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Photoinduced electron transfer
Photoinduced electron transfer
Photoinduced electron transfer
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Schematic of photoinduced electron transfer process

Photoinduced electron transfer (PET) is an excited state electron transfer process by which an excited electron is transferred from donor to acceptor.[1][2] Due to PET a charge separation is generated, i.e., redox reaction takes place in excited state (this phenomenon is not observed in Dexter electron transfer).

Breadth

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Such materials include semiconductors that can be photoactivated like many solar cells, biological systems such as those used in photosynthesis, and small molecules with suitable absorptions and redox states.

Process

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It is common to describe where electrons reside as electron bands in bulk materials and electron orbitals in molecules. 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. It produces an electron in a high energy orbital which can be donated to an electron acceptor. In these respects a photoexcited molecule can act as a good oxidizing agent or a good reducing agent.

Photoinduced oxidation
[MLn]2+ + hν → [MLn]2+*
[MLn]2+* + donor → [MLn]+ + donor+
Photoinduced reduction
[MLn]2+ + hν → [MLn]2+*
[MLn]2+* + acceptor → [MLn]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

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In many photo-productive systems this charge separation is kinetically isolated by delivery of the electron to a lower energy conductor attached to the p/n junction or into an electron transport chain. 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
[MLn]+ + donor+ → [MLn]2+ + donor

Potential induced photon production

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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 light emission.

References

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Photoinduced electron transfer

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from Grokipedia
Photoinduced electron transfer (PET) is a light-triggered photochemical process in which an is transferred from a photoexcited donor to a ground-state acceptor, generating charge-separated states that can drive subsequent chemical reactions. This phenomenon occurs in donor-bridge-acceptor (D-B-A) architectures, either through covalent bonding or supramolecular assembly, and can proceed via oxidative (electron from excited donor to acceptor) or reductive (electron from ground-state donor to excited acceptor). The efficiency of PET is influenced by key factors including the potentials of the donor and acceptor, the distance between them, the surrounding medium, and the driving force of the reaction, as described by theories such as . In natural systems, PET is essential to , where it initiates the conversion of into ; for instance, in bacterial reaction centers like that of Rhodobacter sphaeroides, light absorption by a special pair of bacteriochlorophylls leads to rapid via pheophytin to a acceptor, achieving high quantum efficiency. This process powers the , ultimately enabling ATP synthesis and carbon fixation in and . Beyond biology, PET underpins artificial photosynthetic systems designed to mimic natural processes for production, such as to generate or CO₂ reduction to form carbon-based fuels. Technological applications of PET are prominent in photovoltaic devices, particularly dye-sensitized solar cells (DSSCs), where photoexcitation of a sensitizer adsorbed on a TiO₂ leads to ultrafast injection into the conduction band, facilitating solar-to-electricity conversion with efficiencies exceeding 14% in recent designs. PET also plays a role in organic , for , and molecular , where controlled charge separation enables light-driven switching and information processing. Ongoing focuses on optimizing PET dynamics through nanostructured materials and hybrid systems to enhance energy conversion yields and stability.

Fundamentals

Definition and Scope

Photoinduced (PET) is an that occurs from an electronic generated by the absorption of . In this reaction, excites an in to , enabling the subsequent transfer of the and resulting in charge separation into radical ion pairs. This distinguishes PET from ground-state electron transfer, which proceeds without photoexcitation and typically involves less favorable energetics due to the absence of the excited-state driving force. The scope of PET spans diverse fields, including semiconductors, biological systems, and synthetic molecular architectures, where it facilitates efficient light-to-chemical or electrical energy conversion. In semiconductor applications, such as dye-sensitized solar cells, PET enables rapid electron injection from an excited dye sensitizer into the conduction band of a wide-bandgap oxide like TiO₂, initiating generation. In biological contexts, PET drives charge separation in light-harvesting complexes of photosynthetic organisms, where excited molecules transfer electrons to primary acceptors, powering the conversion of into chemical fuels. Similarly, in synthetic systems like organic , PET at donor-acceptor interfaces generates free charge carriers essential for device efficiency. Successful PET requires careful alignment of key parameters, including compatible redox potentials between donor and acceptor to ensure exergonic electron transfer and overlapping absorption spectra to match the incident light source for effective excitation. Unlike Förster resonance energy transfer (FRET), which transfers excitation energy through dipole-dipole coupling without net charge displacement, PET produces spatially separated charges that can drive subsequent redox chemistry or energy harvesting.

