Charge transport in solid polymer matrixes with redox centers

Precise design of copolymer-conjugated nanocatalysts for active electron transfer

A copolymer-conjugated nanocatalytic system has been designed for active electron transfer. To enhance photoinduced H2 generation, we precisely synthesize ternary random copolymers capable of transferring electrons through phase transitions, extending and shrinking in response to viologen's redox changes within 2 nm distance from the surface of the catalytic nanoparticle.

Solid-Phase Electrosynthesis

ConspectusThe significance of the new synthetic approach is that it can overcome the limitations of conventional methods and produce previously inaccessible polymer structures and materials. The solid-phase synthesis developed by Merrifield in 1964 is widely employed for the synthesis of biological molecules, such as peptides, nucleic acids, and oligosaccharides. Although the variety of iterative reactions available is theoretically implemented for most organic synthesis protocols, they are usually required to have high efficiency against sluggish reaction kinetics at the solid-liquid interface and process with protection and deprotection steps. Generally, unsatisfied reaction dynamics at the solid-liquid interface cannot statistically permit accurate and uniform polymer synthesis of sophisticated structures and functions within an acceptable time scale. To address this challenge, we propose the concept of solid-phase electrosynthesis, which simultaneously enables rapidly surface-initiated uniform electrosynthesis and unidirectional assembly of metallopolymers via kinetically accelerated and statistically allowed iterative growth. In particular, on a self-assembled monolayer (SAM) of the metal complex with electroactive unit A, the iterative monomer with two electroactive units A and B can be alternatively activated by oxidative and reductive potentials for A-A and B-B covalent couplings with the SAM, respectively. This enables topochemical one-by-one additions of the iterative monomers to end-on-oriented self-assembled molecules through alternative redox reactions. Each iterative step is purified by washing. Repeating the same iterative reaction enables further reaction of the unreactive sites on the SAMs and repairs the morphology defects, thereby ensuring the statistically allowed uniform synthesis and fabrication of polymer monolayers. The resulting monolayers exhibit subnanometer-uniform morphology over centimeter-sized areas with crystalline states and show thicknesses similar to theoretical molecular lengths. This demonstrates the unidirectional formation of polymer assemblies, providing a pathway for obtaining highly ordered formation of noncrystalline polymers. The length-controlled electrosynthesis of metallopolymers can be generalized for many types of organic ligands and metal species, enabling quantitative design of the composition and sequence-controlled metallopolymers with the precise relationships of structures and properties. Solid-phase electrosynthesis offers a unique approach to synthesize polymer structures and monolayers with enhanced functionality and superior physical properties, including physical density, modulus, and conductance. Through the utilization of precise and efficient iterative growth, this predictable electrosynthesis, coupled with optical and electrical monitoring, not only expands the scope of current synthetic chemistry but also paves a potential way for the automated generation of optoelectric molecular monolayers with large-area dimensional consistency and enhanced physical performance.

Composition and sequence-controlled conductance of crystalline unimolecular monolayers

Understanding how the charge travels through sequence-controlled molecules has been a formidable challenge because of simultaneous requirements in well-controlled synthesis and well-manipulated orientation. Here, we report electrically driven simultaneous synthesis and crystallization as a general strategy to study the conductance of composition and sequence-controlled unioligomer and unipolymer monolayers. The structural disorder of molecules and conductance variations on random positions can be extremely minimized, by uniform synthesis of monolayers unidirectionally sandwiched between electrodes, as an important prerequisite for the reproducible measurement on the micrometer scale. These monolayers show tunable current density and on/off ratios in four orders of magnitude with controlled multistate and massive negative differential resistance (NDR) effects. The conductance of monolayer mainly depends on the metal species in homo-metal monolayers, while the sequence becomes a matter in hetero-metal monolayers. Our work demonstrates a promising way to release an ultra-rich variety of electrical parameters and optimize the functions and performances of multilevel resistive devices.

Polymeric Design for Electron Transfer in Photoinduced Hydrogen Generation through a Coil-Globule Transition

To realize a renewable energy society, a polymeric system for photoinduced hydrogen generation utilizing a copolymer containing an electron acceptor was designed. In this system, the redox changes of viologen introduced into poly(N-isopropylacrylamide) cause cyclic conformational changes owing to the shifting of the phase transition temperature (PTT). The polymeric coil-globule transitions with hydrophilic/hydrophobic changes accelerate the electron transfer for hydrogen generation. In particular, hydrogen generation using visible-light energy with high efficiency is achieved around the PTT. In contrast to conventional solution systems, our polymeric system enables efficient hydrogen generation in a close molecular arrangement without the aggregation of catalytic nanoparticles. The utilization of conformational changes will provide a new strategy for synthesizing artificial photosynthetic hydrogels that split water to generate both hydrogen and oxygen.

