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Design and Preparation of Zwitterionic Organic Thin Films: Self-Assembled Siloxane-Based, Thiophene-Spaced N-Benzylpyridinium Dicyanomethanides as Nonlinear Optical Materials Antonio Facchetti,† Milko E. van der Boom,‡ Alessandro Abbotto,† Luca Beverina,† Tobin J. Marks,*,‡ and Giorgo A. Pagani*,† Department of Material Science, University of Milano-Bicocca, via Cozzi 53, 20125 Milano, Italy, and Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113 Received June 1, 2001. In Final Form: July 2, 2001 Regioselective alkylation of 1-(4-pyridyl)-2-[5-(dicyanomethanide)thien-2-yl]ethylene sodium salt (2) by a covalently surface-bound benzyl halide (4) produces a polar-ordered zwitterionic thin film (5). Chemisorption of 2 on the coupling layer 4 was monitored by UV-vis spectroscopy, contact-angle measurements, X-ray photoelectron spectroscopy, and second harmonic generation (SHG). Relative to carbanion 2, the film UV-vis spectrum exhibits a red-shifted (∆λ ∼ 170 nm) charge-transfer band, consistent with quaternization of the pyridyl group of 2 by 4. The nonlinear optical properties of the blue films of 5 were measured by (2) ∼ 5.0 × SHG. The average molecular tilt angle of the chromophore was determined to be ∼46°, and χzzz 10-8 esu (∼20 pm/V) was estimated at a fundamental wavelength of 1064 nm.
Introduction The development of second-order nonlinear optical (NLO) materials with adequate optical, thermal, and chemical properties is a topic of much current interest.1-6 Langmuir-Blodgett (LB) film transfer,1,2 polymer poling,3 and self-assembly (SA)4-6 have been widely implemented to obtain organic thin films (i.e., organic mono- and multilayers) with efficient NLO response properties. Chemically synthesized, covalently interlinked SA films generate more stable and robust structures than those assembled by weak dispersion forces such as LB techniques. Chemisorptive siloxane-based SA has been developed by Sagiv and others4,5,7 and is known to yield densely packed organic films on hydrophilic substrates. Most attractive is the fabrication of covalently bound organic chromophore arrays on silicon and related surfaces by SA, since the resulting electro-optic (EO) materials might be readily integrated in device structures.8 To our * To whom correspondence should be addressed. E-mail:
[email protected] or
[email protected]. † University of Milano-Bicocca. ‡ Northwestern University. (1) (a) Ashwell, G. J. J. Mater. Chem. 1999, 9, 1991-2003. (b) Roberts, M. J.; Lindsay, G. A.; Herman, W. N.; Wynne, K. J. J. Am. Chem. Soc. 1998, 120, 11202-11203. (c) Wijekoon, W. M. K. P.; Wijayu, S. K.; Bhawalkar, J. D.; Prasad, P. N.; Penner, T. L.; Armstrong, N. J.; Ezenyilimba, M. C.; Williams, D. J. J. Am. Chem. Soc. 1996, 118, 44804483. (2) (a) Ricceri, R.; Abbotto, A.; Facchetti, A.; Grando, D.; Gabrielli, G.; Pagani, G. A. Colloids Surf., A 1999, 150, 289-296. (b) Ricceri, R.; Neto, C.; Abbotto, A.; Facchetti, A.; Pagani, G. A. Langmuir 1999, 15, 2149-2151. (c) Ricceri, R.; Abbotto, A.; Facchetti, A.; Pagani, G. A.; Gabrielli, G. Thin Solid Films 1999, 340, 218-230. (d) Ricceri, R.; Grando, D.; Abbotto, A.; Facchetti, A.; Pagani, G. A.; Gabrielli, G. Langmuir 1997, 13, 5787-5790. (e) Ricceri, R.; Abbotto, A.; Facchetti, A.; Pagani, G. A.; Gabrielli, G. Langmuir 1997, 13, 4182-4184. (f) Ricceri, R.; Abbotto, A.; Facchetti, A.; Pagani, G. A.; Gabrielli, G. Langmuir 1997, 13, 3434-3437. (3) (a) Ma, H.; Chen, B.; Sassa, T.; Dalton, L. R.; Jen, A. K.-Y. J. Am. Chem. Soc. 2001, 123, 986-987. (b) Shi, Y.; Zhang, C.; Zhang, H.; Bechtel, J. H.; Dalton, L. R.; Robinson, B. H.; Steier, W. H. Science 2000, 288, 199-122. (c) Yitzchaik, S.; Di Bella, S.; Lundquist, P. M.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1997, 119, 2995-3002.
