Langmuir 2006, 22, 1949-1951
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Polyoxometalate-Based Electro- and Photochromic Dual-Mode Devices Shaoqin Liu,† Helmuth Mo¨hwald,† Dirk Volkmer,§ and Dirk G. Kurth*,†,‡ Max Planck Institute of Colloids and Interfaces, Research Campus Golm, 14424 Potsdam, Germany, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and UniVersita¨t Ulm, Anorganische Chemie II, Albert-Einstein-Allee 11, 89081 Ulm, Germany ReceiVed September 1, 2005 Molecular or supramolecular systems capable of electro- and photostimulated color changes are still rare. We present a device design based on an electrostatic complex of a nanoscopic polyoxometalate cluster and a polyelectrolyte. The coating reversibly changes color from transparent to blue by photo- and/or electroinduced stimulation. The choice of the components results in perfect transparency over the entire visible range, a large optical response, reversible operation, and excellent stability.
In applications for optical displays,1 memories,2 and smart windows,3 there is rising demand for coatings that change transparency in response to irradiation and electrochemical potential. There are, however, only few examples of materials that are at the same time photo- and electrochromic.4-8 A wide range of electro- or photochromic materials are available today based on organic molecules9 and polymers,10 inorganic oxides such as WO3 and MO3,11 and coordination compounds12 such as the Prussian blue analogues.13 Despite enormous progress, many systems still show drawbacks in terms of reliability, contrast ratio, write-erase efficiency, response time, costs, and device fabrication. For instance, vacuum deposition has been used to make electro- and photochromic thin film devices, but waterbased fabrication would be cheaper, simpler, environmentally friendly, and more versatile concerning device size and geometry.14 Here, we report on a novel design for a dual-mode photoand electroactive device that combines the ease of fabrication and flexibility of polymeric materials with the high photo- and electrochemical stability of inorganic materials. Early transition-metal polyoxoanions, generally termed polyoxometalates (POMs), represent a unique class of atomically defined nanoscopic clusters with a plethora of sizes, framework topologies, compositions, and properties.15 Generally, POMs are readily prepared in high yield, excellent purity, and large quantities * Corresponding author. Tel: +49(0)331-567 9211. Fax: +49(0)331567 9202. E-mail:
[email protected]. † Max Planck Institute of Colloids and Interfaces. ‡ National Institute for Materials Science. § Universita ¨ t Ulm. (1) (a) Bechinger, C.; Ferrere, S.; Zaban, A.; Sprague, J.; Gregg, B. A. Nature 1996, 383, 608. (b) Kurth, D. G.; Pitarch Lo´pez, J.; Dong, W.-F. Chem. Commun. 2005, 2119. (2) Kawata, S.; Kawata, Y. Chem. ReV. 2000, 100, 1777. (3) Rosseinsky, D. R.; Mortimer, R. J. AdV. Mater. 2001, 13, 783. (4) Peters, A.; Branda, N. R. J. Am. Chem. Soc. 2003, 125, 3404. (5) Miki, S.; Noda, R.; Fukunishi, K. Chem. Commun. 1997, 925. (6) Saika, T.; Iyoda, T.; Honda, K.; Shimidzu, T. J. Chem. Soc., Perkin Trans. 1993, 2, 1181. (7) Kawai, S. H.; Gilat, S. L.; Ponsinet, R.; Lehn, J.-M. Chem.sEur. J. 1995, 1, 285. (8) Kawai, S. H.; Gilat, S. L.; Lehn, J.-M. Chem. Commun. 1994, 1011. (9) Fernandez-Acebes, A.; Lehn, J.-M. Chem.sEur. J. 1999, 5, 3285. (10) Gaupp, C. L.; Zong, K. W.; Schottland, P.; Thompson, B. C.; Thomas, C. A.; Reynolds, J. R. Macromolecules 2000, 33, 1132. (11) Yamase, T. Chem. ReV. 1998, 98, 307. (12) Bernhard, S.; Goldsmith, J. I.; Takada, K.; Abruna, H. D. Inorg. Chem. 2003, 42, 4389. (13) Pyrasch, M.; Tieke, B. Langmuir 2001, 17, 7706. (14) Yao, J. N.; Hashimoto, K.; Fujishima, A. Nature 1992, 355, 624. (15) Mu¨ller, A.; Reuter, H.; Dillinger, S. Angew. Chem., Int. Ed. Engl. 1995, 34, 2328.
