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Photochemistry
Photochemistry
Photochemistry
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Photochemical immersion well reactor (50 mL) with a mercury-vapor lamp

Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400 nm), visible (400–750 nm), or infrared radiation (750–2500 nm).[1]

In nature, photochemistry is of immense importance as it is the basis of photosynthesis, vision, and the formation of vitamin D with sunlight.[2] It is also responsible for the appearance of DNA mutations leading to skin cancers.[3]

Photochemical reactions proceed differently than temperature-driven reactions. Photochemical paths access high-energy intermediates that cannot be generated thermally, thereby overcoming large activation barriers in a short period of time, and allowing reactions otherwise inaccessible by thermal processes. Photochemistry can also be destructive, as illustrated by the photodegradation of plastics.

Concepts

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Photoexcitation is the first step in a photochemical process: the reactant is elevated to a state of higher energy, an excited state.

Grotthuss–Draper law and Stark–Einstein law

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The first law of photochemistry, known as the Grotthuss–Draper law (for chemists Theodor Grotthuss and John W. Draper), states that light must be absorbed by a chemical substance in order for a photochemical reaction to take place. According to the second law of photochemistry, known as the Stark–Einstein law (for physicists Johannes Stark and Albert Einstein), for each photon of light absorbed by a chemical system, no more than one molecule is activated for a photochemical reaction, as defined by the quantum yield.[4][5]

Fluorescence and phosphorescence

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When a substance in its ground state (S0) absorbs light, one electron is excited. This electron maintains its spin. according to the spin selection rule; other transitions would violate the law of conservation of angular momentum. The excitation to a higher singlet state can be from HOMO to LUMO or to a higher orbital, so that singlet excitation states S1, S2, S3... at different energies are possible.

Kasha's rule stipulates that higher singlet states quickly relax by radiationless decay or internal conversion (IC) to S1. Thus, S1 is usually, but not always, the only relevant singlet excited state. This excited state S1 can further relax to S0 by IC, but also by an allowed radiative transition from S1 to S0 that emits a photon; this process is called fluorescence.

Jablonski diagram. Radiative paths are represented by straight arrows and non-radiative paths by curly lines.

Alternatively, it is possible for the excited state S1 to undergo spin inversion and to generate a triplet excited state T1 having two unpaired electrons with the same spin. This violation of the spin selection rule is possible by intersystem crossing (ISC) of the vibrational and electronic levels of S1 and T1. According to Hund's rule of maximum multiplicity, this T1 state would be somewhat more stable than S1.

This triplet state can relax to the ground state S0 by radiationless ISC or by a radiation pathway called phosphorescence. This process implies a change of electronic spin, which is forbidden by spin selection rules, making phosphorescence (from T1 to S0) much slower than fluorescence (from S1 to S0). Thus, triplet states generally have longer lifetimes than singlet states. These transitions are usually summarized in a state energy diagram or Jablonski diagram, the paradigm of molecular photochemistry.

These excited species, either S1 or T1, have a half-empty low-energy orbital, and are consequently more oxidizing than the ground state. But at the same time, they have an electron in a high-energy orbital, and are thus more reducing. In general, excited species are prone to participate in electron transfer processes.[6]

Experimental setup

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Photochemical immersion well reactor (750 mL) with a mercury-vapor lamp

Photochemical reactions require a light source that emits wavelengths corresponding to an electronic transition in the reactant. In the early experiments (and in everyday life), sunlight was the light source, although it is polychromatic.[7] Mercury-vapor lamps are more common in the laboratory. Low-pressure mercury-vapor lamps mainly emit at 254 nm. For polychromatic sources, wavelength ranges can be selected using filters. Alternatively, laser beams are usually monochromatic (although two or more wavelengths can be obtained using nonlinear optics), and LEDs have a relatively narrow band that can be efficiently used, as well as Rayonet lamps, to get approximately monochromatic beams.

Schlenk tube containing slurry of orange crystals of Fe2(CO)9 in acetic acid after its photochemical synthesis from Fe(CO)5. The mercury lamp (connected to white power cords) can be seen on the left, set inside a water-jacketed quartz tube.

The emitted light must reach the targeted functional group without being blocked by the reactor, medium, or other functional groups present. For many applications, quartz is used for the reactors as well as to contain the lamp. Pyrex absorbs at wavelengths shorter than 275 nm. The solvent is an important experimental parameter. Solvents are potential reactants, and for this reason, chlorinated solvents are avoided because the C–Cl bond can lead to chlorination of the substrate. Strongly-absorbing solvents prevent photons from reaching the substrate. Hydrocarbon solvents absorb only at short wavelengths and are thus preferred for photochemical experiments requiring high-energy photons. Solvents containing unsaturation absorb at longer wavelengths and can usefully filter out short wavelengths. For example, cyclohexane and acetone "cut off" (absorb strongly) at wavelengths shorter than 215 and 330 nm, respectively.

Typically, the wavelength employed to induce a photochemical process is selected based on the absorption spectrum of the reactive species, most often the absorption maximum. Over the last years[when?], however, it has been demonstrated that, in the majority of bond-forming reactions, the absorption spectrum does not allow selecting the optimum wavelength to achieve the highest reaction yield based on absorptivity. This fundamental mismatch between absorptivity and reactivity has been elucidated with so-called photochemical action plots.[8][9]

Photochemistry in combination with flow chemistry

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Continuous-flow photochemistry offers multiple advantages over batch photochemistry. Photochemical reactions are driven by the number of photons that are able to activate molecules causing the desired reaction. The large surface-area-to-volume ratio of a microreactor maximizes the illumination, and at the same time allows for efficient cooling, which decreases the thermal side products.[10]

Photochemical reactions

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Examples of photochemical reactions

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Organic photochemistry

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Main article: Organic photochemistry

Examples of photochemical organic reactions are electrocyclic reactions, radical reactions, photoisomerization, and Norrish reactions.[19][20]

Norrish type II reaction

Alkenes undergo many important reactions that proceed via a photon-induced π to π* transition. The first electronic excited state of an alkene lacks the π-bond, so that rotation about the C–C bond is rapid and the molecule engages in reactions not observed thermally. These reactions include cis-trans isomerization and cycloaddition to other (ground state) alkene to give cyclobutane derivatives. The cis-trans isomerization of a (poly)alkene is involved in retinal, a component of the machinery of vision. The dimerization of alkenes is relevant to the photodamage of DNA, where thymine dimers are observed upon illuminating DNA with UV radiation. Such dimers interfere with transcription. The beneficial effects of sunlight are associated with the photochemically-induced retro-cyclization (decyclization) reaction of ergosterol to give vitamin D. In the DeMayo reaction, an alkene reacts with a 1,3-diketone reacts via its enol to yield a 1,5-diketone. Still another common photochemical reaction is Howard Zimmerman's di-π-methane rearrangement.

In an industrial application, about 100,000 tonnes of benzyl chloride are prepared annually by the gas-phase photochemical reaction of toluene with chlorine.[21] The light is absorbed by chlorine molecules, the low energy of this transition being indicated by the yellowish color of the gas. The photon induces homolysis of the Cl-Cl bond, and the resulting chlorine radical converts toluene to the benzyl radical:

Cl2 + hν → 2 Cl·
C6H5CH3 + Cl· → C6H5CH2· + HCl
C6H5CH2· + Cl· → C6H5CH2Cl

Mercaptans can be produced by photochemical addition of hydrogen sulfide (H2S) to alpha olefins.

Inorganic and organometallic photochemistry

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Coordination complexes and organometallic compounds are also photoreactive. These reactions can entail cis-trans isomerization. More commonly, photoreactions result in dissociation of ligands, since the photon excites an electron on the metal to an orbital that is antibonding with respect to the ligands. Thus, metal carbonyls that resist thermal substitution undergo decarbonylation upon irradiation with UV light. UV-irradiation of a THF solution of molybdenum hexacarbonyl gives the THF complex, which is synthetically useful:

Mo(CO)6 + THF → Mo(CO)5(THF) + CO

In a related reaction, photolysis of iron pentacarbonyl affords diiron nonacarbonyl (see figure):

2 Fe(CO)5 → Fe2(CO)9 + CO

Select photoreactive coordination complexes can undergo oxidation-reduction processes via single electron transfer. This electron transfer can occur within the inner or outer coordination sphere of the metal.[22]

Types of photochemical reactions

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Here are some different types of photochemical reactions-

  • Photo-dissociation: AB + hν → A* + B*
  • Photo induced rearrangements, isomerization: A + hν → B
  • Photo-addition: A + B + hν → AB + C
  • Photo-substitution: A + BC + hν → AB + C
  • Photo-redox reaction: A + B + hν → A− + B+

Historical

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Although bleaching has long been practiced, the first photochemical reaction was described by Trommsdorff in 1834.[23] He observed that crystals of the compound α-santonin when exposed to sunlight turned yellow and burst. In a 2007 study the reaction was described as a succession of three steps taking place within a single crystal.[24]

Santonin Photochemical reaction

The first step is a rearrangement reaction to a cyclopentadienone intermediate (2), the second one a dimerization in a Diels–Alder reaction (3), and the third one an intramolecular [2+2]cycloaddition (4). The bursting effect is attributed to a large change in crystal volume on dimerization.

