Oxygen bridged nitroanilines for quadratic nonlinear optics

Article abstract of DOI:10.1016/S0022-2860(02)00538-0

Several nitro aromatic compounds bridged by an oxygen atom have been synthesized and their linear and nonlinear optical properties have been investigated. In one of the compounds (1), a powder SHG efficiency of 6.2 times of urea was observed while its absorption lies in the UV region. The highest molecular hyperpolarizability β, measured was 230 x 10^-30 for a compound (8f) with increased conjugation. Thermal stability of these compounds has been checked using differential scanning calorimetry and the decomposition temperature (T_d) was found to be high and lying between 266-298 °C. These molecules have potential importance as thermally stable, visible-transparent second order NLO materials.

Full text of DOI:10.1016/S0022-2860(02)00538-0

Journal of Molecular Structure 645 (2003) 51–59
www.elsevier.com/locate/molstruc
Oxygen bridged nitroanilines for quadratic nonlinear optics
Ramanathan Sudharsanama, Srinivasan Chandrasekarana,1, PuspenduKumar Dasb,
*
aDepartment of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India
bDepartment of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
Received 22 July 2002; accepted 2 October 2002
Abstract
Several nitro aromatic compounds bridged by an oxygen atom have been synthesized and their linear and nonlinear optical
properties have been investigated. In one of the compounds (1), a powder SHG efficiency of 6.2 times of urea was observed
while its absorption lies in the UV region. The highest molecular hyperpolarizability b, measured was 230 £ 10230 for a
compound (8f) with increased conjugation. Thermal stability of these compounds has been checked using differential scanning
calorimetry and the decomposition temperature (Td) was found to be high and lying between 266–298 8C. These molecules
have potential importance as thermally stable, visible-transparent second order NLO materials.
q 2002 Elsevier Science B.V. All rights reserved.
Keywords: Nitro aromatic compounds; Azo compounds; Ether linkage; Nonlinear Optics; Second harmonic generation
1. Introduction
NLO response. The magnitude of the response can
easily be altered by changing the donor, acceptor or
the p-backbone. Although large molecular hyperpo-
larizability, b has been realized in some organic
compounds, a large number of them crystallize in a
centrosymmetric fashion in the solid state in which
case the crystals do not exhibit macroscopic second
order nonlinearity, x (2). In an alternate strategy, a
bulky group such as an alkyl group has been
introduced in a donor acceptor aromatic compound,
e.g. para-nitroaniline [3–7] or nitropyridine [8] to
achieve noncentrosymmetric packing in the solid
state. In fact, such attempts have met with a fair
amount of success in identifying functional groups
capable of inducing polar crystal structure. Inclusion
complexes containing a guest NLO chromophore
inside the cavity of a host such as, molecular sieves
[9], cyclodextrin [10], were developed to achieve
Over the past decade, due to its applications in
optical communication and processing [1], quadratic
nonlinear optics has attracted interest of a large
number of researchers from all over the world in the
search of molecules as well as materials with large
microscopic and macroscopic quadratic nonlinear
optical (NLO) response. Considerable amount of
progress has been made in finding new organic
molecules with large second order polarizability,
b[2–4]. Dipolar molecules containing a donor and
an acceptor across a p-conjugation bridge, have
generally been recognized for large second order
*
Corresponding author. Fax: þ91-80-360-1552.
E-mail address: pkdas@ipc.iisc.ernet.in (P.K. Das).
Honorary Professor, Jawaharlal Nehru Centre for Advanced
1
Scientific Research, Jakkur, Bangalore.
0022-2860/03/$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(02)00538-0

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R. Sudharsanam et al. / Journal of Molecular Structure 645 (2003) 51–59
Fig. 1. Oxygen atom bridged nitrobenzenes derivatives.
acentric space groups in the solid state. ‘Lambda-
shaped’ and ‘roof-shaped’ [11] molecules in which
two independent NLO chromophores have been
locked in a specific arrangement, have also been
tested. For example, N,N0-bis(4-nitrophenyl) metha-
nediammine (NMDA) in which two nitroanilines have
been connected by a methylene bridge tend to adopt a
noncentrosymmetric structure.
