Shape-Directed Patterning and Surface Reaction of Tetra-diacetylene

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Shape-Directed Patterning and Surface Reaction of Tetra-diacetylene Monolayers: Formation of Linear and Two-Dimensional Grid Polydiacetylene Alternating Copolymers Yan Yang and Matthew B. Zimmt* Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States S Supporting Information *

ABSTRACT: Side chains containing two diacetylene units spaced by an odd number of methylene units exhibit pronounced “bumps” composed of 0.3 nm steps, in opposite directions, at odd and even side-chain positions. In densely packed self-assembled monolayers, the bis-diacetylene bumps stack into each other, similar to the stacking of paper cups. Bisdiacetylene side chain structure and associated packing constraints can be tailored by altering the bump width, direction, side-chain location, and overall side-chain length as a means to direct the identities and alignments of adjacent molecules within monolayers. Scanning tunneling microscopy (STM) at the solution−HOPG interface confirms the high selectivity and fidelity with which bis-diacetylene bump stacking directs the packing of shape-complementary side chains within one-component monolayers and within two-component, 1-D self-patterned monolayers. Drop cast or moderately annealed monolayers of anthracenes bearing two bis-diacetylene side chains assemble single domains as large as 105 nm2. Light-induced cross-linking of two-component, 1-D patterned monolayers generates linear polydiacetylene alternating copolymers (A-B-)x and ‑B‑ 2-D grid polydiacetylene alternating copolymers (A‑B‑ ‑B‑A‑B‑)x that covalently lock in monolayer structure and patterns.

1. INTRODUCTION The preparation of molecular films with micrometer-tonanometer scale variation1 of composition and functionality is of interest for applications in electronics,2 energy capture,3 and sensing.4,5 The self-assembly of molecular components can produce two-dimensional monolayer films with local6,7 and, in some cases, long-range crystalline ordering.8,9 Recent advances have achieved spontaneous assembly of multicomponent monolayer films that, using cocrystallization10−14 and host− guest15−18 strategies, exhibit periodic composition/functionality variation on the nanoscale. Further development of these selfpatterning monolayers for applications requires the incorporation of molecular components capable of both directing the assembly patterning and providing ways to lock in film structure.19−21 Complementary interactions derived from hydrogen bonding,22,23 coordinate covalent bonding,24 dipolar interactions,13 and van der Waals interactions can supply the driving force needed to direct patterned self-assembly.25 Under the proper conditions, these interactions are sufficiently reversible to enable self-assembly processes to access 26 intended, thermodynamically favored, compositionally patterned monolayers. However, locking in monolayer structure and patterns requires the use of stronger, more permanent interactions among the assembly components.19−21 Covalent cross-linking of diacetylene27 and acetylene units has been used to transform molecular monolayers into 1D and 2D polymers.28,29 Here we demonstrate the use of diacetylene © XXXX American Chemical Society

units for both functions: to direct the morphology and compositional patterning of self-assembled monolayers and to lock in monolayer structure and patterns by conversion to 1D and 2D polymers. Many monolayer systems access densely packed arrangements in which shape-matched components maximize their van der Waals interactions.30 Aliphatic chains incorporating diacetylene functional groups exhibit kinked shapes whose packing constrains the identities of neighboring molecules within a monolayer.31 The packing selectivity of kinked diacetylene chains is sufficient to drive the assembly of monolayer networks exhibiting giant pores10,32 or 1-D patterns spanning more than 20 nm.14 Despite the packing constraints associated with kinked side chains, some alkadiacetylene−aryl systems access multiple packing morphologies that disrupt the long-range order of self-assembled monolayer films.33 In addition, our efforts to lock in patterned monolayer structures by cross-linking27 side chains of these alkadiacetylene−aryl monolayers have not succeeded. Molecular modeling studies of these monolayers’ morphological promiscuity suggested that side chains with two diacetylene kinks, one at an odd and one at an even position, could enhance the packing fidelity. When adsorbed on a planar surface, such bis-diacetylene side chains Received: September 1, 2015 Revised: October 20, 2015

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DOI: 10.1021/acs.langmuir.5b03295 Langmuir XXXX, XXX, XXX−XXX

