Synthesis of nanostructures based on 1,4- and 1,3,5-phenylethynyl units with π-extended conjugation. Carbon networks dendrimer base units

Article abstract of DOI:10.1016/S0040-4039(03)01137-7

A convenient and efficient synthesis of 3,5-di(silylethynyl)phenylacetylene and p-[3,5-di(silylethynyl)phenylethynyl]phenylacetylene has been carried out. These compounds serve to prepare nanometer-sized conjugated 1,4- and 1,3,5-phenylethynyl oligomers,

Full text of DOI:10.1016/S0040-4039(03)01137-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 4831–4834
Synthesis of nanostructures based on 1,4- and
1,3,5-phenylethynyl units with p-extended conjugation.
Carbon networks dendrimer base units
J. Gonzalo Rodr´ıguez* and J. Esquivias
Departamento de Qu´ımica Orga´nica, Facultad de Ciencias, Universidad Auto´noma, Cantoblanco, 28049 Madrid, Spain
Received 6 March 2003; revised 28 April 2003; accepted 3 May 2003
Abstract—A convenient and efficient synthesis of 3,5-di(silylethynyl)phenylacetylene and p-[3,5-di(silylethynyl)phenylethynyl]-
phenylacetylene has been carried out. These compounds serve to prepare nanometer-sized conjugated 1,4- and 1,3,5-phenylethynyl
oligomers, by means of cross-coupling with a convenient haloaryl derivative, catalysed by palladium(II), in excellent yields. The
phenylethynyl homologues show fluorescence emission, the wavelength of which is displaced by approximately 20 nm by each
phenylethynyl unit increasing the conjugate chain. © 2003 Elsevier Science Ltd. All rights reserved.
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 nanostructures based on
1,4-di(ethynylphenyl) and 1,3,5-tri(ethynylphenyl) units,
which are molecular modules to prepare stable and
geometrically well defined nanostructures.
The construction of controlled 1,4-di(ethynylphenyl)
and 1,3,5-tri(ethynylphenyl) conjugated nanostructures,
has been undertaken. The diethynyl or triethynyl sub-
stituted benzenes show a variety of geometric possibili-
ties, due to their hexagonal structure; 1,4-linear,
1,3-trigonal, and 1,2-angular structures (60°).
Conjugated systems containing 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 properties,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.
p-Di(iodophenyl)acetylene (2) was obtained in practi-
cally quantitative yield, from the correponding E/Z
ethenyl mixture 1, by bromine addition followed by
dehydrobromination with potassium tert-butoxide, at
room temperature (Scheme 1). The ethenyl derivative
(1) was satisfactorily obtained by a Wittig reaction
between p-(iodophenyl) carboxaldehyde and the p-
(iodobenzyl)(triphenyl)phosphonium ylide, prepared in
Scheme 1.
Keywords: trigonal geometry; 1,3,5-tri(phenylethynyl) nanostructures; p-extended conjugation; Sonogashira reaction; dendrimer base units.
* Corresponding author. E-mail: gonzalo.rodriguez@uam.es
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01137-7

