Self-Assembly Polymorphism: Solvent-Responsive Two-Dimensional

Article pubs.acs.org/JPCC

Self-Assembly Polymorphism: Solvent-Responsive Two-Dimensional Morphologies of 2,7-Ditridecyloxy-9-fluorenone by Scanning Tunneling Microscopy Li Xu, Xinrui Miao,* Bao Zha, and Wenli Deng* College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, P. R. China S Supporting Information *

ABSTRACT: Two-dimensional self-assembly of 2,7-ditridecyloxy-9-fluorenone (F-OC13) is investigated by scanning tunneling microscopy (STM) in solvents with different polarities and functional groups on a high oriented pyrolytic graphite surface. The STM images reveal that the self-assembly of F-OC13 is strongly solvent-dependent. 1-Phenyloctane can coadsorb on the self-assembly of F-OC13, and the structural transformation of the adlayer from the linear structure to alternate lamella can be observed with the decrease of the concentration. At the 1-octanol/HOPG interface, only a wellordered linear pattern is obtained. The intermolecular hydrogen bonding between the 1-octanoic acid and the F-OC13 molecule is responsive for the formation of butterfly configuration. When n-tridecane or n-tetradecane is used as solvent, a regular alternate pattern is formed under high concentrations, and a coadsorbed lamellar structure is observed under low concentrations. Furthermore, when the sample with use of the methanol, dichloromethane, or toluene as solvent is observed within one hour, a denser-packed structure appears. After the sample is placed more than three hours, in methanol and dichloromethane, a regular alternate pattern is formed corresponding to the result using n-tridecane or n-tetradecane as a solvent under high concentration. In toluene, the alternated pattern is similar with that in 1-phenyloctane at low concentration. The solvent induced self-assembly polymorphism is discussed in terms of factors of the polarity of the F-OC13 molecule and the nature of the solvent. The results provide a new objective to fabricate and control molecular nanopatterns based on the polar group in the molecule.

1. INTRODUCTION Up until now, functional groups have been widely exploited because they give an excellent tenability over the strength and symmetry of the nanostructures that one would like to design and control.1−5 Self-assembly6−8 is considered a relative effortless method to tailor molecular building patterns with nanometer-scale precision by scanning tunneling microscopy (STM). The process is sensitive to a delicate balance among intermolecular, molecule−substrate, and molecule−solvent interactions.9−12 Usually weak and reversible forces such as hydrogen bonding,13−15 van der Waals force,16,17 electrostatic,18 dipolar interaction,19−21 and attraction22−24 are adopted to fabricate self-assembled systems. Chemical nature of solvent (aromatic interaction, odd−even effect, saturated and unsaturated solvent, and alkyl chain length) and properties of solvent (solubility, hydrophilic and hydrophobic properties, polarity, and viscosity) have been used as the strategies to tune the molecular structures at the liquid/ solid interface.25 Recently, solvent-induced polymorphism has been explored. In most cases, solvent molecules can coadsorb in the molecular network, which depends on the size and shape of the solvent molecules26,27 as well as the hydrogen bonding.28,29 In addition, sometimes the solution could affect the molecular coverage and the self-assembly pattern in one building block.30−32 Our previous studies have investigated that © 2012 American Chemical Society

the formation of the self-assembled motifs of 1,3,5-tris(10ethoxycarbonyldecyloxy)benzene (TECDB) molecules with three ester alkoxy chains could be controlled by solvents30 and the honeycomb network was obtained only with coadsorption of a guest template. The structural transition of TECDB was attributed to the polarity of the ester alkoxy chains. Then, we further investigated the self-assembly of 2,7bis(10-ethoxycarbonyldecyloxy)-9-fluorenone (BEF) with polar conjugated moiety and the side chains.33 We found that the solution concentration could affect the formation of linear structure and cyclic network due to the intermolecular dipole− dipole interaction. In this contribution, we systematically study the solventinduced self-assembly of 2,7-ditridecyloxy-9- fluorenone (FOC13, Scheme 1) at the liquid or gas/high oriented pyrolytic graphite (HOPG) interface. 1-Phenyloctane was used as a solvent with aromatic moiety. 1-Octanol and 1-octanoic acid are polar alkylated solvents. n-Tridecane and n-tetradecane are aliphatic solvents. Methanol and dichloromethane were used as strong polar and volatile solvents. Our precious study has identified that the dipole−dipole interaction between the Received: March 13, 2012 Revised: July 5, 2012 Published: July 9, 2012 16014

dx.doi.org/10.1021/jp302422a | J. Phys. Chem. C 2012, 116, 16014−16022

The Journal of Physical Chemistry C

Article

solution of F-OC13 in methanol, dichloromethane, or toluene, only a tetramer pattern is obtained. After the sample was placed more than 3 h, a mixed nanopattern was observed. To the best of our knowledge, this is the first report on tuning assembled morphologies at an interface as a result of the dipole−dipole interaction between the conjugated groups through varying the solvent and solution concentration. The current results are of general interesting and importance for dipolar interaction studies at interfaces and provide a new object of study toward controlling and fabricating two-dimensional (2D) patterns.

