Self-Assembled Chromophoric NLO-Active Structures. Second

Oct 30, 1996 - Stephen B. Roscoe,Shlomo Yitzchaik,Ashok K. Kakkar, andTobin J. ... with eosin B. In the case of iodide and sulfanilate substitution fo...
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Langmuir 1996, 12, 5338-5349

Self-Assembled Chromophoric NLO-Active Structures. Second-Harmonic Generation and X-ray Photoelectron Spectroscopic Studies of Nucleophilic Substitution and Ion Exchange Processes on Benzyl Halide-Functionalized Surfaces Stephen B. Roscoe, Shlomo Yitzchaik, Ashok K. Kakkar, and Tobin J. Marks* Department of Chemistry and the Materials Research Center, Northwestern University, Evanston, Illinois 60208-3113

Zuyan Xu, Tongguang Zhang, Weiping Lin, and George K. Wong* Department of Physics and Astronomy and the Materials Research Center, Northwestern University, Evanston, Illinois 60208-3112 Received February 5, 1996X The progress and extent of nucleophilic substitution and ion exchange reactions of self-assembled chromophoric monolayers are studied by X-ray photoelectron (XPS) and second harmonic generation (SHG) spectroscopy. Self-assembled monolayers prepared from 2-[4-(chloromethyl)phenyl]ethyl trichlorosilane (1) on glass substrates are susceptible to nucleophilic substitution of ∼90% of the surface-confined benzylic chloride functionalities with the “hypernucleophile” 4-(dimethylamino)pyridine; however, only ∼60% of the densely packed benzyl chloride groups undergo reaction with the high-β chromophore precursor 4′[4-[N,N-bis(3-hydroxypropyl)amino]styryl]pyridine (2a). Quaternization of a benzylic monolayer with this molecule yields a monolayer having a bulk second-order NLO response (χ(2)) of 3 × 10-7 esu at λ0 ) 1064 nm, corresponding to a near-maximum chromophore coverage of ∼2 × 1014 molecules/cm2. The kinetics of this substitution reaction and associated structural modifications are studied in real time by in situ polarized SHG techniques, which reveal non-Langmuirian kinetics and a rapidly increasing chromophore tilt angle with increasing coverage. The quaternization kinetics can be fit to a phenomenological biexponential rate equation with k′1 ≈ 2 × 10-2 L mol-1 s-1 and k′2 ≈ 2 × 10-3 L mol-1 s-1 and to a coverage-dependent activation energy model (EA ) E0 + Ebθ), yielding a perturbative energy Eb of 6-8 kJ mol-1. Both models are compatible with increasing repulsive interactions between chromophores at high coverages. The charge-compensating chloride counterions within monolayers having dense chromophore packing can be ion exchanged with iodide, up to a maximum of ∼40% of available chloride ions. The introduction of larger anions (sulfanilate, ethyl orange, eosin B) is observed in less densely packed films; however, the ion exchange process is completely inhibited in monolayers capped with a siloxane overlayer. In all cases, exchange of the chloride leads to significant increases in the second-harmonic generation efficiency, up to 45% on exchange with eosin B. In the case of iodide and sulfanilate substitution for chloride, the increase in the second-order response upon ion exchange is attributable to the incoming anion assuming a position within the monolayer microstructure different from that of the displaced anion.

Introduction Various approaches have been developed for the formation of films having nanoscale dimensions/features on solid surfaces, including Langmuir-Blodgett transfer, chemical and physical vapor deposition, and molecular self-assembly. Among these, self-assembly is especially attractive for the reliable synthesis of functional, structurally stable, molecular films, as a consequence of the intrinsic versatility of molecular chemistry and mild deposition conditions possible.1-3 Although, of necessity, the most detailed investigations of self-assembly have centered on long chain alkanethiol and alkylsilane derivatives, the potential for formation of relatively ordered, denselypacked arrays of electronically functional molecules was recognized early and has provided considerable impetus to the field.4,5 Indeed, the process of modifying surfaces X Abstract published in Advance ACS Abstracts, September 15, 1996.

(1) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press Inc.: San Diego, 1991. (2) Bell, C. M.; Yang, H. C.; Mallouk, T. E. Materials Chemistry: An Emerging Discipline; American Chemical Society: Washington, DC, 1995; p 212. (3) Kumar, A.; Abbott, N. L.; Kim, E.; Biebuyck, H. A.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219.

S0743-7463(96)00109-6 CCC: $12.00

with self-assembled layers terminated with reactive groups amenable to further modification has been used to form a remarkable variety of thin-film materials, including surface-confined proteins and other biomolecules,6-9 biomimetic crystalline oxides,10-12 metallic films,13-16 NLO-active multilayers,17-19 chemical sensors,20-25 and others. The ability to fabricate heteromul(4) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (5) DuBois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (6) Amador, S. M.; Pachence, J. M.; Fischetti, R.; McCauley, J. P., Jr.; Smith, A.B., III; Blasie, J. K. Langmuir 1993, 9, 812. (7) Stenger, D. A.; Georger, J. H.; Dulcey, C. S.; Hickman, J. J.; Rudolph, A. S.; Nielsen, T. B.; McCort, S. M.; Calvert, J. M. J. Am. Chem. Soc. 1992, 114, 8435. (8) Lopez, G. P.; Albers, M. W.; Schreiber, S. L.; Carroll, R.; Peralta, E.; Whitesides, G. M. J. Am. Chem. Soc. 1993, 115, 5877. (9) Naydenova, S.; Petrov, A. G.; Yarwood, J. Langmuir 1995, 11, 3435. (10) Bunker, B. C.; Rieke, P. C.; Tarasevich, B. J.; Campbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Virden, J. W.; McVay, G. L. Science 1994, 264, 48. (11) Shin, H.; Collins, R. J.; De Guire, M. R.; Heuer, A. H.; Sukenik, C. N. J. Mater. Res. 1995, 10, 699. (12) Shin, H.; Collins, R. J.; De Guire, M. R.; Heuer, A. H.; Sukenik, C. N. J. Mater. Res. 1995, 10, 692. (13) Wasserman, S. R.; Biebuyck, H.; Whitesides, G. M. J. Mater. Res. 1989, 4, 886.

