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Systems
Photoresponsive Self-Assembled Systems Jinling Cheng PhD Thesis University of Groningen ISBN: 978-94-034-1463-8 eISBN: 978-94-034-1462-1
Print: Ipskamp Printing, Enschede, the Netherlands
The work described in this thesis was carried out at the Stratingh Institute for Chemistry, in compliance with the requirements of the Graduate School of Science (Faculty of Science and Engineering, University of Groningen).
This work was financially supported by the European Research Council, the
Photoresponsive Self-Assembled
Systems
PhD thesis
to obtain the degree of PhD at the University of Groningen
on the authority of the Rector Magnificus prof. E. Sterken
and in accordance with
the decision by the College of Deans. This thesis will be defended in public on
Friday 15 February 2019 at 12.45 hours
by
Jinling Cheng
born on 27 September 1989Chapter 1
Photochromic Self-Assembly Systems
1
Chapter 2
Bidirectional Photomodulation of Surface Tension in Langmuir Film
37
Chapter 3
Influence of Substituents on Molecular Motors in Photoresponsive
Cholesteric Liquid Crystals
57
Chapter 4
Controlling Orientation of Liquid Crystals at Aqueous-Cholesteric Liquid
Crystal Interfaces
93
Chapter 5
Photoinduced Helical Inversion of Cholesteric Liquid Crystals in Cells with
Inhomogeneous Thickness
123
Chapter 6
Dynamic Control of Chirality and Self-Assembly of Double-Stranded
Helicates with Light
129
Chapter 7
Towards a Photoswithable Mesogen
159
Summary
175
Samenvatting
179
Photochromic Self-Assembly Systems
In this introductory chapter, a literature overview on assembly processes and self-assembled architectures that can be controlled by external stimuli is given. In the first part, we deal with several strategies to construct self-assembled systems based on noncovalent interactions. The second part presents a brief overview on different classes of photochromic compounds (molecular switches and motors) and their switching mechanisms. The third part focuses on the incorporation of photochromic molecules into self-assembling modules that provide photoresponsive multicomponent self-assembled systems. Finally, an outline of the thesis is given.
micro-scale. These self-assembled materials have shown great potential for applications in many fields, including microelectronics,6–9 medicine and biotechnology,8,10 sensors,6,11–13 catalysis14,15, solar energy conversion16,17 and fuel cells16–18.
Furthermore, an important challenge for supramolecular chemists is to control molecular self-assembly in a responsive way by applying external stimuli such as electricity, pH, redox potential, magnetic field, ultrasound waves and light. In particular, control of molecular self-assembly by a light stimulus has many advantages. For instance, light stimulus is not only noninvasive with a high level of remote spatiotemporal resolution19 and fast reactivity (photochemical reactions occur in fs to ps),20,21 but is also widely utilized in natural and living systems such as human vision and photosynthesis in plants.22,23 The choice of wavelengths, polarization, and intensity of light is also important in the control of light-induced transformations. Some specific applications of such materials are often referred to “smart materials” including microfluidic devices,24,25 motors,20,21,26,27 actuators28,29 and controlled drug release systems10,30,31. The basic idea of manipulating supramolecular self-assembled structures to gain adaptive behavior by external light input has already been put forward by Shinkai et al. in 1987.32 Self-complementary azobenzene derivatives functionalized on one end and on the other by a crown ether were designed. The acceptor-guest binding properties of these molecules can be controlled by light due to the trans-cis isomerization of the azobenzene photochromic moiety.
Scheme 1.1 The main framework of this thesis.
In this chapter, we highlight some recent progress on photoresponsive self-assembled systems including (i) self-assembly of hydrogels controlled by photoresponsive host–guest
motif, (iii) photoresponsive liquid crystalline materials inducing color changes and micro-object rotation, (iv) reversible switching of conductance of molecular junctions and (v) photomanipulation of amphiphile assemblies. Before introducing these systems, we will briefly introduce the strategies generally applied for the construction of photoresponsive self-assembled systems involving the process of self-assembly and viable photochromic moieties with a focus on materials pertinent to this thesis (Scheme 1.1).
1.2 Examples of Self-Assemblies
Molecular self-assembly is a spontaneous self-association of small molecular components through intermolecular noncovalent interactions such as hydrogen bonding,33–35 hydrophobic interactions,3,36–39 π−π stacking,40–45 van der Waals forces,46,47 metal−ligand complexation,2,48–50 electrostatic interactions5,35,51,52, etc.
Early studies on self-assembly mainly have focused on molecular self-assembly in bulk solution, for example, the formation of micelle,3,53,54 vesicles,3,54 fibers,45,54–56 helicates,2,57–59 and liquid crystals.60,61 Interfacial self-assembly has drawn considerable attention over the past decades, because of its unique and ingenious roles for constructing materials at interfaces, including self-assembled monolayers,62–65 Langmuir–Blodgett films62,66–69 and liquid crystal droplets.70–72
1.2.1 Langmuir-Blodgett Films
The layer of molecules on a water surface is termed as Langmuir monolayer, and after transfer the monolayer from the water surface to solid substrate, it is called a
Langmuir-Blodgett (LB) film.66,73 This technique is named after Irving Langmuir and Katharine B. Blodgett. Langmuir was awarded the Nobel Prize for Chemistry in 1932 for his investigation of surface chemistry. Much of the current interest in LB films is based on the pioneering work of Kuhn, who used LB methods to control the position and orientation of functional molecules within complex assemblies, an elegant early example of an approach which is currently known as “supramolecular assembly”. 74
Basic Concepts of Langmuir-Blodgett Films
In order to form a stable Langmuir monolayer, it is necessary to use a substrate which is insoluble in water but soluble in a volatile solvent like chloroform, acetonitrile, benzene. LB-compatible materials consist of two fundamental parts, a hydrophilic “head” and a hydrophobic “tail”.66,73,75 The hydrophilic head, typically contains a strong dipole moment and is capable of hydrogen bonding, for example, −OH, −COOH, −NH2, etc. The hydrophobic
tail usually consists of a long aliphatic chain. Such molecules with spatially separated hydrophilic and hydrophobic regions are defined as amphiphiles. Typical examples of LB-compatible materials are fatty acids with long an alkane chain, like stearic acid, arachidic acid, etc., as shown in Figure 1.1.