Scheme 1. Chemical Structure of 2,7-Ditridecyloxy-9fluorenone (F-OC13)

fluorenone moieties existed during the assembly.33 The carbonyl of F-OC13 not only has polarity resulting in the intermolecular dipole−dipole interaction but can form hydrogen bonding with electron donating groups. We expect that, by varying the solvent and changing the concentration, the dipoleinduced nanopattern of F-OC13 could be controlled. The results in this work show that the structural change is observed with the decreasing of solution concentration in 1-phenyloctane. The coadsorbed solvent molecules of 1-octanoic acid result in the formation of butterfly pattern of F-OC13 due to the hydrogen bonding between the carbonyl and the carboxyl. However, the self-assembly of F-OC13 can not be influenced by the 1-octanol solvent. n-Tridecane and n-tetradecane solvents coadsorb in the F-OC13 monolayer due to the spatial effect. When the sample is observed within one hour after dropping a

2. EXPERIMENTAL SECTION 2,7-Ditridecyloxy-9-fluorenone (F-OC13) used in this study was synthesized as described in the literature.34 1-Phenyloctane (Aldrich), 1-octanol (Aldrich), 1-octanoic acid (TCI), ntridecane (TCI), n-tetradecane (TCI), methanol (TCI), dichloromethane (Aldrich), and toluene (TCI) were used as received. The samples were prepared by depositing a droplet (∼2 μL) of solution containing F-OC13 on a freshly cleaved atomically flat surface of HOPG (quality ZYB, Digital Instruments, Santa Barbara, CA). All of the images obtained at the liquid/solid interface were recorded within 3 h after dropping a solution of the F-OC13 in order to avoid the

Figure 1. (a−c) Series of STM images of F-OC13 self-assembly in 1-phenyloctane on HOPG surface at different concentrations: (a) 1.3 × 10−4 mol·L−1 (Vbias = 890 mV, It = 460 pA), (b) 4.3 × 10−5 mol·L−1 (Vbias = 850 mV, It = 510 pA), and (c) 7.2 × 10−6 mol·L−1 (Vbias = 785 mV, It = 480 pA). These images show the typical transition from a pure linear pattern to a hybrid linear structure when the F-OC13 concentration is decreased. (d−f) High-resolution images of F-OC13 self-assembly in 1-phenyloctane at different concentrations. (g−i) Molecular models illustrating the selfassembly patterns based on the STM images in panels d−f, respectively. 16015



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with the side chains parallel to the fluorenone group, while the side chains in the other two molecules are twisted (Figure 1g). The unit cell parameters are determined to be a = 2.3 ± 0.2 nm, b = 2.3 ± 0.1 nm, and α = 64 ± 1°. When the solution concentration decreases (∼10−5 mol·L−1), the monolayer becomes more complex. Figure 1b,e represents STM images of F-OC13 monolayer physisorbed on the HOPG surface. Again, dense-packed trimers as single linear structure are observed as indicated as green arrows in Figure 1b. Two other kinds of self-assembly patterns (double linear and triplex linear patterns labeled as blue arrows and pink arrows in Figure 2b, respectively) are observed. These three kinds of structures

excessive solvent evaporation on the self-assembled pattern and ensure the stability of the observed structure. Different solution concentrations (from ∼10−4 mol·L−1 to ∼10−6 mol·L−1) of FOC13 in different solvents were probed to evaluate the concentration-dependent structural change of the 2D monolayer. The images of F-OC13 molecule in methanol, dichloromethane, or toluene were obtained within one hour after dropping a solution of F-OC13 and placing the sample more than 3 h, respectively. Molecular models of the assembled structures were built by Materials Studio 4.4. The model of monolayer was constructed by placing the molecules according to the intermolecular distances and angles obtained from the analysis of STM images. STM measurements were performed on a Nanoscope IIIa Multimode SPM (Digital Instruments, Santa Barbara, CA) at ambient conditions. The tips were mechanically cut from Pt/Ir wires (90/10). All the images were recorded with the constant current mode and shown without further processing. The tunneling parameters are given in the corresponding figure captions. Different tips and samples were used to check the reproducibility and to exclude the image artifact caused by the tips or the samples. For deep analysis, recording of the selfassembled image was followed by imaging the HOPG lattice under the same experimental conditions according to lower bias voltage. The images were recorded for drift using the recorded HOPG images for calibration purposes, which allowed a more accurate unit cell determination.

