Dipolar Control of Monolayer Morphology on Graphite: Self-Assembly

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Dipolar Control of Monolayer Morphology on Graphite: Self-Assembly of Anthracenes with Odd Length Diether Side Chains Wenjun Tong, Xiaoliang Wei, and Matthew B. Zimmt* Department of Chemistry, Brown UniVersity, ProVidence, Rhode Island 02912 ReceiVed: June 14, 2009; ReVised Manuscript ReceiVed: July 28, 2009

The monolayers self-assembled at the solution-graphite interface by 18 1,5-bis(2,X-dioxaalkyl) anthracene derivatives bearing odd length side chains are studied using scanning tunneling microscopy and molecular mechanics (MM) simulations. 1-D molecular tapes are the key and common building block of the many morphologies self-assembled by these compounds. The dominant monolayer morphology and the extent of monolayer polymorphism depend critically on the position of the X-ether group and side chain length. MM simulations of the compounds’ different morphologies are useful tools with which to dissect, explain, and predict morphological variation and polymorphism as a function of side chain structure. The simulations reveal that the dependence on X-ether group position arises from dipolar repulsions between ether groups in adjacent tapes, dipolar attractions between ether and anthracene C-H groups in adjacent tapes, and geometric distortions of the side chains. Introduction One of many interesting challenges in self-assembly is developing methods for controlled generation of patterned structures.1 Patterned structures may function as prepared2 or may serve as templates for elaboration.3 As part of a program directed toward development of self-patterning monolayers and their use as high-resolution templates, the morphology of some 1,5-bis-(aliphatic ether)-anthracene monolayers on graphite (HOPG) were characterized by STM.4,5 Additional studies were performed on anthracenes bearing linear diether side chains containing odd numbers of heavy atoms. The positions of the ether oxygen atoms were chosen to generate repulsive dipole interactions between one or two pairs of ether groups in adjacent physisorbed molecules.4 These initial studies revealed that side chains with slight differences in ether group position or length produced a variety of morphologies and vastly different extents of polymorphism.6 For templating applications of patterned monolayers, uncontrolled polymorphism represents a serious complication. Moreover, none of the initially studied compounds with odd length side chains showed the morphological response to inter-side-chain dipole repulsions7 that has been used to promote patterned monolayer assembly.8 In an effort to (i) understand the origins of the morphology changes, (ii) identify side chains that control morphology and polymorphism, and (iii) determine whether odd length self-repulsive side chains afford patterning competent molecules, a series of 18 compounds was prepared and their self-assembled monolayers on HOPG characterized by STM. The complete set of compounds evinced a number of empirical trends relating side chain structure to morphology and polymorphism. The origins of these trends were probed using molecular mechanics (MM) simulations. Molecular dynamics and MM minimization have been used to investigate monolayer energetics and polymorphism,9,10 mobility and phase transitions,11 and the influence of substrate, multilayers, and solvent on morphologies assembled by alkanes12 and other compounds.10 * To whom correspondence should be addressed. E-mail: e-mail: [email protected].

In most simulations, adsorbate-adsorbate interactions are treated using classical force fields augmented by charges from semiempirical or ab initio calculations. Adsorbate-substrate interactions are modeled using Steele potentials,13 either with or without image charges.10 Recent studies also explore different models to investigate solvent’s effects on monolayer structure.10c While inclusion of substrate and solvent provides more realistic modeling, informative conclusions have been drawn from calculations using only adsorbate-adsorbate interactions.9 The latter approach was used with these compounds to probe how intermolecular contacts vary with morphology and whether different side chain structures promote or hinder specific interactions. The simulations confirm 1D-tapes as the monolayer building blocks, reveal the presence of “bulge”-“notch” intertape alignments in all morphologies and point to important electrostatic interactions, including stabilization derived from aryl C-H · · · O contacts and destabilization arising from intertape dipolar repulsions. Remarkably, self-assembly energies derived from the simulations correctly predict the dominant morphology for 11 of 12 compounds that generate very different selfassembly energies in different morphologies. Moreover, for four of the seven compounds exhibiting significant polymorphism, the simulations predict similar self-assembly energies for the relevant morphologies. Although the simulations’ dominant morphology predictions are less accurate for these latter compounds, the simulations’ overall predictive relevance is noteworthy given their simplicity. The simulation insights combined with the experimental trends provides strategies for controlling morphology and eliminating polymorphism. Experimental Methods Scanning tunneling microscopy data were acquired using a Digital Instruments NanoScope STM interfaced to a Digital Instruments NanoScope IIIa controller. All data were 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

