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Characterization of a Novel Water-Soluble 3,4,9,10-Perylenetetracarboxylic Diimide in Solution and in Self-Assembled Zirconium Phosphonate Thin Films Rodrigo O. Marcon, Jean G. dos Santos, Karla M. Figueiredo, and Sergio Brochsztain* Centro Interdisciplinar de InVestigac¸ a˜ o Bioquı´mica, UniVersidade de Mogi das Cruzes, AVenida Dr. Caˆ ndido XaVier de Almeida Souza, 200, Mogi das Cruzes, SP, 08780-911, Brazil ReceiVed August 25, 2005. In Final Form: NoVember 21, 2005 The properties of N,N′-bis(2-phosphonoethyl)-3,4,9,10-perylenetetracarboxylic diimide (PPDI), a water-soluble perylene dye, have been studied in solution and in thin films. Absorption spectra showed that PPDI exists in the monomeric form in water/ethanol (1:1) and water/dimethyl sulfoxide (DMSO) (3:7) mixtures, but forms dimers in water and higher aggregates in ethanol. The PPDI monomer is highly fluorescent, in contrast to the dimers and aggregates, which are nonfluorescent. The monomer/dimer equilibrium was conveniently followed in a water/ethanol (7:3) mixture by varying the dye concentration. An equilibrium constant of K ) 1.25 × 105 M-1 was estimated for the dimerization process in this solvent mixture. The addition of cetyl trimethylammonium bromide (CTAB), a cationic surfactant, to aqueous solutions of PPDI resulted in the dissociation of the dimers, showing that the dye was incorporated into the micellar phase. Self-assembled thin films of PPDI were grown on both silica gel particles and flat surfaces, using zirconium phosphonate chemistry. The growth of multilayered films on flat surfaces was monitored by ellipsometry (silicon substrates) and UV/Vis spectroscopy (quartz slides), and was linear with the number of deposition cycles. No fluorescence was detected from the PPDI films, and the absorption spectra of the films were quite similar to the spectrum of the compound in ethanol, indicating that the dye molecules were stacked in the films. Mixed monolayers containing PPDI and N,N′-bis(2-phosphonoethyl)-1,4,5,8-naphthalenediimide (PNDI) on quartz were also prepared. Monolayers obtained by codeposition from solutions containing both PPDI and PNDI were richer in PPDI, even when the solution contained a large excess of the naphthalene derivative, showing that π-stacking of PPDI is an important driving force in the formation of the films.
Introduction The 3,4,9,10-perylenetetracarboxylic diimides (PDIs) form a class of highly fluorescent compounds with excellent photochemical stability.1 Because of these properties, PDIs have been used in a variety of applications, such as in xerography,2 solar cells,3 molecular switches,4 laser dyes,5 and in electroluminescent6 and electrochromic7 devices. The use of PDIs in molecular photonics and electronics requires the incorporation of the molecules in supramolecular assemblies with controlled geometry. In this regard, literature reports describe the incorporation of PDIs in sol-gel matrixes,8 lipid vesicles,9 calixarenes,10 den* To whom correspondence should be addressed. E-mail: brochsztain@ umc.br. Tel.: (55) (11) 47987102. Fax: (55) (11) 47987068. (1) (a) Rademacher, A.; Markle, S.; Langhals, H. Chem. Ber. 1982, 115, 29272934. (b) Ford, W. E.; Kamat, P. V. J. Phys. Chem. 1987, 91, 6373-6380. (2) Popovic, Z. D.; Loutfy, R. O.; Hor, A.-M. Can. J. Chem. 1985, 63, 134139. (3) (a) Panayotatos, P.; Parikh, D.; Sauers, R.; Bird, G.; Piechowski, A.; Husain, S. Sol. Cells 1986, 18, 71-84. (b) Angadi, M. A.; Gosztola, D.; Wasielewski, M. R. J. Appl. Phys. 1998, 83, 6187-6189. (4) O’Neil, M. P.; Niemczyk, M. P.; Svec, W. A.; Gosztola, D.; Gaines, G. L., III; Wasielewski, M. R. Science 1992, 257, 63-65. (5) (a) El-Ebeid, Z. M.; El-Daly, S. A.; Langhals, H. J. Phys. Chem. 1988, 92, 4565-4568. (b) Sadrai, M.; Hadel, L.; Sauers, R. R.; Husain, S.; Krogh-Jespersen, K.; Westbrook, J. D.; Bird, G. R. J. Phys. Chem. 1992, 96, 7988-7996. (6) (a) Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (b) Ranke, P.; Bleyl, I.; Simmerer, J.; Haarer, D.; Bacher, A.; Schmidt, H. W. Appl. Phys. Lett. 1997, 71, 1332-1334. (c) Posch, P.; Thelakkat, M.; Schmidt, H.-W. Synth. Met. 1999, 102, 1110-1112. (7) (a) Lu, W.; Gao, J. P.; Wang, Z. Y.; Qi, Y.; Sacripante, G. G.; Duff, J. D.; Sundararajan, P. R. Macromolecules 1999, 32, 8880-8885. (b) Lee, S. K.; Zu, Y.; Herrmann, A.; Geerts, Y.; Mullen, K.; Bard, A. J. J. Am. Chem. Soc. 1999, 121, 3513-3520. (8) (a) Burgdorff, C.; Lohmannsroben, H.-G.; Reisfeld, R. Chem. Phys. Lett. 1992, 197, 358-363. (b) Schneider M.; Mullen, K. Chem. Mater. 2000, 12, 352-362. (9) Karolin, J.; Johansson, L. B.-A.; Ring, U.; Langhals, H. Spectrochim. Acta, Part A 1996, 52, 747-753.
