Microwave-assisted synthesis and photochromic properties of new azo-imidazoles

Article abstract of DOI:10.1016/j.dyepig.2017.04.053

A convenient, one-pot three-component synthesis of new photochromic azo-imidazoles (1a-1h) from the corresponding azo dyes (2a-2h), benzil and ammonium acetate under microwave irradiation were described and their photochromic properties were investigated.

Full text of DOI:10.1016/j.dyepig.2017.04.053

Accepted Manuscript
Microwave-assisted synthesis and photochromic properties of new azo-imidazoles
Nosrat O. Mahmoodi, Somayyeh Rahimi, Meysam Pasandideh Nadamani
PII:
S0143-7208(17)30237-1
10.1016/j.dyepig.2017.04.053
DYPI 5950
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 7 February 2017
Revised Date: 23 April 2017
Accepted Date: 25 April 2017
Please cite this article as: Mahmoodi NO, Rahimi S, Pasandideh Nadamani M, Microwave-assisted
synthesis and photochromic properties of new azo-imidazoles, Dyes and Pigments (2017), doi: 10.1016/
j.dyepig.2017.04.053.
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ACCEPTED MANUSCRIPT
Microwave-assisted synthesis and photochromic properties of new azo-imidazoles
Nosrat O. Mahmoodi , Somayyeh Rahimi, Meysam Pasandideh Nadamani
Department of Chemistry, Faculty of Science, University of Guilan, P.O. Box 41335-1914, Rasht, Iran
Abstract
A convenient, one-pot three-component synthesis of new photochromic azo-imidazoles (1a-
1h) from the corresponding azo dyes (2a-2h), benzil and ammonium acetate under
microwave irradiation were described and their photochromic properties were investigated.
The structures of the new azo-imidazoles were confirmed by UV-Vis, FT-IR and NMR
spectroscopies and elemental analysis. The properties and photochromic structural behavior
relationship (PSBR) of these compounds have been analyzed. All the compounds exhibit
good photochromic behavior upon irradiation with alternative UV and visible light.
Keywords: Azo-imidazole, Azo dyes, Imidazole, Microwave, Photochromism.
1. Introduction
Photochromic compounds have attracted considerable attention due to their extensive
application in display and anti-counterfeiting systems, optical storage and memory devices,
and optical molecular switches [1–9]. Hence, in the past decade, a large number of
photochromic compounds with different photoresponsive units have been synthesized and
their properties have been investigated [10–13]. Azo dyes are an important and large class of
synthetic organic colorants used in many practical applications [14,15]. The photochromism
and thermochromism of aromatic azo dyes make them suitable as optical sensors, nonlinear
optical chromophores and molecular memory storage [16–20]. A representative family of
azobenzenes photochromic molecules, induced by alternating irradiation with ultraviolet and
* Corresponding author. Tel.: +981313233262; fax: +981313233262.
E-mail address: mahmoodi@guilan.ac.ir, nosmahmoodi@gmail.com (N. O. Mahmoodi).
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blue lights, conducting E to Z and vice versa photoisomerization via azo π–π* and azo n–π*
excitations, respectively [21]. Regarding synthesis of heterocyclic azo dyes with thiophene,
pyrrole and azoles, several reports recently have been published. These azo dyes were used as
optical switching, second harmonic generation, chemo-sensing, organic sensitized solar cells
and memories [22–24].
The imidazole unit is a key building block that has been widely used in medicine, ionic
liquids, anion sensors, as well as electronic and optical materials [25–29]. Hence, it convinces
us to incorporate an imidazole unit into an azo dye, it is anticipated that the azo-imidazoles
with excellent photochromic properties will be obtained.
Because of the advantages of short reaction time, high efficiency, improving reaction yields,
and reducing thermal degradation byproducts, microwave-assisted synthesis has been
successfully and routinely applied in organic and medicinal chemistry [30-33]. In
continuation of our interest in the development of photochromic compounds [34–39] here, for
the first time we report the coupling of the photoactive azo aldehydes (2a-2h), benzil and
NH4OAc in convenient, one-pot three-component synthesis under microwave irradiation to
azo-imidazoles (Scheme 1) and investigate their PSBR. The results indicate that upon
irradiation of 1a-1h with UV/Vis light they can be easily isomerized to their corresponding
photoisomers.
