4-Nitrophenylhydrazone crystals with large quadratic nonlinear optical response by optimal molecular packing

Article abstract of DOI:10.1021/cg200320f

We investigate a novel nonlinear optical nitrophenylhydrazone (NPH) crystal, 3,4,5-trimethoxybenzaldehyde-4-nitrophenylhydrazone (TMO-NPH). Compared with many acentric NPH crystals having rod-shaped heads reported previously, the TMO-NPH molecule maintain

Full text of DOI:10.1021/cg200320f

ARTICLE
pubs.acs.org/crystal
4-Nitrophenylhydrazone Crystals with Large Quadratic Nonlinear
Optical Response by Optimal Molecular Packing
Eun-Young Choi,‡,† Pil-Joo Kim,‡ Mojca Jazbinsek,§ Jong-Taek Kim,^ Yoon Sup Lee,^
Peter G€unter,§ Soon W. Lee,|| and O-Pil Kwon*,‡
‡Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Korea
§Nonlinear Optics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland
^Department of Chemistry and School of Molecular Science (BK 21), Korea Advanced Institute of Science and Technology
(KAIST), Daejeon 305-701, Korea
Department of Chemistry (BK21), Sungkyunkwan University, Natural Science Campus, Suwon 440-746, Korea
ABSTRACT: We investigate a novel nonlinear optical nitrophenylhydrazone
(NPH) crystal, 3,4,5-trimethoxybenzaldehyde-4-nitrophenylhydrazone (TMO-
NPH). Compared with many acentric NPH crystals having rod-shaped heads
reported previously, the TMO-NPH molecule maintains the acentric intermo-
lecular hydrogen bond synton through hydrazone and nitro groups but has
distinct space filling characteristics due to a different shape of the headgroup, the
disk-shaped trimethoxybenzaldehyde. We investigated the crystal structure and
its characteristics, as well as details of the microscopic and the macroscopic
nonlinear optical properties by quantum chemical calculations using density
functional theory (DFT) and powder second-harmonic generation at nonreso-
nant conditions. For comparison, DA-NPH (4-dimethylaminobenzaldehyde-4-
nitrophenylhydrazone) having a rod-shaped head and known nonlinear optical
properties is also investigated by the same analysis. TMO-NPH crystals have the
monoclinic space group symmetry Pn and exhibit a large macroscopic optical nonlinearity due to a favorable molecular packing with
a very high order parameter cos3(θp) = 0.90, while DA-NPH exhibits a lower order parameter cos3(θp) = 0.33. The diagonal
effective hyperpolarizability tensor component of TMO-NPH crystals is 30% higher than the largest tensor component of DA-NPH,
even though the microscopic molecular susceptibility of TMO-NPH molecule is only about half of the one of DANPH.
applications that require a high diagonal nonlinear susceptibility
element, χ(ii2i ), such as most often used in electro-optics1 and
terahertz wave generation.9 For example, DA-NPH (4-dimethy-
laminobenzaldehyde-4-nitrophenylhydrazone, see Figure 1a)
crystals exhibit a very large nonresonant diagonal nonlinear
optical susceptibility element, χ(121)1(ꢀ2ω,ω,ω) = 280 pm/V,
but an even larger off-diagonal element, χ(222)1(ꢀ2ω,ω,ω) = 320
pm/V measured at 1.9 μm fundamental wavelength.4,10 This is
because DA-NPH molecules are not aligned perfectly parallel, as
shown in Figure 2.4,10 The reason for such an alignment is,
besides hydrogen-bonding interactions, related to the shape of
DA-NPH molecules. During crystallization, molecules with a
rod-shaped head (dimethylaminobenzaldehyde, see Figure 1a)
adopt a bent shape to fill the space with a relatively large angle of
more than 90ꢀ between the neighboring molecules. A parallel
1. INTRODUCTION
Acentric organic crystals1,2 are useful materials for quadratic
nonlinear optical applications such as frequency conversion,
terahertz-wave generation, and ultrafast electro-optics.3 A challenging
topic for organic crystals exhibiting a large macroscopic nonlinear
