Search

Graphene-based nanostructures exhibit electronic properties that are not present in extended graphene. For example, quantum confinement in carbon nanotubes and armchair graphene nanoribbons leads to the opening of substantial electronic bandgaps that are directly linked to their structural boundary conditions (1,2). Nanostructures with zigzag edges are expected to host spinpolarized electronic edge states and can thus serve as key elements for graphene-based spintronics (3). The edge states of zigzag graphene nanoribbons (ZGNRs) are predicted to couple ferromagnetically along the edge and antiferromagnetically between the edges (4), but direct observation of spin-polarized edge states for zigzag edge topologies--including ZGNRs--has not yet been achieved owing to the limited precision of current top-down approaches (5-10). Here we describe the bottom-up synthesis of ZGNRs through surface-assisted polymerization and cyclodehydrogenation of specifically designed precursor monomers to yield atomically precise zigzag edges. Using scanning tunnelling spectroscopy we show the existence of edge-localized states with large energy splittings. We expect that the availability of ZGNRs will enable the characterization of their predicted spin-related properties, such as spin confinement (11) and filtering (12,13), and will ultimately add the spin degree of freedom to graphene-based circuitry., Graphene nanoribbons with armchair edges (AGNRs) can be synthesized using a bottom-up approach based on surface-assisted polymerization and subsequent cyclodehydrogenation of oligophenylene precursor monomers (14). On-surface synthesis has been applied [...]

Graphene nanoribbons--narrow and straight-edged stripes of graphene, or single-layer graphite--are predicted to exhibit electronic properties that make them attractive for the fabrication of nanoscale electronic devices (1-3). In particular, although the two-dimensional parent material graphene (4,5) exhibits semimetallic behaviour, quantum confinement and edge effects (2,6) should render all graphene nanoribbons with widths smaller than 10 nm semi-conducting. But exploring the potential of graphene nanoribbons is hampered by their limited availability: although they have been made using chemical (7-9), sonochemical (10) and lithographic (11,12) methods as well as through the unzipping of carbon nanotubes (13-16), the reliable production of graphene nanoribbons smaller than 10 nm with chemical precision remains a significant challenge. Here we report a simple method for the production of atomically precise graphene nanoribbons of different topologies and widths, which uses surface-assisted coupling (17,18) of molecular precursors into linear polyphenylenes and their subsequent cyclodehydrogenation (19,20). The topology, width and edge periphery of the graphene nanoribbon products are defined by the structure of the precursor monomers, which can be designed to give access to a wide range of different graphene nanoribbons. We expect that our bottom-up approach to the atomically precise fabrication of graphene nanoribbons will finally enable detailed experimental investigations of the properties of this exciting class of materials. It should even provide a route to graphene nanoribbon structures with engineered chemical and electronic properties, including the theoretically predicted intraribbon quantum dots (21), superlattice structures (22) and magnetic devices based on specific graphene nanoribbon edge states (3)., Figure 1 sketches the basic graphene nanoribbon (GNR) fabrication steps for the prototypical armchair ribbon6 of width N = 7 obtained from 10,10'-dibromo-9,9'-bianthryl precursor monomers. Thermal sublimation ofthe monomers onto [...]

The active steering of the pathways taken by chemical reactions and the optimization of energy conversion processes (1-3) provide striking examples of the coherent control of quantum interference through the use of shaped laser pulses. Experimentally, coherence is usually established by synchronizing a subset of molecules in an ensemble (4-7) with ultra-short laser pulses [8]. But in complex systems where even chemically identical molecules exist with different conformations and in diverse environments, the synchronized subset will have an intrinsic inhomogeneity that limits the degree of coherent control that can be achieved. A natural--and, indeed, the ultimate--solution to overcoming intrinsic inhomogeneities is the investigation of the behaviour of one molecule at a time. The single-molecule approach (9,10) has provided useful insights into phenomena as diverse as biomolecular interactions (11-13), cellular processes (14) and the dynamics of supercooled liquids (15) and conjugated polymers (16). Coherent state preparation of single molecules has so far been restricted to cryogenic conditions (17), whereas at room temperature only incoherent vibrational relaxation pathways have been probed (18). Here we report the observation and manipulation of vibrational wave-packet interference in individual molecules at ambient conditions. We show that adapting the time and phase distribution of the optical excitation field to the dynamics of each molecule results in a high degree of control, and expect that the approach can be extended to achieve single-molecule coherent control in other complex inhomogeneous systems., For few-atom molecules, coherent control schemes can be designed on the basis of theory (19). For more complex systems, such as large (bio-)molecules at ambient conditions, ab initio quantum mechanical [...]

