Design, synthesis, and characterization of the electrochemical, nonlinear optical properties, and theoretical studies of novel thienylpyrrole azo dyes bearing benzothiazole acceptor groups

Article abstract of DOI:10.1016/j.tet.2011.05.053

Two series of related donor-acceptor conjugated heterocyclic azo dyes based on the thienylpyrrole system, functionalized with benzothiazol-2-yl (5-6) or benzothiazol-6-yl acceptor groups (7) through an NN bridge, have been synthesized by azo coupling usin

Full text of DOI:10.1016/j.tet.2011.05.053

Tetrahedron 67 (2011) 5189e5198
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Design, synthesis, and characterization of the electrochemical, nonlinear optical
properties, and theoretical studies of novel thienylpyrrole azo dyes bearing
benzothiazole acceptor groups
,
a
a
b
b
M. Manuela M. Raposo a , M. Cidalia R. Castro , A. Maurício C. Fonseca , Peter Schellenberg , M. Belsley
*
ꢀ
a Center of Chemistry, University of Minho, Campus of Gualtar, 4710-057 Braga, Portugal
b Center of Physics, University of Minho, Campus de 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:
Two series of related donoreacceptor conjugated heterocyclic azo dyes based on the thienylpyrrole system,
functionalized with benzothiazol-2-yl (5e6) or benzothiazol-6-yl acceptor groups (7) through an N]N
bridge, have been synthesized by azo coupling using 1-alkyl(aryl)thienylpyrroles (1) and benzothiazolyl
diazonium salts (2e4) as coupling components. Their optical (linear and first hyperpolarizability), elec-
trochemical, and thermal properties have been examined. Optimized ground-state molecular geometries
and estimates of the lowest energy single electron vertical excitation energies in dioxane solutions were
obtained using density functional theory (DFT) at the B3LYP/6-31þG(d,p) level. Hyper-Rayleigh scattering
(HRS) in dioxane solutions using a fundamental wavelength of 1064 nm was employed to evaluate their
second-order nonlinear optical properties. Of these systems, the benzothiazol-2-yl-diazenes 5e6 exhibit
Received 4 March 2011
Received in revised form 9 May 2011
Accepted 11 May 2011
Available online 20 May 2011
Dedicated to the Centenary of the Portu-
guese Chemical Society
Keywords:
Heterocyclic azo dyes
Thienylpyrrole
Benzothiazole
Nonlinear optical (NLO) materials
Reverse polarity
First hyperpolarizability (b)
Thienylpyrroles as auxiliary donors
Redox properties
the largest first hyperpolarizabilities (
b
¼460e660ꢀ10ꢁ30 esu, T convention) compared to benzothiazol-6-
yl-diazenes 7 (
b
¼360e485ꢀ10ꢁ30 esu, T convention). Good to excellent thermal stabilities were also
obtained for all azo dyes (235e317 ꢂC). This multidisciplinary study showed that modulation of the optical
and electronic properties can be achieved by introduction of the benzothiazole acceptor group in the
thienylpyrrole system through position 2 or 6 of the benzothiazole heterocycle.
Ó 2011 Elsevier Ltd. All rights reserved.
Density functional theory (DFT)
Thermal stability
1. Introduction
an important example. It has been shown by theoretical and
experimental studies, that the use of heteroaromatics, such as
Aryl and (hetero)aryl diazenes have long been used as dyes and
colorants, but more recent applications have sought to exploit their
use as versatile precursors in several organic reactions, the ability
to photo switch the geometry and associated electronic and optical
properties of these molecules for various applications including the
fabrication of second harmonic generation chromophores,
memories and switches,1,2 materials for organic solar cells,3 and
chemosensors.4
pyrrole, thiophene or (benzo)thiazole, in which electrons are more
easily delocalizable compared to aryl derivatives, can lead to
enhanced hyperpolarizabilities.5e7
One of the major challenges in the design and optimization of
the second-order polarizability of organic NLO materials consists in
finding substituents with an optimal combination of their donor/or
acceptor strength for a given parent chromophore. Although a large
variety of donor, acceptor, and spacer groups have been used for the
design of NLO chromophores, the use of strong donating moieties,
such as electron-rich heteroaromatic rings has seldom appeared
in the NLO literature.1a,7,8 Even rarer are examples of azo dyes
bearing pyrrole or thienylpyrrole systems as donor moieties func-
tionalized with aryl or heteroaryl diazene acceptor systems spe-
cifically designed for nonlinear optical and photochromic
Heterocycles by themselves can also play an important role as
versatile building blocks in several areas of advanced materials
chemistry, second-order nonlinear optical (NLO) applications being
* Corresponding author. Tel.: þ351 253 604381; fax: þ351 253 604382; e-mail
address: mfox@quimica.uminho.pt (M.M.M. Raposo).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.05.053

5190
M.M.M. Raposo et al. / Tetrahedron 67 (2011) 5189e5198
applications.1a,b,8a,8g On the other hand, several investigators
reported studies concerning the use of benzothiazole heterocycle
as acceptor group in NLO chromophores.5b,6aec,9
2. Results and discussion
2.1. Synthesis
Recently we have designed, synthesized, and characterized
several series of donoreacceptor substituted
p
-conjugated systems
Usually pushepull benzothiazole derivatives reported in the
in which we have used 1-alkyl(aryl)thienylpyrroles as donor moi-
eties functionalized with several acceptor groups (dicyanovinyl,
tricyanovinyl, benzothiazolyl, benzimidazolyl, aryldiazene, and
thiazolyldiazene). Evaluation of the electrochemical and optical
(linear and nonlinear) properties of these novel donoreacceptor
systems revealed that they could be used as potential candidates for
second-order NLO8 and photochromic applications.8g,10 Moreover,
our experimental and theoretical results concerning the auxiliary
donor/acceptor effect of electron-rich and electron-deficient het-
literature have normal polarity,9aec,9e,9gꢁq with the exception of
ꢀ
the three articles published recently by Hrobarik et al. concerning
the theorectical studies9d,j and the synthesis of a few examples of
structurally simple benzothiazole derivatives having reverse
polarity.9f The reverse polarity of pushepull heterocyclic NLO
chromophores are specially important when an additional
electron-withdrawing group is attached to the benzothiazole ring.
Additionally, as the strength of the withdrawing group substituted
in the benzothiazole heterocycle is increased, the resulting increase
in the NLO response becomes more sensitive to its position in the
benzothiazole ring.
erocycles on pushepull thienylpyrrole p-conjugated systems have
shown that the position of the acceptor groups, such as dicyano-
vinyl or electron-deficient heterocycles (e.g., benzothiazole or
benzimidazole) on the thienylpyrrolyl system can have a clear
influence on the electronic and optical properties of these hetero-
cyclic chromophores.
The effects of several molecular modifications were examined
with the aim of understanding this structureeproperty relation-
ship. The results9d,f,j suggested that the most promising de-
rivatives in terms of optimal NLO-phore properties (highest
quadratic molecular hyperpolarizabilities) are benzothiazole de-
In view of our recent results8,10 and other previous
studies1a,4g,9d,9f we decided to investigate the effect of different
benzothiazole acceptor groups linked to the thienylpyrrole (1)
donor system through an azo bridge on the electronic and optical
properties of the novel heterocyclic azo dyes. Thus two novel series
of azobenzothiazolyl thienylpyrroles 5e7 were designed, synthe-
sized and their electrochemical and optical (linear and nonlinear)
properties were evaluated.
rivatives containing acceptor groups on position 2 and a
p-donor
styryl or heteroalkenyl group in position 6 of the benzothiazole
ring. Having in mind these theoretical studies and also our recent
work8 we decide to synthesize two types of donoreacceptor
benzothiazole derivatives with different dipole moment di-
rections in order to evaluate the effect of different geometries of
the benzothiazole dyes on their thermal, electrochemical, and
optical properties: compounds with normal polarity in which the
thiazole moiety is linked to the azo bridge through the C-2 of the
heterocycle (5e6) and chromophores with reverse polarity9f in
which the benzothiazole heterocycle in linked to the azo bridge
through C-6 (7).
The aim of the present work is to demonstrate that combined
tuning of optical and electrochemical properties of heterocyclic azo
derivatives 5 and 7 can be achieved by functionalization of the
thienylpyrrole (1) systems with heteroaromatic benzothiazoles
linked to the p-conjugated system/donor moiety through different
positions of the benzothiazole ring (2 or 6). To this end, two series
of structurally related chromophores have been designed and
investigated. One is based on thienylpyrrolyl-benzothiazol-2-yl-
diazenes 5e6 (normal polarity) and the other on thienylpyrrolyl-
benzothiazol-6-yl-diazenes 7 (reverse polarity). The influence of
the different regiochemistries of the heterocyclic acceptor group
was investigated by combined experimental and theoretical studies
of the electronic, linear, and nonlinear optical properties of these
heterocyclic azo dyes.
In addition, the azo dyes 5 and 7 were compared to their similar
thienylpyrrole derivatives functionalized with phenyldiazene 8a,8g
thiazoyldiazene 9aec,8g and benzothiazole 10aeb8d moieties
(Fig. 1). First, the influence of an additional N]N bridge on the
electronic and optical properties has been studied by comparing
Thienylpyrroles 1aed11 have been used as coupling components
together with benzothiazolyl diazonium salts 2e4 in order to
prepare heterocyclic azo dyes 5e7 (Scheme 1). Therefore,
diazotation of 2-aminobenzothiazole, 6-aminobenzothiazole, and
2-amino-6-methylbenzothiazole with NaNO2 in HCl at 0e5 ꢂC in
water gave the corresponding diazonium salts 2e4, which reacted
immediately with thienylpyrroles 1aed in acetonitrile at 0 ꢂC given
benzothiazolyldiazenes 5e7 in fair to good yields (10e81%). Higher
yields (60e81%) were obtained in the synthesis of thienylpyrrole
azo dyes 7aec bearing the azo bridge substituted on position 6 of
the benzothiazole heterocycle probably due to the higher stability
of diazonium salt 4 when compared to 2.
