Impact of Local Molecular Environment on the Decay of Image Potential States

Image potential-derived states of upright chemisorbed benzoate molecules on a Cu(110) surface have been measured with scanning tunneling spectroscopy. The widths of image-derived features for these adsorbates are sensitive to the presence of coexisting flat-lying benzoate molecules. This dependence is attributed to the different couplings between image potential state wave functions and bulk metal states that result from differences in substrate-mediated intermolecular interactions. The comparative trends are semiquantitatively modeled using a dielectric continuum approach

Modification of Molecular Spin Crossover in Ultrathin Films

Scanning tunneling microscopy and local conductance mapping show spin-state coexistence in bilayer films of Fe­[(H₂Bpz₂)₂bpy] on Au(111) that is independent of temperature between 131 and 300 K. This modification of bulk behavior is attributed in part to the unique packing constraints of the bilayer film that promote deviations from bulk behavior. The local density of states measured for different spin states shows that high-spin molecules have a smaller transport gap than low-spin molecules and are in agreement with density functional theory calculations

Charge-Transfer-Induced Magic Cluster Formation of Azaborine Heterocycles on Noble Metal Surfaces

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

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