The role of redox hopping in metal–organic framework electrocatalysis

A perspective on redox hopping charge transport through metal organic frameworks and its role in driving efficient electrocatalysis.

Design of Polymer Networks Involving a Photoinduced Electronic Transmission Circuit toward Artificial Photosynthesis

Many strategies have been explored to achieve artificial photosynthesis utilizing mediums such as liposomes and supramolecules. Because the photochemical reaction is composed of multiple functional molecules, the surrounding microenvironment is expected to be rationally integrated as observed during photosynthesis in chloroplasts. In this study, photoinduced electronic transmission surrounding the microenvironment of Ru(bpy)3(2+) in a polymer network was investigated using poly(N-isopropylacrylamide-co-Ru(bpy)3), poly(acrylamide-co-Ru(bpy)3), and Ru(bpy)3-conjugated microtubules. Photoinduced energy conversion was evaluated by investigating the effects of (i) Ru(bpy)3(2+) immobilization, (ii) polymer type, (iii) thermal energy, and (iv) cross-linking. The microenvironment surrounding copolymerized Ru(bpy)3(2+) in poly(N-isopropylacrylamide) suppressed quenching and had a higher radiative process energy than others. This finding is related to the nonradiative process, i.e., photoinduced H2 generation with significantly higher overall quantum efficiency (13%) than for the bulk solution. We envision that useful molecules will be generated by photoinduced electronic transmission in polymer networks, resulting in the development of a wide range of biomimetic functions with applications for a sustainable society.

Linker-based control of electron propagation through ferrocene moieties covalently anchored onto insulator-based nanopores derived from a polystyrene-poly(methylmethacrylate) diblock copolymer

This paper reports the effects of linker length on electron propagation through ferrocene moieties covalently anchored onto insulator-based cylindrical nanopores derived from a cylinder-forming polystyrene-poly(methylmethacrylate) diblock copolymer. These nanopores (24 nm in diameter, 30 nm long) aligned perpendicular to an underlying gold electrode were modified via esterification of their surface COOH groups with OH-terminated ferrocene derivatives having different alkyl linkers (FcCO(CH(2))(n)OH; n = 2, 5, 15). Cyclic voltammograms were measured in 0.1 M NaBF(4) at different scan rates to assess the efficiency of electron propagation through the ferrocene moieties. The redox peaks of the anchored ferrocenes were observed at nanoporous films decorated with FcCO(CH(2))(15)OH and FcCO(CH(2))(5)OH, but not at those with FcCO(CH(2))(2)OH. Importantly, the higher electron propagation efficiency was observed in the use of the longer linker, as shown by the apparent diffusion coefficients (ca. 10(-12) cm(2)/s for n = 15; ca. 10(-13) cm(2)/s for n = 5; no electron propagation for n = 2). The observed electron propagation resulted from electron hopping across relatively large spacing that was controlled by the motion of anchored redox sites (bounded diffusion). The longer linker led to the larger physical displacement range of anchored ferrocene moieties, facilitating the approach of the adjacent ferrocene moieties within a distance required for electron self-exchange reaction. The linker-based control of redox-involved electron propagation on nanostructured, insulating surfaces will provide a means for designing novel molecular electronics and electrochemical sensors.

Quenching of singlet photoexcited state of water soluble phthalocyanines and porphyrins by viologens interacting electrostatically

Quenching of photoexcited anionic and water-soluble phthalocyanines and 5,10,15,20-tetraarylporphyrins by viologens has been investigated. It was confirmed that the quenching of the singlet photoexcited state takes place, and that the mechanism is mostly a static one due to electrostatic interaction between the donor and the acceptor. The static mechanism was analyzed by curve-fitting of the relative emission intensity vs viologen concentration resulting in four kinds of mechanisms composed of static quenching accompanied partly by a dynamic one. The static mechanism was classified into two types: one mechanism is due to 1:n electrostatic interaction of the anionic sensitizer and the cationic acceptor, and other is a Perrin type for which the acceptor is incorporated into the quenching sphere around the sensitizer according to a Poisson distribution. The effect of micelles for the quenching was also studied including the effect of viologen with a long alkyl chain. The ionic micelles either incorporated or repulsed the ionic sensitizer and accepter resulting in either static quenching or prohibition of the quenching. Cationic phthalocyanines and porphyrins were also examined for the reaction with cationic viologens.

