Synthesis and characterization of new liquid crystals containing a non-activated indolinobenzospiropyranyl group. Part 4*[1]

Article abstract of DOI:10.1080/15421400490434892

A series of new liquid crystalline compounds, 1′,3′,3′- Trimethylspiro[2H-1-benzopyran-2,2′-indolin/-6-yl 4-(4′- alkoxyphenylazo)benzoate, SP-APAB 1a ～ 1f, has been synthesized. Their liquid crystallines were subjected to thermal analysis on a differential scanning calorimeter (DSC), to texture of phases on a polarizing microscope and to phase transition on an X-ray diffraction mid electro-optical measurement. Most of compounds examined exhibit monotropic nematic and SmA liquid crystal phases on cooling from isotropic liquid. Suprisingly, SP-APAB 1c is shown to exhibit monotropic SmC phase in addition. X-ray diffraction study of the layer spacing confirmed that there was a layer spacing in the range of two theta from 2.25 to 2.45 A on the temperature near the phase transition from, SmA to SmC.

Full text of DOI:10.1080/15421400490434892

Mol. Cryst. Liq. Cryst., Vol. 411, pp. 119=[1161]–126=[1168], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490434892
SYNTHESIS AND CHARACTERIZATION OF NEW
LIQUID CRYSTALS CONTAINING A
NON-ACTIVATED INDOLINOBENZOSPIROPYRANYL
ꢀ
GROUP. PART 4 [1]
S.-R. Keum, J.-T. Shin, and S.-H. Lee
Department of New Material Chemistry, Korea University, Chochiwon,
South Korea 339-800
S.-T. Shin
Department of Physics, Korea University, Chochiwon, Chung-Nam,
South Korea 339-800
0
0
0
A series of new liquid crystalline compounds, 1 ,3 ,3 -Trimethylspiro[2H-1-ben-
0
0
zopyran-2,2 -indolin]-6-yl 4-(4 -alkoxyphenylazo)benzoate, SP-APAB 1a ꢁ 1f,
has been synthesized. Their liquid crystallines were subjected to thermal
analysis on a differential scanning calorimeter (DSC), to texture of phases
on a polarizing microscope and to phase transition on an X-ray diffraction
and electro-optical measurement. Most of compounds examined exhibit mono-
tropic nematic and SmA liquid crystal phases on cooling from isotropic liquid.
Suprisingly, SP-APAB 1c is shown to exhibit monotropic SmC phase in
addition. X-ray diffraction study of the layer spacing confirmed that there
˚
was a layer spacing in the range of two theta from 2.25 to 2.45 A on the
temperature near the phase transition from SmA to SmC.
Keywords: indolinobenzospiropyran dye; liquid crystal optical switch; Nematic; Non-activated
spiropyran; phase transition; Smectic A and Smectic C phase
INTRODUCTION
Photochromic materials [2,3] which can be reversibly written and read by
light dominate the technology in our digital age. Organic photochromic
compounds have obvious potential for such light-controlled devices.
This research was supported by grant No.1999-2-114-001-3 from the interdisciplinary
research program of the KOSEF.
Address correspondence to S.-R. Keum, Dept. of New Materical Chemistry, Korea
University, Chochiwon, South Korea 339-800.
119=[1161]

1
20=[1162]
S.-R. Keum et al.
Among these materials, photochromic spiropyran dyes have attracted wide
attention to many applications in optical switching, high-density optical
data storage, holographic system and optical computing.
Although we have examined the synthesis and characterization of sev-
eral series of liquid crystalline compounds containing a non-activated spir-
opyran unit as a chiral unit, a limited number of LC materials containing a
non-activated spiropyran were found to exhibit a monotropic SmC phase
ꢀ
[
4,5]. These materials, exhibiting a SmC phase after resolution, hold
considerable potential as candidates for photo-resolvable dopants in UV-
transparent nematic and polymeric nematic liquid crystal phases [6,7].
Irradiation of a suitable racemic chiral dopant in an aligned nematic liquid
crystal with circularly polarized light would induce a cholesteric phase,
whereas irradiation of the induced cholesteric phase with unpolarized light
of the same wavelength would restore the nematic phase by photoracemi-
zation of the dopant. Because the light-induced interconversion between a
spiropyran (SP)-an achiral merocyanine (MC) can be sensed by the change
in the optical rotatory power of the liquid crystal, spiropyrans may serve as
the basis for the liquid crystal optical switch. For the development of a
liquid crystal optical switch (LCOS) based on the photoresolution [6], we
have further expanded our research into SmC mesogens incorporating a
non-activated spiropyran unit.
In this report we describe the synthesis and phase transition studies of
0
0
0
0
0
a series of 1 ,3 ,3 -trimethylspiro[2H-1-benzopyran-2,2 -indoline]-6-yl 4-(4 -
alkyloxyphenylazo) benzoate, SP-APAB 1aꢁ1f.
MATERIALS AND EXPERIMENTAL METHODS
The DSC thermograms of the compounds were obtained using a DuPont
9
10 Thermal Analyzer calibrated with indium under N atmosphere at a
2
ꢂ
ꢃ1
heating=cooling rate of 10C min . The optical textures and thermal transi-
tions were achieved using a Nikon Labophot-2 polarizing microscope
equipped with a Mettler FP82HT hot stage. An X-ray diffractometer,
3
kW-8eV, was applied for studying the phase transition on temperatures.

