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A Tale of Alkyl Chains: Chain-Length Effect Directed Formation of Complex Self-Assembly Behaviors at Liquid/Solid Interface for Unsymmetrically Substituted Fluorenone Derivatives Yi Hu, Kai Miao, Xinrui Miao, and Wenli Deng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00231 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 19, 2018

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The Journal of Physical Chemistry

A Tale of Alkyl Chains: Chain-Length Effect Directed Formation of Complex Self-Assembly Behaviors at Liquid/Solid Interface for Unsymmetrically Substituted Fluorenone Derivatives Yi Hu, Kai Miao, Xinrui Miao and Wenli Deng* College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China Tel: +86 20-22236708 E-mail: [email protected]

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Structural diversity induced by chain-length effect in the field of two-dimensional selfassembly has gained immense attention due to its potential application in nanoscience and crystal engineering. Via modifying the two side chains in a certain molecule by one carbon atom and gradually increasing the alkyl chain length, seven fluorenone derivatives (F−CnCn+1, n = 11−17) were synthesized. At the 1-octanoic acid/graphite interface, diverse nanostructures of Hexamer-I, Tetramer, Dimer, Alternate-I, Hexamer-II and Alternate-II were recorded. The arrangement for the two side chains which differ from each other only by one carbon atom was discussed from the viewpoint of thermodynamics and kinetics. Alkyl chain in the same length were speculated to show selective identification during the self-assembly process, which was favored in consideration of dense packing and maximizing the molecular interplay. Three forces of dipole−dipole, hydrogen bonding and van der Waals interactions cooperatively or competitively exert their roles on stabilizing the assembled monolayers. For the purpose of further understanding the self-assembly mechanisms, we performed force field calculations, which revealed that the strength of hydrogen bonds was related to the arrangement of the fluorenone units while the van der Waals interaction showed a close relationship with the alkyl chain length. This work displays an efficient method on fabricating complex self-assembly networks, and we believe that it will promote the study of chain-length effect in supramolecular chemistry and interfacial science.


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1. INTRODUCTION Two-dimensional (2D) molecular self-assembly has be considered as a significant approach for constructing ordered nanoscale structures and molecule-based devices.1-4 In the past decades, investigation of the complex self-assemblies for organic molecules at the liquid/solid or air/solid interface has attracted a lot of attention in the field of supramolecular chemistry, physical chemistry, crystal engineering, interfacial and material science.2,5-9 Therefore, fabrication, induction and regulation of diverse nanostructures on surface are still great challenges which are worthy of more scientific devotions. π-Conjugated molecules can self-assemble into monolayers on surface via physisorption, with their largest face flatly lying on the substrate, which is facilitated for monitoring the intermolecular bonding types.10,11 As has been previously reported, hydrogen bonds, van der Waals (vdWs) interaction, dipolar interaction and metal coordination are the common driving forces during the self-assembly process.12-18 The assembled structures are sensitive to the change of experimental conditions, thus the solvent, concentration, temperature, substrate and voltage are the factors that exert important roles on determining the self-assembly configurations.19-25 Moreover, except for these factors, chain-length effect also seems to be an efficient method on inducing structural diversity, owing to the possibility of regulating the molecule–molecule, molecule–solvent and molecule–substrate vdWs interactions.1,26-28 Chain-length effect is traditionally investigated through gradually prolonging the alkyl chains

by

one

carbon

atom.

For

example,

Tobe

et

al.

synthesized

hexadehydrotribenzo[12]annulene (DBA) derivatives which were substituted by different alkyl chains.1,2,29,30 They found that based on the same building blacks, Kagomé, honeycomb, linear and host-guest networks formed. The changeable supramolecular structures depending on the alkyl chain length were explained by Miyake and co-workers, from the balance between enthalpy and entropy terms.31 They characterized the self-assembly behaviors of N,N-bis(n3 ACS Paragon Plus Environment


alkyl)naphthalenediimides derivatives, with the alkyl chain length span from C3 to C18. As a consequence, they recorded structural transition from lamellar to honeycomb, then to lamellar again. However, other new methods which can more accurately changing the alkyl chain length seem to be valuable and need to be developed. Fluorenone derivatives substituted by a single or two same side chains have been studied in our group using scanning tunneling microscopy (STM) as the visualization tool. We found that the assembled structures are closely dependent on the alkyl chains.22,32-37 Herein, we use a series of fluorenone derivatives (F–CnCn+1, n = 11−17) that are substituted by two alkyl chains, as shown in Scheme 1. Different from the previous reports, the two side chains are modified Scheme 1. Chemical structures for the F–CnCn+1 derivatives and the solvent of 1-octanoic acid.