© 1996 American Chemical Society

Benzyl Halide-Functionalized Surfaces

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Scheme 1. Schematic Depiction of the Deposition Chemistry for the Layer-by-Layer Assembly of Chromophoric Multilayer Films

tilayer structures by sequential chemisorption of different molecular species is of particular relevance, since this is a prerequisite for the construction of complex functional device structures.26,27 A wide variety of terminal functionalities have been incorporated into monolayer films, and a similarly great variety of chemical transformations applied to them.28-32 In particular, the ability to selectively modify the terminal functionality of a molecule after adsorption onto a surface allows introduction of terminal groups which may not in general be compatible with the self-assembling group. For example, the highly reactive SiCl3 functionality has been used for deposition of monolayers possessing a terminal alkyl bromide which can then be displaced by subsequent reaction with nucleophiles to yield trichlorosilyl-incompatible alkylamine- or alkanethiol-functionalized surfaces.33 Compared to alkyl and aryl functionalities, the benzyl halide group offers the attraction of enhanced reactivity34 as well as compatibility with the trichlorosilyl (14) Goss, C. A.; Charych, D. H.; Majda, M. Anal. Chem. 1991, 63, 85. (15) Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 116, 6636. (16) Potochnik, S. J.; Pehrsson, P. E.; Hsu, D. S. Y.; Calvert, J. M. Langmuir 1995, 11, 1841. (17) Li, D.; Ratner, M. A.; Marks, T. J.; Zhang, C.; Yang, J.; Wong, G. K. J. Am. Chem. Soc. 1990, 112, 7389. (18) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485. (19) Lin, W.; Yitzchaik, S.; Lin, W.; Malik, A.; Durbin, M. K.; Richter, A. G.; Wong, G. K.; Dutta, P.; Marks, T. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 1497. (20) Zimmerman, S. C.; Kwan, W.-S. Angew. Chem., Int. Ed. Engl. 1995, 34, 2404. (21) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 1329. (22) Sun, L.; Johnson, B.; Wade, T.; Crooks, R. M. J. Phys. Chem. 1990, 94, 8869. (23) Steinberg, S.; Tor, Y.; Sabatini, E.; Rubenstein, I. J. Am. Chem. Soc. 1991, 113, 5176. (24) Bharathi, S.; Yegnaraman, V.; Prabhakra Rao, G. Langmuir 1995, 11, 666. (25) Kepley, L. J.; Crooks, R. M.; Ricco, R. J. Anal. Chem. 1992, 64, 3191. (26) Freeman, T. L.; Evans, S. D.; Ulman, A. Langmuir 1995, 11, 4411. (27) Ulman, A. Organic Thin Films and Surfaces: Directions For The Nineties; Academic Press: New York, 1995. (28) Collins, R. J.; Sukenik, C. N. Langmuir 1995, 11, 2322. (29) Tillman, N.; Ulman, A.; Elman, J. F. Langmuir 1989, 5, 1020. (30) Kurth, D. G.; Bein, T. Langmuir 1993, 9, 2965. (31) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (32) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529. (33) Balachander, N.; Sukenik, C. N. Langmuir 1990, 6, 1621. (34) March, J. Advanced Organic Chemistry: Reactions, Mechanisms and Structure; John Wiley and Sons: New York, 1992, p 341.

group, and thus a molecule containing both benzyl chloride and trichlorosilyl groups is a suitable “coupling” linkage between hydroxylated surfaces and a subsequent nucleophilic reagent. This coupling route places few restrictions on the nucleophilic species, which need not be compatible with the trichlorosilyl group and can more readily be rendered multifunctional in character and thus amenable to further elaboration at some remote site in the adsorbed molecule. Recent efforts in this laboratory have focused on forming optically functional multilayer films by initial deposition (Scheme 1, step i) of a benzyl halide-terminated trichlorosilane (1).17,19,35-43 The benzyl position is susceptible to subsequent nucleophilic attack by a chromophore precursor, such as the 4-aminostilbazole 2a, to form an anchored, oriented, cationic stilbazolium layer (step ii). This quaternization reaction is of particular importance from the standpoint of nonlinear optical (NLO) material design, since, by forming an electron-deficient center, it also constitutes the last step in the synthesis of a conjugated donor-acceptor NLO-active chromophore of large hypercalcd ) 946 × 10-30 cm5 esu-1 at λ0 ) 1064 polarizability (βzzz nm using the reliable ZINDO/SOS formalism44 ). The structural restrictions imposed by a surface reaction are exploited advantageously here, inhibiting the formation of electrostatically favored antiparallel arrangements of the molecular dipoles. Instead, the NLO-active chromophores are necessarily organized acentrically (in an approximately parallel arrangement), which is optimal for efficient bulk optical second-harmonic generation (35) Allan, D. S.; Kubota, F.; Orihashi, Y.; Li, D.; Marks, T. J.; Zhang, T. J.; Lin, W.; Wong, G. K. Polym. Prepr. 1991, 32 (2), 86. (36) 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. (37) Kakkar, A. K.; Yitzchaik, S.; Roscoe, S. B.; Kubota, F.; Allan, D. S.; Marks, T. J.; Lin, W.; Wong, G. K. Langmuir 1993, 9, 388. (38) Roscoe, S. B.; Yitzchaik, S.; Kakkar, A. K.; Marks, T. J.; Lin, W.; Wong, G. K. Langmuir 1994, 10, 1337. (39) Kakkar, A. K.; Yitzchaik, S.; Roscoe, S. B.; Marks, T. J.; Lin, W.; Wong, G. K. Thin Solid Films 1994, 242, 142. (40) Yitzchaik, S.; Kakkar, A. K.; Roscoe, S. B.; Orihashi, Y.; Marks, T. J.; Lin, W.; Wong, G. K. Mol. Cryst. Liq. Cryst. 1994, 240, 9. (41) Roscoe, S. B.; Kakkar, A. K.; Marks, T. J.; Malik, A.; Durbin, M.; Lin, W.; Wong, G. K. Langmuir, in press. (42) (a) Marks, T. J.; Ratner, M. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 155. (b) Yitzchaik, S.; Marks, T. J. Acc. Chem. Res. 1996, 29, 197. (43) (a) Lin, W.; Lin, W.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 8034. (b) Lin, W.; Lee, T.-L.; Lyman, P. F.; Lee, J.; Bedzyk, M. J.; Marks, T. J. J. Am. Chem. Soc., in press. (44) (a) Albert, I. D. L.; Marks, T. J.; Ratner, M. A. J. Phys. Chem. 1996, 100, 9714. (b) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Chem. Rev. 1994, 94, 195. (c) Di Bella, S.; Marks, T. J.; Ratner, M. A. J. Am. Chem. Soc. 1994, 116, 4440.

