Origins of Offset-Stacking in Porous Frameworks

Parallel-displaced pi-stacking in the benzene dimer and larger polycyclic aromatic hydrocarbons is driven by competition between dispersion and exchange-repulsion interactions. The present work examines whether the same is true in porous frameworks that exhibit stacking interactions, including the [18]annulene dimer, porphyrin dimer, and several models of the covalent organic framework known as COF-1. Interaction energies and their components are computed using extended symmetry-adapted perturbation theory along twodimensional scans representing slip-stacking. As in the polycyclic aromatic hydrocarbons studied previously, we find that the van der Waals interaction potential (defined as the sum of dispersion and Pauli repulsion) drives the system into a slip-stacked geometry. Electrostatics is a relatively small component of the total interaction energy. In the case of COF-1, the van der Waals potential drives the conformational preference whether or not a solvent molecule intercalates into the framework, although the presence of the guest (mesitylene) molecule substantially limits the low-energy slip-stacking configurations that are available. Even when the COF-1 pore is empty, a modest lateral offset of less than or similar to 1.5 angstrom is preferred, which is small compared to the pore size.

Automated summary

This study investigates the interaction energies and components in porous frameworks and finds that the van der Waals interaction is the main driving force for parallel-displaced pi-stacking, while electrostatics plays a minor role. The presence of a guest molecule limits the available slip-stacking configurations in COF-1.