Nonconventional Symmetric Double-Side Electrochromic Devices Employing a Nafion Conductive Layer to Unlock Superior Durability.

In this work, we present a methodology to create an effective novel double-sided symmetric architecture of solid-state electrochromic devices. This principally new nonconventional configuration provides access to novel electrochromic systems that could be applicable for the creation of smart double-side signage, smart boards, nonemissive displays, and other smart interactive devices that change their color upon application of a voltage. The proposed configuration is based on the assembly of two identical electrochromic materials facing each other through an opaque optical separator. As a proof of concept, we use an electrochromic material based on bis(4'-(pyridin-4-yl)-2,2':6',2″-terpyridine) iron complex, covalently immobilized on screen-printed surface-extended ITO support. The symmetric configuration allows for a drastic enhancement of the overall stability of the device due to both attenuation of the counter electrode polarization and minimization of electrolyte decomposition. A nontransparent ion-permeable separator, in turn, allows observing the color change of only one of the electrodes by cutting off the optical contribution of the electrode located behind it. Further functionalization of the electrochromic material with a thin layer of Nafion is a beneficial strategy to significantly boost up long-term durability of the devices. Applying a layer of Nafion to the electrochromic material results in an increase in ionic conductivity within the device and ensures better retention of electrochromic molecules on the surface, thus minimizing device decomposition during long-term electrochemical cycling. An electrochromic device that bears Nafion-functionalized electrodes can operate (i) in the dual-side mode, where both sides demonstrate effective electrochromic performance; or (ii) in a one-side manner, where only one side of the device changes color. Notably, when operating in the one-side mode, the device withstands 70,000 cycles, after which the performance of the device can be resumed by simply turning the device to the other side (via switching the polarity of the electrodes).

Flexible electrochemical aptasensor for cortisol detection in human sweat.

This communication demonstrates an electrochemical DNA aptasensor for the detection of cortisol in human sweat. The aptasensor was fabricated via layer-by-layer assembly on stretchable polydimethylsiloxane (PDMS) coated with conductive nanoporous carbon nanotube-cellulose nanocrystals (CNC/CNT) film using a linker to a cortisol specific DNA aptamer. The flexible cortisol aptasensor had a dynamic range of 2.5-35 ng mL-1. The aptasensor precision was determined to be 2.7% relative standard deviation (%RSD) across the concentration dynamic range. The aptasensor was determined to have a limit of detection (LOD) of ∼ 1.8 ng mL-1. The aptasensor was demonstrated to have high selectivity to cortisol and was unresponsive to interfering species including glucose, sodium lactate, and ß-estradiol. The aptasensor was successfully evaluated for the detection of cortisol in human sweat indicative of its high specificity.

Post-Synthetic Color Tuning of the Ultra-Effective and Highly Stable Surface-Confined Electrochromic Monolayer: Shades of Green for Camouflage Materials.

We report here on the strategy for the preparation of a series of electrochromic (EC) materials in green shades designed for camouflage purposes. This top-down post-synthetic modification provides access to new EC materials by fine modulation of the color of the surface-confined metalorganic monolayer pre-deposited on indium tin oxide screen-printed supports. Selective on-surface N-quaternization of the outer pyridine unit of the EC metal complex covalently embedded onto an enhanced surface area electrode results in a bathochromic shift of the absorbance signal as well as visual color change from blue to different shades of green. When assembled into solid-state EC devices (ECDs), the materials demonstrate high color differences between colored and bleached states and significant differences in optical density. Upon electrochemical switching, the ECDs initially featuring different shades of green become yellowish or clay. The accessible gamut of colors, fulfilling the requirements for chameleon-like camouflage materials, is able to mimic conditions of various natural environments including forests and sands. Notably, ECDs demonstrate high long-term durability (95% retention of the performance after 3300 cycles), fast coloration (0.6-1.1 s), and bleaching (1.2-3.3 s) times and outstanding coloration efficiencies of 1018-1513 cm2/C. Importantly, post-synthetic N-quaternization/color tuning does not deteriorate the performance of the resulting EC materials and devices as judged by cyclic voltammetry, spectroelectrochemistry, and electrochemical impedance spectroscopy. This work adds to the limited number of reports that explore color tuning of EC molecular layers via on-surface modification with the aim to access new non-symmetric materials. Notably, the facile and straightforward technology presented here allows the creation of green-colored EC materials that are difficult to prepare in other ways.

