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2005, 109, 1063-1066 Published on Web 12/31/2004
Evidence of Strong Hydration and Significant Tilt of Amphiphilic [2]Rotaxane Molecules in Langmuir Films Studied by Synchrotron X-ray Reflectivity Kasper Nørgaard,† Jan O. Jeppesen,*,‡ Bo W. Laursen,§ Jens B. Simonsen,† Markus J. Weygand,# Kristian Kjaer,# J. Fraser Stoddart,*,§ and Thomas Bjørnholm*,† Nano-Science Center, UniVersity of Copenhagen, UniVersitetsparken 5, DK-2100 KøbenhaVn Ø, Denmark, Department of Chemistry, UniVersity of Southern Denmark, CampusVej 55, DK-5230 Odense M, Denmark, Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark, and California NanoSystems Institute and Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, 405 Hilgard AVenue, Los Angeles, California 90095-1569, USA ReceiVed: NoVember 10, 2004; In Final Form: December 14, 2004
Surface sensitive X-ray techniques have been used to elucidate the structures of amphiphilic [2]rotaxane and dumbbell monolayers at the air/water interface. The [2]rotaxanes were found to adopt highly hydrated tilted and/or folded conformations on the water surface largely due to the hydrophilic nature of their tetracationic ring component. This conformation was less pronounced in monolayers of the dumbbell precursors. Increasing the surface pressure resulted in an expansion of [2]rotaxane monolayers in the vertical direction and decreased hydration.
Redox-controlled switching of bistable [2]catenanes and [2]rotaxanes, wherein an electron deficient tetracationic cyclophane moves between two electron-donating recognition sites, has been the subject of intense investigations1 in recent times. One of the potentially useful attributes of these mechanically interlocked molecular switches is the finding2 that LangmuirBlodgett monolayers of amphiphilic variants of these bistable molecules can act as electronically controllable memory devices when incorporated into solid-state crossbar circuits. Experimental data suggest3 that the switching behavior of these crossbar devices, at least those employing Si/SiO2 or semiconducting carbon nanotube bottom electrodes4,5 and vapor deposited Ti/Al top electrodes, relies on the intrinsic electromechanical properties of the amphiphilic bistable [2]rotaxane molecules. Yet, the detailed (super)structure of the active molecular monolayers present in these devices is not understood, and even less is known about the overall superstructural response during the device operation. This lack of detailed (super)structural information is not a specific problem for molecular electronic devices incorporating bistable [2]catenanes and [2]rotaxanes, but rather a general problem6 for the type of soft organic materials sandwiched as one-molecule thick layers between metallic/semiconductor electrodes. Complications arise for several reasons, which include (i) the small number of molecules, e.g., 107-103, in a typical crossbar device, (ii) the minute areas, e.g., 108-104 nm2, occupied by the monolayers in the devices, and (iii) the difficulties of probing through the electrode materials. These complications provide a compelling reason for studying more accessible model systems which may provide a * Corresponding author. E-mail:
[email protected]. † University of Copenhagen. ‡ University of Southern Denmark. § UCLA. # Risø.
10.1021/jp0448494 CCC: $30.25
better understanding of the (super)structure and dynamics occurring in the more intricate molecular electronic devices. Herein, we describe our investigations of the (super)structure of amphiphilic bistable [2]rotaxane molecules in Langmuir monolayers, which form the basis of the crossbar devices, using highly sensitive X-ray techniques. Synchrotron source X-ray radiation is a useful tool7 for obtaining structural information at the molecular scale. In the present investigation, two complementary X-ray techniques were used to study the (super)structure of Langmuir monolayers of some amphiphilic bistable [2]rotaxanes at the air/water interface employing conditions resembling those used in the preparation2d of the crossbar memory devices: they were (i) grazing incidence X-ray diffraction (GIXD), which establishes the in-plane crystalline structure, and (ii) specular X-ray reflectivity, which probes the out-of-plane electron density profile of the Langmuir film.8 The structural formulas of the amphiphilic bistable [2]rotaxanes9,10 14+ and 24+, which we have focused our attention on in this investigation, are shown in Figure 1. The [2]rotaxane 14+ consists of (i) a tetracationic cyclophane component11s cyclobis(paraquat-p-phenylene)scontaining two π-electrondeficient bipyridinium units and (ii) a linear rodlike segment that includes two different matching π-electron-rich recognition sites for the cyclophane, viz., a monopyrrolotetrathiafulvalene (MPTTF) unit and a 1,5-dioxynaphthlalene (DNP) moiety: the rodlike segment is terminated by a hydrophilic dendritic stopper at one end and by a hydrophobic tetraarylmethane stopper at the other, thus rendering the molecule amphiphilic and enabling it to form Langmuir monolayers at the air/water interface.2 In solution, the cyclophane is located predominantly12 around the MPTTF recognition site, it being the stronger of the two π-electron donating units in the molecule. Oxidation of 14+, with excess of oxidant, yields the hexacationic species 16+ (not © 2005 American Chemical Society
1064 J. Phys. Chem. B, Vol. 109, No. 3, 2005
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Figure 1. Graphical representation and structural formulas for the amphiphilic bistable [2]rotaxane 14+, the amphiphilic “monostable” [2]rotaxane 24+, and its amphiphilic dumbbell precursor 3.
