James N. O'Shea

Experimental evidence for sub-3-fs charge transfer from an aromatic adsorbate to a semiconductor

The ultrafast timescale of electron transfer processes is crucial to their role in many biological systems and technological devices. In dye-sensitized solar cells, the electron transfer from photo-excited dye molecules to nanostructured semiconductor substrates needs to be sufficiently fast to compete effectively against loss processes and thus achieve high solar energy conversion efficiencies. Time-resolved laser techniques indicate an upper limit of 20 to 100 femtoseconds for the time needed to inject an electron from a dye into a semiconductor, which corresponds to the timescale on which competing processes such as charge redistribution and intramolecular thermalization of excited states occur. Here we use resonant photoemission spectroscopy, which has previously been used to monitor electron transfer in simple systems with an order-of-magnitude improvement in time resolution, to show that electron transfer from an aromatic adsorbate to a TiO2 semiconductor surface can occur in less than 3 fs. These results directly confirm that electronic coupling of the aromatic molecule to its substrate is sufficiently strong to suppress competing processes.

Vernier templating and synthesis of a 12-porphyrin nano-ring

Templates are widely used to arrange molecular components so they can be covalently linked into complex molecules that are not readily accessible by classical synthetic methods. Nature uses sophisticated templates such as the ribosome, whereas chemists use simple ions or small molecules. But as we tackle the synthesis of larger targets, we require larger templates—which themselves become synthetically challenging. Here we show that Vernier complexes can solve this problem: if the number of binding sites on the template, nT, is not a multiple of the number of binding sites on the molecular building blocks, nB, then small templates can direct the assembly of relatively large Vernier complexes where the number of binding sites in the product, nP, is the lowest common multiple of nB and nT (refs 8, 9). We illustrate the value of this concept for the covalent synthesis of challenging targets by using a simple six-site template to direct the synthesis of a 12-porphyrin nano-ring with a diameter of 4.7 nm, thus establishing Vernier templating as a powerful new strategy for the synthesis of large monodisperse macromolecules.

Self-assembled aggregates formed by single-molecule magnets on a gold surface

The spontaneous ordering of molecules into two-dimensional self-assembled arrays is commonly stabilized by directional intermolecular interactions that may be promoted by the addition of specific chemical side groups to a molecule. In this paper, we show that self-assembly may also be driven by anisotropic interactions that arise from the three-dimensional shape of a complex molecule. We study the molecule Mn(12)O(12)(O(2)CCH(3))(16)(H(2)O)(4) (Mn(12)(acetate)(16)), which is transferred from solution onto a Au(111) substrate held in ultrahigh vacuum using electrospray deposition (UHV-ESD). The deposited Mn(12)(acetate)(16) molecules form filamentary aggregates because of the anisotropic nature of the molecule-molecule and molecule-substrate interactions, as confirmed by molecular dynamics calculations. The fragile Mn(12)O(12) core of the Mn(12)(acetate)(16) molecule is compatible with the UHV-ESD process, which we demonstrate using near-edge X-ray adsorption fine-structure spectroscopy. UHV-ESD of Mn(12)(acetate)(16) onto a surface that has been prepatterned with a hydrogen-bonded supramolecular network provides additional control of lateral organization.

Square, Hexagonal, and Row Phases of PTCDA and PTCDI on Ag−Si(111) × R30°

We have investigated the ordered phases of the perylene derivatives perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and the imide analogue PTCDI on the Ag−Si(111) × R30° surface using scanning tunneling microscopy. We find that PTCDA forms square, hexagonal, and herringbone phases, which coexist on the surface. The existence of a square phase on a hexagonal surface is of particular interest and is a result of a near commensurability between the molecular dimensions and the surface lattice. Contrast variations across the square islands arise from PTCDA molecules binding to different sites on the surface. PTCDI on Ag−Si(111) × R30° forms extended rows, as well as two-dimensional islands, both of which are stabilized by hydrogen bonding mediated by the presence of imide groups. We present models for the molecular arrangements in all these phases and highlight the role of hydrogen bonding in controlling this order.

N 1s x-ray absorption study of the bonding interaction of bi-isonicotinic acid adsorbed on rutile TiO2(110)

N 1s x-ray absorption spectra of bi-isonicotinic acid (2,2′-bipyridine–4,4′-dicarboxylic acid) on rutile TiO2(110) have been studied experimentally and quantum chemically. Differences between multilayer and monolayer spectra are explained by the adsorbate bonding to the substrate. A connection to the electronic coupling in dye-sensitized electrochemical devices is made.

