Organization of a New Tetraalkynyl Porphyrin by the Langmuir

Apr 17, 1999 - A new tetraalkynyl porphyrin: the meso-tetra(trimethylsilylethynyl)-β-tetra(acetic acid)porphyrin, abbreviated as H2P1, has been synth...
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Langmuir 1999, 15, 3653-3660

3653

Organization of a New Tetraalkynyl Porphyrin by the Langmuir-Blodgett Technique Fernande Da Cruz,† Franck Armand,*,† Pierre-Antoine Albouy,‡ Martine Nierlich,† and Annie Ruaudel-Teixier† CEA/DSM/DRECAM/Service de Chimie Mole´ culaire, CE Saclay, F-91191 Gif sur Yvette Cedex, France, and Laboratoire de Physique des Solides, Baˆ timent 510, Universite´ de Paris-Sud, F-91405 Orsay Cedex, France Received November 30, 1998. In Final Form: February 25, 1999 A new tetraalkynyl porphyrin: the meso-tetra(trimethylsilylethynyl)-β-tetra(acetic acid)porphyrin, abbreviated as H2P1, has been synthesized and organized by the Langmuir-Blodgett technique. When spread alone on pure water, it spontaneously stacks in a herringbone manner. Different methods have been used to break the π-π interactions and favor a flat-on anchoring of the macrocycle onto the water surface. Stable Langmuir and Langmuir-Blodgett films have been thus obtained and characterized by UV and IR spectroscopies and X-ray diffraction. A first method consists in introducing divalent cations into the subphase. It leads to an almost planar arrangement with a tilt angle of the macrocycle of ca. 15°. Another approach consisting in mixing a H2P1/tricosylamine complex with methyl eicosanoate was also tried. An optimal proportion of 1:4:10 was found, leading to a tilt angle of ca. 25°.

1. Introduction The ability to control the structural organization at a molecular level is quite often an essential step in building active organic macroscopic structures. Indeed, depending on how the molecules are arranged in the material, some specific properties, derived from their molecular structure, can be lost, be enhanced,1 or even be the base for new macroscopic features.2 Nature often organizes organic compounds in a particular way through crystallization; however, the energy minimization that is the driving mechanism in crystal formation often leads to materials in which the specific molecular property is not optimally expressed because of inadequate molecular packing. Moreover, the fabrication of crystals in thin layers which is required for many applications is a very challenging task. The Langmuir-Blodgett (LB) approach, which is at the crossroad between organization techniques and thin layer building techniques, provides a set of new tools for better control of the molecular arrangement.3 The long-term goal to which the present paper contributes is the fabrication of a conjugated bidimensional polymer of molecular thickness.4 This requires molecular elements presenting at the same time (i) a large overlapping of π electrons, (ii) organization sites that allow the prearrangement of the elements in a planar configuration, and (iii) coupling sites for the formation of the planar * To whom correspondence should be addressed. Tel.: 33 1 69 08 51 34. Fax: 33 1 69 08 66 40. E-mail: [email protected]. cea.fr. † CEA/DSM/DRECAM/Service de Chimie Mole ´ culaire. ‡ Universite ´ de Paris-Sud. (1) Nicoud, J. F.; Twieg, R. J. In Nonlinear Optical Properties of Organic Molecules and Crystals Vol. 1; Chemla, D. S., Zyss, J., Eds.; Academic Press: London, 1987; p 227. (2) (a) De´noyer, F.; Come`s, R.; Garito, A.; Heeger, A. J. Phys. Rev. Lett. 1975, 35, 445. (b) Kagoshima, S.; Anzai, H.; Kajimura, K.; Ishiguro, T. J. Phys. Soc. Jpn. 1975, 39, 1143. (3) (a) Kuhn, H.; Mo¨bius, D. In Investigations of Surfaces and Interfaces, Part B; Rossiter, B. W., Baetzold, R. C., Eds.; John Wiley & Sons: New York, 1993; p 375. (b) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (c) Petty, M. C. Langmuir-Blodgett Films. An Introduction; Cambridge University Press: Cambridge, 1996. (4) Lefe`vre, D.; Porteu, F.; Balog, P.; Roulliay, M.; Zalczer, G.; Palacin, S. Langmuir 1993, 9, 150.

Chart 1

conjugated macromolecule. Porphyrins possess the adequate geometry and structure to be used as building blocks for such a conjugated polymer and further exhibit specific electronic and luminescent properties. A great interest for their organization by the LB technique5 lies in their ability to be symmetrically functionalized by various substituents. The present paper focuses on the organizational aspect of a new highly substituted porphyrin, H2P1, designed with the aim of building a two-dimensional (2D) polymer based on a planar square grid pattern. As represented on Chart 1, H2P1 is a meso-tetraalkynylporphyrin bearing four acetic functions on the β positions of the pyrrole moieties. The acetylenic groups may provide a route to polymer formation while the carboxylic acid substituents will allow some control of the organization of the macrocycles. 2. Experimental Section 2.1. Materials. Methyl eicosanoate and zinc acetate were obtained from Aldrich and used without further purification. (5) (a) Arnold, D. P.; Manno, D.; Micocci, G.; Serra, A.; Tepore, A.; Valli, L. Langmuir 1997, 13, 5951. (b) Grieve, M. B.; Richardson, T.; Anderson, H. L.; Bradley, D. D. C. Thin Solid Films 1996, 285, 648.

