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Nanoscale Consecutive Self-Assembly of Thin-Film Molecular Materials for Electrooptic Switching. Chemical Streamlining and Ultrahigh Response Chromophores Milko E. van der Boom,† Peiwang Zhu,† Guennadi Evmenenko,‡ Joshua E. Malinsky,† Wenbin Lin,† Pulak Dutta,‡ and Tobin J. Marks*,† Department of Chemistry, Department of Physics and Astronomy, and the Materials Research Center, Northwestern University, Evanston, Illinois 60208-3113 Received October 16, 2001. In Final Form: February 11, 2002 A broadly applicable approach to formation of self-assembled organic electrooptic superlattices for highspeed switching is reported. This two-step “one-pot” method involves (i) layer-by-layer covalent self-assembly of intrinsically acentric monolayers of a new high-hyperpolarizability donor-acceptor aminophenylbenzothiazolpyridinium chromophore (β(0.65 eV)calcd ) 1620 × 10-30 cm5 esu-1) on hydrophilic substrates and (ii) in-situ chromophore deprotection concurrent with self-limiting “capping”/planarization of each chromophore layer with octachlorotrisiloxane. The resulting organic films are characterized using a combination of physicochemical methodolgies including synchrotron X-ray specular reflectivity, angledependent polarized second harmonic generation spectroscopy, optical spectrometry, X-ray photoelectron spectroscopy, and advancing contact angle measurements. The superlattices exhibit very large secondorder responses, χ(2) ≈ 370 pm/V, and a large macroscopic electrooptic coefficient, r33 ≈ 120 pm/V, is estimated at λ0 ) 1064 nm.

Polar organic solids can exhibit far higher EO (electrooptic) coefficients (r33) and lower dielectric constants () than conventional inorganic EO materials, suggesting the possibility of inexpensive organic high-speed optical switches with digital level operating voltages and very large bandwidths (e.g., compare modulator figures of merit, n3r33/: 10-120 pm/V for self-assembled superlattices (SASs) and poled-polymers vs 8.7 pm/V for LiNbO3).1-6 Sophisticated poled-polymer modulators are rapidly approaching or even exceeding the performance of traditional inorganic devices in terms of bandwidth and operating voltage. Indeed, poled-polymer modulators operating at telecommunication wavelengths (1.3 and 1.55 †

Department of Chemistry. Department of Physics and Astronomy. * To whom correspondence should be addressed. E-mail: [email protected]. ‡

(1) (a) Wu¨rthner, F.; Wortmann, R.; Meerholz, K. Chem. Phys. Chem. 2002, 3, 17-31. (b) Eldada, L.; Shacklette, L. W. IEEE J. Quantum Electron. 2000, 6, 54-67. (c) Samijn, C.; Verbiest, T.; Persoons, A. Macromol. Rapid Commun. 2000, 21, 1-15. (d) Dalton, L. R.; et al. J. Mater. Chem. 1999, 9, 1905-1920. (e) Steier, W. H.; et al. Chem. Phys. 1999, 245, 487-506. (f) Saadeh, H.; Yu, D.; Wang, L. M.; Yu, L. P. J. Mater. Chem. 1999, 9, 1865-1873. (g) Marder, S. R.; Kippelen, B.; Jen, A. K. Y.; Peyghambarian, N. Nature (London) 1997, 388, 845-851. (h) Long, N. J. Angew. Chem., Int. Ed. 1995, 34, 21-38. (i) Chem. Rev. special issue on Optical Nonlinearities in Chemistry, Burland, D. M., Ed.; 1994, 94, 1-278. (2) (a) Zhang, H.; Oh, M.-C.; Szep, A. Steier, W. H.; Zhang, C.; Dalton, L. R. Appl. Phys. Lett. 2001, 78, 3136-3138. (b) Lee, S.-S.; Garner, S. M.; Chuyanov, V.; Zhang, H.; Steier, W. H.; Wang, F.; Dalton, L. R.; Udupa, A. H.; Fetterman, H. R. IEEE J. Quantum Electron. 2000, 36, 527-532. (c) Shi, Y.; Zhang, C.; Zhang, H.; Bechtel, J. H.; Dalton, L. R.; Robinson, B. H.; Steier, W. H. Science 2000, 288, 119-122. (d) For a recent review, see: Dalton, L. R. Opt. Eng. 2000, 39, 589-595. (3) (a) Evans, O. R.; Lin, W. Chem. Mater. 2001, 13, 3009-3017. (b) Kanazawa, A.; Ikeda, T.; Abe, J. Angew. Chem., Int. Ed. 2000, 39, 612615. (c) Coe, B. J. Chem. Eur. J. 1999, 5, 2464-2471. (d) Saadeh, H.; Wang, L.; Yu, L. J. Am. Chem. Soc. 2000, 122, 546-547. (e) Lin, W.; Wang, Z.; Ma, L. J. Am. Chem. Soc. 1999, 121, 11249-11250. (f) Johal, M. S.; Cao, Y. W.; Chai, X. D.; Smilowitz, L. B.; Robinson, J. M.; Li, T. J.; McBranch, D.; Li, D.-Q. Chem. Mater. 1999, 11, 1962-1965. (g) Lin, W.; Evans, O. R.; Xiong, R.-G.; Wang, Z. J. Am. Chem. Soc. 1998, 120, 13272-13273. (h) Di Bella, S.; Fragala`, I.; Ledoux, I.; Diaz-Garcia, M. A.; Marks, T. J. J. Am. Chem. Soc. 1997, 119, 9550-9557.

