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Functional Control on the 2D Self-Organization of Phenyleneethynylenes† K. Yoosaf,‡ A. R. Ramesh,‡ Jino George,‡ C. H. Suresh,§ and K. George Thomas*,‡ Photosciences and Photonics Group, and Computational Modeling and Simulation Section, National Institute for Interdisciplinary Science and Technology (CSIR) (formerly, Regional Research Laboratory), TriVandrum 695 019, India ReceiVed: March 1, 2009; ReVised Manuscript ReceiVed: April 21, 2009
Two-dimensional self-organization of a series of phenyleneethynylenes was investigated, at ambient conditions, by varying the length of alkoxy chain and introducing functional groups at the terminal positions using highresolution scanning tunneling microscopy (STM). The model phenyleneethynylene molecule, which does not possess any functional groups, self-organizes into wire like structures on surface. High-resolution STM imaging revealed that molecules are arranged in a skewed 1D fashion. The spacing between the molecular wires was successfully modulated by replacing hexyloxy (C6) chains with dodecyloxy (C12) chains. The initial step of the formation of all the molecular assemblies involves the alkyl CH · · · acetylenic π interactions (CH · · · π) leading to the organization of molecules as two types of strips. These strips further interlock to two types of 2D organizations. The hydroxyl as well as aldehyde groups present at the terminal positions of the phenyleneethynylene molecules play an important role in the interlocking process. An end-to-end assembly was observed in the case of phenyleneethynylene molecule possessing hydroxyl groups at the terminal positions, which is attributed to the intermolecular hydrogen bonding between the strips. The adsorption of molecules with two faces results in enantiomeric 2D structures and these aspects were investigated using molecular modeling studies. Introduction Organization of molecules on surfaces plays a crucial role in the fabrication of optoelectronic devices such as organic light emitting diodes, thin film transistors, and photovoltaic cells.1,2 The efficiency of these systems is often controlled by the way in which molecules are arranged on surfaces. Among the various methods adopted, self-organization of molecules has been emerged as an efficient strategy for the creation of well-defined molecular architectures.3,4 Molecules when adsorbed onto the surface experience several modes of interactions, which include the relatively weak van der Waals interactions between the molecules and probable electronic interactions of the adsorbate with the surface. Scanning tunneling microscopy (STM) is the most outstanding tool for atomic scale resolution imaging and manipulation of molecules on surfaces.5 An understanding on the molecular level arrangements of physisorbed layers using STM can provide insight on various molecular interactions.5–9 Photoactive as well as electroactive molecules assembled on surfaces form the basic building blocks in various optoelectronic devices. Efforts have been made in recent years to understand the molecular level organization of these systems on surfaces and their responses as a function of external stimuli.10 Some of the recent examples include the light induced changes on the organization of photochromic molecules such as azobenzene, stilbene, diarylethene, and cinnamate when adsorbed on various surfaces.11–16 The overall structure and stability of the molecular assembly on the surface is determined by various strong and weak intermolecular interactions such as dipole-dipole interactions, hydrogen bonding, coordinate bonding, host-guest †
Part of the “Hiroshi Masuhara Festschrift”. * Corresponding author. Tel: +91-471-2515364. Fax: +91-471-2490186. E-mail:
[email protected]. ‡ Photosciences and Photonics Group. § Computational Modeling and Simulation Section.
