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Langmuir 2006, 22, 26-28
Molecular Layer-by-Layer Self-Assembly of Water-Soluble Perylene Diimides through π-π and Electrostatic Interactions Tingji Tang,† Jiangqiang Qu,‡ Klaus Mu¨llen,‡ and Stephen E. Webber*,† Department of Chemistry and Biochemistry and Center for Nano and Molecular Science, The UniVersity of Texas at Austin, Austin, Texas 78712, and Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany ReceiVed October 13, 2005. In Final Form: NoVember 5, 2005 A layer-by-layer deposition process has been carried out for two oppositely charged water-soluble perylene diimide dyes without the use of intervening polyelectrolyte layers. The strong π-π interactions between the perylene moieties help stabilize the layers and simultaneously diminish the fluorescence quantum yield of the array without strongly affecting the absorption or fluorescence spectra. There is an alternation of fluorescence intensity according to which perylene species is on the outer layer, which is interpreted as the effect of facile energy transfer between the perylenes.
Conventionally, the layer-by-layer (LBL) process is based on the sequential adsorption of polycations and polyanions from dilute aqueous solution onto a solid support as a consequence of the electrostatic interaction and complex formation between oppositely charged polyelectrolytes.1 Additionally, dendrimers, proteins, clays, and nanoparticles have been utilized for tuning the internal structure of the functional films, a tactic that has great potential in nanoelectronic, optoelectronic, magnetic, and chemical or biosensing technologies.2,3 Precise sterochemical and positional control of the functional species at the molecular level has not been achieved by the LBL self-assembly technique to our knowledge. In this letter, we report the fabrication of self-assembled thin films of perylene diimides carrying four positive or negative charges (p-PDI and n-PDI, respectively; see Figure 1) on quartz substrates using molecular layer-by-layer (MLBL) deposition from aqueous solution.4 The reasons for choosing PDIs as the building block are the following: (1) Isolated PDI moieties have a large extinction coefficient in the visible, a high fluorescent quantum yield and very good photostability, and are good candidates for photon-harvesting arrays (photovoltaic cells) and OLEDs. (2) The perylene structure will very likely form assemblies because of the strong π-π interaction between them,5 and the π stacking is expected to facilitate energy * Corresponding author. E-mail:
[email protected]. † The University of Texas at Austin. ‡ Max-Planck-Institut fu ¨ r Polymerforschung. (1) (a) Decher, G.; Hong, J. Makromol. Chem. Symp. 1991, 46, 321. (b) Decher, G. Science 1997, 277, 1232-1237. (2) (a) Schmitt, J.; Decher, G.; Dressik, W. J.; Brandow, S. L.; Geer, R. E.; Shashidbar, R.; Calvert, J. M. AdV. Mater. 1997, 9, 61. (b) Gao, M. Y.; Richter, B.; Kirstein, S.; Mo¨hwald, H. J. Phys. Chem. B 1998, 102, 4096-4103. (c) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (d) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (e) Caruso, F.; Mo¨hwald, H. J. Am. Chem. Soc. 1999, 121, 6039. (3) (a) Wang, Y.; Tang, Z. Y.; Correa-Duarte, M. A.; Liz-Marza’n, L. M.; Kotov, N. A. J. Am. Chem. Soc. 2003, 125, 2830. (b) Tsukruk, V. V. AdV. Mater. 2001, 13, 95. (c) Hiller, L.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59. (d) Tang, Z.; Wang, Y.; Kotova, N. A. Langmuir 2002, 18, 7035. (e) Park. J.; Hammond, P. T. AdV. Mater. 2004, 6, 520. (f) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Chem. Mater. 2001, 13, 1076. (g) Wang, B. Q.; Rusling, J. F. Anal. Chem. 2003, 75, 4229. (4) Other authors have used chromophores with four charges to construct polyelectrolyte layers but not with oppositely charged molecular species. See, for example, (a) Van Patten, P. G.; Shreve, A. P.; Donohoe, R. J. J. Phys. Chem. B 2000, 104, 5986-5992. (b) Tedeschi, C.; Caruso, F.; Mo¨hwald, H.; Kirstein, S. J. Am. Chem. Soc. 2000, 122, 5841-5848. (5) (a) Dobrawa, R.; Lysetska, M.; Ballester, P.; Gru¨ne, M.; Wu¨rthner, F. Macromolecules 2005, 38, 1315-1325. (b) Wu¨rthner, F. Chem. Commun. 2004, 1564.
