Organization of Cationic Porphyrins in Mixed LangmuirâBlodgett Films

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Organization of Cationic Porphyrins in Mixed Langmuir-Blodgett Films. An Absorption and Steady-State Fluorescence Study Maria Isabel Viseu,* Ame´lia M. Gonç alves da Silva, Patrıćia Antunes, and Sı´lvia M. B. Costa Centro de Quı´mica Estrutural, Complexo I, Instituto Superior Te´ cnico, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal Received December 10, 2001. In Final Form: March 4, 2002 Two cationic porphyrins, the amphiphilic meso-tetra(4-N-stearylpyridyl)porphine (PO1) and the hydrophilic meso-tetra(4-N-methylpyridyl)porphine (TMPyP or PO2), were incorporated in ionic diluent matrixes containing sodium hexadecyl sulfate (SHS) and dioctadecyldimethylammonium bromide (DODAB) at the air-water interface and in Langmuir-Blodgett films. All systems containing PO1 show a preferential interaction of SHS with DODAB, instead of SHS with PO1. The cationic system PO1/SHS/DODAB at a 1:4:4 molar ratio behaves as the separate PO1 plus SHS + DODAB monolayers and forms H-type porphyrin aggregates at high π. The neutral system PO1/SHS/DODAB at a 1:8:4 molar ratio separates into PO1 + 4SHS plus SHS + DODAB monolayers, forming a PO1 + 4SHS bilayer at high π in which the porphyrin macrocycles can adopt “planar” and “nonplanar” conformations. The quenching efficiency of the PO1 fluorescence was found to be generally higher in the cationic, rather than neutral, monolayers and also to increase with the porphyrin surface concentration and number of deposited layers. Systems containing PO2 are generally not very stable at the air-water interface. The system PO2/4SHS forms H-type PO2 aggregates (or H-dimers) at low π and dissolves in water at high π. The system PO2/4SHS/4DODAB separates into the soluble PO2 plus the neutral SHS + DODAB monolayer. Only the neutral system PO2/8SHS/4DODAB is quite stable at the air-water interface because of favorable (cooperative) electrostatic and hydrophobic interactions.

Introduction Porphyrins and phthalocyanines have recently been the subject of considerable research because of their use in molecular metals,1 semiconductors,2 nonlinear optics,3 and catalysis.4 Porphyrins, in particular, are molecules that are structurally similar to chlorins that perform important biological functions, such as chlorophyll a. Therefore, the optical and electrical properties of porphyrins have also been extensively studied in connection with biological modeling.5,6 The proximity and relative orientation of natural porphyrins in biological systems are very important for performing such functions as light-induced charge separation or energy transduction. Therefore, monomolecular films formed at the air-water interface or transferred onto solid substrates by the Langmuir-Blodgett (LB) technique constitute important model systems for investigating the nature of the interactions between porphyrin molecules.7 Functionalized ionic porphyrins containing long hydrophobic chains at the periphery of the conjugated * Corresponding author: Maria Isabel Viseu. Corresponding address: Centro de Quı´mica Estrutural, Complexo I, Instituto Superior Tećnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal. Telephone: 00-351-21-8419389. Fax: 00-351-21-8464455. E-mail: [email protected]. (1) Hoffman, B. M.; Ibers, J. A. Acc. Chem. Res. 1983, 16, 15. (2) Nevin, W. A.; Chamberlain, G. A. J. Appl. Phys. 1991, 69, 4324. (3) Norwood, R. A.; Sounik, J. R. Appl. Phys. Lett. 1992, 60, 295. (4) Kobayashi, N.; Janda, P.; Lever, A. B. P. Inorg. Chem. 1992, 31, 5172. (5) Schouten, P. G.; Warman, J. M.; Haas, M. P.; Fox, M. A.; Pan, H. L. Nature 1991, 353, 736. (6) Liu, C. Y.; Pan, H. L.; Fox, M. A.; Bard, A. J. Science 1993, 261, 897. (7) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience: New York, 1966.

π-electron system, such as alkylpyridynium porphyrins with varying numbers and lengths of chains are ideal candidates for the formation of Langmuir monolayers at the air-water interface and for LB deposition.8,9 Indeed, these films are stabilized relative to those formed, for example, by neutral tetraphenylporphyrins because the ionic groups interact strongly with water and reduce porphyrin aggregation by electrostatic repulsions. An example is the case of 5-(4-N-octadecylpyridyl)-10,15,20tri-p-tolyl porphyrin, for which the orientation of the rings relative to the water surface, the rigidity and orientation of the chains, and the electronic energy transfer have been analyzed at the air-water interface and in LB films.10-13 The incorporation of diluent molecules, such as ionic and nonionic surfactants, into Langmuir monolayers and LB films of porphyrins is also of current research interest because of the possibility of controlling the molecular orientation, aggregation, and other structural features of porphyrins by changing the molar ratio of the components.14-17 (8) Hann, R. A. Molecular Structure and Monolayer Properties. In Langmuir-Blodgett Films; Roberts, G. G., Ed.; Plenum Press: New York, 1990. (9) Petty, M. C. Langmuir-Blodgett Films; Cambridge University Press: Cambridge, U.K., 1996. (10) Zhang, Z.; Verma, A. L.; Yoneyama, M.; Nakashima, K.; Iriyama, K.; Ozaki, Y. Langmuir 1997, 13, 4422. (11) Zhang, Z.; Verma, A. L.; Nakashima, K.; Yoneyama, M.; Iriyama, K.; Ozaki, Y. Langmuir 1997, 13, 5726. (12) Zhang, Z.; Nakashima, K.; Verma, A. L.; Yoneyama, M.; Iriyama, K.; Ozaki, Y. Langmuir 1998, 14, 1177. (13) Verma, A. L.; Zhang, Z.; Tamai, N.; Nakashima, K.; Yoneyama, M.; Iriyama, K.; Ozaki, Y. Langmuir 1998, 14, 4638. (14) Vandevyver, M.; Barraud, A.; Raudel-Texier; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571. (15) Gru¨niger, H.; Mo¨bius, D.; Meyer, H. J. Chem. Phys. 1983, 79, 3701. (16) Anikin, M.; Tkachenko, N. V.; Lemmetyinen, H. Langmuir 1997, 13, 3002.

