Synthesis and characterization of novel second-order NLO-chromophores bearing pyrrole as an electron donor group

Article abstract of DOI:10.1016/j.tet.2012.07.082

Two series of novel push-pull 1-(4-(thiophen-2-yl)phenyl)-1H-pyrroles 3-5 were designed to explore the consequence of using different electron accepting moieties linked to the thiophene at the arylthiophene bridge or to the pyrrole heterocycle, which plays the role of donor group. Compound 2 showed a different reactivity behavior in the presence of the Vilsmeier reagent or with tetracyanoethylene (TCNE) giving compounds 4a and 4b functionalized, respectively, at the 2 or on the 3-position of the pyrrole heterocycle. Their optical (linear and first hyperpolarizability), electrochemical, and thermal properties have been examined. 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, thiobarbituric acid derivative 5b functionalized in the thiophene ring exhibits the largest first hyperpolarizability (β=2480×10^-30 esu, T convention) compared to the corresponding compound 4c substituted on the pyrrole heterocycle (β=290×10^-30 esu, T convention). Good to excellent thermal stabilities were also obtained for push-pull compounds 4 and 5 (270-288 °C). This multidisciplinary study shows that modulation of the optical and electronic properties can be achieved by introduction of the acceptor groups in the thiophene of the arylthiophene bridge. The measured molecular first hyperpolarizabilities and the observed electrochemical behavior are quite sensitive to the position of acceptor group on the heterocyclic system (on the thiophene or on the pyrrole ring) as well as the strength of the acceptor moieties. Moreover, the combination of their good nonlinearity and high thermal stability make them good candidates for second-order nonlinear optical applications.

Full text of DOI:10.1016/j.tet.2012.07.082

Tetrahedron 68 (2012) 8147e8155
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis and characterization of novel second-order NLO-chromophores bearing
pyrrole as an electron donor group
a
b
a
a,
*
ꢀ
M. Cidalia R. Castro , M. Belsley , A. Maurício C. Fonseca , M. Manuela M. Raposo
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:
Received 31 May 2012
Received in revised form 18 July 2012
Accepted 24 July 2012
Available online 31 July 2012
Two series of novel pushepull 1-(4-(thiophen-2-yl)phenyl)-1H-pyrroles 3e5 were designed to explore
the consequence of using different electron accepting moieties linked to the thiophene at the arylth-
iophene bridge or to the pyrrole heterocycle, which plays the role of donor group. Compound 2 showed
a different reactivity behavior in the presence of the Vilsmeier reagent or with tetracyanoethylene
(TCNE) giving compounds 4a and 4b functionalized, respectively, at the 2 or on the 3-position of the
pyrrole heterocycle. Their optical (linear and first hyperpolarizability), electrochemical, and thermal
properties have been examined. Hyper-Rayleigh scattering (HRS) in dioxane solutions using a funda-
mental wavelength of 1064 nm was employed to evaluate their second-order nonlinear optical proper-
ties. Of these systems, thiobarbituric acid derivative 5b functionalized in the thiophene ring exhibits the
Keywords:
Pushepull heterocyclic systems
Pyrrole
Thiophene
Reactivity studies
Auxiliary donor heterocycle
First hyperpolarizability (b)
Hyper-Rayleigh scattering (HRS)
Redox properties
largest first hyperpolarizability (
b
¼2480ꢀ10ꢁ30 esu, T convention) compared to the corresponding
compound 4c substituted on the pyrrole heterocycle (
b
¼290ꢀ10ꢁ30 esu, T convention). Good to excellent
thermal stabilities were also obtained for pushepull compounds 4 and 5 (270e288 ꢂC). This multidis-
ciplinary study shows that modulation of the optical and electronic properties can be achieved by in-
troduction of the acceptor groups in the thiophene of the arylthiophene bridge. The measured molecular
first hyperpolarizabilities and the observed electrochemical behavior are quite sensitive to the position of
acceptor group on the heterocyclic system (on the thiophene or on the pyrrole ring) as well as the
strength of the acceptor moieties. Moreover, the combination of their good nonlinearity and high
thermal stability make them good candidates for second-order nonlinear optical applications.
Ó 2012 Elsevier Ltd. All rights reserved.
Thermal stability
1. Introduction
compounds allows easy optimization of their structural character-
istics, which is necessary in order to maximize the NLO properties.
Over the last three decades there has been an intense research
effort aimed at improving the nonlinear optical response of
pushepull type organic chromophores, motivated by their strong
potential to make fundamental improvements in organic modern
communication technology, e.g., ultrafast image-processing, optical
data processing, transmission, and storage.
Several studies have demonstrated that organic materials have
ultrafast response times, low dielectric constants, are easier to
tailor or process, and possess stronger NLO responses when com-
pared to inorganic compounds. These advantages are in large part
due to their optical properties derived from the high electronic
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.1
A combination of synthetic innovations2 reinforced by theoret-
ical chemical calculations3 and confirmed by structural and optical
characterizations has shown that the usage of easily delocalized
five-membered heteroaromatic rings (usually thiophene, furan,
thiazole, and imidazole) instead of benzene ring results in an en-
hanced molecular nonlinear response as quantified through the
first molecular hyperpolarizability b.
polarization of the
distortions of the atoms in the crystal lattice, which is a slower
process. The ease of synthesis and functionalization of organic
p
-electrons of the molecules instead of the
Although a large variety of donor, acceptor, and spacer groups
have been used for the design of NLO-chromophores, the use of the
pyrrole heterocycle as conjugated bridge and/or as a strong donor
moiety has rarely appeared in the NLO literature before 20053aec,4
probably due to the difficulty of their synthesis. Even rare are
examples of donoreacceptor pyrrole derivatives specifically
designed for second-order NLO applications3a,b,4a,c
* 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 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.07.082

8148
M.C.R. Castro et al. / Tetrahedron 68 (2012) 8147e8155
Initial work developed in our research group, concerning
2. Results and discussion
2.1. Synthesis
pushepull functionalized thienylpyrroles,5 paved the way for
several authors to synthesize and evaluate the first hyper-
polarizability
b for several chromophores bearing pyrrole as p-
bridge or as auxiliary donating group.6
During the past years we have investigated the reactivity of
thienylpyrrole systems. Electrophilic substitution reactions of
thienylpyrroles were found to be very selective. The reactivity of
these systems has been demonstrated with the use of electrophilic
reactions (Vilsmeier formylation, tricyanovinylation, azo coupling)
producing derivatives with the electrophile substituted primarily
on the pyrrole ring.5,7d,e,8
Recently, in continuation of our work, we have designed, syn-
thesized, and characterized, several series of donoreacceptor
substituted 1-alkyl(aryl)-2-thienylpyrroles as donor moieties and/
p-bridge functionalized with different acceptor groups (benzo-
thiazolyl, benzimidazolyl, aryldiazene, and (benzo)thiazolyldia-
zene). 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 NLO and photochromic applications.7 Furthermore, our ex-
perimental and theoretical results concerning the auxiliary donor/
acceptor effect of electron-rich and electron-deficient heterocycles
In order to prepare two series of 1-(4-(thiophen-2-yl)phenyl)-
1H-pyrroles 2e5 functionalized selectively on the thiophene or on
the pyrrole heterocycles different synthetic methodologies were
applied (Schemes 1 and 2). Having in mind our earlier results5a,b,8
we decided to study the reactivity of the 1-(4-(thiophen-2-yl)
phenyl)-1H-pyrrole 2 through electrophilic substitution aiming for
the preparation of pyrrole derivatives functionalized selectively on
the pyrrole heterocycle. In first place, the 4-bromophenylpyrrole
precursor 1 (Ref. 9) was prepared through PaaleKnorr reaction
from 2,5-dimethoxy-tetrahydrofurane and 4-bromoaniline in high
yield (93%). Suzuki cross-coupling reactions of 1 with thiophen-2-
yl-boronic acid and 5-formyl-thiophen-2-yl-boronic acid gave, re-
spectively, compounds 2 and 3 in 32 and 61% yields. Compound 2
was used as precursor for the reactivity studies with several elec-
trophiles. Therefore, Vilsmeier formylation of compound 2, with
DMF/POCl3, proceeded selectively in position-2 of the pyrrole ring
to form the corresponding formyl-pyrrole 4a in 59% yield. On the
other hand, tricyanovinylation reaction of 2 with TCNE in DMF
using a similar previously reported procedure5b gave compound 4b
bearing the tetracyanovinyl group on position-3 of the pyrrole ring
in 43% yield. A possible explanation for the different regiose-
lectivity observed for compound 2 with TCNE could be probably
due to the possible steric influence of the bulky tricyanovinyl-
ethylene as well as their pre-coordination to the substrate.
