Solvent Effects on Supramolecular Networks Formed by Racemic Star

May 14, 2008 - Two-dimensional networks of star-shaped oligofluorene ... investigated using scanning tunneling microscopy (STM) in solvents with diffe...
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J. Phys. Chem. C 2008, 112, 8649–8653

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Solvent Effects on Supramolecular Networks Formed by Racemic Star-Shaped Oligofluorene Studied by Scanning Tunneling Microscopy Yibao Li,†,‡ Zhun Ma,§ Guicun Qi,† Yanlian Yang,† Qingdao Zeng,† Xiaolin Fan,*,‡ Chen Wang,*,† and Wei Huang*,| National Center of Nanoscience and Technology, Beijing 100080, People’s Republic of China, College of Chemistry and Life Science, Gannan Normal UniVersity, Ganzhou 341000, Jiangxi ProVince, People’s Republic of China, Institute of AdVanced Materials, Fudan UniVersity, 220 Handan Road, Shanghai 200433, People’s Republic of China, and Institute of AdVanced Materials, Nanjing UniVersity of Posts and Telecommunications, Nanjing, 210003, People’s Republic of China ReceiVed: NoVember 27, 2007; ReVised Manuscript ReceiVed: March 2, 2008

Two-dimensional networks of star-shaped oligofluorene end-capped with carboxylic groups (StOF-COOH3) are investigated using scanning tunneling microscopy (STM) in solvents with different polarities and functionality on graphite surface. The high-resolution STM images show that the assembly of StOF-COOH3 is strongly solvent dependent. Well-ordered porous honeycomb networks are revealed at the octanoic acid/ graphite and 1,2,4-tricholrobenzene/graphite interfaces, while an irregular and densely packed structure is observed at the 1-phenyloctane/graphite interface. In n-tetradecane, an intermediate state of the coexistence of a disordered and honeycomb structure is identified on the surface. High resolution STM images revealed two different hydrogen-bonded networks for StOF-COOH3 molecules with 1-octanoic acid as the solvent, including homochiral domains and heterochiral domains. The stabilization of the networks consisting of chiral species at the graphite surface is suggested to be possibly associated with the asymmetrical polar environments because of the ordered arrangements of coadsorbed polar solvent molecules. 1. Introduction Two-dimensional (2D) hydrogen bonded networks on surfaces demonstrate the unique potential of tailoring supramolecular assemblies. In the recently reported works, several types of the hydrogen-bonded two-dimensional nanoporous networks have been obtained by adsorption on highly oriented pyrolytic graphite (HOPG) surfaces. Rigid supramolecular networks formed by 1,3,5-benzenetricarboxylic acid (trimesic acid (TMA)),1,2 perylene tetra-carboxylic di-imide (PTCDI) and melamine3 were shown to accommodate single or clusters of guest molecules with nearly unchanged geometries. On the other hand, building units with flexible segments such as alkyl chains would enable appreciable flexibility of the networks, as shown in several types of star-shaped molecules such as 1,3,5-tris(10carboxydecyloxy) benzene (TCDB),4 (1,3,5-tris[(E)-2-(3,5-didecyloxyphenyl)-ethenyl]-benzene (TSB35),5 and dehydrobenzo[12]annulene (DBA) derivatives.6 A pressing issue in the design of the molecular nanoporous architectures is tuning the cavity size and symmetry that are inherent to the networks in a controlled manner. In order to gain knowledge on controlling the cavity size and symmetry of the network cavities, various approaches have been explored, including molecular structural designs and, more recently, solvent induced polymorphism.7,8 For the latter effect, it has been shown as an effective method to stabilize the molecular networks with carboxyl-terminated building units by tuning the * To whom correspondence should be addressed. E-mail: fanxl@ gnnu.edu.cn (X.F.), [email protected] (W.H.), [email protected] (C.W.). † National Center of Nanoscience and Technology. ‡ Gannan Normal University. § Fudan University. | Nanjing University of Posts and Telecommunications.

