Optically transparent and thermally stable nonlinear optic chromophores featuring a thieno[2,3-b]thiophene donor

Article abstract of DOI:10.1016/j.tet.2007.07.059

A thieno[2,3-b]thiophene core has been utilized as a π-donor component to design two series of push-pull thienothiophenes by introducing various acceptor groups either via olefinic or aza-spacers. The molecules show a UV-visible cut-off wavelength below t

Full text of DOI:10.1016/j.tet.2007.07.059

Tetrahedron 63 (2007) 10011–10017
Optically transparent and thermally stable nonlinear optic
chromophores featuring a thieno[2,3-b]thiophene donor
a,*
a
Sabir H. Mashraqui, Shailesh Ghadigaonkar, Mohamed Ashraf, A. Sri Ranjini,
a
b
b
Sampa Ghosh and P. K. Das
b
a
Department of Chemistry, University of Mumbai, Vidyanagari, Santacruz-E, Mumbai 400 098, Maharashtra, India
b
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
Received 21 February 2007; revised 2 July 2007; accepted 19 July 2007
Available online 26 July 2007
Abstract—A thieno[2,3-b]thiophene core has been utilized as a p-donor component to design two series of push–pull thienothiophenes by
introducing various acceptor groups either via olefinic or aza-spacers. The molecules show a UV–visible cut-off wavelength below the second
harmonic generation (SHG) at l/2 of 532 nm, thereby conforming to the nonlinearity–transparency trade-off. Second order molecular non-
ꢁ
30
linearity, b measured by Hyper-Rayleigh scattering technique was found in the range of 9.58–47.66ꢀ10
esu, while the Kurtz powder tech-
nique produced signals of the order of 0.43–1.02 U. Thermal decomposition temperatures measured by differential scanning calorimetry
ꢃ
revealed decomposition temperaturesꢂ275 C, indicating high thermal stability.
Ó 2007 Elsevier Ltd. All rights reserved.
1
. Introduction
D–p–A chromophores, including dithienyl push–pull sys-
tems have been extensively investigated, many showing
2
1,22
Materials exhibiting large second order nonlinear optic
NLO) properties are in demand due to their potential appli-
cations in photonic devices for telecommunications and
optical information processing.
optical nonlinearity, NLO materials must also possess trans-
parency–nonlinearity trade-off and good thermal stability to
good NLO characteristics.
perpolarizability have also been reported for NLO systems
Moderate levels of first hy-
(
2
3–25
derived from thieno[3,2-b]thiophenes.
In the context
we recently de-
scribed the first application of thieno[2,3-b]thiophene in the
1
–3
26,27
While exhibiting high
of our interest in organic NLO systems,
design of a novel NLO system by incorporating this nucleus
within an unsymmetrically functionalized cyclophane.
4
–8
28
be viable for device applications. The realization in 1970s
that organic chromophores might be more versatile than
their inorganic counterparts in terms of high molecular non-
linearity, ultrafast response time, and low costs has stimu-
lated unprecedented development in the field of organic
Presently, we have extended the scope of thieno[2,3-b]thio-
phene toward the creation of push–pull structures with
a view to investigate their potential in the NLO field.
9
–11
based NLO systems.
2. Results and discussion
Although, increasing p-conjugation generally translates into
higher molecular hyperpolarizability, the advantage is often
offset by lower stability and serious compromise on optical
transparency. One of the recent strategies to contain these
drawbacks is to replace aromatics with less resonance
stabilized heteroaromatic nuclei as the p-bridges in donor–
Most known organic NLO phores utilize aromatics or hetero-
aromatic rings as the p-conjugation bridges. However, in
contrast, we decided to exploit the 10p electron rich
thieno[2,3-b]thiophene (TT) core in capacity as the donor
component and to introduce various p-electron acceptors
at the C2 position of TT through olefinic or aza-spacers to
access the corresponding push–pull chromophores, 4a–f
and 7a–d, respectively. We envisaged that the high lying
1
2–15
p–acceptor (D–p–A) systems.
The hetero-ring chro-
mophores, compared to their aromatic counterparts, not
only possess higher hyperpolarizability but they also display
1
6–20
29
better thermal stability.
In particular, thiophene based
HOMO of thienothiophene would effectively interact
with the low lying LUMO of the p-attached acceptors to
generate sufficiently polarized frameworks necessary for
exhibiting the NLO effects. Further, it was anticipated that
in the absence of an additional donor chromophore, the intra-
molecular charge transfer (ICT) transitions in 4a–f and 7a–d
should be restricted below the simple harmonic generation
Keywords: Nonlinear optics; Push–pull thieno[2,3-b]thiophenes; Synthesis;
UV–visible; First hyperpolarizability; Hyper-Rayleigh scattering tech-
nique; Kurtz powder method; Thermal decomposition.
