Green-to-Red Electrochromic Fe(II) Metallo-Supramolecular Polyelectrolytes Self-Assembled from Fluorescent 2,6-Bis(2-pyridyl)pyrimidine Bithiophene

Spectro-electrochemical study of iron and ruthenium bis-terpyridine complexes with methyl viologen-like subunits as models for supramolecular polymers

Metallo-supramolecular polyelectrolytes (MEPE) have a variety of attractive properties concerning electrochromism, spin-crossover, rheology, and cell differentiation. Previous studies suggest that these polynuclear structures can be regarded as an assembly of individual subunits and mononuclear complexes can act as models. In this study, we synthesize a monotopic and a ditopic terpyridine ligand with pyridinium units as well as the corresponding iron and ruthenium MEPEs and their mononuclear counterparts. UV-vis studies show that the mononuclear complexes have similar absorption properties to MEPEs. Furthermore, all complexes and MEPEs exhibit electrochromic behavior. Yet only the MEPEs can be deposited on different substrates using a layer-by-layer approach which makes them attractive for applications as electrochromic devices. However, the low solubility particularly of the ruthenium MEPE, renders characterization in solution impractical. Hence, the use of mononuclear complexes with similar monotopic ligands as presented herein can act as a first instance to evaluate the properties of corresponding MEPEs, facilitating the development of metallo-supramolecular materials.

Tunable Capacitive Behavior in Metallopolymer-based Electrochromic Thin Film Supercapacitors

Volumetric capacitance is a more critical performance parameter for rechargeable power supply in lightweight and microelectronic devices as compared to gravimetric capacitance in larger devices. To this end, we report three electrochromic metallopolymer-based electrode materials containing Fe²⁺ as the coordinating metal ion with high volumetric capacitance and energy densities in a symmetric two-electrode supercapacitor setup. These metallopolymers exhibited volumetric capacitance up to 866.2 F cm^-3 at a constant current density of 0.25 A g^-1. The volumetric capacitance (poly-Fe-L2: 544.6 F cm^-3 > poly-Fe-L1: 313.8 F cm^-3 > poly-Fe-L3: 230.8 F cm^-3 at 1 A g^-1) and energy densities (poly-Fe-L2: 75.5 mWh cm^-3 > poly-Fe-L1: 43.6 mWh cm^-3 > poly-Fe-L3: 31.2 mWh cm^-3) followed the order of the electrical conductivity of the metallopolymers and are among the best values reported for metal-organic systems. The variation in the ligand structure was key toward achieving different electrical conductivities in these metallopolymers with excellent operational stability under continuous cycling. High volumetric capacitances and energy densities combined with tunable electro-optical properties and electrochromic behavior of these metallopolymers are expected to contribute to high performance and compact microenergy storage systems. We envision that the integration of smart functionalities with thin film supercapacitors would warrant the surge of miniaturized on-chip microsupercapacitors integrated in-plane with other microelectronic devices for wearable applications.

Dinuclear 2,4-di(pyridin-2-yl)-pyrimidine based ruthenium photosensitizers for hydrogen photo-evolution under red light

In this study, we report two dinuclear Ru(II) complexes C1 and C2 and compare them to their mononuclear analogues Ref1 and Ref2. The dinuclear species exhibit a much stronger absorption, longer excited-state lifetimes and higher luminescence quantum yields than the mononuclear complexes. In addition, C1 and C2 are easier to reduce. An estimation of the driving forces for the electron transfer processes relevant to photocatalytic hydrogen evolution suggests that C1 and Ref2 possess similar activity as photosensitizer (PS). Yet, the improved photophysical properties of C1 make it a more promising candidate for hydrogen evolution. In hydrogen evolution experiments, C1 indeed exhibits increased activity as PS, however, the catalytic system loses its activity after only a few hours. C2 is less active than the mononuclear complexes despite its superior photophysical properties. This observation is attributed to a lack of driving force for the electron transfer towards the catalyst. Further studies of the dinuclear complex C1 show that it is indeed the PS, which decomposes under the catalytic conditions, presumably due to the electron transfer towards the catalyst being the rate-limiting step.