10.1021/la981663l CCC: $18.00 © 1999 American Chemical Society Published on Web 04/17/1999

3654 Langmuir, Vol. 15, No. 10, 1999 Tricosylamine, trimethylsilylpropynal,6 and 3-carboxymethylpyrrole7 were synthesized in the laboratory. All the porphyrins were characterized by IR spectroscopy (Perkin-Elmer IFTS 1725X spectrometer), UV-visible spectrophotometry (Perkin-Elmer UV/VIS/NIR-Lambda 900 spectrophotometer), MALDI mass spectrometry (Voyager-Elite type spectrometer), and proton NMR (Bruker WM-200-SY, 200 MHz spectrometer). Synthesis of 3-Methoxycarbonylmethylpyrrole. 3-Carboxymethylpyrrole (1.5 g, 12.2 mmol) and concentrated sulfuric acid (1 mL) were heated at reflux in methanol for 24 h. After cooling, the resulting solution was first concentrated by nearly complete methanol evaporation and then diluted with water. The solution was alkalinized by adding sodium carbonate and the desired product extracted with dichloromethane. The organic phase was washed with water and dried on magnesium sulfate. The ester was recovered by solvent evaporation as a slightly brown liquid and used without further purifications (yield: 95%). IR (cm-1): 3398 (N-H), 1728 (CdO), 2953, 1437, 1284, 1202, 1157, 1071, 1061, 1014, 962, 772, 748, 715, 636. 1H NMR (CDCl3): 8.30 ppm (broad, 1H, NH), 6.73 ppm (m, 2H, H2 and H5 pyrrole), 6.17 ppm (m, 1H, H4 pyrrole), 3.70 ppm (s, 3H, CH2CO2CH3), 3.54 ppm (s, 2H, CH2CO2CH3). Synthesis of the Free Base meso-Tetra(trimethylsilylethynyl)porphyrin (H2P). The molecule has been synthesized by condensation of trimethylsilylpropynal on unsubstituted pyrrole according to a published procedure.8 Single crystals have been grown by slow evaporation of a chloroform solution at 5 °C. Synthesis of the Free Base meso-Tetra(trimethylsilylethynyl)β-tetra(methoxycarbonylmethyl)porphyrin (H2P2). An equimolar mixture of (3-methoxycarbonylmethyl)pyrrole (1.52 g; 10.9 mmol) and trimethylsilylpropynal (1.38 g; 10.9 mmol) in dried dichloromethane (1 L) was cooled to -30 °C and kept under nitrogen. BF3,OEt2 (168 µL, 1.37 mmol) was added and the solution was stirred first at -30 °C for 3 h and then overnight at room temperature. An equivalent molar amount of dicyano-dichlorobenzoquinone (DDQ; 2.48 g; 10.9 mmol) was added and the mixture was stirred at room temperature for 1 h more. Solvent evaporation yielded a dark solid from which the porphyrin was extracted by filtration on silica gel using Et2O as the eluent. Further purification by chromatography on silica gel using dichloromethane as the eluent gave the porphyrin as a blue solid (yield: 10%). IR (cm-1, KBr): 2955 (C-H), 2137 (CtC), 1737 (CdO), 1250, 852 (Si-C). UV-vis λmax(nm) (log  (CHCl3) (dm3 mol-1 cm-1)): 462 (5.51), 574 (3.98), 613 (4.49), 651 (3.50), 716 (3.73). MALDI-MS (m/z): 984.8 (MH+). 1H NMR (CDCl3): 9.37 ppm (m, 4 H, Hβ), 5.19 ppm (m, 8H, CH2CO2CH3), 3.70 ppm (m, 12 H, CH2CO2CH3), 0.58 ppm (m, 36 H, Si(CH3)3), -1 to -2 ppm (m, 2 H, NH). Synthesis of the Free Base meso-Tetra(trimethylsilylethynyl)β-tetra(carboxymethyl)porphyrin (H2P1). H2P2 (80 mg; 0.08 mmol) was dissolved in strickly anhydrous chloroform and kept under argon. Trimethylsilyliodide (117 µL; 0.82 mmol) was added and the mixture heated at 80 °C in a sealed tube under argon for 48 h. After solvent evaporation, the residue was dissolved in acetone and hydrolyzed by adding a few drops of water and subsequent stirring for 4 h. After solvent evaporation and drying under vacuum, a blue solid is recovered with an almost quantitative yield. IR (cm-1, KBr): 2958 (C-H), 2138 (CtC), 1714 (CdO), 1251, 849 (Si-C). UV-vis λmax (nm) (log  (CHCl3/ EtOH-9/1) (dm3 mol-1 cm-1)): 461 (5.41), 573 (3.83), 613 (4.39), 651 (3.37), 717 (3.61). MALDI-MS (m/z): 929.0 (MH+). 1H NMR (D6 acetone): 9.43 ppm (m, 4 H, Hβ), 5.36 ppm (m, 8H, CH2CO2CH3), 0.74 ppm (m, 36 H, Si(CH3)3), -2 ppm (m, 2 H, NH). 2.2. Preparation of Langmuir and Langmuir-Blodgett Films. The layers were built on a fully automated LB 105 Atemeta trough. The pressure/area isotherms of the Langmuir films were performed at room temperature by imposing the surface pressure by steps of 2 mN/m; the area per molecule recorded at each step corresponds to an equilibrium value. Deposition was performed on calcium fluoride slides for IR and UV-visible spectroscopic (6) Kruithof, K.; Schmitz, R.; Klumpp, G. Tetrahedron 1983, 39, 3073. (7) KaKushima, M.; Hamel, P.; Frenette, R.; Rokach, J. J. Org. Chem. 1983, 48, 3214. (8) Anderson, H. L. Tetrahedron Lett. 1992, 33, 1101.

