Photoredox Catalysis Mediated by Tungsten(0) Arylisocyanides

Abundant Transition Metal Based Photocatalysts for Red Light-Driven Photocatalysis

Photocatalysis emerges as an efficient and versatile tool for the preparation of organic compounds via the development of new methodologies and new photosensitizers. Mostly UV and blue light irradiation are used for such reactions. Red light is low-energy light, it is less harmful and has more penetration depth. Hence red light-driven photocatalysis would be more suitable for preparing value-added products. Red-absorbing photosensitizers are mostly based on rare and expensive metals. In this review, we describe the recent developments on Earth-abundant transition metal-based photosensitizers (W(0), Mo(0), Cr(0), Fe(III), Cu(I), Zn(II)) and their applications in red light-driven photocatalysis. Photocatalysis using both electron transfer and energy transfer processes is discussed. Three different red light-induced reactions such as direct monophotonic excitation, sensitized triplet-triplet annihilation upconversion (sTTA-UC), and dual red light photocatalysis are presented. Various organic transformations such as reductive dehalogenation and detosylation, reduction of diazonium salts, C─C coupling via C─H activation, oxidation of aryl boronic acids to phenols, polymerization reactions, cross dehydrogenative couplings, α-cyanation of tertiary amines, Barton decarboxylation have been carried out using abundant photosensitizers and red light.

Base-Promoted Homolytic Aromatic Substitution (BHAS) Reactions and Hydrodehalogenations Driven by Green Light and an Iron(III)-NHC Photoredox Catalyst

An Fe(III)-NHC complex has been employed for the green light driven catalysis of base-promoted homolytic aromatic substitution (BHAS) reactions. Tributylamine was used as a sacrificial electron donor, together with potassium carbonate as base in dimethyl sulfoxide as solvent. In contrast to previously studied photocatalysts, the excited Fe(III)-NHC complex is not reducing the arylhalide substrates. Instead, the latter are activated by α-aminoalkyl radicals formed upon reductive quenching of the photocatalyst by tributylamine. Avoiding strongly reducing photocatalysts as well as strong base, these mild reaction conditions allowed for the expansion of the substrate scope to accommodate also aldehyde and ester substituents. 100 % conversion was obtained after 48 h of irradiation. In this way a wide variety of cyclized products and their corresponding hydrodehalogenated products were obtained as isolated and pure compounds, in the vast majority of cases.

A Blessing and a Curse: Remote Ligand Functionalization Modulates 3MLCT Relaxation in Group 6 Tricarbonyl Complexes

We recently reported a molecular design for carbonylpyridine molybdenum(0) complexes that unlocks long-lived luminescent and photoactive charge-transfer states. Here, we translate this strategy to chromium(0), and tungsten(0) and report three fully characterized tricarbonyl metal(0) complexes featuring a tripodal ligand with a remote n-butyl substituent in the backbone. All complexes show phosphorescence in the red to near-infrared spectral region from metal-to-ligand charge-transfer excited states. Surprisingly, the alkyl chain significantly affects excited state relaxation: lifetimes are shortened in solution but extended in the solid state by one order of magnitude compared to the molybdenum(0) complex with a methyl substituent. Temperature-dependent luminescence and NMR spectroscopy in combination with quantum chemical calculations reveal the reasons for these disparate effects. The n-butyl substituent distorts the metal coordination geometry. The resulting structural flexibility flattens the potential energy surfaces in solution, which lowers the barrier for the population of distorted metal-centered states and facilitates nonradiative relaxation. In the solid state, the rigidified alkyl chain separates neighboring molecules, which reduces self-quenching. Our study sheds light on the relationship between structure and excited state relaxation to inform the development of photoactive complexes based on earth-abundant metals.

