Effects of halogenation on the ionized and excited states of free-base and zinc porphyrins

Ab initio electronic structure theory has been applied to study the effects of β-halogenation on the structure, ionization potentials (IP), and electronic spectra of free-base (PH2) and zinc porphyrins (ZnP). Electronic and infrared (IR) spectra of β-octahalogenated free-base (β-PX8H2; X=F, Cl, Br) and zinc porphyrins (β-ZnPX8; X=F, Cl, Br) are predicted using TDDFT, B3LYP, and CIS methods. Computations of the excitation energies and IPs are also carried out using structures obtained with the B3LYP density functional. Valence IPs of PH2 and ZnP increase significantly upon β-halogenation. Except for the fluoro group, all β-halogen auxochromes slightly redshift the ultraviolet (UV) and visible bands of the free-base and zinc porphyrin chromophores. β-halogenation significantly redshifts the B bands with increasing magnitude upon going down the period. In addition, we gauge the effects of β-halogenation on the thermodynamic stability of ZnP by calculating the metal binding energies for β-ZnPX8. We found that all β-ZnPX8 have lower metal binding energies than the corresponding binding energy predicted for ZnP. The metal binding energies for zinc porphyrins are significantly larger (∼2 eV) than the H2 elimination enthalpies of the corresponding free-base analogs.