Da Cruz et al. studies and silicon or fused silica slides for X-ray diffraction measurements, with a transfer speed of 0.5 cm‚min-1. The molecules were spread from chloroform solutions (concentration: 10-4 M) on Millipore water. 2.3. Characterization Techniques. X-ray reflectivity measurements at the air/water interface were performed on a reflectometer operating in the angular dispersive mode.9 It is essentially characterized by the absence of any moving part. A conventional X-ray powder diffractometer equipped with specific collimation slits was used for measuring the reflectivity of transferred films. X-ray diffraction in transmission was performed on a diffractometer described elsewhere.10 In this last case, deposition was made on a 20 µm thick silicon wafer so as to minimize substrate scattering. The KR wavelength of copper was used in all cases (λ ) 1.542 Å) The X-ray crystallographic experiment on H2P was realized on a single crystal introduced into a thin-walled Lindeman glass tube. Data were collected at room temperature on a Nonius CAD4 diffractometer equipped with a graphite monochromator (λ ) 0.71073 Å). The cell parameters were obtained by a least-squares refinement of the setting angles of 25 reflections with θ between 8° and 12°. Three standard reflections were measured every hour: a decay was observed (0.6% in 27 h) and a linear correction made. The data were corrected for Lorentz polarization effects. The structure was solved by direct method11 and refined by full matrix least squares on F2 with anisotropic thermal parameters. Hydrogen atoms were introduced at calculated positions and constrained to ride on their parent carbon atoms. All calculations were made with the SHELXTL11 program package on a O2 R10000 SGI computer.

3. Results and Discussion 3.1. Synthesis. The free base meso-tetra(trimethylsilylethynyl)-β-tetra(methoxycarbonylmethyl)porphyrin, H2P2, has been prepared following a general method reported by Lindsey12 for the synthesis of free base porphyrins. It consists in reacting trimethylsilylpropynal with 3-methoxycarbonylmethylpyrrole in the presence of BF3,OEt2 as the catalyst. After oxidation of the resulting porphyrinogen with DDQ, pure porphyrin H2P2 was obtained in a good yield (10%) for this kind of compound. Treatment with trimethylsilyliodide in strictly anhydrous conditions followed by hydrolysis gave H2P1, in almost quantitative yield while the direct condensation of the aldehyde with the acetic acid pyrrole gave an impure H2P1 with a very low yield (0.5%). The UV-visible spectra of H2P1, H2P2, and H2P are very similar; compared to an unsubstituted porphyrin, all the absorption bands are red-shifted. These results suggest that the electronic communication between the macrocycle aromatic system and the triple bonds mentionned by Anderson for H2P is also present for H2P1 and H2P2. The steric hindrance due to the introduction of substituents on the β positions of the porphyrin does not seem to induce significant changes in the macrocycle geometry. Moreover, whereas a unique molecular peak is observed on mass spectra for H2P1 and H2P2, NMR spectra present multiplet signals for all the protons of the molecules. It shows that each of the porphyrins is formed by a statistical mixture of four regioisomers. 3.2. Langmuir-Blodgett Study of H2P1 Spread Alone on Pure Water. Langmuir Film. The π/A iso(9) (a) Naudon, A.; Chihab, J.; Goudeau, P.; Minmault, J. J. Appl. Crystallogr. 1989, 22, 460. (b) Albouy, P.-A.; Valerio, P. Supramol. Sci. 1997, 4, 191. (10) Albouy, P.-A. J. Phys. Chem. 1994, 98, 8543. (11) Method SHELXS86, Sheldrick 1985 and program SHELXTL93, Sheldrick 1993 for the refinement of crystal structures. University of Go¨ttingen, Germany. (12) (a) Lindsey, J. S.; Schreiman, I. C.; Hsu, H. C.; Kearney, P. C.; Marguerettaz, A. M. J. Org. Chem. 1987, 52, 827. (b) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828.

