Computational analysis and experimental verification of donor-acceptor behaviour of berberine, and its co-oligomers and co-polymers with ethylenedioxythiophene.

The donor-acceptor (D-A) type of conjugated polymers has emerged as the paradigm of the third generation of electronically conducting polymers demonstrating improved infrared activity and intrinsic electronic conductivity. Judicious selection of donor (D) and acceptor (A) monomers for copolymerization can further fine-tune these properties. Notably, for such refinement, natural compounds provide many conjugated molecules with various functional groups. Berberine cation (Ber+) found in Coscinium fenestratum has extensive conjugation and contains both an electron deficient isoquinolium A moiety and electron-rich D-type methylenedioxy and methoxy groups. The incorporation of natural products in electronic materials is a novel area of research which opens a wide scope for future electronic and optoelectronic devices. Investigation of their fundamental properties via computer simulations is therefore important. In this study, quantum chemical calculations are performed using density functional theory (DFT) to investigate the electronic and optical properties of oligomers of Ber+ and 3,4-ethylenedioxythiophene (EDOT) and to explore the possibilities for homo-polymerization of Ber+ and its copolymerization with EDOT. It has been revealed that homo-polymerization is not favoured but copolymerization with EDOT is possible. As such, Ber+ was copolymerized with EDOT and the copolymers formed by electro-polymerization are extensively characterised and the D-A behaviour of the copolymers verified. Furthermore, the theoretical predictions have been compared with the experimental data.

Vesicles, fibres, films and crystals: A low-molecular-weight-gelator [Au(6-thioguanosine)2]Cl which exhibits a co-operative anion-induced transition from vesicles to a fibrous metallo-hydrogel.

We describe a simple coordination compound of Au(I) and 6-thioguanosine, [Au(6-tGH)2]Cl, that has a rich self-assembly chemistry. In aqueous solution, the discrete complex assembles into a supramolecular fibre and forms a luminescent hydrogel at concentrations above about 1 mM. Below this concentration, the macromolecular structure is a vesicle. Through appropriate control of the solvent polarity, the gel can be turned into a lamellar film or crystallised. The molecular structure of [Au(6-tGH)2]Cl was determined using single crystal X-ray diffraction, which showed bis-6-thioguanosine linearly coordinated through the thione moiety to a central Au(I) ion. In the vesicles, the photoluminescence spectrum shows a broad, weak band at 550 nm owing to aurophilic interactions. Co-operative self-assembly from vesicle to fibre is made possible through halogen hydrogen bonding interactions and the aurophilic interactions are lost, resulting in a strong photoluminescence band at 490 nm with vibronic structure typical of an intraligand transition. The vesicle-fibre transition is also revealed by a large increase of ellipticity in the circular dichroism spectrum with a prominent peak near 390 nm owing to the helical structure of the fibres. Atomic force microscopy shows that at the same time as fibres form, the sample gels. Imaging near the vesicle-fibre transition shows that the fibres form between vesicles and a mechanism for the transition based on vesicle collisions is proposed.

Berberine isolation from Coscinium fenestratum: optical, electrochemical, and computational studies.

Berberine was extracted from Coscinium fenestratum (tree turmeric) and purified by column chromatography. The UV-Vis absorption spectroscopy of berberine was studied in acetonitrile and aqueous media. TD-DFT calculations employing the B3LYP functional were found to reproduce the general features of the absorption and emission spectra correctly. The electronic transitions to the first and second excited singlet states involve a transfer of electron density from the electron donating methylenedioxy phenyl ring to the electron accepting isoquinolium moiety. An estimate of the electrochemical gap (2.64 V) was obtained from microelectrode voltammetry and good agreement was found with quantum chemical calculations using the cc-pVTZ basis set and the B3LYP, CAM-B3LYP and wB97XD functionals. The calculations indicate spin density of the radical dication is delocalised over the molecule. These basic data are useful for assessment of the synthesis of donor-acceptor polymeric materials employing oxidative polymerization or co-polymerisation of berberine.

Hierarchical self-assembly in an RNA-based coordination polymer hydrogel.