A facile gelator based on phenylalanine derivative is capable of forming fluorescent Zn-metallohydrogel, detecting Zn2+ in aqueous solutions and imaging Zn2+ in living cells

Supramolecular hydrogels are attracting soft materials with potential applications. In this study, we synthesized a facile gelator (named 2-QF) based on phenylalanine derivative with a Quinoline group. 2-QF can assemble to form hydrogels at room temperature in different colors under low pH values. Moreover, 2-QF was triggered to form a yellow metallohydrogel (2-QF-Zn) at high pH by the coordination between 2-QF and Zn²⁺. 2-QF-Zn metallohydrogel showed excellent multi-stimuli responsiveness, especially the reversible "on-off" luminescence switching, as induced by base/acid. In addition, at a low concentration, 2-QF can selectively and visibly identify Zn²⁺ through fluorescence enhancement, and can detect Zn²⁺ at physiological pH as a chemosensor. Remarkably, 2-QF and 2-QF-Zn exhibited an excellent biocompatibility without cell cytotoxicity, and 2-QF is able to penetrate live HeLa cells and image intracellular Zn²⁺ by a turn-on fluorescent response, which makes it a potential candidate for biomedical applications.

Ligand-Controlled Electrochromic Diversification with Multilayer Coated Metallosupramolecular Polymer Assemblies

Designing surface-confined molecular systems capable of expressing changes in functional properties as a result of slight variations in chemical structure under the influence of an external stimulus is of contemporary interest. In this context, we have designed three tetraterpyridine ligands with variations in their core architecture (phenyl vs tetraphenylethynyl vs bithiophene) to create spray-coated electrochromic assemblies of iron(II)-based metallosupramolecular polymer network films on transparent conducting oxide substrates. These assemblies exhibited molecular permeability and spectroelectrochemical properties that are in turn dictated by the ligand structure. Electrochromic films with high coloration efficiencies (up to 1050 cm²/C) and superior optical contrast (up to 76%) with a concomitant color-to-color redox transition were readily achieved. These functional switching elements were integrated into sandwich-type electrochromic cells (CE up to 641 cm²/C) that exhibited high contrast ratios of up to 56%, with attractive ON-OFF ratios, fast switching kinetics, and high operational stability. Every measurable spectroelectrochemical property of the films and devices is an associated function of the ligand structure that coordinates the same metal ion to different extents. While exhibiting a ligand-structure induced differential metal coordination leading to porosity and spectroelectrochemical diversification, these assemblies allow the creation of electrochromic patterns and images by a simple spray-coating technique.

Iron (II) Metallo-Supramolecular Polymers Based on Thieno[3,2-b]thiophene for Electrochromic Applications

Four new bis(tpy) unimers with different linkers between the thieno[3,2-b]thiophene-2,5-diyl central unit and terpyridine-4′-yl (tpy) end-groups: no linker (Tt), ethynediyl (TtE), 1,4-phenylene (TtPh) and 2,2′-bithophene-5,5′-diyl (TtB) are prepared, characterized, and assembled with Fe²⁺ ions to metallo-supramolecular polymers (Fe-MSPs). The Fe-MSP films prepared by spin-casting on Indium Tin Oxide (ITO) glass are characterized by atomic force microscope (AFM) microscopy, cyclic voltammetry, and UV/vis spectroscopy and studied for their electrochromism and effect of the unimer structure on their electrochromic performance. Of the studied MSPs, Fe-Tt shows the highest optical contrast as well as coloration efficiency (CE = 641 cm² C⁻¹) and the fastest optical response. This makes it an excellent candidate for possible use in electrochromic devices.

Substituted 2,4-Di(pyridin-2-yl)pyrimidine-Based Ruthenium Photosensitizers for Hydrogen Photoevolution under Red Light

The photocatalytic reduction of water to form hydrogen gas (H₂) is a promising approach to collect, convert, and store solar energy. Typically, ruthenium tris(bipyridine) and its many derivatives are used as photosensitizers (PSs) in a variety of photocatalytic conditions. The bis(terpyridine) analogues, however, have only recently gained attention for this application because of their poor photophysical properties. Yet, by the introduction of electron-donating or -withdrawing groups on the terpyridine ligands, the photophysical and electrochemical properties can be considerably improved. In this study, we report a series of nonsymmetric 2,6-di(pyridin-2-yl)pyrimidine ligands with peripheral pyridine substituents in different positions and their corresponding ruthenium(II) complexes. The presence of the pyrimidine ring stabilizes the lowest unoccupied molecular orbital, leading to a red-shifted emission and prolonged excited-state lifetimes as well as higher luminescence quantum yields compared to analogous terpyridine complexes. Furthermore, all complexes are easier to reduce than the previously reported bis(terpyridine) complexes used as PSs. Interestingly, the pyridine substituent in the 4-pyrimidine position has a greater impact on both the photophysical and electrochemical properties. This correlation between the substitution pattern and properties of the complexes is further investigated by using time-dependent density functional theory. In hydrogen evolution experiments under blue- and red-light irradiation, all investigated complexes exhibit much higher activity compared to the previously reported ruthenium(II) bis(terpyridine) complexes, but none of the complexes are as stable as the literature compounds, presumably because of an additional decomposition pathway of the reduced PS competing with electron transfer from the reduced PS to the catalyst.

