Dipolar Side Chain Control of Monolayer Morphology: Symmetrically

Feb 10, 2009 - Mostofa Kamal Khan and Pudupadi R. Sundararajan. The Journal of Physical Chemistry B 2011 115 (27), 8696-8706. Abstract | Full Text HTM...
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Langmuir 2009, 25, 2913-2923

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Dipolar Side Chain Control of Monolayer Morphology: Symmetrically Substituted 1,5-(Mono- and diether) Anthracenes at the Solution-HOPG Interface Wenjun Tong, Yanhu Wei,† Kurt W. Armbrust,‡ and Matthew B. Zimmt* Department of Chemistry, Brown UniVersity ProVidence, Rhode Island 02912 ReceiVed NoVember 17, 2008. ReVised Manuscript ReceiVed December 27, 2008 Scanning tunneling microscopy (STM) is used to determine the 2-D unit cell parameters of monolayers selfassembled by twelve symmetrical, 1,5-bis(linear aliphatic ether side chain) anthracenes at the solution-graphite interface. The standard morphology assembled by 1,5-bis(alkyloxymethyl) anthracenes consists of single-lamella domains containing columns of anthracene cores alternating with columns of interdigitated, aliphatic side chains. Adjacent side chains within the aliphatic columns adsorb in antiparallel orientations. The terminal methyl (ω-position) of each side chain lies in registration with the 2-positions of its two neighboring chains ((ωT2)-packing). Anthracenes with diether side chains can generate repulsive or attractive dipole-dipole interactions between proximate ethers of adjacent aliphatic chains. Anthracenes bearing eVen length side chains with oxygens at the 2- and ω-1 positions or at the 3- and ω-2 positions do not assemble (ωT2)-packed monolayers. Repulsive dipolar interactions between ethers in adjacent side chains raise the energy of (ωT2) morphologies. These “self-repulsive” side chains drive assembly of (ωT1)- or (ωT3)-packed morphologies, which enjoy stabilizing dipolar interactions between ethers in adjacent side chains. In stark contrast, anthracenes bearing odd length diether side chains assemble (ωT2)-packed morphologies, regardless of whether adjacent chains suffer zero, one, or two sets of proximate dipole-dipole repulsions. The intrinsic energy gap from (ωT2)- to non-(ωT2)-packed morphologies of odd length side chain anthracenes is, apparently, larger than for even length side chain anthracenes. Overall, the twelve compounds self-assemble seven different morphologies. Distinguishing morphologies, understanding polymorphism within the monolayers, and evaluating the morphological consequences of side chain dipolar interactions is facilitated by viewing the monolayers as assemblies of 1-D, molecular tapes.

Introduction Numerous compounds spontaneously assemble 2D-crystalline monolayers at the solution-graphite interface. An extensive literature reports and analyzes the influence of molecular topology and functionality on the morphologies of self-assembled monolayers.1 A smaller number of studies probe the influence of solvent or of substrate on monolayer morphology.2,3 In 1996, Rabe and co-workers reported the self-assembly of cocrystalline monolayers from mixtures of hydrogen bond donating isophthalic acids and * Corresponding author. E-mail: [email protected]. † Current address: Department of Chemical and Biological Engineering Northwestern University, Evanston, IL 60208. ‡ Current address: Department of Chemistry Massachusetts Institute of Technology, Cambridge, MA 02139. (1) (a) Walba, D. M.; Stevens, F.; Clark, N. A.; Parks, D. C. Acc. Chem. Res. 1996, 29, 591–97. (b) Claypool, C. L.; Faglioni, F.; Goddard, W. A., III; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B. 1997, 101, 5978–95. (c) Giancarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491–501. (d) Yin, S.; Wang, C.; Xu, Q.; Lei, S.; Wan, L.; Bai, C. Chem. Phys. Lett. 2001, 348, 321–28. (e) Plass, K. E.; Kim, K.; Matzger, A. J. J. Am. Chem. Soc. 2004, 126, 9042–53. (f) Tao, F.; Bernasek, S. L. J. Phys. Chem. B 2005, 109, 6233–38. (g) Zell, P.; Mc¸gele, F.; Ziener, U.; Rieger, B. Chem. Eur. J. 2006, 12, 3847–57. (h) Mu, Z.; Wang, Z.; Zhang, X.; Zhang, X.; Ye, K.; Wang, Y. J. Phys. Chem. B 2004, 108, 19955–59. (2) (a) Mamdouh, W.; Uji-i, H.; Ladislaw, J. S.; Dulcey, A. E.; Percec, V.; De Schryver, F. C.; De Feyter, S. J. Am. Chem. Soc. 2006, 128, 317–25. (b) Umemura, K.; Fujita, K.; Ishida, T.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. I 1998, 37, 3620–25. (c) Tahara, K.; Furukawa, S.; Uji-i, H.; Uchino, T.; Ichikawa, T.; Zhang, J.; Mamdouh, W.; Sonoda, M.; De Schryver, F. C.; De Feyter, S.; Tobe, Y. J. Am. Chem. Soc. 2006, 128, 16613–25. (d) Li, C.-J.; Zeng, Q.-D.; Wang, C.; Wan, L.-J.; Xu, S.-L.; Wang, C.-R.; Bai, C.-L. J. Phys. Chem. C 2003, 107, 747–50. (e) Kaneda, Y.; Stawasz, M. E.; Sampson, D. L.; Parkinson, B. A. Langmuir 2001, 17, 6185–95. (f) Giancarlo, L. C.; Fang, H.; Rubin, S. M.; Bront, A. A.; Flynn, G. W. J. Phys. Chem. B 1998, 102, 10255–63. (g) Magonov, S. N.; Wawkuschewski, A.; Bar, G.; Zo¨nnchen, P.; Cantow, H.-J. Thin Solid Films 1994, 243, 419–24. (3) Pokrifchak, M.; Turner, T.; Pilgrim, I.; Johnston, M. R. ; Hipps, K. W. J. Phys. Chem. C 2007, 111, 7735–40.

hydrogen bond accepting pyrazines.4 Subsequently, numerous studies of multicomponent monolayers were reported.5 Three limiting types of mixed monolayers self-assemble: (i) phase separated, (ii) randomly mixed, and (iii) cocrystalline.5 Randomly mixed monolayers have been used to characterize electronic properties of diluent molecules, to probe monolayer exchange and desorption kinetics and to template monolayer morphology.6 Cocrystalline monolayers have potential as templates for directing nanoparticle adsorption5f,7 and for sensing and technology applications. The design and morphology control of such monolayers pose particular challenges. Symmetric and unsymmetric 1,5-(linear side chain substituted) anthracenes readily assemble monolayers on graphite.3,5d,8 Side chain structure and molecular symmetry influence the assembled (4) Eichhorst-Gerner, K.; Stabel, A.; Moessner, G.; Declerq, D.; Valiyaveettil, S.; Enkelmann, V.; Mu¨llen, K.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 1492–95. (5) (a) Yang, X.; Mu, Z.; Wang, Z.; Zhang, X.; Wang, J.; Wang, Y. Langmuir 2005, 21, 7225–7229. (b) De Feyter, S.; Larsson, M.; Schuurmans, N.; Verkuijl, B.; Zoriniants, G.; Gesquie`re, A.; Abdel-Mottaleb, M. M.; van Esch, J.; Feringa, B. L.; van Stam, J.; De Schryver, F. Chem. Eur. J. 2003, 9, 1198–1206. (c) Plass, K. E.; Engle, K. M.; Cychosz, K. A.; Matzger, A. J. Nano Lett. 2006, 6, 1178– 1183. (d) Wei, Y.; Tong, W; Zimmt, M. B. J. Am. Chem. Soc. 2008, 130, 3399– 405. (e) Tao, F.; Bernasek, S. L. Surf. Sci. 2007, 601, 2284–2290. (f) Lei, S.; Tahara, K.; Feng, X.; Furukawa, S.; De Schryver, F. C.; Mu¨llen, K.; Tobe, Y.; De Feyter, S. J. Am. Chem. Soc. 2008, 130, 7119–29. (6) (a) Padowitz, D. F.; Sada, D. M.; Kemer, E. L.; Dougan, M. L.; Xue, W. A. J. Phys. Chem. B 2002, 106, 593–98. (b) Papadantonakis, K. M.; Brunschwig, B. S.; Lewis, N. S. Langmuir 2008, 24, 857–61. (c) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292(5525), 2303–2307. (d) Hallba¨ck, A.-S.; Poelsema, B.; Zandvliet, H. J. W. ChemPhysChem 2007, 8, 661–665. (e) Wakamatsu, S.; Fujii, S.; Akiba, U.; Fujihira, M. Nanotechnology 2004, 15, S137-S141. (7) (a) Pinto, Y. Y.; Le, J. D.; Seeman, N. C.; Musier-Forsyth, K.; Taton, T. A.; Kiehl, R. A. Nano Lett. 2005, 5, 2399. (b) Zhang, J.; Liu, Y.; Ke, Y.; Yan, H. Nano Lett. 2006, 6, 248–51.

