pubs.acs.org/Langmuir © 2009 American Chemical Society
Nanostructured Aggregates of meso-Tetramesitylporphyrin on Solid Substrate Palanisamy Kalimuthu and S. Abraham John* Department of Chemistry, Gandhigram Rural University, Gandhigram, Dindigul, Tamilnadu, India-624 302 Received May 28, 2009. Revised Manuscript Received September 22, 2009 We report aggregated nanostructure formation of water insoluble meso-tetramesitylporphyrin (MTMP) induced by hydrogen chloride (HCl) vapor on a glass plate. The formation of aggregated nanostructures of MTMP was confirmed by UV-vis, time-resolved emission spectral techniques, atomic force microscopy (AFM), and high resolution transmission electron microscopy (HR-TEM). Exposure of HCl vapor to a thin film of MTMP coated on a glass plate resulted in a 35 nm red shift of the Soret band in contrast to the case of MTMP thin film. AFM images showed the formation of MTMP aggregates induced by HCl vapor with the size of 50-170 nm in height and an average width of 120 nm. Protonation of MTMP coated on the glass plate by HCl molecules triggered the formation of aggregated nanostructures on the glass plate. Interestingly, it was found that the aggregates were not reverted back to the original morphology of MTMP thin film when it was deprotonated by exposure to ammonia vapor. The fluorescent lifetime of the MTMP thin film was decreased from 11.48 to 0.78 ns when it was exposed to HCl vapor, indicating the formation of MTMP aggregates on the glass surface.
1. Introduction Molecular aggregates have received much interest in recent years because of their involvement in many fundamental physiochemical as well as biological processes.1-6 Many of the unique properties of aggregates arise from the fact that the constituent molecules are electronically coupled through the interaction between transition dipoles so that optical excitation of the chromophore produces a state that is delocalized over several monomer units.7 Delocalization of the initial excitation frequently causes aggregates to have line widths that are considerably narrower than those of their constituent monomers, making them attractive for high-density information storage.8,9 Among the different aggregates studied to date, porphyrin aggregates have received considerable attention because of their interesting structural and photophysical properties in addition to their large photostability and models for artificial solar energy capture as in photosynthesis.10-12 The water-soluble 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) has been the most studied molecule by various researchers, since it forms aggregates simply under acidic conditions and also in the presence of different *To whom correspondence should be addressed. Telephone: 91-4512452371. Fax: 91-451-2453071. E-mail:
[email protected]. (1) Saga, Y.; Akai, S.; Miyatake, T.; Tamiaki, H. Bioconjugate Chem. 2006, 17, 988–994. (2) Burda, K.; Hrynkiewicz, A.; Kooczek, H.; Stanek, J.; Strzaka, K. Biochim. Biophys. Acta 1995, 1244, 345–350. (3) Blankenship, R. E.; Olson J. M.; Miller, M. In Anoxygenic Photosynthetic Bacteria; Blankenship, R. E., Madigan, M. T., Bauer, C. E., Eds.; Kluwer Academic Publishers: Dordrecht, 1995; p 399. (4) Pullerits, T.; Sundstrom, V. Acc. Chem. Res. 1996, 29, 381–389. (5) Tsuda, A.; Osuka, A. Science 2001, 293, 79–82. (6) de Witte, P. A. J.; Castriciano, M.; Cornelissen, J. J. L. M.; Scolaro, L. M.; Nolte, R. J. M.; Rowan, A. E. Chem.;Eur. J. 2003, 9, 1775–1781. (7) Kitahama, Y.; Kimura, Y.; Takazawa, K. Langmuir 2006, 22, 7600–7604. (8) Knapp, E. W. Chem. Phys. 1984, 85, 73–82. (9) Ishimoto, C.; Tomimuro, H.; Seto, J. Appl. Phys. Lett. 1986, 49, 1677–1679. (10) Kobayashi, T. Mol. Cryst. Liq. Cryst. 1998, 22, 301–304. (11) Okamura, M. Y.; Feher, G.; Nelson, N. In Photosynthesis; Govindjee, Ed.; Academic Press: New York, 1982; pp 195-272. (12) Kuhlbrandt, W. Nature 1995, 374, 497–498.
