Two-dimensional Molecular Arrays Research Articles

Forces and Flows at Cell Surfaces.

Flow is an important physiological signal and modulates a variety of cell functions. However, the molecular mechanisms that cells use to sense flow have remained surprisingly opaque. The first steps in flow sensing are likely to occur at the plasma membrane, the fluid barrier between the inside and outside of a cell. This membrane is an organized, two-dimensional molecular array that has both solid and fluid properties. The mobility of membrane proteins and lipids is constrained by complex interactions with the cytoskeletal protein network that supports the membrane. Physiologically relevant flows can only generate tiny forces on individual proteins, smaller than those from thermal noise. However, cells could overcome this problem by sensing micron-scale concentration gradients of extracellular membrane proteins. This is possible in cell plasma membranes because their particular physical state allows flow to sort membrane proteins laterally over the extracellular surface.

Self-assembled two-dimensional nanoporous molecular arrays and photoinduced polymerization of 4-bromo-4'-hydroxybiphenyl on Ag(111).

Self-assembled two-dimensional molecular arrays and photoinduced polymerization of 4-bromo-4'-hydroxybiphenyl on Ag(111) were studied using low-temperature scanning tunneling microscopy combined with density functional theory calculations. Square-like self-assembled structures of 4-bromo-4'-hydroxybiphenyl stabilized by intermolecular hydrogen and halogen bonds were transformed into hexagonal nanopores of biphenyl biradicals by 266 nm UV laser irradiation at 80 K. The biradicals further coupled to each other and formed covalently linked polyphenylene polymer chains at room temperature.

Consecutive Thiophene-Annulation Approach to π-Extended Thienoacene-Based Organic Semiconductors with [1]Benzothieno[3,2-b][1]benzothiophene (BTBT) Substructure

We describe a new synthetic route to the [1]benzothieno[3,2-b][1]benzothiophene (BTBT) substructure featuring two consecutive thiophene-annulation reactions from o-ethynyl-thioanisole substrates and arylsulfenyl chloride reagents that can be easily derived from arylthiols. The method is particularly suitable for the synthesis of unsymmetrical derivatives, e.g., [1]benzothieno[3,2-b]naphtho[2,3-b]thiophene, [1]benzothieno[3,2-b]anthra[2,3-b]thiophene, and naphtho[3,2-b]thieno[3,2-b]anthra[2,3-b]thiophene, a selenium-containing derivative, [1]benzothieno[3,2-b][1]benzoselenophene. It also allows us to access largely π-extended derivatives with two BTBT substructures, e.g., bis[1]benzothieno[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']dithiophene and bis[1]benzothieno[2,3-d:2',3'-d']naphtho[2,3-b:6,7-b']dithiophene (BBTNDT). It should be emphasized that these new BTBT derivatives are otherwise difficult to be synthesized. In addition, since various substrates and reagents, o-ethynyl-thioanisoles and arylthiols, respectively, can be combined, the method can be regarded as a versatile tool for the development of thienoacene-based organic semiconductors in this class. Among the newly synthesized materials, highly π-extended BBTNDT afforded very high mobility (>5 cm(2) V(-1) s(-1)) in its vapor-deposited organic field-effect transistors (OFETs), which is among the highest for unsubstituted acene- or thienoacenes-based organic semiconductors. In fact, the structural analyses of BBTNDT both in the single crystal and thin-film state indicated that an interactive two-dimensional molecular array is realized in the solid state, which rationalize the higher carrier mobility in the BBTNDT-based OFETs.

Synthesis, crystal structure and photophysical properties of lanthanide coordination polymers of 4-[4-(9H-carbazol-9-yl)butoxy]benzoate: the effect of bidentate nitrogen donors on luminescence

A new aromatic carboxylate ligand, 4-[4-(9H-carbazol-9-yl)butoxy]benzoic acid (HL), has been synthesized by the replacement of the hydroxyl hydrogen of 4-hydroxy benzoic acid with a 9-butyl-9H-carbazole moiety. The anion derived from HL has been used for the support of a series of lanthanide coordination compounds [Ln = Eu (1), Gd (2) and Tb (3)]. The new lanthanide complexes have been characterized by a variety of spectroscopic techniques. Complex 3 was structurally authenticated by single-crystal X-ray diffraction and found to exist as a solvent-free 1D coordination polymer with the formula [Tb(L)(3)](n). The structural data reveal that the terbium atoms in compound 3 reside in an octahedral ligand environment that is somewhat unusual for a lanthanide. It is interesting to note that each carboxylate group exhibits only a bridging-bidentate mode, with a complete lack of more complex connectivities that are commonly observed for extended lanthanide-containing solid-state structures. Examination of the packing diagram for revealed the existence of two-dimensional molecular arrays held together by means of CH-π interactions. Aromatic carboxylates of the lanthanides are known to exhibit highly efficient luminescence, thus offering the promise of applicability as optical devices. However, due to difficulties that arise on account of their polymeric nature, their practical application is somewhat limited. Accordingly, synthetic routes to discrete molecular species are highly desirable. For this purpose, a series of ternary lanthanide complexes was designed, synthesized and characterized, namely [Eu(L)(3)(phen)] (4), [Eu(L)(3)(tmphen)] (5), [Tb(L)(3)(phen)] (6) and [Tb(L)(3)(tmphen)] (7) (phen = 1,10-phenanthroline and tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline). The photophysical properties of the foregoing complexes in the solid state at room temperature have been investigated. The quantum yields of the ternary complexes 4 (9.65%), 5 (21.00%), 6 (14.07%) and 7 (32.42%), were found to be significantly enhanced in the presence of bidentate nitrogen donors when compared with those of the corresponding binary compounds 1 (0.11%) and 3 (1.45%). Presumably this is due to effective energy transfer from the ancillary ligands.

