porphine in Pure and Mix - American Chemical Society

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Langmuir 2000, 16, 1196-1204

Organization of meso-Tetra(4-N-stearylpyridyl)porphine in Pure and Mixed Monolayers at the Air/Water Interface and in Langmuir-Blodgett Films Ame´lia M. Gonç alves da Silva,* Maria Isabel Viseu, Anantha Malathi, Patrıćia Antunes, and Sı´lvia M. B. Costa Centro de Quı´mica Estrutural, Complexo I, Instituto Superior Te´ cnico, 1049-001 Lisboa, Portugal Received June 23, 1999. In Final Form: September 17, 1999 The molecular packing of meso-tetra(4-N-stearylpyridyl)porphine tetra-p-toluenesulfonate (PO) was investigated at the air/water interface and in Langmuir-Blodgett (LB) films in progressively more complex systems, namely, the pure component PO, PO mixed with sodium hexadecyl sulfate (SHS), and PO mixed with SHS and stearic acid (SA). The LB films formed at selected surface pressures have been analyzed by UV-visible absorption spectroscopy. The long transition in the pure PO monolayer was ascribed to changes in the orientation of PO bases, from parallel to oblique orientation, toward the water surface. At the interface, the PO/4SHS mixture exhibits two transitions: a low surface pressure transition, attributed to a closer arrangement favoring electrostatic interactions; and a high surface pressure transition, ascribed to the formation of an interdigitated double layer. The PO/4SHS/4SA system exhibits the high surface pressure transition in a short range of areas. This behavior was ascribed to the coexistence of two mechanisms of long chain dense packing: the filling of PO/4SHS bases by SA molecules and the double layer interdigitation of PO/4SHS. On increasing the SA content in the ternary mixture, the transition disappears and a solid condensed monolayer, induced by the long chain packing, begins at low surface pressures.

Introduction The organization of chlorophyll a in a photosynthetic membrane plays a key role in the quantum efficiency of the energy, electron, and transport processes in photosynthesis. Porphyrins are structurally similar to chlorins, which have been extensively studied in connection with important biological functions. Therefore, the optical and electrical properties of porphyrin derivatives are important for the biological modeling and construction of molecular devices.1-3 The architecture of supramolecular organizates or complex monolayers and multilayers with the desired molecular structure has been a big challenge over the recent years. A large variety of assembled systems have shown cooperative effects and are, in general, viewed as good candidates for the study of noncovalent hydrophobic and/or electrostatic interactions. Control of molecular orientation and packing of porphyrin in molecular organized systems is required in fundamental research and potential applications. The Langmuir-Blodgett method has been widely used to incorporate porphyrin into molecular assemblies with wellcontrolled compositions, structures, and thicknesses.4-13 * To whom correspondence should be addressed. E-mail: [email protected]. (1) Dolphin, D., Ed. The Porphyrins; Academic Press: New York, 1978. (2) Schouten, P. G.; Warman, J. M.; Haas, M. P.; Fox, M. A.; Pan, H. L. Nature 1991, 353, 736. (3) Liu, C. Y.; Pan, H. L.; Fox, M. A.; Bard, A. J. Science 1993, 261, 897. (4) Roberts, G. G., Ed. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (5) Kroon, J. M.; Sudholter, E. J. R.; Schening, A. P. H. J.; Nolte, R. J. M. Langmuir 1995, 11, 214. (6) De´sormeaux, A.; Max, J. J.; Leblanc, R. M. J. Phys. Chem. 1993, 97, 6670. (7) Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuruda, S.; Sugi, M.; King, L. G.; Crossley, M. J. J. Phys. Chem. 1993, 97, 12862.

Another important requirement is that porphyrin should retain its physicochemical characteristics after incorporation in multilayer Langmuir-Blodgett (LB) films. Typically, the incorporation of porphyrin in rigid films results in lateral phase separation and π-π aggregation. Thus, it is essential that the porphyrin molecules do not aggregate in the assemblies, since aggregation provides alterations in the optical properties. To prepare LB films, stable monolayers of porphyrin are required to form at the air/water interface before the transfer onto the substrate.4 It is now well established that unsubstituted tetraphenylporphyrin complexes do not form stable monolayers in the pure form at the air/ water interface.14 Long-chain amphiphilic substituents on the phenyl groups seem to be necessary for the formation of stable monolayers.12,15 Additional stabilization is provided when the cromophore is incorporated in a surfactant matrix.12,16-18 Other porphyrins, such as the unsubstituted pyridylporphyrin derivatives, which are water soluble due to the high hydrophilicity of the (8) Romanovskii, Y. V.; Personov, R. I.; Samoilenko, A. D.; Holliday, K.; Wild, U. P. Chem. Phys. Lett. 1992, 197, 373. (9) Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.; King, L. G.; Crossley, M. J. Langmuir 1995, 11, 4056. (10) Arnold, D. P.; Manno, D.; Micocci, G.; Serra, A.; Tepore, A.; Valii, L. Langmuir 1997, 13, 5951. (11) Aramata, K.; Kamachi, M.; Takahashi, M.; Yamagishi, A. Langmuir 1997, 13, 5161. (12) Song, X.; Miura, M.; Xu, X.; Taylor, K. K.; Majunder, S. A.; Hobbs, J. D.; Cesarano, J.; Shelnutt, J. A. Langmuir 1996, 12, 2019. (13) Liu, J.; Xu, L.; Shen, S.; Zhou, Q.; Li, T.; Xu, H. J. Photochem. Photobiol. A: Chem. 1993, 71, 275. (14) Bull, R. A.; Bulkowski, J. E. J. Colloid Interface Sci. 1983, 92, 1. (15) De´sormeaux, A.; Ringuet, M.; Leblanc, R. M. J. Colloid Interface Sci. 1991, 147, 57. (16) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsay, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1989, 111, 1344. (17) Vandevyver, M.; Barraud, A.; Ruaudel-Texier, A.; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571. (18) Efimov, A. V.; Anikin, M.; Tkachenko, N. V.; Mironov, A. F.; Lemmetyinen, H. Chem. Phys. Lett. 1998, 289, 572.

