J. Phys. Chem. C 2010, 114, 20783–20792
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Morphology Control and Monolayer Patterning with CF2 Groups: An STM Study† Wenjun Tong,‡ Yi Xue, and Matthew B. Zimmt* Department of Chemistry, Brown UniVersity, ProVidence, Rhode Island 02912, United States ReceiVed: July 28, 2010; ReVised Manuscript ReceiVed: September 19, 2010
Difluoromethylene (CF2) groups introduce large dipole moments (2 D) into aliphatic chains with minimal perturbation of chain geometry and shape. Proper placements of CF2 and other dipolar groups within aliphatic chains generate dipole-dipole repulsions or attractions between neighboring chains of self-assembled monolayers. The ability of CF2 groups to direct packing, morphology, and patterning of self-assembled monolayers is tested using six, 1,5-(aliphatic side chain substituted)anthracene derivatives with one CF2 group per linear side chain. STM is used to characterize the monolayer morphology assembled on graphite from phenyl octane solution containing one or two of these anthracene compounds. Dipolar interactions between CF2 and ether groups {CF2/ether} within contacting, eVen length side chains drive assembly of a morphology not normally observed from anthracene monolayers (ωT3 packed, AA tape, C-2-C). However, {CF2/ether} interactions are not sufficient to disrupt the normal, more stable monolayer morphology (ωT2 packed, AA tape, C-2-C) assembled by anthracenes with odd length side chains. By contrast, the larger dipolar interactions generated between pairs of CF2 groups in contacting chains {CF2/CF2} overwhelm the normal packing of anthracenes with odd length side chains and drive assembly of a previously unobserved monolayer morphology (ωT3 packed, AA′ tape, C-2-C). The potent {CF2/CF2} dipolar interactions also drive assembly of patterned (AB) monolayers from solution mixtures of dipolar “complements”; two anthracenes (A and B) whose complementary dipolar group placements generate stabilizing electrostatic interactions when packed next to each other using the normal alignment and morphology of odd length side chains (ωT2, “AA” tape, C-2-C). The favorable packing (ωT2) and highly stabilizing dipolar interactions between CF2 side chain complements produce only patterned monolayers from 1:1 solution mixtures; no domains exhibiting the single component morphologies have been observed in STM studies. Molecular mechanics simulations of monolayer sections on graphene sheets confirm the critical role {CF2/CF2} dipolar interactions exert in driving the morphology and patterning of these anthracenes’ monolayers. Introduction Directing the self-assembly of structured and functional materials is an important challenge for nano and material science.1 Driving force derived from interactions integrated over macromolecular2 and macroscopic3 length scales promotes selfassembly of structured materials. Driving force and selectivity derived from pairwise specific, molecular interactions promotes structural and compositional organization on nano-micrometer length scales, as demonstrated by DNA Origami.4 Molecule scale and functional group specific interactions promote formation and influence the morphology of monolayers self-assembled on surfaces. Developing orthogonal sets of group specific interactions should enable control of monolayer composition, structure and, ultimately, function. Although there are numerous examples of crystalline monolayers having structurally complex5 or multicomponent6 unit cells, only a handful of these systems allow control over structural elements in the 2-D crystal. One very successful system uses double crossover nucleic acids to generate complex, 2-D, compositionally patterned monolayer grids.7 A second system, employing 1,5-[linear side chain]substituted anthracenes, yielded 1-D compositionally patterned monolayers driven by ether-ether dipolar interactions.8 The small magnitude of ether dipoles limited their application in †
Part of the “Mark A. Ratner Festschrift”. * To whom correspondence should be addressed. E-mail:
[email protected]. ‡ Current Address: Brown University Department of Molecular Biology, Cell Biology and Biochemistry, Providence, RI 02912.
monolayer patterning. In addition, subtle geometric changes coupled to the side chain positions of ether oxygen atoms produced complex monolayer polymorphism. This manuscript describes a more powerful approach to dipolar patterning of (anthracene) monolayers using the CF2 group. The CF2 group’s dipole moment is larger than an ether’s and the steric similarity of CF2 and methylene (CH2) improves control of monolayer patterning and morphology. 1-D tapes are the primary building block of monolayers selfassembled from 1,5-[linear side chain]-substituted anthracenes.10 Each molecule within a 1-D tape makes extensive, side chain-side chain contacts with two adjacent molecules in the tape. In the absence of overwhelming dipolar interactions, 1-D tapes exhibit “ωT2 packing”; the terminal heavy atom of each side chain (ω) lies in registration with the second heavy atom of the side chain from an adjacent molecule in the tape (Scheme 1).11 ωT2 packing of eVen length side chains forces adjacent molecules in a tape to adsorb via opposite enantiotopic faces (A or A′) and yields nearly perpendicular alignment of adjacent anthracene cores (i.e., forms AA′ tape and monolayer morphology). ωT2 packing of odd length side chains cause adjacent molecules in a tape to adsorb via the same enantiotopic face and produces parallel alignment of adjacent anthracene cores (AA tape and monolayer morphology). The presence or absence of ωT2 packed tapes can be determined using STM to visualize the long axis orientation of anthracenes in the monolayer.11
10.1021/jp107068a 2010 American Chemical Society Published on Web 10/13/2010
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SCHEME 1: (Top) ωT2 Packing of A-[162]2 Even Length Side Chains; (bottom) ωT2 Packing of A-[132]2 Odd Length Side Chainsa
a ωT2 packing of A-[162]2 even length side chains produces a 1-D, AA′ tape in which adjacent tape molecules adsorb via opposite enantiotopic faces.9 The long axis of adjacent anthracene cores are aligned nearly perpendicular. ωT2 packing of A-[132]2 odd length side chains produces a 1-D AA tape, with adjacent tape molecules adsorbed via the same enantiotopic face. Adjacent anthracenes’ long axes are aligned parallel. The yellow line superposed on each tape highlights ωT2 side chain packing.
