Synthesis of carbon dendron nano-chains with π-extended conjugation based on linear 1,4-phenylethynyl and 1,5-naphthylethynyl subunits

Article abstract of DOI:10.1016/S0040-4039(03)01615-0

A convenient and efficient synthesis of 3,5-di(trimethylsilylethynyl)phenylacetylene and p-[3,5-di(trimethylsilylethynyl)-1-phenylethynyl]phenylacetylene and the naphthylethynyl homologues terminal acetylenes of 5-(N,N-dimethylamino)naphthylethyne have been carried out. These terminal acetylene compounds serve to prepare nanometer-sized conjugated 1,3,5-tri(ethynylphenyl)benzene and 1,3,5-tri(ethynylnaphthyl)benzene oligomers, by means of heterocoupling with 1,3,5-triiodobenzene, catalysed by palladium, in excellent yields. Both the ethynylphenyl or ethynylnaphthyl homologues chains show fluorescence emission radiation, with important quantum yield.

Full text of DOI:10.1016/S0040-4039(03)01615-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6375–6378
Synthesis of carbon dendron nano-chains with p-extended
conjugation based on linear 1,4-phenylethynyl and
1,5-naphthylethynyl subunits
J. Gonzalo Rodr´ıguez,a,* J. Luis Tejedor,a Jorge Esquiviasa and Cristina D´ıazb
aDepartamento de Qu´ımica Orga´nica, Facultad de Ciencias, Universidad Auto´noma, Cantoblanco, 28049 Madrid, Spain
bDepartamento de Qu´ımica F´ısica, Facultad de Ciencias, Universidad Auto´noma, Cantoblanco, 28049 Madrid, Spain
Received 25 June 2003; revised 27 June 2003; accepted 27 June 2003
Abstract—A convenient and efficient synthesis of 3,5-di(trimethylsilylethynyl)phenylacetylene and p-[3,5-di(trimethylsilylethynyl)-
1-phenylethynyl]phenylacetylene and the naphthylethynyl homologues terminal acetylenes of 5-(N,N-dimethy-
lamino)naphthylethyne have been carried out. These terminal acetylene compounds serve to prepare nanometer-sized conjugated
1,3,5-tri(ethynylphenyl)benzene and 1,3,5-tri(ethynylnaphthyl)benzene oligomers, by means of heterocoupling with 1,3,5-tri-
iodobenzene, catalysed by palladium, in excellent yields. Both the ethynylphenyl or ethynylnaphthyl homologues chains show
fluorescence emission radiation, with important quantum yield.
© 2003 Elsevier Ltd. All rights reserved.
Introduction
Conjugated systems integrated by 1,2-, 1,3-, 1,4-
diethynyl and 1,3,5-triethynylphenyl units have been
used in the preparation of aromatic monomers bearing
multiple ethynyl groups with electroluminescence prop-
erties,10 poly(phenylenevinylene) with fluorescence
properties as well as dendrimers11 with poly(yne)
chain.12 The triple bonds play the role of a wire while
the polyaromatic systems behave as an energy relay
subunit.
The synthesis and characterisation of nanometer-sized
conjugated molecules of precise length and constitution,
are of wide-spread interest, due to their inherent syn-
thetic flexibility, which permits the design of molecular
architectures with important properties.1,2 Molecules
showing p extended conjugation, in general exhibit high
thermal stability and can present electroconductive,
magnetic and optical properties.3 A variety of potential
applications such as artificial photosynthesis,4 photo-
catalysis,5 molecular photovoltaic cells,6 molecular
informatics,7 and optoelectronic devices,8,9 are begin-
ning to emerge from this new field of research.
We now report the controlled synthesis of a novel
family of rigid conjugated dendron nanostructures
with a well defined geometry and the fluorescence prop-
erties.
Scheme 1. Reagents and conditions: (i) 4-(p-iodophenyl)-2-methylbut-3-yn-2-ol, PdCl2(PPh3)2, Cu2I2, Et3N; (ii) NaOH, toluene at
reflux.
Keywords: trigonal geometry; 1,3,5-tri(ethynylphenyl)benzene and 1,3,5-tri(ethynylnaphthyl)benzene nanostructures; p-extended conjugation;
1,3,5-triiodobenzene; Sonogashira reaction; dendron base units.
* Corresponding author. E-mail: gonzalo.rodriguez@uam.es
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01615-0

