Towards Dynamic Control of Wettability by Using Functionalized Altitudinal Molecular Motors on Solid Surfaces

Photo-responsive functional materials based on light-driven molecular motors

In the past two decades, the research and development of light-triggered molecular machines have mainly focused on developing molecular devices at the nanoscale. A key scientific issue in the field is how to amplify the controlled motion of molecules at the nanoscale along multiple length scales, such as the mesoscopic or the macroscopic scale, or in a more practical perspective, how to convert molecular motion into changes of properties of a macroscopic material. Light-driven molecular motors are able to perform repetitive unidirectional rotation upon irradiation, which offers unique opportunities for responsive macroscopic systems. With several reviews that focus on the design, synthesis and operation of the motors at the nanoscale, photo-responsive macroscopic materials based on light-driven molecular motors have not been comprehensively summarized. In the present review, we first discuss the strategy of confining absolute molecular rotation into relative rotation by grafting motors on surfaces. Secondly, examples of self-assemble motors in supramolecular polymers with high internal order are illustrated. Moreover, we will focus on building of motors in a covalently linked system such as polymeric gels and polymeric liquid crystals to generate complex responsive functions. Finally, a perspective toward future developments and opportunities is given. This review helps us getting a more and more clear picture and understanding on how complex movement can be programmed in light-responsive systems and how man-made adaptive materials can be invented, which can serve as an important guideline for further design of complex and advanced responsive materials.

Laser-Assisted Direct Grafting of Poly(ethyleneimine) on Poly(methyl methacrylate)

Demand for direct chemical modification of functional material on a surface is increasing in various fields. A new approach for a functionalized surface is investigated by applying a conventional laser in order to generate chemical activation by photothermal energy. Poly(ethyleneimine) (PEI), with a high density of amino groups, is chemically grafted on poly(methyl methacrylate) (PMMA) by irradiation of a CO₂ laser (10.6 μm). Laser parameters such as power, scan rate, and focal length are observed to play an important role in order to introduce effective photothermal energy for the chemical reaction between PEI and PMMA. By optimization of laser parameters, the amide compound is produced as a result of the reaction of amine from PEI and the ester of PMMA successfully. The PMMA surface modified with PEI is analyzed by XPS and TOF-SIMS to identify the functional groups. Furthermore, the surface is characterized in terms of wettability, adhesion force, and surface charge for various applications. Finally, reaction with dye and metal on the amine-terminated PMMA shows promising results in supplying a selective and reliable functional substrate.

Photoresponsive porous materials

Molecular machines, switches, and motors enable control over nanoscale molecular motion with unprecedented precision in artificial systems. Integration of these compounds into robust material scaffolds, in particular nanostructured solids, is a fabrication strategy for smart materials with unique properties that can be controlled with external stimuli. Here, we describe a subclass of these structures, namely light-responsive porous materials metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and porous aromatic frameworks (PAFs) appended with molecular photoswitches. In this review, we provide an overview of a broad range of light-responsive porous materials focusing on potential applications.

Spectroscopic study of the pH dependence of the optical properties of a water-soluble molecular photo-switch

In this report we present a UV-Vis spectroscopic study of the pH dependent optical properties of 4,4'-azobenzene dicarboxylic acid in aqueous solution. A combination of chemical (acid-base) and light stimuli is combined to demonstrate that the system undergoes two types of optical switching cycles. At neutral pH the azobenzene undergoes photo-induced cis-trans isomerisation. Upon pH reduction the UV-Vis spectra show changes consistent with aggregation of the azobenzene photo-switch. The photo-responsive behaviour is dependent on the pH and conformation of the azobenzene. The optical properties of the system are dependent on the mode of pH reduction and the isomeric cis/trans composition of the photo-switch, showing hierarchical control of self-assembly.

Light-Activated Organic Molecular Motors and Their Applications

Molecular motors are at the heart of cellular machinery, and they are involved in converting chemical and light energy inputs into efficient mechanical work. From a synthetic perspective, the most advanced molecular motors are rotators that are activated by light wherein a molecular subcomponent rotates unidirectionally around an axis. The mechanical work produced by arrays of molecular motors can be used to induce a macroscopic effect. Light activation offers advantages over biological chemically activated molecular motors because one can direct precise spatiotemporal inputs while conducting reactions in the gas phase, in solution and in vacuum, while generating no chemical byproducts or waste. In this review, we describe the origins of the first light-activated rotary motors and their modes of function, the structural modifications that led to newer motor designs with optimized rotary properties at variable activation wavelengths. Presented are molecular motor attachments to surfaces, their insertion into supramolecular structures and photomodulating materials, their use in catalysis, and their action in biological environments to produce exciting new prospects for biomedicine.

Design, Synthesis, Computational and Biological Evaluation of 4-Amino-3,5-dimercapto-1,2,4-triazole Surface Functionalized Gold Nanoparticles

Gold nanoparticles (AuNPs) are an obvious choice for rapid advance in nanotechnology due to their amenability of synthesis, functionalization and less toxicity. Functionalization of AuNP surface with 4-amino-3,5-dimercapto-1,2,4-triazole (ADMT) ligand as ADMT-AuNPs was investigated with the aim to probe the suitability of innovative product to develop new antibacterial and anticancer strategies. Various characterization studies like UV-spectra, Zeta size, Zeta potential, XRD, SEM, TEM and FTIR results of AuNPs and ADMT-AuNPs have been performed to study the structural and electronic properties. The studies revealed that the functionalized nanoparticles are highly crystalline in nature with the sizes ranging between 20-22 and 50-55 nm for AuNPs and ADMT-AuNPs, respectively with FCC structures. The characterization data reveals that the synthesized nanoparticles are stable and presence of strong interactions between the metallic surface and the organic ligand. Further, ADMT-AuNPs showed good antibacterial activity against Gram-positive and Gram-negative bacteria. MTT assay exhibited the cell viability with an IC50 value of 45.32 % v/v for ADMT-AuNPs against breast adenocarcinoma (MCF-7) cell lines. Molecular characterization i.e., in silico docking analysis helped in identifying and organizing the structural similarity/diversity at the molecular level. The in silico study indicated that the structure S1a has good glide score and glide energy for H-bonding among the possible conformations against bacterial and breast cancer protein. Molecular docking studies confirmed the introduction of conformational changes that are essential to surpass the potential energy barriers of ADMT-AuNPs for biocompatibility and proved that they hold a promising future in the medical field.

