Letter pubs.acs.org/JPCL
Controllable Orientation of Ester-Group-Induced Intermolecular Halogen Bonding in a 2D Self-Assembly Bao Zha, Meiqiu Dong, Xinrui Miao,* Kai Miao, Yi Hu, Yican Wu, Li Xu, and Wenli Deng* College of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China S Supporting Information *
ABSTRACT: Halogen bonding with high specificity and directionality in the geometry has proven to be an important type of noncovalent interaction to fabricate and control 2D molecular architectures on surfaces. Herein, we first report how the orientation of the ester substituent for thienophenanthrene derivatives (5,10-DBTD and 5,10-DITD) affects positive charge distribution of halogens by density functional theory, thus determining the formation of an intermolecular halogen bond and different self-assembled patterns by scanning tunneling microscopy. The system presented here mainly includes heterohalogen X···OC and X···S halogen bonds, H···Br and H···O hydrogen bonds, and I···I interaction, where the directionality and strength of such weak bonds determine the molecular arrangement by varying the halogen substituent. This study provides a detailed understanding of the role of ester orientation, concentration, and solvent effects on the formation of halogen bonds and proves relevant for identification of multiple halogen bonding in supramolecular chemistry.
H
multiple halogen bonding utilizing molecular self-assembly to achieve controllable motifs in a comparatively facile manner, which may open a new avenue for understanding the competition of intermolecular XBs with more complexity. In order to fully utilize halogen bonding as a design unit in 2D supramolecular self-assemblies, it is important to get deep insight into the relative strength of the various noncovalent interactions involving halogens and other donor groups (O and S) and to identify halogen-bonding configurations and noncovalent bonding sites. To achieve this goal, the strategy employed in this study entailed designing a system that presents the importance of thiophene and ester groups in the presence of halogen (Br or I) atoms in thienophenanthrene (TP) derivatives (Figure 1a) to evaluate the preference of heterohalogen X···OC and X···S over homohalogen X···X interactions along with other weak (H···X and H···O) hydrogen bonds (HBs) by a combined STM and DFT calculation. As a building block core, we used TP, a symmetric planar triangle, because (1) its planarity and rigidity are advantageous to form a coplanar assembly, (2) different substituents can be introduced at three vertex positions, and (3) it had been applied in polymer light-emitting devices and solar cells due to its photoelectric property.19,20 In addition, the introduction of two alkyl chains could stabilize the adlayers by interdigitation of side chains and molecule−substrate interactions at the liquid−solid interface. A straightforward approach to achieve a better understanding of the relative importance of multiple halogen bonds entails examining the structures of closely related
alogen substituents, especially the heavier halogens (Br and I), exhibit a positively charged cap (or σ-hole) along the axes of C−X bonds and a negative ring-like charge distribution along vectors perpendicular to these bonds. Therefore, the halogen bond (XB) occurs when there is a net attractive interaction between an electrophilic region associated with a halogen (X) atom in a molecule and a nucleophilic region in the same or another molecule due to its high specificity and directionality in the geometry.1 The significant role of XBs in building a supramolecular assembly in 3D crystal engineering has been vastly investigated and is welldocumented.2−6 Moreover, several groups have carried out theoretical calculations to elucidate the physical nature and propensity of halogen bonding and related σ-hole interactions.7−10 However, because of the limitation of crystal growth and calculation ability, cooperation and competition of multiple halogen bonds in complex systems by increasing the number of XB donor and/or acceptor sites on the starting molecular modules are strategies and have not been studied widely. Recently, scanning tunneling microscopy (STM) has proven to be a powerful approach for direct identification of relatively complex halogen bonding and delicate tailoring of structural patterns by designing elements on solid surfaces. The 2D molecular self-assembly has been utilized to understand the nature of homohalogen intermolecular interactions like Cl···Cl, Br···Br, and I···I in ultrahigh vacuum (UHV)11−16 and heterohalogen intermolecular interactions like I···N in ambient conditions.17 Most previous studies mentioned above mainly exhibited the construction of various structural motifs by simple intermolecular halogen bonds. However, to the best of our knowledge, a controllable mechanism of the XB by substituent, solvent, and concentration through STM has not been reported widely to date.18 It is therefore of utmost interest to explore © XXXX American Chemical Society
Received: July 10, 2016 Accepted: August 2, 2016
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Figure 1. (a) Chemical structure of TP derivatives: 5,10-DBTD and 5,10-DITD. (b) Calculated electrostatic potential maps of 5,10-DBTD and 5,10-DITD under vacuum, shown by blue (positive) and red (negative) regions. The hexadecyloxy side chains were replaced by the methoxy groups. (c) Illustration of possible intermolecular interactions involving X atoms.
