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Langmuir 2007, 23, 13146-13150
pH-Induced Interconversion between J-Aggregates and H-Aggregates of 5,10,15,20-Tetrakis(4-sulfonatophenyl)porphyrin in Polyelectrolyte Multilayer Films Yuya Egawa,* Ryosuke Hayashida, and Jun-ichi Anzai Graduate School of Pharmaceutical Sciences, Tohoku UniVersity, Aramaki, Aoba, Sendai 980-8578, Japan ReceiVed July 1, 2007. In Final Form: October 3, 2007 We fabricated a layer-by-layer (LbL) film composed of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and poly(allylamine) (PAA) and investigated its pH response by UV-visible spectrometry. When the (PAA/TPPS)5PAA film was immersed in a pH 1.5 solution, J-aggregate bands were observed at 484 and 691 nm. Above pH 3.0, the J-aggregates were completely dissociated and an H-aggregate band was observed at 405 nm. The interconversion between the J-aggregates and H-aggregates in the LbL film was repeatable and controllable by changing the pH of the solutions.
1. Introduction Recently, there has been considerable interest in the functionalities of self-assembled supramolecular structures.1,2 In biological systems, it is known that self-organized tetrapyrroles play a key role in light-harvesting and electron transfer.3,4 Some synthetic porphyrins form aggregates so that they are considered a desirable candidate as building blocks for self-assembled nanostructures.5 In general, the aggregate formations of porphyrins have been studied in solution, in which porphyrins sometimes aggregate as ordered precipitates.6,7 However, thin films containing self-assembled structures are highly desirable for practical use, which can be applied to nonlinear optical materials,8 organic solar cells,9 and sensor devices.10 Aggregates of dye molecules can be formed in thin films prepared by a layer-by-layer (LbL) deposition technique.11-14 The LbL deposition technique has recently attracted much attention because of advantages over the other methods for preparing thin films, such as the ease of fabrication and wide applicability. The LbL deposition is based on a repeated adsorption * Corresponding author. Tel: +81-22-795-6844. Fax: +81-22-795-6840. E-mail:
[email protected]. (1) Hamley, I. W.; Castelletto, V. Angew. Chem. Int. Ed. 2007, 46, 44424455. (2) Davis, A. P.; Sheppard, D. N.; Smith, B. D. Chem. Soc. ReV. 2007, 36, 348-357. (3) Saga, Y.; Akai, S.; Miyatake, T.; Tamiaki, H. Bioconjugate Chem. 2006, 17, 988-994. (4) Burda, K.; Hrynkiewicz, A.; Kooczek, H.; Stanek. J.; Strzaka, K. Biochim. Biophys. Acta 1995, 1244, 345-350. (5) Marks, T. J. Science 1985, 227, 881-889. (6) Schwab, A. D.; Smith, D. E.; Rich, C. S.; Young, E. R.; Smith, W. F.; de Paula, J. C. J. Phys. Chem. B 2003, 107, 11339-11345. (7) Wang, Z.; Medforth, C. J.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 15954-15955. (8) Collini, E.; Ferrante, C.; Bozio, R.; Lodi, A.; Ponterini, G. J. Mater. Chem. 2006, 16, 1573-1578. (9) Hasobe, T.; Imahori, H.; Fukuzumi, S.; Kamat, P. V. J. Mater. Chem. 2003, 13, 2515-2520. (10) Fujii, Y.; Hasegawa, Y.; Yanagida, S.; Wada, Y. Chem. Commun. 2005, 3065-3067. (11) Araki, K.; Wagner, M. J.; Wrighton, M. S. Langmuir 1996, 12, 53935398. (12) Sun, Y.; Zhang, X.; Sun, C.; Wang, Z.; Shen, J.; Wang, D.; Li, T. Chem. Commun. 1996, 2379-2380. (13) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 22242231. (14) Van Patten, P. G.; Shreve, A. P.; Donohoe, R. J. J. Phys. Chem. B 2000, 104, 5986-5992.
