Aminostilbazolium derivatives substituted by hydroxyethyl groups for second-order nonlinear optics

Article abstract of DOI:10.1080/15421406.2011.566081

As DAST (1-methyl-4-{2-[4-(dimethylamino)phenyl]ethenyl}pyridinium p-toluenesulfonate) derivatives for improved crystal stability and inhibition of crystal water inclusion, 1-(2-hydroxyethyl)-4-(2-{4-[bis(2-hydroxyethyl)amino] phenyl}ethenyl)pyridinium salts 2 (2a-2l) and 1-(2-hydroxyethyl)-4-{2-[4- (diethylamino)phenyl]ethenyl}pyridinium salts 3 (3a-3l) were synthesized and their properties were investigated. Salts 3 were found to have no crystal water, and the cation with one hydroxyethyl group was effective to eliminate water inclusion in the crystals. Although the melting points of 2 were lower than those of the corresponding 3, decomposition temperatures of these chromophores were above 250°C indicating their good thermal stability. Crystals of m-nitrobenzenesulfonate salt 3f was obtained without crystal water and showed SHG activity. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421406.2011.566081

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Aminostilbazolium Derivatives
Substituted by Hydroxyethyl Groups for
Second-Order Nonlinear Optics
Takahiro Ikeda a , Yui Tadaki a , Ryo Funajima a , Yoko Tatewaki a &
Shuji Okada a
a Graduate School of Science and Engineering, Yamagata University,
Jonan, Yonezawa, Japan
Version of record first published: 16 May 2011.
To cite this article: Takahiro Ikeda , Yui Tadaki , Ryo Funajima , Yoko Tatewaki & Shuji Okada (2011):
Aminostilbazolium Derivatives Substituted by Hydroxyethyl Groups for Second-Order Nonlinear Optics,
Molecular Crystals and Liquid Crystals, 539:1, 142/[482]-147/[487]
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Mol. Cryst. Liq. Cryst., Vol. 539: pp. 142=[482]–147=[487], 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2011.566081
Aminostilbazolium Derivatives Substituted
by Hydroxyethyl Groups for Second-Order
Nonlinear Optics
TAKAHIRO IKEDA, YUI TADAKI, RYO FUNAJIMA,
YOKO TATEWAKI, AND SHUJI OKADA
Graduate School of Science and Engineering, Yamagata University,
Jonan, Yonezawa, Japan
As DAST (1-methyl-4-f2-[4-(dimethylamino)phenyl]ethenylgpyridinium p-toluene-
sulfonate) derivatives for improved crystal stability and inhibition of crystal water
inclusion, 1-(2-hydroxyethyl)-4-(2-f4-[bis(2-hydroxyethyl)amino]phenylgethenyl)-
pyridinium salts 2 (2a–2l) and 1-(2-hydroxyethyl)-4-f2-[4-(diethylamino)phenyl]-
ethenylgpyridinium salts 3 (3a–3l) were synthesized and their properties were
investigated. Salts 3 were found to have no crystal water, and the cation with one
hydroxyethyl group was effective to eliminate water inclusion in the crystals.
Although the melting points of 2 were lower than those of the corresponding 3,
decomposition temperatures of these chromophores were above 250ꢀC indicating
their good thermal stability. Crystals of m-nitrobenzenesulfonate salt 3f was
obtained without crystal water and showed SHG activity.
Keywords Aromatic compound; DAST; ionic dye; organic crystal; second-order
nonlinear optics
1. Introduction
Utilization of organic second-order nonlinear optical (NLO) crystals as electro-optic
devices [1] or terahertz-wave emitters has been extensively investigated [2,3]. Among
the organic NLO crystals, 1-methyl-4-f2-[4-(dimethylamino)phenyl]ethenylg-
pyridinium p-toluenesulfonate (DAST) [4–6] is one of the promising materials
[1,2,7–10]. However, its crystal growth should be strictly controlled, and the yield
of the crystals with both an appropriate size and a good optical quality is not so
high. In addition, the obtained crystals are affected by water to be second-order
NLO inactive crystals [11]. These points should be improved for the DAST deriva-
tives in the next generation besides enhancement of the NLO susceptibility. In our
previous studies, we have investigated synthesis of pure DAST [12], DAST deriva-
tives with different counter anions [13,14] and DAST derivatives with extended
p-conjugation in the cation part [15,16]. In this study, we introduced hydroxyethyl
Address correspondence to Shuji Okada, Department of Organic Device Engineering,
Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa
992-8510, Japan. E-mail: okadas@yz.yamagata-u.ac.jp
142=[482]