interactions, and alkyl chain interdigitations.6,17–20 In the absence of strong intermolecular interactions, interdigitation of alkyl chains plays a crucial role in the formation of molecular assembly on the surface. For example, self-organization of dehydrobenzo[12]annulene derivatives is highly dependent on their geometry, and interdigitation of alkyl chains plays a major role in their assembly.21–23 Polycyclic aromatic hydrocarbons such as coronene which possess high charge-carrier mobility among discotic liquid crystals showed close packed hexagonal structures on surfaces such as graphite, Au, and Ag.24–27 Noncovalent interactions such as hydrogen bonding (e.g., terephthalic acid, isophthalic acid, and trimesic acid) 28–35 and metal-organic coordination36 networks play an important role in the molecular self-assembly due to their high selectivity and directionality. Phenyleneethynylenes are another interesting class of molecular systems, which possess a rigid rod structure and are widely used in the design of functional molecular materials.37–39 This class of molecules is proposed as an ideal linker for electronic communication due to the cylindrical symmetry of the acetylene unit, which maintains the π electron conjugation at any degree of rotation.40–46 Recent studies on the selforganization of phenyleneethynylenes, from our group47 and others,48–58 have revealed that these systems can well organize on a surface through noncovalent intermolecular interactions. Based on our studies, we have concluded that various noncovalent interactions (particularly CH · · · π) drive the phenyleneethynylene into wire-like structures on 2D surfaces.47 The main objective of the present investigation is to modulate the 2D organization of this class of molecular system by varying the length of the alkoxy chain and introducing proper functional moieties. Herein we report the self-organization of a few phenyleneethynylenes (1-5 in Chart 1) on a highly oriented pyrolitic graphite (HOPG) surface, (i) by replacing hexyloxy
10.1021/jp901884v CCC: $40.75 2009 American Chemical Society Published on Web 05/18/2009
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CHART 1: Structures of Various Phenyleneethynylene Derivatives under Investigation
(C6) chains with dodecyloxy (C12) chains and (ii) by introducing hydroxyl and aldehyde groups at the terminal positions. The various intermolecular interactions leading to the molecular assembly were investigated, with the aid of molecular modeling. Results and Discussion We have probed earlier the 2D organization of a model phenyleneethynylene molecule (1) on HOPG surface. The existence of two types of molecular arrangements arising from two different modes of alkyl CH · · · acetylenic π (CH · · · π) interactions were observed.47 CH · · · π interactions in phenyleneethynylene 1 result in the formation of molecular strips, which further interlocks into two types of 2D oganization (type-I and type-II arrangements). These aspects were discussed in our earlier communication.47 In the present study, we have further investigated the organization of a series of phenyleneethynylenes on the HOPG surface by replacing hexyloxy (C6) chains with dodecyloxy (C12) chains and introducing hydroxyl and aldehyde groups at the terminal positions (Chart 1). The samples were prepared through drop casting to achieve an organization with thermodynamic control, and it was found that the phenyleneethynylenes (1-5) self-organizes as domains on HOPG surface. We have analyzed these domains by varying the scan size and specific domains were located by changing the X and Y positions of the sample. All of the images presented in Figures 1-6 showed well-defined bright rod like structures having an average length of 1.8 ( 0.1 nm. The electron tunneling along the longitudinal axis of phenyleneethynylene aromatic units is high due to the large electron density of the π-cloud, compared to the alkyl regions. The length of bright rod-like structures in the STM images is in good agreement with the molecular length of the phenyleneethynylene core (1.844 nm) calculated from the X-ray crystal structure42 of the model compound (1). Thus, the bright rod-like structures are identified as the phenyleneethynylene core of 1-5. Due to the large energy difference between the electronic states of the aliphatic chain and the Fermi level of the substrate, the alkyl chain regions of 1-5 appeared as dark.17,19,59,60 Effect of Alkoxy Chain Length. STM imaging was carried out by replacing hexyloxy (C6) chains with dodecyloxy (C12)