Lanthanide-Based Coordination Polymers Assembled from Derivatives of 3,5-Dihydroxy Benzoates: Syntheses, Crystal Structures, and Photophysical Properties

Two new aromatic carboxylic acids, namely, 3,5-bis(benzyloxy)benzoic acid (HL1) and 3,5-bis(pyridine-2-ylmethoxy)benzoic acid (HL2), have been prepared by replacing the hydroxyl hydrogens of 3,5-dihydroxy benzoic acid with benzyl and pyridyl moieties, respectively. The anions derived from HL1 and HL2 have been used for the support of a series of lanthanide coordination compounds [Eu(3+) = 1-2; Tb(3+) = 3-4; Gd(3+) = 5-6]. The new lanthanide complexes have been characterized on the basis of a variety of spectroscopic techniques in conjunction with an assessment of their photophysical properties. Lanthanide complexes 2, 4, and 6, which were synthesized from 3,5-bis(pyridine-2-ylmethoxy)benzoic acid, were structurally authenticated by single-crystal X-ray diffraction. All three complexes were found to exist as infinite one-dimensional (1-D) coordination polymers with the general formula {[Ln(L2)(3)(H(2)O)(2)]·xH(2)O}(n). Scrutiny of the packing diagrams for 2, 4, and 6 revealed the existence of interesting two-dimensional molecular arrays held together by intermolecular hydrogen-bonding interactions. Furthermore, the coordinated benzoate ligands serve as efficient light harvesting chromophores. In the cases of 1-4, the lowest energy maxima fall in the range 280-340 nm [molar absorption coefficient (ε) = (0.39-1.01) × 10(4) M(-1) cm(-1)]. Moreover, the Tb(3+) complexes 3 and 4 exhibit bright green luminescence efficiencies in the solid state (Φ(overall) = 60% for 3; 27% for 4) and possess longer excited state lifetimes than the other complexes (τ = 1.16 ms for 3; 1.38 ms for 4). In contrast to the foregoing, the Eu(3+) complexes 1 and 2 feature poor luminescence efficiencies.

Two-dimensional molecular array of porphyrin derivatives with bright and dark spots as a model of two-digit molecular-dot memory

Scanning tunneling microscopic (STM) observation of an array of 5,10,15,20-tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin molecules deposited on a Au(1 1 1) surface revealed dark and bright spots within the regular two-dimensional pattern, which are due to conformational diversity of the adsorbed molecules caused by distortion of the porphyrin core. With this structure, two-digit information can be input with ca. 2-nm interval, which would correspond to an information density of 25 terabit/cm 2 if exploited in a memory device.

New aspects of porphyrins and related compounds: self-assembled structures in two-dimensional molecular arrays

The advanced state of development of molecular design and synthetic chemistry of porphyrins and related molecules makes these compounds good candidates for technological appli cations, in which well characterized and designed structures and properties are required. In particular, 2-dimensional molecular level control of porphyrin array structures should reveal new aspects of nanotechnology. In this review, recent research on porphyrin assemblies, including 2-dimensional porphyrin arrays, is described with emphasis on phenol- and quinone-substituted tetrapyrrole units. A series of research aimed at developing strategies for preparation of porphyrin molecular arrays, where several novel aspects of molecular arrays, including phase transitions, ordered 2-D phase boundaries, and hydrogen-bonding networks, are introduced.

Regular Two-Dimensional Molecular Array of Photoluminescent Anderson-type Polyoxometalate Constructed by Langmuir−Blodgett Technique

A regular two-dimensional photoluminescent array of Anderson-type polyoxometalates (POMs) was constructed as built-up Langmuir-Blodgett (LB) films. LB films of hexatungstoantimonate (SbW(6)) and -manganate (MnW(6)) were successfully fabricated by using dimethyldioctadecylammonium (DODA) as cationic partner, while hexamolybdochromate (CrMo(6)) was unsuccessful. Specular X-ray reflectivity (SXR) and grazing incidence X-ray diffraction (GIXD) measurements revealed that both SbW(6)/DODA and MnW(6)/DODA LB films exhibited well-ordered layers consisting of periodic arrangement of the planar-structured Anderson-type molecules. Surprising periodicity was observed in SbW(6)/DODA LB film, in which the distance between SbW(6) and DODA layers was 4.40 nm, and SbW(6) anions would form a two-dimensional square lattice with a length of 1.4 nm. SbW(6)/DODA LB films exhibited photoluminescence at 77 K, while emission spectra were observed at room temperature for SbW(6) solid.

