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: Utilizing light is a smart way to fuel chemical transformations as it allows to selectively focus the energy on certain molecules. Many reactions involving electronically excited species proceed via open-shell intermediates enabling novel and unique routes to expand the hitherto used synthetic toolbox in organic chemistry. The direct conversion of non-prefunctionalized, less activated compounds is a highly desirable goal to pave the way towards a more sustainable and atom-economic chemistry. Photoexcited closed-shell anions have been shown to reach extreme potentials in single electron transfer reactions and reveal unusual excited state reactivity. It is therefore surprising, that their use as reagent or as photocatalyst is limited to a few examples. In this review, we briefly discuss the characteristics of anionic photochemistry, highlight pioneering work and show recent progress which has been made by utilizing photoexcited anionic species in organic synthesis.


Introduction
Initial attention to the versatile reaction modes of photoexcited organic anions and their special spectroscopic behaviour was drawn by the early reviews of Fox [1] and Tolbert [2] . Since then, other excellent publications followed, summarizing the photochemistry of excited organic anions with focus on their photoreductive properties and underlining the peculiarities of anionic organic molecules in photochemistry. [3,4] Compared to the neutral species, the absorption of an organic anion is usually redshifted, which facilitates the selective excitation in complex mixtures and often allows to use visible light. Along with the enhanced electron-electron repulsion found in anionic molecules, negatively charged species are expected to act as particularly potent electron donors from their photoexcited states. In addition, a single electron transfer from an anionic donor to a neutral acceptor gives rise to a neutral radical and a radical anion. These species are free of attracting forces and are able to diffuse freely, which suppresses back electron transfer (BET) reactions resulting in higher reaction efficiencies. Organic anions can be easily formed in presence of base and their rather long excited state lifetimes distinguish them from radical anions.
Excited anionic species are also utilized in key photochemical steps in biology. For instance, in an ATP-driven process, the excited oxyluciferin anion causes the bioluminescence of fireflies. [5] Moreover, phototrophic organisms show locomotory movement upon stimulus of light. The photoactive yellow protein (PYP) encloses the anionic trans-para-coumaric acid as blue-light photoreceptor. Subsequent trans-cis isomerization of the excited chromophore induces a conformational change of the protein leading to a biological signal transduction. [6] The enzymemediated repair of photodamaged DNA is another well-known example dealing with excited anions in living cells. A crucial step is the photoinduced electron transfer from the excited cofactor flavin adenine dinucleotide (FADH − ) which provides an electron for the light-driven repair catalyzed by photolyases. [7,8] Literally, the last decade has been a very exciting time in terms of photochemistry and many novel chemical transformations have been developed which complement the available synthetic toolbox. We are sure that, inspired by nature and the herein presented examples, the photochemistry of closed-shell anions will be further developed towards the generation of ever stronger light-activated reductants and novel reaction modes. In this review, we briefly summarize key spectroscopic and electrochemical properties of organic anions and provide an overview of the versatile photochemistry of anionic species with a special focus on recent examples of organic anions used as photocatalysts or as light-activated reagents.

Spectroscopic Properties of Organic Anions
The chemistry of molecules excited by light is initiated by the absorption of a photon and thus, we will start with discussing the peculiarities of the absorption spectra for closed-shell anions. Compared to their neutral precursors, organic anions usually experience a significant bathochromic shift in their absorption spectra and pronounced absorption bands can be attributed to , * transitions. The narrowed gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO, see Figure 1) causing the red-shift can be primarily explained by the increased shielding of the core due to an imbalance of charges. The strength of the electric field is reduced and electrons in the HOMO sense much weaker attracting forces. As a result, the spatial distribution of electrons becomes more diffuse as if the conjugation length is extended. [1,9] The absorption of organic anions is also affected by size and nature of the countercation, solvent polarity and ion pairing effects in solution. In non-polar or weakly polar solvents, contact ion pairs are formed and the properties of the anionic species are strongly influenced by the character of the countercation. In contrast, the increased solubility of ions in polar solvents, induced by aligning molecular dipoles, causes solvent-separated REVIEW or free ion pairs and the mutual ionic interaction is diminished. In general, an increase in solvent polarity and/or ionic radius of the countercation results in a bathochromic shift of the absorption, which can be attributed to a destabilization of the ground state ion pair. This destabilization effect is less pronounced in the excited state. [1,4] Similarly, the emission of excited organic anions is usually influenced by solvent polarity and countercation. The fluorescence decay of sodium 2-naphtholate was studied in different solvents. [10] For polar protic and polar aprotic solvents, a mono-exponential fluorescence decay was observed. However, in polar protic MeOH the fluorescence lifetime was remarkably decreased and the emission spectrum was blue-shifted compared to polar aprotic DMF or DMSO, which the authors attribute to a stabilization of the anion ground state caused by strong hydrogen bonding of the solvent. In weakly polar THF contact ion pairs and solvent separated ion pairs of 2-naphtholate and Na + coexist and cause a bi-exponential fluorescence decay due to varying fluorescence lifetimes. Upon addition of crown-ether to the system, a mono-exponential decay was recorded suggesting that sodium cations are complexed and the ion pairs formed with naphtholate are solvent separated in nature. Owing to the lack of ground state stabilization in solvent separated or free ion pairs, lifetimes similar to experiments in polar aprotic solvents were found in presence of a crown-ether.
The nature of ion pairing might also affect the efficiency of bimolecular electron transfer processes. Tamaoki and co-workers studied the quantum yield for the photodissociation of a benzene diazonium salt 1 with 9,10-dimethoxyanthracene-2-sulfonate (2) being the visible-light-absorbing counteranion (Scheme 1). [11] The photodecomposition of the benzene diazonium cation 1 initiated by PET from the excited anion 2 was found to be six-times higher in CHCl3 compared to MeCN. The difference in reactivity of the diazonium salt between the solvents was explained by the different nature of ion pairs formed. The weakly polar solvent CHCl3 promotes a fast reaction due to the close proximity of 1 and 2 in a tight ion pair. Solvent separated loose ion pairs in polar MeCN allowed to measure a distinct fluorescence lifetime. Upon excitation in polar media the anionic donor needs to initially encounter a cationic acceptor to trigger the photodecomposition and hence increased lifetimes are recorded. For a more comprehensive discussion of ion-pairing and solvent effects, we refer to several excellent reports. [4,10,[12][13][14] Scheme 1. The rate of photoinduced electron transfer is influenced by the solvent polarity: fast in CHCl3 (tight pair), slow in MeCN (loose pair)

