Article pubs.acs.org/JACS
Electrochromic Metallo-Organic Nanoscale Films: Fabrication, Color Range, and Devices Neta Elool Dov,† Sreejith Shankar,†,‡ Dana Cohen,† Tatyana Bendikov,§ Katya Rechav,§ Linda J. W. Shimon,§ Michal Lahav,*,† and Milko E. van der Boom*,† †
Department of Organic Chemistry and §Department of Chemical Research Support, Weizmann Institute of Science, 7610001 Rehovot, Israel S Supporting Information *
ABSTRACT: In this study, we demonstrate a versatile approach for the formation of electrochromic nanoscale assemblies on transparent conductive oxides on both rigid and flexible substrates. Our method is based on the application of alternating spin-coated layers of well-defined metal polypyridyl complexes and a palladium(II) salt to form electrochemically addressable films with a high chromophore density. By varying the central metal ion of the polypyridyl complexes (Os, Ru, and Fe) and their ligands and by mixing these complexes, coatings with a wide range of colors can be achieved. These coatings cover a large area of RGB color space. The coloration intensities of these nanoscale films can be tuned by the number of deposition steps. The materials have very attractive ON/OFF ratios, electrochemical stabilities, and coloration efficiencies. Reversible color-to-colorless and color-to-color transitions were demonstrated, and the films were further integrated into sandwich cells.
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INTRODUCTION The stimuli-induced modulation of physicochemical properties in synthetic materials has marked a watershed moment in the field of functional supramolecular chemistry. Molecules and materials that are responsive to light, heat, electricity, pH, and chemicals have been extensively studied over the past few years.1−5 Among the different stimuli-responsive approaches, electrically stimulated optical switching has been viewed as a potential and powerful strategy for the realization of smart optoelectronic materials and devices.6−10 Increasing demand for smart windows, improved display technologies, optical memory devices, and sensors has generated an overwhelming interest in such materials. Electrochromism (EC), whereby a material changes its absorbance or reflectance through an electrochemical redox process, offers the possibility of reversibly and persistently modulating the material’s optical response. Notably, in the context of extended energy demands and environmental concerns, EC materials and coatings can provide an important technology to benefit energy efficiency, through dynamic control of indoor lighting and temperature.11,12 The choice of practical EC materials has been limited by the industrial demands for high durability, high contrast ratios and coloration efficiencies, short switching and response times, and low operating voltages. Commendable progress has been made in the development of conjugated polymers and inorganic oxides as electrochromic materials.13−15 Despite the great strides in this field, however, the design of suitable electrochromic materials (e.g., metal-oxide films, conductive polymers, and liquid crystals) remains a challenging task. Excellent contrast ratios (80%) have © 2017 American Chemical Society
been reported for tungsten oxide, and even larger values have been reached in commercial electrochromic metal oxides. Although metal oxides deposited as thin films on conducting electrodes are among the best-studied materials to date,16,17 their application has been limited by high production costs related to vacuum processing. Solution-processable materials include Prussian Blue and various organic polymers.9,18 However, these materials generally suffer from low optical contrast or insufficient stability. Antiglare automotive mirrors based on liquid electrochemical cells19 are still not suitable for large-scale devices because of the possibility of leakage. Color versatility and tunability, multicolor electrochromism, and the processability/ fabrication of these materials on rigid, flexible, foldable, stretchable, and even wearable substrates are also related challenges. Moreover, molecular assemblies, especially homogeneous metallo-organic films and their fabrication into electrochromic devices (ECDs), have not been extensively investigated.20−23 Therefore, despite the large number of reports, significant efforts are still needed to develop high-performance and efficient electrochromic materials and to advance these devices and interactive smart systems beyond academic interest. We have reported that the alternating deposition of polypyridyl complexes of cobalt, iron, ruthenium, osmium, and palladium dichloride from solution results in linear and exponentially growing molecular assemblies.24 The overall reaction is essentially a three-component process, in which the Received: April 30, 2017 Published: July 13, 2017 11471
DOI: 10.1021/jacs.7b04217 J. Am. Chem. Soc. 2017, 139, 11471−11481