See also

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Photochemistry

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from Grokipedia
Photochemistry is the branch of chemistry that studies chemical , isomerizations, and physical transformations induced by the absorption of , particularly in the visible and regions of the . This field explores how photons excite molecules to higher states, enabling processes that differ fundamentally from due to the involvement of electronically excited . The foundational principles of photochemistry are outlined by two key laws. The Grotthuss-Draper law states that a photochemical reaction can only occur if light is absorbed by the chemical system, emphasizing absorption as a prerequisite for any light-induced change. Complementing this, the Stark-Einstein law (also known as the law of photochemical equivalence) posits that each absorbed activates exactly one , linking the quantum nature of light to molecular excitation. These laws underpin the quantitative analysis of photochemical efficiency through metrics like , which measures the number of product molecules formed per absorbed and can range from less than 1 to over 10^6 in chain reactions. Historically, photochemistry emerged from early observations of light-induced changes, such as the 1801 discovery by Cruickshank of the explosive reaction between hydrogen and chlorine gas under sunlight, which laid groundwork for understanding radical chain mechanisms. The field advanced significantly in the early through the work of Giacomo Ciamician, often called the "father of ," who conducted pioneering sunlight-driven syntheses at the and advocated for sustainable ". Albert Einstein's 1912 derivation of the Stark-Einstein law during the development of quantum theory further formalized the discipline. Photochemistry has profound applications across natural and technological domains. In biology, it drives photosynthesis, where plants convert into chemical bonds via light-harvesting complexes, initiating in . Industrially, it enables like the Paterno-Büchi reaction for [2+2] cycloadditions forming oxetanes, and supports for efficient bond formations under mild conditions. Additional uses include modeling, UV stabilization of materials, waste remediation through , and historical innovations like silver halide-based .

Fundamental Principles

Grotthuss-Draper Law

The Grotthuss-Draper law, also known as the first law of photochemistry, states that only absorbed by a or substance can initiate a photochemical reaction; wavelengths that are transmitted or reflected have no chemical effect. This principle underscores that photochemical activity depends entirely on the absorption process, where photons must interact with the electronic structure of the reactant to induce change. The law originated from the work of Theodor Grotthuss, who in 1817 proposed that only absorbed light rays could produce chemical alterations, based on his experiments with light propagation through colored media and its effects on silver salts. Independently, rediscovered and formalized this concept in 1841 through studies on the photochemical reaction between and , where he demonstrated that the was proportional to the absorbed light intensity rather than the incident light. These foundational observations established absorption as the prerequisite for photochemistry, predating modern quantum mechanical interpretations. The implications of the Grotthuss-Draper law are profound for designing selective photochemical processes, as reactions occur only at wavelengths matching the molecule's absorption , enabling wavelength-specific control in synthetic applications. For instance, this selectivity explains why certain photocatalysts or photosensitizers are chosen based on their overlap with available light sources. A classic example is in , where absorbs red and blue wavelengths to drive the , while green light is reflected and thus ineffective in initiating the photochemical splitting of . Following absorption, the efficiency of the resulting reaction is quantified by , which measures product formation per absorbed .

Stark-Einstein Law

The Stark-Einstein law, also known as the photochemical equivalence law, states that for every quantum of absorbed by a chemical , only one is activated to undergo a primary photochemical process, such as excitation to a higher electronic state or dissociation into fragments. This principle establishes a direct, one-to-one correspondence between the number of absorbed photons and the number of molecules participating in the initial light-induced event. Formulated by in 1912, the law drew inspiration from Max Planck's quantum theory of radiation and built upon earlier ideas proposed by in 1908, applying quantum concepts to photochemical reactions for the first time. Building on the Grotthuss-Draper law as the prerequisite for light absorption to initiate reactions, the Stark-Einstein law quantifies the efficiency of this absorption in terms of molecular activation. The law is mathematically expressed through the quantum yield Φ\Phi, which relates the number of molecules undergoing the primary photochemical event to the number of photons absorbed: Φ=number of molecules reacted in the primary processnumber of photons absorbed\Phi = \frac{\text{number of molecules reacted in the primary process}}{\text{number of photons absorbed}} For simple primary processes without subsequent amplification, Φ=1\Phi = 1, meaning the number of reacting molecules equals the number of absorbed photons; however, the formulation accommodates deviations via Φ\Phi, allowing for values greater than 1 in cases where a single initiates reactions that propagate beyond the initial event. A classic illustration of the law is the of (HI) gas, where absorption of a single in the appropriate range (typically around 200–300 nm) excites an HI molecule, leading to the primary cleavage of the H-I bond into H and I atoms, with a Φ≈1\Phi \approx 1 for this dissociation step. This example demonstrates the law's one-to-one equivalence under conditions where no chain propagation occurs.

Quantum Yield

In photochemistry, the , denoted as Φ\Phi, is defined as the number of defined events—such as molecules undergoing reaction, products formed, or other specified photochemical outcomes—occurring per absorbed by the system. This metric quantifies the efficiency of light utilization in photochemical processes and is formally expressed as Φ=number of eventsnumber of photons absorbed.\Phi = \frac{\text{number of events}}{\text{number of photons absorbed}}. The concept builds on the Stark-Einstein law, which establishes that primary excitation events involve one molecule activated per absorbed photon (Φ=1\Phi = 1), but quantum yield accounts for subsequent secondary processes that may enhance or diminish overall efficiency. Several factors influence the value of Φ\Phi, including temperature, solvent environment, excitation wavelength, molecular architecture, and the presence of quenchers or competing deactivation pathways such as non-radiative decay. For instance, higher temperatures or protic solvents can promote vibrational relaxation or hydrogen bonding, reducing Φ\Phi by favoring energy dissipation over reaction, while specific wavelengths may align better with absorption bands to optimize yield. Quantum yields typically range from less than 1, as seen in fluorescence quenching where excited states return to the ground state without productive reaction, to greater than 1 in processes involving amplification, such as chain reactions in photopolymerization. To determine Φ\Phi, the rate of the photochemical event is compared to the rate of absorption, with the latter quantified via actinometry. A widely used standard is the actinometer, which undergoes photoreduction from Fe(III) to Fe(II) with a known Φ≈1.2\Phi \approx 1.2–$1.4$ in the UV range (200–500 nm), allowing accurate measurement of incident flux by spectrophotometric analysis of the ferrous ion product. Representative examples illustrate the range of Φ\Phi values. In simple photodissociation, such as the cleavage of a single bond upon absorption, Φ≈1\Phi \approx 1, directly embodying the primary step efficiency of the Stark-Einstein law without significant secondary contributions. Conversely, in the photochlorination of , radical chain leads to Φ≈106\Phi \approx 10^6, where a single initiates a cascade of and termination steps, dramatically amplifying product formation.

Excited States and Emission Processes

Light Absorption and Excitation

In photochemistry, the absorption of light serves as the initial trigger for molecular excitation, consistent with the Grotthuss-Draper law, which states that only absorbed radiation can initiate photochemical changes. When a molecule in its ground electronic state, denoted as S₀ (a singlet state where all electrons are spin-paired), encounters a photon of appropriate energy, the photon is absorbed, promoting a single electron from a filled molecular orbital to an unoccupied higher-energy orbital. This vertical transition occurs on a femtosecond timescale, adhering to the Franck-Condon principle, and results in the formation of an electronically excited singlet state, typically S₁ or higher (S₂, S₃, etc.), depending on the photon's energy. The Jablonski diagram illustrates this process, depicting energy levels for ground and excited states with horizontal lines representing vibrational sublevels within each electronic state; absorption corresponds to upward arrows from S₀ to Sₙ (n ≥ 1), without immediate relaxation details. The nature of the electronic transition determines the of absorbed and the molecule's suitability for photochemical activation. Common transitions in organic molecules include π → π*, where an moves from a bonding π orbital to an antibonding π* orbital, often observed in conjugated systems like alkenes or aromatic compounds, leading to strong absorption in the UV-visible range. Another prevalent type is the n → π*, involving promotion of a non-bonding (e.g., from oxygen or lone pairs) to a π* orbital, which typically requires lower energy and thus occurs at longer wavelengths, as seen in carbonyl compounds. In coordination compounds, d → d* transitions dominate, particularly in complexes, where electrons shift within the split d-orbital manifold, influenced by field effects and resulting in color and photochemical reactivity. Excited states possess distinct properties compared to the , including higher content that weakens bonds and enhances reactivity; for instance, in π → π* excitations, the promotion of electrons from bonding to antibonding orbitals reduces , facilitating processes like dissociation or rearrangement. Singlet excited states, such as S₁, have short lifetimes on the order of nanoseconds due to rapid vibrational relaxation and to lower vibrational levels within the same electronic state. From these singlets, can occur, a non-radiative where a spin flip converts the excited electron's spin, transforming the (S₁) to a (T₁), which has parallel spins and lower . This spin-forbidden transition is enabled by spin-orbit coupling, an interaction between the electron's spin and orbital , often enhanced by heavy atoms or specific molecular symmetries. The efficiency of light absorption is quantified by the molar absorptivity (ε), a measure of how strongly a absorbs at a given , typically in units of L mol⁻¹ cm⁻¹. The extent of absorption in a sample follows the Beer-Lambert law, which describes the decrease in light intensity (I) through a medium of path length l and concentration c: I=I0×10−εclI = I_0 \times 10^{-\varepsilon c l} where I₀ is the incident intensity and absorbance A = ε c l = -log₁₀(I/I₀); this law underpins the quantitative analysis of photochemical systems by relating absorbance to molecular concentration and excitation probability. High ε values, often exceeding 10⁴ L mol⁻¹ cm⁻¹ for allowed transitions like π → π*, ensure effective photon capture even at low concentrations.

