Rate-determining factors in the chain polymerization of molecules initiated by local single-molecule excitation

Nanometer-scale patterning of hard and soft interfaces: from photolithography to molecular-scale design

Many areas of modern materials chemistry, from nanoscale electronics to regenerative medicine, require design of precisely-controlled chemical environments at near-molecular scales on both hard and soft surfaces.

Plenty of Room at the Top: A Multi-Scale Understanding of nm-Resolution Polymer Patterning on 2D Materials

Lamellar phases of alkyldiacetylenes in which the alkyl chains lie parallel to the substrate represent a straightforward means for scalable 1-nm-resolution interfacial patterning. This capability has the potential for substantial impacts in nanoscale electronics, energy conversion, and biomaterials design. Polymerization is required to set the 1-nm functional patterns embedded in the monolayer, making it important to understand structure-function relationships for these on-surface reactions. Polymerization can be observed for certain monomers at the single-polymer scale using scanning probe microscopy. However, substantial restrictions on the systems that can be effectively characterized have limited utility. Here, using a new multi-scale approach, we identify a large, previously unreported difference in polymerization efficiency between the two most widely used commercial diynoic acids. We further identify a core design principle for maximizing polymerization efficiency in these on-surface reactions, generating a new monomer that also exhibits enhanced polymerization efficiency.

Impact of the Air Atmosphere on Photoinduced Chain Polymerization in Self-Assembled Monolayers of Diacetylene on Graphite

We have studied the effect of the reaction environment on the photoinduced chain polymerization in self-assembled monolayers of a diacetylene compound 10,12-pentacosadiyn-1-ol on graphite, using scanning tunneling microscopy. Comparing the polymerization behaviors in air and in vacuum, we show that the polymer generation efficiency is considerably enhanced under vacuum because of suppressed collisional quenching of the photoexcited radicals by oxygen molecules. We also find that the polymer chain length tends to increase in vacuum as a result of the inhibition of deactivation of reactive species for the polymer chain growth due to oxidation.

The deconvolution analysis of ATR-FTIR spectra of diacetylene during UV exposure

We performed a detailed deconvolution analysis of ATR-FTIR peaks of a common diacetylene, 10,12-tricosadiynoic acid (TRCDA) during the polymerization and the blue-to-red transition. Based on the analysis and the solvent dependence on the IR signals, we found that the triple peak from CC stretching mode that has been previously suspected as a consequence of Fermi resonance is rather associated with the macromolecular assembly of TRCDA. Besides these CC triple peaks, we found that the background in the region increased during the UV exposure due to the CC signals from polymers. In addition, the anisotropic compression during polymerization was also detected, which supports the proposed interpretation of X-ray data reported previously. These results are the benefits from the deconvolution analysis.

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.

Confined polydiacetylene polymerization reactions for programmed length control

Polydiacetylene polymers of defined lengths are formed from self-assembled precursors inside nanocorrals created within grafted graphite substrates. A scanning tunneling microscope tip is used to nanoshave corrals at the liquid-solid interface allowing orientationally controlled supramolecular self-assembly of linear diacetylene molecules. Electrical pulses trigger topological one-dimensional polymerization reactions that are confined by the nanocorral template dimensions.

Hierarchically Patterned Noncovalent Functionalization of 2D Materials by Controlled Langmuir-Schaefer Conversion

Noncovalent monolayer chemistries are often used to functionalize 2D materials. Nanoscopic ligand ordering has been widely demonstrated (e.g., lying-down lamellar phases of functional alkanes); however, combining this control with micro- and macroscopic patterning for practical applications remains a significant challenge. A few reports have demonstrated that standing phase Langmuir films on water can be converted into nanoscopic lying-down molecular domains on 2D substrates (e.g., graphite), using horizontal dipping (Langmuir-Schaefer, LS, transfer). Molecular patterns are known to form at scales up to millimeters in Langmuir films, suggesting the possibility of transforming such structures into functional patterns on 2D materials. However, to our knowledge, this approach has not been investigated, and the rules governing LS conversion are not well understood. In part, this is because the conversion process is mechanistically very different from classic LS transfer of standing phases; challenges also arise due to the need to characterize structure in noncovalently adsorbed ligand layers <0.5 nm thick, at scales ranging from millimeters to nanometers. Here, we show that scanning electron microscopy enables diynoic acid lying-down phases to be imaged across this range of scales; using this structural information, we establish conditions for LS conversion to create hierarchical microscopic and nanoscopic functional patterns. Such control opens the door to tailoring noncovalent surface chemistry of 2D materials to pattern local interactions with the environment.

