Keywords

3.1 Synthesis

Bis(terpyridine)s are widely used as ditopic ligands for metallo-supramolecular polymer synthesis because of their high coordination ability. In particular, transition metal ions with an octahedral coordination such as Fe(II) and Ru(II) fit into the tridentate coordination sites on the ligands. Similar to the Fe(II)-based polymers, Ru(II)-based metallo-supramolecular polymers (polyRuL1-5) are prepared by the 1:1 complexation of a Ru(II) salt and bis(terpyridine)s (L1-5) (Fig. 3.1) [1]. The complexation conditions are different from those for the Fe(II)-based polymers. The typical synthetic procedure is as follows. Equimolar amounts of L1 and RuCl2(DMSO)4 are stirred at 130 °C in argon-saturated absolute ethylene glycol (ca. 10 mL of solvent per mg of L1) for 24 h. After the solution is cooled to room temperature, THF is added until the solution becomes colorless. The precipitated polymer is collected by filtration, washed twice with THF, and then dried under vacuum overnight to give polyRuL1 (>95%). The complexation in the polymer synthesis is confirmed by the color change of the reaction mixture to red, because the complexes have MLCT absorptions.

Fig. 3.1
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a Ru(II)-based metallo-supramolecular polymers (polyRuL1-5)

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In general, the chain length of metallo-supramolecular polymers in solution is difficult to determine because the position of equilibrium involving the coordination of the polymer changes with the polymer chain length. However, the linear polymer structure of polyRuL1 can be observed as bright threads in an atomic force microscopy (AFM) image of solid-state polyRuL1 cast on a Si substrate from the dilute methanol solution. The observed height (1–2 nm) indicates that the bright threads are single polymer chains or small bundles of several polymer chains, because the polymer chain width estimated by molecular modeling is about 1 nm. In the AFM image, the polymer chain length is more than 500 nm. Since the distance between two Ru ions in the polymer is estimated to be about 1.6 nm by molecular modeling, the calculated degree of polymerization (DP) should be more than 300, which corresponds to a molecular weight of 2.1 × 105 Da.

3.2 Red Electrochromism

Similar to polyFeL1-5, the optical properties of polyRuL1-5 are related to the MLCT absorption. In the UV-vis spectra of polyRuL1-5, the MLCT absorption of the Ru(II) complex moieties appears around 520 nm. The maximum wavelength (λmax) and absorption coefficient (ε) are summarized in Table 3.1. Similar to the Fe(II) polymers, the introduction of a biphenyl group into the ligand as a spacer leads to a blue shift of the absorption. The introduction of methoxy groups into the ligand causes a large decrease in ε and a red shift of λmax by 20 nm. The introduction of bromo groups results in a large decrease in ε and a blue shift of λmax. The different substituent effects on λmax and ε between polyFeL1-5 and polyRuL1-5 depend on subtle differences in the structures of the metal complexes. As an overall tendency, the MLCT absorption in polyRuL1-5 appears at a shorter wavelength and has a higher ε than that in polyFeL1-5, probably because of the stronger π-backbonding of L1-5 to Ru(II) than to Fe(II) and a stronger dynamic chelate effect of L1-5 to Ru(II) than to Fe(II).

Table 3.1 Optical and electrochemical properties of Ru(II)-based metallo-supramolecular polymers (polyRuL1-5)
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In cyclic voltammograms (CVs) of polyRuL1-5, a redox wave is observed owing to the redox of Ru(II)/(III). The results clearly show the shift to a lower oxidative potential by the introduction of electron-releasing groups (methoxy groups) and a shift to a higher oxidative potential by the introduction of electron-withdrawing groups (bromo groups), because the strong electron-donating groups stabilize the oxidized Ru(III) state and the electron-withdrawing groups destabilize it. The trend is quite similar to that observed in polyFeL1-5.

The film of polyRuL1-5 on an ITO glass shows electrochromic properties from red to colorless when an oxidative potential is applied to the film in an electrolyte solution, because Ru(II) is oxidized to Ru(III) electrochemically and the MLCT absorption disappears. The colorless film shows the original red color when a reductive potential is applied to the film.

