Adiabatic electron transfer

[1] [2] Electron transfer during a collision between an oxidant and a reductant occurs adiabatically on a continuous potential energy surface.

labelled D (for “donor”) and A (for “acceptor”) become a distance R apart, either through collisions, covalent bonding, location in a material, protein or polymer structure, etc.

Adiabatic electron-transfer theory stresses that intricately coupled to such charge transfer is the ability of any D-A system to absorb or emit light.

Figure 2 sketches what happens if light is absorbed by just one of the chemical species, taken to be the charge donor.

[5] The inverse of this process is also used to make organic light-emitting diodes (OLEDs).

), Hush showed[2] that the rate of light absorption (and hence charge separation) is given from the Einstein equation by This theory explained[2] how Prussian blue absorbes light, creating[6] [7][8] [9][10] the field of intervalence charge transfer spectroscopy.

Adiabatic electron transfer is also relevant to the Robin-Day classification system, which codifies types of mixed valence compounds.

is not small: charge is not localized on just one chemical species but is shared quantum mechanically between two Ru centers, presenting classically forbidden half-integral valence states.

[13] that the critical requirement for this phenomenon is Adiabatic electron-transfer theory stems from London's approach to charge-transfer and indeed general chemical reactions[14] applied by Hush using parabolic potential-energy surfaces.

[15][16] Hush himself has carried out many theoretical and experimental studies of mixed valence complexes and long range electron transfer in biological systems.

Hush's quantum-electronic adiabatic approach to electron transfer was unique; directly connecting with the Quantum Chemistry concepts of Mulliken, it forms the basis of all modern computational approaches to modeling electron transfer.

It also leads seamlessly[21] to understanding electron-transfer transition-state spectroscopy pioneered by Zewail.

When electron transfer occurs during collisions of the D and A species, the coupling is typically large and the “adiabatic” limit applies in which rate constants are given by transition state theory.

[4] In biological applications, however, as well as some organic conductors and other device materials, R is externally constrained and so the coupling set at low or high values.

In the weak-coupling (“non-adiabatic”) limit, the activation energy for electron transfer is given by the expression derived independently by Kubo and Toyozawa[22] and by Hush.

[16] Using adiabatic electron-transfer theory,[23] in this limit Levich and Dogonadze then determined the electron-tunneling probability to express the rate constant for thermal reactions as[24] This approach is widely applicable to long-range ground-state intramolecular electron transfer, electron transfer in biology, and electron transfer in conducting materials.

It also typically controls the rate of charge separation in the excited-state photochemical application described in Figure 2 and related problems.

In that work,[25] he also derived the standard expression for the solvent contribution to the reorganization energy, making the theory more applicable to practical problems.

Use of this solvation description (instead[4] of the form that Hush originally proposed[16]) in approaches spanning the adiabatic and non-adiabatic limits is often termed “Marcus-Hush Theory”.

Adiabatic electron-transfer theory is also widely applied [2] in Molecular Electronics.

Fig. 1. Electron transfer occurs between donor (D) and acceptor (A) species separated by distance R that may be found in many forms in both condensed phases and the gas phase. Internal structure, external structure, or chance collisions provide interconnection between the species. Upon electron transfer, the structure of the local chemical environments involving D and A change, as does the polarization these species induce on any surrounding media.
Fig. 2. When the donor species absorbs light energy, it goes into a high-energy excited state, generating significant changes to its local chemical environment and the polarization of its external environment. These environments facilitate coupling between the donor and acceptor, which drives photochemical charge separation with a rate given by Eqn. (3) in the weak-coupling limit. This rate is also dependent on the energy required to rearrange the atoms to the preferred local geometry and environment polarization of the charge-separated state D + -A − and the energy change associated with charge separation.
Fig. 3. Light energy is absorbed by the donor and acceptor, initiating intervalence charge transfer to directly convert solar energy into electrical energy as D + -A − . In the weak-coupling limit, the coupling , reorganization energy , and the free energy change control the rate of light absorption (and hence charge separation) via Eqn. (1).