Single crystal preparation of DAST for terahertz-wave generation

Article abstract of DOI:10.1080/15421400601021513

DAST (1-methyl-4-{2-[4-(dimethylamino)phenyl]ethenyl}pyridinium p-toluenesulfonate) is a promising crystalline material for electro-optic applications and THz-wave generation. DAST powder for crystal growth was prepared by two synthesis routes including the conventional one and we confirmed that purity of DAST powder became indistinguishable after proper recrystallization even using conventional synthesis process. The highest solubility of DAST was found for methanol-acetonitrile mixture, and DAST crystal growth in this mixture or methanol-ethanol mixture gave thicker crystals than those grown in methanol. Thus, morphological control of DAST single crystals was found to be possible by varying solvents.

Full text of DOI:10.1080/15421400601021513

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Single Crystal Preparation
of DAST for Terahertz-Wave
Generation
Yoshihiro Mineno a , Takeshi Matsukawa a , Satoshi
Ikeda a , Tetsuo Taniuchi a , Hachiro Nakanishi a ,
Shuji Okada b , Hiroaki Adachi c , Masashi Yoshimura
c , Yusuke Mori c & Takatomo Sasaki c
a Institute of Multidisciplinary Research for
Advanced Materials, Tohoku University, Katahira,
Sendai, Japan
b Department of Polymer Science and Engineering,
Faculty of Engineering, Yamagata University,
Yonezawa, Yamagata, Japan
c Department of Electrical Engineering, Osaka
University, Suita, Osaka, Japan
Version of record first published: 12 Mar 2007.
To cite this article: Yoshihiro Mineno , Takeshi Matsukawa , Satoshi Ikeda , Tetsuo
Taniuchi , Hachiro Nakanishi , Shuji Okada , Hiroaki Adachi , Masashi Yoshimura ,
Yusuke Mori & Takatomo Sasaki (2007): Single Crystal Preparation of DAST for
Terahertz-Wave Generation, Molecular Crystals and Liquid Crystals, 463:1, 55/
[337]-61/[343]
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Mol. Cryst. Liq. Cryst., Vol. 463, pp. 55=[337]–61=[343], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400601021513
Single Crystal Preparation of DAST for
Terahertz-Wave Generation
Yoshihiro Mineno
Takeshi Matsukawa
Satoshi Ikeda
Tetsuo Taniuchi
Hachiro Nakanishi
Institute of Multidisciplinary Research for Advanced Materials,
Tohoku University, Katahira, Sendai, Japan
Shuji Okada
Department of Polymer Science and Engineering, Faculty of
Engineering, Yamagata University, Yonezawa, Yamagata, Japan
Hiroaki Adachi
Masashi Yoshimura
Yusuke Mori
Takatomo Sasaki
Department of Electrical Engineering, Osaka University,
Suita, Osaka, Japan
DAST (1-methyl-4-{2-[4-(dimethylamino)phenyl]ethenyl}pyridinium p-toluenesul-
fonate) is a promising crystalline material for electro-optic applications and
THz-wave generation. DAST powder for crystal growth was prepared by two syn-
thesis routes including the conventional one and we confirmed that purity of DAST
powder became indistinguishable after proper recrystallization even using
conventional synthesis process. The highest solubility of DAST was found for
methanol-acetonitrile mixture, and DAST crystal growth in this mixture or
methanol-ethanol mixture gave thicker crystals than those grown in methanol.
Thus, morphological control of DAST single crystals was found to be possible by
varying solvents.
Keywords: crystal growth; DAST; ionic dye; organic crystal; second-order nonlinear
optics; THz-wave generation
Address correspondence to Shuji Okada, Department of Polymer Science and
Engineering, Faculty of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa,
Yamagata, 992-8510 Japan. E-mail: okadas@yz.yamagata-u.ac.jp
55=[337]