Photoinduced Electron Transfer
Electron transfer reactions from electronically excited states of molecules are among the earliest photochemical reactions reported. [15] Photoexcited molecules exhibit increased reduction and oxidation potentials compared to their ground state and the resulting excited state potentials can be estimated, according to the free enthalpy change of a PET, by measuring the ground state potentials E1/2 and the transition energy E0,0 (see Figure 2). [16] In polar organic solvents the electrostatic work term usually contributes little to the free enthalpy change and is frequently omitted. [17] PET from a neutral excited-state donor ( * D) to a neutral ground-state acceptor (A) causes a charge separation, resulting in a pair of radical ions. In contrast, the PET from an anionic excited-state donor to a neutral acceptor can be considered as a charge shift, generating products that are free of electrostatic attraction and expected to diffuse freely (Scheme 2). Hence, the lost channel of a back electron transfer, which would regenerate the initial non-excited status quo, is less competitive in a chargeshift process. [18] Scheme 2. Charge separation with neutral donor (left) and charge shift with anionic donor (right).
An anionic molecule is considered as a superior electron donor compared to its neutral parent. Both repulsion between electrons and the shielding from the nucleus are increased. As a consequence, the excess negative charge facilitates the removal of an electron. Experimentally, this becomes apparent when solvated electrons are expelled from organic anions in a biphotonic process using energy-rich UV light [19] in glassy matrices (77 K) or pulsed high-energy lasers [20,21] in alkaline aqueous solution. Working with visible-light LEDs and in common organic solvents however, renders the photo-ejection of an electron unlikely to occur and hence under these conditions electron transfer reactions are prevailing. We recently demonstrated that 9-anthrone and its derivatives are easily deprotonated in presence of carbonate base to form colored anions (e.g. ANT − , Figure 2), which upon visible-light excitation turn into remarkably strong reductants. [22] Cyclic voltammetry measurements in alkaline DMSO revealed that the anionic ground state is already a good reductant, as the excess charge is removed easily due to resonance stabilization of the resulting radical. In sharp contrast, the dianions of fluorescein FL 2− or eosin Y (EY 2− ) show a significantly decreased tendency towards electrochemical oxidation in alkaline MeOH and hence, the resulting excited state oxidation potentials are only moderate (cf., Table 1, Entry 5-7). [23] Scheme 3. Formation of the eosin Y radical trianion upon PET in presence of triethanolamine (top). Intramolecular PET from the amino group causing selfquenching of the fluorescence.

REVIEW
Furthermore, it was reported that FL 2− and EY 2− , although being present as ground state dianions, are easily reduced upon photoexcitation in basic solutions containing triethanolamine or phenol to form radical trianions (Scheme 3, top). [24][25][26][27][28] Walt and co-workers attached an amino group on the benzoate scaffold of fluorescein NH2-FL 2− and found that the fluorescence quantum yield dropped by almost a factor of 60. They explained this observation by an intramolecular PET from the nitrogen lone pair to the fluorescein scaffold (Scheme 3, bottom). A similar fluorescence quantum yield with respect to non-modified FL 2− was however recorded when adjusting the pH of the solution to a value around the pKa of the aromatic amine. Due to protonation of the amine, the nitrogen lone pair is no longer available for intramolecular PET resulting in increased fluorescence. [29] In 1991, Soumillion et al. showed that the fluorescence of the excited anion of the xanthene dye resorufin is quenched in presence of 2naphtholate and the formation of a radical dianion of resorufin was proposed. [18] The moderate reducing abilities of negatively charged xanthene dyes (e.g. EY 2− , FL 2− ) can be explained by an overwhelming contribution of the electron-deficient conjugated system to the overall electronic properties. Thus, to obtain strongly reducing excited anions, a facile single electron oxidation is crucial (cf., Table 1, Entry 5-6 show similar values for E0,0 but differ significantly in their ground state and excited state oxidation potentials).

Photoredox Catalysis
During the last decade, impressive progress has been made in the field of synthetic photoredox catalysis and many novel transformations, which were previously inaccessible, have been developed. Photoexciting a molecule changes the electron distribution in the molecular orbitals resulting in both increased oxidizing-and reducing abilities of the excited species compared to the ground state (see Figure 2). These redox properties can be fine-tuned by attaching electron donating or withdrawing substituents. [30][31][32] Up to now, a variety of photocatalysts have been reported which are often classified regarding their composition into polypyridyl transition metal complexes, [33] organic dyes [17] or polyoxometalates [34] (POMs). In addition, heterogenous organic semiconductors were successfully employed as photocatalysts. [35] Their intrinsic photophysical properties like excited state redox potential, absorption of light or the excited state lifetime define the scope and limitations of chemical reactions. Selected examples of organic photocatalysts are depicted in Figure 2. The photochemistry of the non-charged donor-acceptor dyad 4CzIPN covers a broad electrochemical range (see Table 1, Entry 3). Bearing a versatile excited-state reduction and oxidation potential, it is often used to replace precious and toxic Ru-or Ir-polypyridyl complexes. [31,36] However, to convert less activated substrates via photoinduced single electron transfer, the frontiers need to be pushed towards higher excited state potentials. Recently, it was shown that photoexcited, electron-rich N-arylphenothiazines (e.g. PTH) act as very strong reductants but these compounds do not absorb in the visible range and hence UV-light is necessary which might interfere with other reaction components. Large Stokes shifts were found for the substituted N-arylphenothiazines which result in high transition energy values (cf., Table 1, Entry 4). [32] Apart from commonly used neutral organic dyes, molecules with a charged or an open-shell ground state or both were found to significantly increase achievable excited state potentials and allowed to widen the substrate scope for photoinduced electron transfer reactions (Scheme 4). Several organic dyes form stable and coloured radical anions in presence of suitable sacrificial donors via PET and hence, enable a subsequent second excitation (see Scheme 4, A). [37][38][39][40] The versatile photochemistry of excited radical anions allowed to convert various (hetero)aryl halides in coupling reactions and has been subject of several reviews. [41][42][43][44] Very recently, this strategy was reported to promote Birch-type reductions of benzene derivatives upon visible-light irradiation. [45] In contrast, the formation of super-oxidants has been reported upon photoexcitation of stable, chemically generated phenothiazine radical cations (see Scheme 4, B). [46] Furthermore, electron transfer from photoexcited doublet states of neutral radicals has been studied. [47][48][49][50][51] The acridine radical ACR • was recently found to act as an extremely potent photoreductant upon excitation with blacklight (see Scheme 4, C). [52] Although enabling high redox potentials, the photochemistry of excited open-shell species suffers from short lifetimes which are usually in the picosecond range. [52][53][54] As the photochemistry of open-shell molecules is beyond the scope of this review, the interested reader is referred to cited literature.
Photoreactions using catalytic amounts of closed-shell cations were found to be synthetically very useful (Scheme 4, D). The pioneering work of Fukuzumi and co-workers [55] paved the way for plenty of publications using acridinium-based donor-acceptor dyads as strongly oxidizing photocatalysts. [17,[56][57][58][59] Moreover, a new benchmark regarding the excited state potential was set by using pyrylium-, quinolinium-or diazapyrenium salts as extremely powerful photooxidants. [17] Among other cationic dyes, the photoexcited pyrylium-or acridinium salts (e.g. TPT + and ACR + , Figure 2) are strong oxidants in their excited states and found widespread synthetic applications. [17,[60][61][62][63][64][65][66][67] Surprisingly, in contrast to the wealth of reports dealing with photoexcited cations, the photochemistry of closed-shell anions received far less attention although it constitutes the logical equivalent (Scheme 4, E). Hence, in the following section the ability of anionic photocatalysts to drive challenging transformations is underlined based on selected examples. Due to their moderate redox-potentials and the wealth of available reviews, reactions of anionic xanthene dyes like eosin Y, rose bengal or fluorescein are not discussed herein. [17,[68][69][70] Furthermore, examples where anionic groups are mainly installed to increase the solubility of the sensitizer (e.g. 9,10-anthraquionone sulfonate salts) in polar media without changing its reactivity in a significant manner are excluded.

