Synthesis and physico-chemical properties of highly conjugated azo-aromatic systems. 4-(azulen-1-yl)-pyridines with mono- and bis azo-aromatic moieties at C3-position of azulene

Article abstract of DOI:10.1016/j.dyepig.2011.02.008

Highly conjugated azo-aromatic systems have been prepared in high to moderate yields by linking mono- and bis-azo aromatic fragments to 4-(Rn-azulen-1-yl)-2,6-dimethyl-pyridine. The synthesized π-extended systems have been studied by NMR spectroscopy, UV-

Full text of DOI:10.1016/j.dyepig.2011.02.008

Dyes and Pigments 91 (2011) 55e61
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Dyes and Pigments
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Synthesis and physico-chemical properties of highly conjugated azo-aromatic
systems. 4-(azulen-1-yl)-pyridines with mono- and bis azo-aromatic
moieties at C3-position of azulene
,
b
,
a
c
Alexandru C. Razus a, Simona Nica a , Liliana Cristian , Matei Raicopol ,
*
Liviu Birzan a, Andreea Eugenia Dragu a
a Institute of Organic Chemistry “C.D. Nenitzescu”, 202 B Splaiul Independentei, Bucharest 060023, Romania
b “Petru Poni” Institute of Macromolecular Chemistry, 41 A, Grigore-Ghica Voda Alley, Iasi 700847, Romania
c University “Politehnica” of Bucharest, Faculty of Applied Chemistry and Materials Science, “C.D. Nenitzescu” Organic Chemistry Department, 1-7 Polizu, Bucharest 011061, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly conjugated azo-aromatic systems have been prepared in high to moderate yields by linking
mono- and bis-azo aromatic fragments to 4-(Rn-azulen-1-yl)-2,6-dimethyl-pyridine. The synthesized
Received 13 December 2010
Received in revised form
24 January 2011
Accepted 21 February 2011
Available online 9 March 2011
p
-extended systems have been studied by NMR spectroscopy, UVeVis and electrochemistry. Systematic
increase of the conjugation along the azobenzene skeleton has affected the spectral properties of the
azophenyl substituted 4-(azulen-1-yl)-pyridine. The synthesized compounds exhibit a bathochromic
shift of the visible absorption maxima with the increase of the conjugating skeleton and introduction of
an electron-withdrawing group. The electrochemical behavior revealed a high stability toward oxidation
owing to the higher polarization induced by the azulenylpyridine moiety.
Keywords:
Arylazo chromophores
Diazene azulenylpyridine
UVeVis spectroscopy
Solvatochromism
Ó 2011 Elsevier Ltd. All rights reserved.
Electrochemistry
Pushepull compounds
1. Introduction
molecule versatile for designing a large variety of pushepull
compounds with NLO properties [6,7]. Moreover, azulene-based
Molecules with various functionalities possess peculiar prop-
erties, and their facile preparation is one of the most challenging
goals for synthetic chemistry. The unusual electronic and optical
properties of azulenes have led to a significant interest in the design
and synthesis of new materials incorporating this aromatic system
[1e7]. Azulene, isomer of naphthalene, is a nonbenzenoid aromatic
hydrocarbon with a characteristic deep blue color. The color arises
from the easy electronic transition from the highest occupied
molecular orbital to the lowest unoccupied antibonding orbital.
Substitution of the ring can alter the wavelength of the absorption
bands resulting in different colors for different azulene derivatives.
The electron drift from the seven-membered ring toward the five-
membered ring forming an intrinsic dipolar structure makes this
compounds have found applications as molecular switchers [8,9],
liquid crystals [10,11] or in optical data storage devices [12].
As part of our on going interest in developing methodologies for
polyfunctionalization of azulenes, we have investigated the
synthesis of series of 2,6-dimethyl-4-(azulen-1yl) pyranylium and
pyridinium salts, as well as the corresponding pyridines [13,14].
These compounds represent valuable synthons for synthesis of
highly conjugated aromatic systems with potential NLO properties
and other technical applications. In addition, pyridylazulenes are
useful for constructing novel analytical reagents with utility as
colorimetric reaction indicators [1e4]. Therefore, we proposed to
take advantage of the reactivity of the azulene moiety of these
heteroarylazulene derivatives and to further extend the
p electronic
system by substitution of the azulenyl fragment with an azobenzene
functionality. This azo chromophores possesses numerous spec-
troscopic and photophysical properties, in particular strong elec-
tronic absorption in the UV and Vis regions of the spectrum [15] and,
because of the planarity of the azo bridge versus stilbenes or other
* Corresponding author. Institute of Organic Chemistry “C.D. Nenitzescu”, 202 B
Splaiul Independentei, Bucharest 060023, Romania. Tel.: þ40 21 316 79 00; fax: þ40
21 312 16 01.
similar systems, they contribute to larger
p transmission effects
E-mail address: snica@cco.ro (S. Nica).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.02.008

56
A.C. Razus et al. / Dyes and Pigments 91 (2011) 55e61
leading to higher optical activity [16,17]. Therefore, we expect that,
the obtained more complex chromophores possess interesting
spectroscopic and photophysical properties, mainly strong elec-
tronic absorption in the visible region of the spectrum. Thus, we
report herein the synthesis, spectroscopic investigation and sol-
vatochromic behavior of new (2,6-dimethyl-pyridin-4-yl)-azulen-
1-yl diazenes with azulen-1-yl moiety substituted in position 3.
