Controlling the Crystallization of Porous Organic Cages: Molecular Analogs of Isoreticular Frameworks Using Shape-Specific Directing Solvents

Small structural changes in organic
molecules can have a large
influence on solid-state crystal packing, and this often thwarts attempts
to produce isostructural series of crystalline solids. For metal–organic
frameworks and covalent organic frameworks, this has been addressed
by using strong, directional intermolecular bonding to create families
of isoreticular solids. Here, we show that an organic directing solvent,
1,4-dioxane, has a dominant effect on the lattice energy for a series
of organic cage molecules. Inclusion of dioxane directs the crystal
packing for these cages away from their lowest-energy polymorphs to
form isostructural, 3-dimensional diamondoid pore channels. This is
a unique function of the size, chemical function, and geometry of
1,4-dioxane, and hence, a noncovalent auxiliary interaction assumes
the role of directional coordination bonding or covalent bonding in
extended crystalline frameworks. For a new cage, CC13, a dual, interpenetrating pore structure is formed that doubles
the gas uptake and the surface area in the resulting dioxane-directed
crystals.