Dipolar groups in the side chains can destabilize ωT2 packing and drive alternate tape packing. For example, side chains with ether oxygen atoms at the n and (ω + 1 - n) positions or at the n and (ω + 3 - n) positions generate dipolar repulsions when ωT2 packed; pairs of ether dipoles in adjacent side chains align antiparallel, one position out of registration. The dipolar repulsion between these side chain ether groups changes to dipolar attraction upon assembly of ωT1 or ωT3 packed tapes.8 The latter side chain alignments produce parallel dipoles, either in registration or out of registration by two side chain positions. However, ωT1 or ωT3 packing of the diether side chains affords less favorable van der Waals and/or steric interactions within and between tapes. The incongruity of the dipolar and van der Waals/steric interactions for any one ωT# side chain alignment produces a nonmonotonic dependence of tape packing and monolayer morphology on the lengths of n,(ω + 1 - n)-diether side chains. Anthracenes bearing eVen length diether side chains with two sets of dipole repulsions per pair of ωT2 packed side chains do not assemble ωT2 packed, AA′ tapes and monolayers. These even length, “self-repulsive” side chains promote assembly of AA morphology monolayers built from ωT1 or ωT3 packed AA tapes.8b By contrast, odd length diether side chains promote assembly of AA monolayers built from ωT2 packed tapes irrespective of whether this side chain alignment yields zero, one, or two sets of ether dipole repulsions per pair of adjacent side chains. STM experiments and calculations indicate that the extent of dipolar repulsion needed to force odd length side chains to forego ωT2 tape packing (i.e., to switch from AA to AA′ tapes) is significantly larger than the repulsion needed to induce eVen length side chains to switch from AA′ (ωT2) to AA (non-ωT2) packed tapes.12 Selfrepulsive, di-, and even triether side chains provide insufficient driving force to change the alignments of adjacent, odd length side chains.13 The dipole moments of ether groups, 1.2-1.3 D,14 generate moderate dipole-dipole repulsions and attractions between appropriately aligned, diether side chains.15 The oxygen atom’s small van der Waals radius, relative to CH2, produces a notch in and a concave bend of the ether side chain. These shape perturbations promote numerous stacking alignments of adjacent tapes which promotes monolayer polymorphism. The CF2 group is a potentially useful alternative to the ether group for dipolar applications in monolayers. Its dipole moment, 2.2 D, is larger and provides correspondingly larger dipole-dipole interactions. The CF2 group is slightly larger than a CH2 group, thus, producing a bump and a slight convex bend in the side chain.
Tong et al. To test the CF2 group’s ability to control tape packing and monolayer structure, six 1,5-disubstituted-anthracenes bearing side chains with one CF2 and a benzylic ether group were prepared and their monolayer morphologies characterized using STM; A-[132,F-12,12]2, A-[142,F-13,13]2, A-[152,F-8,8]2, A-[152,F-9,9]2, A-[172,F-9,9]2, and A-[172,F-10,10]2.9 Within ωT2 packed tapes of A-[132,F-12,12]2 or A-[142,F-13,13]2, the CF2 and benzylic ether dipoles in contacting side chains are aligned antiparallel, one position out of registration ({CF2/ether} interactions). ωT2 packed tapes assembled from each of the other four compounds align the CF2 dipole in adjacent side chains antiparallel and one position out of registration ({CF2/CF2} interactions). Following characterization of the six compounds’ single component monolayer morphologies, monolayers assembled from A-[152,F-8,8]2/A[152,F-9,9]2 mixtures and of A-[172,F-9,9]2/A-[172,F-10,10]2 mixtures were characterized by STM. The results of these studies demonstrate the potency of {CF2/CF2} dipolar interactions for control of monolayer morphology and patterning. Experimental Section 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 from 5-15 mg of compound in 250 µL of phenyl octane (Aldrich, 98%) at 20 °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 (15 - 20 °C). A solution drop (5 µL) was deposited on a recently cleaved HOPG surface. Samples were imaged immediately or after annealing for 2-5 h. The STM tip was engaged through the solution and scanned in constant height mode. After monolayers appeared and thermal drift minimized, data was collected in constant current mode, using feedback parameters specified in each image. Tip scan velocities were in the range 0.20-1.2 µm/s. Multiple samples of each compound were prepared and imaged to evaluate tape packing, monolayer morphology, and unit cells. For a few compounds, high resolution data could be obtained only in constant height mode. Thermal drift could not be completely eliminated during data acquisition. Small or moderate thermal drift distortions do not prevent morphology evaluation. To determine the unit cell of each morphology, thermal drift effects were removed using a program that solves for the x- and y-thermal drift velocities in consecutively captured images obtained using opposite slow scan directions. The program minimizes differences in the two scans’ unit cell parameters.13 This correction is valid if thermal drift velocities remain relatively constant in consecutive scans. Reported unit cell parameters (Supporting Information) are averages of thermal drift corrected STM data from three or more independently prepared and scanned sets of samples. STM scanner x- and y-calibration was performed prior to monolayer formation using sequentially captured HOPG scans (10 nm scale) corrected for thermal drift. 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 and oxygen atoms). The superscript
Morphology and Patterning with CF2 Groups CHART 1
numbers prior to an atom label specify the side chain position(s) of ether oxygen atom(s) relative to the anthracene core. The numbers after the superscript F specify the side chain positions of fluorine atoms. Typical synthetic procedures and spectral data from all new compounds are provided in the Supporting Information.
J. Phys. Chem. C, Vol. 114, No. 48, 2010 20785 SCHEME 2: C-2-C Morphologies Arising from ωT2 Stacking (Inter-Tape Alignment) of ωT2 Packed Tapesa
a Anthracene cores in the aryl columns make contacts at their H-3/ H-4 corners. (top) ωT2 packed AA tapes of A-[132,F-12,12]2. (bottom) ωT2 packed AA’ tapes of A-[142,12]2.10b