knowledge, in all such films prepared to date, the chromophoric NLO unit is present as a cation (pyridinium salt), and the neutrality of the film is ensured by an external halide or sulfonate counteranion.4,5 The zwitterionic blue dyes, 1-[N-(alkyl)-4-pyridinio)2-[5-(dicyanomethanide)thien-2-yl]ethene (1; alkyl ) (a) (4) (a) Pagani, G. A.; Abbotto, A.; Beverina, L.; Bradamante, S.; Facchetti, A.; van der Boom, M. E.; Marks, T. J. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.), in press. (b) van der Boom, M. E.; Evmenenko, G.; Dutta, P.; Marks, T. J. Adv. Funct. Mater., in press. (c) Evmenenko, G.; van der Boom, M. E.; Kmetko, J.; Dugan, S. W.; Marks, T. J.; Dutta, P. Chem. Phys., in press. (d) van der Boom, M. E.; Richter, A. G.; Malinsky, J. E.; Dutta, P.; Marks, T. J. Chem. Mater. 2001, 13, 15-17. (e) van der Boom, M. E.; Richter, A. G.; Malinsky, J. E.; Dutta, P.; Marks, T. J.; Lee, P. A.; Armstrong, N. R. Polym. Mater. Sci. Eng. 2000, 83, 160-161. (f) van der Boom, M. E.; Richter, A. G.; Malinsky, J. E.; Dutta, P.; Marks, T. J. Science Highlights; NSLS Activity Report; Corwin, M. A., Ehrlich, S. N., Eds.; Brookhaven Science Associates, Inc.: Upton, NY, 1999; Vol. 2, pp 47-49. (g) Lin, W.; Lee, T.-L.; Lyman, P. F.; Lee, J.; Bedzyk, M. J.; Marks, T. J. J. Am. Chem. Soc. 1997, 119, 2205-2211. (h) Lin, W.; Lin, W.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 8034-8042. (i) Yitzchaik, S.; Marks, T. J. Acc. Chem. Res. 1996, 29, 197-202. (j) Marks, T. J.; Ratner, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 155-173. (k) Li, D.-Q.; Ratner, M. A.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 7389-7390. (5) (a) Hung, W.; Helvenston, M.; Casson, J. L.; Wang, R.; Bardeau, J.-F.; Lee, Y.; Johal, M. S.; Swanson, B. I.; Robinson, J. M.; Li, D.-Q. Langmuir 1999, 15, 6510-6514. (b) Li, D.-Q.; Swanson, B. I.; Robinson, J. M.; Hoffbauer, M. A. J. Am. Chem. Soc. 1993, 115, 6975-6980. (6) (a) Bakiamoh, S. B.; Blanchard, G. J. Langmuir 2001, 17, 34383446. (b) Neff, G. A.; Helfrich, M. R.; Clifton, M. C.; Page, C. J. Chem. Mater. 2000, 12, 2363-2371. (c) Flory, W. C.; Mehrens, S. M.; Blanchard, G. J. J. Am. Chem. Soc. 2000, 122, 7976-7985. (d) Hanken, D. G.; Naujok, R. R.; Gray, J. M.; Corn, R. M. Anal. Chem. 1997, 69, 240-248. (e) Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 116, 6636-6640. For an example of metal ion-based centrosymmetric SA, see (f) Doron-Mor, H.; Hatzor, A.; Vaskevich, A.; van der Boom-Moav, T.; Shanzer, A.; Rubinstein, I.; Cohen, H. A. Nature 2000, 406, 382385. (7) (a) Moaz, R.; Sagiv, J. Langmuir 1987, 3, 1034-1044. (b) Moaz, R.; Yam, R.; Berkovic, G.; Sagiv, J. Thin Films; Ulman, A., Ed.; Academic: San Diego, CA, 1995; Vol. 20, pp. 41-66. (c) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074-1087. (d) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (e) van der Veen, N. J.; Flink, S.; Deij, M. A.; Egberink, R. J. M.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 2000, 122, 6112-6113. (f) Richter, A. G.; Yu, C.-Y.; Datta, A.; Kmetko, J.; Dutta, R. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2000, 61, 607-615.