10.1021/jp905563q CCC: $40.75  2009 American Chemical Society Published on Web 09/09/2009

Dipolar Control of Monolayer Morphology on Graphite CHART 1

compound in 250 µL of phenyl octane (Aldrich, 98%) at 22 °C. Solutions were diluted (adding 0-250 µL of 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 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 minimized, data were 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. Four to 10 samples of each compound were prepared and imaged. Between 400 and 1000 tape-tape contacts were analyzed to characterize the morphology distributions of each compound. Thermal drift could not be completely eliminated during data acquisition. Small or moderate thermal drift distortions do not affect morphology evaluation. To determine the unit cell of each morphology, thermal drift effects were removed by transforming consecutive pairs of up and down scans using a program that solves for the x- and y-thermal drift velocities that minimize differences in the two scans’ unit cell parameters. This transformation is valid if thermal drift velocities are constant in consecutive scans. Reported unit cell parameters (available in Supporting Information) are averages of thermal drift corrected STM data from two or more independently prepared and scanned 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 compounds14 discussed in this manuscript are presented in Chart 1 along with the side chain abbreviations and a cartoon illustrating (ωT2)-intratape packing and (ωT8)intertape packing. The side chain abbreviations use a nonsuperscript number to specify chain length (number of carbon and oxygen atoms). The superscript numbers specify the ether oxygen side chain positions, relative to the anthracene core. The syntheses of and spectral data from all new compounds are provided in the Supporting Information.

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17105 Results and Discussion 1-D tapes (Figure 1) are the key building block of all monolayer morphologies self-assembled by the anthracenes in Chart 1. Within each tape, the side chains of adjacent molecules lie in registration and are ωT2 packed, i.e., a group (CH2 or O) at the nth position of one side chain lies in registration with the group at the (ω + 2 - n)th position of the contacting side chain. For odd length side chains, ωT2 packing produces tapes in which each molecule adsorbs to graphite using the same prochiral face (AA tape). Molecules in adjacent tapes may adsorb via this same prochiral face or via the opposite prochiral face (i.e., A*A* tapes alternating with AA tapes). Groups at odd side chain positions (benzylic CH2 ) 1-position) constitute the interior of odd-core (oc) AA tapes. Groups at even side chain positions define much of the oc-tape periphery. The purple lines running parallel to the 1-D tape axis in Figure 1 help identify (i) portions of tape molecules that extend farthest from the tape center (bulges) and (ii) vacant regions within the tape that lie close to the tape periphery (notches). A tape alignment that fits all bulges on each tape’s periphery into notches of adjacent tapes affords closer tape packing and produces larger intertape van der Waals stabilization. A morphology assembled with this tape alignment should incur greater cohesive energy and appear with increased probability in the self-assembled monolayers.15 Substituting an oxygen atom for a side chain CH2 group perturbs both the shape and the electrostatic properties of the molecule: (i) the smaller van der Waals volume of an ether oxygen, relative to a CH2 group, introduces a “notch” in the side chain van der Waals surface; (ii) the ether oxygen bends the side chain, producing a preferred Cβ-O-Cβ′ angle of 172-174° compared to a Cβ-C-Cβ′ angle of 180° for a poly(methylene) side chain; (iii) the larger electronegativity of oxygen localizes negative charge on the ether oxygen and spreads smaller, positive charges onto the four R-hydrogen atoms. These molecular perturbations alter the structures of the 1-D tapes, in both generic and position specific ways, and modify the energetics of 1-D tape stacking in the various morphologies. Generically, shorter C-O bond lengths reduce registration between adjacent side chains. Optimal side chain packing is likely to increase the Cβ-O-Cβ angle, with a concomitant increase of angle strain. The parity of a side chain ether’s position (i.e., odd vs even) modulates the conversion of “perturbed” molecular properties into altered tape structure. An even-position ether oxygen lies on the periphery of an oc-AA tape. It bends the chain’s ends toward the tape periphery, which augments the bulge at the chain ω - 1 position. Both the oxygen “notch” and the negatively charged oxygen lie on the tape periphery where they may interact directly with an adjacent tape. By contrast, an odd-position ether oxygen lies in the interior of an oc-tape. The oxygen “notch” sits inside the tape and cannot accommodate adjacent tape bulges. The negatively charged oxygen also lies inside the tape. Only the smaller, distributed R-H positive charges lie along the tape periphery. An oddposition oxygen bends the chain’s ends toward the tape interior, thus reducing the “bulge” at the chain ω - 1 position and slightly widening the tape near the oxygen. Parity effects on tape structure are not constant; they vary with the ether’s location along the side chain. Thus, it is difficult to predict how side chain ether position will alter tape structure and the energetics of tape packing into various monolayer morphologies. Monolayer Morphologies Assembled by AA Tapes. The corner-to-corner (C-2-C) morphology appears where contacting side chains from adjacent oc-AA tapes pack using the same