Results and Discussion CF2 Bearing Side Chains Assemble C-2-C Morphologies. STM images show that each of the six compounds assembles single morphology monolayers (Figure S1).16 This behavior contrasts with the extensive monolayer polymorphism of 1,5substituted anthracenes bearing diether side chains. Monolayers assembled by A-[152,8]2, the diether analog of A-[152,F-8,8]2, contain two dominant morphologies; monolayers assembled by A-[172,10]2 (analogous to A-[172,F-10,10]2) contain three prevalent morphologies.12 Polymorphism in these anthracene monolayers arises from stacking17 adjacent 1-D tapes with different intertape side chain alignments. Replacing the diether side chains’ nonbenzylic ether oxygen by a CF2 group produces a single morphology in which intertape side chain alignment is identical to intratape side chain alignment. This gives a “corner-to-corner” morphology (C-2-C) in which anthracene cores from adjacent tapes contact each other at their H-3/H-4 corners (Scheme 2). The resulting anthracene columns alternate with alkyl columns consisting of ωT# aligned, antiparallel side chains. The anthracenes columns appear “taller” in constant current STM images. C-2-C morphology can arise by stacking either AA or AA′ tapes (Scheme 2). Anthracenes with {CF2/Ether} Repulsions in ωT2 Packed Tapes. A-[132,F-12,12]2 self-assembles C-2-C monolayers with parallel aligned anthracene cores in adjacent aryl columns (Figure 1a). Although the 3 × 2 dot tunneling pattern characteristic of anthracene is not evident in many molecules, the parallel edge-shapes of all the “high tunneling” (yellow) columns confirm parallel anthracene alignments and the monolayer’s assembly from AA 1-D tapes (e.g., Scheme 2, top). The dark, oval regions bordering the anthracene columns correspond to locations of the side chain 12-CF2 groups (CPK overlay, Figure 1a). CF2 units are less effective than CH2 units at promoting electron tunneling and appear dark in STM scans.18 The unit cell (green box) parameters (a (along aryl column) ) 1.04 nm, b ) 2.41 nm (between aryl columns), R ) 95°; Table S1) are consistent with ωT2 tape packing of 13-atom-long side chains.19 The dipolar repulsion generated by two pairs of adjacent,
antiparallel {CF2/ether} dipoles20 per pair of contacting side chains (Scheme 2, top) is not sufficient to disrupt ωT2 packing within or between the AA tapes of A-[132,F-12,12]2 monolayers on HOPG. Lengthening each side chain by one CH2 group yields the even length homologue A-[142,F-13,13]2. The STM data (Figure 1b) reveals parallel aligned anthracene cores in adjacent columns of this C-2-C monolayer in contrast to the perpendicular alignment of nearest anthracene columns expected for ωT2 packed even length side chains. The A-[142,F-13,13]2 monolayers are assembled from non-ωT2 packed, AA tapes. Dipolar repulsion generated by two pairs of adjacent, antiparallel {CF2/ ether} dipoles per pair of ωT2 packed side chains (e.g., as in Figure 1c) is sufficient to make the AA′ tape (monolayer) higher energy than the observed AA tape with parallel {CF2/ether} dipoles (Figure 1d). The unit cell parameters of the C-2-C, AA monolayer (a ) 1.02 nm, b ) 2.78 nm, R ) 98°) indicate assembly of ωT3 packed AA tapes.21 The interactions of {CF2/ether} dipole pairs produce the same tape packing results as observed previously for {ether/ether} dipole pairs: ωT2 tape packing of odd length side chains and ωT3 packing of even length side chains. Despite the larger CF2 dipole moment, the interactions of the {CF2/ether} pairs are insufficient to prevent ωT2 packing of odd length side chains. The larger physical size of the CF2 group, compared to an ether oxygen, does alter the lowest energy mode of tape stacking; giving ωT2 stacked, C-2-C morphologies from compounds with a CF2 group in each side chain compared to ωT8 stacked, head-to-head morphologies from the diether side chain compounds.10 Anthracenes with {CF2 /CF2} Repulsions in ωT2 Packed Tapes: A-[172,F-9,9]2. Molecules bearing a single dipolar group at the (ω + 1)/2 or at the (ω + 3)/2 position of odd length side chains generate “self-repulsive” side chain interactions in ωT2 packed tapes; the dipoles in contacting side chains are adjacent and aligned antiparallel. For 17-atom long side chains, 9- or 10-position dipoles satisfy this self-repulsion criterion. STM
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Figure 1. (a) Constant current STM image (11 × 11 nm) of the monolayer assembled on HOPG by A-[132,F-12,12]2 in phenyl octane at 15 °C. (0.1 nA, 1000 mV, 7.6 Hz). CPK models are superposed on molecules contributing to the same and to adjacent aryl columns. The dark ovals bordering the aryl columns correspond to positions of the 12-CF2 groups (yellow atoms in CPK models). (b) Constant current STM image (11 × 11 nm) of the monolayer assembled from A-[142,F-13,13]2 on HOPG at 16 °C (0.1 nA, 1000 mV, 7.6 Hz). CPK models are superposed on molecules within the same and in adjacent aryl columns. (c) CPK model of the ωT2 packed A-[132,F-12,12]2 monolayer. Red/yellow arrows superposed at the ether (upper) and CF2 (lower) dipoles exhibit these dipoles’ adjacent, antiparallel alignment20 in contacting chains. d) CPK model of ωT3 packed A-[142,F13,13]2 monolayer. Red/yellow arrows superposed at the CF2 (upper) and ether (lower) dipoles are parallel and two side chain positions out of registration.
Figure 2. (a) Constant current STM image (14 × 14 nm) of the monolayer assembled on HOPG by A-[172,F-9,9]2 in phenyl octane at 17 °C (0.1 nA, 1000 mV, 7.6 Hz). CPK models are superposed on molecules contributing to the same and to adjacent aryl columns. The dark “furrow” near the center of each alkyl lamellae corresponds to the placement of the 9-CF2 group (yellow atoms in CPK models). (b) CPK model of the ωT2 packed, C-2-C monolayer showing the CF2 groups’ antiparallel alignment, one position out of registration, within each alkyl lamella and AA tape. (c) CPK model of the ωT3 packed, C-2-C monolayer showing the CF2 groups’ parallel alignment, two positions out of registration, within each alkyl lamella and AA′ tape.
images of A-[172,F-9,9]2 monolayers exhibit nearly perpendicular alignments of anthracenes in adjacent aromatic columns (Figure 2a). The monolayer is comprised of AA′ tapes rather than the AA tapes expected for ωT2 packing of odd length side chains. The assembly of AA′ tapes and monolayers from A-[172,F-9,9]2 indicates that ωT1 or ωT3 tapes and monolayers are lower in energy than ωT2 tapes and monolayers. This is the first
compound for which repulsions between odd length, ωT2 packed side chains driVe assembly of AA′ tapes and monolayers. The ωT2 packed, AA tapes of A-[172,F-9,9]2 generate dipolar, and possibly steric, repulsions between antiparallel CF2 groups one position out of registration in adjacent side chains (Figure 2b). By contrast, both ωT1 and ωT3 packed A-[172,F-9,9]2 tapes establish attractive interactions between parallel CF2 dipoles in
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Figure 3. (a) Constant height STM image (10 × 10 nm) of the monolayer assembled on HOPG by A-[172,F-10,10]2 in phenyl octane at 16 °C (0.1 nA, 850 mV, 15 Hz). CPK models are superposed on molecules contributing to the same and to adjacent aryl columns. The dark, narrow “furrow” near the center of each alkyl lamellae corresponds to the 10-CF2 locations (yellow atoms in CPK models). (b) Constant current STM image (30 × 30 nm) of A-[172,F-10,10]2 exhibiting alternating tunneling patterns in neighboring aryl columns (0.1 nA, 1000 mV, 12.5 Hz). (c) CPK model of the ωT2 packed, C-2-C, monolayer showing the CF2 groups’ antiparallel alignment, one position out of registration, within each AA tape. (d) CPK model of the ωT3 packed, C-2-C monolayer showing the CF2 groups’ parallel, collinear alignment within each alkyl lamella and AA′ tape.