Organization of a New Tetraalkynyl Porphyrin

Figure 1. π/A isotherms of H2P1 (solid line) and H2P2 (dotted line) on pure water.

therms of the tetraacid porphyrin H2P1 and the corresponding ester H2P2 are shown in Figure 1. The very stiff pressure rise observed for H2P2, between 2 and 22 mN/ m, suggests a stable solid phase. In contrast, the isotherm of H2P1 presents a smoother behavior during compression. At low surface pressure, the area per molecule is much larger for H2P1 than for H2P2: 225 Å2 for H2P1 and 90 Å2 for H2P2 at 2 mN/m. However, the area for both molecules becomes quite similar at higher pressures: 85 Å2 for H2P1 and 70 Å2 for H2P2 at 20 mN/m. Obviously, the presence of acid substituents has an impact on the behavior of the porphyrin macrocycle at the air/water interface. These results suggest that the interaction between the carboxylic acid functions and water results in a rather flat orientation of the macrocycle at low pressure (computed macrocycle area: ca. 300 Å2). However, the hydrophilicity of the carboxylic functions is not high enough to stabilize this orientation at higher pressures. To get a better understanding of the molecular organization in H2P1 Langmuir films, X-ray reflectivity spectra were recorded at a surface pressure of 20 mN/m. The experimental curve could be fitted on the basis of a simple one-slab electronic density model. A slab thickness of 14 Å was found with an electron density of 0.353 electrons/ Å3. This leads to 420 electrons/molecule, assuming a molecular area of 85 Å2. This value is markedly lower than the calculated number of electrons per molecule (490 e-). This discrepancy could be ascribed to a slight penetration of the macrocycles into the water subphase. The film thickness corresponding to a flat-on orientation can be evaluated to ca. 4 Å, in contradiction with the experimental data: this points to a tilted organization of the macrocycles (see below). Langmuir-Blodgett Films. X-ray reflectivity experiments have been performed on a 24 layer LB film deposited at 20 mN/m with a transfer rate close to unity. The observation of well-defined Kiessig fringes is consistent with a regular film deposition and points to a total thickness of 317 Å, that is, 13.2 Å/layer. This value is close to the one observed on the corresponding Langmuir film, which suggests that the molecular organization is preserved during transfer. An X-ray diffraction transmission pattern has been recorded with a sample consisting of 100 layers deposited on both sides of a 20 µm thick silicon slide. The pattern presented on Figure 2 reveals two diffraction rings (arrows) corresponding to in-plane characteristic distances of 16.7

Langmuir, Vol. 15, No. 10, 1999 3655

Figure 2. X-ray diffraction pattern in a transmission geometry realized on a 100 layer LB film of H2P1 deposited onto a 20 µm silicon slide.

Figure 3. UV-visible spectra of a 10-4 M concentrated solution of H2P1 in chloroform + 2% EtOH (solid line) and of a H2P1 45 layer LB film (dotted line).

and 4.9 Å. The large-angle ring associated with this last distance is rather broad, indicating short-range order. It can be attributed to the stacking distance between macrocycles. An IR spectroscopic study on LB films transferred onto calcium fluoride slides indicates that the carboxylic functions are not ionized. The nonpolarized UV-visible absorption spectra of a H2P1 LB film and a H2P1 chloroform solution are compared in Figure 3. The absorption bands of the LB film are broader and blueshifted. According to the excitonic coupling theory13 of the electronic transitions in the porphyrin π system, the blue shift of the Soret band is consistent with a parallel stacking of the macrocycles. The band broadening suggests either very closely packed chromophores or the occurrence of different molecular conformations. The orientation of the macrocycles is determined by out-of-plane and in-plane UV-visible dichroism14 experiments. Dichroic ratios calculated for the Soret band show an in-plane isotropy and a tilt angle between the molecular planes and the substrate surface of 38°. IR dichroism14 measured on the macrocycle deformation vibration (1156 cm-1) yields a very close value of 35°. (13) (a) Adachi, M.; Yoneyama, M.; Nakamura, S. Langmuir 1992, 8, 2240. (b) Hunter, C.; Sanders, J.; Stone, A. Chem. Phys. 1989, 133, 395. (14) Vandevyver, M.; Barraud, A.; Ruaudel-Teixier, A.; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571.

