARTICLES
Molecular Mechanism of Anomalous Increase in the Helical Pitch of Cholesteric Liquid
Crystals Induced by Achiral Dopants
Masatoshi Kidowaki, Masaya Moriyama, Momoyo Wada, and Nobuyuki Tamaoki*
Molecular Function Group, Institute for Materials & Chemical Process,
National Institute of AdVanced Industrial Science and Technology (AIST),
Tsukuba Central 5, 1-1-1, Higashi, Tsukuba, Ibaraki, 305-8565, Japan
ReceiVed: April 8, 2003; In Final Form: September 2, 2003
A dopant-induced anomalous increase in the helical pitch of dicholesteryl 10,12-docosadiynedioate 1 is
explained by generation of smectic clusters in the cholesteric phase where the dopants act as promoters of
smectic domains. The dopants, having a mesogenic core and the proper length of alkyl chains on both their
molecular sides, such as 4,4′-dialkylazobenzenes, 4,4′-dialkylazoxybenzenes and 4,4′-dialkylbiphenyls,
considerably increase the pitch. However, dopants having benzene or a binaphthyl structure in the molecular
center, an alkyl chain at one side of the molecules and Z-isomers of azobenzene and azoxybenzene derivatives,
slightly decrease the pitch by acting as impurities that decrease the transition temperature of the host. An
X-ray diffraction study of the cholesterics revealed that the pitch increases with an increase in size and amount
of the smectic domains induced by the appropriate dopants.
Introduction
of the melting point due to impurities. In the former case,
although there has been no convincing experimental evidence,
the modification of macroscopic chirality would be explained
by chiral intermolecular interaction between a chiral solute and
its nearest host molecules through modification of both their
stackings and conformations.9-12 When a chiral solute is
dissolved in nematics at the limit of a low concentration, the
induced helical pitch P is correlated with the concentration C
of the solute by the empirical equation.13,14
Cholesteric liquid crystals (CLCs) have attracted great interest
due to their unique macroscopic chirality originating from the
helical arrangement of the molecules. The molecular arrange-
ment is spontaneously formed when the mesophasic compounds
are intrinsically chiral or are doped with a chiral compound.
The helical superstructure of the CLCs is characterized by the
helical pitch and handedness. The reflection colors raised by
interference of light due to the periodic molecular arrangement
are related to the helical pitch through the following equation.
P
-1 ) âC
λmax ) pn
The proportionality constant â is referred to as the helical
twisting power (HTP). In the latter case, an achiral solute in
the CLC medium acts as an impurity, resulting in depression
of the apparent transition temperature of the systems, leading
to isothermal modification of the helical pitch. Therefore, the
increase or decrease in the helical pitch depends on the sign of
the thermal characteristics (dP/dT) of the host CLCs. The
theoretical studies of the thermodynamic properties of nematic
systems, which can be applied to cholesterics, have been done
by adapting the Flory-Huggins theory with minor modifi-
cations.15-17 The change in the nematic-isotropic phase transi-
tion temperature at an infinitely low guest concentration is a
function of the heat of transition of the nematic host and the
activity coefficient of the dopant in both the nematic and
isotropic phases. The strong relevance of thermodynamic
properties of the nematics with the size, shape, and chemical
nature of the solute molecules has been accounted for in
consideration with thermodynamic parameters.18,19
Here, λmax, p, and n are the wavelength of maximum reflection,
the helical pitch length, and the average refractive index,
respectively. The helical pitch of the CLC is very sensitive to
external stimuli such as temperature, pressure, and electrical
and magnetic fields.1 Due to this nature, the CLC has been
intensively studied from both fundamental and application points
of view. Such applications include refractive displays,2 light
modulators,3-5 color recording media,6,7 and temperature sen-
sors.8
In addition to the stimuli mentioned above, a molecule
incorporated in the systems as a solute is another important
factor contributing to modification of the helical pitch. This
effect would be classified roughly into two categories based on
the way it affects the helicity of the systems. One is a chirality
transfer from a chiral guest to the mesophasic host, as is the
typical case with the nematics involving a chiral dopant. Another
is an isothermal phase perturbation as typified by depression
Photochemical modification of the macroscopic chirality using
photoactive dopants has been extensively investigated,20 because
the systems offer new possibilities in the preparation of optical
* Corresponding author. Tel: +81-29-861-4671. Fax: +81-29-861-4673.
E-mail: n.tamaoki@aist.go.jp.
10.1021/jp034916e CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/15/2003

Helical Pitch Increase by Achiral Dopants
J. Phys. Chem. B, Vol. 107, No. 44, 2003 12055
Figure 1. Structures of cholesteric hosts and dopants.
components. In our previous paper,21-25 we have reported on
the specific properties and availability for full color recording
devices of glass-forming CLCs. In the course of the study, we
have found that addition of a small amount of 4,4′-di-n-
alkylazobenzene derivatives considerably increases the helical
pitch of dicholesteryl docosa-10,12-diyndioate (1) and that the
variation has a strong correlation with the length of the alkyl
chains of azobenzenes and with the ratio of the photoisomers,24
which is not accounted for by the impurity effect mentioned
above.
diameter; 0.01-mm wall thickness) and positioned on a hot stage.
The samples were heated to their isotropic phase, subsequently
cooled to the mesophase, and then exposed to a radiation beam
for 15 s with a 150-mm camera length. A high-pressure mercury
lamp (Ushio, 500 W) with appropriate glass filters (for irradia-
tion at 366 nm: Toshiba Glass Co., UV-35 + UVD-36C) was
employed as the irradiation source.
Materials. All solvents and reagents were obtained from
commercial sources and used without further purification unless
otherwise noted.
Cholesteric Liquid Crystals. Preparation of dicholesteryl
10,12-docosadiynedioate 1 was previously reported.7,24 Cho-
lesteryl chloride (CC) and cholesteryl nonanoate (CN) were
purchased and purified through flash column chromatography
before using.
Dopants. Azo Derivatives (a-n, b-n, c-n, and d-n).