hydrophobicity of the solvents. It was also postulated that coadsorption of solvent molecules may also contribute to stabilizing the porous networks. Such a solvent hydrophobicity effect was also illustrated in an earlier study on the selective adsorption and assembling of the triacontane/triacontanol mixtures.9 These explorations have shed light on the needs for optimizing the preparation strategies of molecular networks capable of guest-molecule inclusions. One of the plausible venues for designing the cavity symmetry could be associated with the symmetry of the building units. It has been reported that the symmetry of the constituting molecular building units could have significant impacts on the properties of the molecular architectures, such as mesophase temperature range and molecular alignment behavior.10–12 For example, the substitution symmetry effect in supramolecular networks has recently been reported, showing discernible impacts on the assembling characteristics as well as thermal behavior. In another closely related topic, it has been recognized that adsorption to surfaces could induce chiral molecular conformations and subsequently lead to chiral assembling characteristics.13 Such chirality effects intrinsic to the adsorbed molecules may provide a complimentary approach to the attachment of chiral functional groups to the building units. As the result, the cavity symmetry could be affected. The assembling of supramolecular structures at the liquid-solid interface is controlled by many factors, such as adsorbatesubstrate interaction, intermolecular hydrogen bonding, molecular symmetry, and so forth. At the liquid-solid interface, complex processes and interactions (such as solvent-adsorbate, solvent-substrate, adsorbate-substrate, and adsorbate-adsorbate interactions) exist,9,13–15 and the solvent is believed to play a key role in the assembling processes, especially for the formation of surface nanoporous molecular networks. Recently, we have

10.1021/jp711221u CCC: $40.75  2008 American Chemical Society Published on Web 05/14/2008

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Figure 1. Chemical structures of the molecules: (a) StOF-COOH3, (b) 1,2,4-trichlorobenzene (TCB), (c) 1-octanoic acid, (d) n-tetradecane, (e) 1-phenyloctane.

synthesized a series of rigid star-shaped oligofluorene endcapped with a different number of carboxylic groups, and the effect of molecular symmetry on the two-dimensional assembling structure was studied.16 Here, we focus on the solvent effect on the assembling of one of the derivatives end-capped with three carboxylic groups (StOF-COOH3) (the molecular structure is shown in Figure 1). This molecule is observed to form a well-ordered, highly porous honeycomb pattern at the octanoic acid/graphite interface.16 The structure of StOF-COOH3 represents an expanded analogue of trimesic acid and 1,3,5benzenetribenzoic acid (BTB) which have been previously reported to form nanoporous networks.7,8 The main difference is associated with the attachment of six alkyl chains to the fluorene segments and the possible chiral isomerization when adsorbed on the surface. The addition of the hydrophobic segments to the molecular cores is expected to enhance the effects of hydrophobicity of the solvents, providing an opportunity to further study the interactions between the solvent and the solute molecules. It could be noted that the previously published work on the assembling characteristics of StOF was mainly dedicated to the molecular symmetry due to different numbered end-capped carboxylic groups.16 It was also postulated in that study that solvent effect may be keen to the formation of molecular networks showing discernible chirality, though it was not able to resolve the coadsorption of solvent molecules in that study. Because of the existence of high defect density, it was considered relatively reliable to identify only the domains with 3-fold symmetry cavities, while the domains with 6-fold symmetry could not be exclusively identified because of the possible interference from the adjacent molecular defects. Such difficulties have been overcome in the subsequent studies in search of the explicit solvent effects by optimizing the sample preparation conditions to obtain relatively large areas of homogeneous domains which are essential to differentiate the cavity symmetries. In the current study, solvents with different polarities and functionality are used, and their impacts on the

assembling characteristics are pursued. The results presented in this work allow us to reliably identify the existence of the cavities with 6-fold symmetry in the solvent of 1-octanoic acid. Furthermore, the evidence of the coadsorbed solvent molecules of 1-octanoic acid and n-tetradecane within the cavity of molecular networks could be obtained. We believe the observed results in this work are complementary to the previously published work. 2. Experimental Section The materials used in the experiments are shown in Figure 1. The StOF-COOH3 was synthesized according to the method described in our previous report.16 All solvents were purchased from Aldrich and used without further purification. STM measurements were performed with a Nanoscope IIIa (Veeco Metrology, U.S.A.) with mechanically formed Pt/Ir (80/20) tips. A droplet (2 µL) of solution was dropped on a freshly cleaved highly oriented pyrolytic graphite (HOPG; grade ZYB, Veeco Metrology, U.S.A.) surface and investigated by STM immediately. All images were recorded in constant current mode. The specific tunneling conditions are given in the corresponding figure captions. The molecular models were built with a HyperChem software package. 3. Results and Discussion The StOF-COOH3 molecule is an analogue to widely investigated TMA, which is capable of forming nanoporous 2D networks to act as hosts for host-guest architectures.17 It is of interest to further explore such molecules that have similar structures to TMA, but with different core size and the attachment of six alkyl chains to the fluorene segments, in order to tune the interactions with both the substrate and the solvent. We have reported the assembly of StOF-COOH3 at the octanoic acid/graphite interface16 and, at the same time, discussed the possible stabilization effect of the solvent for this highly porous network. In that report, the coadsorption effect