by a small difference of one carbon atom. Moreover, the chains are gradually increased for the purpose of exploring chain-length effect. To our knowledge, this method for accurate modification on the side chains has never been reported in the field of 2D self-assembly. In this present work, at the 1-octanoic acid/highly oriented pyrolytic graphite (HOPG) interface, under the similar concentration, we recorded diverse networks. F–C12C13 self-assembled into chiral Hexamer-I, Tetramer, and achiral Dimer structures. For F–C13C14 and F–C14C15, they were observed to pack into Alternate-I and Hexamer-II patterns. As the side chain increased to n = 15–17, we observed another nanostructure of Alternate-II. During the self-assembly process, the alkyl chains in the same length were speculated to be packed together, which was favored in consideration of increasing the molecule–molecule vdWs interaction. Since the electron 4 ACS Paragon Plus Environment

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affinity of the carbonyl group in the fluorenone core, the fluorenone derivatives show strong polarity.27,35,37 Therefore, in the self-assembled adlayers, molecules tend to arrange in an antiparallel way to offset the whole dipole moments. Hydrogen bonding interaction between the adjacent fluorenone units is another force which contributes to stabilizing the assembled structures. In order to reveal the self-assembly mechanism, we performed force filed calculation for the binding energies of hydrogen bonds and vdWs interaction. On account of the energy-related property, we also conducted differential scanning calorimetry (DSC) performance to further explore the chain-length effect from the viewpoint of thermal analysis. In general, this work presents a systematic study on chain-length effect, and we believe that it will have implication in inducing, investigating, and realizing the diverse 2D self-assembly in supramolecular chemistry. 2. EXPERIMENTAL SECTION The F–CnCn+1 derivatives were synthesized by separately connecting the alkyl chains to the fluorenone cores, using a method as we have previously reported.34,38 The solvent was purchased from Tokyo Chemical Industry and used without purification. All of the solutions in this work are in the similar concentration of 1.3−1.7 × 10−4 M. The STM experiments were performed in ambient conditions (temperature: 18−23 °C, humidity: 45−50%), on the HOPG (quality ZYB, Bruker, USA) surface, using an Agilent system. The tips were mechanically cut from Pt/Ir (80%/20%) wire. The STM images were recorded under constant current mode and shown without further processing. Materials Studio 7.0 was used to construct the molecular models and the force field calculation. The DSC experiments were conducted with a scan rate of 10 °C min−1 for heating and cooling traces (NETZSCHDSC 200F3). 3. RESULTS



Ordered self-assembled structures for F–CnCn+1 (n = 12–17) spontaneously formed as a drop of solution was applied onto the HOPG surface. However, for F–C11C12, no assemblies were observed under the same experimental condition, even though we tried different samples. This is attributed to the length of the alkyl chains. For molecules with shorter side chains, it is a common phenomenon that molecules cannot self-assemble on the substrate via physisorption because of the limited molecule–substrate interaction.27,38 3.1 Chiral and achiral self-assemblies for F–C12C13. The self-assembled monolayer for F–C12C13 at the 1-octanoic acid/HOPG interface contains three kinds of domains, as indicated from No. 1 to 3 in the large-scale STM image in Figure 1 and S1. The fluorenone cores in the STM image appear as bright dots, while the alkyl chains are recorded as dark lines, owing to their different tunneling efficiency.39-41 According to the number of molecules in the basic aggregations, these three networks were named as HexamerI, Tetramer and Dimer structures.

Figure 1. Chemical structure for F–C12C13 and its large-scale STM image at the 1-octanoic acid/HOPG interface. No. 1 to 3 represent the Hexamer-I, Tetramer and Dimer structures, respectively. Scanning parameters: It = 200 pA, Vbias = −220 mV. The Hexamer-I configuration is chiral, as shown in Figure 2a, from which we can clearly see two domains constructed by hexamers in reverse chirality. Figure 2b and 2c are the high6 ACS Paragon Plus Environment

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resolution STM images. The six molecules in the hexamers are arranged into normal S-like or anti-S-like shapes, as indicated by the red and green bold lines in Figure 2d. If we divide the molecules in the hexamer into two trimers, we found that they are packed in an antiparallel mode. Moreover, in the trimers, molecules are closely arranged, directing to the same orientation. This kind of arrangement is favored when the factors of hydrogen bonds and

Figure 2. (a) Large-scale STM image of the Hexamer-I structure for F–C12C13. (b and c) Highresolution STM images for the left-handed and right-handed Hexamer-I structures. The threefold symmetry axis for the HOPG lattice are marked by the black arrows. The blue and pink rectangles represent the interdigitated and non-interdigitated alkyl chains, respectively. The yellow circles represent the uncovered areas between the adjacent alkyl chains. (d) Magnified images and structural models for the S-like and anti-S-like hexamers. The red and green bold lines are used to show the reverse chirality. The C−H···O═C hydrogen bonds are illustrated by black dotted lines. Dipolar pairs in which the dipole moments can be minimized to zero are indicated by arrows in the same color. (e and f) Proposed structural models for the left-handed and right-handed Hexamer-I configurations. Scanning parameters: It = 200 pA, Vbias = −300 mV.



