Principal Component Analysis of Surface-Enhanced Raman Scattering Spectra Revealing Isomer-Dependent Electron Transport in Spiropyran Molecular Junctions: Implications for Nanoscale Molecular Electronics.

The characterization of single-molecule structures could provide significant insights into the operation mechanisms of functional devices. Structural transformation via isomerization has been extensively employed to implement device functionalities. Although single-molecule identification has recently been achieved using near-field spectroscopy, discrimination between isomeric forms remains challenging. Further, the structure-function relationship at the single-molecule scale remains unclear. Herein, we report the observation of the isomerization of spiropyran in a single-molecule junction (SMJ) using simultaneous surface-enhanced Raman scattering (SERS) and conductance measurements. SERS spectra were used to discriminate between isomers based on characteristic peaks. Moreover, conductance measurements, in conjunction with the principal component analysis of the SERS spectra, clearly showed the isomeric effect on the conductance of the SMJ.

Switchable Chemoselectivity of Reactive Intermediates Formation and Their Direct Use in A Flow Microreactor.

A chemoselectivity switchable microflow reaction was developed to generate reactive and unstable intermediates. The switchable chemoselectivity of this reaction enables a selection for one of two different intermediates, an aryllithium or a benzyl lithium, at will from the same starting material. Starting from bromo-substituted styrenes, the aryllithium intermediates were converted to the substituted styrenes, whereas the benzyl lithium intermediates were engaged in an anionic polymerization. These chemoselectivity-switchable reactions can be integrated to produce polymers that cannot be formed during typical polymerization reactions.

Electrical detection of ppb region NO2 using Mg-porphyrin-modified graphene field-effect transistors.

The trace detection of NO2 through small sensors is essential for air quality measurement and the health field; however, small sensors based on electrical devices cannot detect NO2 with the desired selectivity and quantitativity in the parts per billion (ppb) concentration region. In this study, we fabricated metalloporphyrin-modified graphene field-effect transistors (FETs). Mg-, Ni-, Cu-, and Co-porphyrins were deposited on the graphene FETs, and the transfer characteristics were measured. With the introduction of NO2 in the ppb concentration region, the FETs of pristine graphene and Ni-, Cu-, and Co-porphyrin-modified graphene showed an insufficient response, whereas the Mg-porphyrin-modified graphene exhibited large voltage shifts in the transport characteristics. This indicates that Mg-porphyrin acts as an adsorption site for NO2 molecules. An analysis of the Dirac-point voltage shifts with the introduction of NO2 indicates that the shifts were well-fitted with the Langmuir adsorption isotherm model, and the limit of detection for NO2 was found to be 0.3 ppb in N2. The relationship between the mobility and the Dirac-point voltage shift with the NO2 concentration shows that the complex of NO2 and Mg-porphyrin behaves as a point-like charge impurity. Moreover, the Mg-porphyrin-modified graphene FETs show less response to other gases (O2, H2, acetic acid, trimethylamine, methanol, and hexane), thus indicating high sensitivity for NO2 detection. Furthermore, we successfully demonstrated the quantitative detection of NO2 in air, which is near the environmental standards. In conclusion, the results of the Mg-porphyrin-modified graphene FETs enable a rapid, easy, and selective detectability.

Mechanical switching of current-voltage characteristics in spiropyran single-molecule junctions.

The electrical properties of a single-molecule junction of spiropyran are investigated through the break junction (BJ) method, and the current-voltage (I-V) characteristics are switched from rectified to symmetric through mechanical stimulus. This phenomenon indicates isomerization from spiropyran to merocyanine. In addition, an increase in the conductance associated with isomerization is observed.

Macroscopic Change in Luminescent Color by Thermally Driven Sliding Motion in [3]Rotaxanes.

Systematic investigation of rotaxane structures has revealed a rational design for thermally driven switching of their macroscopic properties. At low temperature, the luminophore is insulated by the macrocycles and displays monomer emission, whereas at high temperature, the luminophore is exposed owing to a change in the macrocyclic location distribution and interacts with external molecules, affording a thermally driven luminescent color change with high reversibility and responsiveness. This macroscopic switching through efficient thermal sliding was made possible by appropriate tuning of both the macrocycle-luminophore interactions within the rotaxane and the coupling between the excited luminophore and external molecules in an exciplex. The ability to switch properties by a simple and clean thermal stimuli should expand the utilization of rotaxanes as components of thermally driven molecular systems.