The diffusional and orientational behavior of single terrylene diimide derivative (TDI) dyes molecules diffusing in cetyltrimethylammoniumbromide (CTAB)-templated pores of hexagonal mesoporous films are analyzed. The results have shown that the diffusion of TDI in the mesoporous film is more complicated due to the presence of adsorption sites in which the TDI molecules is occasionally trapped.

The effects of different surfactant solutions on the photophysics of various watersoluble perylene diimides (WS-PDIs) are studied. The surfactants are found to increase the fluorescence quantum yield and lifetime of these WS-PDIs.

A newly developed adsorption process for a site- and orientation-selective anchoring of prototypical molecular building blocks on appropriate templates is described. The various data-sets obtained from the analysis shows that the specific adsorption angle observed is only due to the adsorption on the kink sites, as it requires the lowest energy.

The largest chemically monodisperse molecules to date, up to 28nm at 271.6 kDa for the sixth generation is synthesized using the divergent polyphenylene dendrimer synthesis. Self-assembly is inextricably linked with the synthesis as well as analysis and gives hitherto access to tremendous 3D nanostructural control.

A new terrylene diimide-based dye (WS-TDI) that is soluble in water was synthesized and its photophysical properties are investigated. A single molecule study of WS-TDI in polymeric films of PVA and PMMA reveals excellent photostability with respect to photobleaching, far above the photostability of other common water-soluble dyes, such as oxazine-1, sulforhodamine-B, and a water-soluble perylenediimide derivative.

The magnetic fields in substituted hexabenzocoronenes (HBC) are effective for producing oriented films of discotic liquid-crystalline materials, which allows the investigation of structure-property relationships in magnetically aligned films formed in field-effect transistor structures in a noninvasive, contact-free way. Since the magnetic field alignment is applicable to a wide range of self-assembled systems, this approach can be used for optimizing the properties of functional materials.

The structural evolution of hexakis(n-dodecyl)-peri-hexabenzocoronene (HBC-C12) on an n-C50H102 monolayer acting as a template is investigated. The results constitute an important step toward controlling the growth and structure of highly ordered monolayers of functional conjugated molecules by modifying the graphite surface with an n-alkane monolayer of appropriate chain length.

Polyphenylene dendrimers possessing a defined number of keto groups in the dendritic scaffold is synthesized by using benzophenone-functionalized tetraphenylcyclopentadienone branching unit. Results reveal a declocalization of the charge/spin into the polyphentlene dendritic arms leading to a stabilization of the ions/radicals.

The structure of a thin zone-cast film of the hexa-n-dodecyl-substituted hexa-peri-benzocovonene (HBC) was investigated using grazing incidence X-ray diffraction is discussed. It is found that the relatively long side chains of dodecyl were in an ordered interdigitated state.

Single-molecule fluorescence spectroscopy is performed on a number of multi-peryleneimide substituted polyphenylene dendrimers for the purpose of investigating the parameters, which influence the efficiency of the dendrimers as single-photon sources at room temperature. Analysis of fluorescence intensity, lifetime, and interphoton arrival time distribution revealed that all measured first-generation dendrimers behave as single-photon emitters.

We use single-molecule fluorescence lifetimes to probe dynamics of photoinduced reversible electron transfer occurring between triphenylamine (donor) and perylenediimide (acceptor) in single molecules of a polyphenytenic rigid dendrimer embedded in polystyrene. Here, reversible electron transfer in individual donor-acceptor molecules results in delayed fluorescence that is emitted with a high photon count rate. By monitoring fluorescence decay times on a photon-by-photon basis, we find fluctuations in both forward and reverse electron transfer spanning a broad time range, from milliseconds to seconds. Fluctuations are induced by conformational changes in the dendrimer structure as well by polystyrene chain reorientation. The conformational changes are related to changes in the dihedral angle of adjacent phenyl rings located in the dendritic branch near the donor transferring the charge, a torsional motion that results in millisecond fluctuations in the 'through-bond' donor-acceptor electronic coupling. Polymer chain reorientation leads to changes in the local polarity experienced by the donors and to changes in the solvation of the charge-separated state. As a result, switching between different donor moieties within the same single molecule becomes possible and induces fluctuations in decay time on a time scale of seconds.

