Conformational Pseudopolymorphism and Orientational Disorder in

The relative size of the two alkyl groups in a molecule affected the frequency of orientational ..... Not shown are models lacking symmetry elements o...
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Langmuir 2005, 21, 647-655

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Conformational Pseudopolymorphism and Orientational Disorder in Two-Dimensional Alkyl Carbamate Crystals Kibum Kim, Katherine E. Plass, and Adam J. Matzger* Department of Chemistry and the Macromolecular Science and Engineering Program, The University of Michigan, 930 North University, Ann Arbor, Michigan 48109-1055 Received July 7, 2004. In Final Form: October 8, 2004 The structures of self-assembled physisorbed monolayers of alkyl carbamates were examined with atomic detail by scanning tunneling microscopy at the liquid-solid interface. Through systematic variation of molecular structure, the factors determining the two-dimensional crystal packing and dynamics of alkyl carbamate monolayers were isolated. Two different conformational pseudopolymorphs on the surface were observed and their order of stability was varied by changing the length of the alkyl groups. The relative size of the two alkyl groups in a molecule affected the frequency of orientational flipping within a column, which in turn, exerts an influence on the relative orientation of the two-dimensional crystalline domains. These phenomena were explained on the basis of the preferred hydrogen-bonding geometry of the carbamate functional group and the different degree of van der Waals interaction for each form.

Introduction Understanding of the structure of physisorbed monolayers formed at the liquid-solid interface has advanced rapidly in recent years.1-4 Scanning tunneling microscopy (STM) is the method of choice in most research examining these systems because of its benefits of both high spatial and temporal resolution.5 By using STM, packing structures of molecules on surfaces can be directly visualized in real time often with atomic resolution. Physisorbed monolayers provide an opportunity to pattern surfaces with features at the low end of the nanoscale, a region that is difficult to access with other patterning techniques. To fully exploit this opportunity for the design of functional surfaces, the relationship between the molecular structure and surface packing mode must be determined. As in the case of three-dimensional crystals, predictable intermolecular interactions between molecules, dubbed supramolecular synthons,6 often provide important clues in * Author to whom correspondence should be addressed. Fax: (734) 615-8553. E-mail: [email protected]. (1) Giancarlo, L. C.; Flynn, G. W. Annu. Rev. Phys. Chem. 1998, 49, 297-336. Giancarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491-501. De Feyter, S.; Gesquie`re, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mu¨llen, K. Acc. Chem. Res. 2000, 33, 520-531. De Feyter, S.; De Schryver, F. C. Chem. Soc. Rev. 2003, 32, 139-150. Cousty, J.; Van, L. P. Phys. Chem. Chem. Phys. 2003, 5, 599-603. Hoeppener, S.; Wonnemann, J.; Chi, L. F.; Erker, G.; Fuchs, H. Chemphyschem 2003, 4, 490-494. Qiu, X. H.; Wang, C.; Zeng, Q. D.; Xu, B.; Yin, S. X.; Wang, H. N.; Xu, S. D.; Bai, C. L. J. Am. Chem. Soc. 2000, 122, 5550-5556. Olson, J. A.; Bu¨hlmann, P. Anal. Chem. 2003, 75, 1089-1093. (2) Zhang, H. M.; Xie, Z. X.; Mao, B. W.; Xu, X. Chem. Eur. J. 2004, 10, 1415-1422. (3) 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. (4) Stawasz, M. E.; Parkinson, B. A. Langmuir 2003, 19, 1013910151. (5) Buchholz, S.; Rabe, J. P. J. Vac. Sci. Technol., B 1991, 9, 11261128. 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. Eur. J. 2000, 6, 3739-3746. De Feyter, S.; Larsson, M.; Schuurmans, N.; Verkuijl, B.; Zoriniants, G.; Gesquiere, A.; AbdelMottaleb, M. M.; van Esch, J.; Feringa, B. L.; van Stam, J.; De Schryver, F. Chem.-Eur. J. 2003, 9, 1198-1206. Baker, R. T.; Mougous, J. D.; Brackley, A.; Patrick, D. L. Langmuir 1999, 15, 4884-4891. Stevens, F.; Beebe, T. P. Langmuir 1999, 15, 6884-6889. (6) Desiraju, G. R. Crystal engineering: the Design of Organic Solids; Elsevier: Amsterdam, New York, 1989. Desiraju, G. R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2311-2327.