chromatography was done using TLC grade Acme
silicagel. Melting points were measured using a Buchi
B-540 instrument; spectroscopic measurements were
made in the following instruments; UV–Vis: Hitachi
U-3400; FT-IR; Perkin–Elmer-781; 1H: JEOL
300 MHz, BRUKER 200 MHz (NMR chemical shifts
are reported in d values in parts per million down field
from the internal reference tetramethyl silane);
LRMS: JEOL JMS-DX303 (only principal molecular
fragments are reported). Elemental analysis were
performed using a Carlo Erba elemental analyser
model—1106. Monomers: N,N-Dimethyl-3-nitroani-
line [12] 9c, 3-nitrophenylpiperidine [13] 9e, 4-(3-
nitrophenylazo)-N,N-dimethylaniline [14] 9f, were
synthesized according to literature procedures.
In this paper, we present a strategy, wherein two
NLO moieties have been linked by an ether linkage.
We have taken simple chromophores such as nitro-
benzenes and paranitroanilines to arrive at structures 1
and 2, respectively (Fig. 1). Steric interaction between
the two nitrobenzene moities would force the molecule
to be nonplanar and thus may lead to a noncentrosym-
metric packing in the solid state.
2.1.1. Bis(2-acetamido-4-nitrophenyl) ether (8a)
This compound was synthesized according to
procedure reported by Randall et al. [15]. Bis(2-
acetamidophenyl) ether (7) (26 g, 92.0 mmol) was
dissolved in hot acetic acid (39 ml) and poured slowly
into cold concentrated sulfuric acid (60 ml). A
nitrating mixture of concentrated nitric acid (18 ml)
and concentrated sulfuric acid (24 ml) was slowly
added while the temperature was maintained below
10 8C. The reaction mixture was then allowed to warm
up to room temperature and poured over ice. The
product was filtered, washed with water and dried.
The product was recrystallized from methyl cello-
solve to yield 8a as yellow crystals. Yield: 35.4 g
(55%); mp: 261 8C, lit. [15] mp: 258 8C; IR (nujol,
cm21): 3400, 1680, 1530, 1340.
2. Experimental
2.1. Synthesis
Para-nitrofluorobenzene 10 and para-nitrophenol
11 were obtained from commercial sources. Com-
mercial grade solvents were distilled prior to use.
DMF was shaken with NaOH and distilled over CaO.
Toluene was distilled with sodium/benzophenone and
stored over sodium wire. DMSO was dried by
standing over CaSO4 overnight and filtered. The
filtered solvent was fractionally distilled over CaH2.
n-butyl amine was dried over KOH. Analytical thin
layer chromatography was carried out on Merck
precoated silicagel 60F-254, 0.25 mm glass plates.
Visualisation of spots was achieved by one or more of
the following techniques: (a) UV illumination, (b)
exposure to iodine, or, (c) immersion of the plate in a
10% solution of phosphomolybdic acid in ethanol
followed by heating to ,200 8C. Column chroma-
tography was carried out using 60–120 and 100–200
mesh Acme silicagel and basic alumina. Flash
This compound originally thought to have the
structure 2a, turned out to have the structure 8a (based
on single crystal X-ray diffraction).
2.1.2. Bis(2-amino-4-nitrophenyl) ether (8b)
The mixture of bis(2-acetamido-4-nitrophenyl)
ether (8a) (8.1 g, 0.021 mol) and 70% (w/w) sulphuric
acid (40 ml) was heated at reflux for 20–30 min. or

R. Sudharsanam et al. / Journal of Molecular Structure 645 (2003) 51–59
53
until the test sample remains clear upon dilution with
2–3 times of its volume of water. The hot solution
was then poured into cold water (500 ml) and a
precipitate of the bis(2-amino-5-nitrophenyl) ether
(8b) was obtained by adding an excess of 10% sodium
hydroxide solution or concentrated ammonia solution.
The mixture was cooled in ice water. The yellow
crystalline precipitate was filtered at the pump,
washed thoroughly with water and dried. The product
was recrystallised three times from methyl cellosolve
to give golden crystals of 8b. Yield: 5.7 g (95%), mp:
245–248 8C (decomp.), lit. [15] mp 248–250 8C; IR
(nujol, cm21): 3490–3440, 3390–3370, 1510, 1340;
1H NMR (CDCl3, 200 MHz): d 7.71 (d, J ¼ 2.6 Hz, 2
H, Ar H), 7.60 (dd, J ¼ 9 Hz, J ¼ 2.7 Hz, 2 H, Ar H)
6.64 (d, J ¼ 4.8 Hz, 2 H, Ar H), 4.21 (s, 2 H, NH2).