heating until complete dissolution of the amine hydrochloride. The
resulted solution was cooled at 0 ꢁC on ice bath, while some solid
started to precipitate. To this mixture, NaNO2 (0.9 mmol, 60 mg)
dissolved in a minimum amount of water was added slowly with
vigorous stirring. The stirring was continued at low temperature for
15 min. Then, it has been added to an ice-cooled solution of 4-(Rn-
azulen-1-yl)-2,6-dimethyl-pyridine (0.87 mmol) and potassium
acetate (0.6 g) dissolved in methanol (50 ml). The solution was
stirred on ice bath for 30 min and then was neutralized with NaOH,
10% aqueous solution and extracted with DCM. The organic layer
was washed with water and dried over Na2SO4. After removal of the
solvent, the crude product was purified by column chromatography
on alumina using as eluent hexane: DCM (3:1).
2. Experimental
2.1. Materials and instrumentation
The azo-coupling was performed using commercially available
aromatic amines. Dichloromethane (DCM) was dried over
calcium hydride. Acetonitrile, spectroscopic purity for cyclic vol-
tammetry was dried and kept over molecular sieves. For the column
chromatography basic alumina (IIeIII Brockman grade) was used.
4-(Azulen-1-yl)-2,6-dimethyl-pyranylium perchlorate, 4-(4,6,8-tri-
methyl-azulen-1-yl)-2,6-dimethyl-pyranylium perchlorate and
their corresponding pyridine derivatives were obtained as described
in the literature [13]. The numbering of atom positions used for the
characterization of the products is described in Scheme 2.
Melting points were determined with Koehler Automatic
Melting Point Range Apparatus (K90190). Elemental analyses were
performed with Perkin Elmer CHN 240B analyzer. UVeVis spectra
were recorded on Varian Cary 100 spectrophotometer. NMR spectra
in CDCl3 containing TMS as internal standard were recorded on
Bruker Avance DRX4 (1H: 400 MHz, 13C: 100.62 MHz) spectrom-
2.2.1.1. (E)-[3-(2,6-dimethyl-pyridin-4-yl)azulen-1-yl]-phenyl-diaze-
ne (3). This compound was synthesized from 4-azulen-1-yl-2,6-
dimethyl-pyridine (202 mg, 0.87 mmol) with diazonium salt of
aniline (81 mg, 0.87 mmol) following the general procedure. It was
obtained as greenish brown solid; mp 150e152 ꢁC. UVeVis (meth-
anol):
(CDCl3):
l
(log
2.66 (s, 6H, 2-Me, 6-Me), 7.24 (s, 2H, 3-H, 5-H), 7.39 (t,1H,
3
) 237 (4.40), 324 (4.39), 425 (4.32) nm. 1H NMR
d
J ¼ 9.6 Hz, 50-H), 7.41 (t, J ¼ 7.5 Hz, 1H, 400-H), 7.48 (t, 1H, J ¼ 10.0 Hz,
70-H), 7.56 (t, 2H, J ¼ 7.6 Hz, 300-H, 500-H), 7.82 (t,1H, J ¼ 10.0 Hz, 60-H),
8.03 (dAB, 2H, J ¼ 7.6 Hz, 200-H, 600-H), 8.44 (s, 1H, 20-H), 8.59 (d, 1H,
J ¼ 9.6 Hz, 40-H), 9.44 (d,1H, J ¼ 9.6 Hz, 80-H) ppm; 13C NMR (CDCl3):
d
24.6 (Me-2,6),121.0 (CH-3,5),122.4 (CH-200,600),124.6 (CH-20),125.3
(Cq), 127.1 (CH-50), 127.3 (CH-70), 129.1 (CH-300,500), 129.3 (CH-400),
129.6 (Cq), 130.1 (Cq), 136.2 (CH-80), 137.0 (CH-40), 139.5 (Cq), 140.1
(Cq),140.5 (CH-60),142.9 (Cq),144.8 (Cq),154.1 (Cq),158.1 (Cq) ppm.
ESI-MS: m/z (%) ¼ 338 (100) [Mþ1]. Anal. Calcd. for C23H19N3
(337.42): C, 81.87; H, 5.68; N,12.45; found: C, 81.85; H, 5.62; N,12.48.
eter; chemical shifts (d) are expressed in ppm, and J values are given
in Hz. Mass spectra were recorded with Varian 1200L Quadrupole/
MS/MS spectrometer by direct injection in ESI, positive mode.
Electrochemical measurements were performed using a Princeton
Applied Research model 273 potentiostat and CorrWare software
(Scribner Associates). Cyclic voltammetry was performed in argon
atmosphere in an undivided cell, using a platinum disk working
electrode (2 mm in diameter), a platinum gauze auxiliary electrode
and a silveresilver ion reference electrode. All potentials are
reported against the Ag/Agþ 0.01 M reference electrode. Potentials
were scanned independently from 0 to ꢀ2.5 V and from 0 to þ2.5 V
with 100 mV/s scan rate. The electrochemical measurements were
carried out on 2 mM concentration using anhydrous acetonitrile
containing 0.1 M tetrabutyl ammonium perchlorate as supporting
electrolyte. HOMO and LUMO orbital energies were calculated with
MOPAC-9 package using AM1 approach.
