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Self-Assembly Polymorphism of Regioisomeric Diketopyrrolopyrrole Based #-Conjugated Organic Semiconductors Xinrui Miao, Jinxing Li, Lei Ying, Fabien Silly, Chun-shan Che, Gang Kong, and Wenli Deng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08701 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 2, 2019

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The Journal of Physical Chemistry

Self-Assembly Polymorphism of Regioisomeric Diketopyrrolopyrrole Based π-Conjugated Organic Semiconductors Xinrui Miao,*,†,§ Jinxing Li,† Lei Ying,‡ Fabien Silly,*,§ Chunshan Che,† Gang Kong,† Wenli Deng† College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640,

†

People’s Republic of China. Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials

‡

and Devices, South China University of Technology, Guangzhou 510640, People’s Republic of China. §TITANS,

SPEC, CEA, CNRS, Université Paris-Saclay, CEA Saclay, Gif sur Yvette, F-91191, France

Corresponding authors：Xinrui Miao ([email protected]) Fabien Silly ([email protected])

Abstract Two-dimensional molecular assemblies of two structural isomers p-DBPy and d-DBPy containing a diketopyrrolopyrrole (DPP) unit relevant to the field of organic electronics are investigated on graphite surface in different solvents (phenyloctane and octanoic acid) using scanning tunneling microscopy (STM). These two molecules only differ in the position of pyridyl N-atoms. The solution concentration and solvent effects on the morphology of adlayer are observed. Both kinds of molecules could exhibit obviously different molecular conformations and packing patterns, which demonstrate that the variation of the N-atom position in the pyridyl leads to different intermolecular interactions. Positioning the pyridyl N-atoms proximal to the central conjugated core could change the molecular conformation into a banana shape by bending the molecule, however, the d-DBPy molecule in which pyridyl N-atoms are distal to the central conjugated cores keep a linear shape in the monolayers. The slight difference in molecular chemical structure significantly influences the final morphology of the films. This systematic research work on the self-assembly of regio-isomeric DPPbased semiconductors on surfaces could provide guidance for the rational design of optoelectronic materials and the control of the morphology of the films. 1

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Introduction Diketopyrrolopyrrole-containing molecules are high-performance organic industrial pigments. They have been widely explored with diverse applications, such as organic solar cells,1 organic thin-film transistors,2,3 and organic photovoltaics.4-8 The structure of DPP-based materials can be modified with various aromatic substituents on the carbon atoms and side chains on the nitrogen atoms to tune their electronic properties of these materials.9,10 The co-planarity, crystallinity, energy levels, and intermolecular interactions in thin films can be precisely tailored by modifying these flanked units. Mastering the process of two-dimensional (2D) molecular self-assembly from molecular scale is essential for synthesizing organic materials and engineering novel 2D materials with optimized performance for organic devices.11-15 A slight change in preparation conditions can have a drastic effect on intermolecular interactions. For example, molecular structure, solvent, concentration, and temperature may lead to significant different self-assembled nanostructures.16-20 The resulting nanostructure of the self-assembled process is usually governed by the subtle balance between numerous weak intermolecular interactions including van der Waals,21,22 hydrogen bond,23,

24

halogen

band,25,26 dipole-dipole interaction,27,28 and π-π stacking.29,30 Microscopic molecular orientation plays a critical role in the optoelectronic properties of materials and the organic semiconducting devices.31-34 It is therefore essential to explore the influence of specific parameters on the formation of organic self-assembled adlayers. Regioisomeric structures that contain the same conjugated molecular backbone are of particular interest to understand the fundamental structure-property relationship, because the substituents in different topological geometries may induce different molecular orbital distributions, molecular dipoles, and intermolecular interactions. The molecular arrangements resulting from these interactions can possess distinct electrical and optoelectronic properties that can be exploited in various device applications.6,35,36 In addition, the solution concentration and solvent nature may also affect the molecular arrangement by changing molecular conformation and packing density. Different techniques are currently used to investigate the structure of the films, for example, STM, AFM,37-39 TEM,7,40 and grazing-incidence wide-angle X-ray scattering (GIWAXS).41,42 Among these techniques, STM allows the atomic scale characterization of two-dimensional molecular packing at room temperature and at the solid/liquid interface, which is essential to assess the intermolecular interactions.17,43-46 We have recently reported the effect of pyridyl orientation on the molecular conformation, optical property, and film morphology of the regioisomeric DPP-based derivatives (p-DBPy and d-DBPy, Figure 1).47 It was found that the solid powder and the solution displayed different colors, indicating the solvent influenced the 2


