  Prussian blue was the world's first synthetic dye. Its structural, optical
and magnetic properties have led to many applications in technology and
medicine, and provide paradigms for understanding coordination polymers,
framework materials and mixed-valence compounds. The intense red absorption of
Prussian blue that characterises chemical and physical properties critical to
many of these applications is now shown to arise from localised intervalence
charge transfer transitions within two chromophoric variants (ligand isomers)
of an idealised "dimer" fragment {(NC)5FeII}(mu-CN){FeIII(NC)3(H2O)2}. This
fragment is only available in modern interpretations of the material's crystal
structure, with the traditional motif {(NC)5FeII}(mu-CN){FeIII(NC)5} shown not
to facilitate visible absorption. Essential to the analysis is the
demonstration, obtained independently using absorption and magnetic circular
dichroism spectroscopies, that spectra of Prussian blues are strongly
influenced by particle size and (subsequent) light scattering. These
interpretations are guided and supported by density functional theory
calculations (CAM-B3LYP), supplemented by coupled cluster and Bethe-Salpeter
spectral simulations, as well as electron paramagnetic resonance spectroscopy
of Prussian blue and a model molecular dimeric ion [Fe2(CN)11]6-.