10.1021/la0117909 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/21/2002

Absorption and Fluorescence of Porphyrins in LB Films

Figure 1. Surface pressure-area (π-A) isotherms on a pure water subphase at 298 K for the systems 4SHS/4DODAB (0), PO1 (1), PO1/4SHS (2), PO1/4SHS/4DODAB (3), and PO1/ 8SHS/4DODAB (4). On the abscissa scale, “area per PO1” refers only to systems 1-4 containing PO1; for system 0, the abscissa represents the mean area of 8 molecules (4 DODAB + 4 SHS molecules). Curves 3′ and 4′ are the calculated sums (in area) of curves 1 + 0 and 2 + 0, respectively. The inset represents the chemical structure of PO1 [meso-tetra(4-N-stearylpyridyl)porphine], and the open arrows near the π scale indicate the target surface pressures used in LB deposition.

In a previous paper,18 the highly symmetric meso-tetra(4-N-stearylpyridyl)porphine (PO1, a tetracationic, amphiphilic porphyrin represented in the inset of Figure 1 above) was investigated at the air-water interface, either pure or in mixed films containing the surfactant sodium hexadecyl sulfate (SHS), in the absence and presence of stearic acid (SA). The four stearyl chains enable the pure porphyrin to form by itself a Langmuir monolayer at the water surface that can be transferred onto a solid substrate. The selective electrostatic binding of the cationic porphyrin to the anionic surfactant SHS was studied therein, and by increasing the proportion of SA it was possible to assess the role of hydrophobic versus electrostatic interactions. Because these interactions are reflected in the optical properties of PO1, surface pressure-area (π-A) isotherm measurements at the air-water interface were complemented by UV-visible absorption spectroscopy of LB films transferred onto quartz substrates. A parallel investigation was performed in our laboratory with the hydrophilic porphyrin meso-tetra(4-N-methylpyridyl)porphine (TMPyP or PO2) in mixed films containing SHS and different proportions of SA.19 PO1 and PO2 have nearly identical macrocycles, but the long stearyl chains of PO1 are substituted by four methyl groups in PO2 (see the inset of Figure 7 below). The water-soluble PO2 cannot form a stable monolayer by itself, but can be included in monolayers containing oppositely charged surfactants, such as SHS19 or L-R-dimyristoylphosphatidic acid (DMPA).20-22 The purpose of the present paper is to complement the previous studies by exploring the changes in molecular (17) Efimov, A. V.; Anikin, M.; Tkachenko, N. V.; Mironov, A. F.; Lemmetyinen, H. Chem. Phys. Lett. 1998, 289, 572. (18) Gonç alves da Silva, A. M.; Viseu, M. I.; Malathi, A.; Antunes, P.; Costa, S. M. B. Langmuir 2000, 16, 1196. (19) Gonç alves da Silva, A. M.; Viseu, M. I.; Romaõ, R.; Costa, S. M. B., manuscript submitted. (20) Martıń, M. T.; Prieto, I.; Camacho, L.; Mo¨bius, D. Langmuir 1996, 12, 6554. (21) Prieto, I.; Camacho, L.; Martıń, M. T.; Mo¨bius, D. Langmuir 1998, 14, 1853. (22) Prieto, I.; Martıń-Romero, M. T.; Camacho, L.; Mo¨bius, D. Langmuir 1998, 14, 4175.

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conformation and aggregation of PO1 and PO2, in Langmuir monolayers and LB films, induced by the presence of a catanionic surfactant matrix containing SHS and dioctadecyldimethylammonium bromide (DODAB). The synergistic behavior of the mixture SHS/DODAB in different proportions has been studied previously at the air-water interface.23-25 In the present work, two ternary systems of PO1, SHS, and DODAB (or PO2, SHS, and DODAB) with molar ratios of 1:4:4 and 1:8:4 are investigated. The conclusions drawn from π-A isotherm measurements at the air-water interface are compared with UV-visible absorption and fluorescence spectra of LB films. In the present paper, a comparison between the behavior of systems containing the water-soluble PO2 or the amphiphilic PO1 provides further information on the roles played by intermolecular interactions (hydrophobic plus electrostatic) versus covalent binding of long alkyl chains. The results are discussed in terms of porphyrin aggregation, conformational changes of the macrocycle, phase separation within the mixtures, and (for systems containing PO1) intra- or interlayer porphyrin self-quenching. Experimental Section Materials. meso-Tetra(4-N-stearylpyridyl)porphine tetra-ptoluenesulfonate (PO1), from Dojin (Japan), was a kind gift from Professor I. Yamazaki. meso-Tetra-(4-N-methylpyridyl)porphine tetra-p-toluenesulfonate (PO2) was also obtained from Dojin, and meso-porphyrin IX dimethyl ester was purchased from Sigma. Sodium hexadecyl sulfate (SHS), 99% pure, from Merck, was kindly provided by Professor M. Vela´zquez. Dioctadecyldimethylammonium bromide (DODAB), with purity higher than 98%, was purchased from Fluka. The solvents chloroform and ethanol were of spectroscopic grade (Uvasol) and were obtained from Merck. Porphyrins, surfactants, and solvents were used as obtained, without further purification. The water used in the subphase was distilled twice and purified with the Millipore Milli-Q system. Sample Preparation. Separate stock solutions of DODAB and SHS were prepared in chloroform and ethanol, respectively. Each spreading solution was obtained by adding precisely measured volumes of SHS and/or DODAB stock solutions to the appropriate amount of the solid porphyrin and diluting to the final volume with chloroform and ethanol; the final spreading solvent was always a 4:1 (v/v) mixture of chloroform and ethanol. Surface Pressure-Area (π-A) Measurements. π-A isotherm measurements were carried out at 298.2 ( 0.1 K on a computer-controlled KSV 5000 Langmuir-Blodgett system (KSV Instruments, Helsinki, Finland) installed in a laminar flow hood. The procedure is detailed elsewhere.18 Langmuir-Blodgett (LB) Deposition. The films were deposited onto hydrophilic substrates (quartz slides) as described before18 at surface pressures below and above the main transitions of the π-A isotherms. In the expanded regime, only a monolayer could be deposited onto each face of the substrate because of the weak hydrophobic interactions among the loosely packed chains. In the condensed regime, several layers could generally be transferred by using an odd number of withdrawing/dipping strokes. The transfer ratios obtained were not uniform and in most cases were greater than unity, especially in the case of systems with PO2. In the multilayer deposition, the transfer ratio obtained in the first (withdrawing) stroke was generally higher than that obtained in the following strokes. This was likely due to some instability of the monolayers at the air-water interface, leading to relaxation effects that were not corrected for. Therefore, the reproducibility of the absorption spectra of the LB films was carefully checked. It was found that the spectral (23) Gonç alves da Silva, A. M.; Viseu, M. I.; Campos, C. S.; Rechena, T. Thin Solid Films 1998, 320, 236. (24) Gonç alves da Silva, A. M.; Viseu, M. I. Colloids Surf. A: Physicochem. Eng. Aspects 1998, 144, 191. (25) Viseu, M. I.; Gonç alves da Silva, A. M.; Costa, S. M. B. Langmuir 2001, 17, 1529.

