Synthesis of poly(aryleneethynylene)s bearing glucose units as substituents

Article abstract of DOI:10.1039/b207753a

A series of poly(aryleneethynylene)s functionalized with acetylated glucopyranosyl units were synthesized by the Pd-catalyzed reaction of trimethylsilylethynyl derivatives with aromatic halides in the presence of silver oxide.

Full text of DOI:10.1039/b207753a

Synthesis of poly(aryleneethynylene)s bearing glucose units as
substituents†
Francesco Babudri,a Donato Colangiuli,a Paolo A. Di Lorenzo,a Gianluca M. Farinola,a Omar Hassan
Omarb and Francesco Naso*a
a Dipartimento di Chimica, Università degli Studi di Bari, via Orabona, 4, I-70126 Bari, Italy
b CNR ICCOM, Dipartimento di Chimica, Università degli Studi di Bari, via Orabona, 4, I-70126, Italy.
E-mail: naso@area.ba.cnr.it; Fax: 0039 080 5442924; Tel: 0039 080 5442073
Received (in Cambridge, UK) 7th August 2002, Accepted 14th November 2002
First published as an Advance Article on the web 2nd December 2002
A series of poly(aryleneethynylene)s functionalized with
acetylated glucopyranosyl units were synthesized by the Pd-
catalyzed reaction of trimethylsilylethynyl derivatives with
aromatic halides in the presence of silver oxide.
Moreover, PAEs functionalized with monosaccharide units may
act as multivalent glycosides ligands for proteins and peptides.
Various dendritic or linear non-conjugated polymeric structures
have been recently proposed for this purpose.14 However, up to
now, no example has been reported concerning conjugated
polymers with this function. The consequence of the protein–
polymer interaction may be the quenching of the intense
fluorescence emission of these polymers,6 which can be
exploited in fabricating specific sensors for the interacting
protein.
On the basis of these promising potentialities and in the
framework of our efforts dealing with the synthesis and
characterization of various classes of conjugated polymers,15we
considered it of interest to investigate a synthetic strategy for
carbohydrate substituted PAEs.
We have found that the PAEs 1a and1b (Scheme 1), can be
build up by cross-coupling between the trimethylsilyl derivative
2 and the diiodo arenes 3a and 3b, respectively (ESI†).
As shown in Scheme 1, the polymerization was performed
adopting the Pd-catalyzed coupling reaction of trimethylsilyle-
thynyl derivatives with aromatic halides in the presence of
silver oxide16 (Scheme 1). As shown by a comparative
preliminary work, this methodology, with respect to the classic
Cassar–Heck–Sonogashira cross-coupling process, is known to
produce a lower amount of compounds deriving from oxidative
homocoupling reaction of the ethynyl derivative, and has been
successfully applied to the synthesis of different PAEs.16
In our case, the 1H and 13C NMR spectra of the copolymer 1b
demonstrates the complete absence of structural defects that
could originate from the oxidative homocoupling reaction of the
monomer 2. In order to obtain other glucose substituted PAEs,
the monomer 2 was also coupled with diiodotetrafluorobenzene
3c and dibromothiophene 3d, affording, in high yields,
copolymers 1c and 1d (Scheme 1).
Conjugated polymers represent a wide class of organic
compounds with properties of electric conductors1 or semi-
conductors2 and, in the last two decades, they have attracted the
interest of the scientific community for their promising
applications in various electronic devices,3 such as light-
emitting diodes (LEDs),3a plastic lasers,3b and photovoltaic
cells.3cAmong the different typologies of conjugated polymers,
which have come into focus in the last few years, poly-
(aryleneethynylene)s (PAEs) are considered one of the most
representative classes.4 LEDs,5 sensors,6 molecular wires,7 and
polarizers for LC displays8 have been fabricated using PAEs as
active semiconducting materials, and, for this reason, a great
multiplicity of molecular architectures have been produced.
An interesting structural modification lacking in the wide
view of PAEs, and, in general, in the field of conducting
polymers, is represented by conjugated materials having
carbohydrate units as substituents. The few examples reported
are related to polythiophenes9 or polyazulenes10 with
D-
glucopyranose pendant groups, obtained by oxidative or
electrochemical polymerization of suitable monomers, or to
sugar-coated polydiacetylene11 obtained by UV polymerization
reaction of amphiphilic butadyines in liposomes or Langmuir–
Schaefer films. This is rather surprising in view of the interest
which is usually attributed to polymers, mainly of the
poly(phenylenevinylene) family (PPVs), presenting various
types of chiral nonracemic substituents.12
On the other hand, the introduction of monosaccharide
functionalities as substituents in the chain of conjugated
materials may induce solubility properties and features of
potential interest in various fields of application of semi-
conducting organic polymers, such as LEDs emitting polarized
light deriving from the presence of chiral nonracemic groups.13
Molecular mass values are reported in Table 1 together with
reactions yields, UV, and fluorescence data.
The lower molecular weight of copolymer 1a, compared with
that of 1b, may be due to steric hindrance of monosaccharide
substituents on the nearest benzenic rings that may prevent the
growth of the polymeric chains. The higher molecular weight
value measured in the case of polymer 4 (Scheme 2) supports
this hypothesis.
† Electronic supplementary information (ESI) available: experimental
procedures and related references, and IR and NMR spectra of all the
compounds. See http://www.rsc.org/suppdata/cc/b2/b207753a/
Scheme 1 Polymerization reaction.
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CHEM. COMMUN., 2003, 130–131
This journal is © The Royal Society of Chemistry 2003

