Push-pull azo-chromophores containing two fused pentatomlc heterocycles and their nonlinear optical properties

Article abstract of DOI:10.1002/ejoc.200900218

We have developed procedures for the synthesis of push-pull azo-chromophores containing the s-triazolo[3,4-b]thiadiazole heterocycle compatible with the presence of electron-acceptor groups (nitrophenyl group) as substituents on both the triazole and thia

Full text of DOI:10.1002/ejoc.200900218

FULL PAPER
DOI: 10.1002/ejoc.200900218
Push–Pull Azo-Chromophores Containing Two Fused Pentatomic Heterocycles
and Their Nonlinear Optical Properties
Roberto Centore,*[a] Sandra Fusco,[a] Andrea Peluso,[b] Amedeo Capobianco,[b]
Matthias Stolte,[c] Graziano Archetti,[c] and Hans-Georg Kuball*[c]
Keywords: Chromophores / Heterocycles / EOA spectroscopy / Isomerization / Nonlinear optics / Density functional
calculations
We have developed procedures for the synthesis of push–pull
azo-chromophores containing the s-triazolo[3,4-b]thiadiazole
heterocycle compatible with the presence of electron-ac-
ceptor groups (nitrophenyl group) as substituents on both the
triazole and thiadiazole rings. The linear and non-linear op-
tical properties of the synthesized chromophores have been
characterized by the EFISH technique, electro-optical ab-
sorption spectroscopy and DFT calculations. The combined
results indicate a significant modulation of the properties of
the chromophores depending on the substitution pattern on
the heterobicycle. Fast UV-driven trans Ǟ cis and slower
thermally driven cis Ǟ trans isomerizations around the N=N
bond were observed and characterized for the chromo-
phores.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
high electronic excitation energies, which, according to the
well-known two-state model,[7] lowers the second-order
NLO response. A simple way to obviate this shortcoming
is to substitute phenylene with heteroaromatic rings due to
the smaller aromatic stabilization energy of the heterocycle
as compared with benzene. It has been experimentally
shown that the presence of heterocycles significantly en-
hances the non-linear optical response of conjugated push–
pull chromophores.[8]
The most studied chromophores contain single-ring aro-
matic heterocycles with one or more heteroatoms (e.g., pyr-
idine,[9] pyrrole,[10] thiophene,[11] imidazole,[12] thiazole,[13]
and oxadiazole[14]). Examples of chromophores containing
two aromatic fused rings, one pentatomic heterocycle and
one hexatomic non-heterocycle, have also been reported
(benzimidazole,[15] benzoxazole[16] and benzothiazole[17]).
Chromophores containing two fused rings that are both
heteroaromatic are more rare, in particular, those in which
both rings are pentatomic.[18] In the search for new het-
eroaromatic compounds suitable for quadratic NLO appli-
cations we have started a general investigation of push–pull
chromophores containing two fused rings that are both het-
eroaromatic.
Introduction
Heterocycles play a prominent role in modern chemistry:
They are important building blocks in supramolecular
chemistry,[1] in biomimetic compounds for advanced phar-
maceutical/medical applications[2] and in advanced materi-
als, such as conducting polymers,[3] OFETs,[4] organic solar
cells[5] and materials that exhibit non-linear optical (NLO)
response.
The emergence of photonic technologies in telecommuni-
cations has led to a growing demand for NLO materials.[6]
In this field, organic compounds are technologically privi-
leged in principle, compared with inorganics because they
exhibit large non-linearities with ultra-fast responses, and
their ease of handling makes possible their integration with
existing silicon-based technology.[6]
NLO effects depend on the ability of the molecular elec-
tronic cloud to show an electric-field-dependent polarizabil-
ity. Accordingly, attention has been mainly focused on or-
ganic compounds possessing long conjugated domains
(comprising polyene/phenylene backbones) end-capped
with electron-donor and -acceptor groups to induce the
necessary directional bias, the so-called push–pull chromo-
phores.[6] Phenylene-based chromophores usually exhibit
In this paper we report a study of chromophores contain-
ing the fused s-triazolo[3,4-b]thiadiazole heterocycle
(Scheme 1). This heterocycle has been investigated recently
in biochemistry and has a diverse biological activity.[19]
Some of its features suggest that this heterocycle is interest-
ing as a building block for NLO-active chromophores.
a) The high number of electronegative nitrogen atoms and
the sulfur atom are expected to increase the quadratic mo-
lecular NLO response[8] and to provide the whole chromo-
[a] Department of Chemistry “Paolo Corradini”,
University of Naples “Federico II”,
Via Cinthia, 80126 Naples, Italy
[b] Department of Chemistry, University of Salerno,
Via S. Allende, 84081 Salerno, Italy
[c] Department of Chemistry-Physical Chemistry,
Technische Universität Kaiserslautern,
Erwin-Schrödinger Str., 67663 Kaiserslautern, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200900218.