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2. EXPERIMENTAL METHODS

19 to 40 °C over a 30 min period, held at the target temperature for the reported annealing time and allowed to cool to room temperature (typically 30 min). STM images were acquired by engaging the STM tip through solution and scanning in constant height or constant current mode, with tip scan velocities in the range of 0.20−0.60 μm/s. Thermal drift in collected data was corrected as described in the Supporting Information. Reported unit cell parameters are averages of thermal-drift-corrected STM images from three or more independently prepared and imaged samples. Analyses of domain interface densities were performed on STM images collected from independent samples and monolayer regions. In each local region, three images were collected by adding 150% of the scan size to the X and Y offsets. The HOPG substrate was then shifted 1 to 2 mm so that the tip accessed macroscopically distinct regions. A minimum of four distinct regions was scanned per HOPG substrate. Monolayer irradiation was performed using 254 nm light from a model UVG-54 Mineralight lamp (115 V, 60 Hz, 2.2 mW/cm2 of 254 nm at 3 in.; Ultraviolet Products, Inc., San Gabriel, CA, USA). The HOPG substrate with drop cast or annealed monolayer was placed on an aluminum-foil-covered surface roughly 7 cm directly below the lamp and irradiated for the reported time duration. Imaging began 5− 10 min postirradiation.

Synthesis routes to and characterizations of all compounds are reported in the Supporting Information. Scanning tunneling microscopy images were acquired with a Digital Instruments MS-10 STM interfaced with a Digital Instruments NanoScope IIIa controller. Data was collected at the solution−graphite interface (HOPG, ZYB grade, Momentive Performance, Strongsville, OH) with mechanically cut 80/20 Pt/Ir tips (0.25 mm, Goodfellow, Oakdale, PA) or 87/13 Pt/Rh tips (0.25 mm, Omega Engineering, Stamford, CT). Singlecompound solutions were prepared by dissolving 2−4 mg of compound in 1 mL of phenyl octane (Alpha Aesar). These concentrated solutions (2−5 mM) were stored in the freezer. Lower-concentration solutions (0.125 to 1 mM) were prepared prior to STM experiments and stored at 19 °C. Two-component solutions were prepared by mixing equal molarity solutions (e.g., 26-6(5) and 26-10(5), Chart 1) to give a total concentration of 0.125 or 0.25 mM. Drop-cast samples were prepared by depositing 2 to 3 μL of anthracene solution on a freshly cleaved HOPG surface. Imaging started 5 to 10 min after drop casting. Annealed samples were prepared by depositing 6−10 μL of solution on a dimple cut in the center of an aluminum chamber. The graphite substrate was placed on top of the drop with a freshly cleaved face in contact with solution. A tightly fitting aluminum lid was applied to minimize solvent evaporation during annealing. The sample was annealed using a programmable heating block. The sample temperature was raised from

3. RESULTS AND DISCUSSION a. Bumped Bis-diacetylene Side-Chain Design: Structural Parameters to Test. A bumped side chain at an anthracene 1 or 5 position has five structural parameters that alter the bump structure and function: (i) bump height, (ii) bump displacement direction, (iii) bump width, (iv) bump location along the side chain, and (v) side-chain length. (i) A bump consisting of two diacetylene units separated by an odd number of methylene units is 0.3 nm high. If needed, the bump height can be increased using two triynes or tetraynes. An even number of CH2 units between diacetylenes creates a staircaseshaped side chain (not studied here). (ii) For 1- or 5-position side chains, the bump displacement extends in, i.e., toward the anthracene center ring, if the diacetylene closer to the anthracene is located at an odd side-chain position (Chart 1). The displacement direction is out if the inner diacetylene unit is at an even side position. (iii) The bump width increases 0.25 nm for each two additional methylenes (odd total number) inserted between the side-chain diacetylene units. Adjusting the bis-diacetylene bump width may be of use for complex patterning and may help destabilize unintended packing

exhibit sizable “bumps” that serve as formidable packing constraints. Stacking geometrically congruent bis-diacetylene bumps, in a manner analogous to stacking paper cups, offers substantially greater van der Waals contacts than alternate packing alignments. Bump width, direction, height, and chain location are structural elements that can be exploited as shapebased “codes” to direct the assembly of complexly patterned films exhibiting nanometer features. Herein, we demonstrate the efficacy of bis-diacetylene side chains, with varied structures, to direct the monolayer packing and self-patterning of molecularly thin films assembled from tetra-diacetylene anthracenes. Regions of these bump-directed, 1-D patterned monolayers are transformed into linear alternating polydiacetylene copolymer34,35 (A-B-)x or into 2-D grid alternating ‑B‑ polydiacetylene copolymer (A‑B‑ ‑B‑A‑B‑)x by light-induced crosslinking of stacked bis-diacetylene units. Bis-diacetylene bumps realize the two important functions of directing and locking in patterned monolayer assembly.