chains, to investigate the influence of the alkoxy chain length on the molecular organization of phenyleneethynylenes. The corresponding STM current images of dodecyloxy derivative (2) and an illustration of various molecular interactions leading to skewed type molecular packing are presented in Figure 1. As anticipated, the distance between the two adjacent molecular wires in the dodecyloxy (C12) derivative has increased to 2.2 nm, which is necessary to accommodate the -OC12H25 chain in its extended conformation. In the case of hexyloxy derivative 1, we have earlier reported that the distance between the two parallel wires is 1.3 nm.47 On the basis of STM and molecular modeling studies, we have found that in the case of the dodecyloxy derivative 2, two types of CH · · · π interactions exist when phenyleneethynylenes interact along a-strip, as observed in 1. With respect to dodecyloxy groups, the alkyl chain of the adjacent molecules can interact with the acetylenic moiety, either from the ortho or meta position (cf. Figure 1), leading to the formation of two types of a-strips (x and x′). These x and x′ strips further interlocks into type-I and type-II arrangements, respectively. The interaction from the ortho position in x provides more room for interlocking. Therefore in the type-I arrangement, the end phenyl group of one molecule can be interlocked up to the acetylenic region of the neighboring strip. In contrast, interaction from the meta position in x′ does not allow effective interlocking in type-II packing. A close analysis of STM images further showed that the angle of orientation of phenyleneethynylene core (bright features) with respect to the a axis is 70° for Figure 1A (type-I) and 84° for Figure 1B (typeII). In order to have a better understanding, the type-I and typeII arrangements were optimized at the AM1 level of the semiempirical method with a pack of six molecules.61 The angle of orientation (66° for type-I and 86° for type-II) of the molecules along the a-strip in the AM1 optimized geometries (Figure 1C and 1D) matches well with the experimentally obtained values from STM images. These studies clearly suggest that the spacing between the molecular wires can be further fine-tuned by the appropriate choice of the alkoxy groups. Chiral Molecular Arrangements. In domains having a type-I arrangement, phenyleneethynylenes always orient at 70° with respect to the a axis. Careful analysis of various domains
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Figure 1. Schematic representation of the formation of a-strip by two possible modes of alkyl CH · · · acetylenic π interactions (x and x’) and further interlocking results in the 2D organization (y and y′) of 2. STM current images showing the (A) type-I and (B) type-II arrangements; scan-size: 5 × 5 nm2; Vbias ) -1602 mV; It ) 324 pA. AM1 geometry optimized structures of (C) type I and (D) type II arrangements.
having type-I arrangement resulted in an important observation: one can further classify them into type-IA and type-IB domains in which phenyleneethynylenes are arranged in clockwise and counterclockwise orientations, as illustrated by arrows in Figure 2. Type-1A and type-1B arrangements are mirror images, which are nonsuperimposable. Such surfaces with a chiral signature, obtained by the adsorption of molecular systems, are of interest from both fundamental and practical points of view. It is well understood that chirality is preserved when two-dimensional (2D) crystals are formed through the adsorption of chiral molecules on the surface.62 In a recent study, Meijer and coworkers have demonstrated the self-organization of chiral oligo(p-phenylenevinylene) molecules on 2D surfaces into enantiomeric pure domains.63 The chiral arrangement of achiral molecules when adsorbed on the surface arises from the confinement of molecules in two dimensions, which removes the mirror symmetry in the plane of the substrate. Such molecules are said to be prochiral, which on adsorption on the surface can lead to equal amounts of mirror
image enantiomers. Recent research reports have shown that adsorption of prochiral molecules on surfaces lead to enantiomeric domains through symmetry breaking.64 Various reports on the formation of enantiomeric domains include (i) diarylethenes,65 (ii) 1,5-bis(3-thiaalkyl)anthracenes,66 (iii) oligo phenyleneethynylenes,67 and (iv) star-shaped phenyleneethynylenes.53 The enantiomeric phase separation in such systems has been ascribed to the lateral mass transport in combination with intermolecular chiral recognition at domain boundaries. Linderoth and co-workers investigated the chiral ordering and conformational dynamics of three oligo(phenyleneethynylene) derivatives, using time-resolved STM on a Au(111) surface.67 These molecular systems vary in geometry but contain the same functional moieties. Authors have observed two levels of chirality (organizational and intramolecular) resulting from the adsorption of these molecules onto the surface. Interestingly, these systems undergo thermally induced conformational changes due to the rotation of the bulky molecular end group, which
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Figure 2. STM current images of 2 illustrating enantiomeric assembly: (A) type-IA arrangement, Vbias ) -872 mV; It ) 717 pA and (B) type-IB arrangement, Vbias ) -872 mV; It ) 717 pA.
