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Langmuir 2007, 23, 11972-11976
Highly Stable 3,4,9,10-Perylenediimide Radical Anions Immobilized in Robust Zirconium Phosphonate Self-Assembled Films Rodrigo O. Marcon and Sergio Brochsztain* Centro Interdisciplinar de InVestigac¸ a˜ o Bioquı´mica, UniVersidade de Mogi das Cruzes, AV. Dr. Caˆ ndido XaVier de Almeida Souza 200, Mogi das Cruzes-SP 08780-911, Brazil ReceiVed August 27, 2007. In Final Form: September 23, 2007 Self-assembled thin films of 3,4,9,10-perylenediimides (PDIs) containing up to 50 PDI layers were grown on quartz slides using the zirconium phosphonate technique. When the films were immersed in aqueous solutions of the sodium dithionite reducing agent, in situ reduction of the dye was observed, generating a purple film containing PDI radical anions. The PDI radical anions formed within the films were rather stable, persisting for several minutes in the presence of atmospheric oxygen. Atomic force microscopy (AFM) images showed that the film surface was rather smooth and pinhole-free.
1. Introduction 3,4,9,10-Perylenediimides (PDIs) are well-known fluorescent dyes with high thermal stability and photostability that have been widely used as industrial pigments.1,2 PDI derivatives have also been known as one of the best n-type organic semiconductors,3 with applications in optoelectronic devices such as transistors,4 solar cells,5 and light-emitting diodes.6 Most of these properties are related to the facile reduction of PDI derivatives, giving relatively stable radical anion species. The electron acceptor character is a common property of aromatic diimides7 and arises from the strong electron-withdrawing power of the imide groups. Miller et al.8 have reported in a series of articles the spectroscopic and electrochemical properties of radical anions generated by the reduction of 1,4,5,8-naphthalenediimides (NDI) with sodium dithionite. Surprisingly, there has been much less information published about PDI radical anions, in spite of the fact that the PDIs, as a class of dyes, have been widely studied in the last few * Corresponding author. E-mail:
[email protected]. Phone: (55) (11) 47987102. Fax: (55) (11) 47987068. (1) Langhals, H. Heterocycles 1995, 40, 477. (2) Kazmaier, P. M.; Hoffmann, R. J. Am. Chem. Soc. 1994, 116, 9684. (3) (a) Wu¨rthner, F. Chem. Commun. 2004, 1564. (b) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.; Bredas, J.-L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436. (c) Gregg, B. A.; Cormier, R. A. J. Am. Chem. Soc. 2001, 123, 7959. (d) Struijk, C. W.; Sieval, A. B.; Dakhorst, J. E. J.; van Dijk, M.; Kimkes, P.; Koehorst, R. B. M.; Donker, H.; Schaafsma, T. J.; Picken, S. J.; van de Craats, A. M.; Warman, J. M.; Zuilhof, H.; Sudholter, E. J. R. J. Am. Chem. Soc. 2000, 122, 11057. (4) (a) Xu, B.; Xiao, X.; Yang, X.; Zang, L.; Tao, N. J. Am. Chem. Soc. 2005, 127, 2386. (b) Chesterfield, R. J.; McKeen, J. C.; Newman, C. R.; Ewbank, P. C.; da Silva Filho, D. A.; Bredas, J.-L.; Miller, L. L.; Mann, K. R.; Frisbie, C. D. J. Phys. Chem. B 2004, 108, 19281. (c) Malenfant, P. R. L.; Dimitrakopoulos, C. D.; Gelorme, J. D.; Kosbar, L. L.; Graham, T. O.; Curioni, A.; Andreoni, W. Appl. Phys. Lett. 2002, 80, 2517. (5) (a) Gregg, B. A. J. Phys. Chem. B 2003, 107, 4688. (b) Schmidt-Mende, L.; Fechtenkotter, A.; Mullen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (c) Angadi, M. A.; Gosztola, D.; Wasielewski, M. R. J. Appl. Phys. 1998, 83, 6187. (d) Panayotatos, P.; Parikh, D.; Sauers, R.; Bird, G.; Piechowski, A.; Husain, S. Solar Cells 1986, 18, 71. (6) (a) Lu, W.; Gao, J. P.; Wang, Z. Y.; Qi, Y.; Sacripante, G. G.; Duff, J. D.; Sundararajan, P. R. Macromolecules 1999, 32, 8880. (b) Ranke, P.; Bleyl, I.; Simmerer, J.; Haarer, D.; Bacher, A.; Schmidt, H. W. Appl. Phys. Lett. 1997, 71, 1332. (7) (a) Gosztola, D.; Niemczyk, M. P.; Svec, W.; Lukas, A. S.; Wasielewski, M. R. J. Phys. Chem. A 2000, 104, 6545. (b) Tauber, M. J.; Kelley, R. F.; Giaimo, J. M.; Rybtchinski, B.; Wasielewski, M. R. J. Am. Chem. Soc. 2006, 128, 1782. (c) van der Boom, T.; Evmenenko, G.; Dutta, P.; Wasielewski, M. R. Chem. Mater. 2003, 15, 4068. (d) Lee, S. K.; Zu, Y.; Herrmann, A.; Geerts, Y.; Mullen, K.; Bard, A. J. J. Am. Chem. Soc. 1999, 121, 3513. (e) Viehbeck, A.; Goldberg, M. J.; Kovac, C. A. J. Electrochem. Soc. 1990, 137, 1460. (8) (a) Penneau, J.-F., Stallman, B. J.; Kasai, P. H.; Miller, L. L. Chem. Mater. 1991, 3, 791. (b) Miller, L. L.; Mann, K. R. Acc. Chem. Res. 1996, 29, 417.