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Scheme 1. General synthetic route for synthesis of azo-imidazoles
2. Experimental
2.1. Materials and apparatus
All reagents were purchased from Fluka, Merck and Aldrich and used without further
purification. Azo dyes precursors, 2a-2h, were prepared according to the well-known
literature procedure [40]. Melting points were measured by electrothermal 9100s apparatus
and were uncorrected. FT-IR spectra were recorded on a Bruker Vecter 22 in the region of
1
400–4000 cm−1 in KBr pellets. The HNMR and 13C NMR spectra were obtained from
solution DMSO-d6 and CDCl3 with TMS as an internal standard Bruker Avance III (400
MHz) spectrometer. The absorption spectra of azo-imidazoles were measured by Shimadzu
UV-2100 spectrophotometer in the range 200–800 nm (c = 2 × 10−4 M, cell path length 1
cm). Spectral data were obtained using quartz cuvettes with 1 cm optical path length before
and after UV irradiation for various times. A light at 254 nm from a low-pressure Hg lamp
was used for the photoisomerization from the trans-enol photoisomer to the cis-keto
photoisomer. The CHN analyses were performed on a Vario-ELIII elemental analyzer. High
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resolution mass spectra (HRMS) were obtained on a Finnigan MAT 95, EI: 70 eV, R:10000.
Microwave-assisted reactions were carried out on a commercially available monomode
microwave unit (CEM Discover) in a 25-mL round-bottom flask. The machine consists of a
continuous focused microwave power delivery system with operator selectable power output
from 0 to 300 W. Reactions were performed in a 25-mL round-bottom flask. The temperature
of the contents of the vessel was monitored using an IR sensor located underneath the
reaction vessel. Primary microwave irradiation of 230 W was used, the temperature being
ramped from rt to reflux.
2.2 Synthesis
2.2.1. General procedure for the synthesis of azo-imidazoles 1a-1h
Azo day 2a-2h (1 mmol), benzil (0.210 g, 1 mmol) and NH4OAc (0.310 g, 4 mmol) in AcOH
glacial (4 ml) were heated in a stirred microwave open-vessel (230 W) for appropriate time
1.5-2.5 min see Table 2. The progress of the reaction was monitored by TLC (eluent: n-
hexane:EtOAc, 6:2). After completion of the reaction distilled H2O was added to the mixture
and the precipitated solid was collected by filtration, and recrystallized from EtOH or
EtOH/CHCl3 to give azo-imidazoles 1a-1h in high yields 82-96% see Table 2.
2.3. Spectral data for the synthesis of azo-imidazoles
2.3.1. 2-(4,5-Diphenyl-1H-imidazol-2-yl)-4-(phenyldiazenyl)phenol (1a)
Yield: 87%, mp 191-192 °C, orange crystal; FT-IR (KBr, υ/cm-1): 3059 (aromatic C–H
stretch), 1603 (C=N stretch), 1496 (N=N stretch), 1439, 1383, 1269 (C–O stretch), 767, 693
(aromatic out of plane bend). 1H NMR (400 MHz, DMSO-d6) δ (ppm): 7.20 (d, J = 8.8 Hz, 1H),
7.45 (br, 6H), 7.54-7.64 (m, 7H), 7.87-7.92 (m, 3H), 8.79 (d, J = 2.4 Hz, 1H), 13.46 (br, 1H), 13.79
(br, 1H). 13C NMR (100 MHz, DMSO-d6) δ (ppm): 113.7, 118.4, 121.8, 122.6, 124.3, 126.9-129.7
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(br, 3 arom C), 127.9, 129.2, 129.9, 131.3, 145.5, 145.6, 152.6 and 160.3. Anal. Calcd. For
C27H20N4O: C, 77.87; H, 4.84; N, 13.45%. Found: C, 77.91; H, 4.92; N, 13.43%. HRMS: m/z
for C27H20N4O; calculated 416.1574; found: 416.1576.