2.1.4. Bis(N-butylamino-4-nitrophenyl) ether (8d)
A solution of the n-butaraldehyde (0.063 ml,
0.69 mmol) in 2-methoxy ethanol/THF (1.6 ml; 3:2)
and 3 M sulfuric acid (1.63 ml, 4.8 mmol) was added
dropwise to a conical flask containing bis(2-amino-5-
nitrophenyl) ether (8b) (0.1 g, 0.344 mmol) in
2-methoxy ethanol/THF (5 ml, 2:1). While adding n-
butaraldehyde solution, powdered NaBH4 (approxi-
mately 0.05 g, 1.3 mmol) was added continuously as
it was consumed under vigorous magnetic stirring at
0 8C. The mixture was then allowed to warm up to RT
over 90 min, diluted with water (2 ml) and made
strongly alkaline with sodium hydroxide pellets
(cooling). The basic solution was extracted with t-
butylmethyl ether (3 £ 10 ml). The organic extract
was dried over anhydrous sodium sulfate and solvent
was removed by distillation. The product (8d) from
the crude residue was chromotographed on silicagel
column using mixtures of petroleum ether and EtOAc.
Yield: 0.020 g (15%), mp: 79.1 8C, IR (film, cm21):
3432, 3020, 2961, 2932, 2874, 1625, 1578, 1531and
1343; 1H NMR (CDCl3, 200 MHz):d 7.52 (br, m, 4 H,
Ar H) 6.69 (d, J ¼ 8.4 Hz, 2 H, Ar H), 4.38 (t, 2 H,
NH), 3.22 (br, m, 4 H, CH2), 1.70 (br, m, 4 H, CH2),
1.4 (br, m, 4H, CH2), 0.94 (br, m, 6H, CH3); LRMS
(m/z): 402 (Mþ), 207 (base peak).
2.1.3. Bis(2-N,N dimethylamino-4-nitrophenyl) ether
(8c)
To a stirred solution of 3 M sulfuric acid (5.74 ml,
17.24 mmol) and 40% aqueous formaldehyde (3.8 ml,
4.13 mmol) in an Erlenmeyer flask was added the
slurry of bis(2-amino-4-nitrophenyl) ether (8b) (1 g,
3.4 mmol) and finely crushed sodium borohydride
(1.82 g, 4.82 mmol) in THF (48 ml) slowly by
maintaining the temperature of the reaction mixture
between 210 and 2 20 8C. After the completion of
addition, reaction mixture was made strongly alkaline
by adding solid NaOH pellets. The supernatent
solution was decanted (and saved) and the solid
mass was mixed with water (10 ml), and the resulting
solution was extracted with ether (3 £ 10 ml). The
ether extract was mixed with decanted THF solution.
The combined organic solution was washed with
saturated NaCl solution and dried over anhydrous
NaSO4. The organic solvent was evaporated and the
crude product was chromotographed on silicagel
column using petroleum ether and EtOAc mixture as
eluent to isolate compound 8c as a solid. It was
recrystallized from petroleum ether (60–80 8C) and
EtOAc mixture. Yield: 1.08 g (91%) mp: 134.7 8C. IR
(nujol, cm21): 1515, 1344, 1225, 1192; 1H NMR
(CDCl3, 200 MHz): d 7.78 (d, J ¼ 2.6 Hz, 2 H, Ar H),
7.67 (dd, J ¼ 8.9 Hz, J ¼ 2.6 Hz, 2 H, Ar H), 6.74 (d,
J ¼ 9 Hz, 2 H, Ar H), 2.84 (s, 12 H, CH3); LRMS
(m/z):346 (Mþ), 179 (base peak); Analysis (%): Calc.
for C16H18N4O5:C, 55.55; H, 5.20; N, 16.18;
Found:C, 55.59; H, 5.19; N, 16.98.