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.

A fast-switching electrochromic device with a surface-confined 3D metallo-organic coordination assembly

Demonstrated herein is a fast (<1 s)-switching solid-state electrochromic device (t = 0.49 s for coloration and 0.90 s for bleaching), fabricated with a novel imidazolium-linked [Fe(terpyridine)2]2+ chromophore-based surface-confined three dimensional metallo-organic coordination assembly. The device also exhibits promising electrochromic attributes such as high coloration efficiency (η = 275 cm2 C-1), moderate operating voltage (from -2 V to +3.2 V) and transmittance contrast (ΔT = 40%), and high cycling stability (up to 4500 cycles).

Integrated Molecular Logic Using a Multistate Electrochromic Platform

Herein, we present an approach that integrates molecular logic functions using surface-confined metallo-organic assemblies. These assemblies are electrochromic and mimic the behaviour of logic elements. The logic elements are addressed individually by electrochemical methods, and their outputs are simultaneously read-out optically by UV/Vis absorption spectroscopy. The versatility of our setup is demonstrated by the integration of two multi-component assemblies; each acting as ternary logic elements. We used also a laminated cell configuration to demonstrate color-to-color and color-to-transparent transitions. This concept offers a route for the future development of devices with multiple logic states.

Metallohydrogel with Tunable Fluorescence, High Stretchability, Shape-Memory, and Self-Healing Properties

Aiming at the problem that the reported smart optical metallohydrogels were limited with poor mechanical properties, we reported here a novel smart optical metallohydrogel (Al-hydrogel) with excellent elongation, shape-memory ability, self-healing property, and controllable fluorescence intensity. The Al-hydrogel was obtained by the HHPMA-Al³⁺ and carboxylate-Al³⁺ coordination after one-pot micellar copolymerization of acrylic acid (AAc), acrylamide (AAm), and hydrophobic arylhydrazone-based ligand (HHPMA). This hydrogel was able to extend up to 5000% of its original length without fracture. Its emission intensity was tunable by OH^-/H⁺ or Zn²⁺/AAc and increased by 500% with 0.1 M OH^- or Zn²⁺. Its tunable fluorescence enabled us to repeatedly pattern it. A reversible system consisting of Fe³⁺/H⁺, was implemented to control the shape of the Al-hydrogel, endowing the Al-hydrogel with shape-memory ability. This highly stretchable and multifunctional Al-hydrogel has potential applications in information transmission, wearable devices, and flexible sensors.

The first example of an asymmetrical μ-oxo bridged dinuclear iron complex with a terpyridine ligand

The reaction of Fe(iii) ions with a terpyridine ligand L in the presence of chlorides and independent of conditions results in the formation of μ-oxo bridged dinuclear [FeLCl(μ-O)FeCl₃] and the mononuclear complex [FeLCl₂].

Supramolecular strategy for smart windows

Supramolecular strategy-based materials are outlined and their applications for fabricating smart windows are summarized for future exploration of ideal smart windows.

Improving Electrochromic Cycle Life of Prussian Blue by Acid Addition to the Electrolyte

In this study, we examined the cyclic stability of Prussian blue (PB) films in electrolytes with acid. The cyclic stabilities of the PB films were investigated in K⁺ based electrolytes with different values of solution pH. The acidified KCl solution can significantly improve the durability of the film. Among the three pH values tested, the KCl solutions (pH = 2.15 and pH = 3.03) showed better performance. Furthermore, we investigated the cyclic stabilities of the PB films in LiClO₄/PC electrolyte containing different acids. We found that the cyclic stability of PB film was significantly improved when a small amount of acetic acid was dissolved in LiClO₄/PC electrolyte. The PB film exhibited stable optical modulation after up to 20,000 cycles in LiClO₄/PC electrolyte containing acetic acid—a much higher result than those of some literatures. This suggests that the addition of acetic acid to LiClO₄/PC electrolyte can promote the development of PB-based devices with improved stability.