The structures of thienylpyrrolyl-benzothiazol-2-yl-diazenes
5e6 and thienylpyrrolyl-benzothiazol-6-yl-diazenes 7 were un-
ambiguously confirmed by their analytical and spectral data.
thienylpyrrolyl-benzothiazol-2-yl-diazenes
5 with the corre-
sponding thienylpyrrolyl-benzothiazoles 10.8d Furthermore com-
parison of thienylpyrrolyl-thiazolyldiazenes 9aec8g with
compounds 5aec showed the higher acceptor capacity of the
benzothiazole electro-deficient heterocycle when compared to the
thiazole ring.
2.2. Electronic structure analysis
The structures and charge transfer transitions of the heterocy-
clic azo dyes were first analyzed by 1H NMR spectroscopy (Table 1)
and cyclic voltammetry (Table 2).
Second, the effect of the substitution of a phenylazo moiety by
the heterocyclic benzothiazolyldiazene acceptor system have been
studied by comparison of thienylpyrrolyl-benzothiazol-6-yl-dia-
zene 7a with 8a8g (Fig. 1).
For example in the 1H NMR spectra of thienylpyrrole de-
rivatives 5aeb and 6a functionalized with a benzothiazol-2-yl-
diazene moiety on the 2-position of the pyrrole ring exhibit two
N
S
S
S
N
N
N
a
b
c
R1 = n-Pr
1 = 4-MeOC6H4
R1 = 4-FC6H4
N
N
N
N
N
S
R
S
R1
R1
R1
8a
9a-c
10a-b
Fig. 1. Structure of thienylpyrrole derivatives 8a, 9aec, and 10aeb.8d,8g

M.M.M. Raposo et al. / Tetrahedron 67 (2011) 5189e5198
5191
3
N
2
4´
N
3´
CH3
N
+
4
N
S
+N2
S
N
S
N2
N
S
5´´
4´´
S
5
N
N
N
N
R1
N
R1
S
S
CH3CN/CH3COOH
R1
3´´
CH3CN/CH3COOH
oC
6
CH3
0
oC
7
5a-d
0
6a
1a
R
1 = n-Pr
b
c
d
R1 = 4-MeOC6H4
R1 = 4-FC6H4
R1 = 1-Naphthyl
N
+N2
CH3CN/CH3COOH
oC
4
0
S
7
S
7a
R
1 = n-Pr
S
N
2
N
7b R1 = 4-MeOC6H4
7c R1 = 4-FC6H4
N
R1
N
5
4
Scheme 1. Synthesis of benzothiazole azo dyes 5e7 through azo coupling reaction of thienylpyrroles 1 with benzothiazolyl diazonium salts 2e4.
doublets at about 6.89e7.13 and 7.12e7.27 ppm were detected
with coupling constants of 4.6e4.9 Hz indicating the presence of
two adjacent protons (40-H and 30-H) at the corresponding pyrrole
moiety. In the 1H NMR spectra of thienylpyrrole azo dye 5d four
signals at about 7.51 (double triplet), 7.61 (double triplet), 7.84
(double doublet), and 7.90 (double doublet) were detected and
were attributed, respectively, to the 5, 6, 4, and 7-H protons in the
benzothiazole ring.
benzothiazol-6-yl-diazenes 7 four signals were detected at about
8.07e8.18 (doublet), 7.76e8.06 (double doublet), 8.33e8.53 (dou-
blet), and 9.30e9.31 (singlet). These signals were attributed, re-
spectively, to the 4, 5, 7, and 2-H protons in the benzothiazole ring.
The 1H NMR chemical shifts reflect a charge separation in the-
ground state, consequently the analysis of these data in
donoreacceptor compounds bearing benzothiazole electron-
deficient heterocycles, such as benzothiazolyldiazenes 5e7 also
In the 1H NMR spectrum of derivative 6a bearing a 6-methyl-
benzothiazol-2-yl- moiety an additional singlet at 2.86 ppm was
detected due to the methyl group attached to C6. Moreover, the
signals attributed to the 5-H and 7-H in the benzothiazole system
were detected as a double doublet at 7.35 ppm and as a doublet at
7.79 ppm, respectively, with coupling constants of 8.3 and 2.0 Hz for
5-H and 2.0 Hz for 7-H. On the other hand for thienylpyrrolyl-
confirms their pushepull character with a significant intra-
molecular charge transfer (ICT) from the donor thienylpyrrole
moiety to the acceptor benzothiazole groups (Table 1).12 The effect
of the substitution of a phenyl group for dye 8a by a benzothiazole
heterocycle (e.g., 5a) is noteworthy. All the protons of the thie-
nylpyrrole 5a (30-H and 40-H, and 300, 400, and 500-H) were shifted to
higher chemical shifts (e.g., 40-H and 30-H
d
¼6.89 and 7.15 ppm,
Table 1
Chemical shifts of protons of the heterocyclic azo dyes 5e7a
Compd
R1
4-H
5-H
6-H
7-H
30-H
40-H
300-H
400-H
500-H
5a
5b
5c
5d
6a
n-Pr
7.99e8.02
7.90
7.90
7.84
7.88
7.46
7.40
7.41
7.51
7.35
7.54
7.99e8.02
7.96
7.96
7.90
7.79
7.15
7.27
7.27e7.29
7.40e7.43
7.12
6.89
7.13
7.13
7.32e7.34
6.87
7.61
7.29
7.27e7.29
7.25
7.59
7.32
7.79
7.56
7.59
7.40e7.43
7.80
4-MeOC6H4
4-FC6H4
1-Naphthyl
n-Pr
7.45e7.54
7.45e7.52
7.61
7.08e7.14
7.10e7.13
6.96e6.98
7.32
d
Compd
R1
2-H
4-H
5-H
7-H
30-H
40-H
300-H
400-H
500-H
7a
7b
7c
n-Pr
4-MeOC6H4
4-FC6H4
9.30
9.31
9.31
8.18
8.07
8.08
8.06
7.76
7.76
8.53
8.33
8.34
6.83
7.00e7.02
7.00
6.58
6.87
6.87
7.35
7.00e7.02
7.00e7.04
7.19e7.22
7.00e7.02
7.00e7.04
7.59
7.42e7.46
7.45
a
Proton NMR measurements carried out in deuterated acetone at 400 MHz, with chemical shift values in ppm. The numbering scheme of protons is shown in Scheme 1.
Table 2
Electrochemical data and theoretical values for the HOMO and LUMO energies for compounds 5e7, 8a, and 9a
Reductiona
Oxidationa
HOMO DFTb
LUMO DFTb
Band gap (eV)
1
2
3
c
Compound
ꢁ E1/2 (V)
ꢁ E1/2 (V)
ꢁ E1/2 (V)
Epa (V)
Exp.
Theory
5a
5b
5c
5d
6a
7a
1.36
1.33
1.35
1.36
1.41
1.81
1.73
1.77
1.95
1.51
2.08
1.97
2.03
2.07
2.22
2.40
2.26
2.29
__
__
__
__
2.87
__
__
__
__
__
__
0.70
0.66
0.69
0.72
0.67
0.57
0.55
0.54
0.56
0.59
ꢁ5.6226
ꢁ5.7018
ꢁ5.7418
ꢁ5.7236
ꢁ5.5693
ꢁ5.6071
ꢁ5.5344
ꢁ5.5802
ꢁ5.4190
ꢁ5.5502
ꢁ3.0716
ꢁ3.1320
ꢁ3.1633
ꢁ3.1535
ꢁ3.0403
ꢁ2.6841
ꢁ2.7176
ꢁ2.7478
ꢁ2.5744
ꢁ2.9676
2.06
1.99
2.04
2.02
2.08
2.38
2.28
2.31
2.51
2.10
2.5510
2.5698
2.5785
2.5701
2.5290
2.9230
2.8168
2.8324
2.8426
2.5826
7b
7c
8a8g
9a8g
2.25
a
Measurements made in dry dimethyformamide containing 1.0 mM of the indicated compound 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 versus the ferrocinium/ferrocene-couple. E1/2 corresponds to the reversible process. Epa correspond to the anodic
peak potentials.
b
Calculated by DFT using B3LYP/6-31þG(d,p) in dimethylformamide.
c
EHOMO¼ꢁ(4.39þEpa) (eV) and ELUMO¼ꢁ(1E1/2þ4.39) (eV).

5192
M.M.M. Raposo et al. / Tetrahedron 67 (2011) 5189e5198
respectively) when compared to the corresponding phenyldiazene
azo dye 8a (e.g., 40-H and 30-H
d
¼6.58 and 6.88 ppm, respectively)8g
indicating a decrease of the electron density due to the stronger
electron-withdrawing power, a higher charge-demand cx, of the
benzothiazole ring, allowing a more efficient charge transfer from
the donor to the acceptor group. On the other hand compounds 5 in
which the electron-poor C-2 of the benzothiazole heterocycle is
linked to the azo bridge, exhibit higher chemical shifts for all pro-
tons of the thienylpyrrole moiety (30-H, 40-H, 300-H, 400-H, and 500-H)
compared to derivatives 7 in which the benzothiazole heterocycle
is linked to the azo bridge through position 6 indicating a more
efficient charge transfer from the donor to the acceptor in com-
pounds 5. A possible explanation for these results is that in com-
pounds 5 the electron-poor C2 of the 1,3-heteroaromatic thiazole
moiety is at the acceptor end of the chromophore corresponding to
the ‘matched’ configuration in which the electron-poor C2 is at the
acceptor end of the molecule.
