text: [Ru(bpy)] absorbs ultraviolet and visible light. Aqueous solutions of [Ru(bpy)]Cl are orange due to a strong MLCT absorption at 452 ± 3 nm (extinction coefficient of 14,600 Mcm). 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 excited state relaxes to the ground state by emission of a photon or non-radiative relaxation. The quantum yield is 2.8% in air-saturated water at 298 K and the emission maximum wavelength is 620 nm.  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)] has both stronger oxidizing and reducing properties than its ground state.  This situation arises because the excited state can be described as an Ru complex containing a bpy radical anion as a ligand.  Thus, the photochemical properties of [Ru(bpy)] are reminiscent of the photosynthetic assembly, which also involves separation of an electron and a hole.
  [Ru(bpy)] has been examined as a photosensitizer for both the oxidation and reduction of water.  Upon absorbing a photon, [Ru(bpy)] converts to the aforementioned triplet state, denoted [Ru(bpy)]*.  This species transfers an electron, located on one bpy ligand, to a sacrificial oxidant such as peroxodisulfate (SO).  The resulting [Ru(bpy)] is a powerful oxidant and oxidizes water into O and protons via a catalyst.  Alternatively, the reducing power of [Ru(bpy)]* 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 EDTA or triethanolamine is provided to return the Ru(III) back to Ru(II).
  Derivatives of [Ru(bpy)] 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)] and its derivatives to fabrication of optical chemical sensors is arguably one of the most successful areas so far.