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J. G. Rodr´ıguez, J. Esqui6ias / Tetrahedron Letters 44 (2003) 4831–4834
situ from the appropriate phosphonium salt with potas-
sium tert-butoxide in toluene.
heterocoupling between 1,3,5-tribromobenzene and 2-
methyl-3-butyn-2-ol, in the presence of palladium(II),
affords 4 (mp 52–54°C, 80%), which was coupled with
trimethylsilylacetylene, in triethylamine at reflux in the
presence of palladium(II), to give 5 (mp 85–86°C) in
good yield. The selective deprotection of the propar-
gylic group was carried out with powdered sodium
hydroxide in dry toluene at reflux, giving the terminal
acetylene 6 (transparent oil), in practically quantitative
yield (Scheme 2). Heterocoupling between 1,4-
diiodobenzene and 2-methyl-3-butyn-2-ol, catalysed by
a palladium(II), gives 7 (mp 89–90°C) (Scheme 3).
Compound 6 was heterocoupled with 7, to give 8 (mp
95–97°C). The selective deprotection of the propargylic
group affords 9 in practically quantitative yield. Treat-
ment of 5 with sodium hydroxide in toluene with small
amounts of water or in THF–methanol (4:1) produces a
mixture of all the possible terminal acetylene com-
pounds. Thus, under dry conditions, solid sodium
hydroxide enables only proton abstraction and acetone
elimination of the propargylic group, while in the pres-
ence of water or methanol a nucleophile attack of the
−OR anion to the silicon atom (R=H or CH3) occurs
with Csp-SiMe3 cleavage.13
The synthesis of molecular units of trigonal geometry
was undertaken starting from 1,3,5-tribromobenzene.
Thus, 3,5-[di(2-methyl-2-hydroxybutynyl)phenyl]ethyne
was obtained by heterocoupling of 1,3,5-tribromobenz-
ene and 2-methyl-3-butyn-2-ol in the presence of palla-
dium(II). 1,3,5 - Tris(2-methyl - 2 - hydroxy - 3 - butynyl)-
benzene, was obtained in 80% of yield. However, 3,5-
[di(2-methyl-2-hydroxybutynyl)phenyl]ethyne (3) (mp
95–97°C) was isolated pure in low yield (12%) by
deprotection with powdered sodium hydroxide in dry
toluene at reflux (Scheme 2).13 Moreover, due to the
low solubility of these propargylic derivatives, another
synthetic methodology was undertaken.
An alternative was the synthesis of the doubly pro-
tected molecular units 6 and 9, which were employed to
obtain nanostructures with a trigonal geometry. The
Thus, the double heterocoupling reaction between p-
diiodobenzene and 6 (or 9), in presence of palladium-
(II), gives 10 (mp 219–220°C) (or 11, mp 270–273°C) in
practically quantitative yield. Both compounds were
deprotected by treatment with potassium carbonate in
THF/MeOH (4:1) to give quantitatively the terminal
tetraacetylene 12 (or 13) (Scheme 4).
Moreover, the phenylethynyl homologous compound
14 was prepared by double heterocoupling between
1,2-di(p-iodophenyl)ethyne (2) and the terminal acetyle-
nes 6, catalysed by palladium(II), in practically quanti-
tative yield (Scheme 5). Compound 14 (mp 220–223°C)
was completely deprotected to the terminal tetra-
acetylene 15 by treatment with potassium carbonate in
THF–MeOH (4:1).
Scheme 2. Reagents and conditions: (i) 2-Methyl-3-butyn-2-ol,
Cl2Pd(Ph3P)2, Cu2I2, NEt3; (ii) NaOH, toluene at reflux; (iii)
trimethylsilylethyne, Cl2Pd(Ph3P)2, Cu2I2, NEt3, rt.
Scheme 3. Reagents and conditions: (i) 2-Methyl-3-butyn-2-ol, Cl2Pd(PPh3)2, Cu2I2, NEt3, rt; (ii) 6 and 7, Cl2Pd(PPh3)2, Cu2I2,
NEt3, rt; (iii) NaOH, toluene, at reflux.
Scheme 4. Reagents and conditions: (i) p-Diiodobenzene, Cl2Pd(PPh3)2, Cu2I2, NEt3.

J. G. Rodr´ıguez, J. Esqui6ias / Tetrahedron Letters 44 (2003) 4831–4834
4833
Scheme 5. Reagents and conditions: (i) Cl2Pd(PPh3)2, Cu2I2, NEt3.
Scheme 6. Reagents and conditions: (i) Cl2Pd(PPh3)2, Cu2Cl2, NEt3.
Table 1. UV–vis and fluorescence spectra data of conjugated compounds 10, 11, 14, 16 and 17
Compd
UV–visa (CH2Cl2) umax (nm)
m (M−1 cm−1
)
Fb (CH2Cl2) umax (nm)
Ffc
10
11
14
16
17
348
354
344
352
365
44900
103600
76300
33800
65100
355, 373
394, 413
376, 394
377, 398
397, 418
0.65
0.72
0.68
0.60
0.81
a All spectra were recorded at room temperature.
b All fluorescence spectra were recorded at room temperature at c$10−8 M.
c The fluorescence quantum yield was determined in dichloromethane relative to 2-aminopyridine in 0.1N H2SO4.
Acknowledgements
On the other side, the synthesis of a parallel trigonal-
linear growing structure chain was undertaken for hete-
rocoupling of 6 (or 9) with 1,5-diiodonaphthalene.
We are grateful to Dr. Cristina Diez for the fluores-
cence quantum yield determination. We are undebted
to CICYT of Spain for financial support (Project No.
PB97-0060)
Hence, the reaction in the presence of palladium(II),
gives 16 (mp 228–230°C) (or 17, mp 271–273°C) in
quantitative yield (Scheme 6).
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