3.3 Fe(II)/Ru(II)-Based Heterometallo-Supramolecular Polymers

Heterometallo-Supramolecular polymers are metallo-supramolecular polymers containing more than two metal ion species. The introduction of more than two metal ion species to a metallo-supramolecular backbone itself is not difficult. The polymers may be obtained by mixing different metal salts and a ditopic ligand in solution. However, precise control of the metal sequence is quite difficult, because simple mixing gives a polymer with the metal ions randomly introduced. To control the position of the two metal ion species in the polymer, we focused on the different reaction conditions for metal ions with the ligand. If the reaction conditions for the ligand are significantly different between the two metal ion species, the polymer with two metal ion species introduced alternately can be prepared by two-step complexations of the metal ions with the ligand (Fig. 3.2a) [2].

Fig. 3.2
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a Synthetic strategy for heterometallo-supramolecular polymers. b Synthesis of Fe(II)/Ru(II)-based heterometallo-supramolecular polymers (polyFe0.5Ru0.5L1)

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At first, the 1:2 complex of one metal with a ligand is prepared under the complexation conditions; then the polymer with two metal ion species introduced alternately is obtained by the complexation of the 1:2 complex with another metal ion species. In the two-step complexations, the complexation with the more severe reaction conditions should be the first complexation. If the complexation with the milder reaction conditions is selected as the first complexation, exchange between the two metal ion species can occur during the second complexation with the more severe reaction conditions. According to this synthetic strategy, a series of Fe/Ru-based heterometallo-supramolecular polymers are synthesized by the stepwise coordination of Fe(II) and Ru(II) ions to L1. The molar ratio of Fe(II) to Ru(II) in the polymer can be controlled by changing the molar ratio in the polymerization. The polymer with the 1:1 molar ratio of Fe(II) to Ru(II) (polyFe0.5Ru0.5L1) is shown in Fig. 3.2b.

The molar ratio of Fe(II) to Ru(II) in the heterometallo-supramolecular polymer can be changed by changing the seeding molar ratio of the metal salts. The polymers with the 3:1, 1:1, and 1:3 molar ratios of Fe(II) to Ru(II) (polyFe0.75Ru0.25L1, polyFe0.5Ru0.5L1, and polyFe0.25Ru0.75L1, respectively) are synthesized by seeding the calculated molar ratios of Fe(BF4)2 to RuCl2(DMSO)4 (Table 3.2). The preparation is as follows. First, L1 and RuCl2(DMSO)4 are added in argon-saturated absolute ethylene glycol (EG) and stirred at 130 °C for 24 h. After the reaction, Fe(BF4)2 dissolved in EG is added into the reaction mixture and stirred at 80 °C for 24 h. Following the same purification method used for polyRuL1, the polymers are obtained in >90% yields. Molecular weight of the polymers couldn’t be determined by GPC measurement because of the decomplexation in the GPC column.

Table 3.2 Seeding molar ratios of Fe(II), Ru(II), and L1 in the synthesis of Fe(II)/Ru(II)-based heterometallo-supramolecular polymers
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The redox potentials (E1/2) of Fe(II)/(III) and Ru(II)/(III) in the polymers are determined by cyclic voltammetry (CV) and summarized in Table 3.3. The heterometallo-supramolecular polymers have two redox waves based on the redox of Fe(II)/(III) and Ru(II)/(III). The data show that the redox potential of Fe(II)/(III) is positively shifted as the ratio of Ru ions in the polymers increases, and that of Ru(II)/(III) is negatively shifted as the ratio of Fe ions in the polymers increases. These shifts clearly suggest that the intramolecular metal–metal interactions occur between the adjacent Fe and Ru ions through the π-conjugated spacer in the ligand.

Table 3.3 Oxidation and reduction potentials of Fe(II)/Ru(II)-based heterometallo-supramolecular polymers
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3.4 Multi-Color Electrochromism