by simple (one-pot) self-assembly reactions starting from cheap, commercially available inorganic precursors. The ability to reversibly accept and release a large number of electrons under marginal structural rearrangement is one of the most promising Value-adding properties of POMs in materials science.16 The reduced POMs frequently display a deep-blue color. In the presence of counterions that can donate a proton, such as alkylammonium, pyridinium, or anilinium, the irradiation of POM crystals by UV light results in a colored, long-lived chargeseparated state.17-19 Previously, we explored electrostatic layer-by-layer selfassembly20 to place POMs into ultrathin multilayers.21 The coating is made by repetitive deposition of oppositely charged POMs and polyelectrolytes. This versatile and simple process offers precise structural control and appears particularly attractive for the creation of high-quality, large-area, uniform coatings on arbitrarily shaped substrates.22 The permeable and transparent polyelectrolyte matrix is ideally suited for electro- and photoinduced switching. We chose the negatively charged, transparent Preyssler-type POM, (NH4)14[NaP5W30O110] (P-POM, Scheme 1) because it exhibits excellent electrochromic performance in terms of optical contrast, stability, and reversibility.23 Therefore, we anticipate equally good performance of this compound in photochromic applications. As the polyelectrolyte component, we employ poly(4-vinylpyridine) (P4VP) as a proton reservoir needed for the photochromic response and as a matrix to immobilize the clusters. Multilayer formation on a cushion of polyethyleneimine (PEI) and polystyrene (PSS) is readily effected by dipping the substrate repeatedly into aqueous solutions containing P4VP (pH 3) or P-POM (pH 5-6). Multilayer formation is confirmed by UVvis spectroscopy, cyclic voltammetry (CV), and X-ray reflectivity (XRR). Figure 1 shows UV-vis absorption spectra of (P-POM/ (16) (a) Pope, M. T. In Mixed Valence Compounds; Brown, D. B., Ed.; D. Reidel: Dordrecht, The Netherlands, 1980; p 365. (b) Pope, M. T. Prog. Inorg. Chem. 1991, 39, 181. (17) Arnaud-Neu, F.; Schwing-Weill, M.-J. Bull. Soc. Chim. Fr. 1973, 3225. (18) Yamase, T.; Prokop, P.; Arai, Y. J. Mol. Struct. 2003, 656, 107. (19) Yamase, T.; Suga, M. J. Chem. Soc., Dalton Trans. 1989, 4, 661. (20) Decher, G. Science 1997, 277, 1232. (21) (a) Caruso, F.; Kurth, D. G.; Volkmer, D.; Koop, M. J.; Mu¨ller, A. Langmuir 1998, 14, 3462. (b) Kurth, D. G.; Volkmer, D.; Ruttorf, M.; Richter, B.; Mu¨ller, A. Chem. Mater. 2000, 12, 2829. (22) (a) Liu, S.; Kurth, D. G.; Bredenkotter, B.; Volkmer, D. J. Am. Chem. Soc. 2002, 124, 12279. (b) Liu, S.; Kurth, D. G.; Volkmer, D. Chem. Commun. 2002, 976. (23) Liu, S.; Kurth, D. G.; Mo¨hwald, H.; Volkmer, D. AdV. Mater. 2002, 14, 225.
10.1021/la0523863 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/27/2006
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Scheme 1. (Left) Structure of the Negatively Charged Preyssler Cluster, (NH4)14[NaP5W30O110] [P-POM], and Poly(4-vinylpyridine) [P4VP] and (Right) Schematic Section of the Self-Assembled Multilayera
a Adhesion of the components is primarily attributed to electrostatic interactions of the oppositely charged species.
Figure 1. UV-vis absorption spectra of a PEI/PSS/P4VP(P-POM/ P4VP)m multilayer on quartz as a function of the number of layers, m () 1-8). The inset shows the absorption maxima at 282, 256, and 202 nm as a function of m, confirming linear and uniform multilayer growth.
