Copper(I)-Pyrazolate Complexes as Solid-State Phosphors: Deep-Blue Emission through a Remote Steric Effect

We describe a novel manifestation of rigidochromic behavior in a series of tetranuclear Cu(I)-pyrazolate (Cu(4)pz(4)) macrocycles, with implications for solid-state luminescence at deep-blue wavelengths (<460 nm). The Cu(4)pz(4) emissions are remarkably sensitive to structural effects far from the luminescent core: when 3,5-di-tert-butylpyrazoles are used as bridging ligands, adding a C4 substituent can induce a blue shift of more than 100 nm. X-ray crystal and computational analyses reveal that C4 units influence the conformational behavior of adjacent tert-butyl groups, with a subsequent impact on the global conformation of the Cu(4)pz(4) complex. Emissions are mediated primarily through a cluster-centered triplet ((CC)-C-3) state; compression of the Cu-4 cluster into a nearly close-packed geometry prevents the reorganization of its excited-state structure and preserves the (CC)-C-3 energy at a high level. The remote steric effect may thus offer alternative strategies toward the design of phosphors with rigid excited-state geometries.