Showing papers by "J. Fraser Stoddart published in 2007"

A 160-kilobit molecular electronic memory patterned at 10 11 bits per square centimetre

Abstract: The primary metric for gauging progress in the various semiconductor integrated circuit technologies is the spacing, or pitch, between the most closely spaced wires within a dynamic random access memory (DRAM) circuit. Modern DRAM circuits have 140 nm pitch wires and a memory cell size of 0.0408 mum(2). Improving integrated circuit technology will require that these dimensions decrease over time. However, at present a large fraction of the patterning and materials requirements that we expect to need for the construction of new integrated circuit technologies in 2013 have 'no known solution'. Promising ingredients for advances in integrated circuit technology are nanowires, molecular electronics and defect-tolerant architectures, as demonstrated by reports of single devices and small circuits. Methods of extending these approaches to large-scale, high-density circuitry are largely undeveloped. Here we describe a 160,000-bit molecular electronic memory circuit, fabricated at a density of 10(11) bits cm(-2) (pitch 33 nm; memory cell size 0.0011 microm2), that is, roughly analogous to the dimensions of a DRAM circuit projected to be available by 2020. A monolayer of bistable, [2]rotaxane molecules served as the data storage elements. Although the circuit has large numbers of defects, those defects could be readily identified through electronic testing and isolated using software coding. The working bits were then configured to form a fully functional random access memory circuit for storing and retrieving information.

Photo-driven molecular devices

Abstract: In this critical review, we discuss switching of the light-powered bistable rotaxanes and catenanes and highlight the practical applications of some of these systems. Photoactive molecular and supramolecular machines are comprised of two parts-1) a switching element, based on noncovalent interactions within the recognition units, which is responsible for executing mechanical movement, and 2) a light-harvesting unit which utilizes light to control the competitive interactions between the recognition sites. We also survey another class of molecular devices, namely molecular rotary motors--i.e., those that behave like their macroscopic counterparts--in which photochemically and thermally induced mechanical movement relies on isomerizations of a pivotal C=C bond, leading to a rotation of the top propeller part with respect to the stationary bottom part of the helical shaped chiral molecule. (146 references.).

Template-directed synthesis employing reversible imine bond formation

Abstract: The imine bond – formed by the reversible condensation of an amine and an aldehyde – and its applications as a dynamic covalent bond in the template-directed synthesis of molecular compounds, will be the focus of this tutorial review. Template-directed synthesis – or expressed another way, supramolecular assistance to covalent synthesis – relies on the use of reversible noncovalent bonding interactions between molecular building blocks in order to preorganise them into a certain relative geometry as a prelude to covalent bond formation to afford the thermodynamically preferred product. The use of this so-called dynamic covalent chemistry (DCC) in templated reactions allows for an additional amount of reversibility, further eliminating potential kinetic products by allowing the covalent bonds that are formed during the template-directed reaction to be ‘proofread for errors’, thus making it possible for the reaction to search out its thermodynamic minimum. The marriage of template-directed synthesis with DCC has allowed chemists to construct an increasingly complex collection of compounds from relatively simple precursors. This new paradigm in organic synthesis requires that each individual piece in the molecular self-assembly process is preprogrammed so that the multiple recognition events expressed between the pieces are optimised in a highly cooperative manner in the desired product. It offers an extremely simple way of making complex mechanically interlocked compounds – e.g., catenanes, rotaxanes, suitanes, Borromean rings and Solomon knots – from relatively simple precursors.

Design and Optimization of Molecular Nanovalves Based on Redox-Switchable Bistable Rotaxanes

Abstract: Redox-controllable molecular nanovalves based on mesoporous silica nanoparticles have been fabricated, using two bistable [2]rotaxanes with different spacer lengths between their recognition sites as the gatekeepers. Three different linkers with varying chain lengths have been employed to attach the bistable [2]rotaxane molecules covalently to the silica substrate. These nanovalves can be classified as having IN or OUT locations, based on the positions of the tethered bistable [2]rotaxanes with respect to the entrances to the nanopores. The nanovalves are more efficient when the bistable [2]rotaxane-based gatekeepers are anchored deep within (IN) the pores than when they are attached closer to (OUT) the pores' orifices. The silica nanopores can be closed and opened by moving the mechanically interlocked ring component of the bistable [2]rotaxane closer to and away from the pores' orifices, respectively, a process which allows luminescent probe molecules, such as coumarins, tris(2-phenylpyridine)iridium, a...