Fluorescence

Fluorescence represents a key deactivation pathway in photochemistry, involving the radiative transition from the first excited (S₁) to the ground (S₀) with the emission of a . This spin-allowed process follows light absorption and excitation to S₁, typically occurring on a timescale of hundreds of picoseconds to tens of nanoseconds, depending on the molecule's structure and environment. The emitted photon's energy is generally lower than that of the absorbed photon, resulting in a red-shifted . This phenomenon, known as the , arises from rapid vibrational relaxation within the S₁ state to its lowest vibrational level before emission, dissipating excess energy non-radiatively. The efficiency of fluorescence is characterized by the quantum yield, denoted as Φ_f, defined as the ratio of the number of photons emitted to the number of photons absorbed by the . Values of Φ_f range from near zero to unity, with high yields observed in rigid molecular structures that restrict non-radiative decay pathways, such as or ; conversely, Φ_f decreases in flexible s or those subject to by external agents like oxygen or heavy atoms. In photochemical studies, fluorescence serves as a probe for excited-state dynamics, revealing timescales and pathways of energy dissipation through time-resolved measurements. It is particularly valuable in applications like dyes, where high Φ_f and tunable emission enable efficient and , as seen in coumarin-based systems used for tunable visible s. Representative examples include , which displays bright blue in under excitation due to its rigid structure, and fluorescein, a employed as a tracer in hydrological and environmental photochemistry to monitor flow and dispersion without significant photochemical degradation.

Phosphorescence

Phosphorescence refers to the emission of light from a in its lowest triplet (T₁) returning to the ground (S₀), typically following from a photoexcited . This radiative process is spin-forbidden due to the change in spin multiplicity from triplet to singlet, resulting in emission lifetimes ranging from milliseconds to seconds, in contrast to the much faster nanosecond-scale from . The slow decay rate of phosphorescence arises from the prohibition of direct T₁ to S₀ transitions, allowing triplet states to persist long after excitation ceases. However, at , is frequently quenched by non-radiative decay pathways, such as thermal vibrational relaxation or , which compete with emission; it is more readily observed at low temperatures (e.g., 77 K) or in rigid media like glassy solvents or matrices that suppress molecular motions. The (Φ_p), defined as the ratio of photons emitted via phosphorescence to those absorbed, is typically low (<0.1) in simple organic molecules due to dominant non-radiative deactivation. In engineered organic phosphors, such as those employed in phosphorescent organic light-emitting diodes (OLEDs), Φ_p can be enhanced to 0.2–0.6 through strategies like incorporating heavy atoms (e.g., iodine or bromine) to boost intersystem crossing efficiency and suppress quenching. Triplet states accessed via phosphorescence play a key role in photochemical reactions by facilitating triplet-triplet energy transfer, known as sensitization, where the long-lived T₁ state donates energy to ground-state acceptors, enabling photocatalysis of otherwise inert substrates. A representative example is benzophenone, which undergoes efficient intersystem crossing to its T₁ state and emits characteristic green phosphorescence at wavelengths around 440–520 nm. This property makes benzophenone useful in rigid media for applications like glow-in-the-dark materials, where sustained emission provides persistent luminescence after excitation.

Experimental Techniques

Laboratory Setup

Photochemical experiments require specialized laboratory setups to deliver controlled light exposure while maintaining reaction integrity and operator safety. Essential components include light sources, reaction vessels, monitoring tools, and safety protocols, designed to handle the unique demands of photoinduced processes such as excitation and energy transfer. Light sources are selected based on the required wavelength range, with ultraviolet (UV) light predominant for most organic photochemical reactions due to its ability to excite molecules to reactive states. Medium-pressure mercury lamps (e.g., 75–450 W) emit a broad spectrum including key lines at 254 nm, 310 nm, and 365 nm, often filtered to isolate wavelengths above 280 nm or 330 nm for targeted applications. Xenon arc lamps (150–500 W) provide continuous output across UV and visible regions, simulating solar conditions when paired with AM 1.5G filters, while light-emitting diodes (LEDs) offer monochromatic emission (e.g., 365 nm UV or 455 nm blue, 3–75 W) with narrow bandwidths (10–30 nm) for precise, energy-efficient irradiation. Reaction vessels must transmit the excitation light without absorption or degradation, typically using quartz for full UV transparency (down to 200 nm) in immersion wells or tubes (e.g., 10–16 mm diameter, 4 mm wall thickness) that allow internal cooling via circulating water jackets. Pyrex glass serves for visible light or longer UV wavelengths (>300 nm), as in cylindrical vessels or vials (2–100 mL), preventing thermal side reactions through immersion in cooled baths. These setups ensure uniform illumination and efficient heat dissipation. Safety measures are paramount given the hazards of UV radiation and reactive intermediates. Operators use protective eyewear and full-body shielding to block UV exposure, with enclosures featuring interlocks that shut off lamps upon access. Inert atmospheres, such as or purging (10 mL/min flow), exclude oxygen to prevent of excited states and oxidative byproducts, often achieved via gloveboxes or sealed systems with sparging. Monitoring tools verify consistent conditions, including photometers or integrating spheres to measure light intensity (e.g., in mW/cm²) and calibrate against quantum yield standards like actinometry for reproducible delivery. Temperature control via water baths (0–70 °C) or fans avoids thermal interference, ensuring photochemical specificity over thermal pathways. Setups operate at various scales for versatility: batch modes in vials or wells (mmol quantities, 15–24 h ) suit exploratory work, while continuous configurations using microreactors (0.45–111 mL volumes) enhance efficiency and scalability to gram or outputs through higher throughput and reduced light path lengths.

Spectroscopic Methods

Spectroscopic methods play a crucial role in probing photochemical processes by characterizing light absorption, excited-state lifetimes, transient intermediates, and product formation. These techniques span steady-state and time-resolved approaches, providing quantitative data on electronic transitions, quantum efficiencies, and reaction kinetics essential for understanding phototransformations. Ultraviolet-visible (UV-Vis) is fundamental for recording absorption spectra that identify the electronic transitions responsible for photochemical excitation. It measures the molar absorptivity and at specific wavelengths, enabling the determination of quantum yields by quantifying the fraction of absorbed photons that lead to reaction or emission. In practice, UV-Vis data ensure low (typically <0.1) to avoid inner filter effects, as used in comparative methods for accurate yield calculations. Time-resolved methods extend these insights to dynamic processes. Flash photolysis achieves nanosecond resolution by delivering a short laser pulse (e.g., 10 ns from Nd-YAG or ) to generate transient species, followed by broadband detection of their absorption or emission spectra. This technique reveals the formation and decay of short-lived intermediates, such as triplets or radicals, in reactions like ketone cleavage. Femtosecond laser spectroscopy targets ultrafast excitation dynamics, using pulses on the order of 100 fs to initiate and probe events like vibrational wave packet evolution and nonadiabatic transitions. Pump-probe configurations track processes such as bond dissociation in ethylene or proton migration in acetylene, offering sub-picosecond temporal resolution for primary photochemical steps. Emission spectroscopy focuses on radiative deactivation pathways. Steady-state fluorimeters excite samples with monochromatic light and collect fluorescence at right angles, integrating spectra to compute the fluorescence quantum yield (Φ_f), the ratio of photons emitted to those absorbed, which quantifies singlet-state efficiency. Measurements at varying concentrations ensure linearity, with corrections for refractive index differences between samples and standards. Phosphorescence detection requires low-temperature conditions (e.g., <80 K in rigid matrices) to suppress thermal quenching of triplet states, enabling observation of delayed emission from intersystem crossing. This approach measures phosphorescence quantum yields and lifetimes, distinguishing triplet-mediated photochemistry from fluorescence. Electron paramagnetic resonance (EPR) spectroscopy identifies radicals and other paramagnetic transients in photochemical reactions. Continuous-wave or time-resolved EPR detects hyperfine splittings and g-factors of species like hydroxyalkyl radicals from ketone photolysis, even under high-temperature (up to 720 K) and pressure (up to 150 bar) conditions, providing structural and kinetic details. Infrared (IR) spectroscopy captures vibrational signatures of photochemical changes, with time-resolved variants monitoring bond stretches or bends in intermediates post-excitation. Step-scan or dispersive IR techniques resolve dynamics in reactions involving carbonyl or olefin transformations, complementing UV-Vis by probing ground- and excited-state geometries. Actinometry standardizes photon flux measurements for reproducible quantum yield determinations. Potassium ferrioxalate serves as a robust chemical actinometer in the 250–500 nm range, where UV irradiation reduces Fe(III) to Fe(II) with a quantum yield of 1.20–1.26 at 254–366 nm. The standard procedure involves irradiating 3 cm³ of a 0.006 M solution in 0.5 M sulfuric acid under stirring, transferring a 1 cm³ aliquot to a 10 cm³ flask with 4 cm³ of 0.1% 1,10-phenanthroline and 0.5 cm³ buffer, and diluting to the mark. Measure absorbance at 510 nm (ε = 11,100 M⁻¹ cm⁻¹) after color development. Photon flux is then calculated as q = (ΔA × V₁ × V₃) / (Φ × ε × V₂ × l × t), where V₁ = 3 cm³ (irradiated volume), V₂ = 1 cm³ (aliquot volume), V₃ = 10 cm³ (total volume), l = 1 cm, and t = irradiation time, ensuring linearity with exposure time.