Preparation of 4,4′-dialkylazobenzenes a-n were previously
reported.7,24 4,4′-Dialkoxyazoxybenzenes d-n were obtained
commercially. 4,4′-Dialkoxyazobenzenes b-n were synthesized
from 4,4′dihydroxyazobenzene, which was prepared according
to the literature.38
4-Dodecylazobenzene. To a mixture of 2.5 g (9.6 mmol) of
4-dodecylaniline and 1.2 g (9.8 mmol) of nitrobenzene was
added 1.0 g of NaOH over 30 min at 180 °C. After stirring for
more 15 min, the mixture was cooled to room temperature and
neutralized with dilute hydrochloric acid. The product extracted
with ethyl ether was purified through flash chromatography
(silica gel, dichloromethane-hexane (1:1)) and recrystallized
from ethanol to give 0.92 g (27%) as yellow crystals (mp. 72
°C). 1H NMR(CDCl3, 270 MHz) δ 0.68-2.02 (72H, m), 2.21-
In this paper, we discuss the mechanism of change in the
helical pitch induced by a solute. For this purpose, we employed
a variety of dialkyl compounds as solutes including azobenzene,
azoxybenzene, biphenyl, binaphthyl, benzene, etc., as shown
in Figure 1, and investigated the correlation of the helical pitch
with the structure of molecules added to the CLCs. Furthermore,
X-ray diffraction studies were conducted to observe the
microscopic structure of the CLC systems, and the mechanism
of change in the helical pitch induced by the dopants is
discussed.
Experimental Methods
General Methods. The NMR spectra were measured by a
JEOL GX 270 or LA 600 spectrometer. The transmission spectra
were recorded on UV-visible spectrophotometers, Hewlett-
Packard 8453. X-ray diffraction patterns were measured using
a Rigaku diffractometer (type 4037) with graded d-space
elliptical side-by-side multilayer optics, monochromated Cu KR
radiation (40 kV, 30 mA), and an imaging plate (R-Axis IV).
The samples were placed in quartz capillary tubes (1.5-mm

12056 J. Phys. Chem. B, Vol. 107, No. 44, 2003
Kidowaki et al.
2.32 (8H, m), 2.58 (2H, t), 2.69 (2H, t), 4.61(1H, m), 5.37 (1H,
m), 7.24 (2H, d), 7.32 (2H, d), 7.84 (2H, d), 7.94 (2H, d). Anal.
Calcd. for C24H34N2: C, 82.23; H, 9.78; N, 7.99. Found C,
82.51; H, 9.95; N, 7.77.
dodecane and 0.34 g (14.7 mmol) of magnesium in 15 mL of
dry ether for 2 h at room temperature. After refluxing for 12 h,
the reaction was terminated by the addition of 50 mL of dilute
hydrochloric acid, and the product was extracted with 50 mL
of ether, washed with a saturated aqueous solution of NaCl and
dried over MgSO4. The solvent was removed and the collected
solid was recrystallized from acetone to give 1.95 g (62%) of
colorless solid of mp. 77-78 °C. 1H NMR (CDCl3, 600 MHz)
δ 0.88 (6H, t), 1.25-1.37 (36H, m), 1.61-1.67 (4H, m), 2.63
(4H, t), 7.23 (4H, d), 7.49 (4H, d). Anal. Calcd. for C36H58: C,
88.24; H, 11.76. Found C, 88.62; H, 11.73.
4,4′-Dihydroxyazobenzene. This was obtained by recrystal-
lization from ethanol and hexane in 26% yield as reddish brown
1
crystals; mp. 218-219 °C. H NMR(DMSO- d6, 600 MHz) δ
6.90 (4H, d), 7.70 (4H, d), 10.14 (2H, s). Anal. Calcd. for
C12H10N2O2‚H2O: C, 62.07; H, 5.17; N, 12.07. Found C, 61.89;
H, 5.19; N, 11.78.
4,4′-Di-n-octyloxyazobenzene (b-8). This was prepared from
0.40 g (1.9 mmol) of 4,4′-dihydroxyazobenzene, 0.72 g (3.8
mmol) of 1-n-octylbromide, and 0.52 g (3.8 mmol) of K2CO3
by means of the Williamson reaction in 8 mL of N,N-
dimethylformamide (DMF) at 75 °C. After stirring for 24 h,
the product was extracted with dichloromethane, washed with
water, and recrystallized from hexane and ethyl acetate to give
0.39 g (48%) of reddish brown crystals of mp. 99-100 °C. 1H
NMR(CDCl3, 600 MHz) δ 0.89 (6H, t), 1.25-1.49 (20H, m),
1.78-1.85 (4H, m), 4.03 (4H, t), 6.98 (4H, d), 7.85 (4H, d).
Anal. Calcd. for C28H42N2O2: C, 76.67; H, 9.65; N. 6.39. Found
C, 76.56; H, 9.80; N, 6.41.
4,4′-Di-n-nonyloxyazobenzene (b-9). This was prepared in a
way similar to that for b-8 in 55% yield; mp. 104-105 °C. 1H
NMR(CDCl3, 600 MHz) δ 0.89 (6H, t), 1.25-1.49 (24H, m),
1.78-1.85 (4H, m), 4.03 (4H, t), 6.98 (4H, d), 7.85 (4H, d).
Anal. Calcd. for C30H46N2O2: C, 77.21; H, 9.93; N. 6.00. Found
C, 77.53; H, 10.14; N, 5.99.
4,4′-Di-n-dodecyloxybiphenyl (f-12). This was prepared from
2.0 g (10.7 mmol) of 4,4′-dihydroxybiphenyl, 5.4 g (21.4 mmol)
of 1-dodecylbromide, and 3.0 g (21.4 mmol) of K2CO3 by means
of the Williamson reaction in 13 mL of DMF at 65 °C. After
stirring for 10 h, the product was extracted with dichlo-
romethane, washed with water, and recrystallized from dichlo-
romethane to give 4.9 g (82%) of colorless plate crystals of
mp. 114-115 °C. 1H NMR (CDCl3, 600 MHz) δ 0.88 (6H, t),
1.25-1.38 (32H, m), 1.43-1.49 (4H, m), 1.76-1.82 (4H, m),
3.98 (4H, t), 6.93 (4H, d), 7.45 (4H, d). Anal. Calcd. for
C36H58O2: C, 88.24; H, 11.76. Found C, 88.62; H, 11.73.
4,4′-Di-n-heptyloxybiphenyl (f-7). Same procedure as for the
synthesis of f-12. White plate crystals in 79% yield; mp. 121-
1
122 °C. H NMR (CDCl3, 600 MHz) δ 0.90 (6H, t), 1.30-
1.38 (12H, m), 1.43-1.49 (4H, m), 1.76-1.82 (4H, m), 3.98
(4H, t), 6.94 (4H, d), 7.46 (4H, d). Anal. Calcd. for C26H38O2:
C, 81.62; H, 10.01. Found C, 81.34; H, 9.91.