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the light absorption by these complexes in the visible region. The complexes were designed to rapidly form on-surface molecular assemblies (MAs) by coordination with a late transition metal (Figure 1C). The three or six vinylpyridyl moieties allow for rapid cross-linking with PdCl2 to generate polymeric threedimensional networks with high chromophore densities. The precursor PdCl2(PhCN)2 was used, as the two benzonitrile ligands are readily replaced by the pyridine-binding sites of complexes 1−4. The structural similarities and coordination chemistries of these electrochromic complexes allow for the spontaneous incorporation of a mixture of complexes with different functionalities into these molecular assemblies. This approach has the advantages that (i) no adjustments are required in the assembly procedures, (ii) a large color gradient can be achieved with a small set of building blocks, and (iii) the coatings are electrochemically addressable within the same chemical environment and a narrow potential window. The metallo-organic assemblies MA1−MA4, MA1·2, and MA1·3 (Figure 1C) were prepared using iterative spin-coating of solutions of PdCl2(PhCN)2 and the metal polypyridyl complexes (1−4) or equimolar mixtures of two different metal complexes (1·2 and 1·3) on transparent conductive oxides (TCOs). These commercially available doped metal oxides were supported on glass or flexible poly(ethylene terephthalate) (PET) substrates (up to 6 cm × 6 cm; Figure S2A). We first dropcasted a tetrahydrofuran (THF) solution of PdCl2(PhCN)2 (3.0 mM) onto a freshly cleaned TCO substrate that was subsequently spun at 500 rpm for 10 s and then at 1000 rpm for 30 s. This metal-salt deposition step on the TCO is crucial; spincoating the complexes (1−4) did not afford stable films. It is known that d8 palladium centers can bind through the surface hydroxyl groups onto the metal-oxide substrate to form a dense layer.33 This layer can be used for successive binding of the polypyridyl complexes 1−4. Excess palladium on the surface is likely to induce immediate cross-linking of the polypyridyl complexes. X-ray photoelectron spectroscopy (XPS) analysis revealed a Sn/Pd ratio of 6.1 (without washing) and 89.5 (after washing). The difference reflects the amount of Pd chemically bound to the metal-oxide surface versus the amount of physisorbed material. After 80 s, the PdCl2-modified substrate was, without washing, exposed to the 1:1 DCM:MeOH solution of the corresponding complex. For the formation of MA1, MA2, MA4, and MA1·2, the deposition cycles were repeated 18 times. For the formation of MA3 and MA1·3, the deposition cycles were repeated 12 times. After each cycle, the films were immersed in acetone to remove physisorbed materials, if any, and dried under a gentle stream of air (Figure 2). Photographs of a matrix of 30 MAs on fluorine-doped tin oxide- (FTO-) coated glass (2 cm × 1 cm) are shown in Figure 3A. The purple color palette corresponds to the assemblies based on complexes 1 and 2 and a combination of the two complexes (MA1·2), whereas the red color palette consists of assemblies based on complexes 3 and 4 and a combination of complexes 1 and 3 (MA1·3). The intensity of the colors is a function of the number of deposition cycles (1−18). The colors of the MAs were identified by their red−green−blue (RGB) values (Table S1) and are graphically presented in RGB color space in Figure 3B. The MAs were characterized by UV/vis spectroscopy, angleresolved X-ray photoelectron spectroscopy (XPS), optical microscopy, focused ion beam (FIB) scanning electron microscopy (SEM), atomic force microscopy (AFM), electrochemistry, and spectroelectrochemistry (SEC). Representative data for MA1 are shown in Figure 4. Figure 4A shows a set of
self-propagating molecular assemblies (SPMAs) store excess of the d8 palladium salt. The palladium salt is used to coordinatively bind polypyridyl complexes from solution onto the surface of the assembly. These materials are electrochromic with high cycling, excellent thermal stabilities and coloration efficiencies, short response times, and low power consumption.20,25 An earlier work by Rubinstein and Bard showed that polypyridyl complexes can be embedded into polymeric matrixes for electrogenerated chemiluminescence.26 Electropolymerization of vinyl-substituted polypyridyl complexes pioneered by Murray, Meyer, and Abruña has resulted in electrochromic metallopolymeric coatings.27−30 The groups of Kurth and Higuchi have extensively studied the formation, structure, and properties of metallosupramolecular coordination polymers fabricated by metal-ion-induced selfassembly of di- and tritopic ligands in solution.31,32 Their modular approach allows for the control of the structure and electrochromic properties from the molecular to the macroscopic level. Large-area thin films of high optical quality can be produced by solution dip-coating on transparent conducting electrodes. In this study, we demonstrate a highly versatile fabrication process that allows for the formation of homogeneous, highchromophore-density coatings on transparent conductive oxides (TCOs). The versatility is shown by both the intensity and diversity of colors that can be achieved through the incorporation of different chromophores in a single assembly. These coatings are deeply colored in their ground state and become transparent upon electrochemical oxidation. We also demonstrate the design of an electrochromic coating that exhibits a color-to-color transition. Some of these nanoscale-thick coatings (210−300 nm) have high coloration efficiencies up to 474 cm2/C, switching stabilities up to ∼4000 cycles, and ON/OFF ratios as high as 64%. Furthermore, low-voltage -operable (from −2.5 V to +3 V) rigid and flexible devices have been fabricated using the new metallo-organic coatings as functional switching elements.
