On the dipole moments and first-order hyperpolarizability of N,N-bis(4-bromobutyl)-4-nitrobenzenamine

Article abstract of DOI:10.1016/j.saa.2007.10.044

The nonlinear optical molecule N,N-bis(4-bromobutyl)-4-nitrobenzenamine was synthesized. The ground state dipole moment was determined by the Debye-Guggenheim method. A solvent mixture of acetonitrile and toluene was used for the solvatochromic determination of the excited state dipole moment. Excited state has a high value for the dipole moment which indicated a higher degree of charge transfer from the donor to the acceptor moiety on excitation by light. The first hyperpolarizability (β_{i j k}) of the molecule was evaluated assuming the two level model of the first hyperpolarizability.

Full text of DOI:10.1016/j.saa.2007.10.044

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Spectrochimica Acta Part A 70 (2008) 1227–1230
On the dipole moments and first-order hyperpolarizability of
N,N-bis(4-bromobutyl)-4-nitrobenzenamine
V.C. Kishorea, R. Dhanyab, K. Sreekumarc, Rani Josephb, C. Sudha Karthaa,∗
a Department of Physics, Cochin University of Science and Technology, Cochin 682022, India
b Department of Polymer Science & Rubber Technology, Cochin University of Science and Technology, Cochin 682022, India
c Department of Applied Chemistry, Cochin University of Science and Technology, Cochin 682022, India
Received 14 October 2007; received in revised form 26 October 2007; accepted 31 October 2007
Abstract
The nonlinear optical molecule N,N-bis(4-bromobutyl)-4-nitrobenzenamine was synthesized. The ground state dipole moment was determined
by the Debye–Guggenheim method. A solvent mixture of acetonitrile and toluene was used for the solvatochromic determination of the excited
state dipole moment. Excited state has a high value for the dipole moment which indicated a higher degree of charge transfer from the donor to the
acceptor moiety on excitation by light. The first hyperpolarizability (βijk ) of the molecule was evaluated assuming the two level model of the first
hyperpolarizability.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Dipole moments; First hyperpolarizability; Solvatochromism
1. Introduction
For a molecule to show nonlinear optical (NLO) properties,
the basic requirements are polarizability, asymmetric charge
distribution and acentric crystal packing [5]. On a molecu-
lar level, the parameters that can be changed are the relative
electron affinities of the donor and acceptor groups and the
length and nature of the conjugated segment connecting the
donor to the acceptor. The presence of a strong dipole moment
in the ground state is an indication that the molecule has
an asymmetric charge distribution. In this paper, determina-
tion of the ground state dipole moment, excited state dipole
moment and the first hyperpolarizability of the NLO molecule
N,N-bis(4-bromobutyl)-4-nitrobenzenamine are discussed in
detail.
Synthesis of molecules with large first order hyperpolar-
izabilities is important due to their potential applications in
nonlinear optics. Many components for optical signal process-
ing and communication devices can be made out of materials
possessing high electrooptic coefficients [1]. It is already recog-
nized that organic molecules with electron donor and acceptor
groups possess high nonlinear optical properties. High values of
the first order hyperpolarizabilities, βijk , gives rise to large sec-
ond order effects, which has applications in electrooptic devices,
photorefractive materials [2], second harmonic generation, etc.,
provided that non-centrosymmetry is maintained in the bulk.
Most of the molecules with large first hyperpolarizability also
have a strong permanent dipole moment and an ensemble of
such molecules will tend to align antiparallel in pairs, leaving
no macroscopic second order susceptibility. Poling techniques
[3] are usually employed to break the centrosymmetry of the
systems under study. Polymers doped with such chromophores
have shown to possess high electrooptic (EO) coefficients [4].
2. Experimental
2.1. Synthesis of
N,N-bis(4-bromobutyl)-4-nitrobenzenamine—BBN
The molecule was synthesized according to the scheme
[Fig. 1]. 5.02 g (0.0363 mol, SD Fine, 99% pure) para-
nitroaniline and 8.96 g (0.8453 mol, SD Fine 99% pure) sodium
carbonate in 60 ml water was stirred at 98 ◦C. To this solution
10 ml 1,4-dibromobutane (0.08456 mol, Lancaster, 98% pure)
was added drop wise from a pressure equalizing funnel and
∗
Corresponding author.
E-mail address: csk@cusat.ac.in (C. Sudha Kartha).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.10.044