Red light excitation: illuminating photocatalysis in a new spectrum

Red-light-activated photocatalysis has become a powerful approach for achieving sustainable chemical transformations, combining high efficiency with energy-saving, mild conditions. By harnessing the deeper penetration and selectivity of red and near-infrared light, this method minimizes the side reactions typical of higher-energy sources, making it particularly suited for large-scale applications. Recent advances highlight the unique advantages of both metal-based and metal-free catalysts under red-light irradiation, broadening the range of possible reactions, from selective oxidations to complex polymerizations. In biological contexts, red-light photocatalysis enables innovative applications in phototherapy and controlled drug release, exploiting its tissue penetration and low cytotoxicity. Together, these developments underscore the versatility and impact of red-light photocatalysis, positioning it as a cornerstone of green organic chemistry with significant potential in synthetic and biomedical fields.

Efficient photoredox catalysis in C-C cross-coupling reactions by two-coordinated Au(I) complex

Photocatalysis provides a versatile approach to redox activation of various organic substrates for synthetic applications. To broaden the scope of photoredox catalysis, developing catalysts with strong oxidizing or reducing power in the excited state is imperative. Catalysts that feature highly cathodic oxidation potentials and long lifetimes in their excited states are particularly in demand. In this research, we demonstrate the catalytic utility of two-coordinate Au(I) complex photocatalysts that exhibit an exclusive ligand-to-ligand charge-transfer (LLCT) transition in C-C cross-coupling reactions between N-heterocycles and (hetero)aryl halides, including redox-resistant (hetero)aryl chlorides. Our photocatalysis system can steer reactions under visible-light irradiation at a catalyst loading as low as 0.1 mol% and exhibits a broad substrate scope with high chemo- and regioselectivity. Our mechanistic investigations provide direct spectroscopic evidence for each step in the catalysis cycle and demonstrate that the LLCT-active Au(I) complex catalysts offer several benefits, including strong visible-light absorption, a 210 ns-long excited-state lifetime without short-lived components, and a 91% yield in the production of free-radical intermediates. Given the wide structural versatility of the proposed catalysts, we envision that our research will provide useful insights into the future utilization of the LLCT-active Au(I) complex for organic transformations.

Low-valent-tungsten catalysis enables hydroboration of esters and nitriles

In the research presented herein, low-valent-tungsten-catalyzed hydroboration of esters and nitriles was investigated. Aromatic and aliphatic substrates were smoothly reduced to corresponding alcohol derivatives and N,N-diborylamines in the presence of W(CO)4(NCMe)2. Valuable derivatives were conveniently accessed by introducing a further functionalization process to crude hydroboration mixtures in one pot.

Reversible Photoinduced Ligand Substitution in a Luminescent Chromium(0) Complex

Light-triggered dissociation of ligands forms the basis for many compounds of interest for photoactivated chemotherapy (PACT), in which medicinally active substances are released or "uncaged" from metal complexes upon illumination. Photoinduced ligand dissociation is usually irreversible, and many recent studies performed in the context of PACT focused on ruthenium(II) polypyridines and related heavy metal complexes. Herein, we report a first-row transition metal complex, in which photoinduced dissociation and spontaneous recoordination of a ligand unit occurs. Two scorpionate-type tridentate chelates provide an overall six-coordinate arylisocyanide environment for chromium(0). Photoexcitation causes decoordination of one of these six ligating units and coordination of a solvent molecule, at least in tetrahydrofuran and 1,4-dioxane solvents, but far less in toluene, and below detection limit in cyclohexane. Transient UV-vis absorption spectroscopy and quantum chemical simulations point to photoinduced ligand dissociation directly from an excited metal-to-ligand charge-transfer state. Owing to the tridentate chelate design and the substitution lability of the first-row transition metal, recoordination of the photodissociated arylisocyanide ligand unit can occur spontaneously on a millisecond time scale. This work provides insight into possible self-healing mechanisms counteracting unwanted photodegradation processes and seems furthermore relevant in the contexts of photoswitching and (photo)chemical information storage.