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RESULTS AND DISCUSSION We used three different metal ions and two bipyridine-derived ligands to demonstrate the scope of achievable colors, utilizing only a small library of functional components. The electrochromic components of our coatings are structurally well-defined iron, ruthenium, and osmium polypyridyl complexes.20,24,25 Their molecular structures (1−4; Figure 1A) and the singlecrystal structures of two isomers of complex 1 are shown in Figures 1B and S1. The asymmetric unit contains two independent molecules, one of which is discretely disordered. The molecule without disorder is found with full occupancy in the 1-mer-Λ conformation. The second molecular site is disordered with both 1-mer-Λ and 1-fac-Λ conformations, with refined occupancies of approximately 59% and 41%, respectively. The space groups’ center of symmetry generates the enantiomeric isomers 1-mer-Δ and 1-fac-Δ. Thus, each unit cell contains, on average, three molecules of the meridional isomer and one of the facial isomer, indicating a ratio of about 3:1 in favor of the meridional isomer. These metal complexes are known to undergo reversible oneelectron redox processes.20,24,25,27−30 The metal-to-ligand charge transfer (MLCT) in the ground state (M2+) of these complexes is reflected in their high molar extinction coefficients (ε > 2.1 × 104 M·cm−1) and distinct colors (1, purple; 2, grayish; 3, orange; 4, bordeaux). Upon oxidation of the metal center (M2+ → M3+), MLCT becomes prohibited, and ligand charge transfer (LCT) becomes dominant. This effect results in a dramatic reduction of 11472
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Figure 1. Molecular and single-crystal structures of the electrochromic complexes and schematic of the thin films. (A) Molecular structures of polypyridyl complexes 1−4.20,24,25 (B) Single-crystal structures of the 1-facial and 1-meridional isomers. The crystal structures are displayed in ORTEP views using thermal ellipsoids set at the 50% probability level. Hydrogen atoms and PF6− anions are omitted for clarity. Color code: black, carbon; blue, nitrogen; yellow, iron. (C) Schematic of molecular assemblies (MAs) consisting of polypyridyl complexes 1−4 cross-linked with palladium dichloride on a transparent conductive oxide (TCO). For simplicity, the structure of MA2 is not drawn.
Figure 2. Fabrication of the electrochromic molecular assemblies (MAs). The MAs were formed by depositing alternating layers of PdCl2 and complexes 1−4 or a combination thereof by spin-coating (n = 2−18). Transparent conductive oxides (TCOs) on glass and poly(ethylene terephthalate) (PET) were used as substrates.
∼2.7 × 1016 molecules/cm2. The XPS spectrum of [MA1|FTO/ glass] showed the two characteristic peaks of the 3d orbitals of Pd(II) at 337 eV (3d5/2) and 342 eV (3d3/2), the single peak of the 1s orbital of N at 399 eV, and the two characteristic peaks of the 2p orbitals of Fe(II) at 708 eV (2p3/2) and 720 eV (2p1/2) (Figure 4B). The Pd/Fe ratio was found to be ∼2.7, nearly twice
ex situ absorption spectra of MA1 recorded after various numbers of deposition cycles (n ≤ 18). Plotting the absorption intensity of the MLCT at λmax = 573 nm versus the number of deposition cycles corroborated the linear trend in growth behavior (Figure 4A, inset). The molecular density was found to be high and was roughly estimated to be 11473
