Acentric nonlinear optical N-benzyl stilbazolium crystals with high environmental stability and enhanced molecular nonlinearity in solid state

Article abstract of DOI:10.1039/c0ce00456a

We have developed a new cation core structure, N-benzyl stilbazolium nonlinear optical chromophore with a non-polar benzyl group to achieve acentric molecular ordering in the crystalline state. New N-benzyl stilbazolium crystal, BP3 (N,N-dimethylamino-N′-2,5-dimethylbenzyl-stilbazolium p-toluenesulfonate), exhibits an acentric crystal structure with the monoclinic P2₁ phase with a large macroscopic optical nonlinearity of 540 times the powder second harmonic generation efficiency of urea at the non-resonant wavelength of 1.9 m. Compared to conventional rod-shaped N-alkyl stilbazolium crystals, an enhanced hyperpolarizability of the chromophores β₀ in the solid state can be utilized in bent-shaped N-benzyl stilbazolium crystals. This is attributed to the decreased inter-chromophore interactions due to the larger chromophore-chromophore separation induced by the bulky and bent-shaped N-benzyl group, so-called site-isolation effect. Moreover, by introducing the non-polar dimethylbenzyl group, BP3 crystals show a high environmental stability: they exhibit almost one order of magnitude smaller solubility in water than conventional stilbazolium crystals and also do not form a hydrated centrosymmetric phase even if crystallized in water-containing solvents. We have grown good optical quality crystals large enough for optical characterization. With as-grown BP3 crystals without additional polishing and cutting procedures we have demonstrated THz generation by optical rectification using 180 fs pulses at the pump wavelength of 836 nm.

Full text of DOI:10.1039/c0ce00456a

CrystEngComm
www.rsc.org/crystengcomm
Volume 13 | Number 2 | 21 January 2011 | Pages 373–710
COVER ARTICLE
Kwon et al.
Acentric nonlinear optical N-benzyl stilbazolium crystals with high
environmental stability and enhanced molecular nonlinearity in
solid state

PAPER
www.rsc.org/crystengcomm | CrystEngComm
Acentric nonlinear optical N-benzyl stilbazolium crystals with high
environmental stability and enhanced molecular nonlinearity in solid state†
Pil-Joo Kim,a Jae-Hyeok Jeong,a Mojca Jazbinsek,b Seong-Ji Kwon,b Hoseop Yun,c Jong-Taek Kim,d
d
Yoon Sup Lee, In-Hyung Baek, Fabian Rotermund, Peter Gunter and O-Pil Kwon
c
c
b
a
*
€
Received 27th July 2010, Accepted 2nd September 2010
DOI: 10.1039/c0ce00456a
We have developed a new cation core structure, N-benzyl stilbazolium nonlinear optical chromophore
with a non-polar benzyl group to achieve acentric molecular ordering in the crystalline state. New
N-benzyl stilbazolium crystal, BP3 (N,N-dimethylamino-N0-2,5-dimethylbenzyl-stilbazolium
p-toluenesulfonate), exhibits an acentric crystal structure with the monoclinic P21 phase with a large
macroscopic optical nonlinearity of 540 times the powder second harmonic generation efficiency of
urea at the non-resonant wavelength of 1.9 mm. Compared to conventional rod-shaped N-alkyl
stilbazolium crystals, an enhanced hyperpolarizability of the chromophores b0 in the solid state can be
utilized in bent-shaped N-benzyl stilbazolium crystals. This is attributed to the decreased inter-
chromophore interactions due to the larger chromophore–chromophore separation induced by the
bulky and bent-shaped N-benzyl group, so-called site-isolation effect. Moreover, by introducing the
non-polar dimethylbenzyl group, BP3 crystals show a high environmental stability: they exhibit almost
one order of magnitude smaller solubility in water than conventional stilbazolium crystals and also do
not form a hydrated centrosymmetric phase even if crystallized in water-containing solvents. We have
grown good optical quality crystals large enough for optical characterization. With as-grown BP3
crystals without additional polishing and cutting procedures we have demonstrated THz generation by
optical rectification using 180 fs pulses at the pump wavelength of 836 nm.
contrast to many studies with various counter anions, only few
cation core structures that resulted in an acentric molecular
ordering based on Coulomb interactions leading to large
macroscopic nonlinearities have been reported, typically N-alkyl
and N-aryl stilbazolium salts. Finding core structures for acentric
molecular ordering in crystalline state is the starting point of
crystal engineering, with the aim to obtain crystals with opti-
mized optical and processing properties, as well as a high
stability.