adjacent side chains; ωT1 packing yields collinear CF2 dipoles. ωT3 packing offsets the CF2 dipoles by two side chain positions (Figure 2c). For A-[172,F-9,9]2, dipolar repulsions (ωT2 packing) and attractions (ωT1 or 3 packing) of proximate CF2 groups shift the respective morphologies’ energies enough to make ωT2 packed, AA tapes (normally observed) higher in energy than non-ωT2 packed, AA′ tapes (not previously assembled by anthracenes with odd length side chains). The unit cell parameters of the A-[172,F-9,9]2 C-2-C monolayer (a ) 0.95 nm, b ) 5.54 nm, R ) 87°) are consistent with ωT3 tape formation. The AA′ tapes formed by ωT3 packed A[172,F-9,9]2 and by ωT2 packed A-[182]2 have identical registrations of side chain atoms, so the compounds’ C-2-C, AA′ monolayers should have comparable unit cell parameters. Although A-[182]2 has not been prepared, its unit cell parameters can be estimated from the C-2-C, AA′ monolayer assembled by A-[162]2 on HOPG (a ) 1.03 nm, b ) 5.29 nm, R ) 91°). As the unit cell b-axis of an AA′ monolayer is coincident with the side chains, the b-axis length of A-[182]2 should be ∼0.25 nm longer (i.e., two CH2 units) than of A-[162]2. This extrapolated b-axis length, 5.54 nm, matches the measured b-axis length of A-[172,F-9,9]2. The smaller a-axis length in the A-[172,F-9,9]2 unit cell may result from reduced steric repulsion between anthracene hydrogen atoms (2 and 3 positions) and the terminal methyl groups of adjacent side chains in the ωT3 packed monolayer (compare the terminal methyl/anthracene proximity in Scheme 2, bottom and Figure 2c). Finally, the A-[172,F-9,9]2 STM images exhibit a broad, somewhat zigzag pattern of dark “furrows” near the middle of each aliphatic lamella. The furrows correspond to the locations of the lower conductivity CF2 groups.18 The furrow width and zigzag shape are consistent with
ωT3 packed tapes, in which CF2 groups from adjacent side chains are two positions out of registration (Figure 2a,c, CPK overlay). Anthracenes with {CF2/CF2} Repulsions in ωT2 Packed Tapes: A-[172,F-10,10]2. In analogy to [172,F-9,9] side chains, ωT2 packed [172,F-10,10] side chains should produce dipolar repulsion between adjacent chains’ 10-position CF2 groups. Constant height STM scans of A-[172,F-10,10]2 monolayers adsorbed on HOPG exhibit nearly perpendicular anthracene orientations within adjacent aryl columns (Figure 3a).22 In larger scale images (Figure 3b), the perpendicular anthracene orientations of adjacent aryl columns generate distinctive, albeit poorly resolved, STM patterns. The alternating aryl column patterns show this C-2-C monolayer consists of non-ωT2 packed, AA′ tapes rather than the ωT2 packed, AA tapes usually assembled by odd length side chains.11 ωT2 packed, AA tapes of A-[172,F-10,10]2 generate dipolar repulsion between adjacent, antiparallel CF2 groups within contacting side chains (Figure 3c).23 By contrast, ωT3 packed A-[172,F-10,10]2 tapes establish attractive interactions between parallel, collinear CF2 dipoles in adjacent side chains (Figure 3d). The energy difference between these different {CF2/ CF2} dipolar interactions drives A-[172,F-10,10]2 to assemble C-2-C monolayers built from ωT3 packed, AA′ tapes. The unit cell parameters of the A-[172,F-10,10]2 C-2-C monolayer (a ) 0.96 nm, b ) 5.64 nm, R ) 91°) are in reasonable agreement with those determined for A-[172,F-9,9]2 and with the values expected for the A-[182]2 C-2-C AA′ monolayer (vide supra). As shown in Figure 3d, a ωT3 packed, C-2-C monolayer produces linearly aligned CF2 groups. This is the likely origin of the narrow, linear, dark “furrow” in the middle of each alkyl lamella (Figure 3a). The four dark “spots” bordering each anthracene lie next to the terminal methyl groups of the four
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Figure 4. (a) CPK model of a C-2-C monolayer section assembled from ωT2 packed, patterned AA (AB) tapes of A-[172,F-9,9]2 and A-[172,F-10,10]2. The purple line runs parallel to one ωT2 packed tape. “9” indicates an aryl column containing A-[172,F-9,9]2. “10” indicates an aryl column containing A-[172,F-10,10]2. (b) Constant current STM image (13 × 13 nm) of the monolayer assembled on HOPG by a 1:1 solution mixture of A-[172,F-9,9]2 and A-[172,F-10,10]2 in phenyl octane at 18 °C (0.12 nA, 850 mV, 7.6 Hz). The unit cell (green) contains two molecules. CPK models are not superimposed on the image as the aryl columns could not be assigned, unambiguously, to A-[172,F-9,9]2 or A-[172,F-10,10]2. (c) CF2 furrows are visible in a constant current STM image (15 × 7.5 nm) recorded (0.12 nA, 780 mV, 8.7 Hz) a few minutes prior to the image in Figure 4b. The furrow locations allow assignment of aryl column contents (9 or 10).
neighboring alkyl chains (Figure 3a, CPK overlay). Side chain ωT3 packing leaves vacancies adjacent to the chain 1- and 2-positions. Each vacancy lowers the local conductivity and yields a dark spot between the chain terminus and the anthracene. These dark spot are additional evidence supporting assembly of ωT3 packed AA′ tapes. It is worth noting that 10-position CF2 groups lie on the exteriors of ωT2 packed, AA tapes (Figure 3c). Antiparallel CF2 groups on opposite edges of the same ωT2 packed tape suffer no steric interactions (Figure 3c). Thus, the destabilization of A-[172,F-10,10]2 ωT2 packed, AA tapes must be dipolar, not steric, in origin. C-2-C stacking of ωT2 packed, AA tapes may produce contact, and possibly steric repulsion, between CF2 groups on adjacent tapes’ exterior edges (Figure 3c). If no alternate tape stacking alignments (e.g., ωT8) are possible for AA tapes of A[172,F-10,10]2, CF2-CF2 steric repulsions may play an indirect role in destabilizing ωT2 tape assembly. Monolayer Patterning with CF2 Groups: Mixed Monolayers of A-[172,F-9,9]2 and A-[172,F-10,10]2. Dipolar interactions between side chain CF2 groups induce assembly of C-2-C, ωT3 packed, AA′ tape monolayers from either A-[172,F-9,9]2 or A-[172,F-10,10]2. The nearly orthogonal anthracene alignments in adjacent aryl columns confirm the presence of AA′ tapes in these single component monolayers. The [172,F-9,9] and [172,F-10,10] side chains were designed to be dipolar complements;8a ωT2 packing of each side chain with its complement affords stabilizing, parallel, collinear alignments of their CF2 dipoles (Figure 4a) in addition to optimal side chain van der Waals interactions (i.e., ωT2 packing). Ideally, ωT2 packing of the complementary side chains of A-[172,F-9,9]2 and A-[172,F-10,10]2 should afford the highest stabilization and drive assembly of AA (AB)24 tapes and a patterned monolayer (Figure 4a). STM scans of the C-2-C monolayers self-assembled by 1:1 solution mixtures of A-[172,F-9,9]2 and A-[172,F-10,10]2 exhibit parallel aligned anthracenes in all aryl columns (Figure 4b). This monolayer must contain both compounds because monolayers assembled by the individual compounds exhibit AA′ domains. The observed morphology is consistent with self-