10.1021/la990802b CCC: $19.00 © 2000 American Chemical Society Published on Web 11/27/1999

Porphyrin Packing in Monolayers

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the anionic surfactant sodium hexadecyl sulfate (SHS) and with SHS and stearic acid (SA) with several proportions of SA. The paper is organized in the following way: Surface pressure isotherms of the pure PO and the mixtures with SHS and SA were studied and are described in part I. The corresponding monolayers were transferred onto quartz slides and the electronic absorption spectra of LB films are presented in part II. The packing and orientation of porphyrin at the air-water interface and in the films, suggested by the combination of the data, is proposed in the Discussion section, by the use of possible schematic arrangements. Experimental Section Figure 1. Chemical structure of meso-tetra(4-N-stearylpyridyl) porphine (PO).

pyridinium, can also form stable monolayers when mixed with long-chain anionic surfactants.19-23 Due to electrostatic interactions, these porphyrin derivatives will be surrounded by the negatively charged surfactants. Again, long hydrophobic chains, linked to the pyridinium group, decrease the water solubility and stabilize the porphyrin monolayer at the air/water interface.24 Several studies have shown that the number and length of the alkylpyridyl groups on the porphyrin ring can influence the orientation and packing of the porphyrin molecule.25-28 The purpose of this work was to study the selective binding and association of a functionalized positively charged porphyrin with a negatively charged surfactant. Furthermore, since the addition of a long-chain fatty acid in the buildup of the molecular assemblies can considerably affect the orientation of the porphyrin molecule included in the monolayer and in the LB film, this work provided an opportunity to assess the interplay of the hydrophobic/electrostatic interactions reflected in the porphyrin’s optical properties. In this paper, the meso-tetra(4-N-stearylpyridyl)porphine (PO), presented in Figure 1, was investigated at the air/water interface. Cationic pyridinium, replacing the phenyl group, plays two important roles on the porphyrin film stability: due to the increase of porphyrin hydrophilicity, it enhances the interaction with the water subphase and it reduces aggregation due to the electrostatic repulsion between the positive charges in the porphyrin ring.27 Furthermore, the four stearyl chains on the pyridinium groups allow the PO to form, on its own, a stable monolayer at the interface, which can be transferred onto a solid substrate. In this study, the PO was investigated either in the pure form or mixed with (19) Gregory, B. W.; Vaknin, D.; Gray, J. D.; Ocko, B. M.; Stroeve, P.; Cotton, T. M.; Struve, W. S. J. Phys. Chem. 1997, 101, 2006. (20) Gregory, B. W.; Vaknin, D.; Cotton, T. M.; Struve, W. S. Thin Solid Films 1996, 284-285, 849. (21) Prieto, I.; Martıń Romero, M. T.; Camacho, L.; Mo¨bius, D. Langmuir 1998, 14, 4175. (22) Martin, M. T.; Mo¨bius, D. Thin Solid Films 1996, 284-285, 663. (23) Prieto, I.; Camacho, L.; Martıń, M. T.; Mo¨bius, D. Langmuir 1998, 14, 1853. (24) Fukushima, H.; Taylor, D. M.; Morgan, H. Langmuir 1995, 11, 3523. (25) Nagamura, T.; Koga, T.; Ogawa, T. J. Photochem. Photobiol. A: Chem. 1992, 66, 119. (26) Zhang, Z.; Verma, A. L.; Yoneyama, M.; Nakashima, K.; Iriyama, K.; Ozaki, Y. Langmuir 1997, 13, 4422. (27) Schening, A. P. H. J.; Hubert, D. H. W.; Feiters, M. C.; Nolte, R. J. M. Langmuir 1996, 12, 1572. (28) Kroon, J. M.; Sudho¨lter, E. J. R.; Schening, A. P. H. J.; Nolte, R. J. M. Langmuir 1995, 11, 214.