shown in Figure 1), resulting from the removal of two electrons from the MPTTF unit. This oxidation, which can be affected either chemically of electrochemically, of the MPTTF unit is accompanied in solution by the movement of the cyclophane from this unit to the DNP moiety. By contrast, in the [2]rotaxane 24+, the cyclophane has to be located on the DNP moiety even in its “reduced” form, i.e., when the MPTTF unit is neutral, on account of the presence of a blocking ethylthio group situated between the two recognition sites, thus preventing the cyclophane from moving onto the MPTTF unit. Hence, it serves as a reference compound for the oxidized form 16+ of the switchable [2]rotaxane. The neutral compound 3, the dumbbell precursor to the [2]rotaxane 24+, also contains the DNP and MPTTF recognition sites. Chloroform solutions of the compounds were spread onto the pure water surface of a Langmuir trough and GIXD and X-ray reflectivity were measured at several positions along the compression isotherm to follow the (super)structural evolution of the resulting monolayers as a function of surface pressure and molecular area. Chemical oxidation of 14+ was achieved using Fe(ClO4)3 as the oxidant (see Supporting Information for details). As expected for such intricate and flexible molecules, the GIXD measurements did not reveal any in-plane crystallinity in the monolayers of any of the rotaxanes studied at either of the measured surface pressures. The electron density profiles of selected samples, resulting from fitting13 the measured reflectivity data (see Supporting Information), are shown in Figure 2. Combined with the molecular areas deduced from the Langmuir isotherms, these data give a quantitative account (∼10% accuracy) of the total electron distribution in the monolayer molecules normal to the water surface. The area above the horizontal dotted line in Figure 2 corresponds to the number of electrons in one molecule at the given surface pressure (Π ) 20 mN/m). The shaded area represents the approximate position of the interface to bulk water and was chosen at the point on the curve where the electron density is 3% larger than that for bulk water. The discrepancy between these two lines (indicated in light blue in Figure 2) is caused by the inclusion of a substantial number of water molecules (∼60, corresponding to 28% of the total electron density) from the subphase into the hydrophilic parts of the [2]rotaxane 14+,
Figure 2. Synchrotron X-ray reflectivity measurements at the air/water interface: (a) electron density profile of the [2]rotaxane 14+, inverted from the reflectivity data for the starting compound (solid green line): (b) electron density profiles of the [2]rotaxane 24+ (red line) and its dumbbell precursor 3 (black line). The areas above the horizontal dotted lines correspond to the number of electrons in one molecule at the given molecular area (where Π ) 20 mN/m), and the shaded section illustrates the approximate position of the interface to the bulk water.
Figure 3. Schematic illustration of the proposed monolayer structure of the amphiphilic bistable [2]rotaxane 14+ at the air/water interface.