Phenolate Catalyzed Oxyarylation of Olefins with Aryl Halides
The low pKa value of phenol, caused by the charge-stabilizing effect of the benzene ring, allows facile deprotonation in presence of base to afford the phenolate, which is able to undergo photochemical reactions under visible-light irradiation. Xia and coworkers examined several 4-phenylphenol derivatives as potential photocatalysts for the visible-light oxyarylation of olefins upon photoreduction of aryl halides (Scheme 5). [71] 4-Phenylphenol PhPH bearing bulky tert-butyl groups adjacent to the phenolic alcohol (see Scheme 6) showed the highest catalytic efficiency and the corresponding oxyarylated products 5 formed in presence of aryl halides 3, olefins 4 and TEMPOH could be isolated in moderate to good yields. Remarkably, the estimated REVIEW excited state oxidation potential of *PhPH − ( * = −3.16 V vs. SCE) also allowed to convert more inert and electron rich aryl bromides and chlorides in presence of 4-methoxystyrene. Following the developed procedure, the authors present a broad scope of tolerated (hetero)aryl bromides and iodides including polyaromatic hydrocarbons, pyridines, indoles, quinolines, thiophen, thianaphthene and benzofuran. The reaction scope of tolerated olefins comprises various styrenes, aliphatic olefins, allylic sulfonamide and alcohol derivatives, enol ethers as well as 1,1-and 1,2-disubstituted olefins. In addition, the method allowed for intramolecular cyclization reactions using aryl iodides and for the late-stage modification of pharmaceuticals. Noteworthy, the use of TEMPOH as H-atom donor and radical trap seems to be crucial due to the weak nature of the O−H bond and the high stability of the aminoxyl radical formed. The proposed reaction mechanism involves the deprotonation of the phenol PhPH by base and a PET from the photoexcited *PhPH − to the aryl halide 3.1. Upon cleavage of the halide anion, the resulting aryl radical is trapped by the olefin 4.1 causing a carbon centered radical 3.1b • . Hydrogen atom transfer between the oxidized species of the catalyst and TEMPOH recovers PhPH and causes the stable radical TEMPO • . The oxyarylation product 5.1 is formed upon radical-radical coupling (Scheme 6). The formation of a ground-state electron donor acceptor complex (EDA) between phenolate anion and aryl halide was excluded by UV-vis measurements. Fluorescence quenching experiments and isolated TEMPO-trapping adducts of the aryl radical intermediate support the mechanistic hypothesis. Moreover, a radical clock experiment suggests the formation of a benzylic radical, whereas intramolecular trapping experiments disprove the involvement of a benzylic carbocation formed upon oxidation of the radical 3.1b • . Scheme 6. Proposed mechanism for the phenolate catalyzed oxyarylation of olefins via the generation of aryl radicals.

Naphtholate-Catalyzed Dehalogenation and Detosylation
The first studies of the photochemical behavior of 2naphtholate anion NA − date back to 1989, when the countercation, temperature and solvent were systematically evaluated regarding effects on the luminescence lifetime and the absorption and emission maxima. [10] In the same year, Soumillion and co-workers demonstrated the application of the naphtholate anion in the photocatalyzed defunctionalization of 2-chloronaphthalene and 4chlorobiphenyl (6.2-3) in degassed, alkaline MeOH (Scheme 7, left). [72] Scheme 7. Scope of the NA − -catalyzed dechlorination (left) and desulfonylation reactions (right).
This concept was further extended in a heterogenous approach where 2-hydroxynaphthoic acid was covalently anchored to a silica surface via amidation reaction. The efficiency of the dechlorination however was significantly decreased. [73] The substrate scope was later broadened to mono-and dichloronitrobenzenes. [74] In addition, NA − was shown to catalyze the detosylation of sulfonamides in presence of excess NaBH4 as terminal reductant (Scheme 7, right). [75] Following this procedure, 2-phenylethylamine (9.2) and N-methylbenzylamine (9.5) were obtained in quantitative yield starting from the respective sulfonamides. Although a stoichiometric amount of 2-naphthol (NA) was utilized, the catalyst could be efficiently regenerated. The proposed reaction mechanism suggests the deprotonation of NA to form the naphtholate NA − . Upon excitation with blacklight the photoexcited state of *NA − is oxidatively quenched by either aryl chloride or sulfonamide, which causes the formation of NA • and an arene radical anion. After cleavage of the respective anionic leaving group (Cl − or 4-Me(C6H4)SO2 − ), an aryl-or nitrogen centered radical is formed respectively. Abstraction of a hydrogen atom from the solvent affords the defunctionalized REVIEW arene. The N-centered radical converts to the amine via H-atom abstraction from either the solvent or NaBH4. To close the catalytic cycle, NA • is transformed to NA via hydrogen atom abstraction from the solvent or NaBH4, followed by subsequent deprotonation (Scheme 8). Recently, a zwitterionic visible-light-absorbing benzimidazolium naphtholate BINA was successfully employed in photocatalytic deiodination and desulfonylation reactions in presence of a combined electron and hydrogen atom donor 10 (see Scheme 10). [76] The cationic benzimidazolium moiety can be considered as separated from the naphtholate, due to the tilted structure that prevents -conjugation. The photocatalytic activity was studied using different solvents with attributed Lewis-basic or Lewis-acidic character estimated by donor and acceptor numbers. The authors concluded that Lewis-basic solvents cause tight interactions with the Lewis-acidic benzimidazolium moiety, whereas the electronic properties of the Lewis-basic naphtholate anion are less governed, resulting in an increased electron donating ability. The best results (Scheme 9) were found using DMF as solvent. Utilizing 10 mol% of catalyst BINA and 1.2 eq. of 10 enabled the formation of cyclized 12 in 82% yield. A lower catalyst loading of only 1 mol% resulted in full conversion of the iodoarene 11, however the product yield was lowered (69%). In addition to the cyclization of iodoarene, the photocatalytic reactivity was demonstrated based on the reductive desulfonylation of tertiary sulfonamides 13 and β-ketosulfones 15. The respective secondary amines and desulfonylated ketones were obtained in good yields. The proposed photocatalytic cycle is depicted in Scheme 10. Upon photoexcitation (λ > 390 nm), the zwitterionic excited state catalyst *BINA ( * = −2.08 vs. SCE) reduces 11 via PET. Subsequent cleavage of iodide followed by fast 5-exo-trig cyclization affords the primary radical 11b • . The oxidized photocatalyst BINA • is regenerated in presence of a sacrificial reductant 10 ( 1/2 = +0.34 V vs. SCE) via single electron transfer to give the radical cation 10 •+ , which acts as hydrogen atom donor to form 12 and in turn is converted to the cation 10 + . In presence of other terminal reductants e.g. Hantzsch ester ( 1/2 = +0.93 V vs. SCE) no product was formed as the higher ground state oxidation potential renders an electron transfer towards BINA • endergonic. Scheme 10. Proposed catalytic cycle for the radical cyclization of iodoarene in presence of photoexcited benzimidazolium naphtholate.
In previously published work, photoexcited 1,3-dimethyl-2hydroxynaphthylbenzimidazoline (BIA-H.1) was found to convert N-sulfonamides and N-sulfonylamines into the respective desulfonylated products. [77] Based on these results, Hasegawa et al. further developed the catalytic system depicted in Scheme 10 by utilizing the in situ reduction of benzimidazolium aryloxides (BIA) in presence of readily available boron hydride donors to generate the anionic species BIA-H − . [78] Scheme 11. Proposed photocatalytic cycle for the desulfonylation reported by Hasegawa et al. In addition to the reported electron donor and hydrogen atom donor abilities of the benzimidazoline scaffold (cf., Scheme 10, 10), the resulting benzimidazoline aryloxides BIA-H − are equipped with a photoredox active unit, the aryloxide moiety. Reductant, H-atom donor and photocatalyst are thus combined in one molecule. Various benzimidazoline aryloxides BIA-H.1-5 (Scheme 11) were synthesized and characterized regarding their spectroscopic and electronic properties. [78] The calculated excited state oxidation potential for BIA-H.1 − ( * = −2.71 V vs. SCE) was found to be significantly enhanced compared to the zwitterionic species BINA, allowing the conversion of less activated substrates. The elaborated protocol was used for the reductive desulfonylation of N-sulfonylindoles, -amides, -amines, and sulfonyl ketones, affording the unprotected secondary amines as well as the -defunctionalized ketones in good to excellent yield (Scheme 12). For the desulfonylation of -carbonyls, a less reactive hydride donor PicBH3 was used to avoid the direct reduction of the carbonyl group. Remarkably, utilizing the developed photocatalytic protocol allowed to convert diphenylsulfonamide (17.4) and dibenzylsulfonamide (17.5) almost quantitatively in 24 and 48 hours, respectively. Note that both substrates exhibit a challenging reduction potential ( 1/2 < −2 V vs. SCE). All synthesized catalysts BIA.1-5 were successfully tested in the desulfonylation reaction of N-tosylindole 17.1 but BIA.1 (or BINA, cf. Scheme 10) showed superior catalytic activity. Changing the light-source from a xenon lamp (500 W, λ ˃ 390 nm) to a white LED (7.3 W) afforded comparable product yields, but the reaction time increased. No product was formed in the absence of photocatalyst and only traces were found in absence of hydride donor or light. Regarding the mechanism, the authors propose the in situ formation of BIA-H − via nucleophilic attack of a hydride on the benzimidazolium moiety of BIA. Excitation with either Xe lamp or white LED renders the catalyst a strong photo-reductant and allows PET onto the substrate. The open-shell fragment formed upon N-S or C-S bond-rupture abstracts a hydrogen atom from the photocatalyst BIA-H • which is turned into a biradical BIA: and, upon intramolecular single electron transfer, the benzimidazolium BIA is regenerated. Eventually, a hydride transfer activates the catalyst for another catalytic cycle (Scheme 11). The acidic hydroxy group on the aryl oxide is easily deprotonated and enables to directly employ the benzimidazoline BIA-H instead of the betaine BIA as catalyst. In that case, the addition of base (sodium carbonate or butoxide) increased the reaction efficiency significantly, indicating a facile deprotonation of BIA-H.