Materials. meso-Tetra(4-N-stearylpyridyl)porphine tetra-ptoluenesulfonate (PO) was obtained from Dojin (Japan) and was a kind gift from Professor I. Yamazaki. Mesoporphyrin IX dimethyl ester was purchased from Sigma. Sodium hexadecyl sulfate (SHS) was obtained from Merck Darmstadt with 99% purity, while stearic acid (SA), as specially pure, was purchased from BDH. The solvents chloroform and ethanol were of spectroscopic grade (Uvasol) from Merck. Amphiphiles and solvents were used as purchased, without further purification. Water used in the subphase was distilled twice and purified with the Millipore Milli-Q system in order to obtain a resistivity higher than 18 MΩ cm. Separate stock solutions of SA and SHS were prepared in chloroform and ethanol, respectively. Mixed solutions of PO, SHS, and SA were prepared by adding precisely measured volumes of SHS and SA stock solutions to the adequate amount of PO and diluting to a final volume with chloroform and ethanol; the final spreading solvent was always a 4:1 (v/v) mixture of chloroform + ethanol. Surface Pressure-Area Measurements. Surface pressure-area (π-A) isotherm measurements were carried out on a KSV 5000 Langmuir-Blodgett system (KSV Instruments, Helsinki) installed in a laminar flow hood. The surface tension measurements were performed with the Wilhelmy plate technique. A symmetrical compression of the monolayer was achieved with two barriers, moving at a rate of 10 mm min-1, unless otherwise specified. The temperature of the subphase was maintained at 25.0 ( 0.1 °C, except when the temperature effect was studied in the range from 15 to 35 °C. For the π-A measurements, precisely measured volumes of the convenient solution were spread on the water surface, using a SGE microliter syringe. The π-A isotherms were measured several times, and different initial solutions were used. LB Deposition. Silica substrates were cleaned by immersion in concentrated chromic-sulfuric acid mixture for several minutes and were subsequently thoroughly rinsed with Milli-Q water, immersed in ultrapure water for several hours, and finally dried in a nitrogen flow before using. The substrates were clamped parallel to the barriers and immersed in the subphase before spreading the monolayer material. After complete evaporation of the solvent, the floating layer was compressed until the target surface pressure. After a 5 min relaxation period, the deposition was performed at constant surface pressure with a dipping rate of 5 mm min-1. UV-Visible Absorption Studies. The absorption spectra of LB films were obtained in the transmission mode using a Jasco V-560 UV-visible spectrophotometer. The wavelength region of 300-700 nm was systematically scanned, although only the Soret band was analyzed. Background scattering from the films was corrected by subtracting the “apparent absorption” spectrum of a clean quartz slide (blank), recorded in the same operational conditions as the sample films.

Results Part I. Air-Water Interface. Surface PressureArea Isotherms. The molecular packing and association of PO was investigated in progressively more complex monolayers at the air-water interface: the single-

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Table 1. Areas per PO Molecule (Å2) and Surface Pressures (mN/m) for Systems 1-5 no. of system PO/SHS/SA chains A0ei A0eii A0c A50 1 2 3 4 5

Figure 2. Surface pressure-area isotherms on a pure water subphase, at 25 °C, for systems: PO (1); PO/4SHS (2); PO/ 4SHS/4SA (3); PO/4SHS/8SA (4); and PO/4SHS/16SA (5). The inset represents the area per PO molecule at π ) 50 mN/m as a function of the number of hydrocarbon chains per PO. The open symbol stands for system 1.

component PO monolayer; the mixture of PO with SHS in a 1:4 molar ratio (PO/4SHS); and finally, the threecomponent monolayer (PO/4SHS/xSA), with x ) 4, 8, and 16. The surface pressure, π, as a function of the area per PO molecule, A, is compared in Figure 2 for the five systems. PO (System 1). PO, carrying four positive charges and four stearyl chains, forms an expanded monolayer at low surface pressures and a condensed monolayer at high surface pressures (curve 1). The transition from the expanded to the condensed state, corresponding to a considerable decrease of the area occupied per molecule, starts at 34 mN/m and evolves to a long plateau at 33 mN/m. The values of the extrapolated limiting molecular areas at low (A0e) and high (A0c) surface pressures are respectively 500 and 150 Å2. PO/4SHS (System 2). When PO is mixed with the anionic surfactant SHS at 1:4 molar ratio, the charge of the PO molecule is neutralized. The first compression isotherm (curve 2) exhibits three regions separated by two transitions: the first plateau at 9 mN/m corresponds to the transition from an expanded regime ei to an intermediate region eii; the second plateau at 22 mN/m corresponds to the transition from eii to a condensed region c. The condensed film collapses at a very high surface pressure, 68 mN/m. The condensing effect introduced by the oppositely charged surfactant SHS at low π is clearly shown by the deviation of regions ei and eii of system 2 to lower areas than the pure PO (curve 1) at the same π. PO/4SHS/4SA (System 3). When stearic acid (SA) is added to the previous mixture at a 1:4:4 molar ratio (curve 3), there is only one transition from the expanded to the condensed region, at 23 mN/m. Curves 2 and 3 are very close at low surface pressures, but curve 3 deviates to higher areas at high π. These facts indicate that, at low π the SA molecules do not contribute to the total monolayer area, remaining at the interface without occupying an extra area in the monolayer. However, they have an effective contribution at high π. Furthermore, the presence of SA molecules seems to hinder the transition observed in the previous system at 9 mN/m. PO/4SHS/8SA (System 4). When added in a higher proportion (1:4:8) the four SA extra molecules occupy an additional area of 100 Å2 at low surface pressures (curve 4). Instead of the plateau observed in the previous systems, a gradual transition occurs, characterized by a lower isotherm slope in the range 24-32 mN/m. The extrapolated area at the condensed region is 350 Å2 per PO