*
Corresponding author. Tel.: +91 22 26526091; fax: +91 22 26528547;
e-mail: sh_mashraqui@yahoo.com
0
040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.07.059

10012
S. H. Mashraqui et al. / Tetrahedron 63 (2007) 10011–10017
(l/2 of 532 nm) to avoid reabsorption of the simple harmonic
generation. Herein, we describe the synthesis, optical proper-
ties, and thermal stability of 4a–f and 7a–d.
refluxing benzene. In order to enhance the NLO properties,
4e was quaternized with CH I to form the pyridinium salt 4f
in excellent yield.
3
2.1. Synthesis of push–pull thienothiophene chromo-
phores
As shown in Scheme 2, aza-coupling of diazonium salts
derived from a few acceptor amines (6a–d) with thienothio-
phene 1 smoothly led to the synthesis of deep yellow to red
aza-conjugated systems 7a–d in good yields. The successful
diazonium coupling clearly implies the high p-electron
donating ability of the thieno[2,3-b]thiophene ring and
constitutes the first report on the aza-coupling of this ring
system. The structures of target NLO phores 4a–f and 7a–d
have been fully characterized by elemental analysis
and spectral data (see Section 4).
The synthetic approaches used to synthesize the first series
of TT–p–A, 4a–f are depicted in Scheme 1. The starting
material, 2-formyl-3,4-dimethylthieno[2,3-b]thiophene 2
was readily prepared in multigram quantity by formylation
of known thieno[2,3-b]thiophene 1 using our recently de-
2
8
scribed protocol. Wittig condensation of 2 with phosphoni-
um salts 3a and 3b yielded cis/trans mixtures from which the
desired, major E isomers 4a and 4b were readily isolated by
recrystallization. Dicyanovinyl substituted TT, 4c was pre-
i) HCl / NaNO , 0°C
2
2
8
pared via the standard Knoevenagel protocol in excellent
yield and tricyanovinyl chromophore was introduced via
the lithiation protocol (treatment of 1 with n-BuLi, followed
by reaction with TCNE) to access tricyanovinyl analog 4d.
Thienothiophene chromophore 4e carrying a 4-vinylpyridyl
group was prepared in two steps by condensing 2 with 4-pic-
oline under LDA/THF conditions, followed by dehydration
of the resulting carbinol 5 with p-toluenesulfonic acid in
+ Ar-NH2
N
ii) AcOH / AcONa
S
S
S
S
N
Ar
2
6
a : Ar = -C H -4-Cl
6
4
7a : Ar = -C H -4-Cl
6
4
6b : Ar = -C H -4-NO
7b : Ar = -C H -4-NO
6
4
2
6 4 2
6
c : Ar = 2-thiazolyl
7c : Ar = 2-thiazolyl
7d : Ar = -C H -4-CO H
6d : Ar = -C H -4-CO H
6
4
2
6
4
2
Scheme 2. Synthesis of push–pull thienothiophenes 7a–d.
DMF / POCl3
CHO
1
,2-Dichloroethane
S
S
S
S
2
1
-
+
NO2 Br
(3a)
Ph3P
S
S
4
NO2
CN
K CO / MeOH,
2
3
a
-
+
CNBr
(3b)
Ph3P
CHO
K CO / MeOH,
S
S
2
3
S
S
2
4b
CH2(CN)2 / Pyridine
CN
S
S
c
NC
CN
4
NC
NC
CN
CN
CN
n-BuLi /
1
-10° C to RT
S
S
NC
4d
OH
4-Picoline / LDA
2
C H / PTSA
6 6
-10° to 0° C
N
S
S
S
S
N
5
4e
CH3I / MeCN
+
-
S
S
N CH3 I
4f
Scheme 1. Synthesis of push–pull thienothiophenes 4a–f.

S. H. Mashraqui et al. / Tetrahedron 63 (2007) 10011–10017
10013
2
.2. Electronic spectra
4
e
0
0
.8
.7
4d
4b
The UV–visible spectra of p-conjugated thieno[2,3-b]thio-
phenes, 4a–f and 7a–d recorded in CHCl3 solvent are
shown in Table 1. For illustration, UV–visible spectra for
0.6
.5
0.4
4
c
0
4a
4
a–f and 7a–d are depicted in Figures 1 and 2, respectively.
The main absorption bands for 4a–f and 7a–d are located in
the range of 367–442 nm and 382–431 nm, respectively,
depending on the nature of the p-spacer and the attached
electron acceptor groups. These absorption bands are attrib-
utable to the ICT transitions. Consistent with the increase in
the number of electron withdrawing –CN groups, the ICT
bands in the cyano series, 4b–d are red shifted in the order
4f
0
0
.3
.2
0.1
.0
0
250
300
350
400
450
500
550
4
b<4c<4d. The absorption maxima for the tricyano com-
Wavelength (nm)
pound 4d and that of the 4-nitrostyryl analog 4a appear at
nearly similar lmax (around 407 nm), thereby suggesting
their comparable electron withdrawing properties. Conver-
sion of neutral 4e to ionic N-quaternized salt 4f led to a bath-
ochromic shift of the ICT band from 367.5 nm in 4e to
3
Figure 1. UV–visible spectra of 4a–f in CHCl .