on pushepull thienylpyrrole
that the position of the acceptor groups such as dicyanovinyl or
electron-deficient heterocycles, e.g., (benzo)thiazole or
p-conjugated systems have shown
benzimidazole on the thienylpyrrolyl system can have a clear in-
fluence on the electronic and optical properties of theses hetero-
cyclic chromophores.
We were therefore motivated to extend these studies and ex-
plore the potential of two new series of pushepull 1-(4-(thio-
phen-2-yl)phenyl)-1H-pyrroles 3e5 bearing pyrrole as donor
group and dicyanovinyl, tricyanovinyl, and thiobarbituric acid as
acceptor moieties linked to the arylthiophene spacer or to the
pyrrole ring.
The aim of this work is to demonstrate that combined tuning of
optical and electrochemical properties of donoreacceptor thio-
phen-2-yl-phenyl-pyrroles 3e5 can be achieved by functionaliza-
tion of these systems with acceptor groups linked to the pyrrole
ring (4) or to arylthiophene bridge (3 and 5). Consequently, two
series of structurally related chromophores have been designed
and the influence of the different position of the acceptor groups on
1-(4-(thiophen-2-yl)phenyl)-1H-pyrroles 4 and 5 was studied by
combined experimental studies of the electronic, linear, and non-
linear optical properties of these heterocyclic systems.
Despite the large number of donoreacceptor heterocyclic sys-
tems showing NLO properties reported in the literature, the present
concept of combining the donor properties of pyrrole heterocycle
linked to an arylthiophene bridge functionalized with strong ac-
ceptor groups has not, to the best of our knowledge, been pre-
viously communicated in the literature.
Additionally the thiobarbituric derivative 4c functionalized on
the pyrrole ring was prepared through Knoevenagel reaction of al-
dehyde 4a with 1,3-diethyl-2-thiobarbituric in 39% yield (Scheme 1).
The structures of pyrroles 4aec were unambiguously confirmed
by their analytical and spectral data. The 1H NMR spectra of com-
pounds 4a and 4c exhibit two sets of doublets and an apparent
triplet due to protons 4, 3, and 5, respectively. In the case of com-
pounds 4a and 4c functionalized with the acceptor groups on the 2-
position of the pyrrole ring the signal with a higher chemical shift is
S
N
O
N
N
MeO
OMe
4c
O
H2N
Br
O
AcOH / reflux
N
1
S
Br
O
N
S
NEt3
DCM / reflux
N
Pd(PPh3)4 / Na2CO3
DME / 80ºC
(HO)2B
S
O
CN
CHO
N
NC
NC
TCNE
DMF / POCl3
N
N
S
S
DMF / r.t.
S
4b
2
4a
Scheme 1. Synthesis of pyrroles 2 and 4aec.

M.C.R. Castro et al. / Tetrahedron 68 (2012) 8147e8155
8149
Pd(PPh3)4 / Na2CO3
DME / 80ºC
malononitrile
EtOH / reflux
CN
CN
N
N
N
S
S
CHO
5a
H
3
(HO)2B
CHO
S
Br
O
NEt3
DCM / reflux
N
S
1
N
O
O
N
N
S
S
N
5b
O
Scheme 2. Synthesis of 1-(4-(thiophen-2-yl)phenyl)-1H-pyrroles 3 and 5a,b.
Table 1
a doublet of doublets attributed to 3-H with coupling constants in
the range of 3.3e3.6 Hz and 1.2e1.5 Hz. Protons 4 in compounds 4a
and 4c appear also as doublet of doublets with coupling constants
in the range of 3.3e3.6 Hz and 2.7e2.8 Hz. On the other hand, for
compound 4b the signal with a higher chemical shift appears as an
apparent triplet with a coupling constant of 1.5 Hz, which was at-
tributed to 2-H.
Knoevenagel condensation of precursor 3 with malononitrile
and 1,3-diethyl-2-thiobarbituric gave compounds 5a,b selectively
functionalized with the acceptor groups on the thiophene ring in 43
and 65% yields, respectively (Scheme 2).
Derivatives 2 and 5 in which the pyrrole ring is functionalized
only in position-1 showed two 1H NMR signals at about
6.38e6.41 ppm and 7.12e7.17 ppm. Both signals appear as multi-
plets. These signals were attributed, respectively, to protons 3 and 4
and 2 and 5 in the pyrrole moiety. In all of the 1H NMR spectra of
derivatives 3 and 5a,b two doublets at about 7.42e7.56 ppm and
7.76e7.87 ppm with coupling constants of about 3.9e4.2 Hz were
detected. These signals were attributed, respectively, to the 4 and 3-
H protons of the thiophene moiety.
Chemical shifts of protons of the arylthiophene derivatives 5a and 610 in deuterated
chloroform at 400 MHz, with chemical shift values in parts per million
Compd
4-H
3-H
30-Hþ50-H
20-Hþ60-H
5a
H
2´
S
3´
CN
CN
7.45
7.80
7.48
7.75
N
N
3
4
5´ 6´
6
H
2´
3´
CN
CN
7.28
7.63
6.64
7.60
S
3
4
5´ 6´
donor group exhibits signals that are shifted upfield relative to the
corresponding pyrrole derivative 5a indicating the weaker donor
ability of the pyrrole heterocycle when compared to an N,N-dia-
lkylamino group. The electronic data obtained can be explained by
realizing that the pyrrole electron pair is involved in the aromatic
system and, thus, not available for delocalization to the acceptor
dicyanovinyl group.
2.2. Electronic structure analysis
The structures and charge-transfer transitions of the pyrrole
derivatives were first analyzed by 1H NMR spectroscopy and cyclic
voltammetry.
The 1H NMR chemical shifts reflect a charge separation in the
ground state, consequently the analysis of these data in
donoreacceptor compounds such as pyrroles 4 and 5 bearing
dicyanovinyl, tricyanovinyl, and thiobarbituric acid moieties also
We used cyclic voltammetry to investigate the redox behavior of
compounds 2e5 and obtain their HOMO and lowest occupied
molecular orbital (LUMO) energy levels. The redox properties of
compounds were measured using 0.1 mol dmꢁ3 DMF/TBABF4 so-
lutions (Table 2). Compounds 2e5 show an irreversible oxidation
process associated with the oxidation of the pyrrole group. The
oxidation potentials are influenced by the acceptor group
substituted on the pyrrole or on the thiophene ring. Therefore,
derivatives 3e5 functionalized with strong acceptor groups exhibit
an anodic shift compared to the unsubstituted precursor 2. For
example, in their reduction traces the cyclic voltammetry of com-
pound 2 exhibits two quasi reversible one-electron reduction
processes at ꢁ2.23 and ꢁ2.55 V. On the other hand compound 5a
showed reduction potentials at ꢁ1.40 and ꢁ2.00 V. Derivatives
bearing tricyanovinyl (4b), dicyanovinyl (5a) or thiobarbituric acid
group (5b) have a different electrochemical behavior. As an ex-
ample, Fig. 1 shows the cyclic voltammetry waves recorded for
compounds 4b and 5a.
confirms their pushepull character with
a significant intra-
molecular charge transfer (ICT) from the donor pyrrole heterocycle
to the acceptor groups and a high polarizability of the whole
donoreacceptor derivatives. This interpretation is supported by the
observation that the chemical shifts of the protons in compounds
5a,b bearing stronger acceptor groups on the arylthiophene spacer,
exhibit signals that are downfield relative to the unsubstituted
derivative 2 indicating CT from the donor to the acceptor group. On
the other hand, stronger acceptor groups substituted on position 2
or 3 of the pyrrole ring on series of compounds 4 also lead to upfield
signals for all protons thus again demonstrating the ease of electron
communication within the whole heterocyclic system.
