Photoinduced Gelation by Stilbene Oxalyl Amide Compounds

Snežana Miljanić*, Leo Frkanec, Zlatko Meić, and Mladen Žinić. Laboratory of Analytical ..... Itsuro Tomatsu , Ke Peng , Alexander Kros. Advanced...
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Photoinduced Gelation by Stilbene Oxalyl Amide Compounds Snezˇana Miljanic´,*,† Leo Frkanec,‡ Zlatko Meic´,† and Mladen Zˇ inic´‡ Laboratory of Analytical Chemistry, Faculty of Science, University of Zagreb, Strossmayerov trg 14, 10000 Zagreb, Croatia, and Laboratory of Supramolecular and Nucleoside Chemistry, Rudier Bosˇ kovic´ Institute, Bijenicˇ ka 54, 10002 Zagreb, Croatia Received November 16, 2004. In Final Form: January 14, 2005 Oxalyl amide derivatives bearing 4-dodecyloxy-stilbene as a cis-trans photoisomerizing unit were synthesized. The trans derivative acted as a versatile gelator of various organic solvents, whereas the corresponding cis derivative showed a poor gelation ability or none at all. In diluted solution (c ) 2.0 × 10-5 mol dm-3, ethanol), the cis isomer was photochemically converted into the trans isomer within 4 min. Depending on the radiation wavelength, the trans isomer was stable or liable to photodecomposition. When exposed to irradiation, a concentrated solution of the cis isomer (c ) 2.0 × 10-2 mol dm-3, ethanol) turned into a gel. The FT-Raman, FT-IR, and 1H NMR spectra demonstrated that the gelation process occurred because of a rapid cis f trans photoisomerization followed by a self-assembly of the trans molecules. Apart from the formation of hydrogen bonding between the oxalyl amide parts of the molecules, confirmed by FT-IR spectroscopy, it was assumed that the π-π stacking between the trans-stilbene units of the molecule and a lipophilic interaction between long alkyl chains were the interactions responsible for gelation.

Introduction Recently, a design of small organic molecules capable of forming a gel with water and various organic solvents has been the subject of numerous studies. Particularly attractive seems to be the synthesis of gelators with a gelation ability that can be affected by external stimuli such as heat, light, magnetic or electric fields, pH, or chemical reactions.1 Gels are commonly prepared by heating the gelator in an appropriate solvent until the substance dissolves, followed by cooling of the clear solution. There are very few examples of gelation that do not require a heating process.2-12 To a promising group of organic gelators capable of gelation at room temperature belong compounds whose interconversion can be induced photochemically. Photoinduced changes in the gelator structure4-7 or gelator geometry8-12 affect its gelation properties. There is a growing interest in the design of * To whom correspondence should be addressed. Phone: +3851-4819283. Fax: +385-1-4818458. E-mail: [email protected]. † University of Zagreb. ‡ Rud ier Bosˇkovic´ Institute. (1) Feringa, B. L.; van Delden, R. A.; Koumura, N.; Geertsema, E. M. Chem. Rev. 2000, 100, 1789-1816. (2) Suzuki, M.; Nakajima, Y.; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K. Org. Biomol. Chem. 2004, 2, 1155-1159. (3) Suzuki, M.; Nakajima, Y.; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K. Langmuir 2003, 19, 8622-8624. (4) Lucas, L. N.; van Esch, J.; Kellogg, R. M.; Feringa, B. L. J. Chem. Soc., Chem. Commun. 2001, 759-760. (5) de Jong, J. J. D.; Lucas, L. N.; Kellogg, R. M.; van Esch, J. H.; Feringa, B. L. Science 2004, 304, 278-281. (6) Ahmed, S. A.; Sallenave, X.; Fages, F.; Mieden-Gundert, G.; Mu¨ller, W. M.; Mu¨ller, U.; Vo¨gtle, F.; Pozzo, J. L. Langmuir 2002, 18, 70967101. (7) Ayabe, M.; Kishida, T.; Fujita, N.; Sada, K.; Shinkai, S. Org. Biomol. Chem. 2003, 1, 2744-2747. (8) Geiger, C.; Stanescu, M.; Chen, L.; Whitten, D. G. Langmuir 1999, 15, 2241-2245. (9) Murata, K.; Aoki, M.; Nishi, T.; Ikeda, A.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1991, 1715-1718. (10) de Loos, M.; van Esch, J.; Kellogg, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 2001, 40, 613-616. (11) Moriyama, M.; Mizoshita, N.; Yokota, T.; Kishimoto, K.; Kato, T. Adv. Mater. 2003, 15, 1335-1338. (12) Frkanec, L.; Jokic´, M.; Makarevic´, J.; Wolsperger, K.; Zˇ inic´, M. J. Am. Chem. Soc. 2002, 124, 9716-9717.

