Photoinduced electron transfer is an electron transfer

Electron transfer

Electron transfer is the process by which an electron moves from an atom or a chemical species to another atom or chemical species...

 which occurs when certain photoactive materials interact with light.

Breadth

Such materials include semiconductors that can be photoactivated like many solar cells, biological systems such as those used in photosynthesis

Photosynthesis

Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

, and small molecules with suitable absorption

Absorption (electromagnetic radiation)

In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...

s and redox states.

Process

It is common to describe where electrons reside as electron bands in bulk materials and electron orbital

Electron orbital

An electron orbital may refer to:* An atomic orbital, describing the behaviour of an electron in an atom* A molecular orbital, describing the behaviour of an electron in a molecule- See also :...

s in molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s. For the sake of expedience the following description will be described in molecular terms. When a photon excites a molecule an electron in a ground state orbital can be excited to a higher energy orbital. This excited state leaves a vacancy in a ground state orbital that can be filled by an electron donor

Electron donor

An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process....

. It produces an electron in a high energy orbital which can be donated to an electron acceptor

Electron acceptor

An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....

. In these respects a photoexcited molecule can act as a good oxidizing agent

Oxidizing agent

An oxidizing agent can be defined as a substance that removes electrons from another reactant in a redox chemical reaction...

 or a good reducing agent

Reducing agent

A reducing agent is the element or compound in a reduction-oxidation reaction that donates an electron to another species; however, since the reducer loses an electron we say it is "oxidized"...

.

Photoinduced Oxidation

[ML_n]²⁺ + hν → [ML_n]²⁺*

[ML_n]²⁺* + donor → [ML_n]⁺ + donor⁺

Photoinduced Reduction

[ML_n]²⁺ + hν → [ML_n]²⁺*

[ML_n]²⁺* + acceptor → [ML_n]³⁺ + acceptor^-

The end result of both reactions is that an electron is delivered to an orbital that is higher in energy than where it previously resided. This is often described as a charge separated electron-hole pair when working with semiconductors.

In the absence of a proper electron donor or acceptor it is possible for such molecules to undergo ordinary fluorescence emission. The electron transfer is one form of photoquenching.

Subsequent Processes

In many photo-productive systems this charge separation is kinetically isolated by delivery of the electron to a lower energy conductor

Electrical conductor

In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...

 attached to the p/n junction or into a electron transport chain

Electron transport chain

An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

. In this case some of the energy can be captured to do work. If the electron is not kinetically isolated thermodynamics will take over and the products will react with each other to regenerate the ground state starting material. This process is called recombination and the photon's energy is released as heat.

Recombination of Photoinduced Oxidation

[ML_n]⁺ + donor⁺ → [ML_n]²⁺ + donor

Potential induced photon production

The reverse process to photoinduced electron transfer is displayed by light emitting diodes (LED) and chemiluminescence. Where potential gradients are used to create excited states that decay by emitting by light.