Magnetically Induced Current Densities in Zinc Porphyrin Nanoshells

The molecular structures
of porphyrinoid cages were obtained by
constructing small polyhedral graphs whose vertices have degree-4.
The initial structures were then fully optimized at the density functional
theory (DFT) level using the generalized gradient approximation. Some
of polyhedral vertices were replaced with Zn–porphyrin units
and their edges were replaced with ethyne or butadiyne bridges or
connected by fusing two neighboring Zn–porphyrin units. Molecule 1 is an ethyne-bridge porphyrinoid nanotube, whose ends are
sealed with a Zn–porphyrin. Molecule 2 is the
corresponding open porphyrinoid nanotube. Molecule 3 is
a clam-like porphyrinoid cage, whose shells consist of fused Zn–porphyrins,
and the two halves are connected via butadiyne bridges. Molecule 4 is a cross-belt of fused Zn–porphyrins, and molecule 5 is a cross-belt of Zn–porphyrins connected with butadiyne
bridges. The magnetically induced current density of the optimized
porphyrinoid cages was calculated for determining the aromatic character,
the degree of aromaticity and the current-density pathways. The current-density
calculations were performed at the DFT level with the gaugeincluding
magnetically induced currents (GIMIC) method using the B3LYP hybrid
functional and def2-SVP basis sets. Calculations of the current densities
show that molecule 2 sustains a paratropic ring current
around the nanotube, whereas sealing the ends as in molecule 1 leads to an almost nonaromatic nanotube. Fusing porphyrinoids
as in molecules 3 and 4 results in complicated
current-density pathways that differ from the ones usually appearing
in porphyrinoids. The aromatic character of molecules 4 and 5 changes upon oxidation. The neutral molecule 4 is antiaromatic, whereas the dication is nonaromatic. Molecule 5 is nonaromatic, and its dication is aromatic.