  We have performed a structural analysis of the Ni K-edge XANES spectrum of a
square planar coordination complex, Ni-tetramesitylporphyrin, Ni(II)TMP, in
dilute toluene solution. The fit of the spectrum was carried out in the frame
of the full multiple scattering (FMS) approach, via the MXAN program, starting
from a muffin-tin (MT) form of the molecular potential. We have applied the
MXAN analysis to the time-resolved XANES difference spectrum obtained between a
100 ps life-time photoexcited state of Ni(II)TMP (a relaxed triplet state, T1,
obtained by exciting the Soret band) and the ground state, S0. From our direct
simulation of the laser on/laser off difference spectrum, we obtain the
fraction of T1 population (0.41-0.63) A in agreement with optical transient
absorption determination, and a 0.05-0.1 A elongation of the d(Ni-N) and
d(Ni-C) distances. The overall uncertainty on these results mostly depends on
uncertainty on the chemical shift between the T1 and S0 state, that is assumed
a priori in the differential analysis. Still our results are in good agreement
with previous EXAFS data (obtained on a reconstructed absolute spectrum of the
T1 state). Our best fit corresponds to no chemical shift, 54% fraction of T1
state, and either a (0.04 +/- 0.01) elongation of the average d(Ni-N) or a
(0.06 +/- 0.01) elongation of the average d(Ni-pyrrol) when pyrrol rings are
treated as perfectly rigid bodies. This work demonstrates that XANES
spectroscopy can be used to provide structural information on square-planar
transition metal compounds by analysing the metal K-edge via MS method under
the MT approximation, and that XANES differential analysis can be successfully
applied to study electronic/structural relationships in transient photoexcited
species from time-resolved experiments.
