Shape-Directed Patterning and Surface Reaction of Tetra-diacetylene Monolayers: Formation of Linear and Two-Dimensional Grid Polydiacetylene Alternating Copolymers

Polymerization of Physisorbed Molecular Monolayers via Overhanging Alkynyl Chains: Characterization of Polymerization Kinetics and Monolayer Durability

Monolayers self-assembled by triphenyleneethynylene (TPE) compounds bearing two terminal alkynyl chains were polymerized by Glaser-Hay (G-H) alkyne coupling at the acetonitrile-HOPG interface. The alkynyl chains extend into the solution due to the monolayer's dense-packed morphology. Reacting substructures that have no morphology-determining roles is a potential strategy for preserving monolayer morphology throughout polymerization. Monolayer G-H reaction kinetics and polymerized monolayer durability were characterized by using mass spectrometry and fluorescence. Fourier transform ion cyclotron resonance (FTICR) mass spectrometry (MS) and time-of-flight (TOF) MS were used to identify TPE-oligomers in the monolayer and to track the monolayer populations of TPE-monomer, -dimer, and -trimer as a function of G-H reaction duration. Comparison of the observed kinetics to a Monte Carlo simulation provided evidence of step-growth polymerization. The durability of polymerized monolayers depended strongly on the length of the alkynyl chains linked by G-H reaction. Polymerized T6y monolayers (O(CH2)3C≡CH alkynyl chains) desorbed minimally during 16-h immersion in 90 °C o-dichlorobenzene (oDCB), whereas polymerized T8y (O(CH2)5C≡CH alkynyl chains) and polymerized T11y (O(CH2)8C≡CH alkynyl chains), desorbed 33 and 60%, respectively, of their TPE units after 4 h in 90 °C oDCB. All the polymerized monolayers are much more durable than unpolymerized monolayers, which desorb quantitatively from HOPG when rinsed with 25 μL of oDCB. Polymerized T6y monolayer is a highly durable anchor that may be adapted to build multilayer structures "permanently" attached to the HOPG surface. The alkynyl chain length dependence may be useful for tuning polymerized TPE monolayer durability for specific applications.

Identification of Topotactic Surface-Confined Ullmann-Polymerization

On-surface Ullmann coupling is an established method for the synthesis of 1D and 2D organic structures. A key limitation to obtaining ordered polymers is the uncertainty in the final structure for coupling via random diffusion of reactants over the substrate, which leads to polymorphism and defects. Here, a topotactic polymerization on Cu(110) in a series of differently-halogenated para-phenylenes is identified, where the self-assembled organometallic (OM) reactants of diiodobenzene couple directly into a single, deterministic product, whereas the other precursors follow a diffusion driven reaction. The topotactic mechanism is the result of the structure of the iodine on Cu(110), which controls the orientation of the OM reactants and intermediates to be the same as the final polymer chains. Temperature-programmed X-ray photoelectron spectroscopy and kinetic modeling reflect the differences in the polymerization regimes, and the effects of the OM chain alignments and halogens are disentangled by Nudged Elastic Band calculations. It is found that the repulsion or attraction between chains and halogens drive the polymerization to be either diffusive or topotactic. These results provide detailed insights into on-surface reaction mechanisms and prove the possibility of harnessing topotactic reactions in surface-confined Ullmann polymerization.

Synthesis on inert surfaces

Extraordinarily robust extended covalent organic nanostructures with unprecedented structures and intriguing chemical and electronic properties are currently synthesized on metal surfaces. Envisaged electronic applications, for instance in field effect transistors or sensors, however, demand insulating supports. To obviate the need for a cumbersome post-synthetic transfer from the metal growth surface to the target substrate, synthesis directly on inert surfaces is highly desirable. Albeit reversible polycondensations are broadly established on inert graphite surfaces, carbon-carbon (C-C) coupling remains mostly elusive. Thermally activated coupling on weakly interacting supports suffers from the "desorption problem", that is the premature desorption of reactants due to increased reaction barriers, which becomes even worse on inert surfaces due to diminished desorption barriers. Consequently, C-C coupling on inert surfaces requires new paradigms. We propose either photochemical coupling or activation of monomers prior to deposition as possible alternatives, discuss the current state-of-the-art and identify future challenges.

