© Copyright 2003 American Chemical Society
SEPTEMBER 2, 2003 VOLUME 19, NUMBER 18
Letters Kinetic and Thermodynamic Forms of a Two-Dimensional Crystal Kibum Kim, Katherine E. Plass, and Adam J. Matzger* Department of Chemistry and the Macromolecular Science and Engineering Program, The University of Michigan, Ann Arbor, Michigan 48109-1055 Received February 16, 2003. In Final Form: April 25, 2003 The first examples of self-assembled physisorbed monolayers containing a carbamate functional group have been imaged with atomic resolution by scanning tunneling microscopy. Two different conformational pseudopolymorphs and a clear structural transition between these packing structures are observed. Each of these minima is characterized by a different degree of van der Waals and electrostatic interactions. In addition, one of these modifications shows a pseudoperiodic change of column direction resulting from disordering of the packing structure associated with orientational flipping of the molecules. The phase transition behavior can be understood in the context of Ostwald’s rule of stages, a widely observed phenomenon in three-dimensional crystallizations.
Introduction Physisorbed monolayers are proving to be excellent models for three-dimensional crystallization phenomena. They can be regarded as two-dimensional crystals sharing common structural characteristics such as packing motifs1-3 and reactivity4,5 with three-dimensional crystals. Scanning tunneling microscopy (STM) has been extensively used for the study of the structure and dynamics of physisorbed monolayers because it offers direct information on local packing and defect structure with atomic resolution.6-8 By contrast, X-ray crystallographic studies of three-dimensional crystals generally provide space-
averaged information. Another advantage of STM is that atomic resolution is obtained with relatively fast temporal resolution; thus, changes in molecular packing and structure are directly observable in situ9-12 without relying on indirect methods such as vibrational spectroscopy, thermal analysis, or morphology observation, which are often employed in the study of structural reorganization in three-dimensional crystals. These advantages make the study of physisorbed monolayers with STM suitable not only for elucidating the role of weak intermolecular interactions in crystal packing but also for studying the kinetics of formation of these motifs.
* Corresponding author. E-mail:
[email protected]. Fax: (734) 615-8553.
(7) Claypool, C. L.; Faglioni, F.; Goddard, W. A.; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5978-5995. (8) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424-427. (9) Gesquie`re, A.; Abdel-Mottaleb, M. M.; De Feyter, S.; De Schryver, F. C.; Sieffert, M.; Mu¨llen, K.; Calderone, A.; Lazzaroni, R.; Bre´das, J. L. Chem.sEur. J. 2000, 6, 3739-3746. (10) Padowitz, D. F.; Messmore, B. W. J. Phys. Chem. B 2000, 104, 9943-9946. (11) Stawasz, M. E.; Sampson, D. L.; Parkinson, B. A. Langmuir 2000, 16, 2326-2342. (12) Stabel, A.; Heinz, R.; Rabe, J. P.; Wegner, G.; De Schryver, F. C.; Corens, D.; Dehaen, W.; Siiling, C. J. Phys. Chem. 1995, 99, 86908697.
(1) Kim, K.; Matzger, A. J. J. Am. Chem. Soc. 2002, 124, 8772-8773. (2) Azumi, R.; Gotz, G.; Debaerdemaeker, T.; Bauerle, P. Chem.s Eur. J. 2000, 6, 735-744. (3) De Feyter, S.; Gesquie`re, A.; Wurst, K.; Amabilino, D. B.; Veciana, J.; De Schryver, F. C. Angew. Chem., Int. Ed. 2001, 40, 3217-3220. (4) Abdel-Mottaleb, M. M. S.; De Feyter, S.; Gesquie`re, A.; Sieffert, M.; Klapper, M.; Mu¨llen, K.; De Schryver, F. C. Nano Lett. 2001, 1, 353-359. (5) Okawa, Y.; Aono, M. Nature 2001, 409, 683-684. (6) Giancarlo, L. C.; Flynn, G. W. Annu. Rev. Phys. Chem. 1998, 49, 297-336.
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Figure 1. (a) STM image of 1 revealing two different domains coexisting on HOPG. High-resolution image of the bent domain (b) and of the linear domain (c) obtained from separate imaging sessions. The overlaid molecular structures are taken from the energy minimized periodic structures in the absence of substrate. The bent domain shows a pseudoperiodic change of the column direction resulting from flipping of molecules. This phenomenon is generally more frequent in bent domains than in linear domains. Arrows denote the direction of the column. Optimized geometries are shown for a dimeric subunit of bent (d) and linear (e) domains.