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R. Centore, H.-G. Kuball et al.
FULL PAPER
phore with a high dipole moment, which is another impor-
tant requirement for obtaining acentric polar materials by
the poling process. b) The planar geometry of the two fused
rings allows good π-electron conjugation. c) The bent ge-
ometry of the bonds in the 2- and 5-positions is expected
to confer on the chromophore a more oblate ellipsoid shape
than prolate which, again, is reported to improve poling
performances and hence NLO properties.[20]
Results and Discussion
A versatile synthetic route to the s-triazolo[3,4-b]thiadia-
zole heterocycle is reported in Scheme 3; it is based on the
cyclization between the 4-aminotriazole-3-thiol I and a car-
boxylic acid in the presence of a dehydrating agent, for ex-
ample, polyphosphoric acid (PPA).[19c]
Scheme 1. Structure of the s-triazolo[3,4-b]thiadiazole heterocycle.
Scheme 3. Synthesis of the s-triazolo[3,4-b]thiadiazole heterocycle.
A general synthesis of the 4-aminotriazole-3-thiol I has
In a recent paper we reported the synthesis of a chromo-
phore containing the heterocycle of Scheme 1 (with an ad- been described by Hoggarth[22] and is shown in Scheme 4.
ditional electron-poor pyridine ring attached to the fused The carboxylic acid RCOOH is converted (by standard
system at the 5-position) and some derived polymers.[21] In methods) into the hydrazide, which is treated with carbon
this report we describe the synthesis and full physicochemi- disulfide and KOH in absolute ethanol to afford the dithio-
cal characterization, including NLO measurements, of three carbazate potassium salt. This is converted into the 4-ami-
new chromophores containing this heterocycle in which notriazole-3-thiol in the final step by hydrazinolysis with
more powerful electron-acceptor groups have been intro- elimination of H2O and H2S.
duced, for example, 4-nitrophenyl and 4-pentafluorophenyl.
In summary, to synthesize the s-triazolo[3,4-b]thiadiazole
Moreover, the effect of introducing the acceptor onto the heterocycle (Scheme 1), carboxylic acids RCOOH and
triazole or thiadiazole ring has also been investigated. The RЈCOOH, hydrazine and CS2 are needed as starting materi-
structures of the new chromophores are reported in als. The general route depicted in Schemes 3 and 4, and,
Scheme 2 together with the structure of the previously re- in particular, the hydrazinolysis step of Scheme 4, is clearly
ported chromophore 3.
inadequate if oxidizing groups are present in the molecule,
The chromophores were synthesized with two hydroxy for example, a nitro group. Because the nitro group is
groups to allow their use as monomers in polymerization strongly electron-withdrawing it is widely used in push–pull
reactions. The properties of the polymers will be reported NLO active compounds and therefore it is important to de-
in a separate publication.
velop synthetic routes compatible with the presence of this
Scheme 2. Structures of the synthesized chromophores.
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Push–Pull Azo-Chromophores
and 2, as shown in Scheme 5. We note that in 1 and 2 the
electron-withdrawing group is attached to the fused hetero-
cycle through the thiadiazole ring.
Chromophore 4 can be synthesized in a similar way by
using p-aminobenzoic acid in the cyclization step provided
the intermediate, 4-amino-5-(4-nitrophenyl)-1,2,4-triazole-
3-thiol (Ib), is available (Scheme 6). However, Ib cannot be
obtained by the route shown in Scheme 4 because of re-
duction of the nitro group during the hydrazinolysis step.
Thus, a different synthetic path was followed (Scheme 7).
