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

Challenges and Future Perspectives in Photocatalysis: Conclusions from an Interdisciplinary Workshop

Photocatalysis is a versatile and rapidly developing field with applications spanning artificial photosynthesis, photo-biocatalysis, photoredox catalysis in solution or supramolecular structures, utilization of abundant metals and organocatalysts, sustainable synthesis, and plastic degradation. In this Perspective, we summarize conclusions from an interdisciplinary workshop of young principal investigators held at the Lorentz Center in Leiden in March 2023. We explore how diverse fields within photocatalysis can benefit from one another. We delve into the intricate interplay between these subdisciplines, by highlighting the unique challenges and opportunities presented by each field and how a multidisciplinary approach can drive innovation and lead to sustainable solutions for the future. Advanced collaboration and knowledge exchange across these domains can further enhance the potential of photocatalysis. Artificial photosynthesis has become a promising technology for solar fuel generation, for instance, via water splitting or CO2 reduction, while photocatalysis has revolutionized the way we think about assembling molecular building blocks. Merging such powerful disciplines may give rise to efficient and sustainable protocols across different technologies. While photocatalysis has matured and can be applied in industrial processes, a deeper understanding of complex mechanisms is of great importance to improve reaction quantum yields and to sustain continuous development. Photocatalysis is in the perfect position to play an important role in the synthesis, deconstruction, and reuse of molecules and materials impacting a sustainable future. To exploit the full potential of photocatalysis, a fundamental understanding of underlying processes within different subfields is necessary to close the cycle of use and reuse most efficiently. Following the initial interactions at the Lorentz Center Workshop in 2023, we aim to stimulate discussions and interdisciplinary approaches to tackle these challenges in diverse future teams.

Factors Controlling Cage Escape Yields of Closed- and Open-Shell Metal Complexes in Bimolecular Photoinduced Electron Transfer

The cage escape yield, i.e., the separation of the geminate radical pair formed immediately after bimolecular excited-state electron transfer, was studied in 11 solvents using six Fe(III), Ru(II), and Ir(III) photosensitizers and tri-p-tolylamine as the electron donor. Among all complexes, the largest cage escape yields (0.67-1) were recorded for the Ir(III) photosensitizer, showing the highest potential as a photocatalyst in photoredox catalysis. These yields dropped to values around 0.65 for both Ru(II) photosensitizers and to values around 0.38 for the Os(II) photosensitizer. Interestingly, for both open-shell Fe(III) complexes, the yields were small (<0.1) in solvents with dielectric constant greater than 20 but were shown to reach values up to 0.58 in solvents with low dielectric constants. The results presented herein on closed-shell photosensitizers suggest that the low rate of triplet-singlet intersystem crossing within the manifold of states of the geminate radical pair implies that charge recombination toward the ground state is a spin-forbidden process, favoring large cage escape yields that are not influenced by dielectric effects. Geminate charge recombination in open-shell metal complexes, such as the two Fe(III) photosensitizers studied herein, is no longer a spin-forbidden process and becomes highly sensitive to solvent effects. Altogether, this study provides general guidelines for factors influencing bimolecular excited-state reactivity using prototypical photosensitizers but also allows one to foresee a great development of Fe(III) photosensitizers with the 2LMCT excited state in photoredox catalysis, providing that solvents with low dielectric constants are used.

Management of Triplet States in Modified Mononuclear Ruthenium(II) Complexes for Enhanced Photocatalysis

Controlling the interplay between relaxation and charge/energy transfer processes in the excited states of photocatalysts is crucial for the performance of artificial photosynthesis. Metal-to-ligand charge-transfer triplet states (3MLCT*) of ruthenium(II) complexes are broadly implemented for photocatalysis, but an effective means of managing the triplets for enhanced photocatalysis has been lacking. Herein, We proposed a strategy to considerably prolong the triplet excited-state lifetime by decorating a ruthenium(II) phosphine complex (RuP-1) with pendent polyaromatic hydrocarbons (PAHs). Systematic studies demonstrate that in RuP-4 decorated with anthracene, sub-picosecond electron transfer from anthracene to 3MLCT* leads to a charge-separated state that can mediate the formation of the intra-ligand triplet state (3IL) of anthracene, resulting in an exceptionally long excited-state up to several milliseconds. This triplet management strategy enables impressive photocatalytic reduction of CO2 to CO with a turnover number (TON) of 404, an optimized quantum yield of 43 % and 100 % selectivity, which is the highest reported performance for mononuclear photocatalysts without additional photosensitizers. RuP-4 also catalyzes photochemical hydrogen generation under argon. This work opens up an avenue for regulating the excited-state charge/energy flow for the development of long-lived 3IL multi-functional mononuclear photocatalysts to boost artificial photosynthesis.

