C5H10â: An Efficient Strategy on Fabricating ... - ACS Publications

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One Chain Fixed, One Chain Modified by −C5H10−: An Efficient Strategy on Fabricating Structural Diversity for 2D Self-Assembly Yi Hu, Kai Miao, Li Xu,* Xinrui Miao, and Wenli Deng* College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China S Supporting Information *

ABSTRACT: Fabrication of diverse self-assembled structures at the liquid/solid interface has been a topic receiving immense attention in 2D crystal engineering. We designed a series of 2-pentadecyloxy-7-alkoxy-9fluorenone (F−C15Cn) molecules via fixing one chain and prolonging the other chain by five carbon atoms (−C5H10−) and explored their selfassembly behaviors at the 1-octanoic acid/HOPG interface. We successfully obtained six nanostructures for F−C15, F−C15C5, F−C15C10, F−C15C15, and F−C15C20. These assembled configurations were driven by noncovalent forces of dipole−dipole, van der Waals, and hydrogen bonding interactions. Moreover, we performed force field calculation to reveal the involved binding energies. The results showed that the strength of hydrogen bonds were related to the arrangement of hydrogen bonds donor and acceptor: As the alkyl chain in the 7-positon became longer, the van der Waals interactions were accordingly stronger. As a further step of exploring how the alkyl chain length affects the crystalline properties, we investigated these fluorenone derivatives in bulk phase during thermal process. Except for F−C15, the molecules displayed different liquid crystal phases, which indicated their potential application as functional materials. In general, these results are evidence that accurate modification of the alkyl chains can be regarded as an efficient strategy on inducing structural diversity in surface confined system.

1. INTRODUCTION Controlling and inducing the self-assembled nanostructures in the field of two-dimensional (2D) crystal engineering is a subject that has received intense attention.1−3 In recent decades, structural diversity for organic molecules on solid surfaces has been widely reported, using scanning tunneling microscopy (STM) as the visualization tool.4−8 These surfaces are used as templates or as functional elements which have potential application for catalysis, sensing, crystal growth, organic electronic devices, and so on.9−11 For the purpose of fabricating diverse nanostructures, 2D self-assembly has been recognized as a promising methodology based on the bottomup principle in nanoscience and nanotechnology.12−15 Molecular self-assembly is a process which is sensitive to the delicate balance among the noncovalent interactions, including hydrogen bonds, π−π stacking, dipolar interaction, van der Waals (vdWs) interaction, and so on.16−19 Moreover, the selfassembled networks are able to be regulated by changing the concentration,18,20 solvent,5,21 voltage,22 temperature,23,24 the chain length,17,25 and functional groups.26,27 Among all of these strategies, chain-length effect has always been considered as an efficient approach because it is associated with not only the regulation of diverse nanostructures but also control of the molecule−molecule and molecule−substrate vdWs interactions. The traditional method for chain-length effect mainly focuses on gradual elongation of the chains, together with the odd−even effect.28−30 Furthermore, by © XXXX American Chemical Society

unsymmetrical modification of the side chains, molecules show the capability of chiral recognition and separation. For example, De Feyter and co-workers synthesized a series of alkoxylated dehydrobenzo[12] annulene derivatives. When the molecules were substituted by chains which were of different lengths, they successfully obtained two chiral nanoporous packings that were identical in their packing density but differ in their symmetry.12 When the molecules were substituted by chiral chains, they described a chiral phenomenon in which the initial majority handedness of the self-assembled networks were able to be amplified or reversed to produce a homochiral surface.3 Such results indicate that modifications on the alkyl chains show the possibility to be utilized for inducing diverse 2D nanostructures. Therefore, a more comprehensive and accurate modification specialized on the alkyl chains based on a series of molecules seems to be a meaningful work which will contribute to the study of self-assembly in supramolecular chemistry. For this reason, we focus on regulation of the selfassembled structures via progressively changing the alkyl chains. Fluorenone derivatives have been intensively explored as the building blocks in our group because of their strong ability of forming diverse self-assembled structures which were induced by the functional groups,20,31,32 modification of the end groups Received: July 5, 2017 Revised: September 2, 2017 Published: September 7, 2017 A

DOI: 10.1021/acs.jpcc.7b06585 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C of the alkyl chains,18,33 competition between the noncovalent forces,34 and type of the solvent.35,36 Moreover, fluorenone derivatives with a thiophene group in the alkyl chain have been reported by Demadrille et al. They explored these compounds with polarized optical microscopy (POM), and the results showed that these compounds possess liquid crystal (LC) property.37 On the basis of our previous work, we were motivated to design and synthesize molecules by accurate modification of the side chains via changing the chain length which were in an arithmetic sequence. The molecules used in this present work are 2-pentadecyloxy-7-alkoxy-9-fluorenone (F−C15Cn), as shown in Scheme 1. First, a pentadecyloxy chain

of force field calculations, the underlying mechanisms of these assembled structures were illustrated from the viewpoint of their binding energies. As a further step of investigating these five compounds toward crystal engineering and comparing the alkyl chains induced crystalline differences, we characterized their POM properties. F−C15, which is substituted by only one alkyl chain, is not an LC molecule, while the other four molecules with two alkyl chains are LCs. In general, this work demonstrates the significance of modification of alkyl chain on inducing structural diversity for molecular networks in the surface-confined monolayers, and we believe that our results will promote the study of interface science.

