Pi-electron System Research Articles

Structure, Antioxidant Activity and Antimicrobial Study of Light Lanthanide Complexes with p-Coumaric Acid.

This paper presents the results of a study of the effects of the lanthanide ions Ce3+, Pr3+, Nd3+ and Sm3+ on the electronic structure and antioxidant and biological (antimicrobial and cytotoxic) properties of p-coumaric acid (p-CAH2). Structural studies were conducted via spectroscopic methods (FTIR, ATR, UV). Thermal degradation studies of the complexes were performed. The results are presented in the form of TG, DTG and DSC curves. Antioxidant properties were determined via activity tests against DPPH, ABTS and OH radicals. The reducing ability was tested via CUPRAC assays. Minimum inhibitory concentrations (MICs) of the ligand and lanthanide complexes were determined on E. coli, B. subtilis and C. albicans microorganisms. The antimicrobial activity was also determined using the MTT assay. The results were presented as the relative cell viability of C. albicans, P. aeruginosa, E. coli and S. aureus compared to controls and expressed as percentages. In the obtained complexes in the solid phase, lanthanide ions coordinate three ligands in a bidentate chelating coordination mode through the carboxyl group of the acid. Spectroscopic analysis showed that lanthanide ions increase the aromaticity of the pi electron system of the ligand. Thermal analysis showed that the complexes are hydrated and have a higher thermal stability than the ligand. The products of thermal decomposition of the complexes are lanthanide oxides. In the aqueous phase, the metal combines with the ligand in a 1:1 molar ratio. Antioxidant activity tests showed that the complexes have a similar ability to remove free radicals. ABTS and DPPH tests showed that the complexes have twice the ability to neutralise radicals than the ligand, and a much higher ability to remove the hydroxyl radical. The abilities of the complexes and the free ligand to reduce Cu2+ ions in the CUPRAC test are at a similar level. Lanthanide complexes of p-coumaric acid are characterised by a higher antimicrobial capacity than the free ligand against Escherichia coli bacteria, Bacillus subtilis and Candida albicans fungi.

Understanding the electronic pi-system of 2D covalent organic frameworks with Wannier functions

We investigate a family of hexagonal 2D covalent organic frameworks (COFs) with phenyl and biphenyl spacer units and different chemical linker species. Chemical trends are elucidated and attributed to microscopic properties of the pi-electron-system spanned by atomic p_z-orbitals. We systematically investigate the electronic structure, delocalization of electronic states, effects of disorder, bond torsion, and doping, and correlate these with variable pi-conjugation and nucleus-independent chemical shift (NICS) aromaticity. Molecular orbitals are obtained from maximally localized Wannier functions that have sigma- and pi-character, forming distinct sigma- and pi-bands for all valence states. The Wannier-orbital description goes beyond simple tight-binding models and enables a detailed understanding of the electronic topology, effective electronic coupling and delocalization. It is shown that a meaningful comparison between COFs with different chemical elements can only be made by examining the entire pi-electron system, while a comparison of individual bands (e.g., bands near the Fermi energy) can be a insufficient to derive general design rules for linker and spacer monomer selection. We further identify delocalized states that are spread across tens or hundreds of pores of the 2D COFs and analyze their robustness against structural and energetic disorders like out-of-plane rotations of molecular fragments, different strength of energetic disorder and energetic shifts due to chemical doping.

Anisotropic Stark effect of carbon monoxide: emergent orbital cooperativity

ABSTRACT Applying external electric fields to molecules gives rise to spectral shifting and splitting, a phenomenon known as the Stark effect. However, a fundamental question of how electronic structures of molecules are modified by electric fields is still not well understood. By applying electric fields to a carbon monoxide molecule, herein we have successfully addressed the fundamental question at orbital scales and discovered that the Stark effect exhibits anisotropic characters depending on the direction of the electric fields with respect to the molecular axis. Based on the fact that applying electric fields along the molecular axis always preserved the orthogonality between the sigma and pi electrons, we found that orbital resemblance-based cooperativity can only operate within either the sigma system in which sigma electrons somehow prefer to resemble each other or the pi electron system in which the 1π electrons experience polarization-based self-resemblance. However, switching the electric field vertical to the molecular axis breaks down the orthogonality between the sigma system and pi electron systems, opening up electronic channels that allow σ electron systems to resemble π electrons. Such orbital cooperativity represents a new physical effect beyond the conventional Stark effect. Moreover, we have found that applying electric fields to the molecule would modify its molecular orbital diagram, depending on the directions of the electric fields; the electric field along the carbon-to-oxygen direction basically retains the MO diagram of the free CO molecule, with noticeable intra-orbital electron redistributions, whereas the oxygen-to-carbon electric field does create new states of molecular orbital contributions.

