Surface Patterning of Benzenecarboxylic Acids: Influence of Structure

Article pubs.acs.org/JPCC

Surface Patterning of Benzenecarboxylic Acids: Influence of Structure, Solvent, and Concentration on Molecular Self-Assembly Gina M. Florio,*,†,‡ Kimberly A. Stiso,† and Joseph S. Campanelli† †

Department of Chemistry and ‡Department of Physics, St. John’s University, Queens, New York 11439, United States ABSTRACT: We investigated the molecular self-assembly of pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid), trimellitic acid (1,2,4-benzenetricarboxylic acid), and 1,3,5-benzenetriacetic acid at the liquid/graphite interface to assess how the number and orientation of carboxylic acid groups modulate network formation. In addition, we studied the roles solvent composition and solution concentration play in the self-assembly of pyromellitic acid and trimellitic acid by investigating each in various alkanoic acid/alkanol solvents and solvent mixtures under saturated and diluted conditions. For pyromellitic acid, three distinct, ordered monolayer structures were observed, depending on the solvent type and solution concentration. Trimellitic acid was observed to form highly disordered monolayers due to the molecule’s inherent asymmetry, whereas high-symmetry networks are observed from the symmetric building block pyromellitic acid. No solvent or concentration effects were observed for trimellitic acid monolayers. Self-assembly was not observed for 1,3,5-benzenetriacetic acid, likely due to an insufficient adsorption energy and difficulty in forming extended hydrogen-bonded networks because of its nonplanar molecular structure. Density functional theory geometry optimizations and relaxed potential energy surface scans were used to explore the conformational preferences of isolated analogue molecules, benzoic acid and phthalic acid (1,2-benzenedicarboxylic acid), providing a qualitative picture of surface adsorption and network formation. The observed network structures and solvent/ concentration effects are rationalized using basic principles of thermodynamics and equilibrium.

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INTRODUCTION Molecular self-assembly at the liquid/solid interface is the spontaneous ordering of molecules, under equilibrium conditions, into well-defined networks, joined together by noncovalent bonds. In the absence of kinetic effects, molecular self-assembly is driven by minimization of the Gibbs free energy of the system (solution/monolayer/substrate), and thus a balance of enthalpic and entropic factors upon network formation.1,2 The total entropy of the system is partitioned among rotational, translational, vibrational, and conformational degrees of freedom. At a given temperature, the entropy is expected to decrease upon monolayer formation, due primarily to loss of rotational and translational entropy when molecules are confined in ordered arrays at the interface.1,2 The enthalpic contribution to the free energy of a physisorbed system is typically dominated by favorable adsorbate−substrate interactions; however, the details of patterns formed by the molecules are often dictated by the nature and strength of the intermolecular forces.3,4 Understanding the forces that initiate and control self-assembly is of practical interest because the spontaneous formation of highly ordered films on surfaces provides a synthetic route for formation of functional nanoscale structures.1,5 The design of supramolecular architectures relies on the precise ability to control the collective noncovalent intermolecular interactions.5−7 Intermolecular hydrogen-bonding (Hbonding) is a common motif used to orchestrate self-assembly due to their high relative strength and directionality. The Hbonding capacity of the carboxylic acid functionality is © 2012 American Chemical Society

commonly employed in two- and three-dimensional crystal engineering due to its ability to serve as H-bond donor and acceptor, as well as their preference for the formation of strong, centrosymmetric H-bonded dimers. Benzenecarboxylic acids have been investigated extensively due to their ability to form a multitude of well-ordered, twodimensional, H-bonded networks on planar surfaces. Molecular self-assembly has been reported for isophthalic acid (1,3benzenedicarboxylic acid),8−11 terephthalic acid (1,4-benzenedicarboxylic acid),8,12−14 and trimesic acid (1,3,5-benzenetricarboxylic acid)11,13,15−23 on graphite or gold surfaces. Selfassembly is not observed for phthalic acid (1,2-benzenedicarboxylic acid).8,24 The number and position of the carboxylic acid moieties on the benzene ring alters the morphology of the extended network, producing linear tapes,8,10,24 zigzag tapes,8,10,24 honeycomb (aka chicken wire),11,13,15−23 or rosettes (flower, superflower).11,13,15−23 The network structure can also be modulated by attaching alkyl chains to the benzene ring,10 changing the size of the aromatic core,10,25−27 the presence of a solvent,17,19,20,25,28 solvent concentration,21 changes in temperature,29 and varying the surface potential.11,14,22,30 Depending on the surface composition (e.g., Cu, Fe) and environmental conditions (e.g., high temperature, surface potential), metal−organic coordination networks and vertically aligned molecules can be observed,11,14,16,22,27,30−37 Received: February 24, 2012 Revised: July 23, 2012 Published: July 25, 2012 18160

dx.doi.org/10.1021/jp301840g | J. Phys. Chem. C 2012, 116, 18160−18174

The Journal of Physical Chemistry C

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Table 1. Summary of Solute/Solvent Combinations Studied and Resultsa solvent

a

solute

pentanoic acid

hexanoic acid

heptanoic acid

octanoic acid

nonanoic acid

pentanol undecanol

PMA TMLA BTA

CP1, O CP1, D NA

CP1 CP1, D −

CP1 CP1, D −

CP1 CP1, D −

CP1 CP2 CP1, D −

CP1 CP1, D −

CP1 CP1, D −

pentanoic acid/pentanol 1:1 mixture

pentanoic acid/undecanol 1:1 mixture

CP1 CP1, D −

CP1 CP1, D −

Key: CP1 = close-packed structure 1; CP2 = close-packed structure 2; O = open; D = disordered; NA = no assembly; dash (−) = not studied.

groups. The role solvent composition and solution concentration play in the molecular self-assembly is also addressed.

often involving the complexation of deprotonated carboxylate acid groups (i.e., carboxylate ions, COO−). Vertically aligned benzenecarboxylic acids are not observed under ambient conditions at the liquid/graphite interface given the low reactivity of graphite and its preference for π-stacking. Historically, our interest in benzenecarboxylic acid selfassembly was sparked by the observation of solvent-induced pseudopolymorphism in trimesic acid (TMA) self-assembly on graphite in a series of alkanoic acid solvents by Lackinger and co-workers.17 Two different types of monolayers, the so-called chicken wire and flower structures, were observed to form depending on the chain length of the solvent. Interestingly, both monolayers were porous, exhibiting a periodic arrangement of voids. We found the lack of a high-density surface structure, such as the postulated superflower structure,15 intriguing and set about to understand the factors responsible for self-assembly in TMA at the liquid/graphite interface,38 as well as the related benzenecarboxylic acid derivatives reported here. Our hypothesis is that solution concentration and equilibrium thermodynamics are responsible for the lack of a close-packed monolayer structure.38 High-density structures have been observed for TMA on gold in ultrahigh vacuum, in which the chicken wire and flower structures are simply two of a series of networks observed as a function of surface coverage.23 More recently, concentration effects have been observed in STM images of TMA on graphite in heptanoic acid, octanoic acid, and nonanoic acid solutions.21 Here we exploit the H-bonding properties of carboxylic acid moieties to form two-dimensional arrays of benzenecarboxylic acid derivatives at the liquid−graphite interface under ambient conditions. Scanning tunneling microscopy (STM) was used to examine the structure of H-bonded networks formed by two different moleculespyromellitic acid (1,2,4,5-benzenetetracarboxylic acid) (PMA) and trimellitic acid (1,2,4-benzenetricarboxylic acid) (TMLA), shown below. PMA was studied using a variety of alkanoic acid and alkanol solvents and solvent mixtures to determine how the presence of four carboxylic acid groups modulates the assembly structure and the observation of solvent-induced polymorphism (Table 1). PMA was previously reported to self-assemble at the liquid/graphite interface;8 however, no STM images or information on the network structure was published. TMLA has three carboxylic acid groups asymmetrically disposed about the benzene ring, thereby allowing a direct comparison with the threefold symmetric TMA. No previous work on self-assembly of TMLA has been reported at the liquid/solid interface, whereas it is known to induce ordered metal−organic coordination networks on copper surfaces under ultrahigh vacuum.35 We also studied solutions containing 1,3,5-benzenetriacitic acid (BTA); however, no network formation was observed. The defining role molecular structure has on network formation in benzenecarboxylic acids (or lack thereof) is addressed in terms of the number, position, and orientation of carboxylic acid