Mesostructured silica supports for functional materials and molecular machines

Abstract: Mesostructured silica thin films and particles prepared by surfactant-templated sol–gel techniques are highly versatile substrates for the formation of functional materials. The ability to deliberately place molecules possessing desired activities in specific spatially separated regions of the nanostructure is an important feature of these materials. Such placement utilizes strategies that exploit the physical and chemical differences between the silica framework and the templated pores. As an example of placement of pairs of molecules, donor and acceptor molecules can be targeted to different regions of mesostructured thin films and energy transfer between them can be measured. The results not only demonstrate the spatial separation but also are used as a molecular ruler to measure the average distance between them. Mesostructured silica is also an excellent support for molecular machines. Molecules that undergo large amplitude motion, when attached to the silica, can function as impellers and nanovalves when activated by light, electrical (redox) and chemical (pH, competitive binding) energy. Derivatized azobenzene molecules, attached to pore walls by using one of the placement strategies, function as impellers that can move other molecules through the pores. Rotaxanes and pseudorotaxanes, placed at pore entrances, function as gatekeepers that can trap and release molecules from the pores when stimulated. Deliberately placed functional molecules on and in mesostructured silica offer many possibilities for both fundamental studies on the nanoscale and for applications in fields as diverse as fluidics, biological drug delivery and controlled release.

A redox-driven multicomponent molecular shuttle.

Abstract: A multicomponent [2]rotaxane designed to operate as a molecular shuttle driven by light energy has been constructed, and its properties have been investigated. The system is composed of (1) a light-fueled power station, capable of using the photon energy to create a charge-separated state, and (2) a mechanical switch, capable of utilizing such a photochemically generated driving force to bring about controllable molecular shuttling motions. The light-fueled power station is, in turn, a dyad comprising (i) a pi-electron-accepting fullerene (C60) component and (ii) a light-harvesting porphyrin (P) unit which acts as an electron donor in the excited state. The mechanical switch is a redox-active bistable [2]rotaxane moiety that consists of (i) a tetrathiafulvalene (TTF) unit as an efficient pi-electron-donor station, (ii) a dioxynaphthalene (DNP) unit as a second pi-electron-rich station, and (iii) a tetracationic cyclobis(paraquat-p-phenylene) (CBPQT4+) pi-electron-acceptor cyclophane, which encapsulates the better pi-electron-donating TTF station. Diethylene glycol spacers were conveniently introduced between the electroactive components in the dumbbell-shaped thread to facilitate the template-directed synthesis of the [2]rotaxane. A modular synthetic approach was undertaken for the overall synthesis of this multicomponent bistable [2]rotaxane, beginning with the syntheses of the P-C60 dyad unit and the two-station TTF-DNP-based [2]rotaxane separately, using conventional synthetic methodologies. These two components were finally stitched together by an esterification to afford the target rotaxane. Its structure was characterized by 1H NMR spectroscopy and mass spectrometry as well as by UV-vis-NIR absorption spectroscopy and voltammetry. The observations reflect remarkable electronic interactions between the various units, pointing to the existence of folded conformations in solution. The redox-driven shuttling process of the CBPQT4+ ring between the two competitive electron-rich recognition units, namely, TTF and DNP, was investigated by electrochemistry and spectroelectrochemistry as a means to verify its operational behavior prior to the photophysical studies related to light-driven operation. The oxidation process of the TTF unit is dramatically hampered in the rotaxane, thereby reducing the efficiency of the shuttling motion. These results confirm that, as the structural complexity increases, the overall function of the system no longer depends simply on its "primary" structure but also on higher-level effects which are reminiscent of the secondary and tertiary structures of biomolecules.

Clicked interlocked molecules

Abstract: The CuI-catalyzed Huisgen 1,3-dipolar cycloaddition, popularized as “click chemistry,” is one of the latest acquisitions to the synthetic arsenal for the making of mechanically interlocked molecula...

Functionally rigid bistable [2]rotaxanes.

Abstract: Two-station [2]rotaxanes in the shape of a degenerate naphthalene (NP) shuttle and a nondegenerate monopyrrolotetrathiafulvalene (MPTTF)/NP redox-controllable switch have been synthesized and characterized in solution. Their dumbbell-shaped components are composed of polyether chains interrupted along their lengths by (i) two π-electron-rich stationstwo NP moieties or a MPTTF unit and a NP moietywith (ii) a rigid arylethynyl or butadiynyl spacer situated between the two stations and terminated by (iii) flexibly tethered hydrophobic stoppers at each end of the dumbbells. This modification was investigated as a means to simplify both molecular structure and switching function previously observed in related bistable [2]rotaxanes with flexible spacers between their stations and incorporating a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring. The nondegenerate MPTTF-NP switch was isolated as near isomer-free bistable [2]rotaxane. Utilization of MPTTF removes the cis/trans isomerization that characterizes the tet...