(Invited) Improved Charge and Exciton Transport in Polymer-Removed SWCNT Thin Films: Implications for Photovoltaic and Thermoelectric Energy Harvesting

The chemical and physical structure of semiconducting single-walled carbon nanotubes (SWCNTs) results in optical and electronic properties of promise for a wide variety of applications. Until quite recently the presence of metallic SWCNT impurities has hampered efforts to gain a deeper understanding of their true potential, with the additional complication that most commercially available materials contain tens of different chiral species. Significant effort has been devoted to elegant enrichment strategies aimed at extracting tailored semiconducting SWCNT species from the raw soot, from the use of subtly tunable surfactant interactions to the exploitation of specific DNA sequences. However, conjugated polymers, typically based on the fluorene moiety, appear to show the greatest promise with regards to their high selectivity and viability for scalable manufacturing approaches. Unfortunately, the van de Waals forces between the pi-electron systems of the polymer and SWCNT that enable the selective extraction of semiconducting SWCNTs with high purity also make removal of the polymer difficult. Since these polymers typically have a wide bandgap they act as an insulating coating on the surface of the individual SWCNTs within functional networks, inhibiting the transport of energy in the form of excitons and/or charge carriers. Here we demonstrate approaches aimed at replacing the strongly-bound polymers with variants that can be removed using simple solution-based chemical strategies, resulting in networks with vastly improved energy transport properties. We show that removal of the polymer results in a significant enhancement of the charge carrier mobility and electrical conductivity in the SWCNT networks, allowing optimization of the thermoelectric properties for both p-type and n-type transport. Finally, we extend the approach to samples strongly enriched in a single chiral SWCNT species, which allows us to employ transient spectroscopic techniques to probe enhanced exciton transport through the SWCNT network with high spectral fidelity. Our studies highlight a methodology by which high-performance SWCNT thin films can be prepared that could realize their potential for electronic and optoelectronic applications.

Stable molecular switches

Molecular Junctions Many single-molecule current switches have been reported, but most show poor stability because of weak contacts to metal electrodes. Jia et al. covalently bonded a diarylethene molecule to graphene electrodes and achieved stable photoswitching at room temperature (see the Perspective by Frisbie). The incorporation of short bridging alkyl chains between the molecule and graphene decoupled their pielectron systems and allowed fast conversion of the open and closed ring states. Science , this issue p. [1443][1]; see also p. [1394][2] [1]: /lookup/doi/10.1126/science.aaf6298 [2]: /lookup/doi/10.1126/science.aag0827

Covalently bonded single-molecule junctions with stable and reversible photoswitched conductivity.

Through molecular engineering, single diarylethenes were covalently sandwiched between graphene electrodes to form stable molecular conduction junctions. Our experimental and theoretical studies of these junctions consistently show and interpret reversible conductance photoswitching at room temperature and stochastic switching between different conductive states at low temperature at a single-molecule level. We demonstrate a fully reversible, two-mode, single-molecule electrical switch with unprecedented levels of accuracy (on/off ratio of ~100), stability (over a year), and reproducibility (46 devices with more than 100 cycles for photoswitching and ~10(5) to 10(6) cycles for stochastic switching).

(Invited) Pyrrole-Based Polycyclic Pi-Electron Systems with Various Redox States

We have recently reported a series of pyrrole-fused azacoronenes. Due to the interior nitrogen atoms and various resonance forms, all azacoronenes showed multiple oxidation states with different electronic properties. On the basis of these results, a pyrrole-fused azacoronene functionalized with carbonyl groups was designed and synthesized. From the ESR and crystal structure analyses, the azacoronenes revealed spin-delocalized diradical nature in the neutral state. The CV results showed multistep oxidation and reduction peaks. The band gap is very small and the absorption spectra tailed near-infrared region. Pi-extended systems over the previous azacoronenes were also obtained using a similar synthetic strategy. Some crystal structure analyses revealed highly contorted structures. In the presentation, we will report the synthesis, structures, and basic physical properties.

Photocontrollable NO-releasing compounds and their biological applications.