One of the most widely observed structural transition phenomena in three-dimensional crystals is Ostwald’s rule of stages.13,14 This rule, which describes initial metastable structure formation and subsequent conversion to the most stable structure, is applicable to crystallizations of organic as well as inorganic compounds.15 However, the experimental observations are generally made on macroscopic crystals (i.e. growth of one crystal morphology at the expense of another) although the rate at which the conversion occurs will clearly depend on microscopic nucleation events. The study of the phase transformation of two-dimensional crystals in physisorbed monolayers gives excellent examples of studying these phenomena at the molecular level.11,12 We report herein the structure of a self-assembled monolayer of a carbamate that crystallizes in a metastable form and subsequently transitions to a more stable structure: a demonstration of Ostwald’s rule of stages in a two-dimensional crystal. The transition between the two structures is directly followed with atomic resolution, yielding detailed information concerning the structural reorganization accompanying this change. The organization of carbamate two-dimensional crystals has not been previously studied with STM despite their strong intermolecular hydrogen bonding, which is responsible for the excellent mechanical properties of polyurethanes and has significant potential for directing supramolecular assembly.
Experimental Section Scanning Tunneling Microscopy. A Nanoscope E STM (Digital Instruments) was used for all imaging. The tips were made from Pt/Ir wire by mechanical cutting, and highly oriented pyrolytic graphite (HOPG, SPI-1 grade, Structure Probe Inc.) was used as a substrate for monolayer formation. A nearly saturated phenyl octane solution of 1 was placed on freshly cleaved HOPG to obtain a self-assembled monolayer of 1. The tips were shaped in situ by applying short voltage pulses, and the quality of the tips was verified by scanning the HOPG surface under the monolayer at reduced bias voltage. All images are unfiltered. STM imaging was performed at ambient conditions, and typical STM settings include 200-400 pA of current and 600-1000 mV of bias voltage (sample positive). To watch the initial formation of the monolayer, a solution of 1 was applied while the scanning tunneling microscope was scanning a bare HOPG surface, and the monolayer was observed immediately. Synthesis of 1. 1-Tetradecanol (0.801 g, 3.74 mmol) was added to octadecyl isocyanate (0.92 g, 3.11 mmol) in a flask under a nitrogen atmosphere and maintained at 110 °C for 12 h. The product was recrystallized from 500 mL of hot acetone. Yield: 944 mg (59.5%). Mp: 74.5-75 °C. IR: ν ) 3360, 2957, 2920, 2850, 1685, 1528, 1470, 1258, 1242, 1140, 719 cm-1. 1H NMR (300 MHz, CDCl3): δ 0.86 (t, 6H), 1.23-1.55 (m, 56H), 3.13 (q, 2H), 4.01 (t, 2H), 4.57 (br s, 1H). 13C NMR (100 MHz, CDCl3): δ 14.1, 22.7, 25.9, 26.7, 29.1, 29.3, 29.5, 29.6, 29.7, 30.0, 31.9, 41.0, 64.9, 156.8. Anal. Calcd for C33H67NO2: C, 77.73; H, 13.24; N, 2.75. Found: C, 77.77; H, 13.37; N, 2.78. MS (CI, CH4): m/z 510.6 [MH+].
Results and Discussion (13) Ostwald, W. Z. Phys. Chem. 1897, 22, 289-330. (14) Zhang, G. G. Z.; Gu, C. H.; Zell, M. T.; Burkhardt, R. T.; Munson, E. J.; Grant, D. J. W. J. Pharm. Sci. 2002, 91, 1089-1100. (15) Mullin, J. W. Crystallization, 4th ed.; Butterworth-Heinemann: Oxford, Boston, 2001.
Compound 1, octadecylcarbamic acid tetradecyl ester, spontaneously forms a physisorbed monolayer on HOPG. The STM image in Figure 1a shows domains with two different structures during the initial 30 min of monolayer
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Figure 2. STM images (40 × 40 nm2) of a physisorbed monolayer of 1 obtained consecutively over the same region. The time interval between each frame is 16.7 s. The magnified image (a) shows that the lower half is a linear domain and the upper half is a bent domain. Expansion of the linear domain into the bent domain (b) and eventual disappearance of the bent domain (c) are completed in