results in the switching between chiral and nonchiral forms or between opposite surface enantiomers. In order to investigate these aspects, different possibilities of interaction of achiral molecule 2 on the surface were analyzed using molecular modeling studies. The STM data in combination with molecular modeling calculations (Figures 2 and 3, respectively), indicate that the alkoxy side chains prefer an extended conformation. The molecule in this conformation has a symmetric planar structure with C2h point group (symmetry elements: S2, C2, Cs, and Ci). In the present case, the two faces of 2 are interchangeable only by the action of (i) symmetry plane, (ii) center of symmetry, or (iii) the alternating axis of symmetry and not by any other axis of symmetry. Hence, the two faces of 2 are enantiotopic and prochiral. The adsorption of molecule 2 onto the planar HOPG substrate removes its center of symmetry, mirror symmetry, and alternating axis of symmetry. Hence the two modes of adsorption of 2 on the HOPG surface, with either faces, results in energetically similar but nonsuperimposable mirror images (Figure 3B,B′). During the formation of a-strips, the interaction of the second molecule of 2 having similar enantiotopic face is thermodynamically more favored. This results in an ordered and compact arrangement with relatively strong intermolecular interactions (Figure 3C,C′). 2D organization is formed through the interlocking of two similar enantiomeric a-strips, which are thermodynamically favorable (Figure 3D,D′). Various steps leading to the 2D mirror image arrangements are (a) the formation of enantiomeric systems on to the surface (B,B′), (b) enantiomeric a-strip formation through the alkoxy interdigitation (C,C′), and (c) enantiomeric domains formation (D,D′) resulting from the interlocking of a-strips. The steps presented in Figure 3 were optimized at the AM1 level of the semiempirical method61 (note that the underlying HOPG surface is not optimized). Thus, the adsorption of molecule with two faces result in enantiomeric 2D structures as observed in the STM images. Effect of Functional Group. Another objective of the present investigation is to modulate the self-organization of molecules on surfaces by varying the functional groups. With this view, STM current images of phenyleneethynylene derivatives bearing hydroxyl and aldehyde groups at the terminal positions (3-5 in Chart 1) were recorded. The various intermolecular interactions leading to their assembly were analyzed with the aid of molecular modeling. Role of the Hydoxyl Group. STM current image of the hydroxyl substituted phenyleneethynylene possessing hexyloxy chains (3) at large scan size (75 × 75 nm2) is presented in Figure 4A. As in the previous cases, parallel wire-like structures, having
a regular spacing of 1.3 nm, were observed. A close analysis of the image of 3 at smaller scan size showed several similarities with that of unsubstituted phenyleneethynylene derivatives 1 and 2: (i) formation of a parallel strip like arrangement along the a axis (a-strip) and (ii) a 1D arrangement along the b axis (b-strip). In contrast to the skewed 1D arrangement observed in the case of 1 and 2, compound 3 showed an end-to-end arrangement. The end-to-end organization observed in the present case arises from the hydrogen bonding interaction of the hydroxyl groups present at the terminal positions of the phenyleneethynylene. The bright rods, assigned as phenyleneethynylenes, are arranged in an end-to-end fashion along the b axis with a small gap between the molecules. The small gap between the molecules, corresponding to the hydrogen bonded regions, appeared less bright in STM images (with slight steps). This may be due to the fact that the CH2OH group at the terminal positions of 3 possesses lower electron density and hence less conducting. A close analysis of the STM current images showed the presence of two types of organizations (Figure 4A) with respect to the a axis (labeled as type-I and type-II). Real time zoomed images of two regions showing different types of organization are presented in panels B and C in Figure 4. In both cases, molecules are arranged parallel along the a axis; however, they are oriented ∼60° in type-I and ∼85° in type-II arrangement. From these results, it is clear that the two organizations arise from the two possible modes of interdigitation of hexyloxy chains. These aspects were further investigated through molecular modeling experiments. Molecular modeling experiments were performed by geometry optimization (AM1 level) with a pack of six molecules, corresponding to each assembly.61 The difference in molecular interaction along a-strip results in two types of arrangements. As in the case of unsubstituted compounds, CH · · · π hydrogen bonding, through ortho and meta positions, is the primary intermolecular interaction leading to type-I and type-II in 3 (Figure 4, panels D and E, respectively). Interdigitation of alkoxy chains and the resulting CH · · · O hydrogen bonding interaction between alkyl CH and hexyloxy oxygen, further stabilizes the type-I arrangement. A careful analysis of the geometry optimized structures presented in Figure 4, panels D and E, revealed an end-to-end arrangement due to OH · · · OH hydrogen bonding interaction (between CH2OH groups) along b-strip. Based on molecular modeling experiments, it was found that the phenyleneethynylene core makes an angle of 61° in type-I and 85° in type-II assembly (with respect to the a axis), which is in good
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Figure 3. Various energy optimized geometries of 2 and their assemblies: (A and A′) two enantiotopic faces of 2 in gas phase (which are superimposable mirror images through 180° rotation or reflection); (B and B′) adsorption of 2 on to the surface with the two faces resulting in the formation of enantiomeric systems (which are nonsuperimposable mirror images); (C and C′) formation of enantiomeric a-strip through the alkoxy interdigitation; (D and D′) interlocking of a-strips into enantiomeric domains.
