Emerging Activation Modes and Techniques in Visible-Light-Photocatalyzed Organic Synthesis

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

Abstract

Visible light photocatalysis has evolved into a promising mild and sustainable strategy to access radicals. This field unlocks formerly challenging or even previously inaccessible organic transformations. In this review, an overview of some lesser-known modes of photochemical activation of organic molecules and several emerging techniques within the versatile field of visible light photocatalysis are discussed. These are illustrated by selected photocatalytic reactions, with particular attention given to the reaction mechanism.

1 Introduction

2 Advanced Photoactivation Modes

2.1 Photoinduced Hydrogen-Atom Transfer

2.2 Proton-Coupled Electron Transfer

2.3 Electron Donor-Acceptor Photoactivation of Organic Substrates

2.4 Excited-State Transition Metal Catalysis

3 Emerging Techniques

3.1 Dual Catalysis

3.2 Excited Radical Ion Photocatalysis

3.3 Upconversion Strategies and Other Two-Photon Mechanisms

3.4 Red and Near-Infrared Photocatalysis

4 Conclusions and Outlook

Biographical Sketches

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.

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.

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

Visible light photocatalysis is a rapidly growing field in organic synthesis and has emerged as a mild alternative to the use of high-energy UV photons, allowing the development of previously inaccessible novel radical-based organic transformations. The most frequently employed photocatalysts are precious metal complexes of Ru or Ir, whose electronic properties can be tuned by ligand manipulations. However, organic dyes have emerged as more sustainable alternatives and their redox properties can likewise be modified by structural modification. Furthermore, a trend towards the development of heterogeneous catalysts seeks to increase catalyst lifespan and reusability.

The most common activation modes are photoinduced electron transfer (PET) or photoinduced energy transfer (EnT) (Scheme [1]). The mechanism of PET or EnT starts with the absorption of light of appropriate energy, causing a photocatalyst (PC) to promote an electron, hereby generating a high-energy singlet state (S₁). This S₁ state generally undergoes intersystem crossing towards a long-living triplet state (T₁). The resulting excited-state photocatalyst (*PC) can interact in various ways with organic substrates. Despite PET and EnT being ubiquitous in modern visible-light-mediated photocatalysis, substrates can be photocatalytically activated in other ways, even without involving a photocatalyst. Since its revival about 15 years ago, visible light photocatalysis has matured into a diverse field consisting of various activation modes and links with other types of catalysis such as transition metal catalysis, organocatalysis, and electrocatalysis providing novel opportunities for organic synthesis. Furthermore, efforts are being made to unlock photocatalysis by harnessing less energetic parts of the visible spectrum. This has inherent advantages in terms of light penetrability which is an important factor when implementing these processes in chemical development and production or when dealing with biological tissue.

As visible light photocatalysis is becoming an important part of the synthetic toolbox of organic chemists, it is not surprising that a number of reviews and perspectives have recently appeared in the literature. However, most of them are either predominantly focused on a particular photoactivation mode (e.g., photoinduced electron transfer,[1] energy transfer,[2] hydrogen-atom transfer,[3] proton-coupled electron transfer,[4] electron donor-acceptor photoactivation,[5] excited-state transition-metal catalysis[6]) or discuss the synthetic applications of a specific class of photocatalyst (e.g., transition-metal-based complexes,[1b] [7] organic dyes,[1c,8] semiconductors[9]) in organic synthesis in great detail. This review provides both an accessible introduction to the more advanced photocatalytic activation modes as well as some emerging techniques. The basics of photocatalysis are covered in a complementary introductory review.[10]

This review first describes advanced activation modes in organic photocatalysis, with particular attention on the explanation of the reaction mechanisms (Section 2). Subsequently, a selection of emerging techniques in photocatalysis such as dual catalysis, excited radical ion chemistry, triplet-triplet annihilation upconversion, and near-infrared (NIR)/deep red light photocatalysis are discussed in Section 3. The objective of this review is to introduce some lesser-known concepts to (synthetic) chemists who are familiar with the basic concepts of photocatalysis and the most common activation modes (PET, EnT). Although the core principles behind the discussed activation modes in photocatalysis are disclosed, an in-depth discussion of the (extensive) literature illustrating applications of these techniques is outside the scope of this review, and for this readers are directed to state-of-the-art reviews and books. The synthetic examples included illustrating the activation modes and emerging techniques are either seminal to the field or particularly interesting and detailed from a mechanistic point of view.

Advanced Photoactivation Modes

Upon absorption of a photon, a photocatalyst (PC) promotes to its excited state. The excited-state photocatalyst (*PC) can engage with the reactants in different ways. The most common photoactivation modes involved in organic synthesis are photoinduced electron transfer (PET) and photoinduced energy transfer (EnT) which were covered in a separate basic introductory review on photocatalysis.[10] However, some other advanced activation modes of organic molecules exist that are covered in this section and these do not always involve a photocatalyst: photoinduced hydrogen-atom transfer (HAT) (Section 2.1), proton-coupled electron transfer (PCET) (Section 2.2), electron donor-acceptor (EDA) photoactivation (Section 2.3), and excited-state transition-metal (*TM) catalysis (Section 2.4).

Photoinduced Hydrogen-Atom Transfer

Photoinduced hydrogen-atom transfer (HAT) reactions provide unique opportunities in organic synthesis and allow regioselective C(sp³)–H functionalization of organic molecules (Scheme [2]).[3] In the HAT activation mode, a photocatalyst exploits the energy of a photon to trigger the selective homolytic cleavage of C(sp³)–H bonds of various hydrogen donors, including amides, aldehydes, ethers, and even alkanes, to obtain carbon-centered radicals. These radical intermediates are used to access carbon-carbon and carbon-heteroatom bonds via reaction with specific reactants. As depicted in Scheme [2]A, upon light absorption, the photocatalyst (PC) promotes to its excited state (*PC). Subsequently, it abstracts a hydrogen atom from a substrate to form an alkyl radical, which then undergoes radical reactions with a reactant. The catalytic cycle is subsequently closed through the removal of a proton and an electron, or a hydrogen atom (back-HAT) from the photocatalyst to the radical intermediates formed during the process. So far, the number of available photocatalysts for HAT reactions has been quite limited, being restricted to the families of aromatic ketones, polyoxometalates (e.g., decatungstate anion, [W₁₀O₃₂]^4–),[11] and uranyl salts [UO₂]²⁺ (Scheme [2]B).

The HAT process can be considered as a subclass of proton-coupled electron transfer (PCET, see Section 2.2), where the photocatalyst itself harbors the abstracted proton. The ease of hydrogen atom abstraction in simple alkanes normally follows the bond dissociation energies (BDE), so that abstraction occurs at 3° > 2° > 1° C–H bonds (Figure [1]A).[11] [12] However, for molecules containing polar groups, the use of the BDE values is not reliable, as polar effects in the homolytic bimolecular abstraction process transition state (S_H2 TS) play an important role in the selectivity of hydrogen abstraction by the decatungstate anion (Figure [1]B).[11] For example, hydrogen atom abstraction is disfavored in α-positions of electron-withdrawing groups, while it is especially favored in α-positions of electron-donating groups. The origin of this effect lies in the charge separation upon hydrogen atom abstraction when using the electronegative oxygen atoms of the decatungstate anion. A partial positive charge is imparted upon the carbon atom delivering the hydrogen atom, which is destabilized by α-electron-withdrawing groups (EWGs) and stabilized by α-electron-donating groups (EDGs). These effects only influence the positions close to polar functional groups. Farther from these, sterics influence the site-selectivity of the HAT process, since tetrabutylammonium decatungstate (TBADT) is a very large molecule (MW = 3320.24 g/mol) (Figure [1]B). For example, cyclohexanone did not show hydrogen atom abstraction in the α-position of the ketone, as these positions are disfavored due to polar effects. Only the β- and γ-methylenes were feasible reaction sites, with a statistical distribution. The substitution pattern of substituted cyclohexanones causes changes in this selectivity, with the presence of a 4-methyl causing increased γ-selectivity (80:20, which is 16:1 γ/β statistically distributed based on the number of hydrogens). The presence of the steric hindrance in 4-tert-butylcyclohexanone or 3,3,5-trimethylcyclohexanone completely suppressed hydrogen atom abstraction, as there is no viable way for the large decatungstate anion to approach the hydrogen atoms.

In an early report by C. L. Hill, published in 1993, alkanes were activated with TBADT under UV light irradiation.[13] The resulting carbon-centered radical adds to unactivated alkenes constructing novel C–C bonds. In this conceptually new C(sp³)–H bond activation, selectivity was limited to the BDE effect (3° > 2° > 1°) as no functional groups were present in the substrates. The influence of functional groups on this interesting process remained unstudied for many years.