The colors of polyFeL1 and polyRuL1 in solution are blue and red, because the complementary color of the MLCT absorption at 585 or 513 nm in the Fe(II)- or Ru(II)-terpyridine complex moieties is seen, respectively. Since the Fe(II)/Ru(II)-based heterometallo-supramolecular polymers (polyFe0.75Ru0.25L1, polyFe0.5Ru0.5L1, and polyFe0.25Ru0.75L1) have both absorptions, the polymers display different colors (bluish purple, purple, and reddish purple, respectively) based on the different absorbance ratios of the two MLCT absorptions. Interestingly, the heterometallo-supramolecular polymers show multi-color electrochromism by changing the applied potential from 0 to 1.1 V versus Ag/Ag+. The multi-color electrochromism in a polyFe0.5Ru0.5L1 film (purple at 0 V vs. Ag/Ag+, orange at 0.9 V, and light green at 1.2 V) was investigated by in situ UV–vis spectral measurements, which monitored the absorbance changes while applying a potential. At 0.7 V versus Ag/Ag+, only the MLCT absorption at 585 nm attributed to the Fe(II) complex moieties decreased slightly. At 0.8 V versus Ag/Ag+, the absorption decreased greatly, and that of Ru(II) at 508 nm also began to decrease. At 0.9 V versus Ag/Ag+, the absorption of Fe(II) almost disappears, but that of Ru(II) still remains. Finally, the absorption of Ru(II) totally disappears at 1.1 V versus Ag/Ag+. Since the oxidation potentials of Fe(II) and Ru(II) ions in the polymer are different, the two MLCT absorptions disappear stepwise due to the electrochemical oxidation of Fe(II) to Fe(III) and the subsequent oxidation of Ru(II) to Ru(III). This electrochromic performance is summarized in Table 3.4. The polymer films show high optical contrast and very fast response times: the transmittance changes (ΔT) at 508 and 585 nm are 68% and 37%, respectively, and the color changes end at 0.4 and 1.5 s for coloring and bleaching, respectively. In addition, high durability for repeated color changes is confirmed by applying 0 and 0.9 V repeatedly with only 0.7 and 1.8% charge loss in 5,000 and 10,000 cycles, respectively.

Table 3.4 Electrochromic properties of a polyFe0.5Ru0.5L1 film when switched between 0 and 0.9 V, and 0 and 1.1 V at 5 s intervals
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3.5 Flexible Electrochromic Devices

Fe(II)- and Ru(II)-based metallo-supramolecular polymers are soluble in polar solvents such as water and methanol. The polymer film can be prepared using spin coating and spray coating on an ITO electrode. Inkjet printing is another method of preparing the polymer film. Mixed color films of blue and red are prepared on ITO substrates (ITO glass: 6.8 Ω/□; flexible ITO-PEN (polyethylene-naphthalate): 35 Ω/□) by inkjet printing different ratios of polyFeL1 and polyRuL1 (4/0, 3/1, 2/2, 1/3, 0/4) (Fig. 3.3a) [3]. A 1.0 wt% methanol solution of each polymer is prepared and diluted with an equal volume of deionized water to reduce nozzle clogging problems in the inkjet-printing process. The inks are placed in two cartridges and printed one after another in the printing process for the color-mixing thin films. Droplets 70 μm in diameter are ejected from a nozzle 50 μm in diameter at a speed of 1.01 m/s and a frequency of 500 Hz with a dot spacing of 50 μm for all printed patterns. The substrate is heated at 35 °C during printing. The devices are fabricated with a transparent solid-state electrolyte thin film (Fig. 3.3b). The electrolyte solution is prepared by mixing poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP), 1-ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl) amide (EMIBTI), and acetone (weight ratio: 1:4:7). The solution is poured into a glass Petri dish and dried overnight in a vacuum oven at 70 °C for 24 h. The dried thin film (500 μm thick) is then peeled off and cut to the proper size for the device fabrication.

Fig. 3.3
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a Dot images of inkjet printing with polyFeL1 and polyRuL1 at different ratios from 4/0 to 0/4. b A flexible solid-state device structure with a printed electrochromic layer

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Cyclic voltammograms of the inkjet-printed polymer films show only small changes in the redox potentials of Fe(II)/(III) and Ru(II)/(III) when the molar ratio of polyFeL1 and polyRuL1 is changed in the printing (Table 3.5), unlike the case of Fe/Ru-based heterometallo-supramolecular polymers, in which Fe and Ru ions electronically interact through the π-conjugated ditopic ligand. It is considered that most of the two polymers exist individually in the polymer film prepared by the inkjet-printing. However, the slight shifts of redox potential shown in Table 3.5 indicate that a small amount of block-copolymer with both Fe and Ru ions was formed due to the equilibrium reaction in solution during the inkjet-printing. The solid-state devices show multicolored electrochromic behavior. The effect of bending the electrochromic devices is investigated using flexible solid-state devices with polyFeL1 or polyRuL1 (Table 3.6). The use of PEN with higher resistance than ITO as the electrode results in longer response times (tcoloring and tbleaching). When the properties of the devices are compared between the flat and bent states, the performance in the bent state is worse than that in the flat state, but the properties in the bent state are still good.

