Detailed study of layer-by-layer self-assembled and dip-coated electrochromic thin films based on metallo-supramolecular polymers

Near-IR Electrochromic Film with High Optical Contrast and Stability Prepared by Oxidative Electropolymerization of Triphenylamine Modified Terpyridine Platinum(II) Chloride

Terpyridine (TPY) platinum(II) chloride with a triphenylamine (TPA) group was successfully synthesized. The strong intramolecular Donor(TPA)-Acceptor(TPY) interaction induced the low-energy absorption band, mixing the spin-allowed singlet dπ(Pt)→π*(TPY) metal-to-ligand charge transfer (MLCT) with the chloride ligand-to-metal charge transfer (LMCT) and chloride ligand-to-ligand (TPY) charge transfer (LLCT) transitions, to bathochromically shift to λmax = 449 nm with significant enhancement and broadening effects. Using the cyclic voltammetry method, its oxidative electropolymerization (EP) films on working Pt disk and ITO electrodes were produced with tunable thickness and diffusion controlled redox behavior, which were characterized by the SEM, EDS, FT-IR, and AC impedance methods. Upon applying +1.4 V voltage, the sandwich-type electrochromic device (ECD) with ca. 290 nm thickness of the EP film exhibits a distinct color transformation from red (CIE coordinates: L = 50.75, a = 18.58, b = 5.69) to dark blue (CIE coordinates: L = 45.65, a = -1.35, b = -12.49). Good electrochromic (EC) parameters, such as a large optical contrast (ΔT%) of 78%, quick coloration and bleaching response times of 2.9 s and 1.1 s, high coloration and bleaching efficiencies of 278.0 and 390.5 C-1·cm2, and good cycling stability (maintains 70% of the initial ΔT% value after 3200 voltage switching cycles), were obtained.

Flexible Electrochromic Device on Polycarbonate Substrate with PEDOT:PSS and Color-Neutral TiO2 as Ion Storage Layer

Electrochromic (EC) windows on glass for thermal and glare protection in buildings, often referred to as smart (dimmable) windows, are commercially available, along with rearview mirrors or windows in aircraft cabins. Plastic-based applications, such as ski goggles, visors and car windows, that require lightweight, three-dimensional (3D) geometry and high-throughput manufacturing are still under development. To produce such EC devices (ECDs), a flexible EC film could be integrated into a back injection molding process, where the films are processed into compact 3D geometries in a single automized step at a low processing time. Polycarbonate (PC) as a substrate is a lightweight and robust alternative to glass due to its outstanding optical and mechanical properties. In this study, an EC film on a PC substrate was fabricated and characterized for the first time. To achieve a highly transmissive and colorless bright state, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) was used as the working electrode, while titanium dioxide (TiO₂) was used as the counter electrode material. They were deposited onto ITO-coated PC films using dip- and slot-die coating, respectively. The electrodes were optically and electrochemically characterized. An ECD with a polyurethane containing gel electrolyte was investigated with regard to optical properties, switching speed and cycling behavior. The ECD exhibits a color-neutral and highly transmissive bright state with a visible light transmittance of 74% and a bluish-colored state of 64%, a fast switching speed (7 s/4 s for bleaching/coloring) and a moderately stable cycling behavior over 500 cycles with a decrease in transmittance change from 10%to 7%.

Pathway-Dependent Coordination Networks: Crystals versus Films

We demonstrate the formation of both metallo-organic crystals and nanoscale films that have entirely different compositions and structures despite using the same set of starting materials. This difference is the result of an unexpected cation exchange process. The reaction of an iron polypyridyl complex with a copper salt by diffusion of one solution into another resulted in iron-to-copper exchange, concurrent ligand rearrangement, and the formation of metal–organic frameworks (MOFs). This observation shows that polypyridyl complexes can be used as expendable precursors for the growth of MOFs. In contrast, alternative depositions of the iron polypyridyl complex with a copper salt by automated spin coating on conductive metal oxides resulted in the formation of electrochromic coatings, and the structure and redox properties of the iron complex were retained. The possibility to form such different networks from the same set of molecular building blocks by “in solution” versus “on surface” coordination chemistry broadens the synthetic space to design functional materials.

