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Counterion Effects in [Ru(bpy)3](X)2-Photocatalyzed Energy Transfer Reactions
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Cite this: JACS Au 2024, 4, 8, 3049–3057
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https://doi.org/10.1021/jacsau.4c00384
Published July 24, 2024

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Abstract

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Photocatalysis that uses the energy of light to promote chemical transformations by exploiting the reactivity of excited-state molecules is at the heart of a virtuous dynamic within the chemical community. Visible-light metal-based photosensitizers are most prominent in organic synthesis, thanks to their versatile ligand structure tunability allowing to adjust photocatalytic properties toward specific applications. Nevertheless, a large majority of these photocatalysts are cationic species whose counterion effects remain underestimated and overlooked. In this report, we show that modification of the X counterions constitutive of [Ru(bpy)3](X)2 photocatalysts modulates their catalytic activities in intermolecular [2 + 2] cycloaddition reactions operating through triplet–triplet energy transfer (TTEnT). Particularly noteworthy is the dramatic impact observed in low-dielectric constant solvent over the excited-state quenching coefficient, which varies by two orders of magnitude depending on whether X is a large weakly bound (BArF4–) or a tightly bound (TsO–) anion. In addition, the counterion identity also greatly affects the photophysical properties of the cationic ruthenium complex, with [Ru(bpy)3](BArF4)2 exhibiting the shortest 3MLCT excited-state lifetime, highest excited state energy, and highest photostability, enabling remarkably enhanced performance (up to >1000 TON at a low 500 ppm catalyst loading) in TTEnT photocatalysis. These findings supported by density functional theory-based calculations demonstrate that counterions have a critical role in modulating cationic transition metal-based photocatalyst potency, a parameter that should be taken into consideration also when developing energy transfer-triggered processes.

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Introduction

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Photocatalysis under visible-light irradiation has witnessed tremendous successes in recent years in a wide variety of applications (1−7) and has notably initiated a paradigm shift in organic synthesis. (8−19) Within the design of efficient molecular photocatalytic systems, the architecture of metal-based photocatalysts can easily be tuned to efficiently achieve the desired photochemical properties required to promote specific transformations. (20) Thanks to their strong absorption in the visible region, rapid intersystem crossing, and long-lived triplet excited state, polypyridyl ruthenium(II) and cyclometalated iridium(III) complexes are the most widely explored classes of photoredox catalysts and photosensitizers. (21) The common method to manipulate excited-state properties of such photocatalysts relies on modifying the ligand scaffold around the metal center: a modus operandi that has provided access to a plethora of photocatalysts covering a broad range of redox potentials and triplet excited state energies. (22,23) Importantly, a large majority of these Ir(III) and Ru(II) complexes, but also of emerging earth-abundant transition metal-based luminescent complexes, (24−31) are cationic species associated with counterion(s) to ensure charge balance of the molecular photocatalysts. Nevertheless, while photocatalytic reaction cycles are usually modeled considering the properties of the sole organometallic cation, the true nature of the photocatalyst is that of a salt in which the anions could play a role beyond the one of mere spectators. (32−38) In this respect, recent reports have brought to light the influence of counterion identity on the efficiency of single electron transfer (SET) processes operated by cationic iridium and ruthenium photoredox catalysts (Figure 1A). (39−42) However, despite the fact that both SET and energy transfer (EnT) photocatalyses rely on the triplet metal-to-ligand charge-transfer (3MLCT) excited state of the photocatalyst, counterion effects on EnT processes have not been demonstrated yet. In triplet–triplet energy transfer (TTEnT) processes, a burgeoning field of research with a wide range of synthetic applications, (43−46) the photocatalyst is excited by the direct absorption of visible light and subsequently transfers its triplet excited state energy to a desired substrate, opening otherwise elusive relaxation channels and photoreactive routes, leading to original chemical transformations. (47) Herein, we show that modification of the X counterions in the emblematic [Ru(bpy)3](X)2 complex series has a dramatic impact on their photophysical properties and photocatalytic activity in TTEnT. Specifically, we demonstrate that large noncoordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF4–) anions are highly beneficial, giving rise to much higher photocatalytic activity (up to >1000 TON) in intermolecular [2 + 2] cycloaddition reactions of olefins to yield cyclobutanes (Figure 1B). (48) We rationalize the superior photocatalytic performance of [Ru(bpy)3](BArF4)2 over its congeners on the basis of experimental and density functional theory (DFT) studies that indicate a favorable increase in the energy of both the 3MLCT and the metal-centered (3MC) excited states, which are respectively responsible for a larger energy transfer driving force and an improved photostability. Our results provide fundamental insights into how counterions impact cationic transition metal-based photocatalyst potency in triplet energy transfer-activated reactions, a topic of great interest but surprisingly overlooked so far in the literature.

Figure 1

Figure 1. Counterion dependence in photochemical processes. (A) Previously reported counterion effects on single electron transfer (SET). (B) Counterion effects in [Ru(bpy)3](X)2-catalyzed intermolecular [2 + 2] cycloaddition enabled by energy transfer. Bimolecular quenching rate constant (kq). 3,5-Bis(trifluoromethyl)benzenesulfonate (ArFSO3–). Turnover number (TON).

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Results and Discussion

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As a model reaction to evaluate the impact of counterion effect in TTEnT photocatalysis, we selected the [2 + 2]-cycloaddition of α,β-unsaturated 2-acyl imidazole 1a with styrene 2a using [Ru(bpy)3](X)2 as a triplet sensitizer under visible-light irradiation. The choice of the reaction has been made in light of recent studies on “triplet activation” of 1a, (49−52) in particular that of Yoon and co-workers, (50) who showed that the presence of trifluoromethanesulfonic acid as a Brønsted acid cocatalyst is strictly necessary to efficiently promote this [Ru(bpy)3]2+-catalyzed [2 + 2] photocycloaddition in acetonitrile. The effect was justified by the fact that the protonated acyl imidazole substrate is a better energy acceptor for the triplet excited state of [Ru(bpy)3](Cl)2*. We therefore assumed that by modifying the counterions of [Ru(bpy)3](X)2 photosensitizers and assessing their ability to promote the aforementioned [2 + 2] photocycloaddition in the absence of any additive, we could acquire the first evidence of counterion effects in TTEnT photocatalysis. The results of the initial screening of a series of Ru(II) photosensitizers for the intermolecular [2 + 2] cycloaddition between 1a and 2a are summarized in Table 1. Under the standard conditions, using 2.5 mol % of [Ru(bpy)3](X)2 as a sensitizer under blue-light irradiation, the rates of the photocycloaddition performed in either acetonitrile or dichloromethane as solvent were compared after 2 h of reaction. [Ru(bpy)3](Cl)2 proved incompetent as a photosensitizer to promote the desired transformation, affording a very low 5% yield of the cyclobutane products in acetonitrile and displaying a complete lack of catalytic activity in dichloromethane, and was therefore left out for the rest of the experimental study. (50,53,54) On the other hand, using p-toluenesulfonate (TsO–), trifluoromethanesulfonate (TfO–), hexafluorophosphate (PF6–), or BArF4– anions proved beneficial, and similar yields (∼30%) of the desired cyclobutane products were obtained in acetonitrile after 2 h. More interestingly, a striking difference in reaction rates was observed in dichloromethane depending on the counterion identity. While up to 43% yield of the [2 + 2]-cycloaddition products was obtained after 2 h of irradiation using the [Ru(bpy)3](BArF4)2 photocatalyst, the reaction proceeded to 26% yield with [Ru(bpy)3](PF6)2, 22% with [Ru(bpy)3](TfO)2, and only 8% yield with [Ru(bpy)3](TsO)2.
Table 1. Initial Screening of Counterion Effects on EnTa
X– =Cl–TsO–TfO–PF6–BArF4–
yielda (in CH3CN)531303029
yieldb (in CH2Cl2)ndc8222643
a

Reaction conditions: 1a (0.1 mmol), 2a (1 mmol), [Ru(bpy)3](X)2 (2.5.10–3 mmol), degassed solvent (2 mL), and blue LED irradiation (λmax = 460 nm) for 2 h under Ar.

b

Yield in % of 3a+3a′ determined by 1H NMR using 1,3,5-trimethoxybenzene as an internal standard.

c

Products 3 were not detected by 1H NMR.

In order to gain a better understanding of the parameters influencing the catalytic performances of this series of [Ru(bpy)3](X)2 photosensitizers, their optical properties were recorded in both acetonitrile and dichloromethane solutions. In acetonitrile, characterized by relatively high dielectric constant (εr ∼ 37) impeding ion association, the properties of the ground state and [Ru(bpy)3]2+* excited state of solvated [Ru(bpy)3](X)2 salts should not be significantly affected by the nature of the counterion. Indeed, all complexes exhibited a similar metal-to-ligand charge-transfer (MLCT) absorption band at λmax = 451 nm with similar molar extinction coefficients (15,000 M–1 cm–1) and demonstrated superimposable emission spectra with λem = 605 nm (Figure 2A). In dichloromethane (εr ∼ 9), on the other hand, while the impact of counterion identity on absorption spectra was modest with only a slight increase of molar extinction coefficients from the more coordinating TsO– anion to the least coordinating BArF4–, (55) differences were more significant in emission spectroscopy (Figure 2B). (56) First, the emission of the TsO– complex is characterized by a maximum at 604 nm/2.053 eV, and then the TfO– and PF6– complexes showed a slight hypsochromic shift (λem = 598 nm/2.073 eV and 596 nm/2.080 eV, respectively) and finally the complex with the largest and least coordinating BArF4– ions exhibited the most blue-shifted emission at λem = 575 nm/2.156 eV, up by 29 nm/0.103 eV. This relative ordering was satisfactorily reproduced by DFT ΔSCF calculations (Table S24). In dichloromethane of low dielectric constant, the emission energy of ion pairs thus seems to correlate with the charge density of the counterions. In fact, in the MLCT state, the transferred electron is localized on a single bipyridine ligand inducing a dipole moment in the [(bpy•–)RuIII(bpy)2]2+* excited state of C2 symmetry, (57−59) which can be stabilized by anion–dipole interaction with the paired anions positioned away from the formally reduced ligand (Figure 2C). The extent of this stabilization is inversely correlated with the counterion size. (60) The blue-shifted emission of [Ru(bpy)3](BArF4)2 reflects its increased 3MLCT energy, which is 0.103 eV/2.4 kcal/mol higher than the triplet energy of [Ru(bpy)3](TsO)2, which could in part account for the improved performance of the BArF4– photocatalyst by increasing the driving force for energy transfer from its triplet state (ET = 49.7 kcal/mol) (61) to the acyl imidazole substrate 1a (ET = 47.6 kcal/mol). (50) The GS-3MLCT gaps for [Ru(bpy)3](X)2 complexes have been computed as the difference in their electronic energies at their respective equilibrium geometries in dichloromethane, as reported in Table S23. They follow a trend that perfectly reflects the experimental observations: the gap goes from 46.6 kcal/mol for [Ru(bpy)3](BArF4)2 to 45.4 kcal/mol for [Ru(bpy)3](PF6)2, 44.4 kcal/mol for [Ru(bpy)3](TfO)2 and 44.1 kcal/mol for [Ru(bpy)3](TsO)2. These values correspond to the electrostatically favorable adjacent arrangement of the counterions, i.e., counterions in neighboring quadrants and remote from the formally reduced bpy ligand, as opposed to an unfavorable opposite arrangement where counterions would be diametrically opposed with respect to the metal center and with one counterion in the immediate vicinity of the reduced bpy ligand (ΔEadj/opp = 0.6–3.7 kcal/mol) (Figure 2D and Table S23). Noteworthy is the 3MLCT state of [Ru(bpy)3](BArF4)2 with an adjacent arrangement of the counterions that is slightly different from the others. Due to their bulkiness and specific geometry, the BArF4– counterions are pinching both neutral bpy ligands in a clip-like configuration (Figure 2E, Figure S23 and the SI for related discussions and additional visual representations).

Figure 2

Figure 2. Counterion effect on absorption (solid lines) and emission (dashed lines, λex = 450 nm) properties of [Ru(bpy)3](X)2 in acetonitrile (A) and in dichloromethane (B) at 25 °C. (C) 3MLCT state of [Ru(bpy)3](X)2. (D) Opposite and adjacent arrangements of counterions in calculated 3MLCT states of [Ru(bpy)3](X)2 shown for X– = PF6– in ball and stick (left) and space-filling (right) models (and Figure S22 for X– = Cl–). (E) Specific arrangement of counterions in the calculated 3MLCT state of [Ru(bpy)3](BArF4)2.

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To gain further insight into the influence of counterion on energy transfer performance, we conducted Stern–Volmer experiments of intermolecular deactivation of the triplet excited states of ruthenium complexes by an increasing concentration of substrate 1a as a quencher. (62) First, in agreement with the very similar catalytic activities observed in acetonitrile (vide supra, Table 1), all excited complexes demonstrated the same ability to be quenched by 1a with a Stern–Volmer rate constant (Ksv) of ∼8.2 M–1 (Figure 3A). In sharp contrast, Stern–Volmer plots measured in dichloromethane evidenced large differences depending on the identity of the counterions (Figure 3B). The quenching rate constant of the photoexcited catalysts by the substrate increased by a factor of 25 from the TsO– to the BArF4– counterion, also in line with the markedly superior photocatalytic performance of the latter. Then, their 3MLCT excited-state lifetimes (τo) were measured in dichloromethane and once again a significant influence of the counterions was established. The BArF4– complex evidenced the shortest excited-state lifetime (το = 231 ns), which is almost four times shorter than that of the TsO– complex (το = 804 ns). This behavior is usually rationalized by a reduced energy gap between the 3MLCT excited state and the low-lying metal-centered (3MC) excited states of [Ru(bpy)3]2+, which facilitates the 3MLCT-3MC internal conversion enabling more efficient nonradiative deactivation pathways, (63,64) as reflected in the decreased luminescence quantum yield (Φlum, Table 2). The better performance of the [Ru(bpy)3](BArF4)2 photocatalyst is all the more remarkable that its excited state lifetime is significantly reduced, which decreases the probability of encountering the substrate for productive energy transfer. The actual quenching rate constant (kq) for this series of [Ru(bpy)3](X)2 photosensitizers could be accessed thanks to the Stern–Volmer relationship (Ksv = kq τo), and remarkably, a decrease by two orders of magnitude between the large and noncoordinating BArF4– and the small coordinating TsO– was determined, highlighting the impressive superiority of [Ru(bpy)3](BArF4)2 to efficiently transfer its triplet energy to the α,β-unsaturated 2-acyl imidazole substrate 1a. This reactivity could be explained by the higher tendency of the bulky BArF4– counterions to dissociate from their cation to form solvent-separated ion pairs, (40,65) thus unshielding the [Ru(bpy)3]2+ cation in its triplet excited state and facilitating collisions with the substrate for effective triplet–triplet energy transfer. (66,67)

Figure 3

Figure 3. Stern–Volmer plots for excited-state quenching of [Ru(bpy)3]X)2 by 1a in acetonitrile (A) and in dichloromethane (B). Excited-state lifetime of [Ru(bpy)3](X)2 in deaerated CH2Cl2 (C).

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Table 2. Counterion Dependence of the Optical Properties and Quenching Constants of [Ru(bpy)3](X)2 in Dichloromethanea
X–ε (M–1·cm–1)λem (nm)Ksv (M–1)τ0 (ns)bkq (106 M–1 s–1)ckr (104 s–1)dknr (106 s–1)eΦlum
TsO–12,4506040.468040.578.711.160.070
TfO–15,3405982.406943.467.921.360.055
PF6–16,0205963.346754.957.411.410.050
BArF–17,53057512.8623155.679.964.230.023
a

Photophysical data were obtained in dry, deaerated CH2Cl2 at 293 K. ε is the molar extinction coefficient at the lowest-energy absorption band maximum, and λem is the wavelength of the emission band maximum.

b

λexc = 455 nm.

c

kq = Ksv/τ0.

d

kr = Φlum/τ0.

e

knr = (1 – Φ lum)/τ0.

Another parameter that plays an important role in photocatalysis is the photostability of catalysts. (63) Nevertheless, [Ru(bpy)3]2+ complexes are recognized for their relatively low photostability in nonpolar solvents such as dichloromethane. In fact, photodecomposition of [Ru(bpy)3]2+ is known to operate through photodissociation of a bipyridine (bpy) ligand, a phenomenon that becomes significant in solvent of low dielectric constant with strongly coordinating anions that can substitute the bpy ligand leading to photocatalytically inactive Ru(bpy)2X2 complexes such as Ru(bpy)2Cl2. (53,54) Therefore, the photostability of our series of [Ru(bpy)3](X)2 complexes was investigated by monitoring the luminescence intensity of CH2Cl2 solutions irradiated with blue LEDs (λmax = 460 nm). The decrease of luminescence intensity (I/I0) as a function of irradiation time is presented in Figure 4. As expected, [Ru(bpy)3](TsO)2 with the most coordinating counterion in the series evidenced the lowest photostability, leading to almost complete decomposition (>85%) after only 10 min of irradiation. A significantly lower decomposition rate was measured for [Ru(bpy)3](TfO)2 and [Ru(bpy)3](PF6)2, which maintained 25 and 40% of their initial luminescence after 1 h of irradiation, respectively. Interestingly, [Ru(bpy)3](BArF4)2 exhibited the highest photostability being roughly twice as robust as [Ru(bpy)3](PF6)2. The superior photostability of [Ru(bpy)3](BArF4)2 compared to [Ru(bpy)3](PF6)2, both involving noncoordinating ions, is of note given its increased 3MLCT state energy. DFT calculations were performed to optimize the lowest MLCT and MC states for the PF6– and BArF4– complexes (and the bare cation as a reference, SI, since all our previous calculations on [Ru(bpy)3]2+ were conducted using MeCN as implicit solvent). (68−71) Indeed, several types of MC states may be invoked, whose electronic structures differ in the nature of the populated dσ* orbital. As a global picture, when populating a dz2-like orbital, two major Ru–N elongations will occur involving two pyridines from separate ligands in tris(bidentate) complexes such as Ru(bpy)32+, leading to what we termed MCtrans states. On the contrary, if a dx2–y2-like orbital is populated, three or four major elongations will occur, two of which involving the same ligand, leading to MCcis states. For this reason, photoinstability issues due to ligand loss have been suggested to involve preferably dx2–y2-based MCcis states while nonradiative deactivation has been suggested to proceed preferably through dz2-based MCtrans states (Figure 5). (68,70,71) The calculated Jablonski diagram (Figure 6) shows that MCcis states (red dots) are higher in energy and better separated from MCtrans states (green dashes) for the BArF4– complex compared to the PF6– analogue, in line with the higher photostability of the former. Besides, since the 3MLCT lifetime is long enough to allow relaxation of the anions, (40) the globally lowest state, labeled as 3MLCT-adj and presenting adjacent anions, is the one that should be considered as the most populated. The gap between 3MLCT-adj and 3MCtrans-adj is smaller for [Ru(bpy)3](BArF4)2 (5.3 kcal/mol) than for [Ru(bpy)3](PF6)2 (7.0 kcal/mol), in agreement with the shorter MLCT lifetime of the BArF4– complex (vide supra Figure 3C and Table 2).

Figure 4

Figure 4. Photostability of [Ru(bpy)3](X)2 complexes in dichloromethane (8 mM) at 25 °C under 50W blue LED (λmax = 460 nm) irradiation.

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Figure 5

Figure 5. Schematic representation of 3MC states of [Ru(bpy)3]2+ with elongation along either dx2–y2 (3MCcis, left) or dz2 (3MCtrans, right).

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Figure 6

Figure 6. Diagram schematizing the relative electronic energies of optimized GS (black), 3MLCT (blue lines), 3MCcis (red dots), and 3MCtrans (green dashes) states for [Ru(bpy)3](BArF4)2 and [Ru(bpy)3](PF6)2 complexes. Multiple states of the same color indicate various anion arrangements, such as adjacent (adj) and opposite (opp) (see text, Tables S22, S23, S25 and the SI).

