Photolithographically-Patterned Electroactive Films and

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Photolithographically-Patterned Electroactive Films and Electrochemically Modulated Diffraction Gratings Kirk S. Schanze,* Troy S. Bergstedt, Brain T. Hauser, and Carla S. P. Cavalaheiro† Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200 Received June 28, 1999. In Final Form: September 17, 1999 This manuscript presents an overview of work directed toward the creation of arbitrary micron-scale patterns of electroactive polymer films by the application of photolithography. A brief overview of work by other groups in the field is followed by a detailed description of work from our own labs which resulted in the development of methods to pattern a variety of different electroactive materials, including Ru- and Os-polypyridine complexes, viologen-based polymers, and a low-potential polythiophene. The discussion provides detail on the photolithographic methodology. In addition, the spatially-patterned films are characterized by using optical and scanning electron microscopy. The voltammetric and spectroelectrochemical properties of several of the lithographically-patterned films are also presented. Finally, photolithography is applied to fabricate electrochromic optical diffraction gratings. The diffraction efficiency of these electroactive gratings can be modulated by an electrochemical stimulus. The properties and mechanism for operation of the electrochromic gratings are described.

I. Introduction Electrodes that are chemically modified with monolayer, multilayer, or polymeric thin films have become increasingly important in basic research and in technological applications.1-3 Diverse areas such as corrosion,4 electrocatalysis,5-12 energy conversion,13-16 chemical and biological sensing,17 and electrochromic displays18,19 have been impacted by the advent of convenient methods for * Corresponding author. E-mail: [email protected]. † Permanent address: Departimento De Fı´sico-Quı´mica, IQSP/ USP, Saõ Carlos, SP, Brazil. (1) Abrunã, H. D., Skotheim, T. A., Ed.; Marcel Dekker: New York, 1988; Vol. 1, p 97. (2) Abrunã, H. D. Coord. Chem. Rev. 1988, 86, 135-189. (3) Murray, R. W. Molecular Design of Electrode Surfaces; Wiley and Sons: New York, 1992. (4) Grundmeier, G.; Reinartz, C.; Rohwerder, M.; Stratmann, M. Electrochim. Acta 1998, 43, 165-174. (5) Offord, D. A.; Sachs, S. B.; Ennis, M. S.; Eberspacher, T. A.; Griffin, J. H.; Chidsey, C. E. D.; Collman, J. P. J. Am. Chem. Soc. 1998, 120, 4478-4487. (6) Hutchison, J. E.; Postlethwaite, T. A.; Chen, C. H.; Hathcock, K. W.; Ingram, R. S.; Ou, W.; Linton, R. W.; Murray, R. W.; Tyvoll, D. A.; Chng, L. L.; Collman, J. P. Langmuir 1997, 13, 2143-2148. (7) Leasure, R. M.; Ou, W.; Moss, J. A.; Linton, R. W.; Meyer, T. J. Chem. Mater. 1996, 8, 264-273. (8) O’Toole, T. R.; Meyer, T. J.; Sullivan, B. P. Chem. Mater. 1989, 1, 574-576. (9) Younathan, J. N.; Wood, K. S.; Meyer, T. J. Inorg. Chem. 1992, 31, 3280-3285. (10) Collombdunandsauthier, M. N.; Deronzier, A.; Ziessel, R. Inorg. Chem. 1994, 33, 2961-2967. (11) Chardon-Noblat, S.; Deronzier, A.; Ziessel, R.; Zsoldos, D. J. Electroanal. Chem. 1998, 444, 253-260. (12) Caix, C.; ChardonNoblat, S.; Deronzier, A.; Ziessel, R. J. Electroanal. Chem. 1996, 403, 189-202. (13) Hagfeldt, A.; Gratzel, M. Chem. Rev. 1995, 95, 49-68. (14) Thompson, D. W.; Kelly, C. A.; Farzad, F.; Meyer, G. J. Langmuir 1999, 15, 650-653. (15) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Meyer, G. J. Langmuir 1999, 15, 731-737. (16) Moss, J. A.; Stipkala, J. M.; Yang, J. C.; Bignozzi, C. A.; Meyer, G. J.; Meyer, T. J.; Wen, X. G.; Linton, R. W. Chem. Mater. 1998, 10, 1748-1750. (17) Ricco, A. J.; Crooks, R. M. Acc. Chem. Res. 1998, 31, 200-200. (18) Reddinger, J. L.; Reynolds, J. R. Macromolecules 1997, 30, 673675. (19) Kumar, A.; Reynolds, J. R. Macromolecules 1996, 29, 76297630.

producing chemically tailored electrochemical interfaces. Because of the importance of modified electrodes to science and technology, a variety of methods have been developed for preparing electrodes that are chemically modified with materials that are redox or electronically conductive.1,2,20 Most work in the area of modified electrodes has focused on approaches to producing films that are spatially homogeneous in the plane defined by the electrode surface.1-3 However, within the past decade a number of groups have devised methods for modifying electrodes with electroactive films that are microscopically patterned within the plane defined by a surface.7,21-50 These methods (20) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers, 2nd ed.; Marcel Dekker: New York, 1998. (21) Okano, M.; Itoh, K.; Fujishima, A.; Honda, K. Chem. Lett. 1986, 469-472. (22) Okano, M.; Itoh, K.; Fujishima, A.; Honda, K. J. Electrochem. Soc. 1987, 134, 837-841. (23) Okano, M.; Kikuchi, E.; Itoh, K.; Fujishima, A. J. Electrochem. Soc. 1988, 135, 1641-1645. (24) Yoneyama, H.; Kawai, K.; Kuwabata, S. J. Electrochem. Soc. 1988, 135, 1699-1702. (25) Zhang, H. T.; Bebel, J. C.; Hupp, J. T. J. Electroanal. Chem. 1989, 261, 423-429. (26) Wuu, Y. M.; Fan, F. R. F.; Bard, A. J. J. Electrochem. Soc. 1989, 136, 885-886. (27) Gould, S.; Otoole, T. R.; Meyer, T. J. J. Am. Chem. Soc. 1990, 112, 9490-9496. (28) Gould, S.; Gray, K. H.; Linton, R. W.; Meyer, T. J. Inorg. Chem. 1992, 31, 5521-5525. (29) Leasure, R. M.; Moss, J. A.; Meyer, T. J. Inorg. Chem. 1994, 33, 1247-1248. (30) Gould, S.; Leasure, R. M.; Meyer, T. J. Chem. Br. 1995, 31, 891893. (31) Angelopoulos, M.; Shaw, J. M.; Lee, K. L.; Huang, W. S.; Lecorre, M. A.; Tissier, M. J. Vac. Sci. Technol., B 1991, 9, 3428-3431. (32) Yang, R.; Naoi, K.; Evans, D. F.; Smyrl, W. H.; Hendrickson, W. A. Langmuir 1991, 7, 556-558. (33) Vandyke, L. S.; Brumlik, C. J.; Martin, C. R.; Yu, Z. G.; Collins, G. J. Synth. Met. 1992, 52, 299-304. (34) Cai, S. X.; Kanskar, M.; Nabity, J. C.; Keana, J. F. W.; Wybourne, M. N. J. Vac. Sci. Technol., B 1992, 10, 2589-2592. (35) Huang, W. S. Polymer 1994, 35, 4057-4064. (36) Lowe, J.; Holdcroft, S. Macromolecules 1995, 28, 4608-4616. (37) Lowe, J.; Holdcroft, S. Synth. Met. 1997, 85, 1427-1430. (38) Yu, J. F.; Abley, M.; Yang, C.; Holdcroft, S. J. Chem. Soc., Chem. Commun. 1998, 1503-1504. (39) Wu, Y.; Pfennig, B. W.; Bocarsly, A. B.; Vicenzi, E. P. Inorg. Chem. 1995, 34, 4262-4267.

