Observation of Hierarchical Chiral Structures in 8-Nitrospiropyran

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2007, 111, 6973-6977 Published on Web 06/07/2007

Observation of Hierarchical Chiral Structures in 8-Nitrospiropyran Monolayers Tian Huang, Zhenpeng Hu, Bing Wang, Lan Chen, Aidi Zhao, Haiqian Wang, and J. G. Hou* Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed: February 12, 2007; In Final Form: April 23, 2007

The adsorption and self-organization of racemic mixture of 8-nitrospiropyran (SP8) molecules on Au(111) surfaces was studied by scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV). The SP8 enantiomers, in spite of their low-symmetric and nonplanar molecular structures, formed well-ordered monolayers on Au(111). In the monolayers, we found two types of enantiomorphous, i.e., mirror-imaged, 2D chiral domains, denoted as λ and δ phases. Both phases consist of periodically packed chiral quatrefoils. In the λ domain, the quatrefoils are counterclockwise folded, while in the δ domain, the quatrefoils are clockwise folded. High-resolution STM images revealed that each chiral quatrefoil contains four heterochiral dimers and that each dimer is composed of two antiparallelly packed homochiral SP8 molecules. Therefore both of the two mirror-imaged 2D chiral structures are not chirally pure but racemic 2D crystals. A domain boundary, which serves as the glide reflection line between a λ domain and a δ domain, was also observed along the [112h] direction of the Au(111) substrate.

Introduction Self-organization of racemic mixtures of chiral molecules into ordered structures on solid surfaces has attracted increasing research interest in recent years.1-4 Such a process provides a facile and economical procedure to create chiral surfaces, which have potential applications in enantioselective heterogeneous catalysis.5-7 Moreover, the study of the chiral expression in twodimensional (2D) systems is also helpful for a comprehensive understanding of the crystallization behavior of chiral molecules in three-dimensional (3D) systems. In a 2D molecular system based on a solid substrate, the molecular orientations and conformations are significantly affected by the substrate due to the interactions between the molecules and the substrate as well as the intermolecular interactions.4 In addition, the spatial freedom of the molecules on the substrate is also confined, therefore the chiral expression of molecules in 2D system is expected to have its unique properties distinct from which in a 3D system.8 With the technique of the scanning tunneling microscopy (STM), it is possible to characterize the 2D chiral structures at a single molecular scale,9 which may give much detailed information to reveal the chemical and physical properties behind. In previous studies, various surface structures formed by racemic mixtures of chiral molecules have been observed.10-27 It has been reported that the racemic mixture underwent spontaneous resolution into the 2D conglomerates, i.e., the chirally pure domains.10-17 This is in agreement with the theoretical prediction that the presence of surface enhances the chiral discrimination between enantiomers.28 There are also some examples in which the adsorbed racemic mixtures formed 2D domains consisting of equal number of left-handed (S-) and right-handed (R-) * Corresponding author. E-mail: [email protected].

10.1021/jp071193g CCC: $37.00

molecules on surfaces (i.e., the racemates).16-20 Moreover, 3D achiral molecules may become chiral upon adsorption due to the reduction of the symmetry. Deposition of these 2D chiral molecules will produce equal numbers of the 2D enantiomers on the surface, and these 2D racemic mixtures can either separate to 2D conglomerates29-36 or form 2D racemates.15,32,34,37 Recently, some novel chiral structures formed by the racemic mixtures of chiral molecules (including both 3D chiral molecules and 2D chiral molecules) have been reported. These structures are in contrast to the conventional 2D conglomerates or racemates and do not have any 3D analogue. For instance, racemic molecules have been found to separated into chiral clusters (each contains a fixed number of homochiral molecules, i.e., chirally identical molecules),21-24,37-41 and the homochiral clusters further constructed enantiomorphous 2D domains, which means the 2D domains contain two types of mirror-imaged structures.23,24 It was also found that the racemic mixtures underwent a “quasi-phase separation” into one-dimensional chirally pure rows.25,26 Very recently, Fasel et al. reported that the racemic heptahelicene formed two racemic domains with enantiomorphous structures instead of separating to 2D conglomerates, and the heterochiral molecular pairs represented truly chiral units in the racemic domains.27 These examples indicate the complexity of chiral expression even in the 2D molecular systems.42 Yet, it is still unavailable to predict whether a racemic mixture of certain chiral molecules will separate into conglomerates or form racemic crystals upon crystallization. In this paper, we present the chiral expression of the 8-nitrospiropyran (SP8) molecules in a monolayer on Au(111). SP8 is an isomer of 6-nitrospiropyran (SP6). Recently, we found that the racemic mixture of SP6 molecules formed orientational disordered 2D structures undergoing quasichiral separation upon adsorption on Au(111) surface.43 The main structural difference © 2007 American Chemical Society

