NANO LETTERS
Chiral Symmetry Breaking in Two-Dimensional C60−ACA Intermixed Systems
2005 Vol. 5, No. 11 2207-2211
Bo Xu,‡,† Chenggang Tao,§,† William G. Cullen,§ Janice E. Reutt-Robey,*,‡ and Ellen D. Williams§ Department of Chemistry and Biochemistry and Department of Physics, UniVersity of Maryland, College Park, Maryland 20742 Received July 21, 2005; Revised Manuscript Received September 8, 2005
ABSTRACT We have demonstrated a method for fabricating C60 overlayers with controlled spacing and chirality by reactive coadsorption with the aromatic molecule acridine-9-carboxylic acid (ACA). Structural control is achieved by the mismatched symmetries of the coadsorbates, as well as specific intermolecular and adsorbate−substrate interactions. The resulting supramolecular structure has a C60 period nearly three times as large as the normal C60 2D packing of 1 nm and exists in enantiopure domains with robust chirality.
The development of chiral surfaces offers new approaches to chiral synthesis and active devices. Chiral surfaces, produced by chiral molecular adsorption,1-6 electrodeposition,7 or slicing chiral single crystals,8-12 provide a platform for heterogeneous enantioselective reactions, with the advantage of easier product separation and reactivation. For device applications, chiral surfaces exhibit enantiospecific properties, such as electron-molecule interactions,13 polarization-dependent photoemission,14 and nonlinear optical response.1,15 The latter are especially interesting in the burgeoning field of organic optoelectronic and integrated photonic materials.16,17 Here we demonstrate how a mixture of C60 and ACA can be organized into chiral phases through a self-assembly process,18,19 even though the constituent molecules themselves are achiral. The assembly mechanism suggests a general strategy for designing chiral aromate surfaces, which are particularly desirable for potential optoelectronic applications. Adsorption-induced surface chirality generally falls into two classes: (1) the adsorption of chiral molecules on an achiral surface (direct-transfer chirality), and (2) the adsorption of molecules that are achiral in 3D but become chiral when confined to the 2D surface because of symmetry breaking imposed by the surface (surface-confined chirality). In the latter case, even though the individual adsorbed molecules become chiral, the surface phase often remains achiral because of the formation of a racemate (equal population of the two enantiomers). Only if molecule-molecule * Corresponding author. E-mail:
[email protected]. † These authors contributed equally to this work. ‡ Department of Chemistry and Biochemistry. § Department of Physics. 10.1021/nl051415r CCC: $30.25 Published on Web 09/28/2005
© 2005 American Chemical Society
and molecule-substrate interactions favor an enantiopure phase will surface chirality result.20,21 By coadsorption of C60 and ACA, we introduce a new method for generating surface chirality. In this method, the interactions among two achiral molecular species of very different symmetry, as well as the adsorbate-substrate interactions, act to break the symmetry of the individual components. With its Ih symmetry classification, C60 appears a most unlikely candidate for chiral symmetry breaking. Nonetheless, when coadsorbed with a C2V symmetric ACA, interactions between C60 and ACA result in the assembly of a chiral phase. In this case, the C60 molecules form reactive complexes with ACA that nucleate into enantiopure domains with a specific chirality. Samples were prepared by sequential deposition of ACA (390 K) and C60 (650 K) on an epitaxially ordered Ag(111) thin film grown on a mica substrate.22 C60 was deposited from an effusion cell in the UHV measurement system (base pressure ∼5 × 10-11 Torr) and ACA from a resistively heated Knudsen cell in a load-lock connected to the UHV chamber (base pressure ∼8 × 10-9 Torr). During the experiment, ca. 0.4 ML ACA and 0.4 ML C60 were deposited sequentially on the clean Ag(111) surface and imaged using an Omicron VT-STM in the same UHV chamber. All of the STM images shown in this paper were taken at room temperature. Low-temperature (40 K) STM studies (see the Supporting Information) were used to provide additional information about specific molecular configurations as noted. The highly symmetrical C60 molecule is illustrated in Figure 1a. Upon adsorption on metal surfaces, individual C60 molecules are mobile and have no preferential orientation unless cooled to low temperature.23,24 An ordered phase of
Figure 1. Self-assembly of C60 and ACA supramolecular structures. (a and b) Chemical structure of C60 and ACA, respectively. (c) STM image of distinct self-organizing phases produced by sequential deposition of ACA (0.4 ML) and C60 (0.4 ML) on a room-temperature Ag(111) substrate: Phase I, (2x3 × 2x3)R30° C60/Ag(111); Phase II, (4 0, 2 4) ACA/Ag(111); Phase III, 2D ACA gas/Ag(111); and Phase IV, C60 and ACA intermixed phase. Two domains of Phase IV are symmetric with respect to the Ag [112h] direction. Constantcurrent (40 pA) images were acquired with a -0.8 V sample bias.