Aminostilbazolium Derivatives for Nonlinear Optics
143=[483]
groups instead of all or a part of methyl groups of the DAST cation to study about
the effect of hydroxy groups for crystal stability and crystal water inclusion. Intro-
duced hydroxy groups are also useful for further molecular modification. The
bromide salts and the anion-exchanged compounds were prepared and their proper-
ties were investigated. Although preparation and luminescent property of the
nanocomposite of 2a with zinc sulfide has been reported [17], crystals of the
anion-exchanged salts were not investigated so far.
2. Experimental
Aminostilbazolium derivatives were synthesized according to Figure 1. 4-[N,N-
bis(2-hydroxyethyl)amino]benzaldehyde 1 was prepared by the Vilsmeier formylation
referring to the literature [18]. This reaction starting from 2,20-(phenylimino)-
diethanol gave the targeting aldehyde 1 in 82% yield. Condensation reaction [19] of
the resulting aldehyde and 4-(diethylamino)benzaldehyde with 1-(2-hydroxyethyl)-
4-methylpyridinium bromide afforded bromide salts 2a and 3a in 71% and 63% yields,
respectively. Anion-exchange reaction of 2a and 3a using silver sulfonates [14]
resulted in the corresponding sulfonates in the yields from 9% to almost quantitative.
The salts obtained were purified by recrystallization from methanol-ethanol mixed
solvent.
1H NMR spectra were recorded for deuterated methanol or chloroform solution
on a JEOL ECX-400 spectrometer. IR spectra were obtained for KBr-pelletized
samples by using a Horiba FT-210 spectrometer. UV-vis spectra in methanol
were recorded on a Jasco V-560 spectrophotometer. Thermogravimetric and differ-
ential thermal analyses (TG=DTA) were performed using a Seiko Instruments
TG=DTA220 apparatus with heating rate of 10ꢀC=min. Elemental analyses were
performed using a Perkin-Elmer 2400II analyzer at Elementary Analysis Labora-
tory, Department of Polymer Science and Engineering, Yamagata University.
X-ray crystallographic analysis was carried out using a Rigaku RAXIS-RAPID
diffractometer. Second-order NLO activity of the crystals was verified by
Figure 1. Synthesis scheme of DAST derivatives with hydroxyethyl groups. Percentages in
parentheses are the compound yields, in which ND indicates that the pure compounds were
not isolated and the accurate yields could not be determined.

144=[484]
T. Ikeda et al.
second-harmonic generation (SHG) of green emission at 532 nm when a Q-switched
Nd:YAG laser at 1064 nm (Lotis LS-2137U) was irradiated to the powder samples.
3. Results and Discussion
All compounds were successfully synthesized and the structures were confirmed by
1H NMR spectra. When original salts were contained in the anion-exchanged crys-
tals as impurity, recrystallization was repeated and the yield decreased. The yield of
the anion-exchanged salts with high solubility was also tend to be low. Figure 2
shows UV-vis absorption spectra of 2a and 3a. By replacing the substituents
attached to the amino group from ethyl to hydroxyethyl groups, the absorption
maximum (kmax) was blue-shifted by 12 nm. This result indicates that the hydro-
xyethyl groups decrease electron-donating ability of the amino group compared with
the ethyl groups.
The data on thermal analyses of 2a and 3a are displayed in Figure 3. They only
showed one endothermic peak corresponding to the melting points, and there was
no weight loss below the melting points. This result suggests that these crystals
contained no crystal water, and it was also confirmed by the elemental analysis.
These chromophores were found to have high thermal stability because weight loss
of these compounds was not observed up to around 250ꢀC. Melting points of the
crystals are summarized in Table 1. Crystals of 2d, 2f and 2i were not obtained by
recrystallization from methanol-ethanol mixed solvent because of their high solu-
bility for the solvent. Although their recrystallization was also tried using methanol
mixed with chloroform or THF or ethyl acetate, oily residues were obtained. Since
high affinity for water was considered in these salts, an experiment to remove water
from 2f as an example was performed. The oily residue of 2f was refluxed in toluene
using a Dean-Stark apparatus, and water was eliminated to give solid material.
When the resulting solid was exposed in air, solid became wet. Namely, water in
air seemed to be easily adsorbed in the compound. For 2e and 3l, impurity mainly
1
originated from the silver arenesulfonate salts was confirmed by H NMR, and it
was hard to remove by the recrystallization process. Among the compounds
obtained as crystals, 2b, 2c, 2g and 2l contained crystal water under ambient recrys-
tallization conditions, and 2b crystals with crystal water were deliquescent. When
these crystals with crystal water were heated at 100ꢀC for 2 h under reduced pressure
to dehydrate, crystal water in 2c and 2g could be eliminated but that in 2b and 2l
Figure 2. UV-vis spectra of 2a and 3a in methanol.