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include laser writing,21-25,33,44 e-beam writing,34,47 photolithography,7,27-31,36-38,40,41 microcontact printing,48 inkjet printing,45,49 and holography.42 Although these efforts have been motivated primarily by interest in basic science, it is possible to envision a number of technologically important uses for surfaces that are modified with microscopically patterned redox- or electronically-conducting polymer films such as fabrication of microelectronic or -photonic devices 31,41 or microsensor arrays.29 Several years ago we initiated a research program that is directed toward development of photolithographic methods for creating arbitrary51,52 microscopic patterns of electrochemically active polymer films on electrode surfaces.53-55 Initially our work focused on the fabrication and electrochemical characterization of photolithographically-patterned polymer films consisting of redox or conducting polymers. However, during the course of our studies we recognized that it is possible to utilize the micropatterned, electrochemically-active films to create optical diffraction gratings that feature a diffraction efficiency that can be modulated with an electrochemical stimulus. Thus, our work has been expanded to include studies directed at exploring the effect of diffraction efficiency modulation using gratings comprised of electroactive polymers. The studies of electroactive gratings carried out to date focus on defining their response to an electrochemical stimulus and on developing an understanding of the underlying basis for the observed optical switching effects. However, in the longer term we envision the possible application of photolithographically-patterned electroactive materials to allow the fabrication of (electro)chemically-stimulated optical switching devices, photonic crystal materials where the “photonic band gap” can be tuned electrochemically,56,57 or holograms that can be switched “on and off” by an electrochemical stimulus. The objective of this article is to provide an overview of our work in the area of photolithographic patterning of electrochemically active polymers and their application (40) Bocarsly, A. B.; Chang, C. C.; Wu, Y.; Vicenzi, E. P. J. Chem. Educ. 1997, 74, 663-667. (41) Renak, M. L.; Bazan, G. C.; Roitman, D. Adv. Mater. 1997, 9, 392-395. (42) Gigli, G.; Rinaldi, R.; Turco, C.; Visconti, P.; Cingolani, R. App. Phys. Lett. 1998, 73, 3926-3928. (43) Lidzey, D. G.; Bradley, D. D. C.; Martin, S. J.; Pate, M. A. IEEE J. Quantum Electron. 1998, 4, 113-118. (44) Wong, T. K. S.; Gao, S.; Hu, X.; Liu, H.; Chan, Y. C.; Lam, Y. L. Mater. Sci. Eng., B 1998, 55, 71-78. (45) Pede, D.; Serra, G.; De Rossi, D. Mater. Sci. Eng., C 1998, 5, 289-291. (46) Kobayashi, N.; Teshima, K.; Hirohashi, R. J. Mater. Chem. 1998, 8, 497-506. (47) Persson, S. H. M.; Dyreklev, P.; Inganas, O. Adv. Mater. 1996, 8, 405-408. (48) Huang, Z. Y.; Wang, P. C.; MacDiarmid, A. G.; Xia, Y. N.; Whitesides, G. Langmuir 1997, 13, 6480-6484. (49) Chang, S.-C.; Bharathan, J.; Yang, Y.; Helgeson, R.; Wudl, F.; Ramey, M.; Reynolds, J. R. Appl. Phys. Lett. 1998, 73, 2561-2563. (50) Smela, E. J. Micromech. Microeng. 1999, 9, 1-18. (51) By arbitrary we mean that the electroactive materials can be patterned in the plane of the electrode surface into any microstructure defined by the user, regardless of the shape of the electrode. Note that Wrighton and co-workers pioneered the use of microelectrode arrays to produce (nonarbitrary) patterns of electroactive materials (ref 52). (52) Natan, M. J.; Wrighton, M. S. Prog. Inorg. Chem. 1989, 37, 391494. (53) Hauser, B. T.; Bergstedt, T. S.; Schanze, K. S. J. Chem. Soc., Chem. Commun. 1995, 19, 1945-1946. (54) Bergstedt, T. S.; Hauser, B. T.; Schanze, K. S. J. Am. Chem. Soc. 1994, 116, 8380-8381. (55) Schanze, K. S.; Bergstedt, T. S.; Hauser, B. T. Adv. Mater. 1996, 8, 531. (56) Joannopoulos, J. D. Photonic Crystals: Molding the Flow of Light; Princeton University Press: Princeton, NJ, 1995. (57) Yoshino, K.; Kawagishi, Y.; Tatsuhara, S.; Kajii, H.; Lee, S.; Fujii, A.; Ozaki, M.; Zakhidov, A. A.; Vardeny, Z. V.; Ishikawa, M. Microelectron. Eng. 1999, 47, 49-53.

Schanze et al. Scheme 1

to the construction of optical diffraction gratings. Most of the work that is described herein has not been previously published, and experimental details are available as Supporting Information. II. Background: Related Work on Photolithographic Patterning of Electroactive and/or Conjugated Polymers II.A. Redox Polymers. To the best of our knowledge, Hupp and co-workers were the first to report a method for creating arbitrary microscopic patterns of redox polymers on conducting substrates.25 Their method takes advantage of the photovoltage produced by band gap illumination of a semiconductor electrode to selectively electropolymerize redox-active transition metal complexes in areas that are being illuminated. For example, 200 µm wide lines of the redox polymer poly-Ru(vbpy)32+ (vbpy ) 4-vinyl-4′-methyl2,2′-bipyridine) were created by using a Kr+ laser (647 nm) to illuminate a single-crystal wafer of p-type silicon immersed in a solution of the Ru(vbpy)32+ monomer. Due to the significant photovoltage produced by band gap illumination of p-Si (+0.5 V), Ru(vbpy)32+ undergoes cathodic electropolymerization selectively in regions that are illuminated by the laser. By focusing the laser to a small spot (φ ≈ 100 µm), it is possible to write “lines” of the polymer on the electrode surface. Although the method is general in terms of the types of electroactive monomers that can be polymerized, the spatial resolution is limited to 100 µm, presumably because of competing polymerization processes that occur in the dark regions of the semiconductor electrode. More recently, Meyer and co-workers devised a general strategy for producing arbitrary micropatterns of electropolymerized films on conducting substrates.7,27-30 Meyer’s strategy relies on the photochemical lability of pyridyl ligands in complexes of the type [(bpy)RuII(py)2]2+, where bpy is 2,2′-bipyridine or a related bidentate ligand and py is a substituted pyridine ligand. Thus, in the presence of a suitable nucleophile such as halide (X-) or dimethylthiocarbamate (dmdtc-), [(bpy)RuII(py)2]2+ and related complexes undergo photosubstitution of the pyridyl ligands to afford the corresponding mono- and di-substituted complexes [(bpy)RuII(py)(X)]+ and [(bpy)RuII(X)2].58 Interestingly, in dry films of electropolymerized [(bpy)2Ru(vpy)2]2+ the 4-vinylpyridine (vpy) ligands remain photolabile. Visible light photolysis of a dry film of poly[(bpy)2Ru(vpy)2]2+ which has been presoaked in a solution containing dmdtc- leads to an increase in the film’s solublity. This change in solubility is believed to arise because irradiation induces photosubstitution of the vpy ligands by dmdtc-, as shown in Scheme 1. Since the (bpy)2RuII unit is a cross-link in the poly-[(bpy)2Ru(vpy)2]2+ matrix, photolysis decreases the cross-link density, thereby increasing film solubility. Photolysis of poly-[(bpy)2Ru(vpy)2]2+ films through a contact photolithographic mask followed by development of the latent image with a suitable solvent allows formation of arbitrary micropatterns with (58) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444-2453.

