Donor−Acceptor-Promoted Gelation of Polyaromatic Compounds

Polymer Phase-Transition Behavior Driven by a Charge-Transfer Interaction. Shogo Amemori , Kenta Kokado , Kazuki Sada. Angewandte Chemie International...
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Donor-Acceptor-Promoted Gelation of Polyaromatic Compounds Dan Rizkov, Jenny Gun, Ovadia Lev,* Ron Sicsic, and Artem Melman* The Institute of Chemistry, The Hebrew University of Jerusalem, Edmond E. Safra Givat Ram Campus, Jerusalem, 91904 Israel Received August 8, 2005. In Final Form: September 29, 2005 Low molecular mass organogels are nonconventional polymeric structures in which a minute amount of low molecular weight compound can reversibly gelify the whole solution without forming covalent bonds between the monomers. In this article, we demonstrate that certain electron acceptors (taking dinitrobenzoates as model compounds) that are incapable of gelifying the solvent on their own can assemble as much as a 15-16-fold larger amount of polyaromatic hydrocarbons (PAHs) and form two-component donor-acceptor organogels in different solvents. At the core of the long-range order stand donor-acceptor pairs. We assess our claims by detailed 1H NMR, spectrophotometry, fluorescence, and time-resolved fluorescence methods. The thermodynamics of the gelation process is described on the basis of temperature dependent 1H NMR studies. We believe that, in this case, 1H NMR provides direct quantification of the dissolved concentrations of the different species and therefore provides a direct way to measure the enthalpy, entropy, and free energy associated with gel formation.

Introduction Low molecular weight organogels attract considerable attention because of their diverse self-assembly,1 their potential value in pharmaceutics,2 optoelectronics,3 and separation technology,4 as well as their ability to template advanced ceramics5 and conductive polymers.6 Their thermal reversibility and thixotropy are attractive for microfluidic applications.4a Most of the reported organogels to date are based on single building block monomers. The scope of low molecular weight organogels can be further enlarged by forming gels from two types of monomers. First-generation dualcomponent organogels could be formed from each of the two components alone, and the use of two-monomer combinations merely improves their structure and, at times, alters their gel morphology.7 This has only a limited effect on the range of functionalities that could be * Corresponding authors. E-mail: [email protected] (A.M.); [email protected] (O.L.). (1) (a) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 12371247. (b) van Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263-2266. (c) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 31333159. (2) (a) Anand, B.; Pisal, S. S.; Paradkar, A. R.; Mahadik, K. R. J. Sci. Ind. Res. 2001, 60, 311-318. (b) Couffin-Hoarau, A. C.; Motulsky, A.; Delmas, P.; Leroux, J. C. Pharm. Res. 2004, 21, 454-457. (c) Kantaria, S.; Rees, G. D.; Lawrence, M. J. Int. J. Pharm. 2003, 250, 65-83. (3) (a) Ishi-i, T.; Hirayama, T.; Murakami, K.; Tashiro, H.; Thiemann, T.; Kubo, K.; Mori, A.; Yamasaki, S.; Akao, T.; Tsuboyama, A.; Mukaide, T.; Ueno, K.; Mataka, S. Langmuir 2005, 21, 1261-1268. (b) Yabuuchi, K.; Marfo-Owusu, E.; Kato, T. Org. Biomol. Chem. 2003, 1, 3464-3469. (4) (a) Mizrahi, S.; Gun, J.; Kipervaser, Z. G.; Lev, O. Anal. Chem. 2004, 76, 5399-5404. (b) Trivedi, D. R.; Ballabh, A.; Dastidar, P. Chem. Mater. 2003, 15, 3971-3973. (c) Bhattacharya, S.; Krishnan-Ghosh, Y. Chem. Commun. 2001, 185-186. (5) (a) Jung, J. H.; Shinkai, S.; Shimizu, T. Chem. Mater. 2003, 15, 2141-2145. (b) Moreau, J. J. E.; Vellutini, L.; Man, M. W. C.; Bied, C.; Dieudonne, P.; Bantignies, J. L.; Sauvajol, J. L. Chem.sEur. J. 2005, 11, 1527-1537. (c) Jung, J. H.; Kobayashi, H.; van Bommel, K. J. C.; Shinkai, S.; Shimizu, T. Chem. Mater. 2002, 14, 1445-1447. (6) Hatano, T.; Bae, A. H.; Takeuchi, M.; Fujita, N.; Kaneko, K.; Ihara, H.; Takafuji, M.; Shinkai, S. Chem.sEur. J. 2004, 10, 50675075. (7) Partridge, K. S.; Smith, D. K.; Dykes, G. M.; McGrail, P. T. Chem. Commun. 2001, 319-320. (b) Jeong, Y.; Friggeri, A.; Akiba, I.; Masunaga, H.; Sakurai, K.; Sakurai, S.; Okamoto, S.; Inoue, K.; Shinkai, S. J. Colloid Interface Sci. 2005, 283, 113-122.

incorporated into the gel skeleton. However, recently a few examples of “true” two-component gels, whereby both components are essential for the formation of the twocomponent donor-acceptor organogels, were reported. Rudkevich reported reversible, two-component organogels based on the carbon dioxide covalent anchoring of amine functionalities.8 McPherson and co-workers reported twocomponent gels incorporating an anionic surface active agent and phenolic hydrogen donors.9 Recently, Lu and co-workers reported binary tartaric acid-substituted stilbazole organogels formed by hydrogen bonding.10 Closer to the current publication, Maitra and co-workers11 reported organogels that are formed by donor-acceptor interactions between bile acid-bound pyrene or guaiazulene and trinitrofluorenone. In all these two-component donor-acceptor organogels, 1:1 stoichiometry was favorable for gel formation, and close to 1:1 stoichiometry between the monomers of the dual two-component donoracceptor organogels was preserved. One of the attractive properties of two-component donor-acceptor organogels is their ability to assemble organic moieties in close proximity without forming a precipitate. This may bear potential applications in optics and electrochemistry because close packing of monomers improves conductivity and may provide improved and even nonlinear optical properties. In this article, we show that dinitrobenzoate gelators can be used to pack a wide range of polyaromatic compounds in close proximity by twocomponent donor-acceptor organogel formation. We show that the two monomer precursors form two-component donor-acceptor organogels principally by donor-acceptor interactions in which both components are essential for gel formation. We further show that both the donor and (8) (a) Xu, H.; Rudkevich, D. M. J. Org. Chem. 2004, 69, 8609-8617. (b) Xu, H.; Rudkevich, D. M. Chem.sEur. J. 2004, 10, 5432-5442. (9) (a) Singh, M.; Tan, G.; Agarwal, V.; Fritz, G.; Maskos, K.; Bose, A.; John, V.; McPherson, G. Langmuir 2004, 20, 7392-7398. (b) Xu, X. D.; Ayyagari, M.; Tata, M.; John, V. T.; McPherson, G. L. J. Phys. Chem. 1993, 97, 11350-11353. (10) Bao, C. Y.; Lu, R.; Jin, M.; Xue, P. C.; Tan, C. H.; Liu, G. F.; Zhao, Y. Y. Org. Biomol. Chem. 2005, 3, 2508-2512. (11) Babu, P.; Sangeetha, N. M.; Vijaykumar, P.; Maitra, U.; Rissanen, K.; Raju, A. R. Chem.sEur. J. 2003, 9, 1922-1932.

