Synthesis of conjugated 2-arylethynyl and 2-arylethenyl thiophene structures with optical properties

Article abstract of DOI:10.1016/j.tetlet.2004.05.104

Conjugated mono(arylethynyl)oligothiophene structures have been obtained starting with (E)-[1-(2′-thienyl)-2-(p-phenyl)]ethyne (E)-7 and 2-[p-(iodophenyl)ethynyl]thiophene 8. Conjugated nanostructures were synthesized by oxidative coupling between the ter

Full text of DOI:10.1016/j.tetlet.2004.05.104

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 5685–5688
Synthesis of conjugated 2-arylethynyl and 2-arylethenyl thiophene
structures with optical properties
*
ꢀ
J. Gonzalo Rodrıguez, Antonio Lafuente and Laura Rubio
Departamento de Quꢀımica Orgꢀanica, Universidad Autꢀonoma, Cantoblanco, 28049 Madrid, Spain
Received 25 March 2004;revised 18 May 2004;accepted 19 May 2004
Available online 10 June 2004
Abstract—Conjugated mono(arylethynyl)oligothiophene structures have been obtained starting with (E)-[1-(20-thienyl)-2-(p-
phenyl)]ethyne (E)-7 and 2-[p-(iodophenyl)ethynyl]thiophene 8. Conjugated nanostructures were synthesized by oxidative coupling
between the terminal acetylenes (E)-7 and 8 to give, respectively, 1,4-di[(2-p-(iodophenyl)ethynyl)thienyl]-1,3-butadiyne and 1,4-
di[(2-p-iodophenylethynyl)thienyl]-1,3-butadiyne. All the thiophene derivatives synthesized show an important fluorescence radia-
tion emission, with a bathochromic shift, which increases with the conjugation of the chain.
Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction
varying the classical donor and acceptor groups on the
conjugate system,8 and by the extension of the conju-
gation between donor and acceptor moieties.9;10
One strategy used to improve the nonlinear optical
properties is the insertion of five-member heteroaro-
matic rings in the conjugate backbone by increasing the
first molecular hyperpolarizability b. It has been shown
that highest b-values can be achieved by replacing ben-
zenoid rings with easily delocalizable thiophene moie-
ties.1
The 2-ethynyl and 2,5-di(ethynyl)thiophene units are
good starting candidates for preparing end-capped thi-
enyl oligomers and conjugated structures with angular
geometry (about 155°). The para-connection of two
acetylene units guarantees the conjugate electronic
communication. Cyclic oligomers with nanometric
diameter, integrated by 2,5-thienyl and para-phenyl-
ethynyl rings, have been recently reported.11 A series of
2-thienyl polyenes has been synthesized and analyzed
spectroscopically in n-alkane solutions at liquid helium
temperature.12
The thiophene-based compounds have attracted great
attention, mainly because they show intrinsic electronic
properties such as: luminescence,2 redox,3 and charge-
transport.4 Thiophene oligomers emit light across the
entire visible range. Moreover, the compounds are very
stable, easy to functionalize, and soluble in most organic
solvents.5 Thienyl oligomer dendrimers with p-extended
conjugate chains have shown that the p–pà electronic
conduction band decreases from periphery to heart with
an increase in the fluorescence quantum yield.6
We now report the synthesis of conjugate 2-thienyl ring
end-capped structures, which are attractive and prom-
ising by their fluorescence properties.
2. Results and discussion
Conjugated thienyl systems have been recently devel-
oped with enhanced thermal and chemical stability.7 In
the area of electro-optical applications, the improve-
ment in electronic strength effect was obtained by
We carried out the syntheses of (E,E)-1,4-di(20-thienyl)-
1,3-butadiene, (E,E)-2 and 1,4-di(2-thienyl)-1,3-buta-
diyne (4),13 which permit a comparison between the
polarity and polarizability effect of the conjugate thienyl
systems formed by double or triple bonds.