2.1.5. Bis(2-piperidino-4-nitrophenyl) ether (8e)
A solution of the glutaraldehyde (25% water
solution, 0.38 ml, 2.9 mmol) in 2-methoxy etha-
nol/THF (1.6 ml; 3:2) and 6 M sulfuric acid
(1.63 ml, 4.8 mmol) was added dropwise to a conical
flask containing bis(2-amino-5-nitrophenyl) ether
(8b) (0.1 g, 0.344 mmol) in 2-methoxy ethanol/THF
(6 ml, 2:1). While adding glutaraldehyde solution,
powdered NaBH4 (approximately 0.27 g, 7.2 mmol)
was added as soon as the former was consumed, under
vigorous magnetic stirring at 0 8C continuously. The
reaction mixture was worked up as described earlier to
yield 8e as a solid. Yield: 0.129 g (88%), mp: 116 8C;
IR (thin film, cm21): 3019, 1522, 1343, 1215, 756,
669; 1H NMR (CDCl3, 300 MHz): d 7.88 (d,
J ¼ 2.7 Hz, 2 H, Ar H), 7.80 (dd, J ¼ 8.7 Hz,
J ¼ 2.7 Hz, 2 H, Ar H), 6.85 (d, J ¼ 9 Hz, 2 H, Ar
H), 3.8 (br, m, 4 H, CH2), 1.53 (br, m, 6 H, CH2);
LRMS (m/z): 426 (Mþ), 204 (base peak); Analysis
(%: Calc. for C22H16N4O4, C, 61.99; H, 6.10; N,
13.14; Found C, 61.69; H, 6.17; N, 12.57.

54
R. Sudharsanam et al. / Journal of Molecular Structure 645 (2003) 51–59
2.1.6. 4,40-[Oxybis(3-nitro-o-phenyleneazo)]bis(N,N-
dimethyl-aniline) (8f)
This compound was synthesised from bis(2-
acetamido-4-nitrophenyl) ether (8a), according to
procedure reported by Randall et al. [15] Bis(2-
amino-4-nitrophenyl) ether (8b) (1.1 g, 3.8 mmol)
was dissolved in 5 N hydrochloric acid (10 ml) and
cooled to 0–5 8C by adding ice. A solution of sodium
nitrite (1 N, 8 ml, 8.0 mmol) was added slowly at 0–
5 8C and stirring was continued for 1 h. N,N-
dimethylaniline (1 g, 8.3 mmol) was dissolved in
dilute hydrochloric acid (10 ml) and was added in one
portion. The pH of the solution was adjusted to about
five with sodium acetate. The solid separated was
filtered, air-dried and recrystallized from toluene.
Yield:2.1 g (100%), mp: 217 8C, lit. [15] mp: 217–
219 8C; IR (film, cm21): 1590, 1520, 1340; 1H NMR
(CDCl3, 200 MHz): d 8.60 (d, J ¼ 2.8 Hz, 2 H, Ar H),
8.20 (dd, J ¼ 8.6 Hz, J ¼ 3.2 Hz, 2 H, Ar H), 7.54 (d,
J ¼ 9.2 Hz, 2 H, Ar H), 7.35 (d, J ¼ 3.4 Hz, 2 H, Ar
H), 6.59 (d, J ¼ 9.4 Hz, 2 H, Ar H), 3.01 (s, 6 H,
CH3). This compound originally thought to have the
structure 2f, turned out to have the structure 8f.
Fig. 2. ORTEP diagram of Bis(2-N,N-dimethylamino-4-nitro)ether
8c.
angle and torsion angles will be deposited in the
Cambridge Structural Data Base.
2.3. Hyperpolarizability measurements
The first hyperpolarizability, b values of all the
molecules have been measured using the hyper-
Rayleigh scattering (HRS) technique [17,18]
in solution at 1064 nm. Chloroform was used
as the solvent, and the b values were obtained
using PNA in chloroform as an external reference
(b ¼ 17 £ 10230 esu) [19].
