Molecular modeling of highly peripheral substituted Mg- and Zn-porphyrins

The MNDO/PM3 semiempirical method was used for predicting nonplanar distort ion modes in highly substituted porphyrins. Octaethyl-tetraphenylporphyrin (MOETPP) and dodecaphenylporphyrin (MDPP) (M=H-2, Mg and Zn) were obtained as minimum energy structures and exist in saddle-shape conformations. Optim ized meso-substituted tetra-tert-butylporphyrin (MTtBuP) (M=Mg, Zn) adopts a nonplanar ruffled structure. Atomic deviations from planarity and geometr ical variations induced by ruffling are in good agreement with reported dat a in previous crystallographic studies and molecular mechanic calculations of highly substituted Ni, Zn, Cu and Co porphyrins. The present semiempiric al calculations confirm that the macrocycle deformations respond to steric repulsions of peripheral substituents. The degree of deformation does not o nly depend on the sterical crowding of the substituents, but it is also ver y sensitive to the size of the metal located into the macrocycle cavity. La rge Zn metal gives rise to a flattening of the macrocycle, whereas small Mg metal favors a marked ruffling. In the last case the small size of Mg meta l is enough by itself for disturbing the unsubstituted Mg-porphyrin into a saddle conformer. The PM3 method used here predicts very accurately the typ ical nodal patterns: a(1u) and a(2u), for the HOMOs in porphyrins. The reve rsal of a(2u)/a(1u) ordering has been investigated in previous papers as a function of either electron-withdrawing or -donating character of the subst ituents placed in different peripheral positions. The present results revea l that nonplanar macrocycle distortion affects the ordering of the two high est occupied molecular orbitals. The saddle form exists in an a(1u)/a(2u) ( HOMO/HOMO-1) orbital structure, while the ruffled deformation raises the en ergy of the a(2u) orbital due to the large meso-carbon deviations. These re sults suggest that possible transitions between conformers induced under ex citations may give rise to important electronic differences between ground and excited states of the nonplanar porphyrins.