Photoinduced electron transfer (PET) is an excited state

electron transfer Electron transfer (ET) occurs when an electron relocates from an atom, ion, or molecule, to another such chemical entity. ET describes the mechanism by which electrons are transferred in redox reactions. Electrochemical processes are ET reactio ...

process by which an excited electron is transferred from donor to acceptor."Organic and Inorganic Photochemistry" V. Ramamurthy and Kirk S. Schanze 1998 Marcel Dekker Due to PET a charge separation is generated, ''i.e.'',

redox Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...

reaction takes place in excited state (this phenomenon is not observed in

Dexter electron transfer Dexter electron transfer (also called Dexter electron exchange and Dexter energy transfer) is a fluorescence quenching (fluorescence), quenching mechanism in which an Excited state, excited electron is transferred from one molecule (a Electron don ...

Breadth

Such materials include

semiconductors A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping levels ...

that can be photoactivated like many

solar cells A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect.

, biological systems such as those used in

photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...

, and small molecules with suitable

absorption Absorption may refer to: Chemistry and biology *Absorption (biology), digestion **Absorption (small intestine) *Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials *Absorption (skin), a route by which su ...

s and redox states.

Process

It is common to describe where electrons reside as

electron band In solid-state physics, the electronic band structure (or simply band structure) of a solid describes the range of energy levels that electrons may have within it, as well as the ranges of energy that they may not have (called ''band gaps'' or '' ...

s in bulk materials and

electron orbitals 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 * Electron configuration, the arrangement of electro ...

molecule A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...

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 In chemistry, an electron donor is a chemical entity that transfers electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process. An obsolete definition equated an electron dono ...

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

electron acceptor An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. Electron acceptors are oxidizing agents. The electron accepting power of an electron acceptor is measured by its redox potential. In the ...

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

oxidizing agent An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ''electron donor''). In ot ...

or a good

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are common reducing agents include hydrogen, carbon ...

. :Photoinduced oxidation : L_nsup>2+ + hν → L_nsup>2+* : L_nsup>2+* + donor → L_nsup>+ + donor⁺ :Photoinduced reduction : L_nsup>2+ + hν → L_nsup>2+* : L_nsup>2+* + acceptor → L_nsup>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

. 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 Conductor or conduction may refer to: Biology and medicine * Bone conduction, the conduction of sound to the inner ear * Conduction aphasia, a language disorder Mathematics * Conductor (ring theory) * Conductor of an abelian variety * Cond ...

attached to the p/n junction or into an

electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules which transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...

. 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 : L_nsup>+ + donor⁺ → L_nsup>2+ + donor

Potential induced photon production

The reverse process to photoinduced electron transfer is displayed by

light emitting diodes A light-emitting diode (LED) is a semiconductor device that Light#Light sources, emits light when Electric current, current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of pho ...

(LED) and

chemiluminescence Chemiluminescence (also chemoluminescence) is the emission of light (luminescence) as the result of a chemical reaction, i.e. a chemical reaction results in a flash or glow of light. A standard example of chemiluminescence in the laboratory se ...

, where potential gradients are used to create excited states that decay by light emission.

References

{{Reflist Photochemistry Chemical reactions Optoelectronics