This paper reports several significant findings. First, PMA and TMLA are observed to form two-dimensional, selfassembled, H-bonded networks at the liquid−graphite interface. Three distinct packing structures (polymorphs) are observed for PMA: two high-density structures and at least one porous network. The observation of the porous structure is correlated with the chain length of the solvent, in conjunction with the solution concentration. No solvent effects are observed in TMLA monolayers. Molecular symmetry is shown to play a significant role in network formationasymmetric TMLA generates highly disordered films, whereas the high symmetry of PMA give rises to highly ordered, symmetric monolayers. Self-assembly is not observed for BTA, presumably due to its nonplanar geometry. Comparison of our results for PMA, TMLA, and BTA with planar benzenecarboxylic acid derivatives (e.g., TMA, terephthalic acid, 1 2−14 ,2 4, 39 isophthalic acid9−11,24,40,41) allows us to further assess the effects of molecular conformation on self-assembly. Finally, the STM studies are augmented by quantum chemical calculations on analogue molecules, benzoic acid and phthalic acid (1,2benzenedicarboxylic acid), providing qualitative information regarding the structural and energetic properties of isolated benzenecarboxylic acid derivatives and instructing our understanding of surface adsorption and network formation.

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METHODS I. Experimental Section. All STM experiments were performed using a Digital Instruments Nanoscope IIIa scanning tunneling microscope (Veeco Instruments) operating under ambient conditions between 16 and 20 °C. STM tips were mechanically cut from platinum iridium wire (Alfa Aesar, Pt:Ir 90:10 wt %). The basal plane of a highly oriented pyrolytic graphite crystal, HOPG (0001), was used as the substrate (Momentive Performance Materials, ZYB grade). Prior to solution deposition, bare graphite was freshly cleaved and imaged to verify the calibration of the piezoelectric tube scanner and the quality of the tip. Three different benzenecarboxylic acid solutes were studied: (1) pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid, Fluka, >97%), (2) trimellitic acid (1,2,4-benzenetricarboxylic acid, Aldrich, 99+%), and (3) 1,3,5-benzentriacetic acid (Fluka, 18161



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≥97.0%). All solutes and solvents were used without purification. PMA and TMLA solutions were prepared in a variety of solvents and solvent mixtures: the homologous series of alkanoic acids (pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, and nonanoic acid), undecanol, pentanol, a 1:1 mixture of undecanol and pentanoic acid, and a 1:1 mixture of undecanol and heptanoic acid. BTA was studied only in pentanoic acid solutions. Table 1 summarizes this information. For scanning in pentanol, STM tips were insulated with thermoplastic adhesive to minimize spurious solution current. Benzenecarboxylic acids are solids under standard atmospheric conditions and are sparingly soluble in moderately polar or nonpolar solvents due to strong intermolecular forces (e.g., Hbonding, π-stacking) in the 3D crystals. Solubility information for these specific derivatives could not be found in the literature. To determine the solubility of PMA in pentanoic acid, a calibration curve was prepared using the peak absorption at 293 nm from the UV/vis spectra of three different solutions of known concentration, and the absorption intensity was assumed to vary linearly as a function of solution concentration. We found the solubility of PMA in pentanoic acid to be 0.86 mM at room temperature. By comparison, the solubility of TMA is 1.3 mM in pentanoic acid and decreases monotonically with increasing solvent chain length.2 For STM imaging, all solutions were prepared at what was assumed to be saturation, with a large excess of the solid in a few milliliters of solvent, followed by sonication in an ultrasonic bath for approximately 30 min, to aid in solubilizing the carboxylic acids. Postsonication, the solutions were left unperturbed to equilibrate for at least 30 min to a few days prior to deposition for imaging. Recent work on trimesic acid self-assembly in alkanoic acid solutions emphasizes the need for sonication.21,28 For network formation, 5−10 μL of supernatant solution was deposited onto the graphite surface and STM measurements were performed in situ. Each solution was scanned on numerous days and with different tips to ensure reproducibility. Most STM images were obtained in constant current (topographic) mode. Typical scanning conditions are ±500− 1000 mV bias voltage, 100−450 pA tunneling current, and a scan rate of ∼7 Hz. Specific conditions are given for individual images in the figure captions, with the sign of the bias voltage referenced to the sample. No change in image contrast was observed upon changing the polarity of the tip−sample bias. Raw STM images were background corrected using a first-order flattening and smoothed by applying a low-pass filter one time, unless otherwise specified in the image caption. Quantitative information on the monolayer structure was obtained from the STM images. Using section analysis, the center-to-center distance between bright features (i.e., individual molecules) along a line of 5−15 features was measured, providing an initial averaged value of the intermolecular spacing. All spatial data reported are averages of multiple measurements from images obtained with different tips on different days. The standard deviation of the measurement is reported as its error. Unit cell parameters were obtained by processing the STM images using a demonstration version of the Scanning Probe Image Processor (SPIP) software package (Image Metrology). Within SPIP, raw data files are processed by plane-fitting and then using the linear autocorrelation method. Error was introduced to the unit cell measurements (reported in Table 2) because the demonstration version of SPIP reduces the image resolution from the full 512 × 512 pixels to 128 × 128 pixels.42

Table 2. Unit Cell Parameters for Pyromellitic Acid (PMA) in Different Solvents solvent pentanoic acida pentanoic acidb hexanoic acid heptanoic acid octanoic acidc nonanoic acidd pentanol pentanoic acid/ undecanol 1:1 mixture

a (nm)

b (nm)

γ (deg)

area (nm2)

molecules per unit cell

0.97 1.90 1.29 1.32 − 1.38 1.25 0.95

0.88 1.77 1.31 1.20 − 1.40 1.40 1.12

86.9 118 88.1 98.3 − 85.1 74.1 97.4

0.852 2.97 1.69 1.57 − 1.92 1.68 1.06

1 3 1 1 − 2 1 1

a

Close-packed structure. bOpen structure. cUnit cell measurements not made due to low resolution. dOffset dimer structure.

II. Computational. Density functional theory (DFT) calculations were performed on isolated analogue benzenecarboxylic acids benzoic acid and phthalic acid, shown below, to determine their structural preferences and estimate energy barriers associated with conformational isomerization. Benzoic acid was studied to benchmark the quality of the calculated structures and internal rotation barriers with respect to literature values. We assume that the gross features of the conformational landscapes of o-benzenecarboxylic acid molecules (PMA, TMLA, phthalic acid) will be largely the same; therefore, we exploited their similarity in structure to reduce the size of the molecule, and thereby the calculation time. All calculations were performed using Becke’s three-parameter exchange functional,43 the Lee, Yang, and Parr correlation functional,44 and the 6-311G basis set within the Gaussian 03 suite of programs45 on a PC. Hartree−Fock geometry optimizations were first performed on phthalic acid to generate reliable starting structures for the DFT calculations.