Table 1 Molecular mass values and lmax data for PAEs 1a-e, 4
Polymer
Yields (%)
Mnab (Da)
Mwac (Da)
Mw/Mn
UV-Vis lmaxd(nm)
Fluorescence lmaxd (nm)
1a
1b
1c
1d
1e
4
86
86
90
90
99
95
5000
19500
4200
2600
—
6900
32500
14700
16000
—
1.4
1.7
3.5
6.4
—
448
421
392
427
418e
433
459
464
437
472
459e
455
11650
27500
2.4
a Determined by gel permeation chromatography (GPC) with uniform polystyrene standards and THF as solvent. b Number average molecular mass. c Weight
average molecular mass. d In chloroform solution. e In methanol solution.
may prevent the aggregate formation, even in the presence of
relatively high concentration of non polar solvent.
In conclusion, we have succeeded in setting up a procedure
for the preparation of a series of poly(aryleneethynylene)s
substituted with glucose units, which may find useful applica-
tions in various fields of organic materials based devices. The
advantages of our procedure are represented by (i) the
versatility deriving from the possibility of placing the sugar
moiety either on the disilyl derivative or on the aromatic
diiodides (ii) the experimental simplicity, and (iii) the high
yields in products which do not present structural defects due to
homocoupling.
This work was financially supported by Ministero dell’Uni-
versità e delle Ricerca Scientifica e Tecnologica, Rome (Project
‘Sintesi di materiali organici per applicazioni ottiche’ L.488
19/12/92, Piano ‘Materiali Innovativi’).
Scheme 2 Synthesis of polymer 4.
Indeed, in the latter polymer the butadyine units give a greater
distance between the two nearest aromatic rings in the
polymeric chain than the acetylenic groups. However, steric
hindrance cannot be the only factor affecting the length of the
polymer. A lower solubility of the growing chains may also lead
to a lower degree of polymerization.
The hydrolysis of 1b, easily conducted by an alkali (MeO2/
MeOH) treatment, led to the corresponding deacetylated
polymer 1e, soluble in polar solvents (methanol, dimethyl
sulfoxide) and moderately soluble in water.
Notes and references
‡ In mixtures with chloroform concentration higher than 60% the polymer
1e is insoluble.
Recently, aggregate and excimer formation in a mixture of
solvent/non solvent (chloroform/methanol) has been proved for
alkyl substituted PAEs by absorption and fluorescence spectros-
copy.17 Indeed, we have noticed a similar behaviour for the PAE
1b. The absorption spectra of this polymer in a mixture of these
two solvents starting from 60% up to 90% of methanol in
chloroform showed the appearance of a sharper second red
shifted band (469 nm), which could be ascribed to polymer
aggregate formation as a consequence of the non solvent
(methanol) presence (Fig. 1). On the contrary, for the depro-
tected polymer 1e we did not found a similar tendency to
aggregate in solution. Polymer 1e is almost completely
insoluble in chloroform, which acts in this case as non solvent.
Absorption spectra in mixtures of methanol/chloroform from
90% to 60% of methanol are almost identical.‡ The strong
interaction between the hydroxy groups on 1e and methanol
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Fig. 1 Absorption spectra of polymer 1b in various mixtures of chloroform/
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