Time-resolved single-crystal diffraction performed with synchrotron radiation shows that the $53(1)\mu \mathrm{s}$ phosphorescent state, generated in the crystalline phase of trimeric $\{[3,5\mathrm{\text{−}}({\mathrm{CF}}_3{)}_2\mathrm{\text{Pyrazolate}}]\mathrm{Cu}{\}}_3$ molecules by exposure to 355 nm of light at 17 K, is due to the formation of an excimer rather than the shortening of the intramolecular $\mathrm{Cu}\cdots \mathrm{Cu}$ distances within the trimeric units, or the formation of a continuous chain of interacting molecules. One of the intermolecular $\mathrm{Cu}\cdots \mathrm{Cu}$ distances contracts by 0.56 Å from 4.018(1) to 3.46(1) Å, whereas the interplanar spacing of the trimers is reduced by 0.65 Å from 3.952(1) to 3.33(1) Å. Density-functional theory calculations support the formation of a $\mathrm{Cu}\cdots \mathrm{Cu}$ bond through the intermetallic transfer of a Cu $3d$ electron to a molecular orbital with a large $4p$ contribution on the reacting Cu atoms.

• Received 20 January 2005

DOI:https://doi.org/10.1103/PhysRevLett.94.193003