optical susceptibility is the molecular design forming noncen-
trosymmetric molecular packing with a high order parameter in
the crystalline state.2 In organic crystals, molecular packing is
mainly influenced by the following two factors: intermolecular
interactions and space filling characteristics, that is, functional
groups and the shape/size of the constituent molecules. The
widely investigated nitrophenylhydrazone (NPH) derivatives
exhibit an especially high tendency for forming a noncentrosym-
metric molecular order in the crystalline state due to hydrazone
and nitro functional groups and non-rod bent shape.4ꢀ8 Main
intermolecular interactions of many acentric NPH crystals4ꢀ8
present strong hydrogen bonds between the hydrazone and the
alignment of DA-NPH molecules would potentially increase the
(2)
111
diagonal susceptibility element χ by a factor of 3, which would
be beyond the best value measured to date in organic crystals.11
nitro group (ꢀNdNꢀH OꢀNꢀ) with a short distance of
3 3 3
about 2.3 Å, which most often leads to a λ-shaped packing.5
Although many acentric NPH crystals exhibit a relatively large
macroscopic nonlinearity, the molecular ordering in the crystal-
line state is not optimal for second-order nonlinear optical
Received: March 15, 2011
Revised:
May 3, 2011
Published: May 03, 2011
r
2011 American Chemical Society
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Figure 1. The chemical structure of NPH derivatives having a rod-
shaped head (DA-NPH and DH-NPH) (a) and having a disk-shaped
head (TMO-NPH) (b).
It is therefore interesting to investigate the possibilities for
optimal packing with NPH derivative molecules.
In this work, we investigate a different nitrophenylhydrazone
crystal, TMO-NPH (3,4,5-trimethoxybenzaldehyde-4-nitrophenyl-
hydrazone), and relate its properties to previously investigated
NPH crystals. The TMO-NPH molecule was chosen since it
maintains the main supramolecular interactions through hydra-
zone and nitro groups but introduces a different shape of the
head by using a disk-shaped trimethoxybenzaldehyde (see
Figure 1b). Although the synthesis and a rough estimation of
the macroscopic resonant nonlinearity at a wavelength of 1.3 μm
have been previously reported,12 the crystal structure and its
characteristics, as well as a more detailed evaluation of the
microscopic and the macroscopic nonlinear optical properties
at nonresonant conditions have not been investigated yet. We
report on the theoretical analysis by quantum chemical calcula-
tions using density functional theory (DFT) and powder second-
harmonic generation at the nonresonant wavelength of 1.9 μm to
evaluate the microscopic and the macroscopic optical nonlinea-
rities of TMO-NPH crystals. For comparison, DA-NPH with a
rod-shaped head and known nonlinear optical properties is also
investigated by the same analysis. TMO-NPH crystals have the
monoclinic space group symmetry Pn and exhibit a large macro-
scopic optical nonlinearity with optimal molecular packing for
electro-optics and terahertz-wave generation.
Figure 2. The λ-shaped molecular packing of the DA-NPH crystal. The
main hydrogen bonds between the NꢀH OꢀN groups are illu-
3 3 3
strated with the dotted lines. The solid and the dotted vectors present
the direction of the maximum first hyperpolarizability, βmax, as deter-
mined by finite-field calculations and the polar axis of the crystal,
respectively. The angle between the polar axis of the crystal and the
hyperpolarizabilities βmax of the molecules is θp ≈ 46.6ꢀ.
8.09 (d, 2H, J = 9.2, C6H4), 7.93 (s, 1H, NH), 7.54 (d, 2H, J = 8.8,
C6H4), 7.09 (d, 2H, J = 8.0, C6H4), 6.74 (d, 2H, J = 8.8, C6H4), 2.96 (s,
6H, N(CH3)2). Elemental analysis for C15H16N4O2: (%) calcd C 63.37,
H 5.67, N 19.71, O 11.25; found C 63.31, H 5.64, N 19.72.
