Synthesis and optical properties of conjugated N,N-dimethyl and thienyl end-capped 2,5-(arylethynyl)thiophene oligomer structures

Article abstract of DOI:10.1016/j.tetlet.2004.07.123

End-capped (N,N-dimethylaminophenyl) and 2′-thienylethynyl 2,5-thiophene oligomer structures were synthesized by heterocoupling between the terminal acetylenes such as: p-(N,N-dimethylaminophenyl)ethyne (3) [or 1-(p-(N,N-dimethylaminophenyl)-2-p-(ethynylp

Full text of DOI:10.1016/j.tetlet.2004.07.123

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 7061–7064
Synthesis and optical properties of conjugated N,N-dimethyl and
thienyl end-capped 2,5-(arylethynyl)thiophene oligomer structures
*
J. Gonzalo Rodr ´ı guez, Antonio Lafuente, Laura Rubio and Jorge Esquivias
Departamento de Qu ´ı mica Org a´ nica, Universidad Aut o´ noma, Cantoblanco, 28049-Madrid, Spain
Received 26 May 2004; revised 23 July 2004; accepted 27 July 2004
Available online 13 August 2004
0
Abstract—End-capped (N,N-dimethylaminophenyl) and 2 -thienylethynyl 2,5-thiophene oligomer structures were synthesized by
heterocoupling between the terminal acetylenes such as: p-(N,N-dimethylaminophenyl)ethyne (3) [or 1-(p-(N,N-dimethylaminophenyl)-
0
0
2
diiodothiophene, catalyzed by the Cl
-p-(ethynylphenyl)ethyne, 4]; p-(b-ethenyl-2 -thienyl)phenylethyne (E-9) [or p-(b-ethynyl-2 -thienyl)phenylethyne, 11], and 2,5-
0
0
2
Pd(PPh
phenyl] -1-ethynyl]thiophene oligomers were prepared by heterocoupling between 3 ,5 -di[(trimethylsilylethynyl)phenyl] -1-eth-
3
)
2
/CuI system, in good to excellent yields. The 2,5-di[(3 ,5 -di(trimethylsilylethynyl)-
0
0
x
x
yne (n = 0–2) terminal acetylenes and 2,5-diiodothiophene, in excellent yields. The terminal acetylenes were efficiently prepared
by a specific protection-deprotection methodology. All the ethynylphenyl compounds obtained show fluorescence radiation emis-
sion, with a bathochromic shift of the wavelength that increases with the chain conjugation.
Ó 2004 Elsevier Ltd. All rights reserved.
1
. Introduction
tion. Thus, cyclic oligomers with nanometre diameter,
integrated by 2,5-thienyl and para-phenylethynyl rings,
can be prepared.
1
1
The insertion of thiophene in the linear ethynylphenyl
conjugated chain, gives a higher electron delocalization
to the molecule. In general, the conjugated molecules,
integrating the thiophene rings and the end-capped
1
We now report the synthesis of conjugated 2,5-thio-
phene ring with the end-capped 2-ethenyl, 2-ethynyl
thienyl and N,N-dimethylaminophenyl moieties, which
are attractive and promising by their fluorescence
properties.
N,N-dimethylaminophenyl moiety, of precise length
2
,3
and constitution, exhibit high thermal stability and
show intrinsic electronic properties such as: lumines-
4
5
6
cence, redox, and charge transport. Moreover, the
compounds are very stable, easy to functionalize, and
soluble in most organic solvents. The improvement of
electronic strength effect was obtained by varying the
classical donor and acceptor groups on the conjugated
7
2. Results and discussion
The syntheses of 2,5-di(phenylethynyl)thiophene struc-
tures with p-extended conjugation having the N,N-
dimethylamino donor group has been undertaken.
Hence, the linear terminal acetylenes p-(N,N-dimethyl-
aminophenyl)ethyne (3) and 1-(p-N,N-dimethylamino-
phenyl)-2-(p-ethynylphenyl) ethyne (4) were prepared,
starting of p-(N,N-dimethylamino)iodobenzene (1).
8
system, and the extension of the conjugation between
9
donor and acceptor moieties.