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V.C. Kishore et al. / Spectrochimica Acta Part A 70 (2008) 1227–1230
2.3. Determination of excited state dipole moment (μe)
The excited state dipole moment shows the extent of charge
transfer taking place after excitation by radiation of sufficient
energy. If the difference between the ground and excited state
dipole moments is large, the charge transfer is also large, pro-
vided, the bond length remains unaltered upon excitation. The
determination of excited state dipole moment is very impor-
tant and there are many methods proposed in the literature. It
has been shown that the dynamics of photophysical properties
depends on the solvent in which the molecule is dissolved [9,10].
A simple method is to analyze the solvent polarity dependence
of the absorption and emission maxima (solvatochromism) of
the molecule under consideration to extract μe [11].
The absorption and fluorescence spectra of the molecule were
measured in a series of solution mixtures of toluene and acetoni-
trile. The spectra are shown in [Fig. 2] for a 50:50 mixture of
toluene and acetonitrile. These spectra were measured using a
Jasco V-570 spectrophotometer and a Fluoromax-3 Fluorimeter.
Both absorption and fluorescence showed bathochromic shifts
with increasing solvent polarity, or the molecule exhibited pos-
itive solvatochromism [12].
Fig. 1. Synthesis of N,N-bis(4-bromobutyl)-4-nitrobenzenamine.
refluxed for 12 h. After cooling down the reaction mixture, the
product was allowed to solidify. Subsequent filtration yielded
the crude product, it was washed with distilled water and puri-
fied by column chromatography using benzene:ethyl acetate
mixture. Bright yellow crystals of N,N-bis(4-bromobutyl)-4-
nitrobenzenamine was obtained. Dried and recrystallized from
ethanol. Yield = 89% M.P. = 160 ◦C.
2.2. Determination of ground state dipole moment (μg)
The figure of merit of nonlinear optical chromophores is
highly dependent on the ground state dipole moment and the
first hyperpolarizability of the molecule [6]. The refractive index
modulation achievable in a photorefractive composite is depen-
dent on the figure of merit [7] and in turn on the dipole moment of
the material. The ground state dipole moment can be determined
by the Debye–Guggenheim method, where the actual measure-
ment is the static dielectric constant of a series of solutions of
varying weight fraction of the molecule in a non-polar solvent.
The value is then extrapolated to infinite dilution to calculate the
dipole moment using Eq. (1).
For the determination of the excited state dipole moment μe,
the Stoke’s shift υa − υf is calculated from the absorption and
fluorescence spectra. Here υa and υf denote the absorption and
fluorescence band maxima, respectively, in cm−1
.
3. Results and discussion
In the Debye–Guggenheim method, toluene was used as
the non-polar solvent. To calculate dielectric constant, several
concentrations of the molecule in toluene was prepared and
filled between the plates of a parallel plate capacitor. Toluene
(ꢀr = 2.379) was used for the standardization of the measure-
ment. The variation of dielectric constants with weight fraction
of the solute is shown in [Fig. 3].
ꢀ
ꢁ
3ε0kT
NA
9
M
∂ꢀ
μ2g =
,
(1)
2
0
ρ
∂ω
(ε + 2)(n0 + 2)
0
0
where M is the molar mass of the molecule, ω is the weight frac-
tion, ε0, n0 and ρ0 are the dielectric constant, refractive index
and the density of pure solvent, respectively. NA is the Avogadro
constant and ε0 is the vacuum permittivity. The term (∂ꢀ/∂ω)0
represents the variation of dielectric constant with weight frac-
tion at infinite dilution.
The parameter ∂ꢀ/∂ω obtained from the graph was used in
Eq. (1) along with all other known or measured parameters
The above equation assumes that the refractive index of
a solution of the molecule has negligible dependence on its
weight fraction [8]. In the present work, the refractive index
of the solutions were measured using an Atago DRM2 refrac-
tometer and the variation with weight fraction was found
to be negligibly small, so that the equation can be safely
applied. Measurement of the capacitance was done using a
HP 4277A LCZ meter operating at a frequency of 10 kHz.
The capacitor consisted of two parallel aluminium sheets
of active area 4 cm2, clamped using teflon insulators. Mea-
surement of capacitance was carried out with varying plate
separation. A similar measurement was done using toluene
between the plates. The ratio of the slopes of the capaci-
tance with inverse of the plate separation for the two cases
give the value of the dielectric constant of the solution under
study.
Fig. 2. Absorption and fluorescence spectra of the sample in 50:50 mixture of
toluene and acetonitrile.