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.

Aminomethylations of electron-deficient compounds-bringing iron photoredox catalysis into play

The α-functionalisation of N-containing compounds is an area of broad interest in synthetic chemistry due to their presence in biologically active substances among others. Visible light-induced generation of nucleophilic α-aminoalkyl radicals as reactive intermediates that can be trapped by electron-deficient alkenes presents an attractive and mild approach to achieve said functionalisation. In this work, [Fe(iii)(phtmeimb)2]PF6 (phtmeimb = phenyl(tris(3-methylimidazol-2-ylidene))borate), an N-heterocyclic carbene (NHC) complex based on Earth-abundant iron, was used as photoredox catalyst to efficiently drive the formation of α-aminoalkyl radicals from a range of different α-trimethylsilylamines and their subsequent addition to a number of electron-deficient alkenes under green light irradiation. Mechanistic investigations elucidated the different reaction steps of the complete photocatalytic cycle. In terms of yields and substrate scope, we show that [Fe(iii)(phtmeimb)2]PF6 can compete with noble metal photoredox catalysts, for instance outcompeting archetypal [Ru(bpy)3]Cl2 under comparable reaction conditions, illustrating that iron photocatalysts can efficiently facilitate photoredox reactions of synthetic value.

Establishing the origin of Marcus-inverted-region behaviour in the excited-state dynamics of cobalt(III) polypyridyl complexes

Growing interest in the use of first-row transition metal complexes in a number of applied contexts-including but not limited to photoredox catalysis and solar energy conversion-underscores the need for a detailed understanding of their photophysical properties. A recent focus on ligand-field photocatalysis using cobalt(III) polypyridyls in particular has unlocked unprecedented excited-state reactivities. Photophysical studies on Co(III) chromophores in general are relatively uncommon, and so here we carry out a systematic study of a series of Co(III) polypyridyl complexes in order to delineate their excited-state dynamics. Compounds with varying ligand-field strengths were prepared and studied using variable-temperature ultrafast transient absorption spectroscopy. Analysis of the data establishes that the ground-state recovery dynamics are operating in the Marcus inverted region, in stark contrast to what is typically observed in other first-row metal complexes. The analysis has further revealed the underlying reasons driving this excited-state behaviour, thereby enabling potential advancements in the targeted use of the Marcus inverted region for a variety of photolytic applications.

Time-Resolved X-ray Emission Spectroscopy and Synthetic High-Spin Model Complexes Resolve Ambiguities in Excited-State Assignments of Transition-Metal Chromophores: A Case Study of Fe-Amido Complexes

To fully harness the potential of abundant metal coordination complex photosensitizers, a detailed understanding of the molecular properties that dictate and control the electronic excited-state population dynamics initiated by light absorption is critical. In the absence of detectable luminescence, optical transient absorption (TA) spectroscopy is the most widely employed method for interpreting electron redistribution in such excited states, particularly for those with a charge-transfer character. The assignment of excited-state TA spectral features often relies on spectroelectrochemical measurements, where the transient absorption spectrum generated by a metal-to-ligand charge-transfer (MLCT) electronic excited state, for instance, can be approximated using steady-state spectra generated by electrochemical ligand reduction and metal oxidation and accounting for the loss of absorptions by the electronic ground state. However, the reliability of this approach can be clouded when multiple electronic configurations have similar optical signatures. Using a case study of Fe(II) complexes supported by benzannulated diarylamido ligands, we highlight an example of such an ambiguity and show how time-resolved X-ray emission spectroscopy (XES) measurements can reliably assign excited states from the perspective of the metal, particularly in conjunction with accurate synthetic models of ligand-field electronic excited states, leading to a reinterpretation of the long-lived excited state as a ligand-field metal-centered quintet state. A detailed analysis of the XES data on the long-lived excited state is presented, along with a discussion of the ultrafast dynamics following the photoexcitation of low-spin Fe(II)-Namido complexes using a high-spin ground-state analogue as a spectral model for the 5T2 excited state.

