Theoretical characterization of proton-induced spectral shifts in Schiff base porphyrins

The roles that substituents and protonation play in the electronic structure and spectrum of the Schiff base of magnesium 4-vinyl-8-formylporphine (MgPSB) have been investigated by ab initio self-consistent-field molecular orbital and configuration interaction calculations, using a floating spherical Gaussian orbital basis. The red shift of the visible band of the parent magnesium porphine (MgP) due to the presence of electron-withdrawing vinyl and Schiff base substituents is shown to arise from a small but significant destabilization of the highest occupied MpG π-orbital brought about by its conjugative interaction with the π-orbitals of the substituents. Changes in the ground-state electron density, however, result from larger perturbations of lower lying MgP orbitals. Protonation of the Schiff base leads to a dramatic differential stabilization of the high-lying π*-orbital of the Schiff base moiety, giving rise to new orbital interactions with low-lying MgPπ*-orbitals that significantly alter the π*-orbital structure of MgPSB. These changes, which affect only the excited states, are shown to be largely responsible for the additional proton-induced red shift (approx. 1000 cm-1) of the visible band the the unique doublet structure of the Soret band observed experimentally. 23 references, 5 figures, 1 table