title: Ultrafast structural rearrangements in the MLCT excited state for copper(I) bis-phenanthrolines in solution.
abstract: Ultrafast excited-state structural dynamics of [Cu(I)(dmp)(2)](+) (dmp = 2,9-dimethyl-1,10-phenanthroline) have been studied to identify structural origins of transient spectroscopic changes during the photoinduced metal-to-ligand charge-transfer (MLCT) transition that induces an electronic configuration change from Cu(I) (3d(10)) to Cu(II) (3d(9)). This study has important connections with the flattening of the Franck-Condon state tetrahedral geometry and the ligation of Cu(II)* with the solvent observed in the thermally equilibrated MLCT state by our previous laser-initiated time-resolved X-ray absorption spectroscopy (LITR-XAS) results. To better understand the structural photodynamics of Cu(I) complexes, we have studied both [Cu(I)(dmp)(2)](+) and [Cu(I)(dpp)(2)](+) (dpp = 2,9-diphenyl-1,10-phenanthroline) in solvents with different dielectric constants, viscosities, and thermal diffusivities by transient absorption spectroscopy. The observed spectral dynamics suggest that a solvent-independent inner-sphere relaxation process is occurring despite the large amplitude motions due to the flattening of the tetrahedral coordinated geometry. The singlet fluorescence dynamics of photoexcited [Cu(I)(dmp)(2)](+) were measured in the coordinating solvent acetonitrile, using the fluorescence upconversion method at different emission wavelengths. At the bluest emission wavelengths, a prompt fluorescence lifetime of 77 fs is attributed to the excited-state deactivation processes due to the internal conversion and intersystem crossing at the Franck-Condon state geometry. The differentiation between the prompt fluorescence lifetime with the tetrahedral Franck-Condon geometry and that with the flattened tetrahedral geometry uncovers an unexpected ultrafast flattening process in the MLCT state of [Cu(I)(dmp)(2)](+). These results provide guidance for future X-ray structural studies on ultrafast time scale, as well as for synthesis toward its applications in solar energy conversion.
authors.affiliation: Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439, USA.
authors.forename: George B
authors.initials: GB
authors.lastname: Shaw
authors.forename: Christian D
authors.initials: CD
authors.lastname: Grant
authors.forename: Hideaki
authors.initials: H
authors.lastname: Shirota
authors.forename: Edward W
authors.initials: EW
authors.lastname: Castner
authors.forename: Gerald J
authors.initials: GJ
authors.lastname: Meyer
authors.forename: Lin X
authors.initials: LX
authors.lastname: Chen