Basic Concepts and Activation Modes in Visible-Light-Photocatalyzed Organic Synthesis

Dedicated to Professor Alain Krief on the occasion of his 80^th birthday

Abstract

Visible light photocatalysis has established itself as a promising sustainable and powerful strategy to access reactive intermediates, i.e. radicals and radical ions, under mild reaction conditions using visible light irradiation. This field enables the development of formerly challenging or even previously inaccessible organic transformations. In this tutorial review, an overview of the essential concepts and techniques of visible-light-mediated chemical processes and the most common types of photochemical activation of organic molecules, i.e. photoredox catalysis and photosensitization, are discussed. Selected photocatalytic alkene functionalization reactions are included as examples to illustrate the basic concepts and techniques with particular attention given to the understanding of their reaction mechanisms.

1 Introduction

2 Photocatalysts

3 Photophysical and Electrochemical Properties

3.1 Excited-State Energy

3.2 Ground-State Redox Potentials

3.3 Excited-State Redox Potentials

3.4 Local Absorbance Maximum for Lowest Energy Absorption

3.5 Excited-State Lifetime

3.6 [Ru(bpy)₃]²⁺ as a Case Study

3.7 Basic Laws and Equations of Photochemistry and Photocatalysis

3.8 Common Terminology in Photochemistry and Photocatalysis

4 Activation Modes in Photocatalysis

4.1 Photoinduced Electron Transfer

4.2 Photoinduced Energy Transfer

5 Conclusions and Outlook

Biosketches

Karthik Gadde obtained his Bachelor of Science degree from Bangalore University and a Master of Science degree from Kuvempu University (India). From 2012–2015 he worked as a project researcher under the guidance of Prof. Kandikere Ramaiah Prabhu in the Department of Organic Chemistry at the Indian Institute of Science Bangalore. In 2016 he moved to the University of Antwerp in Belgium for his doctoral studies working under the guidance of Prof. Bert U.W. Maes and Prof. Kourosch Abbaspour Tehrani. Gadde obtained his Ph.D. degree in Chemistry in February 2022. His doctoral dissertation research centered on the development of sustainable catalytic methods employing (Lewis) acid and photocatalysis. He is currently working as a postdoctoral researcher with Prof. Maes on the C–H functionalization of aza-heterocycles. His research interests are focused on photocatalysis in organic synthesis and heterocyclic and medicinal chemistry.

Dries De Vos obtained his Bachelor and Master of Science degrees in Chemistry from the University of Antwerp in 2018 and 2020, respectively. He conducted his master’s thesis project in the group of Prof. Carsten Bolm at the RWTH Aachen University (Germany) via an Erasmus exchange­. De Vos obtained a prestigious Ph.D. fellowship fundamental research of the Science­ Foundation (FWO-Flanders) in Belgium and is currently pursuing his Ph.D. in Organic Chemistry under the guidance of Prof. Bert U. W. Maes at the University of Antwerp. His current research focuses on the development of visible-light-mediated photocatalytic reactions for organic synthesis.

Bert U. W. Maes obtained his Ph.D. in Organic Chemistry at UAntwerp in 2001 and subsequently received a Post-Doctoral Fellowship of the Science Foundation (FWO-Flanders) in Belgium. He worked at the École Normale Supérieure in Paris (mechanisms in catalysis) with Prof. Anny Jutand (CNRS). Maes was appointed Assistant Professor (Docent) in 2003 and since 2016 holds a Full Professorship (Gewoon Hoogleraar) of Organic Chemistry in the Department of Chemistry at UAntwerp. In the period 2009–2019 he was a Research Professor of the Special Research Fund (BOF). In 2019 Maes was awarded a Collen-Francqui Research Professorship by the Francqui Foundation. In 2015–2016 he acted as chairman of the Department of Chemistry. His research interests cover the fields of heterocyclic chemistry, organometallic chemistry, homogeneous catalysis, and sustainable chemistry. Maes is the spokesman of CASCH, one of the Excellence Centers of UAntwerp. He is an editor of Topics in Heterocyclic Chemistry and an editorial board member of SynOpen, Advances in Heterocyclic Chemistry, and ACS Sustainable Chemistry & Engineering.

Introduction

Light-driven chemical transformations have recently received extensive attention and emerged as an important and useful platform in organic synthesis for the construction of complex molecular architectures. Organic photochemistry is based upon the excitation of a molecule by a suitable photon, promoting a substrate (Sub) from its ground state to an electronically excited state (*Sub, the excited state usually denoted by an asterisk) (Scheme [1]A). These excited-state molecules possess a distinct electronic configuration compared to their ground state, which can alter the spin-multiplicity and bond strength, and exhibit unique chemical properties and reactivity. Traditionally, most photochemical reactions were induced by the direct excitation of organic molecules using high-energy ultraviolet (UV, <380 nm) light irradiation as most organic molecules, except for extended conjugated systems, do not absorb in the visible region (Vis, 400–780 nm) of the electromagnetic spectrum (Figure [1]). Fascinating examples of UV-mediated photoreactions are [2+2] cycloadditions yielding strained cyclobutanes in a single step[1] and Norrish-type photoreactions which allow for the homolytic cleavage of C–C bonds (Scheme [2]).[2] Despite remarkable advances in UV-light-mediated photochemical processes,[2a] they often suffer from some major drawbacks, including (i) the requirement of specialized equipment and glassware (quartz/Pyrex), (ii) the use of more expensive light sources relative to visible light sources, and (iii) undesirable side reactions and product decomposition due to high-energy UV radiation, which are difficult to predict and control. Moreover, direct solar irradiation cannot induce UV-mediated photochemical reactions on Earth’s surface, because its short wavelength (290–380 nm) contribution is only approximately 5%, after filtering by the ozone layer. In contrast, around 45% of the solar spectrum contains visible light, and the remaining major part (around 50%) of the solar spectrum is infrared (IR) radiation. In this regard, the use of visible light photons as an energy source in photochemical processes is a powerful alternative strategy to alleviate the obstacles associated with UV-mediated photochemical reactions.

Visible light photons are an inexpensive source of light and can at present be easily produced at a lower energy cost by the use of energy-efficient high power light-emitting diode (LED) technology. Harnessing visible light photons as a traceless reagent in organic synthesis is a sustainable chemical technology to boost a diverse range of chemical transformations.[3] However, most organic molecules, with the exception of extended conjugated systems, do not absorb in the visible region of the electromagnetic spectrum. In other words, the energy of a visible light photon is typically insufficient to cleave bonds and thus induce chemical reactions (e.g., a 440 nm blue photon corresponds to 65 kcal·mol^–1; Table [1]). In visible light photocatalysis, this is circumvented by using catalytic amounts of photoactive molecules, known as photocatalysts, that can absorb visible light photons.[4] Once excited, a photocatalyst can exchange energy (photosensitization)[5] or an electron (photoredox catalysis)[6] with nonabsorbing organic molecules to drive chemical reactions (Scheme [1]B). The tremendous evolution of visible light LED technology has resulted in the development of LEDs featuring both specific wavelengths (narrow emission bands) and varying intensities (which can be easily tuned by changing the current). This allows more selective photochemical reactions via selective excitation of the photocatalyst, therefore avoiding the excitation of nontargeted functional groups or substrates thus minimizing the occurrence of undesired side-product formation. In addition, visible light photocatalysis offers economic and environmental benefits over the classical radical generation approach, which heavily relied on toxic reagents (e.g., Bu₃SnH), hazardous radical initiators (e.g., AIBN [2,2′-azobis(isobutyronitrile)], BEt₃ (triethylborane)), and/or elevated temperatures. Photocatalysis has resulted in a revival of radical chemistry.[7]

In fact, the idea to use visible light photons to induce chemical transformations is not new. Over a century ago, the Italian photochemist Giacomo Ciamician (1857–1922) realized that solar light is a renewable and abundant energy source for performing green chemical reactions.[8] However, only over the last fifteen years has visible-light-mediated photocatalysis emerged as a powerful synthetic strategy to access radicals under (very) mild conditions, enabling the development of various novel and formerly challenging or even impossible synthetic transformations with reduced waste generation and the potential for minimal environmental impact.[7]

As visible light photocatalysis is a rapidly maturing area of research, a number of reviews and perspectives have recently appeared in the literature. However, most of them are either predominantly focused on a particular photocatalytic activation mode (e.g., photoinduced electron transfer,[6] [9] proton-coupled electron transfer,[10] hydrogen-atom transfer,[11] energy transfer,[5] [12] electron donor-acceptor photoactivation,[13] excited-state transition metal catalysis[14]) or discuss the synthetic applications of a specific class of photocatalyst (e.g., transition-metal-based complexes,[9b] [15] organic dyes,[9c] [16] semiconductors[17]) in organic synthesis in great detail. This review, however, aims to introduce the essential basic photochemistry concepts together with the two most common activation modes. First, the fundamental concepts of photochemistry are discussed in Sections 2 and 3, including the basic photochemistry laws and a list of necessary terminology (Sections 3.7 and 3.8). Subsequently, photoinduced electron transfer (PET) and energy transfer (EnT) are introduced as the most widely accessed photocatalytic activation modes of organic molecules (Section 4). The objective of this review is to serve as an accessible guide to understanding basic concepts of synthetic visible light photocatalysis. Particularly aimed at non-experts, the basic concepts introduced herein should prepare the reader for a discussion of advanced activation modes and some specialized emerging techniques within the field of photocatalysis, separately discussed in another review.[18] An in-depth discussion of the (extensive) literature illustrating applications of the activation modes is outside the scope of this review, and readers are directed to state-of-the-art reviews and books on these specialized topics.

Photocatalysts

Visible light photocatalysts (PCs) are photoactive compounds, catalytically added to a reaction mixture in order to convert visible light electromagnetic energy into chemical energy. Upon absorption of a visible light photon, the photocatalyst (PC) accesses its electronically excited state, *PC. Subsequently, the excited photocatalyst (*PC) enables the formation of reactive intermediates, by engaging in photoinduced single electron transfer (SET) or photoinduced energy transfer (EnT) with a suitable organic substrate or reagent to achieve the desired chemical transformation (Scheme [1]B).[3] To this end, an ideal photocatalyst should have (i) a strong absorption of light in the visible region (>400 nm), (ii) an excited-state lifetime ideally longer than one nanosecond (to allow for effective bimolecular electron transfer/energy transfer), (iii) photostability in solution, and (iv) for SET a strong reduction and/or oxidation power relative to their ground states, in order to transfer to or accept an electron from a suitable organic substrate, and for EnT a high triplet excited energy. Depicted in Figure [2] are the structures of various common photocatalysts used in visible-light-mediated photochemical reactions. The most widely used photocatalysts are precious metal complexes, where the metal center is coordinated by organic ligands that can act as electron acceptors (e.g., pyridines). In this regard, ruthenium(II) polypyridyl or cyclometalated iridium(III) complexes have been employed as photocatalysts in the majority of visible-light-mediated photochemical reactions.[9b] These complexes exhibit favorable photophysical and electrochemical properties to drive photochemical reactions such as strong absorption in the visible region (>400 nm), sufficiently long excited-state lifetimes (hundreds of nanoseconds to microseconds), high photostability, high oxidation and reduction potentials of their photo­excited­ states relative to their ground states, and furthermore their properties can be tailored by ligand manipulations in accordance with the parameters of an ideal photocatalyst mentioned before.[19] The photophysical and electrochemical properties of commonly utilized visible light metal-based photocatalysts are given in Table [2]. When visible light is absorbed by precious metal photocatalysts, an electron is promoted from the metal-centered d orbitals to the lowest energy [lowest unoccupied molecular orbital (LUMO)] ligand-centered π*-antibonding orbital (Figure [3]A), hence reaching the excited-state via metal-to-ligand charge-transfer (MLCT). This excited-state complex is prone to transfer an electron or energy to a suitable substrate (Scheme [3]). These and related modes of activation are discussed in detail in Section 4. The MLCT usually refers to the transfer of an electron from a metal-centered d orbital to a π*-antibonding ligand orbital, while the analogous ligand-to-metal charge-transfer (LMCT) represents the transition of a π-bonding orbital electron of the ligand to the metal d orbital.

Iridium and ruthenium are noble metals and rare elements in the earth’s crust, hence making the resulting photocatalysts expensive for large-scale applications in the chemical and pharmaceutical industry. Furthermore, the requirement for precious metals engenders doubt about the sustainability of these catalysts. In this context, the replacement of expensive noble metal complexes with earth-abundant transition-metal-based complexes has received considerable attention in organic synthesis recently.[20] Among earth-abundant transition-metal complexes, copper complexes (e.g., Cu^I coordination compounds bearing phenanthrolines and phosphines as ligands) are a promising and economically attractive alternative to precious metal complexes featuring high luminescence, strong reducing power, and sufficiently long lifetimes in their excited state (Figure [2] and Table [2]).[15b] [21]

On the other hand, the research field of visible light photocatalysis has begun to shift toward organic photocatalysts, which are readily available with the benefits of low cost, decreased toxicity, and biocompatibility.[9c] Notable examples of organic photocatalysts are acridinium[22] and pyrylium salts, cyanoarenes,[22b] [23] and organic dyes such as eosin Y,[24] rose bengal, fluorescein, and rhodamine (Figure [2]).[9c] [16] Because these catalysts are highly conjugated and often planar, they are solvatochromic, reflected in the high importance of solvent choice. However, the wide application of organic photocatalysts is in general limited by their lower photostability and shorter excited-state lifetimes (pico- to nanosecond range) compared to Ru^II or Ir^III complexes (microsecond range). In this context, the systematic tailoring of organic photocatalysts is an ongoing research area to expand the scope of organic chromophores in photocatalytic applications as a cheap and nontoxic alternative to expensive noble metal photocatalysts.[16b] [e] Generally, organic photocatalysts feature (hetero)aromatic scaffolds creating extended conjugated systems and allowing light absorption in the visible region (Figure [2]). The photophysical and electrochemical properties of commonly utilized visible light organic photocatalysts are listed in Table [3]. The electronic properties can be fine-tuned by structural modification of the donor and acceptor moieties of the (hetero)arene scaffold. Unlike transition-metal-based photocatalysts, organic photocatalysts are dominated by the π–π* and n–π* transitions during excitation, which involve unsaturated functional groups (Figure [3]B). When increasing the conjugation of the π-system, a shift in the absorbance maxima to longer wavelengths (bathochromic shift) is observed due to a decrease in the energy gap between the HOMO (highest occupied molecular orbitals) and the LUMO (lowest unoccupied molecular orbitals). A charge-transfer (CT) state exists for some of the organic photocatalysts, analogous to metal-to-ligand charge-transfer (MLCT) states in metal complexes. In this case, charge-transfer occurs between the donor/acceptor moieties of the molecule.

