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Thin-Film Characterization of Neutral π-Associated Assemblies Incorporating Putative Pseudo-Rotaxanes Daniel E. Lynch,*,† Darren G. Hamilton,‡ Nicolas J. Calos,§ Barry Wood,§,| and Jeremy K. M. Sanders‡ School of Natural and Environmental Sciences, Coventry University, Coventry CV1 5FB, U.K., University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, U.K., and Department of Chemistry and Brisbane Surface Analysis Facility, The University of Queensland, Brisbane Q4072, Australia Received November 26, 1998. In Final Form: April 26, 1999 Thin films of 1:1 and 1:2 stoichiometric mixtures of bis(1,5-naphtho)-38-crown-10 (1) with both N,N′di-n-alkylpyromellitic diimide (2) and N,N′-di-n-alkyl-1,4,5,8-naphthalenetetracarboxylic diimide (3) have been prepared using both Langmuir-Blodgett (LB) and thermal evaporation techniques. For thermal evaporation, the substituted n-alkyl chains were C6H13 (2a and 3a, respectively), while for LB deposition, they were C14H29 (2b and 3b, respectively). LB monolayers of different stoichiometric mixtures of both 1.2b and 1.3b were studied using UV-visible spectroscopy, Brewster-angle microscopy, and angle-dependent X-ray photoelectron spectroscopy and revealed that in both cases a stoichiometric mix of electron-donating and electron-accepting aromatic moieties was necessary to achieve good film-forming properties. 1:2 mixtures of 1.2a and 1.3a were then deposited as thin films using thermal evaporation techniques and studied using atomic force microscopy. This showed that films of 1.2a consisted of large spheres e ca. 300 nm in diameter whereas films of 1.3a were found to be relatively uniform, with the maximum surface roughness being less than a third of the total film thickness. Additional electrical conduction measurements made on both LB and thermally evaporated prepared metal/organic/metal devices resulted in only evaporated films of 1.3a giving any measurable resistance. Furthermore, films of this material displayed a Poole-Frenkel conduction mechanism. These experiments have shown that rudimentary electrical devices can be prepared from neutral π-associated assemblies.
1. Introduction An emergent theme of contempary supramolecular chemistry concerns the potential of self-assembly to provide access to discrete, functional molecular assemblies. An undeniably attractive application of such technology would be the development of molecular-scale mimics of electronic components, and much has been written concerning the prospect of organizing such “components” into simple devices1 and, ultimately, molecular-scale versions of integrated circuits.2 One class of supramolecular materials that has been proposed for providing potential vehicles for the transfer of molecular organization is that of topologically complex molecular assemblies.3 The templated syntheses of these systems of interlocked macrocycles, rings on thread molecules, and knotted assemblies are seen as ways of defining spatial relationships between, for example, electron-donating and -accepting species4 and therefore exerting control over electron-transfer processes.5 We have developed supramolecular assembly routes6-9 to a class of neutral * Corresponding author. Tel.: +44 (01203) 83 8019. Fax: +44 (01203) 83 8282. E-mail:
[email protected]. † Coventry University. ‡ University Chemical Laboratory. § Department of Chemistry, The University of Queensland. | Brisbane Surface Analysis Facility, The University of Queensland. (1) Metzger, R. M.; Panetta, C. A. New J. Chem. 1991, 15, 209. (2) Hopfield, J. J.; Onuchic, J. N.; Beratan, D. N. Science 1988, 241, 817. (3) Amabilino D. B.; Stoddart, J. F. Chem. Rev. 1995, 95, 2725. (4) Anelli, P. L.; Ashton, P. R.; Ballardini, R.; Balzani, V.; Delgado, M.; Gandolfi, M. T.; Goodnow, T. T.; Kaifer, A. E.; Philp, D.; Pietraszkiewicz, M.; Prodi, L.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Vicent C.; Williams, D. J. J. Am. Chem. Soc. 1992, 114, 193.