µm) with half-wave voltages (Vπ) as low as 0.8 V represent a major advance.2 Nevertheless, the innovative design and implementation of improved molecule-based EO materials remains an intriguing synthetic challenge which offers the potential to significantly advance the field.3 For example, polar chromophore alignment in polymer matrices must currently be induced near the glass transition temperature using very large and frequently deliterious electric poling fields (g106 V/cm),4 making the incorporation of intrinsically acentric organic films into device structures an attractive alternative.5,7 Recently, SASbased frequency-doubling devices and EO modulators which do not require poling have been reported from this laboratory.5 SAS structures are prepared via layer-bylayer molecular self-assembly (SA). Such templated formation of intrinsically polar arrays of high-β chromophores directly on silicon or related device substrates, which requires neither electric field poling, poling electrodes, nor electrically matched buffer layers and which offers greater degrees of molecular alignment than (4) (a) Ma, H.; Chen, B.; Sassa, T.; Dalton, L. R.; Jen, A. K.-Y. J. Am. Chem. Soc. 2001, 123, 986-987. (b) Davey, M. J.; Lee, V. Y.; Wu, L.-M.; Moylan, C. R.; Volksen, W.; Knoesen, A.; Miller, R. D.; Marks, T. J. Chem. Mater. 2000, 12, 1679-1693. (c) Yitzchaik, S.; Di Bella, S.; Lundquist, P. M.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1997, 119, 2995-3002. (5) (a) Zhao, Y. G.; Wu, A.; Lu, H. L.; Chang, S.; Ho, S. T.; van der Boom, M. E.; Marks, T. J. submitted. (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. 2001, 42, 550-551. (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. For a SA waveguiding SHG device, see: (d) 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. (6) (a) Neff, G. A.; Helfrich, M. R.; Clifton, M. C.; Page, C. J. Chem. Mater. 2000, 12, 2363-2371. (b) Flory, W. C.; Mehrens, S. M.; Blanchard, G. J. J. Am. Chem. Soc. 2000, 122, 7976-7985. (c) Hanken, D. G.; Naujok, R. R.; Gray, J. M.; Corn, R. M. Anal. Chem. 1997, 69, 240-248. (d) Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 116, 66366640. (7) Cresswell, J. P.; Petty, M. C.; Wang, C. H.; Wherrett, B. S.; AliAdib, Z.; Hodge, P.; Ryan, T. G.; Allen, S. Opt. Commun. 1995, 115, 271-275.

