Solvatochromic and Ionochromic Effects of Iron(II)bis(1,10-phenanthroline)dicyano: a Theoretical Study

Solvatochromic and ionochromic effects of the iron(II)bis(1,10-phenanthroline)dicyano (Fe(phen)₂(CN)₂) complex were investigated by means of combined DFT/TDDFT calculations using the PBE and B3LYP functionals. Extended solvation models of Fe(phen)₂(CN)₂ in acetonitrile and aqueous solution, as well as including interaction with Mg²⁺, were constructed. The calculated vertical excitation energies reproduce well the observed solvatochromism in acetonitrile and aqueous solutions, the ionochromism in acetonitrile in the presence of Mg²⁺, and the absence of ionochromic effect in aqueous solution. The vertical excitation energies and the nature of the transitions were reliably predicted after inclusion of geometry relaxation upon aqueous micro- and global solvation and solvent polarization effect in the TDDFT calculations. The two intense UV−vis absorption bands occurring for all systems studied are interpreted as transitions from a hybrid Fe^II(d)/cyano N(p) orbital to a phenanthroline π* orbital rather than a pure metal-to-ligand-charge transfer (MLCT). The solvatochromic and ionochromic blue band shifts of Fe(phen)₂(CN)₂ were explained with preferential stabilization of the highest occupied Fe^II(d)/cyano N(p) orbitals as a result of specific interactions with water solvent molecules or Mg²⁺ ions in solution. Such interactions occur through the CN⁻ groups in the complex, and they have a decisive role for the observed blue shifts of UV−vis absorption bands.