Bipyridine and Friends as Device Components

One of the most common types of ligand in supramolecular photochemistry is the bidentate chelate 2,2 -bipyridine (bpy, 11.1), which represents the first member of the homologous series of polypyridyls that includes 2,2 6, 2 -terpyridine (tpy, 11.3) and carries on up to the helicate-forming sexipyridine (10.120, Section 10.8) and beyond. 

In practice, metal complexes of bpy and phen, such as [Ru(bpy)3] and [Ru(phen)3] +, exhibit long-lived phosphorescent excited states, arising from ligand-centred triplet charge transfer states ( MLCT). Lifetimes are of the order of 10 -10 ns in fluid solution at room temperature. 

In order to take advantage of the simplicity of tpy chemistry, the ligand may be modified in two ways. Firstly, addition of electron acceptors to the 4 position (X) increases the luminescence lifetime markedly (36 ns in the case of S02Me). On the other hand, sophisticated substitution of bpy- and phen-based units can also give linear bridging ligands, as in 11.4. 

---