,10
The 2-ethynyl and 2,5-di(ethynyl)thiophene units are
good starting compounds for preparing end-capped
thienyl oligomers and conjugated structures with angu-
lar geometry. The para-connection of two acetylene
units guarantees the conjugated electronic communica-
Compound 1 was obtained by reductive amination with
formaldehyde and sodium cyanoborohydride in acetic
acid, in practically quantitative yield (mp 105–107°C,
Scheme 1). The heterocoupling reaction between the
iodoarene 1 and 2-methyl-3-butyn-2-ol, in triethylamine
at room temperature, catalyzed by the palladium/copper
system, gives the propargyl derivative 2 in good yield
Keywords: End-capped (N,N-dimethylaminophenyl), thienyl and 1,3,5-
tri(phenylethynyl) 2,5-thiophene oligomers; p-Extended conjugation;
Sonogashira reaction.
*
Corresponding author. Tel.: +34 914974715; fax: +34 914973966;
e-mail: gonzalo.rodriguez@uam.es
0
040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.07.123

7062
J. G. Rodr ı´ guez et al. / Tetrahedron Letters 45 (2004) 7061–7064
OH
H
I
I
i.
ii.
iii.
iv.
iii.
Me
2
N
H
4
NH
2
NMe
2
1
NMe
2
NMe
2
2
3
Scheme 1. Reagents and conditions: (i) NaCNBH
4
, formaldehyde, AcOH; (ii) 2-methyl-3-butyn-2-ol, Cl
Pd(PPh , CuI, NEt
2
Pd(PPh
3
)
2 3
, CuI, NEt ; (iii) NaOH, toluene
at reflux; (iv) 4-(p-iodophenyl)-2-methyl-3-butyn-2-ol, Cl
2
3
)
2
3
.
1
2
(
with powdered sodium hydroxide in toluene at reflux
mp 71–73°C, 85%). Finally, compound 2 was treated
ladium/copper system, in triethylamine at room
temperature, gives 2,5-di[(p-N,N-dimethylaminophenyl-
ethynyl)-p-(phenylethynyl)]thiophene (7) as an orange
solid, mp 227–230°C, in excellent yield (98%, Scheme
temperature, giving the terminal acetylene 3 in practi-
1
2
cally quantitative yield (mp 51–53°C).
2
). The oxidative homocoupling product 8, brown solid,
In the same way, the heterocoupling reaction between
the acetylene 3 and 4-(p-iodophenyl)-2-methyl-3-butyn-
mp >260°C, was detected in very low yield (<2%).
2
-ol, in the presence of the palladium/copper catalyst
On the other hand, the end-capped thienyl conjugated
chains were also prepared. The heterocoupling reaction
system, and successive deprotection with powdered so-
dium hydroxide, in toluene at reflux temperature, gave
4
tative yield (Scheme 1).
1
4
between the thienylethenyl terminal acetylene (E)-9,
1
4
as a white solid, mp 138–140°C, in practically quanti-
(or the thienylethynyl terminal acetylene 11 ) and 2,5-
diiodothiophene, in triethylamine, in the presence of
the palladium/copper catalyst system, afforded conju-
gated 2,5-thiophene structure 10, as a yellow solid, mp
261–262°C (65%) (or 2,5-di(thienylethynyl)thiophene
12, yellow solid, mp 234–235°C, 85%) in good yield
Now, the terminal acetylenes 3 and 4 serve to prepare
the end-capped (N,N-dimethylaminophenyl)-ethynyl-
2
,5-thiophene oligomers.
(
Scheme 3). Compounds 5, 7, 10 and 12 exhibit angular
1
5
Thus, the syntheses of the 2,5-disubstituted thiophene
conjugated compound 5, was carried out by heterocou-
pling between the terminal acetylene 3 and 2,5-
di(iodo)thiophene, in triethylamine at room tempera-
ture, in the presence of the palladium/copper catalyst
system, providing 2,5-di(p-N,N-dimethylaminophenyl-
ethynyl)thiophene (5), as a yellow solid, mp 181–
geometry with an interchain angle about 148°.
