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A combined theoretical and experimental study has been carried out to investigate in detail the electronic structure at the boroxine–gold interface. Tripyrene boroxine adsorbs with a flat geometry on Au(111), and the interaction is driven by dispersion forces. Near-edge X-ray absorption measurements, supported by density functional theory calculations, give strong evidence of the presence of interfacial hybrid unoccupied final states while B 1s resonant photoemission spectroscopy is used to identify a super participator decay process after the B 1s core-hole creation. The charge transfer injection mechanism implied by the B 1s core-hole decay process is accounted for by the calculated energy shift of the lowest unoccupied molecular orbital, which, upon B 1s ionization, partially falls below the Fermi level.

The energy level alignment in the heterojunction formed by tetracene and copper phthalocyanine grown on the Ag(111) substrate has been studied for two different sequences of layer stacking. Filled and empty molecular levels as well as charge transfer states have been characterized by combining ultraviolet and two-photon photoemission spectroscopies. It is shown that the layer in contact with the substrate determines the molecular arrangements of the whole system, thus inducing different interface dipoles at the heterojunction, depending on the stacking sequence. Such dipoles strongly influence the exciton dynamics and charge separation at the interface. The accumulation of the charge carriers in the heterojunction has been observed by measuring the transient shift of core level photoemission lines, which arises after the resonant excitation of either of the two molecular constituents.

We investigate the early stages of the growth of zinc-phthalocyanine on Al(100) using X-ray photoemission spectroscopy (XPS) and low-energy electron diffraction (LEED). Diffraction patterns show a (5 × 5) reconstruction, characteristic of flat-lying molecules forming a long-range-ordered structure with a square unit cell. The degree of ordering (i.e., the average domain size) is increased when the substrate is kept above 100 °C during the deposition. At low coverage (≤1 ML), a sizeable charge transfer from the substrate to the molecules is observed, indicating a strong interaction at the organic–inorganic interface. As a consequence of charge filling of ZnPc LUMO, a self-demetalation of the molecule occurs while the structure of the ligand remains mostly unaffected.

The role of additives and thermal treatment in the formation of donor–acceptor copolymer organic films of PFO–DBT (poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole]) with increased transport properties is addressed by resonant Auger photoemission and core-hole clock spectroscopy, which allows an analysis of the competition between the inner shell core-hole lifetime and the motion of photoexcited electrons on a femtosecond time scale. From the branching of the competing core-hole decay channels, we study the delocalization dynamics of excited electrons over empty molecular orbitals. We find evidence of ultrafast charge-carrier transfer from specific orbitals (LUMO+1) and increased coupling in copolymer assemblies when a solvent additive (1,8 diiodooctane) is added and samples are post-treated with thermal annealing. Relative conformational energies and core-hole spectra were calculated by time-dependent density functional theory.

Electronic structure of 4-hydroxycyanobenzene in the gas phase, thick films, and single crystals has been investigated by X-ray photoemission spectroscopy (XPS) and near edge X-ray absorption fine structure spectroscopy (NEXAFS). We have used resonant photoemission spectroscopy (RESPES) to identify the symmetry and atomic localization of the occupied and unoccupied molecular orbitals for the free molecule. Upon condensation into a thick film, we find XPS energy shifts in opposite directions for the oxygen and nitrogen core levels, consistent with the formation of an intermolecular hydrogen bond. This interaction is also accompanied by a significant spatial distortion of the lowest unoccupied molecular orbital that is displaced from the nitrogen atom, as indicated by the RESPES measurements. Thick films and single crystals display the same dichroism in polarization dependent NEXAFS, indicating that the intermolecular hydrogen bonding also steers the molecular assembly into a preferred molecular orientation.

Charge transfer through noncovalent interactions is crucial to a variety of chemical phenomena. These interactions are often weak and nonspecific and can coexist, making it difficult to isolate the transfer efficiency of one type of bond versus another. Here, we show how core-hole clock spectroscopy can be used to measure charge transfer through noncovalent interactions. We study the model system 1,4-benzenediamine molecules bound on an Au surface through an Au–N donor–acceptor bond as these are known to provide a pathway for electronic conduction in molecular devices. We study different phases of the molecule/Au system and map charge delocalization times from carbon and nitrogen sites on the molecule. We show that charge delocalization across Au–N donor–acceptor bond occurs in less than 500 as. Furthermore, the Au–N bond also enhances delocalization times from neighboring carbon sites, demonstrating that fast charge transfer across a metal–organic interface does not require a covalently bonded system.

The self-assembly of the amino acid L-methionine on Cu(111) was investigated under ultrahigh vacuum (UHV) conditions by scanning tunneling microscopy (STM), helium atom scattering (HAS) and X-ray photoelectron spectroscopy (XPS). The system is strongly influenced by the substrate reactivity and the deposition temperature. The STM and HAS structural analysis yields that, for temperatures below 273 K, the biomolecules assemble in strings oriented with an angle of −10° with respect to the ⟨110⟩ axes of the substrate. For temperatures above 283 K, a regular and ordered one-dimensional (1D) phase arises following an angle of +10° with respect to the same directions. High resolution STM data of this ordered 1D arrangement evidence molecular dimerization and dimer alignment into ordered chains which are commensurate with the Cu(111) atomic lattice. XPS measurements reveal that the high temperature ordered phase consists of an exclusively anionic ensemble with a deprotonated carboxylic group and a neutral amino group, while the low temperature phase is heterogeneously composed of both zwitterionic and anionic species, depending on whether the molecules are immobilized in clusters of dimers on the free terraces or at the low-coordinated adsorption sites of the substrate step-edges. These combined results evidence a structural transformation of the supramolecular assembly which is triggered by a thermally activated process involving the underlying Cu(111) substrate and which carries the intrinsic chiral signature of the adsorbed molecular units.

The structure of ultrathin Cu-phthalocyanine (Cu-Pc) films on the (1 × 2)-Au(110) surface has been studied. The overlayer deposition has been monitored in real time by helium atom scattering (HAS) and low energy electron diffraction (LEED). Throughout the monolayer regime the Cu-Pc molecules are systematically observed to line-up edge-to-edge along the [11̅0] direction of the Au substrate, yielding a commensurate 5-fold periodicity (14.4 Å). Cu-Pc chains deconstruct the 2-fold Au missing row order in the early stage of deposition. A set of higher order periodicities (5-, 7-, and 3-fold) are progressively observed along [001] with increasing Cu-Pc deposition, the 3-fold phase appearing at the monolayer saturation coverage. The corresponding molecular orientation has been studied by variable polarization absorption spectroscopy (XAS), whereas the Au substrate structure has been determined by out-of-plane surface X-ray diffraction. The (5 × 5) phase is found to be rather corrugated, and it exhibits a high degree of long-range order yielding the most prominent diffraction pattern. In the (5 × 5) phase, the Cu-Pc chains are found to lift the underneath missing row reconstruction, being separated by residual Au rows. Similarly, in the more compressed 3-fold monolayer phase, the Cu-Pc molecules were formerly found to lie within a shallow (1 × 3) Au reconstruction [Cossaro, A.; et al. J. Phys. Chem. B2004, 108, 14671]. From comparison of the different deposition stages, as measured in real time by HAS, we can draw a comprehensive picture of the system evolution. In fact, the observed periodicities at different coverage are always formed by an array of Cu-Pc chains in shallow troughs that are equally spaced by a number of uncovered Au rows, as dictated by the Cu-Pc coverage. The growth of Cu-Pc arrays in the submonolayer range is thus driven by an interchain repulsion mechanism.