Article pubs.acs.org/JPCC
Evolution of Metastable Clusters into Ordered Structures for 1,1′Ferrocenedicarboxylic Acid on the Au(111) Surface Ryan D. Brown,† Joseph M. Coman,† John A. Christie,† Ryan P. Forrest,† Craig S. Lent,‡ Steven A. Corcelli,† Kenneth W. Henderson,§ and S. Alex Kandel*,† †
Department of Chemistry and Biochemistry and ‡Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States § Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115, United States ABSTRACT: A series of experiments and electronic structure calculations were performed to identify metastable 1,1′-ferrocenedicarboxylic acid supramolecular structures formed during solution deposition in a vacuum on a Au(111) substrate, as well as to observe their evolution into more stable species under mild annealing conditions. Electrospray ionization mass spectrometry measurments were performed to determine which species are likely to be present in the rapidly evaporating droplet, and these experiments found that a hexamer can exist in solution during deposition, albeit as a metastable species. The molecular clusters present after solution deposition were observed and analyzed using ultrahighvacuum scanning tunneling microscopy, and the initial monolayer contains four basic classes of structures: ordered dimer domains, tilted dimer rows, square tetramers, and rectangular chiral hexamers. Electronic structure calculations indicate that the chiral hexamers consist of a central dimer surrounded by four molecules oriented to form birfurcated hydrogen bonds with other carboxylic acid groups and weaker hydrogen bonds with hydrogens from the aromatic rings. The calculations also indicated that the tetramers are clusters held together by carboxylic acid dimer bonds on each ring oriented perpendicular to each other, and that this conformation is slightly more stable than two dimers for a cluster of four molecules. Annealing this surface at 50 °C for 1 h results in the formation of both isolated tetramers and ordered tetramer rows at the expense of the end-to-end dimer domains, with few chiral hexamers remaining. Further annealing at 50 °C, as well as annealing at 65 °C drives the system to form chiral dimer domains, as well as several other minor structures. Annealing at 75 °C resulted in a dramatic decrease in apparent surface coverage, and most ordered structures existed as large tilted dimer rows, whether isolated or in ordered domains. This drop in surface coverage is likely due to some combination of decomposition of the molecule, desorption, or the growth of three-dimensional crystal structures. The observed coexistence of many forms of ordered dimer structures after annealing indicates that the equilbrium conformation of 1,1′-ferrocenedicarboxylic acid is some array of ordered dimers, and the variety of supramolecular structures present after annealing is an indicator that this system evolves under kinetically controlled growth conditions.
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engineering for directing supramolecular assembly.11,12 Carboxylic acids are good candidates for molecular engineering tools due to their ability to form a variety of supramolecular structures. Carboxylic acids crystals tend to form based upon “ring” dimers, in which each molecule donates and receives a hydrogen bond.13 However, in rare cases carboxylic acids can form cyclic hydrogen-bonding rings of three to six molecules.14−16 Additionally, the presence of weak hydrogen bonding elements adjacent to the carboxylic acid can also lead to deviations from ring dimer bonding, either by forming a ring of bifurcating hydrogen bonds or by adopting a concatenated dimer, or catemer, configuration.14,17 Recently, several studies of organic and organometallic molecules containing hydrogen-bonding functional groups