2.3.2. 2-(4,5-Diphenyl-1H-imidazol-2-yl)-4-(p-tolyldiazenyl)phenol (1b)
Yield: 88%, mp 198-200 °C, orange powder; FT-IR (KBr, υ/cm-1): 3236 (N–H stretch), 3029
(aromatic C–H stretch), 1598 (C=N stretch), 1491 (N=N stretch), 1440, 1383, 1278 (C–O
stretch), 825, 764, 694 (aromatic out of plane bend). 1H NMR (400 MHz, DMSO-d6) δ (ppm):
2.43 (s, 3H), 7.19 (d, J = 8.8 Hz, 1H), 7.42 (d, J = 8.0 Hz, 4H), 7.43 (br, 5H), 7.58 (d, J = 7.2 Hz, 4H),
7.80 (d, J = 8.4 Hz, 2H), 7.88 (dd, J = 8.8, 2.4 Hz, 1H), 8.76 (d, J = 2.4 Hz, 1H), 13.40 (br, 1H), 13.73
(br, 1H). 13C NMR (100 MHz, DMSO-d6) δ (ppm): 21.5, 113.7, 118.3, 121.7, 122.7, 124.0, 127-
129.5 (br, 2 arom C), 127.5, 128.2, 129.1, 130.4, 141.4, 145.6, 145.7, 150.7 and 160.1. Anal. Calcd.
For C28H22N4O: C, 78.12; H, 5.15; N, 13.01%. Found: C, 78.17; H, 5.21; N, 12.97%. HRMS:
m/z for C28H22N4O; calculated 430.1781; found: 430.1786.
2.3.3. 4-((4-Chlorophenyl) diazenyl)-2-(4, 5-diphenyl-1H-imidazol-2-yl)phenol (1c)
Yield: 82%, mp 215-217 °C, dark orange crystal; FT-IR (KBr, υ/cm-1): 1605 (C=N stretch),
1490 (N=N stretch), 1441, 1384, 1274 (C–O stretch), 832, 766, 694 (aromatic out of plane
bend). 1H NMR (400 MHz, DMSO-d6) δ (ppm): 7.20 (d, J = 8.8 Hz, 2H), 7.41-7.45 (br, 6H), 7.57
(d, J = 7.2 Hz, 4H), 7.68 (d, J = 8.4 Hz, 2H), 7.88-7.91 (m, 3H), 8.78 (d, J = 2.4 Hz, 1H), 13.56 (br,
2H). 13C NMR (100 MHz, DMSO-d6) δ (ppm): 113.8, 118.4, 121.7, 124.3, 124.5, 127.5-129 (br, 3
arom C), 128.3, 129.2, 130.1, 135.6, 145.4, 145.5, 151.2 and 160.6. Anal. Calcd. For
C27H19N4OCl: C, 71.92; H, 4.25; N, 12.43%. Found: C, 71.88; H, 4.21; N, 12.47%. HRMS:
m/z for C27H19ClN4O; calculated 450.1209; found: 450.1214.
2.3.4. 2-(4,5-Diphenyl-1H-imidazol-2-yl)-6-methoxy-4-(phenyldiazenyl)phenol (1d)
Yield: 88%, mp 232-234 °C, golden crystal; FT-IR (KBr, υ/cm-1): 3056 (aromatic C–H
stretch), 1602 (C=N stretch), 1471 (N=N stretch), 1390, 1267 (C–O stretch), 762, 692
(aromatic out of plane bend). 1H NMR (400 MHz, DMSO-d6) δ (ppm): 3.96 (s, 3H), 7.41(br, 2H),
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7.46 (t, J = 7.2 Hz, 4H), 7.54 (d, J = 2.0 Hz, 1H), 7.55-7.64 (m, 7H),7.90 (d, J = 7.2 Hz, 2H), 8.51 (d,
J = 2.0 Hz, 1H), 13.55 (br, 2H). 13C NMR (100 MHz, CDCl3) δ (ppm): 56.3, 104.1, 112.2, 113.9,
122.6, 127-129.3 (br, 4 arom C), 127.7, 128.8, 129.2, 130.6, 145.1, 149.8, 150.7 and 152.6. Anal.
Calcd. For C28H22N4O2: C, 75.32; H, 4.97; N, 12.55%. Found: C, 75.38; H, 5.01; N, 12.51%.
HRMS: m/z for C28H22N4O2; calculated 446.1692; found: 416.1695.
