Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “bistability”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,045 records · Page 58Linked to original sources
PubMed1996

Copper(II/I) Complexes of a Hexakis(bipyridyl)cyclotriveratrylene Ligand: A Redox-Induced Conformational Switch.

A series of copper(II) and copper(I) complexes have been synthesized with ligands combining 6-methyl-2,2'-bipyridines with cyclotriveratrylene (CTV) (1) and with catechol (2). The electrochemical, (1)H NMR, and mass spectrometry characterizations of these complexes are described and discussed. The six pendant bipyridines of ligand 1 allow for the formation of two trinuclear copper(I) complexes [(1)Cu(3)](BF(4))(3) differing only in the conformation "vic" or "int" adopted by the ligand to fit the tetrahedral cuprous ions. Similarly, 1 generates two trinuclear copper(II) complexes in which the conformation of the ligand fits the square planar geometry of cupric ions. In both the cuprous and cupric complexes, a conformational equilibrium exists. Ligand 2 bearing two methylbipyridines has proven to be a useful model of the coordinating sites of ligand 1. In this case, two homologous copper(I) complexes are obtained, [(2)Cu]BF(4) and [(2)(2)Cu(2)](BF(4))(2), modeling respectively two possible coordination conformations of ligand 1. With copper(II), ligand 2 yields only one complex [(2)Cu](CF(3)SO(3))(2), which allows for the unambiguous identification of the conformations observed for ligand 1 complexes. The different coordinating modes of ligand 1 in the complexes mentioned are in exchange but exhibit different physical properties, thus representing a new bistable system based on conformational isomerism which exhibits an electrochemical potential hysteresis. An equilibrium constant and thermodynamic data were obtained for this system by variable-temperature cyclic voltammetry. The influence of coordinating vs noncoordinating solvents was also studied.

Journal Article↗
PubMed2003

Molecular quantum-dot cellular automata.

Molecular electronics is commonly conceived as reproducing diode or transistor action at the molecular level. The quantum-dot cellular automata (QCA) approach offers an attractive alternative in which binary information is encoded in the configuration of charge among redox-active molecular sites. The Coulomb interaction between neighboring molecules provides device-device coupling. No current flow between molecules is required. We present an ab initio analysis of a simple molecular system which acts as a molecular QCA cell. The intrinsic bistability of the charge configuration results in dipole or quadrupole fields which couple strongly to the state of neighboring molecules. We show how logic gates can be implemented. We examine the role of the relaxation of nuclear coordinates in the molecular charge reconfiguration.

Journal Article↗
PubMed2003

Multielectron transfer in clusters of polar-polarizable chromophores.

We present a model for interacting polar-polarizable molecules. Exotic collective effects are recognized as being due to supramolecular interactions: in attractive lattices, bistable behavior is found, and, for proper molecular orientation, multielectron transfer represents the primary photoexcitation event.

Journal Article↗
PubMed2003

On the electronic origins of structural isomerism in the iron-sulfur cubane, [(C(5)H(5))(4)Fe(4)S(4)](2+).

Density functional theory provides new insights into the structural isomerism observed in the cyclopentadienyl-capped iron-sulfur cluster, [(C(5)H(5))(4)Fe(4)S(4)](2+). Two distinct, closely spaced minima have been located, a triplet with D(2) symmetry and a C(2)-symmetric singlet, both of which correspond closely to the structure of one of the known crystal forms of the cation. Thus, the structural diversity in these species reflects genuine molecular bistability rather than simple solid-state packing effects. In contrast, no stable D(2)(d)()-symmetric minimum has been located, suggesting that the reported D(2)(d)() symmetry of the cation in [(C(5)H(5))(4)Fe(4)S(4)][PF(6)](2) may be a crystallographic artifact. In the ruthenium analogue, the more diffuse 4d orbitals stabilize the C(2)-symmetric singlet, which is unambiguously the ground state, but the D(2)-symmetric potential energy surface provides a viable low-energy pathway for the dynamic exchange of the Ru-Ru bonds.

Journal Article↗
PubMed2005

Theoretical studies on the magnetic switching controlled by stacking patterns of bis(maleonitriledithiolato) nickelate(III) dimers.