1/0/0 1/4/0 1/4/4 1/4/8 1/4/16

4 8 12 16 24

500 150 80 450 350 170 100 400 260 196 500 350 290 500 435

πc

πt1

πt2

54 30-31 68 9 22 57-58 23 63 24-32 60-62

molecule. This value supports a dense packing of 16 hydrocarbon chains at high surface pressures (350/16 ) 22 Å2 per chain). It seems that the PO base, remaining parallel to the water surface, is filled up by the alkyl chains. PO/4SHS/16SA (System 5). A steep isotherm curve is obtained when the complex unit PO/4SHS is mixed with 16 SA (curve 5). This is a typical condensed monolayer corresponding to the close packing of 24 alkyl chains per PO molecule. It seems that the PO is filled up by SHS and SA in a matrix of exceeding SA: This is in agreement with the “filling in” mechanism described by Gruninger et al.29 and Song et al.12 Summarized in Table 1 are the extrapolated areas from the different regimes, A0ei, A0eii, A0c, the area at 50 mN/m, A50, and the surface pressure at the collapse, πc, and at transitions, πt1, πt2, taken from the monolayer isotherms represented in Figure 2. A common feature of the above mixed systems is the nearly constant contribution of each hydrocarbon chain to the area in the condensed region. The area at 50 mN/m, as a function of the number of hydrocarbon chains per PO, is plotted in the inset of Figure 2. In fact, there is a linear variation with the slope of 20 Å2 per chain, which is a good estimate of the area per alkyl chain in a condensed monolayer. To clarify the type of transition and the kind of reorganization occurring at each plateau presented in Figure 2, the compression-expansion cycles and the temperature effect on systems 1-3 were studied. Compression-Expansion Cycles. System 1. The compression-expansion cycles of the pure PO (system 1) are completely reversible before the transition at 33 mN/m (results not shown) but show strong hysteresis loops when compressed beyond the transition (Figure 3a). After compression up to 40 mN/m, the expansion curve appears deviated to lower areas, approaching the compression curve at low surface pressures. However, the second and following compressions follow the first one, at slightly lower areas, and a pseudoplateau is recovered. All cycles superimpose at high surface pressures indicating no loss of material. Thus, the deviation of the expanded region to lower areas in successive cycles should correspond to a progressive reorganization of the molecules in the monolayer. System 2. All four compression-expansion cycles of system 2 presented in Figure 3b show hysteresis loops, and no plateau is visible in the expansion curves. The expansion isotherm from 40 mN/m (cycle 3) is very steep (except at low π) and the next compression (cycle 4) appears at much lower areas than the first one. However, despite the shorter range of areas, the first plateau appears at nearly the same surface pressure as in the prior compressions. This behavior is independent of the barrier speed (2-10 mm min-1). The disappearance of the second plateau (after compressing the monolayer beyond πt2) is compatible with an irreversible aggregation, such as a double layer (29) Gru¨niger, H.; Mo¨bius, D.; Meyer, H. J. Chem. Phys. 1983, 79, 3701.


Figure 3. Compression-expansion cycles, at 25 °C, for systems: PO (a); PO/4SHS (b); PO/4SHS/4SA (c).

formation, while the recovery of the first plateau is compatible with a more compact arrangement of charges at the water surface. System 3. As in the previous system, the compressionexpansion cycle of system 3 beyond πt is not reversible and the plateau occurring at 23 mN/m in the first compression disappears during recompression (Figure 3c). This means that this transition is of the same kind as that occurring in the previous system, at the same surface pressure. Temperature Effect. System 1. The effect of temperature on the PO isotherm is only significant at the plateau (Figure 4a). As temperature increases, the transition starts at lower π and the plateau becomes longer. The small peak appearing at the beginning of the

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Figure 4. Surface pressuresarea isotherms at 15, 25, and 35 °C (the temperature increase is indicated by the arrow) of systems PO (a), PO/4SHS (b), and PO/4SHS/4SA (c).

transition decreases with temperature. It seems that this transition is favored by temperature as an activated process. System 2. The analysis of the temperature effect on system 2 provides much information (Figure 4b). In fact, the temperature affects the two plateaus differently: the surface pressure of the first plateau, πt1, increases with the temperature, while the second plateau, πt2, is not significantly affected. This is in principle a good indication that the processes occurring at those plateaus have different mechanisms. The increase of πt1 with temperature is the expected variation of a first-order transition