In addition to organic photocatalysts, the use of semiconductors has recently received significant attention as heterogeneous photocatalysts in organic synthesis due to their photochemical stability and reusability.[17] [25] Ease of separation is a common problem for all homogeneous systems. Heterogeneous semiconductors such as (graphitic) carbon nitrides (g-C₃N₄),[26] various metal oxides (e.g., Bi₂O₃), sulfides (e.g., CdS), and selenides (e.g., CdSe) operate by the same SET and/or energy transfer processes as homogeneous photocatalysts. In addition to semiconductors, metal-organic frameworks (MOFs),[27] porous organic polymers (POPs), and covalent organic frameworks (COFs)[28] can also be used as heterogeneous photocatalysts.[25] Although photoactive transition metal complexes or organic dyes can be grafted onto, or be encapsulated within these heterogeneous structures, their advantage is not necessarily limited to enhanced recyclability only.[25] The structural tunability of COFs and MOFs also allows control of light absorption properties of the material, in ways impossible to achieve with classic inorganic semiconductors. Furthermore, metal(s), ligands, or complexes can be used to synthesize MOFs themself, allowing the integration of all parts necessary for an efficient photocatalyst in one specific site. Remarkably, this has been used to create recyclable materials containing multiple metals, with applications in dual catalysis.[29]

Photophysical and Electrochemical Properties

Knowing the properties of a photocatalyst in both the ground and excited states is crucial for designing any novel photocatalytic organic transformation. Hence, the essential photophysical and electrochemical parameters of a photocatalyst are described in this section.

Excited-State Energy

The behavior of a molecule upon absorption of light irradiation is illustrated in a Jablonski diagram (Figure [4]). Typically, when a molecule (PC) is electronically excited by a photon of suitable energy (+hν), an electron is promoted to a higher energy orbital. As per the spin-selection rule, the spin is retained during this transition, thus accomplishing a singlet excited state (S₁) from the singlet ground state (S₀). Multiple singlet excited states (S_n) with different vibrational energies might be accessed, depending on the energy of the photon. However, within picoseconds, all higher-lying excited states relax to the lowest energy, vibrationally equilibrated S₁ (‘first’ singlet) excited state, i.e. *¹PC, in accordance with Kasha’s rule.[32] Michael Kasha’s original statement was ‘the emitting electronic level of a given multiplicity is the lowest excited level of that multiplicity’[32] (in these cases S₁ or T₁). The energy difference between the zeroth vibrational level of the electronic ground state (S₀) and the relevant excited state is called the excited-state energy (E ^S1 _0,0 or E ^T1 _0,0) (Figure [4]). In most metal-based complexes, and even in some organic photocatalysts, the singlet excited state (S₁) species rapidly decays to a lower energy triplet excited-state (T₁) by a spin-forbidden, nonradiative (heat emission) process known as intersystem crossing (ISC) to form triplet species, i.e. *³PC. The probability of such a nonradiative transition is higher if vibrational levels of the two excited states overlap. Furthermore, the presence of heavy atoms increases the viability of intersystem crossing due to spin-orbit coupling, which scales with Z⁴ (Z = atomic number).[33] However, in most organic photocatalysts, the excited species involved in the photocatalytic cycle is usually the singlet (S₁) state.[16b] E _0,0 normally indicates singlet excited states, but it is often used to refer to triplet excited states as well. In this regard, E _0,0 can be abbreviated as E ^S1 _0,0 (the lowest-lying singlet excited state) and E ^T1 _0,0 (the lowest-lying triplet excited state). Triplet states are typically long-lived excited states because the electronic transition towards the S₀ ground state (T₁→S₀) is spin forbidden. In this regard, the multiplicity of an excited state plays a pivotal role in the photophysical properties of a photocatalyst and is the reason why the lifetime of the excited states of metal complexes is in microseconds, whereas organic photocatalysts have to operate in the time frame of nanoseconds (Tables 2 and 3). The lifetime of the triplet state (T₁) is limited by deactivation to the ground state (S₀) through either a radiative (light emission, –hν) process via spin-forbidden phosphorescence or through nonradiative (heat emission) processes. Instead of intersystem crossing to the triplet excited state (T₁), the singlet excited state (S₁) itself can also deactivate to the ground state (S₀), which occurs either via fluorescence (radiative, –hν) or through internal conversion (IC), which is a nonradiative pathway.

The excited-state energy (E _0,0) can be estimated in different ways and is generally expressed in electron volts (eV). The singlet excited-state energy E ^S1 _0,0 can be determined spectroscopically at room temperature from either (i) the wavelength at the intersection between the normalized UV-visible absorption and emission spectra or (ii) the midpoint between the absorption and emission maxima (Figure [5]A). The triplet excited-state energy E ^T1 _0,0 is less trivial to determine because either they do not show room temperature emission or do not possess a crossing point between the absorption and emission spectra. Hence, it is often estimated using the highest energy local maximum in the phosphorescence spectrum obtained at cryogenic temperature (Figure [5]B).[5] [9c] Alternatively, the triplet excited-state energy E ^T1 _0,0 can be predicted via computational methods such as time-dependent density functional theory (TDDFT) calculations.

Ground-State Redox Potentials

The ground-state redox potentials of a photocatalyst are useful to calculate the corresponding excited-state potential values (see Section 3.3) and to predict the thermodynamic feasibility of photoinduced single electron transfer reactions. These values are commonly obtained by cyclic voltammetry (CV) measurements on ground-state photocatalysts or organic substrates.[34] The obtained photocatalyst’s ground-state oxidation potential (E _ox or E _[PC•+/PC]) and reduction potential (E _red or E _{[PC/PC•–]}) values are reported in volt (V) vs. SCE and correspond to the first one-electron oxidation and reduction processes, respectively, as shown in the modified Latimer diagram (Scheme [4]).[35] Importantly, the ground-state potential values of a photocatalyst provide information regarding the feasibility of electron transfer from or towards the substrate/intermediate or a sacrificial electron donor or acceptor to regenerate the ground-state photocatalyst in the catalytic cycle. Note that the oxidation potential of the reduced photocatalyst PC^•– is equal to the reduction potential of the ground-state photocatalyst PC. In other words, the potential necessary to reduce the ground-state photocatalyst to its radical anion is the same as the potential necessary for this radical anion to oxidize back to the ground-state photocatalyst. As this value is measured from the ground-state photocatalyst using cyclic voltammetry,[9c] [35] it is often tabulated as E _red, but we prefer the notation E _1/2 to avoid confusion, as in a photocatalytic cycle the photocatalyst will be re-oxidized in this step. The same reasoning applies to the reduction potential of the oxidized photocatalyst PC^•+.

Excited-State Redox Potentials

The suitability of a photocatalyst for a given photoinduced SET depends on its ground- and excited-state redox potentials. The photoreducing and photooxidizing ability of an electronically excited photocatalyst (*PC) is dictated by its excited-state redox potentials, which in turn impact substrate/sacrificial reagent activation in a photochemical reaction. Typically, the excited-state reduction potential (*E _red or E _{[*PC/PC•–])} and excited-state oxidation potential (*E _ox or E _[PC•+/*PC]) are not directly measured and can be estimated quantitatively in two ways. (i) Comparison of the rates of excited-state electron transfer to a series of stable reactants with known ground-state potentials. (ii) More commonly, by employing equations 1.1 and 1.2 as depicted in Equation 1.[36] The excited-state reduction potential {*E _red or E _{[*PC/PC•–]}) and excited-state oxidation potential (*E _ox or E _[PC•+/*PC]) are calculated using the ground-state redox potentials (E _red and E _ox values) and excited-state energy E _0,0 for a given excited state (either singlet or triplet) catalyst (Equation 1). As appeared in Equations 1.1 and 1.2, w_r is an electrostatic work term that considers the solvent-dependent Coulombic attractions but is typically omitted due to its relative insignificance in polar solvents. The excited-state redox potentials of a photocatalyst are considered benchmarks when evaluating the possibility of a substrate/sacrificial reagent interacting with a photocatalyst in its excited state, which is explained in detail in Section 4.1.

Local Absorbance Maximum for Lowest Energy Absorption

An overlap between the emission spectrum of the light source and the absorption spectrum of the photocatalyst is required for photoexcitation of the photocatalyst.[9c] By measuring the UV-Vis absorption spectrum of the photocatalyst, we can determine the local absorbance maximum for the lowest energy absorption (λ_max) peak of a photocatalyst, which is an important parameter when selecting an appropriate light source of irradiation for a given photocatalyst (Figure [6]). In this context, LEDs have evolved into a powerful technology since the wavelength and the intensity of the emission source can be accurately modulated to closely match the absorption corresponding to the electronic transitions of the photocatalyst and thereby initiate a chemical process in a very selective manner.

Excited-State Lifetime

As mentioned in Section 3.1, the excited-state lifetime (τ) is a crucial parameter when evaluating the suitability of a photocatalyst. After all, obtaining appreciable substrate activation depends on effective photoinduced electron transfer or photoinduced energy transfer. An ideal photocatalyst should possess a long-lived excited state to allow competition between a bimolecular electron transfer or energy transfer and the inherent radiative (light emission, –hν) or nonradiative (heat emission) decay of the excited-state photocatalyst. When the excited-state lifetime is shorter than one nanosecond (decay rate constant k _r > 10⁹ s^–1), the excited photocatalyst (*PC) will decay much faster than the diffusion limit, preventing bimolecular processes. Photocatalysts in their excited triplet state generally possess a lifetime of hundreds of nanoseconds up to microseconds, which is several orders of magnitude longer (T₁→S₀ transition is spin-forbidden) than photocatalysts in their excited singlet state possessing a lifetime in the order of nanoseconds (S₁→S₀ transition is spin-allowed). In transition-metal-based photocatalysts, undesired nonradiative (thermal) deactivation and short-lived excited states might be involved if the ligand-field d-d states can be populated, especially when they lie lower in energy than the desired MLCT state, or if they can be accessed thermally from this state. Designing photocatalysts with longer excited-state lifetimes remains an important field of research in photocatalysis.[35]

[Ru(bpy)₃]²⁺ as a Case Study

If we examine the photophysical and electrochemical properties of tris(2,2′-bipyridyl)ruthenium(II) chloride ([Ru^II(bpy)₃]Cl₂) in detail, it becomes clear why it has been extensively employed as a photocatalyst to generate various radicals in a diverse range of visible-light-mediated photochemical reactions.[15a] [Ru^II(bpy)₃]Cl₂ shows a strong absorption at 452 nm in the visible region of the electromagnetic spectrum, which corresponds to a metal-to-ligand charge-transfer (MLCT) transition (see Figure [6]). After the absorption of visible light, the [Ru^II(bpy)₃]Cl₂ photocatalyst reaches the excited-state, *[Ru^III(bpy)₂(bpy^•– )]Cl₂ via metal-to-ligand charge-transfer (MLCT) by promoting an electron from the metal-centered t _2g orbitals to the lowest energy (LUMO) ligand-centered π*-antibonding orbital (Scheme [5]). Subsequently, the primary singlet metal-to-ligand charge-transfer (¹MLCT) state rapidly undergoes intersystem crossing (ISC) to attain the triplet metal-to-ligand charge-transfer (³MLCT) state. This ³MLCT state possesses a long-lived excited-state lifetime (1100 ns), as decay from the triplet excited state to the singlet ground state S₀ is spin-forbidden. This sufficiently long lifetime allows the excited photocatalyst to engage in bimolecular single electron transfer or energy transfer processes. The ³MLCT state of [Ru^II(bpy)₃]Cl₂ holds an excited electron in a high-energy ligand-centered π* orbital and a ‘hole’ in a metal-centered d-orbital. For this reason, the excited-state photocatalyst *[Ru^II(bpy)₃]Cl₂ is both an oxidator and a reductor: it can act as a reductant by donating a high-energy electron from its ligand π* orbital or act as an oxidant by accepting an electron into its low-energy metal-centered vacancy (Scheme [3]A). Moreover, the resulting reduced [Ru^I(bpy)₃]Cl₂ or oxidized photocatalyst [Ru^III(bpy)₃]Cl₂ can close the photocatalytic cycle by a second SET, making net-redox neutral processes become accessible. Furthermore, the ³MLCT state can likewise partake in an energy transfer with a reaction substrate concomitantly regenerating the photocatalyst (Scheme [3]B). These processes facilitate challenging transformations that are difficult to achieve by traditional thermal synthetic methodologies.

Basic Laws and Equations of Photochemistry and Photocatalysis

The first law of photochemistry (Grotthuss–Draper law) states that the absorption of light is necessary to allow photochemical reactions.

The second law of photochemistry (Stark–Einstein law) states that each photon of light absorbed by a chemical system cannot activate more than one molecule. It means one photon can only activate a single molecule, and thus, a photochemical reaction can be regarded as a one-quantum process. However, it should be noted that the law only applies to primary photochemical processes.

