Electro- and Photochemistry of 13-Membered Azocrowns in Solution and as Langmuir-Blodgett Films Leonid M. Goldenberg,†,‡ Jan F. Biernat,§ and Michael C. Petty*,† School of Engineering and Centre for Molecular Electronics, University of Durham, Durham DH1 3LE, UK, Department of Chemical Technology, Technical University, 80952 Gdansk, Poland, and Institute of Chemical Physics in Chernogolovka, Russian Academy of Science, Chernogolovka, 142432 Moscow region, Russia Received July 15, 1997. In Final Form: December 15, 1997
Electrochemical and photochemical investigations of amphiphilic 13-membered azobenzene crowns have shown that, in solution, Z T E isomerization takes place for both the free crown ether and also for complexes with metal ions. Evidence for the formation of a new 1:2 (ligand:Na+) complex is presented. Monolayer Langmuir-Blodgett (LB) films on indium-tin oxide electrodes exhibited a stable voltammetric response corresponding to the cis isomer. Significant differences were found for both the photochemical behavior and the ion selectivity between the solution and solid phases. Some indications of electrochemcial recognition of Na+ were found for some LB films. This was evident from a shift in the voltammetric potentials and in the peak current versus scan rate dependence.
Introduction Azo and azoxy compounds exist in two isomeric forms, Z and E. For example, the simplest azo compounds azobenzenesexists in these forms. The configurations differ in that the E form is flat,1 with zero dipole moment, while in the Z form the phenyl rings are not coplanar with a resulting dipole moment of 3.0 D.2 In solution, the Z form is less stable and under the influence of a solvent, visible light, or increased temperature is transformed into the E form.3 The conversion of Z azobenzene into E is inhibited in alkaline solutions, while in slightly acidic solution the conversion is very fast. There is now considerable interest in the synthesis and properties of azo and azoxy compounds. This is partly related to the possibilities of exploiting these materials as molecular switches and optical memories. Azo compounds can also be used as the photoactive or redox-active components of films deposited on solid substrates.4 The combination of -NdN- or -N(O)dN- groups with a crown ether can result in an even more interesting class of materials. These may form the basis of highly selective chemical sensors. Over recent years, we have been studying the family of azocrowns 1-4.5 A single crystal of the Z isomer of 1 was identified by chance from a bulk E isomer crystal mass and fully characterized by X-ray studies. Later it was found that Z and E isomers of 13-membered azocrown ethers could be separated.5g Both isomers were found to form a complex with NaI with the formula Na(1)2+‚I- possessing a trans geometry for the coordinated azo group of the ligand.6 Based on 1H NMR spectra,5c,g the E geometry was found for the crystalline bis(tetramethylbutyl) azocrown 3. Both isomers of 3 form well-packed monolayers at the airwater interface5ce,7 and interact with a sodium salt added to the subphase to form an E complex.8 †
University of Durham. Russian Academy of Science. § Technical University, Gdansk. ‡
(1) Brown, C. J. Acta Crystallogr. 1966, 21, 146. (2) Jaffe, H. H.; Yeh, S. J.; Gardher, J. Mol. Spectrosc. 1958, 2, 120. (3) Fischer, E. J. Am. Chem. Soc. 1960, 82, 3249.