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between SP8 and SP6 molecules is the different connecting position of the NO2 group; however, their adsorption behaviors on Au(111) are significantly different. The enantiomers in the SP8 monolayer form two kinds of chiral quatrefoil bases, each composed of four heterochiral SP8 dimers. In each dimer, two antiparallel homochiral SP8 molecules (either SS or RR) have the same adsorption orientation on Au(111). The chiral quatrefoils further form two enantiomorphous 2D chiral domains via chiral organization. Such a behavior is not reported before in other 2D molecule/substrate systems. The adsorption orientations and absolute chirality of SP8 molecules in the monolayer are discussed based on the STM observations, and a packing model of the chiral 2D structure is assumed. Experimental Section The epitaxial Au(111) substrates with well-defined terraces and single atomic steps on mica were prepared by vacuum deposition. Ar+ ion sputtering and annealing (to 600 K) cycles were used to clean and flatten the Au(111) surfaces. The experiments were carried out at a base pressure below 3 × 10-11 Torr using an OMICRON low-temperature scanning tunneling microscope. SP8 molecules in powder form (purchased from Acros Co.) were degassed for weeks and sublimated from a Knudsen cell to a Au/mica substrate at room temperature. The as-deposited sample was then transferred from the preparation chamber into the cryostat of the microscope precooled to 78 K. Constant-current STM images were recorded with an electrochemically etched tungsten tip, which was subjected to a careful cleaning treatment. The thermal drift during imaging was as small as 0.5 nm/h. STM images presented were corrected by planar background subtraction and processed with a low-pass filtering. Results and Discussion An SP8 molecule consists of two substituted indoline and chromene rings connected at a sp3 hybrid carbon atom, as shown in Figure 1a. Here, the sp3 hybrid carbon serves as a chiral center. For SP8 molecules in powder, the barrier between two enantiomers is very small at room temperature, therefore S- and R-SP8 molecules interconvert to each other spontaneously and form a natural racemic mixture.44,45 Deposition of this racemic mixture on the Au(111) surface resulted in highly ordered SP8 monolayers, as shown in Figure 1b. The ordered monolayer of SP8 contains two mirror-imaged chiral 2D structures, i.e., enantiomorphous phases, denoted as λ phase and δ phase. It is found that the chirality of the SP8 monolayer may not be conserved across the two Au(111) steps, but may be randomly performed on different terraces. Figure 2 shows the features of the enantiomorphous λ and δ phases. In both of the λ phase and the δ phase, the main feature is the periodically packed chiral quatrefoils. The unit cells are marked with parallelograms in the λ and the δ phases. Each quatrefoil basis consists of four counterclockwise folded SP8 dimers in the λ phase (λ-quatrefoil, four dashed ellipses in Figure 2a) or four clockwise folded SP8 dimers in the δ phase (δquatrefoil, four dashed ellipses in Figure 2b). The mean lattice parameters of the unit cell are 3.2 ( 0.2 nm and 2.9 ( 0.2 nm with an angle of 95 ( 3°. Parts c and d of Figure 2 display the high-resolution images of λ and δ phases. A single SP8 molecule exhibits an elliptic protrusion with an obliquely attached tail, as illustrated by the pollywog-like marks. It is observed that, in both λ and δ phases, each quatrefoil includes two types of SP8 dimers, as labeled with I, II in Figure 2c and I′, II′ in Figure 2d, respectively. Each dimer contains

Figure 1. (a) Molecular structures and optimized CPK models of lefthanded (S-) and right-handed (R-) SP8. (b) Large-scale STM image of SP8 monolayer on Au(111) surface. The tunneling conditions are Vbias ) -1.8 V and Isetpoint ) 50 pA.