The inverse-square law states that the intensity of light is inversely proportional to the square of the distance from the light source (Equation 2).

According to the Beer–Lambert law, the absorbance of light is correlated with the characteristics and concentration of an absorbing species and the optical path length. It shows that light is transmitted less through solutions with higher concentrations and a longer path length (Equation 3).

According to the Planck–Einstein law, the energy of a photon can be related to the wavelength (or frequency) of the light. The energy of a mole of photons (corresponding to 6.022 × 10²³ photons) is called an Einstein (Equation 4).

Kasha’s rule states that after absorption of a photon, all higher-lying excited states relax to the lowest energy, vibrationally equilibrated S₁ (‘first’ singlet) excited state, i.e. *¹PC.[32] Michael Kasha’s original statement was ‘the emitting electronic level of a given multiplicity is the lowest excited level of that multiplicity’.

The quantum yield Φ is a measure of the efficiency of a primary photochemical event. For example, if after excitation the excited state will decay through the emission of a photon, rather than through a nonradiative mechanism, the fluorescence quantum yield is high. As stated by the Stark–Einstein law, this quantum yield cannot be larger than one (Equation 5.1), as one absorbed photon can only produce one emitted photon.

In photocatalytic literature, the quantum yield Φ is often used as a measure of the efficiency of an overall light-induced radical process (Equation 5.2). In this case, a quantum yield higher than unity is possible when a radical chain process is present. When each absorbed photon exactly leads to one product molecule, Φ = 1. When more molecules are formed than photons absorbed (Φ > 1), there is a contribution of a radical chain process (see Scheme 18).[38] Importantly, this value is distinct from the quantum yield of the initiation of the process, which is a primary photochemical event, and by consequence, Φ is limited to maximally 1.

Often, light-dark experiments are conducted to investigate the necessity of continuous light irradiation for the reaction. If after a period of irradiation the reaction yield still increases in the dark, a radical chain is at play. However, the absence of such continuing reaction is insufficient evidence to exclude the presence of a radical chain, as the radical chains might be short-lived.[38a]

In absence of any molecule to interact with, an excited-state photocatalyst will decay to the ground state by either emission of a photon (fluorescence or phosphorescence), nonradiative processes, or unimolecular photochemical processes such as isomerization, dissociation, or decomposition (photobleaching). The rate of these processes determines the (undisturbed) lifetime τ₀ of the excited state (Equation 6).[39]

In the presence of an organic molecule that can perform electron or energy transfer with the excited photocatalyst, the aforementioned processes are averted and productive transfer can occur, creating the desired radicals for an organic transformation. For this reason, a decrease in the emission of a photocatalyst proves that there is an interaction between the organic molecule and the excited photocatalyst. The organic molecule is in this case denoted a ‘quencher’ of the excited-state photocatalyst.

The Stern–Volmer equation relates the change in emission intensity at a specific wavelength to the concentration of quencher Q present in the system (Equation 7).

With an increasing concentration of a quencher, the emission intensity (which in absence of the quencher is equal to I ₀) will decrease to I _n according to Equation 7. By plotting I ₀/I _n against the concentration of quencher Q, a linear relation is expected, known as the Stern–Volmer plot (Figure [7]). The slope of this plot is called the Stern–Volmer quenching constant K _SV and it gives information about the quenching rate k _q and the (natural) lifetime τ₀ of the excited state. The latter is often a tabulated value for known photocatalysts so that the rate of quenching can be determined.

Common Terminology in Photochemistry and Photocatalysis

Absorption spectrum – The wavelength dependence of the absorption cross-section (or absorption coefficient); usually represented as a plot of absorption cross-section versus wavelength λ (or 1/λ) of the light.

Doublet state – A state having a total electron spin quantum number equal to ½.

Electron transfer – The transfer of an electron from one molecular entity to another, or between two localized sites in the same molecular entity.

Emission spectrum – Plot of the emitted spectral radiant power (spectral radiant exitance) or of the emitted spectral photon irradiance (spectral photon exitance) against a quantity related to photon energy, such as frequency (ν), wavenumber (σ), or wavelength (λ). When corrected for wavelength-dependent variations in the equipment response, it is called a corrected emission spectrum.

Excited state – State of a system with energy higher than that of the ground state. This term is most commonly used to characterize a molecule in one of its electronically excited states, but can also refer to vibrational and/or rotational excitation in the electronic ground state.

Fluorescence – Luminescence which occurs essentially only during the irradiation of a substance by electromagnetic radiation (i.e., S₁ to S₀; see Figure [4]).

Ground state – The state of lowest Gibbs free energy of a system.

Internal conversion – A photophysical process. An isoenergetic radiationless transition between two electronic states of the same multiplicity. When the transition results in a vibrationally excited molecular entity in the lower electronic state, this usually undergoes deactivation to its lowest vibrational level, provided the final state is not unstable to dissociation.

Intersystem crossing – A photophysical process. An isoenergetic radiationless transition between two electronic states having different multiplicities. It often results in a vibrationally excited molecular entity in the lower electronic state, which then usually deactivates to its lowest vibrational level.

Luminescence – Spontaneous emission of radiation from an electronically or vibrationally excited species not in thermal equilibrium with its environment.

Phosphorescence – Long-lived luminescence involving change in spin multiplicity, typically from triplet to singlet or vice versa.

Quencher – A molecular entity that deactivates (quenches) an excited state of another molecular entity, either by energy transfer, electron transfer, or by a chemical mechanism.

Singlet state – A state having a total electron spin quantum number equal to 0.

Triplet state – A state having a total electron spin quantum number equal to 1.

Triplet-triplet energy transfer – Energy transfer from an electronically excited triplet donor to produce an electronically excited acceptor in its triplet state.

Remark: The definitions in this section related to photochemistry and photocatalysis are based on IUPAC’s Gold Book.[41]

Activation Modes in Photocatalysis

Upon absorption of a photon, a photocatalyst PC promotes to its excited state *¹PC. Within picoseconds, all higher-lying excited states relax to the lowest energy vibrationally equilibrated S₁. The S₁ state can subsequently undergo intersystem crossing (ISC) into T₁. Depending on the photocatalyst either the S₁ or T₁ state subsequently engages with the reactants in different ways. The most widely used activation modes involved in organic synthesis are: photoinduced electron transfer (PET) (Section 4.1) and photoinduced energy transfer (EnT) (Section 4.2).

An alkene is a fundamental functional group in organic chemistry, featured in both commodity chemicals produced by the petrochemical industry as well as natural products such as plant-produced terpenes.[42] Furthermore, alkenes can be obtained from renewable alcohols via the elimination of water.[43] In the past few decades, the functionalization of alkenes has received tremendous attention, especially in an anti-Markovnikov fashion,[44] and several novel methods have been developed for the functionalization of alkenes. Difunctionalization via an addition reaction on alkenes represents an attractive atom-economic synthetic strategy to introduce molecular and functional complexity in organic molecules in a single operation, as the double bond of the alkene is sacrificed to form two new single bonds.[45] Therefore, this section presents an overview of PET and EnT and their mechanisms, illustrated by selected literature examples of alkene difunctionalization and hydrofunctionalization reactions. By limiting the examples to substrates featuring the alkene moiety, the focus of the reader is directed toward the photocatalytic activation steps. The reader is referred to cited reviews to find reactions on a plethora of other substrates triggered by these specific activation modes.

Photoinduced Electron Transfer

The photoinduced electron transfer (PET) process, also known as photoredox catalysis, has emerged in the past decade as a powerful tool in synthetic organic chemistry. Through the conversion of visible light energy into an electrochemical potential, various organic transformations can be initiated allowing to achieve novel synthetic methodologies. The PET facilitates a SET process from the excited photocatalyst (*PC) to the organic substrate or sacrificial reagent or the reverse, thus promoting the catalytic generation of reactive radical intermediates under mild conditions. The pioneering work in visible light photoredox catalysis was reported in the 1980s and 1990s, by Deronzier,[46] Fukuzumi,[47] Kellogg,[48] Okada,[49] Oda,[49] Pac,[50] Pandey,[51] Tanaka,[47a] Tomioka,[52] and Willner.[53] However, the beginning of modern photoredox catalysis was marked only two decades later, when the research groups of Yoon, with the report of [2+2] enone cycloadditions,[54] MacMillan, on the asymmetric α-alkylation of aldehydes,[55] and Stephenson, with the report of dehalogenation reactions,[56] independently described three types of photoredox reactions using [Ru(bpy)₃]Cl₂ as a photocatalyst (see Section 3.6). Since then, photoredox catalysis has matured into an essential part of modern synthetic organic chemistry.

The central feature of a photoredox-catalyzed process is a SET from the excited-state photocatalyst to a target substrate/reagent or the reverse (Figures [8] and 9). In general, a SET process involves an electron transfer to an electron-accepting molecule (A ⁿ ) from an electron-donating molecule (D ^m ) (Figure [8]A). This process can be either an inner-sphere electron transfer (ISET) or an outer-sphere electron transfer (OSET), depending on the interaction between the donor (D ^m ) and the acceptor (A ⁿ ) in the transition state. In the inner-sphere mechanism, the donor (D ^m ) and acceptor (A ⁿ ) interact strongly (binding energy >4.8 kcal·mol^–1), while in the outer-sphere mechanism, the donor (D ^m ) and acceptor (A ⁿ ) are weakly interacting (with binding energy 1.0–3.8 kcal·mol^–1) or nonbonded organic compounds (Figure [8]B).[4] With Marcus theory, the kinetics of outer-sphere electron transfer between donor and acceptor species can be calculated.[57] The applications of visible light photocatalysis predominately involve OSET processes, employing transition metal photocatalysts (Ru^II polypyridyl or cyclometalated Ir^III complexes), organic photocatalysts (e.g., eosin Y, rose bengal, fluorescein, 4CzIPN, Mes-Acr⁺-Me ClO₄ ^–, etc.), or (in)organic semiconductors (e.g., Bi₂O₃, graphitic carbon nitride, CdSe), as photocatalysts. Remarkably, some copper complexes can perform an unusual inner-sphere pathway (via ligand dissociation upon photoexcitation) in addition to outer-sphere electron transfer.[21c]

Upon excitation of the photocatalyst, an electron promotes from the HOMO (highest occupied molecular orbital) to the LUMO (lowest unoccupied molecular orbital) and creates a vacancy in the HOMO, thus increasing the electron affinity of the system and allowing for easier oxidation. At the same time, an electron is present in the higher-energy LUMO, decreasing the ionization potential and allowing for easier reduction. This explains the favorable redox properties of the excited state with respect to the ground state (see Figure [9]). After excitation, the photocatalyst, *PC is converted into a deactivated form (PC^•+ or PC^•–) upon interaction with a substrate (or reagent) (A ⁿ or D ^m ) by either donating an electron to the substrate (or reagent) (A ⁿ ) or acquiring an electron from the substrate (or reagent) (D ^m ). Finally, the catalytic cycle can be closed to regenerate the original state of the photocatalyst (PC) by reaction with an intermediate or substrate D/A, which allows the photocatalytic cycle to restart (Scheme [6]). Depending on the redox behavior of the excited photocatalyst, two fundamental catalytic cycles can occur, namely an oxidative and a reductive quenching cycle. In an oxidative quenching catalytic cycle, the excited photocatalyst (*PC) acts as an electron donor (excited-state photoreductant). By donating an electron to the substrate the catalyst is oxidized (PC^•+) and the substrate (or reagent) is reduced. On the other hand, in a reductive quenching catalytic cycle, the excited photocatalyst (*PC) acts as an electron acceptor (excited-state photooxidant). The excited photocatalyst (*PC) accepts an electron from the substrate (or reagent), resulting in the formation of a reduced catalyst (PC^•–) and an oxidized substrate. In both electron transfer processes, the reaction substrates (or reagents) are activated, generating radicals (anions, cations, or neutral radicals) that are able to undergo secondary reactions. Knowing the redox potentials of species participating in a photoredox reaction can help to predict the thermodynamic feasibility of a desired single electron transfer. The redox potential of an electron acceptor (acceptor reduction potential, E _red, measured in volts, V) measures its ability to accept an electron and thereby be reduced (A ⁿ →A ^n–1). Likewise, the redox potential of an electron donor measures how readily that donor will give up an electron, and hence be oxidized (D ^m →D ^{m +1}, donor oxidation potential, E _ox, measured in volts, V).

Species that are readily oxidized possess a more negative oxidation potential whereas more readily reduced species possess a more positive reduction potential. Equation 8.1 connects two standard electrode potential values to the change in standard Gibbs free energy of electron transfer. For thermodynamically favorable SET through a reductive quenching cycle, where a donor quenches the excited-state photocatalyst, a photocatalyst whose excited-state reduction potential is higher than the oxidation potential of the donor should be used (see Equation 8.2). For thermodynamically favorable SET through an oxidative quenching cycle, a photocatalyst should be used whose excited-state oxidation potential is lower than the reduction potential of the acceptor (see Equation 8.3). Note that these are all changes in standard Gibbs free energy, implying standard conditions. Nevertheless, Equation 8 provides a useful estimate of the thermodynamics of a (photo)redox step, as will be illustrated in the examples below.

The applications of visible-light-mediated photoredox catalysis for the difunctionalization of alkenes have gained significant attention in the last few years.[45] Among them, ATRA (atom transfer radical addition) reactions offer a direct method to introduce two new bonds in a single step. The mechanism of ATRA photoredox-mediated functionalization of alkenes is shown in Scheme [7]. As depicted, a radical species R^• is formed by a SET process, where the excited photocatalyst *PC is either reductively or oxidatively quenched by the radical precursor R–X. Generally, the resulting radical species R^• subsequently attacks the (terminal) carbon atom of the alkene to form the most stable carbon-centered radical, which is then either oxidized or reduced by PC^•+ or PC^•– to give the respective cationic or anionic intermediate, creating a photocatalytic cycle. Additionally, the carbon-centered radicals can react with another molecule of R–X providing the product and another radical species R^•, representing a radical propagation. If this pathway is predominant, the photocatalyst merely functions as an initiator for a radical chain, instead of mediating a true photocatalytic cycle.

