Electrochromic Os(II)-Based Metallo-Supramolecular Polymers

Stepwise introduction of three different transition metals in metallo-supramolecular polymer for quad-color electrochromism

Metallo-supramolecular polymers (MSPs) show unique electrochemical and optical properties, that are different to organic polymers, caused by electronic interactions between metals and ligands. For the development of quad-color electrochromic materials, here we report the stepwise introduction of three different transition metal ions into an MSP, utilizing the different complexation abilities of the transition metals. An MSP with Os(II), Ru(II), and Fe(II) (polyOsRuFe) was synthesized via a stepwise synthetic route through the formation of an Os(II) complex first, followed by the introduction of Ru(II) to the Os(II) complex, and finally the attachment of Fe(II) to the Os(II)-Ru(II) complex to produce the polymer. This synthetic procedure was extended to fabricate MSPs that comprised Co(II)/Ru(II)/Os(II) and Zn(II)/Ru(II)/Os(II). The synthesized MSPs showed a broad optical and electrochemical window due to the coupling of three heterometallic segments into the polymer. Introducing acetate anion as the counter anion greatly enhanced the solubility of polyOsRuFe in methanol. A thin film of polyOsRuFe was prepared on ITO/glass by spin-coating the methanol solution, and its reversible quad-color electrochromism was demonstrated.

Synthesis of Bis-Terpyridine-Based Metallopolymers and the Thermoelectric Properties of Their Single Walled Carbon Nanotube Composites

Although the organic and the conventional inorganic thermoelectric (TE) materials have been extensively developed in recent years, the number of cases involving conducting metallopolymers is still quite limited. In view of the versatile coordination capability of the terpyridine fraction and the electron-rich nature of the 3,4-ethylenedioxythiophene moiety, a bis-terpyridine-featured ligand was designed, and a series of metallopolymers were then synthesized. Upon the addition of single-walled carbon nanotube (SWCNT), the TE properties of the resulting metallopolymer-SWCNT composite films were investigated. It was found that metal centres played an important role in affecting the morphology of the thin films, which was a key factor that determined the TE performances of the composites. Additionally, the energy levels of the metallopolymers were feasibly tuned by selecting different metal centres. With the combined effects of a uniform and condensed surface and an optimized band structure, the highest power factor was achieved by the Cu(II)-containing metallopolymer-SWCNT composite at the doping ratio of 75%, which reached 38.3 μW·m-1·K-2.

Synthesis of an Alternated Heterobimetallic Supramolecular Polymer Based on Ru(II) and Fe(II)

A heterobimetallic supramolecular polymer (polyRuFe) with alternately complexed Ru(II) and Fe(II) is prepared following a stepwise synthetic route through harnessing first the strongly binding metal ion Ru(II) and then the weakly binding metal ion Fe(II). A high yield of product is achieved in each step. The heterometal ions are incorporated into the polymer chain in identical coordination environments formed by two 2,2′:6′,2″-terpyridine moieties. Characterization is accomplished by NMR spectroscopy, MALDI–TOF mass spectrometry, UV–Vis spectroscopy, and cyclic voltammetry. PolyRuFe shows a wide optical window (λ = 311–577 nm) and a broad distinct reversible redox nature of two types, originated from the coupling of the two heterometallic segments into the polymer chain. Such characteristics of polyRuFe suggest its potential for various electrochemical and electro-optical applications.

Yellow-to-brown and yellow-to-green electrochromic devices based on complexes of transition metal ions with a triphenylamine-based ligand

Transmissive-to-colored electrochromism has been achieved by combination of MLCT of transition metal complexes with the electrochromic properties of ligand molecules. The color transitions were from yellow to dark brown for the Fe(ii) complex, yellow to orange to bluish-green for the Co(ii) complex and yellow to green for the Zn(ii) complex. By using a metal ion-ligand coordination approach, the self-assembly of hydrazone-based ligands containing a triphenylamine group with appropriate metal salts (FeCl2, Co(ClO4)2 and Zn(BF4)2) produced novel complexes of the general formula [ML2]X2. The isolated complexes were characterized by spectroscopic methods, and the Co(ii) complex also by X-ray diffraction analysis. Thin films of the complexes have been obtained by a spray-coating method and they were used in the construction of electrochromic devices, which showed good electrochromic stability, a high color contrast of 47.5% for Fe(ii), 37.2% for Co(ii) and 33.7% for Zn(ii) complexes and fast coloring and bleaching times.

