Chromium complex has a long luminescent glow

Ruthenium bipyridine and related precious-metal complexes are popular because they exhibit long-lived, redox-active excited states that are useful for making solar cells and light-emitting devices and as sensitizers for photocatalytic organic reactions. Researchers would like to replace the precious metals with more abundant first-row transition metals, such as iron, but they have been waylaid so far because the excited-state luminescence lifetimes are too short. A key to modulating the properties of the complexes to extend the lifetime is designing ligands that better stabilize the metal ions. To that end, Laura A. Büldt, Oliver S. Wenger, and their coworkers at the University of Basel have designed a bulky chelating diisocyanide terphenyl ligand that dramatically improves chromium(0)’s excited-state lifetime, surpassing that of the best iron(II) complexes by two orders of magnitude (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.6b11803). The researchers say the electronic effects of their ligand and the cage structure it forms around the chromium(0) ion provides a room-temperature excited-state lifetime of 2.2 nanoseconds, which exceeds the 37 picoseconds of the previous iron(II) record holder. In addition, they found that the chromium complex is a stronger photoreductant than ruthenium tris(bipyridine). Wenger’s group has tested the chromium complex and an analogous molybdenum complex in photoredox catalysis and is exploring grafting the chromium complex onto semiconductor surfaces to make dye-sensitized solar cells.