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in the monolayer.30 Furthermore, the condensed and expanded films behave differently with varying temperature: the area per PO in the regions ei and eii does not change in the range 15-25 °C, increasing at 35 °C, while for the condensed region (c) the area per PO decreases as temperature increases. This is probably due to the molecular packing in those regions being controlled by different forces. At low π the structural arrangement could be determined by the electrostatic forces between oppositly charged ions at the water surface, while at high π the packing might be dominated by the hydrophobic interactions between the alkyl chains. System 3. The effect of temperature on system 3 (Figure 4c) is similar to that observed on the previous system, except for a stronger deviation to lower areas observed in the condensed region when temperature increases and for the quite different behavior at 35 °C in both the expanded and transition regions. In fact, at 15 and 25 °C, the isotherms coincide at low surface pressures and in the plateau. At 35 °C, a more expanded monolayer forms at low π, and the transition starts at higher surface pressures than at 15 and 25 °C and then decreases to lower values as the area decreases. A similar behavior was observed by Dumaine-Bouaziz et al.31 on a dialkyl-macrocycloureide, studied in the same range of temperatures, 15-35 °C. The plateau, ascribed to a bilayer formation, remained at the same surface pressure regardless of the temperature; a strong hysteretic compression-expansion behavior was observed once the beginning of the plateau had been reached; a less packed monolayer was obtained at 35 °C and at low surface pressures; and a drastic decrease of the molecular area was observed at high surface pressures when the temperature increases. The authors attributed this last unexpected behavior to the squeezing out of the water molecules from the hydration shells surrounding the polar headgroup of the molecules at the air/water interface. The temperature effect on the transition regions of π-A isotherms and the compression-expansion cycles strongly suggest three types of transitions: type I, decrease of πt with temperature and recovery of the plateau in the recompression (transition of system 1); type II, increase of πt with temperature and recovery of the plateau in the recompression (first transition of system 2); type III, independence of πt with temperature and irreversibility of the plateau (second transition of system 2 and transition of system 3). Part II. Absorption Spectra of LB Films. In this work, UV-visible absorption spectra of LB films of systems 1-5 were obtained. The films were deposited onto hydrophilic quartz slides, at surface pressures selected for each regime (below and above the main transitions): at 40 mN/m in the condensed region; at 15 mN/m in the expanded region; and also at 5 mN/m for system 2. Several transfer cycles were performed at the expanded regimes and both parameters, transfer ratio and absorption intensity, indicate that only one layer remains at the hydrophilic substrate. Consequently, the comparison presented herein includes only LB films obtained by one upstroke. As similar nonmetalated (free-base) tetrapyridinium or tetraphenyl porphyrins, PO presents an intense Soret band in the wavelength region of ≈350-500 nm and four Q-bands of much lower intensity in the 500-700 nm (30) Levine, I. N. Physical Chemistry, 4th ed.; McGraw-Hill International Editions, Chemistry Series: Lisbon, 1995; Chapter 7. (31) Dumaine-Bouaziz, M.; More´lis, R. M.; Cordier, D.; Coulet, P. R. Langmuir 1998, 14, 6749.


Figure 5. Absorption spectra of PO/4SHS in one layer thick LB films obtained at 5, 15, and 40 mN/m. (Inset) absorption spectrum at 40 mN/m (1) as a composition of two-component spectra: PO/4SHS LB film, obtained at 5 mN/m (2) and mesoporphyrin IX dimethyl ester (herein in chloroform solution, 3).

region. Due to both the low absorbance and the high scattering of these LB films, the spectra present a low signal/noise ratio in the Q-band region, and therefore only the Soret band was analyzed. Parameters such as the wavelength of maximum absorbance (λmax) and the band wavenumber half-width (ω j 1/2), were used for comparison between the different systems and surface pressures. PO (System 1). Expanded Region. The absorption spectrum of a LB film of PO obtained at 15 mN/m (not shown herein) presents a Soret maximum which is somewhat red shifted (λmax ≈ 442 nm) as compared to the solution spectrum in the spreading solvent (λmax ≈ 434 nm). However, the spectra present similar shapes and half-widths, suggesting the absence of strong interactions in the monolayer film. Therefore, PO is probably in monomeric form in the expanded LB films. Condensed Region. The absorption spectra of PO LB films obtained at 40 mN/m (also not shown herein) are blue shifted (λmax ≈ 430-434 nm) relative to those obtained in the expanded region (λmax ≈ 440 nm). Furthermore, the Soret band is more rounded and wider (ω j 1/2 ) 0.27 × 10-3 nm-1) than in the expanded region (ω j 1/2 ) 0.19 × 10-3 nm-1). It is also wider than that in the solution spectrum (ω j 1/2 ) 0.20 × 10-3 nm-1). These observations suggest the presence of some type of PO aggregates in the condensed film, possibly H-aggregates. These face-to-face or “cardpack” aggregates are formed when the porphyrin rings lie parallel to each other and usually lead to spectral blue shifts relative to the monomer spectrum.32,33 However, the small shift observed in the present case seems to indicate that only a partial superposition of the PO rings is attained. The higher absorbance of the LB films formed at 40 mN/m, as compared with those obtained at 15 mN/ m, also indicates a more dense packing of the porphyrin molecules in the condensed region. PO/4SHS (System 2). Expanded Regions. The absorption spectra of PO/4SHS LB films obtained at 5 and 15 mN/m present Soret maxima at ≈442-443 nm and, except for a small region near 400 nm, are similar to each other (Figure 5) and to the absorption spectra of expanded PO films. Therefore, the presence of SHS does not significantly affect the absorption spectrum of PO, as it also happens in solution (not shown herein). Consequently, we infer that these spectra are mainly due to the monomeric PO, free from intermolecular interactions. (32) Whitten, D. G. The Spectrum 1993, 6 (3), 1. (33) Maiti, N. C.; Mazundar, S.; Periasamy, N. J. Phys. Chem. B 1998, 102, 1528.