π-associated interlocked molecules (Figure 1), the electronaccepting elements of which possess useful electrochemical10 and photochemical properties.11 These aromatic diimides combine the electron-accepting properties of bipyridinium dications, the ubiquitous components of a huge class of charged π-associated interlocked molecules, with a neutral aromatic framework. The absence of potentially charge-carrying counterions from systems prepared with our neutral complementary building blocks could present a considerable advantage in the development of functional molecular assemblies. As a preliminary step toward establishing the viability of our system of components for potential device preparation, we have examined the thin-film-forming properties of supramolecular assemblies based on crown ether 1 and aryl diimides 2 and 3 (Figure 2); these complexes are viewed as simple prototypes for more complex topological systems derived from these components. Similar thin-film studies have been previously performed on charged bipyridinium pseudo-rotaxanes12 and, more recently, on some more (5) For a theoretical analysis of a molecular electronic application of electron-transfer, see: Hopfield, J. J.; Onuchic, J. N.; Beratan, D. N. J. Phys. Chem. 1989, 93, 6350. (6) Hamilton, D. G.; Davies, J. E.; Prodi, L.; Sanders, J. K. M. Chem. Eur. J. 1998, 4, 608. (7) Hamilton, D. G.; Feeder, N.; Prodi, L.; Teat, S. J.; Clegg, W.; Sanders, J. K. M. J. Am. Chem. Soc. 1998, 120, 1096. (8) Try, A. C.; Harding, M. M.; Hamilton, D. G.; Sanders, J. K. M. Chem. Commun. 1998, 723. (9) Hamilton, D. G.; Feeder, N.; Teat, S. J.; Sanders, J. K. M. New J. Chem. 1998, 4, 1019. (10) Viehbeck, A.; Goldberg, M. J.; Kovac, C. A. J. Electrochem. Soc. 1990, 137, 1460. (11) Greenfield, S. R.; Svec, W. A.; Gosztola, D.; Wasielewski, M. R. J. Am. Chem. Soc. 1996, 118, 6767. (12) Ahuja, R. C.; Caruso, P.-L.; Mo¨bius, D.; Philp, D.; Preece, J. A.; Ringsdorf, H.; Stoddart, J. F.; Wildburg, G. Thin Solid Films 1996, 284, 671.
10.1021/la9816508 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/23/1999
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for incorporation into simple electronic devices. The instrumental techniques used to elucidate the thin-film quality of each material in two different solid-state forms were UV-visible spectroscopy, Brewster-angle microscopy, angle-dependent X-ray photoelectron spectroscopy (ADXPS), shallow-angle X-ray reflectometry (SAXR), and atomic force microscopy (AFM). While the complexes used in this study are, in the long term, of limited interest, they do serve as indicators of useful prototypes for further thin-film studies of mechanically interlocked systems. Figure 1. Chemical illustration of an interlocked neutral π-associated catenane.
Figure 2. Chemical illustrations of the naphthoxy crown (compound 1), pyromellitic diimide derivatives (compound 2a,b), and naphthalene diimide derivatives (compound 3a,b).
complex branched bipyridinium [n]rotaxanes13 and benzylic amide [2]catenanes.14 A first step toward the realization of molecular electronic devices involving, for example, π-associated catenanes involves an examination of the solid-state form in which they are to be used. For electrooptic devices, this is typically as thin solid films. Such films can be produced by using a variety of techniques, but several of these, such as spincoating, require the material under investigation to be supported in a polymer matrix. Under such conditions the long-range donor-acceptor ordering of these materials is likely to be compromised. Two technologies which allow fabrication of thin films of pure target materials are the Langmuir-Blodgett (LB) technique and thermal evaporation. Of these two, thermal evaporation is the preferred method for commercial film production, especially because no special considerations need to be observed in the synthesis of the materials under investigation. By contrast, the attachment of long hydrophobic alkyl chains is generally necessary for LB deposition. An implicit assumption of our use of the latter two thin-film fabrication techniques is that the donor-acceptor interactions between the complementary electron-rich and electronaccepting components, which have proved so useful for directing assembly of discrete molecular assemblies, will provide a consistent ordering force for thin-film assembly. In this study we have investigated the thin-film characteristics of two different series of supramolecular assemblies in order to begin to evaluate their potential (13) Amabilino, D. B.; Asakawa, M.; Ashton, P. R.; Ballardini, R.; Balzani, V.; Beˇlohradsky´, M.; Credi, A.; Higuchi, M.; Raymo, F. M.; Shimizu, T.; Stoddart, J. F.; Venturi, M.; Yase, K. New J. Chem. 1988, 4, 959. (14) Fustin, C. A.; Rudolf, P.; Taminiaux, A. F.; Zerbetto, F.; Leigh, D. A.; Caudano, R. Thin Solid Films 1988, 327, 321.
