The synthesis of highly active thiophene ring-containing chromophore components for photonic polymers based on a newly designed route

Article abstract of DOI:10.1039/a904945b

2-Aminothiophene derivatives are the key intermediates for the present synthesis. It is known that the synthesis of 2-aminothiophene is troublesome although it is a rather simple heterocycle. In this work, an early report was newly developed as a basis for the efficient synthesis of thiophene-ring-containing chromophore components for photonic polymers. 2-Amino-5-nitrothiophene and 2-amino-3,5-dinitrothiophene were synthesized in excellent yield. After diazotization, the 2-aminothiophene derivatives were directly treated with N-phenyldiethanolamine to afford two-electron push-pull compounds. A similar styryl compound was also prepared. All of these chromophore molecules have further polymerizable hydroxy groups on one end of the molecule. These compounds are currently showing interesting potential in making highly sensitive, nonlinear optical polymeric materials. The Royal Society of Chemistry 1999.

Full text of DOI:10.1039/a904945b

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The synthesis of highly active thiophene ring-containing
chromophore components for photonic polymers based on
a newly designed route
Shen Yuquan, Zhao Yuxia, Li Zao, Wang Jianghong, Qiu Ling, Liu Shixiong, Zhai Jianfeng
and Zhou Jiayun
Institute of Photographic Chemistry, Academia Sinica, Beijing, 100101, China.
E-mail: shenyq@ipc.ac.cn
Received (in Cambridge, UK) 21st June 1999, Accepted 14th October 1999
2-Aminothiophene derivatives are the key intermediates for the present synthesis. It is known that the synthesis of 2-
aminothiophene is troublesome although it is a rather simple heterocycle. In this work, an early report was newly
developed as a basis for the efficient synthesis of thiophene-ring-containing chromophore components for photonic
polymers. 2-Amino-5-nitrothiophene and 2-amino-3,5-dinitrothiophene were synthesized in excellent yield. After
diazotization, the 2-aminothiophene derivatives were directly treated with N-phenyldiethanolamine to afford two-
electron push–pull compounds. A similar styryl compound was also prepared. All of these chromophore molecules
have further polymerizable hydroxy groups on one end of the molecule. These compounds are currently showing
interesting potential in making highly sensitive, nonlinear optical polymeric materials.
Introduction
Push–pull conjugated systems are a subject which has received
intensive attention because of their potential applications in
modern photonic technology. It is well known that lithium nio-
bate, currently the most commonly used electro-optic material,
has an electro-optic coefficient of 35 pm VϪ1. To produce
materials having optical nonlinearity comparable to or greater
than that of lithium niobate is still an essential target for
organic/polymeric materials research. The most intensively
studied rod-like organic push–pull conjugated system for phot-
onic materials consists of aromatic rings to which electron-
donating (push) and -accepting (pull) groups are attached, and
with a double bond between the aromatic rings as an electron
bridge. Theoretical investigation has concluded that hetero-
Scheme 1
aromatics, such as the thiophene ring, have an important role in
made its synthesis an inviting subject of investigation. There are
several methods for the preparation of 2-aminothiophene
derivatives. For example, as shown in Scheme 1a,6 a 2-halogeno-
thiophene (2-HT) can be converted into the corresponding
primary amino compound by reaction with bis(trimethylsilyl)-
amidocopper (BTSAC), BTSAC can be prepared in situ from
hexamethyldisilazane, n-butyllithium and copper() iodide, by
coupling 2-HT with BTSAC, to give a silyl-protected amine first
(yield 45%), and then, after alcoholysis and distillation, to
afford a 2-aminothiophene derivative.
A conventional method of preparing 2-amino-substituted
thiophenes is starting from the reaction of mercaptoacetalde-
hyde with malononitrile as shown in Scheme 1b.3
The mercaptoacetaldehyde usually is used in its dimer form
and cyanoacetic acid which is the hydrolysis product of malo-
nonitrile is believed to be the real participant in the reaction.