0
0
0
0
0
.7
.6
.5
.4
.3
4
p-acceptor property of the pyridinium ring relative to the
42 nm in 4f. The red shift is in accord with the increased
7a
2
7
neutral pyridyl analog. The following comparisons serve
to indicate that the p-donor ability of thieno[2,3-b]thio-
phene ring is superior than that of the thiophene ring. For
instance, the lmax for 4e and 4f appear at longer wave-
lengths (367 and 442 nm, see Table 1), whereas for the
case of known thiophene analogs, 1-(2-thienyl)-2-(4-pyri-
dyl)ethene and its N-methylpyridinium salt, the absorption
maxima are reported at lower wavelengths at 332 and
7
7
d
b
7c
0.2
0.1
3
0
3
77 nm, respectively.
3
1
0.0
In accord with the literature report, available comparisons
in the TT chromophores indicate that the aza-bridge seems
to offer a better charge redistribution than the olefinic
bridge. Thus, the transition energy for aza-nitrophenyl
system 7b is found to be lower (lmax at 422 nm) than that
observed for the corresponding nitrostyryl analog 4a (lmax
2
50
300
350
400
Wavelength (nm)
450
500
550
Figure 2. UV–visible spectra of 7a–d in CHCl3.
4
07 nm). Also, the excitation energy for 7a, carrying
2.3. Molecular hyperpolarizability and thermal stability
a weakly electron withdrawing 4-chlorophenyl group is
lower (lmax at 382 nm) than that of 4b, possessing a rela-
tively stronger acceptor, 4-cyanophenyl chromophore
The first hyperpolarizability b of the presently synthesized
‘push–pull’ thienothiophenes was experimentally deter-
mined using the Hyper-Rayleigh scattering (HRS) technique
(
(
lmax at 371 nm). It is worth noting that no absorption
lcut-off at 520 nm) is discernible in the region of SHG at
in CHCl
length of 1064 nm by applying the external reference
solution using the fundamental excitation wave-
3
5
32 nm for molecules 5a–f and 7a–d (see Figs. 1 and 2). Ac-
3
2
cordingly, these molecules fairly well conform to the trans-
parency–nonlinearity trade-off.
method using p-nitroaniline as the reference standard.
The b values were derived using the method reported by
d
Table 1. UV–visible, first hyperpolarizability b, static hyperpolarizability b(0), Kurtz powder data, and thermal decomposition T of 4a–f and 7a–d
a
ꢁ30
b(0)ꢀ10ꢁ30 esu
Compounds
CHCl
max (nm)
3
b ꢀ10
esu
Kurtz powder
efficiency (U)
Thermal stability
ꢃ
b
l
T
d
( C)
4
4
4
4
4
4
7
7
7
7
a
407
39.59
9.59
9.58
44.03
10.05
29.01
34.89
40.33
46.86
47.66
14.01
4.31
3.82
15.58
4.62
7.17
13.59
10.44
13.32
17.33
0.57
0.49
1.03
0.78
0.68
0.65
0.43
0.56
0.54
0.70
345
389
325
370
275
363
371
285
435
310
b
371.5
380.5
408
367.5
442
382
422
431
404
2
7
c
d
e
f
a
b
c
d
a
In the external reference method, the measured b values were calibrated against p-nitroaniline (17.8ꢀ10ꢁ30 esu) in chloroform.
b
The values of Kurtz powder method are with respect to urea as 1 U.

10014
S. H. Mashraqui et al. / Tetrahedron 63 (2007) 10011–10017
3
3
Parsoons et al., whereas b(0) were calculated by using
TT NLO phores were measured by differential scanning
calorimetry and the results are cited in Table 1. Interestingly,
3
4
a two state model. Hyperpolarizability values together
with data on powdered SHG efficiency measured by the
Kurtz powder method are summarized in Table 1. 4-Cyanos-
tyryl-TT, 4b and dicyanovinyl-TT, 4c both showed moder-
ate b values of around 9.5ꢀ10
c, the tricyanovinyl chromophore, 4d, in accord with the
powerful acceptor property of tricyanovinyl chromophore,
all the TT chromophores investigated revealed high thermal
stability and T ’s are higher than 275 C. The thermal stabil-
ꢃ
ity of majority of TT NLO phores are superior to that
d
ꢁ
30
esu. In contrast to 4b or
reported for the well-known NLO prototype, DANS
ꢃ
39
4
(T ¼290 C).
d
ꢁ
30
displayed nearly four times higher b of 44.03ꢀ10 esu.
