A new stilbazolium salt with perfectly aligned chromophores for second-order nonlinear optics: 4-N,N-Dimethylamino-4′-N'-methyl- stilbazolium 3-carboxy-4-hydroxybenzenesulfonate

Article abstract of DOI:10.1016/j.dyepig.2011.12.004

A new organic nonlinear optical crystal 4-N,N-dimethylamino-4′-N'- methyl-stilbazolium 3-carboxy-4-hydroxybenzenesulfonate (DSCHS) has been developed with very promising properties for quadratic nonlinear optical applications. DSCHS single crystals with n

Full text of DOI:10.1016/j.dyepig.2011.12.004

Dyes and Pigments 94 (2012) 120e126
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Dyes and Pigments
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A new stilbazolium salt with perfectly aligned chromophores for second-order
nonlinear optics: 4-N,N-Dimethylamino-40-N0-methyl-stilbazolium 3-carboxy-4-
hydroxybenzenesulfonate
,
b
,
c
b
a,
Jianhong Yin a, Liang Li a, Zhou Yang a , Mojca Jazbinsek , Xutang Tao , Peter Günter , Huai Yang
*
*
*
a Department of Materials Physics and Chemistry, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China
b Nonlinear Optics Laboratory, Institute of Quantum Electronics, ETH Zurich, CH-8093 Zurich, Switzerland
c State Key Laboratory of Crystal Material, Shandong University, Jinan 250100, PR China
a r t i c l e i n f o
a b s t r a c t
A new organic nonlinear optical crystal 4-N,N-dimethylamino-40-N0-methyl-stilbazolium 3-carboxy-4-
hydroxybenzenesulfonate (DSCHS) has been developed with very promising properties for quadratic
nonlinear optical applications. DSCHS single crystals with non-centrosymmetric structure have been
obtained from aqueous methanol solution. X-ray crystallographic analysis revealed that the crystal
structure of DSCHS is triclinic P1 with the chromophores aligned perfectly parallel, leading to the
Article history:
Received 30 August 2011
Received in revised form
17 November 2011
Accepted 3 December 2011
Available online 13 December 2011
maximum possible order parameter <cos3
q> ¼ 1 in the crystalline state, which is optimal for electro-
optics, THz-wave generation and field detection applications. Kurtz powder test has shown that
DSCHS exhibits a very large second-order optical nonlinearity, with a 30 percent higher second-harmonic
signal than the well-known organic nonlinear optical crystal 4-N,N-dimethylamino-40-N0-methyl-stil-
bazolium tosylate (DAST).
Keywords:
Nonlinear optical (NLO) materials
Aligned chromophores
Single crystals
Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
Hydrogen bond
Organic dye
Kurtz powder test
1. Introduction
films [5,9], as well as single crystals [2,10]. Among them, polar
organic crystals are of special interest, since large NLO effects can be
There is considerable interest in organic nonlinear optical (NLO)
materials with large second-order optical nonlinearities. This is on
the one hand because of the convenient and easy way in which
their molecular structures, and hence their properties, can be tuned
for various applications [1e3]. On the other hand, the main merits
of organic materials compared with inorganic ones for second-
order NLO applications such as THz generation, electric-field
detection, electro-optic modulation and second-harmonic genera-
tion (SHG) applications are the large macroscopic second-order
achieved due to the high density of chromophores in crystals, as
well as the most stable packing of chromophores in macroscopic
materials, resulting in a superior photochemical and thermal
stability as compared with softer materials such as the widely
studied poled polymers [11]. Furthermore, organic NLO crystals can
be also produced in a bulk form and are therefore not limited to
thin-film applications [10e13]. This is particularly important for
frequency conversion applications such as terahertz-wave genera-
tion, where large interaction lengths are required [14]. State-of-the-
art organic NLO crystal DAST (4-N,N-dimethylamino-40-N0-methyl-
stilbazolium tosylate) and its derivative DSTMS (4-N,N-dimethyla-
mino-40-N0-methyl-stilbazolium 2,4,6-trimethylbenzenesulfonate)
have been recently introduced in commercial THz sources [15],
with high figures of merit and large frequency bandwidths
compared to semiconductor alternatives like ZnTe or GaAs [16,17].
