Third-order nonlinear optical properties and crystal structures of N-(2-nitrobenzalidene)-2,4-dimethylaniline and N-(3-nitrobenzalidene)-2,4- dimethylaniline

Article abstract of DOI:10.1515/znb-2007-1113

N-(2-nitrobenzalidene)-2,4-dimethylaniline (1) and N-(3-nitrobenzalidene)- 2,4-dimethylaniline (2) have been synthesized and characterized by X-ray diffraction analysis. Linear optical characteristics have been evaluated theoretically using the configurat

Full text of DOI:10.1515/znb-2007-1113

Third-order Nonlinear Optical Properties and Crystal Structures of
N-(2-Nitrobenzalidene)-2,4-dimethylaniline and N-(3-Nitrobenzalidene)-
2,4-dimethylaniline
a
b
c
¨
Aslı Karakas¸ , Hu¨seyin Unver , and Ayhan Elmalı
a Department of Physics, Faculty of Arts and Sciences, Selc¸uk University, TR-42049 Campus, Konya,
Turkey
b Department of Physics, Faculty of Sciences, Ankara University, Tandog˘an, TR-06100 Ankara,
Turkey
c Department of Engineering Physics, Faculty of Engineering, Ankara University, Tandog˘an,
TR-06100 Ankara, Turkey
¨
Reprint requests to Dr. Hu¨seyin Unver. Fax: +903122232395. E-mail: unver@science.ankara.edu.tr
Z. Naturforsch. 2007, 62b, 1437 – 1442; received May 30, 2007
N-(2-nitrobenzalidene)-2,4-dimethylaniline (1) and N-(3-nitrobenzalidene)-2,4-dimethylaniline
(2) have been synthesized and characterized by X-ray diffraction analysis. Linear optical characteris-
tics have been evaluated theoretically using the configuration interaction (CI) method. The maximum
one-photon absorption (OPA) wavelengths of the studied compounds are shorter than 450 nm, giving
rise to good optical transparency in the visible and near IR regions. To provide an insight into the
third-order nonlinear optical (NLO) behavior of the title molecules, both dispersion-free (static) and
frequency-dependent (dynamic) linear polarizabilities (α) and second hyperpolarizabilities (γ) at λ =
825 – 1125 nm and 1050 – 1600 nm wavelength ranges have been computed using the time-dependent
Hartree-Fock (TDHF) method. The ab initio computational results on (hyper)polarizabilities reveal
that both compounds exhibit second hyperpolarizabilities with non-zero values, implying micro-
scopic third-order NLO behavior.
Key words: Third-order Optical Nonlinearity, One-photon Absorption, First Hyperpolarizability,
Second Hyperpolarizability, Configuration Interaction
Introduction
In recent years, designing and synthesizing molec-
ular materials with larger (hyper)polarizabilities are
hot research topics [1 – 3], which relate to several
fields, such as physics, medicine and chemistry. Along
with linear and quadratic effects there has been grow-
ing interest in third-order optical nonlinearity. Due to
potential applications in various photonic technolo-
gies, the nonlinear optical (NLO) properties of or-
ganic molecular materials have been the object of in-
tense research [4]. Devices for applications in optical
communications, optical processors, optical switches,
wavelength filters and modulators have been created
using the NLO response properties of organic sys-
Fig. 1. Chemical structure of compounds 1 and 2.
tion to the understanding of the (hyper)polarizabilities
underlying the molecular third-order NLO processes
and the establishment of structure-property relation-
ships.