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molecular conformation. In addition, the optimized structures calculated by density functional theory (DFT) showed that the thienyl groups adopt different orientations towards the pyridyl groups. However, further studies are still keenly required to unravel the molecular self-assembly involved regulating packing fashion of regio-isomers in different solvents and concentrations. Herein, we report the systematic study of the solvent and solution concentration effects on supramolecular 2D orderings of p-DBPy and d-DBPy at the solid-liquid and solid-gas interfaces. STM observations display that the self-assemblies of regioisomeric molecules have obvious solvent and concentration dependence. Different intermolecular interactions determine the 2D molecular packing and further control different morphologies of the films. The results provide deeper insight from molecular scale to understand the molecular conformation and the nature of the morphology formation for the films.

Experimental section p-DBPy and d-DBPy as shown in Figure 1 were synthesized according to the established procedures.47 The solvents (phenyloctane and octanoic acid) used in this work were purchased from TCI and were used without further purification. Highly ordered pyrolytic graphite (HOPG, grade ZYB, Bruker, USA) as the substrate was cleaved to obtain a flat surface. The two molecules were dissolved separately in different solvents with different concentrations (10−3 ~ 10−6 mol L−1). The samples were prepared by depositing a droplet (about 1 μL) of solution onto the HOPG surface. The samples were then studied using STM with the tips immersed into the droplet directly. The samples were investigated at the gas/solid interface using dichloromethane as the solvent. After the solvent was evaporated completely, the STM imaging was conducted at the gas-solid interface. STM measurements were performed on a Nanoscope IIIa Multimode SPM (Bruker, USA) under ambient conditions. The tips were mechanically cut from Pt/Ir wires (80/20). All the STM images were obtained using a constant current mode and shown without further processing. Different tips and samples were used to check the reproducibility of the results. Tunneling parameters are given in the corresponding figure captions. Material studio 7.0 was used to build molecular models for the assembled structures. The models were constructed based on the intermolecular distances, angles, and analysis of the STM results.

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a C6H13

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O

C16H33 N

C6H13

C6H13

O

S

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C16H33

N C16H33

p-DBPy

N

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C6H13

O

d-DBPy

Figure 1. Chemical structures of 2,5-dihexadecyl-3,6-bis(5-(5-hexylthiophen-2-yl)pyridin-2-yl)pyrrolo[3,4c]pyrrole-1,4(2H,5H)-dione

(p-DBPy)

and

2,5-dihexadecyl-3,6-bis(6-(5-hexylthiophen-2-yl)pyridin-3-

yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (d-DBPy). Results Self-Assembled Monolayers of p-DBPy at the 1-Phenyloctane/HOPG Interface

cc

1.0

nm

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nm 1.0

Figure 2. (a) Large-scale (60 × 60 nm2) and (b) high-resolution (20 × 20 nm2) STM images of the adlayer for p-DBPy (c = 5 × 10−4 mol L−1) at the 1-phenyloctane/HOPG interface showing a fish-scale-like pattern. Tunneling condition: Vbias = 620 mV, Iset = 510 pA. (c) Tentative possible space-filling model of molecular packing for the fish-scale-like pattern. Inset shows the intra- and intermolecular interactions. (d) Large-scale (60 × 60 nm2) and (e) high-resolution (30 × 30 nm2) STM images of the adlayer for p-DBPy (c = 2 × 10−6 mol L−1) at the 1-phenyloctane/HOPG interface showing a segmented pattern. Tunneling condition: Vbias = 600 mV, Iset = 560 pA. (f) Tentative space-filling model of molecular packing for the segmented pattern. Inset shows the intra- and intermolecular interactions. 4


