Confined polydiacetylene polymerization reactions for programmed length control

2D Self-assembled molecular networks and on-surface reactivity under nanoscale lateral confinement

Supramolecular self-assembly at surfaces provides a pathway for building chemically customized interfaces. Over the last three decades, research on the role of key parameters such as temperature, solute concentration, and molecular design has enabled a steady increase in the complexity of self-assembled molecular networks (SAMNs) that can thus be created. However, the structure and quality of SAMNs is often determined during the early stages of nucleation and growth. To study and influence self-assembly processes at this deterministic length scale, spatial confinement of molecular adsorbates to well-defined surface patterns with nanoscale lateral dimensions offers exciting possibilities. The aim of this tutorial review is to give an overview of the various ways in which confinement impacts SAMN formation, and how we can use that knowledge to direct assemblies towards desired structures. The possibility to exploit confinement for improved control over on-surface reactions is also contemplated.

Collective radical oligomerisation induced by an STM tip on a silicon surface

Over the past decade, on-surface fabrication of organic nanostructures has been widely investigated for the development of molecular electronic components, catalysts, and new materials. Here, we introduce a new strategy to obtain alkyl oligomers in a controlled manner using on-surface radical oligomerisations that are triggered by electrons between the tip of a scanning tunnelling microscope and the Si(111)√3 ×√3 R30°-B surface. This electron transfer event only occurs when the bias voltage is below -4.5 V and allows access to reactive radical species under exceptionally mild conditions. This transfer can effectively 'switch on' a sequence leading to the formation of oligomers of defined size distribution thanks to the on-surface confinement of the reactive species. Our approach enables new ways to initiate and control radical oligomerisations with tunnelling electrons, leading to molecularly precise nanofabrication.

Controlled individual and partial polyethylene nanofibers with high Tm2 prepared by PPM-supported Cp2TiCl2 catalysts

Controlled individual and partial nanofibrous polyethylenes with high second melting point (T_m2) were obtained through ethylene-confined polymerization using a porous polymer microsphere (PPM)-supported Cp₂TiCl₂ catalyst.

Light-directed trapping of metastable intermediates in a self-assembly process

Self-assembly is a dynamic process that often takes place through a stepwise pathway involving formation of kinetically favored metastable intermediates prior to generation of a thermodynamically preferred supramolecular framework. Although trapping intermediates in these pathways can provide significant information about both their nature and the overall self-assembly process, it is a challenging venture without altering temperature, concentrations, chemical compositions and morphologies. Herein, we report a highly efficient and potentially general method for "trapping" metastable intermediates in self-assembly processes that is based on a photopolymerization strategy. By employing a chiral perylene-diimide possessing a diacetylene containing an alkyl chain, we demonstrated that the metastable intermediates, including nanoribbons, nanocoils and nanohelices, can be effectively trapped by using UV promoted polymerization before they form thermodynamic tubular structures. The strategy developed in this study should be applicable to naturally and synthetically abundant alkyl chain containing self-assembling systems.

High Melting Point of Linear, Spiral Polyethylene Nanofibers and Polyethylene Microspheres Obtained Through Confined Polymerization by a PPM-Supported Ziegler-Natta Catalyst

In this work, different types of polyethylene (linear, spiral nanofibers and microspheres) were obtained via confined polymerization by a PPM-supported Ziegler-Natta catalyst. Firstly, the Ziegler-Natta catalyst was chemical bonded inside the porous polymer microspheres (PPMs) supports with different pore diameter and supports size through chemical reaction. Then slightly and highly confined polymerization occurred in the PPM-supported Ziegler-Natta catalysts. SEM results illustrated that the slightly confined polymerization was easy to obtain linear and spiral nanofibers, and the nanofibers were observed in polyethylene catalyzed by PPMs-1#/cat and PPMs-2#/cat with low pore diameter (about 23 nm). Furthermore, the highly confined polymerization produced polyethylene microspheres, which obtained through other PPM-supported Ziegler-Natta catalysts with high pore diameter. In addition, high second melting point (Tm2: up to 143.3 °C) is a unique property of the polyethylene obtained by the PPM-supported Ziegler-Natta catalyst after removing the residue through physical treatment. The high Tm2 was ascribed to low surface free energy (σe), which was owing to the entanglement of polyethylene polymerized in the PPMs supports with interconnected multi-modal pore structure.

