Size- and Orientation-Selective Encapsulation of C70 by Cycloparaphenylenes

The size- and orientation-selective formation of the shortest-possible C-70 peapod in solution and in the solid state by using the shortest structural unit of an armchair carbon nanotube (CNT), cycloparaphenylene (CPP), has been studied. [10]CPP and [11]CPP exothermically formed 1:1 complexes with C-70, thereby giving the resulting peapods. A vant Hoff plot analysis revealed that the formation of these complexes in 1,2-dichlorobenzene was mainly driven by entropy, whereas the theoretical calculations suggested that the formation of the complex in the gas phase was predominantly driven by enthalpy. C-70 was found to exist in two distinct orientations inside the CPP cavity, namely lying and standing, depending on the specific size of the CPP. The theoretical calculations and the X-ray crystallographic analysis revealed that the interactions between [10]CPP and the short axis of C-70 in its lying orientation were isotropic and similar to those observed between [10]CPP and C-60. However, the interactions between [11]CPP and C-70 in its standing orientation were anisotropic, thereby involving the radial deformation of [11]CPP into an ellipsoidal shape. This induced fit maximized the van der Waals interactions with the long axis of C-70. Theoretical calculations revealed that the deformation occurred readily with low energy loss, thus suggesting that CPPs are highly radially elastic molecules. These results also indicate that the same type of radial deformation should occur in CNT peapods that encapsulate anisotropic fullerenes.