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The heterocoupling of the terminal acetylenes 1–3 with
Discussion
1,3,5-triiodobenzene gave the dendron nanostructures
with a benzene core, which links a terminal 3,5-
di(trimethylsilylethynyl)phenylacetylene, through a con-
jugated linear ethynylphenyl fragment giving 4–6 as
yellow solids in good yield: 4 (118–120°C, 93%); 5
(143–146°C, 85%) and; 6 (199–203°C, 80%) (Scheme 2).
The 1,3,5-triiodobenzene was obtained by direct iodina-
tion giving a mixture of products that, in general, result
in low yield and tedious purification.13 A new practical
synthesis of 1,3,5-triiodobenzene has been developed to
obtain 1,3,5-tri(ethynylphenyl or ethynylnaphthyl)-
benzene derivatives. Thus, by reduction of 1-iodo-3,5-
dinitrobenzene with stannous chloride in ethyl acetate,
was quantitatively isolated 1-iodo-3,5-diaminobenzene
(123–125°C14) which, was transformed to the double
diazonium salt with sodium nitrite/sulfuric acid, at −5
to −10°C, that was followed by treatment with potas-
sium iodide at rt,15 to give 1,3,5-triiodobenzene (184–
186°C, 68%13).
In the same way, the 5-(N,N-dimethylamino)naphthyl-
ethynyl terminal acetylenes 7–9 were synthesised from
7.17 Compounds 8 (145–147°C) or 9 (210–211°C), were
satisfactory prepared by heterocoupling between 7 or 8
with
4-(5-iodo-1-naphthyl)-2-methylbut-3-yn-2-ol,15
catalysed by palladium, giving the propargylic interme-
diates in good yield (87 and 88%, respectively), fol-
lowed by treatment with sodium hydroxide in dry
toluene at reflux temperature, to give the terminal
acetylenes in quantitative yield (Scheme 3).
Now, a new family of compounds have been synthe-
sised starting of the terminal acetylene 3, which was
prepared by heterocoupling between the terminal acetyl-
ene homologue 2 and 4-(p-iodophenyl)-2-methylbut-3-
yn-2-ol, followed by the specific deprotection of the
propargylic group by treatment with powdered sodium
hydroxide in dry toluene at reflux temperature.16 Termi-
nal acetylene 3 was isolated as a brown solid (148–
150°C), in practically quantitative yield (Scheme 1).
The heterocoupling reaction between the terminal
acetylenes 7–9 with 1,3,5-triiodobenzene, catalysed by
palladium,
affords
to
the
5-(N,N-dimethyl-
amino)naphthylethynyl dendron nanostructures 10–12,
as orange solids; compound 10 (193–195°C, 85%); com-
pound 11 (237–238°C, 81%) (Scheme 4). The terminal
acetylene 9 was treated with 1,3,5-triiodobenzene under
Scheme 2. Reagents and conditions: (i) 1,3,5-triiodobenzene, PdCl2(PPh3)2, Cu2I2, Et3N.
Scheme 3. Reagents and conditions: (i) PdCl2(PPh3)2, Cu2I2, Et3N, 4-(5-iodo-1-naphthyl)-2-methylbut-3-yn-2-ol; (ii) NaOH,
toluene reflux.

J. G. Rodr´ıguez et al. / Tetrahedron Letters 44 (2003) 6375–6378
6377
Scheme 4. Reagents and conditions: (i) 1,3,5-triiodobenzene, Cl2Pd(PPh3)2, Cu2I2, Et3N.
Table 1. UV/vis and fluorescence spectra of the dendron structures 4, 5, 6, and 10, 11, 1218
Compd.
UV/visa (CH2Cl2) umax (nm)
m (M−1 cm−1
)
Fb (CH2Cl2) umax (nm)
bf
4
5
6
10
11
12
308
351
347
356
364
370
87400
143700
187500
81400
10650
96800
353, 367
356, 377
377, 398
504
527
530
0.16c
0.48c
0.80c
0.43d
0.21d
<0.10d
a At room temperature.
b At room temperature and [c]$10−8 M.
c Fluorescence quantum yield was in dichloromethane relative to 2-aminopyridine in 0.1N H2SO4.
d Fluorescence quantum yield in dichloromethane relative to quinine sulfate in 1N H2SO4.
the same conditions, but in pyridine, at reflux tempera-
ture to give 12 in low yield (212–214°C, 30%).
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J.G.R. will be always grateful to his father Natalio
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