Direct Porphyrin−Aryl Orbital Overlaps in Some meso-Tetraarylporphyrins

Several meso -tetraarylporphyrins with ortho substituents such as −Cl, −Br, and −NO 2 exemplify a hitherto unsuspected mesomeric interaction between the porphyrin and aryl groups. The interaction consists of direct orbital overlap between the porphyrin π system and the ortho substituents on the aryl groups. Interestingly, both the a 1u and a 2u HOMOs of the porphyrin ring have the correct symmetry for participation in this type of interaction. Nonlocal density functional calculations have been used to characterize and visualize the nature of this interaction. We have also investigated the effects of this interaction on porphyrin valence ionization potentials and the relative stabilities of the A 1u and A 2u cation radical states. An interesting consequence of this interaction is that the unpaired electron spin of porphyrin cation radicals can leak from the porphyrin macrocycle onto the aryl groups, an effect for which there is some experimental support from EPR measurements.