Relaxed potential energy surface (PES) scans were performed on B3LYP/6-311G-optimized benzoic acid and phthalic acid to estimate barrier heights between conformational minima. For the PES of phthalic acid, one of the carboxylic acid groups was specified to rotate about its C(benzene)−C(acid) bond by a full 360° in 18° increments, while all other coordinates, including the other acid group, were allowed to fully optimize. The starting configuration of both carboxylic acid groups was the syn configuration, despite the fact that this is not the global minimum configuration for phthalic acid (v.i.). Since we are interested in relating our computational results to the experimental self-assembly, we ran the PES calculations on a phthalic acid conformation that is likely to result in extended network formation at the liquid/ solid interface (structure 2 of Figure 5). Ortho-substituted benzene molecules such as PMA and TMLA are nonplanar when isolated, and presumably in solution, due to steric hindrance between adjacent acid groups. Because the enthalpy of surface adsorption scales linearly with 18162



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Figure 1. STM topographic images of pyromellitic acid (PMA) in pentanoic acid (a) in the close-packed structure (10 nm × 10 nm, −500 mV, 250 pA, 6.10 Hz) and (b) in the open structure (10 nm × 10 nm, −1000 mV, 100 pA, 10.2 Hz). Bright spots in the images are individual PMA molecules. Section analysis, autocorrelation images, and unit cell measurements are indicated for each network. The intermolecular spacings are those between the molecules indicated, while average values are reported in Table 3. The open structure (b) was observed upon dilution of the stock solution by a factor of 2 prior to deposition.

pentanoic acid, and (3) a second type of close-packed structure that is a minority phase in nonanoic acid. In STM topographs of PMA monolayers, molecular-level resolution is obtained in which the diameter of the bright features (∼0.9 nm) is consistent with the size of an individual PMA molecule (0.88 nm). The dominant morphology of PMA at the liquid−graphite interface is a close-packed, high-density structure observed in all solvents and solvent mixtures investigated. An example STM topographic image of the close-packed morphology of PMA formed in equilibrium with a pentanoic acid solution is shown in Figure 1a. Each bright feature is a single PMA molecule. Section analysis taken along the black line in the image, shown as an inset in Figure 1a, reveals a center-to-center distance between two PMA molecules of 0.88 nm. This nearestneighbor distance is consistent with the average diameter of a single PMA molecule (blue circle) of 0.88 nm. The unit cell determined by the autocorrelation method is a = 0.97 nm, b = 0.88 nm, γ = 86.9° and contains one molecule. Based on the unit cell measurements (Table 2) and section analysis data (Table 3), the close-packed structures exhibited in each solvent/solvent mixture are similar, with the exception of a minority phase observed only in nonanoic acid. Dispersion among the data in Table 2 is likely the result of the analysis method, as described in the Methods section. Alternatively, it is possible that the dispersion reflects a subtle solvent-induced structural change, akin to what we reported for 1-bromalkane self-assembly on graphite.47,48 The close-packed PMA monolayer has a surface density, ρclosed, of 1.17 molecules/nm2. In

the number of heavy atoms in direct contact with the graphite,46 planarity is often the preferred configuration for a physisorbed molecule. In order to maximize favorable surface adsorption interactions and facilitate intermolecular H-bonding for network formation, these molecules may undergo structural changes (to planar or near planar configurations) upon surface adsorption. Calculations were performed to estimate the energy penalty associated with forcing ortho-substituted benzenecarboxylic acid molecules flat. Geometry optimizations were performed on five phthalic acid conformers found to be representative minima on the global PES from the relaxed PES scan. Frequency calculations were performed on these optimized structures to confirm convergence to minima and to find the zero-point contribution to the total energy. Each of the five optimized structures was then forced into a planar configuration and reoptimized with symmetry restrictions to guarantee planarity throughout the calculation (fixing certain dihedral angles while allowing all other degrees of freedom to optimize).38 The difference in energy between the structures in their unrestricted and planar-optimized configurations serves as an upper-limit estimate of the energy penalty associated with the conformation change in the absence of solvent, intermolecular interactions, and the substrate.

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RESULTS

I. Pyromellitic Acid. Three distinct H-bonded networks of PMA were observed via STM imaging at the liquid/graphite interface: (1) a close-packed morphology in each solvent studied, (2) a morphology containing voids seen only in 18163



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Table 3. Intermolecular Spacing and Cavity Length Measured from Section Analyses of STM Images of Pyromellitic Acid (PMA) in a Range of Solvents solvent pentanoic acid pentanoic acid hexanoic acida heptanoic acida octanoic acidb nonanoic acidc pentanola pentanoic acid/undecanol 1:1 mixture

intermolecular spacing (nm) 0.88 0.89 1.16 1.09 − 0.92 1.14 0.84

± ± ± ±

0.03 0.01 0.19 0.04

± 0.01, 1.05 ± 0.03 ± 0.02 ± 0.01

cavity diameter (nm) N/A 1.78 ± 0.01 N/A N/A N/A N/A N/A N/A

a

Measurement from images containing drift, contributing to error in the intermolecular spacing. bMeasurement not made due to low resolution; close-packed structure observed. cIntermolecular spacing within and between dimer pairs (blue and red arrows, respectively, in Figure 3).

many of the STM images, domains are observed with the same packing morphology by different orientation along the graphite surface. The relative orientation between domains is measured to be approximately 80°. While the exact orientation of the monolayer relative to the underlying graphite lattice was not determined explicitly in this work, we expect there to be a distinct epitaxial relationship as there is for TMA.2 In addition to the close-packed structure, a monolayer containing voids was also observed for self-assembly of PMA in pentanoic acid, designated here as the “open structure.” This monolayer has a lower density than the close-packed structure described above, containing periodic arrangements of voids (Figure 1b), reminiscent of the porous structures found for TMA15,17−20,23,38 and 1,3,5-benzenetribenzoic acid.25 The average cavity diameter of the open structure of PMA is 1.78 nm. Section analyses, such as the one taken along the black line in the image, shown as an inset in Figure 1b, reveal an average nearest-neighbor intermolecular spacing (blue scale bar) of 0.89 nm, corresponding to the average diameter of a single PMA molecule (blue circle). The unit cell determined by the autocorrelation method, also shown as an inset in Figure 1.b, is a = 1.90 nm, b = 1.77 nm, γ = 117.7° and contains three molecules. Section analysis performed on the close-packed PMA morphology (Figure 1a) gives the second nearestneighbor distance (red cursors in line profile) as 1.76 nm, the exact length as the cavity in the open structure (Figure 1b), suggesting that the open structure could be related to the closepacked form, less one-quarter of a molecule per unit cell. The unit cell analysis shows that the surface packing density is ρopen = 1.01 molecules/nm2 for the open structure. The open structure is seen only in solutions of pentanoic acid that have been diluted by a factor of 2−10 prior to deposition. High-resolution STM images of the open structure of PMA in pentanoic acid often show evidence of solvent inclusion. Figure 2 depicts one such image in which features are apparent within the cavities (red circle) surrounded by PMA molecules (yellow circle). With an estimated length of 0.75 nm, pentanoic acid is nearly the same size as a PMA molecule (∼0.88 nm), and it is likely that solvent molecules occupy the empty adsorbate site of the open structure. Upon careful inspection of the zoomed in portion of Figure 2, two fuzzy, narrow features are observed on either side of the cavity (within the red circle),

Figure 2. STM topographic image of PMA in pentanoic acid at (a) 20 nm × 20 nm resolution with (b) a 10 nm × 10 nm resolution zoomed inset and (c) the same 10 nm × 10 nm resolution image show as a 3D surface plot. The yellow circle indicates a single PMA molecule, and the orange circle indicates the cavity with solvent present. The image was obtained at −1 V, 100 pA, and 8.72 Hz.

