Synthesis and characterization of novel diazenes bearing pyrrole, thiophene and thiazole heterocycles as efficient photochromic and nonlinear optical (NLO) materials

Article abstract of DOI:10.1016/j.dyepig.2011.02.012

Two series of novel thermally stable second-order nonlinear optical (NLO) and photochromic chromophores have been designed and synthesized. The two series of compounds were based on different combinations of donor groups (pyrrole or thienylpyrrole) which

Full text of DOI:10.1016/j.dyepig.2011.02.012

Dyes and Pigments 91 (2011) 62e73
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and characterization of novel diazenes bearing pyrrole,
thiophene and thiazole heterocycles as efficient photochromic
and nonlinear optical (NLO) materials
,
a
a
b
M. Manuela M. Raposo a , A. Maurício C. Fonseca , M. Cidália R. Castro , M. Belsley ,
*
M. Fátima S. Cardoso b, Luís M. Carvalho c, Paulo J. Coelho c
a Center of Chemistry, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal
b Center of Physics, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal
c Center of Chemistry, University of Trás-os-Montes and Alto Douro, 5001-801 Vila Real, 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 novel thermally stable second-order nonlinear optical (NLO) and photochromic chromo-
phores have been designed and synthesized. The two series of compounds were based on different
Received 12 November 2010
Received in revised form
6 January 2011
Accepted 21 February 2011
Available online 9 March 2011
combinations of donor groups (pyrrole or thienylpyrrole) which act simultaneously as
p-conjugated
bridges, together with diazoaryl or diazothiazolyl as acceptor moieties. Their photochromic and elec-
trochemical behavior were characterized, while hyper-Rayleigh scattering (HRS) was employed to
evaluate their second-order nonlinear optical properties. The results of these studies have been critically
analyzed together with two other related compounds reported earlier from our laboratories in which the
thienylpyrrole system was used as the donor group keeping the functionalized diazoaryl as acceptor
moiety. The measured molecular first hyperpolarizabilities and the observed photochromic behavior
showed strong variations for the different donor systems used (pyrrole or thienylpyrrole) and were also
sensitive to the acceptor strength of the diazenehetero(aryl) moiety.
Dedicated to the Centenary of the
Portuguese Chemical Society.
Keywords:
Heterocyclic azo dyes
Thiazole
Hyper-Rayleigh scaterring (HRS) technique
Nonlinear optical (NLO) materials
Thermal stability
The thienylpyrrole based compounds endowed with extended
p-conjugated bridges and stronger
donor auxiliary effects in comparison to the pyrrole compounds, when coupled to the stronger acceptor
diazo(hetero)aryl groups gave rise to significantly larger hyperpolarizabilities (
b
¼ 274e415 ꢀ 10ꢁ30 esu)
for an incident wavelength of 1064 nm. These compounds also displayed improved photochromic
behavior with very fast response to the visible light stimulus (1.5 s) and fast thermal return to the
original forms (2e3 s).
Photochromism
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
groups linked through a
systems [1]. The efficient intramolecular charge transfer (ICT) along
the -conjugated bridges of these organic systems is particularly
relevant in the development of NLO materials since, amongst other
properties, it can promote large optical nonlinearities and ultra-fast
responses, in particular a nearly instantaneous electronic polariza-
p-conjugated spacer, so called D-p-A
Although the search for organic molecules with strong nonlinear
optical (NLO) response has been the focus of intense research for
many years, their potential for improving a variety of opto-elec-
tronic applications ranging from optical data transmission to
information processing, has only been partially realized. Candidate
molecules for NLO applications should possess large molecular
hyperpolarizabilities and low optical losses within the spectral
region of interest. A general approach for obtaining materials with
important nonlinear optical properties consists in the synthesis
of chromophores involving electron-donor and electron-acceptor
p
tion. For these systems, optimization of the
p-conjugated bridges,
electron-donor and electron-acceptor characteristics of the
substituents are needed to obtain the highest nonlinearities at
a molecular level [2].
One approach currently being explored by many researchers is
to substitute the benzene rings typical of these D-
with an electron rich and/or electron poor aromatic ring that
can act as an auxiliary donor/acceptor while modulating the
p-A systems
p
-
conjugated bridges. Experimental studies have demonstrated
that replacing the benzene ring of a chromophore with easily
* Corresponding author. Tel.: þ351 253 604381; fax: þ351 253 604382.
E-mail address: mfox@quimica.uminho.pt (M.M.M. Raposo).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.02.012

M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 62e73
63
delocalizable five-membered heteroaromatic rings, such as thio-
phene, pyrrole and thiazole, results in an enhanced molecular
hyperpolarizability [3]. Recent theoretical calculations suggest that
heterocyclic rings play a subtle role in the second-order NLO
properties of donoreacceptor compounds. In fact, the increase or
decrease of the molecular nonlinear activity of these hetero-
aromatic systems depends not only on the electronic nature of the
aromatic rings, but also on the location of these heterocycles in the
system [4]. Our research group has recently published experi-
mental and theoretical results concerning the auxiliary donor/
acceptor effect of electron rich and electron-deficient heterocycles
NLO and photochromic applications. Recently our group has
been interested in the synthesis of new heterocyclic azo dyes
prepared through azo coupling reaction using, for the first time,
thienylpyrrole derivatives [13a,13b], 5-alkoxy-2,20-bithiophene
moieties [14], 5-N,N-dialkylamino-2,20-bithiophenes [15a] and
aryldiazonium salts as coupling components. Moreover, the char-
acterization of the thermal, nonlinear optical and photochromic
properties of the novel azo dyes proved that they could be used as
efficient and thermally stable solvatochromic probes, nonlinear
optical [13a,13b,14,15a] and photochromic materials [13c,15b].
Before our recent photochromic studies on azo dyes bearing
thienylpyrrole [13c] and bithiophene [15b] systems, only a few
reports concerning the photochromic properties of heterocyclic azo
dyes were found in the literature [15c,15d,15e]. In our earlier studies,
we have showed that, the kinetics of the azo isomerization reaction
and the amplitude of the absorption variation are strongly depen-
dent on the nature of the heterocyclic system (thienylpyrrole or
bithiophene) and also on the position of the azo linkage on the
bithiophene moiety. While aryldiazene thienylpyrroles [13c] exhibit
thermal Z / E isomerization rates around 0.30 sꢁ1, aryldiazene
on pushepull thienylpyrrole p-conjugated systems. We have found
that the position of the acceptor groups such as dicyanovinyl or
electron-deficient heterocycles (e.g., benzothiazole or benzimid-
azole), on the thienylpyrrolyl system have a clear influence on the
NLO, spectroscopic (singlet and triplet state) and the photophysical
properties in solution (at room and low temperature) [5].
The arylazo derivatives of the “pseudo-stilbene” type are char-
acterized by a strong asymmetric electron distribution, which
results from being substituted at the 4 and 40 positions with elec-
tron-donating and electron-withdrawing groups (called a ‘push/
pull’ substitution pattern). These pseudo-stilbenes have a strong
and broad absorption feature in the visible, indicative of a sizeable
electric dipole moment, possess nonlinear optical properties
(owing to the asymmetric electron distribution), and often have
a superlative photo-switching response, making them strong
candidates for a variety of applications and studies [6]. In this
context, aryl and heteroaryl-azo dyes are a versatile class of organic
compounds that have recently attracted the interest of many
research groups due to their diverse optical applications [7,8].
Particularly, (hetero)arylpyrrole azo dyes are promising candidates
for non linear optical and photochromic applications [3k,3m,9,10].
The photochemical E / Z isomerization of aromatic azo dyes, in
solution or incorporated in polymeric matrices, can be achieved
bithiophenes showed significantly slower rates (0.01e0.04 sꢁ1
indicating more stable Z-isomers [15b].
)
Given our previous results with the above mentioned azo dyes
[13,14,15a,15b] and also other previous studies [3k,3m,12] we
envisaged the use of a heterocycle that has never been considered
for NLO applications in combination of conjugated 1-substituted
pyrrole and 1-(alkyl)arylthienylpyrroles azo dyes. This synthetic
strategy was employed having in mind the increase of the acceptor
strength of the diazene moiety replacing the aryl ring, used earlier
in the acceptor end [13a,13b], by an electron-deficient hetero-
aromatic molecule having reduced aromaticity and electron-
acceptor electronic character such as the thiazole heterocycle
[3e,3f,4a,4b,4c]. Using this heterocycle we sought to improve the
intramolecular electronic delocalization, leading to an enhance-
through visible irradiation (
l
within the broad azo absorption band)
ment of the first hyperpolarizability b and photochromic properties
and since the E-isomer is more stable then the Z-isomer once the
irradiation source is turned off, the later is thermally reconverted
back to the initial E form (Scheme 1). This conversion is accompa-
nied by a change in the maximum absorption wavelength, since the
Z-isomer absorbs at shorter wavelengths, the absorption peak
being typically shift by 50e70 nm [11].
of the new chromophores.