2.2. Single-Crystal Structure. 2.2.1. TMO-NPH. C16H17N3O5,
Mr = 331.33, monoclinic space group Pn (point group m), a = 4.0325(7)
Å, b = 12.6155(17) Å, c = 15.365(2) Å, β = 97.535(9)ꢀ, V = 774.9(2) Å3,
Z = 2, μ = 0.107 mmꢀ1, Dc=1.420 mg/m3, crystal size 0.740 ꢁ 0.540 ꢁ
0.360 mm3, F(000) = 348. Of the 9761 reflections collected in the Θ
range of 2.10ꢀ to 28.32ꢀ on a Bruker CCD diffractiometer, 3317 were
unique reflections (Rint = 0.1805, completeness = 95%). The structure
was solved by the direct methods using SHELXTL package of crystal-
lographic software13 and refined by the full-matrix least-squares techni-
2. EXPERIMENTAL SECTION
que on F2, 285 variables; the final R1= 0.0670, wR2 = 0.1799 for Fo
>
2
2
2σ(Fo ) and R1 = 0.0799, wR2 = 0.1976 for all data. Goodness-of-fit on
2.1. Materials. The NPH materials were prepared according to
literature,4,12 and the chemical identification and the purity was deter-
mined by 1H NMR and elemental analysis. 1H NMR data were recorded
on a Varian 400 MHz. All chemical shifts are reported in ppm (δ) relative
to (CH3)4Si.
2.1.1. 3,4,5-Trimethoxybenzaldehyde-4-nitrophenylhydrazone (TMO-
NPH). 3,4,5-Trimethoxy-benzaldehyde (10 g contains >30% water) and
4-nitrophenylhydrazine (10.06 g, 0.05 mol) was dissolved in methanol
(300 mL). Acetic acid (20 drops) for catalyst was added dropwise into the
solution under N2 conditions. The solution was heated at 70 ꢀC for 2 days.
After the solution was cooled to room temperature, orange-brown powder
was filtered and washed with methanol. The products were recrystallized
in acetone. Yield: 72%. 1H NMR (400 MHz, DMSO-d6, δ): 11.40 (s, 1H,
CH), 8.23 (d, 2H, J = 8.8, C6H4), 8.07 (s, 1H, NH), 7.29 (d, 2H, J = 8.0,
C6H4), 7.15 (s, 2H, C6H2), 3.96 (s, 6H, OMe), 3.81 (s, 3H, OMe).
Elemental analysis for C16H17N3O5: (%) calcd C 58.00, H 12.68, N 5.17;
found C 58.08, H 12.69, N 5.16.
2
F2 = {∑[w(Fo ꢀ Fc2)2]/(n ꢀ p)}1/2 = 1.082, where n is the number of
reflections and p is the total number of parameters refined. All
nonhydrogen atoms were refined anisotropically, and the hydrogen
atoms were included in the final stages of the refinements on calculated
positions bonded to their carrier atoms. The maximum/minimum
residual electron density is 0.198/ꢀ0.231 e Åꢀ3. Further details of
3
the crystal structure investigations may be obtained from the Cambridge
Crystallographic Data Centre [CCDC, 12 Union Road, Cambridge
CB12 1 EZ (U.K.)] on quoting the depository number CCDC 781885.