Metallo-supramolecular polymers derived from benzothiadiazole-based platinum acetylide complexes for fluorescent security application

Metallo-supramolecular polymers with the incorporation of benzothiadiazole-substituted organoplatinum moiety have been successfully constructed. The designed monomer displays intense fluorescence signals, which are severely quenched upon the supramolecular polymerization process. On–off switching of fluorescence can be further exploited for data security materials in response to the chemical stimuli. Accordingly, the resulting supramolecular polymers can be regarded as a novel and efficient candidate toward information processing applications.

Metallo-supramolecular polymers with the incorporation of benzothiadiazole-substituted organoplatinum moiety have been successfully constructed.

Slow magnetic relaxation in a Tb(iii)-based coordination polymer

A Tb(iii)-based coordination polymer (polyTb) was synthesized by complexation of Tb(NO3)3·(6H2O) and 4',4''''-[1,1'-biphenyl]-4,4'-diylbis[6,6''-bis(ethoxycarbonyl)2':6',2''-terpyridine](L). The polymer structure was determined by Job's plots, DFT calculation, and X-ray absorption fine structure (XAFS) measurement. Job's plots indicated that the mole ratio (Tb ion : L) is 1 : 1. The optimized model structures suggested a La model: the LaN6(O[double bond, length as m-dash]C)2 model. The bond distances of La-O and La-N are ∼2.80 Å and 2.60 Å, respectively. The EXAFS fitting indicated that the bond distances of Tb-O and Tb-N are 2.65 Å and 2.95 Å, respectively. polyTb shows field-induced magnetic relaxation in the solid and solution state. The luminescence of polyTb, originating from an f-f transition, was observed (φ = 6.9%). polyTb formed a porous structure on a Si substrate, whereas a fibrous complex structure was formed on glass. polyTb chains are orientated on glass, which were determined by XRD.

Polypyridyl Metallo-Organic Assemblies for Electrochromic Applications

Electrochromic films undergo optical changes in response to a redox stimulus. This intriguing phenomenon can be used for a wide range of applications, including smart windows, sensors, color displays, and memory elements. Despite the rapid progress of late, designing suitable electrochromic materials that offer low-cost production, appealing colors, and pronounced optical contrast with high efficiency, as well as long-term stability remains an engineering challenge. Solid-state metal oxides, liquid crystals, and organic polymers have been for many years the leading candidates, successfully making their way into commercial products. An alternative class of materials relies on metal complexes that can be processed from solution, offer a variety of colors, and have metal-centered stable and reversible redox chemistry. These metallo-organic materials possess a full range of electrochromic properties, including ultrahigh coloration efficiencies, and cyclic stability. Here, some of the recent scientific developments in this field are highlighted.

Redox-Triggered Folding of Self-Assembled Coordination Polymers incorporating Viologen Units

We report the study of stimuli-responsive Zn^II and Fe^II coordination polymers (MC3⁴⁺ or MC2⁴⁺ with M=Fe²⁺ or Zn²⁺ ). These soluble metallopolymers were formed spontaneously by reaction of an organic ligand (C3⁴⁺ or C2⁴⁺ ) with one molar equivalent of metal ions. The C3⁴⁺ and C2⁴⁺ ligands incorporate two chelating terpyridine groups bridged by a redox responsive hinge featuring two viologen units (viologen=N,N'-dialkyl-4,4'-bipyridinium) linked either with propyl (C3⁴⁺ ) or ethyl (C2⁴⁺ ) chains. The viologen units in the polymer chains were reduced (1 e^- per viologen group) either by bulk electrolysis or by visible-light irradiation carried out in the presence of a photosensitizer. The 1 e^- reduction of the viologen units in the MC2⁴⁺ polymers induced a slight decrease in the viscosity of the solutions due to a modification of the overall charge carried by the metallopolymers. In strong contrast, reduction of coordination polymers involving propyl linkers (MC3⁴⁺ ) led to a remarkable increase (≈+400 %) in observed viscosity. This reversible effect was attributed to a folding of the polymer chains triggered by π-dimerization of the photo-generated viologen cation radicals.

Sub-phthalocyanine-incorporated Fe(ii) metallo-supramolecular polymer exhibiting blue-to-transmissive electrochromic transition with high transmittance and coloration efficiency

The preparation of a new Fe(ii) metallo-supramolecular polymer (poly-subPc-Fe) constructed from a terpyridine-functionalized sub-phthalocyanine with axially substituted polyisobutylene is presented.