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'''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 (physics)|ion]].
Polypyridine compounds are multidentate [[ligand|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)<sub>3</sub>]<sup>2+</sup>. 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 [[ion|anion]]. This complex also exhibits [[Fluorescence|photoluminescence]] at room temperature, and its wavelength and lifetime can be tuned by substitution of either bipyridine or dithiolate moieties. Structual control is easier than for [[ruthenium]] complexes due to the square planar structure of the [[platinum]] complex.
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