10.1021/la010806s CCC: $20.00 © 2001 American Chemical Society Published on Web 08/23/2001
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Figure 1. Chromophore precursor 2 and zwitterionic blue dyes 1 [(a) CH3-PDCNTE and (b) C16H33-PDCNTE].
Figure 2. Schematic representation of the consecutive SA of zwitterionic chromophoric films (5) by selective alkylation of the chromophore precursor 2 by a surface-bound benzyl iodide monolayer (4).
CH3-PDCNTE and (b) C16H33-PDCNTE), belong to a new class of negative solvatochromic push-pull molecules exhibiting a negatively charged electron-donating dicyanomethanide group linked via an extended π-electron bridge to the positively charged N-alkylpyridinium acceptor (Figure 1).9 The unique combination of these fragments results in a dramatic increase of the first-order molecular hyperpolarizability (e.g., for 1a, βµ ) 13 450 × 10-48 esu in chloroform at λ ) 1.9 µm),10 making these compounds promising candidates for EO applications. In previous papers, some of us reported various LB films comprised of 1b and related chromophores exhibiting sharp, photobleachable UV-vis absorption bands that may have applications in multifrequency optical data storage.2 However, no NLO characterization of the films was reported. In this current paper, we demonstrate the stepwise formation of intrinsically acentric zwitterionic SA films 5 by 100% regioselective alkylation of the orange bidentate 1-(4-pyridyl)-2-[5-(dicyanomethanide)thien-2-yl]ethylene sodium salt (2) by a covalently surface-bound benzyl halide (4; Figure 2). Experimental Details General Procedures. All reactions were carried out under an inert atmosphere. Solvents were dried over a Na/K alloy, distilled, and degassed before use. The compounds 2 and p-(iodomethyl)phenyldiiodochlorosilane (3) were synthesized according to procedures described elsewhere.5b,9a Sodium lime (8) For a SA-based waveguiding SHG device, see (a) Lundquist, P. M.; Lin, W.; Zhou, H.; Hahn, D. N.; Yitzchaik, S.; Marks, T. J.; Wong, G. K. Appl. Phys. Lett. 1997, 70, 1941-1943. Recently, SA-based EO modulators have been developed in our lab; refer to (b) van der Boom, M. E.; Malinsky, J. E.; Zhao, Y.-G.; Chang, S.; Lu, W. K.; Ho, S. T.; Marks, T. J. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.), in press. (c) Zhao, Y.-G.; Wu, A.; Lu, H.-L.; Chang, S.; Lu, W.-K.; Ho, S.-T.; van der Boom, M. E.; Marks, T. J. Appl. Phys. Lett. 2001, 79, 587-589. (9) (a) Abbotto, A.; Bradamante, S.; Facchetti, A.; Pagani, G. A. J. Org. Chem. 1997, 62, 5755-5765. (b) Bradamante, S.; Facchetti, A.; Pagani, G. A. J. Phys. Org. Chem. 1997, 10, 514-524. (10) (a) Abbotto, A.; Bradamante, S.; Facchetti, A.; Pagani, G. A.; Ledoux, I.; Zyss, J. Mater. Res. Soc. Symp. Proc. 1998, 488, 815-818. (b) Abbotto, A.; Bradamante, S.; Facchetti, A.; Pagani, G. A.; Yuan, L.; Prasad, P. N. Gazz. Chim. Ital. 1997, 127, 165-166.
