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Nucleation and Growth of Molecular Crystals on Self-Assembled Monolayers Lynn M. Frostman, Mamoun M. Bader, and Michael D. Ward' Department of Chemical Engineering and Materials Science, University of Minnesota, 151 Amundson Hall, 421 Washington Avenue S.E., Minneapolis, Minnesota 55455 Received July 30, 1993. I n Final Form: December 3, 199P Self-assembled organosulfur monolayers with different interfacial chemical functionalities influence the heterogeneous nucleation and growth of malonic acid crystals. Nucleation rates of malonic acid (HOOCCHzCOOH), as deduced from the number of crystals per unit area and mean crystal size, are significantlymore rapid on organosulfurmonolayersterminated with carboxylicacid groups ([Aul-S(CH2)11COOH and [Au]-S(CH2)1&0OH) than on corresponding monolayers terminated with methyl, methyl ester, or ethyl ester functionalities. X-ray diffractionreveals that the orientation of malonic acid crystals is affected by the monolayer composition. The malonic acid (001)plane, described by truncated hydrogenbonded chains, is oriented parallel to the [Au]-S(CH2)11COOH monolayer interface. However, this orientation is absent on [Au]-S(CH2)11CHa and [Au]-S(CH2)&OOCH3. The observed behavior is consistent with interfacial stabiliation of malonic acid prenucleation aggregates by interfacial hydrogen bonding and surface polar forces when carboxylic acid monolayers are employed.
Introduction The search for new materials with tailored physical and electronic properties has prompted numerous efforts in the design and synthesis of molecular crystals, whose supramolecular structures result from intermolecular interactions of several types, including van der Waals, hydrogen-bonding, electrostatic, and charge-transfer interactions. Thishas led to "crystal engineering",l a strategy involving the design of crystal packing based on rational molecular approaches, which has proven successful in the synthesis of organic metals and superconductors,2 nonlinear optical materials? and materials with well-defined solid-state reaction pathways.l It also is becoming increasinglyrecognized that crystal engineering approaches may be important in the manufacture of pharmaceutical reagents, in which bioavailability and deliverability of a particular polymorph depend upon ita morphology and crystal packing energy.4 While most efforts in crystal engineering have focused on design of the solid state from the perspective of thermodynamic control of intermolecular interactions, relatively few attempts have been made to elucidate the molecular level events involved in crystallization processes. Better control of nucleation and growth of molecular crystals can have an impact on crystal seeding and the resolution of optically active enantiomers? the crystallization of noncentrosymmetricpolymorphs for
* To whom correspondence should be addressed.
Abstractpublishedin Advance ACS Abstracts, January 16,1994. (1) Schmidt, G. M. J. Pure Appl. Chem. 1971,27,647. (2) (a) Cowan,D. 0.;Wiygul, F. M. Chem. Eng. News 1986,64,28. (b) 0
Williams, J. M.; Beno, M. A.; Wang, H. H.; Leung, P. C. W.; Emge, T. J.; Geiser, U.; Carlson, K. D. Acc. Chem. Res. 1986, 18, 261. (c) Garito, A. F.; Heeger, A. J. Acc. Chem. Res. 1974, 7, 232. (3) Nicoud, J. F.; Twieg, R. J. In Design and Synthesis of Organic Molecular Compounds for Efficient Second-Harmonic Generation; Chemla, D. S.,Zyss, J., Eds.; Academic Prees, Inc.: Orlando, 1987; p 227 and references therein. (4) Byrn, S. R. Solid-state Chemistry ofDrugs; Academic Press, Inc.: New York, 1982. (5) (a) Weissbuch, I.; Addadi, L.; Leiserowitz, L.; Lahav, M. J. Am. Chem. SOC.1988, 110, 561. (b) Addadi, L.; Berkovitch-Yellin, 2.; Weissbuch, I.; van Mil, J.; Shimon, L. J. W.; Lahav, M.; Leiserowitz,L. Angew. Chem.,Znt.Ed. Engl. 1985,24,466. (c) Collet,A.;Brienne, M.-J.; Jacques, J. Chem. Rev. 1980,80,216. (d) Secor,R. M. Chem. Reu. 1963, 63, 297.
