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Langmuir 1996, 12, 854-856
Photoisomerization of a Mesoionic 4,6-Dioxo-1,3-diazine in Langmuir Films
Scheme 1
Karl-Ulrich Fulda,† Ellen Maassen,‡ Helmut Ritter,‡ Rolf Sperber,‡ and Bernd Tieke*,† Institut fu¨ r Physikalische Chemie der Universita¨ t zu Ko¨ ln, Luxemburgerstrasse 116, D-50939 Ko¨ ln, Germany, and Bergische Universita¨ t Wuppertal, Fachbereich 9, Chemie, Gaussstrasse 20, D-42097 Wuppertal, Germany Received February 16, 1995. In Final Form: October 13, 1995
Introduction Photoreactions in oriented systems as for example monomolecular layers and Langmuir-Blodgett (LB) films are of great scientific and practical interest. The scientific interest arises (a) from the possibility to tailor membrane properties such as packing density of the molecules, permeability, optical properties, and surface polarity, and (b) from the link to biological membranes and the photochemical processes taking place in those membranes. The practical interest is based on potential applications in photography and information storage. Known reactions are cis-trans isomerization,1-3 dimerization,4-6 polymerization,7 and headgroup isomerization leading to an increase or decrease of the polarity,8-10 or photochemical cleavage of the polar headgroup.11 This study reports on the photoisomerization of a mesoionic compound in Langmuir films that is accompanied by an irreversible headgroup transition from polar to much less polar. Mesoionic 4,6-dioxo-1,3-diazines are known to undergo an irreversible photoisomerization in solution leading to the formation of a bis(β-lactam) as shown in Scheme 1.12,13 Introduction of an n-octadecyl chain in R leads to the new amphiphilic 4,6-dioxo-1,3-diazine derivative 1 that is able to form stable monomolecular layers at the airwater interface. The Langmuir films were characterized by measuring the surface pressure-area (π-A) isotherms and the photoisomerization into the bis(β-lactam) 2 was studied by ATR-IR spectroscopy of the monolayers after LB-type transfer onto a substrate.
Materials and Methods Compound 1 was prepared from octadecylmalonic acid14 and 1,3-diphenylformamidine15 as described below. Dicyclohexylcarbodiimide (DCC) (puriss.; ∼99%) was purchased from Fluka and used without further purification. All solvents were dried and purified by standard methods.14 Compound 2 was prepared from 1 by photochemical reaction in solution. Synthesis of 5-Octadecyl-1,3-diphenyl-4,6-dioxo-1,3-diazine (1). Well powdered octadecylmalonic acid14 (3.57 g, 10 mmol) is added in small portions over a period of 5 min to a well-stirred solution of 1.96 g (10 mmol) of 1,3-diphenylformamidine15 and 4.12 g (20 mmol) of dicyclohexylcarbodiimide (DCC) in 20 mL of dry dichloromethane. An exothermic reaction occurs. The color of the refluxing reaction mixture turns to a bright yellow while dicyclohexyl urea precipitates. After 1 h of reaction at room temperature the mixture is diluted with 40 mL of dichloromethane. The urea is filtered off and washed with further 20 mL of dichloromethane. The filtrate is evaporated in vacuum to dryness and the oily residue is treated with 50 mL of n-pentane. Crystallization starts within a few minutes and is finished after 2 h at 4 °C. The saponaceous crystals are separated by hydroextraction. Drying at 40 °C in high vacuum for 24 h yields 4.0 g (78%) of pale yellow substance; mp 81-82 °C. IR (KBr): 3055 (C-H, arom), 2915, 2845 (C-H, aliphat), 1695 (sh), 1680 (sh), 1665 (CdO, CdN, arom), 1595, 1555, 1490 (CdC, arom), 760, 690 cm-1 (phenyl (Ph) wagging). Further intensive signals are at 1400, 1360, 1335, and 1250 cm-1. 1H-NMR (400 MHz, CDCl ): δ ) 0.92 (t, J ) 6.7 Hz, 3H, H24); 3 1.28-1.43 (m, 30H, H9-23); 1.59-1.63 (m, 2H, H8); 2.56 (t, J ) 7.7 Hz, 2H, H7); 7.42-7.46 and 7.52-7.55 (2m, 10H, Ph); 8.44 (s, 1H, H2). UV (CHCl3): λmax (log ) ) 363.0 nm (3.156). Anal. Calcd for C34H48N2O2 (516.77): C, 79.02; H, 9.36; N, 5.42. Found: C, 78.68; H, 9.23; N, 5.42.