Photocontrollable nitric oxide (NO)-releasing compounds, or caged NOs, are very useful to expose cells or tissues to NO in a spatiotemporally well-controlled manner, e.g., for precise investigations of biological responses to NO and NO-related signaling pathways. We have developed photocontrollable NO releasers based on two mechanisms: photoinduced isomerization reaction of a dimethylnitrobenzene moiety conjugated with a pi-electron system, and photoinduced electron transfer of a moderately electron-rich N-nitroso aminophenol moiety linked with an antenna dye moiety. In this review, we describe the development of our photoinduced NO releasers based on these mechanisms, and present examples of cellular and ex vivo applications.

Emulating Molecular Orbitals and Electronic Dynamics with Ultracold Atoms

In recent years, ultracold atoms in optical lattices have proven their great value as quantum simulators for studying strongly correlated phases and complex phenomena in solid-state systems. Here we reveal their potential as quantum simulators for molecular physics and propose a technique to image the three-dimensional molecular orbitals with high resolution. The outstanding tunability of ultracold atoms in terms of potential and interaction offer fully adjustable model systems for gaining deep insight into the electronic structure of molecules. We study the orbitals of an artificial benzene molecule and discuss the effect of tunable interactions in its conjugated pi electron system with special regard to localization and spin order. The dynamical time scales of ultracold atom simulators are on the order of milliseconds, which allows for the time-resolved monitoring of a broad range of dynamical processes. As an example, we compute the hole dynamics in the conjugated pi system of the artificial benzene molecule.

On constitutional isomers and tautomers of oxadiazolones and their mono- and disulfur analogues (C2H2N2XY; X, Y = S, O)

The constitutional isomers and tautomers of oxadiazolones, as well as their mono- and disulfur analogues, were calculated at the B3LYP/aug-cc-pVDZ level. Four groups of 30 molecules each were considered: oxadiazolone, oxadiazolthione, thiadiazolone, and thiadiazolthione isomers. The compounds were categorized into six groups according to permutations of three heteroatoms in the five-membered ring. Additionally, each of the constitutional isomer was considered to have five tautomers conserving stable five-membered ring: two NH tautomers, two rotameric OH (or SH) forms and one CH2 tautomer. It appeared that the largest difference between oxadiazolone O and S analogues is produced by the kind of chalcogen atom in the ring, which is strained when the O atom is in the ring while much less strained when the S-atom, of much larger van der Waals radius, is built into the ring. The external chalcogen is only modifying the general energetic factors. The comparison of energetics of analogous groups of molecules with thiadiazole and oxadiazole rings is done in details as well as differences resulting from different external chalcogen atoms are discussed as well. The presence of water surrounding was mimicked with the IEF-PCM implicit water model which did not change general isomer relative stability picture, but for some special cases indicated an extra stability of the forms with external OH or SH groups. The aromaticity monitored by the structural HOMA aromaticity index shows that the systems are not additionally stabilized by pi-electron delocalization. The fair linear correlation between the aromaticity indices of oxadiazolones and oxadiazolthiones shows that the pi-electron system in the studied systems is not sensitive to change of the external chalcogen group.

1,2-ジヒドロ-1,2-アザボリンを含むπ電子系化合物の合成と性質

1,2-ジヒドロ-1,2-アザボリンを含むπ電子系化合物の合成と性質

Organometallic chemistry of extended periodic π-electron systems: hexahapto-chromium complexes of graphene and single-walled carbon nanotubes

We demonstrate the organometallic hexahapto complexation of chromium with graphene, graphite and carbon nanotubes. All of these extended periodic pi-electron systems exhibit some degree of reactivity toward the reagents CrCO)6 and (eta6-benzene)Cr(CO)3, and we are able to demonstrate the formation of (eta6-rene)Cr(CO)3 or (eta6-arene)2Cr, where arene = single-walled carbon nanotubes (SWNT), exfoliated graphene (XG), epitaxial graphene (EG) and highly-oriented pyrolytic graphite (HOPG). We find that the SWNTs are the least reactive presumably as a result of the effect of curvature on the formation of the hexahapto bond; in the case of HOPG, (eta6-HOPG)Cr(CO)3 was isolated while the exfoliated graphene samples were found to give both (eta6-graphene)2Cr, and (eta6-graphene)Cr(CO)3 structures. We report simple and efficient routes for the mild decomplexation of the graphene-chromium complexes which appears to restore the original pristine graphene state. This study represents the first example of the use of graphene as a ligand and is expected to expand the scope of graphene chemistry in connection with the application of this material in organometallic catalysis.

