Aromatic Pathways in Porphycene Derivatives Based on Current-Density Calculations

Magnetically induced current densities are reported for porphycenes at the density functional theory level using gauge-including atomic orbitals, which ensure gauge-origin independence and a fast basis-set convergence of the current densities. We have analyzed the current densities by using the gauge-including magnetically induced current (GIMIC) method. The porphycenes are found to be aromatic. They sustain strong diatropic ring currents. The ring-current pathways have been obtained by integrating the strength of the current density passing selected bonds. The calculations show that the ring current of the porphycenes divides into an outer and inner pathways at the pyrrolic rings. Thus, the ring current involves all 26 pi electrons of the porphycenes, which is similar to the ring current of porphin. No significant local ring currents are sustained by the pyrrolic rings. Dihydroporphycene with four inner hydrogens is found to be antiaromatic with weakly aromatic pyrrolic rings. The six-membered rings in benzoporphycene sustain local paratropic ring currents. The global ring current of dibenzoporphycene divides into an outer and inner pathway at the benzoic rings. Interactions between the inner hydrogen and the neighbor nitrogen are found to be more significant for differences in the H-1 NMR shieldings than variations in global ring-current strengths. We found that the antiaromatic dihydroporphycene has a larger HOMO-LUMO gap but a smaller optical gap than the aromatic porphycene.