The azoxycrown ether 2 has the Z geometry.9 In a sodium iodide complex the substituent location around the -NdN- bond is the same as for the free ligand 2.6 This is probably true for compound 4. Here, we report further studies of the amphiphilic 13membered azo- and azoxycrowns 3 and 4, both in solution and as Langmuir-Blodgett (LB) monolayers deposited on solid supports. The work may be useful for the design of electrode surfaces for electroanalytical chemistry.10 Experimental Section Langmuir-Blodgett film deposition was undertaken in a class 10,000 microelectronics clean room using a Labcon Molecular Photonics minitrough. The details of monolayer behavior at the air-water interface have been reported elsewhere.5c,e Films were transferred onto solid supports at a dipping speed of 5 or 10 mm min-1. Monolayer and multilayer films were built up on quartz and indium tin oxide glass (ITO, sheet resistance 300 Ω per square, from Balzers). To improve the hydrophilic properties of ITO, these substrates were treated prior to LB film transfer by saturated Na2Cr2O7/concentrated H2SO4 solution for ca. 10 s and carefully washed with ultrapure water.11 Optical absorption spectra were recorded by using a Perkin-Elmer Lambda 19 UVvis-near-IR spectrometer. ITO slides with an area between 20 and 30 cm2 were used for LB film transfer. After film deposition, the slides were carefully cut with a diamond-tipped stylus to form several electrodes with contact areas between 0.1 and 0.5 cm2. Spectroscopy and photolysis were carried out in a 1-cm quartz cuvette. The solution was irradiated by using a highpressure mercury lamp (Hanonia, 125 W) with a band-pass glass filter (centered at 365 nm) and cutoff glass filters (>420 nm). An EG&G PARC model 273 potentiostat with an Advanced Bryans XY recorder was used for the electrochemical experiments. Pt mesh was used as the counter electrode and ITO electrodes covered with LB films served as working electrodes. A Pt disk, diameter 0.076 mm, and a glassy carbon disk, diameter 3 mm (Bioanalytical System Incorporated), were the working electrodes for solution electrochemistry. All potentials were recorded versus an Ag/AgCl reference electrode in aqueous solution and versus Ag/Ag+ in organic solutions (corrected to Fc/Fc+ as 0.35 V vs Ag/AgCl). Acetonitrile (Aldrich, HPLC or anhydrous), dimethylacetamide (Fluka, anhydrous), HClO4 (Aldrich, ACS reagent), Bu4NClO4 (Fluka, electrochemical grade), LiClO4 (Fluka, microselect), NaClO4, KClO4, Me4NCl (all Aldrich), and ultrapure water were used for the preparation of electrolyte solutions. (4) (a) Herr, B. R.; Mirkin, C. A. J. Am. Chem. Soc. 1994, 116, 1157. (b) Cadwell, W. B.; Chen, K. M.; Herr, B. R.; Mirkin, C. A.; Hulteen, J. C.; Duyne, R. P. V. Langmuir 1994, 10, 4109. (c) Caldwell, W. B.; Campbell, D. J.; Chen, K.-M.; Herr, B. R. Mirkin, C. A.; Malik, A, Durbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071. (d) Liu, Z. F.; Hashimoto, K.; Fujishima A. Nature 1990, 47, 658. (e) Liu, Z. F.; Loo, B. H.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1991, 297, 133. (f) Liu, Z. F.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. 1992, 96, 1875. (5) (a) Biernat, J. F.; Luboch, E.; Cygan, A.; Simonov, Yu. A.; Dvorkin, A. A.; Muszalska, E.; Bilewicz, R. Tetrahedron 1992, 48, 4399. (b) Biernat, J. F.; Cygan, A.; Luboch, E.; Simonov, Yu. A.; Dvorkin, A. A. J. Incl. Phenom. 1993, 16, 209. (c) Luboch, E.; Biernat, J. F.; Muszalska, E.; Bilewicz, R. Supramol. Chem. 1995, 5, 201. (d) Muszalska, E.; Bilewicz, R. Analyst 1994, 119, 1235. (e) Muszalska, E.; Bilewicz, R.; Luboch, E.; Skwierawska, A.; Biernat, J. F. J. Incl. Phenom. 1996, 26, 47. (f) Biernat, J. F.; Luboch, E.; Skwierawska, A.; Bilewicz, R.; Muszalska, E. Biocyb. Biomed. Eng. 1996, 16, 125. (g) Skwierawska, A.; Luboch, E.; Biernat, J. F.; Kravtsov, V. Ch.; Simonov, Yu. A.; Dvorkin, A. A.; Bel’skii, V. K. J. Incl. Phenom., submitted for publication. (6) Simonov, Yu. A.; Luboch, E.; Biernat, J. F.; Bolotina, N. V.; Zavodnik, V. E. J. Incl. Phenom. 1997, 28, 17. (7) Huesmann, H.; Maack, J.; Mo¨bius, D.; Biernat J. F. Sens. Actuators 1995, B29, 148. (8) Zawisza, I.; Bilewicz, R., in preparation. (9) Biernat, J. F.; Cygan, A.; Luboch, E.; Simonov, Yu. A.; Dvorkin, A. A. J. Incl. Phenom. 1993, 16, 209. (10) Bilewicz, R. Ann. Chim. 1997, 87, 53. (11) Fu, Y.; Ouyang J.; Lever, A. B. P. J. Phys. Chem. 1993, 97, 13753.
