A meta-terphenyl unit was substituted with an isocyanide group on each of its two terminal aryls to afford a bidentate chelating ligand (CNArNC) that is able to stabilize chromium in its zerovalent oxidation state. The homoleptic Cr(CNArNC) complex luminesces in solution at room temperature, and its excited-state lifetime (2.2 ns in deaerated THF at 20 °C) is nearly 2 orders of magnitude longer than the current record lifetime for isoelectronic Fe(II) complexes, which are of significant interest as earth-abundant sensitizers in dye-sensitized solar cells. Due to its chelating ligands, Cr(CNArNC) is more robust than Cr(0) complexes with carbonyl or monodentate isocyanides, manifesting in comparatively slow photodegradation. In the presence of excess anthracene in solution, efficient energy transfer and subsequent triplet-triplet annihilation upconversion is observed. With an excited-state oxidation potential of -2.43 V vs Fc/Fc, the Cr(0) complex is a very strong photoreductant. The findings presented herein are relevant for replacement of precious metals in dye-sensitized solar cells and in luminescent devices by earth-abundant elements.

We report the first homoleptic Mo(0) complex with bidentate isocyanide ligands, which exhibits metal-to-ligand charge transfer ((3) MLCT) luminescence with quantum yields and lifetimes similar to Ru(bpy)3 (2+) (bpy=2,2'-bipyridine). This Mo(0) complex is a very strong photoreductant, which manifests in its capability to reduce acetophenone with essentially diffusion-limited kinetics as shown by time-resolved laser spectroscopy. The application potential of this complex for photoredox catalysis was demonstrated by the rearrangement of an acyl cyclopropane to a 2,3-dihydrofuran, which is a reaction that requires a reduction potential so negative that even the well-known and strongly reducing Ir(2-phenylpyridine)3 photosensitizer cannot catalyze it. Our study thus provides the proof-of-concept for the use of chelating isocyanides to obtain Mo(0) complexes with long-lived (3) MLCT excited states that are applicable to unusually challenging photoredox chemistry.

Newly discovered tris(diisocyanide)molybdenum(0) complexes are Earth-abundant isoelectronic analogs of the well-known class of [Ru(-diimine) 3 ] 2+ compounds with long-lived 3 MLCT (metal-to-ligand charge transfer) excited states that lead to rich photophysics and photochemistry. Depending on ligand design, luminescence quantum yields up to 0.20 and microsecond excited state lifetimes are achieved in solution at room temperature, both significantly better than for [Ru(2,2'-bipyridine) 3 ] 2+. The excited Mo(0) complexes can induce chemical reactions that are thermodynamically too demanding for common precious metal based photosensitizers, including the widely employed fac-[Ir(2-phenylpyridine) 3 ] complex, as demonstrated on a series of light-driven aryl-aryl coupling reactions. The most robust Mo(0) complex exhibits stable photoluminescence and remains photoactive after continuous irradiation exceeding two months. Our comprehensive optical spectroscopic and photochemical study shows that Mo(0) complexes with diisocyanide chelate ligands constitute a new family of luminophores and photosensitizers, which is complementary to precious metal based 4d 6 and 5d 6 complexes and represents an alternative to non-emissive Fe(II) compounds. This is relevant in the greater context of sustainable photophysics and photochemistry, as well as for possible applications in lighting, sensing, and catalysis.

Luminescence from Earth-abundant metal ions in solution at room temperature is a very challenging objective due to the intrinsically weak ligand field splitting of first row transition metal ions, which leads to efficient non-radiative deactivation via metal-centered states. Only a handful of 3d n metal complexes (n ≠ 10) show sizeable luminescence at room temperature. Luminescence in the near-infrared spectral region is even more difficult to achieve as further non-radiative pathways come into play. No Earth-abundant first-row transition metal complexes display emission > 1000 nm at room temperature in solution up to now. Here we report the vanadium(III) complex mer-[V(ddpd) 2 ][PF 6 ] 3 yielding phosphorescence around 1100 nm in valeronitrile glass at 77 K as well as at room temperature in acetonitrile with 1.810 -4 % quantum yield (ddpd = N,N '-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine). In addition, mer-[V(ddpd) 2 ][PF 6 ] 3 shows very strong blue fluorescence with 2 % quantum yield in acetonitrile at room temperature. Our comprehensive study demonstrates that vanadium(III) complexes with d 2 electron configuration constitute a new class of blue and NIR-II luminophores, which complement the classical established complexes of expensive precious metals and rare-earth elements.

Arylisocyanide complexes based on earth-abundant Group 6 d metals are interesting alternatives to photoactive complexes made from precious metals such as Ru , Re , Os , or Ir . Some of these complexes have long-lived MLCT excited states that exhibit luminescence with good quantum yields as well as nano- to microsecond lifetimes, and they are very strongly reducing. Recent studies have demonstrated that Cr , Mo , and W arylisocyanide complexes have great potential for applications in luminescent devices, photoredox catalysis, and dye-sensitized solar cells.