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 Cu4S4 staple, which is appended with four respective carboranes. For Cu-4@ICBT, strain generated by the bulky iodine substituents on the carboranes makes the Cu4S4 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 mu s-long phosphorescence is observed in their crystalline forms. The Cu-4@oCBT and Cu-4@mCBT NCs are green emitting with quantum yields (phi) of 81 and 59%, respectively, whereas Cu-4@ICBT is orange emitting with a phi 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 Cu4S4 structures. Cu-4@oCBT and Cu-4@mCBT are thermally stable up to 400 degrees C. Cu-4@oCBT retained green emission even upon heating to 200 degrees 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.

Automated summary

This study presents a new class of thermally stable tetranuclear copper nanoclusters (Cu-4@oCBT, Cu-4@mCBT and Cu-4@ICBT) with tunable solid-state luminescence. They exhibit bright phosphorescence in their crystalline forms. Cu-4@oCBT and Cu-4@mCBT are green emitting, while Cu-4@ICBT is orange emitting. DFT calculations reveal the nature of their electronic transitions. Cu-4@oCBT and Cu-4@mCBT are thermally stable up to 400 degrees C, while Cu-4@ICBT is structurally flattened.