Narrow Results

Switching properties of metal/polymer/metal junctions is studied under application of twist using a combination of the tight-binding and nonequilibrium Green function calculations. The junction is taken to be composed of a short chain of cis-polyacetylene sandwiched between two ferromagnetic electrodes. It is shown that the current flow across the polymer can effectively be controlled by torsion applied through rotating the the electrodes about the junction's axis. It is found out that twisting the polymer leads to widening of its HUMO–LOMO energy gap which, in consequence, hinders the electron tunneling. The ferromagnetic electrodes are assumed to be single-band and their tight-binding parameters are chosen so as to simulate the ab initio density functional calculations of the band structure of Fe along its [001] crystallographic direction. According to our calculations it is also possible to adjust tunneling magnetoresistance of the junction by applying specific amounts of torsion.

We carry out first-principles calculations based on density functional theory and non-equilibrium Green’s function to investigate the electronic transport properties of a diarylethene-based molecule sandwiched between two Au electrodes. This molecular switch can be reversed between open and close forms by using light stimulation. We analyze the switch behavior of these two forms through the current–voltage curves, transmission spectra and molecular projected self-consistent Hamiltonian. It has been found that the current of the close form is significantly larger than the open form, and there is a large and stable switch ratio in a wide bias window. This result indicates that this molecule can become one of the good candidates for optical molecular switch in the future.

A new inorganic molecule [Co(Hbim)(C₆H₄O₂)(NH₃)₂]₂ that can be used as a new optically durable molecular switch was theoretically designed in the framework of density functional theory. Three stable minima, belonging to ¹A_g, ⁵A₁, and ⁹A_g states, were found in the complex. Theoretically predicted infrared spectra of the complexes showed that a strong peak of NH stretching vibration is observed at 2690, 2120, and 2770 cm^-1 in the ¹A_g, ⁵A₁, and ⁹A_g states, respectively. The apparent red shift of the NH stretching vibration band in the ⁵A₁ state make it possible to distinguish the electronic state from others (¹A_g and ⁹A_g). This means that the complex can be used as a molecular level switch whose memory can be stably read by IR light without any photoreaction process; namely, without memory degradation.

By applying nonequilibrium Green's function (NEGF) formalism combined first-principles density functional theory (DFT), we investigate the electronic transport properties of optical molecular switch based on the naphthopyran molecule with two different single-walled carbon nanotube (SWCNT) electrodes. The molecule that comprises the switch can convert between the closed and open forms upon photoexcitation. Theoretical results show that these two forms exhibit very different conductance properties both in armchair and zigzag junction, which can realize the on and off states of the molecular switch. Meantime, the chirality of the SWCNT electrodes strongly affects the switching characteristics of the molecular junctions. The maximum value of on–off ratio can reach 292 at 1.6 V for the switch with zigzag SWCNT electrodes.

The equilibrium geometry of the 2,5-bis-[2-(pyridin-2-yl)-vinyl]-1$H$-pyrrole calculated at the MP2/6-311$++$G($d$,$p$) level of theory evidences the breaking of one of the components in the three-centered intramolecular hydrogen bond due to the steric strain. For this reason, the three-centered intramolecular hydrogen bonding turns out to be asymmetric interaction involving the major and minor components. However, the reversible switching between these components under an external impact is also possible. Two different stable states with unequal geometric and electronic structure are observed in the derivatives of the 2,5-bis-[2-(pyridin-2-yl)-vinyl]-1$H$-pyrrole. These molecules represent novel molecular switches operating due to the pendulum-like transition between the nonequivalent two-centered components of the overcrowded three-centered intramolecular hydrogen bond. Implantation of hydrogen bond as a unit of the molecular scale device enhances potential of molecular electronics and could serve as a step towards the construction of artificial biological ensembles.

We perform first-principles calculations to investigate the electronic transport properties of chalcone and flavanone molecules sandwiched between graphene electrodes. These two molecules can be reversibly converted between open and closed states induced by pH, and the significant switching behaviors are observed. The currents and switching ratios are influenced by rotating molecules around the $X$ axis, which are discussed by the transmission eigenstates, electrostatic potential distributions and transmission spectra. The observed negative differential resistance effect is explained in chalcone configuration. The results suggest that spatial distributions of molecules will influence the performance of devices, indicating a potential application in future molecular circuits.