10.1021/la803811w CCC: $40.75  2009 American Chemical Society Published on Web 02/10/2009

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morphologies. For example, mixtures of anthracenes with specifically tailored side chains assemble predesigned cocrystalline monolayers.5d,8b Each anthracene molecule influences the identity and adsorption orientation of its neighbors through length, shape, and dipolar interactions acting between their side chains. The most common monolayer morphology positions neighboring, identical length side chains in antiparallel orientations, with the terminal methyl group (ω-position) of one side chain in registration with the second chain atom of neighboring side chains [(ωT2)-packing, Chart 1, top]. Certain side chains, such as the 12-methoxy-dodecyloxymethyl side chain (Me-O-(CH2)12-OCH2-) and the 10-ethoxy-decyloxymethyl side chain (Et-O(CH2)10-O-(CH2)2-) induce different packing.5d,8b These “selfrepulsive” side chains suffer repulsive dipolar interactions between proximal ether groups of neighboring, identical, (ωT2)-packed molecules. The repulsive interactions destabilize (ωT2)-packing and drive assembly of non-(ωT2)-packed morphologies. This manuscript evaluates the generality of using side chain dipolar repulsions to control monolayer morphology at the solution-HOPG (highly ordered pyrolytic graphite) interface. Analyses of monolayer unit cells and molecule packing reveal correlations between mono- and diether side chain structure and the morphologies assembled by symmetric, 1,5-bis-substituted anthracenes. Different positioning of side chain ether groups produce dipolar repulsive, dipolar attractive, or dipolar neutral interactions within (ωT2)-packed assemblies. The repulsions generated by (ωT2)-packing of eVen length, self-repulsive side chains force assembly of non-(ωT2)-packed morphologies. By contrast, anthracenes with odd length, self-repulsive diether side chains assemble (ωT2)-packed morphologies whether there are one or two repulsive ether dipole interactions per side chain pair. This surprising result, along with numerous examples of monolayer polymorphism,9 is analyzed in terms of ether perturbations of side chain shape, charge distribution, and packing.

Tong et al. Chart 1

eliminate this distortion, consecutive pairs of up and down scans were transformed using a program that minimizes differences in the two scans’ unit cell parameters by adjusting the x- and y-thermal drift velocities. This transformation is valid if thermal drift velocities are constant in consecutive scans.10 Reported unit cell parameters and uncertainties were calculated from raw and thermal drift corrected STM data collected from two or more independently prepared and scanned samples. STM scanner x- and y-calibration was performed prior to monolayers studies using sequentially captured HOPG scans (5 or 10 nm scale) corrected for thermal drift.

Experimental Section

Results and Discussion

Scanning tunneling microscopy data was acquired using a Digital Instruments NanoScope STM interfaced to a Digital Instruments NanoScope IIIa controller. All data was collected from the solution-graphite interface (HOPG, ZYB grade, Momentive Performance, Strongsville, OH) using mechanically cut 87/13 Pt/Rh tips (0.25 mm, Omega Engineering, Stamford, CT) or 80/20 Pt/Ir tips (0.25 mm, Goodfellow, Oakdale, PA). Sample solutions were prepared by dissolving 5-15 mg of compound in 250 µL of phenyl octane (Aldrich, 98%) at 22 °C. Solutions were diluted (adding 0-250 µL phenyl octane), filtered (Anatop Plus 0.02 µm filters, Whatman), and equilibrated at the temperature of the STM room (16-20 °C). A solution drop (5 µL) was deposited on a recently cleaved HOPG surface. Samples were imaged immediately or after annealing at 40-45 °C for 1-2 h. The STM tip was engaged through the solution and scanned in constant height mode. After monolayers appeared and thermal drift was minimized, data was collected in constant current mode with feedback parameters specified in each image. Tip scan velocities were in the range 0.20-1.2 µm/s. Thermal drift could not be completely eliminated during data acquisition. Consequently, data collected with opposite slow scan directions (up, down) yielded different unit cell parameters. To

The side chain structures of the C2h symmetry, 1,5-disubstituted anthracene compounds discussed in this manuscript are presented in Chart 1 along with the side chain abbreviations and a cartoon illustrating (ωT2)-packing. The side chain abbreviations use a nonsuperscript number to specify chain length (number of carbon, oxygen, and sulfur atoms). The superscript numbers indicate side chain positions, relative to the anthracene, of ether oxygen atoms or, if the superscript number follows a capital S, of thioether sulfur atoms. The Standard Morphology: (ωT2)-Packing. The monolayers assembled by many 1,5-linear side chain substituted anthracene compounds consist of single-lamella domains11 containing columns of anthracene cores alternating with columns of interdigitated,12 aliphatic side chains.3,8 Neighboring side chains within the aliphatic columns adsorb in antiparallel orientations and are connected to anthracene cores in opposing, flanking aromatic columns. In the “standard” morphology, the terminal methyl group (ω-position) of each linear side chain lies in registration and in van der Waals contact with the group (CH2, O, S) at the 2-position of both neighboring side chains.3,8 This

(8) (a) Wei, Y.; Kannappan, K.; Flynn, G. W.; Zimmt, M. B. J. Am. Chem. Soc. 2004, 126, 5318–22. (b) Wei, Y.; Tong, W.; Wise, C.; Wei, X.; Armbrust, K.; Zimmt, M. J. Am. Chem. Soc. 2006, 128, 13362–63. (9) (a) Buchner, F.; Comanici, K.; Jux, N.; Steinrueck, H.-P.; Marbach, H. J. Phys. Chem. C. 2007, 111, 13531–38. (b) Zhou, H.; Dang, H.; Yi, J.-H.; Nanci, A.; Rochefort, A.; Wuest, J. D. J. Am. Chem. Soc. 2007, 129, 13774–75. (c) Cyganik, P.; Buck, M.; Strunskus, T.; Shaporenko, A.; Wilton-Ely, J. D. E. T.; Zharnikov, M.; Wöll, C. J. Am. Chem. Soc. 2006, 128, 13868–78. (d) Plass, K. E.; Kim, K.; Matzger, A. J. J. Am. Chem. Soc. 2004, 126, 9042–53. (e) Kannappan, K.; Werblowsky, T. L.; Rim, K. T.; Berne, B. J.; Flynn, G. W. J. Phys. Chem. B 2007, 111, 6634–42.