Among NLO molecules, the Schiff bases have
tems. Therefore, organic molecules have been inten- gained special interest for many investigators because
sively studied with respect to their potential appli- of their relatively large molecular hyperpolarizabilities
cations as NLO media [1, 5]. During the investiga- due to delocalization of the π electrons [5, 6]. In gen-
tions of organic molecules for NLO, quantum chem- eral, Schiff base ligands may bear a variety of sub-
ical calculations have made an important contribu- stituents with different electron-donating or electron-
0932–0776 / 07 / 1100–1437 $ 06.00 © 2007 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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1438
A. Karakas¸ et al. · N-(2-Nitrobenzalidene)-2,4-dimethylaniline and N-(3-Nitrobenzalidene)-2,4-dimethylaniline
C29
C3
C4
C19
O4
C20
C21
C24
C23
C13
C25
C26
C5
C18
C17
C2
C22
C7
N1
C12
C11
C30
N3
C28
C16
N4
C1
C8
C6
C15
C27
C14
O1
C10
N2
O3
C4
C9
C5
C9
C8
N1
C14
C3
C2
C7
O2
C6
C10
C12
C11
C1
N2
Fig. 2. Molecular structure of 1 and 2 in the crys-
tal (displacement ellipsoids at the 50 % probability
level; H atoms as spheres of arbitrary radii).
C15
C13
O2
O1
˚
Table 1. Crystal and structure refinement data for 1 and 2.
Table 2. Selected bond length (A), angles (deg), and dihedral
angles (deg) for 1 and 2 with estimated standard deviations
in parentheses.
1
2
Formula
Mr
C15H14N2O2
254.28
C15H14N2O2
254.28
1
2
Cryst. size, mm3
Crystal system
Space group
0.04×0.24×0.50
0.24×0.24×0.50
monoclinic
C2/c
15.701(2)
5.909(1)
28.300(4)
90
94.74(1)
90
2616.6(7)
8
Distances
C1–C2
C1–N2
C9–C14
N2–O2
N2–O1
C8–N1
1.365(2)
1.457(8)
1.493(2)
1.208(4)
1.212(1)
1.421(6)
1.373(2)
1.464(1)
1.496(2)
1.215(1)
1.229(1)
1.419(1)
triclinic
¯
P1
˚
a, A
15.701(2)
5.909(1)
28.300(4)
94.06(1)
94.48(1)
103.32(1)
1298.5(3)
4
˚
b, A
˚
c, A
α, deg
β, deg
γ, deg
Angles
C2–C1–C6
C4–C3–C2
O2–N2–C1
C7–N1–C8
Dihedral angles
O1–N2–C1–C2
C14–C9–C8–N1
C13–C8–N1–C7
C1–C2–C3–C4
122.0(6)
118.8(0)
117.4(9)
116.6(7)
122.7(1)
120.4(1)
118.7(1)
121.9(1)
3
˚
V, A
Z
Dcalcd, g cm−3
µ(MoKα ), cm−1
F(000), e
1.30
0.009
536
1.29
0.009
1072
152.1(1)
5.2(1)
48.4(1)
−0.41(1)
2.2(2)
−176.4(1)
hkl range
((sinθ)/λ)max, A
Refl. measured
Refl. unique
Rint
9, 10, +26
0.63
5204
5182
0.0312
343
19, 0 – 7, +35
0.62
2673
2665
0.0271
172
−4.8(2)
−1
˚
−0.2(2)
on dispersion-free and frequency-dependent linear po-
larizabilities and second hyperpolarizabilities.
