Porosity Prediction through Hydrogen Bonding in Covalent Organic Frameworks

Easy and bulk-scale
syntheses of two-dimensional (2D) covalent
organic frameworks (COFs) represent an enduring challenge in material
science. Concomitantly, the most critical aspect is to precisely control
the porosity and crystallinity of these robust structures. Disparate
complementary approaches such as solvothermal synthesis have emerged
recently and are fueled in part by the usage of different modulators
and acids that have enriched the COF library. Yet, the fundamental
understanding of the integral processes of 2D COF assembly, including
their growth from nucleating sites and the origin of periodicity,
is an intriguing chemical question that needs to be answered. To address
these cardinal questions, a green and easy-to-perform approach of
COF formation has been delineated involving acid-diamine salt precursors.
The role of hydrogen bonding [dav(Namine–H···Oacid); dav signifies the average Namine–H···Oacid distances, i.e., the average distance from the H atom
of the amine to the O atom of the acid] present in the acid-diamine
salts in improving the COFs’ crystallinity and porosity has
further been decoded by thorough crystallographic analyses of the
salt molecules. What is particularly noteworthy is that we have established
the hydrogen-bonding distances dav(Namine–H···Oacid) in the acid-diamine
salts that are pivotal in maintaining the reversibility of the reaction,
which mainly facilitates highly crystalline and porous COF formation.
Moreover, this reactant-structure to the product-quality relationship
has further been utilized for the synthesis of highly crystalline
and porous COFs that are unattainable by other synthetic means.