which are assigned as two weakly bound, and therefore mobile, solvent molecules. A third PMA packing structure was observed as a minority phase for concentrated solutions in nonanoic acid. The network is a close-packed structure consisting of offset pairs of molecules (Figure 3a) and is easily distinguished from the close-packed structure observed for PMA in all other solvents/ solvent mixtures (Figure 3b). In this minority phase, PMA forms linear rows of dimer units that are offset from adjacent rows, as shown by the blue arrows in the image in Figure 3a. The typical close-packed structure of PMA found in all other solvents shows linear rows of PMA molecules in alignment with adjacent rows, indicated by blue arrows in the image in Figure 3b. The section analysis taken along the dashed line in Figure 3a shows two different characteristic distances: the space between the PMA dimer units is 1.08 nm (red arrow), while the space between the molecules within the dimer pair is 0.92 nm (blue arrow). In contrast, section analysis for the closed structure observed in pentanoic acid (Figure 3b) shows uniform separation of 0.85 nm (red arrows) between all molecules in the film. II. Trimellitic Acid (TMLA). TMLA assembly was explored to determine if asymmetric disposition of carboxylic acid groups alters the assembly properties as compared to those monolayers formed by symmetric TMA and PMA molecules. Solutions of TMLA were prepared in the same series of solvents used in the PMA study (Table 1). Acquisition of highresolution images of TMLA proved difficult in all solvents and concentrations investigated. Figure 4 shows representative, 18164



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Figure 3. (a) STM image of PMA at the interface between a saturated solution of PMA in nonanoic acid and graphite (12 nm × 12 nm), acquired at −800 mV bias voltage, 200 pA, 12.2 Hz, and a 60° angle scan angle. (b) STM image of PMA at the interface between a saturated solution of PMA in pentanoic acid and graphite (12 nm × 12 nm), acquired at −800 mV, 220 pA, and 7.63 Hz. Line profiles are also shown and indicate two different intermolecular spacings for assembly in nonanoic acid solutions (a), whereas only one is found in pentanoic acid (b).

on the solubility of TMA in alkanoic acid solvents.2,17 There are several possible explanations for the negative result for BTA, including low solubility, tip effects, monolayer instability on the time scale of the STM imaging, or an inability to form extended networks due to molecular geometry. IV. DFT Calculations. Two different benzenecarboxylic acids, benzoic acid and phthalic acid (structures shown below), were studied using DFT calculations. As the simplest benzenecarboxylic acid derivative, benzoic acid served as the starting point for our calculations and was used to benchmark the quality of our computational results, given its extensive computational and experimental literature. The only benzoic acid conformer observed experimentally under all conditions is planar and has the carboxylic acid group in the synorientation.49 As expected, our calculations predict this to be the global minimum benzoic acid conformation. The zero-point corrected energy difference between the syn- and anti-oriented, planar configurations is 8.18 kcal/mol, which is close to the known energy difference of 6.5 kcal/mol for the syn- and anticonformers of acetic acid in the gas phase.50 From our PES calculation, we find that the barrier to internal rotation of the syn-oriented acid group relative to the plane of the benzene ring to be large (8.76 kcal/mol), given the loss of resonance delocalization between the π-electrons in the phenyl ring and

molecularly resolved STM topographic images of TMLA at the liquid/graphite interface. The size of the bright features in all STM images of TMLA is close to the predicted diameter of a single TMLA molecule (∼0.9 nm). Similar close-packed structures were observed from all solutions of TMLA in the series of alkanoic acid solvents (Figure 4a) and solvent mixtures (Figure 4b). The STM images of TMLA reveal a high degree of disorder, as seen by following the dashed line in Figure 4b and in the variable spacing between molecules in the rows (Figure 4c). Because of the disorder, quantitative section analysis of the monolayers was not possible via the autocorrelation method. Figure 4c shows a 2D section analysis taken along the white line in Figure 4a, where variability is observed in the spacing between individual molecules. The distances measured between pairs of molecules labeled in Figure 4a and c are shown in Table 4. The standard deviation between the individual measurements is an order of magnitude larger than that observed for ordered arrays of PMA (Table 3), indicative of the degree of disorder found in TMLA monolayers. III. 1,3,5-Benzentriacetic Acid (BTA). Despite numerous attempts, self-assembly was not observed for saturated solutions of BTA at the pentanoic acid/graphite interface. BTA was not investigated in other solvents/solvent mixtures, as we expect BTA to be most soluble in the shorter chain length acids, based 18165



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Figure 4. (a) Topographic STM image of a molecular self-assembled monolayer formed by trimellitic acid (TMLA) in heptanoic acid at halfsaturation (12.7 nm × 12.7 nm). The tunneling parameters are +900 mV, 110 pA, and 7.63 Hz. (b) Topographic STM image of a molecular selfassembled monolayer formed by TMLA in a 1:1 mixture of undecanol and heptanoic acid (17 nm × 17 nm). The tunneling parameters are +800 mV, 130 pA, and 7.63 Hz. The high degree of disorder is indicated by the dotted white line, as the line cannot be drawn directly across a row of molecules. (c) Section analysis taken along the white line in the STM image (a), with the position of each molecule labeled according to the image. Corresponding intermolecular distances given in Table 4.

carbonyl group upon out-of-plane rotation. This barrier compares well with other calculations of benzoic acid51 and the known barrier for out-of-plane rotation of the aldehyde group of benzaldehyde, 7.6−7.9 kcal/mol.52 Additional details regarding benzoic acid calculations can be found elsewhere.38 To determine the preferred molecular orientations of orthosubstituted benzenecarboxylic acid derivatives, DFT calculations were performed on phthalic acid, including a series of geometry optimizations and harmonic frequency calculations on stationary points and relaxed potential energy surface scans. Under isolated conditions, PMA is nonplanar due to the presence of two pairs of ortho-oriented carboxylic acid groups. We expect that PMA and phthalic acid will exhibit similar conformational landscapes associated with internal rotations of the ortho-carboxylic acid groups. Figure 5 shows the five lowlying conformational minima identified in this work for phthalic acid, associated with different relative orientations of its orthogonal acid groupsthree of the conformers have both

Table 4. Intermolecular Spacing from the Section Analysis of Trimellitic Acid (TMLA) in Heptanoic Acid Shown in Figure 4a, b molecule paira

intermolecular spacing (nm)

1/2 2/3 3/4 4/5 5/6 6/7 average standard deviation

0.841 1.07 0.920 1.22 1.07 1.07 1.03 0.134b

a

Spacing measured between pairs of molecules labeled in Figure 4a. The standard deviation is an order of magnitude greater than that measured for ordered assemblies (Table 3).

b

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coordinates but one dihedral angle, starting the carboxylic acids syn effectively traps them in this configuration. In addition to the relaxed PES calculation, we have estimated the enthalpic loss associated with forcing representative phthalic acid conformers into planar configurations. A second set of geometry optimizations and vibrational frequency calculations were performed on the five optimized stationary points shown in Figure 5, in which the molecules were locked into a planar configuration using symmetry restrictions. Figure 5 shows the optimized planar geometry of each conformer shown beside its original, unrestricted optimized structure. The change in energy between the unrestricted and restricted geometry optimization, ΔEn, is given in Figure 5. The energy ordering of structures 1−5 changes somewhat upon forcing the structures planar. For structures 1 and 5, the enthalpic penalty for planar adsorption is nonexistent or very small, as ΔE1 is 0.00 and ΔE5 is +3.88 kcal/mol. Significant destabilization is observed for structures 2−4 when forced planar. Structures 2, 3, and 4 are destabilized by +9.86, +8.44, and +7.74 kcal/mol, respectively, relative to their original nonplanar configurations. The relative energy ordering of structures 2−4 in the flat configuration depends on the interior oxygen−oxygen separation of the two acid groups, due to the reduction of electronic repulsion between the lone pairs of electrons on the interior oxygen atoms at large O−O separations (Figure 5). In a planar configuration, 4 exhibits the lowest relative energy and has the largest O−O separation (2.61 Å), relative to somewhat smaller separation and higher ΔE in 3 (2.55 Å) and 2 (2.50 Å). As expected, other bond lengths and dihedral angles change slightly for each structure (1−5) to compensate for the restriction of planarity.38 The energy difference between the phthalic acid conformers (1−5) optimized in their unrestricted and restricted geometries, ΔEn, is used here as an upper-limit estimate of the energy required to flatten the molecule for planar adsorption and extended network formation. For monolayers built off from PMA monomers with two intramolecular bonds, akin to phthalic acid conformers 1 and 4, we expect the enthalpic penalty to planar adsorption to be 0−6 kcal/mol/molecule, while monolayers formed from PMA monomers with two sets of orthogonally oriented, syn-acid groups, akin to phthalic acid conformers 2−4, the penalty is between 4 and 20 kcal/mol/molecule. Note that we assume that the PMA values will be double that of phthalic acid, based on the symmetry of the molecule.