2.2.1.2. (E)-[3-(2,6-dimethyl-pyridin-4-yl)-4,6,8-trimethyl-azulen-1-
yl]-phenyl-diazene, (4). This compound was synthesized from
2,6-dimethyl-4-(4,6,8-trimethyl-azulen-1-yl)-pyridine (240 mg,
0.87 mmol) with diazonium salt of aniline (81 mg, 0.87 mmol)
following the general procedure. It was obtained as brown crystals;
mp 213e214 ꢁC. UVeVis (methanol):
433 (4.38) nm. 1H NMR (CDCl3):
l
(log
3) 253 (4.45), 326 (4.36),
d
2.47 (s, 3H, 60-Me), 2.57 (s, 6H, 2-
Me, 6-Me), 2.64 (s, 3H, 40-Me), 3.41 (s, 3H, 40-Me), 7.03 (s, 2H, 3-H, 5-
H), 7.12 (s, 1H, 50-H), 7.33 (s, 1H, 70-H), 7.37 (t, 1H, J ¼ 7.6 Hz, 400-H),
7.49 (t, 2H, J ¼ 7.2 Hz, 300-H, 500-H), 7.85 (dAB, 2H, J ¼ 7.6 Hz, 200-H, 600-
H), 8.04 (s, 1H, 20-H) ppm; 13C NMR (CDCl3):
d 24.5 (Me-2,6), 28.2
(Me-80), 29.6 (Me-60), 30.4 (Me-40), 121.7 (CH-3,5), 122.4 (CH-200,600),
125.5 (CH-20), 128.9 (CH-400), 129.0 (CH-300,500), 131.5 (Cq),132.4 (CH-
50), 133.3 (CH-70), 134.9 (Cq), 136.2 (Cq), 145.6 (Cq), 147.8 (Cq), 148.8
(Cq), 149.9 (Cq), 150.0 (Cq), 154.1 (Cq), 156.9 (Cq) ppm. ESI-MS: m/z
(%) ¼ 380 (100) [M]. Anal. Calcd for C26H25N3 (379.50): C, 82.29; H,
6.64; N, 11.07; found: C, 82.26; H, 6.63; N, 11.10.
2.2. Synthesis and spectroscopic characterization
2.2.1. General procedure for the synthesis of mono-diazene, 3e6
To a stirred suspension of aromatic amine (0.87 mmol) in water
(10 ml) an aqueous solution of HCl (0.8 ml; 5 M) was added with
2.2.1.3. (E)-[3-(2,6-dimethyl-pyridin-4-yl)azulen-1-yl]-(4-nitro-phenyl)
diazene, (5). This compound was synthesized from 4-azulen-1-yl-
2,6-dimethyl-pyridine (202 mg, 0.87 mmol) with diazonium salt of
p-nitro-aniline (121 mg, 0.87 mmol) following the general proce-
dure. It was obtained as dark brown crystals; mp 247 ꢁC. UVeVis
Rn
(methanol):
(CDCl3):
l (log 3
) 234 (4.65), 302 (4.57), 470 (4.60) nm. 1H NMR
d
2.64 (s, 6H, 2-Me, 6-Me), 7.23 (s, 2H, 3-H, 5-H), 7.49 (t,1H,
X = O+; RN+; N
J ¼ 9.8 Hz, 50-H), 7.62 (t, 1H, J ¼ 9.6 Hz, 70-H), 7.90 (t, 1H, J ¼ 9.8 Hz,
60-H), 8.06 (dABt, 2H, J ¼ 8.8 Hz and 2 Hz, 200-H, 600-H), 8.35 (dABt, 2H,
J ¼ 8.8 Hz and 2 Hz, 300-H, 500-H), 8.39 (s, 1H, 20-H), 8.61 (d, 1H,
X
13
J ¼ 9.6 Hz, 40-H), 9.43 (d, 1H, J ¼ 9.6 Hz, 80-H) ppm; C NMR
(CDCl3):
d
24.5 (Me-2,6), 120.9 (CH-3,5), 122.6 (CH-200,600), 124.6
1: X = N; Rn = H
2: X = N; Rn = 4,6,8-Me3
(CH-20), 124.8 (CH-300,500), 128.7 (CH-50), 128.8 (CH-70), 131.4 (Cq),
136.4 (CH-80), 137.5 (CH-40), 140.9 (Cq), 141.1 (CH-60), 141.8 (Cq),
143.4 (Cq), 144.3 (Cq), 147.4 (Cq), 157.5 (Cq), 158.2 (Cq) ppm. ESI-
Scheme 1.

A.C. Razus et al. / Dyes and Pigments 91 (2011) 55e61
57
(1")
C6H4Y
Rn
N
N
Compound
3: Rn = H; Y = H
4: Rn = 4,6,8-Me3; Y = H
Yield (%)
25
71
(1')
YC6H4N2+Cl-
AcOK in MeOH
5: Rn = H; Y = NO2
6: Rn = 4,6,8-Me3; Y= 4-NO2
89
72
Rn
N
(1)
(1'")
C6H4Y
N
N
YC6H4N2
(1")
N
N
Rn
N
N
N+Cl-
AcOK
1: Rn = H
2: Rn = 4,6,8-Me3
Compound
7: Rn = H; Y = H
8: Rn = 4,6,8-Me3; Y = H
Yield (%)
41
76
in MeOH
(1')
(1)
9: Rn = H; Y = NO2
10: Rn = 4,6,8-Me3; Y= 4-NO2 48
36
N
Scheme 2.
