Polymerization and Characterization of Distyrylbipyridines in Solution

We have synthesized and characterized 4,4'-bis(4-O-methacrylstyryl)-2,2'-bipyridine which forms stable, transferable Langmuir−Schaefer films at the ...
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Langmuir 1996, 12, 2035-2040

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Polymerization and Characterization of Distyrylbipyridines in Solution and Langmuir-Schaefer Films: Effect of Preorganization K. K. Balasubramanian and V. Cammarata* Department of Chemistry, Auburn University, Auburn, Alabama 36849-5312 Received September 11, 1995. In Final Form: January 10, 1996X We have synthesized and characterized 4,4′-bis(4-O-methacrylstyryl)-2,2′-bipyridine which forms stable, transferable Langmuir-Schaefer films at the air-water interface. The Langmuir film shows a liquidsolid phase transition with the respective extrapolated areas of 43 and 32 Å2/molecule. These films were transferred to quartz, Au, and Ge substrates with transfer ratios of 1.0 ( 0.1. Characterization of the resulting multilayer films show that films transferred before the phase transition give relatively isotropic films and strongly anisotropic films from transfer after the phase transition. We have polymerized this material in CHCl3 solutions using both 60Co γ-rays and AIBN, a radical source, and characterized the resulting materials. We have also polymerized Langmuir-Schaefer multilayers using γ-rays. Under the conditions used, γ-irradiation only polymerizes oriented films, leaving isotropic films intact. Radical polymerization of aggregated solutions leads to aggregated polymeric material where γ-irradiation of solutions leads to unaggregated polymeric chains. We conclude that there is a topochemical requirement for polymerization to occur in the 2-D solid state and preorganization before polymerization in solution can lead to new materials.

Organic thin films have many possible real world applications. Organic materials possess both advantages and disadvantages with respect to present inorganic materials. On the positive side, organic materials are easily modified either during the synthetic process (primary structure) or during processing (generally secondary or tertiary structure). On the negative side, organic materials tend to suffer from heat, light and oxygen intolerance.1 Molecular crystals can be quite well organized and cooperative effects between neighbors can greatly enhance material properties such as nonlinear optical susceptibility. However molecular crystals have rather weak intermolecular bonding forces and these materials tend to lose order at relatively low temperatures (a few hundred °C).2 Also these materials tend to be mechanically frail. These weaknesses can be overcome with polymeric materials. The drawback is the general lack of intermolecular order in these materials and the ability to retain order under elevated temperatures and light flux.1 Langmuir-Blodgett-Schaefer (LBS) methods have been used to form thin organized supramolecular arrays of molecular and polymeric materials.3 There are two approaches of making polymeric thin films by the LBS technique. Either the polymerization is accomplished after depositing the organized polymerizable molecules onto a surface4 or deposition is done with preformed polymers.5 The limited solubility of many polymers in organic solvents makes the former method preferable. Since the orientation before and after the polymerization * Address correspondence to this author: e-mail, CAMMAVI@ MAIL.AUBURN.EDU; Telephone, (334) 844-6962; Fax, (334) 8446959. X Abstract published in Advance ACS Abstracts, March 15, 1996. (1) Marder, S. R., Sohn, J. E., Stuckey, G. D., Eds. Materials for Nonlinear Optics: Chemical Perspectives; ACS Symposium Series 455; American Chemical Society: Washington, DC, 1991. (2) Prasad, P. N. Introduction to Nonlinear Optical Effects in Molecules and Polymers; Wiley: New York, 1991. (3) (a) Roberts, G. G., Ed. Langmuir-Blodgett Films; Plenum: New York, 1990. (b) Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (4) Tieke, B.; Lieser, G.; Weiss, K. Thin Solid Films 1983, 99, 95. Lieser, G.; Tieke, B.; Wegner,G. Thin Solid Films 1980, 68, 77. Naegele, D.; Lando, J. B.; Ringsdorf, H. Macromolecules 1977, 10, 1339. Orihashi, Y.; Iwata, R.; Taniguchi, I., Itaya, A. Chem. Mater. 1995, 7, 324.