Synthesis and Acid-Responsiveness of an Insulated π-Conjugated Polymer Containing Spiropyrans in Its Backbone.

A π-conjugated polymer containing spiropyrans (SPs), which could be almost completely converted to protonated merocyanines (MCH⁺) and back to the SP form by adding an acid and a base, respectively, was developed. The insulation of the π-conjugated polymer, referred to as insulated spiropyran-containing poly(p-phenylene ethynylene) (ins-SP-PPE), using permethylated α-cyclodextrins (PM α-CD) suppressed the π-π interaction between the polymer chains containing MCH⁺, and the installation of PM α-CD improved the switching ability of SPs. The polymer exhibited repeatable acidochromism with almost complete conversion between the SP and MCH⁺ forms. Photoluminescence measurements were conducted and the acid-induced luminescence quenching of the polymer in the solution was observed, which stemmed from energy transfer from the PPE to MCH⁺ moieties. In the solid state, the quantum yield of ins-SP-PPE was more than twice that of the uninsulated polymer, which derived from the insulation effects. The acid-induced luminescence quenching was also observed in the solid state.

Facile preparation of hybrid thin films composed of spin-crossover nanoparticles and carbon nanotubes for electrical memory devices.

In this study, organic solvent-dispersible nanoparticles of an FeII-1,2,4-triazole spin-crossover complex were synthesized. On mixing the suspension of the spin-crossover nanoparticles with a solution of single-walled carbon nanotubes (SWCNTs), the nanoparticles were strongly adsorbed on the hydrophobic SWCNT bundles, resulting in hybrid network structures. Variable temperature DC electrical conductivity measurements of the hybrid network thin films demonstrated that the conductivities of the composite films were switched by the spin transition of the nanoparticles.

Nonlinear and Nonsymmetric Single-Molecule Electronic Properties Towards Molecular Information Processing.

This review highlights molecular design for nonlinear and nonsymmetric single-molecule electronic properties such as rectification, negative differential resistance, and switching, which are important components of future single-molecule information processing devices. Perspectives on integrated "molecular circuits" are also provided. Nonlinear and nonsymmetric single-molecule electronics can be designed by utilizing (1) asymmetric molecular cores, (2) asymmetric anchoring groups, (3) an asymmetric junction environment, and (4) asymmetric electrode materials. This review mainly focuses on the design of molecular cores.

Dualism of Sensitivity and Selectivity of Porphyrin Dimers in Electroanalysis.

This work uncovers the application of porphyrin dimers for the use in electroanalysis, such as potentiometric determination of ions. It also puts in question a current perception of an occurrence of the super-Nernstian response, as a result of the possible dimerization of single porphyrins within an ion-selective membrane. To study that, four various porphyrin dimers were used as ionophores, namely, freebase-freebase, Zn-Zn, Zn-freebase, and freebase-Zn. Since the Zn-freebase and freebase-Zn porphyrin dimers carried both anion- and cation-sensitive porphyrin units, their application in ISEs was utilized in both anion- and cation-sensitive sensors. With respect to the lipophilic salt added, both porphyrins dimers were found anion- and cation-sensitive. This allowed using a single molecule as novel type of versatile ionophore (anion- and cation-selective), simply by varying the membrane composition. All anion-sensitive sensors were perchlorate-sensitive, while the cation-selective sensors were silver-sensitive. The selectivity of the sensors depended primarily on the porphyrin dimers in the ion-selective membrane. Furthermore, the selectivity of cation-sensitive dimer based sensors was found significantly superior to the ones measured for the single porphyrin unit based sensors (precursors of the porphyrin dimers). Thus, the dimerization of single porphyrins may actually be a factor to increase or modulate porphyrin selectivity. Moreover, in the case of cation-sensitive sensors, the selectivity vastly depended on the order of porphyrin units in the dimer. This opens a new approach of regulating and adjusting sensitivity and selectivity of the sensor through the application of complex porphyrin systems with more than one porphyrin units with mix sensitive porphyrins.