10.1021/la052155w CCC: $30.25 © 2005 American Chemical Society Published on Web 11/09/2005

Donor-Acceptor Gelation of PAHs Scheme 1. Dinitrobenzoyl Appended Gelators: Intermediate, Rigid and Flexible Gelators (1-3).

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Cholesteryl-derived gels were used as ceramic templates by Shinkai15 and for the production of metal sulfide nanofibers by Lu.16 Anthracenyl and other PAH-appended gelators were also studied, for example, by Pozzo et al.,17 Desvergne,18 and others.19 Experimental Section

the acceptor are active ingredients of the gel and do not act as mere environmental gel promoters. We also refute the possibility of coprecipitation of one of the compounds within the organogel skeleton and show that the two monomers are fully incorporated into the gel skeleton. Although the donor-acceptor interactions are indeed essential for two-component donor-acceptor organogel formation, they are insufficient for gel formation and have to be supported by π-π stacking involving additional polyaromatic hydrocarbon (PAH) molecules. The twocomponent organogels expand the scope of organogels since a large number of polyaromatics bearing different appended functionalities can be used for gel formation with the same acceptor. A simple set of prognostic guidelines can help predict the successful cogelator, a long sought property of gelators, which hopefully will guide the selection or synthesis of cogelators bearing desirable functionalities. In this study we used the dinitrobenzoate moieties of Scheme 1, which are conjugated to cholesterol or to a C-18 alkyl chain. The three test compounds, 1-3, represent three electron acceptors with similar electronegativity but varying flexibility, going from the rigid ester 2 (cholesteryl 3,5-dinitrobenzoate (CDNB)), through a compound of intermediate flexibility (ester 1; cholesteryl[(3,5-dinitrobenzoyl)oxy]acetate (CDNBOA)), to the flexible ester 3 (1-octadecyl 3,5-dinitrobenzoate (ODDNB)). We used a large range of commercially available polyaromatic compounds as electron donor monomers. We denote the acceptor component as the gelator, and we call the donor component the cogelator, although neither forms gel by itself. Cholesterol-containing gelators were thoroughly studied by Whitten,12 Weiss,13 and Shinkai,14 and they are attractive building blocks for gel formation because of the easy perturbation of their crystalline structure. (12) Geiger, C.; Stanescu, M.; Chen, L. H.; Whitten, D. G. Langmuir 1999, 15, 2241-2245. (13) (a) George, M.; Weiss, R. G. Langmuir 2003, 19, 1017-1025. (b) Huang, X.; Terech, P.; Raghavan, S. R.; Weiss, R. G. J. Am. Chem. Soc. 2005, 127, 4336-4344. (14) (a) Gronwald, O.; Snip, E.; Shinkai, S. Curr. Opin. Colloid Interface Sci. 2002, 7, 148-156. (b) Sugiyasu, K.; Fujita, N.; Shinkai, S. Angew. Chem., Int. Ed. 2004, 43, 1229-1233. (c) Kawano, S. I.; Fujita, N.; van Bommel, K. J. C.; Shinkai, S. Chem. Lett. 2003, 32, 12-13.