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root-mean-square roughness (Rrms) of 40 nm for a scan area of 500 × 500 nm (Figure 4E), in good agreement with optical microscope imaging (Figure S2B). The electrochemical properties of the assemblies were evaluated using a three-electrode cell configuration consisting of [MA|FTO/glass, 10 Ω/□], Pt, and Ag/Ag+ wires as the working, counter, and reference electrodes, respectively. Cyclic voltammograms (CVs) of [MA1|FTO/glass] showed the redox characteristics for the Fe2+/3+ couple, with a half-wave potential (E1/2) of 1.01 V and a peak-to-peak separation of 325 mV at a scan rate of 100 mV/s (Figure 4F). The charge density (Q) of 3.23 mC/cm2 indicates a molecular density of 2.1 × 1016 molecules/cm2, which is in good agreement with the UV/vis data (2.7 × 1016 molecules/cm2). Importantly, MA1 was found to be stable for at least 1600 redox cycles (Figures 4F and S2C). An exponential dependence of the current on the scan rate and a linear dependence of the current on the square root of these scan rates were observed (Figures 4G and S2D, S3H−S5H, and S7H− S10H), indicating a process controlled by slow diffusion.34−40 The calculated diffusion coefficients (Df) of MA1−MA4 correlate with the observed switching times (Table 2). The electrode reaction is a complex process that has to involve intercalation or ejection of ions as a mandatory part of the redox processes to maintain electroneutrality. The diffusion coefficient can be related to the movement of the anions (PF6−) in the MAs.39,40 The Df values of MA1 and MA4 and the Df values of MA2 and MA3 differ by about 1 order of magnitude. However, the molecular densities of MA1−MA4 are similar (∼1 molecule/ nm2). The additional three vinylpyridine groups for complex 2 are likely to hamper the diffusion of the anions and explain the lower Df value of MA2. The origin of the lower Df value and higher switching time for MA3 is unclear. Spectroelectrochemical (SEC) measurements revealed prominent differences in the absorbance spectra of the reduced (colored) and oxidized (bleached) states of all of the MAs on FTO/glass (Figure 5A). Response times, defined as the time required to change the color to 95% of ΔTmax, were found to be 1 s (MA1), 1.8 s (MA2), 6.8 s (MA3), 0.5 s (MA4), 0.5 s (MA1·2), and 4.3 s (3) and 8.5 s (1) (MA1·3) (Figure 5A, insets; Figures S2E, S3I−S5I, and S7I−S10I). The two MAs containing the ruthenium complex 3 (MA3 and MA1·3) exhibited slower response times with respect to the applied potentials. These color changes were clearly visible to the naked eye (Figures 5B and S2F; photographs). Interestingly, MA4 underwent a color-tocolor transition; this assembly is dark red in the ground state (Os2+) and becomes yellow upon oxidation (Os3+) as a result of the strong absorbance band at λmax = 410 nm. The MAs have high contrast ratios (ΔT, %) at the λmax values corresponding to their MLCTs: 65% (MA1), 37% (MA2), 50% (MA3), 39% (MA4), 40% (MA1·2), and 57% (MA1·3) (Figure 5B). These values are higher than the ratios reported for many electrochromic metal oxides and are comparable to the contrast ratios of some of the best-performing organic polymers reported by Reynolds and others.7,9,41−44 MA1, MA2, MA4, and MA1·2 were operated under SEC conditions for more than 500 redox cycles. The coatings containing ruthenium complex 3 were found to be less stable. Further characterization data for the MAs are included in the Supporting Information (Figures S3−S10). Laminated electrochromic devices were fabricated on both glass and flexible substrates. The devices based on glass supports with MA1 and MA4 are presented in Figure 6. This setup consisted of (i) [MA|FTO/glass] as the working electrode (bottom), (ii) FTO/glass as the counter and reference electrodes
Figure 3. Color palette of the molecular assemblies (MAs) and corresponding RGB color space, demonstrating color diversity and intensity. (A) Color diversity was obtained by depositing alternating layers of PdCl2 and complexes 1−4 or a combination thereof. The color intensity is a function of the number of deposition cycles (shown at the top are the palettes). (B) Two ellipsoids in RGB color space indicating the molecular assemblies (MAs) with purple and red tones according to standard color palette definitions (Table S1).
