Dynamics of Solvent-Mediated Electron Localization in Electronically Excited Hexacyanoferrate(III)

We have used polarization-resolved UV pump-mid-IR probe spectroscopy to investigate the dynamics of electron hole localization for excited-state ligand-to-metal charge-transfer (LMCT) excitation in Fe(CN)(6)(3-). The initially generated LMCT excited state has a single CN-stretch absorption band with no anisotropy. This provides strong evidence that this initial excited state preserves the octahedral symmetry of the electronic ground state by delocalizing the ligand hole in the LMCT excited state on all six cyanide ligands. This delocalized LMCT excited state decays to a second excited state with two CN-stretch absorption bands. We attribute both peaks to a single excited state because the formation time for both peaks matches the decay time for the delocalized LMCT excited state. The presence of two CN-stretch absorption bands demonstrates that this secondary excited state has lower symmetry. This observation, in conjunction with the solvent-dependent time constant for the formation of the secondary excited state, leads us to conclude that the secondary excited state corresponds to a LMCT state with a localized ligand hole.