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After demonstrating the high photostability of the BArF4– catalyst and its excellent ability to transfer its triplet energy to substrate 1a in dichloromethane, we optimized the conditions for the [2 + 2] photocycloaddition reaction in the presence of styrene 2a as a partner. First, using 2.5 mol % of [Ru(bpy)3](BArF4)2 as photocatalyst and by increasing the reaction concentration from 0.05 to 0.1 M, the rate of the cycloaddition after 2 h of irradiation under blue LEDs (λmax = 460 nm) improved to reach 56% yield of the desired product (Table 3, entries 1 and 2). Interestingly, a decrease of the photocatalyst loading from 2.5 to 1 mol % (Table 3, entry 3) and 0.5 mol % (Table 3, entry 4) did not significantly affect the productivity since very similar results were obtained after 2 h of reaction, allowing to reach a TON above 100 in the latter case. The optimized conditions used a low photocatalyst loading of 0.5 mol % and a reaction concentration of 0.2 M (Table 3, entry 5), which allowed full conversion within 16 h at room temperature and 72% yield of the desired cycloaddition products (Table 3, entry 6). Finally, by further decreasing its catalytic loading and increasing the reaction concentration, we were able to highlight the outstanding catalytic performance of the BArF4– photosensitizer, which achieved an impressive TON of up to 1040 at a very low loading of 500 ppm (Table 3, entry 7, and the SI).
Table 3. Optimization Using [Ru(bpy)3](BArF4)2 as Catalysta
entry[Ru(bpy)3](BArF4)2(mol %)[1a] (mol L–1)time (h)yield (%)bTONc
12.50.0524317
22.50.125622
310.125654
40.50.1254108
50.50.2270140
60.50.21672144
70.051.016521040
a

Reaction conditions: 1a (0.1 mmol), 2a (1 mmol), [Ru(bpy)3](BArF4)2 (0.05 to 2.5 × 10–3 mmol), degassed CH2Cl2 (0.25 to 2 mL), and blue LED irradiation (λmax = 460 nm) for 2 to 16 h under Ar.

b

Yield in % of (3a+3a′) was determined by 1H NMR using 1,3,5-trimethoxybenzene as an internal standard.

c

Turnover number = n(3a+3a′)/nRu.

With the optimized conditions in hand, we then investigated the scope of the intermolecular [2 + 2] cycloaddition reaction by first varying the electronic and steric properties of the β-aryl substituent of the α,β-unsaturated 2-acyl N-methylimidazole 1 (Table 4). Aryl groups with an electron-donating substituent were well tolerated affording the desired products (3b–3e) in good isolated yields. A slight decrease in yield was nevertheless observed when the steric hindrance increased from a methoxy substituent in the para-position (3c) to a meta-(3d) and ortho-position (3e). Substrate 1f bearing the bulky 2-naphthyl group also efficiently engaged in the [2 + 2] cycloaddition with styrene to afford the product 3f in 58% isolated yield. Thienyl and furanyl groups were well tolerated, allowing the formation of products 3g and 3h in reasonable yields. With the exception of the o-iodophenyl group (3l), which induced a very sluggish reaction, substrates bearing an electron-deficient fluoro-, chloro-, bromo-, cyano-, trifluoromethyl, or nitro-aryl group (3i–3p) were all amenable to efficient cycloaddition catalyzed by the [Ru(bpy)3](BArF4)2 photocatalyst. We then investigated the scope of the olefin partner, and the results are shown in Table 5. Aryl-substituted electron-rich and electron-poor styrenes, including sterically congested (2r), were valuable substrates to afford the desired products (3q–3u) in good yields. Gratifyingly, the presence of a methyl group at the α-position of styrene (2v) did not hamper the reactivity. Butadiene and 2,3-dimethylbutadiene are also compatible with the [2 + 2] cycloaddition, allowing the synthesis of cyclobutanes 3w and 3x in good yields and with improved 7:1 selectivity in favor of the trans–trans diastereomer in the latter case. Moreover, the method is not limited to α,β-unsaturated 2-acyl N-methylimidazole 1 since the N-methylimidazole moiety can be replaced by other heteroaromatic groups, such as N-phenylimidazole (4a) or pyridine (4b) with no significant impact on the catalytic performance, as demonstrated by the isolation of products (5a) and (5b) in 72 and 63% yields, respectively (Table 6). Interestingly, energy transfer from the triplet state energy of [Ru(bpy)3](BArF4)2 to chalcone is also operative in CH2Cl2, allowing cycloaddition with styrene to produce the corresponding intermolecular cycloadduct 5c in a reasonable 44% isolated yield.
Table 4. Scope of α,β-Unsaturated 2-Acyl N-Methylimidazolea
a

Reaction conditions: 1 (0.2 mmol), 2a (2 mmol), [Ru(bpy)3](BArF4)2 (1 × 10–3 mmol), degassed CH2Cl2 (1 mL), blue LED irradiation (λmax = 460 nm) for 16 h under Ar.

b

Isolated yield of 3 and 3′.

c

Yield in % of 3 and 3′ determined by 1H NMR using 1,3,5-trimethoxybenzene as an internal standard.

d

Diastereomeric ratio (3:3′) determined by 1H NMR analysis.

Table 5. Scope of Styrene Partnera
a

Reaction conditions: 1a (0.2 mmol), 2 (2 mmol), [Ru(bpy)3](BArF4)2 (1 × 10–3 mmol), degassed CH2Cl2 (1 mL), blue LED irradiation (λmax = 460 nm) for 16 h under Ar.

b

Isolated yield of 3 and 3′.

c

Yield in % of 3 and 3′ determined by 1H NMR using 1,3,5-trimethoxybenzene as an internal standard.

d

Diastereomeric ratio (3:3′) determined by 1H NMR analysis.

Table 6. Scope of α,β-Unsaturated Ketonesa
a

Reaction conditions: 1 (0.2 mmol), 2a (2 mmol), [Ru(bpy)3](BArF4)2 (1 × 10–3 mmol), degassed CH2Cl2 (1 mL), blue LED irradiation (λmax = 460 nm) for 16 h under Ar.

b

Isolated yield of 5 and 5′.

c

Yield in % of 5 and 5′ determined by 1H NMR using 1,3,5-trimethoxybenzene as an internal standard.

d

Diastereomeric ratio (5:5′) determined by 1H NMR analysis.

Conclusions

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In this work, we reveal counterion effects in triplet–triplet energy transfer processes catalyzed by a cationic transition-metal-based photosensitizer in low-polarity solvent. We observe a strong impact of the counterion identity on the excited-state properties of [Ru(bpy)3](X)2 photocatalysts and on their capacity to transfer triplet energy to cinnamoyl-type substrates. The limited stabilization by anion–dipole interaction of the [Ru(bpy)3]2+* excited state in the presence of large BArF4– anions in dichloromethane results in a 3MLCT excited state of reduced lifetime but of higher energy, offering an increased driving force for the energy transfer process. In comparison, the efficiency of the energy transfer drops by two orders of magnitude in the presence of small anions like TsO–. The noncoordinating BArF4– anion is also beneficial for the photostability of the cationic photocatalyst by disfavoring the loss of a bpy ligand, which is a predominant decomposition pathway in solvents of low dielectric constant such as dichloromethane in the presence of coordinating/nucleophilic anions. This modulation is due to the fact that BArF4– anions favor the population of nondissociative 3MCtrans states over dissociative 3MCcis states. Thanks to the excellent photostability and high efficiency in triplet–triplet energy transfer of the [Ru(bpy)3](BArF4)2 photocatalyst in dichloromethane, excellent photocatalytic activities (up to >1000 TON) could be achieved in intermolecular [2 + 2] cycloaddition reactions of broad scope with photocatalyst loading as low as 500 ppm. These results clearly demonstrate that the nature of counterion makes a significant contribution to the potency of cationic photocatalysts, a subtle effect that can also be of prime importance and that has been so far overlooked in the literature regarding photocatalyzed transformations and especially reactions enabled by triplet-triplet energy transfer.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.4c00384.

  • General information; photocatalyst synthesis; substrate synthesis; synthesis of racemate [2+2] cycloaddition products; electrochemical data; photophysical experiments; complementary studies; computational studies; X-ray diffraction; and NMR data (PDF)

  • Cartesian coordinates for all optimized complexes, ground and excited states (ZIP)

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  • Corresponding Authors
  • Authors
    • Juliette Zanzi - LCC−CNRS, Université de Toulouse, CNRS, UPS, Toulouse 31077, France
    • Zachary Pastorel - Institut des Biomolécules Max Mousseron, Université de Montpellier, CNRS, ENSCM, Montpellier 34095, France
    • Carine Duhayon - LCC−CNRS, Université de Toulouse, CNRS, UPS, Toulouse 31077, France;  Orcidhttps://orcid.org/0000-0001-7345-8782
    • Elise Lognon - ITODYS, Université Paris Cité and CNRS, Paris F-75006, France
    • Christophe Coudret - Université de Toulouse, UPS, Institut de Chimie de Toulouse, FR2599, 118 Route de Narbonne, Toulouse F-31062, France
    • Antonio Monari - ITODYS, Université Paris Cité and CNRS, Paris F-75006, France;  Orcidhttps://orcid.org/0000-0001-9464-1463
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by the Centre National de la Recherche Scientifique (CNRS), the Université Toulouse III - Paul Sabatier, the Université de Montpellier, the Région Occitanie Pyrénées-Méditerranée (21012860 “R-SMASH”), and the Agence Nationale de la Recherche (ANR-20-CE07-0021 “SMASH” grant to J.Z., Z.P., Y.C., M.S., and O.B.; ANR-21-CE07-0026 “LYMACATO” grant to Y.C., E. L., and A.M.). The authors thank Charles-Louis Serpentini from Université de Toulouse for excited-state lifetime measurements and INSA Toulouse for luminescence quantum yield measurements. CALMIP is also kindly acknowledged for providing HPC resources (project P18013), as well as the EXplor mesocenter and the national GENCI calculators.