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is important, systematic studies to understand the structural effects on polymerization would facilitate the molecular design of new LBS films having planned structure and properties. Some studies have shown that film areas (hence film structure) can change during the polymerization procedure.6 Traditionally, LB films were made with the chromophores having long alkyl chains which pack well to provide organization within each layer.7 But, since these alkyl chains reduce the density of active groups in each layer and tend to lose order at low temperatures, it is necessary to expand the scope of LBS materials which lack alkyl chains.8,9 Molecular films lacking alkyl chains show high viscosities and do not, in general, transfer well by conventional vertical deposition technique. Horizontal lifting, Langmuir-Schaefer (LS) techniques can sometimes overcome these difficulties.10 To achieve orientation, however, the van der Waals forces aligning alkyl chains will have to be replaced by other intermolecular forces. Inter- and intramolecular aggregation are known to occur in dye molecules and have been systematically investigated by several workers.11 In previous work we have established the ability to form continuous, oriented films based on materials without alkyl chains using Langmuir-Schaefer techniques. Using (5) Yamada, T.; Yokoyama, S.; Kajikawa, K.; Ishikawa, K.; Takezoe, H.; Fukuda, A.; Kakimoto, M.; Imai, Y. Langmuir 1994, 10, 1160. Zhang, H.; Hordon, L. S.; Kuan, S. W. J.; Maccagno, P.; Pease, R. F. W. J. Vac. Sci. Technol. B 1989, 7 (6), 1717. Hisada, K.; Ito, S.; Yamamoto, M. Langmuir 1995, 11, 996. Naito, K J. Colloid Interface Sci. 1989, 131, 218. (6) See for example, Rabe, J. P.; Rabolt, J. F.; Brown, C. A.; Swalen, J. D. Thin Solid Films 1985, 133, 153. (7) Petty, M. C.; Lednev, I. K. J. Phys. Chem. 1995, 99, 4176. Schoondorp, M. A.; Schouren, B. J.; Hulshof, J. B. E.; Feringa, B. L. Langmuir 1992, 8, 1825. Ulman, A.; Scarringe, R. P. Langmuir 1992, 8, 894. Ulman, A. Adv. Mater. 1991, 3, 298. Jones, R.; Tredgold, R. H.; Hodge, P. Thin Solid Films 1983, 99, 25. Ruaudel-Teixier, A.; Barraud, A.; Blebeoch, B.; Roulliay, M. Thin Solid Films 1983, 99, 33. (8) Cammarata, V.; Atanasoska, L.; Miller, L. L.; Kolaskie, C. J.; Stallman, B. J. Langmuir 1992, 8, 876. Kenny, P. W.; Miller, L. L.; Rak, S. F.; Jozefiak, T. H.; Christopfel, W. C.; Kim, J.-H.; Uphaus, R. A. J. Am. Chem. Soc. 1988, 110, 4445. Wegmann, A.; Tieke, B.; Mayer, C. W.; Hilti, B. J. Chem. Soc., Chem. Commun. 1989, 716. Kunitake, T.; Watakabe, A. Thin Solid Films 1990, 186, L21. (9) Balasubramanian, K. K.; Cammarata, V.; Wu, Q. Langmuir 1995, 11, 1658. (10) Langmuir, I.; Schaefer, V. J. J. Am. Chem. Soc. 1938, 60, 1351.

© 1996 American Chemical Society

2036 Langmuir, Vol. 12, No. 8, 1996

styrylbipyridines, we found that the O-acetyl derivative, 1, exhibited the greatest degree of orientation.9 These