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(SHG) and electro-optic response. Monolayers prepared this way display remarkably high bulk second-order nonlinearity, χ(2) ) 4-6 × 10-7 esu, at λ0 ) 1064 nm. The chromophoric monolayer thus formed may be treated with octachlorotrisiloxane (4; Scheme 1, step iii) to transversely “cross-link” and rigidify the monolayer and to regenerate a fresh Si-OH surface on which step i may be repeated, allowing the formation of multilayer structures. This capping step is also expected to minimize propagation of defects into subsequent layers, providing high-quality multilayer growth by an iterative procedure. Repetition of steps i-iii thus leads to multilayer superlattices of precisely defined thickness, high structural regularity, and high SHG response.17,19,37 The modular nature of these self-assembled films allows variation of the components independently, and relatively minor modifications of either the coupling agent or the chromophore architecture lead to substantial changes in monolayer microstructural parameters, as well as in the NLO response.41 For example, the chromophore precursors 5-8 have also been employed in this quaternization reaction, leading to NLO-active monolayers and multilayers of varying structural and NLO response properties.37,40,41,43

This contribution is concerned in part with an extension of our exploratory studies to the ion exchange substitution of the charge-compensating chloride ion in these multilayers, a different process from those previously investigated in that it involves modification of a pre-existing cationic framework rather than use of alternate precursors.44,45 The NLO response of crystalline stilbazolium salts displays a pronounced anion-dependence, and elucidating the SHG response of a homologous series of monolayers should provide further insights into the nature of the cation-anion SHG response relationships.46-48 The rational manipulation of self-assembled film microstructure additionally requires a detailed understanding of the self-assembly process. We also present here an in situ SHG study of the kinetics of the chromophore-forming nucleophilic substitution reaction (Scheme 1, step ii), as well as of the film microstructural evolution during this reaction. Self-assembly reactions have traditionally been studied by ex situ techniques;31,49-57 however, direct (45) (a) Some of the ion exchange data have been communicated previously. See refs 38, 39, and 45. (b) Yitzchaik, S.; Kakkar, A. K.; Roscoe, S. B.; Marks, T. J.; Lundquist, P. M.; Lin, W.; Wong, G. K. Mater. Res. Soc. Symp. Proc. 1994, 328, 27. (46) Meredith, G. R. In Nonlinear Optical Properties of Organic and Polymeric Materials; Williams, D. J., Ed.; ACS Symposium Series 233; American Chemical Society: Washington, DC, 1984; p 27. (47) Marder, S. R.; Perry, J. W.; Shaefer, W. P. Science 1989, 245, 626. (48) Marder, S. R.; Perry, J. W.; Yakymyshyn, C. P. Chem. Mater. 1994, 6, 1137. (49) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (50) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (51) Chen, S. H.; Frank, C. W. In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D. R., Ed.; ACS Symposium Series 447; American Chemical Society: Washington, DC, 1991; p 160. (52) Schwartz, D. K.; Steinberg, S.; Israelachvili, J.; Zasadzinski, J. A. N. Phys. Rev. Lett. 1992, 69, 3354. (53) Mathauer, K.; Frank, C. W. Langmuir 1993, 9, 3446.

Roscoe et al.

