Langmuir 1997, 13, 5161-5167
5161
Orientation of Porphyrin Moieties in Langmuir-Blodgett Films of Tetraphenylporphyrin Vinyl Monomers and Their Polymers Kaoru Aramata,† Mikiharu Kamachi,*,† Masayuki Takahashi,‡ and Akihiko Yamagishi*,‡ Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka 560, Japan, and Graduate School of Science, Division of Biological Sciences, Hokkaido University, Sapporo 060, Japan Received February 26, 1997. In Final Form: June 24, 1997X The monolayer behavior and formation of Langmuir-Blodgett (LB) films were compared between the vinyl monomers of silver(II) tetraphenylporphyrin (AgTPP) and their polymers. In both cases, eicosanoic acid was mixed at [eicosanoic acid]/[AgTPP unit] ) 1.5 ratio to reduce the aggregation of the porphyrins. From the isotherms of the monolayers at an air-water interface, the porphyrin moieties in the monomers occupied a larger area than those in the polymers. It indicated that the porphyrin groups were more compact in the polymers because they were attached to a polymer chain. Applying the electron spin resonance and polarized UV-vis absorption spectroscopies for the LB films deposited on a hydrophobic glass plate, the orientation of the AgTPP moieties was estimated. As a result, three orientations of porphyrin planes existed in the monomer films: the angle between the film normal and the transition dipole (or the porphyrin plane), φ, was 90° for one orientation and 39-59° for the other two orientations. Two orientations existed in the polymer films: φ was 39-59° for these two. The porphyrin whose plane oriented at φ ) 90° was considered to be the one that was squeezed out of the fatty acid layers. Such porphyrin moieties did not exist in the polymer films. These results have shown that the polymer main chain plays an important role in stabilizing the orientation of porphyrin moieties.
Introduction In our laboratory (K.A. and M.K.) many kinds of vinyl polymers with the paramagnetic metal complexes of tetraphenylporphyrin (TPP) in the side chains have been prepared. Through studies by magnetic susceptibility measurements, electron spin resonance (ESR), and UVvis spectroscopies, we have found that the metalloporphyrins interact magnetically and electronically.1-7 In the case of the silver(II) complexes of TPP polymers, for example, they displayed strong antiferromagnetic interaction.2 Notably these interactions were not observed for the monomers. It is important to determine the interacting paths of the metalloporphyrin moieties and the role of polymer chains for understanding the nature of interactions. In order to carry out such studies, it is desirable to prepare the sample in which the orientation of the porphyrins is controlled on a molecular level. There are, however, several difficulties in solid samples such as incrystallizability and reproducibility of the sample preparations. One of the solutions for this is to prepare Langmuir-Blodgett (LB) films of the polymers. These days, many studies of LB films containing porphyrins have been reported.8,9 These papers have revealed that the orientation in porphyrins is really * To whom correspondence should be addressed. † Osaka University. ‡ Hokkaido University. X Abstract published in Advance ACS Abstracts, August 15, 1997. (1) Kamachi, M.; Akimoto, H.; Nozakura, S. J. Polym. Sci., Polym. Lett. Ed. 1981, 20, 3354. (2) Kamachi, M.; Akimoto, H.; Mori, W.; Kishita, M. Polym. J. 1984, 16, 23. (3) Kamachi, M.; Cheng, X. S.; Kida, T.; Kajiwara, A.; Shibasaka, M.; Nozakura, S. Macromolecules 1987, 20, 2665. (4) Kamachi, M.; Cheng, X. S.; Nozakura, S. Fifth Rare Earth Symposium (Tokyo, 1987), Preprints 2B05. (5) Nozakura, S.; Kamachi, M. Makromol. Chem. Suppl. 1985, 12, 255. (6) Kajiwara, A.; Kamachi, M.; Maeda, H. Polym. J. 1991, 23, 343. (7) Kamachi, M.; Kajiwara, A.; Mori, W.; Yamaguchi, K. Mol. Cryst. Liq. Cryst. 1995, 273, 117.