drimers,11 liquid crystals,12 and thin films.13,14 The assembly of the molecules as ultrathin films is a particularly attractive approach, since there are several techniques available, allowing a fine control over film structure at a molecular level. In the case of PDI derivatives, thin film preparation has been achieved mainly by vapor deposition13 or by the Langmuir-Blodgett (LB)14 method. The films obtained by these techniques, however, are stabilized by relatively weak intermolecular forces, such as van der Waals interactions, and are therefore easily disassembled by, for example, exposure to high temperatures or organic solvents. This is a major drawback, since, for most practical purposes, it is essential to have films with high thermal stability and low solubility in most solvents. The zirconium phosphonate (ZP) approach for the growth of self-assembled films is known to produce very stable layered structures,15 with the layers held together by strong electrostatic forces. Moreover, most organic structures that can be function(10) van der Boom, T.; Evmenenko, G.; Dutta, P.; Wasielewski, M. R. Chem. Mater. 2003, 15, 4068-4074. (11) Herrmann, A.; Weil, T.; Sinigersky, V.; Wiesler, U.-M.; Vosch, T.; Hofkens, J.; De Schryver, F. C.; Mullen, K Chem.sEur. J. 2001, 7, 4844-4853. (12) (a) Iverson, I. K.; Casey, S. M.; Seo, W.; Tam-Chang, S.-W.; Pindzola, B. A. Langmuir 2002, 18, 3510-3516. (b) Gregg, B. A.; Cormier, R. A. J. Am. Chem. Soc. 2001, 123, 7959-7960. (13) (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, 4516-4525. (b) Schlettwein, D.; Back, A.; Schilling, B.; Fritz, T.; Armstrong, N. R. Chem. Mater. 1998, 10, 601-612. (c) Schlettwein, D.; Graaf, H.; Meyer, J.-P.; Oekermann, T.; Jaeger, N. I. J. Phys. Chem. B 1999, 103, 3078-3086. (d) Aroca, R.; Del Can˜o, T.; de Saja, J. A. Chem. Mater. 2003, 15, 38-45. (14) (a) Burghard, M.; Fischer, C. M.; Schmelzer, M.; Roth, S.; Hanack, M.; Gopel, W. Chem. Mater. 1995, 7, 2104-2109. (b) Johnson, E.; Aroca, R. Langmuir 1995, 11, 1693-1700. (c) Dutta, A. K.; Kamada, K.; Ohta, K. Langmuir 1996, 12, 4158-4164. (d) Dutta, A. K.; Vanoppen, P.; Jeuris, K.; Grim, P. C. M.; Pevenage, D.; Salesse, C.; De Schryver, F. C. Langmuir 1999, 15, 607-612. (e) Parra, V.; Del Can˜o, T.; Rodriguez-Mendez, M. L.; de Saja, J. A.; Aroca, R. F. Chem. Mater. 2004, 16, 358-364.
10.1021/la052329+ CCC: $33.50 © 2006 American Chemical Society Published on Web 01/17/2006
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Scheme 1. Layer-by-layer Growth of PPDI/ZP Thin Films
Chart 1. Structure of the Compounds Employed in This Work
different substrates. Finally, we study the formation of mixed monolayers containing codeposited PPDI and PNDI.′ Experimental Section
alized with two phosphonate groups at opposite ends of the molecule are readily incorporated into ZP films. Thanks to these characteristics, the ZP technique has been employed successfully for the preparation of thin films containing organic chromophores for optoelectronic devices.16 We recently reported on the growth of very stable self-assembled ZP films containing N,N′-bis(2phosphonoethyl)-1,4,5,8-naphthalenetetracarboxylic diimide (PNDI, Chart 1).17 In the present paper, we extend the approach to thin films containing perylenic diimides (Scheme 1). For this purpose, we synthesized N,N′-bis(2-phosphonoethyl)-3,4,9,10perylenetetracarboxylic diimide (PPDI, Chart 1), a derivative of PDI that was specially designed to be incorporated into ZP structures. We first describe the behavior of PPDI in aqueous solutions, using UV/Vis absorption and fluorescence spectroscopies to study the aggregation of the dye, as well as the influence of micelles on aggregation. In the second part, we describe the preparation of self-assembled thin films of PPDI on (15) (a) Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 618-620. (b) Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem. 1988, 92, 2597-2601. (c) Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420-427. (16) (a) Katz, H. E.; Schilling, M. L.; Chidsey, C. E. D.; Putvinski, T. M.; Hutton, R. S. Chem. Mater. 1991, 3, 699-703. (b) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485-1487. (c) Vermeulen, L. A.; Snover, J. L.; Sapochak, L. S.; Thompson, M. E. J. Am. Chem. Soc. 1993, 115, 11767-11774. (17) (a) Rodrigues, M. A.; Petri, D. F. S.; Politi, M. J.; Brochsztain, S. Thin Solid Films 2000, 371, 109-113. (b) Brochsztain, S.; Rodrigues, M. A.; Demets, G. J. F.; Politi, M. J. J. Mater. Chem. 2002, 12, 1250-1255. (c) Campos, I. B.; Rodrigues, F. A.; Nantes, I. L.; Brochsztain, S. J. Mater. Chem. 2004, 14, 54-60.