It is therefore interesting to develop organic materials with
improved quadratic NLO susceptibility and therefore further
increase the efficiency for applications.
NLO susceptibilities (c
(2)), low dielectric dispersion and ultrafast
response to external electric fields [4,5]. Organic NLO materials are
based on NLO molecules (chromophores), which should be
oriented in a non-centrosymmetric way in a macroscopic material
system in order to exhibit macroscopic second-order NLO response.
There are several different strategies resulting in macroscopic c(2)
organic materials, including ferroelectric liquid crystals [6], self-
assembled films [7], LangmuireBlodgett films [8], poled polymer
Particularly challenging for the development of efficient organic
crystalline c(2) materials is to simultaneously achieve good NLO
properties and good physical properties for the growth of crystals
* Corresponding authors. Tel.: þ86 10 62333759.
E-mail address: yangz@ustb.edu.cn (Z. Yang).
0143-7208/$ e see front matter Crown Copyright Ó 2011 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.12.004

J. Yin et al. / Dyes and Pigments 94 (2012) 120e126
121
suitable for applications. The ways to design molecules with a high
microscopic nonlinearity are now well understood, however, it is
presently not possible to predict their packing into a crystalline
lattice, which is extremely important for their macroscopic NLO
response. The macroscopic response will basically depend on the
number density of chromophores, non-centrosymmetric packing
with a preferably high order parameter, and influence of the
intermolecular interactions. Many methods [7e11] have been
developed to design organic molecules leading to a relatively high
probability for non-centrosymmetric arrangement in a bulk form.
Out of these, the method of using Coulomb interactions of ionic
chromophores to promote crystallization has been proved to be one
of the most effective strategies [7]. For example, DAST (4-N,N-
dimethylamino-40-N0-methyl-stilbazolium tosylate), composed of
the stilbazolium-chromophore cation and tosylate anion is one of
the most well-known NLO materials, which is because of its large
macroscopic NLO susceptibility c(2) ¼ 580 ꢀ 30 pm/V and the
Fig. 1. Molecular diagram of DAST derivatives.
3H, NMe), 3.02(s, 6H, NMe2). Calcd for C23H24N2O6S: C, 60.51; H,
5.30; N, 6.14%. Found: C, 60.53; H, 5.29; N, 6.12%.
4-N,N-dimethylamino-40-N0-methyl-stilbazolium 3-carboxyben
zene-1-sulfonate (DSCS): yield 70%; 1H NMR (400 MHz, DMSO-
electro-optic figure of merit n3r ¼ 455 ꢀ 80 pm/V at 1.55
mm [18,19].
d6):
d
¼ 8.68(d, 2H, J ¼ 6.8 Hz, C5H4N), 8.19(s, 1H, C7H5O5S), 8.04(d,
Nevertheless, theoretical calculations and recent progress have
shown that it is still possible to improve the NLO properties and
crystal-growth characteristics so that they will be superior to those
of DAST. In previous studies, several DAST analogs with a modified
counteranion were synthesized to achieve such an enhancement
[20e24]. Research has shown that even very minor changes in the
structures of counteranion have an important effect on the lattice
stacking [22e27]. Well-designed counteranions can induce non-
centrosymmetric structures having even higher NLO activity than
DAST [22]. This provides nearly endless design possibilities, as we
can achieve the combination of good NLO properties and desirable
physical properties by adjusting the structure of both the anion and
the cation.
2H, J ¼ 6.8 Hz, C5H4N), 7.92(m, 3H, C2H2þC7H5O5S), 7.60(d, 2H,
J ¼ 8.8 Hz, C6H4), 7.46(t, 1H, J ¼ 15.3 Hz, C7H5O5S), 7.18(d, 1H,
J ¼ 16.1 Hz , C2H2), 6.79(d, 2H, J ¼ 8.8 Hz, C6H4), 4.17(s, 3H, NMe),
3.02(s, 6H, NMe2). Calcd for C23H24N2O5S: C, 62.71; H, 5.49; N,
6.36%. Found: C, 62.69; H, 5.27; N, 6.16%.