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At the concentrated concentration (5 × 10−4 mol L−1), p-DBPy molecules form a fish-scale-like pattern, in which the conjugated cores arrange in a line with the curved banana conformation (Figure 2a). From the molecular packing (Figure 2b), it is interesting to note that these alkyl chains attached with the conjugated cores are also not observed during STM imaging for the fish-scale-like rows and are not likely adsorbed on the surface and stretch into the solution, which has been observed for other systems.48-50 This arrangement is attributed to optimize the overall adsorption energy by packing more molecules onto the surface, particularly for the molecules with conjugated cores that are relatively more polarizable than the alkyl chains. The average length of the molecules in banana conformation measured experimentally is 1.85 ± 0.08 nm, which is shorter than the optimized molecular length of 2.12 nm (straight molecular conformation, Figure 1a). This observation indicates that the stability of such structure for p-DBPy is strengthened by interactions beyond the molecule-substrate van der Waals force. The molecules in the fish-scale-like pattern are lying flat on the surface. We suggest that the intramolecular interaction in each molecule and the intermolecular interactions between neighboring conjugated moieties in each lamella control the structural formation. Based on the STM images, a reasonable model is proposed as shown in Figure 2c. STM image analysis indicates that the intramolecular C−H···S and C−H···O=C interactions between the hydrogen of pyridyl and the sulfur of an adjacent thienyl locked the left side of the molecular conformation. In addition, in each lamella one carbonyl group in the DPP core is forming intermolecular C−H···O=C hydrogen bonds with the hydrogen atoms of pyridyl and thienyl cores in neighboring molecules. This intermolecular interaction is stabilizing the conformation of the right side of each p-DBPy molecule (inset in Figure 2c).47 Network parameters units are measured to be a = 1.2 ± 0.1 nm, b = 2.3 ± 0.1 nm, and γ = 85 ± 1º. The calculated nanostructure density is one molecule per 2.75 nm2. After a drop of diluted solution (c = 2 × 10−6 mol L−1) is deposited onto the HOPG surface, STM images show that an ordered segmented organic arrangement of p-DBPy molecule is formed consisting of parallel rows (Figure 2d,e). Each row is composed by the periodic arrangement of tilted dimers and parallel trimers or tetramers (Figure 2e). Molecules are arranged side-by-side in the parallel multimers whereas they are shifted along their axis in the tilted dimers. The high-resolution image (Figure 2e) shows that the building blocks are arranged sequentially (dimer-trimer-dimer-tetramer∙∙∙) and are forming an organic rows. The alkyl chains are not observed in the STM images clearly. Some of chains are most likely not adsorbed on the surface.51 The conjugated cores of p-DBPy molecules display the linear optimized conformation. The average length of each bright organic rod is 2.1 nm, which corresponds to the optimized length of conjugated cores calculated by DFT. According to the distance between neighboring molecules in each trimer or tetramer with 5