films retain their orientation for >6 months at room temperature and several hours at 100 oC before eventually subliming. Also these films show high viscosities and the ability to preserve submicrometer features induced by STM lithography for >3 days.12 Here we present the results of chain extension and making the side chain a polymerizable group. The methacrylate unit has been used as a pendant polymerizable group by several workers in LB films. Monolayers can be made from preformed polymers13 or polymerization can be accomplished after the monolayers are transferred.14 Postpolymerization of LB films using γ-rays has been the focus of a few reports. For example, long alkyl chain acrylates,15,16 and mono- and bipyridines with diacetylene units within the long alkyl chains17 have been deposited as LB films and subjected to γ-irradiation. In this paper, we will show first for 4 in solution that the product of polymerization from γ-irradiation results in straight chain (nonaggregated) polymers where radical initiated solution polymerization leads to an aggregated polymer. This work helps us to interpret the main focus of this paper where we show that we can polymerize an all aromatic LS film and that preorganization is necessary to polymerize it in the solid state. These results underscore the importance of supramolecular order in achieving designed structures and properties. Experimental Section Synthesis. All the chemicals were obtained from Aldrich Chemical Co. and used as received with the exception of chloroform (Fisher Chemical Co.), which was passed through a column of adsorption grade alumina (Fisher Chemical Co.) to remove acidic impurities. NMR spectra were obtained on a Bruker 250 multiprobe spectrometer. Elemental analyses were performed by Atlantic Microlabs. Mass spectrometry data were provided by the Auburn Mass Spectrometry Laboratory. 4,4′-Bis(4-O-methacrylstyryl)-2,2′-bipyridine (4) was synthesized via a modified Perkin reaction9 by refluxing 4,4′-dimeth(11) Sato, T.; Ozaki, Y.; Iriyama, K. Langmuir 1994, 10, 2363. Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1989, 5, 1378. Hall, R. A.; Thistlethwaite, P. J.; Grieser, F.; Kimizuka, N.; Kunitake, T. Langmuir 1994, 10, 2743. Furman, I.; Geiger, H. C.; Whitten, D. G.; Penner, T. L.; Ulman, A. Langmuir 1994, 10, 837. Heesemann, J. J. Am. Chem. Soc. 1980, 102, 2167. (12) Demir, U.; Balasubramanian, K. K.; Cammarata, V.; Shannon, C. J. Vac. Sci. Technol. 1995, B13 (3), 1294. (13) Mumby, S. J.; Swalen, J. D.; Rabolt, J. F. Macromolecules 1986, 19, 1054. Gebrielli, G.; Guarini, G. G. T. J. Colloid. Interface Sci. 1978, 64, 185. Puggelli, M.; Gabrielli, G. J. Colloid. Interface Sci. 1977, 61, 420. Takahashi, T.; Miller, P.; Chen, Y. M.; Samuelson, L.; Galotti, D.; Mandal, B. K.; Kumar, J.; Tripathy, S. K. J. Polym. Sci., B 1993, 31, 165. (14) Laschewsky, A.; Ringsdorf, H.; Schmidt, G. Thin Solid Films 1985, 134, 153. Elbert, R.; Laschewsky, A.; Ringsdorf, H. J. Am. Chem. Soc. 1985, 107, 4134. (15) Puterman, M.; Fort, T., Jr.; Lando, J. B. J. Colloid. Interface Sci. 1974, 47, 705. Banerjie, A.; Lando, J. B. Thin Solid Films 1980, 68, 67. (16) Fukuda, K.; Shibasaki, Y.; Nakahara, H. Thin Solid Films 1983, 99, 87. (17) Tieke, B.; Weiss, K. Colloid Polym. Sci. 1985, 263, 576.