An RNA-based coordination polymer is formed by the aqueous reaction of CuI ions with the thionucleoside enantiomer (-)6-thioguanosine, (6tGH). The resulting polymer, [CuI(µ3-S-thioG)]n1, has a one-dimensional structure based on a [Cu4-S4] core and undergoes extensive hierarchical self-assembly transforming from oligomeric chains → rod → cable → bundle through which a fibrous gel forms, that undergoes syneresis to form a self-supporting mass. The assembly involves the formation of helical cables/bundles and, in combination with the intrinsic photoemission of the polymer, results in the material exhibiting circularly polarised luminescence (CPL).

Circularly polarised luminescence in an RNA-based homochiral, self-repairing, coordination polymer hydrogel.

The aqueous equimolar reaction of Ag(i) ions with the thionucleoside enantiomer (-)6-thioguanosine, ((-)6tGH), yields a one-dimensional coordination polymer {Ag(-)tG} n , the self-assembly of which generates left-handed helical chains. The resulting helicity induces an enhanced chiro-optical response compared to the parent ligand. DFT calculations indicate that this enhancement is due to delocalisation of the excited state along the helical chains, with 7 units being required to converge the calculated CD spectra. At concentrations ≥15 mmol l-1 reactions form a sample-spanning hydrogel which shows self-repair capabilities with instantaneous recovery in which the dynamic reversibility of the coordination chains appears to play a role. The resulting gel exhibits circularly polarised luminescence (CPL) with a large dissymmetry factor of -0.07 ± 0.01 at 735 nm, a phenomenon not previously observed for this class of coordination polymer.

Implementing the donor-acceptor approach in electronically conducting copolymers via electropolymerization.

Electropolymerization has become a convenient method for synthesizing and characterizing complex organic copolymers having intrinsic electronic conductivity, including the donor (D)-acceptor (A) class of electronically conducting polymers (ECPs). This review begins with an introduction to the electrosynthesis of common second-generation ECPs. The information obtainable from electroanalytical studies, charge carriers such as polarons (positive and negative) and bipolarons (positive and negative) and doping will be discussed. The evolutionary chain of ECPs is then presented. ECPs comprising electron-rich D and electron-deficient A moieties have been shown to possess intrinsic electronic conductivity and unique optical and electronic properties. They are third generation ECPs and electropolymerization of mixtures of D and A leads to stoichiometrically controlled block copolymers. These D-A type ECPs are discussed on the basis of selected representative materials. Since the discovery of electropolymerization as a powerful tool to synthesize copolymers of conjugated monomers with a pre-determined ratio of D and A repeat units present in the polymer, the field of D-A type ECPs has grown considerably and the literature available since 2004 to 2021 is summarized and tabulated. Electronic and optical properties of the materials determined by computational chemistry are presented. The data obtained from electrochemical and optical methods are compared with those obtained from computational methods and reasons for discrepancies are given. The literature on the concept of electropolymerization extended to synthesizing triblock and many-block copolymers is reviewed. Finally, applications of D-A polymers in optoelectronic devices (organic solar cells and field-effect transistors) and in bio-imaging are explained quoting appropriate examples.

Models for sensing by nanowire networks: application to organic vapour detection by multiwall carbon nanotube-DNA films.

Electronic sensors for volatile organic compounds have been prepared by drop-casting dispersions of multi-wall carbon nanotubes (MWCNTs) in aqueous solutions ofλ-DNA onto Pt microband electrodes. The MWCNTs themselves show a metal-like temperature dependence of the conductance, but the conductance of DNA/MWCNT composites has an activated component that corresponds to inter-tube tunneling. The resistance of the composite was modelled by a series combination of a term linear in temperature for the nanotubes and a stretched exponential form for the inter-tube junctions. The resistance may increase or decrease with temperature according to the composition and may be tuned to be almost temperature-independent at 67% by mass of DNA. Upon exposure to organic vapours, the resistance of the composites increases and the time-dependence of this signal is consistent with diffusion of the vapour into the composite. The fractional change in resistance at steady-state provides an analytical signal with a linear calibration and the presence of DNA enhances the signal and adjusts the selectivity in favour of polar analytes. The temperature dependence of the signal is determined by the enthalpy of adsorption of the analyte in the inter-tube junctions and may be satisfactorily modelled using the Langmuir isotherm. Temperature and pressure-dependent studies indicate that neither charge injection by oxidation/reduction of the analyte nor condensation of analyte on the device is responsible for the signal. We suggest that the origin of the sensing response is an adsorption of the analyte in the inter-tube regions that modulates the tunneling barriers. This suggests a general route to tuning the selectivity of MWCNT gas sensors using non-conductive polymers of varying chemical functionality.