new drug delivery systems based on organo- or hydrogels containing entrapped bioactive molecules.13-16 As such molecules are often thermally sensitive, the methods for the preparation of gels that avoid the heating process seem to be highly desirable. An obvious solution is the design of photoinduced gelation. Stilbenes are among the most extensively investigated compounds with regard to their unique photochemistry.17 Owing to their photoresponsive conformational changes, they are suitable moieties for the synthesis of “smart” gelators. Despite an apparently simple process of cistrans isomerization, the photochemical properties of stilbenes are still the subject of numerous studies and controversies. The aggregation of trans-stilbene amphiphiles18-22 and the gelation properties of cholesterol transstilbene derivatives23,24 have been thoroughly investigated by Whitten and co-workers. In this work, we report on the design of a photoinduced gelation system. Considering the excellent gelation ability of oxalyl amide gelators,25-27 their derivatives consisting of trans-stilbene and cis-stilbene units were synthesized. (13) Tiller, J. C. Angew. Chem., Int. Ed. 2003, 42, 3072-3075. (14) Zhang, Y.; Gu, H.; Yang, Z.; Xu, B. J. Am. Chem. Soc. 2003, 125, 13680-13681. (15) Yang, Z.; Gu, H.; Zhang, Y.; Wang, L.; Xu, B. Chem. Commun. 2004, 208-209. (16) Friggeri, A.; Feringa, B. L.; van Esch, J. J. Controlled Release 2004, 97, 241-248. (17) Meier, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 1399-1420. (18) Song, X.; Geiger, C.; Furman, I.; Whitten, D. G. J. Am. Chem. Soc. 1994, 116, 4103-4104. (19) Song, X.; Geiger, C.; Leinhos, U.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1994, 116, 10340-10341. (20) Furman, I.; Geiger, H. C.; Whitten, D. G. Langmuir 1994, 10, 837-843. (21) Song, X.; Geiger, C.; Farahat, M.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 12481-12491. (22) Whitten, D. G.; Chen, L.; Geiger, H. C.; Perlstein, J.; Song, X. J. Phys. Chem. B 1998, 102, 10098-10111. (23) Wang, R.; Geiger, C.; Chen, L.; Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 122, 2399-2400. (24) Duncan, D. C.; Whitten, D. G. Langmuir 2000, 16, 6445-6452. (25) Makarevic´, J.; Jokic´, M.; Peric´, B.; Tomisˇic´, V.; Kojic´-Prodic´, B.; Zˇ inic´, M. Chem.sEur. J. 2001, 7, 3328-3341. (26) Makarevic´, J.; Jokic´, M.; Frkanec, L.; Katalenic´, D.; Zˇ inic´, M. J. Chem. Soc., Chem. Commun. 2002, 2238-2239.

10.1021/la047183d CCC: $30.25 © 2005 American Chemical Society Published on Web 02/18/2005

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Table 1. Gelation Ability of Stilbene Oxalyl Amide trans Isomer 1 and cis Isomer 2a solvent

1

2

water methanol ethanol dimethyl sulfoxide ethyl acetate tetrahydrofurane acetone dichloromethane toluene tetralin

I G (10.9) G (3.4) S G (46.9) G (46.4) G (41.2) G (137.6) G (45.3) G (50.6)

I P G (82.4) S S S S S S S

a S, solution; I, insoluble; P, precipitate; G, gel. The number in parentheses is the minimum gelator concentration in mM.

Chart 1. Chemical Structures of Stilbene Oxalyl Amide trans Isomer 1 and cis Isomer 2

The gelation tendencies of the prepared substances toward various organic solvents were examined. The equilibrium of cis-trans isomerization in solution was interpreted on the basis of UV-vis absorption measurements, whereas photoinduced gelation was investigated by means of FTRaman, FT-IR, and 1H NMR spectroscopy. Results and Discussion Gelation Properties. Oxalyl amide derivatives consisting of one or two oxalyl amide moieties coupled to the 4 or 4,4′ position of cis-stilbene and trans-stilbene were synthesized.28 Their gelation properties were tested in water and in various organic solvents. Compounds 1 and 2 showed an expected difference in their gelation behavior (Chart 1, Table 1). trans Isomer 1 formed thermally reversible gels with all the organic solvents used except for dimethyl sulfoxide. It exhibited an excellent gelation tendency toward ethanol, which it could gelatinize at concentrations as low as 0.21 wt % (3.4 mM). In contrast, cis isomer 2 failed to prove its gelating potential with most of the tested solvents owing to its good solubility. Although it was capable of forming a gel with ethanol (5.09 wt %; 82.4 mM), its gelation ability was greatly inferior to that of the trans isomer. Generally, gels had a turbid appearance and were not very stable: the gelator gradually crystallized from the system within 24 h. Intermolecular lipophilic interactions between long alkyl chains, aromatic π-π stacking of the stilbene moieties, and hydrogen bonding involving oxalyl amide fragments were assigned as driving forces for the gelation of organic solvents by the trans isomer. It was assumed that the bent geometry of the cis-stilbene unit hindered (27) Makarevic´, J.; Jokic´, M.; Raza, Z.; Sˇ tefanic´, Z.; Kojic´-Prodic´, B.; Zˇ inic´, M. Chem.sEur. J. 2003, 9, 5567-5580. (28) Miljanic´, S.; Frkanec, L.; Meic´, Z.; Zˇ inic´, M. Manuscript in preparation.

Figure 1. TEM images of 1/ethanol gel: (a) gel network; (b) crystalline form in gel. Magnification 10 000×.

the unidirectional self-assembly of cis molecules in the gel fibers, increasing the solubility of the compounds.9 In view of the well-known phenomenon of cis-trans photoisomerization of stilbene compounds in solution,17 an ethanolic medium was chosen for a photoinduced gelation study. Upon irradiation by a high-pressure mercury lamp (250 nm < λ < 520 nm), the solution of the cis isomer in ethanol (20 mM) turned into a gel. This was explained by a fast isomerization of the cis molecules into trans molecules followed by a formation of the gel network. Prolonged irradiation of the gel did not affect its structure, despite the reported observation of photoreversible formation and destruction of the gel originating from cholesterol stilbene gelators.10 However, the fibrous gel suprastructure formed by cholesterol stilbene molecules contained a significant amount of the solvent, making sufficient room for the rotation of the stilbene phenyl groups.23 Apparently, oxalyl amide stilbene molecules selfassembled in gel fibers in a way that resembles the crystalline form of the substance, preventing reversible photoisomerization. TEM Investigations. Transmission electron microscopy (TEM) revealed that the gel of trans isomer 1 in ethanol consisted of thin, long rodlike fibers (Figure 1a). The gel network was not particularly dense, supporting its sensitivity to mechanical agitation and low stability. Within the gel suprastructure, crystalline forms were observed, confirming the fact that by aging the gelator changed the structure from gel to crystal (Figure 1b). Photoisomerization in Solution. The absorption spectra of trans isomer 1 and cis isomer 2 in ethanol are shown in Figure 2. Both isomers showed strong absorption in the UV region, which was assigned to a π* r π transition of electrons of the stilbene part of the molecule.17 In the spectrum of the trans isomer, an intense band was visible

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Figure 2. Absorption spectra of 1 and 2 in ethanol, c ) 2.0 × 10-5 mol dm-3.