2.2.2. DA-NPH (Phase II). C15H16N4O2, Mr = 284.3, monoclinic, space
group Cc (point group m), a = 6.281(2) Å, b = 28.133(8) Å, c = 8.380(2)
Å, β = 97.87(2)ꢀ, V = 1466.9(4) Å3, Z = 4.4
2.3. Computational Details. All quantum chemical calculations
were performed using the Gaussian 03 program14 with the B3LYP
hybrid functional15 and the 6-311þG(d) basis set. The zero-frequency
hyperpolarizabilities, βmnp, were calculated by the finite field (FF)
method as in ref 8 for the optimized molecular structures (OPT) as
well as for the experimental structures (EXP) determined by the X-ray
2.1.2. 4-Dimethylaminobenzaldehyde-4-nitrophenylhydrazone
1
(DA-NPH). H NMR (400 MHz, DMSO-d6, δ): 11.05 (s, 1H, CH),
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single- crystal analysis. The molecular coordinate system xyz is here
defined so that the molecular plane is along the yz plane and the direction
of the ground-state dipole moment, μg, is parallel to the z-direction
(μg = ꢀμz). The βmnp tensor components were calculated in this system
and subsequently transformed to obtain the value and the direction of the
maximal hyperpolarizability, βmax (i.e., the hyperpolarizability component
parameter, cos3(θp) = 0.33, which is still far from the optimal
molecular packing with cos3(θp) = 1 for optimizing the diagonal
susceptibility element along the polar axis of the crystal.
A different hydrazone derivative, DH-NPH, was developed to
improve the molecular packing of NPH derivatives for quadratic
nonlinear optics (see Figure 1a).12 For DH-NPH, the space-
filling characteristics of the constituting molecules are similar to
those of DA-NPH, but different functional groups alter the main
intermolecular interactions common for DA-NPH and many
other NPH derivatives. DH-NPH molecules have a rod-shaped
head like DA-NPH, but the strong hydrogen bond donor site OH
group at the opposite end of the nitro-group hydrogen bond
acceptor site leads to strong head-to-tail hydrogen bonds in the
crystal, resulting in a favorable acentric molecular packing with
high order parameter, cos3(θp) = 0.80.17
For achieving an improved molecular packing, we choose here
another approach: we keep the main intermolecular interactions
of DA-NPH and many other NPH derivatives and modify the
space-filling characteristics using a different headgroup, disk-
shaped trimethoxybenzaldehyde (see Figure 1b). The investi-
gated TMO-NPH materials were prepared according to ref 12.
For comparison, we also prepared DA-NPH crystals, whose
optical and nonlinear optical properties were previously fully
characterized.4,10 The high purity of investigated materials was
confirmed by 1H NMR and elemental analysis. DA-NPH (phase II)
noncentrosymmetric monoclinic crystals (see Figure 2) having a
red-greenish prism morphology were obtained by the rapid cooling
method in acetonitrile solution.
along the main charge-transfer direction of the molecule). The effective
eff
ijk
hyperpolarizabilities, β in the crystalline state were calculated by
considering all projections of the hyperpolarizability tensor elements βmnp
ofallmoleculesin thecrystallographicsystem bythe so-calledoriented-gas
model16
nðgÞ
3
1
eff
ijk
β
¼
cosðθs Þcosðθs Þcosðθs Þβ
ð1Þ
nðgÞ ∑ ∑
mnp
im
jn
kp
s
mnp
where n(g) is the number of equivalent positions in the unit cell, s denotes
a site in the unit cell, and θsim is the angle between the Cartesian axis i and
the molecular axis m. In this model, the macroscopic susceptibility tensor
eff
elements, χ(ij2k), are related to the effective hyperpolarizabilities, β , as
ijk
ð2Þ
χ
¼ NFijkβeijfkf
ð2Þ
ijk
where N is the number density of the molecules and Fijk is the correction
factor due to intermolecular interactions including the local-field
corrections.
3. RESULTS AND DISCUSSION
3.1. Characteristics of NPH Derivatives. The chemical
structures of the investigated NPH derivatives are shown in
Figure 1. DA-NPH and DH-NPH (dihydroxybenzaldehyde-4-
nitrophenylhydrazone) have a rod-shaped head, dimethylami-
nobenzaldehyde and dihydroxybenzaldehyde, respectively, while
TMO-NPH has a disk-shaped head, trimethoxybenzaldehyde.