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characteristics (absorbance, wavelength of maximum absorbance, etc.) of a given system at a given surface pressure of deposition did not depend on the transfer ratio but only on the number of transfer strokes n. A Y-type transfer of condensed films was then assumed. UV-Visible Absorption Spectra. The absorption spectra of LB films were obtained on a Jasco V-560 UV-visible absorption spectrometer in the transmission mode. Background light scattering was corrected by subtracting from the sample spectra the “apparent absorption spectrum” of a clean quartz slide (recorded under the same operating conditions as the sample films) or the function s ) a + b/λ4 (where the constants a and b were evaluated for each sample by an iterative procedure). Fluorescence Spectra. The steady-state fluorescence spectra of LB films were obtained with a Perkin-Elmer LS-50B luminescence spectrometer. A homemade slide holder, specially designed to orient the substrates at a 30°/60° (excitation/emission) geometry, was used. To obtain relative quenching efficiencies, the operating conditions (such as photomultiplier voltage, excitation and emission slits, etc.) were kept constant in all emission runs. Background light scattering was corrected by subtracting a fraction of the “apparent emission spectrum” of a blank slide, to take into account that the most absorbing samples present less scattering. A low signal/noise ratio was found for most spectra of LB films, and uncertainties as large as ∼1025% of the measured intensities (mainly because of high background scattering and difficulties in reproducing the substrate position in the slide holder) could be estimated from repetitive sample measurements. Consequently, only a semiquantitative evaluation of the relative emission intensity (or quenching efficiency) was made.

Results and Discussion A. Systems Containing the Amphiphilic Porphyrin PO1. A.1. Surface Pressure-Area (π-A) Isotherms. Figure 1 presents the π-A isotherms of two ternary mixtures containing PO1, SHS, and DODAB (with molar ratios of 1:4:4 and 1:8:4) obtained at the air-water interface and 298 K. The π-A isotherms of pure PO1, PO1/4SHS, and SHS/DODAB, analyzed in previous papers,18,23-25 are also reproduced herein and described briefly for comparison. At equimolar proportions, the catanionic surfactant system SHS/DODAB (system 0) is neutral and presents a very steep isotherm (Figure 1, curve 0).23-25 Because of a strong condensing effect (synergistic behavior), the mixture forms a solid-type monolayer with densely packed chains; at the minimum compressibility of the monolayer (for π ≈ 44 mN m-1), the area per chain is ∼20 Å2.25 PO1 (System 1). The amphiphilic PO1 (system 1) is insoluble in water and forms a net cationic Langmuir monolayer at the air-water interface. The long transition at π ≈ 30 mN m-1 between the expanded and condensed states (Figure 1, curve 1) was assigned to a change in orientation of the porphyrin macrocycles from parallel (at low π) to oblique or tilted (at high π) relative to the water surface.18 PO1/4SHS (System 2). At a 1:4 molar ratio, the catanionic system PO1/SHS (system 2) is neutral. The two transitions of the π-A isotherm, found at π ≈ 10 and 24 mN m-1 (Figure 1, curve 2), separate the three monolayer states expanded, intermediate, and condensed. When π is increased, these transitions were assigned to a closer arrangement of charges and chains, resulting in the formation of an interdigitated bilayer with a dense chain packing at high π.18 PO1/4SHS/4DODAB (System 3). In this ternary system, the eight positive charges of PO1 and DODAB exceed the four negative charges of SHS, and therefore, a net cationic monolayer is formed at the air-water interface. Interestingly, the π-A isotherm of this system (Figure 1, curve

Viseu et al.

Figure 2. UV-visible absorption (curves b, c) and emission (curves b′, c′) spectra of LB films of pure PO1 as compared to the corresponding spectra of the porphyrin in ethanol solution (curves a, a′, on arbitrary intensity scales). The films were obtained at π ) 15 (b, b′) or 40 (c, c′) mN m-1, with one layer on each face of the substrate (n ) 1).

3) is very similar to that of system 1 but displaced to higher areas. Furthermore, the transition from the expanded to the condensed state in system 3 has a similar length (∆A3 ≈ 130 Å2, ∆A1 ≈ 150 Å2) and occurs at a similar surface pressure (π3 ≈ 33-35 mN m-1, π1 ≈ 30-31 mN m-1) as in system 1. Because the molar composition of system 3 corresponds to the sum of the compositions of systems 1 (PO1) and 0 (4SHS/4DODAB), the experimental isotherm 3 is compared in Figure 1 with the calculated sum (in area) of curves 1 and 0, which is curve 3′. The proximity of curves 3 and 3′ is compatible with either a phase separation (into systems 1 and 0) or a nearly ideal mixture. The UV-visible absorption and emission spectra presented in section A.2 favor the former hypothesis (phase separation). PO1/8SHS/4DODAB (System 4). In this system, the eight SHS anions neutralize the PO1 and DODAB cations, forming a neutral (catanionic) monolayer. The molar composition of system 4 now corresponds to the sum of the compositions of systems 2 (PO1/4SHS) and 0 (4SHS/ 4DODAB). The π-A isotherm of system 4 (Figure 1, curve 4) also presents two transitions, as in system 2, but it is displaced to higher areas. Again, curve 4 can be compared with curve 4′, the calculated sum (in area) of curves 2 and 0. Curves 4 and 4′ are nearly coincident except at low π, showing that the specific behavior of system 2 remains in the ternary system 4. The hypotheses of ideal mixing versus phase separation (into systems 2 and 0) are considered in section A.2. A.2. UV-Visible Absorption and Fluorescence Spectra of LB Films. UV-visible absorption spectra of the monomeric PO1 in organic solution show an intense Soret band in the wavelength region of ∼350-500 nm and four weaker Q-bands in the ∼500-700 nm region. Fluorescence emission spectra present only two bands in the region of ∼600-800 nm, Q00 and Q01. In ethanol, the wavelength of maximum absorbance (λmax) in the Soret band is 426 nm (Figure 2, curve a, and Table 1), and those of maximum emission intensity (λ00 and λ01) are ∼656 and ∼720 nm, respectively (Figure 2, curve a′). LB deposition was performed at relevant surface pressures below and above the main transitions in the π-A isotherms (see the open arrows near the π scale in Figure 1). The absorption spectral parameters of the Soret band (λmax and full width at half-maximum in wavenumbers, ω j 1/2) of the LB films are given in Table 1 for the different systems and surface pressures.


