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Surface-Induced Selection During In Situ Photoswitching at the Solid/Liquid Interface
Abstract
Here we report for the first time a submolecularly resolved scanning tunneling microscopy (STM) study at the solid/liquid interface of the in situ reversible interconversion between two isomers of a diarylethene photoswitch, that is, open and closed form, self-assembled on a graphite surface. Prolonged irradiation with UV light led to the in situ irreversible formation of another isomer as by-product of the reaction, which due to its preferential physisorption accumulates at the surface. By making use of a simple yet powerful thermodynamic model we provide a quantitative description for the observed surface-induced selection of one isomeric form.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Supplementary resource (1)
... The central point in the design of an efficient photoresponsive monolayer at the liquid/solid interface lies in the choice of the photochromic units. To date, existing systems have been based on the photodecomposition of diazoanthrone [84] or the photoisomerization of thioindigo [85], anthracene dimer [86], azobenzene [87][88][89][90][91][92][93][94][95], stilbene [94,96], diarylethene [97][98][99][100][101][102][103][104], or terthiophene [105] (Fig. 4). ...
... Lateral ester alkyl chains sufficiently stabilized molecular ordering for a similar diarylethene reported recently by Cechini, Hecht, Samorì, and co-workers [99]. A two-step transition was again observed for diarylethene 8, and reversible transition between isomers 8o and 8c was possible (Fig. 11). ...
... Adapted with permission from Reference [99]. of the open-ring isomer 9o, which provided two polymorphs (orderings ␣ and ) depending on its concentration. Irradiation with UV light led to the formation of an annulated-ring isomer 9a that exhibited the characteristic honeycomb molecular ordering ␥. ...
Control over molecular nanostructure is of the utmost importance in bottom-up strategies to create functionalized surfaces for electronic devices and advanced materials. In this context, the study of two-dimensional self-assembled structures consisting of organic molecules on surfaces using scanning tunneling microscopy (STM) has been the subject of intensive research. The formation of stimuli-responsive assemblies, especially photoresponsive ones, on surfaces is attracting interest. Meanwhile, assemblies formed at the liquid/solid interface have been extensively studied using STM from a supramolecular chemistry perspective in order to understand the assembly process of the molecules from the solution phase to the substrate interface. In this review, an overview of advances in photoresponsive supramolecular self-assemblies formed at the liquid/solid interface is given. Recent progress in the analysis of the adsorption process using the nucleation-elongation model of two-dimensional self-assembly will be featured and discussed in the context of photochemical control of the assembly.
... which has a finite value that is independent of a. Importantly, eqn (15) can be numerically evaluated using an explicit molecular model of the SAM and the substrate. 14,29,30 In the limit of sufficiently large supramolecular architectures (a -+N), eqn (8) yields ...
... In fact, since g is proportional to Dm AB , 2D self-assembly will occur if and only if g is lower than zero. In addition, since g is architecture-dependent through both the strength of molecular recognition in the monolayer (E sam 0 ) and the density of packing (A uc 0 ), its numerical evaluation allows one to compare the relative stability of different SAMs, which is useful to explore competitive equilibria at surfaces 29,30 and rationalize the concentration-dependent 2D polymorphism. 25,26,44 To illustrate this aspect, we analyzed the concentration dependence of trimesic acid (TMA) self-assembly, which was shown by the STM to form three distinct monolayers: (1) the porous hexagonal or chickenwire (CHK) architecture; (2) the slightly more dense flower (FLW); and (3) the densely-packed superflower (SFW); see Fig. 3. ...
... 20,23,24 The existence of a chemical potential per unit cell provides a simple expression for the surface free energy of the SAM (eqn (19)), which gives numerical access to its thermodynamic stability in the limit of the ideal gas and the rigid rotor, harmonic oscillator approximations. Remarkably, eqn (19) has already been used by us to rationalize the chain-length dependence of the graphite-exfoliation yield in the presence of fatty acids, 14 provide an interpretation of competitive self-assembly of three diarylethene photoisomers on graphite, 29 and design an improved graphite/graphene surface binder by the perchloro substitution of coronene. 30 Interestingly, this expression of g provides a theoretical foundation to the per molecule, per unit of area approach of Lackinger and coworkers. ...
Molecular self-assembly at surfaces and interfaces is a prominent example of self-organization of matter with outstanding technological applications. The ability to predict the equilibrium structure of the self-assembled monolayer (SAM) is of fundamental importance and would boost the development of bottom-up strategies in a number of fields. Here, we present a self-consistent theory for a first-principle interpretation of 2D self-assembly based on modeling and statistical thermodynamics. Our development extends the treatment from finite-size to infinite supramolecular objects and delineates a general framework in which previous approaches can be recovered as particular cases. By proving the existence of a chemical potential per unit cell, we derive an expression for the surface free energy of the SAM (γ), which provides access to the thermodynamic stability of the monolayer in the limit of the ideal gas approximation and the model of energetics in use. Further manipulations of this result provide another expression of γ, which makes the concentration dependence as well as the temperature dependence of 2D self-assembly explicit. In the limit of the approximations above, this second result was used to analyze competitive equilibria at surfaces and rationalize the concentration-and temperature-dependent polymorphism in 2D. Finally, the theory predicts that there exists a critical aggregation concentration (Ccac) of monomers above which 2D self-assembly can be viewed as a "precipitation" in a solubility equilibrium. Numerical analysis of thirteen model SAMs on graphene shows that the value of Ccac sets an absolute scale of 2D self-assembly propensity, which is useful to compare chemically distinct and apparently unrelated self-assembly reactions.
... Once organized on a surface, photochromic switches can be visualized at a molecular level by scanning tunneling microscopy (STM) [10,[38][39][40] which remains always challenging. Azobenzene and its derivatives are among the photochromic molecules which have been investigated the most on-surface, and which switch between their cis and trans forms both in Ultra High Vacuum (UHV) [41][42][43] and under solid-liquid conditions [44][45][46][47][48]. ...
... A molecular model using the protonated CF bpy-DAE-bpy is proposed in Figure 1c, where plausible hydrogen bonds between the bipyridines are formed by adjusting the nitrogen positions of the molecules. The protonated CF bpy-DAE-bpy isomers form stable intermolecular N···H-N hydrogen bonds [38][39][40] with adjacent molecules from the neighboring stripes at the two terminal bipyridines in Figure 1c. Each bipyridine unit interacts with its adjacent molecule by double N···H-N hydrogen bonds. ...
A series of ligands, which terminate with ditopic bipyridines connected by a variable central bridge, fluorene biEDOT or azo-benzene, have been investigated by high-resolution scanning tunneling microscopy (STM). The bipyridines can form different hydrogen bonds or can coordinate with transition metal ions at the solid/liquid interface. Their interactions allow such molecules to self-assemble on HOPG. The bridges provide redox-active, photoactive, or fluorescent functionalities. Both the bipyridine terminals and the bridges are units able to switch the molecules in terms of structure organization and electronic properties. Multi-functional switches or transitions are thus generated, for example (i) protonation generates a molecular cis-like to trans-like isomerization and switches the nanoscale organization of the molecule on the surface, (ii) Co(II) cations react with the pyridines to generate on-surface coordination complexes, and (iii) changing the central bridge induces different self-organized structures on the surface. A series of bpy-X-bpy molecules showing different molecular isomers, structural phase transitions and coordination configurations have thus been revealed by means of high-resolution STM.
... Once organized on a surface, photochromic switches can be visualized at a molecular level by scanning tunneling microscopy (STM) [10,[38][39][40] which always remains challenging. Azobenzene and its derivatives are among the photochromic molecules which have been investigated the most on-surface, and which switch between their cis and trans forms both in ultra-high vacuum (UHV) [41][42][43] and under solid-liquid conditions [44][45][46][47][48]. ...
... A molecular model using the protonated CF bpy-DAE-bpy is proposed in Figure 1c, where plausible hydrogen bonds between the bipyridines are formed by adjusting the nitrogen positions of the molecules. The protonated CF bpy-DAE-bpy isomers form stable intermolecular N···H-N hydrogen bonds [38][39][40] with adjacent molecules from the neighboring stripes at the two terminal bipyridines in Figure 1c. Each bipyridine unit interacts with its adjacent molecule by double N···H-N hydrogen bonds. ...
Diarylethene is a prototypical molecular switch that can be reversibly photoisomerized between its open and closed forms. Ligands bpy-DAE-bpy, consisting of a phenyl-diarylethene-phenyl (DAE) central core and bipyridine (bpy) terminal substituents, are able to self-organize. They are investigated by scanning tunneling microscopy at the solid–liquid interface. Upon light irradiation, cooperative photochromic switching of the ligands is recognized down to the submolecular level. The closed isomers show different electron density of states (DOS) contrasts, attributed to the HOMO or LUMO molecular orbitals observed. More importantly, the LUMO images show remarkable differences between the open and closed isomers, attributed to combined topographic and electronic contrasts mainly on the DAE moieties. The electronic contrasts from multiple HOMO or LUMO distributions, combined with topographic distortion of the open or closed DAE, are interpreted by density functional theory (DFT) calculations.
... Once organized on a surface, photochromic switches can be visualized at a molecular level by scanning tunneling microscopy (STM) [10,[38][39][40] which remains always challenging. Azobenzene and its derivatives are among the photochromic molecules which have been investigated the most on-surface, and which switch between their cis and trans forms both in Ultra High Vacuum (UHV) [41][42][43] and under solid-liquid conditions [44][45][46][47][48]. ...
... A molecular model using the protonated CF bpy-DAE-bpy is proposed in Figure 1c, where plausible hydrogen bonds between the bipyridines are formed by adjusting the nitrogen positions of the molecules. The protonated CF bpy-DAE-bpy isomers form stable intermolecular N···H-N hydrogen bonds [38][39][40] with adjacent molecules from the neighboring stripes at the two terminal bipyridines in Figure 1c. Each bipyridine unit interacts with its adjacent molecule by double N···H-N hydrogen bonds. ...
Diarylethene is a prototypical molecular switch that can be reversibly photoisomerized between its open and closed forms. Ligands bpy-DAE-bpy, consisting of a phenyl-diarylethene-phenyl (DAE) central core and bipyridine (bpy) terminal substituents, are able to self-organize. They are investigated by scanning tunneling microscopy at the solid-liquid interface. Upon light irradiation, cooperative photochromic switching of the ligands is recognized down to the sub-molecular level. The closed isomers show different electron density of states (DOS) contrasts, attributed to the HOMO or LUMO molecular orbitals observed. More importantly, the LUMO images show remarkable differences between the open and closed isomers, attributed to combined topographic and electronic contrasts mainly on the DAE moieties. The electronic contrasts from multiple HOMO or LUMO distributions, combined with topographic distortion of the open or closed DAE, are interpreted by density functional theory (DFT) calculations.