Balasubramanian and Cammarata ylbipyridine with 4-hydroxybenzaldehyde in the presence of methacrylic anhydride and a trace amount of iodine for 70 h. The crude product was extracted with absolute ethanol (Florida Distilleries Co.). Reprecipitation from boiling ethanol was done twice. 1H NMR (CDCl3): d 8.74 (2H), s 8.64 (2H), d 7.60 (4H), d 7.49 (2H), d 7.45 (2H), d 7.16 (4H), d 7.08 (2H), s 6.37 (2H), s 5.78 (2H), s 2.08 (6H). IR (cm-1): 1731, 1636, 1586, 1545, 1507, 1460, 1375, 1310, 1292, 1218, 1203, 1170, 1131, 1014, 973, 949, 882. Mass spectrometry (EI) M+ ) 528. Elemental analyses: C, 75.18; N, 5.45; H, 5.11 (calculated: C, 77.33; N, 5.31; H, 5.35). 4,4′-Bis(4-O-propanoylstyryl)-2,2′-bipyridine (2). A similar procedure as that for (1) was followed, and propionic anhydride was used instead of acetic anhydride.9 A light tan precipitate was obtained after several reprecipitations of the crude product from boiling ethanol. NMR (CDCl3): d 8.60 (2H), s 8.48 (2H), d 7.49 (4H), s 7.42 (2H), m 7.31-7.35, m 6.99-7.08, q 2.50-2.59 (4H), t 1.18-1.24 (6H). IR (cm-1): 1755, 1636, 1584, 1544, 1507, 1460, 1417, 1375, 1197, 1168, 1141, 1077, 1015, 978, 894, 837. Mass spec. (EI) M+ ) 504. Elemental analyses: C, 75.50, H, 5.75; N, 5.24 (calculated: C, 76.19; H, 5.55; N, 5.05). 4,4′-Bis(4-O-butanoylstyryl)-2,2′-bipyridine (3). Acetic anhydride was replaced by butyric anhydride in the procedure (1) in synthesizing this compound. The repeated precipitation from boiling ethanol gave the desired compound. IR (cm-1): 2966, 2932, 2873, 1749, 1633, 1583, 1544, 1506, 1459, 1415, 1373, 1197, 1168, 1143, 975, 838. Elemental analyses: C, 75.52; H, 6.11; N, 5.45 (calculated C, 76.69; H, 6.01; N, 5.26). Langmuir Trough and Substrates. Langmuir-Schaefer (LS) experiments were performed on a symmetric compression KSV3000 Langmuir-Blodgett trough as described previously.9 Deposition was accomplished via horizontal lifting.9 Ge crystals were sonicated with CHCl3/15% trifluoroacetic acid (TFA) mixture once for 5 min and with CHCl3 twice for 5 min each and airdried. Quartz slides were cleaned with hot piranha solution (3:1 H2SO4/H2O2) for 15 min, rinsed with distilled water thoroughly, and dried in the oven. Au-coated borosilicate glass slides were prepared as described previously.9 Polymerization. 60Co γ-radiation was used to polymerize compound 4 both in solution and in thin films. For solutions the concentration was typically 10 mM in a Ar degassed CHCl3 solution. A borosilicate glass tube sealed with a rubber septum was used to hold the solution during irradiation. Thin films were made as 4 LS layers deposited on Au-coated borosilicate glass or were cast from a CHCl3 solution onto Au-coated glass.11 These glass slides were mounted inside a large glass chamber and purged with Ar gas. The time of exposure in all experiments was 12 h corresponding to 2 Mrads. Thermal polymerization was done by adding 45 mg of AIBN as a radical source to 75 mg of the monomer in 10 mL of CHCl3 or 15% (v:v) TFA/CHCl3 and heated at ∼60 °C for 2 h. The precipitated polymer was washed with CHCl3. The polymer obtained via γ-irradiation was yellow. It was neutralized with a few drops of triethylamine and washed with 2 mL of CHCl3 several times. Similar treatment was used for the polymer made out of monomer/15% TFA/CHCl3/AIBN mixture. Spectroscopy. UV-vis experiments were performed on a Hitachi U-2000 spectrophotometer. Grazing angle reflectance IR (GIR) spectra were recorded on a Mattson RS-1 FTIR spectrometer with a Specac Graseby Reflection accessory set at an 86° angle of incidence. ATR spectra were recorded with a Harrick Twin Parallel Mirror Reflection Variable Angle ATR accessory and a Cambridge Sciences KRS-5 substrate/0.12 µm aluminum wire grid polarizer. All the IR data were collected at a spectral resolution of 4 cm-1 with 500 to 1000 sample scans as previously described.9 Corrections for baseline discrepancies were performed, but no other signal processing (smoothing or filtering) was used.

Results and Discussion Polymerization in the Solution Phase. (a) UVvis. In our previous work with Langmuir-Schaefer films of styrylbipyridines, we showed that these molecules formed H aggregates in solution and a combination of J and H aggregates in films.9 Compound 4 also shows these trends. In Figure 1 is shown the normalized visible spectrum of 4 at low dilution (∼50 nM) and a more

Styrylbipyridine Films

Figure 1. Solution UV-visible spectra of 4 in CHCl3: (A) monomer of concentration of 50 nM; (B) monomer of concentration of 10 µM; (C) γ-ray formed polymer of concentration of ∼50 nM; (D) thermally polymerized 4 of concentration of ∼50 nM.

