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Langmuir Monolayers of a Photoisomerizable Macrocycle Surfactant Eric Karp,† Cory S. Pecinovsky,‡ Michael J. McNevin,‡ Douglas L. Gin,†,‡ and Daniel K. Schwartz*,† Department of Chemical and Biological Engineering and Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed May 9, 2007. In Final Form: June 5, 2007 An amphiphilic photoisomerizable macrocycle has been prepared that forms stable Langmuir monolayers at the air-water interface. The hydrophilic core of the molecule switches between closed and open isomers upon irradiation by the appropriate wavelengths of light. Isotherm measurements, Brewster angle microscope images, and atomic force micrographs (of transferred Langmuir-Blodgett films) suggest a phase transition between a face-on to a tilted edge-on molecular orientation as a function of surface concentration. In the face-on phase, in situ photoisomerization results in a reversible increase in surface pressure due to greater molecular crowding in the open configuration.
Introduction Macrocycles with incorporated azobenzene moieties allow one to control the shape of the macrocycle by exposure to characteristic wavelengths of UV or visible light.1-4 The large geometry change associated with the trans to cis isomerization of azobenzene can have a correspondingly large impact on the geometry of the macrocycle.1 In an early demonstration of this potential, the trans isomer of an azobenzene-crown ether derivative was found to exclude metal cations, while the cis isomer was sufficiently “open” to accommodate them.2-4 Thus, by using rational design, one can tailor a macrocycle to have a “more open” and a “more closed” state accessible reversibly via a light trigger. While the controlled complexation and release of guest molecules has been well-studied in discrete macrocycles in solution, supramolecular assemblies of these shape-changing macrocycles are rare.5 We believe that ensembles of photochromic macrocycles of this type have the potential to act as stimulus-responsive molecular “gates” or “irises”. By modulating between the open and closed forms using a light stimulus, one may be able to control the transport of small molecules through the supramolecular assembly. As such, azobenzene-based macrocycles could have a broad range of applicability, ranging from separations to catalysis. Herein, we present the design and synthesis of an amphiphilic, photochromic macrocycle that we predict has a large, reversible change in diameter upon exposure to UV and visible light. This molecule was found to form a stable Langmuir monolayer at the air-water interface and to undergo a first-order face-on to edgeon phase transition as a function of molecular area. In the faceon phase, photoisomerization from the trans to cis state is observed to induce a fast and reversible change in the measured surface * To whom correspondence should be addressed. Daniel.Schwartz@ colorado.edu, phone: 303-735-0240. † Department of Chemical and Biological Engineering. ‡ Department of Chemistry and Biochemistry.
E-mail:
(1) Norikane, Y.; Kitamoto, K.; Tamaoki, N. J. Org. Chem. 2003, 68, 8291. (2) Shinkai, S.; Honda, Y.; Kusano, Y.; Manabe, O. J. Chem. Soc., Chem. Commun. 1982, 848. (3) Shinkai, S.; Yoshioka, A.; Nakayama, H.; Manabe, O. J. Chem. Soc., Perkin Trans. 2 1990, 1905. (4) Shinkai, S.; Manabe, O. Top. Curr. Chem. 1984, 121, 67. (5) (a) Huesmann, H.; Fujiwara, H.; Luboch, E.; Biernat, J. F.; Mo¨bius, D. J. Inclusion Phenom. Macrocyclic Chem. 2004, 49, 181. (b) Zawisza, I.; Bilewicz, R.; Janus, K.; Sworakowski, J.; Luboch, E.; Biernat, J. F. Mater. Sci. Eng., C 2002, 22, 91. (c) For a review on azomacrocycles, see: Luboch, E.; Bilewicz, R.; Kowalczyk, M.; Wagner-Wysiecka, E.; Biernat, J. F. AdV. Supramol. Chem. 2003, 9, 71-162.