dipole–dipole interaction are taken into account. The O atom from the carbonyl group usually functions as the hydrogen bond acceptor, with the H atoms from the π-conjugated cores as the donor.22,33,42 The C−H···O═C hydrogen bonds formed between the adjacent fluorenone units are indicated by black dotted lines (Figure 2d). The molecules from the adjacent trimers are doubly packed into pairs, as indicated by arrows in the same color in Figure 2d. As a result of this antiparallel arrangement, the total dipole moments of the adlayer are offset within the hexamers. The HOPG lattice can be obtained by changing the bias to −10 mV,43 as depicted by a series of black arrows in the bottom-left of the high-resolution STM images. The alkyl chains extend along the symmetry axis of the HOPG surface, which is able to maximize the molecule−substrate interaction to stabilize the network.28,44-46 Alkyl chains from the adjacent hexamers are partly interdigitated, and partly not, as respectively indicated by the blue and pink rectangles in Figure 2b and 2c. Between the non-interdigitated alkyl chains, the HOPG surface is empty, which appear as a sequence of black areas, as marked by the yellow circles. On account of maximumly increasing the molecule−molecule vdWs interaction, the long chains (–C13H27) were speculated to be interdigitated, instead of the short chains (–C12H25). Another detail observed from the high-resolution STM images is the co-adsorbed solvent molecules. Among the neighboring hexamers, there are some fuzzy areas which seem to be filled by short lines, as indicated by green arrows in Figure 2b and 2c. According to their shape and length, and the principle of densely packing32,47, they are certain to be the 1-octanoic acid molecules. The structural models for the left-handed and right-handed Hexamer-I structures are proposed in Figure 2e and 2f. For ease of distinguishing, the long and short alkyl chains are in orange and blue, and the solvent molecules are in green. Two unit cells are overlaid in the highresolution STM images, with the parameters of a = 4.5 ± 0.1 nm, b = 4.1 ± 0.1 nm, α = 71 ± 1°


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for Figure 2b; a = 4.5 ± 0.1 nm, b = 4.2 ± 0.1 nm, α = 72 ± 2° for Figure 2c. The unit cell contains six molecules, and the calculated area density is 2.9 nm2 per molecule.

Figure 3. (a) Large-scale STM image of the Tetramer structure for F–C12C13. (b and c) Highresolution STM images for the chiral Tetramer structures. The green and pink crescent shapes are used to illustrate the CCW and CW tetramers. (d) Magnified images of the chiral fluorenone units and their corresponded models. The arrows in the same color represent the dipolar pairs which can offset their dipole moments. (e and f) Proposed structural models for the chiral Tetramer networks. Scanning parameters: It = 220 pA, Vbias = −250 mV. Figure 3a displays the large-scale STM image of the Tetramer structure for F–C12C13. According to the direction of the alkyl chains respected to the central line of the tetramer, the tetramers are classified into clockwise (CW) and counter clockwise (CCW), as described in Figure S2. Figure 3b and 3c are the high-resolution STM images. In the tetramers, every two molecules are packed in an antiparallel fashion, which is favored for dipole–dipole interaction, as indicated by the color arrows in Figure 3d. From the STM images we see that all of the alkyl chains from neighboring tetramers are interdigitated. During the self-assembly process, the



alkyl chains showed selective identification, which means that the side chains in the same length tend to be interdigitated. This is favored on account of densely packing and increasing the molecule–molecule vdWs interactions (to be discussed below). In this Tetramer structure, the distance between the neighboring fluorenone cores are too far away, thus hydrogen bonds are impossible. This is an indication that the weak forces of vdWs, dipolar and hydrogen bonding interactions exert their functions through cooperation or competition. However, the stable self-assembled structures are the results of balance among these forces. Based on these analysis and the STM images, the structural models for the CCW and CW Tetramer networks are proposed in Figure 3e and 3f. The long (–C13H27) and short (–C12H25) alkyl chains are in orange and blue, respectively. Two unit cells are overlaid on the highresolution STM images, with the parameters of a = 4.3 ± 0.2 nm, b = 3.2 ± 0.2 nm, α = 61 ± 1° for Figure 3b; a = 4.2 ± 0.1 nm, b = 3.2 ± 0.1 nm, α = 62 ± 2° for Figure 3c. The unit cell contains four molecules, and the area density is calculated to be 3.0 nm2 per molecule.

