Tuning UV Absorption in Imine-Linked Covalent Organic Frameworks via Methylation

Covalent organic
frameworks (COFs) are porous materials with high
surface areas, making them interesting for a large variety of applications
including energy storage, gas separation, photocatalysis, and chemical
sensing. Structural variation plays an important role in tuning COF
properties. Next to the type of the building block core, bonding directionality,
and linking chemistry, substitution of building blocks provides another
level of synthetic control. Thorough characterization and comparison
of various substitution patterns is relevant for the molecular engineering
of COFs via rational design. To this end, we have systematically synthesized
and characterized multiple combinations of several methylated and
non-methylated building blocks to obtain a series of imine-based COFs.
This includes the experimental assignment of the COF structure by
solid-state NMR. By comparing the properties of all COFs, the following
trends were found: (1) upon methylation of the aldehyde nodes, COFs
show increased Brunauer–Emmett–Teller surface areas,
reduced pore collapse, blue-shifted absorbance spectra, and ∼0.2
eV increases in their optical band gaps. (2) COFs with dimethylated
amine linkers show a lower porosity. (3) In tetramethylated amine
linkers, the COF porosity even further decreases, the absorbance spectra
are clearly red-shifted, and smaller optical band gaps are obtained.
Our study shows that methyl substitution patterns on COF building
blocks are a handle to control the UV absorbance of the resulting
frameworks.