At this stage, a comparison can also be made between the 1H
NMR data of the new pyrrole 5a with 2-dicyanovinyl-5-(4-
pyrrolidinophenyl)thiophene 6,10 recently reported by us (Table 1).
The electronic nature of the donor group, N,N-dialkylamino for 6 or
heteroaromatic for 5a had a clear influence on the 1H NMR data of
these compounds. Compound 6 bearing a pyrrolidino moiety as
The electrochemical response of the tricyanovinyl compound 4b
consists of stable cathodic processes. The first reduction process
near ꢁ1.10 V is attributed to the tricyanovinyl group due to its

8150
M.C.R. Castro et al. / Tetrahedron 68 (2012) 8147e8155
Table 2
Electrochemical data for compounds 2e5
Compd
Reductiona
Oxidationa
1
2
3
ꢁ Epc (V)
D
E
b (mV)
ꢁ Epc (V)
D
220
d
Eb (mV)
ꢁ Epc (V)
D
Eb (mV)
Epa (V)
0.88
0.91
0.91
0.93
0.95
0.92
0.98
HOMOc (eV)
ꢁ5.68
LUMOc (eV)
ꢁ2.57
Band gapd (eV)
3.13
3.01
3.06
2.03
2.24
2.32
2.22
2
3
2.23
2.10
2.15
1.10
1.29
1.40
1.24
180
130
63
60
72
d
2.55
2.60
2.56
1.84
2.21
2.00
1.94
d
d
d
ꢁ5.71
ꢁ2.70
4a
4b
4c
5a
5b
ꢁ5.71
ꢁ2.65
70
d
d
70
2.90
d
190
d
ꢁ5.73
ꢁ3.70
ꢁ5.75
ꢁ3.51
d
d
ꢁ5.72
ꢁ3.40
d
d
d
ꢁ5.78
ꢁ3.56
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 versus the ferrocenium/ferrocene couple. Epc and Epa correspond to the cathodic and anodic peak potentials, respectively.
b
D
E¼jEredꢁEoxj.
c
EHOMO and ELUMO were calculated according to the energy level of the ferrocene reference 4.80 eV below vacuum level. EHOMO¼ꢁ(4.80+Epa
) (eV) and
ELUMO¼ꢁ(1Epcþ4.80) (eV).
d
10
Band gap calculated from the difference between ELUMO and EHOMO
.
for 6) that could be due to the stronger electron-donating ability of
pyrrolidino group compared to the pyrrole heterocycle. These re-
sults are in agreement with the previous 1H NMR analysis that
showed increased electron densities for compound 6 compared to
5a.
5a
20 μA
The HOMO and LUMO energies of all compounds were calcu-
lated according to the energy level of the ferrocene reference
(4.8 eV below vacuum level)13,14 Accordingly, the HOMO energy
levels of 2e5 were between ꢁ5.68 and ꢁ5.78 eV, showing the
variations resulting from the presence of different electron-
withdrawing substituents on the pyrrole or on the thiophene het-
erocycles. The LUMO energy levels of 2e5 were all located between
ꢁ2.57 and 3.70 eV thus, the energy of this orbital is strongly
influenced by the presence of electron-withdrawing groups in the
system. In addition, the electrochemical band gaps, estimated from
the difference between the onset potentials for oxidation and re-
duction, were in the range 2.03e3.13 eV. The efficient
donoreacceptor conjugation leads to a lowering of the HOMO and
the LUMO levels in 3 and 5a,b, revealing a more efficient coupling
between the 1-(4-(thiophen-2-yl)phenyl)-1H-pyrrole system and
the formyl, dicyanovinyl, and thiobarbituric acceptor groups.
The analysis of the electrochemical data for compounds 2e5
showed that the functionalization of compound 2 with acceptor
groups leads to two series of pushepull chromophores 4 and 5 and
in both cases the electronic acceptor strength has a strong influence
4b
-1.8
-1.4
-1.0
-0.6
E (V vs. fc+/fc)
Fig. 1. Cyclic voltammograms of compounds 4b and 5a (1.0ꢀ10ꢁ3 mol dmꢁ3) in DMF,
0.1 mol dmꢁ3 [NBu4][BF4] at a vitreous carbon electrode, scan rate 0.1 V sꢁ1
.
on the electronic delocalization of the
Moreover, as expected the stronger acceptor substituents leads to
smaller band gaps values.
p-conjugated systems.
strongest electron-withdrawing nature. The first sight of the re-
duction at ꢁ1.10 V can be related to the to the geminal terminal
(CN)2 groups, whereas the second reduction apparently involves
mainly the cross-conjugated CN group.11
For compounds 5a,b, we observe that the first reduction process
is irreversible (Fig. 1). In these compounds, after the reduction,
negative charges in the anionic species are mainly concentrated in
the vicinity of the electron-withdrawing moieties. For example, in
the vinyl C(H)-atom linked the dicyano group, for 5a, or to other
acceptor groups such as in compound 5b. Therefore those anion
radicals might react between them forming dimmer molecules by
generation of new CeC bonds. Thus the difference in the reduction
behavior for these vinylic derivatives could be attributed to the
presence of a quite labile hydrogen atom.5c,11,12
2.3. Optical properties
Pyrrole derivatives 2e5 showed good solubility in common
polar and non-polar organic solvents such as dioxane, diethyl ether,
ethanol, DMF, and DMSO. The extinction coefficients (
wavelength maxima lmax of compounds 2e5 in several solvents,
3
) and the
are summarized in Table 3 and were compared with the
p values
*
for each solvent, as determined by Kamlet and Taft.15 All chromo-
phores exhibit broad and intense CT absorptions in the visible re-
gion from 306 nm to 465 nm in dioxane solutions. The position of
this band was strongly influenced by the electronic nature of the
acceptor moiety and also by the position of substitution of this
Comparison of compound 4c and 5b bearing a thiobarbituric
acid moiety on the pyrrole or on the thiophene moiety, respectively,
showed that the functionalization of the thiophene ring by the
acceptor group results in a larger shift to less reduction potentials
(e.g., 1Epc¼ꢁ1.29 V for 4c and 1Epc¼ꢁ1.24 V for 5b).
group in the
p-bridge (on the pyrrole or on the thiophene het-
erocycle). Therefore, the absorption maxima (lmax) of pyrrole-
based chromophores 4aec, in dioxane are located at the range of
308e433 nm, on the other hand compounds 3 and 5a,b bearing the
acceptor group on the thiophene moiety exhibit absorption max-
ima (lmax) located at the range of 349e465 nm probably due to
more extensive electron delocalization. As observed earlier for
At this moment another comparison can also be made between
dicyanovinyl derivatives 5a and 6.10 The substitution of a pyrroli-
dino donor group (6) by the pyrrole ring (5a) results in a decrease of
the reduction potentials (e.g., 1Epc¼ꢁ1.40 V for 5a and 1Epc¼ꢁ1.50 V

M.C.R. Castro et al. / Tetrahedron 68 (2012) 8147e8155
8151
Table 3
Solvatochromic data [lmax (nm) and Dnmax (cmꢁ1) of the charge-transfer band] for pyrroles 2e5 in five solvents with
p
*
values by Kamlet and Taft15
Dnmax (cmꢁ1
)
3
(dioxane) (Mꢁ1 cmꢁ1
)
a
Compd
Diethyl ether
1,4-Dioxane (0.55) lmax (nm)
Chloroform
DMSO (1.00) lmax (nm)
(0.54) lmax (nm)
(0.76) lmax (nm)
2
3
305
345
306
400
430
415
461
306
349
308
403
433
416
465
307
350
309
413
441
425
478
310
357
313
417
445
430
479
529
974
731
1019
784
841
815
39,720
32,740
28,420
31,990
45,790
39,180
45,790
4a
4b
4c
5a
5b
a
Dnmax
¼nmax (diethyl ether)
ꢁ
nmax (DMSO)/cmꢁ1
.