For example, in 2015 Ryu, Fagnoni, and co-workers reported the β- or γ-site-selective C(sp³)–H alkylation of aliphatic nitriles using TBADT as the HAT photocatalyst in acetonitrile under light irradiation.[14] The TBADT photocatalyst requires a UV-A light source, usually a 365 nm LED, a Xenon lamp, or a compact fluorescent lightbulb (CFL), which also contains UV-A. The proposed mechanism of this reaction is outlined in Scheme [3]. After absorption of light, the excited decatungstate anion photocatalyst (*[W₁₀O₃₂]^4–) is able to selectively abstract a hydrogen atom from a C(sp³)–H fragment of the aliphatic nitrile, yielding a carbon-centered radical. Subsequently, this radical adds to an electron-deficient alkene, such as dimethyl maleate, leading to a radical in the carbonyl α-position. Subsequent, back-hydrogen atom transfer from the reduced decatungstate anion (H⁺[W₁₀O₃₂]^5–) leads to the alkylated reaction product and regenerates the decatungstate anion photocatalyst. Interestingly, TBADT is commercially available and furthermore easily synthesized from tetrabutylammonium bromide and sodium tungstate in a single step (Scheme [4]).[15]

The selectivity of the reaction in Scheme [3] can be explained using the principles outlined in Figure [1]. Due to the polar effect, the α-methylene is disfavored in all aliphatic nitriles. In butanenitrile, the γ-methyl C–H bond is stronger than the β-methylene C–H bond, explaining the β/γ 91:9 selectivity (Scheme [3]). Pentanenitrile has both β- and γ-methylene C–H bonds, but apparently the disfavorable inductive effect of the cyano group propagates to the β-position, making it relatively less attractive for hydrogen atom abstraction, favoring the γ-position. For 3-methylbutanenitrile and 4-methylpentanenitrile, full regioselectivity at the tertiary C–H bond was achieved due to the BDE difference with the methylene or methyl C–H bonds. Although adiponitrile showed the expected β-selectivity, glutaronitrile showed no reaction, as its β-position is twice disfavored due to the propagating inductive effect of the cyano group.

Proton-Coupled Electron Transfer

Proton-coupled electron transfer (PCET) is a redox process where a proton and an electron originating from one N-H or O-H bond are transferred, often in one concerted elementary step.[4a] It can facilitate photoredox catalytic steps when the corresponding electron transfer (SET) is either thermodynamically unfeasible or kinetically slow. This process can be achieved by the formation of a hydrogen bond between the abstracting proton and a base during the electron transfer. In this case, the proton and the electron travel from one bond of the substrate to two different species, respectively the base and the photocatalyst, which is denoted a ‘multisite’ oxidative PCET-step. Likewise, it is possible that a substrate receives a proton from an acid and an electron from a photocatalyst, therefore labelled a ‘multisite’ reductive PCET-step (Scheme [5]).

Oxidative PCET is complementary to HAT since hydrogen atoms cannot be easily abstracted if the N-H or O-H bond dissociation free energy (BDFE) is too large (Figure [2]).[4] For example, N–H bonds in N-alkyl amides or O–H bonds in aliphatic alcohols or carboxylic acids feature BDFE’s of over 105 kcal·mol^–1. There is no hydrogen atom acceptor that can form a stronger bond, so hydrogen atom abstraction is not thermodynamically feasible. Similarly, for processes where a hydrogen atom is added to the π-system of a substrate, the created vicinal radical destabilizes the new bond significantly, as is the case in ketones, imines, and olefins (BDFE < 35 kcal·mol^–1). Once again, no hydrogen atom donor features weaker bonds than that of the vicinal radical, as complexes with weaker bonds than metal hydrides (BDFE ≥ 50 kcal·mol^–1) are not stable and will spontaneously generate H₂. Therefore, hydrogen atom transfer to double bonds is thermodynamically not favored and hence not feasible. Recently, oxidative and reductive PCET unlocked these transformations, as exemplified by the formation of amidyl and ketyl radicals, respectively (Scheme [5]).[4]

In 2015, the Knowles group reported the photocatalytic carboamination of alkenes via a concerted proton-coupled electron transfer (PCET) strategy under visible light irradiation (Scheme [6]).[16] The reaction proceeds by a reductive quenching cycle. By employing a combination of the [Ir^III{dF(CF₃)ppy}₂(bpy)]PF₆ photocatalyst as electron acceptor and a soluble dibutyl phosphate co-catalyst as a base, oxidative PCET homolysis of an amide N–H bond leads to the formation of the amidyl radical in the first step of the catalytic cycle (oxidative PCET step). Subsequently, the amidyl radical undergoes 5-exo-trig cyclization followed by radical scavenging with methyl vinyl ketone to generate an α-carbonyl radical. Reduction by the reduced Ir^II photocatalyst and protonation of the formed α-carbanion subsequently generates the final desired reaction product.

An example featuring a reductive PCET step was disclosed in 2016 by the Rueping group, who reported an intramolecular reaction of salicylaldehyde derivatives via a concerted PCET strategy under visible light irradiation (Scheme [7]).[17] In this reaction, PCET facilitates the intramolecular addition of ketyl radicals to alkenes. The proposed reaction mechanism proceeds by the reductive quenching of *[Ir^III(ppy)₂(dtbbpy)]PF₆ by a tertiary amine, N,N-diisopropylethylamine (DIPEA), affording Ir^II and an amine radical cation. The resulting Lewis acidic radical cation interacts with the C=O bond of the salicylaldehyde derivative through a two-center/three-electron bond, which facilitates the reduction of the carbonyl of the ketone by Ir^II to a ketyl radical by lowering the energy barrier for the SET process. Alternatively, protonated DIPEA could also partake in PCET. Subsequently, the intramolecular addition of the ketyl radical to the styrene moiety and further hydrogen abstraction yields the desired reaction product.

Electron Donor-Acceptor Photoactivation of Organic Substrates

In recent years, synthetic strategies based on electron donor-acceptor (EDA) complexes have emerged that do not require a photocatalyst to generate radicals. An electron-donating molecule and an electron-accepting molecule can assemble to form a charge-transfer (CT) complex in the ground state and this molecular assembly is described as an electron donor-acceptor (EDA) complex; it absorbs visible light, even when the two original molecules only absorb in the UV region. Subsequently, an electron-transfer event occurs from the donor to the acceptor without the presence of a photocatalyst to form a radical ion pair. Normally, this radical ion pair is in equilibrium with the original donor and acceptor through back-electron transfer (BET), but the expulsion of a leaving group on the acceptor drives the equilibrium towards the radical species, especially if the elimination of the leaving group is an efficient irreversible process (Scheme [8]).[5b] [c] Although seminal examples of EDA complex photochemistry were published in the 20th century, its potential benefits for organic synthesis as part of the broad field of photocatalysis were not explored.

EDA photochemistry was reintroduced as part of modern photocatalysis in 2013, when Tobisu and Chatani described the arylation of (hetero)arenes using diaryliodonium salts as aryl radical sources under [Ir(ppy)₂(bpy)]PF₆ photocatalysis (Scheme [9]).[18] Surprisingly, when pyrroles were used as substrates, the photoredox catalyst could be omitted due to the formation of an EDA complex between the electron-rich pyrroles and the electron-poor diaryliodonium salts. Upon electron transfer, a pyrrole radical cation and aryl radical are formed, concomitantly expelling ArI, which combine to form the most stable Wheland intermediate; deprotonation finally leads to the 2-arylated pyrrole. As expected, the reaction was more efficient if electron-poor diaryliodonium salts were applied, and was completely suppressed for X = OMe.

Independently, and in the same year, the Melchiorre group described an enantioselective α-alkylation of aldehydes in the presence of a chiral pyrrolidine as an asymmetric organocatalyst (Scheme [10]).[19] The mechanism commences with the formation of a chiral enamine from the aldehyde and the chiral pyrrolidine catalyst. Next, an EDA complex could form between the electron-rich chiral enamine and the electron-poor α-bromoacetophenone derivatives or benzyl bromides featuring strong electron-withdrawing groups (Scheme [10]). Upon light absorption, a radical ion pair is formed, where the α-bromoacetophenone or benzyl bromide radical anion quickly expels a bromide anion. The corresponding benzyl or 2-oxo-2-arylethyl radical is still present in a solvent cage with the enamine radical cation and adds to its double bond, forming the iminium cation. Hydrolysis of the iminium sets free the α-alkylated aldehyde concomitantly with the regeneration of the chiral co-catalyst.

Excited-State Transition Metal Catalysis

Recently, visible-light-induced excited-state transition metal catalysis has attracted significant attention for the development of various chemical transformations without the need for an exogenous photocatalyst. Here, metal complexes (Mn, Co, Cu, and Pd) are used that act both as photocatalysts and as reactive catalysts for cross-coupling reactions (Scheme [11]).[20] In particular, palladium catalysis mediated by visible light irradiation has matured into an attractive strategy for inducing SET reactions using unactivated alkyl iodides/bromides.[21] Contrary to classical ground-state Pd⁰-catalyzed reactions that typically proceed by two-electron redox processes, the mechanisms of excited-state Pd catalysis typically involve single electron transfer. Early studies by Gevorgyan,[22] Fu,[23] Yu,[24] Glorius,[25] and others[26] revealed that photoexcitation of Pd⁰ favors the single-electron activation of alkyl halides resulting in the formation of a nucleophilic alkyl radical Pd^IX hybrid intermediate, featuring both radical and classical Pd-type behavior.

For example, in 2017 the Gevorgyan group disclosed a room temperature, visible-light-induced Heck reaction between classically strenuous α-heteroatom-substituted alkyl halides with vinylated (hetero)arenes (Scheme [12]).[22b] Among others, allylic silanes, boronates, germanes, phosphonates, and tosylates could be synthesized from the corresponding α-substituted methyl iodides. Under adapted conditions, making use of the air, moisture, and thermally stable XantPhos Pd G3 Buchwald precatalyst and DIPEA as a base, R¹ = stannanes, phthalimides, and pivalates could also be applied. Interestingly, these substrates failed under classic thermal conditions, making the visible light approach complementary to the well-established classical Heck reaction. The authors attempted a few alkyl bromides, which likewise delivered the desired products albeit in lower yield (39–80%). The majority of the selected alkyl halide substrates cannot undergo β-hydride elimination due to the presence of the α-heteroatom substituent. The authors nevertheless showed that the presence of β-hydrogen atoms is tolerated by their procedure by applying tert-butyl iodide, cyclohexyl iodide, and n-decyl iodide, resulting in comparable yields and superior E/Z selectivity in comparison with thermally induced Heck conditions. Possibly, the mild reaction conditions circumvent the thermal isomerization of the reaction products.