3. RESULTS First, the formation of F-OC13 self-assembled pattern at the 1phenylotane/HOPG interface was probed in order to evaluate the effect of the fluorenone group on the geometry of the 2D networks. In the next step, the spontaneous monolayer formation was repeated in several solvents to elucidate how the solvents determine the structural polymorphism due to the change of the molecule−molecule and molecule−solvent interactions. These solvents were chosen by their different characteristics. 1-Phenyloctane was used as a solvent with aromatic moiety. 1-Octanol and 1-octanoic acid are polar alkylated solvents and have electron donating group. nTridecane and n-tetradecane are aliphatic solvents. Methanol, dichloromethane, and toluene as strong polar solvents were used at the gas/HOPG interfaces. 3.1. Concentration-Dependent Structural Transition in 1-Phenylotane. The monolayer formation of F-OC13 was investigated at the 1-phenyloctane/HOPG interface under different solution concentrations (from 10−4 to 10−6 mol·L−1) to evaluate the effect of the solution concentration on the geometry of the 2D networks. Figure 1 shows the three observed structures and tentative molecular models at different solution concentrations. In the STM images, the fluorenone moieties are observed to be bright gemmiform features due to a high tunneling efficiency. The stripe features between the fluorenone moieties correspond to alkyl chains. At high concentration (>10−4 mol·L−1), clearly a typical linear structure is formed as depicted in Figure 1a,d. Three F-OC13 molecules form a trimer, as shown in Figure 1d. Small bright round dots between the trimers in each lamella can be observed (indicated with green rounds), which are assigned to the phenyl groups of the coadsorbed solvent molecules, indicated in green in the tentative molecular model in Figure 1g. The alkyl chains of 1phenyloctane do not adsorb on the surface and stretch into the solution. In each trimer, one of the F-OC13 molecules arranges

Figure 2. (a,b) Large-scale and high-resolution STM images of F-OC13 self-assembly in 1-octanol on the HOPG surface. Vbias = 795 mV, It = 457 pA. (c) Structural model of the pure linear pattern of F-OC13.

arrange in random, and the triplex linear configuration dominates the surface pattern (Supporting Information, Figure S1a). In the double linear structure, the F-OC13 molecule forms a dimer with a shoulder-to-shoulder fashion and the solvents coadsorb in the voids indicated by green rounds in Figure 2e. The angle between the side chains of F-OC13 is 120°, while in other patterns, the side chains almost parallel each other. In the triplex linear pattern, except for two linear structures with dense-packed trimers, the F-OC13 molecule forms a dimer with a back-to-back fashion and the solvents also coadsorb in the voids. The self-assembled morphology is illustrated in the molecular model (Figure 2h). When the solution concentration decreases to ∼10−6 mol·L−1, the whole scanning area is covered by a uniform pattern as shown in Figure 1c. The alternate pattern with double linear structure and dense-packed linear structure appear alternatively. Only in the large-scale STM image (200 nm), a strip of triplex linear pattern was observed (Supporting Information, Figure S1b). A unit cell is given in Figure 1f, with the parameters of a = 1.9 ± 0.1 nm, b = 5.7 ± 0.2 nm, and α = 78 ± 2°. The model (Figure 1i) is in good agreement with the observed result. The results show that the molecules can form different structures in the same solvent with the change of F-OC13 molecular concentration, in which the solvent could coadsorb 16016



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Figure 3. (a,b) Large-scale and high-resolution STM images of F-OC13 self-assembly in 1-octanoic acid on HOPG surface. Vbias = 820 mV, It = 506 pA. (c) Structural model of the butterfly pattern of F-OC13.

