File:Ruthenium bipyridyl energy level diagram.png|thumb|left|200px|Transitions of [Ru(bpy)3]2+ where MC is metal centered, LC is ligand centered, and MLCT is metal ligand charge transfer. File:Ru(bpy)32+ absorption&emission.png|thumb|left|200px|Absorption and emission spectrum of [Ru(bpy)3]2+ in alcoholic solution at room temperature [Ru(bpy)3]2+ absorbs
ultraviolet and visible light. Aqueous solutions of [Ru(bpy)3]Cl2 are orange due to a strong
MLCT absorption at 452 ± 3 nm (
extinction coefficient of 14,600 M−1cm−1). Further absorption bands are found at 285 nm corresponding to ligand centered π*← π transitions and a weak transition around 350 nm (d-d transition). Light absorption results in formation of an
excited state have a relatively long lifetime of 890
ns in acetonitrile and 650 ns in water. The long lifetime of the excited state is attributed to the fact that it is
triplet, whereas the ground state is a
singlet state and in part due to the fact that the structure of the molecule allows for charge separation. Singlet-triplet transitions are forbidden and therefore often
slow. Like all molecular excited states, the triplet excited state of [Ru(bpy)3]2+ has both stronger oxidizing and reducing properties than its ground state. This situation arises because the excited state can be described as an Ru3+ complex containing a bpy•− radical anion as a ligand. Thus, the photochemical properties of [Ru(bpy)3]2+ are reminiscent of the
photosynthetic assembly, which also involves separation of an
electron and a
hole. [Ru(bpy)3]2+ has been examined as a
photosensitizer for both the oxidation and reduction of water. Upon absorbing a photon, [Ru(bpy)3]2+ converts to the aforementioned triplet state, denoted [Ru(bpy)3]2+*. This species transfers an electron, located on one bpy ligand, to a sacrificial oxidant such as
peroxodisulfate (S2O82−). The resulting [Ru(bpy)3]3+ is a powerful oxidant and oxidizes water into O2 and protons via a
catalyst. Alternatively, the reducing power of [Ru(bpy)3]2+* can be harnessed to reduce
methylviologen, a recyclable carrier of electrons, which in turn reduces protons at a
platinum catalyst. For this process to be catalytic, a sacrificial reductant, such as
EDTA4− or
triethanolamine is provided to return the Ru(III) back to Ru(II). Derivatives of [Ru(bpy)3]2+ are numerous. Such complexes are widely discussed for applications in biodiagnostics,
photovoltaics and
organic light-emitting diode, but no derivative has been commercialized. Application of [Ru(bpy)3]2+ and its derivatives to fabrication of optical chemical
sensors is arguably one of the most successful areas so far. == [Ru(bpy)3]2+ and photoredox catalysis ==