Effect of BN/CC Isosterism on the Thermodynamics of Surface and Bulk Binding: 1,2-Dihydro-1,2-azaborine vs Benzene

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 1,2-dihydro-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.