Facchetti et al. glass and quartz slides (1 × 1 cm2) were cleaned by immersion in a “piranha” solution (H2SO4:30% H2O2, 70:30 v/v) at 80 °C for 1 h (warning: piranha is an extremely strong oxidation reagent). After being cooled to room temperature, the substrates were rinsed repeatedly with deionized (DI) H2O and then sonicated in a solution of DI H2O, 30% H2O2, and NH3 (5:1:1 v/v/v) for 45 min. The substrates were then washed with copious amounts of DI H2O and dried at 115 °C overnight before deposition of coupling agent 3. Aqueous contact angles were measured on a standard tensiometric bench instrument fitted with a Teflon micrometer syringe (Gilmont Instruments, Inc.). Polarized second harmonic generation (SHG) measurements were made in the transmission mode by placing the glass slides in the path of the 1064-nm output of a Q-switched p-polarized light from a Nd:YAG laser operated at 10 Hz with a pulse width of 3 ns. The details of this setup can be found elsewhere.11 UV-vis spectra were recorded with a Varian Cary 1E spectrophotometer. The reagent 1,4-diazabicyclo[2.2.2]octane (DABCO) was supplied by Aldrich and added () 1%) to the chromophore precursor 2 solution in order to avoid photochemical degradation of dye 5 by singlet oxygen.2 Samples were stored in the dark. Growth of Chromophoric Bilayers (5). The glass substrates were functionalized in two steps. (a) Coupling layer formation (4): Under N2, freshly cleaned sodium lime glass was loaded into a Teflon sample holder and immersed in a dry toluene solution of 3 (14 mM) for 1 h at room temperature. The colorless substrates were washed twice with excess dry toluene and acetone and dried at 25 °C under vacuum. (b) Chromophore layer formation (5): Under N2, the functionalized substrates were transferred to a saturated, orange tetrahydrofuran (THF) solution of 2 containing a crystal of DABCO and refluxed for 3 h in the dark. After being cooled to 25 °C, the blue substrates were thoroughly washed with degassed methanol and dried at 25 °C under vacuum. Solvents were transferred using standard cannula techniques.
Results and Discussion Freshly cleaned float glass substrates were treated with a dry toluene solution of 3 at room temperature for 1 h under N2 (Figure 2). The substrates were then washed with dry toluene and acetone and dried in vacuo. Subsequently, the colorless iodobenzyl functionalized substrates were immersed in a saturated THF solution of the sodium salt 2 containing a crystal of DABCO. This base was used to prevent possible photodegradation of the surface-bound chromophore 5 under the reaction conditions. The orange solution was refluxed for 3 h under N2 with rigorous exclusion of light. The solution was then allowed to cool to room temperature, and the blue substrates were rinsed repeatedly with degassed methanol and analyzed by optical (UV-vis) spectroscopy, advancing contact-angle (CA) measurements, X-ray photoelectron spectroscopy (XPS), and angle-dependent polarized SHG. The new SA bilayer 5 adheres strongly to the hydrophilic substrates, as it cannot be removed from the surface by the standard “Scotch tape test” and is insoluble in common organic solvents. Photobleached samples of 5 were obtained after sunlight exposure for a few hours in air. Figure 3A shows the optical spectra of 1b and 2 in solution, the SA film 5, and the corresponding photobleached film. The selective formation of 5 by quaternization of the pyridyl moiety with the anchored benzyl iodide functionality is immediately obvious by the intense blue color of the glass substrates. Indeed, the UV-vis spectrum of 5 exhibits two pronounced features: (i) a large red shift of the charge-transfer band (CTB) to λmax ) 643 nm in comparison with the chromophore precursor 2, where λmax ) 480 nm (acetone), and (ii) a band due to (11) Yitzchaik, S.; Roscoe, S. B.; Kakkar, A. K.; Allan, D. S.; Marks, T. J.; Xu, Z.; Zhang, T.; Lin, W.; Wong, G. K. J. Phys. Chem. 1993, 97, 6958-6960.