Phosphorescent fac-Bis(triarylisocyanide) W(0) and Mo(0) Complexes

Homoleptic W(0) and Mo(0) complexes containing bis(triarylisocyanide) ligands with bulky substituents were synthesized and spectroscopically characterized. Crystallographically determined structures revealed that these complexes are hourglass-like in shape with the tridentate ligands adopting a facial coordination mode to the metal center. These complexes luminesce in fluid solutions and in the solid state. Typically in toluene at 298 K, the two W(0) complexes display the emission maximum (lifetime and quantum yield) at 591 nm (0.83 μs and 0.35) and 628 nm (1.04 μs and 0.39), and similarly, the two Mo(0) complexes display it at 575 nm (0.54 μs and 0.15) and 617 nm (0.56 μs and 0.23). DFT and TDDFT calculations indicated that the low-energy absorption bands of the W(0) and Mo(0) complexes could be metal-to-ligand charge transfer (MLCT) transitions in nature. These complexes exhibited a reversible M+/0 redox couple at -0.70 and -0.63 V vs Fc+/0 for the W(0) complexes and -0.86 and -0.67 V for the Mo(0) complexes. The excited-state reduction potentials were hence estimated to be -2.91 and -2.74 V vs Fc+/0 for the W(0) complexes and -3.10 and -2.81 V vs Fc+/0 for the Mo(0) complexes, indicating that they are potentially strong photoreductants.

Near-Infrared Light-Driven Photocatalysis with an Emphasis on Two-Photon Excitation: Concepts, Materials, and Applications

Efficient utilization of sunlight in photocatalysis is widely recognized as a promising solution for addressing the growing energy demand and environmental issues resulting from fossil fuel consumption. Recently, there have been significant developments in various near-infrared (NIR) light-harvesting systems for artificial photosynthesis and photocatalytic environmental remediation. This review provides an overview of the most recent advancements in the utilization of NIR light through the creation of novel nanostructured materials and molecular photosensitizers, as well as modulating strategies to enhance the photocatalytic processes. A special focus is given to the emerging two-photon excitation NIR photocatalysis. The unique features and limitations of different systems are critically evaluated. In particular, it highlights the advantages of utilizing NIR light and two-photon excitation compared to UV-visible irradiation and one-photon excitation. Ongoing challenges and potential solutions for the future exploration of NIR light-responsive materials are also discussed.

Improved transition metal photosensitizers to drive advances in photocatalysis

To function effectively in a photocatalytic application, a photosensitizer's light absorption, excited-state lifetime, and redox potentials, both in the ground state and excited state, are critically important. The absorption profile is particularly relevant to applications involving solar harvesting, whereas the redox potentials and excited-state lifetimes determine the thermodynamics, kinetics, and quantum yields of photoinduced redox processes. This perspective article focuses on synthetic inorganic and organometallic approaches to optimize these three characteristics of transition-metal based photosensitizers. We include our own work in these areas, which has focused extensively on exceptionally strong cyclometalated iridium photoreductants that enable challenging reductive photoredox transformations on organic substrates, and more recent work which has led to improved solar harvesting in charge-transfer copper(i) chromophores, an emerging class of earth-abundant compounds particularly relevant to solar-energy applications. We also extensively highlight many other complementary strategies for optimizing these parameters and highlight representative examples from the recent literature. It remains a significant challenge to simultaneously optimize all three of these parameters at once, since improvements in one often come at the detriment of the others. These inherent trade-offs and approaches to obviate or circumvent them are discussed throughout.

Photoredox-active Cr(0) luminophores featuring photophysical properties competitive with Ru(II) and Os(II) complexes

Coordination complexes of precious metals with the d6 valence electron configuration such as Ru(II), Os(II) and Ir(III) are used for lighting applications, solar energy conversion and photocatalysis. Until now, d6 complexes made from abundant first-row transition metals with competitive photophysical and photochemical properties have been elusive. While previous research efforts focused mostly on Fe(II), we disclose that isoelectronic Cr(0) gives access to higher photoluminescence quantum yields and excited-state lifetimes when compared with any other first-row d6 metal complex reported so far. The luminescence behaviour of the metal-to-ligand charge transfer excited states of these Cr(0) complexes is competitive with Os(II) polypyridines. With these Cr(0) complexes, the metal-to-ligand charge transfer states of first-row d6 metal complexes become exploitable in photoredox catalysis, and benchmark chemical reductions proceed efficiently under low-energy red illumination. Here we demonstrate that appropriate molecular design strategies open up new perspectives for photophysics and photochemistry with abundant first-row d6 metals.