Control of the reaction conditions is critical in this reaction
route.
the design of efficient, second-order, nonlinear optical mole-
cules,1 and also that the planarity of an azo unit versus the
nonplanarity of stilbenes or other such systems should contrib-
ute to the larger π electron transmission effects and lead to
higher optical activity.2 DR-19 is a widely used typical example
of such chromophores. In order to have highly active core layer
organic materials for optical waveguide devices, we have there-
fore designed and synthesized three similar chromophore mole-
cules (see below) in which one of the phenyl rings was replaced
by a thiophene unit; in the synthesis, a new synthetic route
based on Steinkopf’s report3 was used for the preparation of
the thiophene ring containing an azo chromophore. Our syn-
thetic route has the advantages of good reproducibility and
quite high reaction yield. The general reaction route is shown in
Scheme 1.
Results and discussion
2-Aminothiophene derivatives 4 and 6 are the key intermediates
for the synthesis of the highly optically sensitive thiophene-
ring-containing chromophores. However, 2-aminothiophene is
only stable in an inert atmosphere below its melting point (12–
13 ЊC).5 Heating above its melting point or exposure to air will
result in immediate decomposition. The absence of a practical
method for preparing such a simple and interesting compound
Because of its instability and the very low yield of its prepar-
ation, synthetically it is ideal to avoid direct involvement of this
compound in a practical synthetic scheme. In this synthesis, as
shown in Scheme 2, first, 2-acetamidothiophene 2 instead of
2-aminothiophene was prepared. It seems that substitution
with an electron-withdrawing group on the N atom of its amine
group or at 3- and 5- position of the 2-aminothiophene greatly
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Scheme 2
changes the situation: compounds 2–6 as well as its stannous
bones, for example, for further making polymers like polyesters,
[tin()] chloride double salt are all stable enough, therefore can
be isolated, and be prepared in good yield. Compounds 4 and 6
are novel intermediates.
The presence of the hydroxy groups on the chromophore
molecules is essential for their attachment to polymer back-
polyurethanes, polyimides, and the like. Such hydroxy groups
usually are necessarily protected, by converting them into their
diacetate, for example, N,N-bis(acetoxyethyl)aniline was used
as a coupling component to react with diazonium salts for
preparing azo dyestuffs.7 Regarding the relative stability of
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Table 1 UV–visible spectral data of target compounds
N-phenyldiethanolamine was directly used for the coupling
reaction to form the target azo molecules without protection of
the hydroxy groups and deprotection afterwards. Comparing
with other methods, all the steps shown in Scheme 1 are easy to
handle and are well defined in the Experimental section of this
paper. The synthesized chromophore molecules are very useful
for making a variety of photonic polymers. The preparation of
various photonic polymers based on these chromophoric mole-
cules and their optical properties are under investigation;
details will hopefully be published later.
Å/l
DMF
λ
/
molϪ1
cmϪ1
max
nm
7
583
47 040
Experimental
Instruments employed were IR (BIO-RAD FTS-165), UV-vis
(1601PC Shimadzu), MS (Trio-2000), 1H NMR (Varian Gemini
300 operating at 300 MHz) spectrophotometers, and element
analyzer (Heraeus CHN-RAPID). NMR J-values are given in
Hz. Solutions were dried over anhydrous Na2SO4. Petroleum
spirit refers to the fraction with distillation range 60–90 ЊC.
8
671
581
500
40 060
37 010
37 030
14
2-Nitrothiophene 13
A mixture of 40 g fuming nitric acid and 300 ml of glacial acetic
acid was added dropwise to a solution of thiophene (42 g, 0.50
mol) in 170 ml of acetic anhydride at 10–15 ЊC. The reaction
mixture was stirred for 2.5 h at ambient temperature, then was
poured into 500 g of ice–water with vigorous stirring. After
filtration and drying, a yellow precipitate was obtained (52 g,
81%), mp 40–41 ЊC (lit.,3 41 ЊC); IR 1523, 1332 cmϪ1. The
product was kept in the dark for further reaction.
Disperse
Red-19
2-Acetamidothiophene 2
the hydroxy groups under the diazotizing reaction conditions,
N-phenyldiethanolamine was directly used and was successful
in our synthesis.