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Chart 2

Modeling of the H2P1 Organization. The above presented results have been brought together to elaborate on a model depicted in Chart 2 for the molecular organization of H2P1 molecules in LB films. It can be supposed that the molecules are stacked in a herringbone manner into flat-lying columns. A tilt angle of ca. 38° of the macrocycles with respect to the column axis is demonstrated by the spectroscopic measurements. The 16.7 Å d spacing can be ascribed to an in-plane intercolumnar distance. The layer thickness deduced from the present model is 13.1 Å, in perfect agreement with the experimental determination. The calculated area per molecule is 82 Å2, very close to the value deduced from the isotherm at the transfer pressure (85 Å2). Indeed, the columns must be short-sized as indicated on one hand by the width of the 4.9 Å reflection and the absence of in-plane anisotropy on the other hand. Very few crystal structures of alkynyl porphyrins are reported in the literature.15 Attempts to grow crystals of H2P1 have failed, probably because of the presence of different regioisomers. However, single crystals of the very similar H2P molecules could be obtained and the structure determined by conventional diffraction methods. It is worth discussing some relevant structural features and establishing comparisons to the above introduced model. Crystallographic data of the H2P structure and selected bond distances and angles are listed respectively in Tables 1 and 2. A view of the molecule showing the atomnumbering scheme is shown on Figure 4. The H2P macrocycle is almost flat ((0.04 Å) with a center of symmetry. The triple bonds slightly diverge from this average plane and the angles Cmeso-CtC-Si are not perfectly linear. The macrocycles appear to be stacked and shifted as shown in Figure 5 with an interplane distance of 3.45(2) Å. It is interesting to notice that this organization is quite reminiscent of the model developed for H2P1 in LB films, although H2P does not bear acidic substituents. This indicates that the π-π interactions between the macrocycles are the driving organizational mechanisms. The polarity of the acid functions did allow the elaboration of stable Langmuir films but was not strong enough to prevent the spontaneous π stacking of the molecules. 3.3. Induction of a Flat-on H2P1 Organization. We present here some of the methods that have been used to modify the arrangement of H2P1 at the air-water interface and induce the formation of a planar array of macrocycles. The strategy consists of ionizing the carboxylic substituents in order to break off the π-π interactions and to favor a flat anchoring of the porphyrin on water. However, this cannot be realized directly since the tetracarboxylate porphyrin is soluble into the subphase. H2P1 Spread on Water Containing Zinc Acetate. The first way to proceed consists in introducing cations in the (15) (a) Anderson, H. L.; Wylie, A. P.; Prout, K. J. Chem. Soc., Perkin Trans. 1 1998, 1607. (b) LeCours, S. M.; DiMagno, S. G.; Therien, M. J. J. Am. Chem. Soc. 1996, 118, 11854. (c) Arnold, D. P.; James, D. A.; Kennard, C. H.; Smith, G. J. Chem. Soc., Chem. Commun. 1994, 2131.

Table 1. Crystallographic Data for H2P Crystal Data C40H48N4Si4 697.20 0.60 × 0.60 × 0.20 blue triclinic P-1 5.787(3) 11.657(3) 15.277(5) 88.53(2) 80.12(3) 84.15(3) 1010(1) 1 1.146 1.732 372

formula M crystal size (mm) color crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g cm-3) µ (Mo KR) (cm-1) F(000)

Data Collection θ limits (deg) 1, 22 scan type ω/2θ scan width 0.8 + 0.35 tan θ range h, k, l -6 to 6, -12 to 12, 0 to 16 reflections collected 2658 unique 2293 with I > 3σ(I) 1994 no. of parameters 218 R ) ∑|Fo| - |Fc|/∑|Fo| 0.0408 Rw ) [∑w|Fo| - |Fc|2/∑w(|Fo|)2]1/2 0.1063 max. residual electron density (e/Å3) 0.287 Table 2. Selected Bond Distances (Å) and Angles (deg) for the H2P Crystal Structure N(1)-C(1) N(2)-C(6) C(1)-C(2) C(3)-C(4) C(5)-C(6) C(7)-C(8) C(9)-C(10A)a C(16)-C(17) C(11)-C(12) Si(1)-C(17) Si(1)-C(19) Si(2)-C(12) Si(2)-C(14) C(1)-N(1)-C(4) C(10)-C(11)-C(12) C(5)-C(16)-C(17) a

1.367(3) 1.363(3) 1.418(4) 1.420(4) 1.405(4) 1.332(4) 1.396(4) 1.202(4) 1.205(4) 1.832(3) 1.847(4) 1.831(3) 1.815(5) 109.1(2) 176.6(3) 174.3(3)

N(1)-C(4) N(2)-C(9) C(2)-C(3) C(4)-C(5) C(6)-C(7) C(8)-C(9) C(5)-C(16) C(10)-C(11)

1.365(3) 1.366(3) 1.355(4) 1.399(4) 1.441(4) 1.444(4) 1.441(4) 1.435(4)

Si(1)-C(18) Si(1)-C(20) Si(2)-C(13) Si(2)-C(15) C(6)-N(2)-C(9) C(11)-C(12)-Si(2) C(16)-C(17)-Si(1)

1.848(3) 1.848(4) 1.784(6) 1.809(5) 105.5(2) 171.3(3) 173.1(3)

A ) 2 -x, -y, 1-z.

subphase that could ionize the carboxylic functions. Zn2+ ions have been chosen for different reasons. First, they are bivalent ions and can form ionic bridges between porphyrins: monovalent cations would have certainly led to a soluble moiety; the solubilization of the porphyrins is prevented by the formation of polychelates while their adsorption at the interface is facilitated. Another interest with zinc ions lies in their easy coordination into the porphyrin center. It is already known that the macrocycles arrangement is highly dependent upon the complexed metal;16,17 it is our personal experience that zinc favors a more planar orientation in some substituted phthalocyanines.16 The compression isotherm of H2P1 spread on a zinc acetate solution (10-4 M) is presented in Figure 6, curve b. With the difference in the behavior observed on pure water, a very stable solid phase is obtained at rather low (16) Fouriaux, S.; Armand, F.; Araspin, O.; Ruaudel-Teixier, A. J. Phys. Chem. 1996, 100, 16984. (17) Matsuzawa, Y.; Seki, T.; Ichimura, K. Langmuir 1998, 14, 683.