INTRODUCTION A primary goal of the fields of crystal engineering and molecular self-assembly is to understand how to produce one-, two-, or three-dimensional molecular organization by carefully tailoring the chemical composition, substrate, and growth conditions of the system. The two principal strategies for achieving such ordering are through intermolecular interactions and substratemediated assembly.1−7 A careful choice of chemical functionalities, substrate, and assembly conditions can result in a cookbook-style compendium of ingredients to produce a variety of two- and three-dimensional supramolecular structures.8,9 In addition, the preparation method can influence the ultimate structures produced using these strategies, and this is a complex topic still under investigation. Hydrogen bonding is an attractive method for directing supramolecular assembly due to its relatively high bonding energy, strong directional dependence, and specificity.10 As a result, this bonding motif is widely used in the field of crystal © XXXX American Chemical Society
Received: January 31, 2017 Revised: March 3, 2017 Published: March 3, 2017 A
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solution was delivered to the electrospray capillary at 10 μL/ min using a syringe pump (Princeton Scientific). The spectra were obtained in negative-ion mode, and the operating parameters employed in these experiments were chosen to fully evaporate the droplet solvent, minimize collision-induced dissociation of the supramolecular clusters, and maximize ion extraction from the spray. Typical operating conditions are as follows: a capillary voltage of 3.6 kV, a cone voltage of 25 V, source and desolvation temperatures of 50 °C, and a desolvation gas flow rate of 300 L/h. The mass spectra are all additive and acquired over a 10 min period, with an individual sweep time of 3 s per scan, which corresponds to roughly 100 scans per spectrum. Scanning Tunneling Microscopy. Au(111)-on-mica samples were prepared in vacuum using three cycles of argon-sputtering (20 mA, 15 min) and annealing (400 °C, 15 min) in a UHV sample preparation chamber. The clean sample was allowed to cool to room temperature and then moved to the deposition chamber. Microliter-sized droplets of a 6 mM solution of Fc(COOH)2 were deposited onto the surface using a pulsed-solenoid valve (Parker Instruments 9-series, 0.5 mm nozzle diameter, IOTA ONE driver) in a series of three 500 μs pulses. This deposition procedure results in near monolayer coverage of the surface with only a few instances of multilayer features. The solution and sample were at room temperature throughout this procedure. After the solvent evaporated (the deposition chamber pressure returned to its previous baseline), the sample was transferred to an Omicron LT-STM in a UHV chamber (base pressure 10−10 Torr) and cooled to an operating temperature of 77 K. All STM images were acquired at this temperature in constant-current mode with a tunneling set point of 10 pA and at a +1.00 V tip−sample bias. The annealing steps were carried out by transferring the samples to the sample preparation chamber and annealing to the desired temperature for 1 to 2 h. The sample was then cooled to room temperature before returning it to the LT-STM and cooling to 77 K. Theoretical Calculations. All calculations were performed using the Q-Chem software package.27 Crystal structure geometries were used for individual Fc(COOH)2 molecules.28 To effectively investigate plausible hydrogen bonding motifs, an array of different geometries were constructed algorithmically using different configurations of individual molecules. To construct the tetramer systems, four individual molecules were translated such that their Fe atom was placed at each centroid found in Figure 4b. The cyclopentadienyl (Cp) rings were then rotated to create a double dimer system. Using this double dimer system as a starting point, the Cp rings were then modified iteratively at intervals of 0, 45, and 90 deg. This process was repeated after iteratively flipping the carboxylic acid groups to produce different conformation geometries for individual molecules. After symmetrically equivalent system geometries were filtered, there were 391 876 possible geometries remaining. Single point energy calculations were performed on the 13 044 geometries containing at least six hydrogen bonds within the cluster. Because of time constraints for performing such a large number of calculations, only the Hartree−Fock level of theory was used with a minimal basis set.