Magnetic switchable maleonitriledithiolate (mnt) complexes were studied by density functional theory. The calculations were performed for anion dimers of [RBzPyR'][Ni(mnt)(2)] (RBzPyR' = derivatives of benzylpyridinium) to elucidate magnetostructural correlations and the nature of the weak intermolecular chemical bonding. The calculated results showed that the spin delocalization, favored by the eclipsed stacking and the shorter interlayer distance, was responsible for the diamagnetic character of [1-benzyl-4-aminopyridinium][Ni(mnt)(2)] at low temperature. The weak antiferromagnetic and ferromagnetic interactions were also reproduced for [1-benzyl-4-aminopyridinium][Ni(mnt)(2)] and [1-(4'-fluorobenzyl)pyridinium][Ni(mnt)(2)] at high temperature, respectively. The natural bond orbital analysis suggested that the cooperative effect of the weak intermolecular bondings may be the intrinsic driving force resulting in the switchable property, which is essentially similar to those in organic radicals exhibiting magnetic bistability. Further investigations with varying interlayer distance d, the extent of slippage (slipping distance r and deviation angle alpha), and rotational angle theta suggested that the extent of slippage played an important role in magnetic interactions. Therefore, the abrupt modulation of the extent of slippage in the [Ni(mnt)(2)](-) complexes by external perturbations provided new possibilities for the design of molecular magnetic switching devices.

Bridged Bicyclo Compounds, Heterocyclic↗
PubMed2006

Electronic switching of single silicon atoms by molecular field effects.

We have observed on-off switching of scanning tunneling microscope current flow to silicon adatoms of the Si(111)-(7 x 7) surface that are enclosed within a bistable dimeric corral of self-assembled chlorododecane molecules. These thermally activated oscillations amounted to an order of magnitude change in the current. Theory showed that small changes in molecular configuration could cause alterations in the corralled adatom's electronic energy by as much as 1 eV due to local field effects, accounting for the observed current switching.

Journal Article↗
PubMed2006

Photochromic switching of excited-state intramolecular proton-transfer (ESIPT) fluorescence: a unique route to high-contrast memory switching and nondestructive readout.

Aiming at the high-contrast photochromic switching of fluorescence emission and its perfect nondestructive readout, a polymer film highly loaded with a specific photochromic compound, 1,2-bis(2'-methyl-5'-phenyl-3'-thienyl)perfluorocyclopentene (BP-BTE), and an excited-state intramolecular proton-transfer (ESIPT)-active compound, 2,5-bis(5'-tert-butyl-benzooxazol-2'-yl)hydroquinone (DHBO), was employed in this work. The special class of photochrome, BP-BTE, has negligible absorbance at 415 nm both in the open form and in the 365 nm photostationary state (PSS), and the ESIPT fluorophore, DHBO, emits large Stokes' shifted (175 nm; lambda(max)(abs) = 415 nm, lambda(max)(em) = 590 nm) and enhanced fluorescence (Phi(F)(powder) = 10%, Phi(F)(soln) = 2%). Bistability, high-contrast switching (on/off fluorescence switching ratio >290), nondestructive readout (over 125000 shots), and erasability were all together accomplished in this novel recording medium.

Journal Article↗
PubMed2006

Rotational reorganization of doped cholesteric liquid crystalline films.

In this paper an unprecedented rotational reorganization of cholesteric liquid crystalline films is described. This rotational reorganization results from the conversion of a chiral molecular motor dopant to an isomer with a different helical twisting power, leading to a change in the cholesteric pitch. The direction of this reorganization is correlated to the sign of the change in helical twisting power of the dopant. The rotational reorganization of the liquid crystalline film was used to rotate microscopic objects 4 orders of magnitude larger than the bistable dopants in the film, which shows that molecular motors and switches can perform work. The surface of the doped cholesteric liquid crystalline films was found to possess a regular surface relief, whose periodicity coincides with typical cholesteric polygonal line textures. These surface features originate from the cholesteric superstructure in the liquid crystalline film, which in turn is the result of the presence of the chiral dopant. As such, the presence of the dopant is expressed in these distinct surface structures. A possible mechanism at the origin of the rotational reorganization of liquid crystalline films and the cholesteric surface relief is discussed.