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cationic species (alkaline earth metal ions, monoamines, surfactants, and cationic porphyrins).13-17 Aggregation of porphyrins has been studied mainly in solution.13-17 However, for practical applications, the formation of aggregated structures on solid substrates is highly desirable because they can be applied for the fabrication of organic solar cells,18 nonlinear optical materials,19 and sensor devices.20,21 To date, few reports have been published for J- and H-aggregates of water-soluble TPPS on solid substrates by layer-by-layer assembly,22 casting,23 and Langmuir-Blodgett24 methods. In this Letter, we wish to report the formation of nanostructured aggregates of hydrophobic meso-tetramesitylporphyrin (MTMP) induced by HCl vapor in the solid state. The formation of nanostructured MTMP aggregates was confirmed by UV-visible, time-resolved emission spectral, atomic force microscopy (AFM), and high resolution transmission electron microscopy (HR-TEM) techniques. Exposure of HCl vapor to a thin film of MTMP resulted in a red shift of the Soret band in contrast to the case of the MTMP film. The AFM and TEM images confirmed the formation of nanostructured aggregates on the glass plate. (13) Ohno, O.; Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1993, 99, 4128–4139. (14) Ribo, J. M.; Crusats, J.; Farrera, J. A.; Valero, M. L. J. Chem. Soc., Chem. Commun. 1994, 681–682. (15) Pasternack, R. F.; Schaefer, K. F.; Hambright, P. Inorg. Chem. 1994, 33, 2062–2065. (16) Akins, D. L.; Zhu, H. R.; Guo, C. J. Phys. Chem. 1994, 98, 3612–3618. (17) Maiti, N.; Ravikanth, M.; Mazumdar, S.; Periasamy, N. J. Phys. Chem. 1995, 99, 17192–17197. (18) Hasobe, T.; Imahori, H.; Fukuzumi, S.; Kamat, P. V. J. Mater. Chem. 2003, 13, 2515–2520. (19) Collini, E.; Ferrante, C.; Bozio, R.; Lodi, A.; Ponterini, G. J. Mater. Chem. 2006, 16, 1573–1578. (20) Fujii, Y.; Hasegawa, Y.; Yanagida, S.; Wada, Y. Chem. Commun. 2005, 3065–3067. (21) Kalimuthu, P.; John, S. A. Anal. Chim. Acta 2008, 627, 247–253. (22) Kawasaki, M.; Aoyama, S.; Kozawa, E. J. Phys. Chem. B 2006, 110, 24480– 24485. (23) Rotomskis, R.; Augulis, R.; Snitka, V. J. Phys. Chem. B 2004, 108, 2833– 2838. (24) Furuki, M.; Wada, O.; Pu, L. S.; Sato, Y.; Kawashima, H.; Tani, T. J. Phys. Chem. B 1999, 103, 7607–7612.
Published on Web 10/01/2009
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Interestingly, these aggregates were not reverted back to the morphology of the MTMP film when it was exposed to ammonia vapor. In the present method, there is no possibility for the excessive reagent as an impurity in the formed aggregates compared to the reported methods,13-17,22-24 since the protonating reagent (HCl) is in the gaseous state.
2. Experimental Section Pyrrole and mesitaldehyde were purchased from Aldrich. Pyrrole was distilled prior to use. The compound MTMP was synthesized according to the reported procedure25 and characterized using NMR and MALDI-TOF techniques (1H NMR (CDCl3 300 MHz) δ -2.51 (br, S, 2H, NH), 1.85 (S, 24H, o-CH3), 2.62 (S, 12H, p-CH3), 7.27 (S, 8H m-ArH), 8.61 (S, 8H, β-pyrrole). MS (MALDI-TOF): found m/z = 782.2956, calcd for C56H54N4 = 782.4348). A thin film of MTMP on a glass plate was prepared using 0.01 M MTMP in dichloromethane by the dip coating method. A clean glass plate was immersed in the MTMP solution at a withdrawal rate of 12 cm/min using a homemade stepper and dried at room temperature under nitrogen atmosphere. Standard dry HCl gas (19 ppm) diluted with nitrogen was used for J-aggregate formation. The MTMP thin film was exposed for 1 min to HCl vapor (Supporting Information Figure S1) to obtain the aggregated structure. UV-vis spectra were recorded using a Perkin-Elmer UV-vis Lambda35 spectrophotometer. Fluorescence spectra were recorded using a PerkinElmer LS 55 model fluorimeter. The quantum yield of MTMP thin films and aggregates were determined by comparing the spectral intensities of the lamp and the sample emission using a reported procedure.25 The MTMP was spin coated on 10 mm diameter quartz substrate and placed in the integrating sphere L6020203 (Perkin-Elmer). The MTMP was excited at 438 and 453 nm for MTMP thin film and MTMP thin film exposed to HCl vapor, respectively. A fluorescence lifetime spectrometer (model 5000U, IBH, U.K.) was used to measure the lifetime of the porphyrin emission. The second harmonic (425 nm) output from the Tsunami mode locked picosecond laser and LEDs were used as the exciting sources. Fluorescence decay analysis was carried out by the software provided by IBH (DAS-6) which is based on reconvolution technique and iterative nonlinear least-squares methods. AFM images were obtained using the MultiMode V scanning probe microscope for MTMP coated glass plates before and after exposure to HCl vapor. High resolution transmission electron microscopy (HR-TEM) images were obtained using a JEOL JEM 3010 microscope operating at 200 kV. The structures of the MTMP and protonated MTMP were initially optimized using MOPAC 2000 version 1.0 at the MNDO level of the basis set. Finally, the structures were optimized using the Gaussian 03W, HF method at the 6311-G level of theory.