decades. The few systematic studies reported in the literature about the properties of PDI radical anions employed electrochemical reduction in organic solvents.6a,7a,9 Our group has recently reported the layer-by-layer growth of stable and ordered zirconium phosphonate PDI films (the films will be designated heretofore as PDIZP).10 In this letter, we show that robust PDIZP films with up to 50 layers can be reduced with sodium dithionite in aqueous solution, generating in situ PDI radical anions that are highly stabilized within the films, persisting for several minutes in contact with atmospheric oxygen. This is the first report, to our knowledge, showing the reduction of a PDI derivative immobilized in a self-assembled film using a chemical reducing agent. 2. Experimental Section Materials. All of the reagents and salts used were purchased from Aldrich and used as received. All solvents employed were spectroscopic grade. Aqueous solutions were prepared with deionized water. Quartz slides were supplied by Heraeus. N,N′-Bis(2phosphonoethyl)-3,4,9,10-perylenediimide (PPDI) was synthesized as previously reported.10 Equipment. UV/vis absorption spectra were recorded with a Cary 50 spectrophotometer (Varian). AFM images were recorded with a Shimadzu SPM 9600 scanning probe microscope using dynamic (noncontact) mode with a PPR NCHR-20 cantilever (Nanoworld). The scanner used had a maximum x-y range of 30 µm, and the images were taken using the maximum resolution available (512 pixels × 512 pixels), resulting in a step size of 0.586 nm for the images shown in Figure 4 (300 × 300 nm). Preparation of PDIZP Films. The quartz substrates were modified by treatment with 3-aminopropyltriethoxysilane, followed by POCl3, resulting in a primer layer rich in phosphonate groups. Multilayered PDIZP films were then obtained by exposing the modified substrate alternately to aqueous solutions of Zr4+ and PPDI. The detailed experimental procedure is given in reference 10. Reduction Experiments. Reduction with Na2S2O4, both in solution and in the PDIZP films, was carried out in a quartz cuvette sealed with a screw cap (Hellma model 117.100), equipped with a silicone septum for purging argon with a needle. Stock solutions of sodium dithionite (0.02 M in pH 8 borate buffer) were freshly prepared and used in the same day. For the reduction of the diimide, the (9) (a) Kim, S.-H.; Ko, H. C.; Moon, B.; Lee, H. Langmuir 2006, 22, 9431. (b) Mackinnon, S. M.; Wang, Z. Y. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 3467. (c) Zaban, A.; Diamant, Y. J. Phys. Chem. B 2000, 104, 10043. (10) (a) Marcon, R. O.; Santos, J. G.; Figueiredo, K. M.; Brochsztain, S. Langmuir 2006, 22, 1680. (b) Marcon, R. O.; Brochsztain, S. Thin Solid Films 2005, 492, 30.
10.1021/la702642h CCC: $37.00 © 2007 American Chemical Society Published on Web 10/17/2007
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Figure 1. (A) Absorption spectra showing the layer-by-layer growth of a 50-layer PDIZP film from aqueous solutions of PPDI and Zr4+ on a quartz slide. The spectra shown correspond (from bottom to top) to 4, 10, 16, 24, 30, 44, and 50 layers (considering both sides of the substrate). (Inset) Plot of the absorbance (monitored at 485 nm) as a function of layer number for the 50-layer PDIZP film (9). A similar plot for an 18-layer PDIZP film10 is also shown for comparison (O). The solid lines are the corresponding linear regressions. (B) Absorption spectrum of a PDIZP film with 16 layers (---) in comparison to the spectra of PPDI solutions (6 × 10-6 M) in water/ethanol (1:1 v/v) (-) and pure ethanol (‚‚‚). cuvette containing the PPDI solution or the quartz slide immersed in buffer was purged with argon for 5 min. Aliquots from the Na2S2O4 stock solution were then introduced with a needle, the solution was stirred, and the absorption spectra were recorded as a function of time.