Silica gel 60 F254 plates and silica gel 60 (0.063-0.200 mm) for column chromatography were purchased from Merck (Darmstadt, Germany). Cholesterol was obtained from Sigma (Milwaukee, WI). Chloroform-d (98%), 3,5-dinitrobenzoic acid, dicyclohexylcarbodiimide (DCC), 1-octadecanol, N,N-dimethyl-4pyridinamine (DMAP), tert-butyl bromoacetate, and trifluoracetic acid (TFA) were from Aldrich (Milwaukee, WI). Acetonitrile, dichloromethane (DCM), and acetone were from J. T. Baker (Deventer, Holland). Toluene (CP), ethyl ether (CP), and ethyl acetate (CP) were from Frutarom (Haifa, Israel). Potassium carbonate and sodium bicarbonate were from Riedel-de Hae¨n (Seelze, Germany). Unless otherwise stated, all reagents were analytical grade and were used as received. We used deionized water (conductivity < 0.1 mS/cm) purified by a Seradest SD 2000 system. Analytical Procedures and Instruments. Powder X-ray diffraction was measured with a Bruker AXC automated powder diffractometer (Long focus Cu X-ray tube) at a 2°/min scan rate. 1H NMR was recorded on Bruker DPX-200, Bruker AMX-300, and DRX-400 spectrometers in deuterated chloroform (CDCl3) using the residual solvent peaks for calibration or using added 1,4-dimethoxybenzene as an internal standard. Elemental analysis was performed in the Microanalysis Laboratory of the Hebrew University. Electrospray mass spectrometry was conducted using an LCQ (Thermo-Quest, Finnigan, San Jose´, CA) ion trap mass spectrometer equipped with an electrospray ionization (ESI) interface. The ESI was operated in negative ion mode using a 3.6-kV spray voltage and a 25-V capillary voltage. Scanning electron microscopy (SEM) imaging was performed by a JEOL JXA-8600 superprobe. Synthesis. Cholesteryl[(3,5-dinitrobenzoyl)oxy]acetate) (Ester 1). A suspension of anhydrous K2CO3 (1.4 g, 10 mmol), tert-butyl bromoacetate (1.95 g, 10 mmol), and 3,5-dinitrobenzoic acids (2.33 g, 11 mmol) in dimethylformamide (DMF) (50 mL) was vigorously stirred for 14 h, diluted with ethyl acetate (200 mL), washed with a water (3 × 100 mL) and NaHCO3 solution (100 mL, 5% w/v), dried, and evaporated, providing a residue. The residue was triturated with ether and filtered to afford tert-butyl 3,5dinitrobenzoyloxyacetate as pale yellow crystals (2.84 g, 8.7 mmol, 87%). 1H NMR: 1.49 (s, 9H); 4.85 (s, 2H); 9.20 (s, 2H); 9.25 (s, 1H). Crystals of tert-butyl 3,5-dinitrobenzoyloxyacetate (2.77 g, 8.5 mmol) were dissolved in TFA (20 mL). After 3 h, a spontaneous crystallization provided residue that was filtered and washed with ether to afford pale yellow crystals of 3,5-dinitrobenzoyloxyacetatic acid (2.0 g, 7.5 mmol, 75%). 1H NMR: 5.02 (s, 2H); 9.24 (s, 2H); 9.27 (s, 1H). 3,5-Dinitrobenzoyloxyacetic acid (2.0 g, 7.5 mmol), cholesterol (3.0 g, 7.77 mmol), DMAP (0.1 g, 0.8 mmol), and DCC (1.6 g,7.5 mmol) were suspended in dry DCM at room temperature and kept under stirring overnight. The precipitated crystals of dicyclohexyl urea were separated, the DCM was evaporated, and the product was purified by chromatography on silica gel using toluene eluent. Yield: 4.0 g (85%). (15) (a) Jung, J. H.; Lee, S. H.; Yoo, J. S.; Yoshida, K.; Shimizu, T.; Shinkai, S. Chem.sEur. J. 2003, 9, 5307-5313. (b) Jung, J. H.; Kobayashi, H.; Masuda, M.; Shimizu, T.; Shinkai, S. J. Am. Chem. Soc. 2001, 123, 8785-8789. (16) (a) Xue, P. C.; Lu, R.; Huang, Y.; Jin, M.; Tan, C. H.; Bao, C. Y.; Wang, Z. M.; Zhao, Y. Y. Langmuir 2004, 20, 6470-6475. (b) Xue, P. C.; Lu, R.; Li, D. M.; Jin, M.; Tan, C. H.; Bao, C. Y.; Wang, Z. M.; Zhao, Y. Y. Langmuir 2004, 20, 11234-11239. (17) Pozzo, J.-L.; Clavier, G. M.; Colomes, M.; Bous-Laurebt, H. Tetrahedron 1990, 53, 6377-6390. (18) (a) Shklyarevskiy, I. O.; Jonkheijm, P.; Christianen, P. C. M.; Schenning, A.; Del Guerzo, A.; Desvergne, J. P.; Meijer, E. W.; Maan, J. C. Langmuir 2005, 21, 2108-2112. (b) Terech, P.; Meerschaut, D.; Desvergne, J. P.; Colomes, M.; Bouas-Laurent, H. J. Colloid Interface Sci. 2003, 261, 441-450. (19) Ihara, H.; Yamada, T.; Nishihara, M.; Sakurai, T.; Takafuji, M.; Hachisako, H.; Sagawa, T. J. Mol. Liq. 2004, 111, 73-76.

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mp: 144-145 °C. 1H NMR (200 MHz, CDCl3): 0.68 (s,3H); 0.841.92 (m, 38H); 2.38 (d, J ) 7.2 Hz, 2H); 4.70 (m, 1H); 4.94 (s, 2H); 5.40 (d, J ) 5.2 Hz, 1H); 9.23 (s, 2H); 9.27 (s, 1H). 13C NMR (CDCl3, 100 MHz): 166.04, 162.01, 148.65, 138.97, 132.87, 129.65, 123.15, 122.72, 75.97, 62.30, 56.58, 56.06, 49.89, 42.22, 39.62, 39.43, 37.85, 36.77, 36.46, 36.10, 35.71, 31.81, 31.74, 28.14, 27.93, 24.19, 23.75, 22.74, 22.48, 20.95, 19.20, 18.64, 11.77. Anal. Calcd. for C36H50N2O8: C, 67.69; H, 10.15; N, 4.39. Found: C, 67.91; H, 10.33; N, 4.12. Cholesteryl 3,5-Dinitrobenzoate (Ester 2). DCC (0.32 g, 1.55 mmol) was added to a suspension of 3,5-dinitrobenzoic acids (0.313 g, 1.47 mmol), cholesterol (0.6 g, 1.55 mmol), and DMAP (0.018 g, 0.15 mmol) in dry DCM (30 mL). The reaction mixture was stirred overnight at room temperature. The dicyclohexyl urea crystals were separated, the DCM was evaporated, and the product was purified by LC chromatography on a silica gel column (using toluene eluent) to afford the title compound (0.66 g, 77.3%). mp: 189-190 °C. 1H NMR (200 MHz, CDCl3): 0.68 (s, 3H); 0.831.98 (m, 38H); 2.48 (d, J ) 7.2 Hz, 2H); 4.90 (m, 1H); 5.43 (d, J ) 4.8 Hz, 1H); 9.23 (s, 2H); 9.27 (s, 1H). 13C NMR (CDCl3, 100 MHz): 161.78, 148.55, 138.85, 134.45, 129.34, 123.44, 122.12, 76.96, 56.60, 56.08, 49.95, 42.25, 39.64, 39.44, 37.94, 36.85, 36.55, 36.12, 35.72, 31.86, 31.78, 28.15, 27.94, 24.22, 23.77, 22.75, 22.49, 20.99, 19.27, 18.65, 11.79. Anal.20 Calcd. for C34H48N2O6: C, 70.32; H, 8.33; N, 4.82. Found: C, 70.16; H, 8.29; N, 4.97. 1-Octadecyl 3,5-Dinitrobenzoate (Ester 3). The preparation of this compound was similar to that of ester 2, with 1-octadecanol instead of cholesterol. Yield: 0.90 g (87.9%). mp: 75-77 °C. 1H NMR (400 MHz, CDCl3): 0.88 (t, 3H); 1.25-1.56 (br s, 30H); 1.79-1.86 (m, 2H); 4.45 (t, J ) 6.8 Hz, 2H); 9.15 (s, 2H); 9.23 (s, 1H). 13C NMR (CDCl3, 100 MHz): 162.48, 148.60, 134.11, 129.33, 122.21, 67.08, 31.85, 29.62 (very intense), 29.58, 29.56, 29.49, 29.41, 29.28, 29.14, 28.47, 25.81, 22.61, 14.03. Anal. Calcd. for C25H40N2O6: C, 64.63; H, 8.68; N, 6.03. Found: C, 64.98; H, 8.73; N, 5.89. General Methods for the Preparation of Two-Component Donor-Acceptor Organogels. A suspension of esters 1-3 with PAH (3-15 equiv) in dry acetonitrile was refluxed until a clear yellowish solution was formed. The resultant solution was cooled in an ice-water bath to provide, depending on the concentration and ratio of the components, an opaque yellow or white gel in 1-10 min. Gelation Criteria and Gel Transition Temperature. The inverted vial approach21 was used as a general criterion for gel formation. For gel transition temperature dependence, we used the dropping glass bead approach,21 using a set of glass beads (3 mm in diameter) in 10-mm-diameter vials. A glass bead was carefully placed on top of the gel, and the temperature was slowly raised 1 °C/minute. We used a Eurotherm 2416 temperature controller. The gel transition temperature was determined as the temperature at which the gel was unable to bear the glass bead and it dropped down. Despite the fact that this type of measurement is highly influenced by surface tension and liquid density, it is widely accepted, and therefore it forms a basis for comparison with other gels.

