Intervalence (Charge-Resonance) Transitions in Organic Mixed-Valence Systems. Through-Space versus Through-Bond Electron Transfer between Bridged Aromatic (Redox) Centers

Abstract

Intervalence absorption bands appearing in the diagnostic near-IR region are consistently observed in the electronic spectra of mixed-valence systems containing a pair of aromatic redox centers (Ar •+ /Ar) that are connected by two basically different types of molecular bridges. The through-space pathway for intramolecular electron transfer is dictated by an o-xylylene bridge in the mixed-valence cation radical 3•+ with Ar = 2,5-dimethoxy-p-tolyl (T), in which conformational mobility allows the proximal syn disposition of planar T•+ /T redox centers. Four independent experimental probes indicate the large through-space electronic interaction between such cofacial Ar•+ /Ar redox centers from the measurements of (a) sizable potential splitting in the cyclic voltammogram, (b) quinonoidal distortion of T•+ /T centers by X-ray crystallography, (c) "doubling" of the ESR hyperfine splittings, and (d) a pronounced intervalence charge-resonance band. The through (br)-bond pathway for intramolecular electron transfer is enforced in the mixed-valence cation radical 2a•+ by the p-phenylene bridge which provides the structurally inflexible and linear connection between Ar•+ /Ar redox centers. The direct comparison of intramolecular rates of electron transfer (kET) between identical T•+ /T centers in 3•+ and 2a•+ indicates that through-space and through-bond mechanisms are equally effective, despite widely different separations between their redox centers. The same picture obtains for 3•+ and 2a•+ from theoretical computations of the first-order rate constants for intramolecular electron transfer from Marcus−Hush theory using the electronic coupling elements evaluated from the diagnostic intervalence (charge-transfer) transitions. Such a strong coherence between theory and experiment also applies to the mixed-valence cation radical 7•+ , in which the aromatic redox S center is sterically encumbered by annulation.

Tryptophan-mediated charge-resonance stabilization in the bis -Fe(IV) redox state of MauG

“…A point of critical significance is that the hopping maps predict ET rates greater than 10 7 s −1 for the hopping mechanism. As stated earlier, model studies suggest that rate constants in the range of 10 7 -10 11 s −1 are essential for delocalization of the spin and charge within CR complexes (3)(4)(5). Thus, a hole-hopping mechanism for ET between the hemes mediated by Trp93 can explain the observed CR-transition phenomenon in bis-Fe(IV) MauG in the absence of direct contact between the hemes.…”

“…The former requires parallel alignment and close proximity of the monomers to allow direct interaction between their π-orbitals; the latter requires covalent linkage of the monomers by suitable bridging structures serving as an "electric wire" to efficiently shuttle the spin and charge between them. In both cases, ultrafast and reversible ET with rate constants in the range of 7 -10 11 s −1 frequently is observed and suggested to be essential for delocalization of the spin and charge within the CR complexes (3)(4)(5)27).…”

“…The CR band (hυ CR ) typically is observed in the NIR region, and it is a characteristic signature of resonance stabilization of spin and charge within the CR complex. Kochi and coworkers (3)(4)(5) investigated the ET process between the two monomer units in the CR systems and demonstrated that a CR transition can proceed via through-space or through-bond ET. The former requires parallel alignment and close proximity of the monomers to allow direct interaction between their π-orbitals; the latter requires covalent linkage of the monomers by suitable bridging structures serving as an "electric wire" to efficiently shuttle the spin and charge between them.…”

Computational and spectroscopic studies of organic mixed-valence compounds: where is the charge?

“…Typical redox centers are e.g. triarylamines, [5][6][7][8][9][10] perchlorotriphenylmethyl radicals, [11][12][13] hydrazines, 14,15 dimethoxybenzenes, 16,17 or quinones. 18,19 ET may proceed between the redox centers via the bridge, thermally or optically induced.…”

Mechanically induced intramolecular electron transfer in a mixed-valence molecular shuttle

“…Marcus-Hush theory is now widely applied to the study of electron transfer in a plethora of organic-based mixed-valence systems in order to understand (i) spatial interactions between redox centers (10)(11)(12); (ii) the use of different solvents and counterions (13)(14)(15); and (iii) how the existence of covalent bridges (16,17) affects the kinetics (k ET ) and thermodynamics (ΔG ‡ ) of electron transfer. In this article, we demonstrate how the switching mechanism present in a molecular shuttle-comprised of a bistable [2]rotaxane (R 6þ ) (Fig.…”