On the basis of the observation results and the above analysis, a structural model can be proposed for the F-OC13 morphology in 1-octanoic acid, as shown in Figure 3c. The formation of tetramers might involve hydrogen bonding between the carbonyl group of the F-OC13 molecule and the carboxyl acid of solvent molecule, which plays a key role in creating the featured hydrogen bonded tetramer assembly. The results also demonstrate that the formation of hydrogen bonds between the carboxyl acid and carbonyl group of F-OC13 molecule are easier than that between the hydroxyl group and carbonyl group. 3.4. Self-Assembly in n-Tridecane and n-Tetradecane. Experimentals have been also performed in two hydrophobic solvating hydrocarbons such as n-tridecane and n-tetradecane under low concentrations (10−5−10−6 mol·L−1), respectively, to investigate the 2D molecular ordering of F-OC13. When adsorbed from n-tetradecane, well-ordered lamellar structure was observed on the surface within 2 h after the solution was dropped on the HOPG surface, as shown in Figure 4a. The equal distant bright lines are seen to distribute orderly on the surface. The formation of a zigzag pattern with the domain− domain angle of 120° is due to molecules in two adjacent lamellae matching the 3-fold symmetry of the underlying graphite lattice.37 The detailed self-assembly pattern of F-OC13 can be revealed in the high-resolution STM image (Figure 4b).

with F-OC13 molecules. Molecules with different conformations have different contact areas with the surface, which can result in a difference in adsorption energy, and in turn lead to preferential adsorption.35 In view of the total system energy, densely packed assembly is most frequently favored in which the adsorbate−substrate and adsorbate−adsorbate interactions could be maximized, especially when the intermolecular interaction lacks directionality.36 The average area per molecule (1.58 nm2) in the linear structure (Figure 1d) is larger than that (1.45 nm2) in the hybrid structure (Figure 1f), which elucidates that the lower the concentration, the larger the chance to find a low density pattern. 3.2. Self-Assembly in 1-Octanol. Figure 2a,b shows the representative STM images of a physisorbed monolayer of FOC13 at the 1-octanol/HOPG interface. Similar to the molecular packing observed for F-OC13 in 1-phenyloctane at the high concentration, the linear structure is observed. The voids between the trimer in each lamella can be observed clearly in Figure 2b, which indicates that no other molecules occupy the voids. The solution concentration has no effect on the formation of the self-assembled pattern. Although the hydroxyl group exists in 1-octanol, it can not form a hydrogen bond with the carbonyl group of the F-OC13 molecule. A structural model is proposed for the F-OC13 assembly in 1octanol, as shown in Figure 2c. The unit cell parameters are a = 2.2 ± 0.1 nm, b = 2.3 ± 0.1 nm, and α = 65 ± 1° 3.3. Self-Assembly in 1-Octanoic Acid. Figure 3a,b illustrates STM images recorded for the adlayer of F-OC13 physisorbed at the 1-octanoic acid/HOPG interface. The structure is totally different from the images acquired in 1phenyloctane and 1-octanol solution. Such a nanopattern can be consistently observed at various concentrations. The highresolution STM image (Figure 3b) provides the detailed information for the self-assembly structure. From the molecular arrangement and intermolecular distance, it can be determined that every four F-OC13 molecules form a tetramer that consists of two dimers. Two alkyl chains in an F-OC13 molecule are parallel to each other. The fluorenone groups in each dimer arrange by a shoulder-to-shoulder fashion. Two dimers in one tetramer adopted a back-to-back fashion dislocate and form a butterfly pattern. Note that short chains could be seen in the image (Figure 3b). On the basis of the length and the shape, the short chains are attributed to 1-octanoic acid molecules coadsorbed with the F-OC13 molecules. The unit cell of the 2D assembly is superimposed on the image in Figure 3b with parameters a = 2.7 ± 0.2 nm, b = 3.2 ± 0.1 nm, and α = 60 ± 2°.

Figure 4. (a,b) Large-scale and high-resolution STM images of F-OC13 self-assembly in n-tetradecane on the HOPG surface. Vbias = 798 mV, It = 448 pA. (c) Structural model of the lamellar pattern of F-OC13. 16017



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Figure 5. (a,b) Large-scale and high-resolution STM images of F-OC13 self-assembled in methanol on the HOPG surface. Vbias = 825 mV, It = 450 pA. (c) Structural model of the wavelike pattern of F-OC13.