In 2011, Stephenson and co-workers reported an intermolecular atom-transfer radical addition (ATRA) reaction between activated halides and unactivated alkenes by using an Ir^III photocatalyst, [Ir^III{dF(CF₃)ppy}₂(dtbbpy)]PF₆, under visible light irradiation (Scheme [8]).[58] This method delivered ATRA-products in high yields with unactivated alkenes when the Lewis acid LiBr was added as an additive to activate the diethyl bromomalonate. In terms of substrate scope, the method tolerates free alcohol, ester, ether, silyl ether, halide, enone, and carbamate functional groups. The method is limited to activated bromides and iodides. In addition, this protocol was restricted to nonconjugated terminal alkenes. The mechanism for this reaction is proposed to be a SET via an oxidative quenching pathway (Scheme [8]). Upon irradiation with blue light (435 nm), the photocatalyst [Ir^III{dF(CF₃)ppy}₂(dtbbpy)]PF₆ reaches an excited state, i.e. *[Ir^III{dF(CF₃)ppy}₂(dtbbpy)]PF₆. Subsequently, the photoexcited iridium catalyst reduces diethyl bromomalonate, thus generating a diethyl malonate radical, which adds to an alkene. The resulting carbon-centered radical can abstract a halogen atom from another R¹–X starting material molecule to give the addition product via a radical propagation pathway. On the other hand, the carbon-centered radical can also be oxidized by the oxidized photocatalyst Ir^IV, generating the carbocation and concomitantly Ir^III and hereby closing the photocatalytic cycle. In this case, the addition product is generated by trapping the halide (nucleophile trapping).

Since all relevant redox potentials are known for the example using diethyl bromomalonate as an alkyl radical source, an estimation of the thermodynamic feasibility of the catalytic cycle can be made using Equation 8. The excited Ir^III catalyst has an excited-state oxidation potential *E _ox (Ir^IV/*Ir^III) = –0.89 V vs. SCE and is, by consequence, capable of reducing diethyl bromomalonate (E _red = –0.62 V vs. SCE) since the reduction potential is higher than the oxidation potential. Additionally, the addition of LiBr as a Lewis acid is suggested to ease the reduction of diethyl bromomalonate even further. In the second step of the catalytic cycle, the alkyl radical (E _ox = +0.47 V vs. SCE) must be oxidized towards the carbocation by the Ir^IV species. The reduction potential of the transient Ir^IV species is not tabulated as such, but is measured as the oxidation potential of ground state Ir^III (E _1/2(Ir^IV/Ir^III) = +1.69 V vs. SCE). This redox step is likewise feasible since the reduction potential is higher than the oxidation potential. The denotation E _ox/E _red, although often used by organic photochemists, is often confusing. In reality, only one value E _1/2 exists for one specific reversible redox couple.[9c] [35] For example, the oxidation of the alkyl radical to the alkyl cation was denoted by an E _ox value. Yet, this value is identical to the E _red value of the cation to the radical. Instead of writing E _1/2 next to the alkyl radical, often E _ox is used in order to distinguish from the redox potential belonging to the reduction of the alkyl radical to the alkyl anion (which is a distinct redox couple).

In 2011, Masson and Courant developed an oxyalkylation of enecarbamates using diethyl bromomalonate and alcohols under visible light irradiation (Scheme [9]).[59] With [Ir^III(ppy)₂(dtbbpy)]PF₆ as the photocatalyst under irradiation by a 25 W compact fluorescent lightbulb (CFL), two different possible mechanisms were considered, i.e. *Ir^III/Ir^II (via reductive quenching) or Ir^IV/*Ir^III (via oxidative quenching similar to Scheme [8]). Considering 2 equivalents of a tertiary amine were required, a reductive quenching pathway has been postulated, where photoactivated *Ir^III oxidizes the triethylamine reagent, hereby providing [Ir^II(ppy)₂(dtbbpy)]PF₆, which subsequently oxidizes and hereby reductively generates the alkyl radical from diethyl bromomalonate substrate (Scheme [9]). The addition of the generated electrophilic radical to the β-position of enecarbamates generates α-amido radicals which upon radical propagation with another diethyl bromomalonate molecule provide the corresponding α-bromoamide. Elimination of bromide and subsequent addition of alcohol forms the desired product. Alternatively, the α-amido radical can take the place of triethylamine in the first step of the catalytic cycle, generating the iminium. The method was compatible with a wide variety of carbamate protecting groups (Cbz, Alloc (allyloxycarbonyl), Boc). Additionally, carbamates could be substituted by amides.

The examples provided so far demonstrate the use of robust, photochemically efficient, and bench stable noble metal photocatalysts. However, due to the scarcity of these transition metals they may be too expensive to allow scale-up for industrially relevant applications. Reiser and co-workers addressed this concern by reporting an atom-transfer radical addition (ATRA) of alkyl radicals to alkenes by using [Cu^I(dap)₂]Cl as a photocatalyst under visible light irradiation (Scheme [10]).[60] The authors’ proposed mechanism proceeds through photoexcited *[Cu^I(dap)₂]Cl (*E _ox(Cu^II/*Cu^I) = –1.43 V vs. SCE) which is a strong reductant and transfers an electron to the alkyl halide, resulting in the formation of an alkyl radical and halide. The intermediate obtained via radical addition to the alkene provides a carbon-centered radical, which then reduces the oxidized copper catalyst and generates a carbocation that can be trapped by the halide. Similar to the previously discussed examples with noble transition-metal catalysts (Ru and Ir), a radical propagation pathway cannot be ruled out, although no quantum yield was provided to support this.

Once again, all required redox potentials are available for the reaction between styrene and CBr₄, showing that each step of the catalytic cycle is thermodynamically feasible. [Cu^I(dap)₂]Cl does not possess a tabulated excited-state reduction potential. Only an excited-state oxidation potential is known (*E _ox(Cu^II/*Cu^I) = –1.43 V vs. SCE), and therefore an oxidative quenching cycle is likely. The authors’ proposed mechanism commences through the generation of photoexcited *[Cu^I(dap)₂]Cl, which transfers an electron to CBr₄. This electron transfer is only thermodynamically feasible if E _red(CBr₄) = –0.48 V vs. SCE is less negative than *E _ox(Cu^II/*Cu^I) = –1.43 V vs. SCE, which is in fact the case. The benzylic radical formed by the addition of ^•CBr₃ to styrene then needs to be oxidized by the generated Cu^II species, in order to regenerate [Cu^I(dap)₂]Cl. The electron transfer from the benzylic radical towards [Cu^II(dap)₂]Cl in this second step of the catalytic cycle is thermodynamically feasible, as the oxidation potential of the benzylic radical (E _ox = +0.37 V vs. SCE) is less positive than the reduction potential of [Cu^II(dap)₂]Cl. Once again, the latter is never tabulated as such, but can be found as the oxidation potential of ground-state [Cu^I(dap)₂]Cl (E _1/2(Cu^II/Cu^I) = +0.62 V vs. SCE).

In 2019, Reiser and co-workers reported an atom-transfer radical addition (ATRA) of sulfonyl chlorides to alkenes by using [Cu^I(dap)₂]Cl as a photocatalyst under green light irradiation (Scheme [11]).[61] Most metal-based photocatalysts have ligands which do not decomplex during photocatalysis. Besides outer-sphere electron transfer processes, copper-based photocatalysts are known to undergo ligand exchange, allowing uncommon inner-sphere mechanisms.[62] The proposed mechanism commences with the photoexcitation of [Cu^I(dap)₂]Cl to *[Cu^I(dap)₂]Cl (*E _ox(Cu^II/*Cu^I) = –1.43 V vs. SCE), which is able to reduce sulfonyl chlorides (E _red = –1.37 V vs. SCE for TsCl), resulting in a sulfonyl radical and the [Cu^II(dap)Cl₂] catalyst after decomplexation of the dap ligand. The sulfonyl radical can attack the alkene, forming a C-centered radical. This radical can either abstract a chlorine atom from the copper core, leading to the desired product and regeneration of [Cu^I(dap)₂]Cl, or is able to add to the Cu^II metal creating a Cu^III complex. Finally, reductive elimination of this complex liberates the desired reaction product and concomitantly closes the catalytic cycle. These more specialized copper complexes are sometimes not commercially available or expensive but can actually easily be synthesized. Photocatalyst [Cu^I(dap)₂]Cl was obtained in two steps. The dap ligand is synthesized via Suzuki coupling of 2,9-dichloro-1,10-phenanthroline with 4-methoxyboronic acid. Complexation of dap with CuCl provides [Cu^I(dap)₂]Cl (Scheme [12]).[60]

The use of organic photocatalysts has advantages in terms of cost, availability, and tunability. Furthermore, they may even exceed the capabilities of organometallic photocatalysts. Despite these beneficial features, there are still far fewer reports on their use in photocatalysis. In 2017, the Mariano and Wang groups developed a metal-free decarboxylative hydroformylation of styrenes with diethoxyacetic acid as formylating agent to generate terminal aldehydes in continuous flow under visible light irradiation upon aqueous work-up of the photocatalytic reaction mixture (Scheme [13]).[63] Traditional transition-metal-catalyzed hydroformylation reactions of styrenes involving carbon monoxide deliver branched aldehydes rather than linear aldehydes and require ligands to control the regioselectivity. In contrast, the developed approach relied on radical-based hydroformylation in the presence of a 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) photocatalyst, selectivity producing terminal aldehydes. A reaction mechanism was proposed with a single electron transfer from the cesium diethoxyacetate salt to the singlet excited photocatalyst *[4CzIPN], and subsequent decarboxylation to generate the formyl radical equivalent. This formyl radical equivalent adds to the styrene derivative in an anti-Markovnikov fashion and the reduced photocatalyst [4CzIPN]^•– subsequently donates an electron to the benzyl radical to form the carbanion intermediate. After protonation and acid-catalyzed hydrolysis of the ethoxy groups of the acetal in the work-up, the terminal aldehyde is formed. The hydroformylation reaction was performed under blue LED irradiation in batch and flow and was compatible with a range of substituted styrene derivatives, as well as heteroarene derivatives to generate the aldehydes in 50–90% yield. The reaction time was significantly reduced in flow (2.5 h in flow versus 36 h in batch) and furthermore allowed to scale-up to produce 3.44 g of 2-[4-(trifluoromethyl)phenyl]ethanal from 4-(trifluoromethyl)styrene (87% yield). Interestingly, the 4CzIPN photocatalyst is commercially available and furthermore easily accessible in one step from tetrafluoroisophthalonitrile and carbazole via S_NAr (Scheme [14]). Easy variation of the structure is possible by altering one of the reactants.[16b]

While homogeneous catalysts are interesting, heterogeneous catalysts possess inherent advantages with respect to removal and recyclability. In this context, a number of inorganic and organic semiconductors have been evaluated as photocatalysts. In 2015, Pericàs and Riente reported a visible-light-mediated ATRA of alkyl radicals to alkenes using a nontoxic semiconductor Bi₂O₃ powder as a heterogeneous photocatalyst under the irradiation of a 23 W compact fluorescent lamp (CFL) (Scheme [15]).[64] In contrast to thermal catalysis, where homogeneous catalysts typically cannot be easily substituted by a suitable heterogeneous material, photocatalysis seems more flexible. Particularly semiconductors are promising heterogeneous photocatalysts performing the same SET processes as observed with homogeneous transition-metal or organic photocatalysts.[25] When a semiconductor absorbs light of suitable energy, electrons are excited from its valence band (VB) to its conduction band (CB). This creates both an oxidator and a reductor in the same material: the electron holes (h⁺) in the valence band (VB) are able to oxidize electron donors while the electrons in the conduction band (CB) can reduce electron acceptors via a SET process. Bi₂O₃ has a smaller bandgap (2.1–2.8 eV) than TiO₂ (3.05–3.26 eV) and can absorb visible light without requiring doping. This recyclable photocatalyst was employed with various alkyl bromide substrates by oxidative quenching to give rise to the corresponding ATRA products in moderate to good yields. The proposed mechanism shown in Scheme [15] has recently been updated the Noël and Pericàs groups, who provided theoretical and experimental evidence that the perceived heterogeneous photocatalysis with Bi₂O₃ in the presence of alkyl bromides in DMSO or DMF actually involves a homogeneous Bi_nBr_m species.[65] Instead of a true heterogeneous catalyst, Bi₂O₃ is therefore rather a precatalyst that slowly transforms into the active homogeneous photocatalyst. Still, the use of Bi₂O₃ is advantageous to the direct use of BiBr₃, which is highly hygroscopic.

Photoinduced Energy Transfer

Nature has used energy transfer (EnT) to convert solar energy into chemical energy for billions of years.[66] This process has motivated chemists to develop so-called photosensitization by employing a photocatalyst, a distinct activation mode for organic synthesis.[67] This complements other activation modes in photocatalysis. Organic molecules that cannot directly undergo a SET process with photocatalysts are unreactive under photoredox catalysis. Many organic molecules possess relatively high reduction or oxidation potentials that are incompatible with excited photocatalysts. A strategy to overcome this obstacle is to use excited photocatalysts to convert substrates into their triplet states via energy transfer, which is also known as (photo)sensitization.[5] [12] [68] Upon absorption of a photon, the ground singlet state (S₀) photocatalyst promotes to its lowest singlet excited state (S₁) first, and subsequently achieves its long-lived lowest-energy triplet state (T₁) via intersystem crossing (ISC). Then, the decay of the triplet photocatalyst (*PC, which is the energy donor) from its triplet state to its ground singlet state promotes the singlet ground state of the substrate (Sub, which is the energy acceptor) to its lowest-energy triplet state (*Sub) via a triplet-triplet energy transfer (TTET) process. The result of the energy transfer processes is an electronically excited acceptor (*Sub) molecule together with the regeneration of the electronic ground state of the donor (PC) (Scheme [16]).