Crystallization↗
PubMed2001

Binding studies between tetrathiafulvalene derivatives and cyclobis(paraquat-p-phenylene).

The complexation between a number of different pi-electron donating TTF derivatives and the pi-electron accepting tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT(4+)) has been studied by (1)H NMR and UV-vis spectroscopy. The results demonstrate that the strength of association between the donors (TTF derivatives) and acceptor (CBPQT(4+)) is strongly dependent on the pi-electron donating properties (measured by the first redox potential ) of the TTF derivatives. However, the first redox potential () is not the only factor of importance. The extended pi-surface of the TTF derivatives also exerts a stabilizing influence upon complexation. The kinetics for the complexation-decomplexation were studied using (1)H NMR spectroscopy and are related to the bulkiness of the TTF derivatives. These effects may serve to improve the design of interlocked molecular systems, especially (bistable) molecular switches, in which CBPQT(4+) and a derivatized TTF unit are incorporated.

Journal Article↗
PubMed2002

An acid-base switchable [2]rotaxane.

A chemically addressable, bistable [2]rotaxane, which incorporates a dumbbell-shaped component containing both secondary dialkylammonium and 1,2-bis(pyridinium)ethane recognition sites for its ring component, dibenzo[24]crown-8 (DB24C8), has been assembled. (1)H NMR spectroscopy has demonstrated that deprotonation (and reprotonation) of the secondary dialkylammonium (dialkylamine) recognition site induces the DB24C8 ring to move away from this site to the 1,2-bis(pyridinium)ethane one (and back again) in a discrete manner, particularly when the experiment is performed in CDCl(3) solution.

Journal Article↗
PubMed2003

Electrochiroptical response of 2,2'-(2,2-diarylethenyl)binaphthyl-type electron donors that undergo reversible C-C bond formation/breaking upon two-electron transfer.

2,2'-[2,2-Bis(4-dimethylaminophenyl)ethenyl]biphenyl (1) is a strong electron donor that undergoes oxidative C-C bond formation to give a stable dication rac-2(2+), the 9,10-dihydrophenanthrene derivative substituted with two bis(4-dimethylaminophenyl)methylium chromophores. This dication salt regenerates the starting diolefin 1 by reductive C-C bond breaking, thus realizing a new electrochromic system with high electrochemical bistability and a vivid change in color from yellow to deep blue. Similarly, the binaphthylic diolefin rac-3 and the helicene-type dication rac-4(2+) are interconvertible upon two-electron transfer. Both the UV-vis and CD spectra changed drastically upon electrochemical transformation between optically pure 3 and 4(2+), which represents a new electrochiroptical system.

Journal Article↗
PubMed2005

Complex behavior in the formaldehyde-sulfite reaction.

The formaldehyde-sulfite reaction is an example of an "acid-to-alkali" clock. It displays an induction period, during which the pH varies only slowly in time, followed by a reaction event, during which the pH increases rapidly by several units. When the reaction is performed in a closed (batch) reactor, the clock time is found to increase with a decrease in initial concentrations of formaldehyde and sulfite and an increase in the total initial concentration of S(IV). At long times, following the clock event, there is a slow decrease in pH. In an open (flow) reactor, bistability between a low-pH steady state (pH approximately 6-8) and a high-pH steady state (pH approximately 11) is observed. Additionally, we report the existence of sustained, small-amplitude oscillations in pH in this system. An extended kinetic mechanism reproduces the batch behavior but fails to account for the complex behavior observed in the flow reactor. Possible additional reaction steps are discussed.

Journal Article↗
PubMed2005

Wave patterns driven by chemomechanical instabilities in responsive gels.

The first experimental evidence of a chemomechanical mechanism leading to morphogenetic instabilities is demonstrated experimentally. The system consists of a pH-responsive gel that swells at high pH and shrinks at low pH, and a bistable reaction system exhibiting an acid steady state (pH approximately 2) and an alkaline steady state (pH approximately 10). Within the gel, the steady state selection depends on the gel size. We show that in a constant and uniform nonequilibrium chemical environment, the responsive gel undergoes large amplitude dynamical deformations under the form of travelling contraction waves and complex spatio-temporal volume oscillations. These deformations are coupled to concentration patterns of protons. We present different sequences of dynamical behaviors observed under various controlled chemical conditions. A simple heuristic model is proposed to account for the observations. These experiments open a new route for pattern formation driven by chemical energy, in soft matter systems.

