Supramolecular Chirality in Organogels: A Detailed Spectroscopic

Sep 22, 2010 - This Article addresses the formation of chiral supramolecular structures in the organogels derived from chiral organogelator 1R (or 2R)...
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Supramolecular Chirality in Organogels: A Detailed Spectroscopic, Morphological, and Rheological Investigation of Gels (and Xerogels) Derived from Alkyl Pyrenyl Urethanes Rajat K. Das,† Ramesh Kandanelli,† Juha Linnanto,‡ Kunal Bose,§ and Uday Maitra*,† †

Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, Karnataka, India, ‡ Department of Chemistry, University of Jyv€ askyl€ a, P.O. Box 35, 40351 Jyv€ askyl€ a, Finland, and § Agilent Technologies Nanomeasurements, South Asia Pacific, Bangalore 560048, India Received July 29, 2010. Revised Manuscript Received September 3, 2010

This Article addresses the formation of chiral supramolecular structures in the organogels derived from chiral organogelator 1R (or 2R), and its mixtures with its enantiomer (1S) and achiral analogue 3 by extensive circular dichroism (CD) spectroscopic measurements. Morphological analysis by atomic force microscopy (AFM) and scanning electron microscopy (SEM) were complemented by the measurements of their bulk properties by thermal stability and rheological studies. Specific molecular recognition events (1/3 vs 2/3) and solvent effects (isooctane vs dodecane) were found to be critical in the formation of chiral aggregates. Theoretical studies were also carried out to understand the interactions responsible for the formation of the superstructures.

Introduction Molecular self-assembly and hierarchical growth of complex, functional structures form the basis of all life forms.1 Many biological macromolecules, for example, DNA, R-helices, amylase, and so on, adopt helical conformations. In many cases, additional noncovalent interactions lead to the formation of superstructures which do not necessarily have the same handedness as that in the basic chiral assembly. All these higher order helical structures are based on self-assembly, and many of these processes have important implications in physiological processes. Collagen triple helix is one such example, where the coiling of three left-handed helices results in the formation of a right-handed superhelix.2 Understanding such processes with synthetic systems has gained considerable importance in recent years, primarily because of the ease with which synthetic systems can be designed, manipulated, and studied, with an eventual goal to engineer functional molecules. Also, the intriguing question of the origin of homochirality in nature and the prevalence of L-amino acids and D-sugars over their enantiomers in the fundamental life processes has been a long-standing problem.3 Therefore, probing the mechanism of amplification of chirality4 originating from a small enantiomeric excess or small quantity of a chiral dopant both in synthetic polymers5 as well as in supramolecular assemblies from small organic molecules9,11 assumes significance. One of the ways to express molecular chirality at the supramolecular level is through the “sergeants and soldiers” principle, which *To whom correspondence should be addressed. Fax: (þ) 91 80 2360 0529. E-mail: [email protected]. Also at Chemical Biology Unit, JNCASR, Bangalore. (1) (a) Eyre, D. R. Science 1980, 207, 1315. (b) Bella, J; Eaton, M; Brodsky, B; Berman, H. M. Science 1994, 266, 75. (2) Orgel, J. P. R. O.; Irving, T. C.; Miller, A.; Wess, T. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 9001. (3) (a) Frank, F. C. Biochim. Biophys. Acta 1953, 11, 459. (b) Meierhenrich, U. Amino Acids and the Asymmetry of Life; Springer: Berlin, 2008. (4) Blackmond, D. G. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 5732. (5) (a) Cornelissen, J. J. L. M.; Donners, J. J. J. M.; Gelder, R. d.; Graswinckel, W. S.; Metselaar, G. A.; Rowan, A. E.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. Science 2001, 293, 676. (b) Fujiki, M.; Koe, J. R.; Terao, K.; Sato, T.; Teramoto, A.; Watanabe, J. Polym. J. 2003, 35, 297. (c) Yashima, E.; Matsushima, T.; Okamoto, Y. J. Am. Chem. Soc. 1997, 119, 6345. (6) (a) Green, M. M.; Reidy, M. P. J. Am. Chem. Soc. 1989, 111, 6452. (b) Green, M. M.; Park, J.-W.; Sato, T.; Teramoto, A.; Lifson, S.; Selinger, R. L. B.; Selinger, J. V. Angew. Chem., Int. Ed. 1999, 38, 3138.

