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Conversion of solar energy into chemical energy can also be achieved by photochemical reactions, occurring in a homogeneous or a heterogeneous system.

Photochemical decomposition of water into H2and O2 is an interesting process.

H2O(l) →H2(g) + ½ O2(g)   ∆ G o298 = 238 kJ mol–1

This reaction requires a photosensitizes because water itself does not absorb radiation above 200 nm. Generally, transition metal complexes exhibiting photoredox properties are used as photosensitizers for this reaction.

The main problem however, is that most transition metal complexes involve one electron transfer per photon absorbed, whereas the water spliting reaction,such as,

H2O(l) + 2e– → 2OH–(aq) + H2(g)
2H2O (l) → 4H+ (aq) +O2 (g) + 4e–

involve more than one electron. Ru(bipy)32+ (ruthenium bipyridyl complex ion), is a good photo-sensitizer, and acts as a reducing and oxidizing agent in the excited state. Reductive quenching of Ru(bipy)32+ produces a powerful reductant, Ru(bipy)3+ and oxidative quenching generates a strong oxidant, Ru(bipy)33+.

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