Two distinct underlying mechanisms regarding energy transfer exist: Förster energy transfer (via Coulombic dipole-dipole interactions) and Dexter energy transfer (via exchange interactions) (Figure [10]).[68] In the first mechanism, an electronic oscillation in the excited-state donor D* causes an electronic oscillation in the ground-state acceptor A, leading to its excitation. This is entitled FRET (fluorescence resonance energy transfer). However, for a triplet-triplet energy transfer process, this would necessitate two spin-forbidden events: a triplet to singlet relaxation in the excited donor, and a singlet to triplet excitation in the acceptor. For this reason, the dominating mechanism for photocatalytic activation of organic substrates via energy transfer is the Dexter electron-exchange mechanism between the excited-state donor D* and ground-state acceptor A. In this mechanism, electrons are transferred through orbital-overlap, leading to a net energy transfer. Furthermore, this circumvents the spin-selection rule. A suitable energy transfer photocatalyst should possess (i) a high absorption at the desired wavelength, (ii) an efficient ISC (intersystem crossing) to its triplet state, (iii) a sufficiently long lifetime of the excited triplet state, and (iv) a higher triplet energy than the energy accepting substrate.[12b]

The photochemistry of carbonyl compounds, especially ketones, has been one of the most intensively investigated areas in classical photochemistry since photoexcited ketones are known to induce various types of photochemical transformations. Ketones readily excite to their singlet excited state under light irradiation [acetone: 320 nm; acetophenone: 350 nm; benzophenone: 380 nm; thioxanthone: 420 nm for n–π* via the promotion of an n-electron to the antibonding π-system] and then rapidly convert into a triplet state via intersystem crossing. The relatively longer lifetimes of the triplet state of ketones have allowed the promotion of photochemical transformations via energy transfer (EnT), but also hydrogen-atom transfer (HAT) and photoinduced electron transfer (PET) reactions.

Ketone sensitizers are widely applied as energy transfer organophotocatalysts. An important limitation is the relatively low absorption in the visible region, especially for dialkyl ketones. For example, superstoichiometric amounts of acetone can act as a photosensitizer under UV light irradiation (300 nm). The use of (sub)stoichiometric quantities of diaryl ketones as photosensitizer compensates for these disadvantages, allowing excitation with higher wavelengths (>350 nm) (Table [4]).[12b] Complexes of ruthenium or iridium and certain organic photocatalysts, which can also be used in PET, are found to be more suitable energy transfer photocatalysts. After all, these sensitizers feature strong absorption in the visible (>400 nm) rather than the UV region and generate long-lived charge-transfer (MLCT) excited states (Figure [11]).[4] [69]

Table 4 Excited-State Properties of Carbonyl Photosensitizers

Property of carbonyl	Acetone	Acetophenone	Benzophenone	Thioxanthone
E T (kcal·mol–1)	79	74	69	63
λmax (nm)	320	350	380	420
τ (μs)	47	0.14	50	73
φISC	>0.95	>0.95	>0.95	>0.6

Glorius and co-workers developed the [Ir^III{dF(CF₃)ppy}₂(dtbbpy)]PF₆ sensitization of benzophenone-derived oxime esters. Both with arene and alkane carboxylic acid esters a concerted homolytic N–O cleavage and subsequent decarboxylation were achieved.[70] The use of both the carbon-centered and the nitrogen-centered radical was shown in the carboimination of activated alkenes (R¹ = Ar, EWG) through a radical addition-recombination pathway (Scheme [17]).

The applied benzophenone-based oximes cannot be reduced through a SET from the photocatalyst since their reduction potential is very high (E _red = –2.05 V vs. SCE). In an oxidative quenching cycle, the excited-state oxidation potential *E _ox(Ir^IV/*Ir^III) of the Ir photocatalyst is –0.89 V vs. SCE, while in a reductive quenching cycle, the reduced photocatalyst only reaches E _1/2(Ir^III/Ir^II) = –1.37 V vs. SCE. On the other hand, the triplet energy of the photocatalyst (E _T = 60.8 kcal·mol^–1) is higher than the triplet energy of the benzophenone-derived oxime ester from 3-phenylpropanoic acid (E _T = 45.4 kcal·mol^–1). For this reason, a triplet-triplet energy transfer (TTEnT) from the excited photocatalyst to the benzophenone oxime ester is thermodynamically possible and a feasible reaction pathway.

Maes and co-workers recently developed an organic dye photocatalyzed 1,2-thiosulfonylation of various unactivated alkenes with readily available thiosulfonates under blue LED irradiation (Scheme [18]).[38b] The method tolerates a wide variety of functional groups, including halides, alcohols, esters, nitro, cyano, and carboxylic acid groups, and is particularly attractive with respect to green chemistry by utilizing 9-mesityl-10-methylacridinium perchlorate (Mes-Acr⁺-Me ClO₄ ^–) as an organic photocatalyst in the recommended solvent dimethyl carbonate. The proposed mechanism shows the absence of a SET process, but the involvement of an EnT (energy transfer) process from the excited organo-photocatalyst to the thiosulfonate, providing a sulfonyl and a sulfenyl radical via homolytic cleavage, which then functionalize the alkene. The addition of the sulfonyl radical to an unactivated alkene affords a carbon-centered radical, which is subsequently involved in a radical-radical coupling with the sulfenyl radical, delivering the 1,2-thiosulfonylation product (path a). Alternatively, the carbon-centered radical can subsequently initiate a radical chain process (rather than a catalytic cycle) through the reaction with another thiosulfonate molecule (path b), hereby also generating the reaction product and a sulfonyl radical. In addition, there is a second possible radical chain initiated by sulfenyl radicals reacting with thiosulfonate, generating a sulfonyl radical and disulfide. The sulfonyl radical reacts with alkene and the disulfide acts as the sulfenyl source, finally delivering the 1,2-thiosulfonylation product. These different possible mechanisms can run in parallel and a quantum yield of Φ = 1.9 points to the involvement of a radical chain pathway in addition to a photocatalytic cycle.[38a] The observation of disulfide at least points to its involvement in the reaction scheme. Once again, a photoredox mechanism is impossible. The excited organophotocatalyst cannot donate an electron (only *E _red = +2.08 V vs. SCE is tabulated) and can therefore only perform a reductive quenching cycle. Thiosulfonate (E _red = –1.43 V vs. SCE) cannot be reduced in the second step of the reductive quenching cycle by the reduced photocatalyst [Mes-Acr ^• -Me], since the redox potential associated with its re-oxidation to Mes-Acr⁺-Me is only E _1/2 = –0.57 V vs. SCE. The excited photocatalyst is also too weak an oxidant to oxidize the allylbenzene reactant (E _ox = +2.52 V vs. SCE).

Conclusions and Outlook

The field of visible light photocatalysis has emerged as a powerful synthetic strategy which enables the selective activation and functionalization of small molecules. This led to the development of a variety of novel synthetic methodologies under mild reaction conditions with broad synthetic applications, including key transformations in the preparation of complex natural products. It provides a revival of radical chemistry offering exciting opportunities to achieve both economic and environmental benefits over the classical radical reactions, which highly relies on toxic reagents, hazardous radical initiators, and/or elevated temperatures. Visible light photons can be easily produced at a low energy cost by the use of energy-efficient high-power light-emitting diodes technology, featuring both specific wavelengths and varying intensities.

The most common activation mode in visible light photocatalysis is photoinduced electron transfer, where the excited photocatalyst undergoes SET with substrates to form radical species. The most elegant photoredox catalytic processes are redox neutral. However, net oxidative or net reductive reactions are possible by using sacrificial oxidants or reductants as reagents. Alternatively, the excited photocatalyst could also transfer energy to a reaction substrate, triggering reactivity beyond oxidative and reductive processes, through homolytic bond cleavage.

Ruthenium- and iridium-based complexes have traditionally been employed as photocatalysts in the majority of visible-light-induced photochemical reactions. However, the research field of visible light photocatalysis has recently begun to shift toward the use of noble-metal-free, more sustainable, and low-cost alternative photocatalysts. In this regard, recent intense research efforts have been devoted to developing novel earth-abundant transition-metal-based complexes, organic chromophores, and semiconductors as photocatalysts replacing or complementing the well-established ruthenium and iridium complexes. The most common solvent selected for visible photocatalyzed reactions is acetonitrile. While this is a very widely used solvent in organic chemistry discovery labs its occupational exposure limit is low (20–40 ppm depending on the specific country regulations). It is therefore denoted as a problematic solvent that requires substitution whenever possible.[71] The search to replace it with recommended solvents with respect to green chemistry (health, safety, environment aligned with the Global Harmonized System (GHS)) is therefore highly important to further improve the inherent greenness (catalysis, light energy, room temperature) of the novel photocatalyzed reactions developed. These acetonitrile substitutes should be recyclable and whenever possible renewable considering the majority of waste generated in an organic reaction is solvent based.[72] Though several papers show such solvents (ethyl acetate, dimethyl carbonate)[38b] [70] can be used, more efforts are required here to show the generality. Greenness is more than avoidance of noble metals and the use of benign recyclable (renewable) solvents. A variety of mass-based metrics (atom efficiency, reaction mass efficiency, process mass intensity), as well as qualitative aspects (solvent selection, health, environment and safety aspects, involvement of critical elements, reagent use, energy requirement, and work-up method) need to be considered. These aspects already need to be studied in the discovery phase to avoid reoptimization in chemical development to identify suitable production routes.[72,73] After all, a smooth transfer of novel reactions from discovery to chemical development and finally process chemistry producing chemicals on large scale without or with minimal reoptimization is beneficial.

The reproducibility of newly developed reactions is another important aspect. The assessment of the sensitivity of a given reaction to key parameters (e.g., light intensity, concentration, oxygen, temperature, water content) can greatly aid researchers here, trying to reproduce the work of others on other reactants. Recently developed and easily to apply tools should become embedded in the standard way of working and hereby facilitate this synthetic methodology transfer.[74]

A completely different and emerging field is electrocatalysis which deserves mentioning here due to its similarities with photocatalysis.[75] In electrocatalysis an electrochemical mediator is oxidized or reduced at the electrode. The oxidized or reduced electrocatalyst subsequently reacts with a reactant producing a radical initiating the catalytic cycle.[76] Although direct oxidation or reduction in electrochemistry is theoretically possible, allowing a broad voltage range, this does not provide the required selectivity to execute reactions, therefore requiring the use of an electrocatalyst. Both fields are actually complementary and hold great promise to produce fine and speciality chemicals in a sustainable, scalable, and cost-effective manner.

This review article describes and explains the fundamental concepts and techniques of photochemistry. It introduces photoinduced electron transfer and energy transfer as the most common activation modes, illustrated by selected photocatalytic alkene functionalization reactions. Particular attention is given to the explanation of the reaction mechanisms involved, as this review is intended to serve as an introductory guide to support chemists who are new to the field and want to apply published and/or start developing their own visible light photocatalytic reactions. Furthermore, it lays the foundation for a second review, detailing lesser-known activation modes and some emerging concepts within the field of photocatalysis.[18]

Conflict of Interest

The authors declare no conflict of interest.