Simultaneously, the Fu group proposed that β-hydride elimination of the alkyl halide in a Heck reaction could be suppressed by visible light irradiation (Scheme [13]).[23b] Through excitation, the alkyl-Pd^II species undergoes a geometric change from a square planar to a tetrahedral species, which is in equilibrium with its alkyl radical Pd^ILX hybrid intermediate, thereby preventing true Pd-alkyl bond formation and circumventing undesired β-hydride elimination of the alkyl halide. Fu’s conditions feature lower catalyst and ligand loading, and showed a broad scope of both 1°, 2°, and even 3° alkyl bromides featuring β-hydrogen atoms. Remarkably, an excellent E-selectivity was obtained, similarly to thermal Heck reactions with aryl halides, while thermal Heck reactions of 1° or 2° alkyl halides thermally often delivered mixtures of E/Z isomers and 3° alkyl halides featuring β-hydrogen atoms were thermally even inaccessible. Mechanistically, the authors propose an irradiation-induced ‘single electron transfer oxidative addition’. The ground-state Pd(PPh₃)₂Cl₂ complex has an absorption band of around 350 nm, reaching up to 460 nm. For this reason, a direct light-induced single electron transfer towards the hybrid alkyl radical Pd^I-Br is possible. Furthermore, continuous irradiation might circumvent radical recombination towards a Pd^II-alkyl complex, which is prone to β-hydride elimination.

Besides its potential to solve problems with existing thermal Pd-catalyzed reactions, visible-light-mediated Pd-catalysis can also benefit from the inherent radical character of the R^• Pd^IX hybrid intermediate. For example, in 2016, the Gevorgyan group first generated an aryl radical Pd^I-I hybrid intermediate from the corresponding aryl iodide and Pd⁰ complex and exploited its radical character in the selective oxidation of silyl ethers into silyl enol ethers (Scheme [14]).[27]

The proposed mechanism commences with the excitation of the Pd(0) complex. For the generation of the aryl radical Pd^I-I hybrid, the authors considered two possibilities. Either a direct single electron transfer leads to the aryl radical Pd^I-I hybrid, or a conventional oxidative addition leads to Pd^II, which is subsequently excited to generate the aryl radical Pd^I-I hybrid. Since the authors were unable to synthesize the oxidative addition product, possibly due to the steric hindrance of the bulky silyl group next to the iodide participating in oxidative addition, the first pathway was deemed more likely.

The resulting aryl radical Pd^I-I hybrid intermediate is potent enough to intramolecularly abstract the α-hydrogen atom of the alcohol generating another alkyl Pd^I-I radical hybrid. From here, multiple pathways can lead to the desired product. The Pd^I can recombine with the alkyl radical, forming Pd^II allowing β-hydride elimination towards the desired alkene product. The Pd⁰ catalyst is then regenerated by the reductive elimination of HI. Alternatively, the Pd^I center can perform a direct hydrogen atom abstraction from the β-position of the alcohol to form the desired product directly. A third option is the oxidation of the radical by Pd^I towards the cation, concomitantly generating Pd⁰ (not shown). Next, β-deprotonation leads to the desired reaction product.

This example shows the synergy between classical transition metal catalytic steps (β-hydride elimination) and the radical generating properties of visible light photocatalysis (HAT step). Later, this procedure was generalized for the selective remote β–γ, γ–δ, or δ–ε dehydrogenation of aliphatic (α-iodo)silyl ethers.[28]

An important side-note is the lack of standardization of the used mechanism and terms in these early examples of visible-light-mediated palladium-catalyzed reactions. For example, originally a direct β-hydride elimination was written from the second Pd^I-X hybrid radical, without a recombination step for the reaction in Scheme [12].[22b] Likewise, in Scheme [13], the addition of the hybrid alkyl Pd^I-Br radical to the alkene was written as a ‘radical insertion’, immediately delivering the recombined product without the formation of a second hybrid alkyl Pd^I-Br radical.[23b] For educational purposes, we chose to merge both proposed mechanisms, especially since no detailed studies were conducted by the original authors regarding these steps of the mechanism. Furthermore, many other possible pathways could lead to the alkene, such as a direct hydrogen atom abstraction (Scheme [14]).[27]

Finally, the Fu group denoted the SET from the excited Pd⁰L_n complex to the alkyl bromide as an irradiation facilitated oxidative addition through single-electron transfer.[23b] For all clarity, we deliberately reserved the term ‘oxidative addition’ for the formal two-electron process in classic transition metal catalysis, and chose to denote this step as a single-electron transfer (SET), underlining the similarity with other photocatalytic transformations. It is not always clear whether the ground-state palladium complex first undergoes light absorption, and then undergoes SET with a substrate or vice versa first undergoes a classic oxidative addition, which then is irradiated to form the hybrid Pd^I-X radical. Some authors observe an increased absorption of the catalyst in the presence of the substrate. This could be indicative of a continuous absorption of the hybrid alkyl Pd^I-X radical circumventing its recombination.[23b]

Emerging Techniques

Dual Catalysis

Dual catalysis is the merger of visible light photocatalysis with another type of catalysis, such as transition metal catalysis, (Lewis) acid catalysis, or organocatalysis (Figure [3]).[29] Here, the two catalysts (a photocatalyst and a co-catalyst) work together to provoke a chemical transformation that is not accessible using either catalyst on its own. The combination of photocatalysis with transition metal catalysis is known as metallaphotocatalysis, and it has emerged as a powerful strategy for cross-coupling reactions.[21] [29b] [c] [30] It allows for replacing classical cross-coupling nucleophiles involved in transmetalation by reactants that in situ generate radicals that add to the transition metal complex. While the former process does not change the oxidation state, the latter increases it with +I. Such an alternative pathway is not achievable through conventional transition metal catalysis and enables the selective formation of carbon–carbon and carbon–heteroatom coupled products under mild reaction conditions in an unconventional manner (Scheme [15]).[30`] [b] [c] Various transition metal catalysts, including palladium, gold, rhodium, nickel, copper, cobalt, or iron complexes, have been used. One of the earliest examples of metallaphotocatalysis is in Pd-catalyzed directed C–H functionalization.[31]

Recently, dual photoredox/nickel catalysis has received increased attention in cross-coupling reactions to forge C(sp²)–C(sp²), C(sp³)–C(sp³), and C(sp³)–C(sp²) bonds. Nickel is particularly attractive as it is a base metal that can access a wide range of oxidation states, i.e. 0, +I, +II, +III.[30e] [32] The photocatalytic cycle typically commences by single-electron oxidation of a reactant, generating an alkyl radical through a reductive quenching cycle. Examples of common reactants are alkanecarboxylates, alkylborates or -silicates, or 4-alkyl-substituted Hantzsch esters. A Ni⁰ complex can undergo radical recombination with the aforementioned alkyl radical followed by an oxidative addition with an alkyl/aryl halide, or vice versa. This leads to a Ni^III species that undergoes reductive elimination thus generating the cross-coupled product. The Ni^I complex can consequently be returned to its original Ni⁰ complex by accepting an electron from the reduced photocatalyst, hereby closing both the photocatalytic and transition metal catalytic cycles.

In 2019, the Nevado group developed a dual ruthenium photo- and nickel transition-metal-catalyzed three-component alkene difunctionalization reaction using alkylsilicates and aryl bromides as reactants (Scheme [16]).[33] Alkyl bis(catecholo)silicates are known to undergo facile SET oxidation by a reductive quenching pathway with a variety of photocatalysts producing C(sp³)-centered radicals. In this case, the excited [Ru(bpy)₃]Cl₂ photocatalyst (*E _red = +0.77 V vs. SCE in MeCN) oxidizes alkylsilicates (E _ox = +0.3 to +0.9 V vs. SCE in DMF) delivering the corresponding alkyl radical, which is captured by an alkene substrate to generate the most stable carbon-centered radical. The reaction of this carbon-centered radical intermediate with Ni⁰ generates a transient alkyl-Ni^I complex, which upon oxidative addition with the aromatic halide (Ar-X) produces the key Ar-Ni^III-alkyl intermediate (Scheme [16], path A). Alternatively, the same species can be formed as a result of the oxidative addition of the aromatic halide (Ar-X) onto Ni⁰ followed by radical recombination with the carbon-centered radical intermediate (path B). In both cases, reductive elimination of an Ar-Ni^III-alkyl intermediate subsequently delivers the difunctionalized alkene addition product together with a Ni^I complex. The catalytic cycle is closed by oxidation of the Ru^I species by Ni^I thus regenerating the Ru^II photocatalyst and the active Ni⁰ transition metal catalyst, respectively. Besides tolerating a variety of functional groups and its operational simplicity, this method allowed the transformation of various alkene reactants including acrylates, acrylonitriles, vinylboronic esters, and allylic acetates.

Soon after, the Molander group published a three-component carbodifunctionalization of electron-deficient alkenes using alkyltrifluoroborate radical precursors and aryl bromides as cross-coupling partners (Scheme [17]).[34] The excited [Ir{dF(CF₃)ppy}₂(bpy)]PF₆ photocatalyst (*E _red[*Ir^III/Ir^II] = +1.32 V vs. SCE) oxidizes the organotrifluoroborate (E _ox = +1.26 V vs. SCE for potassium tert-butyltrifluoroborate) to deliver an alkyl radical. The addition of this radical to an electron-deficient alkene substrate subsequently affords a stabilized carbon-centered radical. After capture by a Ni⁰ complex, oxidative addition of the aryl bromide again leads to a Ni^III complex. Reductive elimination delivers the alkene difunctionalization product and Ni^I. The reaction of the reduced photocatalyst with the Ni^I complex regenerates both catalysts. A wide variety of alkyltrifluoroborates and (hetero)aryl bromides were joined across the double bond of vinylboronic esters, and some selected additional other electron-deficient alkenes.