Results Cogelation of PAHs. We used the inverted vial approach to determine the criteria for the selection of aromatic compounds that can be self-assembled in acetonitrile solutions containing the dinitrobenzoyl esters 1-3. Esters 1 and 2 did not gelify acetonitrile by themselves, no matter what concentration was used. These dinitrobenzoyl esters also did not gelify any other organic solvent that was tested in our laboratory (including, for example, methanol, DMSO, acetonitrile, n-butyronitrile, acetone, chloroform, toluene, and isooctane). This could be expected since aromatic compounds possessing electron withdrawing groups are known for their weak π-π (20) Kellie, A. E.; Smith, E. R.; Wade, A. P. Biochem. J. 1953, 53, 578-582. (21) Murata, K.; Aoki, M.; Suzuki, T.; Harada, T.; Kawabata, H.; Komori, T.; Ohseto, F.; Ueda, K.; Shinkai, S. J. Am. Chem. Soc. 1994, 116, 6664-6676.

Rizkov et al. Table 1. Anthracene-Derived Gel Precursors and Their Ionization Energies (eV)

cogelatora anthracene 1-aminoanthracene 1-dimethylamino anthracene 2-aminoanthracene 9-vinylanthracene acridine 9-chloroanthracene 9,10-dibromo anthracene 9-formyl anthracene

minimal gelation ionization gel concentration energy (eV) formation of 1 (mg/mL) 7.43b 6.24c

Yes Yes Yes

5.2 11.0 9.0

6.24d 7.33e 7.99f 7.47d 7.5424 7.6824

Yes Yes No No No No

9.4 8.2

Ester 1-cogelator ration ) 1:6. b Ref 22. c Ref 23. 25. f Ref 26. a

d

Ref 24. e Ref

bonding. If the 3,5-dinitrobenzoyl group is tethered to a rigid cholesterol residue, optimal geometries for the intermolecular binding of the two parts of the gelator molecule can mismatch one another, thus preventing gel formation in esters 1 and 2. Ester 3, which does not contain protruding side obstacles to hinder stacking, gelified acetonitrile at concentrations above 3.5 mg/mL. Table 1 depicts the anthracene derivatives that gelified acetonitrile solutions of ester 1. For our screening studies, we used an identical molar ratio between the anthracenyl compounds and the gelator, r ) 6. The value r is defined as the molar ratio between the cogelator and the gelator. The ionization potentials of the donors are also reported in Table 1 as a measure for their electron donation aptitude. Anthracenyl-containing moieties with ionization potentials lower than or equal to 7.43 eV, that is, those having higher electron donation power than anthracene, gelified mixtures of ester 1 in acetonitrile. In contrast, aromatics that are poorer electron donors, such as acridine and different haloanthracenes, did not form gels with 1 or 2. The minimal gelation concentrations that gelified acetonitrile are also depicted in the Table (for r ) 6). Unexpectedly, larger affinities between the cogelator and ester 1 did not necessarily improve gelation at lower r values. We speculate that the ability to sustain π-π stacking plays a more significant role in cogelation than the stability of the donor-acceptor complex. The lower ability of the less-potent aromatic donors to form π stacking is a contributing factor in the need for high donor concentration to obtain gels. To generalize the pattern that was observed for the anthracenyl cogelators of Table 1, we examined a large range of other PAHs. A similar trend was observed for the four-ring PAH family (chrysene, pyrene, and 1-pyrenomethanol-gelified ester 1-acetonitrile solutions), whereas 1-pyrene aldehyde and 1-pyrenecarbolxylic acid did not yield organogels. Polyaromatic species with a larger number of rings had a greater tendency to gelify acetonitrile-ester 1 solutions. Thus, while neither naphthalene nor 1-aminonaphthalene gelified ester 1-acetonitrile solutions, anthracene, phenanthrene, chrysene, triphenylene, pyrene, and 1,2,5,6dibenzanthracene formed acetonitrile two-component donor-acceptor organogels successfully. Structure and Stability of Anthracene Organogels. We have singled out the anthracene-dinitrobenzoyl gelators for more detailed studies. We had two targets for our study; the first was to describe the properties of the PAH-dinitrobenzoyl two-component organogels, and, on top of that, we wanted to confirm unequivocally the ability to pack the PAHs in close proximity by two-component organogel formation.

