Design, synthesis and evaluation of redox, second order nonlinear optical properties and theoretical DFT studies of novel bithiophene azo dyes functionalized with thiadiazole acceptor groups

Article abstract of DOI:10.1016/j.dyepig.2012.05.014

Two series of novel thermally stable second-order nonlinear optical (NLO) heterocyclic azo dyes 4-5 have been designed and synthesized. The two series of compounds were based on different combinations of acceptor groups (thiadiazole or arylthiadiazole electron-deficient heterocycles) linked to bithiophene which acts at the same time as a donor group and as a π-conjugated bridge. The solvatochromic behavior of azo dyes 4-5 was investigated in several solvents of different polarity, while their thermal stability was evaluated using thermogravimetric analysis. Optimized ground-state molecular geometries and an estimation of the lowest energy single electron vertical excitation energies in DMF solutions were obtained using density functional theory (DFT). Their redox properties were studied by cyclic voltammetry, while hyper-Rayleigh scattering (HRS) was employed to evaluate their second-order nonlinear optical properties. The measured molecular first hyperpolarizabilities and the observed electrochemical behavior showed variations for the different acceptor systems used (thiadiazole or arylthiadiazole) and were also sensitive to the electronic acceptor strength of the substituents (R) linked to thiadiazole or arylthiadiazole heterocycles. Donor-acceptor arylthiadiazole-bithienyl diazenes exhibit the most promising thermal (T_d = 237-305°C) and solvatochromic (Δν = 1117-2503 cm^-1) properties and second order nonlinear optical response (136-226 × 10^-30 esu).

Full text of DOI:10.1016/j.dyepig.2012.05.014

Dyes and Pigments 95 (2012) 392e399
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Design, synthesis and evaluation of redox, second order nonlinear optical
properties and theoretical DFT studies of novel bithiophene azo dyes
functionalized with thiadiazole acceptor groups
M.Cidália R. Castro a, Peter Schellenberg b, M. Belsley b, A.Maurício C. Fonseca a, Sara S.M. Fernandes a,
,
M.Manuela M. Raposo a
*
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
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) heterocyclic azo dyes 4e5
have been designed and synthesized. The two series of compounds were based on different combina-
tions of acceptor groups (thiadiazole or arylthiadiazole electron-deficient heterocycles) linked to
Received 10 April 2012
Received in revised form
10 May 2012
Accepted 15 May 2012
Available online 23 May 2012
bithiophene which acts at the same time as a donor group and as a
p-conjugated bridge. The sol-
vatochromic behavior of azo dyes 4e5 was investigated in several solvents of different polarity, while
their thermal stability was evaluated using thermogravimetric analysis. Optimized ground-state
molecular geometries and an estimation of the lowest energy single electron vertical excitation ener-
gies in DMF solutions were obtained using density functional theory (DFT). Their redox properties were
studied by cyclic voltammetry, while hyper-Rayleigh scattering (HRS) was employed to evaluate their
second-order nonlinear optical properties. The measured molecular first hyperpolarizabilities and the
observed electrochemical behavior showed variations for the different acceptor systems used (thiadia-
zole or arylthiadiazole) and were also sensitive to the electronic acceptor strength of the substituents (R)
linked to thiadiazole or arylthiadiazole heterocycles. Donoreacceptor arylthiadiazole-bithienyl diazenes
Keywords:
Heterocyclic azo dyes
Thiadiazole
Auxiliary donor/acceptor heterocycles
Hyper-Rayleigh scattering (HRS)
Density functional theory (DFT)
Nonlinear optical (NLO) materials
exhibit the most promising thermal (Td ¼ 237e305 ꢀC) and solvatochromic (Dn ¼ 1117e2503 cmꢁ1
)
properties and second order nonlinear optical response (136e226 ꢂ 10ꢁ30 esu).
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
second order nonlinear response of these pushepull organic
molecules can be improved by inclusion of different heterocycles
Heterocyclic azo dyes are versatile chromophores that can
provide bright strong shades tunable for absorption over all of the
visible spectrum [1]. For decades, they have been widely investi-
gated mostly due to their application for coloring textiles and
plastics or in general as dispersed dyes. More recently azo dyes
bearing heterocyclic moieties such as thiophene, pyrrole and azoles
have been used for multiple optical and electronic applications
such as second harmonic generation, optical switching, chemo-
sensing, and organic sensitized solar cells [2].
within the
modulating the conjugation in the
p
-electron bridge, using them both as a means of
p-electron network and to
change the electronic character of the charge transfer by acting as
auxiliary donors or acceptors [3,4]. Previously we reported also
other NLO-active heterocyclic chromophores bearing electron rich
groups (thiophene, pyrole) linked to electron deficient heterocycles
(benzothiazole, benzimidazole and phenanthroline) [5]. However,
despite intensive theoretical and experimental activity it remains at
present unclear what the right strategy will be to optimize NLO
properties.
On the other hand, it is well known that pushepull systems
bearing azole electron-deficient heterocycles can increase the
quadratic molecular nonlinear optical response due to the electro-
negative nitrogen and sulfur atoms [6]. In this respect, five-
membered diazoles, in particular thiadiazole, seems to be an
appropriate acceptor moiety. It possesses two nitrogen atoms and
Due to their well established synthetic routes, they are well
suited candidates for a systematic investigation of how NLO prop-
erties are affected by variations within their p-conjugated system.
In the past years we have been exploring the ways in which the
* Corresponding author. Tel.: þ351 253 604381; fax: þ351 253 604382.
E-mail address: mfox@quimica.uminho.pt (M.M.M. Raposo).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.05.014

M.Cidália R. Castro et al. / Dyes and Pigments 95 (2012) 392e399
393
R
2
N
N
S
S
N
N
R
1
R
S
1
N
S
N
R
2
S
3
S
7
R = H; R = H, CH
1 2
6
R = H, MeO, EtO; R = H, CH , CHO
1 2 3
Fig. 1. Structure of bithienyl-thiazolyl 6 and benzothiazolyl diazenes 7 [3f,4d].
a sulfur atom with different electronic nature and represents
robust and stable heterocycle. Earlier studies on NLO-
chromophores bearing thiadiazole heterocycle showed its poten-
tial application as second order nonlinear optical chromophores [7].
