Article pubs.acs.org/JPCC
Charge-Transfer-Induced Magic Cluster Formation of Azaborine Heterocycles on Noble Metal Surfaces Colin J. Murphy,† Daniel P. Miller,‡ Scott Simpson,§ Andrew Baggett,∥ Alex Pronschinske,† Melissa L. Liriano,† Andrew J. Therrien,† Axel Enders,⊥ Shih-Yuan Liu,*,∥ Eva Zurek,*,‡ and E. Charles H. Sykes*,† †
Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260-3000, United States § Department of Physical and Life Sciences, Alfred State, State University of New York College of Technology, Alfred, New York 14802, United States ∥ Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02467, United States ⊥ Department of Physics and Astronomy, University of NebraskaLincoln, Lincoln, Nebraska 68588, United States ‡
S Supporting Information *
ABSTRACT: We report a combined experimental and theoretical study of the adsorption and assembly of a nitrogen−boron-containing heterocycle, 1,2-dihydro-1,2-azaborine, on Au(111) and Cu(111). Despite the inherent molecular dipole moment, the self-assembly behavior is found to be highly surface dependent, with isolated molecules prevalent on Cu(111) and discrete (“magic”) clusters on Au(111). The ability to form clusters of a particular size can be understood in terms of a balance between attractive intermolecular interactions, including directional B−H···H−N dihydrogen bonding, and repulsive forces from Coulombic interactions between the charged molecules dictated by differences in the charge transfer and Pauli repulsion between the adsorbate and the surface. This work highlights the importance of metal−molecule charge transfer in the adsorption and assembly of dipolar molecules on surfaces and demonstrates that their surface-bound properties cannot be predicted a priori from gas-phase dipole moments alone.
1. INTRODUCTION The phenomenon of “magic clustering,” where particular cluster sizes are energetically favored over all other cluster sizes, has been extensively studied in gas-phase cluster physics.1 Experimentally, magic clusters can be identified by their increased abundance and stability as compared to clusters of other sizes. Magic clusters of metal atoms have been shown to predominately form due to stabilization effects such as geometrical considerations2,3 and electronic shell filling4 or because they represent minima in terms of the Gibbs free energy.5 These established concepts are not easily transferred to molecular clusters, and the situation is even more complex for organic clusters adsorbed on surfaces, where different factors driving cluster formation must be considered.6−15 In the simplest case, such as one in which there are no strong dispersion forces, a cluster of organic molecules can appear to be magic if all functional groups it possesses are already involved in intermolecular chemical bonds so that no attachment points for additional molecules exist. A consequence of molecular adsorption onto surfaces is the restriction of directional intermolecular interactions to two rather than three dimensions. This restriction has been used with functional groups that form directional intermolecular inter© XXXX American Chemical Society
actions, such as hydrogen bonds, to control supramolecular assembly.16,17 Dihydrogen bonds are a subgroup of hydrogen bonds with a comparable bond strength and directionality. To date, their potential to template surface-supported 2D assemblies, including magic clusters, has not been examined.18 Additionally, unlike metal atoms, molecules can possess an intrinsic dipole moment, and their interaction with a supporting substrate can result in charge redistribution between the molecule and the surface, leading to intermolecular interactions differing from those in the gas phase, which can be either attractive or repulsive.14,17,19 This redistribution of charge can modify the magnitude and orientation of an existing molecular dipole,6 can induce electrostatic interactions between neighboring molecules,19 and can, in general, cause substrate-mediated intermolecular interactions.14,17,19 The impact and relative importance of charge transfer and dihydrogen bonds on magic cluster formation is not well understood. It is generally assumed that when intrinsic molecular dipoles are present, dipole− dipole interactions will drive surface-bound assembly. However, Received: December 8, 2015 Revised: January 13, 2016
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DOI: 10.1021/acs.jpcc.5b11970 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
2. EXPERIMENTAL SECTION Scanning Tunneling Microscopy. All STM experiments were performed in an Omicron Nanotechnology GmbH lowtemperature microscope at Tufts University, operating under ultra-high-vacuum conditions (