Keywords: Arylethynyl and arylethenylthiophene structures; p-Exten-
ded conjugation;Fluorescence analysis;Eglinton–Glaser, Cadiot–
Chodkiewicz and Sonogashira reaction.
Compound (E,E)-2 was obtained in good yield (72%) by
the homocoupling reaction of 2-(20-chlorovinyl)thio-
phene (1), catalyzed by zero-valent nickel complexes.
* Corresponding author. Tel.: +34-914974715;fax: +34-914973966;
e-mail: gonzalo.rodriguez@uam.es
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.05.104

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J. G. Rodrıguez et al. / Tetrahedron Letters 45 (2004) 5685–5688
5686
H
O
H
i.
ii.
S
S
S
Cl
S
(E/Z)-1
(E,E)-2
iii.
S
iv.
H
S
S
3
4
t
Scheme 1. Reagents and conditions: (i) Cl2CH2PPh3, KOtBu;(ii) Cl 2Ni(PPh3)2, Zn, THF, rt, 2 h;(iii) KO Bu, THF;(iv) Cu 2Cl2/O2, pyridine.
The 2-chlorovinylthiophene, as the (E/Z)-114 mixture
(57:43), was prepared in good yield by the Wittig reac-
tion between the (chloromethyl)(triphenyl)phosphonium
ylide and 2-thiophenecarboxaldehyde. The homocou-
pling reaction of the (E/Z)-1 mixture was carried out in
the presence of the zero-valent nickel complex, prepared
in situ by reaction of dichloro bis[(triphenyl)phosphine]
nickel and powdered zinc in tetrahydrofuran, giving 1,4-
di(2-thienyl)-1,3-butadiene 2, as a brown solid, mp 140–
143 °C, hexane, only as the (E,E) isomer in good yield
(82%;Scheme 1).
reaction between p-(iodobenzyl)(triphenyl)phospho-
nium ylide and 2-thiophenecarboxaldehyde, giving a (E/
Z)-5 mixture (50:50),16 which was completely isomerized
to E-5, by exposure to sunlight in the presence of a few
iodine crystals (Scheme 2).
The heterocoupling of (E)-5 with 2-methyl-3-butyn-2-ol
in the presence of dichloro bis[(triphenyl)phosphine]
palladium and cuprous iodide, in diethylamine at room
temperature, gives the propargyl derivative (E)-6 in
excellent yield (brown solid, mp 144–147 °C, 94%). By
treatment of (E)-6 with powdered sodium hydroxide in
dry toluene at reflux temperature, the terminal acetylene
(E)-7, was obtained in excellent yield (pale-yellow solid,
mp 117–119 °C, hexane, 92%).
In the same way, 2-ethynylthiophene (3) was obtained in
good yield starting with the (E/Z)-1 mixture in THF
treatment with potassium tert-butoxide at room tem-
perature (brown oil, 87%).15
2-(p-Iodophenylethynyl)thiophene (8) (white solid, 89–
91 °C, hexane), was prepared in quantitative yield from
(E)-5 by bromine addition followed by treatment with
potassium tert-butoxide. The heterocoupling reaction of
8 with 2-methyl-3-butyn-2-ol, catalyzed by the palla-
dium/copper system, gave propargyl intermediate 9 in
good yield (white solid, mp 102–105 °C, 85%), which by
subsequent treatment with powdered sodium hydroxide
in dry toluene afforded terminal acetylene 10,10 yellow
solid, mp 90–92 °C, hexane in excellent yield (94%).
The synthesis of compound 4 has been reported by
means of the Glaser homocoupling of the acetylene 3 in
the presence of Cu2Cl2 in pyridine under an oxygen
atmosphere, giving 1,4-di(thienyl)-1,3-butadiyne (4)
(yellow solid, mp 92–93 °C, hexane), in excellent yield
(91%;Scheme 1).