Bottom-up device fabrication via the seeded growth of polymer-based nanowires.

The bottom-up assembly of nanoelectronic devices from molecular building blocks is a target of widespread interest. Herein we demonstrate an in situ seeded growth approach to produce a nanowire-based electrical device. This exploits the chemisorption of block terpolymer-based seed fibres with a thiophene-functionalised corona onto metal electrodes as the initial step. We then use these surface-bound seeds to initiate the growth of well-defined one-dimensional fibre-like micelles via the seeded growth method known as "Living crystallisation-driven self-assembly'' and demonstrate that they are capable of spanning an interelectrode gap. Finally, a chemical oxidation step was used to transform the nanofibres into nanowires to generate a two-terminal device. This seeded growth approach of growing well-defined circuit elements provides a useful new design tool for bottom-up device fabrication.

transoid and cisoid Conformations in Silver-Mediated Cytosine Base Pairs: Hydrogen Bonding Dictates Argentophilic Interactions in the Solid State.

We have synthesized and structurally characterized examples of the well-known silver-mediated DNA base pair in its simplest possible form, as [AgI-bis-(N3-cytosine)]+. The compounds show differences such as variable coordination geometry, conformation with cisoid and transoid arrangements, and, in one case, intramolecular base pairing. Collectively, these compounds represent three of five permutations of linear/bent coordination geometry and cisoid/transoid arrangement of the cytosine ligands and contain the global minimum conformation as determined by DFT calculations: bent-transoid. Furthermore, these compounds show no argentophilic intercomplex interactions in the solid state, in marked contrast to alkylated and nucleoside analogues, and so do not form the supramolecular 1D "metallo-DNA" duplex observed in those cases but instead form hydrogen-bonded sheets. This marked difference may be attributed, in part, to the additional hydrogen bond donor site at N1 of the nucleobase that ubiquitously participates in intermolecular interactions.

Endocytosis and Lack of Cytotoxicity of Alkyl-Capped Silicon Quantum Dots Prepared from Porous Silicon.

Freely-dissolved silicon quantum dots were prepared by thermal hydrosilation of 1-undecene at high-porosity porous silicon under reflux in toluene. This reaction produces a suspension of alkyl-capped silicon quantum dots (alkyl SiQDs) with bright orange luminescence, a core Si nanocrystal diameter of about 2.5 nm and a total particle diameter of about 5 nm. Previous work has shown that these particles are rapidly endocytosed by malignant cell lines and have little or no acute toxicity as judged by the standard 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay for viability and the Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay for apoptosis. We have extended this work to the CACO-2 cell line, an established model for the human small intestinal mucosa, and demonstrate that neither acute nor chronic (14 days) toxicity is observed as judged by cell morphology, viability, ATP production, ROS production and DNA damage (single cell gel electrophoresis) at doses of 50-200 µ g mL - 1 . Quantitative assessment of the extent of uptake of alkyl SiQDs by CACO-2, HeLa, HepG2, and Huh7 cell lines by flow cytometry showed a wide variation. The liver cell lines (HepG2 and Huh7) were the most active and HeLa and CACO-2 showed comparable activity. Previous work has reported a cholesterol-sensitivity of the endocytosis (HeLa), which suggests a caveolin-mediated pathway. However, gene expression analysis by quantitative real-time polymerase chain reaction (RT-PCR) indicates very low levels of caveolins 1 and 2 in HepG2 and much higher levels in HeLa. The data suggest that the mechanism of endocytosis of the alkyl SiQDs is cell-line dependent.

Addressing the properties of "Metallo-DNA" with a Ag(i)-mediated supramolecular duplex.