As expected, the 4-nitrostyryl-TT, 4a having a similar
3. Conclusion
l
1
as that of 4d, exhibited a comparable b of 39.5ꢀ
esu. The first hyperpolarizability b for the pyridyl-
max
30
ꢁ
0
Push–pull chromophores 4a–f and 7a–d bearing thieno[2,3-
b]thiophene motif as the donor component and p-deficient
aromatics/heteroaromatic as the acceptors have been de-
signed using straightforward synthetic protocols. It is note-
worthy that despite the absence of any additional donor
group, some of the TT NLO phores exhibit substantial
ꢁ
30
TT, 4e was found to be 10ꢀ10
esu, which increased
esu in the corresponding
ꢁ30
almost three times to 29ꢀ10
N-alkylpyridinium salt 4f. This result clearly reflects the
higher acceptor strength of the pyridinium ring vis- ꢀa -vis
the neutral pyridine ring. The static hyperpolarizability
b(0), being a more reliable measure of nonlinear response
ꢁ
30
b up to 48ꢀ10
esu. In accord with our expectations, the
ꢁ
30
was found to be the highest for 4a and 4d (14ꢀ10
and
esu, Table 1) carrying 4-nitrostyryl and tricya-
absence of any additional donor in the TT NLO phores leads
to anticipated restriction of the lICT below the second har-
monic generation, a feature, which is of key importance
for device fabrications. Despite the first hyperpolarizability
values for TT NLO phores being modest by current stan-
dard, their optical transparency and high thermal stability
render push–pull thienothiophenes potentially interesting
materials in NLO applications.
ꢁ
30
1
novinyl chromophores, respectively.
5.6ꢀ10
Among the aza-TT systems, 7a–d, b was found to be in the
range of 34.8–47.7ꢀ10
ꢁ30
esu. With respect to the p-at-
tached acceptor chromophores, the b in the above series fol-
lowed the order 4-Cl–Ph<4-NO–Ph <thiazole<4-HO CPh.
2
Interestingly, the highest b in the aza-TT series is recorded
for the case of 7d although its lmax is lower (404 nm) com-
pared to those of 7b and 7c (422 and 431 nm, respectively).
This observation implies that factor(s) other than molecular
polarizability contribute in part to the nonlinear optic effect.
Consistent with the higher polarizability of the aza-push–
pull systems over the olefinic analogs, we find b for aza-
TT systems, 7a–d to be comparable or better than those
measured for the olefinic-TT, 4a–f. In terms of b(0), NLO
4
. Experimental
4
.1. General
3
5
The chemicals and spectral grade solvents were purchased
from S/D. Fine Chemicals (India) and used as received. IR
spectra were recorded on a Shimadzu FTIR-420 spectropho-
ꢁ
30
phores, 7d showed the highest value of 17.33ꢀ10
esu.
It is worth noting that for a number of TT-based NLO
systems, the b values are substantially higher (up to
1
tometer. H NMR spectra were scanned in CDCl using
3
Bruker 300 MHz spectrometer with TMS as an internal stan-
dard. Coupling constants J are given in hertz. Elemental
analyses were done on Carlo Ebra instrument EA-1108
Elemental analyzer. UV–vis spectra were recorded on Jasco
V-530 UV–vis spectrophotometer. The b values were de-
rived using the Hyper-Rayleigh scattering (HRS) technique
in CHCl solution at the fundamental excitation wavelength
3
of 1064 nm by using p-nitroaniline (17.8ꢀ10 esu in chlo-
roform) as the reference standard. The static hyperpolariz-
ꢁ
30
4
7.66ꢀ10
esu, see Table 1) compared to the well-known
esu). No photo-
decomposition of TT chromophores was noticed since the
samples recovered after repeated laser excitations showed
FTIR spectra are identical to those of the un-irradiated
samples.
ꢁ
30
NLO prototype, p-nitroaniline (17.8ꢀ10
ꢁ
30
3
3
Interestingly, all the presently synthesized systems exhibited
SHG signals by Kurtz powder test with efficiency of the
order of 0.43–1.03 U. Since, crystals suitable for single
X-ray crystal analysis could not be grown for 4a–f and
ability values b(0) were calculated by applying a two state
model as per the expression given in Eq. 1.
3
4
34
ÂÀ
ÁÀ
ÁÃ
4
2
0
2
2
0
2
b=b ¼ ðu
0
Þ = u ꢁ u u ꢁ 4u
ð1Þ
0
7
a–d, we are presently unable to comment on the structural
features responsible for the origin of macroscopic nonlinear-
ity in these systems. Despite the powder efficiency being
modest, the detection of positive SHG signals for NLO
phores 4a–f and 7a–d may indicate non-centrosymmetric
where u is the single photon absorption maximum of the
0
molecule in wave numbers, and u is the laser fundamental.
The hyperpolarizability values are uncorrected for any
possible two photon fluorescence.
3
6
orientation in the solid state. As shown in Table 1, no
apparent correlation is found between the b derived from
the HRS and SHG efficiency obtained by Kurtz test. The
absence of correlation between the two methods may not
be surprising on the ground that unlike the HRS, which is
a solution measurement, the Kurtz method, being a solid
state technique is subjected to wide variations depending
upon the nature of the space group, sizes and quality of crys-
4.1.1. Preparation of 4-nitrostyryl analog 4a. Thienothio-
phene 2-carboxaldehyde 2 (0.196 g, 1 mmol) and triphenyl
phosphonium salt 3a (0.478 g, 1 mmol) were dissolved in
dry methanol, to which anhyd K CO (1 g) was added.
