Flexible and lightweight optoelectronic displays depend on luminophores that emit brightly at red, green, and blue wavelengths, but access to the latter is hindered by the limited availability or high cost of phosphors that can produce “pure blue” emission. Here, we describe solid-state phosphorescence from a series of tetranuclear copper(I)-pyrazolate (Cu4pz4) clusters with high quantum yields and blue emission with color coordinates of [0.15, 0.11]. These emissions are remarkably sensitive to structural effects far from the Cu4 core: When 3,5-di-tert-butylpyrazoles are used as bridging ligands, adding a C4 substituent can cause a blueshift in photoluminescence by over 100 nm. X-ray crystal structures of several Cu4pz4 complexes reveal C4 substitution (H, F, Cl, or Br) to have modest effects on solid-state geometries, yet structure-property trends and time-dependent density functional theory (TD-DFT) calculations show the impact of these remote substituents on phosphorescence to be entirely steric in nature. The size of the C4 unit controls the conformations of adjacent tert-butyl groups and can increase the rigidity of the Cu4pz4 metallacycle, which can diminish Jahn-Teller distortion and prevent the excited-state structure from adopting lower energy states prior to photoluminescence. Such remote steric effects on photophysical processes are unprecedented, and may offer a novel approach toward the design of molecular phosphors with tunable optoelectronic properties.

Tetranuclear copper(I) complexes as highly efficient blue phosphors: Tunable emissions via remote steric effects

ACS Fall 2021

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