The silver-nucleoside complex [Ag(i)-(N3-cytidine)2], 1, self-assembles to form a supramolecular metal-mediated base-pair array highly analogous to those seen in metallo-DNA. A combination of complementary hydrogen-bonding, hydrophobic and argentophilic interactions drive the formation of a double-helix with a continuous silver core. Electrical measurements on 1 show that despite having Ag···Ag distances within <5% of the metallic radii, the material is electrically insulating. This is due to the electronic structure which features a filled valence band, an empty conduction band dominated by the ligand, and a band gap of 2.5 eV. Hence, as-prepared, such Ag(i)-DNA systems should not be considered molecular nanowires but, at best, proto-wires. The structural features seen in 1 are essentially retained in the corresponding organogel which exhibits thixotropic self-healing that can be attributed to the reversible nature of the intermolecular interactions. Photo-reduced samples of the gel exhibit luminescence confirming that these poly-cytidine sequences appropriately pre-configure silver ions for the formation of quantum-confined metal clusters in line with contemporary views on DNA-templated clusters. Microscopy data reveals the resulting metal cluster/particles are approximately spherical and crystalline with lattice spacing (111) similar to bulk Ag.

Enhancing the kinetics of hydrazone exchange processes: an experimental and computational study.

The capacity of hydrazone bonds to readily undergo component exchange processes sees their extensive utilization in dynamic combinatorial chemistry. The kinetics of hydrazone exchange are optimal at pH ∼4.5, which limits the use of hydrazone-based dynamic combinatorial libraries, particularly for biological targets which are only stable at near-neutral pH values. It would thus be advantageous if hydrazone exchange proceeded with faster rates at pH values closer to neutral. We experimentally and computationally evaluated the hypothesis that hydrazones possessing neighbouring acidic or basic functional groups within the carbonyl-derived moitety of the hydrazone would enhance exchange rates. Our work suggests that judiciously placed N- or O-hydrogen bond acceptors within the carbonyl-derived moiety of the hydrazone stabilize transition states via hydrogen bonding interactions, providing a valuable boost to exchange kinetics at near-neutral pH values. We anticipate these findings will be of interest in dynamic combinatorial chemistry, dynamic covalent polymers/materials, functionalized nanoparticles and interlocked molecules, all of which may benefit from hydrazone exchange processes able to operate at near-neutral pH values.

Marine antifouling performance of polymer coatings incorporating zwitterions.

Zwitterionic materials display antifouling promise, but their potential in marine anti-biofouling is still largely unexplored. This study evaluates the effectiveness of incorporating small quantities (0-20% on a molar basis) of zwitterions as sulfobetaine methacrylate (SBMA) or carboxybetaine methacrylate (CBMA) into lauryl methacrylate-based coatings whose relatively hydrophobic nature encourages adhesion of the diatom Navicula incerta, a common microfouling organism responsible for the formation of 'slime'. This approach allows potential enhancements in antifouling afforded by zwitterion incorporation to be easily quantified. The results suggest that the incorporation of CBMA does provide a relatively minor enhancement in fouling-release performance, in contrast to SBMA which does not display any enhancement. Studies with coatings incorporating mixtures of varying ratios of the cationic monomer [2-(methacryloyloxy)ethyl]trimethylammonium chloride and the anionic monomer (3-sulfopropyl)methacrylate, which offer a potentially lower cost approach to the incorporation of anionic and cationic charge, suggest these monomers impart little significant effect on biofouling.

A coordination polymer for the site-specific integration of semiconducting sequences into DNA-based materials.

Advances in bottom-up material design have been significantly progressed through DNA-based approaches. However, the routine integration of semiconducting properties, particularly long-range electrical conduction, into the basic topological motif of DNA remains challenging. Here, we demonstrate this with a coordination polymer derived from 6-thioguanosine (6-TG-H), a sulfur-containing analog of a natural nucleoside. The complexation reaction with Au(I) ions spontaneously assembles luminescent one-dimensional helical chains, characterized as {AuI(µ-6-TG)} n , extending many µm in length that are structurally analogous to natural DNA. Uniquely, for such a material, this gold-thiolate can be transformed into a wire-like conducting form by oxidative doping. We also show that this self-assembly reaction is compatible with a 6-TG-modified DNA duplex and provides a straightforward method by which to integrate semiconducting sequences, site-specifically, into the framework of DNA materials, transforming their properties in a fundamental and technologically useful manner.Integration of semiconducting properties into the basic topological motif of DNA remains challenging. Here, the authors show a coordination polymer derived from 6-thioguanosine that complexes with Au(I) ions to form a wire-like material that can also integrate semiconducting sequences into the framework of DNA materials.

