Au3-to-Ag3 coordinate-covalent bonding and other supramolecular interactions with covalent bonding strength.

An efficient strategy for designing charge-transfer complexes using coinage metal cyclic trinuclear complexes (CTCs) is described herein. Due to opposite quadrupolar electrostatic contributions from metal ions and ligand substituents, [Au(µ-Pz-(i-C3H7)2)]3·[Ag(µ-Tz-(n-C3F7)2)]3 (Pz = pyrazolate, Tz = triazolate) has been obtained and its structure verified by single crystal X-ray diffraction - representing the 1st crystallographically-verified stacked adduct of monovalent coinage metal CTCs. Abundant supramolecular interactions with aggregate covalent bonding strength arise from a combination of M-M' (Au → Ag), metal-π, π-π interactions and hydrogen bonding in this charge-transfer complex, according to density functional theory analyses, yielding a computed binding energy of 66 kcal mol-1 between the two trimer moieties - a large value for intermolecular interactions between adjacent d10 centres (nearly doubling the value for a recently-claimed Au(i) → Cu(i) polar-covalent bond: Proc. Natl. Acad. Sci. U.S.A., 2017, 114, E5042) - which becomes 87 kcal mol-1 with benzene stacking. Surprisingly, DFT analysis suggests that: (a) some other literature precedents should have attained a stacked product akin to the one herein, with similar or even higher binding energy; and (b) a high overall intertrimer bonding energy by inferior electrostatic assistance, underscoring genuine orbital overlap between M and M' frontier molecular orbitals in such polar-covalent M-M' bonds in this family of molecules. The Au → Ag bonding is reminiscent of classical Werner-type coordinate-covalent bonds such as H3N: → Ag in [Ag(NH3)2]+, as demonstrated herein quantitatively. Solid-state and molecular modeling illustrate electron flow from the π-basic gold trimer to the π-acidic silver trimer with augmented contributions from ligand-to-ligand' (LL'CT) and metal-to-ligand (MLCT) charge transfer.

Connecting Solution-Phase to Single-Molecule Properties of Ni(Salophen).

We present a strong correlation of the Ni(salophen) structure and properties measured in single-molecule vs bulk quantities and in ultra high vacuum vs solution phase. Under a scanning tunneling microscope (STM), Ni(salophen) forms a self-assembled monolayer (SAM) on Au(111) at 23 °C with molecular structure identical to that of the X-ray crystallographic measurement. The HOMO and LUMO levels are determined using elastic tunneling spectroscopy at the single-molecule level with confirmation by monolayer-quantity ultraviolet photoelectron spectroscopy (UPS) and by cyclic voltammetry (CV) measurements. The STM-determined HOMO-LUMO gap of 3.28 eV and (HOMO-1)-HOMO gap of 0.36 eV form a new foundation for the selection of hybrid functionals with a simple basis set to be effective in accurately calculating single-molecule Ni(salophen) frontier MO levels. Our results suggest that microscopy-based experiments on a surface, along with free-molecule gas-phase calculations, can provide useful insights into the physical properties of metal(salen) complexes, especially when such direct measurements are not available in solution.

Cooperativity and coverage dependent molecular desorption in self-assembled monolayers: computational case study with coronene on Au(111) and HOPG.

One of the common practices in the literature of molecular desorption is the comparison of theoretically (mostly using DFT) calculated single molecule adsorption energies with experimental desorption energies from studies like temperature programmed desorption (TPD) etc. Comparisons like those do not consider that the experimental desorption energies are obtained via ensemble techniques while theoretical values are calculated at the single molecule level. Theoretical values are generally based upon desorption of a single molecule from a clean surface, or upon desorption of an entire monolayer. On the other hand, coverage dependent molecule-molecule interactions add to and modify molecule-substrate interactions that contribute to the experimentally determined desorption energies. In this work, we explore the suitability of an additive nearest neighbor model for determining general coverage dependent single molecule desorption energies in non-covalent self-assembled monolayers (SAMs). These coverage dependent values serve as essential input to any model attempting to reproduce coverage dependent desorption or for understanding the time dependent desorption from a partially covered surface. This method is tested using a case study of coronene adsorbed on Au(111) and HOPG substrates with periodic DFT calculations. Calculations show that coronene exhibits coverage and substrate dependence in molecular desorption. We found that intermolecular contact energies in the coronene monolayer are not strongly influenced by the HOPG substrate, while coronene desorption on Au(111) exhibits strong cooperativity where the additive model fails.

Surface directed reversible imidazole ligation to nickel(ii) octaethylporphyrin at the solution/solid interface: a single molecule level study.