Synthesis of Ru(II) and Os(II) photosensitizers bearing one 9,10-diamino-1,4,5,8-tetraazaphenanthrene scaffold

The synthesis of eight Ru(II) and Os(II) photosensitizers bearing a common 9,10-disubstituted-1,4,5,8-tetraazaphenanthrene backbone is reported. With Os(II) photosensitizers, the 9,10-diNH2-1,4,5,8-tetraazaphenanthrene could be directly chelated onto the metal center via the heteroaromatic moiety, whereas similar conditions using Ru(II) resulted in the formation of an o-quinonediimine derivative. Hence, an alternative route, proceeding via the chelation of 9-NH2-10-NO2-1,4,5,8-tetraazaphenanthrene and subsequent ligand reduction of the corresponding photosensitizers was developed. Photosensitizers chelated via the polypyridyl-type moiety exhibited classical photophysical properties whereas the o-quinonediimine chelated Ru(II) analogues exhibited red-shifted absorption (520 nm) and no photoluminescence at room temperature in acetonitrile. The most promising photosensitizers were investigated for excited-state quenching with guanosine-5'-monophosphate in aqueous buffered conditions where reductive excited-state electron transfer was observed by nanosecond transient absorption spectroscopy.

Ferromagnetically Coupled Chromium(III) Dimer Shows Luminescence and Sensitizes Photon Upconversion

There has been much progress on mononuclear chromium(III) complexes featuring luminescence and photoredox activity, but dinuclear chromium(III) complexes have remained underexplored in these contexts until now. We identified a tridentate chelate ligand able to accommodate both meridional and facial coordination of chromium(III), to either access a mono- or a dinuclear chromium(III) complex depending on reaction conditions. This chelate ligand causes tetragonally distorted primary coordination spheres around chromium(III) in both complexes, entailing comparatively short excited-state lifetimes in the range of 400 to 800 ns in solution at room temperature and making photoluminescence essentially oxygen insensitive. The two chromium(III) ions in the dimer experience ferromagnetic exchange interactions that result in a high spin (S=3) ground state with a coupling constant of +9.3 cm-1. Photoinduced energy transfer from the luminescent ferromagnetically coupled dimer to an anthracene derivative results in sensitized triplet-triplet annihilation upconversion. Based on these proof-of-principle studies, dinuclear chromium(III) complexes seem attractive for the development of fundamentally new types of photophysics and photochemistry enabled by magnetic exchange interactions.

Electrochemiluminescence of a First-Row d6 Transition Metal Complex

We report the electrochemiluminescence (ECL) of a 3d6 Cr(0) complex ([Cr(LMes)3]; λem=735 nm) with comparable photophysical properties to those of ECL-active complexes of 4d6 or 5d6 precious metal ions. The electrochemical potentials of [Cr(LMes)3] are more negative than those of [Ir(ppy)3] and render the [Cr(LMes)3]* excited state inaccessible through conventional co-reactant ECL with tri-n-propylamine or oxalate. ECL can be obtained, however, through the annihilation route in which potentials sufficient to oxidise and reduce the luminophore are alternately applied. When combined with [Ir(ppy)3] (λem=520 nm), the annihilation ECL of [Cr(LMes)3] was greatly enhanced whereas that of [Ir(ppy)3] was diminished. Under appropriate conditions, the relative intensities of the two spectrally distinct emissions can be controlled through the applied potentials. From this starting point for ECL with 3d6 metal complexes, we discuss some directions for future development.

Red-Light-Promoted Radical Cascade Reaction to Access Tetralins and Dialins Enabled by Zinc(II)porphyrin, A Light-Flexible Catalyst

[5,15-Bis(pentafluorophenyl)-10,20-diphenylporphinato]zinc(II) (1), a metalloporphyrin derivative that was recently reported as an efficient photocatalyst driven by blue LEDs by our group, was found to catalyze a red-light-promoted (630 nm LEDs) radical cascade reaction of N-3-arylpropionyloxyphthalimides with radicophiles including electron-deficient alkenes and alkynes, providing access to a range of functionalized tetralin and dialin derivatives. The radical cascade reaction catalyzed by 1 took place via an oxidative quenching cycle in DMSO, where no sacrificial electron donor was required, uncovering a unique solvent effect capable of promoting the porphyrin catalysis.

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.