Introduction
Organic nonlinear optical crystals1 are very promising for
numerous applications such as integrated photonics2 and
frequency conversion including THz wave generation.3 Among
organic crystals exhibiting the second-order (quadratic) optical
nonlinearity, the benchmark materials are N-substituted stilba-
zolium crystals based on Coulomb interactions with large
macroscopic optical nonlinearities.4–8 Since DAST (N,N-dime-
thylamino-N0-methyl-stilbazolium p-toluenesulfonate) crystal
based on N-methyl-stilbazolium cation has been first reported in
1989,4a many derivatives have been extensively investigated by
using the same cation core structure, N-methyl-stilbazolium with
various arylsulfonate anions:5–7 for example, DSTMS with 2,4,6-
trimethylbenzenesulfonate anion,5 and DSNS with 2-naph-
thalenesulfonate anion.6a Another example of a core structure
leading to the required acentric molecular ordering is N-aryl
stilbazolium:8 for example, DAPSH (N,N-dimethylamino-N0-
phenyl-4-stilbazolium hexafluorophosphate) crystal with
extremely large nonresonant macroscopic nonlinearity.8b In
Although benchmark stilbazolium crystals exhibit a large
macroscopic nonlinearity and good crystal characteristics,
enhancing these physical properties including the environmental
stability is a challenging topic. The salt-type stilbazolium chro-
mophores are for example often dissolved in water and form
a centrosymmetric hydrated phase in the presence of water,9 and
therefore humidity may damage the crystals, as well as limit the
application of various crystal growth techniques. Also, the
theoretical limits for the nonlinearities in stilbazolium salts have
by far not been reached yet.10 This is partially attributed to the
decreased molecular nonlinearities in the solid state due to the
interactions of the chromophores with the neighboring mole-
cules.10 For poled polymers it was shown that a bulky donor
group and bulky isolation groups on the chromophore may help
to decrease such interactions, via the so-called site-isolation
effect,11a which is for polymer systems beneficial to achieve
a higher poling efficiency and therefore a higher macroscopic
nonlinearity.11
aDepartment of Molecular Science and Technology, Ajou University,
Suwon, 443-749, Korea. E-mail: opilkwon@ajou.ac.kr; Fax: +82 31 219
1610; Tel: +82 31 219 2462
bNonlinear Optics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland
cDivision of Energy Systems Research, Ajou University, Suwon, 443-749,
Korea
dDepartment of Chemistry and School of Molecular Science (BK 21),
Korea Advanced Institute of Science and Technology (KAIST), Daejeon,
305-701, Korea
In this work, we develop new cation core structure, N-benzyl
stilbazolium with a non-polar benzyl group for acentric molec-
ular ordering in the crystalline system. New N-benzyl
† CCDC reference number 761254. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0ce00456a
444 | CrystEngComm, 2011, 13, 444–451
This journal is ª The Royal Society of Chemistry 2011

stilbazolium crystal, BP3 (N,N-dimethylamino-N0-2,5-dime-
thylbenzyl-stilbazolium p-toluenesulfonate), exhibits an acentric
crystal structure with the monoclinic P21 space group with
a large macroscopic optical nonlinearity of 540 times the second
harmonic generation efficiency as that of urea at the non-reso-
nant wavelength of 1.9 mm. Compared to conventional
rod-shaped N-alkyl stilbazolium crystals, an enhanced hyper-
polarizability of the chromophores b0 in the solid state is
observed in bent-shaped N-benzyl stilbazolium crystals. More-
over, by introducing the non-polar dimethylbenzyl group, BP3
crystals show a high environmental stability: 9 times smaller
solubility in water than the conventional stilbazolium crystal
DAST. BP3 does not form a hydrated centrosymmetric phase
even if crystallized in water-containing solvents. We have grown
good optical quality crystals and have demonstrated THz
generation by optical rectification of fs pulses with as-grown BP3
crystals without additional polishing and cutting procedures.
methyl groups on the benzyl ring are expected to increase the
asymmetry of the chromophore shape.
The chromophores were synthesized by metathesization of
corresponding N,N-dimethylamino-N0-benzylstilbazolium chlo-
ride salt with p-toluenesulfonate sodium salt in water.5,6 First, the
corresponding N-benzylpicolinium chloride derivatives were
prepared by the reaction of picoline with appropriated chlor-
obenzyl derivatives. N,N-Dimethylamino-N0-benzylstilbazolium
chloride salts were synthesized by a condensation of N-benzyl-
picolinium chloride with dimethylaminobenzaldehyde. The BP
chromophores synthesized in water were recrystallized in meth-
anol by the rapid cooling method for purification and removing
residual water.
