Showing papers by "J. Fraser Stoddart published in 2011"
675 citations
TL;DR: Two types of covalent organic frameworks (COFs) having a porphyrin unit have been synthesized as discussed by the authors, and the two COFs have shown very high charge carrier mobility values (8.1 cm2 V−1 s−1).
Abstract: Two types (imine and boronate) of covalent organic frameworks (COFs) having a porphyrin unit have been synthesized. The two highly crystalline porphyrin COFs (COF-366 and COF-66) display excellent chemical and thermal stability and are permanently porous. Two-dimensional extended layered structures of the two COFs demonstrate very high charge carrier mobility values (8.1 cm2 V−1 s−1).
620 citations
TL;DR: The evolution of various MSNPs is chronicle, advances in the synthesis of novel hybrid SNPs and the various MIMs which have been attached to their surfaces are discussed, and the design of more robust theranostic delivery systems are focused on using mesoporous silica nanoparticles (SNPs).
Abstract: Medicine can benefit significantly from advances in nanotechnology because nanoscale assemblies promise to improve on previously established therapeutic and diagnostic regimes. Over the past decade, the use of delivery platforms has attracted attention as researchers shift their focus toward new ways to deliver therapeutic and/or diagnostic agents and away from the development of new drug candidates. Metaphorically, the use of delivery platforms in medicine can be viewed as the “bow-and-arrow” approach, where the drugs are the arrows and the delivery vehicles are the bows. Even if one possesses the best arrows that money can buy, they will not be useful if one does not have the appropriate bow to deliver the arrows to their intended location.Currently, many strategies exist for the delivery of bioactive agents within living tissue. Polymers, dendrimers, micelles, vesicles, and nanoparticles have all been investigated for their use as possible delivery vehicles. With the growth of nanomedicine, one can env...
539 citations
TL;DR: A synthetic route to monofunctional pillar[5]arenes has been developed, allowing for the creation of a fluorescent sensor for alkylamine binding and the proposed [2]pseudorotaxane nature of the superstructure of the 1:1 host-guest complexes is supported by the template-directed synthesis of a related rotaxane.
Abstract: Alkanediamines serve as neutral guests for the recently discovered host pillar[5]arene. The proposed [2]pseudorotaxane nature of the superstructure of the 1:1 host-guest complexes is supported by the template-directed synthesis of a related [2]rotaxane. A synthetic route to monofunctional pillar[5]arenes has also been developed, allowing for the creation of a fluorescent sensor for alkylamine binding. The precursors to this host could act as starting points for a large library of monofunctional pillar[5]arene macrocycles.
440 citations
TL;DR: The highly selective adsorption of CO(2) by CD-MOF-2, a recently described green MOF consisting of the renewable cyclic oligosaccharide γ-cyclodextrin and RbOH, by what is believed to be reversible carbon fixation involving carbonate formation and decomposition at room temperature is reported.
Abstract: The efficient capture and storage of gaseous CO2 is a pressing environmental problem. Although porous metal–organic frameworks (MOFs) have been shown to be very effective at adsorbing CO2 selectively by dint of dipole–quadruple interactions and/or ligation to open metal sites, the gas is not usually trapped covalently. Furthermore, the vast majority of these MOFs are fabricated from nonrenewable materials, often in the presence of harmful solvents, most of which are derived from petrochemical sources. Herein we report the highly selective adsorption of CO2 by CD-MOF-2, a recently described green MOF consisting of the renewable cyclic oligosaccharide γ-cyclodextrin and RbOH, by what is believed to be reversible carbon fixation involving carbonate formation and decomposition at room temperature. The process was monitored by solid-state 13C NMR spectroscopy as well as colorimetrically after a pH indicator was incorporated into CD-MOF-2 to signal the formation of carbonic acid functions within the nanoporous ...
342 citations
TL;DR: These results demonstrate successful biomolecular-targeted hydrophobic drug delivery carriers that selectively target specific cancer cells and result in enhanced drug delivery and cell mortality.
