Shedding light on the metal-phthalocyanine EXAFS spectra through classical and ab initio molecular dynamics

Extended X-Ray Absorption Fine Structure (EXAFS) theoretical spectra for some 3d transition metal-phthalocyanines-FePc, NiPc, CuPc, and ZnPc-are presented. Their complexity and rigidity make them a good testbed for the development of theoretical strategies that can complement the difficulties present in the experimental spectrum fitting. Classical and ab initio molecular dynamics trajectories are generated and employed as a source of structural information to compute average spectra for each MPc species. The original ZnPc force field employed in the classical molecular dynamics simulations has been modified in order to improve the agreement with the experimental EXAFS spectrum, and the modification strategy-based on MP2 optimized structures-being extended to the rest of MPcs. Both types of trajectories, classical and ab initio, provide very similar results, showing in all cases the main features present in the experimental spectra despite the different simulation timescales employed. Spectroscopical information has been analyzed on the basis of shells and legs contributions, making possible the comparison with the experimental fitting approaches. According to the simulations results, the simple relationships employed in the fitting process to define the dependence of the Debye Waller factors associated with multiple scattering paths with those of single scattering paths are reasonable. However, a lack of multiple backscattering paths contributions is found due to the intrinsic rigidity of the chemical motif (macrocycle). Its consequences in the Debye Waller factors of the fitted contributions are discussed.

Automated summary

This study presents the theoretical spectra of some 3d transition metal-phthalocyanines (FePc, NiPc, CuPc, and ZnPc) using Extended X-Ray Absorption Fine Structure (EXAFS). The complexity and rigidity of these compounds make them suitable for testing theoretical strategies in spectrum fitting. Classical and ab initio molecular dynamics trajectories are generated to calculate average spectra for each MPc species. The results show that both types of trajectories produce similar results to the experimental spectra. Spectroscopical information is analyzed based on shells and legs contributions, allowing for comparison with experimental fitting approaches. The lack of multiple backscattering paths due to the rigidity of the chemical motif is discussed.