PPDI and PNDI were synthesized by condensation of 2-aminoethylphosphonic acid (Aldrich) with 3,4,9,10-perylenetetracarboxylic dianhydride (Aldrich) or 1,4,5,8-naphthalenetetracarboxylic dianhydride (Aldrich), respectively, in molten imidazole, according to the reported procedure.18 All the analytical data for PPDI and PNDI were in agreement with the literature and confirmed that the compounds were pure. Cetyl trimethylammonium bromide (CTAB) (99%), cetyl trimethylammonium chloride (CTAC) (99%), and sodium dodecyl sulfate (SDS) (g 99%) were purchased from Sigma Chemical Co. (St. Louis, MO). CTAB and CTAC were recrystallized from acetone/methanol prior to use. Zyrconyl chloride octahydrate (98%), (3-aminopropyl)triethoxysilane (99%), phosphoryl chloride (>99%), and collidine (99%) were purchased from Aldrich and used as received. Nonporous silica gel Cab-O-Sil L-90 (supplier data: BET surface area: 90 m2/g; average particle diameter: 20 nm) was a gift from Cabot Corporation and was used as received. Polished (100) silicon wafers were obtained from CrysTec GmbH. Quartz slides were supplied by Heraeus. All solvents employed were of spectroscopic grade. Aqueous solutions were prepared with deionized water (Barnstead Easypure RF system, Dubuque, IA). UV/Vis absorption spectra were recorded in a Shimadzu MultiSpec 1501 spectrophotometer. Emission spectra were taken with a Hitachi F 2500 fluorescence spectrophotometer. Ellipsometric measurements were performed with an EL X-01R rotating analyzer ellipsometer (DRE). Centrifugation was performed either in a Himac CR 21E centrifuge or in a Himac CF 15R microcentrifuge (Hitachi). Samples were stirred in a model C24 Incubation Shaker (New Brunswick Scientific). Elemental analyses (C,H,N) of the samples were performed by the Chemical Analysis Laboratory, at the Chemistry Institute, Universidade de Sa˜o Paulo. Absorption and emission spectra of PPDI in different solvents were obtained by diluting aliquots from a stock solution ([PPDI] ) 1 mM in H2O) in standard quartz cuvettes containing the solvent. The PPDI stock solution was prepared by weighing an amount of solid PPDI and diluting it in a volumetric flask with water (containing 2 equivalents NaOH to convert the compound into the water-soluble disodium salt). The concentration profiles of the absorption spectra were obtained by adding aliquots from a stock solution of PPDI (1 mM in water) to either a 1 cm (5 × 10-7 M < [PPDI] < 3 × 10-5 M) or a 1 mm (3 × 10-5 M < [PPDI] < 4 × 10-4 M) path length cuvette containing the desired solvent mixture (water/ethanol or water/dimethyl sulfoxide (DMSO)). To keep the solvent ratio constant, an aliquot of the pure organic solvent was also added to the cuvette following each addition from the stock solution. (18) Marcon, R. O.; Brochsztain, S. Thin Solid Films 2005, 492, 30-34.
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Table 1. Spectroscopic Characteristics of PPDI in Solution and Organized Media media
absorption maxima (nm)a
φFb
aggregation state
water water/ethanol (1:1)d ethanol water/DMSO (3:7)d DMSO CTAB 10 mM zirconated silica gel multilayered filmf
538 (0.53), 499 (1) 527 (1), 492 (0.70), 462c(0.29) 545c(0.28), 474 (1) 529 (1), 493 (0.73), 463c(0.30) 528 (0.76), 492 (1) 532 (1), 495 (0.66), 465 (0.25) 541 (0.56), 500 (1), 484c(0.86) 541c(0.57), 485 (1)
0.17 1 0.0056 0.86 0.022 0.28 0e 0e
dimer monomer higher aggregates monomer dimer monomer stacked stacked
a Values in parentheses are the relative intensities, in reference to the most intense vibrational band (marked in bold type). b Fluorescence quantum yields, relative to the water/ethanol (1:1) solution. c Shoulder. d The proportions are v/v. e Emission was not observed. f On quartz substrates.
