Langmuir 2007, 23, 11611-11616
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Langmuir and Langmuir-Blodgett Monolayers of Molecular Tweezers and Clips P. Degen,*,† T. Optenhostert,† H. Rehage,† C. Verhaelen,‡ M. Lange,‡ J. Polkowska,‡ and F.-G. Kla¨rner‡ Institut fu¨r Physikalische Chemie II, UniVersita¨t Dortmund, Otto Hahn Strasse 6, 44227 Dortmund, Germany, and Institut fu¨r Organische Chemie, UniVersita¨t Duisburg-Essen, 45117 Essen, Germany ReceiVed June 12, 2007. In Final Form: August 10, 2007 Molecular clips and tweezers are able to selectively bind electron-deficient aromatic and aliphatic substrates. By means of pressure-area isotherms and Brewster angle microscopy (BAM), the self-association process and phase behavior of dimethylene-bridged molecular clips and tetramethylene-bridged molecular tweezers each substituted with two acetoxy groups as polar head groups were investigated. In a series of experiments, we observed that the molecular surface area of the clips and tweezers only depended on the skeletal structure and not on the polar groups. The measured areas agreed with the effective molecular diameters of the molecules if the aromatic side walls of the clips or tweezers were assumed to be aligned perpendicularly to the water surface. We compared the phase behavior of the pure molecular clips and tweezers with that of the host-guest complexes of these molecules, which were formed with 1,2,4,5-tetracyanobenzene (TCNB) as the guest molecule. For the clips with a central benzene (I) and naphthalene spacer unit (II), the complex formation with TCNB had no measurable influence on the phase diagrams of the films. We observed, however, a dramatic difference in the BAM images and π-A isotherms between the pure molecular tweezers III and its complex with TCNB (TCNB@III). In addition to the π-A isotherms, we used the surface potential (V)-area (A) isotherms to compare the pure tweezers III with the corresponding complex (TCNB@III). There was a strong difference in the maximum surface potential value for the pure tweezers (450 mV) and that for the complex (300 mV). In additional experiments, we prepared LB layers of such molecules, which were investigated by fluorescence spectroscopy. In comparison to the pure tweezers III, a luminescence emission of charge-transfer (CT) origin was observed for the host-guest complex (TCNB@III) fixed on the solid substrate. It turned out that the spectra were in good agreement with the results observed in chloroform solution.
1. Introduction In recent years, molecular assemblies on solid surfaces have been the subject of extensive studies.1-3 The reason is their potential application in the creation of novel electronic, photoelectronic, and sensing films. Chemical immobilization of small molecules onto a substrate surface is one of the important approaches to prepare molecular assemblies.4 Among the techniques for the deposition of thin films on solid substrates, layer-by-layer deposition and the Langmuir-Blodgett (LB, vertical lift) technique are two of the most promising methods because they enable fine control of the thickness and homogeneity of the monolayer.5,6 Furthermore, the monolayer technique is a useful tool for model studies (mostly at the water surface) because surface pressure/area (π-A) isotherms reflect the intermolecular forces operating in the 2D arrangement of molecules. These data, sometimes referred as two-dimensional phase diagrams, provide information on the molecular packing.7 By extrapolation of the isotherm from the high-pressure regime to zero pressure, the size of the amphiphilic molecules can be estimated assuming a dense packing of the molecules.8 The asymmetric arrangement * To whom correspondence should be addressed. E-mail: patrick.
[email protected]. † Universita ¨ t Dortmund. ‡ Universita ¨ t Duisburg-Essen. (1) McConnell, H. M. Annu. ReV. Phys. Chem. 1991, 42, 171. (2) Mo¨hwald, H. Thin Solid Films 1988, 159, 1. (3) Knobler, C. M.; Desai, R. C. Annu. ReV. Phys. Chem. 1992, 43, 207. (4) Vollhardt, D. AdV. Colloid Interface Sci. 1994, 73, 1. (5) Stine, K. J.; Moore, B. G. In Nano-Surface Chemistry, 1st ed.; Rosoff, M., Ed.; Marcel Dekker: New York, 2001; p 59. (6) Decher, G. Science 1997, 277, 1232. (7) Lavigne, P.; Tancrede, P.; La marche, F.; Max, J. J. Langmuir 1992, 8, 1988.