Figure 4. (a) High-resolution STM image and (b) proposed structural model for the Dimer structure for F–C12C13. Scanning parameters: It = 200 pA, Vbias = −200 mV. The Dimer structure observed in the monolayer for F–C12C13 is achiral. Figure 4a is the high-resolution STM image which displays more structural details. The two molecules in the dimer are also packed in an antiparallel way, with their fluorenone cores adjacent with each other, in a shoulder-by-shoulder mode. As a result, between the fluorenone cores, hydrogen


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bonds are impossible but the dipole–dipole interaction is favored (Figure S3a and S3b). The two alkyl chains stretch along the direction which is perpendicular to the axis of the fluorenone core (Figure S3c). This kind of extension makes it easy for molecules to form dense packing by lamellar arrangement. Based on this ordered linear packing, we judge that vdWs interaction plays the most important role on stabilizing the self-assembled configurations. Figure 4b shows the proposed structural model for this Dimer structure. A unit cell constructed by two molecules is overlaid in Figure 4a, with the parameters of a = 2.1 ± 0.1 nm, b = 3.0 ± 0.1 nm, α = 82 ± 1°, and the calculated area density is 3.1 nm2 per molecule. 3.2 Self-assemblies for F–C13C14. Under the similar concentration, F–C13C14 self-assemble into a configuration which is constructed by alternate molecular rows, as shown in the large-scale STM image in Figure 5a. So this kind of network is named as Alternate-I. Careful observation of the high-resolution STM image in Figure 5b reveals that the Alternate-I structure is built from ordered tetramer and trimer rows.

Figure 5. (a) Large-scale and (b) high-resolution STM images for F–C13C14 at the 1-octanoic acid/HOPG interface. The green arrows indicate the co-adsorbed solvent molecules among the alkyl chains. The red and blue arrows are used to indicate the directions of alkyl chains in the molecular rows constructed by tetramers. (c) Structural model for the Alternate-I structure. Scanning parameters: It = 200 pA, Vbias = −300 mV.



In the tetramer, the molecules are doubly gathered, and the two dimers are arranged in an antiparallel fashion. Moreover, four alkyl chains stretch outside the tetramer and interdigitate with the alkyl chains from the neighboring chains, which is favored for molecular vdWs interaction. However, the other four chains within the tetramer row are not interdigitated. In consideration of maximizing the chain–chain vdWs interaction, the chains outside the tetramers are speculated to be the long ones. Among the tetramers, we see a series of short lines between the short alkyl chains of F–C13C14, as indicated by green arrows. Hence, there is no doubt that they are the 1-octanoic acid molecules. An interesting phenomenon we observed during the scanning process is that the adjacent tetramer rows are alternatively pointing to different directions, as shown by the alternate red and blue arrows in Figure 5b and Figure S4. In the trimer rows, molecules are packed into lines which are not completely straight in long range. Since the alkyl chains beside the trimer rows are all the long ones, it is inevitable that the interdigitation between the short and long chains appears among the tetramer and trimer rows. In this Alternate-I structure, no hydrogen bonds form between the fluorenone cores. Besides, the dipole moments within the tetramers can be offset while no dipole pairs form in the trimer rows. Therefore, the molecular vdWs interaction is the main force on stabilizing the selfassembly monolayer. Figure 5c shows the proposed structural model. A unit cell is overlaid in Figure 5b, with the parameters of a = 2.6 ± 0.2 nm, b = 16.7 ± 0.2 nm, α = 90 ± 1°. The unit cell contains fourteen molecules, and the area density was calculated to be 3.1 nm2 per molecule. 3.3 Self-assemblies for F–C14C15. Large-scale STM images for F–C14C15 reveal that domains in reverse handedness constructed the whole self-assembly adlayer, as shown in Figure 6a and Figure S5. The F–C14C15 molecules gather into continuous S-like and anti-S-like hexamers (Figure 6b and 6c) which are similar


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with the assembled structures for F–C12C13. Then this kind of configuration is named as Hexamer-II.

Figure 6. (a) Large-scale STM image for F–C14C15, showing the co-existent left-handed and right-handed domains. (b and c) High-resolution STM images for the chiral Hexamer-II structures. The co-adsorbed 1-octanoic acid molecules are marked by green arrows. (d) Illustration of the hydrogen bonds and offset of the dipole moments. (e and f) Proposed structure models for the left-handed and right-handed Hexamer-II patterns. Scanning parameters: It = 220 pA, Vbias = −400 mV. In the hexamers, molecules can be divided into two trimers which are antiparallel to each other. This is favored for dipole−dipole interaction. Moreover, in the trimers, C−H···O═C hydrogen bonds form between the neighboring fluorenone units. The dipole offset and hydrogen bonds are respectively indicated by arrows and black dotted lines in the enlarged model in Figure 6d. Among the hexamers, there are also some short lines, which correspond to the co-adsorbed 1-octanoic acid molecules. However, different from that in the Hexamer-I structures, between two hexamers, there are two solvent molecules which are packed into a