DA-NPH crystallizes in different polymorphic forms: form I
with noncentrosymmetric, nonpolar space group P212121, form
II with noncentrosymmetric, polar space group Cc, and form III
with centrosymmetric space group P21/c.4b In this work, we only
consider the polymorph with substantial macroscopic second-
order nonlinearity, which is the phase II polymorph crystallizing
in the polar space group Cc.4 In DA-NPH crystals (phase II), the
hydrazone group, which acts as hydrogen bond donor site, is at
the center of the molecule and the nitro group, which acts as
hydrogen bond acceptor, is at the end of the molecule, as is most
common for NPH derivatives.4ꢀ8 Therefore, due to the position
of the hydrogen bond donor and the acceptor sites, combined
with the specific shape of DA-NPH molecules, they form a λ-
shaped packing with a relatively large angle (>90ꢀ) between the
neighboring molecules (see Figure 2), rather than head-to-tail
hydrogen bonds.4,5 In order to evaluate the relation between the
molecular packing in the crystalline system and the macroscopic
nonlinearity of DA-NPH crystals, we calculated the zero-
frequency hyperpolarizability tensor, βijk, of the DA-NPH (EXP)
molecule by quantum chemical calculation with the finite-field
density functional theory (FF-DFT) method, as well as the
amplitude and the direction of the maximal first-order hyperpo-
larizability, βmax, of the molecule in the crystalline system. In
Figure 2, the dotted and the solid vectors present the directions
of the polar axis of the crystal and the maximum first hyperpolar-
izability, βmax, of the molecules as determined by finite-field
calculations. The angle between the polar axis of the crystal and
the hyperpolarizabilities βmax of DA-NPH molecules is relatively
large, θp = 46.6ꢀ, which leads to a relatively small order
The chemical and physical characteristic data including the
melting temperature, Tm, decomposition temperature, Td, the
maximal first-order hyperpolarizability, βmax, the order para-
eff
meter cos3(θp), and the effective hyperpolarizability tensor, β ,
ijk
are listed in Table 1. In different solvents, the wavelength of
maximum absorption, λmax, of TMO-NPH is similar: 408 nm in
acetone, 403 nm in acetonitrile, and 403 nm in methanol
solution. This means that the trimethoxybenzaldehyde head is
not very sensitive to the environmental conditions, like the
phenolic head of DH-NPH molecules.17 The wavelength of
maximum absorption λmax for TMO-NPH (408 nm) is however
lower than that for DA-NPH (430 nm) in acetone solution.
According to the nonlinearityꢀtransparency trade-off,1 the mo-
lecular nonlinearity of TMO-NPH may be smaller than that of
DA-NPH, which has been confirmed by quantum chemical
calculation. As listed in Table 1, the maximal first-order hyper-
polarizability, βmax, of TMO-NPH (OPT) is about 57% that of
DA-NPH (OPT).
The thermal properties including the melting temperature,
Tm, and the decomposition temperature, Td, were determined by
differential scanning calorimetry (DSC) for a heating rate of
10 ꢀC/min. The Tm and Td were chosen at the peak position and
the initial exothermic position, respectively, in the DSC thermo-
diagram. Figure 3 shows the DSC curves of TMO-NPH and DA-
NPH(II) crystalline powders. Compared with DA-NPH, TMO-
NPH exhibits a higher thermal stability with a higher Td (220 ꢀC
for TMO-NPH and 193 ꢀC for DA-NPH).
