Identification of the Chromophores in Prussian blue

TL;DR

This study reveals that Prussian blue's intense red color arises from localized intervalence charge transfer within specific chromophoric variants, supported by spectroscopic and computational analyses, refining the understanding of its electronic structure.

Contribution

It identifies the specific chromophores responsible for Prussian blue's color, challenging traditional structural motifs and integrating spectroscopic and computational methods for analysis.

Findings

Red absorption is due to localized charge transfer within ligand isomers.

Particle size and light scattering significantly influence spectral properties.

Computational models support the revised structural interpretation.

Abstract

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-.