A new green-to-blue upconversion system with efficient photoredox catalytic properties

[EN] The design and development of new triplet-triplet annihilation upconversion (TTA-UC) systems combining triplet sensitizers with acceptor compounds have attracted considerable interest. In this vein, sensitizers made from purely organic dyes rather than transition-metal complexes appear to be more convenient from an environmental point of view. BODIPYs are a very well-known class of dyes with applications in a widespread range of scientific areas. Owing to the versatility of BODIPYs, we present herein a new asymmetric BODIPY with excellent photophysical properties to be used as an appropriate sensitizer in a bimolecular TTA-UC system. Detailed spectroscopic measurements demonstrated the ability of this new design to sensitize TTA-UC by combination with a suitable acceptor such as 2,5,8,11-tetra-tert-butylperylene (TBPe), allowing a successful conversion of green to blue light. The singlet-excited TBPe so obtained is capable of activating aryl chlorides reductively which initiated the functionalization of N-methylpyrrole (Meerwein-type arylation) and formation of both substituted triarylethylenes (Mizoroki-Heck reaction) and heteroarene phosphonates (photo-Arbuzov reaction). Product yields reveal that our TTA-UC system behaved as a highly efficient photocatalytic entity.We thank the Generalitat Valenciana (project CIDEGENT/2018/044), the Spanish Government (project PID2019-105391GB-C22 funded by MCIN/AEI/10.13039/501100011033), the German Federal Environmental Foundation (DBU, PhD fellowship to T. J. B. Z., grant number 20022/028) and the German Research Foundation (DFG, grant number KE 2313/3-1) for financial support. We also thank Prof. Julia Perez-Prieto for spectroscopic facilities.Pérez-Ruiz, R.; Castellanos-Soriano, J.; Johannes Gutenberg University Mainz; Herrera-Luna, JC.; Jiménez Molero, MC.; C. Kerzig (2023). A new green-to-blue upconversion system with efficient photoredox catalytic properties. Physical Chemistry Chemical Physics. 25(17):12041-12049. https://doi.org/10.1039/d3cp00811h1204112049251

Efficient Energy and Electron Transfer Photocatalysis with a Coulombic Dyad

Photocatalysis holds great promise for changing the way how value-added molecules are currently prepared. However, many photocatalytic reactions suffer from lousy quantum yields, hampering the transition from lab-scale reactions to large-scale or even industrial applications. Molecular dyads can be designed such that the beneficial properties of inorganic and organic chromophores are combined, resulting in milder reaction conditions and improved quantum yields of photocatalytic reactions. We have developed a novel approach for obtaining the advantages of molecular dyads without the time- and resource-consuming synthesis of these tailored photocatalysts. Simply by mixing a cationic ruthenium complex with an anionic pyrene derivative in water a salt bichromophore is produced owing to electrostatic interactions. The long-lived organic triplet state is obtained by static and quantitative energy transfer from the preorganized ruthenium complex. We exploited this so-called Coulombic dyad for energy transfer catalysis with similar reactivity and even higher photostability compared to a molecular dyad and reference photosensitizers in several photooxygenations. In addition, it was shown that this system can also be used to maximize the quantum yield of photoredox reactions. This is due to an intrinsically higher cage escape quantum yield after photoinduced electron transfer for purely organic compounds compared to heavy atom-containing molecules. The combination of laboratory-scale as well as mechanistic irradiation experiments with detailed spectroscopic investigations provided deep mechanistic insights into this easy-to-use photocatalyst class

Multi‐Photon Excitation in Photoredox Catalysis: Concepts, Applications, Methods

The energy of visible photons and the accessible redox potentials of common photocatalysts set thermodynamic limits to photochemical reactions that can be driven by traditional visible‐light irradiation. UV excitation can be damaging and induce side reactions, hence visible or even near‐IR light is usually preferable. Thus, photochemistry currently faces two divergent challenges, namely the desire to perform ever more thermodynamically demanding reactions with increasingly lower photon energies. The pooling of two low‐energy photons can address both challenges simultaneously, and whilst multi‐photon spectroscopy is well established, synthetic photoredox chemistry has only recently started to exploit multi‐photon processes on the preparative scale. Herein, we have a critical look at currently developed reactions and mechanistic concepts, discuss pertinent experimental methods, and provide an outlook into possible future developments of this rapidly emerging area