compounds that differ only in the identity of the halogen substituent. The electrostatic potential at the σ-hole becomes more positive with larger halogens, as shown in Figure 1b, meaning that the strength of the intermolecular interactions can be adjusted by varying the halogen substituent. Halogen− halogen interactions have been termed type-I and type-II (electrophile−nucleophile model) depending on the angular approach of the halogens toward each other (Figure 1c).7 Type-I interactions belong to the van der Waals type. Type-II interactions were understood as attractive interactions between the positive polar region along the X−C axis and the negative equatorial region perpendicular to the X−C axis. According to the charge distribution, two other sorts of intermolecular XB interactions might be formed (Figure 1c). At the onset of this project, we felt that carboxyl oxygen offered the greatest possibility to form relatively strong halogen bonds because the sulfur atom would be spatially and directionally demanding due to its higher sp3 hybridization state. Considering the steric effect on the molecular geometry and molecular closest-packing principle, type-II XBs are not inclined to form in the construction of a supramolecular architecture. To confirm the formation of halogen bond sites, STM experiments were conducted. When a drop of saturated solution of 5,10-DBTD/1-phenylotane was applied onto a freshly cleaved HOPG surface, large organized domains of a vertebra-like motif were observed (Figure 2a and Supporting Information Figures S1a,b and S2a). Bright triangle features correspond to the TP π-cores as a result of their higher tunneling efficiency. Two TP π-cores arrange in an infinite joint-like motif, as the red rectangle indicates in Figure 2a, in an angle-to-angle fashion through bifurcated Br···OC and Br···S XBs. The 5,10-DBTD molecule interacts with the neighboring
Figure 2. (a) High-resolution STM image of 5,10-DBTD showing the vertebra-like pattern at the 1-phenylotane/HOPG interface (concentration: 2.5 × 10−4 M; scan area: 20 × 20 nm2; tunneling parameters: It = 408 pA, Vb = 679 mV). b) High-resolution STM image of 5,10DBTD showing the alternate vertebra-like pattern at the 1-phenylotane/HOPG interface (9.7 × 10−5 M; 20 × 20 nm2; It = 402 pA; Vb = 687 mV). (c,d) Packing models for the observed structures. Insets show the intermolecular interactions. (e,f) Proposed models for 5,10DBTD showing the intermolecular interactions. Red dotted lines indicate XBs, and blue dotted lines indicate HBs. (g−i) Calculated 3D charge density maps of 5,10-DBTD with different configurations of the ester group; the map color scales from −5 (blue) to 5 kcal/mol (red).