B


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Figure 1. STM images of monolayers assembled by 0.25 mM 26-9(3) in phenyloctane at the HOPG surface. (a) 10 nm × 10 nm region of monolayer drop cast at 19 °C with a CPK overlay. The green box marks one unit cell. (b) A 11.7 nm × 11.7 nm region exhibiting a parallel-shift interface.36 (c) A 170 nm × 170 nm single-domain region resulting from annealing the solution−HOPG interface at 40 °C for 1 h.

−750 ± 150 mV, iSET ≈ 100 ± 50 pA), tunneling at anthracene cores exceeds that at diacetylenes, which exceeds that at alkyl chains. Each pair of diacetylene groups separated by three methylene units, i.e., the bump, exhibits a chevron shape. These chevrons are stacked along the midline of each aliphatic column. The two chevron stacks flanking each anthracene column point in opposite directions, as expected given this molecule’s symmetry. With sufficient imaging resolution, the three methylenes between the diacetylenes appear as a low tunneling feature at the chevron apex (Figure 1a). The nearly rectangular unit cell of the monolayer contains two molecules with bump-stacked bis-diacetylene chains, extends 0.94 nm along the anthracene columns, and spans 7.7 nm roughly perpendicular to the anthracene columns (Table 1). Bisdiacetylene bump stacking of [269,11,16,18] side chains directs molecular packing with reasonable selectivity.

alignments in which anthracene 2-, 3-, 6-, or 7-position H− C(sp2)−C(sp2)−H units pack within a bis-diacetylene bump. (iv, v) The bump location and chain length are expected to influence the neighbor selectivity of each side chain. Installation of the inner diacetylene at a side chain’s (ω − (σ + 5))/2 position is predicted to create a self-complementary, bumped side-chain shape, where ω is the side-chain length and σ is the number of sp3 atoms between the diacetylene groups. Selfcomplementary side chains pack optimally with identical side chains as neighbors. Positioning the bis-diacetylene bump at another location is expected to generate a self-incommensurate side chain, i.e., a side chain whose shape prevents it from packing optimally with identical side chains as neighbors. Designing two self-incommensurate side chains that are pairwise shape complementary may be useful for directing 1D patterning.10,14,31,32 A pair of bumped side chains should be optimally shape complementary if they have identical lengths, ω, identical bump widths, σ, and position the inner diacetylene group at the n position of one chain and at the (ω − (σ + 5 + n)) position of the shape-complementary side chain. The impact of bump structural features (ii)−(v) on the morphology, self-patterning, and UV-induced cross-linking of monolayers self-assembled from anthracenes bearing two identical bisdiacetylene side chains is reported below. b. Assembly Directed by Shape Self-Complementary [269,11,16,18] “In-Bump” Bis-diacetylene Side Chains. Bisdiacetylene bumps constrain the close approach of side chains within neighboring molecules of a monolayer. If stacking bisdiacetylene bumps like paper cups is the only way to assemble densely packed monolayers, then bis-diacetylene bumps may be of use to design and control molecule packing alignments and neighbor selectivity in monolayers. For the molecules used here, close packing of shape-complementary side chains should assemble monolayers exhibiting bump-stacked aliphatic columns alternating with aryl columns, and with anthracenes in the nearest aryl columns adsorbing to HOPG via opposite enantiotopic faces. The STM signature of this 2D-racemate morphology is nearly orthogonal long-axis alignments of anthracenes in the nearest aryl columns (Chart 1, dashed lines). A [269,11,16,18] side chain attached at an anthracene 1 position presents an “in”-directed bis-diacetylene bump at its midpoint (Chart 1). The monolayers assembled by drop casting 0.25 mM phenyloctane solutions of A[269,11,16,18]2 (26-9(3)) on HOPG exhibit only racemate morphologies (Figure 1a); anthracenes appear as rectangular, 3 × 2 dot features and have nearly perpendicular long-axis alignments in the nearest aryl columns. For the imaging conditions employed in this study (Vbias ≈