Dicationic Acridinium/Carbene Hybrids as Strongly Oxidizing Photocatalysts

A new design concept for organic, strongly oxidizing photocatalysts is described based upon dicationic acridinium/carbene hybrids. A highly modular synthesis of such hybrids is presented and the dications are utilized as novel, tailor-made photoredox catalysts in the direct oxidative C−N coupling. Under optimized conditions, benzene and even electron-deficient arenes can be oxidized and coupled with a range of N-heterocycles in high to excellent yields with a single low-energy photon per catalytic turnover, while commonly used acridinium photocatalysts are not able to perform the challenging oxidation step. In contrast to traditional photocatalysts, the here reported hybrid photocatalysts feature a reversible two-electron redox system with regular or inverted redox potentials for the two-electron transfer. The different oxidation states could be isolated and structurally characterized supported by NMR, EPR and X-ray analysis. Mechanistic experiments employing time-resolved emission and transient absorption spectroscopy unambiguously reveal the outstanding excited-state oxidation potential of our best-performing catalyst (+2.5 V vs. SCE) and they provide evidence for mechanistic key steps and intermediates

Multiphotonen‐Anregung in der Photoredoxkatalyse: Konzepte, Anwendungen und Methoden

Die Energie sichtbarer Photonen und die zugänglichen Redoxpotentiale üblicher Photokatalysatoren schränken die Anzahl derjenigen Photoreaktionen ein, die durch sichtbares Licht angetrieben werden können. Da UV‐Anregung schädlich ist und häufig Nebenreaktionen auslöst, ist sichtbares Licht oder sogar NIR‐Strahlung vorzuziehen. Die Photochemie befasst sich momentan mit einer zweischneidigen Herausforderung, und zwar mit dem Wunsch, immer thermodynamisch anspruchsvollere Reaktionen mit immer kleineren Photonenenergien durchführen zu können. Das Akkumulieren der Energie zweier energiearmer Photonen kann dazu passende Lösungsansätze liefern. Obwohl die Multiphotonen‐Spektroskopie gut etabliert ist, haben Photoredox‐Synthesechemiker erst kürzlich begonnen, Multiphotonen‐Prozesse präparativ zu nutzen. Hier werfen wir einen kritischen Blick auf kürzlich entwickelte Reaktionen und mechanistische Konzepte, diskutieren einschlägige experimentelle Methoden und geben einen Ausblick auf mögliche zukünftige Entwicklungen dieses jungen Forschungsgebiets

A simple yet stable molybdenum(0) carbonyl complex for upconver-sion and photoredox catalysis

Photoactive complexes with earth-abundant metals have attracted increasing interest in the recent years fueled by the promise of sustainable photochemistry. However, sophisticated ligands with complicated syntheses are oftentimes required to enable photoactivity with non-precious metals. Here, we combine a cheap metal with simple ligands to easily access a photoactive complex. Specifically, we synthesize the molybdenum(0) carbonyl complex Mo(CO)3(tpe) featuring the tripodal ligand tris(pyridyl)ethane (tpe) in two steps with high overall yield. The complex shows intense deep-red phosphorescence with excited state lifetimes of several hundred nanoseconds. Time-resolved infrared spectroscopy and laser flash photolysis reveal a triplet metal-to-ligand charge-transfer (3MLCT) state as lowest excited state. Temperature-dependent luminescence complemented by density functional theory (DFT) calculations suggest thermal deactivation of the 3MLCT state via higher lying metal-centered states in analogy to the well-known photophysics of [Ru(bpy)3]2+. Importantly, we found that the title compound is very photostable due to the lack of labilized Mo–CO bonds (as caused by trans-coordinated CO) in the facial configuration of the ligands. Finally, we show the versatility of the molybdenum(0) complex in two applications: (1) green-to-blue photon upconversion via a triplet-triplet annihilation mechanism and (2) photoredox catalysis for a green-light driv-en dehalogenation reaction. Overall, our results establish tripodal carbonyl complexes as a promising design strategy to ac-cess stable photoactive complexes of non-precious metals avoiding tedious multi-step syntheses