The fluorenone moieties were arranged in a line, and the side chains were stacked by a tail-to-tail fashion. The F-OC13 molecules are observed to array with their molecular axes at an angle of 90 ± 1° with respect to their lamellar axes. A careful observation reveals that two F-OC13 molecules arrange back-toback in which the side chains are parallel to the fluorenone moieties indicated in green in Figure 4b. The fluorenone moiety in the third molecule were inclined toward the side chains indicted by a pink arrow. By counting the number of the chains and the side chain of F-OC13, we find that the ntetradecane molecules coadsorb on the HOPG surface due to the steric effect. The length of the alkoxy chain of F-OC13 is almost equal to that of n-tetradecane (1.7 nm). The unit cell can be defined with a = 2.2 ± 0.1 nm, b = 4.1 ± 0.1 nm, and α = 90 ± 1°. On the basis of the above analysis, a structural model for the coadsorbed pattern is proposed in Figure 4c. When adsorbed from n-tridecane, a similar coadsorbed lamellar pattern can be observed (Supporting Information, S2). 3.5. Self-Assembly of F-OC13 after Solvent Evaporation. In order to decrease the effect of the solvent−molecule interactions on the self-assembled pattern of F-OC13 on HOPG surface, methanol, or dichloromethane was chosen as the strong polar and volatile solvent. Because the solvent can significantly affect the resulting adlayer structure, we create a dry monolayer using methanol or dichloromethane as a solvent to identify the molecule−solvent and molecule−substrate interactions. The same self-assembled pattern is observed in two kinds of solvents (Supporting Information, Figure S3). After the methanol solvent evaporated, we carried out the STM experiment, and the sample was observed within one hour. A wavelike pattern was observed, as shown in Figure 5a. The high-resolution STM image (Figure 5b) shows the structural details. Four F-OC13 molecules form a tetramer as the basic unit of the adlayer, as indicated in green in Figure 5b. The tetramers arrange uniformly and form kinked lamellae. Each tetramer consists of two pairs of F-OC13 molecules indicated in pink in Figure 5c, which arrange with a back-to-back configuration. Careful observation shows that the alkyl chains are parallel with the conjugated moieties and pack interdigitally. No voids are observed in such self-assembled pattern. On the basis of the STM observation, a structural model for the wavelike pattern is proposed in Figure 5c. The parameters of the unit cell outlined in Figure 5b are a = 2.3 ± 0.1 nm, b = 2.4 ± 0.1 nm, and α = 88 ± 1°. After the sample (dichloromethane as the solvent) was stored more than 3 h, a mixed nanopattern was observed. A typical large-scale STM image is shown in Figure 6a. The coexistence of three kinds of structures with different domains

Figure 6. Typical STM images of F-OC13 self-assembly in (a) dichloromethane and (b) toluene on the HOPG surfaces showing the coexistence of different patterns after the sample was placed more than three hours. Scan area: 200 nm × 200 nm. Vbias = 769 mV, It = 473 pA.

labeled as I, II, and III is obtained and covers the whole surface. The same result can be observed using the methanol as the solvent (Supporting Information, Figure S4). The domain boundaries are illuminated by white dashed lines. In domain I, the self-assembled structure is the same with the linear structure observed in 1-octanol. The wavelike pattern occupies the domain III. In domain II, an alternate pattern, which is similar with the configurations of F-OC13 in 1-phenyloctane (Figure 1b,c), is observed. The experiment using toluene as the solvent was also conducted. A typical STM image (Figure 6b) of F-OC13 selfassembly in toluene on the HOPG surface shows the coexistence of different pattern labeled domains I, II, and III. Two kinds of nanopatterns (domains I and II) are the same with the F-OC13 self-assembly using methanol or dichloromethane as the solvent. The alternated structure (domain III) is similar with that at the 1-phenyloctane/HOPG interface under the low concentration, in which the double linear pattern adopts the same arrangenment with that in Figure 7b. From the high-resolution STM image (Supporting Information, Figure S5) of the linear structure, it is found that the voids between the trimers are not occupied by any molecules, which is the same with the F-OC13 self-assembly in 1-octanol. Toluene is a volatile solvent. After the solution was dropped on the HOPG surface, the toluene solvent evaporated immediately. Although toluene has a benzene ring, it can not coadsorb on the HOPG surface. The slight difference of F-OC13 self-assembly in different volatile solvents is that there are one or two linear lamellae in the alternated structure. The results indicate that the self-assembly of F-OC13 without the solvent effect is almost the same. The solvent polarity is an important factor for the kinetics of the assembled process. The wavelike pattern is 16018



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consists of two molecular rows. In lamella B, F-OC13 molecule form linear structure. The high-resolution STM image in Figure 7b gives detailed insight into such a structure. There are two linear lamellae between the doubled lamellae (lamella A), which is different from the structures in Figure 1b,c. In addition, it is noted that the distance between the adjacent dimers indicated in pink is closer than that of the similar structures in Figure 1e,f. The number of alkyl chains observed in lamella A is less than that of F-OC13 molecules, implying that certain tridecane chain of the F-OC13 molecule is possibly oriented toward the gas phase. A unit cell is outlined in Figure 7b with a = 2.3 ± 0.1 nm, b = 9.3 ± 0.2 nm, and α = 80 ± 2°. In the self-assembled process above, the solvent evaporation results in the absence of the molecule−solvent interaction and the structural transition. The microenvironment of the adsorbates can be tuned by the polarity of the solvents; thus, the molecular assembly could be versatile by tuning the molecules−molecule and molecule−substrate interactions.25 However, the solvent polarity is also important for the solubility of the adsorbates, which will influence the kinetics of the assembled process.

