Elsevier

Polyhedron

Volume 49, Issue 1, 25 January 2013, Pages 24-28
Polyhedron

Homoleptic Ru(II) complex with terpyridine ligands appended with terthiophene moieties: Synthesis, characterization and electropolymerization

https://doi.org/10.1016/j.poly.2012.09.056Get rights and content

Abstract

A new Ru(II) complex, [Ru(TTT)2][PF6]2 (TTT = 4′-[(2,2′:5′,2″-terthien-3′-yl)methoxy]-2,2′:6′,2″-terpyridine) has been synthesized and characterized by 1H NMR and UV–Vis spectroscopy and by cyclic voltammetry. The electrosynthesis of the corresponding conductive polymer, P[Ru(TTT)2], is reported, as well as its voltammetric and spectroelectrochemical characterization. The comparison between [Ru(TTT)2][PF6]2 and TTT voltammetric data shows that the presence of the metal ion makes easier the oxidation and the polymerization of the organic framework. On the other hand, UV–Vis data confirm that the presence of the –O–CH2– spacer between coordinating and polymerizing unit (terpyridine and terthiophene, respectively) reduces the electronic delocalization among these moieties, analogously to that observed in the TTT ligand alone. As far as P[Ru(TTT)2] is concerned, the voltammetric characterization evidences the so-called ‘charge-trapping’ phenomena, already observed also in PTTT, too. Moreover, spectroelectrochemical characterization shows a likely overlapping of the π–π polymer and MLCT complex absorption bands, causing a bathochromic shift with respect to PTTT.

Graphical abstract

A new ruthenium(II) complex, [Ru(TTT)2][PF6]2 (TTT = 4′-[(2,2′:5′,2″-terthien-3′-yl)methoxy]-2,2′:6′,2″-terpyridine) was synthesized. The spectroscopic and electrochemical characterization of the terpyridine-coordinated species, the electrosynthesis of the corresponding polymer P[Ru(TTT)2] and its optical and electrochemical characterization are reported.

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Introduction

Polypyridyl-Ru(II) complexes with N-donor ligands, such as bipyridine, 1,10-phenanthroline and terpyridine, have attracted a great interest over the past three decades, due to their peculiar photophysical, photochemical and electrochemical features [1], [2], [3], [4]. Such properties make these species appealing in the fields of photocatalysis, dye sensitized solar cells (DSSCs) and artificial photosynthesis [5], [6], [7], [8]. Especially Ru(II) complexes with bidentate ligands have shown excellent photophysical properties, starting from the highly emissive [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) [9]. However, the main trouble of this class of compounds lies in the fact that they afford chiral complexes, leading to mixtures of diastereomers hard to resolve. On the other hand, the bis-terdentate complex [Ru(tpy)2]2+ (tpy = 2,2′:6′,2″-terpyridine) shows a very weak emission and a short-lived excited state, preventing its application for photochemical devices [10]. Nevertheless, tpy ligand can be easily functionalized in the 4′ position of the central ring, leading to achiral assemblies and allowing the tuning of the photochemical properties of the related complexes. A successful way to properly modify the structure of the tpy ligand consists of the introduction of electron-donating or electron-withdrawing groups in the 4′ position. Especially, five-membered heterocyclic rings, such as thiophene, furane and pyrrole, lead to a more extended delocalization, stabilising the 3MLCT (metal-to-ligand charge transfer) states relative to non-emissive 3MC (metal centred) states, mostly in systems where the heterocyclic ring facilitates a co-planar arrangement [11]. Furthermore, such substitution allows the obtainment of a molecule with two different functions: a coordinating one, due to the tpy fragment, and a polymerizing one, due to the heterocyclic ring. The corresponding multifunctional metal-containing π-conjugated polymers form a very interesting class of materials: the simultaneous presence of a polymer backbone and a metal complex, performing different roles, induces a mishmash of the conductivity deriving from the conjugated framework with the optical, electronic and catalytic function of the metal complex. Models for charge transfer in conducting polymers and metallopolymers were developed since 1990’s [12], [13], [14], [15], [16], accounting for the electronic conduction in the polymer backbone and the ionic conduction in the ionic pores. If the metal complex is characterized by a strong absorption in the visible region, the resulting metallopolymer shows a higher photosensitivity [17]. Moreover, combining Ru(II)-polypyridyl fragments with thiophene-based oligomers allows to obtain mono- or multimetallic arrays suitable for optoelectronic applications and sensitizers for DSSCs [18].

In a previous work [19] we reported the synthesis and the electrochemical characterization of a novel species (4′-[(2,2′:5′,2″-terthien-3′-yl)methoxy]-2,2′:6′,2″-terpyridine, TTT), where the polymerizing terthiophene framework was linked to a terpyridine unit, able to coordinate transition metal ions. The electrochemistry and UV–Vis spectroscopy of the corresponding electro-generated polymer, PTTT, was also reported.

In the present paper we describe the synthesis, the spectroscopic characterization and the electrochemistry of a new homoleptic complex of TTT with Ru(II) ion, [Ru(TTT)2][PF6]2 (Scheme 1).

Section snippets

Materials and methods

All reagents used for the synthesis were purchased from Aldrich and used without prior purification. 1H NMR spectra were recorded on a Varian VXR 300 spectrometer, working at 300 MHz, in CD3CN solution, using the solvent residual signal (1.94 ppm) as internal standard [20]. QTOF/MS spectra were recorded on a QTOF2 instrument by WATERS. Elemental analyses were recorded using a Perkin Elmer 240B elemental analyzer. Electrochemical characterizations were performed with an Autolab PGSTAT 12 system

Synthesis and characterization

The synthesis of [Ru(TTT)2][PF6]2 was first attempted by us starting from RuCl3 and TTT, according to the approach described by Husson et al. [22], based on the extraction of chlorides with AgBF4 and on the subsequent anion exchange of BF4 with a saturated aqueous solution of KPF6. The synthetic route proposed by Newkome and He [23] was then explored: TTT and RuCl3 mixture was refluxed in CH3OH, to obtain the corresponding Ru(TTT)Cl3 derivative; then 4-ethylmorpholine and an excess of NH4PF6

Conclusions

A novel bis-terdentate ruthenium(II) complex, [Ru(TTT)2]2+, where 2,2′:6′,2″-terpyridine was functionalized in the 4′ position with a polymerizing fragment such as 2,2′:5′,2″-terthiophene, was synthesized. The spectroscopic characterization suggests that electronic communication between terthiophene unit and terpyridine-complex is not completely effective, due to the unsaturated spacer (–O–CH2–), and they behave as two electronically distinct fragments. The comparison between ligand polymer,

Acknowledgment

Authors would like to thank Dr. S. Crobu for recording MS spectra.

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