The dithiocarbazate salt was converted into the oxadia-
zole derivative, 2-(4-nitrophenyl)-1,3,4-oxadiazole-5-thiol,
by heating it at reflux in pyridine with the evolution of hy-
drogen sulfide.[22] After several trials with different concen-
trations of hydrazine, we found that the oxadiazole can be
converted into the triazole Ib by reaction with suitably di-
lute hydrazine solutions. In this way, Ib was obtained in
satisfactory yield and purity provided the molar concentra-
tion of hydrazine in the reaction solution was not more
than 0.4 mol/L.
Scheme 4. Synthesis of the 4-amino-1,2,4-triazole-3-thiols.
group. In fact, the oxidizing nature of the nitro group has
been exploited (starting from nitrobenzoic acid and follow-
ing Scheme 4) to achieve, in a single hydrazinolysis step,
ring-closure and reduction of the nitro to the amino group,
so obtaining 4-amino-5-(4-aminophenyl)-1,2,4-triazole-3-
Some physicochemical properties of 1–4 are reported in
thiol (Ia), which is the key intermediate in the synthesis of Table 1. All chromophores show fair thermal stability,
aniline precursors AN1 and AN2 and of chromophores 1 which is an important parameter for second-order NLO
Scheme 5. Synthesis of chromophores 1 and 2.
Scheme 6. Synthesis of chromophore 4.
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FULL PAPER
Scheme 7. Synthesis of 4-amino-5-(4-nitrophenyl)-1,2,4-triazole-3-thiol.
materials. The UV/Vis spectroscopic characterization data a first or pseudo-first order reaction (Figure 2) with k =
are similar for chromophores 1 and 2 and for 3 and 4. For 6.6(2)ϫ10–4 s–1 in DMF and 2.9(1)ϫ10–4 s–1 in 1,4-diox-
3 and 4 the λmax values are significantly redshifted com- ane for 2. For the other chromophores in 1,4-dioxane the
pared with 1 and 2 (Figure 1).
following rate constants were found: k Ͼ 1.7 s–1 for 1, k =
1.00(1)ϫ10–3 s–1 for 3 and k = 1.7(3)ϫ10–4 s–1 for 4. Three
clear isosbestic points show that only two different species
take part in a reversible process (Figure 3). Irradiation with
short-wavelength UV light reduces reversibly the absorption
intensity through the backward reaction, which is at least
one order of magnitude faster. This behaviour is compatible
with the hypothesis that a cis Ǟ trans isomerization around
the N=N bond occurs in the dark after the trans Ǟ cis
reaction has been photochemically induced.[23] The molecu-
lar properties were all measured under equilibrium condi-
tions in the dark; to evaluate the molecular properties the
existence of 100% of the trans isomer has been assumed
because the concentration of the cis form is at least one
order of magnitude smaller, as can be estimated from the
faster backward reaction.
Table 1. Physicochemical properties of the chromophores 1–4.
Td [°C][a]
λmax [nm][b]
1
277
280
309
299
469
468
489
489
2
3[c]
4
[a] Decomposition temperature corresponding to 5% weight loss.
[b] Measured in DMF solution. [c] Data for 3 are taken from
ref.[21]
.
Figure 1. Absorption spectra of 1 (Ϫ), 2 (---), 3 (···), 4 (-·-·-), AN1
(-··-··), AN2 (----), AN3 (···) and AN4 (-·-·-·) in 1,4-dioxane at T =
298 K after reaching equilibrium in the dark. The data for AN3
and 3 are taken from ref.[21]
.
Figure 2. Time-dependent absorbances at different wavelengths:
500 (᭹), 475 (᭡), 450 (᭢), 425 (o), 410 (᭿) and 400 nm (Ɇ) for 2
measured in 1,4-dioxane at 298 K in the dark and the regression
analysis (Ϫ) of the experimentally measured absorptions (the time-
dependence of the EOA spectra shows the same results within ex-
perimental error with the same reaction rate).
As depicted in Figure 1, the λmax values of the precursors
ANn (n = 1, 2, 3, 4) show the same behaviour and thus
the gap between the first excited state of the n and ANn
compounds is approximately constant. Although the UV/
Vis spectra of the ANn compounds do not change with
time after dissolving, the absorbance of the n compounds,
The quadratic NLO activities of the chromophores 1, 2
after quickly dissolving the chromophores in daylight, in- and 4 were measured by the EFISH technique. The results
creases with time upon storing in the dark as a result of reported in Table 2 suggest that the substitution pattern
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Push–Pull Azo-Chromophores
Table 3. EOA data of the chromophores 1–4.
