Synthesis of Supramolecular Organogels Derived ...

This article was downloaded by: [University of Glasgow] On: 10 July 2013, At: 13:06 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/lsyc20

Synthesis of Supramolecular Organogels Derived from Urea and Bisurea Derivatives of Dehydroabietylamine a

a

b

Afshan Aslam , Imran Ali Hashmi , Viqar-uddin Ahmed & Firdous Imran Ali

a

a

Department of Chemistry , University of Karachi , Karachi , Pakistan b

International Center for Chemical and Biological Sciences, University of Karachi , Karachi , Pakistan Published online: 08 Jul 2013.

To cite this article: Afshan Aslam , Imran Ali Hashmi , Viqar-uddin Ahmed & Firdous Imran Ali (2013) Synthesis of Supramolecular Organogels Derived from Urea and Bisurea Derivatives of Dehydroabietylamine, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 43:20, 2824-2831, DOI: 10.1080/00397911.2012.745157 To link to this article: http://dx.doi.org/10.1080/00397911.2012.745157

PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,

Downloaded by [University of Glasgow] at 13:06 10 July 2013

systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/termsand-conditions

Synthetic Communications1, 43: 2824–2831, 2013 Copyright # Taylor & Francis Group, LLC ISSN: 0039-7911 print=1532-2432 online DOI: 10.1080/00397911.2012.745157

SYNTHESIS OF SUPRAMOLECULAR ORGANOGELS DERIVED FROM UREA AND BISUREA DERIVATIVES OF DEHYDROABIETYLAMINE Afshan Aslam,1 Imran Ali Hashmi,1 Viqar-uddin Ahmed,2 and Firdous Imran Ali1 1

Department of Chemistry, University of Karachi, Karachi, Pakistan International Center for Chemical and Biological Sciences, University of Karachi, Karachi, Pakistan


2

GRAPHICAL ABSTRACT

Abstract The gelation ability of diterpenes was investigated by applying aromatic linker steroid strategy. Four new mono (1) and bis-urea (2–4) derivatives of dehydroabietylamine (DAA) (a tricyclic diterpene amine) were synthesized on reaction of respective isocyanates with DAA and characterized through spectroscopic data. Three of these (1, 2, and 4) were obtained as low-molecular-weight organogelators that can form thermally reversible organogels. Supplementary materials are available for this article. Go to the publisher’s online edition of Synthetic Communications1 for full experimental and spectral details. Keywords Bisurea; dehydroabietylamines; diterpenes; organogels; urea

INTRODUCTION In recent years the interests of researchers have been increased in gels[1] because of their wide applications in biomedicine,[2] nanotechnology,[3] and molecular Received October 4, 2012. Address correspondence to Firdous Imran Ali, Department of Chemistry, University of Karachi, Karachi 75270, Pakistan. E-mail: fi[email protected]; fi[email protected]