Co-solvent polarity tuned thermochromic nanotubes of cyclic dipeptide-polydiacetylene supramolecular system

The cooperative non-covalent interactions arising from structurally integrated multiple molecules have emerged as a powerful tool for the creation of functional supramolecular structures. Herein, we constructed cyclic dipeptide (CDP)–polydiacetylene (PDA) conjugate (CDP–DA) by introducing cyclo(l-Phe-l-Lys) to the linear 10,12-pentacosadiynoic acid. Owing to extensive hydrogen bonding characteristics, together with structural chirality of cyclo(l-Phe-l-Lys) and strong π–π stacking diacetylenic template, CDP–DA generated supramolecular nanotubes. The structural visualization using scanning and transmission electron microscopy revealed chloroform/methanol co-solvent polarity tuned morphological transformation of intrinsic lamellar assemblies into nanotubes comprising single-wall and multi-wall structure. The mechanistic understanding by X-ray diffraction patterns confirms bilayer organization in lamellar structure, which forms nanotubes via a gradual lamellar curling-to-scrolling process. The supramolecular CDP–DA nanotubes are transformed into the rigid covalently cross-linked blue-phase polydiacetylene (CDP–PDA) by UV irradiation. Very interestingly, the blue-phase nanotubes display reversible thermochromic changing temperature up to 150 °C with excellent repeatability over a dozen thermal cycles. This work provides an efficient strategy for precise morphological control and aiding the perspective for development in nanostructures for functional devices.

Co-solvent controlled fabrication of thermo-responsive chromogenic nanotubes of a cyclic dipeptide–polydiacetylene supramolecular system.

Photoinduced Reversible Bending and Guest Molecule Release of Azobenzene-Containing Polydiacetylene Nanotubes

Creation of hollow, one-dimensional nanomaterials has gained great recent attention in the chemical and material sciences. In a study aimed at discovering new functional materials of this type, we observed that an amphiphilic diacetylene (DA) derivative, containing an azobenzene moiety and an oligo-ethylene group, self-assembles to form nanotubes and undergoes photopolymerization to form hollow polydiacetylene (PDA) nanotubes with a uniform wall thickness and diameter. The azobenzene-PDA nanotubes are photoresponsive in that on-and-off UV-irradiation leads to a reversible morphological change between straight and bent forms in association with E-Z photoisomerization of the azobenzene group. Owing to the UV-induced structural change feature, the new DA and PDA nanotubes serve as a controlled release material. Accordingly, fluorescent rhodamine B encapsulated inside the nanotubes are effectively released by using repeated on-off UV irradiation. Furthermore, photo-release of rhodamine B was shown to occur in an artemia (brine shrimp).

Graphite and Graphene Fairy Circles: A Bottom-Up Approach for the Formation of Nanocorrals

A convenient covalent functionalization approach and nanopatterning method of graphite and graphene is developed. In contrast to expectations, electrochemically activated dediazotization of a mixture of two aryl diazonium compounds in aqueous media leads to a spatially inhomogeneous functionalization of graphitic surfaces, creating covalently modified surfaces with quasi-uniform spaced islands of pristine graphite or graphene, coined nanocorrals. Cyclic voltammetry and chronoamperometry approaches are compared. The average diameter (45-130 nm) and surface density (20-125 corrals/μm²) of these nanocorrals are tunable. These chemically modified nanostructured graphitic (CMNG) surfaces are characterized by atomic force microscopy, scanning tunneling microscopy, Raman spectroscopy and microscopy, and X-ray photoelectron spectroscopy. Mechanisms leading to the formation of these CMNG surfaces are discussed. The potential of these surfaces to investigate supramolecular self-assembly and on-surface reactions under nanoconfinement conditions is demonstrated.

