Impact of Diimine Ligand on Singlet-to-Triplet Charge Transfer Electroabsorption in Iron and Ruthenium Complexes.

The electroabsorption and absorption spectra of eight homoleptic complexes of the general form [M(LL) 3 ] 2+ where M = Ru, Fe, and LL = 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), and 4,4',-(R) 2 -bpy where R = -OCH 3 , -CF 3 , were quantified at 77 K in a butyronitrile glass. Intense metal-to-ligand charge transfer (MLCT) absorption bands were evident in the visible region. Electroabsorption spectra measured with applied electric fields >0.2 MV/cm were analyzed by the two-state Liptay model. Significant light-induced dipole moment changes of Δ μ ⇀ = 4-13 D were found consistent with a metal-to-ligand charge transfer (MLCT) excited state comprised an electron localized on a single diimine ligand, [M III (LL - )(LL) 2 ]* 2+ , in the initially formed Franck-Condon excited state. A low energy feature evident in the electroabsorption spectra was assigned to a direct singlet-to-triplet MLCT excited state. The identity of the diimine ligand had an unexpected and large impact on these transitions. Analysis relative to the higher energy absorption provides a comparison of spin-allowed and disallowed transitions for first- and second-row transition metal complexes. With the notable exception of [Fe(CF 3 bpy) 3 ] 2+ , the change in dipole moment for the 3 MLCT excited states was less than or equal to that of the 1 MLCT excited states. The charge transfer distances for the iron complexes were generally larger than those for the Ru complexes, a behavior attributed to a smaller degree of iron-diimine coupling in the ground state. A striking result was the sensitivity of the extinction coefficient and spectral profile of the low energy electroabsorption assigned to the identity of the diimine ligand; data that suggests electronic coupling with ligand localized triplet states and high spin metal centered states must be considered when modeling the Franck-Condon excited state.