• References

• 1a Poplata S, Tröster A, Zou Y.-Q, Bach T. Chem. Rev. 2016; 116: 9748
  CrossrefPubMedSearch in Google Scholar
• 1b Mangion IK, MacMillan DW. C. J. Am. Chem. Soc. 2005; 127: 3696
  CrossrefPubMedSearch in Google Scholar
• 1c Liu H.-J, Lee SP. Tetrahedron Lett. 1977; 18: 3699
  CrossrefSearch in Google Scholar
• 2a Kärkäs MD, Porco JA, Stephenson CR. J. Chem. Rev. 2016; 116: 9683
  CrossrefPubMedSearch in Google Scholar
• 2b Bach T, Hehn JP. Angew. Chem. Int. Ed. 2011; 50: 1000
  CrossrefPubMedSearch in Google Scholar
• 2c Nicolaou KC, Gray DL. F, Tae J. J. Am. Chem. Soc. 2004; 126: 613
  CrossrefPubMedSearch in Google Scholar
• 2d Renata H, Zhou Q, Baran PS. Science 2013; 339: 59
  CrossrefPubMedSearch in Google Scholar
• 3 Marzo L, Pagire SK, Reiser O, König B. Angew. Chem. Int. Ed. 2018; 57: 10034
  CrossrefPubMedSearch in Google Scholar
• 4 Bell JD, Murphy JA. Chem. Soc. Rev. 2021; 50: 9540
  CrossrefPubMedSearch in Google Scholar
• 5 Strieth-Kalthoff F, James MJ, Teders M, Pitzer L, Glorius F. Chem. Soc. Rev. 2018; 47: 7190
  CrossrefPubMedSearch in Google Scholar
• 6 Narayanam JM. R, Stephenson CR. J. Chem. Soc. Rev. 2011; 40: 102
  CrossrefPubMedSearch in Google Scholar
• 7 McAtee RC, McClain EJ, Stephenson CR. J. Trends Chem. 2019; 1: 111
  CrossrefPubMedSearch in Google Scholar
• 8 Ciamician G. Science 1912; 36: 385
  CrossrefPubMedSearch in Google Scholar
• 9a Zeitler K. Angew. Chem. Int. Ed. 2009; 48: 9785
  CrossrefPubMedSearch in Google Scholar
• 9b Prier CK, Rankic DA, MacMillan DW. C. Chem. Rev. 2013; 113: 5322
  Search in Google Scholar
• 9c Romero NA, Nicewicz DA. Chem. Rev. 2016; 116: 10075
  CrossrefPubMedSearch in Google Scholar
• 9d Wang C.-S, Dixneuf PH, Soulé J.-F. Chem. Rev. 2018; 118: 7532
  CrossrefPubMedSearch in Google Scholar
• 9e Cauwenbergh R, Das S. Synlett 2022; 33: 129
  Thieme ConnectSearch in Google Scholar
• 10a Miller DC, Tarantino KT, Knowles RR. Top. Curr. Chem. 2016; 374: 30
  CrossrefPubMedSearch in Google Scholar
• 10b Murray PR. D, Cox JH, Chiappini ND, Roos CB, McLoughlin EA, Hejna BG, Nguyen ST, Ripberger HH, Ganley JM, Tsui E, Shin NY, Koronkiewicz B, Qiu G, Knowles RR. Chem. Rev. 2022; 122: 2017
  CrossrefPubMedSearch in Google Scholar
• 11 Capaldo L, Ravelli D, Fagnoni M. Chem. Rev. 2022; 122: 1875
  CrossrefPubMedSearch in Google Scholar
• 12a Zhou Q.-Q, Zou Y.-Q, Lu L.-Q, Xiao W.-J. Angew. Chem. Int. Ed. 2019; 58: 1586
  CrossrefPubMedSearch in Google Scholar
• 12b Strieth-Kalthoff F, Glorius F. Chem 2020; 6: 1888
  CrossrefSearch in Google Scholar
• 13a Wei Y, Zhou Q.-Q, Tan F, Lu L.-Q, Xiao W.-J. Synthesis 2019; 51: 3021
  Thieme ConnectSearch in Google Scholar
• 13b Crisenza GE. M, Mazzarella D, Melchiorre P. J. Am. Chem. Soc. 2020; 142: 5461
  CrossrefPubMedSearch in Google Scholar
• 13c Sumida Y, Ohmiya H. Chem. Soc. Rev. 2021; 50: 6320
  CrossrefPubMedSearch in Google Scholar
• 13d Tasnim T, Ayodele MJ, Pitre SP. J. Org. Chem. 2022; 87: 10555
  CrossrefPubMedSearch in Google Scholar
• 14a Kancherla R, Muralirajan K, Sagadevan A, Rueping M. Trends Chem. 2019; 1: 510
  CrossrefSearch in Google Scholar
• 14b Cheung KP. S, Sarkar S, Gevorgyan V. Chem. Rev. 2022; 122: 1543
  CrossrefPubMedSearch in Google Scholar
• 14c Sarkar T, Shah TA, Maharana PK, Debnath B, Punniyamurthy T. Eur. J. Org. Chem. 2022; in press
  Crossref
• 15a Arias-Rotondo DM, McCusker JK. Chem. Soc. Rev. 2016; 45: 5803
  CrossrefPubMedSearch in Google Scholar
• 15b Hockin BM, Li C, Robertson N, Zysman-Colman E. Catal. Sci. Technol. 2019; 9: 889
  CrossrefSearch in Google Scholar
• 16a Bryden MA, Zysman-Colman E. Chem. Soc. Rev. 2021; 50: 7587
  CrossrefPubMedSearch in Google Scholar
• 16b Vega-Peñaloza A, Mateos J, Companyó X, Escudero-Casao M, Dell’Amico L. Angew. Chem. Int. Ed. 2021; 60: 1082
  CrossrefPubMedSearch in Google Scholar
• 16c Amos SG. E, Garreau M, Buzzetti L, Waser J. Beilstein J. Org. Chem. 2020; 16: 1163
  CrossrefPubMedSearch in Google Scholar
• 16d Lee Y, Kwon MS. Eur. J. Org. Chem. 2020; 2020: 6028
  CrossrefSearch in Google Scholar
• 16e Bobo MV, Kuchta JJ, Vannucci AK. Org. Biomol. Chem. 2021; 19: 4816
  CrossrefPubMedSearch in Google Scholar
• 17a Riente P, Noël T. Catal. Sci. Technol. 2019; 9: 5186
  CrossrefSearch in Google Scholar
• 17b Franchi D, Amara Z. ACS Sustainable Chem. Eng. 2020; 8: 15405
  CrossrefSearch in Google Scholar
• 18 De Vos D, Gadde K, Maes BU. W. Synthesis 2022; in press,
  Thieme Connect
• 19a Singh A, Teegardin K, Kelly M, Prasad KS, Krishnan S, Weaver JD. J. Organomet. Chem. 2015; 776: 51
  CrossrefSearch in Google Scholar
• 19b Koike T, Akita M. Inorg. Chem. Front. 2014; 1: 562
  CrossrefPubMedSearch in Google Scholar
• 20 Maes J, Mitchell EA, Maes BU. W. Base Metals in Catalysis: From Zero to Hero. In Green and Sustainable Medicinal Chemistry: Methods, Tools and Strategies for the 21st Century Pharmaceutical Industry, Chap. 16. Summerton L, Sneddon HF, Jones LC, Clark JH. Royal Society of Chemistry; Cambridge: 2016: 192-202
  CrossrefSearch in Google Scholar
• 21a Paria S, Reiser O. ChemCatChem 2014; 6: 2477
  CrossrefSearch in Google Scholar
• 21b Paria S, Reiser O. Visible Light and Copper Complexes: A Promising Match in Photoredox Catalysis. In Visible Light Photocatalysis in Organic Chemistry. Stephenson CR. J, Yoon TP, MacMillan DW. C. Wiley-VCH; Weinheim: 2018: 233-251
  CrossrefSearch in Google Scholar
• 21c Hossain A, Bhattacharyya A, Reiser O. Science 2019; 364: eaav9713
  CrossrefPubMedSearch in Google Scholar
• 21d Larsen CB, Wenger OS. Chem. Eur. J. 2018; 24: 2039
  CrossrefPubMedSearch in Google Scholar
• 21e Wenger OS. J. Am. Chem. Soc. 2018; 140: 13522
  Search in Google Scholar
• 22a Joshi-Pangu A, Lévesque F, Roth HG, Oliver SF, Campeau L.-C, Nicewicz D, DiRocco DA. J. Org. Chem. 2016; 81: 7244
  CrossrefPubMedSearch in Google Scholar
• 22b Tlili A, Lakhdar S. Angew. Chem. Int. Ed. 2021; 60: 19526
  CrossrefPubMedSearch in Google Scholar
• 23a Speckmeier E, Fischer TG, Zeitler K. J. Am. Chem. Soc. 2018; 140: 15353
  CrossrefPubMedSearch in Google Scholar
• 23b Natarajan P, König B. Eur. J. Org. Chem. 2021; 2021: 2145
  CrossrefSearch in Google Scholar
• 24 Hari DP, König B. Chem. Commun. 2014; 50: 6688
  CrossrefPubMedSearch in Google Scholar
• 25 Gisbertz S, Pieber B. ChemPhotoChem 2020; 4: 456
  CrossrefPubMedSearch in Google Scholar
• 26a Savateev A, Ghosh I, König B, Antonietti M. Angew. Chem. Int. Ed. 2018; 57: 15936
  CrossrefPubMedSearch in Google Scholar
• 26b Savateev A, Antonietti M. ChemCatChem 2019; 11: 6166
  CrossrefSearch in Google Scholar
• 27 Zhang T, Lin W. Chem. Soc. Rev. 2014; 43: 5982
  CrossrefPubMedSearch in Google Scholar
• 28a Geng K, He T, Liu R, Dalapati S, Tan KT, Li Z, Tao S, Gong Y, Jiang Q, Jiang D. Chem. Rev. 2020; 120: 8814
  CrossrefPubMedSearch in Google Scholar
• 28b Yang Q, Luo M, Liu K, Cao H, Yan H. Appl. Catal., B 2020; 276: 119174
  CrossrefSearch in Google Scholar
• 29 Lan G, Quan Y, Wang M, Nash GT, You E, Song Y, Veroneau SS, Jiang X, Lin W. J. Am. Chem. Soc. 2019; 141: 15767
  CrossrefPubMedSearch in Google Scholar
• 30 Knorn M, Rawner T, Czerwieniec R, Reiser O. ACS Catal. 2015; 5: 5186
  CrossrefPubMedSearch in Google Scholar
• 31 Martiny M, Steckhan E, Esch T. Chem. Ber. 1993; 126: 1671
  CrossrefPubMedSearch in Google Scholar
• 32 Kasha M. Discuss. Faraday Soc. 1950; 9: 14
  CrossrefPubMedSearch in Google Scholar
• 33 Rey YP, Abradelo DG, Santschi N, Strassert CA, Gilmour R. Eur. J. Org. Chem. 2017; 2017: 2170
  CrossrefPubMedSearch in Google Scholar
• 34a Elgrishi N, Rountree KJ, McCarthy BD, Rountree ES, Eisenhart TT, Dempsey JL. J. Chem. Educ. 2018; 95: 197
  CrossrefSearch in Google Scholar
• 34b Roth HG, Romero NA, Nicewicz DA. Synlett 2016; 27: 714
  CrossrefSearch in Google Scholar
• 35 Wu Y, Kim D, Teets TS. Synlett 2022; 33: 1154
  Thieme ConnectPubMedSearch in Google Scholar
• 36 Rehm D, Weller A. Isr. J. Chem. 1970; 8: 259
  CrossrefPubMedSearch in Google Scholar
• 37 Campagna S, Puntoriero F, Nastasi F, Bergamini G, Balzani V. Photochemistry and Photophysics of Coordination Compounds: Ruthenium . In Photochemistry and Photophysics of Coordination Compounds I . Balzani V, Campagna S. Springer; Heidelberg: 2007: 117-214
  CrossrefSearch in Google Scholar
• 38a Cismesia MA, Yoon TP. Chem. Sci. 2015; 6: 5426
  CrossrefPubMedSearch in Google Scholar
• 38b Gadde K, Mampuys P, Guidetti A, Ching HY. V, Herrebout WA, Van Doorslaer S, Abbaspour Tehrani K, Maes BU. W. ACS Catal. 2020; 10: 8765
  CrossrefSearch in Google Scholar
• 39a Gaida F, Griesbeck AG, Vollmer M. Photocatalysis: The Principles . In Science of Synthesis: Photocatalysis in Organic Synthesis, Chap. 3. König B. Thieme; Stuttgart: 2019: 3
  Search in Google Scholar
• 39b Kuijpers KP. L, Bottecchia C, Cambié D, Drummen K, König NJ, Noël T. Angew. Chem. Int. Ed. 2018; 57: 11278
  CrossrefPubMedSearch in Google Scholar
• 40 Żamojć K, Bylińska I, Wiczk W, Chmurzyński L. Int. J. Mol. Sci. 2021; 22: 885
  CrossrefPubMedSearch in Google Scholar
• 41 IUPAC Compendium of Chemical Terminology, 2nd ed. McNaught AD, Wilkinson A. Blackwell Scientific; Oxford: 1997. https//goldbook.iupac.org(accessed Sept 15, 2022
  CrossrefSearch in Google Scholar
• 42 Tibbetts JD, Bull SD. Adv. Sustainable Syst. 2021; 5: 2000292
  CrossrefPubMedSearch in Google Scholar
• 43 Besson M, Gallezot P, Pinel C. Chem. Rev. 2014; 114: 1827
  CrossrefPubMedSearch in Google Scholar
• 44 Beller M, Seayad J, Tillack A, Jiao H. Angew. Chem. Int. Ed. 2004; 43: 3368
  CrossrefPubMedSearch in Google Scholar
• 45a Courant T, Masson G. J. Org. Chem. 2016; 81: 6945
  CrossrefPubMedSearch in Google Scholar
• 45b Cao M.-Y, Ren X, Lu Z. Tetrahedron Lett. 2015; 56: 3732
  CrossrefSearch in Google Scholar
• 45c Koike T, Akita M. Chem 2018; 4: 409
  CrossrefSearch in Google Scholar
• 46a Cano-Yelo H, Deronzier A. J. Chem. Soc., Perkin Trans. 2 1984; 1093
  Crossref
• 46b Cano-Yelo H, Deronzier A. Tetrahedron Lett. 1984; 25: 5517
  CrossrefSearch in Google Scholar
• 46c Cano-Yelo H, Deronzier A. J. Chem. Soc., Faraday Trans. 1 1984; 3011
  Crossref
• 46d Cano-Yelo H, Deronzier A. J. Photochem. 1987; 37: 315
  CrossrefSearch in Google Scholar
• 47a Hironaka K, Fukuzumi S, Tanaka T. J. Chem. Soc., Perkin Trans. 2 1984; 1705
  Crossref
• 47b Ishikawa M, Fukuzumi S. J. Chem. Soc., Chem. Commun. 1990; 1353
  Crossref
• 47c Fukuzumi S, Mochizuki S, Tanaka T. J. Phys. Chem. 1990; 94: 722
  CrossrefSearch in Google Scholar
• 48a Hedstrand DM, Kruizinga WH, Kellogg RM. Tetrahedron Lett. 1978; 19: 1255
  CrossrefSearch in Google Scholar
• 48b Van Bergen TJ, Hedstrand DM, Kruizinga WH, Kellogg RM. J. Org. Chem. 1979; 44: 4953
  CrossrefSearch in Google Scholar
• 49a Okada K, Okamoto K, Morita N, Okubo K, Oda M. J. Am. Chem. Soc. 1991; 113: 9401
  CrossrefSearch in Google Scholar
• 49b Okada K, Okubo K, Morita N, Oda M. Tetrahedron Lett. 1992; 33: 7377
  CrossrefSearch in Google Scholar
• 50a Pac C, Ihama M, Yasuda M, Miyauchi Y, Sakurai H. J. Am. Chem. Soc. 1981; 103: 6495
  CrossrefSearch in Google Scholar
• 50b Ishitani O, Pac C, Sakurai H. J. Org. Chem. 1983; 48: 2941
  CrossrefSearch in Google Scholar
• 50c Majima T, Pac C, Nakasone A, Sakurai H. J. Am. Chem. Soc. 1981; 103: 4499
  CrossrefSearch in Google Scholar
• 51 Pandey G, Hajra S, Ghorai MK, Kumar KR. J. Am. Chem. Soc. 1997; 119: 8777
  CrossrefPubMedSearch in Google Scholar
• 52 Tomioka H, Ueda K, Ohi H, Izawa Y. Chem. Lett. 1986; 15: 1359
  CrossrefSearch in Google Scholar
• 53a Willner I, Tsfania T, Eichen Y. J. Org. Chem. 1990; 55: 2656
  CrossrefSearch in Google Scholar
• 53b Mandler D, Willner I. J. Chem. Soc., Perkin Trans. 2 1986; 805
  Crossref
• 53c Mandler D, Willner I. J. Chem. Soc., Chem. Commun. 1986; 851
  Crossref
• 54 Ischay MA, Anzovino ME, Du J, Yoon TP. J. Am. Chem. Soc. 2008; 130: 12886
  CrossrefPubMedSearch in Google Scholar
• 55 Nicewicz DA, MacMillan DW. C. Science 2008; 322: 77
  CrossrefPubMedSearch in Google Scholar
• 56 Narayanam JM. R, Tucker JW, Stephenson CR. J. J. Am. Chem. Soc. 2009; 131: 8756
  CrossrefPubMedSearch in Google Scholar
• 57 Marcus RA. Angew. Chem. Int. Ed. 1993; 32: 1111
  CrossrefPubMedSearch in Google Scholar
• 58 Nguyen JD, Tucker JW, Konieczynska MD, Stephenson CR. J. J. Am. Chem. Soc. 2011; 133: 4160
  CrossrefPubMedSearch in Google Scholar
• 59 Courant T, Masson G. Chem. Eur. J. 2012; 18: 423
  CrossrefPubMedSearch in Google Scholar
• 60 Pirtsch M, Paria S, Matsuno T, Isobe H, Reiser O. Chem. Eur. J. 2012; 18: 7336
  CrossrefPubMedSearch in Google Scholar
• 61 Hossain A, Engl S, Lutsker E, Reiser O. ACS Catal. 2019; 9: 1103
  CrossrefSearch in Google Scholar
• 62 Engl S, Reiser O. Chem. Soc. Rev. 2022; 51: 5287
  CrossrefPubMedSearch in Google Scholar
• 63 Huang H, Yu C, Zhang Y, Zhang Y, Mariano PS, Wang W. J. Am. Chem. Soc. 2017; 139: 9799
  CrossrefPubMedSearch in Google Scholar
• 64 Riente P, Pericàs MA. ChemSusChem 2015; 8: 1841
  CrossrefPubMedSearch in Google Scholar
• 65 Riente P, Fianchini M, Llanes P, Pericàs MA, Noël T. Nat. Commun. 2021; 12: 625
  CrossrefPubMedSearch in Google Scholar
• 66 Mirkovic T, Ostroumov EE, Anna JM, van Grondelle R, Govindjee, Scholes GD. Chem. Rev. 2017; 117: 249
  CrossrefPubMedSearch in Google Scholar
• 67a Yoo W.-J, Tsukamoto T, Kobayashi S. Org. Lett. 2015; 17: 3640
  CrossrefPubMedSearch in Google Scholar
• 67b Heitz DR, Tellis JC, Molander GA. J. Am. Chem. Soc. 2016; 138: 12715
  CrossrefPubMedSearch in Google Scholar
• 68 Neveselý T, Wienhold M, Molloy JJ, Gilmour R. Chem. Rev. 2022; 122: 2650
  CrossrefPubMedSearch in Google Scholar
• 69 Chen D.-F, Chrisman CH, Miyake GM. ACS Catal. 2020; 10: 2609
  CrossrefPubMedSearch in Google Scholar
• 70 Patra T, Bellotti P, Strieth-Kalthoff F, Glorius F. Angew. Chem. Int. Ed. 2020; 59: 3172
  CrossrefPubMedSearch in Google Scholar
• 71 Prat D, Wells A, Hayler J, Sneddon H, McElroy CR, Abou-Shehada S, Dunn PJ. Green Chem. 2016; 18: 288
  CrossrefPubMedSearch in Google Scholar
• 72a McElroy CR, Constantinou A, Jones LC, Summerton L, Clark JH. Green Chem. 2015; 17: 3111
  CrossrefSearch in Google Scholar
• 72b Abou-Shehada S, Mampuys P, Maes BU. W, Clark JH, Summerton L. Green Chem. 2017; 19: 249
  CrossrefSearch in Google Scholar
• 73 Monteith ER, Mampuys P, Summerton L, Clark JH, Maes BU. W, McElroy CR. Green Chem. 2020; 22: 123
  CrossrefPubMedSearch in Google Scholar
• 74 Pitzer L, Schäfers F, Glorius F. Angew. Chem. Int. Ed. 2019; 58: 8572
  Search in Google Scholar
• 75 Novaes LF. T, Liu J, Shen Y, Lu L, Meinhardt JM, Lin S. Chem. Soc. Rev. 2021; 50: 7941
  CrossrefPubMedSearch in Google Scholar
• 76 Sambiagio C, Sterckx H, Maes BU. W. ACS Cent. Sci. 2017; 3: 686
  CrossrefPubMedSearch in Google Scholar