Solvent Effects on Supramolecular Networks of the solvent is considered the most important effect, though no clear coadsorption structure is observed. In this work, we choose four representative solvents, namely, 1-phenyloctane, 1-octanoic acid, 1,2,4-trichlorobenzene, and n-tetradecane, to further elucidate solvent effect on the assembling characteristics of the nanoporous molecular networks. The choice of these solvents was motivated by their different characteristics: 1-phenyloctane as a nonpolar solvent having both aromatic and aliphatic moieties, 1-octanoic acid as a polar protic alkylated solvent, 1,2,4-trichlorobenzene as a polar nonprotic, and ntetradecane as a nonpolar aliphatic solvent. Figure 2a shows StOF-COOH3 at the 1-octanoic acid/graphite interface. Particularly, fine stripes can be observed within the cavities. The orientation of the stripe within the cavities does not have long-range correlations and could be attributed as localized assembling characteristics. Such stripe pattern is consistent with the lamella characteristics typically observed for alkane derivatives18,19 and therefore could be associated with the assemblies of 1-octanoic acid molecules. This has not been observed in our previous experiments,16 possibly because of the difference of tunneling conditions. The existence of the assembled solvent molecules leads to anisotropic local environments at the immediate vicinity of the molecular building units, which could be dramatically different from the random distribution of solvent species. Figure 2b shows StOF-COOH3 at the 1-octanoic acid/graphite interface showing triangularly arranged bright spots, which can be assigned to the fluorene groups of StOF-COOH3. Close examination of the image in Figure 2b reveals the coexistence of two types of domains consisted of cavities with different characteristic edges, corresponding to A (straight edges) and C (bending edges). In domain A, the higher-resolution STM image (Figure 2c) reveals the molecules in the dimer composed of one edge of the honeycomb are on the same line. All of the carboxyl groups of StOF-COOH3 participate in hydrogen bonding by normal dimeric pairing (honeycomb). A proposed model for the molecular arrangement could be proposed according to the STM image (Figure 1f). The unit cell of the two-dimensional (2D) assembly is superimposed on the image with parameters a ) 4.4 ( 0.1 nm, b ) 4.4 ( 0.1 nm, and R ) 60 ( 1.0°, which are consistent with the STM measurements. In addition, one can identify that in domain C (Figure 2d), the six edges of the honeycomb are not equal. The distance between the bright spots on different sides of one edge is also slightly different (Figure 2d). Two molecules in one edge of the honeycomb are slightly tilted with respect to each other. As a measure of the bending behavior, the angle β is determined as about 5.0 ( 1.0°. The unit cell of the 2D assembly is superimposed on the image in Figure 1d with parameters a ) 4.4 ( 0.1 nm, b ) 4.2 ( 0.1 nm, and R ) 59.0 ( 1.5°. As discussed in the previous work,16 because of the symmetry breaking by the substrate when adsorbed onto the surface, two chiral adsorption conformations with mirror symmetry could be induced as schematically shown in Figure 2e. The issue remains whether these two isomers will undergo phase separation to form homochiral domains or will mix well and organize to heterochiral domains. On the basis of the STM observations, two possible configurations are schematically proposed as listed in Figure 2f,g.16 Since the alkyl chains can not be clearly resolved, the above schematics should only be considered as a qualitative illustration. In the first model (Figure 2f), the six molecules composing one honeycomb are the same isomers. The two molecules in one edge of the honeycomb are along the same line. The bonding pattern proposes two O-H · · · O

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Figure 2. (a) STM image (20.4 nm × 20.4 nm) showing 1-octanoic acid coadsorption with StOF-COOH3 at the 1-octanoic acid/graphite interface. The imaging conditions are I ) 316.5 pA and V ) -655.3 mV. (b) An STM image (50.6 nm × 50.6 nm) of StOF-COOH3 at the1-octanoic acid/graphite interface. The imaging conditions are I ) 187.3 pA and V ) -751.0 mV. (c) A high-resolution STM image (14.2 nm × 14.2 nm) of straight characteristics (homochiral domain) at the 1-octanoic acid/graphite interface. The imaging conditions are I ) 149.5 pA and V ) -824.8 mV. (d) A high-resolution STM image (13.5 nm × 13.5 nm) of bending characteristics (heterochiral domain) at the1octanoic acid/graphite interface. The imaging conditions are I ) 149.5 pA and V ) -824.8 mV. (e) Two chiral adsorption conformations. (f) Structural model for straight characteristics (homochiral domain). (g) Structural model for bending characteristics (heterochiral domain).