References

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    Henwood, A. F.; Zysman-Colman, E. Lessons Learned in Tuning the Optoelectronic Properties of Phosphorescent Iridium(III) Complexes. Chem. Commun. 2017, 53, 807– 826,  DOI: 10.1039/C6CC06729H
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    Lessons learned in tuning the optoelectronic properties of phosphorescent iridium(III) complexes
    Henwood, Adam F.; Zysman-Colman, Eli
    Chemical Communications (Cambridge, United Kingdom) (2017), 53 (5), 807-826CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A review. This perspective illustrates our approach in the design of heteroleptic cationic iridium(III) complexes for optoelectronic applications, esp. as emitters in electroluminescent devices. We discuss changes in the photophys. properties of the complexes as a consequence of modification of the electronics of either the cyclometalating (Ĉ N) or the ancillary (N̂ N) ligands. We then broach the impact on these properties as a function of modification of the structure of both types of ligands. We explain trends in the optoelectronic behavior of the complexes using a combination of rationally designed structure-property relationship studies and theor. modeling that serves to inform subsequent ligand design. However, we have found cases where the design paradigms do not always hold true. Nevertheless, all these studies contribute to the lessons we have learned in the design of heteroleptic cationic phosphorescent iridium(III) complexes.
  24. 24
    Larsen, C. B.; Wenger, O. S. Photoredox Catalysis with Metal Complexes Made from Earth-Abundant Elements. Chem. - Eur. J. 2018, 24, 2039– 2058,  DOI: 10.1002/chem.201703602
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    Photoredox Catalysis with Metal Complexes Made from Earth-Abundant Elements
    Larsen, Christopher B.; Wenger, Oliver S.
    Chemistry - A European Journal (2018), 24 (9), 2039-2058CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Photoredox chem. with metal complexes as sensitizers and catalysts frequently relies on precious elements such as ruthenium or iridium. Over the past 5 years, important progress towards the use of complexes made from earth-abundant elements in photoredox catalysis has been made. This review summarizes the advances made with photoactive CrIII, FeII, CuI, ZnII, ZrIV, Mo0, and UVI complexes in the context of synthetic org. photoredox chem. using visible light as an energy input. Mechanistic considerations are combined with discussions of reaction types and scopes. Perspectives for the future of the field are discussed against the background of recent significant developments of new photoactive metal complexes made from earth-abundant elements.
  25. 25
    Hockin, B. M.; Li, C.; Robertson, N.; Zysman-Colman, E. Photoredox Catalysts based on Earth-Abundant Metal Complexes. Catal. Sci. Technol. 2019, 9, 889– 915,  DOI: 10.1039/C8CY02336K
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    Photoredox catalysts based on earth-abundant metal complexes
    Hockin, Bryony M.; Li, Chenfei; Robertson, Neil; Zysman-Colman, Eli
    Catalysis Science & Technology (2019), 9 (4), 889-915CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    A review. Over the last decade, visible light photoredox catalysis has exploded into the consciousness of the synthetic chemist. The principal photocatalysts used are based on rare and toxic Ru(II) and Ir(III) complexes. This crit. review focuses on Earth-abundant metal complexes as potential replacement photocatalysts and summarizes the use of photoactive Cu(I), Zn(II), Ni(0), V(V), Zr(IV), W(0), W(VI), Mo(0), Cr(III), Co(III) and Fe(II) complexes in photoredox reactions. The optoelectronic properties of these complexes and relevant structurally related analogs, not yet used for photoredox catalysis, are discussed in combination with the reaction scope reported for each photocatalyst. Prospects for the future of photocatalyst design are considered.
  26. 26
    Förster, C.; Heinze, K. Photophysics and Photochemistry with Earth-Abundant Metals – Fundamentals and Concepts. Chem. Soc. Rev. 2020, 49, 1057– 1070,  DOI: 10.1039/C9CS00573K
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    Photophysics and photochemistry with Earth-abundant metals - fundamentals and concepts
    Forster Christoph; Heinze Katja
    Chemical Society reviews (2020), 49 (4), 1057-1070 ISSN:.
    Recent exciting developments in the area of mononuclear photoactive complexes with Earth-abundant metal ions (Cu, Zr, Fe, Cr) for potential eco-friendly applications in (phosphorescent) organic light emitting diodes, in imaging and sensing systems, in dye-sensitized solar cells and as photocatalysts are presented. Challenges, in particular the extension of excited state lifetimes, and recent conceptual breakthroughs in substituting precious and rare-Earth metal ions (e.g. Ru, Ir, Pt, Au, Eu) in these applications by abundant ions are outlined with selected examples. Relevant fundamentals of photophysics and photochemistry are discussed first, followed by conceptual and instructive case studies.
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    Wegeberg, C.; Wenger, O. S. Luminescent First-Row Transition Metal Complexes. JACS Au 2021, 1, 1860– 1876,  DOI: 10.1021/jacsau.1c00353
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    Luminescent First-Row Transition Metal Complexes
    Wegeberg, Christina; Wenger, Oliver S.
    JACS Au (2021), 1 (11), 1860-1876CODEN: JAAUCR; ISSN:2691-3704. (American Chemical Society)
    A review. Precious and rare elements have traditionally dominated inorg. photophysics and photochem., but now we are witnessing a paradigm shift toward cheaper and more abundant metals. Even though emissive complexes based on selected first-row transition metals have long been known, recent conceptual breakthroughs revealed that a much broader range of elements in different oxidn. states are useable for this purpose. Coordination compds. of V, Cr, Mn, Fe, Co, Ni, and Cu now show electronically excited states with unexpected reactivity and photoluminescence behavior. Aside from providing a compact survey of the recent conceptual key advances in this dynamic field, our Perspective identifies the main design strategies that enabled the discovery of fundamentally new types of 3d-metal-based luminophores and photosensitizers operating in soln. at room temp.
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    Herr, P.; Kerzig, C.; Larsen, C. B.; Häussinger, D.; Wenger, O. S. Manganese(I) Complexes with Metal-to-Ligand Charge Transfer Luminescence and Photoreactivity. Nat. Chem. 2021, 13, 956– 962,  DOI: 10.1038/s41557-021-00744-9
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    Manganese(I) complexes with metal-to-ligand charge transfer luminescence and photoreactivity
    Herr, Patrick; Kerzig, Christoph; Larsen, Christopher B.; Haussinger, Daniel; Wenger, Oliver S.
    Nature Chemistry (2021), 13 (10), 956-962CODEN: NCAHBB; ISSN:1755-4330. (Nature Portfolio)
    Precious metal complexes with the d6 valence electron configuration often exhibit luminescent metal-to-ligand charge transfer (MLCT) excited states, which form the basis for many applications in lighting, sensing, solar cells and synthetic photochem. Iron(II) has received much attention as a possible Earth-abundant alternative, but to date no iron(II) complex has been reported to show MLCT emission upon continuous-wave excitation. Manganese(I) has the same electron configuration as that of iron(II), but until now has typically been overlooked in the search for cheap MLCT luminophores. Here we report that isocyanide chelate ligands give access to air-stable manganese(I) complexes that exhibit MLCT luminescence in soln. at room temp. These compds. were successfully used as photosensitizers for energy- and electron-transfer reactions and were shown to promote the photoisomerization of trans-stilbene. The observable electron transfer photoreactivity occurred from the emissive MLCT state, whereas the triplet energy transfer photoreactivity originated from a ligand-centered 3π-π* state. [graphic not available: see fulltext].
  29. 29
    de Groot, L. H. M.; Ilic, A.; Schwarz, J.; Wärnmark, K. Iron Photoredox Catalysis–Past, Present, and Future. J. Am. Chem. Soc. 2023, 145, 9369– 9388,  DOI: 10.1021/jacs.3c01000
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    Iron Photoredox Catalysis - Past, Present, and Future
    de Groot, Lisa H. M.; Ilic, Aleksandra; Schwarz, Jesper; Waernmark, Kenneth
    Journal of the American Chemical Society (2023), 145 (17), 9369-9388CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A review. Photoredox catalysis of org. reactions driven by iron has attracted substantial attention throughout recent years, due to potential environmental and economic benefits. In this Perspective, three major strategies were identified that have been employed to date to achieve reactivities comparable to the successful noble metal photoredox catalysis: (1) Direct replacement of a noble metal center by iron in archetypal polypyridyl complexes, resulting in a metal-centered photofunctional state. (2) In situ generation of photoactive complexes by substrate coordination where the reactions are driven via intramol. electron transfer involving charge-transfer states, for example through visible light-induced homolysis. (3) Improving the excited state lifetimes and redox potentials of the charge-transfer states of iron complexes through new ligand design, for example by introducing N-heterocyclic carbene ligands. We seek to give an overview and evaluation of recent developments in this rapidly growing field, and at the same time provide an outlook on the future of iron-based photoredox catalysis.
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    Sinha, N.; Yaltseva, P.; Wenger, O. S. The Nephelauxetic Effect Becomes an Important Design Factor for Photoactive First-Row Transition Metal Complexes. Angew. Chem., Int. Ed. 2023, e202303864  DOI: 10.1002/anie.202303864
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    Chan, A. Y.; Ghosh, A.; Yarranton, J. T.; Twilton, J.; Jin, J.; Arias-Rotondo, D. M.; Sakai, H. A.; McCusker, J. K.; MacMillan, D. W. C. Exploiting the Marcus Inverted Region for First-Row Transition Metal–based Photoredox Catalysis. Science 2023, 382, 191– 197,  DOI: 10.1126/science.adj0612
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    Troian-Gautier, L.; Beauvilliers, E. E.; Swords, W. B.; Meyer, G. J. Redox Active Ion-Paired Excited States Undergo Dynamic Electron Transfer. J. Am. Chem. Soc. 2016, 138, 16815– 16826,  DOI: 10.1021/jacs.6b11337
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    Redox active ion-paired excited states undergo dynamic electron transfer
    Troian-Gautier, Ludovic; Beauvilliers, Evan E.; Swords, Wesley B.; Meyer, Gerald J.
    Journal of the American Chemical Society (2016), 138 (51), 16815-16826CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Ion-pair interactions between a cationic ruthenium complex, [Ru(dtb)2(dea)][PF6]2, C12+ where dea is 4,4'-diethanolamide-2,2'-bipyridine and dtb is 4,4'-di-tert-butyl-2,2'-bipyridine, and chloride, bromide, and iodide are reported. A remarkable result is that a 1:1 iodide: excited-state ion-pair, [C12+, I-]+*, underwent diffusional electron-transfer oxidn. of iodide that did not occur when ion-pairing was absent. The ion-pair equil. consts. ranged 104-106 M-1 in CH3CN and decreased in the order Cl- > Br- > I-. The ion-pairs had longer-lived excited states, were brighter emitters, and stored more free energy than did the non-ion-paired states. The 1H NMR spectra revealed that the halides formed tight ion-pairs with the amide and alc. groups of the DEA ligand. Electron-transfer reactivity of the ion-paired excited state was not simply due to it being a stronger photooxidant than the non-ion-paired excited state. Instead, work term, ΔGw was the predominant contributor to the driving force for the reaction. Natural bond order calcns. provided natural at. charges that enabled quantification of ΔGw for all the atoms in C12+ and [C12+, I-]+* presented herein as contour diagrams that show the most favorable electrostatic positions for halide interactions. The results were most consistent with a model wherein the non-ion-paired C12+* excited state traps the halide and prevents its oxidn., but allows for dynamic oxidn. of a second iodide ion.
  33. 33
    Li, G.; Swords, W. B.; Meyer, G. J. Bromide Photo-oxidation Sensitized to Visible Light in Consecutive Ion Pairs. J. Am. Chem. Soc. 2017, 139, 14983– 14991,  DOI: 10.1021/jacs.7b06735
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    Bromide Photo-oxidation Sensitized to Visible Light in Consecutive Ion Pairs
    Li, Guocan; Swords, Wesley B.; Meyer, Gerald J.
    Journal of the American Chemical Society (2017), 139 (42), 14983-14991CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The titrn. of bromide into a [Ru(deeb)(bpz)2]2+ (Ru2+, deeb = 4,4'-diethylester-2,2'-bipyridine; bpz = 2,2'-bipyrazine) dichloromethane soln. led to the formation of two consecutive ion-paired species, [Ru2+, Br-]+ and [Ru2+, 2Br-], each with distinct photophys. and electron-transfer properties. Formation of the first ion pair was stoichiometric, Keq 1 > 106 M-1, and the second ion-pair equil. was estd. to be Keq 2 = (2.4 ± 0.4) × 105 M-1. The 1H NMR spectra recorded in deuterated dichloromethane indicated the presence of contact ion pairs and provided insights into their structures and were complimented by d. functional theory calcns. Static quenching of the [Ru(deeb)(bpz)2]2+* photoluminescence intensity (PLI) by bromide was obsd., and [Ru2+, Br-]+* was found to be nonluminescent, τ < 10 ns. Further addn. of bromide resulted in partial recovery of the PLI, and [Ru2+, 2Br-]* was found to be luminescent with an excited-state lifetime of τ = 65 ± 5 ns. Electron-transfer products were identified as the reduced complex, [Ru(deeb)(bpz)2]+, and dibromide, Br2•-. The bromine atom, Br•, was detd. to be the primary excited-state electron-transfer product and was an intermediate in Br2•- formation, Br• + Br- → Br2•-, with a second-order rate const., k = (5.4 ± 1) × 108 M-1 s-1. The unusual enhancement in PLI for [Ru2+, 2Br-]* relative to [Ru2+, Br-]+* was due to a less favorable Gibbs free energy change for electron transfer that resulted in a smaller rate const., ket = (1.5 ± 0.2) × 107 s-1, in the second ion pair. Natural at. charge anal. provided ests. of the Coulombic work terms assocd. with ion pairing, ΔGw, that were directly correlated with the measured change in rate consts.
  34. 34
    Morton, C. M.; Zhu, Q.; Ripberger, H. H.; Troian-Gautier, L.; Toa, Z. S. D.; Knowles, R. R.; Alexanian, E. J. C–H Alkylation via Multisite-Proton-Coupled Electron Transfer of an Aliphatic C–H Bond. J. Am. Chem. Soc. 2019, 141, 13253– 13260,  DOI: 10.1021/jacs.9b06834
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    C-H Alkylation via Multisite-Proton-Coupled Electron Transfer of an Aliphatic C-H Bond
    Morton, Carla M.; Zhu, Qilei; Ripberger, Hunter; Troian-Gautier, Ludovic; Toa, Zi S. D.; Knowles, Robert R.; Alexanian, Erik J.
    Journal of the American Chemical Society (2019), 141 (33), 13253-13260CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The direct, site-selective alkylation of unactivated C(sp3)-H bonds in org. substrates is a long-standing goal in synthetic chem. General approaches to the activation of strong C-H bonds include radical-mediated processes involving highly reactive intermediates, such as heteroatom-centered radicals. Herein, we describe a catalytic, intermol. C-H alkylation that circumvents such reactive species via a new elementary step for C-H cleavage involving multisite-proton-coupled electron transfer (multisite-PCET). Mechanistic studies indicate that the reaction is catalyzed by a noncovalent complex formed between an iridium(III) photocatalyst and a monobasic phosphate base. The C-H alkylation proceeds efficiently using diverse hydrocarbons and complex mols. as the limiting reagent and represents a new approach to the catalytic functionalization of unactivated C(sp3)-H bonds.
  35. 35
    Uraguchi, D.; Kimura, Y.; Ueoka, F.; Ooi, T. Urea as a Redox-Active Directing Group under Asymmetric Photocatalysis of Iridium-Chiral Borate Ion Pairs. J. Am. Chem. Soc. 2020, 142, 19462– 19467,  DOI: 10.1021/jacs.0c09468
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    Urea as a Redox-Active Directing Group under Asymmetric Photocatalysis of Iridium-Chiral Borate Ion Pairs
    Uraguchi, Daisuke; Kimura, Yuto; Ueoka, Fumito; Ooi, Takashi
    Journal of the American Chemical Society (2020), 142 (46), 19462-19467CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The development of a photoinduced, highly diastereo- and enantioselective [3 + 2]-cycloaddn. of N-cyclopropylurea with α-alkylstyrenes is reported. This asym. radical cycloaddn. relies on the strategic placement of urea on cyclopropylamine as a redox-active directing group (DG) with anion-binding ability and the use of an ion pair, comprising an iridium polypyridyl complex and a weakly coordinating chiral borate ion, as a photocatalyst. The structure of the anion component of the catalyst governs reactivity, and pertinent structural modification of the borate ion enables high levels of catalytic activity and stereocontrol. This system tolerates a range of α-alkylstyrenes and hence offers rapid access to various aminocyclopentanes with contiguous tertiary and quaternary stereocenters, as the urea DG is readily removable.
  36. 36
    Xu, J.; Li, Z.; Xu, Y.; Shu, X.; Huo, H. Stereodivergent Synthesis of Both Z- and E-Alkenes by Photoinduced, Ni-Catalyzed Enantioselective C(sp3)–H Alkenylation. ACS Catal. 2021, 11, 13567– 13574,  DOI: 10.1021/acscatal.1c04314
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    Stereodivergent Synthesis of Both Z- and E-Alkenes by Photoinduced, Ni-Catalyzed Enantioselective C(sp3)-H Alkenylation
    Xu, Jitao; Li, Zhilong; Xu, Yumin; Shu, Xiaomin; Huo, Haohua
    ACS Catalysis (2021), 11 (21), 13567-13574CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    An enantioselective benzylic C(sp3)-H alkenylation of simple alkylarenes with vinyl bromides via photoinduced nickel catalysis was reported, which allowed the stereodivergent synthesis of both enantioenriched Z- and E-alkenes bearing aryl-substituted, allylic tertiary stereogenic centers. Interestingly, the tunable Z/E-selectivity was achieved by energy transfer catalysis via a judicious choice of the photocatalyst counteranion. This versatile strategy featured simple starting materials, mild reaction conditions, broad substrate scope, divergent Z- and E-selectivity and high enantioselectivities. Moreover, a formal asym. benzylic C(sp3)-H alkylation was also be achieved via a one-pot alkenylation/redn. sequence, providing a complementary strategy to address the notoriously challenging stereochem. control in C(sp3)-C(sp3) bond construction.
  37. 37
    Chapman, S. J.; Swords, W. B.; Le, C. M.; Guzei, I. A.; Toste, F. D.; Yoon, T. P. Cooperative Stereoinduction in Asymmetric Photocatalysis. J. Am. Chem. Soc. 2022, 144, 4206– 4213,  DOI: 10.1021/jacs.2c00063
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    Cooperative Stereoinduction in Asymmetric Photocatalysis
    Chapman, Steven J.; Swords, Wesley B.; Le, Christine M.; Guzei, Ilia A.; Toste, F. Dean; Yoon, Tehshik P.
    Journal of the American Chemical Society (2022), 144 (9), 4206-4213CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Stereoinduction in complex org. reactions often involves the influence of multiple stereocontrol elements. The interaction among these can often result in the observation of significant cooperative effects that afford different rates and selectivities between the matched and mismatched sets of stereodifferentiating chiral elements. The elucidation of matched/mismatched effects in ground-state chem. reactions was a critically important theme in the maturation of modern stereocontrolled synthesis. The development of robust methods for the control of photochem. reactions, however, is a relatively recent development, and similar cooperative stereocontrolling effects in excited-state enantioselective photoreactions have not previously been documented. Herein, we describe a tandem chiral photocatalyst/Bronsted acid strategy for highly enantioselective [2 + 2] photocycloaddns. of vinylpyridines. Importantly, the matched and mismatched chiral catalyst pairs exhibit different reaction rates and enantioselectivities across a range of coupling partners. We observe no evidence of ground-state interactions between the catalysts and conclude that these effects arise from their cooperative behavior in a transient excited-state assembly. These results suggest that similar matched/mismatched effects might be important in other classes of enantioselective dual-catalytic photochem. reactions.
  38. 38
    Girvin, Z. C.; Cotter, L. F.; Yoon, H.; Chapman, S. J.; Mayer, J. M.; Yoon, T. P.; Miller, S. J. Asymmetric Photochemical [2 + 2]-Cycloaddition of Acyclic Vinylpyridines through Ternary Complex Formation and an Uncontrolled Sensitization Mechanism. J. Am. Chem. Soc. 2022, 144, 20109– 20117,  DOI: 10.1021/jacs.2c09690
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    Asymmetric Photochemical [2 + 2]-Cycloaddition of Acyclic Vinylpyridines through Ternary Complex Formation and an Uncontrolled Sensitization Mechanism
    Girvin, Zebediah C.; Cotter, Laura F.; Yoon, Hyung; Chapman, Steven J.; Mayer, James M.; Yoon, Tehshik P.; Miller, Scott J.
    Journal of the American Chemical Society (2022), 144 (43), 20109-20117CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Stereochem. control of photochem. reactions that occur via triplet energy transfer remains a challenge. Suppressing off-catalyst stereorandom reactivity is difficult for highly reactive open-shell intermediates. Strategies for suppressing racemate-producing, off-catalyst pathways have long focused on formation of ground state, substrate-catalyst chiral complexes that are primed for triplet energy transfer via a photocatalyst in contrast to their off-catalyst counterparts. Herein, we describe a strategy where both a chiral catalyst-assocd. vinylpyridine and a nonassocd., free vinylpyridine substrate can be sensitized by an Ir(III) photocatalyst, yet high levels of diastereo- and enantioselectivity in a [2 + 2] photocycloaddn. are achieved through a preferred, highly organized transition state. This mechanistic paradigm is distinct from, yet complementary to current approaches for achieving high levels of stereocontrol in photochem. transformations.
  39. 39
    Farney, E. P.; Chapman, S. J.; Swords, W. B.; Torelli, M. D.; Hamers, R. J.; Yoon, T. P. Discovery and Elucidation of Counteranion Dependence in Photoredox Catalysis. J. Am. Chem. Soc. 2019, 141, 6385– 6391,  DOI: 10.1021/jacs.9b01885
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    Discovery and elucidation of counteranion dependence in photoredox catalysis
    Farney, Elliot P.; Chapman, Steven J.; Swords, Wesley B.; Torelli, Marco D.; Hamers, Robert J.; Yoon, Tehshik P.
    Journal of the American Chemical Society (2019), 141 (15), 6385-6391CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Over the past decade, there has been a renewed interest in the use of transition metal polypyridyl complexes as photoredox catalysts for a variety of innovative synthetic applications. Many derivs. of these complexes are known, and the effect of ligand modifications on their efficacy as photoredox catalysts has been the subject of extensive, systematic investigation. However, the influence of the photocatalyst counteranion has received little attention, despite the fact that these complexes are generally cationic in nature. Herein, we demonstrate that counteranion effects exert a surprising, dramatic impact on the rate of a representative photocatalytic radical cation Diels-Alder reaction. A detailed anal. reveals that counteranion identity impacts multiple aspects of the reaction mechanism. Most notably, photocatalysts with more noncoordinating counteranions yield a more powerful triplet excited state oxidant and longer radical cation chain length. It is proposed that this counteranion effect arises from Coulombic ion-pairing interactions between the counteranion and both the cationic photoredox catalyst and the radical cation intermediate, resp. The comparatively slower rate of reaction with coordinating counteranions can be rescued by using hydrogen-bonding anion binders that attenuate deleterious ion-pairing interactions. These results demonstrate the importance of counteranion identity as a variable in the design and optimization of photoredox transformations and suggest a novel strategy for the optimization of org. reactions using this class of transition metal photocatalysts.
  40. 40
    Earley, J. D.; Zieleniewska, A.; Ripberger, H. H.; Shin, N. Y.; Lazorski, M. S.; Mast, Z. J.; Sayre, H. J.; McCusker, J. K.; Scholes, G. D.; Knowles, R. R.; Reid, O. G.; Rumbles, G. Ion-Pair Reorganization Regulates Reactivity in Photoredox Catalysts. Nat. Chem. 2022, 14, 746– 753,  DOI: 10.1038/s41557-022-00911-6
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    Ion-pair reorganization regulates reactivity in photoredox catalysts
    Earley, J. D.; Zieleniewska, A.; Ripberger, H. H.; Shin, N. Y.; Lazorski, M. S.; Mast, Z. J.; Sayre, H. J.; McCusker, J. K.; Scholes, G. D.; Knowles, R. R.; Reid, O. G.; Rumbles, G.
    Nature Chemistry (2022), 14 (7), 746-753CODEN: NCAHBB; ISSN:1755-4330. (Nature Portfolio)
    Cyclometalated and polypyridyl complexes of d6 metals are promising photoredox catalysts, using light to drive reactions with high kinetic or thermodn. barriers via the generation of reactive radical intermediates. However, while tuning of their redox potentials, absorption energy, excited-state lifetime and quantum yield are well-known criteria for modifying activity, other factors could be important. Here we show that dynamic ion-pair reorganization controls the reactivity of a photoredox catalyst, [Ir[dF(CF3)ppy]2(dtbpy)]X. Time-resolved dielec.-loss expts. show how counter-ion identity influences excited-state charge distribution, evincing large differences in both the ground- and excited-state dipole moment depending on whether X is a small assocg. anion (PF6-) that forms a contact-ion pair vs. a large one that either dissocs. or forms a solvent-sepd. pair (BArF4-). These differences correlate with the reactivity of the photocatalyst toward both reductive and oxidative electron transfer, amounting to a 4-fold change in selectivity toward oxidn. vs. redn. These results suggest that ion pairing could be an underappreciated factor that modulates reactivity in ionic photoredox catalysts.
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    Geunes, E. P.; Meinhardt, J. M.; Wu, E. J.; Knowles, R. R. Photocatalytic Anti-Markovnikov Hydroamination of Alkenes with Primary Heteroaryl Amines. J. Am. Chem. Soc. 2023, 145, 21738– 21744,  DOI: 10.1021/jacs.3c08428
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    For an example of counterion effect in iron photoredox catalysis (LMCT emitter), see:

    Jang, Y. J.; An, H.; Choi, S.; Hong, J.; Lee, S. H.; Ahn, K.-H.; You, Y.; Kang, E. J. Green-Light-Driven Fe(III)(btz)3 Photocatalysis in the Radical Cationic [4 + 2] Cycloaddition Reaction. Org. Lett. 2022, 24, 4479– 4484,  DOI: 10.1021/acs.orglett.2c01779
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    Green-Light-Driven Fe(III)(btz)3 Photocatalysis in the Radical Cationic [4+2] Cycloaddition Reaction
    Jang, Yu Jeong; An, Hyeju; Choi, Seunghee; Hong, Jayeon; Lee, Seung Hyun; Ahn, Kwang-Hyun; You, Youngmin; Kang, Eun Joo
    Organic Letters (2022), 24 (24), 4479-4484CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
    Green-light-driven Fe(btz)3 I·3PF6- photocatalysis for the radical cationic [4+2] cycloaddn. of terminal styrenes and nucleophilic dienes has been investigated. The Fe-MIC (mesoionic carbene) complex forms a ligand-to-metal charge-transfer transition state with relatively high excited-state redn. potentials that can selectively oxidize terminal styrene derivs. Unique multisubstituted cyclohexenes and structurally complex biorelevant cyclohexenes were constructed, highlighting the usefulness of this mild and practical first-row transition metal complex system.
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    Dutta, S.; Erchinger, J. E.; Strieth-Kalthoff, F.; Kleinmans, R.; Glorius, F. Energy Transfer Photocatalysis: Exciting Modes of Reactivity. Chem. Soc. Rev. 2024, 53, 1068– 1089,  DOI: 10.1039/D3CS00190C
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    Energy transfer photocatalysis: exciting modes of reactivity
    Dutta, Subhabrata; Erchinger, Johannes E.; Strieth-Kalthoff, Felix; Kleinmans, Roman; Glorius, Frank
    Chemical Society Reviews (2024), 53 (3), 1068-1089CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Excited (triplet) states offer a myriad of attractive synthetic pathways, including cycloaddns., selective homolytic bond cleavages and strain-release chem., isomerizations, deracemizations, or the fusion with metal catalysis. Recent years have seen enormous advantages in enabling these reactivity modes through visible-light-mediated triplet-triplet energy transfer catalysis (TTEnT). This tutorial review provides an overview of this emerging strategy for synthesizing sought-after org. motifs in a mild, selective, and sustainable manner. Building on the photophys. foundations of energy transfer, this review also discusses catalyst design, as well as the challenges and opportunities of energy transfer catalysis.
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    Großkopf, J.; Kratz, T.; Rigotti, T.; Bach, T. Enantioselective Photochemical Reactions Enabled by Triplet Energy Transfer. Chem. Rev. 2022, 122, 1626– 1653,  DOI: 10.1021/acs.chemrev.1c00272
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    Enantioselective Photochemical Reactions Enabled by Triplet Energy Transfer
    Grosskopf, Johannes; Kratz, Thilo; Rigotti, Thomas; Bach, Thorsten
    Chemical Reviews (Washington, DC, United States) (2022), 122 (2), 1626-1653CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    Review. For mols. with a singlet ground state, the population of triplet states is mainly possible (a) by direct excitation and subsequent intersystem crossing or (b) by energy transfer from an appropriate sensitizer. The latter scenario enables a catalytic photochem. reaction in which the sensitizer adopts the role of a catalyst undergoing several cycles of photon absorption and subsequent energy transfer to the substrate. If the product mol. of a triplet-sensitized process is chiral, this process can proceed enantioselectively upon judicious choice of a chiral triplet sensitizer. An enantioselective reaction can also occur in a dual catalytic approach in which, apart from an achiral sensitizer, a second chiral catalyst activates the substrate toward sensitization. Although the idea of enantioselective photochem. reactions via triplet intermediates has been pursued for more than 50 years, notable selectivities exceeding 90% enantiomeric excess (ee) have only been realized in the past decade. This review attempts to provide a comprehensive survey on the various photochem. reactions which were rendered enantioselective by triplet sensitization.
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    Zhou, Q.-Q.; Zou, Y.-Q.; Lu, L.-Q.; Xiao, W.-J. Visible-Light-Induced Organic Photochemical Reactions through Energy-Transfer Pathways. Angew. Chem., Int. Ed. 2019, 58, 1586– 1604,  DOI: 10.1002/anie.201803102
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    Visible-light-induced organic photochemical reactions through energy-transfer pathways
    Zhou, Quan-Quan; Zou, You-Quan; Lu, Liang-Qiu; Xiao, Wen-Jing
    Angewandte Chemie, International Edition (2019), 58 (6), 1586-1604CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Visible-light photocatalysis is a rapidly developing and powerful strategy to initiate org. transformations, as it closely adheres to the tenants of green and sustainable chem. Generally, most visible-light-induced photochem. reactions occur through single-electron transfer (SET) pathways. Recently, visible-light-induced energy-transfer (EnT) reactions have received considerable attentions from the synthetic community as this strategy provides a distinct reaction pathway, and remarkable achievements have been made in this field. In this Review, we highlight the most recent advances in visible-light-induced EnT reactions.
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    Strieth-Kalthoff, F.; James, M. J.; Teders, M.; Pitzer, L.; Glorius, F. Energy Transfer Catalysis Mediated by Visible Light: Principles, Applications, Directions. Chem. Soc. Rev. 2018, 47, 7190– 7202,  DOI: 10.1039/C8CS00054A
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    Energy transfer catalysis mediated by visible light: principles, applications, directions
    Strieth-Kalthoff, Felix; James, Michael J.; Teders, Michael; Pitzer, Lena; Glorius, Frank
    Chemical Society Reviews (2018), 47 (19), 7190-7202CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Harnessing visible light to access excited (triplet) states of org. compds. can enable impressive reactivity modes. This tutorial review covers the photophys. fundamentals and most significant advances in the field of visible-light-mediated energy transfer catalysis within the last decade. Methods to det. excited triplet state energies and to characterize the underlying Dexter energy transfer are discussed. Synthetic applications of this field, divided into four main categories (cyclization reactions, double bond isomerizations, bond dissocns. and sensitization of metal complexes), are also examd.
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    Strieth-Kalthoff, F.; Glorius, F. Triplet Energy Transfer Photocatalysis: Unlocking the Next Level. Chem. 2020, 6, 1888– 1903,  DOI: 10.1016/j.chempr.2020.07.010
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    Triplet Energy Transfer Photocatalysis: Unlocking the Next Level
    Strieth-Kalthoff, Felix; Glorius, Frank
    Chem (2020), 6 (8), 1888-1903CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
    A review. Energy transfer can leverage the enormous potential of excited-state reactivity. Through "indirect excitation" of substrates, otherwise elusive reactivity modes can be switched on, allowing for, e.g., cycloaddns., fragmentations, rearrangements, or challenging organometallic steps. This perspective recaps almost 70 years of energy transfer in org. chem., highlighting the way it evolved, as well as recent developments in the field of visible-light photocatalysis. Building upon the photophys. fundamentals, diverse applications and directions of energy transfer catalysis are pointed out.
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    Poplata, S.; Tröster, A.; Zou, Y. Q.; Bach, T. Recent advances in the synthesis of cyclobutanes by olefin [2 + 2] photocycloaddition reactions. Chem. Rev. 2016, 116, 9748– 9815,  DOI: 10.1021/acs.chemrev.5b00723
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    Recent Advances in the Synthesis of Cyclobutanes by Olefin [2+2] Photocycloaddition Reactions
    Poplata, Saner; Troester, Andreas; Zou, You-Quan; Bach, Thorsten
    Chemical Reviews (Washington, DC, United States) (2016), 116 (17), 9748-9815CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    The [2+2] photocycloaddn. is undisputedly the most important and most frequently used photochem. reaction. In this review, it is attempted to cover all recent aspects of [2+2] photocycloaddn. chem. with an emphasis on synthetically relevant, regio-, and stereoselective reactions. The review aims to comprehensively discuss relevant work, which was done in the field in the last 20 years (i.e., from 1995 to 2015). Organization of the data follows a subdivision according to mechanism and substrate classes. Cu(I) and PET (photoinduced electron transfer) catalysis are treated sep. in sections and , whereas the vast majority of photocycloaddn. reactions which occur by direct excitation or sensitization are divided within section into individual subsections according to the photochem. excited olefin.
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    Huang, X.; Quinn, T. R.; Harms, K.; Webster, R. D.; Zhang, L.; Wiest, O.; Meggers, E. Direct Visible-Light-Excited Asymmetric Lewis Acid Catalysis of Intermolecular [2 + 2] Photocycloadditions. J. Am. Chem. Soc. 2017, 139, 9120– 9123,  DOI: 10.1021/jacs.7b04363
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    Direct Visible-Light-Excited Asymmetric Lewis Acid Catalysis of Intermolecular [2+2] Photocycloadditions
    Huang, Xiaoqiang; Quinn, Taylor R.; Harms, Klaus; Webster, Richard D.; Zhang, Lilu; Wiest, Olaf; Meggers, Eric
    Journal of the American Chemical Society (2017), 139 (27), 9120-9123CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A reaction design is reported in which a substrate-bound chiral Lewis acid complex absorbs visible light and generates an excited state that directly reacts with a cosubstrate in a highly stereocontrolled fashion. Specifically, a chiral rhodium complex catalyzes visible-light-activated intermol. [2+2] cycloaddns., providing a wide range of cyclobutanes with up to >99% ee and up to >20:1 d.r. Noteworthy is the ability to create vicinal all-carbon-quaternary stereocenters including spiro centers in an intermol. fashion.
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    Sherbrook, E. M.; Jung, H.; Cho, D.; Baik, M.-H.; Yoon, T. P. Brønsted Acid Catalysis of Photosensitized Cycloadditions. Chem. Sci. 2020, 11, 856– 861,  DOI: 10.1039/C9SC04822G
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    Bronsted acid catalysis of photosensitized cycloadditions
    Sherbrook, Evan M.; Jung, Hoimin; Cho, Dasol; Baik, My-Hyun; Yoon, Tehshik P.
    Chemical Science (2020), 11 (3), 856-861CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Authors showed that Bronsted acids can also modulate the reactivity of excited-state org. reactions. Bronsted acids dramatically increase the rate of Ru(bpy)32+-sensitized [2 + 2] photocycloaddns. between C-cinnamoyl imidazoles and a range of electron-rich alkene reaction partners. A combination of exptl. and computational studies supported a mechanism in which the Bronsted acid co-catalyst accelerates triplet energy transfer from the excited-state [Ru*(bpy)3]2+ chromophore to the Bronsted acid activated C-cinnamoyl imidazole. Computational evidence further suggested the importance of driving force as well as geometrical reorganization, in which the protonation of the imidazole decreases the reorganization penalty during the energy transfer event.
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    Jung, H.; Hong, M.; Marchini, M.; Villa, M.; Steinlandt, P. S.; Huang, X.; Hemming, M.; Meggers, E.; Ceroni, P.; Park, J.; Baik, M.-H. Understanding the Mechanism of Direct Visible-Light-Activated [2 + 2] Cycloadditions Mediated by Rh and Ir Photocatalysts: Combined Computational and Spectroscopic Studies. Chem. Sci. 2021, 12, 9673– 9681,  DOI: 10.1039/D1SC02745J
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    Understanding the mechanism of direct visible-light-activated [2 + 2] cycloadditions mediated by Rh and Ir photocatalysts: combined computational and spectroscopic studies
    Jung, Hoimin; Hong, Mannkyu; Marchini, Marianna; Villa, Marco; Steinlandt, Philipp S.; Huang, Xiaoqiang; Hemming, Marcel; Meggers, Eric; Ceroni, Paola; Park, Jiyong; Baik, Mu-Hyun
    Chemical Science (2021), 12 (28), 9673-9681CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    The mechanism of [2 + 2] cycloaddns. activated by visible light and catalyzed by bis-cyclometalated Rh(III) and Ir(III) photocatalysts was investigated, combining d. functional theory calcns. and spectroscopic techniques. Exptl. observations show that the Rh-based photocatalyst produces excellent yield and enantioselectivity whereas the Ir-photocatalyst yields racemates. Two different mechanistic features were found to compete with each other, namely the direct photoactivation of the catalyst-substrate complex and outer-sphere triplet energy transfer. Our integrated anal. suggests that the direct photocatalysis is the inner working of the Rh-catalyzed reaction, whereas the Ir catalyst serves as a triplet sensitizer that activates cycloaddn. via an outer-sphere triplet excited state energy transfer mechanism.
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    Sherbrook, E. M.; Genzink, M. J.; Park, B.; Guzei, I. A.; Baik, M. H.; Yoon, T. P. Chiral Brønsted Acid-Controlled Intermolecular Asymmetric [2 + 2] Photocycloadditions. Nat. Commun. 2021, 12, 5735,  DOI: 10.1038/s41467-021-25878-9
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    Chiral Bronsted acid-controlled intermolecular asymmetric [2 + 2] photocycloadditions
    Sherbrook, Evan M.; Genzink, Matthew J.; Park, Bohyun; Guzei, Ilia A.; Baik, Mu-Hyun; Yoon, Tehshik P.
    Nature Communications (2021), 12 (1), 5735CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Abstr.: Control over the stereochem. of excited-state photoreactions remains a significant challenge in org. synthesis. Recently, it has become recognized that the photophys. properties of simple org. substrates can be altered upon coordination to Lewis acid catalysts, and that these changes can be exploited in the design of highly enantioselective catalytic photoreactions. Chromophore activation strategies, wherein simple org. substrates are activated towards photoexcitation upon binding to a Lewis acid catalyst, rank among the most successful asym. photoreactions. Herein, we show that chiral Bronsted acids can also catalyze asym. excited-state photoreactions by chromophore activation. This principle is demonstrated in the context of a highly enantio- and diastereoselective [2+2] photocycloaddn. catalyzed by a chiral phosphoramide organocatalyst. Notably, the cyclobutane products arising from this method feature a trans-cis stereochem. that is complementary to other enantioselective catalytic [2+2] photocycloaddns. reported to date.
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    In both solvents, an obvious color change from red to violet is rapidly observed, attesting to the premature decomposition of the photosensitizer under the reaction conditions; see: (a)

    Jones, R. F.; Cole-Hamilton, D. J. The Substitutional Photochemistry of Tris(bipyridyl)-Ruthenium(II)Chloride. Inorg. Chim. Acta 1981, 53, L3– L5,  DOI: 10.1016/S0020-1693(00)84722-2
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    and reference (54)

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    Ward, W. M.; Farnum, B. H.; Siegler, M.; Meyer, G. J. Chloride Ion-Pairing with Ru(II) Polypyridyl Compounds in Dichloromethane. J. Phys. Chem. A 2013, 117, 8883– 8894,  DOI: 10.1021/jp404838z
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    Chloride ion-pairing with ruthenium(II) polypyridyl compounds in dichloromethane
    Ward, William M.; Farnum, Byron H.; Siegler, Maxime; Meyer, Gerald J.
    Journal of Physical Chemistry A (2013), 117 (36), 8883-8894CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    Chloride ion-pairing with a series of four dicationic Ru-(II) polypyridyl compds. of the general form [Ru-(bpy)3-x(deeb)x]-(PF6)2, where bpy is 2,2'-bipyridine and deeb is 4,4'-diethylester-2,2'-bipyridine, was obsd. in dichloromethane soln. The heteroleptic compds. [Ru-(bpy)2(deeb)]2+ and [Ru-(bpy)-(deeb)2]2+ were found to be far less sensitive to ligand loss photochem. than were the homoleptic compds. [Ru-(bpy)3]2+ and [Ru-(deeb)3]2+ and were thus quantified in most detail. X-ray crystal structure and 1H NMR anal. showed that, when present, the C-3/C-3' position of bpy was the preferred site for adduct formation with chloride. Ion-pairing was manifest in UV-visible absorption spectral changes obsd. during titrns. with TBACl, where TBA is tetra-Bu ammonium. A modified Benesi-Hildebrand anal. yielded equil. consts. for ion-pairing that ranged from 13 700 to 64 000 M-1 and increased with the no. of deeb ligands present. A Job plot indicated a 2:1 chloride-to-ruthenium complex ratio in the ion-paired state. The chloride ion was found to decrease both the excited state lifetime and the quantum yield for photoluminescence. Nonlinear Stern-Volmer plots were obsd. that plateaued at high chloride concns. The radiative rate consts. decreased and the nonradiative rate consts. increased with chloride concn. in a manner consistent with theory for radiative rate consts. and the energy gap law. Equil. consts. for excited state ion-pairing abstracted from such data were found to be significantly larger than that measured for the ground state. Photophys. studies of hydroxide and bromide ion-pairing with [Ru-(bpy)2(deeb)]2+ are also reported.
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    The ion-pairs have little impact on the properties of the ground state of D3-symmetry, see

    Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer, T. J. Photochemistry of Ru(bpy)32+. J. Am. Chem. Soc. 1982, 104, 4803– 4801,  DOI: 10.1021/ja00382a012
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    Photochemistry of tris(2,2'-bipyridine)ruthenium(2+) ion
    Durham, Bill; Caspar, Jonathan V.; Nagle, Jeffrey K.; Meyer, Thomas J.
    Journal of the American Chemical Society (1982), 104 (18), 4803-10CODEN: JACSAT; ISSN:0002-7863.
    Temp.-dependent lifetime data in CH2Cl are reported for the emitting charge-transfer excited state of (Ru(bpy)32+ (3CT) (bpy = 2,2'-bipyridine) under photochem. (NCS- salt) and nonphotochem. (PF6- salt) conditions. Temp.-dependent lifetime data were also obtained in CH2Cl2 for the salts [Ru(bpy)2(py)2](PF6)2 (py = pyridine) and [Ru(phen)3](PF6)2 (phen = 1,10-phenanthroline) together with temp.-dependent quantum-yield data for photochem. loss of bpy for the salt [Ru(bpy)3](NCS)2 (.vphi.(25°) = 0.068). The obtained data, combined with the data and suggestions made earlier by J. Van Houten and R. Watts (1975, 1978) based on their expts. in H2O suggest a detailed view of the microscopic events which lead to photosubstitution. Initial excitation leads to a charge-transfer state largely triplet in character, 3CT which undergoes thermal activation to give a d-d excited state. The d-d state undergoes further thermal activation by loss of a pyridyl group to give a 5-coordinate intermediate which is square pyramidal in structure. The fate of the intermediate is capture of a sixth ligand, either by a solvent or an anion held close to the activated metal center by ion-pairing or by chelate ring closure to return to Ru(bpy)32+.
  56. 56

    Absorbance and emission spectra of [Ru(bpy)3](Cl)2 could not be recorded in deoxygenated CH2Cl2 due to low solubility and premature decomposition, vide infra references (53) and (54).

    There is no corresponding record for this reference.
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    Kober, E. M.; Sullivan, B. P.; Meyer, T. J. Solvent dependence of metal-to-ligand charge-transfer transitions. Evidence for initial electron localization in MLCT excited states of 2,2’-bipyridine complexes of ruthenium(II) and osmium(II). Inorg. Chem. 1984, 23, 2098– 2104,  DOI: 10.1021/ic00182a023
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    Solvent dependence of metal-to-ligand charge-transfer transitions. Evidence for initial electron localization in MLCT excited states of 2,2'-bipyridine complexes of ruthenium(II) and osmium(II)
    Kober, Edward M.; Sullivan, B. Patrick; Meyer, Thomas J.
    Inorganic Chemistry (1984), 23 (14), 2098-104CODEN: INOCAJ; ISSN:0020-1669.
    Metal-to-ligand charge-transfer (MLCT) absorption bands for the complexes Ru(bpy)32+, Os(bpy)32+, Os(bpy)2(py)22+, Os(bpy)2(MeCN)22+, and Os(bpy)2(1,2-(Ph2P)2C6H4)2+ (bpy = 2,2'-bipyridine) are solvent dependent. The dependence can be interpreted with use of the dielec. continuum theory but for the D3 ions Ru(bpy)32+ and Os(bpy)32+ only if in the excited state the excited electron is localized on a single ligand rather than delocalized over all 3.
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    Maurer, A. B.; Piechota, E. J.; Meyer, G. J. Excited-State Dipole Moments of Homoleptic [Ru(bpy′)3]2+ Complexes Measured by Stark Spectroscopy. J. Phys. Chem. A 2019, 123, 8745– 8754,  DOI: 10.1021/acs.jpca.9b05874
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    Excited-State Dipole Moments of Homoleptic [Ru(bpy')3]2+ Complexes Measured by Stark Spectroscopy
    Maurer, Andrew B.; Piechota, Eric J.; Meyer, Gerald J.
    Journal of Physical Chemistry A (2019), 123 (41), 8745-8754CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The visible absorption and Stark spectra of five [Ru(4,4'-R-2,2'-bipyridine)3](PF6)2 and [Ru(bipyrazine)3(PF6)2 complexes, where R = CH3O-, tert-butyl-, CH3-, H-, or CF3-, were obtained in butyronitrile glasses at 77K as a function of the applied field in the 0.2-0.8 MV/cm range. Anal. of the metal-to-ligand charge-transfer (MLCT) absorption and Stark spectra with the Liptay treatment revealed dramatic light-induced dipole moment changes, Δμ = 5-11 D. Application of a two-state model to the Δμ values provided values of the metal-ligand electronic coupling, HDA = 4400-6600 cm-1, reasonable for this class of complexes. The ground state of these complexes has no net dipole moment and with the RuII center as the point of ref., the dipole moment changes were reasonably assigned to the dipole present in the initially formed MLCT excited state. Further, the excited state dipole moment was sensitive to the presence of electron donating (MeO-, tert-butyl-, CH3-) or withdrawing (CF3-) substituents on the bipyridine ligands, and Δμ was correlated with the substituent Hammett parameters. Hence the data show for the first time that substituents on the bipyridine ligands, that are often introduced to tune formal redn. potentials, can also induce significant changes in the excited state dipole, behavior that should be taken into consideration for artificial photosynthesis applications.
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    Alary, F.; Heully, J.-L.; Bijeire, L.; Vicendo, P. Is the 3MLCT the Only Photoreactive State of Polypyridyl Complexes?. Inorg. Chem. 2007, 46, 3154– 3165,  DOI: 10.1021/ic062193i
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    Is the 3MLCT the Only Photoreactive State of Polypyridyl Complexes?
    Alary, F.; Heully, J.-L.; Bijeire, L.; Vicendo, P.
    Inorganic Chemistry (2007), 46 (8), 3154-3165CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    By means of Δ-SCF and time-dependent d. functional theory (DFT) calcns. on [Ru(LL)3]2+ (LL = bpy = 2,2'-bipyridyl or bpz = 2,2' -bipyrazyl) complexes, we have found that emission of these two complexes could originate from two metal-to-ligand charge-transfer triplet states (3MLCT) that are quasi-degenerate and whose symmetries are D3 and C2. These two states are true min. Calcd. absorption and emission energies are in good agreement with expt.; the largest error is 0.14 eV, which is about the expected accuracy of the DFT calcns. For the first time, an optimized geometry for the metal-centered (MC) state is proposed for both of these complexes, and their energies are found to be almost degenerate with their corresponding 3MLCT states. These [RuII(LL)(η1-LL)2]2+ MC states have two vacant coordination sites on the metal, so they may react readily with their environment. If these MC states are able to de-excite by luminescence, the assocd. transition (ca. 1 eV) is found to be quite different from those of the 3MLCT states (ca. 2 eV).
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    Vining, W. J.; Caspar, J. V.; Meyer, T. J. The Influence of Environmental Effects on Excited-State Lifetimes. The effect of Ion Pairing on Metal-to-Ligand Charge Transfer Excited States. J. Phys. Chem. 1985, 89, 1095– 1099,  DOI: 10.1021/j100253a010
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    The influence of environmental effects on excited-state lifetimes. The effect of ion pairing on metal-to-ligand charge transfer excited states
    Vining, William J.; Caspar, Jonathan V.; Meyer, Thomas J.
    Journal of Physical Chemistry (1985), 89 (7), 1095-9CODEN: JPCHAX; ISSN:0022-3654.
    Excited-state emission and lifetimes are reported for the complexes Os(phen)32+ and Os(4,4'-Ph2phen)32+ (phen = 1,10-phenanthroline; 4,4'-Ph2phen = 4,4'-diphenyl-1,10-phenanthroline) as a function of counterion in CH2Cl2 soln. The changes in nonradiative decay rate consts. vary with changes in emission energy max. as predicted by the energy gap law. The variations in emission energies and through them the decay rates appear to be induced by changes in ion-dipole interactions in the excited state.
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    Triplet excited state energies (ET) were estimated from the emission maxima recorded at room temperature, see:

    Caspar, J. V.; Meyer, T. J. Photochemistry of Ru(bpy)32+. Solvent Effects. J. Am. Chem. Soc. 1983, 105, 5583– 5590,  DOI: 10.1021/ja00355a009
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    Photochemistry of tris(2,2'-bipyridine)ruthenium(2+) ion (Ru(bpy)32+). Solvent effects
    Caspar, Jonathan V.; Meyer, Thomas J.
    Journal of the American Chemical Society (1983), 105 (17), 5583-90CODEN: JACSAT; ISSN:0002-7863.
    The excited-state lifetime of the metal-to-ligand charge-transfer (MLCT) excited state or states of Ru(bpy)32+ (byp = 2,2'-bipyridine) was measured in a series of solvents at different temps. From a combination of lifetime and emission quantum yield measurements, values for kr and knr (kr and knr denote radiative and nonradiative rate const., resp., for decay of MLCT state(s)) were obtained in the series of solvents. From the variations of the various kinetic parameters with solvent the following conclusions are reached: (1) kr is only slightly solvent dependent; (2) the variations in knr and emission energy with solvent are in quant. agreement with the predictions of the energy gap law for radiationless transitions; and (3) the solvent dependence of the kinetic parameters which characterize the MLCT → dd transition can be considered in the context of electron-transfer theory. The implications of solvent effects on the use of Ru(bpy)32+* as a sensitizer are discussed.
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    Buzzetti, L.; Crisenza, G. E. M.; Melchiorre, P. Mechanistic Studies in Photocatalysis. Angew. Chem., Int. Ed. 2019, 58, 3730– 3747,  DOI: 10.1002/anie.201809984
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    Mechanistic Studies in Photocatalysis
    Buzzetti, Luca; Crisenza, Giacomo E. M.; Melchiorre, Paolo
    Angewandte Chemie, International Edition (2019), 58 (12), 3730-3747CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. The fast-moving fields of photoredox and photocatalysis have recently provided fresh opportunities to expand the potential of synthetic org. chem. Advances in light-mediated processes have mainly been guided so far by empirical findings and the quest for reaction invention. The general perception, however, is that photocatalysis is entering a more mature phase where the combination of exptl. and mechanistic studies will play a dominant role in sustaining further innovation. This Review outlines the key mechanistic studies to consider when developing a photochem. process, and the best techniques available for acquiring relevant information. The discussion will use selected case studies to highlight how mechanistic studies can be instrumental in guiding the invention and development of synthetically useful photocatalytic transformations.
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    Schmid, L.; Kerzig, C.; Prescimone, A.; Wenger, O. S. Photostable Ruthenium(II) Isocyanoborato Luminophores and Their Use in Energy Transfer and Photoredox Catalysis. JACS Au. 2021, 1, 819– 832,  DOI: 10.1021/jacsau.1c00137
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    Photostable Ruthenium(II) Isocyanoborato Luminophores and Their Use in Energy Transfer and Photoredox Catalysis
    Schmid, Lucius; Kerzig, Christoph; Prescimone, Alessandro; Wenger, Oliver S.
    JACS Au (2021), 1 (6), 819-832CODEN: JAAUCR; ISSN:2691-3704. (American Chemical Society)
    Ruthenium(II) polypyridine complexes are among the most popular sensitizers in photocatalysis, but they face some severe limitations concerning accessible excited-state energies and photostability that could hamper future applications. In this study, the borylation of heteroleptic ruthenium(II) cyanide complexes with α-diimine ancillary ligands is identified as a useful concept to elevate the energies of photoactive metal-to-ligand charge-transfer (MLCT) states and to obtain unusually photorobust compds. suitable for thermodynamically challenging energy transfer catalysis as well as oxidative and reductive photoredox catalysis. B(C6F5)3 groups attached to the CN- ligands stabilize the metal-based t2g-like orbitals by ~ 0.8 eV, leading to high 3MLCT energies (up to 2.50 eV) that are more typical for cyclometalated iridium(III) complexes. Through variation of their α-diimine ligands, nonradiative excited-state relaxation pathways involving higher-lying metal-centered states can be controlled, and their luminescence quantum yields and MLCT lifetimes can be optimized. These combined properties make the resp. isocyanoborato complexes amenable to photochem. reactions for which common ruthenium(II)-based sensitizers are unsuited, due to a lack of sufficient triplet energy or excited-state redox power. Specifically, this includes photoisomerization reactions, sensitization of nickel-catalyzed cross-couplings, pinacol couplings, and oxidative decarboxylative C-C couplings. Our work is relevant in the greater context of tailoring photoactive coordination compds. to current challenges in synthetic photochem. and solar energy conversion.
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    Soupart, A.; Dixon, I. M.; Alary, F.; Heully, J. L. DFT Rationalization of the Room-Temperature Luminescence Properties of Ru(bpy)32+ and Ru(tpy)22+: 3MLCT–3MC Minimum Energy Path from NEB Calculations and Emission Spectra from VRES Calculations. Theor. Chem. Acc. 2018, 137, 37,  DOI: 10.1007/s00214-018-2216-1
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  • Abstract

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    Figure 1

    Figure 1. Counterion dependence in photochemical processes. (A) Previously reported counterion effects on single electron transfer (SET). (B) Counterion effects in [Ru(bpy)3](X)2-catalyzed intermolecular [2 + 2] cycloaddition enabled by energy transfer. Bimolecular quenching rate constant (kq). 3,5-Bis(trifluoromethyl)benzenesulfonate (ArFSO3–). Turnover number (TON).

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    Figure 2

    Figure 2. Counterion effect on absorption (solid lines) and emission (dashed lines, λex = 450 nm) properties of [Ru(bpy)3](X)2 in acetonitrile (A) and in dichloromethane (B) at 25 °C. (C) 3MLCT state of [Ru(bpy)3](X)2. (D) Opposite and adjacent arrangements of counterions in calculated 3MLCT states of [Ru(bpy)3](X)2 shown for X– = PF6– in ball and stick (left) and space-filling (right) models (and Figure S22 for X– = Cl–). (E) Specific arrangement of counterions in the calculated 3MLCT state of [Ru(bpy)3](BArF4)2.

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    Figure 3

    Figure 3. Stern–Volmer plots for excited-state quenching of [Ru(bpy)3]X)2 by 1a in acetonitrile (A) and in dichloromethane (B). Excited-state lifetime of [Ru(bpy)3](X)2 in deaerated CH2Cl2 (C).

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    Figure 4

    Figure 4. Photostability of [Ru(bpy)3](X)2 complexes in dichloromethane (8 mM) at 25 °C under 50W blue LED (λmax = 460 nm) irradiation.

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    Figure 5

    Figure 5. Schematic representation of 3MC states of [Ru(bpy)3]2+ with elongation along either dx2–y2 (3MCcis, left) or dz2 (3MCtrans, right).

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    Figure 6

    Figure 6. Diagram schematizing the relative electronic energies of optimized GS (black), 3MLCT (blue lines), 3MCcis (red dots), and 3MCtrans (green dashes) states for [Ru(bpy)3](BArF4)2 and [Ru(bpy)3](PF6)2 complexes. Multiple states of the same color indicate various anion arrangements, such as adjacent (adj) and opposite (opp) (see text, Tables S22, S23, S25 and the SI).