The ‘matched’ and ‘un-matched’ configurations of the 1,3-
thiazole moiety are defined by the relative orientations between
dipoles of thiazole and the NLO chromophore as a whole.
Table 2 displays the redox potentials measured in [NBu4][BF4]/
DMF (1 mmol dmꢁ3) by cyclic voltammetry, the theoretical values
for the HOMO and LUMO energies calculated by DFT B3LYP/6-
31þG(d,p) in DMF and the corresponding band gap values for
compounds (5aed, 6a, 7aec, 8a, and 9a).
0.0
5b
7b
-40.0
-1.80
-1.00
E/ V vs. fc+/fc
Fig. 2. Cyclic voltammograms for the first reduction step of compound 5b and 7b
(1.0ꢀ10ꢁ3 mol dmꢁ3) in DMF, 0.1 mol dmꢁ3 [NBu4][BF4] at a vitreous carbon electrode,
between ꢁ0.6 V and ꢁ2.10 V versus fcþ/fc, scan rate 0.1 V sꢁ1
.
The efficient donoreacceptor conjugation leads to a lowering of
the HOMO and the LUMO levels in 5 and 6, revealing more efficient
coupling between the thienylpyrrole system and the benzothia-
zole-2-yl acceptor. In contrast, a weaker donoreacceptor coupling
of the benzothiazole-6-yl moiety 7 brings about an elevation of the
HOMO and the LUMO levels. It is notable that the elevation of
the HOMOs in comparable compounds of set 7 are around 0.1 eV,
the elevation of the LUMOs is much stronger with elevations
around 0.4e0.5 eV. Accordingly the contribution to the larger band
gap in set 7 compared to set 5 is mostly due to the influence on the
LUMOs. These observations are endorsed by the theoretical DFT
analysis of the data, which show the same tendency. It is notable
that the theoretical values for all compounds, the LUMOs are only
slightly lower in energy compared to experiment (w0.1 eV), while
the values for the HOMOs are lower by w0.5e0.6 eV. This indicates
a possible overestimation of conjugation and accordingly an
underestimation of the charge separation in the HOMO.
All molecules showed
a donoreacceptor character with
reversible reduction and irreversible oxidation processes. The irre-
versible oxidation process is associated with the oxidation of the
pyrrole moiety. These results are consistent with previous electro-
chemical studies of other pyrrole and thiophenes derivatives.8a-c,13
In contrast all thienylpyrrole azo dyes (5aed and 6a and 7aec)
bearing a benzothiazole acceptor group linked to the azo bridge
through positions 2 (5aed and 6a) or 6 (7aec) exhibited two
monoelectronic reversible reductions indicating a high electron af-
finity of the chromophores. Compound 5d shows a third reduction
process attributed to the naphthyl group.14 The one-electron stoi-
chiometry for these reductionprocesses is ascertained bycomparing
the current heights with known one-electron redox processes under
identical conditions.8a For all compounds, the two reduction pro-
cesses are associated with the reduction of the (hetero)aromatic azo
moiety. For all azo dyes it was also observed that the reduction po-
tential values of the first process are only slightly influenced by the
substituent on the nitrogen atom of the pyrrole ring. On the other
hand the introduction of a benzothiazole acceptor group linked to
the azo bridge through position 2 (compounds 5 and 6) could sig-
nificantly decrease the reduction potentials while increasing the
oxidation potentials, when compared to derivatives 7 (bearing the
benzothiazole heterocycle linked through the 6-position), e.g., 7b
1E1/2¼ꢁ1.73 V and 5b ꢁ1E1/2¼ꢁ1.33 V, (Fig. 2) and 5a Epa¼0.70 V and
7a Epa¼0.57 V suggesting a stronger electron-accepting ability of
compounds 5. At this moment two other comparisons can also be
made between compound 7a with aryldiazene 8a or between ben-
zothiazole azo dye 5a with the corresponding thiazolyldiazene 9a.
The substitution of an aryldiazene system (8a) by the benzothiazol-
6-yl- moiety (7a) or the substitution of a thiazole heterocycle (9a) by
a benzothiazol-2-yl acceptor group (5a) results in a decrease of the
reduction potentials (e.g.,1E1/2¼ꢁ1.51 V for 9a and 1E1/2¼ꢁ1.36 V for
5a) that could also be due to the stronger electron-accepting ability
of compounds 5a and 7a compared, respectively, to 9a and 8a. These
results are in agreement with the previous 1H NMR analysis that
showed increased electron densities for azo dyes 7 and 9a and de-
creased electron densities for compounds 5.
2.3. Optical properties
Benzothiazolyl azo dyes 5e7 showed good solubility in common
polar and non-polar organic solvents, such as dioxane, diethyl
ether, ethanol, DMF, and DMSO. The extinction coefficients (
the wavelength maxima lmax of compounds 5e7 in several sol-
vents, are summarized in Table 3 and were compared with the
3) and
p
*
values for each solvent, as determined by Kamlet and Taft.16 All
chromophores exhibit broad and intense CT absorptions in the
visible region from 452 nm to 509 nm in dioxane solutions. The N-
aryl substituted chromophores 5bed exhibited a small red-shifted
lmax (7e8 nm) relative to the N-alkyl derivative 5a suggesting
a higher donor strength of aryl derivatives than the alkyl func-
tionalized compound. On the other hand, the position of the linkage
of the benzothiazolyl group to the azo bridge has significant effect
on the electronic absorption property of NLO chromophores. For
example, the ‘matched’ thiazole-based compounds 5aed exhibited
a distinctively red-shifted lmax (46e50 nm) with respect to the ‘un-
matched’ derivatives 7aec. These results agree with the band gap
calculations based on the redox properties (Table 2), which show
increased band gaps for benzothiazol-6-yl azo dyes 7aec and de-
creased band gaps for benzothiazol-2-yl based compounds 5aed.
All the benzothiazole azo dyes exhibited positive sol-
vatochromism (Dnmax¼855e1332 cmꢁ1) from diethyl ether to
DMSO, suggesting the good optical nonlinearities of these chro-
mophores (Table 3).17
The electrochemical band gaps were calculated as described
previously15 from the potentials of the anodic (oxidation of the
thienylpyrrole) and cathodic processes (Table 2). Smaller band gaps
were obtained for compounds 5 and 6 compared to azo dyes 7.
Themolecularfirsthyperpolarizabilitiesbofheterocyclicdiazenes
5e7 were measured by Hyper-Rayleigh scattering (HRS) method18 at
a fundamental wavelength of 1064 nm of a laser beam. Dioxane was

M.M.M. Raposo et al. / Tetrahedron 67 (2011) 5189e5198
5193
Table 3
Solvatochromic data for thienylpyrrole 5e7 in five solvents with
p
*
values by Kamlet and Taft16
Dnmax (cmꢁ1
)
3
(Dioxane)
a
Azo dye
Diethyl ether (0.54)
lmax (nm)
Ethanol (0.54)
lmax (nm)
1,4-Dioxane (0.55)
lmax (nm)
DMF (0.88)
lmax (nm)
DMSO (1.00)
lmax (nm)
(Mꢁ1cmꢁ1
)
5a
5b
5c
5d
6a
7a
7b
7c
491
500
496
498
494
437
450
445
509
519
517
518
504
455
464
458
500
509
507
508
503
451
461
452
511
523
518
523
512
460
469
465
519
530
526
528
520
464
474
468
1099
1132
1132
1141
1013
1332
855
(35,400)
(35,780)
(42,800)
(28,320)
(30,060)
(26,800)
(21,900)
(36,130)
1105
a
Dnmax¼nmax(diethyl ether)ꢁnmax(DMSO)/cmꢁ1.
used as the solvent, and the
b
values were measured against a refer-
have the ‘matched’ orientation of the dipole moment with rele-
vant substitution pattern.6c,9d,21
ence solution of p-nitroaniline (pNA)19 in order to obtain quantitative
values, while care was taken to properly account for possible fluo-
rescence of the dyes (see Experimental section for more details). The
static hyperpolarisability b0 values20 were calculated using a very
simple two-level model neglecting damping. They are therefore only
indicative and should be treated with caution (Table 4).
At this stage, several comparisons could also be made between
the nonlinear optical data of the new benzothiazole azo dyes 5 and
7, with thienylpyrrole derivatives functionalized with aryldiazene
moiety 8a,8g thiazolyldiazene 9aec8g or benzothiazole heterocy-
cles 10aeb,8d recently reported by us (Fig. 1). The results obtained
Table 4
UVevis absorptions,
a
b
and b0 values and Td data for benzothiazolyl azo dyes 5e7 and aryl and thiazolyl azo dyes 8e108d,8g
Azo dye
lmax (nm)
bb (10ꢁ30 esu)
b0 (10ꢁ30 esu)
c
Azo dye
lmax (nm)
b
b(10ꢁ30 esu)
b0 c(10ꢁ30 esu)
d
Td (ꢂC)
5a
5b
5c
5d
6a
7a
7b
7c
500
509
507
508
503
451
461
452
352
590
460
660
575
590
385
485
360
4019
54
33
47
39
50
86
95
82
20
235
250
244
242
241
de
8a
9a
9b
9c
10a
10b
d
419
486
493
491
353
366
225f
390f
370f
470f
150f
200f
d
72
51
41
55
74
93
d
d
d
317
274
d
d
d
pNA
d
d
a
Experimental hyperpolarizabilities and spectroscopic data measured in dioxane solutions.