concentrated CHCl3 solution (∼10 µM). There is a pronounced blue shift of ∼30 nm upon concentration indicating H aggregation in solution. The 10 µM solution spectrum is observed up to concentrations of 1.9 mM indicating that the degree of aggregation is already large.18 This is reversible since upon dilution the band at 320 nm decreases. Addition of trifluoroacetic acid (TFA) to these solutions (15% (v:v)) shifts the band maxima to longer wavelengths. This is consistent with protonation of the pyridine nitrogen.19 For compound 4 the band maxima shifts to 400 nm irrespective of concentration from 50 nM to 2.1 mM. We have polymerized solutions of 4 in two different ways. The first method involves using AIBN as a radical source in a concentrated solution (∼10 mM) of the bipyridine. Control experiments with compound 1 show no precipitation and no change in solution IR or UVvisible spectrum upon exposure to polymerization conditions. However, compound 4 under these conditions forms precipitated material which can be filtered and redissolved at much lower concentrations. This shows that the methacrylate group is necessary for polymer formation and that ethenyl group between the pyridine and phenyl group is not affected. At 50 µM (in concentration of monomer units) as shown in Figure 1, the spectrum of the polymerized material is nearly identical to the H aggregated spectrum of 1 and 4. H aggregation was intact even with the dilution up to 50 nM monomer unit concentration. It appears that under these conditions polymerization stabilizes the H aggregated form. We can rationalize this if the predominate species in solution is H aggregated and polymerization is fast compared to the kinetics of aggregation/dissociation. The tertiary structure of the material would be permanently “locked in” by covalent attachment. The second method of solution polymerization is γ-irradiation of CHCl3 solutions of 4. A precipitated polymer was obtained when 10 mM solution of 4 in CHCl3 was irradiated with a 60Co γ-radiation source for 12 h. The polymer has an absorption maxima at 406 nm indicating that it has been protonated during polymerization. The polymer was then neutralized with Et3N and redissolved in CHCl3. From Figure 1 it is clear that the polymer formed from γ-irradiation has a spectrum that resembles the dilute, unaggregated monomer with a λmax ) 348 nm. The polymer scatters light more than polymers formed (18) Emerson, E. S.; Conlin, M. A.; Rosenoff, A. E.; Norland, K. S.; Rodriguez, H.; Chin, D.; Bird, G. R. J. Phys. Chem. 1967, 71, 2396. (19) Summers, L. A. Adv. Heterocycl. Chem. 1984, 35, 281.

Langmuir, Vol. 12, No. 8, 1996 2037

Figure 2. Transmission IR in KBr pellets of 4: (A) monomer; (B) after thermal polymerization; (C) after γ-ray polymerization.

from AIBN initiation as evidenced from the rising background. It is unclear at present the origin of the scattering although we speculate that it is due to chain size.20 Presently we are undertaking mass spectral studies to elucidate the degree of polymerization. Further experiments on the nature of the polymer are planned. Washio et al. suggest that the mechanism involves eejection producing the CHCl3.+ which can lose H+ and the resulting radical initiate polymerization.21 Even though the monomer concentration during polymerization is 10 mM of 4, the resulting polymer is not aggregated. As shown by the IR below, the distyrylbipyridine unit is intact and only the side chain methacrylate is polymerized. Protonation of both the AIBN and γ-irradiated polymers leads to large band at 398 and 406 nm, respectively. This is consistent with the monomer in solution from highest to lowest concentrations which show broad peaks around 400 nm. Preliminary results on the concentration dependence of fluorescence of the excitation spectra suggest that aggregation also occurs in the protonated species.22 (b) IR Characterization. As shown in Figure 2, the occurrence of polymerization was confirmed from the disappearance of the 1291 and 1320 cm-1 bands and shift in the CdO stretching from 1731 to 1749 cm-1. The 1291 and 1320 cm-1 bands have been assigned to the CdC double bond of the methacrylate group. The disappearance of this band is consistent with polymerization through the methacrylate bond. We also see an increase in the aliphatic C-H stretching region consistent with polymerization. The shift in the CdO frequency can be explained by the removal of the double bond of the methacrylate from conjugation with the carbonyl. Compound 1 for example without π-conjugation shows a CdO stretching at 1760 cm-1. Previous work by Murray et al. had shown that metal complexes of distyrylbipyridine (R ) H and CH3, see 1) could be electropolymerized through the styryl double bond.23 It appears that the ethenyl group between the pyridine and phenyl group is intact since the 966 cm-1 peak assigned to the trans ethenyl C-H wag is still observed. We have prepared bis-4,4′-(phenylethyl)-2,2′bipyridine by hydrogenating distyrylbipyridine24 and the (20) Billmeyer, F. W., Jr. Textbook of Polymer Science; Wiley: New York, 1970. (21) Washio, M.; Tagawa, S.; Tabata, Y. Radiat. Phys. Chem. 1983, 21, 239. (22) Balasubramanian, K. K.; Cammarata, V. Work in progress. (23) Leidner, C. R.; Sullivan, B. P.; Reed, R. A.; White, B. A.; Crimmins, M. T.; Murray, R. W.; Meyer, T. J. Inorg. Chem. 1987, 26, 882.. (24) Sasse, W. H. F.; Whittle, C. P. J. Chem. Soc. 1961, 1347.