head-to-head fashion, as indicated by the green arrows. As a result, −COOH···COOH− hydrogen bonds formed, as shown in the pink circles in Figure 6d. The two alkyl chains beside the 1-octanoic acid molecules are a bit bent, as shown in the enlarged images in Figure 6b and 6c. In most cases, the alkyl chains are packed in straight lines and stretch along the symmetry axis of the graphite lattice.48-51 However, sometimes the alkyl chains may be bent in order to fill the limited space.52 In this Hexamer-II structure, alkyl chains are bent to give more space to accommodate two 1-octanoic acid molecules. In the Hexamer-I structure (Figure 2) for F– C12C13, between the adjacent hexamers, the sole 1-octanoic acid molecule is located between the fluorenone cores, thus there is no need for the alkyl chains to bend themselves. Six long alkyl chains (–C15H31) in every hexamer are interdigitated with the chains from the neighboring hexamers, while the other six short chains are not. According to the high-resolution STM images, the structural models for the left-handed and right-handed Hexamer-II structures are proposed in Figure 6e and 6f. Two unit cells are overlaid in Figure 6b and 6c, with the parameters of a = 4.6 ± 0.1 nm, b = 4.3 ± 0.1 nm, α = 64 ± 2° for Figure 6b; a = 4.6 ± 0.1 nm, b = 4.2 ± 0.1 nm, α = 65 ± 1° for Figure 6c. The unit cell contains six molecules, and the calculated area density is 3.0 nm2 per molecule. 3.4 Self-assemblies for F–C15C16. The self-assembled monolayer for F–C15C16 at the 1-octanoic acid/HOPG interface is constituted from ordered alternative molecular rows, as shown in Figure 7a. Therefore, this network is named as Alternate-II. It is clear to see from the high-resolution STM image in Figure 7b that the F–C15C16 molecules are arranged into loose and dense rows, whose basic units are dimer and trimer, as illustrated by the pink and green crescent shapes. In the dimer row, every two molecules are packed with their fluorenone cores pointing outside the dimer. In the trimer row, the three fluorenone cores direct to different orientations. As a result, dipole offset and hydrogen bonds (Figure S6) are not favored in this Alternate-II


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structure. However, the alkyl chains are interdigitated and stretch along the symmetry axis of the substrate lattice. In consideration of fully interdigitation, alkyl chains in the same length are judged to interdigitate with each other. The structural model for this Alternate-II structure is proposed in Figure 7c. The unit cell that consists five molecules is overlaid in Figure 7b, with the parameters of a = 2.3 ± 0.1 nm, b = 6.0 ± 0.1 nm, α = 86 ± 1°. The area density for this structure is calculated to be 2.8 nm2 per molecule.

Figure 7. (a) Large-scale and (b) high-resolution STM images for F–C15C16. The pink and green crescent shapes are used to indicate the dimer and trimer. (c) Proposed structural model for the Alternate-II structure. Scanning parameters: It = 200 pA, Vbias = −350 mV. 3.5 Self-assemblies for F–C16C17 and F–C17C18. As the side chains were further prolonged, we obtained another two molecules of F–C16C17 and F–C17C18. The self-assembled structures for them are almost the same with the Alternate-II structure for F–C15C16. The alternate dense and loose rows construct the main assembled monolayers, and the basic units are dimers and trimers (Figure S7). However, what is different from F–C15C16 is that in the Alternate-II structures for F–C16C17 and F–C17C18, there are some molecules which are inserted randomly among the dimers and trimers. Furthermore, the dimer and trimer rows are always mixed, which means that in the same linear row, molecules can gather into both of dimers and trimers. In Figure 8, the molecules in the dimers and trimers are indicated by pink and green dots, respectively. Therefore, for F–C15C16, the pink and green dots 15 ACS Paragon Plus Environment


appear regularly (Figure 8a). For F–C16C17 and F–C17C18, both of pink and green dots appear (Figure 8b and 8c) in a molecular row. The other molecules which don’t belong to the dimer or trimer aggregations are indicated by blue dots.

Figure 8. STM images for (a) F–C15C16, (b) F–C16C17 and (c) F–C17C18. The molecules in the dimers and trimers are marked by pink and green dots, respectively. The blue dots represent the molecules which appear randomly among the dimers and trimers. Since the main basic unit for the Alternate-II structures for F–C16C17 and F–C17C18 are dimer and trimer, we measured their unit cells (Figure S7c and S7f). The parameters are a = 2.4 ± 0.2 nm, b = 6.3 ± 0.2 nm, α = 86 ± 1° for F–C16C17; a = 2.4 ± 0.1 nm, b = 6.7 ± 0.1 nm, α = 86 ± 1° for F–C17C18. From these results we see that the distances between the nearer fluorenone cores (value for a) are similar, while the distances between the farther fluorenone cores (value of b) are different, which is attributed to the increased alkyl chains. The area densities for F–C16C17 and F–C17C18 are calculated to be 3.0 and 3.2 nm2 per molecule, respectively. Then a question arises as why the appearance of the dimers and trimers in the AlternateII structures for F–C16C17 and F–C17C18 is not regular. This is definitely caused by the length of the alkyl chains. As we have analyzed, the vdWs interactions between the molecule– molecule and molecule–substrate are the driven forces during the self-assembly process. With the length of side chains increases, the molecule–substrate vdWs interactions becomes stronger, hence can restrict the desorption–adsorption behavior and the in-plane mobility of the 16 ACS Paragon Plus Environment

