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

Luminescence
mechanochromism of the well-known Cu3Pz3-type
(Pz = pyrazolate) complexes is reported here, which is unusual for
this family. Two types of new Cu3Pz3 complexes,
namely Cu3(EBPz)3 (1; EBPz = ethyl-4′-benzoate-3,5-dimethylpyrazolate)
and Cu3(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 (S1) and triplet cluster centered state
(T8) 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 (102 ns scale) of the HE bands.