Walljet electrochemistry: quantifying molecular transport through metallopolymeric and zirconium phosphonate assembled porphyrin square thin films

Electrochemical Rectification of Redox Mediators Using Porphyrin-Based Molecular Multilayered Films on ITO Electrodes

Electrochemical charge transfer through multilayer thin films of zinc and nickel 5,10,15,20-tetra(4-ethynylphenyl) porphyrin constructed via copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) "click" chemistry was examined. Current rectification toward various outer-sphere redox probes is revealed with increasing numbers of layers, as these films possess insulating properties over the neutral potential range of the porphyrin, then become conductive upon reaching its oxidation potential. Interfacial electron transfer rates of mediator-dye interactions toward [Co(bpy)3](2+), [Co(dmb)3](2+), [Co(NO2-phen)3](2+), [Fe(bpy)3](2+), and ferrocene (Fc), all outer-sphere redox species, were measured by hydrodynamic methods. The ability to modify electroactive films' interfacial electron transfer rates, as well as current rectification toward redox species, has broad applicability in a number of devices, particularly photovoltaics and photogalvanics.

Layer-by-layer grown scalable redox-active ruthenium-based molecular multilayer thin films for electrochemical applications and beyond

Here we report the first study on the electrochemical energy storage application of a surface-immobilized ruthenium complex multilayer thin film with anion storage capability. We employed a novel dinuclear ruthenium complex with tetrapodal anchoring groups to build well-ordered redox-active multilayer coatings on an indium tin oxide (ITO) surface using a layer-by-layer self-assembly process. Cyclic voltammetry (CV), UV-Visible (UV-Vis) and Raman spectroscopy showed a linear increase of peak current, absorbance and Raman intensities, respectively with the number of layers. These results indicate the formation of well-ordered multilayers of the ruthenium complex on ITO, which is further supported by the X-ray photoelectron spectroscopy analysis. The thickness of the layers can be controlled with nanometer precision. In particular, the thickest layer studied (65 molecular layers and approx. 120 nm thick) demonstrated fast electrochemical oxidation/reduction, indicating a very low attenuation of the charge transfer within the multilayer. In situ-UV-Vis and resonance Raman spectroscopy results demonstrated the reversible electrochromic/redox behavior of the ruthenium complex multilayered films on ITO with respect to the electrode potential, which is an ideal prerequisite for e.g. smart electrochemical energy storage applications. Galvanostatic charge-discharge experiments demonstrated a pseudocapacitor behavior of the multilayer film with a good specific capacitance of 92.2 F g(-1) at a current density of 10 μA cm(-2) and an excellent cycling stability. As demonstrated in our prototypical experiments, the fine control of physicochemical properties at nanometer scale, relatively good stability of layers under ambient conditions makes the multilayer coatings of this type an excellent material for e.g. electrochemical energy storage, as interlayers in inverted bulk heterojunction solar cell applications and as functional components in molecular electronics applications.

Bimodal mesoporous titanium dioxide anatase films templated by a block polymer and an ionic liquid: influence of the porosity on the permeability

In the present paper, we report the synthesis of bimodal mesoporous anatase TiO2 films by the EISA (Evaporation-Induced Self-Assembly) method using sol-gel chemistry combining two porogen agents, a low molecular weight ionic template and a neutral block copolymer. The surfactant template (C16mimCl) generates non-oriented worm-like pores (8 to 10 nm) which connect the regularly packed ellipsoidal mesopores (15 to 20 nm diameter) formed by an amphiphilic block copolymer of the type poly(isobutylene)-b-poly(ethylene oxide) (PIB-PEO). The surfactant template can also significantly influence the size and packing of the ellipsoidal mesopores. The mesostructural organization and mesoporosity of the films are studied by Environmental Ellipsometry-Porosimetry (EEP), Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) and electron microscopy techniques. Electrochemical characterization is performed to study the permeability of the films to liquid solutions, using two types of probe moieties (K3Fe(III)(CN)6 and Ru(bpy)3(2+)) by the wall-jet technique. An optimum ratio of C16mimCl/PIB-PEO provides anatase films with a continuous bimodal mesopore structure, possessing a permeability up to two times higher than that of the mesoporous films templated by PIB-PEO only (with partially isolated mesopores). When C16mimCl is used in large quantities, up to 20% weight vs. PIB-PEO, large overall porous volume and surface area are obtained, but the mesostructure is increasingly disrupted, leading to a severe loss of permeability of the bimodal films. A dye-sensitized solar cell set-up is used with anatase films as the photoelectrode. The photosensitizer loading and the total energy conversion efficiency of the solar cells using the mesoporous films templated by an optimal ratio of the two porogen agents C16mimCl and PIB-PEO can be substantially increased in comparison with the solar cells using mesoporous films templated by PIB-PEO only.

Functionalized polypyrrole film: synthesis, characterization, and potential applications in chemical and biological sensors

In this paper, we report the synthesis of a carboxyl-functionalized polypyrrole derivative, a poly(pyrrole-N-propanoic acid) (PPPA) film, by electrochemical polymerization, and the investigation of its basic properties via traditional characterization techniques such as confocal-Raman, FTIR, SEM, AFM, UV-vis, fluorescence microscopy, and contact-angle measurements. The experimental data show that the as-prepared PPPA film exhibits a hydrophilic nanoporous structure, abundant -COOH functional groups in the polymer backbone, and high fluorescent emission under laser excitation. On the basis of these unique properties, further experiments were conducted to demonstrate three potential applications of the PPPA film in chemical and biological sensors: a permeable and permselective membrane, a membrane with specific recognition sites for biomolecule immobilization, and a fluorescent conjugated polymer for amplification of fluorescence quenching. Specifically, the permeability and permselectivity of ion species through the PPPA film are detected by means of rotating-disk-electrode voltammetry; the specific recognition sites on the film surface are confirmed with protein immobilization, and the amplification of fluorescence quenching is measured by the addition of a quenching agent with fluorescence microscopy. The results are in good agreement with our expectations.