2824


ORGANOGELS FROM DEHYDROABIETYLAMINE

2825

electronics.[4] On the basis of solvent type, trapped by gelators, these can be classified as hydrogels (water as solvent) and organogels (organic solvent). Organogels are prepared by heating the organogelators in organic solvents and then cooling the solution to room temperature. Gelation occurs because of trapping of the organic solvent by the gelator in cross-linked three-dimensional networks. The gelation of organic solvent is believed to proceed through the self-assembly of the gelator molecules into fibers. The self-assembled fibrous networks of most organogels are formed thermoreversibly by cooling a solution and dissolved by heating the gel above a transition temperature (Tg).[5] The organogelators have been further classified as polymer gelators and low-molecular-weight gelators (give rise to supramolecular organogels). In lowmolecular-weight gelators, only physical interactions are possible, which make them thermally reversible. The interactions that are possible in low-molecular-weight organogelators (LMWOGs) are van der Waal’s interaction, p–p stacking, dipole–dipole interaction, electrostatic interaction, and hydrogen bonding. An organogelator must have at least one moiety that can participate in any of these interactions[7] beside the appropriate hydrophobic part. Therefore, the ability of a molecule to gelate depends upon the ratio of hydrophobic part of the molecule having large surface area to produce van der Waal interaction and=or p–p stacking and polar functionality, providing H-bonding, dipole–dipole, and=or electrostatic interactions. In the past decade, the number of LMWOGs have been reported using naturally occurring molecules including steroidal derivatives,[8] amino acids,[9] fatty acids,[10] carbohydrates,[11] etc., beside various other systems. Weiss et al. were the first to utilize a natural product (cholesterol) to prepare a low-molecular-weight organogelator and introduce the aromatic linker steroid (ALS) system to promote gelation in a molecule. Because of chemical and physical properties of steroids, they have been extensively and successfully utilized to synthesize gelators (especially LMWOGs).[12] Next to steroids, triterpene-based gelators have also been reported.[13] Diterpene-based gelators are hitherto unreported, although they present polarity profiles similar to steroids and possess functional moieties to be chemically altered. We therefore considered investigating diterpene-based gelators employing dehydroabietylamine (DAA). The present studies reports the synthesis, characterization, and gelation test of four new urea and bis-urea derivatives of dehydroabietylamine. RESULTS AND DISCUSSION DAA is a tricyclic diterpene belonging to the abietane series of diterpenoids. Usually these compounds occur in oleoresin of conifers.[14] Resin acid (including abietane and dehydroabitane), constituting a major nonvolatile part of coniferous oleoresin, is thought to be antioxidant protective for these trees. Further, a number of researchers have studied biological activities of abietic acid and dehydroabietic acid and reported them as anticancer,[15] antiulcer,[16] antiviral,[17] antifungal,[18] and anti-inflammatory[19] agents. Usually the ALS strategy utilizes a natural product (steroid or triterpene) as a hydrophobic part linked with an aromatic part or alkyl chain (spacer) through ester,[20] amide,[21] urea, or bisurea[22] to furnish H-bonding.


2826

A. ASLAM ET AL.

Substituted urea molecules are known to exhibit intermolecular H-bonding even in dilute solutions, and therefore urea and bis-urea moieties have been extensively and successfully investigated for the formation of organogels. In the light of these facts, we intended to prepare urea and bis-urea derivatives of DAA and investigate their gelation behavior. Supramolecular organogels of DAA employing the ALS strategy (S stands for steroid and here it can be replaced by D for diterpene) have been prepared by introducing urea or bis-urea as a linker group via reaction of DAA with isocyanates. Urea and bis-urea gelator (1–4) were synthesized by mixing DAA and respective isocyanates in ethanol at room temperature and then stored in a refrigerator overnight (Scheme 1) All gelators were obtained as white solids, the mono-urea derivative (1) formed with 1-naphthyl isocyanate as a gel, which was converted into sol at room temperature. This behavior of 1 may be due to p–p stacking of the naphthyl group that decreases competition of H-bonding between urea moieties and solvent (EtOH). For the preparation of bis-urea derivatives, tolylene-2,4-diisocyanate, 1,3-bis(1isocyanato-1 methyl ethyl benzene), and hexamethylene diisocyante have been used. The confirmation of the formation of compounds 1–4 was done by highresolution mass spectrometry–electrospray ionization (HRMS-ESI) with the [M þH]þ peaks at m=z 455.3058, m=z 745.5418, m=z 739.5869, and m=z 815.6579 corresponding to molecular formulas C31H39N2O, C49H69N4O2, C54H79N4O2, and C48H75 N4O2, respectively. In the infrared (IR) spectrum of each, absence of isocyanate absorption at 2257 cm1 and characteristic absorptions of amide groups around 1640 cm1 and 1560 further confirmed the formation of not only substituted urea derivatives but intermolecular H-bonding as well. The 1H NMR data of all four were

Scheme 1. Synthesis of mono- and bis-urea derivatives of DAA.