understanding surface packing structure. Several research groups have undertaken the task of systematically varying molecular structure or functional groups of molecules in order to elucidate their influence on two-dimensional crystal structure in molecules self-assembled at liquidsolid interfaces.3,7,8,9 Among the many classes of functional groups that can be used to direct the packing of twodimensional crystals, carbamates drew our attention because of their ability to form strong hydrogen bonds, as well as their role in contributing to the excellent mechanical properties of polyurethane surface coatings.10 Initial studies on alkyl carbamate monolayers revealed that kinetic and thermodynamic packing forms, bearing a pseudopolymorphic relationship, (co)exist and that their behavior mirrors certain aspects of three-dimensional crystal packing phenomena.11 However, the generality of these phenomena and the molecular parameters affecting the monolayer structure and dynamics have not been elucidated. As in the case of many previously studied physisorbed monolayers formed at liquid-solid interfaces, the major intermolecular forces contributing to the formation of physisorbed alkyl carbamates are van der Waals12-15 and (7) Cyr, D. M.; Venkataraman, B.; Flynn, G. W.; Black, A.; Whitesides, G. M. J. Phys. Chem. 1996, 100, 13747-13759. Stabel, A.; Heinz, R.; Rabe, J. P.; Wegner, G.; De Schryver, F. C.; Corens, D.; Dehaen, W.; Siiling, C. J. Phys. Chem. 1995, 99, 8690-8697. Giancarlo, L.; Cyr, D.; Muyskens, K.; Flynn, G. W. Langmuir 1998, 14, 1465-1471. De Feyter, S.; Gesquie`re, A.; Klapper, M.; Mullen, K.; De Schryver, F. C. Nano Lett. 2003, 3, 1485-1488. Samori, P.; Fechtenkotter, A.; Jackel, F.; Bohme, T.; Mullen, K.; Rabe, J. P. J. Am. Chem. Soc. 2001, 123, 1146211467. Wei, Y. H.; Kannappan, K.; Flynn, G. W.; Zimmt, M. B. J. Am. Chem. Soc. 2004, 126, 5318-5322. (8) Kaneda, Y.; Stawasz, M. E.; Sampson, D. L.; Parkinson, B. A. Langmuir 2001, 17, 6185-6195. (9) Plass, K. E.; Kim, K.; Matzger, A. J. J. Am. Chem. Soc. 2004, 126, 9042-9053. (10) Oertel, G.; Abele, L. Polyurethane Handbook: Chemistry, Raw Materials, Processing, Application, Properties, 2nd ed.; Hanser: Cincinnati, 1994. Ranney, M. W. Polyurethane Coatings; Noyes Data Corp.: Park Ridge, NJ, 1972. (11) Kim, K.; Plass, K. E.; Matzger, A. J. Langmuir 2003, 19, 71497152. (12) McGonigal, G. C.; Bernhardt, R. H.; Thomson, D. J. Appl. Phys. Lett. 1990, 57, 28-30. (13) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424-427. (14) Groszek, A. J. Proc. R. London, Ser. A 1970, 314, 473-498. (15) Findenegg, G. H.; Liphard, M. Carbon 1987, 25, 119-128.

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hydrogen-bonding interactions.16 The alkyl groups contribute most prominently to van der Waals interaction, a common occurrence in the formation of physisorbed monolayers, and the carbamate groups provide strong hydrogen bonding, which facilitates the formation of monolayers and directs their packing structure in one dimension. These two intermolecular interactions not only cooperate to promote molecular ordering on a surface but also compete to dictate molecular conformation in the twodimensional crystal. In a previous report,11 we demonstrated that octadecylcarbamic acid tetradecyl ester (1814 carbamate) exhibits two pseudopolymorphic packing motifs on graphite. These were proposed, on the basis of computational