of two components through electrostatic interactions, hydrogen bonding, and biological affinities.15-27 For example, the anionic 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and a polycation have been employed to fabricate LbL films. The polycation/TPPS films exhibited the UV-visible absorption spectrum of the aggregates.12-14 The TPPS aggregates are classified into two types: J-aggregates and H-aggregates. The zwitterionic species (H42+TPPS4-) forms J-aggregates, which have been characterized as an edge-to-edge interaction of the constituent H42+TPPS4-.28-31 The cationic nitrogen atoms in the center of the H42+TPPS4- screen the negatively charged SO3- groups, resulting in edge-to-edge stacking. The J-aggregates show an intense narrow absorption band red-shifted with respect to the absorption band of the monomer (434 nm Soret band). Kuba´t et al. reported that the tetraanionic species H2TPPS4- forms H-aggregates in pH 7.0 solution in the presence of cationic dendrimer.31 The arrangement of H-aggregates is featured as a face-to-face interaction of H2TPPS4-. The H-aggregates of TPPS show a blue-shifted Soret band that is not as sharp as that in the J-aggregates. The spectral (15) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210-211, 831-835. (16) Hoshi, T.; Anzai, J.; Osa, T. Anal. Chem. 1995, 67, 770-774. (17) Decher, G. Science 1997, 277, 1232-1237. (18) Clark, S. L.; Montague, M. F.; Hammond, P. T. Macromolecules 1997, 30, 7237-7244. (19) Anzai, J.; Takeshita, H.; Kobayashi, Y.; Osa, T.; Hoshi, T. Anal. Chem. 1998, 70, 811-817. (20) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621-6623. (21) Anzai, J.; Kobayashi, Y.; Nakamura, N.; Nishimura, M.; Hoshi, T. Langmuir 1999, 15, 221-226. (22) Anzai, J.; Kobayashi, Y. Langmuir 2000, 16, 2851-2856. (23) Hoshi, T.; Saiki, H.; Kuwazawa, S.; Tsuchiya, C.; Chen, Q.; Anzai, J. Anal. Chem. 2001, 73, 5310-5315. (24) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301-310. (25) Palumbo, M.; Pearson, C.; Petty, M. C. Thin Solid Films 2005, 483, 114-121. (26) Egawa, Y.; Hayashida, R.; Anzai, J. Anal. Sci. 2006, 22, 1117-1119. (27) Egawa, Y.; Hayashida, R.; Anzai, J. Polymer 2007, 48, 1455-1458. (28) Maiti, N. C.; Mazumdar, S.; Periasamy, N. J. Phys. Chem. B 1998, 102, 1528-1538. (29) Pasternack, R. F.; Fleming, C.; Herring, S.; Collings, P. J.; dePaula, J.; DeCastro, G.; Gibbs, E. J. Biophys. J. 2000, 79, 550-560. (30) Paulo, P. M. R.; Costa, S. M. B. Photochem. Photobiol. Sci. 2003, 2, 597-604. (31) Kuba´t, P.; Lang, K.; Janda, P.; Anzenbacher, P., Jr. Langmuir 2005, 21, 9714-9720.
10.1021/la701957b CCC: $37.00 © 2007 American Chemical Society Published on Web 11/10/2007
pH-Induced InterconVersion of J- and H-Aggregates
changes of the TPPS aggregates are known to be induced depending on the excitonic interactions between the chromophores. The aggregate formation of TPPS is often influenced by various conditions, such as pH, ionic strength, temperature, and the presence of polyelectrolytes.28-31 In this study, we report the pH-induced interconversion between the J-aggregates and H-aggregates of TPPS immobilized in an LbL film. It has been already found that TPPS forms J-aggregates in LbL films.12-14 However, little attention has been paid to the H-aggregates of TPPS in LbL films. The ability to switch between J-aggregates and H-aggregates on a solid surface will have important applications in diverse fields ranging from material science to molecular machines. We used poly(allylamine) (PAA) as the polycation to immobilize anionic TPPS into LbL films. The PAA/ TPPS multilayer films were fabricated on a quartz slide to monitor the UV-visible absorption spectra of the PAA/TPPS films as a function of pH. A quartz crystal microbalance (QCM) was also employed to investigate the properties of the films.
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Figure 1. UV-visible absorption spectra for the deposition of (PAA/ TPPS)nPAA film (n ) 0, 1, 2, 3, 4, 5), recorded after the PAA depositions.
