The 1,4-Phenylenediisocyanide (PDI) Dimer
Collaboration with Yan Li and Giulia Galli (UC Davis)

Non-covalent interactions, such as H-bonding and dispersion, are known to decisively dictate properties in biological contexts. Less often studied—but equally relevant—is the role of these same interactions in surface properties. Self-assembled monolayers have found widespread use in surface applications, most notably their applicability to molecular electronics. The local structure and global morphology of these monolayers, as well as their mere existence, is almost exclusively due to non-covalent interactions.

In this work, we showed that the structure of a PDI monolayer—a prototype for conjugated isocyano-based molecular junctions—can be accurately predicted by dual-basis MP2 methods. Significant results were obtained on two fronts:

• With gas-phase dimer calculations, we benchmarked the dimer interaction in several isomeric forms. Most notably, the isocyano ligands increased the binding energy (relative to the unsubstituted benzene dimer) by more than a factor of three! The binding was also much more configuration-dependent than the benzene dimer.
  
• Adding surface constraints significantly modified the unhindered gas-phase results. By building pairwise-interaction models, we showed that constraining the PDI molecules to surface binding sites led to specific surface morphologies. We also predicted the most stable surface configuration to be a Herringbone structure—a finding that significantly differs from standard DFT approaches.