Aromatic Pathways in Porphycene Derivatives Based on Current-Density Calculations

Abstract

Magnetically induced current densities have been calculated for porphycenes at the density functional theory (DFT) level using gauge-including atomic orbitals to ensure gauge-origin independence and a fast basis-set convergence of the current densities. The current densities have been analyzed by using the gauge-including magnetically induced current (GIMIC) method. The porphycenes are aromatic sustaining strong diatropic ring currents. The ring-current pathways have been determined by integrating the strength of the current density passing selected bonds. The calculations show that the ring current of the porphycenes splits into an outer and inner branch at the pyrrolic rings implying that the ring current involves all 26 π electrons of the porphycenes, which is similar to the ring current of porphyrins. The pyrrolic rings of the aromatic porphycenes do not sustain any significant local ring currents. Dihydroporphycene with four inner hydrogens is antiaromatic with weakly aromatic pyrrolic rings. The annelated benzoic rings in benzoporphycene sustain local paratropic ring-currents, whereas the global ring current of dibenzoporphycene splits into an outer and inner branch at the benzoic rings. Comparison of calculated 1 H NMR shieldings with ringcurrent strengths shows that interactions between the inner hydrogen and the neighbor nitrogen is more significant for differences in the 1 H NMR shieldings than variations in global ring-current strengths. Calculated excitation energies show that the antiaromatic dihydroporphycene has a smaller optical gap than the aromatic porphycene, even though its HOMO-LUMO gap is larger. 1 Introduction Vogel et al. performed systematic studies of novel kinds of aromatic porphyrin structures in the mid-1980s, which led to the discovery of porphycene. 1 Porphycenes are a class of compounds that is reminiscent of porphyrin with four linked pyrrolic rings forming a central 16-membered macroring and an outer annulene ring with 20π electrons. Waluk et al. showed later that the electronic structure and the aromatic properties of porphycenes can

Aromatic Pathways in Porphyrinoids by Magnetically Induced Ring Currents

“…8,11,[111][112][113][114][115][116][117] The calculations show that for aromatic properties, it does not matter whether one has one meso carbon between each pyrrolic ring or all four meso carbons between two rings. 111 The chemistry of porphycenes has also been recently reviewed. 118,119 Current-density calculations show that porphycene has a similar aromatic character as porphyrin.…”

“…105 The porphycene chemistry has been generalized to corrphycene, hemiporphycene, and isoporphycene, 18,[106][107][108][109][110] which have similar aromatic pathways as porphycene. 111 Porphycenes have been studied experimentally and computationally using different approaches. 8,11,[111][112][113][114][115][116][117] The calculations show that for aromatic properties, it does not matter whether one has one meso carbon between each pyrrolic ring or all four meso carbons between two rings.…”

“…111 Porphycenes have been studied experimentally and computationally using different approaches. 8,11,[111][112][113][114][115][116][117] The calculations show that for aromatic properties, it does not matter whether one has one meso carbon between each pyrrolic ring or all four meso carbons between two rings. 111 The chemistry of porphycenes has also been recently reviewed.…”

“…The aromatic pathway splits at the pyrrolic rings into an inner and an outer pathway. 111 At the pyrrolic rings without an inner hydrogen, the ring current is equally divided between the two branches, whereas at the pyrrolic rings with an inner hydrogen about one third takes the inner route (see Figure 16). Dihydroporphycene, which is obtained by adding two inner hydrogens to porphycene, sustains a paratropic ring current along the inner pathway with 20 π electrons and the pyrrolic rings are weakly aromatic.…”

“…Dihydroporphycene, which is obtained by adding two inner hydrogens to porphycene, sustains a paratropic ring current along the inner pathway with 20 π electrons and the pyrrolic rings are weakly aromatic. 111 In dibenzoporphycene, which has two benzoic rings annelated to the long end of porphycene, the main aromatic pathways passes on the outside of the benzoic ring, whereas about 20% of the ring current flows on the inside of it. Benzoporphycene, which has two benzoic rings between the pyrrolic ring on the long side of porphycene, is aromatic, whereas the two annelated benzoic rings sustain paratropic ring currents.…”