Aminostilbazolium Derivatives for Nonlinear Optics
145=[485]
Figure 3. TG=DTA thermograms of 2a and 3a.
could not be. The crystals except the above four salts were obtained without crystal
water by simple recrystallization. In particular, all obtained crystals of 3a derivatives
were found to have no crystal water, and introduction of one hydroxyethyl group per
cation seems to be quite effective to avoid water inclusion in the crystals.
In general, when the number of hydroxy groups increase among compounds
with the same structure except the number of hydroxy groups, melting points
become high, e.g., the melting points of 1-propanol, 1,3-propanediol and 1,2,3-
propanetriol are ꢁ127ꢀC, ꢁ27ꢀC and 20ꢀC, respectively [20]. This is due to increase
in the number of intermolecular hydrogen bonds per molecule. However, in the
present case, 2a with three hydroxy groups showed lower melting point than 3a with
one hydroxy group, and the same relation was found in other salts with the same
anion. In IR spectra, peaks for free O-H stretching vibration are generally observed
at around 3600 cmꢁ1. Since the peaks of 2a and 3a derivatives appeared from
3200 cmꢁ1 to 3300 cmꢁ1, all derivatives were found to form hydrogen bonding.
For 3a, X-ray crystallographic analysis was carried out [21] and the OH group
was found to be near a bromide anion to form hydrogen bonding. Sulfonate deriva-
tives of 3a generally showed the sharper OH peaks in the lower wavenumber region
compared with the corresponding 2a derivatives, indicating that the hydrogen bonds
Table 1. Melting points of crystals
Melting point=ꢀC
a
b
c
d
e
f
g
h
i
j
k
l
b
c
b
b
2
3
142 114a 141a
231 130 228
–
196 189 153
–
–
155a 191
196
–
225 182 173 205
152 189 113a
c
–
aThe crystals obtained by recrystallization included crystal water.
bThe crystals were not obtained by recrystallization from methanol-ethanol mixed solvent.
cPurification could not be completed by recrystallization from methanol-ethanol mixed
solvent.

146=[486]
T. Ikeda et al.
of 3a derivatives were stronger than those of 2a derivatives. In addition, hydro-
xyethyl group with three non-hydrogen atoms seemed to have more flexibility than
ethyl group with two non-hydrogen atoms, and stabilization by intermolecular
hydrogen bonds may be weakened. Therefore, 3a derivatives showed higher melting
points than the corresponding 2a derivatives. The decomposition temperatures were
found to be above 250ꢀC, and all compounds showed good thermal stability as chro-
mophores. Among the synthesized salts with or without crystal water, only m-nitro-
benzenesulfonate salt 3f showed SHG activity. The probability to obtain SHG active
crystals was not so high in this study. However, it is not a serious problem because
SHG activity can be achieved by proper selection of the counter anion.
In conclusion, the aminostilbazolium derivatives having hydroxyethyl groups
accompanied by a variety of counter anions 2a–2l and 3a–3l were synthesized and
their thermal properties, crystal water inclusion and SHG activity were studied.
Obtained crystals of 3a derivatives do not contain crystal water at all although some
derivatives of 2a include crystal water. When three hydroxy groups were introduced
in a cation like 3a derivatives, hydrophilicity became too large to avoid crystal water
inclusion. On the other hand, one hydroxy group in the cation gave good results to
exclude crystal water. The melting points of 3a derivatives were higher than those of
the corresponding 2a derivatives. However, both cationic chromophores have high
thermal stability above 250ꢀC without decomposition. Although a similar chromo-
phore with two hydroxyethyl groups has been incorporated in a polymer structure
[22], 2a derivatives are more suitable for the NLO chlomophores in polymer matrixes
because there are three crosslinkable parts to fix the poled orientation. Crystals of 3f
have SHG activity and no crystal water. Therefore, by extending conjugated system
of 3a cation, high-performance crystals without crystal water will be prepared.
Acknowledgment
This study was partly supported by the program for Strategic Promotion of Innova-
tive Research and Development, JST.
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˚
[21] Crystal data for 3a: C19H25BrN2O, M ¼ 377.32, orthorhombic, Pbca, a ¼ 13.5378(7) A,
3
˚
˚
˚
b ¼ 8.8811(5) A, c ¼ 31.4905(15) A, V ¼ 3786.1(3) A , Z ¼ 8, Dx ¼ 1.324, MoKa
(k ¼ 0.71073 A), l ¼ 2.184 mmꢁ1, T ¼ 293 K, 4313 reflection measured, 1481 [I > 2r(I)]
˚
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