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Figure 3. Pressure-area isotherm at 6 °C of (A) 1, (B) 4, and (C) 3. The spreading solutions were 1 mg/mL in CHCl3 onto 18 MΩ H2O subphase.

966 cm-1 band is not observed, confirming the assignment of this peak. Polymerization of Solid Films. (a) Langmuir Films. Compounds 1-4 are all derived from the basic distyrylbipyridine unit. Previously we found the derivatives that form stable Langmuir films show extrapolated mean molecular areas of 30-35 Å2.9 We thought that simple chain extension of the terminal methyl to ethyl or propyl would lead to similar results. However, as observed in Figure 3, 3 always shows a break in the pressure-area curve on the trough relative to 1. This transition is similar to first-order phase transitions found in other materials.25 2 shows a phase transition upon compression in about half of the experiments. We have varied temperature and compression speed, yet no clear trends have been observed. The collapse to the smaller area phase appears to be quite sensitive to, as yet, undetermined variables. The larger area phases of 2 and 3 have extrapolated areas of ∼30 Å2 consistent with 1. The smaller area phases have mean molecular areas ∼25 Å2 more consistent with limitations by alkyl chains.3 Thus we conclude that either the transstyrylbipyridine or the alkyl chain substituent can dominate the close-packing. This has been observed in other compounds such as azobenzene derivatives with long alkyl chains.26 Compound 4 also shows a break in the pressure-area compression curve but the initial phase is larger than the simple distyrylbipyridine unit. The larger area occupied by the methacrylate unit as compared to the acetyl unit may account for the observed extrapolated area of 43 Å2 in Figure 3. Upon greater compression the isotherm follows a pressure area curve closer in area to 1. The conclusion is that initially the larger methacrylate group can dominate the packing allowing the styrylbipyridine group conformational space. As we will show below this can indeed affect the chemistry of the resulting films. The horizontal lifting method9,10 was used to transfer the films from the water interface to various substrates. Compounds 1, 2, 3, and 4 transferred well to Au, Ge, and quartz substrates. The ratio of film area transferred to substrate area was determined to be 1.0 ( 0.1 and was quite reproducible from 2 to 10 layers.12 As shown in our previous work, the first layer is not always reproducible.9 (b) Polarized ATR IR. Polarized ATR spectra of LS films of 4 were taken before and after the phase transition (25) Kano, K.; Fujii, H.; Uraki, H.; Hashimoto, S. Langmuir 1989, 5, 927. Yanagi, M.; Tamamura, H.; Kurihara, K.; Kunitake, T. Langmuir 1991, 7, 167. Mitchell, M. L.; Dluhy, R. A. J. Am. Chem. Soc. 1988, 110, 712. Dutta, P.; Peng, J. B.; Lin, B.; Ketterson, J. B.; Prakash, M.; Georgopoulos, P.; Ehrlich, S. Phys. Rev. Lett. 1987, 58, 2228. (26) Ahuja, R. C.; Maack, J.; Tachibana, H. J. Phys. Chem. 1995, 99, 9221. Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1989, 5, 1378. Heeseman, J. J. Am. Chem. Soc. 1980, 102, 2167.