Heterogeneous Electron-Transfer Rates for the Reduction of Viologen Derivatives at Platinum and Bismuth Electrodes in Acetonitrile.

The standard heterogeneous rate constants for the reduction of a series of viologen derivatives with a range of inter-ring torsion angles were measured at Bi and Pt electrodes. The electrode potentials for the first one-electron reduction of the viologens vary from -684 mV to -1070 mV vs. Ag/0.01 m Ag+; this enabled a comparison of the behaviour of metallic (Pt) and semi-metallic (Bi) electrodes over a wide range of applied potentials. The differential capacitance (6.5 µF cm-2) of Bi/MeCN,TBAPF6 interfaces at the potential of zero charge (pzc=-0.60 V) is at least an order of magnitude greater than that calculated on the basis of the bulk Bi carrier density (3×1017 cm-3) and the differential capacitance (9.5 µF cm-2) of Pt/MeCN interfaces at their pzc (-0.43 V) is of the same order. The series of viologen derivatives exhibited simple one-electron redox behaviour and showed similar rate constants at Pt (1.8×10-4-1.6×10-3 cm s-1) and Bi electrodes (1.1×10-4-1.9×10-3 cm s-1) after application of the Frumkin correction. These results demonstrate that the density of states at the Bi surface is much higher than in bulk. Finally, the Frumkin-corrected standard rate constants were observed to be inversely correlated with the inter-ring torsion angle of the viologens.

Nanoscale silicon substrate patterns from self-assembly of cylinder forming poly(styrene)-block-poly(dimethylsiloxane) block copolymer on silane functionalized surfaces.

Poly(styrene)-block-poly(dimethylsiloxane) (PS-b-PDMS) is an excellent block copolymer (BCP) system for self-assembly and inorganic template fabrication because of its high Flory-Huggins parameter (χ âˆ¼ 0.26) at room temperature in comparison to other BCPs, and high selective etch contrast between PS and PDMS block for nanopatterning. In this work, self-assembly in PS-b-PDMS BCP is achieved by combining hydroxyl-terminated poly(dimethylsiloxane) (PDMS-OH) brush surfaces with solvent vapor annealing. As an alternative to standard brush chemistry, we report a simple method based on the use of surfaces functionalized with silane-based self-assembled monolayers (SAMs). A solution-based approach to SAM formation was adopted in this investigation. The influence of the SAM-modified surfaces upon BCP films was compared with polymer brush-based surfaces. The cylinder forming PS-b-PDMS BCP and PDMS-OH polymer brush were synthesized by sequential living anionic polymerization. It was observed that silane SAMs provided the appropriate surface chemistry which, when combined with solvent annealing, led to microphase segregation in the BCP. It was also demonstrated that orientation of the PDMS cylinders may be controlled by judicious choice of the appropriate silane. The PDMS patterns were successfully used as an on-chip etch mask to transfer the BCP pattern to underlying silicon substrate with sub-25 nm silicon nanoscale features. This alternative SAM/BCP approach to nanopattern formation shows promising results, pertinent in the field of nanotechnology, and with much potential for application, such as in the fabrication of nanoimprint lithography stamps, nanofluidic devices or in narrow and multilevel interconnected lines.

Metal-conductive polymer hybrid nanostructures: preparation and electrical properties of palladium-polyimidazole nanowires.