agreement with the observed values in the STM images (Figure 4, panels B and C). The length of the alkoxy group was varied by replacing hexyloxy chains (3) with dodecyloxy chains (4) in order to investigate the influence of the alkyl chain length on the selforganization of hydroxyl substituted phenyleneethynylene. STM images observed in the case of 4 revealed an end-to-end arrangement, similar to that of 3. A representative image of the type-II arrangement is shown in Figure 5A and the two consecutive parallel wires (b-strips) were separated by a larger distance of 2.2 nm due to the increase in chain length of the alkoxy group in its extended conformation. Corresponding optimized geometry is presented in Figure 5B. Role of Aldehyde Group. STM imaging of phenyleneethynylene 5 having aldehyde groups at the terminal positions was
carried out under similar conditions and presented in Figure 6. Both type-I and type-II molecular arrangements were observed at smaller scan size (6 × 6 nm2; Figure 6B,C), as in the case of unsubstituted phenyleneethynylenes 1 and 2. In both of the cases, the molecules possess a skewed wire-like arrangement along the b axis and a strip-like arrangement along the a axis. As in the previous cases (1-4), the CH · · · π interaction between two adjacent molecules occurs through the ortho and meta positions. This is obvious from the constant intermolecular distance of 1.3 ( 0.1 nm between two consecutive parallel wires (b-strips). The type-I and type-II arrangements in 5 showed an orientation of 55° and 82°, respectively. The extent of interlocking between the a-strips plays an important role in the 2D self-organization of phenyleneethynylenes. In the case of unsubstituted phenyleneethynylenes, the
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Figure 4. STM current images of 3 on HOPG substrate and the optimized model geometries at AM1 level: (A) Large scan size (75 × 75 nm2) STM image where both type-I and type-II organization separated by a boundary: Vbias ) -1215 mV; It ) 318 pA. (B) type-I organization; scan size 5 × 5 nm2; Vbias ) -966 mV; It ) 360 pA and (C) type-II organization; scan size 10 × 10 nm2; Vbias ) -900 mV; It ) 700 pA. Optimized structures of (D) type-I and (E) type-II assemblies.
Figure 5. (A) STM current images of 4 on HOPG substrate: scan size 7 × 7 nm2; Vbias ) -1384 mV; It ) 348 pA and (B) Optimized geometry at AM1 level for the meta interactions of alkoxy chains and the hydrogen bonding between the terminal hydroxyl groups.
extent of interlocking is more effective in type-I arrangement wherein the CH · · · π hydrogen bonding occurs through the ortho position. In contrast, the STM image of aldehyde substituted phenyleneethynylene molecule 5 showed more efficient interlocking for type-II arrangement wherein the CH · · · π hydrogen bonding occurs through the meta position. This difference in interlocking may be due to the presence of the aldehyde group at the terminal position of 5. In order to have a better understanding on the mode of hydrogen bonding in the interlocking step, we have further carried out the molecular modeling by incorporating various possibilities of the hydrogen
bonding of carbonyl oxygen with the hydrogen atoms on the (i) aromatic ring, (ii) alkyl chain, and (iii) carbonyl group on the adjacent phenyleneethynylene. AM1 optimized structures showed that the interlocking between two strips is controlled by weak hydrogen bonding interaction between the carbonyl oxygen atom and the hydrogen atom of the aromatic ring/ hexyloxy chain. The interaction between the carbonyl oxygen and aromatic CH (meta H with respect to the acetylene group) is more favored in a type-I arrangement (Figure 6D), whereas the interaction between the carbonyl oxygen atoms with the hydrogen atoms of the hexyloxy chain results in a type-II
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Figure 6. STM current images of 5 on HOPG substrate and the molecular modeling geometries: (A) Large scan size STM image; scan size 75 × 75 nm2; Vbias ) -900 mV; It ) 1200 pA. (B) type-I arrangement; scan size 6 × 6 nm2; Vbias ) -500 mV; It ) 432 pA and (C) type-II arrangement; scan size 6 × 6 nm2; Vbias ) -913 mV; It ) 405 pA. AM1 level optimized structures of type-I (D) and type-II (E) assemblies.