Anthrolate-catalyzed Generation of Hydrated Electrons
Goez and co-workers thoroughly investigated the potential use of anionic 9-anthrolate (ANT − ) as a sustainable source for hydrated electrons which are ejected upon laser irradiation. [20] Hydrated electrons are among the strongest reductants [79][80][81] and are capable to directly reduce dinitrogen [82] or carbon dioxide [83] . Approaches to liberate solvated electrons photochemically often rely on high-energetic and harmful UV-C light. Notably, pulsed 355 nm UV-A laser irradiation of ANT − in alkaline aqueous media afforded hydrated electrons via a biphotonic photoionization pathway. The first photon generates the excited anionic species (S1 state) and the absorption of another photon within the excited state lifetime of *ANT − stimulates photo-ejection of a hydrated electron. The catalytic cycle is closed in presence of the ascorbate dianion, which acts as sacrificial reductant recovering the catalyst from its oxidized species ANT • (Scheme 13). The sequence of photoionization and regeneration of the catalyst could be repeated several times until the system was exhausted. At the same time, the initial concentration of the catalyst remained constant, indicating the robustness of anthrolate against an attack of the exceptionally reducing solvated electron. Despite its minute molar absorption coefficient at the wavelength used for exciting the system, the ascorbate dianion Asc 2− was found to slightly contribute in generating hydrated electrons. Scheme 13. The photocatalytic generation of hydrated electrons reported by Goez and co-workers.
A follow-up work of the Goez group [21] focused on the direct photoionization of the ascorbate dianion in absence of catalyst by applying a 355 nm laser pulse. A possible application of solvated electrons generated in this way was demonstrated based on the efficient dechlorination of chloroacetate as a generic pollutant in waste water detoxification.

Activation of Aryl chlorides with 9-Anthrolate
Recently, the photochemical properties and synthetic applications of a series of 9-anthrone derivatives were studied by König and co-workers and the corresponding anions were found to reach remarkable excited-state oxidation potentials. [22]  In solution, anthrone ANT is in equilibrium with its enolic form and is easily deprotonated to give the visible-light-absorbing anthrolate ANT − . The most efficient catalysts examined in that work are depicted in Figure 3.

REVIEW
These photocatalysts were proved successful in catalyzing the C−H arylation reaction of several (hetero)aryl chlorides with electron-rich (hetero)arenes, isocyanides, phosphite and B2pin2 (Scheme 14). In presence of base, anthrone ANT is deprotonated, which causes a red-shift in the absorption spectrum with a new distinct absorption band arising in the visible range. Upon excitation with blue LED light, the strongly reducing excited anion *ANT − is formed (cf., Table 1 TEMPO-trapping experiments confirmed the formation of aryl radical 21.1a• and bicyclic radical 21.1b•. Remarkably, in contrast to other photocatalyzed procedures for aryl halide activation [37][38][39][40]52,84] no sacrificial electron donor (e.g. DIPEA) was necessary and the scope of aryl chlorides as well as tolerated radical trapping reagents could be broadened. In the model reaction, the catalyst loading could be lowered to 5 mol% (92% yield) which indicates a turn-over number greater than 18. In accordance with recently reported photocatalyzed C−H arylation procedures [37][38][39][40]84] it was found that excess of the trapping reagent is crucial for the reaction outcome, as a stoichiometric amount with reference to the aryl halide resulted in significantly decreased product yield. Anthrolates are converted in presence of oxygen yielding the corresponding anthraquinones, thus reactions were carried out under inert atmosphere. Noteworthy, acridone (ACO) afforded the desired arylation product 23.2 in good yield (83%) in nondegassed solvent and in presence of air, indicating an increased stability in presence of oxygen.