3. Results and Discussion 2. Experimental Section 2.1. Materials. An aqueous solution (20%) of PAA (average molecular weight, ca. 10 000) was purchased from Nitto Boseki Co., Ltd. (Tokyo, Japan). TPPS was obtained from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). All other chemicals were of the highest grade available and were used without further purification. 2.2. Apparatus. The UV-visible absorption spectra were recorded by a Shimadzu UV-3100PC spectrometer (Kyoto, Japan) equipped with a quartz glass cell (4.5 × 1 × 1 cm). The fluorescence spectra were measured by a Shimadzu RF-5300PC spectrophotometer. A quartz crystal microbalance (Seiko EG & G, QCA917, QA-A9MPT, Chiba, Japan) was used for the gravimetric analysis of the multilayer thin films. The pH value was monitored by a DKK IOL50 (Tokyo, Japan). 2.3. Preparation of PAA/TPPS Multilayer Films. The LbL films were prepared on the surface of a quartz slide (5 × 1 × 0.1 cm). The quartz slides were first treated in dichlorodimethylsilane (5% solution in toluene) overnight at room temperature to make the surface hydrophobic, and then the slides were washed with acetone and distilled water. To prepare the PAA/TPPS multilayer films, a slide was immersed in a PAA solution (0.10 mg/mL in 150 mM NaCl aqueous solution) for 5 min to deposit the first PAA layer. The slide was then immersed in water for 5 min to remove any excess PAA. The slide was subsequently immersed in a TPPS solution (50 µM in 150 mM NaCl aqueous solution) for 5 min to deposit TPPS and then rinsed in water for 5 min. These processes provide both sides of the quartz slide with a PAA/TPPS bilayer. The second PAA layer was similarly deposited on the surface of the slide modified with the PAA/TPPS bilayer film. The deposition was repeated in order to build up multilayer films. QCM was also employed for assessing the PAA/TPPS multilayer films. The multilayer film was prepared in a similar manner on the surface of a platinum filmcoated quartz resonator (9 MHz). 2.4. pH Response of PAA/TPPS Multilayer Films. The quartz slide modified with the (PAA/TPPS)5PAA multilayer film was used to study the pH response. The UV-visible absorption and fluorescence emission spectra of the LBL film-coated quartz slide were recorded in aqueous solutions with varying pHs. The solutions contained 10 mM sodium acetate, and the pH was adjusted with a small amount of HCl or NaOH solution. During the study of the reversible pH response of the films, the quartz slide modified with (PAA/TPPS)5PAA films was immersed in each pH solution, which was stirred using a small magnetic stirring bar to reduce the aqueous diffusion layer thickness. All experiments were carried out at room temperature (ca. 20 °C).
3.1. Preparation of PAA/TPPS Multilayer Films. Figure 1 shows the typical absorption spectra for the formation of the (PAA/TPPS)nPAA films. The spectra of the film were recorded after depositing the PAA. The absorption of TPPS was increased with the number of depositions. Relatively small amounts of TPPS were adsorbed in the beginning of the film preparation. For subsequent adsorptions, the film mass and thickness tend to increase. A nonlinear film growth has been often seen in the early stage of the LbL film fabrications, probably due to the effects of the surface properties of the solid substrates on which the multilayer film was deposited.25 At first, an absorption peak was observed around 420 nm, which was close to the Soret band of the monomeric TPPS. As the number of deposition cycles increases, the peak broadened and shifted toward lower wavelengths. The absorption peak at 405 nm of the (PAA/TPPS)5PAA film can be identified as the H-aggregates of TPPS, which is blue-shifted from the Soret band of the tetraanionic form (H2TPPS44-, 414 nm).28-31 The absorption peaks arising from the J-aggregates were not detected in this step, although the J-aggregates of TPPS were observed in other types of LbL films.12-14 In this study, the assignment of the type of aggregates is made on the basis of the spectral shifts of the Soret band. The H-type and J-type aggregates show a blue-shift and red-shift of the Soret band, respectively. These relationships between the spectral shifts and types of aggregates can be applied to the allowed transition. In the case of porphyrins, the Soret band is the allowed transition, whereas the Q-band is quasiallowed. Therefore, the prediction of the spectral shifts is applied to the Soret band.28 A Q-band maximum was observed at 653 nm in the (PAA/TPPS)5PAA films, which was red-shifted from a Q-band of the monomeric tetraanionic form (H2TPPS44-, 633 nm).28,30 The red-shift of the Q-band has been observed when TPPS interacts with polycations or surfactants. In particular, the Q-band tends to be above 650 nm in the H-aggregates of TPPS.28,30 The Q-band at 653 nm in the (PAA/TPPS)5PAA films is evidence for the H-aggregates formation. The reason why the H-aggregates are observed in the LbL film is probably due to the effect of PAA. Previous reports showed that a polycation attracts the TPPS through electrostatic interactions to provide opportunities for the ordered assembly of TPPS.29-31 To investigate the effect of PAA, we measured the UV-visible absorption spectra of 1.2 µM TPPS in the presence of 10 µg/mL PAA. In a pH 7.0 solution, on the other hand, the formation of the H-aggregates was suggested from the blue-shift
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Figure 3. UV-visible absorption spectra of TPPS solutions (0.10, 0.20, 0.50, 1.0 µM) in the presence of 0.10 µg/mL PAA, measured in a pH 7.0 solution containing 10 mM sodium acetate.