Control of the Orientational Order and Nonlinear Optical Response of the “Push-Pull” Chromophore RuPZn via Specific Incorporation into Densely Packed Monolayer Ensembles of an Amphiphilic Four-Helix Bundle Peptide: Characterization of the Peptide−Chromophore Complexes

"Push-pull" chromophores based on extended pi-electron systems have been designed to exhibit exceptionally large molecular hyperpolarizabilities. We have engineered an amphiphilic four-helix bundle peptide to vectorially incorporate such hyperpolarizable chromophores having a metalloporphyrin moiety, with high specificity into the interior core of the bundle. The amphiphilic exterior of the bundle facilitates the formation of densely packed monolayer ensembles of the vectorially oriented peptide-chromophore complexes at the liquid-gas interface. Chemical specificity designed into the ends of the bundle facilitates the subsequent covalent attachment of these monolayer ensembles onto the surface of an inorganic substrate. In this article, we describe the structural characterization of these monolayer ensembles at each stage of their fabrication for one such peptide-chromophore complex designated as AP0-RuPZn. In the accompanying article, we describe the characterization of their macroscopic nonlinear optical properties.

Electrothermal Manipulation of Individual Chromophores in Single Conjugated Polymer Chains: Controlling Intrachain FRET, Blinking, and Spectral Diffusion

Single molecule spectroscopy of individual chains of a conjugated polymer opens up deep insight into electronic localization phenomena, which govern the underlying optical properties of these complex and disordered materials. We explore the nature of a single chromophore arising in a delocalized pi-electron system by applying periodic electrothermal perturbations at low temperatures. Brief heating of the chromophore leads to a dramatic increase in the transition line width and is generally accompanied by a random jump of the emission energy. This observation demonstrates that chromophores on a polymer chain are not only defined by structural disorder but also by the subtleties of the local dielectric environment. The effect of thermal perturbation becomes more complex when long polymer chains are considered, which can potentially support the formation of multiple chromophores. Here, a momentary increase in temperature can promote intrachain energy transfer to quenching sites, leading to a strong modulation of emission intensity with temperature. Unexpectedly, such energy transfer can serve to either raise or lower the transition line width and quantum yield of the ensemble with increasing temperature, depending on the specific energetics of the chromophores in the system, which in turn vary with time. The controlled perturbation of both the emission spectrum and the intensity by brief heating of the polymer chain offers insight into possible microscopic origins of fluorescence blinking and spectral diffusion, which ultimately impact on the efficiency and spectral purity of devices.

Effects of Hydrogen Bonding on Vibrational Normal Modes of Pyrimidine

The effects of weak intermolecular interactions on 10 vibrational normal modes of pyrimidine are investigated by Raman spectroscopy and electronic structure computations. Hydrogen-bonded networks of water induce a shift to higher energy in certain normal modes of pyrimidine with increasing water concentration, while other modes are relatively unaffected. Pyrimidine molecules also exhibit weak C-H...N interactions and shifted normal modes upon crystallization. The selective nature of the shifting of normal modes to higher energy allows for definitive assignments of the nearly degenerate nu(8a) and nu(8b) modes with polarized Raman spectroscopy. Natural bond orbital (NBO) analyses indicate that when water molecules donate hydrogen bonds to the nitrogen atoms of pyrimidine, there is significant charge transfer from pyrimidine to water, much of which can be accounted for by substantial decreases in the populations of the nitrogen lone pair orbitals. Despite the overall decrease of electron density in pyrimidine upon complexation with water, there are concomitant changes in NBO populations that polarize the pi-electron system toward the proton acceptor N atoms, as well as contractions of the bonds associated with the N-C-N and C-C-C regions of the pyrimidine ring.

Correlation between proton transfer and 35Cl NQR frequency as well as molecular geometry of chloranilic acid in co‐crystals with some organic bases

Proton transfer in hydrogen-bonded organic co-crystals of chloranilic acid with some organic bases was investigated by nuclear quadrupole resonance (NQR) spectroscopy. The (35)Cl NQR frequencies of chloranilic acid molecule as well as (14)N NQR frequencies of the organic base molecule were measured with the conventional pulse methods as well as double-resonance methods, respectively. The extent of proton transfer in the O...H...N hydrogen bond was estimated from Townes-Dailey analysis of the (14)N NQR parameters. The (35)Cl NQR frequency and molecular geometry of chloranilic acid are correlated to the extent of proton transfer in the protonation process of the organic base molecule. It is shown that the hydrogen bond affects the pi-electron system of chloranilic acid. Geometry dependence of the O...H...N hydrogen bond, i.e. the H-N valence bond order versus the hydrogen-bond geometry correlation is also discussed.