To 1 (24 g, 0.19 mol) dissolved in 408 ml of conc. hydrochloric
acid was added tin powder (40.8 g, 0.34 mol) in portions over a
period of 15 min with vigorous stirring at 40–45 ЊC. Ice–water
may be used when necessary for temperature control. The
mixture was kept for 30 min; after complete disappearance of
the tin powder, the reaction was allowed to continue for another
10 min, then the thus formed complex of SnCl4–aminothio-
phene hydrochloride was filtered out from the reaction mixture,
washed with ethanol–ether (1:1) briefly, and dried. The com-
plex was obtained as a gray-white solid (35 g, 71%). 50 ml of
acetic anhydride–diethyl ether (1:1) was added to 100 ml of an
aqueous solution of the above complex (35 g). At ice-bath tem-
perature, 87 g of 50% aq. sodium hydroxide was added slowly
over 15 min onto the resulting mixture under vigorous stirring,
a white precipitate appearing immediately. After the completion
of NaOH addition, 10 min of additional stirring gave a precipi-
tate, which was filtered off, recrystallized from water, decolored
by use of activated charcoal and dried under vacuum drying
box overnight (15 mmHg, room temp.). Compound 2, a white
solid, was obtained (12 g, 64%), mp 161 ЊC (lit.,3 160–161 ЊC);
The styryl-class compound 14 was obtained based on the
base-catalyzed aldol condensation reaction between the active-
hydrogen component of compound 10, and the carbonyl com-
ponent of compound 12. In this reaction, the formation of the
carbanion intermediate is a key step. Although formation of
the intermediate was not as easy as in the case with no substitu-
tion nitro groups on the thiophene ring, as reported by Jen
et al.,4 compound 10 still offered an acceptable yield of 56% in
the aldol condensation reaction. A yield of 75% was reported in
Jen’s case.
The UV-visible spectral data of 7, 8 and 14 are shown in
Table 1, together with that of Disperse Red-19 for comparison.
The relative importance of the heteroaromatic ring, the
substituent(s) on the ring, and the electron bridge towards
λmax, or in general, towards charge transfer in π-electron con-
jugated systems is clearly shown in Table 1: 8 and 14 are very
similar in their molecular structure except for the difference
in the π-electron bridge between the two rings. Interestingly,
just because of this difference, 8 showed a nearly 100 nm
redshift in λmax compared with 14, and even 7 has a comparable
λmax with 14, despite the fact that 14 has more nitro groups
substituted on its thiophene ring. Anyway, comparing 7 with
DR-19, a redshift of nearly 100 nm occurs because of the
replacement of the phenyl group in DR-19 by the thiophene
ring in 7.
IR(KBr) 1643, 1581 cmϪ1
.
2-Acetamido-5-nitrothiophene 3
Below 5 ЊC, a solution of 2 (7 g, 0.05 mol) in 350 ml of acetic
anhydride, was treated with a mixture of nitric acid (14 ml;
67%) and acetic anhydride (70 ml) over a period of 20–25 min
under vigorous stirring. The mixture was then stirred for an
additional 10 min and then was poured into 420 ml of ice–water
(stirring), followed by the addition of 1400 ml of water. A small
amount of residue was removed from the mixture by filtration.
About 420 g of solid sodium carbonate was added in portions
to the filtrate for neutralization. A yellow-grey precipitate
appeared, and was separated by filtration, washed with water,
and dried under vacuum drying box overnight (15 mmHg,
room temp.) to give 6 g of product. Another 1 g of product
could be got from the filtrate by Et2O extraction and recrystal-
lization from 50% aq. ethanol. Total yield 66%. The product
In summary, considering a moderate delocalization energy of
the thiophene ring system and consequently a higher expected
optical sensitivity, three thiophene-ring-containing push–pull
azo compounds with further polymerizable hydroxy groups
have been synthesized by a newly designed and efficient syn-
thetic route. To avoid the direct involvement of an unstable
intermediate, 2-aminothiophene, which is a simple but elusive
compound, its derivative 2-acetamidothiophene 2 was found to
be pretty stable and could be isolated for further reaction.