a phenomenon that inevitably increases the number of electrons.14 Inspection of the resulting profile for 14+ reveals that the monolayer height is close to 33 Å. This height is notably smaller than the approximately 60 Å expected for an extended molecular conformation of vertically oriented molecules on the water surface. We suggest that this difference is a result of the molecules being folded and/or tilted on the water surface, a situation caused presumably by favorable interactions between the highly charged cyclophane and its counterions with the polar water subphase. Hydrophobic and/or π-π interactions between the free DNP moiety and the cyclophane in either the same molecule or in neighboring molecules, as well as between the cyclophane and the aromatic portions of the hydrophilic stopper, could also influence the packing of the monolayer. This situation is illustrated graphically in an idealized manner in Figure 3. To study the structural evolution of the rotaxane monolayers as a function of the applied surface pressure, X-ray reflectivity was performed at different points along the compression isotherm. A marked increase in the monolayer thickness is observed as the surface pressure is increased. This dependence is illustrated in Figure 4, which shows the electron density
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Figure 4. Electron density profiles for Langmuir films of the amphiphlic bistable [2]rotaxane 14+ derived from synchrotron X-ray reflectivity measurements at the air/water interface at applied surface pressures Π ) 0.5, 4, and 20 mN/m, corresponding to mean molecular areas of 470, 265, and 180 Å2, respectively. The shaded section illustrates the approximate position of the interface to the bulk water.
profiles of 14+ measured at three different surface pressures, Π ) 0.5, 4, and 20 mN m-1, corresponding to mean molecular areas of ∼470, 265, and 180 Å2. Assuming that the monolayer is homogeneous, Figure 4 shows that the thickness of the [2]rotaxane monolayers increases from 22 Å, in very expanded monolayers (Π ) 0.5 mN m-1), to over 23 Å in slightly compressed monolayers (Π ) 4 mN m-1), and ultimately to 33 Å in moderately condensed monolayers (Π ) 20 mN m-1). This progression suggests that this [2]rotaxane, when spread on the water surface, initially adopts a very tilted conformation in which the molecule is more or less lying down with large parts exposed to the water surface. This picture is also supported by the absence of texture in the Brewster angle micrographs of the Langmuir film and the absence of crystallographic order, effects that are otherwise frequently observed in Langmuir films of more hydrophobic molecules that spontaneously form 2-dimensional islands when spread at the water surface. In the case of the sterically encumbered [2]rotaxane 24+, a similar monolayer thickness (ca. 31 Å) and mean molecular area (180 Å2) were found at 20 mN/m (Figure 2, right). However, the detailed electron density profile and, in particular, the degree of hydration differ significantly. Hence, for 24+ where the cyclophane is confined to the “upper” station (DNP), less hydration of the monolayer is observed than for 14+, where the cyclophane is predominantly on the “lower” (MPTTF) station. From comparison of the electron density profiles of 14+ and 24+, it is clearly evident that it is possible to distinguish between the case where the cyclophane is located around the MPTTF recognition site (14+) and when it is located around the DNP recognition site (24+). The monolayer thickness of the dumbbell 3 is around 41 Å, a value much closer to that expected for a stretched and nontilted