4,4′-Di-n-octyloxybiphenyl (f-8). Same procedure as for the
synthesis of f-12. White plate crystals in 75% yield; mp. 117-
4,4′-Di-n-decyloxyazobenzene (b-10). This was prepared in
a way similar to that for b-8 in 72% yield; mp. 101-102 °C.
1H NMR(CDCl3, 600 MHz) δ 0.89 (6H, t), 1.25-1.49 (28H,
m), 1.78-1.85 (4H, m), 4.03 (4H, t), 6.98 (4H, d), 7.85 (4H,
d). Anal. Calcd. for C32H50N2O2: C, 77.68; H, 10.19; N, 5.66.
Found C, 77.93; H, 10.28; N, 5.60.
1
118 °C. H NMR (CDCl3, 600 MHz) δ 0.89 (6H, t), 1.26-
1.38 (20H, m), 1.43-1.49 (4H, m), 1.76-1.82 (4H, m), 3.98
(4H, t), 6.94 (4H, d), 7.46 (4H, d). Anal. Calcd. for C28H42O2:
C, 81.90; H, 10.31. Found C, 81.69; H, 10.31.
4,4′-Di-n-nonyloxybiphenyl (f-9). Same procedure as for the
synthesis of f-12. White plate crystals in 80% yield; mp. 115-
4,4′-Di-n-dodecyloxyazobenzene (b-12). This was prepared
in a way similar to that for b-8 in 81% yield; mp. 107-108 °C.
1H NMR(CDCl3, 600 MHz) δ 0.88(6H, t), 1.25-1.49 (36H,
m), 1.76-1.87 (4H, m), 4.03 (4H, t), 6.98 (4H, d), 7.85 (4H,
d). Anal. Calcd. for C36H58N2O2: C, 78.49; H, 10.61; N, 5.09.
Found C, 78.13; H, 10.46; N, 5.23.
1
116 °C. H NMR (CDCl3, 600 MHz) δ 0.89 (6H, t), 1.26-
1.38 (24H, m), 1.43-1.49 (4H, m), 1.76-1.82 (4H, m), 3.98
(4H, t), 6.94 (4H, d), 7.46 (4H, d). Anal. Calcd. for C30H46O2:
C, 82.14; H, 10.57. Found C, 81.87; H, 10.48.
4,4′-Di-n-decyloxybiphenyl (f-10). Same procedure as for the
synthesis of f-12. White plate crystals in 56% yield; mp. 114-
4,4′-Didodecylazoxybenzene (c-12). The oxidation of a-12
(0.25 g, 0.48 mmol) with hydrogen peroxide (6.0 mL, 30% aq.)
in 4.0 mL of acetic acid and 4.0 mL of chloroform was carried
out at 50 °C for 24 h. After the reaction, the product was
extracted with chloroform and purified through column chro-
matography (silica gel, dichloromethane-hexane (1:2)) to give
0.17 g (64%) as a light yellow powder (mp. 122 °C). 1H NMR
(CDCl3, 270 MHz) δ 0.82-0.90 (6H, m), 1.20-1.40 (36H, m),
1.58-1.70 (4H, m), 2.60-2.72 (4H, m), 7.24-7.30 (4H, m),
8.12 (2H, d), 8.19 (2H, d). Anal. Calcd. for C36H58N2O: C,
80.84; H, 10.93; N, 5.24. Found C, 81.22; H, 11.06; N, 5.19.
Biphenyl Derivatives (e-12 and f-n). 4,4′-Di-n-butyloxy-
biphenyl f-4, 4,4′-di-n-heptyloxybiphenyl f-5 and 4,4′-di-n-
hexyloxybiphenyl f-6 were purchased from Tokyo Chemical
Industry and used without further purification. The other
biphenyl derivatives were synthesized by the coupling reaction
between 4,4′-dibromobiphenyl or 4,4′-dihydroxybiphenyl and
the corresponding n-bromoalkanes.
1
115 °C. H NMR (CDCl3, 600 MHz) δ 0.88 (6H, t), 1.26-
1.38 (16H, m), 1.43-1.49 (4H, m), 1.76-1.82 (4H, m), 3.98
(4H, t), 6.94 (4H, d), 7.45 (4H, d). Anal. Calcd. for C32H50O2:
C, 82.35; H, 10.80. Found C, 82.82; H, 10.81.
4,4′-Di-n-undecyloxybiphenyl (f-11). Same procedure as for
the synthesis of f-12. White plate crystals in 18% yield; mp.
115-116 °C. 1H NMR (CDCl3, 600 MHz) δ 0.88 (6H, t), 1.26-
1.38 (16H, m), 1.43-1.49 (4H, m), 1.76-1.82 (4H, m), 3.98
(4H, t), 6.94 (4H, d), 7.45 (4H, d). Anal. Calcd. for C34H54O2:
C, 82.53; H, 11.00. Found C, 82.75; H, 11.03.
4,4′-Di-n-tridecyloxybiphenyl (f-13). Same procedure as for
the synthesis of f-12. White plate crystals in 81% yield; mp.
116-117 °C. 1H NMR (CDCl3, 600 MHz) δ 0.88 (6H, t), 1.25-
1.38 (16H, m), 1.43-1.49 (4H, m), 1.76-1.82 (4H, m), 3.98
(4H, t), 6.94 (4H, d), 7.45 (4H, d). Anal. Calcd. for C38H62O2:
C, 82.85; H, 11.34. Found C, 83.27; H, 11.37.
4,4′-Di-n-dodecylbiphenyl (e-12). 2.0 g (6.4 mmol) of 4,4′-
dibromobiphenyl and a catalytic amount of [1,2-bis(diphe-
nylphosphino)ethane]dichloronickel(II) were suspended in 30
mL of dry ether under a nitrogen atmosphere. To the suspension
was added 15 mL of THF solution of dodecylmagnesium
bromide, prepared by refluxing 3.2 g (12.8 mmol) of 1-bromo-
4,4′-Di-n-tetradecyloxybiphenyl (f-14). Same procedure as for
the synthesis of f-12. White plate crystals in 63% yield; mp.