4-N,N-dimethylamino-40-N0-methyl-stilbazolium 3,5-dicarbox
ybenzene-1-sulfonate (DSDCS): yield 72%; 1H NMR (400 MHz,
DMSO-d6):
d
¼ 8.68(d, 2H, J ¼ 6.8 Hz, C5H4N), 8.40(s, 1H, C8H5O7S),
8.36(s, 2H, C8H5O7S), 8.04(d, 2H, J ¼ 6.8 Hz, C5H4N), 7.90(d, 1H,
J ¼ 16.1 Hz , C2H2), 7.58(d, 2H, J ¼ 8.8 Hz, C6H4), 7.18(d, 1H,
J ¼ 16.1 Hz, C2H2), 6.79(d, 2H, J ¼ 8.8 Hz, C6H4), 4.17(s, 3H, NMe),
3.02(s, 6H, NMe2). Calcd for C24H24N2O7S: C, 59.49; H, 4.99; N,
5.78%. Found: C, 59.50; H, 5.06; N, 5.83%.
It has been recently observed that beside Coulomb forces,
important intermolecular interactions in DAST and its analogues
present weak hydrogen bonds [25]. In this paper, we report the
synthesis, crystal structure and NLO properties of a series of DAST
derivatives with 3-carboxyl group on the counteranions. The
counteranion functional groups have been chosen to promote
hydrogen bonds in order to better understand their influence on
packing and physical properties of stilbazolium salts. Among them,
3. Results and discussion
3.1. Synthesis of DAST derivatives
DAST derivatives were successfully prepared by a three-step
reaction approach; their structures were confirmed by 1H NMR.
The chemical structures of the three DAST derivatives are shown in
Fig.1. All of these new DAST derivatives possess 3-carboxyl group in
the sulfonate counteranions. One of them, DSDCS possesses two
carboxyl groups while DSCHS possesses one 4-hydroxyl group.
These groups were chosen since they can lead to hydrogen bond
formation, which could effectively influence the crystallographic
packing of the stilbazolium-chromophore cation [25].
DSCHS
(4-N,N-dimethylamino-40-N0-methyl-stilbazolium
3-
carboxy-4-hydroxybenzenesulfonate) shows very high non-
resonant powder second-harmonic generation (SHG) efficiency of
30 percent higher than DAST, which is due to the perfectly parallel
aligned chromophores.
2. Materials and methods
3.2. Physical properties
General Considerations. All reagents were purchased as high
purity from Aldrich and used without further purification. 1H-NMR
spectra were recorded on a Brucker 400 MHz spectrometer on
DMSO-d6 solutions. Thermal analysis was conducted on Per-
kineElmer Pyris 6 DSC spectrometer at a heating rate of 10 ꢁC/min.
DAST derivatives were synthesized via metathesization of 4-N,
N-dimethylamino-40-N0-methyl-stilbazolium iodide with the
sodium salts of counteranions with 3-carboxyl group. The iodide
salt was prepared from 4-picoline and iodomethane, and 4-
(dimethylamino)benzaldehyde in the presence of piperidine as
described in literature [1]. DAST derivatives were then recrystal-
lized from methanol to get high purity materials for crystal growth.
4-N,N-dimethylamino-40-N0-methyl-stilbazolium 3-carboxy-4-
hydroxybenzenesulfonate (DSCHS): yield 75%; 1H NMR (400 MHz,
The melting point, UV absorption peak lmax in solution, and
powder SHG efficiency of the three DAST derivatives are listed in
Table 1 compared to DAST. The melting points of all of the three
DAST derivatives are quite higher than that of DAST and most of
previously studied DAST derivatives [25e27]. These new
Table 1
Physical properties of the three DAST derivatives compared to DAST.