have postulated that cluster formation during aersolized solution-deposition can define the self-organization of the deposited molecules.18−22 The conditions present in either electrospray deposition or pulse-injection of solution into a vacuum system are likely far from equilibrium wherein there can exist temperature and concentration gradients in the droplets not accessible to the traditional solution in or near equilibrium.23−25 These conditions might allow the solutes to sample regions of the configuration space not normally accessible in solution, and thus self-organization can produce metastable species, which then can precipitate out of the droplet during the deposition.26 A previous study of 1,1′-ferrocenedicarboxylic acid (Fc(COOH)2) observed chiral hexamer clusters after pulsed deposition of a methanolic solution on an Au(111) substrate.22 The proposed model for the existence and orientation of chiral Fc(COOH)2 clusters on the Au(111) surface postulated that these clusters formed in solution during deposition, precipitated, then adsorbed on the surface following a random sequential adsorption (RSA) model with minor realignment driven by the gold substrate.22 In order for this model to be sound, following conditions must be true: (1) the hexamer must form as metastable species in the rapidly evaporating droplet, (2) the hexamer must precipitate from solution during deposition (potentially according to Ostwald’s rule), (3) the hexamer must adsorb randomly on the surface following RSA statistics, and (4) the hexamer is kinetically trapped in a chiral conformation with minimal or no surface diffusion after adsorption, with only minor realignment driven by the gold substrate. The third and fourth conditions were inferred from high coverage regions of a Fc(COOH)2 decorated Au(111)/mica substrate, but the presence of the hexamer species in solution and its stability on the surface were not experimentally tested. Herein we present an investigation of the exact nature of these chiral hexamers through a combination of electrosprayionization mass spectrometry (ESI-MS), low-temperature scanning tunneling microscropy (LT-STM) studies, and density functional theory (DFT) calculations. The ESI-MS studies demonstrate that hexamer clusters exhibit enhanced stability in a rapidly evaporating solvent spray, and studies of annealed Fc(COOH)2 monolayers test the stability of the various observed ordered structures on the surface. The STM studies reveal several different supramolecular structures postdeposition, and after annealing these structures evolve into a range of larger, ordered structures, which begin to resemble the bulk crystalline packing motif after the highesttemperature annealing conditions. The fact that this surface appears to undergo a stepwise progression to more stable forms with increasingly higher annealing temperatures is indicative of a system evolving under kinetically controlled growth conditions.2
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EXPERIMENTAL SECTION Electrospray-Ionization Mass Spectrometry. The synthesis of Fc(COOH)2 has been described previously.22 Electrospray-ionization mass spectrometry of Fc(COOH)2 in methanol was performed using a tandem quadrupole detector (Waters TQD) equipped with an electrospray-ionization source and operated under a single mass-spectrometry mode. The
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RESULTS AND DISCUSSION ESI-MS of Fc(COOH)2. A typical ESI-MS spectrum for the 1.7 mM solution of Fc(COOH)2 in methanol is shown in Figure 1. This spectrum is quite different than what has previously been reported in the literature,29 but this experiment B
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H]− cluster was determined by numerically integrating over all peaks resulting from the isotope pattern. The residuals of the natural log of the peak intensity are in Figure 1d). It is clear that the hexamer peak has a much larger deviation from the exponential decay than any other cluster. This difference corresponds to an observed intensity of roughly 40 times greater than the expected intensity, and thus is likely a metastable species in the rapidly evaporating spray. While it is not a given that ionic and neutral clusters are directly comparable, the prevalence of hexamers in both ESI and STM observations argues that the first assumption of our model is reasonable, that hexamers are forming in a droplet undergoing free evaporation. The As-Deposited Surface. The pulse-deposition of a 6 mM solution of Fc(COOH)2 in methanol onto a Au(111) substrate in a high vacuum preparation chamber produces a heterogeneous surface containing a variety of supramolecular structures with various degrees of ordering. The structures observed on this surface should only be those which are either the most stable configuration of Fc(COOH)2 molecules on the surface, or metastable species which have kinetic barriers to reorganization which are larger than the thermal energy available at room temperature. These isolated clusters and ordered structures can be classified as belonging to three broad categories: those comprised of molecular dimers, square tetramers, or chiral hexamers. This combination of clusters and ordered regions is displayed in Figure 2. The majority of
Figure 1. (a) An ESI-MS spectrum including the trimer, tetramer, pentamer, and hexamer families of peaks. The peak position corresponding to [Mn − H]− for each size cluster is noted with a red triangle. The inset is the full scale of the spectrum, with the two largest peaks corresponding the to monomer and dimer anionic clusters. (b) An overlay of the predicted isotope pattern for a [M6 − H]− peak (red) with the actual spectrum (black). The m/z values for each predicted peak by 0.4 amu/e to account for a calibration error. The natural log of each cluster intensity is displayed in (c) as black open circles, and a linear fit, excluding the hexamer outlier, is shown as a red dashed line. The difference between this linear fit and the natural log of each cluster intensity is shown in (d) for each [Mn − H]− peak.