Bis(terpyridine) Iron(II) Functionalized Vertically-Oriented Nanostructured Silica Films: Toward Electrochromic Materials

Recent and potential applications of electrochromic materials include smart windows, optoelectronic devices, and energy conversion. In this study, we have incorporated bis(terpyridine) iron (II) complexes into vertically-oriented silica thin films deposited on indium-tin oxide (ITO) and their electrochromic behavior has been investigated. If 2,2′:6′,2″-terpyridine is commonly used as a ligand for forming metallo-supramolecular assemblies, with the objective to get metal-terpyridine complexes with multiple stable redox states, their simple and reliable arrangement into linear structures enabling effective electronic communication is however more challenging. We propose to overcome this difficulty by generating such complexes within vertical nanochannels on electrode. Terpyridine ligands were firstly immobilized by combining a click chemistry azide/alkyne approach with an electrochemically-assisted self-assembly (EASA) method used to grow an oriented mesoporous silica membrane bearing azide groups which were further derivatized with 4′-ethynyl-terpyridine ligands. The resulting terpyridine-functionalized films were consecutively dipped in an aqueous solution of Fe(BF₄)₂ and then in a solution of terpyridine in acetonitrile to form the bis(terpyridine) iron (II) complexes in situ. The electrochromic properties of the films functionalized at various levels were examined by monitoring the changes in their UV/Vis spectra upon electrochemical oxidation at controlled potential of +1.2 V vs. Ag/AgCl. Due to facile charge delocalization during the Fe²⁺ to Fe³⁺ redox process, the bis(terpyridine) iron (II) functionalized silica films exhibited electrochromic properties by changing from violet to non-colored using TBABF₄ in acetonitrile as an electrolyte. The bis(terpyridine) iron(II) film experienced reversible electrochromic switching by applying +0.5 V in a reverse reduction electrochemical process. The Fe(tpy)₂-functionalized silica thin films displayed a good contrast ratio (ΔT%) of 47% and relatively high coloration efficiency (CE) of about 245 cm²/C with a response time of coloring and bleaching of a few seconds (< 4 s).

Interfacial Coordination Nanosheet Based on Nonconjugated Three-Arm Terpyridine: A Highly Color-Efficient Electrochromic Material to Converge Fast Switching with Long Optical Memory

An electrochromic (EC) hyperbranched coordination nanosheet (CONASH) comprising a three-arm terpyridine (3tpy)-based ligand and Fe(II) ion has been synthesized by interfacial complexation at the liquid-liquid interface. The film can be easily deposited on the desired substrate such as indium tin oxide (ITO) glass. Characterization of CONASH deposited on ITO by microscopic methods reveals the homogeneous nanosheet film with an ∼350 nm thickness after 48 h of reaction. The fabricated solid-state EC device (ECD) undergoes a reversible redox reaction (Fe²⁺ → Fe³⁺) in the potential range of +3 to -2 V in ECDs accompanied with a distinct color change from intense pink to colorless for several switching cycles with a coloration time of 1.15 s and a bleaching time of 2.49 s along with a high coloration efficiency of 470.16 cm² C^-1. Besides, the nonconjugated 3tpy ligand restricts the easy electron redox conduction inside the EC film to enhance the EC memory in open-circuit condition as it shows 50% retention of its colorless state until 25 min. The long EC memory compared to other metallo-supramolecular polymers having a conjugated ligand suggests the potentiality of the 3tpy-Fe CONASH film to be used as a power-efficient EC material for modern display device applications.

Electrochromic Os-based metallo-supramolecular polymers: electronic state tracking by in situ XAFS, IR, and impedance spectroscopies

In this study, the electronic states of Os-based metallo-supramolecular polymers (poly(OsL)²⁺) during electrochromism were tracked by in situ X-ray absorption fine structure (XAFS), infrared (IR), and impedance spectroscopies. The XAFS spectra suggested electronic charge migration in the polymer, and the in situ spectra revealed reversible changes caused by electrochemical redox reactions. The IR spectra of the polymers showed an IVCT band, and we also confirmed the reversible changes by applying a voltage to the redox cell. During the impedance measurements, we found a drastic decrease in the charge transfer resistance (R_CT) of the polymer films near the electrochemical redox potential.

In this study, the electronic states of Os-based metallo-supramolecular polymers (poly(OsL)²⁺) during electrochromism were tracked by in situ X-ray absorption fine structure (XAFS), infrared (IR), and impedance spectroscopies.