Compound
Melting point/ꢁC
lmaxa/nm
Powder SHGb
DSCHS
DSDCS
DSCS
>300
474
474
474
474
1.3
0.1
0
292 ꢀ 1
281 ꢀ 1
256 ꢀ 1
DMSO-d6):
d
¼ 8.68(d, 2H, J ¼ 6.1 Hz, C5H4N), 8.04(d, 3H, C5H4N
DAST
1
þ C7H5O6S), 7.91(d, 1H, J ¼ 16.0 Hz , C2H2), 7.68(d, 1H, J ¼ 8.5 Hz,
C7H5O6S), 7.60(d, 2H, J ¼ 8.0 Hz, C6H4), 7.18(d,1H, J ¼ 16.1 Hz , C2H2),
6.87(d, 1H, J ¼ 8.5 Hz, C7H5O6S), 6.79(d, 2H, J ¼ 8.1 Hz, C6H4), 4.17(s,
a
In methanol solution at room temperature.
b
1907 nm fundamental radiation, compared with DAST, with an experimental
error of less than 10%.

122
J. Yin et al. / Dyes and Pigments 94 (2012) 120e126
a wavelength
l
¼ 1907 nm from an optical parametrical amplifier
pumped by an amplified Ti: sapphire femtosecond laser was used
to screen the NLO activity of the new compound. DSCHS crystalline
material was grinded and sieved to a particle size distribution of
63e90
mm, and then put into a 1.00 mm Hellma UV quartz sample
cell to give a constant sample thickness. The scattered SHG signal at
l
¼ 954 nm was detected by using appropriate filters. The SHG
signals were calibrated with respect to equally prepared DAST
samples. By this method we found that the SHG efficiencies of
DSCHS and DSDCS are 1.3 and 0.1 times as that of DAST, respec-
tively, while DSCS showed no signal pointing to the centrosym-
metric final structure.
3.4. Crystal growth
DSCHS, whose counteranion possesses hydroxyl and carboxyl
groups, was of most interest for crystal growth experiments
because it showed a strong SHG activity 30% higher than DAST.
However, DSCHS is not well soluble even in most polar solvents.
After testing several common solvents and their combinations, we
found that the solubility can be improved by using aqueous
methanol solution with respect to both water and methanol alone.
We compared solvent mixtures of methanol and water with 3:1, 2:1
and 1:1 proportion and found that DSCHS dissolved best using the
last condition.
Fig. 2. Spectra of DSCHS and DAST powders compared to the absorption spectrum of
DAST single crystals [28] along the polar axis. Differently prepared powders were
measured to see the variation in spectra due to scattering.
compounds possess hydroxyl and carboxyl groups, which can form
intermolecular hydrogen bonding, which is possibly one of the
factors contributing to their high melting points. Hydrogen bonds
in DSCHS crystal are discussed in more detail in Section 3.5. The
wavelength of maximal absorption lmax ¼ 474 nm in methanol
solution is the same for all compounds, which is because of the
same NLO chromophore that contributes mainly to the charge
transfer in these compounds. We also measured the spectra of the
solid-state DSCHS in powder form in comparison with DAST, for
that the absorption spectra in resonance have been previously fully
characterized by more advanced methods using optically polished
single crystals [28]. For this, we measured transmission spectra of
thin layers of powders using a PerkineElmer Lambda 9 spectrom-
eter. As shown in Fig. 2, both DSCHS and DAST show a red-shifted
spectrum in solid state compared to solution, but their relative
maxima are not considerably shifted, meaning that the molecular
environment does not considerably change the electronic proper-
ties of the stilbazolium chromophore in the solid state for these two
materials.
The slow cooling and slow evaporation methods are most
suitable for crystal growth of the DSCHS. We performed crystal
growth experiments by using the slow cooling method. The
mixture of methanol and deionized water with 1:1 volume
proportion was prepared as solvent. The DSCHS can be reasonably
well-dissolved in the above mentioned solvent, although the
solubility of 0.2 g/100 ml solvent at 45 ꢁC is still much lower than
the solubility 3.6 g/100 ml of DAST in methanol, which is the
solvent most often employed for the crystal growth of DAST. We
continued adding DSCHS to the solvent to prepare a saturated
solution at 45 ꢁC. After filtering the saturated solution by using
0.2 m
m porosity Millipore filters at 45 ꢁC, they were preheated to
45 ꢁC to ensure that all ingredients were dissolved, then slowly
cooled the solution from 45 ꢁC to room temperature. The temper-
ature was cooled down relatively fast at a rate of 2e3 ꢁC per day
without further optimization. It was found that red color DSCHS
crystals start to nucleate and continue to grow in a plate-like shape
preferentially along all edges in all directions. We filtered the
solution and dried the crystals when the growth had stopped at
room temperature.