Table 1. Monolayer Unit Cell Parameters a, nm

molecule(s) 26-9(3) 28-10(3) 28-9(5) 26-6(5)/ 26-10(5)

experiment MM+ experiment MM+ experiment MM+ experiment MM+

0.94 0.96 0.94 0.94 0.93 0.95 0.94 0.93

± ± ± ± ± ± ± ±

0.04 0.01 0.03 0.00 0.02 0.01 0.05 0.01

α

b, nm 7.68 7.68 7.83 8.25 7.74 8.23 7.48 7.67

± ± ± ± ± ± ± ±

0.10 0.06 0.22 0.03 0.14 0.02 0.15 0.03

88.8 89.3 88.2 89.2 89.3 88.8 88.6 89.1

± ± ± ± ± ± ± ±

0.6 0.4 1.0 0.9 0.7 0.3 0.8 0.3

Drop casting A[269,11,16,18]2 (26-9(3)) solutions with a concentration above 0.5 mM produces multilayer regions on HOPG. Multilayers are detected infrequently from 0.25 mM or lower-concentration solutions. Surveying multiple regions and samples prepared by drop casting 0.25 mM 26-9(3) solutions reveals the infrequent assembly of interfaces between (i) domains in which aryl columns are rotated by 120°, (ii) domains in which aryl columns are parallel but shifted by ∼1/4 of a rectangular unit cell across the interface (parallel shift), and (iii) domains in which aryl columns are shifted and slightly nonparallel across the interface (angled shift). The parallel-shift interface can be imaged at high resolution (Figure 1b), suggesting minimal chain dynamics and reasonable packing.36 In monolayers formed by drop casting 0.25 mM 26-9(3) at 19 °C, each of the three domain interface types (120°, parallel shift, and angled shift) is observed roughly once in every four (100 nm)2 regions scanned. Drop casting lower-concentration 26-9(3) solutions (0.125 mM) on HOPG or annealing 0.125 C


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alignments. The diacetylene chevrons within the nearest aliphatic columns point in opposite directions. Reduced tunneling at the diacetylene chevrons’ midpoint, due to three methylene groups, is more evident for 28-10(3) than for 269(3) monolayers. Each diacetylene unit is aligned nearly parallel to the closest anthracene’s long axis. This contrasts with 26-9(3) monolayers, in which each diacetylene unit is aligned roughly perpendicular to the closest anthracene’s long axis. A rectangular unit cell of the 28-10(3) monolayer contains two molecules with bump-stacked diacetylene chains, spans 0.94 nm along the anthracene columns, and extends 7.8 nm nearly perpendicular to the anthracene columns (Table 1). The latter distance is 0.4 nm smaller than the unit cell length predicted in simulations. The three types of domain interfaces (parallel shift (Figure 2b),36 angled shift (Figure 2b), and 120°) are observed roughly four times more frequently in monolayers drop cast from 2810(3) solutions compared to identical concentration 26-9(3) solutions (Table 2). The more frequent occurrence of parallelshift interfaces for drop cast 28-10(3) monolayers is at odds with modeling that predicts poorer packing and higher energy for this interface in 28-10(3) monolayers than in 26-9(3) monolayers (Supporting Information Table S1). 28-10(3) has four more methylene units than 26-9(3) and should physisorb more strongly to HOPG than does 26-9(3).37 At identical solution concentrations, domain nucleation rates may be larger and monolayer-to-solution exchange rates should be smaller for 28-10(3) than for 26-9(3).38 Thus, the morphologies of dropcast 28-10(3) monolayers may be influenced to a greater extent by assembly kinetics than the morphologies of drop-cast 269(3) monolayers. Annealing the 28-10(3) solution−HOPG interface at 40 °C for 1 h increases the domain size and reduces domain interface occurrence (Table 2, Figure 2c). These observations confirm that assembly kinetics influence the morphologies of 28-10(3) monolayers drop cast at 19 °C. Overall, the out-bump [2810,12,17,19] bis-diacetylene side chain attached to an anthracene 1-position selects the intended, bump-stacked packing alignment with good fidelity, although the density of domain interfaces is slightly higher than for 269(3) monolayers after annealing for 1 h. The larger size of 2810(3) necessitates higher assembly temperatures to make packing thermodynamics, rather than assembly kinetics, the dominant morphology determinant.26 d. Assembly Directed by Self-Complementary [289,11,18,20] “Wider In-Bump” Bis-diacetylene Side Chains. The diacetylene groups in the [289,11,18,20] side chain

or 0.25 mM 26-9(3) solution−HOPG interfaces for 1 h at 40 °C (Figure 1c) reduces the number of domain interfaces found in (100 nm)2 scans (Table 2). These observations indicate that Table 2. Domain Interface Counts and Interface Lengths number of interfaces in 12 (100 nm)2 images {total length (nm) of shift interfaces in the 12 images} molecule

conditionsa

120°

26-9(3)