Table 4
other pyrrole pushepull systems, a bathochromic shift in the
UVevis spectra is observed when stronger acceptor groups are
linked to the heterocyclic system.5,7a,b,7dee For example, the sub-
stitution of a hydrogen for a thiobarbituric acid moiety leads to
a red shift of 127 nm from 306 nm (2) to 433 nm (4c), furthermore
a larger red-shifted lmax (159 nm) was obtained for derivative 5b
bearing the acceptor group on the thiophene ring when compared
with the same compound 2.
Within the two series of compounds 4a, 4c and 3, and 5a,b the CT
bands move to lower energies as the electron accepting ability of the
acceptor moiety increases, in the order CHO<dicyanovinyl
<thiobarbituric acid (Fig. 2). These results agree with the band gap
calculations based on the redox properties (Table 2), which show
decreased band gaps for derivatives 4b, 4c, and 5a,b functionalized
with strong electron acceptor groups.
UVevis absorptions,
b
and b0 values and Td data for pyrrole derivatives 2e5a and
compound 65c
bb (conv T)
b0c (conv T)
d
Td (ꢂC)
Compd
Yield (%)
lmax
(nm)
(10ꢁ30 esu)
(10ꢁ30 esu)
e
e
2
3
32
41
59
43
39
65
65
d
309
349
308
403
433
416
465
503
352
194
199
d
66
34
e
e
4a
4b
4c
5a
5b
610
p-NA
225
290
2200
82
82
725
288
d
277
270
d
2480
474
f
f
d
40
20
d
a
Experimental first hyperpolarizabilities
b and spectroscopic data measured in
dioxane solutions.
b
All compounds are transparent at the 1064 nm fundamental wavelength.
Data corrected for resonance enhancement at 532 nm using the two-level model
c
with b0
¼b
[1ꢁ(lmax/1064)2][1ꢁ(lmax/532)2]; damping factors not included.18
d
Decomposition temperature (Td) measured at a heating rate of 20 ꢂC minꢁ1
under a nitrogen atmosphere, obtained by thermogravimetric analysis (TGA).
1
e
Due to a combination of low signals and fluorescence it was not possible to
measure the
b
value.
f
Due to fluorescence it was not possible to measure the
b
value.
3
0.5
fluorescence contamination at 532 nm. From Table 4 it is clear that
the progression of acceptor substituents from formyl, dicyanovinyl
to thiobarbituric acid results in enhanced nonlinearities as
anticipated from the acceptor strengths of the substituents. As
5a
5b
expected compounds 5b and 4c exhibit the highest
b values.
Moreover values for compounds having the acceptor groups
b
0
on the thiophene ring are 55e62 times greater than p-NA, whereas
the respective b0 are 24e36 times greater. On the other hand
compounds 4 functionalized with acceptor moieties on the pyrrole
300
400
500
λ (nm)
600
Fig. 2. Comparative absorption spectra of 3 and 5a,b in dioxane at room temperature.
ring exhibit smaller
compounds 5. For example, 5b exhibits a
b values when compared to the corresponding
b
value of 2480ꢀ10ꢁ30 esu
while for 4c the
b
value is eight times smaller (290ꢀ10ꢁ30 esu). The
Compounds
2e5
exhibit
positive
solvatochromism
results obtained show that the location of the acceptor group on the
pyrrole or on the thiophene ring alone can dramatically alter the
overall molecular nonlinearity of the system. Pyrrole, being the
most electron-rich five-membered heteroaromatic ring, counter-
acts the electron-withdrawing effect of the acceptor groups,
(
Dnmax¼529e1019 cmꢁ1) with respect to their CT absorption band,
that is, the position of the absorption maximum shifts to longer
wavelengths as the polarity of the solvent increases due to a greater
stabilization of the excited state relative to the ground state with
increasing polarity of the solvent (Table 3).
resulting in a decrease in b. These findings are in accordance with
The molecular first hyperpolarizabilities
b
of compounds 2e5
our recent work related to pushepull thienylpyrroles5c,7a,b where it
was concluded that the increase or decrease of the molecular
nonlinear activity in these systems depends on the electronic na-
ture of the heterocycles as well as the location of the acceptor
groups in these heterocyclic systems.
were measured by hyper-Rayleigh scattering (HRS) method16 at
a fundamental wavelength of 1064 nm of a laser beam. Dioxane was
used as the solvent, and the
b values were measured against a refer-
ence solution of p-nitroaniline (p-NA)17 in order to obtain quantita-
tive values, while care was taken to properly account for possible
fluorescence of the dyes (see Experimental section for more details).
The static hyperpolarizability b0 values18 were calculated using a very
simple two-level model neglecting damping. They are therefore only
indicative and should be treated with caution (Table 4).
The thermal stability of chromophores 2e5 was characterized
by thermogravimetric analysis (Fig. 3). The samples had moderate
to high decomposition temperatures (Td¼199e288 ꢂC), measured
at a heating rate of 20 ꢂC minꢁ1 under a nitrogen atmosphere. Ex-
perimental results indicate that good nonlinearityethermal sta-
bility is well balanced for these chromophores, specially for
It was not possible to measure the
b value for compounds 2
and 4a due to combination of low absolute signals and
a
compounds 5a,b, which possess b values at the range of

8152
M.C.R. Castro et al. / Tetrahedron 68 (2012) 8147e8155
4.2.2. General procedure for the synthesis of pyrroles 2 and 3 through
Suzuki cross-coupling reaction.19 1-(4-Bromophenyl)-1H-pyrrole 1
(0.90 mmol, 200 mg) was coupled to thiophen-2-yl-boronic acid or
5-formyl-thiophen-2-yl-boronic acid (1.12 mmol) in a mixture of
DME (10 ml), aqueous 2 M Na2CO3 (1 ml), and Pd(PPh3)4 (3 mol %)
at 80 ꢂC under nitrogen. The reaction was monitored by TLC, which
determined the reaction time (24e32 h). After cooling, the mixture
was extracted with chloroform (3ꢀ20 ml) and a saturated solution
of NaCl was added (20 ml) and the phases were separated. The
organic phase was washed with water (3ꢀ10 ml) and with a solu-
tion of NaOH (10%) (1ꢀ10 ml). The organic phase obtained was
dried (MgSO4), filtered, and the solvent removed to give a crude
mixture. The crude product was purified through a silica gel
chromatography column using mixtures of chloroform and light
petroleum of increasing polarity to afford the coupled products 2
and 3. Recrystallization from light petroleum/ether gave the pure
compounds.
Fig. 3. Thermal analysis data for compound 5a througꢁh1TGA recorded under a nitrogen
atmosphere, measured at a heating rate of 20 ꢂC min
.