3.3. Kurtz powder test
The second-harmonic generation (SHG) efficiency of DSCHS was
determined by using the Kurtz powder technique [29] in the same
configuration as used in Ref. [22,25] An idler wave with
Fig. 3. Crystal of DSCHS between crossed polarizers in a microscope at the position of the maximum transmission (a) and rotated by 45ꢁ from this position (b).

J. Yin et al. / Dyes and Pigments 94 (2012) 120e126
123
Table 2
Crystallographic data of DSCHS.
Sample
DSCHS
Formula
Formula weight
Crystal system
Space group
Point group
a/Å
C23H24N2O6S
456.50
Triclinic
P1
1
7.1334
8.5394
9.965
67.77
70.65
75.29
1
b/Å
c/Å
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
Z
V/Å3
524.4
173
Tm (ꢁC)
Fig. 5. Oak Ridge thermal ellipsoid program (ORTEP) representation of one ion-pair of
DSCHS at 263 K (Vibrational ellipsoids are shown at the 30% probability level.)
We selected a thin plate crystal and placed it between crossed
polarizers in a microscope (Fig. 3). Rotating the crystal by 45ꢁ
around the direction of light propagation from the orientation with
the highest transmission turned it completely dark, which means
that in this case the main axis of the optical indicatrix (or its
projection to the plane of the crystal plate) was aligned parallel to
the polarization of the transmitted light, and also indicates that the
crystals are single crystalline [18e20,25].
crystals with a non-centrosymmetric structure have been obtained
from aqueous solvent. Moreover, X-ray crystallographic data show
that no water molecules have been included into the crystal
structure of DSCHS.
Crystallographic data shows that beside the Coulomb interac-
tions between the cation and anion parts, hydrogen bonds also play
a role in crystal packing and chromophore orientation [25]. In
DSCHS, the data shows that at least three kinds of hydrogen bonds
have formed. One is the intermolecular hydrogen bond of
O$$$H$$$O between the sulfonic oxygen atoms and carboxyl
oxygen atoms with H$$$O distance of 1.754 Å; the other is intra-
molecular hydrogen bond of O$$$H$$$O between the hydrogen
oxygen atoms and carboxyl oxygen atoms with H$$$O distance of
1.837 Å; the third are the weaker hydrogen bonds of C$$$H$$$O
between the cation layers and the anion layers with H$$$O
distances from 2.242 Å to 2.423 Å (Fig. 6). In DAST, strongest
hydrogen bonds are weak hydrogen bonds between the cation and
the anion layers with H$$$O distances of larger than 2.4 Å. Rela-
tively strong hydrogen bonds may be one of the reason why DSCHS
possesses a high melting point.
3.5. X-Ray crystallographic study
We measured the crystallographic structure of DSCHS by X-ray
analysis of single crystals. The data obtained is listed in Table 2.
DSCHS crystals are triclinic and have a P1 space group (point group
1); its crystal-packing diagram as seen along the crystallographic
axes is shown in Fig. 4. The structure of one ion-pair is shown in
Fig. 5. The chromophore’s non-centrosymmetric alignment is
a prerequisite for second-order NLO effects. In DSCHS the planar
stilbazolium chromophores are aligned perfectly parallel, which
represents the ideal case for adding up the microscopic polariz-
abilities into a macroscopic effect, maximizing the diagonal electro-
optic and NLO coefficients.