Electrodes↗
PubMed2005

Spintronics.

Thin ferromagnetic films can be lithographically patterned and incorporated into magnetoelectronic devices that have applications in digital electronics. Their bistable magnetization states are adaptive to application as memory cells in nonvolatile, magnetic random access memories (MRAM). Prototype MRAM chips are characterized by rapid, low energy switching and excellent durability. This novel approach to integrated memory is expected to have immediate impact in the niche market of high performance, embedded memory. Research in the field is confronting several issues associated with the reproducibility of device behavior and scaling device structures to the nanometer dimensions of competing silicon devices. If successful solutions are found, the technology could challenge large segments of the semiconductor memory market. Another research effort is aimed at the development of a spintronic device with power gain. Such a device would enable new kinds of logic architectures, as well as novel on-chip combinations of memory and logic. Our studies investigate the dynamics of high-speed magnetization switching processes. We are also studying charge and spin transport in high mobility semiconductor heterostructures, with the ultimate goal of creating a spin-injected field effect transistor.

Journal Article↗
PubMed2006

Infrared spectroscopic characterization of [2]rotaxane molecular switch tunnel junction devices.

Langmuir-Blodgett monolayers of a bistable [2]rotaxane were prepared at packing densities of 118, 73, and 54 A(2)/molecule. The monolayers were both characterized via infrared spectroscopy before and after evaporation of a 2 nm film of titanium and incorporated into molecular switch tunnel junction devices. The study suggests that the evaporation process primarily affects portions of the molecule exposed to the metal atom source. Thus, in tightly packed monolayers (73 and 54 A(2)/molecule), only the portions of the [2]rotaxane that are present at the molecule/air interface are clearly affected, leaving key functionality necessary for switching intact. Monolayers transferred at a lower pressure (118 A(2)/molecule) exhibit nonspecific damage and poor switching behavior following Ti deposition. These results indicate that tightly packed monolayers and sacrificial functionality displayed at the molecule/air interface are important design principles for molecular electronic devices.

Journal Article↗
PubMed2006

Spatiotemporal dynamics of the Landolt reaction in an open spatial reactor with conical geometry.

In a previous study, the iodate-sulfite proton autoactivated reaction (Landolt reaction) was shown to exhibit spatial bistability and spatiotemporal oscillations when operated in an open spatial reactor with fixed "thickness", i.e., feed boundary to core distance. Here, we show that the spatial reactors with conical geometry enable one to rapidly probe the sensitivity of the above phenomena over a large range of the "thickness" parameter. This often-neglected parameter in chemical pattern studies plays an important role on the selection and stability of states. We reveal that the quenching capacity of slow diffusing polyacrylate ions on the spatiotemporal oscillations depends on this "thickness". The presented results should be useful for further research on reaction diffusion patterns and chemomechanical structures.

Journal Article↗
PubMed2004

Novel thiophene oligomers containing a redox active hexaarylethane unit.

[structure: see text] Novel thiophene oligomers containing a redox active hexaarylethane unit have been prepared as a new class of molecular wires having a large electrochemical bistability and definitely characterized by using single-crystal X-ray structure analysis and redox potential measurements. They constitute reversible redox pairs with the corresponding bis(thioxanthyl) dications by undergoing C-C bond making/breaking. They show unique electrochromism and a fluorescence change during interconversion.

Journal Article↗
PubMed2006

Locking down the electronic structure of (monopyrrolo)tetrathiafulvalene in [2]rotaxanes.

[reaction: see text] The redox potentials of a highly constrained [2]rotaxane have been measured and used to model the energy of the HOMO of tetrathiafulvalene-based bistable [2]rotaxanes in their two co-conformationally isomeric states. Restrained from co-conformational movements, the measured oxidation and reduction potentials provide insights into the orbital energies and electronic structure of a (monopyrrolo)tetrathiafulvalene unit when encircled by a tetracationic cyclobis(paraquat-p-phenylene) ring.

Journal Article↗