assembly of a patterned monolayer built from ωT2 packed, AB tapes24 whose composition alternates between A-[172,F-9,9]2 and A-[172,F-10,10]2 (Figure 4a). The anthracenes’ characteristic 3 × 2 dot patterns are evident in Figure 4b, but dark “furrows” attributable to the CF2 groups are not evident in this image. Dark “furrows” are evident in other STM images and facilitate assignment of specific aryl columns as A-[172,F-9,9]2 or A[172,F-10,10]2 based on alignments with CF2 groups in the CPK models (Figure 4c). The unit cell of the patterned monolayer contains one molecule of each compound. Its parameters (a ) 0.95 nm, b ) 5.48 nm, R ) 81°) are in reasonable agreement with expectations for an ωT2 packed, AB tape, C-2-C morphology.25 As no AA′ domains were observed in monolayers prepared from 1:1 solution mixtures of A-[172,F-9,9]2 and A[172,F-10,10]2, the formation free energy of the patterned monolayer must be significantly more favorable than the ωT3 packed AA′ monolayer formed by either pure component. CF2 Group Repulsions and Patterning with A-[152,F-8,8]2 and A-[152,F-9,9]2. A single CF2 group at the (ω + 1)/2 or the (ω + 3)/2 position of odd length, ω ) 15 side chains should generate “self-repulsive” side chain interactions in ωT2 packed tapes. Thus, ωT2 packing of either [152,F-8,8] or [152,F-9,9] side chains should generate repulsive dipolar interactions that destabilize a C-2-C, AA tape based monolayer. A switch to ωT3 packing of either side chain should generate attractive dipolar interactions between parallel aligned CF2 groups and stabilize a C-2-C, AA′ tape based monolayer. Additionally, the [152,F-8,8] and [152,F-9,9] side chains are dipolar complements; solution mixtures of A-[152,F-8,8]2 and A-[152,F-9,9]2 are expected to assemble patterned AB tape based, C-2-C monolayers. The monolayer assembled by A-[152,F-8,8]2 (Figure 5a) exhibits two distinct patterns for the anthracenes in neighboring aryl columns. “Bright”, 3 × 2 dot anthracene patterns appearing as horizontal rectangles are barely discernible in the aryl columns marked with asterisks. The “bright” features within the other aryl columns appear as narrow, diagonally aligned objects with little recognizable structure. The pattern alternation between adjacent aryl columns offers weak evidence in support of an AA′ tape
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Figure 5. (a) Constant current STM image (12 × 12 nm) of the ωT3 packed, C-2-C monolayer assembled on HOPG by A-[152,F-8,8]2 in phenyl octane at 21 °C. (0.12 nA, 780 mV, 8.7 Hz). The * marks aryl columns whose dot patterns resemble the typical anthracene core. (b) Constant current STM image (13 × 13 nm) of the monolayer assembled on HOPG by A-[152,F-9,9]2 in phenyl octane at 18 °C (0.1 nA, 800 mV, 10.2 Hz). (c) CPK model of the ωT3 packed, C-2-C monolayer of A-[152,F-9,9]2 showing the parallel, collinear alignment of CF2 groups within each alkyl lamella.
Figure 6. (a) Constant current STM image (12 × 12 nm) of the monolayer assembled on HOPG by a 1:1 solution mixture of A-[152,F-8,8]2 and A-[152,F-9,9]2 in phenyl octane at 18 °C (0.12 nA, 780 mV, 7.6 Hz). Dark “furrows” appearing near the middle of the aliphatic lamellae (upper left to lower right) correspond to locations of the side chain CF2 groups (yellow fluorine atoms in CPK model overlay). (b) CPK model of a C-2-C monolayer section assembled from ωT2 packed, patterned AB tapes of A-[152,F8,8]2 and A-[152,F-9,9]2. The green line runs parallel to one ωT2 packed, interior edge AB tape. The purple line demarcates the edge of adjacent exterior edge, AB tapes. An “8” indicates an aryl column containing A-[152,F-8,8]2. A “9” indicates an aryl column containing A-[152,F-9,9]2.
based monolayer. Dark “furrows” corresponding to CF2 groups are not evident in the STM images of A-[152,F-8,8]2. The aryl column patterns in monolayers assembled by A-[152,F-9,9]2 (Figure 5b) exhibit insufficient structure to discern an AA or AA′ composition of the underlying tapes. However, the A[152,F-9,9]2 monolayer does exhibit dark, linear “furrows” that can be attributed to collinear, parallel CF2 groups in an ωT3 packed monolayer (Figure 5c). The monolayers of A-[152,F-8,8]2 and of A-[152,F-9,9]2 were exceedingly difficult to image by STM. The domains disappeared rapidly during scanning, making it difficult to adjust collection parameters and optimize monolayer resolution.
Monolayers assembled from 1:1 solution mixtures of A[152,F-8,8]2 and A-[152,F-9,9]2 are robust, persistent and easily imaged. The STM scans exhibit parallel aligned anthracenes in all aryl columns (Figure 6a). This C-2-C monolayer must contain both A-[152,F-8,8]2 and A-[152,F-9,9]2 as the single component monolayers consist of AA′ tapes. The observed morphology is consistent with patterned self-assembly of a monolayer containing ωT2 packed, AB tapes. A dark “furrow” present in the middle of each aliphatic lamella marks the locations of side chain CF2 groups (Figure 6b). Additional dark, diagonal stripes (lower left f upper right) highlight the periphery of every second exterior-edge tape10b (purple line in Figure 6b) and
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TABLE 1: Self-Assembly Energies (SAE) for ωT2 and ωT3 Packed C-2-C Morphologiesa energy: ωT2 packed tapes; C-2-C stacking (kcal/mol/molecule; stabilizing < 0) AM1 charges molecule 2,F-12,12
]2 A-[13 A-[142,F-13,13]2 A-[132,12]2b A-[142,13]2b A-[152,F-8,8]2 A-[152,F-9,9]2 A-[152,F-8,8]2 + A-[152,F-9,9]2 A-[172,F-9,9]2 A-[172,F-10,10]2 A-[172,F-9,9]2 + A-[172,F-10,10]2
no AM1 charges
adiabatic SAE per molecule
strain
-17.32 -18.76 -14.75 -16.10 -19.08 -19.27 -23.30 -21.61 -21.76 -25.92
0.85 0.83 0.90 1.00 1.19 1.43 0.85 1.55 1.63 0.92
adiabatic SAE per molecule
strain
-19.83 -21.33 -16.83 -18.15
-23.71 -23.39 -24.40
energy: ωT3 packed tapes; C-2-C stacking (kcal/mol/molecule; stabilizing < 0) AM1 charges
no AM1 charges
adiabatic SAE per moleculec
strain
0.94 0.91 0.81 0.99
-18.88 (-1.56) -20.32 (-1.56) -16.56 (-1.81) -18.06 (-1.96) -21.10 (-2.02) -22.04 (-2.77)
1.50 1.31 0.93
-23.94 (-2.33) -24.85 (-3.09)
adiabatic SAE per moleculec
strain
0.66 0.66 0.93 0.90 1.02 0.90
-18.12 (1.71) -19.46 (1.87) -16.64 (0.19) -18.11 (0.04)
0.69 0.68 1.05 0.98
1.00 1.01
-23.31 (0.40) -23.09 (0.30)
0.99 1.12
a Results of molecular mechanics minimizations: see text. The SAE standard deviation (for the set of eight interior molecules) is less than 5% of the value reported for each compound and morphology. b This diether assembles an H-2-H morphology built from AA tapes (ωT2 for [132,12]; ωT1 for [142,13]). SAE calculations predict the H-2-H morphology is more stable than a C-2-C morphology built from ωT3 packed tapes. c Value in parentheses is ∆(SAE) ) SAE(ωT3) - SAE(ωT2) for each compound.