λeg
µeg
β0
µg
∆µ
[D]
µg·β0
[nm][a]
[D][a] [10–30 esu] [D][b]
[10–48 esu][c]
1
2
3
4
444.2
441.1
467.5
469.6
8.8
8.9
8.9
9.1
73
103[d]
93
3.4
3.6
8.5
9.4
13.1
21.7
14.2
15.6
234
195
766
888
105
[a] Data refer to measurements in 1,4-dioxane; all other data are
measured in 1,4-dioxane and then corrected for the gas phase. [b]
Solvent corrections were made using the Onsager factor f for spher-
ical particles. This approximation is to some extent responsible for
the discrepancies between experimental and theoretical results due
to the elongated and flat shape of the molecules. [c] In accord with
the phenomenological convention (X convention). [d] Overesti-
mated by the applied equation of the two-state model because of
the large angle between µg and the corresponding µeg.
Figure 3. Increase of the absorption of 2 in 1,4-dioxane at T =
298 K in the dark over a period of 305 min. There are at least three
isosbestic points, which indicates the existence of two different ab-
sorbing species.
Table 4. NLO data claculated for chromophores 1–4.
λeg
µeg
µg
β0par
|µg·β0|
Є(µg,β0)
[°]
[nm]
[D]
[D] [10–30 esu][a] [10–48 esu]
1
2
3
4
431
421
454
467
10.7
11.2
11.6
10.4
7.7
8.0
14.9
16.5
56
31
62
75
434
245
927
19
34
9
with the electron-withdrawing group on the triazole ring (4)
gives the highest NLO activity.
1236
11
[a] In accord with the phenomenological convention (X conven-
tion).
Table 2. EFISH data of the chromophores 1, 2 and 4.
[b]
µg·β[a]
µg·β0
(Tables 2 and 3) probably results from the fact that in our
experiments EOA spectroscopy only takes into account the
contribution of the long-wavelength transition to β. The UV
spectra reported in Figure 1 indicate that contributions
from other bands in the shorter wavelength region may also
be important. The spectroscopic data in Table 3 show also
that the λeg values change according to the variation of the
substitution pattern: with the electron-withdrawing group
on the thiadiazole ring the values of λeg are about 20 nm
1
430
270
980
306
186
675
2
4[c]
[a] Measured by the EFISH technique at 1.9 µm in DMF at 298 K
and expressed in 10–48 esu according to the phenomenological con-
vention (X convention).[24] [b] Extrapolated to zero frequency using
the two-level model. [c] Measured on the diacetylated derivative in
chloroform at T = 298 K. The data are not solvent-corrected.
However, EFISH data include both the non-linear prop- shorter and this, too, is consistent with the DFT analysis
erty of the molecule (in the form of β, which is one of the (Table 4). On the other hand, the experimental results of
vector parts of the β tensor) and the ground-state dipole the EFISH and EOA measurements in comparison with the
moment through their dot product. Thus we have carried DFT calculations refer to different situations and thus the
out electro-optical absorption (EOA) measurements[25] and quantitative correlation is less satisfactory. This can be ex-
parallel DFT calculations to extract the important linear plained by the fact that in the DFT calculations we have
and non-linear chromophore properties (Tables 3 and 4). considered only optimized geometries. A few test computa-
For easy comparison, we have also included in Table 3 EOA tions indicate that there are other low-energy conformers
data for the chromophore 3 already published by us.[21]
involving either different conformations of the HO-alkyl
The most striking feature of the EOA measurements is tails and torsions within the conjugated systems (which are
the difference in the ground-state dipole moments de- fully planar in the optimized geometries) with dipole mo-
pending on the substitution pattern on the heterobicycle. In ments lower also by 3–4 D Thus, a statistical average ap-
particular, if the electron-withdrawing group is on the tri- pears to be necessary for a more significant comparison be-
azole ring (3, 4), the dipole moment is more than two times tween the experimental and theoretical data of these chro-
larger than if it is on the thiadiazole ring (1, 2), a result that mophores. In solution, in fact, a distribution of conformers
is qualitatively also found in DFT calculations. Thus, the exists and the EOA and EFISH methods measure the
higher EFISH response of 4, as compared with the isomeric mean, which leads to a smaller µg and thus to a smaller
molecule 1, is due more to the higher dipole moment of 4 µg·β0.
than to a higher β. This trend in the experimental EFISH
Thus, by comparing the DFT with EOA data it can be
data is correctly reproduced both by the EOA measure- noted that DFT overestimates ground-state dipole moments
ments and by DFT results. In particular, the somewhat and underestimates β0 values in such a way that DFT-calcu-
lower values obtained by EOA as compared with EFISH lated values of µg·β0 do follow the trends shown by experi-
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R. Centore, H.-G. Kuball et al.