3.2. Single-Crystal Structures. In order to choose an appro-
priate solvent for the growth of single crystals, we measured the
solubility of TMO-NPH crystals in various solvents. We kept the
solutions with excessive amounts of crystalline powders for 3
days in an oven at 32 ꢀC and then measured the amount of the
dissolved powder. We determined the following solubilities of
TMO-NPH: 2.6 g/100 g acetone, 1.2 g/100 g acetonitrile, and
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Table 1. Chemical and Physical Properties of NPH Derivatives. a
β
max(OPT)
β
max(EXP)
θp(EXP)
order parameter,
λmax (nm)
Tm (ꢀC)
Td (ꢀC)
(10ꢀ30 esu)
(10ꢀ30 esu)
(deg)
cos3(θp)
βeijffk (10ꢀ30 esu)
TMO-NPH
408
430
203
189
220
193
79.5
72.2
14.8
46.6
0.90
0.33
β1eff11 = 65.2; β2eff21 = 4.6
βe1ff11 = 50.9; β2eff21 = 50.2
DA-NPH (II)
138.6
144.7
a The melting temperature, Tm, and the decomposition temperature, Td, are determined by the peak position and the initial exothermic position,
respectively, in the DSC thermodiagram. The maximal first-order hyperpolarizability, βmax, is determined by quantum chemical calculations with
B3LYP/6-311þG* of optimized (OPT) and experimental (EXP) molecules. The angle θp is the angle between the polar axis of the crystalline system
and the maximal first-order hyperpolarizabilities, βmax (charge-transfer direction), of the molecules. The highest diagonal and the highest off-diagonal
eff
effective hyperpolarizability tensor elements, β ꢁ χ(ij2k), were calculated from the hyperpolarizability tensor components βmnp of all molecules by
ijk
considering their orientation in the crystallographic system. λmax is the wavelength of the maximum absorption in acetone solution.
0.4 g/100 g methanol. We chose acetone solvent for the single-
crystal growth.
For single-crystal structural analysis, TMO-NPH single
crystals were grown by the slow evaporation method at 32 ꢀC in
acetone solution. TMO-NPH crystals obtained have the mono-
clinic Pn space group symmetry (point group symmetry m).
Figure 4 shows the experimental molecular structure (EXP) of
the TMO-NPH molecule in the crystalline state and the crystal-
packing diagram of TMO-NPH crystals. In the crystals, TMO-
NPH (EXP) molecule has a planar conformation as shown in
Figure 4a. However, the planarity of the TMO-NPH molecule is
not perfect: the trimethoxybenzaldehyde head is slightly out of
the plane of the nitrophenylhydrazone with a small dihedral angle
between the two phenyl rings of 13ꢀ. Large dihedral angle between
the two aromatic (or heteroaromatic) rings in NPH derivatives is
often accompanied with a large change of the first hyperpolarizability
Figure 3. DSC curves of TMO-NPH and DA-NPH(II) crystalline
powders showing Tm and Td.
of the molecules with respect to the perfectly planar conformation,
which is usually the case of optimized (OPT) molecules obtained by
quantum chemical calculations.8,17 In finite-field calculations, the
The most important difference in the molecular packing of
TMO-NPH crystals compared with DA-NPH crystals is a much
better parallel alignment of the NPH unit, which is essential for
the nonlinear optical properties. In order to clearly determine the
direction of the charge transfer in the crystalline state, we
calculated the direction of the maximal first-order hyperpolariz-
ability, βmax, of TMO-NPH (EXP) by quantum chemical calcu-
lation, which is illustrated in Figure 5. The dotted and the solid
vectors present the direction of the polar axis of the crystal and
βmax of the molecules, respectively. An acentric polar chain is
oriented along the crystallographic c-axis, which is approxi-
mately (about 6ꢀ off in the ac crystallographic plane) along the
polar axis of TMO-NPH crystals. The angle between the polar
axis of the crystal and βmax of the molecules is θp ≈ 14.8ꢀ.
Therefore, TMO-NPH crystals have an almost optimal mo-
lecular packing with a very high order parameter cos3(θp) =
0.90 for quadratic nonlinear optical applications, which is
considerably larger than that for DA-NPH crystals with a
relatively low order parameter cos3(θp) = 0.33. Therefore,
even though the molecular nonlinearity is only half of that for
DA-NPH crystals, TMO-NPH crystals can be expected to
exhibit a large macroscopic nonlinearity, as discussed in the
following section.
maximal first-order hyperpolarizability, βmax, of TMO-NPH (EXP)
is similar to that of TMO-NPH (OPT) with the planar conforma-
tion, as listed in Table 1, indicating that a small dihedral angle
does not considerably alter the molecular nonlinearity in case of
TMO-NPH.
