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
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Abstract
Synthesis, crystallographic characterization,
and molecular self-assembly
of two novel cyclotrimeric gold(I) complexes, Au<sub>3</sub>[3,5-(COOEt)<sub>2</sub>Pz]<sub>3</sub> (Au<sub>3</sub>Pz<sub>3</sub>) and Au<sub>3</sub>[(<i>n</i>-Pr–O)CN(Me)]<sub>3</sub> (Au<sub>3</sub>Cb<sub>3</sub>) 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 Au<sub>3</sub>Pz<sub>3</sub> trimer units
stack with two alternate and symmetrical Au(I)···Au(I)
interactions while the Au<sub>3</sub>Cb<sub>3</sub> 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<sup>–4</sup> M, Au<sub>3</sub>Pz<sub>3</sub> complexes exhibit a single morphology, while Au<sub>3</sub>Cb<sub>3</sub> complexes exhibit polymorphology. Two polymorphs,
one nonporous and the other porous, are observed at 22.0 ± 2.0
°C for Au<sub>3</sub>Cb<sub>3</sub> complexes. A nonporous, low-surface-density
(0.82 molecules/nm<sup>2</sup>) Au<sub>3</sub>Cb<sub>3</sub> nanostructure
forms first and then transforms into a high-density (1.43 molecules/nm<sup>2</sup>) 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 Au<sub>3</sub> trimer orbitals and
HOPG. The multiple-technique approach used herein provides insights
concerning molecule–substrate and molecule–molecule
interactions