Photoionization of tetrapyrrole macrocycles: Porphyrin isomers and corrole tautomers

Abstract

The electronic structures of tetrapyrrole macrocycles: porphyrin’s structural isomers and corrole tautomers have been investigated using high-level ab initio method: Green’s function method (OVGF). The vertical, valence ionization energies were calculated and gas phase photoelectron spectra simulated on the basis of these ionization energies. The Franck-Condon analysis of vibronic effects has also been performed and changes between molecular geometries in neutral and ionic ground state have been discussed. Relative energies (standard formation enthalpies) for isomers have been calculated using composite G4MP2 method. The results help in understanding of how is the electronic structure affected when molecule deviates from planar geometry and what is the influence of changes in ring-ring linkages. We have demonstrated that the electronic structures (reflected in ionization energies) of title compounds (as free, unsubstituted bases) are not significantly perturbed when molecular geometry deviates from planarity. We also discuss the relationship between lowest band ionization energy and aromaticity of porphyrins and corroles.