considerations, to be a kinetic form favored by electrostatic interaction and a thermodynamic form that benefited from more extensive van der Waals interaction. The multiple packing motifs arising from this simple molecule, along with the potential application to surface patterning, led us to investigate the relationship between molecular structure and two-dimensional crystal packing in a series of alkyl carbamates. The relative magnitude of van der Waals and hydrogen-bonding interactions were manipulated by changing the size of the alkyl chains while keeping the number of carbamate groups the same. The relative length of the two alkyl chains, as well as their combined length, was studied and found to affect the packing structures and dynamics in these monolayers. Experimental Section Scanning Tunneling Microscopy. A drop of 1-phenyl octane (Acros Organics) solution containing the compound to be imaged was placed on freshly cleaved highly oriented pyrolytic graphite (HOPG, SPI-1 grade, Structure Probe, Inc.) to obtain a selfassembled monolayer. A nearly saturated solution was used to obtain most STM images in this work. However, when two pseudopolymorphic forms were observed, the solution was diluted until only one form remained on HOPG to compare their relative stability.17 In cases where thermal annealing of the monolayer was carried out, the HOPG with the solution on it was heated to 60 °C for 30 min followed by cooling to room temperature. A Nanoscope E STM (Digital Instruments) was used for all imaging. The tips were made from Pt/Ir wire (Pt/Ir ) 80/20%, California Fine Wire) by mechanical cutting, and the quality of the tips was verified by scanning bare HOPG prior to imaging. All images are unfiltered. Measurements obtained from STM images are corrected against the substrate lattice parameters obtained from multiple HOPG images. STM imaging was performed under ambient conditions with typical settings of between 200 and 400 pA of current and 600-1000 mV of bias voltage (sample positive). Synthesis. All solvents were purchased from Fisher Scientific except ethanol (Pharmco). THF was purified by passage through activated alumina columns. All reagents were used as received and were purchased from Acros Organics except 1-octanol, dibutyltin dilaurate, octadecyl isocyanate, 1-octadecanol, and 1-docosanol (Sigma-Aldrich Co.). Column chromatography was performed on silica gel 60 from EM Science (particle size 0.0400.063 mm). Carbamate names are assigned by designating the number of carbons in the alkyl chain derived from the isocyanate followed by the number from the alcohol. For example, a (16) McGonigal, G. C.; Bernhardt, R. H.; Yeo, Y. H.; Thomson, D. J. J. Vac. Sci. Technol., B 1991, 9, 1107-1110. Lim, R.; Li, J.; Li, S. F. Y.; Feng, Z.; Valiyaveettil, S. Langmuir 2000, 16, 7023-7030. Qian, P.; Nanjo, H.; Yokoyama, T.; Miyashita, T.; Suzuki, T. M. Chem. Commun. 1998, 943-944. Nanjo, H.; Qian, P.; Yokoyama, T.; Suzuki, T. M. Jpn. J. Appl. Phys., Part 1 2003, 42, 6560-6563. Gesquie`re, A.; AbdelMottaleb, M. M. S.; De Feyter, S.; De Schryver, F. C.; Schoonbeek, F.; van Esch, J.; Kellogg, R. M.; Feringa, B. L.; Calderone, A.; Lazzaroni, R.; Bre´das, J. L. Langmuir 2000, 16, 10385-10391. Yin, S. X.; Wang, C.; Xu, Q. M.; Lei, S. B.; Wan, L. J.; Bai, C. L. Chem. Phys. Lett. 2001, 348, 321-328. (17) Brittain, H. G. Polymorphism in Pharmaceutical Solids; M. Dekker: New York, 1999.