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Figure 4. Absorption spectra taken during 300-min irradiation (250 nm < λ < 520 nm) of 1 in ethanol, c ) 2.0 × 10-5 mol dm-3. Table 2. Raman Bands of CdC Double Bond Stretching and Aromatic Ring Vibration in the Spectra of 1 and 2 in the Solid State, Solution, and Gel ν˜ /cm-1 sample 1a 2a 1/ethanolb 2/ethanolb 1/ethanolc 2/ethanolc

Figure 3. Absorption spectra taken during 4-min irradiation (250 nm < λ < 520 nm) of 2 in ethanol, c ) 2.0 × 10-5 mol dm-3.

at 339 nm, while, in the spectrum of the cis isomer, there was a less intense band at 313 nm and a shoulder at 273 nm. Solutions of the trans isomer and cis isomer obeyed Beer’s law in the concentration range from 1.0 × 10-6 to 7.0 × 10-5 mol dm-3 and from 1.0 × 10-6 to 1.0 × 10-4 mol dm-3, respectively. Deviations from linearity of the more concentrated solutions were attributed to the aggregation of molecules. The spectra of cis isomer 2 in ethanol, c ) 2.0 × 10-5 mol dm-3, were recorded during irradiation of the sample by a high-pressure mercury lamp (250 nm < λ < 520 nm) (Figure 3). The isosbestic point at 289 nm indicated the presence of two absorbing species in solution. An increase in intensity and a shift of the absorption band to 339 nm reflected interconversion of the cis isomer into the trans isomer within 4 min. According to some authors,29 rapid cis f trans photoisomerization occurred mainly as a consequence of the short lifetime of the excited singlet states (10-11 s) in cis molecules. Irradiation of the ethanolic solution of the cis isomer with a glass filter (320 nm < λ < 520 nm) resulted in identical spectral changes but at a rate 10 times slower (40 min). When the sample was exposed to daylight, cis f trans isomerization was not observed. Irradiation (250 nm < λ < 520 nm) of trans isomer 1 in ethanol, c ) 2.0 × 10-5 mol dm-3, was followed by a decrease in intensity of the absorption band with time (Figure 4). However, spectral changes indicating the presence of the cis isomer in solution were not observed. Although the photostationary equilibrium for trans f cis isomerization of stilbenoid compounds depends on the (29) Schulte-Frohlinde, D.; Blume, H.; Gu¨sten, H. J. Phys. Chem. 1962, 66, 2486-2491.

CdC

arCsC

ICdC/IarCsC

1632 1621 1636 1628 1633 1622

1601 1607 1603 1607 1603 1607

0.459 1.044 0.671 0.708 0.547 0.929

a Solid state. b Solution (c(1) ) 0.020 mol dm-3, 75 °C; c(2) ) 0.020 mol dm-3). c Gel (c(1) ) 0.020 mol dm-3; c(2) ) 0.100 mol dm-3).

substituents, the medium, and the radiation wavelength,17,29 in this case, we assumed that very intense radiation caused chemical changes of trans isomer 1, most likely photodecomposition. In the normal lived trans singlet state (10-9 s), photodissociation is reported to be a faster process than isomerization.29 When a glass filter was used during irradiation (320 nm < λ < 520 nm), no changes in the absorption spectrum were observed. The absorbance of the trans isomer remained constant throughout the experiment excluding trans f cis photoisomerization as well as decomposition of the trans isomer. Selective excitation of a single isomer was not possible because of the overlapping of the absorption bands of both isomers. Owing to a lower energy state of the trans isomer in comparison to that of the cis isomer,17 a fast cis f trans isomerization was the preferred process in solution. Photoinduced Gelation. Regarding a very intense vibrational band of the highly polarizable double bond, FT-Raman spectroscopy seemed to be a suitable method for photoinduced sol-gel transition study. FT-Raman spectra of trans isomer 1 and cis isomer 2 were taken in the solid state, solution, and gel (Table 2). The most intense bands in the spectra of solid substances at 1632 and 1621 cm-1 were assigned to the stretching of the olefinic double bond, whereas the bands at 1601 and 1607 cm-1 were the result of the aromatic ring vibration for compounds 1 and 2, respectively. Although the ethanol spectrum overlapped most of the isomer spectra, interfering bands were not observed in the wavenumber region of interest, that is, between 1700 and 1550 cm-1. In FT-Raman spectra recorded during irradiation of cis isomer 2 in ethanol, c ) 2.0 × 10-2 mol dm-3, the double bond stretching band at 1628 cm-1 and the phenyl group vibration band at 1607 cm-1 were shifted to 1632 and 1603 cm-1, respectively (Figure 5). These results implied

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Figure 5. Raman spectra taken during 60-min irradiation (250 nm < λ < 520 nm) of 2 in ethanol, c ) 2.0 × 10-2 mol dm-3.