Scheme 1. Chemical Structures for the Fluorenone Derivatives of F−C15, F−C15C5, F−C15C10, F−C15C15, and F−C15C20, and the Solvent of 1-Octanoic Acid

2. EXPERIMENTAL SECTION The fluorenone derivatives were synthesized by separately adding the two alkyl chains to the fluorenone cores, as described in Figure S1. Also, the characterization data confirming the chemical structures and purity of the compounds are shown in Figures S2−S6. The powder X-ray diffraction patterns (XRD) were recorded using a Bruker D8ADVANCE diffractometer with Cu Kα radiation. A step-scan mode was adopted with a sampling time of 0.1 s and a scanning step of 0.02°. The sharp and relatively intense peaks for these compounds are evidence that they are crystalline and in different three-dimensional solid states, as shown in Figure S7. 1-Octanoic acid was purchased from TCI and used without purification. The concentration for the solutions used in this study are 1.4 to 2.5 × 10−4 M. STM experiments were conducted after a drop of solution was deposited onto the HOPG (quality ZYB, Bruker, USA) surface, using a Nanoscope IIIa Multimode SPM (Bruker, USA), under ambient condition (temperature: 15−20 °C, humidity: 45−60%). The tips were mechanically cut from Pt/In wires (80%/20%). Different tips and samples were used for the sake of checking the reproducibility of the results. All of the images were recorded under a constant current mode. The imaging parameters are given in the corresponding figure captions. We used Materials Studio 7.0 to build the structural models. The models were constructed based on the intermolecular distance and angle as well as analysis of the high-resolution STM results. The force field calculations for the binding energies were calculated from four unit cells, using the Forcite package. The charge density for the structural models were calculated using DMol 3 package. The DSC experiments were conducted with a scan rate of 10 °C min−1 for heating and cooling traces (NETZSCH DSC 200F3). The samples were heated to a temperature a bit higher than the melting point, then cooled from the liquid phases to the bulk phases while using POM (iBX51-

(−C15H31) was fixed in the 2-position of the fluorenone core. Then, another alkyl chain (−CnH2n+1) was substituted to the 7position. We changed the chain length by five carbon atoms (−C5H10−); thus, we obtained four fluorenone derivatives: F− C15C5, F−C15C10, F−C15C15, and F−C15C20. Moreover, for the purpose of further exploring the chain-length effect, we removed the chain in the 7-postion by five carbon atoms and obtained another compound, F−C15. Note that in this work, n = 0 refers to F−C15. To our knowledge, exploration of molecules with one chain fixed and the other chain changed in arithmetic sequence has never been reported previously. We demonstrated the self-assembly properties of these fluorenone derivatives at the 1-octanoic/highly oriented pyrolytic graphite (HOPG) interface at similar concentrations. F−C15, F−C15C5, F−C15C10, F−C15C15, and F−C15C20 arranged into six networks of linear I, wave-like, wave−linear, chiral S-like, trimer, and linear II structures, which were regulated by their different alkyl chains. These diverse 2D configurations were governed under the combined functions of hydrogen bonds, dipolar forces and vdWs interactions. Furthermore, with the aid

Figure 1. (a) High-resolution STM image for F−C15 at the 1-octanoic acid/HOPG interface. Concentration: 2.5 × 10−4 M. A series of black arrows indicate the 3-fold symmetry axis of the HOPG surface. Scanning conditions: It = 600 pA, Vbias = 690 mV. (b) Proposed structural model for this linear I pattern. (c) Illustration of the C−H···OC hydrogen bonds between the fluorenone cores. B


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The Journal of Physical Chemistry C

Figure 2. (a and b) Large-scale STM images for F−C15C5 at the 1-octanoic acid/HOPG interface, showing the structural transition from the metastable wave-like structure to the stable wave−linear structure. Concentration: 2.1 × 10−4 M. (c) Proposed structural model for the wave-like structure. (d) High-resolution STM image for the wave−linear structure. The green and blue polygons represent the linear and wave-like rows, respectively. The black arrows are used to shown the 3-fold symmetry axis of the graphite lattice. (e) Proposed structural model for the wave−linear structure. (f) Enlarged image which illustrates the C−H···OC hydrogen bonds between the fluorenone cores and offset of the dipole moments within the adjacent dimers. Scanning conditions: It = 650 pA, Vbias = 710 mV.

