Structures of 17,19-hexatriacontadiyne monolayers on Au(111) studied by infrared reflection absorption spectroscopy and scanning tunneling microscopy

Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis

On-surface synthesis is appearing as an extremely promising research field aimed at creating new organic materials. A large number of chemical reactions have been successfully demonstrated to take place directly on surfaces through unusual reaction mechanisms. In some cases the reaction conditions can be properly tuned to steer the formation of the reaction products. It is thus possible to control the initiation step of the reaction and its degree of advancement (the kinetics, the reaction yield); the nature of the reaction products (selectivity control, particularly in the case of competing processes); as well as the structure, position, and orientation of the covalent compounds, or the quality of the as-formed networks in terms of order and extension. The aim of our review is thus to provide an extensive description of all tools and strategies reported to date and to put them into perspective. We specifically define the different approaches available and group them into a few general categories. In the last part, we demonstrate the effective maturation of the on-surface synthesis field by reporting systems that are getting closer to application-relevant levels thanks to the use of advanced control strategies.

Diacetylene polymerization on a bulk insulator surface

Molecular electronics has great potential to surpass known limitations in conventional silicon-based technologies. The development of molecular electronics devices requires reliable strategies for connecting functional molecules by wire-like structures. To this end, diacetylene polymerization has been discussed as a very promising approach for contacting single molecules with a conductive polymer chain. A major challenge for future device fabrication is transferring this method to bulk insulator surfaces, which are mandatory to decouple the electronic structure of the functional molecules from the support surface. Here, we provide experimental evidence for diacetylene polymerization of 3,3'-(1,3-butadiyne-1,4-diyl)bisbenzoic acid precursors on the (10.4) surface of calcite, a bulk insulator with a band gap of around 6 eV. When deposited on the surface held at room temperature, ordered islands with a (1 × 3) superstructure are observed using dynamic atomic force microscopy. A distinct change is revealed upon heating the substrate to 485 K. After heating, molecular stripes with a characteristic inner structure are formed that excellently match the expected diacetylene polymer chains in appearance and repeat distance. The corresponding density functional theory computations reveal molecular-level bonding patterns of both the (1 × 3) superstructure and the formed striped structure, confirming the assignment of on-surface diacetylene polymerization. Transferring the concept of using diacetylene polymerization for creating conductive connections to bulk insulator surfaces paves the way towards application-relevant systems for future molecular electronic devices.

Self-assembling diacetylene molecules on atomically flat insulators

Single crystal sapphire and diamond surfaces are used as planar, atomically flat insulating surfaces, for the deposition of the diacetylene compound 10,12-nonacosadiynoic acid. The surface assembly is compared with results on hexagonal boron nitride (h-BN), highly oriented pyrolytic graphite (HOPG) and MoS₂ surfaces. A perfectly flat-lying monolayer of 10,12-nonacosadiynoic acid self-assembles on h-BN like on HOPG and MoS₂. On sapphire and oxidized diamond surfaces, we observed assemblies of standing-up molecular layers. Surface assembly is driven by surface electrostatic dipoles. Surface polarity is partially controlled using a hydrogenated diamond surface or totally screened by the deposition of a graphene layer on the sapphire surface. This results in a perfectly flat and organized SAM on graphene, which is ready for on-surface polymerization of long and isolated molecular wires under ambient conditions.

2D Supramolecular networks of dibenzonitrilediacetylene on Ag(111) stabilized by intermolecular hydrogen bonding

The two-dimensional (2D) surface-directed self-assembly of dibenzonitrile diacetylene (DBDA) on Ag(111) under ultrahigh vacuum (UHV) conditions was investigated by combining scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and theoretical simulations based on density functional theory (DFT) calculations. The molecule consists of two benzonitrile groups (-C₆H₄-C[triple bond, length as m-dash]N) on each side of a diacetylene (-C[triple bond, length as m-dash]C-C[triple bond, length as m-dash]C-) backbone. The terminating nitrile (-C[triple bond, length as m-dash]N) groups at the meta position of the phenyl rings lead to cis and trans stereoisomers. The trans isomer is prochiral and can adsorb in the R or S configuration, leading to the formation of enantiomeric self-assembled networks on the surface. We identify two simultaneously present supramolecular networks, termed parallel and chevron phases, as well as a less frequently observed butterfly phase. These networks are formed from pure R (or S) domains, racemic mixtures (RS), and cis isomers, respectively. Our complementary data illustrates that the formation of the 2D supramolecular networks is driven by intermolecular hydrogen bonding between nitrile and phenyl groups (-C[triple bond, length as m-dash]NH-C₆H₃). This study illustrates that the molecular arrangement of each network depends on the geometry of the isomers. The orientation of the nitrile group controls the formation of the most energetically stable network via intermolecular hydrogen bonding.