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Figure 4. SHG intensity (arbitrary units) as a function of fundamental beam incident angle from a float glass slide having a SA bilayer 5 on either side.
transitions within the aromatic rings (aromatic bands, AB) located at λ ) 336 nm. Importantly, no other features are observed, ruling out incorporation of 2 within the film by alkylation of the dicyanomethanide group or formation of molecular aggregates as observed recently for 1a-based LB films.2 The CT band of 5 is readily photobleached by illumination with visible light in air,12 similar to all molecules belonging to the same family of chromophores as 1.9,10 After photodegradation, only the absorption band at λ ) 336 nm is present. In solution, dyes of 1 exhibit similar characteristic CT band and UV absorption transitions as the new surface-bound chromophore 5 (e.g., for 1b, λmax (CTB) ) 614 nm and λmax (AB) ) 328 nm in methanol). Moreover, previously reported LB films2 and spin-coated films13 of 1b exhibit nearly identical CTB and AB absorptions at λmax ) 635 nm/λ max ) 365 nm and λmax ) 652 nm/λmax ) 340 nm, respectively, together with a sharp band at λmax ) 432 nm/λ max ) 429 nm. The blue shifts of these transitions with respect to the CTBs are attributed to the formation of H-type aggregates.2 However, going from pure 1b in solution to the corresponding films induces very different relative intensity ratios of the CTB/AB bands. In fact, while in solution, the CTB is more intense than the AB; in the case of LB, spin-coated, and SA films, the two bands have almost comparable absorbance values. These results might lead to the conclusion that most of the chromophore units in the solid
state are photobleached either during film preparation or during substrate transfer for measurements in the air. However, we have shown that this is not the case (Figure 3B). Dye 1b of a spin-coated film (spectrum e), once dissolved in MeOH, exhibits a spectrum (f) identical to that of the pure chromophore in the same solvent. SA film 5 was found to be less stable to the combined action of light and air than LB (Y-type, 30 layers)2 and spin-coated 1b films.13 In fact, while samples of 5 bleached within a few hours, the LB and spin-coated films are stable for a much longer period of time. However, this result is in full accord with the film structure (Figure 2). Indeed, the dicyanomethanide moieties of the SA film chromophore moleculessthe dye functional groups that are more closely exposed to airsare also the sensitive anionic sites leading to a decrease in environmental stability of the film. The chromophore density of SA film 5 was estimated to be at least ∼1 chromophore/40 Å2 by using 1b ) 79 400 ( 400 L mol-1 cm-1.9a Similar values have been reported for related azobenzene type SA films.4,5,14 LB films of similar zwitterionic molecules have a density of ∼1 chromophore/19 Å2.2e Further evidence for the formation of 5 is obtained from aqueous CA measurements. These show a significant increase in θa from 50 ( 4° for 4 to 80 ( 4° for 5, in accord with a dense chromophoric surface coverage. XPS measurements on 5 reveal the presence of C, N, S, O, Si, and traces of NaI. The observation of NaI is surprising but fully in agreement with the formation of 5. Because all samples were rinsed thoroughly with excess MeOH before XPS analysis, this result suggests that the observed NaI is possibly adsorbed on the film structure by electrostatic interactions. Previous studies have shown that angle-dependent SHG measurements are an excellent tool for characterization of SA [(aminophenyl)azo]pyridinium mono- and multilayer films.4,5 The present polarized SHG measurements were carried out in the transmission mode using the 1064-nm
(12) Abbotto, A.; Bradamante, S.; Facchetti, A.; Pagani, G. A. Unpublished results. (13) Samples were prepared by spin coating quartz substrates at 3000 rpm with a 2 mM solution of 1b in chloroform. Abbotto, A.; Facchetti, A.; Pagani, G. A. Unpublished results.
(14) (a) The SHG-derived tilt angle is close to the so-called magic angle: Simpson, G. J.; Rowlen, K. L. J. Am. Chem. Soc. 1999, 121, 2635-2636. (b) This relationship assumes a narrow distribution of molecular tilt angles: Bloembergen, N.; Pershan, P. S. Phys. Rev. 1962, 128, 606-622.
Figure 3. Transmission optical absorbance spectra (abs; arbitrary units). Panel A: (a) chromophore precursor 2 in acetone, (b) chromophore 1b in methanol, (c) zwitterionic chromophoric film 5, and (d) film 5 after complete photobleaching in air. Panel B: (e) Spin-coated film 1b and (f) spin-coated film 1b dissolved in MeOH.