partially obscure the CF2 furrows. Measured distances between contacting anthracenes are the same with or without an intervening dark stripe, so the latter may be Mo´ire patterns involving the monolayer and HOPG repeats. The unit cell (green parallelogram, Figure 6a) based on interior edge tapes (green line, Figure 6b) contains one molecule of A-[152,F-8,8]2 and one of A-[152,F-9,9]2. The unit cell parameters (a ) 0.98 nm, b ) 5.49 nm, R ) 84°) are in reasonable agreement with the results of molecular mechanics simulation (a ) 0.96 nm, b ) 5.36 nm, R ) 82°). The frail, AA′ tape, C-2-C monolayers assembled by A-[152,F-8,8]2 (and tentatively by A-[152,F-9,9]2) and the robust, “AA” tape, C-2-C monolayers assembled from solution mixtures of A-[152,F-8,8]2 and A-[152,F-9,9]2 constitute reasonable evidence supporting patterned assembly of two component, AB tapes and monolayers by the mixture. Monolayer patterning is driven by dipole-dipole self-repulsions and complement attractions employing the CF2 group. Molecular Mechanics Studies of Self Assembly Energetics. Molecular mechanics26 (MM) minimizations (Hyperchem 8.0)27 were performed on monolayer sections consisting of four AAtapes or AA′-tapes truncated to six anthracenes (Figures 4a and 6b show complete monolayer sections used in simulations). The HOPG surface was modeled as a single layer, graphene sheet 10-20% wider and longer than the monolayer section. Partial charges on the anthracene and side chain atoms were assigned using Mulliken population analysis of AM128 minimized structures. Tapes were stacked and aligned on the graphene sheet in ωT2 or ωT3 packed, C-2-C morphologies. Minimizations were run until the MM energy of the monolayer section plus graphene decreased less than 10-5 kcal/mol in 24 h.29 The graphene sheet was then deleted, the monolayer section saved and its energy determined by a single point MM calculation. The self-assembly energy per molecule for each compound and morphology was calculated in two parts. First, a single molecule from an interior position of the monolayer section was shifted so that it no longer interacted with other molecules in the section. A single point MM energy calculation was performed for this “shifted” assembly. Subtracting the intact monolayer section’s energy from the “shifted” assembly energy affords the “vertical” extraction energy of the shifted anthracene. Next, the strain energy of the shifted molecule was determined as the shifted molecule’s MM energy in the monolayer minimized geometry
minus its MM energy after minimization in isolation. The shifted molecule’s “adiabatic” self-assembly energy was calculated as its strain energy minus half its “vertical” extraction energy.30 Table 1 reports this adiabatic self-assembly energy per molecule (SAE) averaged over the eight interior molecules of each minimized monolayer section. The adiabatic self-assembly energy represents the per molecule heat of formation of a gas phase monolayer with the specified morphology from isolated gas phase molecules. Interactions of the monolayer with HOPG and the solvent are not included in the SAE. The SAE calculations for each pure compound’s monolayer predict the ωT3 packed, C-2-C morphology is more stable than its ωT2 packed, C-2-C morphology (Table 1, columns 2 and 6).31 The predictions agree, qualitatively, with the STM results for five of the six compounds with CF2 groups. The one counter case, A-[132,F-12,12]2, assembles ωT2 packed, C-2-C monolayers. Evaluating SAE while neglecting monolayer interactions with the graphite and solvent could yield incorrect morphology predictions. However, a previous study of polymorphism12 in monolayers of di- and triether side chain anthracenes found that simple MM calculations correctly predict the STM observed morphology when the SAE differences among the morphologies exceeded 0.5 kcal/mol/molecule. For the CF2 bearing anthracenes in Table 1, the SAE differences exceed 1.0 kcal/mol/ molecule. The smallest magnitude ∆(SAE) values are for A-[132,F-12,12]2 and A-[142,F-13,13]2. It is possible that the AM1 derived charge distributions for the CF2 group overestimate {CF2/ether} and {CF2/CF2} repulsions, attractions and, thus, the relative stability, ∆(SAE), of ωT3 and ωT2 packed morphologies. To assess the magnitude of dipole-dipole contributions to the calculations, SAE for the MM minimized geometries were re-evaluated after setting all atomic charges to zero (Table 1, columns 4 and 8). Removing the dipolar terms increases the stabilities of single component, ωT2 packed, C-2-C monolayers (SAE values more negative by -1.6 to -2.6 kcal/mol), presumably due to neglect of dipolar repulsions. Conversely, removing the dipolar terms decreases the stabilities of ωT3 packed, C-2-C monolayers for the CF2 bearing compounds (SAE values more positive by 0.6-1.8 kcal/ mol), most likely due to neglect of attractive dipolar interactions. Without dipolar terms, A-[132,12]2, A-[142,13]2, A-[172,F-9,9]2, and A-[172,F-10,10]2 generate small (e0.4 kcal/mol/molecule) SAE preferences for ωT2 packed, C-2-C monolayers, whereas
Morphology and Patterning with CF2 Groups A-[132,F-12,12]2 and A-[142,F-13,13]2 exhibit large (1.7-1.9 kcal/ mol/molecule) SAE preferences for ωT2 packed, C-2-C monolayers. If the relevant dipolar terms turned out to be only half as large as the calculated AM1/MM dipolar terms, they would be more than sufficient to drive assembly of ωT3 packed monolayers by the first four compounds. The same cannot be said for A-[132,F-12,12]2 and A-[142,F-13,13]2; generating an SAE preference for ωT3 packed monolayers from these two compounds requires much larger dipolar terms to overcome a larger bias toward ωT2 packed monolayers in the absence of dipolar contributions. Qualitatively correct morphology predictions for the latter two compounds requires more quantitative accuracy in the dipole-dipole terms. The SAE calculations also predict that patterned, ωT2 packed, C-2-C monolayers assembled from dipolar complements (A-[172,F-9,9]2/A-[172,F-10,10]2 or A-[152,F-8,8]2/A-[152,F-9,9]2) are 1-2 kcal/mol/molecule more stable than ωT3 packed single component monolayers and ∼4 kcal/mol/molecule more stable than the ωT2 packed single component monolayers. The simultaneous realization of optimal van der Waals and dipolar alignments provides strong driving force for the assembly of patterned monolayers from the gem-difluoride bearing dipolar complementary side chains. Conclusion Long alkyl chains promote self-assembly of monolayers on planar surfaces such as graphite. Interactions of alkyl chains with the graphite and with adjacent, closely packed alkyl chains contribute to the driving force for monolayer assembly. Replacing a methylene group (CH2) by a gem-difluoride group (CF2) confers a large dipole moment (>2 D) to an alkyl chain, with minimal distortion of chain shape and geometry. The interactions of CF2 and other strongly dipolar groups within self-assembled monolayers add significant dipole-dipole terms to the assembly energetics. Dipole placement within each molecule and molecule packing within the monolayer (morphology) determine whether dipole-dipole interactions stabilize or inhibit monolayer formation. Proper placement of CF2 groups within the side chains of 1,5-bis(side chain substituted)anthracene derivatives is an effective means of directing the packing, morphology, and patterning of self-assembled monolayers on graphite. In the absence of dipolar interactions between contacting side chains, these anthracene compounds assemble ωT2 packed, C-2-C morphology monolayers. Introducing two pairs of {CF2/ ether} repulsions per pair of ωT2 packed side chains destabilizes this standard morphology. If these side chains have even lengths, the dipolar destabilization of the ωT2 packed molecules is sufficient to drive formation of ωT3 packed, C-2-C monolayers. The destabilization generated by {CF2/ether} repulsions is not sufficient to override the larger driving force for assembly of ωT2 packed morphologies by anthracenes with odd length side chains. By contrast, arranging a single {CF2/CF2} dipolar repulsion per pair of ωT2 packed, odd length side chains drives assembly of ωT3 packed, C-2-C morphologies. The dipolar destabilization arising from antiparallel CF2 groups one position out of registration in ωT2 packed tapes, combined with dipolar stabilization of parallel, collinear CF2 groups in ωT3 packed side tapes, drives assembly of the ωT3, AA′ tape, C-2-C morphology. The four compounds with a single pair of interacting {CF2/CF2} dipoles between neighboring alkyl chains (A-[172,F-9,9]2, A-[172,F-10,10]2, A-[152,F-8,8]2, A-[152,F-9,9]2) are the first systems that overcome the highly stable, ωT2 packing of anthracenes with odd length side chains. The {CF2/CF2} dipolar interaction is effective for generating self-repulsive side chains and molecules.