nonlinear optical materials, and the crystallization of organic conductors and ferromagnetsS6 Crystallization can be described as a stepwise process in which molecules first self-assembleinto a prenucleation aggregate whose supramolecular structure resembles the crystal structure of the mature crystalline phase toward which it is evolving. The next stage involves growth of the aggregate to a critical size at which its volume free energy begins to dominate unfavorable surface energies, resulting in a sustainable nucleus upon which crystal growth occurs. The relationship between a prenucleation aggregate and a mature crystalline phase suggests that control of the early stages of the crystallization process, when aggregates are present, represents a complementary approach to previous crystal engineering strategies. Elucidation of the critical factors in nucleation of molecular crystals can lead to better control of nucleation and growth rates, crystal size, growth orientation, morphology, defect density, and polymorph identity. It is generally recognized that heterogeneous nucleation on substrate surfaces is more favorable than homogeneous nucleation, as the former can provide interfacial interactions with prenucleation aggregates which lower their surface energy. This results in a smaller critical size of the corresponding nuclei and a more rapid nucleation rate. Several studies indicate this behavior is important for molecular crystals, which have been reported to exhibit enhanced nucleation rates and growth orientation effecta on inorganic single-crystal substrates' and polymer surfaces.8 Recent reports indicate that nucleation of molecular crystals can be influenced by ordered molecular interfaces such as surfaces of organic single crystals,g (6) Ward, M. D. In Electrochemical Aspects of Low-Dimensional Molecular Solids; Bard, A. J., Ed.; Marcel Dekker: New York, 1989; p 181. (7)
(a) Yase, K.; Yamanaka, M.; Sasaki, T.; Inaoka, K.; Okada, M. J. Cryst. Growth 1992,118, 348. (b) Hayashi, S.; Ikuno, H.; Yanagi, H.; Ashida, M. J. Cryst. Growth 1992,123,35. (c) Yanagi, H.;Takemoto, K.; Hayashi,S.;Ashida, M. J.Cryst. Growth 1990,99,1038. (d)Zimmermann, U.; Schnitzler, G.; Karl, N.; Umbach, E. Thin Solid Film 1989,175,85. (e) Mobus, M.; Schreck, M.; Karl, N. Thin Solid Film 1989, 175, 89. (8) (a) Kawaguchi, A.; Okihara, T.; Katayama, K. J. Cryst. Growth 1990,99, 1028. (b)Willems, J.; Willems, 1. Nature 1956, 178,429. (9) (a) Carter, P. W.; Ward, M. D. J . Am. Chem. SOC.1993,115,11621. (b)Gavish, M.; Wang, J.-L.; Eisenstein, M.; Lahav, M.; Leiserowitz, L. Science 1992, 256, 816.
0743-7463/94/2410-0576$04.50/00 1994 American Chemical Society
Nucleation a n d Growth of Molecular Crystals
Langmuir monolayers,l'Jand Langmuir-Blodgett films.lb Another approach to molecular interfaces for crystal growth involves organosulfur monolayers on metals. The significanceof interfacial interactions on these monolayers is evident in several investigations, including promotion of the dropwise condensation of steam," patterning of surfams,12specificbinding of protein^,'^ chemical sensors,14 electrode modification,ls controlled interfacial wetting,16 and construction of molecular surface defects." Previous research has demonstrated that these monolayers can interact specificallywith vapor-phase organic molecules.18 Indeed, it was recently reported that the terminal functional group of an organosulfur monolayer has a pronounced effect on the nucleation and growth of calcium oxalate m0n0hydrate.l~Rational design strategies based on construction of monolayers that resemble a twodimensional plane of a desired crystalline phase may lead to more rapid crystal growth, growth orientation effects governed by the specific substrate-aggregate interactions, control of polymorphism, and possibly the formation of new metastable phases that may be kinetically favored because of interfacial stabilization of their respective aggregates by the monolayer. We herein report our initial investigations of the nucleation and growth of organiccrystals on self-assembled monolayers. Specifically, we descrjbe the crystallization of a simple dicarboxylic acid, malonic acid (HOOCCH2COOH), on organosulfur monolayers functionalized with terminal methyl, carboxylic acid, methyl ester, and ethyl ester groups. Malonic acid was chosen in order to probe the role of hydrogen bonding at the substrate interface during nucleation. We observed that nucleation rates of