In this paper we report a systematic study of two series of
organic chromophores consisting of newly synthesized thie-
nylpyrrole 5e7 and pyrrole azo dyes 9e10. The design and the
synthesis of these chromophores were based on different combi-
nation of electron-withdrawing groups, aryl or thiazole, linked to
Therefore, visible light promotes a E / Z conversion leading to
a decrease of the absorption at the lmax of the E-isomer and when
the irradiation is ceased an absorption increase is observed due to
the thermal back reaction Z / E whose rate constants can be
calculated.
the pyrrole or thienylpyrrole strong p-electron donor moieties
through an azo bridge. Their solvatochromic, electrochemical,
nonlinear optical and photochromic properties along with the
thermal stability have been investigated to better understand the
influence of electron-accepting, electron-donor groups and
p-
Under continuous visible irradiation conditions, the decrease in
the absorbance at the lmax of the E isomer is inversely related to the
kinetic rate of the colouration process (Z / E). Consequently, in
similar systems, an increase in thermal colouration kinetics leads to
lower absorbance variations.
conjugated heterocyclic bridges on the electronic and optical
properties related to the ICT characteristics of the D-
2. Results and discussion
p-A dyes.
Apart from a few recent reports [3k,3m,12] very little experi-
mental work has been focused on the use of pyrroles or thie-
nylpyrroles as building blocks for the synthesis of novel azo dyes for
2.1. Synthesis
We have recently reported the synthesis of thienylpyrroles 1
through the combination of Friedel-Crafts and Lawesson reactions
[16]. These compounds have proved to be versatile substrates in
azo coupling reactions, allowing the preparation of novel donore
acceptor substituted thienylpyrrole phenyldiazenes [13a,13b].
The synthesis of thienylpyrrole 5e7 and pyrrole azo dyes 9e10
is outlined in Schemes 2 and 3. Thiazolyl and aryl amines were
diazotized using NaNO2 in HCl at 0e5 ꢂC and the coupling reaction
of (hetero)aryldiazonium salts 2e4, with 1-alkyl(aryl)-2-(20-
thienyl)pyrroles 1, in acetonitrile/acetic acid at 0 ꢂC, gave rise to the
formation of the corresponding heterocyclic azo derivatives 5aef,
6b, 7a, 9b and 10aec (Schemes 2 and 3). The azo coupling reaction
hυ1
Ar
N
N
N
N
Ar
Ar
Ar
Δ or hυ2
E - isomer
Z - isomer
Scheme 1. Photochemical E / Z isomerization of aromatic azo dyes.

64
M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 62e73
Scheme 2. Synthesis of thienylpyrrole azo dyes 5e7 through azo coupling reaction of thienylpyrroles 1 with thiazolyldiazonium salts 2 and 3 and aryldiazonium salt 4a.
was accomplished selectively at the 2-position [17] of the pyrrole
ring to give thienylpyrroles 5aef, 6b and 7a in good yields
(67e89%). These results are consistent with the greater nucleo-
philicity of the pyrrole ring versus the thiophene ring as has been
shown earlier in the case of azo coupling, formylation and tricya-
novinylation reactions of thienylpyrroles [12,13a,13b,16b,18]. As
expected, higher yields (77e89%) were obtained in the synthesis of
thienylpyrrole azo dyes 5bec and 5eef bearing aryl groups
substituted on 4- or 2,4- position(s) by electron-donating groups
when compared with propyl derivative 5a (47%).
On the other hand, when compared to the yield of dye 10a
(85%), which was obtained from the unsubstituted aryldiazonium
salt, the synthesis of pyrrole azo dyes 10 was achieved in higher
yields for compounds 10b (94%) and 10c (95%) which were
prepared through azo coupling using diazonium cations
substituted by withdrawing groups (CN, NO2) in the 4-position of
the aryl ring (Scheme 3). These results follow from the fact that
the azo coupling is an aromatic electrophilic substitution, there-
fore electron-donor substituents on the pyrrole ring (R1) and/or
electron-withdrawing groups (R2) on the aryldiazonium salt
should facilitate the reaction.
Another comparison could also be made having in mind the
results previously obtained by us in the synthesis of thienylpyrrolyl
aryldiazenes. In this case the introduction of the thiophene ring in
position 2 of the pyrrole heterocycle leads to lower yields for
thienylpyrroles 11bec (81e84%) [13a,13b] compared to pyrroles
10bec (94e95%).
Thienylpyrrole azo dye 7a was synthesized in order to compare
the difference of the electronic and optical properties when a phe-
nylazo moiety is substituted by an azothiazole system (e.g., 5a) and
pyrrole azo dyes 9b and 10aec were synthesized in order to
The structures of azo dyes bearing thienylpyrrole 5e7 or pyrrole
conjugated systems 9e10 were unambiguously confirmed by their
analytical and spectral data.
In the 1H NMR spectra of azo thienylpyrrole derivatives 5aef
and 6b functionalized with a thiazolyldiazene moiety on the 2-
position of the pyrrole ring two doubles at about 7.01e7.15 and
6.77e7.17 ppm were detected with coupling constants of
4.4e4.8 Hz indicating the presence of two adjacent protons (30-H
and 40-H) at the corresponding pyrrole moiety. In the 1H NMR
spectrum of derivative 6b bearing a 5-methyl-thiazole moiety two
signals at 2.43 and 7.54 ppm were detected. Both signals appear as
doublets with a coupling constant of 0.8 Hz. These signals were
attributed respectively, to the methyl group attached to C5 and to
the 4-H, in the thiazole moiety. In all the 1H NMR spectra of thie-
nylpyrrole azo dyes 5aef and 6b three signals at about 7.05e7.08
(multiplet) 7.18e7.52 (double doublet), and 7.46e7.72 (double
doublet) were detected and were attributed respectively, to the 400,
300 and 500-H protons in the thiophene ring.
The 1H NMR spectra confirm also a significant CT from the
pyrrole or thienylpyrrole moieties to the hetero(aryl)azo groups
and a high polarizability of the whole donoreacceptor derivatives.
Therefore, the chemical shifts of azo dyes 10bec bearing stronger
acceptor groups on the diazene moiety, exhibit signals that are
downfield relative to the unsubstituted derivative 10a indicating CT
from the donor to the acceptor. The effect of the substitution of
a phenyl group to the acceptor end for dye 7a by a thiazole
heterocycle (e.g., 5a) is also noteworthy. All the protons of the
thienylpyrrole 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
respectively) when compared to the corresponding phenyldiazene
azo dye 7a (e.g., 40-H and 30-H
d
¼ 6.77 and 7.01 ppm
d
¼ 6.58 and 6.88 ppm respectively)
compare the effect of the
p
-conjugated bridge/donor auxiliary
thus indicating a decrease of the electron density due to the
stronger acceptor ability of the thiazole ring, allowing a more
efficient charge-transfer from the donor to the acceptor group. On
the other hand stronger donor groups substituted on position 1 of
effect of the thiophene heterocycle on the electro optical properties
of azothiazole thienylpyrrole (e.g., 5b) and arylazo thienylpyrroles
11bec (Fig. 1) [13a,13b].
Scheme 3. Synthesis of pyrrole azo dyes 9b and 10aec through azo coupling reaction of pyrrole 8 with thiazolyldiazonium salt 2 and aryldiazonium salts 4aec.

M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 62e73
65
The ICT bands of pyrrole azo dyes 10bec in dioxane solutions are
also red-shifted by 18 nm for 10b (R2 ¼ CN, lmax ¼ 404 nm) and
31 nm for 10c (R2 ¼ NO2, lmax ¼ 417 nm) compared to the ICT band
of the unsubstituted derivative 10a (R2 ¼ H, lmax ¼ 386 nm) indi-
cating that, the electron-accepting abilities of p-substituted phenyl
groups increase, as expected, in the order 10a < 10b < 10c.
On the other hand the electron-donor ability of the substituent
on the pyrrole nitrogen atom has a smaller impact on the UVevis
spectra of thienylpyrrole azo dyes 5aef. For instance, the absorp-
tion maxima of compounds 5a and 5c were shifted from 486 nm
(5a) to 497 nm (5c).
S
N
N
N
R1
R2
11b R1 = 4-MeOC6H4, R2 = CN
11c R1 = 4-MeOC6H4, R2 = NO2
Fig. 1. Structure of diazene thienylpyrroles 11bec [13a,13b].
As anticipated, the introduction of a thiophene ring induces
a significant bathochromic shift on the UVevis spectra of azo
thienylpyrrole 5b and thienylpyrrole derivatives 11bec [13a,13b]
compared to their pyrrole counterparts 9b and 10bec respec-
tively. The difference in lmax values between compound 5b and 9b
in dioxane is 74 nm while for compounds 11bec the difference is
even larger (69e80 nm) when compared to the pyrroles 10bec as
a result of a more extensive electron delocalization (Table 1). These
observations confirm previously obtained results, namely that the
incorporation of thiophene units in pushepull compounds
enhances their charge-transfer properties, and can be explained
considering the bathochromic effect of sulphur, the partial decrease
of aromatic character of the thiophene heterocycle and also the
the pyrrole ring of thienylpyrrole azo dyes leads to highfield signals
thus again demonstrating the easy of electron communication
within the whole heterocyclic system.