Figure 1.2 Schematic of LB trough. The Wilhelmy plate monitors the surface through a microbalance interfaced with a computer.
The measurement of surface pressure (π) as a function of area per molecule (A) in the monolayer films is known as the isotherm characteristics, a conceptual illustration is shown in Figure 1.3. When the barriers are moving close, the pressure of the film increases and the random lying molecules change their order from gas phase (G) to a two-dimensional liquid expanded (LE) state. Upon further compression, the pressure begins to rise more steeply as the LE phase gives way to a more condensed phase in three dimensions. Rather, condensed phases tend to have short-range coherence and are called liquid condensed (LC) phases. If the surface pressure increases further, a much more condensed monolayer in which the film has low compressibility, movability and behaves like a 2D solid, named solid phase (S). A steep rise in surface pressure, and a distinct break in the monolayer isotherm is found at the collapse point as shown in Figure 1.3.66,73,75–77
Figure 1.3 Schematic representation of surface pressure – area per molecule (π -A) isotherm showing different phases of a monolayer at the air-water interface.
Figure 1.4 Deposition scheme for the transformation of the ordered monolayer from the liquid subphase onto solid substrates using the Langmuir-Blodgett technique during up (a) and down (b) strokes.
The term “Langmuir–Blodgett films” traditionally refers to monolayers that have been transferred from an aqueous subphase onto other solid substrates like glass, quartz, wafer or metal plates. There are two deposit strategies: one is horizontal deposition and the other is vertical deposition. Not only monolayers but also multilayer transfer can be achieved by immersion (down stroke) and emersion (upstroke) of the substrate. The substrate can be either highly hydrophilic or hydrophobic. When hydrophilic, the substrate originates below the water surface. After the monolayer has been spread and compressed to the desired transfer pressure, the submerged substrate is withdrawn vertically from the subphase with transfer involving the hydrophilic interactions between the head and the substrate (Figure 1.4a). When hydrophobic, the substrate originates above the water surface. After the monolayer is stabilized, the substrate is dipped through the monolayer and inverted transfer is achieved by hydrophobic interactions. (Figure 1.4b).
Characterization Method
Many analytical techniques are used to study LB films. Typically interest in film characteristics are thickness, interlayer spacing, molecular orientation, packing, film coverage, surface topology, etc. The techniques used to study these parameters are well described in textbooks on LB films,66,73,75,76 here we only briefly introduce some of the methods.
a. Determination of isotherms (in situ)73,76
As described above, the π-A isotherms measured in the LB trough present crucial information for the phase behavior upon monolayer compression. More importantly, the surface pressure and area are the free parameters of a 2D phase diagram and should be seen as the equivalents of pressure and volume in 3D. The isotherm is determined by both the compound and the subphase.
Figure 1.5 Schematic drawing of Brewster angle microscopy together with a Langmuir trough, illustrating its working principles.
Brewster Angle Microscopy (BAM) enables visualization of monolayers in real time, typically at the air-water interface in a Langmuir Trough. By detecting changes in the refractive index of the water surface in the presence of surfactant molecules, BAM measurements provide information on homogeneity, phase behavior and film morphology. It is possible to observe phase changes, phase separation, domain size, shape and packing.
c. X-ray diffraction (ex-situ)80,81
Figure 1.6 Schematic representation of thin film X-ray diffraction principle.
After the monolayers being transferred onto a solid substrate, several methods can be applied to characterize the LB films. X-ray diffraction is one of the most powerful methods used in the studies of the LB film. It works through measuring the diffraction of light of the molecular assembly formed by the material. As shown in Figure 1.6, parallel X-ray radiation beams (incidence and reflected) from periodic structure formed by a well-ordered LB film can be observed. Constructive interface will occur if the path difference between the two beams is an integer multiple of the optical λ, according to the Bragg formula:
d = 𝑛λ 2 sin 𝜃
The interlayer spacing can be determined, and from interlayer spacing, film thickness can be further inferred.
d. Microscopies
Microscopies including atomic force microscopy (AFM),82–86 scanning tunneling microscopy (STM)87 and scanning electron microscope (SEM)73,87 are widely used methods to characterize mono- or multiple layers on surfaces. These microscopic techniques are all capable of providing nanometer-scale information about the material and capable of detecting variations at the surface. The general principle is that a tip is scanning over a sample and the surface structure is recorded due to the interaction. For the discussion of the related scanning technique, see Chapter 2.
For these microscopies, each of them has its unique working mechanism. For AMF, according to the nature of the tip and sample, the tip experiences van der Waals, capillary electrostatic or magnetic forces. For STM, the working principle is based on recording the tunneling current flowing from a metallic tip to a conducting surface, which means STM can only work on conductive samples. In SEM, a focused high energy-electron beam is applied to scan over the sample surfaces.