Unidirectional supramolecular self-assembly inside nanocorrals via in situ STM nanoshaving

Self-assembly of an alkylated diacetylene derivative is spatially confined via in situ scanning tunneling microscopy (STM) nanoshaving inside covalently modified highly ordered pyrolytic graphite (CM-HOPG). In contrast to unconstrained self-assembly that occurs randomly along three thermodynamically equivalent surface lattice directions, spatially confined assemblies are shown to form along chosen substrate orientations. Experimental statistics suggest two mechanisms for breaking the rotational degeneracy of the surface. First, the assembly orientation is biased via lateral confinement inside nanocorrals that do not match the substrate symmetry. Second, an interaction between the assembling molecules and the STM tip during nanoshaving guides 2D crystal nucleation and growth. The results presented here open new possibilities to regulate and orient self-assembled architectures via in situ nanomechanical manipulation techniques and provide mechanistic insights into the process.

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.

Tracking Invisible Transformations of Physisorbed Monolayers: LDI-TOF and MALDI-TOF Mass Spectrometry as Complements to STM Imaging

Triphenyleneethynylene (TPEE) derivatives bearing one long aliphatic chain on each terminal aryl ring and two short aliphatic chains on the central aryl ring (core chains) self-assemble single component and 1-D patterned, two-component, crystalline monolayers at the solution-graphite interface. The monolayer morphology directs the core chains off the graphite, making them accessible for chemical reactions but invisible to imaging by scanning tunneling microscopy (STM). This precludes using STM to monitor transformations of the core chains, either by reaction or solution-monolayer exchange of TPEE molecules. Laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) successfully identifies TPEE compounds within physisorbed monolayers. The LDI-TOF spectra of TPEE monolayer-graphite samples exhibit strong molecular ion peaks and minimal fragmentation or background. LDI-TOF and STM techniques are combined to evaluate monolayer composition and morphology, track solution-monolayer exchange, to identify reaction products and to measure kinetics of chemical reactions at the solution-monolayer interface. LDI-TOF MS provides rapid qualitative evaluation of monolayer composition across a graphite substrate. Challenges to quantitative composition evaluation by LDI-TOF include compound-specific light absorption, surface desorption/ionization and fragmentation characteristics. For some, but not all, compounds, applying matrix onto a self-assembled monolayer increases molecular ion intensities and affords more accurate assessment of monolayer composition via matrix assisted laser desorption/ionization (MALDI) MS. Matrix addition precludes subsequent chemical or STM studies of the monolayer, whereas reactions and STM may be performed at nonirradiated regions following LDI-TOF measurements. LDI- and MALDI-TOF MS are useful complements to STM and are easily implemented tools for study of physisorbed monolayers.

Chemistry at the square nanometer: reactivity at liquid/solid interfaces revealed with an STM

An overview is given of single molecule reactivity at a liquid/solid interface employing a scanning tunneling microscope.

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.

Controllable Orientation of Ester-Group-Induced Intermolecular Halogen Bonding in a 2D Self-Assembly

Halogen bonding with high specificity and directionality in the geometry has proven to be an important type of noncovalent interaction to fabricate and control 2D molecular architectures on surfaces. Herein, we first report how the orientation of the ester substituent for thienophenanthrene derivatives (5,10-DBTD and 5,10-DITD) affects positive charge distribution of halogens by density functional theory, thus determining the formation of an intermolecular halogen bond and different self-assembled patterns by scanning tunneling microscopy. The system presented here mainly includes heterohalogen X···O═C and X···S halogen bonds, H···Br and H···O hydrogen bonds, and I···I interaction, where the directionality and strength of such weak bonds determine the molecular arrangement by varying the halogen substituent. This study provides a detailed understanding of the role of ester orientation, concentration, and solvent effects on the formation of halogen bonds and proves relevant for identification of multiple halogen bonding in supramolecular chemistry.