2.2. UVevisible study of thienylpyrrole 5e7 and for pyrrole
azo dyes 9e10
All compounds were soluble in common organic solvents, such
as diethyl ether, ethanol, dioxane and DMSO. The absorption
spectra data of azo dyes 5, 6e7 and 9e10 in these solvents are
summarized in Table 1. They show an intense lowest energy charge-
transfer absorption band in the UVevis region. The position of this
band was strongly influenced by the structure of the compounds,
increase of the
units [20].
p-overlap between the thiophene and the pyrrole
for example by the type of p-conjugated bridge, by the substitution
pattern in the donor and the acceptor moieties and also by the
electronic nature of the acceptor moiety. The absorption maxima
2.3. Solvatochromic study of azo dyes 5e7 and 9e10
(lmax) of the thienylpyrrole azo dyes 5, 6e7 in dioxane are located
at the range of 419e498 nm as opposed to the range of 386e419 nm
for pyrrole azo dyes 9e10. Dramatic differences in energy occur
upon thiazolylazo or arylazo substitution of thienylpyrroles 1 and
pyrrole 8. For example, the absorption maxima, in ethanol
for thienylpyrrole 1c (lmax ¼ 286.5 nm) [13a] is shifted 219.5 nm
Previous studies have demonstrated that donoreacceptor
substituted thienylpyrroles exhibit a positive solvatochromism
[5,13a,13b]. In order to investigate whether thienylpyrroles 5e7 and
pyrroles 9e10 exhibit the same behavior, we carried out a study of
the absorption spectra for all compounds in four selected solvents
of different solvatation character (diethyl ether, ethanol, dioxane
and DMSO). The wavelength maxima lmax of compounds 5e7 and
upon thiazolylazo substitution (thienylpyrrole azo dye 5c, lmax
¼
506.0 nm) (Table 1).
As observed for other heterocyclic azo dyes, a bathochromic
shift in the UVeVis. spectra is observed when stronger donor and/
or acceptor groups are linked to the heterocyclic system
[13a,13b,14a,14b,15a,19]. Therefore, substitution of an arylazo
system (e.g., 7a) for a thiazolyl azo moiety (e.g., 5a) leads to a red
shift of 67 nm from 419 nm (e.g., 7a) to 486 nm for thiazole azo dye
5a. Due to the electron density deficiency on the ring C atoms, the
thiazole heterocycle acts as an electron-withdrawing group and
also as an auxiliary acceptor; in fact it is a stronger acceptor group
than the phenyl ring [3e,3f,4].
9e10 are listed in Table 1 and were compared with the
each solvent, as determined by Kamlet and Taft [21].
Moderate to large positive solvatochromism
p
* values for
(Dnmax
¼
884e1058 cmꢁ1) was observed moving from diethyl ether to DMSO
solutions for thiazolyl diazene derivatives 5aef and 6b. On the
other hand pyrrole azo dyes 10 exhibit smaller positive sol-
vatochromism (Dnmax ¼ 400e882 cmꢁ1) compared to their thie-
nylpyrrole counterparts [13a].
In agreement with other solvatochromic studies for heteroaryl-
azo dyes, the increase of the electron-withdrawing strength of the
Table 1
Solvatochromic data [lmax (nm) and ymax (cmꢁ1) of the charge-transfer band] for thienylpyrrole 5e7 and pyrrole azo dyes 9e10 in 4 solvents with
p* values by Kamlet and Taft
[21].
Compound
Solvent (p*)
Diethyl ether (0.54)
Ethanol (0.54)
1,4-Dioxane (0.55)
DMSO (1.00)
lmax (nm)
nmax (cmꢁ1
)
lmax (nm)
nmax (cmꢁ1)
lmax(nm)
nmax (cmꢁ1
)
lmax (nm)
nmax (cmꢁ1
)
5a
5b
5c
5d
5e
5f
6b
7a
9b
10a
10b
10c
477
487
488
487
483
482
488
413
414
384
402
413
20,964
20,533
20,491
20,533
20,704
20,747
20,491
24,213
24,154
26,041
24,875
24,213
491
504
506
500
499
498
505
416
417
386
406
417
20,367
19,841
19,763
20,000
20,040
20,080
19,801
24,038
23,980
25,906
24,630
23,980
486
493
497
494
491
489
498
419
419
386
404
417
20,576
20,284
20,121
20,243
20,367
20,450
20,080
23,866
23,866
25,906
24,752
23,980
498
511
514
512
509
504
513
426
427
390
411
429
20,080
19,570
19,455
19,531
19,646
19,841
19,493
23,474
23,419
25,641
24,330
23,331

66
M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 62e73
substituent of the diazo component and/or the increase of the
electron-donating strength of the coupling moiety was found to
cause pronounced bathochromism [13a,14,15a,19]. Therefore, the
pyrrole azo dyes with stronger acceptor groups on the azophenyl
moiety (10bec) and the thienylpyrroles bearing stronger donor
groups substituted on position 1 of the pyrrole ring (e.g., 5c) display
a comparatively larger solvatochromism when compared to the
unsubstituted compound 10a and with the alkyl thienylpyrrole
derivative 5a, respectively. The same trend was observed when
a diazenephenyl moiety (7a, Dnmax ¼ 739 cmꢁ1) was substituted by
a thiazolyldiazene system (5a, Dnmax ¼ 884 cmꢁ1), due to the
electron-deficient nature of the thiazole heterocycle.
These sizable solvatochromic responses are indicative of highly
polarizable p-conjugated structures. These features are key factors
in designing efficient second-order NLO-chromophores [22].
Therefore, the UVevis optical data suggest that the chromophores
5aef and 6b will have greater hyperpolarizabilities than the
pyrroles 10.
Fig. 2. Cyclic voltammogram of compound 5d (1.0
ꢀ
10ꢁ3 mol dmꢁ3
) in DMF,
0.1 mol dmꢁ3 [NBu4][BF4] at a vitreous carbon electrode. a e between ꢁ0.50 V
and ꢁ2.30 V versus fcþ/fc, scan rate 0.1 V sꢁ1; b e between e 0.50 V and ꢁ1.70 V, scans
rate 0.02, 0.05, 0.20 and 0.30 V sꢁ1; c e between ꢁ0.50 V and 0.80 V versus fcþ/fc, scan
2.4. Electrochemical properties of compounds 5e7 and 9e10
rate 0.1 V sꢁ1
.
The redox properties of thienylpyrroles (5aef, 6b and 7a) and
pyrrole azo dyes (9b, 10aec) were studied by cyclic voltammetry in
DMF containing tetrabutylammonium tetrafluoroborate (0.10 M) as
the supporting electrolyte. Table 2 lists the reduction and oxidation
onsets and the electrochemical band gap values.
nitro group. The one-electron stoichiometry for these reduction
processes was ascertained by comparing the current heights with
known one-electron redox processes under identical conditions
[13a,13b,23,24]. For all compounds, the first process was associated
with the reversible reduction of the (hetero)aromatic azo moiety.
For all azo derivatives it was also observed that the reduction
potential values are only slightly influenced by the substituent on
the nitrogen atom of the pyrrole ring or by the introduction of
a thiophene ring on the pyrrole system. Therefore, the difference
between the reduction potentials obtained for thienylpyrroles and
the corresponding pyrrole derivatives are between 0.03 and 0.05 V.
All thienylpyrrole and pyrrole azo dyes undergo oxidation due
to the absence of substituents in 500 position of the thiophene (5aef,
6b and 7a) or in 50 position of the pyrrole (9b and 10aec) groups.
Dimerization via the electrogenerated radical cations has been
shown to usually occur at such positions [25].
The data obtained for the process of the oxidation illustrate that
there is a prominent electrochemical distinction between the
compounds depending on the type of donor and acceptor groups
(Table 2). Contrary to the reduction process, a comparison of the
oxidation potentials obtained for thienylpyrroles and the corre-
sponding pyrrole azo dyes bearing the same group substituted on
position 1, of the pyrrole ring, showed a remarkable difference in
these values due to the effect of the length and the different elec-
Upon azo coupling reactions, the azo dyes bearing thie-
nylpyrrole 5 and pyrrole systems 9e10 display oxidations at more
positive potentials as a consequence of the destabilizing effect of
the electron-withdrawing groups (thiazole or functionalized
phenyl rings) on the diazene moiety. For example pyrrole azo dye
9b displays an oxidation at Epa ¼ 0.69 V, corresponding to a nega-
tive shift of 0.17 V with respect to the unsubstituted pyrrole 8.
All thienylpyrrole azo dyes bearing a thiazole acceptor group
(5aef and 6b) exhibit two monoelectronic reversible reductions
and one oxidation process (Fig. 2). On the other hand, the thie-
nylpyrrole azo dye 7a and the pyrrole azo dyes with aryl end groups
10aeb exhibit, in cathodic scans only one reversible reduction
process. The dye 10c exhibits a similar first reduction process while
the second reduction process is assigned to the reduction of the
Table 2
Electrochemical data for compounds 5e10.
Compound
Reductiona
1E1/2(V)
1.51
1.50
1.48
1.53
1.46
1.46
1.55
1.95
e
Oxidationa
-
-
2Epc(V)
Epa(V)
Band gap(eV) b
tronic nature of the
p-conjugated bridge (e.g., 5b, Epa ¼ 0.57 V and
5a
5b
5c
5d
5e
5f
6b
7a
8
9b
10a
10b
10c
11b [13a]
11c [13a]
2.25
0.59
0.57
0.58
0.60
0.62
0.62
0.53
0.56
0.86
0.69
0.76
0.79
0.81
0.59
0.62
2.10
2.07
2.06
2.13
2.08
2.08
2.08
2.51
e
2.16
2.11
2.27
2.14
2.10
2.25
e
9b Epa ¼ 0.69 V). The observed shifts are a direct consequence of the
extent of donoreacceptor coupling [5a,13a]. The same trend was
observed for pyrroles 10bec compared to thienylpyrroles 11bec
[11a]. Since the oxidation potential is directly related to the ioni-
zation potential or the tendency of losing electron, it is expected
that thienylpyrrole chromophores have a more efficient ICT from
the donor to the acceptor compared to their pyrrole counterparts.