The 1,3-diene derivative (E,E)-2 shows a fluorescence
radiation with a low quantum yield, while the 1,3-diyne
(4) does not show appreciable fluorescence emission in
dichloromethane solvent (Table 1).
The oxidative homocoupling of alkynes (E)-7 or 10
afforded (E,E)-12 or 13, respectively, with an extended
conjugation through the 1,3-butadiyne chain. The oxi-
dative homocoupling of compound (E)-7 in presence of
cuprous chloride, under the Glaser (or Eglinton) con-
ditions, failed. In this case, the 1,3-butadiyne (E,E)-12
was obtained by means of the Cadiot–Chodkiewicz
reaction. Thus, the oxidative bromination of compound
(E)-7 in situ with potassium hypobromide gave (E)-11 in
good yield, which in the presence of cuprous chloride,
furnished (E,E)-12 as a yellow solid, mp > 300 °C, in
moderate yield (40%, Scheme 3).
The conjugated 2-(E)-phenylethenyl- and phenylethyn-
yl-thiophene structures were obtained starting of 2-[p-
(iodophenyl)-20-ethenyl]thiophene E-5 and the ethynyl
analogue 8. Compound E-5 was prepared by the Wittig
Table 1. Wavelengths for the first absorption and fluorescence emis-
sion maxima for the compounds 2, 7, 10, 12, 13 and 14 in CH2Cl2 at
room temperature19
Compd.
UV–vis
max, nm
e
Fluorescence
kmax, nm
Uf
k
(MÀ1 cmÀ1
)
2
381
341
337
382
359
352
25,100
28,400
30,000
78,300
81,250
57,100
431
0.02
0.13
0.14
0.29
0.36
0.30
In contrast, the 1,3-butadiyne derivative 13 was ob-
tained in good yield by oxidative homocoupling reaction
of the terminal acetylene 10 by the Eglinton–Glaser
reaction, catalyzed by cuprous chloride in pyridine at
room temperature under oxygen atmosphere (yellow
solid, mp 239–240 °C, dichloromethane, 71%, Scheme
3). The different behaviour of (E)-7 seems to be due to
7
420, 450
392, 410
427, 454
421
10
12
13
14
390, 410
Fluorescence quantum yield was determined relative to 2-aminopyr-
idine in 0.1 N H2SO4.

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J. G. Rodrıguez et al. / Tetrahedron Letters 45 (2004) 5685–5688
5687
CH3
v.
i.
I
S
I
ii.
iii.
S
iv.
I
8
(E)-5
(E/Z)-5
vii.
vii.
vi.
vi.
S
S
(E)-5
OH
OH
H
(E)-6
(E)-7
8
H
S
S
9
10
Scheme 2. Reagents and conditions: NBS/CCl4;(ii) PPh 3;(iii) KO tBu, Furfural, toluene;(iv) EtOH, sunlight, I 2;(v) Br 2, CCl4, KOtBu;(vi)
PdCl2(PPh3)2, CuI, 2-methyl-3-butyn-2-ol;(vii) NaOH, toluene, at reflux.
S
i.
ii.
(E)-7
S
S
Br
11
(E,E)-12
iii.
S
10
S
13
iv.
S
8 + 10
S
14
Scheme 3. Reagents and conditions: KOBr, H2O/THF;(ii) ( E)-6, Et2NH, NH2OHÆHCl, Cu2Cl2, MeOH;(iii) Cu 2Cl2, O2, pyridine;(iv)
PdCl2(PPh3)2, Cu2I2, HNEt2.
coordination of the double bond to the cuprous salt
catalyst thus inhibiting the intermediate cuprous acetylide
formation.17
454 nm (Table 1). This fact contrasts with the low
quantum yield observed in the 1,3-diene (E,E)-2 versus
the 1,3-diyne 4 (no emission fluorescence). The conju-
gated mono-yne molecule 14 shows two fluorescence
emission bands at 390 and 410 nm with a considerable
quantum yield although lower than that of 1,3-diyne 13.