S0743-7463(97)00211-4 CCC: $14.00
Chart 1
controlled in a film. Tetraphenylporphyrins do not form good LB films by themselves because of their tendency to aggregate with each other. Thus a great deal of effort has been made to reduce the aggregation of porphyrins. As a chemical modification method, for example, alkyl chains are attached to the macrocycles.8 Another simple but effective method to form stable LB films is to mix a fatty acid with a long alkyl chain to the porphyrin.9 In this paper we report the monolayer behavior and formation of LB films of porphyrin monomers (AgAATPP, AgMATPP, and AgVTPP) and their polymers (polyAgAATPP, polyAgMATPP, and polyAgVTPP) whose structures are shown in Chart 1. We have prepared a LB film by mixing eicosanoic acid with porphyrin monomers and polymers. The spectroscopic studies have been performed in order to obtain the orientation of the porphyrins in the film, using ESR and UV-vis absorption spectroscopy. Experimental Section Synthesis: 5-(Acrylamidephenyl)-10,15,20-triphenylporphyrin (H2AATPP) was prepared by the literature synthesis.10 (8) Koon, J. M.; Sudholter, E. J. R.; Schenning, A. P. H. J.; Nolte, R. J. M. Langmuir 1995, 11, 214. (9) Chou, H.; Chen, C.-T.; Stork, K. F.; Xu, X.; Taylor, K. K.; Majumder, S. A.; Hobbs, J. D.; Cesarano, J.; Shelnutt, A. Langmuir 1996, 12, 2019.
© 1997 American Chemical Society
5162 Langmuir, Vol. 13, No. 19, 1997 5-(Methacrylamidephenyl)-10,15,20-triphenylporphyrin (H2MATPP) was prepared in a manner similar to that of H2AATPP. H2MATPP was prepared by reaction between H2TPPNH2 (0.27 g, 0.42 mmol) and methacryloyl chloride (0.44 g, 4.24 mmol) in toluene using 0.89 g of triethylamine as a HCl trap. H2MATPP was obtained as purple microcrystal (120 mg, 40.6%). IR (KBr) 1630 cm-1, ν(CdC), 1670-1690 cm-1, ν(CdO). 1HNMR (CDCl3, 270 MHz) δ -2.75 (s, 2H), 2.18 (s, 3H), 5.60 (s, 1H), 5.97 (s, 1H), 7.72-7.78 (m, 9H), 7.84 (s, 1H), 8.19-8.24 (m, 8H), 8.84-8.88 (m, 8H). Anal. Calcd for C48H35N5O: C, 82.62; H, 5.06; N, 10.04. Found: C, 81.62; H, 5.14; N, 9.61. Fast atom bombardment mass spectrometry (FAB-MS) calcd [M] m/e: 697. Found [M + 1] m/e: 698.5. 5-(Vinylphenyl)-10,15,20-triphenylporphyrin (H2VTPP) was prepared through the method published before.11 Radical polymerization of these porphyrin monomers was performed as described previously.10,11 A benzonitrile solution of monomer and initiator (azobis(isobutylnitrile), AIBN) was sealed in a glass ampule under high vacuum and immersed in a 60 °C thermostat for 50 h. A crude product thus obtained was purified by repeating precipitation from tetrahydrofuran to acetone. 5-(Acrylamidephenyl)-10,15,20-triphenylporphyrinate silver (AgAATPP) was prepared by refluxing H2AATPP (0.23 g, 0.34 mmol) and silver(I) acetate (0.17 g, 1.02 mmol) in THF for 11 h. After the solvent was removed under reduced pressure, the residue was dissolved in dichloromethane. The solution was filtrated to remove silver(0). The filtrate was washed with water and was dried over anhydrous Na2SO4. Crude AgAATPP thus obtained was recrystallized from chloroform-methanol as reddish purple crystals (64.1 mg, 23.9%). Mp > 300 °C. Anal. Calcd for C47H31N5OAg: C, 71.49; H, 3.96; N, 8.87. Found: C, 71.06; H, 3.94; N, 8.73. FAB-MS calcd (Ag ) 107/109) [M] m/e: 788/ 790. Found [M] m/e: 790.3. 5-(Methacrylamidephenyl)-10,15,20-triphenylporphinate silver (AgMATPP) was synthesized through the same method as in case of AgAATPP preparation. H2MATPP (0.25 g, 0.36 mmol) and silver(I) acetate (0.18 g, 1.08 mmol) were used as AgMATPP was obtained as red crystals (244 mg, 84.7%). Mp > 300 °C. Anal. Calcd for C48H33N5OAg: C, 71.82; H, 4.14; N, 8.71. Found: C, 71.82; H, 4.36; N, 8.33. FAB-MS calcd (Ag ) 107/109) [M] m/e: 802/804. Found [M] m/e: 804.4. 5-(Vinylphenyl)-10,15,20-triphenylporphinate silver (AgVTPP) was prepared by the same method as in the case of AgAATPP. H2VTPP (212 mg, 0.332 mmol) and silver(I) acetate (168 mg, 1.01 mmol) were used for this preparation. AgVTPP was obtained as reddish purple crystals (224 mg, 90.5% yield). Mp > 300 °C: Anal. Calcd for C46H30N4Ag: C, 74.00; H, 4.05; N, 7.50. Found: C, 73.29; H, 4.21; N, 7.32. Field desorption mass spectrometry (FD-MS) calcd (Ag ) 107/109) [M] m/e: 745/ 747. Found [M] m/e: 747. Introduction of Ag(II) Ion to Metal-Free TPP Polymers. The procedure was described for the preparation of polyAgVTPP. PolyH2VTPP (Mw 107 000, weight average molecular weight obtained from gel permeation chromatography (GPC) calibrated by standard polystyrene) was used as a