Corresponding Author

Publication History

Received: 13 April 2022

Accepted after revision: 29 August 2022

Accepted Manuscript online:
29 August 2022

Article published online:
27 October 2022

© 2022. Thieme. All rights reserved

Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany

• References

• 1a Poplata S, Tröster A, Zou Y.-Q, Bach T. Chem. Rev. 2016; 116: 9748
  CrossrefPubMedSearch in Google Scholar
• 1b Mangion IK, MacMillan DW. C. J. Am. Chem. Soc. 2005; 127: 3696
  CrossrefPubMedSearch in Google Scholar
• 1c Liu H.-J, Lee SP. Tetrahedron Lett. 1977; 18: 3699
  CrossrefSearch in Google Scholar
• 2a Kärkäs MD, Porco JA, Stephenson CR. J. Chem. Rev. 2016; 116: 9683
  CrossrefPubMedSearch in Google Scholar
• 2b Bach T, Hehn JP. Angew. Chem. Int. Ed. 2011; 50: 1000
  CrossrefPubMedSearch in Google Scholar
• 2c Nicolaou KC, Gray DL. F, Tae J. J. Am. Chem. Soc. 2004; 126: 613
  CrossrefPubMedSearch in Google Scholar
• 2d Renata H, Zhou Q, Baran PS. Science 2013; 339: 59
  CrossrefPubMedSearch in Google Scholar
• 3 Marzo L, Pagire SK, Reiser O, König B. Angew. Chem. Int. Ed. 2018; 57: 10034
  CrossrefPubMedSearch in Google Scholar
• 4 Bell JD, Murphy JA. Chem. Soc. Rev. 2021; 50: 9540
  CrossrefPubMedSearch in Google Scholar
• 5 Strieth-Kalthoff F, James MJ, Teders M, Pitzer L, Glorius F. Chem. Soc. Rev. 2018; 47: 7190
  CrossrefPubMedSearch in Google Scholar
• 6 Narayanam JM. R, Stephenson CR. J. Chem. Soc. Rev. 2011; 40: 102
  CrossrefPubMedSearch in Google Scholar
• 7 McAtee RC, McClain EJ, Stephenson CR. J. Trends Chem. 2019; 1: 111
  CrossrefPubMedSearch in Google Scholar
• 8 Ciamician G. Science 1912; 36: 385
  CrossrefPubMedSearch in Google Scholar
• 9a Zeitler K. Angew. Chem. Int. Ed. 2009; 48: 9785
  CrossrefPubMedSearch in Google Scholar
• 9b Prier CK, Rankic DA, MacMillan DW. C. Chem. Rev. 2013; 113: 5322
  Search in Google Scholar
• 9c Romero NA, Nicewicz DA. Chem. Rev. 2016; 116: 10075
  CrossrefPubMedSearch in Google Scholar
• 9d Wang C.-S, Dixneuf PH, Soulé J.-F. Chem. Rev. 2018; 118: 7532
  CrossrefPubMedSearch in Google Scholar
• 9e Cauwenbergh R, Das S. Synlett 2022; 33: 129
  Thieme ConnectSearch in Google Scholar
• 10a Miller DC, Tarantino KT, Knowles RR. Top. Curr. Chem. 2016; 374: 30
  CrossrefPubMedSearch in Google Scholar
• 10b Murray PR. D, Cox JH, Chiappini ND, Roos CB, McLoughlin EA, Hejna BG, Nguyen ST, Ripberger HH, Ganley JM, Tsui E, Shin NY, Koronkiewicz B, Qiu G, Knowles RR. Chem. Rev. 2022; 122: 2017
  CrossrefPubMedSearch in Google Scholar
• 11 Capaldo L, Ravelli D, Fagnoni M. Chem. Rev. 2022; 122: 1875
  CrossrefPubMedSearch in Google Scholar
• 12a Zhou Q.-Q, Zou Y.-Q, Lu L.-Q, Xiao W.-J. Angew. Chem. Int. Ed. 2019; 58: 1586
  CrossrefPubMedSearch in Google Scholar
• 12b Strieth-Kalthoff F, Glorius F. Chem 2020; 6: 1888
  CrossrefSearch in Google Scholar
• 13a Wei Y, Zhou Q.-Q, Tan F, Lu L.-Q, Xiao W.-J. Synthesis 2019; 51: 3021
  Thieme ConnectSearch in Google Scholar
• 13b Crisenza GE. M, Mazzarella D, Melchiorre P. J. Am. Chem. Soc. 2020; 142: 5461
  CrossrefPubMedSearch in Google Scholar
• 13c Sumida Y, Ohmiya H. Chem. Soc. Rev. 2021; 50: 6320
  CrossrefPubMedSearch in Google Scholar
• 13d Tasnim T, Ayodele MJ, Pitre SP. J. Org. Chem. 2022; 87: 10555
  CrossrefPubMedSearch in Google Scholar
• 14a Kancherla R, Muralirajan K, Sagadevan A, Rueping M. Trends Chem. 2019; 1: 510
  CrossrefSearch in Google Scholar
• 14b Cheung KP. S, Sarkar S, Gevorgyan V. Chem. Rev. 2022; 122: 1543
  CrossrefPubMedSearch in Google Scholar
• 14c Sarkar T, Shah TA, Maharana PK, Debnath B, Punniyamurthy T. Eur. J. Org. Chem. 2022; in press
  Crossref
• 15a Arias-Rotondo DM, McCusker JK. Chem. Soc. Rev. 2016; 45: 5803
  CrossrefPubMedSearch in Google Scholar
• 15b Hockin BM, Li C, Robertson N, Zysman-Colman E. Catal. Sci. Technol. 2019; 9: 889
  CrossrefSearch in Google Scholar
• 16a Bryden MA, Zysman-Colman E. Chem. Soc. Rev. 2021; 50: 7587
  CrossrefPubMedSearch in Google Scholar
• 16b Vega-Peñaloza A, Mateos J, Companyó X, Escudero-Casao M, Dell’Amico L. Angew. Chem. Int. Ed. 2021; 60: 1082
  CrossrefPubMedSearch in Google Scholar
• 16c Amos SG. E, Garreau M, Buzzetti L, Waser J. Beilstein J. Org. Chem. 2020; 16: 1163
  CrossrefPubMedSearch in Google Scholar
• 16d Lee Y, Kwon MS. Eur. J. Org. Chem. 2020; 2020: 6028
  CrossrefSearch in Google Scholar
• 16e Bobo MV, Kuchta JJ, Vannucci AK. Org. Biomol. Chem. 2021; 19: 4816
  CrossrefPubMedSearch in Google Scholar
• 17a Riente P, Noël T. Catal. Sci. Technol. 2019; 9: 5186
  CrossrefSearch in Google Scholar
• 17b Franchi D, Amara Z. ACS Sustainable Chem. Eng. 2020; 8: 15405
  CrossrefSearch in Google Scholar
• 18 De Vos D, Gadde K, Maes BU. W. Synthesis 2022; in press,
  Thieme Connect
• 19a Singh A, Teegardin K, Kelly M, Prasad KS, Krishnan S, Weaver JD. J. Organomet. Chem. 2015; 776: 51
  CrossrefSearch in Google Scholar
• 19b Koike T, Akita M. Inorg. Chem. Front. 2014; 1: 562
  CrossrefPubMedSearch in Google Scholar
• 20 Maes J, Mitchell EA, Maes BU. W. Base Metals in Catalysis: From Zero to Hero. In Green and Sustainable Medicinal Chemistry: Methods, Tools and Strategies for the 21st Century Pharmaceutical Industry, Chap. 16. Summerton L, Sneddon HF, Jones LC, Clark JH. Royal Society of Chemistry; Cambridge: 2016: 192-202
  CrossrefSearch in Google Scholar
• 21a Paria S, Reiser O. ChemCatChem 2014; 6: 2477
  CrossrefSearch in Google Scholar
• 21b Paria S, Reiser O. Visible Light and Copper Complexes: A Promising Match in Photoredox Catalysis. In Visible Light Photocatalysis in Organic Chemistry. Stephenson CR. J, Yoon TP, MacMillan DW. C. Wiley-VCH; Weinheim: 2018: 233-251
  CrossrefSearch in Google Scholar
• 21c Hossain A, Bhattacharyya A, Reiser O. Science 2019; 364: eaav9713
  CrossrefPubMedSearch in Google Scholar
• 21d Larsen CB, Wenger OS. Chem. Eur. J. 2018; 24: 2039
  CrossrefPubMedSearch in Google Scholar
• 21e Wenger OS. J. Am. Chem. Soc. 2018; 140: 13522
  Search in Google Scholar
• 22a Joshi-Pangu A, Lévesque F, Roth HG, Oliver SF, Campeau L.-C, Nicewicz D, DiRocco DA. J. Org. Chem. 2016; 81: 7244
  CrossrefPubMedSearch in Google Scholar
• 22b Tlili A, Lakhdar S. Angew. Chem. Int. Ed. 2021; 60: 19526
  CrossrefPubMedSearch in Google Scholar
• 23a Speckmeier E, Fischer TG, Zeitler K. J. Am. Chem. Soc. 2018; 140: 15353
  CrossrefPubMedSearch in Google Scholar
• 23b Natarajan P, König B. Eur. J. Org. Chem. 2021; 2021: 2145
  CrossrefSearch in Google Scholar
• 24 Hari DP, König B. Chem. Commun. 2014; 50: 6688
  CrossrefPubMedSearch in Google Scholar
• 25 Gisbertz S, Pieber B. ChemPhotoChem 2020; 4: 456
  CrossrefPubMedSearch in Google Scholar
• 26a Savateev A, Ghosh I, König B, Antonietti M. Angew. Chem. Int. Ed. 2018; 57: 15936
  CrossrefPubMedSearch in Google Scholar
• 26b Savateev A, Antonietti M. ChemCatChem 2019; 11: 6166
  CrossrefSearch in Google Scholar
• 27 Zhang T, Lin W. Chem. Soc. Rev. 2014; 43: 5982
  CrossrefPubMedSearch in Google Scholar
• 28a Geng K, He T, Liu R, Dalapati S, Tan KT, Li Z, Tao S, Gong Y, Jiang Q, Jiang D. Chem. Rev. 2020; 120: 8814
  CrossrefPubMedSearch in Google Scholar
• 28b Yang Q, Luo M, Liu K, Cao H, Yan H. Appl. Catal., B 2020; 276: 119174
  CrossrefSearch in Google Scholar
• 29 Lan G, Quan Y, Wang M, Nash GT, You E, Song Y, Veroneau SS, Jiang X, Lin W. J. Am. Chem. Soc. 2019; 141: 15767
  CrossrefPubMedSearch in Google Scholar
• 30 Knorn M, Rawner T, Czerwieniec R, Reiser O. ACS Catal. 2015; 5: 5186
  CrossrefPubMedSearch in Google Scholar
