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 (sic)1.5 & ANGS; is preferred, which is small compared to the pore size.

Automated summary

In this study, the slip-stacking in porous frameworks, such as COF-1, is investigated by computing interaction energies and their components. It is found that the van der Waals interaction potential drives the system into a slip-stacked geometry, while electrostatics only plays a minor role. Even in the absence of solvent molecules, a modest lateral offset is preferred in COF-1.