Significant π-stacking effect between 2,4,6-triphenyl-1-phosphabenzenes

2,4,6-Triphenyl-1-phosphabenzene has a non-planar structure, with the substituent phenyl groups rotated out from the plane of the central ring by about 45°. The rotational barriers are relatively low, and the phenyl groups rotate independently with barriers of about 3 kcal/mol, yielding in two minima (“propeller” and “statistical average”) with nearly identical energy. Altogether, the basically rigid aromatic units form a molecule with increased flexibility. The structural characteristics were compared to the related benzene analog, and also to 2,4,6-triphenyl-1,3,5-triphosphabenzene, as well as to the related nitrogen compounds. It was shown that the steric need of N (characterized by the position of the N lone pair from the ring center) is significantly smaller than that of P, which is slightly larger than the steric need of the CH group. The height of the phenyl rotational barrier in different six-membered rings, decreasing in the order 1,3,5triphosphabenzene > phosphabenzene ~ benzene > pyridine and vanishing completely in 1,3,5triazine, can nicely be explained by this finding. The flexible arrangement of the aromatic rings is found to show advantage for self-assembly by π-stacking effect. For the dimer, a somewhat stronger attractive interaction was calculated than that of the corresponding polycyclic aromatic hydrocarbon (tetracene). The rotational barrier increases in the π-stacked aggregates.