hydrogen bonds, where the StOF-COOH3 molecules are rotated 180° with respect to each other and bonded by a 2-fold hydrogen bond between two carboxylic groups. Thus, the structure appears with 6-fold symmetry, while in the second model (Figure 2g), molecules on the three alternating acmes are the same chiral isomers, but the molecules on the other three acmes are in different conformations. In this model, the two molecules in one edge of the honeycomb are slightly tilted with respect to each other, leading to the observed bending characteristics in

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Figure 3. (a) High-resolution STM image (18.7 nm × 18.7 nm) of StOF-COOH3 at the 1,2,4-trichlorobenzene/graphite interface. The imaging conditions are I ) 177.0 pA and V ) -1.23 V. (b) A highresolution STM image (26.1 nm × 26.1 nm) of StOF-COOH3 in the 1-phenyloctane solvent. The imaging conditions are I ) 320.6 pA and V ) -625.8 mV.

the STM observation in Figure 2d. As specified, in the above measurements, the bending angle β is about 5 ( 1.0°, and the assembly cavity is 3-fold rather than 6-fold symmetric. It should be noted that the difference between the 3-fold and the 6-fold symmetry cavities is indeed not significant. However, we consider the measured bending angle of about 5.0 ( 1.0° for the molecular cavity edges is experimentally reliable to distinguish the two assembling configurations. As mentioned in the introduction, the existence of high defect density in the previously published results16 has hindered the identification of the domains with 6-fold symmetry cavities. Such difficulties could be overcome by optimizing the sample preparation conditions to obtain relatively large areas of homogeneous domains which are essential to differentiate the cavity symmetries. It has reported previously that the most common polymorph of TMA in crystals is the infinite chicken-wire motifs arising from the dimers, which is observed commonly in 90% of the carboxyl containing crystal structures in the Cambridge Structural Database (CSD) analysis.20 This behavior is consistently demonstrated at the solid/liquid interface, as has also been reported recently.12 No hybrid combination of dimeric and trimeric association (flower pattern) is revealed, which exists in certain TMA adlayers.10 But such bending characteristics have not been observed in previously reported studies on solvent effects on supramolecular networks. The characteristic bending of the StOF-COOH3 racemic dimers may be attributed to the asymmetric environment because of coadsorbed polar solvent molecules in ordered arrangements. To elucidate coadsorbed polar solvent effect on the assembling characteristics of the nanoporous networks, we tried other solvents with different polarity and functionality, namely, 1,2,4-trichlorobenzene (TCB), 1-phenyloctane, and n-tetradecane. At the TCB/graphite interface, a well-ordered honeycomb pattern is exclusively revealed, similar as that at the octanoic acid/graphite interface, though the resolution is not as good (Figure 3a). Because of the limited resolution, the symmetry of the assembly could not be undoubtedly defined. The unit cell parameter is determined to be a ) 4.3 ( 0.1 nm, b ) 4.4 ( 0.1 nm, and R ) 60.0 ( 1.5°. However, at the 1-phenyloctane/ graphite interface, StOF-COOH3 forms an amorphous monolayer where the molecules are arranged irregularly, strikingly different from the well-ordered porous honeycomb pattern formed at the 1-octanoic acid/graphite interface (Figure 2b). The bright spots, clearly resolved as triangular-shaped features in the high resolution STM image, are attributed to the aromatic cores adsorbed with its plane parallel to the substrate surface.

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Figure 4. (a) Large-scale STM image (62.9 nm × 62.9 nm) of showing n-tetradecane coadsorption with StOF-COOH3 at the n-tetradecane/ graphite interface. The imaging conditions are I ) 227.7 pA and V ) -946.6 mV. (b) A high-resolution STM image (15.2 nm × 15.2 nm) shows the cocrystallized n-tetradecane molecules inside the cavities clearly. The imaging conditions are I ) 337.7 pA and V ) -789.4 mV.