shortest time for clear STM image. We call this kind of configuration a wave-like structure. However, during the scanning process we found that this wave-like structure was metastable because it disappeared very quickly. The existence time for this wave-like structure was limited such that we could not obtain any high-resolution STM images even though we tried different samples. Instead, the stable phase for F−C15C5 under this experimental condition is shown in Figure 2b, which was observed after the disappearance of the wave-like phase. For the purpose of proving the stability of this wave−linear structure, the sample was continuously scanned for 2 h (Figure S9a) and also scanned 12 hours after it was placed under ambient condition (Figure S9b). Figure 2d is the highresolution STM image that manifest the structural details. The F−C15C5 molecules are arranged into two kinds of rows: One row contains two linear ribbons, and the other row contains a wave-like ribbon, as respectively indicated by the green and blue polygons in Figure 2d. For its distinct arrangement features, this kind of molecular pattern is named wave−linear structure. Careful observation reveals that molecules in both of the linear and wave-like rows are packed every two together, forming orderly aligned dimers. Molecules in the dimer are arranged with their carbonyl groups directing to reverse directions and forming C−H···OC hydrogen bonds. Moreover, the dimers in the wave-like rows are packed in an antiparallel fashion which is favored to offset the dipole moment of the self-assembled area. The linear row contains two ribbons, and the dimers in these two ribbons are also packed in an antiparallel mode, which is favored in consideration of dipole−dipole interaction. The alkyl chains all extend along one of the symmetry axes of the graphite lattice; thus, the molecule−substrate vdWs interaction is maximized. The long chains for F−C15C5 molecules are interdigitated while the short chains are not. A proposed structural model is shown in Figure 2e, and the enlarged image in Figure 2f shows the C−H···OC hydrogen bonds and the

PLINKAMTHMS600) to record their crystalline states. The heating rate was 20 °C min−1, and the cooling rate was 5 °C min−1.

3. RESULTS AND DISCUSSION 3.1. 2D Self-Assemblies. 3.1.1. Self-Assembly for F−C15 at the 1-Octanoic Acid/HOPG Interface. First, we explored the self-assembly of F−C15 which is substituted only by one alkyl chain in the 2-position. As a drop of solution was applied onto the HOPG surface, an ordered adlayer constructed by linear lines formed, as shown in the large-scale STM image in Figure S8. This kind of 2D configuration is a linear I structure. Figure 1a is the high-resolution STM image which displays more structural details. In the linear rows, the F−C15 molecules are packed every three together, with their fluorenone groups in a head-to-back mode. Therefore, C−H···OC hydrogen bonds were believed to exist. The alkyl chains in the same row are densely arranged, extending along one of the symmetry axis of the graphite lattice (as shown by a series of black arrows in the left bottom of Figure 1a). This kind of packing is favored in consideration of molecule−molecule and molecule−substrate vdWs interactions. Figure 1b is the proposed structural model for this linear I structure based on the high-resolution STM image. The enlarged image in Figure 1c indicates the weak hydrogen bonds formed with the H atom from the fluorenone core exerting its role as the donor and O atom from the carbonyl group as the acceptor. The unit cells consist of three molecules are overlaid in Figure 1a,b, with lattice parameters of a = 2.4 ± 0.1 nm, b = 1.3 ± 0.1 nm, and α = 83 ± 2°. The calculated area density is 1.0 nm2 per molecule. 3.1.2. Self-Assembly for F−C15C5 at the 1-Octanoic Acid/ HOPG Interface. As a pentyloxy chain was substituted in the 7position, we obtained completely different self-assembly networks. A monolayer consists curve lines spontaneously formed as a drop of solution was deposited onto the HOPG surface, as shown in Figure 2a, which was recorded within the C


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Figure 3. (a) Large-scale STM image for F−C15C10 at the 1-octanoic acid/HOPG interface. The white dotted lines represent the domain boundaries. Concentration: 1.8 × 10−4 M. (b and c) High-resolution STM images for the two enantiomers which are constructed by S-like and anti-S-like shapes. The red and green bent lines are used to show the chirality of these two structures. The green arrows indicate the coadsorbed 1-octanoic acid molecules among the two adjacent octamers. (d) Illustration of the molecular packing in the basic unit of the S-like and anti-S-like octamers. The dipole pairs which can offset their dipole moments within the octamer are marked by a series of arrows. The C−H···OC and COOH···COOH hydrogen bonds are indicated by black dotted lines in the circles. (e and f) Proposed structural models for the chiral S-like structures. The F−C15C10 and the 1-octanoic acid molecules are in blue and orange. Scanning conditions: It = 590 pA, Vbias = 680 mV.

(about 0.8 nm), they are the coadsorbed solvent molecules of 1-octanoic acid. Moreover, the two 1-octanoic acid molecules are speculated to be arranged with their carboxyl groups in a head-to-head way, which is favored to form COOH···COOH hydrogen bonds to stabilize the self-assembled structure (Figure 3d).16 From the high-resolution STM images, we see that the short chains are not interdigitated. However, the long chains are interdigitated and extend along the symmetry axis of the HOPG lattice (as indicated by the black arrows in the highresolution STM image), which is favored for molecule− molecule and molecule−substrate interactions. The structural models for these chiral S-like structures are proposed in Figure 3e,f. For ease of distinction, the F−C15C10 and 1-octanoic acid molecules are in blue and orange, respectively. Two unit cells are overlaid in Figure 3b,c, and the parameters are a = 6.4 ± 0.2 nm, b = 4.2 ± 0.2 nm, α = 53 ± 1° for Figure 3b and a = 6.4 ± 0.1 nm, b = 4.1 ± 0.1 nm, α = 52 ± 1° for Figure 3c. The unit cells each contain eight molecules, and the calculated area densities are 2.7 and 2.6 nm2 per molecule. 3.1.4. Self-Assembly for F−C15C15 at the 1-Octanoic Acid/ HOPG Interface. As the alkyl chain in the 7-position increases to 15 carbon atoms, there are two same chains that are symmetrically substituted to the fluorenone cores, we obtain another kind of monolayer. Figure 4a is the typical large-scale STM image for F−C15C15 at the 1-octanoic acid/HOPG interface. It is clear that the molecules are arranged into linear rows in several domains. From the high-resolution STM image in Figure 4b, we see that the F−C15C15 molecules are packed every three together, forming trimers. This kind of network is called a trimer structure. According to the shape of the bright dots which are related to the fluorenone cores, the three molecules in the trimer are arranged in head-to-back and backto-back modes, which indicate that only two of them point in the same orientation while the other one points in the reverse orientation. Therefore, C−H···OC hydrogen bonds form