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1229
The excited state dipole moment can be determined by study-
ing the Stoke’s shift as a function of either the bulk solvent
polarity parameter [15,16] or the dimensionless solvent polar-
ity parameter ENT . The latter is considered superior due to the
improved correlation between the shift and the polarity value of
the corresponding solvent. Thus the method proposed by Ravi et
al. [11] was employed for the estimation of μe. In this method,
the Stoke’s shift (υa − υf) was analyzed as a function of the
polarity of the solvent. According to this, the Stoke’s shift is
related to the difference in dipole moments and the Onsager
radii of the molecule and the dye by Eq. (4).
ꢄ
ꢅ
ꢁμ2a3D
ꢁμ2Da3
υa − υf = 11307.6
ETN + Constant
(4)
ꢁμ = μe − μg is the difference between the ground and excited
state dipole moments of BBN and ꢁμD is that of Betaine dye.
In the expression, the Onsager radii of both Betaine dye (aD)
and N,N-bis(4-bromobutyl)-4-nitrobenzenamine (a) enters as a
ratio. The Onsager cavity radius (a) of the synthesized molecule
can be calculated using the density (d) and molecular weight (M)
of the molecule from the relation a3 = (3M/4πdNA) [17]. As
Fig. 3. Variation in dielectric constant with weight fraction of solute. The error
bars correspond to 3%.
to calculate the ground state dipole moment of the molecule.
The calculated ground state dipole moment value is 9.78 Debye
(3.26 × 10−29 Cm). This high value indicates that the molecule
has a tendency to align antiparallel in pairs, so that the applica-
tion of poling techniques would be required to make composites
having good βijk , values.
A binary solvent mixture comprising the polar solvent ace-
tonitrile and the non-polar solvent toluene was used for the
solvatochromic analysis. The analysis was based on the dimen-
sionless solvent polarity parameter ENT proposed by Reichardt
[12], which is given by Eq. (2).
˚
the values of aD and ꢁμD are known (6.2A and 9D, respectively
[12]), the excited state dipole moment of BBN can be evaluated
from the plot of υa − υf with ENT . The plot is shown in Fig. 5.
The value of the excited state dipole moment evaluated using
the above method is 14.95D (4.99 × 10−29) Cm.
Photorefractive materials can be used for beam amplifica-
tion in the two beam coupling geometry. NLO chromophores
for such photorefractive composites must not be absorbing at
the operating wavelength to get an effective two beam coupling
gain. The present molecule is having only low absorption in the
visible region of the spectrum. It has been shown that the length
of the alkyl spacer connecting the donor to the acceptor moiety
has major effect on the value of (βijk ) [18]. This approach of
increasing the value of β has the advantage that the absorption
maxima of the molecule is not getting changed as there is no
change in the effective conjugation length in the molecule. So
the modification of the molecule using different spacer lengths
ET(solvent) − 30.7
ENT
=
,
(2)
32.4
and ET(solvent) = 28591/λmax (nm), where λmax corre-
sponds to the peak wavelength in the red region of the
intramolecular charge transfer absorption of the pyridinium-N-
phenolate betaine dye [12]. The absorption maximum of the dye
in solvent mixtures of varying polarity was experimentally deter-
mined and used for the calculation of ENT . This reduces the error
due to the presence of impurities in the mixture as each solvent
mixture is separately analyzed for its polarity value before use.
The refractive index and the dielectric constant of the mixture
at various proportions were also measured to compute the bulk
solvent polarity parameter F(ε, n) defined [13] by Eq. (3).
ꢂ
ꢃ
(2n2 + 1) (ε − 1) (n2 − 1)
F(ε, n) =
−
,
(3)
(n2 + 2) (ε + 2) (n2 + 2)
where n and ε are the refractive index and the dielectric constant
of the mixture, respectively. The plots of ENT and F(ε, n) ver-
sus the weight fraction of Acetonitrile in the mixture is shown
in Fig. 4. It was found that both plots were nonlinear when
the percentage of toluene was higher, thus for the analysis of
the data, only solvents showing a linear variation of the polar-
ity parameters was used. Detailed physical properties of the
toluene–acetonitrile mixture is available in the literature [14].
Fig. 4. Variation of the polarity parameters with weight fraction of acetonitrile.

1230
V.C. Kishore et al. / Spectrochimica Acta Part A 70 (2008) 1227–1230
Instrument Facility, Punjab and Central Drug Research Institute,
Lucknow, India, for providing NMR and Mass spectral analysis
data in connection with this work.
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3ωe2g(μe − μg)μ2eg
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(5)
2h¯ 2(ωe2g − 4ω2)(ωe2g − ω2)
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The nonlinear optical molecule N,N-bis(4-bromobutyl)-4-
nitrobenzenamine could be synthesized. Determination of the
ground and excited state dipole moment showed that the excited
state has a high value for the dipole moment which indicated a
higher degree of charge transfer from the donor to the acceptor
moiety on excitation by light. The molecule exhibited positive
solvatochromism. The first hyperpolarizability of the molecule
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Acknowledgments
Authors would like to thank DST, Government of India,
for the support through Research Project (Sanction No. -
SR/S2/LOP-20/2003). We also thank Sophisticated Analytical