2200e2480ꢀ10ꢁ30 esu and high decomposition temperatures
(Td¼270e277 ꢂC).
4.2.2.1. 1-(4-(Thiophen-2-yl)phenyl)-1H-pyrrole 2. Pale beige
solid (65 mg, 32%). Mp 215e216 ꢂC. 1H NMR (CDCl3)
d 6.38e6.39 (m,
2H, 3-H and 4-H), 7.11 (dd, 1H, J¼5.2 and 3.6 Hz, 400-H), 7.12e7.14
(m, 2H, 2- and 5-H), 7.29e7.33 (m, 2H, 300-H and 500-H), 7.42 (d, 2H,
J¼9.0 Hz, 20-H and 60-H), 7.68 (d, 2H, J¼9.0 Hz, 30-H and 50-H). 13C
3. Conclusions
In conclusion we have developed two new series of pushepull
pyrrole NLO-heterocyclic systems functionalized with acceptor
groups on the pyrrole (4aec) or on the thiophene heterocycles (3
and 5a,b).
Reactivity studies of compound 2 in the presence of different
electrophiles (Vilsmeier reagent or with tetracyanoethylene)
showed that the strongest and bulky electrophile react in position 3
of the pyrrole heterocycle to produce compound 4b while the
Vilsmeier reagent gives rise to the derivative 4a functionalized on
position 2 of the pyrrole ring.
NMR (CDCl3)
139.9, 143.4. lmax (dioxane)/nm 306 (
(CHCl3):
d 110.6, 119.2, 120.7, 123.1, 124.9, 127.0, 128.1, 131.9,
3
/dm3 molꢁ1 cmꢁ1 39,720). IR
n
3139, 1886, 1605, 1538, 1509, 1333, 1252, 1133, 1079, 854,
819, 703 cmꢁ1. MS (ESI) m/z (%)¼226 ([MþH]þ, 100), 225 (64), 209
(38), 203 (37). HMRS: m/z (ESI) for C14H11NS; calcd 226.0685;
found: 226.0684. Anal. Calcd for C14H11NS: C, 74.63; H, 4.92; N,
6.22; S, 14.23. Found: C, 74.85; H, 5.30; N, 6.55; S, 14.60.
4.2.2.2. 5-(4-(1H-Pyrrol-1-yl)phenyl)thiophene-2-carbaldehyde
3. Orange solid (210 mg, 61%). Mp 205e206 ꢂC. 1H NMR (CDCl3)
By varying the position of linkage of the acceptor moiety the
electrochemical properties of compounds 4 and 5 as well as the
d
6.39e6.41 (m, 2H, 300-H and 400-H), 7.14e7.16 (m, 2H, 200- and 500-
H), 7.42 (d, 1H, J¼4.2 Hz, 4-H), 7.47 (d, 2H, J¼8.9 Hz, 30-H and 50-H),
linear and nonlinear optical properties of these pushepull
p-con-
7.74 (d, 2H, J¼8.9 Hz, 20-H and 60-H), 7.76 (d, 1H, J¼4.2 Hz, 3-H), 9.91
jugated systems can be readily tuned.
(s, 1H, CHO). 13C NMR (CDCl3)
d 111.1, 119.1, 120.6, 123.9, 127.7, 130.2,
Electrochemical studies and characterization of the optical
(linear and nonlinear), and thermal properties for compounds 2e5
indicate that, good nonlinearityethermal stability is well balanced
for chromophores 5a,b, making them good candidates for several
optoelectronic applications such as solvatochromic probes and
second-order nonlinear optical materials.
137.5, 141.3, 142.4, 153.2, 182.7. lmax (dioxane)/nm 349
(
3
/dm3 molꢁ1 cmꢁ1 32,740). IR (CHCl3):
n
3442, 3148, 2945, 1648,
1605, 1536, 1480, 1449, 1330, 1233, 1001, 919, 826, 805, 730 cmꢁ1
.
MS (ESI) m/z (%)¼254 ([MþH]þ, 100). HMRS: m/z (ESI) for
C15H11NOS; calcd 254.0634; found: 254.0630. Anal. Calcd for
C15H11NOS: C, 71.12; H, 4.38; N, 5.53; S, 12.66. Found: C, 71.29; H,
4.57; N, 5.85; S, 13.00.
4. Experimental
4.1. Materials
4.2.3. Synthesis of compound 4a. POCl3 (0.44 mmol) was added to
DMF (0.44 mmol) at 0 ꢂC and the mixture was stirred for 15 min at
0 ꢂC. After this time pyrrole 2 (0.44 mmol) dissolved in DMF (2 ml)
was added dropwise with stirring. The reaction mixture was then
heated for 4 h at 60 ꢂC. The solution was then poured slowly into
10 ml saturated sodium acetate aqueous solution and stirred
30 min. The organic layer was diluted with ether (25 ml), washed
with saturated NaHCO3 aqueous solution (2ꢀ25 ml), and dried with
anhydrous MgSO4. Evaporation of the organic extract under re-
duced pressure gave the crude aldehyde 4a, which was purified
through a silica gel chromatography column using mixtures of
chloroform and light petroleum of increasing polarity to afford al-
dehyde 4a. Recrystallization from light petroleum/ether gave the
pure compound.
Thiophen-2-yl-boronic acid, 5-formyl-thiophen-2-yl-boronic
acid, 4-bromoaniline, and 2,5-dimethoxytetradihydrofuran used as
precursors for the synthesis of pyrroles 1e5 were purchased from
Aldrich and Fluka and used as received. TLC analyses were carried
out on 0.25 mm thick precoated silica plates (Merck Fertigplatten
Kieselgel 60F254) and spots were visualized under UV light. Chro-
matography on silica gel was carried out on Merck Kieselgel
(230e240 mesh).
4.2. Synthesis
4.2.1. Synthesis of Compound 1. Compound 1 was synthesized us-
ing the experimental procedure described earlier.9
4.2.3.1. 1-(4-(Thiophen-2-yl)phenyl)-1H-pyrrole-2-carbaldehyde
4a. Pale beige solid (66 mg, 59%). Mp 215e216 ꢂC. 1H NMR (CDCl3)
d
4.2.1.1. 1-(4-Bromophenyl)-1H-pyrrole 1. Pale brown solid
6.44 (dd, 1H, J¼3.6 and 2.8 Hz, 4-H), 7.11e7.13 (m, 2H, 5-H and 400-
(242 mg, 93%). 1H NMR (CDCl3)
d
6.36e6.38 (m, 2H, 3-H and 4-H),
H), 7.19 (dd, 1H, J¼3.6 and 1.2 Hz, 3-H), 7.34 (dd, 1H, J¼5.2 and
7.05e7.07 (m, 2H, 2-H and 5-H), 7.28 (d, 2H, J¼8.6 Hz, 20-H and 60-
1.2 Hz, 500-H), 7.37e7.39 (m, 3H, 30-H, 50-H and 300-H), 7.70 (d, 2H,
H), 7.55 (d, 2H, J¼8.6 Hz, 30-H and 50-H).
J¼8.6 Hz, 20-H and 60-H), 9.62 (s, 1H, CHO). 13C NMR (CDCl3)
d 111.0,

M.C.R. Castro et al. / Tetrahedron 68 (2012) 8147e8155
8153
122.7, 123.8, 125.5, 126.4, 128.2, 131.1, 132.4, 134.4, 137.8, 143.0,
179.0, 192.5. lmax(dioxane)/nm 308 (
/dm3 molꢁ1 cmꢁ1 28,420). IR
(CHCl3):
of increasing polarity. The fraction containing the purified product
was collected and evaporated under vacuum. Recrystallization from
light petroleum/ether gave the pure compound.