The reaction mixture was refluxed for 24 h whereby the
color of the reaction mixture changed from light yellow
to red. After cooling, the reaction mixture was poured into
2
3
3
7,38
tals.
The thermal decomposition temperatures (T ) of
d

S. H. Mashraqui et al. / Tetrahedron 63 (2007) 10011–10017
10015
water and the precipitated solid filtered off, washed with
water and then crystallized from ethanol to give 4a as yellow
crystals, mp 237–239 C in 80% yield. Anal. Calcd for
(eluant: pet. ether–chloroform 30:70) to give carbinol 5 as
a brown solid, mp 179–181 C in 65% yield. Anal. Calcd
ꢃ
for C H NOS : C, 62.28; H, 5.19; N, 4.84; S, 22.15. Found
ꢃ
C H NO S : C, 60.95; H, 4.13; N, 4.44; S, 20.32. Found:
C, 60.71; H, 4.03; N, 4.40; S, 20.56. IR (KBr; n cm ): 3150,
1
5
15
2
ꢁ
1
C, 62.30; H, 5.32; N, 4.71; S, 22.15. IR (KBr; n cm ): 3200,
3100, 2950, 1600, 1560, 1420, 1410, 1300, 1070, 100, 800,
1
6
13
2 2
ꢁ
1
1
2
8
950, 1620, 1590, 1510, 1480, 1390, 1340, 1105, 940, 870,
40, 820, 750, 720, 690. H NMR (300 MHz; CDCl ): d 2.51
740. H NMR (300 MHz; CDCl ): d 2.2 (3H, s), 2.4 (3H, s),
3
1
3.1 (2H, d, J¼9.5 Hz), 5.2 (1H, t, J¼9.5 Hz), 6.8 (1H, s),
3
1
3
(
7
3H, s), 2.58 (3H, s), 6.89 (1H, s), 6.82 (1H, d, J¼16 Hz),
7.1 (2H, d, J¼6 Hz), 7.3 (1H, s), 8.4 (2H, d, J¼6 Hz).
C
NMR (100 MHz, CDCl ): d 13.061, 15.843, 45.082,
.47 (1H, d, J¼16 Hz), 7.58 (2H, d, J¼8 Hz), 8.20 (2H, d,
3
1
3
J¼8 Hz). C NMR (100 MHz, CDCl ): d 13.355, 15.910,
69.110, 122.980, 124.991, 126.614, 131.022, 135.988,
143.653, 146.321, 147.237, 149.283.
3
1
1
23.719, 124.367, 124.619, 125.183, 126.563, 131.474,
38.292, 144.012.
Carbinol 5 (0.58 mg, 0.5 mmol) was dissolved in dry benz-
ene (10 mL) containing a catalytic amount of p-toluenesul-
fonic acid (50 mg). The reaction was heated on water bath
for 3 h to effect dehydration. The initial yellow color of
the reaction turned gradually reddish indicating the forma-
tion of conjugated product 4e. The reaction was diluted
with water, basified with satd Na CO and extracted with
4
.1.2. Preparation of 4-cyanostyryl analog 4b. Thienothio-
phene 2-carboxaldehyde 2 (0.196 g, 1 mmol) and triphenyl
phosphonium salt 3b (0.458 g, 1 mmol) were reacted as
described above. The crude product was recrystallized
from ethanol to give 4b as yellow crystals, mp 196–198 C
in 75% yield. Anal. Calcd for C H NS : C, 69.15; H,
ꢃ
1
7
13
2
2
3
4
S, 21.48. IR (KBr; n cm ): 3100, 2950, 2200, 1620, 1600,
.41; N, 4.75; S, 21.69. Found: C, 68.89; H, 4.38; N, 4.61;
dichloromethane. The organic extract, after washing with
water was dried over anhyd Na SO and concentrated to
afford the product as a red solid. Purification on SiO column
ꢁ
1
2
4
1
1
NMR (300 MHz; CDCl ): d 2.50 (3H, s), 2.56 (3H, s),
420, 1410, 1390, 930, 860, 840, 810, 720, 680, 640. H
2
(elution with pet. ether–chloroform 50:50) gave 4e as a deep
ꢃ
3
6
.88 (1H, s), 6.76 (1H, d, J¼16 Hz), 7.40 (1H, d,
yellow solid, mp 189–191 C in 90% yield.
J¼16 Hz), 7.53 (2H, d, J¼8 Hz), 7.62 (2H, d, J¼8 Hz).
1
3
C NMR (100 MHz, CDCl ): d 13.209, 15.810, 110.212,
3
Anal. Calcd for C H NS : C, 66.42; H, 4.80; N, 5.17; S,
15 13
2
1
1
19.211, 123.543, 124.214, 125.037, 126.532, 128.637,
32.549, 141.860.