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important as the SA content of the monolayer increases. A red-shifted absorbance (relative to the absorbance of the monomeric species) is frequently attributed to the presence of another type of aggregate, a J-aggregate, where the PO rings lie edge-to-edge in the same plane.32,33 However, in the present expanded films all the experimental evidence (isotherms, absorption spectra) indicates that the PO rings lie closer to each other in the PO/4SHS system than in systems containing stearic acid. Therefore, it is not very likely that J-aggregates are formed in these systems. Another hypothesis could be an incipient protonation of PO by stearic acid, since porphyrin protonation (in the central nitrogen atoms) was observed to produce spectral red shifts.34,35 Condensed Region. The absorption spectra of LB films of systems 3-5 at 40 mN/m show marked differences with each other, but two general trends are observed as the SA content of the system increases (see Figure 6b, where system 2 is included): (i) The species responsible for the Soret maximum at ≈400 nm is progressively diluted, as compared to the monomeric PO absorbing at ≈440 nm. (ii) The shoulder at =480 nm progressively increases. These observations suggest that the SA molecules prevent pyridinium rotation, recovering the absorption spectrum of the “coplanar” PO ring, and favor the PO protonation (should this be the case). Discussion

Figure 6. Absorption spectra of one layer thick LB films of systems PO/4SHS (2), PO/4SHS/4SA (3), PO/4SHS/8SA (4), and PO/4SHS/16SA (5), obtained at 15 mN/m (a) and 40 mN/m (b).

Condensed Region. The absorption spectra of PO/4SHS LB films obtained at 40 mN/m are drastically different from those obtained at 5 and 15 mN/m, due to a strong, new absorption maximum at ≈400 nm (Figure 5). As seen in the inset of Figure 5, this spectrum seems to result mainly from the superposition of two spectra: the monomeric PO spectrum with λmax ≈440 nm, in a smaller proportion; and another with λmax ≈400 nm, in a higher proportion. The strong blue shift of the second spectrum suggests either the presence of PO aggregates (H-type), or the restriction of the π-electron delocalization of the PO ring to the central porphine ring. (The initial π-electron delocalization over all the PO base lying parallel to the water surface in the expanded state can be broken by the rotation of the four pyridinium groups out of the porphine plane, forming a “nonplanar” PO ring.) We favor the second hypothesis, since meso-porphyrin IX dimethyl ester, in which the pyridinium groups are absent, presents a Soret maximum at ≈398 nm in chloroform solution (Figure 5 inset). This is close to the maximum at ≈405 nm observed herein, especially if we are aware of possible differences between LB and solution spectra. PO/4SHS/xSA, with x ) 4, 8, and 16 (Systems 3, 4, and 5). Expanded Region. Absorption spectra of LB films of systems 3-5 at 15 mN/m show λmax ≈441-443 nm, in a similar way to those of systems 1 and 2 in the expanded region (compare Figure 6a, where system 2 is included, with Figure 5). This indicates that the “coplanar” PO is the main species responsible for the absorption. However, some differences may be noted among these spectra: (i) The absorbance decreases as the SA content of the monolayer increases. This is due to the increase in area per PO molecule (recall Figure 2), i.e., to the progressive dilution of PO in the film. (ii) In systems with stearic acid, a shoulder appears at ≈480 nm and becomes more

The results presented in the previous section show that the functionalized porphyrin, PO, which can form expanded and condensed monolayers depending on the surface pressure, significantly changes its pattern upon mixing, in the proper ratio, with a single-chained anionic surfactant. An attempt to interpret the possible structural arrangements and clarify the description of distinct regimes observed in the π-A isotherms of Figure 2 is made by using the illustrative schemes of Figure 7. However, these should be viewed as a proposal and do not exclude other arrangements. PO (System 1). The molecular areas reported in the literature for the base of tetraphenyl porphyrin derivatives range from 160 to 400 Å2, for a flat parallel orientation to the water surface, to about 50-80 Å2, when the porphyrin rings lie perpendicular to the interface.14,25,29,36 This broad range of molecular area is probably due to the variable orientation adopted by the porphyrin ring, which depends on the number, length, and orientation of the attached hydrocarbon chains. In particular, assuming that the porphyrin base lies flat at the interface, the area reported for the porphyrin ring with four methyl (instead of stearyl) groups is 320 Å2 at 8 mN/m.37 For another tetrakisphenylphorphyrin, with four eicosanyl chains and eight acetate groups attached to the porphyrin ring, the molecular area calculated by molecular mechanics calculations is 360 Å2, while the experimental limiting molecular area is 420 Å2.12 Considering the bulky effect of the stearyl chains, it seems reasonable to expect an area between 320 and 360 Å2 for the porphyrin used in the present study. Thus, we (34) Kuhn, H.; Mo¨bius, D. In Investigations of Surfaces and Interfaces - part B; Rossiter, B. W., Baetzold, R. C., Eds.; Physical Methods of Chemistry Series, 2nd ed.; John Wiley & Sons: New York, 1993; Vol. IXB, Chapter 6. (35) Chernia, Z.; Gill, D. Langmuir 1999, 15, 1625. (36) Choudhury, B.; Weedon, A. C.; Bolton, J. R. Langmuir 1998, 14, 6192. (37) Martıń, M. T.; Prieto, I.; Camacho, L.; Mo¨bius, D. Langmuir 1996, 12, 6554.

