Supramolecular chemistry has synthesized large and small molecules which host guest molecules for several decades. What started as a way to mimic of enzymes in nature, has exploded into a sea of materials such as porous coordination polymers, low-density metal-organic frameworks, inclusion compounds, and hydrogen bonded frameworks. We previously designed a layered framework consisting of a metal complex with coordinate covalent ligands. These ligands have peripheral carboxylic acid groups which hydrogen bond to organic pillars containing terminal amines. The layered structure is separated by these pillars, which are closed-packed, creating 1-dimensional channels able to co-crystallize molecules. There is interest in selectively binding molecules for separation, catalysis, molecular recognition or transport. How do guests selectively co-crystallize into the framework? Do properties of guest molecules such as size, shape or electronics dictate preference? By establishing a set of selectivity rules, potential applications appear.

In our pursuit, we devised a new way of coupling a thermogravimetric analyzer to a mass spectrometer using solid-phase microextraction fibers. These two instruments can be used together for a fraction existing coupling cost.

By testing guests of different size and shape, we found large guest molecules will co-crystallize over smaller ones. If a guest is too large, the selectivity can become concentration dependent. Maintaining the size difference between two molecules, we changed to geometric isomers. The framework lost selectivity due to poor guest co-crystallization and low guest inclusion rates.

Next, we tested guest molecules whose size and shape was similar but had different electronics. Aromatic guests with electron donating substituents were preferred over those with electron withdrawing groups. The framework could detect subtle changes in the electronic structure, e.g., substituting chloro- for a methyl. Guests containing anchor points, sigma-hole, were showed preference. Selectivity correlated to physical properties such as boiling point and density of the guests containing electron withdrawing substituents.

Finally, we focused on single co-crystallized guests tested by thermogravimetric analysis, gas chromatography, and powder x-ray diffraction. The preferred guests in the previous study contained electron donating groups and high occupancy. Outliers such as iodobenzene were preferred in competition but had low concentrations as a single guest.