Electronic Structure and Absorption Spectra of Biferrocenyl and Bisfulvalenide Diiron Radical Cations: Detection and Assignment of New Low‐Energy Transitions

Abstract

New transitions: Low‐energy electronic transitions have been detected spectroscopically in the FeII–FeIII mixed‐valent biferrocenyl radical cation, but are absent in the spectra of the neutral analogue. They have been assigned by time‐dependent DFT calculations (squares in figure). Analogous investigations were performed for the bisfulvalenide FeII–FeIII radical cation.magnified imageUV‐visible/near‐IR (NIR)/mid‐IR (MIR) solution, solid‐state, and matrix‐isolation electronic absorption spectra of the FeII–FeIII mixed‐valent homobimetallic compounds biferrocenyl triiodide (1) and 1′,1′′′‐diethylbiferrocenyltriiodide (2) reveal the presence of a low‐energy transition in the MIR region that has not been reported before. The new absorption feature and the known NIR band are both assigned to intervalence charge‐transfer (IVCT) transitions. To obtain insight into the electronic structures of 1 and 2, DFT calculations with the BP86 functionals and different basis sets have been performed. Based on the molecular orbital scheme of cation 1, one band corresponds to the transition between the highest occupied d orbitals on the two iron centers, whereas the other one is assigned to a transition from a lower‐lying d orbital to the d orbital. For comparison, the doubly bridged bisfulvalenide diiron cation (3) has been investigated by optical absorption spectroscopy and DFT calculations. The experimental and theoretical results are discussed with respect to the degree of electron localization/delocalization in these systems.

Functionalised Biferrocene Systems towards Molecular Electronics

“…Such bands are typical of mixed‐valent biferrocenium radical cations, including the ones derived from biferrocene and diethyl biferrocene as reported by Bally and Tuczek and from macrocyclic oligoferrocene rings. [15b], For the former biferrocenium radical cations, two NIR bands with similar energy and extinction coefficients were observed and assigned to IVCT excitations between the symmetric and antisymmetric combinations of the d x ²– y ² Fe 3d orbitals (lower energy band) or from the antibonding combination of the d z ² Fe 3d orbitals to the antisymmetric combination of the d x ²– y ² orbitals. We propose that the NIR bands of the present biferrocenium radical cations are of similar origin but note that our TD‐DFT calculations did not produce any absorption at such low energies.…”

“Tail–Tail Dimerization” of Ferrocene Amino Acid Derivatives

“…In order to assist band assignments, time‐dependent DFT calculations (with the BVP86 functional and the DGDZVP2 basis set24) on geometry‐optimized mononuclear ferrocenium cations [AcNH–Fc] + and [AcNH–Fn–COOMe] + have been performed. Transitions with non‐vanishing intensities are calculated at 1928, 888, and 585 nm ( f = 0.0001, 0.0071, 0.0015) for [AcNH–Fc] + and at 1909, 938, and 590 nm ( f = 0.0001, 0.0062, 0.0010) for [AcNH–Fn–COOMe] + , respectively, which nicely reproduces the experimental spectra (≈ 2200, 759, 551 nm and ≈ 2150, 796, 555 nm; see Supporting Information).…”

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.…”