The effect of CTAB on the absorption spectra of PPDI was studied by adding 25 µL from a 1 mM aqueous stock solution of PPDI to a series of 5 mL volumetric flasks (to give a [PPDI] ) 5 × 10-6 M). Aliquots from a 50 mM CTAB aqueous stock solution were then added, to give the desired [CTAB], and water was added to the flasks to make 5 mL. The solutions were stirred in a shaker for 20 h (25 °C) before reading the absorbance. The same procedure was used for the fluorescence spectra in the presence of CTAB, but using [PPDI] ) 1 × 10-6 M. Preparation of the zirconated silica gel was performed according to a literature procedure.19 A solution of ZrOCl2‚8H2O (0.06 M in 750 mL H2O) was mixed with a suspension of the nonporous silica gel (2.6 g in 150 mL H2O). The resulting suspension was stirred in a shaker for 72 h at 60 °C and then centrifuged (12000 rpm, 20 min). The solid was then washed thoroughly by stirring for 2 h in a shaker with 1 L of water, followed by centrifugation. The washing procedure was repeated seven times, and the zirconated silica was finally dried under vacuum. Deposition of PPDI was carried out by adding 40 mg of the zirconated silica to 15 mL of a 3 mM PPDI solution in water (prepared with the addition of 2 equivalents of NaOH). The mixture was stirred for 2 h at room temperature and then centrifuged (20 min, 12000 rpm). The solid was washed with water, as described above for the zirconated silica, until no PPDI could be detected in the supernatant by absorption spectroscopy (typically five washing cycles were necessary). The PPDI modified silica gel, which showed a red color typical of the dye, was finally dried under vacuum. A reference sample was prepared by the same procedure, but using the original, nonzirconated silica, giving a white material after washing with water. Self-assembled thin films of PPDI were grown using reported methods (Scheme 1).15-17 Quartz and silicon substrates were modified by a cleaning with “piranha” solution (caution: this solution reacts violently with organics) and then exposure to a solution of (3-aminopropyl)triethoxysilane in toluene (1% v/v, 30 min, 60 °C) followed by POCl3 in acetonitrile (0.01 M POCl3 + 0.01 M collidine, 15 min, room temperature). This treatment resulted in a primer layer that was rich in phosphonate groups, which adsorbed a layer of Zr4+ when exposed to an aqueous solution of ZrOCl2 (5 mM, 8 h, room temperature). The zirconated surface was then exposed to an aqueous solution of PPDI (1 mM, 8 h, room temperature), resulting in the deposition of a monolayer of the imide. Multilayered films of PPDI were obtained by repetition of the zirconation and imide deposition steps (Scheme 1). Mixed monolayers were obtained by exposing the zirconated quartz substrates to aqueous solutions containing mixtures of PPDI and PNDI.
Results and Discussion Absorption Spectra of PPDI in Solution. Absorption spectra of PPDI in water, ethanol, and a water/ethanol mixture (1:1 v/v) are shown in Figure 1 (relevant spectral data are presented in Table 1). The spectrum in aqueous ethanol was very similar to that reported for other PDI derivatives in the monomeric form,9,10,20 showing a well-resolved vibrational structure with (19) Hong, H. G.; Sackett, D. D.; Mallouk, T. E. Chem. Mater. 1991, 3, 521527. (20) Ford, W. E. J. Photochem. 1987, 37, 189-204.
Figure 1. Absorption spectra of 6.0 × 10-6 M PPDI in water (solid line), EtOH/water (1:1 v/v) (dashed line), and EtOH (dotted line).
maxima at 527 (λ0-0; ) 5.5 × 104 M-1 cm-1) and 492 nm (λ0-1; ) 3.9 × 104 M-1 cm-1) and a shoulder at 462 nm. The absorption spectrum of PPDI in pure water, however, was quite different from the monomer spectrum, showing an inversion in the intensity of the vibrational peaks. The most intense absorption maximum in water was the λ0-1 vibrational band at 499 nm ( ) 3.3 × 104 M-1 cm-1), whereas the lowest energy vibrational band (λ0-0) was reduced in intensity and red shifted to 538 nm ( ) 1.8 × 104 M-1 cm-1). The spectrum of PPDI in water was very similar to that described for PDI derivatives in the dimeric form9,10,20 and can therefore be assigned to a PPDI dimer. Further changes were noticed in the absorption spectrum of PPDI in pure ethanol, with the most intense absorption peak being shifted further toward lower wavelengths (474 nm, ) 1.9 × 104 M-1 cm-1) and the presence of a longer wavelength shoulder at 545 nm ( ) 5.3 × 103 M-1 cm-1), indicating the formation of higher aggregates in ethanol, as described for other PDI derivatives.20 It can be concluded that PPDI exists as a monomer in water/ EtOH (1:1), as a dimer in pure water and in a higher aggregated state in pure ethanol, as summarized in Table 1. Similar results were obtained with PPDI in water/DMSO mixtures (Table 1). In the case of DMSO, however, the monomeric form predominated in a mixture with the composition 30% water/ 70% DMSO. Furthermore, the spectrum of PPDI in pure DMSO showed the features of the dimer, being very similar to that observed in pure water, in contrast to the spectrum found in pure ethanol (Figure 1), where higher aggregates were formed. The observed trends in the spectra of PPDI in dimeric and aggregated forms indicate that the aromatic rings are in a faceto-face stacked geometry, with the transition dipoles parallel to each other. According to the molecular exciton theory,21 this arrangement would result in splitting the monomer absorption band into two components, one of them red-shifted and the other blue-shifted relative to the monomer band position. In a strictly parallel arrangement, only the blue-shifted transition would be (21) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11, 371-392.
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mixtures, the spectra of the monomer were observed in diluted solutions ([PPDI] < 2 × 10-6 M), whereas the dimer spectra predominated at higher concentrations ([PPDI] > 1 × 10-4 M). The concentration profile of the spectra, when displayed in a molar absorptivity () scale (Figure 2B), showed well-defined isosbestic points at 466, 477, 506, and 545 nm. The results were treated as a single monomer h dimer equilibrium (eqs 1 and 2), and a literature procedure was employed to find the dimerization constant K.20,22 The experimental data fit well to the proposed model with K ) 1.25 × 105 M-1 (details on the calculations are given as Supporting Information).