114-115 °C. 1H NMR (CDCl3, 600 MHz) δ 0.88 (6H, t), 1.25-
1.38 (16H, m), 1.43-1.49 (4H, m), 1.76-1.82 (4H, m), 3.98
(4H, t), 6.94 (4H, d), 7.45 (4H, d). Anal. Calcd. for C40H66O2:
C, 82.98; H, 11.49. Found C, 83.21; H, 11.45.

Helical Pitch Increase by Achiral Dopants
J. Phys. Chem. B, Vol. 107, No. 44, 2003 12057
Binaphthyl Derivatives (g-n and h-12). The synthesis of
4,4′-diethyl-1,1′-binaphthyl g-2 was conducted by the catalytic
coupling reaction of 1-ethylnaphthalene using NaNO2 with CF3-
SO3H.39 The synthesis of 4,4′-dialkyl-1,1′-binaphthyls was
started from the coupling reaction of 1-bromonaphthalene to
afford 1,1′-binaphthyl,40 and then bromination41 was conducted
with bromine. Alkylation was carried out in the same way as
for the dialkylbiphenyls. 4,4′-Didodecyloxy-1,1′-binaphthyl h-12
was synthesized from 1-dodecyloxynaphthalene with aluminum
chloride by means of the Scholl reaction.42
4,4′-Di-n-dodecyloxybenzene (j-12). Colorless solid in 62%
yield. mp. 74-75 °C. 1H NMR (CDCl3, 600 MHz) δ 0.88 (6H,
t), 1.25-1.46 (36H, m), 1.72-1.76 (4H, m), 3.89 (4H, t), 6.82
(4H, s). Anal. Calcd. for C30H54O2: C, 80.65; H, 12.18; O, 7.16.
Found C, 80.45; H, 12.20; O, 7.27.
4,4′-Di-n-tetradecyloxybenzene (j-14). Colorless solid in 24%
yield; mp. 80-81 °C. 1H NMR (CDCl3, 600 MHz) δ 0.88 (6H,
t), 1.25-1.46 (44H, m), 1.72-1.76 (4H, m), 3.89 (4H, t), 6.81
(4H, s). Anal. Calcd. for C34H62O2: C, 81.21; H, 12.43. Found
C, 81.54; H, 12.60.
4,4′-Diethyl-1,1′-binaphthyl (g-2). This was obtained in 49%
yield as colorless crystals; mp. 106-107 °C. 1H NMR (CDCl3,
600 MHz) δ 1.48 (6H, t), 3.22 (4H, q), 7.26 (2H, dd), 7.40-
7.46 (6H, m), 7.49 (2H, dd), 8.15 (2H, d). Anal. Calcd. for
C24H22: C, 92.86; H, 7.14. Found C, 92.57; H, 7.12.
4,4′-Di-n-hexadecyloxybenzene (j-16). Colorless solid in 57%
yield; mp. 86-87 °C. 1H NMR (CDCl3, 600 MHz) δ 0.88 (6H,
t), 1.25-1.46 (52H, m), 1.72-1.76 (4H, m), 3.89 (4H, t), 6.81
(4H, s). Anal. Calcd. for C38H70O2: C, 81.65; H, 12.62. Found
C, 81.498; H, 12.59.
Other Compounds. Chiral agents, R811 and S811 (Merck),
were used as received. 2,6-Di-n-dodecyloxynaphthalene was
synthesized by the same procedure as that for the synthesis of
f-12 under N2 atmosphere. The synthesis of 2,7-di-n-undecy-
loxyfluorene was started from the preparation of 2,7-dihydroxy-
fluorene and conducted by means of the Williamson reaction.
2,6-Di-n-dodecyloxynaphthalene (k-12). The product was
recrystallized from ethyl acetate to give colorless crystals in
27% yield. mp. 89-90 °C. 1H NMR(CDCl3, 600 MHz) δ 0.89-
(6H, t), 1.25-1.51(36H, m), 1.80-1.85(4H, m), 4.04(4H, t),
7.08(2H, s), 7.11(2H, d), 7.60(2H, d). Anal. Calcd. for
C34H56O2: C, 82.20; H, 11.36. Found C, 82.69; H, 11.36.
2,7-Dihydroxyfluorene. The mixture of 4.8 g (15 mmol) of
2,7-dibromofluorene, 0.25 g (1.7 mmol) of CuBr(I), and 88 mL
of NaOH aqueous solution (20%) was sealed into an autoclave
and stirred at 230 °C for 6 h. The mixture was allowed to room
temperature and filtered. The filtrate was acidified with con-
centrated H2SO4 to afford the precipitate. The collected pre-
cipitate was purified through flash chromatography (silica gel,
ethyl acetate) to give a light yellow solid in 8.8% yield. H
NMR (DMSO- d6, 600 MHz) δ 3.61 (2H, s), 6.71 (2H, d), 6.90
(2H, s), 7.46 (2H, d), 9.25 (2H, s).
2,7-Di-n-undecyloxyfluorene (l-11). The product was recrys-
tallized from ethyl acetate and hexane to give light yellow
crystals in 28% yield; mp. 113-114 °C. 1H NMR (CDCl3, 600
MHz) δ 0.88 (6H, t), 1.25-1.51 (32H, m), 1.77-1.82 (4H, m),
3.81 (2H, s), 3.99 (4H, t), 6.88 (2H, d), 7.05 (2H, s), 7.54 (2H,
d). Anal. Calcd. for C35H54O2: C, 82.95; H, 10.74. Found C,
83.32; H, 10.75.
4,4′-Diheptyl-1,1′-binaphthyl (g-7). This was obtained in 62%
yield as colorless crystals; mp. 106-107 °C. 1H NMR (CDCl3,
600 MHz) δ 0.91 (6H, t), 1.32-1.54 (16H, m), 1.82-1.88 (4H,
m), 3.13-318 (4H, m), 7.26-7.28 (2H, m), 7.40-7.51 (8H,
m), 8.15 (2H, d). Anal. Calcd. for C34H42: C, 90.61; H, 9.39.
Found C, 90.45; H, 9.47.
4,4′-Didecyl-1,1′-binaphthyl (g-10). This was obtained in 16%
1
yield as colorless crystals; mp. 42-43 °C. H NMR (CDCl3,
600 MHz) δ 0.89 (6H, t), 1.25-1.54 (28H, m), 1.82-1.88 (4H,
m), 3.13-318 (4H, m), 7.26 (2H, dd), 7.39 (2H, d), 7.41-7.43
(4H, m), 7.48 (2H, dd), 8.13 (2H, d). Anal. Calcd. for C40H54:
C, 89.82; H, 10.18. Found C, 89.58; H, 10.21.
