Liquid Crystals as Organogelators: Liquid Crystals Gelled Organic Liquids

JOURNAL OF OLEO SCIENCE Copyright ©2006 by Japan Oil Chemists’ Society J. Oleo Sci., Vol. 55, No. 10, 545-549 (2006) JOS RAPID PAPER Liquid Crystals as Organogelators: Liquid Crystals Gelled Organic Liquids Kanji KUBO1 , Hajime TAKAHASHI2 and Haruko TAKECHI2 1 School of Dentistry, Health Sciences University of Hokkaido (1757 Kanazawa, Ishikari-Tobetsu, Hokkaido 061-0293, JAPAN) 2 Faculty of Pharmaceutical Sciences, Health Sciences University of Hokkaido (1757 Kanazawa, Ishikari-Tobetsu, Hokkaido 061-0293, JAPAN) Accepted July 3, 2006 (received for review March 22, 2006) Abstract: Gelation tests of rod like liquid crystals (LCs), columnar LCs, and cholesteric LCs rod-like have been employed. 4-Cyanophenyl 4-n-alkoxybenzoates, 4-cyano-4’alkoxybiphenyls, 4,4’-dialkanoyloxybiphenyls, azoxybenzene derivatives, 2,3,6,7,10,11hexaalkoxytriphenylenes, and cholesteryl esters, except for cholesteryl alkyl carbonates and cholesteryl esters with alkenyl side chain, gelled organic liquids. By using these LCs as organogelator, terpene and perfume gels containing 95% or more of terpenes and essential oil could be prepared. Key words: liquid crystal, organogelator, gel, terpene gel, perfume gel 1 Introduction Self-assembled systems are of great significance particularly for their potential application to nanomaterials such as liquid crystals (LCs) and gelators. Numerous studies have been dedicated to the structural investigation to the determination of the molecular aggregation mechanisms. The liquid crystalline compounds more than 90000 are reported till now (1). As a design of the molecule which has mesomorphic properties, it is known that it is desirable that there are an rigid core and flexible alkyl side chains as for the molecular design of liquid crystals. LCs is divided roughly by structures, such as rod like LCs, columnar LCs, and cholesteric LCs (2). On the other hand organogels are of great significance particularly for their potential application to template for materials synthesis, drug delivery, separations, cosmetics, sensors and biomimetics etc (3). The number of organogelators has rapidly increased over 15 years. In the past, new organogelators often have been discovered accidentally and their studies have been dedicated to understanding the relation between the structure of gelators and gelation behaviors (4-8). The aggregation of organogelators into fibrous networks is driven by multiple, weak interactions such as dipole-dipole, van der Waals, hydrogen-bonding and p-stacking interactions. Gelators are generally classified by their driving force for molecular aggregations into two categories of non-hydrogen bond-based and hydrogen bond-based gelators. Amide compounds, such as amino acid and urea, hydroxyl compounds such as 12-hydroxystearic acid and sugars belong to hydrogen bond-based gelators. While anthracene, cholesterol (9), and tropone derivatives (10) belong to non-hydrogen bond-based gelators. Recently we have reported that some liquid crystalline compounds with bitropone and cyanotropone core gelled organic liquids (9-11). We named such Correspondence to: Kanji KUBO, School of Dentistry, Health Sciences University of Hokkaido, 1757 Kanazawa, Ishikari-Tobetsu, Hokkaido 061-0293, JAPAN