Photostable Ruthenium(II) Isocyanoborato Luminophores and Their Use in Energy Transfer and Photoredox Catalysis

Abstract

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 compounds suitable for thermodynamically challenging energy transfer catalysis as well as oxidative and reductive photoredox catalysis. B(C 6 F 5 ) 3 groups attached to the CN – ligands stabilize the metal-based t 2g -like orbitals by ∼0.8 eV, leading to high 3 MLCT 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 respective isocyanoborato complexes amenable to photochemical 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 compounds to current challenges in synthetic photochemistry and solar energy conversion.

Luminescent First-Row Transition Metal Complexes

“…3,22,23,65,77 There is no single recipe that is uniformly applicable to address this issue in different metals and oxidation states, because each d n valence electron configuration has its own characteristics. Fairly general guidelines include however the design of rigid coordination environments to limit vibrational degrees of freedom, 38,39,84,86,87 the enhancement of the ligand field strength to shift MC states to higher energies, 73,77,78,[120][121][122]167 the optimization of coordination geometries to minimize splitting of degenerate states into several sublevels and to maximize energy gaps between individual states, 130,131,138,162 the installment of an inversion center to influence radiative excited-state decay rates, 147 and the introduction of push-and/or pullcharacter to control the energies of charge-transfer states. 73,168 Perhaps less obvious but potentially interesting could be strategies based on optimizing metal−ligand bond covalence to minimize repulsion between spectroscopically relevant dorbitals 148 as well as the exploitation of triplet reservoir and/ or delocalization effects 47 in first-row transition metal-based compounds with covalently attached organic chromophores.…”

“…Decelerating nonradiative excited-state relaxation remains the most challenging part in the development of new emissive metal complexes, particularly in the first row of transition metals, where the ligand field is typically comparatively weak and low-lying MC states can represent effective deactivation channels (Figure b). ,,,, There is no single recipe that is uniformly applicable to address this issue in different metals and oxidation states, because each d n valence electron configuration has its own characteristics. Fairly general guidelines include however the design of rigid coordination environments to limit vibrational degrees of freedom, ,,,, the enhancement of the ligand field strength to shift MC states to higher energies, ,,,− , the optimization of coordination geometries to minimize splitting of degenerate states into several sublevels and to maximize energy gaps between individual states, ,,, the installment of an inversion center to influence radiative excited-state decay rates, and the introduction of push- and/or pull-character to control the energies of charge-transfer states. , Perhaps less obvious but potentially interesting could be strategies based on optimizing metal–ligand bond covalence to minimize repulsion between spectroscopically relevant d-orbitals as well as the exploitation of triplet reservoir and/or delocalization effects in first-row transition metal-based compounds with covalently attached organic chromophores. − Ligand deuteration, a well-known strategy in the field of lanthanide luminophores, is now successfully applied to first-row transition metal emitters …”

“…On the one hand, this includes comparatively abundant second- and third-row transition metals (for example Zr, , Mo, ,,, or W ,,,, ) or f-elements (for example Ce , ). On the other hand, molecular complexes of abundant main group elements are now attracting increasing attention as luminophores and provide additional insight into how nonradiative relaxation can be tamed. − At the same time, further developments of precious metal-based complexes (for example, Ru, ,, Rh, , Re, − Ir, − or Pt − ) have significantly advanced the field of inorganic photophysics and photochemistry. Regardless of whether complexes of d-, f-, or main group elements are considered, the interplay between synthetic, spectroscopic, and computational work seems crucial for the development of new designer luminophores.…”

High Triplet Energy Iridium(III) Isocyanoborato Complex for Photochemical Upconversion, Photoredox and Energy Transfer Catalysis

“…Though there have been several detailed reports on the photophysical and electrochemical properties of isocyanoborato complexes with different metals, 51−64 their photochemical properties, as well as their applications in TTET catalysis and photochemical upconversion, have remained underexplored so far. 65 In addition to unusually challenging applications in TTET catalysis, the new complex permits sensitized triplet−tripletannihilation upconversion (sTTA-UC) into the UV−B region. There have been numerous reports on visible/near-infrared to visible 66−74 as well as a few visible to UV 75−80 upconversion systems, but only a very small number of them can provide emission below 350 nm.…”

Visible-Light Mediated Metal-Free 6π-Photocyclization of N-Acrylamides: Thioxanthone Triplet Energy Transfer Enables the Synthesis of 3,4-Dihydroquinolin-2-ones

“…Applications of visible-light TET processes in synthetic organic chemistry include the activation of transition-metal catalysts, isomerization reactions, as well as cyclization processes  encompassing the large majority of these reports. , Belonging to this latter class, visible-light mediated TET photocyclizations onto arenes are extremely synthetically useful reactions as they facilitate a formal C­(sp 2 )–H/C­(sp 3 )–H hydroarylation process , and thus enable facile access to an array of biologically relevant polycyclic and heteroaromatic ring systems (Figure A). To date, all these reports rely either on the use of transition-metal catalysis (mediated photochemically or thermally) or on nonselective UV light.…”