Scanning tunneling microscopy (STM) is used to study for the first time the reversible binding of imidazole (Im) and nickel(ii) octaethylporphyrin (NiOEP) supported on highly oriented pyrolytic graphite (HOPG) at the phenyloctane/NiOEP/HOPG interface at 25 °C. The ligation of Im to the NiOEP receptor while not observed in fluid solution is readily realized at the solution/HOPG interface. The coordination process scales with increasing Im concentration and can be effectively modeled by the Langmuir isotherm. At room temperature it is determined that the standard free energy of adsorption is ΔGc = -15.8 kJ mol(-1) and the standard enthalpy of adsorption is estimated to be ΔHc ≈ -80 kJ mol(-1). The reactivity of imidazole toward NiOEP adsorbed on HOPG is attributed to charge donation from the graphite stabilizing the Im-Ni bond. This charge transfer pathway is supported by molecular and periodic modeling calculations which indicate that the Im ligand behaves as a π-acceptor. DFT calculations also show that the nickel ion in the Im-NiOEP/HOPG complex is in a singlet ground state. This is surprising since both our calculations and previous experimental studies find a triplet ground state for the five and six coordinated Im-nickel(ii) porphyrins in the gas-phase or in solution. Both the experimental and the theoretical findings provide information that is useful for better understanding of chemical sensing/recognition and catalytic processes that utilize metal-organic complexes adsorbed on surfaces where the reactivity of the metal is moderated by the substrate.

Effect of dispersion on surface interactions of cobalt(II) octaethylporphyrin monolayer on Au(111) and HOPG(0001) substrates: a comparative first principles study.

A density functional theory study of a cobalt(II) octaethylporphyrin (CoOEP) monolayer on Au(111) and HOPG(0001) surfaces was performed under periodic boundary conditions. Calculations with and without dispersion corrections are performed and the effect of van der Waals forces on the interface properties is analyzed. Calculations have determined that the CoOEP molecule tends to bind at the 3-fold and the 6-fold center sites on Au(111) and HOPG(0001), respectively. Geometric optimizations at the center binding sites have indicated that the porphyrin molecules (in the monolayer) lie flat on both substrates. Calculations also reveal that the CoOEP monolayer binds slightly more strongly to Au(111) than to HOPG(0001). Charge density difference plots disclose that charge is redistributed mostly around the porphyrin plane and the first layer of the substrates. Dispersion interactions cause a larger substrate to molecule charge pushback on Au(111) than on HOPG. CoOEP adsorption tends to lower the work functions of either substrate, qualitatively agreeing with the experimental photoelectron spectroscopic data. Comparison of the density of states (DOS) of the isolated CoOEP molecule with that on gold and HOPG substrates showed significant band shifts around the Fermi energy due to intermolecular orbital hybridization. Simulated STM images were plotted with the Tersoff-Hamann approach using the local density of states, which also agree with the experimental results. This study elucidates the role of dispersion for better describing porphyrin-substrate interactions. A DFT based overview of geometric, adsorption and electronic properties of a porphyrin monolayer on conductive surfaces is presented.

Molecular and electronic structure of cyclic trinuclear gold(I) carbeniate complexes: insights for structure/luminescence/conductivity relationships.

An experimental and computational study of correlations between solid-state structure and optical/electronic properties of cyclotrimeric gold(I) carbeniates, [Au3(RN═COR')3] (R, R' = H, Me, (n)Bu, or (c)Pe), is reported. Synthesis and structural and photophysical characterization of novel complexes [Au3(MeN═CO(n)Bu)3], [Au3((n)BuN═COMe)3], [Au3((n)BuN═CO(n)Bu)3], and [Au3((c)PeN═COMe)3] are presented. Changes in R and R' lead to distinctive variations in solid-state stacking, luminescence spectra, and conductive properties. Solid-state emission and excitation spectra for each complex display a remarkable dependence on the solid-state packing of the cyclotrimers. The electronic structure of [Au3(RN═COR')3] was investigated via molecular and solid-state simulations. Calculations on [Au3(HN═COH)3] models indicate that the infinitely extended chain of eclipsed structures with equidistant Au--Au intertrimer aurophilic bonding can have lower band gaps, smaller Stokes shifts, and reduced reorganization energies (λ). The action of one cyclotrimer as a molecular nanowire is demonstrated via fabrication of an organic field effect transistor and shown to produce a p-type field effect. Hole transport for the same cyclotrimer-doped within a poly(9-vinylcarbazole) host-produced a colossal increase in current density from ∼1 to ∼1000 mA/cm(2). Computations and experiments thus delineate the complex relationships between solid-state morphologies, electronic structures, and optoelectronic properties of gold(I) carbeniates.