MS: m/z (%) ¼ 383 (100) [Mþ1]. Anal. Calcd for C23H18N4O2
(382.41): C, 72.24; H, 4.74; N, 14.65; found: C, 72.21; H, 4.76; N,
14.61.
J ¼ 8.8 Hz, 200-H, 600-H), 8.04 (s,1H, 20-H), 8.33 (dAB, 2H, J ¼ 9.2 Hz, 300-H,
500-H) ppm; 13C NMR (CDCl3): 24.5 (Me-2,6), 28.3 (Me-80), 29.7 (Me-
d
60), 30.6 (Me-40), 121.4 (CH-3,5), 122.5 (CH-200,600), 124.8 (CH-300,500),
125.5 (CH-20),132.8 (Cq),134.2 (CH-50),135.0 (CH-70),136.9 (Cq),138.2
(Cq), 146.5 (Cq), 146.8 (Cq), 148.7 (Cq), 149.5 (Cq), 149.6 (Cq), 150.3
(Cq), 157.1 (Cq), 157.8 (Cq) ppm. ESI-MS: m/z (%) ¼ 425 (100) [Mþ1].
Anal. Calcd for C26H24N4O2 (424.49): C, 73.56; H, 5.70; N,13.20; found:
C, 73.52; H, 5.66; N, 13.15.
2.2.1.4. (E)-[3-(2,6-dimethyl-pyridin-4-yl)-4,6,8-trimethyl-azulen-1-
yl]-(4-nitro-phenyl)diazene, (6). This compound was synthesized
from 2,6-dimethyl-4-(4,6,8-trimethyl-azulen-1-yl)-pyridine (240 mg,
0.87 mmol) with diazonium salt of p-nitro-aniline (121 mg,
0.87 mmol) following the general procedure. It was obtained as dark
brown crystals; mp 255e256 ꢁC. UVeVis (methanol):
l
(log
3
) 251
2.2.2. General procedure for the synthesis of bis-diazene, 7e10
To a stirred suspension of 4-phenylazo-phenylamine (1.0 mmol)
in water (15 ml) an aqueous solution of HCl (1.15 ml; 10 M) was
added with heating until the main amount of the amine salt was
dissolved. The resulted slurry solution was cooled at 0 ꢁC and
a solution of NaNO2 (70 mg, 1.0 mmol) dissolved in a minimum
amount of water was added slowly with vigorous stirring. The
stirring was continued at low temperature for 15 min. Then, the
mixture has been added to an ice-cooled solution of 4-(Rn-azulen-
1-yl)-2,6-dimethyl-pyridine (1.0 mmol) and potassium acetate
(3.35 g) in methanol (50 ml). The mixture was stirred at the same
temperature for 1 h, then neutralized with NaOH 10% aqueous
solution and extracted with DCM. The organic layer was washed
with water and dried over Na2SO4. After removal of the solvent, the
crude product was purified by column chromatography on alumina
using as eluent hexane: DCM (3:1), with increasing the polarity of
the eluting mixture. Due to the incomplete separation of the
desired product from the starting amine, second chromatography
using the same conditions was necessary for these compounds.
(4.48), 320 (4.32), 477 (4.48) nm. 1H NMR (CDCl3):
d
2.49 (s, 3H, 60-
Me), 2.58 (s, 6H, 2-Me, 6-Me), 2.68 (s, 3H, 40-Me), 3.41 (s, 3H, 80-Me),
7.01 (s, 2H, 3-H, 5-H), 7.24 (s,1H, 50-H), 7.46 (s,1H, 70-H), 7.90 (dAB,1H,
N
N
N
N
N
N
+
+
N
N
N
A
B
_
C
O
N
O
2.2.2.1. (E)-[3-(2,6-dimethyl-pyridin-4-yl)azulen-1-yl]-[4-[(Z)-phen-
ylazo]phenyl]diazene, (7). This compound was synthesized from
4-azulen-1-yl-2,6-dimethyl-pyridine (233 mg, 1.0 mmol) with
diazonium salt of 4-phenylazo-phenylamine (197 mg, 1.0 mmol)
following the general procedure. It was obtained as black crystals;
N
N
+
mp 125e127 ꢁC. UVeVis (MeOH),
l
(log
3): 235 (4.39), 307 (4.40),
353 (4.34), 471 (4.38) nm. 1H NMR (CDCl3):
d
2.63 (s, 6 H, 2-Me, 6-
N
D
Me), 7.21 (s, 2H, 3-H, 5-H), 7.37 (t, 1H, J ¼ 9.8 Hz, 50-H), 7.53 (t, 2H,
J ¼ 6.9 Hz, 3000-H, 5000-H), 7.47-7.52 (m, 2H, 70-H and 4000-H), 7.79 (t,
1H, J ¼ 9.8 Hz, 60-H), 7.95 (d, 2H, J ¼ 7.2 Hz, 2000-H, 6000-H), 8.07 (dAB
,
Scheme 3.