Beside the methods we have discussed above, there are many other techniques that can be used, such as Infrared spectroscopy (IR), Raman spectroscopy, UV-vis absorption or Fluorescence spectroscopy if the sample is transparent.88
Applications
Current studies of LB films are mainly focused on two areas: 1) a detailed fundamental investigation of the physical nature and structure of Langmuir monolayers and LB films; 2) the applications that take advantage of the ability to prepare thin films with controlled thickness.
For example, in 2001, Swager and co-workers showed the conformation of individual polymers and interpolymer interactions in conjugated polymers which is controlled through the use of designed surfactant Langmuir films.89 They claimed that by mechanically inducing reversible conformational changes of these Langmuir monolayers, the precise interrelationship of the intrinsic optical properties of a conjugated polymer and a single chain's conformation and/or interpolymer interactions could be obtained (Figure 1.7).
Figure 1.7. Conformations and spatial arrangements of polymers 1-4 at the air-water interface and their reversible conversions between face-on, zipper and edge-on structures on the Langmuir trough. Reproduced with permission from ref 89. Copyright 2008 Springer Nature.
LB systems are also used for a variety of other applications, such as OLEDs,90,91 OFETs,92,93 bio-diagnostic devices,94,95 and catalysis.96,97
1.2.2 Liquid Crystals
Since the first discovery of Liquid Crystals (LCs) by Reinitzer who observed colored phenomena occurring in melts of cholesteryl acetate and cholesteryl benzoate in 1888, LC materials have drawn worldwide interest from both academia and industry, resulting in technologies such as the modern LC display technique (Figure 1.8).98–101
Figure 1.8. Timeline of the history of liquid crystal phase applications, from their discovery to the present day. Reproduced with permission from Ref 100. Copyright 2017 the Royal Society of Chemistry.
Basic Concepts of Liquid Crystals98,102–104
Liquid Crystals (LCs), which lie between the crystalline solid state and isotropic liquid state, is a unique state of matter that displays both anisotropic properties such as anisotropy of optical, electrical and magnetic properties of crystals and flow properties of ordinary liquids. Owing to its appearance between two condensed states of matter, the LC phase is often referred as the mesophase. The constituent entities of mesophases are known as mesogens.
Figure 1.9. Geometric shapes of organic compounds forming LC phases: rod-shaped, banana-shaped and Disc-banana-shaped LC.
LC systems can be classified in many ways. Depending on different stimuli conditions, LC materials can be divided into thermotropic and lyotropic LC,103, 104 as the former exhibits a
Figure 1.10 Molecular organizations in common LC phases. a. Smectic phases
The distinctive feature of a smectic mesophase is its stratification. The constituent molecules are arranged in layers and the layers can slide freely over another. Locally, the molecules within each layer are often slightly tilted as the tilting is random over the bulk phase. Depending on the molecular order in layers, a number of different types of smectic phases have been observed like Sm A-C, etc., as shown in Figure 1.10. When placed between glass substrates, smectic phases do not assume a simple arrangement, as the layers become distorted and can slide over each other in order to accommodate the interaction with the substrates.
b. Nematic phase
In a nematic mesophase, molecules possess a long-range orientational order along a preferred direction without long-range order in the positions. In a nematic phase, the molecules are able to rotate around their long axes, and there is no preferential arrangement of them. While optically examining a nematic mesophase under the microscope with cross polarizers, a Schlieren texture of a nematic phase can be typically observed, in which four dark brushes emerge from every point-defect indicating that the director is parallel to the polarizer or analyzer.
c. Cholesteric phase
The cholesteric mesophase is similar to the nematic: it has a long-range orientational order, but no long-range positional order of the centers of mass of molecules. It differs from the nematic mesophase in which the director distribution is precisely changed with additional twisting angles. In a chiral nematic phase formed by rodlike molecules, the direction of helical organization is, on average, perpendicular to the molecular long axis named as the handedness of helical structures which has distinct right- or left-handed helicity. While the pitch (p) is defined as the distance over which the director rotates a full cycle.
Figure 1.11 Chemical structures of commonly used nematic LC molecules and different classes of chiral dopants.
Formally, a nematic LC is a cholesteric LC with an infinite pitch, so that there is no phase transition between nematic and cholesteric mesophases. If nematic materials were doped with chiral molecules, cholesterics can be induced.105,106 The doped LC materials, obtained by dissolving a chiral guest (the dopant) in a nematic host, have many interesting properties. A variety of LC molecules have been developed and have served as LC hosts, as shown in Figure 1.11. The efficiency of a chiral molecule to twist the nematic mesogen into a helical structure is expressed as helical twisting power (HTP) which is highly dependent on the properties of the chiral dopant and the interaction between the achiral host and the chiral dopants. According to the equation:
β = 1
𝑝 ∙ 𝑐 ∙ 𝑒𝑒
Where c is either weight concentration or molar fraction of the chiral dopant, p is the helical pitch. The HTP can be calculated by the helical pitch which can be obtained by the Grandjean- Cano methods by using wedge cells or the lens.107–112 By far the most extensively studied class of chiral dopants consist of inherently chiral molecules, like binaphthyls, biphenyls and helicenes. These compounds are generally characterized to have a chiral plane or axis. Because of their synthetic accessibility and generally high HTP, they are widely applied in LCs research.106
Figure 1.12 A schematic representation of the origin of the Grandjean–Cano disclination lines, which formed when a cholesteric LC is inserted between (a) a plane-convex lens and a glass plate (b) wedge cell and the corresponding texture. Reproduced with permission from Ref 112. Copyright 2011 the Royal Society of Chemistry.