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was pioneered by Green and co-workers to achieve amplified chirality in polymeric systems,6 and later has been extensively applied for dynamic self-assembled structures by Meijer and co-workers7,10 and also by other groups.8 Its applicability was recently demonstrated in low molecular mass organogels (LMOGs).9 In particular, in a recent communication, Hong et al. have described the induction of homochiral helical structures in a supramolecular organogel through this principle.11 This prompted us to disclose the results from our studies on the supramolecular and macroscopic assembly using chiral/achiral organogel systems based on alkyl 1-pyrenyl urethanes. In this contribution, we provide evidence that the transcription of molecular chirality to the supramolecular level in these mixed organogels can occur through the operation of “sergeants and soldiers” principle, but the effect is solvent dependent. We also investigate the effects of doping chiral urethanes on the macroscopic properties of the organogels derived from the achiral urethanes and show that the amplification of chirality can be achieved in xerogels. We have used chiral (1R, 1S, and 2R) and achiral alkyl urethane gelators (3) derived from pyrene and studied the chirality of the supramolecular aggregates in isooctane and dodecane gels. Circular dichroism (CD) spectroscopy12a has been used to probe the chirality in the gels12b-f as well as in (7) (a) Smulders, M. M. J.; Schenning, A. P. H. J.; Meijer, E. W. J. Am. Chem. Soc. 2008, 130, 606. (b) Jonkheism, P.; van der Schoot, P.; Schenning, A. P. H. J.; Meijer, E. W. Science 2006, 313, 80. (c) Wilson, A. J.; Masuda, M.; Sijbesma, R. P.; Meijer, E. W. Angew. Chem., Int. Ed. 2005, 44, 2275. (d) Brunsveld, L.; Schenning, A. P. H. J.; Broeren, M. A. C.; Janssen, H. M.; Vekemans, J. A. J. M.; Meijer, E. W. Chem. Lett. 2000, 292. (8) (a) Lohr, A.; W€urthner, F. Chem. Commun. 2008, 2227. (b) Ishi-I, T.; Kuwahara, R.; Takata, A.; Jeong, Y.; Sakurai, K.; Mataka, S. Chem.;Eur. J. 2006, 12, 763. (c) Ishi-i, T.; Crego-Calama, M.; Timmerman, P.; Reinhoudt, D. N.; Shinkai, S. J. Am. Chem. Soc. 2002, 124, 14631. (9) Ajayaghosh, A.; Varghese, R.; George, S. J.; Vijayakumar, C. Angew. Chem., Int. Ed. 2006, 45, 1141. (10) For a recent review on the amplification of chirality in dynamic supramolecular assemblies, see: Palmans, A. R. A.; Meijer, E. W. Angew. Chem., Int. Ed. 2007, 46, 8948. (11) Nam, S. R.; Lee, H. Y.; Hong, J.-I. Chem.;Eur. J. 2008, 14, 6040. (12) (a) Gottarelli, G.; Lena, S.; Masiero, S.; Pieraccini, S.; Spada, G. P. Chirality 2008, 20, 471. (b) Dawn, A.; Shiraki, T.; Haraguchi, S.; Sato, H.; Sada, K.; Shinkai, S. Chem.;Eur. J. 2010, 16, 3676. (c) Jung, J. H.; Ono, Y.; Shinkai, S. Chem.;Eur. J. 2000, 6, 4552. (d) Li, Y.; Wang, T.; Liu, M. Soft Matter 2007, 3, 1312. (e) Maitra, U.; Mukhopadhyay, S.; Sarkar, A.; Rao, P.; Indi, S. S. Angew. Chem., Int. Ed. 2001, 40, 2281. (f) 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.

Published on Web 09/22/2010

DOI: 10.1021/la1029905

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the xerogels13 of the mixed systems. The thermal and mechanical strengths of these gels were investigated by gel melting experiments and dynamic rheology. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to analyze the morphology of these gels. We have supplemented the experimental results with molecular modeling studies, wherein the origin of helicity in the supramolecular assembly is explained.