23.62. Found C, 66.27; H, 4.64; N, 5.33; S, 23.85. IR
(KBr; n cm ): 3150, 2900, 1620, 1590, 1550, 1480, 1420,
ꢁ1
1
1380, 1290, 1220, 1200, 1020, 990, 940, 800. H NMR
(300 MHz; CDCl ): d 2.50 (3H, s), 2.55 (3H, s), 6.88 (1H,
4.1.3. Preparation of tricyanovinyl analog 4d. A solution
of thienothiophene 1 (0.168 g, 10 mmol) in dry THF
3
s), 6.91 (1H, d, J¼16 Hz), 7.50 (1H, d, J¼16 Hz), 7.32 (2H,
ꢃ
was added under a continuous flow of N . Stirring was con-
tinued for 40 min and then tetracyanoethylene (0.128 g,
13
(
20 mL) was cooled to ꢁ10 C and n-BuLi (1.6 M, 2 mL)
d, J¼6 Hz), 8.55 (2H, d, J¼6 Hz). C NMR (100 MHz,
CDCl ): d 13.748, 15.373, 121.003, 123.437, 124.918,
3
2
125.242, 126.579, 132.785, 137.385, 145.124, 149.241.
1
mmol) dissolved in 10 mL of dry THF was added drop-
wise. Reaction mixture was stirred for 30 min and tempera-
4.1.5. Preparation of N-methyl pyridinium thienothio-
phene analog 4f. Pyridylvinyl analog 4e (0.271 g, 1 mmol)
was dissolved in dry acetonitrile (2 mL) to which an excess
of methyl iodide (0.5 mL) was added. The reaction mixture
was kept at room temperature for 48 h and the separated solid
product was filtered, washed with cold ethanol, and dried un-
ꢃ
ture was allowed to rise to 0 C. The reaction mixture was
concentrated, diluted with water, and extracted with chloro-
form. The organic layer was washed with water and dried
over anhyd Na SO . The crude product obtained upon sol-
2
4
vent removal was purified by chromatography on silica gel
eluant: pet. ether–chloroform 30:70) to afford 4d as deep
ꢃ
(
der vacuum to afford 4f as a reddish solid, mp 334 C (dec) in
quantitative yield. Anal. Calcd for C H NS I: C, 46.49; H,
ꢃ
yellow solid, mp 289–291 C in 25% yield. Anal. Calcd
for C H N S : C, 57.99; H, 2.60; N, 15.61; S, 23.79. Found:
C, 57.81; H, 2.72; N, 15.41; S, 23.91. IR (KBr; n cm ):
1
6
16
2
3.87; S, 15.49; N, 3.39; I, 30.75. Found: C, 46.21; H, 3.69; S,
15.60; N, 3.56; I, 30.70. IR (KBr; n cm ): 3100, 2950, 1640,
1
3 7 3 2
ꢁ1
ꢁ1
3
1
100, 2900, 2200, 1540, 1510, 1490, 1450, 1420, 1370,
260, 1220, 1040, 830, 760, 770. H NMR (300 MHz;
1600, 1580, 1520, 1480, 1420, 1380, 1300, 1280, 1210, 1180,
1150, 1050, 940, 820. H NMR (300 MHz; DMSO): d 2.48
1
1
1
3
CDCl ): d 2.54 (3H, s), 2.57 (3H, s), 6.96 (1H, s). C
NMR (100 MHz, CDCl ): d 16.085, 16.360, 110.160,
(3H, s), 2.68 (3H, s), 4.23 (3H, s), 6.98 (1H, d, J¼16 Hz),
3
d 7.23 (1H, s), 8.25 (1H, d, J¼16 Hz), 8.29 (2H, d,
3
1
3
1
1
12.043, 112.310, 113.271, 126.830, 132.237, 142.944,
47.825, 148.069.
J¼7 Hz), 8.78 (2H, d, J¼7 Hz). C NMR (100 MHz,
DMSO): d 13.748, 15.492, 47.542, 120.446, 123.098,
124.842, 126.776, 130.421, 131.283, 132.557, 135.908,
137.385, 145.154, 152.862.
4.1.4. Preparation of pyridylvinyl analog 4e. To a solution
of LDA (2 M in THF, 1.25 mL) was added a solution of g-pic-
ꢃ
under an atmosphere of N . The reaction mixture was stirred
oline (0.1 mL, 1 mmol) dissolved in dry THF (5 mL) at 0 C
4.2. Synthesis of aza-thienothiophenes 7a–d: typical
procedure
2
for 30 min and a solution of aldehyde 2 (0.196 g, 1 mmol) in
dry THF (10 mL) was added dropwise for 10 min. The reac-
tion mixture was allowed to warm to room temperature and
left overnight. The reaction mixture was poured onto crushed
ice and extracted with dichloromethane, washed with water,
and dried over anhyd Na SO . The crude product obtained
To concd sulfuric acid (4 mL) was added a given aromatic
amine (6a–d) (2.5 mmol) and the reaction was cooled to
0 C. A solution of sodium nitrite (206 mg, 3 mmol) pre-
ꢃ
pared in 2 mL of water was added slowly and the reaction
ꢃ
thieno[2,3-b]thiophene 1 (336 mg, 2 mmol) in 25 mL of
maintained at 0 C for 1 h. To this mixture, a solution of
2
4
on concentration was purified by silica gel chromatography

10016
S. H. Mashraqui et al. / Tetrahedron 63 (2007) 10011–10017
ꢃ
glacial acetic acid was added at 0–5 C with stirring. Sodium
acetate (1.0 g) was added and the reaction was stirred for 2 h.