Meanwhile, for simplification of the preparation procedures,
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was a mixture of 2-acetamido-5-nitrothiophene 3 and 2-
acetamido-3-nitrothiophene. 2-Acetamido-3-nitrothiophene
the addition of the acid mixture has finished; then the diazo
component 6 (1.2 g, 6.35 mmol) was added slowly over a period
of 30 min. The diazotizing reaction was allowed to proceed for
another 30 min. An aqueous solution (43 ml), cooled at 0 ЊC,
containing the coupling component of N-phenyldiethanol-
amine (1.77 g, 9.78 mmol) and conc. hydrochloric acid (1.9 ml)
was then added with stirring. The diazonium reaction was
allowed to continue for another 50 min. The solid was collected
on a Büchner filter under reduced pressure, washed several
times with water, and vacuum dried for 24 h to afford 1.5 g of 8
as a deep blue solid, which was further purified by TLC, with
can be separated by vacuum sublimation at 114 ЊC under
reduced pressure (10 mmHg), mp 163–164 ЊC (lit.,3 165.5–
166.5 ЊC); IR(KBr) 1706, 1544, 1385 cmϪ1. Pure 3 could be
obtained from the remained residue by, first evaporation at
150 ЊC/10 mmHg and then recrystallization from sufficient hot
water; mp 220–222 ЊC (lit.,3 222–223 ЊC).
2-Amino-5-nitrothiophene 4
3 (1.0 g, 5.38 mmol) was deacetylated by its slow addition to 50
ml aq. of KOH (20 g) at 30 ЊC and stirring for 1 h. 25 ml of
water were added to the reaction mixture, which was then
gently neutralized with conc. hydrochloric acid at ice–water
temperature. An orange precipitate appeared immediately. The
precipitated product was collected, and purified by column
chromatography on silica gel with ethyl acetate–petroleum
spirit (2:1) as eluent. A pure yellow product was obtained (0.55
g, 71%), mp oxidized >140 ЊC; IR(KBr) 3406, 3290, 3181, 2925,
1619, 1529, 1479, 1423, 1399, 1331, 1291, 1133 cmϪ1; NMR-
(DMSO-d6): δ 6.0 (d, 1H), 7.75 (d, 1H), 8.1 (s, 2H) [Anal. (%).
Calc. for C4H4N2O2S: C, 33.33; H, 2.78; N, 19.44; S, 22.22.
Found: C, 33.56; H, 2.68; N, 19.06; S, 22.02].
DMF
silica gel as absorbent and ethyl acetate as developer, λmax
671 nm (ε 40 060); IR(KBr) 3399, 3241, 2919, 1602, 1544,
1386, 1293, 1256, 1134 cmϪ1; MS (m/z) 381 (Mϩ); 1H
NMR(DMSO-d6) δ 0.85 (s, 20H), 3.90 (t, 8H), 7.1–7.9 (m, 4H),
8.38 (s, 1H) [Anal.(%). Calc. for (C14H15N5O6S): C, 44.09; H,
3.94; N, 18.37; S, 8.40. Found: C, 44.35; H, 4.07; N, 18.07; S,
8.52].
2-Methyl-5-nitrothiophene 94
To a solution of 2-methylthiophene (15 g, 0.15 mol) in acetic
anhydride (30 g) was added dropwise a mixture of fuming nitric
acid (12 g) and acetic anhydride (24 g) under vigorous stirring at
Ϫ5 to Ϫ10 ЊC. After nitration, the reaction mixture was poured
onto 200 g of ice, neutralized with Na2CO3 powder, and
extracted with diethyl ether. The ethereal solution was dried
with anhydrous MgSO4. The residue after removal of the ether
was first purified by steam distillation, the distillate was again
extracted with diethyl ether, then 9 was separated from the the
residue, after removal of the ether, by vacuum distillation as the
fraction at 104 ЊC/15 mmHg (7 g, 32%), IR 3105, 3083, 3061,
3027, 2919, 2851, 2364, 1715, 1561, 1450, 1357, 1069, 968, 812
2-Acetamido-3,5-dinitrothiophene 5
The above obtained mixture of 2-acetamido-5-nitrothiophene 3
and 2-acetamido-3-nitrothiophene (3.8 g) was slowly added to
24 ml of fuming nitric acid with stirring at 0–5 ЊC. The nitration
mixture was kept for 5–10 min and then 120 ml of ice–water
were added. A yellow precipitate was formed and isolated by
filtration, recrystallized from ethanol, and dried over anhydrous
sodium sulfate, to give 5 (3.6 g, 76%), mp 181–182 ЊC; IR(KBr)
1713, 1549, 1349 cmϪ1; MS 231 (Mϩ).
cmϪ1
.