3. Results and Discussion 3.1. Spectroscopic Characterization. Figure 1A curve a shows the electronic spectrum of MTMP film deposited on a glass plate. It shows a characteristic Soret band at 438 nm and Q bands at 521, 549, 596, and 650 nm. In contrast to the MTMP solution spectrum26 (Supporting Information Figure S2; curve a), the Soret band was broadened and red-shifted by 20 nm for MTMP film (curve a). Such behavior may be due to the aggregate-like partially stacked molecules of highly nonpolar MTMP on the glass plate.27 Surprisingly, when the film was exposed to HCl vapor, not only the Soret band was shifted from 438 to 453 nm but also intensities were found to increase at 453 and 650 nm (curve b). The observed 35 nm red shift in the Soret band with (25) Palsson, L. O.; Monkman, A. P. Adv. Mater. 2002, 14, 757–758. (26) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828–836. (27) Kubat, P.; Lang, K.; Prochazkova, K.; Anzenbacher, P., Jr. Langmuir 2003, 19, 422–428.
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an increase in intensities at 453 and 650 nm, in contrast to the MTMP monomer solution spectrum, is a signature of J-aggregate formation.28,29 The bathochromic shift to 453 nm and increase of absorbance of the Soret band may be attributed to a Frenkel exciton-like transition coupling.30 It is known that when the molecules are closely arranged, the dipole-dipole interaction is much stronger, resulting in a red shift of absorption in the electronic spectrum.31 J-Aggregates are a typical example for this kind of behavior. We examined the formation of MTMP J-aggregates in solution by adding HCl to MTMP in a dichloromethane and methanol (8:2) mixture. Except for a red shift in the Soret band, no increase in the absorbance was observed upon protonation (Supporting Information Figure S2; curve b). This result indicated that MTMP does not form aggregates in solution as does TPPS which is in accordance with a recent report.32 Figure 1A curve c corresponds to the absorption spectrum of MTMP aggregates after exposure to ammonia vapor. It shows a blue shift of the Soret band when compared to that of MTMP film (curve a) and MTMP aggregates (curve b). Exposure of ammonia vapor treated MTMP film on a glass plate to HCl vapor reverts the Soret band position to 453 nm (Figure 1A curve d). Since the absorption peak corresponding to the J-band of the porphyrin was observed at 453 nm instead of 490 nm, as in the case of TPPS J-aggregates,7 we presumed that the formed aggregates do not have much stronger dipole-dipole interaction. Figure 1B shows the steady state emission spectra of MTMP thin film and aggregated structures. The emission intensity at 650 nm of the MTMP thin film exposed to HCl gas was decreased (curve b) in contrast to that of MTMP thin film (curve a), suggesting the formation of aggregates. The emissions between 560 and 620 nm may be attributed to resonance scattering. The increase in emission intensity of the MTMP aggregates for the subsequent exposure to ammonia vapor (curve c) indicates that the aggregate nature of MTMP was decreased but not completely. The fluorescence lifetime of the MTMP coated on the glass surface was calculated using the time correlated single photon counting technique. The fluorescence lifetime of MTMP in dichloromethane was estimated to be 12.92 ns. It was found that the decay of fluorescence emission of the thin film of MTMP was fit into a monoexponential decay and has a lifetime of 11.48 ns before exposing to HCl vapor (Supporting Information Figure S3A). The lifetime was decreased for the same thin film after exposure to HCl vapor. The decay profile of the thin film exposed to HCl vapor was fit into the biexponential decay, and it shows two different lifetimes (0.78 ns (69.10) and 1.35 ns (30.90)). The lowest lifetime corresponds to the aggregated MTMP on glass plate which has the highest relative amplitude (Supporting Information Figure S3B). The