5,10-DBTD units through C−H···Br and C−H···OC HB interactions, which further reinforces this ribbon motif (Figure 2c). Two rows of molecules pack in an opposite orientation (blue arrows indicated) and form the vertebra-like pattern, resulting in the minimized polarity of the adlayer.21 In Figure 2e, possible XBs and HBs are marked by dotted lines formed by three neighboring molecules. In each row, for the dimers, the side chains in one molecule stretch to different directions along the lattice of HOPG, which enables one of the carboxyl groups to be detached from the TP π-core due to steric hindrance, while those in the other molecule stretch toward the same direction, as the purple arrows indicate in Figure 2e. We propose that the orientation of the side chains directly determines the orientation of the carboxyl, resulting in the formation of intermolecular Br···OC XBs. The direction of Br···S halogen bonding is along the C−Br bond toward the positive charge region of the sulfur atom.12,13 Thus, 5,10DBTD exhibits distinct and nonidentical XBs that vary as a function of halogen identity. 3165
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cores of 5,10-DITD pack in an angle-to-angle fashion, and all of the side chains stretch in different directions, indicating the existence of successive intermolecular bifurcated I···OC and I···S XBs (Figure 3e). No intermolecular interactions are formed between the neighboring molecules in different lines indicated by blue arrows in Figure 3a. Upon decreasing the solution concentration, an alternate pattern is formed (Figure 3b and Supporting Information Figures S4 and S5b). Except for the zigzag line, which is the same as that in the honeycomb-like pattern, a dimer structure distributes on both sides of the zigzag line in the opposite direction as the surface chirality. This chirality depends on the side chains of the dimers that interdigitate with those of neighboring lines and contact with the substrate, as well as the intermolecular dipole−dipole interactions. In the dimers, the side chains have the same extensional orientation as those in the zigzag structure (Figure 3d), indicating that only the I···I type of contact bond is formed, as shown in Figure 3f. Just as our predication, type-II I···I XBs were not observed due to the molecular geometry and molecular closest-packing principle. To get insight into the formation mechanism of intermolecular multiple halogen/or hydrogen bonding, we performed DFT calculations on the 3D charge density for 5,10DBTD and 5,10-DITD with different configurations of the ester group, as illustrated in Figures 2g−i and 3g,h. When two carboxyl groups approach the conjugated TP core, their main negative charges are close to the negative charges of X atoms. The positive charge distributes on the top of the sulfur atom (inset in Figure 2g). Once one of the carboxyl groups escapes from the TP π-core, the main negative charge is upward, which results in the enhanced σ-hole of the X atom on the same side. The charge distribution of the X atom on other side of the 5,10DBTD and 5,10-DITD molecules remains about the same (Figures 2h,i and 3h). Thus, in the vertebra-like row for 5,10DBTD, an electrophilic (δ+) region on one Br atom along the C−Br bond axis interacts with the nucleophilic (δ−) carbonyl oxygen, and the nucleophilic (δ−) region in the same Br atom contacts with the slightly positive charge of the sulfur atom to form bifurcated Br···OC and Br···S XBs. At the same time, a nucleophilic (δ−) region on the other Br atom forms a HB with the electrophilic (δ+) hydrogen, and the carboxyl oxygen atom on the same side of the TP core forms HBs with two hydrogen atoms (Figure 2e). This result is in good agreement with our STM finding, in which only one Br atom could form XBs with its neighboring molecule. In the single line row, all of the carbonyl oxygens are accessible to the conjugated cores. When two molecules align in the opposite direction, the positive σhole of Br atoms pointing toward the negative charge of carbonyl oxygens could form a pair of Br···OC XBs. It is noteworthy that in the honeycomb-like and alternated patterns for 5,10-DITD, only one iodine atom in each molecule is involved in the bifurcated I···OC and I···S bonds and I···I contact bond formation, respectively, whereas the other one does not appear to be involved in any intermolecular associations. XBs could be either weak or strong based on participating atoms. Increasing the size and positive nature of a halogen’s σhole dramatically enhances the strength of the electrostatic component of the halogen-bonding interaction. For various electronegative atoms, oxygen atoms show the strongest preference to form XBs, followed by nitrogen atoms, with the weakest preference for sulfur atoms.24 Interestingly, STM reveals that these patterns for 5,10-DBTD are mainly stabilized