Balasubramanian and Cammarata

similar to our earlier work.9 We define the z film direction as perpendicular to the surface while the x and y directions are parallel to the surface. The y direction is perpendicular to the movable barrier as previously reported.9 The dichroic ratio was defined as D ) (Ax + Az)/Ay. From the dichroic ratios we can calculate the tilt angle for each vibrational band. The spectra show different dichroic ratios for each band assigned in Table 1. For compound 4 the band at 1586 cm-1 was assigned to the ring stretch along the pyridine long axis. The bands at 1731 cm-1 and 1218, 1203, and 1170 cm-1 were assigned to the CdO stretch and the C-O-Ph bending frequencies, respectively.27 The umbrella mode of the -CH3 in the methacrylate group was observed at 1375 cm-1. The other band assignments and their respective dichroic ratios are shown in Table 1. Using the tilt angles from Table 1, we determine that the average orientation along the long molecular axis is ∼70° from normal for the film obtained after the phase transition (at a surface pressure, ∏ ) 25 mN/m). The point should be stressed that this is not the only orientation but is most consistent with the data. For films of 4 deposited before the phase transition (at surface pressure, ∏ ) 10 mN/m), however, the dichroic ratios are different. The important IR dipole moments have average tilt angles around 60°. This orientation is close to random (54.7°, the magic angle would be perfectly random). This indicates that the intermediate phase could be liquid-like, which would be consistent with the large conformational possibilities for the styrylbipyridine unit or a disordered solid that can be compressed to greater order. (c) Grazing Angle IR. LS multilayers before and after the phase transition and cast films from CHCl3 solution (where the molecules are randomly oriented) were subjected to γ irradiation and characterized by grazing angle IR spectroscopy (GIR). GIR selects for those vibrations perpendicular to the surface. This spectroscopy can be used to evaluate anisotropy or simply identify thin layers since it has enhanced sensitivity to surface layers.28 In Figure 4 is shown the IR spectra of 4 deposited at a surface pressure, Π ) 25 mN/m, before and after polymerization. Comparing the spectra before and after irradiation, we see a shift in the CdO frequency from 1733 to 1756 cm-1. The 1756 cm-1 peak is broader and a significant intensity from the 1733 cm-1 peak remains. Also the small peaks at 1291 and 1308 cm-1 have decreased significantly. These results indicate that the methacrylate group has been partially polymerized. The peak at 966 cm-1 also decreases in intensity indicating that the ethenyl group between the pyridine and phenyl group may also polymerize. We did not see a change in this peak in the solution polymerization. We speculate that in solution the aggregates are flexible enough that steric factors are important where in the solid film the proximity is forced by compression. The important conclusion from Figure 4 is that polymerization has taken place on the LS film. The above experiment was also done using LS films transferred before the phase transition (at Π ) 10 mN/m) and films cast by evaporating CHCl3 solutions of 4. To ensure that the irradiation conditions were the same, we exposed films deposited at different surface pressures in the same deoxygenated sample container at the same time. As shown in Figure 5, the GIR spectrum of a film deposited before the phase transition, the spectra before and after polymerization are nearly identical. There are no shifts in the CdO region and the bands at 1291, 1308, and 966 (27) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press: New York, 1991. (28) Porter, M. A. Anal. Chem. 1988, 60, 1143a.

Styrylbipyridine Films

Langmuir, Vol. 12, No. 8, 1996 2039 Table 1. ATR Results for 4 before and after the Phase Transitiona

frequency (cm-1)

dichroic ratio (isotropic film)

dichroic ratio (Π ) 25 mN/m)

tilt angle, ϑ (from normal)

dichroic ratio (Π ) 10 mN/m)

tilt angle, ϑ (from normal)

band assignment

1731 1586 1507 1460 1375 1318 1292 1218 1203 1170 1131

1.47 1.19 1.24 1.30 1.37 1.19 1.15 1.26 1.22 1.23 1.20

1.24 0.98 0.96 1.07 1.11 0.95 1.07 0.94 0.96 0.95 0.95

62 73 78 67 66 80 61 85 78 81 80

1.14 1.09 1.09 1.15 1.13 1.08 1.00 n/a 1.11 1.19 1.16

66 61 63 61 65 62 69 n/a 61 57 57

CdO stretch along pyr Ph-O-C str Ph oblique str CH3 umbrella methacryl CdC methacryl CdC Ph-O-C bend Ph-O-C bend Ph-O-C bend Ph-O-C bend?

a

Multilayer films transferred at 6 °C onto a Ge ATR crystal.

Figure 4. Grazing angle IR spectra of a four layer LS film of 4 deposited at Π ) 25 mN/m onto Au-coated glass slides: (A) before exposure to γ-rays and (B) after exposure.

Figure 5. Grazing angle IR spectra of a six layer LS film of 4 deposited at Π ) 10 mN/m onto Au-coated glass slides (A) before exposure to γ-rays and (B) after exposure.

cm-1 are intact. The baseline of the 1733 cm-1 band does broaden insignificantly. We interpret this as little or no polymerization has occurred in this film. Also, no polymerization occurred on the cast film since again there are no band shifts observed in the GIR spectra after irradiation. Much work has been done on the mechanism of polymerization of neat solutions of methyl methacrylate by γ-irradiation.29 Although the radical mechanism is favored, there is evidence for an anionic mechanism.30 (29) Okamura, S., Ed. Recent Trends in Radiation Polymer Chemistry: Advances in Polymer Science, Springer-Verlag: Berlin, 1993. (30) Arai, S.; Kira, A.; Imamura, M. J. Phys. Chem. 1977, 81, 110.

