Macrocyclic Hosts for Fullerenes: Extreme Changes in Binding Abilities with Small Structural Variations

Exploiting the shape and electronic complementarity of C-60 and C-70 with pi-extended derivatives of tetrathiafulvalene (exTTF), we have very recently reported a macrocyclic receptor featuring two exTTF recognizing units which forms 1:1 complexes with C-60 with log K-a = 6.5 +/- 0.5 in chlorobenzene at 298 K. This represents one of the highest binding constants toward C-60 reported to date and a world-record for all-organic receptors. Here, we describe our efforts to fine-tune our macrocyclic bis-exTTF hosts to bind C-60 and/or C-70, 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 fragments and three alkene-terminated chains of different length to achieve macrocycles of different size through ring closing metathesis. Owing to the structural simplicity of our design, all nine receptors could he accessed in a synthetically straightforward manner. A thorough investigation of the binding abilities of these nine receptors toward C-60 and C-70 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 in molecular 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.