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Results and Discussion Solution Studies. The electrochemical and photochemical behavior of compounds 3 and 4 may be determined by Z-E isomerization and by complexation with metal ions. Z-E isomerization is usually triggered thermally or by light. A priori complexes with metal cations can be formed with both Z and E forms and with the reduced (hydrazobenzene) form. However, the binding constants may differ. The existence of 1:1, 2:1, and 1:2 (ligand:M+) complexes must be considered. Electrochemical studies5c showed that the potential for the first reduction of compounds 3 in DMF was -1.49 V vs SCE. According to LB electrochemistry12 this may indicate that compound 3 is in the E form. The electrochemical behavior of azobenzene derivatives has been studied extensively.13 There has been much discussion on the possibility of electrochemical recognition by E-Z isomers. As early as 1953, it was reported13f that isomers were reduced in acidified ethanolic solutions at the same potential, while in alkaline solutions the reduction potentials differed. It was postulated that, at low pH, the electrode reaction is reversible and the Z isomer is always converted to the E form before the final reduction product is obtained. At high pH, the reduction requires an overpotential depending on the isomer. In an even earlier report,13g a difference of reduction potentials of 0.235 V was found. This value corresponds to the free energy difference of 10.8 kcal mol-1 and agrees with the value of 12 kcal mol-1 found by calorimetry.13h Later all these observations were disputed,13c,i,j and no separate peaks for Z-E isomers were seen, even at -80 °C. A small prewave was reported13k at pH 8-9 and ascribed to the reduction of the Z form. Laviron13c observed different reduction potentials but only in alkaline ethanol solutions. Our present studies in dimethylacetamide and acetonitrile exhibited irreversible reduction peak potentials at -1.7 and -1.65 V vs Ag/AgCl (scan rate 1 V s-1) in agreement with the previous results.5c Illumination of the dimethylacetamide solution by UV for 30 min did not significantly change the reduction potentials; longer illumination led to disappearance of the voltammetric peaks (possibly another photochemical pathway). To find out which isomer dominates in solution, we followed the photolysis in acetonitrile by spectroscopy. Previously, photochemical Z-E interconversion was observed in floating layers.7 Here, irradiation by UV light resulted in an enhancement of the Z isomer and irradiation by visible light in an increase of the E form. The spectrum of compound 3 in acetonitrile is characteristic of the E form as the ratio of intensities in UV and visible regions is a distinctive mark of isomers.7 Irradiation of acetonitrile solution of compound 3 by visible light did not show a significant spectral change, again suggesting that compound 3 is in a photostationary state, predominantly the E form. Moreover, irradiation with UV light (Figure 1) resulted in the observation of a photostationary state due to E-Z isomerization. Three isosbestic points (at ca. 410, (12) (a) Liu, Z. F.; Hashimoto, K. Fujishima, A. J. Electroanal. Chem. 1992, 324, 259. (b) Liu, Z. F.; Zhao, C. Y.; Tang M.; Cai, S. M. J. Phys. Chem. 1996, 100, 17337. (13) (a) Wavzonek, S. Frederikson, J. D. J. Am. Chem. Soc. 1955, 77, 3985. (b) Salder J. L.; Bard, A. J. J. Am. Chem. Soc. 1967, 90, 1979. (c) Laviron E.; Mugnier, Y. J. Electroanal. Chem. 1980, 111, 337. (d) Laviron, E. J. Electroanal. Chem. 1984, 169, 29. (e) Flamigni, L.; Moni, L. J. Phys. Chem. 1985, 89, 3702. (f) Hilson, P. J.; Birnbaum, P. P. Trans. Faraday Soc. 1952, 478. (g) Winkel, A.; Siebert, H. Ber. 1941, B74, 670. (h) Hartley, J. Chem. Soc. 1938, 633. (i) Castor, R. C.; Saylor, J. H. J. Am. Chem. Soc. 1953, 75, 1427. (j) Laviron, E.; Mugnier, Y. J. Electroanal. Chem. 1978, 93, 69. (k) Chuang, L.; Fried, I.; Elving, P. J. Anal. Chem. 1965, 37, 1528.
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Figure 1. Absorption spectra of compound 3 (2.1 × 10-5 M) in acetonitrile irradiated with UV light (365 nm) for 0, 80, 200, and 440 s; dashed linesssubsequently irradiated with visible light (>420 nm) for 420 and 1800 s. Inset shows expanded scale.
Figure 2. Absorption spectra of compound 3 (7.8 × 10-5 M) in acetonitrile containing NaClO4 at concentrations of 0, 1.98 × 10-3, 3.92 × 10-2, 5.83 × 10-2, 9.52 × 10-2, and 1.82 × 10-1 M. Inset shows expanded scale.
