Carborane-thiol protected copper nanoclusters: stimuli-responsive materials with tunable phosphorescence

Atomically precise nanomaterials with tunable solid-state luminescence attract global interest. In this work, we present a new class of thermally stable isostructural tetranuclear copper nanoclusters (NCs), shortly Cu(4)@oCBT, Cu(4)@mCBT and Cu(4)@ICBT, protected by nearly isomeric carborane thiols: ortho-carborane-9-thiol, meta-carborane-9-thiol and ortho-carborane 12-iodo 9-thiol, respectively. They have a square planar Cu(4) core and a butterfly-shaped Cu(4)S(4) staple, which is appended with four respective carboranes. For Cu(4)@ICBT, strain generated by the bulky iodine substituents on the carboranes makes the Cu(4)S(4) staple flatter in comparison to other clusters. High-resolution electrospray ionization mass spectrometry (HR ESI-MS) and collision energy-dependent fragmentation, along with other spectroscopic and microscopic studies, confirm their molecular structure. Although none of these clusters show any visible luminescence in solution, bright μs-long phosphorescence is observed in their crystalline forms. The Cu(4)@oCBT and Cu(4)@mCBT NCs are green emitting with quantum yields (Φ) of 81 and 59%, respectively, whereas Cu(4)@ICBT is orange emitting with a Φ of 18%. Density functional theory (DFT) calculations reveal the nature of their respective electronic transitions. The green luminescence of Cu(4)@oCBT and Cu(4)@mCBT clusters gets shifted to yellow after mechanical grinding, but it is regenerated after exposure to solvent vapour, whereas the orange emission of Cu(4)@ICBT is not affected by mechanical grinding. Structurally flattened Cu(4)@ICBT didn't show mechanoresponsive luminescence in contrast to other clusters, having bent Cu(4)S(4) structures. Cu(4)@oCBT and Cu(4)@mCBT are thermally stable up to 400 °C. Cu(4)@oCBT retained green emission even upon heating to 200 °C under ambient conditions, while Cu(4)@mCBT changed from green to yellow in the same window. This is the first report on structurally flexible carborane thiol appended Cu(4) NCs having stimuli-responsive tunable solid-state phosphorescence.