10.1021/la011561m CCC: $22.00 © 2002 American Chemical Society Published on Web 03/27/2002

Molecular Materials for Electrooptic Switching

Figure 1. Two-step layer-by-layer assembly process for intrinsically acentric self-assembled superlattices by (i) covalent chemisorption of 3 from dry toluene solution onto hydrophilic substrates and (ii) in-situ desilylation of 3 concomitant with covalent capping using a dry heptane solution of octachlorotrisiloxane.

poling,6-9 suggests ready integration into semiconductor electronics and into all-organic microphotonic circuits.5,10 We report here a new expeditious approach to SAS materials which considerably simplifies fabrication while offering far higher electrooptic responses than previously achieved. The present two-step synthetic method involves iterative combination of (i) self-limiting polar chemisorption of monolayers composed of a new siloxy-protected high-β donor-acceptor aminobenzothiazolpyridinium chromophore (β(0.65 eV)calcd ) 1620 × 10-30 cm5 esu-1) and (ii) in-situ siloxy group removal concurrent with self-limiting “capping” of each chromophore layer with an octachlorotrisiloxane-derived layer (Figure 1).9a This latter step deposits a robust polysiloxane layer (∼8 Å thick)8,11 essential for stabilizing/planarizing the acentric microstructure via interchromophore cross-linking and regenerates a reactive hydrophilic surface. The iterative twostep chemisorptive SA process can be efficiently carried out in a single reaction vessel using standard cannula techniques, and the resulting multilayer microstructural regularity as well as EO response has been characterized by a full complement of physicochemical techniques: optical spectroscopy (UV-vis), advancing (CA) measurements, synchrotron X-ray reflectivity (XRR), X-ray photoelectron spectroscopy (XPS), and angle-dependent polarized second harmonic generation (SHG) spectroscopy. New chromophore precursor 1 was prepared by diazotization of 6-benzothiazolamine-2-(pyridin-4-yl) 212 (to form an unstable diazonium intermediate),13 immediately followed by coupling with N-phenyldiethanolamine (Figure 2).14 The hydroxyl groups of 1 were then quantitatively derivatized/protected by treatment of 1 with a dry THF solution of tert-butyldimethylchlorosilane in the presence (8) (a) Yitzchaik, S.; Marks, T. J. Acc. Chem. Res. 1996, 29, 197-202. (b) Lin, W.; Lin, W.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 8034-8042. (9) (a) Zhu, P.; van der Boom, M. E.; Evmenenko, G.; Dutta, P.; Marks, T. J. Polym. Prepr. 2001, 42, 579-580. (b) van der Boom, M. E.; Richter, A. G.; Malinsky; J. E.; Dutta, P.; Marks, T. J. Adv. Funct. Mater. 2001, 11, 393-397. (c) van der Boom, M. E.; Evmenenko, G.; Dutta, P.; Marks, T. J. Chem. Mater. 2001, 13, 15-17. (d) Evmenenko, G.; van der Boom, M. E.; Kmetko, J.; Dugans, S. W.; Marks, T. J.; Dutta, P. J. Chem. Phys. 2001, 115, 6722-6727. (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. (10) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1999, 280, 1741-1742. (11) Malinsky, J. E.; Jabbour, G. E.; Shaheen, S. E.; Anderson, J. D.; Richter, A. G.; Marks, T. J.; Armstrong, N. R.; Kippelen, B.; Dutta, P.; Peyghambarian, N. Adv. Mater. 1999, 11, 227-231. (12) (a) Malinsky, J. E. Ph.D. Thesis, Northwestern University, 2000. (b) Kato, T. Japan Patent JP5333390, 1993. (13) Kalatzis, E. J. Chem. Soc. B 1967, 273-277.

