Article pubs.acs.org/JPCB
Shape Amphiphiles in 2‑D: Assembly of 1‑D Stripes and Control of Their Surface Density Yan Yang and Matthew B. Zimmt* Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States S Supporting Information *
ABSTRACT: The morphology of monolayers assembled from mixtures of a shape-amphiphilic molecule, {33,19} = 1((hentriaconta-14,16-diyn-1-yloxy)methyl)-5-((heptadecyloxy)methyl)anthracene, and a symmetric molecule, {192}, at the solution−HOPG interface depends strongly on the components’ solution concentrations and sample annealing history. The kinked alkadiyne side chain, {33}, packs optimally only with antiparallel aligned, {33} side chains. Thus, optimal packing of {33} side chains should assemble “{33} stripes” consisting of two adjacent {33,19} columns with interdigitated {33} chains. The aliphatic {19} side chain of {33,19} can pack with antiparallel aligned {19} side chains from {192} or from {33,19}. Thus, {33} stripes can incorporate as “guests” within {192} “host” monolayers. The composition and morphology of monolayers formed by drop casting solutions of {33,19} and {192} at 19 °C are dominated by assembly kinetics. Short {33} strips are immersed haphazardly in monolayers comprised mostly of {192}. Thermal annealing promotes fuller expression of {33,19}’s shape amphiphilicity and assembly of thermodynamically determined monolayers incorporating 1-D {33} stripes within a 2-D matrix of {192}. Larger solution mole fractions of {192} yield annealed monolayers with nearly constant {33} strip lengths, decreased {33} strip density, and increased {33} strip spacing.
1. INTRODUCTION There is considerable interest in controlling the structures and properties of molecular films.1−6 Scanning microscopies are invaluable for revealing connections between the structures of molecular components and the two-dimensional monolayer films they assemble on surfaces.7 Molecular composition, connectivity, shape, stereochemistry, and conformation modulate the key intermolecular electrostatic and van der Waals interactions that determine monolayer morphology.8 These structural elements have been exploited as design tools to direct self-assembly of complexly patterned, multicomponent monolayers9−13 and to assemble monolayers that function as templates.14−19 Classical amphiphiles, molecules containing both hydrophilic and hydrophobic components, have been used widely in monolayer self-assembly. Some molecules used to assemble patterned or template monolayers are shape amphiphiles,20,21 in that they contain components whose distinct shapes generate dissimilar, preferential interactions with neighboring structures in a monolayer. Aliphatic side chains with different lengths or shapes are structural elements that provide shape based packing selectivity and confer shape amphiphilicity to monolayer forming molecules. The kinked shapes of conjugated alkadiyne chains afford shape criteria for energetically optimal packing with adjacent components in a monolayer, and have been used to direct high precision assembly of monolayers with large pores and of multicomponent, 1-D patterned monolayers.22,23 © 2015 American Chemical Society
Assembly of a four-component, crystalline monolayer with 23 nm spacing between repeating 1-D columns was directed by the lengths and kink locations of two alkadiyne side chains per component. Diyne shape directed assemblies form with remarkable fidelity but require substantial redesign and synthesis to alter spacings between “target” 1-D columns. In an effort to develop 2-D systems capable of adaptable spacing between target columns, a two-component, “shape amphiphile/diluent” strategy was explored (Chart 1). In analogy to micelle assembling amphiphiles, shape amphiphiles were designed (i) to assemble clusters, in this case 1-D stripes, directed by the shape of one side chain and (ii) to integrate into a diluent host, in this case a 2-D monolayer, by virtue of a second side chain’s shape. The diluent molecules, {192}, project identical heptadecyloxymethyl side chains, {19}, in opposite directions from the 1- and 5-positions of an anthracene core. The {192} molecules self-assemble a close-packed “host” monolayer. The shape amphiphiles, {33,19}, project a shape self-complementary, “kinked” alkadiynyl side chain, {33}, and a “linear” {19} side chain in opposite directions from the 1- and 5- positions of anthracene cores. Each kinked {33} side chain packs optimally only with neighboring, antiparallel {33} chains Special Issue: John R. Miller and Marshall D. Newton Festschrift Received: January 11, 2015 Revised: February 28, 2015 Published: March 3, 2015 7740
DOI: 10.1021/acs.jpcb.5b00291 J. Phys. Chem. B 2015, 119, 7740−7748
Article
The Journal of Physical Chemistry B Chart 1. Structures of {33,19} and {192} and Intended Self-Packing of {33} Side Chains to Form Strip(e)s
and can assemble 1-D strip(e)s that are two molecules wide ({33} strip(e) = 1-D target column). The exterior of each {33} stripe projects {19} side chains perpendicular to the 1-D column, allowing integration into the {192} host. If successful, the distribution of distances between {33} stripes and the overall {33} stripe density can be tuned by varying the relative solution concentrations of diluent and amphipile molecules.