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      McAtee, Rory C.; McClain, Edward J.; Stephenson, Corey R. J.
      Trends in Chemistry (2019), 1 (1), 111-125CODEN: TCRHBQ; ISSN:2589-5974. (Cell Press)
      A review. Over the past decade, photoredox catalysis has risen to the forefront of synthetic org. chem. as an indispensable tool for selective small-mol. activation and chem.-bond formation. This cutting-edge platform allows photosensitizers to convert visible light into chem. energy, prompting generation of reactive radical intermediates. In this , we highlight some of the recent key contributions in the field, including elucidating the impact of the chosen light arrays, promoting fundamental cross-coupling steps, selectively functionalizing aliph. amines, engaging complementary mechanistic paradigms, and applications in industry. With such a wide breadth of reactivity already realized, the presence of photoredox catalysis in all sectors of org. chem. is expected for years to come.
    13. 13
      Marzo, L.; Pagire, S. K.; Reiser, O.; König, B. Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?. Angew. Chem., Int. Ed. 2018, 57, 10034– 10072,  DOI: 10.1002/anie.201709766
      13
      Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?
      Marzo, Leyre; Pagire, Santosh K.; Reiser, Oliver; Koenig, Burkhard
      Angewandte Chemie, International Edition (2018), 57 (32), 10034-10072CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review on visible-light photocatalysis has evolved over the last decade into a widely used method in org. synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloaddns., ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temp., and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in org. synthesis. The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chem. and to open up new avenues by accessing reactive species hitherto unknown, esp. by merging photocatalysis with organo- or metal catalysis.
    14. 14
      Michelin, C.; Hoffmann, N. Photosensitization and Photocatalysis─Perspectives in Organic Synthesis. ACS Catal. 2018, 8, 12046– 12055,  DOI: 10.1021/acscatal.8b03050
      14
      Photosensitization and Photocatalysis-Perspectives in Organic Synthesis
      Michelin, Clement; Hoffmann, Norbert
      ACS Catalysis (2018), 8 (12), 12046-12055CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      A review. Photochem. sensitization and photocatalysis have very similar definitions and are closely related. Each of the two terms are preferentially used in different scientific communities. Three types of processes are discussed: (1) sensitization involving energy transfer, (2) photocatalysis in which hydrogen abstraction plays a key role, and (3) photoredox catalysis in which electron transfer is involved. The processes are discussed in connection with [2 + 2] photo-cycloaddns. and C-H activation, which are of particular interest for org. synthesis.
    15. 15
      Twilton, J.; Le, C.; Zhang, P.; Shaw, M. H.; Evans, R. W.; MacMillan, D. W. C. The Merger of Transition Metal and Photocatalysis. Nat. Rev. Chem. 2017, 1, 0052  DOI: 10.1038/s41570-017-0052
      15
      The merger of transition metal and photocatalysis
      Twilton, Jack; Le, Chi; Zhang, Patricia; Shaw, Megan H.; Evans, Ryan W.; MacMillan, David W. C.
      Nature Reviews Chemistry (2017), 1 (7), 0052CODEN: NRCAF7; ISSN:2397-3358. (Nature Research)
      A review. The merger of transition metal catalysis and photocatalysis, termed metallaphotocatalysis, has recently emerged as a versatile platform for the development of new, highly enabling synthetic methodologies. Photoredox catalysis provides access to reactive radical species under mild conditions from abundant, native functional groups, and, when combined with transition metal catalysis, this feature allows direct coupling of non-traditional nucleophile partners. In addn., photocatalysis can aid fundamental organometallic steps through modulation of the oxidn. state of transition metal complexes or through energy-transfer-mediated excitation of intermediate catalytic species. Metallaphotocatalysis provides access to distinct activation modes, which are complementary to those traditionally used in the field of transition metal catalysis, thereby enabling reaction development through entirely new mechanistic paradigms. This Review discusses key advances in the field of metallaphotocatalysis over the past decade and demonstrates how the unique mechanistic features permit challenging, or previously elusive, transformations to be accomplished.
    16. 16
      Romero, N. A.; Nicewicz, D. A. Organic Photoredox Catalysis. Chem. Rev. 2016, 116, 10075– 10166,  DOI: 10.1021/acs.chemrev.6b00057
      16
      Organic Photoredox Catalysis
      Romero, Nathan A.; Nicewicz, David A.
      Chemical Reviews (Washington, DC, United States) (2016), 116 (17), 10075-10166CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. Use of org. photoredox catalysts in a myriad of synthetic transformations with a range of applications was reviewed. This overview was arranged by catalyst class where the photophysics and electrochem. characteristics of each was discussed to underscore the differences and advantages to each type of single electron redox agent. Net reductive and oxidative as well as redox neutral transformations that could be accomplished using purely org. photoredox-active catalysts was highlighted. An overview of the basic photophysics and electron transfer theory was presented in order to provide a comprehensive guide for employing this class of catalysts in photoredox manifolds.
    17. 17
      Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. Photoredox Catalysis in Organic Chemistry. J. Org. Chem. 2016, 81, 6898– 6926,  DOI: 10.1021/acs.joc.6b01449
      17
      Photoredox Catalysis in Organic Chemistry
      Shaw, Megan H.; Twilton, Jack; MacMillan, David W. C.
      Journal of Organic Chemistry (2016), 81 (16), 6898-6926CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
      In recent years, photoredox catalysis has come to the forefront in org. chem. as a powerful strategy for the activation of small mols. In a general sense, these approaches rely on the ability of metal complexes and org. dyes to convert visible light into chem. energy by engaging in single-electron transfer with org. substrates, thereby generating reactive intermediates. In this Perspective, we highlight the unique ability of photoredox catalysis to expedite the development of completely new reaction mechanisms, with particular emphasis placed on multicatalytic strategies that enable the construction of challenging carbon-carbon and carbon-heteroatom bonds.
    18. 18
      Brimioulle, R.; Lenhart, D.; Maturi, M. M.; Bach, T. Enantioselective Catalysis of Photochemical Reactions. Angew. Chem., Int. Ed. 2015, 54, 3872– 3890,  DOI: 10.1002/anie.201411409
      18
      Enantioselective Catalysis of Photochemical Reactions
      Brimioulle, Richard; Lenhart, Dominik; Maturi, Mark M.; Bach, Thorsten
      Angewandte Chemie, International Edition (2015), 54 (13), 3872-3890CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. The nature of the excited state renders the development of chiral catalysts for enantioselective photochem. reactions a considerable challenge. The absorption of a 400 nm photon corresponds to an energy uptake of approx. 300 kJ mol-1. Given the large distance to the ground state, innovative concepts are required to open reaction pathways that selectively lead to a single enantiomer of the desired product. This Review outlines the two major concepts of homogeneously catalyzed enantioselective processes. The first part deals with chiral photocatalysts, which intervene in the photochem. key step and induce an asym. induction in this step. In the second part, reactions are presented in which the photochem. excitation is mediated by an achiral photocatalyst and the transfer of chirality is ensured by a second chiral catalyst (dual catalysis).
    19. 19
      Prier, C. K.; Rankic, D. A.; MacMillan, D. W. C. Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis. Chem. Rev. 2013, 113, 5322– 5363,  DOI: 10.1021/cr300503r
      19
      Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis
      Prier, Christopher K.; Rankic, Danica A.; MacMillan, David W. C.
      Chemical Reviews (Washington, DC, United States) (2013), 113 (7), 5322-5363CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. This review will highlight the early work on the use of transition metal complexes as photoredox catalysts to promote reactions of org. compds. (prior to 2008), as well as cover the surge of work that has appeared since 2008. We have for the most part grouped reactions according to whether the org. substrate undergoes redn., oxidn., or a redox neutral reaction and throughout have sought to highlight the variety of reactive intermediates that may be accessed via this general reaction manifold.
    20. 20
      Teegardin, K.; Day, J. I.; Chan, J.; Weaver, J. Advances in Photocatalysis: A Microreview of Visible Light Mediated Ruthenium and Iridium Catalyzed Organic Transformations. Org. Process Res. Dev. 2016, 20, 1156– 1163,  DOI: 10.1021/acs.oprd.6b00101
      20
      Advances in Photocatalysis: A Microreview of Visible Light Mediated Ruthenium and Iridium Catalyzed Organic Transformations
      Teegardin, Kip; Day, Jon I.; Chan, John; Weaver, Jimmie
      Organic Process Research & Development (2016), 20 (7), 1156-1163CODEN: OPRDFK; ISSN:1083-6160. (American Chemical Society)
      Photocatalytic org. transformations utilizing ruthenium and iridium complexes have garnered significant attention due to the access they provide to new synthetic spaces through new reaction mechanisms. A survey of the photophys. data and the diversity of transformations that may be accomplished utilizing com. available photocatalysts is contained herein.
    21. 21
      Schultz, D. M.; Yoon, T. P. Solar Synthesis: Prospects in Visible Light Photocatalysis. Science 2014, 343, 1239176  DOI: 10.1126/science.1239176
      21
      Solar synthesis: prospects in visible light photocatalysis
      Schultz Danielle M; Yoon Tehshik P
      Science (New York, N.Y.) (2014), 343 (6174), 1239176 ISSN:.
      Chemists have long aspired to synthesize molecules the way that plants do-using sunlight to facilitate the construction of complex molecular architectures. Nevertheless, the use of visible light in photochemical synthesis is fundamentally challenging because organic molecules tend not to interact with the wavelengths of visible light that are most strongly emitted in the solar spectrum. Recent research has begun to leverage the ability of visible light-absorbing transition metal complexes to catalyze a broad range of synthetically valuable reactions. In this review, we highlight how an understanding of the mechanisms of photocatalytic activation available to these transition metal complexes, and of the general reactivity patterns of the intermediates accessible via visible light photocatalysis, has accelerated the development of this diverse suite of reactions.
    22. 22
      Vlcek, A. A.; Dodsworth, E. S.; Pietro, W. J.; Lever, A. B. P. Excited State Redox Potentials of Ruthenium Diimine Complexes; Correlations with Ground State Redox Potentials and Ligand Parameters. Inorg. Chem. 1995, 34, 1906– 1913,  DOI: 10.1021/ic00111a043
      22
      Excited State Redox Potentials of Ruthenium Diimine Complexes; Correlations with Ground State Redox Potentials and Ligand Parameters
      Vlcek, A. A.; Dodsworth, Elaine S.; Pietro, William J.; Lever, A. B. P.
      Inorganic Chemistry (1995), 34 (7), 1906-13CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The relation between charge transfer emission energies and redox potentials was studied for a large and diverse set of ruthenium diimine complexes. An alternative derivation of excited state redox potentials is developed, which related them directly to the corresponding (observable) ground state potentials and allows them to be estd. when the 0-0' emission energy is unknown. The difference between the excited state and corresponding ground state potentials, D, is approx. const. for complexes in which the emission and redn. processes involve bipyridine-like ligands, provided there are no strong specific solvent-solute interactions. Excited state redox potentials may also be obtained directly by using ligand electrochem. parameters, EL(L). EL(L-) values are calcd. here for a no. of reduced ligands.
    23. 23
      Henwood, A. F.; Zysman-Colman, E. Lessons Learned in Tuning the Optoelectronic Properties of Phosphorescent Iridium(III) Complexes. Chem. Commun. 2017, 53, 807– 826,  DOI: 10.1039/C6CC06729H
      23
      Lessons learned in tuning the optoelectronic properties of phosphorescent iridium(III) complexes
      Henwood, Adam F.; Zysman-Colman, Eli
      Chemical Communications (Cambridge, United Kingdom) (2017), 53 (5), 807-826CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A review. This perspective illustrates our approach in the design of heteroleptic cationic iridium(III) complexes for optoelectronic applications, esp. as emitters in electroluminescent devices. We discuss changes in the photophys. properties of the complexes as a consequence of modification of the electronics of either the cyclometalating (Ĉ N) or the ancillary (N̂ N) ligands. We then broach the impact on these properties as a function of modification of the structure of both types of ligands. We explain trends in the optoelectronic behavior of the complexes using a combination of rationally designed structure-property relationship studies and theor. modeling that serves to inform subsequent ligand design. However, we have found cases where the design paradigms do not always hold true. Nevertheless, all these studies contribute to the lessons we have learned in the design of heteroleptic cationic phosphorescent iridium(III) complexes.
    24. 24
      Larsen, C. B.; Wenger, O. S. Photoredox Catalysis with Metal Complexes Made from Earth-Abundant Elements. Chem. - Eur. J. 2018, 24, 2039– 2058,  DOI: 10.1002/chem.201703602
      24
      Photoredox Catalysis with Metal Complexes Made from Earth-Abundant Elements
      Larsen, Christopher B.; Wenger, Oliver S.
      Chemistry - A European Journal (2018), 24 (9), 2039-2058CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Photoredox chem. with metal complexes as sensitizers and catalysts frequently relies on precious elements such as ruthenium or iridium. Over the past 5 years, important progress towards the use of complexes made from earth-abundant elements in photoredox catalysis has been made. This review summarizes the advances made with photoactive CrIII, FeII, CuI, ZnII, ZrIV, Mo0, and UVI complexes in the context of synthetic org. photoredox chem. using visible light as an energy input. Mechanistic considerations are combined with discussions of reaction types and scopes. Perspectives for the future of the field are discussed against the background of recent significant developments of new photoactive metal complexes made from earth-abundant elements.
    25. 25
      Hockin, B. M.; Li, C.; Robertson, N.; Zysman-Colman, E. Photoredox Catalysts based on Earth-Abundant Metal Complexes. Catal. Sci. Technol. 2019, 9, 889– 915,  DOI: 10.1039/C8CY02336K
      25
      Photoredox catalysts based on earth-abundant metal complexes
      Hockin, Bryony M.; Li, Chenfei; Robertson, Neil; Zysman-Colman, Eli
      Catalysis Science & Technology (2019), 9 (4), 889-915CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      A review. Over the last decade, visible light photoredox catalysis has exploded into the consciousness of the synthetic chemist. The principal photocatalysts used are based on rare and toxic Ru(II) and Ir(III) complexes. This crit. review focuses on Earth-abundant metal complexes as potential replacement photocatalysts and summarizes the use of photoactive Cu(I), Zn(II), Ni(0), V(V), Zr(IV), W(0), W(VI), Mo(0), Cr(III), Co(III) and Fe(II) complexes in photoredox reactions. The optoelectronic properties of these complexes and relevant structurally related analogs, not yet used for photoredox catalysis, are discussed in combination with the reaction scope reported for each photocatalyst. Prospects for the future of photocatalyst design are considered.
    26. 26
      Förster, C.; Heinze, K. Photophysics and Photochemistry with Earth-Abundant Metals – Fundamentals and Concepts. Chem. Soc. Rev. 2020, 49, 1057– 1070,  DOI: 10.1039/C9CS00573K
      26
      Photophysics and photochemistry with Earth-abundant metals - fundamentals and concepts
      Forster Christoph; Heinze Katja
      Chemical Society reviews (2020), 49 (4), 1057-1070 ISSN:.
      Recent exciting developments in the area of mononuclear photoactive complexes with Earth-abundant metal ions (Cu, Zr, Fe, Cr) for potential eco-friendly applications in (phosphorescent) organic light emitting diodes, in imaging and sensing systems, in dye-sensitized solar cells and as photocatalysts are presented. Challenges, in particular the extension of excited state lifetimes, and recent conceptual breakthroughs in substituting precious and rare-Earth metal ions (e.g. Ru, Ir, Pt, Au, Eu) in these applications by abundant ions are outlined with selected examples. Relevant fundamentals of photophysics and photochemistry are discussed first, followed by conceptual and instructive case studies.
    27. 27
      Wegeberg, C.; Wenger, O. S. Luminescent First-Row Transition Metal Complexes. JACS Au 2021, 1, 1860– 1876,  DOI: 10.1021/jacsau.1c00353
      27
      Luminescent First-Row Transition Metal Complexes
      Wegeberg, Christina; Wenger, Oliver S.
      JACS Au (2021), 1 (11), 1860-1876CODEN: JAAUCR; ISSN:2691-3704. (American Chemical Society)
      A review. Precious and rare elements have traditionally dominated inorg. photophysics and photochem., but now we are witnessing a paradigm shift toward cheaper and more abundant metals. Even though emissive complexes based on selected first-row transition metals have long been known, recent conceptual breakthroughs revealed that a much broader range of elements in different oxidn. states are useable for this purpose. Coordination compds. of V, Cr, Mn, Fe, Co, Ni, and Cu now show electronically excited states with unexpected reactivity and photoluminescence behavior. Aside from providing a compact survey of the recent conceptual key advances in this dynamic field, our Perspective identifies the main design strategies that enabled the discovery of fundamentally new types of 3d-metal-based luminophores and photosensitizers operating in soln. at room temp.
    28. 28
      Herr, P.; Kerzig, C.; Larsen, C. B.; Häussinger, D.; Wenger, O. S. Manganese(I) Complexes with Metal-to-Ligand Charge Transfer Luminescence and Photoreactivity. Nat. Chem. 2021, 13, 956– 962,  DOI: 10.1038/s41557-021-00744-9
      28
      Manganese(I) complexes with metal-to-ligand charge transfer luminescence and photoreactivity
      Herr, Patrick; Kerzig, Christoph; Larsen, Christopher B.; Haussinger, Daniel; Wenger, Oliver S.
      Nature Chemistry (2021), 13 (10), 956-962CODEN: NCAHBB; ISSN:1755-4330. (Nature Portfolio)
      Precious metal complexes with the d6 valence electron configuration often exhibit luminescent metal-to-ligand charge transfer (MLCT) excited states, which form the basis for many applications in lighting, sensing, solar cells and synthetic photochem. Iron(II) has received much attention as a possible Earth-abundant alternative, but to date no iron(II) complex has been reported to show MLCT emission upon continuous-wave excitation. Manganese(I) has the same electron configuration as that of iron(II), but until now has typically been overlooked in the search for cheap MLCT luminophores. Here we report that isocyanide chelate ligands give access to air-stable manganese(I) complexes that exhibit MLCT luminescence in soln. at room temp. These compds. were successfully used as photosensitizers for energy- and electron-transfer reactions and were shown to promote the photoisomerization of trans-stilbene. The observable electron transfer photoreactivity occurred from the emissive MLCT state, whereas the triplet energy transfer photoreactivity originated from a ligand-centered 3π-π* state. [graphic not available: see fulltext].
    29. 29
      de Groot, L. H. M.; Ilic, A.; Schwarz, J.; Wärnmark, K. Iron Photoredox Catalysis–Past, Present, and Future. J. Am. Chem. Soc. 2023, 145, 9369– 9388,  DOI: 10.1021/jacs.3c01000
      29
      Iron Photoredox Catalysis - Past, Present, and Future
      de Groot, Lisa H. M.; Ilic, Aleksandra; Schwarz, Jesper; Waernmark, Kenneth
      Journal of the American Chemical Society (2023), 145 (17), 9369-9388CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A review. Photoredox catalysis of org. reactions driven by iron has attracted substantial attention throughout recent years, due to potential environmental and economic benefits. In this Perspective, three major strategies were identified that have been employed to date to achieve reactivities comparable to the successful noble metal photoredox catalysis: (1) Direct replacement of a noble metal center by iron in archetypal polypyridyl complexes, resulting in a metal-centered photofunctional state. (2) In situ generation of photoactive complexes by substrate coordination where the reactions are driven via intramol. electron transfer involving charge-transfer states, for example through visible light-induced homolysis. (3) Improving the excited state lifetimes and redox potentials of the charge-transfer states of iron complexes through new ligand design, for example by introducing N-heterocyclic carbene ligands. We seek to give an overview and evaluation of recent developments in this rapidly growing field, and at the same time provide an outlook on the future of iron-based photoredox catalysis.
    30. 30
      Sinha, N.; Yaltseva, P.; Wenger, O. S. The Nephelauxetic Effect Becomes an Important Design Factor for Photoactive First-Row Transition Metal Complexes. Angew. Chem., Int. Ed. 2023, e202303864  DOI: 10.1002/anie.202303864
      There is no corresponding record for this reference.
    31. 31
      Chan, A. Y.; Ghosh, A.; Yarranton, J. T.; Twilton, J.; Jin, J.; Arias-Rotondo, D. M.; Sakai, H. A.; McCusker, J. K.; MacMillan, D. W. C. Exploiting the Marcus Inverted Region for First-Row Transition Metal–based Photoredox Catalysis. Science 2023, 382, 191– 197,  DOI: 10.1126/science.adj0612
      There is no corresponding record for this reference.
    32. 32
      Troian-Gautier, L.; Beauvilliers, E. E.; Swords, W. B.; Meyer, G. J. Redox Active Ion-Paired Excited States Undergo Dynamic Electron Transfer. J. Am. Chem. Soc. 2016, 138, 16815– 16826,  DOI: 10.1021/jacs.6b11337
      32
      Redox active ion-paired excited states undergo dynamic electron transfer
      Troian-Gautier, Ludovic; Beauvilliers, Evan E.; Swords, Wesley B.; Meyer, Gerald J.
      Journal of the American Chemical Society (2016), 138 (51), 16815-16826CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Ion-pair interactions between a cationic ruthenium complex, [Ru(dtb)2(dea)][PF6]2, C12+ where dea is 4,4'-diethanolamide-2,2'-bipyridine and dtb is 4,4'-di-tert-butyl-2,2'-bipyridine, and chloride, bromide, and iodide are reported. A remarkable result is that a 1:1 iodide: excited-state ion-pair, [C12+, I-]+*, underwent diffusional electron-transfer oxidn. of iodide that did not occur when ion-pairing was absent. The ion-pair equil. consts. ranged 104-106 M-1 in CH3CN and decreased in the order Cl- > Br- > I-. The ion-pairs had longer-lived excited states, were brighter emitters, and stored more free energy than did the non-ion-paired states. The 1H NMR spectra revealed that the halides formed tight ion-pairs with the amide and alc. groups of the DEA ligand. Electron-transfer reactivity of the ion-paired excited state was not simply due to it being a stronger photooxidant than the non-ion-paired excited state. Instead, work term, ΔGw was the predominant contributor to the driving force for the reaction. Natural bond order calcns. provided natural at. charges that enabled quantification of ΔGw for all the atoms in C12+ and [C12+, I-]+* presented herein as contour diagrams that show the most favorable electrostatic positions for halide interactions. The results were most consistent with a model wherein the non-ion-paired C12+* excited state traps the halide and prevents its oxidn., but allows for dynamic oxidn. of a second iodide ion.
    33. 33
      Li, G.; Swords, W. B.; Meyer, G. J. Bromide Photo-oxidation Sensitized to Visible Light in Consecutive Ion Pairs. J. Am. Chem. Soc. 2017, 139, 14983– 14991,  DOI: 10.1021/jacs.7b06735
      33
      Bromide Photo-oxidation Sensitized to Visible Light in Consecutive Ion Pairs
      Li, Guocan; Swords, Wesley B.; Meyer, Gerald J.
      Journal of the American Chemical Society (2017), 139 (42), 14983-14991CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The titrn. of bromide into a [Ru(deeb)(bpz)2]2+ (Ru2+, deeb = 4,4'-diethylester-2,2'-bipyridine; bpz = 2,2'-bipyrazine) dichloromethane soln. led to the formation of two consecutive ion-paired species, [Ru2+, Br-]+ and [Ru2+, 2Br-], each with distinct photophys. and electron-transfer properties. Formation of the first ion pair was stoichiometric, Keq 1 > 106 M-1, and the second ion-pair equil. was estd. to be Keq 2 = (2.4 ± 0.4) × 105 M-1. The 1H NMR spectra recorded in deuterated dichloromethane indicated the presence of contact ion pairs and provided insights into their structures and were complimented by d. functional theory calcns. Static quenching of the [Ru(deeb)(bpz)2]2+* photoluminescence intensity (PLI) by bromide was obsd., and [Ru2+, Br-]+* was found to be nonluminescent, τ < 10 ns. Further addn. of bromide resulted in partial recovery of the PLI, and [Ru2+, 2Br-]* was found to be luminescent with an excited-state lifetime of τ = 65 ± 5 ns. Electron-transfer products were identified as the reduced complex, [Ru(deeb)(bpz)2]+, and dibromide, Br2•-. The bromine atom, Br•, was detd. to be the primary excited-state electron-transfer product and was an intermediate in Br2•- formation, Br• + Br- → Br2•-, with a second-order rate const., k = (5.4 ± 1) × 108 M-1 s-1. The unusual enhancement in PLI for [Ru2+, 2Br-]* relative to [Ru2+, Br-]+* was due to a less favorable Gibbs free energy change for electron transfer that resulted in a smaller rate const., ket = (1.5 ± 0.2) × 107 s-1, in the second ion pair. Natural at. charge anal. provided ests. of the Coulombic work terms assocd. with ion pairing, ΔGw, that were directly correlated with the measured change in rate consts.
    34. 34
      Morton, C. M.; Zhu, Q.; Ripberger, H. H.; Troian-Gautier, L.; Toa, Z. S. D.; Knowles, R. R.; Alexanian, E. J. C–H Alkylation via Multisite-Proton-Coupled Electron Transfer of an Aliphatic C–H Bond. J. Am. Chem. Soc. 2019, 141, 13253– 13260,  DOI: 10.1021/jacs.9b06834
      34
      C-H Alkylation via Multisite-Proton-Coupled Electron Transfer of an Aliphatic C-H Bond
      Morton, Carla M.; Zhu, Qilei; Ripberger, Hunter; Troian-Gautier, Ludovic; Toa, Zi S. D.; Knowles, Robert R.; Alexanian, Erik J.
      Journal of the American Chemical Society (2019), 141 (33), 13253-13260CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The direct, site-selective alkylation of unactivated C(sp3)-H bonds in org. substrates is a long-standing goal in synthetic chem. General approaches to the activation of strong C-H bonds include radical-mediated processes involving highly reactive intermediates, such as heteroatom-centered radicals. Herein, we describe a catalytic, intermol. C-H alkylation that circumvents such reactive species via a new elementary step for C-H cleavage involving multisite-proton-coupled electron transfer (multisite-PCET). Mechanistic studies indicate that the reaction is catalyzed by a noncovalent complex formed between an iridium(III) photocatalyst and a monobasic phosphate base. The C-H alkylation proceeds efficiently using diverse hydrocarbons and complex mols. as the limiting reagent and represents a new approach to the catalytic functionalization of unactivated C(sp3)-H bonds.
    35. 35
      Uraguchi, D.; Kimura, Y.; Ueoka, F.; Ooi, T. Urea as a Redox-Active Directing Group under Asymmetric Photocatalysis of Iridium-Chiral Borate Ion Pairs. J. Am. Chem. Soc. 2020, 142, 19462– 19467,  DOI: 10.1021/jacs.0c09468
      35
      Urea as a Redox-Active Directing Group under Asymmetric Photocatalysis of Iridium-Chiral Borate Ion Pairs
      Uraguchi, Daisuke; Kimura, Yuto; Ueoka, Fumito; Ooi, Takashi
      Journal of the American Chemical Society (2020), 142 (46), 19462-19467CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The development of a photoinduced, highly diastereo- and enantioselective [3 + 2]-cycloaddn. of N-cyclopropylurea with α-alkylstyrenes is reported. This asym. radical cycloaddn. relies on the strategic placement of urea on cyclopropylamine as a redox-active directing group (DG) with anion-binding ability and the use of an ion pair, comprising an iridium polypyridyl complex and a weakly coordinating chiral borate ion, as a photocatalyst. The structure of the anion component of the catalyst governs reactivity, and pertinent structural modification of the borate ion enables high levels of catalytic activity and stereocontrol. This system tolerates a range of α-alkylstyrenes and hence offers rapid access to various aminocyclopentanes with contiguous tertiary and quaternary stereocenters, as the urea DG is readily removable.
    36. 36
      Xu, J.; Li, Z.; Xu, Y.; Shu, X.; Huo, H. Stereodivergent Synthesis of Both Z- and E-Alkenes by Photoinduced, Ni-Catalyzed Enantioselective C(sp3)–H Alkenylation. ACS Catal. 2021, 11, 13567– 13574,  DOI: 10.1021/acscatal.1c04314
      36
      Stereodivergent Synthesis of Both Z- and E-Alkenes by Photoinduced, Ni-Catalyzed Enantioselective C(sp3)-H Alkenylation
      Xu, Jitao; Li, Zhilong; Xu, Yumin; Shu, Xiaomin; Huo, Haohua
      ACS Catalysis (2021), 11 (21), 13567-13574CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      An enantioselective benzylic C(sp3)-H alkenylation of simple alkylarenes with vinyl bromides via photoinduced nickel catalysis was reported, which allowed the stereodivergent synthesis of both enantioenriched Z- and E-alkenes bearing aryl-substituted, allylic tertiary stereogenic centers. Interestingly, the tunable Z/E-selectivity was achieved by energy transfer catalysis via a judicious choice of the photocatalyst counteranion. This versatile strategy featured simple starting materials, mild reaction conditions, broad substrate scope, divergent Z- and E-selectivity and high enantioselectivities. Moreover, a formal asym. benzylic C(sp3)-H alkylation was also be achieved via a one-pot alkenylation/redn. sequence, providing a complementary strategy to address the notoriously challenging stereochem. control in C(sp3)-C(sp3) bond construction.
    37. 37
      Chapman, S. J.; Swords, W. B.; Le, C. M.; Guzei, I. A.; Toste, F. D.; Yoon, T. P. Cooperative Stereoinduction in Asymmetric Photocatalysis. J. Am. Chem. Soc. 2022, 144, 4206– 4213,  DOI: 10.1021/jacs.2c00063
      37
      Cooperative Stereoinduction in Asymmetric Photocatalysis
      Chapman, Steven J.; Swords, Wesley B.; Le, Christine M.; Guzei, Ilia A.; Toste, F. Dean; Yoon, Tehshik P.
      Journal of the American Chemical Society (2022), 144 (9), 4206-4213CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Stereoinduction in complex org. reactions often involves the influence of multiple stereocontrol elements. The interaction among these can often result in the observation of significant cooperative effects that afford different rates and selectivities between the matched and mismatched sets of stereodifferentiating chiral elements. The elucidation of matched/mismatched effects in ground-state chem. reactions was a critically important theme in the maturation of modern stereocontrolled synthesis. The development of robust methods for the control of photochem. reactions, however, is a relatively recent development, and similar cooperative stereocontrolling effects in excited-state enantioselective photoreactions have not previously been documented. Herein, we describe a tandem chiral photocatalyst/Bronsted acid strategy for highly enantioselective [2 + 2] photocycloaddns. of vinylpyridines. Importantly, the matched and mismatched chiral catalyst pairs exhibit different reaction rates and enantioselectivities across a range of coupling partners. We observe no evidence of ground-state interactions between the catalysts and conclude that these effects arise from their cooperative behavior in a transient excited-state assembly. These results suggest that similar matched/mismatched effects might be important in other classes of enantioselective dual-catalytic photochem. reactions.
    38. 38
      Girvin, Z. C.; Cotter, L. F.; Yoon, H.; Chapman, S. J.; Mayer, J. M.; Yoon, T. P.; Miller, S. J. Asymmetric Photochemical [2 + 2]-Cycloaddition of Acyclic Vinylpyridines through Ternary Complex Formation and an Uncontrolled Sensitization Mechanism. J. Am. Chem. Soc. 2022, 144, 20109– 20117,  DOI: 10.1021/jacs.2c09690
      38
      Asymmetric Photochemical [2 + 2]-Cycloaddition of Acyclic Vinylpyridines through Ternary Complex Formation and an Uncontrolled Sensitization Mechanism
      Girvin, Zebediah C.; Cotter, Laura F.; Yoon, Hyung; Chapman, Steven J.; Mayer, James M.; Yoon, Tehshik P.; Miller, Scott J.
      Journal of the American Chemical Society (2022), 144 (43), 20109-20117CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Stereochem. control of photochem. reactions that occur via triplet energy transfer remains a challenge. Suppressing off-catalyst stereorandom reactivity is difficult for highly reactive open-shell intermediates. Strategies for suppressing racemate-producing, off-catalyst pathways have long focused on formation of ground state, substrate-catalyst chiral complexes that are primed for triplet energy transfer via a photocatalyst in contrast to their off-catalyst counterparts. Herein, we describe a strategy where both a chiral catalyst-assocd. vinylpyridine and a nonassocd., free vinylpyridine substrate can be sensitized by an Ir(III) photocatalyst, yet high levels of diastereo- and enantioselectivity in a [2 + 2] photocycloaddn. are achieved through a preferred, highly organized transition state. This mechanistic paradigm is distinct from, yet complementary to current approaches for achieving high levels of stereocontrol in photochem. transformations.
    39. 39
      Farney, E. P.; Chapman, S. J.; Swords, W. B.; Torelli, M. D.; Hamers, R. J.; Yoon, T. P. Discovery and Elucidation of Counteranion Dependence in Photoredox Catalysis. J. Am. Chem. Soc. 2019, 141, 6385– 6391,  DOI: 10.1021/jacs.9b01885
      39
      Discovery and elucidation of counteranion dependence in photoredox catalysis
      Farney, Elliot P.; Chapman, Steven J.; Swords, Wesley B.; Torelli, Marco D.; Hamers, Robert J.; Yoon, Tehshik P.
      Journal of the American Chemical Society (2019), 141 (15), 6385-6391CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Over the past decade, there has been a renewed interest in the use of transition metal polypyridyl complexes as photoredox catalysts for a variety of innovative synthetic applications. Many derivs. of these complexes are known, and the effect of ligand modifications on their efficacy as photoredox catalysts has been the subject of extensive, systematic investigation. However, the influence of the photocatalyst counteranion has received little attention, despite the fact that these complexes are generally cationic in nature. Herein, we demonstrate that counteranion effects exert a surprising, dramatic impact on the rate of a representative photocatalytic radical cation Diels-Alder reaction. A detailed anal. reveals that counteranion identity impacts multiple aspects of the reaction mechanism. Most notably, photocatalysts with more noncoordinating counteranions yield a more powerful triplet excited state oxidant and longer radical cation chain length. It is proposed that this counteranion effect arises from Coulombic ion-pairing interactions between the counteranion and both the cationic photoredox catalyst and the radical cation intermediate, resp. The comparatively slower rate of reaction with coordinating counteranions can be rescued by using hydrogen-bonding anion binders that attenuate deleterious ion-pairing interactions. These results demonstrate the importance of counteranion identity as a variable in the design and optimization of photoredox transformations and suggest a novel strategy for the optimization of org. reactions using this class of transition metal photocatalysts.
    40. 40
      Earley, J. D.; Zieleniewska, A.; Ripberger, H. H.; Shin, N. Y.; Lazorski, M. S.; Mast, Z. J.; Sayre, H. J.; McCusker, J. K.; Scholes, G. D.; Knowles, R. R.; Reid, O. G.; Rumbles, G. Ion-Pair Reorganization Regulates Reactivity in Photoredox Catalysts. Nat. Chem. 2022, 14, 746– 753,  DOI: 10.1038/s41557-022-00911-6
      40
      Ion-pair reorganization regulates reactivity in photoredox catalysts
      Earley, J. D.; Zieleniewska, A.; Ripberger, H. H.; Shin, N. Y.; Lazorski, M. S.; Mast, Z. J.; Sayre, H. J.; McCusker, J. K.; Scholes, G. D.; Knowles, R. R.; Reid, O. G.; Rumbles, G.
      Nature Chemistry (2022), 14 (7), 746-753CODEN: NCAHBB; ISSN:1755-4330. (Nature Portfolio)
      Cyclometalated and polypyridyl complexes of d6 metals are promising photoredox catalysts, using light to drive reactions with high kinetic or thermodn. barriers via the generation of reactive radical intermediates. However, while tuning of their redox potentials, absorption energy, excited-state lifetime and quantum yield are well-known criteria for modifying activity, other factors could be important. Here we show that dynamic ion-pair reorganization controls the reactivity of a photoredox catalyst, [Ir[dF(CF3)ppy]2(dtbpy)]X. Time-resolved dielec.-loss expts. show how counter-ion identity influences excited-state charge distribution, evincing large differences in both the ground- and excited-state dipole moment depending on whether X is a small assocg. anion (PF6-) that forms a contact-ion pair vs. a large one that either dissocs. or forms a solvent-sepd. pair (BArF4-). These differences correlate with the reactivity of the photocatalyst toward both reductive and oxidative electron transfer, amounting to a 4-fold change in selectivity toward oxidn. vs. redn. These results suggest that ion pairing could be an underappreciated factor that modulates reactivity in ionic photoredox catalysts.
    41. 41
      Geunes, E. P.; Meinhardt, J. M.; Wu, E. J.; Knowles, R. R. Photocatalytic Anti-Markovnikov Hydroamination of Alkenes with Primary Heteroaryl Amines. J. Am. Chem. Soc. 2023, 145, 21738– 21744,  DOI: 10.1021/jacs.3c08428
      There is no corresponding record for this reference.
    42. 42