Our earlier studies concerning the evaluation of electronic and
optical properties of heterocyclic azo dyes bearing thienylpyrrole
reacted with bithiophene 1 in acetonitrile at 0 ꢀC given thiadiazolyl
diazenes 4e5 in fair to moderate yields (15e58%).
a
All the compounds were completely characterized by 1H and 13
C
NMR, IR, MS, EA or HRMS and the data obtained were in full
agreement with the proposed formulation.
[3] or bithiophene [4]
p-conjugated bridges and simultaneously
2.2. Redox properties
donor groups linked to thiazolyl- (6) or benzothiazolyl- (7) diazene
acceptor moieties, (Fig. 1) reveal that their optical and redox
properties can be modulated by varying the position of the
heterocycle attachment to the diazene group, by altering the elec-
tronic nature of the groups substituted on the heterocyclic diazene
system as well as by tuning the electronic acceptor strength of the
heterocycle on the same diazene moiety.
The redox properties of bithiophene azo dyes 4e5 were studied
by cyclic voltammetry in DMF containing tetrabutylammonium
tetrafluoroborate (1 mmol dmꢁ3) as the supporting electrolyte.
Table 1 lists the reduction and oxidation onsets and the electro-
chemical band gap values, using ferrocene as reference.
All molecules showed
a donoreacceptor character with
In view of these findings and in continuation of our recent work
concerning the synthesis and characterization of heterocyclic azo
dyes as solvatochromic probes, second harmonic generators and
photochromic systems [3,4] we report here on the synthesis and
evaluation of optical (linear and nonlinear), redox and thermal
properties of two series of heterocyclic chromophores consisting of
newly synthesized (aryl)thiadiazolyldiazenes 4e5 bearing bithio-
phene spacers. Additionally, optimized ground-state molecular
geometries and estimation of the lowest energy single electron
vertical excitation energies in DMF solutions for the novel chro-
mophores were obtained using density functional theory (DFT).
reversible reductions and irreversible oxidation processes. The
irreversible oxidation process is associated with the oxidation of
the thiophene moiety. These results are consistent with previous
electrochemical studies of other thiophenes derivatives [3a,c,d,9].
The values for the oxidation process are shifted to more anodic
potentials when stronger acceptor substituent groups are intro-
duced in thiadiazole and arylthiadiazole moieties. All bithiophene
azo dyes 4e5 bearing a thiadiazole and arylthiadiazole acceptor
group linked to the diazene group through position 2 exhibited two
one-electron reversible reductions indicating a high electron
affinity of the chromophores (Fig. 2).
The one-electron stoichiometry for these reduction processes is
ascertained by comparing the current heights with known one-
electron redox processes under identical conditions [3a]. For all
compounds, the two reduction processes are associated with the
reduction of the (hetero)aromatic azo moiety. The introduction of
an arylthiadiazole acceptor group linked to the azo bridge through
position 2 (compound 5a) could slightly decrease the reduction
potentials (compared to the corresponding thiadiazole derivative
4a) while increasing the oxidation potentials, suggesting a higher
electron-accepting ability of compound 5a. It was also observed for
all compounds, that the reduction potential values of the first and
second processes are influenced by the acceptor electronic strength
of the substituent on the thiadiazole (1Epc for 4a < 1Epc for 4b < 1Epc
for 4c) and arylthiadiazole rings (1Epc for 5a < 1Epc for 5b < 1Epc for
5c). Therefore, the substitution on the aryl ring of the arylth-
iadiazole system by fluoro (5b) or nitro (5c) groups results in
a moderate decrease of the reduction potentials (1Epc ¼ ꢁ1.04 V for
2. Results and discussion
2.1. Synthesis
Generally thiophene azo dyes are synthesized through azo
coupling reaction of 2-aminothiophenes with arylamines [1a,8],
however the difficulties of synthesizing 2-aminothiophenes are
well known. In order to overcome these problems, we have recently
developed a novel synthesis methodology for bithienyl diazenes
using as coupling components 2,20-bithiophene, 5-alkoxy- or 5-
N,N-dialkylamino-2,20-bithiophenes and (hetero)aryldiazonium
salts [4a,b,d],
Consequently, 2,20-bithiophene 1 has been used as coupling
component together with thiadiazolyl 2aec and arylthiadiazolyl
3aec diazonium salts in order to prepare heterocyclic azo dyes 4e5
(Scheme 1). Diazotation of 2-amino-thiadiazole and 2-amino-
arylthiadiazole derivatives with NaNO2 in HCl (6 N) at 0e5 ꢀC in
water gave rise to the corresponding diazonium salts 2e3, which
1
5b and Epc ¼ ꢁ0.95 V for 5c) when compared to the reduction
potential obtained for the unsubstituted derivative (5a,
N
N
N
N
R1
N2
N2
S
S
R1
3a R1 = H
3b R1 = F
3c R1 = NO2
2a R1 = H
2b R1 = Br
2c R1 = CF3
N
N
N
N
S
S
S
N
N
S
S
N
R1
N
S
S
S
R1
1
4a-b
5a-c
Scheme 1. Synthesis of bithienyl diazenes 4e5 trough azo coupling reaction of 2,20-bithiophene 1 with thiadiazolyl diazonium salts 2e3.

394
M.Cidália R. Castro et al. / Dyes and Pigments 95 (2012) 392e399
Table 1
Electrochemical data for compounds 4e5.
