Nature of C-60 and C-70 fullerene encapsulation in a porphyrin- and metalloporphyrin-based cage: Insights from dispersion-corrected density functional theory calculations

Abstract

The search for efficient synthetic hosts able to encapsulate fullerenes has attracted attention with regard to the purification and formation of ordered supramolecular architectures. This study of a porphyrin-based cage as an extension of the well-described ExCage(6+) and BlueCage(6+), involving viologen as sidearms, provides an interesting scenario where the oblate C-70 fullerene is preferred in comparison to the spherical C-60. Our results expose the nature of the fullerene-cage interaction involving similar to 50% of dispersion-type interactions evidencing the strong pi MIDLINE HORIZONTAL ELLIPSIS pi surface stacking, with a complementary contribution by the electrostatic and orbital polarization character produced by a charge reorganization with a charge accumulation facing the porphyrin macrocycles and a charge depletion along the equator formed by the viologens sidearms. Interestingly, the central N4H2 ring from each porphyrin contributes to the dispersion term via N-HMIDLINE HORIZONTAL ELLIPSIS pi interactions, which is decreased when the metallate N4Zn is evaluated. Thus, the formation of stable and selective fullerene encapsulation can be achieved by taking into account two main driving forces, namely, (a) the extension of the pi MIDLINE HORIZONTAL ELLIPSIS pi and X-HMIDLINE HORIZONTAL ELLIPSIS pi stacking surface and (b) charge reorganization over the fullerene surfaces, which can be used to control fine tuning of the encapsulation given by the capabilities to introduce more electron-deficient and electron-rich groups within the host cage.