XANES analysis of structural changes in a transient photoexcited state of metalloporphyrin

TL;DR

This study uses XANES spectroscopy and full multiple scattering analysis to investigate transient structural changes in a photoexcited nickel porphyrin complex, revealing elongation of specific bonds and confirming the method's effectiveness.

Contribution

It demonstrates that XANES differential analysis combined with MS approach can effectively determine transient structural changes in square-planar transition metal complexes.

Findings

Elongation of Ni-N and Ni-C bonds in the excited state

Fraction of T1 state population between 41% and 63%

Good agreement with previous EXAFS data

Abstract

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.