It has been reported that DAST absorbs moisture easily to form
a hydrate polymorph with centrosymmetric structure thus without
second-order NLO properties [20]. The same phenomena have also
been found from some other DAST derivatives, for example, DSTMS
[24e26]. This means that for these stilbazolium salts the crystals
must be grown in a completely nonaqueous environment to avoid
forming centrosymmetric structure. In contrary for DSCHS, single
3.6. Discussion
We can estimate the expected powder test efficiency by
considering the crystallographic packing of the chromophores and
their density. In first approximation, we neglect any influence of the
intermolecular interactions on the microscopic nonlinearity
b of
Fig. 4. Crystal-packing diagram: DSCHS with triclinic P1 symmetry viewed along the three crystallographic axes.

124
J. Yin et al. / Dyes and Pigments 94 (2012) 120e126
Fig. 6. The molecules in DSCHS are linked by Coulomb interactions between the ionic parts and by hydrogen bonds that are indicated by dotted lines.
the stilbazolium chromophore. This assumption is very reasonable
considering the similar position of the absorption band observed in
the powder spectrum (see Fig. 2). We also assume that the stilba-
zolium chromophore is essentially one-dimensional, with only one
The perfectly parallel orientation of the chromophores is
optimal to maximize the diagonal NLO susceptibility element. For
phase-matched SHG applications in bulk materials, on the other
hand, a high off-diagonal NLO susceptibility element is preferred,
dominant component
b
of the first-hyperpolarizability tensor along
with the optimal angle q
of about 55ꢁ [30]. Such an orientation
the direction of the long axis of the chromophore, and neglect any
contribution of the counteranion to the nonlinearity. In DAST, the
four chromophores in the unit cell are aligned along the 3a þ b
and along the 3a ꢂ b crystallographic vectors, and therefore make
however reduces the maximal NLO susceptibility element by more
eff
eff
than 60%;
b
z0:38
b
at
q
¼ 55ꢁ, while
b
z
b
at
q
¼ 0ꢁ
122;max
111;max
[30]. Recently, most promising applications of organic NLO mate-
rials are within electro-optics, THz-wave generation and electric-
field detection [9,13,16], for which a maximal possible NLO
susceptibility element is optimal, as in the case of DSCHS.
In the last few years, there have been other DAST derivatives
identified with isomorphous crystal structure as DSCHS, i.e. P1
space group with perfectly parallel chromophore orientation [25],
so it is interesting to compare the observed powder test efficiency
with these compounds. For all DAST derivatives crystallizing in
an angle of about
q
¼ 20ꢁ with respect to the polar axis a. If we
define the Cartesian system with its x axis parallel to a, y axis
parallel to b, and z axis perpendicular to x and y, the effective
hyperpolarizability of the crystalline system of DAST only contains
eff
eff
the following components:
b
¼
b
cos3
q
¼ 0:83
b
and
b
¼
111
221
eff
eff
b
¼
b
¼
b
cos
q
sin2
q
¼ 0:11
b
; according to the so-called
212
122
oriented-gas model [24]. In case of DSCHS,
q
¼ 0ꢁ, and only one
space group P1, the theoretical spatial average hðbeff Þ2i ¼ 0:18b2
,
effective hyperpolarizability component may be considered,
the same as for DSCHS. The number density of the chromophores in
eff
111
these compounds is N ¼ 1.84 ꢃ 1027 mꢂ3, N ¼ 1.79 ꢃ 1027
m
ꢂ3, and
b
¼
b
; in this case the x axis is chosen parallel to the chromo-
N ¼ 1.77 ꢃ 1027 mꢂ3, for DSDMS, DSNS-1, and DSNS-2, respectively,
see also Table 3. This gives the following theoretical powder test
efficiencies relative to DAST: 1.2,1.1, and 1.1 for DSDMS, DSNS-1, and
DSNS-2, respectively, while the experimentally measured values
were 0.7, 0.7 and 1.5 [25]. Since for all of these compounds the
orientation of the chromophores is parallel, we expect that the
relative experimental powder test results are rather accurate, since
no other than diagonal components significantly contribute to the
measured signal and therefore we also expect no contribution of
eventual phase-matched SHG generation.
phore, i.e. approximately along the ea þ 2b crystallographic vector
that is also the polar axis of DSCHS (see Fig. 4c).
