Helical twisting power of optically active spiro compounds with a 3,3′-(4H, 4′H)-spirobi(2H-naphtho[1,2-b]pyran) skeleton

Article abstract of DOI:10.1246/cl.2008.1124

New chiral dopants for nematic liquid crystals, containing a (5)-3,3′-(4H, 4′H)-spirobi(2H-naphtho[1,2-b]pyran) skeleton, were synthesized and their helical twisting powers (HTP) were evaluated. A chiral dopant with two terphenyl structures showed the highest molar HTP value of 50.3 (μm^-1 mol^-1 kg. Copyright

Full text of DOI:10.1246/cl.2008.1124

1124
Chemistry Letters Vol.37, No.11 (2008)
Helical Twisting Power of Optically Active Spiro Compounds
with a 3,30-(4H,40H)-Spirobi(2H-naphtho[1,2-b]pyran) Skeleton
Kenta Tojo, Tatsuya Arisawa, Yoshio Aoki, and Daiyo Terunumaꢀ
Graduate School of Science and Engineering, Saitama University,
255 Shimo-ohkubo, Sakura-ku, Saitama 338-8570
(Received August 18, 2008; CL-080790; E-mail: teru@fms.saitama-u.ac.jp)
New chiral dopants for nematic liquid crystals, containing
a (S)-3,30-(4H,40H)-spirobi(2H-naphtho[1,2-b]pyran) skeleton,
were synthesized and their helical twisting powers (HTP) were
evaluated. A chiral dopant with two terphenyl structures showed
the highest molar HTP value of 50.3 mmꢁ1 molꢁ1 kg.
R
O
O
n
O
O
Chirality is one of the most interesting subjects in the field
of liquid crystals. Chiral nematic liquid crystals with a macro
helical structure are currently used in liquid crystal display
(LCD) devices. Generally, chiral nematic materials consist
of a mixture of achiral host nematic liquid crystals and a chiral
dopant with large helical twisting power (HTP).1,2 A helical
structure is induced in the chiral nematic liquid crystals by
the interaction between the host liquid crystalline molecules
and the chiral dopant. Many optically active compounds with
asymmetric carbons have been synthesized and used as dopants
to induce chiral nematic phases.1–8 It has been reported that
biaryl chiral dopants with axial chirality show large HTP
values.2,9–12 In particular, conformational fixation of 1,10-
binaphthyl-2,20-diol (BINOL) derivatives has been shown to
be effective in achieving large HTP values. In previous studies,
conformational fixation was found to increase the molar HTP
value of BINOL derivatives from 0.38 to 21 mmꢁ1 molꢁ1 kg.2,12
Optically active spiro compounds have a more rigid struc-
ture than axially chiral compounds. Therefore, optically active
spiro compounds have been used as chiral ligands for asymmet-
ric synthesis13 and as chiral dopants for smectic liquid crystalline
mixtures.14 However, the subject of optically active compounds
with a spiro structure remains largely unexplored in the field of
chiral dopants for nematic liquid crystals.
O
O
R
n
−OC8H17
2*: n = 1, R =
3*: n = 2, R = −OC8H17
4*: n = 3, R =
−C7H15
Figure 2. Structures of novel chiral dopants 2ꢀ–4ꢀ.
1) SOCl2 / toluene, reflux
(S)-(-)-1*
2*, 3*, 4*
2) phenols, Et3N / CH2Cl2
Scheme 1. Synthesis of novel chiral dopants 2ꢀ–4ꢀ.
and discuss their potential as chiral dopants for nematic liquid
crystals (2ꢀ–4ꢀ, Figure 2). The rigid structure of the spiro part
is thought to suppress conformational changes. Based on the
interaction of the aromatic rings of the chiral dopants with those
of the host liquid crystalline molecules, these novel dopants
were expected to show large HTP values.
The novel chiral dopants 2ꢀ–4ꢀ were derived from (S)-(ꢁ)-
1ꢀ and phenols (Scheme 1).16 The chiral nematic liquid crystal-
line mixtures were prepared by adding the chiral dopant (1 wt %)
to the host nematic liquid crystal (ZLI-1132, Merck).17 The
helical pitch of the chiral nematic phase was measured using
Cano wedge cells.18 The HTP values were calculated based on
eq 1, where p is the pitch of the chiral nematic phase (in mm)
and c is the mass fraction of the chiral dopant. In order to
describe HTP per molecule, we used the value of molar helical
twisting power (MHTP), as defined in eq 2, where Md is the
molecular weight of the chiral dopant.6
We recently reported the synthesis of a novel optically
active spiro carboxylic acid, 3,30-(4H,40H)-spirobi(2H-naphtho-
[1,2-b]pyran)-6,60-dicarboxylic acid (1ꢀ, Figure 1).15 The two
naphthalene rings of 1ꢀ are fixed by the spiro structure. The
