Synthesis, spectroscopic and electrochemical characterization of Co(II)-terpyridine based metallopolymer
Graphical abstract
Introduction
Polythiophene (PTh) based coatings have attracted tremendous attention in the last decades, thanks to the possible use in many fields, including chemical sensors, solar cells, field effect transistors, anticorrosive coatings, and electroluminescence devices [[1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]]. One of the most promising way to improve the performance of PTh derivatives lies in the introduction of redox metal centres covalently bonded to the polymer backbone [1,5,[12], [13], [14], [15], [16], [17], [18], [19]]. The impulse to develop similar hybrid materials comes from the possibility to combine the well known chemical, optical and electrochemical properties of PTh derivatives with those of metal complexes, taking advantage of the electronic interaction between the organic and inorganic frames. Similar materials, most widely investigated since the beginning of the nineties, are generally referred to as conducting metallopolymers [[14], [15], [16], [17], [18], [19]].
The electron communication between the organic and the inorganic moieties affects the mechanism of intra-chain conduction, which allows charge to flow along the polymer chain [[13], [14], [15], [16], [17], [18], [19]]. Modulation of the chemical nature of the organic and inorganic moieties, as well as of the eventual spacer between them, strongly conditions the physico-chemical properties of the material and, for this reason, constitutes a very appealing item. Due to the great importance of all these aspects, studies devoted to clarify the nature of the electronic interaction between a metal centre within a given ligand set and the polymer backbone, as well as between the metal centres of the same chain, are mandatory [[13], [14], [15], [16], [17], [18], [19]]. Inter-molecular electron transfers also condition heavily the charge percolation throughout the film. However, the contribution of this last process to the overall charge-transfer mechanism is much more difficult to be defined.
To study and address the synthetic effort to the characteristics sought by the material, the combination of results coming from electrochemical, spectroelectrochemical and in situ conductimetric investigations is necessary.
In this article, we report the synthesis of a terthienyl-substituted terpyridine, namely 4'-[5-(7-(thiophen-2-yl)-2,3-dihydrothieno [3,4-b] [1,4]dioxin-5-yl)thiophenyl]-2,2':6,2″-terpyridine (TETtpy - Scheme 1), together with the relevant complexation with Co(II) ions. Terpyridine moiety is chosen because it is suitable to stably bind a number of metal ions [[20], [21], [22], [23], [24], [25]]. The nature of the metal, conditioning the oxidation potential of the metal complex, allows more or less strong electronic interaction between the orbitals of the polymer and of the inorganic units. Moreover, it determines the conductivity and redox properties of the metallopolymer as a whole. The presence of a terpyridine moiety in the α-position of a terthienyl chain, also containing a 3,4-ethylendioxythienyl unit, should induce strong interaction between π-conjugated orbitals of the organic backbone and d-orbitals of the metal centres of the relevant Co(TETtpy)2 metal complex (Scheme 1). The charge flow along the polymer chain occurs by inner sphere mechanism, allowing the communication between adjacent metal centres by orbital overlap via a mutually bridging ligand [16].
Co(II) was chosen as the metal centre, due to the particularly low potential at which [Co(tpy)2]2+ oxidation occurs and to the recognized high stability of such a metal complex both in the reduced and in the oxidized state [26]. The chemical structure of the electrogenerated p[Co(TETtpy)2] deposit was defined by X-ray photoemission and absorption experiments.
Electrochemical, spectroelectrochemical and ac impedance tests allowed us to identify the potentials at which oxidation of the different fragments of the polymer coating occurs and to define the electronic state of the polymer as a whole.
Section snippets
Chemicals
All reagents were purchased from Aldrich and used as received. All organic solvents were anhydrous and the reactions requiring anhydrous conditions, even including electrochemical polymerization, were performed using oven-dried and nitrogen-flushed glassware.
Instrumentation
1H and 13C NMR spectra were recorded with a Bruker FT-NMR DPX200 or AVANCE400 spectrometer; chemical shifts (δ) are reported in part per million (ppm) downfield from TMS as internal standard (s = singlet, d = doublet, t = triplet,
Synthesis of Co(TETtpy)2
TETtpy was obtained by following the synthetic pathway reported in Scheme 1. TET was prepared in two steps as a yellow solid (98% yield) from commercially available 3,4-ethylenedioxythiophene, by suitably modifying the synthesis described by Zhu [28]. TET-CHO was obtained (48% yield) as a fluorescent orange solid, by reaction of TET with POCl3 at 0 °C, followed by quenching with sodium acetate. Treatment of TET-CHO with 2-acetylpyridin and potassium tert-butoxide, first, and with ammonium
Conclusions
A new metallopolymer, consisting of Co(Tpy)2 units included between hexathienyl chains, has been synthesized by electrochemical oxidation of the relevant monomer, obtained through original synthesis. The chemical structure of the deposit was studied by X-ray photoemission and absorption experiments. Electrochemical, spectroelectrochemical and ac impedance tests were performed to define the electronic state of the polymer at the different applied potentials. Results indicate that the presence of
Acknowledgements
The authors acknowledge Elettra Sincrotrone Trieste (https://www.elettra.trieste.it) for the financial and technical support to the execution of the X-ray photoemission and absorption experiments (proposal n. 2007340).
Dr. Diego Pinetti and Dr. Massimo Tonelli of the Centro Interdipartimentale Grandi Strumenti of the Ateneo of Modena and Reggio Emilia are also acknowledged for the acquisition of the mass spectra and of AFM images, respectively.
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