of amphiphilic molecules at the water surface leads to macroscopic electric dipole moments perpendicular to the interface.9 A relation between the molecular dipole moments and the surface potential is given by Demchak and Fort.10 The simultaneous measurement of the surface pressure (π) and the surface potential (V) allows one to get additional information on the film properties.11-14 The monolayer technique also allows studying of the film structure just before coating the solid substrate. Microscopic observation of the textures of Langmuir monolayers is possible, for example, by fluorescence microscopy, Brewster angle microscopy (BAM), and ellipsometric microscopy techniques.15,16 Molecular clips and tweezers are interesting because of their supramolecular properties, like self-assembling and molecular recognition of small electron-deficient guest molecules.17 Molecular recognition is important in many areas of biological and supramolecular chemistry, for example, in antigen-antibody recognition or enzyme-substrate binding.18 The electrostatic potential surface (EPS) of I-IV was calculated to be surprisingly negative on the concave side of each molecule and, hence, (8) Vollhardt, D. AdV. Colloid Interface Sci. 1996, 64, 143. (9) Heinig, P.; Wurlitzer, S.; Steffen, P.; Kremer, F.; Fischer, Th. M. Langmuir 2000, 16, 10254. (10) Demchak, R. J.; Fort, T. J. Colloid Interface Sci. 1973, 46, 191. (11) Hoda, K.; Ikeda, Y.; Kawasaki, H.; Yamada, K.; Higuchi, R.; Shibata, O. Colloids Surf., B 2006, 52, 57. (12) Petrov, J. G.; Andreeva, T. D.; Kurth, D. G.; Mo¨hwald, H. J Phys. Chem. B 2005, 109, 14102. (13) Ahluwalia, A.; Piolanti, R.; De Rossi, D. Langmuir 1997, 13, 5909. (14) Oliviera, O. N.; Bonardi, C. Langmuir 1997, 13, 5920. (15) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (16) He´non, S.; Meunier, J. ReV. Sci. Instrum. 1991, 62, 936. (17) Kla¨rner, F.-G.; Kahlert, B. Acc. Chem. Res. 2003, 36, 919. (18) Atwood, J. L.; Davies, J. E. D.; MacNicol, D. D.; Vo¨gtle, F.; Suslick, K. S. ComprehensiVe Supramolecular Chemistry, Elsevier: Oxford, U.K., 1996.
10.1021/la7017314 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/06/2007
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Figure 1. Structures of the investigated molecular clips and tweezers; I ) clip and IV ) tweezers, each with a p-diacetoxybenzene spacer; II ) clip and III ) tweezers, each with a 1,4-diacetoxynaphthalene spacer.
complementary to the EPS of the electron-deficient substrates, suggesting that the attractive receptor-substrate interaction is here predominantly of electrostatic nature.19,20 These specific host-guest relationships were extensively studied in organic or aqueous solutions by Kla¨rner et al.17,21-23 It could be shown that some of the receptor molecules are highly selective for Nalkylpyridinium ions like NAD+, which is an important cofactor of many redox-active enzymes.24 Because of their hydrophilic and hydrophobic chemical groups, the molecular clips and tweezers studied here have distinct surfaceactive properties. The molecules tend to form, therefore, monomolecular films on the water surface.25 This phenomenon offers the opportunity to investigate molecular recognition processes at fluid surfaces and provides a convenient way to produce molecular assemblies on solid surfaces, for example, by LB techniques. In comparison to former investigations,26 we were interested in the influence of different skeletal structures on the aggregation process and the morphology of these receptors on the water surface. Therefore, we used one clip and one tweezer, each substituted with a central 1,4-diacetoxynaphthalene spacer unit (II an III), and one clip and one tweezer, each substituted with a 1,4-diacetoxybenzene spacer unit (I and IV). The molecular structures are shown in Figure 1. One important aim of our work was the investigation of the behavior of the pure host molecules in comparison to the corresponding host-guest complexes. Former investigations did not show any effect of the host-guest complex formation on the Langmuir film of the clip (I) with different polar groups bound to the central benzene spacer unit and TCNB as the guest molecule.26 We assumed that the small association constant for the complex of the clip I with TCNB was responsible for this finding (Table 1). The association constants in CHCl3 solution for the investigated receptors with the guest TCNB were obtained
Table 1. Association Constants of Different Molecular Clips and Tweezers
a
molecule
Ka [M-1]
I II III IV
140 ( 14a 555 ( 56b 7.3 × 105 c g103
From ref 28. b From ref 29. c From refs 20 and 27.