The molecules could be identified to be orientated randomly in the monolayer, though in some areas dimers it could be observed in which the two molecules are orientated opposite to each other (Figure 3b). These molecules in the dimer are possibly connected by hydrogen bonds between carboxyl groups. No honeycomb structures as expected from intermolecular hydrogen bonding and molecular symmetry were observed. When adsorbed from n-tetradecane, well-ordered honeycomb and disordered structure were observed to coexist on the surface as shown in Figure 4a. In the high resolution STM image (Figure 4b), n-tetradecane molecules can be well-resolved in the honeycomb cavities, and the adjacent molecules in the same cavity are also parallel to each other as in Figure 2a. The length of alkane backbone is measured to be 1.7 ( 0.1 nm, which is consistent with the length of one n-tetradecane molecule. This could serve as the direct evidence of cocrystallization of the solvent molecules. The unit cell parameters of the honeycomb in n-tetradecane are determined to be a ) 4.5 ( 0.1 nm, b ) 4.2 ( 0.1 nm, and R ) 59 ( 1°. When assembled on the surface, in view of the total system energy, densely packed assembly is most frequently favored in which both the adsorbate-substrate and the adsorbate-adsorbate interaction could be maximized, especially when the intermolecular interaction lacks directionality.10,15,21 However, at the solid/liquid interface the situation is normally more complex. As mentioned before, there exist mainly four types of interactions at the solid/liquid interface: solvent-adsorbate, solventsubstrate, adsorbate-substrate, and adsorbate-adsorbate interactions. There always exists competitive adsorption between the solvent and the adsorbate. In principle, the coadsorption of solvent favors formation of porous structures, which helps sustain the porous structure by filling the voids. However, this is not the only factor affecting the formation of porous networks. In our observation, tetradecane shows very strong coadsorption effect but is not the best solvent for porous network formation. In contrast, no ordered coadsorption of TCB is observed despite it being a good solvent in favor of porous network formation. It is known that there exists an equilibrium between the adsorption and the desorption of both the solute and the solvent molecules at the solid/liquid interface. In a poor solvent, the solvophobic effect favors the adsorption of molecules at the solution/solid interface thus leading to a more densely packed structure on the surface. By considering the nature of StOFCOOH3 which contains polar carboxyl groups, the polar solvents such as TCB and 1-octanoic acid are more favorable solvents. The solvophilic effect of these solvents will increase

Solvent Effects on Supramolecular Networks the desorption rate at the solution/solid interface, thus favoring the formation of a porous structure with low surface density. It could be proposed that the mechanism of solvent effect could be understood taking the above factors into account. Because of the symmetry consideration, 1-phenyloctane solvent shows weak coadsorption effect. Also the solvophobic effect of this nonpolar solvent limits the out-of-plane desorption of StOF-COOH3, leading to more densely packed, disordered structures. For tetradecane, the strong coadsorption helps stabilize the highly porous honeycomb network. On the other hand, solvophobic effect favors close packing of the molecules. As the result, tan intermediate packing state is formed which is a state between well-ordered honeycomb and disordered assembly structure. For octanoic acid, the relatively strong coadsorption and solvophilic characteristics help stabilize the porous honeycombs. The stabilization of the solvent molecules within the cavities may be associated with the six alkyl chains attached to the fluorine segments that could assist the ordering of the solvent molecules with alkyl segments. The networks formed with the racemic assembling characteristics in 1-octanoic acid are noticeably different from those formed by molecules with sole chirality. It could be envisaged that the ordered arrangements of alkyl segments of the solvent molecules could introduce localized asymmetrical chemical environments, especially when polar groups are attached to the alkyl segments. Since the orientation of the solvent molecules in the paralleled edges of the hexagonal structures are identical, so there could be a net dipole moment within the hexagonal networks. Therefore, the polarity of the environment could be a control factor to tune the assembly structures. Such asymmetrical distribution of solvent molecules could subsequently affect the monolayer structures as manifested in symmetry and chirality characteristics. 4. Conclusion Star-shaped oligofluorene end-capped with three carboxylic groups (StOF-COOH3) were studied using STM at the solid/ liquid interface. The 2D supramolecular self-assembly of StOFCOOH3 is strongly solvent dependent. In the situation of solvent species containing aliphatic segments, the STM observations provide direct evidence that coadsorption of solvent molecule plays a significant role in stabilizing the molecular networks at the interface. Such coadsorption could lead to a asymmetric local