Organization of a New Tetraalkynyl Porphyrin

Langmuir, Vol. 15, No. 10, 1999 3657

Figure 6. π/A isotherm for H2P1 (a) on pure water (b) on a 10-4 M Zn(OAc)2 subphase, and (c) on water in a mixture with 4 tricosylamine equiv.

Figure 4. Molecular structure of H2P. Symmetry code: (A) 2 - x, -y, 1 - z. Displacement ellipsoids are shown at the 30% probability level: (a) View perpendicular to the macrocycle plane; (b) view along the macrocycle plane.

Figure 5. Packing of H2P projected onto the ab-plane.

pressures, between 4 and 18 mN/m, corresponding to a molecular area of ca. 150 Å2. The film could be transferred onto hydrophilic substrates on upstroke only (Z-type LB film) at a surface pressure of 14 mN/m with a transfer ratio close to unity. Only two Q-bands at 612 and 662 nm are observed on the UV-visible spectrum of a monolayer LB film (see Table 3) which clearly demonstrates the metalation of the macrocycle. The Soret band is thinner than the one observed on a film built on pure water and red-shifted as compared to that of a ZnP1 solution in chloroform. This red shift can be interpreted as an excitonic coupling between coplanar porphyrins.13 The presence of zinc cations seems thus to have limited the π-π aggregation at the air-water interface. The UV-visible spectrum of a ZnP1 multilayer film presents an additional shoulder located at 455 nm, close to the Soret band. The position of this shoulder is reminiscent of the Soret band observed for H2P1 on pure water. Since its intensity increases with the number of layers, it can be related to π-π interactions between adjacent layers. The IR spectrum of the LB films presents simultaneously the CdO vibrations characteristic of the acid (1712

cm-1) and the carboxylate (1604 and 1397 cm-1). A comparison of intensity ratios ν(acid CdO)/ν(CtC) for LB films built on pure water and in the presence of zinc acetate indicates that ca. 40% of the carboxylic functions are ionized. LB films built on a zinc chloride water subphase present the same characteristics which demonstrates that the carboxylate bands are not related to the presence of acetate ions coordinated to the central zinc cation. UV-visible dichroism experiments based on the Soret band of a multilayer LB film indicate that the macrocycles form with the substrate an angle of around 28° (see Table 3). However, the same study on a single-layer LB film shows that the macrocycles are almost flat on the substrate with only a small angle of 15°. The organization of the molecules in a single-layer film seems to be in contradiction with the area per molecule derived from the π/A isotherm (150 Å2). However, this can be understood from the absorption intensities. The extinction coefficient of the molecule oriented flat on the substrate can be derived from the one in solution; the experimental surface density of porphyrin that is obtained from such considerations is 0.0087 Å-2, which is very high compared to the theoretical density of one monolayer of H2P1, 0.0033 Å-2. This result, together with the area per molecule obtained from the π/A isotherm and the angle of the macrocycle derived from the UV-visible dichroism study, suggests the formation of a bilayer of flat molecules at the air-water interface.18 A schematic view of a monolayer LB film obtained from the spreading of H2P1 onto a Zn(OAc)2 subphase is presented in Chart 3a. The larger tilt of the molecular plane observed in mutlilayer LB films suggests that the molecules stack in a herringbone manner when multilayers are formed. The presence of a shoulder observed close to the Soret band in the UV spectra tends to confirm such an interpretation. H2P1 Co-spread with Tricosylamine. Another approach for changing the molecular orientation consists in the introduction of long aliphatic chains into the Langmuir film. It aims at filling the space available above the macrocycles so as to prevent their stacking and facilitate a planar orientation. Tricosylamine appeared as a good candidate for the formation of a perfect mixture as it can interact with the carboxylic acid functions. This interaction could further lead to an ionization through a classical (18) Prieto, I.; Camacho, L.; Martin, M. T.; Mo¨bius, D. Langmuir 1998, 14, 1853.

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Table 3. UV Spectroscopic Data and Dichroism for H2P1 Solutions, Langmuir-Blodgett Monolayers, and Multilayer Built on a Water Subphase, Zn(OAc)2 Subphase, or a Water Subphase in a Mixture with Tricosylamine in a 1:4 Proportion soret band molecule (transfer pressure)

type of sample

λmax (nm)

log  or each layer absorption

FWHM (nm)a

area

H2P1 (20 mN/m)

solution monolayer multilayer

461 451 450

5.32 0.029 0.023

20 42 52

H2P1 on Zn(OAc)2 (14 mN/m)

ZnP1 sol.b monolayer multilayer

467 472 472 (455)

5.28 0.042

17 19

0.80

H2P1/amine (1/4) (20 mN/m)

solution monolayer multilayer

460 470 470 (450)

5.32 0.022

22 22

0.48

a

1.22

Q bands λmax (nm)

macrocycle tilt angle θ

573, 613, 651, 717 573, 615, 650, 718 578, 620, 654, 722

41° 38°

612, 659 612, 662

15° 28°

572, 612, 652, 717 573, 615, 650, 715 573, 612, 650, 716

36°

Full width at half maximum. b A spectrum of ZnP1 solution has been recorded to allow a comparison to films of H2P1 on a zinc subphase.