Figure 5. Summary of the five phthalic acid conformers (a−e) optimized under unrestricted and planar conditions. The energy of each structure is indicated in units of kcal/mol and referenced from the lowest energy conformation in the unrestricted (blue) or planar (red) geometries. The net energy change of each structure from its unrestricted to planar optimization is shown as ΔEn in (a)−(e). The separation between interior oxygen atoms for the flat conformations of 2−4 is given in (b), (c).

acids in the syn-configuration (Figure 5b−d), while the other two have one acid group syn and the other anti (Figure 5a, e). For benzoic acid, the global minimum has the acid in the synconfiguration. The energy ordering of the optimized structures results from a trade-off between the minimization of unfavorable electronic effects and maximization of favorable intramolecular interactions. The lowest energy conformation of phthalic acid is structure 1 (Figure 5a), in which the antioriented carboxylic acid group enables formation of an intramolecular H-bond between the OH of one acid group and the carbonyl oxygen of the other with a H-bond angle of 161°, generating a seven-member ring. The DFT calculations indicate that for low-energy phthalic acid conformers containing intramolecular H-bonds (Figure 5a, e), smaller out-of-plane angles are observed for the orthogonal acid groups, while those conformers without intramolecular H-bonding have them rotated out of the plane of the benzene ring by approximately 36° (Figure 5b−d). Additional structural details on the individual conformations can be found elsewhere.38 The relaxed PES calculation of phthalic acid, in which one acid group was forced to rotate in 18° increments relative to the plane of the benzene ring for a full 360°, was started from structure 2 (Figure 5), in which both acid groups are in the synorientation and have an out-of-plane angle of about 36°. The barriers between conformers are all found to be less than 2 kcal/mol, with the highest energy conformer one in which the carboxylic acid is oriented perpendicular with respect to the plane of the benzene (89.6°), with a relative energy of +1.63 kcal/mol (not zero-point corrected). In addition, the relaxed PES calculation indicates that the free acid group monotonically reorients as a function of the stepped acid group’s orientation. Despite the fact that the molecule was free to optimize all

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DISCUSSION I. Pyromellitic Acid (PMA). A. Molecular Structure and Network Formation. Determining the exact details of the molecular structure of PMA within the network, as well as the intermolecular H-bonding scheme for the monolayer, is complicated by several factors. Because the STM images (Figures 1a, 3b) do not all have atomic-level resolution, they cannot provide unequivocal assignments. Many of the STM studies on small benzenecarboxylic acid molecules in the literature show only molecular-level resolution.2,53 It is well established that the carboxylic acid functional group appears as a region of low tunneling current, rendering it “dark” in STM images, whereas electron-rich regions such as π-systems appear “bright.”53−59 Furthermore, our DFT calculations, in concert with the literature,24,60 suggest that the potential energy landscape for phthalic acid (and by extension that of PMA) is rather complexthere are a series of low-lying conformational minima in which the relative orientation of the acid 18167



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Figure 6. Proposed models for the close-packed structure of PMA on graphite: (a) polymorph A with four doubly H-bonded carboxylic acid groups per molecule and (b) polymorph B with two doubly H-bonded carboxylic acid groups and two intramolecular H-bonds per molecule. Proposed models for the open structure of PMA: (c) polymorph A′ identical to polymorph A with one less molecule per unit cell and (d) polymorph C with four doubly H-bonded carboxylic acid groups per molecule. The green circles indicate favorable H-bonding interactions.

possible for molecules to adsorb in a nonplanar fashion, but only when favorable enthalpic contributions from intermolecular van der Waals forces and π-stacking can compensate for the significantly lower adsorbate−substrate enthalpy, as well as the entropic cost associated with forming the more densely packed monolayer.29,61 For PMA, vertical packing does not seem likely to facilitate enhanced intermolecular forces and the unit cell dimensions do not support a vertical configuration. Thus, we will assume a planar PMA molecule as the starting point for network formation. Guided by our DFT calculations, we hypothesize network structures consistent with the STM images of PMA on graphite. Two different types of network configurations for the closepacked PMA networks are proposed, structures A and B, shown in Figure 6. Network structure A is built from the planar version of PMA in which the ortho-carboxylic acid groups are both syn and oriented in an alternating fashionthe acid groups in the 1 and 4 positions have their CO group pointed up and OH pointed down toward the CO group of the carboxylic acid at the 2 and 4 positions. This PMA conformer is akin to phthalic acid conformer 2 from Figure 5. The key features of this 2D network are ∼120° angles between rows of PMA molecules and all molecules having an identical adsorption orientation. Two similar versions of network A can be conceived by starting from PMA analogues the planar, syn-phthalic acid conformers 3 and 4 of Figure 5. We anticipate these alternate versions having nearly identical lattice parameters and thermodynamic properties (i.e., enthalpic

groups differ, as well as a set of conformers in which the orientation of the atoms within each acid group differ (syn versus anti) (Figure 5). Despite these complications, we can make some general assignments of network structures. We anticipate that, whenever possible, molecules adsorb in a planar configuration, giving rise to the most favorable enthalpy of adsorption and enhanced intermolecular interactions. The STM images are consistent with planar (or near planar) adsorptionno intensity modulation is observed in the section analysis data (Figures 1a, 3), just periodic arrays of round spots of roughly equal size and brightness, with unit cell measurements appropriate for planar adsorption.29 We suggest that the enthalpic penalty associated with flattening out a PMA molecule, between 0 and 20 kcal/mol depending on the conformation (v.s.), can be compensated for by enthalpic gains associated with planar (or near planar) surface adsorption and enhanced intermolecular forces (v.i.). It should be noted that Martsinovich and Troisi report DFT calculations in which phthalic acid is nonplanar in its dimers and trimers and adsorbs on graphite and Au(111) surfaces, flattening somewhat upon adsorption, and forms extended networks.24 While surface adsorption and intermolecular H-bonding are destabilized, the nonplanarity does not prevent network formation.24 Given that the calculations were performed under vacuum conditions, we anticipate that at the liquid/solid interface the solvent effectively smoothes out the potential energy surface,47,48 thereby facilitating structural changes. In addition, STM studies of much larger benzenecarboxylic acids have shown that it is 18168



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Figure 7. (a) High-resolution STM image of PMA in pentanoic acid in the open structure and (b) its corresponding network structure (structure C from Figure 6).

