A study on the aromaticity and conjugation pathways of diheterodiazuliporphyrins and their dications

Abstract The aromaticity of a series of diheterodiazuliporphyrins, both in the neutral and the dicationic states, has been investigated using the topological resonance energy (TRE) method. Our TRE results show that in each of the dicationic states, all the diheterodiazuliporphyrin isomers have greater aromaticity than when in their corresponding neutral states. An evaluation of the local aromaticity and of the main aromatic conjugation pathways has been carried out through the use of the bond resonance energy (BRE) and circuit resonance energy (CRE) indices, both in the neutral and dicationic states. Our BRE and CRE results show that the local aromaticity arising from the small rings is much larger and is a greater contributor to global aromaticity than the aromaticity which arises from the main aromatic conjugation pathway. In their neutral states, the azulene units are the main contributors to global aromaticity and the compounds themselves are stabilized primarily by their 10π‐electron peripheral structures. In their dicationic states however, it is the 6π‐electron tropylium unit structures which are the main contributors to the global aromaticity. The strength of the magnetically induced ring current (RC) has been calculated using the Hückel‐London model. The RC results show that current strength and the direction of the induced RC of the macrocyclic circuit play a crucial role in determining the paratropicity or diatropicity of the compound. The correlation between the observed 1H NMR chemical shift and the RC strength is discussed, and good correlation is obtained. Finally, the main impact factors upon aromaticity and the RCs are clarified.