J. Phys. Chem. C, Vol. 112, No. 23, 2008 8653 environment that could further stabilize the racemic cavity formed by isomeric building units. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China under Grants 0478117, 50573089, 90406019, and 60325412. National Basic Research Program of China (973 Program, Grant 2006CB932100) is also gratefully acknowledged for financial support. References and Notes (1) Dmitriev, A.; Lin, N.; Weckesser, J.; Barth, J. V.; Kern, K. J. Phys. Chem. B 2002, 106, 6907. (2) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M.; Heckl, W. M. Single Molecules 2002, 3, 25. (3) Theobald, J. A.; Oxtoby, N. S.; Philips, M. A.; Champness, N. R.; Beton, P. H. Nature. 2003, 424, 1029. (4) Lu, J.; Lei, S. B.; Zeng, Q. D.; Kang, S. Z.; Wang, C.; Wan, L. J.; Bai, C. L. J. Phys. Chem. B 2004, 108, 5161. (5) Schull, G.; Douillard, L.; Fiorini-Debuisschert, C.; Charra, F.; Mathevet, F.; Kreher, D.; Attias, A. J. Nano Lett. 2006, 6, 1360. (6) Furukawa, S.; Tahara, K.; De Schryver, F. C.; Auweraer, M. V. D.; Tobe, Y.; De Feyter, S. Angew. Chem., Int. Ed. 2007, 46, 2831. (7) Lackinger, M.; Griessl, S.; Heckl, W. M.; Hietschold, M.; Flynn, G. W. Langmuir. 2005, 21, 4984. (8) Kampschulte, L.; Lackinger, M.; Maier, A. K.; K; Kishore, R. S.; Griessl, S.; Schmittel, M.; Heckl, W. M. J. Phys. Chem. B 2006, 110, 10829. (9) Venkataraman, B.; Breen, J. J.; Flynn, G. W. J. Phys. Chem. 1995, 99, 6608. (10) Wu, J.; Pisula, W.; Mu¨llen, K. Chem. ReV. 2007, 107, 718. (11) Arikainen, E. O.; Boden, N.; Bushby, R. J.; Clements, J.; Movaghar, B.; Wood, A. J. Mater. Chem. 1995, 5, 2161. (12) Ruiz, J. P.; Dharia, J. R.; Reynolds, J. R.; Buckley, L. J. Macromolecules. 1992, 25, 849. (13) Li, C. J.; Zeng, Q. D.; Wang, C.; Wan, L. J.; Xu, S. L.; Wang, C. R.; Bai, C. L. J. Phys. B 2003, 107, 747. (14) Shao, X.; Luo, X.; Hu, X.; Wu, K. J. Phys. Chem. B 2006, 110, 1288. (15) (a) Furukawa, S.; Uji-i, H.; Tahara, K.; Ichikawa, T.; Sonoda, M.; De Schryver, F. C.; Tobe, Y.; De Feyter, S. J. Am. Chem. Soc. 2006, 128, 3502. (b) Tahara, K.; Furukawa, S.; Uji-I, H.; Uchino, T.; Ichikawa, T.; Zhang, J.; Mamdouh, W.; Sonoda, M.; De Schryver, F. C.; De Feyter, S.; Tobe, Y. J. Am. Chem. Soc. 2006, 128, 16613. (16) Ma, Z.; Wang, Y. Y.; Wang, P.; Huang, W.; Li, Y. B.; Lei, S. B.; Yang, Y. L.; Fan, X. L.; Wang, C. ACS Nano 2007, 1 (3), 160. (17) Griessl, S. J. H.; Lackinger, M.; Jamitzky, F.; Markert, T.; Hietschold, M.; Heckl, W. A. Langmuir 2004, 20, 9403. (18) Giancarlo, L. C.; Fang, H. B.; Rubin, S. M.; Bront, A. A.; Flynn, G. W. J. Phys. Chem. B 1998, 102, 10255. (19) Giancarlo, L.; Cyr, D.; Muyskens, K.; Flynn, G. W. Langmuir 1998, 14, 1465. (20) Kolotuchin, S. V.; Thiessen, P. A.; Fenlon, E. E.; Wilson, S. R.; Loweth, C. J.; Zimmerman, S. C. Chem.sEur. J. 1999, 5, 2537. (21) De Feyter, S.; De Schryver, F. C. Chem. Soc. ReV. 2003, 32 (3), 139.

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