Abstract: Synthetic methodologies integrating hydrophobic drug delivery and biomolecular targeting with mesoporous silica nanoparticles are described. Transferrin and cyclic-RGD peptides are covalently attached to the nanoparticles utilizing different techniques and provide selectivity between primary and metastatic cancer cells. The increase in cellular uptake of the targeted particles is examined using fluorescence microscopy and flow cytometry. Transferrin-modified silica nanoparticles display enhancement in particle uptake by Panc-1 cancer cells over that of normal HFF cells. The endocytotic pathway for these particles is further investigated through plasmid transfection of the transferrin receptor into the normal HFF cell line, which results in an increase in particle endocytosis as compared to unmodified HFF cells. By designing and attaching a synthetic cyclic-RGD, selectivity between primary cancer cells (BT-549) and metastatic cancer cells (MDA-MB 435) is achieved with enhanced particle uptake by the metastatic cancer cell line. Incorporation of the hydrophobic drug Camptothecin into these two types of biomolecular-targeted nanoparticles causes an increase in mortality of the targeted cancer cells compared to that caused by both the free drug and nontargeted particles. These results demonstrate successful biomolecular-targeted hydrophobic drug delivery carriers that selectively target specific cancer cells and result in enhanced drug delivery and cell mortality.
210 citations
119 citations
TL;DR: It has been proven that the mechanically interlocked nature of the [3]catenanes facilitates the formation of the TTF radical dimers under redox control, allowing an investigation to be performed on these intermolescular interactions in a so-called "molecular flask" under ambient conditions in considerable detail.
Abstract: Two donor−acceptor [3]catenanes—composed of a tetracationic molecular square, cyclobis(paraquat-4,4′-biphenylene), as the π-electron deficient ring and either two tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) containing macrocycles or two TTF-butadiyne-containing macrocycles as the π-electron rich components—have been investigated in order to study their ability to form TTF radical dimers. It has been proven that the mechanically interlocked nature of the [3]catenanes facilitates the formation of the TTF radical dimers under redox control, allowing an investigation to be performed on these intermolecular interactions in a so-called “molecular flask” under ambient conditions in considerable detail. In addition, it has also been shown that the stability of the TTF radical-cation dimers can be tuned by varying the secondary binding motifs in the [3]catenanes. By replacing the DNP station with a butadiyne group, the distribution of the TTF radical-cation dimer can be changed from 60% to 100%. These findings have been established by several techniques including cyclic voltammetry, spectroelectrochemistry and UV−vis−NIR and EPR spectroscopies, as well as with X-ray diffraction analysis which has provided a range of solid-state crystal structures. The experimental data are also supported by high-level DFT calculations. The results contribute significantly to our fundamental understanding of the interactions within the TTF radical dimers.
113 citations
TL;DR: A dynamic smart surface which was constructed by the self-assembly of an azobenzene-functionalised pillar exhibits reversibly responsive morphologies towards UV and visible light as observed by TEM, SEM and AFM.
86 citations
TL;DR: A variety of solution-phase counterion effects are discussed, from the fundamental origins to innovative ways in which these effects are exploited for useful functions.
Abstract: The self-assembly of molecular components into complex superstructures involves the subtle interplay of various noncovalent forces. Charged species are often utilised in self-assembly processes as a result of the favorable π–π, cation–π, electrostatic, and hydrogen bonding interactions that form between these species. Although the counterions associated with these charged species can exert significant effects on the synthesis, stability, and operation of superstructures in solution, rarely are the counterions considered, leading to misinterpretations and misunderstandings of the studied systems. In this tutorial review, we discuss a variety of solution-phase counterion effects, from the fundamental origins to innovative ways in which these effects are exploited for useful functions.
77 citations
TL;DR: Dynamic (1)H NMR spectroscopic investigations performed on one of these donor-acceptor [2]catenanes reveal that the pirouetting motion of the butadiyne-triethylene glycol chain occurs with a dramatically lower activation enthalpy, yet with a much higher negative activation entropy in water, compared to organic solvents.
Abstract: A subset of mechanically interlocked molecules, namely, donor−acceptor [2]catenanes, have been produced in aqueous solutions in good yields from readily available precursors. The catenations are templated by strong hydrophobic and [π···π] stacking interactions, which serve to assemble the corresponding supramolecular precursors, prior to postassembly covalent modification. Dynamic 1H NMR spectroscopic investigations performed on one of these [2]catenanes reveal that the pirouetting motion of the butadiyne-triethylene glycol chain occurs with a dramatically lower activation enthalpy, yet with a much higher negative activation entropy in water, compared to organic solvents. The preparations of mechanically interlocked molecules in water constitute the basis for the future development of complex functional molecular machinery in aqueous environments.