J. Phys. Chem. C, Vol. 114, No. 48, 2010 20791 The {CF2/CF2} dipolar interaction is also an effective tool for monolayer patterning. Solution mixtures (1:1) of dipolar complementary pairs of self-repulsive {CF2/CF2} anthracenes assemble ωT2 packed, 1-D tapes and monolayers. The CF2 group placements in the complementary side chains produces parallel, collinear alignments of their CF2 dipoles when ωT2 packed. The patterned monolayer simultaneously optimizes the dipolar and van der Waals interactions driving monolayer assembly. The selectivity of each self-repulsive molecules for its dipolar complement is so large that no monolayer regions have been found containing single component domains from the (A-[172,F-9,9]2/A-[172,F-10,10]2) complementary pair or from the (A-[152,F-8,8]2/A-[152,F-9,9]2) complementary pair. Molecular mechanics studies of monolayer sections on graphene sheets indicate that {CF2/CF2} dipolar interactions provide the critical driving force required to generate the self-repulsive and dipolar complementary properties of (A-[172,F-9,9]2/A-[172,F-10,10]2) and (A-[152,F-8,8]2/A-[152,F-9,9]2). Similar calculations performed to probe {CF2/ether} dipolar interactions correctly predict the assembly of ωT3 packed monolayers from A-[142,F-13,13]2 but incorrectly predict ωT3 packed monolayers from A-[132,F-12,12]2. The origin of the incorrect prediction in the latter case is not known. Acknowledgment. The authors thank the National Science Foundation (CHE0616474) for financial support of this work and Dr. Vlastimil Fidler for advice and assistance. Supporting Information Available: Synthetic methods and spectral data for all new compounds. Table of 2D unit cell parameters of the compounds’ morphologies determined by tunneling microscopy experiments and from molecular mechanics simulations. Larger scale STM scans of the monolayer assembled by each compound. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Zhang, J.; Tang, Y.; Weng, L.; Ouyang, M. Nano Lett. 2009, 12, 4061–65. (b) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493–497. (c) Li, Y.; Zhang, Q.; Nurmikko, A. V.; Sun, S. Nano Lett. 2005, 5, 1689– 92. (d) Park, S.-G.; Lee, S. Y.; Jang, S. G.; Yang, S.-M. Langmuir 2010, 26, 5295–99. (2) (a) Matsen, M. W.; Bates, F. S. Macromolecules 1996, 29, 7641– 44. (b) Mansky, P.; Harrison, C. K.; Chaikin, P. M.; Register, R. A.; Yao, N. Appl. Phys. Lett. 1996, 68, 2586–88. (c) Mansky, P.; Chaikin, P.; Thomas, E. L. J. Mater. Sci. 1995, 30, 1987–92. (3) Wolfe, D. B.; Snead, A.; Mao, C.; Bowden, N. B.; Whitesides, G. M. Langmuir 2003, 19, 2206–14. (4) (a) Rothemund, P. W. K. Nature 2006, 440, 297–302. (b) Chhabra, R.; Sharma, J.; Ke, Y.; Liu, Y.; Rinker, S.; Lindsay, S.; Yan, H. J. Am. Chem. Soc. 2007, 129, 10304–05. (c) Aldaye, F. A.; Palmer, A. L.; Sleiman, H. F. Science 2008, 321, 1795–99. (5) (a) De Feyter, S.; De Schryver, F. C. J. Phys. Chem. B 2005, 109, 4290–4302. (b) Ahn, S.; Morrison, C. N.; Matzger, A. J. J. Am. Chem. Soc. 2009, 131, 7946–47. (c) De Feyter, S.; De Schryver, F. C. Top. Curr. Chem. 2005, 258, 205–55. (d) Wasserfallen, D.; Fischbach, I.; Chebotareva, N.; Kastler, M.; Pisula, W.; Ja¨ckel, F.; Watson, M. D.; Schnell, I.; Rabe, J. P.; Spiess, H. W.; Mu¨llen, K. AdV. Funct. Mater. 2005, 15, 1585–94. (6) (a) Nath, K. G.; Ivasenko, O.; Miwa, J. A.; Dang, H.; Wuest, J. D.; Nanci, A.; Perepichka, D. F.; Rosei, F. J. Am. Chem. Soc. 2006, 128, 4212– 13. (b) Surin, M.; Samorı`, P.; Jouaiti, A.; Kyritsakas, N.; Hosseini, M. W. Angew. Chem., Int. Ed. 2007, 46, 245–49. (c) Li, S.-S.; Northrup, B. H.; Yuan, Q.-H.; Wan, L.-J.; Stang, P. J. Acc. Chem. Res. 2009, 42, 249–59. (d) 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, 52031. (e) De Feyter, S.; Larrson, M.; Schuurmans, N.; Verkuijl, B.; Zoriniants, G.; Gesquie`re, A.; Abdel-Mottaleb, M. M.; Feringa, B. L.; van Stam, J.; De Schryver, F. D. Chem.sEur. J. 2003, 9, 1198–1206. (f) Llanes-Pallas, A.; Palma, C.-A.; Piot, L.; Belbakra, A.; Listorti, A.; Prato, M.; Samori, P.; Armaroli, N.; Bonifazi, D. J. Am. Chem. Soc. 2009, 131, 509–20. (g) Scudiero, L.; Hipps, K. W.; Barlow, D. E. J. Phys. Chem. B 2003, 107, 2903–09. (h) Lei, S.; Wang, C.; Wan,