... Unraveling the dynamics of self-assembly 16 could provide higher control over key parameters governing the mechanism of molecular self-organization in 2D, thereby permitting to further engineer molecular functionalization 7,17 . Scanning tunneling microscopy (STM) imaging enabled to monitor with subnanometer spatial resolution the kinetics of nucleation and rearrangements taking place in supramolecular adlayers at solid/ liquid interfaces [18][19][20][21] , including light-responsive assemblies composed of photochromic molecules [22][23][24][25][26] . However, the information provided by STM is confined at a length scale of a few tens of square nanometers, thus not suitable to describe population dynamics of self-assembly on macroscopic distances, which involves billions of molecules. ...
... A very different scenario is encountered when the supernatant solution was irradiated with UV light to trigger the SP → MC isomerization. Immediately after irradiation, an ordered assembly could be imaged with sub-molecular resolution (Fig. 2b), displaying a lamellar assembly in which the long alkyl chains lie flat on the HOPG surface, as typically observed for alkyl chains on graphitic surfaces 2,25 . The unit cell of the assembly, as extracted from STM image analysis, is composed of two MC molecules and characterized by lattice parameters a = 1.1 ± 0.1 nm, and b = 3.9 ± 0.1 nm, forming an angle α = 90 ± 2°l eading to an area A = 4.3 ± 0.2 nm 2 . ...
Mastering the dynamics of molecular assembly on surfaces enables the engineering of predictable structural motifs to bestow programmable properties upon target substrates. Yet, monitoring self-assembly in real time on technologically relevant interfaces between a substrate and a solution is challenging, due to experimental complexity of disentangling interfacial from bulk phenomena. Here, we show that graphene devices can be used as highly sensitive detectors to read out the dynamics of molecular self-assembly at the solid/liquid interface in-situ. Irradiation of a photochromic molecule is used to trigger the formation of a metastable self-assembled adlayer on graphene and the dynamics of this process are monitored by tracking the current in the device over time. In perspective, the electrical readout in graphene devices is a diagnostic and highly sensitive means to resolve molecular ensemble dynamics occurring down to the nanosecond time scale, thereby providing a practical and powerful tool to investigate molecular self-organization in 2D.
... [78] Another report showed (by molecular mechanics calculations) that the annulated isomer has a denser packing than that of the c form, which explains its strong surface adsorption and disproportionate dominance on STM images. [79] The irreversibility of the annulated form could make it an unwanted side-reaction from the viewpoints of rewritable optical memories or switches but it could be useful for permanent memories (non-rewritable or write-once memories) similar to applications sought for diarylethenes with very low cycloreversion quantum yields. [80] In a subsequent report, Matsuda's team found that the o form of 1.22 (Figure 1.5a) can have two packing patterns depending on the concentration of the solution. ...
... In fact, most studies in the solid-liquid interface that elucidated molecule-molecule and surface-molecule interactions utilized the non-functional perfluoro head on top of the photochromic core which usually does not interact with the surface in a way that would show contrast in the STM images. [73][74][75][76][77][78][79][80] When Matsuda's team used a terarylene derivative having 2-phenylthiophene moieties instead of perfluorocyclopentene, they were able to develop switching involving four states beyond the usual photochromic switching of the compound (Figure 1.5). [81] Under UHV, the strong dipole of perfluorocyclopentene group was shown to aid in assembly formation [88] but the electronic switching remained mainly dependent on the rearrangement around the aromatic rings. ...
Photoswitching diarylethenes, and their terarylene derivatives, are promising for the next generation optoelectronic devices because of their excellent photochemical properties. To make them viable for miniaturized electronic devices, it is necessary to study this class of molecules at the single molecular level by scanning tunneling microscopy under ultra-high vacuum (UHV STM). This thesis has three parts: 1) development of terarylenes highly sensitive to switching; (2) their modification for STM studies; and 3) results of STM investigations. To be studied at the single molecular level by STM, terarylenes with high switching sensitivity have been selected. These compounds display high quantum yields of up to 100 %. However, the cycloreversion reaction remains low so an alternative route, through a chain-reaction oxidative mechanism, has been sought. In the first part, we show that the efficiency and speed of this reaction may be controlled by attachment of aromatic groups on the reactive carbons. In the second part, we functionalized these molecules for STM studies by attaching tert-butyl and chloride groups. These substituents preserve their excellent photochemical and switching properties while tert-butyl groups show bright contrast in STM images, minimize aggregation of these molecules on the surface, and slightly decouple the molecule from the surface. The chlorine group has been introduced to direct their surface assembly on insulating substrates composed of crystalline NaCl bilayer previously grown over a metallic substrate. In the third part, results of STM are presented. We developed a new bottom-up approach for forming reproducible nanoassemblies of the unmodified terarylene at 77 K. Meanwhile, at 5 K, the terarylene functionalized with tert-butyl groups present different forms on the Ag(111) surface. From the positioning of the high-contrast tert-butyl groups and with the aid of DFT calculations, we assign different conformations of the molecule on the surface. On NaCl/Ag(111), direct visualization of the occupied and unoccupied states could be achieved. This illustrates that for these applications, molecules with appropriate properties can be interesting candidates for STM studies to obtain information at the single molecular level. Such molecules may be redesigned with a consideration of the surface as its mere presence may induce behavior previously unobserved or neglected if they were studied in solution. This thesis opens terarylenes to future applications which require a solid surface.
... As a consequence, the electrical effects measured at the device level could not be rationalized in various cases 12,13 . On the contrary, real-space images of supramolecular assemblies of photochromic molecules have been acquired through scanning tunneling microscopy (STM) [17][18][19][20][21] , but either the photo-induced switching events could not be monitored 18,19 or the specific experimental conditions hampered simple translation and integration in solid-state devices 17,20,21 . ...
... As a consequence, the electrical effects measured at the device level could not be rationalized in various cases 12,13 . On the contrary, real-space images of supramolecular assemblies of photochromic molecules have been acquired through scanning tunneling microscopy (STM) [17][18][19][20][21] , but either the photo-induced switching events could not be monitored 18,19 or the specific experimental conditions hampered simple translation and integration in solid-state devices 17,20,21 . ...
Molecular switches enable the fabrication of multifunctional devices in which an electrical output can be modulated by external stimuli. The working mechanism of these devices is often hard to prove, since the molecular switching events are only indirectly confirmed through electrical characterization, without real-space visualization. Here, we show how photochromic molecules self-assembled on graphene and MoS2 generate atomically precise superlattices in which a light-induced structural reorganization enables precise control over local charge carrier density in high-performance devices. By combining different experimental and theoretical approaches, we achieve exquisite control over events taking place from the molecular level to the device scale. Unique device functionalities are demonstrated, including the use of spatially confined light irradiation to define reversible lateral heterojunctions between areas possessing different doping levels. Molecular assembly and light-induced doping are analogous for graphene and MoS2, demonstrating the generality of our approach to optically manipulate the electrical output of multi-responsive hybrid devices.
... In fact, because γ is proportional to ∆µ AB , 2D self-assembly will occur if and only if γ is lower than zero. In addition, since γ is architecture-dependent through the strength of molecular recognition in the monolayer (E sam ) and the density of packing (A uc ), its numerical evaluation allows one to compare the relative stability of different SAMs, which is useful to explore competitive equilibria at surfaces [189,190] and rationalize the concentration dependent 2D polymorphism [160,162,191] . ...
... In this chapter, the joint experimental and computational effort to image and understand the in situ photoswitching and self-assembly of a diarylethene derivative at the solid/liquid interface is presented. The focus is on the 1,2-bis(2-methyl-5-(4-octadecyloxycar- published in Angewandte Chemie [190] . ...
Molecular self-assembly at surfaces is a prominent example of self-organization of matter with outstanding technological applications. The ability to predict the structure of the self-assembled monolayer (SAM) formed at equilibrium is of great fundamental and technological importance. In this dissertation I present a self-consistent theory for a first-principle interpretation of 2D self-assembly based on modeling and statistical thermodynamics. The developed framework provides access to the thermodynamic stability of the SAM and to its concentration dependence. This allows to study competitive equilibria at surfaces and to rationalize the 2D polymorphism evidenced by scanning probe techniques. The theory predicts the existence of a critical concentration of monomers, which is used to set an absolute scale for the 2D self-assembly propensity. Last, four technological applications are discussed, showing the potentials of the developed framework.
... Self-assembly, the process by which monomers assemble themselves into highly ordered structures, has been studied over decades. (1)(2)(3)(4)(5)(6)(7)(8) Traditionally, self-assembly can be studied thermodynamically (9)(10)(11)(12)(13)(14)(15) by comparing the free energies of monomeric and assembled structures, however, this may fail to reveal the true nature of those complex self-assembly processes which are governed via kinetic control. Therefore, it is important to study the kinetics of self-assembly, which provides the opportunity to understand the underlying mechanism of self-assembly process and further realize the rational design of advanced materials with novel structures through the bottom-up approach. ...
Uncovering slow collective variables (CVs) of self-assembly dynamics is important to elucidate its numerous kinetic assembly pathways and drive the design of novel structures for advanced materials through the bottom-up approach. However, identifying the CVs for self-assembly presents several challenges. First, self-assembly systems often consist of identical monomers and the feature representations should be invariant to permutations and rotational symmetries. Physical coordinates, such as aggregate size, lack the high-resolution detail, while common geometric coordinates like pairwise distances are hindered by the permutation and rotational symmetry challenge. Second, self-assembly is usually a downhill process, and the trajectories often suffer from insufficient sampling of backward transitions that correspond to the dissociation of self-assembled structures. Popular dimensionality reduction methods, like tICA, impose detailed balance constraints, potentially obscuring the true dynamics of self-assembly. In this work, we employ GraphVAMPnets which combines graph neural networks with variational approach for Markovian process (VAMP) theory to identify the slow CVs of the self-assembly processes. First, GraphVAMPnets bears the advantages of graph neural networks, in which the graph embeddings can represent self-assembly structures in a high-resolution while being invariant to permutations and rotational symmetries. Second, it is built upon VAMP theory that studies Markov processes without forcing detailed balance constraint, which addresses the out-of-equilibrium challenge in self-assembly process. We demonstrate GraphVAMPnets for identifying slow CVs of self-assembly kinetics in two systems: aggregation of two hydrophobic molecules and self-assembly of patchy particles. We expect that our GraphVAMPnets can be widely to applied to molecular self-assembly.