Constructing Alternated Heterobimetallic [Fe(II)/Os(II)] Supramolecular Polymers with Diverse Solubility for Facile Fabrication of Voltage-Tunable Multicolor Electrochromic Devices

Metallo-supramolecular polymer (MSP)-based electrochromic devices (ECDs) have drawn much attention because of their variable colors and attractive electrochromic (EC) properties. However, fabrication of voltage-tunable multicolor ECDs using single MSP is yet hard to realize. We anticipated alternate introduction of two different redox-active metal ions in an MSP combined with the adjustment of counteranions could be a solution to fabricate multicolor ECDs. The heterometals will induce color variability upon voltage alteration, and counteranions will help to tune the solubility of MSP in different solvents. In an attempt to fulfill this target, we have synthesized four heterobimetallic supramolecular polymers (HBPs) having different counteranions (BF₄^-, Cl^-, PF₆^-, and OAc^-), in which Fe(II) and Os(II) are alternately complexed by two terpyridine units. To apply as EC material, the HBPs should be soluble in methanol and insoluble in acetonitrile for the preparation of EC film as well as ECDs. However, among the HBPs, only HBP-OAc is found to meet this requirement. The EC behaviors of the spray-coating film of HBP-OAc on an indium tin oxide (ITO)-coated glass substrate are investigated in terms of maximum transmittance contrast, coloration voltage, response time, coloration efficiency, and operational stability, which exhibits reversible multicolor electrochromism (the initial purple color of the film is changed to violet followed by greenish-yellow) upon alteration of the voltage from 0.0 to 0.7 V [required to oxidize the Os(II) ion] and to 1.0 V [required to oxidize the Fe(II) ion]. The film is also integrated into a laminated ECD by using lithium-based gel electrolyte. Finally, as a proof-of-concept, a prototype voltage-tunable multicolor EC display (6 cm × 2.5 cm) is fabricated by using a designed image containing a flower, leaves, and a flower pot, which exhibits six different types of multicolor image upon application of tunable voltages.

Reversible four-color electrochromism triggered by the electrochemical multi-step redox of Cr-based metallo-supramolecular polymers

Four color electrochromism (yellow, magenta, blue, and navy) has been achieved in Cr(iii)-based metallo-supramolecular polymers (polyCr), which were synthesized by 1 : 1 complexation of Cr ions and 1,4-di[[2,2′:6′,2′′-terpyridin]-4′-yl]benzene (L).

Construction of Coordination Nanosheets Based on Tris(2,2'-bipyridine)-Iron (Fe2+) Complexes as Potential Electrochromic Materials

The coordination nanosheets (CONASHs) are emerging as a new class of functional two-dimensional materials, which are one of the most active research areas of chemistry and physics in this decade. Despite the success of various structural and functional CONASHs, the development of a new molecular structure to discover alluring functional CONASHs remains challenging. Herein, we report successful preparation of two novel CONASHs (NBP1 and NBP2) through coordination between one of the unexplored molecular frameworks of bis(2,2'-bipyridine)-based ligands (BP1 and BP2) and Fe²⁺ ions. Using a liquid-liquid interface as a platform, large-scale thin films of multilayer CONASHs have been prepared without any support, which can be deposited onto any desired substrate. Detailed characterization of the CONASHs using various microscopic and spectroscopic techniques reveals homogeneous and flat morphology of nanometer thickness with the quantitative formation of tris(2,2'-bipyridine)-Fe²⁺ complex motifs in the nanosheet frameworks. The color of the films has been tuned from blue to magenta by the suitable molecular design of the ligands. Owing to the insolubility of the CONASH films in any solvent and the presence of redox-active Fe²⁺, we explore the functionality of these nanostructured thin films deposited on indium tin oxide as electrochromic materials. The CONASHs exhibit color-to-colorless and color-to-color electrochromic transitions with attractive response times, switching stabilities, and coloration efficiencies. Finally, we demonstrate solid-state electrochromic devices of the CONASHs operated at a potential range of +2.5 to -2.5 V, which are electrochemically stable for several switching cycles, suggesting that these CONASHs are potential electrochromic materials for next-generation display applications.

Multi-color electrochromism from coordination nanosheets based on a terpyridine-Fe(ii) complex

A new metal complex electrochromic nanosheet with multiple color electrochromism, fast switching speed and excellent cyclic stability was prepared controllably by the liquid–liquid interface self-assembly method.