dipole offset between the adjacent dimers. Besides, according to Figure 2a,b, we found that the rows in the wave-like structure are the same as those wave-like rows in the wave−linear structure. Therefore, based on the model in Figure 2e, we proposed a structural model for the metastable wave-like structure, as shown in Figure 2c. From these models we see that the linear row is denser than the wave-like row; from the viewpoint of densely packing principle, the wave−linear structure is more favored than the wave-like structure. The unit cells for the wave−linear structure are overlaid in Figure 2d,, with the parameters a = 2.0 ± 0.2 nm, b = 8.9 ± 0.2 nm, and α = 89 ± 1°. The unit cell contains eight molecules, and the calculated packing density is 2.3 nm2 per molecule. Since the wave-like structure for F−C15C5 is a metastable phase, it is impossible for us to obtain the high-resolution STM images and give its unit cell information. 3.1.3. Self-Assembly for F−C15C10 at the 1-Octanoic Acid/ HOPG Interface. The F−C15C10 molecules self-assemble into chiral structures, as shown in Figure 3a, in which the monolayer is constructed by domains that are in different chirality. The basic unit for this chiral structure is the S-like or anti-S-like shape, which contains eight F−C15C10 molecules. Figure 3b,c gives the high-resolution STM images for these two enantiomers, and the red and green bent lines are used to indicate their handedness. In this chiral S-like structure, both the S-like and anti-S-like octamers can be regarded as two tetramers that are opposite to each other. The four molecules in the tetramers are arranged in a head-to-back mode, which means their carbonyl groups are pointing in the same direction, and C−H···OC hydrogen bonds form among the neighboring fluorenone cores (Figure 3d). Molecules in the two tetramers are antiparallel thus the dipole moment within the octamer is offset to zero (Figure 3d). From Figure 3b,c, we see two short lines between the adjacent octamers, as indicated by a series of green arrows. According to their shape and length D


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Figure 5. High-resolution STM image for F−C15C20 at the 1-octanoic acid/HOPG interface. Concentration: 1.4 × 10−4 M. Scanning conditions: It = 590 pA, Vbias = 630 mV. The red, blue, yellow, pink, and green circles represent the monomer, dimer, trimer, tetramer, and pentamer aggregations, respectively.

Figure 4. (a) Large-scale and (b) high-resolution STM images for F− C15C15 at the 1-octanoic acid/HOPG interface, showing the trimer structure. Concentration: 1.6 × 10−4 M. Scanning conditions: It = 550 pA, Vbias = 600 mV. (c) Proposed structural model for this trimer structure. (d) Illustration of the C−H···OC hydrogen bonds between the fluorenone cores.

between the two head-to-back fluorenone units. All of the alkyl chains are fully interdigitated and extending along one of the symmetry axis of the HOPG lattice, thus both of molecule− molecule and molecule−substrate vdWs interactions are maximized. Figure 4c shows the proposed structural model for this trimer structure. The enlarged image in Figure 4d displays the hydrogen bonds between the fluorenone cores. The unit cells which contains three molecules are overlaid in Figure 4b,c, with the parameters of a = 2.6 ± 0.2 nm, b = 2.9 ± 0.2 nm, α = 58 ± 1°, and the calculated area density is 2.1 nm2 per molecule. 3.1.5. Self-Assembly for F−C15C20 at the 1-Octanoic Acid/ HOPG Interface. For further exploring how the alkyl chain number affects the structural diversity of this fluorenone system, we increased the number of carbon atoms in the alkyl chain in the 7-position to 20. As a result, we observed monolayers that were constructed by noncontinuous and nonstraight lines, as shown in the large-scale STM image in Figure S10. Figure 5 shows the high-resolution STM image which manifests the structural details. In the nonstraight lines, the F−C15C20 molecules are aggregated in a random fashion. As indicated by the red, blue, yellow, pink, and green circles, molecules are gathered into monomer, dimer, trimer, tetramer, and pentamer, respectively. Nevertheless, there are also other aggregations in the self-assembled adlayer. This kind of arrangement is labeled as linear II. The direct result for these different kinds of packing aggregations are the irregular linear spaces among the alkyl chains, as indicated by green arrows in Figure 5. Even though the alkyl chains from adjacent linear rows are interdigitated, they do not match well with each other to fill all of the HOPG surface because of their random appearance. For the sake of revealing the structural details, we made a statistical histogram which shows the possibility of every aggregations, as shown in Figure 6. In a 50 × 50 nm2 area,

Figure 6. Surface coverage distribution of aggregations with different number of molecules (measuring area: 50 × 50 nm2). N represents number of a certain aggregation. N′ represents total number of molecules in a certain aggregation.

