Low-Energy Bands of Ferrocene−Ferrocenium Dimers: Bandshape Analysis with a Four-Level Two-Mode Vibronic Coupling Configuration Interaction (VCCI) Model Including Asymmetry

Ferrocene−ferrocenium dimers exhibit a double-peak intervalence charge-transfer (IVCT) band in the NIR/MIR region, which is analyzed in terms of a four-level, two-mode vibronic coupling configuration interaction (VCCI) scheme. Besides providing satisfactory fits of the measured spectra, the model also gives electronic and vibronic coupling parameters as well as CI mixing coefficients. A temperature-dependent asymmetry of the potential is introduced in order to describe the temperature dependence of the solid-state spectra and account for complementary Mössbauer data, which indicate a temperature-driven electron localization−delocalization transition in the singly bridged radical cations. The VCCI model is also successfully applied to the doubly bridged Fe(II)−Fe(III) bisfulvalenide dimer, which exhibits a double-peak IVCT transition as well. The VCCI analysis reveals that all dimers have a one-minimum potential in the ground state, leading in the absence of asymmetry to class III behavior (electron delocalization). If electron localization corresponding to class II characteristics occurs, it is due to an asymmetric (but still one-minimum) potential and not, as usual, to a double-minimum potential, explaining the class II−III borderline behavior observed for these systems.