Absorption spectroscopy and time-dependent theoretical calculation of the intervalence band of [FeIIFeIIIBPMP(OPr)2](BPh4)2, a localized mixed-valence compound with a nonlinear metal-bridging ligand- metal core

Abstract

The title compound shows an intervalence absorption band in the near-infrared spectral region with a maximum at 8400 cm^-1 at 80 K, as measured on a crystalline sample. The band has a width of 4750 cm^-1 and is asymmetric. The molecule does not have a linear metal-bridging ligand-metal structure, a distinct difference to the vast majority of mixed-valence compounds studied by absorption spectroscopy. A one-dimensional model using coupled potential energy surfaces and a non-constant transition dipole moment varying along the configurational coordinate Q is presented, consistent with the symmetry of the title compound. We calculate the absorption band shape and its temperature dependence using a time-dependent theoretical approach. Our calculations show that the variation of the dipole moment along Q cannot be neglected in the analysis of the full absorption spectrum.