Size and shape do matter: When dimerized in nonpolar solvents, an equimolar mixture of eleven tetra-urea calixarenes with different wide-rim substituents self-sorts into only six out of 35 different homo- and heterodimers (see picture). Since the calixarene scaffold and the four urea units are the same in all cases, the self-sorting process is driven only by the cooperative action of steric requirements and stoichiometry.
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