Synthetic Bilayers on Mica from Self-Assembly of Hydrogen-Bonded Triazines

Abstract

This study describes organic thin films prepared under a range of conditions from a model series of bis-N-alkyl chloro-triazines functionalized with short alkyl chains from ethyl to hexyl. The pure films were characterized using atomic force microscopy (AFM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). When cast on mica, these compounds assemble as crystalline sheets made up of a synthetic bilayer along the crystallographic ab-plane with an internal hydrogen-bonded domain between external alkyl chains. These micron-scale surfaces stack along the c-axis, and increasing the alkyl chain length results in changes to the crystal morphology from needles to nanoscale plates. Thicker films produce nanoscale, pyramidal stacks of bilayers. Compared to atomically flat mica, a rougher, unetched silicon substrate produced irregular domains in the secondary bilayer. Films of mixtures comprising the ethyl derivative with butyl, pentyl, or hexyl derivative were imaged using time-offlight secondary-ion mass spectrometry (ToF-SIMS) that indicated a trend toward a constant stoichiometry with increasing alkyl chain length. AFM of mixed films on mica showed single bilayers of height <2 nm, with an acceptable correlation to the XRD measurements, supporting a constant stoichiometry. These materials permit easy modification of mica to a micron-scale, atomically flat hydrophobic surface, and the use of mixtures with different alkyl chain lengths suggests a method to improve the quality of functional organic thin films.

Short Fluoroalkanes Suppress a Thermally-Induced Phase Transition in a Layered Organic Crystal

“…Given the similarity in packing between the room-temperature crystal structures of C3 and C3-F, we explored the possibility of producing a mixed crystal as an additional tool for crystal engineering in this system [33]. There is precedence for producing molecular mixtures of these N-hydrocarbon triazines, and they have been shown to form larger, highly ordered bilayers on mica than pure molecular crystals [9]. Hence, preparing mixed molecular crystals could be a useful tool to tune the structural properties of this crystal system.…”

“…These layered systems are more than just conceptualised visualisations of their single crystals. They have been shown to adopt highly ordered, ultrathin bilayer films with controlled thickness on mica and silicon substrates [9]. pid bilayers.…”

“…These layered systems are more than just conceptualised visualisations of their single crystals. They have been shown to adopt highly ordered, ultrathin bilayer films with controlled thickness on mica and silicon substrates [9]. As with lipid bilayers found at interfaces and in biological systems, the alkyl chains in these systems are ordered at low temperatures but show a structural transition on warming as the alkyl chains become disordered.…”