Synthesis of a series of Zinc(II)/freebase porphyrin dimers and trimers with programmable sequences from a common key molecule.

We have developed a new methodology that enables the synthesis of any sequence of metal porphyrin arrays starting from a common key molecule. Using this method, we prepared porphyrin dimers and trimers with varying component unit sequences via consecutive Suzuki coupling reactions using the same key porphyrin compound under the same reaction conditions. The key porphyrin compound was synthesized on a gram scale in one batch, and the coupling reactions afforded the desired oligomers in good yields. Thus, the prepared porphyrin arrays showed unique physical properties depending on the sequence of the central metals. The reaction is potentially applicable for the automated synthesis of porphyrin arrays.

Thin films of spin-crossover coordination polymers with large thermal hysteresis loops prepared by nanoparticle spin coating.

This communication describes the synthesis of spin-crossover nanoparticles, which can disperse in various organic solvents without an excess amount of surfactants. The nanoparticles form homogeneous thin films on substrates by spin coating. The films show abrupt spin transitions with large thermal hysteresis loops.

Surface plasmon Raman scattering studies of liquid crystal anchoring on liquid-crystal-based self-assembled monolayers.

We studied the anchoring of 6CB on a series of self-assembled monolayers (SAMs) with a functional group that mimics that of the nematic liquid crystal (LC). The SAMs were first characterized by wetting, Fourier-transform infrared spectroscopy, and surface potential measurements. We found that, in two of these SAMs, the end group dipoles were oriented close to the normal of the surface and that these promoted homeotropic anchoring. In the case of the other SAM, the dipole was oriented parallel to the surface, and planar anchoring was obtained. Raman scattering by adsorbates on thin metal films is enhanced by the electromagnetic fields of surface plasmon polaritons (SPPs). Despite the inherent polarization of SPPs, there have been few reports in which SPP Raman scattering has been used to study molecular orientation. We have developed optical instrumentation to provide efficient excitation and collection of SPP Raman scattered light using attenuated total reflection geometry. The Kretschmann prism coupling configuration was used to excite SPPs on thin (500 A) gold films with adsorbed SAMs of alkanethiols in contact with thin films (50 microm) of the nematic liquid crystal 4'-hexylbiphenyl-4-carbonitrile (6CB, Merck). The anchoring and orientational wetting of the LC 6CB at the interface with omega-functionalized SAMs was studied using this arrangement. In agreement with the results of previous studies, a high-energy surface (-COOH) was found to promote planar anchoring, whereas a low-energy surface (-CF(3)) was found to induce homeotropic anchoring.

Synthesis and reactivity of six-membered oxa-nickelacycles: a ring-opening reaction of cyclopropyl ketones.