Electronic Structures and Photoredox Chemistry of Tungsten(0) Arylisocyanides

ConspectusThe high energy barriers associated with the reaction chemistry of inert substrates can be overcome by employing redox-active photocatalysts. Research in this area has grown exponentially over the past decade, as transition metal photosensitizers have been shown to mediate challenging organic transformations. Critical for the advancement of photoredox catalysis is the discovery, development, and study of complexes based on earth-abundant metals that can replace and/or complement established noble-metal-based photosensitizers.Recent work has focused on redox-active complexes of 3d metals, as photosensitizers containing these metals most likely would be scalable. Although low lying spin doublet ("spin flip") excited states of chromium(III) and metal-to-ligand charge transfer (MLCT) excited states of copper(I) have relatively long lifetimes, the electronic excited states of many other 3d metal complexes fall on dissociative potential energy surfaces, owing to the population of highly energetic σ-antibonding orbitals. Indeed, we and other investigators have shown that low lying spin singlet and triplet excited states of robust closed-shell metal complexes are too short-lived at room temperature to engage in bimolecular reactions in solutions. In principle, this problem could be overcome by designing and constructing 3d metal complexes containing strong field π-acceptor ligands, where thermally equilibrated MLCT or intraligand charge transfer excited states might fall well below the upper surfaces of dissociative 3d-3d states. Notably, such design elements have been exploited by investigators in very recent work on redox-active iron(II) systems. Another approach, one we have actively pursued, is to design and construct closed-shell complexes of earth-abundant 5d metals containing very strong π-acceptor ligands, where vertical excitation of 5d-5d excited states at the ground state geometry would require energies far above minima in the potential surfaces of MLCT excited states. As this requirement is met by tungsten(0) arylisocyanides, these complexes have been the focus of our work aimed at the development of robust redox-active photosensitizers.In the following Account, we review recent work on homoleptic tungsten(0) arylisocyanides. Originally reported by our group 45 years ago, W(CNAr)₆ complexes have exceptionally large one- and two-photon absorption cross-sections. One- or two-photon excitation produces relatively long-lived (hundreds of nanoseconds to microsecond) MLCT excited states in high yields. These MLCT excited states, which are very strong reductants with E°(W⁺/*W⁰) = -2.2 to -3.0 V vs Fc^[+/0], mediate photocatalysis of organic reactions with both visible and near-infrared (NIR) light. Here, we highlight design principles that led to the development of three generations of W(CNAr)₆ photosensitizers; and we discuss likely steps in the mechanism of a prototypal W(CNAr)₆-catalyzed base-promoted homolytic aromatic substitution reaction. Among the many potential applications of these very bright luminophores, two-photon imaging and two-photon-initiated polymerization are ones we plan to pursue.

Metal-Organic Bichromophore Lowers the Upconversion Excitation Power Threshold and Promotes UV Photoreactions

Sensitized triplet-triplet annihilation upconversion is a promising strategy to use visible light for chemical reactions requiring the energy input of UV photons. This strategy avoids unsafe ultraviolet light sources and can mitigate photo-damage and provide access to reactions, for which filter effects hamper direct UV excitation. Here, we report a new approach to make blue-to-UV upconversion more amenable to photochemical applications. The tethering of a naphthalene unit to a cyclometalated iridium(III) complex yields a bichromophore with a high triplet energy (2.68 eV) and a naphthalene-based triplet reservoir featuring a lifetime of 72.1 μs, roughly a factor of 20 longer than the photoactive excited state of the parent iridium(III) complex. In combination with three different annihilators, consistently lower thresholds for the blue-to-UV upconversion to crossover from a quadratic into a linear excitation power dependence regime were observed with the bichromophore compared to the parent iridium(III) complex. The upconversion system composed of the bichromophore and the 2,5-diphenyloxazole annihilator is sufficiently robust under long-term blue irradiation to continuously provide a high-energy singlet-excited state that can drive chemical reactions normally requiring UV light. Both photoredox and energy transfer catalyses were feasible using this concept, including the reductive N-O bond cleavage of Weinreb amides, a C-C coupling reaction based on reductive aryl debromination, and two Paternò-Büchi [2 + 2] cycloaddition reactions. Our work seems relevant in the context of developing new strategies for driving energetically demanding photochemistry with low-energy input light.