Param. refined
R1/wR2 [I ≥ 2σ (I)]
0.065/0.122
1.247
0.20/−0.30
0.058/0.124
1.017
0.15/−0.20
Goodness-of-fit−(3F2)
Experimental Section
˚
∆ρmax/min, e A
Preparation of compounds 1 and 2
withdrawing groups. Therefore, the effect of electron
donor/acceptor substituents on the second hyperpolar-
izabilities of Schiff base molecules has received much
attention in recent years. The aim of our present study
is twofold: to characterize the newly synthesized Schiff
bases shown in Fig. 1 having electron-donor methyl
groups in ortho and para positions and also accep-
tor nitro groups in ortho and meta positions of 1 and
2, respectively, with spectroscopic (UV/vis) and crys-
tallographic (X-ray diffraction) techniques, and to in-
vestigate the third-order NLO behavior utilizing an ab
initio time-dependent Hartree-Fock (TDHF) procedure
Compound 1 was prepared by condensation of 2-nitro-
benzaldehyde (0.01 mol) and 2,4-dimethylaniline (0.01 mol)
in 150 mL of ethanol. The reaction mixture was stirred for
4 h, and then placed in a freezer for 12 h. The precipitate was
collected by filtration, and then washed with cold ethanol. It
was crystallized from chloroform/n-heptane as yellow crys-
tals. – M. p. 112 ◦C, 2.41 g (95 %) yield. – 1H NMR (DMSO):
δ = 9.60 (s, 1H, Ar–CH=N–Ar), 8.98 – 6.71 (m, 7H, Ar-H),
2.03 (s, 3H, Ar-CH3), 2.01 (s, 3H, Ar-CH3). – C15H14N2O2
(254.28): calcd. C 70.85, H 5.55; found C 70.56, H 5.53.
Compound 2 was synthesized with the same procedure. –
M. p. 128 ◦C, 2.36 g (93 %) yield. – 1H NMR (DMSO):
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A. Karakas¸ et al. · N-(2-Nitrobenzalidene)-2,4-dimethylaniline and N-(3-Nitrobenzalidene)-2,4-dimethylaniline
1439
δ = 9.47 (s, 1H, Ar–CH=N–Ar), 8.51 – 6.57 (m, 7H, Ar-H), The average linear polarizability ꢀαꢁ and third-order hy-
2.02 (s, 3H, Ar-CH3), 2.00 (s, 3H, Ar-CH3). – C15H14N2O2 perpolarizability ꢀγꢁ values were calculated using the follow-
(254.28): calcd.
H 5.51.
C
70.85,
H
5.55; found
C
70.76, ing expressions [12]:
ꢀ
ꢁ
ꢀαꢁ = αxx +αyy +αzz /3
(1)
X-Ray structure determination
ꢂ
ꢀ
ꢁꢃ
1
5
ꢀγꢁ =
γxxxx +γyyyy +γzzzz +2 γxxyy +γxxzz +γyyzz (2)
The data collection for both compounds was performed
on an Enraf-Nonius diffractometer employing graphite-
monochromatized MoKα radiation (λ = 0.71073 A). Data re-
Since α and γ values of the GAMESS output are reported
in a. u., the calculated α and γ values were converted into
the electrostatic units (esu) (1 a. u. α = 0.1482 × 10−24 esu,
1 a. u. γ = 5.0367 × 10−40 esu). To calculate all the (hy-
per)polarizabilities, the origin of the Cartesian coordinate
system (x, y, z) = 0, 0, 0 was chosen at the center of mass
of the compounds 1 and 2.
˚
duction and corrections for absorption and crystal decompo-
sition of compounds 1 and 2 was carried out using the Non-
ius Diffractometer Control Software [7]. The structures were
solved with SHELXS-97 [8] and refined with SHELXL-97 [9].
The positions of the H atoms bonded to C atoms were calcu-
lated, and included in the structure factor calculation using
a riding model. The details of X-ray data collection, struc-
ture solution and structure refinements are given in Table 1.
Bond lengths and angles are listed in Table 2. The molecu-
lar structures with the atom-numbering scheme are shown in
Fig. 2 [10].
Besides, the π → π∗ transition wavelengths (λmax) of the
lowest-lying electronic transition and the oscillator strengths
(f ) of these transitions for the investigated molecules
were studied theoretically by electron excitation configu-
ration interaction using the CIS/6-31G method in GAUS-
SIAN98W [13].
CCDC648357 and 648358 contain the supplementary
crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data request/cif.