56=[338]
Y. Mineno et al.
INTRODUCTION
Organic ionic crystal DAST (1-methyl-4-{2-[4-(dimethylamino)pheny-
l]ethenyl}pyridinium p-toluenesulfonate) has attracted attention for
its large second-order nonlinear optical properties [1], and its single
crystal growth and application for electro-optic devices have been
extensively investigated [2]. After Zhang et al. reported pulsed THz-
wave generation with a broad range spectrum from DAST using
optical rectification [3], DAST was noticed as a crystalline material
for THz-wave generation as well. Different frequency generation was
also applied for DAST to generate tunable THz-waves [4]. The tunable
range is from 2 THz to 30 THz [5], which is quite wide compared with
semiconductors such as gallium phosphide, lithium niobate etc.
Although DAST is a promising THz-wave generation material,
improvement of crystal quality has been an important issue to be
solved.
In this connection, we reexamine the synthesis route of DAST and
recrystallization process was optimized since purity of DAST powder
crystals used for crystal growth seemed to influence on the quality
of the grown crystals. For DAST crystal growth, methanol has been
generally used because of the highest solubility among the conven-
tional organic solvents [6]. Accordingly, solution crystal growth with
slow cooling process in methanol has been mainly utilized to prepare
quality crystals of DAST, and morphological control of crystals was
not so easy due to limited valuable factors. Thus, DAST crystal growth
using methanol, ethanol, acetonitrile, and their mixed solvents was
also investigated.
EXPERIMENTAL
DAST was prepared by the following two routes shown in Figure 1.
Route A is the conventional process to prepare DAST derivatives [7].
At first, c-picoline 1 was reacted with methyl p-toluenesulfonate 2,
and the resulting pyridinium salt 3 was condensed with benzaldehyde
4 to give DAST. In the second route (Route B), 4-bromo-N,N-dimethy-
laniline 5 was coupled with 4-vinylpyridine 6 using a palladium
catalyst [8], and the obtained stilbazol 7 was methylated by using
methyl p-toluenesulfonate 2 to give DAST.
Solubility of DAST was evaluated as follows. At first, enough
amount of DAST was put in a solvent and heated at a certain tempera-
ture. After keeping at the temperature for 6 h and confirming that
DAST powder has still remained, 10 ml of the saturated supernatant
solution was taken out and the solvent was evaporated under a

Single Crystal Preparation of DAST
57=[339]
FIGURE 1 Two synthesis procedures of DAST.
reduced pressure. By weighing the DAST crystals remained, solubility
at the temperature was calculated.
DAST single crystals were grown from its solution by the slow cool-
ing method. General procedure was as follows: In a 500 ml PFA wide-
mouth bottle (Flonchemical NR-191), a certain amount of DAST and
200 ml of solvent were added. The input amount of DAST was set to
soluble amount in the saturated solution at 45ꢀC. In the solution, a
teflon board with straight grooves was sunk to perform the slope
nucleation method [9]. The bottle was tightly sealed by a stopper
and a screw cap and it was put in a programmable temperature-
controlled water bath. The temperature was first kept at 55ꢀC for
10 h and DAST was completely dissolved with stirring. After that,
the temperature was cooled to 45ꢀC for 4 h. Then, the temperature
was lowered with cooling rate of 0.1ꢀC=day in general.
RESULTS AND DISCUSSION
DAST was prepared by the two routes for comparison. In the second
step of Route A, a basic catalyst such as piperidine is added for the con-
densation reaction and water is naturally produced as a byproduct. On
the other hand, in the second step of Route B, catalyst is not necessary