Growth on Metallo-Supramolecular Coordination Polyelectrolyte (MEPE) Stimulates Osteogenic Differentiation of Human Osteosarcoma Cells (MG63) and Human Bone Marrow Derived Mesenchymal Stem Cells

BACKGROUND

Culturing of cells is typically performed on standard tissue culture plates generating growth conditions, which in general do not reflect the native three-dimensional cellular environment. Recent investigations provide insights in parameters, which strongly affect the general cellular behavior triggering essential processes such as cell differentiation. The physical properties of the used material, such as stiffness, roughness, or topology, as well as the chemical composition of the cell-surface interface are shown to play a key role in the initiation of particular cellular responses.

METHODS

We extended our previous research, which identified thin films of metallo-supramolecular coordination polyelectrolytes (MEPEs) as substrate to trigger the differentiation of muscular precursor cells.

RESULTS

Here, we show that the same MEPEs similarly stimulate the osteogenic differentiation of pre-osteoblasts. Remarkably, MEPE modified surfaces also trigger the differentiation of primary bone derived mesenchymal stem cells (BMSCs) towards the osteogenic lineage.

CONCLUSION

This result leads to the conclusion that these surfaces individually support the specification of cell differentiation toward lineages that correspond to the natural commitment of the particular cell types. We, therefore, propose that Fe-MEPEs may be used as scaffold for the treatment of defects at least in muscular or bone tissue.

On-Surface Self-Assembly of Stimuli-Responsive Metallo-Organic Films: Automated Ultrasonic Spray-Coating and Electrochromic Devices

We demonstrate the on-surface formation of homogeneous and uniform electrochromic films via ultrasonic spray coating. This fully automated process is capable of fabricating metallo-organic films on transparent conducting oxides (TCOs) on glass or flexible poly(ethylene terephthalate) (PET) with surface areas of up to 36 cm² and film thicknesses of half a micron. The assembly process involves alternatingly spray-coating dilute solutions of structurally well-defined iron polypyridyl ([Fe(mbpy-py)₃]²⁺) complexes and bis(benzonitrile)palladium dichloride (Pd(PhCN)₂Cl₂) onto conductive substrates, where the latter palladium salt was used as the inorganic cross-linker. The on-surface self-assembled three-dimensional networks are intensely colored and were subsequently integrated into laminated electrochromic devices (ECDs) containing a lithium-based gel electrolyte. The ECDs retain their intense color in the ground state, having a Δ T_max of 40-49% at λ_max ≈ 600 nm, and can be operated for up to 1500 redox cycles. The fluorine-doped tin oxide counter electrode coated with poly(3,4-ethylene-dioxythiophene)polystyrene sulfonate (PEDOT:PSS) as a charge-storage layer resulted in these stable devices. A significant decrease in the potential window of Δ E ≈ 2.5 V was achieved by using a metal grid on PET as the counter electrode. The operation of the electrochromic films is diffusion-controlled, and the diffusion coefficients ( D_f) reflect their molecular densities. During these studies, we found that ClO₄^- is a suitable counterion of the lithium-based electrolytes for optimal ECD performance.

Metallopolymers for advanced sustainable applications

Since the development of metallopolymers, there has been tremendous interest in the applications of this type of materials. The interest in these materials stems from their potential use in industry as catalysts, biomedical agents in healthcare, energy storage and production as well as climate change mitigation. The past two decades have clearly shown exponential growth in the development of many new classes of metallopolymers that address these issues. Today, metallopolymers are considered to be at the forefront for discovering new and sustainable heterogeneous catalysts, therapeutics for drug-resistant diseases, energy storage and photovoltaics, molecular barometers and thermometers, as well as carbon dioxide sequesters. The focus of this review is to highlight the advances in design of metallopolymers with specific sustainable applications.