2827

also in accordance with the expected structures. Gelation of compounds 1–4 was investigated in different polar and nonpolar organic solvents (Table 1). Compounds 1, 3, and 4 formed transparent gels in 1 wt% solution. These gels are stable up to several weeks under ambient conditions. Gelation was confirmed by the test tube inversion method.[23] At 1 wt% gelation occurred after refrigerating samples overnight, since at 5 wt% gels formed on cooling within 5 to 10 min. Gelation occurs on heating the gelator with a solvent followed by cooling, and this cycle of sol-gel can be repeated several times. Solubilities and gelation behaviors of all compounds are remarkably different. Compound 1, a mono-urea derivative containing, a naphthyl group, is soluble in all nonpolar solvents indicated in Table 1: hence gelation did not occur in highly polar solvents (DMF, acetonitrile, and acetone) and in chloroform and tetrohydrofuran (THF). In contrast to mono-urea derivative 1, diurea compounds 2 and 3 are soluble in polar organic solvents and chloroform (Table 1) because of their increased polarity. Although both compounds show similarities in solubility behavior, compound 3 is dissolved in toluene and xylene on heating and formed a gel on cooling at 1 wt%, whereas compound 2 did not. This may be attributed to the increased hydrophobic part (alkyl groups) in compound 3 that results in an increase in intermolecular van der Waal’s interaction beside the flexibility in the structure. Compound 4, a bis-urea organogelator containing an aliphatic spacer group, is soluble in most of the solvents except acetonitrile and acetone. Gelation does occur in the majority of solvents including MeOH and EtOH as expected due to decreased competition for H bonding between solvent and gelator molecules. Gelation was not observed in acetone, acetonitrile, DMF, and THF up to 10 wt% like other gelators. All the gels are thermally reversible, composed of low-molecular-weight organogelators that result in supramolecular assemblies due to noncovalent interactions. Tgel(gel-sol transition temperature) is measured by the ball dropping method. Organogels of compound 3 are thermally stable up to 100 C. This shows that different spacer groups attached to the linker have an impact on the properties of gelation system.

Table 1. Gelation properties at 1 wt% No. 1 2 3 4 5 6 7 8 9 10 11

Solvents

Compound 1

Compound 2

Compound 3

Compound 4

Hexane Toluene Xylene Chloroform Ethyl acetate Methanol Ethanol THF DMF Acetone Acetonitrile

G G G S G G G S S S S

I I I S I PS PS S S S S

I G G S I PS PS S S S S

G G G G G G G S S I I

Notes. G, gel; I, insoluble; PS, partially soluble; S, solution.

2828

A. ASLAM ET AL.


Figure 1. SEM images of xerogels at 1 wt% (1 in ethyl acetate, 3 and 4 in toluene).

The difference in gelation behavior was also evident from morphologies of different gels. The morphologies of 1 wt% organogels were studied using scanning electron microscopy (SEM). SEM image of compound 1(Fig. 1) in ethyl acetate revealed a typical fibrillar network associated with ALS gelators. On the basis of the SEM image, it can be classified as a dry gel with crystalline property as per the explanation of Jeans et al.[22] The SEM image of bis-urea gelators 3 showed large planar sheet-like structures similar to bis-urea derivatives reported earleir. On the other hand, the SEM of bis-urea gelator 4 with a long flexible alkyl chain showed the presence of large planar sheet-like structure along with rod-like structures. Electron micrographs of both gelators 3 and 4 indicating strong H bonding and well-ordered arrangement of molecules,[24] which is further supported by high thermal stability. The SEM images of gels from different solvents were found to be similar, rejecting the possibility of polymorphism. EXPERIMENTAL General Procedure for Synthesis of Urea and Bis-urea Organogelators Dehydroabietylamine (1 mmol) is dissolved in ethanol, and the respective isocyanate (1 mmol) is added. After addition, the reaction mixture is kept in a refrigerator overnight, and no precipitation occurred. On evaporation under reduced pressure, a white solid is obtained in more than 90% yield as pure compound. For further purification, the resulted urea and bis-urea compounds were subjected to column chromatography using hexane–ethyl acetate in order of increasing polarity. Characterization of Organogelator (1) Rf: 0.422 (hexane=ethylacetate, 8:2), white solid, m.p: 145 C, yield: 98.5%; HRMS-ESI: [M þH]þ at m=z 455.3058, calcd. for C31H39N2O: 455.3062; IR (cm1): 3328 (N–H stretching), 2927 (C-H), 2866 (C-H), 1640 (C=O), 1560 (N–H bending), 1460 (CH2 bending), 1381 (CH3 bending); 1H NMR (300 MHz, CDCl3) d ppm: 0.817 (3H, s, H-19), 1.123 (3H, s, H-20), 1.248 (6H, d, J ¼ 6.9 Hz, H-16 & 17), 2.874 (1H, m, H-15), 3.064 (2H, s, H-18), 6.809 (1H, s, H-14), 6.985 (1H, d, J ¼ 8.1 Hz, H-12), 7.104 (1H, d, J ¼ 8.1 Hz, H-11), 7.206 (1H, d, J ¼ 6.9 Hz, H-50 ), 7.318 (1H, t, J ¼ 7.2 Hz, H-60 ), 7.389 (2H, d, J ¼ 7.8 Hz, H-70 & 9’), 7.457 (1H, t, J ¼ 7.2 Hz,