Figure 6. Plot of Raman intensity ratio, ICdC/IarCsC, against time of irradiation (250 nm < λ < 520 nm) of 2 in ethanol, c ) 2.0 × 10-2 mol dm-3.

that the molecules of the trans isomer formed by photoisomerization of cis molecules were organized in a gel network. In addition, the ratio of the intensities of the double bond and the aromatic ring vibration bands was calculated (Table 2). The time-related changes in the intensity ratio pointed out that cis f trans photoisomerization followed by gel formation was completed in 60 min (Figure 6). The ICdC/IarCsC value 0.534 calculated after 1 h of irradiation was in good agreement with the one valid for the gel spectrum, ICdC/IarCsC ) 0.547, proving that the trans molecules formed a gel as a supramolecular structure. With sigmoidal kinetics being characteristic of many growth processes,30 the S-shaped curve was taken to indicate that photoinduced gelation was an autocatalytic cooperative process. Since a cooperative effect of various intermolecular interactions stabilizes the gel network,10,31,32 it was assumed that gelation proceeded cooperatively due to stilbene π-π stacking, oxalyl amide hydrogen bonding, and alkyl chain lipophilic interactions. A distinct molecular structure of the isomers implied that π-π interactions between nearly planar trans-stilbene17 in the formed trans molecules had a key role in the formation of the fibrous gel network. On the contrary, the propeller-type structure of cis-stilbene17 hindered efficient π-π interaction between the cis molecules, preventing their self-assembly in the gel fibers. (30) Moore, J. W.; Pearson, R. G. Kinetics and Mechanism, 3rd ed.; John Wiley & Sons: New York, 1981. (31) Amaike, M.; Kobayashi, H.; Shinkai, S. Bull. Chem. Soc. Jpn. 2000, 73, 2553-2558. (32) Snip, E.; Koumoto, K.; Shinkai, S. Tetrahedron 2002, 58, 88638873.

Figure 7. 1H NMR spectra: (a) 2 in ethanol-d6, c ) 2.0 × 10-2 mol dm-3, before irradiation; (b) gel after 60-min irradiation (250 nm < λ < 520 nm); (c) aggregates of 1 after heating the gel to 75 °C.

In the 1H NMR spectrum of cis isomer 2 in deuterated ethanol, c ) 2.0 × 10-2 mol dm-3, a characteristic signal of cis double bond protons at 6.45 ppm was observed (Figure 7a). After 60-min irradiation of the sample, only solvent signals were present (Figure 7b). The absence of gelator proton signals was due to the suppressed mobility of gelator molecules in the rigid gel network and to long correlation times. By heating the gel to 75 °C, a threedimensional network collapsed to aggregates, resulting in broad peaks with unresolved J-coupling (Figure 7c). However, the signal of the trans double bond protons at 6.98 ppm confirmed the presence of the trans isomer as building-block molecules in gel aggregates. With the gel exposed to radiation for 48 h, the gelator tended to crystallize. It can be assumed that tight packing of the trans-stilbene moieties in gel fibers prevented isomerization as well as photodecomposition of the substance

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This paper has shown that gelators bearing a photoisomerizing unit have a high potential for photoinduced solvent gelation. We believe that similar gelation systems, if they contain a biologically active compound, can be used as drug delivery systems. Due to the thermal instability of bioactive substances, the photoinduced gelation is the method of choice for the production of such gel systems. Experimental Section

Figure 8. FT-IR spectra taken during 60-min irradiation (250 nm < λ < 520 nm) of 2 in ethanol-d6, c ) 2.0 × 10-2 mol dm-3.

observed in a diluted solution. In concentrated systems of compound 1, crystallization was the preferred process over photochemical reactions. In the FT-IR spectra of the solid substances 1 and 2, the amide I (CdO) vibrational bands were obtained at 1688 and 1699 cm-1 for the trans isomer and the cis isomer, respectively. Due to insolubility of the trans isomer in ethanol at room temperature, only the FT-IR spectrum of the cis isomer in solution was taken. The deuterated solvent was used to avoid overlapping of the signals in the CdO stretching region (1740 cm-1 > ν˜ > 1630 cm-1). The band at 1690 cm-1 in the spectrum of the cis isomer in solution was assigned to a free CdO group. Although it was found at a lower wavenumber than the corresponding band in the spectrum of the solid substance, it was assumed that interactions between the CdO group and deuterated ethanol occurred rather than intermolecular oxalyl amide hydrogen bonding. During irradiation of cis isomer 2 in ethanol-d6, c ) 2.0 × 10-2 mol dm-3, the existing amide I band at 1690 cm-1 was split into two bands with the maxima at 1688 and 1707 cm-1 (Figure 8). The band at the lower wavenumber resembled that obtained in the solid state of 1 and therefore was assigned to the hydrogen bonded carbonyl group of the trans isomer, whereas the band at the higher wavenumber was indicative of a free CdO group of the same isomer. In comparison to FT-Raman spectroscopy, where by detection of scattered light only gel was observed, the detection of the transmitted light by FT-IR spectroscopy allowed, in addition to the gel, the identification of free molecules in solution. These results proved that cis f trans photoisomerization took place in solution, whereas simultaneous self-assembly of trans molecules in gel fibers occurred. Moreover, the FT-IR spectroscopic data support the view that intermolecular oxalyl amide-oxalyl amide hydrogen bonding played an important role in gel formation. Conclusion An irreversible photoinduced gelation system based on the conformational changes of a gelator molecule was demonstrated. An expected difference in the gelation abilities of the trans isomer and the cis isomer of the stilbene oxalyl amide compound allowed control of the solvent gelation by light at room temperature. Besides 1H NMR and FT-IR spectroscopy, FT-Raman spectroscopy served as a very useful tool for sol-gel transition study. The results of selected spectroscopic methods indicate that the conversion of cis molecules into trans molecules occurring in irradiated solution leads to the gel network formation.