pressure, suggesting that the modification of the molecular shape due to isomerization increases the crowding of the pendant aliphatic chains. Experimental Details Materials. Chloroform stabilized with 0.5% ethanol (99.8+%; Sigma-Aldrich, St. Louis, MO) was used as the solvent for Langmuir monolayer deposition. Water was purified with a Milli-Q UV+ purification system (Millipore, Bedford, MA). Muscovite mica (purchased from Ted Pella, Inc., Redding, CA) was cut into 10 mm disks and then cleaved just before use. Quartz microscope slides (Chemglass, Vineland, NJ) were cut to rectangular dimensions 8 mm by 4 mm and were also used as a substrate for LangmuirBlodgett films. Sulfuric acid (Fisher Scientific, Fair Lawn, NJ) and hydrogen peroxide (30%; Sigma-Aldrich) were mixed to make a 50/50 piranha solution to clean the quartz slides before use. A small array of LEDs (Opto Technology Inc., Wheeling, IL) outputting 375 nm ultraviolet light at 150 mW was used to drive molecule A to the cis conformeration, while another array of LEDs (Opto Technology Inc., Wheeling, IL) outputting blue light, at 475 nm and 200 mW, was used to drive the cis isomers back to the trans state. Macrocycle Design and Synthesis. In designing this macrocycle, three considerations were addressed: (1) ease of synthesis, (2) amphiphilic character, and (3) a large dimensional change upon light exposure. Work by Fujita6 and co-workers and Stang7 and co-workers provided a synthetic avenue to address (1) and (2). Using Pt2+-py coordination chemistry, they assembled a number of macrocycles ranging in dimensionality from triangles and squares to cages and prisms. Often, the construction of these structures was high-yielding and facile, owing to the bond-forming reaction being under thermodynamic control. Also of interest for our purposes, the macrocycles were frequently highly charged, depending on the number of Pt2+ atoms present. Since our design required the macrocycle to be the hydrophilic part of the amphiphile, we felt this chemistry was suitable for installing the polar “core” of the molecule to which the hydrophobic periphery could be added. In order to address design consideration (3), we envisioned the trans form of the macrocycle having little free space in the interior, while the cis (6) (a) Fujita, M.; Yazaki, J.; Ogura, K. J. Am. Chem. Soc. 1990, 112, 5645. (b) Fujita, M.; Nagao, S.; Iida, M.; Okada, K.; Ogura, K. J. Am. Chem. Soc. 1993, 115, 1574. (c) Fujita, M.; Sasaki, O.; Mitsuhashi, T.; Fujita, T.; Yazaki, J.; Yamaguchi, K.; Ogura, K. Chem. Commun. 1996, 1535. (7) (a) Stang, P. J.; Cao, D. H. J. Am. Chem. Soc. 1994, 116, 4981. (b) Stang, P. J.; Cao, D. H.; Saito, S.; Arif, A. M. J. Am. Chem. Soc. 1995, 117, 6273. (c) Stang, P. J.; Chen, K. J. Am. Chem. Soc. 1995, 117, 1667. (d) Mana, J.; Whiteford, J. A.; Stang, P. J. J. Am. Chem. Soc. 1996, 118, 8731. (e) Whiteford, J. A.; Rachlin, E. M.; Stang, P. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 2524. (f) Fan, J.; White, J. A.; Olenyuk, B.; Levin, M. D.; Stang, P. J.; Fleischer, E. B. J. Am. Chem. Soc. 1999, 121, 2741.
10.1021/la701345e CCC: $37.00 © 2007 American Chemical Society Published on Web 06/21/2007
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Figure 1. All-trans form of azobenzene macrocycle core 1 derived from X-ray crystal structure. After irradiation of 1a with 375 nm light, the calculated all-cis isomer 1b is formed. Since 1 did not form stable monolayers on water, an analogue with increased amphiphilicity 2 was synthesized. This simplified schematic ignores the possibility of the mixed cis-trans photoisomer. form would have a considerably larger diameter. Therefore, the trans isomer was designed to have a rectangle-like shape and the cis form a more square-like shape. The polar macrocycle core unit 1 is shown in Figure 1 and is the product of the above criteria. The assembly of 1 involves four steps from readily available starting materials (see Supporting Information). The crucial macrocyclization step proceeded in quantitative yield and produced the expected dimer, as confirmed by single-crystal X-ray diffraction.8 The all-trans isomer (1a) represents the “closed” form of the macrocycle and was found to have a 0.40 nm distance between the two azo groups in the single-crystal structure. The spacefilling model illustrates that there is only sufficient space in the closed form of 1 to accommodate or allow passage of very small molecules or ions. The geometry depicted for the “open” all-cis form (1b) is the result of a semiempirical PM5 minimization using MOPAC. The diameter as measured between the N atoms of the azo subunits increases from 0.40 to 1.13 nm between the photoisomers 1a and 1b. The larger space in isomer 1b is now sufficient to accommodate or allow the passage of many guest molecules. In a real irradiated sample, we expect that there will be some stationarystate distribution of all-trans, all-cis, and also mixed cis-trans photoisomers. NMR data presented in the Supporting Information suggest that such mixtures are present in solution.9 In order to form stable monolayers, hydrophobic peripheral units were added to 1 by extending the chain length of the triethylphosphine ligands to triundecylphosphine. This was accomplished by synthesizing ditrifluoromethanesulfonatebis(triundecylphosphine)platinum(II) and displacing the triflate ligands with the same azobenzenebipyridine ligand used in the synthesis of 1 (see Supporting Information). Ion exchange with the PF6- counterion was performed to successfully yield 2. The increase in amphiphilicity of 2 proved sufficient to encourage the formation of stable Langmuir monolayers at the air-water interface. Langmuir Monolayer Formation. For Langmuir monolayer deposition, molecule 2 was dissolved in chloroform at a typical concentration of ∼1.0 mg/mL solvent. Langmuir monolayers of compound 2 were prepared by dropwise addition of solution on the (8) CCDC 646228 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. (9) For an interesting example of isomerism in a strained azophane, see: Rau, H.; Rottger, D. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1994, 246, 143.