Basic characterization of N-benzyl stilbazolium molecules and
crystals
Fig. 2 shows the absorption spectra of the N-benzyl stilbazolium
BP chromophores in methanol. The wavelength of maximum
absorption of the investigated BP chromophores is lmax z 487
nm, which is about 12 nm higher than for the N-methyl stilba-
zolium chromophores (for example, DAST lmax ¼ 475 nm).4
According to the well-known nonlinearity-transparency trade-
off;1 N-substituted stilbazolium chromophores with a higher
Results and discussion
Design of new core structure for acentric molecular ordering
The chemical structures of the new N-substituted stilbazolium
derivatives are shown in Fig. 1. The chromophores consist of
a N-substituted pyridinium electron acceptor group and the
dimethylaminophenyl electron donor group. To induce asym-
metry of the chromophores, which is one of the strategies to
promote a noncentrosymmetric structure,12 as well as achieve
molecular nonlinearity generally exhibit a red shift of lmax.
8c N-
Benzyl BP chromophores are therefore expected to exhibit
a higher molecular nonlinearity than N-alkyl stilbazolium chro-
mophores.
In order to investigate the molecular nonlinearity of the new
N-benzyl stilbazolium derivatives in more details, we calculated
the first hyperpolarizability tensor bijk and the maximal first
hyperpolarizability bmax (hyperpolarizability component along
the main charge-transfer direction) of the cation core structures
of DAST and BP3 crystals by quantum chemical calculations14
using the hybrid functional B3LYP15 with the 6-311 + G(d) basis
set. The optimized structures (OPTs) obtained by using the
density functional theory (DFT) of the cations, N,N-dimethyla-
mino-N0-methyl-stilbazolium for DAST and N,N-dimethyla-
mino-N0-2,5-dimethylbenzyl-stilbazolium for BP3 are shown in
Fig. 3 and the calculated results are listed in Table 1. The
calculated molecular hyperpolarizability of the N-benzyl BP3
cation is about 22% higher than that of the N-methyl DAST
a
site-isolation effect,11 N-position on the pyridinium is
substituted with a bent-shaped bulky benzyl group, replacing the
methyl group of conventional rod-shaped N-alkyl stilbazolium
DAST chromophores. Considering the environmental stability
including water-insolubility, not only non-polar benzyl group,
but also non-polar methyl groups on the benzyl ring are intro-
duced for BP2 and BP3. In order to maintain the main supra-
molecular Coulomb interactions of these compounds, we use
non-polar substituents such as benzyl and methyl groups, which
may form only weak intermolecular interactions. This is because
polar substituents on the benzyl ring may form unexpected
additional strong intermolecular interactions such as hydrogen
bonds and dipole–ion interactions, which may diminish the
hyperpolarizability of the molecules in the solid state.10,11,13 In
BP3 chromophore, the ortho and the meta positions of the two
Fig. 1 Chemical structures of N-benzyl pyridinium salt derivatives and
Fig. 2 Absorption spectra of salt molecules in methanol solution: lmax
475 nm for DAST and ꢀ 487 nm for BP chromophores.
¼
their abbreviations.
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CrystEngComm, 2011, 13, 444–451 | 445

Single crystal structure and nonlinear optical properties
Single BP3 crystals for crystal structure analysis by X-ray
diffraction (XRD) were grown from methanol solution by slow
evaporation method at 40 ꢁC in an oven. The BP3 crystals have
an acentric monoclinic crystal structure with P21 space group
(point group 2). Its molecular structure and the crystal packing
are shown in Fig. 4 and 5. While the N-alkyl and N-aryl
stilbazolium part including the dimethylamino group presents
a rod-shape in previously investigated stilbazolium salts,4–8 N-
substituted benzyl group is out of the plane of the stilbazolium
and the BP3 cation possesses a non-rod-shape (see Fig. 4a),
which can induce asymmetry and a different kind of acentric
molecular ordering.
Fig. 3 Representation of the optimized cation structures (OPTs)
obtained by the density functional theory (DFT) using B3LYP/6-311 +
G(d) and the experimental cation structures (EXPs) obtained by the
X-ray diffraction of acentric single crystals of DAST (N,N-dimethyla-
mino-N0-methyl-stilbazolium p-toluenesulfonate) and BP3 (N,N-dime-
thylamino-N0-2,5-dimethylbenzyl-stilbazolium p-toluenesulfonate). The
dotted vectors present the directions of the maximum first hyper-
polarizability bmax of the cations as determined by finite-field calculations
(see Table 1).
