Check out this an incredibly cool host guest complex: the -cycloparaphenylene (CPP) hoop encapsulating C60!1
(Be sure to click on this image to bring up the 3-D interactive structure – as with all structures in my blog!)
1H and 13C NMR and fluorescence quenching spectrometry clearly indicate that this complex is formed when CPP is mixed with C60 in toluene. In fact, when C60 is mixed with a mixture of nanohoops ranging from 8 to 12 phenyl ring, only the CPP hoop complexes with the fullerene. The experimental binding energy is between 38 and 59 kJ mol-1.
M06-2x/6-31G* computations give the structure shown above. The computed binding energy is 173 kJ mol-1, but the computations do not include solvent. So this overestimation might be somewhat due to the difference in gas phase vs. solution complexation.
(Check out this post for other interesting nanohoops.)
(1) Iwamoto, T.; Watanabe, Y.; Sadahiro, T.; Haino, T.; Yamago, S., "Size-Selective Encapsulation of C60 by Cycloparaphenylene: Formation of the Shortest Fullerene-Peapod," Angew. Chem. Int. Ed., 2011, 50, 8342-8344, DOI: 10.1002/anie.201102302