12590
2008, 112, 12590–12593 Published on Web 07/30/2008
Isomeric Discriminating and Indiscriminating Assembly of Adsorbed Oligothiophenes on Ag(110) Takashi Yokoyama,*,† Saki Kurata,† and Shoji Tanaka‡ Department of Nanoscience and Technology, Yokohama City UniVersity, 22-2 Seto, Kanazawa-ku, Yokohama 236-0027, Japan, and Institute for Molecular Science, and Japan Science and Technology Agency (JST-CREST), 38 Nishigo-naka, Myodaiji, Okazaki 444-8585, Japan ReceiVed: June 11, 2008; ReVised Manuscript ReceiVed: July 18, 2008
Selective discriminating and indiscriminating assembly of isomeric oligothiophene derivatives is performed on a Ag(110) surface. The self-assembled structures are directly analyzed by using scanning tunneling microscopy. We demonstrate that the phase separation and phase mixing of s-cis- and s-trans-oligothiophenes can be controlled by shape-complementary intermolecular interactions associated with the overall molecular shapes. The overall shapes of the s-cis- and s-trans-oligothiophenes are tuned by the length of alkyl side chains. The spontaneous separation of stereoisomers through molecular crystallization has been discovered by Pasteur. Recently, this fundamental concept has been applied in two-dimensional systems on surfaces,1-7 in which the phase separation of isomeric molecules has been occasionally observed in selfassembled layers. In particular, scanning tunneling microscopy (STM) has allowed one to successfully determine conformation,8,9 configuration,10 and chirality2-4,11 of constituent molecules within the self-assembled layers. Nevertheless, controlled isomeric selective assembly on surfaces has not been reported so far. Here, we show selective discriminating and indiscriminating assembly of s-cis- and s-trans-oligothiophene isomers on a surface. The isomeric discrimination is controlled by overall molecular shape anisotropy, the degree of which is tuned by alkyl side chain length. We find that the s-cis and s-trans conformational isomers of anisotropic-shaped oligothiophenes are separately ordered into close-packed molecular layers on Ag(110). In contrast, the molecular layers of isotropic-shaped oligothiophenes are composed of intermixed isomers. This work provides a method for controlling the isomeric selective assembly on a surface by the length of the alkyl side chains, which will lead to the rational design and construction of a wide range of molecular architectures on surfaces. R-Oligothiophene is one of the most extensively studied π-conjugated molecules because of its characteristic electronic and optical properties.12,13 Nevertheless, the rotational flexibility at the thiophene-thiophene single bonds causes two distinct conformational forms,14,15 in which the thiophene rings are oriented parallel for an s-cis conformer and antiparallel for an s-trans conformer. The s-trans form has been calculated to be more stable than the s-cis form in energy only by abou 0.5 kcal/ mol.16 The energy barrier for the rotation is about 1.9 kcal/ * To whom correspondence should be addressed. E-mail: tyoko@ yokohama-cu.ac.jp. † Yokohama City University. ‡ Institute for Molecular Science, and Japan Science and Technology Agency.
10.1021/jp805116v CCC: $40.75
Figure 1. Chemical structures of (a) s-trans- and (b) s-cis-R-6T-Si-R, in which R-sexithiophene is substituted with N-silyl groups and various alkyl side chains. The shape anisotropy of the conformational isomers is characterized with the offset ratio b/a. When b/a is close to 1.0, the s-cis and s-trans forms exhibit an almost identical shape.
mol.16 Although the stable s-trans conformer has been exclusively observed in the crystal phase,17 the metastable s-cis conformer of substituted R-oligothiophenes has been partially observed in an adsorbed state.18 In this study, we have used R-sexithiophenes substituted with N-silyl groups and various alkyl side chains (R-6T-Si-R), as shown in Figure 1, which are designed as basic components of single molecular devices.19 In R-6T-Si-R, the two bulky N-silyl substituents protect the π-conjugated backbone against unexpected chemical reactions. The conformational isomerization of R-6T-Si-R is caused by the internal rotation at the central thiophene-thiophene bond, whereas other thiophene-thiophene bonds should be fixed with the s-trans form by the steric hindrance between adjacent bulky substituents. Due to the difference in shape and size of the N-silyl groups and alkyl chains, the s-cis and s-trans isomers of R-6T-Si-R have distinct overall shapes, which are identified from the relative orientations of two N-silyl groups as shown in Figure 1a and b. The s-cis conformer has a rectangular overall shape with C2V symmetry, independent of the length of the alkyl 2008 American Chemical Society
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Figure 2. (a) Large-area STM image (70 nm × 70 nm) at 63 K of self-assembled R-6T-Si-Dod on Ag(110). Two different arrangements of R-6T-Si-Dod with single and double molecular rows coexist on the surface. High-resolution STM images (12 nm × 12 nm) and the molecular models of (b) single and (c) double molecular rows. These arrangements are commensurate with the substrate lattice and further divided by the mirror symmetry with respect to the [11j0] direction of the Ag(110) surface. In these arrangements, s-trans and s-cis forms are separately assembled into single and double molecular rows, respectively. The additional chiral separation of the s-trans forms is observed between the left and right panels in (b).
