Published online by Cambridge University Press: 10 August 2009
Redox coloration and the underlying electronic transitions
Metal coordination complexes show promise as electrochromic materials because of their intense coloration and redox reactivity. Chromophore properties arise from low-energy metal-to-ligand charge-transfer (MLCT), intervalence charge-transfer (IVCT), intra-ligand excitation, and related visible-region electronic transitions. Because these transitions involve valence electrons, chromophoric characteristics are altered or eliminated upon oxidation or reduction of the complex, as touched on in Chapter 1. A familiar example used in titrations is the redox indicator ferroin, [FeII(phen)3]2 + (phen = 1,10-phenanthroline), which has been employed in a solid-state ECD, the deep red colour of which is transformed to pale blue on oxidation to the iron(III) form. Often more markedly than other chemical groups, a coloured metal coordination complex susceptible to a redox change will in general undergo an accompanying colour change, and will therefore be electrochromic to some extent. The redox change – electron loss or gain – can be assigned to either the central coordinating cation or the bound ligand(s); often it is clear which, but not always. If it is the central cation that undergoes redox change, then its initial and final oxidation states are shown in superscript roman numerals, while the less clear convention for ligands is usually to indicate the extra charge lost or gained by a superscripted + or −. As mentioned in Chapter 1, whilst the term ‘coloured’ generally implies absorption in the visible region, metal coordination complexes that switch between a colourless state and a state with strong absorption in the near infra red (NIR) region are now being intensively studied.
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