2. EXPERIMENTAL SECTION Scanning tunneling microscopy data was acquired using a Digital Instruments NanoScope 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) using mechanically cut 80/20 Pt/Ir tips (0.25 mm, Goodfellow, Oakdale, PA). Solutions of each compound were prepared by dissolving 2−3 mg of compound in ∼800 μL of phenyl octane (99%) at 20 °C and then filtered (0.02 μm filters). These concentrated solutions (2−3 mM) were stored at 10 °C. Solutions of lower concentrations (1−0.05 mM) were prepared prior to STM experiments and stored at 19 °C. Solutions with the two component’s combined concentration, CT, equal to 0.25 or 0.38 mM and with a molar ratio of {192} to {33,19}, RS, equal to 1, 2, 4, and 8 were prepared by mixing solutions of {192} and {33,19}. A series of RS = 1 solutions with total concentration CT = 0.5, 0.25, and 0.125 mM were prepared by serial dilution and used to identify conditions producing monolayers with minimal multilayer formation. For drop cast samples, a solution drop (1−3 μL) was deposited on a newly cleaved HOPG surface. Data collection commenced less than 10 min after drop casting. To prepare annealed samples, 5−10 μL of solution was deposited on a dimple cut in the center of an aluminum chamber. The graphite piece was placed on top of the solution with the freshly cleaved side facing down. A close fitting aluminum lid was used to seal the chamber to minimize solvent evaporation during annealing. The chamber was set on a programmable heating block. Typically, 20−30 min was required to heat a sample from room temperature (19 °C) to 45−55 °C. Samples were maintained at the target temperature for the reported annealing period before being cooled to room temperature (30 min minimum). For each sample preparation condition, 10−12 independent STM images were collected from a minimum of three separate sample preparations. The same images were used to determine the two components’ area ratio, the monolayer composition (i.e., {192}:{33,19} mole ratio), {33} strip length distributions, and linear density analyses (see the Supporting Information for the detailed descriptions of each of these procedures).
Figure 1. STM scans of {192} (left, 13 nm × 13 nm) and {33,19} (right, 12.5 nm × 12.5 nm) at the HOPG−phenyloctane interface. The anthracene cores appear as rectangular 3 × 2 dot patterns (yellow). The green parallelogram in each image marks a unit cell: {192} (a = 0.96 nm, b = 3.07 nm, α = 84°; 1 molecule per unit cell), {33,19} (a = 0.96 nm, b = 7.60 nm, α = 87°; 2 molecules per unit cell).
side chains and an adjacent column of anthracene cores. Monolayers of pure {33,19} exhibit a repeating sequence of four, parallel 1-D columns (Figure 1): an aliphatic column of close packed, kinked {33} diyne side chains, a column of anthracene cores, an aliphatic column of close packed, straight {19} side chains, and a second column of anthracene cores. The {33} and {19} side chains segregate into single composition aliphatic columns to maximize all chains’ van der Waals interactions; kinked {33} diyne side chains pack optimally with shape complementary, antiparallel aligned {33} chains, whereas straight {19} side chains pack best next to antiparallel aligned, straight {19} chains. Each {33,19} molecule comprising an anthracene column projects its {33} side chain in the same direction, thus comprising 50% of the adjacent {33} column, and projects its {19} side chain in the opposite direction, comprising 50% of the adjacent {19} column. The objective of preparing mixed monolayers of {192} and {33,19} is to modulate the spacing of the nearest {33} columns (referred to as stripes) by inserting multiple replicas of the {192} two column repeat. Varying the concentration of {192} in solution and in the monolayer will, in principle, translate into control over the density and mean spacing of {33} stripes. 3.1. Monolayer Composition: Impact of Component Solution Ratios, Concentration, and Annealing. The different sizes and shapes of the diluent, {192}, and amphiphile, {33,19}, should generate different equilibrium adsorption constants,24−27 and different adsorption and desorption rate constants to and from HOPG. As a first step in exploring shape amphiphile guided assembly, monolayer composition was determined as a function of the two components’ solution concentration ratio, their total concentration, and sample annealing treatment. Solutions with {192}:{33,19} mole ratios, RS, of 1, 2, 4, and 8 and total concentration CT = C{33,19} + C{192} = 0.25 mM were
3. RESULTS AND DISCUSSION Monolayers of pure {192} exhibit a repeating sequence of two, parallel 1-D columns (Figure 1): a column of close packed {19} 7741
DOI: 10.1021/acs.jpcb.5b00291 J. Phys. Chem. B 2015, 119, 7740−7748
Article
The Journal of Physical Chemistry B drop cast as phenyl octane solutions onto HOPG at 19 °C.28 Imaging commenced within 10 min and was completed within 8 h (Figure 2). The {192}:{33,19} mole ratios of the resulting
RS = 8 may arise, in part, from depletion of {33,19} solution concentration during monolayer assembly.29 It also may indicate a higher order concentration dependence of the {33,19} monolayer incorporation rate than of the {192} incorporation rate. The substantial under representation of {33,19} (RML > RS) in drop cast monolayers is surprising given that {33,19} molecules have larger surface area and should adsorb more strongly (exoergically) to HOPG than {192}.25−27 As such, {33,19} should be over represented in equilibrated monolayers (equilibrated RML should be