Compound Reductiona
Oxidationa Band gapc
(eV)
1
2
ꢁ Epc (V)
D
E
b (mV) ꢁ Epc (V)
D
Eb (mV) Epa (V)
4a
4bd
4c
5a
5b
5c
1.09
0.96
0.84
1.05
1.04
0.95
63
63
61
63
62
61
1.96
1.97
1.67
1.89
1.87
1.46
64
63
70
63
61
63
0.95
0.97
0.99
0.98
1.01
1.04
2.04
1.93
1.83
2.03
2.04
1.99
a
Measurements made in dry DMF containing 1.0 mM in each compounds and
0.10 M [NBu4][BF4] as base electrolyte at a carbon working electrode with a scan rate
of 0.1 V sꢁ1. All E values are quoted in volts vs. the ferrocenium/ferrocene -couple.
Epc and Epa correspond to the cathodic and anodic peak potentials, respectively.
b
D
E ¼ jEred ꢁ Eoxj.
c
EHOMO ¼ ꢁ(4.39 þ Eox) (eV) and ELUMO ¼ ꢁ(Ered þ 4.39) (eV).
Fig. 3. UVevisible absorption normalized spectra of compounds 4a and 5a in dioxane
at room temperature.
d
Additional cathodic irreversible process was observed at ꢁ1.77 V vs. fcþ/fc,
attributed to the loss of bromide [11].
the groups substituted on thiadiazole or on the arylthiadiazole
heterocycles have a significant effect on the electronic absorption
property of compounds 4 and 5. Therefore, azo dyes 4c and 5c
exhibited a red-shifted lmax (21 and 12 nm, respectively) with
respect to the unsubstituted derivatives 4a and 5a.
-5
4.0x10
-5
5a
2.0x10
The wavelength maxima lmax of compounds 4e5 in four
solvents, are summarized in Table 2 and were compared with the
0.0
p
* values for each solvent, as determined by Kamlet and Taft [12].
926e1216 cmꢁ1
) and 5
a
b
c
e
Thiadiazole azo dyes
4
(Dymax
¼
(
Dymax ¼ 950e2503 cmꢁ1) exhibited positive solvatochromism
-5
d
-2.0x10
from diethyl ether to DMSO, suggesting the good optical nonline-
arities of these chromophores (Fig. 4).
The molecular first hyperpolarizabilities
zenes 4e5 were measured by hyper-Rayleigh scattering (HRS)
b of heterocyclic dia-
-5
-4.0x10
method [13] at a fundamental wavelength of 1064 nm of a laser
beam. Dioxane was used as the solvent, and the
b values were
-2.4
-1.8
-1.2
-0.6
measured against a reference solution of p-nitroaniline (pNA) [14]
in order to obtain quantitative values, while care was taken to
properly account for possible fluorescence of the dyes (see exper-
imental section for more details). The static hyperpolarisability b0
values [15] were calculated using a very simple two-level model
neglecting damping. They are therefore only indicative and should
be treated with caution (Table 3).
E/V vs. fc+/fc
Fig. 2. Cyclic voltammograms for the reduction steps of compound 5a
(1.0 ꢂ 10ꢁ3 mol dmꢁ3) in DMF, 0.1 mol dmꢁ3 [NBu4][BF4] at a vitreous carbon elec-
trode, between ꢁ0.5 V and ꢁ2.20 V vs. fcþ/fc, scan rate at 0.02 (a), 0.05 (b), 0.10 (c), 0.20
(d) and 0.30 (e) V sꢁ1
.
From Table 3 it is apparent that the increase of the acceptor
strength of the substituent on 5-position of the thiadiazole ring
(dyes 4aec) resulted in a significant resonant enhancement, with
1Epc ¼ ꢁ1.05 V). On the other hand a higher decrease on the
reduction potential were observed in the case of thiadiazole azo
1
dyes (e.g. 4c, 1Epc ¼ ꢁ0.84 V and 4a Epc ¼ ꢁ1.09 V). The electro-
enhanced
b values accompanied by a red-shifted absorption
chemical band gaps were calculated as described previously [10]
from the potentials of the anodic and cathodic processes
(Table 1). The efficient donoreacceptor conjugation leads to
a lowering of the HOMO and the LUMO levels in compounds 4 and
5, revealing efficient coupling between the bithiophene spacer and
thiadiazolyl or arythiadiazolyl-diazene acceptor groups.
maxima (e.g. 4a, R ¼ H, lmax 475 nm,
b
¼ 153 ꢂ 10ꢁ30 esu; 4c,
R ¼ CF3, lmax 496 nm,
b
¼ 221 ꢂ 10ꢁ30 esu). Also noteworthy is the
effect of the electronic nature of the group that substitutes the aryl
ring at 4-position for compounds 5aec. As expected the increase of
the acceptor strength of the NO2 group (5c) compared to H (5a)
Table 2
2.3. Optical properties
Solvatochromic data [lmax (nm) and Dymax (cmꢁ1) of the charge-transfer band] for
bithiophene azo dyes 4e5 in 4 solvents with
p* values by Kamlet and Taft [12].
(Aryl)thiadiazole azo dyes 4e5 showed good solubility in
common polar and non-polar organic solvents such as diethyl
ether, dioxane, ethanol and DMSO.