F−C15C20 gather together into aggregations that contains different number of molecules (n = 1 to 9). N represents the number for a specific aggregation in the 50 × 50 nm2 area, and N′ represents the total number of molecules for that aggregation. Then, we calculated the surface coverage (as described in the Supporting Information) for all nine kinds of aggregations. From the results, we come to the conclusion that the tetramer shows priority over the other aggregations during the self-assembly process. Then, a question arises as to why F− C15C20 self-assembles into this linear II structure in which molecules are irregularly packed, while the other fluorenone derivatives self-assemble into ordered monolayers. This phenomenon is believed to be caused by the long alkyl chain. Disordered self-assembly networks are always observed for molecules with long side chains, and this phenomenon has been reported several times.19,28,31,38 The longer alkyl chain causes stronger molecule−substrate interaction and thus can restrict the adsorption−desorption behavior and in-plane mobility of the molecules. 3.2. Self-Assembly Mechanisms. It has been widely reported that the spontaneously formed self-assembly monolayer is governed by the delicate balance between various noncovalent forces, such as vdWs, hydrogen bonding, π−π stacking, metal coordination, halogen bonds, and electrostatic interactions.39,40 The stable self-assembly at the liquid/solid E


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Table 1. Lattice Parameters and Calculated Binding Energies for the Self-Assembled Nanostructures Observed at the 1Octanoic Acid/HOPG Interfacea molecule F−C15 F−C15C5 F−C15C10 F−C15C15 F−C15C20

phase

a (nm)

b (nm)

α (deg)

N

SN (nm2)

ΔEH (kJ mol−1)

ΔEV (kJ mol−1)

linear I wave-like wave−linear S-like anti-S-like trimer linear II

2.4 ± 0.1

1.3 ± 0.1

83 ± 2

3 4 8 8 8 3

1.0

−27.9 −15.2 −15.7 −25.1

−41.4 −11.1 −31.1 −61.9

−36.9

−76.9

2.0 6.4 6.4 2.6

± ± ± ±

0.2 0.2 0.1 0.2

8.9 4.2 4.1 2.9

± ± ± ±

0.2 0.2 0.1 0.2

89 53 52 58

± ± ± ±

1 1 1 1

2.3 2.7 2.6 2.1

N represents the number of molecules in the unit cell. SN = unit cell area divided by number of molecules per unit cell area. ΔEH = binding energy of hydrogen bonding interaction. ΔEv = binding energy of vdWs interaction.

a

Except for the linear II structure which is impossible to propose as a unit cell and structural model, the underlying binding energies of the vdWs interactions were calculated to be −11.1 and −31.1 kJ mol−1 for the two self-assembled structures of F− C15C5, − 61.9 kJ mol−1 for F−C15C10, and − 76.9 kJ mol−1 for F−C15C20. These results are summarized in Table 1. From this table, it is clear that when the alkyl chain in the 2-position is fixed (−C15H31) as the alkyl chain in the 7-positon increases (n = 5, 10, and 15) that the molecule−molecule vdWs interaction increases accordingly. Therefore, we conclude that accurate modification of the length of the alkyl chain is an efficient method of controlling the strength of the noncovalent vdWs interaction. Hydrogen bonding interaction is another driving force which exerts important function on formation of different molecular configurations, because of its high selectivity and directionality.18,41,42 In the self-assembled networks for these fluorenone derivatives, hydrogen bonds form between the adjacent fluorenone cores. Moreover, according to the arrangement modes of the fluorenone cores and the positions of the H atoms (labeled as H1, H2, and H3, as shown in Table 2), the hydrogen bonds within these assembled structured are classified into three types, as shown in Figure 7. In type I, the fluorenone cores are arranged in a head-to-head fashion, and hydrogen