Figure 1. Chemical structures of (a) n-PDI and (b) p-PDI.
or charge transport.6 The π stacking can also lead to strong quenching of the fluorescence, as we will see in the following text. Thus, the fluorescence intensity serves as a measure of the extent to which the perylene diimide moieties can be isolated from each other. p-PDI (CAS[817207-11-7]) and n-PDI (CAS[694438-88-5]), whose chemical structures are shown in Figure 1, were synthesized by Mu¨llen et al.7 p-PDI and n-PDI have dimensions of 2.13 × 1.33 × 1.89 and 2.13 × 1.15 × 1.49 nm3 (LWH), respectively. The height (H) dimension reflects the twisting of the side chains and is not relevant to the estimation of coverage presented later. The peak molar extinction coefficients in water were measured to be 3.28 × 104 and 2.97 × 104 M-1 cm-1 for p-PDI (λ ) 591 nm) and n-PDI (λ ) 568 nm), respectively. Beer’s law was verified for these solutions, suggesting the absence of aggregation. Both of these compounds are fluorescent in the aqueous phase (φfln-PDI ) 0.53 and φflp-PDI ) 0.13). However, if n-PDI and p-PDI are mixed in a 1:1 ratio (in pH 6.5 water), then the fluorescence is strongly quenched (by a factor of ca. 13 relative to that of p-PDI at an equivalent optical density, see Figure 2). As will be discussed later, analogous quenching also occurs in the LBL assembly. Unlike most examples of PDI aggregates,8 (6) (a) Malenfant, P. R. L.; Dimitrakopouls, C. D.; Gelmore, J. D.; Kosbar, L. L.; Graham, T. O.; Curioni, A.; Andreoni, W. Appl. Phys. Lett. 2002, 80, 2517. (b) Chesterfield, R. J.; Mckeen, J. C.; Newman, P. R.; Ewbank, P. C.; da Silva Filho, D. A.; Bre´das, J.-L.; Miller, L. L.; Mann, K. P.; Frisbie, C. D. J. Phys. Chem. B 2004, 108, 19281 (7) (a) Qu, J.-Q.; Kohl, C.; Pottek, M.; Mu¨llen, K. Angew. Chem., Int. Ed. 2004, 43, 1528. (b) Kohl, C.; Weil, T.; Qu, J.-Q.; Mu¨llen, K. Chem.sEur. J. 2004, 10, 5297-5310. (8) Wu¨rthner, F.; Thalacker, C.; Diele, S.; Tschierske, C. Chem.sEur. J. 2001, 7, 2245.
10.1021/la052766o CCC: $33.50 © 2006 American Chemical Society Published on Web 12/03/2005
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Figure 2. (Left) UV and (right) fluorescence emission spectra for n-PDI (black line), p-PDI (green line), p-PDI/n-PDI mixed in a 1:1 ratio (red line), and the sum of the p-PDI and n-PDI spectra (blue line).
Figure 3. UV spectra of the monolayer of p-PDI and n-PDI (showing only the odd layers for clarity) and inset showing the absorbance at the absorption peak around 575 nm as a function of the number of monolayers of p-PDI and n-PDI with p-PDI as the first layer.
the absorption spectrum of the 1:1 complex is only slightly modified from the individual species (The simple sums of the p-PDI and n-PDI UV individual absorption and fluorescence spectra are also displayed in Figure 2, labeled as “p-PDI + n-PDI”). The MLBL procedure was carried out by dipping the cleaned quartz substrates alternately into 0.l g/L p-PDI or n-PDI solutions for 10 min, starting with the p-PDI layer. For each cycle, there were three washes with DI water followed by drying with a gentle stream of argon gas. A typical series of absorption spectra for the formation of films on quartz is shown in Figure 3 with the inset showing the absorbance at 577 nm as a function of the number of monolayers of p-PDI and n-PDI. For the first two layers, there is a 5 nm blue shift in the peak positions (Supporting Information). The absence of any significant change of the UV spectra implies that no strongly interacting aggregates were formed.8 For PDI compounds without bay substitution, there is a very strong red shift in the absorption and fluorescence spectra upon aggregation.9 Control experiments dipping only into a p-PDI solution failed to increase the absorbance, even after 10 dipping cycles, which implies that the π-π interaction alone was not sufficient to make the PDI layers. The following rough calculation suggests that only a monolayer of p-PDI or n-PDI was formed for every deposition. The surface density, Γ, of PDI for every deposition in (p-PDI/n-PDI)n films can be calculated using Γ ) [Aλλ-1NA] × 10-3, where Aλ is the absorbance of PDI in the film at a given wavelength, λ is the extinction coefficient of PDI in solution (9) (a) Gregg, B. A.; Sprague, J.; Peterson, M. J. Phys. Chem. B 1997, 101, 5362. (b) Law, K. Y. Chem. ReV. 1993, 93, 449. (c) Liu, S.-G.; Sui, G.; Cormier, R. A.; Leblanc, R. M.; Gregg, B. A. J. Phys. Chem. B 2002, 106, 1307-1315. (d) Wang, W.; Han, J. J.; Wang, L.-Q.; Li, L.-S.; Shaw; W. J.; Li, A. D. Q. Nano Lett. 2003, 3, 455-458.