Toward Fast and Efficient Visible-Light-Driven Molecular Motors: A Minimal Design

A key goal in the development of light-driven rotary molecular motors is to facilitate their usage in biology and medicine by shifting the required irradiation wavelengths from the UV regime to the nondestructive visible regime. Although some progress has been made toward this goal, most available visible-light-driven motors either have relatively low quantum yields or require that thermal steps follow the photoisomerizations that underlie the rotary motion. Here, a minimal design for visible-light-driven motors without these drawbacks is presented and evaluated on the basis of state-of-the-art quantum chemical calculations and molecular dynamics simulations. The design, featuring dihydropyridinium and cyclohexenylidene motifs and comprising only five conjugated double bonds, is found to produce a full 360° rotation through fast photoisomerizations (excited-state lifetimes of ≈170-250 fs) powered by photons with energies well below 3 eV.

The Art of Building Small: From Molecular Switches to Motors (Nobel Lecture)

A journey into the nano-world: The ability to design, use and control motor-like functions at the molecular level sets the stage for numerous dynamic molecular systems. In his Nobel Lecture, B. L. Feringa describes the evolution of the field of molecular motors and explains how to program and control molecules by incorporating responsive and adaptive properties.

Photochemical Immobilization of Polymers on a Surface: Controlling Film Thickness and Wettability

In this manuscript, we demonstrate the control of film thickness and surface wettability in the photochemical immobilization of poly (vinyl alcohol) (PVA) to a self-assembled monolayer (SAM) containing a phthalimide chromophore. Surface attachment is characterized by ellipsometry and contact angle measurements. The wettability of the resulting films is shown to depend on the chemical composition of the polymer. The film thickness is shown to depend on the irradiation time and molecular weight of the polymer. Using a photomask, micropatterns of polymers can be grafted to the SAM. The photopatterned surface can be "developed" by coating with a thin layer of a mixture containing poly (styrene) (PS) and triphenylsulfonium triflate.

Rigid multipodal platforms for metal surfaces

In this review the recent progress in molecular platforms that form rigid and well-defined contact to a metal surface are discussed. Most of the presented examples have at least three anchoring units in order to control the spatial arrangement of the protruding molecular subunit. Another interesting feature is the lateral orientation of these foot structures which, depending on the particular application, is equally important as the spatial arrangement of the molecules. The numerous approaches towards assembling and organizing functional molecules into specific architectures on metal substrates are reviewed here. Particular attention is paid to variations of both, the core structures and the anchoring groups. Furthermore, the analytical methods enabling the investigation of individual molecules as well as monomolecular layers of ordered platform structures are summarized. The presented multipodal platforms bearing several anchoring groups form considerably more stable molecule-metal contacts than corresponding monopodal analogues and exhibit an enlarged separation of the functional molecules due to the increased footprint, as well as restrict tilting of the functional termini with respect to the metal surface. These platforms are thus ideally suited to tune important properties of the molecule-metal interface. On a single-molecule level, several of these platforms enable the control over the arrangement of the protruding rod-type molecular structures (e.g., molecular wires, switches, rotors, sensors) with respect to the surface of the substrate.

Controlling Motion at the Nanoscale: Rise of the Molecular Machines

As our understanding and control of intra- and intermolecular interactions evolve, ever more complex molecular systems are synthesized and assembled that are capable of performing work or completing sophisticated tasks at the molecular scale. Commonly referred to as molecular machines, these dynamic systems comprise an astonishingly diverse class of motifs and are designed to respond to a plethora of actuation stimuli. In this Review, we outline the conditions that distinguish simple switches and rotors from machines and draw from a variety of fields to highlight some of the most exciting recent examples of opportunities for driven molecular mechanics. Emphasis is placed on the need for controllable and hierarchical assembly of these molecular components to display measurable effects at the micro-, meso-, and macroscales. As in Nature, this strategy will lead to dramatic amplification of the work performed via the collective action of many machines organized in linear chains, on functionalized surfaces, or in three-dimensional assemblies.

Hydantoin-based molecular photoswitches

A new family of molecular photoswitches based on arylidenehydantoins is described together with their synthesis and photochemical and photophysical studies. A series of hydantoin derivatives have been prepared as single isomers using simple and versatile chemistry in good yields. Our studies show that the photostationary states of these compounds can be easily controlled by means of external factors, such as the light source or filters. Moreover, the detailed investigations proved that these switches are efficient (i.e., they make efficient use of the light energy, are high fatigue resistant, and are very photostable). In some cases, the switches can be completely turned on/off, a desirable feature for specific applications. A series of theoretical calculations have also been carried out to understand the photoisomerization mechanism at the molecular level.

Review: physico-chemical modification as a versatile strategy for the biocompatibility enhancement of biomaterials

Physico-chemical modification induced improvement in biocompatibility of materials.