Syntheses, structures and second-order nonlinear optical behavior of (E)-N-(4-Nitrobenzylidene)-3,4-dimethylaniline and (E)-N-(3-Nitrobenzylidene)-3, 4-dimethylaniline

Article abstract of DOI:10.1515/znb-2010-0215

(E)-N-(4-Nitrobenzylidene)-3,4-dimethylaniline (1) and (E)-N-(3- nitrobenzylidene)-3,4-dimethylaniline (2) have been synthesized, and their crystal structures have been determined by X-ray diffraction analysis. Linear optical characteristics have been eva

Full text of DOI:10.1515/znb-2010-0215

Syntheses, Structures and Second-order Nonlinear Optical
Behavior of (E)-N-(4-Nitrobenzylidene)-3,4-dimethylaniline
and (E)-N-(3-Nitrobenzylidene)-3,4-dimethylaniline
a
b
a
¨
Hu¨seyin Unver , Aslı Karakas¸ , and Tahsin Nuri Durlu
a Department of Physics, Faculty of Sciences, Ankara University, TR-06100 Tandog˘an, Ankara,
Turkey
b Department of Physics, Faculty of Sciences, Selc¸uk University, TR-42049 Campus, Konya, Turkey
¨
Reprint requests to Dr. Hu¨seyin Unver. E-mail: unver@science.ankara.edu.tr
Z. Naturforsch. 2010, 65b, 185 – 190; received October 27, 2009
(E)-N-(4-Nitrobenzylidene)-3,4-dimethylaniline (1) and (E)-N-(3-nitrobenzylidene)-3,4-dimeth-
ylaniline (2) have been synthesized, and their crystal structures have been determined by X-ray
diffraction analysis. Linear optical characteristics have been evaluated experimentally using UV/Vis
spectroscopy and 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. The ab initio quantum-mechanical cal-
culations (finite field second-order Møller-Plesset perturbation theory) of the investigated molecules
have been carried out to compute the electric dipole moment (µ) and the first hyperpolarizability (β)
values. The ab initio results suggest that the title compounds might have microscopic second-order
nonlinear optical (NLO) behavior with non-zero values.
Key words: X-Ray Diffraction, UV/Vis Spectroscopy, Electric Dipole Moment,
First Hyperpolarizability, Configuration Interaction
Introduction
medicinal and biological chemistry. Recently, the ther-
mochromism, photochromism and NLO properties of
these compounds have found applications in modern
technologies. Because of the structural characteristics
of the Schiff base products (i. e., electron donor and
acceptor groups connected to a π-conjugated chain), it
could be said that they will have potential as NLO or
electro-optical materials [6, 7].
Conjugated organic molecules containing both
donor and acceptor groups are of great inter-
est for molecular electronic devices. Second-order
NLO organic materials that contain stable molecules
with large molecular hyperpolarizabilities in non-
centrosymmetric packing are of great interest for de-
vice applications [8], but according to a statistical
study, the overwhelming majority of achiral molecules
crystallizes centrosymmetrically [9].
Molecular materials with quadratic nonlinear opti-
cal (NLO) properties are currently attracting consider-
able interest [1, 2]. Progress has been made in finding
new organic molecules with large second-order polar-
izability β. Among the materials producing NLO ef-
fects, organic materials are of considerable importance
owing to their synthetic flexibility, large hyperpolar-
izabilities, ultra-fast response times, and high laser
damage thresholds, compared to inorganic materials.
Up to now, several hundreds of donor- and acceptor-
substituted systems which show NLO properties have
been reported. However, owing to difficulties in getting
transparent, good quality crystals of considerable size
only some of them could be used for applications as
in modulators, second harmonic generators and optical
wave guides [3].
