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

Abstract

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 Mossbauer 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.

Metal–Metal Interaction in Fischer Carbene Complexes: A Study of Ferrocenyl and Biferrocenyl Tungsten Alkylidene Complexes

“…26,29 However, the latter absorption is very narrow compared to the corresponding value of Δν 1/2(theo) ( Table 4, Figure SI-7). An assignment to a LMCT or a charge transfer assisted ligand field transition is also not uncommon.…”

“…31) (42) 39) (29) ocenyl; fc = iferrocenyl; er carbene Fc; 3, R = (2, R' = fc; 4, R' = bfc), is reported with the aim of investigating low energy charge transfer transitions between the transition metal carbonyl fragment and the (oxidized) carbene substituents. For this reason, ferrocenyl and biferrocenyl mono-and bismethoxbismethoxycarbene tungsten(0) complexes 1 -4 were prepared and characterized spectroscopically in solution.…”

Boron Dipyrrin Dyes Exhibiting “Push–Pull–Pull” Electronic Signatures

“…The key requirement is that only one of the two appendages becomes activated and thereby hinders further reaction at the opposite terminal; that is, negative cooperativity 1. The result is a change in the optical properties, leading to chromism, modified π‐conjugation pathways and disparate chemical properties that, in certain cases, could lead to intervalence charge‐transfer behaviour 2. A particularly intriguing case comprises a symmetrically substituted dye equipped with two identical electron donors, both of which push electrons towards the central unit and give rise to a “push–pull–push” configuration.…”

“…The cooperative effect1, 2 of the terminal ferrocene units at the semi‐oxidized level, compared to the antagonistic behaviour of the neutral or dicationic derivatives, amplifies the electrochromic properties. Indeed, the push–pull–pull signature provides a simple means for extending the degree of π‐electron conjugation and serves to discriminate the electronic nature of the semi‐oxidized species from the other two redox states.…”

“…[1] The result is a change in the optical properties, leading to chromism, modified p-conjugation pathways and disparate chemical properties that, in certain cases, could lead to intervalence charge-transfer behaviour. [2] A particularly intriguing case comprises a symmetrically substituted dye equipped with two identical electron donors, both of which push electrons towards the central unit and give rise to a "push-pull-push" configuration. To some degree, the two donor groups work against each other, the acceptor has to satisfy both donors and, as a consequence, the full effect of a "push-pull" circuit is not seen.…”