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molecules.1,27,28 Therefore, the irregularly-appeared dimers and trimers for F–C16C17 and F– C17C18 are probably arise from their limited mobility, which means that the self-assembly process stops before they reach a more ordered state. 4. DISCUSSIONS For ease of comparison, the unit cell parameters of the self-assembled structures for F–CnCn+1 are summarized in Table 1. Table 1 Schematic unit cell parameters and the binding energies of hydrogen bonds and vdWs interaction for different nanostructures. molecules

structures

models

a (nm)

b (nm)

α (°)

N

SN

∆EH

∆EV

(nm2)

(kJ mol−1)

(kJ mol−1)

Hexamer-I *

4.5 ± 0.1

4.1 ± 0.1

71 ± 1

6

2.9

–20.2

–48.4

Tetramer *

4.3 ± 0.2

3.2 ± 0.2

61 ± 1

4

3.0

-

–62.5

Dimer

2.1 ± 0.1

3.0 ± 0.1

82 ± 1

2

3.1

-

–30.9

F–C13C14

Alternate-I

2.6 ± 0.2

16.7 ± 0.2

90 ± 1

14

3.1

-

–58.0

F–C14C15

Hexamer-II

4.6 ± 0.1

4.3 ± 0.1

64 ± 2

6

3.0

–21.9

–61.3

F–C12C13

* F–C15C16

Alternate-II

2.3 ± 0.1

6.0 ± 0.1

86 ± 1

5

2.8

–33.1

–49.9

F–C16C17

Alternate-II

2.4 ± 0.2

6.3 ± 0.2

86 ± 1

5

3.0

–34.1

–59.1

F–C17C18

Alternate-II

2.4 ± 0.1

6.7 ± 0.1

87 ± 1

5

3.2

–35.7

–63.6

* The Hexamer-I, Tetramer and Hexamer-II structures are chiral, so the parameters used in this table are for the left-handed and CCW patterns. 4.1 Selective identification for the alkyl chains. During the self-assembly process for mixtures, molecules with the same alkyl chain can identify each other and gather together, forming separated domains.53 For the fluorenone derivatives substituted by different chains, we previously observed that alkyl chains in the same



length are always interdigitated.34 However, in this present work, the two alkyl chains only display a difference of one carbon atom (–CH2–). Due to this tiny difference, it is impossible or super hard to identify the short and long chains in ambient condition. Therefore, the proposed arrangement for these two different alkyl chains is based on analysis of thermodynamics and kinetics. We take the Hexamer-I structure for F–C12C13 for example. According to the packing style of the alkyl chains, there are four types of arrangements, as shown in Figure 9. In this work, we speculate that i) the alkyl chains in the same length are interdigitated; ii) if there are some alkyl chains which are not interdigitated, they should be the short ones. This speculation corresponds with the structure in Figure 9a.

Figure 9. Four types of arrangements for the Hexamer-I structure (F–C12C13). For ease of distinguishing, the fluorenone unit, –C12H25 and –C13H27 chains are in red, blue and orange. The orange, blue and green rectangles are used to indicate different kind of interdigitation between the short and long chains. The spaces beside the –C12H25 chains are marked by red dots and arrows. Firstly, from the viewpoint of thermodynamics, interdigitation between the same alkyl chains is favorable for dense-packing. In Figure 9a and 9b, the area constructed by the –C12H25 and –C13H27 chains are indicated by the blue and orange rectangles. If the long and short chains are interdigitated, as shown in Figure 9c and 9d in the green rectangles, dense-packing is not satisfied, owing to the presence of open spaces beside the short chains (as indicated by red 18 ACS Paragon Plus Environment

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dots). Therefore, the arrangements in Figure 9a and 9b are thermodynamically favored than that in Figure 9c and 9d. Secondly, from the viewpoint of kinetics, the molecular vdWs and dipolar interactions are taken into account. Within the hexamer, molecules in Figure 9a and 9b are packed into antiparallel pairs, as marked by the blue, green and purple arrows. This is favorable to offset the dipole moment of the whole self-assembled structure. However, in Figure 9c and 9d, molecules within the hexamers are not arranged into pairs. Hence, in consideration of dipole– dipole interaction, the arrangements in Figure 9a and 9b also show priority than that in Figure 9c and 9d. The difference between the structures in Figure 9a and 9b is the interdigitation between the alkyl chains. In the Hexamer-I structure, on the same side of the hexamer, we observed from the STM image that only three chains participate in the interdigitation with their neighboring chains. We confirm that the long chains are interdigitated (Figure 9a), instead of the short chains (Figure 9b). The vdWs interaction is directly relevant with the length of the alkyl chains, thus interdigitation between the long chains can cause stronger chain–chain vdWs interaction. So the arrangement in Figure 9a is more favorable than that in Figure 9b. For the strength of vdWs interactions within these four arrangements, we performed forcefield calculation. The binding energies were calculated to be –48.4, –44.4, –41.7 and –42.0 kJ mol– 1