observation of the reacting surface58 provides a more incisive picture of monolayer growth, and such in situ methods are becoming more common.59-67 NLO spectroscopic techniques represent ideal methods for in situ study of the self-assembling quaternization step ii (Scheme 1), since they are sensitive exclusively to the chemisorbed (as opposed to physisorbed) product of the reaction, are noninvasive, and allow real-time evaluation of both the extent of the reaction and the structural evolution of the monolayer by simultaneous assessment of the chromophore orientation with respect to the surface normal. Experimental Section General. The synthesis of the chromophore precursors and coupling agents is reported elsewhere.17,41 The p-methyl analog of the coupling agent (2-(4-tolyl)ethyl trichlorosilane was prepared in 80% yield by the same hydrosilylation procedure that was used for the standard coupling agent.68,69 Product identity was verified by 1H and 13C NMR spectroscopy and mass spectrometry.17,37,41,70 Solvents (Fisher Scientific, Pittsburgh, PA) were predried and distilled from appropriate drying agents under nitrogen. Tetrabutylammonium iodide (electrochemical grade) was supplied by Fluka, AG and was used as received. Pyridine (Aldrich Chemical Co., Milwaukee, WI) was refluxed overnight over molecular sieves (Linde 4A) and distilled under N2 immediately prior to use. Other reagents (Aldrich) were used as received. Multilayer Deposition Techniques. Scheme 1, Steps i-iii. Glass substrates (typically glass microscope slides) were rinsed with aqueous detergent and then several times with deionized (DI) water, followed by heating to 80 °C for 1.0 h in “piranha solution” (concentrated H2SO4/30% H2O2, 70:30). After cooling, the substrates were rinsed copiously with DI water and then further cleaned with an RCA-type (H2O/H2O2/NH3, 5:1:0.5) cleaning protocol, followed by further DI water rinsing. The substrates were then carefully rinsed with acetone (5×), acetone/ toluene, and toluene (5×) before being immersed under inert atmosphere in a solution of coupling agent 1 (5 mL in 100 mL of freshly distilled toluene). Various coupling reaction (step i, Scheme 1) times were employed, with the best results (as assessed by the maximum second-harmonic response of the chromophoric film) being achieved using a dual immersion procedure, in which the samples were removed after 18 h of reaction, rinsed with toluene and acetone for approximately 1 h, and then returned to the coupling solution for a further 18 h of reaction. When (54) Banga, R.; Yarwood, J.; Morgan, A. M.; Evans, B.; Kells, J. Langmuir 1995, 11, 4393. (55) De´jardin, P.; ten Hove, P.; Yu, X. J.; Brash, J. L. Langmuir 1995, 11, 4001. (56) Mohri, N.; Inoue, M.; Arai, Y.; Yoshikawa, K. Langmuir 1995, 11, 1612. (57) Tam-Chang, S.-W.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371. (58) Tolia, A. A.; Williams, C. T.; Weaver, M. J.; Takoudis, C. G. Langmuir 1995, 11, 3438. (59) Guyot-Sionnest, P.; Superfine, R.; Hunt, J. H.; Shen, Y. R. Chem. Phys. Lett. 1988, 144, 1. (60) van Velzen, P. N. T.; Ponjee´, J. J.; Benninghoven, A. Appl. Surf. Sci. 1989, 37, 147. (61) Chen, S. H.; Frank, C. W. Langmuir 1989, 5, 978. (62) Buck, M.; Eisert, F.; Fisher, J.; Grunze, M.; Tra¨ger, F. Appl. Phys. A 1991, 53, 552. (63) Cheng, S. S.; Scherson, D. A.; Sukenik, C. N. J. Am. Chem. Soc. 1992, 114, 5436. (64) Xu, C.; Sun, L.; Kepley, L. J.; Crooks, R. M.; Ricco, R. J. Anal. Chem. 1993, 65, 2102. (65) Acevedo, D.; Bretz, R. L.; Tirado, J. D.; Abrun˜a, H. D. Langmuir 1994, 10, 1300. (66) Mrksich, M.; Sigal, G. B.; Whitesides, G. M. Langmuir 1995, 11, 4383. (67) Karpovich, D. S.; Blanchard, G. J. J. Chem. Educ. 1995, 72, 466. (68) Chuang, V. T. U.S. Patent 3,925,434, 1975; Chem. Abstr. 1975, 84, 469. (69) Kennedy, J. P.; Chang, V. S. C. Polym. Prepr. 1980, 21, 146. (70) p-Methyl coupling agent. 1H NMR (CDCl3) δ: 1.72 (m, 2H, CH2), 2.33 (s, 3H, CH3), 2.85 (m, 2H, CH2), 7.11 (s, 4H, ArH). 13C NMR (CDCl3) δ: 21.02 (CH2), 26.24 (CH2), 27.78 (CH3), 127.74, 129.31, 135.98, 138.30. Mass spectrum (EI) m/z: M•+ ) 252/254/256 (1.00:1.00:0.41, calc 1.00: 0.97:0.32).

Benzyl Halide-Functionalized Surfaces

Figure 1. Schematic of the reaction cell for in situ secondharmonic generation studies (step ii, Scheme 1) and the experimental optical geometry. silanization was complete, the samples were rinsed carefully with toluene and then acetone and then heated at 115 °C for 10 min in air to complete the curing of the siloxane network. The quaternization step ii was carried out by placing the samples in a toluene solution of the appropriate chromophore precursor (2a unless otherwise specified) and stirring at 80 °C under N2 for 5 days. In Situ Monitoring of Deposition Step ii. Float glass circles (1 3/8 in diameter; Kaufman Glass, Wilmington, DE) were cleaned and silanized with the coupling agent as described above. Three different silanization conditions were investigated: 18 h (i1), 64 h (i2), and 2 × 18 h (i3). At the completion of the coupling reaction, the windows were removed from the reaction solution, rinsed twice for 10 min each with toluene, toluene/acetone, and acetone, and then placed in an oven under air at 115 °C for 15 min. The cured windows were immediately affixed to the thermostated, in situ reaction cell (Figure 1). The in situ cell is comprised of a glass vessel of approximately 35 mL volume, fitted with a demountable window, silanized as above, held in place by a modified size 25 Ace-Thred cap, and filled with a 7.0 mM n-propanol solution of chromophore precursor 2a. The reaction temperature was maintained at 60 °C with heating tape, an electronic temperature controller, and a PTFE-coated thermocouple positioned at the reacting surface. Unfortunately toluene, which is the solvent of choice for chromophore deposition, proved unsuitable for in situ measurements, since fluorescence was invariably observed at the point where the laser impinged upon the surface, overwhelming the nascent SHG signal. Ion Exchange Processes. The change in monolayer SHG response due to ion exchange was assessed by measuring film samples before and after the exchange process, while the degree of ion exchange was assessed by comparing the Cl 2p/Si 2p ratio of an exchanged sample to that of a sample which was prepared identically but not subjected to exchange. Iodide exchange was achieved by stirring substrates functionalized with chromophoric monolayers (step ii) in a 0.070 M acetonitrile solution of tetrabutylammonium iodide for 48 h under N2 at 60 °C. The substrates were then rinsed successively three times each with acetonitrile, acetone, and methanol. The other anions were exchanged into the monolayer using a similar procedure with acetonitrile solutions of the sodium salts of ethyl orange (0.006 M, 60 °C, 48 h), eosin B (0.003 M, 60 °C, 48 h) or a methanol solution of the sodium salt of sulfanilic acid (0.05 M, room temperature, 72 h). Transmission SHG Measurements. Second-harmonic generation (SHG) measurements in the transmission mode were carried out using a Q-switched Nd:YAG laser operating at 1064 nm in the p-polarized geometry. No in-plane anisotropy of the SHG signal was observed when the slides were rotated about the film normal, indicating that the molecular orientation of the chromophores has no azimuthal dependence. Varying the input beam incident angle yielded an interference pattern due to the phase difference between the two SHG waves generated at either side during the propagation of the fundamental wave. It is

