Synthesis and properties of 6,6′-dithienyl-4,4′-bipyrimidine and its hetero- and homo-leptic Ru(II) complexes

Abstract

Hetero- and homo-leptic Ru(II) complexes of a new 4,4′-bipyrimidine ligand, th 2bpm (6,6′-di(2″-thienyl)-4,4′-bipyrimidine), have been synthesized and characterized. The parent ligand th 2bpm has electron rich thiophene units on the periphery of a bidentate ligand which is capable of binding to metal ions. The heteroleptic complex of th 2bpm [Ru(bpy) 2th 2bpm] 2+ (bpy = 2,2′-bipyridine) exhibits a Ru-to-bpm metal-to-ligand charge transfer (MLCT) absorption centered at 547 nm and a Ru-to-bpy MLCT absorption centered at 438 nm. The assignment of the low energy absorption is supported by the relative ease of electrochemical reduction of the new complex as compared to [Ru(bpy) 3] 2+. The homoleptic complex, [Ru(th 2bpm) 3] 2+, exhibits a Ru-to-bpm MLCT absorption at slightly higher energy (544 nm). Both complexes are emissive at room temperature in fluid solution and 5 is one of the lowest energy emitters based on tris-bidentate Ru(II) complexes known ( λ max = 770 nm). The luminescence spectra is red-shifted compared to [Ru(bpy) 3] 2+ and this effect is ascribed to the delocalization in the acceptor ligand.