TL;DR: The success of the copper(I) 1,3-dipolar cycloaddition between azides and alkynes, resulting in the triazole ring has inspired the application of other emerging click reactions, for example, Diels-Aldercycloadditions, thiol-ene/yne chemistry, and nitrile N-oxide cycloadDitions, towards the creation of advanced functional supramolecular and mechanostereochemical systems.
Abstract: Supramolecular chemistry and mechanostereochemistry have been major beneficiaries of the concepts and reactions pioneered under the “click chemistry” philosophy. The success of the copper(I) 1,3-dipolar cycloaddition between azides and alkynes, resulting in the triazole ring has inspired the application of other emerging click reactions, for example, Diels–Alder cycloadditions, thiol–ene/yne chemistry, and nitrile N-oxide cycloadditions, towards the creation of advanced functional supramolecular and mechanostereochemical systems. In this Focus Review, recent advances in the use of click chemistry in these fields are highlighted.
TL;DR: Wet stamping allows metal–organic framework (MOF) crystals to be imprinted with micropatterns of various organic chemicals which change their appearance upon contact with specific chemicals, thus reporting the environmental “status” of the crystal.
Abstract: Wet stamping allows metal–organic framework (MOF) crystals to be imprinted with micropatterns of various organic chemicals. Printing the MOFs with photochromic molecules and pH indicators generates stimuli-responsive micropatterns which change their appearance upon contact with specific chemicals (see picture), thus reporting the environmental “status” of the crystal.
TL;DR: It is demonstrated that the BIPY(2+) unit can effectively employ to serve as electrostatic barriers in water in order to gain control over the motions of the CBPQT(4+) ring in both mechanically interlocked and supramolecular systems.
Abstract: The ability to control the kinetic barriers governing the relative motions of the components in mechanically interlocked molecules is important for future applications of these compounds in molecular electronic devices. In this Full Paper, we demonstrate that bipyridinium (BIPY^(2+)) dications fulfill the role as effective electrostatic barriers for controlling the shuttling and threading behavior for rotaxanes and pseudorotaxanes in aqueous environments. A degenerate [2]rotaxane, composed of two 1,5-dioxynaphthalene (DNP) units flanking a central BIPY^(2+) unit in the dumbbell component and encircled by the cyclobis(paraquat-p-phenylene) (CBPQT^(4+)) tetracationic cyclophane, has been synthesized employing a threading-followed-by-stoppering approach. Variable-temperature ^(1)H NMR spectroscopy reveals that the barrier to shuttling of the CBPQT^(4+) ring over the central BIPY2+ unit is in excess of 17 kcal mol^(−1) at 343 K. Further information about the nature of the BIPY^(2+) unit as an electrostatic barrier was gleaned from related supramolecular systems, utilizing two threads composed of either two DNP units flanking a central BIPY^(2+) moiety or a central DNP unit flanked by a BIPY2+ moiety. The threading and dethreading processes of the CBPQT^(4+) ring with these compounds, which were investigated by spectrophotometric techniques, reveal that the BIPY^(2+) unit is responsible for affecting both the thermodynamics and kinetics of pseudorotaxane formation by means of an intramolecular self-folding (through donor–acceptor interactions with the DNP unit), in addition to Coulombic repulsion. In particular, the free energy barrier to threading (Δequation image) of the CBPQT^(4+) for the case of the thread composed of a DNP flanked by two BIPY^(2+) units was found to be as high as 21.7 kcal mol^(−1) at room temperature. These results demonstrate that we can effectively employ the BIPY^(2+) unit to serve as electrostatic barriers in water in order to gain control over the motions of the CBPQT^(4+) ring in both mechanically interlocked and supramolecular systems.
TL;DR: Cyclic voltammetry (CV) reveals that the BDNP38C10 ring can be enticed to pass over the speed bump onto the neutral BIPY0 unit upon the generation of the NpI2− dianion, even although theneutral BIPy0 has presumably little or no affinity for the BD NP38C 10 ring.