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Table 1. Spectral Parameters of PO1 in Ethanol Solution and in LB Films system

π (mN m-1)

λmax (Soret) (nm)

ω j 1/2 (Soret) (cm-1)

spectra in figure

-

426

1550

2

PO1

15 40

1 1-5 7

441-443 431-434 431

1800-1900 2600-2800 3900

2, 6a 2, 3, 6b 3

PO1/4SHS

5 15 40

1 1 1

443 441-443 404, ∼436

1800 1800-1900 (2 bands)

4, 5 4, 6a 4, 5, 6b

PO1/4SHS/4DODAB

15 40

1 1

440 433

1850 2900

6a 6b

PO1/8SHS/4DODAB

15 40

1 1

443 405, ∼439

1800 (2 bands)

6a 6b

PO1 in ethanol

-

n

System 1. Expanded Regime. The absorption spectrum of an expanded LB monolayer of PO1, transferred at π ) 15 mN m-1, is presented in Figure 2 (curve b). The red shift (∼16 nm) of the Soret band relative to the spectrum of the monomeric porphyrin in ethanol (see Table 1) could suggest an edge-to-edge (J-type) aggregation26-31 or a conformational change of the porphyrin macrocycle in the film. In previous work,18 we assumed that PO1 is mainly in the monomeric form; the red shift was attributed to a greater π-electron delocalization in the film than in solution because of the more planar conformation of the macrocycle in the film in contact with the quartz substrate. Indeed, in the expanded films, the porphyrin rings might be constrained to planarity if they interact strongly with the hydrophilic substrate, whereas some rotation (or distortion) of the lateral pyridynium groups out of the central porphine plane might occur in solution. This effect might explain the red shift observed systematically in the spectra of the monomeric PO1 in LB films relative to solution spectra. A similar interpretation (flattening of the macrocycle) was given by Chernia and Gill for the red shift (∼30 nm relative to aqueous solution) observed for TMPyP (PO2) adsorbed on the synthetic clay mineral Laponite.32 However, the possibility of an edge-to-edge aggregation cannot be discarded (see section C below). The fluorescence emission spectrum of the same expanded film of PO1 (Figure 2, curve b′) shows two bands, with λ00 ≈ 680 nm and λ01 ≈ 730 nm. These two maxima are closer, and the spectrum is more poorly resolved than in solution. Condensed Regime. The Soret band of a condensed LB monolayer of PO1 deposited at π ) 40 mN m-1 (Figure 2, curve c) is blue-shifted relative to that of the expanded region (see Table 1). This spectral feature and the much smaller area per PO1 molecule suggest the presence of face-to-face (H-type) aggregates of PO1.18,27-31 These aggregates correspond to a stacked arrangement of the macrocycles, with a tilted alignment of the transition dipole moments relative to the line of molecular centers (26) In J-type aggregation, red shifts result from the interactions between transition dipoles that are nearly parallel to the line of centers of the chromophores, i.e., when the monomers form a linear “head-totail” (or “edge-to-edge”) array.27-31 (27) Czikkely, V.; Fo¨rsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 11. (28) Czikkely, V.; Fo¨rsterling, H. D.; Kuhn, H. Chem. Phys. Lett. 1970, 6, 207. (29) Whitten, D. G. The Spectrum 1993, 6 (3), 1. (30) Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.; Sugi, M.; King, L. G.; Crossley, M. J. J. Phys. Chem. 1993, 97, 12862. (31) Cantor, C. R.; Schimmel, P. R. Biophysical Chemistry. Part II. Techniques for the Study of Biological Structure and Function; W. H. Freeman and Co.: New York, 1998. (32) Chernia, Z.; Gill, D. Langmuir 1999, 15, 1625.

(see section C below).33 As found in aggregates of other porphyrins,34 an enlargement of the H-band is also observed herein relative to the monomer band obtained in the expanded region (see also Table 1).35 Figure 2 illustrates the effect of the surface pressure of deposition (π) on the relative absorption and emission intensities. An increase in π from 15 to 40 mN m-1 leads to an increase in the porphyrin surface concentration (or packing density) and, therefore, to an increase in absorbance. Correcting for the absorbance at the excitation wavelength (440 nm), a strong decrease in emission intensity (or higher fluorescence quenching efficiency) is found at the higher π. This result supports an intralayer quenching mechanism among the porphyrin molecules, because of a smaller intermolecular distance (and presence of tilted H-aggregates)37,38 at π ) 40 mN m-1. Figure 3 shows the effect of the number of deposited layers in the absorption and emission spectral intensities (recall that in the expanded regime only one layer could be transferred). The absorbance is proportional to the number of transfer strokes, n, up to n ) 5 (Figure 3, inset), suggesting that one layer is deposited in each stroke.39 However, the corrected emission intensity decreases with n s or with the number of transferred layers s according to an interlayer quenching mechanism of the porphyrin fluorescence. Zhang et al. also observed intra- and interlayer fluorescence quenching of 5-(4-N-octadecylpyridyl)-10,15,20tri-p-tolyl porphyrin (an amphiphilic porphyrin similar to PO1 but containing only one positive charge and one stearyl chain), deposited on glass or gold-evaporated glass (33) Because the transition moments lie in the plane of the porphyrin macrocycle, the formation of perfect (fully superposed) H-aggregates would imply that the macrocycles stand perpendicularly to the water surface. However, to increase the contact between the hydrophilic pyridinium groups and water, it seems more likely that, along the transition, the porphyrin rings become tilted relative to the interface (at an angle below 90°) and therefore only partially superimposed. (34) Maiti, N. C.; Mazumdar, S.; Periasamy, N. J. Phys. Chem. B 1998, 102, 1528. (35) As happens frequently with J-aggregates, an H-aggregate might, in principle, also present a narrow spectrum, and this has indeed been observed.36 However, polydisperse H-aggregates (in terms of the number of assembled monomers) might have larger bands than the monomer band. This is a common situation, observed, e.g., with cyanines and porphyrins. (36) Kunisawa, T.; Sato, T.; Yonezawa, Y.; Popova, G. V. Thin Solid Films 1997, 311, 267. (37) Perfect (completely superposed) H-aggregates would not emit fluorescence because the transition from the lower S1 state is forbidden.38 Our results, showing a weak fluorescence, seem to support, instead, the formation of tilted H-aggregates, where the angle between the individual transition dipole moments and the line of molecular centers would be smaller than 90° but larger than arccos(1/x3), or 57.4°. (38) McRae, E. G.; Kasha, M. J. Chem. Phys. 1958, 28, 721. (39) A slower increase in absorption is seen thereafter (for n ) 7), but this could be due to a reorganization of the aggregates in the film because a larger width of the band is also observed; see Table 1.