Figure 2. (A) Absorption spectra of 6.0 × 10-6 M PPDI in water/ DMSO mixtures. %H2O (v/v): 30% (curve 1); 60% (curve 2); 70% (curve 3); 80% (curve 4); 90% (curve 5); 100% (curve 6). (B) Absorption spectra of PPDI in 70% water/30% EtOH (v/v) at different dye concentrations (each spectrum was divided by the corresponding concentration). [PPDI] (M): 2.0 × 10-6 (curve 1); 8.2 × 10-6 (curve 2); 2.7 × 10-5 (curve 3); 7.0 × 10-5 (curve 4); 2.0 × 10-4 (curve 5); 4.0 × 10-4 (curve 6).
allowed. Thus, the blue shift observed in the most intense maximum of PPDI (Table 1) from 527 nm in the monomer to 499 nm in water (dimer) and to 474 nm in higher aggregates (ethanol) is consistent with this model. The appearance of redshifted shoulders (at 538 nm in water and 545 nm in ethanol), however, shows that the symmetry-forbidden transitions in the stacked arrangement still had significant oscillator strength, as observed by other authors that studied similar PDI derivatives.10,14b-c,20 It has been proposed that vibronic coupling relieves the symmetry restrictions imposed by the exciton model, which explains the observation of red-shifted absorption shoulders.10 Dimerization of PPDI. The dimerization of PPDI was studied by monitoring the absorption spectra of the dye in water/ethanol and water/DMSO mixtures of varying composition. The spectra of PPDI in water/DMSO mixtures are shown in Figure 2A. Note the gradual change from the monomer spectrum, which predominated in DMSO containing 30% water, to the dimer spectrum in pure water, with the presence of isosbestic points at 481, 513, and 546 nm. A similar behavior was observed when the composition of water/ethanol mixtures was gradually changed from 1:1 (monomer) to pure water (dimer), with isosbestic points at 478, 511, and 539 nm (see Supporting Information). Changing the composition of the solutions from the monomeric state toward pure organic solvents, on the other hand, resulted in profiles without clear isosbestic points (see Supporting Information). The solvent effects observed on PPDI aggregation are in contrast to those observed in the aggregation of most organic dyes, in which the addition of organic solvents to aqueous solutions causes a linear change from dimers to higher aggregates. The behavior of PPDI can be attributed to the amphiphilic character of the molecule, with an apolar core and polar end groups. Thus, the aromatic rings can be solvated by the organic solvent, whereas the phosphonate groups are solvated by water in the mixed solvent, stabilizing the monomer. The monomer/dimer equilibrium was further studied by changing the concentration of PPDI in solvent mixtures of fixed composition. Concentration-dependent spectral changes corresponding to the conversion of the monomer into the dimer were observed in mixtures containing 70% water and 30% ethanol (or 50% water and 50% DMSO), as shown in Figure 2B. In these
M+MhD
(1)
K ) [D]/[M]2
(2)
It is worth comparing the dimerization constant obtained here for PPDI with that reported for the dy(glicyl)imide derivative of PDI,20 a water-soluble diimide with a structure that is closely related to that of PPDI. In that case, a value of K ) 1.0 × 107 M-1 was found in water. These values cannot be directly compared, however, since our measurements were performed in water/ethanol (7:3) mixtures. In pure water, PPDI existed as a dimer in the whole concentration range studied, in contrast to the behavior of the dy(glicyl)imide, which exists as a monomer in highly diluted aqueous solutions.20 It can be concluded, therefore, that PPDI has a higher tendency to dimerization than the corresponding dy(glicyl)imide derivative, when solvents of the same polarity are considered. Fluorescence Spectra of PPDI in Solution. Solutions of PPDI were also studied using fluorescence spectroscopy. Fluorescence spectra of PPDI in water, ethanol, and water/ethanol mixtures are shown in Figure 3. The excitation and emission spectra showed a specular image relationship, and the excitation spectra in all the three solutions were quite similar in shape to the absorption spectra observed for PPDI monomer. No spectral changes corresponding to PPDI dimerization or aggregation were observed in the fluorescence spectra, even in solutions where the absorption spectra (Figure 1) showed that the compound existed as a dimer or in a higher aggregated form. These results can be explained by admitting that PPDI dimer and aggregates are totally nonfluorescent (quantum yield ) 0), as suggested by Ford for the dy(glicyl)imide derivative.20 Therefore, the weak fluorescence observed from water solutions can be regarded as arising from small amounts of PPDI present in the highly fluorescent monomeric form. Even in pure ethanol, a very weak fluorescence could be detected from PPDI (Figure 3), showing that some monomer was present, even in conditions that strongly favored PPDI aggregation. Furthermore, it can be noticed a red shift in the excitation and emission maxima of monomeric PPDI with increasing solvent polarity (Figure 3, Table 2), as expected for a π f π* transition (the high value of PPDI indicates a π f π* transition). The excitation maximum was shifted from 520 nm in ethanol to 528 nm in water/ethanol (1:1) and 532 nm in water. The emission maximum changed from 529 nm in ethanol to 539 nm in water/ ethanol (1:1) and 546 nm in water (Table 2). Studies of PPDI in Micellar Solutions. The effect of the addition of CTAB, a cationic detergent, to aqueous solutions of PPDI is shown in Figure 4. The absorption spectrum of PPDI in the absence of CTAB, corresponding to the dimerized dye, changed to that of the monomer in the presence of concentrated CTAB (Figure 4A). The spectrum in a 10 mM CTAB solution (22) (a) Bergmann, K.; O’Konski, C. T. J. Phys. Chem. 1963, 67, 2169-2177. (b) Selwin, J. E.; Steinfeld, J. I. J. Phys. Chem. 1972, 76, 762-774.
