Polypyridine complex

Polypyridine complexes are compounds in which a polypyridine, such as 2,2'-bipyridine, 1,10-phenanthroline, or 2,2';6'2"-terpyridine, coordinates to a metal ion.

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Polypyridine compounds are multidentate ligands which have characteristic properties, such as photosensitization. The complexes are very stable to light, electricity, and heat because the bonds between the central metal ion and polypyridine ligands are usually very strong; furthermore, the polypyridine itself is generally very stable. The compounds have peculiar optical, electrochemical, and magnetic properties. Some complexes exhibit a strong absorption band in the visible light region, which is called metal-to-ligand charge transfer (MLCT) or ligand-to-metal charge transfer (LMCT). The properties of the complexes can be tuned easily by introducing substituents, for example, electron donation, electron withdrawal, and π-conjugating groups, to the polypyridine moiety. The MLCT absorption band can be shifted, the emission wavelength can be changed, and the emission lifetime can be extended.

A well-known example of a polypyridine complex is rutheniumtris(bipyridine), Ru[(bpy)₃]²⁺. This complex exhibits intense luminescence at room temperature in aqueous solution. Another example is a platinum-bipyridine-dithiolate complex, Pt(bpy)(bdt), in which bdt denotes a 1,2-benzenedithiolate anion. This complex also exhibits photoluminescence at room temperature, and its wavelength and lifetime can be tuned by substitution of either bipyridine or dithiolate moieties. Structural control is easier than for ruthenium complexes due to the square planar structure of the platinum complex.