3. Results and Discussion The PDIZP films were grown by alternately dipping phosphateprimed quartz slides into aqueous solutions of Zr4+ and N,N′bis(2-phosphonoethyl)-3,4,9,10-perylenediimide (PPDI, Figure 1), a water-soluble PDI.10 Film growth was followed by absorption spectroscopy and was found to be linear with the number of deposition cycles up to 50 layers (Figure 1A), showing a regular growth pattern, with the same amount of diimide being deposited in each cycle. The plot of the absorbance (at λmax ) 485 nm) versus layer number for the 50-layer PDIZP film is shown in the inset of Figure 1A, together with the data reported previously by our group for the growth of an 18-layer PDIZP film.10 The two curves show nearly the same slope (within experimental error), showing that the growth of PDIZP films is an easily controllable and reproducible process. The thickness of the 18-
Figure 2. (A) Absorption spectra of 2 × 10-5 M PPDI in a water/ ethanol solution (1:1 v/v) before (-) and after the addition of 9 µL (---) and 150 µL (‚‚‚) aliquots of an aqueous sodium dithionite stock solution (0.02 M Na2S2O4 in pH 8 borate buffer). (B) Absorption spectra of 2 × 10-5 M PPDI in pure ethanol before (-) and after the addition of 30 µL (---) and 190 µL (‚‚‚) aliquots of a Na2S2O4 stock solution. All of the solutions were purged with argon before adding the reducing agent.
layer film has been estimated as 1.85 nm/layer using ellipsometric measurements on PDIZP films grown on silicon.10b The similar slopes observed for the growth of PDIZP films with 18 and 50 layers suggest that that thickness value can be used here to good approximation, giving a total thickness for the 50 layer film of ca. 90 nm (45 nm on each side of the substrate). Using Beer’s law, with the thickness given above as the path length and max ) 1.9 × 104 (value found for PPDI in ethanol solution at λmax ) 473 nm, see below), a density of ca. 3 × 1014 molecules/cm2 can be calculated for each layer, corresponding to a molecular area of roughly 33 Å2 per PDI molecule. In a well-packed organic zirconium phosphonate film, each molecule occupies 24 Å2,11 corresponding to a density of 4 × 1014 molecules/cm2. Therefore, these calculations show that a closely packed PDIZP film was formed, with a high degree of coverage. The absorption spectra of the PDIZP films are characteristic of π-stacked PDI, as can be seen in comparison to the spectra in solution (Figure 1B).7c,10a In water/ethanol mixtures, PPDI shows spectra that can be assigned to monomeric PDI, with a vibronic structure having the longest wavelength band (0 f 0 (11) (a) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618. (b) Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597. (c) Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420.
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Table 1. Absorption Maxima for the PDI Radical Anion in Solution and in PDIZP Films absorption maxima media
λmax1
λmax2
λmax3
water/ethanol (1:1 v/v) (monomer) ethanol (aggregate) zirconium phosphonate film
713 728 733
800 818 819
960 987 989
transition, λmax ) 527 nm) as the most intense band. In pure ethanol, however, an inversion in the relative intensities of the vibronic bands was observed, with the most intense absorption now being the 0 f 2 transition (λmax ) 473 nm), indicating the formation of strongly interacting π stacks (Figure 1B).7c,10a The spectra of the PDIZP films are quite similar to that in ethanol, suggesting the formation of densely packed films with π-stacked PDI molecules. When the 50-layer PDIZP film was exposed to an aqueous sodium dithionite solution, a reduction of the red film took place, giving the purple PDIZP-• radical film. To understand the behavior of the PDIZP film upon reduction better, we carried out reduction studies in solution with PPDI (Figure 2). Figure 2A shows the reduction of PPDI by Na2S2O4 in a water/ethanol mixture, where the diimide exists in the monomeric form. The addition of small amounts of Na2S2O4 (less than 1 equiv) leads to the formation of the corresponding anion radical, with absorption maxima at 713, 800, and 960 nm (Table 1), corresponding closely to the values reported by Wasielewski et al.7a for electrochemically generated PDI-•. However, the addition of a large excess of the reducing agent leads to further reduction, giving the corresponding dianion, which shows no significant absorption at wavelengths longer than 650 nm. The spectra of PDI2- are also in agreement with the reported spectra.7a,9 When the reduction experiments were carried out in pure ethanol, where PPDI is aggregated, rather different results were obtained (Figure 