58
A.C. Razus et al. / Dyes and Pigments 91 (2011) 55e61
2H, J ¼ 8.8 Hz, 200-H, 600-H or 300-H, 500-H), 8.12 (dAB, 2H, J ¼ 8.8 Hz,
300-H, 500-H or 200-H, 600-H), 8.39 (s, 1H, 20-H), 8.54 (d, 1H, J ¼ 9.6 Hz,
H) ppm; 13C NMR (CDCl3): 24.3 (Me-2,6), 28.3 (Me-60), 29.6 (Me-
d
40), 30.5 (Me-80), 121.6 (CH-3,5), 123.2 (CH-2000,6000), 123.5 (CH-
300,500),124.7 (CH-200,600),124.8 (CH-3000,5000),125.6 (CH-20),128,8 (Cq),
130.9 (Cq), 133.6 (CH-50), 134.4 (CH-70), 136.3 (Cq), 137.6 (Cq), 146.6
(Cq), 148.4 (Cq), 148.7 (Cq), 149.3 (Cq), 150.3 (Cq), 152.1 (Cq), 156.0
(Cq), 156.6 (Cq), 156.9 (Cq) ppm. ESI-MS: m/z (%) ¼ 529 (100)
[Mþ1]. Anal. Calcd. for C32H28N6O2 (528.60): C, 72.71; H, 5.34; N,
15.90; found: C, 72.74; H, 5.38; N, 15.86.
40-H), 9.40 (d, 1H, J ¼ 10.0 Hz, 80-H) ppm; 13C NMR (CDCl3):
d 24.6
(Me-2,6), 120.9 (CH-3,5), 122.9 (CH-2000,6000), 123.1 (CH-300,500), 123.9
(CH-200,600), 124.6 (CH-20), 127.7 (CH-50),127.8 (CH-70), 129.1 (CH-
3
000,5000), 130.6 (Cq), 131.1 (CH-4000), 136.3 (CH-80), 137.1 (CH-40), 140.1
(Cq), 140.7 (CH-60), 143.3 (Cq), 144.5 (Cq), 152.7 (Cq), 152.8 (Cq),
155.4 (Cq), 158.1 (Cq) ppm. ESI-MS: m/z (%) ¼ 442 (100) [Mþ1].
Anal. Calcd. for C29H23N5 (441.53): C, 78.89; H, 5.25; N, 15.86;
found: C, 78.84; H, 5.24; N, 15.81.
3. Results and discussions
2.2.2.2. (E)-[3-(2,6-dimethyl-pyridin-4-yl)-4,6,8-trimethyl-azulen-1-
yl]-[4-[(Z)-phenylazo]phenyl]diazene, (8). This compound was
synthesized from 2,6-dimethyl-4-(4,6,8-trimethyl-azulen-1-yl)-
pyridine (275 mg, 1.0 mmol) with diazonium salt of 4-phenylazo-
phenylamine (197 mg, 1.0 mmol) following the general procedure.
It was obtained as black crystals; mp 196e198 ꢁC. UVeVis (MeOH),
3.1. Synthesis
The ability of the azulene system to be involved in electrophile
reactions is given by the enhanced electron density at C1(and/or
C3)-position. Substitution with electron donor or releasing group
increases the reactivity of the azulene moiety, whereas, the pres-
ence of an electron-acceptor group has an opposite effect. Recently,
we have reported the synthesis and physico-chemical properties of
azulenes substituted with heterocyclic rings at C1-position as
described in Scheme 1 [13,14].
Substitution of azulene in 1-position with positive charged
group, as for example 4-pyranylium or 4-pyridinium moiety,
renders more difficult the electrophile substitution at C3-position.
Therefore, we have used as starting materials for azo-coupling
reactions the pyridines 1 and 2 which were generated in high yields
from the corresponding pyranylium perchlorates [14]. The reaction
routes are shown in Scheme 2.
The diazonium salts were generated in situ from the corre-
sponding amine using equimolecular amounts of sodium azotite in
acidic media. According with the used amine, hydrochloric acid of
various concentrations has been used: 5 M aqueous solution for
anilines and concentrated acid (w32%, w10 M) for 4-phenylazo-
phenylamines. The diazonium salts reacted with a solution of
azulenylpyridine 1 or 2 in methanol, in the presence of potassium
acetate. The electrophilicity of the diazonium salts increases in the
following order of the starting amines: aniline < 4-phenylazo-
phenylamine < 4-(4-nitro-phenylazo)-phenylamine < 4-nitro-
aniline. Therefore, the coupling yield increases in the same order
(Scheme 2) generating the azo derivatives 3 < 7 < 9 and 5,
respectively. Substitution of the azulenyl moiety with electron
releasing group, i. e. methyl groups, generally enhances the
nucleophilicity of the azulenyl system, the diazo-coupling reaction
occurring in higher yields. A peculiar exception was observed in the
case of compound 10 which was generated in low yield.