Integration with Flow Chemistry

The integration of photochemistry with flow chemistry has revolutionized synthetic processes by leveraging continuous-flow reactors to overcome limitations inherent in traditional batch setups, such as poor light penetration and scalability issues. In flow systems, photochemical reactions occur in narrow channels or tubes where reactants are pumped continuously, enabling efficient photon delivery and precise control over reaction parameters. This transition from batch to flow, building on basic laboratory configurations, facilitates the handling of light-sensitive transformations at larger scales while minimizing safety risks associated with energetic light sources and reactive species. Key advantages of this integration include enhanced light penetration due to the thin geometry of flow channels, which ensures uniform irradiation throughout the reaction mixture, unlike batch reactors where light attenuation limits efficiency in larger volumes. Precise residence time control allows for optimization of excitation periods, reducing side reactions and improving selectivity, while safer management of hazardous intermediates—such as singlet oxygen or radicals—is achieved through small reaction volumes that prevent accumulation and overheating. Additionally, flow photochemistry supports scalability by enabling "numbering-up" strategies, where multiple parallel reactors increase throughput without altering reaction conditions. Typical setups involve microfluidic reactors constructed from transparent fluorinated ethylene propylene (FEP) tubing, which offers chemical inertness and high UV transmittance, coiled around or integrated with arrays of light-emitting diodes (LEDs) for targeted wavelengths. These systems often use peristaltic or syringe pumps to deliver reactants at controlled flow rates, with the reactor coil immersed in a cooling bath to maintain temperature, ensuring consistent photon flux and mixing. For instance, scalable photooxygenations, such as the conversion of α-terpinene to ascaridole using singlet oxygen generated in situ, benefit from this design by avoiding batch overheating and achieving high yields over extended runs. Despite these benefits, challenges persist, including potential clogging from precipitated solids or viscous mixtures, which can disrupt continuous operation, and ensuring uniform irradiation across varying flow rates. Solutions include modular reactor designs with backpressure regulators to prevent precipitation and the use of wider channels or spinning disc configurations for better mixing and light distribution. Recent advances post-2020 have focused on 3D-printed reactors, enabling customized geometries and integration with specific LED wavelengths for enhanced flexibility; for example, open-source 3D-printed polypropylene reactors paired with Kessil lamps have demonstrated rapid optimization for diverse photochemical transformations.

Types of Photochemical Reactions

Photoisomerization

Photoisomerization refers to a photochemical reaction in which light absorption induces a geometric or structural rearrangement of a molecule, typically involving rotation around a double bond, without the formation or breaking of chemical bonds. This process occurs in the excited state following light absorption, as described in the initial excitation step. The reaction is ultrafast, often completing within picoseconds, due to barrierless potential energy surfaces in the excited state that facilitate torsional motion leading to isomer interconversion. The mechanism begins with photoexcitation of the ground-state molecule to a singlet excited state, such as S1 (ππ* or nπ*), where the double bond character weakens, allowing rotation around the former double bond. This leads to a twisted excited-state intermediate, where the molecule reaches a perpendicular geometry. Isomerization proceeds via nonadiabatic decay through a conical intersection between the excited and ground states, enabling rapid return to the ground-state surface of the new isomer. In azobenzene, for instance, the primary coordinate involves torsion of the CNNC dihedral angle coupled with relaxation of the NNC angle, culminating at a conical intersection near 130° CNNC twist. Stereochemistry in photoisomerization depends on the excited-state multiplicity. Direct photoexcitation typically populates the singlet manifold, resulting in stereospecific inversion, such as cis-to-trans or trans-to-cis conversion via bond rotation in the twisted singlet state. In contrast, triplet-sensitized isomerization, involving intersystem crossing to the triplet state, often proceeds through single-bond rotation, leading to retention of configuration or less stereospecific outcomes, as observed in stilbene derivatives where triplet pathways favor cis-to-trans reversion without full inversion. A prominent example is azobenzene, which undergoes reversible trans-to-cis isomerization upon UV irradiation (around 365 nm), populating the nπ* state, while visible light (around 450 nm) or thermal relaxation drives the cis-to-trans reversion. This photochromic behavior arises from the stable trans ground state and metastable cis form. Another key example is the 11-cis-to-all-trans isomerization of retinal bound to rhodopsin in vertebrate vision, where absorption of a visible photon initiates torsional rotation around the C11-C12 bond, triggering the phototransduction cascade with high efficiency. Quantum yields for photoisomerization are typically around 0.5, reflecting competition between productive isomerization and back-reaction via the same conical intersection, though values vary with wavelength and environment; for instance, rhodopsin's yield reaches approximately 0.65 at optimal wavelengths, underscoring its biological optimization. Applications leverage this reversible switching for functional materials. In molecular motors, azobenzene derivatives enable unidirectional rotation through sequential photoisomerization and thermal helix inversion, powering nanoscale devices in solvents, surfaces, or biological media. Photochromic materials based on azobenzene exhibit light-controlled color changes, finding use in optical data storage, sensors, and smart windows due to their fatigue resistance and tunability.

Photocycloaddition

Photocycloaddition encompasses light-induced pericyclic reactions where unsaturated molecules, such as alkenes or arenes, form new carbon-carbon bonds to yield cyclic products, with prominent examples including [2+2] and [4+4] variants that proceed through excited states and exhibit pronounced stereospecificity. These processes typically involve diradical intermediates and suprafacial geometry, enabling the construction of strained rings under mild conditions. Unlike thermal cycloadditions, photochemical variants relax orbital symmetry restrictions, allowing otherwise forbidden pathways. A key example is the [2+2] photocycloaddition of enones, an extension of Norrish-type photochemistry where an excited triplet state of the enone adds to a ground-state alkene, forming a cyclobutane derivative via a 1,4-diradical intermediate. In the intermolecular Paterno-Büchi reaction, a triplet-excited carbonyl (often an enone or aldehyde) interacts with an alkene, with the oxygen-centered radical adding suprafacially to one face of the double bond, followed by rapid closure of the diradical to an oxetane. This mechanism ensures stereospecific retention of alkene geometry in the product, as the addition occurs on the same face without rotation in the intermediate. Triplet sensitization, such as with acetone, promotes the reaction by efficient energy transfer to populate the reactive triplet state. Quantum yields for [2+2] photocycloadditions are typically low, in the range of 0.01 to 0.1, due to the reversible nature of the diradical intermediate, which can dissociate back to starting materials before cyclization. Acetone sensitization enhances efficiency by generating triplets with near-unity intersystem crossing, though overall yields remain modest without optimization. Another illustrative case is the [4+4] photocycloaddition of anthracene, where excitation to the singlet state leads to dimerization across the central 9,10-positions, forming a cage-like dianthracene via a similar diradical pathway, reversible upon reheating. Regioselectivity in these reactions is dictated by orbital symmetry conservation, as outlined in the Woodward-Hoffmann rules for photochemical processes, which permit suprafacial-suprafacial ([_π_2s + _π_2s]) additions in the excited state—thermally forbidden but photochemically allowed due to promotion of an electron to an antibonding orbital. This symmetry control favors head-to-tail or specific orientations in unsymmetrical substrates, enhancing predictive power for product distribution. Photocycloadditions find applications in synthesizing highly strained polycyclic structures, such as cubanes, where sequential [2+2] additions to laddered polyenes under triplet sensitization yield the cubic framework with its exceptional ring strain energy exceeding 150 kcal/mol. For instance, benzophenone-sensitized irradiation of dimethyl bicyclo[2.2.0]hexadiene derivatives affords functionalized cubane-1,4-dicarboxylates in good yields, serving as scaffolds for pharmaceuticals and materials due to their rigidity and reactivity.