In the previous examples, the photocatalyst caused the formation of radicals through a single electron transfer. However, instances have been reported where the precious metal-based or organic photocatalyst performs energy transfer towards the nickel catalyst.[35] For example, in 2019 the Miyake group reported a visible-light-mediated dual organophoto-/Ni-catalyzed C(sp²)–N cross-coupling between aryl bromides and 1° or 2° amines (Scheme [18]).[35b] Mechanistic studies showed the possibility of a Förster energy transfer from the excited phenoxazine organophotocatalyst to the Ni^II(amine)_XBr₂ complex, which causes an intramolecular electron transfer from the amine ligand to the nickel center, creating a Ni^I species and an amine radical cation. This radical cation can abstract a hydrogen atom from another amine molecule, generating a neutral amine radical. Subsequently, oxidative addition of aryl bromide on Ni^I generates a Ni^III species. Finally, the transition metal catalytic cycle is closed by the radical coupling of the aryl and amine moiety, furnishing the desired product. Under these conditions, a variety of amines were coupled with both electron-poor and electron-rich aryl halides, although the latter suffered from lower yields.

Similar to the trend in conventional photocatalysis, where heterogeneous alternatives for homogeneous catalysts have the potential to ease catalyst recycling, the excitation of nickel complexes by energy transfer can also be achieved from the heterogeneous semiconductor mesoporous graphitic carbon nitride (mpg-CN) (Scheme [19]). In 2021, the König group disclosed a photocatalytic activation and arylation of the C(sp³)–H bond adjacent to the nitrogen atom of amides using aryl bromides and chlorides under dual energy transfer/nickel catalysis.[36] The mechanism commences with the oxidative addition of aryl halide to deliver a Ni^II complex. The excited-state *mpg-CN catalyst partakes in energy transfer with the Ni^II complex, triggering the homolytic cleavage of the Ni^II–X bond, generating a halogen radical and a Ni^I complex. Subsequently, the halogen radical abstracts a hydrogen atom from the amide [BDE(H-Br/Cl) = ca. 88/102 kcal·mol^–1 vs. BDE(α-amino C–H) = 89–94 kcal·mol^–1], generating a new radical that reattaches to the Ni^I complex, regenerating Ni^II. A second EnT promotes reductive elimination finally generating the desired arylated amide, closing the transition metal catalytic cycle. Remarkably, the method tolerates many unprotected functional groups such as primary amides, sulfonamides, carboxylic acids, alcohols, and boronic acids.

Besides the fruitful combination of photocatalysis and nickel catalysis, other types of catalysis have been combined with photocatalysis as well, where both SET and energy transfer can be involved in the photocatalysis.[29b] Enantioselective dual photocatalysis/organocatalysis has, since the seminal work by the MacMillan group, matured into a field of its own.[37] In 2008, the MacMillan group first demonstrated the merger of enamine/iminium organocatalysis and photoredox catalysis for the α-alkylation of aldehydes (Scheme [20]).[38] The mechanism commences with the formation of an enamine from the chiral amine and aldehyde. Next, attack of a photochemically generated alkyl radical on the enamine occurs via the open Si-face, as the Re-face is blocked by the methyl group of the enamine. The resulting α-amino radical is oxidized by the excited photocatalyst *[Ru(bpy)₃]²⁺ in a reductive quenching cycle, generating [Ru(bpy)₃]⁺, which is able to reduce (E _red= –1.33 V vs. SCE in MeCN) alkyl bromides with an electron-withdrawing group in the α-position to the bromine atom (e.g., E _red = –0.49 V vs. SCE in MeCN for bromoacetophenone) to an alkyl radical and a bromide anion. Finally, the resulting iminium is hydrolyzed, liberating the desired alkylated aldehyde, concomitantly closing the organocatalytic cycle. After this pioneering report, the MacMillan group published a plethora of other enantioselective transformations using dual photoredox/enamine catalysis.[39]

Besides the use of enamine catalysis, the combination of photocatalysis with Lewis acid catalysis using chiral ligands has also become a powerful strategy for enantioselective synthesis.[37] The Yoon group developed a catalytic asymmetric [2+2] photocycloaddition of 2′-hydroxychalcones with styrenes, employing [Ru(bpy)₃](PF₆)₂ photocatalyst and a chiral scandium Lewis acid co-catalyst. (Scheme [21]).[40] This method provided a wide variety of chiral cyclobutanes in good yields with moderate diastereoselectivities and excellent enantioselectivities. The Lewis acid catalyst Sc^III activates the 2′-hydroxychalcones, decreasing their excited-state triplet energy (E _T) from 54 kcal·mol^–1 to 33 kcal·mol^–1 (Scheme [21]). Lowering the triplet energy allows the [Ru(bpy)₃](PF₆)₂ photocatalyst (E _T = 49.0 kcal·mol^–1) to selectively sensitize the Lewis acid coordinated substrate via an exergonic energy transfer process. Next, the produced chiral triplet intermediate readily engages in [2+2] photocycloaddition with styrene to afford the corresponding cyclobutane reaction product with high enantioselectivity.

Another type of organocatalysis is HAT (hydrogen-atom transfer) catalysis, which has proven to be particularly important in dual catalysis. The Nicewicz group developed efficient anti-Markovnikov olefin functionalization reactions by exploiting this strategy. For example in 2014, they developed a photoredox-catalyzed anti-Markovnikov addition of mineral acids (e.g., HF and HCl) to styrenes using the 9-mesityl-10-phenylacridinium tetrafluoroborate (Mes-Acr⁺-Ph BF₄ ^–) photocatalyst and a thiol/disulfide organic co-catalyst.[41] Triethylamine trihydrofluoride (Et₃N·3HF) was used as the HF source and 4-nitrophenyl disulfide was selected as the HAT catalyst (Scheme [22]).[41] The excited acridinium catalyst (*E _red = +2.09 V vs. SCE in MeCN) is quenched by the styrene (E _ox = +1.74 V vs. SCE in MeCN) to generate an alkene radical cation that is susceptible to attack by a range of nucleophiles. The initial addition of a nucleophile to the alkene radical cation affords a carbon-centered radical, which is trapped by a thiophenol HAT catalyst to afford hydrofunctionalization product. Thiophenol can be generated in situ from aryl disulfide,[42] as the reduced state of the photocatalyst (E _1/2 = –0.58 V vs. SCE in MeCN) facilitates the electron transfer to the thiyl radical generating a thiolate, which takes up the proton from HF. The reduction potential of the thiyl radical to the anion is not tabulated but is expected to be a small negative number.

Excited Radical Ion Photocatalysis

A variety of catalysts with various excited-state potentials have been designed and successfully applied in a variety of different reaction types since the rebirth of visible light photoredox catalysis. Nevertheless, the excited-state redox potentials of these photocatalysts are directly related to their ground-state potentials and excited-state energy E _0,0. Although these values can be tailored to a certain extent by variations in the catalyst structure, the use of one visible photon and its associated energy imparts an inherent limitation to the accessible potentials. For this reason, potentials beyond –2.00 and +2.00 V vs. SCE remained difficult to achieve. Unfortunately, some redox processes require such harsh conditions and are inaccessible by conventional methods, such as the reduction of aryl chlorides (e.g., E _red = –2.72 V vs. SCE for 1-chloro-4-ethylbenzene)[43] to furnish aryl radicals.

Consecutive photon-induced electron transfer (conPET) is an attractive approach to circumvent these limitations by harnessing the energy of two visible photons in one photocatalytic cycle (Figure [4]A).[44] Upon excitation by a first visible photon, the excited-state photocatalyst is either reductively or oxidatively quenched by a sacrificial electron donor or acceptor. Next, instead of immediately closing the cycle by a second SET, the intermediate species absorbs a second photon, reaching a potent radical cation oxidant or radical anion photoreductant. Interestingly, these highly reducing or oxidizing radical ions can also be generated by the excitation of an electrochemically generated radical ion (Figure [4]B), named electro-mediated photoredox catalysis (e-PRC). Note that these radical ion catalysts work from an excited doublet state, as they feature single unpaired electrons, in contrast with conventional photoredox cycles which work from excited triplet (two unpaired electrons) or singlet (no unpaired electrons) states.

In 2014, the König group reported the dehalogenation of aryl iodides, bromides and even chlorides by employing a perylene bisimide photocatalyst, N,N-bis(2,6-diisopropylphenyl[perylene-3,4,9,10-bis{dicarboximide}]) (PDI), under blue LED irradiation (Scheme [23]).[45] Furthermore, by changing the solvent from DMF to DMSO and upon addition of pyrroles as radical trapping agents, the dehalogenation product could be suppressed in favor of the arylation product. Mechanistic studies show that the excited PDI* (*E _red = –0.37 V vs. SCE) can be reduced by sacrificial triethylamine (E _ox = –0.83 V vs. SCE) to form a colored radical anion PDI^•–, which can absorb a second photon. The resulting highly reducing excited radical anion *PDI^•– is able to perform a single electron transfer to an aryl halide, forming the aryl halide radical anion and regenerating ground-state PDI. After scission of the C–X bond, the aryl radical is liberated, which can either abstract a hydrogen atom from triethylamine radical cation or the solvent generating the dehalogenated product, or undergo C–C bond formation by adding to pyrrole. Here, a subsequent hydrogen atom abstraction is necessary to form the desired product.