Anthrolate Catalyzed C−H Carboxylation of (Hetero)arenes and Styrenes with CO2
Very recently, the visible-light-absorbing, strong photoreductant tetramethoxyanthrolate TMA − ( * = −2.92 V vs. SCE) was utilized to achieve the photocatalytic direct reduction of (hetero)arenes and styrenes to their respective radical anions. [85] The associated nucleophilic character of such electron rich species was exploited in C−H carboxylation reactions with gaseous CO2 affording the aromatic carboxylic-and cinnamic acids in moderate to excellent yields. Among others, non-prefunctionalized naphthalenes, thiophenes, furans, indoles, pyrazoles and styrenes are converted to the corresponding carboxylic acids under exceptionally mild reaction conditions (Scheme 16). An examined gram scale carboxylation of 2cyanothiophene 26.9 illustrates the ease to scale-up this reaction. Moreover, a late-stage C−H carboxylation of a Boc-protected thiophene analogue of propranolol 26.31 is demonstrated following this procedure. Besides CO2, ketones were found to convert to the corresponding tertiary alcohols (26.32-33) following the same approach. Noteworthy, similar transformations usually require stoichiometric amounts of reactive organolithium reagents and are conducted under low temperature (−78 °C). Thus, a former protection of labile functional groups is often required, leading to a multi-step synthesis. [86][87][88] Scheme 16. Photocatalyzed C−H carboxylation of (hetero)arenes and styrenes and hydroxyalkylation of thianaphthene. a Reaction with ketone (10 eq.) and under nitrogen atmosphere in absence of CO2.
The regioselectivity of the carboxylation reaction can be predicted by theoretical means. In contrast to the carboxylation mediated by organometallic reagents, the reported photocatalyzed, redoxneutral insertion of CO2 into non-activated sp 2 -hybridized C−H bonds benefits from increased regioselectivity, giving rise to only one regioisomer 26. 18   The mechanistic hypothesis was supported by time-resolved luminescence quenching experiments of the catalyst *TMA − in presence of (hetero)arenes and styrenes. Tolerated substrates shortened the excited-state lifetime of the photocatalyst and linear Stern-Volmer plots could be developed. Although the direct reduction of CO2 ( 1/2 = −2.21 V vs. SCE) [89] by the excited catalyst is thermodynamically feasible, a DMSO solution saturated with carbon dioxide was found to scarcely affect the excited-state lifetime. Examined substrates that showed quenching of the photoexcited state of the catalyst however failed to give the respective carboxylic acids are considered to exhibit insufficient nucleophilicity when present as radical anions and thus do not react with carbon dioxide. In addition, deuteriumlabelling experiments of 24.21 in presence of D2O and t BuOD respectively, caused the incorporation of deuterium into the reactive C-2 position, which supports the assumption of a basic radical anion intermediate.

Catalytic Reactions of Anionic Metal Complexes
Transition-metal complexes like the Ru(II) polypyridine or the cyclometalated Ir(III) found widespread applications in photocatalysis, as they are photostable, show tuneable redox potentials and their excited state lifetimes are usually durable. In contrast to neutral complexes like fac-Ir(ppy)3 or cationic metalbased sensitizers [e.g. Ru(bpy)3 2+ , Ir(ppy)2(dtbbpy) + ], anionic transition-metal complexes are barely explored, which could be attributed to photodecomposition with monodentate anionic ligands [90] and the shortage of more stable dianionic ancillary ligands available. Godbert and co-workers were able to synthesize and characterize the anionic iridium complex 28.1 with a dianionic orotate ligand (Scheme 18, top). [91] Later on, the complex was modified by exchanging the 2-phenylpyridine ligands with coumarin-derived ligands (28.2) to increase the visible-light absorption. The authors successfully demonstrated the use of 28.2 in visible-light-driven H2 generation which resembled the first example of a photoinduced electron transfer using an anionic Ir(III) sensitizer. [92]  Based on the well-established fac-Ir(ppy)3 Wenger and coworkers utilized a trisulfonated analogue 29 3− (Scheme 18, top), which renders the sensitizer water-soluble and negatively charged, to generate hydrated electrons. [93] A potential use of hydrated electrons in waste water treatment was demonstrated by the degradation of chloroacetate (Scheme 18, 31.1) and benzyltrimethylammonium salt (31.3). In addition, the defluorination of trifluoromethylbenzoate is possible in presence of such a strong reductant (31.2). The catalytic cycle is depicted in Scheme 18 (bottom right). The photocatalyst is excited with a 447 nm collimated diode laser. Remarkably, the absorption of a second photon stimulates the ejection of the electron within the lifetime (≈1.6 µs) of the excited sensitizer. The photocatalyst is then regenerated by either sodium ascorbate or triethanolamine acting as sacrificial electron donors. Compared to the neutral fac-Ir(ppy)3, the excited state oxidation potential of the anionic sensitizer 29 3− ( * = −1.89 V vs. SCE) was found to be slightly increased.
The trianionic, rare-earth-metal catalyst hexachlorocerate(III) [Ce III Cl6] 3− was found to be effective in the reductive dehalogenation of aryl halides 32.1 using UVA light (Scheme 19). [94] This complex is stable to air and moisture and can be generated in situ by mixing CeCl3 and NEt4Cl in acetonitrile. Blacklight irradiation causes a metal centered excited state with very negative potential ( * ≈ −3 V vs. SCE) [95,96] enabling a PET to the aryl halide 32.1 to afford a Ce IV species. Interestingly, the reaction could also be performed with a catalytic amount of CeCl3, owing to the complementary oxidative photochemistry of [Ce IV Cl6] 2− (Scheme 19, right). [97] The addition of toluene (34) as the terminal reductant allowed to close the catalytic cycle in which it is converted to benzyl chloride 34b upon hydrogen atom abstraction and reaction with Cl2 •− .

REVIEW
Scheme 19. Scope of the CeCl3 catalyzed defunctionalization of aryl halides by the in situ formation of [Ce III Cl6] 3− and a conceivable mechanism for the reaction.
In a follow-up work, the developed catalytic protocol was utilized for the photoinduced Miyaura borylation of aryl bromides and chlorides. Schelter and co-workers used diboron esters which functioned as both borylation reagent and terminal reductant to close the catalytic cycle. [98] Various arylboronic ester could be obtained in moderate to good yields starting from substituted (hetero)aryl chloride derivatives (Scheme 20). Notably, Stern-Volmer quenching experiments revealed that both electron deficient and electron rich substrates do quench the luminescence of the cerium catalyst. The authors also demonstrated that a sequential borylation and subsequent Pdcatalyzed cross-coupling reaction of the formed arylboronic ester is possible. This procedure is beneficial as it avoids prior isolation of the boronate ester. Based on spectroscopic investigations and experimental findings, a reaction mechanism was proposed (cf. Scheme 19). The in situ formed [Ce III Cl6] 3− gets photoexcited by blacklight. Upon PET towards 35.1 and loss of Cl − , an aryl radical is formed which reacts with the diboron ester 36.1 to yield the aryl boronic ester 37.1 and a boryl radical B(OR2) • . The oxidized catalyst is regenerated in presence of excess Cl − via photoinduced ligand-to-metal charge transfer giving rise to the radical anion Cl2 •− . A reaction quantum yield  ˃ 1 was found by actinometry indicating a radical chain mechanism however, no product formation within the dark periods of an intermittent-light experiment was observed. The authors consider the boryl radical, which is stabilized in presence of Cl − , to presumably propagate a chain mechanism via reaction with another substrate molecule. Scheme 20. Scope of the [Ce III Cl6] 3− catalyzed Miyaura borylation. a Aryl bromide was used.