3
n
3123, 1766, 1673, 1660, 1533, 1523, 1318, 1303, 1280, 1261,
1238, 1210, 1195, 1091, 1036 cmꢁ1. MS (ESI) m/z (%)¼254 ([MþH]þ,
100). HMRS: m/z (ESI) for C15H12NOS; calcd 254.0634; found:
254.0632. Anal. Calcd for C15H11NOS: C, 71.12; H, 4.38; N, 5.53; S,
12.66. Found: C, 71.29; H, 4.57; N, 5.85; S, 12.88.
4.2.6.1. 2-((5-(4-(1H-Pyrrol-1-yl)phenyl)thiophen-2-yl)methy-
lene)malononitrile 5a. Yellow solid (25 mg, 43%). Mp 221e223 ꢂC.
1H NMR (CDCl3)
d
6.40e6.41 (m, 2H, 300-H and 400-H), 7.15e7.17 (m,
2H, 200-H and 500-H), 7.45 (d, 1H, J¼3.9 Hz, 4-H), 7.48 (d, 2H, J¼9 Hz,
4.2.4. Synthesis of compound 4b. A solution of compound
2
30-H and 50-H), 7.80 (d, 1H, J¼3.9 Hz, 3-H), 7.78 (d, 2H, J¼9 Hz, 20-H
(0.22 mmol, 50 mg) in DMF (1.5 ml) was cooled at 0 ꢂC and then
tetracyanoethylene (0.44 mmol, 57 mg) was added slowly. The re-
action mixture was stirred at room temperature for about 2 h. After
this time the mixture was poured into ice/water (5 ml) and the
organic layer was diluted in chloroform (10 ml), washed with water
(2ꢀ20 ml), and dried with anhydrous MgSO4. Evaporation of the
solvent under reduced pressure gave the crude product, which by
crystallization with chloroform gave the pure product 4b.
and 60-H), 7.81 (s, 1H, C]CH). 13C NMR (CDCl3)
d 111.4, 113.3, 114.1,
118.9, 120.5, 124.4, 127.9, 129.2, 134.2, 140.0, 141.9, 150.5, 155.4. lmax
(dioxane)/nm 416 ( 3179,
3
/dm3 molꢁ1 cmꢁ1 39,180). IR (CHCl3):
n
3051, 2221, 1606, 1530, 1568, 1494, 1325, 1264, 1245, 1210, 1141,
1069, 945, 919, 832, 817, 807, 606 cmꢁ1. Anal. Calcd for C18H11N3S:
Found: C, 71.58; H, 3.69; N,13.72. % CHNS requires: C, 71.74; H, 3.68;
N, 13.94.
4.2.7. Synthesis of compound 5b. A solution of aldehyde
3
4.2.4.1. 1-(1-(4-(Thiophen-2-yl)phenyl)-1H-pyrrol-2-yl)ethene-
1,2,2-tricarbonitrile 4b. Dark orange solid (21 mg, 43%). Mp
(0.20 mmol, 50 mg) and 1,3-diethyl-2-thiobarbituric acid
(0.26 mmol, 52 mg) and triethylamine (0.02 mmol) in dichloro-
methane (5 ml) was refluxed for 4 h. The mixture was poured into
water (20 ml) and extracted with chloroform (2ꢀ50 ml). The or-
ganic layer was dried with anhydrous MgSO4 and evaporated under
reduced pressure to give the crude product, which by crystalliza-
tion with hexane gave the pure product 5b.
257e259 ꢂC. 1H NMR (acetone-d6)
d
7.21(dd, 1H, J¼5.1 and 3.9 Hz,
400-H), 7.29 (dd, 1H, J¼2.7 and 2.1 Hz, 4-H), 7.58 (dd, 1H, J¼5.1 and
1.2 Hz, 500-H), 7.62 (dd, 1H, J¼3.0 and 1.2 Hz, 300-H), 7.76 (dd, 1H,
J¼2.7 and 2.1 Hz, 5-H), 7.86 (2H, d, J¼9.0 Hz 30-H and 50-H), 7.92
(2H, d, J¼9.0 Hz, 20-H and 60-H), 8.38 (t, 1H, J¼1.8 Hz, 2-H). 13C NMR
(CDCl3)
d 81.9, 109.7, 113.2, 113.3, 114.0, 118.9, 121.5, 124.7, 125.7,
126.6, 128.7, 129.1 , 133.5, 133.6, 136.7, 141.9. lmax (dioxane)/nm 403
4.2.7.1. 5-((5-(4-(1H-Pyrrol-1-yl)phenyl)thiophen-2-yl)methy-
lene)-1,3-diethyl-dihydro-2-thioxopyrimidine-4,6(1H,5H)-dione
5b. Red solid (86 mg, 65%). Mp 280e282 ꢂC. 1H NMR (CDCl3)
(
3
/dm3 molꢁ1 cmꢁ1 31,990). IR (Nujol):
n
3570, 2246, 2210, 1653,
1610, 1538, 1506, 1329, 1296, 1266, 1249, 1209, 1075, 978, 930, 841,
828 cmꢁ1. MS (EI) m/z (%)¼326 ([M]þ, 100), 301 (27), 300 (32), 185
(10),121 (22). HMRS: m/z (EI) for C19H10N4S; calcd 326.0626; found:
326.0626. Anal. Calcd for C19H10N4S: C, 69.92; H, 3.09; N, 17.17; S,
9.82. Found: C, 70.02; H, 3.39; N, 17.40; S, 10.05.
d
1.26e1.39 (m, 6H, 2ꢀNCH2CH3), 4.57e4.64 (m, 4H, 2ꢀNCH2CH3),
6.39e6.41 (m, 2H, 300-H and 400-H), 7.16e7.17 (m, 2H, 200-H and 500-
H), 7.47 (d, 2H, J¼8.0 Hz, 30-H and 50-H), 7.56 (d, 1H, J¼4.0 Hz, 4-H),
7.87 (d, 2H, J¼8.0 Hz, 20-H and 60-H), 7.89 (d, 1H, J¼4.0 Hz, 3-H) 8.67
(s, 1H, C]CH). 13C NMR (CDCl3)
d 12.4, 12.5, 43.2, 43.9, 110.3, 111.3,
4.2.5. Synthesis of compound 4c. A solution of aldehyde 4a
(0.20 mmol, 50 mg) and 1,3-diethyl-2-thiobarbituric acid
(0.26 mmol, 52 mg) and triethylamine (two drops) in dichloro-
methane (7 ml) was refluxed for 5 h. The mixture was poured into
water (20 ml) and extracted with chloroform (2ꢀ50 ml). The or-
ganic layer was dried with anhydrous MgSO4 and evaporated under
reduced pressure to give the crude product, which by crystalliza-
tion with petroleum ether gave the pure product 4c.
118.9, 120.4, 124.6, 128.1, 130.2, 136.7, 141.7, 157.3, 149.6, 159.9, 160.2,
161.0, 178.7. lmax (dioxane)/nm 465 (
3
/dm3 molꢁ1 cmꢁ1 45,790). IR
(Nujol): 2350, 1684, 1662, 1606, 1548, 1491, 1413, 1334, 1267, 1247,
1105, 1077, 787 cmꢁ1. MS (ESI) m/z (%)¼436 ([MþH]þ, 20), 387 (11),
386 (39), 382 (20), 381 (70), 360 (29), 359 (84), 354 (14), 353 (44),
342 (37), 341 (100), 331 (14), 314 (14), 285 (12), 268 (27), 267 (89),
240 (14) and 239 (35). HMRS: m/z (ESI) for C23H22N3O2S2; calcd
436.1148; found: 43.1155. Anal. Calcd for C23H22N3O2S2: C, 63.28; H,
5.08; N, 9.62; S, 14.69. Found: C, 63.10; H, 5.30; N, 9.87; S, 14.91.
