A series of hexaphyrins with different meso-carbon atoms and their protonated structures were investigated using density functional theory (DFT) and time-dependent DFT. Frontier molecular orbitals (FMOs), aromaticity, and electronic spectra were investigated systematically before and after protonation. The FMO energy gaps before and after protonation were different for the antiaromatic molecules, while they were only slightly different for the aromatic molecules. By analyzing the electronic spectra of the aromatic molecules, the absorption peaks in the Q-like and B-like bands were not significantly different before and after protonation. However, the absorption peaks of the antiaromatic molecules were clearly different before and after protonation in both the Q-like and B-like bands. [24]Hexaphyrin (1.0.1.0.1.0) has 24 π-electrons and is Hückel antiaromatic. However, the absorption spectrum of protonated [24]hexaphyrin (1.0.1.0.1.0) showed aromaticity. In addition, these conclusions were generally consistent with the FMOs, nucleus-independent chemical shifts, harmonic oscillator model of aromaticity, and absorption spectra. Although protonated [24]hexaphyrin (1.0.1.0.1.0) has 24 π-electrons and is Hückel antiaromatic, it has Möbius aromaticity because of the single-sided Möbius topological structure. This explains why [24]hexaphyrin (1.0.1.0.1.0) has diatropic ring currents in solvent. To the best of our knowledge, this system is the smallest Möbius aromatic molecule among the many uncoordinated extended porphyrins.
Keywords: Aromaticity; Electronic spectrum; Hexaphyrin; Möbius cyclacenes; Protonation; Time-dependent density functional theory.