by 1H NMR and spectrofluorimetric titrations. Typical results of these investigations are summarized in Table 1.27 The association constant for the complex formation of benzene tweezers IV with TCNB is, up to now, unknown. However, the comparison of the association constants determined for the complexes of naphthalene tweezers II and benzene tweezers IV, for example, with p-dicyanobenzene or 7,7,8,8-tetracyano-pquinodimethane (TCNQ) as the guest molecules, allows the estimate of Ka values. For the complexes with the benzene tweezers, the values are smaller by a factor of about 10-100 than those of the corresponding complexes with the naphthalene tweezers, and hence, the association constant of complex TCNB@IV is expected to be in the range of Ka g103 M-1.20,27As the complex of the tweezers (III) with the aromatic substrate TCNB is known to be highly stable (Ka ) 7.3 × 105 M-1), we investigated the influence of this guest molecule on the film structure. In additional experiments, we also measured the molecular space occupied by tweezers III on the water surface in the compressed state. The results give new insights into fundamental processes of self-association and might be useful to develop new chemical sensors on the basis of LangmuirBlodgett films. For this purpose, we have made first attempts to produce LB layers of the tweezers III and its corresponding complex with TCNB. 2. Materials and Methods
(19) Kamieth, M.; Kla¨rner, F.-G.; Diederich, F. Angew. Chem., Int. Ed. 1998, 37, 3303. (20) Kla¨rner, F.-G.; Burkert, U.; Kamieth, M.; Boese, R.; Benet-Buchholtz, J. Chem.sEur. J. 1999, 5, 1700. (21) Kamieth, M.; Burkert, U.; Corbin, P. S.; Dell, S. J.; Zimmermann, S. C.; Kla¨rner, F.-G. Eur. J. Org. Chem. 1999, 11, 2741. (22) Kla¨rner, F.-G.; Burkert, U.; Kamieth, M.; Boese, R. J. Phys. Org. Chem. 2000, 13, 604. (23) Kla¨rner, F.-G.; Benkhoff, J.; Boese, R.; Burkert, U.; Kamieth, M.; Naatz, U. Angew. Chem., Int. Ed. Engl. 1996, 35, 1130. (24) Fokkens, M.; Jasper, C.; Schrader, T.; Koziol, F.; Ochsenfeld, C.; Polkowska, J.; Lobert, M.; Kahlert, B.; Kla¨rner, F.-G. Chem.sEur. J. 2005, 11, 477. (25) Rehage, H.; Achenbach, B.; Kla¨rner, F. G.; Kamieth, M.; Panitzky, J. Tenside, Surfactants, Deterg. 2004, 41, 121. (26) Degen, P.; Rehage, H.; Kla¨rner, F.-G.; Polkowska, J. Colloid Polym. Sci. 2005, 284, 44.
The molecular clips or tweezers as well as their 1:1 mixtures with TCNB were spread on the water surface from a 10-3 M solution in chloroform (p.a. Merck). Similarly, we also spread a 1:1 mixture of the molecular tweezers III and TCNB, whereas the used concentration was 5 × 10-4 M. The water was obtained from a pure water system (Seralpur PRO 90 CN). Synthesis and supramolecular properties of the molecular clips and tweezers are described elsewhere.23,24 The structures of the molecular clips and tweezers are shown in Figure 1. The solubility of the clips and tweezers is less than 0.05 mg/100 mL of water at 20 °C. In our experiments, we did not observe any (27) Marchioni, F.; Juris, A.; Lobert, M.; Seelbach, U. P.; Kahlert, B.; Kla¨rner, F.-G. New J. Chem. 2005, 29, 780.