... This explains its strong surface adsorption and disproportionate dominance on STM images. [73] These studies explained well the strong surface adsorption and disproportionate dominance of the annulated isomer on STM images even at 5 % ratio. [71] The irreversibility of the annulated form could make it an unwanted side-reaction from the viewpoints of rewritable optical memories or switches but it could be useful for permanent memories (non-rewritable or write-once memories) similar to applications sought for diarylethenes with very low cycloreversion quantum yields. ...
The efficient switching that can occur between two stable isomers of diarylethenes makes them particularly promising targets for opto‐ and molecular electronics. To examine these classes of molecules for electronics applications, they have been subjected to a series of scanning tunneling microscopy (STM) experiments which are the focus of this Review. A brief introduction to the chemical design of diarylethenes in terms of their switching capabilities along with the basics of STM are presented. Next, initial STM studies on these compounds under ambient conditions are discussed. An overview of how molecular design affects the isomerization and self‐assembly of diarylethenes on the solid‐liquid interface as investigated by STM is then presented as well as single‐molecule studies under ultra‐high vacuum. The last section presents further prospects for molecular designs in the field.
... Organic photochromic compounds is a widely studied class of molecular switch, which are capable of undergoing efficient and reversible transformations between two states with distinctive physical and chemical properties upon photoirradiation [2][3][4][5]. Among organic photochromic compounds, diarylethenes have been extensively studied for their two isomers that are both thermodynamically stable and their cyclization/cycloreversion reaction occurs fast on the piscosecond time scale [6][7][8]. Owing to these excellent characteristics, diarylethene materials have potential applications in the fields of optical memory media and optical switching devices [7]. In the past decades, a significant progress on design and synthesis of diarylethene compounds by the introduction of specific functional groups has enabled diarylethenes to respond not only to UV and visible lights but also to other special external stimuli [9]. ...
C 33 H 20 F 6 N 2 S 2 , monoclinic, P 2 1 / c (no. 14), a = 13.418(4) Å, b = 13.876(4) Å, c = 15.182(4) Å, β = 98.211(3)°, V = 2797.6(14) Å ³ , Z = 4, Rgt ( F ) = 0.0500, wRref ( F² ) = 0.1456, T = 296(2) K.
... The theory above has been recently used to rationalize a series of competitive equilibria at surfaces and interfaces. In a first application, the 2D self-assembly of three photo isomers of a diarylethene (DAE) photoswitch (open, closed and the annulated by-product) was explored by STM/modeling at the liquid-graphite interface [27]. The experimental results evidenced the formation of topologically distinct SAMs depending on the photo-isomerization state of the building blocks and manifested a surprising affinity of the by-product monolayer for graphite. ...
In molecular self-assembly, hundreds of thousands of freely-diffusing molecules associate to form ordered and functional architectures in the absence of an actuator. This intriguing phenomenon plays a critical role in biology and has become a powerful tool for the fabrication of advanced nanomaterials. Due to the limited spatial and temporal resolutions of current experimental techniques, computer simulations offer a complementary strategy to explore self-assembly with atomic resolution. Here, we review recent computational studies focusing on both thermodynamic and kinetic aspects. As we shall see, thermodynamic approaches based on modeling and statistical mechanics offer initial guidelines to design nanostructures with modest computational effort. Computationally more intensive analyses based on Molecular Dynamics simulations and kinetic network models (KNM) reach beyond it, opening to the rational design of self-assembly pathways. Current limitations of these methodologies are discussed. We anticipate that the synergistic use of thermodynamic and kinetic analyses based on computer simulations will introduce an important contribution to the de novo design of self-assembly.
... with K eq being the adsorption constant (eqn (4)). The above results indicate that at any initial concentration C 0 for which (C 3D ) eq o C cac 3D and (C 2D ) eq o C cac 2D no self-assembly takes place and the monomers will distribute among the 2D and 3D disordered phases according to eqn (6) and (7). However, they also indicate that when C cac 3D in the 3D phase (i.e. ...
In most technological applications involving liquids or gases in interaction with solids, the first event is the adsorption of molecules onto the solid surface. Here, we focus on the theoretical understanding of the adsorption equilibrium at the solid-gas interface. In the limit of physisorption, we find that the adsorption probability is independent of the initial concentration of monomers, whereas it varies with the available volume to surface ratio, which depends on the experimental setup. This theoretical finding is verified numerically by Molecular Dynamics simulations of five small-molecule gases physisorbing on graphene. The simulations provide quantitative estimates of the adsorption free energy, which are used to benchmark analytical and numerical integrations of the corresponding partition functions. The significance of the theoretical result above is analyzed in the context of molecular self-assembly at surfaces and interfaces. Our interpretation indicates that there exist (at least) two distinct pathways for 2D self-assembly, which involve or not the formation of a 2D disordered intermediate. And, it predicts that the critical concentration for self-assembly may be shifted by varying the aspect ratio of the experimental setup.
... Diarylethenes are an important class of photoswitches. Originally developed for optical information storage, they found many applications in materials sciences, nanotechnology, and biomimetic chemistry [31][32][33][34][35][36][37][38][39][40][41][42]. Their most typical architecture is shown in Figure 1A: two heteroaromatic, most often five-membered heterocyclic rings are connected via a cyclopentenyl bridge, which may also be perfluorinated. ...
Diarylethenes are an important class of reversible photoswitches and often claimed to require two alkyl substituents at the carbon atoms between which the bond is formed or broken in the electrocyclic rearrangement. Here we probe this claim by the synthesis and characterization of four pairs of deazaadenine-based diarylethene photoswitches with either one or two methyl groups at these positions. Depending on the substitution pattern, diarylethenes with one alkyl group can exhibit significant photochromism, but they generally show poor stability towards extended UV irradiation, low thermal stability, and decreased fatigue resistance. The results obtained provide an important direction for the design of new efficient DNA photoswitches for the application in bionanotechnology and synthetic biology.
The properties of photokinetics under monochromatic light have not yet been fully described in the literature. In addition, for the last 120 years or so, explicit, handy model equations that can map out the kinetic behaviour of photoreactions have been lacking. These gaps in the knowledge are addressed in the present paper. Several general features of such photokinetics were investigated, including the effects of initial reactant concentration, the presence of spectator molecules, and radiation intensity. A unique equation, standing for a pseudo-integrated rate law, capable of outlining the kinetic behaviour of any photoreaction is proposed. In addition, a method that solves for quantum yields and absorption coefficients of all species of a given photoreaction is detailed. A metric (the initial velocity) has been adopted, and its reliability for the quantification of several effects was proven by theoretical derivation, Runge–Kutta numerical integration calculations and through the model equation proposed. Overall, this study shows that, under monochromatic light, photoreaction kinetics is well described by Φ -order kinetics, which is embodied by a unifying model equation. This paper is aimed at contributing to rationalising photokinetics via reliable, easy-to-use mathematical tools.
Uncovering slow collective variables (CVs) of self-assembly dynamics is important to elucidate its numerous kinetic assembly pathways and drive the design of novel structures for advanced materials through the bottom-up approach. However, identifying the CVs for self-assembly presents several challenges. First, self-assembly systems often consist of identical monomers, and the feature representations should be invariant to permutations and rotational symmetries. Physical coordinates, such as aggregate size, lack high-resolution detail, while common geometric coordinates like pairwise distances are hindered by the permutation and rotational symmetry challenges. Second, self-assembly is usually a downhill process, and the trajectories often suffer from insufficient sampling of backward transitions that correspond to the dissociation of self-assembled structures. Popular dimensionality reduction methods, such as time-structure independent component analysis, impose detailed balance constraints, potentially obscuring the true dynamics of self-assembly. In this work, we employ GraphVAMPnets, which combines graph neural networks with a variational approach for Markovian process (VAMP) theory to identify the slow CVs of the self-assembly processes. First, GraphVAMPnets bears the advantages of graph neural networks, in which the graph embeddings can represent self-assembly structures in high-resolution while being invariant to permutations and rotational symmetries. Second, it is built upon VAMP theory, which studies Markov processes without forcing detailed balance constraints, which addresses the out-of-equilibrium challenge in the self-assembly process. We demonstrate GraphVAMPnets for identifying slow CVs of self-assembly kinetics in two systems: the aggregation of two hydrophobic molecules and the self-assembly of patchy particles. We expect that our GraphVAMPnets can be widely applied to molecular self-assembly.
Irreversible two-photon photorearrangement of 1,2-diarylethenes is a unique process providing access to complex 2a1,5a-dihydro-5,6-dithiaacenaphthylene (DDA) heterocyclic core. This reaction was serendipitously discovered during studies on photoswitchable diarylethenes and was initially considered as a highly undesired process. However, in recent years, it has been recognized as an efficient photochemical reaction, interesting by itself and as a promising synthetic method for the synthesis of challenging molecules. Herein, we discuss the state-of-the-art in studies on this notable process.
Photochromes have proven to be attractive molecules for potential applications not necessary based on their spectacular colour change (such as biomimetic chemistry and photoswitchable nucleosides). Their development was incited by the versatility of compounds that could be prepared, but their applications also depended on their photobehaviour. The latter, conceptually requires a comprehensive description of their kinetics. A field which is somewhat still to be developed since, thus far, a single integrated rate-law (c=f(t), expressing the variation of the concentration with irradiation time) has been analytically established for samples exposed to monochromatic light. Similar integrated rate-law equations have yet to be analytically developed for polychromatic light irradiation.
The present paper investigates the facets of photokinetics of a photochromic closed-form diarylethene derivative (c-DAE) and its variability with the light type impinging on the sample. An integrated rate-law equation has been derived, to describe kinetics under polychromatic light. The findings have then been applied to establish robust methodologies for both monochromatic and polychromatic light actinometries for the visible range. c-DAE is been shown to be a reliable, easily manipulated, actinometer for the 400–600 nm range for both light types. It is an exemplar of a new generation of actinometers that are easy-to-handle, regenerated, reliable, do not require to have a wavelength-invariant quantum yield, useful for both monochromatic and polychromatic light set-ups, and their procedures are based on integrated rate-law equations.