the value for a network in which all of the palladium centers are bound to two pyridine moieties of complex 1. This high palladium content and the observed ratios of N/Fe = 12.2 and N/Pd = 4.6 indicate the inclusion of unreacted cross-linker, PdCl2(PhCN)2. Angle-resolved XPS measurements indicated that the elemental distribution was uniform throughout the molecular assembly. The signal depth (15−20 nm) was much less than the thicknesses of the MAs (210−305 nm). Therefore, XPS measurements were performed on MA1 after various numbers of deposition cycles at θ = 0° (normal) and θ = 45°, where θ is the takeoff angle with respect to the surface normal. No significant changes in the Pd/Fe, N/Fe, and N/Pd ratios were observed regardless of the film thickness or takeoff angle (Table 1). The spatial homogeneity of the MAs was further illustrated by coating of the substrates of dimensions up to 6 cm × 6 cm with the purple MA1 (Figures 4C and S2A). SEM measurements of a cross section of [MA1|FTO/glass] was obtained by milling with a focused ion beam (FIB). Prior to this milling process, the region of the cross section was locally coated with a layer (0.6−0.8 μm) of Pt to prevent damage caused by the ion-beam bombardment of MA1 (Figure 4D,D′). The SEM image of the cross section shows the glass support, FTO, MA1, and the Pt layer. The top view shows the grainy nature of MA1 (Figure 4D). The thickness of the metallo-organic assembly was found to be ∼280 nm (18 deposition cycles), which was much higher than the interfacial roughness of [MA1|FTO/glass]. No apparent defects were observed. AFM imaging showed a similar grain-like morphology with an average grain size of 0.4 μm and a 11474
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Figure 4. Surface characterization of [MA1|FTO/glass] after 18 deposition cycles. (A) Ex situ absorption spectra recorded during the formation of MA1 by alternating deposition cycles of PdCl2(PhCN)2 and complex 1. A bare FTO substrate was used for the baseline (black). Inset: Absorbance intensity of the MLCT band (λmax = 573 nm) vs the number of deposition cycles, showing a linear growth behavior. (B) Normalized X-ray photoelectron spectroscopy (XPS) spectra showing the Fe2+ 2p, N 1s, and Pd2+ 3d regions. (C) Photograph of a 6 cm × 6 cm FTO/glass substrate coated with MA1. (D) SEM image showing the cross section of [MA1|FTO/glass], milled with a 30 keV Ga+ FIB. The Pt coating was used to prevent ion-beam damage. (D′) Magnification of the area marked in panel D and corresponding schematic representation of MA1 after Pt coating. (E) Representative AFM topography image. (F) Cyclic voltammograms (CVs) of the 1st cycle (black trace) and the 1600th cycle (red trace) of [MA1|FTO/glass]. The CVs were recorded at a scan rate of 0.1 V/s in 0.1 M TBAPF6/acetonitrile (ACN). (G) Dependence of the peak current on the scan rate (0.01−1.0 V/s) extracted from the corresponding CVs. Exponential and linear correlations between the peak current and the scan rate (left) and between the peak current and the square root of the scan rate (right), respectively, during oxidation (top) and reduction (bottom) (R2 > 0.99 for all fits).
insulating spacer (Figure 6A; see the Experimental Section for details). These devices were found to have a low and practical operating potential range from −2.5 V to +3 V and switching times of 2 s (MA1) and 10 s (MA4). The electrochromic properties of the devices are evident from the absorption spectra (Figure 6B) and clearly visible to the naked eye (Figures 6D,E and S2G). The devices regained their original reduced states under an open-circuit potential. For instance, for the MA1-based device, it took ∼2 min to reach the initial ground state after complete oxidation, k ≈ 0.032 s−1 (Figure 6C). Even though the potential of the counter electrode is not monitored during an electrochemical or electrochromic process, equal and opposite electrode reactions at both the working and counter electrodes
Table 1. XPS-Derived Elemental Ratios for MA1 at Two Takeoff Angles Pd/Fe
N/Fe
N/Pd
deposition cycle
0°
45°
0°
45°
0°
45°
1 5 10 15 18
2.8 2.6 2.7 2.8 2.7
2.9 2.7 2.8 3.2 2.8
13.6 12.6 12.1 11.6 12.2
14.3 13.0 13.2 13.8 13.4
4.9 4.8 4.7 4.2 4.6
4.9 4.7 4.7 4.3 4.8
(top), (iii) a poly(methyl methacrylate)- (PMMA-) based gel electrolyte, and (iv) double-sided tape (3M 9088) as an 11475
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Journal of the American Chemical Society Table 2. Diffusion Coefficientsa (cm2·s−1) and Response Timesb (s) of the Molecular Assemblies on FTO/Glass MA1 MA2 MA3 MA4
oxidation
reduction
response time
2.24 × 10−8 1.31 × 10−9 5.08 × 10−9 1.05 × 10−8
1.88 × 10−8 1.35 × 10−9 6.84 × 10−9 1.03 × 10−8
1.0 1.8 6.8 0.5
a
These values were derived from the Randles−Sevcik equation: ip = (2.69 × 105)n3/2ACDf1/2ν1/2, where ip is the peak current (A), n is the number of electrons transferred in the redox reaction, A is the area of the electrode (cm2), C is the concentration of the solution (mol·cm−3), Df is the diffusion coefficient (cm2·s−1), and ν is the scan rate (V·s−1). bTime required to change the color to 95% of ΔTmax.