Abstract: The synthesis and the dynamic behavior of a bistable [2]rotaxane, based on a reverse donor–acceptor motif containing naphthalene diimide (NpI) and 4,4′-bipyridinium (BIPY2+) as two electron-deficient stations and bis-1,5-dioxynaphthalene[38]crown-10 (BDNP38C10) as the electron-rich ring, is described. A functionalized tetraarylmethane moiety has been incorporated between the two stations in order to control the free energy barrier for the shuttling of the BDNP38C10 on the dumbbell component. The bistable [2]rotaxane was synthesized using the so-called “threading-followed-by-stoppering” approach and characterized by NMR spectroscopy and mass spectrometry. Initially, the BDNP38C10 ring resides on the NpI station on account of the synthetic approach employed in the synthesis of the bistable [2]rotaxane. 1H NMR spectroscopy was used to follow the equilibration process between the two translational isomers of the bistable [2]rotaxane—namely, NpI ⊂ BDNP38C10 and BIPY2+ ⊂ BDNP38C10. After 72 h, equilibrium was reached with a 3 : 2 ratio of the two translational isomers in favor of the NpI ⊂ BDNP38C10 co-conformation in CD3CN. The rate of relaxation of the crown ether from NpI ⊂ BDNP38C10 back to BIPY2+ ⊂ BDNP38C10 was associated with a rate constant of 2.2 ± 0.3 × 10−5 s−1 (t1/2 = 3.4 h), corresponding to a free energy of activation of 23.8 ± 0.1 kcal mol−1. Cyclic voltammetry (CV) reveals that the BDNP38C10 ring can be enticed to pass over the speed bump onto the neutral BIPY0 unit upon the generation of the NpI2− dianion, even although the neutral BIPY0 has presumably little or no affinity for the BDNP38C10 ring.
TL;DR: The power of constructing complex molecular machines using template-directed protocols is demonstrated, enabling them to make the transition from simple molecular switches to their multistate variants for enhancing information storage in molecular electronic devices.
Abstract: Rotacatenanes are exotic molecular compounds that can be visualized as a unique combination of a [2]catenane and a [2]rotaxane, thereby combining both the circumrotation of the ring component (rotary motion) and the shuttling of the dumbbell component (translational motion) in one structure. Herein, we describe a strategy for the synthesis of a new switchable [3]rotacatenane and the investigation of its switching properties, which rely on the formation of tetrathiafulvalene (TTF) radical π-dimer interactions—namely, the mixed-valence state (TTF{sub 2}){sup +.} and the radical-cation dimer state (TTF{sup +.}){sub 2}—under ambient conditions. A template-directed approach, based on donor–acceptor interactions, has been developed, resulting in an improved yield of the key precursor [2]catenane, prior to rotacatenation. The nature of the binding between the [2]catenane and selected π-electron-rich templates has been elucidated by using X-ray crystallography and UV/Vis spectroscopy as well as isothermal titration microcalorimetry. The multistate switching mechanism of the [3]rotacatenane has been demonstrated by cyclic voltammetry and EPR spectroscopy. Most notably, the radical-cation dimer state (TTF{sup +.}){sub 2} has been shown to enter into an equilibrium by forming the co-conformation in which the two 1,5-dioxynaphthalene (DNP) units co-occupy the cavity of tetracationic cyclophane, thus enforcing the separation of TTF radical-cation dimer (TTF{sup +.}){submore » 2}. The population ratio of this equilibrium state was found to be 1:1. We believe that this research demonstrates the power of constructing complex molecular machines using template-directed protocols, enabling us to make the transition from simple molecular switches to their multistate variants for enhancing information storage in molecular electronic devices.« less
TL;DR: The controllable mechanical movement of either the polyether macrocycle or the tetracationic cyclophane in a doubly bistable water-soluble [2]catenane can be accomplished by using orthogonal external stimuli—redox and amine/acid, respectively—in aqueous media.
Abstract: The controllable mechanical movement of either the polyether macrocycle or the tetracationic cyclophane in a doubly bistable water-soluble [2]catenane can be accomplished by using orthogonal external stimuli—redox and amine/acid, respectively—in aqueous media.
TL;DR: It is argued that the many aspects of beauty in MIMs are relevant, not only to the pleasure chemists derive from their research, but also to the progress of the research itself.
