Polymorphic, Porous, and Host–Guest Nanostructures Directed by Monolayer–Substrate Interactions: Epitaxial Self-Assembly Study of Cyclic Trinuclear Au(I) Complexes on HOPG at the Solution–Solid Interface

Abstract

Synthesis, crystallographic characterization, and molecular self-assembly of two novel cyclotrimeric gold(I) complexes, Au3[3,5-(COOEt)2Pz]3 (Au3Pz3) and Au3[(n-Pr–O)C═N(Me)]3 (Au3Cb3) was studied. Single crystal X-ray crystallography data reveal that both gold(I) complexes have one-dimensional stacking patterns caused by intermolecular Au(I)···Au(I) aurophilic interactions. The Au3Pz3 trimer units stack with two alternate and symmetrical Au(I)···Au(I) interactions while the Au3Cb3 units have three alternating and nonsymmetrical Au(I)···Au(I) interactions. Molecular self-assembly of the gold(I) complexes on the 1-phenyloctane/highly ordered pyrolytic graphite (HOPG) (0001) solution–solid interface is studied with scanning tunneling microscopy (STM). The gold(I) cyclotrimers form epitaxial nanostructures on the HOPG surface. At a concentration of ∼1 × 10–4 M, Au3Pz3 complexes exhibit a single morphology, while Au3Cb3 complexes exhibit polymorphology. Two polymorphs, one nonporous and the other porous, are observed at 22.0 ± 2.0 °C for Au3Cb3 complexes. A nonporous, low-surface-density (0.82 molecules/nm2) Au3Cb3 nanostructure forms first and then transforms into a high-density (1.43 molecules/nm2) porous nanostructure. This is the first time any porous surface nanostructure is reported for an organometallic system. The porous structure is thought to be stabilized by a combination of hydrogen bonding and monolayer–substrate interactions. These pores are utilized to incorporate pyrene into the film, rendering this the first organometallic host–guest system imaged at the solid–solution interface. Molecular and periodic density functional theory (DFT) calculations shed light on the two-dimensional topography and polymorphic self-assembly revealed by STM; these calculations suggest significant electronic hybridization of the Au3 trimer orbitals and HOPG. The multiple-technique approach used herein provides insights concerning molecule–substrate and molecule–molecule interactions.