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Langmuir, Vol. 23, No. 23, 2007 11613 Table 2. Influence of Different Central Spacer Units on the Average Diameter of Molecular Clips and Tweezers in the Solid-Condensed Phase
Figure 2. π-A isotherms of the different compounds I-IV at the air-water interface. solution process of the molecules in water during film compression or expansion. The film properties of the different samples were investigated by means of pressure (π)-area (A) isotherms and BAM. The π-A isotherms were recorded using a Langmuir trough (001BAM) and a continuous Wilhelmy-type measuring system, both constructed by NIMA Technology (Coventry, England). The BAM images were performed with a BAM 2 apparatus constructed by Nanofilm Technology Company in Go¨ttingen, Germany. Extensive description of BAM is summarized in several publications.15,16 The lateral resolution of BAM is limited by the wavelength of the incident laser beam (690 nm). Variation of the time intervals (5 min, 2 h) between the spreading process and the beginning of the compression did not influence the data. Therefore, in all of our π-A isotherm measurements, we started the compression 15 min after spreading. The compression and the expansion velocities were adjusted to 0.2 nm2 molecules-1 min-1. Each π-A isotherm was confirmed by at least two repeated, independent measurements. The temperature was always fixed at 20 °C. Simultaneous to the π-A isotherms, the surface potential (V)-area (A) isotherms were measured via the vibrating capacitor method (using a Kelvin probe Nima KP1). The LB layers were prepared using a LB trough (611) also combined with a Wilhelmy-type measuring system, both constructed by NIMA Technology. We prepared just one single LB layer at a plate angle of 45° from the water surface, and the dipping velocity was adjusted to 10 mm/min. The surface pressure was fixed at 30 mN/m. Fluorescence spectroscopic measurements were performed on a quartz glass substrate by use of a Fluoroscan FL 3095 spectrofluorimeter (Firma: J & M, Aalen, Germany).
3. Results 3.1. Influence of Different Skeletal Structures on the Phase Behavior of Molecular Clips and Tweezers during Film Compression. In this publication, we focused on the influence of the basic aromatic structures of the molecular clips and tweezers I-IV on their phase behavior. For all investigated molecules, the OCOCH3 groups were used as polar substructures. Typical results of π/A isotherms of clip I, clip II, tweezers III, and tweezers IV are summarized in Figure 2. At the beginning of the compression, we observed different effects depending on the chemical structures of the aromatic subunits. If the arene units were organized in a bent belt-type arrangement, as it holds for tweezers III and IV, the π-A isotherms raised at an area of 0.87 nm2. The isotherms of the clips I and II with planar naphthalene side walls did not raise before an area of 0.61 nm2 was reached. These findings could not be explained with the differences in the spatial areas required for the tweezers and clips. The dimensions of the molecular clips and tweezers were calculated
molecular clip
required area [nm2]
I II III IV
0.32 ( 0.04 0.49 ( 0.02 0.49 ( 0.02 0.32 ( 0.03
by means of force field methods (MMFF94, Monte Carlo conformer search) and are depicted in the Supporting Information. If the tweezers and clip molecules were assumed to lie flat on the water surface and the molecules rotated around their principal axis, which seems to be reasonable at the point of pressure lift off, the predicted average areas were different from the experimental data (see Supporting Information). The observed deviations could be the result of specific intermolecular interactions, the distribution of various conformations, and/or aggregate formations. It is also possible that the larger lift off areas for the π-A isotherms of the tweezers III and IV are related to differences in the hydrophilic/hydrophobic character of the moieties. This would change the long-range interaction of the moiety and the aqueous subphase. Previous investigations have shown that watersoluble clips and tweezers form self-assembled dimers.30 We do not expect that similar self-assembled dimers are also formed at the water surface because, for energetic reasons, all polar head groups should dip into the water surface. At the end of the compression steps, no influence of the aromatic side walls on their spatial areas was observed. Experimental values of the area requirements for the molecules were derived from the interception of the linear portion of the solid-condensed phase, which was extrapolated to the regime of zero surface pressure. They are summarized in Table 2. The values show that the spatial area of the molecules only depends on the size of the spacer unit. The area required for the benzene spacer unit of the molecules I and IV (0.32 nm2) are considerably lower than that required for the molecules II and III (0.49 nm2). The results are independent of the structures