4,4′-Didodecyl-1,1′-binaphthyl (g-12). This was obtained in
11% yield as colorless crystals; mp. 55-56 °C. 1H NMR
(CDCl3, 600 MHz) δ 0.88 (6H, t), 1.25-1.54 (36H, m),
1.82-1.88 (4H, m), 3.13-318 (4H, m), 7.26 (2H, dd), 7.39
(2H, d), 7.41-7.43 (4H, m), 7.48 (2H, dd), 8.13 (2H, d). Anal.
Calcd. for C44H62: C, 89.43; H, 10.57. Found C, 89.32; H,
10.49.
1
4,4′-Ditetradecyl-1,1′-binaphthyl (g-14). This was obtained
1
in 70% yield as colorless crystals; mp. 62-63 °C. H NMR
(CDCl3, 600 MHz) δ 0.88 (6H, t), 1.25-1.54 (44H, m), 1.82-
1.88 (4H, m), 3.13-318 (4H, m), 7.25 (2H, dd), 7.38 (2H, d),
7.41-7.43 (4H, m), 7.48 (2H, dd), 8.13 (2H, d). Anal. Calcd.
for C48H70: C, 89.10; H, 10.90. Found C, 89.06; H, 10.84.
1-Dodecyloxynaphthalene. Same procedure as for the syn-
thesis of f-12 under N2 atmosphere. The product was purified
through flash chromatography (silica gel, hexane) to give a
1
colorless liquid in 93% yield. H NMR (CDCl3, 600 MHz) δ
0.88 (6H, t), 1.25-1.59 (18H, m), 1.90-1.95 (4H, m), 4.14
(4H, t), 6.80 (1H, d), 7.34-7.49 (4H, m), 7.78 (1H, d), 8.29
(1H, d). Anal. Calcd. for C10H32O: C, 84.33; H, 10.32. Found
C, 84.33; H, 9.85.
4,4′-Didodecyloxy-1,1′-binaphthyl (h-12). This was obtained
in 18% yield as colorless crystals; mp. 127-128 °C. 1H NMR
(CDCl3, 600 MHz) δ 0.89 (6H, t), 1.25-1.47 (36H, m), 1.95-
2.00 (4H, m), 4.21 (4H, t), 6.90 (2H, d), 7.28 (2H, dd), 7.34-
7.37 (4H, m), 7.44 (2H, dd), 8.38 (2H, d). Anal. Calcd. for
C44H62O2: C, 84.83; H, 10.03; O, 5.14. Found C, 84.75; H,
10.05; O, 5.25.
Results and Discussion
General Properties of Cholesteric Films Containing
Dopants. Cholesteryl ester 1 containing several wt % of dopants
shows a cholesteric phase at 78-116 °C same as pure 1. At
this concentration, the doping hardly affects the transition
temperature of 1 at all. The selective reflection color of 1 with
or without dopants is easily observed by shear stress inducing
the grandjean alignment at their cholesteric temperature. The
temperature dependence of the pitch, dP/dT, for all sample films
is minus, and the wavelength of the maximum reflection
decreases with an increase in temperature, the same as for pure
1. The films of 1 containing dopants are vitrified by quenching
from their cholesteric temperature to 0 °C in ice water. The
obtained glassy state of the CLC is sufficiently stable on an
experimental time scale at ambient temperature.
Benzene Derivatives (i-12 and j-n). These compounds were
synthesized from 4,4′-dibromobenzene or hydroquinone by
methods similar to those for the above-mentioned dialkyl and
dialkoxy derivatives.
4,4′-Di-n-dodecylbenzene (i-12). Colorless solid in 40% yield;
1
mp. 39-41 °C. H NMR (CDCl3, 600 MHz) δ 0.89 (6H, t),
Effect of the Center Structure of Dopants on the Pitch.
To investigate the effect of the center structure of the dopants
on the helical pitch of 1, we employed compounds substituted
with dodecyl groups at either end of the molecules as a dopant.
1.25-1.33 (36H, m), 1.56-1.60 (4H, m), 2.56 (4H, t), 7.08
(4H, s). Anal. Calcd. for C30H54: C, 86.88; H, 13.12, Found C,
87.22; H, 13.70.

12058 J. Phys. Chem. B, Vol. 107, No. 44, 2003
Kidowaki et al.
Figure 2. Absorption spectra of the glassy films of pure 1 and 1
containing 2 wt % of a-12, c-12, e-12, g-12, and i-12, respectively.
The films were quenched from 90 °C, and their thickness was controlled
by a 5-µm spherical silica spacer.
TABLE 1: Wavelength of UV-Visible Absorption Peaks
(λmax) for the Glassy Filmsa (5-µm thickness) of 1 With and
Without 2 wt % of a-12, c-12, e-12, g-12, and i-12
λmax/nm
Figure 3. Change in ∆λmax of 1 as a function of the concentration of
a-12-l-11, respectively, at 90 °C.
dopants
electronic transition
selective reflection
non
507
639
724
540
470
474
a-12
c-12
e-12
g-12
i-12
343, 442
340
261
295, 302
a The films were quenched from 90 °C.
Figure 2 shows the UV-visible absorption spectra of the glassy
films of 1 with and without 2 wt % of a-12, c-12 e-12, g-12,
and i-12, respectively. The absorption bands above 400 nm are
assigned to the selective reflections due to the helical alignment
of the molecules, while those below 400 nm are assigned to
the electronic transitions inherent in the dopants. In case of a
film containing a-12, a weak absorption is observed around 442
nm due to the n-π* forbidden transition, typical for azobenzene
derivatives. Their wavelength of absorption maxima, λmax, is
summarized in Table 1. λmax due to the selective reflection for
the films doped with a-12, c-12, and e-12 is observed at 639,
724, and 540 nm, respectively. These values are longer than
the 507 nm of pure 1, indicating that doping of a-12, c-12, and
e-12 increases the pitch. In the case of e-12, although it is
difficult to obtain reproducible results because the solubility of
e-12 in the host CLC should be low, the direction of the shift
in the reflection band shows the same tendency. On the other
hand, doping of g-12 and i-12 decreases the pitch; their λmax
values are 470 and 474 nm, respectively. The dopants increasing
the pitch have a structural resemblance in their rod shape core,
whereas those with decreasing pitch have a bulky shape or a
small core. In other words, dopants having a mesogenic structure
increase the pitch. In the case of g-12, we should consider the
chiral effect as well as the structural effect due to its axial
chirality. However, the chirality effect can be ignored here
because g-12 is a racemic structure. We will discuss the details
of this effect later.
