W0028
Time-domain Pump-probe XAFS Studies of Photoexcited State
Molecular Structures. Lin X. Chen, George B. Shaw, Tao Liu, Guy Jennings,
Klaus Attenkofer, Chemistry Div. and Material Sciences Div., Argonne National
Laboratory, Argonne, IL 60439, U.S.A.
The high brilliance and the timimg structure of the third
generation synchrotron enable structures of short-lived excited state molecules
to be captured in disordered media.
Molecular structures of the photoexcited
metal-to-ligand-charge-transfer (MLCT) state of
[CuI(dmp)2]+, where dmp is
2,9-dimethyl-1,10-phenanthroline, in toluene and acetonitrile were investigated
by laser pump/x-ray probe X-ray absorption spectroscopy using X-ray pulses from
a third generation synchrotron source. Both X-ray pulse cluster with time span
of 14-ns and single X-ray pulses with 100 ps FWHM were used as probes according
to the excited state lifetimes. The experimental results indicated that the
copper ion in the thermally equilibrated MLCT state in both solvents had the
same oxidation state as the corresponding Cu(II) complex in the ground state and
was found to be penta-coordinate with an average nearest neighbor Cu-N distances
0.04 Å longer in toluene and 0.04 Å shorter in acetonitrile than
that of the ground state [CuI(dmp)2]+. The
results confirmed the previous proposed “exciplex” structure of the
MLCT state in Lewis base solvents, and further revealed that what distinguishes
the MLCT state structures in non-coordinating and coordinating solvents is not
the “exciplex” formation, but the strength of the interactions
between the solvent and the Cu(II)* species in the MLCT state. Other results on
the triplet structures of metalloporphyrins and future direction of
time-resolved XAFS will be discussed.
This work is supported by the Div. of Chemical Sciences,
Office of Basic Energy Sciences, U.S. Dept. of Energy under contracts
W-31-109-Eng-38.