Anions and Polyanions of Oligoindenopyrenes: Modes of Electron Delocalization and Dimerization

A series of pyrene‐based polycyclic aromatic compounds, indeno[cd]pyrene, diindeno[cd,fg]pyrene, diindeno[cd,jk]pyrene, tris‐(tert‐butylindeno[cd,fg,jk])pyrene, and tetrakis‐(tert‐butylindeno[cd,fg,jk,mn])pyrene, were reduced with alkali metals in [D₈]tetrahydrofuran, and the resulting anions were studied by NMR spectroscopy. It was found that the diatropic character of the dianions obtained depends on the number of annulated indeno groups. When one such group is present, a paratropic dianion is obtained, which is similar to the dianion of the parent pyrene; the effect, however, is weak. When more indeno groups are annulated, the dianions become diatropic owing to the greater number of five‐membered rings that can acquire aromatic character as a result of reduction. The ¹H NMR chemical shifts of tetrakis‐(tert‐butylindeno[cd,fg,jk,mn])pyrene in the neutral state show an interesting dependence on concentration that reflects an association of the molecules in solution by π stacking. This phenomenon was not observed for the reduced species. The trianion radicals of tris‐(tert‐butylindeno[cd,fg,jk])pyrene and tetrakis‐(tert‐butylindeno[cd,fg,jk,mn])pyrene undergo reductive dimerization and form bilayered hexaanions.