Understanding the affinity of bis-exTTF macrocyclic receptors towards fullerene recognition

A new series of fullerene receptors based on exTTF macrocycles with alkyl ether chains of increasing length is reported. The novel macrocyclic receptors are able to favourably interact with fullerene C-60 through a synergistic combination of pi-pi, CHMIDLINE HORIZONTAL ELLIPSIS pi and nMIDLINE HORIZONTAL ELLIPSIS pi noncovalent interactions. We identify that the highest affinity towards C-60 recognition is achieved for the host with the tightest fit; that is, the smallest receptor with a cavity large enough to host the buckyball inside (log K-a = 5.2 in chlorobenzene at 298 K). However, besides this expected observation, theoretical calculations evidence that the most stable self-assembling configuration corresponds for all the receptors to an outside-ring binding mode, in which the C-60 guest is out of the cavity of the receptor. The higher stability of this configuration results from the smaller deformation energy it implies for the receptor, and allows to explain the experimental trends in the association constants.