Langmuir, Vol. 12, No. 22, 1996 5341 noteworthy that complete destructive interference was consistently observed, indicating that films on the two sides of the glass are of identical quality. The data were highly reproducible over a range of points on the same sample, and the intensity variation for samples prepared simultaneously was less than (2) (10%. Values of χzzz were obtained by calibration of the SHG data against quartz for self-assembled monolayers of 25 Å assumed thickness.41 The presence of nonaligned and/or physisorbed material was investigated by withdrawing samples from the chromophore reaction solution. SHG and UV-vis spectra were recorded immediately afterward and again after exhaustive rinsing with acetone and toluene and wiping with methanolsoaked cotton swabs. Five samples treated this way to remove physisorbed material exhibited a decrease in optical absorbance of ∼70% after cleaning, consistent with a loss of approximately 2/3 of the total chromophoric material adsorbed on the surface. However, the average drop in SH intensity after wiping was less than 3%, well within the uncertainty of this measurement and demonstrating that the SHG measurement is insensitive to contaminants which do not fulfill the dual requirements of high molecular nonlinearity (β) and chromophore alignment. In Situ SHG Measurements. In situ polarized SHG experiments were carried out by using the 1053 nm output of a Q-switched Nd:YLF laser operating at 400 Hz. Each Q-switched envelope contained approximately 20 mode-locked pulses (∼0.2 mJ per Q-switched pulse) of 35 ps duration. The laser light was focused with a 30 cm lens onto the back surface of a couplinglayer-functionalized glass window in contact with a solution of chromophore precursor. The polarization of the fundamental beam was rotated in 22.5° intervals, and the reflected p-polarized SHG signal was detected with a gated photon-counting system. The macroscopic second-order hyperpolarizability, χ(2), is not readily accessible in the reflection geometry used for this experiment; however, the substrate can be removed and an accurate χ(2) value obtained by measuring the SHG in the transmission geometry, referenced to a quartz standard. X-ray Photoelectron Spectroscopy. XPS measurements were recorded on a VG Scientific Ltd. Escalab Mk II spectrometer using Mg KR irradiation (Al KR for I--exchanged samples) and a pass energy of 50 mV. Survey spectra with a step size of 0.5 eV (5-10 scans) and high-resolution spectra (step size 0.1 eV; 5-200 scans) were recorded for the peaks of interest. Relative abundances were calculated from high-resolution peak areas using the following sensitivity factors (relative to C 1s): Si 2p, 0.9; S 2p, 1.7; Cl 2p, 2.4; I 3d, 19.3.71 The background-subtracted N 1s and Cl 2p peak envelopes were simulated with Gaussian/ Lorentzian component peaks, allowing minor variation in the full width at half height (w1/2) to reflect variations in the chemical environment.72 The asymmetric chlorine peak shape was defined as the sum of two component peaks, corresponding to Cl 2p1/2 and Cl 2p3/2, with the same w1/2, separated by 1.7 eV, and with the relative area 1:2. Binding energies were referenced to the Si 2p peak of SiO2, defined at 103.4 eV.73

Results and Discussion The requirement of bulk noncentrosymmetry is a fundamental symmetry property of SHG and in fact all electro-optical effects originating in the first hyperpolarizability tensor χ(2).42,74,75 Furthermore, at a microscopic level, the generation of significant second-harmonic intensities requires a large molecular hyperpolarizability (β). These requirements can impart a remarkable degree of selectivity to SHG measurements as a method of studying reactions at surfaces, since processes involving highly polarizable dipolar molecules arranged in non(71) Supplied by Kratos Analytical Instruments. (72) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers: The Scienta Database; John Wiley and Sons: Chichester, England, 1992. (73) Niemantsverdriet, J. W. Spectroscopy in Catalysis. An Introduction; VCH Verlagsgesellschaft mbH: Weinheim, 1993. (74) Dalton, L. R.; Harper, A. W.; Ghosn, R.; Steier, W. H.; Ziari, M.; Fetterman, H.; Shi, Y.; Mustacich, R. V.; Jen, A. K.-Y.; Shea, K. J. Chem. Mater. 1995, 7, 1060. (75) Dalton, L. R.; Harper, A. W.; Wu, B.; Ghosn, R.; Laquindanum, J.; Liang, Z.; Hubbel, A.; Xu, C. Adv. Mater. 1995, 7, 519.