Later, several other organic photocatalysts, including rhodamine 6G,[46] 9,10-dicyanoanthracene (DCA),[47] anthraquinone,[48] benzo[ghi]perylene (BPI),[49] and acridinium salts (Mes-Acr⁺)[43] were reported to be efficient reductive conPET catalysts for aryl halide reduction, Birch reduction, and reductive sulfonamide deprotection.

In 2018 and 2020, Wagenknecht and Rombach disclosed the α-alkoxypentafluorosulfanylation of styrenes in the presence of aliphatic alcohols using the oxidative conPET catalyst N-phenylphenothiazine (PTH) under 365 nm LED irradiation (Scheme [24]).[50] The PTH catalyst is excited by 365 nm LED irradiation and performs an initial electron transfer to the SF₆, concomitantly generating SF₆ ^•– and PTH^•+. The SF₆ ^•– fragments into the SF₅ ^• radical and a fluoride anion, which is scavenged by the catalytic borane. PTH^•+ is able to absorb a second photon in order to reach the highly oxidative *PTH^•+ doublet excited state, which is able to oxidize styrene derivatives. The addition of the alcohol to the alkene radical cation, followed by deprotonation and the addition of the SF₅ ^• radical finally leads to the desired difunctionalized product. Although this seminal example employs a UV-A light source and suffers from low yields, it interestingly features a non-sacrificial oxidant to attain the highly oxidizing excited PTH^•+. Earlier, the authors disclosed a related α-fluoropentafluorosulfanylation of styrenes, which featured a more limited scope.[50a]

In 2021, the Wickens group used the same oxidative conPET catalyst for unactivated aryl oxidation under visible light irradiation, allowing the formed aryl radical cation to be trapped with pyrazole nucleophiles (Scheme [25]).[51] Interestingly, the necessary sacrificial oxidant was provided by an oxygen atmosphere. The mechanism commences through the excitation of the PTH catalyst, which is oxidatively quenched by oxygen to form a superoxide anion PTH^•+. The authors found that the presence of superoxide anion caused back-electron donation (BET), inhibiting the reaction. For this reason, a lithium salt co-catalyst was added to encourage disproportionation of superoxide towards peroxide and oxygen, thereby suppressing BET. The PTH^•+ next absorbs a second photon, generating the highly oxidative excited radical cation *PTH^•+, which allows the oxidation of benzene derivatives. The resulting arene radical cations can be trapped by pyrazole. The product could be re-aromatized by deprotonation after SET oxidation. Although this method allows the challenging oxidation of an arene, the reaction required solvent quantities of arene, except for the more electron-rich mesitylene or m-xylene as these have a lower oxidation potential than benzene.

Since true conPET mechanisms require the consecutive absorption of two photons, both the initial photocatalyst and the generated radical ion must absorb in the visible region. Additionally, sacrificial oxidants or reductants are typically used to generate the absorbing radical ions, generating waste. These limitations have sparked the development of mechanistically similar one-electron variants where electrochemical mediators themselves are used as photocatalysts. As part of the much broader field of electro-mediated photoredox catalysis (e-PRC), where electrochemical and photocatalytic steps are merged,[52] the excitation of electrochemically generated radical ions creates species with exceptionally high redox potentials (Figure [5]B).[53] The process is schematically represented by the MO diagrams in Figure [5]A.[52b] In order to generate a highly reducing excited-state radical anion, electrochemical oxidation provides the LUMO (lowest unoccupied molecular orbital) with an electron, hence becoming the SOMO (singly occupied molecular orbital) of the radical anion. By absorption of a photon, an electron is excited from the HOMO (highest occupied molecular orbital) to the SOMO causing SOMO-HOMO inversion in the resulting excited radical anion. A highly oxidative excited-state radical cation can be obtained by electrochemical oxidation, which removes an electron in the HOMO, hence becoming the SOMO of the radical cation. By absorption of a photon, an electron is excited from a lower MO to the vacancy in the SOMO, so that it becomes the HOMO of the excited radical cation, and the ground-state MO logically becomes a SOMO. The result is a high energy excited doublet state.

A practical disadvantage of this electro-mediated photoredox (e-PRC) approach is the requirement of specialized set-ups, which are often custom-built. These are either transparent undivided cells or often divided glass cells with a membrane frit. As a lack of standardization of the reaction set-up might cause reproducibility issues, usually commercially available electrophotochemical set-ups for hydrogen gas production are modified to perform these reactions. Although no sacrificial oxidants or reductants are required with e-PRC, an electrolyte is often necessary, which also increases the process mass intensity when not recycled. Furthermore, meticulous control experiments are needed in order to prove that both the applied potential, the light irradiation, and the applied electro-photocatalyst are essential for the reaction.

Although an early report from 1979–1981 by Moutet and Reverdy proposed that 1,1-diphenylethylene is oxidized to its radical cation by a photoexcited electrochemically generated phenothiazine radical cation[54] leading to a cyclodimeric products, the field stagnated with the emergence of conventional photocatalysis and electrosynthesis. A resurgence in the field began in 2019 with the work of the Lambert group, who disclosed the selective oxidative coupling of benzene and pyrazole derivatives using a trisaminocyclopropenium (TAC) cation electrophotocatalyst (Scheme [26]).[55] The mechanism proceeds by the electrochemical oxidation of the TAC cation to the deep red radical dication, which is able to absorb visible light to create a highly oxidizing photoexcited radical dication (E _red = +3.33 V vs. SCE). This highly oxidizing species is able to accept an electron from benzene via single electron transfer (E _ox = +2.48 V vs. SCE), hence generating the benzene radical cation, which is trapped by the nitrogen-containing 5-membered heterocycles. After C–N bond formation, the radical intermediate is oxidized by electrochemically generated TAC radical dication, thereby generating a cation that can rearomatize after deprotonation. In the method, acetic acid was added as a proton source for the reduction of H⁺ at the counter electrode and LiClO₄ as an electrolyte. The authors show the synergy of electrocatalysis and photocatalysis: if the reaction was attempted purely electrochemically at high voltages (up to +3.00 V) polymerization occurred, resulting in low yields. The electrophotochemically generated TAC radical dication was later employed in other reactions,[56] for example as a potent HAT catalyst, allowing hydrogen atom abstraction in the α-position of ethers.[57] The resulting C-centered radicals were trapped with isoquinolines, other azoles, and electron-poor olefins.

A seminal example of a photochemically excited electrogenerated radical anion was disclosed by Lambert and Lin in 2020.[58] An electrochemical reduction of 9,10-dicyanoanthracene (DCA) forms its radical anion (DCA^•–), which absorbs blue light to generate the highly reducing excited radical anion *DCA^•– (E _ox = –3.20 V vs. SCE), capable of reducing aryl bromides and chlorides. Subsequently, the generated aryl radical could be trapped by a variety of agents. With bis(pinacolato)diboron, borylated products were obtained (Scheme [27]). Interestingly, aryl chlorides bearing an electron-donating moiety could still be reduced (E _red up to –2.90 V vs. SCE for 4-chloroanisole), indicating the extreme reductive power of *DCA^•–, especially compared to other excited radical anions such as *PDI^•–, which could only reduce aryl chlorides bearing electron-withdrawing groups (cf. Scheme [23]).

In 2021, the groups of Barham and König reported a selective C(sp³)–O cleavage of phosphinated alcohols; generating the corresponding alkenes (Scheme [28]).[59] The electrochemical reduction of a substituted naphthalenemonoimide ( ⁿ BuO-NpMI) (E _red = –1.3 V vs. SCE) forms its radical anion ( ⁿ BuO-NpMI^•–) that absorbs blue light to generate the highly reducing excited radical anion * ⁿ BuO-NpMI^•– (E _ox = –3.8 V vs. SCE), which transfers an electron to the phosphinated alcohol (E _red = –2.2 to –2.6 V vs. SCE). The formed radical anion then undergoes C(sp³)–O scission, concomitantly liberating a carbon-centered radical and a phosphinate anion. A second reduction (E _red = –1.4 V vs. SCE) triggers the expulsion of the halogenide which leads to the desired product. Interestingly, (pseudo)halogenide substituents on the arene moiety are well tolerated (see Schemes 23 and 27 for comparison).

The emergence of (electro)photochemically generated excited radical cation/anion chemistry nuanced certain basic principles of photocatalysis. For example, the lifetime of the triplet excited state of the employed photocatalyst is an important parameter to assess the feasibility of conventional bimolecular quenching events. However, for many excited radical anion/cation catalysts generated by conPET or e-PRC, lifetimes in the order of tens to hundreds of picoseconds were determined,[60] which clearly prohibits any diffusion-controlled bimolecular processes. When bimolecular reactivity is nonetheless observed, precomplexation of the substrate with the ground-state radical anion/cation catalyst is most likely to be involved.[59] [61] Unfortunately, finding experimental evidence for this ground-state preassembly remains difficult, as its effect on UV-vis and EPR spectra is often absent.[49] [59] However, when investigating the mechanism of the tri(biphenyl-4-yl)amine (TpBPA) catalyzed N-arylation of pyrazoles (similar to Scheme [26]), the Barham group found spectroscopic evidence of precomplexation between the TpBPA radical cation and arene substrates.[61] The generation of extreme oxidants or reductants in an intimate catalyst-substrate precomplex might furthermore allow redox processes at potentials which would normally not be tolerated by the solvent.[52b]

The necessity of ground-state preassembly opens up a new parameter to optimize excited-state radical ion chemistry. For example, the PDI catalyst (Scheme [23]) was proposed to form an intimate catalyst-substrate assembly, which is stabilized by the incorporation of PDI into a metal-organic framework (MOF).[62] In the MOF, PDI undergoes compact π–π-stacking, which allows a strong interaction with aryl halides. Furthermore, the dense PDI active sites are isolated in the MOF, which further explains its superior activity in comparison with its homogeneous counterpart. This is an example where the heterogenization of a homogeneous catalyst has more advantages than just increasing recyclability.