      For an example of counterion effect in iron photoredox catalysis (LMCT emitter), see:

      Jang, Y. J.; An, H.; Choi, S.; Hong, J.; Lee, S. H.; Ahn, K.-H.; You, Y.; Kang, E. J. Green-Light-Driven Fe(III)(btz)3 Photocatalysis in the Radical Cationic [4 + 2] Cycloaddition Reaction. Org. Lett. 2022, 24, 4479– 4484,  DOI: 10.1021/acs.orglett.2c01779
      42
      Green-Light-Driven Fe(III)(btz)3 Photocatalysis in the Radical Cationic [4+2] Cycloaddition Reaction
      Jang, Yu Jeong; An, Hyeju; Choi, Seunghee; Hong, Jayeon; Lee, Seung Hyun; Ahn, Kwang-Hyun; You, Youngmin; Kang, Eun Joo
      Organic Letters (2022), 24 (24), 4479-4484CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
      Green-light-driven Fe(btz)3 I·3PF6- photocatalysis for the radical cationic [4+2] cycloaddn. of terminal styrenes and nucleophilic dienes has been investigated. The Fe-MIC (mesoionic carbene) complex forms a ligand-to-metal charge-transfer transition state with relatively high excited-state redn. potentials that can selectively oxidize terminal styrene derivs. Unique multisubstituted cyclohexenes and structurally complex biorelevant cyclohexenes were constructed, highlighting the usefulness of this mild and practical first-row transition metal complex system.
    43. 43
      Dutta, S.; Erchinger, J. E.; Strieth-Kalthoff, F.; Kleinmans, R.; Glorius, F. Energy Transfer Photocatalysis: Exciting Modes of Reactivity. Chem. Soc. Rev. 2024, 53, 1068– 1089,  DOI: 10.1039/D3CS00190C
      43
      Energy transfer photocatalysis: exciting modes of reactivity
      Dutta, Subhabrata; Erchinger, Johannes E.; Strieth-Kalthoff, Felix; Kleinmans, Roman; Glorius, Frank
      Chemical Society Reviews (2024), 53 (3), 1068-1089CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Excited (triplet) states offer a myriad of attractive synthetic pathways, including cycloaddns., selective homolytic bond cleavages and strain-release chem., isomerizations, deracemizations, or the fusion with metal catalysis. Recent years have seen enormous advantages in enabling these reactivity modes through visible-light-mediated triplet-triplet energy transfer catalysis (TTEnT). This tutorial review provides an overview of this emerging strategy for synthesizing sought-after org. motifs in a mild, selective, and sustainable manner. Building on the photophys. foundations of energy transfer, this review also discusses catalyst design, as well as the challenges and opportunities of energy transfer catalysis.
    44. 44
      Großkopf, J.; Kratz, T.; Rigotti, T.; Bach, T. Enantioselective Photochemical Reactions Enabled by Triplet Energy Transfer. Chem. Rev. 2022, 122, 1626– 1653,  DOI: 10.1021/acs.chemrev.1c00272
      44
      Enantioselective Photochemical Reactions Enabled by Triplet Energy Transfer
      Grosskopf, Johannes; Kratz, Thilo; Rigotti, Thomas; Bach, Thorsten
      Chemical Reviews (Washington, DC, United States) (2022), 122 (2), 1626-1653CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      Review. For mols. with a singlet ground state, the population of triplet states is mainly possible (a) by direct excitation and subsequent intersystem crossing or (b) by energy transfer from an appropriate sensitizer. The latter scenario enables a catalytic photochem. reaction in which the sensitizer adopts the role of a catalyst undergoing several cycles of photon absorption and subsequent energy transfer to the substrate. If the product mol. of a triplet-sensitized process is chiral, this process can proceed enantioselectively upon judicious choice of a chiral triplet sensitizer. An enantioselective reaction can also occur in a dual catalytic approach in which, apart from an achiral sensitizer, a second chiral catalyst activates the substrate toward sensitization. Although the idea of enantioselective photochem. reactions via triplet intermediates has been pursued for more than 50 years, notable selectivities exceeding 90% enantiomeric excess (ee) have only been realized in the past decade. This review attempts to provide a comprehensive survey on the various photochem. reactions which were rendered enantioselective by triplet sensitization.
    45. 45
      Zhou, Q.-Q.; Zou, Y.-Q.; Lu, L.-Q.; Xiao, W.-J. Visible-Light-Induced Organic Photochemical Reactions through Energy-Transfer Pathways. Angew. Chem., Int. Ed. 2019, 58, 1586– 1604,  DOI: 10.1002/anie.201803102
      45
      Visible-light-induced organic photochemical reactions through energy-transfer pathways
      Zhou, Quan-Quan; Zou, You-Quan; Lu, Liang-Qiu; Xiao, Wen-Jing
      Angewandte Chemie, International Edition (2019), 58 (6), 1586-1604CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Visible-light photocatalysis is a rapidly developing and powerful strategy to initiate org. transformations, as it closely adheres to the tenants of green and sustainable chem. Generally, most visible-light-induced photochem. reactions occur through single-electron transfer (SET) pathways. Recently, visible-light-induced energy-transfer (EnT) reactions have received considerable attentions from the synthetic community as this strategy provides a distinct reaction pathway, and remarkable achievements have been made in this field. In this Review, we highlight the most recent advances in visible-light-induced EnT reactions.
    46. 46
      Strieth-Kalthoff, F.; James, M. J.; Teders, M.; Pitzer, L.; Glorius, F. Energy Transfer Catalysis Mediated by Visible Light: Principles, Applications, Directions. Chem. Soc. Rev. 2018, 47, 7190– 7202,  DOI: 10.1039/C8CS00054A
      46
      Energy transfer catalysis mediated by visible light: principles, applications, directions
      Strieth-Kalthoff, Felix; James, Michael J.; Teders, Michael; Pitzer, Lena; Glorius, Frank
      Chemical Society Reviews (2018), 47 (19), 7190-7202CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      A review. Harnessing visible light to access excited (triplet) states of org. compds. can enable impressive reactivity modes. This tutorial review covers the photophys. fundamentals and most significant advances in the field of visible-light-mediated energy transfer catalysis within the last decade. Methods to det. excited triplet state energies and to characterize the underlying Dexter energy transfer are discussed. Synthetic applications of this field, divided into four main categories (cyclization reactions, double bond isomerizations, bond dissocns. and sensitization of metal complexes), are also examd.
    47. 47
      Strieth-Kalthoff, F.; Glorius, F. Triplet Energy Transfer Photocatalysis: Unlocking the Next Level. Chem. 2020, 6, 1888– 1903,  DOI: 10.1016/j.chempr.2020.07.010
      47
      Triplet Energy Transfer Photocatalysis: Unlocking the Next Level
      Strieth-Kalthoff, Felix; Glorius, Frank
      Chem (2020), 6 (8), 1888-1903CODEN: CHEMVE; ISSN:2451-9294. (Cell Press)
      A review. Energy transfer can leverage the enormous potential of excited-state reactivity. Through "indirect excitation" of substrates, otherwise elusive reactivity modes can be switched on, allowing for, e.g., cycloaddns., fragmentations, rearrangements, or challenging organometallic steps. This perspective recaps almost 70 years of energy transfer in org. chem., highlighting the way it evolved, as well as recent developments in the field of visible-light photocatalysis. Building upon the photophys. fundamentals, diverse applications and directions of energy transfer catalysis are pointed out.
    48. 48
      Poplata, S.; Tröster, A.; Zou, Y. Q.; Bach, T. Recent advances in the synthesis of cyclobutanes by olefin [2 + 2] photocycloaddition reactions. Chem. Rev. 2016, 116, 9748– 9815,  DOI: 10.1021/acs.chemrev.5b00723
      48
      Recent Advances in the Synthesis of Cyclobutanes by Olefin [2+2] Photocycloaddition Reactions
      Poplata, Saner; Troester, Andreas; Zou, You-Quan; Bach, Thorsten
      Chemical Reviews (Washington, DC, United States) (2016), 116 (17), 9748-9815CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      The [2+2] photocycloaddn. is undisputedly the most important and most frequently used photochem. reaction. In this review, it is attempted to cover all recent aspects of [2+2] photocycloaddn. chem. with an emphasis on synthetically relevant, regio-, and stereoselective reactions. The review aims to comprehensively discuss relevant work, which was done in the field in the last 20 years (i.e., from 1995 to 2015). Organization of the data follows a subdivision according to mechanism and substrate classes. Cu(I) and PET (photoinduced electron transfer) catalysis are treated sep. in sections and , whereas the vast majority of photocycloaddn. reactions which occur by direct excitation or sensitization are divided within section into individual subsections according to the photochem. excited olefin.
    49. 49
      Huang, X.; Quinn, T. R.; Harms, K.; Webster, R. D.; Zhang, L.; Wiest, O.; Meggers, E. Direct Visible-Light-Excited Asymmetric Lewis Acid Catalysis of Intermolecular [2 + 2] Photocycloadditions. J. Am. Chem. Soc. 2017, 139, 9120– 9123,  DOI: 10.1021/jacs.7b04363
      49
      Direct Visible-Light-Excited Asymmetric Lewis Acid Catalysis of Intermolecular [2+2] Photocycloadditions
      Huang, Xiaoqiang; Quinn, Taylor R.; Harms, Klaus; Webster, Richard D.; Zhang, Lilu; Wiest, Olaf; Meggers, Eric
      Journal of the American Chemical Society (2017), 139 (27), 9120-9123CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A reaction design is reported in which a substrate-bound chiral Lewis acid complex absorbs visible light and generates an excited state that directly reacts with a cosubstrate in a highly stereocontrolled fashion. Specifically, a chiral rhodium complex catalyzes visible-light-activated intermol. [2+2] cycloaddns., providing a wide range of cyclobutanes with up to >99% ee and up to >20:1 d.r. Noteworthy is the ability to create vicinal all-carbon-quaternary stereocenters including spiro centers in an intermol. fashion.
    50. 50
      Sherbrook, E. M.; Jung, H.; Cho, D.; Baik, M.-H.; Yoon, T. P. Brønsted Acid Catalysis of Photosensitized Cycloadditions. Chem. Sci. 2020, 11, 856– 861,  DOI: 10.1039/C9SC04822G
      50
      Bronsted acid catalysis of photosensitized cycloadditions
      Sherbrook, Evan M.; Jung, Hoimin; Cho, Dasol; Baik, My-Hyun; Yoon, Tehshik P.
      Chemical Science (2020), 11 (3), 856-861CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Authors showed that Bronsted acids can also modulate the reactivity of excited-state org. reactions. Bronsted acids dramatically increase the rate of Ru(bpy)32+-sensitized [2 + 2] photocycloaddns. between C-cinnamoyl imidazoles and a range of electron-rich alkene reaction partners. A combination of exptl. and computational studies supported a mechanism in which the Bronsted acid co-catalyst accelerates triplet energy transfer from the excited-state [Ru*(bpy)3]2+ chromophore to the Bronsted acid activated C-cinnamoyl imidazole. Computational evidence further suggested the importance of driving force as well as geometrical reorganization, in which the protonation of the imidazole decreases the reorganization penalty during the energy transfer event.
    51. 51
      Jung, H.; Hong, M.; Marchini, M.; Villa, M.; Steinlandt, P. S.; Huang, X.; Hemming, M.; Meggers, E.; Ceroni, P.; Park, J.; Baik, M.-H. Understanding the Mechanism of Direct Visible-Light-Activated [2 + 2] Cycloadditions Mediated by Rh and Ir Photocatalysts: Combined Computational and Spectroscopic Studies. Chem. Sci. 2021, 12, 9673– 9681,  DOI: 10.1039/D1SC02745J
      51
      Understanding the mechanism of direct visible-light-activated [2 + 2] cycloadditions mediated by Rh and Ir photocatalysts: combined computational and spectroscopic studies
      Jung, Hoimin; Hong, Mannkyu; Marchini, Marianna; Villa, Marco; Steinlandt, Philipp S.; Huang, Xiaoqiang; Hemming, Marcel; Meggers, Eric; Ceroni, Paola; Park, Jiyong; Baik, Mu-Hyun
      Chemical Science (2021), 12 (28), 9673-9681CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      The mechanism of [2 + 2] cycloaddns. activated by visible light and catalyzed by bis-cyclometalated Rh(III) and Ir(III) photocatalysts was investigated, combining d. functional theory calcns. and spectroscopic techniques. Exptl. observations show that the Rh-based photocatalyst produces excellent yield and enantioselectivity whereas the Ir-photocatalyst yields racemates. Two different mechanistic features were found to compete with each other, namely the direct photoactivation of the catalyst-substrate complex and outer-sphere triplet energy transfer. Our integrated anal. suggests that the direct photocatalysis is the inner working of the Rh-catalyzed reaction, whereas the Ir catalyst serves as a triplet sensitizer that activates cycloaddn. via an outer-sphere triplet excited state energy transfer mechanism.
    52. 52
      Sherbrook, E. M.; Genzink, M. J.; Park, B.; Guzei, I. A.; Baik, M. H.; Yoon, T. P. Chiral Brønsted Acid-Controlled Intermolecular Asymmetric [2 + 2] Photocycloadditions. Nat. Commun. 2021, 12, 5735,  DOI: 10.1038/s41467-021-25878-9
      52
      Chiral Bronsted acid-controlled intermolecular asymmetric [2 + 2] photocycloadditions
      Sherbrook, Evan M.; Genzink, Matthew J.; Park, Bohyun; Guzei, Ilia A.; Baik, Mu-Hyun; Yoon, Tehshik P.
      Nature Communications (2021), 12 (1), 5735CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Abstr.: Control over the stereochem. of excited-state photoreactions remains a significant challenge in org. synthesis. Recently, it has become recognized that the photophys. properties of simple org. substrates can be altered upon coordination to Lewis acid catalysts, and that these changes can be exploited in the design of highly enantioselective catalytic photoreactions. Chromophore activation strategies, wherein simple org. substrates are activated towards photoexcitation upon binding to a Lewis acid catalyst, rank among the most successful asym. photoreactions. Herein, we show that chiral Bronsted acids can also catalyze asym. excited-state photoreactions by chromophore activation. This principle is demonstrated in the context of a highly enantio- and diastereoselective [2+2] photocycloaddn. catalyzed by a chiral phosphoramide organocatalyst. Notably, the cyclobutane products arising from this method feature a trans-cis stereochem. that is complementary to other enantioselective catalytic [2+2] photocycloaddns. reported to date.
    53. 53

      In both solvents, an obvious color change from red to violet is rapidly observed, attesting to the premature decomposition of the photosensitizer under the reaction conditions; see: (a)

      Jones, R. F.; Cole-Hamilton, D. J. The Substitutional Photochemistry of Tris(bipyridyl)-Ruthenium(II)Chloride. Inorg. Chim. Acta 1981, 53, L3– L5,  DOI: 10.1016/S0020-1693(00)84722-2
      There is no corresponding record for this reference.

      and reference (54)

    54. 54
      Ward, W. M.; Farnum, B. H.; Siegler, M.; Meyer, G. J. Chloride Ion-Pairing with Ru(II) Polypyridyl Compounds in Dichloromethane. J. Phys. Chem. A 2013, 117, 8883– 8894,  DOI: 10.1021/jp404838z
      54
      Chloride ion-pairing with ruthenium(II) polypyridyl compounds in dichloromethane
      Ward, William M.; Farnum, Byron H.; Siegler, Maxime; Meyer, Gerald J.
      Journal of Physical Chemistry A (2013), 117 (36), 8883-8894CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      Chloride ion-pairing with a series of four dicationic Ru-(II) polypyridyl compds. of the general form [Ru-(bpy)3-x(deeb)x]-(PF6)2, where bpy is 2,2'-bipyridine and deeb is 4,4'-diethylester-2,2'-bipyridine, was obsd. in dichloromethane soln. The heteroleptic compds. [Ru-(bpy)2(deeb)]2+ and [Ru-(bpy)-(deeb)2]2+ were found to be far less sensitive to ligand loss photochem. than were the homoleptic compds. [Ru-(bpy)3]2+ and [Ru-(deeb)3]2+ and were thus quantified in most detail. X-ray crystal structure and 1H NMR anal. showed that, when present, the C-3/C-3' position of bpy was the preferred site for adduct formation with chloride. Ion-pairing was manifest in UV-visible absorption spectral changes obsd. during titrns. with TBACl, where TBA is tetra-Bu ammonium. A modified Benesi-Hildebrand anal. yielded equil. consts. for ion-pairing that ranged from 13 700 to 64 000 M-1 and increased with the no. of deeb ligands present. A Job plot indicated a 2:1 chloride-to-ruthenium complex ratio in the ion-paired state. The chloride ion was found to decrease both the excited state lifetime and the quantum yield for photoluminescence. Nonlinear Stern-Volmer plots were obsd. that plateaued at high chloride concns. The radiative rate consts. decreased and the nonradiative rate consts. increased with chloride concn. in a manner consistent with theory for radiative rate consts. and the energy gap law. Equil. consts. for excited state ion-pairing abstracted from such data were found to be significantly larger than that measured for the ground state. Photophys. studies of hydroxide and bromide ion-pairing with [Ru-(bpy)2(deeb)]2+ are also reported.
    55. 55

      The ion-pairs have little impact on the properties of the ground state of D3-symmetry, see

      Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer, T. J. Photochemistry of Ru(bpy)32+. J. Am. Chem. Soc. 1982, 104, 4803– 4801,  DOI: 10.1021/ja00382a012
      55
      Photochemistry of tris(2,2'-bipyridine)ruthenium(2+) ion
      Durham, Bill; Caspar, Jonathan V.; Nagle, Jeffrey K.; Meyer, Thomas J.
      Journal of the American Chemical Society (1982), 104 (18), 4803-10CODEN: JACSAT; ISSN:0002-7863.
      Temp.-dependent lifetime data in CH2Cl are reported for the emitting charge-transfer excited state of (Ru(bpy)32+ (3CT) (bpy = 2,2'-bipyridine) under photochem. (NCS- salt) and nonphotochem. (PF6- salt) conditions. Temp.-dependent lifetime data were also obtained in CH2Cl2 for the salts [Ru(bpy)2(py)2](PF6)2 (py = pyridine) and [Ru(phen)3](PF6)2 (phen = 1,10-phenanthroline) together with temp.-dependent quantum-yield data for photochem. loss of bpy for the salt [Ru(bpy)3](NCS)2 (.vphi.(25°) = 0.068). The obtained data, combined with the data and suggestions made earlier by J. Van Houten and R. Watts (1975, 1978) based on their expts. in H2O suggest a detailed view of the microscopic events which lead to photosubstitution. Initial excitation leads to a charge-transfer state largely triplet in character, 3CT which undergoes thermal activation to give a d-d excited state. The d-d state undergoes further thermal activation by loss of a pyridyl group to give a 5-coordinate intermediate which is square pyramidal in structure. The fate of the intermediate is capture of a sixth ligand, either by a solvent or an anion held close to the activated metal center by ion-pairing or by chelate ring closure to return to Ru(bpy)32+.
    56. 56

      Absorbance and emission spectra of [Ru(bpy)3](Cl)2 could not be recorded in deoxygenated CH2Cl2 due to low solubility and premature decomposition, vide infra references (53) and (54).

