Photoinduced Charge Separation within Metallo-supramolecular Wires Built around a [Ru(bpy)₃]²⁺–Bisterpyridine Linear Entity

Abstract

A [Ru(bpy)3](2+)-like complex (L1) bearing two free terpyridine groups at the 5 and 5' positions of the same bipyridine, linked by the rigid and linear 2,5-dimethyl phenylene bridges has been synthesized to open access to two classes of linear molecular wires with photosensitive properties: a bimetallic coordination polymer and an inorganic triad. In this Research Article, we report on the synthesis and characterization of the resulting [{Ru(II_)Fe(II)}n](4n+) alternated bimetallic polymer and the [Co(III_)Ru(II_)Fe(II)](7+) triad based on the building block L1. The [{Ru(II_)Fe(II)}n](4n+) polymer is fully characterized in solution. Cyclic voltammetry and emission lifetime measurements show that the bridging ligand allows interaction between the metal centers in the excited state despite the lack of interactions in the ground state. Under visible irradiation, the polymer can be fully oxidized in the presence of a sacrificial electron acceptor in solution. Thin robust films of the polymer are easily obtained on ITO by a simple electrochemical procedure based on an electroreduction adsorption process. The ITO/[{Ru(II_)Fe(II)}n](4n+)-modified electrode behaves as a photocathode under irradiation in the presence of ArN2(+). The magnitude of the photocurrent is dependent on the film thickness, probably limited by the diffusion of charge in thicker film. On the other hand L1 is also used to construct a well-ordered triad in association with Co(III) and Fe(II) metallic centers as electron acceptor and donor, respectively. The metallic triad is anchored on ITO or on a SiO2 wafer, starting from a terpyridine phosphonate modified surface. AFM images prove the presence of the triad in a linear upward orientation. Irradiation of the ITO/[Co(III_)Ru(II_)Fe(II)](7+) modified surface in the presence of triethanolamine in CH3CN induces the generation of an anodic photocurrent of around 30 μA.cm(-2). The photocurrent density generated by the ITO/[Co(III_)Ru(II_)Fe(II)](7+) electrode, appears to be more stable than in the case of ITO/[{Ru(II_)Fe(II)}n](4n+) because of the presence of the anchoring group. Moreover, this photocurrent magnitude represents an enhancement of 30% compared to our previous triad ( Dalton Trans. 2014 , 43 , 12156 - 12159 ), proving the advantage of a linear and rigid spacer for the construction of such molecular assemblies with photoinduced charge transfer abilities.

Spacer Conjugation and Surface Support Effects in Monolayer Electrochromic Materials

“…26 Among others, terpyridine (tpy)-based ligands have been shown to form stable complexes with a wide range of metal ions in different oxidation states. 7,14,15,27 Since the seminal works by Constable, 28,29 the synthetic methodology of tpy derivatives has been significantly improved. 30−32 In contrast to bpy-based asymmetric ligands that often form a mixture of facial and meridional isomers, tpy-containing ligands are structurally more appealing, because they form well-defined six-coordinate cationic Fe(II), 33 Co(II), 34 Ru(II), 35 and Os(II) 36 complexes where two ligating moieties are situated in an orthogonal fashion.…”

“…Therefore, by adjusting the ligand electronics, both electric and optic responses of the metal complex can be fine-tuned . Incorporating a [Ru­(bpy) 3 ] 2+ motif to modify indium–tin oxide (ITO) with a Co­(III)/Ru­(II)/Fe­(II) triad and irradiating the assembly at a bias of 0.12 V (to maintain a Co­(III) state, thus making it a good electron acceptor toward the excited state of [Ru­(bpy) 3 ] 2+ *), Chauvin observed the generation of an anodic photocurrent in the presence of a tertiary amine as a sacrificial electron donor . The back electron transfer within the ITO/[Co­(II)–Ru­(III)–Fe­(II)] 7+ assemblies is short-circuited either by a fast electron injection from the Co­(II) center to the ITO electrode at 0.12 V and/or by an efficient electron transfer process from the Fe­(II) subunits to the Ru­(III) center (Δ G = −200 mV).…”

Synthesis of an Alternated Heterobimetallic Supramolecular Polymer Based on Ru(II) and Fe(II)

“…The synthesis of heterobimetallic supramolecular polymers is generally achieved by two strategies. One strategy is the introduction of two heterometal ions into the polymer chain in different coordination environments, which is achieved through different binding affinity of the ligand for different metal ions [16]. Another strategy is the introduction of heterometal ions into the MSP chain in identical coordination environments, which is synthetically challenging [14].…”

Controlling the Electronic Contact at the Terpyridine/Metal Interface

“…Subsequently, transition-metal ions and bridging ligands are alternately provided to enable wire growth. Among different bridging ligands, ligands with one or more 2,2′:6′,2″-terpyridine (TP) motives have proven particularly popular since they form reliable metallo-supramolecular wires. − TPs are easily accessed synthetically and have rich coordination chemistry. , High binding constants to different transition metals make them especially useful for the incorporation of multiple metal centers into molecular wires. Using TP derivatives as bridging ligands, the last step of the metallo-supramolecular wire assembly between two nanoelectrodes is often the formation of the electrical contact between one TP group and the counter electrode.…”