In N-alkyl DAST, where the same counter anion is used, as
well as in other DAST derivatives with a different counter anion
and a non-centrosymmetric structure, well defined polar sheets of
stilbazolium chromophores and counter anions can be identified
in the crystal structure.4a,6b In BP3, no such layers are observed,
which means that the N-benzyl stilbazolium substitution essen-
tially changes the characteristic packing tendency of DAST
derivatives. Main supramolecular interactions of acentric BP3
crystal are Coulomb interaction between cations and anions and
hydrogen bonds with S–O/H–C (see Fig. 5). However, as
shown in Fig. 3, the main charge transfer character is still
essentially one-dimensional and the main charge-transfer
Table 1 Calculated results of the finite-field (FF) method with the
optimized (OPT) and the experimental (EXP) cation structures of DAST
and BP3 crystals at B3LYP/6-311 + G(d) level: the zero-frequency
hyperpolarizability tensor bijk (ꢂ10ꢃ30 esu) and the maximum first
hyperpolarizability bmax (ꢂ10ꢃ30 esu)
DAST cation
(OPT)
DAST cation
(EXP)
BP3 cation
(OPT)
BP3 cation
(EXP)
bxxx
bxxy
bxyy
byyy
bxxz
bxyz
byyz
bxzz
byzz
bzzz
ꢃ0.11
ꢃ0.04
0.05
ꢃ0.21
0.03
ꢃ0.05
ꢃ0.06
0.25
3.73
8.12
0.61
2.85
ꢃ2.27
ꢃ96.43
ꢃ2.57
2.40
16.29
30.26
10.37
23.69
49.22
30.58
69.87
85.59
194.06
ꢃ0.42
0.18
0.00
0.38
ꢃ4.85
ꢃ3.89
ꢃ1.28
ꢃ3.86
193.60
193.85
86.30
0.38
ꢃ2.16
ꢃ76.69
65.81
ꢃ2.59
158.76
159.05
bmax
231.78
cation, which shows a good agreement with the measured
wavelength of maximum absorption as discussed above.
The melting temperature of the chromophores was determined
by the differential scanning calorimetry (DSC). The BP crystals
having more methyl groups exhibit a highꢁer melting temperature:
ꢁ
210 C for BP1 with benzyl group, 225 C for BP2 with meth-
ylbenzyl group, and 250 ꢁC for BP3 with dimethylbenzyl group.
To investigate the macroscopic non-resonant optical nonline-
arity we performed the Kurtz and Perry second harmonic
generation (SHG) powder test at the fundamental wavelength of
1.9 mm.16 The second harmonic generation signals of BP crys-
talline powders were compared to that of DAST. The BP3
crystals exhibited a large SHG signal of about 54% as that of
DAST or 540 times as that of urea, while for BP1 and BP2
crystals the SHG signal was absent. From this result we may
expect a noncentrosymmetric crystal structure for BP3 crystals
with well-aligned chromophores, leading to a large macroscopic
nonlinearity. BP3 crystals are therefore very attractive materials
for second-order nonlinear optical applications.
Fig. 4 (a) Molecular structure of the acentric cation core of the BP3
chromophore as determined by the X-ray crystallography. (b) Crystal
packing diagram projected along the a-axis for the monoclinic BP3
crystals. The polar 2-fold axis is along the b crystallographic axis.
446 | CrystEngComm, 2011, 13, 444–451
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explain the observed powder SHG test efficiency of BP3 crystals,
which is more than five times higher compared to the expected
one (i.e. 54% of DAST). One possible reason is a relatively bulky
and bent-shaped group at the end of the chromophore, which
considerably reduces the number density of the BP3 chromo-
phores in the crystalline state, by more than 20% compared to
DAST. Due to the decreased inter-chromophore interactions in
the crystalline system by the site-isolation effect,11a this may be
beneficial for enhancing (or not reducing) the hyperpolarizability
of the chromophores b0 in the solid state. Indeed, for DAST it
was shown that the hyperpolarizabilities in the solid state are
reduced by more than 80% compared to solution because of
intermolecular interactions.10 We can therefore expect even much
higher macroscopic second-order nonlinearities if BP3 chromo-
phores can be packed in a more optimal way by using for
example other counter anions.