All the compounds are transparent at the 1064 nm fundamental wavelength. Values are reported in the T convention assuming a single longitudinal element dominates
b
the hyperpolarizabilty tensor. Estimated uncertainties are 10% of the reported values.
c
Data corrected for resonance enhancement at 532 nm using the two-level model with b0
¼b
[1ꢁ(lmax/1064)2][1ꢁ(lmax/532)2]; damping factors not included 1064 nm.20
d
Decomposition temperature (Td) measured at a heating rate of 20 ꢂC minꢁ1 under a nitrogen atmosphere, obtained by TGA, corresponding to 5% of the decomposition of
the material.
e
f
Compound 7a was obtained as an oil.
Values rescaled from those reported in Ref. 8d,8g to conform to the T convention and to take into account the most recent calibration from Ref. 19
.
From Table 4 it can be seen that the
bearing the benzothiazol-2-yl group linked to the azo bridge
exhibit higher first hyperpolarizabilities (
b
values for azo dyes 5aec
showed that the substitution of a benzene ring (8a) by the
electron-poor benzothiazol-6-yl group (7a) in the diazene moiety,
maintaining the same thienylpyrrole donor system produced
a bathochromic shift on the absorption wavelength maxima and
b
¼460e660ꢀ10ꢁ30 esu)
compared to their benzothiazol-6-yl counterparts 7aec
(
b
¼360e485ꢀ10ꢁ30 esu) showing that the
b
values are clearly
a larger value of the molecular hyperpolarizability b (8a,
influenced by the position of substitution of the benzothiazole
heterocycle to the donor moiety through the azo bridge. How-
ever the extrapolation to zero frequency suggests that these
gains are primarily due to the red shift of the HOMOeLUMO
transition and that in fact the extrapolated static hyper-
polarizabiltities exhibit a trend in the opposite direction favoring
the mistmatched compounds. It is important to note that this
extrapolation is based on a simple two-level model without
damping effects and should be treated with some caution. One
means of investigating this discrepancy would be to carry out
detailed experimental measurements as a function of the in-
cident wavelength over a wide spectral range to reliably ex-
lmax¼419 nm,
b
¼225ꢀ10ꢁ30 esu and 7a, lmax¼451 nm,
b
¼385ꢀ10ꢁ30 esu). Due to the electron density deficiency on the
ring C atoms, the benzothiazole heterocycle acts as electron-
withdrawing group and also as an auxiliary acceptor. Further-
more, the large electronegativity and the lone electron pairs of S
and N atoms in the thiazole moiety together with the extension of
the conjugation length of the
p-electron bridge also lead to an
increase in the molecular hyperpolarizability. On the other hand
substitution of
a
thiazol-2-yl diazene moiety 9aec,
(
b
¼370e470ꢀ10ꢁ30 esu)8g by the benzothiazol-2-yl diazene sys-
tem 5aec while maintaining the same donor thienylpyrrole moi-
eties leads to larger nonlinearities for compounds 5aec,
trapolate to static
b
values, but this is beyond our present
(
b
¼460e660ꢀ10ꢁ30 esu) probably due to the more extensive
capabilities. Previous theoretical studies have indicated that
the relative orientation of the heteroaromatic rings (HR) on the
molecular hyperpolarizability can be interpreted in terms of the
dipole orientation and the substitution pattern in D-HR-A sys-
electron delocalization (see the theoretically estimated oscillator
strengths below) and red shift of the HOMOeLUMO energy gap.
Noteworthy is also the effect of the introduction of an N]N bridge
in benzothiazole azo dyes 5aeb (
pared to benzothiazoles 10aeb (
b
¼460e590ꢀ10ꢁ30 esu) com-
tems. Largest
b
values are predicted for all systems when they
b
¼150e200ꢀ10ꢁ30 esu).8d

5194
M.M.M. Raposo et al. / Tetrahedron 67 (2011) 5189e5198
2.4. Theoretical calculations
Figs. 3e5, below, display some of the obtained optimized lowest
lying singlet state molecular geometries for azo dyes 5aec, 6a, and
7aeb, in dioxane. Only azo dyes 5a and 6a (Fig. 3), are strictly planar
in the ground state, all other chromophores display some out of
plane twisting see, for example, Fig. 4 (5bec) and Fig. 5 (7aeb). It is
noteworthy that azo dyes 5a and 6a also display some of the largest
oscillator strengths and hyperpolarizabilities. In this regard azo dye
7a (Fig. 5) is especially interesting when compared to 5a (Fig. 3).
Optimized ground-state molecular geometries and HOMO-
eLUMO gaps have been calculated using the Gaussian 09 pro-
gram22 employing density functional theory using the B3LYP
functional and the 6-31þG(d,p) basis set both on isolated molecules
in vacuum and in dioxane and DMF solutions. For the latter, the
Polarizable Continuum Model (PCM) using the integral equation
formalism variant (IEFPCM) is used, which is the default in
Gaussian 09.
Using the calculated geometry, time dependent DFT calculations
employing the same functional and the same basis set (option:
td¼(nstates¼6)) were then carried out to determine the vertical
electronic transition energies. The results are summarized in
Table 5 and compared with the experimentally determined values
from the linear absorption measurements.
Fig. 3. Ground-state molecular structures for azo dyes 5a (left) and 6a (right) opti-
mized using B3LYP/6-31þG(d,p).
Table 5
Theoretical results for the energy, corresponding wavelength and oscillator strength
of the lowest singlet transition for azo dyes 5e9
Azo dye
Predicted vertical transitionsa
lmax (nm)b
energy (eV)
l
(nm)
f-valuec
5a
5b
5c
5d
6a
7a
7b
7c
8a
9a
9b
9c
2.4224
2.4413
2.4442
2.4451
2.3949
2.6555
2.5929
2.6087
2.6556
2.4751
2.4956
2.5023
511.83
507.87
507.27
507.07
517.70
466.90
478.17
475.26
466.88
500.93
496.82
495.49
1.5077
1.3742
1.3772
1.3764
1.5691
1.4342
1.1509
1.2266
1.2615
1.1966
1.0715
1.0813
500
509
507
508
503
451
461
452
419
486
493
491
Fig. 4. Ground-state molecular structures for azo dyes 5b (left) and 5c (right) opti-
mized using B3LYP/6-31þG(d,p).
a
Theoretically predicted values for the vertical transitions in dioxane solutions.
Observed wavelength of the linear absorption maximum.
Oscillator strength of the transition.
b
c
Fig. 5. Ground-state molecular structures for azo dyes 7a (left) and 7b (right) opti-
mized using B3LYP/6-31þG(d,p).
Overall the agreement between the theoretically predicted
2.5. Thermal properties of benzothiazole diazenes 5e7
transition wavelengths and the experimentally determined ab-
sorption maxima is quite good. Slight exceptions are the diazenes
functionalized with the n-propyl group in position 1of the pyrrole
ring (5a, 6a, 7a, 8a, and 9a) and the mismatched chromophores
(7aec) for which the theoretical calculations tend to predict
bathochromic shifts that are about 2% greater than those that are
observed experimentally.
The thermal stabilities of azo dyes 5e7 were evaluated by
thermogravimetric analysis (TGA) under a nitrogen atmosphere,
measured at a heating rate of 20 ꢂC minꢁ1. As shown in Table 4 all
compounds exhibit good to excellent thermal stability with de-
composition temperatures varying from 235 to 317 ꢂC. Benzothia-
zol-2-yl azo dyes 5aed and 6a decompose between 235 and 250 ꢂC,
while benzothiazol-6-yl azo dyes 7bec showed an improved
thermal stability by ca. 30e67 ꢂC compared to the corresponding
azo dyes 5bec. In addition, the electronic nature of the substituents
on position 1 of the thienylpyrrole azo dyes 5aed and 7bec have
some influence on their thermal stability being higher for de-
rivatives substituted by the 4-methoxyphenyl group (e.g., 5b,
Td¼250 ꢂC and 7b, Td¼317 ꢂC) (Table 4).
The trends in the theoretical oscillator strengths are consistent
with the above observations concerning the measured hyper-
polarizabilities and suggest that a large part of the variation in the
hyperpolarizabilities can be attributed to the differences in the
amount of charge that participates in the ground state to first
excited state vertical transition. The ‘matched’ azo dyes 5aec
bearing the benzothiazol-2-yl group linked to the azo bridge
possess stronger charge transfer transitions compared to their
‘mismatched’ benzothiazol-6-yl analogs 7aec, the oscillator
strengths of the matched compounds being 5e10% greater than
those of the respective mismatched molecules. The substitution of
a benzene ring (8a) by the electron-poor benzothiazol-6-yl group
(7a) in the diazene moiety increases the oscillator strength by just
over 13% indicating a significant increase in the amount of charge
that participates in the transition. Even more pronounced is the
increase in oscillator strength by over 25% when the thiazol-2-yl
diazene moiety (9aec) is substituted by the benzothiazol-2-yl
diazene system (5aec), suggesting that the benzene ring con-
tributes significantly to enhance the charge mobility of these
molecules.
3. Conclusions
In conclusion we have developed two new series of
benzothiazolyl-thienylpyrrole azo dyes with normal or with re-
verse polarity. By varying the position of linkage of the benzo-
thiazole heterocycle (2 or 6) to the azo bridge, the electrochemical
properties as well as the linear and nonlinear optical properties of
this well-defined asymmetric pushepull p-conjugated systems can
be readily tuned.