of the side walls. If we assume that the molecules rotate around their principal axis, the minimum area of the molecules with a naphthalene spacer unit (II and III) is about 0.62 nm2. This calculated value is based on the distance between the carbon atoms of the methylene bridges at the central spacer unit (Supporting Information) and is larger than the value determined by pressure-area isotherms (0.49 nm2). On the basis of the assumption that the molecules do not rotate around there axis in the solid-condensed state, the measured areas are in good agreement with the molecular distances of all clips and tweezers (Supporting Information). The isotherms of I, III, and IV show plateau regions at about 7 and 25 mN/m. Plateaus at low surface pressures often describe the transition between the liquid-expanded (LE) and the liquidcondensed (LC) phase.12 Other possible reasons for the formation of regions of constant surface pressure are super saturation of the LE, gaseous phases, hysteresis effects, or 2D/3D transitions.7,8 Although the curves are reproducible, only a systematic change of the plateau in the π-A isotherms toward higher pressures, as the temperature increases, can provide unambiguous evidence for a LE/LC transition. A helpful tool to interpret the data of a single π-A isotherm is the Brewster angle microscopy. In the region of the LE-LC (28) Kla¨rner, F.-G.; Polkowska, J.; Panitzky, J.; Seelbach, U. P.; Burkert, U.; Kamieth, M.; Baumann, M.; Wigger, A. E.; Bo¨se, R.; Bla¨ser, D. Eur. J. Org. Chem. 2004, 1405. (29) Lange, M. Diploma Thesis, Universita¨t Essen, 2001. (30) Kla¨rner, F.-G.; Kahlert, B.; Nellesen, A.; Zienau, J.; Ochsenfeld, C.; Schrader, T. J. Am. Chem. Soc. 2006, 128, 4831.
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Figure 3. BAM images of the monolayers of the receptors I-IV during compression. First line ) I; second line ) II.
phase transition, the BAM images should show the presence of well-defined domains. The parameters in Figure 2 show some interesting points which specify substantial changes of the film properties. The corresponding BAM images are summarized in Figure 3. The BAM images presented in Figure 3 do not show the formation of well-defined phase domains. We observed rather unstructured bright areas in addition to a darker phase. The third row (molecule III) suggests a more-fluid-like phase, while the fourth panel (molecule IV) suggests a more-solid-like phase due to the appearance of sharp edges on domains. This must be related to differences in molecular structure between the two tweezers III and IV. In the case where domains or aggregates were formed, these structures were small and below the limit of optical resolution (690 nm). At the present state, we cannot explain the appearance of the plateau regimes on the basis of BAM images alone. The BAM images of II, III, and IV showed that already for zero surface pressure and large areas, the molecules form large aggregates and dense zones. The occurrence of multilayers usually leads to bright BAM images, but we did not observe bright structures during the compression up to surface pressures of about 40 mN/m. After a value of approximately 45 mN/m, we measured a kink in the π-A isotherm and also bright lines in BAM images of the folded molecular films. To specify the phase behavior of the receptors in such molecular films, other methods like ellipsometric, X-ray, or neutron reflectivity could additionally be used. 3.2. Influence of TCNB as a Molecular Guest on the Phase Behavior of Tweezers III. In correlation to previous results,26 we could not see any influence of TCNB on the π-A isotherms and BAM images for the clips I and II. The complexes of the clips with TCNB, however, were less stable than that of the naphthalene tweezers III with TCNB (Table 1). The unknown
Figure 4. Influence of TCNB on the π-A and V-A isotherms of III; black lines ) π-A isotherms; blue lines ) V-A isotherms.
association constant of complex formation between the tweezers IV and TCNB was estimated to be Ka g 103 M-1.20,27 On the basis of these arguments, we only investigated the influence of TCNB on the phase behavior of tweezers III. In addition to the π-A isotherms of the pure tweezers and of the corresponding complex with TCNB, the V-A isotherms were determined (Figure 4). Figure 4 shows a clear difference between the pure tweezers III and the corresponding complex TCNB@III in the π-A isotherms as well as in the V-A isotherms. During the film compression of the pure tweezers, the pressure did not rise before reaching an area of 0.85 nm2. For the complex TCNB@III, however, significant surface pressure values were reached already at molecular areas of about 1.17 nm2. At surface pressures above 25 mN/m, the two isotherms became nearly identical. That means that the size per molecule is independent of complex formation in the concentrated regime. This also points to a perpendicular orientation of the aromatic side walls with respect to the water surface in the solid-condensed state.