Fig. 5 Supramolecular interactions of BP3 crystals: hydrogen-bond
˚
˚
interactions with a O/H–C distance of below 2.5 A and 2.6–2.8 A are
illustrated by thick and thin dotted lines, respectively.
direction (along bmax in Fig. 3) in the non-rod-shaped N-benzyl
BP3 is still along the long axis of the stilbazolium core like it is in
DAST derivatives.
Environmental stability and bulk crystal growth
Environmental stability of water-soluble stilbazolium derivatives
studied previously exhibits some limits for practical applications.
In order to investigate the humidity resistance, the solubility of
BP3 crystals in water was measured. BP3 crystals exhibit 9 times
The main charge transfer direction of the chromophores
makes an angle of about q ¼ 79ꢁ with respect to the polar axis
b of the BP3 crystals. This means that the order parameter cos3 q
of BP3 is considerably reduced compared to DAST with q ¼ 20ꢁ.
We can estimate the relative powder SHG test efficiency of both
crystals by comparing the theoretical figure of merit N2h(beff)2i,
where N is the number density of the chromophores and the
squared effective hyperpolarizability b, averaged over all possible
ꢁ
lower solubility (0.022 mmol per 100 g water) in water at 30 C
than DAST crystals (0.20 mmol per 100 g water). The reduced
solubility in water is attributed to introducing the non-polar
N-benzyl groups with two methyl substituents.
Many examples of stilbazolium derivatives form a hydrated
phase with water.9 DAST crystals form a hydrated phase with
a centrosymmetric structure.9a In order to examine the tendency
of forming hydrated phases, BP3 and DAST crystals were crys-
tallized in water-containing solvents. BP3 and DAST crystals
were recrystallized by rapid cooling method in methanol/water
(1 : 1 and 1 : 2 v/v) and subsequently powder X-ray diffraction
and differential scanning calorimetry were measured. In all
experiments, BP3 crystals did not form any hydrated phase or
other polymorphs showing a practically identical X-ray diffrac-
tion pattern (see Fig. 6) and thermodiagram (not shown), while
DAST crystals formed the centrosymmetric hydrated phase in
the presence of water. This high environmental stability with low
water-solubility without hydrated polymorphs is an advantage
orientations.16 We calculated the effective first-hyper-
eff
polarizability components b in the crystal by considering the
ijk
orientation of the chromophores in the solid state; the results are
listed in the Table 2. For DAST, because of the high order
eff
ijk
eff
parameter, the maximum component of b is diagonal, b
111
¼
¼
161 ꢂ 10ꢃ30 esu, the largest off-diagonal component is b
eff
221
21 ꢂ 10ꢃ30 esu, while all other components are considerably
smaller. On the other hand, for BP3 the off-diagonal component
is the largest b ¼ 45 ꢂ 10ꢃ30 esu, while the diagonal component
eff
112
eff
222
is very small b z 1 ꢂ 10ꢃ30 esu. We can use these results to
estimate the average as described in ref. 16. Assuming the
calculated first-hyperpolarizability values in the gas phase and
their projections in the crystal structure for BP3 and DAST, the
parameter N2h(beff)2i for BP3 is only about 10% of the one for
DAST, N2h(beff)2i y 2000 ꢂ 10ꢃ60 esu2 mꢃ3 for BP3 and
N2h(beff)2i y 19 000 ꢂ 10ꢃ60 esu2 mꢃ3 for DAST. This cannot
Table
2 The components of the non-resonant effective hyper-
polarizability tensor (ꢂ10ꢃ30 esu) in the Cartesian system (x1,x2,x3)
rotated by an angle of 1ꢁ and 26ꢁ for DAST and BP3, respectively, from
the crystallographic a-axis toward the c-axis around the symmetry x2 ¼
b-axis, considering the X-ray crystal structure and the bijk components
calculated for the EXP molecular structures (see Table 1)
eff
b111
eff
b221
eff
b331
eff
b113
eff
b223
eff
b333
DAST
BP3
161
21
ꢃ3.2
eff
0
1.0
ꢃ0.3
eff
eff
eff
b123
b112
b222
b332
Fig. 6 Powder X-ray diffraction patterns of BP3 crystals crystallized in
45
0.8
0.5
0
various solvents including those containing water.
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CrystEngComm, 2011, 13, 444–451 | 447

structure as those grown by the rapid cooling method in the
powder XRD experiments.