General Information. Melting points were determined on a Kofler stage. 1H NMR and 13C NMR spectra were recorded at 300 and 75 MHz, respectively, on a Bruker Avance 300 spectrometer using tetramethylsilane as the internal standard. FT-IR spectra were taken on a Bruker Equinox 55 interferometer. FT-Raman spectra were measured with the same instrument equipped with a FRA 106/S module using a Nd:YAG laser excitation at 1064 nm. A Micromass UK mass spectrometer (model Q-TOF Micro) was used for recording mass spectra. Thinlayer chromatography was performed on silica gel coated Merck 60 F254 silica plates, preparative layer chromatography on silica gel Merck 60 coated glass plates, and preparative column chromatography using Merck 60 silica gel, 0.063-0.200 mm. All chemicals were commercially available and were used without further purification. Solvents were purified and dried according to standard procedures. 1-Dodecyloxy-4-methyl-benzene. Potassium carbonate (11.098 g, 80.3 mmol), p-cresol (5.262 g, 48.7 mmol), and 1-bromododecane (17.50 cm3, 73.0 mmol) were suspended in acetonitrile (100 cm3).33 The reaction mixture was stirred and heated at reflux for 48 h and then poured into water (120 cm3). The mixture was extracted with ether. An organic phase was separated and dried (Na2SO4), and the solvent was evaporated. The oil product was purified by vacuum distillation at 106-156 °C to obtain a colorless oil (12.386 g, 92%; Rf ) 0.38 (petrol ether)).34 1H NMR (300 MHz, CDCl3) δ/ppm: 7.07 (d, J ) 8.58 Hz, 2 H; C3-H, C5-H); 6.79 (d, J ) 8.53 Hz, 2 H; C2-H, C6-H); 3.92 (t, J ) 6.59 Hz, 2 H; OCH2CH2); 2.28 (s, 3H; Ar-CH3); 1.76 (p, J ) 6.99 Hz, 2 H; OCH2CH2CH2); 1.44-1.39 (m, 2 H; CH2CH2CH3); 1.26 (s, 16 H; (CH2)8); 0.88 (t, J ) 6.64 Hz, 3 H; CH2CH2CH3). 13C NMR (75 MHz, CDCl3) δ/ppm: 156.89 (C1); 129.72 (C3, C5); 129.49 (C4); 114.24 (C2, C6); 67.94 (OCH2); 31.82 (CH2CH2CH3); 29.56, 29.54, 29.50, 29.49, 29.31, 29.25, 29.23 (CH2); 25.96 (OCH2CH2CH2); 22.59 (CH2CH2CH3); 20.34 (ar-CH3); 14.02 (CH3). FT-IR (KBr) ν˜ /cm-1: 2925, 2857 (C-H st); 1614, 1512 (arC-C st); 1461 (CH3 δ as); 1292, 1241 (arC-O-Cal st as); 1173 (arC-H δ ip); 1039 (C-O-C st sy); 813 (arC-H δ oop). 1-Bromomethyl-4-dodecyloxy-benzene. To a solution of 1-dodecyloxy-4-methyl-benzene (9.990 g, 36.2 mmol) in carbon tetrachloride (90 cm3), N-bromosuccinimide (1.720 g, 9.6 mmol) and a catalytic amount of dibenzoyl peroxide were added.35 The same amount of N-bromosuccinimide (1.720 g, 9.6 mmol) was added twice more into the mixture at 45-min intervals. The reaction mixture refluxed for 3 h. After cooling to room temperature, the precipitate was filtered off and the solvent was evaporated from the filtrate. The resulting oil mixture was distilled under vacuum at a temperature higher than 130 °C to give a colorless oil (4.631 g, 36%; Rf ) 0.60 (petrol ether/CH2Cl2 1/1)).36 1H NMR (300 MHz, CDCl3) δ/ppm: 7.26 (d, J ) 8.54 Hz, 2 H; C3-H, C5-H); 6.87 (d, J ) 8.61 Hz, 2 H; C2-H, C6-H); 4.45 (s, 2H; CH2Br); 3.95 (t, J ) 6.58 Hz, 2 H; OCH2CH2); 1.77 (p, J ) 6.96 Hz, 2 H; OCH2CH2CH2); 1.45-1.40 (m, 2 H; CH2CH2CH3); 1.26 (s, 16 H; (CH2)8); 0.88 (t, J ) 6.65 Hz, 3 H; CH2CH2CH3). 13C NMR (75 MHz, CDCl3) δ/ppm: 158.65 (C1); 130.14 (C4); 129.30 (C3, C5); 114.28 (C2, C6); 71.36 (CH2Br); 67.94 (OCH2); 31.83 (CH2CH2CH3); 29.57, 29.54, 29.51, 29.48, 29.31, 29.26, 29.18 (CH2); 25.95 (OCH2CH2CH2); 22.60 (CH2CH2CH3); (33) Wang, B.; Wasielewski, M. R. J. Am. Chem. Soc. 1997, 119, 12-21. (34) Kida, S.; Yoshimoto, S.; Masuda, K.; Kaneko, Y.; Kadokura, K. Chem. Abstr. 1987, 107, 87042q, 604. (35) Eldo, J.; Arunkumar, E.; Ajayaghosh, A. Tetrahedron Lett. 2000, 41, 6241-6244. (36) Nozary, H.; Piguet, C.; Tissot, P.; Bernardinelli, G.; Bu¨nzli, J.C. G.; Deschenaux, R.; Guillon, D. J. Am. Chem. Soc. 1998, 120, 1227412288.