Experimental Section Analytical Techniques. Circular dichroism spectra for gels and solutions were recorded on JASCO J-715 and JASCO J-815 CD spectropolarimeters with a spectral bandwidth of 2 nm, a response time of 2 s, and a scan rate of 100 nm/min using a quartz cell of 1 mm path length. Linear dichroism (LD) spectra for gels were recorded on the JASCO J-815 spectropolarimeter using the same quartz cell. Corresponding variable temperature experiments were performed with a PTC-423S/15 Peltier-type temperature controller with a temperature range of -10 to 110 °C and adjustable temperature slope. For our experiments, a temperature slope of 1 °C/min was used. Solid state CD and LD spectra were recorded (JASCO J-815) using a detachable quartz cell of 0.5 mm path length, which consists of two quartz plates, with a groove on one of them. Solid state CD was done on xerogels obtained by drop-casting the hot isooctane solutions. A known amount (100 μL) of the sol (90 °C) was placed at the center of the groove. The gel thus formed was allowed to air-dry for ∼30 min, the cell assembled, and the spectra recorded. The gel samples for AFM were prepared by drop-casting (10 μL) the hot sol (90 °C) onto a freshly cleaved mica surface and allowing it to air-dry for about 1 h. The xerogel samples for SEM studies were prepared as follows: 10 μL of sol was placed over a carbon tape on the metal stub and allowed to air-dry for 12 h followed by vacuum desiccation for 3 h. The sample thus obtained was sputter coated with 10 nm thick gold film and examined with Quanta 200 scanning electron microscope operated at 10 kV. AFM imaging was performed in “soft” tapping mode on a Veeco Dimension 3100 SPM instrument (Veeco Instruments, Santa Barbara, CA) using soft FESP cantilevers (Veeco Probes, nominally 2.8 N/m, 75 kHz). For the inverted tube gel melting experiments, sealed test tubes containing the gels (stabilized for ∼12 h) were kept upside down in a thermostatted water bath and the temperature was raised slowly (∼ 2 °C/min). The temperature at which the gel fell under gravity was identified as the gel melting temperature. Dynamic rheological measurements were done on the gels on a stress-controlled AR 1000 rheometer (TA Instruments) using a plate-plate (hatched) geometry (20 mm diameter, 400 μm geometry gap). The hot sol (∼ 90 °C) was placed over the rheometer plate (kept at 15 °C for the undoped/doped gels of 3 and at 20 °C for all other systems) using a preheated Pasteur pipet and allowed to gel for 3 min, and the top plate was brought down to the required gap (400 μm). The gel was stablilized for 30 min, and the experiments conducted thereafter. The dynamic frequency sweep experiments were carried out at a constant stress of 1 Pa, and the stress sweep experiments were carried out at a constant frequency of 0.5 Hz. For the variable temperature experiments, a heating rate of 1 °C/min was chosen and the experiments were done at a constant frequency of 1 Hz. Synthesis. 1-Nitropyrene, 1-aminopyrene, and compounds 1R, 2R, and 3 were synthesized following a previously reported procedure.19b X-ray Crystallography. Crystallographic data were collected on a Bruker CCD diffractometer using graphite-monochromatized Mo KR radiation (λ=0.71073 A˚), using a SMART software package.14 An empirical absorption correction was applied using (13) The hot isooctane sols (100 μL) are drop-cast on a quartz plate and air-dried to obtain a film on the plate. See the Experimental Section for more details. (14) SMART, version 5.05; Bruker AXS: Madison, WI, 1998. (15) Sheldrick, G. M. SADABS; University of G€ottingen: Germany, 1996.

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Das et al. Chart 1. Molecular Structures of 1R and 2R (left) and the Achiral Gelator 3 (right)

SADABS.15 The crystal structure was solved by the direct method using SIR9216 and refined by the full-matrix least-squares method using SHELXL97.17 Crystal Data for 318. C27H31NO2; fw = 401.53; monoclinic; space group P21/c; a = 19.646(4), b=4.8489(10), c=23.877(5) A˚; β=95.299(4)o; V=2264.9(8) A˚3, Z=4; Fcalcd=1.178 g cm-3; Mo KR radiation (λ = 0.71073 A˚); T = 292(2) K; R1 = 0.0596, wR2 = 0.1127 (I > 2σ (I)); R1 = 0.1498, wR2 = 0.1363 (all data).