Upon decreasing the solution concentration, a single line row appeared (Supporting Information Figures S1c−f and S2b) until a uniform alternate vertebra-like structure was formed (Figure 2b), in which all of the 5,10-DBTD molecules were antialigned, and the side chains located at the same side of the molecules arranged orthogonally with respect to the rows (Figure 2d). Thus, we estimate that all of the carboxyl groups orient toward the conjugated TP cores (Figure 2f). Except for the interchain van der Waals interactions, the Br···OC XBs play a decisive role in the structural formation. Theoretical and experimental data demonstrate that among X atoms the tendency to form strong XBs follows I > Br, while the magnitude of the molecular dipole is expect to show the opposite trend.22,23 Therefore, the larger halogen, iodine, is expected to provide stronger XBs and seems to present additional challenges. To further verify that the intermolecular XBs affect the molecular self-assembly by varying the halogen substituent, the adlayers of 5,10-DITD in 1-phenylotane with different concentrations were investigated. At high concentrations, in most cases, the 5,10-DITD form a honeycomb-like network consisting of two zigzag lines, as shown in Figures 3a, S3, and S5a. The corresponding molecular model is tentatively proposed, as shown in Figure 3c. In each line, all of the TP
Figure 3. High-resolution STM images of 5,10-DITD adlayers at the 1-phenylotane/HOPG interface (scan area: 20 × 20 nm2). (a) Honeycomb-like network (6.6 × 10−4 M; It = 426 pA; Vb = 669 mV); (b) alternate chiral pattern (8.2 × 10−5 M; It = 421 pA; Vb = 671 mV). (c,d) Structural models correspond to the observed structures. Insets show the intermolecular interactions. (e,f) Proposed models for 5,10DITD showing the intermolecular interactions. (g,h) Calculated 3D charge density maps of 5,10-DITD with different configurations of the ester group. The map color scales from −5 (blue) to 5 kcal/mol (red). 3166
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Table 1. Calculated Building Block, Structural Parameters, Interaction Energies, and Topological Properties of the Electron Density at the Critical Points Relative to the X Atom in Different Dimers for the Studied System
noncovalent interactions, which associate with the BCP (Supporting Information Figure S7). The ρCP value represents the accumulation of electron density at the BCP and is also a good indicator for the bond strength.28 In general, the larger the ρCP value, the stronger the bond.29 The ρCP values for X(Br, I)···OC interactions are 0.0864 and 0.2247 e Å −3 , respectively, indicating a stronger XB for the larger halogen. The ρCP value for the C−X···OC interaction is larger than that for the X···S interaction, which shows that the X···OC interaction is obviously stronger as compared to the X···S interaction and accounts for the primary role in the bifurcated complex. Further, the ∇2ρCP values represent the depletion of density at the given points for the XB and HB. In the case of a closed-shell (XB or HB) interaction, ∇2ρCP > 0 indicates that the electronic energy density is locally depleted at the BCP.30 In addition, |VCP|/GCP < 1 and HCP > 0 also demonstrate that the interactions in this complex belong to closed-shell noncovalent interactions. The topological values provide strong evidence for the nature of closed-shell system interactions in these complexes. To better understand and visualize the bonding feature in the complexes, the deformation electron density maps are shown in Figure 4 and Supporting Information Figures S8 and S9. These maps clearly show the nonspherical distribution of electron density around the X atom. This reveals the presence of the charge depletion region at the X atom, which is directed toward the charge concentration region over the carbonyl oxygen atom, facilitating formation of X···OC halogen bonding in the complexes. We propose that the relative orientation of different donor/ acceptor groups and the position of X atoms in the conjugated TP cores are the key factors to form the XB. The strength of halogen-bonding interactions can be modulated by neighboring groups, particularly functionality that serves to increase the electrophilic character of the halogen, which often leads to a