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pyridine 7 and phenothiazinylbutyramido styrylpyridine -8 and 2.8 × 10-8 esu, derivative 8 with χ(2) zzz ∼ 6 × 10 4,5 respectively.
χ(2) zzz χ(2) zyy
Figure 5. Cationic chromophore building blocks (6-8; pyridinium salts) for layer-by-layer formation of acentric, optically functional, SA films.4,5
output of a Q-switched Nd:YAG laser. The characteristic SHG response interference pattern for the bilayer film (Figure 4) demonstrates that the quality and uniformity of the organic film are identical on both sides of the substrate. The data can be fit to eq 1,14 where Ψ is the average orientation angle between the surface normal and the principal molecular tensor component. Film 5 is relatively transparent at the fundamental laser wavelength and, more importantly, does not absorb significantly at 532 nm. Therefore, major χ(2) enhancement due to overlap between the film charge-transfer absorption and the second harmonic wave is reasonably excluded. It is known that such resonant effects can enhance experimental SA film nonlinearities as much as 1 order of -8 magnitude.15 A nonlinear susceptibility, χ(2) zzz ∼ 5.0 × 10 esu (∼20 pm/V), is obtained for 5 by calibrating against quartz. The average orientation angle between the surface normal and the principal molecular tensor component, Ψ ∼ 46° (eq 1), is similar to those determined for azobenzenetype films and derivates (Ψ ∼ 38-42°; Figure 5).4,5 The nonlinear susceptibility at this wavelength of the zwitterionic film 5 is lower than 6-based films (χ(2) zzz ∼ 5.0 × 10-7 esu).4 However, the SHG efficiency of 5 is comparable to recently reported self-assembled monolayers (SAMs) of 4-[N,N-bis(3-hydroxypropyl)amino]phenylethynyl-4′(15) Lundquist, P. M.; Yitzchaik, S.; Zhang, T.; Kanis, D. R.; Ratner, M. R.; Marks, T. J.; Wong, G. K. Appl. Phys. Lett. 1994, 64, 2194-2197.
) 2 cot2 Ψ
(1)
In summary, we have demonstrated here the consecutive formation of a photonically functionalized SAM consisting of a novel zwitterionic chromophore. The UVvis and SHG measurements clearly demonstrate that the blue dye molecules are covalently bound to the surface and are aligned in an acentric fashion with a high chromophore density. Interestingly, the solid-state transmission optical spectra of the zwitterionic dyes 1 depend on the film growth techniques that were used. Both LB2 and spin-coated films13 of 5 show an additional sharp band at λmax ) 432 and 429 nm, respectively, whereas this band attributed to the formation of H-type aggregates2 is not observed in SA films of 5. The 100% regioselectivity of the solution-surface alkylation process is remarkable (4 + 2 f 5). Although 2 exhibits two competing nucleophilic sites, one anionic at the carbanionic carbon of the dicyanomethanide group and one neutral at the pyridine nitrogen atom, selective alkylation of the latter by the surfacebound benzyl iodide 4 occurs. Acentric siloxane-based photonic functionalized SA films are primarily based on salt-type chromophores (Figure 5).4,5 For the above reasons, we believe that zwitterionic-based films of the type represented by 5 provide a new valuable entry for the formation of novel optoelectronic/photonic materials.16 Acknowledgment. This research was supported by the NSF MRSEC program (Grant DMR 9632472 to the Northwestern Materials Research Center), by ARO/ DARPA (DAAD 19-00-1-0368), by MURST, and by CNRProgetto Finalizzato Materiali Avanzati II. We thank Mr. P. A. Lee and Prof. N. R. Armstrong (University of Arizona) for XPS measurements. LA010806S (16) We anticipate that the thiophene spacer might be properly functionalized to generate a new reactive (hydrophilic) surface necessary for iterative multilayer film growth. See (a) Gronowitz, S., Ed. The Chemistry of Heterocyclic Compounds; John Wiley & Sons: New York, 1985; Vol. 44. (b) Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds. Handbook of Conducting Polymers; Dekker: New York, 1997.