Theoretical calculations indicate that significant variations in
the oscillator strength of the ground statedfirst excited vertical

M.M.M. Raposo et al. / Tetrahedron 67 (2011) 5189e5198
5195
transitions are associated with these changes. Experimental Hyper-
Rayleigh scattering measurements report good values for these
benzothiazolyl-thienylpyrrole azo dyes with the best values
500-H), 7.90 (dd, 1H, J¼8.4 and J¼0.8 Hz, 4-H), 7.96 (dd, 1H, J¼8.4 and
b
J¼0.8 Hz, 7-H). 13C NMR (DMSO)
d 55.5, 106.5, 114.5, 114.8, 122.4,
123.1,126.0, 126.4, 127.7,127.8, 128.8, 129.8, 130.7, 131.7, 133.2,139.9,
obtained for the matched chromophores 5.
148.9, 152.6, 160.1, 177.2. lmax(Dioxane)/nm 509 (
3
/dm3 molꢁ1 cmꢁ1
2926, 1512, 1456, 1341, 1324, 1314, 1249, 1222,
35,780). IR (CHCl3):
n
4. Experimental
4.1. Materials
1196,1180,1166,1045,1002, 826, 759, 703 cmꢁ1. MS (microTOF) m/z
(%): 417 ([MþH]þ, 100), 416 (Mþ, 36), 391 (10), 296 (10), 247 (15),
225 (20). HMRS: m/z (MicroTOF) for C22H17N4OS2; calcd 417.0844;
found: 417.0835.
2-Amino-benzothiazole, 6-methyl-2-amino-benzothiazole, and
6-amino-benzothiazole used as precursors for the synthesis of
benzothiazoyldiazonium salts 2e4 were purchased from Aldrich
and Fluka and used as received.
4.2.5. 2-(Benzo[d]thiazol-2-yl)-1-(1-(4-fluorophenyl)-5-(thiophen-
2-yl)-1H-pyrrol-2-yl)-diazene 5c. Violet solid (52 mg, 25%). Mp
199e200 ꢂC. 1H NMR (acetone-d6) 7.10e7.13 (m, 1H, 400-H), 7.13 (d,
d
The synthesis of 1-propyl-thienylpyrrole 1a11b and 1-aryl-thie-
nylpyrroles 1bed11a was described elsewhere. TLC analyses were
carried out on 0.25 mm thick precoated silica plates (Merck Fer-
tigplatten Kieselgel 60 F254) and spots were visualized under UV
light. Chromatography on silica gel was carried out on Merck Kie-
selgel (230e240 mesh).
1H, J¼4.8 Hz, 40-H), 7.27e7.29 (m, 2H, 30-H, 300-H), 7.41 (dt, 1H, J¼7.6
and J¼1.2 Hz, 5-H) 7.45e7.52 (m, 3H, 6-H, 3000-H and 5000-H), 7.59 (dd,
1H, J¼5.2 and J¼1.2 Hz, 500-H), 7.65 (dd, 2H, J¼8.8 and J¼4.8 Hz, 2000
-
H and 6000-H), 7.90 (br d, 1H, J¼7.9 Hz 4-H), 7.96 (br d, 1H, J¼7.9 Hz,
7-H). 13C NMR (acetone-d6)
d
115.1, 116.9, and 117.1 (d, J¼20 Hz,
C300and C500), 122.9, 124.4, 127.0, 127.1, 128.6, 128.9, 129.3, 132.6, and
132.7, (d, J¼8 Hz, C200and C600), 133.0, 133.2, 134.8, 139.8, 149.8,
154.2, 162.8, and 165.3 (d, J¼245 Hz, C400) 178.4. lmax(Dioxane)/nm
4.2. Synthesis
507 (3 n 3073, 2925, 1603,
/dm3 molꢁ1 cmꢁ1 42,800). IR (CHCl3):
1509,1457,1438,1394,1349,1330,1315,1248,1221,1184,1158,1126,
1091, 1050, 1025, 1010, 884, 840, 815, 767, 755 cmꢁ1. MS (microTOF)
m/z (%): 405 ([MþH]þ, 100), 295 (17). HMRS: m/z (MicroTOF) for
C21H14FN4S2; calcd 405.06384; found: 405.06372.
General procedure for the azo coupling of thienylpyrroles 1 with
benzothiazolyl diazonium salts 2e4 to afford azo dyes 5aed, 6a,
and 7aec.
4.2.1. Diazotation of 2-amino-benzothiazole, 6-methyl-2-amino-
benzothiazole, and 6-amino-benzothiazole. Heteroaromatic amines
(1.0 mmol) were dissolved in HCl 6 N (1 mL) at 0e5 ꢂC. 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 ꢂC. The reaction
mixture was stirred for 20 min.
4.2.6. 2-(Benzo[d]thiazol-2-yl)-1-(1-(naphthalen-1-yl)-5-(thiophen-
2-yl)-1H-pyrrol-2-yl)-diazene 5d. Dark red solid (68 mg, 30%). Mp
190e191 ꢂC. 1H NMR (acetone-d6)
d
6.96e6.98 (m, 1H, 400-H), 7.25
(dd, 1H, J¼4.4 and J¼1.2 Hz, 300-H), 7.32e7.34 (m, 3H, 40-H, and 2ꢀ
Naphthyl-H), 7.40e7.43 (m, 3H, 30-H, 50-H, and Naphthyl-H), 7.51 (dt,
1H, J¼7.8 and J¼1.2 Hz, 5-H), 7.61 (dt, 1H, J¼7.8 and J¼1.2 Hz, 6-H),
7.84 (dd, 1H, J¼8.0 and J¼1.6 Hz, 4-H), 7.83e7.86 (m, 2H, 2ꢀ Naph-
thyl-H), 7.90 (dd, 1H, J¼8.0 and J¼1.6 Hz, 7-H), 8.18 (br d, J¼8.4 Hz,
4.2.2. Coupling reaction with thienylpyrroles 1. The diazonium salt
solution previously prepared (1.0 mmol) was added drop wise to
the solution of thienylpyrroles 1 (0.52 mmol) and 2e3 drops of
acetic acid in acetonitrile (10 mL). The combined solution was
maintained at 0 ꢂC for 1 he2 h while stirred and diluted with
chloroform (20 mL), washed with water and dried with anhydrous
MgSO4. The dried solution was evaporated and the remaining azo
dyes were purified by column chromatography on silica with
dichoromethane/n-hexane as eluent.
8
000-H), 8.31e8.35 (m,1H, 2000-H).13C NMR (acetone-d6)
d 106.9,114.9,
122.8,123.1,124.3,126.4,126.9,127.0,127.7,128.3,128.5,128.7,129.1,
129.2,129.3,131.3,132.7,132.9,133.6,134.7,135.2,140.6,150.3,154.1,
178.2. lmax(Dioxane)/nm 508 (
(CHCl3):
3
/dm3 molꢁ1 cmꢁ1 28,320). IR
n
3409, 2924, 2854, 1505, 1455, 1342, 1311, 1260, 1208, 1171,
1110, 1045, 840, 803, 774, 758 cmꢁ1. MS (microTOF) m/z (%): 437
([MþH]þ,100), 436 (Mþ, 27), 380 (15), 378 (16), 359 (15), 318 (7), 247
(10), 225 (28). HMRS: m/z (MicroTOF) for C25H17N4S2; calcd
437.0895; found: 437.09040.
4.2.3. 2-(Benzo[d]thiazol-2-yl)-1-(1-(propyl)-5-(thiophen-2-yl)-1H-
pyrrol-2-yl)-diazene 5a. Dark pink solid (86 mg, 47%) Mp
109e111 ꢂC. 1H NMR (acetone-d6)
d
1.04 (t, J¼7.2 Hz, 3H, CH3),
4.2.7. 2-(6-Methylbenzo[d]thiazol-2-yl)-1-(1-(propyl)-5-(thiophen-
2-yl)-1H-pyrrol-2-yl)-diazene 6a. Violet solid (34 mg, 18%). Mp
1.93e1.99 (m, 2H, CH2), 4.62 (t, J¼7.2 Hz, 2H, NCH2), 6.89 (d, 1H,
J¼4.6 Hz, 40-H), 7.15 (d, 1H, J¼4.6 Hz, 30-H), 7.32 (m, 400-H), 7.46 (dt,
1H, J¼7.7 and J¼1.2 Hz, 5-H), 7.54 (dt, 1H, J¼7.7 and J¼1.2 Hz, 6-H),
7.61 (dd, 1H, J¼3.8 and J¼1.2 Hz, 300-H). 7.79 (dd, 1H, J¼5.1 and
J¼1.2 Hz, 500-H) 7.99e8.02 (m, 2H, 4-H and 7-H). 13C NMR (acetone-
120e121 ꢂC. 1H NMR (acetone-d6)
d
1.03 (t, J¼7.0 Hz, 3H, CH3),
1.92e1.96 (m, 2H, CH2), 2.86 (br s, 3H, Ph-CH3), 4.61 (t, J¼7.0 Hz, 2H,
NCH2), 6.87 (d, 1H, J¼4.6 Hz, 40-H), 7.12 (d, 1H, J¼4.6 Hz, 30-H), 7.32
(dd, J¼5.2 and J¼3.7, 1H, 400-H), 7.35 (dd, 1H, J¼8.3 and J¼2 Hz, 5-H),
7.59 (br d,1H, J¼3.7 Hz, 300-H), 7.79 (d,1H, J¼2 Hz, 7-H), 7.80 (dd,1H,
J¼3.7 and 1.0 Hz, 500-H), 7.88 (d, 1H, J¼8.3 Hz, 4H). 13C NMR (ace-
d6) d 11.4, 25.4, 46.6,106.2,116.3,123.0 (2 overlapped signals),124.4,
127.0, 127.2, 128.9, 129.1, 129.3, 132.9, 134.7, 138.6, 154.32, 178.5.
