Mechanically Triggered Fluorescence/Phosphorescence Switching in the Excimers of Planar Trinuclear Copper(I) Pyrazolate Complexes

Luminescence mechanochromism of the well-known Cu(3)Pz(3)-type (Pz = pyrazolate) complexes is reported here, which is unusual for this family. Two types of new Cu(3)Pz(3) complexes, namely Cu-3(EBPz)(3) (1; EBPz = ethyl-4'-benzoate-3,5-dimethylpyrazolate) and Cu-3(MBPz)(3) (polymorphs 2a-c; MBPz = methyl-4'-benzoate-3,5-dimethylpyrazolate), have been synthesized and characterized. Their crystal structures exhibit a similar chairlike dimer stacking supported by short Cu...Cu contacts, which would facilitate the formation of photoinduced excimers. The dual emission from the organic fluorophore and excimeric copper cluster phosphor is found to undergo mechanically induced intensity switching between their high-energy (HE) and low-energy (LE) bands. Specifically, the relative intensities of crystalline samples are HE > LE, while the ground solid samples show LE > HE, resulting in the overall emission color interchanging between bluish violet and red. This switching can be reversed by application of solvent to the ground samples, presumably due to recrystallization, and also by heating. TD-DFT calculations reveal that the emissive singlet ligand localized state (S-1) and triplet cluster centered state (T-8) lie close in energy (separated by a gap of 0.1788 eV), suggesting the feasibility of dual emission and the possibility of reverse intersystem crossing, consistent with the long fluorescent lifetimes (10(2) ns scale) of the HE bands.