0.25 mM, DC 0.125 mM, DC 0.125 mM, Anl 0.25 mM, DC 0.125 mM, DC 0.125 mM, Anl 0.25 mM, DC 0.125 mM, DC 0.125 mM, Anl

4 0 2 18 17 6 11 12 3

28-10(3)

28-9(5)

parallel shift 3 0 0 8 2 0 0 0 0

{329} {0} {0} {427} {182} {0} {0} {0} {0}

angled shift 2 {142} 0 {0} 0 {0} 10 {482} 8 {434} 3 {61} 5 {222} 0 {0} 0 {0}

a DC = drop cast at 19 °C. Anl = annealed 1 h at 40 °C after drop casting.

26-9(3) monolayer morphologies formed by drop casting at 19 °C are impacted by assembly kinetics and packing thermodynamics, that lowering the 26-9(3) solution concentration 2-fold reduces domain nucleation rates more than it reduces domain growth rates, and that monolayer assembly at 40 °C overcomes kinetic barriers likely associated with molecular desorption and interface remodeling. Annealing enables side-chain packing thermodynamics to exert a greater influence on monolayer morphology (vide infra). With annealing, the [269,11,16,18], inbump bis-diacetylene side chain provides very high fidelity packing selectivity when attached at an anthracene 1 position. c. Assembly Directed by Shape Self-Complementary [2810,12,17,19] “Out-Bump” Bis-diacetylene Side Chains. A [2810,12,17,19] side chain attached at an anthracene 1 position projects an out bump at its midpoint composed of two diacetylenes spaced by three methylene units (Chart 1). As with 26-9(3), close packing of A[2810,12,17,19]2 (28-10(3)) was designed to require side-chain bump stacking. The selfassembly of 28-10(3) was examined to gauge the impact of bump orientation, i.e., out versus in, on the assembly of intended and alternately packed morphologies. Drop casting 0.25 mM or lower-concentration solutions of 28-10(3) produces racemate monolayers (Figure 2a), with the nearest anthracene columns exhibiting nearly perpendicular long-axis

Figure 2. STM images of monolayers assembled by 0.25 mM 28-10(3) in phenyloctane at the HOPG surface. (a) An 11.5 nm × 11.5 nm region of the monolayer drop cast at 19 °C with a CPK overlay. The green box marks one unit cell. (b) A 48 nm × 48 nm region exhibiting a parallel-shift interface36 (upper left) and an angled-shift interface (middle right). (c) A 150 nm × 150 nm single-domain region produced by annealing the solution−HOPG interface at 40 °C for 1 h. D


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Figure 3. STM images of monolayers assembled by 0.25 mM 28-9(5) in phenyloctane at the HOPG surface. (a) 11.5 nm × 11.5 nm region of monolayer drop cast at 19 °C with CPK overlay. The green box marks one unit cell. (b) 80 nm × 80 nm monolayer region for a sample drop cast at 19 °C. (c) 120 nm × 120 nm section of a single domain produced by annealing the solution−HOPG interface at 40 °C for 1 h.

Figure 4. STM images of monolayers at the HOPG surface assembled by phenyloctane solutions of 0.125 mM 26-6(5) and 0.125 mM 26-10(5). (a) 17 nm × 17 nm region of a monolayer drop cast at 19 °C with the CPK overlay. The green box marks one unit cell. (b) 120 nm × 120 nm region with no defects assembled by drop casting at 19 °C. (c) 3 × 3 mosaic of 100 nm × 100 nm scans of monolayer produced by drop casting at 19 °C (0.125 mM total concentration). The individual scans are not corrected for thermal drift. A 120° interface is present in the top left scan.