2829

H-100 ), 7.744 (1H, dd, J ¼ 8.1 & 12.6 Hz, H-110 ), 7.987 (1H, d, J ¼ 8.1 Hz, H-120 ). 13 CNMR (75 MHz, CDCl3) d ppm: 18.46 (C-20), 18.60 (C-2), 18.80 (C-6), 24.05 (C-16), 24.07 (C-17), 25.26 (C-19), 30.19 (C-3), 33.46 (C-15), 36.01 (C-7), 37.34 (C-4), 37.57 (C-10), 38.35 (C-1), 44.99 (C-5), 50.56 (C-18), 122.18 (C-50 ), 122.55 (C-70 ), 123.69 (C-120 ), 124.22 (C-12), 125.72 (C-11), 126.06 (C-110 ), 126.19 (C-100 ), 126.41 (C-14), 126.86 (C-60 ), 128.25 (C-90 ), 133.54 (C-80 ), 134.36 (C-8), 134.76 (C-40 ), 145.39 (C-13), 147.17 (C-9), 157.51 (C-20 ).


Preparation of Organogels For the preparation of the gels, the solvent and the organogelator (1–10 wt%) were heated in a closed vial until a clear solution was obtained. The solution was cooled to room temperature and then kept in a refrigerator. Gelation occured immediately at high concentration (5% w=v). It took 15–20 min at low concentration (1% w=v). Gelation Test The test tube inversion method is used for the gelation experiment in which a test tube containing gel is inverted to check whether the gel flows or not. CONCLUSION To summarize, three new thermoreversible organogels of dehydroabietane have been synthesized employing ALS and A(LS)2 strategy. Urea and bis-urea has been used as linker moieties. It is observed that change in spacer groups, results in variation of gelation behavior, thermal stability, and morphologies. Further, type of linker moiety (urea and bisurea) also influences these properties. SUPPORTING INFORMATION Supplementary data associated with this article, including full experimental details spectroscopic data, and copies of 1H NMR spectra are available at this article’s webpage. ACKNOWLEDGMENT One of the authors is thankful to the Higher Education Commission of Pakistan for providing a fellowship. REFERENCES 1. Abdallah, D. J.; Weiss, R. G. Organogels and low-molecular-mass organic gelators. Adv. Mater. 2000, 12(17), 1237–1247. 2. Sahoo, S.; Kumar, N.; Bhattacharya, C.; Sagiri, S. S.; Jain, K.; Pal, K.; Ray, S. S.; Nayak, B. Organogels: Properties and applications in drug delivery. Designed Monom. Polym. 2011, 14, 95–108.