l
(log
3
): 250 (4.58), 320 (4.42), 353 (sh), 478 (4.57) nm. 1H NMR
2.46 (s, 3H, 80-Me), 2.57 (s, 6H, 2-Me, 6-Me), 2.63 (s, 3H,
(CDCl3):
d
60-Me), 3.41 (s, 3H, 40-Me), 7.02 (s, 2H, 3-H, 5-H), 7.14 (s, 1H, 50-H),
7.35 (s, 1H, 70-H), 7.45-7.54 (m, 3H, 3000-H, 4000-H, 5000-H), 7.93-(d, 2H,
J ¼ 7.6 Hz, 2000-H, 6000-H), 7.95 (d, 2H, J ¼ 9.6 Hz, 300-H, 500-H), 8.04 (d,
2H, J ¼ 9.6 Hz, 200-H, 600-H) 8.05 (s, 1H, 20-H) ppm; 13C NMR (CDCl3):
d
24.5 (Me-2,6), 28.3 (Me-60), 29.6 (Me-40), 30.5 (Me-80), 121.6 (CH-
3,5), 122.4 (Cq), 122.9 (CH-2000,6000), 123.1 (CH-300,500), 123.9 (CH-
200,600), 125.1 (Cq), 125.6 (CH-20), 129.1 (CH-4000), 129.8 (CH-3000,5000),
131.0 (Cq), 132.1 (Cq), 133.1 (CH-50),134.0 (CH-70),135.8 (Cq), 137.1
(Cq), 146.3 (Cq), 148.1 (Cq), 149.0 (Cq), 149.9 (Cq), 150.1 (Cq), 152.3
(Cq), 152.9 (Cq), 155.6 (Cq), 157.0 (Cq) ppm. ESI-MS: m/z (%) ¼ 484
(100) [Mþ1]. Anal. Calcd. for C29H29N5 (483.61): C, 79.47; H, 6.04;
N, 14.48; found: C, 79.44; H, 6.08; N, 14.52.
2.2.2.3. (E)-[3-(2,6-dimethyl-pyridin-4-yl)azulen-1-yl]-[4-[(Z)-(4-ni-
trophenyl)azo]phenyl]diazene, (9). This compound was synthesized
from 4-azulen-1-yl-2,6-dimethyl-pyridine (233 mg, 1.0 mmol) with
diazonium salt of 4-(4-nitro-phenylazo)-phenylamine (242 mg,
1.0 mmol) following the general procedure. It was obtained as dark
violet crystals; mp 294e295 ꢁC. UVeVis (MeOH),
(4.41), 306 (4.44), 356 (4.33), 500 (4.48) nm. 1H NMR (CDCl3):
l
(log
3
): 236
d
2.65
(s, 6H, 2-Me, 6-Me), 7.27 (s, 2H, 3-H, 5-H), 7.46 (t, 1H, J ¼ 9.8 Hz, 50-
H), 7.59 (t, 1H, J ¼ 9.6 Hz, 70-H), 7.87 (t, 1H, J ¼ 10.0 Hz, 60-H), 8.08
(dAB, 2H, J ¼ 9.2 Hz, 2000-H, 6000-H), 8.15 (dAB, 2H, J ¼ 8.8 Hz, 200-H, 300-H
or 500-H, 600-H), 8.16 (dAB, 2H, J ¼ 8.8 Hz, 500-H, 600-H or 200-H, 300-H),
8.41 (dABt, 2H, J ¼ 9.2 Hz and 2.0 Hz, 3000-H, 5000-H), 8.45 (s, 1H, 20-H),
8.61 (d, 1H, J ¼ 9.6 Hz, 40-H), 9.47 (d, 1H, J ¼ 9.2 Hz, 80-H) ppm; 13
C
NMR (CDCl3):
d
24.5 (Me-2,6), 121.0 (CH-3,5), 123.2 (CH-2000,6000),
123.5 (CH-200,600), 124.6 (CH-300,500), 124.7 (CH-20), 124.8 (CH-3000,5000),
128.1 (CH-50), 128.2 (CH-70), 136.4 (CH-80), 137.3 (CH-40), 140.5 (Cq),
140.9 (CH-60),141.1 (Cq),143.6 (Cq),148.7 (Cq),152.5 (Cq),155.9 (Cq),
156.3 (Cq),158.0 (Cq) ppm. ESI-MS: m/z (%) ¼ 487 (100) [Mþ1]. Anal.
Calcd. for C29H22N6O2 (486.52): C, 71.59; H, 4.56; N, 17.27; found: C,
71.62; H, 4.54; N, 17.24.
3.2. 1H-NMR spectra
The comparison between the d values of the heterocycle protons
at C3(5) in the starting azulenylpyridines and in the herein
described compounds shows that the introduction of the azophenyl
group has no influence upon these protons. The azo group is too far
away to have inductive influence and moreover, the conjugation is
interrupt by a sequence of two single bonds. At the same time, the
substitution of the azulenyl moiety with the 4-pyridyl group has no
influence on the phenyl or phenylen protons. Instead, the azulenyl
protons are strongly deshielded in both series of synthesized azo-
compounds. These protons are influenced by both the pyridine ring
and the azo-aryl moiety. Table 1 summarizes the chemical shifts of
the azulenyl protons in the herein described azo compound, as
compared with those in the starting azulenylpyridine 1 and azo-
azulene compounds [6,18]. As it results from Table 1, the 20-H
proton is downfield shifted in compound 3 as compared with the
same proton in the parent compound 1. The deshielding effect of
the 40He60-H protons increases slowly in the order: compound
2.2.2.4. (E)-[3-(2,6-dimethyl-pyridin-4-yl)-4,6,8-trimethyl-azulen-1-
yl]-[4-[(Z)-(4-nitrophenyl)azo]phenyl], (10). This compound was
synthesized from 2,6-dimethyl-4-(4,6,8-trimethyl-azulen-1-yl)-
pyridine (275 mg, 1.0 mmol) with diazonium salt of 4-(4-nitro-
phenylazo)-phenylamine (242 mg, 1.0 mmol) following the general
procedure. It was obtained as dark violet crystals; mp 276e277 ꢁC.
