Felix R. Fischer

Direct Imaging of Covalent Bond Structure in Single-Molecule Chemical Reactions

Watching Organic Reactions Single-molecule studies can overcome the difficulty of inferring the various outcomes of reactions in ensemble measurements. De Oteyza et al. (p. 1434, published online 30 May; see the Perspective by Giessibl) used a variation of noncontact atomic force microscopy in which the imaging tip was derivatized with a single CO molecule to obtain subnanometer-resolution images of conjugated organic molecules undergoing reaction on a silver surface. Different thermally induced cyclization reactions of oligo- (phenylene-1,2-ethynylenes) were observed. Noncontact atomic force microscopy imaged the bond structure of an adsorbed organic reactant and its cyclization products. [Also see Perspective by Giessibl] Observing the intricate chemical transformation of an individual molecule as it undergoes a complex reaction is a long-standing challenge in molecular imaging. Advances in scanning probe microscopy now provide the tools to visualize not only the frontier orbitals of chemical reaction partners and products, but their internal covalent bond configurations as well. We used noncontact atomic force microscopy to investigate reaction-induced changes in the detailed internal bond structure of individual oligo-(phenylene-1,2-ethynylenes) on a (100) oriented silver surface as they underwent a series of cyclization processes. Our images reveal the complex surface reaction mechanisms underlying thermally induced cyclization cascades of enediynes. Calculations using ab initio density functional theory provide additional support for the proposed reaction pathways.

Tuning the band gap of graphene nanoribbons synthesized from molecular precursors.

A prerequisite for future graphene nanoribbon (GNR) applications is the ability to fine-tune the electronic band gap of GNRs. Such control requires the development of fabrication tools capable of precisely controlling width and edge geometry of GNRs at the atomic scale. Here we report a technique for modifying GNR band gaps via covalent self-assembly of a new species of molecular precursors that yields n = 13 armchair GNRs, a wider GNR than those previously synthesized using bottom-up molecular techniques. Scanning tunneling microscopy and spectroscopy reveal that these n = 13 armchair GNRs have a band gap of 1.4 eV, 1.2 eV smaller than the gap determined previously for n = 7 armchair GNRs. Furthermore, we observe a localized electronic state near the end of n = 13 armchair GNRs that is associated with hydrogen-terminated sp(2)-hybridized carbon atoms at the zigzag termini.

Molecular bandgap engineering of bottom-up synthesized graphene nanoribbon heterojunctions

Bandgap engineering is used to create semiconductor heterostructure devices that perform processes such as resonant tunnelling and solar energy conversion. However, the performance of such devices degrades as their size is reduced. Graphene-based molecular electronics has emerged as a candidate to enable high performance down to the single-molecule scale. Graphene nanoribbons, for example, can have widths of less than 2 nm and bandgaps that are tunable via their width and symmetry. It has been predicted that bandgap engineering within a single graphene nanoribbon may be achieved by varying the width of covalently bonded segments within the nanoribbon. Here, we demonstrate the bottom-up synthesis of such width-modulated armchair graphene nanoribbon heterostructures, obtained by fusing segments made from two different molecular building blocks. We study these heterojunctions at subnanometre length scales with scanning tunnelling microscopy and spectroscopy, and identify their spatially modulated electronic structure, demonstrating molecular-scale bandgap engineering, including type I heterojunction behaviour. First-principles calculations support these findings and provide insight into the microscopic electronic structure of bandgap-engineered graphene nanoribbon heterojunctions.

Bottom-up graphene nanoribbon field-effect transistors

Recently developed processes have enabled bottom-up chemical synthesis of graphene nanoribbons (GNRs) with precise atomic structure. These GNRs are ideal candidates for electronic devices because of their uniformity, extremely narrow width below 1 nm, atomically perfect edge structure, and desirable electronic properties. Here, we demonstrate nano-scale chemically synthesized GNR field-effect transistors, made possible by development of a reliable layer transfer process. We observe strong environmental sensitivity and unique transport behavior characteristic of sub-1 nm width GNRs.