Photolithographically-Patterned Electroactive Films Scheme 2

spatial resolution on the order of 30 µm. In a series of publications Meyer and co-workers extended this methodology to a variety of metal complexes that are structurally related to [(bpy)2Ru(vpy)2]2+.7,27-30 Moreover, they demonstrate the use of poly-[(bpy)2Ru(vpy)2]2+ and related photochemically reactive redox polymers as positive tone photoresists to control the patterning of other electropolymerizable monomers such as Ru(vbpy)32+ and Os(vbpy)32+ (vbpy ) 4-vinyl-4′-methyl-2,2′-bipyridine). Bocarsly and co-workers developed a unique approach to the creation of arbitrary micron-scale 2-D patterns of redox-active polymer and oligomer films composed of cyanometalates.39,40 Their approach relies on the photoinduced depolymerization of electropolymerized films consisting of the mixed metal cyanometalate [NC-FeII(CN)4-CN-PtIV(NH3)4]n (Scheme 2). Visible-light photolysis of a water-immersed, indium tin oxide (ITO) electrode that is modified with a thin film of [NC-FeII(CN)4-CN-PtIV(NH3)4]n leads to loss of the cyanometalate polymer from regions that are exposed to light. If the exposure is carried out through a lithography mask, the cyanometalate polymer functions as a positive-tone resist; images with 3 µm resolution have been obtained by using a simple contact exposure method. An interesting extension of this chemistry is afforded if the exposure is carried out with the [NC-FeII(CN)4-CN-PtIV(NH3)4]n film immersed in an aqueous solution of a third transition metal ion such as Ni2+.39 In this case the exposed areas are converted to the mixed metal cyanometalate salt [NiFe(CN)6]-1 (Scheme 2). By carrying out the exposure through a mask, this method allows the fabrication of “designer” surfaces consisting of arbitrary patterns of different redox-active cyanometalates having different redox and optical properties. By applying this method, Bocarsly and co-workers fabricated a full-color, 1.5 cm × 1.0 cm image of the U.S. flag on an ITO electrode with feature sizes on the order of 100 µm.40 II.B. Conjugated Polymers. Because of the significant potential for the use of conjugated (semi)conducting polymers such as polyaniline, polypyrrole, polythiophene, and polyphenylene vinylene (PPV) in electronic and photonic devices, there has been a surge of interest in the development of lithographic methods that can be utilized to fabricate arbitrary micron-scale patterns of these materials on a variety of substrates.21-24,26,31,33-38,41,42,44-50,59 Due to the widespread interest in this field, in the following paragraphs we only review work that is directly relevant to our own research on lithographic patterning of conjugated polymers. The first work that was carried out to create arbitrary 2-D patterns of conjugated polymers was reported by Honda and co-workers.21-23 Using a method similar to that described by Hupp,25 the Honda group used the photovoltage produced by band gap illumination of TiO2 and ZnO semiconductor electrodes to create arbitrary 2-D patterns of electropolymerized films of polypyrrole by illuminating the electrode through a lithography mask.60 (59) Yang, R.; Evans, D. F.; Hendrickson, W. A. Langmuir 1995, 11, 211-213. (60) The Honda group reported the use of the photovoltage effect to selectively carry out electropolymerization in an illuminated region of an electrode surface prior to Hupp’s report.

Langmuir, Vol. 16, No. 2, 2000 797 Chart 1

Scheme 3

Unfortunately, the spatial resolution of their method was limited to 100 µm; it was suggested that the poor resolution may be due to the high rate of the photosensitized electropolymerization process. Holdcroft and co-workers developed several different approaches to create photolithographically defined patterns of poly-3-alkylthiophenes on insulating or conducting substrates.36-38 In their first report they describe fabrication of high-resolution patterns of films composed of the acrylate-functionalized 3-alkylthiophene copolymer PTh-1 (Chart 1).36 These patterns were produced by first irradiating a spin-coated film of PTh-1 through a mask that was in contact with the film-coated substrate followed by development of the latent image with chloroform. Negative-tone images were produced with a spatial resolution of 320 nm, allowing BEE to efficiently absorb 366 nm light. Parts a and b of Figure 3 illustrate optical microscope and SEM images of a photolithographically-patterned poly-VBV/PETA film created using a USAF 1951 reticle. These images indicate that resolution on the order of 10 µm is achieved using contact lithography. The magnified view in Figure 3b clearly shows that there is some residual material (present as a very thin film) that is randomly distributed in regions of the substrate that were not exposed to light; nonetheless, over 90% of the unexposed area is free of material. In addition, a thin film of material is apparent in regions adjacent to exposed areas (i.e., within 2 µm). This material is likely produced by edge diffraction effects and/or by diffusion of radicals from illuminated regions into unexposed areas. Figure 3c illustrates a side-on view of a grating-patterned film of poly-VBV/PETA that was created by using a Ronchi grating mask (100 lines mm-1). This image indicates that the poly-VBV/PETA lines have a thickness of ≈500 nm. The SEM observation is confirmed by profilometry scans carried out on a number of grating-patterned films which indicate typical poly-VBV/PETA film thicknesses of 400500 nm. Viologen compounds typically feature three redox states that can be reversibly accessed in deoxygenated solution: a dication (V2+), a singly-reduced cation radical (V•+) and a doubly-reduced neutral species (V0). A number of electrochemical studies carried out on viologen-containing polymers that are adsorbed onto electrode surfaces indicate that the surface-confined viologen units display similar redox properties to those of the monomer in solution, with some exceptions to be discussed below.65-69 Figure 4 illustrates a typical cathodic cyclic voltammetry scan for a grating-patterned poly-VBV/PETA film on an ITO electrode immersed in 0.1 M (n-Bu)4NPF6/CH2Cl2 (v ) 10 mV s-1). The voltammogram features reversible (65) Lewis, T. J.; White, H. S.; Wrighton, M. S. J. Am. Chem. Soc. 1984, 106, 6947-6955. (66) Dalton, D. F.; Murray, R. W. J. Phys. Chem. 1991, 95, 63836389. (67) Hatozaki, O.; Ohsaka, T.; Oyama, N. J. Phys. Chem. 1992, 96, 10492-10497. (68) Ostrom, G. S.; Buttry, D. A. J. Phys. Chem. 1995, 99, 1523615240. (69) Terrill, R. H.; Hutchison, J. E.; Murray, R. W. J. Phys. Chem. B 1997, 101, 1535-1542.