2.3.5. 2-(4,5-Diphenyl-1H-imidazol-2-yl)-6-methoxy-4-((3-nitrophenyl)diazenyl)phenol (1e)
Yield: 96%, mp 248-250 °C, golden crystal; FT-IR (KBr, υ/cm-1): 1622 (C=N stretch), 1522
(NO2 asymmetric stretch), 1463, 1386, 1345 (NO2 symmetric stretch), 1273 (C–O stretch), 1H
NMR (400 MHz, DMSO-d6) δ (ppm): 3.97 (s, 3H), 7.40 (t, J = 7.4 Hz, 2H), 7.47 (t, J = 7.4 Hz, 4H),
7.58-7.60 (m, 5H), 7.94 (t, J = 8.0 Hz, 1H), 8.39 (t, J = 7.8 Hz, 2H), 8.59 (d, J = 2.0 Hz, 1H), 8.63 (d,
J = 1.6 Hz, 1H). Anal. Calcd. For C28H21N5O4: C, 68.42; H, 4.31; N, 14.25%. Found: C, 68.38;
H, 4.29; N, 14.23%. HRMS: m/z for C28H21N5O4; calculated 491.1624; found: 491.1627.
2.3.6. 2-(4,5-Diphenyl-1H-imidazol-2-yl)-6-methoxy-4-(o-tolyldiazenyl)phenol (1f)
Yield: 95%, mp 277-278 °C, orange-red crystal; FT-IR (KBr, υ/cm-1): 3286 (N–H stretch),
3058 (aromatic C–H stretch), 1603 (C=N stretch), 1478 (N=N stretch), 1450, 1388, 1274 (C–
O stretch), 849, 759, 696 (aromatic out of plane bend). 1H NMR (400 MHz, DMSO-d6) δ
(ppm): 2.73 (s, 3H), 3.95 (s, 3H), 7.33-7.46 (br m, 9H), 7.52 (d, J = 2.0 Hz, 1H), 7.58 (d, J = 6.8 Hz,
13
5H), 8.50 (d, J = 2.0 Hz, 1H), 13.48 (br, 2H). C NMR (100 MHz, DMSO-d6) δ (ppm): 17.7, 56.1,
102.3, 113.0, 115.5, 117.9, 126.5-129.5 (br, 3 arom C), 127.1, 128.3, 129.1, 131.0, 131.9, 137.5,
145.5, 145.8, 149.8, 150.5 and 150.8. Anal. Calcd. For C29H24N4O2: C, 75.63; H, 5.25; N,
12.17%. Found: C, 75.68; H, 5.30; N, 12.14%. HRMS: m/z for C29H24N4O2; calculated
460.1884; found: 460.1889.
2.3.7. 2-(4,5-Diphenyl-1H-imidazol-2-yl)-6-methoxy-4-((4-nitrophenyl)diazenyl)phenol (1g)
Yield: 96%, mp 250-251 °C, dark red crystal; FT-IR (KBr, υ/cm-1): 1615 (C=N stretch), 1515
(NO2 asymmetric stretch), 1384, 1336 (NO2 symmetric stretch), 1251 (C–O stretch), 852,
764, 693 (aromatic out of plane bend). 1H NMR (400 MHz, DMSO-d6) δ (ppm): 3.97 (s, 3H),
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7.40 (t, J = 7.2 Hz, 2H), 7.46 (t, J = 7.2 Hz, 4H), 7.57-7.59 (m, 5H), 8.07 (d, J = 8.8 Hz, 2H), 8.47 (d,
J = 9.2 Hz, 2H), 8.61 (s, 1H), 13.84 (br, 2H). Anal. Calcd. For C28H21N5O4: C, 68.42; H, 4.31; N,
14.25%. Found: C, 68.47; H, 4.42; N, 14.28%. HRMS: m/z for C28H21N5O4; calculated
491.1624; found: 491.1626.