Photoinduced Gelation by Stilbenes 14.03 (CH3). FT-IR (KBr) ν˜ /cm-1: 2924, 2856 (C-H st); 1606, 1509 (arC-C st); 1456 (CH3 δ as); 1303, 1248 (arC-O-Cal st as); 1167 (arC-H δ ip); 1028 (C-O-C st sy); 830 (arC-H δ oop). (4-Dodecyloxybenzyl)-triphenylphosphonium Bromide. The synthesized bromo derivative (4.600 g, 13.0 mmol) and triphenylphosphine (3.728 g, 14.3 mmol) were dissolved in toluene (40 cm3).37 The reaction mixture was stirred at room temperature for 19 h. A white, gelatinous precipitate was filtered off and dried in a vacuum desiccator, resulting in a white, greasy solid (5.540 g, 69%; Rf ) 0 (CH2Cl2)). 1H NMR (300 MHz, CDCl3) δ/ppm: 7.79-7.60 (m, 15 H; ar-CH); 6.99 (d, J ) 8.79 Hz, 2 H; C3-H, C5-H); 6.65 (d, J ) 8.32 Hz, 2 H; C2-H, C6-H); 5.30 (d, J ) 6.83 Hz, 2H; CH2P); 3.85 (t, J ) 6.58 Hz, 2 H; OCH2CH2); 1.73 (p, J ) 6.93 Hz, 2 H; OCH2CH2CH2); 1.40-1.38 (m, 2 H; CH2CH2CH3); 1.26 (s, 16 H; (CH2)8); 0.88 (t, J ) 6.65 Hz, 3 H; CH2CH2CH3). 13C NMR (75 MHz, CDCl3) δ/ppm: 159.06 (C1); 134.77, 134.73, 134.29, 134.16, 132.50, 132.43, 132.24, 132.10, 131.97, 130.03, 129.87, 128.83, 128.58, 128.41, 128.02 (ar-C); 118.29, 118.02, 117.91 (P-C1); 117.15 (C4); 114.64, 114.60 (C2, C6); 67.87 (OCH2); 31.71 (CH2CH2CH3); 29.46, 29.43, 29.40, 29.37, 29.22, 29.14, 29.98 (CH2); 25.81 (OCH2CH2CH2); 22.48 (CH2CH2CH3); 13.93 (CH3). FT-IR (KBr) ν˜ /cm-1: 3055 (arC-H st); 2923, 2853, 2793 (C-H st); 1609, 1510 (arC-C st); 1466 (CH3 δ as); 1303, 1247 (arC-O-Cal st as); 1180, 1160 (arC-H δ ip); 1027 (CO-C st sy); 839 (arC-H δ oop). 4-Dodecyloxy-4′-nitro-trans-stilbene. To a solution of phosphonium salt (5.500 g, 8.9 mmol) in dichloromethane (75 cm3), 4-nitrobenzaldehyde (1.346 g, 8.9 mmol), potassium carbonate (1.353 g, 9.8 mmol), and a few crystals of dibenzo18-crown-6 were added. The Wittig reaction mixture was stirred at room temperature for 7 days. A precipitate was removed by filtration, and a filtrate was evaporated to dryness. The resulting solid was purified by column chromatography (silica gel; CH2Cl2) to obtain a mixture of isomers of 4-dodecyloxy-4′-nitrostilbene (3.317 g, 91%). The crude mixture was recrystallized from hot ethanol to give the trans isomer as yellow crystals (1.194 g, 33%; Rf ) 0.38 (petrolether/CH2Cl2 3/2); mp 80-81 °C). 1H NMR (300 MHz, CDCl3) δ/ppm: 8.22 (d, J ) 8.85 Hz, 2 H; C3′s H, C5′sH); 7.61 (d, J ) 8.81 Hz, 2 H; C2′sH, C6′sH); 7.50 (d, J ) 8.72 Hz, 2 H; C2sH, C6sH); 7.24 (d, J ) 16.39 Hz, 1 H; dCH); 7.02 (d, J ) 16.29 Hz, 1 H; dC′H); 6.94 (d, J ) 8.76 Hz, 2 H; C3sH, C5sH); 4.01 (t, J ) 6.56 Hz, 2 H; OCH2CH2); 1.861.77 (m, 2 H; OCH2CH2CH2); 1.50-1.43 (m, 2 H; CH2CH2CH3); 1.29 (s, 16 H; (CH2)8); 0.90 (t, J ) 6.44 Hz, 3 H; CH2CH2CH3). 13C NMR (75 MHz, CDCl ) δ/ppm: 159.77 (C4); 146.26 (C4′); 3 144.23 (C1′); 132.89 (C2′, C6′); 128.59 (C1); 128.28 (C2, C6); 126.33 (C3′, C5′); 124.02, 123.77 (dCH); 114.76 (C3, C5); 68.03 (OCH2); 31.80 (CH2CH2CH3); 29.54, 29.52, 29.47, 29.45, 29.27, 29.23, 29.10 (CH2); 25.90 (OCH2CH2); 22.57 (CH2CH2CH3); 14.00 (CH3). FTIR (KBr) ν˜ /cm-1: 2922, 2851 (CsH st); 1589 (NO2 st as); 1510 (NdO st); 1469 (CH3 δ as); 1342 (NO2 st sy); 1271, 1250 (arCs OsCal st as); 1175, 1110 (arCsH δ ip); 1024 (CsOsC st sy); 842 (arCsH δ oop); 669 (NO2 δ). 4-Dodecyloxy-4′-nitro-cis-stilbene. After crystallization of the trans isomer, a mixture of geometrical isomers remained in ethanolic solution. Separation of 0.100 g of the isomer mixture by preparative layer chromatography (silica gel; petrolether/CH2Cl2 3/2) resulted in the isolation of yellow solids of the trans isomer (0.036 g, 36%; Rf ) 0.38 (petrolether/CH2Cl2 3/2); mp 80-81 °C) and of the cis isomer (0.064 g, 64%; Rf ) 0.54 (petrolether/CH2Cl2 3/2); mp 41-42 °C). 1H NMR (300 MHz, CDCl3) δ/ppm: 8.08 (d, J ) 8.78 Hz, 2 H; C3′sH, C5′sH); 7.41 (d, J ) 8.71 Hz, 2 H; C2′sH, C6′sH); 7.13 (d, J ) 8.62 Hz, 2 H; C2sH, C6sH); 6.77 (d, J ) 8.89 Hz, 2 H; C3sH, C5sH); 6.73 (d, J ) 13.03 Hz, 1 H; dCH); 6.50 (d, J ) 12.11 Hz, 1 H; dC′H); 3.93 (t, J ) 6.50 Hz, 2 H; OCH2CH2); 1.82-1.72 (m, 2 H; OCH2CH2CH2); 1.46-1.39 (m, 2 H; CH2CH2CH3); 1.26 (s, 16 H; (CH2)8); 0.88 (t, J ) 6.61 Hz, 3 H; CH2CH2CH3). 13C NMR (75 MHz, CDCl3) δ/ppm: 158.90 (C4); 146.30 (C4′); 144.64 (C1′); 133.48 (C2′, C6′); 130.09, 129.46 (dCH); 128.10 (C1); 126.18 (C2, C6); 123.48 (C3′, C5′); 114.35 (C3, C5); 67.94 (OCH2); 31.81 (CH2CH2CH3); 29.55, 29.53, 29.49, 29.47, 29.29, 29.24, 29.13 (CH2); 25.94 (OCH2CH2); 22.58 (CH2CH2CH3); 14.01 (CH3). FT-IR (KBr) ν˜ /cm-1: 2919, (37) Chen, L.; Geiger, C.; Perlstein, J.; Whitten, D. G. J. Phys. Chem. B 1999, 103, 9161-9167.