Examples of the as-grown bulk BP3 crystals grown by the slow
cooling method and the slow evaporation method are shown in
Fig. 8a and b. The growth of good quality crystals with side
lengths of up to 4 mm is relatively easy even without optimizing
the temperature gradients or evaporation rates, in contrast e.g. to
other stilbazolium derivatives such as N-aryl stilbazolium
derivatives,17 and relatively large size crystals can be grown by
both slow cooling and slow evaporation.
The X-ray diffraction shows that the large surface of the
crystals is the (001) face. As-grown BP3 crystals exhibit a good
optical quality, as well as nice and flat surfaces. Fig. 8c shows the
transmission spectra with polarized light of an un-polished as-
grown BP3 crystal for light polarization parallel and perpen-
dicular to the polar b-axis as presented in Fig. 8b. The crystal is
transparent between 800 nm and 2100 nm with a small absorp-
tion band at about 1700 nm, which is most probably due to the
overtones of the C–H stretching vibrations. We obtained a rela-
tively good transmission in the transparent region. This shows
that the quality and parallelism of the crystal surfaces are quite
good even without polishing or cutting these as-grown samples.
Fig. 7 Solubility as a function of temperature for BP3 in methanol.
for some practical applications and also for applying various
crystal growth techniques.
While having an extremely low solubility in water, BP3 crystals
exhibit a solubility in methanol high enough for bulk crystal
growth. Fig. 7 shows the solubility in methanol for BP3 crystals,
which is in the same order of magnitude as for DAST crystals
(2.9 g BP3 per 100 g methanol compared to 3.7 g DAST/100 g
methanol at 40 ꢁC). Since the hydrated phase does not form, we
can grow BP3 crystals not only by the slow cooling method, but
also by the slow evaporation method. Bulk DAST crystals
cannot be easily grown by the slow evaporation method because
of the easy formation of the hydrated phase. BP3 crystals grown
by the slow evaporation method exhibit the identical crystal
THz generation
We demonstrate THz pulse generation in an as-grown unpol-
ished 560 mm thick BP3 crystal using a 1 kHz repetition rate
Ti:Sapphire regenerative amplifier (Coherent Inc., Legend) as the
pump source, delivering 180 fs pulses at 836 nm. The output
beam from the amplifier was divided into the pump and the
probe beam for the generation and the detection of THz pulses,
respectively.18 Single pulse energies of up to 380 mJ were used for
Fig. 8 BP3 bulk crystals grown by (a) the slow cooling method with the
cooling rate of 1 ꢁC per day from 40 ꢁC and (b) the slow evaporation
method at 40 ꢁC. The large surface is the (001) face. (c) Transmission
spectra of an unpolished 560 mm thick BP3 crystals (b) grown by the slow
evaporation method. The light was incident normal to the (001) as-grown
face with polarization parallel or perpendicular to the polar b-axis.
Fig. 9 THz pulse generated in a 560 mm thick BP3 crystal using 180 fs
pump pulses with the pulse energy of 380 mJ and repetition rate of 1 kHz
at 836 nm. (a) Time trace of the generated THz electric field and (b) the
corresponding spectrum.
448 | CrystEngComm, 2011, 13, 444–451
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the THz pulse generation by optical rectification. The pump
data were recorded on a Varian 400 MHz. All chemical shifts are
reported in ppm (d) relative to (CH3)4Si. The products were
prepared by the following synthetic routes and dried during 2
hours at 60 ꢁC in a vacuum oven prior to characterization.
polarization was close to the a-axis for the effective use of the
largest off-diagonal nonlinear optical susceptibility component
(2)
c . After the generation of the THz waves in the BP3 crystal, the
211
pump pulses were filtered out by using a Si wafer. The generated
THz pulses were subsequently collimated and focused into the
ZnTe detection crystal by using a telescope and off-axis parabolic
mirrors. The electric field of the generated THz pulses was
measured in a 1 mm thick h110i ZnTe crystal by electro-optic
sampling technique, where the detection crystal was rotated
around the propagation direction to obtain the maximum THz
signal. All the measurements were performed at room tempera-
ture.