Polyoxometalates as Photocatalysts
Polyoxometalates (POMs) are a class of widely studied molecular metal oxide anions. Their robustness upon irradiation renders them attractive candidates as catalysts. The discussion of POM photocatalysis will be limited herein to recent, selected examples of the decatungstates, routinely employed as sodium (NaDT) or tetrabutylammonium salt (TBADT, see Scheme 22). Hence, for a comprehensive study of POM chemistry we refer the interested reader to excellent reviews. [99][100][101][102][103] Despite being negatively charged, these metal oxide anions act as strong oxidants from their excited states. This rare feature might be explained analogously to what was discussed for eosin Y and fluorescein (vide supra). Tungsten is present in its highest oxidation state (+VI) while the negative charge is centered on the oxygen atoms of the cluster, rendering the metal center highly electron poor and prone to reduction. Upon photoexcitation, a ligand to metal charge transfer (O → M) is proposed, generating a relaxed excited state cluster [W10O32] 4−* which is easily reduced ( * = +2.44 V vs. SCE) [99] . Besides electron transfer reactions, excited decatungstate found widespread interest for its ability to abstract hydrogen atoms from non-activated C(sp 3 )−H bonds. Fagnoni, Ryu and co-workers summarized site-selective C−H functionalizations of alkanes, alcohols, ethers, ketones, amides, esters, nitriles and pyridylalkanes by using decatungstate and explained the observed regioselectivities based on polar and steric effects. [104] In 2018, MacMillan and co-workers demonstrated the powerful merger of anionic decatungstate photocatalysis and transition metal-catalyzed cross-coupling. [105] Based on this methodology, a copper/decatungstate dual catalytic approach was recently developed enabling the C(sp 3 )−H trifluoromethylation of various biorelevant compounds including natural products and medicinal agents in moderate to good yield (Scheme 21). [106] Note, that the introduction of a CF3-group into drug molecules often improves pharmacokinetic properties and is therefore of interest. In case of pyrrolidine (40.2) selectivity for the CF3-functionalization is achieved upon protonation of the amine resulting in stronger and less hydridic α-C−H bonds and thus enabling reactivity at the distal position. Regioselective functionalization was found at the benzylic (40.5-40.6, 40.9) or sterically most accessible, electron-rich C(sp 3   Wu and co-workers disclosed the oxidant-free, site-and Eselective dehydrogenative alkenylation of alkanes or aldehydes with alkenes by combining decatungstate HAT photocatalysis and cobaloxime catalysis. [107] This dual-catalytic strategy enables efficient and direct alkenylation of C−H bonds with hydrogen gas being the sole by-product. A broad range of alkanes and aldehydes could be alkenylated. Notably, aryl halides (Cl, Br, I) alkyl bromides, alkenes and alkynes were tolerated which enables subsequent orthogonal functionalization via transitionmetal catalysis. Moderate to good regioselectivity was observed for alkane substrates 42.13-14 and 42.19. In addition, the concept could be employed to the late-stage alkenylation of natural products (Scheme 23). Another example for a light-mediated asymmetric C−H functionalization was recently demonstrated by Pu-Sheng Wang and co-workers. [108] Upon hydrogen atom abstraction by TBADT, an alkyl, benzyl or allyl radical adds to an exocyclic enone and the resulting -carbonyl radical regenerates the photocatalyst via hydrogen atom transfer. In the enantioselective step, the formed enol-intermediate is protonated by an aligned chiral spiro phosphoric acid generating a stereocenter in -carbonyl position.
Based on the synergy of decatungstate HAT catalysis and nickel catalysis, Wang and co-workers demonstrated the acylation of aryl halides and -bromo acetates with aromatic and aliphatic aldehydes and the resulting aromatic ketones could be obtained in moderate to good yield. [109] In a similar fashion, the group of Zheng disclosed very recently the direct C−H arylation of 10.1002/ange.202009288

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Angewandte Chemie REVIEW aldehydes enabled by merging decatungstate HAT photocatalysis and palladium cross-coupling catalysis. [110] Applying this methodology allowed for the efficient linkage of various (hetero)aryl bromides, iodides and triflates with aromatic and aliphatic aldehydes. Moreover, TBADT was shown to promote H/D exchange reactions of formyl C−H and a wide range of hydridic C(sp 3 )−H bonds in a synergistic system comprised of HAT photocatalyst and thiol catalyst. In presence of D2O, this protocol allowed for the regioselective incorporation of deuterium into pharmaceutical relevant molecules and drug precursors. [111] Furthermore, a few examples are known where polyoxometalates equipped with binding sites on the cluster shell or in presence of co-catalysts participate in reductive CO2 activation or H2 generation. [34]

Excited Anionic Compounds as Reagents
Besides using a light-harvesting anionic catalyst as demonstrated in Section 2, chemical reactions can also be promoted via a direct photoexcitation of anionic reagents which will be discussed in the following part.

Excited State Phenolate as Photoreductant
Recently, Xia and co-workers made use of the remarkable excited-state potential of the phenolate 52.2 ( * = −2.48 V vs. SCE) in a Heck-type arylation reaction promoted by blue LED light. [112] Scheme 27. Substrate scope of the Heck-type arylation reaction reported by Xia and co-workers; a E/Z > 19:1.
The synthetic utility was demonstrated based on the arylation of methyl 4-hydroxycinnamate 52.2 with various (hetero)aryl halides 51.1 (Scheme 27). In addition, other derivatives of cinnamic acid (52.2-13, 52.19), and flavonoids (52.16-18, 52.20) were shown to react smoothly via the generated aryl radical to afford the respective arylation products (53.1 & 54.1) in moderate to good yields. Remarkably, as the proposed mechanistic cycle is redoxneutral, no sacrificial electron donor is necessary. Besides electron deficient aryl iodides, the scope includes electron rich as well as electron neutral derivatives. In contrast, arylation products formed with less activated aryl bromides and chlorides are only shown with activated, electron deficient arenes. The E/Z ratios of the formed arylation products are high for most of the isolated compounds. The mild reaction conditions allowed to convert complex, biologically active substrates like chlorogenic acid, esculin and scutellarin. Upon deprotonation of the phenolic OH group, the absorption spectrum of 52.2 in DMSO is shifted towards longer wavelength enabling direct excitation of the phenolate 52. The resulting radical anion 51.2b •− is assumed to either initiate a radical chain mechanism by reducing another equivalent of 51.2 which affords the desired Heck-type arylation product 53.2 (Path b), or is converted to the latter in presence of the phenoxy radical 52.2 • via direct hydrogen atom transfer or electron transfer followed by a proton shift (Path a).
Melchiorre and co-workers have recently demonstrated how phenolate can elicit the generation of perfluoroalkyl radicals via single electron transfer. [113] The developed method allows for the direct perfluoroalkylation and trifluoromethylation of phenols bearing electron withdrawing substituents 57.2-15 (Scheme 29). In presence of the non-nucleophilic base 1,1,3,3tetramethylguanidine (TMG) the absorption spectrum of salicylaldehyde (55.2) is red-shifted and no change was observed upon addition of the perfluoroalkyl iodide 56.1, excluding the formation of a ground state EDA complex. The base-induced bathochromic shift allowed for the use of a CFL bulb as light source. Using a 300 W Xe lamp with cut-off filter (λ > 385 nm) still allowed to form the product however in slightly decreased yield. The proposed mechanism of this transformation (Scheme 30) starts with a SET from the photoexcited phenolate *55.1 − to 56.1. Subsequent reductive cleavage of iodine gives rise to a perfluoroalkyl radical 56.1a • . Monitoring the product distribution over the reaction time revealed that o-and p-alkylated products are formed as intermediates and are further converted to bifunctionalized ortho, para-adducts. Non-substituted or methoxy-substituted phenols as well as nitrophenols failed to convert. Employing phenol 55.12 bearing electron withdrawing groups in ortho and para position afforded the mono-alkylated product as sole isomer. The demonstrated scope of perfluoroalkyl iodides comprises C8, C6, C4 and C1 chains (57.12-15).