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Figure 3. Effect of the number of deposition strokes (n) on the spectra of pure PO1 for n ) 1, 3, 5, and 7 (curves a, b, c, d, respectively, for absorption and a′, b′, c′, d′, respectively, for emission). The films were obtained at π ) 40 mN m-1. Inset: Absorbance at the Soret maximum as a function of n.

Figure 5. UV-visible absorption spectra of a condensed LB monolayer of PO1/4SHS (obtained at π ) 40 mN m-1, curve a), an expanded LB monolayer of PO1/4SHS (obtained at π ) 5 mN m-1, curve b), and a chloroform solution of meso-porphyrin IX dimethylester (curve c). The absorbance scale on the right refers to the spectra of the LB films and that on the left to the solution spectrum. The structural schemes above represent the “planar” (at right) and “nonplanar” (at left) conformations of the porphyrin macrocycle. Figure 4. UV-visible absorption (curves a, b, c, d) and emission (curves a′, b′, c′, d′) spectra of LB films of PO1/4SHS as a function of the surface pressure of deposition (π) and number of strokes (n). The films were obtained at π ) 5 (a, a′), 15 (b, b′), and 40 (c, c′) mN m-1 with n ) 1 and at π ) 40 mN m-1 with n ) 5 (d, d′).

substrates.11,13 These authors interpreted this fluorescence quenching as being due to electronic energy transfer. On the other hand, because of the semiquantitative nature of our results, it is not possible to propose a mechanistic interpretation for the present fluorescence quenching data. System 2. Expanded and Intermediate Regimes. In the expanded and intermediate regimes of the PO1/4SHS monolayer, the presence of SHS does not significantly alter the characteristic features of the absorption spectrum of pure PO1. Indeed, curves a and b of Figure 4 are similar to curve b of Figure 2 (also see Table 1). Therefore, these spectra were also assigned to the monomeric PO1.18 Condensed Regime. Absorption spectra of PO1/4SHS LB films transferred at π ) 40 mN m-1 (Figure 4, curves c and d) present a new absorption maximum at ∼405 nm (λ2), in addition to the band at ∼440 nm (λ1). The similarity in λmax between the porphine spectrum (Figure 5, curve c) and the new band (λ2 in curve a of Figure 5) suggests that this latter spectrum might be due to a conformation of the PO1 macrocycle where the π-electron delocalization is restricted to the central porphine ring. This effect might be caused, for example, by a rotation of the pyridinium groups out of the porphine plane, as illustrated by the macrocycle conformation on the left of Figure 5. The two bands at λ1 and λ2 were therefore assigned, respectively, to coplanar and nonplanar conformations of the porphyrin macrocycle, which seem to coexist in the film bilayer.18

Results obtained in our laboratory with some LB films of two other porphyrins, TMPyP (PO2)19 and meso-tetra(4sulfonatophenyl)porphine (PO3),40 also show an absorption maximum λ2 at ∼405 nm, in addition to the maximum λ1 corresponding to the planar macrocycle. Even though λ1 varies considerably with the porphyrin structure (∼440, ∼430, and ∼420 nm, for PO1, PO2, and PO3, respectively), λ2 is nearly constant at ∼405 nm, suggesting that this band results from a common chromophore in the three porphyrinsslikely the central porphine ring. A different interpretation was, however, proposed for the appearance of a band at ∼398 nm (in aqueous solution) for a tetra-(pyridyl)porphine in which the pyridinium groups are separated from the central porphine ring by methylene chain spacers, preventing the delocalization of the positive charges onto the ring by direct coupling.41 This band is blue-shifted relative to the monomer band at 412 nm, and so it was assigned to a porphyrin faceto-face (or H-type) dimer.41 A special type of face-to-face aggregation (instead of nonplanarity of the macrocycle) might also be occurring in the present systems. Indeed, the band with the absorption maximum at λ2 appears in systems PO1/4SHS, PO2/4SHS/4SA, and PO3/4DODAB, which are all neutral monolayers containing eight alkyl chains per PO molecule. These systems form a porphyrin bilayer in the condensed regime. Consequently, it is possible that, within the bilayer, the porphyrins are aggregated in such a way that it is the central porphine ring that is responsible for the absorption band at ∼405 (40) Gonç alves da Silva, A. M.; Romaõ, R.; Viseu, M. I.; Costa, S. M. B. Instituto Superior Tećnico, Lisboa, Portugal. Unpublished results. (41) Kano, K.; Fukuda, K.; Wakami, H.; Nishiyabu, R.; Pasternack, R. F. J. Am. Chem. Soc. 2000, 122, 7494.


Figure 6. UV-visible absorption and emission spectra of LB films of systems 1-4: (1) pure PO1, (2) PO1/4SHS, (3) PO1/ 4SHS/4DODAB, and (4) PO1/8SHS/4DODAB. The films were obtained either in the expanded regime at π ) 15 mN m-1 (Figure 6a) or in the condensed regime at π ) 40 mN m-1 (Figure 6b) with n ) 1.

nm. This can happen in a stacked bilayer, instead of a bilayer where only the chains are in contact (see Figure 10A below, for system PO1/8SHS/4DODAB). However, we still favor the former hypothesis (nonplanar conformation of the macrocycle) because H-aggregation, as happens in system 1 at high π, seems to cause a much smaller blue shift (from ∼440 to ∼430 nm) than the one observed herein (from λ1 ≈ 440 to λ2 ≈ 405 nm). The effects of the surface pressure of deposition π and number of strokes n on the fluorescence quenching efficiency, in system 2, are qualitatively the same as those observed for system 1, as shown in Figure 4. Indeed, an intralayer quenching of the PO1 fluorescence was found (compare the emission curves a′ and c′ corresponding, respectively, to LB films deposited at π ) 5 and 40 mN m-1, using the same excitation wavelength, 440 nm), as well as an interlayer quenching (now compare the emission curves c′ and d′ corresponding to condensed LB films with n ) 1 and 5, respectively). Systems 3 and 4. Expanded Regime. Figure 6a presents the absorption and emission spectra of LB films of systems 3 and 4, deposited in the expanded regime (at π ) 15 mN m-1) with n ) 1. Systems 1 and 2 are included herein for a global comparison. All absorption spectra are similar in shape and halfwidth (see also Table 1) and show the same wavelength of maximum absorption, ∼440-443 nm. This behavior is characteristic of the monomeric PO1 with a coplanar macrocycle. The main effect of adding 4SHS/4DODAB to the expanded system 1 or 2 is a decrease in absorbance, because of porphyrin dilution in the film. Indeed, the absorbance ratios at 440 nm of systems 1/3 and 2/4 (∼1.8