• 31 Martiny M, Steckhan E, Esch T. Chem. Ber. 1993; 126: 1671
  CrossrefPubMedSearch in Google Scholar
• 32 Kasha M. Discuss. Faraday Soc. 1950; 9: 14
  CrossrefPubMedSearch in Google Scholar
• 33 Rey YP, Abradelo DG, Santschi N, Strassert CA, Gilmour R. Eur. J. Org. Chem. 2017; 2017: 2170
  CrossrefPubMedSearch in Google Scholar
• 34a Elgrishi N, Rountree KJ, McCarthy BD, Rountree ES, Eisenhart TT, Dempsey JL. J. Chem. Educ. 2018; 95: 197
  CrossrefSearch in Google Scholar
• 34b Roth HG, Romero NA, Nicewicz DA. Synlett 2016; 27: 714
  CrossrefSearch in Google Scholar
• 35 Wu Y, Kim D, Teets TS. Synlett 2022; 33: 1154
  Thieme ConnectPubMedSearch in Google Scholar
• 36 Rehm D, Weller A. Isr. J. Chem. 1970; 8: 259
  CrossrefPubMedSearch in Google Scholar
• 37 Campagna S, Puntoriero F, Nastasi F, Bergamini G, Balzani V. Photochemistry and Photophysics of Coordination Compounds: Ruthenium . In Photochemistry and Photophysics of Coordination Compounds I . Balzani V, Campagna S. Springer; Heidelberg: 2007: 117-214
  CrossrefSearch in Google Scholar
• 38a Cismesia MA, Yoon TP. Chem. Sci. 2015; 6: 5426
  CrossrefPubMedSearch in Google Scholar
• 38b Gadde K, Mampuys P, Guidetti A, Ching HY. V, Herrebout WA, Van Doorslaer S, Abbaspour Tehrani K, Maes BU. W. ACS Catal. 2020; 10: 8765
  CrossrefSearch in Google Scholar
• 39a Gaida F, Griesbeck AG, Vollmer M. Photocatalysis: The Principles . In Science of Synthesis: Photocatalysis in Organic Synthesis, Chap. 3. König B. Thieme; Stuttgart: 2019: 3
  Search in Google Scholar
• 39b Kuijpers KP. L, Bottecchia C, Cambié D, Drummen K, König NJ, Noël T. Angew. Chem. Int. Ed. 2018; 57: 11278
  CrossrefPubMedSearch in Google Scholar
• 40 Żamojć K, Bylińska I, Wiczk W, Chmurzyński L. Int. J. Mol. Sci. 2021; 22: 885
  CrossrefPubMedSearch in Google Scholar
• 41 IUPAC Compendium of Chemical Terminology, 2nd ed. McNaught AD, Wilkinson A. Blackwell Scientific; Oxford: 1997. https//goldbook.iupac.org(accessed Sept 15, 2022
  CrossrefSearch in Google Scholar
• 42 Tibbetts JD, Bull SD. Adv. Sustainable Syst. 2021; 5: 2000292
  CrossrefPubMedSearch in Google Scholar
• 43 Besson M, Gallezot P, Pinel C. Chem. Rev. 2014; 114: 1827
  CrossrefPubMedSearch in Google Scholar
• 44 Beller M, Seayad J, Tillack A, Jiao H. Angew. Chem. Int. Ed. 2004; 43: 3368
  CrossrefPubMedSearch in Google Scholar
• 45a Courant T, Masson G. J. Org. Chem. 2016; 81: 6945
  CrossrefPubMedSearch in Google Scholar
• 45b Cao M.-Y, Ren X, Lu Z. Tetrahedron Lett. 2015; 56: 3732
  CrossrefSearch in Google Scholar
• 45c Koike T, Akita M. Chem 2018; 4: 409
  CrossrefSearch in Google Scholar
• 46a Cano-Yelo H, Deronzier A. J. Chem. Soc., Perkin Trans. 2 1984; 1093
  Crossref
• 46b Cano-Yelo H, Deronzier A. Tetrahedron Lett. 1984; 25: 5517
  CrossrefSearch in Google Scholar
• 46c Cano-Yelo H, Deronzier A. J. Chem. Soc., Faraday Trans. 1 1984; 3011
  Crossref
• 46d Cano-Yelo H, Deronzier A. J. Photochem. 1987; 37: 315
  CrossrefSearch in Google Scholar
• 47a Hironaka K, Fukuzumi S, Tanaka T. J. Chem. Soc., Perkin Trans. 2 1984; 1705
  Crossref
• 47b Ishikawa M, Fukuzumi S. J. Chem. Soc., Chem. Commun. 1990; 1353
  Crossref
• 47c Fukuzumi S, Mochizuki S, Tanaka T. J. Phys. Chem. 1990; 94: 722
  CrossrefSearch in Google Scholar
• 48a Hedstrand DM, Kruizinga WH, Kellogg RM. Tetrahedron Lett. 1978; 19: 1255
  CrossrefSearch in Google Scholar
• 48b Van Bergen TJ, Hedstrand DM, Kruizinga WH, Kellogg RM. J. Org. Chem. 1979; 44: 4953
  CrossrefSearch in Google Scholar
• 49a Okada K, Okamoto K, Morita N, Okubo K, Oda M. J. Am. Chem. Soc. 1991; 113: 9401
  CrossrefSearch in Google Scholar
• 49b Okada K, Okubo K, Morita N, Oda M. Tetrahedron Lett. 1992; 33: 7377
  CrossrefSearch in Google Scholar
• 50a Pac C, Ihama M, Yasuda M, Miyauchi Y, Sakurai H. J. Am. Chem. Soc. 1981; 103: 6495
  CrossrefSearch in Google Scholar
• 50b Ishitani O, Pac C, Sakurai H. J. Org. Chem. 1983; 48: 2941
  CrossrefSearch in Google Scholar
• 50c Majima T, Pac C, Nakasone A, Sakurai H. J. Am. Chem. Soc. 1981; 103: 4499
  CrossrefSearch in Google Scholar
• 51 Pandey G, Hajra S, Ghorai MK, Kumar KR. J. Am. Chem. Soc. 1997; 119: 8777
  CrossrefPubMedSearch in Google Scholar
• 52 Tomioka H, Ueda K, Ohi H, Izawa Y. Chem. Lett. 1986; 15: 1359
  CrossrefSearch in Google Scholar
• 53a Willner I, Tsfania T, Eichen Y. J. Org. Chem. 1990; 55: 2656
  CrossrefSearch in Google Scholar
• 53b Mandler D, Willner I. J. Chem. Soc., Perkin Trans. 2 1986; 805
  Crossref
• 53c Mandler D, Willner I. J. Chem. Soc., Chem. Commun. 1986; 851
  Crossref
• 54 Ischay MA, Anzovino ME, Du J, Yoon TP. J. Am. Chem. Soc. 2008; 130: 12886
  CrossrefPubMedSearch in Google Scholar
• 55 Nicewicz DA, MacMillan DW. C. Science 2008; 322: 77
  CrossrefPubMedSearch in Google Scholar
• 56 Narayanam JM. R, Tucker JW, Stephenson CR. J. J. Am. Chem. Soc. 2009; 131: 8756
  CrossrefPubMedSearch in Google Scholar
• 57 Marcus RA. Angew. Chem. Int. Ed. 1993; 32: 1111
  CrossrefPubMedSearch in Google Scholar
• 58 Nguyen JD, Tucker JW, Konieczynska MD, Stephenson CR. J. J. Am. Chem. Soc. 2011; 133: 4160
  CrossrefPubMedSearch in Google Scholar
• 59 Courant T, Masson G. Chem. Eur. J. 2012; 18: 423
  CrossrefPubMedSearch in Google Scholar
• 60 Pirtsch M, Paria S, Matsuno T, Isobe H, Reiser O. Chem. Eur. J. 2012; 18: 7336
  CrossrefPubMedSearch in Google Scholar
• 61 Hossain A, Engl S, Lutsker E, Reiser O. ACS Catal. 2019; 9: 1103
  CrossrefSearch in Google Scholar
• 62 Engl S, Reiser O. Chem. Soc. Rev. 2022; 51: 5287
  CrossrefPubMedSearch in Google Scholar
• 63 Huang H, Yu C, Zhang Y, Zhang Y, Mariano PS, Wang W. J. Am. Chem. Soc. 2017; 139: 9799
  CrossrefPubMedSearch in Google Scholar
• 64 Riente P, Pericàs MA. ChemSusChem 2015; 8: 1841
  CrossrefPubMedSearch in Google Scholar
• 65 Riente P, Fianchini M, Llanes P, Pericàs MA, Noël T. Nat. Commun. 2021; 12: 625
  CrossrefPubMedSearch in Google Scholar
• 66 Mirkovic T, Ostroumov EE, Anna JM, van Grondelle R, Govindjee, Scholes GD. Chem. Rev. 2017; 117: 249
  CrossrefPubMedSearch in Google Scholar
• 67a Yoo W.-J, Tsukamoto T, Kobayashi S. Org. Lett. 2015; 17: 3640
  CrossrefPubMedSearch in Google Scholar
• 67b Heitz DR, Tellis JC, Molander GA. J. Am. Chem. Soc. 2016; 138: 12715
  CrossrefPubMedSearch in Google Scholar
• 68 Neveselý T, Wienhold M, Molloy JJ, Gilmour R. Chem. Rev. 2022; 122: 2650
  CrossrefPubMedSearch in Google Scholar
• 69 Chen D.-F, Chrisman CH, Miyake GM. ACS Catal. 2020; 10: 2609
  CrossrefPubMedSearch in Google Scholar
• 70 Patra T, Bellotti P, Strieth-Kalthoff F, Glorius F. Angew. Chem. Int. Ed. 2020; 59: 3172
  CrossrefPubMedSearch in Google Scholar
• 71 Prat D, Wells A, Hayler J, Sneddon H, McElroy CR, Abou-Shehada S, Dunn PJ. Green Chem. 2016; 18: 288
  CrossrefPubMedSearch in Google Scholar
• 72a McElroy CR, Constantinou A, Jones LC, Summerton L, Clark JH. Green Chem. 2015; 17: 3111
  CrossrefSearch in Google Scholar
• 72b Abou-Shehada S, Mampuys P, Maes BU. W, Clark JH, Summerton L. Green Chem. 2017; 19: 249
  CrossrefSearch in Google Scholar
• 73 Monteith ER, Mampuys P, Summerton L, Clark JH, Maes BU. W, McElroy CR. Green Chem. 2020; 22: 123
  CrossrefPubMedSearch in Google Scholar
• 74 Pitzer L, Schäfers F, Glorius F. Angew. Chem. Int. Ed. 2019; 58: 8572
  Search in Google Scholar
• 75 Novaes LF. T, Liu J, Shen Y, Lu L, Meinhardt JM, Lin S. Chem. Soc. Rev. 2021; 50: 7941
  CrossrefPubMedSearch in Google Scholar
• 76 Sambiagio C, Sterckx H, Maes BU. W. ACS Cent. Sci. 2017; 3: 686
  CrossrefPubMedSearch in Google Scholar