58=[340]
Y. Mineno et al.
and only the desired product is obtained, indicating that purer DAST
is obtained just after the final reaction. This synthesis route was used
to prepare DAST thin films by the chemical vapor deposition [10]. At
first, non-solvent reaction was performed in the second step of Route
B. However, the yield of the step was low (34%). When chloroform
was used as solvent in this step, the yield increased up to 84%. The
total yield from the starting compound in Route A was 77%, which
is slightly higher than that of Route B (67%). As was expected, the
DAST crystal purity just after the final reaction of Route B was higher
than those of Route A, which was evaluated by elemental analysis.
However, after recrystallizing from methanol and then from ethanol,
the purity of DAST crystals obtained from both routes became the
same. Thus, we confirmed that conventional Route A can be used to
prepare pure DAST when its proper recrystallization is performed.
Figure 2 shows the solubility curves of DAST for methanol, ethanol,
acetonitrile and their mixed solvents at around 45ꢀC. The order of the
solubility in pure solvents is methanol >> ethanol > acetonitrile.
Since dielectric constants (e) of methanol, ethanol and acetonitrile at
25ꢀC are 32.7, 24.6 and 35.9, respectively, solubility of DAST in these
solvents cannot be explained only by e. Hydroxyl protons of alcohol
seem to be important for solvation. However, methyl group of meth-
anol is also important to increase solubility of DAST since DAST solu-
bility in water is less than that of methanol. When methanol and
ethanol was mixed in 1:1 volume ratio, the solubility became inter-
mediate between that of methanol and ethanol, indicating that solva-
tion ability was averaged. On the other hand, when acetonitrile was
mixed with methanol or ethanol in 1:1 volume ratio, solubility of DAST
increased about twice or more of that in pure alcohols. Especially,
methanol-acetonitrile binary system shows the highest solubility of
DAST among the solvent investigated so far. Figure 3 displays vari-
ation of the solubility at 45ꢀC depending on the content of methanol
in the methanol-acetoritrile mixed solvent. Solubility of DAST in the
3:1 mixture was quite augmented up to 64.5 gLꢁ1, which was already
near to the maximum solubility of 68.0 gLꢁ1 observed in 1:1 mixture.
Further increase of acetonitrile content in the mixed solvent decreased
DAST solubility. Increase of DAST solubility in methanolic solution by
adding other solvent with larger e than methanol was also observed in
methanol-water mixture [11].
Referring to the solubility curves of DAST, we performed its crystal
growth in these solutions by slow cooling from 45ꢀC. From methanol
solution, we obtained DAST single crystals with a plate-like shape,
in which crystallographic a-axis (the polar axis) and b-axis are on
the developed plane and c-axis is nearly along the thickness direction

Single Crystal Preparation of DAST
59=[341]
FIGURE 2 Solubility curves of DAST in pure methanol, ethanol, acetonitrile
and their mixed solvents: & methanol; 4 ethanol; acetonitrile; & methanol-
ꢀ
ethanol (1:1); ~ ethanol-acetonitrile (1:1);
methanol-acetonitrile (1:1).
.
[12]. However, pure ethanol and acetonitrile solutions and ethanol-
acetonitrile mixed solution only gave tiny crystals less than 1 mm in
side. From methanol-acetonitrile mixed solution, we obtained thicker
crystals. This morphological change is important for applications. In
order to generate THz-waves, the laser was input perpendicular to
the developed ab-plane so far. In this case, the pass length of the inci-
dent beam is limited by the thickness of the crystal. However, thick
DAST crystals may be able to generate THz-waves by passing through
the laser beam along b-axis, i.e., parallel to the ab-plane. In this case,
we can use more interaction length of the crystal. From methanol-
ethanol mixture, thick crystals were also obtained. Rai et al. reported
DAST crystal growth in methanol-ethanol mixture and similar tend-
ency was described [13]. Although the dimension of the developed face
became smaller compared with the case of methanol solution, it seems
to be improved by controlling nucleation and crystal growth rate.

60=[342]
Y. Mineno et al.
FIGURE 3 Solubility of DAST in methanol-acetonitrile binary system at 45ꢀC.
Terahertz-wave generation properties of the crystals were not deterio-
rated by changing the solvent from methanol to methanol-ethanol
mixture when appropriate crystal growth conditions were selected.
FIGURE 4 DAST crystals obtained from (a) methanol solution and (b) methanol-
acetonitrile (1:1) solution. Bars indicate 1-mm length.

Single Crystal Preparation of DAST
61=[343]
In conclusion, we confirmed that DAST crystalline powder as a start-
ing material for crystal growth can be purified by recrystallization
from methanol and ethanol irrespective of synthesis routes. Highest
solubility of DAST among organic solvents was found in methanol-
acetonitrile mixture. By using this mixture or methanol-ethanol
mixture as solvents for DAST crystal growth, we obtained its crystals
thicker than those from methanol. These findings show possibility of
single-crystal morphological control of DAST and its derivatives [14]
and the resulting expansion of their device applications is expected.
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