Abstract: Mechanically interlocked objects are ubiquitous in our world. They can be spotted on almost every scale of matter and in virtually every sector of society, spanning cultural, temporal, and physical boundaries the world over. From art to machinery, to biological entities and chemical compounds, mechanical interlocking is being used and admired every day, inspiring creativity and ingenuity in art and technology alike. The tiny world of mechanically interlocked molecules (MIMs), which has been established and cultivated over the past few decades, has connected the ordinary and molecular worlds symbolically with creative research and artwork that subsumes the molecular world as a miniaturization of the ordinary one. In this review, we highlight how graphical representations of MIMs have evolved to this end, and discuss various other aspects of their beauty as chemists see them today. We argue that the many aspects of beauty in MIMs are relevant, not only to the pleasure chemists derive from their research, but also to the progress of the research itself.
TL;DR: Five donor-acceptor oligorotaxanes made up of dumbbells composed of tetraethylene glycol chains, interspersed with three and five 1,5-dioxynaphthalene units, and terminated by 2,6-diisopropylphenoxy stoppers, have been prepared by the threading of discrete numbers of cyclobis(paraquat-p-phenylene) rings, followed by a kinetically controlled stoppering protocol that relies
Abstract: Five donor–acceptor oligorotaxanes made up of dumbbells composed of tetraethylene glycol chains, interspersed
with three and five 1,5-dioxynaphthalene units, and terminated by 2,6-diisopropylphenoxy stoppers, have been prepared by the threading of discrete numbers of cyclobis(paraquat-p-phenylene) rings, followed by a
kinetically controlled stoppering protocol that relies on click chemistry. The well-known copper(I)-catalyzed
alkyne–azide cycloaddition between azide functions placed at the ends of the polyether chains and alkyne-bearing
stopper precursors was employed during the final kinetically controlled template-directed synthesis of the five oligorotaxanes, which were characterized subsequently by ^1H NMR spectroscopy at low temperature (233 K) in
deuterated acetonitrile. The secondary structures, as well as the conformations, of the five oligorotaxanes were unraveled by spectroscopic comparison with the dumbbell and ring components. By focusing attention on the changes in
chemical shifts of some key probe protons, obtained from a wide range of low-temperature spectra, a picture emerges of a high degree of folding within the thread protons of the dumbbells of four of the five oligorotaxanes—the fifth oligorotaxane represents a control compound in effect—
brought about by a combination of C-H···O and π–π stacking interactions between the p-electron-deficient bipyridinium
units in the rings and the π-electron-rich 1,5-dioxynaphthalene units and polyether chains in the
dumbbells. The secondary structures of a foldamer-like nature have received further support from a solid-state superstructure of a related [3]pseudorotaxane and density functional calculations performed thereon.
TL;DR: In this article, a threading-followed-by-stoppering approach was used to synthesize bistable main-chain poly[n] rotaxanes for molecular switch tunnel junction devices.
Abstract: Electrochemically switchable bistable main-chain poly[n]rotaxanes have been synthesised using a threading-followed-by-stoppering approach and were incorporated into solid-state, molecular switch tunnel junction devices. In contrast to single-station poly[n]rotaxanes of similar structure, the bistable polymers do not fold into compact conformations held together by donor–acceptor interactions between alternating stacked π-electron rich and π-electron deficient aromatic systems. Films of the poly[n]rotaxane were incorporated into the devices by spin-coating, and their thickness was easily controlled. The switching functionality was characterised both (1) in solution by cyclic voltammetry and (2) in devices containing either two metal electrodes or one metal and one silicon electrode. Devices with one silicon electrode displayed hysteretic responses with applied voltage, allowing the devices to be switched between two conductance states, whereas devices containing two metal electrodes did not exhibit switching behaviour. The electrochemically switchable bistable poly[n]rotaxanes offer significant advantages in synthetic efficiency and ease of device fabrication as compared to bistable small-molecule [2]rotaxanes.
TL;DR: Surface-enhanced Raman spectroscopy was used to monitor the response of a self-assembled monolayer of a tetrathiafulvalene (TTF) derivative on a gold film-over-nanosphere electrode, resulting in surface-enhancing resonance Raman scattering (SERRS).