Donor-Acceptor Gelation of PAHs

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Figure 1. Dependence of the minimal gelator concentration for gel formation as a function of r in molar anthracene-dinitrobenzoyl moieties. Symbols correspond to numbers in Scheme 1. Photographs of freeze-dried and wet yellow and white anthracene-ester 1 gels are depicted. Composition of the yellow gel: r ) 4, 10 mg/mL ester 1 in acetonitrile. Composition of the white gel: r ) 8, 10 mg/mL ester 1 in acetonitrile.

Figure 1 depicts the range of permissible concentrations that allow organogel formation from acetonitrile, 1-3 dinitrobenzoyl gelators, and anthracene. Only the minimal concentration for successful gelation is reported by the different curves in Figure 1; higher concentrations pertaining to the area above each curve always gelified acetonitrile, provided that the gel precursors could be fully dissolved in acetonitrile. The resulting gels were, in all cases, stable for at least 12 h at 0 °C and melted at a high temperature below acetonitrile’s boiling point (81.5 °C). Agitation or elevation of the temperature resulted in a gradual decomposition of the gel and crystallization of the components. The minimal gelation concentration curve of the rigid ester 2 follows a monotonic decreasing function, and gel formation is always favored when the concentration is increased. In contrast, the less rigid ester 1 curve shows a well-defined local minimum around r ) 4, suggesting the preferential formation of a ester 1-anthracene complex with this ratio. A visual inspection of the gels revealed that the nonmonotonic curves of Figure 1 were accompanied by a transition between two types of gels that had different color and morphology. Gels that were prepared with r < 5 were yellow and exhibited lower stability compared to the gels with r > 5, which were white. Interestingly, the yellow color of the low-r gels was preserved after freeze-drying and was sustained even after the dried gels were kept for several weeks under ambient conditions. Figure 1 presents colored pictures of gels before and after freeze-drying. The morphology of the freezedried gels was studied by Hi-Res and environmental SEM, which revealed a rodlike structure for the low-r gels (Figure 2a) and a transition to a striplike structure (Figure 2b) for the high anthracene-to-ester 1 gels. However, the micrographs represent the morphology of the dried gels, which may differ substantially from that of the wet gels, despite the color resemblance. In fact, X-ray diffraction studies of the wet gels revealed only trace crystallinity (frame 3). We attribute the small anthracene peaks to artifacts caused by the way we conducted the X-ray diffraction studies. These studies were performed in an open beaker containing the wet gel.

Figure 2. SEM micrographs of freeze-dried gels obtained from an acetonitrile solution of 15.4 mM gelator 1 (r ) 4 (right) and r ) 6 anthracene-ester 1). The insets show magnified sections. Bars denote 50 µm (main figures) and 1 µm (insets).

During several minutes of data acquisition, there was some minute evaporation of acetonitrile, which is the source of the observed traces of crystallinity. In contrast, the X-ray study of the freeze-dried gel (frame 4) revealed well-defined crystal phases of anthracene (frame 1) and ester 1 crystals (frame 2).

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Figure 3. X-ray powder diffraction of (1) anthracene precipitate, (2) ester 1 precipitate, and (3) anthracene-ester 1 gel (prepared from 10 (mg ester 1)/(mL acetonitrile); r ) 8). (4) The freeze-dried product of 3. The peak at θ ) 26° is caused by the Mylar cover film.

Figure 4. Gel-to-sol transition points of anthracene-ester 1 gels in acetonitrile; curves 1-3 correspond to 10, 13, and 20 mg/mL of ester 1.

Gel Transition by the Dropping Bead Approach. The gel transition temperature was determined by the dropping glass bead approach.27 The dependence of the melting temperature of ester 1-anthracene gels as a function of the concentration of ester 1 over a wide range of r values is depicted in Figure 4, which shows increased gel stability with increased gelator concentration and gel formation at lower r values when the concentration of the gel formers is increased. The figure shows that a high concentration stabilizes the gel, as does a large r value. The figure also indicates a sharp transition temperature increase at certain r levels, which depends on the concentration of the gelator. However, transition temperatures that are close to the boiling point of acetonitrile should be regarded cautiously. Nevertheless, despite the uncertainty associated with the near-boiling-point gel transitions, one may note that the gel transition takes place at higher r values for lower concentrations of gels. A more detailed description of the enthalpy of the gel transition is given in the discussion section. Gel Stoichiometry. Figure 1 shows an important feature of the two-component donor-acceptor organogels. A few milligrams per milliliter of the dinitrobenzoyl gelators impart order to a much larger (up to 15 times larger) amount of PAHs. It was therefore important to verify that the phenomenon is generic and that it can be (22) Lias, S. G.; Bartmess, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17, Supplement 1. (23) Bauschlicher, C. W.; Langhoff, S. R. Mol. Phys. 1999, 96, 471. (24) Masnovi, E. A.; Seddon, E. A.; Koshi, J. K. Can. J. Chem. 1984, 62, 2552. (25) Shirota, Y.; Nagata, J.; Nakano, Y.; Nogami, T.; Mikawa, H. J. Chem. Soc., Perkin Trans. 1 1977, 14-18. (26) Xue, W.; Zapien, D.; Warshawsky, D. Chem. Res. Toxicol. 1999, 12, 1234-1239. (27) Takahashi, A.; Sakai, M.; Kato, T. Polymer. J. 1980, 12, 335341.

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Figure 5. Minimal gelation concentration of anthracene (A), phenanthrene (B), and chrysene (C) cogelators with ester 1 as a function of r, the PAH-ester 1 mole ratio.