2.1.7. Synthesis of Bis(4-Nitrophenyl) ether (12)
A stirred solution of para-fluoronitrobenzene 10
(0.5 g, 0.59 mmol), para-nitrophenol 11 (1 g,
7.18 mmol) and potassium carbonate (1.4 g) in dry
DMSO (5 ml) was heated at 140 8C for 4 h. The
solution was cooled to room temperature and was
poured into ice water (10 ml). The resulting solid was
filtered and washed well with water. The compound
was recrystallized from ethanol. Yield: 0.93 g
(100%). mp: 146 8C, lit. [16]143–144 8C; IR (KBr,
cm21): 1580, 1510, 1490, 1340 and 1240
3. Results and discussion
Synthesis of compounds 1, 2a–b has been reported
in the literature by Randall et al. [15] following the
route presented in Scheme 1. Compound 2a was
identified as the product of nitration of compound 7 and
product of hydrolysis of 2a was characterized as 2b.
Following the same procedure for the synthesis of 2a
and 2b we isolated the two compounds whose melting
points matched with those reported in the literature
[15]. A reductive alkylation [12] of alleged 2b was
carried out with formaldehyde and sodium borohy-
dride to get the N-dimethylated derivative 2c. Since
this compound was not reported in the literature
spectral and analytical data for this compound were
collected. The compound showed IR (nujol,
2.2. Crystal structure determination
X-ray data were collected in a computer controlled
Enraf-Nonius CAD-4 diffractometer with graphite-
monochromated Mo Ka radiation at 20 8C. Data were
corrected for Lorentz and polarization effects and the
position of the heavy atoms were determined by
Patterson methods using SHELXS86. The remaining
atoms were found by Fourier analysis using SHELXL-
93. Hydrogen atoms are not shown in Fig. 2. The
crystal data and experimental details are shown in the
Table 1. Atomic coordinates, bond lengths, bond
1
cm2):1515, 1344, 1225, 1192 and H NMR (CDCl3,
200 MHz): d 7.78 (d, J ¼ 2.6 Hz, 2 H, Ar H), 7.67 (dd,
J ¼ 8.9 Hz, J ¼ 2.6 Hz, 2 H, Ar H), 6.74 (d, J ¼ 9 Hz,
2 H, Ar H), 2.84. While it was not unambiguous,

R. Sudharsanam et al. / Journal of Molecular Structure 645 (2003) 51–59
55
Table 1
the spectral data of 2c could also fit in with an isomeric
structure 8c. Since this compound turned out to be
crystalline, a single crystal was grown from a solution
of ethylacetate/petroleum ether (60–80 8C) and
the crystal structure was solved by X-ray diffraction.
The ORTEP diagram of this compound is given in
Fig. 2. Surprisingly in this compound, the nitro groups
were found to be para to the bridged oxygen atom and
not to the amino groups. This is contrary to the
structure proposed by Randall et al. [15]. Thus, the
compound originally thought to be 2c turned out to
have the structure, 8c. Obviously, in the nitration of
compound 7, the actual compound isolated by Randall
et al was 8a rather than 2a. Therefore, all the
compounds thought to have structure 2 turned out to
be derivatives of 8. These structures are shown in
Scheme 2. The presence of only the meta isomer is also
supported by differential scanning calorimetry (DSC).
Only one sharp endothermic peak was obtained
corresponding to the melting of the meta isomer in
the nitrated product (mp ¼ 261 8C, the DSC thermo-
gram is shown later). Due to poor solubility of 8a and
8b in organic solvents, several attempts to grow single
crystals of these compounds failed.
Crystal data and structure refinement for 8c
Empirical formula
Formula weight
Temperature
C16H18N4O5
346.34
293(2) K
˚
Wavelength (Mo Ka)
Crystal system
0.71073 A
Triclinic
P 2 1
Space group
˚
Unit cell dimensions
a ¼ 8.947(2) A, a ¼ 65.19(3)8;
˚
b ¼ 9.936(4) A, b ¼ 81.86(2)8;
˚
c ¼ 10.808(3) A, g ¼ 77.12(2)8.
3
˚
Volume
849.0(5) A
Z
2
Density (calculated)
Absorption coefficient
F(000)
1.355 mg/m3
0.103 mm21
364
Crystal size
0.18 £ 0.2 £ 0.22 mm3
Theta range for data collection 2.08–26.978.