P4VP)m multilayers as a function of the number of layers, m. The flatness and negligible absorbance of the multilayer in the visible range from 400 to 800 nm indicate a high coating quality without significant scattering or reflection losses. A surface roughness below 2 nm and the appearance of Kiessig fringes in XRR (not shown) confirm the excellent uniformity of the multilayers. The absorbance at 282, 202, and 256 nm increases steadily with the number of layers, confirming the irreversible adsorption of both P-POM and P4VP and uniform and linear multilayer growth (Figure 1 inset). On the basis of UV-vis data, the average surface coverage of P-POM per layer amounts to (2.2 ( 0.1) × 10-10 mol/cm2,24 and that of P4VP monomers amounts to (5.7 ( 0.6) × 10-9 mol/cm2. Compared to the charge of P-POM (-14/ POM), the multilayer contains approximately a 2-fold excess of pyridine groups.25 Figure 2 (top) shows UV-vis spectra of a PSS(P4VP/P-POM)40 multilayer on a single-sided indium tin oxide (ITO)-coated quartz substrate before and after electrochemical reduction. The entire device consists of the coated ITO substrate, buffer solution, and a Pt counter electrode mounted in a closed quartz cuvette. Initially, the film is perfectly transparent in the visible range. Upon electrochemical reduction, we observe a strong absorption band with a maximum at approximately 700 nm. Using the ability of P-POM to undergo multiple reversible redox steps, we can fine tune the transparency of the window by the applied potential. The six spectra shown in Figure 2 are recorded at 0.0, -0.9, (24) From the UV-vis spectra the surface coverage, Γ, can be calculated by Γ ) (NAAλ)/2mλ, where NA is Avogadro’s constant, Aλ is the absorbance, and λ is the isotropic molar extinction coefficient at wavelength λ. The isotropic molar extinction coefficients are 282 nm(P-POM) ) 8.8 × 104 M-1 cm-1 and 256 nm (P4VP) ) 3700 M-1 cm-1 per monomer unit. (25) Hoogeveen, N. G.; Stuart, M. A. C.; Fleer, G. J.; Boehmer, M. R. Langmuir 1996, 12, 3675.
Figure 2. (a) UV-vis spectra of a PSS(P4VP/P-POM)40 multilayer on an indium tin oxide (ITO)-coated quartz substrate before and after reduction. The transparency of the window is readily controlled by the applied potential. The six spectra are recorded at 0.0, -0.9, -1.5, -1.7, -1.9, and -2.1 V, giving rise to 0, 2, 15, 60, 94, and 100% coloration. (b) Time-dependent photochromic coloration of the multilayer. A low-intensity UV lamp was used to measure the spectra changes. After 4 h of irradiation, no further change in coloration is observed.
-1.5, -1.7, -1.9, and -2.1 V, giving rise to 0, 2, 15, 60, 94, and 100% coloration with respect to saturation coloration. Even though the coating is very thin, the absorbance (0.28 at 700 nm) is sufficient to allow visual perception of the electrochromic coloration. Under exclusion of air, the blue color of the device remains for many hours (memory effect) if the power source is disconnected. Bleaching is effected by applying a potential of -0.2 V. The time response determined by stepping the potential and simultaneously measuring the absorbance is comparable, within a few seconds, to that of the system prepared with poly(allylamine hydrochloride) reported previously.23 Figure 2 (bottom) shows the change in the optical properties of the multilayer under irradiation with UV light under ambient conditions.26 The time response of photochromic coloration depends, of course, on the power of the irradiation source. A 100 W tungsten halogen lamp causes instant coloration. Therefore, a low-intensity UV lamp is used in these experiments to resolve the spectral changes as a function of exposure time. An absorbance band centered at approximately 700 nm arises during UV irradiation. The similarity of the two sets of spectra and the overall change in the optical density are of comparable magnitude, indicating that all immobilized clusters participate in the coloration process irrespective of the stimulus (electrical/optical). The absorbance per layer amounts to approximately 0.006 in both cases. The molar extinction coefficient of the six-electron reduced form of P-POM is approximately 2.7 × 104 dm3 mol-1 cm-1. Therefore, the concentration of electro- or photoreduced POM amounts to 2.2 × 10-10 mol/cm2, indicating that almost all P-POM anions in the thin film participate in coloration. It is noteworthy to mention that under an inert atmosphere the photoirradiated samples maintain their blue color for many hours. (26) The optical response of the device is corrected for a single-sided substrate to allow direct comparison with the electrochemically induced response.
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Figure 3. P-POM/P4VP multilayer can be colored by either irradiation with UV light (i f ii) or by reduction (i f iii). Bleaching is effected by irradiation with visible light (ii f i) or oxidation (iii f i). In addition, photoinduced coloration (i f ii) can be followed by oxidation (ii f iv), or electrochemically induced coloration (i f iii) can be followed by bleaching with visible light (iii f iv). Also shown are the corresponding spectra (left). The center shows a representative photograph of a colored window.