Figure 4. Absorption spectra of a glassy film of 1 containing 2 wt %
of d-12 before and after photoirradiation, respectively. The photoirra-
diation was performed with a 366-nm light saurce at 90 °C.
increase in the concentration. These dopants are classified into
two types, increasing the pitch (a-12-f-12, k-12, and l-11) and
decreasing it (g-n-j-n). In the former case, the ∆λmax increases
nonlinearly and the difference in ∆λmax among the dopants
becomes clear with an increase in the concentration. It should
be noted that the shift rises to the near-infrared region at higher
concentration and that such large values could not be attained
without dopants. In the latter case, the shift in λmax is small
compared to the former, even at the higher concentration, and
∆λmax decreases in a monotonic manner. These results indicate
that the center structure plays a decisive role in changing the
pitch. Corresponding to the structural effect, it may worth
pointing out that the photochemical isomerization of the dopants
induces a change in the helical pitch as reported for azobenzene
derivatives in our previous paper.24 Exposing a film of 1
including d-12 to UV-light induces a shift in the reflection band
from 725 to 515 nm and a decrease in the electronic transition
band (361 nm) of d-12 (Figure 4). The shorter wavelength shift
is induced by the photoisomerization of d-12 from the E to Z
isomer because the Z isomer has a bent core structure.
The effect is more pronounced when the change in λmax of
the reflection bands is expressed as a function of the concentra-
tion of the dopants. We extended our survey of dopants to the
molecules having alkoxy groups as their side groups and
naphthalene (k-12) and fluorene (l-11) derivatives. The shift in
reflection bands is represented by ∆λmax that is obtained by
subtracting the λmax of pure 1 from the λmax of 1 with the dopants
(Figure 3). The absolute values of ∆λmax increase with an
Effect of Alkyl Chain Length of Dopants on the Pitch.
We have investigated the effect of alkyl chain length of the
dopants on the pitch of 1, as shown in Figure 5. Dependence
of the chain length is clearly observed for the films of 1
containing b-n, d-n, and f-n, and their plots show maximum

Helical Pitch Increase by Achiral Dopants
J. Phys. Chem. B, Vol. 107, No. 44, 2003 12059
Figure 7. Change in ∆λmax for the films of 1 containing R811, S811,
and their racemic mixture as a function of concentration at 90 °C.
Figure 5. Change in ∆λmax for the films of 1 containing 2 wt % of
a-n,24 b-n, d-n, f-n, g-n, and j-n as a function of the alkyl chain
length, n, (CnH2n+1) at 90 °C.
Figure 8. Change in ∆λmax for the cholesteric mixture containing 2
wt % of f-n as a function of the alkyl chain length at 32 °C (circles),
35 °C (squares), 40 °C (triangles).
the amount of which roughly corresponds to the sum of the
shifts due to half portions of each enantiomer. To explain this
result, we should consider both chirality and structural effects.
These enantiomeric dopants have mesogenic cores and alkyl
chains on both sides, which indicates that the dopants increase
the pitch of 1 on the basis of the above results from the achiral
dopants. In general, to consider the chirality effect, we can adopt
the additivity law into the behavior of the pitch. When a chiral
dopant induces the same sense as the host cholesterics, their
pitch will decrease. Conversely, when the sense is opposite, the
pitch will increase. R811 is a case of the opposite sense, while
S811 is a case of the same sense. R811 increases the pitch by
both structural and chirality effects. However, S811 hardly
affects the pitch due to the opposing effects of the structure
and chirality. It is known that when the 3â chain length of the
steroid ring system is long enough, the compounds become left-
handed cholesterics, such as cholesteryl nonanoate and choles-
teryl oleate.26 The observation of the reflection light with circular
polarized plates shows that the samples reflect L-circular
polarized light. Circularly polarized light having the same
handedness as the cholesterics will be reflected. Therefore, 1
has a left-handed sense, which is consistent with the above
explanation of the effect of the chiral dopants.
Doping Effect on Low Molecular Weight Cholesteric
Mixture. We conducted a study of dopant-induced change in
the pitch with the classical low molecular weight cholesteric
mixture of cholesteryl chloride (CC) and cholesteryl nonanoate
(CN). We chose the mixing ratio, CC/CN ) 18:82 because the
mixture shows the reflection bands in the visible region and a
negative sign of dP/dT the same as 1. The significant depen-
dence of the alkyl chain length of the doped f-n and
temperature on the pitch is observed for the cholesteric mixture
(Figure 8) as observed for 1. The plots at 32 and 35 °C show
maximum peaks at carbon number of 7 and 6, respectively. The
Figure 6. Change in ∆λmax of 1 with 2 wt % of a-12-j-12 as a function
of temperature.
values at 9, 9, and 11 of carbon numbers (n), respectively. In
contrast, the dependence is hardly observed for the films with
the dopants (g-n, j-n) which decrease the pitch. The depen-
dence of the pitch on the alkyl chain is not due to the difference
in concentration resulting from the difference in their molecular
weight. d-3 and f-4 decrease in the pitch even though they have
mesogenic cores, indicating that the alkyl chain length is an
important factor as well as the center structure. It should be
mentioned that 4-dodecylazobenzene decreases the pitch, al-
though it has a mesogenic core and a dodecyl group. To increase
the pitch, the dopants should have the proper length of alkyl
chains at both sides of the molecules.
Effect of Temperature on Doping. The doping-induced
variation in λmax at various temperatures is plotted in Figure 6.
The absolute values of ∆λmax of all films decrease with an
increase in temperature. Above 100 °C, the change in pitch is
hardly observed for any cholesteric mixture. The effect of doping
on the pitch is apparent below 100 °C. This result shows that
the effect of the dopants diminishes with an increase in
temperature and has a threshold around 100 °C. Because the
threshold temperature is dopant-independent, the temperature
depends only on the thermal properties of the host cholesterics.