The aim of our present study is twofold: to char-
The effect of electron donor or acceptor substituents acterize the newly synthesized Schiff bases shown in
on the first hyperpolarizability of Schiff bases has re- Fig. 1 having donor and acceptor substituents with
ceived a great deal of attention in recent years [4, 5]. spectroscopic (UV/Vis) and crystallographic tech-
Benzylideneanilines are an important class of Schiff niques, and to compute the electric dipole moments µ
bases which have been widely used in coordination, and first hyperpolarizabilities β by an ab initio finite
c
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¨
186 H. Unver et al. · (E)-N-(4-Nitrobenzylidene)-3,4-dimethylaniline and (E)-N-(3-Nitrobenzylidene)-3,4-dimethylaniline
λmax
λexp Table 1. Calculated (λmax,
420
328
242
nm) and measured (λexp, nm)
maximum UV/Vis absorp-
tion wavelengths of 1 and 2.
1
2
410.12
321.74
243.33
404.56
314.56
239.02
398
348
220
1
2
parent above 450 nm. The absence of the absorption
above 450 nm in the visible regime might enable the
achievement of microscopic NLO response with non-
zero values.
Description of the crystal structures
Fig. 1. Formulae of 1 and 2.
The crystal structures of the title molecules 1 and 2
(Fig. 1) have been determined. Both compounds crys-
tallize in centrosymmetric space groups. Their molec-
ular structures in the solid state are shown in Fig. 2.
The two molecules are not planar. For 1, the two Schiff
base moieties (C1–C7, O1, O2, N1) [planar with a
field second-order Møller-Plesset perturbation (FF
MP2) method. Our interest is not only to predict µ
and β values of the title molecules, but also to inves-
tigate the effect of the nature and the position of the
substituents on the microscopic NLO response with
quantum-mechanical calculations.
˚
maximum deviation of 0.070(1) A for the O1 atom]
and (N2, C8–C15) [planar with a maximum devia-
˚
tion of 0.049(1) A for the C14 atom] are inclined at
an angle of 9.97(6)◦. Similarly, for 2, the two Schiff
Results and Discussion
base moieties (C1–C7, O1, O2, N2) [planar with a
UV/Vis spectroscopy studies
˚
maximum deviation of 0.026(1) A for the O1 atom]
and (N1, C8–C15) [planar with a maximum devia-
tion of 0.027(1) Afor the C14 atom] are inclined at
Utilizing obtained data by UV/Vis spectroscopy
studies, it is possible to determine the optimal work-
ing wavelengths of second-order NLO regions. Al-
bert et al. [10] have reached the conclusion that with
the correct substitution of the push-pull system in the
porphyrin ring, characterized by strong intramolecu-
lar π → π∗ charge transfer transitions found through
UV/Vis spectral analysis, some specific electronic and
structural properties of this system could produce
high NLO responses. Zhou et al. [11] have found
that the λmax results of novel para-phenylenealkyne
macrocycles are not improved with the odd number
of units, even with 10 units, the value of λmax be-
ing 360 nm. Although the λmax was estimated to be
shorter than 450 nm in a large enough sample, a strong
increase in the hyperpolarizability value is obtained.
˚
The UV/Vis spectra of 1 and 2 have been recorded in
the range of 190 – 1100 nm. The maximum absorption
wavelengths (λexp) obtained from the UV/Vis spectral
analyses in tetrahydrofuran (THF) for 1 and cyclohex-
ane for 2 are listed in Table 1. The two compounds
have three absorption bands in the near UV/Vis region
Fig. 2. The molecular structures of 1 (top) and 2 (bottom)
in the solid state (displacement ellipsoids are drawn at the
below 450 nm. As seen in Table 1, there is no absorp- 50 % probability level; H atoms are presented as spheres of
tion above 450 nm, i. e. both molecules remain trans-
arbitrary radii).
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¨
H. Unver et al. · (E)-N-(4-Nitrobenzylidene)-3,4-dimethylaniline and (E)-N-(3-Nitrobenzylidene)-3,4-dimethylaniline 187
Table 2. Calculated all static β components and βtot (×10−30 esu) values of 1 and 2.