The large difference in the binding affinity of tetramethoxy-p-benzoquinone between 5,15-cis-bis(ureidophenyl)porphyrin and its zinc-porphyrin analogue enables construction of a reversible system in which tetramethoxy-p-benzoquinone moves between 5,15-cis-bis(ureidophenyl)porphyrin and the zinc-porphyrin as a quinone shuttle and a nitrate ion works as a chemical switch.

A donor-σ-acceptor compound incorporating tetrathiafulvalene and tetraphenylporphyrin units was synthesized. The structures of the target compound and its intermediates have been characterized by ¹H NMR, ESI MS, UV-vis, IR and mp. The results of UV-vis and cyclic voltammetry of the compound indicated negligible intramolecular charge transfer interaction in its ground state. The fluorescence and fluorescence lifetime of the compound were reduced, compared to tetraphenylporphyrin, which evidently indicated that photoinduced electron transfer occurred from the tetrathiafulvalene (TTF) unit to the tetraphenylporphyrin unit in the excited state. The fluorescence intensity of the compound could be reversibly modulated by sequential oxidation and reduction of the TTF unit using electrochemical and chemical methods. Therefore, a new fluorescence molecular switch based on TTF and porphyrin units was established.

The reversible control of dual (or multiple) states of molecules and supramolecules has attracted much attention for molecular switch, sensor, memory, logic gate, and molecular machine applications. In this paper, a ligand-induced sliding system using a complementary porphyrin-phthalocyanine dimer accompanied by a large fluorescence change is described. An imidazole-appended porphyrinatozinc-phthalocyaninatomagnesium complex 5 was synthesized by the palladium-catalyzed coupling reaction of the corresponding porphyrin and phthalocyanine moieties. The imidazole group of 5 coordinated to the magnesium ion of another 5 was formed spontaneously and dominantly to give a stacked dimer 5ST in non-coordinating solvents, such as chloroform, even at submicromolar concentration (< 10^-6 M). When an appropriate amount of dimethyl sulfoxide (DMSO) was added to the 5ST solution, the stacked dimer was transformed almost quantitatively into the extended dimer 5Ex. This transformation was monitored by UV-vis and fluorescence spectroscopies. In the UV-vis spectra, the porphyrin's Soret band is split characteristically because of excitonic coupling between the two closely located porphyrins in 5Ex. At the same time, the fluorescence intensity of 5Ex at 700 nm (ex. 414 nm) increased by a factor of 28 compared with that of 5ST. Removal of the DMSO molecules by extraction with water from the system regenerated 5ST almost completely from 5Ex, and the fluorescence was quenched again. The reversible system can be repeated by cycles of addition and removal of DMSO. The present system can be classified as a coordination-induced sliding system accompanied by a large fluorescence change, and may be applied as a fluorescence-based molecular switch.

During the 1980s the structure–function relationship of nonsteroidal estrogens and antiestrogens using an a prolactin gene target in primary cultures of rat pituitary cells created basic models for the estrogen receptor (ER)–mediated modulation of estrogen-induced prolactin synthesis. However, the cloning and sequencing of the human ER, made possible by the creation of monoclonal antibodies to the ER, created a new dimension in our understanding of the mechanics of estrogen and antiestrogen action. Although the whole human ER has not been crystallized with ligands that binds to the ligand-binding domain (LBD), the crystallization and resolution of estrogen and antiestrogen LBD complexes are consistent and informative. Simply stated, an array of different complexes are formed with selective ER modulators (SERMs) and a pure antiestrogen causes considerable structural disruption of the LBD. This chapter tells the story of how the shape of the ligand–ER complex programs the interaction with coactivators for estrogen actions or becomes the signal for the destruction of the complex.

The following sections are included:

• General introduction
• Rho family and the molecular switch mechanism
• Regulation of the Rho family GTPases
• Conclusions
• References

Considering the important issues concerning food, environment, and energy that humans are facing in the 21st century, humans mostly depend on plants. Unlike animals which move from an inappropriate environment, plants do not move, but rapidly sense diverse environmental changes or invasion by other organisms such as pathogens and insects in the place they root, and adapt themselves by changing their own bodies, through which they developed adaptability. Whole genetic information corresponding to the blueprints of many biological systems has recently been analyzed, and comparative genomic studies facilitated tracing strategies of each organism in their evolutional processes. Comparison of factors involved in intracellular signal transduction between animals and plants indicated diversification of different gene sets. Reversible binding of Ca²⁺ to sensor proteins play key roles as a molecular switch both in animals and plants. Molecular mechanisms for signaling network of environmental sensing and adaptation in plants will be discussed with special reference to Ca²⁺ as a key element in information processing.