Push-pull bithiophene azo-chromophores bearing thiazole and benzothiazole acceptor moieties: Synthesis and evaluation of their redox and nonlinear optical properties

Article abstract of DOI:10.1016/j.dyepig.2011.05.007

Three series of bithiophene azo dyes functionalized with thiazol-2-yl or benzothiazol-2-yl-diazene acceptor moieties were synthesized through azo coupling reaction using 2,2′-bithiophene, 5-alkoxy-2,2′- bithiophenes, 5-N,N-dialkylamino-2,2′-bithiophenes a

Full text of DOI:10.1016/j.dyepig.2011.05.007

Dyes and Pigments 91 (2011) 454e465
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Dyes and Pigments
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Pushepull bithiophene azo-chromophores bearing thiazole and benzothiazole
acceptor moieties: Synthesis and evaluation of their redox and nonlinear
optical properties
,
a
b
a
M. Manuela M. Raposo a , M. Cidália R. Castro , M. Belsley , A. Maurício C. Fonseca
*
a Center of Chemistry, University of Minho, Campus of Gualtar, 4710-057 Braga, Portugal
b Center of Physics, University of Minho, Campus of Gualtar, 4710-057 Braga, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Three series of bithiophene azo dyes functionalized with thiazol-2-yl or benzothiazol-2-yl-diazene
acceptor moieties were synthesized through azo coupling reaction using 2,20-bithiophene, 5-alkoxy-2,20-
bithiophenes, 5-N,N-dialkylamino-2,20-bithiophenes and thiazolyl- and benzothiazolyl diazonium salts
as coupling components. The 5-alkoxy-2,20-bithiophene precursors yielded the 5-thiazolylazo-50-alkoxy-
2,20-bithiophenes, while the azo coupling reaction of 5-N,N-dialkylamino-2,20-bithiophenes with the
thiazolyl diazonium salt gave 4-thiazolyl-azo-5-N,N-dialkylamino-2,20-bithiophenes. A different reac-
tivity behavior was observed for 2,2-bithiophene with thiazolyl diazonium salts which gave rise to the
expected azo dyes together with several arylation products. The redox behavior, thermal stability, and
the first hyperpolarizability of the novel chromophores were evaluated through cyclic voltammetry,
thermogravimetric analysis (TGA) and hyper-Rayleigh scattering (HRS) respectively. By varying the
position of the thiazolyldiazene acceptor group on the bithiophene system, the electrochemical behavior
Received 15 March 2011
Received in revised form
2 May 2011
Accepted 10 May 2011
Available online 27 May 2011
Dedicated to the Centenary of the Portu-
guese Chemical Society.
Keywords:
Heterocyclic azo dyes
Bithiophene
as well as the optical (linear and nonlinear) properties of the donoreacceptor p-conjugated systems can
(Benzo)thiazole
readily be tuned. Pushepull 5-thiazolylazo-50-alkoxy-2,20-bithiophenes exhibit the most promising
redox and the solvatochromic properties and second-order nonlinear optical response. The redox and the
optical properties also show notable variations for the different heterocyclic spacers and were also
sensitive to the electronic acceptor strength of the (benzo)thiazolyldiazene moieties.
Ó 2011 Elsevier Ltd. All rights reserved.
Auxiliary donor/acceptor heterocycles
Hyper-Rayleigh scattering (HRS)
Nonlinear optical (NLO) materials
1. Introduction
imidazole have been investigated as diazo components for
a potential use in optical data storage materials. Diazo compounds
Heterocyclic azo dyes are a versatile class of colored compounds
that have attracted the interest of many research groups, in part
because of their multiple industrial applications in the fields of
textiles, papers, leather, additives, foodstuffs, cosmetics, holo-
graphic data storage materials, laser materials, xerography, laser
printing and as materials for organic solar cells and chemosensors
[1]. In addition, other heterocyclic azo dyes have found uses as
organic second-order nonlinear optical materials (NLO) suitable for
applications such as harmonic generation and optical switching [2].
Previously the two lowest singlet excitation energies of 18 azo
dyes have been studied by ab initio quantum-chemical methods
within the second-order polarization propagator approximation
(SOPPA) by Åstrand et al.. Various combinations of five-membered
rings such as furan, thiophene, pyrrole, oxazole, thiazole, and
with two heterocyclic five-membered rings are found to have
p
/
p* excitation energies corresponding to laser wavelengths in
the region 450e500 nm whereas one five-membered ring and
a phenyl group as diazo components have excitation wavelengths
in the 400e435 nm region. The results of these theoretical chem-
ical calculations suggest that they could be used as suitable azo
components in optical data storage materials. In particular, it was
shown that azo dyes with two five-membered rings (e.g. thiophene,
pyrrole and thiazole) as diazo components can, in principle, be
obtained with excitation energies corresponding to laser wave-
lengths considerably longer than for azobenzenes, which would be
desirable for some optical data storage devices.
Prior theoretical and experimental and studies [1,2aec] have
stimulated a strong interest in our research group to engage in the
synthesis and characterization of novel heterocyclic azo dyes
bearing thiophene, pyrrole and thiazole heterocycles. The optical
and electronic characterization of several novel chromophores
previously synthesized by us, confirmed that they possess the
* Corresponding author. Tel.: þ351 253 604381; fax: þ351 253 604382.
E-mail address: mfox@quimica.uminho.pt (M.M.M. Raposo).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.05.007

M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 454e465
455
essential characteristics necessary for use as efficient sol-
vatochromic probes, nonlinear optical and photochromic materials
[3,4], supporting the main conclusions of Åstrand et al. theoretical
studies described above.
bithienyl-aryldiazenes. In this manuscript, we report on the use
of the hyper-Rayleigh scattering technique to characterize the
second-order optical nonlinearities of some of these azo dyes and
compare them with the novel bithienyl-thiazolyldiazenes. The
thienylpyrrole azo dye 12c (Fig. 2) was also synthesized in order to
Our compounds were obtained through an azo coupling reac-
tion using bithiophene [5] or thienylpyrroles derivatives [6] and
aryl or heteroaryldiazonium salts as coupling components. We have
recently employed this synthetic methodology to obtain thie-
nylpyrrole derivatives functionalized with phenyldiazene [3a,b]
and thiazolyldiazene groups [3d], and bithiophene chromophores
bearing phenyldiazene [4aec] acceptor moieties. The electro-
chemical, thermal and optical properties of these compounds
indicate that they have good potential to be used in applications as
nonlinear optical [3d,4a,b] and photochromic materials [3c,d,4d].
In view of these facts and as a continuation of our recent studies
on the synthesis and characterization of heterocyclic azo dyes for
optical applications, we considered it worthwhile to synthesize
bithiophene derivatives functionalized with thiazolyl- and benzo-
thiazolyldiazene moieties and evaluate their electrochemical,
thermal and optical (linear and nonlinear) properties.
compare the difference of the p-conjugated bridge/donor auxiliary
effect of the bithiophene system with thienylpyrrole heterocyclic
spacer [3d] on the electronic and optical properties of these
pushepull compounds. Alternatively, thiazolyl- and benzothiazolyl
diazonium salts were chosen as coupling components for the
synthesis of heterocyclic azo dyes 4e7 having in mind that the
substitution of a phenyl group by electron-deficient 2-thiazole or
2-benzothiazole heterocycle in a donoreacceptor organic chro-
mophore significantly enhances the respective first hyper-
polarizability values,
b [8,9].
Therefore, 2,20-bithiophene 1a, 5-alkoxy-1bec and 5-N,N-dia-
lkylamino-2,20-bithiophenes 1dee [5] have been used as coupling
components together with thiazolyl 2aec and benzothiazolyl dia-
zonium salts 3 in order to prepare heterocyclic azo dyes 4e7
(Scheme 1, Table 1). Diazotation of 2-aminothiazole derivatives and
2-aminobenzothiazole with NaNO2 in HCl (6 N) at 0e5 ꢀC in water
gave rise to the corresponding diazonium salts 2e3, which reacted
immediately with bithiophenes 1aee in acetonitrile at 0 ꢀC given
thiazolyldiazenes 4e6 and benzothiazolyldiazene 7 in fair to good
yields (9e80%). Generally, higher yields were obtained in the
synthesis of thiazolyldiazenes 4e6, probably due to the higher
stability of diazonium salts 2 when compared to 3.
2. Results and discussion
2.1. Synthesis
Usually thiophene azo dyes are synthesized through azo
coupling reaction of 2-aminothiophenes with arylamines [2a,j,7],
although the difficulties of synthesizing 2-aminothiophenes are
well known. To overcome these difficulties, we have recently
The thienylpyrrole azo dye 12c (Fig. 2) was also synthesized
with yield of 61% under the same experimental conditions
described above using 1-(4-methoxyphenyl)-2-thieno-2-yl-pyrrole
11 [6] and thiazol-2-yl-diazonium salt 2c as coupling components.
A higher yield was obtained for 12c compared to 6a probably due
to the stronger donor electronic character of thienylpyrrole as
coupling component compared to 2,20-bithiophene, making it more
activated to react through an electrophilic aromatic substitution
such the azo coupling reaction. 1-(4-Methoxyphenyl)-2-thieno-2-
yl-pyrrole exhibits an analogous reactivity with thiazol-2-yl-dia-
zonium salt 2c when compared to the reaction with aryldiazonium
salts [3a,b]. As a result, the corresponding azo dye 12c was obtained
having the thiazolyldiazene moiety substituted at 5-position of the
pyrrole ring.
developed
a novel methodology of synthesis for bithienyl-
aryldiazenes using as coupling components 5-alkoxy- or 5-N,N-
dialkylamino-2,20-bithiophenes and aryldiazonium salts. In our
recent work, 5-alkoxy- and 5-N,N-dialkylamino-2,20-bithiophenes
were shown to be highly reactive with aryldiazonium salts. More-
over, we also demonstrated that the position of the electrophilic
substitution on the bithiophene moiety, depends on the electronic
nature of the group substituted on 5-position of the bithiophene
system (alkoxy or N,N-dialkylamino). For instance, azo coupling
reaction of 5-alkoxy-2,20-bithiophenes with aryldiazonium salts
gave 50-phenylazo-5-alkoxy-2,20-bithiophenes 8 [4a,b]. On the
other hand, the coupling reaction of the 5-N,N-dialkylamino-2,20-
bithiophenes under the same reaction conditions occurs in the
activated 4-position to yield 4-phenylazo-5-N,N-dialkylamino-
bithiophenes 9e10 [4c] (Fig. 1). Despite the steric hindrance, the
4-position is still favored for the electrophilic reaction, as compared
to the 50-position.
