Langmuir 2000, 16, 7545-7547
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Polymerization of a Diacetylene Dicholesteryl Ester Having Two Urethanes in Organic Gel States Nobuyuki Tamaoki,* Satoru Shimada, Yuji Okada, Abdelhak Belaissaoui, Grzegorz Kruk, Kiyoshi Yase, and Hiro Matsuda National Institute of Materials and Chemical Research, Higashi 1-1, Tsukuba, Ibaraki 305-8565, Japan Received April 7, 2000. In Final Form: June 6, 2000
A cholesteryl ester unit is seen in many cholesteric liquid crystals1 or in some organic gelators.2 The van der Waals and dipole-dipole interactions between cholesteryl ester units may play an important role in stabilizing the supramolecular structures. We recently showed that some dicholesteryl esters with relatively high molecular weight formed a stable solid maintaining cholesteric molecular ordering by rapid cooling from a cholesteric phase.3 When the temperature at which rapid cooling started was changed, various cholesteric colors were fixed in the solid state of the single compound. In the course of the study on the relationship between chemical structures and stability of supramolecular ordering, we tried to introduce urethane units in a dicholesteryl ester containing a diyne group. In this note, we report that an obtained compound not only shows a liquid crystalline state that turns into a stable solid state but also forms a polymerizable organic gel. Compound 1 is a mono diacetylene compound with a symmetrical structure containing two cholesteryl ester
units at the ends and a urethane group between a diyne and each cholesteryl goup.4 Compound 1 showed a (1) Gibson, H. W. In Liquid Crystals; Saeva, F. D., Ed.; Marcel Dekker: New York, 1979; p 117. (2) (a) Lin , Y.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (b) Mukkamala, R.; Weiss, R. G. Langmuir 1996, 12, 1474. (c) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Kanamori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664. (d) Inoue, K.; Ono, Y.; Kanekiyo, Y.; Ishi-I, T.; Yoshihara, K.; Shinkai, S. J. Org. Chem. 1999, 64, 2933. (3) (a) Tamaoki, N.; Parfenov, A.; Masaki, A.; Matsuda, H. Adv. Mater. 1997, 9, 1102. (b) Tamaoki, N.; Kruk, G.; Matsuda, H. J. Mater. Chem. 1999, 9, 2381. (c) Kruk, G.; Tamaoki, N.; Matsuda, H.; Kida, Y. Liq. Cryst. 1999, 26, 1687. (4) Preparation of 1. A mixture of 4,19-dioxo-5,18-dioxa-3,20-diazadocosa-10,12-diynedioic acid (1.3 g, 3.3 mmol) obtained by the saponification of a corresponding dibutyl ester,5 N,N′-dicyclohexylcarbodiimide (1.4 g, 6.6 mmol), and 4-(dimethylamino)pyridine (80 mg, 0.66 mmol) in dry CH2Cl2 (20 mL) was stirred under refluxing for 30 min. After the mixture was cooled to room temperature, powder cholesterol (2.6 g, 6.6 mmol) was added. The mixture was stirred at room temperature for 1 h and refluxed for 2.5 h. The reaction mixture was filtered. The CH2Cl2 solution was washed with saturated aqueous NaHCO3 solution and then with water. After removal of CH2Cl2 under vacuum, the soluble part in hexane/CH2Cl2 (1/1) was taken and dried. The compound was purified by reprecipitation using CH2Cl2 and MeOH. The yield was 10% (360 mg). White powder: IR (KBr) νmax: 3350, 2948, 2868, 1748, 1729, 1698, 1537, 1468, 1376, 1366, 1280, 1201, 1053, 779, 737 cm-1; 1H NMR (270 MHz, CDCl3) δ: 0.64-2.35 (m, aliphatic CH), 3.93 (d, J ) 5.1 Hz, 4H, -CO-CH2), 4.10 (t, J ) 6.2 Hz, 4H, -O-CH2), 4.68 (m, 2H, -OCH), 5.14 (m, 2H, -NH), 5.38 (m, 2H, dCH); ES-MS (positive) m/z 1133.9 [M + H]+. Elemental analysis gave satisfactory data.
Figure 1. DSC thermograms at heating and cooling rates of 2 °C min-1 for 1.