Effect of nanointegration on photoinduced hydrogen-generating nanogel systems

The nanointegration mechanism for photoinduced hydrogen nanogenerators using nanogels is described. By spatially integrating poly(N-isopropylacrylamide-co-Ru(bpy)(3)) nanogels as a photosensitizer and Pt nanoparticles as a catalyst, a mechanism using electrostatic interactions and the shrinking behavior of the thermosensitive polymer network is revealed. In addition, to evaluate the sensitivity to exterior energy, light, and heat, the integrated nanospace is controlled by using thermosensitive nanogels, which drastically shrink above the volume phase transition temperature. Such nanospatial control of multiple kinds of functional molecules in a photochemical reaction is important for the realization of artificial photosynthetic systems.

Interpretation of electron diffusion coefficient in organic and inorganic semiconductors with broad distributions of states

The carrier transport properties in nanocrystalline semiconductors and organic materials play a key role for modern organic/inorganic devices such as dye-sensitized (DSC) and organic solar cells, organic and hybrid light-emitting diodes (OLEDs), organic field-effect transistors, and electrochemical sensors and displays. Carrier transport in these materials usually occurs by transitions in a broad distribution of localized states. As a result the transport is dominated by thermal activation to a band of extended states (multiple trapping), or if these do not exist, by hopping via localized states. We provide a general view of the physical interpretation of the variations of carrier transport coefficients (diffusion coefficient and mobility) with respect to the carrier concentration, or Fermi level, examining in detail models for carrier transport in nanocrystalline semiconductors and organic materials with the following distributions: single and two-level systems, exponential and Gaussian density of states. We treat both the multiple trapping models and the hopping model in the transport energy approximation. The analysis is simplified by thermodynamic properties: the chemical capacitance, C(mu), and the thermodynamic factor, chi(n), that allow us to derive many properties of the chemical diffusion coefficient, D(n), used in Fick's law. The formulation of the generalized Einstein relation for the mobility to diffusion ratio shows that the carrier mobility is proportional to the jump diffusion coefficient, D(J), that is derived from single particle random walk. Characteristic experimental data for nanocrystalline TiO(2) in DSC and electrochemically doped conducting polymers are discussed in the light of these models.

Ruthenium polypyridyl chemistry; from basic research to applications and back again

Since the mid 1970's interest in the chemistry and applications of ruthenium polypyridyl complexes has increased steadily. In this perspective, the development of this area is tracked and discussed taking into account new scientific developments as well as novel applications. The interaction between basic and applied research is of particular importance and selected examples are highlighted.

Investigation of novel mediators for a glucose biosensor based on metal picolinate complexes

The metal complexes [Os(byp)(2)(pic)](+) and [Ru(byp)(2)(pic)](+) where byp is 2,2'-bipyridine and HPic is o-picolinic acid were synthesised and characterised using spectroscopic and electrochemical techniques. These complexes were then evaluated as mediators for a glucose oxidase (GOx)-based biosensor. Results demonstrate the electrocatalytic behaviour of both metal couples towards regeneration of the flavoprotein GOx (FADH(2)) group, when co-immobilised with glucose oxidase. Surface immobilisation was achieved by potential cycling in aqueous solutions of the metal complexes at a glucose oxidase (GOx)/Nafion modified electrode. This proved successful in terms of catalytic efficiency and stability of redox sites. Kinetic parameters associated with both enzymatic and mediator reactions were estimated and the stability/performance properties of the sensor were tested.

Voltammetric and spectroelectrochemical characterization of a water-soluble viologen polymer and its application to electron-transfer mediator for enzyme-free regeneration of NADH

A water-soluble polyxylylviologen (PXV(2+)) was characterized with a view to making use of it as a redox electron-transfer (ET) mediator. Cyclic voltammetric and spectropotentiometric studies showed (i) that PXV(2+) gives two redox waves centering at -0.40 and -0.83 V (vs. Ag/AgCl (3.3 mol dm(-3) KCl)) and (ii) that the lifetime of its monocation radical (PXV(+.)) is two orders of magnitude greater than that of the well-utilized dimethyl viologen monocation radical. Subsequently, the reaction of the PXV(2+/+.) couple with NAD(+) was evaluated in the similar manners as above. On the basis of this evaluation and the bioluminescence assay using bacterial NADH/FMN oxidoreductase and luciferase, it was shown (i) that the PXV(2+/+.) couple functions as a useful electron-transfer mediator and (ii) that PXV(+.) reacts with NAD(+), leading to generation of the enzymatically active NADH, in the absence of any reductases.