The 5-alkoxy- and 5-N,N-dialkylamino-2,20-bithiophenes also
exhibit a similar reactivity in azo coupling reactions with thiazolyl-
and benzothiazolyl diazoniumsalts when compared to aryldiazonium
salts [4a,b]. In effect, azo coupling of 5-alkoxy-2,20-bithiophenes 1bec
with thiazolyl diazonium salts 2aeb gave 50-thiazolylazo-5-alkoxy-
2,20-bithiophenes 4bec and 5bec and azo coupling reaction of
Having in mind these earlier results we were motivated to study
the reactivity of the azo coupling reaction of 5-alkoxy-, 5-N,N-
dialkyamino-bithiophenes and also the unsubstituted bithiophene
with heterocyclic thiazolyl- and benzothiazolyl diazonium salts.
In previous studies, we have synthesized and studied the sol-
vatochromic and the redox properties of 5-N,N-dialkyamino
R2
3´ 4´
N
S
N
4
N
S
S
N
5´´
3´´ 4´´
S
N
S
R2
13 R2 = H
15a R2 = CHO
12a,12c
R2 a = H; c = CHO
OMe
R2
N
N
CHO
R2
S
S
S
S
N
N
S
R1
R1
N
N
S
S
N
S
S
N
CH3
S
9d R1 = NEt2; R2 = CN
N
S
N
8 R1 = MeO, EtO; R2 = 3-CO2H, 4-CN, 4-NO2
S
CHO
10e R1 = Piperidino; R2 = CN
15b
14
CH3
Fig. 1. Structure of 50-phenylazo-5-alkoxy-2,20-bithiophenes 8 [4a,b] and 4-phenylazo-
5-N,N-dialkylamino-bithiophenes 9d and 10e [4c].
Fig. 2. Structure of azo dyes 12a [3d] and 12c and arylation products 13e15.

456
M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 454e465
4
N
N
R2
+N2
N
S
S
+N2
4
N
S
N
3´
5
R2
N
S
S
S
6
N
2a
3
R1
R1
5´´
3´´ 4´´
R1
N
S
N
S
S
S
7
1a
1d-e
1a-e
7a
4d R1 = NEt2; R2 = H
4e R1 = Piperidino; R2 = H
+N2
4´´
1a-c
S
R2
2a-c
3´´
4
N
S
N
R1
S
N
R2
S
3´ 4´
4a R1 = R2 = H
4b R1 = MeO; R2 = H
4c R1 = EtO; R2 = H
5a R1 = H; R2 = Me
5b R1 = MeO; R2 = Me
5c R1 = EtO; R2 = Me
R1 a = H; b = MeO; c = EtO; d = NEt2; e =piperidino
R2 a = H; b = Me; c = CHO
6a R1 = H; R2 = CHO
Scheme 1. Synthesis of bithienyl diazenes 4e7 trough azo coupling reaction of 2,20-bithiophene derivatives 1aee with thiazolyl 2aec and benzothiazolyl diazonium 3 salts.
5-N,N-dialkylamino-2,20-bithiophenes 1dee with thiazolyl diazo-
nium salt 2a, gave selectively, 4-thiazolylazo-5-N,N-dialkylamino-
2,20-bithiophenes 4dee (Scheme 1). These results can be explained by
taking in account several factors such as the increase of the electron
density at the 4-position of the bithiophene moiety due to the 5-N,N-
dialkylamino-substituents keeping in mind that the thiazolyl diazo-
nium salt are not sterically a bulky species.
However an unexpected reactivity was observed for the azo
coupling reaction of 2,20-bithiophene 1a with thiazolyl diazonium
salts 2aec. This suggests that, under conditions in which thiazolyl
diazonium salts 2aec couples with 5-alkoxy- and 5-N,N-dia-
lkylamino-2,20-bithiophenes to yield the corresponding azo dyes
4bee and 5bec the same ions interact with bithiophene giving
complex reaction mixtures constituted by several deeply colored
compounds (TLC). Therefore, using the same reaction conditions
that were applied for the preparation of 5-alkoxy- and 5-N,N-dia-
lkylamino-bithiophenes (0.5 equiv. of bithiophene 1a and 1 equiv.
of diazonium salts 2aec) all the coupling reactions gave the
expected azo dyes 4a, 5a and 6a in 23,10 and 15% yields respectively,
together with the arylation products 14 (17%) and traces (3%) of 15a
(Fig. 2). The arylation compounds 14 and 15a were isolated and
completely characterized by the usual spectroscopic techniques.
In order to study the reactivity of bithiophene with thiazolyl
diazonium salts 2aec under different conditions, equimolar
amounts of both coupling components were used. In these condi-
tions more complex reaction mixtures were obtained and the yields
of the expected azo dyes 4a and 6a decreased markedly from 23 to
10% (4a) or from 15 to 4% (6a). Concurrently we also observed an
increase of the yields of the arylation products. The by-product 13
was obtained at a 5% yield in a mixture with the corresponding azo
dye 4a (10%). In the case of the azo coupling with diazonium salt 2c,
the pure azo dye 6a was obtained at 4% yield together with a frac-
tion which seems to be a mixture (1H NMR and HPLCeMS) of the
arylation products 15a and 15b in a 63% yield. An opposite behavior
was observed for azo dye 5a; in this case the yield increased from
10 to 21% together with the decrease of the arylation product 14
from 17 to 6% yield (Table 1). 1H NMR and HPLCeMS were used in
order to identify the components of the fractions that were not
possible to separate by column chromatography. The arylation
product 13 (5%) was identified in a mixture together with the
corresponding azo dye 4a (10%). On the other hand both arylation
products 15aeb were recognized in a mixture at a yield of 63%.
These unexpected arylation products 13e15 probably arise because
the same diazonium salts 2aec can interact also with the bithio-
phene with evolution of nitrogen to give the arylation products
13e15 [10a] (Fig. 2).
Table 1
Yields and Td data for bithiophene azo dyes 4e7, thienylpyrrole azo dye 12c and
arylation products 13e15.a
c
Bithiophene/
thienylpyrrole
Heterocyclic
azo dye
R1
R2
Yielda
(%)
Td
(ꢀC)
1a
1b
1c
1d
1e
1a
1b
1c
1a
1a
11
1a
1a
1a
4a
4b
4c
4d
4e
5a
5b
5c
6a
7a
12c
13
H
MeO
EtO
NEt2
Piperidino
H
MeO
EtO
H
H
H
H
H
23, 10b
36
11
80
200
200
231
d
H
10
223
241
274
214
231
283
200
e
CH3
CH3
CH3
CHO
e
10, 21b
42
9
15, 4b
H
e
9
CHO
H
CH3
CHO
61
H
H
H
5b
17, 6b
e
In fact, Tedder et al. reported earlier the arylation of thiophene
under conditions in which 2,4-dinitrobenzenediazonium salts
couple with anisole to yield the corresponding azo dye. Using
thiophene as coupling component, these investigators obtained
the arylation coupling product 5-(2,4-dinitrophenyl)thiophene
[10a]. However when the thiophene ring was substituted by
t-butyl, phenyl and dimethyl groups the azo coupling using the
same diazonium salt gave the corresponding azo-thiophenes
[10b].
14
15a and 15b
3 (15a),
e
63b (15a þ 15b)
a
Yield for azo dyes 4e7 and for arylation products 13e15 when 0.5 equiv. of
bithiophene and 1.0 equiv. of thiazolyl diazonium salt 2a were used.
b
Yield for azo dyes 4ae6a and for arylation products 13e15 when 1.0 equiv. of
bithiophene and 1.0 equiv. of thiazolyl diazonium salt 2a were used.
c
Decomposition temperature (Td) measured at a heating rate of 20 ꢀC minꢁ1
under a nitrogen atmosphere, obtained by TGA.
d
Compound 4d was obtained as an oil.

M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 454e465
457
The fact that arylation occurs with bithiophene derivatives
suggests that there is a competition between arylation and azo
coupling reactions; if the nucleus is sufficiently activated (e.g. by
alkoxy or N,N-dialkylamino groups) azo coupling is preferred.
(multiplet), 7.52 (double triplet), 7.87 (double doublet), and 8.13
(double doublet) were also detected and were attributed respec-
tively, to the 5, 6, 4 and 7-H protons in the benzothiazole ring.
For thienylpyrrolyl-azo dye 12c functionalized with a thiazo-
lyldiazene moiety on the 20-position of the pyrrole ring two
doublets at 7.00 and 7.44 ppm were detected with coupling
constants of 4.6 Hz indicating the presence of two adjacent protons
(40-H and 30-H) at the corresponding pyrrole moiety.
2.2. Electronic structure analysis
The electronic structures of the heterocyclic azo dyes were first
analyzed by 1H NMR spectroscopy (Table 2) and cyclic voltammetry
(Table 3).
