  As a new class of two-dimensional (2D) materials, 2D covalent organic
frameworks (COFs) are proven to possess remarkable electronic and magnetic
properties. However, their mechanical behaviours remain almost unexplored. In
this work, taking the recently synthesised dimethylmethylene-bridged
triphenylamine (DTPA) sheet as an example, we investigate the mechanical
behaviours of 2D COFs based on molecular dynamics simulations together with
density functional theory calculations. A novel phase transformation is
observed in DTPA sheets when a relatively large in-plane compressive strain is
applied to them. Specifically, the crystal structures of the transformed phases
are topographically different when the compressive loading is applied in
different directions. The compression-induced phase transformation in DTPA
sheets is attributed to the buckling of their kagome lattice structures and is
found to have significant impacts on their material properties. After the phase
transformation, Young's modulus, band gap and thermal conductivity of DTPA
sheets are greatly reduced and become strongly anisotropic. Moreover, a large
in-plane negative Poisson's ratio is found in the transformed phases of DTPA
sheets. It is expected that the results of the compression-induced phase
transformation and its influence on the material properties observed in the
present DTPA sheets can be further extended to other 2D COFs, since most 2D
COFs possess a similar kagome lattice structure.