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Figure 3. (A) Excitation spectra (λem ) 580 nm) of PPDI (1.0 × 10-6 M) in H2O/EtOH (1:1) (solid line), H2O (dotted line), and EtOH (inset). (B) Emission spectra (λex ) 490 nm) of PPDI (1.0 × 10-6 M) in H2O/EtOH (1:1) (solid line), H2O (dotted line), and EtOH (inset).
Figure 4. (A) Absorption spectra of PPDI (5 × 10-6 M) in water (solid line) and in the presence of 0.1 mM CTAB (dotted line) and 10 mM CTAB (dashed line). (B) Emission spectra (λex ) 490 nm) of PPDI (1 × 10-6 M) in water and in the presence of varying concentrations of CTAB. [CTAB] (M): 0 (curve 1); 1.0 × 10-5 M (curve 2); 2.5 × 10-5 (curve 3); 1.5 × 10-4 (curve 4); 1.0 × 10-3 (curve 5); 3.9 × 10-3 (curve 6); 1.0 × 10-2 (curve 7). Curves 1-4 (emission intensity decreasing) are represented by dotted lines; curves 5-7 (emission intensity increasing) are represented by solid lines. Table 2. Fluorescence Data for Monomeric PPDI solution
excitation maxima (nm)a
emission maxima (nm)a
ethanol water/ethanol (1:1)c water CTAB 10 mM
454 (0.30), 486 (0.69), 520 (1) 461 (0.28), 492 (0.69), 528 (1) 464 (0.28), 496 (0.68), 532 (1) 463 (0.28), 495 (0.69), 532 (1)
529 (1), 575 (0.34), 625 (sh)b 539 (1), 583 (0.45), 631 (sh)b 546 (1), 587 (0.54), 637 (sh)b 542 (1), 586 (0.47), 634 (sh)b
a Values in parentheses are the relative intensities, in reference to the most intense vibrational band (marked in bold type). b Shoulder. c The proportions are v/v.
was quite similar to that seen in Figure 1 for water/ethanol mixtures, with the main peak at 532 nm and satellite bands at 495 and 465 nm (Table 1). These results show that PPDI was incorporated into the micellar phase, with the consequent dissociation of the dimer. The presence of SDS (an anionic detergent), on the other hand, did not cause any changes in the absorption spectra of PPDI in water, even at high detergent concentrations, showing that the dye was not incorporated into SDS micelles. These results show that PPDI has affinity to positive micelles, but not to negative micelles, as would be expected if the electrostatic factors were dominant, since the imide bears negatively charged phosphonate groups. Similar trends have been
observed for the effect of CTAB and SDS micelles on PNDI (Chart 1) and other 1,4,5,8-naphthalenetetracarboxylic diimides.23 Curiously, the presence of CTAB in low concentrations (1 × 10-4 M) induced a higher aggregation state of PPDI (Figure 4A), in contrast to that observed in high CTAB concentrations. Considering that the critical micelle concentration (CMC) reported for CTAB is 9 × 10-4 M,24 it seems that the observed phenomenon occurred at sub-CMC concentrations, probably due to the (23) Figueiredo, K. M.; Marcon, R. O.; Campos, I. B.; Nantes, I. L.; Brochsztain, S. J. Photochem. Photobiol., B 2005, 79, 1-9. (24) Fendler, J. H. Membrane Mimetic Chemistry; John Wiley & Sons: New York, 1982; Chapter 2.
Water-Soluble PDI in Solution and in ZP Thin Films
formation of premicellar aggregates containing stacked PPDI molecules (dimers, trimers, etc.). It is well-known that CTAB, because of the presence of quaternary ammonium groups, forms ion pairs with phosphate and phosphonate groups,24 which could help in stabilizing the premicellar aggregates. The emission spectra of PPDI in the presence of CTAB, shown in Figure 4B, are in agreement with the formation of premicellar aggregates. The addition of CTAB in concentrations well below the CMC (∼1 × 10-5 M) resulted in strong fluorescence quenching. Virtually no fluorescence could be detected from solutions containing CTAB in the range of 5 × 10-5 to 5 × 10-4 M, suggesting that even the small amounts of PPDI monomers present in water solution (which are responsible for the observed emission in the absence of CTAB) were incorporated into the nonfluorescent aggregates. The use of CTAC instead of CTAB gave similar results (see Supporting Information), ruling out the participation of bromide ions from CTAB in the quenching of PPDI fluorescence. When the [CTAB] was increased above the CMC, the fluorescence increased again (Figure 4B) to give highly fluorescent solutions at high [CTAB], indicating the incorporation of PPDI into the micelles in the monomeric form. The emission maxima observed in Figure 4B support the above conclusion (Table 2). In curves 1-3, at low [CTAB], fluorescence maxima at 546 and 587 nm were observed, corresponding to the emission of monomeric PPDI in water. In spectra 5-7, however, the maxima were blue-shifted to 542 and 586 nm, respectively, corresponding to the emission of micelle-incorporated PPDI monomers (Table 2). The observed blue shifts are consistent with emission from the less polar micellar environment, considering the π f π* character of the transition. The emission maxima of PPDI in concentrated CTAB are actually very close to those observed