Dimensionality and Molecular Packing Control of Covalent Organic Frameworks through Pendant Group Design.

Tuning the dimensions and molecular packing geometry of crystalline organic frameworks and polymers represents an important challenge for reticular chemistry. Here we show that for extended structures made of 1,3,6,8-tetrakis(4-aminophenyl)pyrene (PyTTA) linked with methoxy group functionalized terephthalaldehyde aldehydes, simple substituents on the aldehyde linker can have profound structure directing effects due to noncovalent interactions. Specifically, reacting 2,3-dimethoxyterephthalaldehyde with PyTTA gives a 2D covalent organic framework with unique AA-inclined-AA stacking and bilayer pyrene motifs, whereas reacting 2,5-dimethoxyterephthalaldehyde with PyTTA gives a 1D crystalline polymer with AB stacking and isolated pyrene motifs. Both materials show high crystallinity, allowing for atomic resolution structure determination using three-dimensional electron diffraction, and the similarity of their fluorescence properties shows the electronic structures of pyrene-based extended structures mostly depends on the in-plane structures, which is supported by density functional theory calculations. These two pyrene-based extended structures also show different fluorescence responses to organic vapors due to different pore environments. The current work shows the potential of noncovalent interactions in the reticular design of functional organic materials.