are required for the redox-balanced and reversible functioning of an electrochromic device. However, in our sandwich cells, no redox species was introduced at the counter electrode, and hence, the counter electrode reaction was undefined. However, a reduction reaction at the counter electrode is required to allow oxidation of the electrochromic film at the working electrode. Because no dedicated ion-storage layer was used, either the gel electrolyte or parts of the counter electrode had to have been reduced.45−47 We previously observed that traces of oxygen or water can cause instability of redox-active films.48,49 The diffusion of products of uncontrolled reactions in the gel could well explain the observed re-reduction of the electrochromic MAs under an open-circuit potential.50,51 SEC measurements indicated that this device was stable for at least 75 redox cycles (Figure 6D). The MA4-based device exhibited a color-to-color transition from bordeaux to yellow and was stable for at least 50 redox cycles (Figure 6E). The yellow color was due to the strong absorption band at λmax = 410 nm. These two devices had high contrast ratios (measured as ΔT) at the λmax values of the MLCT band: 50% (MA1) and 32% (MA4). We tested the electrochemical and spectroelectrochemical properties of MA1 deposited on flexible substrates of 30 Ω/□ ITO-coated PET in an electrochemical cell (Figure 7A−D) and as an EC device (Figure 7E−G). The EC properties were demonstrated when the [MA1|ITO/PET] electrode was in its planar state as well as when it was in its bent state with a radius of curvature (Rc) of 3.7 cm. Absorption spectra corresponding to the consecutive oxidation and reduction of the planar (ΔT = 61%) and bent (ΔT = 36%) substrates showed clear differences in the reduced (colored) and oxidized (bleached) states (Figure 7C), as was shown for the coatings on glass (Figure 5A). SEC measurements indicated that [MA1|ITO/PET] was stable for at least 300 redox cycles with a switching time of 7.5 s for a color transition on a 1 cm × 5 cm substrate. The intensity of the MLCT band of [MA1|ITO/PET] was not significantly affected by repeated bending of the substrate (20 cycles) in intervals of 2 min (Rc = 3.7 cm) (Figure 7D). Flexible devices (2 cm × 6 cm) could be switched with a switching time of ∼30 s for each color transition (Figure 7E−G) with ΔT values of 21% and 35% for the planar and bent configurations, respectively (Figure 7G). Changes in the ΔT value of the device by bending were likely caused by damage to the TCO, which is known to crack.52 Changes in the porosity of the MAs might also play a role.
Figure 5. Spectroelectrochemical (SEC) performance of the molecular assemblies (MAs) on FTO/glass in an electrolyte solution. (A) Absorption spectra corresponding to two consecutive oxidation and reduction cycles of (from top to bottom) MA1, MA2, MA1·2, MA3, MA1·3, and MA4. Bare FTO substrates were used for the baseline (black). Insets: Dependence of the contrast ratio (ΔT) on the switching time. (B) Top: Photographs of the colored and bleached states of the MAs. Note that MA4 exhibited a colorto-color transition. Bottom: Spectroelectrochemistry (SEC) using doublepotential steps: 0.4−1.6 V (MA1, λmax = 573 nm), 0.4−1.8 V (MA2, λmax = 589 nm), 0.4−2.0 V (MA1·2, λmax = 583 nm), 0.7−1.7 V (MA3, λmax = 490 nm), 0.4−1.8 V (MA1·3, λ1 = 495 nm, λ3 = 470 nm, and 0.2−1.4 V (MA4, λmax = 514 nm). The two traces represent the spectroelectrochemical stability of the MAs. For MA1·3, the two traces represent the cycling of the two different metal centers. The stability of MA1·3 is shown in Figure S9. For the formation of MA1, MA2, MA4, and MA1·2, the deposition cycles were repeated 18 times. For the formation of MA3 and MA1·3, the deposition cycles were repeated 12 times. The coloration efficiencies (CEs in cm2/C) were found to be as follows: MA1, 207; MA2, 184; MA1·2, 138; MA3, 208; MA1·3 with Fe, 226; MA1·3 with Ru, 474; and MA4, 230.