The main supramolecular interactions between the TMO-
NPH molecules in the crystalline state are strong hydrogen
bonds between the hydrazone and the nitro groups, N(2)-
H
O(2)ꢀN(1) with O H distances of about 2.3 Å and
3 3 3
3 3 3
C(7)-H O(2)ꢀN(1) with O H distances of about 2.4 Å,
3 3 3
3 3 3
and form an acentric λ-shaped packing (i.e., polar acentric layer),
as is characteristic for DA-NPH as well as many other NPH
derivatives4ꢀ8 (see Figure 4b). The polar acentric layers are
stacked one by one with strong hydrogen bonds between the
closest methoxy groups in the upper and the lower layer,
OꢀCH3 OꢀCH3 with O H distances of about 2.4, 2.7,
3 3 3
3 3 3
and 2.8 Å, as shown in Figure 4c.
The number density of TMO-NPH molecules in the crystal-
line state is due to the larger size of the molecule, about 5% lower
than that of DA-NPH. However, TMO-NPH molecules are
packed more closely than DA-NPH molecules without any
residual void in the crystalline state (cell volume 774.9 Å3 and
predicted volume based on van der Waals radii 801.3 Å3 for
TMO-NPH; cell volume 1466.8 Å3 and predicted volume based
on van der Waals radii 1437.2 Å3 for DA-NPH),18 which is due to
the strong hydrogen bonds and the shape of the head. As
commented earlier, molecular packing in the crystalline state
could be changed by two strong factors, the intermolecular
interactions and the space filling characteristics.
3.3. Macroscopic Nonlinearity with Optimal Molecular
Packing. The macroscopic nonlinear optical properties of
NPH crystals have been evaluated theoretically by calculating
eff
(2)
the effective hyperpolarizability tensor, β ꢁ χ in the crystal-
ijk
ijk
lographic system8 and experimentally by the powder second
harmonic generation (SHG) experiment19 at a nonresonant
fundamental wavelength of 1.9 μm.
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Figure 4. (a) Molecular structure of TMO-NPH molecule in the crystalline state, (b) crystal packing diagram of TMO-NPH crystals (the main
supramolecular interactions between the molecules are strong hydrogen bonds of NꢀO(2) HꢀN(2) with O H distances of about 2.3 Å, which
3 3 3
3 3 3
are presented by the thick dotted lines; selected weak hydrogen bonds are presented with the thin dotted lines), and (c) schematic illustration of the
interlayer hydrogen-bond interactions.
Figure 5. Schematic illustration of an acentric polar chain of TMO-NPH crystals projected along the a-axis. The solid and the dotted vectors present the
directions of the maximum first hyperpolarizability, βmax, as determined by finite-field calculations and the polar axis of the crystal, respectively. The angle
between the polar axis of the crystal and the hyperpolarizabilities βmax of the molecules is θp ≈ 14.8ꢀ.
eff
ijk
The effective hyperpolarizability tensor, β , was calculated
components βmnp calculated by the finite-field method of the
according to eq 1 considering all hyperpolarizability tensor
EXP molecules and the orientation of the molecules in the
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crystal.8 The resulting highest diagonal and off-diagonal effective
shaped trimethoxybenzaldehyde. We have investigated and
analyzed the crystal structure and its characteristics, as well as
the details of the microscopic and the macroscopic nonlinear
optical properties by quantum chemical calculations using den-
sity functional theory (DFT) and powder second-harmonic
generation at nonresonant conditions. TMO-NPH crystals have
the monoclinic space group symmetry Pn and exhibit a large
macroscopic optical nonlinearity due to a highly aligned molec-
ular packing with a very high order parameter, cos3(θp) = 0.90,
while DA-NPH crystals exhibit a lower order parameter, cos3-
(θp) = 0.33. The diagonal effective hyperpolarizability tensor
component of TMO-NPH crystals is 30% higher than the highest
component of DA-NPH, even though the molecular suscept-
ibility for TMO-NPH is only about half of that for DA-NPH.
