Shedding Light on the Structure of a Photoinduced Transient Excimer by Time-Resolved Diffraction

Time-resolved single-crystal diffraction performed with synchrotron radiation shows that the 53(1) μs phosphorescent state, generated in the crystalline phase of trimeric [3,5-(CF₃)₂Pyrazolate]Cu₃ 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 Cu⋯Cu distances within the trimeric units, or the formation of a continuous chain of interacting molecules. One of the intermolecular Cu⋯Cu distances contracts by 0.56 {} from 4.018(1) to 3.46(1) {}, whereas the interplanar spacing of the trimers is reduced by 0.65 {} from 3.952(1) to 3.33(1) {}. Density-functional theory calculations support the formation of a Cu⋯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.