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centrosymmetric arrays will exhibit a considerably higher response than those which do not involve symmetry breaking.76,77 This selective response of SHG is in marked contrast to many other surface analysis techniques, such as surface acoustic wave or quartz crystal microbalance measurements, which respond without discrimination to the total quantity of physisorbed and chemisorbed material deposited on the surface. The combination of SHG and optical spectroscopy has proven to be a powerful method for detecting self-assembled chromophoric monolayers in the presence of physisorbed overlayers of the chromophore precursor or other oxidized byproducts (see Experimental Section for details). The SHG experiment can also be exploited in an analogous manner to provide accurate kinetic information on the self-assembly process. It will be seen that the quaternization reaction (Scheme 1, step ii) is ideally suited for in situ investigations, involving the formation of a high-β chromophore in a necessarily noncentrosymmetric arrangement. The pronounced difference in calculated molecular hyperpolarizability (β) between chromophore 3a and stilbazole precursor 2a calcd ) 946 × 10-30 cm5 esu-1 at 1064 nm for 3a vs 33 × (βzzz 10-30 cm5 esu-1 for 2a)78-81 ensures that the SHG response can be assigned exclusively to the quaternized reaction product, and thus studying the SH intensity as a function of reaction time and conditions yields a direct picture of the self-assembled stilbazolium monolayer evolution. Furthermore, evolution of the molecular orientation can be assessed in real time by monitoring the dependence of the SH response on the input polarization. The assumptions of a dominant βzzz along the principal molecular axis (verified theoretically)78-81 and a relatively narrow distribution of molecular orientations allow calculation of the average angle (ψ j ) by which the chromophore is tilted from the surface normal.82,83 Once the chromophoric monolayers have assembled, the interesting question arises as to whether they can be further modified chemically, not at the terminus but at sites within the monolayer. The chemisorption/quaternization reaction generates a monolayer-bound cation, accompanied by a charge-compensating chloride ion, which may be accessible to ion exchange processes with exogenous anions. Although photochemical and electrochemical reactions within monolayers are well characterized,84-90 few investigations have addressed the constraints under which charged chemical reagents penetrate ordered saltlike monolayers.91-93 The question of whether such reactions are sensitive to the molecular dimensions of the incoming reagent and the available free volume within the (76) Shen, Y. R. Surf. Sci. 1994, 299/300, 551. (77) Corn, R. M.; Higgins, D. A. Chem. Rev. 1994, 94, 107. (78) We thank Dr. D. R. Kanis for these calculations. (79) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Chem. Rev. 1994, 94, 195. (80) Kanis, D. R.; Ratner, M. A.; Marks, T. J. Int. J. Quantum Chem. 1992, 43, 61. (81) Kanis, D. R.; Ratner, M. A.; Marks, T. J.; Zerner, M. C. Chem. Mater. 1991, 3, 19. (82) Zhang, T. G.; Zhang, C. H.; Wong, G. K. J. Opt. Soc. Am. B 1990, 7, 902. (83) Preliminary data have previously been communicated in part. See ref 36. (84) Calvert, J. M. J. Vac. Sci. Technol. B 1993, 11, 2155. (85) Hockett, L. A.; Creager, S. E. Langmuir 1995, 11, 2318. (86) Chan, K. C.; Kim, T.; Schoer, J. K.; Crooks, R. M. J. Am. Chem. Soc. 1995, 117, 5875. (87) Rozsnyai, L. F.; Wrighton, M. S. Langmuir 1995, 11, 3913. (88) Lewis, M.; Tarlov, M. J. Am. Chem. Soc. 1995, 117, 9574. (89) Batcheldor, D. N.; Evans, S. D.; Freeman, T. L.; Ha¨ussling, L.; Ringsdorf, H.; Wolf, H. J. Am. Chem. Soc. 1994, 116, 1050. (90) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323. (91) Maoz, R.; Sagiv, J. Langmuir 1987, 3, 1034. (92) Maoz, R.; Sagiv, J. Langmuir 1987, 3, 1045. (93) Robinson, G. N.; Freedman, A. Langmuir 1995, 11, 2600.

Roscoe et al.

Figure 2. (a) Chlorine 2p region of the XPS spectra of the bare coupling agent film (step i, Scheme 1) fit with two component peaks as described in the Experimental Section. The data at the bottom are the Cl 2p region of a monolayer of the p-methyl analog. (b) Chlorine 2p region of the coupling agent film after being treated with (dimethylamino)pyridine (step ii, Scheme 1), fit with two pairs of component peaks.

monolayer21 can be addressed in part by choosing anions of varying molecular dimensions and studying exchange processes involving monolayers having differing chromophore surface densities. Furthermore, understanding cation-anion interaction effects on the SH response of ordered, ion-exchanged films is relevant to understanding the response of crystalline NLO materials, in which wide variations in SHG efficiency are sometimes observed for homologous, yet structurally diverse, series of cationic chromophores paired with different anions.48 XPS Studies of Chromophore Self-Assembly. Information on monolayer surface chemistry can be obtained by X-ray photoelectron spectroscopy (XPS), which provides both elemental composition and information on the chemical state of the constituent atoms. This is particularly useful here, where a benzylic chlorine is being displaced to yield a chloride anion. XPS spectra of films comprised only of the benzyl chloride coupling layer reveal the expected features due to C 1s, O 1s, and Si 2s and 2p photoelectrons, as well as an asymmetric Cl 2p peak at 199.9 eV, consistent with covalently bound chlorine.94,95 The peak asymmetry is due to the slightly different Cl 2p1/2 and Cl 2p3/2 binding energies and can be simulated by a doublet of relative area 1:2, separated by 1.7 eV (Figure 2a). Films prepared with the p-methyl analog of the standard coupling agent, (2-(4-tolyl)ethyl)trichlorosi(94) Yamamoto, Y.; Toyota, E.; Konno, H. Bull. Chem. Soc. Jpn. 1991, 64, 1398. (95) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin Elmer: Eden Praire, MN, 1992; p 63.