Visible-light-promoted Arylation of Azaallyl Anions
Chruma and co-workers demonstrated how irradiation of the colored azaallyl anion 58.1 − with visible light notably increases its excited state oxidation potential. [114] In presence of strong bases (pKa conjugated acid > 32), the formed 2-azaallyl anion acts as super-electron-donor in the dark [115] and had been successfully employed in the functionalization of non-activated aryl iodides and tertiary alkyl halides.
The accessible substrate scope could be extended by employing visible light causing enhanced reduction potentials and allowed for the conversion of non-activated bromo-and chloro-(hetero)arenes 59.1, which are present in large excess with reference to 58.1. The regioselectivity of the arylation reaction is moderate and product mixtures of 60.1 and 61.1 are usually obtained (Scheme 31).

Synthesis of Pyrazoles via Irradiation of N-centered Hydrazone Anions
Zhu and co-workers reported a series of substituted hydrazones 62.1 which are able to undergo cyclization in presence of base, affording pyrazole derivatives 63.1 mediated by sunlight. [116] The UV-vis spectrum of the anionic hydrazone exhibits a significant red-shift compared to the neutral parent, enabling the use of visible light to accomplish the cyclization reaction. Selected examples of formed pyrazoles are depicted in Scheme 33.

REVIEW
cleavage of a tosyl radical, yields the pyrazole 63.1. Decreased yield is obtained when conducting the reaction under N2 atmosphere or in presence of the radical trap TEMPO, indicative for the latter mechanistic proposal. Notably, the reactions were also shown to operate in water, however resulted in decreased yields. Scheme 34. Proposed photoinduced reaction mechanism towards pyrazole formation.

Utilizing Phthalimide Anions for H-Atom Abstraction
Already in 1988 the exceptionally high ability of the excited phthalimide anion *64 − to abstract hydrogen atoms from alcoholic solutions was recognized. [117] This procedure was further developed and could be extended to ethers, alkylbenzenes and amines, affording the reductive addition products with phthalimide (Scheme 35). [118] The use of 4-methylanisole afforded a product mixture (66.8a-b) as H-atom abstraction is possible from the methoxy group or in benzylic position. In alkaline solution, phthalimide 64 is in equilibrium with its conjugate base 64 − . The photoinduced electron transfer from *64 − to ground-state phthalimide is a thermodynamically favourable process. Thus, the authors propose the phthalimidyl radical 64 • as the hydrogen atom abstracting intermediate, which evolves from the excited anion *64 − upon PET towards phthalimide 64. Remarkably, the electrophilic radical 64 • is able to activate C−H bonds possessing high bond dissociation energies (e.g. t BuOH, Ediss = 100 ± 2 kcal•mol -1 ) [119] and upon hydrogen abstraction, phthalimide 64 and the alkyl radical 65.9 • are formed. Radicalradical coupling between the phthalimide radical anion 64 •− and the carbon-centered radical 65.9 • affords the addition product 66.9 (Scheme 36).
[2+2] photocycloaddition of excited phthalimide anion with alkenes. Remarkably, common approaches to form benzo-fused sevenmembered sultam derivatives are multistep reactions and rely on the use of toxic organotin hydrides [123] or expensive Pd catalysts [124] . A mechanism was proposed based on experimental and computational studies, suggesting the prevailing population of the S2 state upon irradiation of the saccharin anion 69 − . The computed data indicate a fast deactivation into the first singlet state. Presumably, the key step towards benzosultam formation is a nucleophilic attack of the nitrogen of the excited state saccharin anion to the alkene. Moreover, no evidence for an azetidine intermediate (cf., 64.3a − , Scheme 38) resulting from [2+2] cycloaddition of saccharin and alkene was found neither in experiment nor in computational analysis. The C−C bond formation between carbonyl group and alkene is expected to occur in the ground state. Regioselectivity is gained due to the kinetic preference of the nucleophilic nitrogen atom to attack at the terminal, sterically less hindered side (Scheme 40). Scheme 40. Proposed reaction mechanism for the light-promoted formation of benzosultams.

Organic Anions involved in Donor-Acceptor Complexes
Organic anions are also reported to form ground-state electron donor-acceptor (EDA) complexes with electron deficient species usually accompanied by the appearance of a new redshifted charge-transfer absorption band. During the last years, EDA photochemistry has become increasingly popular. Among others, we highlight herein three examples to demonstrate the concept of organic anions participating in EDA complex formation. For a more detailed study we refer to recent excellent reviews. [125,126] The aromatic perfluoroalkylation of α-cyano arylacetates 72.1 developed by Melchiorre and co-workers [127] is mediated by visible light (CFL 23 W) although neither enolate 72.1 − nor perfluoroalkyl iodide 56.1 or TMG show absorbance in that range of light. Mixing all the reagents together however results in a colored solution featured by a strong bathochromic shift in the absorption spectrum indicative for the formation of an EDA complex. Irradiation of p-substituted substrates allowed to perfluoroalkylate α-cyano arylacetates selectively in ortho position. A mixture of regioisomers was obtained when m/o-substituted substrates were employed. In accordance with the proposed homolytic aromatic substitution (HAS) pathway lower yields were obtained with electron deficient arenes. Following the developed protocol, the substrate scope could be extended including heteroarenes and α-cyano phenylketone (Scheme 41). Control experiments revealed that the formed product inhibits the reaction as the forming enolate 73.1 − outperforms the absorbance of the EDA complex. This issue was addressed by utilizing a biphasic system consisting of tetradecafluorohexane and MeCN, which allowed for higher yields and a shorter reaction time.