Compound Solvent ( *)
p
Diethyl ether
(0.54)
Ethanol
(0.54)
1,4-Dioxane
(0.55)
DMSO
(1.00)
All chromophores exhibit broad and intense CT absorptions in
the region of 475e508 nm in dioxane. Thiadiazole being an
electron-deficient five-membered heteroaromatic ring, will act as
acceptor group and will exhibit also an auxiliary acceptor effect
when linked to withdrawing groups [6aec]. Moreover, due to
a larger electronic delocalization, (aryl)thiadiazole azo dyes 5
exhibit bathochromic shifts when compared to the corresponding
thiadiazole derivatives 4 (Fig. 3). The acceptor electronic strength of
lmax
nmax
lmax nmax
(nm) (cmꢁ1
lmax nmax
(nm) (cmꢁ1
lmax nmax
(nm) (cmꢁ1
)
(nm) (cmꢁ1
)
)
)
4a
4b
5a
5b
5c
466
487
487
491
495
21,459 483
20,534 498
20,534 504
20,367 502
20,202 518
20,704 475
20,080 490
19,841 496
19,920 498
19,305 508
21,053 494
20,408 510
20,161 515
20,080 515
19,685 565
20,243
19,608
19,417
19,417
17,699

M.Cidália R. Castro et al. / Dyes and Pigments 95 (2012) 392e399
395
Fig. 5. Thermal analysis data for compound 5c through TGA recorded under a nitrogen
atmosphere, measured at a heating rate of 20 ꢀC minꢁ1
Fig. 4. UVevisible absorption normalized spectra of compound 4a in four solvents of
different polarity (diethylether, dioxane, ethanol and DMSO) at room temperature.
.
results both in red-shifted absorption maxima (12 nm) and a reso-
some impact on the thermal stability of the compounds. Therefore,
the nitro arylthiadiazolyl azo dye 5c (Fig. 5) and the bromo
substituted thiadiazolyl derivative 4b are the most stable showing
higher decomposition temperatures.
nance enhanced
b value for arylthiadiazole azo dye 5c
(b
¼ 226 ꢂ 10ꢁ30 esu).
Comparison of the
b values for arylthiadiazole azo dye 5a with
thiadiazole azo dye 4a also showed that the substitution of a thia-
diazolyl-diazene for a arylthiadiazolyl-diazene acceptor moiety
enhances the first order hyperpolarizability from 153 ꢂ 10ꢁ30 to
170 ꢂ 10ꢁ30 esu at 1064 nm, probably due a greater electronic
delocalization. In the probable resonance structures, the distance
between the charges in 5a is longer compared to 4a, which results
in a greater electric moment of the first diazene, increasing the
value of the molecular hyperpolarizability.
A comparison could also be made between dyes 4a and 5a and
bithiophene thiazolyl azo dye (6, R1 ¼ R2 ¼ H, lmax ¼ 477 nm in
dioxane) and benzothiazolyl-diazene (7, R1 ¼ R2 ¼ H, lmax ¼ 493 nm
in dioxane) reported by us recently (Fig. 1) [4d]. The results
obtained previously suggest that the electron-deficient thiazole
and benzothiazole heterocycles have a larger acceptor strength
Arylthiadiazole azo dye 5a exhibit also an improved stability by
ca 26 ꢀC compared to the corresponding thiadiazole derivative 4a.
2.4. Theoretical calculations
Theoretical calculations were carried out using density func-
tional theory to model the ground state molecular geometries and
energies, the HOMOeLUMO energy gaps as well as the optical
absorption spectra. The class of bithienyl-(aryl)thiadiazole azo dyes
investigated here possess three rotatable single bonds present
throughout the conjugated system indicated as A, B and C in (Fig. 6).
However, these bonds may have a higher isomerization energy
compared to isolated single bonds as they are part of the conju-
gated electronic system.
Therefore eight structurally distinct isomeric configurations are
possible in thermal equilibrium [16,17]. Here, we use the nomen-
clature from [16] according to which c and t stands for cisoid and
transoid, respectively. All calculations were performed using the
than thiadiazole and arylthiadiazole systems. As a result
b values of
the thiazolyl- 6a (
b
¼ 172 ꢂ 10ꢁ30 esu) and benzothiazolyl- 7a
(
b
¼ 207 ꢂ 10ꢁ30 esu) diazenes are larger than the corresponding
values for thiadiazole (4a,
5a (
b
¼ 153 ꢂ 10ꢁ30 esu) and arylthiadiazole
b
¼ 170 ꢂ 10ꢁ30 esu) dyes.
For optoelectronic applications, the thermal stability of organic
materials is critical for device stability. For that reason the thermal
properties of the chromophores 4e5 were investigated by ther-
mogravimetric analysis underꢁa1nitrogen atmosphere, measured at
C
B
A
N
N
N
S
a heating rate of 20 ꢀC min
(Table 3). All chromophores are
N
S
S
thermally stable with decomposition temperatures varying from
221 to 305 ꢀC. In both series of derivatives the electronic nature of
the group substituted on the 5-position of the thiadiazole ring
(4aec) or in 4-position of the aryl ring (5aec) does seem to have
R1
Fig. 6. Rotatable bonds within the conjugated system responsible for the different
isomers.
Table 3
Experimental data for bithiophene azo dyes 4e5.a
b (10ꢁ30 esu)
b0c (10ꢁ30 esu)
d
Td (ꢀC)
Heterocyclic azo dye
R1
Yield (%)
lmax (nm) (
3 )
b
4a
4b
4c
5a
5b
5c
H
Br
CF3
C6H4
4-FC6H4
4-NO2C6H4
e
19
21
15
58
34
16
e
475 (19,980)
490 (36,410)
496 (28,440)
496 (23,400)
498 (36,140)
508 (12,590)
352
153
193
221
170
136
226
25 ꢃ 2.3
23 ꢃ 1.8
23 ꢃ 1
221
244
e
19 ꢃ 1
237
248
305
e
21 ꢃ 1
15.4 ꢃ 0.5
8.5
pNA
16.9 [14]
a
Experimental hyperpolarizabilities and spectroscopic data measured in dioxane solutions.
All the compounds are transparent at the 1064 nm fundamental wavelength.
b
c
Data corrected for resonance enhancement at 532 nm using the two-level model with b0
¼
b
[1 ꢁ (lmax/1064)2][1 ꢁ (lmax/532)2]; damping factors not included 1064 nm
[15].
d
Decomposition temperature (Td) measured at a heating rate of 20 ꢀC minꢁ1 under a nitrogen atmosphere, obtained by TGA.