Site-Specific Substitutional Boron Doping of Semiconducting Armchair Graphene Nanoribbons

A fundamental requirement for the development of advanced electronic device architectures based on graphene nanoribbon (GNR) technology is the ability to modulate the band structure and charge carrier concentration by substituting specific carbon atoms in the hexagonal graphene lattice with p- or n-type dopant heteroatoms. Here we report the atomically precise introduction of group III dopant atoms into bottom-up fabricated semiconducting armchair GNRs (AGNRs). Trigonal-planar B atoms along the backbone of the GNR share an empty p-orbital with the extended π-band for dopant functionality. Scanning tunneling microscopy (STM) topography reveals a characteristic modulation of the local density of states along the backbone of the GNR that is superimposable with the expected position and concentration of dopant B atoms. First-principles calculations support the experimental findings and provide additional insight into the band structure of B-doped 7-AGNRs.

Orthogonal dipolar interactions between amide carbonyl groups

Orthogonal dipolar interactions between amide C=O bond dipoles are commonly found in crystal structures of small molecules, proteins, and protein–ligand complexes. We herein present the experimental quantification of such interactions by employing a model system based on a molecular torsion balance. Application of a thermodynamic double-mutant cycle allows for the determination of the incremental energetic contributions attributed to the dipolar contact between 2 amide C=O groups. The stabilizing free interaction enthalpies in various apolar and polar solvents amount to −2.73 kJ mol−1 and lie in the same range as aromatic-aromatic C–H⋯π and π–π interactions. High-level intermolecular perturbation theory (IMPT) calculations on an orthogonal acetamide/N-acetylpyrrole complex in the gas phase at optimized contact distance predict a favorable interaction energy of −9.71 kJ mol−1. The attractive dipolar contacts reported herein provide a promising tool for small-molecule crystal design and the enhancement of ligand–protein interactions during lead optimization in medicinal chemistry.

Local Electronic and Chemical Structure of Oligo-acetylene Derivatives Formed Through Radical Cyclizations at a Surface

Semiconducting π-conjugated polymers have attracted significant interest for applications in light-emitting diodes, field-effect transistors, photovoltaics, and nonlinear optoelectronic devices. Central to the success of these functional organic materials is the facile tunability of their electrical, optical, and magnetic properties along with easy processability and the outstanding mechanical properties associated with polymeric structures. In this work we characterize the chemical and electronic structure of individual chains of oligo-(E)-1,1′-bi(indenylidene), a polyacetylene derivative that we have obtained through cooperative C1–C5 thermal enediyne cyclizations on Au(111) surfaces followed by a step-growth polymerization of the (E)-1,1′-bi(indenylidene) diradical intermediates. We have determined the combined structural and electronic properties of this class of oligomers by characterizing the atomically precise chemical structure of individual monomer building blocks and oligomer chains (via noncontact atomic force microscopy (nc-AFM)), as well as by imaging their localized and extended molecular orbitals (via scanning tunneling microscopy and spectroscopy (STM/STS)). Our combined structural and electronic measurements reveal that the energy associated with extended π-conjugated states in these oligomers is significantly lower than the energy of the corresponding localized monomer orbitals, consistent with theoretical predictions.

Synergistic Enhancement of Electrocatalytic CO2 Reduction with Gold Nanoparticles Embedded in Functional Graphene Nanoribbon Composite Electrodes

Regulating the complex environment accounting for the stability, selectivity, and activity of catalytic metal nanoparticle interfaces represents a challenge to heterogeneous catalyst design. Here we demonstrate the intrinsic performance enhancement of a composite material composed of gold nanoparticles (AuNPs) embedded in a bottom-up synthesized graphene nanoribbon (GNR) matrix for the electrocatalytic reduction of CO2. Electrochemical studies reveal that the structural and electronic properties of the GNR composite matrix increase the AuNP electrochemically active surface area (ECSA), lower the requisite CO2 reduction overpotential by hundreds of millivolts (catalytic onset > -0.2 V versus reversible hydrogen electrode (RHE)), increase the Faraday efficiency (>90%), markedly improve stability (catalytic performance sustained over >24 h), and increase the total catalytic output (>100-fold improvement over traditional amorphous carbon AuNP supports). The inherent structural and electronic tunability of bottom-up synthesized GNR-AuNP composites affords an unrivaled degree of control over the catalytic environment, providing a means for such profound effects as shifting the rate-determining step in the electrocatalytic reduction of CO2 to CO, and thereby altering the electrocatalytic mechanism at the nanoparticle surface.