Kim et al. carbamate synthesized from hexadecyl isocyanate and 1-octanol is designated as the 16-8 carbamate. Synthesis of 18-14 carbamate was reported elsewhere.11 Synthesis of Hexadecylcarbamic Acid Octyl Ester (16-8 carbamate). 1-Octanol (0.243 g, 1.87 mmol) was added to a mixture of hexadecyl isocyanate (0.500 g, 1.87 mmol) and dibutyltin dilaurate (4.00 µL, 6.72 µmol) in anhydrous THF (20 mL). The mixture was heated to reflux under a nitrogen atmosphere for 12 h. The solvent was removed by rotary evaporation. The residue was recrystallized from 70 mL of hot hexanes and purified by column chromatography (hexanes/ethyl acetate ) 8:1) to yield a white powder (0.123 g, 17%). mp 59.460.2 °C; IR (KBr): 3339, 2957, 2920, 2850, 1686, 1535, 1469, 1268, 1251, 1146, 720 cm-1; 1H NMR (400 MHz, CDCl3, δ): 0.88 (brt, 6H, J ) 6.6 Hz), 1.26 (m, 36H), 1.48 (m, 2H), 1.60 (m, 2H), 3.16 (td, 2H, J ) 6.5, 6.5 Hz), 4.03 (t, 2H, J ) 6.5 Hz), 4.63 (brs, 1H); 13C NMR (100 MHz, CDCl3, δ): 13.98, 14.01, 22.55, 22.60, 25.81, 26.68, 29.01, 29.14, 29.19, 29.22, 29.29, 29.48, 29.51, 29.58, 29.62, 29.96, 31.72, 31.86, 40.94, 64.85, 156.87; Anal. Calcd for C25H51NO2: C, 75.51; H, 12.93; N, 3.52; Found: C, 75.32; H, 13.16; N, 3.37; ESI-MS m/z 420.2 [M+Na+]. Synthesis of Hexadecylcarbamic Acid Tetradecyl Ester (16-14 carbamate). 1-Tetradecanol (0.401 g, 1.87 mmol) was added to a mixture of hexadecyl isocyanate (0.500 g, 1.87 mmol) and dibutyltin dilaurate (5.00 µL, 8.40 µmol) in anhydrous THF (20 mL) under a nitrogen atmosphere and heated to reflux for 9 h. The solvent was removed by rotary evaporation and the mixture was recrystallized from 70 mL of hot hexanes twice to yield a white powder (0.578 g, 64%). mp 74.1-75.0 °C; IR (KBr): 3339, 2920, 2850, 1686, 1539, 1469, 1265, 1251, 1147, 720 cm-1; 1H NMR (400 MHz, CDCl , δ): 0.88 (brt, 6H, J ) 6.6 Hz), 1.25 3 (m, 48H), 1.48 (m, 2H), 1.58 (m, 2H), 3.15 (td, 2H, J ) 6.4, 6.3 Hz), 4.03 (t, 2H, J ) 6.5 Hz), 4.60 (brs, 1H); 13C NMR (100 MHz, CDCl3, δ): 14.07, 22.66, 25.87, 26.73, 29.06, 29.29, 29.34, 29.53, 29.57, 29.63, 29.65, 29.68, 30.01, 31.90, 40.97, 64.84, 156.77; Anal. Calcd for C31H63NO2: C, 77.27; H, 13.18; N, 2.91; Found: C, 77.34; H, 13.11; N, 3.16; ESI-MS m/z 504.4 [M+Na+]. Synthesis of Octadecylcarbamic Acid Octadecyl Ester (18-18 carbamate). 1-Octadecanol (0.744 g, 2.75 mmol) was added to a mixture of octadecyl isocyanate (0.739 g, 2.50 mmol) and dibutyltin dilaurate (1.00 µL, 1.69 µmol) in a flask under a nitrogen atmosphere and maintained at 120 °C for 7 h. Recrystallization from 300 mL of hot ethanol twice yielded a white powder (1.30 g, 88%). mp 83.8-84.6 °C; IR (KBr): 3332, 2958, 2924, 2871, 2851, 1684, 1534, 1463, 1310, 1272, 1249, 1148, 1130, 1050, 988, 782, 728 cm-1; 1H NMR (400 MHz, CDCl3, δ): 0.89 (brt, 6H, J ) 6.8 Hz), 1.26 (m, 60H), 1.49 (m, 2H), 1.60 (m, 2H), 3.16 (td, 2H, J ) 6.3, 6.0 Hz), 4.04 (t, 2H, J ) 6.7 Hz), 4.60 (brs, 1H); 13C NMR (100 MHz, CDCl3, δ): 14.09, 22.68, 25.88, 26.74, 29.07, 29.30, 29.35, 29.55, 29.59, 29.65, 29.69, 30.03, 31.92, 40.99, 64.86, 156.78; Anal. Calcd for C37H75NO2: C, 78.52; H, 13.36; N, 2.47; Found: C, 78.75; H, 13.62; N, 2.28; ESI-MS m/z 588.7 [M+Na+]. Synthesis of Octadecylcarbamic Acid Docosyl Ester (1822 carbamate). 1-Docosanol (0.365 g, 1.12 mmol) was added to a mixture of octadecyl isocyanate (0.300 g, 1.02 mmol) and dibutyltin dilaurate (2.00 µL, 3.38 µmol) in anhydrous THF (20 mL) under a nitrogen atmosphere. The mixture was heated to reflux for 7 h. Removal of the solvent by rotary evaporation and recrystallization of the mixture from 100 mL of hot ethanol twice yielded a white powder (0.567 g, 85%). mp 85.3-86.2 °C; IR (KBr): 3337, 2954, 2916, 2848, 1685, 1534, 1471, 1272, 1256, 1243, 1146, 1045, 1018, 782, 717 cm-1; 1H NMR (400 MHz, CDCl3, δ): 0.89 (brt, 6H, J ) 6.6 Hz), 1.26 (m, 68H), 1.49 (m, 2H), 1.60 (m, 2H), 3.16 (td, 2H, J ) 6.5, 6.4 Hz), 4.04 (t, 2H, J ) 6.6 Hz), 4.60 (brs, 1H); 13C NMR (100 MHz, CDCl3, δ): 14.03, 22.62, 25.82, 26.69, 29.02, 29.24, 29.31, 29.50, 29.53, 29.60, 29.64, 29.97, 31.87, 40.95, 64.87, 156.89; Anal. Calcd for C41H83NO2: C, 79.16; H, 13.45; N, 2.25; Found: C, 79.50; H, 13.35; N, 2.27; MS (CI, CH4): m/z 622.6 [MH+]. Molecular Modeling. Molecular mechanics modeling was carried out with the COMPASS force field18 as implemented in Cerius2 version 4.2 from Accelrys, Inc. In a typical modeling experiment, the optimized geometry of an isolated single molecule (18) Sun, H. J. Phys. Chem. B 1998, 102, 7338-7364.