(10) Yurov, V. Yu.; Klimov, A. N. ReV. Sci. Instrum. 1994, 65, 1551–57. (11) (a) Yablon, D. G.; Wintgens, D.; Flynn, G. W. J. Phys. Chem. B 2002, 106, 5470–5475. (b) Askadskaya, L.; Rabe, J. P. Phys. ReV. Lett. 1992, 69, 1395– 1398. (12) (a) Samori, P.; Fechtenkotter, A.; Reuther, E.; Watson, M. D.; Severin, N.; Mullen, K.; Rabe, J. P. AdV. Mater. 2006, 18, 1317. (b) Stabel, A.; Heinz, R.; Rabe, J. P.; Wegner, G.; De Schryver, F. C.; Corens, D.; Dehaen, W.; Sueling, C. J. Phys. Chem. 1995, 99, 8690. (c) Tahara, K.; Furukawa, S.; Uji-i, H.; Uchino, T.; Ichikawa, T.; Zhang, J.; Mamdouh, W.; Sonoda, M.; De Schryver, F. C.; De Feyter, S.; Tobe, Y. J. Am. Chem. Soc. 2006, 128, 16613.

Dipolar Side Chain Control of Monolayer Morphology

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Scheme 1. (Left): (ωT1)-Packing of A-[152] Molecules Overlaps Side Chain ω-Methyl Hydrogens with Hydrogens of the Neighboring Anthracene Core. (Right): (ωT2)-Packing of A-[152] Side Chains Avoids ω-Methyl-Anthracene Steric Repulsions

Scheme 2. A Molecular Mechanics Simulation of A-[112]2 in the Standard Morphology: (ωT2) Side Chain Packing and C-2-C Anthracene Packinga

a The yellow ellipse highlights contacts of the 3-, 4-, and 7-, 8-hydrogens in adjacent anthracenes. The yellow parallelogram marks a unit cell. The blue arrow connects the centers of adjacent anthracenes within one aryl column and lies on the unit cell a-axis. The green arrow connects anthracene centers in adjacent aromatic columns and lies on the unit cell b-axis. The common origin of the a- and b-axis arrows is the unit cell corner that defines the unit cell angle.13 The horizontal black line (bottom) marks the side chain vector. The orange line (left corner) marks the anthracene long axis.

“(ωT2)-packing” [Chart 1, top] avoids steric repulsions between ω-methyl and anthracene hydrogens that arise in (ωT1)-packing (Scheme 1). At the same time, (ωT2)-packing provides each side chain with greater van der Waals contacts and stabilization than (ωT3)-packing. (ωT2)-Packing of all side chains within the aliphatic columns produces a “corner-to-corner” (C-2-C) alignment of anthracenes within the aromatic columns (Scheme 2). The 3- and 4-hydrogens of each anthracene abut the 7- and 8-hydrogens of an adjacent anthracene. Side chain (ωT2)-packing in combination with anthracene C-2-C packing constitutes the “standard” morphology. The precise structure of each crystalline morphology can be specified using a 2-D unit cell. Scheme 2 defines the unit cell parameters (two lengths, one angle13) and two key directions within each molecule: the side chain vector and the anthracene long axis. As previously reported8a for A-[15S3]2 and A-[14S3]2, (ωT2)packing produces an “odd-even” effect14 on the adsorption (13) The unit cells of odd-chain length anthracene derivatives are parallelograms with two different angle values. The unit cell angle is selected at the corner where rotation of the b-axis toward the a-axis is in the same direction as a bending motion that moves a side chain (anthracene 1-position) toward the anthracene 8-position (red arrow, Scheme 2).

orientations of molecules in adjoining anthracene columns. 1,5Substituted anthracenes are prochiral and adsorb to HOPG via one of two enantiotopic faces. For aliphatic columns assembled from odd length side chains, (ωT2)-packing forces molecules in adjoining anthracene columns to adsorb via the same enantiotopic face (Scheme 1, right). Thus, anthracenes flanking an “odd length” aliphatic column adsorb with parallel alignment of their long axes, and monolayers assembled from symmetric, 1,5-bis(odd length-side chain) anthracenes adsorb all molecules within one domain via the same enantiotopic face. The resulting monolayer morphology is a 2-D analogue of a conglomerate crystal (referred to here as AA morphology). By contrast, (ωT2)packed aliphatic columns assembled from eVen length side chains force molecules in adjoining anthracene columns to adsorb via opposite enantiotopic faces (Scheme 1, left). The long axes of surface-adsorbed anthracene enantiomers intersect in a 60-80° angle. The presence of equal numbers of both enantiomers produces a 2-D morphology analogous to a racemate crystal (referred to here as AA* morphology). The odd-even effect on monolayer morphology has been observed for all 1,5-disubstituted anthracenes lacking self-repulsive side chains (vide infra). The effect is exemplified by the monolayers self-assembled from the monoether side chain compounds A-[112]2, A-[132]2, A-[152]2, and A-[162]2. For A-[112]2 (Figure 1A), each anthracene core appears as an oblong, high tunneling region exhibiting a 3 × 2 pattern of dots. All anthracene cores are aligned parallel, indicating that molecules in this domain physisorb using the same enantiotopic face. A unit cell (Figure 1A) for this AA, C-2-C monolayer is drawn with corners at the anthracenes’ centers of symmetry. The shortest center to center distance between anthracenes is 0.97 nm within an aromatic column (unit cell a-axis) and is 2.00 nm between adjacent aromatic columns (unit cell b-axis). The unit cell angle is 84° (Table 1).13,15 An alternative dissection of anthracene monolayers into “1-D tapes”16 of molecules is helpful for understanding packing, dipolar effects, and polymorphism. Each side chain within a linear, 1-D tape packs in registration with one side chain from one tape neighbor. The displacement vectors between tape neighbors’ anthracene centers are constant and aligned roughly parallel to the side chain vectors. The two possible linear, AA tape building blocks of the A-[112]2 monolayer are superimposed on the STM image in Figure 1A. Within the tape extending from top left to bottom right, adjacent tape molecules’ side chains contact each other along their edges closer to the anthracenes’ center rings. This is referred to as an interior edge, AA tape. In the tape (14) (a) Hibino, M.; Sumi, A.; Tsuchiya, H.; Hatta, I. J. Phys. Chem. B 1998, 102, 4544–47. (b) Yablon, D. G.; Wintgens, D.; Flynn, G. W. J. Phys. Chem. B 2002, 106, 5470–75. (c) Tao, F.; Bernasek, S. L. Chem. ReV. 2007, 107, 1408–53. (15) The anthracene long axis of each A-[112]2 lies at an angle of 37 ( 2° relative to the a-axis. The side chain vectors (Scheme 2) are rotated 83 ( 4° from the unit cell a-axis and 46 ( 6° from the anthracene long axes. (16) Zell, P.; Mo¨gele, F.; Ziener, U.; Rieger, B. Chem. Eur. J. 2006, 12, 3847– 57.