Compound
βxxx
βxxy
βxyy
βyyy
βxxz
βxyz
βyyz
βxzz
βyzz
βzzz
βtot
1
2
13.043
1.813
1.152
−0.839
−0.776
−0.381
−0.013
0.002
0.015
−0.002
−0.013
−0.008
−0.001
12.277
2.423
0.271
−0.464
−0.093
0.032
−0.042
0.004
0.001
an angle of 4.13(5)◦. The torsion angles C2–C1–N1–
O2 [3.0(2)◦] and C6–C1–N1–O1 [2.9(2)◦] for 1 and
C6–C1–N2–O1 [1.5(2)◦] and C2–C1–N2–O2 [0.4(2)◦]
for 2 indicate a slight deviation of the nitro groups from
the plane of their benzene ring.
Table 3. The ab initio-calculated electric dipole moments µ
(Debye) and dipole moment components for 1 and 2.
Compound
µx
µy
µz
µ
1
2
6.073
2.952
1.728
4.632
−0.001
6.314
5.497
0.214
Examination of the bond lengths in 1 and 2 sug-
gests that there is an extended series of at least par-
tially conjugated π bonds through the entire molecules.
Except for the N1–C1 (for 1), N2–C1 (for 2) bonds
connecting the nitro and phenyl groups, all the other
bonds between non-H atoms show π+σ character. The
bond length C4–C7, N2–C8 for 1 and C5–C7, N1–C8
for 2 are shorter than typical single σ bonds, and in
good agreement with related compounds in the liter-
ature [12]. The bond lengths C4–C7, C7–N2, N2–C8
(for 1) and C5–C7, C7–N1 and N1–C8 (for 2) in the
chains are similar for both compounds. The Csp2–N
bonds associated with the nitro groups are clearly sin-
gle bonds, while the bond length C7–N2 for 1 and
the spectroscopic absorbance at the appropriate wave-
length. Thus, the wavelengths obtained by UV/Vis
spectral analysis can be helpful in planning the synthe-
sis of promising NLO materials [16]. Since it is nec-
essary to know the transparency region, the electronic
absorption spectral studies of compounds designed to
possess NLO properties are important. In this paper,
the vertical transition energies from the ground state
to each excited state have been computed, giving one-
photon absorption (OPA), i. e., the UV/Vis spectrum.
The calculated wavelengths (λmax) for the maximum
OPA of 1 and 2 are shown in Table 1. Both molecules
have three OPA peaks in their spectrum. As can be
seen from Table 1, the optical spectra exhibit three rel-
atively intense bands involving π →π∗ transitions cen-
tered between 244 and 411 nm for 1 and between 240
and 405 nm for 2. The values of all absorption maxima
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. It is also seen
from Table 1 that our computations on UV/Vis absorp-
tion wavelengths are reasonably in accord with the ex-
perimental values.
It is shown that 1 and 2 have large non-zero µ val-
ues (see Table 3). Because the studied compounds
are polar molecules with non-zero dipole moments,
these µ values in Table 3 yield non-zero βtot values.
The higher dipole moment values are associated, in
general, with a larger projection of βtot quantities [17].
The dipoles may oppose or enhance one another or,
at least, bring the dipoles into or out of the required
net alignment necessary for NLO properties such as
βtot values [18]. Therefore, the µ values in Table 3
may be responsible for enhancing and decreasing the
βtot values in Table 2. Methyl and nitro groups are
effective donor-acceptor substituents for enhancing
the static first hyperpolarizabilities of 1 and 2. These
˚
C7–N1 for 2 are the same (1.254(1)A), and show a par-
tial double bond character [12, 13] which is also evi-
dence for conjugation. In the phenyl ring, bond lengths
lie close to that of the standard arene C–C bonds,
˚
at 1.39 A, but with one being much shorter [C1–C6
˚
˚
1.369(2) A for 1 and C1–C2 1.360(2) A for 2] and
˚
one being much longer [C2–C3 1.403(2) A for 1 and
˚
C2–C3 1.393(2) A for 2] than this average [14]. The
quinoid character of the phenyl ring in these molecules
is actually rather typical for rings bearing electron
donor and acceptor substituents in para positions, and
this feature is considered important in potential NLO
compounds [15].