Applications of red light photoredox catalysis in organic synthesis

Photoredox catalysis has emerged as an efficient and versatile approach for developing novel synthetic methodologies. Particularly, red light photocatalysis has attracted more attention due to its intrinsic advantages of low energy, few health risks, few side reactions, and high penetration depth through various media. Impressive progress has been made in this field. In this review, we outline the applications of different photoredox catalysts in a wide range of red light-mediated reactions including direct red light photoredox catalysis, red light photoredox catalysis through upconversion, and dual red light photoredox catalysis. Due to the similarities between near-infrared (NIR) and red light, an overview of NIR-induced reactions is also presented. Lastly, current evidence showing the advantages of red light and NIR photoredox catalysis is also described.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

Many coordination complexes and organometallic compounds with the 4d⁶ and 5d⁶ valence electron configurations have outstanding photophysical and photochemical properties, which stem from metal-to-ligand charge transfer (MLCT) excited states. This substance class makes extensive use of the most precious and least abundant metal elements, and consequently there has been a long-standing interest in first-row transition metal compounds with photoactive MLCT states. Semiprecious copper(I) with its completely filled 3d subshell is a relatively straightforward and well explored case, but in 3d⁶ complexes the partially filled d-orbitals lead to energetically low-lying metal-centered (MC) states that can cause undesirably fast MLCT excited state deactivation. Herein, we discuss recent advances made with isoelectronic Cr⁰, Mn^I, Fe^II, and Co^III compounds, for which long-lived MLCT states have become accessible over the past five years. Furthermore, we discuss possible future developments in the search for new first-row transition metal complexes with partially filled 3d subshells and photoactive MLCT states for next-generation applications in photophysics and photochemistry.

Photosensitized Activation of Diazonium Derivatives for C-B Bond Formation

Aryl diazonium salts are ubiquitous building blocks in chemistry, as they are useful radical precursors in organic synthesis as well as for the functionalization of solid materials. They can be reduced electrochemically or through a photo-induced electron transfer reaction. Here we provide a detailed picture of the ground and excited-state reactivity of a series of 9 rare and earth abundant photosensitizers with 13 aryl diazonium salts, which also included 3 macrocyclic calix[4]arene tetradiazonium salts. Nanosecond transient absorption spectroscopy confirmed the occurrence of excited-state electron transfer and was used to quantify cage-escape yields, i.e. the efficiency with which the formed radicals separate and escape the solvent cage. Cage-escape yields were large; increased when the driving force for photo-induced electron transfer increased and also tracked with the C-N2 + bond cleavage propensity, amongst others. A photo-induced borylation reaction was then investigated with all the photosensitizers and proceeded with yields between 9 and 74%.

Properties of Amine-Containing Ligands That Are Necessary for Visible-Light-Promoted Catalysis with Divalent Europium

We describe a study of the influence of amine-containing ligands on the photoredox-relevant properties of Eu^II toward the rational design of Eu^II-containing catalysts for visible-light-promoted photoredox reactions. We report our observations of the effects of the degree of functionalization of amines, denticity, and macrocylic ligands on the absorbance of Eu^II. Ligands that contain secondary amines bathochromically shift the absorbance of EuCl₂ relative to ligands that contain primary or tertiary amines. Similarly, ligands of larger denticity have a larger bathochromic shift of the absorbance than ligands of smaller denticity. We observed that macrocyclic ligands have a larger effect on the absorbance of EuCl₂ than nonmacrocyclic ligands. Also, we report the photoredox reactivity of four new Eu^II-containing complexes. These observations are potentially influential in understanding the ligand properties that promote the use of Eu^II in visible-light-promoted photoredox catalysis.