The wedge cell constitutes of two pieces of substrates which are treated to have planar alignment and assembled to form a wedge with an opening angle α. When a cholesteric LC is filled into the wedge cell, domains with twisting angles being multiples of p are formed and the declination lines at the boundaries of the domains can be measured as indicated in Figure 1.12a. With a known wedge angle α, the pitch can be calculated as:
𝑝
2= 𝑑 tan 𝛼
A similar strategy to determine the pitch based on the observation of the disclination lines that appear when cholesteric material is placed between a flat substrate which is appropriately rubbed to obtain the necessary alignment and a plane convex lens with a diameter(R) as illustrated in Fig. 1.12b. From the distance between these lines, one can obtain the cholesteric pitch as
𝑝 =𝑟𝑛+1
2 − 𝑟
𝑛2
𝑅𝑙𝑒𝑛𝑠 Alignment and Corresponding Textures113,114
For all practical applications of LC materials, a uniform director orientation is essential. For both fundamental studies and technological applications, it is necessary to align the LCs along a desired direction over macroscopic scales so that anisotropic properties are obtained. Commonly employed LC alignments are homeotropic alignment (director is perpendicular to the substrate surface), homogeneous alignment (director is parallel to the substrate surface and oriented along a single direction), and tilted alignment (director is tilted with respect to the substrate surface normal) as shown in Figure 1.13. In addition, other types of alignments such as twisted and hybrid alignment are also used in device applications.
Figure 1.13 Typical alignment of LC systems (a) planar alignment, (b) vertical alignment and corresponding textures (c and d). Reproduced with permission from Ref 114. Copyright 2003 Wiley-VCH, Weinheim.
a. Planar alignment
To achieve planar alignment of LCs, several methods can be adopted. One of the most popular techniques is the use of thin, rubbed polymer layers on a solid glass substrate. Generally, the polymers, like polyimides or nylon, are spin-coated onto the substrates and the substrates are subsequently rubbed. The rubbing method is often performed by use of a velvet cloth, giving rise to grooves, which causes the director to be aligned along this preferred direction. One of the most commonly observed textures of the cholesteric phase under planar alignment is the so-called oily streaks texture, as indicated in Figure 1.13c. b. Homeotropic alignment
To achieve homeotropic alignment, the substrate plates are generally coated with the amphiphilic surfactants. Such materials are dissolved in suitable solvents like chloroform, acetonitrile etc. and deposited on the substrate. After evaporation of the solvent, the substrate is covered with a thin and transparent film of the surfactant. An ideal method to deposit amphiphilic molecules controllably on solid substrates is the LB technique as we discussed before. One of the most commonly observed textures of the cholesteric phase with the homeotropic alignment is called a fingerprint texture (Figure 1.13d). Dark stripes appear, whenever the local director is oriented along the direction of light propagation as depicted in Figure 1.13d. For the cholesteric LCs, the periodicity of the equidistant stripe pattern is given by p/2 and can thus be used to determine the helical pitch.
Characterization of LC103,116
Liquid crystalline materials are often characterized by a set of complementary techniques to avoid ambiguities in the phase identifications.
a. Polarizing optical microscopy (POM)
Polarizing optical microscopy (POM) is the first technique employed to detect the presence of LC phases. Due to the birefringence of LCs, polarized light can be transited and recorded. LCs, organize themselves in structures that are relatively well defined but still with some defects. These defects, when viewed by POM, exhibit characteristic optical textures for
thermometers in each of the sample holders as shown in Figure 1.14.117 In this experiment, different types of phase transitions in one-component systems are observed.
Figure 1.14. DSC curves of a selected sample at rates of 10 oC/min; 2nd heating and cooling runs are shown. Cr – crystal phase; N – nematic phase; SmC – smectic C phase; SmI – smectic I phase; SmX – smectic X phase; I – isotropic phase. Reproduced with permission from Ref 102. Copyright 1999 Artech House.
c. X-Ray scattering
Figure 1.15 A schematic image represents the X-ray diffraction of different LC materials. Copyright 2011 Springer.
X-ray diffraction (XRD) is another extensively used technique, which is used to determine LC phase structures and phase symmetries. As we discussed in the section of LB characterizations, the well-known Bragg’s law forms the basis of XRD as an experimental technique for structure determination. The position and the intensity profile (or line shape) of a Bragg peak contain information regarding not only the inter-planar spacing of the organization but also the size, correlation volume and spatial-orientational distribution of specific structure planes in distorted or multi-domain samples as indicated in Figure 1.15. However, LCs often exhibit weak and broad peaks that are smaller in number than reflections from typical crystalline solids.
In addition to the above three general techniques, recently other methods like electro-optical studies,118,119 dielectric measurement,120,121 scanning probe122,123 and electron microscopy124,125 techniques have been used to fully characterize and understand the detailed molecular organization and dynamics at nanoscale in relatively complex LC phases. Applications
The properties of LC materials make them attractive in a variety of fields, especially in electronic optical displays. Furthermore, the need for lightweight, power-efficient displays revitalized the research of electro-optical properties of LCs.99–101 Apart from optical displays, LC have now found use in gas-liquid chromatography,126,127 optical visualization,128–130 switchable windows131,132 and many other applications.