A simple, convenient method for the formation of hybrid metal/conductive polymer nanostructures is described. Polyimidazole (PIm) has been templated on λ-DNA via oxidative polymerisation of imidazole using FeCl3 to produce conductive PIm/DNA nanowires. The PIm/DNA nanowires were decorated with Pd (Pd/PIm/DNA) by electroless reduction of PdCl4(-2) with NaBH4 in the presence of PIm/DNA; the choice of imidazole was motivated by the potential Pd(II) binding site at the pyridinic N atom. The formation of PIm/DNA and the presence of metallic Pd on Pd/PIm/DNA nanowires were verified by FTIR, UV-vis and XPS spectroscopy techniques. AFM studies show that the nanowires have diameters in the range 5-45 nm with a slightly greater mean diameter (17.1 ± 0.75 nm) for the Pd-decorated nanowires than the PIm/DNA nanowires (14.5 ± 0.89 nm). After incubation for 24 h in the polymerisation solution, the PIm/DNA nanowires show a smooth, uniform morphology, which is retained after decoration with Pd. Using a combination of scanned conductance microscopy, conductive AFM and two-terminal measurements we show that both types of nanowire are conductive and that it is possible to discriminate different possible mechanisms of transport. The conductivity of the Pd/PIm/DNA nanowires, (0.1-1.4 S cm(-1)), is comparable to the PIm/DNA nanowires (0.37 ± 0.029 S cm(-1)). In addition, the conductance of Pd/PIm/DNA nanowires exhibits Arrhenius behaviour (E(a )= 0.43 ± 0.02 eV) as a function of temperature in contrast to simple Pd/DNA nanowires. These results indicate that although the Pd crystallites on Pd/PIm/DNA nanowires decorate the PIm polymer, the major current pathway is through the polymer rather than the Pd.

Metal-enhanced luminescence of silicon quantum dots: effects of nanoparticles and molecular electron donors and acceptors on the photofading kinetics.

Alkyl-capped silicon quantum dots (SiQDs) show enhanced luminescence when drop cast as films on glass slides in mixtures with Ag or Au nanoparticles or the electron donor ferrocene (Fc). Metal enhancement of quantum dot photoluminescence (PL) is known to arise from a combination of the intense near-field associated with the surface plasmon of the metal on the rate of absorption and the decrease in the lifetime of the excited state. Here we present evidence that an additional factor is also involved: electron transfer from the metal to the quantum dot. Under CW irradiation with an argon ion laser at 488 nm, SiQDs undergo a reversible photofading of the PL as the particles photoionize. A steady-state condition is established by the competition between photoionization and electron-hole recombination. The fading of the initial PL I0 to the steady-state value I∞ can be modelled by a simple first order decay with a lognormal distribution of rates, which reflects the heterogeneity of the sample. In the presence of Ag and Au nanoparticles, the modal rate constants of photofading increase by factors of up to 4-fold and the ratio I0/I∞ decreases by factors up to 5-fold; this is consistent with an increase in the rate of electron-hole recombination facilitated by the metal nanoparticles acting as sources of electrons. Further support for this interpretation comes from the enhancement in PL observed in photofading experiments with films of SiQDs mixed with Fc; this compound is a well-known one-electron donor, but shows no plasmon band which complicates the estimation of PL enhancement with Ag NPs.

Mechanical and optical properties of ultralarge flakes of a metal-organic framework with molecular thickness.

The isolation of 2D-materials is already a success for graphene, graphene oxide, boron nitride and a few clays or metal chalcogenides, however despite the fact that some of them show very interesting physical properties, they lack useful functionalities. Metal-Organic Frameworks (MOFs) are multifunctional materials showing a wide range of physical and chemical properties that can be structurally designed by suitable selection of their building-blocks. This strategy may allow the production of layers with a variety of useful electronic and molecular recognition functionalities. Herein we isolate 2D-MOF flakes with areas of hundreds of square microns and an excellent control of the molecular thickness (from single up to ca. 50 layers). The samples exhibit such good photoluminescence and mechanical properties as to allow free-standing characterization of few layers' flakes.

Enhanced Raman and luminescence spectra from co-encapsulated silicon quantum dots and Au-Ag nanoalloys.

We report an approach to enhance simultaneously luminescence and SERS signals with a single excitation wavelength by co-encapsulating silicon quantum dots and Au-Ag alloy nanoparticles encoded with Raman reporter molecules inside polymeric nanoparticles. The SERS-luminescence enhancement exploits the large Stokes shift of silicon quantum dots, which allows 'room' for the display of a Raman spectrum.