Stereochemistry in self-assembled encapsulation complexes: constellational isomerism.

A previously uncharacterized form of stereochemistry, constellational isomerism, is described. The isomerism arises from different arrangements of small-molecule guests in the space of a self-assembled, cylindrical host. The cylindrical host detains three molecules each of CHCl(3), 1,2-dichloroethane, or isopropyl chloride. The exchange of guests in and out of the host is slow on the NMR time scale. The dimensions of the capsular host and the sizes of the guests hinder the mobility of molecules inside, and separate NMR signals are seen for guests at the ends of the capsule and those near its center. When two different guests are encapsulated, the spectra show up to four additional species: two sets of constellational isomers. In every pairwise combination of the three guests, all isomers could be identified. The equilibrium distributions of isomers depended on the concentrations of the guests in the bulk solution. The relative stability of the constellational isomers was a function of the polarity of the guest molecule and its ability to interact with the components of the capsule. The different arrangements represent information, and some possibilities for their use in data storage are proposed.