cholesteric liquid crystalline phase between 101 and 133 °C upon heating. Upon cooling, the liquid crystalline phase of 1 observed below 128 °C was solidified, forming a glassy state around 50 °C. A DSC thermogram is shown in Figure 1. A second-order transition is clearly seen around 50 °C in the figure. Heating the glass regenerated the normal liquid crystalline phase. Crystals were never formed after it first melted. Compound 1 gelated nonpolar solvents such as cyclohexane or mixtures of hexane and dichloromethane. A minimum concentration of 1 that gelated the whole of the solvent was 5.3 and 2.6 mmol dm-3 in cyclohexane and the mixture of hexane and dichloromethane (19:1 volume ratio), respectively. The gels formed with the mixtures of hexane and dichloromethane were transparent, while those with cyclohexane were slightly turbid. The gels changed the color from colorless to dark blue, maintaining a gel state upon irradiation of UV light (a 500-W highpressure Hg lamp). This color change of 1 in the gel state is quite different from that for microcrystals of 1, where the color is changed from white to pink or red upon irradiation. Figure 2 shows photographs of the cyclohexane gel before and after photoirradiation. Figure 3 shows a change of absorption spectra for the cyclohexane gel during the photo irradiation. The absorption band with peaks at 630 and 582 nm is observed after photoirradiation. The absorption spectra after irradiation can be attributed to the exciton band of polydiacetylene.6 Polymerization of 1 is also confirmed by the formation of an insoluble part to dichloromethane after irradiation. Addition of dichloromethane to irradiated cyclohexane gel afforded an insoluble red precipitate that was filtered off and dried. The yield of polymerization was estimated by weighing the precipitate. In Figure 4 the yield of the polydiacetylene is plotted against the exposure time. In the first stage, polymerization proceeded rapidly up to about 10% and then reached 17% after 5 min. The longer irradiation gradually induced bleaching of the blue color, maybe due to the photochemical destruction of the π-conjugated main chain of the obtained polydiacetylene. Energy-filtering transmission electron microscopy (EFTEM) for the unstained specimens revealed that a fibril structure was formed in the gel, and the structure was not basically changed after polymerization (Figure 5). (5) Preziosi, A. F.; Prusik, T.; Baughman, R. H. U.S. Patent 4735745, 1988; Chem. Abstr. 1989, 110, 74127. (6) Ba¨ssler, H. In Polydiacetylene; Cantow, H.-J., Ed.; Advances in Polymer Science 63; Springer-Verlag: Berlin, 1984; p 1 and references therein.
10.1021/la000522s CCC: $19.00 © 2000 American Chemical Society Published on Web 07/28/2000
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Notes
Figure 2. Photographs of a cyclohexane gel containing 5.3 mmol dm-3 of 1 before (left) and after photoirradiation for 1 min (right).
Figure 3. Absorption spectra of cyclohexane gels containing 5.3 mmol dm-3 of 1 before and after photoirradiation. The cell width was 1 mm.
Figure 4. Time conversion curve for the photopolymerization of 1 in a gel state. Cyclohexane gels containing 5.3 mmol dm-3 of 1 were irradiated in quartz cells with 1-mm thickness.
Photopolymerization of 1 increased the stability of the gel state. The Tgel for cyclohexane gel ([1] ) 5.3 mmol dm-3) was 45 °C before polymerization. The polymerized gel maintained the shape after heating it to the boiling point of cyclohexane (80.7 °C), although it changed the color from blue to red and shrank, releasing some amount of the solvent at high temperature. Polymerization of diacetylene derivatives proceeds in a topochemical manner typically in a crystalline state7 or (7) Wegner, G. Z. Naturforsch 1969, 24B, 824.
Figure 5. EF-TEM pictures of cyclohexane gel containing 1 (unstained). Left, unirradiated; right, photoirradiated.
in a two-dimensional crystal like LB films.8 Quite recently, an example of photopolymerization of bis(diacetylene) in a gel state9 and of mono diacetylene in a fibril state10,11 were reported. In these examples, peaks of the obtained absorption band for the polydiacetylene obtained in the gel state did not reached over 600 nm, in contrast to the result shown in the present study. This difference in the position of the absorption band is interpreted in terms of different resonance contributions or changes of planarity of the π conjugate between that in the obtained polydiacetylenes in the present study and that in the former studies.12 In addition to that, the yield of the polymer from 1 in the gel state is enhanced from 5.1%, which is observed in the former study using bis(diacetylene), to 17%. Compound 1 contains a urethane group on both sides (8) Tieke, B.; Lieser, G.; Wegner, G. J. Polym. Sci. Polym. Chem. Ed. 1979, 17, 1631. (9) Inoue, K.; Ono, Y.; Kanekiyo, Y.; Hanabusa, K.; Shinkai, S. Chem. Lett. 1999, 429. (10) Masuda, M.; Hanada, T.; Yase, K.; Shimizu, T. Macromolecules 1998, 31, 9403. (11) Oshikiri, T.; Kasai, H.; Katagi, H.; Okada, S.; Oikawa, H.; Nakanishi, H. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 1999, 337, 25. (12) Enkelmann, K. In Polydiacetylene; Cantow, H.-J., Ed.; Advances in Polymer Science 63; Springer-Verlag: Berlin, 1984; p 91.
Notes
of the diyne group. We have already observed that the compound where the urethane group of 1 is replaced with a trimethylene neither forms gels nor polymerizes in a crystalline phase.3a It is concluded that intermolecular hydrogen bonding between urethane groups on both sides of the diyne group in 1 make the gel state stable13 and the (13) Mizoshita, N.; Kutsuna, T.; Hanabusa, K.; Kato, T. Chem. Commun. 1999, 781.
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distance between diyne groups and the angle between the diacetylene rod and the stacking axis in the gel state preferable for the 1,4-addition reaction. Acknowledgment. We thank Mr. M. Masuda for helpful discussions on the measurements of properties of gels. LA000522S