Figure 2. UV-visible absorption spectra of TPPS solutions (1.2 µM) in a pH 7.0 solution (a) and a pH 1.0 solution (b) in the absence and presence of PAA (10 µg/mL).
of the Soret band (H2TPPS44-, 414 nm) to 402 nm upon the addition of PAA (Figure 2a). In a pH 1.0 solution, the Soret band (H42+TPPS44-, 435 nm) was shifted to 490 nm by the PAA addition, suggesting the J-aggregate formation (Figure 2b). These results support the fact that the PAA induces the aggregate formation of TPPS in the LbL films. It is known that the stoichiometry of the TPPS and polycation is important for the formations of TPPS aggregates.30 We investigated the spectrum of TPPS in the presence of 0.10 µg/mL PAA (1.8 µM in monomer unit) in the pH 7.0 solution as a function of the TPPS concentration (Figure 3). When 0.10-0.20 µM of TPPS was added, the Soret band was observed around 400 nm, which corresponded to the H-aggregates.28,30 Above 0.50 µM TPPS, an absorption peak appeared at 413 nm, which is close to that of the monomeric H2TPPS44-.28-31 On the basis of these results, it is likely that the TPPS/PAA ratio is too high to form aggregates during the early stage of the PAA/TPPS film fabrication. As the number of deposition cycles increased, the H-aggregates are formed in the PAA/TPPS films with the TPPS/PAA ratio decreasing. The amount of TPPS taken into the (PAA/TPPS)5PAA film was estimated to be 4.2 × 10-9 nmol cm-2, assuming that the molar absorption coefficient of the immobilized TPPS at 405 nm could be approximated by that of the H-aggregates (1.2 × 105 M-1 cm-1 at 402 nm), which were observed in the 1.2 µM TPPS solution in the presence of 10 µg/mL PAA at pH 7.0 (Figure 2a). The QCM was also employed to observe the growth of the PAA/TPPS assemblies. The resonance frequency in the QCM decreased by 2.6 kHz. Considering that the area of the quartz
Figure 4. UV-visible absorption spectra of (PAA/TPPS)5PAA multilayers in different pH solutions (pH 1.5, 2.0, 3.0, 7.0), measured in an aqueous solution containing 10 mM sodium acetate.
resonator was 0.2 cm2, we calculated that the total loading of (PAA/TPPS)5PAA is 1.4 × 10-5 g cm-2 on the basis of the fact that the deposition of 1.068 ng of a substance on the 9 MHz resonator induces a -1 Hz change in the frequency.32 The thickness of the (PAA/TPPS)5PAA film can be estimated to be about 120 nm by assuming that the density of the assembly is ∼1.2 g cm-3.13 3.2. pH Response of the PAA/TPPS Multilayer Films. Figure 4 shows the UV-visible absorption spectra of the (PAA/ TPPS)5PAA film in different pH solutions. The spectra changed depending upon the pH value of the solution in which the filmcoated slide was immersed. In a pH 1.5 solution, there were absorption peaks at 484 and 691 nm, which correspond to the J-aggregate bands.12-14 The absorption bands of the J-aggregates decreased as the pH rose, and in the pH 3.0 solution, a new absorption peak of the H-aggregates appeared at 405 nm, which remained virtually unchanged at pH 7.0. An important finding of the present study is that the J-aggregates are directly converted to H-aggregates. Previously, Fujii et al. reported the pH response of TPPS anchored on mesoporous TiO2.10 The J-aggregate bands were observed when the TiO2 was immersed in an acidic solution, while the H-aggregates were not found in the neutral solution. The absorption peak was observed at 414 nm in the neutral solution, which was assigned to the monomeric H2TPPS44-. They (32) Sauerbrey, G. Z. Phys. 1959, 155, 206-222.
pH-Induced InterconVersion of J- and H-Aggregates
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Figure 5. Schematic arrangements of TPPS in the LbL film. H-aggregates in a pH 7.0 solution and J-aggregates in a pH 1.5 solution.