two equivalent but antiparallelly positioned single SP8 patterns, which are superimposable by surface rotational symmetry manipulation. In dimer I and dimer I′, the two single SP8 patterns are face-to-tail coupled (FTC), while in dimer II and dimer II′ the two single SP8 patterns are back-to-back coupled (BBC). The dimers I and I′ are enantiomorphous, so are the dimers II and II′. It is also noticed that the dimer I and the dimer II (or the dimer I′ and the dimer II′′) are not mirror images of each other. This is much more clearly confirmed by the image shown in Figure 2e. Figure 2e shows the STM image of the δ phase obtained under a special STM tip condition. In the image, the patterns of SP8 molecules reflect the geometric contours of the molecules, but not intramolecular details. Each SP8 molecule appears as a pollywog-like shape with a length of ∼1.3 nm, and the two SP8 molecules within each dimer lie down antiparallelly. The distance between the two bright spots in the dimer I′ is shorter than that in the dimer II′, indicating that the two dimers are neither identical nor mirror images of each other. These observations strongly imply that each dimer consists of two antiprallelly arranged homochiral SP8 molecules (either SS or RR), whereas the SP8 molecules in the dimer I (II) has different chirality with the SP8 molecules in the dimer I′ (II′). Therefore, the chirality of the quatrefoil basis not only comes from the folding direction of the dimers (either counterclockwise or clockwise) but also originates from the dimers themselves that construct the quatrefoils (either SS or RR). Moreover, for

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Figure 2. STM images of two enantiomorphous SP8 domains. (a) λ phase and (b) δ phase. Each dashed-ellipse indicates a dimer of SP8. Four dimers form a counterclockwise folded quatrefoil (in λ phase) or a clockwise folded quatrefoil (in δ phase). The enantiomorphous unit cells for λ and δ phase are plotted by the solid lines. (c and d) Highresolution STM images of the λ phase and the δ phase, respectively. The pollywog-like marks denote SP8 molecules lying down in different ways. (e) STM image of δ phase under a special tip condition. The tunneling conditions are Vbias ) -1.8 V and Isetpoint ) 50 pA for (a) and (b), Vbias ) -0.8 V and Isetpoint ) 50 pA for (c) and (d), and Vbias ) -1.5 V and Isetpoint ) 50 pA for (e).

a quatrefoil, the single SP8 patterns from different types of dimers are also enantiomorphous. That is, both λ-quatrefoil and δ-quatrefoil contain two SS and two RR dimers. The enantiomorphous domains λ and δ are racemates rather than the conglomerates because each of them contains equal amounts of S- and R-SP8 molecules. An STM image reflects the contribution of the local density of states (LDOS) of the sample below the applied voltages.46,47 To understand the patterns of the molecules, we need to consider their electronic states. Because Au(111) is a relatively inert substrate and the SP8 molecule does not have any functional groups that can form strong chemical bonds with gold, we suggest that the existence of the substrate may not significantly change the electronic distribution of the SP8 molecules. Therefore, we perform a simple DFT calculation on an isolated gas-phase SP8 molecule to help interpret the STM images. Figure 3a shows the calculated frontier orbitals of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of a free SP8 molecule.48 We may see that the HOMO is mainly localized on indoline moiety, and the LUMO is mainly localized on the chromene moiety. The calculated HOMO-LUMO gap is about 1.6 eV. Note that the STM images presented in Figure 2 were negatively biased; then, the observed images were mainly contributed by the occupied states, which are related to the distribution of the electronic states

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Figure 3. (a) Calculated HOMO and LUMO of a free S-SP8 molecule. (b) STM images and proposed molecular models of the four chiral dimers (I, II, I′, and II′). (c) Assumed adsorption model of an S-SP8.

at HOMO of the SP8 molecule. Comparing the STM images with the calculated HOMO, we may find that the protrusions of the single SP8 patterns well reflect the HOMO of a free SP8, while the obliquely attached tail may be contributed by the chromene ring due to a certain extension of the electronic states on it, as indicated with the arrows in Figure 3a. Although we have made efforts, unfortunately, we have not been able to well image the SP8 monolayer at positive biases. On the basis of the above analysis, we propose a model for the four types of SP8 dimers. As shown in Figure 3b, the FTC-I dimer is composed of two S-SP8 molecules (SS), while the BBC-II dimer is composed of two R-SP8 molecules (RR). Accordingly, the FTC-I′ dimer and the BBC-II′ dimer are composed of two R-SP8 molecules (RR) and two S-SP8 (SS) molecules, respectively. Here, the two molecules in each dimer lie down antiparallelly. Thus the FTC-I-SS (BBC-II-RR) dimer and the FTC-I′-RR (BBC-II′-SS) dimer are mirror images of each other. In this model, S-SP8 and R-SP8 have the same orientation on the substrate except for their different chirality. This assumption is based on the STM observations that all the SP8 molecules exhibited equivalent or mirrored patterns on the surface. Figure 3c shows the proposed molecular orientation of an S-SP8 on the Au(111) substrate. In this configuration, SP8 molecule is supported by two methyl groups and a NO2 group on Au(111). Both of the orthogonal aromatic rings (indoline moiety and chromene moiety) are tilted and resemble an X shape in the side view (1) of Figure 3c. The NO2 group is likely to form relatively strong bond with the gold substrate,49 which may further stabilize the adsorption configuration. In this case, the tilted indoline ring and the upward-pointed methyl group may represent the elliptic protrusion, and the edge of the tilted chromine ring contributes the obliquely attached tail feature in the STM patterns. Because SP8 has neither strong chemical bond

