ARTICLE pubs.acs.org/JACS Macrocyclic Hosts for Fullerenes: Extreme Changes in Binding Abilities with Small Structural Variations.

bS Supporting Information ABSTRACT: Exploiting the shape and electronic complementarity of C60 and C70 with π-extended derivatives of tetrathiafulvalene (exTTF), we have very recently reported a macrocyclic receptor featuring two exTTF recognizing units which forms 1:1 complexes with C60 with log Ka = 6.5 ( 0.5 in chlorobenzene at 298 K. This represents one of the highest binding constants towardC60 reported to date and aworld-record for all-organic receptors.Here,wedescribe our efforts to fine-tune our macrocyclic bis-exTTF hosts to bind C60 and/ or C70, through structural variations. On the basis of preliminary molecular modeling, we have explored p-xylene, m-xylene, and 2,6-dimethylnaphthalene as aromatic spacers between the two exTTF frag-ments and three alkene-terminated chains of different length to achievemacrocycles of different size through ring closingmetathesis. Owing to the structural simplicity of our design, all nine receptors couldbe accessed in a synthetically straightforwardmanner. A thorough investigationof the binding abilities of these nine receptors toward C60 and C70 has been carried out by means of UV-vis titrations. We have found that relatively small variations in the structure of the host lead to very significant changes in affinity toward the fullerene, and in some cases even in the stoichiometry of the associates. Our results highlight the peculiarities of fullerenes as guests inmolecular recognition. The extreme stability of these associates in solution and the unique combination of electronic and geometrical reciprocity of exTTF and fullerenes are the main features of this new family of macrocyclic hosts for fullerenes. 1