Iron(III) Carbene Complexes with Tunable Excited State Energies for Photoredox and Upconversion

Substituting precious elements in luminophores and photocatalysts by abundant first-row transition metals remains a significant challenge, and iron continues to be particularly attractive owing to its high natural abundance and low cost. Most iron complexes known to date face severe limitations due to undesirably efficient deactivation of luminescent and photoredox-active excited states. Two new iron(III) complexes with structurally simple chelate ligands enable straightforward tuning of ground and excited state properties, contrasting recent examples, in which chemical modification had a minor impact. Crude samples feature two luminescence bands strongly reminiscent of a recent iron(III) complex, in which this observation was attributed to dual luminescence, but in our case, there is clear-cut evidence that the higher-energy luminescence stems from an impurity and only the red photoluminescence from a doublet ligand-to-metal charge transfer (2LMCT) excited state is genuine. Photoinduced oxidative and reductive electron transfer reactions with methyl viologen and 10-methylphenothiazine occur with nearly diffusion-limited kinetics. Photocatalytic reactions not previously reported for this compound class, in particular the C-H arylation of diazonium salts and the aerobic hydroxylation of boronic acids, were achieved with low-energy red light excitation. Doublet-triplet energy transfer (DTET) from the luminescent 2LMCT state to an anthracene annihilator permits the proof of principle for triplet-triplet annihilation upconversion based on a molecular iron photosensitizer. These findings are relevant for the development of iron complexes featuring photophysical and photochemical properties competitive with noble-metal-based compounds.

Stimulus-Responsive Copper Complex Nanoparticles Induce Cuproptosis for Augmented Cancer Immunotherapy

Cuproptosis, an emerging form of programmed cell death, has received tremendous attention in cancer therapy. However, the efficacy of cuproptosis remains limited by the poor delivery efficiency of copper ion carriers. Herein, copper complex nanoparticles (denoted as Cu(I) NP) are developed that can efficiently deliver copper complex into cancer cells to induce cuproptosis. Cu(I) NP demonstrate stimulus-responsive release of copper complexes, which results in mitochondrial dysfunction and promotes the aggregation of lipoylated dihydrolipoamide S-acetyltransferase (DLAT), leading to cuproptosis. Notably, Cu(I) NP not only induce cuproptosis, but also elicit robust immune responses to suppress tumor growth. Overall, this study provides a promising strategy for cuproptosis-based cancer therapy.

Luminescent and Photoredox-Active Molybdenum(0) Complexes Competitive with Isoelectronic Ruthenium(II) Polypyridines

Ruthenium(II) complexes with chelating polypyridine ligands are among the most frequently investigated compounds in photophysics and photochemistry, owing to their favorable luminescence and photoredox properties. Equally good photoluminescence performance and attractive photocatalytic behavior is now achievable with isoelectronic molybdenum(0) complexes. The zero-valent oxidation state of molybdenum is stabilized by carbonyl or isocyanide ligands, and metal-to-ligand charge transfer (MLCT) excited states analogous to those in ruthenium(II) complexes can be established. Microsecond MLCT excited-state lifetimes and photoluminescence quantum yields up to 0.2 have been achieved in solution at room temperature, and the emission wavelength has become tunable over a large range. The molybdenum(0) complexes are stronger photoreductants than ruthenium(II) polypyridines and can therefore perform more challenging chemical reductions. The triplet nature of their luminescent MLCT states allows sensitization of photon upconversion via triplet-triplet annihilation, to convert low-energy input radiation into higher-energy output fluorescence. This review summarizes the current state of the art concerning luminescent molybdenum(0) complexes and highlights their application potential. Molybdenum is roughly 140 times more abundant and far cheaper than ruthenium, hence this research is relevant in the greater context of finding more sustainable alternatives to using precious and rare transition metals in photophysics and photochemistry.

Electrocatalytic Asymmetric Nozaki-Hiyama-Kishi Decarboxylative Coupling: Scope, Applications, and Mechanism

The first general enantioselective alkyl-Nozaki-Hiyama-Kishi (NHK) coupling reactions are disclosed herein by employing a Cr-electrocatalytic decarboxylative approach. Using easily accessible aliphatic carboxylic acids (via redox-active esters) as alkyl nucleophile synthons, in combination with aldehydes and enabling additives, chiral secondary alcohols are produced in a good yield with high enantioselectivity under mild reductive electrolysis. This reaction, which cannot be mimicked using stoichiometric metal or organic reductants, tolerates a broad range of functional groups and is successfully applied to dramatically simplify the synthesis of multiple medicinally relevant structures and natural products. Mechanistic studies revealed that this asymmetric alkyl e-NHK reaction was enabled by using catalytic tetrakis(dimethylamino)ethylene, which acts as a key reductive mediator to mediate the electroreduction of the CrIII/chiral ligand complex.