(Figure 6c, d); however, structure A′ has four undercoordinated carboxylic acid groups about the cavity perimeter, while C has none. Solvent is expected to coadsorb and stabilize PMA monolayers containing periodic arrangements of voids,26,29 as is observed in high-resolution STM images of porous PMA networks (Figures 2, 7). We can estimate the enthalpic gain, ΔH, on a per molecule basis associated with PMA adsorption and H-bonding in the different network structures proposed in Figure 6, using an average interaction energy of −1.67 kcal/mol per heavy atom in direct contact with the graphite surface, based on molecular mechanics calculations of other benzenecarboxylic acids on model graphite surfaces (Table 5).26,29 This value agrees well with the interaction energy of −1.5 kcal/mol per heavy atom determined from temperature-programmed desorption measurements of n-alkane derivatives on graphite.46 The adsorption energy for a single, planar PMA molecule on graphite is estimated to be −30.1 kcal/mol. Additional enthalpic gains arise upon monolayer formation due to H-bonding, van der Waals interactions, and other favorable intermolecular interactions. Here, we only consider H-bonding, where the enthalpy associated with intermolecular, cyclic H-bond dimerization is taken to be −14.3 kcal/mol (−60 kJ/mol) and the formation of linear, intramolecular H-bonds is taken to be −2.63 kcal/mol (−11 kJ/mol).26,29 In the absence of solvent coadsorption, ΔH ranges from −58.7 kcal/mol for polymorph A′ to −87.3 kcal/ mol for A and C (Table 5). As expected, structures A and C have the most favorable adsorption enthalpies because each have PMA molecules interacting with four other PMA molecules through four cyclic H-bonded dimers. However, when solvent coadsorption in the cavity is considered, structure C is expected to have the most favorable adsorption enthalpy (−111 kcal/mol), far surpassing network A. Structure A′ also becomes significantly stabilized with solvent coadsorption, both with and without solute−solvent H-bonding, having ΔH values comparable to network A. Despite the relatively low enthalpic gain of −64.0 kcal/mol for structure B, we are hesitant to rule it out because it is based on the global minimum conformation for the isolated PMA molecule, with little energy barrier to flattening out. We can also estimate the entropic loss associated PMA adsorption on a per molecule basis due to loss of translational

gains and entropic losses) as A. Network structure B is based on the global minimum phthalic acid conformer, structure 1 in Figure 5, and allows for a near rectangular alignment of rows of PMA molecules. In network structure A, each PMA is Hbonded to four other PMAs in the H-bonded dimer configuration (Figure 6a), wherein each H-bonded dimer forms an eight-member ring extending the delocalization of the π-electrons in the benzene ring and carbonyl group with those of the neighboring molecule. In contrast, the presence of the two intramolecular H-bonds limits each PMA molecule in network B to H-bond to only two other PMAs in the cyclic dimer configuration (Figure 6b). However, structure B starts from the global minimum conformation, with little energy barrier to flattening out. Interestingly, none of the studies of two-dimensional self-assembly of ortho-benzoic acids consider monolayer structures arising from monomers containing intramolecular H-bonds.8,24 The STM experiments indicate that close-packed PMA monolayers are nearly rectangular; however, given the symmetry of the molecule, we expect that any close-packed network built via cyclic H-bonding in the dimer configuration would have a unit cell angle of ∼120°. It is only when we consider a PMA configuration with intramolecular H-bonds that we can conceive of networks with smaller angles. In addition to close-packed monolayers, PMA also exhibits an open structure in pentanoic acid solutions diluted by a factor of 2−5 from saturation prior to deposition. The unit cell dimensions of the open structure are a = 1.90 nm, b = 1.77 nm, and γ = 117.7°, and it contains three molecules. From STM images of the open structure containing submolecular resolution, we can discern the network packing structure C (Figure 6d), consisting of a trimeric base unit in which three PMA molecules are associated via cyclic, H-bonded dimers. Figure 7 shows one such high-resolution image in which three different adsorption orientations are apparent, indicated by the green, red, and yellow boxes encasing individual PMA molecules. For STM images of the open structure obtained without submolecular resolution, it is impossible for us to rule out alternate network structures, such as A′ (Figure 6c), which is analogous to the close-packed network structure A (Figure 6a), less one-quarter of a molecule per unit cell. Network structures A′ and C have similar distances and surface densities 18169



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Table 5. Estimated Enthalpic Contribution, ΔH, on a Per Molecule Basis to the Gibbs Free Energy for Monolayer Formation for Each PMA Network Proposed in Figure 6 structure

ΔH (kcal/mol)

A

−87.3

B

−64.0

C

−87.3

A′

−58.7

C + 2 5Ab

−111

A′ + 2 5Ab

−82.1

A′ + 2 5A (HB)c

−96.4

four acid groups versus three is responsible for the shift to closed networks as the preferred configurations at the liquid/ graphite interface. As indicated by the DFT calculations presented here, as well as in the literature,24,38,60,64 the potential energy surface of an isolated benzenecarboxylic acid molecule is quite rugged, consisting of numerous low-lying minima related via conformational isomerization. It is striking that despite the fact that the molecular PES becomes significantly more complex as the number of carboxylic acid groups attached to the benzene ring increases, the number of distinct self-assembled monolayers at the liquid/graphite does not. There are far fewer network structures observed here for PMA as compared to TMA, in which a range of network structures has been reported.15,17,21,23,28 Again, this speaks to the strength of the intermolecular H-bonding in these resonance delocalized, cyclic dimer configurations. Given the fact that majority of the molecules in benzoic acid solutions are found in the cyclic, Hbonded dimer configuration, with a known binding energy of about −16 kcal/mol, we expected that the majority of PMA molecules exist as dimers (or higher order clusters) in solution, possessing equally large binding energies. It is highly probable that self-assembled monolayer formation of PMA at the liquid/ solid interface proceeds via adsorption of dimers (or higher order clusters) from solution, followed by reorganization and extended network formation at the interface. The nucleation, growth, and evolution of benzenecarboxylic acid monolayers at the liquid/solid interface are subjects warranting further experimental and theoretical exploration. B. Solvent and Concentration Effects on Self-Assembly. Both the nature of the solvent and the solution concentration are observed to influence molecular self-assembly of PMA at the liquid/graphite interface. In concentrated (sonicated) solutions prepared in all solvents and solvent mixtures studied here (Table 1), PMA self-assembles into one of two closepacked structures (Figures 1a, 3). Interestingly, the offset dimer network structure observed for PMA (Figure 3a) is only observed as a minority phase in nonanoic acid solutions, while all other solvents and solvent mixtures show only the rectangular form seen in Figures 1a and 3b. Upon dilution of concentrated PMA solutions prepared in pentanoic acid by a factor of 2−10, a new network structure is observed. Highresolution STM images suggest the network structure C (Figures 2, 7). This concentration effect was only observed for pentanoic acid solutions. Since we anticipate that PMA follows the same overall solubility trend as TMA in alkanoic acids, where the solubility decreases with increasing alkanoic acid chain length,17 we would expect porous networks to be favored in the longer chain length solvents, particularly upon dilution. Our observation of a solvent-specific concentration effect suggests that open network formation for PMA on graphite in the other solvents/mixtures is thermodynamically inaccessiblethe enthalpic gain associated with surface adsorption and self-assembly is insufficient to compensate for the entropic loss.26,29 In the absence of a concentration effect for PMA assembly in the other solvents, we are left to presume that the size match between pentanoic acid and PMA enables the solvent to adsorb, rendering the open structure thermodynamically stable.26 Longer chain acids may not fit in the pores in the all-trans configuration, rendering them partially adsorbed, or fully adsorbed but in a nonlinear manner that does not allow for optimal packing and limits the number of solvent molecules in the pores. Each scenario results in less favorable enthalpy

formulaa ΔH = −30.1 kcal/mol + (0.5 × mol) ΔH = −30.1 kcal/mol + (0.5 × mol) + (2 × −2.63 kcal/mol) ΔH = −30.1 kcal/mol + (0.5 × mol) ΔH = −30.1 kcal/mol + (0.5 × mol) ΔH = −30.1 kcal/mol + (0.5 × mol) + (2 × −11.7 kcal/mol) ΔH = −30.1 kcal/mol + (0.5 × mol) + (2 × −11.7 kcal/mol) ΔH = −30.1 kcal/mol + (0.5 × mol) + (2 × −11.7 kcal/mol)