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of imidazole. Subsequent reaction with 1-iodo-n-propyl3-trimethoxysilane in dry THF at elevated temperatures results in formation of the new aminophenylbenzothiazolpyridinium iodide chromophore 3, which was fully characterized by conventional analytical techniques. Freshly cleaned float glass and single-crystal Si(111) substrates contained in a Teflon sample holder were treated with a dry toluene solution of 3 (1.0 mM) under N2 at 80 °C for 18 h in an anaerobic reaction vessel. The functionalized substrates were then sequentially washed with dry toluene, acetone, and methanol, and dried in vacuo. The resulting monolayers strongly adhere to hydrophilic substrates, cannot be removed by the “Scotch tape” decohesion test, and are insoluble in common organic solvents. The formation of the 3-derived monolayer on glass substrates is immediately obvious by the red color of the film. Indeed, solution phase optical spectra of 3 in solution and the SA film exhibit a similar bathochromic shift of the charge-transfer (CT) band to λmax ) 497 nm (methanol) and λmax ) 505 nm (solid state), respectively, in comparison to the chromophore precursor 5, where λmax ) 453 nm. Importantly, no film spectral features are observed which would suggest formation of 3-based molecular aggregates as sometimes occurs for LB films.15 Thus, the chromophoric NLO unit is present as a pyridinium cation with electroneutrality ensured by the halide counteranion. The CT transition of the SA film is slowly photobleached by illumination with visible light in air for several days so that samples must be protected accordingly. Photochemical degradation of surface-bound donoracceptor conjugated thiophene-based chromophores by singlet oxygen is a well-documented process.16 The chromophore density, Ns, of the SA film is estimated to be ∼1.7 × 1014 molecules/cm2 (≈60 Å2/chromophore) using A/(2) with  ) 36.3 × 103 mol-1 cm-1, where A is the measured absorbance.17 Similar values have been reported for related SA azobenzene and zwitterionic films.8,9,16 Polarized angle-dependent SHG measurements on float glass slides coated on either side with a 3-based monolayer were carried out at λ0 ) 1064 nm in the transmission mode. The characteristic SHG interference pattern shown in Figure 3 demonstrates that the quality and uniformity of the acentric films are identical on either side of the glass substrate. These data yield (with the appropriate caveats18) an average chromphore orientation angle of ∼45° with respect to the surface normal. Aqueous contact angle (CA) measurements on the functionalized substrates reveal hydrophobic surfaces (θa ∼ 79°) in accord with formation of densely packed hydrophobic organic films, exposing only the trialkylsiloxy protecting groups at the outer surface. XRR measurements on a 3-based monolayer reveal (i) a film thickness of 24.3 ( 0.4 Å, (ii) a surface roughness, σfilm-air ∼3.3 Å, and (iii) a chromophore surface density of ∼50 Å2/molecule (inset, Figure 3). The XRRderived surface roughness is comparable to that of (14) (a) The procedures used are nearly identical to those used for the preparation of the analogous 4-[[4-[N,N-bis((tert-butyldimethylsiloxy)ethyl)amino]phenyl]azo]-1-n-propyl-3-trimethoxysilane-pyridinium iodide salt.8b,9a,b Details are provided in the Supporting Information. (15) Ricceri, R.; Neto, C.; Abbotto, A.; Facchetti, A.; Pagani, G. A. Langmuir 1999, 15, 2149-2151. (16) Facchetti, A.; van der Boom, M. E.; Abbotto, A.; Beverina, L.; Marks, T. J.; Pagani, G. A. Langmuir 2001, 17, 5939-5942. (17) The absorbance is divided by 2 because the substrates were coated with the SA film on both sides. See: Moon, J. H.; Shin, J. W.; Kim, S. Y.; Park, J. W. Langmuir 1996, 12, 4621-4624. (18) This treatment assumes a relatively narrow distribution of orientation angles: (a) Simpson, G. J.; Rowlen, K. L. J. Am. Chem. Soc. 1999, 121, 2635-2636. (b) Bloembergen, N.; Pershan, P. S. Phys. Rev. 1962, 128, 606-622.

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Figure 2. Synthesis of chromophore 1 and the corresponding high-hyperpolarizability n-propyl-3-trimethoxysilane-pyridinium iodide salt 3.

Figure 3. SHG response at λ0 ) 1.06 µm as a function of fundamental beam incident angle from a float glass slide having a 3-based monolayer on either side. Inset: normalized X-ray reflectivity plotted vs wave vector for a 3-based monolayer.

azobenzene-based monolayers (∼5.1 Å) and to that of highly ordered, self-assembled octadecyltrichlorosilane films on silicon,9a,b,19 demonstrating that the present selflimiting film growth process results in smooth, wellorganized films. We find that reaction of octachlorotrisiloxane (34 mM) in dry heptane for 30 min with the hydrophobic 3-derived surface (and presumably adventitious adsorbed H2O)20a effects in-situ chromophore deprotection and formation of a reactive, hydrophilic polysiloxane surface (Figure 1). Octachlorotrisiloxane (a strong Lewis acid) reacts instantly with trace H2O, forming HCl. The latter likely effects deprotection of the chromophore layer and may also catalyze polysiloxane condensation.20b,c In support of this hypothesis, we find that reaction of trialkylsilyl-protected chromophore films with solutions of ethanolic HCl or nBu N+F- in THF also results in rapid protecting group 4 removal as argued by aqueous CA measurements (∆θa ) (19) (a) Bierbaum, K.; Kinzler, M.; Wo¨ll, Ch.; Grunze, M.; Ha¨hner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512-518. (b) Ohtake, T.; Mino, N.; Ogana, K. Langmuir 1992, 8, 2081-2083. (c) Tidswell, I. M.; Ocko, B. M.; Pershan, P. S.; Wasserman, S. R.; Whitesides, G. M.; Axe, J. D. Phys. Rev. B 1990, 41, 1111-1128. (20) (a) Silberzan, P.; Le´ger, L.; Ausserre´, D.; Benattar, J. J. Langmuir 1991, 7, 1647-1651. (b) Green, T. W.; Wuts, P. G. M. Protective Groups in Organic Synthesis, 2nd ed.; John Wiley & Sons: New York, 1991. (c) Plueddemann, E. P. Silane Coupling Agents; 2nd ed.; Plenum Press: New York, 1991.