396
M.Cidália R. Castro et al. / Dyes and Pigments 95 (2012) 392e399
Table 4
The energy values for the HOMO and LUMO and the HOMOeLUMO energy gap for each compound averaged over the results for the eight possible isomers as shown in
Supplementary Table S1 for compound 4a. The values given for DEmax/min refer to the spread between the largest and smallest values of energy obtained amongst the eight
isomers of each compound.
Oscillator strengthc
a
a
b
a
a
E(HOMO) DMF/eV
E(LUMO) DMF/eV
D
E(HO-LU) DMF/eV
DEmax/min DMF/eV
E(HOMO) vac/eV
E(LUMO) vac/eV
D
E(HO-LU)b vac/eV
4a
4b
4c
5a
5b
5c
ꢁ6.0781
ꢁ6.1046
ꢁ6.3208
ꢁ5.9844
ꢁ5.9855
ꢁ6.0662
ꢁ3.4612
ꢁ3.5482
ꢁ3.7033
ꢁ3.4685
ꢁ3.4740
ꢁ3.6591
2.6168
2.5563
2.6174
2.5158
2.5115
2.4072
0.0302
0.0553
0.1273
0.0397
0.0288
0.0639
ꢁ6.1397
ꢁ6.2070
ꢁ6.3722
ꢁ5.9577
ꢁ5.9996
ꢁ6.2373
ꢁ3.3594
ꢁ3.4896
ꢁ3.6730
ꢁ3.3040
ꢁ3.3589
ꢁ3.6707
2.7803
2.7174
2.6992
2.6537
2.6407
2.5667
1.1667
1.3016
1.0981
1.5112
1.5117
1.5207
a
Calculated by DFT using the B3LYP functional and the 6-311 þ G(2d,2p) basis set.
b
c
D
E ¼ ELUMO ꢁ EHOMO
.
Oscillator strength of the transition in DMF, averaged over the eight possible isomers.
Gaussian 09 program [18] using Density Functional Theory (DFT)
with the B3LYP functional and various basis sets.
compared with the equilibrium structures calculated with the same
level of theory. The energy range of isomerization barriers for all
calculations can be found in the supplementary information. To
summarize, the range for the single barrier crossings range from
around 3 to 18 kcal/mol. The conversion rate k is given by simple
We performed three sets of calculations. In the first set we
analyzed the ground state molecular geometry, the ground state
energy and the HOMOeLUMO gap for the different isomers by DFT
using the B3LYP functional and the 6-311 þ G(2d,2p) basis set. All
calculations were done for isolated molecules in vacuo and in DMF.
For modeling the solvent, a Polarizable Continuum Model (PCM)
using the integral equation formalism variant (IEFPCM) was utilized
which is the default in Gaussian 09. In the second set of calculations
the vertical transition energies for these ground state geometries
and the corresponding oscillator strengths were calculated using
the same method and basis set with Time Dependent DFT (TDDFT)
by inclusion of the 6 lowest excited states: td ¼ (nstates ¼ 6). As an
example, the results for the 8 isomers of compound 4a in DMF are
given in Table S1 in the supplementary information. Results for all
of the isomeric forms of the other structures can also be found in
there.
In Table 4, the values obtained by averaging over the different
isomers for each compound is given. A comparison with the
experimental HOMOeLUMO gaps obtained from the cyclic vol-
tammetry (Table 1) indicates that the calculated values are roughly
0.6 eV larger. This difference is primarily due to an underestimation
of the HOMO energy, which is around 0.7 eV more negative
compared to the voltammetric measurements, while the calculated
LUMO energies are approximately 0.1 eV more negative than the
experimental measurements. This situation is similar to results
obtained for recently investigated thienylpyrrole azo dyes bearing
benzothiazole groups [3c]. Interestingly the theoretical values
differ more from the experimental values for compounds 4 than for
derivatives 5 although these include an extra benzene ring.
Although the calculated oscillator strengths for azo dyes 5, with
kinetic theory as k ¼ A(T) exp(ꢁ
DE/kT). Assuming a preexponential
for an elementary reaction of A(T) ¼ 1013 sꢁ1 and the highest
calculated isomerization barrier of 18 kcal/mol, one gets a lifetime
for the corresponding isomer of 0.5 s at 298 K. Even with a smaller
value for the pre-exponential, for example due to some concerted
mechanism, room temperature thermal energies are sufficient to
access all of the possible isomeric forms. Although the solvent
model employed does not account for solvent friction, the barriers
are probably not much influenced by a low viscosity solvent such
as DMF.
Preliminary computational work was also done for calculation
of the hyperpolarizabilites of the compounds. As a general obser-
vation, the calculated DC hyperpolarizabilities are up to a factor of
four lower compared to the frequency dependent hyper-
polarizabilities at the second harmonic wavelength of 532 nm. This
is strongly indicative of close resonance at the second harmonic of
the scattering wavelength and consequently of a strong slope of the
hyperpolarizability function. Furthermore DC as well as frequency
dependent hyperpolarizabilities vary by a factor of more than 3
between different isomers, although the HOMO, LUMO and oscil-
lator strength values remain nearly constant. This suggests that the
statististical distribution amongst isomers could dramatically affect
the values of the measured hyperpolarizabilties.
3. Conclusions
We have synthesized two new series of azo dyes, based on
their extra benzene ring in the
p-conjugate bridge are significantly
bithiophene as donors and simultaneously as p-conjugated bridges
higher, this difference is not reflected in the measured hyper-
polarizability values (Table 3). Instead the main trend observed in
the measured values is a reduction in the dc limit hyper-
polarizability for compounds 5 due to a slight shift of the absorp-
tion maximum to the red presumably due to the increase in the
and thiadiazolyl and arylthiadiazolyl diazene acceptor moieties.
The new compounds were synthesized from commercially avail-
able bithiophene and (hetero)aromatic amines. Simple work-up
procedures produce fair to moderate yields of these derivatives.
Extensive characterization of the optical (linear and nonlinear)
redox and thermal properties was carried out. Experimental hyper-
length of the p-conjugated bridge.