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waves at E1/2 ) 0.0 (VBV2+/•+) and E1/2 ) -0.45 V (VBV•+/0) due to sequential one-electron reduction of the viologen units. Careful inspection of the voltammogram in Figure 4 reveals several important features. First, the peak currents for the cathodic and anodic waves for the VBV•+/0 couple are substantially larger than those for VBV2+/•+. Second, the current waves for the VBV2+/•+ couple are broad and “tail” significantly, while those for VBV•+/0 are sharper, are more symmetrical, and “tail” less. These voltammetric properties indicate that charge transport in the poly-VBV/PETA film is considerably slower for the VBV2+/•+ couple compared to that for VBV•+/0. The diminished current peak amplitudes for VBV2+/•+ arise because of slow charge and/or ion transport; consequently, VBV2+/•+ reduction is incomplete on the time scale of the cathodic scan. A variety of different viologen-based redox polymers produce a similar voltammetric response compared to that of photolithographically-patterned poly-VBV/PETA films.65-74 Specifically, it is generally seen that the voltammetric response of the V2+/•+ couple indicates slower charge transport compared to that for the V•+/0 couple. Effective diffusion coefficients for charge transport have been determined for three different viologen-containing polymers, and in each case charge transport is 10- to 20fold lower for the V2+/•+ couple compared to that for V•+/0.65-71 The difference in charge-transport rates for the two mixed-valence states is attributed to several factors, including: (1) a larger work term for the V2+/•+ couple compared to that for V•+/0 couple due to electrostatic repulsion; (2) larger ion-pairing effects for the V2+/•+ couple which may impede ion transport; (3) a film shrinkageinduced increase of the effective redox unit concentration for the V•+/0 couple.65-67 The important point is that our observations, which imply slower charge transport for the VBV2+/•+ couple relative to that for the VBV•+/0 couple in the VBV/PETA films, are intrinsic to viologen-based polymer systems and are not an artifact unique to the VBV/PETA acrylate matrix that is created by photopolymerization. Additional insight into the nature of the redox states in the VBV/PETA films comes from spectroelectrochemistry carried out on grating-patterned poly-VBV/PETA films. Figure 5 illustrates difference absorption spectra (relative to the spectrum of the fully oxidized polymer)75 obtained for a poly-VBV/PETA film at potentials negative of E1/2 for the VBV2+/•+ and VBV•+/0 couples (E ) -0.3 V and -0.7 V, respectively). A significant feature is evident from close inspection of the absorption spectrum at E ) -0.3 V, which is nominally for the film in the “VBV•+ state”. The spectrum features absorption maxima at 375 and 550 nm with a shoulder at 394 nm. This spectrum is inconsistent with that of a monomeric viologen cation radical (V•+) which exhibits absorption bands at 395 and 605 nm.76 By contrast, the spectrum obtained for the polyVBV/PETA film at E ) -0.3 V is consistent with the dimer (70) Bookbinder, D. C.; Wrighton, M. S. J. Electrochem. Soc. 1983, 130, 1080-1087. (71) Dominey, R. N.; Lewis, T. J.; Wrighton, M. S. J. Phys. Chem. 1983, 87, 5345-5354. (72) Smith, D. K.; Lane, G. A.; Wrighton, M. S. J. Am. Chem. Soc. 1988, 92, 2616-2628. (73) Johansen, O.; Loder, J. W.; Mau, A. W.-H.; Rabani, J.; Sasse, W. H. F. Langmuir 1992, 8, 2577-2581. (74) Schlenoff, J. B.; Laurent, D.; Ly, H.; Stepp, J. Adv. Mater. 1998, 10, 347-349. (75) Since the fully oxidized poly-VBV/PETA film is transparent for λ > 350 nm, the difference spectra shown in Figure 5 closely correspond to the true absorption of the film in two reduced states. (76) Kosower, E. M.; Cotter, J. C. J. Am. Chem. Soc. 1964, 86, 55245527.

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Figure 5. Spectroelectrochemistry of a grating-patterned polyVBV/PETA film (100 lines mm-1). The film is supported on an ITO working electrode, the electrolyte is 0.1 M (n-Bu)4NPF6/ CH2Cl2, and a Pt wire auxiliary electrode and an Ag/AgCl quasireference electrode are used. Difference spectra relative to the film in the fully oxidized state. Solid line: Eapp ) -0.3 V (V•+ state). Dashed line: Eapp ) -0.7 V (V0 state).

dication formed by stacking of two viologen cation radicals, that is, 2 V•+ f [V]22+.76 Since dimerization is believed to occur via face-to-face π-stacking between two cation radicals,73,77,78 the observation that the dimer absorption dominates the spectrum of reduced poly-VBV/PETA implies that in the film the viologen moieties (1) are in close proximity and/or (2) have enough mobility to diffuse together to form the π-stack. The spectral signature of [V]22+ is observed for other electropolymerized viologens70,72 and for viologen multilayer films prepared by electrostatic self-assembly,74 and therefore our observations on poly-VBV/PETA films are not unique. The fact that dimerization occurs concomitant with the VBV2+/•+ reduction may also contribute to the relatively broad voltammetric waves observed for the first reduction process, since when dimer formation is possible, the reduction potential for formation of the cation radical is shifted to more positive potentials.72,78 Repeated cathodic cyclic voltammetry scans of gratingpatterned poly-VBV/PETA films in 0.1 M (n-Bu)4NPF6/ DMF induce a loss of the electrochemical response of the VBV units (the peak current is decreased by ≈50% after 10 scans through the first wave). For reasons that will become apparent, we believe that the diminished electrochemical response occurs because the VBV dissolves from the matrix. Interestingly, the stability of the electrochemical response of the poly-VBV/PETA film is substantially improved when the electrolyte is changed to (n-Bu)4NCl. This difference is related to the solubilities of the viologen salts; the PF6- salt of VBV2+ is readily soluble in organic solvents while the Cl- salt is virtually insoluble. Thus, when the poly-VBV/PETA film is immersed into (n-Bu)4NCl/DMF, PF6- to Cl- anion exchange occurs at the VBV sites in the film. Due to the low solubility of the VBV chloride salt in the solvent, the VBV redox sites remain dispersed in the polymer. The significance of these observations relates to the structure and morphology of the poly-VBV/PETA film. The loss of electrochemical response from the films in (nBu)4NPF6 electrolyte signals that some or all of the VBV is not covalently bound to the photopolymer. This is not surprising, given the substantially greater reactivity of acrylate monomers compared to styrene monomers in free (77) Imabayashi, S.; Kitamura, N.; Tazuke, S.; Tokuda, K. J. Electroanal. Chem. 1988, 243, 143-160. (78) Tang, X.; Schneider, T. W.; Walker, J. W.; Buttry, D. A. Langmuir 1996, 12, 5921-5933.

Figure 6. Scheme used for the photolithography-electropolymerization method.

radical polymerization.79 It is reasonable to assume that poly-VBV/PETA consists of a cross-linked matrix (or gel) created by polymerization and cross-linking of PETA. Much of the VBV is likely dispersed within (but not covalently bound to) the PETA matrix. III.C. Photolithography-Electropolymerization: Patterned Electropolymerized Films. Despite having relatively good success with the photolithographic patterning and electrochemistry of poly-VBV/PETA films, we were still disappointed with the comparatively poor stability of the films to electrochemical cycling. Given the superb electrochemical response and long-term stability that are characteristic of films prepared by electropolymerization of redox-active monomers,61,80,81 we decided to develop a method that would allow the use of photolithography to pattern electropolymerized films. The method that was developed is illustrated in Figure 6. This method, which we dub “PLEP” utilizes poly-methylphenylsilane (p-MPS) as a positive-tone resist.82-84 The key to the success of PLEP lies in (1) the excellent spatial resolution obtained with a p-MPS resist82 and (2) the fact that p-MPS is insoluble in typical polar solvents (CH3CN, DMF) used for electropolymerization.84 PLEP was used to fabricate 2-D micropatterns of the redox polymers formed by reductive electropolymerization of [(bpy)2Ru(vpy)2]2+ and [Os(vbpy)3]2+. Studies by a number of groups demonstrate that cathodic cycling of an electrode dipped into an electrolyte solution of either of these complexes leads to formation of a stable, redoxactive film of the metallopolymer.2,61,80 It is believed that the electropolymerization occurs by C-C bond formation between the ligand-based anion radicals formed by reduction of the metal complexes (i.e., [(bpy)2Ru(vpy)(vpy•-)]+ and [Os(vbpy)2(vbpy•-)]+).80 (79) Odian, G. Principles of Polymerization, 3rd ed.; Wiley-Interscience: New York, 1991. (80) Calvert, J. M.; Schmehl, R. H.; Sullivan, B. P.; Facci, J. S.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1983, 22, 2151-2162. (81) Pickup, P. G.; Murray, R. W. J. Am. Chem. Soc. 1983, 105, 45104514. (82) Miller, R. D.; Hofer, D.; Fickes, G. N.; Willson, C. G.; Marinero, E.; Trefonas, P., III; West, R. Polym. Eng. Sci. 1986, 26, 1129-1134. (83) Miller, R. D.; Michl, J. Chem. Rev. 1989, 89, 1359-1410. (84) Tachibana, Y.; Sakurai, Y.; Yokoyama, M. Chem. Lett. 1994, 1119-1122.