in water/ethanol mixtures (Table 2), suggesting that the dye is located inside the micelle in an environment with polarity similar to that of aqueous ethanol. Thin Films of PPDI on Silica Gel Particles. According to literature reports, silica gel surfaces can be coated with a layer of zirconium cations by stirring the silica with aqueous solutions of zirconium salts.19,25 When zirconated silica gel, prepared by stirring nonporous silica gel with zirconyl chloride, was stirred with aqueous solutions of PPDI (1 mM), deposition of PPDI occurred, as evidenced by the red color acquired by the silica. The color was persistent even after washing the silica several times with water, indicating strong binding of the dye. Elemental analysis of the PPDI-modified silica gel gave 2.06% of carbon, a considerable increase compared to the nontreated silica gel (0.23% C), confirming that the sample was loaded with the imide. If the zirconation step was omitted, however, the silica bleached to the original white color upon washing with water, indicating that no binding of PPDI occurred (this sample gave 0.53% C, which is similar to the nontreated silica). It can be concluded, therefore, that electrostatic attractions between Zr4+ and phosphonate groups are responsible for the strong binding of PPDI onto the silica surface. A simple account using the carbon percent given above and the surface area of the silica gel (90 m2/g, according to the supplier) gives a coverage degree of ∼4 × 1013 PPDI molecules/cm2 on the surface of the modified silica gel. The available area per molecule in ZP films of organic molecules is about 25 Å2, so that a compact monolayer would have 4 × 1014 molecules/cm2.15 (25) (a) Gao, W.; Reven, L. Langmuir 1995, 11, 1860-1863. (b) Gao, W.; Dickinson, L.; Grozinger, C.; Morin, F. G.; Reven, L. Langmuir 1996, 12, 2, 6429-6435. (c) Neff, G. A.; Page, C. J.; Meintjes, E.; Tsuda, T.; Pilgrim, W.-C.; Roberts, N.; Warren, W. W., Jr. Langmuir 1996, 12, 238-242.
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Figure 5. Absorption spectra of PPDI films with 2, 4, 6, 8, 10, 12, 14, 16, and 18 layers on quartz (considering both sides of the substrate). Inset: Absorption spectrum of an aqueous suspension of silica gel containing a monolayer of adsorbed PPDI (the spectrum was baseline corrected to compensate for the scattering due to the silica).
Therefore, the surface coverage obtained with the PPDI-modified silica gel was about 10% of a complete monolayer. The low degree of coverage obtained could be due to the direct zirconation method employed here. Higher coverage degrees are expected for surfaces primed with (3-aminopropyl)triethoxysilane and POCl3 prior to the zirconation step26 (this method was used for PPDI on flat surfaces; see below). The absorption spectrum of an aqueous suspension of the PPDI-modified silica gel is shown in the inset of Figure 5 (the absorption parameters are given in Table 1). The spectrum is quite similar to that obtained in homogeneous solutions of PPDI in water (Figure 1), suggesting that PPDI adsorbed on the silica surface in an aggregated form (probably as dimers). Moreover, no fluorescence could be detected from the modified silica, confirming the aggregation of the surface-bound molecules. Thus, it can be concluded that π-stacking interactions are quite strong, resulting in the formation of islands of surface aggregates, even though the surface density of adsorbed imide molecules is low. Thin Films of PPDI on Quartz and Silicon Substrates. Layer-by-layer growth of self-assembled PPDI films was performed on phosphonate-primed substrates, according to Scheme 1. Film growth on quartz slides was monitored by UV/ Vis absorption spectroscopy (Figure 5 and Table 1). The spectra of PPDI in the films were similar to those obtained in ethanol solution (Figure 1), indicating that the imide was in an aggregated state in the films. Similar spectral changes have been reported for other PDI derivatives incorporated in LB films.14c,e Accordingly, no fluorescence was detected from the films, as expected for stacked aromatic rings. Thus, all evidence suggests that PPDI formed islands of surface aggregates, as noted above for PPDI on silica gel. A linear increase in absorbance with the number of deposited layers was observed (Figure 6A), indicating the formation of regular films, with the same amount of material being deposited in each cycle, as observed for other organic chromophores in ZP films.16 Films were also prepared on silicon surfaces. In this case, film growth was monitored by optical ellipsometry (Figure 6B). The ellipsometric thickness was found to be linear with the number (26) (a) Keller, S. W.; Johnson, S. A.; Brigham, E. S.; Yonemoto, E. H.; Mallouk, T. E. J. Am. Chem. Soc. 1995, 117, 12879-12880. (b) Kohli, P.; Blanchard, G. J. Langmuir 2000, 16, 695-701.