Self-assembled diacetylene molecular wire polymerization on an insulating hexagonal boron nitride (0001) surface

The electrical characterization of single-polymer chains on a surface is an important step towards novel molecular device development. The main challenge is the lack of appropriate atomically flat insulating substrates for fabricating single-polymer chains. Here, using atomic force microscopy, we demonstrate that the (0001) surface of an insulating hexagonal boron nitride (h-BN) substrate leads to a flat-lying self-assembled monolayer of diacetylene compounds. The subsequent heating or ultraviolet irradiation can initiate an on-surface polymerization process leading to the formation of long polydiacetylene chains. The frequency of photo-polymerization occurrence on h-BN(0001) is two orders of magnitude higher than that on graphite(0001). This is explained by the enhanced lifetime of the molecular excited state, because relaxation via the h-BN is suppressed due to a large band gap. We also demonstrate that on-surface polymerization on h-BN(0001) is possible even after the lithography process, which opens up the possibility of further electrical investigations.

Selection of conformational states in surface self-assembly for a molecule with eight possible pairs of surface enantiomers

Chiral adsorption of a complex molecule with as many as eight possible pairs of surface enantiomers is investigated by STM and the selection of enantiomers is understood by statistical analysis and DFT modelling.

Odd or even? Monolayer domain size depends on diyne position in alkadiynylanthracenes

1,5-(Alkadiynyl)anthracenes self-assemble single component and multicomponent monolayers at the solution-HOPG interface. An alkadiynyl chain's kinked shape constrains the molecular structures with which it can close-pack. This affords rudimentary molecular recognition that has been used to direct self-assembly of 1-D patterned, multicomponent monolayers. The unit cell building blocks of single- and multicomponent alkadiynylanthracene monolayers repeat with high fidelity for 100s of nanometers along the side chain direction. Unit cell repeat fidelity along the orthogonal, anthracene column direction of the monolayer depends on diyne location within the side chain; even-position diyne side chains produce high fidelity of unit cell repeats and wider domain widths along the anthracene columns, whereas odd-position diyne side chains produce more frequent domain interfaces that disrupt the anthracene columns. Alkadiynylanthracene monolayers may be viewed as stacks of 1-D molecular tapes. 1-D tape molecular composition, sequence, and intratape side chain alignment are dictated by shape complementarity of the kinked alkadiynyl side chains. Stacking alignments of adjacent 1-D tapes are controlled by shape matching of tape peripheries and determine repeat fidelity along the anthracene columns. Tapes stacked with a constant intertape alignment comprise crystalline domains that repeat along the anthracene columns. The 1-D tapes formed by anthracenes with odd-position diynes have triangle wave peripheries that close-pack in multiple stacking alignments. This reduces unit cell repeat fidelity and decreases the widths of crystalline domains along the anthracene columns. Even-position diyne side chains form 1-D tapes with trapezoid wave peripheries that close-pack in only one stacking alignment. This generates higher stacking fidelity, larger domain widths, and fewer domain interfaces along the anthracene columns of even-position diyne monolayers. Even- and odd-position diyne monolayers exhibit comparable densities of interfaces between enantiotopic domains and between domains aligned along different graphite symmetry axes. These interfaces likely arise through collisions of independently nucleated/growing domains and persist for lack of kinetically competent pathways that interconvert or merge the domains.

Controlled chain polymerisation and chemical soldering for single-molecule electronics

Single functional molecules offer great potential for the development of novel nanoelectronic devices with capabilities beyond today's silicon-based devices. To realise single-molecule electronics, the development of a viable method for connecting functional molecules to each other using single conductive polymer chains is required. The method of initiating chain polymerisation using the tip of a scanning tunnelling microscope (STM) is very useful for fabricating single conductive polymer chains at designated positions and thereby wiring single molecules. In this feature article, developments in the controlled chain polymerisation of diacetylene compounds and the properties of polydiacetylene chains are summarised. Recent studies of "chemical soldering", a technique enabling the covalent connection of single polydiacetylene chains to single functional molecules, are also introduced. This represents a key step in advancing the development of single-molecule electronics.