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Figure 1. Constant current STM scans of symmetric, monoether side chain anthracene monolayers assembled on HOPG from phenyl octane solutions. The unit cell (cyan box) angle is located at the common origin of the a-axis (blue) and b-axis (green) arrows. CPK models have been superimposed on each image. (A) A-[112]2 (100 pA, 1.0 V, 21 nm × 21 nm). The inset provides an expanded view (3.5 nm × 3.5 nm). CPK models (upper left f lower right) are superimposed on an “interior edge tape” and on an “exterior edge tape” (upper right f lower left). (B) A-[132]2 monolayer (200 pA, 0.7 V, 8 nm × 8 nm). (C) A-[152]2 monolayer (167 pA, 0.94 V, 12 nm × 12 nm). (D) A-[162]2 monolayer (75 pA, 1.0 V, 12 nm × 12 nm). Table 1. Monolayer Unit Cell Parameters (measured): Anthracenes with Monoether Side Chains compound 2

A-[11 ]2 A-[132]2 A-[152]2 A-[162]2 a

morphology

length (b) (nm)

width (a) (nm)

angle (γ) (deg)

molecules per unit cell

AA, ωT2, C-2-C, p2 AA, ωT2, C-2-C, p2 AA, ωT2, C-2-C, p2 AA*, ωT2, C-2-C, pgg

2.00 ( 0.09 2.27 ( 0.11 2.63 ( 0.08 5.29 ( 0.13

0.97 ( 0.02 0.96 ( 0.04 0.96 ( 0.04 1.03 ( 0.06

84 ( 3 94 ( 4 72 ( 3 91a ( 2

1 1 1 2

For a pgg monolayer, the unit cell angle is 90°. The average value determined from multiple STM scans is within experimental uncertainty of 90°.

running from upper right to lower left, adjacent tape molecules’ side chains make contact via their edges closer to the anthracenes’ exterior rings. This is referred to as an exterior edge, AA tape. The periphery of the exterior edge tape has a distinct saw-tooth shape, whereas the interior edge tape has a smoother periphery. (ωT2)-Stacking of side chains from adjacent interior edge (or exterior edge) (ωT2)-packed AA tapes produces the standard AA, C-2-C morphology, with all molecules in a domain adsorbing via the same enantiotopic face. A-[132]2 and A-[152]2 self-assemble AA, C-2-C morphology monolayers on HOPG (Figure 1B,C) that are similar to the A-[112]2 monolayer. The unit cells of all three compounds (Table 1) exhibit p2 plane group symmetry and similar a-axis widths (0.96-0.97 nm). The measured unit cell lengths (b-axis) increase 0.27-0.36 nm for a two methylene elongation of the side chain (0.25 nm). The unit cell angle and other angular measures of the monolayer vary nonmonotonically with chain length.17 These

angular changes contribute to the measured b-axis distance variation with side chain. Molecular mechanics minimizations of monolayer sections on graphene sheets reproduce the measured a-axis distances, are in qualitative agreement with the trend in b-axis distances (0.23-0.27 nm b-axis increase per (CH2)2), but do not predict significant changes in unit cell angles or molecular geometry with side chain length.18 Overall, the monolayers assembled by A-[112]2, A-[132]2, and A-[152]2 exemplify variations of the standard AA, C-2-C morphology driven by (ωT2)-packing of odd length side chains.19 The C-2-C monolayer self-assembled by A-[162]2 on HOPG exemplifies the standard AA* morphology induced by (ωT2)packing of eVen length side chains (Figure 1D). Anthracenes in (17) Angles (in degrees) listed in the order A-[112]2, A-[132]2, A-[152]2. (i) unit cell angle (b-axis to a-axis): 84, 94, 72; (ii) a-axis to anthracene long axis: 37, 29, 38; (iii) a-axis to side chain vector: 83, 78, 80; (iv) anthracene long axis to side chain vector: 46, 50, 62; (v) side chain vector to b-axis: 12, 9, 8.

Dipolar Side Chain Control of Monolayer Morphology Scheme 3. The Vertical Distance between Side Chains in the Same Molecule Is “A”. The Vertical Distance between Adjacent Side Chains in One Aliphatic Column Is “B”a

a Starting from the upper left side chain of the central anthracene (0), the two pairs of up and down arrows generate identical vertical displacements (B-A) to the lower side chain of the top molecules in the left and right anthracene columns. Thus, the side chains are parallel to the unit cell b-axis (green arrow). The unit cell a-axis (blue arrow) is perpendicular to the side chains. This produces a unit cell (red) with 90° angles and pgg plane group symmetry. The cyan anthracenes constitute a 1D AA* tape.

adjacent aromatic columns adsorb via opposite enantiotopic faces, with their long axes aligned, alternately, at a +30° or a -30° angle with respect to the anthracene column repeat (unit cell a-axis). The 3 × 2 dot patterns of some anthracenes are obscured by partial overlap with neighboring aromatic cores. The side chain vectors in all aliphatic columns are parallel to the unit cell b-axis. The angle between the anthracene column repeat vector (a-axis) and the side chain vector is nearly 90°. The center to center separation of neighboring anthracene columns is 2.65 ( 0.06 nm. However, the unit cell contains two molecules, one of each surface 2-D enantiomer, producing a 5.3 nm × 1.0 nm unit cell with a 91° angle. Although the statistical evaluation of unit cell parameters from multiple data sets indicates a nearly rectangular unit cell, a simplified geometrical model predicts an exactly rectangular unit cell (Scheme 3) and a monolayer with pgg plane group symmetry. An AA* monolayer can be dissected into 1D AA* tapes. The tape composition alternates between the two 2D-enantiomers and generates a jagged periphery (cyan molecules, Scheme 3). The challenge of close-packing a jagged periphery may contribute to the rarity of polymorphism in AA* monolayers (vide infra). Finally, molecular mechanics simulations of the A-[162]2 monolayer predict slightly longer and thinner unit cells (5.57 nm, 0.95 nm) than observed by STM.18a The unit cell surface area per molecule for the simulated and measured A-[162]2 monolayers are 2.65 and 2.72 nm2, respectively. Extrapolating the measured unit cell areas of A-[112]2 (1.93 nm2), A-[132]2 (2.18 nm2), and A-[152]2 (2.40 nm2) predicts a unit cell area per molecule of 2.53 nm2 for a hypothetical, AA, C-2-C morphology monolayer from A-[162]2. This hypothetical A-[162]2 AA unit cell area is smaller than the simulated or experimental A-[162]2 AA* unit cell areas. Thus, for anthracenes with 2-position monoether side chains, (ωT2)-packing in AA, C-2-C morphologies assembles denser monolayers than (ωT2)-packing in AA*, C-2-C, morphologies. The lower density of AA* monolayers impacts the preferred morphology of self-repulsive diether monolayers (vide infra). (18) (a) Molecular mechanics minimizations were performed on monolayer sections containing five anthracene columns, each with four molecules. Complete molecules were used in the center three columns. The outer side chain was replaced by hydrogen in the eight molecules in the anthracene columns on the two edges. The monolayer section of desired morphology was assembled, adsorbed on a rectangular graphene sheet 14.1 nm × 6.63 nm (3527 carbons) and energy minimized using a conjugate gradient (Polar-Ribieri) algorithm and the MM+ force field within Hyperchem. The minimizations did not achieve the termination condition (RMS gradient of 0.01 kcal/mol-Å). (b) Calculated unit cell parameters: A-[112]2 (2.17 nm, 0.95 nm, 81°), A-[132]2 (2.40 nm, 0.96 nm, 82°), A-[152]2 (2.67 nm, 0.98 nm, 83°). (19) The supporting information discusses details of repetitive STM height modulations within the STM data from A-[112]2, A-[132]2, and A-[152]2 on HOPG.