Figure 4. Specular X-ray reflectivity (XRR) measurements on 3-based superlattices. Left y-axis: film thickness (Å) as a function of the number of bilayers (+). The solid line is the fit by linear regression with R2 ) 0.9964 for 1-4 bilayers, indicating T ) 28.6 ( 0.6 Å × n. Right y-axis: relative film roughness, σfilm-air/Tfilm, as a function of the number of bilayers (b). The solid line is drawn as a guide to the eye.

-25°). It will be seen that the result of the two-step insitu deprotection methodology is thin, microstructurally regular polysiloxane-capped chromophore films (vide infra). Previous studies established that reaction of analogous azobenzene-based SA films with nBu4N+Fresults in rapid siloxy protecting group removal.9 Covalent Si3O2Cl8 capping then yields a net film thickness increase of ∆T ∼ 5.4 Å (∆T ) Tdeprotection + Tcapping ) -2.6 + 8 Å).9a In excellent agreement with these results, in-situ deprotection and capping of the present 3-based films with octachlorotrisiloxane results in an average film thickness increase of 5.5 ( 1.2 Å (to 28.6 ( 0.6 Å in the present case, Figure 4). The linear dependence of the XRR-derived film thickness on the number of bilayers (Figure 4) unambiguously demonstrates that equal densities of chromophore molecules are successively deposited in each self-assembled bilayer. The XRR-derived “footprint” of ∼50 Å2/chromophore for each bilayer is in good agreement with that observed for monolayer 3 by XRR and UV-vis measurements (vide supra) and is in agreement with the footprint found for azobenzene-based superlattices.8,9 Apparently, in-situ protecting group removal and capping of each “deprotected” chromophore layer with octachlorotrisilox-