J. Phys. Chem. B, Vol. 109, No. 3, 2005 1065 backbone conformation. Clearly, the degree of hydration of 3 is much less (ca. 15% of the total electron density) than that for 14+. The limiting area of 3 on the water surface at a surface pressure Π of 20 mN/m is also much smaller than that for the [2]rotaxanes, i.e., approximately 80 Å2 for 3 as opposed to 180 Å2 for 14+ and 24+. Hence, the absence of the tetracationic cyclophane results in a noticeably different molecular arrangement and degree of hydration in Langmuir layers, suggesting a closer packed monolayer with a less tilted/folded conformation. In an attempt to elucidate the structural response to oxidation, the electron density profiles for oxidized Langmuir films of 14+, supposedly containing the species 16+, were found to differ only insignificantly from those obtained for the starting (unoxidized) state 14+. This observation indicates that no net reorganization of the electrons takes place in the vertical direction. Hence, movement of the cyclophane, if any, would have to either (i) occur predominantly in the plane of the film or (ii) be accompanied by an opposite rearrangement of the water molecules and counterions. To address these issues, experiments employing longer and less hydrophilic rotaxanes are in progress. In summary, we provide, by use of surface sensitive synchrotron X-ray techniques, evidence for strongly tilted and/ or folded conformations for the rotaxane molecules in the Langmuir films of 14+ and 24+, which have been used2d in the preparation of molecular crossbar memory devices. The folded conformation is presumably a result of the hydrophilic nature of the tetracationic cyclophane and its counterions. These observations are also supported by recent Langmuir-Blodgett studies15 of other related amphiphilic bistable [2]rotaxanes. Furthermore, we have established that the monolayers are highly hydrated and show no in-plane crystalline order, and that it is possible, based on electron density profiles, to resolve and differentiate between (super)structures of closely related [2]rotaxanes (14+ vs 24+). The electron density profiles may serve as important future references for molecular modeling of the switching mechanism in amphiphilic rotaxanes in condensed Langmuir films. This work is currently in progress.16 Acknowledgment. This research was funded by the Danish Natural Science Research Council (SNF, projects #21-03-0317, #21-03-0014, and #21-02-0414) in Odense, Denmark, and by the Defense Advanced Research Projects Agency (DARPA) in the United States. We thank HASYLAB at DESY in Hamburg for beam time at beam line BW1 and DANSYNC for financial support. Supporting Information Available: The synthesis of 24+ and 3, Langmuir film details and X-ray reflectivity data. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Balzani, V.; Credi, A.; Mattersteig, G.; Matthews, O. A.; Raymo, F. M.; Stoddart, J. F.; Venturi, M.; White, A. J. P.; Williams, D. J. J. Org. Chem. 2000, 65, 1924-1936. (b) Jeppesen, J. O.; Perkins, J.; Becher, J.; Stoddart, J. F. Org. Lett. 2000, 2, 3547-3550. (c) Jeppesen, J. O.; Perkins, J.; Becher, J.; Stoddart, J. F. Angew. Chem., Int. Ed. 2001, 40, 1216-1221. (d) Tseng, H.-R.; Vignon, S. A.; Stoddart, J. F. Angew. Chem., Int. Ed. 2003, 42, 1491-1495. (e) Jeppesen, J. O.; Nielsen, K. A.; Perkins, J.; Vignon, S. A.; Di Fabio, A.; Ballardini, R.; Gandolfi, M. T.; Venturi, M.; Balzani, V.; Becher, J.; Stoddart, J. F. Chem. Eur. J. 2003, 9, 2982-3007. (f) Yamamoto, T.; Tseng, H.-R.; Stoddart, J. F.; Balzani, V.; Credi, A.; Marchioni, F.; Venturi, M. Collect. Czech. Chem. Commun. 2003, 68, 14881514. (g) Tseng, H.-R.; Vignon, S. A.; Celestre, P. C.; Perkins, J.; Jeppesen, J. O.; Di Fabio, A.; Ballardini, R.; Gandolfi, M. T.; Venturi, M.; Balzani, V.; Stoddart, J. F. Chem. Eur. J. 2004, 10, 155-172. (2) (a) Collier, C. P.; Mattersteig, G.; Wong, E. W.; Luo, Y.; Beverly, K.; Sampaio, J.; Raymo, F. M.; Stoddart, J. F.; Heath, J. R. Science 2000,