Moreover, the heterocoupling between the conjugated
terminal acetylenes 13–15 (trigonal–linear geometry)
1
6
and 2,5-diiodothiophene was carried out. Hence, the
heterocoupling between the terminal acetylene 13
(n = 1), (or 14, n = 2, or 15, n = 3) and 2,5-di(iodo)thio-
phene, in triethylamine at room temperature, catalyzed
by the palladium/copper system, gives the conjugated
compound 16, as a yellow solid, mp 139–140°C (97%),
in excellent yield, (or 17, yellow solid, mp 218–220°C,
95%, or 18, yellow solid, mp 291–293°C, 95%, Scheme
4).
1
6
2
83°C, in excellent yield (98%, Scheme 2); 1,3-butadiyne
(2%), was also isolated as a yellow solid, mp 233–
34°C. Compound 6 results by oxidative homocoupling
of the terminal acetylene 3 in the presence of the catalyst
1
3
system. . The same heterocoupling reaction between
the terminal acetylene 3 and 2,5-dibromothiophene as
the haloarene, only gives the 1,3-butadiyne derivative 6.
The UV–visible spectra of the conjugated compounds 5
and 7, 10 and 12 and 16–18 show a bathochromic effect
on the absorption wavelength and a strong increasing on
The heterocoupling reaction between 2,5-di(iodo)thio-
phene and the terminal acetylene 4, catalyzed by the pal-
S
Me
2
N
n
n
NMe
2
n = 0, 5 (98%)
n = 1, 7 (98%)
i.
Me
2
N
H
n
+
n = 0,
n = 1,
3
4
Me
2
N
NMe
2
n
n
n = 0, 6 (<2%)
n = 1, 8 (<2%)
2 3 2 3
Scheme 2. Reagents: (i) 2,5-Diiodothiophene, Cl Pd(PPh ) , CuI, NEt .

J. G. Rodr ı´ guez et al. / Tetrahedron Letters 45 (2004) 7061–7064
7063
i.
S
H
10
S
)
E)-9
S
S
i.
S
H
S
1
48º
11
S
S
1
2
Scheme 3. Reagents: (i) 2,5-Diiodothiophene, Cl
2
3 2 3
Pd(PPh ) , CuI, NEt .
Me
3
Si
Me
3
Si
SiMe
3
S
i.
H
n
n
n
Me
3
Si
n = 0, 13
n = 1, 14
n = 2, 15
n = 0, 16
n = 1, 17
n = 2, 18
Me
3
Si
SiMe
3
Scheme 4. Reagents: (i) 2,5-Diiodothiophene, Cl
2
3 2 3
Pd(PPh ) , CuI, NEt .
the molar extinction coefficient (e), Table 1. Moreover
the 2,5-thiophene ring included in the linear ethynylphenyl
conjugated chain shows slightly higher e values but an
important decreasing of the quantum yield with respect
to the linear 1,4-phenyl ring included in the conjugated
dimethylaminophenyl)ethynyl and the ethenylphenyl-
ethynyl chains show a unique emission band (com-
pounds 5, 7 and 10); (d) The emission wavelength bands
for the 1,4-(N,N-dimethylaminophenyl) (5, 7) and for
the
3,5-di(trimethylsilylethynyl)phenyl
end-capped
1
6
ethynylphenyl chain.
homologous exhibit a bathochromic shift for each ethyn-
ylphenyl unit in the conjugated chain, respectively.
All the 2,5-disubstituted thiophene conjugated struc-
tures show fluorescence radiation emission, the wave-
length and quantum yield are summarized in Table 1.
Some considerations can be remarked: (a) There are a
significant increase in the quantum yield of the radiation
emission with the ethynylphenyl units in the chain, com-
pounds 5 and 7 and 16–18; (b) There is an important in-
crease in the quantum yield for the triple versus double
bond connecting with the thiophene ring unit, com-
pounds 12 and 10, respectively; (c) The ethynylphenyl
chains show two fluorescence wavelength emission
bands (compounds 12 and 16–18), while the 1,4-(N,N-
Hence, new 2,5-di(ethynylphenyl)thiophene conjugated
oligomers were satisfactorily obtained by means of the
Sonogashira reaction using a 2,7-dihalothiophene (Br,
I). The yields are excellent and the iodo derivative shows
highest versatility. All the conjugated compounds show
fluorescent properties.