Table 3.5 Oxidation and reduction potentials of polyFeL1 and polyRuL1 in inkjet-printed polymer films
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Table 3.6 Electrochromic properties of flexible solid-state devices with polyFeL1 or polyRuL1
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3.6 Luminescence

PolyRuL1-5 have the ligand-centered (LC) and MLCT absorptions (Table 3.7). Luminescence appears owing to the excitation of the LC and MLCT bands, but the quantum yield (Φlum) is low (<10−6) at room temperature (Table 3.8). At 77 K, however, a strong luminescence from the excited MLCT absorption is observed [4]. The unsubstituted polyRuL1 exhibits the highest efficiency (Φlum: 2.47 × 10−2) among the polymers. The electron-withdrawing bromo-substituted polyRuL5 shows at least a threefold stronger luminescence with Φlum than the electron-releasing methoxy-substituted polyRuL3. PolyRuL3 shows a lower emission energy than polyRuL1, whereas polyRuL5 exhibits a higher energy. The luminescence lifetimes (τ), which are measured by the time-correlated single-photon counting technique using a picosecond diode laser as the exciter, are on a nanosecond timescale at room temperature. The luminescence lifetimes at 77 K are on a microsecond timescale.

Table 3.7 UV-vis spectral data for polyRuL1-5 at room temperature
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Table 3.8 Emission data for polyRuL1-5 at room temperature and 77 K
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3.7 Electrochemical Switching of Emission

To electrochemically switch the emission, a solid-state device with a polyRuL1 film is fabricated using a gel electrolyte and two transparent ITO electrodes (Fig. 3.4a) [5]. A polyRuL1 film is prepared by spin-coating the polymer in a propanol/methanol (1:1) solution (5 g/L) on an ITO substrate. The gel electrolyte is made by mixing poly(methyl methacrylate) (PMMA), propylene carbonate, and lithium perchlorate. In the device, the MLCT absorption is observed at 500 nm, and the absorption disappears by applying 2.5 V between the two ITO electrodes of the device. The colorless state is restored to the original state by applying −2.5 V. The electrochromism is caused by the electrochemical redox of Ru ions. The Ru(II) complex has a MLCT absorption, but the Ru(III) complex does not show this absorption. The original redox potential of Ru(II)/(III) in polyRuL1 is 0.95 V versus Ag/Ag+, but the application of a more oxidative potential is required to oxidize Ru(II) to Ru(III) in the device due to the internal resistance of the device. In addition, since the internal resistance is different among the devices, the voltage required to oxidize Ru(II) to Ru(III) changes for each device.

Fig. 3.4
figure 4

a Colored state (left) and colorless state (right) of a solid-state device with polyRuL1. b A proposed mechanism of electrochemical switching of emissions in the device

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Photoluminescence in a solid-state device using polyRuL1 is measured using microscopic spectroscopy at room temperature. When a green laser (532 nm) for the excitation of the MLCT absorption in the Ru(II) complex moieties is focused on the polymer film in the device using an objective lens, a luminescent peak at around 720 nm is observed in the photoluminescence spectrum. The emission from the polymer film is collected with an objective lens and the laser light is removed by a notch filter. The emission is detected by a Si detector with a spectrometer (all spectra are taken to account for the background and quantum efficiency of a Si detector). The luminescent peak is attributed to the emission from the 3MLCT states of the Ru(II) complexes, because an electron excited by the laser light (532 nm) transits from the 1MLCT band of the Ru(II) complexes to the 3MLCT band via intersystem crossing and radiatively decays to the ground state (Fig. 3.4b). The intersystem crossing often occurs because of the strong spin-orbit coupling in heavy atoms such as Ru(II). In this device, the MLCT absorption disappears at 1.5 V, which means that Ru(II) ions in polyRuL1 are totally oxidized to Ru(III) at 1.5 V. The emission also disappears at this voltage and can be almost restored to its original intensity by applying the reverse voltage (−1.5 V). The quenched and emitted states are reversibly switched by applying the two voltages alternately. The quenching is likely caused by the disappearance of the MLCT absorption based on the oxidation of Ru(II) to Ru(III).