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the parallel packing fashion (about 1.0 nm) as the purple lines indicated in Figure 2e, no intermolecular π-π stacking exist. The dark space between adjacent lamellae as the green lines indicated in Figure 2e is about 1.0 nm, which is occupied by desorbed short alkyl chains. The tilted molecules in each dimer lock each other and connect with the neighboring cores of each trimer or tetramer as the red rectangle indicates in Figure 2e. A corresponding molecular model is proposed based on the STM images as presented in Figure 2f. Inset model shows the intermolecular C−H···S and C−H···N might be the dominated forces to stabilize such molecular packing (inset in Figure 2e). The measured unit cell parameters are a = 4.5 ± 0.1 nm, b = 13.8 ± 0.1 nm, and γ = 74 ± 1º. The calculated nanostructure density is one molecule per 5.43 nm2. Dichloromethane is now selected as a solvent to explore the self-assembly of p-DBPy at the gas-solid interface. After the solution was deposited on the HOPG surface, the dichloromethane evaporated rapidly. The similar densely-packed fish-scale-like pattern is observed. The organic layer appears to be rather fragile and easily disrupted by the scanning tip (Supporting information, Figure S1). Such densely-packed fish-scalelike pattern was also observed (Supporting information, Figure S2), when the hexadecane was used as the solvent. In contrast with dichloromethane, the adlayer appears to be more stable in hexadecane. The nanostructure is thus more stable at the solid-liquid interface than at the gas-solid interface. Self-Assembled Monolayer of p-DBPy at the 1-Octanoic Acid/HOPG Interface

c

Figure 3. (a) Large-scale (100 × 100 nm2) and (b) high-resolution (30 × 30 nm2) STM images of the adlayer for p-DBPy (c = 2 × 10−6 ~ 5 × 10−4 mol L−1) at the 1-octanoic acid/HOPG interface showing the molecular arrangement. Tunneling condition: Vbias = 590 mV, Iset = 530 pA. (c) Tentative space-filling model of molecular packing. Upon adsorption at the 1-octanoic acid/HOPG interface, STM shows that p-DBPy molecules form a largeorganized arrangement (Figure 3a) that differs from the structures observed previously in Figure 2. The largescale STM image in Figure 3a reveals that the main building block of this structure is a molecular trimer (three side-by-side molecules). Molecular tetramers (four side-by-side molecules) are locally observed. These 6


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building blocks are aligned. The red arrows in Figure 3a highlight the lines of tetramers. A high-resolution STM image of the trimer arrangement is presented in Figure 3b. This arrangement is independent of the solution concentration. A packing model reflecting the molecular ordering of conjugated cores for p-DBPy is shown in Figure 3c. Similar with the molecular packing in Figure 2e, the conjugated cores are flat on the graphite surface completely with a face-on geometry and the long side-chains are exposed to the liquid phase.52 The measured unit cell is superimposed on Figure 3b with a = 3.4 ± 0.1 nm, b = 3.9 ± 0.1 nm, and γ = 74 ± 1º. The calculated nanostructure density is one molecule per 4.25 nm2. Self-Assembled Monolayers of d-DBPy at the 1-Phenyloctane/HOPG Interface

c

f 1.5

n row m row I II

Figure 4. (a) Large-scale (60 × 60 nm2) and (b) high-resolution (20 × 20 nm2) STM images of the monolayer for d-DBPy (c = 2 × 10−6 mol L−1) at the 1-phenyloctane/HOPG interface showing a linear pattern. Tunneling condition: Vbias = 590 mV, Iset = 570 pA. (c) Tentative space-filling model of molecular packing for the linear structure. Inset shows the possible weak intermolecular hydrogen bonds. (d) Large-scale (60 × 60 nm2) and (e) high-resolution (30 × 30 nm2) STM images of the monolayer for d-DBPy (c = 5 × 10−4 mol L−1) at the 1phenyloctane /HOPG interface showing the fishbone-like pattern. Tunneling condition: Vbias = 650 mV, Iset = 550 pA. (f) Tentative space-filling model of molecular packing for the fishbone-like structure. In order to explore the significance of the slight change of the molecular chemical structure on the molecular packing, the self-assembly of d-DBPy where the orientation of pyridyl unit is distal to the central DPP unit was systematically investigated by STM. STM images of the self-assembly of d-DBPy at low concentrations at the 1-phenyloctane/HOPG interface are presented in Figure 4a,b. d-DBPy molecules form 7