270, and 245 nm) are evident, with π f π* absorption (300-365 nm) decreasing and n f π* (420-460 nm) increasing in good agreement with monolayer studies.7 Compound 3 in an ion-selective electrode is known to be highly selective to Na+ ions.5c But high sodium selectivity in a membrane electrode does not imply a high stability for the complex.14 Earlier spectrophotometrical studies of complexation for the unsubstituted analogue 115 in acetonitrile resulted in a log K value of 4.10 for the complexation with LiI. Corresponding values for NaI and KI could not be determined due to the small spectral changes. In contrast, our results exhibit a significant spectral change under complexation with Na+ (Figure 2, note that we used NaClO4). There are clear isosbestic points at ca. 460, 300, and 245 nm. We have treated the data according to a simplified equation for the formation of the 1:1 complex, described by us earlier,16 K
3 + Na+ 798 3(Na+)
(A0 - A)/(A - Ai) ) K[Na+]
where A0 and Ai are the absorbances at zero and infinite salt concentrations, respectively. This resulted in reasonably linear plots with some deviation from linearity at high Na+ concentrations (Figure 3a). The stability constant appears to be wavelength dependent. The values for binding constants together with the regression coefficients are collated in Table 1. Deviation from linearity (low regression coefficient at some wavelengths) may indicate a more complex behavior. A similar approach was therefore used to fit the data to the formation of 2:1 (ligand:Na+) and 1:2 complexes. The former did not provide a good fit to the experimental data (negative value of stability constant, data not shown), while the 1:2 complex shows a better fit than for the 1:1 complex (Figure 3b and Table 1). We have previously observed unusual behavior of sulfonamidocrown ethers, where one metal cation is located inside the cavity, and the second is attached as a “co-cation” outside the cavity.17 However, (14) (a) Biernat, J. F. Izv. Akad. Nauk Mold. (ser. Fiz. Tiechnika), in press. (b) Biernat, J. F.; Luboch, E. 9th International Symposium on Molecular Recognition and Inclusion, Lyon, 7-12 Sept, 1996; p D32. (15) Shiga, M.; Nakamura, H.; Tanagi, M.; Ueno, K. Bull. Chem. Soc. Jpn. 1984, 57, 412. (16) (a) Dieing, R.; Morisson, V.; Moore, A. J.; Goldenberg, L. M.; Bryce, M. R.; Raoul, J.-M.; Petty, M. C.; Garin, J.; Saviron, M.; Lednev, I. K.; Hester, R. E.; Moore, J. N. J. Chem. Soc., Perkin Trans. 2 1996, 1587. (b) Lednev, I. K.; Petty, M. C. Adv. Mater. Opt. Electron. 1995, 5, 137. (c) Lednev, I. K.; Petty, M. C. J. Phys. Chem. 1995, 99, 4176.
Figure 3. Plots for (A0 - A)/(A - Ai) versus (a) [Na+] and (b) [Na+]2 for different wavelengths. A0, absorbance of uncomplexed compound, and Ai, absorbance of fully complexed compound. Data from Figure 2. Table 1. Binding Constants for the Complexes of Compound 3 with Na+ wavelength, nm 210 245 270 360 520
there is some experimental evidence18 that, in the solid state, unsubstituted compound 1 forms a 2:1 complex with Na+. Photolysis experiments with the complexed compound 3 (Figure 4) showed the same behavior as for the
Chemistry of 13-Membered Azocrowns
Figure 4. Absorption spectra of compound 3 (2.1 × 10-5 M) in acetonitrile containing 8.3 × 10-2 M NaClO4 and irradiated with UV light (365 nm) for 0, 120, and 260 s.
Figure 5. Comparison of absorption spectra for uncomplexed (solid line) and complexed with Na+ (dashed line) compound 3 before and after (lower lines) UV irradiation (365 nm). Data from Figures 1 and 4.