The self-assembly of bolaamphiphiles comprised of a central photochromic dithienylethene (DTE) chromophore was investigated in aqueous media. Irradiation at 254 nm induced a conversion from the open to closed states of the DTE chromophores. Whereas, in water, irradiation produced a photostationary state of 20 : 80 (open/closed), in methanol the ratio improved to 10 : 90 (open/closed). The open → closed transition was accompanied by the formation of 1D nanofibers during incubation in darkness. This journal is
A tri-stable structural switching between different polymorphisms is presented in 2D molecular assembly of 5-(benzyloxy)-isophthalic acid derivative (BIC-C12) at the liquid/solid interface. The assembled structure of BIC-C12 is sensitive to the applied voltage between the STM tip and the sample surface. A compact lamellar structure is exclusively observed at positive sample bias, while a porous honeycomb structure or a quadrangular structure is preferred at negative sample bias. Selective switching between the lamellar structure and the honeycomb structure or the quadrangular structure is realized by controlling the polarity and magnitude of the sample bias. The transition between the honeycomb structure and the quadrangular structure is, however, absent in the assembly. This tri-stable structural switching is closely related to the molecular concentration in the liquid phase. This result provides insights into the effect of external electric field on molecular assembly and benefits design and construction of switchable molecular architectures on surface.
We report the synthesis of a novel C3-symmetrical multiphotochromic molecule bearing three azobenzene units at positions 1,3,5 of the central phenyl ring. The unique geometrical design of such a rigid scaffold enables the electronic decoupling of the azobenzene moieties to guarantee their simultaneous isomerization. Photoswitching of all azobenzenes in solution was demonstrated by means of UV-Vis absorption spectroscopy and high performance liquid chromatography (HPLC) analysis. Scanning tunnelling microscopy investigations at the solid-liquid interface, corroborated by molecular modelling, made it possible to unravel the dynamic self-assembly of such systems into ordered supramolecular architectures, by visualising and identifying the patterns resulting from three different isomers, thereby demonstrating that the multi-photochromism is retained when the molecules are confined in two-dimensions.
Self‐assembled monolayers of a π‐expanded oligothiophene macrocycle have been photoisomerized between their E,E and Z,Z diastereomers at the interface between octanoic acid solutions and highly oriented pyrolytic graphite (HOPG). The switching process proceeds in situ at the solid‐liquid interface and was followed by scanning tunneling microscopy (STM). Upon illumination with light at 365 nm (546 nm), a monolayer of Z,Z‐8mer (E,E‐8mer) photoisomerizes to the E,E‐8mer (Z,Z‐8mer) form with changes in 2D‐hexagonal packing. These findings provide insight toward the design of photoresponsive surfaces with desirable optoelectronic and structural (host‐guest) properties.
Self‐assembled monolayers of a π‐expanded oligothiophene macrocycle have been photoisomerized between their E,E and Z,Z diastereomers at the interface between octanoic acid solutions and highly oriented pyrolytic graphite (HOPG). The switching process proceeds in situ at the solid‐liquid interface and was followed by scanning tunneling microscopy (STM). Upon illumination with light at 365 nm (546 nm), a monolayer of Z,Z‐8mer (E,E‐8mer) photoisomerizes to the E,E‐8mer (Z,Z‐8mer) form with changes in 2D‐hexagonal packing. These findings provide insight toward the design of photoresponsive surfaces with desirable optoelectronic and structural (host‐guest) properties.
Photoswitching materials are building blocks of next generation optoelectronic devices which may require molecule deposition on a solid substrate. However, molecule properties change upon adsorption due to surface-molecule interactions and symmetry considerations. Scanning tunneling microscopy (STM) and density functional theory (DFT) offer techniques to address interactions of functional molecules down to the single-molecular level on solid substrates. In this paper, we present a combined STM and DFT study of a tert-butyl functionalized terarylene molecule on Ag(111) and NaCl(001)/Ag(111) at ~5 K. Tert-butyl groups aided in identifying three conformations of the compound upon adsorption on the surface. DFT calculations showed that two of these conformations refer to different adsorption geometries of the trans conformation in the gas phase. The other was assigned to the non-reactive cis conformation. For the first time, this conformation was isolated and imaged at the single-molecular level. Calculations further showed that aside from the electronic structure of the molecule, methyl groups sticking out of the surface are the origin of bright spots observed on the STM. On NaCl(001)/Ag(111), only the trans conformation was found and the mapping of occupied and unoccupied states of terarylenes was accomplished for the first time.
Two-dimensional supramolecular assemblies of a series of 2,7-bis (10-n-alkoxycarbonyl-decyloxy)-9-fluorenone derivatives (BAF-Cn, n = 1, 3−6) consisting of polar fluorenone moieties and ester alkoxy chains were investigated by scanning tunneling microscopy on highly oriented pyrolytic graphite surfaces. The chain-length effect was observed in the self-assembly of BAF-Cn. Self-assembly of BAF-C1 was composed of a linear I pattern, where the side chains adopted a fully interdigitated arrangement. As the length of side chains increased, the coexistence of linear I pattern and cyclic pattern for the self-assembly of BAF-C3 was observed. Further increasing the length of alkoxy chain (n = 4−6), another linear II structure was observed in the BAF-Cn monolayer, in which the side chains in adjacent rows arranged in a tail-to-tail configuration. It is reasonable to conclude that not only the van der Waals forces but also the dipole−dipole interactions from both fluorenone cores and ester alkoxy chains play critical roles in the self-assemblies of BAF-Cn. Our work now gives a detailed insight into the effect of intermolecular dipole−dipole and van der Waals interactions on the monolayer morphology of fluorenone derivatives.
Bottom-up approaches allow for molecular-level control of engineering new nanostructures. For the next generation molecular circuits, machines, and phase-change materials, supramolecular structure formation and interactions must be investigated on a two-dimensional solid substrate. Scanning tunneling microscopy (STM) provides not only a method to address such supramolecular structures at the molecular level on a solid surface but also to locally control and manipulate such structures. In this article, we show that a terarylene molecule, a sub-family of photoswitching diarylethenes that promise a lot of applications from molecular switches to photoelectronics, assembles upon application of a bias voltage pulse from an STM tip. We show that molecules organize themselves in order to align their dipole moment to follow those of the electric field induced in the tunneling junction. The 2D assembly are stabilized by π - π stacking interactions and van der Waals interactions. This expands the repertoire of currently available bottom-up techniques for fabrication of supramolecular nanostructures.
The development of nonprecious electrochemical catalysts for water splitting is a key step to achieve a sustainable energy supply for the future. Molybdenum disulfide (MoS2) has been extensively studied as a promising low-cost catalyst for hydrogen evolution reaction (HER), whereas HER is only catalyzed at the edge for pristine MoS2, leaving a large area of basal plane useless. Herein, on-surface self-assembly is demonstrated to be an effective, facile, and damage-free method to take full advantage of the large ratio surface of MoS2 for HER by using multiscale simulations. It is found that as supplement of edge sites of MoS2, on-MoS2 M(abt)2 (M = Ni, Co; abt = 2-aminobenzenethiolate) owns high HER activity, and the self-assembled M(abt)2 monolayers on MoS2 can be obtained through a simple liquid-deposition method. More importantly, on-surface self-assembly provides potential application for overall water splitting once the self-assembled systems prove to be of both HER and oxygen evolution reaction activities, for example, on-MoS2 Co(abt)2. This work opens up a new and promising avenue (on-surface self-assembly) toward the full exploitation of the basal plane of MoS2 for HER and the preparation of bifunctional catalysts for overall water splitting.
The use of a bottom-up approach to the fabrication of nanopatterned functional surfaces, which are capable to respond to external stimuli, is of great current interest. Herein, the preparation of light-responsive, linear supramolecular metallopolymers constituted by the ideally infinite repetition of a ditopic ligand bearing an azoaryl moiety and Co(II) coordination nodes is described. The supramolecular polymerization process is followed by optical spectroscopy in dimethylformamide solution. Noteworthy, a submolecularly resolved scanning tunneling microscopy (STM) study of the in situ reversible trans-to-cis photoisomerization of a photoswitchable metallopolymer that self-assembles into 2D crystalline patterns onto a highly oriented pyrolytic graphite surface is achieved for the first time. The STM analysis of the nanopatterned surfaces is corroborated by modeling the physisorbed species onto a graphene slab before and after irradiation by means of density functional theory calculation. Significantly, switching of the monolayers consisting of supramolecular Co(II) metallopolymer bearing trans-azoaryl units to a novel pattern based on cis isomers can be triggered by UV light and reversed back to the trans conformer by using visible light, thereby restoring the trans-based supramolecular 2D packing. These findings represent a step forward toward the design and preparation of photoresponsive “smart” surfaces organized with an atomic precision.
With singlet oxygen (¹O2) as the active agent, photodynamic therapy (PDT) is a promising technique for the treatment of various tumors and cancers. But it is hampered by the poor selectivity of most traditional photosensitizers (PS). In this review, we present a summary of controllable PDT implemented by regulating singlet oxygen efficiency. Herein, various controllable PDT strategies based on different initiating conditions (such as pH, light, H2O2 and so on) have been summarized and introduced. More importantly, the action mechanisms of controllable PDT strategies, such as photoinduced electron transfer (PET), fluorescence resonance energy transfer (FRET), intramolecular charge transfer (ICT) and some physical/chemical means (e.g. captivity and release), are described as a key point in the article. This review provide a general overview of designing novel PS or strategies for effective and controllable PDT.
The application of supramolecular chemistry on solid surfaces represents an exciting field of research that continues to develop in new and unexpected directions. This review highlights recent advances in the field which range from the fundamental aspects of the thermodynamics of self-assembly through to the development of new materials with potential application as new materials. The unique aspects of working on solid surfaces are highlighted and advances in the assembly of many component systems and highly complex fractal-like and quasicrystalline systems discussed. The unique features of working in the surface-based environment and the utilisation of scanning probe microscopies as a primary characterisation tool are highlighted.
In the field of extreme miniaturization of electronic devices, it is necessary to study candidate molecules at the single molecular level by scanning tunneling microscopy (STM). This necessitates molecules to harbor specific functionalization to control molecule-surface interactions to avoid molecular diffusion on the surface and molecule-molecule interactions to prevent aggregation of molecules. In this context, we present the synthesis and photochromism of terarylene derivatives with tert-butyl and chloride groups. We show that these groups preserve the photochemical and switching properties of molecules. DFT calculations on the conformation of the molecules predict the presence of several conformations during vacuum deposition. Initial STM experiments show that tert-butyl groups give images with a better contrast to identify the molecules on surface without any aggregation.
Organic/inorganic interfaces play dominant roles in the formation of organic thin films and the performance of organic electronic devices. Preparing a high-quality and high-stability interfacial organic layer on the inorganic substrate is currently challenging and understanding the self-assembly mechanism of the interfacial layer (IL) in depth at a molecular level is thus essential. In this work, by studying the self-assembly of perylene-3,4,9,10-tetracarboxylic dianhydride IL on graphene, we unveil that intermolecular H-bonds can considerably widen the nucleation area, heighten the stability-metastability critical temperature, promote the nucleation speed, and guide the nucleation direction. We further demonstrate that such positive effects on the IL self-assembly are of generality. Moreover, it is found that IL can transform into a well-ordered crystal from an amorphous state through suitable thermal treatment or molecule coverage control. Our work highlights that fabricating H-bond networks is desirable for the synthesis of robust and high-quality IL, and points out feasible routes to improve the quality of poor IL.