It was also observed, that the decrease of the acceptor group
ability on the aryldiazene moiety of pyrroles 10aec results in
a negative shift of the oxidation potential (e.g., 10a Epa ¼ 0.76 V and
10c Epa ¼ 0.81 V). On the other hand, compound 7a with an azo-
phenyl moiety, exhibits only a slight cathodic shift due to the
weaker withdrawing acceptor strength of the phenyl ring
compared to the thiazole heterocycle.
e
1.53
1.90
1.59
1.30
1.74
1.35
e
2.22
2.66
2.38
2.11
2.33
1.97
e
e
1.93
e
1.83
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 ferrocinium/ferrocene-couple.
E1/2 corresponds to the reversible process. Epc and Epa correspond to the cathodic
and anodic peak potentials, respectively.
Electrochemical band gaps were calculated as described previ-
ously from the potentials of the anodic (oxidation of the thie-
nylpyrrole or pyrrole groups) and cathodic processes [20d,26].
b
EHOMO ¼ 4.39 þ Eox (eV) and ELUMO ¼ Ered þ 4.39 (eV).

M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 62e73
67
The analysis of the electrochemical data for compounds 5e7 and
9e10 showed that several factors influence the electronic nature of
From Table 3 it can be seen that the NLO-chromophores 5aef, 6b
and 11bec exhibit moderate to good molecular nonlinearities as
the
p-conjugated systems leading to a decrease of the band gap
their b values are 7e25 times higher that of the well known pNA
values:
molecule for an incident laser wavelength of 1064 nm (the corre-
sponding b0 values are 2e7 times higher than that of pNA).
Earlier, some of us reported the synthesis, solvatochromic and
electrochemical properties of aryldiazene thienylpyrrole deriva-
tives 11bec [13a]. The first hyperpolarizabilities of these
compounds were now also studied in order to compare their values
with those of the pyrrole azo dyes 10bec. The introduction of
i) the strength of the donor group linked to position 1 of the
pyrrole ring (e.g., 5a, 2.10 eV; 5c, 2.06 eV);
ii) the strength and electronic nature of the acceptor group
linked to the diazene system (e.g., 10a, 2.66 eV; 10c, 2.11 eV or
7a, 2.51 eV; 5a, 2.10 eV);
iii) the length of the
2.07 eV).
p
-conjugated bridge (e.g., 9b, 2.22 eV; 5b,
a thiophene ring on the azo dyes 11bec leads to higher b values
(11bec;
counterparts (10bec;
b
¼ 345e415 ꢀ 10ꢁ30 esu) when compared to their pyrrole
b
¼ 84e128 ꢀ 10ꢁ30 esu). As expected, the
The results clearly show that there is a much more efficient
coupling between the thienylpyrrole and pyrrole systems and the
thiazole acceptor as compared to the aryl acceptor groups. The
same trend was observed for the thienylpyrrole thiazolylazo dye
5b, (
b
¼ 156 ꢀ 10ꢁ30 esu) when compared to the corresponding
thiazolylazo pyrrole derivative 9b (
b
¼ 80 ꢀ 10ꢁ30 esu). We attri-
efficient donoreacceptor conjugation leads to
a
raising and
bute this as being due to the more extensive electron
delocalization.
lowering of the HOMO and LUMO levels, respectively. In contrast
a weaker donoreacceptor coupling provokes the opposite effect:
the lowering of the HOMO level and the a raising of the LUMO level.
The measured HOMO/LUMO values and how they are influenced
by the electronic groups are consistent with the spectroscopic
measurements.
Moreover, compounds functionalized with 2,4-dimethox-
yphenyl- (5c), 4-fluorophenyl- (5e) and 4-bromophenyl- (5f)
groups, on position 1 of the pyrrole ring exhibit larger nonlinear-
ities as compared to the other thienylpyrrole azo dyes. It is also
noteworthy the effect of the functionalization of the thiazole ring
with a methyl group (e.g., 6b), which increases the
b values from
156 ꢀ 10ꢁ30 esu for compound 5b to 274 ꢀ 10ꢁ30 esu for 6b.
2.5. Non-linear optical properties and thermal stability
of thienylpyrrole 5e7 and pyrrole azo dyes 9e10
From Table 3 it can be seen that the
b values of pyrrole azo dyes
10b (
b
¼ 84 ꢀ 10ꢁ30 esu) and 10c (
b
¼ 128 ꢀ 10ꢁ30 esu) are strongly
influenced by the strength of the acceptor group (CN < NO2)
substituted on the arylazo moiety. As expected, the same trend
(H < CN < NO2) was observed for the corresponding thie-
We have used the hyper-Rayleigh scattering (HRS) method [27]
to measure the first hyperpolarizability
b of thienylpyrrole 5e7 and
pyrrole azo dyes 9e10 using the 1064 nm fundamental wavelength
of a laser laser beam. Dioxane was used as the solvent, and the
nylpyrroles derivatives 7a, 11b and 11c. Indeed, the
b values
increased from 95 ꢀ 10ꢁ30 esu for 7a (R ¼ H) to 345 ꢀ 10ꢁ30 esu for
11b (R ¼ CN) reaching the higher value of 415 ꢀ 10ꢁ30 esu for 11c
(R ¼ NO2).
b
values were measured against a reference solution of p-nitro-
aniline (pNA) [28] in order to obtain quantitative values, while care
was taken to properly account for possible fluorescence of the dyes
(see Experimental section for more details). The static hyper-
polarizability b0 values were calculated using a very simple two-
level model neglecting damping. They are therefore only indicative
and should be treated with caution (Table 3).
The comparison of the first hyperpolarizabilities for compounds
5a and 7a also showed that the electron-deficient thiazole
heterocycle has a larger acceptor strength than the phenyl ring, as
the
b
value of the thiazolyldiazene 5b (
b
¼ 164 ꢀ 10ꢁ30 esu) was
almost two times larger than the corresponding value for the
phenyldiazene 7a (
electron density on the ring C atoms, the thiazole heterocycle acts
as electron-withdrawing group and also as an auxiliary acceptor.
Furthermore, the large electronegativity and lone electron pairs of S
and N atoms in thiazole together with the extension of the conju-
b
¼ 95 ꢀ 10ꢁ30 esu). Due to the deficiency of
Table 3
UVevis absorptions,
b and b0 values and Td data for thienylpyrrole 5e7 and pyrrole
azo dyes 9b and 10aeca.
d
Td (ꢂC)
Entry
Azo dye
lmax(nm)
b
b(10ꢁ30 esu)
b0c(10ꢁ30 esu)
1
2
3
4
5
6
5a
5b
5c
5d
5e
5f
6b
7a
486
493
497
494
491
489
498
419
419
386
404
417
473
497
352
164
156
175
129
198
190
274
95
21 ꢃ 15
16 ꢃ 6
17 ꢃ 2
14 ꢃ 2
23 ꢃ 4
23 ꢃ 4
27 ꢃ 5
31 ꢃ 3
26 ꢃ 6
ee
239
259
273
220
206
206
227
ef
gation length of the
p-electron bridge also lead to an increase in
molecular hyperpolarizability, showing that thiazole derivatives
are a good choice for NLO applications [3e,3f,3i,3j,4a,4b,4c].
The thermal stabilities of the resulting azo dyes 5e7 and 10e11
were evaluated by thermogravimetric analysis (TGA) under
a nitrogen atmosphere, measured at a heating rate of 20 ꢂC minꢁ1
.
7
8
9
10
11
12
13
6
As shown in Table 3 all compounds exhibit good to excellent
thermal stability with decomposition temperatures varying from
206 to 288 ꢂC for thienylpyrrole azo dyes 5aef, 6b and 11bec.
Pyrrole derivatives 11bec decompose between 216 and 227 ꢂC,
while thienylpyrrole azo dyes (e.g., 11bec) showed an improved
thermal stability by ca 59e62 ꢂC. In addition, the electronic nature
of the substitutents on position 1 of the thienylpyrrole azo dyes
5aef have some influence on their thermal stability being higher
for R1 ¼ 2,4-diOMePh, Td ¼ 273 ꢂC, (Table 3) (Fig. 3).
9b
80
ef
10a
10b
10c
11b
11c
pNA
ee
ef
84
30 ꢃ 11
42 ꢃ 4
58 ꢃ 2
41 ꢃ 2
8.5
216
227
288
286
e
128
345
415
16.9 [28]
a
Experimental hyperpolarizabilities and spectroscopic data measured in dioxane
solutions.
b
c
All the compounds are transparent at the 1064 nm fundamental wavelength.
Data corrected for resonance enhancement at 532 nm using the two-level
model with b0
¼
b
[1 ꢁ (lmax/1064)2][1 ꢁ (lmax/532)2]; damping factors not
2.6. Photochromic properties of thienylpyrrole 5e7 and pyrrole
azo dyes 9e10
included 1064 nm [29].
d
Decomposition temperature (Td) measured at a heating rate of 20 ꢂC minꢁ1
under a nitrogen atmosphere, obtained by TGA.
e
The photochromic properties of azo dyes 5aef, 6b, 7a and
It was not possible to measure the
b
due to the high amount of fluorescence.
Compounds 7a, 9b and 10a were obtained as oils.
pyrrole derivatives 9b and 10aec were studied in 2.0 ꢀ 10ꢁ5
M
f

68
M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 62e73
Table 4
Spectrokinetic properties under continuous visible irradiation: maximum wave-
length of absorption (lmax), maximum absorbance (Amax), absorbance variation
Abs), thermal colouration rates (kD) and half-time life (t1/2) of azo dyes 5aef, 6b,
7a, 10aec, and 11c in acetone.