uncomplexed compound. However, under irradiation with visible light we did not observe any spectral changes (at least on a similar time scale to that for the uncomplexed compound). This indicates better stability of Z-3 in complexed form, which can be explained by the fact that the electron density on -NdN- is higher for cis-azobenzene than for trans.19 Monolayer complexation studies also showed that isomerization is possible in the presence of Na+.7 We should note that both E and Z isomers exhibited a bathochromic shift of the π f π* transition and a hypsochromic shift of the n f π* transition under complexation with Na+ (Figure 5). As deduced before from the spectroscopic results, we have compound 3 in the predominantly E form in acetonitrile solution. A positive shift of the reduction peak potential of about 250 mV was observed on addition of Na+ to the dimethylacetamide solution (data not shown). After complexation with Na+, the compound is still in the E-3 form according to our spectroscopic results. The anodic shift is associated with the complexation, as the complex should be easier to reduce than the neutral compound. LB Films Studies. Monolayer behavior at the airwater interface for compounds 3 and 4 has been described previously.5c,e Compound 3 can easily be transferred onto a solid substrate at a surface pressure of 20 mN m-1, either as a monolayer or as Z-type multilayers, with a transfer (17) Biernat, J. F.; Bochenska, M.; Bradshaw, J. S.; Koyama, H.; Lindh, G.; Lamb, J. D.; Christensen, J. J.; Izatt, R. M. J. Incl. Phenom. 1987, 5, 729. (18) Simonov, Y. A.; Biernat, J. F., personal communication. (19) Badger, G. M.; Lewis, G. E. J. Chem. Soc. 1953, 2147.
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Figure 6. CV response for an LB monolayer of compound 3 on an ITO electrode in 0.1 M MClO4 aqueous solutions, scan rate 0.05 V s-1. (a) Li+, dashed line; Na+, dotted line; K+, dashed-dotted line; Me4N+, solid line; (b) H+. Arrows indicate scan direction.
ratio of 1 ( 0.1 and 0.8 ( 0.1, respectively. The CV response for monolayer films on ITO electrodes was recorded in different electrolyte solutions (Figure 6). Stable, reproducible electrochemical behavior was observed for all these films. The coverage by electroactive species, Γ, estimated by graphical integration of background-corrected CV peaks was 0.05-0.12 nmol cm-2. This is lower than expected from molecular area data (0.23 nM cm-2 at a dipping pressure of 20 mN m-1 and 0.17 nM cm-2 at a molecular area extrapolated to zero surface pressure). Possible reasons include the error in calculation of the electrode area, the electrochemical irreversibility manifested by a large peak separation, even at slow scan rate, the electrode roughness and monolayer rigidity leading to a situation where not all molecules are in direct contact with the electrode surface, and diffusion limitations in hydrophobic films. However, as the Γ values calculated at 25 mV s-1 and 1 V s-1 were similar (at least for some electrolytes, but not for NaClO4), diffusion limitations may not be significant. Another possible explanation for the low Γ value is that compound 3 in LB film form might exist as a mixture of Z and E isomers, with the E form exhibiting a very broad reduction at more cathodic potentials.12 However, application of the method developed by Liu et al.,12a where the amount of the E form was estimated by scanning to more negative potentials and detecting the increase of the reoxidation peak, did not show any change in the reoxidation peak (data not shown). This indicates that we have predominantly the Z form in the LB film. The values of the electrochemical potentials correspond well to the CV response observed for another amphiphilic azobenzene derivative in the Z form.12 These also correspond to the behavior of the films of compound 3 adsorbed on a Hg electrode.5e As the floating monolayer was predominantly in the E form,7 we suggest that we have an “aggregation state induced” E-Z conversion. No change could be observed in the CV response either after heating the film at 70 °C for 1 h (to accelerate thermal cis-trans conversion) or following illumination by visible light for 1 h. This suggests that the metastable cis form is stable in the LB film. Despite special precautions taken to keep the LB films in the dark (UV-free fluorescence tubes in a clean room), a change in the cis-type CV response was not observed, even after several weeks. Furthermore, illumination by UV light for 1 h did not change the CV response, again indicating that we have compound 3 in the Z form in the LB film. A difference in the CV response in acidic and
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Table 2. CV Data for the Monolayer Films of Compound 3 on ITO Electrodes in Different Electrolytes electrolyte, 0.1 M
HClO4 LiClO4 NaClO4 KClO4 Me4NCl
0.02 -0.28 -0.36 -0.30 -0.28
0.37 0.28 0.26 0.31 0.31
65 180 180 220 190
85 110 110 120 105
At 50 mV s-1; half-height width.
Figure 7. Plots of peak separation versus log(scan rate) for LB monolayers of compound 3 on ITO electrodes. Electrolytes are aqueous 0.1 M MClO4 where M is indicated.