In this Review article pioneering work and recent achievements in the emerging research area of on-surface chemistry is discussed. On-surface chemistry, sometimes also called two-dimensional chemistry, shows great potential for bottom-up preparation of defined nanostructures. In contrast to traditional organic synthesis, where reactions are generally conducted in well-defined reaction flasks in solution, on-surface chemistry is performed in the cavity of a scanning probe microscope on a metal crystal under ultrahigh vacuum conditions. The metal first acts as a platform for self-assembly of the organic building blocks and in many cases it also acts as a catalyst for the given chemical transformation. Products and hence success of the reaction are directly analyzed by scanning probe microscopy. This Review provides a general overview of this chemistry highlighting advantages and disadvantages as compared to traditional reaction setups. The second part of the Review then focuses on reactions that have been successfully conducted as on-surface processes. On-surface Ullmann and Glaser couplings are addressed. In addition, cyclodehydrogenation reactions and cycloadditions are discussed and reactions involving the carbonyl functionality are highlighted. Finally, the first examples of sequential on-surface chemistry are considered in which two different functionalities are chemoselectively addressed. The Review gives an overview for experts working in the area but also offers a starting point to non-experts to enter into this exciting new interdisciplinary research field.
For more than three decades the scanning tunnelling microscope (STM) has proven to be an indispensable tool to image molecules adsorbed at a surface at the highest detail possible. In addition to simply imaging molecules, STM can also be applied to monitor dynamic surface phenomena, including chemical reactions. By studying reactions at a surface at the single molecule level, unique information about reaction mechanisms can be obtained which remains hidden when conventional ensemble techniques are used. Many STM studies of chemical reactions have been performed in extreme environments like ultrahigh vacuum or high pressure chambers, but these are far removed from conditions in which most chemical and biological processes take place, i.e., in a liquid at ambient atmospheres. This feature paper highlights the developments in the relatively unexplored research area of investigating chemical reactions with an STM at a liquid/solid interface under ambient conditions. Covalent couplings between molecules, light-induced isomerisations, reactions under electrochemical control, and complex multistep processes and catalysis are discussed.
Fabrication of structural diversity in self-assembled monolayers has gained considerable attention, not only due to its significance in interface science but also because of its potential application in designing nanomaterials. Here, systematical characterization of the molecular self-assembly of tri-substituted anthraquinone derivatives at the 1-octanoic acid/HOPG interface is provided. Different nanopatterns of Linear I, Butterfly-like, and Linear II are recorded for 1,2,4-A-3OC16 by gradually changing the solution concentration. Diverse polymorphs are also obtained by alkyl chain elongation, meaning that 1,2,4-A-3OC15,17,18 adopt Dimer-Linear, Linear III, and Linear IV structures, respectively. Dipole–dipole, hydrogen bonding, and van der Waals interactions are the dominant forces on forming the stable adlayers. Also, the coadsorption of the solvent molecules is a common phenomenon, ascribing to space matching and gain of enthalpy. Moreover, the 1,2,4-A-3OC17,18 self-assembled into monolayers consists of both ordered and chaotic domains. This loss of regularity is attributed to the large number and long length of the side chains, for entropy reasons. This work provides important insight into the fabrication of complex molecular self-assembly and the exploration of monolayers at the liquid/solid interface.
A solid-liquid interface is a unique environment for the construction of two-dimensional molecular assemblies as a bottom-up approach towards functional surfaces. Scanning tunneling microscopy (STM) has proven itself as an excellent tool to characterize such surfaces at the molecular level, by means of visualization. Many rules of design for surface, solvent and chemical structure of the adsorbants have been established, but methods to externally manipulate surface assemblies after their formation are still under development. This feature article deals with these manipulation methods at the solid-liquid interface and evaluates, at the molecular level, the effects of temperature variation, irradiation with light, applied electric or magnetic fields, mechanical manipulation with the STM tip, and shear flow of the liquid phase.
The fluorescence spectra and photostability under 532 nm laser excitation of four different common photoswitches (an azobenzene, spiropyran, indolylfulgide, and a diarylperfluorocyclopentene) were investigated in a silica microstructured optical fibre. An example of each photoswitch was examined in solution and physically adsorbed to the silica fibre surface. This comparison was made to define fluorescence behaviour in these two states and to determine which photoswitch has the best performance in this light intense microenvironment. The azobenzene and the spiropyran switches demonstrated the strongest fluorescence response and the least degradation of the fluorescence signal.
Photoresponsive behavior of systems in equilibrium at liquid/solid interface can be modulated through the design of photochromic core structures and the process of self-assembly. In this study, we investigated the effects of altering the substitution position of a thienyl group on two-dimensional (2-D) molecular ordering composed of photochromic diarylethenes (DAEs) at a liq-uid/highly oriented pyrolytic graphite (HOPG) interface using scanning tunneling microscopy (STM). We found that both the open- and the closed-ring isomers of a 3-thienyl-type DAEs formed stripe-patterned orderings on a HOPG substrate at simi-lar concentrations and that the process of self-assembly on a 2-D surface was highly cooperative for both isomers. In the case of 2-thienyl-type DAE, similar stripe-patterned ordering was observed only for the open-ring isomer, but no ordering was observed for the closed-ring isomer. Upon irradiation with UV and visible light, reversible three-state photoswitching over the formation/disappearance of 2-D ordering was observed using the 3-thienyl-type DAE. The instability of the ordering of the 2-thienyl-type closed-ring isomer was rationalized by the bend angle of the DAE core framework. Fundamental under-standing of the relationship between the photoresponsive behavior of 2-D molecular ordering and molecular structures has been deepened through quantitative analyses of the concentration dependence of surface coverage and computational studies that included molecular mechanics/molecular dynamics (MM/MD) calculations.
A new fluorescent chemosensor for the sequential recognition of Fe3+ and cysteine has been constructed based on photochromic diarylethene. It exhibits sequential recognition of Fe3+ and cysteine via an “on–off–on” molecular switch. The title compound can selectively and sensitively recognize Fe3+ in methanol, causing quenching of the fluorescence. Then, the formed complex was found to be a fluorescent chemosensor for cysteine with an increase in the fluorescence intensity. A detection limit as low as 4.09 × 10−8 mol L−1 for Fe3+ and 2.89 × 10−8 mol L−1 for cysteine was obtained. Moreover, a two input INHIBIT logic gate was fabricated by using Fe3+ and cysteine as inputs and taking I439 as the output.
Nano-fabrication is an issue gained extensive attention in molecular engineering. Thus we intensively probed surface-based 2D self-assembly of 2-hydroxyanthraquinone (2-HA) derivatives by scanning tunneling microscopy (STM). During the STM process, two interesting nanostructures extremely resembled Chinese knot and wheat, thus they were denoted as Knot-like and Wheat-like patterns for legibility. Moreover, careful observation suggests that the Knot-like structure is chiral while the Wheat-like structure is achiral. Systematic analysis indicates that these two arrangements are mainly dominated by synergistic forces of dipole−dipole and hydrogen bonding interactions. To the best of our knowledge, the dipole induced chirality and achirality has been rarely reported, and the synergistic forces of dipole−dipole and hydrogen bonding interactions on dominating 2D assembly have never been proposed. In addition, structural transition between the Knot-like and Wheat-like configurations can be regulated by concentration and solvent as the alkyl chain length changes. Note that the phase transformation is in most cases incomplete. A summary of surface coverage for 2-HA-OCn (n = 12, 14, 16, 18, 20) molecules shows the general trend that Knot-like structure is preferred in polar solvent and under low concentration, while Wheat-like structure takes priority in nonpolar solvent and under high concentration. Besides, 2-HA-OCn (n = 11, 13, 15, 17) molecules adopted Wheat-like´ pattern which differs from the Wheat-like pattern in the relative orientation of adjacent ribbons, ascribing to minimum of steric repulsion between the interdigitated alkyl chains. The study presents efficient strategies on manipulation of chiral and achiral nanostructures, and the results are believed to be of significance to the fields of 2D self-assembly and interface science.
Providing a quantitative understanding of the thermodynamics involved in molecular adsorption and self-assembly at nanostructured carbon material is of fundamental importance and finds outstanding applications in the graphene era. Here, we study the effect of edge perchlorination of coronene, a prototypical polyaromatic hydrocarbon, on the binding affinity for the basal planes of graphite. First, by comparing the desorption barrier of hydrogenated vs. perchlorinated coronene measured by temperature programmed desorption we quantify the enhancement of chlorine substitution on the strength of physisorption at the single-molecule level. Then, by a thermodynamic analysis of the corresponding monolayers based on force-field calculations and statistical mechanics we show that perchlorination decreases the free energy of self-assembly not only enthalpically (by enhancing the strength of surface binding) but also entropically (by decreasing the surface concentration). The functional advantage of a chemically modulated 2D self-assembly is demonstrated in the context of the molecule-assisted liquid-phase exfoliation of graphite into graphene.
Molecular photoswitches have attracted much attention in biological and materials contexts. Despite the fact that existing classes of these highly interesting functional molecules have been heavily investigated and optimized, distinct obstacles and inherent limitations remain. Considerable synthetic efforts and complex structure-property relationships render the development and exploitation of new photoswitch families difficult. Here, we focus our attention on acylhydrazones: a novel, yet underexploited class of photochromic molecules based on the imine structural motif. We optimized the synthesis of these potent photoswitches and prepared a library of over 40 compounds, bearing different substituents in all four crucial positions of the backbone fragment, and conducted a systematic study of their photochromic properties as a function of structural variation. This modular family of organic photoswitches offers a unique combination of properties and the compounds are easily prepared on large scales within hours, through an atom-economic synthesis, from commercially available starting materials. During our thorough spectroscopic investigations, we identified photoswitches covering a wide range of thermal half-lives of their (Z)-isomers, from short-lived T-type to thermally stable P-type derivatives. By proper substitution, excellent band separation between the absorbance maxima of (E)- and (Z)-isomers in the UV or visible region could be achieved. Our library furthermore includes notable examples of rare negative photochromic systems, and we show that acylhydrazones are highly fatigue resistant and exhibit good quantum yields.