interface is a dynamic process which depends on the adsorption−desorption equilibrium of the molecules. For this fluorenone system, dipole−dipole interaction, hydrogen bonds, and vdWs interactions between the adsorbate−adsorbate and adsorbate−substrate are the driving forces. Dipolar interaction is a well-recognized weak force inducing structural diversity in the field of 2D self-assembly. Fluorenone derivatives show strong polarity because of the electron affinity of the fluorenone cores.19 In general, the polar molecules tend to arrange into antiparallel or collinear modes, and as a result, the dipole−dipole interaction is maximized.35 The dipole pairs in which the total dipole moments can be offset to zero were observed in both of the wave-like and wave−linear structures for F−C15C5. In the chiral S-like structure for F−C15C10, the molecules are packed every four together and form two antiparallel tetramers. As a consequence, the dipole moment is minimized to zero in the self-assembled area. However, in the trimer structure for F−C15C15, molecules in the trimer are arranged with two of them pointing in the same direction, while the other one points in the reverse direction. Thus, dipolar interaction is not favored. In the linear II structure for F− C15C20, the molecules are packed in a collinear fashion but lack regularity, and this is also impossible for dipolar interaction. These cases for F−C15C15 and F−C15C20 are indications that the dipole−dipole interaction is not the factor that is always required. We attribute this to the elongation of the side chains, which contribute a lot to increasing the molecule−molecule vdWs interaction. Fully interdigitated alkyl chains which extend along the main symmetry axis of the HOPG surface are in most cases the absolute tendency for the self-assembly networks. This is a phenomenon driven by the maximization of both of the adsorbate−adsorbate and adsorbate−substrate vdWs interactions. In the linear I structure for F−C15, even though the only alkyl chains are not interdigitated, they spin to an angle (about 112°, see Figure S11) and are packed adjacent with each other; thus, the distance between two neighboring rows are decreased. This is also a kind of dense packing which is favored for enthalpy reason. We conducted force field calculation (as described in the Supporting Information) to reveal further the strength and binding energy of vdWs interaction involved in this fluorenone system. The result shows that the vdWs interaction within the linear I structure for F−C15 is relatively strong (compared with the other vdWs interactions which are to be discussed) and is calculated to be −41.4 kJ mol−1. In the self-assembled configurations of wave-like and wave−linear (F− C15C5), chiral S-like (F−C15C10), trimer (F−C15C15), and linear II (F−C 15 C 20 ) structures, the alkyl chains are interdigitated, and they matched well with the graphite lattice.

Table 2. Summary for the Types of C−H···OC Hydrogen Bonds Involved in the Representative Nanostructures at the Liquid/HOPG Interfacea

a

N represents the number of molecules in the unit cell. N1, N2, and N3 represent the number of hydrogen bonds formed from the H1, H2 and H3 atoms and carbonyl groups. N1m, N2m, and N3m refer to the average number of hydrogen bonds per molecule. F


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and wave−linear structures. In type II, the two fluorenone cores are directing to the same orientation. Moreover, according to their center lines, the two fluorenone units show malposition by some extent, and hydrogen bonds form among the H2 and the O atoms (Figure 7c). This kind of hydrogen bonds are found in the chiral S-like structures. In type III, the two fluorenone cores are arranged in a head-to-back fashion, and hydrogen bonds form among the two H3 and one O atoms (Figure 7e). This kind of hydrogen bond exists in the linear I, chiral S-like, and trimer structures. These are indications that the nanostructures for these fluorenone derivatives are associated with the types of hydrogen bonds. The number, type, and density of different intermolecular hydrogen bonds involved in the self-assembled structures are summarized in Table 2. To reveal further the bonding features of these complex hydrogen bonds types, we compiled their charge density maps, as shown in Figure 7b,d,f. These characterization data show that the charge concentration regions over the carbonyl groups direct to the charge depletion regions near the H atoms, which can facilitate the formation of C−H···OC hydrogen bonds. For the linear I, wave-like, wave−linear, chiral S-like, and trimer structures, the binding energies for the hydrogen bonds were calculated to be −27.9, −15.2, −15.7, −25.1, and −36.9 kJ mol−1, respectively (as summarized in Table 1). From these results we conclude that (i) different types of hydrogen bonds cause different strength for these forces and (ii) hydrogen bonds that are in the same type also differ from each other in their strength because of the different packings in the assembled nanostructures. Besides, as we observed the coadsorption of solvent molecules among the chiral S-like structures, we speculated that the two 1-octanoic acid molecules were packed in a head-to-head fashion and that COOH···COOH hydrogen bonds then formed. This kind of hydrogen bonding interaction was calculated to be −22.1 kJ mol−1, which indicated that it was strong (compared with the C−H···OC hydrogen bonds) enough to stabilized the assembled monolayer. The F−C15C5 molecules self-assembled into both wave-like and wave−linear structures. However, during the scanning process we found that the wave-like structure was a metastable

Figure 7. (a, c, e) Molecular models and (b, d, f) charge density maps for the dimer configurations which form three types of hydrogen bonds. The blue and red parts represent the positive and negative contours.

bonds form among the two H1 and the two O atoms (Figure 7a). This kind of hydrogen bond commonly exists in wave-like

Figure 8. Schematic representation of the self-assembled structures for F−C5, F−C15C5, F−C15C10, F−C15C15, and F−C15C20. The fluorenone cores are represented by the green-yellow shapes. The red dots represent the carbonyl groups. The black lines refer to the side alkyl chains. G


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Therefore, we explored the phase transition behaviors from the viewpoint of thermal analysis. Figure 9a shows the differential