FULL PAPER
mental EFISH and EOA measurements. It can also be
pointed out that, with the only exception of 2, the trend in
the DFT-calculated β0 values is in agreement with EOA
data. Concerning, in particular, the evaluation of the hyper-
polarizabilities β0 by EOA spectroscopy, it should be re-
membered that collinearity of the CT transition moment
and ground-state dipole moment vectors (µeg and µg), as
well as of the β and µg vectors, has been used here as an
approximation. This assumption is strictly true only for lin-
ear chromophores, whereas the chromophores investigated,
and in particular 1 and 2, have a bent shape, which allows
the existence of transition and dipole moments that are not
parallel to each other. The apparent anomaly of 2 can be
explained, at least in part, by the larger angle between the
β0 and µg vectors (Table 4).
Conclusions
A family of quadratically NLO-active azo-chromophores
containing the 10-electron heterocycle s-triazolo[3,4-b]thia-
diazole has been prepared. Synthetic strategies allowing the
substitution of an electron-withdrawing group (nitrophenyl)
on the triazole or thiadiazole ring have successfully been
developed. EFISH and EOA measurements as well as DFT
calculations have shown that different substitution patterns
on the heterobicycle strongly affect the linear and non-
linear electronic properties, in particular, the ground-state
electric dipole moment. Both the theoretical calculations
and EOA results conform with EFISH in their relative de-
gree of NLO properties within the series of compounds.
A careful analysis of the results of the DFT calculations
and the EOA spectra allows the following conclusions to be
drawn concerning the different properties of the chromo-
phores in relation to the substitution pattern on the hetero-
bicycle. The transition moment of the CT transition (µeg) is
approximately parallel to the long axis of the diazobenzene
subunit for all the chromophores. The angle between ∆µ =
µe – µg and µg, however, is larger for 2 (1) and smaller for
4 (3) (Figure 4). Furthermore, the different orientation of
the heterobicycle relative to its polar substituent causes dif-
ferently oriented dipole moments in the ground and excited
states for 2 (1) and 4 (3). This different orientation is re-
sponsible for the different interaction of the two local chro-
mophores, the diazo group and the heterocycle. The en-
largement of the conjugation length in 4 (3) leads, in com-
parison with 2 (1), to an enhancement of the absorption
intensity and to a shift of the absorption maxima. This
leads to an essential difference between these two classes of
molecules. Whereas in 4 (3) the total intensity of the CT
band contributes to the vector β0 and thus to µg·β0, for 2
(1) the contribution is reduced because of the larger angle
between the transition moment and the orientation axis of
the heterocycle due to the greater bending in these mole-
cules (Figure 4). For 2 (1) the CT band contributes only in
part to µg·β0 and the vector β0 is oriented differently with
respect to the molecular frame compared with 4 (3). Thus,
chromophores with bent structures can be suitable for qua-
dratic NLO applications,[20] but only if the bending does
not introduce moments that are oblique to each other and
this effect dominates.
Experimental Section
Potassium 2-(4-Nitrobenzoyl)hydrazinecarbodithioate: Potassium
hydroxide (6.615 g, 117.9 mmol), 4-nitrobenzoic acid hydrazide
(12 g, 66.2 mmol) and absolute ethanol (132 mL) were stirred at
room temperature under N2 in a round-bottomed flask equipped
with a condenser. Carbon disulfide (12 mL, 200 mmol) was then
added. An orange solid immediately formed and absolute ethanol
(97 mL) was added. After stirring for 1 h, further carbon disulfide
was added (same quantity as the initial addition) and the mixture
was allowed to stir overnight at room temperature. The orange so-
lid was filtered, washed with cold absolute ethanol and dried in
1
vacuo at 70 °C; yield 16.12 g (88%); m.p. 155 °C (dec.). H NMR
(300 MHz, [D6]DMSO, 25 °C): δ = 8.07 (d, J = 8.4 Hz, 2 H), 8.28
(d, J = 8.6 Hz, 2 H), 9.77 (s, 1 H), 10.64 (s, 1 H) ppm.