2. Experimental Section Bis(1,5-naphtho)-38-crown-10 (1),6 N,N′-di-n-hexylpyromellitic diimide (2a) and N,N′-di-n-hexyl-1,4,5,8-naphthalenetetracarboxylic diimide (3a) were prepared according to literature procedures.15 The tetradecyl analogues (2b and 3b, respectively) were prepared using analogous procedures from condensation reactions of the appropriate aryl dianhydride with n-tetradecylamine in hot dimethylformamide. All starting materials were purchased from Aldrich. Choices of chain length for the electron-accepting diimide derivatives were made to satisfy criteria of solubility in the case of the hexyl derivatives used in the vapor deposition work and hydrophobicity in the case of tetradecyl derivatives used in the fabrication of Langmuir films. Samples for thermal evaporation studies were prepared by dissolving molar ratioed (either 1:1 or 1:2) amounts of the crown and the diimides in dichloromethane followed by the removal of the solvent under reduced pressure to yield the adduct. Surface pressure-area (π-A) isotherms for compounds 2b and 3b, and both 1:1 and 1:2 mixtures of 1.2b and 1.3b were run on a Nima Technology series 2000 L-B trough. LB monolayers were spread from chloroform solutions (ca. 0.1 mg mL-1) onto pure water subphases and then deposited on (a) both sides of hydrophilic quartz substrates for UV-vis measurements and (b) hydrophilic (111)-cut silicon wafers for ADXPS studies at a compressed surface pressure of 15 mN m-1 (13 mN m-1 for the 1:1 mixture of 1.3b) on the upstroke by passing the substrate through the floating monolayer at 25 mm min-1. Langmuir monolayers of 1:1 and 1:2 mixtures of complexes 1.2b and 1.3b were also studied using a MiniBAM (Brewster-angle microscope) from Nano Film Technology GmbH. UV-vis measurements were made on a Unicam UV-4 UV-visible spectrometer. AFM images were collected using a Burleigh ARIS 3300 Personal AFM. Thermally evaporated films of the 1:2 mixtures of 1.2a and 1.3a were prepared in a vacuum of 10-6 mbar using an Edwards 306 coating unit and were deposited on (a) hydrophilic quartz substrates for UV-vis and AFM surface studies and (b) hydrophilic glass slides (half-coated with a thin film of tin) for electrical conduction and SAXR measurements. To assess the structural integrity of the thermally evaporated films, the organic material was washed from representative slides with dichloromethane. Mass spectral analysis with a linked liquid chromatography-electrospray mass spectrometer revealed the presence of the crown ether and the appropriate dialkyl diimide derivative, confirmed by matching their elution times and mass spectral signatures with the original samples. High-field 1H NMR (500 MHz) of the same film extracts again confirmed the identity of the components of the films, and integration of the spectra confirmed retention of the crown:diimide ratio (either 1:1 or 1:2) used in preparing the films. Both this ratio and the lack of evidence for breakdown products, using either (15) Hamilton, D. G.; Prodi, L.; Feeder, N.; Sanders, J. K. M. J. Chem. Soc., Perkin Trans. 1 1999, 1057.