Synthesis of BPnCl. For BP3Cl, 2,5-dimethylbenzyl chloride
(0.1556 mol, 15.25 ml) and 4-picoline (0.1556 mol, 23 ml) were
dissolved in acetonitirile (150 ml) and the mixture was stirred for
2 days at 70 ꢁC. The resulting mixture was cooled to room
temperature. After cooling, the precipitate was filtered off and
then washed with diethyl ether. White-pink solid product was
obtained and dried in vacuum oven to remove solvent. The solid
product (0.137 mol, 34 g) and 4-dimethyl aminobenzaldehyde
(0.137 mol, 20.5 g) were dissolved in methanol (300 ml). The
solution was completely dissolved with catalyst piperidine
(0.0274 mol, 2.7 ml). The resulting solution was stirred under
Fig. 9a shows the time trace of the THz electric field generated
in the BP3 crystal. Quasi-single cycle and sub-ps THz pulses were
successfully generated with 836 nm pumping. This wavelength
corresponds to the operation wavelength of widely spread
femtosecond laser amplifiers. The spectrum of the generated THz
pulses is shown in Fig. 9b. The center frequency of the spectrum
was 0.84 THz. Note that the two stronger absorption bands
appeared at around 0.6 and 1.2 THz, which are attributed
partially to ambient water vapor as well as optical phonon
resonance in BP3 material, similar as observed in DAST at 1.1
THz.
ꢁ
reflux for 24 h at 80 C and cooled to room temperature. After
adding ethanol and diethyl ether, black purple precipitate was
filtered off and washed with diethyl ether. For BP1 and BP2, an
analogous procedure was employed by using the corresponding
starting materials, benzyl chloride for BP1Cl and 4-methylbenzyl
chloride for BP2Cl.
N,N-Dimethylamino-N0-benzyl-stilbazolium chloride (BP1Cl).
Yield ¼ 63%, 1H-NMR (400 MHz, CD3OD): d ¼ 8.61 (d, 2H, J ¼
7.2 Hz, C5H4N), 7.96 (d, 2H, J ¼ 6.8 Hz, C5H4N), 7.84 (d, 1H,
J ¼ 16.0 Hz, CH), 7.60 (d, 2H, J ¼ 8.8 Hz, C6H4), 7.45 (m, 5H,
C6H5), 7.08 (d, 1H, J ¼ 15.6 Hz, CH), 6.78 (d, 2H, J ¼ 8.8 Hz,
C6H4), 5.62 (s, 2H, CH2), 3.07 (s, 6H, NMe).
Conclusions
We have developed a new cation core structure, N-benzyl stil-
bazolium nonlinear optical chromophore with a non-polar
benzyl group to achieve acentric molecular ordering in the
crystalline state. New N-benzyl stilbazolium crystal, BP3 (N,N-
N,N-Dimethylamino-N0-4-methylbenzyl-stilbazolium chloride
(BP2Cl). Yield ¼ 51%, 1H-NMR (400 MHz, CD3OD): d ¼ 8.59
(d, 2H, J ¼ 6.8 Hz, C5H4N), 7.95 (d, 2H, J ¼ 7.2 Hz, C5H4N),
7.82 (d, 1H, J ¼ 16.4 Hz, CH), 7.59 (d, 2H, J ¼ 8.8 Hz, C6H4),
7.34 (d, 2H, J ¼ 8.0 Hz, C6H4CH3), 7.27 (d, 2H, J ¼ 7.6 Hz,
C6H4CH3), 7.07 (d, 1H, J ¼ 15.6 Hz, CH), 6.77 (d, 2H, J ¼ 9.2
Hz, C6H4), 5.56 (s, 2H, CH2), 3.07 (s, 6H, NMe), 2.36 (s, 3H,
Me).
dimethylamino-N0-benzylstilbazolium
p-toluenesulfonate),
exhibits an acentric crystal structure with monoclinic P21 phase
with a large macroscopic optical nonlinearity of 540 times larger
second harmonic generation efficiency than in urea at a non-
resonant wavelength of 1.9 mm. Compared to the conventional
rod-shaped N-alkyl stilbazolium, an enhanced hyper-
polarizability of the chromophores b0 in the solid state can be
utilized in bent-shaped N-benzyl stilbazolium crystals. This is
attributed to the decreased inter-chromophore interactions due
to the larger chromophore–chromophore separation (i.e., site-
isolation effect) induced by the bulky and bent-shaped N-benzyl
group. Moreover, by introducing the non-polar dimethylbenzyl
group, BP3 crystals show a high environmental stability: they
possess 9 times smaller solubility in water than conventional
stilbazolium crystals and at the same time do not form a hydrated
centrosymmetric phase even if crystallized in water-containing
solvents. We have successfully demonstrated THz generation
with as-grown BP3 crystals without performing any polishing
and cutting procedures by optical rectification using 180 fs pulses
at the pump wavelength of 836 nm. Therefore, new cation core
structure, N-benzyl stilbazolium crystals are very promising
materials for second-order nonlinear optical applications.
