Reversible photoswitchable wettability in noncovalently assembled multilayered films

Reversible and irreversible photoinduced changes in surface wettability were observed in noncovalently assembled multilayered films. The multilayered films studied were fabricated from a self-assembled monolayer (SAM) consisting of 4-(10-mercaptodecyloxy)pyridine-2,6-dicarboxylic acid on gold, Cu(II) ions complexed to the pyridine head group of the SAM, and either cis- (film 1) or trans- (film 2) stilbene-4,4'-dicarboxylic acid complexed to the Cu(II) ions. Irradiation of film 1 at wavelengths corresponding to the absorption band of the cis-stilbene isomer resulted in an irreversible chemical change and an irreversible increase in wettability, as indicated by surface contact angle and grazing incidence IR measurements. However, no evidence for cis-/trans-photoisomerization was observed. Films 3 and 4, similar to films 1 and 2 in that they consist of an underlying SAM, an intermediate layer consisting of Cu(II) ions, and either cis- or trans-stilbene-4,4'-dicarboxylic acid as the capping ligand, were fabricated with a mixed SAM that contained both 4-(10-mercaptodecyloxy)pyridine-2,6-dicarboxylic acid and 4-tert-butylbenzenethiol. Irradiation of these films at wavelengths corresponding to stilbene isomer absorption bands resulted in reversible cis- to trans- (film 3) and trans- to cis- (film 4) photoisomerization and reversible switching of the surface wettability between a low wetting state (cis-stilbene) and a high wetting state (trans-stilbene). The difference in observed behavior between films 1 and 2 and films 3 and 4 is attributed to the greater surface spacing afforded by the mixed monolayer, which allows greater conformational flexibility and lowers the steric barriers to isomerization.     

Correlation between the structure and wettability of photoswitchable hydrophilic azobenzene monolayers on silicon

Photoresponsive monolayers of hydrophilically substituted azobenzenes have been prepared by reaction on aminosilane monolayers on silicon surfaces. Grafting densities in the 0.2-1.0 molecule/nm(2) range were determined by X-ray reflectometry. The monolayers exhibit reversible photoisomerization, switching from a more hydrophilic trans state to a less hydrophilic cis state upon UV irradiation, in contrast with the usual behavior of most azobenzene monolayers that switch from a less to a more hydrophilic state. This indicates that the wettability is not dominated by the change in the dipole moment of the azobenzene moiety but originates from variations in the composition of the outer surface of the monolayers resulting from the reorientation of the substituent groups. The light-driven change in the water contact angle correlates linearly with the grafting density but remains small. However, the wettability contrast can be increased by forcing the molecules to stand in an improved vertical orientation, either by densifying the underlying aminosilane monolayer or by filling the voids left at the bottom of the layer of grafted azobenzene molecules.     

Photo-switched wettability on an electrostatic self-assembly azobenzene monolayer

A simple electrostatic self-assembly technique was used to fabricate a photo-switched azobenzene monolayer, on which superhydrophobicity and a large reversible CA change could be realized.     

Time-dependent organization and wettability of decanethiol self-assembled monolayer on Au(111) investigated with STM

A detailed study on the time-dependent organization of a decanethiol self-assembled monolayer (SAM) at a designed solution concentration onto a Au(111) surface has been performed with scanning tunneling microscopy (STM). The SAMs were prepared by immersing Au(111) into an ethanol solution containing 1 microM decanethiol with different immersion times. STM images revealed the formation process and adlayer structure of the SAMs. It was found that the molecules self-organized into adlayers from random separation to a well-defined structure. From 10 s, small domains with ordered molecular organization appeared, although random molecules could be observed on Au(111) at the very initial stage. At 30 s, the SAM consisted of uniform short stripes. Each stripe consisted of sets of decanethiol mainly containing eight molecules. With the immersion time increasing, the length of the stripes increased. At 5 min, the alkyl chains overlapped each other between the adjacent stripes, indicating the start of a stacked process. After immersing Au(111) in decanethiol solution for 3 days, a densely packed adlayer with a (radical 3 x radical 3)R30 degrees structure was observed. The formation process and structure of decanethiol SAMs are well related to sample preparation conditions. The wettability of the decanethiolate SAM-modified Au(111) surface was also investigated.     

Novel photoswitchable rotaxanes

A photoresponsive rotaxane based on the photoheterolysis of an acridane unit which is at the same time a bulky end group has been developed.     

Superparamagnetic iron oxide nanoparticles with photoswitchable fluorescence

The incorporation of superparamagnetic iron oxide nanoparticles with sulfur-oxidized diarylethene molecules resulted in a novel multifunctional nanosystem, in which the fluorescent performance and flocculation and dispersion are reversibly switched by light irradiation and external magnetic field, respectively.     