Langmuir, Vol. 18, No. 15, 2002 5777

and ∼1.9, respectively) agree with the corresponding ratios of PO1 surface concentration at π ) 15 mN m-1 (∼1.6 and ∼1.9, respectively). When the porphyrin charge is neutralized by SHS, a reduction in fluorescence quenching is observed (Figure 6a). Indeed, in the pair of systems 1 and 2, the emission intensity (corrected for the absorption at the excitation wavelength, 440 nm) is higher in the neutral system 2 than in the cationic system 1. A lower quenching efficiency is also found in system 4 (neutral) as compared to system 3 (cationic). Condensed Regime. Figure 6b compares the absorption and emission spectra of condensed LB films of systems 3 and 4, deposited at π ) 40 mN m-1 with n )1, with those of systems 1 and 2, presented before. The absorption spectrum of the cationic system 3 is blue-shifted (λmax ≈ 433 nm) relative to that of the expanded regime, presenting a close resemblance to the spectrum of system 1 at high π (see Table 1). This feature suggests the presence of porphyrin H-aggregates in system 3, as observed in system 1, and supports the likelihood of the phase separation of system 3 into its component subsystems 1 and 0 (instead of ideal mixing, where aggregation would disappear). This behavior shows the relevance of hydrophobic interactions, which are stronger for the pair DODAB/SHS (where the chains can pack very closely)25 than for the pair PO1/4SHS (where the chains pack rather loosely, except when the bilayer is formed).18 Two Soret absorption maxima appear in system 4: the strongest one at λ2 ≈ 405 nm (the “porphine spectrum”, corresponding to the nonplanar macrocycle conformation) and the second, as a shoulder, at λ1 ≈ 440 nm (the coplanar conformation). Again, the similarity in shape of the spectra of systems 2 and 4 is remarkable and supports a phase separation of system 4 into its subsystems 2 and 0. The similarity in behavior presented by systems 1 and 3, on one hand, and by systems 2 and 4, on the other hand, means that SHS interacts preferentially with DODAB, up to charge compensation, and that only the excess of SHS molecules interacts with PO1. The emission spectra of monolayers 1-4 in the condensed regime (Figure 6b) show a decrease in the fluorescence quenching efficiency when the porphyrin charge is neutralized by SHS, as was observed in the expanded regime. Indeed, in the pair of systems 1 and 2, the emission intensity (corrected for excitation at 440 nm) is higher in the neutral system 2 than in the cationic system 1, indicating a lower quenching efficiency in the former system. The same effect (but not so strong) is found for the pair of systems 3 and 4. B. Systems Containing the Hydrophilic Porphyrin PO2. B.1. Surface Pressure-Area Isotherms. Figure 7 presents the π-A isotherms obtained at the air-water interface and 298 K for monolayers containing the watersoluble porphyrin PO2 instead of PO1: pure PO2 (curve 1′), PO2/4SHS (curve 2′), and PO2/SHS/DODAB at molar ratios of 1:4:4 (curve 3′) and 1:8:4 (curve 4′). The isotherm for the equimolar surfactant system SHS/DODAB is also included for comparison (curve 0). PO2 (System 1′). Because of the absence of long alkyl chains, PO2 is hydrophilic and does not form a Langmuir monolayer by itself at the air-water interface. Indeed, as the film is compressed, it becomes progressively dissolved into the subphase (curve 1′ is an example of an “isotherm” obtained for this system, but it is not reproducible). However, PO2 can be included in monolayers containing oppositely charged surfactants, such as SHS, or in catanionic monolayers, such as SHS + DODAB, as described next.

5778


Figure 7. Surface pressure-area (π-A) isotherms on a pure water subphase at 298 K for the systems 4SHS/4DODAB (0), PO2 (1′), PO2/4SHS (2′), PO2/4SHS/4DODAB (3′), and PO2/ 8SHS/4DODAB (4′). On the abscissa scale, “area per PO2” refers only to systems 1′-4′, containing PO2; for system 0, the abscissa represents the mean area of 8 molecules (4 DODAB + 4 SHS molecules). Curve 4′′ is the calculated sum (in area) of curves 2′ and 0. The inset represents the chemical structure of PO2 [meso-tetra(4-N-methylpyridyl)porphine], and the open arrows near the π scale indicate the target surface pressures used in LB deposition.

PO2/4SHS (System 2′). The neutral (catanionic) mixture PO2/4SHS is retained at the air-water interface by electrostatic interactions between the porphyrin cations and SHS anions. This system forms an expanded monolayer (curve 2′) showing a transition at π ≈ 33 mN m-1 (i.e., at surface pressures similar to those at which pure PO1 presents its own transition because of tilting of the porphyrin rings). However, the PO2/4SHS monolayer collapses along the transition, probably because of dissolution of the separate components into the subphase. The isotherm of this system is analyzed in another paper.19 PO2/4SHS/4DODAB (System 3′). This system consists of a net cationic monolayer containing 12 chains per porphyrin molecule with a molar composition corresponding to the sum of the compositions of systems 1′ (pure PO2) and 0 (4SHS/4DODAB). The similarity between the isotherm of this system (curve 3′) and that of SHS/DODAB (curve 0) suggests a phase separation of system 3′ into its component subsystems 1′ (the soluble PO2) and 0 (which is retained at the interface). We concluded that PO2, if adsorbed below or dissolved into the subphase, does not affect the equimolar SHS/DODAB monolayer significantly. As found for the ternary systems containing PO1, this result emphasizes the preferential interaction of SHS with DODAB, rather than with PO2. PO2/8SHS/4DODAB (System 4′). In this system, the eight SHS anions neutralize the PO2 and DODAB cations, forming a catanionic monolayer with a molar composition corresponding to the sum of the compositions of systems 2′ (PO2/4SHS) and 0 (4SHS/4DODAB). The isotherm of system 4′ (curve 4′) is shifted to lower areas and shows a higher collapse surface pressure than curve 4′′ (which is the sum, in area, of curves 2′ and 0). This isotherm also presents a very small transition as compared to that of system 2′. All of these factors indicate the miscibility of systems PO2/4SHS and SHS/DODAB at the interface. The miscibility and stabilization of system 4′ might be due to its neutrality and to the existence of 16 alkyl surfactant chains per porphyrin molecule, which, in a condensed state, occupy approximately the same interfacial area (∼300-320 Å) as a porphyrin ring lying parallel

Viseu et al.

