Abstract
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.
Highlights
We demonstrate the solvent-free amorphous-to-cocrystalline transformations of nanoscale molecular films
We demonstrated the use of Raman spectroscopy to follow cocrystallization processes in thin films involving halogen bonding
In situ microscopic studies of on-surface transformation suggest that the process begins with thermally induced motion and rearrangement of the tetrahedral polypyridyl molecules on the surface
Summary
An on-surface amorphous-to-cocrystalline transformation has been demonstrated using the consecutive solvent-free deposition of tetrahedral polypyridyl molecules and a fluorinated haloarene. In situ microscopic studies of on-surface transformation suggest that the process begins with thermally induced motion and rearrangement of the tetrahedral polypyridyl molecules on the surface This motion and rearrangement allows the penetration of the fluorinated haloarene from the gas phase into the amorphous organic film. In situ Raman spectroscopy measurements showed that the diffusion process relies on physicochemical interactions between the haloarene and the organic film and involves a halogen-bonded amorphous metastable phase. The on-surface cocrystal acquires a noncentrosymmetric crystal structure exhibiting second-order nonlinear optical properties These cocrystals are grown under conditions favoring thermodynamic stability; in an open system where the vapor pressure of haloarene above the crystal is reduced, they spontaneously transform to an amorphous phase with loss of haloarene over time
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