Light-Driven Hydrodefluorination of Electron-Rich Aryl Fluorides by an Anionic Rhodium-Gallium Photoredox Catalyst

An anionic Rh-Ga complex catalyzed the hydrodefluorination of challenging C-F bonds in electron-rich aryl fluorides and trifluoromethylarenes when irradiated with violet light in the presence of H₂ , a stoichiometric alkoxide base, and a crown-ether additive. Based on theoretical calculations, the lowest unoccupied molecular orbital (LUMO), which is delocalized across both the Rh and Ga atoms, becomes singly occupied upon excitation, thereby poising the Rh-Ga complex for photoinduced single-electron transfer (SET). Stoichiometric and control reactions support that the C-F activation is mediated by the excited anionic Rh-Ga complex. After SET, the proposed neutral Rh⁰ intermediate was detected by EPR spectroscopy, which matched the spectrum of an independently synthesized sample. Deuterium-labeling studies corroborate the generation of aryl radicals during catalysis and their subsequent hydrogen-atom abstraction from the THF solvent to generate the hydrodefluorinated arene products. Altogether, the combined experimental and theoretical data support an unconventional bimetallic excitation that achieves the activation of strong C-F bonds and uses H₂ and base as the terminal reductant.

The forgotten reagent of photoredox catalysis

Visible light powers an ever-expanding suite of reactions to both make and break chemical bonds under otherwise mild conditions. As a reagent in photochemical synthesis, light is obviously critical for reactivity but rarely optimized other than in light/dark controls. This Frontier Article presents an overview of recent research that investigates the unique ways light may be manipulated, and its unusual interactions with homogeneous transition metal and organic photocatalysts.

Manganese(I) Complex with Monodentate Arylisocyanide Ligands Shows Photodissociation Instead of Luminescence

Recently reported manganese(I) complexes with chelating arylisocyanide ligands exhibit luminescent metal-to-ligand charge-transfer (MLCT) excited states, similar to ruthenium(II) polypyridine complexes with the same d6 valence electron configuration used for many different applications in photophysics and photochemistry. However, chelating arylisocyanide ligands require substantial synthetic effort, and therefore it seemed attractive to explore the possibility of using more readily accessible monodentate arylisocyanides instead. Here, we synthesized the new Mn(I) complex [Mn(CNdippPhOMe2)6]PF6 with the known ligand CNdippPhOMe2 = 4-(3,5-dimethoxyphenyl)-2,6-diisopropylphenylisocyanide. This complex was investigated by NMR spectroscopy, single-crystal structure analysis, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) measurements, IR spectroscopy supported by density functional theory (DFT) calculations, cyclic voltammetry, and time-resolved as well as steady-state UV-vis absorption spectroscopy. The key finding is that the new Mn(I) complex is nonluminescent and instead undergoes arylisocyanide ligand loss during continuous visible laser irradiation into ligand-centered and charge-transfer absorption bands, presumably owed to the population of dissociative d-d excited states. Thus, it seems that chelating bi- or tridentate binding motifs are essential for obtaining emissive MLCT excited states in manganese(I) arylisocyanides. Our work contributes to understanding the basic properties of photoactive first-row transition metal complexes and could help advance the search for alternatives to precious metal-based luminophores, photocatalysts, and sensors.

Red Light-Based Dual Photoredox Strategy Resembling the Z-Scheme of Natural Photosynthesis

Photoredox catalysis typically relies on the use of single chromophores, whereas strategies, in which two different light absorbers are combined, are rare. In photosystems I and II of green plants, the two separate chromophores P₆₈₀ and P₇₀₀ both absorb light independently of one another, and then their excitation energy is combined in the so-called Z-scheme, to drive an overall reaction that is thermodynamically very demanding. Here, we adapt this concept to perform photoredox reactions on organic substrates with the combined energy input of two red photons instead of blue or UV light. Specifically, a Cu^I bis(α-diimine) complex in combination with in situ formed 9,10-dicyanoanthracenyl radical anion in the presence of excess diisopropylethylamine catalyzes ca. 50 dehalogenation and detosylation reactions. This dual photoredox approach seems useful because red light is less damaging and has a greater penetration depth than blue or UV radiation. UV-vis transient absorption spectroscopy reveals that the subtle change in solvent from acetonitrile to acetone induces a changeover in the reaction mechanism, involving either a dominant photoinduced electron transfer or a dominant triplet-triplet energy transfer pathway. Our study illustrates the mechanistic complexity in systems operating under multiphotonic excitation conditions, and it provides insights into how the competition between desirable and unwanted reaction steps can become more controllable.