Figure 8. UV-visible absorption (curve b) and emission (curve b′) spectra of an LB film of PO2/4SHS, obtained at π ) 30 mN m-1 with n ) 1, as compared to the corresponding spectra of PO2 in ethanol solution (curves a and a′, on arbitrary intensity scales).

to the water surface. The absence of chains covalently linked to the PO2 macrocycle (unlike the case of the PO1 stearyl chains) also favors molecular mobility and monolayer miscibility (see section C below). B.2. UV-Visible Absorption and Fluorescence Spectra of LB Films. PO2 differs from PO1 only in the methyl, instead of stearyl, groups. Therefore, the two porphyrins show quite similar UV-visible absorption and fluorescence spectra in solution (compare curves a and a′ presented in Figures 2 and 8 for PO1 and PO2, respectively, in ethanol). LB deposition was performed at the surface pressures indicated by the open arrows near the π scale in Figure 7. The absorption spectral parameters of the Soret band (λmax and ω j 1/2) of LB films containing PO2 for the different systems and surface pressures are given in Table 2 and compared with those of PO2 in an ethanol solution. System 2′. The system PO2/4SHS could only be deposited in the expanded regime, with a monolayer on each substrate surface. The absorption spectra of these films are blue-shifted when the surface pressure of deposition is increased from 15 to 30 mN m-1 (Table 2). At 30 mN m-1, a Soret λmax at ∼433 nm is obtained (Figure 8, curve b), which is considerably less red-shifted relative to the spectrum in ethanol than is the spectrum of PO1 at ∼443 nm. Therefore, it is possible that this system, even in the expanded state, contains a small percentage of porphyrin H-aggregates or H-dimers that increases slightly with π (see Figure 10B below). It was also found that the wavenumber half-width of the band (∼2000-2100 cm-1) is larger than that in ethanol (Table 2) and is also slightly larger than that obtained in LB films where the monomeric form of PO2 prevails (∼1750 cm-1).19 The two emission bands Q00 and Q01 overlap to a greater extent than in the spectra obtained in ethanol because of the considerable red shift of the Q00 band (compare curves b′ and a′ in Figure 8). System 3′. The absorption spectra of LB films of PO2/ 4SHS/4DODAB transferred at different surface pressures (not presented herein) showed only a residual porphyrin absorbance. According to π-A isotherm data, this result means that the preferential SHS/DODAB interaction, forming a neutral monolayer, excludes the cationic PO2, which dissolves into the subphase. Therefore, on LB deposition, almost no porphyrin can be retained by the quartz substrate.


Langmuir, Vol. 18, No. 15, 2002 5779

Table 2. Spectral Parameters of PO2 in Ethanol Solution and in LB Films

a

ω j 1/2 (Soret) (cm-1)

π (mN m-1)

n

λmax (Soret) (nm)

PO2 in ethanol

-

-

424

1400

8

PO2

a

a

-

-

-

PO2/4SHS

15 15 30 30

1 5c 1 5c

434 433 432-433 432

1750 1850 2000-2050 2100-2150

8 -

PO2/4SHS/4DODAB

15, 30, 50

1

(429-431)b

-

-

PO2/8SHS/4DODAB

10 30 50 50

1 1 1 5

435 430-431 428-429 427-429

1800 1700-1850 1950-2350 1600-2450

9 9 9

system

spectra in figure

No monolayer. b Residual absorbance. c Corresponds to five strokes, but only one layer is transferred to the substrate.

Figure 9. UV-visible absorption (curves a, b, c) and emission (curves a′, b′, c′) spectra of LB films of PO2/8SHS/4DODAB as a function of the surface pressure of deposition (π) and number of strokes (n). The films were obtained at π ) 30 (a, a′) and 50 (b, b′) mN m-1 with n ) 1 and at π ) 50 mN m-1 with n ) 5 (c, c′).

System 4′. The π-A isotherm of the PO2/8SHS/ 4DODAB monolayer shows only a mild transition, with no marked differences in slope from low to high surface pressures (recall Figure 7, curve 4′). This means that the packing and/or aggregation within the monolayer change slowly on compression. Accordingly, absorption spectra of LB films of this system show only a slight enlargement and blue shift of the band when the surface pressure of deposition is increased from 10 to 50 mN m-1 (see Table 2). This result probably means that a small percentage of porphyrin H-aggregates (or H-dimers) is formed at the higher surface pressure (see Figure 10B below). An interlayer quenching of the PO2 fluorescence was observed in this system. Indeed, in Figure 9, the absorbance ratio for LB films with n ) 5 and 1 (curves c and b) is higher than the corresponding ratio of emission intensity (curves c′ and b′). On the other hand, an intralayer mechanism of fluorescence quenching could not be verified with the present data. C. Organization of PO1 and PO2 in the Films. In Figure 10, the proposed packing and orientation of PO1 and PO2 in the systems studied so far is illustrated using schematic (lateral) representations of the component molecules at the air-water interface or on the quartz surface. For the systems containing PO1, because systems 3 and 4 separate into systems 1 and 2, respectively (plus the equimolar SHS/DODAB monolayer, system 0), only the monolayers 3 and 4 are pictured herein (Figure 10A). For systems containing PO2, the monolayers 2′, 3′, and 4′ are illustrated (Figure 10B).