highest lifetime may be attributed to the residual protonated MTMP on the glass plate (the fluorescence lifetime of the protonated MTMP in dichloromethane was estimated to be 1.94 ns). The decrease in the lifetime after treatment with HCl vapor clearly suggested the formation of aggregates on the glass surface. The aggregated structure of the MTMP thin film after exposure to HCl vapor was subsequently exposed to ammonia vapor, and the fluorescence lifetime of the film was found to be increased to 10.65 ns with monoexponantial decay (Supporting Information Figure S3C). The increase of the (28) Jelly, E. E. Nature 1936, 138, 1009–1010. (29) Scheibe, G. Angew. Chem. 1936, 49, 563. (30) McRae, E. G.; Kasha, M. In Physical Processes in Radiation Biology; Augenstein, L., Mason, R., Rosenberg, B., Eds.; Academic Press: New York, 1964; pp 23-42. (31) Mizuguchi, J. J. Appl. Phys. 1998, 84, 4479–4486. (32) Okada, S.; Segawa, H. J. Am. Chem. Soc. 2003, 125, 2792–2796.
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Figure 1. (A) Absorption spectra of (a) MTMP film on glass plate, (b) MTMP film after exposed to HCl vapor, (c) the same film (b) after exposure to ammonia vapor, and (d) subsequent exposure of (c) to HCl vapor. (B) Fluorescence spectra of (a) MTMP film on glass plate, (b) MTMP film after exposure to HCl vapor, and (c) the same film (b) after exposure to ammonia vapor.
Figure 2. AFM (top-down and 3D) images of (A) MTMP film on glass plate, (B) nanostructure of MTMP thin film after exposure to HCl gas, and (C) MTMP aggregates after exposure to ammonia vapor.
lifetime indicated that the aggregate nature of MTMP was reduced. The fluorescence quantum yields of the films before and after HCl treatment and also after NH3 treatment were calculated. The quantum yield of the MTMP film after exposure to HCl vapor was decreased from Φfilm = 0.89 to Φfilm/HCl= 0.06. On the other hand, with subsequent treatment of the film with NH3 vapor, the quantum yield was increased (Φfilm/HCl/ammonia = 0.82) due to deprotonation of MTMP. 3.2. Structural Investigation. The morphology of the MTMP film was examined by AFM and HR-TEM techniques. The AFM image of MTMP film showed that it was uniformly covered as a thin film with the thickness of 5 nm on the glass plate (Figure 2A and Supporting Information Figure S4A section 12416 DOI: 10.1021/la9027783
graph). When this thin film was exposed to HCl vapor, an aggregated structure was obtained (Figure 2B). The 3D view (Figure 2B) indicated that the nanostructures of MTMP aggregates are grown vertically in contrast to the case of the MTMP thin film. Inspection of cross-sectional value of Figure 2B revealed that the height of the TPPS nanostructures was found to be 50170 nm with an average width of 120 nm (Supporting Information Figure S4B). When the MTMP thin film was exposed to HCl vapor, protonation occurs on the tertiary nitrogen atoms of the MTMP ring. Such protonated molecules of the uniformly deposited thin film were moved toward the nucleation center, which is the MTMP molecule wherein the first protonation has occurred, to form MTMP aggregates (Figure 2B). It has been shown that Langmuir 2009, 25(21), 12414–12418
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Figure 3. HR-TEM images of (A) nanostructure of MTMP thin film after exposure to HCl gas, (B) close view of (A), and (C) MTMP aggregates after exposure to ammonia vapor.
Figure 4. Optimized structures of MTMP and protonated MTMP molecules.
Figure 5. Proposed brick stone model of MTMP aggregate.
the porphyrin molecules are mobile on solid surfaces.33,34 The driving force behind this movement of MTMP molecules is the appearance of strong electrostatic attractive force between positively charged tertiary nitrogen atoms of protonated MTMP (33) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M. V.; Hecht, S. Nat. Nanotechnol. 2007, 2, 687–691. (34) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619–621.