* Author to whom correspondence should be addressed. † Institut fu ¨ r Physikalische Chemie der Universita¨t zu Ko¨ln. ‡ Bergische Universita ¨ t Wuppertal. (1) Kano, K.; Tanaka, Y.; Ogawa, T.; Shimomura, M.; Okahata, Y. Chem. Lett. 1980, 421. (2) Miyata, A.; Unuma, Y.; Higashigaki, Y. Bull. Chem. Soc. Jpn. 1991, 64, 1719. (3) Yabe, A.; Kawabata, Y.; Niino, H.; Tanaka, M.; Ouchi, A.; Takahashi, H.; Tamura, S.; Tagaki, W.; Nakahara, H.; Fukuda, K. Chem. Lett. 1988, 1. (4) Regen, S. L.; Yamaguchi, K.; Samuel, N. K. P.; Singh, M. J. J. Am. Chem. Soc. 1983, 105, 6354. (5) Koch, H.; Laschewsky, A.; Ringsdorf, H.; Teng, K. Makromol. Chem. 1986, 187, 1843. (6) Yabe, A.; Kawabata, Y.; Ouchi, A.; Tanaka, M. Langmuir 1987, 3, 405. (7) See for example: Tieke, B. In Polymerization in Organized Media; Paleos, C. M., Ed.; Gordon and Breach: Philadelphia, 1992; p 105 and references cited therein. (8) Mo¨bius, D.; Bu¨cher, H.; Kuhn, H.; Sondermann, J. Ber. BunsenGes. Phys. Chem. 1969, 73, 845. (9) Holden, D. A.; Ringsdorf, H.; Haubs, M. J. Am. Chem. Soc. 1984, 106, 4531. (10) Haubs, M.; Ringsdorf, H. New J. Chem. 1987, 11, 151. (11) Fuhrhop, J. H.; Bartsch, H.; Fritsch, D. Angew. Chem., Int. Ed. Engl. 1981, 20, 804. (12) Gotthardt, H.; Riegels, M. Chem. Ber. 1987, 120, 445. (13) Ritter, H.; Sperber, R.; Weisshuhn, C. M. Macromol. Chem. Phys. 1994, 195, 3823-3834.
0743-7463/96/2412-0854$12.00/0
Synthesis of 4-Octadecyl-2,6-diphenyl-2,6-diazabicyclo[2.2.0]hexane-3,5-dione (2). A well-degassed solution of 2.07 g (4.0 mmol) of 1 in 280 mL of dry acetonitrile is irradiated with a medium pressure mercury lamp (600 W) for 10 h (5 cm distance between lamp and reaction vessel, Pyrex glass filter). The solution is evaporated to dryness in vacuum and the oily residue is treated with small amounts of ether and petroleum ether. The precipitate is recrystallized from petroleum ether to yield 1.8 g (87%) of colorless crystals after drying in high vacuum; mp 9293 °C. IR (KBr): 3050, 2910, 2845 (C-H), 1760, 1740 (CdO), 1600, 1500, 1470 (aromat CdC), 1380, 1365, 1180, 1085, 750 (Phwagging). 1H-NMR (400 MHz, CDCl ): δ ) 0.94 (t, J ) 6.8 Hz, 3H, H24); 3 1.32-1.47 (m, 30H, H9-23); 1.66 (t, J ) 7.9 Hz, 2H, H8); 2.20 (t, J ) 8.1 Hz, 2H, H7); 6.10 (s, 1H, H1); 7.14-7.17, 7.34-7.38, and (14) Organikum, 15th ed.; VEB Verlag der Wissenschaften, Berlin. (15) Oxley, P.; Peak, D. A.; Short, W. F. J. Chem. Soc. 1948, 1616.
© 1996 American Chemical Society
Notes
Langmuir, Vol. 12, No. 3, 1996 855
Figure 1. π-A Isotherms of the amphiphilic 4,6-dioxo-1,3diazine 1 (T ) 5 and 15 °C) and the isomeric bis(β-lactam) 2 (T ) 15 °C) on pure water. 7.45-7.47 (3m, 10H, Ph). Anal. Calcd for C34H48N2O2 (516.77): C, 79.02; H, 9.36; N, 5.42. Found: C, 79.30; H, 9.42; N, 5.40.