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Self-Attractive and Self-Repulsive Side Chains. One or more ether groups within a side chain can generate attractive, repulsive, or negligible electrostatic interactions with ether dipoles in adjacent side chains. Adjacent chain ether interactions are attractive if the dipoles are parallel and nearly colinear. The interaction is repulsive if the dipoles are proximate and aligned antiparallel. Ether dipoles’ orientations, relative placements and interactions are determined by side chain structure and by monolayer morphology. For example, a morphology switch from (ωT2)- to (ωT1)-packing flips the relative directions and alters the separations and interaction of adjacent chain dipoles (Scheme 4). Thus, characterizing side chain interactions as attractive or repulsive requires a morphological context. Given that (ωT2)packing is the prevalent morphology in the absence of dipolar interactions, side chains are designated as dipolar self-attractive or dipolar self-repulsive based on interactions produced in (ωT2)packed morphologies. For (ωT2)-packing of identical chains, an ether dipole at the nth position of one side chain experiences repulsive interactions with an ether dipole at the (ω+3-n) or at the (ω+1-n) position of an adjacent side chain20 (antiparallel dipoles, offset one side chain position21a). Thus, an (ωT2)-packed monolayer assembled from A-[ωn,ω+3-n]2 or from A-[ωn,ω+1-n]2 generates two repulsive dipole-dipole interactions per pair of adjacent side chains.22 Benzylic ethers (n ) 2) generate repulsive dipole-dipole interactions with an ether at the (ω-1)-position of an adjacent chain. Thus, an (ωT2)-packed monolayer assembled from A-[ω2,ω-1]2 generates two repulsive dipole-dipole interactions, one at each end, per pair of adjacent side chains. If these side chain dipole-dipole repulsions are sufficiently destabilizing, the molecules may not assemble organized monolayers or may assemble monolayers that are not (ωT2)-packed. Non-(ωT2)packed morphologies should be detectable by STM. The following sections analyze the morphologies assembled by anthracenes bearing self-repulsive side chains. The results reveal a surprising dependence of tape packing (ωTx) on chain length (odd or even). Attractive dipole-dipole interactions between (ωT2)-packed chains arise for ether dipoles at the nth position of one side chain and an ether dipole (i) at the (ω+2-n) position of adjacent chains (dipoles parallel and colinear) or (ii) at the (ω+4-n) or (ω-n) position (dipoles parallel, offset two side chain positions21b) of adjacent chains.23 Whereas, repulsive dipole interactions may induce morphology changes,21c attractive dipole (20) For an arbitrary side chain packing morphology (ωTx), self-repulsive dipole-dipole interactions arise between an ether dipole at one side chain’s n-position and a second ether dipole at either the (ω+x+1-n) or (ω+x-1-n) position (antiparallel, offset one side chain position) of the adjacent chain. (21) (a) For two ethers in adjacent side chains that are offset one side chain position, a line connecting the oxygen atoms makes a 13° angle (oxygens on outer edges of adjacent chains, O-O distance ) 0.54 nm) or a 19° angle (oxygens on inner edges of adjacent chains, O-O distance ) 0.42 nm) with the ether dipoles. Both geometries generate dipole-dipole repulsions, but molecular mechanics and AM1 calculations indicate that the latter arrangement is 0.17-0.29 kcal/mol higher in energy than the former. (b) For two ethers in adjacent side chains that are offset two side chain positions, a line connecting the oxygens makes a 30° angle with the ether dipoles. This geometry is stabilizing. Molecular mechanics and AM1 calculations indicate that this offset dipole arrangement is slightly less stabilizing (0.14-0.32 kcal/mol) than a parallel colinear arrangement of two ether dipoles. (c) Molecular mechanics and AM1 calculations predict that a pair of parallel, colinear ether dipoles in adjacent chains lies 0.9-2.2 kcal/mol lower in energy than a pair of antiparallel ether dipoles offset one position in adjacent chains. (See the Supporting Information.) (22) Odd length side chains with an ether oxygen at the n ) (ω+3)/2 position or at the n ) (ω+1)/2 position generate one dipole-dipole repulsion between adjacent (ωT2)-packed chains. (23) For an arbitrary side chain packing morphology (ωTx), self-attractive dipole-dipole interactions between an ether dipole at one side chain’s n-position and a second ether dipole at the (ω+x-n) position (parallel, colinear) or at the (ω+x+2-n) or the (ω+x-2-n) positions (parallel, offset two side chain positions) of an adjacent chain.

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Scheme 4. (Left): (ωT2)-Packing of A-[162,15] Side Chains Establishes Antiparallel, Nearly Colinear Alignments of Proximate Ether Dipoles (Arrows) in Adjacent Chains. The Resulting Electrostatic Interactions Are Destabilizing. (Right): (ωT1)-Packing of A-[162,15] Side Chains Produces Colinear, Parallel Alignments of Proximate Ether Dipoles in Adjacent Chains. The Resulting Dipolar Interactions Are Stabilizing

Table 2. Monolayer Unit Cell Parameters (measured): Even Length Diether Side Chains compound 2,12

A-[14 ]2 A-[142,13]2 A-[162,15]2 A-[162,15]2 (alt.) A-[163,14]2

morphology

length (b) (nm)

width (a) (nm)

angle (γ) (deg)

molecules per unit cell

AA*, ωT2, C-2-C, pgg AA, ωT1, H-2-H, p2 AA, ωT1, C-2-C, p2 AA, ωT1, H-2-H, p2 AA, ωT1, H-2-H, p2 AA, ωT3, 14-shift, p2

4.98 ( 0.11 2.41 ( 0.08 2.51 ( 0.09 2.06 ( 0.05 2.54 ( 0.08 2.20 ( 0.04

0.94 ( 0.04 1.10 ( 0.07 0.94 ( 0.05 1.15 ( 0.03 1.15 ( 0.03 1.18 ( 0.02

92 ( 2 126 ( 3 76 ( 1 101 ( 2 127 ( 2 79 ( 2

2 1 1 1 1 1

interactions are not expected to disrupt (ωT2)-packing or to induce qualitative changes relative to the standard morphology. Even Length Self-Repulsive Side Chains: A-[162,15]2 In the absence of dipolar interactions, anthracenes with even length side chains self-assemble AA* morphology monolayers. However, the side chains of A-[162,15]2 are self-repulsive for (ωT2)packing. The low resolution STM scan of A-[162,15]2 in Figure 2A shows identical, zigzag-shaped, tunneling patterns in each anthracene column, indicative of an AA morphology. The higher resolution scan in Figure 2B reveals parallel aligned, oblong (3 × 2 dot) patterns for the anthracenes. No AA* morphology domains are observed from A-[162,15]2. Thus, the two dipolar repulsions between ethers near the ends of adjacent chains raise the energy of an AA*, C-2-C morphology above the energy of

Figure 2. Constant current STM images of A-[162,15]2. (A) 20 nm × 20 nm (30 pA, 1.0 V) scan of C-2-C morphology. (B) 11 nm × 11 nm (80 pA, 0.75 V) scan of C-2-C morphology. The unit cell (cyan), a-axis (blue) and b-axis (green) directions are superimposed on the STM data. (C) 8 nm × 8 nm (90 pA, 0.9 V) section of H-2-H morphology with superimposed unit cell (cyan). (D) 18 nm × 18 nm (80 pA, 0.80 V) scan of a mixed domain containing H-2-H (a-axis, cyan bar) and C-2-C (a-axis, black bar) morphologies.