Therefore, TMO-NPH crystals are interesting materials for
second-order nonlinear optical applications, for which a high
diagonal susceptibility element is required, such as electro-optics
and terahertz-wave generation.
eff
hyperpolarizability tensor elements, β , are listed in Table 1.
ijk
The relative values of the largest effective hyperpolarizability
elements for DA-NPH listed in Table 1 are very well correlated
(2)
with the rigorous susceptibility measurements giving χ
-
111
(ꢀ2ω,ω,ω) = 280 pm/V and χ(222)1(ꢀ2ω,ω,ω) = 320 pm/V at
the nonresonant fundamental wavelength of 1.9 μm.10 In the
Cartesian coordinate system for TMO-NPH crystals, the axis 1 is
along the polar axis of the crystal (about 6ꢀ from the crystal-
lographic c axis) and the axis 2 is along the symmetry b axis. All
other effective hyperpolarizability tensor elements of TMO-
NPH crystals are very small and can be neglected. The diagonal
component of TMO-NPH crystals (βe1ff11 = 65.2 ꢁ 10ꢀ30 esu) is
eff
30% higher than the highest element of DA-NPH (β111
=
50.9 ꢁ 10ꢀ30 esu), even though the molecular susceptibility for
TMO-NPH is only about half that of DA-NPH. This is because
of the more aligned packing of TMO-NPH molecules with a very
high order parameter.
The macroscopic nonlinearities have been screened by the
Kurtz and Perry second-harmonic powder test19 using a non-
resonant fundamental wavelength of 1.9 μm. The relative
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: opilkwon@ajou.ac.kr.
second-harmonic generation (SHG) efficiency of TMO-NPH
exp
and DA-NPH crystalline powder is η
= 10% and
TMOꢀNPH
exp
η
= 76%, respectively, relative to the powder of one of
DAꢀNPH
Present Addresses
†Korea Science Academy of KAIST, Busan 614-822, Korea.
the best organic salts, DAST (N,N-dimethylamino-N0-methyl-
stilbazolium p-toluenesulfonate).20 The measured SHG effi-
ciency of DA-NPH powder, despite the predicted lower
diagonal coefficient, is higher because (i) it has a high diagonal
’ ACKNOWLEDGMENT
eff
111
eff
221
β
and a similarly high off-diagonal component β
Table 1) and the off-diagonal components β
(see
This work has been supported by Midcareer Researcher
Program (2010-0027743) and Priority Research Centers Program
(2010-0028294) through the National Research Foundation of
Korea (NRF) funded by the Ministry of Education, Science and
Technology.
eff
221
contribute
additionally to the powder test result and (ii) the off-diagonal
components βe2ff21 contribute about 50% more to the powder test
eff 19
111
result than the diagonal components β
.
We can to some
extent quantify different contributions by considering that the
measured powder SHG efficiency is proportional to the squared
number density of the molecules and the squared effective
hyperpolarizability components, averaged over all possible or-
ientations, that is, N2Æ(βeff)2æ.19 The spatial average can be
calculated from the βeijffk components by taking into account the
point-group symmetry of the particular crystal.19 The values of
N2Æ(βeff)2æ were normalized to the values for DAST calculated
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theor
respect to DAST: η
NPH
= 29% for TMO-NPH and ηDA-
TMOꢀNPH
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molecule compared with DA-NPH.
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We have investigated a novel nonlinear optical nitrophenylhy-
drazone crystal, 3,4,5-trimethoxybenzaldehyde-4-nitrophenylhy-
drazone (TMO-NPH). Compared with previously investigated
acentric DA-NPH crystals having a rod-shaped head, the TMO-
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through hydrazone and nitro groups, but distinct space-filling
characteristics due to a different shape of the head group, disk-
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dx.doi.org/10.1021/cg200320f |Cryst. Growth Des. 2011, 11, 3049–3055

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