The arylation products were also characterized by 1H NMR
spectra. For example, for compound 14 two doublets at 2.58 and
2.60 ppm were observed with coupling constants of 0.6 Hz which
were attributed to the 5-Me groups and two doublets were also
observed at 7.64 and 7.70 ppm with a coupling constant of 1.2 Hz
due to the two 4-H in the thiazole rings. Three double doublets
were also detected at 7.10, 7.41 and 7.44 and were attributed
respectively, to the 400, 500 and 300-H in the thiophene ring. A singlet
at 7.96 ppm was also observed due to the 30-H in the first thiophene
ring. The ESI HRMS indicates that [M þ 1]þ of 389 m/z can be
attributed to the molecular formula C16H12N4S4 (389.0019).
The study of the NMR chemical shifts in pushepull derivatives
bearing thiazole 4e6 and benzothiazole 7a electron-deficient
heterocycles can also confirm their pushepull character with
a significant intramolecular charge transfer (ICT) from the donor
bithiophene moiety to the acceptor (benzo)thiazole groups
(Table 2). In particular, the effect of the substitution of a thiazolyl
group for dye 4a by a benzothiazole heterocycle (e.g. 7a) is note-
worthy. All the protons of the bithiophene system in 7a (30-H and
40-H, and 300, 400and 500-H) were shifted to higher chemical shifts
The structures of bithiophene azo dyes 4e7 were unambiguously
confirmed by their analytical and spectral data. For example in the 1H
NMR spectrum of 500-alkoxy-5-thiazolylazo-2,20-bithiophene deriv-
atives 4bec, in CDCl3, two signals at about 6.21e6.23 and
7.11e7.12 ppm, were detected. Both signals appear as doublets with
coupling constants of 4.2 Hz indicating the presence of two adjacent
protons in a di-substituted thiophene ring. These signals were
attributed to the 400-H and 300-H protons respectively. On the other
hand in the 1H NMR spectrum of 5-N,N-dialkylamino-4-thiazolylazo-
2,20-bithiophene derivatives 4dee, in CDCl3, one signal at about
7.57e7.59 ppm was detected as a singlet indicating the presence of
only one proton in a trisubstituted thiophene ring. This signal was
attributed to the 3’-H proton. For the same bithiophene azo
dyes, 4dee, three signals at about 6.99e7.04 ppm (multiplet),
7.03e7.05 ppm (double doublet) and 7.20e7.22 ppm (double
doublet) were detected. These signals were attributed respectively, to
the 400, 300 and 500-H protons at the second thiophene ring.
In the 1H NMR spectra of benzothiazolyldiazene 7a functional-
ized with a benzothiazol-2-yl-diazene moiety on the 50-position of
the thiophene ring four additional signals at about 7.42e7.47
(e.g. 40-H and 30-H
to the corresponding thiazolyldiazene azo dye 4a (e.g. 40-H and 30-H
d
¼ 7.37 and 7.91 ppm respectively) as compared
Table 2
Chemical shifts of protons of the heterocyclic azo dyes 4e7 and 12c.a
Comp.
R1
R2
5-H
4-H
40-H
30-H
300-H
400-H
500-H
4a
4b
4c
4d
4e
5a
5b
5c
H
MeO
EtO
NEt2
Piperidino
H
MeO
EtO
H
H
H
H
H
7.32
7.34
7.75
7.03
7.98
7.95
7.95
7.75
7.77
7.66
7.64
7.63
8.52
7.87
8.35
7.81
7.75
7.33
e
7.31
7.14
7.13
7.59
7.57
7.29
7.10
7.10
7.40
7.37
7.44
7.42
7.12
7.11
7.03e7.04
7.04e7.05
7.39
7.11
7.09
7.10
6.23
6.21
7.00e7.04
6.99
7.09
6.21
6.20
7.13
7.39
e
e
7.20
7.22
7.37
e
H
7.04e7.05
e
CH3
CH3
CH3
CHO
e
e
e
e
7.75
7.70
7.70
7.94
7.91
7.00
e
6a
7a
12c
7.50
7.42e7.47
7.14
7.47
7.42e7.47
7.38
H
e
7.42e7.47
e
7.12
7.01e7.02
CHO
a
Proton NMR measurements carried out in deuterated chloroform at 400 MHz. The numbering scheme of protons is shown in Scheme 1 and Fig. 2.
Table 3
Electrochemical data for compounds 4e7 and 12c.
Compound
Reductiona
Oxidationa
1
2
c
ꢁ Epc (V)
DE
b (mV)
ꢁ Epc (V)
D
Eb (mV)
1Epa (V)
ꢁEHOMOc(eV)
ꢁELUMO (eV)
Band gapd (eV)
4a
4b
4c
4d
4e
5a
5b
5c
1.23
1.26
1.26
1.74
1.67
1.29
1.31
1.61
0.86
1.08
0.98
63
63
62
61
60
63
54
60
63
54
65
2.04
1.99
2.07
2.21
2.36
2.09
2.05
2.13
1.61
1.93
1.69
90
95
93
__
__
63
100
95
60
85
__
0.80
0.61
0.59
0.32
0.31
0.77
0.70
0.64
0.98
0.93
0.76
5.19
5.00
4.98
4.71
4.70
5.16
5.09
5.03
5.37
5.32
5.15
3.16
3.13
3.13
2.65
2.72
3.10
3.08
2.78
3.53
3.31
3.41
2.03
1.87
1.85
2.06
1.98
2.06
2.01
2.25
1.84
2.01
1.74
6a
7a
12c
a
Measurements made in dry DMF containing 1.0 mM in each compounds and 0.10 M [NBu4][BF4] as base electrolyte at a carbon working electrode with a scan rate of
0.1 V sꢁ1. All E Values are quoted in volts vs the ferrocinium/ferrocene-couple. Epc and Epa correspond to the cathodic and anodic peak potentials, respectively.
b
D
E ¼ jEred ꢁ Eoxj.
c
EHOMO ¼ ꢁ(4.39 þ Eox) (eV) and ELUMO ¼ ꢁ(Ered þ 4.39) (eV).
d
Calculated form the difference between the onset potentials for oxidation and reduction.

458
M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 454e465
d
¼ 7.31 and 7.81 ppm respectively) thus indicating a decrease of
value of the reduction potential of compounds 4d and 4e to more
negative values compared to the unsubstituted 4a and the alkoxy
derivatives 4bec (e.g. 4a, 1Epc ¼ ꢁ1.23 V; 4c, 1Epc ¼ ꢁ1.26 V and 4e;
1Epc ¼ ꢁ1.67 V). The substitution of the thiazole ring by the methyl
group has little influence on the values of reduction potentials of
compounds 5aec. In contrast, the potentials are strongly influ-
enced by the electronic acceptor nature of the formyl group. As
a consequence, the difference between the reduction potential
values obtained for the first process of 4a and 6a is 370 mV showing
a high anodic shift due to the substitution of the thiazole ring by
a stronger acceptor group (Fig. 3). Moreover, the reduction potential
of the second process of the reduction reflects also the effect of the
electronic nature of the R2 group substituted on the thiazole
heterocycle.
the electron density due to the stronger electron-withdrawing
power. That is a higher charge-demand cx, of the benzothiazole
ring [11] provokes a more efficient charge transfer from the donor
to the acceptor group.
A similar effect was observed in the chemical shifts for all
protons of the bithiophene system 6a (e.g. 40-H and 30-H
d
¼ 7.40
and 7.94 ppm respectively) bearing a thiazole ring functionalized
with a formyl group when compared to thiazolyldiazene 4a. On the
other hand azo dyes 4bec bearing electron-donating alkoxy groups
substituted on position 50 of the bithiophene system exhibit high
field signals for all protons of the bithienyl and thiazolyl moieties
demonstrating the easy electron communication within the whole
heterocyclic system.
A similar behavior was observed for 4-thiazolylazo-5-N,N-dia-
lkylamino-2,20-bithiophenes 4dee functionalized with the
stronger electron-donating N,N-dialkylamino groups substituted
on the ortho position to the thiazolyldiazene acceptor moieties
which exhibit lower chemical shifts for all protons of the bithienyl
and thiazolyl moieties. Noteworthy are the lower chemical shifts
The substitution of a thiazole heterocycle (4a) by a benzothia-
zole acceptor group (7a) on the diazene system results in a decrease
of the reduction potential while increasing the oxidation potential
suggesting a stronger electron-accepting ability of compound 7a
compared to 4a. These results are in agreement with the 1H NMR
analysis that showed increased densities for azo dye 4a and
decreased density for compound 7a.
On the other hand, comparison of compounds 6a and 12c
bearing the same acceptor moiety (5-formyl-thiazol-2-yl-diazene)
but different heterocyclic p-spacers, showed that the thie-
values for the thiazole protons (4-H and 5-H,
7.03e7.05 ppm respectively) for compounds 4dee compared to
d
¼ 7.75e7.77 and
compound 4a (4-H and 5-H
d
¼ 7.98 and 7.32 ppm respectively).
The 1H NMR analysis for thienylpyrrole azo dye 12c also
confirms its pushepull character. Moreover, the thienylpyrrole
derivative exhibit lower chemical shifts for all protons of the thie-
nylpyrrole group and also for the thiazole ring (e.g. 30-H, 40-H and 4-
nylpyrrole system (12c) has a greater donor ability compared to the
bithienyl group which results in a decrease of the oxidation
potential from 0.98 to 0.76 V. At the same time it was also observed
a cathodic shift of the reduction potential (e.g. 6a 1Epc ¼ ꢁ1.23 V and
H
d
¼ 7.00, 7.44 and 8.35 ppm respectively) compared to all
1
bithiophene azo dye 6a protons (e.g. 30-H, 40-H and 4-H,
d
¼ 7.40,
12c Epc ¼ ꢁ0.98 V) for compound 6a. These results are also in
7.94 and 8.52 ppm respectively) demonstrating its stronger elec-
tron donor character and the easy electron communication through
the entire conjugated system.
agreement with the reactivity and the 1H NMR analysis which
showed increasing electronic densities for the thienylpyrrole
protons compared to the bithienyl protons.
