Helical inversion reaction pathway for $\pi $/$\pi $ stacking in aromatic compounds

Polyaromatics have the $\pi $/$\pi $ stacking interaction, which shows that two aromatic units preferentially interact in a parallel-displaced orientation, such as the crystal packing of organic molecules. Recently, $\pi $/$\pi $ stacking compounds have become of interest in the photocatalysis solor energy transformational materials. In particular, the stable configurations of neutral and cationic $o$-phenylenes have helical tightly packed $n$-phenylenes with $\pi $/$\pi $ stacking interactions. To investigate helical inversion pathways, we have studied theoretically the stable and transition-state geometries of neutral and cationic $o$-phenylenes using the density functional theory method. We have found that $o$-phenylenes follow an inversion reaction pathway with three transition-states in which the configuration of each phenylene is inverted. This means that $o$-phenylenes cannot convert from right- to left-handed geometry at once; however, we suggest a step-by-step reaction pathway through the three transition-state structures.