Hence, the conjugated 2-ethynylthiophene 1,3-diyne (13)
exhibits a higher than expected quantum yield of the
fluorescence emission as compared with 2-ethenylthio-
phene 1,3-diyne (E,E-12) or the mono-yne compound
(14), (Table 1).
The 1,3-diyne 13 was analyzed by mass spectrometry
(MALDI-TOF technique) using a laser radiation at
337 nm, in a complete volatilization of the sample.
Hence, at the laser irradiation time, the topo-oligomer-
ization of 13 was detected on the mass spectrum,18 giv-
ing a mixture of oligomers together with the 1,3-diyne
monomer 13 (73%), in the following distribution: dimer
21%;trimer 4%;tetramer 1% and the 1,3-diyne penta-
mer in traces.
A conjugate homologous structure 14 was synthesized
as a conjugate reference structure to compare the optical
properties of the mono-yne and the 1,3-diyne structure
13. Compound 14 was obtained by heterocoupling
reaction of 8 with the terminal acetylene 10, catalyzed by
the palladium/copper system, in low yield (yellow solid,
mp 210–212 °C, dichloromethane, 48%, Scheme 3).
Acknowledgements
We are indebted to CICYT of Spain (project PB97-
0060).
The ethynyl conjugate 1,3-diyne molecule 13 shows a
unique fluorescence emission band at 421 nm with
higher quantum yield than the ethenyl conjugate 1,3-
diyne 12, which displays two emission bands at 427 and
References and notes
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given (1H NMR was registered in CDCl3 at 300 MHz, and
the chemical shifts are given in d with TMS as an internal
reference and constants coupling J are given in Hz).
Compound 2, 1H NMR: 7.17 (d, 2H, J ¼ 4:4 Hz);7.02
(d, 2H, J ¼ 6:9 Hz);6.99 (dd, 2H, J ¼ 6:9 and 4.4 Hz);
6.78 and 6.68 (d, 2H, J ¼ 17:0 Hz). Compound 7, 1H
NMR: 7.40 (d, 2H, J ¼ 8:6 Hz);7.39 (d, 2H, J ¼ 8:6 Hz);
7.24 (d, 1H, J ¼ 16:1 Hz);7.22 (d, 1H, J ¼ 5:0 Hz);7.08
(d, 1H, J ¼ 3:4 Hz);7.00 (dd, 1H, J ¼ 5:0 and 3.4 Hz);
6.88 (d, 1H, J ¼ 16:1 Hz);3.15 (s, 1H). Compound 10, 1H
NMR: 7.46 (s, 4H);7.32 (d, 1H, J ¼ 4:8 Hz);7.30 (d, 1H,
J ¼ 3:2 Hz);7.03 (dd, 1H, J ¼ 4:8 and 3.2 Hz);3.19 (s,
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1
1H). Compound 12, H NMR: 7.45 (d, 2H, J ¼ 8:6 Hz);
7.40 (d, 2H, J ¼ 8:6 Hz);7.24 (d, 1H, J ¼ 16:1 Hz);7.21
(d, 1H, J ¼ 5:0 Hz);7.09 (d,1H, J ¼ 3:4 Hz);7.01 (dd, 1H,
J ¼ 5:0 and 3.4 Hz);6.88 (d, 1H, J ¼ 16:1 Hz). Compound
ꢀ
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1
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13, H NMR: 7.49 (s, 8H);7.33 (d, 2H, J ¼ 5:4 Hz);7.30
(d, 2H, J ¼ 3:8 Hz);7.03 (dd, 2H, J ¼ 5:4 and 3.8 Hz).
Compound 14, 1H NMR: 7.49 (s, 8H);7.33 (d, 2H,
J ¼ 5:2 Hz);7.30 (d, 2H, J ¼ 3:7 Hz);7.03 (dd, 2H,
J ¼ 5:2 and 3.7 Hz).