Article pubs.acs.org/JPCC
Effect of BN/CC Isosterism on the Thermodynamics of Surface and Bulk Binding: 1,2-Dihydro-1,2-azaborine vs Benzene Colin J. Murphy,† Andrew W. Baggett,‡ Daniel P. Miller,§ Scott Simpson,§,∥ Matthew D. Marcinkowski,† Michael F. G. Mattera,† Alex Pronschinske,† Andrew Therrien,† Melissa L. Liriano,† Eva Zurek,*,§ Shih-Yuan Liu,*,‡ and E. Charles H. Sykes*,† †
Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02467, United States § Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260, United States ∥ School of Science, Penn State Erie, The Behrend College, 4205 College Drive, Erie, Pennsylvania 16563, United States ‡
S Supporting Information *
ABSTRACT: The chemistry of organoboron compounds has long been dominated by their high reactivity in synthetic organic chemistry. Recently, the incorporation of boron as a structural element in compounds has led to an increased diversity of organic compounds. A promising method of boron incorporation is BN/CC isosterism, where the replacement of a CC unit of the ubiquitous arene, benzene, with the isolectronic BN unit results in azaborine compounds whose properties are intermediate between benzene and borazine. These conjugated boron−nitrogen-containing heteroatom compounds show potential for use as charge transport materials in organic electronic devices in which the molecule−contact interface is a crucial factor of device performance. Therefore, to gain a fundamental understanding of the interaction of azaborines with two common metals, we examined 1,2-dihydro-1,2-azaborine and benzene desorption from Au(111) and Cu(111) by temperature-programmed desorption (TPD). Scanning tunneling microscopy imaging and theoretical calculations aided in the interpretation of the TPD results. Comparison between TPD spectra of 1,2-dihydro-1,2-azaborine and benzene allowed us to benchmark our experiments with literature values for benzene and to accurately quantify the magnitude of both molecule−molecule and molecule−surface interaction strengths. TPD spectra of 1,2-dihydro-1,2-azaborine show three well-defined adsorption states exist on each surface, assigned to mono-, bi-, and multilayers. The multilayer desorption energy of azaborine was found to be approximately 46 kJ/mol, about 4 kJ/mol larger than benzene and the increase is related to both dihydrogen bonding and dipole−dipole interactions. The bilayer formed by 1,2dihydro-1,2-azaborine is less dense than that formed by benzene, with 0.7 molecules in the bilayer per each molecule in the monolayer on each surface. Importantly, in terms of application, azaborine did not decompose on either Cu or Au surfaces. Our data also reveal that a delicate balance of molecule−surface and molecule−molecule interactions dictate adsorption energetics in the submonolayer regime.
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INTRODUCTION Boron−nitrogen-containing heteroatom compounds comprise a novel structural motif for organic compounds.1−3 Potential applications of these compounds lie in biomedical research,4 hydrogen storage,5−7 and materials science.8 A particular subgroup of interest is azaborines, which are isosteres of benzene where two carbon atoms are substituted with a boron atom and a nitrogen atom, resulting in a compound that exhibits properties intermediate between benzene and borazine. Three structural isomers of azaborine exist, of which 1,2dihydro-1,2-azaborine (from here on, abbreviated as 1,2azaborine) is the most thermodynamically stable.9,10 1,2Azaborine is the focus of this paper and its chemical structure is shown in Figure 1a. The synthesis,11,12 microwave structure,13 and spectroscopic properties14,15 of 1,2-azaborine © XXXX American Chemical Society
have recently been described, yet information on its bulk and surface binding is lacking. To exploit the novel properties of azaborines as arene surrogates in potential applications involving electrode materials, the fundamentals of their intermolecular interactions and binding to metal surfaces must first be understood. The addition of boron into conjugated systems has been shown to increase luminescence efficiency and improve lightemitting properties of a material.16 Intermolecular interactions, Special Issue: Steven J. Sibener Festschrift Received: December 18, 2014 Revised: April 3, 2015
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DOI: 10.1021/jp5126427 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C in particular π-stacking, have been shown to direct the structure of these conjugated systems. The typical method to chemically stabilize the Lewis acidic boron centers is through steric screening with bulky aryl substituents. These groups can block π-stacking in the solid state and impact charge transport. The Lewis acidic boron centers in azaborine molecules are stabilized through orbital interactions with the lone pair on the adjacent nitrogen atom. This less bulky configuration facilitates πstacking with neighboring molecules.8,17,18 This allows the properties of 1,2-azaborine to be compared to those of the ubiquitous arene, benzene. Adsorption of benzene on coinage metals (Cu, Ag, Au) has been thoroughly examined by surface science and computational techniques.19−27 Benzene has a dipole moment of zero, and all of its hydrogen atoms are weakly acidic. In contrast, the dipole of 1,2-azaborine has been computationally determined as 2.1 D,15 potentially leading to strong dipole−dipole intermolecular interactions. Additionally, the boron-bonded hydrogen atom is hydridic and the nitrogenbonded hydrogen atom is protic,11 giving rise to a strong intermolecular BH···HN dihydrogen bond.28,29 The intermolecular effects of these dihydrogen bonds have been examined by theoretical methods in the related molecule, 1,4-dihydro-1,4azaborine, where the interactions were similar to those observed in standard hydrogen bonds.30 Dihydrogen bonds have been shown to direct crystal- and liquid-phase structures.31 These intermolecular interactions are expected to result in stronger attraction between 1,2-azaborine molecules, resulting in higher boiling and sublimation energies than benzene. Our work examining the desorption of 1,2-azaborine and benzene across a range of initial coverages from submonolayer to multilayers on Au and Cu surfaces allows for the interrogation of both intermolecular and molecular− surface interactions. Benzene adsorption on Au(111) and Cu(111) surfaces has been studied by TPD,23,24 STM,21,32 photoemission spectroscopy,20 and theoretical methods,33,34,48 and any impact of the isosterization will be apparent in comparison to these studies. Benzene adsorbs intact at temperatures