Results and Discussion A chiral urethane (1R) that formed gels in hydrocarbon solvents was described by us a few years ago.19 Force field optimized assembly of 1R had suggested a right-handed helical organization at the supramolecular level. The initial results led us to undertake a more detailed study with this molecule, its homologue 2R, and the structurally related achiral analogue 3 in an attempt to understand the transcription of chirality from the molecular to the supramolecular and finally to the macroscopic domain (Chart 1). Self-Assembly of 1R and 2R. The chiral urethanes 1R and 2R showed similar aggregation properties. Isooctane and n-dodecane gels (∼30 mM) of these compounds exhibited CD signals,20 indicating the existence of chiral aggregates (Figure 1a).21 A variable temperature experiment indicated that as the gel was progressively heated, the CD signal decreased and completely disappeared as the gel melted (Figure 1b), suggesting that supramolecular chirality was responsible for the CD signal. Xerogel films of 1R and 2R obtained from isooctane also showed intense CD signals, indicating that the helical organization was retained in the solid state (Figure 1c). The two bands observed in the CD spectrum of the xerogel correspond to the absorption spectrum of the film (Supporting Information Figure S1). The aggregation behavior of 1R in isooctane, from its drop-cast films (obtained from a constant volume), was then investigated as a function of concentration. The critical gelation concentration of 1R in isooctane is ∼25 mM. An isooctane solution of 1R (3 mM) was CD silent, suggesting the absence of chiral supramolecular aggregates in this solution. However, an air-dried film derived from an isooctane solution of 1R (3 mM) showed a strong CD signal, clearly showing that, as the solvent evaporates, 1R begins to form chiral aggregates. The CD intensity remained more or less constant as the concentration of 1R used to prepare the film was increased from 3.1 to 12.5 mM (Figure 1d). This was attributed to the partial precipitation of 1 from the solution at these concentrations as the (16) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (17) Sheldrick, G. M. SHELXS97 and SHELXL97; University of G€ottingen: Germany, 1997. (18) CCDC 720854 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre at www.ccdc.cam.ac.uk/data_request/cif. (19) (a) Maitra, U.; Potluri, V. K.; Sangeetha, N. M.; Babu, P.; Raju, A. R. Tetrahedron: Asymmetry 2001, 12, 477. (b) Babu, P.; Sangeetha, N. M.; Vijaykumar, P.; Maitra, U.; Rissanen, K.; Raju, A. R. Chem.;Eur. J. 2003, 9, 1922. (20) However, due to high optical density of the gel, the position of the CD band for the native gel was not reliable. (21) The LD spectra for all the samples studied by CD spectroscopy were negligible. See the Supporting Information for details.

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Article Table 1. Gel to Sol Transition Temperatures (“Gel Melting”)

Figure 1. (a) CD spectra of isooctane gels of 1R (solid line) and 2R (dotted line) at 293 K (1 mm path length). (b) Variable temperature experiment (heating) on an isooctane gel of 1R (31 mM). (c) CD spectra of the xerogel films of 1R (solid line) and 2R (dotted line) obtained from isooctane (initial conc. ∼ 31 mM). (d) Variation of the ellipticity (at 362 nm) of xerogel films of 1R (closed squares) and 2R (open circles) as a function of concentration. The solid line (for 1R) and the dashed line (for 2R) are obtained by employing adjacent averaging with 2 points and, as such, provide visual aid to the pattern of the dependence of CD on the concentration of 1R or 2R. (Error: 10-15% for dried film CDs.)