A photoswitchable rotaxane with a folded molecular thread

Novel [2]rotaxanes containing the tetracationic cyclophane cyclobis(paraquat-4,4-biphenylene) and a dumbbell-shaped molecular thread incorporating a photoactive diarylcycloheptatriene station as well as a photoinactive anisol station have been synthesized with yields of nearly 50 % by the alkylative endcapping method. The rotaxane was transformed into the related rotaxane incorporating a diaryl tropylium unit by electrochemical oxidation. The precursor of the cycloheptatrienyl rotaxane, the related pseudorotaxane, and the rotaxanes incorporating the diarylcycloheptatriene and the corresponding tropylium unit were characterized by (1)HNMR spectroscopy and UV/Vis spectroscopy. According to the NMR spectra, both the cycloheptatriene and the tropylium rotaxane possess a folded conformation enabling the tetracationic cyclophane to interact with two stations. The diarylcycloheptatriene station is incorporated inside the cavity of the cyclophane and the anisol station resides alongside the bipyridinium unit of the cyclophane. In contrast, the anisol station is inside the cyclophane in the tropylium rotaxane. The exchange between both conformations can be achieved by introducing the methoxy leaving group into the cycloheptatriene ring; the tropylium rotaxane is generated by photoheterolysis of this methoxy-substituted rotaxane, which reacts thermally back to the cycloheptatriene rotaxane, thus closing the switching cycle. These induced conformational changes achieve a so-called molecular machine.     

The first porous MOF with photoswitchable linker molecules

We synthesized a porous twofold interpenetrated MOF [Zn(2)(NDC)(2)(1)] (coined CAU-5) using 3-azo-phenyl-4,4'-bipyridine (1), 2,6-naphthalenedicarboxylic acid, and Zn(NO(3))(2)·6H(2)O. The azo-functionality protrudes into the pores, and can be switched, by irradiation with UV light (365 nm), from the thermodynamically stable trans-isomer to the cis-isomer. Back-switching was achieved thermally and with an irradiation wavelength of λ(max) = 440 nm.     

Photo-induced Alignment Behavior of Azobenzene-containing Polymer Films with Different Cross-linking Degree

Three series of amorphous copolymers containing azobenzene groups with various substituents and certain amounts of crosslinkable acrylic groups were prepared. The cross-linked polymer films were obtained by thermal polymerization of the acrylic groups in the copolymers, during which, by controlling the time of cross-linking reaction, the films can be made with different cross-linking degree (from 0 to 32%, which was monitored by FT-IR spectra measurement). Photo-induced alignment process of the films was performed under irradiation with linearly polarized light at 442 nm, and the effect of cross-linking degree on the photo-induced alignment rate was investigated. The dynamics of the photo-induced alignment was analyzed with biexponential curve fitting. The photo-induced alignment rate and the maximum transmittance of the films decreased because of the cross-linking. Furthermore, for the cross-linked samples, it was found that their saturated value of transmittances keep constant after repeated "writing" and "erasing" cycles. The findings reveal that the cross-linking of the film can effectively restrain the phototactic mass transport of azopolymer during irradiation by polarized light. The relationship between the cross-linking degree and the photo-induced alignment behavior of azopolymer is discussed in detail.     

Azobenzene-containing bent-core liquid crystals: an overview

ABSTRACT

Azo-functionalised materials are of special interest due to their photochromic nature, i.e. reversible trans–cis isomerisation upon photoirradiation. The combination of photosensitivity and liquid crystalline properties in the same molecule allows the material to be exploited for optical and optoelectronic devices. Azobenzene-based bent-core liquid crystals (BCLCs) have attracted considerable attention in recent years due to their rich mesomorphism. In this review, the main research directions and different molecular structures of bent-core molecules incorporating azobenzene unit and its subtype the so-called hockey-stick molecules are summarised. Additionally, azobenzene-based U-shaped molecules, hydrogen-bonded bent-shaped liquid crystalline materials and some selected examples of two different types of photoswitchable mesogenic dimers are provided. The nature, number and position of the lateral substitutions able to modify the phase behaviour of such BCLCs, affording in turn interesting liquid crystalline phases are discussed. Finally, the isomerisation process of these photosensitive BCLCs in solutions or in mesophases under the effect of UV–visible irradiation is summarised.     

Heading to photoswitchable magnets

The review concerns the strategic approaches to the photomodulation of the magnetic properties of molecular hybrid compounds combining magnetic and optical properties. The approaches developed include photocontrol of intra- and intermolecular magnetic coupling and manipulation of the effect of the photochromic sublattice on the bulk behavior of molecular magnets. In the framework of the first approach, we consider photoinduced charge-transfer phase transitions, changes in the spin state of magnetic centers, photoswitching of intramolecular exchange interactions between magnetic centers connected by a photochromic bridge, and generation of high-spin organic molecules forming high-spin groups due to intermolecular exchange interactions. In the framework of the second approach, we discuss hybrid polyfunctional compounds combining magnetic and photochromic sublattices in the same crystal lattice, as well as intercalation of organic photochromes into voids or inter-layer space of organic magnetics (or vice versa). Creation of such materials allows not only several functions to be combined in the same lattice, which is important for reducing the size of hardware components, but also these properties to be controlled and modified through synergistic effects. The results of basic research suggest that novel materials for various practical applications can be created using principles of crystal chemical engineering. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 704–721, April, 2008.     