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lane, display no chlorine signals, indicating that all of the Si-Cl functionalities are consumed in the present selfassembly process. Thus, the chlorine signal in the XPS spectra of 1 can be assigned solely to benzylic chlorine atoms.

The quaternization of carefully prepared benzyl chloride films was initially studied with the well-known pyridinebased “hypernucleophile” 4-(dimethylamino)pyridine.96 The calculated molecular hyperpolarizability, β, of the resultant chromophore is low in comparison to that of the stilbazolium structure 3a, and accordingly the monolayer exhibits no detectable SHG response. The success of the quaternization is demonstrated by the appearance of a nitrogen 1s peak in the XPS spectrum and a shift in the position of the chlorine 2p peak. The chlorine peak in the quaternized samples moves to lower energy (199.0 eV), consistent with the formation of a charge-compensating chloride anion.95 However, a shoulder is consistently observed on the high-energy side of the peak (Figure 2b) in duplicate experiments. Simulations with two asymmetric peaks indicate that ∼10% of the benzylic chlorine atoms remain unreacted. The presence of unreacted chlorine atoms is consistent with other studies in which the reaction of surface-constrained benzyl chlorides with sodium iodide was investigated.97 Incomplete reaction in that case was explained by steric hindrance, where the incoming nucleophile has greater steric demands than the chlorine to be displaced. Such effects should be of even greater significance here, where larger molecular nucleophiles are employed. The nitrogen 1s region reveals a symmetric peak at 401.5 eV, consistent with a tetravalent, cationic nitrogen atom (Figure 3a). Interestingly, the full width at half maximum (w1/2) of the N 1s peak (2.3 eV) is comparable to that obtained with identical instrumental parameters using pyridine as the nucleophile and can be fit to a Gaussian line shape, indicating that the two aminopyridine nitrogen atoms are in similar electronic environments. In contrast to the results obtained with 4-(dimethylamino)pyridine, the XPS spectrum of monolayers of 3a displays a broad, asymmetric N 1s peak (w1/2 ) 3.5 eV), which can be deconvoluted into two symmetric peaks of unequal intensity separated by ∼2 eV (Figure 3b). In this case, the regiochemistry of attack was verified by independent solution-phase NMR experiments, which clearly indicate quaternization of the pyridine nitrogen rather than the amine, and thus these peaks can be assigned to distinct cationic pyridinium and neutral amino nitrogen atoms.98,99 ZINDO-SOS calculations indicate (96) Ho¨fle, G.; Steglich, W.; Vorbru¨ggen, H. Angew. Chem., Int. Ed. Engl. 1978, 17, 569. (97) Koloski, T. S.; Dulcey, C. S.; Haralson, Q. J.; Calvert, J. M. Langmuir 1994, 10, 3122. (98) Moses, P. R.; Wier, L. M.; Lennox, J. C.; Finklea, H. O.; Lenhard, J. R.; Murray, R. W. Anal. Chem. 1978, 50, 576.

Figure 3. N 1s region of the XPS spectra of chromophoric monolayers (step ii, Scheme 1) prepared with (a) (dimethylamino)pyridine and (b) chromophore 2a as nucleophiles.

that the stilbazolium HOMO is localized principally on the aniline ring100 with minimal charge density on the pyridinium nitrogen, which is in contrast to the aminopyridinium cation in which the HOMO has significant electron density on the quaternary nitrogen.101

The Cl 2p region in chromophoric 3a films having large χ(2) responses is also broad and can be deconvoluted to yield two doublets of relative intensity ∼6:4, indicating that the quaternization reaction proceeds to ∼50-60% under the present self-assembly conditions. The lower degree of quaternization compared to that observed for (dimethylamino)pyridine is presumably a consequence of the reduced nucleophilicity of the stilbazole chromophore. However, when the coupling layer is prepared with more dilute solutions of 1, or for shorter reaction times (e8 h), the Cl region of the quaternized product displays a significantly smaller shoulder at high binding energy corresponding to benzylic chlorine. Such coupling layers doubtless have considerably lower molecular surface densities, and thus there is less steric hindrance to (99) Bierbaum, K.; Kinzler, M.; Wo¨ll, C.; Grunze, M.; Ha¨hner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512. (100) (a) Di Bella, S.; Fragala, I.; Ratner, M. A.; Marks, T. J. Chem. Mater. 1995, 7, 400. (b) For experimental optical spectroscopic evidence in Langmuir-Blodgett systems, see: Young, M. C.; Jones, R.; Tredgold, R. H.; Lu, W. X.; Ali-Adib, Z.; Hodge, P.; Abbasi, F. Thin Solid Films 1989, 182, 319. (101) We thank Dr. A. Israel for these calculations.

5344 Langmuir, Vol. 12, No. 22, 1996

Figure 4. Time-dependent in situ SHG data for the quaternization reaction (step ii, Scheme I; only the square root of the Ip-p signal is presented) for samples prepared under different prefunctionalization conditions (step i). The curves are a guide to the eye.

Roscoe et al.

more densely packed, benzyl chloride monolayer. The average chromophore tilt angle ψ j follows a qualitatively similar time dependence to that of the overall SHG signal (Figure 5); however, the maximum ψ j is reached significantly earlier in the assembly process than is the maximum signal intensity. The maximum tilt angle also exhibits a minor but significant dependence on the prefunctionalization time, with the i3 conditions, which lead to the maximum SHG signal, also leading to the maximum tilt angle. The measured rate of the in situ reaction should accurately represent the rate of the quaternization reaction at the surface, since the time scale of the quaternization reaction is sufficiently long, and the solution concentration sufficiently high, that diffusion effects can be neglected. This was confirmed by ceasing stirring during one run, which led to no discernible change in the reaction profile. The degree of reaction, θ, can be calculated from eqs 1 and 2, where ψ j is the angle by which calc 3 ( χ(2) j βzzz s )⊥⊥⊥ ) Ns cos ψ