Figure 8. Plots of oxidation peak current versus scan rate for LB monolayers of compound 3 on an ITO electrode; electrolytes are aqueous 0.1 M MClO4 where M is indicated.
neutral solution is associated with a pH dependence of the azobenzene electrochemistry (2e/2H transfer in neutral and acidic solutions) as shown for a reduction peak in azobenzene LB films,12 shifted by 86 and 63 mV per pH unit, respectively. For instance, the pH values for 0.1 M HClO4, NaClO4, and LiClO4 solutions were 1.4, 5.8, and 6.4, respectively. Reduction peaks for different electrolytes are collated in Table 2. It is notable that the peak separation (Figure 7) and peak half-height width (Table 2) were smaller at lower pH. In all the electrolyte solutions studied, we observed an increase in the peak separation with scan rate (Figure 7), the logarithmic dependence indicating an electrochemically irreversible process. The CV response in different electrolytes showed a linear dependence of the peak current on the scan rate (Figure 8), which is expected for a surface electrochemical reaction. However, deviation from linearity was observed at higher scan rates, indicating diffusion limitation. The greatest deviation was with Na+ (current saturates at lower scan rates) and the smallest with K+. Peak half-height widths, even for acidic solutions, were higher than theoretically predicted for two-electron transfer (45 mV), suggesting
Figure 9. CV response for a monolayer of compound 3 on an ITO electrode in 0.1 M LiClO4 aqueous, scan rate 0.05 V s-1, with different concentrations of Na+ added: dashed line, 1.3 × 10-2 Na+; dotted line, 3.75 × 10-2 Na+.
Figure 10. Shift of oxidation potential Ep,a, reduction potential Ep,c formal potential E0, and peak separation ∆Ep versus Na+ concentration. Data from Figure 8.
strong intermolecular interactions (ref 20 and references therein). From Figure 6, it is evident that the biggest difference in reduction potential was observed between Li+- and Na+-containing electrolytes, which may be related to complexation with Na+. A small cathodic shift in the Na+-containing electrolyte was also observed for an unsubstituted analogue of 1-2 accumulated on the Hg electrode.5e Therefore, we studied the dependence of the CV response in 0.1 M LiClO4 on the addition of NaClO4 (Figure 9). A change of electrochemical behavior with a negative shift of the reduction peak potential was observed. This is opposite to what would be expected for the complexation with Na+. Moreover, the difference in pH for these two solutions should account for about 50 mV of the positive shift. We do not have any explanation for this behavior. The change of potentials and peak separation versus concentration of Na+ are plotted in Figure 10. The oxidation peak potential changes only slightly and the largest difference is observed for the reduction peak potential and peak separation. We tried to treat these data using the approach for the case where two separate peaks cannot be observed.16a The best fit was obtained by using the peak separation ∆E as a variable in the model (Figure 11). However, the slope of this fit (1.5) does not correspond to the expected figure of 1 for 1:1 complex formation or 2 for 1:2 complex formation. This suggests a more complex behavior. To summarize, we have found some indications of electrochemical cation (Na+) recognition in our LB films. This is shown by a shift of the redox potentials and in the peak current-scan rate dependence. However, the mechanism for this process remains unclear and further work is necessary to ascertain if the films may be exploited as sensors. An attempt to study complexation by spectroscopy was unsuccessful. No spectral changes were observed for a 10-layer film, even after 1 h in 0.2 M NaClO4 solution. (20) Daifuku, H.; Aoki, K.; Tokuda, K.; Matsuda, H. J. Electroan. Chem. 1985, 183, 1.
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at 0.39 V vs Ag/AgCl in 0.1 M HClO4). However, the CV response was less developed than for compound 3 and the electroactivity was lower. This compound was therefore not studied any further. Conclusions
Figure 11. Logarithmic plots x/(1 - x) versus Na+ concentration, where x is the change of peak separation ∆E divided by the change at maximum Na+ concentration.
We did not attempt to follow the photochemical reaction in the LB films by spectroscopy as it was already noted that there were no changes in the electrochemistry of the film after illumination. Monolayers of compound 4 were also transferred on an ITO electrode with a transfer ratio close to unity. The electrochemical response was similar to that observed for compound 3 (reduction peak at 0 V and reoxidation peak
Electrochemical and photochemical investigations of amphiphilic 13-membered azocrown 3 have shown that in solution Z-E isomerization takes place for both the free crown ether and also its complexes with metal ions. Evidence of the formation of a new 1:2 (ligand:Na+) complex is presented. LB films were transferred onto solid substrates. Monolayer films deposited onto ITO were stable in the cis form. This is thought to be a result of “aggregation state induced” cis-trans conversion. Some indications of electrochemical recognition of Na+ by an LB monolayer of compound 3 have been found. Acknowledgment. L.M.G. thanks the University of Durham and the Russian Foundation for Fundamental Research (project 97-03-32268a) for financial support. J.B. thanks KBN (grant 3TO9A15508) for financial support. LA970790Z