Effect of Chirality of Dopants. The effect of chiral dopants
on the pitch of 1 is shown in Figure 7. R811 and S811 are
chiral dopants exerting an effect in the opposite direction on
the induced cholesteric pitch of nematics, with R811 having a
right-handed and S811 having a left-handed sense. R811
increases ∆λmax with an increase in concentration, while S811
hardly affects the shift in reflection band. The film doped with
their racemic mixture shows a small shift to a longer wavelength,

12060 J. Phys. Chem. B, Vol. 107, No. 44, 2003
Kidowaki et al.
Figure 10. Effect of the alkyl chain length on the half-width (circles)
and the relative intensity (squares) of the small-angle X-ray diffraction
peaks for 1 containing 2 wt % of a-n at 90 °C (closed markers) and
110 °C (open markers). The lines are visual guides.
Figure 9. X-ray diffraction patterns for pure 1 (a) and 1 containing 2
wt % of a-11 (b) at 90 °C. Inset is the magnification of the small angle
peaks.
values of ∆λmax decrease with an increase in the temperature.
At 40 °C, the dependence of the alkyl chain length is hardly
observed. The alkyl chain length of f-n giving a maximum
change in the pitch in the cholesteric mixture CC/CN is shorter
than that in 1.
X-ray Diffraction Study of the Cholesteric Phase. X-ray
diffraction patterns of unaligned samples of pure 1 and 1 with
2 wt % of a-11 are shown in Figure 9. There are major
diffraction peaks at two diffraction angles. The broader peaks
at 17.1° (d ) 5.18 Å) are ascribed to the interaction of
neighboring, parallel molecules. The peaks at 3.15° (d ) 28.03
Å) would be due to the order parameter fluctuations of the
cholesteric regions from and to the smectic-like arrangement,
which is termed cybotactic domains.27-31 The interlayer dis-
tances are slightly smaller compared to half of the estimated
molecular length of 1 (L ) 60.21 Å). The value (d/L ) 0.47)
suggests that 1 forms an intercalated structure in the transitional
smectic clusters.32,33 The smaller angle peak for 1 containing
a-11 is larger and sharper than that of pure 1. The sharpness of
the peaks can be represented by full widths at half-maximum
(fwhm), and the values for pure and doped 1 are 0.414° and
0.325°, respectively. These results indicates that doping of a-11
promotes the generation of smectic clusters in the cholesterics
because the intensity of the peaks depends on the total number
of scattering centers in the samples, and the fwhm can be
connected with the correlation lengths of the periodic structure
by 2/∆Q, where ∆Q is fwhm when the diffraction intensity is
plotted as function of the scattering vector Q () (4π sin θ)/
λ).27,30 To obtain further information on the dopant-induced
smectic clusters in the cholesterics, X-ray diffraction measure-
ment was performed for 1 containing 2 wt % of azobenzenes,
a-n, at 90 °C and 110 °C. The half width and relative intensity
of the smaller angle peaks are plotted as a function of alkyl
chain length in Figure 10. A significant dependence of the fwhm
and relative intensity on the alkyl chain length is observed for
the samples measured at 90 °C, and the values show the
minimum and maximum at a carbon number of 11. However,
the dependence is not observed for those values measured at
110 °C. These results mean that smectic domains are most
pronounced at a-11 and that the induction of smectic clusters
depends on the alkyl chain length of the dopants. This situation
(dependence on both temperature and alkyl chain length) is
similar to the phenomenon of dopant-induced modification of
the helical pitch as mentioned above.
Figure 11. Relation between the wavelength of maximum reflections
and the parameters (half-width, circles; and relative intensity, squares)
of small-angle X-ray diffraction peaks for 1 containing 2 wt % of a-n
at 90 °C. These data are extracted from the results of Figures 5 and 6.
in λmax brings about an increase in the intensity of the small
angle diffraction peaks and a decrease in their half-width. This
means that the cholesterics with longer pitch have larger size
and a greater number of smectic clusters. In other words, smectic
clusters would cause an increase in the helical pitch. There have
been some reports mentioning the relationship between the
cholesteric pitch and smectic clusters in the cholesterics.34-37
The anomalous increase in the cholesteric pitch near the
cholesteric-smectic transition temperature has been explained
by the formation of the smectic clusters. In the present case,
the smectic domains are induced by a slight amount of dopants
having the proper molecular structure, leading to an anomalous
increase in the pitch because the molecules in the smectic
clusters align parallel to each other without twisting. It should
be stressed that this phenomenon is different from the impurity
effect which leads to a change in the phase transition temperature
of the host CLCs. The fact that the change in the pitch is very
sensitive to alkyl chain length supports this mechanism. The
shape and size of the dopants affect their ability to induce
smectic clusters because the smectic alignment is a layered
structure where the molecules are packed closely and aligned
in a parallel manner. In this case, the compatibility of the
molecular length between the guest dopants and host cholesterics
plays a crucial role. As shown in Figures 5 and 10, b-9, d-9,
and f-11 have a molecular length causing a strong interaction
with 1, and their estimated molecular lengths show similar
values, 32.94, 33.12, and 36.45 Å, respectively. In case of the
cholesteric mixture CC/CN, the length of the alkyl chains of
f-n causing a maximum change in the pitch is observed at 6
or 7, and their estimated molecular lengths are 24.25 and 26.52
Å, respectively. It seems that the difference in the molecular
length of f-n giving a maximum change in the pitch between
The effect of alkyl chain length of the dopants on both the
wavelength of selective reflections and the X-ray peaks of the
cholesterics shown in Figures 5 and 10, respectively, offers an
explicit relation between the helical pitch and cybotactic clusters.
The relation is more clearly shown in Figure 11. The increase

Helical Pitch Increase by Achiral Dopants
J. Phys. Chem. B, Vol. 107, No. 44, 2003 12061
(8) Gleeson, H. F. In Handbook of Liquid Crystals; Demus, D., Goodby,
J., Gray, G. W., Spiess, H. W., Vill, V., Eds.; Wiley-VCH: Weinheim,
1998; Vol. 1, Chapter 9, p 823.
(9) Gottarelli, G.; Hibert, M.; Samori, B.; Solladie, G.; Spada, G. P.;
Zimmermann, R. J. Am. Chem. Soc. 1983, 105, 7318.
(10) Gottarelli, G.; Spada, G. P. Mol. Cryst. Liq. Cryst. 1985, 123, 377.