malonic acid are significantly more rapid on organosulfur monolayers terminated with carboxylicacid groups ((AulS ( C H h C 0 O H and [AUI-S(CH~)I~COOH) than on corresponding monolayers terminated with methyl, methyl ester, or ethyl ester functionalities. The monolayer composition also strongly influencesthe growth orientation of the malonic acid crystals. The observed nucleation and growth orientation behavior is consistent with interfacial interactions, including hydrogen bonding and polar forces, between the monolayers and malonic acid prenucleation aggregates during the nucleation process. (10) (a) Heywood, B. R.; Mann, S. J. Am. Chem. SOC.1992,114,4681. (b) Mann, S.;Heywood, B. R.; Rajam, 5.; Walker, J. B. A. J. Phys. D Appl. Phys. 1991,24,164. (c)Popovitz-Biro, R.; Lahav,M.; hiserowitz, L. J. Am. Chem. SOC.1991,113,8943. (d) Gavish, M.; Popovitz-Biro, R.; Lahav, M.; Leiserowitz, L. Science 1990,260, 973. (e) Landau, E. M.; Wolf, S. G.; Levanon, M.; Leieerowitz, L.; Lahav, M.; Sagiv,J. J. Am. Chem. SOC.1989,111,1436. (11) Black", L. C. F.; Dewar, M. J. S.J. Chem. SOC.1957, 171. (12) (a) Lopez, G. P.; Biebuyck, H. A.; Frisbie, C. D.; Whitesides, G. M. Science 1993,260,647. (b) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. SOC.1992,114,9188. (c)Abbott, N. L.;Folkers, J. P.; Whitesides, G. M. Science 1992,257,1380. (d) Laibinis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesides, G. M. Science 1989, 245,846. (13) (a) Prime, K. L.; Whitesides, G. M. Science 1991,252,1164. (b) Haussling, L.; Ringsdorf, H.; Schmitt, F.-J.; Knoll, W. Langmuir 1991, 7, 1837. (14) Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1992,64, 3191. (16)(a) Malem, F.; Mandler, D. Anal. Chem. 1993,65,37. (b) Cheng, Q.;Brajter-Toth,A. Anal. Chem. 1992,64,1998. (c)Tarlov, M. J.; Bowden, E. F. J. Am. Chem. SOC.1991,113, 1847. (d) Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. J. Electroanal. Chem. 1987, 219, 366. (16) (a) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Am. Chem. SOC. 1990,112,670. (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. (17) (a) Folkers, J. P.; Laibmis, P. E.; Whitesides, G. M. Langmuir 1992,8,1330. (b) Laibmis, P. E.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991,7,3167. (c) Bain, C. D.; Whitesides, G. M. J. Am. Chem. SOC.1989,111,7164. (18)Sun, L.; Kepley, L. J.; Crooks, R. M. Langmuir 1992,8, 2101. (19)Campbell,A.A.;Fryxell,G.E.;Gratf,G.L.;Rieke,P.C.;Tar~vich, B. J. Scanning Microsc. 1993, 7, 423.
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Experimental Section Materials. 16-Mercaptohexadecanoicacid, 12-mercaptododecanoic acid, methyl 12-mercaptododecanate, and ethyl 12mercaptododecanate were prepared using modified published procedures.20 Hexadecyl mercaptan (92 % ) and dodecyl mercaptan (98%) were obtained from Aldrich and used without further purification. Malonic acid (99%, Aldrich) was recrystallized from acetone prior to use. Substrate Preparation and Crystal Growth. Substrates for crystal growth and X-ray diffraction studies consisted of mica (MicaNewYorkCorp.), cutinto0.25-in. squares,cleavedto -0.1mm thickness, and coated with lo00 A of gold (99.95 %, Aesar) by evaporative methods. Gold films for infrared reflection absorption spectroscopy (IRRAS) were prepared on glass microscope slides (1.5 in. X l in.) cleaned in piranha solution (30% H202/70% concentrated H2SO& washed sequentially with deionized water and absolute ethanol, and dried with filtered air (caution: piranha solution reacts violently with organic materials and should not be stored in closed containers). The glass slides were then transferred to the evaporation chamber for coating with a 7 5 4 chromium (99.99%, Aesar) adhesion layer followed by 2000 A of gold. Monolayers were formed by overnight immersion of the fresh gold substrates in 1mM ethanolicsolutions of the thiols. Bare gold control surfaces were stored in absolute ethanol. Sessile drop contact angles were measured in air with 18-MQ deionized water using a Rame'-Hart contact angle goniometer. Values reported represent the average of at least two measurements. The substrates were removed from the dipping solutions,rinsed with copious amounts of absolute ethanol, and blown dry with filtered dry air. Comparative experiments were performed by attaching three substrates to a glass stage in an inverted position 1.5 cm above the malonic acid in a custom-made sublimation chamber. The chamber was then evacuated for 5min toa pressure of