4-Amino-5-(4-aminophenyl)-1,2,4-triazole-3-thiol (Ia): Potassium 2-
(4-nitrobenzoyl)hydrazinecarbodithioate (21.21 g, 71.81 mmol) was
suspended in ethanol (106 mL) and hydrazine monohydrate
(37.2 mL) was added. The solution obtained was heated at reflux
for 5 h. The initial red solution turned green and evolution of gas
was observed (H2S, according to the lead acetate paper test). After
5 h at reflux, the solution was poured into cold water (100 mL)
acidified at pH = 5 with 37% HCl to yield 23.83 g of a white solid;
m.p. 265 °C; yield 5.953 g (40%). 1H NMR (300 MHz, [D6]DMSO,
25 °C): δ = 5.54 (s, 2 H), 6.41 (m, 2 H), 7.51 (m, 2 H), 13.42 (s, 1
H) ppm.
2-(4-Nitrophenyl)-5-(4-aminophenyl)[1,2,4]triazolo[3,4-b][1,3,4]thia-
diazole (AN1): A mixture containing Ia (4.000 g, 19.30 mmol), p-
nitrobenzoic acid (3.225 g, 19.30 mol) and polyphosphoric acid
(PPA; ca. 102 g) was heated whilst stirring. The temperature of the
mixture was gradually increased from 100 to 200 °C over 20 min to
give a brown paste. The reaction was continued for 5 h. Then, the
Figure 4. Relative orientations of the ground- and excited-state dipole moments, transition moments and dipole moment differences for
2 (left) and 4 (right). All vectors have been normalized.
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Push–Pull Azo-Chromophores
brown paste was poured into water and ice (200 mL) and formed
a yellow solid. By increasing the pH of the suspension up to 5 using
an aqueous NaOH solution, the colour of the suspension turned
from yellow to brown. The brown solid was recovered by filtration
and washed with 10 wt.-% aqueous Na2CO3. After filtration and
abundant washing with water, a brown crystalline solid was col-
lected and dried in an oven at 120 °C for 4 h; yield 5.420 g (83%);
m.p. 250 °C. H NMR (300 MHz, [D6]DMSO, 25 °C): δ = 5.73 (s,
2 H), 6.69 (d, J = 8.4 Hz, 2 H), 7.92 (d, J = 8.6 Hz, 2 H), 8.22 (d,
J = 8.8 Hz, 2 H), 8.35 (d, J = 8.8 Hz, 2 H) ppm.
150 mL of a solution obtained by mixing hydrazine monohydrate
(7.2 mL, 148.4 mmol) and water (391 mL) were heated at reflux in
a round-bottomed flask. After 10 h, a further 150 mL of the same
hydrazine/water solution was added and the mixture was heated at
reflux for 4 h. After this period the remaining hydrazine/water solu-
tion was added and the reaction mixture heated at reflux for 4 h.
Then the reaction solution was acidified at pH = 4 with a 37%
HCl solution. The yellow solid obtained was filtered, washed with
cold water and dried in an oven; m.p. 182 °C; yield 0.978 g (46%).
1H NMR (300 MHz, [D6]DMSO, 25 °C): δ = 5.84 (s, 2 H), 8.34
(m, 4 H), 14.16 (s, 1 H) ppm.
1
2-(2,3,4,5,6-Pentafluorophenyl)-5-(4-aminophenyl)[1,2,4]triazolo-
[3,4-b][1,3,4]thiadiazole (AN2): This compound was obtained by the
same synthetic procedure used for the preceding compound but
using pentafluorobenzoic acid instead of 4-nitrobenzoic acid.
