Study on the aromaticity of pyrrole-modified subporphyrins containing nitrogen units

This study designed 15 novel compounds through N-unit modification of the imine-type pyrrole ring in free-base subporphyrins and systematically investigated their overall aromaticity. The phenomenon of stability changes upon the introduction of nitrogen atoms was elucidated through the topological charge stabilisation rule. Using the topological resonance energy (TRE) index for prediction, it was found that the introduction of two nitrogen atoms into the imine-type pyrrole ring significantly affects the overall aromaticity of the compound. Simultaneously, bond resonance energy (BRE) and the circuit resonance energy index (CRE) are utilised to assess the aromatic contribution of local components, including π bonds and topological circuits, to the molecule. The results indicate that the introduction of nitrogen atoms has a more pronounced effect on the five-membered ring system than the 14π electron conjugation pathway, thereby emerging as the primary contributor to aromaticity. After estimating ring current intensity (RC) via the Hückel-London model, the local aromaticity trends measured by TRE indices showed a reasonable correspondence with the directions of the ring currents. However, discrepancies occasionally arose between the local aromaticity strengths predicted by RC and those obtained from TRE results. Finally, the observed relationship between 1H-NMR chemical shifts and ring current intensity was discussed.