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Figure 1. STM images of 16-8 carbamate with overlaid molecular model (a) 20 × 20 nm2 image showing the packing structure of 16-8 carbamate. The white arrow designates the column direction. (b) 300 × 300 nm2 image showing multiple domains of the monolayer of 16-8 carbamate. The relative orientations of the domains are multiples of 60°. Chart 1. The Structure of Alkyl Carbamates Synthesized and Imaged by STM

was obtained in the extended-chain conformation followed by the construction and optimization of a periodic model. The periodic model was based on a monoclinic unit cell with unique c axis and was built in a way such that a layer of the model through the a-b plane represents the arrangement of molecules observed by the STM. The non-90° unit cell angle is in the a-b plane. The c axis of the model, which is the distance between molecular layers, was set at 30 Å and was fixed during structure optimization. The length of the c axis was verified not to affect the computational result by calculating the energy of the structure with different lengths of this axis.

Results and Discussion To investigate the relationship between alkyl carbamate structure and two-dimensional crystal structure, carbamates in Chart 1 were synthesized and imaged by STM. The monolayers of each of these are discussed below in order of increasing alkyl chain length. STM Imaging and Packing Structure of 16-8 Carbamate. Imaging of 16-8 carbamate, the shortest carbamate studied, revealed the two-dimensional periodic structure of this molecule on HOPG, as shown in Figure 1a,b. An energy-minimized molecular model based on the observed STM image is overlaid onto the monolayer to aid visualization of the packing geometry. Formation of a columnar structure is observed, and the direction of the column is indicated with a white arrow in Figure 1a. This arrangement accommodates both hydrogen bonding between carbamate functional groups along the column direction and van der Waals interaction between alkyl chains. Each column is composed of molecules that are

bent at their carbamate groups (approximately 150° between alkyl chains). Accordingly, the packing geometry of 16-8 carbamate is designated as a bent structure to emphasize the orientation of the alkyl chains on either side of the functional group. This bent structure is the geometry adopted by carbamates in a previously determined single-crystal X-ray structure of an alkyl dicarbamate19 and is computed to be the electrostatically favored mode of interaction between carbamate functional groups.11 The computed and measured unit cell parameters shown in Table 1 agree well for this monolayer when the unit cell is set such that there are two molecules related by an inversion center, and the a axis is parallel to the short alkyl chain of the carbamates. One direct consequence of the bent geometry of 16-8 carbamate is its effect on the packing distances between alkyl chains. The preferred packing structure of alkyl chains on HOPG is a dense parallel packing as exemplified in the monolayers of long n-alkane molecules.3,12,13 However, in the bent geometry, it is not possible for both the short and the long alkyl chains to have dense parallel packing due to the hydrogen bonding geometry of the carbamate functional group. In fact, as depicted in Figure 2, the shorter alkyl chains adopt a parallel but not closely packed structure while the intermolecular distance between longer alkyl chains is compressed. Thus, the bent geometry involves a loss of intermolecular alkyl chain interactions in order to accommodate a more favorable intermolecular hydrogen-bonding geometry for the carbamate functional group. In understanding the structure of monolayers on surfaces, the effect of substrate must be considered.2,8,20 For example, when the molecule has long alkyl chains, HOPG can dictate the orientation of the domains in the monolayer by aligning the alkyl chains along its hexagonal (19) Alimova, L. L.; Atovmyan, E. G.; Filipenko, O. S. Kristallografiya 1987, 32, 97-101. (20) Giancarlo, L. C.; Fang, H. B.; Rubin, S. M.; Bront, A. A.; Flynn, G. W. J. Phys. Chem. B 1998, 102, 10255-10263. Cincotti, S.; Rabe, J. P. Appl. Phys. Lett. 1993, 62, 3531-3533. Jung, T. A.; Schlittler, R. R.; Gimzewski, J. K. Nature 1997, 386, 696-698.

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Table 1. Experimental and Computed Unit Cell Parameters and Angle around the Carbamate Group for the Monolayers of Alkyl Carbamates Investigateda carbamate 16-8 (bent) 16-14 (bent)b 16-14 (linear) 18-14 (bent) 18-14 (linear)b 18-18 (flipped bent)a 18-18 (linear) 18-22 (bent) 18-22 (linear)b

STM model STM model STM STM model STM model STM model STM model STM STM model

a (nm)

b (nm)

R (°)

carbamate angle (°)

6.75 ( 0.50 6.65 8.70 ( 0.64 8.18 9.22 ( 0.67 8.42 ( 0.08 8.56 9.02 ( 0.08 9.25 10.46 ( 0.76 10.15 10.53 ( 0.77 10.26 10.74 ( 0.97 11.17 ( 0.83 11.28

0.51 ( 0.04 0.50 0.47 ( 0.03 0.50 0.46 ( 0.03 0.46 ( 0.01 0.50 0.48 ( 0.01 0.46 0.98 ( 0.08 0.93 0.46 ( 0.03 0.46 0.44 ( 0.03 0.48 ( 0.03 0.45

78.7 ( 7.2 86.6 86.6 ( 2.1 87.4 89.9 ( 2.2 88.3 ( 0.9 89.7 87.6 ( 0.9 88.3 85.4 ( 2.7 81.5 84.9 ( 3.1 88.6 86.8 ( 4.9 80.7 ( 2.0 88.7

158.0 ( 3.9 155.7 154.1 ( 3.8 158.6 154.0 ( 1.0 156.3 157.4 ( 3.9 156.1 153.0 ( 4.2

a The a, b, and R parameters are chosen to contain four molecules in the unit cell for the flipped bent form of 18-18 carbamate and two molecules for all others. The carbamate angle is the angle adopted between the two alkyl chains connected to the functional group. The carbamate angles of the linear forms were not measured since the two alkyl chains are essentially parallel. b More stable packing for this alkyl carbamate.