Abstract: Surface-enhanced Raman spectroscopy (SERS) was used to monitor the response of a self-assembled monolayer (SAM) of a tetrathiafulvalene (TTF) derivative on a gold film-over-nanosphere electrode. The electrochemical response observed was rationalized in terms of the interactions between TTF moieties as the oxidation state was changed. Electrochemical oxidation to form the monocation caused the absorbance of the TTF unit to coincide with both the laser excitation wavelength and the localized surface plasmon resonance (LSPR), resulting in surface-enhanced resonance Raman scattering (SERRS). The vibrational frequency changes that accompany electron transfer afford a high-contrast mechanism that can be used to determine the oxidation state of the TTF unit in an unambiguous manner.
TL;DR: Electrochemical experiments indicate that the one-electron reduced bipyridinium radical cation in the dumbbell components of the [2]rotaxanes serves as an additional recognition site for the two-Electron reduced CBPQT(2(˙+)) diradical cationic ring.
Abstract: A synthetic approach to the preparation of [2]rotaxanes (1–5·6PF6) incorporating bispyridinium derivatives and two 1,5-dioxynaphthalene (DNP) units situated in the rod portions of their dumbbell components that are encircled by a single cyclobis(paraquat-p-phenylene) tetracationic (CBPQT4+) ring has been developed. Since the π-electron-deficient bispyridinium units are introduced into the dumbbell components of the [2]rotaxanes 1–5·6PF6, there are Coulombic charge–charge repulsions between these dicationic units and the CBPQT4+ ring in the [2]rotaxanes. Thus, the CBPQT4+ rings in the degenerate [2]rotaxanes exhibit slow shuttling between two DNP recognition sites on the 1H NMR time-scale on account of the electrostatic barrier posed by the bispyridinium units, as demonstrated by variable-temperature 1H NMR spectroscopy. Electrochemical experiments carried out on the [2]rotaxanes 1·6PF6 and 2·6PF6 indicate that the one-electron reduced bipyridinium radical cation in the dumbbell components of the [2]rotaxanes serves as an additional recognition site for the two-electron reduced CBPQT2(˙+) diradical cationic ring. Under appropriate conditions, the ring components in the degenerate rotaxanes 1·6PF6 and 2·6PF6 can shuttle along the recognition sites – two DNP units and one-electron reduced bipyridinium radical cation – under redox control.
TL;DR: The intermolecular template-directed synthesis, separation and characterisation of two constitutional isomers that are self-complexing donor-acceptor [1]rotaxanes has been achieved by click chemistry.
TL;DR: Large crystals have also been nucleated, as indicated by the presence of a bas-relief of the original pattern on one surface of the crystal, suggesting that it is possible to transfer the template surface pattern onto a single crystal of a MOF.
Abstract: The controlled growth of metal-organic frameworks (MOFs) over surfaces has been investigated using a variety of surface analytical techniques. The use of microcontact printing to prepare surfaces, patterned with regions capable of nucleating the growth of MOFs, has been explored by employing copper-catalyzed alkyne-azide cycloaddition (CuAAC) to pattern silicon wafers with carboxylic acids, a functional group that has been shown to nucleate the growth of MOFs on surfaces. Upon subjecting the patterned silicon surfaces to solvothermal conditions, MOF thin films were obtained and characterized subsequently by AFM, SEM, and grazing-incidence XRD (GIXRD). Large crystals (∼0.5 mm) have also been nucleated, as indicated by the presence of a bas-relief of the original pattern on one surface of the crystal, suggesting that it is possible to transfer the template surface pattern onto a single crystal of a MOF.
TL;DR: The hypothesis that the extra stability of one translational isomer over the other is because of the influence of the enforced side-on donor–acceptor interactions brought about by both π-electron-rich recognition sites being part of a macrocyclic polyether is supported.