In a third set of calculations, the activation energies for the
respective isomerizations were computed. Only isomerizations
around one bond at a time were considered. A first optimization
was done assuming a geometry rotated by 90ꢀ with respect to the
ground state geometry around one of the three isomerizable bonds
and with all the other internal coordinates variable for optimiza-
tion. Using the approximate transition state coordinates obtained
from this calculation, a transition state optimization employing the
Synchronous Transit-Guided Quasi-Newton (STQN) algorithm [19]
was employed.
Rayleigh scattering measurements report good
b values for these
heterocyclic azo dyes with the best values obtained for pushepull
derivatives 4c and 5c for an incident wavelength of 1064 nm.
Using the simple two-level model to extrapolate to the zero
frequency limit, chromophores
4 studied here have hyper-
polarizabilities that range from 29% to 37% of the apparent limit
identified by Kuzyk [20] while those of series 5 display a normal-
ized response that is between 11% and 15% of the apparent limit for
the DC hyperpolarizability. The fundamental model of Kuzyk would
predict that by adding six effective electrons contributed by the
The calculations were done using B3LYP/6-31 þ G(d,p) and the
benzene ring to the
p-conjugate bridge, the NLO response of
compounds 5 should increase by a factor of 1.7 ¼ (20/14)3/2. Instead
option opt ¼ QST3. The resulting transition state energies were

M.Cidália R. Castro et al. / Dyes and Pigments 95 (2012) 392e399
397
only a small shift of the absorption peak to the red is observed
without any increase in the NLO response.
(CDCl3)
179.1. lmax (Dioxane)/nm 475 (
film): n 3078, 2385, 1524, 1497, 1448, 1341, 1222, 1206, 1189, 1151,
d
124.9, 127.3, 128.5, 128.8, 136.5, 138.7, 147.0, 151.9, 156.1,
3
/dm3 molꢁ1 cmꢁ1 19,980). IR (Liquid
The theoretical calculations on basis of DFT reproduce the
experimental values relatively well. The largest deviation is an
underestimation of the HOMO energy, which is around 0.7 eV more
negative compared to experiment, similar to previous work.
For the first time we have considered the presence of different
isomers due to rotation around three single bonds within the
conjugated system, and showed by computation of the transition
state energy that indeed all isomers are accessible at room
temperature. Furthermore the ground state energies of the
different species are almost identical. That leads to the conclusion
that the isomers are present in thermal equilibrium in equal
concentration. Although the linear absorption parameters such has
HOMO and LUMO energies and absorption wavelength varied only
marginally between the different isomers, the calculated hyper-
polarizabilities, varied by a factor of three. This may explain in part
why it is typically very difficult to theoretically estimate the first
hyperpolarizabilties of large heterocycle compounds to better than
a factor of two in solution.
1045, 846, 724 cmꢁ1. MS (EITOF) m/z (%) ¼ 278 ([M]þ, 13), 251 (18),
223 (79), 179 (13),177 (12),165 (29),146 (13),127 (40),121 (100), 96
(8). HMRS: m/z (EITOF) for C10H6N4S3; calcd 277.9755; found:
277.9757.
4.2.1.2.2. (E)-2-(2,20-bithiophen-5-yldiazenyl)-5-bromo-1,3,4-
thiadiazole (4b). Dark brown solid (41 mg, 21%). Mp 196e197 ꢀC. 1H
NMR (CDCl3)
d
7.13 (dd, 1H, J ¼ 5.0 and 3.6 Hz, 400-H), 7.37 (d, 1H,
J ¼ 4.4 Hz, 40-H), 7.47 (dd, 1H, J ¼ 5.0 and 1.2 Hz, 500-H), 7.49 (dd, 1H,
J ¼ 3.6 and 1.2 Hz, 300-H), 7.89 (d,1H, J ¼ 4.4 Hz, 30-H).13C NMR (CDCl3)
d
125.2,127.6,128.8,128.9,136.3,139.3,139.9,147.9,155.9,181.2. lmax
(Dioxane)/nm 490 ( n
/dm3 molꢁ1 cmꢁ1 36,410). IR (Liquid film):
3
3422, 2718,1639,1449,1368, 848, 811, 701, 545 cmꢁ1. MS (EITOF) m/z
(%) ¼ 358 (Mþ81Br, 7), 356 (Mþ79Br, 7), 277 (30), 223 (48), 191 (21),
180 (13), 179 (37), 178 (18), 166 (17), 165 (40), 153 (13), 146 (21), 136
(11), 127 (59), 125 (12), 123 (13), 122 (14), 121 (100). HMRS: m/z
(EITOF) for C10H851BrN4S3; calcd 357.8839; found: 357.8831.
4.2.1.2.3. (E)-2-(2,20-bithiophen-5-yldiazenyl)-5-trifluoromethyl-
1,3,4-thiadiazole (4c). Dark pink solid (29 mg, 15%). Mp 180e183 ꢀC.
4. Experimental
1H NMR (CDCl3)
d
7.20 (dd, 1H, J ¼ 4.8 and J ¼ 3.3 Hz, 400-H), 7.48 (d,
1H, J ¼ 5.3 Hz, 40-H), 7.59e7.61 (m, 2H, 300-H and 500-H), 7.86 (d, 1H,
4.1. Materials
J ¼ 5.3 Hz, 30-H). lmax(Dioxane)/nm 496 (
3
/dm3 molꢁ1 cmꢁ1 28,440)
IR (CHCl3):
n 3419, 3100, 3077, 2925, 2359, 1633, 1461, 1450, 1330,
1,3,4-Thiadiazol-2-amine, 2-amino-5-bromo-1,3,4-thiadiazole,
2-amino-5-(trifluoromethyl)-1,3,4-thiadiazole, 2-amino-5-phenyl-
1302, 1262, 1170, 1147, 1100, 1035, 850, 815, 746, 729, 706 cmꢁ1. MS
(EITOF) m/z (%) ¼ 346 ([M]þ, 17), 330 (16), 250 (24), 223 (38), 192
(17),179 (12),178 (30),177 (15),165 (31),161 (25),149 (54),146 (16),
127 (40), 127 (38), 123 (24), 121 (100), 112 (30). HMRS: m/z (EITOF)
for C11H5N4F3S3; calcd 345.9628; found: 345.9629.
