A team of Swiss scientists has developed the first chromium complex that emits light in the NIR-II (1000 to 1700 nm) range. The scientists achieved an NIR-II luminescence of 1067 nm by strengthening the metal-ligand bond covalence in the complex. The approach weakened the mutual repulsion between d-electrons in the emissive excited state, lowering the energy enough to cause NIR-II luminescence to occur in the new trivalent chromium (CrIII) complex. In many complexes, both electrons in a covalent bond come from the ligand. Some of these metal-ligand bonds have a partially covalent character, which reduces the energy of certain excited states, causing the emitted radiation to have a longer wavelength. This phenomenon has been observed in polypyridine ligands, which cause the ruby-red emission of CrIII in complexes to shift into the NIR-I range. Researchers in Switzerland report the first chromium complex to emit in the NIR-II range. The development supports in vivo imaging, among other applications. Courtesy of Wiley. To increase the covalence of the metal-ligand bond and thus further increase the wavelength, University of Basel researcher Narayan Sinha enveloped the chromium in a different type of ligand. Sinha switched from classic polypyridine ligands to a newly tailored, charged, tridentate chelate ligand. The new ligand has three binding sites that allow it to grip the central metal ion like a pincer. In the new complex, the CrIII ion is surrounded on all sides by two tridentate, charged, chelate ligands to form an octahedral shape. This results in a radically altered, unusual electronic structure with a high electron density. In the axial direction, charge transfer takes place from the ligands to the metal, but in the equatorial plane of the octahedron, charge transfer moves from the metal to the ligands. The team believes that the combined “push” and “pull” of the ligand-to-metal and metal-to-ligand interactions strongly influences the spectroscopically relevant electrons of the CrIII complex — and that this could be key to the complex’s ability to emit light in the NIR-II wavelength. The NIR-II wavelength is especially useful for in vivo imaging because it can penetrate deep into tissue. Many materials that emit NIR light are based on expensive or rare metal complexes. Although scientists have developed less expensive alternatives for emitting light in the NIR-I range, NIR-II-emitting complexes made of nonprecious metals remain extremely rare. Previous ligand design strategies have focused on optimizing the ligand field strength, the researchers said. The current work increases metal-ligand bond covalence to shift the ruby-like 2E emission of CrIII to 1067 nm at 77 K. The team’s design principle for luminescent CrIII complexes demonstrates that increasing the metal-ligand bond covalence through combined π-donor and π-acceptor interactions allows emission color tuning and can lead to NIR-II luminescence. The research was published in Angewandte Chemie (www.doi.org/10.1002/anie.202106398).