N,N-Dimethylamino-N0-2,5-dimethylbenzyl-stilbazolium chlo-
ride (BP3Cl). Yield ¼ 43%, 1H-NMR (400 MHz, CD3OD): d ¼
8.48 (d, 2H, J ¼ 6.8 Hz, C5H4N), 7.95 (d, 2H, J ¼ 6.8 Hz,
C5H4N), 7.84 (d, 1H, J ¼ 16.4 Hz, CH), 7.60 (d, 2H, J ¼ 9.2 Hz,
C6H4), 7.19 (s, 2H, C6H3Me2), 7.10 (s, 1H, C6H3Me2), 7.08 (d,
1H, J ¼ 16.4 Hz, CH), 6.78 (d, 2H, J ¼ 8.8, C6H4), 5.63 (s, 2H,
CH2), 3.07 (s, 6H, NMe), 2.34 (s, 3H, Me), 2.27 (s, 3H, Me).
Synthesis of chromophore BPn. For example of BP3, BP3Cl
(2.64 mmol, 1 g) was completely dissolved in boiling water (100
ml). The p-toluenesulfonic acid sodium salt (7.92 mmol, 1.54 g)
was dissolved in water (10 ml) and dropwise added into the
solution of BP3Cl. The solution was cooled to room temperature
and precipitate is filtered off and washed with water. The filtrate
ꢁ
was dried for 1 h at 100 C in vacuum oven. The final product
Experimental
was obtained by recrystallization in methanol.
Synthesis
N,N-Dimethylamino-N0-benzyl-stilbazolium p-toluenesulfonate
(BP1). Yield ¼ 53%, 1H-NMR (400 MHz, CD3OD): d ¼ 8.60 (d,
2H, J ¼ 7.2 Hz, C5H4N), 7.95 (d, 2H, J ¼ 6.8 Hz, C5H4N), 7.83
All chemicals were purchased from commercial suppliers, mainly
1
from Aldrich, and used without further purification. H-NMR
This journal is ª The Royal Society of Chemistry 2011
CrystEngComm, 2011, 13, 444–451 | 449

(d, 1H, J ¼ 16.4 Hz, CH), 7.69 (d, 2H, J ¼ 8.0, C6H4SO3ꢃ), 7.59
(d, 2H, J ¼ 8.8 Hz, C6H4), 7.45 (m, 5H, C6H5), 7.21 (d, 2H, J ¼
8.4 Hz, C6H4), 7.07 (d, 1H, J ¼ 16.0 Hz, CH), 6.77 (d, 2H, J ¼ 8.8
Hz, C6H4), 5.61 (s, 2H, CH2), 3.07 (s, 6H, NMe), 2.36 (s, 3H,
Me). Elemental analysis for C29H30N2O3S: (%) calcd C 71.58, H
6.21, N 5.76, S 6.59; found: C 71.50, H 6.45, N 5.48, S 6.06%.
Acknowledgements
This work has been supported by the National Research Foun-
dation of Korea Grant (NRF) funded by the Korea government
(MEST) (No. 2009-0084918), Priority Research Centers
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Education, Science and
Technology (2010-0028294), and by the Swiss National Science
Foundation.
N,N-Dimethylamino-N0-4-methylbenzyl-stilbazolium p-tolue-
nesulfonate (BP2). Yield ¼ 77%, 1H-NMR (400 MHz, CD3OD):
d ¼ 8.56 (d, 2H, J ¼ 6.8 Hz, C5H4N), 7.94 (d, 2H, J ¼ 7.2 Hz,
C5H4N), 7.82 (d, 1H, J ¼ 16.0 Hz, CH), 7.69 (d, 2H, J ¼ 8.0,
C6H4SO3ꢃ), 7.59 (d, 2H, J ¼ 9.2 Hz, C6H4), 7.34 (d, 2H, J ¼ 8.0,
C6H4CH3), 7.27 (d, 2H, J ¼ 8.0 Hz, C6H4CH3), 7.21 (d, 2H, J ¼
8.4 Hz, C6H4SO3ꢃ), 7.06 (d, 1H, J ¼ 16.0 Hz, CH), 6.77 (d, 2H,
J ¼ 8.8, C6H4), 5.55 (s, 2H, CH2), 3.07 (s, 6H, NMe), 2.36 (s, 6H,
Me). Elemental analysis for C30H32N2O3S: (%) calcd C 71.97, H
6.44, N 5.60, S 6.40; found: C 71.94, H 6.43, N 5.56, S 6.43%.
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