All bithienyl-thiazolyldiazenes 4e7 showed a donoreacceptor
character with reversible reduction and irreversible oxidation
processes. The irreversible oxidation process is associated with the
oxidation of the thiophene moiety [3a,12] and the two reduction
process are associated with the reduction of heterocyclic azo
moiety. These values are comparable with those reported previ-
ously for other similar compounds [13,14].
Results show that the electronic nature of substituent group
(R1 ¼ H, alkoxy, N,N-dialkylamino) in bithiophene moiety has
a significant influence on the value of oxidation potential of this
system. The substitution of the bithiophene system by stronger
electron-donating groups such as N,N-dialkylamino leads to
a significant shift of the cathodic potential values (e.g. azo dyes 4d
and 4e). In contrast, the electronic donor or acceptor nature of the
substituent (R2 ¼ H, CH3, CHO) on the thiazole ring does not
influence significantly the value of oxidation potential of
compounds, 4a, 5a and 6a.
At this moment another comparison can also be made between
bithienyl azo dye 6a and thienylpyrrolyl-azo dye 12c. Here, the
substitution of the bithiophene system (6a) by the thienylpyrrole
spacer (12c) maintaining the same acceptor 5-formyl-thiazol-2-yl-
diazene group results in a cathodic shift of the reduction potential
(e.g. 6a Epc ¼ ꢁ1.23 V and 12c Epc ¼ ꢁ0.98 V) for compound 6a.
Comparison of the reduction potentials between compound 12c
and thienylpyrrole-thiazolyldiazene 12a [3d] reported earlier by us,
showed that the increase of the acceptor electronic ability of the
formyl group substituted on the thiazole heterocycle leads also to
a reduction of the potential of compound 12c.
1
1
For azo dyes 4d and 4e it was also observed that the function-
alization of the 5-N,N-dialkylamino-bithiophenes at 4-position by
the diazenethiazolyl group destabilizes the
p-conjugated system,
All synthesized bithienyl azo dyes bearing a thiazole acceptor
group exhibited two monoelectronic reductions. The one-electron
stoichiometry for these reduction processes is ascertained by
comparing the current heights with known one-electron redox
processes under identical conditions [15]. The first process is
reversible and the second process partially reversible, except for
molecule 5a, which underwent a second reversible process and
molecules 4dee which exhibit irreversible processes. Compounds
4aec showed reversible reduction peaks, with similar onset
potentials between ꢁ1.23 and ꢁ1.26 V, which were assigned to the
reduction of the thiazolyldiazene moiety. This shows that for these
compounds the substitution of the bithiophene system by groups
with different electronic character has no influence on the reduc-
tions potentials. However, the reduction potentials values are
significantly influenced by the substituent on 50-position of the
bithiophene moiety. The N,N-dialkylamino substituents shifts the
Fig. 3. Cyclic voltammograms of compound 4a and 6a in DMF e 0.1 mol dmꢁ3 [NBu4]
[BF4] at a vitreous carbon electrode, scan rate 0.1 V sꢁ1
.

M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 454e465
459
probably due to a decrease in the planarity of the structures,
leading to the irreversibility of the second reduction.
On the basis of these potentials values, we estimated the HOMO
and LUMO energy levels from the potentials of the anodic
and cathodic processes [16,17]. The resulting HOMO energy levels
of compounds 4e7 were located within a reasonable range
of 4.70e5.37 eV, with variations that reflect different amounts of
intramolecular charge transfer (ICT) resulting from the presence of
the electron donors with different electro-donating abilities [18,19].
The LUMO energy levels of compounds 4e7 were located within
a reasonable range of 2.65e3.53 eV and were significantly greater
than those of aryldiazene thienylpyrroles [3a,20]. In addition, the
electrochemical band gaps of 4e7 were estimated from the differ-
ence between the potential values for oxidation and reduction
[12,21,22]. The corresponding values were in the range of
1.85e2.06 eV slightly larger than their respective optical band gaps.
Fig. 4. UVevisible absorption normalized spectra of compounds 4a and 4b in dioxane
at room temperature.
2.3. Optical properties
Bithiophene 4e7 and thienylpyrrole 12c azo dyes showed good
solubility in common polar and non-polar organic solvents such as
dioxane, diethyl ether, ethanol and DMSO. The extinction coeffi-
exhibits a bathochromic shift of 16 nm compared to thiazole azo
dye 4a (Fig. 6).
On the other hand thiazole, due to its electronic nature, coun-
teracts the electron-donating effect of the methyl group, resulting
in a smaller bathochromic shift of lmax (6e9 nm) for compounds
5aec compared to thiazolyl azo dyes 4aec. These results agree with
the general trend of band gap estimations based on the redox
properties (Table 3), displaying increased band gaps for azo dyes 4a
and 5a and decreased band gaps for compounds 4bec, 5b and 6a
functionalized with stronger donor and acceptor groups on the
bithiophene and thiazole moieties respectively.
cients (3) in dioxane and wavelength maxima lmax of compounds
4e7 and 12c in four solvents, are summarized in Table 4 and were
compared with the p* values for each solvent, as determined by
Kamlet and Taft [23].
All chromophores exhibit broad and intense CT absorptions in
the region of 477e539 nm in dioxane. The 5-alkoxy-2,20-bithio-
phene azo dyes 4b and 5b exhibited a red-shifted lmax (36e41 nm)
relative to unsubstituted derivatives 4a and 5a due to the stronger
donor strength of alkoxy groups (Fig. 4).
Heterocyclic azo dyes tend to display a larger solvatochromic
effect than azobenzene dyes due to stronger polarization of their
electronic system, especially in the excited state. Similar sol-
vatochromic behavior was reported for other azo dyes containing
thiophene, pyrrole and (benzo)thiazole fragments [2a,f,j,3a,b,d,4aec].
All thiazole 4e6, 12c and benzothiazole 7a azo dyes exhibited
a large positive solvatochromism (Dymax ¼ 814e1619 cmꢁ1) from
diethyl ether to DMSO, an effect typically associated with good
optical nonlinearities (Table 4, Fig. 7) [24]. In comparison, 5-N,N-
dialkylamino-2,20-bithienyl azo dyes 4dee substituted by the thiazo-
lyldiazene moiety on the 4-position of the bithiophene spacer have
a smaller Dymax (757e814 cmꢁ1) than their bithiophene and 5-alkoxy-
2,20-bithiophene 4b analogs Dlmax (1079e1619 cmꢁ1) suggesting
The thienylpyrrole azo dye 12c also exhibits a red-shift of lmax
by 24 nm compared to the bithiophene derivative 6a due to the
stronger auxiliary donor effect of the thienylpyrrole spacer (Fig. 5).
It was also observed that the donor ability of the methyl group
on the thiazole ring has a smaller impact on the UVevis spectra of
compounds 5aec. On the other hand, the acceptor electronic
strength of the formyl group substituted on the thiazole hetero-
cycle has a significant effect on the electronic absorption property
of thiazolyl azo dye 6a. Thiazole being an electron-deficient five-
membered heteroaromatic ring, will exhibit an auxiliary acceptor
effect when linked to withdrawing groups [8,11a]. Therefore, azo
dye 6a exhibited a distinctively red-shifted lmax (38 nm) with
respect to the unsubstituted derivative 4a. Due to the electron
density deficiency on the ring C atoms, the benzothiazole hetero-
cycle acts also as an electron-withdrawing group [11a]. Moreover,
due to a larger electronic delocalization, benzothiazole azo dye 7a
larger
The molecular first hyperpolarizabilities
diazenes 5e7, 12c and the arylation products 14 and 15a were
b
values for the latter more planar chromophores.
b
of novel heterocyclic
Table 4
Solvatochromic data [lmax (nm) and Dymax (cmꢁ1) of the charge-transfer band] for bithiophene and thienylpyrrole azo dyes 4e7 and 12c in 4 solvents with p* values by Kamlet
and Taft [23].
a
Azo dye
Diethyl ether
(0.54) lmax (nm)
Ethanol (0.54)
lmax (nm)
1,4-Dioxane
(0.55) lmax (nm)
DMSO (1.00)
lmax (nm)
Dymax
e (Dioxane)
(cmꢁ1
)
(Mꢁ1 cmꢁ1
)
4a
4b
4c
4d
4e
5a
5b
5c
460
509
509
504
509
468
509
510
501
482
534
483
525
524
520
525
490
526
526
520
500
553
477
518
513
514
520
483
519
522
515
493
539
497
536
531
524
531
501
540
535
535
517
565
1619
1079
814
754
814
1408
1127
916
1268
1315
1028
(30,240)
(28,980)
(33,690)
(19,970)
(23,120)
(33,060)
(36,130)
(29,600)
(30,610)
(27,990)
(39,720)
6a
7a
12c
a
Dymax ¼ ymax (diethyl ether) e ymax (DMSO)/cmꢁ1
.

460
M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 454e465
Fig. 7. UVevisible absorption normalized spectra of compound 4a in four solvents of
different polarity (diethylether, dioxane, ethanol and DMSO) at room temperature.
Fig. 5. UVevisible absorption normalized spectra of compounds 6a and 12c in dioxane
at room temperature.
indicative of a reduction in the effective conjugation due to steric
effects, due perhaps to the N,N-dialkylamino-groups on the series
4dee molecules not being coplanar with the rest of the molecule,
thereby limiting the conjugation that can be achieved [28].
At this stage, a comparison can also be made between the
nonlinear optical data of the new 50-N,N-dialkylamino-2,20-bithio-
phene chromophores 4dee bearing a thiazolyldiazene moiety with
similar azo dyes functionalized with an aryldiazene group at
4-position of the 5-N,N-dialkylamino-2,20-bithiophene system (9d,
10e) (Fig. 1). Earlier, we reported the synthesis, solvatochromic and
electrochemical properties of these 5-N,N-dialkylamino-bithienyl-
measured by hyper-Rayleigh scattering (HRS) method [25] using
the 1064 nm fundamental wavelength of a q-switched Nd:YAG
laser. Dioxane was used as the solvent, and the
b values were
measured against a reference solution of p-nitroaniline (pNA) [26]
in order to obtain quantitative values, while care was taken to
properly account for possible fluorescence of the dyes (see
Experimental section for more details). The static hyper-
polarisability b0 values [27] were calculated using a very simple
two-level model neglecting damping. They are therefore only
indicative and should be treated with caution (Table 5).