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linear rows of closely packed conjugated moieties. Molecules are arranged side-by-side but they are shifted along their main axis. In contrast with the STM images of p-DBPy molecules, the alkyl chains of d-DBPy molecules are clearly visible. STM reveals that the long chains of molecules in neighboring lamellae are interdigitated and the short chains arrange tail-to-tail. Each unit cell contains one molecule and the unit cell parameters are determined to be are a = 1.6 ± 0.1 nm, b = 3.2 ± 0.1 nm, and γ = 85 ± 1°. The calculated nanostructure density is one molecule per 5.10 nm2. On the basis of the experimentally derived unit cell parameters, a structural model can be constructed (Figure 4c). The model suggests that the organic nanostructure is stabilized by weak –C-H···O=C hydrogen bonds (see inset shown in Figure 4c). Figure 4d and 4e show STM images of the d-DBPy self-assembly at high concentrations. The molecules adopt a fishbone-like pattern. Two types of linear rows (labelled row I and row II in Figure 4e) can be observed in the STM images. The relative orientation of the molecules differs in the rows I and II. Moreover, the clear (row I) and fuzzy (row II) appearances of the conjugated cores in the image change alternatively from rowto-row due to the two different molecular orientations.53 The average length of the bright rods (2.15 ± 0.05 nm) is close to the extended molecular length (calculated to be 2.08 nm using Materials Studio 7.0). The distance separating neighboring molecules in each row indicated by red lines is about 1.5 nm. This is larger than a typical π−π stacking distance (0.35 nm),29, 54 demonstrating that no intermolecular π−π stacking and other intermolecular interactions exist in the monolayer. The d-DBPy molecules are lying flat on the HOPG surface. Compared with low concentration measurements, the molecular alkyl chains are not observed in the fishbone-like arrangement. The angle of the neighboring lamellae is 120° resulting from the well accordance with the graphite lattices. The molecular model (Figure 4f) shows the molecular packing. The conformation of d-DBPy is linear, indicating the formation of intramolecular N···S interaction. The unit cell parameters are measured to be a = 1.9 ± 0.1 nm, b = 6.8 ± 0.3 nm, and γ = 86 ± 1°. Each unit cell contains two molecules and the nanostructure density is one molecule per 4.30 nm2. Such fishbone-like pattern was also observed at the hexadecane/HOPG interface (Supporting information, Figure S3). Self-Assembled Monolayer of d-DBPy at the 1-Octanoic Acid/HOPG Interface d-DBPy assembly at the 1-octanoic acid/HOPG interface is now investigated. The molecules are forming a stable wavy pattern on the graphite surface. Neighboring molecules are connecting with each other in a headand-head fashion and are forming dimers. Molecular chains are visible in the high-resolution STM images, Figure 5b. The C16 alkyl chains are lying straight along the cores of the neighboring molecules. A molecular model of the wavy-like pattern for d-DBPy based on the STM image is shown in Figure 5c. The measured unit cell parameters of the 2D structure are determined to be a = 2.9 ± 0.1 nm, b = 3.2 ± 0.1 nm, and γ = 82 ± 8


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1°. The nanostructure density is one molecule per 4.59 nm2. Molecular dimers appear to be stabilized by–CH···S bonds. STM images show that the octanoic acid molecules do not coadsorb in the monolayer. It should be noted that the wavy structure is also be observed at the gas-solid interface when dichloromethane is used as solvent at low concentrations (Supporting information, Figure S4).