The reaction mixture was then poured into water and neutral-
ized with aq sodium carbonate. The resulting precipitate was
filtered, washed with water, dried, and purified by silica gel
column chromatography giving orange to reddish solid.
2. Handbook of Optics IV, Fibre Optics and Nonlinear Optics;
Bass, M., Enoch, J. M., Stryland, E. W. V., Wolfe, W. L.,
Eds.; McGraw-Hill: New York, NY, 2001.
3. Prasad, P. N.; Ulrich, D. R. Nonlinear Optical and
Electroactive Polymers; Plenum: New York, NY, 1988.
4. Williams, D. H. Angew. Chem., Int. Ed. Engl. 1984, 23,
6
90–703.
4.2.1. Synthesis of aza-thienothiophenes 7a. Yield: 65%,
mp 180–183 C. Anal. Calcd for C H N S Cl: C, 54.81;
5. Marder, S. R.; Cheng, L. T.; Tiemann, B. G.; Friedli, A. C.;
Blanchard-desce, M.; Perry, J. W.; Skindhoj, J. Science 1994,
263, 511–514.
6. Chemla, D. S.; Zyss, J. Nonlinear Optical Properties of
Organic Molecules and Crystals; Academic: New York, NY,
1987; Vols. 1 and 2.
ꢃ
H, 3.59; N, 9.13; S, 20.88; Cl, 11.58. Found: C, 54.65; H,
1
4 11 2 2
ꢁ
1
3
2
1
.50; N, 9.01; S, 20.78; Cl, 11.50. IR (KBr; n cm ): 3092,
915, 1606, 1575, 1519, 1493, 1342, 1267, 1142, 1109,
1
065, 989, 892, 833, 730, 682. H NMR (300 MHz;
CDCl ): d 2.45 (3H, s), 2.85 (3H, s), 6.80 (1H, s), 7.30
3
7. Chen L. T.; Tam W.; Fiering A.; Rikkene G. L. J. Nonlinear
Optical Properties of Organic Nonlinear Materials, III;
Khanarian, G., Ed.; Proc. SPIE 1990; Vol. 203, p 1337.
8. Characterization Techniques and Tabulation for Organic
Nonlinear Optical Materials; Kuzyk, M. G., Dirk, C. W.,
Eds.; Marcel Dekker: New York, NY, 1998.
1
3
(
(
2H, d, J¼7.80 Hz), 7.65 (2H, d, J¼7.80 Hz). C NMR
100 MHz, CDCl ): d 14.225, 15.633, 120.129, 123.991,
3
1
1
24.769, 126.772, 129.199, 129.577, 130.566, 132.508,
36.686.
4
mp 205–207 C. Anal. Calcd for C H N O S : C, 52.99;
.2.2. Synthesis of aza-thienothiophenes 7b. Yield: 69%,
ꢃ
9. Bahl, A.; Grahn, W.; Stadler, S.; Feiner, F.; Bourhii, G.;
Brauchle, C. H. R.; Reisner, A.; Jones, P. G. Angew. Chem.,
Int. Ed. Engl. 1995, 107, 1587–1590.
10. Lambert, C.; Schmalzlin, E.; Meer Holz, K.; Brauchle, C. H. R.
Chem.—Eur. J. 1998, 4, 512–514.
1
4 11 3 2 2
H, 3.47; N, 13.25; S, 20.19. Found: C, 52.78; H, 3.40; N,
ꢁ
1
1
1
3.02; S, 20.02. IR (KBr; n cm ): 3095, 2920, 1588,
518, 1324, 1149, 1105, 879, 857, 776, 752. H NMR
1
(
300 MHz; CDCl ): d 2.52 (3H, s), 2.80 (s, 3H), 6.75 (1H,
3
11. Breitung, E. M.; Shu, C. F.; Mcmahon, R. J. J. Am. Chem. Soc.
2000, 122, 1154–1160.
12. Varanasi, P. R.; Jen, A. K. Y.; Chandrasekhar, J.; Namboothiri,
I. N. N.; Rathna, A. J. Am. Chem. Soc. 1996, 118, 12443–
1
3
s), 7.21 (2H, d, J¼8.0 Hz), 7.80 (2H, d, J¼8.0 Hz). C
NMR (100 MHz, CDCl ): d 14.234, 15.639, 120.218,
1
3
27.182, 129.681, 132.583, 136.789.
