Noncovalent Bonding Caught in Action: From Amorphous to Cocrystalline Molecular Thin Films

We
demonstrate the solvent-free amorphous-to-cocrystalline transformations
of nanoscale molecular films. Exposing amorphous films to vapors of
a haloarene results in the formation of a cocrystalline coating. This
transformation proceeds by gradual strengthening of halogen-bonding
interactions as a result of the crystallization process. The gas–solid
diffusion mechanism involves formation of an amorphous metastable
phase prior to crystallization of the films. In situ optical microscopy shows mass transport during this process, which
is confirmed by cross-section analysis of the final structures using
focused ion beam milling combined with scanning electron microscopy.
Nanomechanical measurements show that the rigidity of the amorphous
films influences the crystallization process. This surface transformation
results in molecular arrangements that are not readily obtained through
other means. Cocrystals grown in solution crystallize in a monoclinic
centrosymmetric space group, whereas the on-surface halogen-bonded
assembly crystallizes into a noncentrosymmetric material with a bulk
second-order nonlinear optical response.