Results and Discussion
Description of the crystal structure
Schiff base compounds have been under investiga-
tion during the last years because of their potential
applicability in optical communications and many of
Theoretical calculations
The theoretical computations involve the determination of
dispersion-free and frequency-dependent linear polarizabil- them were shown to have NLO behavior [14– 16].
ity and second hyperpolarizability tensor components of the
title compounds using the following methods:
The title molecules 1 and 2 are not planar. For 1, the
two Schiff base moieties (C1–C7, O1, O2, N1) (pla-
Before the calculation of static and dynamic (hy-
per)polarizabilities for the investigated molecules, the ge-
ometries were optimized on the ab initio restricted closed-
shell Hartree-Fock level. The optimized structures were used
to compute the linear polarizabilities and third-order hyper-
polarizabilities at ω frequencies with a 6-311+G(d, p) ba-
sis set. There are many well-established and tested compu-
tational codes to compute accurately the NLO properties of
rather large molecular systems. Some of them are important
˚
nar with a maximum deviation of 0.080(1) A for the
O2 atom) and (N1, C8–C15) (planar with a maximum
˚
deviation of 0.076(1) A for the N1 atom) are inclined
at an angle of 7.76(1)◦. Compound 2 has two indepen-
dent molecules in the asymmetric unit. The two phenyl
rings connected by the C=N imino moiety are inclined
at angles of 31.6(1)◦ and 31.2(1)◦, respectively.
The crystal structure of both 1 and 2 is stabi-
as valuable theoretical tools used for the computation of (hy- lized by weak intramolecular and intermolecular
per)polarizabilities. The ab initio TDHF method is the most
useful one among the computational procedures [4]. Calcu-
lations of α(0;0) and γ(0;0,0,0) at ω = 0, and α(−ω; ω) and
γ(−3ω; ω, ω, ω) at ω = 0.05512, 0.04050, 0.04336, 0.02848
atomic units (a. u.) (i. e. at λ = 825, 1125, 1050, 1600 nm
wavelengths), often used laser frequencies in third-harmonic
generation (THG) measurements, were carried out using the
TDHF method implemented in the GAMESS program [11]. In
the γ definitions mentioned above, the first one describes the
static third-order hyperpolarizabilities, and the second rep-
resents the hyperpolarizability for frequency tripling, called
the THG process.
hydrogen bonding [17 – 19]. Intramolecular hydrogen
˚
bonds occur between C7–H7A···O1 (2.723(2) A),
˚
C14–H14A···N1 (2.798(2) A), C22–H22A···O3
˚
˚
(2.737(2) A), C29–H29A···N3 (2.754(2) A) atoms
˚
in 1 and between C14–H14A···N1 (2.826(2) A) in 2
(Fig. 2). The sum of the van der Waals radii of the O
˚
and N atoms (3.07 A) is significantly larger than the in-
tramolecular O···N hydrogen bond length [20]. There
is also intermolecular hydrogen bonding between
˚
˚
C14···O4 (3.582(2) A) and C10···O4 (3.605(2) A) in 1
˚
and between C13···O2 (3.511(2) A), (symmetry code:
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1440
A. Karakas¸ et al. · N-(2-Nitrobenzalidene)-2,4-dimethylaniline and N-(3-Nitrobenzalidene)-2,4-dimethylaniline
Table 3. Calculated maximum UV/vis absorption wave- Table 5. Selected components of the frequency-dependent
α(−ω; ω) and ꢀαꢁ(−ω; ω) (×10−24 esu) values at ω (in
lengths (λmax, nm) and oscillator strengths (f ) of compounds
1 and 2.
a. u.) laser frequencies for 1 and 2.
1
2
1
2
— ω —
λmax
416.31
386.66
259.50
243.60
419.40
390.05
278.24
251.34
f
2.1113
1.9877
1.5633
1.0008
2.0010
1.6450
1.6020
1.1260
0.05512
32.48
33.45
0.04050
31.91
32.97
0.04336
32.01
0.02848
31.54
32.74
αxx
αyy
αzz
1
2
33.04
1
2
18.09
17.43
17.96
17.30
17.98
17.33
17.88
17.23
Table 4. Some selected components of the static α(0; 0) and
ꢀαꢁ(0; 0) [×10−24 esu] values of compounds 1 and 2.