θ)

Figure 5. Average chromophore tilt angle ψ j vs reaction time for the quaternization step (step ii, Scheme 1). Error bars represent the dispersity in 10-20 measurements (the dashed lines are a guide to the eye).

quaternization. This is consistent with the progressive reduction in film SH response intensity (and therefore chromophore coverage) observed as the coupling reaction time is reduced39 (see also the in situ experiments below). In Situ SHG Studies of Chromophore Self-Assembly. Monitoring the reflected p-polarized SHG signal generated from the inside face of the prefunctionalized window of the in situ cell (Figure 1) throughout the course of step ii, Scheme 1 at 60 °C, yields a response which increases rapidly over the first 30-50 h and then reaches a plateau (Figure 4). Continuing for up to 200 additional hours results in no further change in the SHG signal. Samples prepared using different coupling reaction times (step i in Scheme 1; i1 ) 18 h, i2 ) 64 h, i3 ) 2 × 18 h) display significantly different final intensities, although the curves retain the same general shape. The variation in the final second-harmonic intensity with prefunctionalization time (step i) is doubtless related to differing surface coverages of the benzyl chloride monolayer, where the greater SHG signal observed for longer prefunctionalization times (step i, Scheme 1) implies a greater final chromophore surface density. The observation that optimum deposition requires multiple immersions in step i is unusual but not unprecedented, having been reported previously for monolayers of phenoxyalkyl silanes.102 Benzylic films prepared with long single immersions (up to 30 h) typically yield advancing aqueous contact angles of ∼74°,40 while the present dual immersion samples yield consistently higher values of ∼85°, further indicative of a more stable, and presumably (102) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136.

Ns(t) Ns(t ) ∞)

)

(xI2ω p-p)t

j )t)∞ (cos3 ψ

(xI2ω p-p)t)∞

(cos3 ψ j )t

(1)

(2)

the chromophore principal dipole direction (approximately corresponding to the molecular axis) is tilted from the surface normal, Ns(t) is the chromophore surface density at time t, Ns(t ) ∞) corresponds to the final chromophore 2ω surface density, and Ip-p is the second-harmonic intensity at time t, which is proportional to the square of the 103 In surface second-order optical nonlinearit, (χ(2) s (t)⊥⊥⊥). work to be discussed elsewhere, we show that stilbazolium surface densities derived from eq 1 are in good agreement with anion surface densities determined by standing wave X-ray techniques.43b The standard and simplest model for adsorption is the Langmuir isotherm (dθ/dt ) k(1 θ), where θ is the surface coverage),104a which assumes that adsorption occurs at specific sites on a uniform surface and is limited to a monolayer and that there are no interactions between adsorbed species. Plots of -ln(1 θ) against time (Figure 6) are clearly nonlinear, indicating that standard Langmuirian kinetics do not provide a good description of the self-assembling quaternization process.33 The introduction of a second exponential term to the standard Langmuir equation yields eq 3, which provides

θ ) 1 - (R1e -k1t + R2e-k2t)

(3)

a clearly better fit to the experimental coverage in all (103) (a) In principle, coverage-dependent variations in local field effects can compromise the reliability of eqs 1 and 2. However, in the present study, the typical surface densities (e1.9 × 1014 molecules/cm2) and tilt angles (38-44°) encountered should place the dependence of j) χ(2) on surface coverage in the “linear regime”.103b In cases where (ψ is larger, the low surface coverage should again ensure linear 103b (b) Cnossen, G.; Drabe, K. E.; Wiersma, D. A., J. Chem. behavior. Phys. 1992, 97, 4512. (c) Equation 1 pragmatically assumes a narrow distribution in Ψ. In principle, deviations might be detected in the line shapes of linear optical spectra taken as a function of coverage.103d Such effects are not obvious in spectra of the present systems.103e (d) Evans, C. E.; Song, Q.; Bohn, P. W. J. Phys. Chem. 1993, 97, 12302. (e) Lundquist, P. M.; Yitzchaik, S.; Zhang, T.; Kanis, D. R.; Ratner, M. A.; Marks, T. J.; Wong, G. K. Appl. Phys. Lett. 1994, 64, 2194. (104) (a) Atkins, P. W. Physical Chemistry, 4th ed.; W. H. Freeman and Co.: New York, 1990; p 885. (b) A biexponential model yields a significantly better fit (higher correlation coefficients) than monoexponential and stretched-exponential models, while a triexponential model yielded no further improvement. Observed rate constants are expressed in units of inverse seconds, since initial surface benzyl chloride coverages are variable.

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Figure 6. Plot of -ln(1 - θ) vs time for the quaternization step (step ii, Scheme 1) under different prefunctionalization conditions (step i: (a) i1 ) 18 h; (b) i2 ) 64 h; (c) i3 ) 2 × 18 h). Langmuir kinetics would be demonstrated by a straight line fit. The curves are drawn as guides to the eye.

three cases (Figure 7 and Table 1)104b and allows derivation of “fast” and “slow” rate constants (Table 2). Note that the derived pairs of rate constants are identical within experimental error for all prefunctionalization conditions and that R1 + R2 ≈ 1 for all three data sets. The absolute adsorption rate constants k′ for such a model can be estimated from eq 4,60 where [C] is the solution concentration of chromophore precursor 2a (Table 2).

(1 - θ) ) Re-k′1t[C] + (1 - R)e-k 2′ t[C]

(4)

The adsorption of long-chain n-alkanoic acids on silver surfaces and n-alkanethiols on gold surfaces generally follows Langmuir kinetics,61,62,67,105 although the presence of two growth rates has been reported in one system.50 Similarly, the adsorption of octadecyltrichlorosilane on various surfaces has been reported to generally follow Langmuir kinetics,59,63 with a brief (