hot sol was cooled to room temperature.22 In this concentration range, as the solvent evaporates to form the film, partial aggregation and partial precipitation appear to take place. However, at higher concentrations, a net increase in the Cotton effect was observed, since aggregation over precipitation is increasingly favored. Similar dependence of ellipticity on concentration was also observed for the xerogel films of 2R from isooctane, further confirming the similar mode of aggregation of the two molecules in the xerogels (Figure 1d, open circles). With this understanding of the self-assembly processes for the chiral gelators 1R and 2R, we shall now discuss the effect of enantiomeric excess (ee) of 1 on the supramolecular chirality of the aggregates. Majority Rule Experiment in Xerogels (from Isooctane) of 1R/1S. Molecular chirality has a profound influence on the growth and stability of self-assembled fibrous network.23 It has often been reported that, in the case of a chiral gelator, the pure enantiomer is a better gelator than the corresponding racemates,24 but this is not always true.25 When both the enantiomers are present in the system, they tend to interact and form aggregates, which may or may not lead to assemblies with high aspect ratio. The former case leads to fibers and eventually gels, whereas the latter frequently results in platelets26 leading to precipitation. They may also form conglomerates, that is, spontaneously resolve and (22) We have observed that, up to a concentration of around 6 mM, 1 remains in solution. However, at higher concentrations, which are below the CGC (e.g., 12.5 and 18.7 mM), cooling the hot sol to room temperature results in suspensions. (23) (a) Brizard, A.; Oda, R.; Huc, I. Top. Curr. Chem. 2005, 256, 167. (b) Smith, D. K. Chem. Soc. Rev. 2009, 38, 684. (24) (a) Boettcher, C.; Schade, B.; Fuhrhop, J.-H. Langmuir 2001, 17, 873. (b) Bhattacharya, S.; Ghanashyam Acharya, S. N.; Raju, A. R. Chem. Commun. 1996, 2101. (c) Hanabusa, K.; Yamada, M.; Kimura, M.; Shirai, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1949. (d) de Loos, M.; van Esch, J.; Kellog, R. M.; Feringa, B. L. Angew. Chem., Int. Ed. 2001, 40, 613.   c, M.; Pozzo, J.-L.; Fages, F.; Mieden-Gundert, G.; V€ogtle, (25) Caplar, V.; Zini F. Eur. J. Org. Chem. 2004, 4048. (26) Fuhrhop, J.-H.; Boettcher, C. J. Am. Chem. Soc. 1990, 112, 1768. (27) Spector, M. S.; Selinger, J. V.; Singh, A.; Rodriguez, J. M.; Price, R. R.; Schnur, J. M. Langmuir 1998, 14, 3493.

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composition of the isooctane gels

Tgel (°C)

1R (30 mM) 2R (30 mM) 3 (30 mM) 1R (80% ee) 1R (60% ee) 1R / 1S (racemate) 3 (27 mM)/1R (3 mM) 3 (27 mM)/2R (3 mM)

65 60 60 60 58 55 56 61

Figure 2. CD spectra of xerogels obtained from the isooctane sols of 1R/1S (in different proportions).

form separate aggregates.27 In that case, the racemate is generally able to form gels. In our present study, we observed that the racemic mixture of 1R and 1S was able to gel isooctane (total concentration ∼ 30 mM), although the thermal stability was significantly less (55 °C) compared to the gel from pure 1R or 1S (65 °C) (see Table 1). That the racemic gel is thermally less stable is indicative of possible coaggregate formation of the enantiomers, which can compete with conglomeration process and eventually affect the strength of the gel. However, the CD spectra could not be studied in detail as a function of ee in the isooctane gels, since the absorbance of the gel was very high. In dodecane, the gelation behavior was different. Although the gelator with 80% ee formed a mechanically weaker gel in dodecane compared to the pure enantiomer (see rheological data later), the racemic mixture produced a solution, instead of a gel, in the same solvent. This suggests that subtle effects that the change of solvent imparts on the mode of aggregation influence the interaction between the enantiomers.28 The CD spectra of the xerogels were probed as a function of the ee of 1R/1S. The hot isooctane sols of 1R/1S (in varying ratios, with the total concentration kept constant at ∼30 mM) were dropcast on a quartz plate, and the air-dried films were investigated by CD spectroscopy. The CD signal intensity reduced drastically as the 1R (or, 1S) gel was doped with only 10 mol % of the other enantiomer (Figure 2), indicating that there was no chiral amplification in this system through the “majority rule” effect.29 Since the chirality of the dried assembly did not show a linear dependence on the ee, it implied that 1R/1S formed a coassembly in the dried state, with no conglomerate present. From these discussions, we can conclude that there is no chiral amplification in the self-assembly of 1, at least in the xerogels.30 In view of this, it is important to address   c, (28) (a) Makarevic, J.; Jokic, M.; Raza, Z.; Stefani c, Z.; Kojic-Prodic, B.; Zini M. Chem.;Eur. J. 2003, 9, 5567. (b) Oda, R.; Huc, I.; Candau, S. J. Angew. Chem., Int. Ed. 1998, 37, 2689. (29) For chirality amplification through “majority rule” effect, see, for example: (a) Green, M. M.; Garetz, B. A.; Munoz, B.; Chang, H.; Hoke, S.; Cooks, R. G. J. Am. Chem. Soc. 1995, 117, 4181. (b) van Gestel, J.; Palmans, A. R. A.; Titulaer, B.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 2005, 127, 5490. (30) No attempt was made to quantify the ee in the liquid phase at different concentrations, since the CD data discussed in the context of ee were recorded for the xerogel samples.

