The structure and nonlinear optical properties of 3-n-propylamide-4-(4- hexyloxyphenylethynyl)-nitrobenzene

Article abstract of DOI:10.1016/S0022-2860(98)00401-3

We have synthesized the tolane derivative 3-n-propylamide-4-(4- hexyloxyphenylethynyl)-nitrobenzene 1 and examined its second-order nonlinear properties and crystal structure. Compound 1 crystallizes in the space group Clc1 with a = 27.667(6), b = 4.910(2), c = 20.856(3) A, β = 128.36(3)°and Z = 4, and shows a high second-order nonlinear optical efficiency. It has an intermolecular hydrogen bond between C=O and N-H in the amide group. This hydrogen bond plays an important role in forming a non-centrosymmetric structure appropriate for nonlinear optical materials.

Full text of DOI:10.1016/S0022-2860(98)00401-3

Journal of Molecular Structure 471 (1998) 139±143
The structure and nonlinear optical properties of 3-n-propylamide-
4-(4-hexyloxyphenylethynyl)-nitrobenzene
a,
b
a,1
a
*
Midori Kato , Kimiko Kobayashi , Masaaki Okunaka , Nami Sugita ,
Masashi Kiguchia, Yoshio Taniguchia,2
aAdvanced Research Laboratory, Hitachi, Ltd., Hatoyama, Saitama 350-03, Japan
bThe Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako-shi, Saitama 351-01, Japan
Received 23 February 1998; accepted 3 April 1998
Abstract
We have synthesized the tolane derivative 3-n-propylamide-4-(4-hexyloxyphenylethynyl)-nitrobenzene 1 and examined its
second-order nonlinear properties and crystal structure. Compound 1 crystallizes in the space group C1c1 with a ˆ 27.667(6),
Ê
b ˆ 4.910(2), c ˆ 20.856(3) A, b ˆ 128.36(3)8 and Z ˆ 4, and shows a high second-order nonlinear optical ef®ciency. It has
anintermolecularhydrogenbondbetweenCyOandN±Hintheamidegroup.Thishydrogenbondplaysanimportantroleinforming
a non-centrosymmetric structure appropriate for nonlinear optical materials. q 1998 Elsevier Science B.V. All rights reserved
Keywords: Nonlinear optics; Tolan; Hydrogen bond; Second-harmonic generation (SHG)
1. Introduction
vanish. The N±H stretching bond frequency, which is
sensitive to the extent of hydrogen bonding of a group
of tolane derivatives, has been examined, and the rela-
tion found between the frequency and the second-
order nonlinear optical activity [7]. A tolane deriva-
tive, ethyl 2-(4-benzyloxyphenylethynyl)-5-nitro-
benzene-1-carbamate 2 (4-nitro-2-ethoxyamido-4 0-
benzyloxytolane), has been studied and it was found
that one phase of compound 2 has an intermolecular
hydrogen bond and shows second-order nonlinear
optical activity [8, 9]. We have synthesized a new
tolane derivative, 3-n-propylamide-4-(4-hexyloxy-
phenylethynyl)-nitrobenzene 1 (4-nitro-2-n-propyl-
amide-4 0-hexyloxytolane, C24H28N2O4). The nonlinear
optical study and the structural analysis of compound
1 could provide evidence supporting the hypothesis
that the intermolecular hydrogen bond causes tolane
derivatives to show macroscopic second-order optical
nonlinearity.
Many studies of new materials for nonlinear optics,
especially about organic molecules, have been
performed [1]. Among them tolane derivatives are
regarded as promising materials for nonlinear optical
use because of their large intramolecular charge
transfer through the p-electron system with donor±
acceptor groups and a low cut-off wavelength [2].
Accordingly, intensive studies have been conducted
[2±5]. However, tolane derivatives tend to crystallize
in centrosymmetric space groups [6], which causes
the macroscopic second-order optical nonlinearity to
* Corresponding author; e-mail: kato@harl.hitachi.co.jp
1 Present address: Production Engineering Research Laboratory,
Hitachi Ltd., 292 Yoshida-cho, Totsuka-ku, Yokohama 244, Japan.
2 Present address: Department of Functional Polymer Science,
Faculty of Science and Technology, Shinshu University, Ueda,
Nagano 386, Japan.
0022-2860/98/$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.
PII S0022-2860(98)00401-3