/dm3 molꢁ1 cmꢁ1 35,400). IR (CHCl3):
n
tone-d6)
d 11.4, 21.6, 25.4, 46.5, 106.1, 116.1, 122.7, 124.1, 128.7,
lmax(Dioxane)/nm 500 (
3
128.8, 128.9, 129.3, 133.0, 134.9, 137.5, 138,1, 147,9, 152.4, 177.6.
2919, 1506, 1454, 1366, 1341, 1314, 1189, 1158, 1132, 1046, 838, 757,
728, 702 cmꢁ1. MS (microTOF) m/z (%)¼353 ([MþH]þ, 100), 352
(Mþ, 30), 323 (6), 255 (37). HMRS: m/z (MicroTOF) for C18H17N4S2;
calcd; 353.09016; found: 353.08891.
lmax(Dioxane)/nm 503 (
3
/dm3 molꢁ1 cmꢁ1 30,060). IR (CHCl3):
n
3851, 3646, 2925, 1731, 1597, 1505, 1455, 1436, 1413, 1342, 1311,
1260, 1230, 1208, 1186, 1171, 1110, 1045, 1017, 969, 841, 803, 774,
759, 727 cm ꢁ1. MS (microTOF) m/z (%): 366 ([M]þ,18), 338 (20), 309
(18), 296 (30), 206 (40), 164 (80), 163 (100), 136 (18), 121 (35), 110
(21), 97 (31). HMRS: m/z (MicroTOF) for C19H18N4S2; calcd
366.0973; found: 366.0989.
4.2.4. 2-(Benzo[d]thiazol-2-yl)-1-(1-(4-methoxyphenyl)-5-(thio-
phen-2-yl)-1H-pyrrol-2-yl)-diazene 5b. Violet solid (58 mg, 27%).
Mp 161e162 ꢂC. 1H NMR (acetone-d6)
d 3.99 (s, 3H, OCH3), 7.08e7.14
(m, 1H, 400-H), 7.13 (d, 1H, J¼4.9 Hz, 40-H), 7.21 (dd, 2H, J¼9.2 Hz, 3000
-
H and 5000-H), 7.27 (d, 1H, J¼4.9 Hz, 30-H), 7.29 (dd, 1H, J¼3.6 and
J¼1.2 Hz, 300-H), 7.40 (dt, 1H, J¼7.6 and J¼1.2 Hz, 5-H), 7.45e7.54
(m, 4H, 6-H, 300-H, 2000-H and 6000-H), 7.56 (dd,1H, J¼5.3 and J¼1.2 Hz,
4.2.8. 2-(Benzo[d]thiazol-6-yl)-1-(1-(propyl)-5-(thiophen-2-yl)-1H-
pyrrol-2-yl)-diazene 7a. Orange solid (148 mg, 81%). Mp
105e107 ꢂC. 1H NMR (acetone-d6)
d
0.93 (t, 3H, J¼7.5 Hz, CH3),

5196
M.M.M. Raposo et al. / Tetrahedron 67 (2011) 5189e5198
1.80e1.92 (m, 2H, CH2), 4.58 (t, 2H, J¼7.5 Hz, NCH2), 6.58 (d, 1H,
J¼4.7 Hz, 40-H), 6.83 (d, 1H, J¼4.7 Hz, 30-H), 7.19e7.22 (m, 1H, 400-H),
7.35 (dd, 1H, J¼3.5 and 1.2 Hz, 300-H), 7.59 (dd, 1H, J¼5.0 and 1.2 Hz,
500-H), 8.06 (dd,1H, J¼8.7 and 1.8 Hz, 5-H), 8.18 (d,1H, J¼8.7 Hz, 4-H),
8.53 (d, 1H, J¼1.8 Hz, 7-H). 9.30 (s, 1H, 2-H). 13C NMR (acetone-d6)
500 ꢂC. 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.
d
11.4, 25.5, 46.0, 101.2, 113.3, 117.1, 120.6, 124.4, 127.1, 127.2, 128.7,
132.9, 134.1, 135.8, 148.3, 152.2, 154.8, 156.9. lmax(Dioxane)/nm 451
(3
/dm3 molꢁ1 cmꢁ1 26,800). IR (CHCl3):
n
3103, 3071, 2963, 2931,
2873,1790,1693,1593,1546,1537,1501,1463,1432,1403,1366,1343,
1328, 1293, 1247, 1224, 1207, 1178, 1142, 1116, 1081, 1035, 955, 936,
912, 844 cmꢁ1. MS (EI-TOF) m/z (%)¼352 ([M]þ, 75), 206 (30), 203
(69), 191 (100), 177 (53), 163 (50), 134 (52), 121 (68). HMRS: m/z
(MicroTOF) for C18H16N4S2; calcd 352.0816; found: 352.0807.
4.2.9. 2-(Benzo[d]thiazol-6-yl)-1-(1-(4-methoxyphenyl)-5-(thio-
phen-2-yl)-1H-pyrrol-2-yl)-diazene 7b. Orange solid (175 mg, 81%).
Mp 172e174 ꢂC. 1H NMR (acetone-d6)
d 3.96 (s, 3H, OCH3), 6.87 (d,
1H, J¼4.4 Hz, 40-H), 7.00e7.02 (m, 3H, 30-H, 300-H, and 400-H), 7.16 (d,
4.4. Solvatochromic study
2H, J¼8.0 Hz, 3000-H and 5000-H), 7.42e7.46 (m, 3H, 500-H, 2000-H, and
000-H), 7.76 (dd, 1H, J¼8.8 and J¼2.1 Hz, 5-H), 8.07 (dd, 1H, J¼8.8 Hz,
The solvatochromic study was performed using 10ꢁ4 mol dmꢁ3
solutions of dyes 5e7 in several solvents at room temperature using
a Shimadzu UV/2501PC spectrophotometer.
6
4-H), 8.33 (d,1H, J¼2.1 Hz, 7-H), 9.31 (s,1H, 2-H). 13C NMR (acetone-
d6)
d 55.9, 101.5, 112.4, 115.0, 117.6, 120.1, 124.4, 126.8, 127.2, 128.2,
130.4, 131.5, 134.4, 135.3, 135.7, 150.3, 152.1, 154.9, 157.1, 161.1.
lmax(Dioxane)/nm 461 (
3
/dm3 molꢁ1 cmꢁ1 21,900). IR (CHCl3):
n
4.5. Nonlinear optical measurements using the Hyper-Rayleigh
scattering (HRS) method
2955, 2924, 2854, 1515, 1462, 1378, 1297, 1251, 1206, 1168, 1035,
1018, 872, 836, 779, 722 cmꢁ1. MS (EI-TOF) m/z (%): 416 ([M]þ, 100),
373 (16), 270 (97), 255 (40), 210 (18), 134 (21), 121 (77). HMRS: m/z
(EI-TOF) for C22H16N4OS2; calcd 416.0766; found: 416.0782.
Hyper-Rayleigh scattering (HRS) was used to measure the first
hyperpolarizability
b of response of the molecules studied. The
experimental set-up for Hyper-Rayleigh measurements employed
a q-switched Nd:YAG laser and is similar to the one presented by
Clays and Persoons.18 Details of the experimental procedure used
have been previously published.15a We emphasize that particular
care was taken to avoid reporting artificially high first hyper-
polarizabilities by using a pair of interference filters to estimate and
correct for the presence of a possible contamination of the Hyper-
Rayleigh signal by molecular fluorescence near 532 nm. Further
cautions include normalizing the Hyper-Rayleigh signal at each
pulse using the second harmonic signal from a 1 mm quartz plate to
compensate for fluctuations in the temporal profile of the laser
pulses due to longitudinal mode beating and the filtering of the
solutions, using a 0.2 mm porosity filter, to avoid spurious signals
from suspended impurities.
Dioxane was used as a solvent for all measurements. The small
Hyper-Rayleigh signal that arises from dioxane was taken into ac-
count according to the expression
4.2.10. 2-(Benzo[d]thiazol-6-yl)-1-(1-(4-fluorophenyl)-5-(thiophen-
2-yl)-1H-pyrrol-2-yl)-diazene 7c. Orange solid (49 mg, 30%). Mp
191e192 ꢂC. 1H NMR (acetone-d6)
d
6.87 (d, 1H, J¼4.5 Hz, 40-H), 7.00
(d, 1H, J¼4.5 Hz, 30-H) 7.00e7.04 (m, 2H, 300-H and 400-H), 7.41 (t, 2H,
J¼8.8 Hz, 3000-H and 5000-H), 7.45 (dd, 1H, J¼5.1 and J¼1.2 Hz, 500-H),
7.61 (dd, 2H, J¼8.8 and 4.8 Hz, 2000-H and 6000-H), 7.76 (dd, 1H, J¼8.8
and J¼2.0 Hz, 5-H), 8.08 (d, 1H, J¼9.2 Hz, 4-H), 8.34 (d, 1H, J¼1.6 Hz,
7-H). 9.31 (s, 1H, 2-H). 13C NMR (acetone-d6) dd 101.4, 112.6, 112.6,
and 116.8 (d, J¼23 Hz, C3000 and C5000), 117.7, 120.2, 124.4, 127.0, 127.2,
128.3, 132.5, and 132.6 (d, J¼9 Hz, C2000 and C6000) 134.1, 134.20,
134.23, 134.8, 135.8, 150.1, 152.2, 155.1, 157.2, 162.4, and 164.9 (d,
J¼245 Hz, C4000). lmax(Dioxane)/nm 452 (
3
/dm3 molꢁ1 cmꢁ1 36,130).