2830

A. ASLAM ET AL.

3. Makeiff, D. A.; Carlin, R. Nanostructured organogel networks self-assembled from alkylated isophthalic acid and benzimdizalone compounds. Nanotech. 2011, 1, 648–649. 4. Luis, J. P.; Laukhin, V.; Pino, A. P.; Gancedo, J. V.; Rovira, C.; Laukhina, E.; Amabilino, D. B. Supramolecular conducting nanowires from organogels. Angew. Chem., Int. Ed. Engl. 2007, 46(1–2), 238–241. 5. (a) George, M.; Weiss, R. G. Molecular organogels: Soft matter comprised of lowmolecular-mass organic gelators and organic liquids. Acc. Of Chem. Res. 2006, 39(8), 489–497; (b) Huang, X.; Raghavan, S. R.; Terech, P.; Weiss, R. G. Distinct kinetic pathways generate organogel networks with contrasting fractality and thixotropic properties. J. Am. Chem. Soc. 2006, 128, 15341–15352. 6. Maity, G. C. Low-molecular-mass gelators of organic liquids. J. Phys. Sci. 2007, 11, 156–171. 7. Rameshbabu, K.; Zou, L.; Kim, C.; Urbas, A.; Li, Q. Self-organized photochromic dithienylcyclopentene organogels. J. Mater. Chem. 2011, 21, 15673–15677. 8. (a) Ostuni, E.; Kamaras, P.; Weiss, R. G. Novel x-ray method for in situ determination of gelator strand structure: Polymorphism of cholesteryl anthraquinone-2-carboxylate. Angew. Chem., Int. Ed. Engl. 1996, 35, 1324–1326; (b) Lin, Y. C.; Weiss, R. G. Liquidcrystalline solvents as mechanistic probes, 24: A novel gelator of organic liquids and the properties of its gels. Macromolecules 1987, 20, 414–417; (c) Murata, K.; Aoki, M.; Nishi, T.; Ikeda, A.; Shinkai, S. New cholesterol-based gelators with light- and metalresponsive functions. J. Chem. Soc., Chem. Commun. 1991, 1715–1718. 9. (a) Motulsky, A.; Lafleur, M.; Hoarau, A. C. C.; Hoarau, D.; Boury, F.; Benoit, J. P.; Leroux, J. C. Characterization and biocompatibility of organogels based on L-alanine for parenteral drug delivery implants. Biomaterials 2005, 26, 6242–6253; (b) de Vries, E. J.; Kellogg, R. M. Small depsipeptides as solvent gelators. J. Chem. Soc., Chem. Commun. 1993, 238–240; (c) Hanabusa, K.; Tange, J.; Taguchi, Y.; Koyama, T.; Shirai, H. Small molecular gelling agents to harden organic liquids: Alkylamide of N benzyloxycarbonyl-Lvalyl-L-valine. J. Chem. Soc., Chem. Commun. 1993, 390–392; (d) Aggeli, A.; Bell, M.; Boden, N.; Keen, J. N.; Knowles, P. F.; McLeish, T. C. B.; Pitkeathly, M.; Radford, M. Responsive gels formed by the spontaneous self-assembly of peptides into polymeric beat-sheet tapes. Nature 1997, 386, 259–262. 10. Tachibana, T.; Mori, T.; Hori, K. Chiral mesophases of 12-Hydroxyoctadecanoic acid in jelly and in the solid state, I: A new type of lyotropic meophase in jelly with organic solvents. Bull. Chem. Soc. Jpn. 1980, 53, 1714–1719. 11. (a) Yamasaki, S.; Tsutsumi, H. The phase transition in the gel state of the 1,3:2,4-di-obenzylidene-D-sorbitol=ethylene glycol system. Bull. Chem. Soc. Jpn. 1994, 67, 2053– 2056; (b) Hafkamp, R. J. H.; Feiters, M. C.; Nolte, R. J. M. Organogels from carbohydrate amphiphiles. J. Org. Chem. 1999, 64, 412–426; (c) Beginn, U.; Keinath, S.; Moller, M. New carbohydrate amphiphiles, 2: Gel formation and gel morphologies. Macromol. Chem. Phys. 1998, 199, 2379–2384. 12. Terech, P.; Gebel, P.; Ramasseul, R. Molecular rods in a zinc(II) porphyrin=cyclohexane physical gel: Neutron and x-ray scattering characterizations. Langmuir 1996, 12, 4321–4323. 13. (a) Lima, P. F. C.; Liub, X. Y.; Kanga, L.; Hoa, P. C. L.; Chan, S. Y. Physicochemical effects of terpenes on organogel for transdermal drug delivery. Int. J. Pharmaceutics. 2008, 358, 102–107; (b) Lima, P. F. C.; Liub, X. Y.; Kanga, L.; Hoa, P. C. L.; Chan, Y. W.; Chan, S. Y. Limonene GP1=PG organogel as a vehicle in transdermal delivery of haloperidol. Int. J. Pharmaceutics 2006, 311, 157–164. 14. Do, H. S.; Park, J. H.; Kim, H. J. Synthesis and characteristics of photoactivehydrogenated rosin epoxy methacrylate for pressure-sensitive adhesives. J. Appl. Polym. Sci. 2009, 111, 1172–1176.