E-mail: kubo-k@hoku-iryo-u.ac.jp Journal of Oleo Science ISSN 1345-8957 print / ISSN 1347-3352 online http://jos.jstage.jst.go.jp/en/ 545 K. Kubo, H. Takahashi and H. Takechi a molecule ‘organogelling LC’. As some organogelling LCs, cholesteryl benzoate derivatives (9), bitropone derivatives (10), 2-(3,4,5-trialkoxybenzoylamino)-5cyanoropones (11, 12), hexaazatriphenylenes (13), adiketonato copper complex (14), octa(dodecyl)tetrapyrazinoporphyrazine (15), 3,4,5-trialkoxybenzoylamines (16), etc. have been reported. In development of LCs, various organogelators have been found. This might mean that most LCs can be used as an organogelator. Here we report the organogelation ability of some typical LCs such as 4-cyanophenyl 4-n-alkoxybenzoates (1_n), 4-cyano-4’- alkoxybiphenyls (2_n), 4,4’-dialkanoyloxybiphenyls (3_n), azoxybenzene derivatives (4a-c), 2,3,6,7,10,11-hexaalkoxytriphenylenes (5_n), and cholesteryl esters (6a-h). 2 Experimental 2 1 Materials Azoxybenzene derivatives (4a-c) and cholesteric esters (6a-h) were reagent grade (Tokyo Kasei Co.) and were used without further purification. The synthetic procedure for the other compounds has already been described in our previous papers (17-19). For the gelation test, the synthetic compounds which gave satisfactory elemental analysis data were used. The results of the elemental analyses of 1_n, 2_n, 3_n, and 5_n are as follows. 1_12; Found: C, 76.43; H, 8.14; N, 3.42%. Calcd for C26H33NO3: C, 76.62; H, 8.16; N, 3.44%. 1_16; Found: C, 77.48; H, 8.64; N, 3.06%. Calcd for C30H41NO3: C, 77.71; H, 8.91; N, 3.02%. 2_12; Found: C, 82.48; H, 9.42; N, 3.70%. Calcd for C25H33NO: C, 82.60; H, 9.15; N, 3.85%. 2_16; Found: C, 83.28; H, 9.82; N, 3.40%. Calcd for C29H41NO: C, 83.00; H, 9.85; N, 3.34%. 3_7; Found: C, 76.60; H, 8.73%. Calcd for C28H38O4: C, 76.68; H, 8.73%. 3_11; Found: C, 78.53; H, 9.86%. Calcd for C36H54O4: C, 78.50; H, 9.88%. 3_15; Found: C, 79.78; H, 10.64%. Calcd for C44H70O4: C, 79.71; H, 10.64%. 5_8; Found: C, 79.18; H, 11.12%. Calcd for C 66 H 108 O 6 : C, 79.46; H, 10.91%. 5_12; Found: C, 79.88; H, 12.02%. Calcd for C90H156O6: C, 81.02; H, 11.79%. 5_16; Found: C, 81.98; H, 12.59%. Calcd for C114H204O6: C, 81.95; H, 12.31%. 2 2 Gelation of Organic Liquids with LCs LCs (30 mg) were weighed accurately into a screw cap vial. An amount of 0.3 cm 3 organic liquid (methanol, ethanol, 1-butanol, 1-hexanol, 1-octanol, 1decanol, n-hexane, n-octane, n-decane, n-dodecane, ntetradecane, n-hexadecane, acetonitrile, acetone, chloroform, ethyl acetate, salad oil) was added. The screw cap of the vial was closed and heated at 120 until a clear solution was obtained. The vial was left at 25 in an incubator for one hour. The gelation was considered successful when upon inversion there was no fluid running down the walls of the vial. 2 3 Preparation of Terpene and Perfume Gels LCs (1_16, 3_15, 5_12, 6a_17: 50 mg) were weighed accurately into a screw cap vial. An amount of 1.0 cm3 geraniol or la was added. The screw cap of the vial was closed and heated at 120 until a clear solution was Scheme 1 546 J. Oleo Sci., Vol. 55, No. 10, 545-549 (2006) Liquid Crystals as Organogelators Table 1 Gelation Tasts of 1-6. 