Efficient production of [n]rotaxanes by using template-directed clipping reactions.

Abstract: In this article, we report on the efficient synthesis of well defined, homogeneous [n]rotaxanes (n up to 11) by a template-directed thermodynamic clipping approach. By employing dynamic covalent chemistry in the form of reversible imine bond formation, [n]rotaxanes with dialkylammonium ion (-CH(2)NH(2)(+)CH(2)-) recognition sites, encircled by [24]crown-8 rings, were prepared by a thermodynamically controlled, template-directed clipping procedure, that is, by mixing together a dumbbell compound containing a discrete number of CH(2)NH(2)(+)CH(2)- ion centers with appropriate amounts of a dialdehyde and a diamine to facilitate the [n]rotaxane formation. A 21-component self-assembly process is operative during the formation of the [11]rotaxane. The oligomeric dumbbells containing CH(2)NH(2)(+)CH(2)- ion recognition sites were prepared by a stepwise protocol. Several of the dynamic [n]rotaxanes were converted into their kinetically stable counterparts, first by reduction ("fixing") of imine bonds with the BH(3).THF complex, then by protonation of the complex by addition of acid.

Bifunctional [c2]daisy-chains and their incorporation into mechanically interlocked polymers.

Abstract: A strategy for the formation of mechanically interlocked polymers is presented. Ring-closing olefin metathesis has been shown to provide a very high yielding route to [c2]daisy-chains suitably functionalized to allow their one-step conversion to bisolefins which can be used as monomers in ADMET polymerizations to afford mechanically interlocked polymers. Metathesis, in two different guises is making a hitherto unreachable goal in synthesis a reality.

Modular synthesis and dynamics of a variety of donor-acceptor interlocked compounds prepared by click chemistry.

Abstract: A series of donor-acceptor (2)-, (3)-, and (4)rotaxanes and self- complexes ((1)rotaxanes) have been synthesized by a threading-followed- by-stoppering approach, in which the precursor pseudorotaxanes are fixed by using Cu I -catalyzed Huisgen 1,3-dipolar cycloaddition to attach the required stoppers. This alternative approach to forming rotaxanes of the donor-accept- or type, in which the donor is a 1,5-di- oxynaphthalene unit and the acceptor is the tetracationic cyclophane cyclo- bis(paraquat-p-phenylene), proceeds with enhanced yields relative to the tried and tested synthetic strategies, which involve the clipping of the cyclo- phane around a preformed dumbbell containing p-electron-donating recogni- tion sites. The new synthetic approach is amenable to application to highly convergent sequences. To extend the scope of this reaction, we constructed (2)rotaxanes in which one of the phen- ylene rings of the tetracationic cyclo- phane is perfluorinated, a feature which significantly weakens its associa- tion with p-electron-rich guests. The activation barrier for the shuttling of the cyclophane over a spacer contain- ing two triazole rings was determined to be (15.5 � 0.1) kcal mol � 1 for a de- generate two-station (2)rotaxane, a value similar to that previously mea- sured for analogous degenerate com- pounds containing aromatic or ethyl- ene glycol spacers. The triazole rings do not seem to perturbthe shuttling process significantly; this property bodes well for their future incorpora- tion into bistable molecular switches.

Structural and co-conformational effects of alkyne-derived subunits in charged donor-acceptor [2]catenanes.

Abstract: Four donor−acceptor [2]catenanes with cyclobis(paraquat-p-phenylene) (CBPQT4+) as the π-electron-accepting cyclophane and 1,5-dioxynaphthalene (DNP)-containing macrocyclic polyethers as π-electron donor rings have been synthesized under mild conditions, employing Cu+-catalyzed Huisgen 1,3-dipolar cycloaddition and Cu2+-mediated Eglinton coupling in the final steps of their syntheses. Oligoether chains carrying terminal alkynes or azides were used as the key structural features in template-directed cyclizations of [2]pseudorotaxanes to give the [2]catenanes. Both reactions proceed well with precursors of appropriate oligoether chain lengths but fail when there are only three oxygen atoms in the oligoether chains between the DNP units and the reactive functional groups. The solid-state structures of the donor−acceptor [2]catenanes confirm their mechanically interlocked nature, stabilized by [π···π], [C−H···π], and [C−H···Ο] interactions, and point to secondary noncovalent contacts between 1,3-butadiyne and ...