Figure 7. (a,b) Large-scale and high-resolution STM images of F-OC13 self-assembly in n-tetradecane on the HOPG surface after the sample was placed more than two hours. Vbias = 775 mV, It = 467 pA.

formed at the beginning of the sample prepartion. At that time, the effect of solvent−molecule interactions can not be neglected. Such alternate self-assembled structure (domain II in Figure 6a) was also observed in n-tridecane (Supporting Information, S6) and n-tetradecane, when the samples were placed more than two hours. After a part of the solvent (n-tridecane or ntetradecane) evaporates, solvent molecules can not further coadsorb on the HOPG surface. The self-assembly configuration of F-OC13 changes into a more densely packed pattern. Under high concentrations (∼saturated) of F-OC13 in ntridecane or n-tetradecane, the alternate configuration is also observed. Figure 7a is a typical high-resolution STM image depicting the dense-packed alternate structure of domain II (Figure 6a) obtained after a part of the n-tetradecane evaporated. Alternate lamellae labeled as A and B is formed. In lamella A, each lamella

4. DISCUSSION Table 1 summarizes the adsorption structures and characteristic parameters of the 2D self-assembled patterns of physisorbed adlayers of F-OC13 in the different solvents used in this study. The side chains of F-OC13 in each lamella arranged densely with the average distance between adjacent alkyl chains along a row is 0.46 nm. In most cases, the side chains pack along one axis of the HOPG lattice (Supporting Information, Figure S7). 4.1. Effect of the Fluorenone Moiety with Polarity. Why can F-OC13 molecule form such diversiform structures?

Table 1. Different Adsorption Structures and Unit Cell Parameters of All Ordered Structures Observed for F-OC13 in Different Solvents

a

The sample was observed by STM within one hour after dropping a solution of F-OC13. bThese images were recorded after the sample was placed more than 3 h. 16019



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which will be discussed later. If the solvents contain alkyl chains, aromatic rings, and hydrogen bonding donors or acceptors, the solvents can be adsorbed at the liquid/solid interface via van der Waals interactions with the substrate, hydrogen bonding, or van der Waals interactions with the adsorbates. The solvent coadsorption could improve the stability of the supramolecular assembled structure, especially for the porous network, allowing for the construction of 2D multicomponent self-assembled architectures and also immobilizing and imaging of small solute molecules that can not be imaged in monomolecular adlayers owing to their high mobility.25,39 The above three kinds of solvent coadsorptions can be observed in the F-OC13 assembly. At the 1-phenyloctane/ HOPG interface, the phenyloctane molecules occupy the voids of F-OC13 nanostructures. Only the aromatic rings of phenyloctane can be observed, and the side chains stretch into the liquid phase. This result implies that the space constraint is the main reason for the phenyloctane coadsorption. At the n-tridecane or n-tetradecane/HOPG interface under low concentrations, to maximize the substrate coverage, which is favored for enthalpic reasons, conjugated moieties form trimers and arrange densely. So the n-tridecane or n-tetradecane molecules coadsorb on the side chain lamella, giving rise to a closely packed structure and favorable van der Waals interactions between the alkyl chains. This observation is interpreted from geometric considerations in which the ntridecane or n-tetradecane molecule could fit into the voids left by the side chains of the F-OC13 molecule. For the molecules with alkyl side chains, the interdigitation of the side chain is energetically favored, while for the systems that have both adsorbate and solvent chain−chain interactions, the coadsorption decreases the van der Waals interactions due to the decreased density of CH2 units upon replacement of the FOC13 side chain with solvent molecules. The side chains of FOC13 in adjacent lamellae arrange by a tail-to-tail configuration instead of interdigitally, indicating that the van der Waals interactions between the side chain and the coadsorbed solvent molecule enhance the stability of the adlayer. At the 1-octanoic acid/HOPG interface, because the carboxyl groups of the 1-octanoic acid can form hydrogen bonds with the carbonyl of F-OC13, the F-OC13 form tetramers with a butterfly pattern, which is stabilized by the hydrogen bonding. Although the hydroxyl group exists in 1-octanol, it can not form hydrogen bonds with the carbonyl of F-OC13. At the 1-octanol/ HOPG interface, no coadsorption can be observed, and the FOC13 forms a similar self-assembled pattern with that at the 1phenyloctane/HOPG interface under high concentrations. This result further demonstrates that the molecule with a carboxyl group forms more easily hydrogen bonding with the carbonyl of the fluorenone moiety. 4.3. Morphology Change: The Nature of the Solvent. As discussed above, solvent coadsorption of the 1-phenyloctane, n-tridecane or n-tetradecane, and 1-octanoic acid could induce the self-assembly polymorphism according to the different solvent−molecule interactions including space effect, van der Waals interaction, and hydrogen bonding. The question arises as to why, in 1-octanol, methanol, or dichloromethane, no solvent molecules are coadsorbed, while the self-assembled structures are obviously different. For the solvent-induced polymorphism without solvent coadsorption, the solvent effects are more important in tuning the molecular structure only by