1
2
4.05
4.00
4.04
3.99
4.04
3.99
4.03
3.98
αxx
33.56
34.64
αyy
17.81
17.17
αzz
4.05
4.02
ꢀαꢁ
ꢀαꢁ
1
2
18.21
18.29
17.97
18.09
18.01
18.12
17.82
17.98
1
2
18.47
18.61
Table 6. All static γ(0; 0, 0, 0) components and
ꢀγꢁ(0; 0, 0, 0) (×10−37 esu) values for 1 and 2.
0.5 + x, 0.5 + y, z) atoms of neighboring molecules in
2 (Fig. 2).
γxxxx
724.00
804.79
γyyyy
28.79
13.55
γzzzz
0.14
0.36
γxxyy
18.41
30.46
γxxzz
9.15
10.70
γyyzz
1.12
1.21
ꢀγꢁ
1
2
162.06
180.69
Computational results and discussion
Quantum chemistry calculations have shown to be
useful in the description of the relationship among the
electronic structures of the systems and their NLO
response [21]. NLO techniques are among the most
structure sensitive methods to study molecular struc-
tures and assemblies [22]. The determination and anal-
ysis of the NLO properties of molecular systems with
theoretical methods have greatly progressed during the
past years. Actually, to accurately compute NLO prop-
erties of rather large molecular systems, there are well-
tested computational codes.
It can be very helpful in the investigation of NLO
materials to check, apart from NLO responses, also
the spectroscopic absorbance in the appropriate wave-
length. Thus, the wavelengths obtained by UV/vis
spectral analysis can be helpful in planning the syn-
thesis of promising NLO materials [23]. Since it is
necessary to know the transparency region, electronic
absorption spectral studies of compounds designed to
possess NLO properties are important. In this paper,
the vertical transition energies and oscillator strengths
from the ground to the excited states have been com-
puted, giving OPA, i. e., the UV/vis spectrum. The cal-
culated wavelengths (λmax) and oscillator strengths ( f)
for both molecules are located in the UV region with
wavelengths shorter than 450 nm, implying a good op-
tical transparency in the visible region.
One could determine the hyperpolarizability tensors
of molecules using a suitable computational approach.
These tensors describe the response of molecules to an
external electric field. At the molecular level, the NLO
properties are determined by their dynamic hyperpo-
larizabilities. TDHF is a procedure generally used to
find out approximate values and can help understand-
ing both static and dynamic hyperpolarizabilities of or-
ganic molecules. We present here a comprehensive ab
initio study of the NLO properties of the title molecules
using the TDHF method. In this study, in addition
to the static linear polarizabilities α(0; 0) and second
hyperpolarizabilities γ(0; 0, 0, 0), the following pro-
cessess for dynamic (hyper)polarizabilities have been
considered: frequency-dependent linear polarizabili-
ties α(−ω; ω), THG γ(−3ω; ω, ω, ω). Some signifi-
cant calculated magnitudes of the static and frequency-
dependent linear polarizabilities and second hyperpo-
larizabilities are shown in Tables 4 and 7, respectively.
Virtually high-order NLO effects in organic mole-
for the maximum OPA of the investigated molecules cules originate from a strong intramolecular donor-
are shown in Table 3. Molecules 1 and 2 have four OPA acceptor interaction. The dipolar aromatic molecules
peaks in their spectrum. As can be seen in Table 3, the possessing an electron donor group and an electron
optical spectra exhibit four relatively intense bands in- acceptor group contribute to large optical nonlinear-
volving π → π∗ transitions centered between 244 and ity arising from the intramolecular charge transfer be-
416 nm for compound 1 and between 252 and 419 nm tween the two groups of opposite nature. π-Conjugat-
for compound 2. The values of all absorption maxima ed molecules with a donor and an acceptor will not
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A. Karakas¸ et al. · N-(2-Nitrobenzalidene)-2,4-dimethylaniline and N-(3-Nitrobenzalidene)-2,4-dimethylaniline
1441
Table 7. Selected components of the frequency-dependent
γ(−3ω; ω, ω, ω) and ꢀγꢁ(−3ω; ω, ω, ω) (×10−37 esu) val-
ues at ω (in a. u.) laser frequencies calculated with THG pro-
cess for 1 and 2.