(Figure 9a–9d), from which we see that the structure in Figure 9a is the strongest. Therefore, the structure in Figure 9a is the most preferred one in both of thermodynamics

and kinetics. Based on similar speculations, we proposed a series of structural models, as have described above. 4.2 Self-assembly mechanisms: cooperation and competition between the weak forces. The stable self-assembled network on substrate is the result of balance between cooperation and competition among all kinds of weak forces. For the fluorenone system, as we have previously reported that dipole–dipole interaction is in most cases favored27,32,35,37,47, owing to



the electron affinity of the carbonyl group in the fluorenone cores.27,35 Moreover, weak hydrogen bonds between the neighboring fluorenone cores usually form, which contribute to the stabilization of the whole system.22,33,38 However, in this work, dipole–dipole and hydrogen bonding interactions are not always favored. When competition happen among them and vdWs interaction, the latter one is the most preferred.

Figure 10. (a) Densely-packed model and (b) dipolar interaction favored models for the Alternate-II structure for F–C15C16. 4.2.1 Dipole–dipole interaction Molecule pairs which can offset the whole dipole moments around the self-assembled structures were formed only in networks for F–C12C13 and F–C14C15, as shown in Figure 2d, 3d, 6d and S3b. In the other networks, dipolar interaction is not favored. This is attributed to the tiny modification (one-atom-difference) between the two alkyl chains. The alkyl chains in the same length tend to gather together for the sake of forming dense packing, which will sometimes break the possibility of forming dipole pairs. For example, we speculated that in the Alternate-II structure, the alkyl chains in the same length are interdigitated, as shown in Figure 10a. If the dipole–dipole interaction is considered, we proposed another structure, as shown in


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Figure 10b. The dipole pairs are indicated by arrows in the same color. In the trimer row, the odd number of molecules in the aggregation can cause a solo molecule without a partner to offset the dipole moment. As a result of the interdigitation between the short and long chains, there will be an one-atom-space beside the end of the short chains, as indicated by the pink dots. For dense packing, this is not favored. Moreover, the distance between molecular rows in Figure 10b is always determined by the long chain. The value of d2 is larger than d1. So it is clear that the structure in Figure 10a is denser than that in Figure 10b in their area densities, thus it is reasonable that the former one is favored than the latter one.

Figure 11. Illustrations of the two types of hydrogen bonds. 4.2.2 Hydrogen bonds Hydrogen bonding interaction is formed between the neighboring fluorenone cores. The O atom from the carbonyl group acts as the acceptor. According to the position of the H atom which acts as the donor, the hydrogen bonds in this work are classified into two types, as shown in Figure 11. Type I was found in the Hexamer-I and Alternate-II structures; Type II was found in Hexamer-I and Hexamer-II structures. For the other networks, no hydrogen bonds formed. In order to further reveal the hydrogen bonds behind these self-assembled structures, we performed force field calculation. The binding energies were calculated to be –20.2 and –21.9 kJ mol–1 for the Hexamer-I and Hexamer-II patterns. For the Alternate-II pattern, there is only a tiny difference among F–C15C16, F–C16C17 and F–C17C18, which were calculated to be –33.1, –34.1 and –35.7 kJ mol–1. This is because of their same self-assembly structures and same types for the hydrogen bonds. These calculated results are summarized in Table 1. 21 ACS Paragon Plus Environment


4.2.3 VdWs interaction The alkyl chains extend along the symmetry axis of the HOPG surface, which is favored for maximizing the molecule–substrate vdWs interaction. As we have analyzed that the side chains in the same length tend to interdigitate with each other for sake of dense packing. As a result, the molecular distance is minimized, and molecule–molecule vdWs interaction is enhanced. In Figure 9, the identification between the alkyl chains is discussed from the viewpoint of increasing the molecular vdWs interaction. We also performed force field calculation for the vdWs interactions within the self-assembled structures, as summarized in Table 1. For F– C12C13, we observed three networks of Hexamer-I, Tetramer and Dimer, and the binding energies were calculated to be –48.4, –62.5 and –30.9 kJ mol–1. The huge difference among them is caused from their different arrangement types. For F–C13C14 and F–C14C15, the binding energies for vdWs interaction were calculated to be –58.0 and –61.3 kJ mol–1. F–C15C16, F– C16C17 and F–C17C18 self-assembled into the same nanostructure of Alternate-II. Since the vdWs interaction is related to the molecules, we judge that as the alkyl chain length increases, the molecule–molecule vdWs interaction will increase. Corresponding with our judgement, the strength for their vdWs interactions were calculated to be gradually enhanced, which were – 49.9, –59.1 and –63.6 kJ mol–1. 4.3 Chain-length effect Chain-length effect is a factor which received intense attention on inducing complex selfassembly structures.31,54 Herein, we put forward a new method for tiny modification on the alkyl chains. As a consequence of the different length in the alkyl chains for these seven molecules and the one-atom-difference between the two chains in a certain molecule, we successfully fabricated six nanostructures. Compared with the fluorenone derivatives substituted by two same side chains27,35, the most valuable aspect in this work is that we


