Ultrafast structural rearrangements in the MLCT excited state for copper(I) bis-phenanthrolines in solution

George B. Shaw, Christian D. Grant, Hideaki Shirota, Edward W. Castner, Gerald J. Meyer, Lin X. Chen

Research output: Contribution to journalArticle

142 Citations (Scopus)

Abstract

Ultrafast excited-state structural dynamics of [CuI(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) (3d10) 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 [CuI(dmp)2]+ and [CuI(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 [CuI(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.

Original languageEnglish
Pages (from-to)2147-2160
Number of pages14
JournalJournal of the American Chemical Society
Volume129
Issue number7
DOIs
Publication statusPublished - Feb 21 2007

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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