Cation-Directed Self-Assembly of Macrocyclic Diacetylene for Developing Chromogenic Polydiacetylene

The cation-directed self-assembly process has emerged as a fascinating approach for constructing supramolecular architectures and manifested a diverse range of assembly related applications. Herein, we synthesized a macrocyclic structure containing bis-amidopyridine and photopolymerizable diacetylene template, PyMCDA. Owing to the metal coordination affinity of bis-amidopyridine and the π-π stacking characteristic of diacetylene template and complementary to the cyclic molecular framework, Cs⁺-directed organic nanotubes are generated via unidirectional self-assembly of PyMCDA. The monomeric PyMCDA nanotubes are transformed into the covalently cross-linked chromogenic polydiacetylene nanotubes (PyMCPDA-Cs⁺) by UV-promoted topochemical polymerization. The result of a metal-ligand coordination characteristic, geometric parameters in solid-state assemblies, and topochemical polymerization behavior reveals a generation of Cs⁺ ion inserted nanotubes. Interestingly, PyMCDA-Cs⁺ nanotubes display thermochromic property with a brilliant blue-to-red color transition.

Covalently Linked Perylene Diimide-Polydiacetylene Nanofibers Display Enhanced Stability and Photocurrent with Reversible FRET Phenomenon

Because of their unique structural and optical properties, 1D perylene diimide (PDI) derivatives have gained attention for use in optoelectronic devices. However, PDI-containing self-assembled supramolecular systems often are of limited use because they have supramolecular architectures that are held together by weak noncovalent π-π stacking, hydrogen bonding, and hydrophobic interactions. As a result, they are intrinsically unstable under solution-processing conditions. To overcome this limitation, a polydiacetylene (PDA)-based strategy is developed to construct a solvent-resistant and stable PDI assembly. For this purpose, first the monomer PDI-BisDA is generated, in which two polymerizable diacetylene (DA) units are covalently linked to a PDI core. Importantly, 254 nm UV irradiation of self-assembled PDI-BisDA nanofibers forms solvent-resistant and stable PDI-PDA fibers. Owing to the presence of PDA, the generated polymer fibers display an increased photocurrent. In addition, the existence of PDA and PDI moieties in the fiber leads to the occurrence of switchable on-off fluorescence resonance energy transfer (FRET) between the PDI and reversibly thermochromic PDA chromophores.

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.

The impact of grafted surface defects on the on-surface Schiff-base chemistry at the solid-liquid interface

We investigate the effect of covalently modified graphitic surfaces on the formation of single-layer covalent organic frameworks (sCOFs) at the solid-liquid interface. The impact of different densities of grafted aryl species was tested on the on-surface synthesis of three distinct imine-based 2D sCOFs. The grafted aryl species that act as defects provide steric barriers to the progress of the Schiff-base reaction, and can be locally removed to start the 2D polymer healing process. This unique strategy provides a general approach to study in situ this dynamic covalent on-surface chemistry.

Zipping and unzipping monolayers: switchable monolayer oligomerization and adhesion via thiol–disulfide interconversion

Triphenyleneethynylene (TPE) monolayers at the solution–HOPG interface are oligomerized by the oxidation of pendant thioethers to form disulfide cross-links. Subsequent disulfide reduction unzips oligomers to form monomeric TPE monolayer with pendant thiols.

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.

Exploring the science of thinking independently together: Faraday Discussion Volume 204 - Complex Molecular Surfaces and Interfaces, Sheffield, UK, July 2017

The 2017 Faraday Discussion on Complex Molecular Surfaces and Interfaces brought together theoreticians and experimentalists from both physical and chemical backgrounds to discuss the relevant applied and fundamental research topics within the broader field of chemical surface analysis and characterization. Main discussion topics from the meeting included the importance of "disordered" two-dimensional (2D) molecular structures and the utility of kinetically trapped states. An emerging need for new experimental tools to address dynamics and kinetic pathways involved in self-assembled systems, as well as the future prospects and current limitations of in silico studies were also discussed. The following article provides a brief overview of the work presented and the challenges discussed during the meeting.