To understand the relationship between molecular chemical structure and self-assembly pattern on the HOPG surface, the self-assembly of 2,7-ditridecyloxy-9-fluorene (F′-OC13), without the carbonyl in the conjugated moiety, was studied. The main difference in two molecules is whether the carbonyl in the conjugated moiety exists. Figure 8a is a typical high-resolution

Figure 8. (a) High-resolution STM image of 2,7-ditridecyloxy-9fluorene (F′-OC13) self-assembled at the 1-phenyloctane/HOPG interface. Vbias = 726 mV, It = 460 pA. (b) Proposed molecular model for the 2D packing of F′-OC13.

STM image showing the adlayer of F′-OC13 on the HOPG surface. It can be seen that the molecules form an ordered lamellar structure with alternating molecular backbone rows and alkyl rows. No other self-assembled pattern is observed. In the molecular backbone row, the rigid moieties arrange in a line and take a reverse orientation with neighboring molecular rows, which is very similar with the linear arrangement of the 2,7ditetradecyloxy-9-fluorene (DTF) molecule.33 The proposed mode (Figure 8b) is in good agreement with the observed results from STM images. For F′-OC13, although the side chain is the same with that of F-OC13, the carbonyl is substituted by the methylene. The molecular polarity decreases drastically. The van der Waals interaction from the interdigitation of alkyl chain dominates the pattern in F′-OC13 assembly. Therefore, owing to the difference in molecular chemical structure, the self-assembly of F-OC13 presents multiple structures, whereas the self-assembly of F′-OC13 forms only one morphology at different solvent/HOPG interfaces. As the structural models shown in Table 1, the fluorenone moieties look like gemmiform shape and display an unconformable with contact each other. However, a regularity of the arrangement can be found. The F-OC13 molecules form a dimer with the back-to-back fashion or a trimer with the shoulder-to-shoulder configuration. This result demonstrates that a strong interaction between the fluorenone moieties resulting from their strong polarity. These packed patterns could eliminate the polarity of single molecule and lead to the lowest energy. In addition, the main interactions in F-OC13 assembly also include the van der Waals interactions from the interdigitation of alkyl chain and between the molecule and the HOPG surface, associated with solvent−molecule interaction. The competition of these interactions induces the self-assembly polymorphism of F-OC13. The formation of dimers or trimers where the conjugated moieties are clustered is one of the efficient ways to optimize their interactions.26 4.2. Effect of Solvent Coadsorption. The structure and orientation of the F-OC13 assembly are clearly solventdependent. Even if no solvent molecules are coadsorbed, the self-assembly of F-OC13 in different solvents is varied obviously, 16020



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driving force for the molecular assembly is associated with the minimization of Gibbs free energy, which is composed of an enthalpic gain and entropic loss. The enthalpic gain results from attractive adsorbate−adsorbate and adsorbate−substrate interactions, while the entropy is generally weakened when molecules associate or adsorb on a surface. The energy difference among different polymorphs determines the system sensitivity toward concentration: the larger the energy difference, the more dramatic the concentration dependence. Our system is very complex because the effect factors of solvent polarity and coadsorption have to be considered. Attempts to follow the recently published thermodynamic method remained futile, no concentration and solvent codependence among the coverage of different structures according to the proposed formalism was found.32,43,44 Thus, our results could display a general observed trend for concentration induced selfassembled polymorphism at the 1-phenyloctane/HOPG interface: higher solution concentration leads to a more densely packed arrangement (Table 1). At the gas/solid interface, after dropping a solution of FOC13 on the HOPG surface, the STM experiment was conducted immediately. Only a wavelike pattern was obtained and the structural transition could not be observed. In addition, after the sample was placed more than three hours, the wavelike pattern also exists, and the other two patterns appear. This result indicates that the wavelike pattern is stable kinetically and thermodynamically. The linear and alternated patterns are stable thermodynamically. The densely packed wavelike pattern with the lowest molecular density (1.38 nm2 per molecule) has the lowest energy. The formation of a wavelike pattern is associated with the polarity of the residual solvent.