by two types of XBs (Br···OC and Br···S) and HBs (H···Br and H···O) (inset in Figure 2c,d), whereas the self-assembly of 5,10-DITD is dominated by similar I···OC and I···S XBs and I···I interactions (inset in Figure 3c,d). To gain deeper insight into the intermolecular binding mechanism within these assembled patterns, we further employed DFT calculations for different dimers (Supporting Information Figure S6 and Table 1). As shown in Table 1, the most striking difference between bifurcated XBs is the significantly shorter I···OC XB of 2.55 Å in dimer IV versus the Br···OC XB of 2.84 Å in dimer I. This is likely a reflection of the enhanced electrostatic attraction between iodine and oxygen. It is well-established in studies of halogen bonding that the ideal C−X−O (OC) angle is 180°.25,26 This value stems from the location of the electropositive region of the halogen (i.e., the σ-hole) directly opposite the C−X bond. Only the C−Br−O (OC) angle in dimer III is close to 180° (178.2°), indicating that the halogen bond is the strongest with the lowest energy (−9.43 kcal/mol). However, the observed C−X−O (OC) angles for 5,10-DBTD and 5,10DITD in dimers I and IV are 166.8 and 164.0°, respectively. The obvious different geometry is formed because for the following reasons: (1) the orientation of the carboxyl enhanced the positive charge distribution of the X atom; (2) the coexistence of weak C−Br···S XBs could induce a little bit of deflection for the angle of C−X−O (OC). The binding energy for HBs in dimer II (−6.20 kcal/mol) is lower than those of the XBs in dimer I (−3.85 kcal/mol) and dimer IV (−4.42 kcal/mol), indicating that the hydrogen bonding is the primary driving force (HB > XB) to stabilize the 5,10-DBTD adlayer.25 Theoretical calculations based on the quantum theory of atoms in molecules using the Multiwfn program confirms the presence of a bond critical point (BCP) for the weak interactions.7,27 Table 1 tabulates the topological values of all 3167
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obtained at high concentrations31 and triangle Br···Br···H interactions in the double line structure (Figure 5b,d) at low concentrations play dominate roles in the structural formation. The positive charge of 5,10-DBTD distributes orthogonally, which induces the formation of main intermolecular interactions in angle-to-angle styles (Figures 2g and 3g). However, the dispersive positive charge distribution for 6,9DBTD (inset in Figure 5a) is likely to result in the formation of intermolecular HBs and Br···Br interactions. These results demonstrate that the formation of hetero-XB has strong directionality and hydrogen bonding is stronger than halogen bonding in the Br-substituted complex. In the next step, we explore the solvent effect to identify the formation regulation of XBs, in which 1-octanoic acid not only acts as a dispersant but provides a donor and acceptor unit to participate in the bond formation by coadsorption. The assemblies of 5,10-DBTD at the 1-octanoic acid/HOPG interfaces with different concentrations were the same with those at the 1-phenylotane/HOPG interfaces (Supporting Information Figure S11). However, 5,10-DITD formed hierarchically self-assembled high-level clusters at a relative low concentration, as shown in Figure 6a. The pentamer, side-
Figure 4. Deformation charge density maps in the plane for all dimer complexes. Positive contours are shown with blue lines, and negative contours are shown with red lines. (a−c) 5,10-DBTD; (d,e) 5,10DITD.
corresponding increase in halogen-bonding ability.25,26 6,9Position Br-substituted 6,9-DBTD molecules could not form a hierarchical XB in 1-phenylotane at any conditions (Figures 5 and S10), resulting from the far distance between the bromine atom and the carboxyl group in one molecule. The intermolecular triangle Br···Br homo-XB and Br···H HB and the C−H···OC HB in the tetramer pattern (Figure 5a,c)
Figure 6. (a,b) Small-scale (42 × 42 nm2) and high-resolution (20 × 20 nm2) STM images of 5,10-DITD adlayers at the 1-octanoic acid/ HOPG interface (4.3 × 10−5 M; It = 468 pA; Vb = 682 mV). (c) Proposed molecular packing model. The inset shows the bifurcated I··· OC and I···HO− bonds. (d) DFT-calculated dimer for the molecule−solvent interactions. (e) The 2D deformation density map of the dimer offers an intuitive explanation for the coadsorbed geometry.