an AA, non-(ωT2), C-2-C morphology. For A-[162,15]2, the AA* morphology is not the ground state (lowest energy) despite having even length side chains. The unit cell parameters (Table 2) of the AA, C-2-C domains assembled by A-[162,15]2 are consistent with (ωT1)-packing rather than (ωT3)-packing: the unit cell b-axis length (aromatic column separation) is 0.14 nm shorter than one-half the A-[162]2 b-axis length. The side chain vector of A-[162,15]2 is displaced less than 10° from the C-2-C b-axis, similar to the alignment found for the odd chain length monoethers (vide supra). (ωT3)-Packing would have produced a unit cell b-axis length larger than half-of A-[162]2. As shown in Scheme 4, (ωT1)-packing of A-[162,15]2 establishes stabilizing, parallel, colinear alignments of proximate dipoles in neighboring chains instead of the repulsive dipole alignments incurred by (ωT2)-packing.24 In contrast to the monoether compounds discussed above, monolayers formed from A-[162,15]2 exhibit two different AA morphologies. In the second polymorph,9 the long axes of anthracenes within the same aromatic column are nearly colinear (Figure 2C). The unit cell parameters of this “head-to-head” (H-2-H) morphology are listed in Table 2. The a-axis of the H-2-H unit cell is nearly parallel to the anthracene long axis. Thus, the unit cell a-axis width is larger in H-2-H than in C-2-C. The side chain vectors in H-2-H nearly bisect one of the unit cell angles (Figure 2C, 3B), so the unit cell b-axis length is much shorter for H-2-H than for C-2-C. Given the exceedingly different unit cell shapes and parameters of the H-2-H and C-2-C polymorphs, it was surprising that many domains seamlessly integrated both (Figure 2D). Neither the a-axes nor the b-axes of the previously specified C-2-C and H-2-H unit cells are coincident (Figure 2D, 3B). However, a more angled form of the H-2-H unit cell (Table 2, 127° unit cell angle) shares a common b-axis with the C-2-C unit cell (Figure 3B). The common b-axis affords structural compatibility between these A-[162,15]2 polymorphs and allows assembly of mixed morphology domains. The C-2-C and H-2-H morphologies of A-[162,15]2 can be dissected into (ωT1)-packed, interior-edge tapes (Figure 3A). (24) (ωT1)-Packing forces the side chain terminal methyl groups into close contact with the center ring of neighboring column anthracenes (Schemes 1, 4). The STM data for A-[162,15]2 indicate a 40 ( 6° angle between the side chain vector and the anthracene long axis, compared to a 62 ( 4° angle for A-[152]2. To accomodate (ωT1)-packing, it appears that each A-[162,15]2 side chain bends away from its anthracene’s center ring and toward its long axis. This distortion moves the side chain methyls away from the center rings of anthracenes in adjacent columns.

Dipolar Side Chain Control of Monolayer Morphology

Figure 3. (A) CPK model of an interior-edge tape formed by A-[162,15]2. Yellow arrows point to anthracene H-3 bulges. Red arrows indicate O-2 notches. Blue arrows indicate methyl-anthracene notches. (B) The right three tapes pack in the C-2-C morphology (H-3 bulges in adjacent tape methyl-anthracene notches), characterized by the yellow unit cell with a 76° angle. The left four tapes pack in the H-2-H morphology (H-3 bulges in adjacent tape O-2 notches). The dark blue box outlines the H-2-H unit cell with a 101° angle. The red box outlines the alternate H-2-H unit cell (127° angle) which shares its b-axis (red-yellow dashed line) with the C-2-C unit cell.

The side chain ether dipoles within each (ωT1)-packed tape are parallel and colinearly aligned (Scheme 4). Neighboring anthracenes within a tape lie along the b-axis common to the C-2-C and 127° H-2-H unit cells. The periphery of each tape is relatively flat. However, the anthracene H-3 and H-7 hydrogens bulge slightly from the tape periphery (Figure 3A), and there are two types of notches along the periphery: (i) a small gap between the terminal methyl of each side chain and the adjacent anthracene and (ii) the absence of side chain hydrogens at the benzylic ether oxygen (side chain 2-position).25 The C-2-C and H-2-H morphologies differ by which notches in one tape align with the anthracene H-3 (H-7) bulges of the adjacent tape. The C-2-C morphology arises where adjacent, (ωT1)-packed tapes stack with H-3 (H-7) bulges projecting into the neighboring tape’s “methyl-anthracene notches”. This morphology gains additional electrostatic stabilization from parallel and colinear aligned ether dipoles in adjacent tapes. The H-2-H morphology arises where the anthracene H-3 (H-7) bulges of one tape project into the O-2 notches of the adjacent tape. The proximity of aryl hydrogen and ether oxygens may stabilize the H-2-H polymorph through weak, intertape C(aryl)-H · · · O interactions.26 Ether dipoles from adjacent tapes are not proximate in the H-2-H morphology.27 Experimentally, the C-2-C and H-2-H morphologies appear with similar probability, suggesting comparable free energies. Even Length Self-Repulsive Side Chains: A-[163,14]2 The ether oxygens within the side chains of A-[163,14]2 are predicted to generate repulsive interactions in (ωT2)-packed, AA* morphologies. STM scans of A-[163,14]2 assembled on HOPG (Figure 4) exhibit parallel aligned anthracenes in all columns. Thus, A-[163,14]2 assembles an AA morphology rather than the (25) The oxygens at side chain 15 positions do not yield peripheral notches, as these atoms do not lie on the tape exterior. (26) (a) Taylor, R.; Kennard, O. J. Am. Chem. Soc. 1982, 104, 5063–70. (b) Desiraju, G. R. Chem. Commun. 2005, 2995–3001. (c) Vidal, F.; Delvigne, E.; Stepanow, S.; Lin, N.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2005, 127, 10101–06. (d) Meier, C.; Ziener, U.; Landfester, K.; Weihrich, P. J. Phys. Chem. B 2005, 109, 21015–27. (27) For the C-2-C and H-2-H morphologies of A-[162,15]2, interior edge packed side chains (tapes) are (ωT1)-packed. For C-2-C, contacting side chains from adjacent tapes are also (ωT1)-packed. For H-2-H, contacting side chains from adjacent tapes are (ωT7)-packed.

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Figure 4. Constant current STM images of A-[163,14]2. (A) 16.5 nm × 16.5 nm (0.1 nA, 1.0 V) scan of 14-shift morphology. (B) 10 nm × 10 nm (90 pA, 0.95 V) scan of 14-shift morphology exhibiting parallel aligned, 3 × 2 dot anthracene patterns in each aromatic column. The side chain vector extends from the lower left to the upper right of the unit cell (cyan).

Figure 5. (A) CPK model of A-[163,14]2 monolayer consisting of (ωT3)packed tapes (purple lines border one tape). Adjacent tapes are stacked with the anthracene H-3 (H-7) bulges of one tape aligned into O-14 notches of adjacent tapes (14-shift morphology). The unit cell (yellow) parameters (Table 2) agree with those from molecular mechanics minimizations18a (b-axis 2.24 nm, a-axis 1.20 nm, angle 77°). (B) Detail of (ωT3)-packed A-[163,14]2 monolayer sections in C-2-C (top) and 14-shift (bottom) morphology. The yellow ellipses highlight differences in the side chain methyls’ steric crowding in C-2-C and 14-shift morphologies.