Moreover, the reaction mixture was initially held at 150 °C for 1 h
before further increasing the temperature to avoid sublimation of
2-(4-Aminophenyl)-5-(4-nitrophenyl)[1,2,4]triazolo[3,4-b][1,3,4]thia-
diazole (AN4): Ib (0.900 g, 3.794 mmol), 4-aminobenzoic acid
(0.520 g, 3.794 mol) and ca. 30 g of PPA were allowed to react for
4 h at 200 °C, increasing gradually the temperature from 100 to
200 °C over 20 min. After 4 h of reaction, the green paste obtained
was poured into a mixture of water and ice (100 mL). A green-
yellow solid formed. On addition of aqueous NaOH up to pH =
5, the solid changed from green-yellow to orange. After filtering
and washing with 10 wt-.% aqueous sodium carbonate a dark-
orange solid was finally collected; m.p. 249 °C (dec.); yield 1.053 g
1
the acid; yield 70%; m.p. 280 °C (dec). H NMR (300 MHz, [D6]
DMSO, 25 °C): δ = 5.71 (s, 2 H), 6.63 (d, J = 8.2 Hz, 2 H), 7.82
(d, J = 8.8 Hz, 2 H) ppm.
2–(4-Nitrophenyl)-5-(4-{4-[N,N-bis(2-hydroxyethyl)amino]phenyl-
azo}phenyl)[1,2,4]triazolo[3,4-b][1,3,4]thiadiazole (1): For the syn-
thesis of 1 and 2 the same synthetic procedure was adopted: AN1
(3.000 g, 8.867 mmol) was suspended in water (20 mL) whilst stir-
ring at 0–4 °C. A 37% solution of HCl (4.6 mL) was added. So-
dium nitrite (0.667 g, 9.67 mol), dissolved in the minimum amount
of water, was added dropwise to the mixture. The addition of nitrite
caused the suspension to become dark. After 40 min after the last
addition of nitrite, the suspension containing the diazonium salt
was poured into an aqueous solution (100 mL) containing anhy-
drous sodium acetate (5.000 g) and N,N-bis(2-hydroxyethyl)aniline
(1.607 g, 8.867 mmol). A dark-red precipitate was obtained imme-
diately. After stirring for 1 h at room temperature, the solid was
filtered and recrystallized from DMF/H2O to give 1 as a dark-red
1
(82%). H NMR (300 MHz, [D6]DMSO, 25 °C): δ = 6.25 (s, 2 H),
6.66 (d, J = 8.8 Hz, 2 H), 7.67 (s, J = 8.8 Hz, 2 H), 8.37 (dd, 4
H) ppm.
2-(4-{4-[N,N-Bis(2-hydroxyethyl)amino]phenylazo}phenyl)-5-(4-nitro-
phenyl)[1,2,4]triazolo[3,4-b][1,3,4]thiadiazole (4): The synthetic pro-
cedure involves a diazotization/coupling strategy analogous to that
used for 1 and 2; m.p. 299 °C (dec); yield 55%. 1H NMR
(300 MHz, [D6]DMSO, 25 °C): δ = 3.46 (m, 8 H), 4.70 (br. s, 2 H),
6.70 (d, J = 8.7 Hz, 2 H), 7.63 (d, J = 7.8 Hz, 2 H), 7.76 (d, J =
8.4 Hz, 2 H), 8.02 (d, J = 7.8 Hz, 2 H), 8.29 (d, J = 8.7 Hz, 2 H),
8.41 (d, J = 8.1 Hz, 2 H) ppm. C25H22N8O4S (530.568): calcd. C
56.59, H 4.18, N 21.12; found C 56.40, H 4.34, N 20.87%.
1
microcrystalline solid; m.p. 277 °C (dec.); yield 2.164 g (46%). H
Physical Measurements: The thermal behaviour of the synthesized
compounds was studied by DSC (Perkin–Elmer Pyris, scanning
rate 10 °C/min, nitrogen flow), optical observations (Zeiss Axios-
cop polarizing microscope, Mettler FP90 heating stage) and ther-
NMR (300 MHz, [D6]DMSO, 25 °C): δ = 3.57 (s, 8 H), 4.85 (m, 2
H), 6.80 (d, J = 8.0 Hz, 2 H), 7.70 (d, J = 8.8 Hz, 2 H), 7.86 (d, J
= 8.6 Hz, 2 H), 8.24–8.40 (m, 6 H) ppm. C25H22N8O4S (530.568):
calcd. C 56.59, H 4.18, N 21.12; found C 56.36, H 4.29, N 20.95.
1
mogravimetric analysis (Mettler TG50, air atmosphere). H NMR
spectra were recorded with Varian and Gemini spectrometers op-
erating at 200 or 300 MHz.