are separated by five methylene units. Self-assembly of A[289,11,18,20]2 (28-9(5)) was studied to evaluate whether wider in-bumps in the center of the side chain provide bettter packing selectivity than the narrower, three methylene bumps in 26-9(3) and 28-10(3). Drop-cast 0.25 mM 28-9(5) solutions assemble 2D-racemate morphology monolayers (Figure 3a). The bis-diacetylenes appear as stacked columns of trapezoidal chevrons, with the nearest chevrons pointing in opposite directions. As observed with 26-9(3), each diacetylene unit is aligned nearly perpendicular to the long axis of the closest anthracene. The low tunneling region within each sidechain chevron is wider for [289,11,18,20] than for [269,11,16,18] or [2810,12,17,19]. A rectangular 28-9(5) unit cell contains two molecules and extends for 0.93 nm along the anthracene column and 7.7 nm roughly perpendicular to the anthracene column (Table 1). The latter distance is 0.5 nm smaller than the unit cell length predicted in simulations. Drop-cast monolayers assembled from 0.25 mM solutions of 28-9(5) exhibit 3 times more 120° interfaces and angled-shift interfaces than appear in drop-cast 26-9(3) monolayers (Table 2). Relative to 26-9(3), increased physisorption produced by the four additional methylene units37,38 in 28-10(3) and in 289(5) increases the impact of assembly kinetics on monolayer morphologies formed by 19 °C drop casting. Interestingly, parallel-shift interfaces were not observed in any images of 289(5) drop-cast monolayers. The wider, five methylene inbumps of 28-9(5) side chains disfavor assembly and/or the persistence of parallel-shift interfaces to a greater extent than the narrower, three methylene in-bump side chains in 26-9(3) and 28-10(3) (Supporting Information).36 Thermal annealing of the 28-9(5) solution−HOPG interface at 40 °C for 1 h

increases the monolayer domain size and significantly reduces the incidence of angle-shift and 120° interfaces (Table 2, Figure 3b). With moderate thermal annealing, the wider in-bump of [269,11,16,18] side chains direct the assembly of the intended, bump-stacked packing alignment with excellent fidelity. e. One Dimensionally Patterned Monolayer Assembly Directed by Pairs of Shape Complementary, “Wider Out-Bump” Bis-diacetylene Side Chains. Self-stacking of bis-diacetylene bumps conferred good packing selectivity to molecules bearing shape self-complementary side chains. Side chains with bis-diacetylene bumps located away from the sidechain center are shape self-incommensurate; self-stacking of these chains is possible but yields nonoptimal monolayer packing. If the side-chain bump is located too close to the aryl core, then bump stacking forces the alkyl terminus of every chain to desorb from HOPG (Chart 1). If the side-chain bump is located too far from the aryl core, bump stacking cannot afford van der Waals contacts and packing stabilization of sidechain regions closest to the core (Chart 1). Consequently, monolayers assembled by a shape self-incommensurate compound are destabilized relative to monolayers assembled by shape-complementary side chains. This should provide the driving force for two compounds bearing shape-complementary, bumped bis-diacetylene side chains (Chart 1) to assemble two-component, compositionally patterned monolayers in preference to less-stable single-component domains. When attached at the anthracene 1 position, [266,8,15,17] and [2610,12,19,21] side chains present out-bumps with five methylenes between the diacetylene units. Each side chain is shape self-incommensurate, but the pair of chains is shape complementary. An isomeric [26] side chain with bisE


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Figure 5. (a) 50 nm × 50 nm STM image of a 1:1 26-6(5)/26-10(5) monolayer (CT = 0.25 mM, drop cast) after 5 s of UV irradiation at the phenyloctane−HOPG interface. The green line marks polymerized diacetylenes from the 26-6(5) 6-position/26-10(5) 19-position stack. The magenta line marks polymerized diacetylenes from the 26-10(5) 10-position/26-6(5) 15-position stack. (b) Proposed cross-linking yielding polydiacetylene 2-D grid alternating copolymer. Green and magenta polydiacetylene units correspond to enhanced tunneling stripes marked by green and magenta lines, respectively, in (a). (c) 30 nm × 50 nm STM image of a UV-irradiated (5 s) 26-6(5)/26-10(5) monolayer exhibiting a 40nm-long enhanced contrast feature indicating the formation of a linear polydiacetylene from a 26-6(5) 6-position/26-10(5) 19-position diacetylene stack.