Figure 2. Molecular model of the two-dimensional packing of 16-8 carbamate. Longer alkyl chains are close packed with each other, and shorter alkyl chains do not densely pack due to the angle around the carbamate functional group and its preferred hydrogen-bonding geometry. Color code: carbon, gray; hydrogen, white; nitrogen, blue; oxygen, red.

lattice directions3,4,13,14,21 through a commensurate interaction between the alkyl chain and HOPG. The relative orientation of 16-8 carbamate to the HOPG lattice was studied by imaging the substrate in situ by reducing the bias voltage and increasing the tunneling current. The STM images indicate that all domains observed have their longer alkyl chains aligned with the HOPG lattice. Accordingly, the shorter alkyl chains, due to the ∼150° angle around the carbamate functional group, are not aligned with the HOPG lattice. This arrangement is adopted to maximize the overall commensurateness of the alkyl chains with the underlying HOPG. Supporting the notion that only one type of alkyl chain aligns with the HOPG lattice direction is the relative orientation observed between different domains in large-scale images. The domain angles, angles between the column directions of different domains, are measured to be exclusively 60°, 120°, and 180° (Figure 1b). When all the domains have their longer alkyl chains aligned with the HOPG lattice direction, the domain angles can be only multiples of 60° because of the 6-fold symmetry of the substrate lattice. STM Imaging and Packing Structure of 16-14 carbamate. STM images of the monolayer of 16-14 carbamate are presented in Figure 3a,b. Domains with two different packing structures were observed: one with the bent geometry, as observed for 16-8 carbamate (Figure 3a), and the other one with the two alkyl chains approximately parallel to each other (Figure 3b). We designate the latter motif as the linear geometry to reflect the relative geometric relationship of the two alkyl chains. The position of the carbamate functional group in Figure 3a is not readily apparent because the image contrast of both the functional group and the end of the molecule are similar. However, STM images of the domain boundary

(see Supporting Information) provide this information because the boundary coincides with the end of a molecule, and therefore, the position of the carbamate group can be unambiguously assigned. The observed molecular geometry is represented by an energy-minimized model overlaid on the STM image in Figure 3a. The bent domains, which occupy a considerably larger area than the linear domains in this monolayer, share the same characteristics observed in the monolayer of 16-8 carbamate, including the angle between the two alkyl chains and alignment of alkyl chains with the substrate lattice. However, there are two prominent characteristics in this image that merit discussion: the relative topography of the two alkyl chains on a given molecule and the waviness of the column edges. As for the relative topography, alkyl chains designated as A in Figure 3a show a zigzag pattern of hydrogens commonly observed for packing of physisorbed nalkanes,3,13,22 while the other alkyl chains, designated as B, exhibit an almost linear pattern without discernible hydrogen atoms. Topographically, the alkyl chain with a lower resolution (B) is higher than the other by 0.05 ( 0.02 nm. This phenomenon likely arises from a difference in the orientation of the alkyl chains.23 The threedimensional crystal structure of an alkyl dicarbamate19 shows that the alkyl chains around a carbamate functional group are not coplanar. Instead, the planes containing the backbones of the two alkyl chains are rotated by 25° relative to each other.24 This gives rise to a difference in the orientation of the hydrogens relative to the STM tip, resulting in the different shape of the two alkyl chains of 16-14 carbamate observed in Figure 3a.25 Although the variation of topography and resolution between the two alkyl chains are most prominent in the bent geometry of (21) Patrick, D. L.; Cee, V. J.; Morse, M. D.; Beebe, T. P. J. Phys. Chem. B 1999, 103, 8328-8336. (22) Faglioni, F.; Claypool, C. L.; Lewis, N. S.; Goddard, W. A. J. Phys. Chem. B 1997, 101, 5996-6020. (23) Cai, Y. U.; Bernasek, S. L. J. Am. Chem. Soc. 2003, 125, 16551659. (24) The angle is measured between the mean plane of each alkyl chain. (25) The rotation between alkyl chains is not accurately modeled by the molecular mechanics method employed in this study. However, ab initio computations on an isolated molecule of N-ethyl carbamic acid finds that the C-N bond between nitrogen and the carbon R to the carbamate functional group does not prefer a trans geometry but rather a staggered conformation: Sun, H. Macromolecules 1993, 26, 59245936.

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Figure 3. STM images (20 × 20 nm2) of 16-14 and 18-14 carbamate with overlaid molecular models. (a) Bent domain of 16-14 carbamate. The column edge of this monolayer is wavy due to the flipping of this molecule. The circle indicates a void space between molecules. (b) Linear domain of 16-14 carbamate. The two alkyl chains have a parallel geometry favorable for van der Waals interaction between them. This domain type occupies a small area of this monolayer. (c) Bent domain of 18-14 carbamate. The bent domains disappear and transform to linear domains after prolonged imaging. (d) Linear domain of 18-14 carbamate.