1
2448.
4.2.3. Synthesis of aza-thienothiophenes 7c. Yield: 70%,
mp 168–171 C. Anal. Calcd for C H N S : C, 47.31; H,
13. Rao, V. P.; Cai, Y. M.; Jen, A. K. Y. J. Chem. Soc., Chem.
Commun. 1994, 1689–1690.
14. Morley, J. O. J. Chem. Soc., Faraday Trans. 1991, 18, 3009–
3013.
15. Carella, A.; Castaldo, A.; Centore, R.; Fort, A.; Sirigu, A.;
Tuzi, A. J. Chem. Soc., Perkin Trans 2 2002, 1791–1795.
16. Raimundo, J. M.; Blanchard, P.; Ledoux-Rak, I.; Hierie, R.;
Michaux, L.; Roncali, J. Chem. Commmun. 2000, 1597–1598.
17. Jen, A. K. Y.; Rao, V. P.; Dorst, K. J.; Wong, K. Y.; Cava, M. P.
J. Chem. Soc., Chem. Commun. 1994, 2057–2058.
ꢃ
.22; N, 15.05; S, 34.41. Found: C, 47.15; H, 3.10; N,
1
1 9 3 3
3
1
1
1
ꢁ
1
4.88; S, 34.24. IR (KBr; n cm ): 3094, 2920, 2800, 1601,
580, 1500, 1480, 1440, 1420, 1380, 1340, 1310, 1295,
240, 1150, 1140, 1040, 1020, 880, 870, 798, 740, 720. H
1
NMR (300 MHz; CDCl ): d 2.52 (3H, s), 2.75 (3H, s), 6.90
3
(
1H, s), 7.20 (1H, d, J¼3.9 Hz), 7.55 (1H, d, J¼3.9 Hz).
1
3
C NMR (100 MHz, CDCl ): d 13.426, 15.178, 121.873,
3
1
25.455, 132.261, 141.899, 143.792, 145.831.
1
8. Jen, A. K. Y.; Cai, Y.; Bedworth, P. V.; Marder, S. R. Adv.
Mater. 1997, 9, 132–135.
4.2.4. Synthesis of aza-thienothiophenes 7d. Yield: 60%,
mp 190–193 C. Anal. Calcd for C H N O S : C, 56.96;
ꢃ
H, 3.79; N, 8.86; S, 20.25. Found: C, 56.72; H, 3.69; N,
19. Wu, X.; Wu, J.; Liu, Y.; Jen, A. K. Y. Chem. Commun. 1999,
1391–1392.
20. He, M.; Leslie, T. M.; Sinicropi, J. A. Chem. Mater. 2002, 14,
4662–4668.
1
5 12 2 2 2
ꢁ
1
8
1
.76; S, 20.05. IR (KBr; n cm ): 3466, 2920, 1682, 1601,
575, 1417, 1391, 1343, 1285, 1144, 862, 797, 773. H
1
NMR (300 MHz; DMSO): d 2.40 (3H, s), 2.60 (3H, s), 6.80
(
21. Raimundo, J. M.; Blanchard, P.; Gallego-Planas, N.; Mercier,
N.; Ledoux-Rak, I.; Hierie, R.; Roncali, J. J. Org. Chem.
2002, 67, 205–207.
22. Zhang, J. X.; Dubois, P.; Jerome, R. J. Chem. Soc., Perkin Trans
2 1997, 1209–1216.
1H, s), 7.05 (2H, d, J¼7.9 Hz), 7.85 (2H, d, J¼7.9 Hz),
1
3
1
1
1
2.80 (1H, s). C NMR (100 MHz, CDCl ): d 13.334,
3
5.212, 122.177, 124.742, 130.542, 131.643, 132.173,
39.647, 142.281, 145.563, 154.045, 154.995, 166.626.
2
3. Prim, D.; Krisch, G. J. Chem. Soc., Perkin Trans 1 1994, 2603–
606.
2
Acknowledgements
24. Leriche, P.; raimundo, J. M.; Turbiez, M.; Monroche, V.;
Allain, M.; Sauvage, F. C.; Roncali, J.; Frere, P.; Skabara,
P. J. J. Mater. Chem. 2003, 6, 1324–1332.
25. Blenkle, M.; Boldt, P.; Brauchle, C.; Grahn, W.; Ledoux, I.;
Nerenz, H.; Stadler, S.; Wichern, J.; Zyss, J. J. Chem. Soc.,
Perkin Trans 2 1996, 1377–1384.
S.H.M. and P.K.D. thank the C.S.I.R., New Delhi for a re-
search grant. Thanks are also due to Prof. A. Q. Contractor,
I.I.T, Mumbai for providing the thermal stability data.
2
6. Mashraqui, S. H.; Mistry, H.; Subramanian Sundaram, J.
Heterocycl. Chem. 2006, 43, 917–923.
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. Molecular Nonlinear Optics: Materials, Physics and Devices;
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