The powder test efficiency depends on the number density of the
chromophores N and the squared effective
b components, averaged
over all possible orientations hðbeff Þ2i [29]. In case that only two
eff
eff
eff
eff
effective components
b
and
b
¼
b
¼
b
are non-zero,
111
221
212
122
1
105
hðbeff Þ2i can be expressed as [29] hðbeff Þ2i ¼
ð19ð
b
Þ2þ
eff
111
eff eff
eff
221
13
b
b
þ 44ð
b
Þ2Þ, which gives hðbeff Þ2iDAST
¼
0:14b2 for
111 221
DAST and hðbeff Þ2iDSCHS ¼ 0:18
b
2. The number density of chromo-
The large differences observed in experimental powder SHG
tests are interesting because this indicates that intermolecular
phores is in DSCHS very similar as in DAST, N ¼ 1.91 ꢃ 1027 mꢂ3
.
Therefore we expect the non-resonant powder test efficiency of
DSCHS, which is proportional to N2hðbeff Þ2i, to be about 30% larger
than the one of DAST (1000 times that of urea) [3], if the molecular
nonlinearity in the crystalline state is similar for both compounds.
Since the powder size for this measurement was much larger than
the coherence length of these compounds, which is in the order of
Table 3
Comparison of DAST derivatives crystallizing in the space group P1.
Compound
DSCHS
DSDMS
DSNS-1
DSNS-2
N (1027 mꢂ3
)
1.91
1.3
1.3
1.84
1.2
0.7
1.79
1.1
0.7
1.77
1.1
1.5
a
Theoretical Powder SHGb
Measured Powder SHGc
magnitude of 1 mm, we also do not expect any substantial influence
of the powder size. The experimental powder test efficiency
perfectly agrees with our estimation, which confirms that the larger
nonlinearity of DSCHS crystals compared to DAST is primarily due to
the more optimal packing of the chromophores.
a
The number density of the chromophores.
b
Non-resonant conditions neglecting intermolecular interactions, relative to
DAST.
c
1907 nm fundamental radiation, relative to DAST.

J. Yin et al. / Dyes and Pigments 94 (2012) 120e126
125
interactions play an important role in stilbazolium salts, and may
enable to considerably enhance the molecular nonlinearity in the
crystalline state, as also predicted for single-molecule molecular
crystals [31]. For DAST, this has been suggested when comparing the
measured macroscopic NLO susceptibility to the calculated one
considering the molecular hyperpolarizability and using the
oriented-gas model and the two-level model [32]. However, these
models are greatly simplified in particular for strongly anisotropic
dielectric systems such as stilbazolium salts. Therefore it is inter-
esting to directly compare macroscopic crystals that contain the
same NLO chromophore aligned in the same direction, but only
having a different molecular environment of counteranion mole-
cules, which is the case of crystals listed in Table 3. In Ref. [25],
possible reasons for different measured values for the powder test
efficiency of DSDMS, DSNS-1, and DSNS-2 were discussed. Since for
DSNS-2 with the highest powder test efficiency the chromophore
density is the lowest, one possible reason is that the larger chro-
mophore separation will less reduce their molecular nonlinearity
[25]. With our new compound DSCHS we see that this is not
necessarily the case, since the density of the chromophores is in
DSCHS considerably larger than in isomorphous DSDMS and DSNS-
1 compounds, still it seems that the molecular nonlinearity is
enhanced compared to these compounds. The influence of
hydrogen bond formation of the stilbazolium chromophore with
the surrounding counter ions was also investigated [25]. For DSDMS
and DSNS-1 the strongest intermolecular hydrogen bonds involve
the Returned Overseas Chinese Scholars, the National Natural
Science Fund for Distinguished Young Scholar (Grant no.
51025313), and by the Swiss National Science Foundation.
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In summary, a series of new organic NLO stilbazolium salts with
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Acknowledgments
This work was supported by the National Natural Science
Foundation (grant no. 50803005), the Fundamental Research Funds
for the Central Universities, the Scientific Research Foundation for

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J. Yin et al. / Dyes and Pigments 94 (2012) 120e126
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