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) mu s phosphorescent state, generated in the crystalline phase of trimeric {[3,5-(CF3)(2)Pyrazolate]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 Cu center dot center dot center dot Cu distances within the trimeric units, or the formation of a continuous chain of interacting molecules. One of the intermolecular Cu center dot center dot center dot Cu distances contracts by 0.56 angstrom from 4.018(1) to 3.46(1) angstrom, whereas the interplanar spacing of the trimers is reduced by 0.65 angstrom from 3.952(1) to 3.33(1) angstrom. Density-functional theory calculations support the formation of a Cu center dot center dot center dot 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.