Figure 2. Schematic representation of the constant area isomerization experiment. surface of pure water (Millipore Milli-Q UV+) contained in a custombuilt Teflon Langmuir trough or a NIMA 611 trough. All monolayer depositions were performed at room temperature and ambient conditions. The surface area of the trough was adjusted by means of a motor-driven barrier, allowing compression or expansion of the monolayer. A compression speed of 11 ( 1 cm2/min was used. The surface pressure was monitored with a filter paper Wilhelmy plate and an R&K electrobalance. In Situ Photoisomerization Experiments. A PTFE Petri dish with a diameter of 10 cm was used as a small trough for constant area isomerization experiments. The 1 mg/mL solution was deposited carefully on Millipore water contained in the Petri dish, and the surface pressure was measured using a NIMA Wilhelmy plate balance. After deposition, the monolayer was allowed to relax to a constant surface pressure. For isomerization experiments, sufficient solution was deposited so that the monolayer of molecule 2 relaxed to a surface pressure of ∼6.1 mN/m. The monolayer was then exposed to 375 nm radiation emitted by a small LED array, and the surface pressure was monitored continuously (Figure 2) as it changed in response to irradiation. Once the surface pressure stabilized again, the UV LED array was turned off, and an array of blue visible LEDs was turned on, driving the molecules back to their trans conformation. The surface pressure change was then recorded until it stabilized back at its original value. Langmuir Blodgett Film Transfer. The Langmuir-Blodgett (LB) films were transferred by vertical dipping to quartz and mica substrates in a NIMA 611 trough. In all cases, the substrate was
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immersed just below the water surface prior to monolayer compression, and one layer was transferred on the upstroke (at a rate of 2 cm/min), while feedback control of the barriers was used to maintain the Langmuir film at the desired surface pressure. For UV-vis spectroscopy experiments, quartz slides (purchased from Fisher Scientific) were cut so their surface area totaled 8 cm2. The slides were cleaned with piranha solution before use. The surface pressure was held constant at 11 mN/m during dipping; this surface pressure was chosen for consistency with the constant area photoisomerization experiments. The dipping speed was set at 5 mm/min for all transfers. For AFM experiments, a freshly cleaved circular mica substrate, with a total surface area of 0.66 cm2, was dipped at a surface pressure of 11 mN/m. Transfer ratios of approximately 0.96 were observed during transfer to mica substrates. UV-vis Absorbance Studies. An Agilent 8453 spectrophotometer was used to obtain the UV-vis spectra of the LB films on the quartz substrate. The quartz slides were held in the beam path of the UV-vis spectrometer, with a small slide holder purchased from THOR Labs. The integration time was set at 20 s, in order to improve the signal-to-noise ratio. After a spectrum was obtained, the LB films were irradiated by a UV LED array, at a distance of about 5 cm for 20 min. Another UV-vis spectrum was then taken to determine the extent of photoisomerization that had occurred. Isomerization experiments of molecule 2 were also performed in bulk solution. A quartz cuvette filled with chloroform was used to blank the Agilent 8453. A 50 µL aliquot of the 1 mg/mL solution was added to the chloroform-filled cuvette, and an initial spectrum was obtained. The UV light was turned on and placed approximately 5 cm from the cuvette, and spectra were obtained in 5 min intervals until the absorbance band at 380 nm decreased to a consistent value. Atomic Force Microscopy Studies. Atomic force microscope (AFM) imaging was performed with a Digital Instruments (now Veeco, Santa Barbara, CA) Nanoscope III MMAFM instrument, equipped with an E scanner. All images were obtained in tapping mode using etched silicon probes (Nanodevices, Inc., Santa Barbara, CA) with fundamental resonance frequencies (fo) of 300 kHz. These cantilevers were 125 µm in length and have a spring constant of (k) of 40 N/m, width of 45 µm, and thickness of 4 µm. Phase and height images were captured simultaneously at scan rates of 1.2-2.35 Hz. Images were acquired from at least 3 macroscopically selected areas of each sample, and the reported height differences are a result of the general trends found in these areas. These images were minimally flattened. The domain heights and sizes were determined from statistical analysis of 25 cross sections obtained from 5 independent images. Brewster Angle Microscopy Studies. A monolayer of molecule 2 was spread on an ultrapure water surface in a PTFE trough and then examined with a custom-built Brewster angle microscope (BAM)10,11 as described previously.12 BAM images were taken at multiple surface pressures, 4.5 mN/m, 9.8 mN/m, 11.1 mN/m, and 17.0 mN/m. These pressures were chosen to emphasize certain characteristics of different regions along the isotherm.