From Table 5 it can be seen that the
b values for azo dyes 4aec
aryldiazenes [4c]. The first hyperpolarizabilities
b, of these
bearing the thiazolyldiazene group linked at the 50-position of the
compounds were now also measured. The results obtained suggest
that the electron-deficient thiazole heterocycle has a larger acceptor
strength than the phenyl ring, even when it is substituted by
bithiophene system exhibit higher first hyperpolarizabilities
(
b
¼ 172e307 ꢂ 10ꢁ30 esu) as compared to their 5-N,N-dialkykamino-
birhiophene counterparts 4dee
(b
¼ 133e148 ꢂ 10ꢁ30 esu). The
results obtained showed that, the substitution of the diazophenyl
group on the 4-position on the bithiophene moiety functionalized
with N,N-dialkylamino groups on the 5-position (stronger donor
groups compared to alkoxy) produced smaller values of the first
Table 5
UVevis absorptions,
b and b0 values for the novel azo dyes 4e7, 12c, the arylation
products 14 and 15a and 5-N,N-dialkylamino-bithienyl-aryldiazenes 9d and 10e [4c]
and thienylpyrrole-thiazolyldiazene 12a [3d].a
molecular hyperpolarizability
b
than the 5-alkoxy-2,20-bithiophene
bb (10ꢁ30 esu)
b0 (10ꢁ30 esu)
c
azo dyes having the same diazophenyl groups substituted in the
Azo dye
lmax (nm)
50-position of the bithiophene moiety (e.g. 4c, R1 ¼ EtO,
b
¼ 307 ꢂ
4a
4b
4c
4d
4e
5a
5b
5c
477
518
513
514
520
483
519
522
515
493
509
510
486
539
533
396
352
172
291
307
148
133
166
294
150
286
207
101
125
164
610d
71
27 ꢃ 3
12 ꢃ 1
10ꢁ30 esu and 4d, R1 ¼ NEt2,
b
¼ 148 ꢂ 10ꢁ30). This might be
16 ꢃ 1
7.5 ꢃ 0.8
4.5 ꢃ 0.5
23 ꢃ 2
11 ꢃ 1
4.2 ꢃ 0.4
14 ꢃ 1
6a
7a
23 ꢃ 2
9d [4c]
10e [4c]
12a [3d]
12c
14
6.9 ꢃ 0.6
8.2 ꢃ 0.8
21 ꢃ 15
12 ꢃ 1
0.2 ꢃ 0.02
26 ꢃ 10
8.5
15a
pNA
67
16.9 [26]
a
Experimental hyperpolarizabilities and spectroscopic data measured in dioxane
solutions.
b
c
All the compounds are transparent at the 1064 nm fundamental wavelength.
Data corrected for resonance enhancement at 532 nm using the two-level
model with b0
¼
b
[1 ꢁ (lmax/1064)2][1 ꢁ (lmax/532)2]; damping factors not
included 1064 nm [27].
d
Value obtained by making measurements at various concentrations and
extrapolating to infinite dilution in order to correct for the absorption of the
generated second harmonic light as it propagates through the solution to the
detection system.
Fig. 6. UVevisible absorption normalized spectra of compounds 4a, 6a and 7a in
dioxane at room temperature.

M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 454e465
461
Fig. 8. Thermal analysis data for compounds 4c (left) and 5b (right) through TGA recorded under a nitrogen atmosphere, measured at a heating rate of 20 ꢀC minꢁ1
.
a strong acceptor group such as cyano, as the
b
value of the thia-
For optoelectronic applications, the thermal stability of organic
materials is critical for device stability. Therefore, the thermal
properties of the chromophores 4e7 and 12c were investigated by
thermogravimetric analysis under aꢁ1 nitrogen atmosphere,
measured at a heating rate of 20 ꢀC min (Table 1). All the chro-
mophores are thermally stable with decomposition temperatures
varying from 200 to 283 ꢀC. For the thiazolyldiazenes 4e6 the
electronic nature of the group substituted on the 5-position of the
thiazole ring does seem to have some impact on the thermal
stability of the compounds. The 5-methyl-thiazolyldiazenes 5aec,
are the most stable showing higher decomposition temperatures
(Fig. 8).
zolyldiazenes 4dee
(b
¼ 133e148 ꢂ 10ꢁ30 esu) are larger than
the corresponding values for phenyldiazenes 9d and 10e
(
b
¼ 101e125 ꢂ10ꢁ30 esu). Most of this effect appears to be due to
the red shift of the absorption maxima and
a subsequent
enhancement of the nonlinear response as the generated second
harmonic light approaches the charge-transfer transition energy.
From Table 5 it is apparent as well that the increase of the donor
strength of the of the substituent on 50-position of the bithiophene
system (dyes 4aec) also resulted in
a significant resonant
enhancement, with enhanced values accompanied by a red-
b
shifted absorption maxima (e.g. 4a, R ¼ H, lmax 477 nm,
b
¼ 172 ꢂ10ꢁ30 esu; 4c, R ¼ EtO, lmax 513 nm,
b
¼ 307 ꢂ 10ꢁ30 esu).
Benzothiazole azo dye 7a exhibits also an improved stability by
Also noteworthy is the effect of the electronic nature of the
ca 83 ꢀC compared to the corresponding thiazole derivative 4a.
group that substitutes the thiazole ring at 5-position. It was
observed that, the increase of the acceptor strength of the CHO
group (6a) compared to H (4a), results both in red-shifted
3. Conclusions
absorption maxima and a resonance enhanced
b
value for bithio-
In conclusion we have synthesized and characterized several
novel bithiophene and thienylpyrrole azo dyes bearing thiazole or
benzothiazole acceptor groups linked to the diazene group through
position 2 of the thiazole moiety.
By varying the heterocyclic spacer (bithiophene or thie-
nylpyrrole), the electronic nature of the donor and acceptor groups
linked to the bithiophene and thiazole moieties respectively, or the
position of substitution of the thiazolyldiazene acceptor moiety on
the bithiophene system, the thermal and the electrochemical
properties as well as the optical (linear and nonlinear) properties of
phene azo dye 6a (
b
¼ 286 ꢂ 10ꢁ30 esu). A similar effect was
observed for thienylpyrrole azo dyes 12a and 12c. Therefore
compound 12c having a stronger acceptor moiety exhibits a higher
beta value (610 ꢂ 10ꢁ30 esu) for incident light at 1064 nm as
compared to the unsubstituted derivative 12a (164 ꢂ10ꢁ30 esu)
[3d]. Due to the deficiency of electron density on the ring C atoms,
the thiazole heterocycle acts as electron-withdrawing group and
also as an auxiliary acceptor leading to an increase in molecular
hyperpolarizability [8,9a,11a]. Conversely, the methyl group linked
to 5-position of the thiazole ring 5a counters the withdrawing
effect of the thiazole ring leading to lower beta value for 5a
pushepull
p-conjugated systems can be tuned. More interesting
redox properties and the largest first hyperpolarizabilities were
observed for thienylpyrrole azo dye 12c and bithiophene azo dyes
4bec, 5b and 6a functionalized with donor alkoxy groups in
50-position of the bithiophene spacer and formyl acceptor group
substituted in the 5-position of the thiazole heterocycle. Due to
their good first order hyperpolarizability and redox properties
together with their good thermal stability these compounds are
attractive novel heterocyclic NLO-chromophores.
(b
¼ 166 ꢂ 10ꢁ30 esu).
Comparison of the
b values for thienylpyrrole azo dye 12c with
bithiophene azo dye 6a showed also that the substitution of
a bithiophene spacer by a thienylpyrrole heterocyclic bridge
enhances the first order hyperpolarizability from 286 ꢂ 10ꢁ30 to
610 ꢂ 10ꢁ30 esu at 1064 nm probably due to the stronger auxiliary
donor effect of the pyrrole heterocycle compared to the thiophene
ring [15,29]. These results are in agreement with the redox and the
NMR studies described above.
4. Experimental
Benzothiazole azo dye 7a exhibits a larger
b
value (207 ꢂ 10ꢁ30
)
compared to the corresponding thiazole chromophore 4a probably
due a greater electronic delocalization. In the probable resonance
structures, the distance between the charges in 7a being longer
compared to 4a, the electric moment of the first diazene must be
greater, increasing the value of the molecular hyperpolarizability.
4.1. Materials
4.1.1. 2-Aminothiazole, 2-amino-5-methylthiazole, 2-amino-5-
formylthiazole and 2-aminobenzothiazole were used as precursors
for the synthesis of aryldiazonium salts 2e3 and 2,2-bithiophene 1a
were purchased from Aldrich and Fluka and used as received
The synthesis of bithiophenes 1bee [5] and thienylpyrrole 11 [6]
has been described elsewhere. TLC analyses were carried out on
0.25 mm thick precoated silica plates (Merck Fertigplatten Kieselgel
The study of the first order hyperpolarizability
b for the aryla-
tion product 15a without the N]N bridge exhibits a much lower
lmax (396 nm) and beta values (67 ꢂ 10ꢁ30 esu) when compared to
the corresponding azo dye 6a (lmax ¼ 515 nm,
b
¼ 286 ꢂ 10ꢁ30 esu).

462
M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 454e465
60F254) and spots were visualized under UV light. Chromatography
on silica gel was carried out on Merck Kieselgel (230e240 mesh).