was performed using a higher concentration and lower cone voltage, and an attempt was made to minimize the presence of Na+ and thus reduce the presence of sodiated species. There is a general trend of decreasing peak intensity with increasing cluster size, and for all clusters up to hexamers there are also peaks present that correspond to sodium substitution, potassium substitution, and solvent adducts. The presence of sodium and potassium in the mass spectra is most likely due to residual contamination from liquid-chromatography mass spectrometry experiment buffers also performed on this particular instrument. As the cluster size increases, the relative intensity of the peaks corresponding to these alkali ion substitutions and solvent adducts increases relative to the ionized cluster. There is a deviation from this trend of decreasing intensity with increasing cluster size for the hexamer peak (located at approximately 1640 amu/e in Figure 1), and the hexamer has no peaks that are above the noise level corresponding to alkali-atom substitutions. The intensity of the negatively charged hexamer peak is almost as large as that of the negatively charged tetramer peak; it is also much larger than the pentamer and heptamer peaks, both of which are barely resolved from the noise of the baseline. Previous investigations of magic number clusters characterized such clusters using a scaling factor based on a three point fit of adjacent peaks assuming an exponential30 or linear31 decay of peak intensity with cluster size. The semilog plot in Figure 1 shows a linear dependence of log(intensity) on cluster size; that is, the Fc(COOH)2 cluster peak intensities decay exponentially with increasing cluster size. The peak intensity of each [Mn −
Figure 2. (a) A 321 Å × 315 Å topography image of the Fc(COOH)2 decorated surface as deposited. (b) The same area with examples of dimer rows (green), dimer arrays (blue), tetramers (yellow to orange), and chiral hexamers (pink to purple).
observed clusters appear to have a dimer structure as their basis, but there are numerous occurrences of the chiral hexamer and tetramer clusters. Before discussing the specific aspects of each structure, it is useful to consider the bulk crystal structures of Fc(COOH)2. Each of the three reported polymorphs of the bulk crystal is comprised of molecular dimers, with the relative orientation of these dimers being the main difference between these reported crystalline structures.28,32,33 Given this, it seems reasonable to assume that the equilibrium structure of a twodimensional lattice on a surface would also be composed of Fc(COOH)2 dimers. The structure and orientation of the chiral hexamers are the same as was originially reported,22 but in coexistence with the other clusters. Previously, the centroids of the Fc(COOH)2 Cp rings were well established, but the exact orientation of the molecules could not be discerned from the imaging. Electronic structure calculations were performed to generate a plausible model for the exact conformation of the individual molecules within the chiral hexamers, using these centroid positions as a C
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Figure 3. Six lowest energy hexamer conformations, in order from most to least stable (left to right, top to bottom). The hydrogen bonding contacts for the most stable structure are given in blue. The relative energy of each conformation is listed above the structure.
constraint, and constraining all carboxylic acid groups to be oriented within the cluster. Thirty-one structures were within 5kBT of the most stable conformation, and the six most energetically favorable candidates from these calculations are depicted in Figure 3. The common theme of these structures is that the central dimer of the cluster is surrounded by molecules with carboxylic acid groups oriented to form weaker hydrogen bonds with hydrogens from the Cp rings, and which also allow for the formation of birfurcated hydrogen bonds with the carboxylic acid groups of the central dimer. The tetramers are square clusters, with distances of 8.5 ± 0.5 Å and 8.3 ± 0.5 Å between the centroids (Figure 4). These distances are likely slightly reduced due to image warping or calibration errors, as they match measurements of dimers from the same data sets, even though carboxylic acid dimerization would result in a 9.1 Å distance. Certainly, the tetramer structure is not consistent with a defective hexamer (i.e., two vacancies on the base hexamer structure). Since the base structure of the tetramer does not appear to propagate beyond the four molecules, it is reasonable to assume that the carboxylic acid groups are all engaged in dimer type bonding with other carboxylic acid groups within the cluster; specifically, a symmetric structure where two side-by-side CpCOOH dimers are linked via dimerization of the COOH groups on the other Cp rings of the sandwich compound, which will be referred to as the square binding motif. Hartree−Fock level calculations confirm this proposed structure. Figure 5 gives the stucture and relative energies of the three lowest energy supramolecular conformations for the tetramer cluster, all of which are consistent with the two sideby-side CpCOOH dimers. The lowest-energy conformation for two isolated dimers is also given in Figure 5, and it is nearly 0.4 eV higher in energy than those of the square-bonding motif. Despite these results, the tetramer is likely a metastable species given the absence of any such structure in the reported bulk crystal structures.28 The ordered dimer domains present in Figure 2 were also observed on the ferrocene carboxylic acid, Fc(COOH), decorated surface.19 The size of these ordered structures varies from tens of molecules to arrays containing more than a thousand Fc(COOH)2 molecules, as seen in Figure 6. A recent
Figure 4. (a) A composite image of the tetramers observed in Figure 2a is presented, with a plot of the average (blue dot) and each individual positions (red dot) of the molecular centroids for these tetramers (b). The blue ellipses are the standard deviation of the centroid position for each molecule in the tetramer.