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Figure 7. (a) Cathodic-sweep cyclic voltammogram produced during PLEP fabrication of a grating-patterned poly-[Os(vbpy)3]2+ film. The ITO working electrode (area ) 0.0396 cm2) is coated with a grating-patterned film of p-MPS. The working electrode is immersed in a 0.1 M (n-Bu)4NPF6/CH3CN electrolyte solution that contains 1.0 mM [Os(vbpy)3]2+. The potential is swept 9 times at 100 mV s-1. The reversible waves are due to vbpy-ligand-based reductions. (b and c) Anodic-sweep cyclic voltammetry of grating-patterned films of poly-[(bpy)2Ru(vpy)2]2+ (b) and poly-[Os(vbpy)3]2+ (c) on an ITO working electrode (area ) 0.0396 cm2) immersed in 0.1 M (n-Bu)4NPF6/ CH3CN with a Pt wire auxiliary electrode and an Ag/AgCl quasireference electrode. Scan rates (in order of increasing current): 10, 20, 50, and 100 mV s-1. The reversible waves are due to the MII/III couples.

In a typical PLEP procedure, an ITO electrode is coated with a 250-300 nm thick film of p-MPS84 by spin-coating the polymer from toluene. The p-MPS-coated slide is then exposed for 15 min to near-UV light through a contact lithography mask, and then the latent positive-tone image is developed by immersion in 2-propanol. The ITO electrode that is coated with the patterned p-MPS resist is then immersed into 0.1 M (n-Bu)4NPF6/CH3CN that contains 1.0 mM Os(vbpy)32+ and the potential of the electrode is cycled 9 times at 100 mV s-1 from -0.8 to -1.6 V. Figure 7a illustrates the CV response of an ITO electrode coated with a grating-patterned film of p-MPS (100 lines mm-1) that is immersed into a solution of Os(vbpy)32+. The increase in the peak currents with each successive scan is characteristic of the growth of the electroactive poly-Os(vbpy)32+ film on the electrode surface.61 After the electropolymerization is complete, the p-MPS film is removed by submersing the ITO electrode in CHCl3. Profilometry scans of photolithographicallypatterned films of poly-[(bpy)2Ru(vpy)2]2+ and poly-[Os(vbpy)3]2+ indicate that a 100 nm film thickness is typically obtained by the PLEP method.

Schanze et al.

Figure 8 illustrates optical microscope images of films of p-MPS, poly-[(bpy)2Ru(vpy)2]2+, and poly-[Os(vbpy)3]2+ fabricated by PLEP using a USAF 1951 target mask for exposure of the p-MPS resist. Figure 8a shows that the resolution of the p-MPS resist film is nearly diffractionlimitedswhen it is viewed at high magnification, features with sizes on the order of 5 µm are clearly resolved. By contrast, the spatial resolution obtained for poly-[(bpy)2Ru(vpy)2]2+ and poly-[Os(vbpy)3]2+ is lower. Features with sizes smaller than 7 µm × 30 µm created using the USAF 1951 target are missing or distorted. Experience with many films fabricated by using PLEP indicates that very small areas of the electropolymerized material tend to lift off of the ITO substrate because of poor adhesion. The fact that film-substrate adhesion is the ultimate limit to resolution is supported by the observation that it is possible to create well-resolved grating patterns of poly-[(bpy)2Ru(vpy)2]2+ and poly-[Os(vbpy)3]2+ having line widths of 5 µm. In this case adhesion does not pose a problem because of the large area under the individual polymer lines in the grating (i.e., the polymer lines in the grating are approximately 5 µm × 3000 µm).85 Grating-patterned films of poly-[(bpy)2Ru(vpy)2]2+ and poly-[Os(vbpy)3]2+ fabricated by PLEP display an excellent electrochemical response. As seen in Figure 7b and c, films of both complexes feature reversible waves for the MII/III couples. In each case the peak currents vary approximately linearly with scan rate as expected for surface-confined redox units. The peak potentials shift somewhat with scan rate; however, this effect is likely due to the comparatively high resistance of the ITO electrode. Integration of the current under the MII/III waves for a number of gratingpatterned poly-[(bpy)2Ru(vpy)2]2+ and poly-[Os(vbpy)3]2+ films indicates that surface coverage values of Γ ) 1-2 × 10-8 mol cm-1 are typical for films fabricated by PLEP.86 These surface coverages are consistent with 100-200 monolayers of material, and assuming a film thickness of ≈1 nm per layer, the electrochemical results are in rough agreement with the film thickness values obtained by profilometry.87 IV. Photolithographic Patterning of Conjugated Polymers: PMDOT-Acrylate A second avenue that we have pursued in our work on micropatterning of redox-active materials has been to develop a photolithographic method for patterning a thiophene-based conjugated polymer. As noted in section II, a number of other groups have developed methods to pattern conjugated polymers.21-24,26,31,33-38,41,42,44-50,59 However, our work is unique in that it focuses on a specific thiophene derivative, poly-3,4-dialkoxythiophene (PEDOT). This polymer is of interest because it undergoes p-doping at significantly lower anodic potentials compared to unsubstituted poly-thiophene or poly-3-alkylthiophenes, and consequently PEDOT is considerably more stable to repeated electrochemical cycling between the p-doped and undoped forms.19,88-91 Moreover, undoped PEDOT has a (85) Adhesion of the polymer film to the substrate is a cooperative process; therefore, the greater the area of a given polymer region, the better the adhesion. The smaller regions in the USAF 1951 target have relatively small areas (e.g., 7 µm × 30 µm ) 210 µm2), while the lines in the grating pattern have significantly greater area (i.e., 5 µm × 3000 µm ) 15 000 µm2). Thus, we attribute the improved adhesion of the polymer lines in the grating pattern to the larger area under a line. (86) Attempts to prepare thicker films by cycling for a longer period of time invariably lead to a decrease in the spatial resolution. (87) Denisevich, P.; Abrunã, H. D.; Leidner, C. R.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1982, 21, 2153-2161. (88) Pei, Q.; Zuccarello, G.; Ahlskog, M.; Igana¨s, O. Polymer 1994, 35, 1347-1351.


