Magnesium(II) polyporphine: The first electron-conducting polymer with directly linked unsubstituted porphyrin units obtained by electrooxidation at a very low potential
Introduction
Porphyrin and porphyrin-based species represent the object of intensive studies during several last decades, with thousands publications appeared each year [1]. Owing to their chemical stability and specific physical and chemical properties these materials have been proposed for various applications in catalysis and electrocatalysis, non-linear optics, luminescent devices, microelectronic elements, etc. Besides, porphyrins play an important role in numerous biomolecules.
Terminologically, the porphyrin family is based on the principal building block, porphine ring composed from 4 pyrrole groups linked in their 2,5 positions by methylene bridges. All 4 nitrogen atoms of pyrroles are oriented inside the ring and they may associate either two hydrogen atoms giving free-base porphine (Fig. 1a), or a metal cation for metalloporphines, e.g. Mg2+ for magnesium porphine (Fig. 1b). Principally because of the easier synthetic availability, the dominant majority of studies deal with porphyrins, i.e. porphines with partial or full substitution at these carbon positions, by aromatic, aliphatic or more sophisticated functionalized groups.
In view of the aforementioned promising properties, porphyrins have been incorporated into numerous polymeric or oligomeric materials, to provide them with new functionalities. Towards this goal, three main approaches have been developed.
The first approach pertains to porphyrins, which are not included into the conjugation network of the polymer. These macrocycles can be linked via (conjugated or saturated) covalent bonds to a polymerizable fragment, as pyrrole, aniline or thiophene [2], [3]. Another possibility to incorporate porphyrins into a solid matrix is to trap, during the polymerization process (owing to electrostatic interactions), porphyrin-bearing ionic groups such as COO−, SO3−, NR3+ or pyridinium [2], [4]. Metalloporphyrins can also be linked together by axial coordination of bridging ligands such as bipyridine and DABCO on the metal cations, thus providing so-called “shish-kebab” coordination polymers [5].
The second approach concerns copolymers or co-oligomers with alternating porphyrin and aryl, thiophene, aniline or ethynyl units inside the principal conjugated chain [6]. They have been obtained by electrochemical oxidation of the corresponding monomer [7], [8], [9] or by chemical synthesis [10], [11].
The third approach consists in synthesizing solute oligoporphyrins as linear or branched chains in which the neighboring porphyrin units are directly linked in meso or/and β positions while other meso positions of the monomer were made non-reactive owing to protecting groups (aryl, alkyl, etc.) surrounding the ring [12], [13], [14], [15], [16], [17], [18].
To our best knowledge, no polymers (neither homopolymers nor copolymers) with porphine building blocks, i.e. completely unprotected porphyrins have been synthesized and no polymer systems with incorporated porphine moieties are known.
Assuming that a totally unsubstituted porphyrin should undergo very efficient oxidative C–C coupling between porphine units, we have very recently described the redox reactivity of magnesium porphine (MgP) [19]. It has been shown that it was possible to generate directly linked porphine oligomers in dichloromethane solution by applying a potential corresponding to the first oxidation potential of MgP. At the end of the electrolysis, a deep purple material was systematically deposited on the platinum anode. Interested by this intriguing material, we have discovered that it is possible to electropolymerize MgP on an electrode surface.
Thus, the goal of our actual study was to synthesize electroactive films of this type by means of the electrochemical oxidation of the magnesium(II) porphine monomer, MgP (Fig. 1b). The choice of the coordinated cation, Mg(II), stems from the easier synthetic access to the porphine family recently reported for MgP by Lindsey's group [20]. Moreover, among free-base and metalloporphyrins, magnesium porphyrins exhibit one of the lowest first oxidation potential [21] which is of particular interest to avoid “overoxidation” of the electrogenerated material, well-known for electroactive/conducting polymers. In addition, magnesium porphyrins are involved in photosynthetic process [22], [23] and thus polymeric films of MgP may be expected to possess interesting photophysical properties.
The first results on the electropolymerization of MgP and the characterization of the new synthesized electroactive materials are presented below.
Section snippets
Experimental
MgP monomer was synthesized according to Lindsey's procedure [20]. The data (1H NMR, 13C NMR, UV–visible absorption, and MALDI-TOF) were consistent with those in [20].
Acetonitrile (AN) for polymerization and cyclic voltammetry (CV) studies of the film in a monomer-free solution was HPLC grade (Prolabo) with initial water content <0.02% used without further treatment. Tetrahydrofuran (THF) for partial polymer film dissolution (Acros Organics) was distilled over metallic Na + benzophenon (Merck).
Monomer oxidation and film formation
If the potential of a bare electrode (Pt, glassy carbon, ITO) in contact with the MgP-containing electrolyte solution in AN is swept in the positive direction one can see several successive oxidation waves (Fig. 2a), the peak potential of the first wave being at about 0.4 V (Fig. 2a and b). The presentation of these data in the form of a “reduced voltammogram” (Fig 2a) where the current is divided by the monomer concentration, c, and by the square root of the scan rate, , demonstrates the
Conclusion
The synthesized intriguing material, “poly(magnesium(II)-porphine)” (pMgP), belongs to electroactive polymers, possessing characteristic electronically conducting states, with mobile holes (“polarons”, “bipolarons”) at high oxidation levels and mobile “electrons” for a strongly reduced state, separated by the non-electroactivity range in which the material is electronically non-conducting. This behavior is related expectedly to the transition between a molecular organization with small dihedral
Acknowledgment
The authors are thankful J.M. Barbes, Yu.G. Gorbunova and P.D. Harvey for stimulating discussions. We are deeply grateful to J. Cox, Z. Galus, J. Heinze, V.M. Mirsky, R. Seeber, O.A. Semenikhin, M. Skompska and V. Tsakova for the thorough analysis of the primary version of the manuscript and the valuable comments on its improvement. The financial support of CNRS, Conseil Régional de Bourgogne, Université de Bourgogne and the Russian Foundation for Basic Research (project no. 09-03-01172a) is
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