low-temperature study of Fc(COOH)2 on coinage metals used noncontact AFM and STM with a CO functionalized tip to characterize this structure as rows of dimers with intercalated Fc(COOH) 2 molecules with their Cp rings oriented perpendicular to the surface.34 This stucture is likely not the equilibrium structure of Fc(COOH)2 on Au(111) given that there are molecules not forming hydrogen-bonds with other molecules, whereas in the bulk crystal all molecules form D
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motif, it is a viable candidate for the equilibrium monolayer structure on this surface. Annealed Sample. The question of the relative stability of the structures present after pulse-deposition can be addressed by annealing the surface and then imaging. If the annealing temperature is sufficient for overcoming diffusional barriers and activation energies for molecular reorganization of the clusters, then metastable species should evolve into more stable forms, if not the actual equilibrium structure. As stated in previous work, the most plausible explanation for the presence of chiral hexamers is that they form in solution during deposition, while the exact origins of the tetramer species and dimer arrays is not as clear. Two samples were prepared at different times using the same procedure, and then imaged and annealed at different conditions to explore the relative stability of the molecular structures observed after deposition. The first sample was annealed at 50 °C for 1 h, imaged, then annealed at 75 °C for 1 h and imaged. The annealing steps for the second sample consisted of a 1 h 50 °C anneal, a 2 h 50 °C anneal, and finally a 1 h 65 °C anneal. These experiments resolve questions regarding the relative stability of the structures observed after pulsed deposition and indicate that every feature other than the tilted dimer rows are actually metastable species (whether formed in solution and then kinetically locked upon adsorption, or grown under kinetic control on the surface). For both samples, the images obtained after a 1 h at 50 °C anneal contradicts our assumption regarding the relative stability of the tetramer and dimer array structures. This annealing step resulted in a diminished frequency and size of dimer arrays, although they are still present, and a dramatic increase in the number of observed tetramers. The tetramers not only appear more frequently, but also align into rows of rotated, and slightly compressed clusters, as is evident in Figure 7. This observation means that tetramers are a species that can form on the surface, and thus their presence is not solely attributed to cluster formation in the solvent. Few chiral hexamers are present; they are observed less frequently and are mainly present on the boundaries of large domains of other ordered structures. Of the initial species present, possibly only the linear dimer chains were unaffected by this annealing step. It is tempting to suggest, based on these images, that the tetramer is a more stable species than the chiral hexamer or dimer, but that is not necessarily true. It is more likely that these tetramer structures are simply a kinetically trapped metastable configuration that is part of a growth process that will result in a structure comprised of dimers. Another structure which is present after the initial annealing step is an ordered domain consisting of rows of three dimers with faint features between these dimer triplets, as seen in Figure 8. The dim features between the dimer triplet rows are Fc(COOH)2 molecules with the Cp ring perpendicular to the surface, which is similar to observations from the FcCOOH quasicrystal.18 These dimer domains are chiral supramolecular ordered structures; the formation of chiral supramolecular configurations by achiral molecules is a consequence of the loss of the horizontal mirror plane due to the presence of the surface, and is a common feature of two-dimensional molecular assembly at interfaces.5,6,35,36 Further annealing at 50 and 65 °C produces a surface on which the chiral dimer domain is the dominant structure. This change corresponds to a decrease in the frequency of tetramer row structures and the absence of end-to-end dimer arrays, while the tilted dimer rows are still present. Given this
Figure 5. Three lowest energy tetramer conformations with COOH dimer bonds 90° apart and the lowest energy tetramer conformation comprised of two isolated dimers. The relative energies are listed above each structure.