Computational results and discussion
A computational approach allows the determination
of molecular NLO properties as an inexpensive way
to design molecules by analyzing their potential be-
fore synthesis. In addition to the well-known empirical
rules to estimate qualitatively the microscopic nonlin-
ear response in organic molecules, especially ab initio
MP2 calculations allow a more accurate prediction of
the NLO activity.
It can be very helpful in the investigation of NLO donor (-CH3) and acceptor (-NO2) groups thus affect
materials to check, apart from NLO responses, also the second-order optical nonlinearity. βtot values of the
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¨
188 H. Unver et al. · (E)-N-(4-Nitrobenzylidene)-3,4-dimethylaniline and (E)-N-(3-Nitrobenzylidene)-3,4-dimethylaniline
Table 4. Crystal structure data for 1 and 2.
title molecules largely depend on the positioning of
substituents. The βtot value for 1 with a nitro group in a
para position is much larger than that of the value for 2
with a nitro group in a meta position (see Table 2). It is
important to stress that in these βtot investigations we
do not take into account the effect of the field on the
nuclear positions, i. e. we evaluate only the electronic
components of βtot.
Compound
Formula
1
2
C15H14N2O2
254.28
monoclinic
P21/n
C15H14N2O2
254.28
Formula weight
Crystal system
Space group
monoclinic
C2/c
Crystal dimension, mm3
0.06×0.08×0.50 0.06×0.18×0.30
˚
a, A
9.403(1)
10.369(1)
13.392(2)
95.30(1)
1300.1(1)
4
15.548(2)
6.130(1)
27.403(6)
93.40(2)
2607.2(3)
8
1.30
0.1
1072
52.8
˚
b, A
˚
c, A
β, deg
3
Conclusions
˚
V, A
Z
Dcalcd., g cm−3
µ(MoKα ), mm−1
F(000), e
1.30
0.1
536
1 and 2 have been synthesized for the study of
their second-order optical nonlinearities. Their struc-
tures have been investigated by X-ray diffraction mea-
surements. We have presented computational stud-
ies showing that the title compounds possess second-
order NLO behavior. In order to test the microscopic
second-order nonlinearity, the electric dipole moments
and first hyperpolarizabilities of 1 and 2 have been
calculated using an ab initio methodology with a 6-
311+G(d,p) basis set in the FF approach. In the com-
putations of all these properties, the MP2 perturba-
tion theory has provided adequate electron correla-
tion effects. With ab initio FF MP2 computations, it
is also possible to predict the βtot value accurately for
a given molecular structure. The non-zero µ values of
the examined Schiff bases show that 1 and 2 have mi-
croscopic first hyperpolarizabilities with non-zero val-
ues obtained by the numerical second-derivative of the
electric dipole moment according to the applied field
strength. The computational results also reveal that
the substituent positions play a significant role regard-
ing the NLO properties of both compounds. The OPA
characterizations of 1 and 2 have been obtained both
theoretically (CI method) and experimentally (UV/Vis
spectroscopy). According to the results on the lin-
ear optical behavior, the values of 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), in good agreement with
the experimental results.
2θmax, deg
52.8
11, +12, +16
2649 / 2636
hkl range
19, +7, +34
2642 / 2635
Refl. indep. / obs.
(I ≥ 2σ(I))
Ref. parameters
max (∆/sigma)
215
0.001
172
0.003
0.061 / 0.125
1.035
0.14 / −0.21
R (I ≥ 2σ(I))/Rw (all data) 0.062 / 0.119
Goodness-of-fit on F−23
1.037
0.27 / −0.29
˚
∆ρfin (max/min), e A
spectra were measured using a Perkin Elmer Lambda 2 se-
ries spectrophotometer with 1.0 cm quartz cells.