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Figure 7. Proposed schemes for the structural arrangements of systems 1-5: PO (blue); SHS (red); SA (yellow). Top view of the molecules with the alkyl chains perpendicular to the water surface, except for the two schemes 1c (perspective views).

will adopt 340 Å2 as an approximate benchmark. This value is intermediate from those obtained by extrapolation of the expanded (500 Å2) and condensed (150 Å2) monolayers of PO. This means that at low surface pressures

the charged porphyrin bases lie parallel to the interface and somewhat apart from each other (scheme 1e), in agreement with the absorption of the monomeric PO at 15 mN/m. A different molecular orientation or a multilayer


structure is adopted in the condensed region. A multilayer structure is compatible with the low area per PO but is not very plausible due to the recovery of the transition during the next compressions. On the other hand, the small blue shift (≈10 nm) observed in the absorption spectrum at 40 mN/m relative to that obtained in the expanded regime at first sight, might suggest the presence of H-aggregates. However, the bulky stearyl chains, which can adopt different relative orientations, may prevent the formation of perfect H-aggregates. Thus, it is more likely that the PO bases adopt a partially superimposed “card pack” structure, with a tilted orientation, relative to the water surface (scheme 1c shows two orientations for the alkyl chains). This edge-on interaction of all the PO bases with the water subphase allows the recovery of the expanded monolayer after the expansion. PO/4SHS (System 2). The condensing effect of SHS on the PO monolayer is illustrated by scheme 2ei when compared to 1e. The area per PO is lower in the presence of SHS than in the pure PO monolayer, although the limiting area per PO molecule of 450 Å2 is still higher than 340 Å2. The first plateau at 9 mN/m can be viewed as a reorganization of the molecules toward a more dense packing of opposite charges at the interface and hydrophobic chains in the upward direction (see scheme 2eii). Finally, the condensed state at high surface pressures was ascribed to a bilayer structure (scheme 2c), formed at the second plateau occurring at 22 mN/m. The assumption of a bilayer formation is based on two arguments: the irreversibility of the second plateau and the limiting area at the transition starting point (275 Å2, πt2) is approximately twice the transition “end point” (144 Å2, πt2). This value was found by the extrapolation of region 2c to πt2. The driving force of this double layer would be the alkyl chain dense packing. Thus, region 2c would correspond to an irreversible double layer, with the PO bases of the upper layer on the top and the alkyl chains interdigitated with those of the first layer: 16 alkyl chains per two PO molecules in the bilayer (see scheme 2c). This structure should provide high cohesion and stability to the film at high surface pressures, in agreement with the very high collapse surface pressure. A similar packing of 16 alkyl chains above one PO ring lying parallel to the water subphase was proposed by Song et al.12 (one molecule of uncharged porphyrin with 4 eicosanyl chains + 12 SA molecules). The absorption spectrum obtained at 40 mN/m (Figure 5) agrees with this interpretation, if we assume the following asymmetry of the bilayer: (i) The layer lying in contact with the quartz substrate (the one which was in contact with the aqueous subphase) should have predominantly coplanar PO rings, interacting strongly with the slide surface; this layer would then be responsible for the Soret shoulder at ≈440 nm (the “coplanar” PO). (ii) The layer with the inverse orientation, lying away from the quartz surface and interacting mainly with the former layer by their interdigitated chains, could have the pyridynium groups rotated out of the porphine plane; it would thus be responsible for the Soret maximum at ≈400 nm (the “nonplanar” PO ring). The thickness of LB layers is usually a very useful parameter to check the bilayer formation at the air-water interface. However, we think that in the present system this parameter might not be conclusive if the thickness of the interdigitated double layer approaches the thickness of the single layer in order to maximize the hydrophobic interaction. PO/4SHS/xSA, with x ) 4, 8, and 16. System 3. As suggested by the isotherms, and illustrated in scheme 3e, the SA molecules entering in the open structure of the