Figure 6. A 273 Å × 273 Å topography image of as-deposited Fc(COOH)2 on Au(111). The molecules have formed a large, organized domain of dimers. The gold 22 × 3 reconstruction is visible beneath the ordered molecules. The top inset is a composite image of the dimer domain, with two dimers highlighted in green and blue and the unit cell outlined in red. The bottom inset is the Fourier transform of the ordered dimer domain.
dimers.28 The images from this study are very similar to those obtained by Berger et al. without the CO-functionalized STM tip, so while the molecules with Cp rings oriented perpendicular to the surface are not visible, it is still the correct assignment for this domain in our images. The other observed dimer structure is a tilted dimer row highlighted in Figure 2b, and these rows also have a different supramolecular structure than the end-to-end dimer array. Since all molecules form hydrogen-bonding dimers in this E
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annealing steps, including organized arrays of tetramers and dimers, but that the chiral dimer domain is the most frequently observed structure. The chiral dimer domain is not the likely equilibrium structure, though. The surface after a 1 h 75 °C anneal changes drastically, essentially only containing tilted dimer rows and small, isolated groups of clusters. Figure 9 shows a large array of
Figure 7. STM images obtained after a 1 h 50 °C anneal. (a) A 400 Å × 400 Å topography image almost exclusively containing disordered tetramers. The fainter features of this image are due to a double tip. (b) A 400 Å × 400 Å topography image containing both disordered tetramer clusters and the edge of a tetramer row domain. The underlying striped feature is adsorbed hydrocarbon contamination from the preparation chamber. (c) A 250 Å × 250 Å image of the tetramer row domain. (d) A composite image of the tetramer rows from (c) is on top, with the positions of each cluster relative to the centroid of two tetramers (red) and their average position (blue) are plotted below.
Figure 9. A 450 Å × 450 Å topography image containing a large linear tilted dimer domain representative of the ordered Fc(COOH)2 present after a 1 h 75 °C anneal. The inset on the left is a composite image of these dimer rows, and the inset on the right is the Fourier transform of the ordered region central to this image.
these tilted dimer rows. These are likely the most stable structure of Fc(COOH)2 on a Au(111) surface, given that it is composed entirely of dimers and persists after the highest temperature anneal, while other ordered structures have disappeared entirely. However, it is difficult to make definitive pronouncements on relative stability because the overall surface coverage dropped significantly after this highest temperature annealing stage. There are three likely explanations for this occurring, and any combination of these options is a possibility. First, molecules could have desorbed during the annealing process, but this is unlikely given the low annealing temperature, and that no similar changes were observed after the lower temperature annealing stages. Second, it is possible that an “elephant graveyard” of molecules exists on the surface; that is, the absence of molecules in these images is accounted for by their presence in regions covered by multilayer structures that were not imaged (or potentially were not imageable). Finally, there is a possibility that some decomposition of the molecules occurred during the annealing process, and there is evidence for this happening in images obtained after the 75 °C annealing step. Figure 10 contains regions with honeycomb lattice structures which are not present in images acquired after lower temperature annealing steps, and this structure is likely some decomposition product. While there is no evidence in our imaging of Fc(COOH)2, or FcCOOH, decomposing after pulse-deposition on room temperature Au(111), there is evidence that ferrocene dissociatively adsorbs at room temperature on Au(111).37 It is possible that a temperature of 75 °C is sufficient decompose adsorbed Fc(COOH)2 on Au(111). The honeycomb feature appears to be atomic in nature, having a lattice constant of 4.4 Å, and thus adjacent atoms separated by
Figure 8. (a) A 500 Å × 477 Å topography image obtained after a 1 h 50 °C anneal. This region contains clusters and two chiral dimer domains (highlighted in blue). (b) A 300 Å × 287 Å image obtained after a total of 3 h annealing time at 50 °C and 1 h at 65 °C containing both enantiomers of the chiral dimer domain structure. The composite images of the enantiomer of the left and right domains of panel (b) are shown in (c) and (d), respectively. The Fourier transform of the larger domain in panel (b) is displayed in panel (e).
