Polymorphism in supramolecular chiral structures of R- and S-alanine on Cu(1 1 0)

Abstract

A comprehensive study of the local and supramolecular adsorption structures created by the chiral R- and S-enantiomers of alanine on the Cu(1 1 0) surface has been conducted using a multi-technique approach, including reflection absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). Over the entire 300–470 K temperature range studied, the amino acid is found to adsorb as an alaninate species with a local chiral adsorption motif. However, this singular preference of local chemical form contrasts sharply with the supramolecular organisation at the surface where polymorphism is exhibited. This polymorphic behaviour arises from subtle and dynamic changes in the bonding, orientation and adsorption footprints of individual molecules, leading to alterations in the molecule–metal, intermolecular and metal–metal interactions that dictate self-assembly. Thus, at low coverage, a single disordered phase is observed but at higher coverage, three other temperature dependent phases occur. At room temperature, a two-dimensional equivalent of a ‘nematic’ phase is constructed from short single- and double-chain chiral assemblies that possess a preferred chiral orientation but no long range periodicity. This ‘nematic’ phase acts as a precursor to a highly ordered chiral supramolecular assembly, created at 430 K, that consists of regular arrays of size- and shape-defined chiral clusters. This phase possesses global organisational chirality with only one chiral domain observed for each enantiomer. For both the ‘nematic’ and the highly ordered chiral phase, the organisation for the R-enantiomer is the mirror image of that seen for the S-enantiomer, i.e., there is chirality transfer from the nanoscale to the macroscale. By 470 K, both R- and S-alanine form an achirally organised (3 × 2) structure that appears to be the thermodynamically favoured phase for the alanine/Cu(1 1 0) system. The supramolecular organisation and chirality of the various structures are discussed in terms of the molecular chirality and footprint chirality of the alaninate, together with possible intermolecular interactions and reconstructions of the underlying metal surface atoms. A number of candidate models for the system are suggested, but it is clear that a full understanding of this complex adsorption system will only emerge from further careful, high level experimental and computational efforts that currently remain a challenge.