140
M. Kato et al. / Journal of Molecular Structure 471 (1998) 139±143
Fig. 1. SHEW signals of compound 1 and m-NA.
2. Experimental
We prepared 4-hexyloxycinnamic acid by a reaction
of 4-hexyloxy-benzaldehyde, which was prepared
from 4-hydroxybenzaldehyde and n-hexyl bromide,
with malonic acid. The acid was treated with bromine,
followed by elimination of HBr in two steps, which
resulted in the formation of 4-hexyloxyphenyl-
acetylene. 3-n-Propylamide-4-bromonitrobenzene was
Fig. 2. The title molecule with displacement ellipsoids plotted at the
50% probability level.
Table 1
Crystallographic data of compound 1
prepared by the reaction of 4-bromo-3-aminonitroben-
zene with butyrylchloride. To form 3-n-propylamide-
4-(4-hexyloxyphenylethynyl)-nitrobenzene 1, 3-n-
propylamide-4-bromonitrobenzene and 4-hexyloxy-
phenylacetylene were reacted using CuI and PdCl2
(PPh3)2 in NEt3. The obtained material has a yellow
color. Recrystallization was from ethanol, and the
melting point was 398.5 K.
We examined the nonlinear optical properties of
this compound by second-harmonic generation with
the evanescent wave (SHEW) technique. The details
of this technique are described elsewhere [10]. It has
an advantage of providing reasonable results even
with powder samples because the phase-matchability
of the sample does not affect the result. The experi-
ment was performed with the fundamental light of a
Nd:YAG laser (l ˆ 1064 nm). The fundamental light
was guided into a rutile prism at an incident angle
FWD
408.48
Crystal system
Space group
Unit cell dimensions
Monoclinic
C1c1
Ê
a (A)
27.667(6)
4.910(2)
20.856(3)
128.36(3)
2221.6(5.5)
4
Ê
b (A)
Ê
c (A)
bb(8)
Unit cell volume U (A )
Ê 3
Z
Dx (mg m23
)
1.221
2013
No. of unique re¯ections
No. of re®ned parameters
Criterion for observed
re¯ections
384
|Fo| . 4s(|Fo|)
R
0.058
Computer and programs
FACOM M-1800,
UNICS-III program system