observation, it is reasonable to assume that the chiral dimer domains represent a more stable structure than the hexamer and tetramer clusters, the dimer array, and the tetramer rows. However, it is important to reinforce that a variety of minor supramolecular structures still exists on this surface after these F
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calculations, have carboxylic acid dimers on the top and bottom rings of the Cp ring offset by 90°. Annealing this surface at 50 °C for 1 h produces more tetramers and ordered tetramer rows, which then form chiral dimer domains and tilted dimer rows at higher temperature annealing conditions or longer annealing steps. This system appears to undergo a stepwise growth mechanism indicative of a kinetically controlled process. The important role of deposition technique in both the observed supramolecular structure and the system’s evolution into more complex structures could potentially become an exploitable property of nonequilibrium assemblies in crystal engineering.
Figure 10. (a) A 350 Å × 350 Å image after a 1 h 75 °C anneal containing chiral hexamers, tetramers, and tilted dimer rows. The honeycomb lattice is highlighted in blue. (b) A 350 Å × 350 Å image with ordered rows both two and three molecules across. Both images contain features not originating from intact Fc(COOH)2 molecules, which is indicative of molecular degradation.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Steven A. Corcelli: 0000-0001-6451-4447 S. Alex Kandel: 0000-0001-8191-1073
2.5 Å, and this is within the range observed for the growth of Fe atom monolayers on Au(111).38 However, none of the reported structures for Fe on Au(111) are honeycomb lattices, so the exact identity of this feature is not definitively assigned. It is worth noting that the images in Figure 10 contain numerous chiral hexamers and tetramers, in addition to few or isolated tilted dimer rows. The remaining hexamer and tetramer clusters are largely found on top of the hydrocarbon impurity on our surface, which might just serve as a site at which the few remaining clusters and molecules stick after diffusing across the surface. Figure 10b contains tilted dimer rows which are three molecules wide, which is not consistent with the dimer bonding motif present in all other dimer rows in that the top and bottom COOH groups are bonding to molecules in different adjacent rows. Given the large drop in surface coverage of the adsorbed Fc(COOH)2 and the evidence of molecular degradation after heating, it is difficult to definitively assign any ordered structure as the most stable after the 75 °C anneal, and thus the actual equilibrium two-dimensional assembly on a Au(111) surface remains unknown. Despite this fact, several features of this system are clear. Direct injection of Fc(COOH)2 into high vacuum produces a range of metastable structures on the Au(111) surface. The chiral hexamers almost certainly form in solution, precipitate, and then adsorb onto the surface, while the other observed structures likely grow on the gold surface after deposition. Mild annealing results in the stepwise evolution of increasingly stable supramolecular structures, the most stable of which appear to be conformational polymorphs of Fc(COOH)2 dimers.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Support for this work has been provided by the National Science Foundation (NSF Grant No. CHE-1507213). J.M.C. was supported by the American Chemical Society’s Project SEED endowment. Electrospray ionization mass spectrometry was performed at the University of Notre Dame Mass Spectrometry and Proteomics Facility, and we would like to acknowledge B. Boggess for many useful discussions.
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REFERENCES
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CONCLUSIONS A combined study involving electrospray-ionization mass spectrometry, scanning tunneling microscopy, and electronic structure calculations has demonstrated that direct injection of a solution of Fc(COOH)2 on Au(111) in a high vacuum chamber forms a variety of metastable structures, both in solution and on the surface, which evolves into more complex supramolecular structures under mild annealing temperatures. The chiral hexamers observed after deposition are metastable species formed in the rapidly evaporating droplet and have an internal structure consisting of a central dimer surrounded by four Fc(COOH)2 molecules oriented to form both weaker and bifurcated hydrogen bonds. Square tetramers likely formed on the Au(111) surface, and based on the imaging and G
DOI: 10.1021/acs.jpcc.7b00996 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.7b00996 J. Phys. Chem. C XXXX, XXX, XXX−XXX