maintained with other PAHs as well. A second point that has to be addressed is to what extent the precursor composition indeed reflects the composition and stoichiometry of the gel skeletons. Generality. Figure 5 depicts the minimal concentration of gelator 1 that can sustain gels of anthracene, phenanthrene, and chrysene. From our point of view, the most striking feature is that, for all three gels, the donoracceptor pairs can immobilize an additional 14-15 PAH molecules. In fact, the upper bound depicted in this figure is an underestimate of the ordering capability because it reflects the solubility of the PAHs in boiling acetonitrile. The phenomenon by which a minute amount of nucleation centers impart long-range order to a manyfold larger amount of substrate is known in both colloid and surface science. Self-assembled thin films,28 epitaxial growth of crystals, and bicomponent micellar structures are common examples,29 but we are not aware of a parallel phenomenon in gels in which a few molecules assemble as many as 15 more molecules onto them without covalent bonding. This phenomenon is inspiring, recalling that most of the PAH molecules are remote and not in touch with the twocomponent donor-acceptor organogel cornerstones: the donor-acceptor pairs. Relationship between the Composition of the Gel and its Precursors. To prove that the charge-transfer pairs indeed organize the PAHs in a long-range, ordered manner, we needed three additional pieces of information: (1) quantification of the dissolved PAHs after gel formation, (2) supporting evidence that an additional PAH phase is not created within the gel, and (3) proof that a charge-transfer complex is indeed formed. We believe that the way in which we answered these questions is as illuminating as the final conclusion, and therefore we elaborate on the properties of the new two-component donor-acceptor organogels. Dissolved PAH. The quantification of the dissolved gel components anthracene and gelator 1 was based on 1 H NMR studies. The composition and properties of the resultant yellow, 1:5 and white, 1:6 ester 1-anthracene CD3CN gels were studied by 1H NMR spectroscopy. The experiment started from the -20 °C gels, and the temperature was elevated consecutively in 10 °C increments, with a 20-min isothermal delay after each change. The 1H NMR spectra of Figure 6 were taken at the end of each isothermal time delay, immediately before the consecutive temperature increase. Figure 6 shows peaks (28) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives Wiley-VCH: Weinheim, Germany, 1995. (29) Lyklema, J. Fundamentals of Interface and Colloid Science; Academic Press: London, 1995.

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Figure 6. 1H NMR (400 mHz) spectra of anthracene-ester gels in deuterated acetonitrile as a function of temperature (temperature from top: 70, 60, 50, 40, 30, 20, and 0 °C). (a) yellow r ) 5 gel, (b) white r ) 6 gel. Ester 1 concentration ) 15.6 mM.

of the anthracene at 7.5, 8.1, and 8.6 ppm; peaks of ester 1 at 4.5, 5.0, 5.5, and 9.0 ppm; and the 6.8 ppm peak of the 1,4-dimethoxybenzene internal standard. The data show a gradual decrease in all peaks of the gel components at lower temperatures. The gradual decrease in the 1H NMR peaks starting near the gelating temperature was studied by McPherson,9 Whitten,30 and others.31 Diffusion 1H NMR studies provided the diffusion coefficients of anthracene and ester 1, giving D ) 1.0 × 10-5 and 2.3 × 10-5 cm2/sec, respectively, at 20 °C. These values were found to be very close to the observed 1H NMR values that were measured in a homogeneous CD3CN solution: 1.15 × 10-5 and 2.6 × 10-5 cm2/sec. This clearly indicates that, for the studied gels, the 1H NMR peaks pertain almost exclusively to the solution phase, and the 1H NMR signals of the gel-bound species are averaged off by gel heterogeneity. A similar conclusion was reached for other gels, although in those cases, the cluster in the porous gels probably played a more significant role.30,31c,d The practically identical diffusion coefficient is of importance because it implies that 1H NMR can be used to quantify, in this case, the free, inbound gel components. The 1H NMR studies of Figure 6 allow for the direct calculation of the fraction of anthracene and ester 1 in the gel skeleton and in the liquid state as a function of the temperature. These are delineated in Figure 7a for the yellow gel (and very similar results were obtained for the white gels). Figures 7a shows that, at the temperature range of -20 to 0 °C, practically all of the anthracene and ester 1 is (30) Duncan, D. C.; Whitten, D. G. Langmuir 2000, 6, 6445-6452. (31) For example, (a) Kral, V.; Pataridis, S.; Setnicka, V.; Zaruba, K.; Urbabnova, M.; Volka, K. Tetrahedron 2005, 61, 5499-5506. (b) Caplar, V.; Zinic, M.; Pozzo, J. L.; Fages, F.; Mieden-Gundert, G.; Vogtle, F. Eur. J. Org. Chem. 2004, 19, 4048-4059. (c) Makarevic, J.; Jokic, M.; Peric, B.; Tomisic, V.; Kojic-Prodic, B.; Zinic, M. Chem.sEur. J. 2001, 7, 3328-3341. (d) Makarevic, J.; Jokic, M.; Raza, Z.; Stefanic, Z.; KojicProdic, B.; Zinic, M. Chem.sEur. J. 2003, 9, 5567-5580.

integrated in the gel skeleton, which further supports our claim that the anthracene does not form a separate gel phase but is rather an integral part of the gel skeleton. However, noting that the 1H NMR spectrum reflects the concentrations of the dissolved species in equilibrium with the gel skeleton allows us to decipher the pertinent thermodynamic values of gel transition by the procedure that was used for monocomponent gels by Makarevic et al.31c,d A quantitative analysis of the 1H NMR results is presented in the discussion section. Optical Properties and Proof of Charge-Transfer Complex Formation. The intensive yellow color of the organogels formed from faintly yellow ester 1 and anthracene suggests strong donor-acceptor interactions. UV-vis spectroscopy studies supported this assumption, at least for the low-r gels. Figure 8 delineates a set of spectra of a 1:4 ester 1-anthracene gel containing 60 mM anthracene in acetonitrile. The temperature of the gel was gradually raised (using 10 °C steps and a 30-min delay at each step), and a full UV-vis optical spectrum was taken just before the consequent step change. When the gel was melted, the temperature was gradually lowered again, and the spectra were recorded until the solution became turbid. The continuous lines (from the top, i.e., from the low-temperature, turbid gels) represent spectra of the gel state, and the dashed lines, starting from the high temperature (70 °C) at the bottom, represent the transparent solution. The turbidity of the gels is manifested in the elevated background optical density (OD), which is indeed lowered gradually at elevated temperature because of the partial dissolution of the gel, as discussed in the context of the 1H NMR studies. Additionally, we observed that the gel spectra are characterized by a wave that is initiated around 500 nm. This wave is absent in the absorption spectra of the solutions. The height of the absorption wave is increased as the temperature goes down. At T ) 20 °C, we could

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Figure 7. (a) Mole fraction of anthracene and gelator in the gel phase (dashed line) and in the liquid phase (solid line) as a function of temperature for yellow, r ) 5 gel; (b) van’t Hoff dependence of a yellow, r ) 5 gel; (c) van’t Hoff dependence of a white, r ) 6 gel.