The photophysical properties of a new water-soluble fluorescent dye are investigated. Hydrophilic substituents in the bay region are attached for this purpose.

Two kinds of carbon nanotubes (CNTs) are selected to investigate hole-edge reactivity such as single-wall carbon nanotubes (SWNTs) and single-wall carbon nanohorns (SWNHs). The study is helpful to the understanding of hole-edge chemistry and would enable the production of well-designed carbonaceous materials with high functionality.

The electron-transfer process in mono- (NlP1) and multichromophoric (NlP3) triphenylamine core dendrimers carrying one and three peryleneimide electron accepter chromophores, are characterized using the combination of transient absorption and fluorescence experiments performed on a femtosecond to nanosecond time scale.

Addition of arylithium followed by ring closure with boron trifluoride produced a blue-emitting ladder-type pentaphenylene. Bromination followed by reductive polymerization with nickel (0) gave new high molecular mass polymers, which show efficient blue emission with a very small Stokes shift.

Large dentritic oilgophenylenes containing benzene and tetraphenylmethane cores are transformed through oxidative cyclodehydrogenation to novel propeller-shaped molecules with large polycyclic aromatic hydrocarbon units in a polymer analogous approach. Structure analysis is performed by a combination of MADI-TOF mass spectrometry, UV/vis, fluorescence, and Raman spectroscopy using solid-state preparation methods.

We demonstrate measurements of the efficiency of competing Forster-type energy-transfer pathways in single bichromophoric systems by monitoring simultaneously the fluorescence intensity, fluorescence lifetime, and the number of independent emitters with time. Peryleneimide end-capped fluorene trimers, hexamers, and polymers with interchromophore distances of 3.4, 5.9, and on average 42 nm, respectively, served as bichromophoric systems. Because of different energy-transfer efficiencies, variations in the interchromophore distance enable the switching between homoenergy transfer (energy hopping), singlet-singlet annihilation, and singlet-triplet annihilation. The data suggest that similar energy-transfer pathways have to be considered in the analysis of single-molecule trajectories of donor/acceptor pairs as well as in natural and synthetic multichromophoric systems such as light-harvesting antennas, oligomeric fluorescent proteins, and dendrimers. Here we report selectively visualization of different energy-transfer pathways taking place between identical fluorophores in individual bichromophoric molecules.

The directional Forster resonance energy transfer (FRET) process that takes place in single molecules of a first (T1P4) and a second (T2P8) generation of a perylenemonoimide (P)-terrylenediimide (T)-based dendrimer in which the chromophores are separated by rigid polyphenylene arms is studied using a single-molecule spectroscopy. Simultaneous emission from donor and acceptor chromophore is observed when high excitation powers are applied on either single T1P4 or T1P8 molecules.

The synthesis of a polyphenylene dendrimer carrying three perylenemonoimide dyes and one biotin group is presented. The specific binding of the dendrimer-detergent complexes carrying a single biotin group to the protein Streptavidin is demonstrated using a magnetic bead assay.

The time-resolved fluorescence kinetics of a first and a second generation dendrimer with peryleneimide chromophores at the rim and a terrylenediimide chromophore in the core were investigated. Analysis of the in-the-bay area substituted terrylenediimide model compound reveals subsets of molecules showing different relaxation pathways of the terrylene core.

The facile color tuning of fluorene-based polymers is discussed. The perylene dyes are attached to polyfluorene for this purpose and the methods of incorporating the dyes in the polymer mainchain are described.

A complex between poly(ethylene oxide)-block-poly(L-lysine) and a hexa-peri-hexabenzocoronene is examined. It is found that poly(L-lysine) blocks wherein each helix is surrounded by six discotic columns of hexa-peri-hexabenzocoronene forms alpha-helical secondary structure.

The electron-transfer process of a first generation dendrimer with a triphenylamine core substituted with one peryleneimide chromophore at the rim (N1P1) was investigated by steady-state and time-resolved spectroscopic techniques in two different solvents of medium and low polarity. Single photon counting experiments showed a fast charge separation and thermally activated back reaction, which is uncommon for polyaryl bridge or long distance through-space electron transfer.