aromatic ring. It can be seen from Tables 4 and 5 that
static and dynamic polarizabilities of both compounds
are not much affected by changing the substituent po-
sitions. We have found rather similar polarizabilities
for compounds 1 and 2. However, with a great pos-
sibility, the nitro group at the meta position enhances
the static and dynamic third-order optical nonlinear-
ity. Thus, ꢀγꢁ values of compound 2 have been calcu-
lated to be larger than for compound 1 (see Tables 6
and 7). Compounds 1 and 2 may thus have micro-
scopic third-order NLO behavior with non-zero (hy-
per)polarizabilities. It is also important to stress that,
in the above α and γ calculations, we did not take into
account the effect of the field on the nuclear positions,
i. e. we evaluated only the electronic components of
ꢀαꢁ and ꢀγꢁ.
— ω —
0.05512
4304.85
2960.11
0.04050
1373.24
1483.26
0.04336
1526.92
1619.60
0.02848
1191.17
1249.30
γxxxx
γyyyy
γzzzz
γxxyy
γxxzz
γyyzz
ꢀγꢁ
1
2
1
2
124.18
34.51
46.30
19.89
51.09
21.41
35.23
16.51
1
2
−0.10
−0.22
−0.21
−0.26
−0.18
−0.25
−0.24
−0.27
1
2
334.88
53.32
47.36
39.63
58.26
42.02
24.78
33.62
1
2
5.42
6.00
2.74
4.81
2.79
4.78
4.19
6.80
1
2
3.19
1.71
1.75
1.50
1.87
1.59
1.47
1.29
1
2
1010.52
658.22
306.43
325.09
343.00
355.84
257.85
270.89
Conclusion
The title molecules have been synthesized, and their
structures have been determined by X-ray diffraction.
We have presented results of computational studies
showing how the title compounds can possess third-
order optical nonlinearity. To test the microscopic
third-order NLO behavior computation of the OPA
wavelengths, linear and second (hyper)polarizabilites
of the studied compounds may be considered rather
adequate. According to the results of the calculation
on the linear optical behavior, the electronic transition
wavelengths are estimated to be shorter than 450 nm,
implying good optical transparency in the visible and
near-IR region (450– 900 nm). The ab initio calcu-
lated non-zero (hyper)polarizability values imply that
the synthesized Schiff bases might have microscopic
third-order NLO behavior.
display second-order NLO activity if they possess a
center of symmetry. However, such centrosymmetric
molecules might have third-order optical nonlinearity
like the compounds studied here. More NLO chro-
mophores can be envisaged using multiple donor and
acceptor groups linked to different units of π conju-
gations. Although there are a few exceptions, a group
may be an electron acceptor in one condition and an
electron donor in another, depending on the situation.
These acceptor-donor groups are generally attached to
conjugated systems to generate NLO materials with
large molecular hyperpolarizability in order to tailor
their transparency. For the design of new organic ma-
terials such as Schiff bases one has to investigate the
influence of donor-acceptor substituent positions upon
the (hyper)polarizability values of these compounds;
for instance at the meta position, the NLO behavior re-
acts most strongly on various substituents. Compounds
1 and 2 contain the same donor (-CH3) in ortho and
para positions at one end and an acceptor substituent
(-NO2) at the other differing only in the position at the
Acknowledgement
This work was supported by the Turkish State of Planning
˙
Organization (DPT), TUBITAK and Selc¸uk University under
grant numbers 2003-K-12019010-7, 105T132, 2003/030, re-
spectively.
¨
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