Eight-Electron Superatomic Cu₃₁ Nanocluster with Chiral Kernel and NIR-II Emission

Abstract

Owing to the inherent instability caused by the low Cu(I)/Cu(0) half-cell reduction potential, Cu(0)-containing copper nanoclusters are quite uncommon in comparison to their Ag and Au congeners. Here, a novel eight-electron superatomic copper nanocluster [Cu 31 (4-MeO-PhC≡C) 21 (dppe) 3 ](ClO 4 ) 2 ( Cu 31, dppe = 1,2-bis(diphenylphosphino)ethane) is presented with total structural characterization. The structural determination reveals that Cu 31 features an inherent chiral metal core arising from the helical arrangement of two sets of three Cu 2 units encircling the icosahedral Cu 13 core, which is further shielded by 4-MeO-PhC≡C – and dppe ligands. Cu 31 is the first copper nanocluster carrying eight free electrons, which is further corroborated by electrospray ionization mass spectrometry, X-ray photoelectron spectroscopy and density functional theory calculations. Interestingly, Cu 31 demonstrates the first near-infrared (750–950 nm, NIR-I) window absorption and the second near-infrared (1000–1700 nm, NIR-II) window emission, which is exceptional in the copper nanocluster family and endows it with great potential in biological applications. Of note, the 4-methoxy groups providing close contacts with neighboring clusters are crucial for the cluster formation and crystallization, while 2-methoxyphenylacetylene leads only to copper hydride clusters, Cu 6 H or Cu 32 H 14 . This research not only showcases a new member of copper superatoms but also exemplifies that copper nanoclusters, which are nonluminous in the visible range may emit luminescence in the deep NIR region.