Photoredox Reactions

Photoredox reactions are photochemical processes driven by single-electron transfer (SET) from or to a photoexcited catalyst, generating radical ions that enable redox transformations under mild conditions with visible light. These reactions harness the ability of excited-state species to participate in electron donation or acceptance, expanding accessible redox potentials beyond those of ground-state species and facilitating the formation of reactive intermediates without harsh reagents. The general mechanism involves photoexcitation of the catalyst to a higher-energy state, followed by SET to produce a radical species from the substrate; subsequent steps, such as radical coupling or further electron transfer, propagate the cycle and regenerate the ground-state catalyst. For instance, in ruthenium-based systems, [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) absorbs visible light (~450 nm) to form the metal-to-ligand charge-transfer [Ru(bpy)3]2+, which can act as either a one-electron oxidant or reductant. This excitation shifts the effective potentials dramatically: the ground-state [Ru(bpy)3]3+/2+ couple is +1.29 V vs. SCE, and [Ru(bpy)3]2+/+ is -1.33 V vs. SCE, but the enables reduction at -0.81 V vs. SCE ([Ru(bpy)3]2+/ [Ru(bpy)3]3+) or oxidation at +0.77 V vs. SCE ([Ru(bpy)3]+/*[Ru(bpy)3]2+), corresponding to the photon's input of approximately 2.05 eV. These shifts allow mild activation of substrates with high potentials, such as unactivated C-H bonds or alkyl halides. Transition metal complexes, particularly polypyridyl ruthenium and iridium derivatives like [Ru(bpy)3]2+ and [Ir(ppy)3] (ppy = 2-phenylpyridine), serve as robust catalysts due to their long-lived excited states (lifetimes ~1 μs) and tunable photophysical properties. Organic dyes, such as , offer cost-effective alternatives with similar SET capabilities; 's excited exhibits a of -1.11 V vs. SCE and oxidation potential of +0.78 V vs. SCE, enabling metal-free photoredox processes. These catalysts typically operate at low loadings (0.1–5 mol%) and are compatible with diverse solvents and functional groups. Representative applications include C-H functionalization, where enables selective activation of inert bonds; for example, the iridium-catalyzed C-H arylation of electron-rich heteroarenes with diaryliodonium salts proceeds via SET-generated aryl radicals, achieving yields up to 90% under visible light. In cross-coupling reactions, visible-light-driven processes facilitate decarboxylative couplings, such as the ruthenium-catalyzed union of α-amino acids with aryl chlorides to form α-arylated amines, bypassing traditional and proceeding at with high efficiency. These transformations highlight photoredox's role in sustainable synthesis. Catalytic cycles in photoredox reactions often yield quantum efficiencies greater than 1, with values exceeding 10 reported in radical chain mechanisms where a single initiates multiple turnovers through steps.

Organic Photochemistry

Norrish Reactions

The Norrish reactions represent foundational processes in , particularly for carbonyl compounds such as ketones and aldehydes, where UV irradiation induces homolytic bond cleavage or hydrogen abstraction in the . Named after Ronald G. W. Norrish, these reactions typically proceed via the triplet n→π* following from the initial singlet excitation. They exemplify the generation of reactive radical intermediates under mild conditions, contrasting with thermal methods that often require harsh reagents. Type I Norrish reactions involve α-cleavage, where the bond between the carbonyl carbon and the adjacent α-carbon breaks homolytically, yielding an acyl radical and an alkyl radical. This process is prevalent in simple aliphatic ketones and is favored in the gas phase due to reduced recombination. For instance, in acetone vapor, at wavelengths around 313 nm leads to the formation of methyl and acetyl radicals, which can further disproportionate to , , and . Quantum yields for this primary cleavage in acetone approach unity (Φ ≈ 1) under gas-phase conditions at or higher temperatures above 100°C, though they decrease in solution due to solvent effects. Type II Norrish reactions proceed via intramolecular abstraction of a γ-hydrogen by the excited carbonyl oxygen, forming a 1,4-biradical intermediate that can either cleave to an and (which tautomerizes to a ) or undergo Yang photocyclization to a cyclobutanol. The efficiency depends on the conformational accessibility of the γ-hydrogen, with triplet-state abstraction being dominant. A classic example is , where UV excitation (e.g., at 313 nm) generates a biradical leading primarily to and propene, alongside minor cyclobutanol products. Quantum yields for Type II processes vary with conformation and solvent but can approach unity (Φ ≈ 1) in aqueous media for rigid systems like , reflecting efficient hydrogen transfer. These reactions hold synthetic utility in generating radicals under ambient conditions, avoiding the need for high temperatures or toxic initiators common in traditional radical chemistry. They serve as radical clocks to probe reaction kinetics, such as biradical lifetimes and rearrangement rates, enabling precise mechanistic studies in photochemistry.

Paterno-Büchi Reaction

The Paternò–Büchi reaction is a photochemical [2+2] cycloaddition between an excited carbonyl compound, such as an aldehyde or ketone, and an alkene, resulting in the formation of a four-membered oxetane ring. First reported in 1909 by Emanuele Paternò and Giuseppe Chieffi using sunlight to irradiate benzaldehyde with 2-methyl-2-butene, the reaction produces oxetanes as the primary products, though early mechanistic understanding was limited. In 1954, George Büchi and coworkers confirmed the oxetane structure through independent studies on similar substrates, establishing the reaction's utility and prompting its naming in his honor. This intermolecular process serves as a key tool in organic synthesis for constructing strained heterocycles, particularly when direct excitation of the carbonyl at wavelengths around 250–350 nm is employed or when triplet sensitization is used to enhance efficiency. The mechanism proceeds via the triplet excited state of the carbonyl (n,π* configuration), which is populated either by direct photoexcitation or with compounds like (E_T ≈ 74 kcal/mol). The triplet carbonyl oxygen adds to one end of the , generating a 1,4- intermediate; this species then undergoes rapid and bond formation to yield the , often in a stereospecific manner that preserves the alkene's cis or trans geometry. While a concerted pathway has been debated for singlet states, experimental evidence from stereochemical outcomes and studies supports the stepwise mechanism for triplet-mediated reactions. Quantum yields are generally modest, ranging from 0.01 to 0.1, reflecting competing deactivation pathways like or non-radiative decay, though improves selectivity by avoiding singlet reactivity. A classic example involves the of with styrene, affording a 2,2-disubstituted where the carbonyl carbon bonds to the terminal carbon and the oxygen to the benzylic position. Regiochemistry is influenced by electronics: with electron-rich alkenes (e.g., ethers or styrenes), the forms preferentially with oxygen addition to the more substituted carbon, enhancing stability through delocalization. The resulting are versatile intermediates, susceptible to thermal ring-opening under mild conditions (often 100–150°C), which cleaves the C–O bond to generate γ-hydroxy carbonyls or related fragments. Applications of the Paternò–Büchi reaction extend to natural product synthesis, exemplified by Thorsten Bach's enantioselective total synthesis of (-)-grandisol, a boll weevil pheromone, where an intramolecular variant with a tethered alkene formed the core oxetane scaffold in high diastereoselectivity. Additionally, the reaction informs photoprotection studies by modeling UV-induced oxetane formation between carbonyl photosensitizers (e.g., in skin or environmental contexts) and nucleobases like thymine in DNA, revealing pathways for dimer repair or degradation that mitigate photochemical damage.

Stilbene Isomerization

Stilbene isomerization serves as a prototypical example of photoinduced cis-trans isomerization in conjugated organic molecules, providing insights into excited-state dynamics relevant to broader processes. The process involves the reversible conversion between trans-stilbene and cis-stilbene upon light absorption, driven primarily by (UV) irradiation for the trans-to-cis direction and visible light for the reverse. The mechanism begins with π→π* excitation from the ground state (S0) to the first excited singlet state (S1) of the stilbene molecule. In the S1 state, rotation occurs around the central C=C double bond, leading to twisting and eventual crossing to the ground state via a conical intersection, resulting in isomerization. This singlet pathway dominates, with a minor, slower contribution from the triplet state accessed via intersystem crossing. The for trans-to-cis is approximately 0.55 in solution at , reflecting efficient torsional motion in the . The reverse cis-to-trans process has a lower of about 0.35, influenced by competing and . Spectroscopically, trans-stilbene exhibits a strong absorption band centered around 300 nm, corresponding to its extended conjugation, while cis-stilbene absorbs at shorter wavelengths near 260 nm due to steric hindrance. These distinct features enable selective excitation and have established stilbene as a standard chemical actinometer for quantifying UV fluxes in photochemical experiments, leveraging its well-characterized s. In experimental studies, the is often monitored via , as cis-stilbene emits weakly from its S1 state, whereas trans-stilbene undergoes non-radiative twisting, resulting in negligible fluorescence. Applications of stilbene extend to the design of photoswitches for molecular electronics, where derivatives like stiff-stilbenes offer high quantum yields (>0.5) and thermal stability for reversible switching in devices. Additionally, the ultrafast dynamics mimic those in vision pigments, such as the in , serving as a simplified model for studying polyene in biological systems.