Formation of an orthogonal supramolecular polymer on HOPG surface was demonstrated for the first time by means of scanning probe microscope. Atomic force microscope was employed to characterize the variation of both the thickness and the topography of the film formed from (1) monomer 1, (2) monomer 1/Zn2+, and (3) monomer 1/Zn2+/crosslinker 2, respectively. Scanning Tunneling microscope was used to monitor the self-assembly behavior of monomer 1 itself as well as 1/Zn2+ ions binary system on graphite surface, further testifying the formation of linear polymer via coordination interaction at the single molecule level. These results given by the strong surface characterization tool, scanning probe microscopy, confirm the formation of the orthogonal polymer on graphite surface, which is of great significance for fabricating complex superstructure on surfaces.
Mutual exclusivity in the nature of forward and reserve isomerization pathways holds promise for predictably controlling responses of photoswitchable materials according to molecular structure or external stimuli. Herein we have characterized the E/Z photoisomerization mechanisms of the visible-light-triggered switch 1,2-dithienyl-1,2-dicyanoethene (4TCE) in chlorobenzene with ultrafast transient absorption spectroscopy. We observe that switching mechanisms occur exclusively by relaxation through electronic manifolds of different spin multiplicity: trans-to-cis isomerization only occurs via electronic relaxation within the singlet manifold on a timescale of 40 ps; in contrast, cis-to-trans isomerization is not observed above 440 nm, but occurs via two rapid ISC processes into and out of the triplet manifold on timescales of ~2 ps and 0.4 ns, respectively, when excited at higher energies (e.g. 420 nm). Observation of ultrafast ISC in cis-4TCE is consistent with photoinduced dynamics of related thiophene-based oligomers. Interpretation of the photophysical pathways underlying these isomerization reactions is supported by the observation that cis-to-trans isomerization occurs efficiently via triplet-sensitized energy transfer, whereas trans-to-cis isomerization does not. Quantum-chemical calculations reveal that the T1 potential energy surface is barrierless along the coordinate of the central ethylene dihedral angle (θ) from the cis Franck-Condon region (θ = 175°) to geometries that are within the region of the trans ground-state well; furthermore, the T1 and S1 surfaces cross with a substantial spin-orbital coupling. In total, we demonstrate that E/Z photoswitching of 4TCE operates by multiplicity-exclusive pathways, enabling additional means for tailoring switch performance by manipulating spin-orbit couplings through variations in molecular structure or physical environment.
The self-assembly of multiple molecular components into complex supramolecular architectures is ubiquitous in nature and constitutes one of the most powerful strategies to fabricate multifunctional nanomaterials making use of the bottom-up approach. When spatial confinement in two-dimensions on a solid substrate is employed, this approach can be exploited to generate periodically ordered structures from suitably designed molecular tectons. In this manuscript we demonstrate that physisorbed directional periodic arrays of monometallic or hetero bimetallic coordination polymers can be generated on HOPG surface by combinations of suitably designed directional organic tecton or metallatecton based on a porphyrin or Ni(II) metallaporphyrin backbone bearing both a pyridyl and a terpyridyl units acting as coordinating sites for CoCl2. The periodic architectures were visualized at the solid/liquid interface with a sub-molecular resolution by scanning tunneling microscopy (STM) and corroborated by combined density functional and time-dependent density functional theory (DFT and TD-DFT) calculations. The capacity to nanopattern the surface for the first time with two distinct metallic centers exhibiting different electronic and optical properties is a key step towards the bottom-up construction of robust multicomponent, thus, multifunctional molecular nanostructures and nanodevices.
We have investigated photoinduced ordering transformation of a photochromic terthiophene derivative by scanning tunneling microscopy (STM) at the trichlorobenzene/highly oriented pyrolytic graphite (HOPG) interface. The open-ring and annulated isomers of the terthiophene formed two-dimensional molecular orderings with different patterns while the closed-ring isomer did not form any ordering. The ordering of the open-ring isomer exhibited polymorphism depending on the concentration of supernatant solution. Upon UV light irradiation to a solution of the open-ring isomer or the closed-ring isomer, ordering composed of the annulated isomer was irreversibly formed. Upon visible light irradiation or thermal stimulus to the closed-ring isomer, two kinds of ordering composed of the open-ring isomer were formed due to the polymorphism. By controlling photochromism and polymorphism among four states made of three photochemical isomers, four-state three-step transformation was achieved by in situ photoirradiation from a solution of the closed-ring isomer (no ordering) into the ordering composed of the open-ring isomer (ordering α and β) followed by the ordering composed of the annulated isomer (ordering γ).
When applying photochromic switches as functional units in light-responsive materials or devices, an often disregarded yet crucial property is their resistance to fatigue during photoisomerization. In the large family of diarylethene photoswitches, formation of an annulated isomer as a byproduct of the photochromic reaction turns out to prevent the desired high reversibility for many different derivatives. To overcome this general problem, we have synthesized and thoroughly investigated the fatigue behavior of a series of diarylethenes, varying the nature of the hetaryl moieties, the bridging units, and the substituents. By analysis of photokinetic data, a quantification of the tendency for byproduct formation in terms of quantum yields could be achieved, and a strong dependency on the electronic properties of the substituents was observed. In particular, substitution with 3,5-bis(trifluoromethyl)phenyl or 3,5-bis(pentafluorosulfanyl)phenyl groups strongly suppresses the byproduct formation and opens up a general strategy to construct highly fatigue-resistant diarylethene photochromic systems with a large structural flexibility.
Manganese porphyrins have been extensively investigated as model systems for the natural enzyme cytochrome P450 and as synthetic oxidation catalysts. Here, we report single-molecule studies of the multistep reaction of manganese porphyrins with molecular oxygen at a solid/liquid interface, using a scanning tunnelling microscope (STM) under environmental control. The high lateral resolution of the STM, in combination with its sensitivity to subtle differences in the electronic properties of molecules, allowed the detection of at least four distinct reaction species. Real-space and real-time imaging of reaction dynamics enabled the observation of active sites, immobile on the experimental timescale. Conversions between the different species could be tuned by the composition of the atmosphere (argon, air or oxygen) and the surface bias voltage. By means of extensive comparison of the results to those obtained by analogous solution-based chemistry, we assigned the observed species to the starting compound, reaction intermediates and products.
Organic semiconductors are suitable candidates for printable, flexible and large-area electronics. Alongside attaining an improved device performance, to confer a multifunctional nature to the employed materials is key for organic-based logic applications. Here we report on the engineering of an electronic structure in a semiconducting film by blending two molecular components, a photochromic diarylethene derivative and a poly(3-hexylthiophene) (P3HT) matrix, to attain phototunable and bistable energy levels for the P3HT's hole transport. As a proof-of-concept we exploited this blend as a semiconducting material in organic thin-film transistors. The device illumination at defined wavelengths enabled reversible tuning of the diarylethene's electronic states in the blend, which resulted in modulation of the output current. The device photoresponse was found to be in the microsecond range, and thus on a technologically relevant timescale. This modular blending approach allows for the convenient incorporation of various molecular components, which opens up perspectives on multifunctional devices and logic circuits.
The adsorption of large organic molecules on surfaces has recently been the subject of intensive investigation, both because of the molecules’ intrinsic physical and chemical properties, and for prospective applications in the emerging field of nanotechnology. Certain complex molecules are considered good candidates as basic building blocks for molecular electronics and nanomechanical devices. In general, molecular ordering on a surface is controlled by a delicate balance between intermolecular forces and molecule–substrate interactions. Under certain conditions, these interactions can be controlled to some extent, and sometimes even tuned by the appropriate choice of substrate material and symmetry. Several studies have indicated that, upon molecular adsorption, surfaces do not always behave as static templates, but may rearrange dramatically to accommodate different molecular species. In this context, it has been demonstrated that the scanning tunnelling microscope (STM) is a very powerful tool for exploring the atomic-scale realm of surfaces, and for investigating adsorbate–surface interactions. By means of high-resolution, fast-scanning STM unprecedented new insight was recently achieved into a number of fundamental processes related to the interaction of largish molecules with surfaces such as molecular diffusion, bonding of adsorbates on surfaces, and molecular self-assembly. In addition to the normal imaging mode, the STM tip can also be employed to manipulate single atoms and molecules in a bottom–up fashion, collectively or one at a time. In this way, molecule-induced surface restructuring processes can be revealed directly and nanostructures can be engineered with atomic precision to study surface quantum phenomena of fundamental interest. Here we will present a short review of some recent results, several of which were obtained by our group, in which several features of the complex interaction between large organic molecules and metal surfaces were revealed. The focus is on experiments performed using STM and other complementary surface-sensitive techniques.
Self-assembly is one of the most important concepts of the 21st century. Strikingly, despite the rational design of molecules for biological and pharmaceutical applications is rather well established, only few are the attempts to formally refine predictions of self-assembly in material science. In the present tutorial review, we encompass some of the most significant efforts towards the systematic study of (thermodynamically stable) self-assembly. We discuss experimental and computer-simulated self-assembly events in hard-matter, soft-matter and higher symmetry architectures under the common framework of partition functions. In this framework, we endeavor to correlate state-of-the-art chemical design, programming and/or engineering of reversible (thermal and chemical equilibrium) self-assembly with knowledge of the underlying partition function landscape in a step towards quantitative predictions and ab initio molecular design.
Recently, by mastering either top-down or bottom-up approaches, tailor-made macromolecular nano-objects with semiconducting properties have been fabricated. These engineered nanostructures for organic electronics are based on conjugated systems predominantly made up of sp²-hybridized carbon, such as graphene nanoribbons. Here, we describe developments in a selection of these nanofabrication techniques, which include graphene carving, stimulus-induced synthesis of conjugated polymers and surface-assisted synthesis. We also assess their potential to reproduce chemically and spatially precise molecular arrangements, that is, molecular blueprints. In a broad context, the engineering of a molecular blueprint represents the fabrication of an integrated all-organic macromolecular electronic circuit. In this Perspective, we suggest chemical routes, as well as convergent on-surface synthesis and microfabrication approaches, for the ultimate goal of bringing the field closer to technology.
Graphene nanoribbons-narrow and straight-edged stripes of graphene, or single-layer graphite-are predicted to exhibit electronic properties that make them attractive for the fabrication of nanoscale electronic devices. In particular, although the two-dimensional parent material graphene exhibits semimetallic behaviour, quantum confinement and edge effects should render all graphene nanoribbons with widths smaller than 10 nm semiconducting. But exploring the potential of graphene nanoribbons is hampered by their limited availability: although they have been made using chemical, sonochemical and lithographic methods as well as through the unzipping of carbon nanotubes, the reliable production of graphene nanoribbons smaller than 10 nm with chemical precision remains a significant challenge. Here we report a simple method for the production of atomically precise graphene nanoribbons of different topologies and widths, which uses surface-assisted coupling of molecular precursors into linear polyphenylenes and their subsequent cyclodehydrogenation. The topology, width and edge periphery of the graphene nanoribbon products are defined by the structure of the precursor monomers, which can be designed to give access to a wide range of different graphene nanoribbons. We expect that our bottom-up approach to the atomically precise fabrication of graphene nanoribbons will finally enable detailed experimental investigations of the properties of this exciting class of materials. It should even provide a route to graphene nanoribbon structures with engineered chemical and electronic properties, including the theoretically predicted intraribbon quantum dots, superlattice structures and magnetic devices based on specific graphene nanoribbon edge states.
