Visualizing NMR-shielding effect in fullerene-ZnPc aggregates: Characteristic patterns of ZnP-based hosts and encapsulation nature from DFT calculations

Abstract

The formation of supramolecular aggregates incorporating C60 fullerenes can be followed and characterized by nuclear magnetic resonance (NMR) measurements. Here, we unravel the particular patterns provided by zinc-porphyrin (ZnP)-bridged dimers, where the aromatic character of each ZnP unit leads to an enhanced shielding region for the closest fullerene atoms, denoting a slight shielding effect for the equatorial atoms. The nature of the stabilization is discussed and compared to a single ZnP-C60 aggregate and a ZnP-dimer (ZnP2-C60) model, with a significant contribution from noncovalent π-π interactions, allowing us to address the role of bridging chains. The experimental 13C-NMR spectrum of C60 in a bridged ZnP dimer shows a single peak owing to the constant tumbling inside the host, which averages the different groups of carbon atoms. The calculations in a static scenario reveal information concerning the local chemical environment underlying the observed shift in relation to isolated C60. We expect that the current approach can be useful to rationalize and predict the origin of the NMR shift upon the formation of host-guest aggregates involving small and large host species.