Reducing the resistance for the use of electrochemical impedance spectroscopy analysis in materials chemistry.

Electrochemical impedance spectroscopy (EIS) is a highly applicable electrochemical, analytical, and non-invasive technique for materials characterization, which allows the user to evaluate the impact, efficiency, and magnitude of different components within an electrical circuit at a higher resolution than other common electrochemical techniques such as cyclic voltammetry (CV) or chronoamperometry. EIS can be used to study mechanisms of surface reactions, evaluate kinetics and mass transport, and study the level of corrosion on conductive materials, just to name a few. Therefore, this review demonstrates the scope of physical properties of the materials that can be studied using EIS, such as for characterization of supercapacitors, dye-sensitized solar cells (DSSCs), conductive coatings, sensors, self-assembled monolayers (SAMs), and other materials. This guide was created to support beginner and intermediate level researchers in EIS studies to inspire a wider application of this technique for materials characterization. In this work, we provide a summary of the essential background theory of EIS, including experimental design, signal responses, and instrumentation. Then, we discuss the main graphical representations for EIS data, including a scope of the foundation principles of Nyquist, Bode phase angle, Bode magnitude, capacitance and Randles plots, followed by detailed step-by-step explanations of the corresponding calculations that evolve from these graphs and direct examples from the literature highlighting practical applications of EIS for characterization of different types of materials. In addition, we discuss various applications of EIS technique for materials research.

Multichromic Monolayer Terpyridine-Based Electrochromic Materials.

The article describes novel electrochromic materials (ECMs) that are based on a monolayer consisting of two or three isostructural metal complexes of 4'-(pyridin-4-yl)-2,2':6',2''-terpyridine simultaneously deposited on surface-enhanced support. The support was made by screen printing of indium tin oxide (ITO) nanoparticles on ITO-glass and has a surface area sufficient for a monolayer to give color visible to the naked eye. The ability to separately electrochemically address the oxidation state of the metal centers on the surface (i.e., Co2+/Co3+, Os2+/Os3+, and Fe2+/Fe3+) provides an opportunity to achieve several distinct color-to-color transitions, thus opening the door for constructing monolayer-based multicolor ECMs.

Ligand Impact on Monolayer Electrochromic Material Properties.

In this study, we present a range of efficient highly durable electrochromic materials that demonstrate excellent redox and lifetime stability, sufficient coloration contrast ratios, and the best-in-class electron-transfer constants. The materials were formed by anchoring as little as a monolayer of predefined iron complexes on a surface-enhanced conductive solid support. The thickness of the substrate was optimized to maximize the change in optical density. We demonstrate that even a slight change in molecular sterics and electronics results in materials with sufficiently different properties. Thus, minor changes in the ligand design give access to materials with a wide range of color variations, including green, purple, and brown. Moreover, ligand architecture dictates either orthogonal or parallel alignment of corresponding metal complexes on the surface due to mono- or bis-quaternization. We demonstrate that monoquaternization of the complexes during anchoring to the surface-bound template layer results in redshifts of the photoabsorption peak. The results of in-solution bis-methylation supported by density functional theory calculations show that the second quaternization may lead to an opposite blueshift (in comparison with monomethylated analogs), depending on the ligand electronics and the environmental change. It is shown that the variations of the photoabsorption peak position for different ligands upon attachment to the surface can be related to the calculated charge distribution and excitation-induced redistribution. Overall, the work demonstrates a well-defined method of electrochromic material color tuning via manipulation of sterics and electronics of terpyridine-based ligands.