IR (CHCl3): n 3062, 2354,1522,1463,1434,1383,1330, 844, 836, 783,
698 cmꢁ1. Anal. Calcd for C21H13FN4S2: C, 62.36, H, 3.24, N, 13.85.
Found: C, 62.04, H, 3.30, N, 13.46.
4.3. Instruments
ꢀ
D
E
D
Eꢁ
I2 ¼ G Nsolvent bs2olvent þ Nsolute b2solute Iu2
u
NMR spectra were obtained on a Varian Unity Plus Spectrometer
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 ꢂC. All chemical shifts are given in
parts per million using dH Me4Si¼0 ppm as reference and J values are
given in hertz. Assignments were made by comparison of chemical
shifts, peak multiplicities and J values and were supported by spin
decoupling-double resonance and bidimensional heteronuclear
HMBC and HMQC correlation techniques. IR spectra were de-
termined on a BOMEM MB 104 spectrophotometer using KBr discs.
UVevis absorption spectra (200e800 nm) were obtained using
a Shimadzu UV/2501PC spectrophotometer. Mass spectrometry
analyses were performed at the ‘C.A.C.T.I.-Unidad de Espectrometria
de Masas’ at the University of Vigo, Spain. 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 minꢁ1 flow rate. All samples were subjected to
a 20 ꢂC minꢁ1 heating rate and were characterized between 25 and
where the factor G is an instrumental factor that takes into account
the detection efficiency (including geometrical factors and linear
absorption or scattering of the second harmonic light on its way to
the detector) and local field corrections. The concentrations of the
solutions under study were chosen so that the corresponding Hy-
per-Rayleigh signals fell well within the dynamic range of detection
system. We have assumed that molecules could be treated as linear
donoreacceptor chromophores with a first hyperpolarizability
tensor dominated by a single longitudinal element, b333 b, asso-
¼
ciated with the charge transfer axis. In this case the measured Hy-
per-Rayleigh signal is related to the value of the dominant tensor
qffiffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
element through the relation
b
¼
h
b2HRSi ¼
h
b2ZZZi þ hb2XZZ
i
were the angle brackets denote an average for the random
molecular orientations in the solution. Here we have adopted
a laboratory reference frame in which the incident fundamental
light propagates along the X axis and was vertically polarized along
the Z direction, while no polarization discriminationwas carried out

M.M.M. Raposo et al. / Tetrahedron 67 (2011) 5189e5198
5197
Wiley: New York, NY, 1985; (e) Shu, C.-F.; Wang, Y.-K. J. Mater. Chem. 1998,
8, 833.
in the detection arm, which is orientated along the laboratory frame
Y axis. To calibrate our system a reference solution of p-nitroaniline
(pNA) dissolved in dioxane at a concentration of 1ꢀ10ꢁ2 mol dmꢁ3
(external reference method). Kaatz and Shelton19a have measured
the value of b333 of pNA in dioxane at 1064 nm to be 40ꢀ10ꢁ30 esu
using the so-called Taylor convention for the first hyper-
polarizability.19b This value has been corrected by a factor of 1.88 for
the most recent calibration factor of the Hyper-Rayleigh scattering
signal of CCl4, which was used as a reference.23
7. (a) Comprehensive Heterocyclic Chemistry; Katritzky, A. R., Rees, C. W., Eds.;
Pergamon: Oxford, 1984; (b) Bradamante, S.; Facchetti, A.; Pagani, G. A. J. Phys.
Org. Chem. 1997, 10, 514; (c) Facchetti, A.; Abbotto, A.; Beverina, L.; van der
Boom, M. E.; Dutta, P.; Evmenenko, G.; Pagani, G. A.; Marks, T. J. Chem. Mater.
2003, 15, 1064; (d) Abbotto, A.; Beverina, L.; Bradamante, S.; Facchetti, A.;
Klein, C.; Pagani, G. A.; Redi-Abshiro, M.; Wortmann, R. Chem.dEur. J. 2003, 9,
1991; (e) Abbotto, A.; Beverina, L.; Bradamante, S.; Facchetti, A.; Pagani, G. A.;
Bozio, R.; Ferrante, C.; Pedron, D.; Signorini, R. Synth. Met. 2003, 14, 4996; (f)
Facchetti, A.; Beverina, L.; van der Boom, M. E.; Dutta, P.; Evmenenko, G.;
Pagani, G. A.; Marks, T. J. J. Am. Chem. Soc. 2006, 128, 2142; (g) Davies, J. A.;
Elangovan, A.; Sullivan, P. A.; Olbricht, B. C.; Bale, D. H.; Ewy, T. R.; Isborn, C.
M.; Eichinger, B. E.; Robinson, B. H.; Reid, P. J.; Li, X.; Dalton, L. R. J. Am. Chem.
Soc. 2008, 130, 10565; (h) Abbotto, A.; Beverina, L.; Manfredi, N.; Pagani, G. A.;
Archetti, G.; Kuball, H. G.; Wittenburg, C.; Heck, J.; Holtmann, J. Chem.dEur. J.
2009, 15, 6175.
Acknowledgements
ˇ
~
Thanks are due to the Fundac¸ ao para a Ciencia e Tecnologia
8. (a) Raposo, M. M. M.; Sousa, A. M. R. C.; Fonseca, A. M. C.; Kirsch, G. Tetrahedron
2005, 61, 8249; (b) Raposo, M. M. M.; Sousa, A. M. R. C.; Kirsch, G.; Ferreira, F.;
Belsley, M.; Matos Gomes, E.; Fonseca, A. M. C. Tetrahedron 2005, 61, 11991; (c)
Raposo, M. M. M.; Sousa, A. M. R. C.; Kirsch, G.; Cardoso, P.; Belsley, M.; Matos
Gomes, E.; Fonseca, A. M. C. Org. Lett. 2006, 8, 3681; (d) Batista, R. M. F.; Costa,
S. P. G.; Malheiro, E. L.; Belsley, M.; Raposo, M. M. M. Tetrahedron 2007, 63,
4258; (e) Batista, R. M. F.; Costa, S. P. G.; Belsley, M.; Raposo, M. M. M. Tetra-
hedron 2007, 63, 9842; (f) Pina, J.; Seixas de Melo, S.; Batista, R. M. F.; Costa, S. P.
G.; Raposo, M. M. M. Phys. Chem. Chem. Phys. 2010, 12, 9719; (g) Raposo, M. M.
M.; Fonseca, A. M. C.; Castro, M. C. R.; Belsley, M.; Cardoso, M. F. S.; Carvalho, L.
M.; Coelho, P. J. Dyes Pigments 2011, 91, 62.
(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.
We acknowledge the computational support of the Project
SeARCH (Services and Advanced Research Computing with HTC/
HPC clusters), funded by FCT under contract CONC-REEQ/443/EEI/
2005.
9. For some recente examples see: (a) Batista, R. M. F.; Costa, S. P. G.; Raposo, M.
M. M. Tetrahedron Lett. 2004, 45, 2825; (b) Lacroix, P. G.; Padilla-Martínez, I. I.;
ꢀ
Lopez, H. S.; Nakatani, K. New J. Chem. 2004, 28, 542; (c) Martínez-Rubio, M.;
ꢂ
Velasco, D.; Brillas, E.; Julia, L. Tetrahedron 2004, 60, 285; (d) Hrobarik, P.;
Zahradnik, P.; Fabian, W. M. F. Phys. Chem. Chem. Phys. 2004, 6, 495; (e) Lopez-
ꢀ
ꢀ
Calahorra, F.; Molinos-Gomez, A.; Vidal, X.; Maymo, M.; Velasco, D.; Martorell,
ꢀ
ꢀ
ꢀ
J.; Lopez-Calahorra, F. Tetrahedron 2005, 61, 9075; (f) Hrobarik, P.; Sigmundova,
I.; Zahradník, P. Synthesis 2005, 600; (g) Cui, Y.; Qian, G.; Chen, L.; Wang, Z.;
Gao, J.; Wang, M. J. Phys. Chem. B 2006, 110, 4105; (h) Costa, S. P. G.; Batista, R.
M. F.; Cardoso, P.; Belsey, M.; Raposo, M. M. M. Eur. J. Org. Chem. 2006, 17, 3938;
(i) Coe, B. J.; Harris, J. A.; Hall, J. J.; Brunschwig, B. S.; Hung, S.-T.; Libaers, W.;
Clays, K.; Coles, S. J.; Horton, P. N.; Light, M. E.; Hursthouse, M. B.; Garín, J.;
Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2011.05.053. These data in-
clude MOL files and InChiKeys of the most important compounds
described in this article.
