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

Abstract

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.

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Conjugated polymers represent a wide class of organic compounds with properties of electric conductors¹ or semiconductors² 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 semiconducting organic polymers, such as LEDs emitting polarized light deriving from the presence of chiral nonracemic groups.¹³ 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†).

Scheme 1 Polymerization reaction.

As shown in Scheme 1, the polymerization was performed adopting the Pd-catalyzed coupling reaction of trimethylsilylethynyl 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).

Molecular mass values are reported in Table 1 together with reactions yields, UV, and fluorescence data.

Table 1 Molecular mass values and λ_max data for PAEs 1a-e, 4

Polymer	Yields (%)	M n ^a ^b (Da)	M w ^a ^c (Da)	M w/Mn	UV-Vis λmax^d(nm)	Fluorescence λmax^d (nm)
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.
1a	86	5000	6900	1.4	448	459
1b	86	19500	32500	1.7	421	464
1c	90	4200	14700	3.5	392	437
1d	90	2600	16000	6.4	427	472
1e	99	—	—	—	418^e	459^e
4	95	11650	27500	2.4	433	455

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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.

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 (MeO⁻/MeOH) treatment, led to the corresponding deacetylated polymer 1e, soluble in polar solvents (methanol, dimethyl sulfoxide) and moderately soluble in water.

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 spectroscopy.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 deprotected 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 may prevent the aggregate formation, even in the presence of relatively high concentration of non polar solvent.

Fig. 1 Absorption spectra of polymer 1b in various mixtures of chloroform/methanol.

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 applications 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’Università e delle Ricerca Scientifica e Tecnologica, Rome (Project ‘Sintesi di materiali organici per applicazioni ottiche’ L.488 19/12/92, Piano ‘Materiali Innovativi’).

Notes and references

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Footnotes

† 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/

‡ In mixtures with chloroform concentration higher than 60% the polymer 1e is insoluble.

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