Imaging single-molecule reaction intermediates stabilized by surface dissipation and entropy

Chemical transformations at the interface between solid/liquid or solid/gaseous phases of matter lie at the heart of key industrial-scale manufacturing processes. A comprehensive study of the molecular energetics and conformational dynamics that underlie these transformations is often limited to ensemble-averaging analytical techniques. Here we report the detailed investigation of a surface-catalysed cross-coupling and sequential cyclization cascade of 1,2-bis(2-ethynyl phenyl)ethyne on Ag(100). Using non-contact atomic force microscopy, we imaged the single-bond-resolved chemical structure of transient metastable intermediates. Theoretical simulations indicate that the kinetic stabilization of experimentally observable intermediates is determined not only by the potential-energy landscape, but also by selective energy dissipation to the substrate and entropic changes associated with key transformations along the reaction pathway. The microscopic insights gained here pave the way for the rational design and control of complex organic reactions at the surface of heterogeneous catalysts.

Design of living ring-opening alkyne metathesis.

Despite the tremendous impact of alkene ring-opening metathesis polymerization (ROMP) on the design and the synthesis of polymers and functional materials, examples for the analogous alkyne ROMP remain scarce. This imbalance can be rationalized by the limited availability of commercially available catalysts and the lack of alkyne metathesis monomers. We present here the synthesis and a systematic study of the ROMP reactivity of an easily functionalized alkyne precursor derived from a ring-strained dibenzo[a,e][8]annulene. We find that both tungstenand molybdenumbased catalysts yield high-molecular-weight polymers with alternating alkane and alkyne linkages along the poly(orthophenylene) backbone. A systematic study of different phenols and alcohols as the activating ligands in the trialkoxymolybdenum(VI) alkylidyne elucidates the design rules to create living alkyne ROMP. Previously we found that the ROMP reaction of 5,6didehydrodibenzo[a,e]cyclooctatetraene proceeds with Schrock s tungsten catalyst. Unfortunately this monomer cannot be easily derivatized because of its poor thermal and photochemical stability. The poor stability also leads to high polydispersity and a non-living polymeric system. To overcome this limitation we substituted the olefin bridge in the 5,6-didehydrodibenzo[a,e]cyclooctatetraene with a saturated ethyl linker, 1a–c (Scheme 1). These strained alkynes (1a–c) can be conveniently synthesized on a multigram scale starting from the readily available 1,2-bis-(3-methoxyphenyl)ethanes 2a–c as depicted in Scheme 1 (synthetic details are in the Supporting Information). Reaction of 2a–c with tetrachlorocyclopropene and AlCl3 yields the intermediate aromatic dichlorocyclopropenes that were hydrolyzed in situ to give the cyclopropenones 3a–c. Cleavage of the methyl ethers and subsequent alkylation of the hydroxy groups in 4a–c with solubilizing alkyl chains provides 5a–c. Photochemical decarbonylation of the cyclopropenone produces the substituted [8]annulenes 1a–c in 77–89% yield. These monomers are far superior to other strained alkynes used in ROMP because they are indefinitely stable under ambient conditions and exposure to light. The structure of 1a–c is consistent with its previously reported derivatives. The ring strain stored in the compression of the triple bond angles from 1808 to 1558 (Figure 1) is Scheme 1. Reaction conditions: a) AlCl3, tetrachlorocyclopropene, CH2Cl2, 78 8C to 24 8C, 60–81%; b) BBr3, CH2Cl2, 78 8C, 99%; c) NaH, C12H25Br, DMF, 24 8C, 51–63%; d) THF/MeOH, hn, 24 8C, 77– 89%.