(11) Gottarelli, G.; Spada, G. P.; Varech, D.; Jacques, J. Liq. Cryst. 1986,
1, 29.
1 and the cholesteric mixture originates from the difference in
their molecular length. The small-angle X-ray diffraction peak
due to the smectic-like alignment in the CC/CN mixture is at
3.43° (d ) 25.72 Å). This layer distance is slightly shorter than
28.03 Å in 1 and consistent with the molecular length of f-6
and f-7.
(12) Gottarelli, G.; Osipov, M. A.; Spada, G. P. J. Phys. Chem. 1991,
95, 3879.
(13) Baessler, M.; Laronge, T. M.; Labes, M. M. J. Chem. Phys. 1969,
51, 3213.
Conclusions
These results lead to the conclusion that the dopants with a
mesogenic core and the proper length of alkyl chains induce
an anomalous increase in the helical pitch of the cholesterics
through the induction of smectic clusters. The shift in the
reflection band induced by several wt % of dopants reaches the
infrared region, which is impossible to obtain for pure host
cholesterics. An X-ray diffraction study of the cholesterics
elucidated that the doping of appropriate solutes promotes the
generation and growth of the domain acting as a smectic core.
The increase in both the amount and size of the smectic domains
is attributed to the increase in the pitch. The most effective
molecular length of the dopants to increase the pitch depends
on the host cholesterics, which reflects that the layer structures
of the smectic domains mainly consist of host cholesterics.
The information on the mechanism of the dopant-induced
change in the cholesteric pitch here is of great importance in
molecular modeling to achieve optimum tailored properties of
the cholesteric systems.
(14) Sackmann, E. J. Am. Chem. Soc. 1971, 93, 7088.
(15) Kronberg, B.; Patterson, D. J. Chem. Soc., Faraday Trans. 2 1976,
72, 1687.
(16) Kronberg, B.; Glison, D. F. R.; Patterson, D. J. Chem. Soc., Faraday
Trans. 2 1976, 72, 1673.
(17) Luckhurst, G. R.; Gray, G. W. The Molecular Physics of Liquid
Crystals; Academic Press: London, 1979.
(18) Kronberg, B.; Patterson, D. J. Chem. Soc., Faraday Trans. 1 1981,
77, 1223.
(19) Croucher, M. D.; Patterson, D. J. Chem. Soc., Faraday Trans. 1
1981, 77, 1237.
(20) Tamaoki, N. AdV. Mater. 2001, 13, 1135, and references therein.
(21) Mallia, V. A.; Tamaoki, N. J. Mater. Chem. 2003, 13, 219.
(22) Tamaoki, N.; Aoki, Y.; Moriyama, M.; Kidowaki, M. Chem. Mater.
2003, 15, 719.
(23) Moriyama, M.; Tamaoki, N. Chem. Lett. 2001, 1142.
(24) Moriyama, M.; Song, S.; Matsuda, H.; Tamaoki, N. J. Mater. Chem.
2001, 11, 1003.
(25) Tamaoki, N.; Song, S.; Moriyama, M.; Matsuda, H. AdV. Mater.
2000, 12, 94.
(26) Kozawaguchi, H.; Wada, M. Jpn. J. Appl. Phys. 1975, 14, 651.
(27) Albertini, G.; Dubini, B.; Melone, S.; Ponzibossi, M. G. Mol. Cryst.
Liq. Cryst. 1981, 70, 1447.
(28) Bobrovsky, A. Y.; Boiko, N. I.; Shibaev, V. P. Liq. Cryst. 2000,
27, 57.
(29) De Vries, A. Mol. Cryst. Liq. Cryst. 1985, 131, 125.
(30) Francescangeli, O.; Laus, M.; Galli, G. Phys. ReV. E 1997, 55, 481.
(31) McMillan, W. L. Phys. ReV. A 1972, 6, 936.
(32) Marcelis, A. T. M.; Koudijs, A.; Klop, E. A.; Sudholter, E. J. R.
Liq. Cryst. 2001, 28, 881.
Acknowledgment. M.K. thanks New Energy and Industrial
Technology Development Organization (NEDO) for a fellow-
ship.
References and Notes
(33) Blatch, A. E.; Fletcher, I. D.; Luckhurst, G. R. Liq. Cryst. 1995,
18, 801.
(34) Ania, F.; Stegemeyer, H. Mol. Cryst. Liq. Cryst. Lett. 1985, 2, 67.
(35) Chen, S.; Wu, J. J. Mol. Cryst. Liq. Cryst. 1982, 87, 197.
(36) Chilaya, G. S.; Lisetski, L. N. Mol. Cryst. Liq. Cryst. 1986, 140,
243.
(37) Voss, J.; Sackmann, E. Z. Naturforsch. 1973, 28a, 1496.
(38) Ruslim, C.; Ichimura, K. J. Mater. Chem. 1999, 9, 673.
(39) Tanaka, M.; Nakashima, H.; Fujiwara, M.; Ando, H.; Souma, Y.
J. Org. Chem. 1996, 61, 788.
(1) Coles, H. In Handbook of Liquid Crystals; Demus, D., Goodby,
J., Gray, G. W., Spiess, H. W., Vill, V., Eds.; Wiley-VCH: Weinheim,
1998; Vol. 2A, Chapter 4, p 382.
(2) Yang, D. K.; Doane, J. W.; Yaniv, Z.; Glasser, J. Appl. Phys. Lett.
1994, 64, 1905.
(3) Broer, D. J.; Lub, J.; Mol, G. N. Nature 1995, 378, 467.
(4) Hikmet, R. A. M.; Kemperman, H. Nature 1998, 392, 476.
(5) Jacobs, S. D.; Cerqua, K. A.; Marshall, K. L.; Schmid, A.;
Guardalben, M. J.; Skerrett, K. J. J. Opt. Soc. Am. B Opt. Phys. 1988, 5,
1962.
(6) Maxein, G.; Mayer, S.; Zentel, R. Macromolecules 1999, 32, 5747.
(7) Tamaoki, N.; Parfenov, A. V.; Masaki, A.; Matsuda, H. AdV. Mater.
1997, 9, 1102.
(40) Maigrot, N.; Mazaleyrat, J. P. Synthesis-Stuttgart 1985, 317.
(41) Hutchins, L. G.; Pincock, R. E. J. Org. Chem. 1980, 45, 2414.
(42) Clowes, G. A. J. Org. Soc. C 1968, 2519.