phase, and it always disappeared within a very short time. Instead, the wave−linear structure was the stable one. These results illustrate that the wave−linear structure has priority than the wave-like structure. This can be explained from the viewpoints of kinetics and densely packing principle. In both of these two structures, dipole−dipole interaction is favored because the molecules are arranged into pairs in which the dipole moment is offset to zero. The C−H···OC hydrogen bonds also form among the fluorenone units. From the force field calculation results, we see that the binding energy of hydrogen bonds for these two structures are similar (−15.2 and −15.7 kJ mol−1). However, the binding energy of vdWs interactions for them show big differences, which are −11.1 and −31.1 kJ mol−1. These are indications that the wave−linear structure is more favored than the wave-like structure in kinetics, which means that in consideration of their driving forces the former one is more stable. Even though it is impossible to calculate the area density of wave-like structure, from the proposed structural models we clearly see that the linear rows are denser than the wave-like rows. Thus, the wave−linear structure is more favored than the wave-like structure for enthalpy reasons. 3.3. Chain-Length Effect. 2D supramolecular selfassembled structures have been widely reported to be dependent on the alkyl chain length. 2,17,19 This is a phenomenon which resulted from the controllable molecule− molecule and molecule−substrate interaction. The dependence of self-assembled networks on the chain length is presented schematically in Figure 8. Through fixing one chain and gradually increasing the other chain length by five carbon atoms, structural diversity was successfully obtained. Another phenomenon resulted from the gradually increased alkyl chain is the diverse packing modes of the alkyl chains. In the linear I structure for F−C15, the only alkyl chains are packed in collinear, spin to an angle (respect to the fluorenone core), and chains from different linear rows are not interdigitated. As the alkyl chain in the 7-position increased to −C5H11, in both the wave-like and wave−linear structures the long alkyl chains for F−C15C5 are fully interdigitated while the short chains are not. Moreover, a distinct feature for these nanostructures is that the short and long alkyl chains are separated. For F−C15C10, the short chains are not interdigitated. However, in the basic unit of octamer, 12 long chains take part in the interdigitation with the chains from the neighboring octamers, and the other four chains do not. Another feature is that the short and long chains are mixed, meaning that they together construct the chain ribbons in the assembled networks. The direct result of this kind of interdigitation is the curved molecular ribbons, which appear as a series of S-like shapes. Since the curved ribbons have two reverse rotation directions, two nanostructures possessing reverse chirality are formed in the monolayer. When the alkyl chain in the 7-position increases to −C15H31 and −C20H41, all of the alkyl chains are interdigitated. From these we conclude that as the alkyl chain length increases the accordingly increased chain−chain vdWs interaction plays an increasingly strong role on affecting the packing of the chains by facilitating the interdigitation of the alkyl chains which is favored in consideration of dense packing. As we have calculated the binding energies involved in the self-assembled structures and as self-assembly is a process from a chaotic phase to an ordered phase which is also related to energy, we believe that chain-length effect induced structural diversity must have relationship with phase transition process.

Figure 9. (a) DSC thermalgrams for the fluorenone derivatives of F− C5, F−C15C5, F−C15C10, F−C15C15, and F−C15C20. The above and below lines correspond to the heating and cooling traces, respectively. (b) Dependence of the phase-transition temperature (melting point) for fluorenone derivatives on the number of carbon atoms in alkyl chains in the 7-positon.

scanning calorimetry (DSC) thermograms for F−C5, F−C15C5, F−C15C10, F−C15C15, and F−C15C20. The peak temperatures in the horizontal ordinate represent the phase transition temperatures. It is clear to see from the DSC curves that as the alkyl chain length changes the peak positions display an obvious difference which is originated from different energy required to form a uniform phase.43 The dependence of melting points for these fluorenone derivatives on the alkyl chain length is shown in Figure 9b. Furthermore, the DSC curves for F−C15C10 display two apparent peaks, indicating that there are at least two phases for F−C15C10 during its thermal process. This can also be regarded as its underlying structural diversity in bulk phase. Motivated by these phase transition differences, we explored this fluorenone system toward its crystalline property. 3.4. Chain-Length-Induced Crystalline Difference in Bulk Phase. The two phase transition peaks in the DSC results indicate that the F−C15C10 molecule possesses liquid crystalline property. The lower and higher phase-transition temperatures correspond to the clearing and melting points, respectively. POM is a good method for studying the crystallization characters during the thermal process. Furthermore, investigation of the crystalline properties in bulk phase is also beneficial to the study of their potential application in the field of interfacial materials. Even though the length of alkyl chain in the 7-position for these fluorenone derivatives display a difference of five carbon atoms, we need to note that F−C15 molecule is different from F−C15C5, F−C15C10, F−C15C15, and F−C15C20 because of the different alkyl chain number. The difference of one chain causes H


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Figure 10. POM images for F−C15 recorded at different time intervals during the trace of cooling, showing the growth of spherulitic texture. Temperature: 60, 58, 55, 53, 52, and 46 °C for (a−f), respectively. Magnification: 500×.

Figure 11. POM images for fluorenone derivatives on the trace of cooling. (a and b) LC schlieren texture and crystalline phase for F−C15C5. (c and d) LC schlieren texture and crystalline phase for F−C15C15. (e and f) LC corrugated texture and crystalline phase for F−C15C20. (g−i) Phase transition process from LC schlieren texture, to LC schlieren and spherulitic textures, then to crystalline phase for F−C15C10. Temperature: 58 and 46 °C for (a and b); 79 and 63 °C for (c and d); 76 and 59 °C for (e and f); 80, 78, and 61 °C for (g−i). Magnification: 500×.