changing the environment of adsorbates. Clearly, the nature of the solvent must play a key role. The solvent effect on the polymorphism could be induced by the solvent polarity, solvophobic effect, and so on.40 The polarity of the solvents is the most crucial factor that affects the molecular assembly for the molecules with polar functional groups, especially with the hydrogen bond donors or acceptors. A possible explanation for this variation in the F-OC13 monolayer is proposed in view of the dynamics, that is, the solvents with higher dielectric constants enable stabilization and fast deposition rate of the nucleation species with more polar groups. Then, the more polar the solvent, the more densely packed and more oblique the structure.41 In the strong polar solvent (methanol or dichloromethane), the molecular density (S = 1.38) is higher than that in the 1-octanol (S = 1.53). It is noted that the wavelike pattern (Figure 5) has the highest molecular density in all the adlayers of F-OC13. Because of the sensitivity of the dipole−dipole interaction to the polarity of the environment, the solvent polarity is crucial for tuning the configurations and the dipole−dipole interaction between the conjugated moieties of F-OC13. In general, for π-electron conjugated compounds with alkyl chains, the solvent−molecule interaction can be described in terms of solvophobic and solvophilic effects. In this system, in nonpolar solvents (1-phenyloctane, n-tridecane, and ntetradecane), the self-assembled pattern could transform with the change of the concentration, and the alternated structure is observed due to the solvophobic effect. The solvophilic effect between the hydrophilic 1-octanol or 1-octanonic acid molecules and the hydrophilic F-OC13 molecules should stabilize their coadsorption. In fact, coadsorption could observe in 1-octanonic acid due to the strong hydrogen bonding between the carboxyl and carbonyl group. No coadsorption was observed in 1-octanol due to strong van der Waals interaction between the alkyl chains of F-OC13. When the solvent was evaporated completely or the nonpolar molecule acts as the solvent (no coadsorption), the molecule− solvent interaction is inexistence or very weak. A balance between intermolecular van der Waals interaction of side chains and the dipole−dipole interaction of fluorenone groups mainly dominates the molecular self-assembled pattern. Methanol or dichloromethane has a high volatile rate. The self-assembly process could be completed within a shorter period of time, which results in the formation of three kinds of morphologies on the HOPG surface. This result also demonstrates that these self-assembled patterns are all stable and that the dominated structure is associated with the solvent effect. 4.4. Thermodynamics. Self-assembly at the solid−liquid interface is controlled by the balance of adsorbate−adsorbate, adsorbate−substrate, adsorbate−solvent, and solvent−substrate interactions. The assembly process is dynamic and depends on the adsorption−desorption equilibrium.42 In our experiments, no structural transition at the liquid/solid interface was observed during the scanning, which indicates that all the configurations are stable thermodynamically. To understand the effect of F-OC13 concentration on the polymorphs in 1phenyloctane, the adsorption−desorption process under thermodynamic control was proposed. The adsorption− desorption equilibrium determined the surface coverage ratio of the single, double, and triple linear structures. The concentration dependency of the self-assembly of F-OC13 can therefore be understood as arising from the different stabilities and molecular densities of different polymorphs. In general, the

5. CONCLUSIONS The self-assembly adlayers of F-OC13 on the HOPG surface in several different solvents are investigated by STM. The structure and orientation of the F-OC13 assembly are clearly solvent-dependent. The balance between the dipole−dipole interaction of the fluorenone moieties and the van der Waals interaction of the side chain has an important effect on the formation of self-assembly polymorphism. Moreover, the polarity of the solvent significantly affects the self-assembly pattern of F-OC13 due to the existence of carbonyl polar group. Solvent coadsorption of the 1-phenyloctane, n-tridecane or ntetradecane, and 1-octanoic acid could induce the self-assembly polymorphism according to the different solvent−molecule interactions including space effect, van der Waals interaction, and hydrogen bonding. The results will further guide us to investigate the self-assembly of other fluorenone derivatives with different chain length in order to seek for a detailed selfassembly essence and regularity.

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ASSOCIATED CONTENT

S Supporting Information *

Additional STM images of F-OC13 in different solvents. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: (+86)020-22236708. E-mail: [email protected] (X.M.); [email protected] (W.D.). 16021



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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports from the National Program on Key Basic Research Project (2012CB932900 and 2009CB930604), the National Natural Science Foundation of China (21103053, 91023002, and 51073059), and the Fundamental Research Funds for the Central Universities (2011ZM0004) are gratefully acknowledged.

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Self-Assembly Polymorphism: Solvent-Responsive Two-Dimensional

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