Monolayer Formation and Deposition. For monolayer studies and formation of LB films a commercially available film balance (Lauda FW-1) equipped with a film lift was used. Monolayers were spread from solutions of the compound in chloroform (spectroscopic grade, concentration 0.5-1 mg/mL) onto a pure water subphase (Milli-Q plus, T ) 15 °C). In some experiments, a buffer solution at pH 6 containing KH2PO4 (0.09 mol/L) and NaOH (0.01 mol/L) was used. π-A isotherms were measured at a compression and expansion rate of 15.0 cm2 min-1. LB films were built up on zinc selenide crystals or quartz plates hydrophobized with trichlorooctadecylsilane prior to the transfer process. Conditions for transfer were optimum at a surface pressure of 12.5 mN/m, a subphase pH value of 6.0, and a subphase temperature of 5 °C. The dipping rate was 2 mm min-1. Photoisomerization. UV irradiation in solution was carried out using a medium-pressure mercury lamp (Hanau, TQ 750 equipped with pyrex filter). For UV irradiation at the air-water interface, a 6 W Philips low-pressure mercury lamp with maximum intensity at λ ) 366 nm equipped with a UG 1 filter (K. Benda, Wiesloch) was used. Monolayers were irradiated after compression to a surface pressure of 12.5 mN/m. The distance between monolayer and lamp was 5 cm. The irradiated film area was about 100 cm2. Neither a heat filter was used nor was care taken to exclude oxygen from the gas phase. LB films were also irradiated at a distance of 5 cm between sample and lamp. Spectroscopic Measurements. IR spectra (KBr dispersions) and ATR-IR spectra were measured using a Nicolet 5 PC FT-IR spectrometer. Zinc selenide crystals (50 × 10 × 3 mm) coated with 20 double layers on each side were used.
Results and Discussion Spreading Behavior at the Air-Water Interface. If spread at the air-water interface, 1 forms a monomolecular layer. In Figure 1, the monolayer is characterized by surface pressure-area (π-A) isotherms measured at 5 and 15 °C. The isotherm at 15 °C shows a gradual rise of the surface pressure as the area A per molecule is decreased to less than 0.7 nm2. When the surface pressure has reached a value of 37 mN m-1, the monolayer collapses. Up to the collapse point, the isotherm is fully reversible and the monolayer is very stable, i.e. the surface pressure remains constant, when the film area is kept constant. Isotherms at 5 and 15 °C are very similar except that the
Figure 2. FTIR spectra of 4,6-dioxo-1,3-diazine 1 and the isomeric bis(β-lactam) 2 in KBr dispersion. 2 was prepared upon UV photoirradiation of 1 in solution.
area per molecule is slightly smaller at 5 °C. The large A value of 0.38 nm2 at the collapse point suggests that the molecules are present in a fluid state. Possibly the molecules attain a dense packing of the headgroups, but it is unlikely that also the long alkyl chains are densely packed. A relevant structure model is shown in Figure 1. The photoproduct 2 prepared in solution does not form a monomolecular layer at the air-water interface. This can be concluded from the surface-area diagram also shown in Figure 1. Compression is accompanied by a steep rise in surface pressure, but the A value is unreasonably small. Obviously, the isomerized headgroup is not hydrophilic enough in order to sufficiently interact with the aqueous subphase. Formation of Langmuir-Blodgett Films. LB films of the mesoionic 4,6-dioxo-1,3-diazine (1) could be build up on silanized quartz or glass supports and on zinc selenide crystals at a surface pressure of the monolayer of 12.5 mN/m. Deposition occurred during the down- and upstroke at a constant transfer ratio of nearly 1 indicating the formation of Y-type LB films. Photoirradiation of the Monomolecular Layer. To study the photoisomerization at the air-water interface, a monolayer of 1 was spread on an aqueous subphase at 5 °C, compressed to a π-value of 12.5 mN/m, and irradiated with the UV lamp. A slight reduction of the film area was observed which varied from monolayer to monolayer between 5 and 15%. Occasionally, the photoirradiation led to a spontaneous collapse of the monolayer. Since the monolayer was kept under constant surface pressure, the spontaneous collapse was easily detectable by a sudden and progressive decrease of the surface area. In order to prove that a photoisomerization had taken place, the irradiated monolayers were transferred onto solid supports and characterized by infrared spectroscopy. While LB films of nonirradiated monolayers could be easily built up
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Langmuir, Vol. 12, No. 3, 1996
Figure 3. ATR-IR spectra of LB films built up on either side of zinc selenide supports: spectrum a, 20 layers of 1 deposited without UV irradiation; spectrum b, 20 layers of 1 UV irradiated at the air-water interface and then deposited; (c) spectrum of the photoproduct obtained by subtracting the spectral contribution of 1 from spectrum b.