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Figure 5. Single-crystal structure of the complex
[email protected] Figure 7. Typical BAM images of the monolayers of III with TCNB observed for different angles of the analyzer. The surface pressure was fixed to 18 mN/m.
Figure 6. BAM images of the monolayers of III without TCNB (a) and with TCNB (b).
As expected, the V-A isotherms rose a little bit earlier than the corresponding π-A isotherms. This holds for the pure tweezers and the complex. The characteristics of the V-A isotherms are in good agreement with previous investigations in several publications31 and was not discussed in detail here. The surface potential of the pure tweezers III sharply increased below ∼0.95 nm2 and reached a value of 350 mV at the initial increase of the surface pressure. During the increase of the surface pressure, the surface potential V slightly increased to a maximum value of 380 mV. For the corresponding complex TCNB@III, the surface potential increased below ∼1.45 nm2 and was less sharp than that for the pure tweezers. The beginning of the increase was omitted for clearness of the figure and for a better comparability between the two π-A and V-A isotherms. The surface potential for the complex reached a value of 250 mV at the initial increase of the surface pressure, and after this, it remained constant until the compression was completed. The reduction of the surface potential in the corresponding complex TCNB@III is reasonable if we take into account that the electrondeficient guest molecule TCNB was placed inside of the electronrich cavity of the receptor molecule, as it is shown in the singlecrystal structure of the complex TCNB@III (Figure 5). (31) Dhanabalan, A.; Gaffo, L.; Barros, A. M.; Moreira, W. C.; Oliveira, O. N. Langmuir 1999, 15, 3944.
The different behavior of the complex TCNB@III in comparison with the pure tweezers III was also observed by the BAM images. Figure 6 shows typical images of the molecular tweezers (left column) and the complex (right column) for several compression states, respectively. The BAM images clearly differ for the complex TCNB@III and the pure tweezers III. At the beginning of the compression at low surface pressure, large unshaped aggregates for the pure tweezers could be recognized, whereas the complex showed a network of fractal structures. At medium surface pressures, the pure tweezers showed fluctuating, tape-like structures which were randomly distributed. In the presence of TCNB, we observed two kinds of regions with different brightness. However, the structures of these regions had a fractal shape. The different brightness in the separated regions was probably caused by the different long-range orientation order. Figure 7 shows the BAM images of the monolayer of the complex TCNB@III at a surface pressure of 15 mN/m with different angles of the analyzer. In the depicted images, the inversion of the brightness, which is caused by the optical anisotropy of the domains, is assigned by a black circle. At an analyzer angle of 180°, the brightness of the marked structure was only a bit lower than that for the adjacent area. When tuning the analyzer up to an angle of 100°, the brightness of the adjacent areas increased while the framed area became darker. An inversion of the brightness occurred in the area between 100 and 90°. In the last picture at 80°, the original dark structure became bright. 3.3. Langmuir-Blodgett (LB) layer of III and TCNB@III: Fluorescence Emission Properties. The separate components III and TCNB are colorless in chloroform solution and also on the water surface. When both species were present in equimolar concentration, the solution became yellow. The absorption and emission properties of the complex TCNB@III and the pure tweezers III in chloroform solution were extensively described,22,27 and the emission spectrum of the complex clearly differs from the spectrum of the pure tweezers. On the basis of these observations, we measured the emission spectra of the LB layer of the pure tweezers III and of the previously prepared
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Figure 8. Emission spectra of the pure tweezers III and the complex TCNB@III (room temperature, glass substrate, LB transfer pressure 30 mN/m).