metal free polymer. A 282 mg portion of the polymer (0.440 mmol in terms of monomer units) and silver(I) acetate (747 mg, 4.47 mmol) were dissolved in 200 mL of tetrahydrofuran. This solution was refluxed under stirring for 46 h. After the reaction mixture was filtered, the filtrate was concentrated in vacuo. The residue was reprecipitated from THF to acetone. PolyAgVTPP was obtained at 80.5% yield (265 mg). The molecular weight was determined to be Mw ) 71 000 and Mw/Mn ) 3.18 by GPC. PolyAgAATPP was polyAgMATPP were synthesized in the same manner and their molecular weights were Mw ) 18 000, Mw/Mn ) 1.91 for polyAgAATPP and Mw ) 24 000, Mw/Mn ) 1.80 for polyAgMATPP. Eicosanoic acid (CH3(CH2)18COOH, Wako Chemicals) was recrystallized from ethanol. Chloroform (analytical grade) was used as a spreading solvent. Water used as a subphase was purified from a Millipore Milli-Q system. Preparation of Thin Films. Langmuir-Blodgett (LB) films were formed using a USI FSD-110, FSD-23 Langmuir trough. (10) Kajiwara, A.; Aramata, K.; Nomura, S.; Morishima, Y.; Kamachi, M. Chem. Lett. 1992, 95. (11) Kajiwara, A.; Aramata, K.; Kamachi, M.; Sumi, K. Polym. J. 1994, 26, 215.
Aramata et al. The trough width was 10 cm. The isotherm measurements and deposition of the LB films were performed at 20 ( 1 °C. A chloroform solution of the porphyrin monomers and their polymers and eicosanoic acid at molar ratio of [eicosanoic acid]/ [AgTPP unit] ) 1.5 was spread onto an aqueous subphase. After 20 min, the monolayers were compressed at a controlled rate (Vc ) 20 cm2/min) to the expected pressure, and the pressure was held constant automatically during the transfer of the monolayers. The vertical dipping method was used for the LB films at a dipping speed of 20 mm/min for upward and downward strokes. As substrates, we used silanized quartz plates or cover glasses. The quartz plates were dipped in a chromium acid mixture overnight. After the plates were washed with water, silanization of the plates was performed using diphenyldichlorosilane (DDS) purchased from Shin-etsu Silicon Chemicals (Japan). A silanization solution was prepared by adding 1.1 mL of DDS of 46.5 mL of toluene. Silanization was performed at room temperature by dipping the plates in the toluene solution for 3 h. The glass slides for ESR measurements were immersed in mixed acid overnight and then rinsed with water. Silanization of these was done in the same manner as for the case of the quartz plates. Measurement. The X-band ESR spectra were recorded with a JEOL JES-RE1X ESR spectrometer at room temperature. The obtained LB film was placed in the ESR cavity with the film surface either perpendicular to or parallel with the external magnetic field. The X-ray diffraction patterns were measured using a RIGAKU CN2182D6 diffractometer equipped with a Cu KR source at room temperature. The scattering and radiation slits were both 1/2°. GPC analysis was carried out in tetrahydrofuran by the TOSOH CO-8011 system using TSK gel. As detectors, TOSOH UV-8010 and TOSOH RI-8012 were used. The molecular weights of the obtained polymers were calibrated by standard polystyrene. UV-vis absorption spectra were obtained for four-layered LB films and chloroform solutions with a Shimadzu UV-160A spectrophotometer. To investigate the anisotropy of absorption spectra, the films were mounted at the incident angle of 45° or 90° using polarized light with the electric vector parallel with or perpendicular to the dipping direction. The curve fitting of the absorption spectra was performed for the polarized spectra using the procedure reported by Azumi et al.12 (1) The abscissa of each spectrum is reduced to a wavenumber unit in the region of (22-26) × 103 cm-1. (2) Each absorption spectrum was assumed to be expressed as the sum of Lorentzian peaks or Gaussian peaks. For the porphyrin monomer, three Lorentzian peaks
Abs(ν) ) K1/[1 + 4{(ν - K2)/K3}2] + K4/[1 + 4{(ν - K5)/K6}2] + K7/[1 + 4{(ν - K8)/K9}2] (1) and for the polymer, two Gaussian peaks
Abs(ν) ) K1 exp{-(ν - K2)2/K3} + K4 exp{-(ν - K5)2/K6} (2) were assumed. Where ν represents a wavenumber. The parameters K1 - K9 (or K6) were determined by the least-squares fitting method. (3) The values K1-K9 (or K1-K6) were optimized for the p-polarized spectra. (4) The values K1, K4, and K7 were optimized for the s-polarized spectra using the values K2, K3, K5, K6, K8, and K9 obtained for the corresponding p-polarized spectra. (5) The values K1, K4, and K7 were used to obtain the dichroric ratio, As/Ap, of each peak.
