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

Abstract

The MNDO/PM3 semiempirical method was used for predicting nonplanar distortion modes in highly substituted porphyrins. Octaethyl-tetraphenylporphyrin (MOETPP) and dodecaphenylporphyrin (MDPP) (M = H₂, Mg and Zn) were obtained as minimum energy structures and exist in saddle-shape conformations. Optimized meso-substituted tetra-tert-butylporphyrin (MTtBuP) (M = Mg, Zn) adopts a nonplanar ruffled structure. Atomic deviations from planarity and geometrical variations induced by ruffling are in good agreement with reported data in previous crystallographic studies and molecular mechanic calculations of highly substituted Ni, Zn, Cu and Co porphyrins. The present semiempirical calculations confirm that the macrocycle deformations respond to steric repulsions of peripheral substituents. The degree of deformation does not only depend on the sterical crowding of the substituents, but it is also very sensitive to the size of the metal located into the macrocycle cavity. Large 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 metal is enough by itself for disturbing the unsubstituted Mg-porphyrin into a saddle conformer. The PM3 method used here predicts very accurately the typical nodal patterns: a_1u and a_2u, for the HOMOs in porphyrins. The reversal of a_2u/a_1u ordering has been investigated in previous papers as a function of either electron-withdrawing or -donating character of the substituents placed in different peripheral positions. The present results reveal that nonplanar macrocycle distortion affects the ordering of the two highest occupied molecular orbitals. The saddle form exists in an a_1u/a_2u (HOMO/HOMO-1) orbital structure, while the ruffled deformation raises the energy of the a_2u orbital due to the large meso-carbon deviations. These results suggest that possible transitions between conformers induced under excitations may give rise to important electronic differences between ground and excited states of the nonplanar porphyrins.