Synthesis and Characterization of New Liquid Crystallines
121=[1163]
The bulk sample was aligned by slowly cooling from the isotropic phase in
the presence of a ꢁ2.5 kG magnetic field produced by a pair of rare-earth
permanent magnets placed inside the oven. The x-ray scattering for phase
transitions were performed on a 18-kW rotating anode source, and two-cir-
cle and four-circle goniometers with a pair of Ge(111) crystals used as
monochromator and analyzer. The bulk sample was aligned slowly by cool-
ing from the temperature at isotropic phase to the temperature at smectic
phase in the presence of 50 kG strong magnetic field produced in the en-
vironment of liquid helium. The temperature was kept stable less than
5
mK. The temperature and data were controlled by computer using GPIB.
The twisted nematic (TN) cell and the 4 micron cell with planar alignment
were made by the general method by using a commercial align-materials,
ꢂ
RN1199, whose pretilt angle was ꢁ1. Two side 90 twisted rubbing and
one side rubbing treatment were applied for the TN cell and 4 micron cell
with planar alignment, respectively, and our compound filled slowly at the
temperature of the isotropic phase. The thickness of the cell gap was con-
trolled uniformly by a 4.5 mm spacer for optical study.
The detailed synthesis of these LC compounds will be published else-
where [8].
RESULTS AND DISCUSSION
0
0
0
A series of new liquid crystalline compounds, 1 ,3 ,3 -trimethylspiro[2H-
0
0
1
-benzopyran-2,2 -indolin]-6-yl 4-(4 -alkoxyphenylazo)benzoate, SP-APAB
a–1f are synthesized in 48–71% yield via DCC esterification of
1
0
0
0
0
6
-hydroxy-1 ,3 ,3 -trimethylspiro[2H-1-benzopyran-2,2 -indolin] 4 with the
0
0
corresponding 4-(4 -alkoxyphenylazo)benzoic acid 3. These 4-(4 -alkoxy-
phenylazo)benzoic acid 3 were obtained from a diazo coupling reaction of
4
-aminobenzoic acid with sodium nitrite=HCl in the presence of phe-
nol=NaOH, followed by alkylation in EtOH using the corresponding iodo-
alkanes, as shown in the Scheme 1.
The mesophase characterization of liquid crystal phases exhibited by
SP-APABwere studied by differential scanning calorimeter (DSC), optical
polarizing microscopy, the X-ray diffraction method and the electro-optical
method. The DSC thermograms of SP-APAB 1a-1f exhibited a meso-
morphic behavior on the first cooling and the subsequent cycle. Most of
the compounds (except 1c) formed both monotropic nematic and SmA
phases, whereas 1c formed a nematic, SmA and SmC phases, interestingly.
From the DSC thermogram of 1c, the first and second weak peak at 113
ꢂ
and 100C on cooling were identified to be a nematic and SmA phase,
respectively. Upon lowering the temperature further, the next transition
ꢂ
at 95C corresponding to the appearance of SmC phase was revealed.

1
22=[1164]
S.-R. Keum et al.
SCHEME 1
ꢀ
The fourth strong peak at 90 C was found to be the crystal phase. Other
compounds exhibited an isotropic!nematic!SmA phase sequence with
decreasing temperature.
As a result of the polarizing microscopy study for 1c showed a nematic,
SmA and SmC phase on cooling from the isotropic phase, as shown in
Figure 1.
In order to confirm the phases obtained by thermal fluctuation from DSC
and by texture from optical microscopy for SP-APAB 1e, we used a normal
twisted nematic ( TN) cell with 4.5 mm cell gap and a 4 micron cell with pla-
nar alignment applied one side rubbing with the same cell gap. The TN cell
ꢀ
showed the white state at 200C without voltage, but showed the black state
at the same temperature with 15 V voltage. It can be understood a nematic
ꢀ
FIGURE 1 Optical textures of 1c: (a) nematic phase at 113C, (b) SmA phase at
ꢀ ꢀ
99C, (c) SmC phase at 97C on cooling; magnification 200x. (See COLOR PLATE III)