16-14 carbamate, this feature is observed in the bent domains of all alkyl carbamates studied. Regarding the waviness of the column edges, it is readily observed that the column edges of 16-14 carbamate are distorted compared to the straight boundaries of 16-8 carbamate. The waviness of the column edge is associated with the alternation of the column direction. When the lengths of alkyl chains in a given column (A or B) are precisely compared, it is observed that the 16- and 14carbon alkyl chains are mixed together. This originates from frequent flipping of molecules around the hydrogen bonds formed between carbamate groups giving rise to changes in the column direction, as illustrated in Figure 5. This phenomenon is not observed for 16-8 carbamate. If flipping occurs at a certain position in a column, the

adjacent column often flips accordingly to conform to the column edge. However, in cases where propagation to the next column does not occur, small void spaces between carbamate molecules are created, as denoted by a circle in Figure 3a. Although the flipped packing structure is prevalent for 16-14 carbamate, it was not clear initially whether this is an energetically more preferred motif or if its formation is a kinetic outcome. To produce the thermodynamically most stable arrangement, the monolayer was thermally annealed before imaging (see Experimental Section), typically producing domains reaching over 500 × 500 nm2. A significant reduction in flipping frequency was observed, demonstrating that the nonflipped structure is the preferred form for 16-14 carbamate.

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Figure 4. STM images (20 × 20 nm2) of 18-18 and 18-22 carbamate with overlaid molecular models. (a) Bent domain of 18-18 carbamate. Overlaid molecular model consists of six molecules with flipped geometry. (b) Linear domain of 18-18 carbamate. This form disappears in monolayers formed from dilute solution. (c) Bent domain of 18-22 carbamate imaged with coexisting linear domains. The zigzag-shaped column edge is due to the flipping of molecular orientation in the bent domain. (d) Linear domain of 18-22 carbamate. The two alkyl chains are parallel to each other, and the linear domains occupy most of the surface area.

Imaging of the underlying HOPG reveals that only one alkyl chain of a given 16-14 carbamate molecule is aligned with the HOPG lattice direction. Since a given column (A or B) has both longer and shorter alkyl chains mixed, no single alkyl chain type is preferred to align with the substrate. Supporting this notion is the largerscale STM images that show domains meeting at a variety of angles (see Supporting Information); in contrast to 16-8 carbamate, the domain angles of 16-14 carbamate are centered not only on multiples of 60° but also around on 30°, 90°, and 150°. This is a result of having two alkyl chains with an ∼150° angle between them, either of which can align with the substrate. Coexisting with the bent domains, and occupying considerably less area, are linear domains where the two

alkyl chains on 16-14 carbamate are parallel, as shown in Figure 3b. In contrast to the bent conformation, the linear arrangement allows both of the alkyl chains to close pack, providing better van der Waals interactions between the molecules. Also, in relation to the substrate, all alkyl chains in the monolayer can align with the HOPG lattice, providing better interaction between the monolayer and the substrate.26 These two effects combine to provide a better van der Waals interaction for the linear geometry, albeit at the cost of imposing a less-favorable hydrogenbonding geometry on the carbamate functional group.11 (26) Consistent with both alkyl chains aligning with the substrate, it is found that the domain angles for all linear packing motifs are multiples of 60° for the carbamates studied here, including 16-14 carbamate.

Conformational Pseudopolymorphism and Orientational Disorder

STM imaging of nearly saturated solutions of 16-14 carbamate always resulted in the formation of both bent and linear domains, even after thermal annealing, thereby complicating the identification of the thermodynamically preferred form. However, imaging of sufficiently dilute solutions provided the exclusive formation of bent domains, proving that the linear geometry is the less stable form. STM Imaging and Packing Structure of 18-14 carbamate. The 18-14 carbamate gave rise to bent and linear domains on HOPG (Figure 3c,d).11 In terms of chemical structure, 18-14 carbamate has one longer alkyl chain than 16-14 carbamate, increasing the difference in length between the two alkyl chains on a molecule from two carbons to four carbons. The molecular packing geometry in the bent and the linear domains is similar to that of 16-14 carbamate. However, the flipping of molecular orientation is relatively rare in 18-14 carbamate. Both (A-B)-(A-B)- and (A-B)-(B-A)-type packing is observed in Figure 3c. These relationships between neighboring columns are very close in energy (computed to be 16-14 carbamate . 18-14 carbamate ≈ 18-22 carbamate . 16-8 carbamate. In fact, the flipped packing is more prevalent than nonflipped packing in the monolayer of 18-18 carbamate and is found to be more stable through thermal annealing experiments on this monolayer. Molecular mechanics computations27 employing 16-8 and (27) Periodic models were employed in the modeling of the flipped monolayer packing. Each unit cell was chosen to include four molecules to accommodate a flipped pair of molecules and inversion center.

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Figure 6. Optimized molecular models of each alkyl carbamate with overlaid unit cells. (a) 16-8 carbamate, (b) bent form of 16-14 carbamate, (c) bent form of 18-14 carbamate, (d) linear form of 18-14 carbamate, (e) linear form of 18-18 carbamate, (f) flipped bent form of 18-18 carbamate, and (g) 18-22 carbamate. The above models incorporate an inversion relationship between molecular columns. This is what is observed in most alkyl carbamate monolayers. Not shown are models lacking symmetry elements or containing a pseudo-glide plane between molecular columns. These alternative arrangements occur randomly and infrequently within the monolayers.