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Figure 4. Fluorescence spectra from the double layer of p-PDI/ n-PDI with p-PDI as the first layer (only odd layers shown for clarity) and inset showing the fluorescence intensity as a function of double layer p-PDI/n-PDI (diamonds correspond to n-PDI as the outside layer and the circles correspond to p-PDI as the outside layer). The lines between points illustrate the order of deposition (e.g., p-PDI first, followed by n-PDI).
(M-1 cm-1) at λ, and NA is Avogadro’s number.10 Using the average molar extinction coefficient of 3.0 × 104 M-1 cm-1 at 578 nm for p-PDI and n-PDI, an average surface PDI surface density of 3.64 × 1013 molecules per cm2/layer is obtained, which corresponds to an average area per PDI of 2.78 nm2. Assuming a single PDI molecule occupies an area of approximately 2.62 (L × W) nm2 (the average from the earlier estimate of molecular dimensions), the average surface coverage is ∼94%, which reflects the close packing of PDI molecules in each monolayer. Figure 4 displays the fluorescence spectra of films (excitation wavelength at 540 nm; only the odd layers are shown) with the inset showing the fluorescence intensity as a function of the number of PDI monolayers. For all of the fluorescence spectra, only one peak can be observed, and the shape and wavelength maxima are very similar to those of the solution-phase spectra.5,7 The peaks are slightly blue shifted for the first two PDI monolayers and thereafter have the same peak position at 614 nm, which is consistent with the UV spectra. Note that the fluorescence intensity decreased after the first double layer, indicating the presence of strong fluorescence quenching with the absorption of the second PDI double layer. This is consistent with the quenching observed by p-PDI and n-PDI complexation in solution (see above). After that, the fluorescence increased linearly with the number of PDI monolayers. This is an important observation because it demonstrates that no additional quenching defects are generated as additional layers are deposited. It can be seen that the intensity when n-PDI is on the outside layer is higher than for p-PDI as the outside layer. The fact that the fluorescence intensity is sensitive to the outermost layer suggests that excitation energy is transferred to the outer layer before fluorescence occurs (e.g., we hypothesize that the outer layer is acting as an energy trap regardless of which PDI species is present, although we cannot propose a mechanism for this effect). If there were no energy transfer, then the fluorescence intensity should converge to a simple average of the contribution from each species because our film is optically thin (e.g., all chromophores have an equal probability of being excited). By carefully comparing the fluorescence intensity of the films with an aqueous solution of p-PDI contained in a small-pathlength cell with a similar OD (the thin cell was used to eliminate any optical artifacts in the front-face fluorescence mode), we find that the fluorescence intensity for the film is approximately (10) (a) Caruso, F.; Kurth, D. G.; Volkmer, D.; Koop, M. J.; Mu¨ller, A. Langmuir 1998, 14, 3462. (b) Li. D.; Swanson, B. I.; Robison, J. M.; Hoffbauer, M. A. J. Am. Chem. Soc. 1993, 115, 6975.
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13 times lower than for the solution species, just as was found for the 1:1 complexes formed in the solution phase. In conclusion, we have demonstrated that it is possible to fabricate p-PDI/n-PDI monolayer assemblies by the MLBL technique presumably via the combination of π-π and electrostatic interactions. The strongly quenched fluorescence is evidence for π stacking, and the dependence of the fluorescence intensity on the outermost layer suggests efficient energy transfer between the PDI moieties. Ongoing work has demonstrated that the extent of π stacking can be diminished by inserting an inert layer between the p-PDI and n-PDI molecules without eliminating facile energy transfer.11 We think that this technique could be (11) Tang, T. Ongoing experiments.
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applied for making films for future applications in OLED or other organic-photonic devices from the p- or n-PDI species used in these studies. Acknowledgment. S.E.W. thanks the Welch Foundation Program (grant F-356) for its financial support of this work. K.M. acknowledges the support of the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 625 and Schwerpunktprogramm Organische Feldeffekttransistoren). Supporting Information Available: Absorption and fluorescence values of λmax as a function of the number of PDI layers. This material is available free of charge via the Internet at http://pubs.acs.org. LA052766O