Langmuir, Vol. 16, No. 3, 2000 1203

PO/4SHS at low surface pressures do not introduce any additional area to scheme 2ei and prevent transition 2ei f 2eii of system 2, in system 3. The scheme proposed for the condensed state (see scheme 3c) assumes the coexistence of two mechanisms of long-chain dense packing: as the surface pressure increases the SA molecules completely fill the space above part of the PO/4SHS complexes and the remaining PO/4SHS complexes, free of SA, undertake the formation of an interdigitated double layer at the plateau. The irreversibility of this transition, occurring at the same surface pressure as transition 2eii f 2c, corroborates the interdigitated double layer formation. This plateau is shorter than that in system 2 because in this case only one-fourth of PO/4SHS present need to climb the first layer and invert the initial orientation in order to reach the close packing of 16 alkyl chains per PO. A two-phase monolayer, composed by pure SA and a complete interdigitated structure of PO/4SHS, could also explain the transition but not the absence of the first plateau at 9 mN/m. Thus, a mixed model is proposed: an interdigitated double layer coexisting with a monolayer of PO/4SHS filled above by SA. System 4. The SA molecules, which contribute to the total area of the monolayer at low surface pressures (scheme 4e), as the surface pressure increases, are progressively obliged to “fill-in” the open space above the PO/4SHS until a dense monolayer of 16 hydrocarbon chains per PO (scheme 4c) is formed. On the basis of this model, it seems that the value of A0c ) 350 Å2 at this regime, 4c, is a good estimate of the area occupied per PO lying parallel at the water surface. In fact, this value is close to the benchmark adopted (340 Å2). Another favorable argument is the alignment of regions 3e and 4c: region 4c is the extrapolation of 3e to high surface pressures. (At the starting point of transition 3e f 3c, the area of the monolayer is in the range of the area of 4c.) System 5. The structure illustrated in schemes 5e and 5c can be viewed as the PO/4SHS/8SA unit, dissolved in a matrix of SA. Assuming that the Soret maximum at ≈400 nm is associated with the restricted π-electron delocalization of the “nonplanar” PO ring, the spectra presented in Figure 6b indicate that a significant amount of this structure exists in system 3, in accordance with scheme 3c of Figure 7, and only traces of it may be found in system 4. However, it is likely that the real LB films are more complex and “heterogeneous” than those suggested by the models. Taking into account these cartoon models, we go further on the interpretation of the three transition types observed. Thus, we can conclude that the first transition on system 2 (type II) is promoted by electrostatic interactions between opposite charges, while the second transition in the same system and the transition in system 3 (type III) are promoted by hydrophobic interactions. When the number of alkyl chains per PO increases, the hydrophobic interactions become dominant, begining at low π, and the charge reorganization could be masked or hindered and consequently the low π transition disappears. The irreversibility of transition type III corroborates the double layer formation as discussed above. The reversibility of transition types I and II is compatible with first-order transitions interpreted by the Clapeyron equation,30 applied to the monolayer, dπ/dT ) ∆H/T∆A, where ∆A stands for the area change at the transition (∆A < 0 for both transitions) and ∆H represents the enthalpy change, ∆H * 0. For the first transition of system 2 (2ei f 2eii, type II), dπ/dT is positive (the surface pressure of the transition increases with temperature). This indicates an exothermic process (∆H < 0), the Coulombic and

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hydrophobic interactions are stronger in 2eii than in 2ei. For the transition of system 1 (1e f 1c, type I), dπ/dT is negative (π deacreases with T). This indicates an endothermic process (∆H > 0) and can be explained as follows: The compaction of PO bases and chains suggested in scheme 1c is anticipated by the partial breaking of the interaction of cationic PO base with the water subphase. In the condensed state, only two pyridinium per PO interact with the water and the other two are away from the water. This process and the approximation of the positively charged PO bases should be endothermic (∆H1 > 0), while the compaction of chains should be exothermic (∆H2 < 0). The global transition is endothermic if |∆H1| > |∆H2|, and then the πt will decrease with temperature. A similar transition behavior (dπ/dT < 0) is described in the literature as a “re-entrant” behavior.38,39 Conclusion Pure PO can form stable monolayers at the air/water interface, and LB films, one layer thick at least, can be prepared in expanded and condensed states. In expanded films, PO does not aggregate. The PO rings lie parallel to the substrate with the alkyl chains in the upward direction. The presence of some PO aggregates, possibly H-type aggregates, was observed in the condensed state. A significant condensing effect was observed at the air/ water interface at very low π, when the positively charged (38) Ibn-Elhaj, M.; Mo¨hwald, H.; Cherkaoui, M. Z.; Zniber, R. Langmuir 1998, 14, 504. (39) Plehnert, R.; Schro¨ter, J. A.; Tschierske, C. Langmuir 1999, 15, 3773.

PO is mixed with the anionic SHS. Furthermore, still in the range of low surface pressures (9 mN/m), a closer arrangement of charges at the interface is induced by electrostatic interactions. This interaction of PO with SHS prevents PO aggregation and does not affect the absorption spectrum of the monomeric PO in the pure expanded film. At high surface pressures, PO/4SHS forms an interdigitated double layer in order to reach the close packing of the alkyl chains. The absorption spectra of these films are drastically affected with a new absorption maximum at 400 nm. This is probably due to the rotation of the pyridinium rings, located in the upper layer away from the substrate, out of the plane of the porphine ring, disrupting the π-electron delocalization over the pyridinium rings. When the content of SA in the mixture increases, the interlocking disappears and the absorption spectrum of the coplanar PO is recovered. The ensemble of the data presented shows the importance of electrostatic forces in the ionic functionalized molecules and the increasing and prevalent contribution of hydrophobic interactions upon addition of neutral surfactants to the molecular organizates. Acknowledgment. This work was supported by project PRAXIS XXI 2/2.1/QUI/443/94. S.M.B. Costa expresses her gratitude to Professor I. Yamazaki for making the use of the stearylpyridyl porphine possible during her stay in Okazaki (Japan) and subsequently in Portugal. P. Antunes acknowledges Grant PRAXIS XXI /BIC/17209/98. LA990802B

porphine in Pure and Mix - American Chemical Society

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