Langmuir, Vol. 21, No. 7, 2005 2759 2851 (CsH st); 1653 (CdC st); 1592 (NO2 st as); 1509 (NdO st); 1474 (CH3 δ as); 1339 (NO2 st sy); 1254 (arCsOsCal st as); 1177, 1108 (arCsH δ ip); 857 (arCsH δ oop); 669 (NO2 δ). Ethyl-N-[4-(4′-dodecyloxy)-trans-stilbene]-oxamate (1). To a solution of an isomer mixture of 4-dodecyloxy-4′-nitrostilbene (1.420 g, 3.5 mmol) in acetone (9 cm3), an aqueous solution (2 cm3) of ammonium chloride (0.351 g, 6.6 mmol) was added.38 When the mixture was heated to boiling, the oil bath was removed and zinc (0.700 g, 10.7 mmol) was added in small portions. After the reaction subsided, an additional amount of zinc (0.350 g, 5.3 mmol) was added. The reaction mixture refluxed for 20 h. A precipitate was filtered off while hot, and the filtrate was evaporated to dryness. The crude residue was suspended in water, and 2 M NaOH was added dropwise until a pH of 10 was reached, followed by extraction of the suspension with dichloromethane. An organic phase was separated and dried (Na2SO4), and the solvent was evaporated to an orange solid (1.116 g, 86%). The crude amine (1.116 g, 2.9 mmol) was dissolved in dichloromethane (70 cm3), and triethylamine (0.45 cm3, 3.2 mmol) was added into the solution.39 The mixture was cooled (-10 °C) under an argon atmosphere. Ethyl oxalyl chloride (0.32 cm3, 2.9 mmol) was added into the cooled mixture, and stirring was continued at -10 °C and at room temperature for 30 min and for 20 h, respectively. The reaction mixture was washed with 2 M HCl. An organic phase was separated and dried (Na2SO4), and the solvent was evaporated to a light brown solid. By preparative layer chromatography (silica gel; CH2Cl2) were isolated pale yellow solids of trans isomer 1 (0.140 g, 10%; Rf ) 0.33 (CH2Cl2); mp 146-148 °C) and of cis isomer 2 (0.597 g, 43%; Rf ) 0.38 (CH2Cl2); mp 79-80 °C). 1H NMR (300 MHz, CDCl3) δ/ppm: 8.88 (s, 1 H; NH); 7.63 (d, J ) 8.62 Hz, 2 H; C3sH, C5sH); 7.49 (d, J ) 8.64 Hz, 2 H; C2sH, C6sH); 7.43 (d, J ) 8.70 Hz, 2 H; C2′sH, C6′sH); 6.98 (dd, 2 H; dCH); 6.89 (d, J ) 8.68 Hz, 2 H; C3′sH, C5′sH); 4.43 (q, J ) 6.47 Hz, 2 H; OCH2CH3); 3.97 (t, J ) 6.55 Hz, 2 H; OCH2CH2); 1.83-1.74 (m, 2 H; OCH2CH2); 1.44 (t, J ) 7.13 Hz, 3 H; OCH2CH3); 1.27 (s, 18 H; (CH2)9); 0.88 (t, J ) 6.61 Hz, 3 H; CH2CH2CH3). 13C NMR (75 MHz, CDCl3) δ/ppm: 160.54 (COOEt); 158.54 (C4′); 153.18 (ArsCONH); 134.73 (C4); 134.70 (C1); 129.29 (C1′); 127.93 (dCH); 127.20 (C2′, C6′); 126.46 (C2, C6); 124.95 (dCH); 119.47 (C3, C5); 114.29 (C3′, C5′); 67.63 (OCH2); 63.22 (OCH2CH3); 31.42 (CH2CH2CH3); 29.15, 29.13, 29.09, 29.07, 28.90, 28.84, 28.78 (CH2); 25.55 (OCH2CH2); 22.18 (CH2CH2CH3); 13.59 (OCH2CH3); 13.50 (CH3). FT-IR (KBr) ν˜ /cm-1: 3375 (NsH st); 2919, 2850 (CsH st); 1720 (CdO st, ester); 1688 (CdO st, amide I); 1608 (arCsC); 1535 (NsCdO st sy, amide II); 1516 (NH δ); 1473 (CH3 δ as); 1384 (CsN st); 1300 (CsO st); 1270, 1255 (arCs OsCal st as); 1174, 1113 (arCsH δ ip); 1026 (CsOsC st sy); 836 (arCsH δ oop). ES-MS m/z: 480.3107 ([M + H]+). Anal. Calcd for C30H41NO4 (479.638): C, 75.12; H, 8.62; N, 2.92. Found: C, 75.25; H, 8.74; N, 3.15. Ethyl-N-[4-(4′-dodecyloxy)-cis-stilbene]-oxamate (2). 1H NMR (300 MHz, CDCl3) δ/ppm: 8.84 (s, 1 H; NH); 7.50 (d, J ) 8.59 Hz, 2 H; C3sH, C5sH); 7.28 (d, J ) 8.67 Hz, 2 H; C2sH, C6sH); 7.16 (d, J ) 8.66 Hz, 2 H; C2′sH, C6′sH); 6.75 (d, J ) 8.72 Hz, 2 H; C3′sH, C5′sH); 6.49 (dd, 2 H; dCH); 4.42 (q, J ) 7.14 Hz, 2 H; OCH2CH3); 3.93 (t, J ) 6.56 Hz, 2 H; OCH2CH2); 1.81-1.72 (m, 2 H; OCH2CH2); 1.43 (t, J ) 7.13 Hz, 3 H; OCH2CH3); 1.26 (s, 18 H; (CH2)9); 0.88 (t, J ) 6.65 Hz, 3 H; CH2CH2CH3). 13C NMR (75 MHz, CDCl3) δ/ppm: 161.02 (COOEt); 158.40 (C4′); 153.69 (ArsCONH); 134.99 (C4); 134.94 (C1); 130.17 (dCH); 130.06 (C2′, C6′); 129.71 (C2, C6); 129.24 (C1′); 127.65 (dCH); 119.50 (C3, C5); 114.24 (C3′, C5′); 67.97 (OCH2); 63.74 (OCH2CH3); 31.92 (CH2CH2CH3); 29.67, 29.64, 29.60, 29.42, 29.35, 29.29 (CH2); 26.06 (OCH2CH2); 22.69 (CH2CH2CH3); 14.12 (OCH2CH3); 14.01 (CH3). FT-IR (KBr) ν˜ /cm-1: 3365 (NsH st); 2922, 2850 (CsH st); 1730 (CdO st, ester); 1699 (CdO st, amide I); 1607, 1591 (arCsC); 1534 (NsCdO st sy, amide II); 1508 (NH δ); 1470 (CH3 δ as); 1368 (CsN st); 1293 (CsO st); 1249 (arCs OsCal st as); 1179, 1114 (arCsH δ ip); 1024 (CsOsC st sy); 833 (arCsH δ oop). ES-MS m/z: 480.3116 ([M + H]+). Anal. Calcd for C30H41NO4 (479.638): C, 75.12; H, 8.62; N, 2.92. Found: C, 75.29; H, 8.69; N, 3.09. (38) Boyer, J. H.; Alul, H. J. Am. Chem. Soc. 1959, 81, 2136-2137. (39) Cha, X.; Ariga, K.; Onda, M.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 11833-11838.