side chains. In contrast, the s-trans conformer includes an offsetrectangular shape with C2h symmetry, which originates from the different dimensions between the N-silyl and alkyl groups. To evaluate the degree of the deviation from the rectangular shape, we define the offset ratio, b/a, where a and b indicate the dimensions of the N-silyl and alkyl groups, respectively (see Figure 1a). The offset ratio can be systematically controlled by changing the alkyl side chain length. Here, we used a series of R-6T oligomers with ethyl (Et), propyl (Pro), hexyl (Hex), and dodecyl (Dod) side chains, in which the offset ratio, b/a, varies from 0.8 for the ethyl chains (R ) Et) to 2.8 for the dodecyl chains (R ) Dod). Imaging and isomeric identification of R-6T-Si-R were carried out with a STM operated at 63 K under an ultrahigh-vacuum (UHV) condition. A single-crystal Ag(110) surface was used as a substrate that was cleaned by repeated cycles of Ar+
sputtering and annealing. The molecules were deposited by sublimation from a Knudsen cell in UHV onto the clean Ag(110) surface held at room temperature. The sample was subsequently transferred to the cold STM stage without thermal annealing. All STM images were acquired in a constant-current mode of 5.0 pA at 63 K. Figure 2a shows a STM image of R-6T-Si-Dod with an offset ratio of b/a ) 2.8 on the Ag(110) surface, in which a selfassemble molecular monolayer is completed. Two different arrangements are clearly observed in the molecular layer, which are composed of single or double molecular rows with an interchain spacing of about 2.4 or 4.6 nm as shown in Figure 2b and c, respectively. We also find that each molecular row is divided further into two orientations due to the mirror symmetry with respect to the [11j0] direction of the Ag(110) surface. In the STM images, the bright protrusions are assigned as the bulky
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N-silyl groups,18 and the dodecyl side chains with extended conformations appear only faintly. On the other hand, the central thiophene wire cannot be resolved. The s-cis and s-trans conformers can be identified from the relative positions of the two bright protrusions (N-silyl groups) within a molecule. From these results, the arrangements are well understood as a result of the phase separation of the s-cis and s-trans conformational isomers of R-6T-Si-Dod. As depicted in the assembled model in Figure 2b, the s-trans conformer is formed into the single molecular rows, where the paired dodecyl side chains of R-6TSi-Dod are interdigitated between neighboring molecular rows. This assembly results from a head-to-tail contact between the N-silyl groups and dodecyl chains. In contrast, the double molecular rows are associated with a preferential head-to-head contact between adjacent N-silyl groups (or dodecyl side chains) of the s-cis conformers, leading to the enlarged inter-row spacing, as shown in Figure 2c. These superstructures are commensurate with the Ag(110) surface and are described by the matrix notion
(
5 -3 2 8
)
and
(
5 3 -2 8
)
with respect to the substrate lattice for each mirror domain of the s-trans forms and
(
-1 16 6 -1
)
( ) 1 16 6 1
and
for the s-cis forms. In these assemblies, the central thiophene wires are almost aligned in the chain directions, and the packing density of 0.19 molecules/nm2 for the s-trans phase is almost identical with that of 0.18 molecules/nm2 for the s-cis phase. Since both conformers are closely packed in the self-assembled layers, the spontaneous phase separation into the isomerically pure domains should be associated with the shape-complementary van der Waals intermolecular interactions arising from the different overall shapes between the s-cis and s-trans conformers. Furthermore, we have found that the s-trans conformer is additionally classified into two enantiomeric forms, which are characterized as prochirality induced by the adsorption to the surface. Between the mirror-symmetric domains in Figure 2b, the offset rectangles of the constituent molecules cannot be superimposed by the lateral rotation or translation, so that they are chiral on the surface. In contrast to the additional chiral separation of the s-trans forms, the s-cis conformer is achiral on the surface. It contains the mirror symmetry even in the adsorption state, and thereby, each orientation of the molecular rows in Figure 2c is composed of an identical isomer. The discriminating assembly changes dramatically when an isotropic-shaped R-6T-Si-Pro is used. The offset ratio b/a of R-6T-Si-Pro is approximately 1.0, which indicates identical rectangular shapes of the s-cis and s-trans forms. As shown in Figure 3a, we have observed that only a single domain of R-6TSi-Pro is assembled on the Ag(110) surface, although two orientations arising from the substrate mirror symmetry are still formed. The superstructures are described by the matrix notion