compounds may easily be converted to its ester derivatives.
In this paper, we report the HTP values of derivatives of 1ꢀ
HO
O
HTP ¼ ðpcÞꢁ1
MHTP ¼ HTP ꢂ Md ꢂ 10ꢁ3
ð1Þ
ð2Þ
O
O
The helical senses of the chiral nematic phases were deter-
mined by the contact method using a reference mixture of the
host liquid crystal and cholesteryl nonanoate, which features a
left-handed helix (minus sense). The helical pitch, HTP and
MHTP values of the new chiral dopants are summarized in
O
OH
Figure 1. Structure of (S)-(ꢁ)-1ꢀ.
Copyright ꢀ 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.11 (2008)
1125
Table 1. HTP and MHTP values of 2ꢀ–4ꢀ
the size of their aromatic moieties. We have indicated that these
optically active spiro compounds can be used as chiral dopants
for nematic liquid crystals.
Chiral
dopant
Optical Helical pitcha HTPb
MHTPc
n
R
purity/%
/mm
/mmꢁ1 /mmꢁ1 molꢁ1 kg
2ꢀ
3ꢀ
4ꢀ
1
2
3
–OC8H17
–OC8H17
–C7H15
97.9
>99:9
>99:9
ꢁ4:63
ꢁ2:42
ꢁ2:14
ꢁ21:1
ꢁ38:2
ꢁ46:1
ꢁ17:9
ꢁ38:2
ꢁ50:3
References and Notes
1
2
3
4
5
6
7
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Y. Aoki, K. Matsushima, T. Taroura, T. Hirose, H. Nohira,
Mol. Cryst. Liq. Cryst. 2003, 398, 189.
aMeasurement method: Cano wedge cell; measurement temperature: room
temperature. bHTP (mmꢁ1) = ðpcÞꢁ1
; p: helical pitch (mm), c: weight
ratio of chiral dopant (c: 0.01). cMHTP (mmꢁ1 molꢁ1 kg) = HTP ꢂ Mr ꢂ
10ꢁ3; Mr: molecular weight of the chiral dopant, host L.C.: ZLI-1132
(Merck).
Y. Aoki, S. Nomoto, T. Hirose, H. Nohira, Mol. Cryst. Liq.
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2*
4*
Figure 3. PM3-optimized conformations of 2ꢀ and 4ꢀ.
´
10 R. Holzwarth, R. Bartsch, Z. Cherkaoui, G. Solladie,
Chem.—Eur. J. 2004, 10, 3931.
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16 Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp./journals/chem-
lett/index.html.
Figure 4. Image of the interaction between the chiral dopant
molecule and the host liquid crystalline molecule. a) In the case
of 2ꢀ. b) In the case of 4ꢀ.
Table 1. All the novel chiral dopants derived from (S)-(ꢁ)-1ꢀ
induced a minus-sense helix and showed large MHTP values.
The absolute MHTP values of the new chiral dopants were
in the order 4ꢀ > 3ꢀ > 2ꢀ. The value of 4ꢀ was the largest, at
50.3 mmꢁ1 molꢁ1 kg; this is larger than those generally observed
for chiral dopants with asymmetric carbons,1–8 and was compa-
rable with or larger than those of chiral dopants with axial asym-
metric chirality.2,9–12,19 It is thought that the large MHTP values
are due to conformational fixation of asymmetric chirality by the
spiro structure. It is suggested that the phenyl rings of the chiral
dopants interact with the aromatic rings of the host liquid crys-
talline molecule via ꢀ–ꢀ interactions. As biphenyl and terphenyl
moieties are rigid, rod-like structures, it is expected that chiral
dopants with rod-like moieties readily align in the same direction
as the host liquid crystalline molecules, resulting in efficient
interaction between the chiral dopants and the host liquid
crystalline molecules (Figure 3 and 4). Therefore, the absolute
MHTP value was proportional to the number of phenyl rings.
In conclusion, novel chiral dopants with a (S)-3,30-(4H,40H)-
spirobi(2H-naphtho[1,2-b]pyran) skeleton induced minus-sense
helices and showed large MHTP values. It was also found that
the MHTP values of the chiral dopants were proportional to
17 The host liquid crystalline mixture (ZLI-1132, Merck) con-
sists of 4-(4-propylcyclohexyl)benzonitril (24 wt %), 4-(4-
pentylcyclohexyl)benzonitril (36 wt %), 4-(4-heptylcyclo-
hexyl)benzonitril (25 wt %), and 4-{4-(4-pentylcyclo-
hexyl)phenyl}benzonitril (15 wt %). The host liquid crystal-
line mixture shows a nematic liquid crystal phase in the
range ꢁ6 to 70 ꢃC.
´
18 R. Cano, Bull. Soc. Fr. Mineral. Cristallogr. 1968, 91, 20.
19 For example, the MHTP values of the imine chiral dopant
with two (R)-naphthylethylamine moieties3 is 40.0 mmꢁ1
molꢁ1 kg, that of the tetrasubstituted BINOL derivative9 is
19.7 mmꢁ1 molꢁ1 kg, and that of the optically active biphenol
derivative10 is 50.0 mmꢁ1 molꢁ1 kg.
Published on the web (Advance View) October 4, 2008; doi:10.1246/cl.2008.1124