Molecular engineering of microporous crystals: (I) New insight into the formation process of open-framework aluminophosphates

A reverse temporal evolution crystallization process was applied to the structures of open-framework aluminophosphates to seek the structural information of the specific specie that a single crystal grew from. This process allowed us to obtain the exact structural information of the guest and inorganic species at the very early stage of the formation process. In practice, virtual spheres centered at the crystallographically unique guest species were introduced to detect the relative position (close contacts) of the nearby inorganic species in the three-dimensional (3-D) space. The radius of the sphere was varied from 4.0 to 5.0 Å, or longer. The close contacts of the guest and the inorganic species within the sphere were determined. These close contacts form a core unit with specific configuration and hydrogen-bond network (if available) for each aluminophosphate. On the basis of the analysis of the hydrogen-bond network within the core unit, we propose that there are basic building units of monomer, dimer, and trimer of aluminum and phosphor species at the very early stage of the formation process. The relative positions of the guest and inorganic species within the core unit contained the initial structural information of each aluminophosphate at the onset of the crystallization. This new insight into the formation process of the aluminophosphates is applicable to other crystallization system and will guide the study on the formation mechanism of crystals by providing clear structural information of the target species formed at the beginning of the crystallization.