Brightly emissive octahedral Cu₄X₄ clusters showing polymorphic-dependent and mechanochromic phosphorescence

A series of [Cu₄X₄L₂] clusters (X = Cl, Br, and I) with an octahedral Cu₄X₄ core have been synthesized using bis(<i>t</i>-butyl)(2-pyridyl)phosphine (L1) and bis(2-pyridyl)-<i>t</i>-butylphosphine (L2). At room temperature, these clusters exhibit bright ³(M+X)LCT phosphorescence (<i>λ</i>_max = 514-570 nm) with quantum yields of up to 84% and lifetimes of 3-22 μs. Equally important, polymorphic-dependent and mechanochromic luminescence phenomena have been revealed for the first time in octahedral Cu₄X₄ clusters. Thus, two polymorphs of [Cu₄Br₄(L1)₂] exhibit green and yellow-green phosphorescence with different emission efficiencies and lifetimes. The cluster [Cu₄I₄(L2)₂] features two emission bands at 530 and 600 nm, which probably belong to ³(M+X)LCT and ³CC phosphorescence. When the powder of [Cu₄I₄(L2)₂] is ground, these bands gradually merge into an averaged band at 575 nm, and when a small amount of CH₃CN is added, the two original bands are recovered.