One of the great challenges in surface chemistry is to assemble aromatic building blocks into ordered structures that are mechanically robust and electronically interlinked--i.e., are held together by covalent bonds. We demonstrate the surface-confined growth of ordered arrays of poly(3,4-ethylenedioxythiophene) (PEDOT) chains, by using the substrate (the 110 facet of copper) simultaneously as template and catalyst for polymerization. Copper acts as promoter for the Ullmann coupling reaction, whereas the inherent anisotropy of the fcc 110 facet confines growth to a single dimension. High resolution scanning tunneling microscopy performed under ultrahigh vacuum conditions allows us to simultaneously image PEDOT oligomers and the copper lattice with atomic resolution. Density functional theory calculations confirm an unexpected adsorption geometry of the PEDOT oligomers, which stand on the sulfur atom of the thiophene ring rather than lying flat. This polymerization approach can be extended to many other halogen-terminated molecules to produce epitaxially aligned conjugated polymers. Such systems might be of central importance to develop future electronic and optoelectronic devices with high quality active materials, besides representing model systems for basic science investigations.
The use of metal-organic frameworks (MOFs) so far has largely relied on nonspecific binding interactions to host small molecular
guests. We used long organic struts (~2 nanometers) incorporating 34- and 36-membered macrocyclic polyethers as recognition
modules in the construction of several crystalline primitive cubic frameworks that engage in specific binding in a way not
observed in passive, open reticulated geometries. MOF-1001 is capable of docking paraquat dication (PQT2+) guests within the macrocycles in a stereoelectronically controlled fashion. This act of specific complexation yields quantitatively
the corresponding MOF-1001 pseudorotaxanes, as confirmed by x-ray diffraction and by solid- and solution-state nuclear magnetic
resonance spectroscopic studies performed on MOF-1001, its pseudorotaxanes, and their molecular strut precursors. A control
experiment involving the attempted inclusion of PQT2+ inside a framework (MOF-177) devoid of polyether struts showed negligible uptake of PQT2+, indicating the importance of the macrocyclic polyether in PQT2+ docking.
Supramolecular chemistry has a very large impact on chemistry of current interest and the use of non-covalent but directional forces is appealing for the construction of 'supramolecular architectures'. The invention of scanning probe microscopy techniques has opened new doorways to study these concepts on surfaces. This review deals with recent progress in the study of two-dimensional supramolecular self-assembly on surfaces probed by scanning tunneling microscopy, with a special emphasis on structure, dynamics and reactivity of hydrogen bonded systems.
The fabrication methods of the microelectronics industry have been refined to produce ever smaller devices, but will soon reach their fundamental limits. A promising alternative route to even smaller functional systems with nanometre dimensions is the autonomous ordering and assembly of atoms and molecules on atomically well-defined surfaces. This approach combines ease of fabrication with exquisite control over the shape, composition and mesoscale organization of the surface structures formed. Once the mechanisms controlling the self-ordering phenomena are fully understood, the self-assembly and growth processes can be steered to create a wide range of surface nanostructures from metallic, semiconducting and molecular materials.
Photochromic systems can convert light energy into mechanical energy, thus they can be used as building blocks for the fabrication of prototypes of molecular devices that are based on the photomechanical effect. Hitherto a controlled photochromic switch on surfaces has been achieved either on isolated chromophores or within assemblies of randomly arranged molecules. Here we show by scanning tunneling microscopy imaging the photochemical switching of a new terminally thiolated azobiphenyl rigid rod molecule. Interestingly, the switching of entire molecular 2D crystalline domains is observed, which is ruled by the interactions between nearest neighbors. This observation of azobenzene-based systems displaying collective switching might be of interest for applications in high-density data storage.
• scanning tunnel microscopy
• molecular switches
• photochromic system
• data storage
The development of nanoscale masking for particle deposition is exceedingly important to push the future of nanoelectronics beyond the current limits of lithography. We present the first example of ordered hexagonal covalent nanoporous structures deposited in extended arrays of near monolayer coverage across a Ag(111) surface. The networks were formed from the deposition of the reagents from a heated molybdenum crucible between 370 and 460 K under ultrahigh vacuum (UHV) onto a cleaned Ag(111) substrate and imaged using a scanning tunneling microscope (STM). Two surface covalent organic frameworks (SCOFs) are presented; the first is formed from the deposition of 1,4-benzenediboronic acid (BDBA) and its dehydration to form the boroxine-linked SCOF-1, the second is formed from the co-deposition of BDBA and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) to form a dioxaborole-linked SCOF-2 network. The networks were found to produce nanoporous structures of 15 A for SCOF-1 and 29 A for SCOF-2, which agreed with theoretical pore sizes determined from DFT calculations. Both SCOFs were found to have exceptional thermal stability, maintaining their structure until approximately 750 K, which was found to be the polymer degradation temperature from thermal gravimetric analysis (TGA).
Understanding the energetics of molecular adsorption at nanostructured carbon materials opens up to outstanding applications in the postgraphene era. The accurate determination of the adsorbate binding affinity remains however a challenge both experimentally and computationally. Here, we report on the determination of the desorption energy of 25 chemically diverse compounds from graphite using computational methods at different levels of theory: empirical force fields (FF), semiempirical quantum mechanics (SQM), and density functional theory (DFT). By comparing the computational predictions with literature temperature-programmed desorption (TPD) experiments we found that the dispersion-corrected semiempirical method PM6-DH+ yields desorption energies in quantitative agreement with the experiments with an average error of 1.25 kcal mol–1. The discovery of a fast and accurate approach to molecular adsorption at surfaces prompted us to search the chemical space of the adsorbate to optimize the binding affinity for graphene. We found that polarizable groups containing sulfur and halogen atoms significantly enhance the interaction strength with the substrate. In particular, we predict that per-chlorination of aliphatic hydrocarbons doubles the desorption energy from graphene in vacuum. The efficiency of the PM6-DH+ calculations, which allows for screening libraries of compounds, guided the design of potentially improved graphene surfactants, which are commercially available.
A unique photoinduced change in the molecular ordering of a photochromic diarylethene derivative having amide groups was investigated at the liquid/highly oriented pyrolytic graphite (HOPG) interface by scanning tunneling microscopy (STM). Ordering of the diarylethene was stabilized by the hydrogen-bonded network of amide groups. The open- and the closed isomers formed characteristic stripe-patterned orderings; a different hydrogen-bonding network was formed in each case. In situ UV irradiation resulted in the exclusive formation of a third molecular ordering γ that is composed of the photoirreversible annulated isomer. No photoisomerized molecule was detected in the 2-D orderings, i.e., the open-ring isomer in the ordering of the closed-ring isomer and vice versa, despite the similarity in their molecular structures.
Chemical bond rearrangement during the phototransformation induces electronic as well as geometrical structure changes of the molecules. The molecular structure changes can be applied to various photonic devices. The electronic structure changes can be applied to optical memory media and various photoswitching devices. On the other hand, the geometrical structure changes can be applied to light-driven actuators and others. The photochromic diarylethene was serendipitously discovered during the course of study on photoresponsive polymers a quarter of a century ago. Various types of polymers having photoisomerizable chromophores, such as spiropyran, azobenzene, or stilbene, in the side or main chains, have been prepared in an attempt to change their conformation by photoirradiation.
A supramolecular approach for producing homogeneous dispersions of graphene nanosheets in various solvents is described by A. Ciesielski, P. Samorì, and co-workers on page 1691. Comparative studies of the liquid-phase exfoliation of graphene in the presence of linear alkanes of different lengths terminated by a carboxylic-acid head group reveal that they molecules act as graphene dispersion-stabilizing agents during exfoliation. The efficiency of exfoliation in terms of the concentration of exfoliated graphene is found to be proportional to the length of the fatty acid.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Dynamic covalent chemistry relies on the formation of reversible covalent bonds under thermodynamic control to generate dynamic combinatorial libraries. It provides access to numerous types of complex functional architectures, and thereby targets several technologically relevant applications, such as in drug discovery, (bio)sensing and dynamic materials. In liquid media it was proved that by taking advantage of the reversible nature of the bond formation it is possible to combine the error-correction capacity of supramolecular chemistry with the robustness of covalent bonding to generate adaptive systems. Here we show that double imine formation between 4-(hexadecyloxy)benzaldehyde and different α,ω-diamines as well as reversible bistransimination reactions can be achieved at the solid/liquid interface, as monitored on the submolecular scale by in situ scanning tunnelling microscopy imaging. Our modular approach enables the structurally controlled reversible incorporation of various molecular components to form sophisticated covalent architectures, which opens up perspectives towards responsive multicomponent two-dimensional materials and devices.
Endowing both solvent independency and excellent thermal bistability, the benzobis(thiadiazole)-bridged diarylethene system provides an efficient approach to realize extremely high photocyclization quantum yields (Φo-c, up to 90.6 %) by both separating completely pure anti-parallel conformer and suppressing intramolecular charge transfer (ICT). Large hindrance, high efficiency: A novel benzobis(thiadiazole)-bridged diarylethene system shows excellent thermal bistability and extremely high, solvent-independent photocyclization quantum yields (Φo-c, up to 90.6 %). It is characterized by a complete separation of the anti-parallel conformer and the suppression of intramolecular charge transfer (ICT). EWG=electron-withdrawing group.
A photoresponsive self-assembly on a 2-D surface was investigated by scanning tunnelling microscopy (STM). The open-ring isomer of a diarylethene derivative showed an abrupt ordering formation at a critical concentration, which was successfully reproduced by a cooperative model based on Langmuir-type adsorption.
Opening light: Two-dimensional pores are formed by the self-assembly of azobenzene-functionalized triangular building blocks on graphite at the liquid-solid interface. These pores can selectively host a guest molecule. The pore size can be reversibly changed by irradiation at different wavelength which changes the number of guest molecules that are adsorbed.