by-side hexamer, and separate hexamer were observed, as the rectangles indicate. From the high-resolution STM image (Figure 6b), we can find some shorter rods, as the red arrows indicate, attributing to the coadsorbed 1-octanoic acid molecules due to the molecule−solvent bifurcated I···OC XBs and I···HO− HBs. The molecular model is proposed in Figure 6c. Most of the unbonded iodine atoms participate in the bond formation except for a single molecule indicated by blue arrows in Figure 6b. The fact that this coassembly with 1-octanoic acid was not witnessed for 5,10-DBTD shows that the energetics involved in the stability of the multicomponent monolayer is extremely
Figure 5. (a) High-resolution STM image of 6,9-DBTD showing the tetramer pattern at the 1-phenylotane/HOPG interface (concentration: 7.9 × 10−4 M; scan area: 20 × 20 nm2; It = 415 pA; Vb = 671 mV). The inset indicates the calculated 3D charge density map of 6,9DBTD, and the map color scales from −5 (blue) to 5 kcal/mol (red). (b) High-resolution STM image of 6,9-DBTD showing the double line pattern at the 1-phenylotane/HOPG interface (concentration: 2.5 × 10−6 M; scan area: 20 × 20 nm2; It = 422 pA; Vb = 668 mV). (c,d) Structural models that correspond to the two different observed patterns. 3168
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The Journal of Physical Chemistry Letters subtle.32,33 Although the van der Waals radii of bromine and iodine atoms differ by 0.13 Å,34 we consider that the apparently minute difference is not the main factor for the solvent effect because the coadsorption of 1-octanoic acid with a brominesubstituted phenanthrene derivative was obtained previously.20 Therefore, we deduce that the absent coadsorbed structure in the self-assembly of 5,10-DBTD is for the following reasons: (1) compared with the 5,10-DITD assembly, only a few bromine atoms for 5,10-DBTD are not involved in the bond formation and the number of the nonbonded bromine atoms reduces with decreasing concentration; (2) the molecule− solvent binding energies for the bifurcated I···OC and I··· HO− bonds (−4.32 kcal/mol) are higher than that of intermolecular hydrogen bonding (−6.20 kcal/mol) for 5,10DBTD (dimer II); therefore, the solvent could not change the intermolecular HB interactions for 5,10-DBTD. DFT calculations (Figure 6d) show that the molecule−solvent binding energy of −4.32 kcal/mol is similar to the energy found in dimer IV (−4.42 kcal/mol) and lower than that of dimer V (−1.28 kcal/mol) for 5,10-DITD, which could be one of the reasons to form the coadsorbed pattern. The 2D deformation density map of the dimer (Figure 6e) offers a direct visualizing bonding feature in the coadsorbed geometry. Thus, exact fit of the geometric shapes and the molecule−solvent interaction are required and responsible for the induction of different-level clusters. Our attempt to utilize other HB donors, such as 1octanol, to fabricate a similar network failed, which indicates that the molecule−solvent I···OC XB is a key interaction to form the coadsorbed pattern. In conclusion, by combing STM imaging and DFT calculations, we have for the first time reported from this study that X···OC interactions are preferred over X···S interactions and X···X contacts in this system, but incorporation of a bromine group results in a H···Br HB, H···OC HBs, and an iodine group related to I···I contacts. The conjugated rings are now more electron-rich due to the presence of electrondonating carboxyl oxygen atoms. The orientation of an electron-withdrawing OC group could enhance the positive potential of the X atom and the strength of XBs. Consequently, as controllable halogen bonding becomes better understood, the engineering of strong and directional XBs should emerge as valuable design units in the fabrication of functional organic materials. Additionally, the complementary nature of halogen and hydrogen bonding in 2D self-assemblies is a yet underinvestigated area of supramolecular chemistry, especially for the 2D molecular self-assembly.
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ACKNOWLEDGMENTS
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REFERENCES
Financial support from the National Natural Science Foundation of China (21573077, 21103053, 51373055), the National Program on Key Basic Research Project (2012CB932900), and the Fundamental Research Funds for the Central Universities (SCUT) are gratefully acknowledged. We thank Prof. Weijun Jin and Dr. Weizhou Wang for useful discussion about the formation of halogen bonds and calculations.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01508. Detailed description of STM experimental conditions, synthesis, additional STM images, and DFT calculations (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (X.M.). *E-mail:
[email protected] (W.D.). Notes
The authors declare no competing financial interest. 3169
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DOI: 10.1021/acs.jpclett.6b01508 J. Phys. Chem. Lett. 2016, 7, 3164−3170