Cyclopropanecarboxaldehyde (1 a), cyclopropyl methyl ketone (1 b), and cyclopropyl phenyl ketone (1 c) were reacted with [Ni(cod)(2)] (cod = 1,5-cyclooctadiene) and PBu(3) at 100 degrees C to give eta(2)-enonenickel complexes (2 a-c). In the presence of PCy(3) (Cy = cyclohexyl), 1 a and 1 b reacted with [Ni(cod)(2)] to give the corresponding mu-eta(2):eta(1)-enonenickel complexes (3 a, 3 b). However, the reaction of 1 c under the same reaction conditions gave a mixture of 3 c and cyclopentane derivatives (4 c, 4 c'), that is, a [3+2] cycloaddition product of 1 c with (E)-1-phenylbut-2-en-1-one, an isomer of 1 c. In the presence of a catalytic amount of [Ni(cod)(2)] and PCy(3), [3+2] homo-cycloaddition proceeded to give a mixture of 4 c (76%) and 4 c' (17%). At room temperature, a possible intermediate, 6 c, was observed and isolated by reprecipitation at -20 degrees C. In the presence of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr), both 1 a and 1 c rapidly underwent oxidative addition to nickel(0) to give the corresponding six-membered oxa-nickelacycles (6 ai, 6 ci). On the other hand, 1 b reacted with nickel(0) to give the corresponding mu-eta(2):eta(1)-enonenickel complex (3 bi). The molecular structures of 6 ai and 6 ci were confirmed by X-ray crystallography. The molecular structure of 6 ai shows a dimeric eta(1)-nickelenolate structure. However, the molecular structure of 6 ci shows a monomeric eta(1)-nickelenolate structure, and the nickel(II) 14-electron center is regarded as having "an unusual T-shaped planar" coordination geometry. The insertion of enones into monomeric eta(1)-nickelenolate complexes 6 c and 6 ci occurred at room temperature to generate eta(3)-oxa-allylnickel complexes (8, 9), whereas insertion into dimeric eta(1)-nickelenolate complex 6 ai did not take place. The diastereoselectivity of the insertion of an enone into 6 c having PCy(3) as a ligand differs from that into 6 ci having IPr as a ligand. In addition, the stereochemistry of eta(3)-oxa-allylnickel complexes having IPr as a ligand is retained during reductive elimination to yield the corresponding [3+2] cycloaddition product, which is consistent with the diastereoselectivity observed in Ni(0)/IPr-catalyzed [3+2] cycloaddition reactions of cyclopropyl ketones with enones. In contrast, reductive elimination from the eta(3)-oxa-allylnickel having PCy(3) as a ligand proceeds with inversion of stereochemistry. This is probably due to rapid isomerization between syn and anti isomers prior to reductive elimination.

Novel cyanoterphenyl self-assembly monolayers on Au(111) studied by ellipsometry, x-ray photoelectron spectroscopy, and vibrational spectroscopies.

The self-assembled monolayers (SAMs) of two asymmetric disulfides derivatives (namely, LC1 and LC2) were prepared on Au(111). The disulfides contain a pure alkyl chain and an alkyl chain terminated by a cyanoterphenyl group. LC1 and LC2 differ by the way the cyanoterphenyl group is attached onto the alkyl chain: it is expected to be aligned with the alkyl chain in the case of LC1 and perpendicular to it in the case of LC2 (T shape). The consequences in terms of surface coverage, chemical composition, and molecular conformation of the two SAMs are studied using ellipsometry, x-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (RAIRS), and broadband femtosecond sum-frequency generation (SFG). A model of coverage and tilt angle based on ellipsometry and XPS results shows that the SAM "manages" the large size of the terphenyl group by lowering the terphenyl containing chain coverage and by increasing the tilt. In the case of LC2, the disulfide breaks during molecular assembly, less terphenyl chains adsorb than pure alkyl chains, and the overall chain coverage is smaller than for LC1. RAIRS and SFG results show that these differences in surface coverage correspond to a drastically different orientation of the terphenyl axis, which lies nearly parallel to the surface for LC2, while it is tilted by approximately 28 degrees for LC1. This shows that the terphenyl group takes much more space on the surface in the case of LC2 and explains why the terphenyl coverage is found smaller for LC2. The anomalous SFG relative intensities observed in the region of CH stretch between CH2 and CH3 modes, and symmetric and antisymmetric modes, show that the chains are not in the fully stretched, all-trans conformation, LC2 being probably more distorted than LC1. These distorsions allow the molecules to occupy the space available below the large terphenyl group. The relative intensities of symmetric and antisymmetric modes are discussed qualitatively for some typical molecular conformations and orientations of the alkyl chain.

Optical and surface morphological properties of polarizing films fabricated from a chromonic dye by the photoalignment technique.

The inherent chromonic lyotropic liquid crystalline properties of a dye have been manipulated to fabricate multi-axial micropolarizing thin films by means of the photoalignment technique. The dye aqueous solution is deposited on a photopatterned polymer film to result in the macroscopic alignment of the dye molecules, followed by drying at ambient temperature. The solid polarizing dye layers thus produced exhibit very a high contrast ratio and degree of polarization in the region of visible light. Addition of a small amount of surfactant to the dye solution is a prerequisite for the generation of a nematic chromonic phase and for the formation of homogeneous thin dye layers on the polymer film. The correlation between the optical and surface morphological properties of the dye layers is discussed.