UVeVis (MeOH),
l (log 3): 250 (4.58), 320 (4.42), 353 (sh), 478
(4.57) nm. 1H NMR (CDCl3): 2.48 (s, 3H, 60-Me), 2.59 (s, 6H, 2-Me,
d
6-Me), 2.67 (s, 3H, 40-Me), 3.44 (s, 3H, 80-Me), 7.04 (s, 2H, 3-H, 5-H),
7.20 (s, 1H, 50-H), 7.41 (s, 1H, 70-H), 7.98 (dAB, 2H, J ¼ 8.4 Hz, 2000-H,
6
000-H), 8.03 (dAB, 2H, J ¼ 8.8 Hz, 200-H, 600-H), 8.07 (s, 1H, 20-H), 8.10
(dAB, 2H, J ¼ 8.4 Hz, 300-H, 500-H), 8.38 (dAB, 2H, J ¼ 9.2 Hz, 3000-H, 5000
-

A.C. Razus et al. / Dyes and Pigments 91 (2011) 55e61
59
Table 1
Azulenyl protons (d, ppm).
Entry
Compound
Protons
20
40
50
60
70
80
1
2
3
4
5
6
1
7.95
8.30
8.44
8.39
8.35
8.39
8.30
8.39
8.59
8.61
8.33
8.54
7.15
7.37
7.39
7.49
7.34
7.37
7.57
7.80
7.82
7.90
7.75
7.79
7.17
7.45
7.51
7.62
7.51
7.49
8.52
9.36
9.44
9.43
9.36
9.40
Az(1)N2Pha
3
5
4-PhN2PhN2Azb
7
a
Reference [5].
Reference [17].
b
1 < Az(1)N2Ph (entry 2) < compound 3. However, the shift toward
a lower field is more pronounced for the 70-H proton and especially
for the 80-H proton which is dramatically deshielded. It is notable
that, close value of d
for the 70-H and 80-H protons were found for Az
(1)N2Ph and compound 3. These results indicate a reduced con-
jugative contribution of the pyridine and phenyl-azo moieties upon
the chemical shifts of the 40-He60-H azulenyl protons. The high
deshielding of the 20-H, 70-H and especially 80-H protons for Az(1)
N2Ph [18] and compound 3 can be attributed to the anisotropy of
the magnetic field generated by the phenylazo group. Introduction
of an electron-withdrawing group in the phenylazo fragment,
compound 5, causes a very small shielding effect of the 20-H proton
as compared with compound 3. From Table 1 it also results the
similarity between the
d values of the protons belonging to the 1-
azulenyl-2-[4-(phenyldiazenyl)phenyl]diazenes (entry 5) and the
herein described compounds with 4-pyridyl attached to the azu-
lene, for example compound 7. One might also expect a little
influence of the magnetic field of the pyridine ring on the protons
Fig. 1. Bathochromic shift of the visible absorption maxima owing to the methyl
substitution of the azulenyl moiety (up) and the influence of the para-nitro substitu-
tion of the phenyl ring, respectively (down).
20-H, 70-H and 80-H which differentiates the
d values of the
compounds 1 and 3 from previously described Az(1)N2Ph and 4-
PhN2PhN2Az, respectively.
picture). However, the attachment of a nitro group in the para-
position of the benzene ring, compound 3 toward 5 and 7 toward 9
causes a bathochromic shift of around 45 nm for the first pair of
compounds and 29 nm for the last one (Fig. 1, down picture).
Furthermore, a bathochromic shift is also observed when the para-
phenyl-azo chromophore is inserted in the molecule. Thus, passing
from compounds 3e6 to 7e10 the Dlmax values exceed 30 nm.
The structural analysis of compound 3 (Scheme 3) reveals that,
beside the structures A and C, where C is also present in Az(1)N2Ph,
the herein described compound allows the conjugation between
3.3. Electronic behavior
The absorption maxima of the low energy charge transfer
transition, lmax, for the compound 3e10, recorded in different
solvents, are reported in the Table 2. As expected, the lmax for the
parent compounds, 1 and 2, (369 nm and 350 nm) [14] lies lower
than that of the corresponding compounds with the phenyl-azo
chromophores, 3 and 4 (425 nm and 433 nm). At the same time,
substitution of the azulene in Az(1)N2Ph and 4-PhN2PhN2Az with
the 4-(2,6-dimethyl-pyridil) moiety, as in compounds 3 and 7,
enhances the values of lmax with 10 nm [19] and 45 nm [18],
respectively.
the p-electronic system of the azulenyl and heterocycle moieties as
in structure B. Both structures B and C contribute to the extension
of the electronic conjugation and produces the bathochromic shift
of the lmax in the case of compound 3 as compared with Az(1)N2Ph.
The conjugation is more extended in the case of the second azo
group and this is reflected by the shift of the lmax to higher
wavelength in compounds 7e10 versus compounds 3e6. The strong
bathochromic shift observed in compounds with a nitro group 5, 6,
9 and 10 is explained by the high contribution of the structure of
type D.
As it results from Table 2, the bathochromic shift increases in
series 3 < 4 < 5 < 6 and 7 < 8 < 9 < 10. In methanol solution, the
Dlmax resulted by methyl substitution of the azulenyl moiety,
compound 3 toward 4 and 7 toward 8, is under 10 nm (Fig. 1, upper
The MOPAC calculations for HOMO and LUMO orbital energy
seem to confirm the observed experimental results. Thus, by
Table 2
Absorption maxima (lmax in nm) in the visible region in solvents of different
polarities.
Table 3
Solvent
3
4
5
6
7
8
9
10
MOPAC calculations.