When a photon of sufficient energy is absorbed by a conventional photocatalyst, it is excited towards a high singlet state S_n (n > 1). Subsequently, through internal conversion the catalyst ends up in the lowest singlet excited state S₁, which can undergo intersystem crossing towards the triplet excited state T₁.[8b] Considering the chemistry of excited radical cation/anions, one could expect the first doublet state D₁ to be the active state in accordance with Kasha’s rule. Interestingly, multiple instances have been reported where quenching of a higher excited D-state is deemed responsible for extreme oxidation/reduction events.[49] [59] [61] This ‘anti-Kasha’ behavior can be discerned from conventional lowest excited-state mechanisms by comparison of the calculated D₁ energy with the potential necessary for the observed redox step. Alternatively, one could irradiate the radical ion with such a wavelength so that only a selectively excitation from D₀ to D₁ is possible. If the transformation is not observed, this must mean that higher excited states are at play.

For example, in 2020 the Miyake group reported a benzo[ghi]perylene (BPI) photocatalyzed Birch reduction under 405 nm LED irradiation (Scheme [29]).[49] Mechanistic studies showed that hydroxide anions form a covalent adduct to the BPI catalyst, and this then absorbs visible light triggering the expulsion of a hydroxide radical and the creation of a BPI^•– radical anion. This is a relatively strong reductant (E _red = –1.24 V vs. SCE), but insufficiently reducing for a Birch reduction. Hence, the authors proposed a conPET-type mechanism where an excited-state radical anion (*BPI^•–) is formed. TD-DFT calculations proposed that the blue photons could excite the radical anion up to the sixth excited D-state, while the D₁ state is of insufficient energy to allow electron transfer to the solvent, necessary for the formation of solvated electrons. Surprisingly, this must mean that the electron transfer occurs from a D_n state with n = 2–6, a direct violation of Kasha’s rule. Concerning the short lifetime of the excited *BPI^•–, a ground-state preassembly between [BPI-OH]^– or BPI^•– with the benzene substrates could explain the occurrence of the reaction. However, since the authors did not find any UV-vis evidence for this association, their working hypothesis is that the excited state *BPI^•– releases a solvated electron. This is a very rapid process requiring only ca. 11 ps in methanol, which might circumvent the necessity of ground-state preassembly of the BPI^•– or [BPI-OH]^– with the benzene derivatives.

Upconversion Strategies and Other Two-Photon Mechanisms

ConPET and related e-PRC strategies allow the formation of excited radical anions/cations that possess exceptional redox potentials.[63] Although it unlocks the photocatalytic activation of conventionally unreactive substrates using a single catalyst, the excited radical species is often very short-lived. For this reason, bimolecular quenching usually requires high quencher concentrations or the presence of ground-state pre-aggregates. As a consequence, alternative mechanisms allowing high redox potentials to be reached are of interest.

In this sense, sensitized triplet–triplet annihilation upconversion (sTTA-UC) is a thoroughly investigated mechanism that harnesses the energy of two photons to achieve a high energy state (Figure [6]).[64] A photosensitizer (Sens) is excited by a visible photon to reach a singlet excited state (¹Sens), which quickly undergoes intersystem crossing (ISC) to the lowest lying triplet state ³Sens. Next, a triplet-triplet energy transfer (TTET) to a co-catalyst with a lower lying triplet state, called the annihilator (Ann), causes its excitation to the triplet state (³Ann) whilst regenerating the ground-state sensitizer (Sens). Subsequently, a second energy transfer occurs, known as triplet-triplet annihilation (TTA), where the energy of two triplet excited-state annihilators (³Ann) are combined to generate one higher singlet state annihilator (¹Ann) and one ground-state annihilator (Ann). This highly energetic excited state can be used for substrate activation. Since the high energy singlet state of the annihilator (¹Ann) is populated, this process is an example of ‘upconversion’, as in the absence of bimolecular processes, a higher energetic photon is emitted by fluorescence than the ones originally used to excite the sensitizer. In absence of any quencher, this is a visual means to assess the occurrence of upconversion.

Some energetic requirements guide the choice of sensitizer and annihilator.[65] Firstly, the singlet excited state of the sensitizer (¹Sens) must lie below that of the annihilator. If this is not the case, the annihilator can access its singlet state directly by photon absorption. Secondly, the triplet state energy of the sensitizer (³Sens) must be higher than that of the annihilator, so that efficient exergonic energy transfer between ³Sens and the annihilator (Ann) can occur. This way, the singlet and triplet excited states of the sensitizer lie between the singlet and triplet excited states of the annihilator. Thirdly, the sum of the energy of two triplet excited states of the annihilator (³Ann) should at least be equal to the singlet excited-state energy of the annihilator (¹Ann), so that TTA is possible.

When blue light is used, sTTA-UC can be utilized to drive photochemical reactions normally requiring ultraviolet irradiation. However, upconversion from green-to-blue or NIR-to-visible (see Section 3.4) has also been described.[64b] One drawback of sTTA-UC is the extreme oxygen sensitivity of the upconverted species, requiring strict inert conditions. This has been addressed by performing reactions in organogel matrices.[66]

Seminal work in the use of sTTA-UC for organic transformations was conducted in 2015 by Pérez-Ruiz and co-workers, who reported an sTTA-UC based dehalogenation of aryl bromides by using superstoichiometric butane-2,3-dione as sensitizer and 2,5-diphenyloxazole as annihilator.[67] This early method suffered from relatively low yields, limited scope, and required the use of a high-energy pulsed laser but showed the potential of sTTA-UC to activate normally inaccessible aryl bromides. In 2018–2020, aryl radicals were successfully trapped with 5-membered heteroarenes.[68] For example, furans and thiophenes could be arylated by aryl bromides through the combination of BOPHY­ photosensitizer and 9,10-diphenylanthracene (DPA) as annihilator (Scheme [30]).[68b] In contrast to the early work, a low-energy blue laser pointer could be used to excite the BOPHY dye. Although large excesses of furan or thiophene were still required, these could be easily recovered by chromatography. Nevertheless, some scope examples suffered from dehalogenation, which limited the selectivity. The authors proposed an sTTA-UC mechanism, where the blue laser selectively excites the BOPHY dye at 450 nm. After the intersystem crossing of ¹BOPHY* into ³BOPHY, a rapid TTEnT with DPA leads to the formation of large amounts of triplet ³DPA*. Upon collision, two ³DPA* species undergo triplet-triplet annihilation forming the high-energy ¹DPA*, which is strongly reducing. A single electron transfer (SET) from ¹DPA* to aryl bromide generates DPA^•+ and an aryl bromide radical anion that undergoes rapid scission to bromide and aryl radical which is subsequently trapped by furan or thiophene. Single electron transfer (SET) from the allyl radical intermediate to DPA^•+ generates carbocationic species that can rearomatize after deprotonation and concomitantly close the catalytic cycle.

In 2021, the Wenger group published a thorough investigation of the mechanism behind the hydrodehalogenation of selected aryl halides through sensitization-induced electron transfer (Scheme [31]).[63] fac-Ir(ppy)₃ was applied as a photosensitizer, 2,7-di-tert-butylpyrene ( ^t BuPyr) as annihilator and N,N-dimethylaniline (DMA) as terminal reductant. They confirmed an sTTA-UC mechanism where the triplet ³ fac-Ir(ppy)₃* catalyst performs a TTEnT towards ^t BuPyr resulting in the formation of ^3t BuPyr*. Next, triplet-triplet annihilation (TTA) of two ^3t BuPyr* causes upconversion towards the lowest-lying singlet excited state ^1t BuPyr*, and this performs one-electron oxidation of the donor DMA towards DMA^•+, hence forming the pyrene radical anion ^t BuPyr^•–. This pyrene radical anion subsequently reduces the aryl halide, concomitantly regenerating ^t BuPyr. The aryl halide radical anion undergoes scission, forming the aryl radical which performs hydrogen atom abstraction, supposedly from DMA^•+. This example, together with the previous one illustrates that both oxidative and reductive quenching cycles are possible, starting from upconverted species.