Results Surface Pressure-Area Isotherms of the LB Films. When we used the pure materials of porphyrin monomers and their polymers, we could not obtain reversible surface pressure-molecular area (π-A) curves. This was because the porphyrins had too high of a tendency to aggregate (12) Azumi, R.; Matsumoto, M.; Kawabata, Y.; Kuroda, S.; Sugi, M.; King, L. G.; Crossley, M. J. J. Phys. Chem. 1993, 97, 12862.
Orientation of Porphyrin Moieties in LB Films
Langmuir, Vol. 13, No. 19, 1997 5163 Scheme 1
Figure 1. π-A isotherms of mixed monolayers at [eicosanoic acid]/[AgTPP unit] ) 1.5. AgTPP polymer (;) and monomers (- - -): (a) AgAATPP and its polymer; (b) AgMATPP and its polymer; (c) AgVTPP and its polymer.
with each other. Therefore we mixed eicosanoic acid to the porphyrin solution at the molar ratio of [eicosanoic acid]/[AgTPP unit] ) 1.5-5. In this region we could obtain good LB films displaying reversible π-A curves. The results of the isotherms are shown in Figure 1. The x axis denotes the area per AgTPP moieties. In both cases of the monomers and the polymers, the monolayer collapsed at the surface pressures higher than 40 mN/m. At the surface pressure above 40 mN/m, the particles of porphyrin clusters were seen floating on the water surface with the unaided eye. In cases of monomers, the π-A curves were reversible when the compression was stopped below 40 mN/m. In the case of the polymers, even if the pressures were maintained under 40 mN/m, there were differences between the first and second compression curves. The third and further compression curves were the same as the second one. We interpreted this behavior in terms of the assumption that the change in the π-A isotherms of the polymers came from the irreversible aggregation of the porphyrin side chains. That is, during the first compression, the porphyrins in the side chains aggregated intramolecularly, which did not recover even after the layer was expanded. When the layer was compressed again, the lift-off area at which the pressure began to rise from zero became a little smaller than the first one. This was because the main chains of the polymers in the layers could not expand as wide as before (see Scheme 1). This effect of aggregation disappeared when the pressure
became high and the polymer chains were compressed more compactly. On comparison of the π-A curves between the monomer and its polymer, the isotherm of monomers revealed a wider molecular area than the polymers at a given surface pressure. This implied that the AgTPP units in the polymers could not spread as wide as the monomer. Among monomers, AgAATPP, AgMATPP, and AgVTPP, the lift-off area decreased in the order AgAATPP > AgMATPP > AgVTPP. This order is identical with that of the hydrophilicity of the main chains: acrylamide group > methacrylamide group > vinyl group. When the group attached to AgTPP became more hydrophilic, AgTPP tended to aggregate to a lesser extent and expanded water in the layer. In the porphyrin polymers, this effect was not as clear as that in the monomers. We attempted to transfer the mixed monolayer onto a substrate at a constant surface pressure. For the polymers, the monolayer was transferred at 20 mN/m as a LB film of the Y-type with the transfer ratio of 0.9-1. For the monomers, however, the monolayer as transferred at the same surface pressure with the very low transfer ratio (