Langmuir, Vol. 16, No. 2, 2000 803

Figure 8. Optical microscope images of films created during PLEP procedure. (a) Image of p-MPS film on ITO electrode. Polymer is in gray region (background)slines and boxes are areas of electrode that are not coated with p-MPS film. (b) Electropolymerized film of poly-[(bpy)2Ru(vpy)2]2+ on ITO electrode. Polymer is in the lighter colored areas in the image. (c) Electropolymerized film of poly-[Os(vbpy)3]2+ on ITO electrode. Polymer is in the lighter colored areas in the image. Scheme 4

comparatively low band gap and absorbs strongly in the mid-visible.88,90 This feature makes the material uniquely suited for the optical modulation studies that we have carried out with electroactive gratings (this work is described in section V below). The material that is the basis for our work is the acrylate ester-substituted copolymer PMDOT-Acr/Dec (Scheme 4). This polymer is prepared by FeCl3-initiated polymerization of a mixture of the monomers MDOT-Acr and MDOTDec.92-94 In a typical photolithographic procedure, a CHCl3 solution of PMDOT-Acr/Dec (c ) 30 mg mL-1) and BEE

(c ) 1 mg mL) is spin-cast onto an ITO substrate. The resulting film-coated substrate is brought into contact with a photolithography mask and the assembly is exposed to

(89) Sankaran, B.; Reynolds, J. R. Macromolecules 1997, 30, 25822588. (90) Sotzing, G. A.; Reynolds, J. R.; Steel, P. J. Adv. Mater. 1997, 9, 795-797. (91) Irvin, J. A.; Reynolds, J. R. Polymer 1998, 39, 2339-2347.

(92) The monomers MDOT-Acr and MDOT-Dec were prepared by acylation of the alcohol-substituted thiophene monomer using acryloyl chloride and decanolyl chloride. The alcohol-substituted thiophene monomer was prepared by a modified literature procedure (refs 93 and 94).

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Figure 10. Cyclic voltammetry of (a) an electropolymerized film of MDOT-Acr and (b) a grating-patterned film of polyPMDOT-Acr/Dec. Conditions for both scans: ITO working electrode, 0.1 M (n-Bu)4ClO4/CH3CN electrolyte, Pt wire auxiliary electrode, Ag/AgCl quasi-reference electrode, scan rate ) 100 mV s-1.

Figure 9. Optical microscope images of a PMDOT-Acr/Dec film on an ITO electrode: (a) Film patterned using a USAF 1951 reticle. The large bars next to the 1 in the lower righthand corner of the image are ≈35 µm in length. (b) Gratingpatterned film with the spatial frequency 100 lines mm-1 (i.e., the width of 1 line + 1 gap ) 10 µm).

near-UV light for 1-2 min. The latent image is then developed by immersing the exposed film in 1,2-dichlorobenzene. The resulting photopolymer pattern is a negative-tone image of the lithography mask. The decrease in solubility that occurs upon light exposure arises from cross-linking of the polymer via the acrylate side groups.36 Figure 9a shows an optical microscope image of a PMDOTAcr/Dec pattern produced using the USAF 1951 mask. This image shows PMDOT-Acr/Dec features with sizes 800 nm). The low-energy absorption band of the p-doped material is consistent with the mid-gap absorption bands that occur in the near-IR due to the bipolaron states of the doped polymer.88,90 (95) The electropolymerized film of poly-MDOT-Acr was obtained by immersing an ITO electrode into a 0.1 M (n-Bu)4NClO4/CH3CN solution of MDOT-Acr (c ) 5 mM and 10 potential cycles from -1.0 to -1.8 V).


Langmuir, Vol. 16, No. 2, 2000 805 Chart 3

Figure 11. Spectroelectrochemistry of a grating-patterned film of poly-PMDOT-Acr/Dec on an ITO working electrode. Difference spectra are recorded relative to the spectrum of the undoped film. Spectra are obtained at the following applied potentials (in order of decreasing absorbance at 540 nm): -0.2, -0.1, 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 0.9 V relative to the Ag/AgCl quasi-reference. Inset: Plot of ∆A versus Eapp at 634 nm.

A point that will become important later concerns the complex behavior of the spectroelectrochemical changes that occur for PMDOT-Acr/Dec in the red. The inset to Figure 11 reveals that the absorptivity at 633 nm initially decreases with increasing anodic bias, reaches a minimum absorbance at Eapp ≈ +0.4 V, and then increases with applied potential. This complex change in absorptivity apparently arises because at lower applied potential oxidation preferentially occurs on polymer segments with lower oxidation potential. Since segments with lower oxidation potential also have a smaller band gap, bleaching of the visible absorption band at lower applied potentials occurs preferentially in the red. Then, with increasing applied potential, relatively less conjugated chains become oxidized and the absorption bleaching blue-shifts. Additionally, the absorbance in the red increases as the near-IR transition of the bipolaron state gains intensity.88,90 V. Electrochemically Modulated Diffraction Gratings During the course of our work on micropatterned redoxactive polymers, it became apparent that photolithography could be applied to fabricate electrochromic optical diffraction gratings consisting of the redox polymers. The diffraction efficiency (DE)96 of a grating depends on the optical properties (the absorptivity k and the refractive index n) of the grating material. Since k and n vary with the redox state of a material, we reasoned that it would be possible to use an electrochemical stimulus to modulate the DE of a redox-active grating. This proves to be the case: photolithographic gratings fabricated using any of the materials described in the preceding sections diffract and the diffraction efficiency can be modulated by redox switching.97-101 While the studies outlined herein have (96) The diffraction efficiency is defined as the intensity of the firstorder diffracted beam (I1) divided by the intensity of the incident beam (I0), that is, DE ) I1/I0. (97) Several other groups have reported the fabrication of gratings that can be switched by an electronic or electrochemical stimulus (refs 98-101). (98) Zhang, J.; Carlen, C. R.; Palmer, S.; Sponsler, M. B. J. Am. Chem. Soc. 1994, 116, 7055-7063. (99) Lindquist, R. G.; Kulick, J. H.; Nordin, G. P.; Jarem, J. M.; Kowel, S. T.; Friends, M. Opt. Lett. 1994, 19, 670-672. (100) Whitney, D. H.; Ingwall, R. T. SPIE Vol. 1213 Photopolymer Device Physics, Chemistry, and Applications 1990, 1213, 18-26.

focused on cataloging the properties of the electroactive optical diffraction gratings, our long-range objectives in this area are to use the technology to fabricate electrochemically switchable holograms and photonic crystals. Dubbed “molecular venetian blinds” by Bocarsly and co-workers,39 electrochromic diffraction gratings provide an interesting variation on the electrochromic effect whereby a change in the n value of the electroactive grating material is used to modulate the intensity of a diffracted beam of light (rather than using the change in k to modulate a transmitted beam). While the use of electrochemically stimulated changes in n or k to modulate the intensity of an optical beam is conceptually similar, there is an important distinction between the two processes. Specifically, when the intensity of a beam of light is attenuated by an increase in the k value of a material, the optical energy is dissipated in the film as thermal energy (i.e., optical loss occurs). By contrast, when the intensity of a diffracted beam is attenuated by changing the n value of a grating material, there is no net energy loss; rather the excess optical energy is coupled into the undiffracted (zeroth order) beam. V.A. Theory of Optical Diffraction Gratings. Chart 3 illustrates features essential to a transmission diffraction grating.102,103 In one form a transmission grating consists of a substrate coated with a thin film of a material that is patterned in the plane of the substrate (x-axis, Chart 3). The patterned film creates a spatially periodic modulation of the complex index of refraction (n(x,λ)),

n(x,λ) ) n(x,λ) + ik(x,λ)

(1)

where n(x,λ) and k(x,λ) are, respectively, the real and imaginary components of n at position x along the grating. Both k and n are functions of the wavelength of light (λ); note that k(x,λ) is the absorptivity of the film at wavelength λ and position x.104 A plane wave, monochromatic beam incident on the grating normal to the substrate’s surface will be diffracted, and the output intensity will be distributed among a set of diffracted beams (m ) 0, (1, (2, (3, ...). The diffracted beams appear at angles φm with respect to the normal, as defined by the grating (101) Sainov, S.; Mazakova, M.; Pantcheva, M.; Tontchev, D. Mol. Cryst. Liq. Cryst. 1987, 152, 609-615. (102) Tomlinson, W. J.; Chandross, E. A. Adv. Photochem. 1980, 12, 201-281. (103) Collier, R. J.; Burckhardt, C. B. Optical Holography; Academic Press: New York, 1971. (104) The absorptivity k is a unitless number and is related to the optical density (OD) of a film by the expression k ) (2.3λ OD)/(4πT), where T is the film thickness.