Dynamic donor-acceptor [2]catenanes.

Abstract: Donor–acceptor [2]catenanes based on cyclobis(paraquat-p-phenylene) as the π-acceptor ring have been used prominently in the construction of functional molecular devices. We report here their thermodynamically controlled synthesis from isolated π-donor and π-acceptor rings under the catalytic influence of tetra butylammonium iodide. The initial nucleophilic attack of iodide ion, which opens up the π-acceptor ring, is followed by complexation to the π-donor ring and the subsequent catenation of the π-donor ring by the π-acceptor ring [2]catenane. The reaction is general in scope and proceeds in high yields, without giving rise to side-products.

Designing bistable [2]rotaxanes for molecular electronic devices

Abstract: The development of molecular electronic components has been accelerated by the promise of increased circuit densities and reduced power consumption. Bistable rotaxanes have been assembled into nanowire crossbar devices, where they may be switched between low- and high-conductivity states, forming the basis for a molecular memory. These memory devices have been scaled to densities of 10 11 bits cm K2 , the 2020 node for memory of the International Technology Roadmap for Semiconductors. Investigations of the kinetics and thermodynamics associated with the electromechanical switching processes of several bistable [2]rotaxane derivatives in solution, self-assembled monolayers on gold, polymer electrolyte gels and in molecular switch tunnel junction devices are consistent with a single, universal switching mechanism whose speed is dependent largely on the environment, as well as on the structure of the switching molecule. X-ray reflectometry studies of the bistable rotaxanes assembled into Langmuir monolayers also lend support to an oxidatively driven mechanical switching process. Structural information obtained from Fourier transform reflection absorption infrared spectroscopy of rotaxane monolayers taken before and after evaporation of a Ti top electrode confirmed that the functionality responsible for switching is not affected by the metal deposition process. All the considerable experimental data, taken together with detailed computational work, support the hypothesis that the tunnelling current hysteresis, which forms the basis of memory operation, is a direct result of the electromechanical switching of the bistable rotaxanes.

Toward electrochemically controllable tristable three-station [2]catenanes.

Abstract: Encouraged by the prospect of producing an electrochemical, color- switchable red-green-blue (RGB) dye compound, we have designed, synthe- sized, and characterized two three-sta- tion (2)catenanes. Both are composed of macrocyclic polyethers containing three p-electron-rich stations, which act as recognition sites for a p-electron-de- ficient tetracationic cyclophane. The molecular structures of the two three- station (2)catenanes were characterized fully by mass spectrometry and 1 H NMR spectroscopy. To anticipate the relative occupancies of the three stations in each (2)catenane by the cy- clophane, model compounds with the same constitutions in the vicinity of the stations were synthesized. The relative ground-state populations of the three stations occupied in both (2)catenanes were estimated from the thermody- namic parameters for 1:1 complexes between all these model compounds and the cyclophane, obtained from iso- thermal titration calorimetry (ITC). The electrochemical and electrome- chanical properties of the three-station (2)catenanes were analyzed by cyclic voltammetry (CV), differential pulse voltammetry (DPV), and spectroelec- trochemistry (SEC). The first three-sta- tion (2)catenane was found to behave like a bistable system, whereas the second can be described as a quasi-tri- stable system.

Rotaxanes and Catenanes by Click Chemistry

Abstract: Copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition between terminal alkynes and azides – also known as the copper (Cu)-catalyzed Azide-Alkyne Cycloaddition (CuAAC) – has been used in the syntheses of molecular compounds with diverse structures and functions, owing to its functional group tolerance, facile execution, and mild reaction conditions under which it can be promoted. Recently, rotaxanes of four different structural types, as well as donor/acceptor catenanes, have been prepared using CuAAC, attesting to its tolerance to supramolecular interactions as well. In one instance of a rotaxane synthesis, the catalytic role of copper has been combined successfully with its previously documented ability to preorganize rotaxane precursors, i.e., form pseudorotaxanes. The crystal structure of a donor/acceptor catenane formed using the CuAAC reaction indicates that any secondary [p ··· p] interactions between the 1,2,3-triazole ring and the bipyridinium p-acceptor are certainly not destabilizing. Finally, the preparation of robust rotaxane and catenane molecular monolayers onto metal and semiconductor surfaces is premeditated based upon recent advances in the use of the Huisgen reaction for surface functionalization.