Figure 8. Spectra of r ) 4, anthracene-ester 1 gels in acetonitrile (solid lines), and dissolved concentrations at the same composition in acetonitrile (dotted). Spectra correspond to T ) 0, 10, 20, 30, 40, 50, 60, and 70 °C from top).

retrieve the spectra of the gel and the homogeneous solution. Correcting for the background absorbance, by subtracting the 600-nm OD from both spectra and then subtracting the solution spectrum from the gel spectrum, gives the net absorption effect of gelation. This subtraction process is detailed in Figure 9a. The figure clearly shows that the product of the subtraction process gives an absorbance peak rather than a wave, as expected of chargetransfer process. However, the appearance of a new gelation-induced peak is still considered insufficient evidence to prove a charge-transfer process; a peak wavelength shift induced by changing the dielectric constant of the solvent is considered better proof of a charge-transfer mechanism. Fortunately we could repeat the test using acetone as a solvent. The dielectric constant of acetone is  ) 20.56, which is significantly lower than that of acetonitrile,  ) 35.9 (data pertain to 25 °C).32 Figure 9b,c shows the chargetransfer peak reconstruction in acetone and in a 1:1 (v) acetone-acetonitrile solution. The spectra were taken under identical conditions (r ) 6 and T ) 20 °C), although the concentration of the gelator was 10 and 15.4 mg/mL (32) Gota, H.; Okamoto, Y.; Yashima, E. Macromolecules 2002, 35, 4590.

Figure 9. Reconstructed gelation-induced peak of yellow, r ) 4 anthracene-ester 1 gels in acetonitrile (a), acetone (b) and an acetone/acetonitrile 1:1 mixture (c). The three curves in each frame correspond to the gel spectrum (upper curve), the solution spectrum, and the reconstructed, subtracted spectrum induced by the gelation process.

in the acetone and in the mixed solvents, respectively. In all three frames of Figure 9, the upper curves denote the gel spectra (after subtraction of the 600-nm background), and the lower monotonically increasing curves depict solution composition. The reconstructed peak is clear in all cases, and it indeed undergoes the expected hypsochromic shift (from approximately 430 to 415 and 400) as the dielectric constant of the solution is decreased. Solvents with a higher dielectric coefficient tend to stabilize the

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Figure 10. (a) Emission spectra of crystalline anthracene (dominant peaks 3 and 4) and anthracene dissolved in acetonitrile (dominant peaks 1, 2, and 3). (b) Emission spectra of yellow r ) 4 and white, r ) 8 anthracene-ester 1 gels in acetonitrile. Excitation wavelength: 350 nm.

excited, charge-separated state and thus lower the energy barrier for charge separation. Fluorescence Spectra and Time-Resolved Fluorescence. To prove unequivocally that the gel is indeed a single phase and that the anthracene is entirely incorporated into the gel and did not form another separate solid phase, we needed an additional independent method of analysis. The X-ray studies of Figure 3 suggest that, even if a separate anthracene phase is formed, its concentration should be very small. However, powder X-ray spectra are not quantitative, and, despite the fact that we can explain the small peaks of anthracene crystals (by solvent evaporation), the truth of the matter is that the anthracene diffraction peaks are still there. To provide more conclusive evidence of the absence of a crystalline phase, we studied the fluorescence and the time-resolved fluorescence of the ester 1-anthracene gels. Emission spectra of the organogels, solution phase, and anthracene crystals were measured by a Cary Eclipse fluorimeter in a quartz cuvette turned 45° to excitation and emission beams. Figure 10 depicts the emission spectra (350-nm excitation) of crystalline anthracene and a dilute ester 1-anthracene solution (1.5 and 6 mmol, respectively). Five different peaks can be distinguished in the spectra, and they are marked by consecutive 1-5 numerals. Peaks 1 and 2 (385 and 400 nm, respectively) are the most dominant in the solution phase, and peaks 3 and 4 (425 and 445 nm, respectively) are dominant in the crystalline phase. Peak 5 (470 nm) is weak in both spectra. Figure 10b depicts the emission spectra of yellow (r ) 4) and white (r ) 8) gels in acetonitrile (the concentration of the gelator was, in both cases, 15.6 mM). Frame b shows that peaks 1 and 2 are more dominant for the r ) 4 gel, whereas peak 3 becomes more dominant for the r ) 8 gel, as if increasing the r value produces gels that are closer to the crystalline phase. However, since all four dominant peaks were present in both spectra, we could not rule out the possibility that a minor amount of crystalline anthracene phase still precipitated within the gel, which was what we set out to prove, and for that we had to resort to an additional tool. Time-resolved fluorescence provides a powerful tool to distinguish between different phases and crystallinities. We have carried out time-resolved fluorescence studies of the four mixtures of Figure 10 with excitation at 350 nm and measuring emission at 400 nm, and the data are

Figure 11. Time domain emission signals of (a) 1.5 mM ester 1 and anthracene solution in acetonitrile (τ ) 2.49 ns) and the emission signals of a blank experiment and crystalline anthracene (τ ) 16.9 ns). (b) yellow, r ) 4 anthracene-ester 1 gel (τ ) 2.59) and white, r ) 8 anthracene-ester 1 gel (τ ) 2.84 ns).

presented in Figure 11. Time-resolved light emission was measured using a spectrometer (Edinburgh Instruments, FLS 920) with a cooled photomultiplier detection system, connected to a computer (F900 v. 6.3 software). Before sample measurements, a background run with no sample was done, and all spectral results were background corrected and integrated. Measurements were made after the sample was inserted into a standard four clear-sided 10-mm cuvette filled with 3.3 mL of reaction solution. Frame a shows that the excited state of the dissolved anthracene has a significantly shorter lifetime compared to the crystalline phase (τ ) 2.49 ns vs 16.9 ns), which provides a very powerful tool to distinguish between the two phases. The emission time domains of the yellow, r ) 4 (containing 15.6 mM of ester 1 and 60 mM of anthracene), and white, r ) 8 (15.6 and 120 mM), gels were rather similar, providing τ ) 2.59 and 2.84 ns, respectively (frame b). More significantly, the gel emission signals showed simple, single-exponential decay and did not exhibit any change of slope, which should have accompanied the presence of a minute amount of crystalline anthracene. Discussion We set out to prove that a fairly large excess of PAH molecules can be ordered in PAH-dinitrobenzoyl ester gels without forming a precipitate. We believe that the fluorescence studies (of Figures 10 and 11), especially when supported by X-ray powder diffraction studies, have proven conclusively that anthracene did not precipitate. When we add the 1H NMR data, which showed that the anthracene was also not present in the solution phase, we can satisfactorily conclude that the anthracene is embedded in the gel phase. The nature of the anthracene packing is beyond the scope of this article; however, it is clear that the donor-acceptor interactions between the PAH and the gelator are pertinent cornerstones of the PAH-electron acceptor gels. We have proven acceptor-donor adduct formation by three different methods: yellow color forma-