PO1/4SHS/4DODAB (and pure PO1), at low π, presents the Soret maximum λ1 at ∼440 nm characteristic of the monomeric and planar porphyrin. A special type of J-aggregates of PO1 (edge-to-edge aggregates), likely induced by the hydrophobic interaction between stearyl chains (which could enhance an exciton interaction between the aligned transition dipole moments of different monomers), might also be present. These aggregates could explain the more red-shifted absorption compared to that typical of monomeric PO2 at ∼430 nm. At high π, the much smaller area per PO1 molecule and the blue-shifted absorption maximum (∼430 nm) suggest the presence of porphyrin H-type aggregates, in which the macrocycles are stacked, probably in a tilted orientation relative to the quartz surface. PO1/8SHS/4DODAB (and PO1/4SHS), at low π, shows the same spectral features as the previous system at low π. The Soret maximum λ1 at ∼440 nm is characteristic of the monomeric (planar) porphyrin and/or edge-to-edge aggregates. At high π, the smaller area per PO1 molecule (about one-half of the area before the transition, in system 2) indicates the formation of a porphyrin bilayer. To achieve a dense chain packing, the chains should be interdigitated. Two types of bilayer might then be formed, a bilayer in which only the chains are in contact or a bilayer with stacked macrocycles. In the former case, the new absorption maximum λ2 at ∼405 nm could be due to a nonplanar conformation of the porphyrin macrocycle (recall Figure 5), while some of the porphyrin molecules still show the typical spectrum of the planar conformation with λ1 at ∼440 nm. In the case of a stacked bilayer, the strong interaction between the macrocycles might explain the band at ∼405 nm. The system PO2/4SHS, at low π, presents a Soret maximum λ1 at ∼430 nm. This band, which is less redshifted than the corresponding one for PO1/4SHS at ∼440 nm, might be due to the monomeric and planar porphyrin (without any edge-to-edge aggregation or transition dipole interaction, as possibly happens in PO1). Alternatively, porphyrin H-dimers might appear as the surface pressure increases. This system could not be transferred to quartz substrates at high π (above the transition). PO2/4SHS/4DODAB shows almost the same π-A isotherm as the neutral surfactant system SHS/DODAB, because it separates into this system and pure PO2, which dissolves into the subphase. Finally, PO2/8SHS/4DODAB forms a dense monolayer even at low π and is quite stable at the air-water interface. It does not separate into its component subsystems, PO2/ 4SHS and SHS/DODAB. The porphyrin appears to be

5780


Viseu et al.

Figure 10. Proposed schemes for the structural arrangements of systems (A) 3 and 4 with PO1 and (B) 2′, 3′, and 4′ with PO2. Lateral sketch of the molecules in Langmuir monolayers at the air-water interface or in LB monolayers at the quartz surface.

essentially monomeric, but at high π (∼50 mN m-1), some H-dimers possibly appear. From the ternary systems studied herein, PO2/8SHS/ 4DODAB is the only one that presents miscibility at all surface pressures. Indeed, the other systems undergo phase separation, and the system PO2/4SHS/4DODAB even excludes pure PO2 into the subphase. The instability of the latter system, which is cationic and contains only 12 surfactant alkyl chains per PO2 molecule, is likely due to both repulsive electrostatic interactions and weak hydrophobic interactions. In the cationic monolayer PO1/ 4SHS/4DODAB, which has 16 alkyl chains per PO1 molecule and thus a strong hydrophobic interaction, phase separation possibly results from repulsive electrostatic interactions. Finally, the neutral system PO1/8SHS/ 4DODAB, which contains 20 chains per PO1 molecule, should present strong electrostatic and hydrophobic interactions. However, the existence of covalent bonds between the stearyl chains and pyridinium groups, restricting the porphyrin mobility in the film, probably leads to the observed phase separation. Therefore, the miscibility of the PO2/8SHS/4DODAB monolayer seems to result from both its global neutrality and the presence of 16 noncovalently bonded alkyl chains per PO2 molecule. These chains can form a dense, solid-type monolayer above each porphyrin macrocycle without the need for the macrocycles to aggregate. D. Comparison with PO/SHS/SA Systems. It is interesting to compare the behavior of the present ternary systems, PO/SHS/DODAB (where PO is PO1 or PO2), with those containing stearic acid (SA) instead of DODAB, which were studied previously.18,19 In all systems, charge neutralization of the monolayer was first obtained by using

four SHS molecules for each PO molecule. However, whereas the PO/SHS/SA mixtures studied previously remain globally neutral, the present systems containing DODAB are cationic or neutral, making the comparison somewhat indirect. In the PO/4SHS/xSA systems, where the addition of SA to the PO/4SHS monolayer increases mainly the hydrophobic interactions, the mixing of the three components is favorable in all cases, regardless of the porphyrin used and the value of x. For x g 8 (with PO1) or x g 12 (with PO2), the SA molecules “fill in” the open structure of PO/ 4SHS. On the other hand, the addition of DODAB to the PO/ 4SHS monolayer alters both the hydrophobic and electrostatic interactions, and so the organization and behavior of the ternary monolayer depend strongly on the interface composition. In the PO/SHS/DODAB systems, both the PO and DODAB cations compete for interactions with the SHS anions. Because the equimolar complex SHS/DODAB optimizes electrostatic and hydrophobic interactions at the interface,24,25 PO or PO/4SHS is segregated (except in the system PO2/8SHS/4DODAB discussed above), leading to the observed phase separation in these ternary systems. Concluding Remarks The orientation, aggregation, and planarity of the macrocycles of the tetracationic porphyrins PO1 and PO2 were investigated in mixed catanionic monolayers and LB films containing SHS and DODAB. The above characteristics were found to depend on several factors, mainly surface pressure; charge neutralization (electrostatic effect); total number of alkyl chains per porphyrin molecule


(hydrophobic effect); and nature of the bonds, covalent or intermolecular, between the chains and the pyridinium rings. The amphiphilic PO1 is found essentially in the monomeric (and planar) form at low π in all systems studied. The two ternary systems containing PO1 (PO1/4SHS/ 4DODAB and PO1/8SHS/4DODAB) exhibit the separate identities of PO1 and PO1/4SHS, respectively, excluding the catanionic surfactant matrix SHS/DODAB because of the preferential interaction between these surfactants. Therefore, at high π, the system PO1/4SHS/4DODAB forms PO1 H-type aggregates, whereas the system PO1/ 8SHS/4DODAB forms a PO1 + 4SHS bilayer. In systems containing PO1, the porphyrin fluorescence was self-quenched by intra- and interlayer mechanisms, while charge neutralization by SHS was found to reduce the quenching efficiency.

Langmuir, Vol. 18, No. 15, 2002 5781

Monolayers containing the hydrophilic PO2 are generally less stable than those with similar compositions containing PO1. Only the system PO2/8SHS/4DODAB is quite stable at the air-water interface and miscible at all surface pressures, because of cooperative electrostatic and hydrophobic interactions and the absence of covalent bonds between the porphyrin macrocycles and the surfactant alkyl chains. Acknowledgment. The authors acknowledge Ms. R. Romaõ for performing some experimental work. S.M.B.C. expresses her gratitude to Professor I. Yamazaki for the use of the stearylpyridylporphine during her stay in Okazaki, Japan, and subsequently in Portugal. This work was supported by Project PRAXIS XXI 2/2.1/QUI/443/94. P.A. acknowledges the Grant PRAXIS XXI/BIC/17209/98. LA0117909

Organization of Cationic Porphyrins in Mixed LangmuirâBlodgett Films

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