M. Kato et al. / Journal of Molecular Structure 471 (1998) 139±143
141
Table 2
Selected bond lengths of compound 1
grown from an ethanol solution by the cooling
method. The obtained single crystal has a thin plate
shape. Data collection of X-ray diffraction was carried
out with CAD-4 Software, the cell re®nement with
CAD-4 Software, and data reduction with CAD-4
Software (all from Enraf Nonius). The program used
to solve the structure was MULTAN78 [11] and the
program used to re®ne the structure was UNICS-III
[12].
Crystallographic data (excluding structure factors)
for the structure in this paper have been deposited
with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC 101883. Copies
of the data can be obtained, free of charge, on applica-
tion to: CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax: +44-1223-336033; e-mail: deposit@ccdc,-
cam.ac.uk.
Ê
Length (A)
Ê
Length (A)
Bond
Bond
O(20)±C(16)
O(23)±N(21)
O(24)±C(25)
N(15)±C(16)
N(21)±C(5)
C(1)±C(8)
1.221(0.006) O(22)±N(21)
1.220(0.010) O(24)±C(12)
1.434(0.010) N(15)±C(3)
1.219(0.006)
1.357(0.007)
1.411(0.005)
1.356(0.006) N(15)±H(N15) 0.906(0.060)
1.480(0.009) C(1)±C(2)
1.178(0.009) C(2)±C(3)
1.418(0.006) C(3)±C(4)
1.388(0.006) C(5)±C(6)
1.381(0.010) C(8)±C(9)
1.417(0.007) C(9)±C(14)
1.368(0.009) C(11)±C(12)
1.394(0.007) C(13)±C(14)
1.526(0.006) C(17)±C(18)
1.536(0.008) C(25)±C(26)
1.529(0.013) C(27)±C(28)
1.474(0.009) C(29)±C(30)
1.433(0.009)
1.401(0.009)
1.376(0.008)
1.378(0.010)
1.438(0.008)
1.393(0.010)
1.402(0.011)
1.383(0.009)
1.507(0.009)
1.520(0.008)
1.538(0.009)
1.528(0.010)
C(2)±C(7)
C(4)±C(5)
C(6)±C(7)
C(9)±C(10)
C(10)±C(11)
C(12)±C(13)
C(16)±C(17)
C(18)±C(19)
C(26)±C(27)
C(28)±C(29)
3. Results and discussion
such that the fundamental light was totally re¯ected at
the interface between the prism and sample. The
second-harmonic (SH) power in re¯ection was
detected and compared with that from the reference
material, meta-nitroaniline (m-NA).
The SHEW signal of 1 is represented as circles in
Fig. 1 with the signal of m-NA represented as
diamonds. The solid curves represent the ®tting
curves. The value of the susceptibility is calculated
as a ®tting parameter and the value is compared with
Single crystals of 1 for X-ray diffraction were
Table 3
Selected bond angles of compound 1
Bonds
Angle (8)
Bonds
Angle (8)
C(12)±O(24)±C(25)
C(3)±N(15)±H(N15)
O(22)±N(21)±O(23)
O(23)±N(21)±C(5)
C(1)±C(2)±C(3)
117.9(0.4)
122.9(3.3)
123.7(0.7)
118.7(0.5)
119.5(0.4)
118.9(0.6)
121.2(0.5)
117.5(0.6)
118.9(0.4)
118.2(0.4)
177.1(0.7)
122.6(0.5)
120.6(0.7)
115.1(0.4)
119.7(0.6)
121.5(0.5)
123.8(0.4)
113.3(0.5)
107.1(0.5)
112.0(0.6)
112.7(0.6)
C(3)±N(15)±C(16)
C(16)±N(15)±H(N15)
O(22)±N(21)±C(5)
C(2)±C(1)±C(8)
126.5(0.4)
109.6(3.1)
117.6(0.6)
172.0(0.6)
121.6(0.6)
117.4(0.5)
121.4(0.4)
117.3(0.6)
123.8(0.6)
120.3(0.6)
119.2(0.7)
118.2(0.5)
120.5(0.5)
125.2(0.7)
119.6(0.7)
123.0(0.4)
113.1(0.4)
111.0(0.6)
110.8(0.6)
117.5(0.7)
C(1)±C(2)±C(7)
C(3)±C(2)±C(7)
N(15)±C(3)±C(2)
C(2)±C(3)±C(4)
N(15)±C(3)±C(4)
C(3)±C(4)±C(5)
N(21)±C(5)±C(4)
C(4)±C(5)±C(6)
N(21)±C(5)±C(6)
C(5)±C(6)±C(7)
C(2)±C(7)±C(6)
C(1)±C(8)±C(9)
C(8)±C(9)±C(10)
C(10)±C(9)±C(14)
C(10)±C(11)±C(12)
O(24)±C(12)±C(13)
C(12)±C(13)±C(14)
O(20)±C(16)±N(15)
N(15)±C(16)±C(17)
C(17)±C(18)±C(19)
C(25)±C(26)±C(27)
C(27)±C(28)±C(29)
C(8)±C(9)±C(14)
C(9)±C(10)±C(11)
O(24)±C(12)±C(11)
C(11)±C(12)±C(13)
C(9)±C(14)±C(13)
O(20)±C(16)±C(17)
C(16)±C(17)±C(18)
O(24)±C(25)±C(26)
C(26)±C(27)±C(28)
C(28)±C(29)±C(30)

