Long-Range Alignment of Liquid Crystalline Small Molecules on Metal-Organic Framework Micropores by Physical Anchoring

Abstract In this study, we are the first to observe the alignment behaviors of various assembling molecules with different feature sizes and shapes, including rod-, disc-, and column-shaped molecules, on zeolitic imidazolate framework-8 (ZIF-8) surfaces. Among the various rod-shaped 4-alkyl-4′-cyanobiphenyls (nCBs), those with short alkyl chains with less than nine hydrocarbons (e.g., 5CB, 6CB, 8CB, and 9CB) were vertically aligned on the ZIF-8 surface, while nCBs with long alkyl chains with more than 10 hydrocarbons (e.g., 10CB and 11CB) were randomly oriented on the surface. Unlike rod-shaped small molecules, the disc- and column-shaped molecules were randomly oriented on the ZIF-8 surface. A molecular dynamic simulation revealed that the vertical alignment of rod-shaped molecules on the ZIF-8 surface can be attributed to physical anchoring by the ZIF-8 micropores. Most portions of the alkyl chain of nCB with shorter alkyl chain lengths (n ≤ 9) can be inserted into the ZIF-8 aperture, while nCBs with longer alkyl chain lengths (n > 9) cannot be adequately anchored in the ZIF-8 aperture. According to these results, we believe that MOF materials with micropores can offer an effective tool for controlling the orientation of various functional small molecules.