1,3,4-thiadiazole,
2-amino-5-(4-fluorophenyl)-1,3,4-thiadiazole
and 2-amino-5-(4-nitrophenyl)-1,3,4-thiadiazole were used as
precursors for the synthesis of heterocyclic diazonium salts 2e3
and 2,2-bithiophene 1 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 visu-
alized under UV light. Chromatography on silica gel was carried out
on Merck Kieselgel (230e240 mesh).
4.2.1.2.4. (E)-2-(2,20-bithiophen-5-yldiazenyl)-5-phenyl-1,3,4-
thiadiazole (5a). Dark pink solid (60 mg, 58%). Mp 215e217 ꢀC. 1H
NMR (CDCl3)
d
7.13 (dd, 1H, J ¼ 4.8 and 3.6 Hz, 400-H), 7.37 (d, 1H,
J ¼ 4.4 Hz, 40-H), 7.45 (dd, 1H, J ¼ 4.8 and 0.8 Hz, 500-H), 7.47 (dd, 1H,
J ¼ 3.6 and 0.8 Hz, 300-H), 7.51-7.54 (m, 3H, 3-H, 4-H and 5-H), 7.88
(d, 1H, J ¼ 4.4 Hz, 30-H), 8.06 (dd, 2H, J ¼ 6.8 and 1.6 Hz, 2- and 6-H).
13C NMR (CDCl3)
d 124.9, 127.2, 128.1,128.3, 128.7,129.2,130.3,131.7,
4.2. Synthesis
136.6, 138.0, 146.6, 156.4, 168.3, 178.3. lmax(Dioxane)/nm 496
(
3
/dm3 molꢁ1 cmꢁ1 23,400). IR (liquid film):
n
3433, 2121, 1640,
4.2.1. General procedure for the azo coupling of bithiophenes 1 with
thiadiazole 2aec and arylthiadiazole 3aec diazonium salts to
afford azo dyes 4aec and 5aec
1451, 1403, 1330, 1223, 1042, 798, 511 cmꢁ1. MS (EITOF) m/z
(%) ¼ 354 ([M]þ, 11), 296 (14), 223 (40), 180 (17), 179 (28), 178 (14),
177 (79), 153 (12), 148 (12), 146 (10), 136 (12), 135 (14), 127 (38), 122
(13), 121 (100), 104 (28), 103 (21), 77 (12), 74 (45). HMRS: m/z
(EITOF) for C16H10N4S3; calcd 354.0068; found: 354.0069.
4.2.1.2.5. (E)-2-(2,20-bithiophen-5-yldiazenyl)-5-(4-
4.2.1.1. Diazotation of 2-amino-1,3,4-thiadiazole, 2-amino-5-bromo-
1,3,4-thiadiazole, 2-amino-5-(trifluoromethyl)-1,3,4-thiadiazole, 2-
amino-5-phenyl-1,3,4-thiadiazole, 2-amino-5-(4-fluorophenyl)-1,3,4-
thiadiazole and 2-amino-5-(4-nitrophenyl)-1,3,4-thiadiazole. Hetero-
aromatic amines (1.0 mmol) were dissolved in HCl 6 N (1 mL) at
0e5 ꢀC. A mixture of NaNO2 (1.0 mmol) in water (2 mL) was slowly
added to the well-stirred mixture of the thiazole solution at 0e5 ꢀC.
The reaction mixture was stirred for 10 min at 0e5 ꢀC.
fluorophenyl)-1,3,4-thiadiazole (5b). Dark pink solid (53 mg, 53%).
Mp 220e221 ꢀC. 1H NMR (CDCl3)
d
7.14 (t, 2H, J ¼ 8.4 Hz, 3-H and 5-
H), 7.16e7.18 (m, 1H, 400-H), 7.28 (dd, 1H, J ¼ 3.6 and 1.2 Hz, 300-H),
7.44 (d, 1H, J ¼ 5.4 Hz, 40-H), 7.54 (dd, 1H, J ¼ 5.2 and 1.2 Hz, 500-H),
7.81 (d, 1H, J ¼ 5.4 Hz, 30-H), 7.91 (dd, 2H, J ¼ 7.2 and 5.2 Hz, 2-H and
6-H). 13C NMR (CDCl3)
J ¼ 7.0 Hz, C2 and C6), 127.9, 128.5, 129.0, 129.9, 130.0, 130.1, 130.3,
132.3, 136.3, 161.5, 164.3 (d, J ¼ 251 Hz, C4), 167.0. lmax(Dioxane)/
d
116.3 (d, J ¼ 21 Hz, C3 and C5), 126.3 (d,
4.2.1.2. Coupling reaction with bithiophenes 1. The diazonium salt
solution previously prepared (1.0 mmol) was added dropwise to the
solution of bithiophene 1 (0.52 mmol) in acetonitrile (10 mL) and
2e3 drops of acetic acid. The combined solution was maintained at
nm 498 ( n 3411, 2088,
/dm3 molꢁ1 cmꢁ1 36,140). IR (liquid film):
3
1640, 1597, 1508, 1449, 1423, 1396, 1326, 1272, 1234,1223, 1191,
1154, 1092, 1062, 1043, 852, 839, 803, 722. 710,672 cmꢁ1. MS
(EITOF) m/z (%) ¼ 372 ([M]þ, 14), 223 (53), 195 (83), 181 (55), 180
(13), 179 (25), 178 (12), 165 (13), 153 (22), 149 (15), 146 (13), 139
(98), 136 (13), 127 (42), 122 (32), 121 (100), 95 (19), 94 (17), 75 (17),
74 (52). HMRS: m/z (EITOF) for C16H9N4FS3; calcd 371.9973; found:
371.9972.