C60 forms even at low coverage, with a (2x3 × 2x3)R30° structure on Ag(111).25 Individual ACA molecules (Figure 1b) also retain reflection symmetry and thus are achiral, when adsorbed on the surface as free molecules, given the expected symmetric occupancy of the hydrogen atom in the carboxyl group.26,27 At room temperature, ACA molecules form a 2D gas phase on the surface when the coverage is low (0.3 ML), an ordered ACA structure appears, which can be indexed in matrix notation as (4 0, 2 4) with respect to the Ag(111) lattice (definition of Ag lattice vectors is shown in Figure 3a). This ACA ordered phase is in dynamic equilibrium with the 2D gas phase. In the ordered structure, hydrogen bonds between nitrogen atoms and hydroxyl hydrogen atoms link ACA molecules into parallel chains that extend over 100 nm to span the widths of Ag(111) terraces.28 In this structure, the molecular long axis is parallel to the low-symmetry [112h] 2208
direction of Ag(111). Because the symmetry of the carboxyl group is broken by the hydrogen bonding, individual molecules in this higher density phase are chiral. However, the ordered phase itself is achiral as the result of a racemic molecular arrangement. The structures of the coadsorbed ACA and C60 monolayers show a striking dependence on deposition sequence. If C60 molecules are deposited first, then the C60 and ACA molecules form phase-separated domains identical to those of the single-component monolayers described above25,28 (see the Supporting Information). However, when the deposition sequence is reversed, four distinct structures are observed simultaneously, as shown in Figure 1c. Three of the phases are identical to those of the pure C60 and ACA structures (labeled as I, II, and III in Figure 1c, respectively). However, a new and striking structure is also observed, in which there appears to be one C60 per unit cell (labeled as IV in Figure 1c). Two Nano Lett., Vol. 5, No. 11, 2005
Figure 2. STM images of rotational domains with identical chirality of the intermixed C60 and ACA supramolecular phase. (a) A hexagonal Chinese-checkers structure (size 60 × 60 nm2, sample voltage -0.8 V, tunneling current 80 pA). (b) A higher resolution image of the boundary between adjacent domains reveals additional features identified as ACA molecules (size 20 × 20 nm2, sample voltage -0.8 V, tunneling current 40 pA). An outline drawing highlighting these molecular features is superposed on the STM image, and one unit cell is drawn for each of the domains showing the 60° rotation.