Hemoglobin-driven iron-directed assembly of gold nanoparticles.

The ability to form complex 3D architectures using nanoparticles (NPs) as the building blocks and complex macromolecules that direct these assemblies remains a challenging objective for nanotechnology. Here we report results in which the partial substitution of classical Turkevich citrate-capped gold NPs by a novel, heteroaromatic ligand (L) results in NPs able to form coordination-driven assemblies mediated by free or protein-bound iron ions. The morphology of these assemblies can be tuned depending on the source of iron. To prove the concept, classical citrate and novel NPs were reacted with iron-containing protein hemoglobin (Hb). To diminish the influence of possible electrostatic interactions of native Hb and gold NPs, the reaction was performed at the isoelectric point of Hb. Moreover, thiol groups of Hb were protected with p-quinone to exclude thiol-gold bond formation. As expected, citrate-capped gold NPs are well dispersed in functionalized Hb, while L-functionalized NPs form assemblies. The blue shift of the Soret band of the functionalized Hb, when reacted with novel NPs, unambiguously confirms the coordination of a NP-anchored heteroaromatic ligand with the heme moiety of Hb. Coarse-grained molecular dynamics of this system were performed to gain information about aggregation dynamics and kinetics of iron- and hemoglobin-templated assemblies of L-NPs. A multi-scale simulation approach was employed to extend this model to longer time scales. The application of this model towards novel coordination-based assemblies can become a powerful tool for the development of new nanomaterials.

Ultrastable Gold Nanoparticles Modified by Bidentate N-Heterocyclic Carbene Ligands.

Highly stable gold nanoparticles (Au NPs) functionalized by bidentate N-heterocyclic carbene (NHC) ligands have been synthesized by top-down and bottom-up approaches. A detailed study of the effect of alkylation, denticity, and method of synthesis has led to the production of NHC-stabilized nanoparticles with higher thermal stability than bi- and tridentate thiol-protected Au NPs and than monodentate NHC-stabilized NPs. Importantly, bidentate NHC-protected NPs also displayed unprecedented stability to external thiol, which has been an unsolved problem to date with all nanoparticles. Thus, multidentate NHC ligands are an important, and as yet unrecognized, step forward for the preparation of high stability nanomaterials.

Terpyridine-Based Monolayer Electrochromic Materials.

Novel electrochromic (EC) materials were developed and formed by a two-step chemical deposition process. First, a self-assembled monolayer (SAM) of 2,2':6',2″-terpyridin-4'-ylphosphonic acid, L, was deposited on the surface of a nanostructured conductive indium-tin oxide (ITO) screen-printed support by simple submerging of the support into an aqueous solution of L. Further reaction of the SAM with Fe or Ru ions results in the formation of a monolayer of the redox-active metal complex covalently bound to the ITO support (Fe-L/ITO and Ru-L/ITO, respectively). These novel light-reflective EC materials demonstrate a high color difference, significant durability, and fast switching speed. The Fe-based material shows an excellent change of optical density and coloration efficiency. The results of thermogravimetric analysis suggest high thermal stability of the materials. Indeed, the EC characteristics do not change significantly after heating of Fe-L/ITO at 100 °C for 1 week, confirming the excellent stability and high EC reversibility. The proposed fabrication approach that utilizes interparticle porosity of the support and requires as low as a monolayer of EC active molecule benefits from the significant molecular economy when compared with traditional polymer-based EC devices and is significantly less time-consuming than layer-by-layer growth of coordination-based molecular assemblies.

Simple direct formation of self-assembled N-heterocyclic carbene monolayers on gold and their application in biosensing.

