Self-Assembled Chromophoric NLO-Active Monolayers. X-ray Reflectivity and Second-Harmonic Generation as Complementary Probes of Building Block-Film Microstructure Relationships Stephen B. Roscoe, Ashok K. Kakkar, and Tobin J. Marks* Department of Chemistry and the Materials Research Center, Northwestern University, Evanston, Illinois 60208-3113 Amit Malik, Mary K. Durbin, Weiping Lin, George K. Wong,* and Pulak Dutta* Department of Physics and Astronomy and the Materials Research Center, Northwestern University, Evanston, Illinois 60208-3112 Received February 24, 1995. In Final Form: June 4, 1996 X The microstructural changes in chromophoric self-assembled monolayers induced by modifications in the architecture of the molecular components are studied by X-ray reflectivity and polarized second- harmonic generation (SHG) spectroscopy. Monolayers prepared by chemisorption/quaternization of the chromophore precursor 4-[N,N-bis(3-hydroxypropyl)amino]styryl-4′-pyridine (2a) on preassembled films of the coupling agents (p-ClCH 2C6H4)(CH2)2SiCl3 (1a), (p-ClCH2C6H4)(CH2)2SiCl2CH3 (1b), and (p- ClCH 2C6H4)(CH2)2SiCl(CH3)2 (1c) display a progressive reduction in measured film thickness from 26.4 to 21.8 to 19.6 Å and in optical second-harmonic response,  (2) (λ0 ) 1064 nm), from 3.0 × 10 -7 to 1.7 × 10 -7 to 0.8 × 10 -7 esu, respectively, indicating reduced chromophore surface density with increasing methyl substitution on the silicon of the coupling agent. The chromophoric groups in the film also experience a slight increase in tilt angle with respect to the surface normal, from 37° to 41° to 43°, with increased methylation of Si, indicating that the reduction in thickness is primarily due to microstructural changes in other regions of the film. Monolayers prepared under slightly different conditions with coupling agent 1a and chromophore precursor 2a or the analogous alkynyl chromophore precursor [4-[N,N-bis(3- hydroxypropyl)amino]phenyl]ethynyl-4′-pyridine (2b), and then capped with octachlorotrisiloxane, display a decrease in measured film thickness from 20.5 to 15.1 Å, an increase in monolayer-air interface width, and a reduction in  (2) from 2.0 × 10 -7 to 0.6 × 10 -7 esu when 2b is used in place of 2a. Penetration of the capping layer into the film microstructure is suggested by a substantial increase in film electron density to ∼70% of silicon and an increased measured interface width. Introduction Molecule-based self-assembled thin films have attracted considerable attention in recent years, not only for their inherent significance in the modification of surfaces but also because of potential technological applications of these artificially-structured materials. 1,2 A variety of adsorbate/ substrate systems have been investigated, including disulfides and thiols on gold, 3 as well as silanes, 4 metal phosphonates, 5 and carboxylic acids 6 on various oxides. Physical properties such as tribology 7 and especially wettability 8 of straight-chain alkanethiol and alkylsilane films have been intensively studied, but there have been few investigations of the properties of monolayers com- posed of branched, 9 aromatic, 10 or optically functional molecules. For both fundamental scientific and techno- logical reasons, there is a need to deposit well-ordered and well-characterized films with predictable physical, chemical, and, depending on the desired function, elec- tronic and optical properties. Such properties are strongly dependent on the microstructure of the monolayer, and a full understanding of the relationship between the molecular architecture of the adsorbate, the microstruc- ture of the film, and the macroscopic physical properties X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) (a) Ulman, A. An Introduction To Ultrathin Organic Films; Academic Press: San Diego, CA, 1991; Part 3. (b) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (c) 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. (2) (a) Fontaine, P.; Goguenheim, D.; Deresmes, D.; Vuillaume, D.; Garet, M.; Rondelez, F. Appl. Phys. Lett. 1993, 62, 2256. (b) Amador, S. M.; Pachence, J. M.; Fischetti, R.; McCauley, J. P., Jr.; Smith, A. B. III; Blasie, J. K. Langmuir 1993, 9, 812. (c) Vaknin, D.; Kjaer, K.; Ringsdorf, H.; Blankenburg, R.; Piepenstock, M.; Diederich, A.; Lo ¨sche, M. Langmuir 1993, 9, 1171. (d) Calvert, J. M. J. Vac. Sci. Technol. B 1993, 11, 2155. (3) See, for example : (a) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (b) Bain, C. D.; Troughton, E. B.; Tao, Y-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (c) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (4) (a) Parikh, A. N.; Allara, D. L.; Azouz, A. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577. (b) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101. (c) Wasserman, S. R.; Tao, Y.-T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (d) Maoz, R.; Sagiv, J. Langmuir 1987, 3, 1034. (e) Maoz, R.; Sagiv, J. Langmuir 1987, 3, 1045. (5) (a) Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 116, 6636. (b) Zeppenfeld, A. C.; Fiddler, S. L.; Ham, W. K.; Klopfenstein, B. J.; Page, C. J. J. Am. Chem. Soc. 1994, 116, 9158. (c) Cao, G.; Hong, H-G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420 and references therein. (6) (a) Allara, D. L.; Atre, S. V.; Elliger, C. A.; Snyder, R. G. J. Am. Chem. Soc. 1991, 113, 1852. (b) Chen, S. H.; Frank, C. W. Langmuir 1989, 5, 978. (c) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (7) (a) Nakagawa, T.; Ogawa, K.; Kurumizawa, T. Langmuir 1994, 10, 525. (b) Ru ¨ he, J.; Novotny, V. J.; Kanazawa, K. K.; Clarke, T.; Street, G. B. Langmuir 1993, 9, 2383. (8) (a) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (b) Montgomery, M. E. Jr.; Green, M. A.; Wirth, M. J. Anal. Chem. 1992, 64, 1170. (c) Barrat, A.; Silberzan, P.; Bourdieu, L.; Chatenay, D. Europhys. Lett. 1992, 20, 633. (d) Tidswell, I. M.; Rabedeau, T. A.; Pershan, P. S.; Folkers, J. P.; Baker, M. V.; Whitesides, G. M. Phys. Rev. B 1991, 44, 10869. (9) Offord, D. A.; Griffin, J. H. Langmuir 1993, 9, 3015. (10) (a) Dufor, C. N.; Turner, D. C.; Georger, J. H.; Peek, B. M.; Stenger, D. A. Langmuir 1994, 10, 148. (b) Mathauer, K.; Frank, C. W. Langmuir 1993, 9, 3002. (c) Mathauer, K.; Frank, C. W. Langmuir 1993, 9, 3446. (d) Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N.; J. Am. Chem. Soc. 1991, 113, 4121. (e) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136. 4218 Langmuir 1996, 12, 4218-4223 S0743-7463(95)00142-9 CCC: $12.00 © 1996 American Chemical Society