Edge-on adsorption of multi-chain functional alkanes stabilizes noncovalent monolayers on MoS2

Diyne phospholipids adsorb edge-on on MoS₂, producing stable monolayers with large, ordered domains, despite low alkane adsorption enthalpies on MoS₂.

Standing, lying, and sitting: translating building principles of the cell membrane to synthetic 2D material interfaces

Lessons can be drawn from cell membranes in controlling noncovalent functionalization of 2D materials to optimize interactions with the environment.

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.

Two-dimensional self-assembly of diacetylenic acid derivatives and their light-induced polymerization on HOPG surfaces

The effect of UV illumination time on the polymerization efficiency and the structural change of DA-25.

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.

Photochemical Reactions in Self-Assembled Organic Monolayers Characterized by using Scanning Tunneling Microscopy

Research on the supramolecular self-assembly behavior at interfaces is of great importance to improving the performance of nanodevices that are based on optical functional materials. In this Minireview, several photoinduced isomerization and polymerization reactions in self-assembled organic monolayers on surfaces are discussed. Typical organic molecules contain azobenzene, alkynyl, or olefins groups. The feature surface base is a highly oriented pyrolytic graphite (HOPG) surface or a gold surface. Scanning tunneling microscopy (STM) is used as a strong tool to characterize new species' structures before and after illumination.

Defect-Tolerant Aligned Dipoles within Two-Dimensional Plastic Lattices

Carboranethiol molecules self-assemble into upright molecular monolayers on Au{111} with aligned dipoles in two dimensions. The positions and offsets of each molecule's geometric apex and local dipole moment are measured and correlated with sub-Ångström precision. Juxtaposing simultaneously acquired images, we observe monodirectional offsets between the molecular apexes and dipole extrema. We determine dipole orientations using efficient new image analysis techniques and find aligned dipoles to be highly defect tolerant, crossing molecular domain boundaries and substrate step edges. The alignment observed, consistent with Monte Carlo simulations, forms through favorable intermolecular dipole-dipole interactions.

Nanojunction between fullerene and one-dimensional conductive polymer on solid surfaces

Bottom-up creation of huge molecular complexes by covalently interconnecting functional molecules and conductive polymers is a key technology for constructing nanoscale electronic circuits. In this study, we have created an array of molecule-polymer nanojunctions from C60 molecules and polydiacetylene (PDA) nanowires at designated positions on solid surfaces by controlling self-assemblies and intermolecular chemical reactions of molecular ingredients predeposited onto the surfaces. In the proposed method, the construction of each nanojunction spontaneously proceeds via two types of chemical reactions: a chain polymerization among self-assembled diacetylene compound molecules for creating a single PDA nanowire and a subsequent cycloaddition reaction between the propagating forefront part of the PDA backbone and a single C60 molecule adsorbed on the surface. Scanning tunneling microscopy has proved that the C60 molecule is covalently connected to each end of the π-conjugated PDA backbone. Furthermore, the decrease in the energy gap of the C60 molecule in nanojunctions is observed as compared with that of pristine C60 molecules, which is considered to be due to the covalent interaction between the PDA edge and the C60 molecule.

On-surface single molecule synthesis chemistry: a promising bottom-up approach towards functional surfaces

In this review, we introduce recent progress on surface synthesis and focus on supramolecular self-assembled structures driven by several typical chemical reactions at solid surfaces, with the aid of scanning tunneling microscopy (STM). We also emphasize the relationship between the non-covalent self-assembly and surface reactivity, by which we hope to find an effective way for further controllable nano-manufacture.