(D
Azo dye
lmax (nm)
Amax
DAbs
kD(sꢁ1) (%)
t1/2 (s)
5a
487
0.592
0.121 (20.4%)
0.120 (22%)
0.147 (20.5%)
0.135 (22%)
0.142 (21.2%)
1.4 (98)
0.5
0.072 (2)
0.62 (99)
0.08 (1)
1.3 (82)
0.33 (18)
1.1 (81)
0.33 (19)
0.97 (87)
0.3 (13)
5b
5c
5d
5e
499
500
495
493
0.542
0.659
0.612
0.669
1.1
0.65
0.75
0.82
5f
6b
493
499
0.511
0.459
0.100 (19.6%)
0.036 (7.8%)
0.63 (100)
1.25 (97)
0.031 (3)
0.069 (43)
0.026 (57)
0.0193 (100)
5.2 ꢀ 10ꢁ4 (100)
0.0028 (100)
0.0095 (100)
0.33
1.1
0.83
7a
420
0.523
0.277 (53%)
20
9b
10a
10b
10c
420
388
405
420
500
0.38
0.53
0.472
0.38
0.42
0.19 (50%)
0.29 (54%)
0.31 (66%)
0.21 (55%)
0.11 (26)
36
1330
237
70
Fig. 3. Thermal analysis data for azo dye 5a throughꢁT1GA recorded under a nitrogen
atmosphere, measured at a heating rate of 20 ꢂC min
.
11c [13a]
2.1
acetone solutions by irradiating them with visible light
> 420 nm) from a 150 W ozone free xenon lamp equipped with
(l
a water filter and a long-pass filter Schott GG 420 at 20 ꢂC and
simultaneously monitoring absorbance of the solutions at their
corresponding wavelength of maximum absorption. The irradiation
of the thienylpyrrole azo dye 5aef, 6b, 7a solutions led to a very fast
and pronounced decrease of the maximum absorbance at longer
wavelengths and, at the same time, an increase in the band located
at 380e400 nm, indicating the transformation of the more stable E-
isomer to the Z-isomer. The change in the visible spectra of dye 5a is
depicted in Fig. 4. When the irradiation was stopped the inverse
situation was observed, the band at 380 nm decreased and the band
at 487 nm increased.
The absorbance variation observed under visible irradiation of
diazenethiazolyl thienylpyrroles 5aef varies from 0.10 to 0.15
absorbance units corresponding to a loss of 20e22% of the initial
absorbance. The procedure was subsequently repeated and the
behavior was fully reproducible indicating that under these
experimental conditions no noticeable degradation occurred. For
methylthiazole azo dye 6b the decrease in the absorption was
considerably lower (7.8%) while for the diazenephenyl azo dye 7a
and pyrrole azo dyes 9b, 10aec and 11b the absorption variations
were significantly higher (50e66%) (Table 4).
less than 1.5 s a photostationary equilibrium was attained. For
pyrrole azo dyes 10aec the decolouration rate was much slower
and the photostationary equilibrium was attained only after
35e90 s of irradiation. When the irradiation ceased the system
returned to its initial highly coloured state with different rates
depending on the dye structure. The kinetics of the decolouration
and colouration process of dyes 5a and 7a are shown in Fig. 5.
The kinetics of the thermal Z / E back reaction (colouration, in
the dark, at 20 ꢂC) for the thiazolyl thienylpyrrole azo dyes 5aef
and 6b were very fast (kD ¼ 1.4e0.62 sꢁ1) but effectively inde-
pendent of the nature of the substituent present on the pyrrole ring
(t1/2 (Z-isomer) between 0.5 and 1.1 s). The high thermal Z / E
reaction rate observed with these compounds and the limited
magnetic stirring of the solution, leads to a considerable oscillation
of the absorbance at the photostationary equilibrium (Fig. 5,
compound 5a), which is less significant for slower systems like
compound 7a (Fig. 5).
Pyrrole azo dyes 10aec showed very slow kinetics of the
thermal back Z / E isomerization (9.5 ꢀ 10ꢁ3 to 5.2 ꢀ 10ꢁ4
s
ꢁ1),
indicating a relatively high stability of the Z-isomer (t1/2
¼
For the new thiazole azo dyes 5aef and 6b the decrease in the
absorbance, due to the E / Z transformation, was very fast and in
70e1330 s). For these compounds the switching rates between the
two isomers are quite slow.
From the comparative analysis of these thienylpyrrole and
pyrrole azo dyes we can conclude that the substitution of the
thiazole ring by a phenyl ring [comparison between compounds 5a
(t1/2 ¼ 0.5 s) and 7a (t1/2 ¼ 20 s)] leads to a more stable Z species as
indicated by the significant reduction of the thermal back reaction.
The removal of the thiophene ring on diazenethiazolyl dyes
[comparison between compounds 5b (t1/2 ¼ 1.1 s) and 9b (t1/
1.0
0.8
487
0.6
¼ 36 s)] has a similar effect. The same trend was also observed for
2
diazenearyl pyrrole 10c when compared to the corresponding
thienylpyrrole 11c.
0.4
380
The introduction of substituents with increased acceptor
strength on the diazenearyl moiety of pyrrole dyes 10aec, has
a strong effect on the kinetics of the thermal back isomerization.
Therefore, the presence of a stronger acceptor group such as nitro
leads clearly to faster kinetics (e.g., 10a, R ¼ H, t1/2 ¼ 1330 s; 10c,
R ¼ NO2, t1/2 ¼ 70 s). This effect, already observed with diazenearyl
thienylpyrroles [11c], can also be seen by comparing compounds 7a
(R ¼ H, t1/2 ¼ 20 s) and 11c [11c] (R ¼ NO2, t1/2 ¼ 2.1 s).
0.2
0.0
300
400
500
600
700
Wavelength (nm)
Fig. 4. Absorption spectra of dye 5a under visible irradiation (———) and in the dark
(dd) in acetone.

M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 62e73
69
7a
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
5a
0.5
0.4
0.3
0.2
0.1
0.0
0
5
10
15
20
25
30
0
2
4
6
8
10
12
14
Time (min)
Time (min)
Fig. 5. Visible irradiation/dark cycles for azo dyes 5a and 7a.
Overall within this set of compounds the thienylpyrrole bearing
diazenethiazolyl moieties 5aef and 6b showed the best photo-
chromic behaviour: fast decouloration promoted by visible light
stimulus (1.5 s) with a relatively high absorbance variation (20%)
and very fast thermal back colouration (2e3 s) to the initial state,
thus performing fast reproducible cycles.
aryldiazonium salts 2-4 and 1-(40-methoxyphenyl)pyrrole, were
purchased from Aldrich and Fluka and used as received.
The synthesis of 1-alkyl-thienylpyrrole 5a [16b] and 1-aryl-
thienylpyrroles 5bef [16a] was described elsewhere. TLC analyses
were carried out on 0.25 mm thick precoated silica plates (Merck
Fertigplatten Kieselgel 60F254) and spots were visualised under UV
light. Chromatography on silica gel was carried out on Merck Kie-
selgel (230e240 mesh).
3. Conclusions
We have synthesized two new series of azo dyes, based on the 1-
alkyl(aryl)thienylpyrrole and 1-(4-methoxyphenyl)pyrrole as
4.2. Synthesis
donors and simultaneously as
p-conjugated bridges, and dia-
4.2.1. General procedure for the azo coupling of thienylpyrroles
1 with thiazolyl-diazonium salts 2e3 and aryldiazonium salt
4a to afford azo dyes 5aef, 6b and 7a
4.2.1.1. Diazotisation of 2-aminothiazole, 2-amino-5-methylthiazole
and aniline. Primary aromatic and heteroaromatic amines
(1.0 mmol) were dissolved in HCl 6N (1 mL) at 0 e 5 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 0 e 5 ꢂC. The reaction
mixture was stirred for 10 min.
zenearyl and diazenethiazolyl acceptor moieties. Extensive char-
acterization of the optical (linear and nonlinear) electrochemical,
thermal and photochromic properties was carried out. The new
compounds were synthesized from easily available thienylpyrroles
1 and low cost, commercially available (hetero)aromatic anilines
and pyrrole 8. Simple work-up procedures produce moderate to
excellent yields of these derivatives.
Despite the large number of donoreacceptor systems showing
NLO and photochromic properties reported in the literature, the
present concept of combining the donor properties of electron-rich
pyrrole and thienylpyrrole heterocyclic systems with the electron-
deficient thiazole moiety in azo dye derivatives has not, to the best
of our knowledge, been previously communicated in the literature.
This multidisciplinary study shows that the electronic nature of
the withdrawing group substituted on the azo moiety and the type
4.2.1.2. Coupling reaction with thienylpyrroles 1. The diazonium salt
solution previously prepared (1.0 mmol) was added dropwise to the
solution of thienylpyrroles 1 (0.52 mmol) in acetonitrile (10 mL)
and 2e3 drops of acetic acid. The combined solution was main-
tained at 0 ꢂC for 1e2 h while stirred and then diluted with chlo-
roform (20 mL), washed with water and dried with anhydrous
MgSO4. The dried solution was evaporated and the remaining azo
dyes purified by column chromatography on silica with dichoro-
methane/n-hexane as eluent.
of p-conjugated system (pyrrole or thienylpyrrole) of the different
synthesized azo dyes, has a remarkable influence on the electronic,
optical (linear and nonlinear optical), photochromic and thermal
properties of these donoreacceptor systems. The NLO and the
photochromic properties of these heteroaromatic azo dyes, in
particular the kinetics of the back isomerization, can be modulated
using thienylpyrrole sytems instead of pyrrole derivatives and/or
through the introduction of different diazene (hetero)aryl moieties.