Controlling the Photophysical Properties of a Series of Isostructural d6 Complexes Based on Cr0, MnI, and FeII

Development of first-row transition metal complexes with similar luminescence and photoredox properties as widely used RuII polypyridines is attractive because metals from the first transition series are comparatively abundant and inexpensive. The weaker ligand field experienced by the valence d-electrons of first-row transition metals challenges the installation of the same types of metal-to-ligand charge transfer (MLCT) excited states as in precious metal complexes, due to rapid population of energetically lower-lying metal-centered (MC) states. In a family of isostructural tris(diisocyanide) complexes of the 3d6 metals Cr0, MnI, and FeII, the increasing effective nuclear charge and ligand field strength allow us to control the energetic order between the 3MLCT and 3MC states, whereas pyrene decoration of the isocyanide ligand framework provides control over intraligand (ILPyr) states. The chromium(0) complex shows red 3MLCT phosphorescence because all other excited states are higher in energy. In the manganese(I) complex, a microsecond-lived dark 3ILPyr state, reminiscent of the types of electronic states encountered in many polyaromatic hydrocarbon compounds, is the lowest and becomes photoactive. In the iron(II) complex, the lowest MLCT state has shifted to so much higher energy that 1ILPyr fluorescence occurs, in parallel to other excited-state deactivation pathways. Our combined synthetic-spectroscopic-theoretical study provides unprecedented insights into how effective nuclear charge, ligand field strength, and ligand π-conjugation affect the energetic order between MLCT and ligand-based excited states, and under what circumstances these individual states become luminescent and exploitable in photochemistry. Such insights are the key to further developments of luminescent and photoredox-active first-row transition metal complexes.

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.

Structural Control of Highly Efficient Thermally Activated Delayed Fluorescence in Carbene Zinc(II) Dithiolates

Luminescent metal complexes based on earth abundant elements are a valuable target to substitute 4d/5d transition metal complexes as triplet emitters in advanced photonic applications. Whereas CuI complexes have been thoroughly investigated in the last two decades for this purpose, no structure-property-relationships for efficient luminescence involving triplet excited states from ZnII complexes are established. Herein, we report on the design of monomeric carbene zinc(II) dithiolates (CZT) featuring a donor-acceptor-motif that leads to highly efficient thermally activated delayed fluorescence (TADF) with for ZnII compounds unprecedented radiative rate constants kTADF =1.2×106  s-1 at 297 K. Our high-level DFT/MRCI calculations revealed that the relative orientation of the ligands involved in the ligand-to-ligand charge transfer (1/3 LLCT) states is paramount to control the TADF process. Specifically, a dihedral angle of 36-40° leads to very efficient reverse intersystem-crossing (rISC) on the order of 109  s-1 due to spin-orbit coupling (SOC) mediated by the sulfur atoms in combination with a small ΔES1-T1 of ca. 56 meV.

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.

First-Row d6 Metal Complex Enables Photon Upconversion and Initiates Blue Light-Dependent Polymerization with Red Light

Photosensitizers for sensitized triplet-triplet annihilation upconversion (sTTA-UC) often rely on precious heavy metals, whereas coordination complexes based on abundant first-row transition metals are less common. This is mainly because long-lived triplet excited states are more difficult to obtain for 3d metals, particularly when the d-subshell is only partially filled. Here, we report the first example of sTTA-UC based on a 3d6 metal photosensitizer yielding an upconversion performance competitive with precious metal-based analogues. Using a newly developed Cr0 photosensitizer featuring equally good photophysical properties as an OsII benchmark complex in combination with an acetylene-decorated anthracene annihilator, red-to-blue upconversion is achievable. The upconversion efficiency under optimized conditions is 1.8 %, and the excitation power density threshold to reach the strong annihilation limit is 5.9 W/cm2 . These performance factors, along with high photostability, permit the initiation of acrylamide polymerization by red light, based on radiative energy transfer between delayed annihilator fluorescence and a blue light absorbing photo-initiator. Our study provides the proof-of-concept for photon upconversion with elusive first-row analogues of widely employed precious d6 metal photosensitizers, and for their application in photochemical reactions triggered by excitation wavelengths close to near-infrared.