Self-organized porphyrinic materials

The self-assembly and self-organization of porphyrins and related macrocycles enables the bottom-up fabrication of photonic materials for fundamental studies of the photophysics of these materials and for diverse applications. This rapidly developing field encompasses a broad range of disciplines including molecular design and synthesis, materials formation and characterization, and the design and evaluation of devices. Since the self-assembly of porphyrins by electrostatic interactions in the late 1980s to the present, there has been an ever increasing degree of sophistication in the design of porphyrins that self-assemble into discrete arrays or self-organize into polymeric systems. These strategies exploit ionic interactions, hydrogen bonding, coordination chemistry, and dispersion forces to form supramolecular systems with varying degrees of hierarchical order. This review concentrates on the methods to form supramolecular porphyrinic systems by intermolecular interactions other than coordination chemistry, the characterization and properties of these photonic materials, and the prospects for using these in devices. The review is heuristically organized by the predominant intermolecular interactions used and emphasizes how the organization affects properties and potential performance in devices.

Electrochemically assisted self-assembly of mesoporous silica thin films

Supported mesostructured thin films are of major importance for applications in optical, electrochemical and sensing devices. However, good performance is restricted to mesostructured phases ensuring good accessibility from the film surface, which would be straightforward with cylindrical pores oriented normal to the underlying support, but this remains challenging. Here, we demonstrate that electrochemistry is likely to induce self-assembly of surfactant-templated (organo)silica thin films on various conducting supports, homogeneously over wide areas. The method involves the application of a suitable cathodic potential to an electrode immersed in a surfactant-containing hydrolysed sol solution to generate the hydroxyl ions that are necessary to catalyse polycondensation of the precursors and self-assembly of hexagonally packed one-dimensional channels that grow perpendicularly to the electrode surface. The method is compatible with controlled and localized deposition on heterogeneous supports, opening the way to electrochemically driven nanolithography for designing complex patterns of widely accessible mesostructured materials.

Construction of amperometric immunosensors based on the electrogeneration of a permeable biotinylated polypyrrole film

The construction of amperometric immunosensors to cholera antitoxin immunoglobulins were shown to have improved sensitivity when the cholera toxin B subunit biorecognition entity was linked to an electrogenerated biotinylated polypyrrole film copolymerized with pyrrole-lactobionamide monomer. The copolymer exhibits greater film permeability than biotinylated polypyrrolic or polyphenolic films for the permeation of electroactive species. Hence, when the presence of the HRP marker of the immunoassay was determined using hydroquinone, the production of electroactive quinone was shown to permeate faster to the electrode, thus providing a faster response time.

Liquid/liquid interface polymerized porphyrin membranes displaying size-selective molecular and ionic permeability

Thin polymeric membranes have been formed by liquid/liquid interfacial copolymerization of a sterically demanding tetraphenylporphyrin derivative having reactive phenol substituents and a second porphyrin having reactive acid chloride groups. The out-of-plane steric demand is created by 3,5-hexoxyphenyl groups positioned at two of the four meso carbons of the porphyrin ring. The bulky substituents were designed to create local pockets and extended pores within the resulting ester-linked copolymer. Quantitative measures of molecular and ionic transport were obtained by placing membranes over microelectrodes and recording voltammetric responses from redox-active probes. The membranes were found to be permeable to small molecules and ions, but blocking toward larger ones, displaying a sharp size cutoff at a probe diameter of ca. 3.5 A. Molecular transport can be modulated by axially ligating pore-blocking moieties to available porphyrin metal centers.

Permeable nonaggregating porphyrin thin films that display enhanced photophysical properties

Porphyrins bearing bulky alkoxyphenyl substituents at two of the four meso-positions and phenyl phosphonates at the other two have been prepared and used as building blocks for layer-by-layer assembly of conductive-glass-supported thin films via zirconium phosphonate chemistry. Thin-film characterization shows that the addition of sterically demanding 2,6-di(n-hexoxy)phenyl substituents to the meso-positions of the porphyrin skeleton can successfully prevent molecular aggregation. Both absorption and emission studies of multilayer thin films provide strong evidence that the new compounds have the ability to form thin films in which very little molecular (chromophore) interaction is present, relative to porphyrins that are not sterically hindered. Furthermore, the films are found to be permeable to selected small redox probes but blocking toward larger ones. Taken together, the sharp absorption spectra, increased emission yields, and permeability are expected to be advantageous for various materials-based applications such as photovoltaics and sensors.

Irreversible or reversible self-assembly procedures yield robust zirconium (iv)-porpyrinphosphonate cones or μm-long fibers of monomolecular thickness

Stepwise, irreversible self-assembly of porphyrinphosphonates by zirconium(IV) produces cones of 20 nm height and similar widths. Side-on growth cannot be prevented. Reversible fiber growth without metal ions gave micrometer long fibers on mica of 2 nm width, because charge repulsion allowed only for end-on growth.