4.2.5.1. 1,3-Diethyl-dihydro-5-(((1-(4-thiophen-2-yl)phenyl)-1H-
pyrrol-2-yl)methylene)-2-thioxopyrimidine-4,6(1H,5H)-dione
4c. Brown solid (33 mg, 39%). Mp 230e232 ꢂC. 1H NMR (acetone-
4.3. Instruments
d6)
d
1.24e1.28 (m, 6H, 2ꢀNCH2CH3), 4.50e4.52 (m, 4H,
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 sup-
ported by spin decoupling-double resonance and bidimensional
heteronuclear HMBC and HMQC correlation techniques. IR spectra
were determined on a BOMEM MB 104 spectrophotometer using
KBr discs. UVevis 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.dUnidad de Espec-
trometria 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 character-
ized between 25 and 500 ꢂC. All melting points were measured on
2ꢀNCH2CH3), 6.76 (dd, 1H, J¼3.3 and 2.7 Hz, 4-H), 7.17 (dd, 1H,
J¼4.5 and 3.6 Hz, 400-H), 7.58 (d, 2H, J¼8.7 Hz, 30-H and 50-H), 7.60
(dd, 1H, J¼5.1 and 1.2 Hz, 500-H), 7.68 (dd, 1H, J¼3.0 and 1.2 Hz, 300-
H), 7.85 (tap,1H, J¼1.8 Hz, 5-H), 7.98 (d, 2H, J¼8.7 Hz, 20-H and 60-H),
8.32 (s, 1H, C]CH), 8.80 (dd, 1H, J¼3.3 and 1.5 Hz, 3-H). lmax (di-
oxane)/nm 433 ( n 1691, 1657,
/dm3 molꢁ1 cmꢁ1 45,790). IR (CHCl3):
3
1557, 1282, 1106, 1044, 824, 784 cmꢁ1. MS (ESI) m/z (%)¼436
([MþH]þ, 23). HMRS: m/z (ESI) for C23H22N3O2S2; calcd 436.1148;
found: 43.1132. Anal. Calcd for C23H22N3O2S2: C, 63.28; H, 5.08; N,
9.62; S, 14.69. Found: C, 63.47; H, 5.22; N, 9.85; S, 14.35.
4.2.6. Synthesis of compound 5a. To a solution of malononitrile
(0.24 mmol, 0.016 g) and aldehyde 3 (0.19 mmol, 0.50 mg) in eth-
anol (10 ml) was added piperidine (one drop). The solution was
stirred at reflux for 3 h. After cooled to room temperature the
mixture was evaporated to dryness and the organic layer was
extracted with chloroform (2ꢀ20 ml) and dried over anhydrous
MgSO4. The product was purified through a silica gel column
chromatography using mixtures of chloroform and light petroleum

8154
M.C.R. Castro et al. / Tetrahedron 68 (2012) 8147e8155
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 elec-
trode, and a silver-wire pseudo-reference electrode were employed
for cyclic voltammetric measurements. The concentration of the
compound was 1 mmol dmꢁ3 and 0.1 mol dmꢁ3 [NBu4][BF4] was
used as the supporting electrolyte in dry N,N-dimethylformamide
orientated along the laboratory frame Y axis. To calibrate our sys-
tem a reference solution of p-nitroaniline (p-NA) dissolved in di-
oxane at a concentration of 1ꢀ10ꢁ2 mol dmꢁ3 (external reference
method). Kaatz and Shelton17a have measured the value of b333 of
p-NA in dioxane at 1064 nm to be 40ꢀ10ꢁ30 esu using the so-called
Taylor convention for the first hyperpolarizability.17b This value
includes a correction factor of 1.88 that accounts for the most recent
measurement of the CCl4 hyper-Rayleigh scattering signal, which
was used as a reference.20
Acknowledgements
solvent. The cyclic voltammetry was conducted usually at 0.1 V sꢁ1
,
ˇ
or at different scan rates (0.02e0.50 V sꢁ1), for investigation of scan
rate influence. The potential is measured with respect to ferroce-
nium/ferrocene as an internal standard.
~
The author wish to thank the Fundac¸ ao para a Ciencia e Tecnologia
(Portugal) and FEDER-COMPETE for financial support through the
Centro de Química and Centro de FísicadUniversidade do Minho,
Projects PTDC/QUI/66251/2006 (FCOMP-01-0124-FEDER-007429),
PTDC/CTM/105597/2008 (FCOMP-01-0124-FEDER-009457), PEst-C/
QUI/UI0686/2011 (F-COMP-01-0124-FEDER-022716) and a Ph.D.
grant to M.C.R.C. (SFRH/BD/78037/2011). 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 Scien-
tific Re-equipment, contract REDE/1517/RMN/2005 with funds from
POCI 2010 (FEDER) and FCT.
4.4. Solvatochromic study
The solvatochromic study was performed using 10ꢁ4 mol dmꢁ3
solutions of compounds 2e5 in several solvents at room temper-
ature using a Shimadzu UV/2501PC spectrophotometer.
4.5. Nonlinear optical measurements using the hyper-
Rayleigh scattering (HRS) method
References and notes
Hyper-Rayleigh scattering (HRS) was used to measure the first
1. (a) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Chem. Rev. 1994, 94, 195; (b) Verbiest,
T.; Houbrechts, S.; Kauranen, M.; Clays, K.; Persoons, A. J. Mater. Chem. 1997, 7,
2175; (c) Cho, M. J.; Choia, D. H.; Sullivan, P. A.; Akelaitis, A. J. P.; Dalton, L. R.
Prog. Polym. Sci. 2008, 33, 1013; (d) Benight, S. J.; Bale, D. H.; Olbricht, B. C.;
Dalton, L. R. J. Mater. Chem. 2009, 19, 7466; (e) Dalton, L. R.; Sullivan, P. A.; Bale,
D.; Olbricht, B.; Davies, J.; Benight, S.; Kosilkin, I.; Robinson, B. H.; Eichinger, B.
E.; Jen, A. K.-Y. Organic Electro-optic Materials Understanding Structure/Func-
tion Relationships Critical to the Optimization of Electro-Optic Activity In Or-
ganic Thin Films for Photonic Applications; Herman, W., Foulger, S., Eds.; ACS
Symposium Series; American Chemical Society: Washington, DC, 2010; Vol.
1039, Chapter 2, pp 13e33; (f) Dalton, L. R.; Sullivan, P. A.; Bale, D. H. Chem. Rev.
2010, 110, 25 and references cited.
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 et al.16 Details of the experimental procedure used have been
previously published.10 We emphasize that particular care was
taken to avoid reporting artificially high first hyperpolarizabilities
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
2. For some examples see: (a) Dirk, C. W.; Katz, H. E.; Schilling, M. L.; King, L. A.
Chem. Mater. 1990, 2, 700; (b) Rao, V. P.; Jen, A. K.-Y.; Wong, K. Y.; Drost, K. J.
Tetrahedron Lett. 1993, 34, 1747; (c) Jen, A. K.-Y.; Rao, V. P.; Wong, K. Y.; Drost, K.
J. J. Chem. Soc., Chem. Commun. 1993, 90; (d) Rao, V. P.; Jen, A. K.-Y.; Wong, K. Y.;
Drost, K. J. J. Chem. Soc., Chem. Commun. 1993, 1118; (e) Miller, R. D.; Lee, V. Y.;
Moylan, C. R. Chem. Mater. 1994, 6, 1023; (f) Chou, S.-S. P.; Sun, D.-J.; Lin, H.-C.;
Yang, P.-K. Tetrahedron Lett. 1996, 37, 7279; (g) Shu, C.-F.; Tsai, W.-J.; Chen, J.-Y.;
Jen, AK.-Y.; Zhang, Y.; Chen, T.-A. Chem. Commun. 1996, 2279; (h) Shu, C.-F.;
a 0.2
impurities.