2.3.8. 2-(4,5-Diphenyl-1H-imidazol-2-yl)-4-((4-fluorophenyl)diazenyl)-6-methoxyphenol (1h)
Yield: 92%, mp 230-232 °C, golden crystal; FT-IR (KBr, υ/cm-1): 1599 (C=N stretch), 1465,
1387, 1271 (C–O stretch), 842, 766, 695 (aromatic out of plane bend). 1H NMR (400 MHz,
DMSO-d6) δ (ppm): 3.95 (s, 3H), 7.41 (br, 2H), 7.43-7.48 (m, 6H), 7.52 (d, J = 2.4 Hz, 1H), 7.58 (d,
13
J = 7.2 Hz, 4H), 7.96 (dd, J = 9.0, 5.4 Hz, 2H), 8.49 (d, J = 2.0 Hz, 1H), 13.61 (br, 2H). C NMR
(100 MHz, DMSO-d6) δ (ppm): 56.3, 102.4, 102.8, 113.1, 116.8, 117.0, 117.4, 124.8, 124.9, 128.3,
127.5-129.5 (br, 1 arom C), 129.2, 144.8, 145.8, 149.3, 149.9 and 162.6. Anal. Calcd. For
C28H21N4O2F: C, 72.40; H, 4.56; N, 12.06%. Found: C, 72.47; H, 4.61; N, 12.05%. HRMS:
m/z for C28H21FN4O2; calculated 464.1593; found: 464.1599.
3. Results and discussion
In continuation to our prior works [34–39], herein, o-vanillin or salicylaldehyde in the first
step was coupled with the diazonium chloride obtained from aniline derivatives to give azo
dyes 2a-2h. In the next efforts, the azo-imidazoles 1a-1h were synthesized via three-
component reactions between benzil, NH4OAc and azo dye precursor under microwave
irradiation in the presence of AcOH glacial as solvent and organocatalyst in short reaction
times, as shown in Scheme 1 and Table 2. The prepared azo-imidazoles 1a-1h exhibited
photoisomerization conversion in the CHCl3 solution.
To optimize reaction condition for the synthesis of azo-imidazoles, the one-pot three-
component reaction of azo dye 2a (1 mmol), benzil (1 mmol) and NH4OAc (4 mmol) was
selected as the model reaction. Using different types of catalysts e.g. under reflux (entries 1-6
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in Table 1) and microwave (entry 7 in Table 1) conditions the reaction proceeded slowly.
When reaction performed in the presence of AcOH glacial as the solvent and catalyst under
reflux condition afforded the azo-imidazole 1a in good yield (Entry 8). When the reaction
was run in the presence of AcOH glacial under microwave irradiation the best result was
obtained (entry 9). The goal of a reaction optimization is to select the best inputs to achieve, a
shorter reaction time, high yield and simple workup procedure as shown in entry 9.
Table 1. Optimization of the synthesis of 1a
Catalyst
(10 mol%)
Time
(min)
Entry
Solvent
Condition
Yield%
15
1
2
EtOH
EtOH
ZnCl2
KSF
Reflux
Reflux
720
720
44
30
40
10
10
3
4
6
7
EtOH
CHCl3
EtOH
EtOH
KSF@Zn
KSF@Zn
p-TSA
Reflux
Reflux
720
720
720
30
Reflux
NiCl2
MW (230W)
8
AcOH
-
Reflux
360
82
9
AcOH
-
MW (230W)
2
87
In order to evaluate the generality of the process, different azo dyes 2a-2h were used in the
synthesis of azo-imidazoles 1a-1h under optimized reaction condition (Table 2). As shown,
in Table 2 the expected products were obtained in short reaction time (1.5–2.5 min) with high
yields without formation of any byproducts. The structures of prepared compounds were
confirmed by analyzing their spectral characteristics.
The FT-IR spectra of 1a-1h indicate the absence of C=O stretching vibration of azo aldehyde
precursors 2a-2h and benzil and appearance of a new C=N absorption band in the range of
1598-1622 cm-1 confirm the formation of imidazole ring.
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Table 2. Microwave-assisted synthetic of photochromic azo-imidazoles 1a-1h
Product
Color
Recrystallization
mp (°C)
Yield%
87
Time (min)
orange
orange
2
EtOH
191-192
198-200
1a
1b
2.5
EtOH/CHCl3
88
82
88
96
95
96
92
dark orange
golden
2
2.5
1.5
2
EtOH
EtOH
215-217
232-234
248-250
277-278
250-251
230-232
1c
1d
1e
1f
golden
EtOH/CHCl3
EtOH/CHCl3
EtOH
orange red
dark red
golden
2
1g
1h
1.5
EtOH/CHCl3
Primarily AcOH glacial activated the carbonyl groups of either benzil or aldehyde to
facilitate a nucleophilic attack of the nitrogen of NH3 on the activated carbonyl group 2a to
produce intermediate 5. Intermediate 5 in condensation with benzyl forms intermediate 6,
which in turn rearrange to the azo-imidazole 1a via [1,5]-H shift (Scheme 2).