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SUMMARY AND CONCLUSIONS We have introduced a versatile assembly method based on spincoating to generate diversely colored nanoscale coatings, whose color intensities can be readily controlled by adjusting the 11476
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Figure 6. Structure and operation of electrochromic devices. (A) Schematic representation of the device structure on FTO/glass. The gel electrolyte consisted of 70:20:7:3 wt % acetonitrile (ACN)/propylene carbonate (PC)/poly(methyl methacrylate) (PMMA)/trifluoromethylsulfonamide lithium salt. A piece of double-sided tape (3M 9088) was used as the spacer. (B) Absorption spectra corresponding to the oxidized (gray) and reduced (purple) states of an MA1-based device. The baseline (black) corresponds to the absorbance of a device lacking the electrochromic MA1. (C) Decay of the transmittance of an MA1-based device under an open-circuit potential. Inset: Logarithmic plot showing a linear fit (R2 = 0.99). (D) Spectroelectrochemistry (SEC) monitored at λmax = 573 nm, using a potential range between −2.5 V and +3 V and a pulse width of 4 s. Inset: Photographs of the colored and bleached states of an MA1-based device (4 cm × 4 cm). (E) SEC monitored at λmax = 510 nm, using a potential range from −2.5 V to +3 V and a pulse width of 20 s. Inset: Photographs of the colored and bleached states of an MA4-based device (2 cm × 2 cm). The deposition cycles were repeated 18 times for both MA1 and MA4.
coatings. The materials homogeneously cover the relatively rough surface of metal-oxide substrates up to 6 cm × 6 cm, without detectable electrical short circuits. This spin-coating
number of deposition cycles or by applying a potential. We believe that this new method is an attractive and powerful alternative for the preparation of electrochromic organic 11477
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Figure 7. Spectroelectrochemistry (SEC) of [MA1|ITO/PET 30 Ω/□] (A−D) in an electrolyte solution and (E−G) as a sandwich cell using a gel electrolyte. (A) Schematic representation of the electrochemical cell. Pt wire and Ag/Ag+ were the counter (CE) and reference (ref) electrodes, respectively. The working electrode (WE) was [MA1|ITO/PET]. (Top) Planar substrate, (bottom) bent substrate. (B) Photographs of the colored and bleached states: (top) planar and (bottom) bent with a radius of curvature (Rc) of 3.7 cm. (C) Left: Absorption spectra corresponding to the consecutive oxidation and reduction of (top) planar and (bottom) bent substrates. Spectra corresponding to two consecutive cycles of oxidation and reduction are shown. Bare ITO/PET substrates were used for the baseline (black). Right: SEC at λmax = 589 nm for the (top) planar and (bottom) bent substrates. Double-potential steps of 0.4 and 1.8 V were used at a pulse width of 20 s. (D) Maximum absorbance intensity of the MLCT bands obtained during multiple bending cycles with an interval of 2 min. Planar, purple; bent, green. (E) Schematic representation of the flexible devices (WE, 2 cm × 6 cm; CE, 3 cm × 6 cm; active area = 1.75 cm × 4 cm). The electrodes were ITO/PET substrates, and the spacer was a piece of double-sided tape (3M 9088). (F) Photographs of the device in its colored and bleached states: (top) planar and (bottom) bent with Rc = 3.7 cm. (G) Absorption spectra corresponding to the consecutive oxidation and reduction of (left) planar and (right) bent devices. The baselines (black) correspond to the absorbance of bare devices lacking MA1. The deposition cycles were repeated 18 times.
the metallo-organic assemblies. Some of the new MAs that lack the organic monolayer have contrast ratios more than 20% higher than those of the previously reported films prepared by layer-bylayer dip-coating.20,25 The direct deposition of the MAs onto the metal-oxide surfaces under ambient conditions in air eliminates
approach is about 7−8 times faster than layer-by-layer dip-coating methods involving the use of densely packed organic monolayers covalently immobilized on the metal-oxide-coated electrodes.20,25 These monolayers hamper efficient electron-transfer processes, thereby limiting the electrochromic characteristics of 11478
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Article
Journal of the American Chemical Society
