Elsevier

European Polymer Journal

Volume 42, Issue 11, November 2006, Pages 2940-2948
European Polymer Journal

Synthesis and characterization of a π-conjugate, covalent layered network derived from condensation polymerization of the 4,4′-bipyridine-cyanuric chloride couple

https://doi.org/10.1016/j.eurpolymj.2006.07.025Get rights and content

Abstract

Nucleophilic substitutions of the reactive chlorine atoms in cyanuric chloride by the bridging 4,4′-bipyridine in refluxing toluene lead to quaternarization of the latter and the subsequent formation of a π-conjugate, covalent layered network. The network is composed of central 1,3,5-triazine units with 4,4′-bipyridinium rings covalently attached and balanced by the released chloride ions. Due to the extremely high electron deficiency of the triazine rings, the material undergoes partial reduction by its compensated chloride ions resulting in a radical concentration of 1 × 1020 spin g−1, according to EPR quantitative analysis. In this instance, the radicals provide stability to the organic network by minimizing its electron deficiency. The material exhibits thermal and electrochemical stability, as evidenced by thermal gravimetric analysis (TGA) and cyclic voltammetry techniques. As such, the π-conjugate organic material displays low band gaps and electrical conductivity in the range of 10−4–10−5 S cm−1 at room temperature.

Introduction

Engineering of organic networks defines a rapidly emerging discipline in materials science due to its both fundamental and practical importance [1], [2], [3], [4], [5]. The aim of this process is namely to establish connections between molecular units towards supramolecular architectures with certain functionalities in the solid-state. In particular, suitable organic molecules of the same or different kind are assembled into intriguing solid-state networks via hydrogen bond or covalent interactions between the constituent parts. In this context, 1,3,5-triazine derivatives have been widely used in the design of two-dimensional organic networks on the basis of their structural diversity, molecular symmetry, π-interaction abilities and aptitude to form hydrogen bonds [1], [2], [3], [4], [5]. A well-known member of this family is cyanuric chloride, of which the reactivity of the chlorine atoms allows the preparation of a series of triazine derivatives that serve as unique building units in a variety of layered organic materials. Although there is an extensive literature on the engineering of hydrogen bonded layered networks based on 1,3,5-triazine rings [1], [2], [3], however, only few reports refer to covalent layered analogues, with most representative examples being the cases of carbon nitride C3N4 and melamine-cyanuric chloride adducts [4], [5].

On the other hand, there is an increased research interest in the design and synthesis of novel low band gap, π-conjugated organic networks that will further expand the gallery of available polymeric materials, such as polyanilines, polythiophenes, polybenzothiazioles and polyisonapthenes [6], [7], [8]. The extensive delocalization of the p orbitals within these conjugated systems, which builds up a characteristic electronic band structure, alongside with the presence of a certain radical population, as evidenced by EPR and magnetic susceptibility measurements [9], give rise to characteristic UV–visible transitions and inherited electronic properties [10]. As a result the particular systems hold promise in the field of photovoltaics and electronic devices.

Along this lines, we report here the synthesis and characterization of a π-conjugate organic material based on a covalent 4,4′-bipyridine-cyanuric chloride layered network that undergoes partial reduction by its chloride counterions towards the generation of stabilizing free radicals. To the best of our knowledge, covalent networks based on 1,3,5-triazine and 4,4′-bipyridine rings have not been reported or studied yet. The structure, morphology, stability and solid-state electronic properties of the radical-rich, π-conjugate organic material has been unveiled with a variety of techniques. The material displays low band gaps and electrical conductivity in the range of 10−4–10−5 S cm−1 at room temperature. The method is anticipated to give access to a wider class of similar derivatives by varying the structure of the starting reagents, e.g. type of the bridging diamine.

Section snippets

Materials

All chemical and solvent reagents employed in this work were of analytical grade and used as received. Cyanuric chloride (C3N3Cl3) and anhydrous 4,4′-bypyridine (C10H8N2) were supplied from Fluka. Sodium lauryl sulfate (C12H25OSO3Na) was purchased from Aldrich.

Synthesis

In a typical preparation, 1.1 g cyanuric chloride was dissolved in 50 mL toluene (∼95 w/v% solubility). The solution was filtered off and to the clear solution 1.5 g anhydrous 4,4′-bypyridine was added to afford a bulk precipitate. The slurry

Synthesis

The colored organic product is synthesized in an almost quantitative yield by nucleophilic substitutions of the chlorine atoms by 4,4′-bypiridine in the proper molar ratio under reflux in toluene. Fig. 1 illustrates a schematic view of the 2-D conjugated network, assuming a co-planar orientation for the triazine and the bipyridine building units. Accordingly, the network consists of central triazine molecules bridged by bipyridinium moieties in a triangular symmetry. Although triazine

Conclusions

The quantitative condensation polymerization between cyanuric chloride and 4,4′-bipyridine gives rise to a layered network composed of triazine central cores covalently attached with bridging 4,4′-bipyridine rings in triangular fashion. The high electron deficiency of the triazine rings induces partial reduction of the network by its chloride counterions and the subsequent formation of stable free radicals. The radicals confer stability to the network by minimizing its electron deficiency. As a

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