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tion, hypsochromic change in the excitation energy with lower dielectric solvent (which actually proves chargetransfer complex formation for the yellow gel), and systematic studies of the gelation of anthracenyl compounds, which illustrated that only electron donors can act as cogelators of esters 1-3 acetonitrile solutions. Our studies relied extensively on the 1H NMR spectroscopy data, and these deserve a more thorough discussion. A similar treatment for monogelators was reported by Makarevic.31c,d A simple description of the gel dissolution is given by eq 1:

νAnAn + Gelator a Gel

(1)

in which An is the cogelator, for example, anthracene. By assigning unit activity for the gel phase and taking the concentration in the solution phase to be equal to the dissolved concentrations of the gelator and the anthracene, one may formally express the Gibbs free energy of gel dissolution by eq 2:

∆G° ) -RT ln K ) -RT ln[An]νAn[gelator]

(2)

The van’t Hoff equation can then be used on the condition that the enthalpy of the reaction is approximately constant, giving

∆H0 ≈ -R

d ln K d(1/T)

(3)

Plotting the natural logarithm of the concentration product of the dissolved free anthracene and ester 1 against the absolute temperature reciprocal indeed gives straight lines for the yellow (Figure 7b) as well as for the white gels (Figure 7c) with very high correlation coefficients: R2 ) 0.997 and 0.999 for the yellow and white gels, respectively. From the slopes of the curves of eqs 1 and 3 and from the T ) 25 °C equilibrium concentration for the free anthracene and ester 1, one can derive the equilibrium constants and standard Gibbs free energies for the yellow (r ) 5) and white (r ) 6) gels. For the r ) 5 gel, we obtained ∆H° ) 207.1 kJ/mol, ∆G° ) 68.4 kJ/ mol, and ∆S° ) 465 J/(mol K). For the r ) 6 gel, we obtained ∆H° ) 228.9 kJ/mol, ∆G° ) 77.8 kJ/mol, and ∆S° ) 507 J/(mol K). Interestingly, the driving force for gelation at room temperature remained almost constant, which is also manifested in the almost identical melting points of the r ) 5 and r )6 gels (lowest curve of Figure 4), whereas the entropy of dissolution increased significantly because of the larger number of molecules that are dissolved in the r ) 6 gels compared to that in the r ) 5 gels. As expected, the entropy change also constitutes the major driving force for dissolution. It was interesting to compare the enthalpy change in gel formation by the NMR prediction and by the falling bead approach (Figure 4). It is customary, for example,33 to derive the enthalpy of the gel transition by plotting the logarithm of the concentrations against the reciprocal of the transition temperatures times the gas constant, giving (33) Brotin, T.; Utermohlen, R.; Fages, F.; Bouas-Laurent, H.; Desvergne, J. P. Chem. Commun. 1991, 416-417.

the transition enthalpy. This, however, applies well only to single-component gels. For two-component gels, one has to rely on eq 1, taking into account the gel stoichiometry and assuming unity activity for the gel. For the sake of comparison, we used the gel transition temperatures to calculate the equilibrium constant K(νNA) for the dissolution of anthracene-ester 1 gels of four different stoichiometries, with νNA ) 3-6. For the higher νNA values, the transition temperatures were too close to the acetonitrile boiling point and were therefore less reliable. For example, for νNA ) 3, K ) 0.18 × 10-6, 0.54 × 10-6, and 2.88 × 10-6 for the concentrations 10, 13, and 20 mg/mL ester 1, respectively, used in Figure 4. We plotted the van’t Hoff dependency (for each r), which gave reasonably straight lines with the correlation coefficients (R2 ) 0.98, 0.98, 0.91, and 0.94 for νNA) 3-6, respectively). The gel transition enthalpies derived by this procedure were 60.7, 86.4, 75.1, and 76.5 kJ/mol, respectively. The scatter of the derived enthalpies and the lack of trend along the stoichiometric series of compositions reflect the large experimental error in this procedure. The use of only 3 points for each van’t Hoff plot is also an error-contributing factor. The gel transition enthalpies were only about 30% of the melting enthalpies derived based on the 1H NMR studies. We believe that the large deviation reflects the different reactions involved. The 1H NMR data pertain to the true melting of the gel and are based on more meaningful thermodynamic values (actual concentration of the reactants). The gel transition temperature, on the other hand, marks a transition from a gel to a weaker gel or, at most, to a colloidal state, and therefore it reflects a change from a well-defined state to one that is thermodynamically less clearly defined and certainly different from the melt state. Therefore, the assigned enthalpy values for this transition are unavoidably biased and considerably smaller than the true melting enthalpies. Indeed, the 1H NMR-based enthalpies gave three times larger values compared to the gel transition enthalpies based on the dropping bead approach. We believe that a straightforward assignment of ∆H/R based on the slope of the gelator concentration against the reciprocal of the transition temperature makes no sense, and therefore it was not presented here. Concluding Remarks Donor-acceptor complexes composed of PAHs and dinitrobenzoate esters can self-assemble a manyfold larger amount of PAHs. A rule of thumb indicates that threering PAHs that exhibit an ionization potential lower than 7.43 eV form a sufficiently stable cornerstone for the gelation of acetonitrile solutions. Full incorporation of the PAH molecules into the gel has been proven by 1H NMR, X-ray, and time-resolved fluorescence. Acknowledgment. We are grateful to the European Union for partial funding under the Aquachem network (contract MRTN-CT-2003-503864), for the financial support of the Israel Science Foundation (ISF, Grant 176/ 02-1), and for a Ministry of Science grant for the promotion of infrastructure research. LA052155W