8 × −14.3 kcal/ 4 × −14.3 kcal/ 8 × −14.3 kcal/ 4 × −14.3 kcal/ 8 × −14.3 kcal/ 4 × −14.3 kcal/ 6 × −14.3 kcal/

a

The enthalpy change is estimated for a single molecule as the sum of the PMA adsorbate−substrate binding energy of −1.67 kcal/mol per heavy atom (18 × −1.67 kcal/mol = −30.1 kcal/mol), contribution(s) due to H-bonding, and adsorbate−substrate binding enthalpy of solvent molecules, where applicable (7 × −1.67 kcal/mol = −11.7 kcal/mol for pentanoic acid). Intermolecular H-bonding in the cyclic dimer arrangement contributes −14.3 kcal/mol (−60 kJ/mol) per dimer, while linear OH...O bonds contribute −2.63 kcal/mol (−11 kJ/ mol). The enthalpy of H-bonding in the cyclic dimer arrangement is calculated as the number of H-bonds of this type, times the dimerization energy, times a factor of 0.5 to correct for double counting of the pairwise interactions. bEstimate includes two pentanoic acid (5A) molecules. cEstimate includes two pentanoic acid (5A) molecules H-bonded to PMA in the cyclic dimer configuration.

and rotational degrees of freedom using the method of Whitesides and co-workers62 as implemented by the Lackinger group,26,29 where ΔStot ≈ ΔStrans + ΔSrot. We find that ΔStrans = −0.0433 kcal/mol and ΔSrot = −0.0325 kcal/mol, for a total entropic loss of −0.0758 kcal/mol for PMA adsorption, on par with that observed for other benzenecarboxylic acids.26,29 We expect the entropic loss to be worse for networks with the highest packing density (e.g., A and B); however, considering solvent coadsorption will lead to essentially identical entropic loss for all structures.29 Without accurate packing densities for the various structures proposed in Figure 6, we cannot scale ΔH and ΔS on a per unit area basis, making it impossible to compare their Gibbs free energy and perform a more complete quantitative thermochemical analysis. Future theoretical analysis on the proposed monolayers is planned to address this issue. For benzenetricarboxylic acids, such as TMA, 1,3,5benzenetribenzoic acid (BTB), and 1,3,5-tris[4′-carboxyl(1,1′biphenyl-4-yl)]benzene (TCBPB), porous network are routinely observed.2,17,25,26,39,63 Here we find that the addition of a fourth acid group to the benzene ring in PMA effectively shuts down the possibility of forming open networks, except under the special conditions of dilution in a solvent of matching length. This effect is not related to the change in molecular weight, and thus increased adsorption enthalpy, of the benzenecarboxylic acid by addition of the fourth acid group to the benzene ring, as BTB and TCBPB are much larger than TMA and still exhibit the tendency to form porous networks. We suggest that in PMA the increased intermolecular interactions associated with multiple H-bond formation via 18170



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Figure 8. Example network structures for TMLA illustrating (a) ordered and (b) disordered configurations.

intramolecular H-bonding is expected to be larger than that for benzenetricarboxylic acids, we suggest that the only way an open network will form is if the solvent can readily stabilize cavity formation. In this case, the solvent whose chain length best matches the diameter of a single PMA molecule is pentanoic acid. In contrast, the solvent effect observed for PMA in nonanoic acid is somewhat mysterious. The offset-dimer morphology observed as a minority phase for PMA in nonanoic acid resembles the assembly of isophthalic acid (1,3-benzenedicarboxylic acid)8,10,11 and TMA,28 at the liquid/graphite interface, in which zigzag tapes of molecules joined by the cyclic Hbonded dimer motif are observed. It is unclear as to why this morphology is only observed for PMA in nonanoic acid solutions. Perhaps nonanoic acid stabilizes a different conformation of PMA in solution, thereby seeding a different network configuration at the liquid−graphite interface. Without an in-depth, theoretical study of the surface adsorption and network formation, it is difficult to propose an assignment of the offset-dimer structure. II. Trimellitic Acid (TMLA). A. Molecular Structure and Network Formation. The disorder observed in images of TMLA monolayers are in contrast with STM images of symmetric molecules such as PMA (Figures 1−3), TMA, and terephthalic acid (1,4-benzenedicarboxylic acid) which form monolayers that are highly periodic. Despite the fact that a highly ordered network structure can be conceived (Figure 8a), STM images show rows of TMLA molecules that appear wavy, indicative of changes in the orientation of individual molecules relative to their neighbors along a row (Figure 8b). Intermolecular spacing measured from a section analysis of the STM image of TMLA in heptanoic acid reveals large deviations (Table 4), as compared to the ordered assemblies of PMA (Table 3). Since there is no site-specific binding for TMLA on graphite, we expect randomization in the orientation of TMLA molecules upon adsorption (Figure 8b). Coupled with the molecule’s inherent asymmetry, we expect that nucleation and growth of monolayer networks occur randomly, giving rise to the observed disorder in the STM images. Again,

changes upon monolayer formation, as compared with a closepacked PMA network, making the open network thermodynamically unfeasible. Our result suggests a delicate balance between enthalpic and entropic contributions to the total free energy of monolayer formation in PMA, as compared to TMA, BTB, and other benzenecarboxylic acids. Given that the open structure was observed only when PMA solutions were diluted by as little as a factor of 2 from saturation, it is interesting to determine how close the concentration of PMA at saturation in pentanoic acid is to that required form complete surface coverage. Assuming complete coverage of a 10 mm × 10 mm piece of graphite in a close-packed structure, the number of molecules required to cover the surface is 1.2 × 1014 molecules, corresponding to about 10 μL of a 0.02 mM solution. A 10 μL droplet of a saturated solution of PMA in pentanoic acid (0.86 mM) contains 5.2 × 1015 molecules. Thus, at saturation, there is over 40 times more PMA in solution than required for close-packed monolayer formation. Dilution by a factor of 2−10 causes a shift from complete surface coverage in the close-packed morphology to partial surface coverage in the open structure; however, we anticipate that there is still sufficient PMA in solution (4−20 times that required for complete surface coverage) to form close-packed networks while meeting the condition for thermodynamic equilibrium. For the solution/ monolayer system to remain in thermodynamic equilibrium, the concentration of solute in the solution phase can never go to zero. Therefore, the existence of open structures in the lowconcentration regime is really not surprisingit is the best the system can do enthalpically to cover the surface while maintaining the condition for equilibrium. Concentrationinduced polymorphism, where less densely packed monolayer structures form in diluted solutions, has been observed in previously.21,29,65−67 What is surprising here is that the open network was only observed for diluted solutions of PMA prepared in pentanoic acid. This indicates that while concentration plays a significant role as to what surface network(s) are observed, the identity of the solvent is relevant. In the case of PMA, where the enthalpic gain associated with 18171