2831

15. (a) Hee, S. K.; Hyun-Mi, O.; Sung-Kyu, C.; Dong, C. H.; Byoung-Mog, K. Anti-tumor abietane diterpenes from the cones of Sequoia sempervirens. Bioorg. Med. Chem. Lett. 2005, 15, 2019–2021; (b) Minami, T.; Wada, S.; Tokuda, H. Potential antitumorpromoting diterpenes from the cones of Pinus luchuensis. J. Nat. Prod. 2002, 65(12), 1921–1923; (c) Rao, X.; Song, Z.; He, L. Synthesis and antitumor activity of novel a-aminophosphonates from diterpenic dehydroabietylamine. Heteroatom Chem. 2008, 19(5), 512–516. 16. Wada, H.; Kodato, S.; Kawamori, M. Chem. Pharm. Bull. 1985, 33(4), 1472–1487. 17. (a) Tada, M.; Chiba, K.; Okuno, K.; Ohnishi, E.; Yoshii, T. Antiviral diterpenes from Salvia officinalis. Phytochemistry 1994, 35, 539–542; (b) Batista, O.; Simoes, M. F.; Duarte, A.; Valdivia, M. L.; De La Torre, M. C.; Rodriguez, B. An antimicrobial abietane from the root of Plectranthus hereroensis. Phytochemistry 1995, 38, 167–169. 18. Gigante, B.; Santos, C.; Silva, A. M.; Curto, M. J. M.; Nascimento, M. S. J.; Pinto, E.; Pedro, M.; Cerqueira, F.; Pinto, M. M.; Duarte, M. P.; Laires, A.; Rueff, J.; Goncalves, J.; Pegadod, M. I.; Valdeirad, M. L. Catechols from abietic acid: Synthesis and evaluation as bioactive compounds. Bioorg. Med. Chem. 2003, 11, 1631–1638. 19. (a) Shishido, K.; Got, K.; Miyoshi, S.; Takaishi, Y.; Shibuya, M. Synthetic studies on diterpenoid quinones with interleukin-1 inhibitory activity: Total synthesis of ()- and (þ)-triptoquinone A. J. Org. Chem. 1994, 59, 406–414; (b) Zheng, Y. L.; Lin, J. F.; Lin, C. C.; Xu, Y. Anti-inflammatory effect of triptolide. Acta Pharm. Sin. 1994, 15, 540–543. 20. Upadhyay, K. K.; Tiwari, C.; Khopade, A. J.; Bohidar, H. B.; Jain, S. K. Sorbitan ester organogels for transdermal delivery of sumatriptan. Drug Dev. Ind. Pharm. 2007, 33(6), 617–25. 21. Hanabusa, K.; Tanaka, R.; Suzuki, M.; Kimura, M.; Shirai, H. Excellent gelators for organic fluids: Simple bolaform amides derived from amino acids. Adv. Mat. 1997, 9(14), 1095–1097. 22. (a) Loos, M. D.; Esch, J. V.; Kellogg, R. M.; Feringa, B. L. Chiral recognition in bis-ureabased aggregates and organogels through cooperative interactions. Angew. Chem. Int. Ed. 2001, 40(3), 613–616; (b) Jeong, Y.; Hanabusa, K.; Masunaga, H.; Akiba, I.; Miyoshi, K.; Sakurai, S.; Sakurai, K. Solvent=gelator interactions and supramolecular structure of gel fibers in cyclic bis-urea=primary alcohol organogels. Langmuir. 2005, 21, 586–594. 23. Esch, J. V.; Kellogg, R. M.; Feringa, B. L. Di-urea compounds as gelators for organic solvents. Tetrahedron Lett. 1997, 38(2), 281–284. 24. Lam, S. T.; Wang, G. X.; Yam, V. W. W. Luminescent metallogels of alkynylrhenium(I) tricarbonyl diimine complexes. Organometallics 2008, 27, 4545–4548.