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Figure 3. π-A isotherms for compound 2b (-‚-), the 1:1 mixture of 1.2b (- - -), and the 1:2 mixture of 1.2b (s). The molecule in the molecular area is defined as the putative 1:1 or 1:2 adduct complex.
technique, suggest minimal decomposition of the organic materials during film preparation. Furthermore, after evaporation no organic residue was detected in the evaporating boat. Thermally evaporated metallic tin films (thickness of ca. 100 nm) were prepared in a vacuum of 10-6 mbar using an Edwards 306A coating unit. SAXR patterns for the determination of thin-film thicknesses were recorded on a Philips PW1700 system diffractometer (Cu KR X-radiation). The electron spectrometer employed for ADXPS studies was a Physical Electronics Industries PHI model 560, using a model 25-270 AR cylindrical mirror analyzer (CMA).16 This double-pass CMA rotatable drum device has a fixed sample-analyzer-source geometry, thus avoiding errors due to misalignment which may occur from tilting of the sample.17 Multiple sweeps of the 1s peaks of C, N, and O with binding energies of 285.0, 398.0, and 532.0 eV, respectively, from the monolayers and the Si 2p peaks at 101.8 and 105.4 eV from the substrates were used for the analyses. Spectra relative to the sample surface were collected at 87.7, 69.9, 51.5, and 34.5° for all samples. Integrated intensities were normalized for instrumental sensitivity factors. Mean-free paths for the photoelectrons through each of the media were calculated according to the formulas of Ashley.18 Using functions of the form given by Marshbanks et al.,19 a set of functions describing the angle-dependent relative intensities of each of the C, N, O, and Si signals was derived for monolayers of compounds 2b and 3b and 2:1 mixtures of 1.2b and 1.3b. A “trial-and-error” least-squares fitting of the observed data was undertaken, using free variables of elemental scale factors, the percentage coverage of the Si surface, and the thickness of each layer. Additionally, the presence of the surface oxide layer on silicon served as an internal standard to verify the accuracy of the reported organic thicknesses. Previously reported values for a SiO2 layer on silicon lie within the range 18 ( 2 Å19,20 3. Results and Discussion 3.1. Langmuir Studies of 1.2b. The π-A isotherms for compound 2b and the 1:1 and 1:2 mixtures of 1.2b are shown in Figure 3; at the concentration of the spreading (16) Seah, M. P.; Mathieu, H. J. Rev. Sci. Instrum. 1985, 56, 703. (17) Hofmann, S.; Sanz, J. M. Surf. Interface Anal. 1984, 6, 75. (18) Ashley, J. C. IEEE Trans. Nucl. Sci., NS-27 1980, 1454. (19) Marshbanks, T. L.; Jugduth, H. K.; Deglass, W. N.; Franses, E. I. Thin Solid Films 1993, 232, 126. (20) Sarapatka, T. J. Thin Solid Films 1993, 226, 219.
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solution, films of the pure crown ether 1 were soluble in the subphase and upon compression developed no significant surface pressure. While significant differences between compressed monolayers of pure 2b and the isotherms of the donor-acceptor mixes are observed, little difference is noted between the isotherms of the 1:1 and 1:2 mixtures of 1.2b, with film collapse in both cases occurring at a surface area of 68 ( 2 Å mol-1 at a surface pressure of 20 ( 1 mN m-1. Accordingly, Langmuir monolayers were studied at 15 mN m-1 for both observation using a MiniBAM camera and deposition using absorption or X-ray photoelectron spectroscopy. The closely related molecular areas for the 1:1 and 1:2 mixtures indicate that both materials have similar limiting-area molecular arrangements. Considering the constituent molecules in both cases, the area is determined by a diimide molecule plus a naphthoxy ring system. This would suggest that, in the case of the 1:1 mixture, most of the diimide motifs are not bound within the dinaphthocrown ether (the slight decrease in the molecular area for the 1:1 isotherm is likely a result of a small degree of inclusion of 2b). Of course, the 1:2 mixture will suffer no change in the overall molecular area if the second diimide moiety resides within the dinaphthocrown ether, which is the situation observed in the 1:2 