Dynamic Mechanically Interlocked Dendrimers: Amplification in Dendritic Dynamic Combinatorial Libraries

Abstract: In the context of constructing nonclassical mechanically interlocked dendrimers by employing a convergent templation procedure, the “clipping” thermodynamic approach has been explored to introduce sterically bulky Frechet-type dendrons with successive generations [G0] to [G3] onto a trivalent ammonium ion core using a seven-component self-assembly via imine bond formation. Four generations of mechanically interlocked dendrimers up to a molecular weight over 8800 Da were synthesized in a one-pot reaction by simply mixing the seven components together. The dendrimers form in excellent yield (>90%). The mechanically interlocked core of the [G0]−[G2] dendrimers can be modified and transformed into kinetically stable dendrimers by reduction of the imine bonds with borane−tetrahydrofuran complex. Moreover, the dynamic nature of the thermodynamically controlled self-assembly process is employed to obtain three dynamic combinatorial libraries of dendrimers by the treatment of the dendrons [G0]−[G3] with the compl...

Equilibrating dynamic [2]rotaxanes.

Abstract: Upon mixing and dehydration, 2,6-diformylpyridine and 2,2‘-oxybis(ethylamine) form a dynamic combinatorial library of at least nine members. Through hydrogen bonding and other intermolecular interactions, templating dumbbell molecules select one macrocyclic member of the library, at the expense of all the others, to create [2]rotaxanes. These rotaxanes, however, retain the dynamic character of the library, since a diformylpyridine analogue can exchange with the macrocyclic component in solution. In addition, crystallization of the mixture surprisingly furnishes only the [24]crown-8-like macrocycle on its ownevidence of a kinetic selection process occurring between phase transitions.

A molecular plug-socket connector.

Abstract: A monocationic plug−socket connector that is composed, at the molecular level, of three components, (1) a secondary dialkylammonium center (CH2NH2+CH2), which can play the role of a plug toward dibenzo[24]crown-8 (DB24C8), (2) a rigid and conducting biphenyl spacer, and (3) 1,4-benzo-1,5-naphtho[36]crown-10 (BN36C10), capable of playing the role of a socket toward a 4,4‘-bipyridinium dicationic plug, was synthesized and displays the ability to act as a plug−socket connector. The fluorescent signal changes associated with the 1,5-dioxynaphthalene unit of its BN36C10 portion were monitored to investigate the association of this plug−socket connector with the complementary socket and plug compounds. The results indicate that (1) the CH2NH2+CH2 part of the molecular connector can thread DB24C8 in a trivial manner and (2) the BN36C10 ring of the connector can be threaded by a 1,1‘-dioctyl-4,4‘-bipyridinium ion only after the CH2NH2+CH2 site is occupied by a DB24C8 ring. The two connections of the three-compone...

Switching surface chemistry with supramolecular machines.

Abstract: Tethered supramolecular machines represent a new class of active self-assembled monolayers in which molecular configurations can be reversibly programmed using electrochemical stimuli. We are using these machines to address the chemistry of substrate surfaces for integrated microfluidic systems. Interactions between the tethered tetracationic cyclophane host cyclobis(paraquat-p-phenylene) and dissolved π-electron-rich guest molecules, such as tetrathiafulvalene, have been reversibly switched by oxidative electrochemistry. The results demonstrate that surface-bound supramolecular machines can be programmed to adsorb or release appropriately designed solution species for manipulating surface chemistry.

Making Molecular Borromean Rings. A Gram-Scale Synthetic Procedure for the Undergraduate Organic Lab.

Abstract: Borromean rings (BRs) have long fascinated scholars of all disciplines for their wide cultural appeal and unique topology. Undergraduate students are no exception and so we have modified our published experimental procedure for the synthesis of molecular BRs to turn it into a lab instruction experiment suitable for undergraduate students to pursue in an organic chemistry laboratory course. Herein, we describe a procedure that requires seven 4-hour blocks of time to allow an undergraduate student to prepare the molecular BRs on a gram-scale in 90% yield. Just as important as engaging students in the BRs is the fact that the making of the molecular BRs incorporates several important, yet nonetheless overlooked, areas of chemistry. They include synthetic organic, physical organic, inorganic and metallo-organic chemistry, supramolecular, and dynamic covalent chemistry, all packaged up under the same umbrella in one project.