complex TCNB@III on a glass substrate. Figure 8 illustrates the emission bands reflected by the LB layer of the pure molecular tweezers III and by the CT adduct TCNB@III. In chloroform solution, the pure components III and TCNB showed the typical π-π* fluorescence emission expected for such aromatic species. The emission bands for tweezers III were structured and showed only a small blue shift upon lowering the temperature.27 This emission band could also be observed for the LB layer of the tweezers III as a broad, unstructured band at approximately 370 nm. The wavelength at which the emission band appeared was in good agreement with the results obtained in chloroform solutions. We could, however, not detect a structured emission band as described for the chloroform solution.27 The emission spectra of a LB layer of the previously prepared complex TCNB@III showed an emission band at 540 nm. Such a broad and unstructured band was also observed for the TCNB@III complex in chloroform solution. This phenomenon was extensively described by Marchioni et al.27 The shape, lifetime (920 ns for chloroform solution), and band shift upon lowering the temperature of this new emission band are characteristic for a charge-transfer (CT) emission.32,33 The results showed that we were able to transfer the complex TCNB@III from the CHCl3 solution to the water surface and afterward to the solid substrate without destroying the molecular structures.
4. Conclusions In a series of experiments, we used π-A isotherms and BAM images to investigate monolayers of different molecular clips and tweezers. We were interested in investigating the influence of different molecular groups on the phase diagrams of monomolecular films. We were able to show that at the beginning (32) Kla¨rner, F.-G.; Kahlert, B.; Boese, R.; Bla¨ser, D.; Juris, A.; Marchioni, F. Chem.sEur. J. 2005, 11, 3363. (33) Jones, G., II. In Photoinduced Electron Transfer, Part A; Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, The Netherlands, 1998.
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of the film compression, the structure of the aromatic side walls had an influence on the π-A isotherms. In the regime of high concentrations, we observed marked influence of the central spacer units of the surface-active compounds. The molecular surface areas of the two clips and the two tweezers, determined from pressure-area isotherms, were in good agreement with the theoretical values. This holds under the assumption that the arene side walls were oriented perpendicular to the water surface. That is an important prerequisite for the use of the molecules as a synthetic receptor at interfaces. We could show by π-A and V-A isotherms and BAM that the complexation of TCNB by the molecular tweezers III had distinct influence on the phase behavior and surface potential of the monolayers. For the surface pressure π, this was especially observed in the regime of low surface pressures. The reduction of the surface potential of the monolayer of the tweezers caused by the complexation of TCNB up to 120 mV (or 28.5%) gave a new and simple opportunity to investigate such host-guest interactions at the water surface. In comparison to the pure tweezers III, the complex with TCNB formed a network of a fractal structure at low surface pressures. In more concentrated regimes, we observed an optical anisotropy between two different regions in the Langmuir film. In the solid-condensed state, however, we could not detect any influence of the complex formation on the π-A isotherms. At these conditions, the guest molecules did not lead to significant changes of the phase diagrams. This result confirms that the complex is oriented like the pure tweezers (with arene side walls perpendicular to the water surface) and that TCNB is placed inside of the cavity of the tweezers molecule. In additional experiments, we prepared LB layers in order to investigate the possibility of forming molecular sensors on solid substrates. We transferred the compressed film of the pure tweezers III and the previously prepared complex TCNB@III onto solid glass substrates. Afterward, we could investigate the optical properties of such LB layers. We found marked differences between the LB layers of the pure tweezers III and the complex TCNB@III in their fluorescence emission spectra. The emission spectra of the LB monolayer of the complex TCNB@III showed a substantial charge-transfer (CT) emission band at 540 nm, which is in good agreement with experiments performed in organic solvents like chloroform.27 The results are certainly an encouragement for a future application of molecular clips and tweezers as molecular sensors. By the immobilization of molecular tweezers and clips on a substrate surface, we should be able to detect electrondeficient aromatic and aliphatic guest molecules by fluorescence spectroscopy and surface potential measurements of their hostguest complexes. This gives us the opportunity to prepare a photo- and electroresponsive sensor for many biologically interesting molecules like NAD+. Acknowledgment. Financial support for this work through grants from the “Deutsche Forschungsgemeinschaft” DFG Re 681/15-1 is gratefully acknowledged. Supporting Information Available: Calculation of the dimensions of the molecular clips and tweezers. This material is available free of charge via the Internet at http://pubs.acs.org. LA7017314