Historical Development

The earliest documented observations of light-induced processes, precursors to modern understanding of photoinduced (PET), date back to the . In 1805, Seekamp reported the photooxidation of under light, demonstrating how light could drive the decomposition of organic compounds in the presence of metal ions. Similarly, in 1830, Döbereiner observed the photooxidation of by iron(III ions in , providing further evidence of light-mediated in simple chemical systems. These findings, though not interpreted in terms of electron transfer at the time, laid the groundwork for recognizing photochemical reactions. In the mid-20th century, the concept of electron flow in biological systems gained prominence, particularly in research from 1949 to 1959. Eugene Rabinowitch's comprehensive works, including his 1956 treatise on photosynthetic transport, helped establish the framework for light-driven electron movement across membranes. This period also saw the discovery of non-cyclic by Daniel Arnon in 1954, which demonstrated ATP synthesis coupled to from to NADP+ under illumination, solidifying the role of sequential electron transfers in energy conversion. Rabinowitch's theoretical framework helped conceptualize the separation of two light reactions, highlighting PET as central to oxygenic . Theoretical advancements transformed PET from empirical observations to a predictive science. Rudolph A. Marcus's seminal 1956 paper introduced a quantum mechanical framework for rates, accounting for solvent reorganization and electronic coupling, which became foundational for understanding PET kinetics. This work, expanded in subsequent publications, earned Marcus the 1992 for contributions to electron transfer theory. In synthetic chemistry, Vincenzo Balzani and Franco Scandola's 1991 book Supramolecular Photochemistry formalized PET mechanisms in designed molecular assemblies, emphasizing vectorial electron transfer in multicomponent systems for mimicking natural processes. Post-2000 developments have integrated PET into , focusing on . Daniel G. Nocera's research advanced molecular catalysts, such as the cobalt-phosphate in 2008, enabling light-driven with high turnover numbers. His lab's hybrid systems, combining molecular catalysts with semiconductors or biological components, achieved efficient charge separation for production, with ongoing refinements in stability and reported through 2025.

Theoretical Framework

Basic Mechanism

Photoinduced electron transfer (PET) begins with the excitation step, where a is absorbed by either the donor (D) or the acceptor (A) , promoting an from the (S₀) to an excited singlet (S₁) or triplet (T₁) state. This process generates a highly reactive species, such as D* or A*, with sufficient to drive . For instance, the excitation can be represented as D + hν → D*, where the excited donor possesses a higher , facilitating subsequent interactions. The initial transfer occurs in two primary modes: photooxidation, where the excited donor donates an to the ground-state acceptor (D* + A → D⁺ + A⁻), or photoreduction, where the excited acceptor accepts an from the ground-state donor (A* + D → A⁻ + D⁺). These processes create charge-separated states essential for further dynamics in PET systems. In metal complexes, specific examples illustrate these mechanisms; for photooxidation, excitation of a metal complex followed by electron donation to an acceptor yields [MLₙ]²⁺ + hν → [MLₙ]²⁺* → [MLₙ]³⁺ + acceptor⁻, as observed in ruthenium polypyridyl systems, such as [Ru(bpy)₃]²⁺* reducing methylviologen to form [Ru(bpy)₃]³⁺ + MV⁺•. Similarly, for photoreduction, the excited complex accepts an electron from a donor: [MLₙ]³⁺ + hν → [MLₙ]³⁺* → [MLₙ]²⁺ + donor⁺, for example in cobalt(III) ammine complexes quenched by alcohols. Key factors influencing the initiation of PET include orbital overlap between the donor and acceptor, which enables efficient electron tunneling, and solvent effects that modulate the excited-state lifetimes, typically ranging from 10⁻⁹ to 10⁻⁶ s. Polar solvents can stabilize charge-separated intermediates but may also quench excited states through interactions, while nonpolar environments often extend lifetimes by reducing such quenching. These elements ensure the excited state persists long enough for transfer to compete with other deactivation pathways.

Rate Theories

The classical framework for quantifying the rates of photoinduced (PET) is , which models the process as a non-adiabatic transition between donor and acceptor states following photoexcitation. Developed by , this theory posits that the electron transfer rate depends on the overlap of vibrational wavefunctions in the initial and final states, accounting for nuclear reorganization in the surrounding medium. The rate constant kETk_{ET} for such transfers is expressed as: kET=2πℏ∣V∣214πλkBTexp⁡[−(λ+ΔG∘)24λkBT]k_{ET} = \frac{2\pi}{\hbar} |V|^2 \frac{1}{\sqrt{4\pi \lambda k_B T}} \exp\left[ -\frac{(\lambda + \Delta G^\circ)^2}{4\lambda k_B T} \right]
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