significant features of this new structure are noted. First, the distance between neighboring C60 molecules is 2.65 nm, in comparison with the 1-nm nearest-neighbor spacing of C60 in the pure (2x3 × 2x3)R30° C60/Ag(111) phase. Second, there are two domains of phase IV that demonstrate mirror symmetry with respect to the [112h] directions of Ag(111) as indicated in Figure 1c. The structure is determined to be (2x21 × 2x21)R ( 10.9°, or (10 2, -2 8) and (8-2, 2 10) in matrix notation with respect to the Ag(111) surface. STM images of this mixed phase reveal triangular domains, which are frequently arranged into a hexagonal “Chinese-checkers” pattern (Figure 2a). Higher resolution images (Figure 2b) show that in addition to the dominant feature in each unit cell (identified as the C60 molecule), there are additional rod-shaped features visible between the C60 molecules. These rod-shaped features are similar in size and shape to the appearance of individual ACA molecules, observed after cooling the 2D ACA gas phase from room temperature to 40 K (see the Supporting Information). With the help of C60 vacancies, the otherwise occluded ACA molecules are imaged directly. Their positions are highlighted with gray-shading in Figure 2b, with the extrapolated ACA positions under the C60 molecules also demonstrated. The ACA molecules are arranged into two types of “molecular pinwheel” trimers, labeled as S and R in Figure 2b, with different orientations with respect to the underlying Ag(111) lattice and with specific chiralities. In Figure 2, each C60 molecule is located atop an R trimer and immediately bounded by three open S ACA pinwheels inside the domain. We define the domain where C60 sits atop an R trimer as S chirality (because the S trimer is exposed). The six triangular domains shown in Figure 2a and the two shown in Figure 2b all have S chirality. STM images of both enantiomeric phases, R and S, are presented in Figure 3 with a chemically reasonable molecular Nano Lett., Vol. 5, No. 11, 2005
model. In Figure 3b, C60 molecules always occupy the valley of an S trimer and leave the R trimer empty, whereas in Figure 3d occupancy is reversed. As a result, the two phases in Figure 3b and 3d are of R and S chirality, respectively. These two phases demonstrate mirror symmetry with respect to the [11h0] direction of Ag(111). Low-temperature STM images indicate that C60 molecules are positioned with opposing hexagonal rings parallel to the surface (see the Supporting Information), consistent with the threefold symmetry of the underlying ACA structure. The ACA molecules in the underlying trimer are positioned without centrosymmetry with respect to the C60 to accommodate the close spacing between molecules. This is the origin of the chiral symmetry breaking. Given this observed symmetry, the trimer ACA molecules are suggested to orient with the ring nitrogen atoms pointing inward and the carboxyl groups pointing outward, providing primary hydrogen bonds between carboxyl groups and secondary hydrogen bonds between ring nitrogen atoms and phenyl hydrogen atoms. The resulting spatial array of the ACA molecules alone (e.g., without the C60 or the Ag substrate) would be described by the plane symmetry group P31M. However, because of the enantioselective C60 positioning, the symmetry group is lowered to P3, with symmetric operations of 120° rotation centered on the C60 molecules, the center points of the unoccupied trimers, as well as corners of the unit cells drawn in the figures. The enantiomerically pure phase can exist in two nonequivalent rotational domains: a rotation of 60° will give a nonsuperposable domain, whereas a rotation of 120° will give a superposable domain. The walls between the homochiral domains are found to be straight and separated by distinct structures of the ACA molecules, as highlighted in Figure 2. Using our proposed molecular model, these domain boundaries are naturally stabilized by H-bond formation between the ACA molecules at the edges of the 2209
Figure 3. Proposed structural models and STM images of the C60-ACA intermixed supramolecular structures for the R chiral surface (a and b) and S chiral surface (c and d). For reference, a unit cell is indicated in each panel, along with the matrix notation of the domain. Ag(111) lattice vectors are shown in a. Constant-current (40 pA) STM images (18 × 18 nm2) were acquired with a -0.8 V negative sample bias.