Moreover, these new derivatives bearing thiazole moieties exhibit
improved photochromic properties compared to the previously
described diazenearyl azo dyes. In conclusion, the thienylpyrrole
derivatives 5aef and 6b, are endowed with good first hyper-
polarizabilities, good thermal stabilities and excellent photo-
chromic properties making them interesting candidates as
prospective second-order NLO materials, as novel molecular
switches or memory and recording devices.
1-(1-(Propyl-5-(thiophen-2-yl)-1H-pyrrol-2-yl)-2-(thiazol-2-
yl))diazene (5a). Dark red solid (74 mg, 47%). Mp 68e69 ꢂC. 1H NMR
(Acetone-d6)
d
0.98 (t, 3H, J ¼ 7.6 Hz, CH3), 1.85e1.94 (m, 2H, CH2),
4.54 (t, 2H, J ¼ 7.6 Hz, NCH2), 6.77 (d, 1H, J ¼ 4.4 Hz, 40-H), 7.01 (d,
1H, J ¼ 4.4 Hz, 30-H), 7.27e7.29 (m, 1H, 400-H), 7.52 (dd, J ¼ 4.0 and
1.2 Hz, 300-H), 7.54 (d, 1H, J ¼ 3.6 Hz, 5-H), 7.72 (dd, 1H, J ¼ 5.2 and
1.2 Hz, 500-H), 7.92 (d, 1H, J ¼ 3.6 Hz, 4-H). 13C NMR (Acetone-d6)
d
11.4, 25.4, 46.3, 105.0, 115.2, 120.0, 128.2, 128.4, 129.1, 133.3, 136.5,
144.2, 147.5, 179.9. lmax(Dioxane)/nm 486 (
25,400). IR (Nujol)
3
/dm3 molꢁ1 cmꢁ1
n
/cmꢁ1 3075, 1619, 1504, 1341, 1297, 1214, 1162,
1135, 1017, 874, 851, 775, 721, 621. MS (microTOF) m/z (%) ¼ 303
([M þ H]þ, 100), 207 (2). HMRS: m/z (MicroTOF) for C14H15N4S2;
calcd 303.0738; found: 303.0733.
1-(1-(4-Methoxyphenyl)-5-(thiophen-2-yl)-1H-pyrrol-2-yl)-
2-(thiazol-2-yl)diazene (5b). Violet solid (146 mg, 77%). Mp
4. Experimental
124e126 ꢂC. 1H NMR (Acetone-d6)
d 4.00 (s, 3H, OCH3), 7.00 (d, 1H,
4.1. Materials
J ¼ 4.4 Hz, 40-H), 7.04e7.07 (m, 1H, 400-H), 7.14 (d, 1H, J ¼ 4.4 Hz, 30-
H), 7.17e7.18 (m, 3H, 300-H, 3000-H and 5000-H), 7.40e7.42 (m, 3H, 5-H,
2000-H and 6000-H), 7.49 (dd, 1H, J ¼ 4.8 and 1.2 Hz, 500-H), 7.85 (d, 1H,
Aniline, 4-cyanoaniline, 4-nitroaniline, 2-aminothiazole, 2-
amino-5-methylthiazole used as precursors for the synthesis of
J ¼ 3.2 Hz, 4H). 13C NMR (Acetone-d6)
d 55.9, 104.8, 113.9, 115.0,

70
M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 62e73
120.2,127.8,128.3,129.6,131.4,133.7,130.0,144.1,149.5,161.3,179.8.
lmax(Dioxane)/nm 493 (
/dm3 molꢁ1 cmꢁ1 25,830). IR (Nujol)
/cmꢁ1 2954, 1609, 1515, 1403, 1334, 1297, 1243, 1220, 1195, 1174,
Mp 118e120 ꢂC. 1H NMR (Acetone-d6)
d
2.43 (d, 3H, J ¼ 0.8 Hz, 5-
3
CH3), 3.95 (s, 3H, OCH3), 6.95 (d, 1H, J ¼ 4.8 Hz, 30-H), 7.03e7.05 (m,
1H, 400-H), 7.07 (d, 1H, J ¼ 4.8 Hz, 40-H), 7.12 (dd, 1H, J ¼ 3.6 and
J ¼ 1.2 Hz, 300-H), 7.15 (d, 2H, J ¼ 8.8 Hz, 3000-H and 5000-H), 7.39 (d, 1H,
J ¼ 8.8 Hz, 2000-H and 6000-H), 7.47 (dd, 1H, J ¼ 4.0 and J ¼ 1.2 Hz, 500-
n
1145, 1044, 1022, 1002, 877, 852, 772, 711, 614. MS (microTOF) m/z
(%): 367 ([M þ H]þ, 100), 303 (6), 221 (4), 207 (8). HMRS: m/z
(MicroTOF) for C18H15N4OS2; calcd 367.0687; found: 367.0682.
1-(1-(2,4-Dimethoxyphenyl)-5-(thiophen-2-yl)-1H-pyrrol-2-
yl)-2-(thiazol-2-yl)diazene (5c). Violet solid (171 mg, 83%). Mp
H), 7.54 (d, 1H, J ¼ 0.8 Hz, 4-H). 13C NMR (Acetone-d6)
d 12.6, 55.9,
104.1, 113.6, 115.0, 127.6, 128.0, 128.3, 129.7, 131.4, 133.8, 135.5, 137.4,
142.2, 149.5, 161.2, 177.6. lmax(Dioxane)/nm 498 (3
/dm3 molꢁ1 cmꢁ1
125e126 ꢂC. 1H NMR (Acetone-d6)
d
3.72 (s, 3H, OCH3), 3.96 (s, 3H,
24,820). IR (Nujol) n
/cm-1 2960, 1511, 1301, 1246, 1196, 1169, 1031,
OCH3), 6.74 (dd, 1H, J ¼ 8.8 and J ¼ 2.8 Hz, 5000-H), 6.84 (d, 1H,
J ¼ 4.8 Hz, 30-H), 7.05 (d, 1H, J ¼ 4.4 Hz, 3000-H), 7.05e7.08 (m, 1H, 400-
H), 7.14 (d, 1H, J ¼ 4.8 Hz, 40-H), 7.28 (dd, 1H, J ¼ 3.6 and J ¼ 1.2 Hz,
300-H), 7.34 (d,1H, J ¼ 8.8 Hz, 6000-H), 7.36 (d,1H, J ¼ 3.6 Hz, 5-H), 7.46
(dd, 1H, J ¼ 5.2 and J ¼ 1.2 Hz, 500-H), 7.83 (d, 1H, J ¼ 3.6 Hz, 4-H). 13C
966, 892, 835, 773, 722. MS (EI) m/z (%): 351 ([Mþ1]þ, 5), 350 ([Mþ,
20), 270 (60), 253 (25), 121 (37) HMRS: m/z (EI) for C19H16N4S2O;
calcd: 380.0766; found: 380.0770.
2-Phenyl-(1-(1-propyl-5-(thiophen-2-yl)-1H-pyrrol-2-yl))
diazene (7a). Orange oil (102 mg, 67%). 1H NMR (CDCl3)
d 0.98 (t,
NMR (Acetone-d6)
d
56.3, 56.0, 79.2, 100.2, 105.8, 113.6, 118.6, 119.8,
3H, J ¼ 7.6 Hz, CH3), 1.85e1.94 (m, 2H, CH2), 4.51 (t, 2H, J ¼ 7.6 Hz,
NCH2), 6.58 (d, 1H, J ¼ 4.4 Hz, 40-H), 6.88 (d, 1H, J ¼ 4.4 Hz, 30-H),
7.13e7.16 (m, 1H, 400 -H), 7.24 (dd, 1H, J ¼ 4.0 and J ¼ 1.2 Hz, 300-H)
7.37e7.40 (m, 2H, 4-H and 500-H), 7.48 (t, 2H, J ¼ 7.2 Hz, 5-H) 7.85
(dd, 2H, J ¼ 5.2 and J ¼ 1.2 Hz, 5000-H), 7.87 (dd, 2H, J ¼ 1.0 and
127.3, 128.1, 128.2, 132.2, 133.8, 138.5, 144.0, 148.9, 158.3, 163.1,
178.0. lmax(Dioxane)/nm 497 (
/dm3 molꢁ1 cmꢁ1 21,960). IR (Nujol)
/cmꢁ1 2954, 1588, 1515, 1402, 1338, 1329, 1301, 1256, 1242, 1221,
3
n
1210, 1197, 1160, 1001, 878, 836, 791, 723. MS (microTOF) m/z (%):
397 ([M þ H]þ, 100), 359 (2), 259 (2), 233 (4). HMRS: m/z (micro-
TOF) for C19H17N4O2S2; calcd 397.0793; found: 397.0787.