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molecules and chloride ions. Figure 2C shows the AFM images of the MTMP aggregates after exposure to ammonia gas. It shows that the aggregates were deformed to small sized nanostructures (20 nm in height and 150 nm in width; Supporting Information Figure S4C) but not to thin film morphology as observed for MTMP thin film in Figure 2A. Figure 3 shows the HR-TEM images of the MTMP film and that after exposure to HCl and ammonia vapors. These images DOI: 10.1021/la9027783
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indicate that after exposure to HCl vapor of a MTMP thin film, nanostructured aggregates are formed (Figure 3A and B). With the subsequent exposure of the formed aggregated nanostructures to ammonia vapor, the aggregates were dispersed significantly as shown in Figure 3C. The observed results are consistent with the results obtained from AFM and spectral studies (vide supra). We have also optimized the structures of MTMP and protonated MTMP using ab initio method, and these are shown in Figure 4. In MTMP, all four mesityl groups of MTMP are lying in the plane perpendicular to the plane of the porphyrin ring. Upon protonation, the planarity of the porphyrin ring was destroyed and converted into saddle conformation, which is due to the fact that the positively charged protonated tertiary nitrogen atoms move away from the plane of the porphyrin ring. It is likely that the deposited film of MTMP has aggregate-like partially stacked molecules.27,35 When the MTMP film was exposed to HCl vapor, MTMP molecules were protonated at tertiary nitrogen atoms with conformational change. As predicted in the ab initio studies, the mesityl groups of MTMP twisted away upon protonation and facilitated the effective molecular stacking in addition to the electrostatic attraction between protonated tertiary nitrogen atoms and chloride ions, which leads to aggregate formation. Thus, MTMP aggregates are stabilized by the electrostatic interaction between the positively charged tertiary nitrogen atoms and negatively charged chloride ions. When aggregates were exposed to ammonia vapor, the molecules regain the planarity of the porphyrin ring due to the removal of HCl molecules and the MTMP aggregated state was demolished significantly. The existence of the aggregated state even after the removal of HCl molecules can be explained by the fact that the molecules in the solid state cannot be dispersed to that extent like in solution. This is the reason for the blue shift of the Soret band after the removal of HCl molecules from the aggregates (Figure 1A curve c). Based on the spectral, AFM, (35) Lidzey, D. G.; Bradley, D. D. C.; Virgili, T.; Armitage, A.; Skolnick, M. S. Phys. Rev. Lett. 1999, 82, 3316–3319.
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HR-TEM, and ab initio studies, we proposed the brick-stone model36,37 for the formed nanostructured J-aggregates of MTMP on glass substrate (Figure 5). We assumed that the saddle structured protonated MTMP molecules were slipped over each other as shown in Figure 5.7,23
4. Conclusion In conclusion, we have demonstrated the formation of nanostructures of hydrophobic MTMP aggregates on a glass plate induced by HCl vapor. The formed MTMP aggregates showed a 35 nm red shift in Soret band absorption with an increase in absorbance when compared to the case of MTMP thin films. The fluorescent intensity and lifetime of the MTMP thin film were decreased when it was exposed to HCl vapor. The size of the MTMP nanostructures was found to be 50-170 nm in height with an average width of 120 nm. The removal of HCl molecules from the MTMP J-aggregates by exposure to ammonia vapor reduces the aggregate nature significantly. We have proposed the bickstone model for the formed MTMP aggregates based on the spectral, AFM, TEM, and ab initio studies. Acknowledgment. P.K. thanks the University Grants Commission, New Delhi for the award of Senior Research Fellowship. We thank Prof. P. Ramamurthy, National Centre for Ultrafast Processes, Madras University for recording time correlated single photon counting measurements. We thank the Department of Science and Technology, New Delhi for financial support under Nanomission (No. SR/NM/NS-28/2008). Supporting Information Available: Flow cell setup for the exposure of MTMP thin film to vapors, expanded views of Figure 2, and fluorescence decay profile of MTMP film before and after exposure to HCl gas. This material is available free of charge via the Internet at http://pubs.acs.org. (36) Misawa, K.; Kobayashi, T. In J-Aggregates; Kobayashi, T., Ed.; World Scientific: Singapore, 1996; p 41-65. (37) Egorov, V. V.; Alfimov, M. V. Phys.-Usp. 2007, 50, 985–1029.
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