1066 J. Phys. Chem. B, Vol. 109, No. 3, 2005 289, 1172-1175. (b) Collier, C. P.; Jeppesen, J. O.; Luo, Y.; Perkins, J.; Wong, E. W.; Heath, J. R.; Stoddart, J. F. J. Am. Chem. Soc. 2001, 123, 12632-12641. (c) Pease, A. R.; Jeppesen, J. O.; Stoddart, J. F.; Luo, Y.; Collier, C. P.; Heath, J. R. Acc. Chem. Res. 2001, 34, 433-444. (d) Luo, Y.; Collier, C. P.; Jeppesen, J. O.; Nielsen, K. A.; DeIonno, E.; Ho, G.; Perkins, J.; Tseng, H.-R.; Yamamoto, T.; Stoddart, J. F.; Heath, J. R. ChemPhysChem 2002, 3, 519-525. (3) (a) Flood, A. H.; Ramirez, R. J. A.; Deng, W.-Q.; Muller, R. P.; Goddard, W. A., III.; Stoddart, J. F. Aust. J. Chem. 2004, 57, 301-322. (b) Tseng, H.-R.; Wu, D.; Fang, N. X.; Zhang, X.; Stoddart, J. F. ChemPhysChem 2004, 5, 111-116. (c) Steuerman D. W.; Tseng H.-R.; Peters A. J.; Flood A. H.; Jeppesen J. O.; Nielsen K. A.; Stoddart J. F.; Heath, J. R. Angew. Chem. Int. Ed. 2004, 43, 6486-6491. (4) (a) Diehl, M. R.; Steuerman, D. W.; Tseng, H.-R.; Star, A.; Celestre, P. C.; Stoddart, J. F.; Heath, J. R. ChemPhysChem 2003, 4, 1335-1339. (b) Yu, H.; Luo, Y.; Beverly, K.; Stoddart, J. F.; Tseng, H.-R.; Heath, J. R. Angew. Chem., Int. Ed. 2003, 42, 5706-5711. (5) Both semiconductors (see refs 2-4) and metals, see: (a) Chen, Y.; Ohlberg, D. A. A.; Li, X.; Stewart, D. R.; Williams, R. S.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart, J. F.; Olyniek, D. L.; Anderson, E. Appl. Phys. Lett. 2003, 82, 1610-1612. (b) Chen, Y.; Jung, G.-Y.; Ohlberg, D. A. A.; Li, X.; Stewart, D. R.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart, J. F.; Williams, R. S. Nanotechnology 2003, 14, 462-468. (c) Stewart, D. R.; Ohlberg, D. A A.; Beck, P. A.; Chen, Y.; Williams, R. S.; Jeppesen, J. O.; Nielsen, K. A.; Stoddart, J. F. Nano Lett. 2004, 4, 133-136 have been used as the bottom electrode for molecular electronic devices incorporating bistable [2]catenanes and [2]rotaxanes. Recently, it has been established that devices incorporating metals as the bottom electrode do not switch as a result of a molecular-based process. Rather, the observed bistability is probably a result of reversible filament growth between the two electrodes. This result does not pertain, however, to devices operating at low bias voltages ((2 V) and incorporating semiconductors (polysilicon, for example) as the bottom electrode. To date, all of the available experimental evidence suggests that these devices switch as a consequence of a molecularly based electromechanical mechanism.
Letters (6) (a) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550-1552. (b) Vilan, A.; Shanzer, A.; Cahen, D. Nature 2000, 404, 166-168. (c) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnel, J. D.; Stapleton, J. J.; Price, D. W., Jr.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303-2307. (d) Metzger, R. M. Chem. Rev. 2003, 103, 3803-3834. (e) Dinglasan, J. A. M.; Baley, M.; Park, J. B.; Dhirani, A.-A. J. Am. Chem. Soc. 2004, 126, 6491-6497. (7) Als-Nielsen, J.; McMorrow, D. Elements of Modern X-ray Physics; Wiley: New York, 2001. (8) Jensen, T. R.; Kjaer K. In NoVel Methods to Study Interfacial Layers; Mo¨bius, D., Miller R., Eds.; Elsevier: Amsterdam, 2001; p 205. (9) Jeppesen, J. O.; Nygaard, S.; Vignon, S. A.; Stoddart, J. F. Eur. J. Org. Chem., in press. (10) The synthesis of 1‚4PF6 will be reported in ref 9. The synthesis of 2‚4PF6 and its component dumbbell 3 are described in the Supporting Information. (11) Asakawa, M.; Dehaen, W.; L’abbe´, G.; Menzer, S.; Nouwen, J.; Raymo, F. M.; Stoddart, J. F.; Williams, D. J. J. Org. Chem. 1996, 61, 9591-9595. (12) The 1H NMR spectrum (CD3CN, 298 K) recorded of 14+ revealed that 14+ exists as ∼85% of the co-conformer where CBPQT4+ is located around the MPTTF unit and 15% of the co-conformer where CBPQT4+ is located around the DNP moiety. (13) Pedersen, J. S.; Hamley, I. W. J. Appl. Crystallogr. 1994, 27, 3649. (14) Integration of the light blue area in Figure 2 yields the number of excess electrons in the monolayer, i.e., electrons not originating from the [2]rotaxane. These can be converted to the number of water molecules by simple division (one water molecule contains 10 electrons). Similar calculations were performed using values of F/Fwater ) 1.02, 1.04, and 1.05 to determine the position of the interface to bulk water. These calculations yielded the same quantitative results as F/Fwater ) 1.03 except for a small perturbation in the number of water molecules included in the monolayer. (15) Lee, I. C.; Curtis, F. W.; Yamamoto, T.; Tseng, H.-R.; Flood, A. H.; Stoddart, J. F.; Jeppesen, J. O. Langmuir 2004, 20, 5809-5828. (16) Jang, S. S.; Goddard, W. A., III; et al. Work in progress.