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equation

φm ) sin-1(mλ/d)

(2)

where λ is the optical wavelength, d is the grating period, and m is the diffracted order. The diffraction efficiency (DE) of the grating is defined by eq 3

DE ) Idiff/I0

(3)

where Idiff is the sum of the intensity in all of the diffracted beams (i.e., m ) (1, (2, etc.) and I0 is the incident light intensity. Fayer and co-workers showed that the diffraction efficiency is given approximately by eq 4 for a grating produced by a sinusoidal variation of k and/or n.105,106

[

DE(λ) ) exp -

](

)

2.3 OD(λ) πT {∆k(λ)2 + ∆n(λ)2} cos θ λ cos θ (4)

In eq 4 ∆k(λ) and ∆n(λ) are, respectively, the difference in the peak-null values of k and n at wavelength λ, OD(λ) is the average absorptivity of the grating at λ, T is the grating thickness, and θ is the Bragg angle. Equation 4 reveals that the DE is proportional to the square of the refractive index and absorptivity modulation terms. For a transmission grating that consists of an electroactive (and/or conducting) polymer film immersed in an electrolyte solution, ∆k(λ) and ∆n(λ) correspond to the difference in the absorptivity and refractive index of the film and electrolyte solution at wavelength λ. It is wellknown that switching the electrochemical potential applied to an electroactive polymer induces a change in the k(λ) and n(λ) values of the material.107-110 Given this fact, one expects that the diffraction efficiency of a transmission grating that is comprised of an electroactive material will change if the redox (or doping) state of the material is switched. Equation 5 allows one to calculate the change in the diffraction efficiency (∆DE) for a grating consisting of a material that undergoes a change in its optical properties (here we discard the λ index for DE, OD, k, and n).

∆DE )

(

){ [

]

2.3 ODf πT exp (∆kf2 + ∆nf2) λ cos θ cos θ 2.3 ODi exp (∆ki2 + ∆ni2) cos θ

[

]

}

(5)

In this equation the subscripts i and f refer, respectively, to the initial and final states of the grating material (i.e., before and after an electrochemically-induced switch in redox state). The plots in Figure 12 were computed using eq 5 by assuming that the grating material initially has T ) 200 nm, OD ) 0, and n ) 1.564, is immersed in a liquid with n ) 1.430, and has a probe wavelength ) 633 nm.111 The dashed line is calculated by assuming that (105) Fayer, M. D. Annu. Rev. Phys. Chem. 1982, 33, 63-87. (106) Nelson, K. A.; Casalegno, R.; Miller, R. J. D.; Fayer, M. D. J. Chem. Phys. 1982, 77, 1144-1152. (107) Monk, P. M. S.; Mortimer, R. J.; Rosseinsky, D. R. Electrochromism: Fundamentals and Applications; VCH: Weinheim, 1995. (108) Redondo, A.; Ticianelli, E. A.; Gottesfeld, S. Synth. Met. 1989, 29, E265-E270. (109) Gottesfeld, S. Electroanal. Chem. 1989, 15, 143-265. (110) Redondo, A.; Ticianelli, E. A.; Gottesfeld, S. Mol. Cryst. Liq. Cryst. 1988, 160, 185-203. (111) These parameters correspond to a poly-VBV/PETA grating immersed in DMF solvent.

Figure 12. Calculated change in the diffraction efficiency (∆DE, expressed as %) versus the change in the refractive index or the change in the optical density. Lines are calculated using eq 5; see text.

only the absorptivity of the grating changes (i.e., ∆ni ) ∆nf)112 while the solid line is calculated by assuming that only the refractive index changes (i.e., ∆ki ) ∆kf). These calculations clearly demonstrate that the diffraction efficiency of a grating is considerably more sensitive to changes in the refractive index of the grating material than to changes in absorptivity. Thus, a change in n of 0.001 is expected to induce a >1% change in DE, while a change in OD of 0.025 or greater is required to elicit a similar response in the DE. V.B. Relationship between Electrochromism and an Electrochemically-Induced Change in Refractive Index: The Kramers-Kronig Relation. As noted above, it is well-known that many materials exhibit a change in absorptivity concomitant with a change in redox state.107 However, it is less appreciated that the refractive index of a material will also change with the redox state (in part) due to the close relationship between the absorptivity and the refractive index.108-110 The KramersKronig relation (eq 6) allows one to calculate the change in refractive index (∆n) that is induced by a change in the absorptivity of a material.113,114

∆n(ω′) )

c π

∆R(ω)

∫0∞ω2 - ω′2 dω

(6)

In eq 6 ∆R is the change in the absorption coefficient (units ) cm-1) and ω ) 2πc/λ. To provide the reader with an understanding of the relationship between ∆n and ∆R, eq 6 was solved numerically using a simulated difference absorption spectrum (∆R(ω)) as input. The simulated difference spectrum assumes that the material of interest is initially transparent, and then after some perturbation (i.e., a change in the redox state) it features a single (Gaussian-shaped) absorption band centered at ν ) 20 000 cm-1 (∆ν1/2 ) 1500 cm-1) with Rmax ) 104 cm-1 (see Figure 13a).115 The wavelength dependence of the refractive index change (∆n) that is induced by the appearance of the new absorption band is calculated with eq 6 and is also shown in Figure 13a. This simulation reveals that the absorption (112) An OD ) 0.1 corresponds to a 100 nm thick film that contains a 1.0 M loading of a chromophore with ) 10 000 M-1 cm-1. This is consistent with a poly-VBV/PETA film in which the viologen units are in the V•+ redox state. (113) Wooten, F. Optical Properties of Solids; Academic Press: New York, 1972. (114) Olbright, G. R.; Peyghambarian, N. App. Phys. Lett. 1986, 48, 1184-1186. (115) An absorption coefficient of 104 cm-1 corresponds to a material that contains a chromophore with ) 105 M-1 cm-1 at a concentration of 1.0 M.