1-(1-(3,4,5-Trimethoxyphenyl)-5-(thiophen-2-yl)-1H-pyrrol-
2-yl)-2-(thiazol-2-yl)diazene (5d). Violet solid (155 mg, 70%). Mp
J ¼ 7.2 Hz, 200-H and 600-H). 13C NMR (CDCl3)
d 11.4, 25.0, 45.6,
104.6, 115.2, 120.0, 128.2, 128.4, 129.1, 133.3, 136.5, 144.2, 147.5.
lmax(Dioxane)/nm 419 (3
/dm3 molꢁ1 cmꢁ1 18,300). IR (Liquid film)
n
/cmꢁ1 2959, 1594, 1495, 1368, 1345, 1032, 846, 765, 694. MS (EI)
182e184 ꢂC. 1H NMR (Acetone-d6)
d
3.85 (s, 6H, 2 ꢀ OCH3), 3.88 (s,
m/z (%): 295 (Mþ, 50), 203 (66), 162 (40), 121 (65). HMRS: m/z (EI)
3H, OCH3), 6.84 (br s, 2H, 2000-H and 6000-H), 7.02 (d, 1H, J ¼ 4.4 Hz, 30-
H), 7.07e7.10 (m, 1H, 400-H), 7.15 (d, 1H, J ¼ 4.4 Hz, 40-H), 7.21 (dd,
1H, J ¼ 4.0 and J ¼ 1.0 Hz, 300-H), 7.42 (d, 1H, J ¼ 3.6 Hz, 5-H), 7.52
(dd, 1H, J ¼ 5.2 Hz, J ¼ 1.0 Hz, 500-H), 7.83 (d, 1H, J ¼ 3.6 Hz, 4-H). 13C
for C17H17N3S; calcd: 295.1143; found: 295.1141.
4.2.2. General procedure [30] for the azo coupling of pyrrole 8 with
diazonium salts 2 and 4a to afford azo dyes 9b and 10a
NMR (Acetone-d6)
d
56.8, 60.9, 104.6, 108.4, 113.9, 120.3, 128.0,
4.2.2.1. Diazotisation of aniline and 2-aminothiazole. To a suspen-
sion of the (hetero)aromatic amines (0.86 mmol) in water (2 mL)
was added concentred HCl (2.5 mmol, 0.21 mL) until the mixture
was homogeneous. The solution was cooled and kept at 0 e 5 ꢂC in
an ice bath and a solution of NaNO2 (0.89 mmol) in water (1 mL) was
slowly added to the well-stirred mixture of the thiazole solution at
0 e 5 ꢂC. The reaction mixture was stirred during 20e30 min.
The diazonium salt solution previously prepared (0.86 mmol)
was added drop wise to the solution of pyrrole 8 (1 mmol) and
pyridine (1 mL, 1.25 mmol) in methanol (20 mL). The combined
solution was maintained at 0 e 5 ꢂC for 5 h while stirred. After this
time the resulting mixture was evaporated under vacuum to
dryness. The residue was purified by column chromatography
eluting with a mixture of dichloromethane and hexane (1:1).
1-(1-(4-Methoxyphenyl)-1H-pyrrol-2-yl)-2-(thiazol-2-yl)dia-
128.3, 128.4, 132.3, 133.5, 137.74, 140.1, 144.1, 149.1, 154.5, 179.9.
lmax(Dioxane)/nm 494 (
/dm3 molꢁ1 cmꢁ1 27,100). IR (Nujol)
/cmꢁ1 3115, 3101, 3075, 2956, 1748, 1598, 1535, 1505, 1485, 1423,
3
n
1407, 1365, 1343, 1315, 1284, 1242,1222, 1193, 1185, 1167, 1143, 1123.
MS (microTOF) m/z (%): 427 ([M þ H]þ, 100), 233 (2). HMRS: m/z
(MicroTOF) for C20H19N4O3S2; calcd 427.0899; found: 427.0893.
1-(1-(4-Fluorophenyl)-5-(thiophen-2-yl)-1H-pyrrol-2-yl)-2-
(thiazol-2-yl)diazene (5e). Violet solid (158 mg, 86%). Mp
120e124 ꢂC. 1H NMR (Acetone-d6)
d
7.02 (d, 1H, J ¼ 4.4 Hz, 40-H),
7.06e7.09 (m, 1H, 400-H), 7.16e7.17 (m, 3H, 30 and 300-H and 500-H),
7.39e7.44 (m, 3H, 5-H, 3000-H and 5000-H), 7.53 (dd, 1H, J ¼ 4.8 and
J ¼ 1.2 Hz, 500-H), 7.58 (dd, 2H, J ¼ 9.0 and J ¼ 5.2 Hz, 2000-H and 6000-
H), 7.84 (d, 1H, J ¼ 3.2 Hz, 4-H). 13C NMR (Acetone-d6)
d 104.9, 114.2,
116.6 and 116.9 (d, J ¼ 23 Hz), 120.4, 128.1, 128.4, 132.5 and 132.6 (d,
J ¼ 9 Hz), 133.4, 133.4, 133.4, 137.8, 144.2, 149.3, 162.6 and 165.1 (d,
zene (9b). Orange oil (73 mg, 30%). 1H NMR (CDCl3)
d 3.89 (s, 3H,
J ¼ 246 Hz), 179.7. lmax(Dioxane)/nm 491 (
3
/dm3 molꢁ1 cmꢁ1
OCH3), 6.55e6.57 (m, 1H, 40-H), 7.02 (d, 2H, J ¼ 9.3 Hz, 300-H and 500-
H), 7.18e7.20 (m, 2H, 30-H and 50-H), 7.35e7.41 (m, 3H, 4-H, 200-H
and 600-H), 7.87 (d, 1H, J ¼ 3.6 Hz, 3-H). lmax(Dioxane)/nm 419
25,300). IR (Nujol) n
/cmꢁ1 1510, 1306, 1217, 1200, 1147, 1044, 968,
839. MS (microTOF) m/z (%): 355 ([M þ H]þ, 100), 337 (4), 300 (3),
233 (2). HMRS: m/z (MicroTOF) for C17H12FN4S2; calcd 355.0487;
found: 355.0482.
(3 n
/dm3 molꢁ1 cmꢁ1 17,600). IR (Liquid film) /cmꢁ1 2965, 1731, 1407,
1339, 1304, 1243, 1216, 1170, 1141, 996, 898, 828, 743, 728. MS (EIþ):
m/z (%) ¼ 285 ([Mþ1]þ, 93), 264 (10). HMRS: m/z (EIþ): for
C14H12N4OS, calcd. 285.0800, found 285.0805.
1-(1-(4-Bromophenyl)-5-(thiophen-2-yl)-1H-pyrrol-2-yl)-2-
(thiazol-2-yl)diazene (5f). Violet solid (186 mg, 86%). Mp
172e174 ꢂC. 1H NMR (Acetone-d6)
d
7.00 (d, 1H, J ¼ 4.4 Hz, 30-H),
1-(1-(4-Methoxyphenyl)-1H-pyrrol-2-yl)-2-(phenyl)diazene
7.07e7.10 (m, 1H, 400-H), 7.16 (m, 2H, 300-H and 50-H), 7.45 (d, 1H,
J ¼ 3.6 Hz, 5-H), 7.50 (d, 2H, J ¼ 8.8 Hz, 2000-H and 6000-H), 7.54 (dd,1H,
J ¼ 5.2 and J ¼ 1.2 Hz, 500-H), 7.83 (d, 2H, J ¼ 8.8 Hz, 3000-H and 5000-H),
(10a). Orange oil (205 mg, 85%). 1H NMR (CDCl3)
d 3.90 (s, 3H,
OCH3), 6.45e6.48 (m, 1H, 40-H) 6.95e6.96 (m, 1H, 50-H), 7.00 (d, 2H,
J ¼ 9.0 Hz, 300-H and 400-H), 7.19e7.21 (m, 1H, 30-H), 7.27e7.43 (m,
5H, 3-H, 4-H, 5-H, 200-H, and 500-H), 7.70e7.73 (m, 2H, 2-H and 6-H).
7.88 (d, 1H, J ¼ 3.6 Hz, 4-H). 13C NMR (Acetone-d6)
d 105.0, 114.5,
120.5, 123.7, 128.2, 128.5, 132.4, 133.0, 133.2, 136.6, 137.5, 144.2,
13C NMR (CDCl3)
d
55.5, 99.9, 111.1, 113.9, 122.2, 127.0, 127.3, 128.9,
129.5, 131.8, 146.6, 153.0, 158.7. lmax(Dioxane)/nm 386
/dm3 molꢁ1 cmꢁ1 15,700). IR (Liquid film) /cmꢁ1 2957, 2934,
149.1, 179.5. lmax(Dioxane)/nm 489 (
3
/dm3 molꢁ1 cmꢁ1 31,840). IR
(Nujol)
n
/cmꢁ1 1736, 1402, 1340, 1302, 1218, 1212, 1194, 1169, 1141,
(3
n
1043, 1022, 1004, 967, 897, 876, 850, 835, 768. MS (microTOF) m/z
(%): 417 ([M þ H]þ 81Br, 100), 415 ([M þ H]þ 79Br, 97), 259 (8), 233
(13). HMRS: m/z (microTOF) for C17H1281BrN4S2; calcd 416.9687;
found: 416.9681.
1-(1-(4-Methoxyphenyl)-5-(thiophen-2-yl)-1H-pyrrol-2-yl)-
2-(5-methylthiazol-2-yl)diazene (6b). Violet solid (176 mg, 89%).
2908, 2836, 2053, 1879, 1632, 1612, 1589, 1515, 1480, 1463, 1454,
1442,1301, 1250, 1202, 1109, 1093, 1071, 1035, 996, 913, 872, 832,
801. MS (EI): m/z (%) ¼ 277 (Mþ, 100), 276 (65), 172 (51), 157 (17).
HMRS: m/z (EI) for C17H15N3O, calcd. 277.1215, found 277.1210.