Scheme 2. Proposed mechanism for the synthesis of azo-imidazoles 1a-1h
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Photoisomerization of the azo-imidazoles 1a-1h in CHCl3 upon irradiation with 254 nm UV
light were studied. All the compounds exhibit good photochromic behavior upon irradiation
with alternative UV and visible light (See supporting data). The UV-Vis spectra of 1a
varying with the irradiation time and given in Fig. 1. Before UV irradiation the weak band at
244 nm and the strong band at 324 nm were observed. After UV-light irradiation, the
absorption intensity at 324 nm gradually was decreased and a new absorption band at 537 nm
was appeared. Prior to UV irradiation, trans-enol-1a photoisomer dominates in the solution
and strong absorption band centered at 324 nm was observed. Under UV irradiation, an
intramolecular proton transfer is induced and convert a pale yellow trans-enol-1a
photoisomer to purple cis-keto-1a΄ and/or quinone-hydrazone-1a˝ photoisomers via trans-
keto-1, cis-enol-1 or both photoisomers. Beyond any reasonable doubt, the obtained
isosbestic point reveals the equilibrium establishment of 1a between trans-enol-1a and
quinone-hydrazone-1a˝ photoisomers (Scheme 3).
H
N
O
Ph
Ph
N
H
N
N
trans-keto-1
υ 2
h
O
N
O
H
H
N
H
N
H
N
O
Ph
N
Ph
Ph
N
Ph
N
H
Ph
N
H
N
N
N
N
Ph
trans-enol-1a
cis-keto-1a'
quinone-
hydrazone-
1a''
O
H
Ph
Ph
N
N
H
N
N
cis-enol-1
Scheme 3. Photochromic reaction 1a in CHCl3 solution
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All compounds were shown very high thermal back relaxation and the purple or blue color of
compounds by overnight staying in dark at room temperature returns back to initial color
(cis→trans). Repeated irradiation showed little sign of degradation at least up to 20 cycles.
The UV–Vis absorbance spectra of 1g with para nitro substitution in CHCl3, before light
irradiation, the weak band at 244 nm and broad absorption bands at 302, 405 (due to NO2
group) were observed. This photoisomer under UV irradiation, the absorption intensity at 302
and 405 nm gradually decreased and a new absorption band at 537 nm was appeared, the
color was changed from yellow to purple solution (Fig. 1, right).
2.5
UV-light (254 nm)
Irradiation time
UV-light (254 nm)
Irradiation time
2
255 s
225 s
195 s
165 s
135 s
120 s
105 s
90 s
75 s
60 s
45 s
35 s
25 s
15 s
10 s
5 s
180 s
160 s
140 s
120 s
100 s
90 s
80 s
70 s
60 s
50 s
40 s
30 s
20 s
10 s
0 s
2
1.5
1
1.5
isosbestic
1
isosbestic
isosbestic
isosbestic
0.5
0.5
0
0 s
0
230
330
430
530
630
230
330
430
530
630
Wavelength (nm)
Wavelength (nm)
Fig. 1 The UV-Vis spectra of 1a (left), 1g (right) before and after UV light irradiation for
various times in CHCl3 (c = 2.0 × 10-4 M, 293 K).
4. Conclusions
Here, we incorporate an imidazole unit into an azo dye via three-component reaction between
azo dye, benzil and NH4OAc under microwave irradiation to obtained azo-imidazoles 1a-1h.
Short reaction time, simple workup procedure and use of inexpensive condition and solvent
are advantages of the proposed procedure. All the azo-imidazoles exhibit excellent positive
photochromic behavior upon irradiation under UV light irradiation.
Acknowledgments
We thank the Research Committee of University of Guilan for their support of this study.
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Highlight
One-pot three-component synthesis of new photochromic azo-imidazoles
Properties and photochromic structural behavior relationship (PSBR)
Excellent positive photochromic behavior
azo-imidazoles synthesis under microwave irradiation
1