Since the serendipitous event that led to the first synthesis of a molecule by the hands of Man in 1826, the creation of molecular matter depended for 150 years on linking together molecules from other molecular building blocks with the help of strong covalent bonds. The advent of supramolecular chemistry in the last decades of the 20th century has provided chemists with a wealth of new possibilities to synthesize molecular structures and materials that are held together by relatively weak, non-covalent interactions, such as hydrogen bonding, π–π stacking, electrostatic and van der Waals interactions. Using nature as a source of inspiration, the creation of even more complex supramolecular architectures has recently become possible by applying the concept of hierarchical self-assembly, i.e. the non-covalent organization of molecules and macromolecules which takes places over distinct multiple levels, in which the assembly processes gradually decrease in strength. This review will focus on some recent discoveries in the field of spontaneous hierarchical organization of synthetic amphiphiles, disk-like molecules and concave building blocks into well-defined nano-sized assemblies.
Introduction of isopropyl substituents to 2 and 2' positions of benzothiophene rings of bis(1-benzothiophen-3-yl)hexa-fluorocyclopentene increased the population of anti-parallel conformation, and enhanced the quantum yield of the cyclization reaction.
Beside normal cyclization/ring-opening photochromic reactions, a side reaction to produce a photostable by-product took place when 1,2-bis(2-methyl-5-phenyl-3-thienyl)perfluorocyclopentene was irradiated in deaerated hexane with ultraviolet light.
Synthetic polymers exhibit diverse and useful properties and influence most aspects of modern life. Many polymerization methods provide linear or branched macromolecules, frequently with outstanding functional-group tolerance and molecular weight control. In contrast, extending polymerization strategies to two-dimensional periodic structures is in its infancy, and successful examples have emerged only recently through molecular framework, surface science and crystal engineering approaches. In this Review, we describe successful 2D polymerization strategies, as well as seminal research that inspired their development. These methods include the synthesis of 2D covalent organic frameworks as layered crystals and thin films, surface-mediated polymerization of polyfunctional monomers, and solid-state topochemical polymerizations. Early application targets of 2D polymers include gas separation and storage, optoelectronic devices and membranes, each of which might benefit from predictable long-range molecular organization inherent to this macromolecular architecture.
Photochromic molecules provide an intriguing and relatively untapped alternative to traditional materials utilized in organic memory devices. Here, we review recent progress in the implementation of photochromic molecules in electrically-addressed organic memory devices. Recent results for a lightemitting photochromic organic diode are highlighted in the context of multifunctional devices with the ability to simultaneously operate as multilevel memory, signage and display elements. Furthermore, a set of design rules for successful implementation of photochromic compounds in organic memory devices are suggested.
Part 1 From molecular to supramolecular chemistry: concepts and language of supramolecular chemistry. Part 2 Molecular recognition: recognition, information, complementarity molecular receptors - design principles spherical recognition - cryptates of metal cations tetrahedral recognition by macrotricyclic cryptands recognition of ammonium ions and related substrates binding and recognition of neutral moelcules. Part 3 Anion co-ordination chemistry and the recognition of anionic substrates. Part 4 Coreceptor molecules and multiple recognition: dinuclear and polynuclear metal ion cryptates linear recognition of molecular length by ditopic coreceptors heterotopic coreceptors - cyclophane receptors, amphiphilic receptors, large molecular cage multiple recognition in metalloreceptors supramolecular dynamics. Part 5 Supramolecular reactivity and catalysis: catalysis by reactive macrocyclic cation receptor molecules catalysis by reactive anion receptor molecules catalysis with cyclophane type receptors supramolecular metallo-catalysis cocatalysis - catalysis of synthetic reactions biomolecular and abiotic catalysis. Part 6 Transport processes and carrier design: carrier-mediated transport cation-transport processes - cation carriers anion transport processes - anion carriers coupled transport processes electron-coupled transpoort in a redox gradient proton-coupled transport in a pH gradient light-coupled transport processes transfer via transmembrane channels. Part 7 From supermolecules to polymolecular assemblies: heterogeneous molecular recognition - supramolecular solid materials from endoreceptors to exoreceptors - molecular recognition at surfaces molecular and supramolecular morphogenesis supramolecular heterogeneous catalysis. Part 8 Molecular and supramolecular devices: molecular recognition, information and signals - semiochemistry supramolecular photochemistry - molecular and supramolecular photonic devices light conversion and energy transfer devices photosensitive molecular receptors photoinduced electron transfer in photoactive devices photoinduced reactions in supramolecular species non-linear optical properties of supramolecular species supramolecular effects in photochemical hole burning molecular and supramolecular electronic devices supramolecular electrochemistry electron conducting devices - molecular wires polarized molecular wires - rectifying devices modified and switchable molecular wires molecular magnetic devices molecular and supramolecular ionic devices tubular mesophases. (Part contents).
Among the many significant advances within the field of supramolecular chemistry over the past decades, the development of the so-called ‘‘dynamers’’ features a direct relevance to materials science. Defined as ‘‘combinatorial dynamic polymers’’, dynamers are constitutional dynamic systems and materials resulting from the application of the principles of supramolecular chemistry to polymer science. Like supramolecular materials in general, dynamers are reversible dynamic multifunctional architectures, capable of modifying their constitution by exchanging, recombining, incorporating components. They may exhibit a variety of novel properties and behave as adaptive materials. In this review we focus on the design of responsive switchable monolayers, i.e. monolayers capable to undergo significant changes in their physical or chemical properties as a result of external stimuli. Scanning tunneling microscopy studies provide direct evidence with a sub-nanometre resolution, on the formation and dynamic response of these self-assembled systems featuring controlled geometries and properties.
Scanning tunneling microscopy (STM) was used to study physisorbed monolayers of 5-octadecyloxyisophthalic acid (C18ISA) and 5-[ω-(4‘-dodecyloxy-4-azobenzeneoxy)dodecyloxy]isophthalic acid (C12(AZO)C12ISA) at a liquid/graphite interface. The acquired STM images exhibit the molecular packing of the monolayers with submolecular resolution. Monolayers formed by codeposition of solvent molecules and C18ISA or C12(AZO)C12ISA molecules are found when 1-octanol and 1-undecanol are used as a solvent. The cis and trans isomers of C12(AZO)C12ISA, the reagent and reaction product of a reversible photoinduced reaction, are simultaneously present and are both structurally identified at the liquid/graphite interface.
Functionalized molecules represent the central issue of molecular nanotechnology. Scanning tunnelling microscopy (STM) is a powerful method to investigate such molecules, because it allows us to image them with sub-molecular resolution when adsorbed on a surface and can be used at the same time as a tool to manipulate single molecules in a controlled way. Such studies permit deep insight into the conformational, mechanical and electronic structure and thus functionalities of the molecules. In this review, recent experiments on specially designed molecules, acting as model systems for molecular nanotechnology, are reviewed. The presented studies focus on key functionalities: lateral rolling and hopping motion on a supporting surface, the switching behaviour of azobenzene derivatives by using the STM tip and the controlled reactivity of molecular side groups, which enable the formation of covalently bound molecular nanoarchitectures.
Up or down? Making use of self-assembly, the graphite surface is decorated with upright oriented photochromic units organized by a terfluorene platform. The azobenzenes within the monolayer could be converted from the trans to the cis form leading to substantial and reversible structural reorganization, as revealed by scanning tunneling microscopy (STM) studies.
Materials with a pre-programmed order at the supramolecular level can be engineered with a sub-nanometer precision making use of reversible noncovalent interactions. The intrinsic ability of supramolecular materials to recognize and exchange their constituents makes them constitutionally dynamic materials. The tailoring of the materials properties relies on the full control over the self-assembly behavior of molecular modules exposing recognition sites and incorporating functional units. In this review we focus on three classes of weak-interactions to form complex 2D architectures starting from properly designed molecular modules: van der Waals, metallo-ligand and hydrogen bonding. Scanning tunneling microscopy studies will provide evidence with a sub-nanometer resolution, on the formation of responsive multicomponent architectures with controlled geometries and properties. Such endeavor enriches the scientist capability of generating more and more complex smart materials featuring controlled functions and unprecedented properties.
By means of STM, direct evidence on the sub-nanometer scale of a dynamer operating at surfaces has been provided. Octadecyl guanine (G) was reversibly interconverted at the solid–liquid interface between two highly ordered supramolecular motifs, that is, hydrogenbonded ribbons and G4-based architectures, upon subsequent addition of [2.2.2]cryptand, potassium picrate (K+(pic)-), and trifluoromethanesulfonic acid. The visualization of such supramolecular interconversion at the solid–liquid interface opens new avenues towards understanding the mechanism of formation and function of complex nucleobase architectures such as DNA or RNA. Furthermore, the in situ reversible assembly and reassembly between two highly ordered supramolecular structures at the surfaces represents the first step towards the generation of nanopatterned responsive architectures.
Eine äußerst wirksame Selbstorganisation kleiner organischer Gelbildner in verschiedenen Lösungsmitteln führt zu einem verflochtenen Netzwerk und verwandelt dabei die Flüssigkeit in ein Gel. Die neuere Entwicklung organischer Gelbildner führte u. a. zu maßgeschneiderten Gelbildnern für überkritisches CO2. Solche Stoffe kann man darüber hinaus auch als Template zum Aufbau neuartiger Materialien verwenden und als Gele, die auf einen externen Auslöser („Trigger”;) reagieren. Das Bild zeigt schematisch, wie die viskoelastischen Eigenschaften eines schaltbaren organischen Gels beeinflusst werden können.
Wenn aus Feinden Freunde werden: Die hierarchische Selbstorganisation – durch konzertierte Wirkung verschiedener Kräfte auf ihren charakteristischen Längenskalen – ermöglicht die Bildung von funktionellen supramolekularen Architekturen mit hohem Ordnungsgrad sowohl auf der nano- als auch auf der makroskopischen Ebene. Anwendungen solcher Systeme in der Elektronik, Katalyse und Medizin stehen in Aussicht.
On-surface self-condensation of 1,4-benzenediboronic acid was previously shown to yield extended surface-supported, long-range-ordered two-dimensional covalent organic frameworks (2D COFs). The most important prerequisite for obtaining high structural quality is that the polycondensation (dehydration) reaction is carried out under slightly reversible reaction conditions, i.e., in the presence of water. Only then can the subtle balance between kinetic and thermodynamic control of the polycondensation be favorably influenced, and defects that are unavoidable during growth can be corrected. In the present study we extend the previously developed straightforward preparation protocol to a variety of para-diboronic acid building blocks with the aim to tune lattice parameters and pore sizes of 2D COFs. Scanning tunneling microscopy is employed for structural characterization of the covalent networks and of noncovalently self-assembled structures that form on the surface prior to the thermally activated polycondensation reaction.