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Gelation Experiments. A suspension of the gelator (5 mg) in a measured volume of an appropriate solvent was heated to boiling and then cooled to room temperature. Depending on the solvent, a clear solution remained or a precipitate or a gel formed. A minimum gelation concentration was estimated by adding 100-µL volume portions of the solvent into suspension until a low-viscosity solution, instead of a gel, formed during the cooling of the hot solution. TEM Investigation. Transmission electron micrographs were taken on an EM-10 Zeiss transmission electron microscope from a small amount of the unstained gel sample (1/ethanol, c ) 2.0 × 10-2 mol dm-3) put on a carbon coated grid (copper, 100 mesh). Photoisomerization in Solution. UV-vis absorption measurements were carried out with a Varian spectrophotometer (model CARY 3). Conventional quartz cells (10 mm × 10 mm) were used throughout. Spectra were recorded during 300-min irradiation of the sample (c ) 2.0 × 10-5 mol dm-3, ethanol). The sample was irradiated by an Elektrokovina high-pressure mercury lamp (type Ballast DHM 520-250) without a filter (250 nm < λ < 520 nm) or with a glass filter (320 nm < λ < 520 nm). Photoinduced Gelation. FT-Raman spectra of the sample (cis compound 2, c ) 2.0 × 10-2 mol dm-3, ethanol) in a quartz cell (10 mm × 10 mm) were recorded during 90-min irradiation (250 nm < λ < 520 nm). To obtain a spectrum of trans compound

Miljanic´ et al. 1 in solution (c ) 2.0 × 10-2 mol dm-3, ethanol), the cell was thermostated at 75 ( 1 °C by means of a Medingen Dresden thermostat (model U3). All FT-Raman spectra were corrected by subtracting the solvent spectrum. FT-IR spectra of the sample (cis compound 2, c ) 2.0 × 10-2 mol dm-3, ethanol-d6) were recorded at the same time intervals of irradiation (250 nm < λ < 520 nm) as the FT-Raman spectra. A calcium fluoride cell with a thickness of 0.5 mm was used for handling the sample. FT-IR spectra were corrected by subtracting the solvent spectrum. 1H NMR spectra of the sample (cis compound 2, c ) 2.0 × 10-2 mol dm-3, ethanol-d6) were taken before and after 60-min irradiation (250 nm < λ < 520 nm) in a quartz tube (diameter 5 mm) at the 300-MHz resonance frequency. The spectrum at 75 °C was obtained by heating the sample in the instrument.

Acknowledgment. This research was supported by the Croatian Ministry of Science, Education and Sport, Project Nos. 0119641 and 0098053. The authors thank Dr. N. Ljubesˇic´, Rudier Bosˇkovic´ Institute, Zagreb, Croatia, for the TEM experiments and Dr. M. Cindric´, PLIVA, Zagreb, Croatia, for the MS experiments. LA047183D