case. Observation of the filmforming process of the two mixed films using the MiniBAM shows that the 1:1 monolayer compresses as a uniform film up to 15 mN m-1. On decompression this film breaks apart into small islands, which upon recompression close up to a uniform film once again. The 1:2 film also compresses uniformly up to 15 mN m-1 but upon decompression expands as a well-ordered solid film, only slowly breaking apart at the edges; recompression of this film also occurs uniformly. Equivalence of donor and acceptor subunits is achieved for a 1:2 ratio of crown to diimide, potentially allowing the formation of extended donoracceptor stacks. It is notable that a radical improvement in film properties is noted when this ratio is reached. The UV-visible absorption spectra for monolayers of 2b and the 1:1 and 1:2 mixed films of 1.2b are shown in Figure 4. Aside from increased absorption in the aromatic region, little change is observed across the UV region for the 1.2b mixtures. The charge-transfer peaks that give rise to the characteristic colors of the donor-acceptor complexes are known to be rather weak and are not discerned in the two mixed films. However, although weak, the absorption maxima of the films are consistent with those of solutions of unambiguously interlocked molecules such as those shown in Figure 1.6 For ADXPS analysis the LB films dealt with in this study were considered as consisting of four layers. These layers were assigned nominal compositions for the purpose of calculating average electron densities and photoelectron penetration depths. The four layers are designated as (a) the alkyl group, represented as (CH2)n, (b) the aromatic diimide and diimide/crown ether headgroup, represented as CaHbNcOd (exact chemical formula for each case is listed in Table 1), (c) the oxidized Si surface, the “actual” substrate for the monolayer, and (d) an infinitely thick elemental Si layer. ADXPS depth profile thicknesses for these layers are listed in Table 1 and reveal that at 15 mN m-1 films of pure 2b deposit with the aromatic groups essentially parallel to the substrate with the alkyl chains arranged above, whereas 2:1 mixed films of 1.2b show an increased headgroup thickness but no overall increase in the total film thickness. However, the coverage factor is vastly improved in the mixed film. These thicknesses alone provide insufficient evidence to indicate the orientation of the crown ether/diimide headgroups which, as a
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Figure 5. π-A isotherms for compound 3b (-‚-), the 1:1 mixture of 1.3b (- - -) and the 1:2 mixture of 1.3b (s).
Figure 4. UV-visible absorption spectra for two monolayers of (a) compound 2b (lower curve) and the 1:2 mixture of 1.2b and (b) the 1:1 mixture of 1.2b. Table 1. ADXPS Thickness Data (Å) and Percentage Surface Coverage for Monolayers of Compounds 2b and 3b and the 1:2 Mixtures of 1.2b and 1.3b compound 2b
alkyl (CH2)n 13.0 ( 1.0
1.2b
7.5 ( 1.5
3b
6.0 ( 1.0
1.3b
6.5 ( 0.5
headgroup
SiO2
coverage
4.0 ( 2.0 (C10H2N2O4) 9.0 ( 2.0 (C52H40N4O16) 8.5 ( 1.5 (C28H8N4O8) 14.5 ( 0.5 (C60H44N4O16)
17.0 ( 1.0
78 ( 3%
17.5 ( 0.5
95 ( 1%
19.0 ( 0.5
88 ( 2%
17.5 ( 0.5 100 ( 1%
consequence, would indicate the direction of the chargetransfer (CT) stacks through which electrical conduction is expected. However, the thicknesses reported for the overlayer tetradecane suggest that in the two cases listed above the alkyl chains are inclined parallel to the surface and do not form densely packed Langmuir films. With this arrangement the headgroups may not be able to form connected donor-acceptor arrangements. 3.2. Langmuir Studies of 1.3b. The π-A isotherms for compound 3b and the 1:1 and 1:2 mixtures of 1.3b are shown in Figure 5; different isotherms are obtained in each case. Observation of the 1:1 mixture using the MiniBAM reveals that the film compresses as a sparsely packed monolayer with visible holes up to a surface pressure of 15 mN m-1. Increasing the pressure from 15 mN m-1 leads to a slight decrease in surface pressure as the film reorganizes to close up the holes. A uniform monolayer results. Decompression through this pressurearea regime reveals that the bulk of the monolayer remains intact with a little fracturing only at the edges; upon