4.2.1.6. 5-N,N-Diethylamino-4-(thiazol-2-yl-azo)-2,2’-bithiophene
(4d). Pink oil (80%). 1H NMR (CDCl3)
d
1.39 (t, 6H, J ¼ 7.2 Hz,
2 ꢂ CH3), 3.82 (q, 4H, J ¼ 7.2 Hz, 2 ꢂ CH2), 7.00-7.04 (m, 1H, 4’’-H),
7.03 (d, 1H, J ¼ 3.6 Hz, 5-H), 7.03-7.04 (m, 1H, 3’’-H), 7.20 (dd, 1H,
J ¼ 5.2 and 1.2 Hz, 5’’-H), 7.59 (s, 1H, 3’-H), 7.75 (d, 1H, J ¼ 3.6 Hz,
4.2. Synthesis
4-H). 13C NMR (CDCl3)
d 12.1, 50.6, 109.6, 113.7, 116.1, 121.9, 123.4,
124.7, 127.7, 136.4, 136.8, 141.9, 143.7, 161.2, 180.1. lmax(Dioxane)/nm
4.2.1. General procedure for the azo coupling of bithiophenes 1aee
and thienylpyrrole 11 with thiazolyl-2aec and benzothiazolyl-3
diazonium salts to afford azo dyes 4aee, 5aec, 6a, 7a and 12c
4.2.1.1. Diazotation of 2-aminothiazole, 2-amino-5-methylthiazole,
2-amino-5-formylthiazole and 2-aminobenzothiazole. Heteroaromatic
amines (1.0 mmol) were dissolved in HCl 6 N (1 mL) at 0e5 ꢀC.
A mixture of NaNO2 (1.0 mmol) in water (2 mL) was slowly added to
the well-stirred mixture of the thiazole solution at 0e5 ꢀC. The
reaction mixture was stirred for 10 min.
514 ( 3449, 3075, 2975,
3
/dm3 molꢁ1 cmꢁ1 19,970). IR (Liquid film):
n
2931, 1538, 1492, 1464, 1435, 1359, 1332, 1291, 1263, 1216, 1173,
1150, 1138, 1075, 1015, 878, 833, 818, 784, 749, 700, 622 cmꢁ1. MS
(ESI) m/z (%) ¼ 349 ([M þ H]þ, 100), 295 (9), 249 (4), 223 (10).
HMRS: m/z (ESI) for C15H17N4S3; calcd 349.0609; found: 349.0610.
4.2.1.7. 5-Piperidino-4-(thiazol-2-yl-azo)-2,2’-bithiophene (4e). Dark
pink solid (10%). Mp 151e152 ꢀC 1H NMR (CDCl3)
d 1.80e1.85 (m, 6H,
3 ꢂ CH2), 3.95-3.98 (m, 4H, 2 ꢂ NCH2), 6.99 (dd,1H, J ¼ 5.2 and3.6 Hz,
4’’-H), 7.04-7.05 (m, 2H, 5 and 3’’-H), 7.22 (dd, 1H, J ¼ 5.2 and 1.2 Hz,
5’’-H), 7.57 (s, 1H, 3’-H), 7.77 (d, 1H, J ¼ 3.6 Hz, 4-H). 13C NMR (CDCl3)
4.2.1.2. Coupling reaction with bithiophenes 1 and thienylpyrrole
11. The diazonium salt solution previously prepared (1.0 mmol)
was added dropwise to the solution of bithiophenes 1 or thie-
nylpyrrole 11 (0.52 mmol) in acetonitrile (10 mL) and 2e3 drops of
acetic acid. The combined solution was maintained at 0 ꢀC for 1e2 h
while stirred and then diluted with chloroform (20 mL), washed
with water and dried with anhydrous MgSO4. The dried solution
was evaporated and the remaining azo dyes purified by column
chromatography on silica with dichloromethane/n-hexane as
eluent.
d
23.9, 25.7, 54.4, 113.8, 116.3, 122.3, 123.5, 124.8, 127.7, 136.9, 137.3,
142.0, 162.6, 179.9 lmax(Dioxane)/nm 520
23,120). IR (Liquid film): 3082, 2917, 2849, 2362, 1535, 1507, 1491,
(3
/dm3 molꢁ1 cmꢁ1
n
1462, 1438, 1388, 1336, 1292, 1245, 1221, 1174, 1145, 1017, 998, 910,
877, 857, 832, 819, 754, 718, 701, 663, 622 cmꢁ1. MS (ESI) m/z (%) ¼
361 ([M þ H]þ,100). HMRS: m/z (ESI) for C16H16N4S3; calcd 360.0537;
found: 361.0610.
4.2.1.8. 2-(5-Methylthiazol-2-yl)-1-(5-(thiophen-2-yl)thiophen-2-yl)
diazene (5a). The first compound eluted from the column chro-
matography was diazene 5a as a pink solid (10%). Mp 166e167 ꢀC.
4.2.1.3. 2-(Thiazol-2-yl)-1-(5-(thiophen-2-yl)thiophen-2-yl)diazene
(4a). Dark pink solid (23%). Mp 130e131 ꢀC. 1H NMR (CDCl3)
d 7.10
1H NMR (CDCl3)
d
2.53 (d, 3H, J ¼ 1.2 Hz, CH3), 7.09 (dd, 1H, J ¼ 5.2
(dd, 1H, J ¼ 5.0 and J ¼ 3.8 Hz, 4’’-H), 7.31 (d, 1H, J ¼ 4.0 Hz, 3’-H),
7.32 (d, 1H, J ¼ 3.6 Hz, 5-H), 7.39 (dd, 1H, J ¼ 4.8 and 1.0 Hz, 5’’-H),
7.42 (dd, 1H, J ¼ 3.8 and 1.0 Hz, 3’’-H), 7.81 (d, 1H, J ¼ 4.0 Hz, 4’-H),
and 3.6 Hz, 4’’-H), 7.29 (d, 1H, J ¼ 4 Hz, 3’-H), 7.37 (dd,1H, J ¼ 5.2 and
1.2 Hz, 5’’-H), 7.39 (dd, 1H, J ¼ 3.6 and 1.2 Hz, 3’’-H), 7.66 (d, 1H,
J ¼ 1.2 Hz, 4-H), 7.75 (d, 1H, J ¼ 4 Hz, 4’-H). 13C NMR (CDCl3)
d 30.9,
7.98 (d, 1H, J ¼ 3.6 Hz, 4-H), 13C NMR (CDCl3)
d 120.7, 124.5, 126.5,
127.6, 128.5, 136.4, 136.8, 143.7, 144.6, 156.5, 176.4. lmax(Dioxane)/
124.3,126.2,127.2,128.5,135.4,136.7,136.9,142.1,143.7,156.7,174.4.
3
/dm3 molꢁ1 cmꢁ1 33,060). IR (Liquid film):
nm 477 (3 n 3105, 1640,
/dm3 molꢁ1 cmꢁ1 30,240). IR (Liquid film):
lmax(Dioxane)/nm 483 (
3062, 2358, 1500, 1447, 1427, 1335, 1213, 1129, 1041, 842, 791 cmꢁ1
.
1503, 1483, 1450, 1406, 1305, 1244, 1222, 1212, 1136, 1044, 878, 846,
801, 753 cmꢁ1. MS (ESI) m/z (%) ¼ 278 ([M þ H]þ, 20). HMRS: m/z
(ESI) for C11H7N3S3; calcd 277.9875; found: 277.9876.
Anal. calcd. for C12H9N3S3 (291,42): C, 48.60; H, 3.29; N, 14.16; S,
32.96%; found C, 48.88; H, 3.53; N, 14.41; S, 32.94%.
The second compound eluted was the diazene 14 as a dark pink
solid (17%). Mp 211e213 ꢀC. 1H NMR (CDCl3)
d
2.58 (d, 3H, J ¼ 1.2 Hz,
4.2.1.4. 1-(5-(5-Methoxythiophen-2-yl)thiophen-2-yl)-2-(thiazol-2-
yl)diazene (4b). Dark pink solid (36%). Mp 147e149 ꢀC. 1H NMR
CH3), 2.60 (d, 3H, J ¼ 1.2 Hz, CH3), 7.10 (dd,1H, J ¼ 5.2 and 3.6 Hz, 4’’-
H), 7.41 (dd, 1H, J ¼ 5.2 and 1.2 Hz, 5’’-H), 7.44 (dd, 1H, J ¼ 3.6 and
1.2 Hz, 3’’-H), 7.64 (d, 1H, J ¼ 1.2 Hz, 4-H or 4’-H), 7.70 (d, 1H,
J ¼ 1.2 Hz, 4’-H or 4-H), 7.96 (br s, 1H, 3’’-H). 13C NMR (CDCl3)
(CDCl3)
d
3.97 (s, 3H, OCH3), 6.23 (d,1H, J ¼ 4.2 Hz, 4’’-H), 7.12
(d, 1H, J ¼ 4.2 Hz, 3’’-H), 7.14 (d, 1H, J ¼ 4 Hz, 3’-H), 7.34 (d, 1H,
J ¼ 3.6 Hz, 5-H), 7.75 (d, 1H, J ¼ 4 Hz, 4’-H), 7.95 (d, 1H, J ¼ 3.6 Hz,
4-H). 13C NMR (CDCl3)
d 60.5, 105.6, 120.2, 122.8, 123.2, 125.5,
d
12.20, 13.14, 123.45, 126.78, 127.87, 128.48, 136.63, 137.81, 139.43,
140.55, 157.87, 174.95. lmax(Dioxane)/nm 533 (3
/dm3 molꢁ1 cmꢁ1
136.7, 143.7, 145.9, 155.2, 168.7, 176.7. lmax(Dioxane)/nm 518
(
3
/dm3 molꢁ1 cmꢁ1 28,980). IR (Liquid film):
n
3163, 2033, 1552,
46,230). IR (Liquid film): 2921, 2851, 1501, 1456, 1416, 1320, 1284,
1221, 1158, 1132, 1063, 900 cmꢁ1. MS (ESI) m/z (%) ¼ 389 ([M þ 1]þ,
100), 381 (5), 295 (10). HMRS: m/z (ESI) for C16H12N4S4; calcd
389.0018; found: 389.0019.