1686 Langmuir, Vol. 22, No. 4, 2006
Figure 6. (A) Absorbance of PPDI films (monitored at 485 nm) as a function of the number of deposition cycles. (B) Ellipsometric thickness (using a refractive index of n ) 1.64) for the growth of PPDI multilayer films as a function of the number of deposition cycles. Measurements were taken at the end of each deposition cycle, i.e., after immersion in the Zr4+ solution followed by immersion in the imide solution. Solid lines represent linear regressions of the experimental points.
of layers, in agreement with the absorption data, giving a slope of 1.85 nm/layer. If a length of 20.6 Å (PPDI + Zr) is assumed,18 a tilt angle of 26° of the PPDI molecules relative to the surface normal can be estimated. This shows that the PPDI molecules are standing nearly upright with the long axis slightly tilted relative to the surface normal, in contrast to that observed with thin films of PDI prepared by vapor deposition, in which the molecules are usually oriented parallel to the substrate surface.13 In the case of the LB films of PDI reported in the literature, the molecules were most often oriented with the long axis tilted relative to the surface normal.14 Johnson and Aroca,14b using reflection absorption infrared spectroscopy, determined tilt angles close to 60° for a series of PDI derivatives. Thus, the ZP approach seems to produce films with the long axis more perpendicular to the surface than other techniques. Mixed Monolayers of PPDI/PNDI. Aggregation of PPDI in the films resulted in fluorescence quenching, and is therefore undesirable for several applications. To avoid surface aggregation, codeposition of PPDI with the naphthalenic imide PNDI (Chart 1) on quartz surfaces was attempted. The strategy was to dilute the perylenic imide in the films with the naphthalenic analogue. The absorption spectra of the mixed monolayers obtained are shown in Figure 7. The spectra of the original solutions used for deposition, containing both PPDI and PNDI, are also shown in Figure 7. It can be noticed that all monolayers had about the same stoichiometry (estimated as 10:1 PPDI/PNDI, based on solution values), regardless of the composition of the solution. Remarkably, the perylenic imide was always present in excess in the monolayers, even when the naphthalenic derivative was present in great excess in the solution used for deposition. A similar behavior was found by Horne and Blanchard27 for the codeposition of two different organic bis(phosphonates). They (27) Horne, J. C.; Blanchard, G. J. J. Am. Chem. Soc. 1996, 118, 1278812795.
Marcon et al.
Figure 7. (A) Absorption spectra (0.1 cm path length) of the solutions used for monolayer deposition: 1.5 × 10-4 M PPDI + 3.0 × 10-4 M PNDI in water (dotted line); 5.5 × 10-5 M PPDI + 4.8 × 10-4 M PNDI in water (dashed line); 6.6 × 10-5 M PPDI + 1.1 × 10-4 M PNDI in water/ethanol (1:1) (solid line). (B) Absorption spectra of the mixed monolayers obtained from the solutions above (on quartz substrates).
observed preferential adsorption of an aromatic bithiophene bis(phosphonate) derivative from a mixed solution containing 1,6hexanediyl bis(phosphonic acid). Moreover, it can be noticed that the spectra of PPDI in the films correspond to a stacked form, even when the deposition was carried out from water/ethanol (1:1), where the compound was in the monomeric state. A comparison of the spectra of PNDI in solution to those in the films shows an inversion in the ratio of the vibrational bands (Figure 7), similar to that observed with PPDI, indicating that the naphthalenic imide was also stacked in the monolayers. It can be concluded that the two imides did not mix in the films, but rather formed segregated islands of surface aggregates. Other authors have reported a similar phase separation in coevaporated PDI/phthalocyanine mixed films13d and in PDI/stearic acid LB films.14d It can be noticed in the solution spectra of the mixtures (Figure 7A) that PPDI and PNDI do not associate in solution (the spectra of the mixtures are the sum of the individual spectra), in agreement with the segregation observed in the films. From the above observations, it can be concluded that the adsorption of PPDI is much faster than that of PNDI, resulting in monolayers that are richer in the perylene analogue. Since both imides have the same ethylphosphonate end groups available for binding (Chart 1), electrostatic attractions between phosphonate and Zr4+ should not be the only factor determining the rate of deposition. In the case of water solutions, increased electrostatic interactions between dimers in solution and the zirconated surface might contribute for the fast growth of PPDI layers, but this is not possible for aqueous ethanol solutions, where PPDI is monomeric. It is most likely that, once the first PPDI molecule is attached to the surface, strong π-stacking interactions between perylene rings leads to a cooperative lateral growth of surface islands. The smaller ring system of PNDI has a lower tendency to aggregate (note that PNDI is monomeric in water, which is not the case for PPDI), and therefore PNDI adsorption is slower, and the islands formed are confined to the area not occupied by PPDI.
Conclusions PPDI is a very versatile organic molecule with outstanding chromophoric properties. It has the advantage of being water
Water-Soluble PDI in Solution and in ZP Thin Films
soluble, allowing studies in aqueous solutions, in contrast to most PDI derivatives, which are rather insoluble compounds. On the other hand, very stable and insoluble films can be obtained from PPDI using the ZP approach, what is very important for device manufacturing. The absorption spectra of PPDI are very sensitive to the aggregation state of the dye, allowing structural studies of the compound in different environments. Strong π-stacking interactions, however, result in dye aggregation in the films, with consequent fluorescence quenching, which could be a disadvantage for some applications. We are presently trying to find the conditions to obtain fluorescent films containing nonaggregated PPDI.
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Acknowledgment. This work was supported by grants from Brazilian agency FAPESP (grant N° 99/07114-2 and 05/511044). R.O.M. and J.G.S. wish to acknowledge FAPESP for the M.Sc. (03/10192-2) and IC (02/01138-1) scholarships, respectively. S.B. thanks FAEP for a research fellowship. Supporting Information Available: Spectra showing the solvent effects on the aggregation of PPDI (complement for Figure 2A) and the detailed calculations used to determine the dimerization constant for PPDI. This material is available free of charge via the Internet at http://pubs.acs.org. LA052329+