However, the mechanism of these systems is not always clear-cut. For example, in 2017 the König group introduced a similar system [Ru(bpy)₃]²⁺/pyrene (Pyr)/DIPEA, which allowed aryl radical formation from aryl bromides/chlorides. These aryl radicals could be trapped by N-methylpyrrole and other radical trapping agents.[69] The originally proposed one-photon mechanism entailed a sensitization induced electron transfer (SenI-ET), where the photoexcited Ru(bpy)₃ ²⁺ catalyst performed TTEnT to pyrene (Scheme [32]). The generated triplet ³Pyr* was proposed to be reduced by DIPEA to form the highly reducing pyrene radical anion (E(Pyr/Pyr^•–) = –2.1 V), capable of reducing aryl bromides/chlorides. However, this reduction of triplet-excited pyrene (³*E _red = –0.1 V vs. SCE) by DIPEA (E _ox = +0.9 V vs. SCE) was contested by Ceroni and Balzani as too thermodynamically unfeasible, and an alternative two-photon sensitization-induced triplet-triplet annihilation upconversion (sTTA-UC) mechanism similar to that in Scheme [31] was proposed.[70] Here, ³Pyr* is still formed by a conventional TTEnT with the excited photocatalyst *[Ru^II(bpy)₃]Cl₂, however, instead of a direct reduction by DIPEA, a TTA towards ¹Pyr* was proposed. Next, a SET between ¹Pyr* (¹*E _red = +1.2 V vs. SCE) and DIPEA (E _ox = +0.9 V vs. SCE) is thermodynamically feasible and could form the pyrene radical anion (Pyr^•–). The König group responded, acknowledging the possibility of TTA, but also noted that ¹Pyr* generated by TTA is sufficiently reducing to reduce aryl halides (¹*E _ox = –2.1 V vs. SCE, identical to that of Pyr^•–).[71] This would mean that the presence of DIPEA is unnecessary, something that has been disproven experimentally. Furthermore, ³Pyr* (*E _red = –0.1 V vs. SCE) could still be reduced by photoreduced [Ru^I(bpy)₃]Cl₂ (E(Ru^II/Ru^I) = –1.33 V vs. SCE), which is known to be generated from *[Ru^II(bpy)₃]Cl₂ in a reductive quenching cycle with DIPEA (Scheme [32]).[71] The authors conclude that, besides these, multiple thermodynamically viable pathways are possible, with or without TTA, and detailed mechanistic studies are required to elucidate the ‘true’ mechanism.

In 2020, Moore and co-workers showed spectroscopically that one dominant pathway was present.[72] Starting from ³Pyr*, the rate of bimolecular TTA was approximately one order of magnitude slower than the rate of bimolecular single electron transfer with photoreduced [Ru^I(bpy)₃]Cl₂.[72] TTA together with other non-radiative processes accounts for less than 5% of the consumption of ³Pyr* in the presence [Ru^I(bpy)₃]Cl₂. Hence, the revised mechanism of König holds: the highly reducing Pyr^•– is generated by the reduction of ³Pyr* (*E _red = –0.1 V vs. SCE) by [Ru(bpy)₃]⁺ (E _red(Ru^II/Ru^I) = –1.33 V vs. SCE), generated in a separate reductive quenching cycle from *[Ru(bpy)₃]²⁺ and DIPEA. Pyr^•– is able to reduce aryl halides (E(Pyr/Pyr^•–) = –2.1 V) towards their aryl radicals, concomitantly closing the organocatalytic cycle. Notably, both the EnT as SET behavior of the [Ru(bpy)₃]²⁺ photocatalyst are essential for this reaction.

The core difference between the fac-Ir(ppy)₃/ ^t BuPyr/DMA system (Scheme [31]) and the [Ru(bpy)₃]²⁺/Pyr/DIPEA system (Scheme [32]) can be explained by the redox potentials of the photocatalyst and sacrificial reductant. For the case of fac-Ir(ppy)₃ in Scheme [31], the excited-state reduction potential of *fac-Ir(ppy)₃ is a lot less positive (*E _red = +0.31 V vs. SCE) in comparison to that of Ru(bpy)₃ ²⁺ (*E _red = +0.77 V vs. SCE).[63] The sacrificial reductant (DMA) has an oxidation potential (E _ox= +0.74 V vs. SCE) which thermodynamically does not allow electron transfer from DMA to the *fac-Ir(ppy)₃ catalyst (*E _red = +0.31 V vs. SCE). However, this transfer is possible for the combination of [Ru(bpy)₃]Cl₂ (*E _red = +0.77 V vs. SCE) and DIPEA (E _ox = +0.9 V vs. SCE), causing the generation of the reduced photocatalyst [Ru^I(bpy)₃]Cl₂, which quenches ³Pyr*. Although even in this case *E _red < E _ox, this is most probably a consequence of not working under standard conditions, for which these values are tabulated. The thermodynamic evaluation is by consequence always only an estimation. In the case of the reaction with fac-Ir(ppy)₃, ^3t BuPyr* is not quenched by a reduced photocatalyst and can partake in triplet-triplet annihilation instead. DMA can only interact with the singlet excited ^1t BuPyr* annihilator to generate its radical anion ^t BuPyr^•–.

Red and Near-Infrared Photocatalysis

The emergence of visible light photocatalysis provided a remarkable solution for classical high energy UV-mediated photochemical reactions, allowing the development of novel synthetic methods under mild conditions. Nevertheless, visible light photocatalysis is plagued by two limitations: (i) the low penetration of visible light in reaction media limits the scale-up processes toward industrial applications, and (ii) biological and medicinal applications of photocatalysis are limited due to the opacity of biological tissues to visible light. While the development of flow technology is an effective solution for (i), it does not provide a solution for (ii).[73] The use of near-infrared (NIR) and deep red (DR) light photocatalysis is a promising solution based on its deeper penetration in various media (including tissue).[74]

An indirect way to harness red or NIR photons is the use of an sTTA-UC system (see Section 3.3.). Here, a low-energy red or NIR photon excites the photosensitizer (Sens), which transfers its energy to an annihilator (Ann). Next, triplet-triplet upconversion creates a high-energy singlet state ¹Ann*, which fluoresces upconverted photons in the absorption region of a conventional photocatalyst (Figure [7]).[64] This strategy has been used by Congreve, Rovis, and Campos to excite organophotocatalysts Eosin Y or Rose Bengal (NIR-to-yellow/green)[75] and by Wenger in combination with the noble metal catalyst [Ru(bpy)₃]²⁺ (red-to-blue).[76] Although this creates a fairly complicated three-catalyst system, it can be used with photocatalysts with known absorption features and redox potentials. Furthermore, the photocatalyst can be grafted onto an upconverting nanoparticle, exemplified by Glorius and Ravoo who functionalized a NaYF₄:Yb nanoparticle with a modified [Ru(bpy)₃]²⁺ catalyst.[77]

In some examples, the presence of an external conventional photocatalyst is not necessary, as the singlet excited state of the annihilator (¹Ann) can immediately interact with a substrate inducing the desired transformation. For example, the cyclization of dienyl azides to pyrroles can be induced by NIR light (Scheme [33]) in the presence of an sTTA-UC system consisting of the photosensitizer PtTPTNP and the annihilator TTBP (Figure [7]).[75] The triplet energy of ³TTBP* (E _T = ca. 35 kcal·mol^–1) is insufficient to induce TTEnT with the azide substrate (E _T = ca. 45 kcal·mol^–1), and its reduction potential (*E _ox(³TTBP*)= –0.78 V vs. Ag/AgCl in MeCN) is likewise insufficient to reduce the substrate (E _red = –1.55 V vs. Ag/AgCl in MeCN). For these reasons, the authors propose the involvement of an sTTA-UC system which generates the higher energy singlet state ¹TTBP* capable of reducing the dienyl azide, triggering cyclization. The resulting radical anion is oxidized by the TTBP radical cation, allowing the formation of the corresponding pyrrole (Scheme [33]).[75] Concurrently, the authors show that ¹TTBP is sufficiently potent to reduce methyl 2-bromo-2-phenylacetate (E _red = –1.58 V vs. Ag/AgCl in MeCN).

The energy of two photons could also be harnessed in absence of an upconversion strategy, where two NIR photons simultaneously excite the photocatalyst in a single step. Winkler, Gray, and co-workers showed an intramolecular arylation of pyrrole with an aryl iodide using a tungsten(0) arylisocyanide complex with a large two-photon absorption cross-section (Scheme [34]).[78] Since two-photon absorption is a non-linear process, high light intensities are usually required. For this reason, the authors employed a high-energy fs-pulsed 810 nm laser.

Interestingly, the two photons do not have to be absorbed by one type of catalytic species. In 2022, the Wenger group reported a red-light-mediated reductive deprotection of tosyl-protected phenols and nitrogen-containing substrates using dual photocatalyst excitation, employing both the base metal photocatalyst [Cu(dap)₂]Cl and the organic redox catalyst DCA (Scheme [35]).[79] Through two different reaction pathways, a highly reducing DCA excited radical anion is capable of reducing the tosyl group, liberating the free alcohol or amine. Both mechanisms commence with the excitation of [Cu(dap)₂]Cl, using a 623 nm LED. In the first mechanism, a conventional oxidative quenching cycle occurs, where the triplet excited ³[Cu^I(dap)₂]Cl (*E _ox(Cu^II/*Cu^I) = –1.43 V vs. SCE) reduces DCA (E _red = –0.93 V vs. SCE) to its radical anion DCA^•–. Concomitantly the oxidized photocatalyst [Cu^II(dap)₂]Cl is generated which can be reduced by the sacrificial electron donor DIPEA, hereby closing the photoredox cycle. However, DCA^•– can also be generated through sensitization-induced electron transfer. In this case, the excited ³[Cu^I(dap)₂]Cl (E _T = 2.05 eV) performs exergonic triplet-triplet energy transfer with DCA (E _T = 1.8 eV), which in its ground state is an insufficiently strong oxidant to perform PET (E _red = –0.93 V vs. SCE) with DIPEA (E _ox = +0.75 V vs. SCE). However, the resulting triplet excited ³*DCA possesses a higher reduction potential (*E _red = 0.87 V vs. SCE) and can be reduced to DCA^•– by DIPEA. Both competing pathways eventually lead to the formation of DCA^•–, and depending on the reaction conditions, the excited ³[Cu^I(dap)₂]Cl catalyst will rather perform PET or TTEnT with DCA. For example, in MeCN the authors estimated that PET is dominant, while in acetone TTEnT is dominant. DCA^•– features strong absorption bands around 642 and 706 nm, and can be excited by 623 nm irradiation to form *DCA^•–. *DCA^•– possesses an estimated excited-state redox potential of ca. –2.6 V and could successfully reduce a range of substrates with reduction potentials up to –2.3 V vs. SCE. Although excited radical ions typically possess a lifetime in the picosecond range,[60a] this is possibly mediated by a precomplexation of DCA^•– with the substrate.