diacetylene units at the 8 and 17 positions, [268,10,17,19], would be shape self-complementary. Thus, the diacetylene bumps in [266,8,15,17] and [2610,12,19,21] are shifted by two positions, in opposite directions, from side-chain center. Drop casting a single-component, 0.25 mM solution of A[266,8,15,17]2 (266(5)) onto HOPG produces no STM evidence of stable monolayer assembly. Likewise, no monolayer is observed upon drop casting a single-component, 0.25 mM solution of A[2610,12,19,21]2 (26-10(5)) onto HOPG. However, if a drop of 0.25 mM 26-6(5) solution is added to the 26-10(5) solution−HOPG interface, then a patterned monolayer containing both components is observed immediately (Figure 4a). Reversing the order in which the two compounds are drop cast or drop casting 1:1 solutions of 26-6(5) and 26-10(5) (each 0.125 mM) yields identical 2D racemate monolayers in which the centers of the stacked bis-diacetylene chevrons lie closer to every other anthracene column. The nearly rectangular unit cell contains one molecule of 26-6(5), one molecule of 26-10(5), and extends 0.94 nm along the anthracene column and 7.5 nm perpendicular to the anthracene column (Table 1). Larger-scale STM scans of the two-component monolayer (Figure 4b) exhibit an alternating pattern of narrow and wide high-tunneling-contrast columns. The narrow lines arise from tunneling through anthracene cores of the 26-10(5) columns. The wider lines are due to tunneling through anthracene cores of the 26-6(5) columns and through the nearby columns of stacked bis-diacetylenes. Remarkably, monolayers assembled by 19 °C drop casting of a 1:1 26-6(5)/26-10(5) solution (0.125 mM total concentration) exhibit remarkably few defects: a 3 × 3 array of 100 nm × 100 nm STM scans contains one or possibly two 120° interfaces (Figure 4c). No parallel-shift or angled-shift interfaces have been observed in any drop-cast monolayers assembled from these two components. The shapecomplementary pair, 26-6(5)/26-10(5), each of which is shape self-incommensurate, affords excellent neighbor selectivity (long-range 1D-pattern assembly) and packing selectivity (single morphology) under 19 °C drop-cast conditions. These results demonstrate that side chains with off-center bisdiacetylene bumps are exceedingly potent design elements for the self-assembly of compositionally patterned monolayers.

f. One-Dimensional and Two-Dimensional Polymers via Diacetylene Cross-Linking. Cross-linking of all side chains in these tetra-diacetylene monolayers would be a means to lock in molecular patterns and film structure by conversion to a physisorbed two-dimensional polymer.19−21,28 The aliphatic columns in monolayers assembled by 26-9(3), 2810(3), 28-9(5) and the 26-6(5)/26-10(5) pair contain two stacks of diacetylene units separated by three or five methylene units. Diacetylene polymerization27 has been demonstrated in reduced-mobility media, such as crystals,39 thin films,40 and monolayers.28,29 It requires appropriate alignment, spacing, and residual mobility of the diacetylene units. Polydiacetylene formed by diacetylene oligomerization in monolayers often appears with enhanced tunneling contrast in STM scans.28,29 Our prior attempts to polymerize alkadiacetylene-substituted anthracene monolayers on HOPG were unsuccessful; UV irradiation resulted in the disappearance of monolayer from STM images or in unperturbed monolayers, with no regions of enhanced STM tunneling contrast. Despite prior failures, the prospect of transforming 1-D-patterned monolayers into compositionally patterned 2-D polymers prompted irradiation studies of the 26-6(5)/26-10(5) monolayer. Five second UV irradiation of this monolayer at the phenyloctane−HOPG interface produced enhanced tunneling stripes in line with diacetylene stacks. Figure 5a shows a 26-nm-long enhanced tunneling stripe (green line) within a diacetylene stack composed of 6-position diacetylenes from the 26-6(5) anthracene column to the stripe’s left alternating with 19position diacetylenes from the 26-10(5) column to the stripe’s right. Two additional enhanced tunneling stripes, 9 and 10 nm long, are present in the aliphatic column immediately to the right of this same 26-10(5) anthracene column. These stripes are in line with a diacetylene stack composed of 10-position diacetylenes from the 26-10(5) anthracene column alternating with 15-position diacetylenes from the 26-6(5) anthracene column to the stripes’ right. Cross-linking diacetylene stacks on both sides of a central 26-10(5) anthracene column yields a section of polydiacetylene 2-D grid alternating copolymer, roughly 20 nm high and 10 nm wide, with a ∼50 kDa molecular weight. The grid section’s horizontal arms (Figure 5b) capture the monolayer’s 1-D pattern: 26-10(5) cores within the central F


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Figure 6. STM images of a 0.125 mM 28-9(5) monolayer following 5 s of UV irradiation. (a) 26 nm × 26 nm molecular resolution image of a dropcast monolayer containing an 11-nm-high contrast stripe. (b) 120 nm × 120 nm image of a monolayer following 1 h of annealing at 40 °C and irradiation. Three collinear, high tunneling stripes extend 10, 71, and 15 nm. (c) 110 nm × 110 nm image following 1 h of annealing at 40 °C and irradiation. Two high tunneling stripes extend 80 and 46 nm. (d) 16 nm × 70 nm image exhibiting two high tunneling features within adjacent diacetylene stacks of the same lamella.