Accepted Manuscript
Angewandte Chemie REVIEW protocol allowed to convert both electron rich and poor thiophenols under visible-light irradiation and in presence of caesium carbonate (Scheme 43). Remarkably, tolerated aryl halides are not limited to activated, electron-deficient arenes, as thioethers were formed with iodobenzene and toluene; however, a prolonged reaction time was required (20-24 h).
Remarkably fast coupling reactions (1 h) were observed between electron deficient aryl halides and electron rich thiophenols. In addition, benzylic halides revealed to convert similarly. Following the developed protocol allowed for the mild and efficient latestage functionalization of pharmaceutically active compounds. Scheme 44. Proposed reaction mechanism for the C-S cross-coupling reaction of thiophenols and aryl halides via the formation of a visible-light-absorbing EDA complex.
The formation of an EDA complex 74.2 − -EDA between thiophenolate 74.2 − and aryl halide 75.2 was confirmed by UV-vis spectroscopy and TD-DFT calculations. The arising chargetransfer absorption band allows to initiate the reaction with visible light via an electron transfer from the thiolate anion to the aryl halide, followed by cleavage of the halide anion. The formed thiyland aryl radical combine to afford the C−S cross-coupled product 76.2 (Scheme 44).
Based on the perfluoroalkylation of alkenes and alkynes, it was recently shown that the anionic counterpart involved in the EDA complex formation can be utilized catalytically. [129] In presence of base, 2-bromophenol (BrPhOH) was found to promote the visible-light-mediated 1,2-addition of fluoroalkyl iodides to alkenes and alkynes. Noteworthy, although a significant amount of product was formed in the reaction of allylbenzene 77.2 and ethyl iododifluoroacetate 56.6 in absence of phenol catalyst, the yield could be doubled using a catalytic amount of BrPhOH. The use of a more polar solvent gave rise to Heck-type coupling products. Allylphenols, acting themselves as catalyst, could be

Organic Anions promoting the Radical-Nucleophilic Substitution (SRN1) Reaction
In the course of SRN1 reactions, radicals and radical anions are formed as intermediates and chain mechanisms are likely to occur. Proposed for the first time in the 1960s, [130,131] the reaction affords nucleophilic substitution on aromatic and aliphatic compounds and tolerates a wide scope of nucleophiles and substrates. [132] Initiation is commonly achieved by photoinduced electron transfer from an electron-rich anionic nucleophile to an electron-poor acceptor, leading to the open-shell nucleophile Nu • and a radical anion. EDA complex formations between nucleophile and substrate are reported and allow to initiate SRN1 reactions by using less-energetic light. [125] Upon mesolytic bond cleavage, the resulting radical R • is trapped by the nucleophile and forms a radical anion. A single electron transfer from the radical anion [R-Nu] •− to the acceptor R-X affords the desired substitution product along with another radical anion [R-X] •− , which enables the propagation of a chain reaction (Scheme 46), provided that this SET is thermodynamically favourable.

REVIEW
Scheme 46. General reaction mechanism for the radical-nucleophilic substitution (SRN1) reaction and for the photoinitiated base-promoted homolytic aromatic substitution reaction (photo-BHAS).
Closely related to the concept of the light-induced SRN1 reaction is the photoinitiated base-promoted homolytic aromatic substitution reaction (photo-BHAS), affording C−H arylated products starting from aryl or alkyl halides in presence of a strong base (e.g. KO t Bu or NaH). The reactive intermediate R • is proposed to add to the arene forming an aromatic radical, which is converted into the respective radical anion by deprotonation and eventually gives the arylated product upon SET to propagate the chain reaction. In absence of further additives, it has recently been shown that the dimsyl anion can be excited by visible light and plays a pivotal role for initiating the reaction (see Scheme 46). [133] The initiation of the BHAS reaction was also reported by other photo-activation modes e.g. through PET from an iridium sensitizer to R-X, or upon light-excitation of an in-situ formed photosensitive complex between KO t Bu and phenanthroline. [134,135] Non-nucleophilic bases are commonly employed to avoid the competing SRN1 reaction pathway. Lightmediated substitutions following the SRN1 reaction with organic anions as nucleophiles have been studied extensively and were subject of recent reviews [4,132,[136][137][138][139][140] and thus will not be further discussed herein.

Direct Photodecarboxylation of Carboxylates
In presence of light, various organic carboxylates are known to undergo photodecarboxylation (PDC) affording CO2 and either a carbanion intermediate (heterolytic cleavage) or an alkyl radical intermediate in combination with a solvated electron (homolytic cleavage). Meiggs et al. [141] performed flash photolysis of sodium phenyl acetate and could proof the formation of a benzyl radical intermediate by transient absorption spectroscopy. The formation of toluene, besides polyacids and bibenzyl, may suggest a competing heterolytic bond cleavage mechanism. Reaction pathways via high-energetic carbanion or radical intermediates are favoured in compounds bearing stabilizing substituents. Hence, PDC is often observed upon irradiation of dissociated aryl acetic acids 81.1, causing intermediates which benefit from benzylic stabilization (Scheme 47). The light-mediated decomposition of carboxylates has been covered in detail in various reviews and thus is beyond the scope of this work. [142][143][144] Photobases are important initiators of photopolymerization processes. Xanthone and thioxanthone acetic acids (81.7-8) form carbanions upon decarboxylation and have recently received interest as amine free alternatives to enable efficient thiol-epoxy polymerization. [145,146] Scheme 47. General scheme for the photodecarboxylation of dissociated aryl acetic acids and prominent examples.

Sulfite Anions used in Photoreactions
The ability of cheap and available sulfite salt to generate hydrated electrons upon irradiation renders its use attractive (Scheme 48). The method was successfully applied for the photodegradation of hazardous halogenated pollutants like monochloroacetic acid [147] 83 and perfluorooctanesulfonate [148] . However, harmful highenergetic UV-light (254 nm) is necessary to photoexcite sulfite anions and the process efficiency suffers in more complex media due to light attenuation by scattering or competing absorption of other organic compounds including the solvent.

Summary and Outlook
Organic anions and light are a perfect combination to achieve challenging synthetic transformations as either reagents or photocatalysts. Compared to a corresponding neutral molecule, the absorption spectrum of the negatively charged anion usually exhibits a bathochromic shift and often fluorescence is exclusively observed for the anionic species. This allows photochemical conversions with less-energetic light, in many cases visible light. Fluorescence quenching studies enable the verification of interactions between substrates and the excited chromophore. The seminal work of Soumillion and co-workers in this field and their excellent review [33] demonstrated early the potential of organic anions as strong photoreductants in the dechlorination of arenes and the desulfonylation of sulfonamides using excited 2naphtholate. The oxygen-centered radicals of photoexcited anionic decatungstates allow to break strong C(sp 3 )−H bonds of non-prefunctionalized alkanes to form new carbon bonds. Synergistic approaches of HAT and transition-metal-catalysis have recently found widespread interest and also enabled asymmetric reactions. In addition to the use of anions as photocatalysts, excited anions found applications as strong reductants to activate a reaction partner via PET followed by a subsequent conversion of both open-shell intermediates. Examples are the arylation of azaallylanions or the Heck-type 10.1002/ange.202009288

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Angewandte Chemie REVIEW arylation of vinylphenols. Photoexcited organic anions allow cyclization reactions yielding pyrazoles or participate in ringexpanding reactions. Moreover, organic anions serve as potent electron-rich donor molecules for the formation of light-absorbing EDA complexes.
Overall, the use of photoexcited anions harbours enormous potential for applications in synthetic organic chemistry. We observe an increasing research interest in applying photoexcited anions as catalysts or reagents and hope that this review will stimulate more contributions to this yet underexplored but emerging field, which holds promise for many more exciting applications in organic synthesis.