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Figure 4. (a) STM image containing a domain boundary between the λ and δ domains. (b) Higher magnified image of (a). The domain boundary is along the [112h] direction of the Au(111) substrate. The lattices in λ phase can be superposed to those in δ after a glide reflection symmetry manipulation (with the offset of ∼1.1 nm) along the boundary. At the domain boundary the type I (I′) dimers of λ (δ) domains are destroyed and leave the S-SP8 (R-SP8) monomers. (c) A proposed model for λ and δ domains separated by a domain boundary on Au(111).

with the substrate nor strong intermolecular interaction between adsorbates, we suggest that the formation of the novel 2D chiral structure is dominated by the interadsorbate van der Waals as well as C-H‚‚‚O34,37-39 and C-H‚‚‚π interactions.40,50 It should be noticed that the above interpretation of the adsorption structures is only a simplified treatment. Although the calculated HOMO of the gas-phase molecule with the proposed orientation qualitatively represents the main feature of the STM patterns, a further theoretical analysis taking the substrate into consideration is still necessary for a comprehensive understanding of the adsorption of the SP8 molecules on Au(111). Chiral dimers consisting of homochiral enantiomers have been observed previously for several different chiral molecules on surfaces.21,23,40 The formation of the chiral dimers was attributed to the chiral recognition process. For SP8, because the spontaneous thermal enantiomerization can take place between two enantiomers even at room temperature, we cannot exclude the possibility of the chiral assimilation mechanism, which involves a chirality flipping event within the dimer during its formation.40 The results presented above only involve the domains either in λ phase or in δ phase. It is interesting to know the structure of the domain boundary between the two enantiomorphous phases. Figure 4a gives an STM image of the SP8 monolayer containing a domain boundary between the λ phase and the δ phase. The boundary is along the [112h] direction of the Au(111) substrate. In the magnified image shown in Figure 4b, the λ phase (upper part) and the δ phase (lower part), separated by the boundary marked by the arrow, can be much clearly seen. The lattice vector a (a′) in the λ (δ) domain is tilted about 44° from the domain boundary, i.e., the [112h] direction of the Au(111) substrate. Each domain can be obtained by applying a glide reflection symmetry manipulation (with the offset of ∼1.1 nm) to its mirror domain along the domain boundary. At the domain boundary, the type FTC-I (FTC-I′) dimers in λ (δ)

domains are broken, forming SP8 monomers in both sides of the boundary. Figure 4c gives a packing model of SP8 at the boundary according to the STM observation. The formation of 2D chiral structures in the SP8 monolayer described above represent a novel chiral expression of racemic enantiomers at the solid surface. The molecular chirality of the SP8 molecule is expressed by the formation of chiral dimers (I, II, I′, and II′) and then enantiomorphous 2D domains (λ and δ). The chirality transfer from chiral dimers into 2D chiral networks, similar to the hierarchical assembly process.24,51 However, four heterochiral dimers form a quatrefoil-like basis which acts as truly chiral unit in two enantiomorphous 2D chiral networks (λ and δ). Because the two enantiomorphous lattices also have mirrorlike registries with respect to the Au(111) lattice, the formation of λ and δ domains is energetically equivalent and equally probable, indicating the racemic nature of the overall monolayer. Conclusion In summary, we studied the self-organization and the chiral expression of SP8 molecules on Au(111) surface with STM. The racemic mixture of SP8 molecules formed a well-ordered monolayer on Au(111) surface. In the monolayer, two enantiomorphous 2D chiral phases (λ and δ) formed by periodically packed chiral quatrefoils (λ-quatrefoil and δ-quatrefoil) were observed simultaneously in ordered domains. Lattice parameters of a ) 3.2 ( 0.2 nm and b ) 2.9 ( 0.2 nm and their included angle of 95 ( 3° are obtained. Every λ-quatrefoil contains two FTC-SS dimers and two BBC-RR dimers, while every δ-quatrefoil contains two FTC-RR dimers and two BBC-SS dimers. In each dimer, two antiparallel homochiral SP8 molecules (either SS or RR) have the same adsorption orientation on Au(111). Therefore, the λ-quatrefoil and the δ-quatrefoil are mirror images of each other. Both λ and δ domains are racemates because