Figure 6. Fluorescence emission spectra of (PAA/TPPS)5PAA multilayers in different pH solutions (pH 1.5, and 7.0), measured in an aqueous solution containing 10 mM sodium acetate (excitation at 405 nm).
reported a pH-dependent reversible conversion of TPPS between the monomeric form and J-aggregates. On the other hand, our reported result is the first observation of the pH-induced interconversion of TPPS between the J-aggregates and Haggregates. Figure 5 shows a schematic representation of the pH-induced interconversion of the TPPS aggregates. Figure 6 shows the fluorescence emission spectra of the (PAA/ TPPS)5PAA film. When the film was immersed in a pH 7.0 solution, the film showed a single emission peak at 668 nm. The shape of the spectra in a pH 7.0 solution closely resembles that of the previously reported H-aggregate systems.28 In contrast, virtually no fluorescence was observed in a pH 1.5 solution. Thus, the film can act as a fluorescence on-off switch under varying pHs. 3.3. Reversible pH Response of PAA/TPPS Multilayer Film. Figure 7a shows the changes in absorbance at 405 nm when the film is alternately immersed in pH 1.5 and 7.0 solutions. The absorption peak arising from the H-aggregates at 405 nm increased when the film was immersed in the pH 7.0 solutions, and it decreased when the film was immersed in the pH 1.5 solutions. The change in absorbance at 405 nm was very fast (within 5 seconds) and reversible. These results suggest that the formation and dissociation of the H-aggregates are quick and reversible. The absorbance intensity remained unchanged in each pH solution after the repeated immersions, which means that the (PAA/ TPPS)5PAA film is stable in the pH 1.5 and 7.0 solutions. Figure 7b shows the changes in absorbance at 484 nm arising from the J-aggregates. When the (PAA/TPPS)5PAA film was immersed in the pH 1.5 solution, the absorbance at 484 nm slowly increased. These results indicate that the J-aggregate formation requires a longer time when the film was immersed in the pH 1.5 solutions. Previously, we studied the pH response
Figure 7. The changes in absorbance of the (PAA/TPPS)5PAA film at 405 nm (a) and 484 nm (b), when the film was alternately immersed in pH 1.5 (0-12 min) and pH 7.0 solutions (12-24 min). The data during the replacing of solutions are omitted for clarity.
of Brilliant Yellow in LbL films.26,27 The immobilized dye showed a quick spectral change when the film was immersed in a weakly acidic solution. Thus, the diffusion of protons may not be the rate-controlling factor of the J-aggregate formation of the immobilized TPPS. The kinetics of the J-aggregate formation of TPPS has been studied in solution.29 The delay of the J-aggregate formation was seen even in the homogeneous systems, depending on the conditions. In order to accelerate the rate of J-aggregate formation, we tried to use a more acidic solution because it is known that an acidic environment enhances the J-aggregate formation of TPPS. However, the (PAA/TPPS)5PAA film peeled off the quartz slide when the film was immersed in a pH 1.0 solution. Further work is needed to find the appropriate conditions for a quick J-aggregate formation. When the (PAA/TPPS)5PAA film was immersed in a pH 7.0 solution, the decrease in absorbance at 484 nm was fast (within 5 s). Combining the results of Figure 7a,b, when the (PAA/TPPS)5PAA film is immersed in a pH 7.0 solution, the J-aggregate dissociation and H-aggregate formation simultaneously and quickly occur. When the (PAA/TPPS)5PAA film is immersed in a pH 1.5 solution, it is likely that the H-aggregates quickly dissociate and the J-aggregates form relatively slowly.
4. Conclusion We have fabricated the PAA/TPPS film and investigated its pH response. In a pH 1.5 solution, the spectrum of the film
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exhibited absorption peaks at 484 and 691 nm, which correspond to the J-aggregates of TPPS. Above pH 3.0, an absorption maximum was observed at 405 nm, which corresponds to the H-aggregates of TPPS. This is the first report of the direct interconversion between the J-aggregates and H-aggregates in LbL films. We confirmed that the interconversion of the
Egawa et al.
J-aggregates to H-aggregates was repeatable and controllable by changing the pH. The ability to switch between the J-aggregates and H-aggregates on a solid surface will have important applications in diverse fields. LA701957B