The formation of organic films on gold employing N-heterocyclic carbenes (NHCs) has been previously shown to be a useful strategy for generating stable organic films. However, NHCs or NHC precursors typically require inert atmosphere and harsh conditions for their generation and use. Herein we describe the use of benzimidazolium hydrogen carbonates as bench stable solid precursors for the preparation of NHC films in solution or by vapour-phase deposition from the solid state. The ability to prepare these films by vapour-phase deposition permitted the analysis of the films by a variety of surface science techniques, resulting in the first measurement of NHC desorption energy (158±10 kJ mol(-1)) and confirmation that the NHC sits upright on the surface. The use of these films in surface plasmon resonance-type biosensing is described, where they provide specific advantages versus traditional thiol-based films.

Mechanistic Aspects of Aryl-Halide Oxidative Addition, Coordination Chemistry, and Ring-Walking by Palladium.

This contribution describes the reactivity of a zero-valent palladium phosphine complex with substrates that contain both an aryl halide moiety and an unsaturated carbon-carbon bond. Although η(2) -coordination of the metal center to a C=C or C≡C unit is kinetically favored, aryl halide bond activation is favored thermodynamically. These quantitative transformations proceed under mild reaction conditions in solution or in the solid state. Kinetic measurements indicate that formation of η(2) -coordination complexes are not nonproductive side-equilibria, but observable (and in several cases even isolated) intermediates en route to aryl halide bond cleavage. At the same time, DFT calculations show that the reaction with palladium may proceed through a dissociation-oxidative addition mechanism rather than through a haptotropic intramolecular process (i.e., ring walking). Furthermore, the transition state involves coordination of a third phosphine to the palladium center, which is lost during the oxidative addition as the C-halide bond is being broken. Interestingly, selective activation of aryl halides has been demonstrated by adding reactive aryl halides to the η(2) -coordination complexes. The product distribution can be controlled by the concentration of the reactants and/or the presence of excess phosphine.

Highly selective directed arylation reactions via back-to-back dehydrogenative C-H borylation/arylation reactions.

Dimeric rhodium N-heterocyclic carbene complexes are demonstrated to be effective catalyst precursors for directed C-H borylation reactions at room temperature. The reactions are highly selective for mono-borylation and can be combined with a one-pot Suzuki-Miyaura coupling to give C-H arylation products with exclusive selectivity for mono-arylation without the requirement for steric blocking groups.

Ultra stable self-assembled monolayers of N-heterocyclic carbenes on gold.

Since the first report of thiol-based self-assembled monolayers (SAMs) 30 years ago, these structures have been examined in a huge variety of applications. The oxidative and thermal instabilities of these systems are widely known, however, and are an impediment to their widespread commercial use. Here, we describe the generation of N-heterocyclic carbene (NHC)-based SAMs on gold that demonstrate considerably greater resistance to heat and chemical reagents than the thiol-based counterparts. This increased stability is related to the increased strength of the gold-carbon bond relative to that of a gold-sulfur bond, and to a different mode of bonding in the case of the carbene ligand. Once bound to gold, NHCs are not displaced by thiols or thioethers, and are stable to high temperatures, boiling water, organic solvents, pH extremes, electrochemical cycling above 0 V and 1% hydrogen peroxide. In particular, benzimidazole-derived carbenes provide films with the highest stabilities and evidence of short-range molecular ordering. Chemical derivatization can be employed to adjust the surface properties of NHC-based SAMs.

Dioxygen adducts of rhodium N-heterocyclic carbene complexes.

Rhodium complexes functionalized by N-heterocyclic carbene ligands react with dioxygen to form adducts. Depending on the specifics of the ancillary ligands, oxygen binds to Rh either as a peroxide to form a fully oxidized Rh(III) complex, or as singlet dioxygen in a Rh(I) square planar complex. We have shown through analysis of a series of compounds, some previously published and some novel, that the presence of additional ligands that would support the formation of an octahedral geometry, as typically found with Rh(III) complexes, is critical for formation of the peroxide. In addition, we have demonstrated through DFT studies, that the potential energy surface with regard to the O-O bond length is relatively shallow, which provides a rationale for the distribution of bond lengths observed for apparently similar complexes analyzed by crystallography.