2–(2,3,4,5,6-Pentafluorophenyl)-5-(4-{4-[N,N-bis(2-hydroxyethyl)-
amino]phenylazo}phenyl)[1,2,4]triazolo[3,4-b][1,3,4]thiadiazole (2):
The compound obtained in this case was a red microcrystalline
solid that decomposes at about 280 °C. The yield was the same as
The conventional EFISH technique was used to determine experi-
mentally the non-linearity µg·β0 of the chromophores, where µg is
the ground-state permanent dipole moment of a molecule and β
is the vector part of the quadratic hyperpolarizability tensor of a
molecule. The light source was a Q-switched Nd:Yag laser the emis-
sion of which at 1.06 µm was shifted to 1.907 µm by stimulated
Raman scattering in a high-pressure hydrogen cell. The measure-
1
for 1. H NMR (300 MHz, [D6]DMSO, 25 °C): δ = 3.57 (s, 8 H),
4.82 (m, 2 H), 6.83 (d, J = 8.8 Hz, 2 H), 7.73 (d, J = 8.8 Hz, 2 H),
7.89 (d, J = 8.8 Hz, 2 H), 8.29 (d, J = 8.2 Hz, 2 H) ppm.
C25H18N7O2F5S (575.521): calcd. C 52.17, H 3.15, N 17.04; found
C 52.32, H 3.37, N 17.08.
ments were calibrated relative to a quartz wedge: the experimental
4-Amino-5-(4-nitrophenyl)-1,2,4-triazole-3-thiol (NO2-triazole)
Quartd
value of d
= 1.2ϫ10–9 esu at 1.06 µm was extrapolated to
11
1.11ϫ10–9 esu at 1.907 µm. Measurements were performed ne-
glecting the electric field dependence of molecular properties and
allowing for Kleinman symmetry.[26] Measurements were per-
formed in DMF solution for 1 and 2 and in chloroform solution
for the diacetylated derivative of 4. The results given in the text are
in accord with the phenomenological convention (X conven-
tion).[24]
Step 1. Synthesis of 2-(4-Nitrophenyl)-1,3,4-oxadiazole-5-thiol: Po-
tassium 2-(4-nitrobenzoyl)hydrazinecarbodithioate (8.110 g,
27.46 mmol) and pyridine (52 mL) were heated at reflux for 4 h.
The suspension containing an orange solid was then poured into
water (73 mL) and a white solid formed. This was filtered and the
mother liquor acidified with a 37% HCl solution until pH = 4. The
yellow solid formed was collected and recrystallized from ethanol/
water to give yellow needle-shaped crystals; m.p. 207 °C (dec.);
Dipole moments of the ground state µg and the dipole moment
differences ∆µ = µe – µg (µe is the excited-state dipole moment) of
the chromophores were determined by electro-optical absorption
(EOA) spectroscopy by which the difference of absorption of a
solution with [εE(φ,ν)] and without [ε(ν)] an external applied electric
field E is measured with light polarized parallel (φ = 0°) and per-
1
yield 4.107 g (67%). H NMR (300 MHz, [D6]DMSO, 25 °C): δ =
8.00 (d, J = 7.7 Hz, 2 H), 8.22 (d, J = 8.7 Hz, 2 H), 14.02 (br. s, 1
H) ppm.
Step 2. 4-Amino-5-(4-nitrophenyl)-1,2,4-triazole-3-thiol (Ib): 2-(4-
Nitrophenyl)-1,3,4-oxadiazole-5-thiol (2.001 g, 8.960 mmol) and
Eur. J. Org. Chem. 2009, 3535–3543
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3541

R. Centore, H.-G. Kuball et al.
FULL PAPER
pendicular (φ = 90°) to the direction of E.[27] For uniaxial phases,
induced in a solution by both an alternating and a constant electric
field of about 3ϫ106 V m–1, the dichroism εE(φ,ν) – ε(ν) depends
on the orientational order of the molecules due to their ground-
state dipole moment µg, the shift of the absorption band pro-
portional to the dipole moment difference ∆µ and the electric-field
dependence of the electric-transition dipole moment µeg(E). UV/
Vis spectra, required for the evaluation of the integral absorption
(ϰ µeg2), were recorded with a Perkin–Elmer Lambda 900 spectro-
photometer at 298 K.
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Received: March 2, 2009
Published Online: June 16, 2009
Eur. J. Org. Chem. 2009, 3535–3543
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
3543