Terthiophene-appended Ru(II)-terpyridine complexes: nature of the tether and luminescent behaviour.

Polypyridyl-Ru(II) complexes have attracted a great interest due to their photophysical, photochemical and electrochemical properties [1,2], making them appealing for application in the field of catalysis, dye sensitized solar cells and artificial photosynthesis [3,4]. Notably, [Ru(bpy)3]2+ (bpy = 2,2’-bipyridine) complexes show outstanding photophysical and electrochemical properties [5]. However, the synthesis of Ru(bpy)3-core species often affords chiral complexes as mixture of diastereomers hard to resolve. On the other hand, the bis-terdentate [Ru(tpy)2]-core (tpy = 2,2’:6’,2”-terpyridine) complexes are easily obtained as a pure form, but the simple species [Ru(tpy)2]3+ shows a very weak emission and a short-lived excited state, preventing its application in photochemical devices [6]. Nevertheless, tpy ligand can be functionalized by electron-withdrawing and electron-donating substituents in order to modify its structure, allowing the tuning of the properties of the related complexes. Especially, 5-membered heterocyclic rings as thiophene lead to an extended delocalization and stabilize 3MLCT states relative to the non-emissive 3MC ones, in particular in co-planar systems [7]. Moreover, functionalization with heterocyclic rings allows the substituted tpy i) to act as a coordinating ligand towards transition metal ions through the N-donor centres, and ii) to polymerize, along the 5-memberd rings. Then, it is possible to obtain multifunctional metal-containing π-conjugated polymers, combining the conductivity properties of the organic framework with the optical, electronic and catalytic properties of the metal complex. In this contest, we addressed our research interest to the study of terthiophene-appended-terpyridine ligands [8] and of the corresponding Ru(II) complexes [9], where the 2,2’:5’,2”-terthiophene unit, having at least one α-position free in order to polymerization happen, is linked to tpy unit by a saturated or unsaturated spacer. In particular, here we report a comparison in optical and electrochemical properties at varying the tether between the terthiophene (polymerizing) and the tpy (coordinating) moieties.