Abstract: In donor–acceptor mechanically interlocked molecules that exhibit bistability, the relative populations of the translational isomers—present, for example, in a bistable [2]rotaxane, as well as in a couple of bistable [2]catenanes of the donor–acceptor vintage—can be elucidated by slow scan rate cyclic voltammetry. The practice of transitioning from a fast scan rate regime to a slow one permits the measurement of an intermediate redox couple that is a function of the equilibrium that exists between the two translational isomers in the case of all three mechanically interlocked molecules investigated. These intermediate redox potentials can be used to calculate the ground-state distribution constants, K. Whereas, (i) in the case of the bistable [2]rotaxane, composed of a dumbbell component containing π-electron-rich tetrathiafulvalene and dioxynaphthalene recognition sites for the ring component (namely, a tetracationic cyclophane, containing two π-electron-deficient bipyridinium units), a value for K of 10 ± 2 is calculated, (ii) in the case of the two bistable [2]catenanes—one containing a crown ether with tetrathiafulvalene and dioxynaphthalene recognition sites for the tetracationic cyclophane, and the other, tetrathiafulvalene and butadiyne recognition sites—the values for K are orders (one and three, respectively) of magnitude greater. This observation, which has also been probed by theoretical calculations, supports the hypothesis that the extra stability of one translational isomer over the other is because of the influence of the enforced side-on donor–acceptor interactions brought about by both π-electron-rich recognition sites being part of a macrocyclic polyether.
TL;DR: In this article, a diarylethylene (DAE) ligand was used to cross-link CdSe quantum dots (QDs) within electrical junctions with photoswitchable conductivity.
Abstract: This paper describes the use of a diarylethylene (DAE) ligand, which adopts structures that are sensitive to the wavelength of light, to cross-link films of CdSe quantum dots (QDs) within electrical junctions with photoswitchable conductivity. These QD-DAE films are deposited on indium-tin-oxide/poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (ITO/PEDOT:PSS) electrodes and have eutectic Ga-In top-contacts. The photocurrent density of the cross-linked QD films is enhanced by a factor of 6.5 (averaged over all applied voltages) when the DAE ligand is switched from its open, non-conductive form (by illumination with 500–650 nm light) to its closed, conductive form (by illumination with 300–400 nm light). This enhancement is accomplished by changing the inter-particle electronic coupling, not the inter-particle distance. Identical QD films cross-linked with dibenzenedithiol ligands have a photoconductivity that is insensitive to the wavelength of light.
TL;DR: Computational studies reveal that, on account of the stiffness of the structures, the vibrational frequencies of characteristic low-frequency modes decrease more slowly with increasing ring diameter than do the lowest optical excitation energies.
Abstract: Toroidal carbon nanotubes (TCNTs), which have been evaluated for their potential applications in tera- hertz communication systems, provide a challenge of some magnitude from a purely scientific perspective. A design approach to TCNTs, as well as a clas- sification scheme, is presented based on the definition of the six hollow sec- tions that comprise the TCNT, slicing each of them to produce a (possibly creased) planar entity, and projecting that entity onto a graphene lattice. As a consequence of this folding approach, it is necessary to introduce five- and seven-membered rings as defect sites to allow the fusing together of the six segments into final symmetric TCNTs. This analysis permits the definition of a number of TCNT geometry families containing from 108 carbons up to much larger entities. Based on density functional theory (DFT) calculations, the energies of these structural candi- dates have been investigated and com- pared with (60)fullerene. The structures with the larger tube diameters are com- puted to be more stable than C60, whereas the smaller diameter ones are less stable, but may still be within syn- thetic reach. Computational studies reveal that, on account of the stiffness of the structures, the vibrational fre- quencies of characteristic low-frequen- cy modes decrease more slowly with in- creasing ring diameter than do the lowest optical excitation energies. It was found that this particular trend is true for the "breathing mode" vibra- tions when the diameter of the tubes is small, but not for more flexible toroi- dal nanotubes with larger diameters.
TL;DR: Monitoring the fate of the rotaxanes' charge transfer absorption bands provides evidence for preferential mechanical degradation of a midsection rotaxane unit as compared to a terminal rotAXane entity as a consequence of mechanical forces accumulating in the central region of the polymer chain.
TL;DR: The formation of both [2]- and [7]pseudorotaxanes, which are obtained by mixing of a dibenzylammonium derivative with mono- and hexakis-adducts of [60]fullerene bearing malonato-benzo[25]crown-8 rings, has been monitored in dichloromethane by both 1D and 2D (1)H NMR spectroscopies.
TL;DR: In this article, the stoppers on the dumbbell components, the solvents, and the counterions can affect the conformations adopted by the [2]rotaxanes in the solid state.