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

Abstract

A series of [Cu4X4L2] clusters (X = Cl, Br, I) with an octahedral Cu4X4 core has been synthesized using bis(t-butyl)(2-pyridyl)phosphine (L1) and bis(2-pyridyl)-t-butylphosphine (L2). At room temperature, these clusters exhibit a bright 3(M+X)LCT phosphorescence (λmax = 514–570 nm) with the quantum yields up to 84% and lifetimes of 3–22 µs. Equally important, polymorphic-dependent and mechanochromic luminescence phenomena have been revealed for the first time for 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.