Kinetic and Mechanistic Study of COF-1 Phase Change from a Staggered to Eclipsed Model upon Partial Removal of Mesitylene

The kinetics of the irreversible phase conversion of covalent organic frameworks-1 (COF-1) has been investigated using time-resolved, in situ environmental X-ray diffraction (EXRD) and modeled with the Avrami-Erofe'ev model. Tightly fitting mesitylene solvent is found to be present in both the AB staggered and AA eclipsed polymorphs, which plays a key role in the phase change. Solid-state NMR (SSNMR) showed the presence of discrete dipolar coupling between residual mesitylene solvent and the framework in both polymorphs, indicative of a host-guest adsorptive interaction. Binding energy calculations indicate two different adsorbed mesitylene configurations in the AB and AA phases, both with short distances to the framework pore walls to generate the observed dipolar coupling. The mechanism of phase change has been illustrated using molecular dynamics simulations and was found to be a displacive transition from AB staggered to AA eclipsed COP-I structures, which was made possible due to low in-plane shear modulus of 2D COFs. Our findings highlight the polymorphic nature of COF-1 material mediated by the interactions with guest molecules and the irreversibility for polymorph formation and conversion.