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

The structure and properties of metallo-supramolecular polyelectrolytes (MEPEs) self-assembled from rigid 2,6-bis(2-pyridyl)pyrimidine and the metal ions Fe^II and Co^II are presented. While FeL1-MEPE (L1 = 1,4-bis[2,6-bis(2-pyridyl)pyrimidin-4-yl]benzene) is deep blue, FeL2- and CoL2-MEPE (L2 = 5,5'-bis[2,6-bis(2-pyridyl)pyrimidin-4-yl]-2,2'-bithiophene) are intense green and red in color, respectively. These novel MEPEs display a high extinction coefficient and solvatochromism. Ligand L2 shows a high absolute fluorescence quantum yield (Φ_f = 82%). Viscosity and static light-scattering measurements reveal that the molar masses of these MEPEs are in the range of 1 × 10⁸ g/mol under the current experimental conditions. In water, FeL1-MEPE forms a viscous gel at 20 °C (c = 8 mM). Thin films of high optical quality are fabricated by dip coating on transparent conducting indium tin oxide (ITO) glass substrate. Optical, electrochemical, and electrochromic properties of the obtained MEPEs are presented. Green to red and blue to colorless electrochromism is observed for FeL2-MEPE and FeL1-MEPE, respectively. The results show that the electrochromic properties are affected by the ligand topology. The Fe-MEPEs show a reversible redox behavior of the Fe^II/Fe^III couple at 0.86 and 0.82 V versus Fc⁺/Fc and display an excellent redox cycle stability under switching conditions. FeL2-MEPE in its oxidized state exhibits a broad absorption band covering the near-IR region (ca. 1500 nm) due to the ligand-to-metal charge transfer transition originating due to charge delocalization in the bithiophene spacer.

A Thiophene-Containing Conductive Metallopolymer Using an Fe(II) Bis(terpyridine) Core for Electrochromic Materials

Three Fe(II) bis(terpyridine)-based complexes with thiophene (Fe(L1)₂), bithiophene (Fe(L2)₂), and 3,4-ethylenedioxythiophene (Fe(L3)₂) side chains were designed and synthesized for the purpose of providing two terminal active sites for electrochemical polymerization. The corresponding metallopolymers (poly-Fe(Ln)₂, n = 2 or 3) were synthesized on indium tin oxide (ITO)-coated glass substrates via oxidative electropolymerization of the thiophene-substituted monomers and characterized using electrochemistry, X-ray photoelectron spectroscopy, UV-vis spectroscopy, and atomic force microscopy. The film poly-Fe(L2)₂ was further studied for electrochromic (EC) color-switching properties and fabricated into a solid-state EC device. Poly-Fe(L2)₂ films exhibit an intense MLCT absorption band at 596 nm (ε = 4.7 × 10⁴ M^-1 cm^-1) in the UV-vis spectra without any applied voltage. Upon application of low potentials (between 1.1 and 0.4 V vs Fc⁺/Fc), the obtained electropolymerized film exhibited great contrast with a change of transmittance percentage (ΔT%) of 40% and a high coloration efficiency of 3823 cm² C^-1 with a switching time of 1 s. The film demonstrates commonplace stability and reversibility with a 10% loss in peak current intensity after 200 cyclic voltammetry cycles and almost no loss in change of transmittance (ΔT%) after 900 potential switches between 1.1 and 0.4 V (vs Fc⁺/Fc) with a time interval of 0.75 s. The electropolymerization of Fe(L2)₂ provides convenient and controllable film fabrication. Electrochromic behavior was also achieved in a solid-state device composed of a poly-Fe(L2)₂ film and a polymer-supported electrolyte sandwiched between two ITO-coated glass electrodes.

Anion-induced palladium nanoparticle formation during the on-surface growth of molecular assemblies

We demonstrate a process that results in the formation of palladium nanoparticles during the assembly of molecular thin films. These nanoparticles are embedded in the films and are generated by a chemical reaction of the counter anions of the molecular components with the metal salt that is used for cross-linking these components.

Efficient CO2 capture by metallo-supramolecular polymers as fillers to fabricate a polymeric blend membrane

Novel fillers based on metallo-supramolecular polymers were incorporated into PEBA2533 to obtain blend membranes, showing excellent CO₂ permeability and CO₂/N₂ selectivity.