Index ranges
0 , ¼ h , ¼ 11, 212
, ¼ k , ¼ 12, 213
, ¼ l , ¼ 13
3943
Reflections collected
Independent reflections
3700 [R(int) ¼ 0.0187]
Completeness to theta ¼ 26.978 99.9%
Refinement method
Full-matrix least-squares on F 2
Data/restraints/parameters
Goodness-of-fit on F 2
3700/0/299
1.014
Final R indices [I . 2sigma(I )] R1 ¼ 0.0465, wR2 ¼ 0.1166
R indices (all data)
R1 ¼ 0.0816, wR2 ¼ 0.1364
Now that we have synthesized substituted diaryl
ether derivatives 8 rather than the originally planned
2, it was decided to prepare a few more derivatives in
Extinction coefficient
Largest diff. peak and hole
0.007(3)
˚
0.192 and 20.218 e A
23
Scheme 1.

56
R. Sudharsanam et al. / Journal of Molecular Structure 645 (2003) 51–59
Scheme 2.
this series and investigate their linear and nonlinear
optical properties. Each free amine in compound 8b
was monobutylated [13] by n-butyraldehyde and
sodium borohydride in the presence of 3 M sulfuric
acid to prepare compound 8d. Further, the nitrogen
atom in the free amine of compound 8b was converted
to be part of a piperidine ring by reductive alkylation
[13] using 25% glutaraldehyde and sodium borohy-
dride to give compound 8e. The piperidine ring in 8e
is one of the strongest donors employed in this study.
The conjugation length between the donor and the
acceptor was increased from amine functionality by
diazotizing the free amine in compound 8b with
sodium nitrite, 5 N HCl at 0 8C and coupled with N,N-
dimethylaniline to give compound 8f. These reactions
are given in Scheme 3. Similarly, bis(4-nitropheny-
l)ether 12 without the amino groups was synthesized
in quantitative yield by the reaction of p-fluoro
nitrobenzene 10 with p-nitrophenol 11 in DMSO at
140 8C. (Scheme 4). The linear and nonlinear optical
properties of these new sets of compounds were then
examined and compared with those of the correspond-
ing monomers.
3.1. Linear optical properties
The electronic spectra of all the compounds have
been recorded in chloroform and results are listed in
Table 2. A representative UV spectrum of the
oxygen atom bridged molecules is shown in Fig. 3.
Only one peak has been observed for nitrobenzene
at 259 nm. When two nitrobenzene units were
bridged by an oxygen atom at ortho positions as
in 1a, a new absorption band appeared at 309.8 nm.
Scheme 3.

R. Sudharsanam et al. / Journal of Molecular Structure 645 (2003) 51–59
57
Scheme 4.
However, when the nitro groups were placed para to
3.2. Powder SHG studies
the bridged oxygen atom only one band at 304 nm
was observed as seen in 12. Due to the mesomeric
(2M) effect of the nitro group the primary benzene
Compound 1 shows SHG efficiency of 6.2 times of
that of urea. This is the highest value obtained among
the oxygen atom bridged compounds studied here.
Unfortunately, several attempts to grow single crystal
of this compound were not successful. Dimers 8 show
low or no SHG when the size of the amine group is
small. Higher SHG efficiency compared to the
monomers was observed when the size of the amine
group was increased as in 8d–8f. The implication is
that steric constraints provided by large groups that
are ortho to the ether linkage, lead to noncentrosym-
metric packing arrangement. It also points to the fact
that the presence of large substituents ortho to the
bridged oxygen atom is essential for higher SHG
efficiency.
1
1La ˆ A1g transition (203 nm) shifts to the red in 1
and 12. Compound 1 has a lcutoff wavelength
(the wavelength at which the absorption becomes
zero on the longer wavelength side is taken as
lcutoff) in the UV region, which is conducive for
application. Normally three bands are observed in
the donor-acceptor meta disubstituted benzene
compounds [20,21]. All the three bands arise due
to the three primary transitions in benzene but they
are red-shifted because of substitution. However,
only two bands were resolved when the donor
strength is high as seen for 9c–f. In the case of the
dimers, 8, the lone pairs on the bridge oxygen atom
interact with the ring p-electrons and displace the
primary transitions in the benzene ring. The peak
positions are also affected by the nature of the
amine group. No correlation was observed between
the lmax’s of the dimers. This result is in sharp
contrast to what happens in 1, 4- donor–acceptor
disubstituted benzenoid compounds, where CT
exists from the donor to the acceptor via the
benzene ring. This absence of the CT band could
be due to the presence of the bridged oxygen atom,
which contributes to the CT and weak interactions
between the amine and the nitro groups positioned
in the meta-position with respect to each other. In
compounds 8f and9f, there is a pronounced red shift
in lmax, which may very well be due to an increased
conjugation between the donor and the acceptor.