Acknowledgements
We are indebted to CICYT of Spain (project PB97-
0060).
Table 1. Wavelengths for the first absorption and fluorescence
17
emission maxima for the compounds 5, 7, 10, 12, 16, 17 and 18 in
CH Cl at room temperature
2
2
References and notes
ꢀ
1
ꢀ1
)
Compd
UV–vis
max (nm)
e (M cm
Fluorescence
U
f
k
kmax (nm)
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a
b
a
a
a
a
a
5
7
385
393
367
379
355
379
377
57,500
104,320
21,850
44,000
46,900
78,900
113,000
456
512
440
0.23
0.30
0.10
0.34
0.20
0.42
0.54
1
1
1
1
1
0
2
6
7
8
2
398, 423
391, 410
421, 447
429, 454
a
Fluorescence quantum yield was determined relative to 2-aminopyr-
idine in 0.1N H SO
Fluorescence quantum yield was determined relative to quinine sulf-
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3. (a) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605; (b) Irie, M.
Chem. Rev. 2000, 100, 1685; (c) Delaire, J. A.; Nakatani,
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Phys. Phys. Chim. Biol. 1998, 65, 1155.
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2
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. (a) Barbarella, G. Chem. Eur. J. 2002, 8, 5072; (b) Donat-
0
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1
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in CDCl at 300MHz, and the chemical shifts are given in d
3
with TMS as an internal reference and constants coupling J
are given in Hz): Compound 5, H NMR: 7.38 (d, 4H,
1
7
8
J = 8.0Hz); 7.05 (s, 2H); 6.64 (d, 4H, J = 8.0Hz); 2.99 (s,
12H). Compound 7, H NMR: 7.46 (br s, 8H); 7.43 (d, 4H,
1
11249.
. (a) Abbotto, A.; Bradamante, S.; Facchetti, A.; Pagani, G.
A. J. Org. Chem. 1997, 62, 5755; (b) Cheng, L. T.; Tam,
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J = 8.0Hz); 7.16 (s, 2H); 6.66 (d, 4H, J = 8.0Hz); 3.00 (s,
12H). Compound 10, H NMR: 7.45 (s, 8H); 7.35 (d, 1H,
1
J = 4.8Hz), 7.27 (d, 2H, J = 15.9Hz); 7.15 (d, 2H,
J = 3.8Hz), 7.10 (s, 2H); 7.02 (dd, 2H, J = 4.8 and
1
9
. Cheng, L. T.; Tam, W.; Marder, S. R.; Stiegman, A. E.;
Rikken, G.; Spangler, C. W. J. Phys. Chem. 1991, 95,
3.8Hz), 6.90 (d, 2H, J = 15.9Hz). Compound 12,
H
NMR: 7.49 (s, 8H); 7.32 (d, 2H, J = 4.8Hz); 7.31 (d, 2H,
J = 4.3Hz); 7.18 (s, 2H); 7.03 (dd, 2H, J = 4.8 and 4.3Hz).
Compound 16, H NMR: 7.55 (d, 4H, J = 1.6Hz); 7.53 (t,
1
0643.
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1
1
1
1
1
7
2H, J = 1.6Hz); 7.14 (s, 2H); 0.24 (s, 36H). Compound 17,
H NMR: 7.56 (d, 4H, J = 1.6Hz); 7.53 (t, 2H, J = 1.6Hz);
1
3
7.51 (d, 4H, J = 8.7Hz); 7.46 (d, 4H, J = 8.7Hz); 7.18
(s, 2H); 0.24 (s, 36H). Compound 18, H NMR: 7.56 (d,
1
4H, J = 1.6Hz); 7.53 (t, 2H, J = 1.6Hz); 7.52 (br s, 8H);
7.51 (d, 4H, J = 8.9Hz); 7.47 (d, 4H, J = 8.9Hz); 7.18
(s, 2H); 0.25 (s, 36H).