corrugated (Figure 11e) textures for F−C15C5, F−C15C15, and F−C15C20 at temperatures of 58, 79, and 76 °C, respectively. The LC textures for F−C15C5 and F−C15C15 are the same. When the temperature decreases further, the LC textures are replaced by the crystalline phases, as shown in Figure 11b,d,f which correspond to the temperatures of 46, 63, and 59 °C, respectively. The DSC data in Figure 9a (n = 10) show that both of the heating and cooling traces display two phase transition peaks illustrating that the F−C15C10 melts from crystalline solid to liquid crystalline phase then to isotropic liquid. The POM image in Figure 11g displays the LC phase of schlieren texture recorded at 80 °C on the trace of cooling. However, as the temperature decreases, the spherulitic texture appears. Figure 11h is the POM image that shows the coexistence of schlieren and spherulitic textures at 78 °C. Several seconds later, the schlieren texture disappeared, and the thin film showed complete spherulitic texture which resembled

great property difference. The POM results show that F−C15 is not a LC molecule, while the others are all LCs. From the DSC curves in Figure 9a (the curves for n = 0), we see that F−C15 displays only one phase transition peak in both of its heating and cooling traces. As shown in Figure 10, during the process of cooling, we observed the phase transition from liquid state to crystalline state. Figure 10a−f shows the optical micrographs for F−C15 recorded at different time intervals, and they show the growth of spherulitic texture. For the other fluorenone derivatives which are substituted by two side chains, we observed their LC phases during both of the processes of heating and cooling. However, the DSC results show that only F−C15C10 display two obvious phase transition peaks, indicating that the melting point and clearing point for the other fluorenone derivatives are too close to each other that their peaks are crossed. By gradually decreasing the temperature, we recorded the LC phases of schlieren (Figure 11c) and I


Article

The Journal of Physical Chemistry C snakeskin (Figure S12). In the process of gradually cooling, crystalline phase was observed (Figure 11i) at the temperature of 61 °C. The POM images that display the rapid phase transition from schlieren LC texture to spherulitic LC texture, then to crystalline solid are shown in Figure S13. The appearance of two LC phases for F−C15C10 is a specific feature when compared with the other fluorenone derivatives. In general, these results proved that the chain-length effect is an efficient approach to induce structural diversity in crystal engineering, toward phase transition in both of 2D selfassembly and bulk state. We believe that our results are also strong evidence that the asymmetrically substituted fluorenone derivatives have the potential to be used as LCs, which is a topic attracting a lot of attention in the field of functional materials.44−48

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 020-22236708. *E-mail: [email protected]. ORCID

Yi Hu: 0000-0003-1073-7009 Xinrui Miao: 0000-0002-6727-7720 Wenli Deng: 0000-0001-7930-8742 Notes

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The authors declare no competing financial interest.

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CONCLUSIONS Using the method of fixing one chain and accurately modifying the length of the other chain by five carbon atoms, we explored the chain-length effect on the base of fluorenone derivatives of F−C5, F−C15C5, F−C15C10, F−C15C15, and F−C15C20. We successfully obtained diverse 2D assembled networks which are linear I (F−C5), wave-like and wave−linear (F−C15C5), chiral S-like (F−C15C10), trimer (F−C15C15), and linear II (F− C15C20) structures. These nanostructures were formed under the driving forces of dipole−dipole interaction, hydrogen bonds, and vdWs interactions. According to the packing modes of the fluorenone cores and the positions of H atoms which act as the C−H···OC hydrogen bonds donors, the hydrogen bonds in this work were classified into three types. The nanostructures for these fluorenone derivatives were associated with these types, which result in different binding energies and different strength of intermolecular C−H···OC hydrogen bonds. Alkyl chain length also have a strong impact on affecting the molecule−molecule and molecule−substrate vdWs interactions. Even though the side chains for F−C15 are not interdigitated, they spin to an angle with respect to the fluorenone cores, and the molecule−molecule distance was minimized. As a result, the underlying molecule−molecule vdWs interaction was strong enough to stabilize the monolayer. For the F−C15C5, F−C15C 10, F−C 15C15, and F−C15C20 molecules, as the alkyl chain length increased, the strength of molecule−molecule vdWs interactions were calculated to be accordingly increased. These results proved that accurate modification of the alkyl chains can be used as an efficient approach on regulating the intermolecular forces and inducing structural diversity. As a further step, the chain-length effect was studied from the viewpoint of crystal engineering in bulk phase. Using POM as the visualization tool, we found that F−C15 is not a LC molecule, while the F−C15C5, F−C15C10, F−C15C15, and F−C15C20 molecules display diverse LC phases. These are indications that the chain-length effect can also be used in controlling the crystalline properties during the thermal process. On the whole, this study provides an efficient strategy for fabricating diverse self-assembled polymorphs. We believe that the results will be of significance to understanding the alkyl chain length effects, and will also be a significant step toward molecular self-assembly in supramolecular chemistry.

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Details on synthesis, NMR characterization data, XRD results, STM experiments, surface coverage and binding energy calculations, models, and POM images (PDF)

ACKNOWLEDGMENTS This work is supported by the Natural Science Foundation of China (21573077, 21403072, and 51373055). South China University of Technology (SCUT) and Katholieke Universiteit Leuven (KU Leuven) are gratefully acknowledged. Y.H. acknowledges receipt of a scholarship from China Scholarship Council (CSC).

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