Figure 6. UV-vis spectra of LS films and films evaporated from CHCl3 solution onto quartz slides: (A) LS film of four layers thick deposited at Π ) 10 mN/m; (B) LS film of four layers thick deposited at Π ) 25 mN/m; (C) a film evaporated from CHCl3 solution.

Fukuda et al. favor the radical mechanism in LB films of long chain alkyl acrylates.16 Also we should note that overexposure to high-energy irradiation is known to lead to polymer scission in methacrylate polymers which may limit the extent of polymerization.29 The solution polymerizations, however, lead to protonated polymers while the solid state polymerization leads to neutral polymers. For protonated styrylbipyridines we find an intense band at ∼1619 cm-1, which is also observed in the solution polymer from γ-irradiation before neutralization by Et3N. This band is not observed in the GIR spectrum of the LS polymer giving evidence that it is unprotonated. Since the products are different, we infer that the mechanism is different from the CHCl3 solution to the neat monomer. (d) UV-vis of LS Films. To elucidate the differences in the solid state polymerization, we obtained the UVvis spectra of LS films of 4 deposited on quartz. In Figure 6 we show the differences in the electronic spectra of films deposited at surface pressures of Π ) 10 and 25 mN/m compared to an evaporated, randomly oriented film. The randomly oriented film appears to be composed of a number of peaks consistent with different order J and H aggregates similar to those reported earlier for 1.9 From above, monomers of 4 have an absorption maximum at 347 nm, which is seen here as a shoulder on the higher aggregate peaks. The LS films also appear to be composed of different H aggregates. Two peaks are observed at 300 and 320 nm. The 320 nm peak is at the same energy as the >20 µM solutions. In the less compact phase (Π ) 10 mN/m), the 320 nm peak is weak; presumably it is only a minority species. We speculate that given the intensities of the two peaks, the fact that polymerization occurs in solution, and the partial polymerization of 25 mN/m films, the 320 nm peak is the species that polymerizes. Cur-

2040 Langmuir, Vol. 12, No. 8, 1996

rently, we are investigating the three-dimensional structure of those polymers. These experiments show that preorganization is necessary for polymerization to occur in these films. While this is an important result it is not unprecedented. Solid state polymerizations such as trans-cinnamic acid and diacetylenes do have topological requirements.31 To our knowledge, though, no one has used γ-ray polymerization of all aromatic LBS films to show this. Conclusions 4,4′-Bis(4-O-methacrylstyryl)-2,2′-bipyridine forms stable, transferable Langmuir-Schaefer films. Transfer before the phase transition gives a relatively isotropic film, and a strongly anisotropic film was obtained after the (31) Addadi, L.; van Mil, J.; Lahav, M. J. Am. Chem. Soc. 1982, 104, 3422. Koch, H.; Laschewsky, A.; Ringsdorf, H.; Teng, K. Makromol. Chem. 1986, 187, 1843. Lieser, G.; Tieke, B.; Wegner, G. Thin Solid Films 1980, 68, 77. Tanaka, Y.; Nakayama, K.; Iijima, S.-I.; Shimizu, T.; Maitani, Y. Thin Solid Films 1985, 133, 165.

Balasubramanian and Cammarata

phase transition. In solution and in the solid state this material can be polymerized through the methacrylate group. γ-Irradiation only polymerizes oriented films, leaving isotropic films intact. Radical polymerization of H-aggregates leads to H aggregated polymeric material where γ-irradiation of solutions leads to unaggregated polymeric chains. From the spectroscopic data it is concluded that preorganization is necessary for polymerization to occur in the 2-D solid state and preorganization before polymerization in solution can lead to new materials. Acknowledgment. Acknowledgment is made to The Petroleum Research Foundation, for financial support of this research. The authors thank Mr. R. Knight and the Auburn University Nuclear Science Center for assistance with γ-irradiation experiments. We also thank Dr. George Goodloe of the Auburn Mass Spectrometry lab for providing assistance with mass spectra. LA9507491