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Figure 1. Portion of a ωT2, odd-core AA tape extracted from a MM simulation of an A-[152]2 monolayer section on HOPG. Each chain’s terminal (ω-position) carbon lies in registration with the adjacent molecule’s 2-position oxygen (yellow line). All tape molecules adsorb via the same prochiral face (A). The inset (upper right) shows a molecule oriented to adsorb via the opposite prochiral face (A*). The purple lines are parallel to the tape axis (unit cell b-axis) and help define bulges (red arrows, anthracene 3-H and side chain ω - 1 CH2) and notches (green arrows, side chain 2-oxygen and ω-methyl-anthracene gap).

Figure 2. C-2-C morphology self-assembled by A-[192,11]2 on HOPG. (A) CPK model (11.3 nm × 5.65 nm) of a MM minimized monolayer section (vide infra). The molecules within one oc-AA tape are highlighted in green. The red vertical line (left) highlights the ωT2 side chain alignment within and between tapes. A C-2-C unit cell is marked with a white (a-axis) and yellow (b-axis) parallelogram. (B) Constant current STM image (11.0 nm × 5.5 nm) of A-[192,11]2 from within a C-2-C domain assembled from phenyl octane solution (0.85 V, 0.2 nA). Each anthracene appears as a 3 × 2 dot pattern (yellow). A C-2-C unit cell is marked in cyan.

ωT2 registration found within each tape. ωT2 tape packing aligns the anthracene 3-H bulges from one tape into the ω-methyl-anthracene notches of the adjacent tapes. The side chains extend nearly perpendicular to the C-2-C unit cell a-axis, which runs parallel to the inversion centers of contacting anthracenes in adjacent tapes. Adjacent tape anthracenes’ make contact via their 3- and 4-position hydrogen atoms (corners). The head-to-head (H-2-H) morphology appears where contacting side chains from adjacent oc-AA tapes are ωT8 packed. Each oc-AA tape retains ωT2 packing. However, the anthracene 3-H bulges from each ωT2 tape extend into side chain 2-O notches in the adjacent tapes. This places the ω-position of each chain in registration with the 8-position of the adjacent tape. Anthracenes in adjacent tapes make contacts via their 2- and 3-position hydrogen atoms (head). The side chains extend nearly parallel to the H-2-H unit cell b-axis and form a 130° angle with the unit cell a-axis. Compared to C-2-C packing of the oc-AA tapes,

Figure 3. H-2-H morphology self-assembled by A-[152,8]2 on HOPG. (A) CPK model (10. nm × 5.0 nm) of a MM minimized monolayer section (vide infra). The molecules within adjacent oc-AA tapes exhibit ωT8 registration, which aligns each anthracene 3-H bulge into a side chain 2-O notch of the adjacent tape. An H-2-H unit cell is marked in yellow. (B) Constant current STM image (11.0 nm × 5.5 nm) from within an H-2-H domain assembled from phenyl octane solution (0.65 V, 0.2 nA). The 3 × 2 dot patterns (yellow) of most anthracenes are visible. The H-2-H unit cell is marked in cyan.