0
ꢀC for 1e2 h while stirred and then diluted with chloroform
(20 mL), washed with water and dried with anhydrous MgSO4.
The dried organic solution was evaporated and the remaining azo
dyes purified by column chromatography on silica with
dichoromethane/n-hexane as eluent.
4.2.1.2.1. (E)-2-(2,20-bithiophen-5-yldiazenyl)-1,3,4-thiadiazole
(4a). Dark pink solid (41 mg, 16%). Mp 166e167 ꢀC. 1H NMR (CDCl3)
4.2.1.2.6. (E)-2-(2,20-bithiophen-5-yldiazenyl)-5-(4-nitrophenyl)-
1,3,4-thiadiazole (5c). Dark brown solid (28 mg, 16%). Mp
d
7.13 (dd, 1H, J ¼ 5 and 3.7 Hz, 400-H), 7.37 (d, 1H, J ¼ 4.4 Hz, 40-H),
7.46 (dd, 1H, J ¼ 5.0 and 1.2 Hz, 500-H), 7.48 (dd, 1H, J ¼ 3.7 and
1.2 Hz, 300-H), 7.90 (d, 1H, J ¼ 4.4 Hz, 30-H), 9.08 (s, 1H, 5-H), 13C NMR
232e234 ꢀC. 1H NMR (CDCl3)
d
7.20 (dd, 1H, J ¼ 5.2 and 3.6 Hz, 400-
H), 7.29 (dd, 1H, J ¼ 3.6 and 1.2 Hz, 300-H), 7.48 (d, 1H, J ¼ 5.4 Hz, 40-

398
M.Cidália R. Castro et al. / Dyes and Pigments 95 (2012) 392e399
H), 7.58 (dd, 1H, J ¼ 5.2 and 1.2 Hz, 500-H), 7.85 (d, 1H, J ¼ 5.4 Hz, 30-
H), 8.10 (dd, 1H, J ¼ 7.2 and 2 Hz, 2- and 6-H), 8.32 (dd, 1H, J ¼ 6.8
1 ꢂ 10ꢁ2 mol dmꢁ3 (external reference method). The hyper-
polarizability of pNA dissolved in dioxane is known from EFISH
measurements carried out at the same fundamental wavelength.
Special care has been taken to use the correct convention [21].
and 2 Hz, 3- and 5-H). 13C NMR (CDCl3)
d 116.2, 116.4, 126.4, 127.9,
128.5, 128.9, 129.8, 129.9, 130.1, 130.3, 132.3, 136.3, 161.5, 163.1,
165.6, 167.0. lmax(Dioxane)/nm 508 (
/dm3 molꢁ1 cmꢁ1 12,590). IR
(Liquid film): 3583, 3087, 1996, 1596, 1560, 1521, 1500, 1459, 1449,
3
All solutions were filtered (0.2 mm porosity) to avoid spurious
n
signals from suspended impurities. The small hyper Rayleigh signal
that arises from dioxane was taken into account. We took particular
care to avoid reporting artificially high hyperpolarizibilities due to
a possible contamination of the hyper Rayleigh signal by molecular
fluorescence near 532 nm. Measurements were carried out using
two different interference filters with different transmission pass
bands centered near the second harmonic at 532 nm allowing us to
estimate and correct for any fluorescence emitted near 532 nm.
1230, 1420, 1400, 1376, 1351, 1337, 1257, 1221, 1188, 1105, 1048, 988,
916, 897, 870, 860, 780 cmꢁ1. MS (EITOF) m/z (%) ¼ 400 ([M þ H]þ,
100), 395 (14), 374 (10), 373 (13), 372 (72), 343 (25), 339 (11), 338
(20), 272 (16), 234 (13), 206 (21). HMRS: m/z (EITOF) for
C16H9N5O2S3; calcd 399.99911; found: 399.99753.
4.3. Instruments
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 ꢀC. All chemical shifts are given in
ppm using dH Me4Si ¼ 0 ppm as reference and J values are given in
Hz. Assignments were made by comparison of chemical shifts, peak
multiplicities and J values and were supported by spin decoupling-
double resonance and bidimensional 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 Shi-
madzu UV/2501PC spectrophotometer. Mass spectrometry anal-
yses were performed at the “C.A.C.T.I. -Unidad de Espectrometria de
Masas” at the University of Vigo, Spain. Thermogravimetric analysis
of samples was carried out using a TGA instrument model Q500
from TA Instruments, under high purity nitrogen supplied at
a constant 50 mL minꢁ1 flow rate. All samples were subjected to
a 20 ꢀC minꢁ1 heating rate and were characterized between 25 and
500 ꢀC. All melting points were measured on a Gallenkamp melting
point apparatus and are uncorrected. Cyclic voltammetry (CV) was
performed using a potentiostat/galvanostat (AUTOLAB/PSTAT 12)
with the low current module ECD from ECO-CHEMIE and the data
analysis processed by the General Purpose Electrochemical System
software package also from ECO-CHEMIE. Three electrode-two
compartment cells equipped with vitreous carbon-disc working
electrodes, a platinum-wire secondary electrode and a silver-wire
pseudo-reference electrode were employed for cyclic voltam-
metric measurements.
Acknowledgments
Thanks are due to the Fundação para a Ciência e Tecnologia
(Portugal) and FEDER-COMPETE for financial support through the
Centro de Química and Centro de Física e Universidade 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 PhD
grant to M. C. R. Castro (SFRH/BD/78037/2011). The NMR spec-
trometer 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. The computations were
executed on the SeARCH cluster at UMinho (Services and Advanced
Research Computing with HTC/HPC clusters), funded by FCT under
contract CONC-REEQ/443/EEI/2005.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.dyepig.2012.05.014.
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