Results and Discussion Monolayer Phase Behavior. Langmuir monolayers of 2 were found to be stable on a water surface. A representative surface pressure vs molecular area (π-A) isotherm is shown in Figure 3. BAM images associated with various regions of the isotherm are also shown in Figure 3a-d. At molecular areas of >5.0 nm2, the surface pressure is negligible, indicating coexistence with a two-dimensional (2D) vapor phase. The surface pressure rises gradually at molecular areas in the range ∼5.0 nm2 to 4.0 nm2. For comparison, the characteristic molecular dimensions of the macrocycle are significantly smaller; the approximate area of the “closed” core is estimated to be 1.3 nm2, and the closepacked area of the 12 aliphatic chains totals ∼2.3 nm2. Therefore, (10) Henon, S.; Meunier, J. ReV. Sci. Instrum. 1991, 62, 936. (11) Honig, D.; Mobius, D. J. Phys. Chem. 1991, 95, 4590. (12) Ignes-Mullol, J.; Schwartz, D. K. Langmuir 2001, 17, 3017.
Figure 3. Langmuir isotherm of 2 and the corresponding BAM images showing the changes in surface morphology with increasing pressure: (a) Liquid expanded phase; (b,c) liquid expanded-solid coexistence; (d) solid phase.
in this large area phase, each chain occupies at least twice its close-packed area, consistent with a disordered arrangement. We therefore label this phase “liquid-expanded” (LE) in analogy with conventional nomenclature of disordered phases in Langmuir monolayers. The BAM images of the monolayer in this region (Figure 3a) are featureless and relatively dark, also consistent with the LE designation. In the LE region of the isotherm, we hypothesize that 2 is lying with the long axis of the core parallel to the water surface, thereby maximizing the contact of the polar regions of the macrocycle with the subphase. These polar regions are the two ends of the core, with a +2 charge and two counterions associated with each Pt atom. As the monolayer is compressed further, the isotherm displays an inflection point and then a pseudo-plateau region in the range ∼4.0 nm2 to ∼2.5 nm2. This sort of isotherm feature is generally associated with a first-order phase transition, and the BAM images are consistent with this conclusion. Figure 3b,c shows the coexistence of small bright fibrillar domains with the darker LE phase. This suggests that the small domains represent thicker parts of the monolayer. As the monolayer is compressed across this coexistence region, the bright domains occupy a greater fraction of the surface area, consistent with the lever rule associated with a first-order transition. At even smaller molecular areas, these bright domains are sintered tightly together (see Figure 3d), and the surface pressure rises significantly until collapse into the third dimension occurs at approximately 1.0 nm2. The anisotropic shape of the condensed phase domains coupled with the fact that they sinter, instead of coalescing, suggests that this phase has attributes more consistent with a 2D solid rather than a dense liquid or a liquid crystal phase. Therefore, we designate the more condensed phase as the “solid” (S) phase. The fact that the S phase is stable at molecular areas well below 2.0 nm2 makes a face-on orientation unlikely (we recall that the close-packed area of the 12 aliphatic chains per molecule is approximately 2.3 nm2). This suggests a possible tilted, edge-on or end-on orientation in the S phase. Figure 4 shows a schematic
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Figure 4. Schematic representations of molecular orientations in the LE and S phases upon increasing surface pressure.