recompression these defects close up once again to form a rigid monolayer. Decompression below 15 mN m-1 results in separation of the film into large rigid islands; upon recompression these isolated islands behave as for the initial compression with visible fractures up to 15 mN m-1. The 1:2 mixed film of 1.3b shows behavior similar to that of the 1:2 mixed film of 1.2b in that the film compresses as a uniform monolayer up to 15 mN m-1, while upon decompression the film only slowly expands with breaks appearing at the edges. However, a noticeable difference between the two 1:2 mixed systems is that 1.3b expands at a much slower rate than 1.2b. The recompressed 1:2 ratio films of 1.3b proved suitable for deposition with a compressed molecular area of 66 ( 2 Å mol-1 and a surface pressure of 18 ( 1 mN m-1 before collapse. The UV-visible absorption spectra for monolayers of 3b and the mixed films of 1.3b are shown in Figure 6. Notably the absorption maximum in the 1:1 film is less than half that for the 1:2 film. This indicates that, per unit area, less material is being deposited in the 1:1 case, a view which is supported by the observation of rigid holes in the compressed film below surface pressures of 15 mN m-1. These results show that there is a significant difference in the film-forming properties of the 1:1 and 1:2 mixtures of 1.3b and, by analogy with the 1.2b systems, it seems likely that in the case of the 1:1 mixed 1.3b film the naphthalene diimide units reside predominantly within the crown system, and therefore a fluid monolayer is not observed. Introduction of a second diimide molecule induces complementary intermolecular interactions because sufficient units are now available to form extended donor-acceptor stacks. Therefore, in this system, as for 1.2b, the 1:2 mixture is more suitable for LB deposition. ADXPS depth profile thicknesses are listed in Table 1 and reveal that at 15 mN m-1 films of pure 3b deposit with the diimide moiety inclined parallel to the substrate with the alkyl chains arranged above. In contrast, the films of the 1:2 mixture show an increased headgroup thickness, but with little change in the overlying alkyl chain thickness. In this respect the total thicknesses and the percentage coverages are significantly different for the two ratios examined, highlighting the improved LB characteristics of the 1:2 mixed system. 3.3. Thermal Evaporation Studies. The principal conclusion from the Langmuir studies is that it is necessary to have a stoichiometric mix of electron-donating and electron-accepting aromatic moieties to achieve good filmforming properties, i.e., 1:2 ratios of crown to diimide. Accordingly, thermally evaporated films of both 1:1 and
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Figure 8. Representative surface AFM picture of a thermally evaporated film of the 1:2 mixture of 1.3a.
Figure 6. UV-visible absorption spectra for two monolayers of (a) compound 3b (lower curve) and the 1:2 mixture of 1.3b and (b) the 1:1 mixture of 1.3b.
Figure 7. Representative surface AFM picture of a thermally evaporated film of the 1:2 mixture of 1.2a.
1:2 molar amounts of 1.2a and 1.3a were prepared to ascertain the optimum film-forming conditions. Thermally evaporated films of 1.2a (in both 1:1 and 1:2 mixtures) formed as translucent pale yellow layers which visibly scattered light, particularly UV-blue wavelengths. Investigation of the surface roughness of these films using AFM (Figure 7) shows that thermally evaporated layers of 1.2a consist of large spheres (large with respect to the size of the molecules under investigation) e ca. 300 nm in diameter, an observation which accounts for the scattering of UV-blue light. Thermal evaporation of both 1:1 and 1:2 mixtures of 1.3a produces transparent redpink layers which have considerably less surface roughness than the previous example. Figure 8 shows a representative AFM picture of the surface of a 90-nmthick thermally evaporated layer of 1:2 mixed 1.3a; the maximum surface roughness is less than a third of the
Figure 9. UV-visible absorption spectrum of a 90-nm-thick thermally evaporated film of the 1:2 mixture of 1.3a.