AA* morphology anticipated based on chain length. As found for its isomer, A-[162,15]2, the two sets of dipolar repulsions per pair of (ωT2)-packed [163,14] side chains elevate the AA* morphology to a nonground state. The AA monolayer assembled by A-[163,14]2 is distinct from both the C-2-C and the H-2-H morphologies assembled by A-[162,15]2. The long axes of A-[163,14]2 anthracenes are rotated 45° from the unit cell a-axis, similar to the angle found in C-2-C morphologies. However, the unit cell a-axis width for A-[163,14]2, 1.18 nm, is much larger than for C-2-C morphologies and slightly larger than for the colinear aligned anthracenes in the A-[162,15]2 H-2-H morphology (Table 2). Additionally, it appears that [163,14] side chains intervene between adjacent anthracenes in each aromatic column. Analyses of STM scans reveal the side chain vectors in A-[163,14]2 form an angle of 70 ( 6° with the attached anthracenes’ long axes and lie within 5° of the unit cell’s long diagonal (Figure 4B). The two adjacent side chains spanning this long diagonal are attached to the opposing anthracene at these unit cell corners (Figure 5a). The separation of these two anthracenes, 2.69 ( 0.06 nm, suggests (ωT3)-packing of side chains, in contrast to (ωT1)-packing (2.51-2.54 nm) found for A-[162,15]2. (ωT3)Packing of the diagonal anthracenes’ side chains establishes two sets of parallel ether dipoles, offset by two side chain positions. This electrostatic interaction stabilizes the (ωT3)-packed tapes (Figure 5a, purple borders) compared to the repulsive dipolar interactions incurred with (ωT2)-packing. The 3-position oxygen

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Figure 6. Constant current STM images of A-[142,13]2. (A) 8 nm × 8 nm (210 pA, 0.73 V) scan of H-2-H morphology. The anthracene cores appear as 3 × 2 dot patterns. The H-2-H unit cell with a 126° angle is indicated (cyan). (B) 50 nm × 50 nm (210 pA, 0.65 V) scan of a mixed domain displays the prevalence of H-2-H (linearly aligned anthracenes) compared to C-2-C (step) morphologies.

tilts each 163,14 side chain toward the anthracene center ring.28 (ωT1)-Packing of A-[163,14]2 would force terminal methyl groups even closer to the adjacent anthracene than for A-[162,15]2. This increased steric repulsion contributes to the preference for (ωT3)over (ωT1)-packing within A-[163,14]2 AA tapes. The periphery of an (ωT3)-packed, interior edge tape assembled from A-[163,14]2 has two sets of bulges: (i) the anthracene H-3 (H-7) hydrogens and (ii) the terminal methyl groups. The tape periphery also has two sets of notches: (i) gaps between the side chain methyls and adjacent anthracenes and (ii) the absence of side chain hydrogens at the 14-position ether oxygens. Stacking the anthracene H-3 (H-7) bulges of one tape into the methyl-anthracene notches of an adjacent tape (C-2-C morphology) forces the methyl bulges against the adjacent tape’s anthracene H-2 hydrogens (Figure 5B, top). The methyl groups must “bend back” to permit approach of adjacent tapes. By contrast, stacking the anthracene H-3 (H-7) bulges of one tape into the O-14 notches of the adjacent tape (14-shift morphology) minimizes steric repulsions by positioning the methyl groups in a gap within the anthracene columns (Figure 5B, bottom). For 14-shift morphology, each A-[163,14]2 participates in a total of four C(aryl)-H · · · O interactions with molecules of adjoining tapes. As no C-2-C domains of A-[163,14]2 are observed by STM, the (ωT3)-packed, 14-shift morphology appears to afford a lower free energy.29 Even Length Self-Repulsive Side Chains: A-[142,13]2 The STM scans of A-[142,13]2 in Figure 6 display nearly exclusive assembly of AA, H-2-H morphology domains. Less than 5% of the monolayer exhibits AA, C-2-C morphology and no regions exhibit AA* morphology. As the 13-oxygen lies on the side chain’s interior edge, 13-shift morphologies cannot assemble. AA interior edge tapes, either (ωT1)- or (ωT3)-packed, establish stabilizing dipolar interactions between adjacent [142,13] side chains in contrast to dipole repulsions that would arise in (ωT2)packed AA* tapes. Once again, dipolar interactions drive assembly of AA morphologies for anthracenes with self-repulsive, even length side chains. Table 2 lists parameters of the A-[142,13]2 H-2-H unit cell whose b-axis is nearly parallel to the side chains and is shared by the C-2-C unit cell. The H-2-H unit cell, a-axis width (1.10 (28) AM1 calculations indicate the angle formed by the side chain vector and the anthracene long axis is 67° for A-[163,14]2 and 60° for A-[162,15]2. Calculated C-C-C angles are close to 111°. Calculated C-C-O angles for A-[162,15]2 and A-[163,14]2 range from 106 to 109°. Thus, the side chain vector tilts toward the chain edge bearing an ether oxygen. A 3-position oxygen (side chain interior edge) tilts the chain vector toward the anthracene center ring. A 2-position oxygen (side chain exterior edge) tilts the chain vector toward the anthracene long axis. (29) Unit cell parameters for A-[163,14]2 C-2-C morphology from molecular mechanics simulation:18a (b-axis 2.51 nm, a-axis 1.04 nm, 89°).

Figure 7. 10 nm × 10 nm (200 pA, 0.37 V) constant current STM image of A-[142,12]2. Hazed, rounded rectangles have been superposed on two anthracenes exhibiting nearly complete 3 × 2 dot patterns (2nd aryl column from top) and on two anthracenes exhibiting incomplete patterns (3rd column). CPK models are superposed on the second and third anthracene columns.

nm) and the unit cell angle are similar to values for the H-2-H form of A-[162,15]2. The A-[142,13]2 H-2-H unit cell b-axis length, 2.41 nm, is 0.14 nm larger than the b-axis repeat of A-[132]2 and 0.15 nm shorter than the b-axis repeat for the similarly angled unit cell of A-[162,15]2. These comparisons do not allow an unambiguous assignment of A-[142,13]2 side chain packing as (ωT1) or (ωT3). However, simulations of (ωT1)-packed, A-[142,13]2 H-2-H domains closely match the experimental H-2-H unit cell parameters, suggesting that the chains are (ωT1)packed.30 (ωT1)-Packing affords small methyl-anthracene notches within an interior edge tape. Anthracene H-3 bulges of one (ωT1)packed tape do not fit deeply into the compacted notches of an adjacent tape. This increases the distance between C-2-C stacked tapes, which reduces van der Waals interactions and disfavors the C-2-C morphology. By contrast, H-2-H tape stacking utilizes O-2 notches and affords small intertape separations. A-[142,13]2 monolayers contain very few C-2-C stacked tapes. The analogue, A-[162,15]2, assembles monolayers with comparable fractions of H-2-H and C-2-C packed tapes. The longer chains of A-[162,15]2 may have greater flexibility to accommodate the anthracene H-3 bulges and maintain short intertape distances. If this supposition is valid, the fraction of C-2-C tape stacking should increase in longer chain analogues, such as A-[182,17]2. Even Length Self-Attractive Side Chains: A-[142,12]2 (ωT2)Packing of [142,12] side chains establishes parallel alignments, offset by two side chain positions, of the 2- and 12-oxygen dipoles in adjacent chains. Thus, side chain length and side chain dipolar interactions both predict assembly of (ωT2)-packed, AA* monolayers by A-[142,12]2. The STM scan in Figure 7 confirms AA* monolayer formation by A-[142,12]2. The measured unit cell parameters (Table 2) are consistent with values from simulations of (ωT2)-packed monolayers (length ) 5.03 nm, width ) 0.94 nm, angle ) 92°). Many anthracenes in the A-[142,12]2 monolayer scans do not show complete 3 × 2 dot patterns. Two of the more complete patterns are highlighted at (30) Unit cell paremters for A-[142,13]2 on HOPG simulation:18a (a) AA, ωT1, H-2-H (2.43 nm, 1.17 nm, 123°); (b) AA, ωT3, H-2-H (2.66 nm, 1.19 nm, 126°).