Quantum-chemistry simulations of second-harmonic and sum-frequency generation of organic layers

By using quantum-chemistry approaches, the second-order nonlinear optical responses of molecules and two-dimensional molecular arrays containing the p-nitroaniline chromophore are evaluated in order to highlight key features of the simulation of the second-harmonic and sum-frequency generation spectra of organic layers. For the electronic component, which dominates the second-harmonic generation response and which constitutes the weakly frequency-dependent background contribution to the vibrational sum-frequency generation phenomenon, the time-dependent Hartree–Fock scheme based on the semi-empirical AM1 parameterization is suitable for predicting the microscopic responses as well as for accounting for the surrounding effects within a simple multiplicative scheme. For the vibrational resonant part of the sum-frequency generation response, ab initio density functional theory approaches turn out to be necessary for locating the resonances and estimating their intensities.

A Novel Organic Conductor with Two-Dimensional Molecular Array by the “Edge-to-Edge” Donor Interaction

Abstract The title donor, where SM-PDT is 2-[4,5-bis(selenomethyl)-1,3-dithiol-2-ylidene]-5-(pyran-4-ylidene)-1,3,4,6-tetrathiapentalene, formed semiconducting 1:1 salts with various anions. The crystal structure analysis of (SM-PDT)PF6(PhCl)x has revealed that an edge-to-edge chalcogen interaction gave two-dimensional sheet-like structure.

Structural Studies on Phosphatidylcholine Liposomes in Various Phases Using Atomic Force Microscopy

Structural studies on phosphatidylcholine liposomes in various phases were carried out using atomic force microscopy (AFM) by applying a newly proposed fixation method under aqueous conditions. With soybean lecithin liposomes in air at room temperature (liquid crystalline phase), only irregular images were observed under magnification. On the other hand, with dipalmitoylphosphatidylcholine (DPPC) liposomes in air at room temperature (gel phase), regular two-dimensional molecular arrays were observed. With dimyristoylphosphatidylcholine (DMPC) liposomes in water at 18° C (sub-gel phase) on mica substrates immobilized by centrifugation, various types of ripple structures were seen. It was shown that our fixation method using centrifugation is suitable for the AFM observation of artificial membranes under aqueous conditions.

Dipole Orientation Distributions in Langmuir−Blodgett Films by Planar Waveguide Linear Dichroism and Fluorescence Anisotropy

A method to determine the orientation distribution of fluorescent molecules in a thin, substrate-supported film is described. Attenuated total reflection spectrometry on the surface of a planar waveguide is used to measure absorption LD, from which the mean dipole tilt angle in the film is obtained. Steady-state fluorescence anisotropy is measured in a total internal reflection geometry on a film supported on fused silica but prepared under otherwise identical conditions. The angular distribution about the mean can be recovered from the anisotropy measurement by modeling the distribution as a probability density that is specified by two adjustable parameters. The method was tested on Langmuir−Blodgett (LB) films of arachidic acid doped with the fluorescent amphiphiles DiI and BODIPY. In the DiI-doped films, the mean tilt angle was 75° from the surface normal. Assuming a Gaussian distribution, the standard deviation was 12°, indicating a high degree of macroscopic order. In the BODIPY-doped films the distribution was 59 ± 17°, which indicates a less ordered assembly. Larger angular distributions were calculated using a step function model. The results show that dipoles in the headgroup region of arachidic acid LB films are more ordered than dipoles in the alkyl chain region. The method should prove useful in studying relationships between assembly technique, structure, and function in two-dimensional molecular arrays.

Tailored charge transfer complex based on two-dimensional molecular assemblies

For tailoring charge transfer (CT) complexes in bilayer membranes, single-chain bilayer-forming amphiphiles, having a hydrophilic viologen head and a hydrophobic aromatic segment, such as an azobenzene or a biphenyl group, were prepared. Bilayer membranes with a titled molecular orientation were formed when the amphiphiles which had five carbon atoms in the spacer methylene chain between the viologen head and the aromatic segment were dissolved in water. A visible absorption band at 460 nm, which was attributed to the CT interaction between the viologen head and the biphenyl segment of the adjacent molecule in the two-dimensional molecular array, was newly observed in the bilayer membrane.

Scanning Tunneling Microscope Observation of a Polar Liquid Crystal and Its Computer Simulation*1

The scanning tunneling microscope (STM) constructed in the laboratory was utilized to observe the molecular arrangement of a new type of liquid crystalline molecule, 5-(p-dodecyloxyphenyl)pyrazine-2-carbonitrile (DOPPC), which has large dipole moments along the molecular axis. The DOPPC molecules adsorbed on a graphite substrate showed several different STM images with regular two-dimensional molecular arrays. They revealed a novel interdigitated double-row structure, differing from the single-row and the double-row structures proposed for the STM images of cyanobiphenyl liquid crystals. Moreover, a computer calculation was conducted based on electrostatic multipole-multipole interaction and simulated the most energetically preferable molecular arrangement, which agreed well with the observed STM image.