A submolecularly resolved scanning tunneling (STM) investigation of the self-assembly of different enantiomerically pure (S) or (RS) hydrocarbon-substituted hexabenzocoronenes (HBCs) with and without a phenylene spacer into monolayers highly ordered at the graphite-solution interface is reported. The HBC layer exhibit properties of a molecular-level machine, where the stimulated switch between two different positions with respect to the substrate is controlled by intermolecular interactions.

It has been possible to develop a new synthetic method leading to asymmetrically substituted polyphenylene dendrimers. This work also involved demonstrating the synthesis of a multichromophore made up of perylene and terrylene chromophores.

Polyphenylene dendrimers with many peryleneimide donor chromophores and a terryleneimide acceptor trap chromophore were studied to determine intramolecular energy hopping and energy trapping.

This review is focused on the analysis of the macromolecular and supramolecular structures obtained upon the use of benzene as a building block. The presence or the lack of the pi-conjugation resulting from the aromatic benzene unit repetition in different structural motifs has been surveyed and discussed.

It has been possible to establish synthetic approaches to two nonaggregating polyfluorenes using pentaphenylene dendrons as shielding and solubilizing groups. Some research was undertaken into their electronic properties.

Four series of conjugated oligomers are examined by AM1-CAS-CI method with the purpose to describe the adiabatic lowest one-and two-photon excited states. For the one-photon exciton there is an increase from p-phenyleneethynylene (PE) to p-phenylenevinylene (PV) to p-phenylene (PP), but these fall too short to explain the Stokes' shift in the latter.

A new study uses time-resolved fluorescence excitation spectroscopy to investigate the correlation between ground state conformation and excited state dynamics in a multichromophoric dendrimer.

A study was conducted to analyze the pi-pi packing in, and investigate the structure and dynamics of, hexa-n-dodecyl-hexa-peri-hexabenzocoronene using 1H magic-angle spinning and 1H double-quantum, magic-angle spinning nuclear magnetic resonance spectroscopy. Three distinct aromatic resonances were resolved in the single-quantum magic-angle spinning spectrum for the crystalline phase. Experimental methodology and other results are discussed.

The photoluminescence, electrochemistry, UV-vis spectrophotometry and electrogenerated chemiluminescence of perylenedicarboxylic imide, perylenetetracarboxylic diimide, terrylenetetracarboxylic diimide and quaterrylenecarboxylic diimide were investigated. All compounds undergo two reversible one-electron reductions and one reversible one-electron oxidation reaction. The first reduction potential shifts to less negative values and the potential for oxidation to less positive values for the diimide series with increasing size of the aromatic core.

Research was conducted to examine the synthesis of the bay region diketone 2,3:10,11-dibenzoperylene-1,12-dione. The dianion was synthesized in two steps from 9-bromophenanthrene. The chemical shift difference observed for the C6 and C1 carbons and protons provide evidence for the high charge localization at only one bay region of the dianion. Results suggest that an unusual charge 'Umpolung' observed to exist upon reduction of a polycyclic aromatic hydrocarbon system is essential for the synthesis of bay region quinones.

An attempt to examine dipole properties of the lowest excited singlet state for the oligorylene series (1Ag, 1B3u, and 2Ag) using the semiempirical PM3 model with configuration interaction showed that the lowest vertical one-photon transition to 1B3u is below the two-photon 2Ag. Order in the excited electronic states are conditioned by structural changes, which reverse the lowest adiabatic excited states for larger oligomers.

The Diels-Alder reaction of o-quinodimethanes to buckminsterfullerene C60 is described. The reaction produces carboxy-disubstituted adduct mixture which affords thermally stable ester and amine derivatives via the carboxylic acid chloride and active ester routes, respectively. The purification and characterization of the derivatives are presented.

Heterocyclic 1, 4-benzothiazino(2,3-b)phenothiazine is prepared by a process involving Smiles rearrangement and nitrene-induced cyclization. 1H-NOE experiments exhibit the rearrangement-induced 'migration' of the alkyl substituents. X-ray crystallography shows the presence of two hydrogen-bonded solvent molecules per donor molecule. The pentacene core is distorted. Two crystallographically different molecules are found per single crystal.

The mechanism of the intramolecular valence isomerization of 2,2'-distyrylbiphenyl in a reductive environment is investigated by spectroscopic, preparative and electrochemical methods. Results show that the reduction of 2,2'distyrylbiphenyl involves a complex rearrangement sequence which may result to 9,10-dibenzyl-9, 10-dihydrophenanthrene or to its cyclobutane derivative, depending on the conditions of the final quenching step.