Molecular-based single crystal surfaces as functional substrates for directed metal binding

Abstract

Molecular-based crystals provide faces offering specific chemical groups, ordered in a spatially well-defined array, dependent of the crystal space group. These surfaces therefore offer potential to be exploited as templates for directing material binding in order to construct regular arrays of technologically useful nanomaterials, e.g. preformed nanoparticles, metal clusters formed insitu. The dimensions typically associated with molecular crystalline materials (i.e. unit cell, spacing between repeating chemical groups in crystal structure) are typically on the angstrom scale. This offers potential for the construction of patterned arrays of materials with resolutions that may exceed the capabilities of current conventional lithographic techniques which can typically produce feature sizes down to 10 nm (for 2015, ITRS). Towards this, a series bis-pyridyl derivatives containing metal binding sites, separated by a range of spacing groups (differing in length, rigidity and aromaticity), have been prepared and characterized. Single crystal X-ray diffraction has been used to determine the molecular orientation with respect to each crystal face, and hence assess the usefulness as a substrate for metal deposition. A study into how to further influence the crystal structure, and therefore optimize the likelihood of producing surfaces with the desired binding site pattern, was conducted using a crystal engineering approach to generate polymorphs of previously synthesized compounds by varying the crystallization conditions of solvent and temperature. Models for metal binding have been prepared in the form of ligand-metal complexes and have been studied by X-ray crystallography. Reactions involving silver or zinc salts with the prepared bis-pyridyl ligands afford complexes which identify the likely metal binding sites(s) of the organic compounds and also provide details of the effect of binding with respect to ligand conformation. This information is used to predict if there will be a strong interaction between metal ions and the organic single crystal surfaces, where iii minimal changes in geometry are desired, so the pre-grown crystal substrates are left unaffected by metal deposition. AFM (atomic force microscopy) and XPS (X-ray photoelectron spectroscopy) have been used to study a range of phosphine crystal surfaces before and after metal deposition. AFM provides topological analysis of surfaces and XPS provides chemical analysis. AFM experiments on the different bare crystal faces show a range of rough, smooth and stepped surfaces. It has also been shown here that XPS experiments can be performed on different faces of an organic single crystal, with potential to distinguish between faces based on peak intensity. Similar AFM and XPS experiments have also been performed on crystals after gold nanoparticle deposition and with deposition of gold ions in different oxidation states. These experiments show clear preferential binding to specific crystal faces in line with predictions made from viewing their crystal structures.