1514, 1377, 1350, 1305, 1268, 1226, 1169, 1152, 1066, 1049, 971, 892,
874, 772 cmꢁ1. MS (ESI) m/z (%) ¼ 308 ([M þ H]þ, 100). HMRS: m/z
(ESI) for C12H10N3OS3; calcd; 307.9983 found: 307.9980.
4.2.1.9. 1-(5-(5-Methoxythiophen-2-yl)thiophen-2-yl)-2-(5-methyl
4.2.1.5. 1-(5-(5-Ethoxythiophen-2-yl)thiophen-2-yl)-2-(thiazol-2-yl)
thiazol-2-yl)diazene (5b). Dark pink solid (42%). Mp 265e266 ꢀC. 1H
diazene (4c). Dark pink solid (11%). Mp 128e130 ꢀC. 1H NMR
NMR (CDCl3)
d
2.53 (d, 3H, J ¼ 1.2 Hz, CH3), 3.96 (s, 3H, OCH3), 6.21
(CDCl3)
d
1.47 (t, 3H, J ¼ 7.2 Hz, CH3), 4.47 (q, 2H, J ¼ 7.2 Hz, CH2),
(d, 1H, J ¼ 4.4 Hz, 4’’-H), 7.10 (d, 1H, J ¼ 4.4 Hz, 3’-H), 7.11 (d, 1H,
6.21 (d, 1H, J ¼ 4.2 Hz, 4’’-H), 7.11 (d, 1H, J ¼ 4.2 Hz, 3’’-H), 7.13 (d,
1H, J ¼ 3.8 Hz, 3’-H), 7.33 (d, 1H, J ¼ 3.8 Hz, 4’-H), 7.75 (d, 1H,
J ¼ 3.6 Hz, 5-H), 7.95 (d, 1H, J ¼ 3.6 Hz, 4-H). 13C NMR (CDCl3)
J ¼ 4.4 Hz, 3’’-H), 7.64 (d, 1H, J ¼ 1.2 Hz, 4-H), 7.70 (d, 1H, J ¼ 4.4 Hz,
4’-H). 13C NMR (CDCl3)
d 12.9, 60.4, 105.6, 122.7, 123.3, 125.2, 135.9,
136.1, 141.7, 145.3, 155.5, 168.5, 174.6. lmax(Dioxane)/nm 519
d
14.6, 69.8, 106.4, 120.2, 122.7, 123.1, 125.5, 136.7, 143.7, 146.1,
/dm3 molꢁ1 cmꢁ1 36,130). IR (Liquid film): 3579, 3085, 2933, 2897,
155.1, 167.8, 176.7. lmax(Dioxane)/nm 513
33,690). IR (Liquid film): 3243, 2980, 1721, 1547, 1506, 1485, 1465,
1445, 1414, 1389, 1344, 1311, 1275, 1228, 1209, 1139, 1108, 1060,
1027, 992, 876 cmꢁ1. MS (ESI) m/z (%) ¼ 322 ([M þ H]þ, 100), 295
(34), 224 (5). HMRS: m/z (ESI) for C13H12N3S3; calcd 322.0137;
found: 322.0137.
(3
/dm3 molꢁ1 cmꢁ1
(3
1513, 1480, 1451, 1432, 1317, 1226, 1135, 1065, 1049, 972, 859, 788,
764 cmꢁ1. MS (EI) m/z (%) ¼ 321 ([M]þ, 10), 278 (25), 246 (27), 221
(41), 209 (17), 195 (100), 168 (29), 152 (16), 141 (15), 122 (13), 71
(14). HMRS: m/z (EI) for C13H11N3OS3; calcd 321.0064; found:
321.0068.
n

M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 454e465
463
4.2.1.10. 1-(5-(5-Ethoxythiophen-2-yl)thiophen-2-yl)-2-(5-methyl
1186, 1149, 1089, 1047, 1019, 998 cmꢁ1. MS (ESI) m/z (%) ¼ 395
([M þ H]þ, 21),268 (5), 256 (11), 255 (19), 254 (100), 239 (4), 211 (4).
HMRS: m/z (ESI) for C19H15N4O2S2; calcd 395.0631; found:
395.0629.
thiazol-2-yl)diazene (5c). Dark pink solid (9%). Mp 146e147 ꢀC. 1H
NMR (CDCl3)
d
1.46 (d, 3H, J ¼ 0.8 Hz, CH3), 2.52 (t, 3H, J ¼ 7.2 Hz,
OCH2CH3), 4.16 (q, 2H, J ¼ 7.2 Hz, OCH2CH3), 6.20 (d,1H, J ¼ 4 Hz, 4’’-
H), 7.09 (d, 1H, J ¼ 4 Hz, 3’’-H), 7.10 (d, 1H, J ¼ 4.2 Hz, 3’-H), 7.63 (d,
1H, J ¼ 0.8 Hz, 4-H), 7.70 (d, 1H, J ¼ 4.2 Hz, 4’-H). 13C NMR (CDCl3)
4.3. Instruments
d
12.9, 14.6, 69.8, 106.4, 122.6, 123.2, 125.3, 135.9, 136.1, 141.7, 145.4,
155.4, 167.6, 174.6. lmax(Dioxane)/nm 522
(3
/dm3 molꢁ1 cmꢁ1
NMR spectra were obtained using a Varian Unity Plus Spec-
trometer at an operating frequency of 300 MHz for 1H NMR and
75.4 MHz for 13C NMR or a Bruker Avance III 400 at an operating
frequency of 400 MHz for 1H NMR and 100.6 MHz for 13C NMR
using the solvent peak as internal reference at 25 ꢀC. All chemical
shifts are given in ppm using dH Me4Si ¼ 0 ppm as reference and J
values are given in Hz. Assignments were made by comparison of
chemical shifts, peak multiplicities and J values and were supported
by spin decoupling-double resonance and bidimensional hetero-
nuclear HMBC and HMQC correlation techniques. IR spectra were
determined on a BOMEM MB 104 spectrophotometer using KBr
discs. UVevisible absorption spectra (200e800 nm) were obtained
using a Shimadzu UV/2501PC spectrophotometer. Mass spectrom-
etry analyses were performed at the “C.A.C.T.I. e Unidad de
Espectrometria de Masas” at the University of Vigo, Spain. Elec-
trospray ionization mass spectra were recorded using a Thermo-
Finnigan LCQ Deca XP Plus quadrupole ion trap instrument on
samples diluted in ethanol/water (1:1 v/v). Thermogravimetric
analysis of samples was carried out using a TGA instrument model
Q500 from TA Instruments, under high purity nitrogen supplied at
a constant 50 mL min‑1 flow rate. All samples were subjected to
a 20 ꢀC minꢁ1 heating rate and were characterized between 25 and
500 ꢀC. All melting points were measured on a Gallenkamp melting
point apparatus and are uncorrected. Cyclic voltammetry (CV) was
performed using a potentiostat/galvanostat (AUTOLAB/PSTAT 12)
with the low current module ECD from ECO-CHEMIE and the data
analysis processed by the General Purpose Electrochemical System
software package also from ECO-CHEMIE. Three electrode-two
compartment cells equipped with vitreous carbon-disc working
electrodes, a platinum-wire secondary electrode and a silver-wire
pseudo-reference electrode were employed for cyclic voltam-
metric measurements. The concentration of the compounds were
1 mmol dmꢁ3 and 0.1 mol dmꢁ3 [NBu4][BF4] was used as the sup-
porting electrolyte in dry N,N-dimethylformamide solvent. The
cyclic voltammetry was conducted usually at 0.1 V sꢁ1, or at
different scan rates (0.02e0.50 V sꢁ1), for investigation of scan rate
influence. The potential is measured with respect to ferrocinium/
ferrocene as an internal standard.
29,600). IR (Liquid film): 3148, 2925, 1552, 1519, 1509, 1476, 1463,
1429, 1394, 1361, 1336, 1315, 1284, 1266, 1253, 1224, 1210, 1170,
1164, 1047, 1024, 887, 834, 799, 780, 768, 748 cmꢁ1. MS (EI) m/z
(%) ¼ 335 ([M]þ, 10), 278 (26), 250 (14), 246 (29), 225 (21), 223 (10),
196 (100), 168 (25), 140 (21), 71 (15) . HMRS: m/z (EI) for
C14H13N3OS3; calcd 335.0022; found: 335.0022.
4.2.1.11. 2-(5-Formylthiazol-2-yl)-1-(5-(thiophen-2-yl)thiophen-2-
yl)diazene (6a). The first compound eluted from the column
chromatography was the 5-(thiazol-2-yl)-2,20-bithiophene 15a as
an yellow solid (3%). Mp 183e184 ꢀC. 1H NMR (CDCl3)
d 7.08 (dd,1H,
J ¼ 5.2 and 3,8 Hz, 4’’-H), 7.20 (d, 1H, J ¼ 3.9 Hz, 3’-H), 7.32-7.35 (m,
2H, 3’’-H and 5’’-H), 7.60 (d, 1H, J ¼ 3.9 Hz, 4’-H), 8.34 (s, 1H, 4-H),
10.02 (s, 1H, CHO). lmax(Dioxane)/nm 396 (3
/dm3 molꢁ1 cmꢁ1
11,190). IR (Liquid film): 2925, 2811, 1667, 1517, 1499, 1424, 1412,
1359,1275,1219,1153, 1067,1050, 914, 882, 867, 845, 792, 716 cmꢁ1
.
MS (ESI) m/z (%) ¼ 300 ([M þ Na]þ, 10).
