Photoionization of tetrapyrrole macrocycles: Porphyrin isomers and corrole tautomers

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.