H-2-H packing generates four fewer CH2-CH2 intertape van der Waals contacts per pair of side chains. H-2-H packing is stabilized by electrostatic attractions involving the δ+ anthracene 3-H and the δ- 2-O atoms comprising the bulge - notch fit (vide infra). Comparable electrostatic stabilization is not generated by C-2-C packing. The C-2-C and H-2-H morphologies arise by packing adjacent oc-AA tapes with different registrations. C-2-C tape packing along one edge of a tape does not enforce C-2-C tape packing along the other edge. Many of the compounds in Chart 1 selfassemble domains in which strips of two to six H-2-H packed tapes alternate with two to six C-2-C packed tapes. These morphologies, which vary from predominantly H-2-H packed (Figure 4) to predominantly C-2-C packed, are classified as “mixed”. The number of adjacent H-2-H or C-2-C packed tapes per strip varies within each mixed domain. Herringbone (HB) morphologies arise where adjacent tapes adsorb to the graphite using opposite prochiral faces (AA tapes

Dipolar Control of Monolayer Morphology on Graphite

Figure 4. Constant current, STM image (1.0 V, 0.1 nA) of the “mixed” morphology self-assembled on HOPG (25 nm × 25 nm) by A-[192,8]2 in phenyloctane. The cyan bar indicates the a-axis direction for H-2-H packed oc-AA tapes. The black bar indicates the a-axis direction for C-2-C packed oc-AA tapes. The number of contiguous C-2-C and H-2-H packed tapes within each strip vary throughout the image.

Figure 5. oc-AA tape HB morphology self-assembled by A-[132,8]2 on HOPG. (A) CPK model (10. nm × 5.0 nm) of a MM minimized monolayer section (vide infra). The unit cell is marked in yellow. (B) Constant current STM image (10.0 nm × 5.0 nm) from within an ocHB domain assembled from phenyl octane solution (1.0 V, 0.1 nA). The 3 × 2 dot patterns (yellow) of some anthracenes are visible. The oc-HB unit cell is marked in cyan.

alternate with A*A* tapes). The anthracene 3-H bulges in each tape extend into the midchain ether notches of the adjacent tapes. For some compounds, attaining this anthracene 3-H-side chain O alignment forces a change of tape structure (non-ωT2 packed oc-tapes or ωT2 packed even-core (ec)-tapes). STM Determined Morphology Distributions as a Function of Side Chain Structure. Many of the compounds in Chart 1 self-assemble monolayers with domains exhibiting different

