Brightly Phosphorescent Trinuclear Copper(I) Complexes of Pyrazolates:  Substituent Effects on the Supramolecular Structure and Photophysics

Synthetic details, solid-state structures, and photophysical properties of a group of trimeric copper(I) complexes containing pyrazolate ligands are described. The reaction of copper(I) oxide and the fluorinated pyrazoles [3-(CF₃)Pz]H, [3-(CF₃),5-(Me)Pz]H, and [3-(CF₃),5-(Ph)Pz]H leads to the corresponding trinuclear copper(I) pyrazolates, {[3-(CF₃)Pz]Cu}₃, {[3-(CF₃),5-(Me)Pz]Cu}₃, and {[3-(CF₃),5-(Ph)Pz]Cu}₃, respectively, in high yield. The {[3,5-(i-Pr)₂Pz]Cu}₃ compound was obtained by a reaction between [Cu(CH₃CN)₄][BF₄], [3,5-(i-Pr)₂Pz]H, and NEt₃. These compounds as well as {[3,5-(Me)₂Pz]Cu}₃ and {[3,5-(CF₃)₂Pz]Cu}₃ adopt trimeric structures with nine-membered Cu₃N₆ metallacycles. There are varying degrees and types of intertrimer Cu···Cu interactions. These contacts give rise to zigzag chains in the fluorinated complexes, {[3-(CF₃)Pz]Cu}₃, {[3-(CF₃),5-(Me)Pz]Cu}₃, {[3-(CF₃),5-(Ph)Pz]Cu}₃, and {[3,5-(CF₃)₂Pz]Cu}₃, whereas the nonfluorinated complexes, {[3,5-(Me)₂Pz]Cu}₃ and {[3,5-(i-Pr)₂Pz]Cu}₃ form dimers of trimers. Out of all the compounds examined in this study, {[3-(CF₃),5-(Ph)Pz]Cu}₃ has the longest (3.848 Å) and {[3,5-(Me)₂Pz]Cu}₃ has the shortest (2.946 Å) next-neighbor intertrimer Cu···Cu distance. The Cu···Cu separations within the trimer units do not vary significantly (typically 3.20−3.26 Å). All of these trinuclear copper(I) pyrazolates show bright luminescence upon exposure to UV radiation. The luminescence bands are hugely red-shifted from the corresponding lowest-energy excitations, rather broad, and unstructured even at low temperatures, suggesting metal-centered emissions owing to intertrimer Cu···Cu interactions that are strengthened in the phosphorescent state. The {[3-(CF₃),5-(Ph)Pz]Cu}₃ compound exhibits an additional highly structured phosphorescence with a vibronic structure corresponding to the pyrazolyl (Pz) ring. The luminescence properties of solids and solutions of the trimeric compounds in this study show fascinating trends with dramatic sensitivities to temperature, solvent, concentration, and excitation wavelengths.