Hexane
Dioxane
Ethyl acetate
Acetonitrile
Acetone
DMF
Methanol
Chloroform
424
430
427
428
428
432
425
431
432
436
434
434
434
438
433
437
460
443
443
447
476
456
470
481
470
450
450
453
481
461
477
490
467
449
446
448
475
459
471
479
477
458
455
451
485
465
478
491
e
e
505
501
498
501
514
500
511
518
515
508
516
531
512
529
Compounds
3
4
5
6
7
8
9
10
HOMO (eV)
LUMO (eV)
ꢀ8.21 ꢀ7.99 ꢀ8.53 ꢀ8.26 ꢀ8.02 ꢀ7.90 ꢀ8.14 ꢀ8.07
ꢀ1.40 ꢀ1.33 ꢀ1.81 ꢀ1.76 ꢀ1.49 ꢀ1.40 ꢀ1.80 ꢀ1.77
D
HOMO/LUMOa 6.81
Absolute values.
6.66
6.72
6.50
6.53
6.50
6.34
6.30
a

60
A.C. Razus et al. / Dyes and Pigments 91 (2011) 55e61
0.001
0.001
0
4
3
0
-0.001
-0.001
-1.5
E (V)
-2.5
-0.5
-2.5
-1.5
E (V)
-0.5
0.001
0.0005
5
6
0
0
-0.001
-2.5
-0.0005
-1.5
E (V)
-0.5
-2.5
-1.5
E (V)
-0.5
Fig. 2. Cyclic voltammograms showing the reduction potentials for compounds 3e6 (2 mM) in acetonitrile containing Bu4NClO4 (0.1 M) as supporting electrolyte.
extension of the electronic conjugation, the LUMO energy
decreases, and in turn reduces the HOMO-LUMO gap (Table 3) and
moves the absorption band at higher wavelengths. This fact is
better reflected by the substitution with nitro groups.
Besides the structurally characteristics, for technical applica-
tions, the compounds must possess also other properties. Thus, in
order to be used as NLO-materials, the compounds should exhibit
good hyperpolarizability reflected by efficient solvatochromism.
Several classes of azulenyl-containing compounds were already
studied from this point of view [6,7]. Therefore, we were also
interested on the solvatochromic behavior of the obtained
compounds. Table 2 compares the values of lmax for the mono- and
bis-azo azulenylpyridines in solvents of different polarities. Slight
effects on the absorption bands are to be seen between the solu-
tions of the dyes 3e8, which is consistent with the complex
structure of the studied compounds. Thus, the influence of the
solvent on the electronic molecular system is reduced due the
presence of two moieties which can act as pushepull systems (see
Scheme 3).
The redox behavior of the compounds represents another
important feature toward mutual donoreacceptor electronic
influence in the herein described compounds. With the aim to
0.0025
0.005
0
0
0
2
3
0
1
2
3
1
E (V)
E (V)
0.001
0.0025
0
0
0
1
2
3
0
1
2
3
E (V)
E (V)
Fig. 3. Cyclic voltammograms showing the oxidation potentials for compounds 3e6 (2 mM) in acetonitrile containing Bu4NClO4 (0.1 M) as supporting electrolyte.

A.C. Razus et al. / Dyes and Pigments 91 (2011) 55e61
61
Table 4
polarization, the azulenylpyridine diazene exhibit high oxidation
potential.
Values of reduction and oxidation peak potentials (V) obtained by cyclic
voltammetry.
Compound
Ered
Eox
Acknowledgment
Az(1)N2Ph
ꢀ1.54, ꢀ1.67
0.62
0.91
0.63
1.00
0.96
3
4
5
6
ꢀ1.54, ꢀ1.87
S. Nica gratefully acknowledges the financial support of the
European Social Funde“Cristofor I. Simionescu” Postdoctoral
Fellowship Programme (ID POSDRU/89/1.5/S/55216), Sectoral Oper-
ational Programme Human Resources Development 2007e2013.
ꢀ1.71, ꢀ2.06
ꢀ1.12, ꢀ1.55, ꢀ1.70
ꢀ1.23, ꢀ1.58, ꢀ1.91
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compounds was studied on 2 mM acetonitrile solutions containing
0.1 M tetrabutyl ammonium perchlorate as supporting electrolyte.
The obtained values for the reduction and oxidation potentials are
summarized in Table 4.
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4. Conclusions
Mono- and bis-azo azulenylpyridines were synthesized in good
to moderate yields by diazo-coupling reaction of the correspond-
ing azulenylpyridines with diazonium salts. The azo-coupling was
facilitated by both, inductive effect of the grafted methyl groups of
the azulenyl moiety and the conjugation effect of the nitro
substituent with the aromatic ring in the diazonium intermediate.
The structure of the compounds was assigned by spectroscopic
and elemental analysis. Several considerations on the influence of
the structure of the azo-dyes and the NMR and UVeVis spectra
were made. Thus, it was established that the azoic substituent has
a higher participation in the electronic conjugation as compared
with the contribution of the heterocycle. Due to the charge delo-
calization by substituting the azulenyl moiety in both 10 and 30
positions, an important deshielding of the azulenyl protons in both
series of azo-compounds was observed showing a strong conju-
gation mainly along the azo-bridges. The solvatochromic behavior
of both mono- and bis-azo diazene showed that by changing the
solvent polarity, the lmax value is less influenced. Owing to higher