Chart 3

Table 4. X-ray Diffraction Study of a H2P1/ Tricosylamine 1:4 Langmuir Film for Different Surface Pressuresa surface pressure (mN/m)

area per H2P1/tricosylamine complex (Å2)

no. of electrons per complex

film thickness (Å)

10 20 30

245 205 128

1803 1365 658

23 21 17

a The area per complex is data derived from the π-A isotherm while the number of electrons per complex and the film thickness are derived from the fit of the reflectivity curves on the basis of a simple 1 slab electron density model.

acid-base reaction,19 increasing the probability of a planar anchoring. Systematic LB experiments were realized with a 1:4 mixture of H2P1 and tricosylamine. UV spectra of H2P1 only and H2P1/tricosylamine solutions are identical. However, an IR spectrum obtained after evaporation of a H2P1/tricosylamine solution de(19) Porteu, F.; Palacin, S.; Ruaudel-Teixier, A.; Barraud, A. J. Phys. Chem. 1991, 95, 7438.

posited on a calcium fluoride slide demonstrates a proton exchange between the acid functions and the amine groups: the CdO vibration of the acid form at 1714 cm-1 has completely disappeared while the two bands at 1574 and 1367 cm-1 characteristic for the carboxylate are present. The amine-to-porphyrin association is further confirmed by the π/A isotherm shown in Figure 6, curve c. It cannot be interpreted by a mere juxtaposition of H2P1 domains and amine domains inside the film. This shape, which is similar to the one observed for amphiphilic macrocycles,20 is consistent with the presence of a complex between a carboxylate H2P1 and four tricosylammonium moieties. However, the area per molecule is too small for a flatoriented macrocycle. The Langmuir film was characterized by X-ray reflectivity at surface pressures of 10, 20, and 30 mN/m. An adjustment of reflectivity curves on the basis of a simple one-slab electron density model was performed. The measurements, shown on Table 4, indicate that the film thickness and the electron density decrease when the surface pressure increases. These results are unexpected and could suggest a partial solubilization of the porphyrins in the subphase. To further understand this phenomenon, the Langmuir film has been transferred at 10, 20, and 26 mN/m on upand downstroke with a transfer rate close to unity. The proton exchange between the carboxylic acid and the amine groups is confirmed by IR analysis of the transferred films. The position at 470 nm of the Soret band in the UV spectrum of a monolayer suggests an absence of aggregation. However, as was observed on LB films built with H2P1 on the zinc acetate subphase, the spectrum of multilayers does present the 470 nm Soret band with the 450 nm sideband which is characteristic of π-π aggregated porphyrins. The area under the Soret band slightly increases for increasing transfer surface pressures which (20) Porteu, F. Ph.D. Thesis, Paris VI University, Paris, France, 1991.

Organization of a New Tetraalkynyl Porphyrin

Figure 7. Compression isotherms obtained for various mixtures, H2P1/tricosylamine/methyl eicosanoate, in proportions 1/4/x with x ) 0-14.

shows that the macrocycles do not dissolve into the subphase during compression. The results of the X-ray experiments realized at the air-water interface are understood if one considers that, on compression, the macrocycles penetrate deeper into the subphase and contribute less and less to the electronic density and thickness of the monolayer. The porphyrin tilt obtained for a film transferred at 20 mN/m is 36°, which is similar to the result obtained for a pure porphyrin film (see Table 3). The porphyrin density in a monolayer, calculated from the area under the Soret band, is 2.3 times higher for the pure film than for the mixture; this is consistent with a comparison of the area per molecule at 20 mN/m observed in both cases at the air-water interface. These results converge to a molecular organization inside the film as presented in Chart 3b. Since no diffraction peaks could be obtained from lowangle X-ray studies, the LB films do present a rather bad lamellarity certainly due to low packing of the aliphatic chains. Increasing the density of the aliphatic part of the Langmuir film could improve its quality and induce a flatter orientation of the macrocycles. On the basis of this idea, mixed films with H2P1 and a higher proportion of amines (1/8) were realized. The results, based on UV dichroism, indicate an even more tilted orientation of the macrocycles (from 40° to 50°), suggesting that the free amines are not present over the macrocycle but remain in direct contact with the water surface (see Chart 3c). This is certainly due to a too strong hydrophilic character of the amine group. H2P1 Co-spread with Tricosylamine and Methyl Eicosanoate. The results obtained with tricosylamine alone could be explained by a bad filling of the available space by the alkyl tails. To check this hypothesis, an aliphatic compound bearing a low-polarity substituent was added to the spreading solution.17,21 The compression isotherms of mixed films obtained by spreading 1:4:x solutions of H2P1/tricosylamine/methyl eicosanoate are presented in Figure 7. For x larger than 4, the curves display the same behavior characterized by a break in the slope at ca. 18 mN/m. Above this pressure, the area per molecule remains almost unchanged. This suggests that the area variations are first dictated by the macrocycle and, then, by the (21) Martin, M. T.; Prieto, I.; Camacho, L.; Mo¨bius, D. Langmuir 1996, 12, 6554.

