A Tris(diisocyanide)chromium(0) Complex Is a Luminescent Analog of Fe(2,2′-Bipyridine)32+

A meta-terphenyl unit was substituted with an isocyanide group on each of its two terminal aryls to afford a bidentate chelating ligand ((CNAr3NC)-Ar-tBu) that is able to stabilize chromium in its zerovalent oxidation state. The homoleptic Cr((CaAr3NC)-Ar-tBu)(3) complex luminesces in solution at room temperature, and its excited-state lifetime (2.2 ns in deaerated THF at 20 degrees 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((CNAr3NC)-Ar-tBu)(3) 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.