Synthesis and Characterization of New Liquid Crystallines
123=[1165]
ꢂ
phase at 200C. In the case of the 4 micron cell with planar alignment at
ꢂ
1
50C, there was no change of transmittance even though we applied the
electric field on the range of 0 ꢁ40 V=mm. It meant that molecular orien-
tation was perpendicular to the layer, which indicated a SmA phase. When
we rotated the cell at 150C without applying the electric field under the
two crossed polarizers, the highest transmittance (white state) was from
ꢂ
ꢂ
rotating 45 at angle of the black state. From these results, we confirmed
this compound only has a SmA phase.
The SmC for 1c was identified further by the X-ray diffraction method.
Peak positions on three different temperatures were almost same which
meant phase transition was not occurred yet, but intensity decreased when
temperature decreased near the phase transition temperature because of
starting the molecular fluctuations when phase will start to change as
shown in Figure 2. There was only one layer spacing on these temperatures
ꢂ
in the range of two theta from 2.25 to 2.45 and their peak position on two
ꢂ
˚
theta was 2.35 which could be concerned molecular length, 37.6 A. The full
width half maximum(FWHM) was very narrow and didn’t change on these
three temperatures. It indicated one of smectic phases. The similar results
to confirm the phase transition was given in Figure 3 drawn the intensity
against two theta on other three temperatures. At the SmA to the chiral
smectic phase transition, the intensity and the layer spacing, which corre-
FIGURE 2 Intensity versus two theta on three different temperatures just above
SmC phase.

1
24=[1166]
S.-R. Keum et al.
FIGURE 3 Intensity versus two theta on three different temperatures near tem-
perature of the phase transition from SmA to the SmC.
sponded the peak position obtained by two theta scan, decreased as shown
ꢂ
ꢂ
in Figure 3. The peak position at 98.79C was 2.35 on two theta scan which
were same value obtained at the higher temperature, but the peak position
ꢂ
at other two lower temperatures, 98.3 and 97.80C, was each shifted to lar-
ger than 2.36, which gave a clue of molecular tilt to the layer and a tran-
FIGURE 4 Layer spacing versus temperature.

Synthesis and Characterization of New Liquid Crystallines
125=[1167]
TABLE 1 Phase Transition Temperatures of the Synthesized LC Compounds, 1a–1f
Cr
SmC
SmA
N
I
1a
1b
1c
1d
1e
1f
.
.
.
.
.
.
103
106
–
–
.
–
–
–
–
–
.
.
.
.
.
.
117
115
100
108
105
100
.
.
.
.
.
.
128
126
113
122
118
111
.
.
.
.
.
.
95.3
98.5
–
102
62
–
92
–
sition from SmA to SmC phase. Intensity at the these temperatures were
almost constant because of finishing the molecular fluctuation which
induced phase transition. Smectic layer spacing d calculated from Bragg
condition, 2d sin h¼k; h is angle of peak position on two theta scan and k
is wavelength to electromagnetic radiation of 8 keV quantum energy, is
shown in Figure 4 as a function of temperature. The value of d obtained
in a particular run did not depend on sample history and on the time spent
at any temperature. The behavior of d and phase change in the two smectic
regions is in quantitative agreement with the other results. Figure 4 showed
a phase transition from smectic A (SmA) to chiral smectic (SmC)
ꢂ
atꢁ98.6C which was the temperature occurred an abrupt change on layer
ꢂ
spacing due to molecular tilt to the layer. The layer spacing d below 98.5C
which is region of SmC phase, is quite constant. The tilt angle at the chiral
smectic phase isꢁ5 degree. Since this tilt angle is quiet small, one could
say that it may be a smectic C phase.
It was found that a few compounds exhibited the enantiotropic nematic
phase by repeated measurements. Regardless of chain length, most com-
pounds examined in this study were transformed to amorphous solids
which slowly recrystallized at room temperature within a few hours to sev-
eral days. The transition temperatures and the phases for ABP-SPAB
1
aꢁ1f(n¼4ꢁ9) are given in Table 1.
REFERENCES
[
[
[
[
1] Part 3: Keum, S.-R., Lee, M.-J., Yoo, B.-W., & Shin, S.-T. (2001). Dyes and Pigments,
5
2] D u¨ rr, H. (1990). Photochromism-molecules and System; H. Bouas-Laurent., Ed.;
1(1), 15.
Elsevier; Amsterdam.
3] Keum, S.-R., Lee, M.-J., Swansburg, S., Buncel, E., & Lemieux, R. P. (1998). Dyes and
Pigments, 39, 383.
4] Swansburg, S., Choi, Y-. K., Keum, S.-R., Buncel, E., & Lemieux, R. P. (1998). Liquid
Crystals, 24, 341.

1
26=[1168]
S.-R. Keum et al.
[
[
[
[
5] Keum, S.-R., Lee, M.-J., & Shin, S.-T. (2001). Liquid Crystals, 28(11), 1587.
6] Lemieux, R. P. & Schuster, G. B. (1993). J. Org. Chem., 58, 100.
7] Gray, G. W. & Goodby, J. W. G. (1984). Smectic Liquid Crystals, Leonard Hill: London.
8] Keum, S.-R., Lee, M.-J., Lee, J.-J., & Jang, H.-R. (2002). Dyes and Pigments, in press.