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serve to lower the Gibbs free energy of the system enough to favor open networks; the enthalpy change associated with forming open networks of TMLA may not be large enough to offset the change in entropy associated with monolayer formation. Under such conditions, no monolayer formation is expected, as the driving force for self-assembly is minimization of the system’s free energy. 3.1,3,5-Benzenetriacetic Acid (BTA). Despite several attempts to investigate monolayer formation at the interface of saturated solutions of BTA in pentanoic acid via STM imaging, no monolayers were observed. There are several possible explanations for the negative result. First, BTA might only be sparingly soluble in penatnoic acid and therefore there may not be enough present molecules in solution to form the monolayer while still meeting the condition for thermodynamic equilibrium (i.e., that the BTA concentration in solution not go to zero). It would be interesting to investigate this molecule under ultrahigh vacuum, where equilibrium conditions do not apply and poor intermolecular and adsorbate−substrate interactions are exceedingly more favorable than the vacuum (which nature is known to abhor). Second, it is possible that self-assembly occurs, but that we did not observe the monolayer due to insufficient sampling or the quality of the tip. In our experience, monolayers formed at the liquid/ graphite interface by other benzenecarboxylic acid derivatives are observed instantaneously upon engaging in their solutions with the appropriate “molecular” tunneling conditions. Since atomic resolution of graphite was observed prior to solution deposition, as well as under solution conditions, we can also rule out the possibility of tip effects. We hypothesize that the lack of a stable self-assembled monolayer is due to the molecule’s structurethe presence of a methylene unit between the benzene ring and the acid group forces the molecule to be nonplanar. Unlike the other benzenecarboxylic acid studied, BTA’s nonplanar structure is problematic for several reasons. The magnitude of favorable van der Waals interactions between the adsorbate and substrate decreases rapidly as the distance between the molecule and graphite increases (r−6), resulting in substantially diminished interaction energy when a molecule adsorbs in a nonplanar configuration, as compared to the same molecule lying flat. Similarly, nonplanar BTA would experience diminished π-stacking interactions with the graphite. It is possible that BTA can undergo a conformational change upon surface adsorption, such that the benzene ring is nominally flat and in contact with the graphite; however, the geometric restrictions imposed by the methylene group between the ring and the acid moiety would force the acid groups up out of the plane of the benzene ring, rendering the formation of extended surface networks difficult, if not impossible. Orthogonally oriented acid groups decouple the carbonyl π-electrons from those of the benzene ring, resulting in the loss of resonance delocalization in the monomer. In planar benzenecarboxylic acid derivatives (e.g., TMA), further electronic delocalization occurs upon formation of cyclic H-bonded dimers or trimers in the extended surface networks, as well as enhanced π-stacking with the graphite surface, which are two additional sources of monolayer stabilization. Theoretical study is warranted to further address the structural consequences for the lack of monolayer formation in BTA on graphite.

these results provide an experimental basis for testing theoretical models of physisorption and self-assembled monolayer nucleation, growth, and equilibrium configurations at the liquid/graphite interface. TMLA has a pair of ortho-oriented acid groups, unlike the threefold symmetric TMA. As with PMA, we anticipate that the molecule undergoes a change in conformation upon adsorption to a planar configuration, with the estimated energy barrier to isomerization of ∼2−10 kcal/mol/molecule. However, it is very difficult to obtain images of TMLA, and the resultant networks are not well ordered. It is likely that the disorder prevents the molecules from forming tightly packed networks, and as a result, TMLA monolayers have weaker intermolecular forces between rows of molecules as compared to a more densely packed structures observed for TMA or PMA. Indeed, the lack of self-assembly for phthalic acid at the liquid/solid interface has been attributed to its inability to form densely packed monolayers due to the presence of ortho-oriented acid groups.8 In their extensive modeling study of benzenedicarboxylic acids on model surfaces, Martsinovich and Troisi24 report that MM3 force field calculations of perfectly ordered two-dimensional structures can be generated for terephthalic acid (para), isophthalic acid (meta), and phthalic acid (ortho) in which the molecules associate through cyclic H-bonded dimers in linear (terephthalic acid) and zigzag (isophthalic acid and phthalic acid) tapes. In these tapes, the interaction energy between the molecules is calculated to be on par with that of benzoic acid dimer (−16.0 kcal/mol) for each of the three benzenedicarboxylic acids, whereas the interaction energy between H-bonded, one-dimensional chains is found to be smaller (∼1−3 kcal/mol) and to decrease in the order terephthalic acid (para) > isophthalic acid (meta) > phthalic acid (ortho).24 Thus, a plausible explanation for the lack of a stable self-assembled monolayer for phthalic acid at the liquid/ graphite interface is that the weak interchain interactions due to steric effects associated with ortho-substituted acid groups, and the subsequent low surface density, do not serve to lower the Gibbs free energy of the system. Furthermore, the fact that TMLA does assemble at the liquid/solid interface indicates that addition of a third carboxylic acid group to a molecule with a pair of orthogonal acid groups provides enough stabilization energy via intermolecular H-bonding to lower ΔG for monolayer formation, but not quite enough to form ordered, periodic networks as in the case of TMA and PMA. Given the seemingly random orientation of TMLA molecules in the monolayer network, it might be interesting to apply a random tiling analysis to the STM images, provided it is possible to achieve higher imaging resolution, to determine if the monolayer is entropically stabilized.68 B. Solvent and Concentration Effects on Self-Assembly. No solvent- or concentration-dependence is observed for monolayers of TMLA formed at the liquid/graphite interface. It is surprising that porous networks are not observed for diluted TMLA solutions. While it is possible that open networks do exist and we simply failed to observe them, we were rigorous and methodical in our search. We postulate that open networks do not exist based on thermodynamic considerations. Structurally, TMLA lacks the symmetry (and planarity) of TMA and the fourth carboxylic acid group of PMA, which likely renders the TMLA monolayer significantly destabilized, as compared to TMA and PMA. The absence of low-density structures is reflective of weaker intermolecular interactions in TMLA networks with respect to TMA and PMA that do not 18172



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Notes

CONCLUSIONS Using scanning tunneling microscopy imaging under ambient conditions, we investigated the molecular self-assembly of PMA and TMLA at the liquid−graphite interface, providing additional examples of H-bonded networks formed by benzenecarboxylic acids. Our work shows that the number of carboxylic acid groups, as well as their position on the benzene ring, modulates the structure and number of networks observed. One of the most important findings of the present work is that concentration, in addition to solvent composition, plays a definitive role as to what H-bonded network is observed for a given benzenecarboxylic acid at the liquid−graphite interface. Close-packed H-bonded networks were routinely observed for PMA when the concentration of molecules in solution was far in excess of the number of molecules required to cover the surface completely. We anticipate close-packed networks to be preferred thermodynamically because the enthalpic gain associated with their enhanced adsorbate−substrate and intermolecular interactions outweighs the entropic cost of covering the surface in ordered arrays of molecules, giving rise to a two-dimensional analogue of the principle of closepacking,69 as anticipated. Porous networks were observed for PMA at the liquid/solid interface upon dilution of concentrated solutions by as little as a factor of 2. Coverage-dependent assembly of PMA at the liquid/solid interface provides further alignment between the self-assembly of benzenecarboxylic acid derivatives on inert surfaces under solvent and vacuum conditions.18,21,23,38 For PMA, the stability afforded by four acid groups involved in multiple, cooperatively strengthened H-bonds enables formation of close-packed structures under all solvent conditions investigated, with the exception of diluted solutions prepared in pentanoic acid. In this case, porous networks are thought to be stabilized by coadsorption of solvent molecules, because the length of a pentanoic acid molecule matches exactly the diameter of PMA. STM images of the open structure provide evidence of solvent inclusion. The ability of the pentanoic acid molecule to H-bond to the acid groups of the molecules forming the cavity also lends stability to the open structure. On occasion, coexistence of the open and closed networks was observed in pentanoic acid solutions, suggesting they are close in energy and that the concentration of PMA in solution is large enough to support patches of the close-packed network while maintaining the condition for equilibrium. Molecular symmetry was shown to play an important role in network formation of the benzenecarboxylic acids. For TMLA, the asymmetry associated with having two acid groups adjacent in the 1 and 2 positions (ortho), with the third at 4, gives rise to disordered networks, in contrast the periodic networks formed by PMA and TMA. Molecular asymmetry induced disorder in the H-bonded networks is attributed to randomized adsorption orientations and nucleation of network formation. Finally, no self-assembly was observed for solutions containing BTA, likely due to its inability to form a nearly planar geometry for favorable adsorption energy on graphite and the facilitation of network formation.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge funding through the Faculty Start-up Award Program of the Camille and Henry Dreyfus Foundation and the Clare Boothe Luce Program of the Henry Luce Foundation. The authors thank Dr. Steven Graham and Jordan Snaider for their assistance. G.M.F. thanks the participants and organizers of the Faculty Writing Initiative program of the University Writing Center and the Center for Teaching and Learning at St. John’s University for facilitating an environment conducive to scholarly writing and stimulating discussion.

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Surface Patterning of Benzenecarboxylic Acids: Influence of Structure

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