Preparation of compounds 1 and 2
(E)-N-(4-Nitrobenzylidene)-3,4-dimethylaniline (1) was
prepared by condensation of 4-nitrobenzaldehyde (0.02 mol)
and 3,4-methylaniline (0.02 mol) in 75 mL of ethanol. The
reaction mixture was stirred for 3 h, and then placed in a
freezer for 6 h. The yellow precipitate was collected by fil-
tration, and then washed with cold ethanol. – C15H14N2O2
(254.28): calcd. C 70.85, H 5.95, N 11.02; found C 70.14,
H 5.26, N 10.74. – IR (KBr, cm−1): ν = 3058 w, 2911 –
2878 m, 1638 s (C=N), 160 – 1522 s (C=C), 1348 s (C–N).
Compound 2 was synthesized analogously. – C15H14N2O2
(254.28): calcd. C 70.85, H 5.95, N 11.02; found C 70.38,
H 5.12, N 10.94. – IR (KBr, cm−1): ν = 3052 w, 2912 m,
1598 s (C=N), 1510 s (C=C), 1333 s (C–N).
X-Ray structure determination
The data collection for both compounds was performed
on an Enraf-Nonius diffractometer employing graphite-
monochromatized MoKα radiation (λ = 0.71073 A). Data
reduction and corrections for absorption and crystal decom-
Experimental Section
˚
Reagents and techniques
3-Nitrobenzaldehyde, 4-nitrobenzaldehyde, 3,4-dimeth- position (0.7 %) of 1 and (1.1 %) of 2 were achieved us-
ylaniline and ethanol were purchased from Merck (Ger- ing the Nonius Diffractometer Control Software [19]. The
many). CNH analyses were performed on a Leco CHNS- structures were solved by SHELXS-97 [20] and refined with
932 analyzer. Infrared absorption spectra were obtained with SHELXL-97 [21]. All non-hydrogen atom parameters were
a Perkin Elmer BX II spectrometer on KBr discs. UV/Vis refined anisotropically. H atoms for 1 were located in their
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¨
H. Unver et al. · (E)-N-(4-Nitrobenzylidene)-3,4-dimethylaniline and (E)-N-(3-Nitrobenzylidene)-3,4-dimethylaniline 189
idealized positions with C–H distances in the range 0.93 – cessor with 512 MB RAM and Microsoft Windows as the
˚
0.96 A. The positions of H atoms for 2 were refinded freely. operating system.
The details of the X-ray data collection, structure solution
We report βtot (total first hyperpolarizability) values for
and structure refinements are given in Table 4. The molecu- the examined compounds. The components of the first
lar structures with the atom numbering scheme are shown in hyperpolarizability and the complete equation for calcu-
Fig. 2 [22].
lating the magnitude of the first hyperpolarizability from
GAUSSIAN98W output are described in ref. [18].
CCDC 743294 and 743295 contain the supplementary
Since the β values of GAUSSIAN98W outputs are reported
in atomic units (a. u.), the calculated β values have been
converted into electrostatic units (esu) (1 a. u. = 8.6393 ×
10−33 esu). To calculate the electric dipole moments and the
hyperpolarizabilities, the origin of the cartesian coordinate
system (x, y, z) = (0, 0, 0) has been chosen at the centers of
mass of 1 and 2.
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.
Theoretical calculations
As the first step of electric dipole moment and static first
hyperpolarizability calculations, the geometries have been
optimized in the ab initio restricted and unrestricted Hartree-
Fock levels for 1 and 2, respectively. Then, the electric dipole
moments and first hyperpolarizability tensor components of
the investigated compounds have been calculated using the
FF MP2 method at 6-311+G(d, p) polarized and diffused ba-
sis set level, which has been found to be more than adequate
for obtaining reliable trends in the hyperpolarizability val-
ues. All µ and β computations have been performed using
GAUSSIAN98W [23] on an Intel Pentium IV 1.7 GHz pro-
Besides, the π →π∗ transition wavelengths (λmax) of the
lowest-lying electronic transition for 1 and 2 have been cal-
culated by the electron excitation configuration interaction
using the CIS/6-31G method in GAUSSIAN98W.
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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