1_12 Methanol Ethanol Butanol Hexanol Octanol Decanol Hexane Octane Decane Dodecane Tetradecane Hexadecane Acetonitrile Acetone Chloroform Ethyl Acetate Salad Oil 2_16 3_7 3_11 3_15 4a 4b 4c Cr Cr G G G G G G G G G G Cr Sol Sol Sol G Cr Cr G G G G G G G G G G G Cr Sol Cr G Insol G G G G G Cr Cr Cr G G G G Sol Sol Sol G Insol G G G G G Cr Cr G G G G G Cr Sol Cr G Cr Cr G G G G Cr Cr G G G G Cr Cr Sol Sol G Cr G G G G G Cr G G G G G G Cr Sol Cr G Cr G G G G G Cr Cr G G G G Cr Cr Sol Cr G Cr Cr Cr Cr Cr G Insol Insol Insol Insol Insol Insol Cr Sol Sol Sol G Sol Insol Cr Cr Cr G Cr Cr Cr Cr Cr Cr Cr Cr Sol Cr G G G G G G G Insol Insol Insol Insol G G G Sol Sol Sol G 5_12 5_16 6a_6 6a_8 6a_9 6a_11 6a_13 6a_15 6a_16 Insol Cr G G G G Sol Sol Sol Sol Sol Sol Insol G Sol Sol Sol Insol Insol G G G G Sol Sol Cr Cr G G Insol G Sol G G InSol InSol Cr G G G Sol Cr Cr Cr G G InSol Cr Sol Cr G Insol Cr G G G G Cr Cr Cr Cr Cr Cr Cr Cr Sol Cr G Insol Cr G Cr Sol Cr Sol Sol Sol Sol Sol Sol Cr Cr Sol Sol Cr Insol Cr G G Sol Sol Sol Sol Sol Sol Sol Sol Cr Cr Sol Cr Cr Insol Cr G G G G Sol Sol Sol Sol Sol Sol Cr Cr Sol Cr G Insol Cr G G G G Sol Sol Sol Cr Cr Cr Cr Cr Sol G G Insol Cr G G G G Cr Cr Cr Cr Cr G Cr G Sol G G Insol Cr G G G G Cr Cr Cr Cr Cr Cr Cr G Sol G G 6a_17 Methanol Ethanol Butanol Hexanol Octanol Decanol Hexane Octane Decane Dodecane Tetradecane Hexadecane Acetonitrile Acetone Chloroform Ethyl Acetate Salad Oil 2_12 5_8 Methanol Ethanol Butanol Hexanol Octanol Decanol Hexane Octane Decane Dodecane Tetradecane Hexadecane Acetonitrile Acetone Chloroform Ethyl Acetate Salad Oil 1_16 6b 6c 6d 6e 6f 6g 6h_5 6h_7 6h_9 Insol Insol G G G G Cr G G G G G Cr G Sol G G Insol Cr Sol Sol Sol Sol Sol Sol Sol Sol Sol Sol Cr Sol Sol Sol Sol Insol Cr Sol Sol Sol Sol Sol Sol Sol Sol Sol Sol Cr Cr Sol Sol Sol Insol G G G Cr Cr Cr Cr Cr Cr Cr Cr Cr Cr Sol Cr Cr Insol G G G G G Cr Cr Cr Cr Cr Cr Cr Cr Sol Cr Cr Insol G G G G G Sol Sol Sol Sol Sol Sol Cr Cr Sol Sol G Insol G G Sol Sol Sol Sol Sol Sol Sol Sol Cr Cr Sol Sol Sol Sol Insol Cr Cr Cr Cr Cr Sol Sol Sol Sol Sol Cr Cr Cr Sol Sol Sol Insol Cr Cr Sol Sol Sol Sol Sol Sol Sol Sol Sol Cr Sol Sol Sol Sol Insol Cr Cr Cr Cr Cr Sol Sol Sol Sol Sol Sol Cr Cr Sol Sol Cr Cr: Crystallization, Sol: Solution, Insol: Insoluble, G: Gelation, 30 mg 1-6 / 0.3 cm3 organic liquid 547 J. Oleo Sci., Vol. 55, No. 10, 545-549 (2006) K. Kubo, H. Takahashi and H. Takechi obtained. The vial was left at 25 one hour. 3 in an incubator for Results and Discussion Various representative LCs such as 4-cyanophenyl 4n-alkoxybenzoates (1_n), 4-cyano-4’-alkoxybiphenyls (2_n), 4,4’-dialkanoyloxybiphenyls (3_n), azoxybenzene derivatives (4a-c), 2,3,6,7,10,11-hexaalkoxytriphenylenes (5_n), and cholesteryl esters (6a-h) were prepared. Gelation abilities of rod like LCs (1-4), columnar LCs (5) and cholesteric LCs (6a-h) for a range of organic liquid were examined by dissolving 30 mg of compound in 0.3 cm3 of the desired liquid under heating. Upon cooling to 25 , a gel, precipitate or clear solution was observed, depending on the liquid. The results of gelation tests were shown in Table 1. Interestingly, rod like LCs gelled various organic liquids. The smectic A (SmA) LCs such as 4-cyanophenyl 4-n-alkoxybenzoates (1_n) and 4-cyano-4’-alkoxybiphenyls (2_n) and SmB LCs such as 4,4’-dialkanoyloxybiphenyls (3_n) gelled alcohols, hydrocarbons, acetonitrile, and salad oil. 4,4’-Dialkoxyazoxybenzenes (4a, b) gelled 1-decanol and salad oil, while the ethoxycarbonyl derivative (4c) gelled alcohols (C1-C10) and hydrocarbons, acetonitrile, and salad oil. The gelation ability of azoxybenzenes (4) depended on the nature of the connection group between an azoxybenzene core and alkyl side chains. These results should be a first report that rod like LCs can be used as a organogelator. Columnar LCs (5_n) gelled alcohols, hydrocarbons, acetone, ethyl acetate, and salad oil. However LC (5_8) did not gelate with the hydrocarbons, since LC (5_8) is dissolved in hydrocarbon. Cholesteric LCs (6a_n, 6d-g), except for cholesteryl esters with alkenyl side chain (6b, 6c) and cholesteryl alkyl carbonates (6h_n), gelled organic liquids. Cholesteryl esters (6a_n) with long chain alkyl group gelled many kinds of organic liquids than ones with short chain alkyl group. However the gelation behaviours of columnar LCs and cholesteric LCs have already been reported (9-17). As an application to new materials of organogelators, terpene and perfume gels were prepared. In perfume, fragrance and deodorant goods, water-soluble gelators such as carragheenan, agar, collagen, gellan gum and gelatin etc have been used for the gelations of water containing terpenes, essential oils and perfumes. So, the purity of the oils in the gels is low and most of components of these goods are water (3). Fortunately, under the conditions of gel-organic solution (50 mg / 1 cm3), LCs (1_16, 3_15, 5_12, 6a_17) could gel the terpenes and essential oil such as citronellol, geraniol, and lavender oil (Fig. 1). 4 Conclusion In conclusion, almost all the LCs except for cholesteryl alkyl carbonates and cholesteryl esters with alkenyl side chain gelled some organic liquids. This result means that most LCs may have gelation ability for organic solution. From the study of liquid crystals, many organogelators will be synthesized and found out in the future. While we succeeded in preparation of the perfume gels containing 95% or more of terpenes and essential oil. Their gels will be utilized as fragrance and Fig. 1 Gels of (a) 1_16-citronellol, (b) 3_15-citronellol, (c) 5_12citronellol, (d) 6a_17-citronellol, (e) 1_16-lavender oil, (f) 3_15-lavender oil, (g) 5_12-lavender oil, (h) 6a_17-lavender oil. All concentration is 5.0 wt%. 548 J. Oleo Sci., Vol. 55, No. 10, 545-549 (2006) Liquid Crystals as Organogelators deodorant agents. Furthermore these organogelators might be applied to organic analysis kit (20) for liquid organic component. 10. Acknowledgment 11. This work was partially supported by the funds from Ministry of Education, Science, Sports and Culture of Japan. 12. 13. References 1. V. VILL, LiqCryst 4.4-Database of Liquid Crystalline Compounds, Fujitsu Kyushu System Engineering (FQS), Fukuoka (2003). 2. A. MORI and K. KUBO, Liquid Crystals in Materials Organic Chemistry (M. IYODA, ed.), Asakura Syoten, Tokyo, pp. 71-110 (2002). 3. K. KUBO and A. MORI, Synthesis and Gelation Properties of N,N’-bis(3,4,5-trialkoxy)benzoylurea: Terpene and Perfume Gels, Chem. Lett., Vol. 34, 1250-1251 (2005). 4. P. TERECH and R.G. WEISS, Low-Molecular Mass Gelators of Organic Liquids and the Properties of their Gels, Chem. Rev., Vol. 97, 3133-3159 (1997). 5. K. HANABUSA and H. 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