Letters they are composed of an equal number of S and R enantiomers. A domain boundary along the [112h] direction of the Au(111) substrate is also observed between the λ phase and the δ phase. At the domain boundary the type FTC-I (FTC-I′) dimers in λ (δ) domains are broken, forming SP8 monomers at both sides of the boundary. Acknowledgment. This work was supported by National Basic Research Program of China (2006CB922001) and by the National Natural Science Foundation of China (50121202, 50532040, 20473077, 20573099), Chinese Academy of Sciences. References and Notes (1) Walba, D. M.; Stevens, F.; Clark, N. A.; Parks, D. C. Acc. Chem. Res. 1996, 29, 591-597. (2) Giancarlo, L. C.; Flynn, G. W. Acc. Chem. Res. 2000, 33, 491501. (3) De Feyter, S.; Gesquiere, A.; Abdel-Mottaleb, M. M.; Grim, P. C. M.; De Schryver, F. C.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mullen, K. Acc. Chem. Res. 2000, 33, 520-531. (4) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201-341. (5) Jannes, G., Dubois, V., Eds. Chiral Reactions in Heterogeneous Catalysis; Plenum: New York, 1995. (6) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376-379. (7) Baddeley, C. J. Top. Catal. 2003, 25, 17-28. (8) Raval, R. Curr. Opin. Solid State Mater. Sci. 2003, 7, 67-74. (9) Lopinski, G. P.; Moffatt, D. J.; Wayner, D. D.; Wolkow, R. A. Nature 1998, 392, 909-911. (10) Fang, H. B.; Giancarlo, L. C.; Flynn, G. W. J. Phys. Chem. B 1998, 102, 7311-7315. (11) Xu, Q. M.; Wang, D.; Wan, L. J.; Wang, C.; Bai, C. L.; Feng, G. Q.; Wang, M. X. Angew. Chem., Int. Ed. 2002, 41, 3408-3410. (12) Fasel, R.; Parschau, M.; Ernst, K. H. Angew. Chem., Int. Ed. 2003, 42, 5178-5181. (13) Wang, D.; Xu, Q. M.; Wan, L. J.; Bai, C. L.; Jin, G. Langmuir 2003, 19, 1958-1962. (14) Han, M. J.; Wang, D.; Hao, J. M.; Wan, L. J.; Zeng, Q. D.; Fan, Q. H.; Bai, C. L. Anal. Chem. 2004, 76, 627-631. (15) Mamdouh, W.; Uji-i, H.; Dulcey, A. E.; Percec, V.; De Feyter, S.; De Schryver, F. C. Langmuir 2004, 20, 7678-7685. (16) Yablon, D. G.; Wintgens, D.; Flynn, G. W. J. Phys. Chem. B 2002, 106, 5470-5475. (17) Mamdouh, W.; Uji-i, H.; Gesquiere, A.; De Feyter, S.; Amabilino, D. B.; Abdel-Mottaleb, M. M. S.; Veciana, J.; De Schryver, F. C. Langmuir 2004, 20, 9628-9635. (18) De Feyter, S.; Gesquiere, A.; Wurst, K.; Amabilino, D. B.; Veciana, J.; De Schryver, F. C. Angew. Chem., Int. Ed. 2001, 40, 3217-3220. (19) Chen, Q.; Frankel, D. J.; Richardson, N. V. Surf. Sci. 2002, 497, 37-46. (20) Kunitake, M.; Hattori, T.; Miyano, S.; Itaya, K. Langmuir 2005, 21, 9206-9210. (21) Kuhnle, A.; Linderoth, T. R.; Hammer, B.; Besenbacher, F. Nature 2002, 415, 891-893. (22) Kuhnle, A.; Linderoth, T. R.; Besenbacher, F. J. Am. Chem. Soc. 2003, 125, 14680-14681. (23) Yan, H. J.; Wang, D.; Han, M. J.; Wan, L. J.; Bai, C. L. Langmuir 2004, 20, 7360-7364.

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