3.3. First hyperpolarizability
First hyperpolarizability, b of all the compounds
weremeasuredinchloroformbytheHRStechniqueand
alsocalculatedbythesemiempirical AM1method. The
scattering signal was dispersed through a monochro-
mator and checked for two/multiphoton fluorescence
contribution. No significant fluorescence contribution
was observed for any compound. The results are listed
in Table 2. The intrinsic hyperpolarizability, b0 values
were derived from the well known two-state model
[22].Theresultsshowagoodagreementinthetrendinb
values obtained from measurements and calculations.
Among the dimers 8a–e (compounds with similar
conjugation length), 8e shows the highest

58
R. Sudharsanam et al. / Journal of Molecular Structure 645 (2003) 51–59
Table 2
Absorption maximum lmax/nm, molar extinction coefficient 1max /M21cm21 at lmax, cut-off wavelength lcut-off/nm, molecular
hyperpolarizabilities bcal, bHRS, b0 /10230 esu, calculated ground state dipole moment mcal/D, powder SHG (X U) and decomposition
temperature Td/oC for compounds, 1, 8, 9 and 12
S.No
1
l
max (nm)
1
max (102)
l
cutoff (nm)
mg (D)
b
cal (AM1)
b
HRS/10230 (esu)
bo/10230 (esu)
SHG £ U
Td (8C)
309.8
253.4
259.6
288.8
243.0
325.6
275.2
243.0
368.6
251.4
358.6
278.2
243.4
388.2
263.2
407
53
122
96
397
6.9
2.5
6.0
7.0
3.6
6.2
382
400
3.1
6.4
3.5
4.3
5.0
9.4
L
0
8a
9a
830
14.4
288
292
280
1400
15
387
8.4
1.8
17.0
9.6
0
59
169
84
8b
9b
448
443
6.1
5.9
2.8
2.8
22
11
10.6
5.3
0
0
239
17
42
1092
64
8c
9c
8d
494
511
484
6.1
6.2
6.5
4.3
3.3
8.2
16.6
19.2
26.2
6.7
6.7
0.1
0
319
12
256.6
396.2
259.2
1962
26
10.0
0.7
266
298
101
9d
8e
6.3
6.1
2.6
8.9
377.4
267.2
392.6
257.8
449.8
276.2
434.4
261.4
304.2
68
297
11
488
512
577
554
395
36
15.6
11.4
53.8
50.5
8.7
0.4
La
9e
8f
9f
12
5.5
7.6
8.8
3.1
2.9
7.6
29.2
1872
529
314
296
182
186
230
182
14.2
0.95
0.4
0
288b
24.7
1.8
values are obtained by AM1 method using the MOPAC package.
a
L ¼ liquid sample.
b
By TGA.
b ¼ 36 £ 10230 esu. This could be due to the strong
donating ability of piperidine compared to other amine
donors. The highest b ¼ 230 £ 10230 esu was
observed for compound 8f which is ascribed to the
increasedconjugationlengthbetweenthedonorandthe
acceptor. The hyperpolarizability of 8f is ca. 20%
higher than that of the monomer 9f.
3.4. Thermal stability
Thermal stability of the dimers 8a–e was checked
by measuring the decomposition temperature (Td)
under nitrogen atmosphere using scanning calorime-
try and the results are listed in Table 2. The DSC
Fig. 3. UV-visible absorption spectra of 8a and 9a in chloroform.

R. Sudharsanam et al. / Journal of Molecular Structure 645 (2003) 51–59
59
trimers of NLO chromophores show promise for large
microscopic as well as macroscopic NLO response.