The back reaction to the transparent state is induced by thermal or optical activation.27 Heating to 100 °C causes fast and homogeneous bleaching of the blue film. Repeated irradiation and heating cycles of the multilayer reveal good reversibility. Most importantly, the window stays perfectly transparent after coloration and bleaching. The back reaction can also be induced by optical stimulation that is by irradiation with visible light. Even after more than 1500 cycles no signs of photofatigue of the active components are registered. Both electro- and photochromism can be combined in a single device. Figure 3 shows the different states of the device using either electro- or photoinduced stimulation or both modes in succession. Also shown are the corresponding UV-vis spectra of all four possible states. The spectra were recorded after several switching cycles were preformed to ensure constant operation conditions and comparability. UV irradiation (i/ii) or reduction (i/iii) gives rise to results similar to those presented above. The shoulder at 400 nm is associated with a photochromic response of the ITO substrate. Note that photo- and electroinduced switching have different efficiencies. Spectra i/ii/iv show the optical changes after coloration with UV irradiation and bleaching by electrochemical oxidation corresponding to the sequence i f ii f iv on the left-hand side. Finally, spectra i/iii/iv show the change in the optical properties after coloration by reduction and bleaching by irradiation with visible light (sequence i f iii f iv).28 The spectra demonstrate that the device supports electroand photochromic switching. Interconversion is reversible and fast, and under laboratory conditions, we did not detect any signs of decomposition or photofatigue, demonstrating the fidelity of the device. In summary, we demonstrate the construction of an electroand photochromic device with promising technical performance prepared by simple environmentally friendly techniques with the advantage of water-based processing including large-area coatings, patterns, and multicomponent systems. The windows are adaptive in that their optical properties respond to UV (27) Kittel, C. Introduction to Solid State Physics, 8th ed.; John Wiley & Sons: New York, 2004. (28) In these cases, the final step is most likely associated with redox-induced side reactions involving either solvent or matrix molecules. (29) (a) Dickman, M. H.; Gama, G. J.; Kim, K.-C.; Pope, M. T. J. Cluster Sci. 1996, 7, 551. (b) Creaser, I.; Heckel, M. C.; Neitz, R. J.; Pope, M. T. Inorg. Chem. 1993, 32, 1573. (30) Kurth, D. G.; Bein, T. Langmuir 1995, 11, 3061.
radiation. The final transparency, however, can be fine tuned by applying a potential to increase or decrease coloration as desired.
Methods (NH4)14[NaP5W30O110]‚31H2O was prepared according to literature procedures.29 The substrates and the multilayers were prepared according to published procedures.22 The following concentrations and dipping times were used: poly(ethylenimine) (PEI, MW 50 000, Aldrich), 10-2 mol/L, 20 min; poly(styrenesulfonate) (PSS, MW 70 000, Aldrich), 10-3 mol/L (pH 5-6), 10 min; poly(vinyl pyridine) (P4VP, MW 50 000, Aldrich), 10-3 mol/L (pH 3.0, CNaCl ) 0.5 M), 10 min; P-POM, 5 × 10-4 mol/L (pH 5-6), 20 min. After each dipping cycle, the substrate was rinsed with deionized water. After each second layer, the substrate was dried under Ar flow. Quartz-coated substrates were irradiated either with a handheld 4 W UV lamp at a distance of 6 cm or with a 100 W water-cooled quartz tungsten halogen lamp at a distance of 20 cm without focusing optics; to eliminate UV radiation, a glass cover slip was put on both sides of the sample. All measurements were performed under ambient conditions. UV-visible spectra were recorded with a Varian Cary 50. For X-ray reflectance (XRR) measurements (STOE&CIE, Darmstadt, Germany, λ ) 1.54 Å), polished (100) silicon wafers were employed. Electrochemical measurements were performed in a single-compartment cell with a standard three-electrode configuration: Ag/AgCl/KCl (c ) 3 mol/L) as the reference electrode, a platinum gauze of large surface area as the counter electrode, and the bare or coated ITO slides as the working electrode. The cleaned ITO-coated glass electrodes were functionalized with 3-aminopropyltrimethoxysilane.30 Spectroelectrochemical experiments were performed by measuring absorption spectra and monitoring the absorbance at 700 nm as a function of time during chronoamperometry with potential steps of 0 to -2.1 V versus the Pt electrode. Acknowledgment. S.L. thanks the Alexander von Humboldt Foundation for a research fellowship. D.G.K. thanks the Volkswagen Foundation for financial support. Supporting Information Available: Experimental details of the electrostatic layer-by-layer self-assembly and instrumentation as well as cyclic voltammograms and absorbance curves. This material is available free of charge via the Internet at http://pubs.acs.org. LA0523863