16-14 carbamate were used to illuminate the factors leading to this trend (Figure 5b,c). When 16-8 carbamate flips, there is a loss of van der Waals contact between the successive molecules in a column, as can be seen at the edge of the column in Figure 5c.28 This effect is relatively small for 16-14 carbamate since the lengths of the alkyl chains are similar. The loss of van der Waals contact in these monolayers can be partially regained by filling up the void space with interdigitated adjacent columns. However, application of BFDH theory29 to these twodimensional crystals predicts that the growth rate of a domain is much faster along the column propagation direction than along the alkyl chain on the basis of the shape of the unit cell.30 Therefore, the interaction along the fast-growing direction is more important in determining the orientation of an alkyl carbamate than the interaction with the adjacent columns. As the carbamate becomes more asymmetric, the flipped structure is less preferred due to the loss of van der Waals interaction at (28) In addition, flipping of alkyl carbamates gives rise to lessfavorable epitaxial interaction. For example, in the case of 16-8 carbamate, the shorter alkyl chain becomes aligned with the HOPG lattice instead of the longer alkyl chain as a result of flipping leading to decreased substrate-molecule interaction. (29) Donnay, J. D. H.; Harker, D. Am. Mineral. 1937, 22, 446-467. (30) In fact, this higher rate of growth along the column direction is experimentally observed; if the growth rate of a domain were similar for column direction and alkyl chain direction, the size of a domain along the alkyl chain direction would be much longer than along the column direction. Experimentally, in the STM images, the size of a domain is usually similar for both directions or longer for column propagation directions.

the edge. Partially offsetting these effects and stabilizing the flipped packing motif is another factor of particular importance for 18-18 carbamate. The flipped form offers equivalent packing distances for both of the alkyl chains of the alkyl carbamate, providing dense packing for both of them. For a symmetric long alkyl carbamate such as 18-18 carbamate, the loss of van der Waals interaction at the edge normally associated with flipping is negligible, and so a flipped structure becomes the global energy minimum. Packing Motif Dependence on Molecular Structure. Bent and linear molecular conformations give rise to pseudopolymorphic packing structures in the physisorbed monolayers of alkyl carbamates. The 16-8 carbamate exclusively formed the bent packing structure, whereas the other carbamates exhibited both bent and linear motifs. Prolonged observation of monolayers assembled from dilute solution elucidated the thermodynamic relationship between the two forms of each alkyl carbamate studied. The bent form was found to be the more stable packing structure for 16-8, 16-14, and 1818 carbamate, whereas the linear form was more stable for 18-14 and 18-22 carbamate. Optimized structures of each alkyl carbamate are shown in Figure 6 with overlaid unit cells. It was previously proposed that the bent form enables an optimal hydrogen-bonding geometry for the carbamate functional group at the cost of decreased alkyl chain interaction, whereas the linear form maximizes van der Waals interaction between alkyl chains while reducing

Conformational Pseudopolymorphism and Orientational Disorder

hydrogen bonding.11 This model therefore suggests that the alkyl chain length will determine which conformation is adopted, with the linear form being favored for carbamates having longer alkyl chains. The observed packing structures agree well with this prediction. The bent form is favored for 16-8 and 16-14 carbamate, and the linear form is adopted for 18-14 and 18-22 carbamate. However, 18-18 carbamate adopts a bent geometry, albeit one with a high degree of flipping, even though its alkyl chains are longer than 18-14 carbamate. A distinctive feature of the monolayer of 18-18 carbamate is the high frequency of the orientational flipping of the molecule that originates from its symmetric structure; the flipped bent geometry of 18-18 carbamate does not lose van der Waals interaction at the edge and offers dense packing for both of the alkyl chains, eliminating the driving force for the transition to linear packing. Conclusion The structures of alkyl carbamate two-dimensional crystals were studied as a function of molecular structure. Most molecules gave rise to more than one type of monolayer, and these have a pseudopolymorphic relationship. The packing behavior varied remarkably when even small changes in the length of the alkyl chains were made. The relative size of the two alkyl groups on an alkyl carbamate, as well as their combined length, affects not only the selection of pseudopolymorph but also the

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occurrence of flipping within a monolayer. These effects extend beyond the molecular scale because the relative orientation of domains on HOPG is affected by the relationship between alkyl chain length and the packing geometry. This series of molecules provides a particularly lucid example of the subtle effects that can lead to kinetic and thermodynamic stabilization of different forms of a monolayer. As models for polyurethane surface coatings, especially when employed with fillers such as carbon black, these oligomers provide a molecular-level picture of the importance of molecular structure, solution concentration, and annealing in controlling the surface structure adopted. We are currently expanding these studies to encompass molecules with more than one carbamate to assess the effect of adding additional hydrogen-bonding groups on the competition between alkyl group close packing and optimal electrostatic interaction. Acknowledgment. This work was supported by the National Science Foundation under Grant No. CHE0316250. Supporting Information Available: Domain boundary images of 16-14 and 18-18 carbamate and large-scale images of 16-14 and 18-18 carbamate. This material is available free of charge via the Internet at http://pubs.acs.org. LA048299C