Heteroleptic rhodium NHC complexes with pyridine-derived ligands: synthetic accessibility and reactivity towards oxygen.

The synthesis, structure determination and oxidative stability of novel Rh-NHC complexes which feature pyridine-derived ligands have been described. All complexes described herein were synthesized from common dinuclear precursors of general structure [Rh(NHC)(L)Cl](2), where L is a monodentate olefin. We demonstrate that the use of these precursors is critical for the formation of all complexes since related cyclooctadiene containing precursors ([Rh(NHC)(COD)Cl]) were completely unreactive under identical conditions. We further demonstrate that complexes with the general formula [Rh(NHC)(olefin)(Py)Cl] or ([Rh(NHC)(BiPy/Phen)Cl]) are extremely sensitive to oxygen, reacting initially to give an adduct with dioxygen, and then decomposing further. The series of compounds and their oxidation products gave a remarkable range of colours which may be useful in the preparation of colourometric oxygen sensors.

Double single-crystal-to-single-crystal transformation and small-molecule activation in rhodium NHC complexes.

Three gases, one crystal: rhodium NHC complexes undergo back-to-back single-crystal-to-single-crystal transformations by selective nonreversible ligand exchange reactions. Slow diffusion of O(2) converts a dinitrogen complex into a dioxygen complex, and CO subsequently replaces O(2).

Long-range through-bond heteronuclear communication in platinum complexes.

Four analogous platinum stilbene- and stilbazole-based complexes exhibit unusual long-range heteronuclear spin-spin coupling in solution. Single crystal analysis and NMR experiments show that the (19)F, (31)P, and (195)Pt nuclei communicate over large distances (0.9-1.3 nm) through bond rather than through space. Spin-spin couplings between (195)Pt and (19)F over seven bonds and between (31)P and (19)F over eight bonds are observed with (7)J(PtF) = 2.9 Hz and (8)J(PF) = 11.8 Hz. Remarkably, a very large spin coupling between (195)Pt and (19)F over six bonds ((6)J(PtF) = 40.1 Hz) is also observed in a structurally related pyridinium complex. Experimental and gNMR (version 5.0) simulated (19)F{(1)H}, (31)P{(1)H}, and (195)Pt{(1)H} spectra of the complexes reveal a three-spin AMY system (A = (31)P, M = (31)P, Y = (19)F) or a five-spin AMY(3) flanked by a four-spin AMXY or a six-spin AMXY(3) system (X = (195)Pt), respectively. Density functional theory calculations at the PBE0/SDD level of theory show a pi-conjugated metal-ligand network, which may contribute to the experimentally observed spin-spin interactions.

Directing aryl-I versus Aryl-Br bond activation by nickel via a ring walking process.

Activation of a strong aryl-Br bond of a halogenated vinylarene by nickel(0) is demonstrated in the presence of aryl-I containing substrates. eta2-Coordination of Ni(PEt3)2 to the C=C moiety of halogenated vinylarenes is kinetically preferable and is followed by an intramolecular aryl-halide bond activation process. This "ring-walking" process is quantitative and proceeds under mild reaction conditions in solution. Mechanistic studies indicate that the metal insertion into the aryl-halide bond is not the rate-determining step. The reaction obeys first-order kinetics in the eta2-coordination complexes with almost identical activation parameters for Br and I derivatives. The ring-walking process is kinetically accessible as shown by density functional theory (DFT) calculations at the PBE0/SDB-cc-pVDZ//PBE0/SDD level of theory.

Molecular assembly of a 3D-ordered multilayer.

Combining strong metal-ligand coordination and pi-pi interactions affords a 3D-ordered molecular-based multilayer. The organization of the assembly is apparent from the optical properties and X-ray reflectivity.