Nevertheless, instances have been reported where direct near-infrared (NIR) absorption occurs without the necessity for a two-photon mechanism. Rovis, Joe, and co-workers recently reported direct near-infrared (NIR) photocatalysis using osmium(II) polypyridyl complexes as NIR photocatalysts for various synthetic applications.[80] Indeed, Os^II-polypyridyl complexes display a strong-orbit coupling (SOC) that allows direct access to a long-lived triplet (T₁) via S₀ to T₁ excitation (Figure [8]). This spin-forbidden excitation minimizes the energy loss since no intersystem crossing (ISC) accompanied by energy loss is required. For this reason, NIR irradiation is sufficiently energetic to cause direct excitation to T₁. At the same time, the Os(tpy)₂ ²⁺ molar extinction coefficient (ε) is very low (30 times lower than [Ru(bpy)₃]²⁺) illustrating its improved light penetration in solution, allowing reaction scale-up.

In 2022, Rovis and co-workers reported a catalyst library of polypyridyl complexes of osmium with spectroscopic and electrochemical properties facilitating the direct adoption of NIR photocatalysis.[81] For example, they applied the [Os(bptpy)₂](PF₆)₂ as NIR photocatalyst and Umemoto reagent as CF₃ ^• radical source for the previously established visible-light-mediated chlorotrifluoromethylation[82] of alkenes (Scheme [36]).[80] Interestingly, the alkene chlorotrifluoromethylation product was obtained in 81% yield under NIR lamp irradiation using 0.01 mol% [Os(bptpy)₂](PF₆)₂, while the original report employing 1 mol% [Ru(bpy)₃](PF₆)₂ under blue light irradiation achieved a lower yield of 64%.[80] In general, the use of the deeper penetrating NIR light has the potential to improve yields with lower catalyst loading, while allowing easier reaction scale up, and circumventing the decomposition of higher intensity light sensitive reactants or intermediates. The principle behind this NIR-mediated photoredox catalytic cycle is identical to those mediated by visible light. In the first step of the catalytic cycle, Umemoto reagent (E _red = –0.75 V vs. Cp₂Fe in MeCN) is reduced by the excited *[Os^II(bptpy)₂](PF₆)₂ photocatalyst (*E _ox(Os^III/*Os^II) = –1.18 V vs. Cp₂Fe in MeCN). In the second step, the benzylic radical (E _ox = –0.01 V vs. Cp₂Fe in MeCN) can be oxidized by [Os^III(bptpy)₂](PF₆)₂ (E _ox(Os^III/Os^II) = +0.59 V vs. Cp₂Fe in MeCN), which forms the carbocation and regenerates the ground-state photocatalyst. The cation subsequently reacts with TMSCl providing the chloride. The authors furthermore showed an easy synthesis of NIR-photocatalyst [Os(bptpy)₂](PF₆)₂ via terpyridine ligand complexation with OsCl₃ hydrate (Scheme [37]).[80]

However, OsCl₃ belongs to the same category as IrCl₃ with respect to price and scarcity. For this reason, efforts are being made to design base metal or organic photocatalysts capable of absorbing deep red[79] [83] or NIR light (Figure [9]).[84] This is similar to what has happened with the transition of blue light absorbing transition metal complexes into organophotocatalysts.

For example, Goddard and co-workers developed a NIR-light photocatalyzed aza-Henry reaction of N-aryl-tetrahydroisoquinolines using a commercial cyanine organophotocatalyst, cy764 (Figure [9], Scheme [38]).[84a] The organophotocatalyst, once excited by 810 nm light, is able to oxidize N-aryl-tetrahydroisoquinoline to its corresponding radical cation (*E _ox = 0.62 V vs. SCE). The photocatalytic cycle is closed by sacrificial oxygen generating a superoxide anion and this oxidizes the radical cation to the iminium via hydrogen atom abstraction. Finally, an addition of the nucleophile forms the desired α-functionalized products. The authors furthermore showed the versatility of cy746 by its application in a range of other NIR-light-mediated photocatalytic transformations featuring both oxidative and reductive processes. For example, Umemoto’s reagent could be reduced by cy746 under an inert atmosphere. Furthermore, cy746 can generate singlet oxygen in an energy transfer cycle with oxygen.

In metals, the electron density of the free electrons oscillates with respect to the metal cation lattice. This collective oscillation is quantized and can be excited by electromagnetic radiation. When these quantized oscillations exist at the surface of metal particles, they are called surface plasmons. Plasmonic nanoparticles (PNP) are Au or Ag-containing nanoparticles which, once irradiated with NIR light, undergo localized surface plasmon resonance (LSPR) and produce hot electrons and hot holes.[85] These hot electrons can be transferred to the antibonding orbitals of adsorbed organic substrates, decreasing bond strengths. However, photothermal heating caused by the NIR-light irradiation might also contribute to this ‘plasmonic catalysis’, making the distinction between the contribution of hot-electron/holes and photothermal effects difficult.

For example, the oxidation of p-aminothiophenol to p,p′-dimercaptoazobenzene was possible by the NIR irradiation of a gold nanoparticle film by a 785 nm low-intensity (0.03 mW) laser (Scheme [39]).[86] The mechanism was proposed to commence with the adsorption of p-aminothiophenol on the gold surface. Next, NIR irradiation causes the generation of hot electron/hole pairs on the metal surface. This hot hole can capture an electron from the HOMO of the absorbed molecule generating the amine radical cation. The hot electrons can be quenched by oxygen which, with the generated protons, will finally provide water. The amine radical cation is in equilibrium with its neutral radical via deprotonation, and this can dimerize towards its hydrazine derivative. Further oxidation steps furnish the target diazo compound.[87]

The property of these PNPs to exhibit plasmon excitations in the NIR region makes them an intrinsically interesting heterogeneous alternative for the noble metal homogeneous osmium-based photocatalysts. However, a number of model transformations, such as a Suzuki reaction using an Au-Pd nanostructure[88] or an azide-alkyne click reaction using an Au@Cu₂O[89] were carried out with PNPs containing metals that are known catalysts for these transformations in the absence of light irradiation, thereby confounding mechanistic considerations. Additionally, for each case, mechanistic studies are required to prove that the ‘plasmonic catalysis’ is not limited to a thermal effect.

Conclusions and Outlook

Visible light photocatalysis has emerged as a versatile way of accessing radicals allowing the activation of a wide range of substrates under mild conditions. Besides single-electron transfer or energy transfer, other photocatalytic activation modes have been developed using either a catalytic amount of a photocatalyst or even no external photocatalyst at all, i.e. the in situ formation of visible light absorbing complexes. Furthermore, transition metal complexes themselves can act as photocatalysts, allowing mechanisms exhibiting both classical transition metal catalysis steps, and radical behavior. By employing a co-catalyst along with a photocatalyst (so-called dual catalysis) the scope of the application of photocatalysis for organic synthesis has recently widely broadened, combining either single-electron transfer or energy transfer with other types of catalysis such as transition metal catalysis or organocatalysis.

Unfortunately, these excited photocatalysts suffer from limitations in accessing potentials beyond –2.00 and +2.00 V vs. SCE, hereby limiting synthetic applications. New directions are therefore focused on the expansion of the redox windows of the photocatalysts. Consecutive two-photon excitation (conPET) by excitation of the corresponding radical anion or cation provides an exciting solution using only one photocatalytic species and is closely related to the excitation of an electrochemically generated radical ion (e-PRC) of a mediator leading to extremely oxidizing or reducing species. These excited radical ions possess short lifetimes that do not allow bimolecular reactivity. A successful reaction with a substrate should therefore involve a precomplex. Within this precomplex, potentials are tolerated which normally would not be compatible with the solvent.

Alternatively, sensitization-induced triplet-triplet annihilation upconversion (sTTA-UC) can generate highly potent singlet excited states by harnessing the energy of two triplet states, but requires both a photosensitizer and annihilator. Furthermore, unconventional two-photon mechanisms could be at play, where either a photocatalyst partakes both in PET and EnT or two distinct photocatalysts each absorb one photon, overall harnessing two photons in one reaction. Challenging reactivity, such as the reduction of haloarenes or the oxidation of unactivated arenes, has been achieved.

Despite these important advances, visible light photocatalysis suffers from low penetration in reaction media, which inherently limits the scale-up of the processes toward chemical development and production in industrial applications. Although the progress made in flow technology seems to be an effective solution for scale-up with low penetrating visible light, the recent development of near-infrared (NIR) and deep red (DR) light photocatalysis provides another attractive solution. Besides scale-up, this enabling technique allows in vivo biological photoredox applications due to its deeper penetration in various media (including tissue).

This review article presents some lesser-known activation modes and discusses several emerging concepts and techniques of photochemistry, with particular attention to the reaction mechanisms at play. We believe that this review will serve as an introductory guide to support chemists who are familiar with SET and EnT and want to broaden their horizons within the versatile field of photocatalysis.

Conflict of Interest

The authors declare no conflict of interest.

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Corresponding Author

Publication History

Received: 16 July 2022

Accepted after revision: 16 September 2022

Accepted Manuscript online:
16 September 2022

Article published online:
29 November 2022

© 2022. Thieme. All rights reserved

Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany

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