2-Methyl-3,5-dinitrothiophene 10
2-Amino-3,5-dinitrothiophene 6
9 (2 g, 14 mmol) was added in small portions with stirring to a
mixture of fuming nitric acid (8 g) and conc. sulfuric acid (8 g)
at a carefully controlled temperature of 5–10 ЊC. The reaction
was continued for 30 min and then the mixture was poured onto
30 g of ice. The crystals were filtered off, and recrystallized from
ethanol. Compound 10 was obtained (2 g, 76%), mp 99–100 ЊC;
IR 3442, 3111, 1555, 1511, 1329, 1103, 1040, 889, 818, 731, 675
cmϪ1; 1H NMR[(CD3)2CO] δ 2.90 (s, 3H), 8.39 (s, 1H); MS (m/z)
188 (Mϩ), 173 (Mϩ Ϫ 15), 171, 95, 69, 51, 44.
2-Acetamido-3, 5-dinitrothiophene 5 (3.0 g, 1.51 × 10Ϫ2 mol)
was added with stirring to 30 ml of 50% sulfuric acid and the
mixture was stirred for 4 h at 100 ЊC. After cooling, it was
poured onto 300 ml of ice–water with vigorous stirring. A
yellow precipitate appeared and was collected, washed with
water, and dried under vacuum drying box overnight (15
mmHg, room temp.). The crude product was further purified by
column chromatography on silica gel, with ethyl acetate–
petroleum spirit (3:1) as eluent, to give product 6 (1.3 g, 53%),
mp 179–180 ЊC, IR(KBr) 3354, 3274, 3185, 1530, 1353 cmϪ1
[Anal. (%). Calc. for (C4H3N3O4S): C, 25.40; H, 1.59; N, 22.22;
S, 16.93. Found: C, 25.86; H, 1.61; N, 21.71; S, 16.81].
O,OЈ-Diacetyl-N-phenyldiethanolamine 11
A mixiture of N-phenyldiethanolamine (25.0 g, 0.138 mol),
acetic anhydride (31.0 g, 0.31 mol), and pyridine (25 g, 0.356
mol) was heated to reflux for 2 h under nitrogen atmosphere.
The resulting solution was cooled and concentrated on a rotary
evaporator and then vacuum distilled (160 ЊC/1 mmHg) to
afford 11 as a pale golden oil (34.3 g, 93%), IR 3451, 2962, 2896,
4-Diethanolaminobenzene-azo-2-(5-nitrothiophene) 7
7 was prepared by a similar procedure to the synthesis of 8
(see below). UV-vis: λmaxDMF 583 nm (ε 47 040); IR(KBr) 3400–
3100, 1600, 1514, 1491, 1329, 1259, 1156, 1050 cmϪ1; MS (m/z)
1738, 1599, 1507, 1380, 1229, 1036, 750, 696 cmϪ1
.
1
336 (Mϩ); H NMR(DMSO-d6) δ 3.35 (s, 4H, NCH2), 3.65 (s,
4H, OCH2), 6.95 (d, 2H, ArH), 7.60 (s, 1H, ArH), 7.77 (d, 2H,
ArH), 8.18 (s, 1H, ArH) [Anal. (%). Calc. for (C14H16N4O4S): C,
50.00; H, 4.76; N, 16.67; S, 9.52. Found: C, 49.77, H, 4.82; N,
16.20; S, 9.38].
O,OЈ-Diacetyl-4-formyl-N-phenyldiethanolamine 12
Phosphoryl trichloride (22.0 g, 0.144 mol) was added dropwise
at 0 ЊC to 100 ml of N,N-dimethylformamide (DMF), and the
reaction mixture was stirred at 0 ЊC for 2 h. A DMF (100 ml)
solution containing the diacetate 11 (34.32 g, 0.129 mol) was
added slowly. The reaction mixture was heated to 90 ЊC for 3 h.