c

Figure 5. (a) Large-scale (50 × 50 nm2) and (b) high-resolution (20 × 20 nm2) STM images of the monolayer for d-DBPy (c = 2 × 10−6 ~ 5 × 10−4 mol L−1) at the 1-octanoic acid/HOPG interface showing the wavy-like pattern. Tunneling condition: Vbias = 620 mV, Iset = 590 pA. (c) Tentative space-filling model of molecular packing for the wavy structure. In order to display the packing of the conjugated cores, the adsorbed side chains were not shown in the model. The inset shows the intermolecular C-H···S bonds in each dimer. Discussion STM observations display that the self-assemblies of two molecules significantly depend of the solvent nature and solution concentration. At the 1-phenyloctane/graphite interface, p-DBPy molecules adopt different molecular conformations at different solution concentrations. At high concentrations p-DBPy molecules adopt a twisted banana-like conformation. This conformation results from the intramolecular C−H···S and C−H···O=C bonds and intermolecular C−H···O=C hydrogen bonds. At low concentrations, the molecules are forming a segmented pattern with a linear molecular conformation. The molecular conformation of d-DBPy molecule do not change with the solution concentration, in contrast with p-DBPy. The side chains of d-DBPy adsorb on the surface to form the linear pattern at low concentrations. This arrangement is stabilized by the intermolecular van der Waals forces and weak intermolecular C−H···O=C hydrogen bonds. At high concentrations, d-DBPy molecules form the fishbone-like pattern without obvious intermolecular interactions between the conjugated cores. At the 1-octanoic acid/graphite surface, p-DBPy molecules self-assembly into a dislocated segment pattern stabilized mainly by the molecule-substrate interactions, because no intermolecular interactions between the conjugated cores are observed in the adlayer. d-DBPy molecules form a wavy-like structure stabilized by intermolecular C-H···S hydrogen bonds. 9



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The outcome of the self-assembled nanostructure is controlled by a complex balance of solvent-molecule, molecule-molecule and molecule-substrate interactions. STM results indicate that polymorphism is observed and induced by the molecular structure, solvent, and concentration. The intermolecular hydrogen bonds between the conjugated cores prefer to be formed for p-DBPy adlayer in the nonpolar solvents (1-phenyloctane and hexadecane) and for p-DBPy monolayer in polar solvent (1-octanoic acid). The solvent could stabilize the monolayer without coadsorption, so the observed structures in different solvents represent the thermodynamically most stable phases. This indicates the polarity of the solvents might affect the intermolecular interaction and further attribute to the structural formation. It is well-known that molecules tend to form the nanostructure with high molecular surface density to maximize the surface coverage for enthalpic reason. The self-assembly of two molecules only show solution concentration dependence in nonpolar solvent (1-phenyloctane and hexadecane). Obviously, the molecular density at high concentrations is higher than at low concentrations: for p-DBPy, fish-scale-like pattern (one molecule per 2.75 nm2) >segmented pattern (one molecule per 5.43 nm2); for d-DBPy, fishbone-like structure (one molecule per 4.30 nm2) >linear pattern (one molecule per 5.10 nm2). At high concentrations, the lifting of the alkyl chains towards the liquid phase allow more molecules to adsorb on the surface to maximize the interactions between the conjugated cores and the graphite substrate. The identification of molecular conformation of two molecules in solvent and solid state can be utilized to explain some physical properties. Firstly, the solubility of these two molecules is low, especially when the ambient temperature is below 10 ºC. At the low concentration, the side chains of the p-DBPy and d-DBPy molecules could adsorb on the surface, thus we propose that the molecule-substrate interactions play an important role in determining the structural formation. In addition, the molecules incline to form the patterns with low molecular densities. In order to increase the solution concentration, we added more molecules in the solution and heated the solution to enhance the molecular solubility. Then the solution colors for p-DBPy and d-DBPy change from purple black and orange color in solid powder into the similar purple red color, respectively.47 The color change indicated the molecular conformation has been changed. Especial for the pDBPy, the distinct change of the color between the solution and the solid state demonstrates the molecular conformations are different in these two phases. STM results identify the change of molecular conformation of p-DBPy. Thus, we can suggest that the conformation for the conjugated core of p-DBPy is linear in solution, however, the molecular conformation is curve in solid states (solid powder and film). However, for d-DBPy, the color for the solution and solid state has not obviously significant change, indicating the molecules keep the same linear molecular conformation. The STM results are accordance with their physical property. 10


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d-DBPy p-DBPy

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J-aggregation Figure 6. (a,b) UV-vis absorption spectra of p-DBPy and d-DBPy molecules as thin films and in toluene. (c,d) Polarized optical microscopy (POM) images showing the aggregations of p-DBPy and d-DBPy on mica surfaces. Insets are the AFM images showing the morphology of the films (scan area: 8 × 8 μm2). The samples were prepared with an air-drying method. (e,f) Proposed models showing the aggregation modes.