Molecular Materials for Electrooptic Switching

ane does not lead to a significant decrease of chromophore density by irreversible cleavage of surface anchoring Si-O bonds. The XRR-derived film roughness/film thickness (σfilm-air/Tfilm) as a function of the number of bilayers decreases to ∼15% (∼2.5 nm) for a five-bilayer film (Figure 4). The present multilayer film has an average electron density of ∼0.42 times the single-crystal Si electron density, FSi ) 0.71 e Å-3, and is nearly identical to the XRR-derived electron density of octadecyltrichlorosilanebased SA films.19 Contact mode AFM measurements on a one-bilayer sample reveal a smooth, featureless surface with an rms surface roughness of ∼1.1 nm for 1 × 1 µm2 scan areas, which is in good agreement with the XRR measurements, σfilm-air ∼ 1.2 nm. Measurements on a fourbilayer sample reveal slight grain formation with an rms surface rougness of only ∼0.9 nm for 1 × 1 µm2 scan areas.21 XPS measurements on a five-bilayer sample reveal the presence of Si, S, C, O, N, I, and Cl. The inclusion of Cl in the films is probably via an anion exchange process. Upon capping layer formation/condensation, the chromophore salt layer likely undergoes Cl/I exchange with the evolved HCl. Chloride counteranions in uncapped SA monolayers are known to undergo exchange with various anions (e.g., iodide, sufanilate, ethyl orange, and eosin).22 For a structurally regular polar multilayer, the SHG intensity (I2ω) is expected to scale quadratically with the number of chromophore deposition cycles, because the incident light wavelength (1.06 µm) is large compared to the SA film thickness.24 In the present system, the observed linear dependence and zero intercept of (I2ω)1/2 and the HOMO-LUMO CT absorbance at λmax ≈ 500 nm on the number of bilayers demonstrates that approximately equal densities of chromophores with uniformly polar orientation chromophores are deposited in each cycle, while maintaining structural regularity (Figure 5). Remarkably, for similar chromophore densities, the present multilayer films exhibit ∼3.5 times the 532 nm SHG response, I2ω, of known azobenzene-based SAS structures which have similar optical absorption characteristics (λmax ≈ 500 nm in both cases), suggesting similar degrees of resonant enhancement.8,9,24 These 3-based SASs exhibit very large nonlinear susceptibilities and estimated macroscopic EO coefficients, χ(2) ∼ 370 pm/V and r33 ∼ 120 pm/V at 1064 nm,25 respectively, indicating that the thermally robust SASs are competitive in terms of electrooptic response with the highest efficiency chromophoric LB films and poled polymers reported to date1-7 and are significantly more efficient than inorganic EO systems such as LiNbO3.25 (21) For an example of scanning force microscopy studies of group IV metal-coordination-based SA superlattices, see: Hatzor, A.; Moav, T.; Cohen, H.; Matlis, S. Libman, J.; Vaskevich, A.; Shanzer, A.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 13469-13477. (22) Roscoe, S. B.; Yitzchaik, S.; Kakkar, A.; Marks, T. J. Langmuir 1996, 12, 5338-5349. (23) Lundquist, P.; Yitzchaik, S.; Zhang, T.; Kanis, D. R.; Ratner, M. A.; Marks, T. J.; Wong, G. K. Appl. Phys. Lett. 1994, 64, 2194-2197. (24) The macroscopic EO coefficient, r33 ≈ 120 pmV-1 at λ ) 1064 nm (2) is estimated from the relationship r33 ≈ (l × χzzz )/n4.1d (25) For a review, see: Wooten, E. L.; Kissa, K. M.; Yi-Yan, A.; Murphy, E. J.; Lafaw, D. A.; Hallemeier, P. F.; Maack, D.; Attanasio, D. V.; Fritz, D. J.; McBrien, G. J.; Bossi, D. E. IEEE Selected Top. Quantum Electron. 2000, 6, 69-81.

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Figure 5. Optical transmission and second harmonic generation (SHG) data. Left x-axis: absorption at λmax ) 505 nm as a function of the number of bilayers (+). Right x-axis: square root of the SH intensity (I2ω) as a function of the number of bilayers (9).

Conclusions Azobenzene and stilbazolium-based acentric SAS EO structures can be assembled by a laborious, iterative threestep conventional procedure (including spin-coating and vacuum oven treatment) which is limited to volatile chromophore precursors.8 The two-step assembly technique reported here using a new siloxane-functionalized high-β chromophore demonstrates the efficiency of the “one-pot” approach to assembling organic superlattices from solution and is useful for assembly of new types of photonically/electronically functional multilayers.9,23 This streamlined two-step single reactor process results in structurally regular multilayers with each subunit ∼2.8 nm thick. The very large surface densities (Nmax ∼50 Å2/ chromophore) and high degrees of net polar orientation of the large hyperpolarizability chromophore result in very high estimated EO responses (r33 ∼ 120 pm/V at 1064 nm). Further computationally aided chemical modification of the molecular building blocks to increase first hyperpolarizabilities and reactivity (i.e., utilization of reactive -SiCl3 functionalized chromophores) to increase deposition rates will allow systematic optimization of the materials characteristics, film growth, and modulator performance. Several of these advances will be reported shortly.5 Acknowledgment. Research was supported by the NSF MRSEC program (Grant DMR 0076077 to the Northwestern Materials Research Center), by ARO/ DARPA (DAAD 19-00-1-0368), and by Nanovation Technologies, Inc. XRR data were obtained at beamline X23B of the National Synchrotron Light Source, which is supported by the U.S. Department of Energy. We thank Mr. P. A. Lee and Prof N. R. Armstrong (U. of Arizona) for XPS measurements and Dr. I. L. D. Albert for the semiempirical β calculations. Supporting Information Available: Details describing the synthesis of 1 and 3 and the fabrication of superlattices. This material is available free of charge via the Internet at http://pubs.acs.org. LA011561M