Azobenzene-containing polyamic acid with excellent Langmuir-Blodgett-Kuhn film formation behavior suitable for all-optical switching

To generate command surfaces for all-optical switching, highly ordered polymeric Langmuir-Blodgett-Kuhn (LBK) multilayers were fabricated onto silicon substrates and gold-coated optical glass slides from novel azobenzene-bearing polyamic acid systems. Pronounced Bragg peaks and well-defined Kiessig fringes observed in the X-ray reflectivity measurement for samples on silicon substrates indicate that these films possess a regularly repeated Y-type LBK multilayer structure and ultrasmooth surfaces. Fourier transform infrared (FT-IR) spectra taken by grazing incidence reflection suggest specific orientations of the functional groups in the layers. The excellent film-forming properties of the polyamic acid allow for a smooth buildup of several hundreds of layers of the LBK films onto gold-coated glass slides, which in turn allows for determining the geometrical thickness and the anisotropic refractive indices of the films by using optical waveguide spectroscopy. Interestingly, the probe laser beam induced a distinct fluorescence signal from the films, which remained even after the film underwent a thermal imidization process at 160 degrees C for 8 h in vacuo. LBK films fabricated from these compounds can be successfully applied for all-optically switching the alignment of liquid crystals by irradiation with light of different wavelengths.     

Photoresponsive surfaces with controllable wettability

In this paper, current progress in the area of photoresponsive surfaces with controllable wettability is reviewed, including mainly surface conversion between wetting and anti-wetting, prepared from inorganic oxides (e.g., titanium dioxide, zinc oxide, and tungsten oxide) or/and photoactive organic molecules (e.g., azobenzene, and spiropyran), and movement of liquid droplets driven by molecular machines (e.g., molecular shuttles such as rotaxanes). Photoresponsive controllable wettability originates from a transition between the bistable states of photoresponsive materials. The exploration of the basic mechanisms provides a basis for the construction of novel smart responsive surfaces.     

Non-covalent assembly of a photoswitchable surface

A noncovalently bound multilayered thin film in which individual layers are linked by metal ligand interactions undergoes a photochemically initiated permanent change in surface wettability. The film consists of three separate layers: a SAM on gold of 4-[(10-mercaptodecyl)oxy]pyridine-2,6-dicarboxylic acid, a layer of Cu(II) ions that are deposited onto the SAM and bind symmetrically in the site provided by the two carboxylate groups and the pyridyl nitrogen atom, and a layer of cis-2,2'-dipyridylethylene, which caps the Cu(II) layer by complexation through both pyridyl nitrogen atoms (Film I). Photoexcitation of the film in chloroform at 300 nm leads to substantial cis-trans isomerization as indicated by conductivity, impedance, grazing incidence IR, and contact angle measurements. The latter show a decrease in contact angle (increase in wettability) of 17 degrees , which is attributed to exposure of both the underlying Cu(II) layer and one of the pyridyl ring nitrogen atoms following isomerization to the trans isomer.     

Dynamic contrast with reversibly photoswitchable fluorescent labels for imaging living cells

Interrogating living cells requires sensitive imaging of a large number of components in real time. The state-of-the-art of multiplexed imaging is usually limited to a few components. This review reports on the promise and the challenges of dynamic contrast to overcome this limitation.

Interrogating living cells requires sensitive imaging of a large number of components in real time.     

Colloidal wettability probed with X-ray microscopy

Colloids (colloidal particles or nanoparticles) and their in-situ characterizations are important topics in colloid and interface science. In-situ visualization of colloids with X-ray microscopy is a growing frontier. Here, after a brief introduction on the method, we focus on its application for identifying nanoscale wettability of colloidal particles at fluid interfaces, which is a critical factor in colloidal self-assembly. We discuss a quantitative study on colloidal wettability with two microscopic methods: (i) X-ray microscopy by visualizing natural oil–water interfaces and (ii) confocal microscopy by visualizing fluorescently-labeled interfaces. Both methods show consistent estimation results in colloid–fluid interfacial tensions. This comparison strongly suggests a feasibility of X-ray microscopy as a promising in-situ protocol in colloid research, without fluorescent staining. Finally, we address a prospect of X-ray imaging for colloid and interface science.     

Rational construction of self-assembly azobenzene derivative monolayers with photoswitchable surface properties

hoto-responsive azobenzene (ABZ) derivatives with different end groups (R) were employed to construct selfassembled monolayers (SAMs) on silicon substrates. The SAMs based on hydrophilic (4-(4'-aminophenylazo) benzoic acid, ABZ-COOH) show excellent reversible photoswitching performance with a large contact angle change of 35° under optimized process.     

Photoinduced Reversible Solid-to-Liquid Transitions and Directional Photofluidization of Azobenzene-containing Polymers

Photoinduced reversible liquefaction and solidification of polymers enable processing and healing of polymers with light. Some azobenzene-containing polymers(azopolymers) exhibit two types of photoinduced liquefaction properties: photoinduced reversible solid-toliquid transition and directional photofluidization. For the first type, light switches the glass transition temperature(Tg) values of azopolymers and induces reversible solid-to-liquid transitions. For the second type, polarized light gu...