that alter side-chain shape and can be used to design and control neighbor identity and 1-D pattern formation within a monolayer. The long bis-diacetylene side chains employed in this study, with 26 and 28 nonhydrogen atoms, promote strong physisorption to HOPG. Consequently, assembly kinetics impact the monolayer morphology formed by solution dropcasting compounds at 19 °C. The intrinsic packing selectivity conferred by bis-diacetylene bumps is expressed more fully following gentle annealing (40 °C, 1 h) of the solution−HOPG interface for anthracenes bearing two long side chains. Bump-directed self-assembly produces two stacks of closely packed diacetylenes within each aliphatic lamella. Brief UV irradiation of the solution−tetradiacetylene monolayer−HOPG interface induces diacetylene cross-linking, forming hightunneling-contrast polydiacetylenes up to 80 nm long, but only for compounds with five methylene units between diacetylenes. Irradiation of the 1-D-patterned monolayers assembled by the 26-6(5)/26(10(5) pair produces linear, alternating polydiacetylene copolymers and sections of 2-D grid, alternating polydiacetylene copolymers. Bis-diacetylene bumps incorporating five methylenes provide the patterning and cross-linking capabilities required to self-assemble and covalently lock in patterned monolayers.

aryl column are covalently bonded to 26-6(5) cores in both flanking aryl columns. Polydiacetylene backbones constitute the grid section’s vertical arms. Five to ten percent of the 26-6(5)/26-10(5) monolayer regions imaged subsequent to 5 s UV irradiation contain at least one enhanced tunneling stripe, 10−50 nm long, in line with either a 26-6(5) 6-position diacetylene/26-10(5) 19position diacetylene stack (Figure 5c) or a 26-10(5) 10position/26-6(5) 15-position diacetylene stack. These isolated stripes indicate the formation of a linear, alternating 26-6(5)26-10(5) polydiacetylene copolymer, i.e., either the green or the magenta polydiacetylene in Figure 5b.34,35 UV irradiation of 28-9(5) monolayer samples also yielded enhanced tunneling stripes, 10 to 80 nm long, in line with 9position/18-position diacetylene stacks (Figure 6a−c). These stripes indicate the formation of linear polydiacetylenes. Adjacent monomers utilize their 9-position or 18-position diacetylene group, alternately, to form the conjugated backbone. Five to ten percent of regions imaged following 5 s of UV irradiation exhibited one or more enhanced tunneling stripes superposed on diacetylene stacks. A few images appear to indicate the cross-linking of both diacetylene stacks within a short region of the same aliphatic column (Figure 6d). Although monolayers assembled from compounds with five methylenes between diacetylene units (28(9)-5 and 26-6(5)/ 26(10(5)) undergo photoinduced diacetylene polymerization, monolayers assembled from compounds with three methylenes between diacetylene units (26-9(3) or 28(10)-3) exhibit no enhanced tunneling regions following UV irradiation. The failure to detect enhanced tunneling stripes in the latter monolayers suggests a significant difference in the cross-linking activity of the three and five methylene spaced diacetylene systems. Studies to understand structural effects on crosslinking, to increase the cross-linking extent, and to characterize the polymers formed are ongoing.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03295. Synthesis methods and spectral data for all new compounds, STM sample preparation and acquisition protocols, UV irradiation procedure, molecular mechanics simulations and evaluation of monolayer self-assembly energies, and a model used to evaluate Boltzmann ratios of parallel-shift interface units to domain unit cells (PDF)

4. CONCLUSIONS Installation of bis-diacetylene bumps into aliphatic side chains is an effective strategy for controlling the identities and packing alignments of neighboring molecules within a monolayer. The van der Waals stabilization generated by packing side chains with bump-stacked bis-diacetylene units provides strong thermodynamic bias for the assembly of designed monolayers. The width, direction, and placement of the 0.3 nm bisdiacetylene bump represent independent structural elements

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AUTHOR INFORMATION

Corresponding Author

*Tel: (401)863-2909. Fax: (401)863-2594. E-mail: mbz@ brown.edu. Notes

The authors declare no competing financial interest. G


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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE1058241) for partial support of this work.

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