      There is no corresponding record for this reference.
    57. 57
      Kober, E. M.; Sullivan, B. P.; Meyer, T. J. Solvent dependence of metal-to-ligand charge-transfer transitions. Evidence for initial electron localization in MLCT excited states of 2,2’-bipyridine complexes of ruthenium(II) and osmium(II). Inorg. Chem. 1984, 23, 2098– 2104,  DOI: 10.1021/ic00182a023
      57
      Solvent dependence of metal-to-ligand charge-transfer transitions. Evidence for initial electron localization in MLCT excited states of 2,2'-bipyridine complexes of ruthenium(II) and osmium(II)
      Kober, Edward M.; Sullivan, B. Patrick; Meyer, Thomas J.
      Inorganic Chemistry (1984), 23 (14), 2098-104CODEN: INOCAJ; ISSN:0020-1669.
      Metal-to-ligand charge-transfer (MLCT) absorption bands for the complexes Ru(bpy)32+, Os(bpy)32+, Os(bpy)2(py)22+, Os(bpy)2(MeCN)22+, and Os(bpy)2(1,2-(Ph2P)2C6H4)2+ (bpy = 2,2'-bipyridine) are solvent dependent. The dependence can be interpreted with use of the dielec. continuum theory but for the D3 ions Ru(bpy)32+ and Os(bpy)32+ only if in the excited state the excited electron is localized on a single ligand rather than delocalized over all 3.
    58. 58
      Maurer, A. B.; Piechota, E. J.; Meyer, G. J. Excited-State Dipole Moments of Homoleptic [Ru(bpy′)3]2+ Complexes Measured by Stark Spectroscopy. J. Phys. Chem. A 2019, 123, 8745– 8754,  DOI: 10.1021/acs.jpca.9b05874
      58
      Excited-State Dipole Moments of Homoleptic [Ru(bpy')3]2+ Complexes Measured by Stark Spectroscopy
      Maurer, Andrew B.; Piechota, Eric J.; Meyer, Gerald J.
      Journal of Physical Chemistry A (2019), 123 (41), 8745-8754CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      The visible absorption and Stark spectra of five [Ru(4,4'-R-2,2'-bipyridine)3](PF6)2 and [Ru(bipyrazine)3(PF6)2 complexes, where R = CH3O-, tert-butyl-, CH3-, H-, or CF3-, were obtained in butyronitrile glasses at 77K as a function of the applied field in the 0.2-0.8 MV/cm range. Anal. of the metal-to-ligand charge-transfer (MLCT) absorption and Stark spectra with the Liptay treatment revealed dramatic light-induced dipole moment changes, Δμ = 5-11 D. Application of a two-state model to the Δμ values provided values of the metal-ligand electronic coupling, HDA = 4400-6600 cm-1, reasonable for this class of complexes. The ground state of these complexes has no net dipole moment and with the RuII center as the point of ref., the dipole moment changes were reasonably assigned to the dipole present in the initially formed MLCT excited state. Further, the excited state dipole moment was sensitive to the presence of electron donating (MeO-, tert-butyl-, CH3-) or withdrawing (CF3-) substituents on the bipyridine ligands, and Δμ was correlated with the substituent Hammett parameters. Hence the data show for the first time that substituents on the bipyridine ligands, that are often introduced to tune formal redn. potentials, can also induce significant changes in the excited state dipole, behavior that should be taken into consideration for artificial photosynthesis applications.
    59. 59
      Alary, F.; Heully, J.-L.; Bijeire, L.; Vicendo, P. Is the 3MLCT the Only Photoreactive State of Polypyridyl Complexes?. Inorg. Chem. 2007, 46, 3154– 3165,  DOI: 10.1021/ic062193i
      59
      Is the 3MLCT the Only Photoreactive State of Polypyridyl Complexes?
      Alary, F.; Heully, J.-L.; Bijeire, L.; Vicendo, P.
      Inorganic Chemistry (2007), 46 (8), 3154-3165CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      By means of Δ-SCF and time-dependent d. functional theory (DFT) calcns. on [Ru(LL)3]2+ (LL = bpy = 2,2'-bipyridyl or bpz = 2,2' -bipyrazyl) complexes, we have found that emission of these two complexes could originate from two metal-to-ligand charge-transfer triplet states (3MLCT) that are quasi-degenerate and whose symmetries are D3 and C2. These two states are true min. Calcd. absorption and emission energies are in good agreement with expt.; the largest error is 0.14 eV, which is about the expected accuracy of the DFT calcns. For the first time, an optimized geometry for the metal-centered (MC) state is proposed for both of these complexes, and their energies are found to be almost degenerate with their corresponding 3MLCT states. These [RuII(LL)(η1-LL)2]2+ MC states have two vacant coordination sites on the metal, so they may react readily with their environment. If these MC states are able to de-excite by luminescence, the assocd. transition (ca. 1 eV) is found to be quite different from those of the 3MLCT states (ca. 2 eV).
    60. 60
      Vining, W. J.; Caspar, J. V.; Meyer, T. J. The Influence of Environmental Effects on Excited-State Lifetimes. The effect of Ion Pairing on Metal-to-Ligand Charge Transfer Excited States. J. Phys. Chem. 1985, 89, 1095– 1099,  DOI: 10.1021/j100253a010
      60
      The influence of environmental effects on excited-state lifetimes. The effect of ion pairing on metal-to-ligand charge transfer excited states
      Vining, William J.; Caspar, Jonathan V.; Meyer, Thomas J.
      Journal of Physical Chemistry (1985), 89 (7), 1095-9CODEN: JPCHAX; ISSN:0022-3654.
      Excited-state emission and lifetimes are reported for the complexes Os(phen)32+ and Os(4,4'-Ph2phen)32+ (phen = 1,10-phenanthroline; 4,4'-Ph2phen = 4,4'-diphenyl-1,10-phenanthroline) as a function of counterion in CH2Cl2 soln. The changes in nonradiative decay rate consts. vary with changes in emission energy max. as predicted by the energy gap law. The variations in emission energies and through them the decay rates appear to be induced by changes in ion-dipole interactions in the excited state.
    61. 61

      Triplet excited state energies (ET) were estimated from the emission maxima recorded at room temperature, see:

      Caspar, J. V.; Meyer, T. J. Photochemistry of Ru(bpy)32+. Solvent Effects. J. Am. Chem. Soc. 1983, 105, 5583– 5590,  DOI: 10.1021/ja00355a009
      61
      Photochemistry of tris(2,2'-bipyridine)ruthenium(2+) ion (Ru(bpy)32+). Solvent effects
      Caspar, Jonathan V.; Meyer, Thomas J.
      Journal of the American Chemical Society (1983), 105 (17), 5583-90CODEN: JACSAT; ISSN:0002-7863.
      The excited-state lifetime of the metal-to-ligand charge-transfer (MLCT) excited state or states of Ru(bpy)32+ (byp = 2,2'-bipyridine) was measured in a series of solvents at different temps. From a combination of lifetime and emission quantum yield measurements, values for kr and knr (kr and knr denote radiative and nonradiative rate const., resp., for decay of MLCT state(s)) were obtained in the series of solvents. From the variations of the various kinetic parameters with solvent the following conclusions are reached: (1) kr is only slightly solvent dependent; (2) the variations in knr and emission energy with solvent are in quant. agreement with the predictions of the energy gap law for radiationless transitions; and (3) the solvent dependence of the kinetic parameters which characterize the MLCT → dd transition can be considered in the context of electron-transfer theory. The implications of solvent effects on the use of Ru(bpy)32+* as a sensitizer are discussed.
    62. 62
      Buzzetti, L.; Crisenza, G. E. M.; Melchiorre, P. Mechanistic Studies in Photocatalysis. Angew. Chem., Int. Ed. 2019, 58, 3730– 3747,  DOI: 10.1002/anie.201809984
      62
      Mechanistic Studies in Photocatalysis
      Buzzetti, Luca; Crisenza, Giacomo E. M.; Melchiorre, Paolo
      Angewandte Chemie, International Edition (2019), 58 (12), 3730-3747CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. The fast-moving fields of photoredox and photocatalysis have recently provided fresh opportunities to expand the potential of synthetic org. chem. Advances in light-mediated processes have mainly been guided so far by empirical findings and the quest for reaction invention. The general perception, however, is that photocatalysis is entering a more mature phase where the combination of exptl. and mechanistic studies will play a dominant role in sustaining further innovation. This Review outlines the key mechanistic studies to consider when developing a photochem. process, and the best techniques available for acquiring relevant information. The discussion will use selected case studies to highlight how mechanistic studies can be instrumental in guiding the invention and development of synthetically useful photocatalytic transformations.
    63. 63
      Schmid, L.; Kerzig, C.; Prescimone, A.; Wenger, O. S. Photostable Ruthenium(II) Isocyanoborato Luminophores and Their Use in Energy Transfer and Photoredox Catalysis. JACS Au. 2021, 1, 819– 832,  DOI: 10.1021/jacsau.1c00137
      63
      Photostable Ruthenium(II) Isocyanoborato Luminophores and Their Use in Energy Transfer and Photoredox Catalysis
      Schmid, Lucius; Kerzig, Christoph; Prescimone, Alessandro; Wenger, Oliver S.
      JACS Au (2021), 1 (6), 819-832CODEN: JAAUCR; ISSN:2691-3704. (American Chemical Society)
      Ruthenium(II) polypyridine complexes are among the most popular sensitizers in photocatalysis, but they face some severe limitations concerning accessible excited-state energies and photostability that could hamper future applications. In this study, the borylation of heteroleptic ruthenium(II) cyanide complexes with α-diimine ancillary ligands is identified as a useful concept to elevate the energies of photoactive metal-to-ligand charge-transfer (MLCT) states and to obtain unusually photorobust compds. suitable for thermodynamically challenging energy transfer catalysis as well as oxidative and reductive photoredox catalysis. B(C6F5)3 groups attached to the CN- ligands stabilize the metal-based t2g-like orbitals by ~ 0.8 eV, leading to high 3MLCT energies (up to 2.50 eV) that are more typical for cyclometalated iridium(III) complexes. Through variation of their α-diimine ligands, nonradiative excited-state relaxation pathways involving higher-lying metal-centered states can be controlled, and their luminescence quantum yields and MLCT lifetimes can be optimized. These combined properties make the resp. isocyanoborato complexes amenable to photochem. reactions for which common ruthenium(II)-based sensitizers are unsuited, due to a lack of sufficient triplet energy or excited-state redox power. Specifically, this includes photoisomerization reactions, sensitization of nickel-catalyzed cross-couplings, pinacol couplings, and oxidative decarboxylative C-C couplings. Our work is relevant in the greater context of tailoring photoactive coordination compds. to current challenges in synthetic photochem. and solar energy conversion.
    64. 64
      Soupart, A.; Dixon, I. M.; Alary, F.; Heully, J. L. DFT Rationalization of the Room-Temperature Luminescence Properties of Ru(bpy)32+ and Ru(tpy)22+: 3MLCT–3MC Minimum Energy Path from NEB Calculations and Emission Spectra from VRES Calculations. Theor. Chem. Acc. 2018, 137, 37,  DOI: 10.1007/s00214-018-2216-1
      There is no corresponding record for this reference.
    65. 65
      Strauss, S. H. The Search for Larger and More Weakly Coordinating Anions. Chem. Rev. 1993, 93, 927– 942,  DOI: 10.1021/cr00019a005
      65
      The search for larger and more weakly coordinating anions
      Strauss, Steven H.
      Chemical Reviews (Washington, DC, United States) (1993), 93 (3), 927-42CODEN: CHREAY; ISSN:0009-2665.
      A review with >140 refs. including discussion of anions such as tetraphenylborate, carba-closo-dodecaborate, pentafluorooxotellurate, etc.
    66. 66

      For a recent study highlighting the critical role of cage escape in photoredox reactions, see:

      Wang, C.; Li, H.; Bürgin, T. H.; Wenger, O. Cage escape governs photoredox reaction rates and quantum yields. Nat. Chem. 2024, 16, 1151– 1159,  DOI: 10.1038/s41557-024-01482-4
      There is no corresponding record for this reference.
    67. 67
      Goodwin, M. J.; Dickenson, J. C.; Ripak, A.; Deetz, A. M.; McCarthy, J. S.; Meyer, G. J.; Troian-Gautier, L. Factors that Impact Photochemical Cage Escape Yields. Chem. Rev. 2024, 124, 7379– 7464,  DOI: 10.1021/acs.chemrev.3c00930
      There is no corresponding record for this reference.
    68. 68
      Soupart, A.; Alary, F.; Heully, J. L.; Elliott, P. I. P.; Dixon, I. M. Exploration of Uncharted 3PES Territory for [Ru(bpy)3]2+: A New 3MC Minimum Prone to Ligand Loss Photochemistry. Inorg. Chem. 2018, 57, 3192– 3196,  DOI: 10.1021/acs.inorgchem.7b03229
      68
      Exploration of Uncharted 3PES Territory for [Ru(bpy)3]2+: A New 3MC Minimum Prone to Ligand Loss Photochemistry
      Soupart, Adrien; Alary, Fabienne; Heully, Jean-Louis; Elliott, Paul I. P.; Dixon, Isabelle M.
      Inorganic Chemistry (2018), 57 (6), 3192-3196CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      We have identified a new 3MC state bearing two elongated Ru-N bonds to the same ligand in [Ru(bpy)3]2+. This DFT-optimized structure is a local min. on the 3PES. This distal MC state (3MCcis) is destabilized by less than 2 kcal/mol with respect to the classical MC state (3MCtrans), and energy barriers to populate 3MCcis and 3MCtrans from the 3MLCT state are similar according to nudged elastic band min. energy path calcns. Distortions in the classical 3MCtrans, i.e., elongation of two Ru-N bonds toward two different bpy ligands, are not expected to favor the formation of ligand-loss photoproducts. On the contrary, the new 3MCcis could be particularly relevant in the photodegrdn. of Ru(II) polypyridine complexes.
    69. 69
      Soupart, A.; Alary, F.; Heully, J.-L.; Dixon, I. M. On the Possible Coordination on a 3MC State Itself? Mechanistic Investigation Using DFT-Based Methods. Inorganics 2020, 8, 15,  DOI: 10.3390/inorganics8020015
      69
      On the possible coordination on a 3MC state itself? mechanistic investigation using DFT-based methods
      Soupart, Adrien; Alary, Fabienne; Heully, Jean-louis; Dixon, Isabelle M.
      Inorganics (2020), 8 (2), 15CODEN: INORCW; ISSN:2304-6740. (MDPI AG)
      Understanding light-induced ligand exchange processes is key to the design of efficient light-releasing prodrugs or photochem. driven functional mols. Previous mechanistic investigations had highlighted the pivotal role of metal-centered (MC) excited states in the initial ligand loss step. The question remains whether they are equally important in the subsequent ligand capture step. This article reports the mechanistic study of direct acetonitrile coordination onto a 3MC state of [Ru(bpy)3]2+, leading to [Ru(bpy)2(κ1-bpy)(NCMe)]2+ in a 3MLCT (metal-to-ligand charge transfer) state. Coordination of MeCN is indeed accompanied by the decoordination of one pyridine ring of a bpy ligand. As estd. from Nudged Elastic Band calcns., the energy barrier along the min. energy path is 20 kcal/mol. Interestingly, the orbital anal. conducted along the reaction path has shown that creation of the metallic vacancy can be achieved by reverting the energetic ordering of key dσ* and bpy-based π* orbitals, resulting in the change of electronic configuration from 3MC to 3MLCT. The approach of the NCMe lone pair contributes to destabilizing the dσ* orbital by electrostatic repulsion.
    70. 70
      Soupart, A.; Alary, F.; Heully, J.-L.; Elliott, P. I. P.; Dixon, I. M. Theoretical Study of the Full Photosolvolysis Mechanism of [Ru(bpy)3]2+: Providing a General Mechanistic Roadmap for the Photochemistry of [Ru(N^N)3]2+-Type Complexes toward Both Cis and Trans Photoproducts. Inorg. Chem. 2020, 59, 14679– 14695,  DOI: 10.1021/acs.inorgchem.0c01843
      70
      Theoretical Study of the Full Photosolvolysis Mechanism of [Ru(bpy)3]2+: Providing a General Mechanistic Roadmap for the Photochemistry of [Ru(N̂ N)3]2+-Type Complexes toward Both Cis and Trans Photoproducts
      Soupart, Adrien; Alary, Fabienne; Heully, Jean-Louis; Elliott, Paul I. P.; Dixon, Isabelle M.
      Inorganic Chemistry (2020), 59 (20), 14679-14695CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      A complete mechanistic picture for the photochem. release of bipyridine (bpy) from the archetypal complex [Ru(bpy)3]2+ is presented for the first time following the description of the ground and lowest triplet potential energy surfaces, as well as their key crossing points, involved in successive elementary steps along pathways toward cis- and trans-[Ru(bpy)2(NCMe)2]2+. This work accounts for two main pathways that are identified involving (a) two successive photochem. reactions for photodechelation, followed by the photorelease of a monodentate bpy ligand, and (b) a novel one-photon mechanism in which the initial photoexcitation is followed by dechelation, solvent coordination, and bpy release processes, all of which occur sequentially within the triplet excited-state manifold before the final relaxation to the singlet state and formation of the final photoproducts. For the reaction between photoexcited [Ru(bpy)3]2+ and acetonitrile, which is taken as a model reaction, pathways toward cis and trans photoproducts are uphill processes, in line with the comparative inertness of the complex in this solvent. Factors involving the nature of the departing ligand and retained "spectator" ligands are considered, and their role in the selection of mechanistic pathways involving overall two sequential photon absorptions vs. one photon absorption for the formation of both cis or trans photoproducts is discussed in relation to notable examples from the literature. This study ultimately provides a generalized roadmap of accessible photoproductive pathways for light-induced reactivity mechanisms of photolabile [Ru(N̂ N)(N̂ N')(N̂ N'')]2+-type complexes. A full roadmap is provided for the multistep mechanism of photoinduced ligand loss from tris(diimine)ruthenium(II) complexes and the formation of cis and trans bis(solvento) photoproducts. Two major pathways have been identified. One involves two sequential photonic excitations through the formation of an intermediate mono(solvento) product (red route). The other is a novel one-photon pathway (blue route) in which ligand dechelation, coordination of the solvent, and ligand loss all occur in the triplet excited state.
    71. 71
      Eastham, K.; Scattergood, P. A.; Chu, D.; Boota, R. Z.; Soupart, A.; Alary, F.; Dixon, I. M.; Rice, C. R.; Hardman, S. J. O.; Elliott, P. I. P. Not All 3MC States Are the Same: The Role of 3MCcis States in the Photochemical N∧N Ligand Release from [Ru(bpy)2(N∧N)]2+ Complexes. Inorg. Chem. 2022, 61, 19907– 19924,  DOI: 10.1021/acs.inorgchem.2c03146
      There is no corresponding record for this reference.

  • Supporting Information

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    • General information; photocatalyst synthesis; substrate synthesis; synthesis of racemate [2+2] cycloaddition products; electrochemical data; photophysical experiments; complementary studies; computational studies; X-ray diffraction; and NMR data (PDF)

    • Cartesian coordinates for all optimized complexes, ground and excited states (ZIP)


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