Inorganic and Organometallic Photochemistry

Metal-to-Ligand Charge Transfer

Metal-to-ligand charge transfer (MLCT) is a photochemical process in coordination compounds where absorption of promotes an from the metal d-orbitals to the π* orbitals of a , resulting in formal reduction of the metal center and oxidation of the ligand. This excitation, often denoted as d → π*, creates a charge-separated state that can drive subsequent reactivity, such as electron transfer to external acceptors or donors. In typical systems, low-oxidation-state metals paired with π-acceptor ligands, like polypyridyls, facilitate this transition, with the metal oxidation state increasing (e.g., Ru(II) to Ru(III)) upon excitation. The of MLCT transitions features intense absorption bands in the visible or near-UV region, arising from the strong of the allowed charge-transfer process and the spatial separation of the and . These bands are broad due to vibrational progressions and typically exhibit molar absorptivities exceeding 50,000 L mol⁻¹ cm⁻¹, enabling efficient light harvesting. A prototypical example is the tris(2,2'-bipyridine) complex, [Ru(bpy)3]2+, which undergoes MLCT excitation upon visible light absorption, leading to an orange emission from the triplet MLCT state at approximately 620 nm. The lifetime of this 3MLCT manifold is around 600 ns at in , during which back from the reduced bpy to Ru(III) competes with productive processes. The emission quantum yield is approximately 0.04, reflecting efficient radiative decay balanced by nonradiative pathways. These MLCT properties underpin applications in dye-sensitized solar cells, where ruthenium polypyridyl sensitizers like [Ru(bpy)3]2+ derivatives inject electrons into substrates upon photoexcitation, achieving power conversion efficiencies up to 11% in optimized devices. In , the long-lived charge-separated states enable single-electron transfer to organic substrates, facilitating bond formation under mild conditions.

Ligand Photodissociation

Ligand photodissociation refers to the photoinduced cleavage of metal-ligand (M-L) bonds in coordination complexes, primarily driven by excitation into ligand field (LF) states. Upon absorption of , typically in the UV-visible , electrons are promoted from metal d-orbitals to higher-energy d* antibonding orbitals (d→d* transitions), which weakens the M-L σ-bonds along specific axes, facilitating homolytic or heterolytic dissociation. This process contrasts with substitution by enabling selective labilization of ligands trans to weak-field groups, as described by the Adamson rule for d³ and low-spin d⁶ systems. The resulting coordinatively unsaturated species often undergo rapid solvent coordination, such as aquation in aqueous media. A classic example is the photoaquation of chloropentamminecobalt(III), [Co(NH₃)₅Cl]²⁺, under UV irradiation, yielding [Co(NH₃)₅(H₂O)]³⁺ and Cl⁻ via LF excitation around 350 nm. Similarly, chromium(III) ammine complexes like [Cr(NH₃)₆]³⁺ exhibit efficient upon visible light absorption (e.g., 460 nm), producing [Cr(NH₃)₅(H₂O)]³⁺ with a (Φ) of approximately 0.32, independent of temperature and excitation wavelength within the LF bands. Quantum yields for such processes generally range from 0.1 to 1, varying with the metal, ligand set, and excitation energy; for instance, Cr(III) systems often show higher Φ (0.1–0.6) compared to Co(III) (∼0.01–0.1) due to longer-lived excited states in d³ configurations. Several factors influence the efficiency and selectivity of photodissociation. Solvent molecules assist in stabilizing the departing and entering , enhancing aquation rates in protic media like . Back-bonding from filled metal d-orbitals to π*-orbitals of ligands (e.g., CO or bpy) can stabilize M-L bonds, reducing Φ, whereas σ-donor ligands like NH₃ promote dissociation by populating antibonding orbitals. Excitation wavelength is critical, as higher-energy LF bands (e.g., ⁴T₂g ← ⁴A₂g in Cr(III)) lead to more distortive excited states, increasing bond lability. These reactions find applications in synthetic for controlled photosubstitution, allowing the preparation of otherwise inaccessible complexes via selective ligand exchange. In solar fuel research, ligand photodissociation in complexes serves as a model for initiating catalytic cycles, such as oxidation or CO₂ reduction, by generating active sites for substrate binding in photocatalytic systems.

Semiconductor Photocatalysis

Semiconductor photocatalysis involves the use of semiconductor materials, such as (TiO₂), to drive chemical reactions through light-induced charge separation in heterogeneous systems. Upon absorption of photons with energy exceeding the bandgap, electrons are excited from the valence band to the conduction band, generating electron-hole pairs. TiO₂ anatase, a prototypical photocatalyst, has a bandgap of 3.2 eV, limiting its response primarily to light. The photogenerated holes in the valence band act as strong oxidants, typically oxidizing water or donor species to produce hydroxyl radicals or oxygen, while electrons in the conduction band serve as reductants, reducing acceptors like oxygen or protons to form or . The positions of the conduction and valence bands relative to standard redox potentials determine the feasibility of specific reactions. For anatase TiO₂ at pH 0, the conduction band edge lies at approximately -0.29 V vs. normal hydrogen electrode (NHE), sufficiently negative to drive hydrogen evolution (0 V vs. NHE), while the valence band edge at +2.91 V vs. NHE enables water oxidation (+1.23 V vs. NHE). These band alignments allow TiO₂ to straddle the water potentials, facilitating overall into hydrogen and oxygen. Unlike homogeneous photoredox reactions involving molecular catalysts, photocatalysis relies on solid-state band-to-band transitions for sustained charge separation at interfaces. Photocatalytic efficiency is often limited by rapid electron-hole recombination, resulting in low quantum yields, typically less than 1% for undoped TiO₂ in processes like degradation or . Recombination occurs within picoseconds at surface defects or bulk traps, reducing the lifetime of charge carriers available for reactions. Doping strategies, such as nitrogen incorporation into TiO₂, narrow the bandgap and introduce mid-gap states that suppress recombination while extending absorption into the , enhancing quantum yields by factors of 2–10 under visible light. For instance, N-doped TiO₂ exhibits improved charge separation, leading to higher photocatalytic rates compared to pristine material. Key examples include , first demonstrated by Fujishima and Honda in 1972 using irradiated TiO₂ electrodes to evolve oxygen and hydrogen from water under bias, marking the onset of research. Another prominent application is the degradation of organic pollutants, where TiO₂ generates to mineralize dyes, pesticides, and emerging contaminants like pharmaceuticals in wastewater, achieving near-complete removal under UV illumination. In , enables the self-cleaning of air and by degrading volatile organic compounds and heavy metal ions at ambient conditions, with TiO₂-based systems deployed in large-scale pilots for municipal . For , post-2010 advances in semiconductors, such as perovskites (e.g., CH₃NH₃PbI₃), have introduced tunable bandgaps (1.5–2.3 eV) and superior charge mobility, enabling efficient CO₂ reduction to and unassisted with solar-to-hydrogen efficiencies exceeding 10% in hybrid devices. These materials complement traditional oxides by harvesting visible light more effectively, advancing scalable production.

Applications and Historical Development

Synthetic Applications

Photochemistry has emerged as a powerful tool in synthetic chemistry, enabling reactions under mild conditions that enhance efficiency and selectivity in both organic and inorganic synthesis. One key advantage is the ability to conduct transformations at , as light excitation provides the necessary energy without requiring thermal heating, thereby minimizing energy consumption and preventing decomposition of sensitive substrates. Additionally, photochemical processes often exhibit superior stereocontrol due to the unique reactivity of excited states, allowing precise control over stereoisomers in complex molecule assembly, such as in the selective formation of ε-diaminotruxillic acids with complete . These methods also avoid the use of toxic reagents by leveraging as a traceless activator, often employing earth-abundant photocatalysts like or , which reduces and aligns with sustainable practices. In , photochemistry facilitates the production of valuable compounds through selective isomerizations and couplings. A classic example is the of to vitamin D2, where irradiation induces a 6-electron conrotatory ring-opening in the B-ring of ergosterol, yielding previtamin D2 that thermally isomerizes to vitamin D2; this process has been industrially scaled for nutritional supplements, achieving high conversion efficiencies under controlled UV exposure. In pharmaceutical intermediate synthesis, enables efficient C-N bond formation, such as the decarboxylative amination of alkyl carboxylic acids with amines using iridium/ dual , providing access to complex C(sp³)-N linkages with broad substrate scope and mild conditions suitable for late-stage functionalization of drug candidates. Scalability in photochemical synthesis has been advanced through integration with continuous-flow reactors, which improve penetration, mixing, and dissipation for industrial production. For instance, Merck has developed flow photochemistry processes for pharmaceutical intermediates, including a scalable bromination step yielding over 100 kg/day with 91% purity and 94% yield via a numbering-up approach, demonstrating enhanced productivity and safety over batch methods. Recent developments in the 2020s emphasize principles and optimization techniques to further enhance synthetic efficiency. Photochemical processes have shown significant reductions in the E-factor—a metric quantifying waste per unit of product—with photoredox reactions achieving up to 50% lower E-factors compared to traditional methods by minimizing solvent use and byproduct generation, as seen in optimized catalytic cycles. approaches are increasingly applied to optimize reaction parameters, such as predicting ideal wavelengths for maximal quantum yields in photocatalyst design, accelerating the discovery of efficient synthetic routes. In inorganic synthesis, photochemical deposition offers precise control over nanomaterial fabrication. This technique involves light-induced reduction of metal precursors onto substrates, enabling the creation of uniform metal nanoparticles or supported catalysts; for example, UV-mediated deposition of or silver nanoparticles from aqueous solutions produces size-tunable nanostructures (5–50 nm) with high monodispersity, useful for catalytic applications without high-temperature processing.