Ordered monomolecular layers as a template for the regular arrangement of gold nanoparticles

Ordered arrays of metal nanoparticles are important for nanoelectronic and nanophotonic applications. Here, we report the formation of self-assembled arrays of gold nanoparticles on molecular layers of diacetylene compounds on a MoS2(0001) substrate. The arrangement of gold nanoparticles is observed using scanning tunneling microscopy. When gold is deposited on a self-assembled monolayer of 10,12-nonacosadiynoic acid or 10,12-octadecadiynoic acid on a MoS2(0001) substrate, the ordered array of diacetylene moieties in the molecular layer serves as a template for the formation of ordered arrays of gold nanoparticles. In contrast, when gold is deposited on a pristine MoS2(0001) surface or on a molecular layer of stearic acid, the gold nanoparticles are randomly distributed on the surface. It is found that the arrangement of gold nanoparticles is largely determined by the deposition rate; faster deposition results in more ordered arrays of gold nanoparticles. Our observations confirm the role of unsaturated π systems in molecules acting as a template for the regular arrangement of gold nanoparticles; this work will open up new possibilities for interfacial nanoarchitectonics.

Self-organized and cu-coordinated surface linear polymerization

We demonstrate a controllable surface-coordinated linear polymerization of long-chain poly(phenylacetylenyl)s that are self-organized into a "circuit-board" pattern on a Cu(100) surface. Scanning tunneling microscopy/spectroscopy (STM/S) corroborated by ab initio calculations, reveals the atomistic details of the molecular structure, and provides a clear signature of electronic and vibrational properties of the poly(phenylacetylene)s chains. Notably, the polymerization reaction is confined epitaxially to the copper lattice, despite a large strain along the polymerized chain that subsequently renders it metallic. Polymerization and depolymerization reactions can be controlled locally at the nanoscale by using a charged metal tip. This control demonstrates the possibility of precisely accessing and controlling conjugated chain-growth polymerization at low temperature. This finding may lead to the bottom-up design and realization of sophisticated architectures for molecular nano-devices.

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.

Substrate-, wavelength-, and time-dependent plasmon-assisted surface catalysis reaction of 4-nitrobenzenethiol dimerizing to p,p'-dimercaptoazobenzene on Au, Ag, and Cu films

In this article, we experimentally investigate the substrate, wavelength, and time dependence of the plasmon-assisted surface-catalyzed dimerization of 4-nitrobenzenethiol to form p,p'-dimercaptoazobenzene on Au, Ag, and Cu films. We provide direct experimental evidence that surface plasmon resonance plays the most important role in these surface-catalyzed reactions. It is found that the reaction is strongly dependent on the substrate, the wavelength of the laser, and the reaction timescales. Our experimental results revealed that optimal experimental conditions can be rationally chosen to control (accelerate or restrain) this reaction. The experimental results are also confirmed by theoretical calculations.

Deposition and photopolymerization of phase-separated perfluorotetradecanoic acid-10,12-pentacosadiynoic acid Langmuir-Blodgett monolayer films

Mixed monolayer surfactant films of perfluorotetradecanoic acid and the photopolymerizable diacetylene molecule 10,12-pentacosadiynoic acid were prepared at the air-water interface and transferred onto solid supports via Langmuir-Blodgett (LB) deposition. The addition of the perfluoroacid to the diacetylene surfactant results in enhanced stabilization of the monolayer in comparison with the pure diacetylene alone, allowing film transfer onto a solid substrate without resorting to addition of cations in the subphase or photopolymerization prior to deposition. The resulting LB films consisted of well-defined phase-separated domains of the two film components, and the films were characterized by a combination of atomic force microscope (AFM) imaging and fluorescence emission microscopy both before and after photopolymerization into the highly emissive "red form" of the polydiacetylene. Photopolymerization of the monolayer films resulted in the formation of diacetylene bilayers, which were highly fluorescent, with the apparent rate of photopolymerization and the fluorescence emission of the films being largely unaffected by the presence of the perfluoroacid.