and looked quite homogeneous, the irradiated monolayers could only be transferred with great difficulty and the LB films appeared rather inhomogenous. Characterization of the Photoproduct by Infrared Spectroscopy. In Figure 2, IR spectra of 1 and the bis(β-lactam) 2 dispersed in KBr are shown. Spectra are different in the wavenumber region between 1770 and 700 cm-1, while the C-H stretching vibrations between 3000 and 2800 cm-1 are essentially identical. The shift of the CdO stretching frequencies from 1695, 1680, and 1665 cm-1 in unirradiated 1 up to 1760 and 1740 cm-1 in irradiated 2 is especially suited to indicate the successful isomerization into the bis(β-lactam)13 (see also the experimental part for an assignment of the intense bands to vibrational modes). In Figure 3, ATR-FTIR spectra of LB films built up on zinc selenide crystals are shown. The upper spectrum (spectrum a) originates from a 20 layer LB film of nonirradiated 4,6-dioxo-1,3-diazine (1). Band positions and intensities are somewhat different from the bulk material shown in Figure 2. For example, the CdO stretching mode only exhibits a single strong band at 1675 cm-1. This can be ascribed to the unique orientation of the molecules in the LB film. Spectrum b stems from a 20 layer LB film
Notes
built up from a monolayer that had previously been UV irradiated at the air-water interface for a period of 4 h. Compared with spectrum a, several new bands emerged while the original bands are still present though partly at a lower intensity. Especially striking is the occurrence of new bands at 1775 and 1730 cm-1 which can be ascribed to the CdO stretching mode of the bis(β-lactam) 2 indicating a partial photoisomerization of 1. In order to obtain the pure spectrum of the photoproduct, spectrum a of the starting material was subtracted from spectrum b in such a quantity that the CdO stretching mode of 1 at 1675 cm-1 completely disappeared. The resulting differential spectrum (c) indicates strong bands at 1775, 1730, 1600, 1500, 1385, and 1080 cm-1 which are comparable with those of the bulk bis(β-lactam) 2 isotropically dispersed in KBr (Figure 2). Slight differences in band positions and intensities may be ascribed to the anisotropic orientation of 2 in the LB film. A rough estimation of the band intensities of 1 and 2 in spectrum b allows the conclusion that the concentration of isomer 2 does not exceed 30-40%. Efforts to increase the isomer concentration upon prolonged UV irradiation failed so far. Instead, hydrolysis occurred as it could be derived from the appearance of the intense NH-stretching vibration at 3300 cm-1. Photoisomerization in Langmuir-Blodgett Films. It was also tried to isomerize 1 after transfer onto the substrate. Ten layers were deposited on each side of the zinc selenide support at a surface pressure of 12.5 mN/m and UV irradiated for 3 h. However, ATR-IR spectra monitored after UV irradiation indicated that the typical bands of 2 were only very weakly apparent indicating a photoconversion of only a few percent. Summary and Conclusions Our studies show that the mesoionic 4,6-dioxo-1,3diazine 1 forms monomolecular layers at the air-water interface. UV irradiation of the monolayer causes a partial photoisomerization into the bis(β-lactam) 2, which is considerably less polar. The low headgroup polarity has two consequences: Firstly, 2 cannot be directly spread at the air-water interface and, secondly, UV isomerization at the air-water interface destabilizes the monolayer so that it tends to collapse spontaneously. As a consequence, transfer onto solid supports is extremely difficult and leads to rather inhomogeneous LB films. The polarity change is comparable with the photoisomerization of polar 1-iminopyridinium ylides into the less polar 1,2-diazepines that has been reported to occur in monolayers, vesicles, and polymers.10 Similarly, the isomerization of the mesoionic 4,6-dioxo-1,3-diazines may also occur in vesicles. Photoisomerization in amorphous polymers has already been demonstrated recently.13 Acknowledgment. We thank M. Schober, Institut fu¨r Anorganische Chemie der Universita¨t zu Ko¨ln, for kindly recording the ATR-IR spectra. LA950120Y