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obtained chiral domains. So this is a proof that modification on molecule by one-atomdifference is an efficient strategy on inducing structural diversity.

Figure 12. (a) DSC thermograms for compounds of F–CnCn+1 (n = 11–17) recorded at a scanning rate of 10 °C min–1. The upper and lower lines are for the heating and cooling traces, respectively. (b) Dependence of phase transition temperature on the length of the side chains. The self-assembly is a process for molecules to get ordered from a chaotic phase. As this phase transition in nanoscale is related to energy, we were motivated to explore their phase transition in bulk states. We conducted a series of DSC experiments for F–CnCn+1 on the traces of heating and cooling, as shown in Figure 12a. The peak in the curves are related to the phase transition temperature. During the trace of heating, we can see that sometimes, there are two peaks, which correspond to the clearing and melting points. The dependence of melting points on the length of alkyl chains are shown in Figure 12b. As we have calculated and shown in Table 1, as the alkyl chain length increases, the binding energy for vdWs interaction doesn’t display a single change. In Figure 12b, melting points for F–CnCn+1 also displayed a non-single 23 ACS Paragon Plus Environment


change. F–C15C16, F–C16C17 and F–C17C18 self-assembled into the same Alternate-II structure, and as their alkyl chain length increases, their binding energies for vdWs interaction increase. This phenomenon corresponds well with their melting points, which display a gradual increase. Since the difference in melting point originates from different energy required to form a uniform phase,55 we believe that these DSC results must have relationship with the binding energies and are helpful for further understanding the chain-length effect. 5. CONCLUSIONS In conclusion, we have systematically explored chain-length effect by i) accurately modifying the two alkyl chains in a certain molecule by one-atom-difference; and ii) gradually increasing the length of alkyl chains for a series of fluorenone derivatives. We successfully recorded complex self-assembled structures for F–CnCn+1 (n = 12–17) at the liquid/HOPG interface. F– C12C13 self-assembled into Hexamer-I, Tetramer and Dimer structures. As the alkyl chain length increases, we observed Alternate-I and Hexamer-II patterns for F–C13C14 and F–C14C15. F–C15C16, F–C16C17 and F–C17C18 molecules were packed into the same Alternate-II networks. In these assembled structures, the alkyl chains in the same length showed selective identification and tended to interdigitate with each other, facilitating dense packing which is favored in thermodynamics. On account of this kind of interdigitation, molecule–molecule vdWs interaction is the driven force on stabilizing the self-assembled monolayers. Moreover, dipolar interaction and hydrogen bonds also exert their roles during the formation of stable self-assembled structures. According to the position of H atoms in the fluorenone unit, the hydrogen bonds were classified into two type, which determined the strength of the hydrogen bonds. As the length of the alkyl chain increased, the binding energy for the vdWs interaction didn’t display single change, which was caused by the different kind of networks. However, for the molecules which self-assembled into the same configuration, the vdWs interaction was


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calculated to be gradually stronger as the side chains were prolonged. This work emphasizes the chain-length effect and its utility on fabricating diverse nanostructures, and we believe that it will promote the study of 2D self-assembly in the field of supramolecular chemistry and interfacial science. ASSOCIATED CONTENT Supplementary Information This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work is supported by the Natural Science Foundation of China (21573077, 51373055). Y. H. acknowledges receipt of a scholarship from China Scholarship Council (CSC). South China University of Technology (SCUT) and Katholieke Universiteit Leuven (KU Leuven) are gratefully acknowledged. A portion of experiments were carried out in Professor Steven De Feyter’s group in KU Leuven, thus we sincerely thank Prof. De Feyter. REFERENCES (1) Tahara, K.; Furukawa, S.; Uji-i, H.; Uchino, T.; Ichikawa, T.; Zhang, J.; Mamdouh, W.; Sonoda, M.; De Schryver, F. C.; De Feyter, S.; Tobe, Y. Two-Dimensional Porous



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Tale of Alkyl Chains: Chain-Length Effect-Directed ... - ACS Publications

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