Dioxane was used as a solvent for all measurements. The small
mm porosity filter, to avoid spurious signals from suspended
ꢀ
ꢀ
Wang, Y.-K. J. Mater. Chem. 1998, 8, 833; (i) Hrobarik, P.; Sigmundova, I.;
Zahradník, P. Synthesis 2005, 600; (j) Zajac, M.; Hrobarik, P.; Magdolen, P.;
hyper-Rayleigh signal that arises from dioxane was taken into ac-
count according to the expression
ꢀ
ꢀ
Foltínova, P.; Zahradník, P. Tetrahedron 2008, 64, 10605; (k) Kulhanek, J.; Bures,
F. Beilstein J. Org. Chem. 2012, 8, 25 and references cited.
3. (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) Ra, C. S.; Kim, S. C.; Park, G. J. Mol. Struct. (Theochem) 2004,
677, 173; (e) Hrobarik, P.; Zahradnik, P.; Fabian, W. M. F. Phys. Chem. Chem. Phys.
ꢀ
D
E
D
Eꢁ
I2 ¼ G Nsolvent bs2olvent þ Nsolute b2solute Iu2
u
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
hyper-Rayleigh signals fell well within the dynamic range of de-
tection system. We have assumed that molecules could be treated
ꢀ
ꢀ
2004, 6, 495; (f) Benkova, Z.; Cernusak, I.; Zahradnik, P. Struct. Chem. 2006, 17, 287;
ꢀ
ꢀ
(g) Benkova, Z.; Cernusak, I.; Zahradník, P. Mol. Phys. 2006, 104, 2011.
4. (a) Morley, J. O. J. Chem. Soc., Faraday Trans. 1991, 87, 3009; (b) Bradamante, S.;
Facchetti, A.; Pagani, G. A. J. Phys. Org. Chem. 1997, 10, 514; (c) Groenendaal, L.;
Bruining, M. J.; Hendrickx, E. H. J.; Persoons, A.; Vekemans, J. A. J. M.; Havinga, E.
E.; Meijer, E. W. Chem. Mater. 1998, 10, 226; (d) Facchetti, A.; Abbotto, A.; Bev-
erina, L.; van der Boom, M. E.; Dutta, P.; Evmenenko, G.; Marks, T. J.; Pagani, G. A.
Chem. Mater. 2002, 14, 4996; (e) 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; (f) Audebert, P.; Kamada, K.; Matsunaga, K.; Ohta, K. Chem. Phys.
Lett. 2003, 367, 62; (g) Abbotto, A.; Beverina, L.; Bradamante, S.; Facchetti, A.;
Klein, C.; Pagani, G. A.; Redi-Abshiro, M.; Wortmann, R. Chem.dEur. J. 2003, 9,
1991; (h) Abbotto, A.; Beverina, L.; Bradamante, S.; Facchetti, A.; Pagani, G. A.;
Bozio, R.; Ferrante, C.; Pedron, D.; Signorini, R. Synth. Met. 2003, 14, 4996.
5. (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.
as linear donoreacceptor chromophores with
polarizability tensor dominated by a single longitudinal element,
b333 , associated with the charge-transfer axis. In this case the
measured hyper-Rayleigh signal is related to the value of the
a first hyper-
¼b
dominant
tensor
element
through
the
relation
qffiffiffiffiffiffiffiffiffiffiffiffiffi
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
b
¼
h
b2HRSi ¼
h
b2ZZZi þ hb2XZZi 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
discrimination was carried out in the detection arm, which is
6. (a) 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; (b) Li, Q.; Lu, C.; Zhu, J.; Fu, E.; Zhong, C.; Li, S.;
Cui, Y.; Qin, J.; Li, Z. J. Phys. Chem. B 2008, 112, 4545; (c) Abbotto, A.; Beverina, L.;

M.C.R. Castro et al. / Tetrahedron 68 (2012) 8147e8155
8155
Manfredi, N.; Pagani, G. A.; Archetti, G.; Kuball, H. G.; Wittenburg, C.; Heck, J.;
Holtmann, J. Chem.dEur. J. 2009, 15, 6175; (d) Ma, X.; Liang, R.; Yang, F.; Zhao,
Z.; Zhang, A.; Song, N.; Zhou, Q.; Zhang, J. J. Mater. Chem. 2008, 18, 1756; (e) Ma,
X.; Ma, F.; Zhao, Z.; Song, N.; Zhang, J. J. Mater. Chem. 2009, 19, 2975; (f) Ma, X.;
Ma, F.; Zhao, Z.; Song, N.; Zhang, J. J. Mater. Chem. 2010, 20, 2369.
7. (a) Batista, R. M. F.; Costa, S. P. G.; Malheiro, E. L.; Belsley, M.; Raposo, M. M. M.
Tetrahedron 2007, 63, 4258; (b) Batista, R. M. F.; Costa, S. P. G.; Belsley, M.;
Raposo, M. M. M. Tetrahedron 2007, 63, 9842; (c) 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; (d) 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; (e)
Raposo, M. M. M.; Castro, M. C. R.; Schellenberg, P.; Fonseca, A. M. C.; Belsley, M.
Tetrahedron 2011, 67, 5189.
11. Oliva, M. M.; Casado, J.; Raposo, M. M. M.; Fonseca, A. M. C.; Hartmann, H.;
ꢀ
Hernandez, V.; Navarrete, J. T. L. J. Org. Chem. 2006, 71, 7509.
12. Fitzner, R.; Reinold, E.; Mishra, A.; Mena-Osteritz, E.; Ziehlke, H.; Korner, C.; Leo,
€
K.; Riede, M.; Weil, M.; Tsaryova, O.; Weib, A.; Uhrich, C.; Pfeiffer, M.; Bauerle, P.
Adv. Funct. Mater. 2011, 21, 897.
13. Zhou, E.; Wei, Q.; Yamakawa, S.; Zhang, Y.; Tajima, K.; Yang, C.; Hashimoto, K.
Macromolecules 2010, 43, 821.
14. Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Bassler, H.; Porsch, M.;
€
Daub, J. Adv. Mater. 1995, 551.
15. 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.-L. M.; Abraham, M. H.; Taft, R. W. J. Am. Chem.
Soc. 1977, 99, 6027.
16. (a) Clays, K.; Persoons, A. Rev. Sci. Instrum. 1992, 63, 3285; (b) Clays, K.; Per-
8. (a) Raposo, M. M. M.; Sousa, A. M. R. C.; Fonseca, A. M. C.; Kirsch, G. Tetrahedron
2006, 62, 3493; (b) Raposo, M. M. M. Recent Developments in the Chemistry of
2-Thienylpyrroles: Synthesis, Reactivity and Applications In Targets in Hetero-
cyclic Systems: Chemistry and Properties; Attanasi, O. A., Spinelli, D., Eds.; Royal
Society of Chemistry: London, 2008; Vol. 11, pp 122e154.
soons, A. Phys. Rev. Lett. 1991, 66, 2980.
17. (a) Kaatz, P.; Shelton, D. P. J. Chem. Phys. 1996, 105, 3918; (b) Reis, H. J. Chem.
Phys. 2006, 125, 014506.
18. (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.
19. Costa, S. P. G.; Batista, R. M. F.; Cardoso, P.; Belsey, M.; Raposo, M. M. M. Eur. J.
Org. Chem. 2006, 17, 3938.
9. Delen, Z.; Lahti, P. M. J. Org. Chem. 2006, 71, 9341.
10. Herbivo, C.; Comel, A.; Kirsch, G.; Fonseca, A. M. C.; Belsley, M.; Raposo, M. M.
M. Dyes Pigments 2010, 86, 217.
20. Pyatt, R. D.; Shelton, D. P. J. Chem. Phys. 2001, 114, 9938.