Study of the effect of aliphatic and π-conjugated systems on the photophysical properties of polypyridinic Ruthenium II complexes as potential semiconductor materials for iTMC type LEC

The photophysical properties of polypyridinic Ruthenium complexes, as potential semiconductor materials for iTMC (ionic transition metal complex) type LECs have been studied. Substituted 2,2′-bipyridine ligands were used to study the effects of conjugated and aliphatic chains on the properties of the complexes, especially on the photophysical properties. Specifically, the N^N type ligands 4,4′-bis[2-hydroxy-2-(phenyl)ethyl]-2,2′-bipyridine (1), 4,4′-bis(α-styrene)-2,2′-bipyridine (2) and 4,4′-diphenylethyl-2,2′-bipyridine (3) were synthesized, and used to prepare the corresponding [RuII(bpy)2(N^N)](PF6)2 complexes. All three ligands contain a phenyl group as substituent for bpy, but with different residues as bridges between both: ligands 1 and 3 have free rotating connecting groups, while 2 is more rigid due to the styryl double bond. From the achieved results it was observed that, as expected, a conjugation on the ligand produces complexes with bands shifting toward lower energy regions, due to the electronic communication between the phenyl and bipyridine groups. On the contrary, in the absence of this conjugation, as is the case of the complexes with ligands 1 and 3, absorptions and emissions bands are very similar to the corresponding complex with unsubstituted bpy. Therefore, complexes with ligands 1 and 3 seem to be promising for LEC devices, due to the free rotations of the connecting aliphatic chain. This should preserve the properties as emission color, and advantages of Ru(bpy)32+ photoluminescence, but increasing the efficiency when used in a device, avoiding crystallization and diminishing self-quenching processes.