After cooling, the solution was poured onto 2 l of ice–water
containing 60 g (5 equiv.) of Na2CO3. The mixture was stirred
for 24 hours and the resulting solid was collected by filtration
under reduced pressure to give 12 as a pale brown solid and
which was used without further purification (31.1 g, 82%), mp
39 ЊC; IR 2991, 2819, 2746, 1737, 1723, 1675, 1598, 1561, 1525,
4-Diethanolaminobenzene-azo-2-(3,5-dinitrothiophene) 8
Sodium nitrite (0.482 g, 6.96 mmol) was added slowly, with
stirring, to conc. sulfuric acid (5.7 ml) at ice-bath temperature.
To facilitate the dissolution process, the temperature may be
raised to 30 ЊC for a while, then lowered to below 5 ЊC again; a
mixture of propionic acid (3.2 ml) and acetic acid (19 ml) was
added. During the addition of the acid mixture, the temper-
ature of the reaction mixture may rise but must be kept not
higher than 15 ЊC. The solution should be cooled to 0 ЊC when
1409, 1227, 1173, 1116, 1044, 979, 903, 820, 710, 597 cmϪ1
NMR(CD3OD) δ 2.14 (s, 6H), 3.76 (t, J 6.03, J 6.00, 4H), 4.28
.
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(t, J 5.85, J 5.97, 4H), 6.93–7.76 (dd, J 8.87, J 9.09, 4H), 9.65
1315, 1267, 1182, 1046, 820, 725 cmϪ1; NMR(DMSO-d6) δ 3.53
(s, 1H).
(t, 4H), 3.54 (t, 4H), 6.78–7.56 (dd, J 9.81, J 9.91, 4H), 7.73–
7.80 (dd, 2H), 8.46 (s, 1H); MS (m/z) 379 (Mϩ), 348 (Mϩ Ϫ 31);
CH3OH
2-{[4-Bis(acetoxyethyl)phenyl]ethenyl}-3,5-dinitrothiophene 13
λmax
530 nm [Anal: Calc. for (C16H17N3O6S): C, 50.65; H,
4.52; N, 11.08; S, 8.43. Found: C, 50.61; H, 4.50; N, 10.98; S,
8.41].
Re-distilled pyrrolidine (2 drops) and 10 (0.94 g, 5 mmol) were
added to a solution of 12 (2.2 g, 7.5 mmol) in 40 ml of THF.
The mixture was refluxed for 20 h. THF was then removed
under reduced pressure, the residue was recrystallized from
methanol, and 13 was obtained as shining black crystals (1.3 g,
56%), mp 169–170 ЊC; IR 3446, 3110, 2975, 2928, 2848, 1738,
1584, 1538, 1382, 1318, 1296, 1183, 1166, 817 cmϪ1; NMR-
[(DMSO-d6)] δ 1.98 (s, 6H), 3.70 (t, 4H), 4.20 (t, 4H), 6.87–7.62
(dd, J 8.82, 4H), 7.68–7.87 (dd, J 16.0, 2H), 8.45 (s, 1H); MS
(m/z) 463 (Mϩ, 4.62%), 87 (100%).
Acknowledgements
We are grateful to the National Science Foundation of China
and the Chinese National 863 Program Committee for financial
support.
References
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3 V. Steinkopf, Justus Liebigs Ann. Chem., 1913, 403, 17.
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2-{[4-(Diethanolamino)phenyl]ethenyl}-3,5-dinitrothiophene 14
K2CO3 (4 g, 28 mmol) was added with stirring to a solution of
13 (1.0 g, 2.15 mmol) in a mixture of THF (40 ml) and water
(100 ml) at 40 ЊC. The reaction was conducted for 30 h. Then
the mixture was concentrated by rotary evaporator. The
aqueous residue was extracted with chloroform (3 × 100 ml),
and the combined organic layers were washed with water
(3 × 150 ml), and dried with anhydrous Na2SO4. The crude
product was further separated by column chromatography on
silica gel as absorbent and ethyl acetate–methanol (10:1 v/v) as
eluent. Pure 14 as black crystals was obtained (0.36 g, 44%), mp
218 ЊC; IR 3384, 3110, 2927, 1737, 1612, 1537, 1522, 1382,
Paper 9/04945B
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