total thickness of the film. The UV-visible absorption spectrum of this film is shown in Figure 9. The AFM and UV-visible results are essentially the same for thermally evaporated films of the 1:1 mixture. In this study the difference between the two diimide systems is evident in that the larger naphthalene diimide produces good quality, uniform thermally evaporated thin films whereas those involving the smaller benzene diimide afford films of generally poorer quality. Other alternating donor-acceptor ring systems21,22 similar to benzene diimide produce identical films when fabricated using thermal evaporation techniques. The fact that a simple structural alteration can improve the thermal evaporation characteristics of these organics is important to note. 3.4, Electrical Conduction Studies. Devices to test the electrical conduction of both LB and thermally evaporated thin films were constructed as metal/organic/ metal layers. The preferred metal was tin because it has a conducting oxide, and the metal ions have limited mobility through the organic layer. LB film devices of both 1.2b and 1.3b (1:2 mixtures) were constructed using several multilayer thicknesses; in all cases no significant resistance was measured. Control of the CT stack direction under these conditions is an added problem which could potentially limit optimum device performance. Problems also arose with thermally evaporated films of 1.2a (both 1:1 and 1:2 mixtures) and the 1:1 mixed 1.3a, which did not display any significant resistance above that of the (21) Hamilton, D. G.; Lynch, D. E.; Byriel, K. A.; Kennard, C. H. L. Aust. J. Chem. 1997, 50, 439. (22) Hamilton, D. G.; Lynch, D. E.; Byriel, K. A.; Kennard, C. H. L.; Sanders, J. K. M. Aust. J. Chem. 1998, 51, 441.
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respond to a change in rotaxane/catenane structure, in line with the potential data storage mechanism proposed by Stoddart for systems of this type.23 If this were to prove the case, it would represent a method of both reading and writing to and from some property of the rotaxane/ catenane molecule. However, to develop a switching device, the use of asymmetrical metal electrodes is required, as demonstrated by Sakai et al. in studies of LB films of polyimide.24
Figure 10. Current (I) vs voltage (V) plot for a ca. 100-nmthick thermally evaporated thin film of the 1:2 mixture of 1.2a layered between two 100-nm-thick tin thin-film electrodes.
Figure 11. Plot of log(I) vs xV for the data presented in Figure 10.
“bare” tin electrode. In this case nonuniform coverage of the electrode surface is most likely the reason for electrical shorts. However, devices constructed using the 1:2 mixture of 1.3a were successfully produced using thermal evaporation, and the results for a ca. 100-nm-thick thin film are shown in Figure 10. These current vs voltage characteristics, when plotted as log(I) vs xV (Figure 11), are consistent with a Poole-Frenkel mechanism involving localized trapping sites for charge carriers. These localized sites are perhaps associated with the aromatic moieties of the stacked rotaxane systems and could conceivably
4. Conclusions In this study our goal was to determine the thin film characteristics of two types of neutral pseudo-rotaxanes and to begin to assess the suitability of these materials as compounds for fabrication into a rudimentary electrical device. The importance of such preliminary surface studies for identifying suitable molecular scale electronic materials has been highlighted in the concluding remarks in a recent paper by Tour.25 In this study we intentionally left the diimide moieties as individual molecules so that different stoichiometric mixtures of electron-donating and electron-accepting aromatic systems could be produced and studied. Although the eventual LB fabricated electrical devices were flawed, these studies did highlight the importance of equal mixing of aromatic donor and acceptor moieties. Thermal evaporation studies also demonstrated that a naphthalene diimide complex could be fabricated into a metal/organic/metal device which displayed a Poole-Frenkel conduction mechanism. Collectively, these results support our view that studies of more topologically complex systems based on these building blocks are now feasible. Acknowledgment. The authors acknowledge financial support from the School of Natural and Environmental Sciences (Coventry University, Coventry, U.K.) and the Engineering and Physical Sciences Research Council (U.K.) and thank J. Lovering for preparing the thermally evaporated samples, C. Lovering for performing the AFM experiments, and B. Hollyoak for the collection of SAXR data. ADXPS measurements were performed at the Brisbane Surface Analysis Facility (Queensland, Australia). Prof. Ian R. Peterson is thanked for helpful discussion. LA9816508 (23) Ashton, P. R.; Goodnow, T. T.; Kaifer, A. E.; Reddington, M. V.; Slawin, A. M. Z.; Spencer, N.; Stoddart, J. F.; Vicent, C.; Williams, D. J. Angew. Chem. 1989, 101, 1404; Angew. Chem., Int. Ed. Engl. 1989, 28, 1396. (24) Sakai, K.; Kawada, H.; Takamatsu, O.; Matsuda, H.; Eguchi, K.; Nakagiri, T. Thin Solid Films 1989, 179, 137. (25) Tour, J. M.; Kozaki, M.; Seminario, J. M. J. Am. Chem. Soc. 1998, 120, 8486.