Dipolar Side Chain Control of Monolayer Morphology

Figure 8. Constant current STM images of A-[112,7]2. (A) 25 nm × 25 nm (100 pA, 1.0 V) scan of the herringbone morphology and a C-2-C defect (diagonal stripe). The green (blue) arrows and dashed lines indicate AA (A*A*) tapes. This scan was not corrected for thermal drift. (B) 7 nm × 7nm (80 pA, 1.0 V) scan showing anthracene cores and side chain hydrogens, along with the unit cell (cyan). CPK models from a molecular mechanics minimized monolayer built from AA and A*A* exterior edge tapes (diagonal: upper left to lower right) are superposed on the STM data along with a unit cell (black) corresponding to the experimental unit cell (cyan).

the top of the unit cell. The A and A* columns exhibit distinct, incomplete dot patterns, but both column patterns repeat after two anthracenes. For each A-[142,12]2 aliphatic column, all ether dipoles point in the same direction, producing a macroscopic dipole. Although the dipoles from adjacent aliphatic columns point in opposite directions, it may be possible to sense or to use these net column dipoles. Of the molecules reported here, only A-[142,12]2 and A-[132,12]2 (vide infra) have net, nonzero dipoles within an aliphatic column. The assembly of AA* monolayers by A-[142,12]2 demonstrates that even length, diether side chains do not generically disfavor (ωT2)-packed morphologies. Rather, the assembly of AA monolayers by A-[142,13]2, A-[162,15]2, and A-[163,14]2 results from specific side chain placements of ether groups. The switch away from (ωT2)-packing by these latter three, even length compounds implicates a critical role of ether group interactions in determining monolayer morphologies: dipolar interactions of ether groups in neighboring chains drive formation of non-(ωT2)-packed, interior edge, AA tapes from anthracenes bearing even length, self-repulsive side chains. Side chain shape (bends, bulges, and notches), ether dipolar interactions, and C(aryl)-H · · · O interactions jointly determine the preferred packing of adjacent AA tapes and the overall morphology (C-2-C, H-2-H, or X-shift) of the self-assembled monolayers. Odd Length Self-Repulsive Side Chains: A-[112,6]2 and A-[112,7]2 In the absence of dipolar interactions, (ωT2)-packing of odd length side chains assembles AA morphology monolayers. A dipole positioned at the (ω+1)/2 position or at the (ω+3)/2 position of an odd length side chain experiences repulsive interactions with a dipole in an identical, adjacent (ωT2)-packed chain.22 The odd length side chain anthracenes A-[112,6]2 and A-[112,7]2 were studied to determine if a single, self-repulsive interaction per pair of adjacent side chains is sufficient to induce an AA to AA* morphology change. No STM images of organized monolayers were obtained from phenyl octane solutions of A-[112,6]2 at concentrations ranging from 1 mg/mL to saturation. The failure to detect monolayers is not a consequence of selfrepulsion as assembly of (ωT1)- or (ωT3)-packed AA*

Langmuir, Vol. 25, No. 5, 2009 2921

morphologies would produce dipolar attraction between adjacent side chains. In contrast to A-[112,6]2, A-[112,7]2 readily forms monolayers (vide infra). Subsequent STM studies of A-[162,7,11]2 also realized monolayer self-assembly, whereas no monolayers were detected from A-[162,6,11]2. It appears that side chains with ethers at the 2- and 6-positions incur difficulty assembling organized monolayers. Both ether groups curve the side chain toward the anthracene long axis27 which may interfere with side chain packing and assembly of stable monolayers. Low resolution STM scans of the A-[112,7]2 monolayer selfassembled from phenyloctane solutions on HOPG (Figure 8A) reveal a herringbone-like arrangement of the anthracene cores, which appear as elongated, high tunneling regions. Four vertically aligned anthracenes are the nearest neighbors of each horizontally aligned anthracene. The extended defect running diagonally in the bottom half of Figure 8A resembles C-2-C packing of two AA tapes. Similarly aligned (AA) tapes constitute every second diagonal column (green arrows). The intervening diagonal columns (blue arrows) are related by a glide plane to the adjacent AA tapes and are their 2D enantiomers (A*A* tapes). The herringbone morphology exhibits pgg plane group symmetry. Within each diagonal tape, the displacement of adjacent anthracenes’ centers, ∼1 nm, is similar to that found for the exterior edge tapes of A-[112]2 (Figure 1a) and much larger than for the A-[112]2 interior edge tapes. The hydrogen atoms appearing in higher magnification scans of A-[112,7]2 (Figure 8B, orange dots within dark regions) are difficult to assign to specific side chains. Molecular mechanics simulations (i) reproduce the herringbone pattern observed by STM (Figure 8B), (ii) clarify the side chain origin of hydrogens, and (iii) confirm that the monolayer assembles as alternately stacked AA and A*A* exterior edge tapes. The simulated unit cell parameters (b ) 2.22 nm, a ) 1.81 nm, 89°) are in reasonable agreement with the experimentally determined unit cell (Table 3). Each exterior edge tape in the A-[112,7]2 monolayer is (ωT2)packed, thus generating electrostatic repulsion between the 7-position ethers. However, the 7-position oxygens, with their partial negative charges, are farther apart (0.544 nm18a) in an A-[112,7]2 exterior edge tape (Figure 8B) than they are in a (ωT2)packed interior edge tape (0.415 nm21a). The main electrostatic repulsion within this exterior edge tape arises from the proximity (g0.26 nm) of δ+ CH2 groups at the 6- and 8- positions of adjacent chains. Ether groups distribute partial positive charge among two R-carbons, four R-hydrogens and two β-carbons. Close approach of adjacent side chains’ weakly positive, ether R-hydrogens in an A-[112,7]2 exterior edge tape incurs less repulsion than close approach of partially negative, ether oxygens in an interior edge tape.21a Thus, the exterior edge herringbone tape suffers less dipolar repulsion than would be generated by an interior edge tape. In addition, the herringbone morphology stacks each 7-position oxygen next to a 4-(or 8-)anthracene hydrogen from an adjacent tape. C(aryl)-H · · · O electrostatic interactions provide additional stabilization of this morphology. While this discussion of monolayer energetics is qualitative, the observed facts are that A-[112,7]2 (i) assembles exterior edge, (ωT2)-packed tapes, (ii) does not switch to AA*, (ωT1)- or (ωT3)-packed C-2-C morphologies, (iii) assembles racemic

Table 3. Monolayer Unit Cell Parameters (measured): Odd Length Diether Side Chains compound A-[112,7]2 A-[132,7]2 A-[132,12]2 A-[152,9]2

morphology AA,A*A*, AA, ωT2, AA, ωT2, AA, ωT2,

ωT2, HB, pgg C-2-C, p2 H-2-H, p2 C-2-C, p2

length (b) (nm)

width (a) (nm)

angle (γ) (deg)

molecules per unit cell

2.28 ( 0.07 2.28 ( 0.06 2.30 ( 0.08 2.54 ( 0.09

1.77 ( 0.06 1.02 ( 0.04 1.23 ( 0.05 0.95 ( 0.04

87 ( 2 91 ( 2 133 ( 4 95 ( 2

2 1 1 1

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Figure 10. Plot of measured unit cell area versus side chain length. Morphology key shapes: 0 ) C-2-C, ] ) H-2-H, 4 ) 14-shift, / ) exterior edge herringbone. Colors: unshaded ) (ωT2) with no dipolar interactions, yellow ) (ωT2) with attractive dipolar interactions, gray ) (ωT2) with repulsive dipolar interactions, red ) (ωT1) with attractive dipolar interactions, blue ) (ωT3) with attractive dipolar interactions. A least-squares regression line for A-[112]2, A-[132]2, and A-[152]2 is provided. Figure 9. Constant current STM scans of monolayers assembled by self-repulsive, odd chain length compounds. (A) A-[132,7]2: 11 nm × 11 nm (200 pA, 0.55 V) AA, C-2-C domain. (B) A-[132,12]2: 11 nm × 11 nm (200 pA, 0.9 V) AA, H-2-H domain. (C) A-[152,9]2: 9 nm × 9 nm (60 pA, 1.0 V) AA, C-2-C domain.

herringbone morphologies (adjacent AA and A*A* tapes), (iv) forms AA, C-2-C, (ωT2)-packed regions as monolayer defects (