Technology Ltd. (Hong Kong, China). FTO-coated glass substrates were cleaned by sonication in ethanol for 10 min, dried under a stream of N2, and subsequently cleaned for 20 min with UV and ozone in a UVOCS cleaning system (Montgomery, PA). The substrates were then rinsed with tetrahydrofuran (THF), dried under a stream of N2, and oven-dried at 130 °C for 2 h. ITO-coated PET substrates were cleaned by being immersed for 30 s in each ethanol and acetone and then dried under a stream of air. A Laurell spin-coater, model WS-400A-6NPP/ LITE, was used for the formation of the molecular assemblies (MAs). UV/Vis Spectroscopy. UV/vis spectra were recorded on a Cary 100 spectrophotometer. The absorbance was measured using the Cary WinUV−Scan application program, version 3.00 (182) by Varian (200−800 nm), whereas the transmittance was measured using the Cary WinUV−Kinetics application program, version 3.00 (182) by Varian. Bare substrates were used to compensate for the background absorption. X-ray Photoelectron Spectroscopy. XPS measurements were carried out on FTO/glass substrates (2.0 cm × 2.0 cm) with a Kratos AXIS ULTRA system, using a monochromatic Al Kα X-ray source (hυ = 1486.6 eV) at 75 W and detection pass energies ranging between 20 and 80 eV. Curve-fitting analysis was based on Shirley or linear background subtraction and application of Gaussian−Lorenzian line shapes. Atomic Force Microscopy. AFM images were recorded using a Solver P4710 microscope (NT-MDT, Moscow, Russia) operating in semicontact scanning mode, with ∼100-μm silicon cantilevers having a resonant frequency of 70−90 kHz. The roughness values, Rrms, were obtained from 500 nm × 500 nm images, using Nova 1.0.26 RC1 software. Focused Ion Beam Microscopy. SEM images were recorded using a Helios 600 FIB/SEM dual-beam microscope (FEI), operating at 5 keV. The images were taken at the surface of the samples and at cross sections that were milled with a 30 keV Ga+ focused ion beam (FIB). The sample was first locally coated with a 150−200-nm-thick layer of Pt using electron-beam-assisted deposition, which was followed by the ionbeam-assisted deposition of a 500−600-nm-thick layer of Pt. The Pt coating protects the MA layer from ion-beam damage, providing a clean edge of the cross section. Electrochemical Characterization. Electrochemical experiments were carried out using a CHI660A or CHI760E electrochemical workstation. The following configuration of the electrochemical cell was used: FTO-coated glass or ITO-coated PET substrates (size of 2 cm × 2 cm, 4 cm × 4 cm, 2 cm × 6 cm, 3 cm × 6 cm, or 6 cm × 6 cm) served as the working electrode, Ag/Ag+ was used as the reference electrode, and a Pt wire was used as the counter electrode. Tetrabutylammonium hexafluorophosphate (TBAPF6) in ACN (0.1 M) was used as the supporting electrolyte. The electrochemical measurements of the devices were carried out with FTO-coated glass or ITO-coated PET substrates (2 cm × 2 cm, 4 cm × 4 cm, 6 cm × 2 cm, or 6 cm × 6 cm) serving as the working electrode and a corresponding bare substrate as the reference and counter electrodes. Crystallization of Complex 1. Single crystals of complex 1 suitable for X-ray analysis were obtained at room temperature, upon slow evaporation of a 2 mL CH2Cl2/MeOH (1:1 v/v) solution containing ∼1 mg of the complex. X-ray Crystal Structure Analysis of Complex 1. Crystal data: C54H45FeN9 + 2(PF6), purple plates, 0.30 × 0.2 × 0.01 mm3, triclinic, space group P1,̅ a = 15.2797(11) Å, b = 21.3748(15) Å, c = 21.7718(15) Å, α = 74.223(10)°, β = 77.054(10)°, γ = 79.381(11)° from 3462 reflections, T = 100(2) K, V = 6610.9(9) Å3, Z = 4, Fw = 1165.78, Dc = 1.171 Mg·m−3, μ = 0.348 mm−1. Data collection and processing: Rigaku XtaLab diffractometer, Pilatus 200 K detector, MicroMax003 Mo Kα (λ = 0.71073 Å), −15 ≤ h ≤ 15, −21 ≤ k ≤ 21, −21 ≤ l ≤ 21, frame scan width = 0.50°, scan speed 1.0° per 80 s, typical peak mosaicity 0.70°, 55317 reflections collected, 13716 independent reflections (Rint = 0.1349). The data were processed with Rigaku CrysAlisPro. Solution and ref inement: Structure solved with SHELXT-2013.63 Full-matrix leastsquares refinement based on F2 with SHELXL-2013 on 1546 parameters with 208 restraints gave final R1 = 0.0776 (based on F2) for data with I > 2σ(I) and R1 = 0.1296 on 13716 reflections. Goodness of fit on F2 = 0.937, largest electron density peak = 1.291 e·Å−3, largest hole = −0.314 e·Å−3. The crystallographic data have been deposited at the
the need to initially generate these monolayers using timeconsuming silane chemistry under anhydrous conditions over 2−3 days. The coatings reported here are about 7-fold thicker and have a high chromophore density and, therefore, have considerably lower optical transparency in the ground state while exhibiting a better optical transparency in the oxidized state. For thinner metallo-organic films (
