Hydrogen-bonded multilayers of self-assembling silanes: structure elucidation by combined Fourier transform infra-red spectroscopy and X-ray scattering techniques

Abstract

Hydrogen-bonded multilayer stacks of laterally interconnected long-chain silanes are a new class of synthetic self-assembling thin film organizates endowed with a somewhat unusual combination of structural and dynamic characteristics. In this paper we present experimental results obtained from a combined Fourier transform infra-red (FTIR) spectroscopic and X-ray scattering study, on the basis of which it is possible to derive a rather detailed picture of some of the main features of the microstructure of these novel multilayer films and their monolayer precursors. The films are shown to be composed of discrete monolayers, coupled to each other in a flexible, non-epitaxial manner, via interlayer multiple hydrogen bonds. The hydrocarbon tails assume a perpendicular average orientation on the layer planes and form a ‘rotator phase’-like hexagonal lattice with a lateral packing density of ca. 21 Å 2 per molecule and a positional coherence length of ca.70 Å. Extensive lateral coupling of the silane head groups appears to be responsible for the high structural robustness and defect self-healing capability of these films, while the interlayer hydrogen bonding accounts for the facile post-assembly intercalation of various polar guest species into their vertically expandable interlayer polar regions. As the SiO bond is too short to permit extensive intralayer polymerization under the steric constraints imposed by the compact packing of the perpendicularly oriented hydrocarbon tails, the observed high interconnectivity of the silane head groups is rationalized in terms of a dynamic equilibrium model involving continuous redistribution of the SiO bonds within a two-dimensional network of oligomeric siloxane and silanol species. This model of dynamic equilibriation of siloxane linkages can also help to explain other intriguing properties of such silane monolayers.