Environmental and Biological Uses

Photocatalytic processes play a crucial role in environmental remediation, particularly through the degradation of organic pollutants such as dyes and pesticides using semiconductor materials like titanium dioxide (TiO₂) under solar irradiation. In these systems, TiO₂ absorbs ultraviolet light from the sun to generate electron-hole pairs that drive the oxidation of contaminants, breaking them down into harmless byproducts like CO₂ and H₂O. For instance, modified TiO₂ photocatalysts have demonstrated high efficiency in degrading textile dyes and agricultural pesticides in wastewater, with complete mineralization often achieved within hours of solar exposure. This approach leverages abundant solar energy for sustainable pollution control, as detailed in semiconductor photocatalysis mechanisms where TiO₂'s bandgap excitation initiates reactive oxygen species formation. Another key environmental application involves the photochemical formation of in the , which acts as both a and an oxidant. arises from photochemical reactions between nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) in the presence of , where UV photons photolyze NO₂ to produce oxygen atoms that react with O₂ to form O₃. This process, while essential for understanding air quality, contributes to formation in urban areas and affects ecosystems through on and aquatic life. In biological systems, photochemistry underpins vital life processes, most notably , where light absorption by drives through the Z-scheme. In this mechanism, (PSII) absorbs to oxidize water, releasing O₂ and electrons that flow via and cytochrome b₆f to (PSI), where another excitation reduces NADP⁺ to NADPH, powering carbon fixation. The Z-scheme's sequential energy drops ensure efficient charge separation despite low quantum yields, enabling plants to convert into chemical bonds at scales that sustain global ecosystems. Photochemistry also enables therapeutic applications in biology, such as (PDT) for , which relies on to generate upon light activation. In PDT, a non-toxic accumulates in tumor cells and, when irradiated with visible light in the presence of ground-state oxygen, undergoes to produce triplet states that transfer energy to O₂, yielding cytotoxic (¹O₂) that damages cellular components like and proteins. This selective ROS generation has led to clinical approvals for treating various cancers, with quantum yields for ¹O₂ production often exceeding 0.5 in optimized systems. Energy applications of photochemistry include dye-sensitized solar cells (DSSCs), pioneered as Grätzel cells, which mimic by using a ruthenium-based adsorbed on TiO₂ to harvest visible light. Upon absorption, the injects electrons into the TiO₂ conduction band, regenerating via an iodide/triiodide electrolyte, achieving power conversion efficiencies up to 15% under standard solar conditions. Similarly, for employs semiconductors to split H₂O into H₂ and O₂ using , with recent advances reaching solar-to-hydrogen efficiencies over 9% through bandgap-engineered materials that enhance charge separation and surface . Quantum aspects highlight the efficiency trade-offs in natural photochemical systems, where low quantum yields—such as the ~0.1 for CO₂ fixation in —are compensated by the high of , approximately 1000 W/m² at Earth's surface. This balance allows biological processes to operate effectively despite inherent losses from non-radiative decay and antenna inefficiencies, a principle echoed in artificial systems where optimization boosts overall productivity. Challenges in these applications include photostability, where photocatalysts often degrade under prolonged illumination due to recombination or structural breakdown, limiting long-term viability in environmental and energy contexts. Recent advances, particularly post-2022, have addressed this through metal-organic frameworks (MOFs) as photocatalysts, which offer tunable bandgaps and high surface areas for enhanced stability and efficiency in pollutant degradation and H₂ evolution, with some composites showing sustained activity over hundreds of cycles.

Key Historical Milestones

The development of photochemistry began in the early with foundational observations on the interaction between and matter. In 1817, Theodor von Grotthuss proposed that only absorbed by a can induce a reaction, establishing the first law of photochemistry and suggesting a chain-like propagation of 's effects within the absorbing medium. This insight, derived from studies on the photodecomposition of and other salts, laid the groundwork for understanding photochemical selectivity. Two decades later, in 1841, experimentally validated and refined this principle through quantitative measurements on the photochemical reduction of silver halides, demonstrating that the rate of reaction is proportional to the intensity of absorbed light rather than incident light. This confirmation, known today as the Grotthuss-Draper law, shifted photochemistry from qualitative observations to a more empirical . The quantum foundation of photochemistry emerged in the early . In 1912, extended his light quantum hypothesis to photochemical processes, formulating the law of photochemical equivalence, which posits that each absorbed excites one molecule, linking light's particle nature to chemical activation. This principle, independently supported by , enabled the calculation of quantum yields and revolutionized the quantitative analysis of light-induced reactions. During the 1930s, Ronald George Wreyford Norrish advanced the understanding of organic photochemical mechanisms through systematic studies of ketone photolysis, identifying key pathways such as alpha-cleavage and intramolecular hydrogen abstraction, now classified as Norrish Type I and Type II reactions. These discoveries provided mechanistic insights into radical formation and rearrangement in organic systems, bridging early laws with complex molecular behaviors. The mid-20th century saw conceptual tools and applications expand photochemistry's scope. In the 1960s, Jablonski diagrams became a standard representation for depicting electronic excited states, radiative and non-radiative transitions, and energy transfer processes, facilitating the interpretation of fluorescence and phenomena. A pivotal application followed in 1972 with the Honda-Fujishima effect, where Kenichi Honda and Akira Fujishima demonstrated photoelectrochemical on electrodes under ultraviolet light, inaugurating the field of for energy conversion. From the to the , photochemistry experienced a revival through the development of , leveraging complexes to mediate single-electron transfer under visible light. A landmark contribution came in 2008 from David W. C. MacMillan, who merged with organocatalysis to enable asymmetric alpha-alkylation of aldehydes, demonstrating mild conditions for enantioselective C-C bond formation and inspiring a surge in synthetic applications. In the 2020s, photochemistry has integrated with , notably through visible-light-driven organocatalysis using metal-free sensitizers like organic dyes and hypervalent iodine compounds for selective transformations, enhancing in . Concurrently, approaches have begun simulating excited-state dynamics, with variational quantum eigensolvers applied to model photochemical reaction pathways in molecules like diarylethenes, promising accurate predictions beyond classical computational limits.