The second compound eluted from the column chromatography
was the diazene 6a as a dark pink solid (15%). Mp 210e211 ꢀC. 1H
NMR (CDCl3)
d
7.13 (dd, 1H, J ¼ 5.2 and 3,8 Hz, 4’’-H), 7.41 (d, 1H,
J ¼ 4.4 Hz, 3’-H), 7.47 (dd, 1H, J ¼ 5.2 and 0.8 Hz, 5’’-H), 7.50 (dd, 1H,
J ¼ 3.8 and 0.8 Hz, 3’’-H), 7.94 (d, 1H, J ¼ 4.4 Hz, 4’-H), 8.52 (s, 1H,
4-H), 10.05 (s, 1H, CHO). 13C NMR (CDCl3)
d 125.4, 127.6, 128.9, 136.6,
138.3, 139.4, 147.9, 151.7, 156.3, 181.3, 182.8. lmax(Dioxane)/nm 515
(3
/dm3 molꢁ1 cmꢁ1 30,610). IR (Liquid film): 3058, 2842, 1658, 1504,
1449, 1417, 1316, 1279, 1262, 1141, 1058, 893, 873, 850, 814, 723,
643 cmꢁ1. MS (microTOF) m/z (%) ¼ 305 ([M]þ, 14), 277 (33), 194
(18), 181 (82), 179 (28), 165 (16), 146 (21), 128 (58), 127 (100), 121
(68). HMRS: m/z (MicroTOF) for C12H7N3OS3; calcd 304.9751;
found: 304.9760.
4.2.1.12. 2-(Benzo[d]thiazol-2-yl)-1-(5-(thiophen-2-yl)thiophen-2-
yl)diazene (7a). Pink solid (9%). Mp 274e275 ꢀC. 1H NMR (CDCl3)
d
7.12 (dd, 1H, J ¼ 4.8 and 3.6 Hz, 4’’-H), 7.37 (d, 1H, J ¼ 4.4 Hz, 3’-H),
7.42-7.47 (m, 3H, 5-H, 3’’-H and 5’’-H), 7.52 (dt, 1H, J ¼ 7.2 and
1.4 Hz, 6-H), 7.87 (dd, 1H, J ¼ 7.8 and 1.4 Hz, 4-H), 7.91 (d, 1H,
J ¼ 4.4 Hz, 4’-H), 8.13 (dd, 1H, J ¼ 7.8 and J ¼ 1.4 Hz, 7-H). 13C NMR
(CDCl3)
d 122.1, 124.6, 124.9, 126.6, 126.9, 127.1, 128.0, 128.7, 134.5,
136.7, 137.7, 146.1, 152.9, 156.5, 174.9. lmax(Dioxane)/nm 493
4.4. Optical studies
(3
/dm3 molꢁ1 cmꢁ1 27,990). IR (Liquid film): 3430, 3077, 1503, 1482,
1448, 1430, 1359, 1334, 1251, 1224, 1183, 1153, 1046, 848, 802, 722,
692 cmꢁ1. MS (ESI) m/z (%) ¼ 328 ([M þ H]þ, 100), 313 (5), 299 (9),
296 (7), 295 (42). HMRS: m/z (ESI) for C15H10N3S3; calcd 328.0032;
found: 328.0031.
4.4.1. Solvatochromic study
The solvatochromic study was performed using 10ꢁ4 M solu-
tions of dyes 4e7 and 12c in several solvents at room temperature.
4.4.2. Nonlinear optical measurements using the hyper-Rayleigh
scattering (HRS) method [25]
4.2.1.13. 1-(1-(4-Methoxyphenyl)-5-(thiophen-2-yl)-1H-pyrrol-2-yl)-
2-(5-formylthiazol-2-yl)diazene (12c). Dark pink solid (98 mg, 61%).
Hyper-Rayleigh scattering (HRS) was used to measure the first
hyperpolarizability b of response of the molecules studied. The
Mp 151e153 ꢀC. 1H NMR (CDCl3)
d
3.96 (s, 3H, OCH3), 7.00 (d, 1H,
J ¼ 4.6 Hz, 4’-H), 7.01-7.02 (m, 1H, 4’’-H), 7.06 (d, 2H, J ¼ 8.8 Hz, 3000
and 5000-H), 7.14 (dd, H, J ¼ 4.0 and 0.8 Hz, 3’’-H), 7.28 (d, 2H,
J ¼ 8.8 Hz, 2000and 6000-H), 7.38 (dd, 1H, J ¼ 5.2 and 0.8 Hz, 5’’-H), 7.44
(d, 1H, J ¼ 4.4 Hz, 3’-H), 8.35 (s, 1H, 4-H), 9.89 (s, 1H, CHO). 13C NMR
experimental set-up for hyper-Rayleigh measurements has been
previously described in reference [13] and is similar to the one
presented by Clays et al. [25] The incident laser beam came from
a Q-switched Nd:YAG laser operating at a 10 Hz repetition rate
with approximately 10 mJ of energy per pulse and a pulse duration
(FWHM) close to 12 ns at the fundamental wavelength of 1064 nm.
The incident beam had a vertical polarization was weakly focused
(beam diameter w0.5 mm) into the solution contained in a 5 cm
long cuvette. The hyper-Rayleigh signal at right angles to the
(CDCl3)
d 55.6, 109.2, 114.6, 115.9, 116.2, 116.4, 116.6, 128.0, 128.6,
129.0, 129.7, 131.1, 132.1, 135.6, 142.4, 151.3, 160.7, 182.6, 184.4
lmax(Dioxane)/nm 539 (
3
/dm3 molꢁ1 cmꢁ1 30,610). IR (Liquid film):
3102, 3076, 3009, 2933, 2837, 2742, 1888, 1662, 1608, 1587, 1511,
1471, 1442, 1420, 1393, 1378, 1337, 1308, 1276, 1250, 1233, 1204,

464
M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 454e465
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incident beam was collimated using a high numerical aperture lens
passed through an interference filter centered at the second
harmonic wavelength (532 nm) before being detected by a photo-
multiplier (Hamamatsu model H9305-04). The current pulse from
the photomultiplier was integrated using a Stanford Research
Systems gated box-car integrator (model SR250) with a 20 ns gate
centered on the temporal position of the incident laser pulse.
The hyper-Rayleigh signal was normalized at each pulse using
the second harmonic signal from a 1 mm quartz plate to compen-
sate for fluctuations in the temporal profile of the laser pulses due
to longitudinal mode beating. Dioxane was used as a solvent, and
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the
b values were calibrated using a reference solution of p-nitro-
aniline (pNA) [26] also dissolved in dioxane at a concentration of
1 ꢂ10ꢁ2 mol dmꢁ3 (external reference method). The hyper-
polarizability of pNA dissolved in dioxane is known from EFISH
measurements [26] carried out at the same fundamental wave-
length. The concentrations of the solutions under study were
chosen so that the corresponding hyper-Rayleigh signals fell well
within the dynamic range of both the photomultiplier and the box-
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2006;47:3711;
(d) Raposo MMM, Fonseca AMC, Castro MCR, Belsley M, Cardoso MFS,
Carvalho LM, et al. Dyes Pigm 2011;91:62.
car integrator. All solutions were filtered (0.2 mm porosity) to avoid
spurious signals from suspended impurities. The small hyper-
Rayleigh signal that arises from dioxane was taken into account
and particular care to avoid reporting artificially high hyper-
polarizabilities due to a possible contamination of the hyper-
Rayleigh signal by molecular fluorescence near 532 nm using two
different interference filters with different transmission pass bands
centered near the second harmonic at 532 nm (for more details
consult reference [13]). The polarization of the detected signal was
not discriminated and the values reported in Table 5 assume that
the first hyperpolarizability tensor is dominated by a single longi-
tudinal element as is common for strong linear charge pushepull
charge-transfer molecules.
[4] (a) Raposo MMM, Ferreira AMFP, Belsley M, Moura JCVP. Tetrahedron 2008;
64:5878;
(b) Raposo MMM, Ferreira AMFP, Belsley M, Matos Gomes E, Moura JCVP.
Mater Sci Forum 2008;587e588:268;
(c) Raposo MMM, Ferreira AMFP, Amaro M, Belsley M, Moura JCVP. Dyes Pigm
2009;83:59;
(d) Coelho PJ, Carvalho LM, Moura JCVP, Raposo MMM. Dyes Pigm 2009;82:
130.
[5] Raposo MMM, Kirsch G. Heterocycles 2001;55:1487.
[6] Raposo MMM, Sampaio AMBA, Kirsch G. Synthesis 2005;2:199.
[7] For some examples see (a) Hallas G, Towns AD. Dyes Pigm 1997;33:319;
(b) Hallas G, Towns AD. Dyes Pigm 1997;35:219;
(c) Hallas G, Choi J-H. Dyes Pigm 1999;42:249;
(d) Yuquan S, Yuxia Z, Zao L, Jianghong W, Ling Q, Shixiong L, et al. J Chem Soc
Perkin Trans 1; 1999:3691;
Acknowledgments
(e) Ledoux I, Zyss J, Barni E, Barolo C, Diulgheroff N, Quagliotto P, et al. Synth
Met 2000;115:213;
(f) Yuxia Z, Zhao L, Ling Q, Jianfen Z, Jiayun Z, Yuquan S, et al. Eur Polym J
2001;37:445.
Thanks are due to the Fundação para a Ciência e Tecnologia
(Portugal) and FEDER for financial support through the Centro de
Química and Centro de Física-Universidade do Minho, Project
PTDC/QUI/66251/2006 (FCOMP-01-0124-FEDER-007429), Project
PTDC/CTM/105597/2008 with funding from COMPETE/FEDER and
a research grant to M. C. R. Castro (UMINHO/BI/142/2009). The NMR
spectrometer Bruker Avance III 400 is part of the National NMR
Network and was purchased within the framework of the National
Program for Scientific Re-equipment, contract REDE/1517/RMN/
2005 with funds from POCI 2010 (FEDER) and FCT.
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