The 2D molecular self-assembly results could also be used to interpret the self-organization of molecules in solution and as thin films. Figure 6a shows the UV-vis spectra of the molecular thin films, whereas the UV-vis spectra of the molecules in solutions are presented in Figure 6b. We find that the absorption peaks of p-DBPy and d-DBPy in the films are red-shift relative to those in the solution (Figure 6b), which can be explained as molecular J-aggregation.55-56 POM and AFM results (Figure 6c,d) show that the morphologies of the films are fiber-like and terrace-like aggregations, indicating the molecular packing modes for two kinds of molecules are different. According the molecular conformations and packing modes observed by STM at 11



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the gas-solid interface, we speculate that except for the π- π stacking, P-DBPy form the fibers in the film owing to the intra- and intermolecular interactions, so the J-aggregation is formed when the p-DBPy molecules stack by a head-to-tail arrangement with the banana-like conformation (Figure 6e). d-DBPy molecules with the linear molecular conformation form the terraces with the J-aggregation by a head-to-tail packing (Figure 6f). J-aggregation is formed when the molecules stack by a head-to-tail fashion.56 According to the models, the film of p-DBPy molecule grows fast along one-dimensional directions as the pink arrows indicated due to the intermolecular interactions, while the d-DBPy film extends towards two-dimensional directions resulting from the π-π stacking between the lamellae. Because the films are prepared with relative high solution concentration, the aggregation of molecules in the films are similar with 2D molecular self-assembly obtained at high solution concentration. Thus, the STM results could interpret the nature of molecular packing and morphologies of the films from the molecular scale. The morphology of the films also display that the position of N atoms in the molecules could induce different intermolecular interactions and further control the molecular packing. The results demonstrate that regio-isomerism is useful strategy to fabricate films with completely different morphology. Conclusion In summary, we have carried out a systematical investigation of 2D self-assembled monolayers for the regio-isomeric DPP derivatives (p-DBPy and d-DBPy) formed at 1-phenyloctane /HOPG interface and 1octanoic acid/HOPG interface using STM, respectively. Our aim is to explore how the slight difference in position of N-atoms in pyridyl influences the molecular packing. It is interesting to find that the orientation of pyridyl unit relative to the DPP moiety has absolutely impact on the resulting self-assembled patterns. At the 1-phenyloctane/graphite surface, the conjugated cores of p-DBPy molecules adopt different conformations at different solution concentrations resulting from different intermolecular interactions. Although the conjugated moieties of d-DBPy display the same conformations, d-DBPy forms different nanopatterns at different solution concentrations, in which the side chains of d-DBPy exhibit different adsorbed fashions due to the molecular packing density. At the 1-octanoic acid/graphite surface, the p-DBPy and d-DBPy self-assembly into a dislocated segmented and wavy-like pattern, respectively, because of the polarity of the solvent. What is learn from STM results is therefore relevant for understanding the structure and local environment in the active layers of organic optoelectronic devices.

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ASSOCIATED CONTENT Supplementary Information Available: Additional STM images. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Financial supports from the National Natural Science Foundation of China (51373055, 21573077), the Natural Science Foundation of Guangdong Province (2018A030313452), and the Fundamental Research Funds for the Central Universities (SCUT) are gratefully acknowledged. X. Miao acknowledges the receipt of the China Scholarship Council (CSC, 201706155092). This work is supported by a public grant overseen by the French National Research Agency (ANR) as part of the “Investissements d’Avenir” program (Labex NanoSaclay: ANR-10-LABX-0035). REFERENCES 1.

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p-DBPy linear to curved shape

1-phenyloctane

n- hexadecane

d-DBPy keep linear shape


Self-Assembly Polymorphism of Regioisomeric ... - ACS Publications

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