J. Phys. Chem. C, Vol. 113, No. 39, 2009 17107 morphologies. Table 1 characterizes the qualitative distributions of morphologies observed from STM studies. For each compound, a morphology is classified as “major” (+++) if it appears as extended, pure domains (>15 adjacent tapes) and comprises more than 70% of the observed monolayers. A morphology is “minor” (++) if it appears as small, pure domains (5-10 adjacent tapes) and comprises 10-25% of the observed monolayers. A morphology is classified as “defect” (+) if it appears only as isolated occurrences of the smallest number of adjacent tapes required to define the morphology (two oc-AA tapes for H-2-H or C-2-C; three tapes for HB). A number of trends are evident in Table 1. (i) Side chains with ether groups at the 2- and X-positions, where X is odd, promote assembly of C-2-C monolayers with small numbers of defects but no minor morphology. The majority of adjacent tapes are ωT2-packed in these compounds’ monolayers. (ii) (2,8)- and (2,10)-diether side chains promote assembly of monolayers with a large percentage of ωT8-packed tapes, either as H-2-H or mixed domains. The fractions of H-2-H and C-2-C packed tapes depend on the presence or absence of ether oxygen repulsions between C-2-C packed tapes (vide infra). (iii) (2,12)diether side chains promote assembly of C-2-C monolayers, with H-2-H packed tapes appearing as minor or defect morphologies. The exception, A-[132,12]2, suffers two strong intertape repulsions between C-2-C packed tapes (vide infra). The morphological proclivities of compounds with (2,8)and (2,10)- diether side chains are complex. H-2-H is the major morphology assembled by A-[112,8]2 and A-[152,8]2, whereas A-[132,8]2, assembles a HB morphology. A-[172,8]2 monolayers exhibit no extended pure domains that qualify as a “major” morphology. Moderate width C-2-C strips (∼10 tapes) are separated by a pair of H-2-H packed tapes or by small strips of mixed tapes. A-[192,8]2 monolayers also lack extended domains that qualify as a major morphology. Instead, moderate width strips of C-2-C, of H-2-H, and of mixed packed tapes alternate randomly. Mixed morphologies are the major component in A-[252,8]2 monolayers. The mixed domains consist of regions with excess H-2-H packed tapes, as well as regions with excess C-2-C packed tapes. Minor pure domains with C-2-C packing and minor pure domains with H-2-H packing are observed also. The major morphologies assembled by A-[172,10]2 and A-[192,10]2 are “H-2-H rich” mixed; with strips of three to four H-2-H packed tapes alternating with two C-2-C packed tapes. A-[212,10]2, primarily assembles C-2-C domains, with minor mixed domains. The general trends and exceptions will be discussed within the context of results from MM studies. MM Studies of Monolayer Energetics. MM16 minimizations were performed on monolayer sections consisting of four ocAA tapes truncated to six anthracenes.17 The HOPG surface was modeled as a single layer, graphene sheet 10-15% wider and longer than the monolayer section. Partial charges on anthracene and side chain atoms were assigned on the basis of Mulliken population analysis of AM118 minimized structures. Tapes were stacked and aligned on the graphene sheet in geometries approximating the STM observed C-2-C, H-2-H, or HB morphologies. Minimizations were run until the MM energy of the monolayer section plus graphene decreased less than 10-5 kcal/mol in 24 h.19 The graphene sheet was deleted, the monolayer section saved and the monolayer 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 to a position more than 2

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TABLE 1: Summary of Observed Morphologiesa and Intertape Repulsions for C-2-C Tape Packingb molecule 2,7

A-[11 ]2 A-[132,7]2 A-[152,9]2 A-[172,9]2 A-[192,11]2 A-[212,11]2 A-[132,12]2 A-[212,12]2 A-[232, 12]2

C-2-C intertape repulsions C-2-C H-2-H mix 1 1 1 1 1 1 2 1 1

+ +++ +++ +++ +++ +++ +++ +++

HB

molecule

+++ A-[11 ]2 A-[132,8]2 + A-[152,8]2 A-[172,8]2 A-[192,8]2 A-[252,8]2 A-[172,10]2 + + A-[192,10]2 ++ A-[212,10]2 2,8

+ + + + +++ ++ ++

C-2-C intertape repulsions C-2-C H-2-H 3* 1 1 0 0 0 1 1 0

mix

HB

+++ + ++ ++ ++ ++ ++ ++ +++

+++ +++ + ++ ++

+ ++ + +++ +++ ++ +++ ++ ++

a +++ indicates major morphology; ++ indicates minor morphology; + indicates defect morphology. b * indicates intertape dipolar repulsions between ether groups three side chain positions out of registration. All other intertape repulsions are between ether groups one side chain position out of registration.

TABLE 2: Calculated Self-Assembly Energies (SAE) and Components for C-2-C and H-2-H Morphologiesa C-2-C energies (kcal/mol/molecule - stabilizing