Langmuir, Vol. 15, No. 10, 1999 3659

Figure 8. Area per H2P1 macrocycle as a function of the number of ester equivalents, for different surface pressures.

formation of a rigid film of aliphatic chains. This interpretation is supported by a study of the evolution of the area per H2P1 as a function of x, for various surface pressures (see Figure 8). At 40 mN/m, it increases linearly with an increment of 19 Å2/added ester, which corresponds to the typical cross section of an aliphatic chain. At this high pressure, the area is obviously controlled by the aliphatic chains alone. At 30 mN/m, a similar behavior is observed for x > 6; with a smaller ester content, the increment is 9 Å2/added ester. The evolution at 20 mN/m is even more significant: the area increase is almost negligeable up to x ) 6 and a slope of 19 Å2/added ester is not reached before x ) 10. These results can be interpreted as follows: the area of the complex tricosylamine/H2P1 is 200 Å2 at 20 mN/m. After subtraction of the cross section of the four amines, a surface area of ca. 120 Å2 remains available. The macrocycle can thus accommodate approximatively 6 ester molecules without orientational change. As more ester molecules are added, the macrocycle progressively flattens on the water surface so as to increase the available area. For x > 10, the additional molecules remain located between the complexes. Indeed, there are 14 aliphatic chains per H2P1 for x ) 10, which corresponds to an area of 280 Å2 very close to the area of a flat-lying porphyrin (300 Å2). The Langmuir film could be transferred at transfer pressures of 16 and 30 mN/m with a transfer ratio close to unity (Y-type transfer). The macrocycle orientation was deduced from UV and IR dichroism measurements. The tilt angle was shown to be insensitive to the transfer pressure. On the other hand, it depends markedly upon the ester content,22 decreasing from 36° in the absence of the ester to 25° for x ) 10. It remains unchanged at higher ester content. The aliphatic chain orientation could be deduced by an analysis of the 2917 cm-1 CH vibration band. For x > 6, the corresponding dichroic ratio does not depend on the ester content nor on the transfer pressure. An average tilt angle of 20° is found for the orientation of the aliphatic chains with respect to the normal to the substrate. X-ray reflectivity measurements were performed on a sample made of 29 layers of a 1:4:12 mixture transferred at 30 mN/m. As shown in Figure 9, well-defined (22) (a) Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.; Sugi, M.; King, L. G.; Crossley, M. J. J. Am. Chem. Soc. 1992, 114, 10662. (b) Azumi, R.; Matsumoto, M.; Kuroda, S.; King, L. G.; Crossley, M. J. Langmuir 1995, 11, 4056.

3660 Langmuir, Vol. 15, No. 10, 1999

Da Cruz et al.

assuming an area of 300 Å2 at the transfer pressure. After subtraction of the H2P1 and amine contribution, 2071 electrons remain that can be attributed to ca. 11 ester molecules, very close to the nominal composition. A model of the film organization derived from the above-discussed results is shown in Chart 3d. 4. Conclusion

Figure 9. X-ray reflectivity curve obtained with a 29 layerthick film of H2P1/tricosylamine/methyl eicosanoate (proportion: 1:4:12; transfer pressure: 30 mN/m; fused silica substrate). The smaller angle zone is shown in insert together with a calculated curve.

Kiessig fringes are visible together with three Bragg reflections. Actually, the rapid intensity drop at angles higher than 4° is only due to the high surface roughness of the silica substrate. A bilayer periodicity of 66.4 Å and a film thickness of 967 Å is found. This last determination is in very close agreement with the value computed from the stacking periodicity (962 Å). The zone near the total reflection could be adjusted to obtain a precise measurement of the electronic density of the film23 (see insert). An experimental value of 0.335 e-/Å3 leads to 3337 electrons

Polysubstituted macrocycles are very well suited to the different approaches offered by the LB technique for building monolayers at the air-water interface: in particular, reactive substituents open the way for easy modifications of the molecular organization inside the film. The present study suggests that an increase in the substituent polarity is potentially effective in breaking the π-π interactions between macrocycles. In the present case, this strategy gave poor results to induce a flat-on orientation of the macrocycle. On the contrary, the complexation of the porphyrin center with an ion which can be further hydrated appeared as a more effective method. In other words, central anchoring of H2P1 on the water surface was more efficient than peripheral anchoring. The role of space filling above the macrocycle when alkyl chains are present must also be outlined. This work presently focuses on the deprotection and coupling of the acetylenic substituents in order to build a bidimensionnal conjugated polymer. LA981663L (23) Schalchli, A.; Benattar, J.-J.; Licoppe, C. Europhys. Lett. 1994, 26, 271.