References

  1. https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/photchem.htm
  2. https://research.cbc.osu.edu/turro.1/wp-content/uploads/2017/10/Chapter1.pdf
  3. https://chemistry.as.miami.edu/_assets/pdf/murthy-group/absorption--emission.pdf
  4. https://chemistry.as.miami.edu/_assets/pdf/murthy-group/4-history-of-photo-volman.pdf
  5. https://news.rpi.edu/luwakkey/3063
  6. https://catalog.princeton.edu/catalog/99125422543006421
  7. https://pubs.rsc.org/en/content/articlelanding/2016/pp/c5pp00445d
  8. https://chemistry-europe.onlinelibrary.wiley.com/doi/full/10.1002/elsa.202160006
  9. https://pubs.acs.org/doi/10.1021/acs.chemrev.1c00332
  10. https://www.sciencedirect.com/science/article/abs/pii/S0009261401005140
  11. https://goldbook.iupac.org/terms/view/Q04991
  12. https://www.nature.com/articles/s41467-024-50366-1
  13. https://pubs.rsc.org/en/content/articlehtml/2015/sc/c5sc02185e
  14. https://pubs.rsc.org/en/content/articlehtml/2018/pp/c7pp00401j
  15. https://royalsocietypublishing.org/doi/10.1098/rspa.1956.0102
  16. https://magadhmahilacollege.org/wp-content/uploads/2020/04/photochemistry_and_jablonski_diagram_M.sc_II_Sem.pdf
  17. https://chemistry.as.miami.edu/_assets/pdf/jablonski-3.pdf
  18. https://nowgonggirlscollege.co.in/attendence/classnotes/files/1628057189.pdf
  19. https://pitrelab.okstate.edu/images/Lecture_6_-_The_Photochemistry_of_Rubpy32.pdf
  20. https://doyle.chem.ucla.edu/wp-content/uploads/2020/07/50-3d-d-Excited-States-of-NiII-Complexes-Relevant-to-Photoredox-Catalysis.pdf
  21. https://projects.iq.harvard.edu/files/sasselovlab/files/acs.jpclett.1c01384.pdf
  22. https://pmc.ncbi.nlm.nih.gov/articles/PMC11360185/
  23. https://geekztrainerblog.files.wordpress.com/2016/08/unit-iii-photochemistry-and-spectroscopy.pdf
  24. https://ocw.mit.edu/courses/5-61-physical-chemistry-fall-2017/14b9fa61bd38c5f243c9db2048c60680_MIT5_61F17_lec34.pdf
  25. https://micro.magnet.fsu.edu/primer/techniques/fluorescence/fluorescenceintro.html
  26. https://chemistry.as.miami.edu/_assets/pdf/murthy-group/c-3-absorption--emission-ch-4-.pdf
  27. https://pubs.acs.org/doi/10.1021/acs.chemrev.6b00491
  28. https://nvlpubs.nist.gov/nistpubs/jres/80A/jresv80An3p421_A1b.pdf
  29. https://pmc.ncbi.nlm.nih.gov/articles/PMC11755655/
  30. https://pubs.usgs.gov/of/1984/0234/report.pdf
  31. https://pmc.ncbi.nlm.nih.gov/articles/PMC9450115/
  32. https://chemistry.coe.edu/piper/posts/groves-quantum/lecture_slides/Lecture_19_ElectronicTransitionsAndPhotoChemistry_CHEM361B.pdf
  33. https://www.edinst.com/wp-content/uploads/2020/07/Triplet-TA-App-Note_v1.pdf
  34. https://pubs.acs.org/doi/10.1021/acs.jpcc.7b04529
  35. https://pubs.aip.org/aip/jcp/article-pdf/55/3/1323/18875343/1323_1_online.pdf
  36. https://www.riken.jp/en/news_pubs/research_news/rr/20211227_2/
  37. https://doi.org/10.1021/acs.chemrev.1c00332
  38. https://doi.org/10.1021/acs.chemrev.5b00707
  39. https://pubs.acs.org/doi/10.1021/jp001015m
  40. https://pmc.ncbi.nlm.nih.gov/articles/PMC8193824/
  41. https://www.chem.uci.edu/~dmitryf/manuals/Fundamentals/Quantum%20yield%20determination.pdf
  42. https://pubs.acs.org/doi/10.1021/ac3036487
  43. https://vlab.amrita.edu/?sub=2&brch=191&sim=608&cnt=1
  44. https://pubs.acs.org/doi/10.1021/acsphyschemau.5c00022
  45. https://pubs.acs.org/doi/10.1021/acs.jpclett.2c01119
  46. https://www.chem.uci.edu/~dmitryf/manuals/Fundamentals/Fluorescence%20Spectroscopy.pdf
  47. https://scholarworks.bgsu.edu/cgi/viewcontent.cgi?article=1230&context=spectrum
  48. https://pubs.acs.org/doi/10.1021/jp963202j
  49. https://kgroup.du.edu/resources/IUPAC-Actinometers04.pdf
  50. https://pubs.acs.org/doi/10.1021/j150619a015
  51. https://doi.org/10.1021/acs.oprd.8b00213
  52. https://www.vapourtec.com/blog/what-flow-photochemistry/
  53. https://link.springer.com/article/10.1007/s41981-021-00168-z
  54. https://pubs.acs.org/doi/10.1021/acs.oprd.8b00213
  55. https://pmc.ncbi.nlm.nih.gov/articles/PMC6187735/
  56. https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2023.1244043/full
  57. https://pubs.acs.org/doi/10.1021/acs.oprd.4c00458
  58. https://link.springer.com/article/10.1007/s41981-025-00362-3
  59. https://advanced.onlinelibrary.wiley.com/doi/10.1002/adsc.202500133
  60. https://link.springer.com/article/10.1007/s41981-023-00278-w
  61. https://www.sciencedirect.com/topics/chemistry/photoisomerization
  62. https://www.nature.com/articles/s41467-018-06971-y
  63. https://pubs.acs.org/doi/10.1021/cr00003a007
  64. https://www.sciencedirect.com/science/article/pii/0047267080800207
  65. https://pubs.rsc.org/en/content/articlelanding/2012/cs/c1cs15179g
  66. https://pmc.ncbi.nlm.nih.gov/articles/PMC2891588/
  67. https://www.pnas.org/doi/10.1073/pnas.2536769100
  68. https://www.nature.com/articles/srep41145
  69. https://www.nature.com/articles/s41467-022-33177-0
  70. https://pubs.acs.org/doi/10.1021/acs.chemrev.5b00723
  71. https://www.sciencedirect.com/science/article/abs/pii/S0009261419303720
  72. https://pubs.rsc.org/en/content/articlehtml/2023/ob/d3ob00231d
  73. https://pubs.acs.org/doi/10.1021/cr300503r
  74. https://pubs.rsc.org/en/content/articlelanding/2014/cc/c4cc00751d
  75. https://pubs.acs.org/doi/10.1021/acs.chemrev.1c00311
  76. https://pubs.rsc.org/en/content/articlelanding/2015/sc/c5sc02185e
  77. https://doi.org/10.1039/b307997j
  78. https://pubmed.ncbi.nlm.nih.gov/21222478/
  79. https://www.sciencedirect.com/science/article/pii/S1389556717300886
  80. https://pmc.ncbi.nlm.nih.gov/articles/PMC6269742/
  81. https://pubs.acs.org/doi/10.1021/ja01033a067
  82. https://pubs.aip.org/aip/jcp/article/100/12/8856/472839/The-new-photoisomerization-mechanism-of-stilbene
  83. https://pubs.acs.org/doi/10.1021/ja01046a053
  84. https://pubs.acs.org/doi/10.1021/ja00951a014
  85. https://link.springer.com/article/10.1039/b606727c
  86. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_%28Physical_and_Theoretical_Chemistry%29/Spectroscopy/Electronic_Spectroscopy/Metal_to_Ligand_and_Ligand_to_Metal_Charge_Transfer_Bands
  87. https://pubs.aip.org/aip/cpr/article/3/2/021303/2836222/Emergence-of-ligand-to-metal-charge-transfer-in
  88. https://www.sciencedirect.com/science/article/abs/pii/S0010854598001878
  89. https://application.wiley-vch.de/books/sample/3527335609_c01.pdf
  90. https://pubs.acs.org/doi/10.1021/acs.jpca.4c04914
  91. https://doras.dcu.ie/19086/1/Karen_F_Mongey_20130617162730.pdf
  92. https://www.mdpi.com/2304-6740/6/2/52
  93. https://onlinelibrary.wiley.com/doi/10.1155/2012/291579
  94. http://www.hgcollege.edu.in/uploadfiles/PG2%20Inorganic%20Photochemistry%20III_BS.pdf
  95. https://www.sciencedirect.com/science/article/abs/pii/S0016236122021573
  96. https://www.intechopen.com/chapters/77955
  97. https://www.sciencedirect.com/science/article/abs/pii/S0169433214010629
  98. https://pubs.acs.org/doi/10.1021/jz300071j
  99. https://pubs.rsc.org/en/content/articlelanding/2016/nj/c5nj03478g
  100. https://www.nature.com/articles/238037a0
  101. https://www.sciencedirect.com/science/article/pii/S2666821122000230
  102. https://eprints.whiterose.ac.uk/id/eprint/178658/8/UNSDG6_TiO2-photocatalyst-final-R1-revised-without-tracked-changes-P1.pdf
  103. https://pubs.acs.org/doi/10.1021/acsaem.2c02680
  104. https://doi.org/10.1002/ejoc.202301312
  105. https://pmc.ncbi.nlm.nih.gov/articles/PMC7508054/
  106. https://pmc.ncbi.nlm.nih.gov/articles/PMC10132167/
  107. https://pubs.rsc.org/en/content/articlelanding/2023/nr/d3nr02475j
  108. https://iwaponline.com/wst/article/88/6/1495/97423/A-review-of-the-photocatalytic-degradation-of
  109. https://pubs.acs.org/doi/10.1021/acsomega.4c00887
  110. https://www.epa.gov/ground-level-ozone-pollution/ground-level-ozone-basics
  111. https://csl.noaa.gov/assessments/ozone/2010/twentyquestions/Q2.pdf
  112. https://pmc.ncbi.nlm.nih.gov/articles/PMC1069689/
  113. https://www.life.illinois.edu/govindjee/ZSchemeG.html
  114. https://pubs.rsc.org/en/content/articlelanding/2024/nr/d3nr05801h
  115. https://pmc.ncbi.nlm.nih.gov/articles/PMC11510636/
  116. https://pmc.ncbi.nlm.nih.gov/articles/PMC6261913/
  117. https://www.nature.com/articles/nchem.1861
  118. https://pubmed.ncbi.nlm.nih.gov/36600066/
  119. https://pmc.ncbi.nlm.nih.gov/articles/PMC3442578/
  120. https://royalsocietypublishing.org/doi/10.1098/rstb.2016.0376
  121. https://pmc.ncbi.nlm.nih.gov/articles/PMC10055341/
  122. https://www.mdpi.com/1420-3049/28/18/6681
  123. https://pmc.ncbi.nlm.nih.gov/articles/PMC10760874/
  124. https://onlinelibrary.wiley.com/doi/10.1002/anie.198911931
  125. https://pubs.acs.org/doi/10.1021/acs.chemrev.6b00378
  126. https://pubs.rsc.org/en/content/articlelanding/2016/ob/c6ob00842a
  127. https://www.science.org/doi/10.1126/science.1161976
  128. https://pubs.acs.org/doi/10.1021/acs.jctc.3c01378
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