Figure 1.16. Promotional figure for a commercially available LC thermometer sticker. Reproduced with permission from Ref 133. Copyright 2001 Wiley-VCH Verlag GmbH & Co. KGaA. An important property is that cholesteric LCs reflect light with a wavelength equal to their pitch according to the equation:
λ = 𝑛̅ 𝑝
As the pitch is sensitive to temperature variations, the color of the reflected light depends on the temperature. It is therefore possible to read the temperature just by looking at the color of the thermometer. By mixing different compounds, a device for several temperature ranges has been constructed.133 This technique has been used in the medical field. For example, this thermometer is used to measure body temperature with 0.1 °C resolution.
1.2.3 Helicates
As we discussed above, self-assembled complexes are due to noncovalent bond interactions via hydrogen bonding, π-π stacking, electrostatic interactions, etc. There is a different approach to build assembled materials based on metal coordination.2,49,134 In particular, transition metal helicates are one of the most basic units of metallosupramolecular chemistry and they have been studied extensively (see also the introduction of Chapter 6).135–137 Helicate complexes illustrate how self-assembly can be fully used in the specific formation of architecturally complex assemblies, directed by the cooperation between different parameters such as the stereoelectronic preference of the metal ions, the disposition of binding sites in the ligands and a range of binding strengths. By taking advantage of the relatively rigid yet dynamic nature of the metal-ligand interactions, the design and construction of discrete two-dimensional (2D) and three-dimensional (3D) metallosupramolecular architectures via coordination-driven self-assembly have received increasing attention like metal-organic frameworks (MOF),49,134,138,139 ladders,140,141 helices,135,136 polygons,142,143 polyhedral,144 knots145,146 and so on.
Double Helicates57,136,137,147
In 1953, the double helix structure of DNA was disclosed, and since then, the double helix has rightfully become a fascinating theme in chemistry and material science. In 1987, Lehn reported one of the first examples of ‘‘inorganic double helices’’, i.e. a class of CuI polynuclear metal complexes in which two linear polypyridine ligands are wrapped around
Figure 1.17. Schematic presentation of (a) double-stranded helicates formed by complexation of copper(I) cations and bipyridines, (b) triple-Stranded helicates formed by complexation of Fe(Ⅲ), Al(Ⅲ) or Ni(II) and bipyridines, (c) triple-Stranded helicates formed by complexation of Fe (III) and Rhodotorulic acid. Reproduced with permission from Ref 137, 138 and 148. Copyright 2002 AAAS, Copyright 2012 American Chemical Society and Copyright 1978 American Chemical Society.
Generally, nitrogen-donor molecules play an important role in the chemistry of helicates. The reason might be that nitrogen, which is sp2-hybridized and is embedded in an aromatic or any other unsaturated system, is an excellent donor for the coordination to metal ions as shown in Figure 1.17a. In this structure, bipyridine ligand adopts an S-type arrangement, 2,2′-bipyridines which bear substituents in the 6- and/or 6′-positions are ideal to form pseudo tetrahedral 2:1 metal complexes and at the same time, Cu(Ⅰ) metal centers adopt a tetrahedral coordination geometry. In the presence of a ligand L-L, the CuI ion will form a stable dinuclear double-stranded helicate [CuI2(L-L)2] 2+.
Triplet Helicates135,136,148,149
Triple-stranded helical coordination complexes have been known since 1970s. The steric information contained in the oligo-bipy strands based on bipy units connected in 6,6’-orientation is described to yield double helices on combining metal ions undergoing tetrahedral coordination. However, the steric effects due to the 6,6’-substituents hinder the binding of the metal ions of octahedral coordination geometry, therefore, 2,2′-bipyridine derivatives, which bear substituents in the 4- or 5-position of the pyridine units are potential ligands for triple-stranded helicates (as indicated in Figure 1.17b). At the same time, the metal ions should adopt octahedral coordination sites, for instance, iron(Ⅲ), aluminum(Ⅲ), nickel(Ⅱ) as well as silver(Ⅰ). With such ligands and octahedral metal ions, triple-stranded
molecular machines, self-assembled polypeptides, metal-organic nano-assemblies,164–166 micro-/nano-tubes,162,167 ionic liquid crystals.168,169 These materials are playing increasingly important roles in various emerging and interdisciplinary areas of researches, ranging from physics, chemistry, material science to biology and nanotechnologies.
1.3 Photochromic Moieties
Most photoresponsive self-assembled materials possess photoswitchable moieties in their building blocks in order to induce morphological transformation. From this viewpoint, photochromism is another key element in the design of smart materials.
Photochromism is used for the reversible photo-induced transformation of a molecule between at least two isomers, whose absorption spectra and other physical properties are distinctly different.170,171 Photochromic compounds have been used as molecular switches or motors providing key building blocks in data storage, optoelectronic and optical devices21,171 A competent photochromic compound should exhibit excellent characteristics such as fast response, high quantum yields in absorption and isomerization, stability in both isomers, distinct spectra profiles, large Stokes’ shift and so on. Among the photochromes, those in which the switching process involves a reversible electrocyclisation process or E/Z isomerization have received most attention.21,170,172–177
Figure 1.18 Schematic presentations of the main families of photochromic compounds and their reversible photo-induced transformations.
1.3.1 Photoinduced Electrocyclization Systems
In this section, photoswitchable processes based on ring opening and closing reactions such as diarylethenes, spiropyrans and fulgimides will be briefly discussed.