ꢀ
ꢀ
Orduna, J. Chem. Mater. 2006, 18, 5907; (j) Hrobarik, P.; Sigmundova, I.;
Zahradník, P.; Kasak, P.; Arion, V.; Franz, E.; Clays, K. J. Phys. Chem. C 2010, 114,
ꢀ
22289; (k) Razus, A. C.; Birzan, L.; Surugiu, N. M.; Corbu, A. C.; Chiraleu, F. Dyes
Pigments 2007, 74, 26; (l) Chen, L.; Cui, Y.; Qian, G.; Wang, M. Dyes Pigments
ꢀ
2007, 73, 338; (m) Zajac, M.; Hrobarik, P.; Magdolen, P.; Foltínova, P.; Zahradník,
P. Tetrahedron 2008, 64, 10605; (n) Quist, F.; Christophe, M. L.; Velde, V.; Didier,
D.; Teshome, A.; Asselberghs, I.; Clays, K.; Sergeyev, S. Dyes Pigments 2009, 81,
203; (o) Coe, B. J.; Foxon, S. P.; Harper, E. C.; Harris, J. A.; Helliwell, M.; Raftery, J.;
Asselberghs, I.; Clays, K.; Franz, E.; Brunschwig, B. S.; Fitch, A. G. Dyes Pigments
References and notes
1. (a) Facchetti, A.; Abbotto, A.; Beverina, L.; van der Boom, M. E.; Dutta, P.; Ev-
menenko, G.; Marks, T. J.; Pagani, G. A. Chem. Mater. 2002, 14, 4996; (b) Au-
debert, P.; Kamada, K.; Matsunaga, K.; Ohta, K. Chem. Phys. Lett. 2003, 367, 62;
(c) Trofimov, B. A.; Markova, M. V.; Morozova, L. V.; Schmidt, E. Y.; Senotrusova,
E. Y.; Myachina, G. F.; Myachin, Y. A.; Vakul’skaya, T. I.; Mikhaleva, A. I. Polym.
Sci. B 2007, 49, 292; (d) Schmidt, E. Y.; Ushakov, I. A.; Zorina, N. V.; Mikhaleva,
A. I.; Trofimov, B. A. Mendeleev Commun. 2011, 21, 36 and references cited
therein.
ꢀ
ꢀ
ꢀ
ꢃ
2009, 82, 171; (p) Hrobarikova, V.; Hrobarik, P.; Gajdos, P.; Fitilis, I.; Fakis, M.;
ꢀ
Persephonis, P.; Zahradník, P. J. Org. Chem. 2010, 75, 3053; (q) Andreu, R.; Galan,
E.; Garín, J.; Orduna, J.; Alicante, R.; Villacampa, B. Tetrahedron Lett. 2010,
51, 6863.
10. Coelho, P. J.; Carvalho, L. M.; Fonseca, A. M. C.; Raposo, M. M. M. Tetrahedron
Lett. 2006, 47, 3711.
11. (a) Raposo, M. M. M.; Sampaio, A. M. B. A.; Kirsch, G. Synthesis 2005, 2, 199; (b)
Raposo, M. M. M.; Sousa, A. M. R. C.; Fonseca, A. M. C.; Kirsch, G. Tetrahedron
2006, 62, 3493.
2. (a) Towns, A. D. Dyes Pigments 1999, 42, 3; (b) Yesodha, S. K.; Pillai, C. K. S.;
Tsutsumi, N. Prog. Polym. Sci. 2004, 29, 45; (c) Astrand, P.-O.; Sommer-Larsen, P.;
ꢁ
Hvilsted, S.; Ramanujam, P. S.; Bak, K. L.; Sauer, S. P. A. Chem. Phys. Lett. 2000,
325, 115; (d) Wang, Y.; Ma, J.; Jiang, Y. J. Phys. Chem. A 2005, 109, 7197; (e) Yager,
K. G.; Barrett, C. J. J. Photochem. Photobiol., A 2006, 182, 250; (f) Nabeshima, Y.;
Shishido, A.; Kanazawa, A.; Shiono, T.; Ikeda, T.; Hiyama, T. Chem. Mater. 1997, 9,
1480; (g) Zhu, Z.; Wan, Y.; Lu, Y. Macromolecules 2003, 36, 9585.
3. For some recent examples in pyrrole azo dyes see: (a) Mikroyannidis, J. A.;
Sharma, G. D.; Sharma, S. S.; Vijay, Y. K. J. Phys. Chem. C 2010, 114, 1520; (b)
Mikroyannidis, J. A.; Roy, M. S.; Sharma, G. D. J. Power Sources 2010, 195, 5391;
(c) Mikroyannidis, J. A.; Tsagkournos, D. V.; Sharma, S. S.; Kumar, A.; Vijay, Y. K.;
Sharma, G. D. Sol. Energy Mater. Sol. Cells 2010, 94, 2318.
4. For some recent examples in pyrrole azo dyes see: (a) Mohr, G. J.; Nezel, T.;
Spichiger, U. E. Anal. Chim. Acta 2000, 414, 181; (b) DiCesare, N.; Lakowicz, R. C.
Org. Lett. 2001, 3, 3891; (c) Mohr, G. J. Chem. Commun. 2002, 2646; (d) Wagner-
Wysieka, E.; Luboch, E.; Kowalczyk, M.; Biernat, J. F. Tetrahedron 2003, 59, 4415;
(e) Gunnlaugsson, T. J.; Leonard, P.; Murray, N. S. Org. Lett. 2004, 6, 1557; (f)
Zhang, D.; Zhang, M.; Liu, Z.; Yu, M.; Li, F.; Yi, T.; Huang, C. Tetrahedron Lett.
2006, 47, 7093; (g) Trofimov, B. A.; Schmidt, E. Y.; Mikhaleva, A. I.; Vasiltsov, A.
M.; Zaitsev, A. B.; Smolyanina, N. S.; Senotrusova, E. Y.; Afonin, A. V.; Ushakov, I.
A.; Petrushenko, K. B.; Kazheva, O. N.; Dyachenko, O. A.; Smirnov, V. V.;
Schmidt, A. F.; Markova, M. V.; Morozova, L. Eur. J. Org. Chem. 2006, 4021; (h)
Cheng, Y.-F.; Liu, Z.-Q.; Shi, M.; Zhao, Q.; Li, F.-Y.; Yi, T.; Huang, C.-H. Chin. J.
Chem. 2007, 25, 616.
12. (a) Abbotto, A.; Bradamante, S.; Pagani, G. A. J. Org. Chem. 1996, 61, 1761; (b)
ꢀ
ꢀ
ꢀ
Hrobarik, P.; Horvath, B.; Sigmundova, I.; Zahradník, P.; Malkina, O. L. Magn.
Reson. Chem. 2007, 45, 942.
13. (a) Ogura, K.; Yanay, H.; Miokawa, M.; Akazome, M. Tetrahedron Lett. 1999, 40,
8887; (b) Chen, Y.; Harrison, W. T. A.; Imrie, C. T.; Ryder, K. S. J. Mater. Chem.
2002, 12, 579.
14. Dincer, H. A.; Sener, M. K.; Koca, A.; Gul, A.; Kocak, M. B. Electrochim. Acta 2008,
53, 3459.
15. (a) Herbivo, C.; Comel, A.; Fonseca, A. M. C.; Kirsch, G.; Belsley, M.; Raposo, M.
M. M. Dyes Pigments 2010, 86, 217; (b) O’Connor, M. J.; Yelle, R. B.; Linz, T. M.;
Haley, M. M. C. R. Chim. 2009, 12, 385.
16. (a) Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983,
48, 2877; (b) Kamlet, M. J.; Abboud, J.-M.; Abraham, M. H.; Taft, R. W. J. Am.
Chem. Soc. 1977, 99, 6027.
ˇ
€
€
17. See for example. (a) Bossard, G.; Knopfle, P.; Pretre, P.; Gunter, P. J. Appl. Phys.
1992, 71, 1594; (b) Kim, O.-K.; Fort, A.; Barzoukas, M.; Blanchard-Desce, M.;
Lehn, J.-M. J. Mater. Chem. 1999, 9, 2227 and references cited.
18. (a) Clays, K.; Persoons, A. Rev. Sci. Instrum. 1992, 63, 3285; (b) Clays, K.; Per-
soons, A. Phys. Rev. Lett. 1991, 66, 2980.
19. (a) Kaatz, P.; Shelton, D. P. J. Chem. Phys. 1996, 105, 3918; (b) Reis, H. J. Chem.
Phys. 2006, 125 ARTN 014506.
20. (a) Oudar, J. L. J. Chem. Phys. 1977, 67, 446; (b) Oudar, J. L.; Chemla, D. S. J. Chem.
Phys. 1977, 66, 2664; (c) Zyss, J.; Oudar, J. L. Phys. Rev. A 1982, 26, 2016.
21. (a) Ra, C. S.; Kim, S. C.; Park, G. J. Mol. Struct.-Theochem 2004, 677, 173; (b) Nandi,
P. K.; Panja, N.; Ghanty, T. K. J. Phys. Chem. A 2008, 112, 4844.
22. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;
Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.;
Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.;
5. (a) Dirk, C. W.; Katz, H. E.; Schilling, M. L.; King, L. A. Chem. Mater. 1990, 2, 700;
(b) Miller, R. D.; Lee, V. Y.; Moylan, C. R. Chem. Mater. 1994, 6, 1023; (c) Rao, V. P.;
Jen, AK.-Y.; Wong, K. Y.; Drost, K. J. Tetrahedron Lett. 1993, 34, 1747.
6. (a) Varanasi, P. R.; Jen, AK.-Y.; Chandrasekhar, J.; Namboothiri, I. N. N.; Rathna,
A. J. Am. Chem. Soc. 1996, 118, 12443; (b) Albert, I. D. L.; Marks, T. J.; Ratner, M. A.
J. Am. Chem. Soc. 1997, 119, 6575; (c) Breitung, E. M.; Shu, C.-F.; McMahon, R. J.
J. Am. Chem. Soc. 2000, 122, 1154; (d) Gilchrist, T. L. Heterocyclic Chemistry;

5198
M.M.M. Raposo et al. / Tetrahedron 67 (2011) 5189e5198
Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Ha-
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O; Foresman,
J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.1; Gaussian:
Wallingford CT, 2009.
segawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers,
E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, N.
J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.;
€
23. Pyatt, R. D.; Shelton, D. P. J. Chem. Phys. 2001, 114, 9938.