Size Affects Shape: Porous molecular framework materials can adopt a different phase when guest molecules absorb and uniformly distort the framework. Usually the framework returns to its original shape when the guests desorb. Sakata et al. (p. 193 ) noted that because surface stress drives this process, it might be avoided in smaller crystals. Indeed, a flexible porous coordination polymer, [Cu ₂ (dicarboxylate) ₂ (amine)] _n , could retain the structure induced by guest molecules such as methanol if crystallites were made sufficiently small (submicrometer scale) and did so to a greater degree as the crystallite dimensions decreased.

Two porous coordination polymers (PCPs) with different pore surface functionality were integrated into one single crystal by face-selective epitaxial growth, leading to BAB-type block PCP crystals with the core crystal A between the second crystals B.