(
5 3 -1 4
)
and
(
5 -3 1 4
)
for each mirror domain. In the high-resolution STM image of Figure 3b, it is obvious that achiral s-cis and prociral s-trans conformers coexist, which are identified by the relative positions of the paired protrusions. Whereas these conformers are randomly distributed within the constitutive lattice, the assembled structure appears to be well ordered, commensurate
Figure 3. (a) Large-area STM image (40 nm × 40 nm) at 63 K of self-assembled R-6T-Si-Pro with the size ratio of b/a ) 1.0 on Ag(110). (b) High-resolution STM image (12 nm × 12 nm) and the molecular models of R-6T-Si-Pro on Ag(110). Two different orientations of molecular rows originate from the mirror symmetry of the substrate. Within the assembled layer, achiral s-cis and prochiral s-trans conformers are randomly distributed, leading to the formation of isomerically mixed domain.
with the substrate lattice. This leads to the formation of the isomerically mixed domains through the isomeric indiscriminating assembly of R-6T-Si-Pro. Furthermore, by using shorter ethyl chains, the isotropic rectangular shape is slightly modified in R-6T-Si-Et, where the offset ratio b/a is about 0.8. In this case, both isomerically mixed and s-trans pure domains are observed on the surface, as shown in Figure 4, whereas the s-cis pure domains are not observed. These results indicate that the isomeric discrimination is controllable as a function of the offset ratio. Since the selectivity of the isomeric discrimination is correlated with the offset ratio b/a of the overall molecular shape, the head-to-head and head-to-tail intermolecular interactions are expected to be almost identical. Both intermolecular connections are actually formed in the assembled structures of R-6T-Si-R. In addition, for anisotropic R-6T-Si-Hex with b/a ) 1.7, the
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Figure 4. Summary of the discriminating and indiscriminating assemblies of R-6T-Si-R on Ag(110) as a function of the alkyl side chain length.
s-trans pure domain is associated with the head-to-head contact (see Supporting Information, Figure S2), unlike the head-totail contact for R-6T-Si-Dod. Since the head-to-head and headto-tail contacts are changed by the delicate balance of the alkyl side chain length, the quasi-isotropic intermolecular interactions are suggested. Thus, the isomerically discriminating and indiscriminating assemblies of R-6T-Si-R should arise from the simple shape-complementary effects, which are precisely tunable with the alkyl side chain length. The phase separation and mixing of the isomers are controlled by the overall molecular shapes as characterized by the offset ratio of b/a. In addition, we have also observed similar selective discriminating assemblies of R-6T-Si-R on Au(111), indicating that the atomic species and symmetry of the substrate surface play a minor role in the selective assembly. The isomeric and/or chiral selective assembly on surfaces is of central importance in the rational construction of the functional molecular layers. Since the conformational flexibility in large functional molecules generally induces various isomeric and chiral forms, the rational control of the alkane-assisted shape complementarity will provide an important advance in designing molecular nanoarchitectures on surfaces. Acknowledgment. This work was, in part, supported by Grants in Aid from the Japanese Society for the Promotion of Science and by the Grants for Strategic Research Project of Yokohama City University. Supporting Information Available: The high-resolution STM images of a series of R-6T-Si-R (R ) Et, Pro, Hex, Dod) on Ag(110). This material is available free of charge via the Internet at http://pubs.acs.org.
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