adjacent domains (see the Supporting Information). This provides further support for the proposed model and accounts for the commonly observed Chinese-checkers structure shown in Figure 2a. The model also allows a natural mechanism for the production of the two different enantiopure domains. Because this intermixed phase only forms when C60 is deposited on an ACA-preoccupied surface, it is plausible that C60 molecules first induce local ordering of an ACA trimer, with random selection of one of the trimer rotational orientations, which in turn is registered with respect to the underlying Ag(111) lattice. After the initial chirality-creation event, subsequent assembly of ACA molecules is constrained by the hydrogen-bonding interactions to maintain the defined chirality. It is interesting that the subsequent C60 molecules never occupy the ACA trimer of opposite chirality inside one domain even though the spacing is large enough to avoid steric C60-C60 repulsion. It is likely that the initial symmetrybreaking event also induces configurational differences between the two ACA trimers. The STM observations show that in the C60-occupied trimers (both S and R chirality), the long axes of component ACA molecules are pointed along the [112h] directions of Ag(111), whereas for C60-free trimers, the long axes of component ACA molecules are pointed 2210
along the [11h0] directions. The different interaction with the Ag(111) lattice may break the energy degeneracy of the trimers and make the latter energetically unfavorable for C60 occupation. Consequently, the S and R chiral phases show preferred domain orientations with respect to the substrate. The substrate thus plays a decisive role in setting the orientation of chiral domains. Previous observations of surface chirality (class 1 and class 2 systems) have been restricted to the adsorption of single chiral or symmetry-breaking achiral molecules. In the present mixed-phase system, surface chirality is introduced by coadsorption of the decidedly achiral C60, which nucleates chirality by the enantioselective positioning over a racemic mixture of ACA molecules. For the intermixed ACA and C60 system, the resulting extended chirality is limited only by the finite domain size. The chiral C60-ACA phase provides a model for developing chiral aromate surfaces for technical applications. For these particular aromate components, the distance between neighboring C60 molecules is 2.65 nm, dictated by the packing of ACA molecules. The possibility of shrinking or expanding this separation by the choice of aromate (e.g., isonicotinic acid or pentanoic acid) offers the intriguing possibility of tuning the lattice constant. Moreover, because Nano Lett., Vol. 5, No. 11, 2005
C60 can be modified by the subsequent deposition of calixerene moieties,29 opportunity abounds to adjust the molecular landscape in the vicinity of the periodic chiral centers. Mixed phase systems thus offer a new strategy for generating chiral surfaces that offers potential flexibility in tuning both the physical and electronic properties of the chiral structure, as will be needed for nanoarchitecture30 and device applications.16,31 Acknowledgment. We thank P. DeShong and J. Davis for discussions. This work has been supported by the National Science Foundation under the MRSEC grant DMR00-80008 and under CHE-01-36401. Supporting Information Available: (1) STM image of separated phases when C60 was deposited before ACA, (2) low-temperature high-resolution STM image of C60 and ACA intermixed structure demonstrating the threefold symmetry of C60, (3) STM image showing the 2D ACA gas phase following a temperature quench to 40 K, and (4) magnified STM image of juxtaposed ACA-C60 60° rotational domains with the proposed molecular packing model, including molecular registration to the Ag lattice. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Mulligan, A.; Lane, I.; Rousseau, G. B. D.; Johnston, S. M.; Lennon, D.; Kadodwala, M. Angew. Chem., Int. Ed. 2005, 44, 1830. (2) Humblot, V.; Lorenzo, M. O.; Baddeley, C. J.; Haq, S.; Raval, R. J. Am. Chem. Soc. 2004, 126, 6460. (3) Fasel, R.; Parschau, M.; Ernst, K. H. Angew. Chem., Int. Ed. 2003, 42, 5178. (4) Spillmann, H.; Dmitriev, A.; Lin, N.; Messina, P.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2003, 125, 10725. (5) Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval, R. Nature 2000, 404, 376. (6) Kuhnle, A.; Linderoth, T. R.; Hammer, B.; Besenbacher, F. Nature 2002, 415, 891. (7) Switzer, J. A.; Kothari, H. M.; Poizot, P.; Nakanishi, S.; Bohannan, E. W. Nature 2003, 425, 490.
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