2B). The addition of small amounts of dithionite resulted in the formation of the anion radical, but the spectra of PDI-• in ethanol show bands that were broadened and red-shifted (λmax ) 728, 818, and 987 nm) relative to the spectra of PDI-• in water/ethanol mixtures (Table 1). Similar results were observed by Tauber et al.,7b who noticed a red shift in the absorption maxima of PDI radical anions upon dimer formation. Moreover, the addition of a large excess of Na2S2O4 did not lead to dianion formation. It can be concluded that the aggregation state of the dye did not change upon reduction (i.e., monomeric PPDI-• was formed in water/ethanol mixtures whereas aggregated PPDI-• was generated in pure ethanol). Furthermore, aggregation stabilizes the anion radical species, probably because of spin pairing,8 thus preventing further reduction in ethanol solution. When the 50-layer PDIZP film was immersed in a sodium dithionite solution under an argon atmosphere, reduction occurred, as noted by the color change (Figure 3). The PDIZP film, which initially presented a reddish color, turned deep purple, evidencing the formation of reduced species within the film (Figure 3A). The reduction was followed as a function of time by absorption spectroscopy (Figure 3B), showing a smooth conversion to the radical anion form with absorption maxima at 733, 819, and 989 nm (Table 1). The reduction of the PDIZP film was a rather slow process, taking several minutes to complete, in contrast to the behavior in solution where reduction takes place immediately upon mixing the reagents. Furthermore, the spectrum of PDIZP-• was quite similar to that of PPDI-• in ethanolic solution (Figure 2B and Table 1), corresponding to diimide radical anions in the stacked form. The formation of PDIZP2- was not observed, even when a large excess of Na2S2O4 was employed, in agreement
Figure 3. (A) Photograph showing the 50-layer PDIZP film before the addition of the reducing agent (left), after 15 min of immersion in the Na2S2O4 solution (center), and after complete reoxidation by admitting air into the cuvette (right). (B) Absorption spectra as a function of time for the reduction of the 50-layer PDIZP film immersed in an aqueous sodium dithionite solution (0.13 mM Na2S2O4 in pH 8 borate buffer). The black line is the initial spectrum (before adding dithionite), and the other spectra were registered at 54, 90, 126, 180, 270, 360, 450, 540, and 630 s after dithionite addition (from bottom to top at 733 nm). (Inset) Time dependence of the reduction showing the consumption of ground-state PDIZP at 484 nm and the formation of PDIZP-• at 733, 819, and 989 nm.
with the observations in ethanol, confirming the stabilization of PDI-• in the films by stacking. When air was admitted into the cuvette, the radical-containing 50-layer film was slowly reoxidized back to the ground state (Figure 3A) within ca. 20 min under stirring. In contrast, when air was admitted into homogeneous solutions containing PPDI-•, reoxidation took place immediately upon mixing the solution. The 50-layer PDIZP film studied here has been submitted to several such reduction/reoxidation cycles with no sign of fading, always giving back the initial absorption spectrum, with no sign of dye decomposition or desorption from the film. The solutions where the film was immersed remained colorless and dye-free (according to the absorption spectra) during the redox experiments (Figure 3A). The great stability observed for the PDI radical anions immobilized in zirconium phosphonate films can be mainly attributed to two factors, namely, ring stacking, resulting in spin pairing, and the highly organized zirconium phosphonate framework. The pairing of organic radicals to form diamagnetic π dimers is a common phenomenon that has been widely studied in the case of the viologens. Miller et al. have demonstrated the
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Figure 4. Dynamic mode AFM image of a representative section of the 50-layer PDIZP film, displayed in height 3D view (A) and top view (B). (C) Model showing a side view of the film, displaying surface profiles extracted from representative AFM images: quartz substrate and PDIZP films with 9 and 25 layers (from bottom to top), considering only one side of the quartz slide. The line showing the surface profile of the 50-layer PDIZP film in part C corresponds to the horizontal line in part B. The film thicknesses were deduced from ellipsometric measurements on PDIZP films on silicon substrates.10b