Photoredox Catalysis Mediated by Tungsten(0) Arylisocyanides in 1,2-Difluorobenzene

We have studied the photochemical cyclization of 1-(2-iodobenzyl)-pyrrole (IBP) and 1-(2-bromobenzyl)-pyrrole (BBP) to 5H-pyrrolo[2,1-a]isoindol catalyzed by W(CNDipp)₆ (CNDipp = 2,6-diisopropylphenylisocyanide) in 1,2-difluorobenzene (DFB). Irradiation (445 nm) of W(CNDipp)₆ (5 mol %) in DFB solution converted 78% of IBP (50 mM) to product after 1 h (16 turnovers). Addition of tetra-n-butyl ammonium hexafluorophosphate (TBAPF₆) (0.2 M) to the DFB solution led to rapid photoinduced disappearance of W(CNDipp)₆ but, remarkably, did not inhibit photochemical cyclization of IBP, indicating that IBP cyclization could be driven by a nonluminescent photocatalyst.

Two-photon-absorbing ruthenium complexes enable near infrared light-driven photocatalysis

One-photon-absorbing photosensitizers are commonly used in homogeneous photocatalysis which require the absorption of ultraviolet (UV) /visible light to populate the desired excited states with adequate energy and lifetime. Nevertheless, the limited penetration depth and competing absorption by organic substrates of UV/visible light calls upon exploring the utilization of longer-wavelength irradiation, such as near-infrared light (λ_irr > 700 nm). Despite being found applications in photodynamic therapy and bioimaging, two-photon absorption (TPA), the simultaneous absorption of two photons by one molecule, has been rarely explored in homogeneous photocatalysis. Herein, we report a group of ruthenium polypyridyl complexes possessing TPA capability that can drive a variety of organic transformations upon irradiation with 740 nm light. We demonstrate that these TPA ruthenium complexes can operate in an analogous manner as one-photon-absorbing photosensitizers for both energy-transfer and photoredox reactions, as well as function in concert with a transition metal co-catalyst for metallaphotoredox C-C coupling reactions.

Tuning the Electrochemical and Photophysical Properties of Osmium-Based Photoredox Catalysts

The use of low-energy deep-red (DR) and near-infrared (NIR) light to excite chromophores enables catalysis to ensue across barriers such as materials and tissues. Herein, we report the detailed photophysical characterization of a library of OsII polypyridyl photosensitizers that absorb low-energy light. By tuning ligand scaffold and electron density, we access a range of synthetically useful excited state energies and redox potentials.

1 Introduction

1.1 Scope

1.2 Measuring Ground-State Redox Potentials

1.3 Measuring Photophysical Properties

1.4 Synthesis of Osmium Complexes

2 Properties of Osmium Complexes

2.1 Redox Potentials of Os(L)2-Type Complexes

2.2 Redox Potentials of Os(L)3-Type Complexes

2.3 UV/Vis Absorption and Emission Spectroscopy

3 Conclusions

Luminescent chromium(0) and manganese(I) complexes

In this Frontier article, recently discovered chromium(0) and manganese(i) complexes emitting from metal-to-ligand charge transfer (MLCT) excited states are highlighted. Chelating isocyanide ligands give access to this new class of 3d⁶ emitters with MLCT lifetimes in (or close to) the nanosecond regime in solution at room temperature. Although the so far achievable luminescence quantum yields in these open-shell complexes are yet comparatively low, the photophysical properties of the new chromium(0) and manganese(i) isocyanides are reminiscent of those of well-known ruthenium(ii) polypyridines. Our findings provide insight into how undesired nonradiative MLCT deactivation in 3d⁶ complexes can be counteracted, and they seem therefore relevant for the further development of new luminescent first-row transition metal complexes based on iron(ii) and cobalt(iii) in addition to chromium(0) and manganese(i).

In this Frontier article, recently discovered chromium(0) and manganese(i) complexes emitting from metal-to-ligand charge transfer (MLCT) excited states are highlighted.