Figure 5. (a) AFM image of a Langmuir-Blodgett monolayer of 2 transferred in the coexistence region. (b) Cross-sectional height data associated with the horizontal line drawn in (a).
representation of face-on (LE phase) and tilted (S phase) orientations. AFM experiments were performed in order to obtain quantitative information about the thickness difference of the monolayer in the LE and S phases. The monolayer was transferred in the coexistence region, at 10.5 mN/m, with transfer ratios of
approximately unity. Domains of similar lateral dimensions as those seen in BAM were observed with AFM (see Figure 5). The height difference between these domains and the surrounding lower areas were measured and found to be 1.5 ( 0.4 nm. Given that the length of the macrocycle core is approximately 2.5 nm, the measured height difference is consistent with a face-on orientation of the LE phase and a tilted end-on orientation in the S phase as illustrated in Figure 4. Photoisomerization. Once we hypothesized that the macrocycle is lying flat on the water surface at low surface pressures, with the “pore” of the molecule parallel to the water surface, we were most interested in investigating the light-responsive properties in the LE phase. In terms of controlling transport at the interface with the monolayer, this parallel orientation is the most useful one. In addition, it is at these lower pressures that the macrocycle has the maximum free volume available for the isomerization. Since the cis isomer is calculated to take up 1.2 nm2 more area, in the absence of sufficient free volume, conversion to the cis isomer could be prohibitively high in energy. While keeping the monolayer at a constant area, the monolayer was exposed to first 375 nm light, followed by 470 nm light. The 375 nm light causes the trans to cis isomerization and shape change of the macrocyclic core depicted in Figure 1. The polar core area of the macrocycle is estimated to increase from 1.3 nm2/molecule to 2.5 nm2/molecule upon conversion of both azo bonds to the cis form. This area increase is manifested as a surface pressure increase upon exposure to UV light (Figure 6). With a starting surface pressure of 7 mN/m, exposure to 375 nm light caused an increase to 7.9 mN/m, reaching a steady state over the course of 3-4 min. The reverse process was triggered
Figure 6. Reversible change in surface pressure with exposure to UV and visible light.
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by irradiation with 470 nm light, which indicates the reversible nature of the system.
Conclusions
Performing the same experiment at very low surface pressures (0.0-1.0 mN/m) did not result in a measurable increase in pressure upon 375 nm irradiation. It seems likely that, at these low pressures, there is sufficient free space between macrocycles and/or the intermolecular interactions are sufficiently governed by the large hydrophic periphery that the change in core area does not result in a significant surface pressure increase. The relatively small change in surface pressure upon isomerization (∼1 mN/m) is attributed to the relatively loose molecular packing in the LE phase, where the molecular repulsion is dominated by weak interactions between disordered aliphatic chains. However, we cannot rule out the possibility that conversion to the open cis conformer is anomalously low in the monolayer configuration compared to the conversion in solution. Ex situ isomerization experiments on LB monolayers transferred to quartz slides (see Supporting Information) displayed a conversion to the cis isomer of only ca. 10%, as measured by UV-vis spectroscopy.13 In comparison, ca. 80% conversion to cis isomer was achieved in dilute acetone solution. However, since conversion is often highly dependent upon the molecular environment, it is not clear that we can extrapolate the results from the transferred films on quartz to the more mobile monolayers on the water surface. Clearly, in situ UV-vis experiments of the Langmuir monolayer on the water surface would be required to fully resolve this issue. (13) Brode, W. R.; Gould, J. H.; Wyman, G. M. J. Am. Chem. Soc. 1952, 74, 4641.
In summary, an amphiphilic azobenzene-containing macrocycle has been synthesized and characterized in Langmuir monolayers. Pt-pyridine coordination chemisty proved to be an efficient way of installing the polar macrocycle core. The azobenzenes present in the core produce a shape change of the macrocycle upon irradiation with light. Appropriate design of the core allowed two states to be accessible: a more “open” state (cis isomer) and a more “closed” state (trans isomer). Stable Langmuir monolayers were formed with this shape-changing macrocycle, and a phase transition from an expanded to a condensed state was observed. In this transition, AFM and BAM data support the hypothesis of the macrocycle core lying flat with the long axis parallel to the water surface at low surface pressures and tilting on end as the surface pressure increases. UV irradiation of the macrocycle at low surface pressures in its “flat” orientation results in a reversible surface pressure increase, a result of the increase in area of the “open” cis isomer. Thus, these materials can be thought of a potential photoswitchable barrier to transport at the interface. Currently, studies on controlling the evaporation of water through the monolayer by light-mediated transport are underway. Acknowledgment. This work was supported by National Science Foundation Awards DMR-0213918 (EK and DKS) and DMR-0111193 (CSP and DLG). Supporting Information Available: Experimental Section. This material is available free of charge via the Internet at http://pubs.acs.org. LA701345E