Diarylethenes 172–177
Diarylethene (Figure 1.18a) and its derivatives stand out as a highly efficient class of reversible photochromic compounds, as the light induced reversible photocyclization between their unconjugated colorless open-form and conjugated colored closed-form and these systems show excellent fatigue-resistance. Diarylethenes are classified as P-type photochromes as the closed form is thermally stable and hardly return to open-form in the dark at room temperature. The crucial features of diarylethenes are summarized as follows
1) Both the open and closed forms are thermally stable. 2) High fatigue resistance.
3) Visualizations of the isomerization by color.
Due to the above properties, diarylethenes are promising candidates for fabricating responsive supramolecular architectures and optical memory devices.
Fulgimides178,179
Another important class of photochromes, fulgimides (Figure 1.18c), have also been studied widely. Like diarylethenes, under UV irradiation, the open-form of fulgimides can undergo a ring closing process, resulting in the formation of a closed form, which is thermally stable and can be triggered to reverse by visible light irradiation.
spiropyran and merocyanine; 3) Remarkable difference in emission behavior as spiropyran is less emissive, merocyanine exhibits strong emission. Beyond above differences, the stimulus-reversible isomerization is sensitive to multiple triggers like solvents, metal ions, pH, redox potential and temperature which is truly impressive, and as a consequence, spiropyrans are robust materials and capable of fulfilling the requirements of several applications.
1.3.2 Photoinduced Central Double Bond Isomerization
In this section, photochromic systems based on double-bond trans-cis isomerization like azobenzene, stilbene and overcrowded alkenes will be briefly discussed.
Azobenzenes172,182
Azobenzenes (Figure 1.18d) are the most popular photochromes. In contrast to photochromism based on ring opening/closing process, azobenzene and its derivatives undergo a trans–cis isomerization of the central double bond. Generally, UV irradiation induces the thermal stable trans form to isomerize to the cis form. Most azobenzenes are classified as T-type photochromes as the cis form is thermally unstable and can return to
trans form in the dark at room temperature. The trans-cis isomerization of the azobenzene
moiety represents a model photochemical process in which one stereoisomer is favored thermally and the other stereoisomer is favored photochemically.
Photoinduced isomerization of azobenzenes also proceeds with a large structural change as reflected by the dipole moment and the geometry. Furthermore, the high efficiency in the isomerization process makes azobenzenes one of the most widely used systems.
Stilbenes171,172,183
Stilbenes (Figure 1.18e) consist of two phenyl rings separated by a C-C double bond, where isomerization occurs exclusively by rotation like azobenzene. Unlike azobenzenes, cis stilbene does not have an n-π* transition, besides, the photoexcitation of cis-stilbene generates the cyclic product, which cannot be reversed to the initial trans from. As a consequence, this drawback seriously limits the application of stilbenes.
Among such chromophores, stiff-stilbene and derivatives are useful owing to their unique characteristics. First, stiff-stilbene can adopt either a cis or trans configuration with respect to its central double bond. Second, the high activation barrier between the two isomers (∼43 kcal·mol−1, corresponding to a half-life of ∼109 y at 300 K) makes thermal E/Z
the photoisomerization of either isomer is high (50%). Fourth, the stiff-stilbene core is readily substituted using well-established synthetic methods.
Molecular Motors184–188
Since our group reported the first light-driven molecular motor with unidirectional 360° rotation in 1999, molecular motors based on overcrowded alkenes are attracting more attentions as a new family of photochromic materials with great potentials.188
Figure 1.19 Four-stage rotary cycle of first-generation molecular motor 1 and second-generation motor 2.
The first-generated motors are featured with two identical halves connected by the central double bond, where the rotational axial is locating. Due to the steric hindrance, the central double bond is forced out of planarity, adopting the molecule with opposite (M) or (P) helical structures. The unidirectional rotation involves four discrete steps, in which one half of the molecule rotates with respect to the other half around the central double bond. The rotary process involves two photochemical isomerization steps: step 1, stable (P,P)-trans-1 to unstable (M,M)-cis-1; and step 3, stable (P,P)- cis-1 to less stable (M,M)-trans-1, and each photochemical step is followed by an irreversible thermally activated helix inversion: step 2, less stable (M,M)-cis-1 to stable cis-1; step 4, less stable (M,M)-trans-1 to stable
(P,P)-trans-1.
The disadvantage of first generation motor is that the energy barriers for the two thermodynamically downhill helix inversion are different, therefore, a full cycle rotation requires multiple conditions (Figure 1.19a). The second-generation motors with symmetric stator were designed to address this issue. This design can simplify the isomerization process while keeping the unidirectional rotation.184,185
Bistricyclic ethylene189,190
Apart from the molecular motors, light-driven switching of non-helical overcrowded alkenes has been rarely explored. As shown in Figure 1.18g, the six-membered ring of the symmetric overcrowded bistricyclic ethylene rotates around the central double bond under UV irradiation, and the geometry changes from anti-folded to folded. The reverse syn-folded to anti-syn-folded isomerization in principle can also be triggered by photoisomerization of the central C–C double bond, however, the expected photoisomerization is not observed
viewpoint, the photoresponsive self-assembled structure of a water-soluble polymer based on interaction between the azobenzene arylic acid (pAC12Azo) guest with curdlan modified
with an α-cyclodextrin (CD-CUR) host reported by Harada et al. is unique because the photochromic molecule is used as a “photoresponsive key”(Figure 1.20).193,194 They utilized photo-tunable binding affinities of pAC12Azo guests with α-cyclodextrin (α-CD) host, as
trans-pAC12Azo shows higher binding affinity to α-CD compared to the cis-isomer.