Azo pyrroles 10bec were synthesized through azo coupling of
pyrrole
8 with aryldiazonium salts 4bec using the same

M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 62e73
71
experimental procedure described above in sub-sections 4.2.1.1.
and 4.2.1.2.
1-(1-(4-Methoxyphenyl)-1H-pyrrol-2-yl)-2-(4-cyanophenyl)
diazene (10b). Dark green solid (283 mg, 94%). Mp 126e128 ꢂC. 1H
4.4. Solvatochromic study
The solvatochromic study was performed using 10ꢁ4
solutions of dyes 5e7 and 9e10 in several solvents at room
temperature.
M
NMR (CDCl3)
d
3.89 (s, 3H, OCH3), 6.48e6.50 (m, 1H, 40-H), 6.93 (dd,
1H, J ¼ 5.6 and 2 Hz, 50-H), 7.00 (d, 2H, J ¼ 9.0 Hz, 300-H and 500-H),
7.26e7.27 (m, 1H, 30-H), 7.39 (d, 2H, J ¼ 9.0 Hz, 200-H and 600-H),
4.5. Photochromic measurements
7.65e7.73 (m, 4H, 2, 3, 5 and 6-H). 13C NMR (CDCl3)
d 55.5, 101.0,
111.7, 114.0, 118.9, 122.6, 127.3, 128.6, 131.3, 133.0, 147.0, 155.7, 158.9.
lmax(Dioxane)/nm 404 (
/dm3 molꢁ1 cmꢁ1 19,880). IR (Liquid film)
/cmꢁ1IR (Nujol)
For measurements of lmax, Aeq and kD under continuous visible
irradiation, 2.0 ꢀ 10ꢁ5 M acetone solutions were used. Irradiation
experiments were made using a CARY 50 Varian spectrophotom-
eter coupled to a 150 W ozone free xenon lamp (6255 Oriel
Instruments). The light from the UVevis lamp was filtered using
a water filter (61945 Oriel Instruments) and a long-pass filter
(Schott GG 420) at 20 ꢂC and carried to the spectrophotometer
holder, perpendicular to the monitoring beam using a fibre-optic
system (77654 Oriel Instruments). A thermostated (20 ꢂC) 10 mm
quartz cell containing the sample solution (3.5 mL) and equipped
with magnetic stirring was used. Three spectrokinetic parameters,
normally quoted when describing the properties of photochromic
compounds, were evaluated: maximum wavelength of absorption
3
n
n 2955, 2225, 1605, 1515, 1345, 1248, 1201, 1174,
1033, 833, 728 cmꢁ1. MS (EIþ) m/z (%): 302 (Mþ, 100), 301 (60), 172
(68), 118 (38). HMRS: m/z (EIþ) for C18H14N4O; calcd: 302.1168;
found: 302.1169.
1-(1-(4-Methoxyphenyl)-1H-pyrrol-2-yl)-2-(4-nitrophenyl)
diazene (10c). Brown solid (306 mg, 95%). Mp 173e174 ꢂC. 1H NMR
(CDCl3)
d
3.83 (s, 3H, OCH3), 6.57e6.59 (m, 1H, 40-H), 6.94 (dd, 1H,
J ¼ 4.4 and 1.5 Hz, 50-H), 7.09 (d, 2H, J ¼ 9.0 Hz, 300-H and 500-H), 7.50
(d, 2H, J ¼ 9.0 Hz, 200-H and 600-H), 7.69e7.70 (1H, m, 30-H), 7.74 (d,
2H, J ¼ 9.0 Hz, 2-H and 6-H), 8.30 (d, 2H, J ¼ 9.0 Hz, 3-H and 5-H).
13C NMR (DMSO-d6)
d 55.5, 101.6, 112.3, 114.2, 122.4, 125.1, 127.4,
130.7, 146.8, 147.0, 147.1, 156.7, 158.6. lmax(Dioxane)/nm 417
(lmax), thermal colouration rates (kD) and maximum absorbance
(3 n
/dm3 molꢁ1 cmꢁ1 17,600). IR (liquid film) /cmꢁ1 2955, 1197, 1168,
attained at lmax (Amax). The colouration kinetics was then studied in
the dark. The thermal coloration curves were analysed evaluating
the fitting of the experimental data to the mono-exponential
equation:
1033, 892, 834, 771, 722. MS (EI) m/z (%): 322 (Mþ, 30), 292 (100),
172 (34). HMRS: m/z (EI) for C17H14N4O3; calcd: 322.1066; found:
322.1070.
AðtÞ ¼ A1eꢁkt þ A0
4.3. Instruments
where A(t) is the absorbance at lmax at any instant t, A1 a propor-
tional factor, k the thermal colouration rate and A0 the absorbance
in the dark when time approaches infinity. The model was found to
accurately fit our data when the quadratic residual errors were 10ꢁ6
or less.
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
ppm using dH Me4Si ¼ 0 ppm as reference and J values are given in
Hz. Assignments were made by comparison of chemical shifts, peak
multiplicities and J values and were supported by spin decoupling-
double resonance and bidimensional heteronuclear HMBC and
HMQC correlation techniques. IR spectra were determined on
a BOMEM MB 104 spectrophotometer using KBr discs. UVevisible
absorption spectra (200e800 nm) were obtained using a Shimadzu
UV/2501 PC 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 constaꢁn1t
50 mL minꢁ1 flow rate. All samples were subjected to a 20 ꢂC min
heating rate and were characterized between 25 and 500 ꢂC. All
melting points were measured on a Gallenkamp melting point
apparatus and are uncorrected. Cyclic voltammetry (CV) was per-
formed 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 voltammetric
measurements. The concentration of the compounds were
1 mmol dmꢁ3 and 0.1 mol dmꢁ3 [NBu4][BF4] was used as the
supporting 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.
4.6. Nonlinear optical measurements using the hyper-Rayleigh
scattering (HRS) method27
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 is similar to
the one presented by Clays et al. [27] The incident laser beam came
from a Q-switched Nd:YAG laser operating at a 10 Hz repetition rate
with approximately 10 mJ of energy per pulse and a pulse duration
(FWHM) close to 12 ns at the fundamental wavelength of 1064 nm.
The incident power could be varied using a combination of a half
wave-plate and Glan polarizer. The incident beam was weakly
focused (beam diameter w0.5 mm) into the solution contained in
a 5 cm long cuvette. The hyper-Rayleigh signal was collimated
using a high numerical aperture lens passed through an interfer-
ence filter centred at the second harmonic wavelength (532 nm)
before being detected by a photomultiplier (Hamamatsu model
H9305-04). The current pulse from the photomultiplier was inte-
grated using a Stanford Research Systems gated box-car integrator
(model SR250) with a 25 ns gate centred on the temporal position
of the incident laser pulse. The hyper-Rayleigh signal was
normalized at each pulse using the second harmonic signal from
a 1 mm quartz plate to compensate for fluctuations in the temporal
profile of the laser pulses due to longitudinal mode beating.
Dioxane was used as solvent, and the b values were calibrated using
a reference solution of p-nitroaniline (pNA) [28] also dissolved in
dioxane at a concentration of 1 ꢀ 10ꢁ2 mol dmꢁ3 (external refer-
ence method). The hyperpolarizability of pNA dissolved in dioxane
is known from EFISH measurements carried out at the same
fundamental wavelength. The concentrations of the solutions

72
M.M.M. Raposo et al. / Dyes and Pigments 91 (2011) 62e73
under study were chosen so that the corresponding hyper-Rayleigh
signals fell well within the dynamic range of both the photo-
multiplier and the box-car integrator. All solutions were filtered
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ꢂ
ꢀ
SNBAWB ꢁ SWBANB
u
S2NB
¼
¼
TNB
ANB
TNBAWB ꢁ TWBANB
SLBTNB ꢁ SNBTLB
ꢂ
ꢀ
SFNB
TNBAWB ꢁ TWBANB
u
Here S2NB is the hyper-Rayleigh scattering contribution to
the signal, i.e., the signal that would have been measured using the
“narrow” band filter if there were no fluorescence present. The
fluorescence contribution to the signal measured using the narrow
band interference filter is SFNB. The signals SNB and SWB are the actual
signals measured (after correction for the solvent contribution)
using the “narrow” (CVI model F1.5-532-4) and “wide” (CVI model
F03-532-4) band interference filters. The transmissions TNB and TWB
are respectively the transmission of the “narrow” and “wide” band
interference filters at the second harmonic wavelength (47.6% and
63.5%), ANB and AWB represent the area under the respective filter’s
transmission curve. The respective transmission curves were
obtained using a dual-beam spectrophotometer with slits adjusted
to give 0.1 nm resolution. We obtained values of 1.29 nm and
2.18 nm for ANB and AWB respectively.
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Thanks are due to the Fundação para a Ciência e Tecnologia
(Portugal) and FEDER for financial support through the Centro de
Química and Centro de Física- Universidade do Minho, Project
PTDC/QUI/66251/2006 (FCOMP-01-0124-FEDER-007429), Project
PTDC/CTM/105597/2008 with funding from COMPETE/FEDER and
research grants to M. C. R. Castro (UMINHO/BI/142/2009) and M. F.
S. Cardoso (UMINHO/BII/249/2009). The NMR spectrometer Bruker
Avance III 400 is part of the National NMR Network and was
purchased within the framework of the National Program for
Scientific Re-equipment, contract REDE/1517/RMN/2005 with
funds from POCI 2010 (FEDER) and FCT.
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