formation of such dimers in the case of NDI radical anions.8 The present results show that the reduction proceeds only to the radical anion stage in ethanolic solution (Figure 2B), even in the presence of a large excess of dithionite, suggesting a great stabilization of the PDI radicals by stacking. Furthermore, the slow redox processes observed for the 50layer PDIZP film suggest a compact, well-organized film structure, resulting in the slow diffusion of the species involved (S2O42- and O2) through the film. Zirconium phosphonate films consist of organic/inorganic layers whose structure is determined by the bonding requirements of the metal, being quite insensitive to the nature of the organic group.11 Any organic moiety with a cross-sectional area of less than 25 Å2 can be accommodated within ZP films. Because the calculated cross-sectional area for a PDI ring is close to this limiting area, we were not sure whether PPDI would form regular, structured films. The results presented here, however, suggest the formation of good-quality, defectfree films. To further support this conclusion, the 50-layer PDIZP film was studied by dynamic mode atomic force microscopy (AFM). The AFM images in Figure 4 show a surface dominated by grains approximately 100 nm wide and between 4 and 8 nm high. The film surface was generally smooth and pinhole-free, within the resolution of the equipment (step size of 0.586 nm, see Experimental Section). The average surface roughness is much smaller than the total film thickness, as seen in the simulation of the film profile based on the AFM images, together with previously reported ellipsometric measurements (the surface
roughness of an 18-layer film is also shown in the figure). In conclusion, the AFM images suggest a well-ordered, compact film without large defects through which the reducing/oxidizing agents could quickly diffuse to reach the inner layers of the film. Therefore, in spite of their small size, the dithionite anion and oxygen have to diffuse through an intricate network of small pores to reach the film depths.
4. Conclusions and Future Developments All of the evidence presented here suggests that PDIZP films are compact, stable, and well-organized. The capacity to stabilize diimide radicals that absorb light in virtually all of the visible range makes PDIZP films quite suitable for a variety of applications, such as solar energy collectors, chromic sensors, and conducting films. Other authors have prepared PDI thin films using vapor deposition,12 the Langmuir-Blodgett (LB) method13 or by layer-by-layer electrostatic attraction.14 The films (12) (a) Conboy, J. C.; Olson, E. J. C.; Adams, D. M.; Kerimo, J.; Zaban, A.; Gregg, B. A.; Barbara, P. F. J. Phys. Chem. B 1998, 102, 4516. (b) Schlettwein, D.; Back, A.; Schilling, B.; Fritz, T.; Armstrong, N. R. Chem. Mater. 1998, 10, 601. (c) Schlettwein, D.; Graaf, H.; Meyer, J.-P.; Oekermann, T.; Jaeger, N. I. J. Phys. Chem. B 1999, 103, 3078. (d) Aroca, R.; Del Can˜o, T.; de Saja, J. A. Chem. Mater. 2003, 15, 38. (13) (a) Burghard, M.; Fischer, C. M.; Schmelzer, M.; Roth, S.; Hanack, M.; Gopel, W. Chem. Mater. 1995, 7, 2104. (b) Johnson, E.; Aroca, R. Langmuir 1995, 11, 1693. (c) Dutta, A. K.; Kamada, K.; Ohta, K. Langmuir 1996, 12, 4158. (d) Dutta, A. K.; Vanoppen, P.; Jeuris, K.; Grim, P. C. M.; Pevenage, D.; Salesse, C.; De Schryver, F. C. Langmuir 1999, 15, 607. (e) Parra, V.; Del Can˜o, T.; Rodriguez-Mendez, M. L.; de Saja, J. A.; Aroca, R. F. Chem. Mater. 2004, 16, 358.
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obtained by those techniques, however, are stabilized by relatively weak intermolecular forces and therefore are not well organized, being easily disassembled by exposure to high temperatures or organic solvents. The PDIZP films reported here, however, are very stable, resisting to high temperatures boiling organic solvents and aqueous solutions (up to pH ∼11), with no sign of dye desorption or decomposition. We are presently measuring the conductivity of dried PDIZP films reduced with different loadings of dithionite because the (14) (a) Tang, T.; Qu, J.; Mu¨llen, K.; Webber, S. E. Langmuir 2006, 22, 7610. (b) Tang, T.; Qu, J.; Mu¨llen, K.; Webber, S. E. Langmuir 2006, 22, 26.
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literature data have shown that the conductivity in diimide-based semiconductors is increased in mixed-valence stacks containing both neutral molecules and radical anions.7b,8 Acknowledgment. This work was supported by grants from Brazilian agencies FAPESP (grant nos. 99/07114-2, 05/ 51104-4, and 04/08850-4) and CNPq (grant no. 400618/20044). S.B. thanks FAEP for a research fellowship. Special thanks go to Professors Fla´vio Aparecido Rodrigues and Nelson Dura´n for the use of the AFM. LA702642H