Figure 13. Application of the Kramers-Kronig relation (eq 6, see text) to compute the change in the refractive index that results from the change in the absorptivity. In each plot the difference absorptivity (∆R vs λ) is plotted as a dash-dot-dot line with the axis on the left, and the difference refractive index (∆n vs λ) is plotted as a solid line with the axis on the right. (a) Simulated difference absorption spectrum with λmax ) 500 nm and plot of ∆n versus λ computed using eq 6. (b) Difference absorption spectrum for V2+ f V•+ reduction and plot of ∆n versus λ computed using eq 6. (c) Difference absorption spectrum for V•+ f V0 reduction and plot of ∆n versus λ computed using eq 6. (d) Difference absorption spectrum for a PMDOT-Acr/Dec film (p-doped minus undoped polymer) and plot of ∆n versus λ computed using eq 6.

band induces a significant increase in the refractive index of the material that extends well to the red of the absorption band.106 Moreover, ∆n is null at the absorption band maximum and then becomes negative on the blue side of the absorption band.106 An important point is that if the sign of ∆R is changed (i.e., an absorption band bleaches due to a perturbation), then the sign of ∆n also changes. Thus, when an absorption band is bleached (by a change in the redox state, for example), then the refractive index will increase on the blue side and decrease on the red side of the bleached absorption band. V.C. Electrochemical Response of Poly-VBV/PETA Diffraction Gratings: Experimental Data. A 633 nm laser beam is diffracted by a grating-patterned film of poly-VBV/PETA (100 lines mm-1).64 The poly-VBV/PETA grating is optically transparent at 633 nm when the film is in the V2+ state; therefore, diffraction arises from the refractive index modulation (∆n) created by the gratingpatterned polymer film (i.e., the poly-VBV/PETA film acts as a phase grating). Because the thickness of a typical poly-VBV/PETA grating is small compared to the grating spacing (T ≈ 200 nm, d ) 10 µm), the grating is in the “thin” regime102 and therefore multiple orders of diffraction are observed (for a typical film, 6-8 orders of diffracted light are observed). Tests carried out on many poly-VBV/ PETA gratings (V2+ oxidation state) immersed in 0.1 M (n-Bu)4NCl/DMF (n ) 1.43) indicate that an absolute diffraction efficiency of 4% is typical.

Langmuir, Vol. 16, No. 2, 2000 807

Figure 14. Data from the study of the electrochemical modulation of the diffraction efficiency for a poly-VBV/PETA grating (100 lines mm-1). The grating is supported on an ITO electrode (area ) 0.0396 cm2) and immersed in a 0.1 M (nBu)4NCl/DMF electrolyte. A Pt auxiliary electrode and an Ag/ AgCl quasi-reference electrode are used. The probe beam is the 633 nm output of a frequency-stabilized, single-mode He-Ne laser. (a) Applied potential versus time. (b) Intensity of the first-order diffracted beam (I1) versus time. The beam intensity is expressed as the % change relative to the initial intensity. (c) Intensity of the transmitted beam (I0) versus time. The beam intensity is expressed as the % change relative to the initial intensity. (d) Plot of current at the working electrode versus time.

The diffraction efficiency of the poly-VBV/PETA grating can be modulated by switching the redox state of the viologen units in the film. To examine this effect, a series of experiments were carried out in which photodiodes are positioned to monitor the intensity of the undiffracted and first-order diffracted beams (I0 and I1, respectively) from a 633 nm laser beam that is incident on a poly-VBV/ PETA grating supported on an ITO electrode. In a typical experiment I0 and I1 are monitored while the applied potential is scanned or stepped. Figure 14 illustrates data acquired during an experiment where the relative diffraction efficiency of a poly-VBV/PETA grating is monitored while the electrode potential is stepped between +0.39 and -0.15 V (V2+ and V•+ states). Panel a illustrates the applied potential, while panels b, c, and d show respectively I1, I0, and the current passed at the electrode. As can be seen from panel b, on the first stepped potential cycle the intensity of the diffracted beam (I1) increases by 40% when the film is reduced to the V•+ state. Upon reoxidation the diffracted intensity decreases again; however, it does not recover fully to the initial intensity. On subsequent cycles I1 reproducibly oscillates from +10% to +40%. By contrast, the intensity of the undiffracted beam (I0, panel c) decreases by 15% when the film is reduced to the V•+ state and then recovers fully to its initial intensity when the film is reoxidized. The origin of the potential-induced changes in I1 and I0 will be discussed below; however, at this point there are

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Figure 15. Data from the study of the electrochemical modulation of the diffraction efficiency for a poly-VBV/PETA grating (100 lines mm-1). The grating is supported on an ITO electrode (area ) 0.0396 cm2) and immersed in a 0.1 M (nBu)4NCl/DMF electrolyte. A Pt auxiliary electrode and an Ag/ AgCl quasi-reference electrode are used. The probe beam is the 633 nm output of a frequency-stabilized, single-mode He-Ne laser. (a) Applied potential versus time. (b) Intensity of the first-order diffracted beam (I1) versus time. The beam intensity is expressed as the % change relative to the initial intensity. (c) Intensity of the transmitted beam (I0) versus time. The beam intensity is expressed as the % change relative to the initial intensity. (d) Plot of current at the working electrode versus time.

several significant features with regard to the data in Figure 14 that we point out. First, it is clear from the plots of I1 and I0 that the optical properties of the films respond slowly to a stepped change in applied potential. The slow response of the optical signals is mirrored by the current transients (panel d) which indicate that charge (or ion) propagation in the films is slow. This observation is consistent with the cyclic voltammetry data (Figure 4) which suggests that charge (or ion) transport in polyVBV/PETA is slow. Another feature of note concerns the hysteresis in the diffracted beam intensity (I1) after the first cycle. We believe that the hysteresis arises because initially the polycation film contains PF6-, but PF6- to Clion exchange occurs on the first reduction cycle (the electrolyte is 0.1 M (n-Bu)4NCl/DMF). Apparently PF6to Cl- ion exchange results in a slight increase in the refractive index of the film in the fully oxidized (V2+) state and consequently the “baseline” intensity of the diffracted beam is slightly higher compared to that before the first step potential cycle. Figure 15 illustrates data from an experiment where the relative diffraction efficiency of a poly-VBV/PETA grating is monitored while the electrode potential is stepped between -0.15 V and -0.60 V (V•+ and V0 states). The applied potential initially is +0.39 V (V2+ state); then the electrode potential is stepped to -0.15 V (V•+ state) and held for 2 min. Consistent with the results presented above, I1 increases and I0 decreases during this initial V2+

Schanze et al.

f V•+ reduction step. The electrode potential is then repeatedly stepped between -0.15 and -0.6 V (V•+ and V0 states, respectively). Concomitant with the V•+ f V0 reduction, I1 decreases and I0 increases; moreover, when the poly-VBV/PETA film is fully reduced (E ) -0.60 V), I0 nearly recovers to the value observed when the film is in the V2+ state. A feature that is obvious from inspection of the data shown in Figure 15 is that the dynamic response of the optical properties of poly-VBV/PETA is considerably faster when the film is switched between the V•+ and V0 states. The current transients (panel d, Figure 14) mirror the improved dynamics and support the premise made on the basis of the cyclic voltammetry that charge and/or ion transport is considerably faster in the films for the V•+/0 couple than for the V2+/•+ couple. V.D. Origin of Electrochemically Modulated Diffraction in Poly-VBV/PETA Gratings. The most interesting facet of these experiments is that redox-state switching induces significant changes in the diffraction efficiency of the grating. Moreover, the phase of the DE change is opposite for the V2+ f V•+ and V•+ f V0 reduction processes. (In the former, the DE increases concomitant with reduction, and in the latter the DE decreases with reduction.) Several terms could give rise to the redoxinduced changes in DE: (1) the change in film absorptivity (∆k) that is caused by the electrochromic effect; (2) the change in refractive index that is caused by the change in film absorptivity and counterion and solvent content; (3) the change in thickness and density that occurs concomitant with film reduction or oxidation.65 While it is reasonable to assume that all of these factors play a role in the observed effect, it seems likely that a single term is predominant. We first consider the origin of the redox-state-induced changes in DE by examining data for the V2+/•+ couple (Figure 14). The data in panel c indicate that the transmitted beam (I0) decreases in intensity by 15% concomitant with V2+ f V•+ reduction, which signals that the OD of the film at 633 nm increases by ≈0.07 (i.e., ∆OD ) 0.07). However, the DE simulation displayed in Figure 12 clearly shows that a change in OD of 0.07 for a grating will induce a

Photolithographically-Patterned Electroactive Films and

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