Acknowledgements
We thank Prof. S. Ramakrishnan for allowing us to
use his calorimeter. We thank Dr M. Nethaji and Ms
Anitha Thomas for helping us in solving the crystal
structure. The work described here was supported by
CSIR, Govt of India.
References
Fig. 4. DSC thermograms of compounds 8a–e.
[1] M.M. Fejer, Physics Today (1994) 25.
[2] S.R. Marder, L.T. Cheng, B.G. Tiemann, J. Chem. Soc.,
Chem. Commun. (1992) 672.
thermograms of dimers 8a–e are displayed in Fig. 4.
There are two types of peaks in the thermogram. One
is an endothermic peak due to the melting of the
sample and the other is an exothermic peak corre-
sponding to the decomposition of the sample. The
decomposition temperatures for all the compounds lie
in the range 266–298 8C. This temperature range is
rather high and renders these compounds suitable for
processing at elevated temperatures. Due to evapor-
ation of the monomers 9 while heating, we could not
record Td of 9a–f.
[3] A.K-Y Jen, V.P. Rao, K.Y. Wong, K.J. Drost, J. Chem. Soc.,
Chem. Commun. (1990) 90.
[4] M. Barzoukas, M. Blanchard-Desce, D. Josse, J.-M. Lehn, J.
Zyss, Chem. Phys. (1989) 323.
[5] B.M. Levine, C.G. Bethea, C.D. Thurmond, R.T. Lynch, J.L.
Bernstein, J. Appl. Phys. 50 (1979) 2523–2986.
[6] G.F. Lipscomb, A.F. Garito, R.S. Narang, J. Chem. Phys. 75
(1981) 1509.
[7] G.F. Lipscomb, A.F. Garito, R.S. Narang, Appl. Phys. Lett. 75
(1981) 663.
[8] R. Twieg, A. Azema, K. Jain, Y.Y. Cheng, Chem. Phys. Lett.
92 (1982) 208.
[9] S.D. Cox, T.E. Gier, G.D. Stucky, J. Bierlein, J. Am. Chem.
Soc. 110 (1988) 2987.
[10] Y. Wang, D.F. Eaton, Chem. Phys. Lett. 120 (1985) 441.
[11] M. Blanchard-Desce, S.R. Marder, M. Barzoukas, Compre-
hensive Supramolecular Chemistry, first ed., 10, 1996, p. 833.
[12] A.G. Giumanini, G. Chiavari, M.M. Musiani, P. Rossi,
Synthesis (1980) 743.
4. Conclusion
In summary, we have synthesized a series of
oxygen atom bridged meta nitroaniline dimers 8
which were originally thought to have structure 2. The
linear and nonlinear optical properties of these
compounds have been measured and compared with
those of the meta monomers 9. The highest b value,
230 £ 10230 esu was obtained for compound 8f and
this value is ca. 20% higher than that of the monomer
9f. The SHG efficiency of compound 1 is about 6.2
times that of urea, while the compound remains
transparent in the visible region. Some of the oxygen-
bridged dimers show relatively higher SHG activity
than the monomers 9. The results presented here are
encouraging and oxygen atom bridged dimers and
[13] A. Verardo, G. Giumanini, G. Favret, P. Strazzolini, Synthesis
(1991) 447.
[14] F. Gerson, E. Heilbronner, Helv. Chim. Acta 45 (1962) 42.
[15] J.J. Randall, C.E. Lewis, P.M. Slagan, J. Org. Chem. 27 (1962)
4098.
[16] M.J. Rarick, R.Q. Brewster, F.B. Dains, J. Am. Chem. Soc. 55
(1933) 1289.
[17] K. Clays, A. Persoons, Phys. Rev. Lett. 66 (1991) 2980.
[18] P.C. Ray, P.K. Das, J. Phys. Chem. 99 (1995) 14414.
[19] T. Kodaira, A. Watanabe, O. Ito, M. Matsuda, K. Clays, A.
Persoons, J. Chem. Soc. Faraday Trans. 93 (1997) 3039.
[20] H.E. Ungnade, J. Am. Chem. Soc. 76 (1984) 5133.
[21] L. Doub, J.M. Vandenbelt, J. Am. Chem. Soc. 77 (1955)
4535.
[22] J.L. Oudar, D.S. Chemla, J. Chem. Phys. 66 (1977) 2664.