Excited-state charge transfer dynamics in systems of aromatic adsorbates on TiO2 studied with resonant core techniques

Resonant core spectroscopies are applied to a study of the excited electron transfer dynamics on a low-femtosecond time scale in systems of aromatic molecules (isonicotinic acid and bi-isonicotinic acid) adsorbed on a rutile TiO 2 (110) semiconductor surface. Depending on which adsorbate state is excited, the electron is either localized on the adsorbate in an excitonic effect, or delocalizes rapidly into the substrate in less than 5 fs (3 fs) for isonicotinic acid (bi-isonicotinic acid). The results are obtained by the application of a variant of resonant photoemission spectroscopy.

Hydrogen-bond induced surface core-level shift in pyridine carboxylic acids

Intermolecular hydrogen bonding in thick films of pyridine carboxylic acids (isonicotinic; picolinic and bi-isonicotinic acid) evaporated onto rutile TiO2(1 1 0) has been investigated with X-ray ph ...

X-ray absorption and photoemission spectroscopy of zinc protoporphyrin adsorbed on rutile TiO2(110) prepared by in situ electrospray deposition.

Zinc-protoporphyrin, adsorbed on the rutile TiO(2)(110) surface, has been studied using photoemission spectroscopy and near-edge absorption fine structure spectroscopy to deduce the nature of the molecule-surface bonding and the chemical environment of the central metal atom. To overcome the difficulties associated with sublimation of the porphyrin molecules, samples were prepared in situ using ultrahigh vacuum electrospray deposition, a technique which facilitates the deposition of nonvolatile and fragile molecules. Monolayers of Zn protoporphyrin are found to bond to the surface via the oxygen atoms of the deprotonated carboxyl groups. The molecules initially lie largely parallel to the surface, reorienting to an upright geometry as the coverage is increased up to a monolayer. For those molecules directly chemisorbed to the surface, the interaction is sufficiently strong to pull the central metal atom out of the molecule.

Conformation and Packing of Porphyrin Polymer Chains Deposited Using Electrospray on a Gold Surface

Conjugated porphyrin polymers have stimulated great interest due to their potential applications in nonlinear optics, light harvesting and nanoscale charge transport. As with many other organic materials, interfacial properties are likely to play an important role in their applications in molecular electronics. However, it has not so far been possible to study these effects due to the difficulty in preparing suitable monolayers, since the relevant polymers and oligomers cannot be sublimed. A question of particular interest relates to the influence of the flexibility of such a large molecule on the ordering within interfacial regions. We have investigated the adsorption of two oligomers, a porphyrin tetramer (P4, N= 4; see Figure 1 for structural diagrams), a hexamer (P6, N= 6), and a polymer Pn (N = 30–50) on the Au(111) surface using scanning tunneling microscopy (STM). The porphyrin units have long octyloxy side chains to promote solubility in organic solvents. Our experiments are performed at room temperature under ultrahigh vacuum (UHV) conditions (base pressure 5 10 11 Torr) and we use UHV electrospray deposition (UHV-ESD) to transfer the oligomers and polymers directly from solution onto a surface. In our approach to UHV-ESD, a volatilized mixture of solvent and solute molecules is produced in atmosphere by electrospray. This mixture enters the UHV system through a small aperture and is passed through a series of differentially pumped chambers, to the Au(111) substrate (for further details see Supporting Information). UHV-ESD and related approaches have been used to introduce nanotubes, fullerenes, dye molecules, and polymers into a UHV environment. Images acquired after deposition of a sub-monolayer coverage of P6 (Figure 2) show that, despite their large size, the porphyrin oligomers diffuse on the surface and form Figure 1. a) Structure of porphyrin oligomers and polymers. b) P6 molecule with the trihexylsilyl end groups truncated to trimethylsilyl groups for clarity.

Mechanical Stiffening of Porphyrin Nanorings through Supramolecular Columnar Stacking

Solvent-induced aggregates of nanoring cyclic polymers may be transferred by electrospray deposition to a surface where they adsorb as three-dimensional columnar stacks. The observed stack height varies from single rings to four stacked rings with a layer spacing of 0.32 ± 0.04 nm as measured using scanning tunneling microscopy. The flexibility of the nanorings results in distortions from a circular shape, and we show, through a comparison with Monte Carlo simulations, that the bending stiffness increases linearly with the stack height. Our results show that noncovalent interactions may be used to control the shape and mechanical properties of artificial macromolecular aggregates offering a new route to solvent-induced control of two-dimensional supramolecular organization.