arrangement (Figure 6E). The interaction in type-II arrangement results in a more effective interlocking between two a-strips. Conclusion Phenyleneethynylene derivatives form well-organized assemblies on HOPG surface and their organization was modulated by (i) by replacing hexyloxy (C6) chains with dodecyloxy (C12) chains and (ii) introducing functional groups at the terminal positions. The initial step of the self-organization in all the systems under investigation is the formation of a-strip, assisted by the CH · · · π interaction. The CH · · · π interaction can occur through ortho and meta positions leading to the formation of type-I and type-II arrangements. The distance between two molecules in a-strip can be varied by increasing the alkoxy chain length. Interlocking of a-strips results in the formation of 2D assembly through the weak intermolecular interactions and the functional groups at the terminal positions of phenyleneethynylenes play a decisive role in the way in which they interlock. In the case of unsubstituted phenyleneethynylenes (1 and 2), the extent of interlocking of a-strips is more effective in type-I arrangement. In contrast, aldehyde substituted phenyleneethynylene molecule 5 showed more efficient interlocking in the type-II arrangement resulting from the interaction between the carbonyl oxygen atoms of aldehyde group and hydrogen atom of hexyloxy chain. Interestingly an end-to-end organization was observed in the case of hydroxyl substituted phenyleneethynylenes (3 and 4) through OH · · · OH hydrogen bonding interaction between a-strips. These results clearly indicate that it is possible fine-tune the arrangement of the a-strip by varying the length of the alkoxy chain and the b-strip by introducing proper functional groups. Experimental Section Samples for STM investigations were prepared by drop casting 0.1 mM solution of 1-5 in 1,2-dichlorobenzene on to
a freshly cleaved HOPG surface, followed by drying in air for ∼12 h. STM images were acquired in the constant-height mode on a multimode scanning probe microscope (Nanoscope IV controller, Digital Instruments). Electrochemically etched Pt/Ir wire (80:20) was used as the STM tip. Since the imaging was carried out at the ambient condition, minor dynamic/conformational changes can occur for these molecules during imaging. These aspects can be further investigated by measuring the current as a function of time in the open loop configuration as reported by Kockmann and co-workers.68 Acknowledgment. The authors thank CSIR, UGC, and Department of Science and Technology, Govt. of India (SP/ S5/NM-75/2002) and NWP023 for financial support. This is contribution No. NIIST-PPG-281 from NIIST, Trivandrum, India. Dedicated to Professor Hiroshi Masuhara. References and Notes (1) Koch, N. Chem. Phys. Chem. 2007, 8, 1438–1455. (2) Carroll, R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378–4400. (3) Lehn, J.-M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4763–4768. (4) Hermann, B. A.; Scherer, L. J.; Housecroft, C. E.; Constable, E. C. AdV. Funct. Mater. 2006, 16, 221–235. (5) Samorı`, P. J. Mater. Chem. 2004, 14, 1353–1366. (6) Plass, K. E.; Grzesiak, A. L.; Matzger, A. J. Acc. Chem. Res. 2007, 40, 287–293. (7) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671–679. (8) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600–1615. (9) De Feyter, S.; De Schryver, F. Top. Curr. Chem. 2005, 258, 205– 255. (10) Morgenstern, K. Acc. Chem. Res. 2009, 42, 213–223. (11) Vanoppen, P.; Grim, P. C. M.; Rucker, M.; De Feyter, S.; Moessner, G.; Valiyaveettil, S.; Mullen, K.; De Schryver, F. C. J. Phys. Chem. 1996, 100, 19636–19641. (12) Feng, C. L.; Zhang, Y.; Jin, J.; Song, Y.; Xie, L.; Qu, G.; Jiang, L.; Zhu, D. Surf. Sci. 2002, 513, 111–118.
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