142
M. Kato et al. / Journal of Molecular Structure 471 (1998) 139±143
that of m-NA. Although the susceptibility is a tensor,
we can obtain only one value from the powder sample,
the averaged value of every tensor element over the
whole solid angle. The details of the averaging are
described in the Ref. [6]. The averaged susceptibility
of m-NA is 11 pm/V using the value 0.364 pm/V of d11
of quartz [13]. Using this, the averaged value of 1 is
estimated to be 23 ^ 2 pm/V, which is twice as large as
that of m-NA. This is of the same order as 2 [9].
The results of the X-ray structure analysis are
summarized in Table 1. The selected bond lengths
and the angles are summarized in Tables 2 and 3.
The molecular structure of 1 is shown in Fig. 2. It is
designed as a p-electron system with donor and
acceptor groups suitable for large intramolecular
charge-transfer. The two benzene rings are planar
and the mean planes of the two-ring systems are
inclined at 2.3(2)8, making them almost parallel.
Thus, p-electrons can spread over the tolane structure
and the charge can be easily transferred between the
nitro and hexyloxy groups.
Fig. 3. Crystal structure projected along the c-axis. For clarity, only
half of the atoms in the unit cell are drawn. The hydrogen bonds are
shown as dashed lines.
Fig. 4. Packing of the molecules viewed along (a) b-axis and (b) c-axis.

M. Kato et al. / Journal of Molecular Structure 471 (1998) 139±143
143
The hexyloxy group is in almost the same plane as
the benzene rings except for C(29) and C(30). The
conformation change at the end of the group is caused
by the crystal packing. The plane formed by C(26)±
C(27)±C(28) in the hexyloxy group is tilted from the
mean plane of the two benzene rings at 20.7(6)8 and at
22.4(6)8.
The amide group N(15)±C(16)±O(20) forms a
plane tilted by 38.6(3)8 from the mean plane of
the two benzene rings. Thus the amide group is
located between molecules stacked parallel along
the b-axis, permitting formation of the hydrogen
bond (Fig. 3). The distance between O(20) and
N(15) of the neighbor molecule by one unit cell
material 3-n-propylamide-4-(4-hexyloxyphenylethy-
nyl)-nitrobenzene, examined its second-order
nonlinear optical properties, and determined its struc-
ture. Its second-order nonlinear optical susceptibility
was found to be twice as large as that of m-NA, and the
structure was very similar to other tolane derivatives,
which also have large SHG ef®ciency. The intermole-
cular hydrogen bond was found to be formed in the
amide group, suggesting that an amide group is
required for the tolane derivatives to be SH active.
Acknowledgements
We would like to thank Dr. Angel Alvarez-Larena
of Universidad de Autonoma de Barcelona for his
useful discussions.
Ê
along the b-axis is 2.967(5) A. The hydrogen
¼
bond N(15)±H(N15) O(20) is formed between
the neighboring molecules. The angle for the
hydrogen bond was found to be 146.4(4)8. It has
been indicated that tolane derivatives with hydrogen
bonding between molecules tend to have high
nonlinear optical susceptibilities [7] because the
hydrogen bonding is expected to break the centrosym-
metry structure. Thus, for this derivative 1, which
shows large SHG ef®ciency as shown below, hydrogen
bonding plays an important role in its second-order
nonlinear properties.
The mean plane of the tolane structure, which is
formed by the two benzene rings and the triple bond
between C(1) and C(8), is tilted from the ab, ca, and
bc planes at 83.8(1), 42.3(1) and 117.0(1)8, respec-
tively. The mean planes of tolane of the two mole-
cules in symmetry (x, 2 y, z 1 1/2) are tilted 87.7(1)8,
as shown in Fig. 4.
References
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Organic Molecules and Crystals. Academic Press, Orlando,
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Y. Taniguchi, J. Mater. Chem. 7 (1997) 705.
As expected from the SHEW results, the molecular
structure and packing of 1 are very similar to those of
2 [8]. Thus, the molecular packing of 1 and 2, which
contains the intermolecular hydrogen bond, is the key
to have a large second-order nonlinear optical
susceptibility.
[9] M. Kato, M. Kiguchi, J. Appl. Phys. 81 (1997) 550.
[10] M. Kiguchi, M. Kato, N. Kumegawa, Y. Taniguchi, J. Appl.
Phys. 75 (1994) 4332.
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M.M. Woolfson, MULTAN. A Program for the Automatic
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4. Conclusion
[13] Handbook of Lasers, Chemical Rubber Co., Cleveland, OH,
1971, p. 497.
We have synthesized the novel nonlinear optical