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L.; Bai, C. J. Phys. Chem. B 2004, 108, 1173–75. (i) Lu, J.; Lei, S. B.; Zeng, Q. D.; Kang, S. Z.; Wang, C.; Wan, L. J.; Bai, C. L. J. Phys. Chem. B 2004, 108, 5161–65. (j) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029–31. (k) De Feyter, S.; Miura, A.; Yao, S.; Chen, Z.; Wu¨rthner, F.; Jonkheijm, P.; Schenning, A. P. H. J.; Meijer, E. W.; De Schryver, F. C. Nano Lett. 2005, 5, 77–81. (l) Nath, K. G.; Ivasenko, O.; MacLeod, J. M.; Miwa, J. A.; Wuest, J. D.; Nanci, A.; Perepichka, D. F.; Rosei, F. J. Phys. Chem. C 2007, 111, 16996– 17007. (m) Zhao, M.; Deng, K.; Jiang, P.; Xie, S.-S.; Fichou, D.; Jiang, C. J. Phys. Chem. C 2010, 114, 1646–1650. (n) Tao, F.; Bernasek, S. L. Surf. Sci. 2007, 601, 2284–90. (o) Zell, P.; Mo¨gele, F.; Ziener, U.; Rieger, B. Chem.sEur. J. 2006, 12, 3847–57. (p) Plass, K. E.; Engle, K. M.; Cychosz, K. A.; Matzger, A. J. Nano Lett. 2006, 6, 1178–83. (7) (a) LaBean, T. H.; Yan, H.; Kopatsch, J.; Liu, F.; Winfree, E.; Reif, J. H.; Seeman, N. C. J. Am. Chem. Soc. 2000, 122, 1848–60. (b) Le, J. D.; Pinto, Y.; Seeman, N. C.; Musier-Forsyth, K.; Taton, T. A.; Kiehl, R. A. Nano Lett. 2004, 4, 2343–47. (c) Liu, D.; Wang, M.; Deng, Z.; Walulu, R.; Mao, C. J. Am. Chem. Soc. 2004, 126, 2324–25. (8) (a) Wei, Y.; Tong, W.; Zimmt, M. B. J. Am. Chem. Soc. 2008, 130, 3399–405. (b) Wei, Y.; Tong, W.; Wise, C.; Wei, X.; Armbrust, K.; Zimmt, M. B. J. Am. Chem. Soc. 2006, 128, 13362–63. (9) A and A′ refer to anthracenes adsorbed via opposite enantiotopic faces. See the Experimental Section and Chart 1 for explanations of the shorthand used to describe side chain structure. (10) (a) Zell, P.; Mo¨gele, F.; Ziener, U.; Rieger, B. Chem.sEur. J. 2006, 12, 3847–57. (b) Tong, W.; Wei, Y.; Armbrust, K.; Zimmt, M. B. Langmuir 2009, 25, 2913–23. (11) Wei, Y.; Kannappan, K.; Flynn, G. W.; Zimmt, M. B. J. Am. Chem. Soc. 2004, 126, 5318–22. (12) Tong, W.; Wei, X.; Zimmt, M. B. J. Phys. Chem. C 2009, 113, 17104–13. (13) Wenjun, T. Ph.D. Thesis, Brown University, Providence, R.I., 2009. (14) Spurr, R. A.; Zeitlin, H. J. Am. Chem. Soc. 1950, 72, 4832. (15) (a) Molecular mechanics minimizations using AM1 atom charges estimate the attractive interaction between parallel, in registration ether dipoles as-0.9 kcal/mol and the repulsive interaction between anti-parallel ether dipoles one position out of registration as +2 kcal/mol. AM1 estimates are roughly half as large. (b) Supporting information reference 10b. (16) A-[142,F-13,13]2 monolayers contain rare defects in which two adjacent tapes are ωT8 stacked (head-to-head, H-2-H). (17) In this manuscript, “packing” refers to the assembly of 1-D tapes from individual molecules. “Stacking” refers to the assembly of 2-D domains from 1-D tapes. (18) (a) Gesquie`re, A.; Abdel-Mottaleb, M. M.; De Schryver, F. C.; Sieffert, M.; Mu¨llen, K. Langmuir 1999, 15, 6821–24. (b) Gesquie`re, A.;
Tong et al. Abdel-Mottaleb, M. M.; De Feyter, S.; De Schryver, F. C.; Sieffert, M.; Mu¨llen, K.; Calderone, A.; Lazzaroni, R.; Bre´das, J.-L. Chem.sEur. J. 2000, 6, 3739–46. (19) The measured unit cell lengths for the C-2-C monolayers of A-[132]2 and A-[132,7]2 are 2.2910b and 2.28 nm, respectively. (20) The phrase “adjacent, anti-parallel dipoles” describes dipolar groups in contacting side chains that are one side chain position out of registration. The dipole moments are anti-parallel and offset by ∼0.12 Å along the mean side chain direction. (21) The MM+ mininimized unit cell parameters for ωT3 packed [142,F-13,13] a ) 0.94 nm, b ) 2.70, R ) 79°. ωT3 packing of [142,F-13,13] side chains should yield similar unit cell length along b as from ωT2 packing of [152,X] side chains. The measured (calculated) b cell lengths for C-2-C stacked, [152,X] AA tapes are 2.53 nm (2.67 nm)12 for A-[152,8]2, 2.56 nm (2.66 nm)12 for A-[152,9]2 and 2.63 nm for A-[152]2.10b (22) Constant current STM scans of A-[172,F-10,10]2 monolayers did not yield images with sub-molecular resolution. (23) The side chain 10-position lies on the exterior edge of ωT2 packed, AA tapes. Thus, physical overlap between 10-position CF2 groups do not generate intra-tape steric repulsions for ωT2 packed, AA tapes of A-[172,F-10,10]2. Steric repulsion might arise between CF2 groups of adjacent, ωT2 packed, AA tapes. (24) A tape whose composition alternates between two molecules (e.g., A-[172,F-9,9]2 and A-[172,F-10,10]2) that adsorb via the same enantiopic face is referred to as an AB tape. (25) A-[172,9]2 asssembles an ωT2 packed, AA tape, C-2-C monolayer. The parameters of a “two molecule” unit cell from this monolayer (analogous to a two molecule, A-[172, F-9,9]2/A-[172, F-10,10]2 AB unit cell) are (a ) 0.93 nm, b ) 5.54 nm, R ) 85° ). (26) Burkert, U., Allinger, N. L., Eds. Molecular Mechanics; ACS Monograph, No. 177, 1982. (27) Hyperchem 8.0, CyberChem, Inc., 1115 NW 4th Street, Suite 2, Gainesville, FL 32601. (28) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902–09. (29) RMS gradient limit of 10-3 kcal/mol · Å was not achieved for any monolayer minimization. The minimizations required 8-15 days depending on starting position and side chain length. (30) The factor of one half partitions the “vertical” extraction energy equally between the shifted molecule and its neighbors. (31) SAE calculations for 1,5-disubstituted anthracenes with monoether or diether side chains, with one dipole-dipole repulsion per side chain pair, predict the ωT2 packed, C-2-C morphology lies at lower energy than the ωT3 packed C-2-C morphology.13
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