Figure 1.20 Photoresponsive material with CD-CUR and pC12Azo upon irradiating with UV
light (365 nm) and visible light (430 nm) or heating at 60 °C. a) Chemical structure of the CD-CUR and photoresponsive pC12Azo interconversion. b) Schematic representation of a
photoresponsive sol−gel transition material with CD-CUR and pC12Azo upon irradiating with UV light (365 nm) and visible light (430 nm) or heating at 60 °C. Reproduced with permission from Ref 193. Copyright 2014 American Chemical Society.
A mixture of CD-CUR and pC12Azo in an aqueous solution forms a supramolecular hydrogel.
Irradiation of the polymer-hydrogel with UV light (365 nm) resulted in a gel-to-sol conversion as a result of lower binding affinity of photogenerated cis- pAC12Azo to α-CD. Subsequent
irradiation of the photogenerated solution with visible light (430 nm) or heating (60 oC) again could convert it to gelation (within 2 min). The association constant of α-CD with the trans- pAC12Azo (Ka= 1100 M-1) is much larger than that with the cis- pAC12Azo (Ka= 4.1 M-1). These
results indicate that the control of the association and dissociation between the α-CD unit and the azobenzene unit by irradiation affected the phase transition.
Figure 1.21. Design of polyMOCs with photoswitchable topology (a) Schematic illustration of photo-regulated interconversion between two different network topologies. (b) Chemical structure of the photoresponsive polymer ligand and a schematic of MOC interconversion. Reproduced with permission from Ref. 195. Copyright 2018 Springer Nature.
Another example is the photoresponsive self-assembly of polymer hydrogel based on metal-organic cages (polyMOC) which feature two bis-pyridyl dithienylethene groups reported by Johnson et al195 as indicated in Figure 1.21. In polyMOCs, the network junctions are nanoscale metalxligandy (MxLy) cages of defined shape, stoichiometry and the MOC structure
defines the polyMOC topology and bulk properties. Due to the photoresponsive ring opening/closing behavior of diarylethene unit, these ligands could form MOCs that could reversibly switch between small Pd3L6 rings and large Pd24L48 rhombicuboctahedra by
irradiating with green and ultraviolet (UV) light, respectively. This photoswitching behavior produces coherent changes in several network properties, including branch functionality, junction fluctuations, defect tolerance, shear modulus, stress-relaxation behavior and self-healing.
1.4.2 Photoresponsive Liquid Crystals
The stimuli responsive characteristics of LCs with the combination of photochromic compounds and self-assemblies could potentially yield multiple functional materials with novel and/or enhanced properties.
Figure 1.22. Features of a light-driven molecular motor as a dopant in E7: (a) structure of the motor. (b) Color change with time upon irradiation with 365 nm light (up) and during thermal helix inversion (down). (c) Glass rod rotating on the liquid crystal during irradiation with ultraviolet light. Reproduced with permission from Ref 197, 201 and 202. Copyright 2006 Springer Nature. Copyright 2006 American Chemical Society. Copyright 2002 National Academy of Sciences.
For example, a second generation motor was adopted as a chiral dopant to induce photoresponsive cholesteric LCs as shown in Figure 1.22.197–202 This motor has high compatibility within the LC matrix which contributes to the high HTP of this material. Upon light irradiation, the cholesteric LC film displayed tunable color over the entire visible spectrum. However, the most appealing characteristic of the system is that upon photochemical and thermal isomerization, cholesteric LCs exhibit handedness inversion. In addition, the photochemical and thermal isomerization drives the rotation of a microscale object on the top of LC film. This is a classic example of converting light energy to mechanical work enabled by a nanoscale molecular motor (more details can be seen in Chapter 3).197 (For illustration examples of responsive LC-polymer please see the work of prof. Dr. Nathalie Katsonis, prof. Dr. Dirk J. Broer and ect.)
1.4.3 Photoresponsive Self-Assembled Monolayers
Self-assembled monolayers (SAMs) are spontaneously organized assemblies of molecules formed by adsorption from solution or the gas phase onto the surface of solids or liquids. By introducing photochromic compounds into SAMs, surface properties, like surface wettability,203 biological adhesion,204,205 electron conductivity,206 etc., can be controlled with a light stimulus.
Figure 1.23. Current changes of the silicon (111) surface modified with diarylethene due to photoisomerization by alternating UV and visible light irradiation. Reproduced with permission from Ref. 207. Copyright 2011 American Chemical Society.
For example, Nishihara et al. reported light-controlled reversible switching conductance of molecular junctions constructed with a diarylethene monolayer functionalized silicon-(111) surface, where the closed-form diarylethene on the silicon surface exhibited higher electrical conductance than its open-form as illustrated in Figure 1.23.207 The light induced reversible switching of conductance presumably resulted from the differences in molecular geometry and in π-conjugation between ring-open and ring-closed forms. UV irradiation at 313 nm of the devices, at molecule level, resulted in the formation of closed-form, and for the device, an increase of the initial current of the silicon surface was observed. Irradiation with visible light at 578 nm for 2 h reversed the current back to the value of the initial state.
1.4.4 Photoresponsive Amphiphiles
Here, as a new class of photoresponsive assembled materials, we introduce self-assembled amphiphiles whose properties or structures can be controlled due to morphology changes of the constituent building blocks. There is a growing interest in self-assembled amphiphiles because of their potential in biologically relevant applications like drug delivery.208,209 Manipulation of self-assembled amphiphilic systems by external light irradiation thus allows the fabrication of smart nanomaterials and responsive functions.