Structural Characterization of Acylimine-Containing Blue and Red Chromophores in mTagBFP and TagRFP Fluorescent Proteins

We determined the 2.2 A crystal structures of the red fluorescent protein TagRFP and its derivative, the blue fluorescent protein mTagBFP. The crystallographic analysis is consistent with a model in which TagRFP has the trans coplanar anionic chromophore with the conjugated pi-electron system, similar to that of DsRed-like chromophores. Refined conformation of mTagBFP suggests the presence of an N-acylimine functionality in its chromophore and single C(alpha)-C(beta) bond in the Tyr64 side chain. Mass spectrum of mTagBFP chromophore-bearing peptide indicates a loss of 20 Da upon maturation, whereas tandem mass spectrometry reveals that the C(alpha)-N bond in Leu63 is oxidized. These data indicate that mTagBFP has a new type of the chromophore, N-[(5-hydroxy-1H-imidazole-2-yl)methylidene]acetamide. We propose a chemical mechanism in which the DsRed-like chromophore is formed via the mTagBFP-like blue intermediate.

Spectroscopic Detection of Tetracationic Porphyrin H-Aggregation on Polyanionic Matrix of Inorganic Polyphosphate

Self-assembly of tetracationic porphyrin TMPyP(4+) onto polyanionic matrix of inorganic polyphosphate (PPS) in aqueous solutions has been studied in a wide range of molar phosphate-to-dye ratios using techniques of polarized fluorescence, absorption, resonance Raman spectroscopy and static light scattering. The binding of TMPyP(4+) to PPS is characterized by the binding constant of 3 x 10(5) M(-1) and the cooperativity parameter of about 150. The fluorescence quenching of the bound TMPyP(4+) evidences the stacking of the porphyrine chromophores. Under the stoichiometric binding ratio TMPyP(4+) forms extended continuous face-to-face aggregates (so-called H-aggregates) which manifest themselves by a blue shift (12 nm) and a large hypochromisity (51%) of the Soret absorption band. Each face-to-face TMPyP(4+) stack is formed with participation of four PPS chains. Formation of such columnar aggregates is promoted by the ability of PPS chains to take a helix conformation where negative charges are arranged along two oppositely situated rows with intercharge distance of 0.36 nm which corresponds to the thickness of the porphyrin pi-electronic system. The ability of each PPS strand to be template for formation of two porphyrin stacks results in the integration of the adjacent stacks into higher-order aggregates which dimension was estimated from the fluorescence polarization data.

Non-Traditional Aromatic Topologies and Biomimetic Assembly Motifs as Components of Functional Pi-Conjugated Oligomers

This article will highlight our recent work using conjugated oligomers as precursors to electroactive polymer films and self-assembling nanomaterials. One area of investigation has focused on nonbenzenoid aromaticity in the context of charge delocalization in conjugated polymers. In these studies, polymerizable pi-conjugated units were coupled onto unusual aromatic cores such as methano[10]annulene. This article will also show how biologically-inspired assembly of molecularly well-defined oligopeptides that flank pi-conjugated oligomers has resulted in the aqueous construction of 1-dimensional nanomaterials that encourage electronic delocalization among the pi-electron systems.

Mössbauer spectroscopy and magnetic transition ofλ−(BETS)2FeCl4

The compound lambda-(BETS)2FeCl4 provides an effective demonstration of the interaction of pi-conduction electron and d-electron localized moment systems in molecular crystalline materials where antiferromagnetic insulating and magnetic field induced superconducting states can be realized. The metal-insulator transition has been thought to be cooperative, involving both the itinerant pi- electron and localized d-electron spins where antiferromagnetic order appears in both systems simultaneously. However, recent specific heat data has indicated otherwise [Akiba et al., J. Phys. Soc. Japan 78,033601(2009)]: although the pi-electron system orders antiferromagnetically and produces a metal-insulator transition, a "mysterious" paramagnetic d-electron state remains. We report 57Fe Mossbauer measurements that support the paramagnetic model, provided the d-electron spins remain in a fast relaxation state below the transition. From the measured hyperfine fields, we also determine the temperature dependence of the pi-d electron exchange field.