Figure 1.24. Behavior of photoresponsive amphiphilic copolymers. (a) structure and photo-behavior of spiropyran based amphiphile unit (b) Scheme and photographs for the Nile red encapsulated micelles (I) upon UV irradiation at 365 nm, (II) then visible-light irradiation at 530 nm, and (III) reloading of Nile red. Reproduced with permission from Ref. 210. Copyright 2012 American Chemical Society
A set of amphiphilic random copolymers has been constructed directly by hydrophilic tails, a hydrophobic main chain, and functional spiropyran monomers (Figure 1.24).210 The copolymers were able to self-assemble in water to polymeric micelles with spiropyran incorporated into its backbone. As a result, the reversible transfer between the ring closed-spiropyran and opened-merocyanine by UV and visible light irradiation triggered the morphology change of the amphiphilic copolymer. As expected, reversible disruption and regeneration characteristics upon UV and visible light is achieved. In addition, the hydrophobic dye Nile red, which served as a model for drug delivery, was encapsulated within the polymeric micelle, its releasing and reloading could be controlled by exposure to UV and visible light, respectively.
1.4.5 Other Photoresponsive Self-Assembled Systems
Apart from the aforementioned examples, many other systems have been reported, such as photoswitchable ion channels and receptors,211–213 photocontrolled peptides and proteins,214,215 photoresponsive metal–organic nanoassemblies (metal–organic molecular cages and metal–organic frameworks),216–219 etc.
1.5 Conclusion
Self-assembly is now being intensively studied in chemistry, physics, biology, and materials engineering and has become an important ‘‘bottom-up’’ approach to create intriguing structures for different applications. Self-assembly is not only a practical approach for creating a variety of nanostructures, but also shows great superiority in building hierarchical
molecular self-assembly in bulk solution, including the resultant dye aggregates, liposomes, vesicles, liquid crystals, gels, helicates, MOFs or at the interface such as self-assembled monolayers, Langmuir–Blodgett films and liquid crystal droplets.
Stimuli-responsive materials offer considerable challenges in the area of material science. The photoresponsive systems provide the motivation for developing corresponding artificial systems using self-assembly to address issues such as quantum efficiency, selectivity, cooperativity and amplification. The use of light as an external stimulus has particular advantages because it can function in remote distance with very precise control in space and time. A practical strategy for developing photoresponsive materials is to utilize molecules that can undergo considerable change in geometries upon photo-irradiation, consequently, the photochemical and photophysical properties of the self-assembled systems can be modulated.
The incorporation of photochromic molecules into diverse networks and resulting functional materials provides them with intriguing photoresponsive properties, which is expected to stimulate the development of advanced photochromic devices with novel, multiple functionalities. It is to be expected that photoresponsive self-assembled systems are going to play an important role in developing smart materials for applications in various fields.
1.6 Outline of the Thesis
This thesis is exclusively focused on adopting photoresponsive molecules in self-assembled systems, where the molecules can operate in a cooperate fashion for responsive functions. In chapter 2, an amphiphile with central bis(thiaxanthylidene) unit bearing hydrophilic tetraethylene glycol and hydrophobic alkyl tails is described in order to develop a stable Langmuir monolayer at the air-water interface as well as the mixed monolayer containing both the amphiphile and DPPC. Langmuir film surface pressure behavior are examined systematically in response to UV irradiation and found to be dramatically influenced by the packing of the molecules. Our investigation suggests that, besides the modification of molecular structure upon irradiation, the packing mode within the Langmuir film can be used to tune the properties of the whole system.
An investigation of LC materials endowed with molecular motors is presented in Chapters 3, 4 and 5. In Chapter 3, a family of second-generation motors with different substituents at various positions were synthesized and used as dopants in E7 host. It was found that the geometrical changes altered the intermolecular association and compatibility between the chiral dopants and the NLC molecules, which have an important influence on HTP. Under irradiation with UV light, photoisomerization of the motor-units led to changing of the cholesteric pitch and HTP; their reversible decay and reassembly was evidenced upon sequential UV irradiation and thermal relaxation, respectively. Based on the variation of HTP, we could control the macroscopic rotational motion of micro-sized glass rods by irradiation and thermal relaxation. In chapter 4, the interfacial interaction between an aqueous layer and cholesteric LC is investigated. The helical pitch of the cholesteric LC and concentrations of surfactant (SDS) in the aqueous phase are two key elements for the aqueous-LC interaction as a series of reproducible and distinct surface-driven distortions are observed. The helical pitch can be addressed by doping different concentration of molecular motors, but also photochemical and thermal isomerization of molecular motors upon UV irradiation allow the aqueous-LC interaction to be regulated by light irradiation. Chapter 5 is focused on
In chapter 7, the design and properties of molecular motors based on overcrowded alkenes and 8OCB are described. 8OCB is a well-known liquid crystal monomer. The aim of the study in this chapter is to obtain materials with the combined properties of 8OCB and motor units. The 8OCB moiety is capable of forming a mesogenic phase whereas the motor unit is known for its photoresponsive behavior. The self-assembled behavior of the compound is studied using polarizing optical microscopy (POM) as well as wide-angle X-ray scattering (WAXS). The phase transitions of the materials in heating and cooling cycles were further confirmed by differential scanning calorimetry (DSC).
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