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Figure 3. (a) CD spectra of isooctane gel of 1R (solid line), isooctane gel of 3 doped with 10 mol % 1R (dotted line), 3 mM isooctane solution of 1R (dashed line); the total concentration in the gels was maintained constant at ∼30 mM. (b) Ellipticity at 362 nm as a function of concentration of 1R in the drop-cast films of 3/1R (open circles). The control with 1R (closed squares) is shown for comparison. The total concentration in the gel is kept constant at ∼30 mM. (c) Ellipticity at 362 nm as a function of concentration of 2R in the drop-cast films of 3/2R (open circles). The control with 2R (closed squares) is shown for comparison. The total concentration in the gel is kept constant at ∼30 mM. In both (b) and (c), the solid line (for controls with 1R and 2R, respectively) and the dashed lines are obtained by employing adjacent averaging with two points and, as such, provide visual aid to the pattern of the dependence of CD on the concentration of the chiral dopant (Error: 10-15% for solid state CD.)

if we can achieve an amplification of chirality in the mixed gels and xerogels of the structurally similar chiral urethanes 1R or 2 and achiral 3. This is the topic of the next section. Amplification of Chirality in Isooctane Gels and Xerogels. Can molecule 1R or 2R be used to modulate supramolecular helicity in an organogel made from the structurally similar achiral urethane 3? Modeling studies indicated that 3 can also form helical supramolecular aggregates like its chiral analogues 1R and 2R. We envisaged that doping with chiral 1 or 2 should be able to bias the system to adopt a preferred helical arrangement at the supramolecular level. Indeed, when the isooctane gel of achiral 3 (∼30 mM) was doped with 10 mol % (∼3 mM) of 1R, a strong positive CD signal was observed (Figure 3a). A control experiment with a 3 mM solution of 1R did not show any CD signal, indicating that the chirality in the mixed assembly can be attributed to the operation of the “sergeants and soldiers” principle, and this phenomenon can be regarded as an amplification of chirality.31 Similar results were obtained when 2R was used as the dopant. This is expected since both 1R and 2R have shown similar aggregation behavior (vide supra). Surprisingly, doping of the n-dodecane gels of 3 with the chiral urethanes failed to produce any CD signal (Supporting Information Figure S3b), indicating that the successful transfer of molecular chirality of the dopant to the mixed assembly leading to an eventual helical bias depends on the solvent. Xerogel films obtained from mixtures of achiral/chiral urethanes in various proportions in isooctane were also studied by CD spectroscopy.32 For the mixed assembly of 3/1R, interesting observations were made. At ∼20 mol % doping (∼6 mM of 1R), the dried film showed negligible CD (Figure 3b). Since pure 1R formed (31) It is also possible that 1R forms a chiral homoassembly and the gel of 3 provides the fibrous structure on which 1R can aggregate. (32) We observed some variations in the absolute values of the maxima for the solid state CD spectra from one experiment to another, and there was 10-15% error in the observed values, which varied depending on the composition of the films.

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a chiral homoassembly in the xerogel when made from this concentration (vide supra), the observed result indicates that the formation of chiral aggregates was disrupted at this concentration. This suggests that as the concentration of 1R is increased gradually, it forms a coassembly with 3 instead of forming a separate domain. However, at this stage, the amount of 1R is not large enough to induce a net chirality in the coassembly. At higher levels of doping (e.g., 9.4 and 12.5 mM 1R, i.e., 30 and 40 mol % 1R), the films showed a strong negative CD signal (open circles, Figure 3b), indicating that the supramolecular assembly adopts the reverse helical organization (left-handed) with respect to the parent homoassembly of 1R (the homoassembly of 1R is right-handed).19 Although 3 is still present in excess, 1R is now able to “chirally bias” the process of supramolecular aggregation. Since the selfstacking of 1R leads to a right-handed helical assembly (vide supra), it is reasonable to believe that it is the coassembly of 1R and 3 that is responsible for the observed negative Cotton effect. This suggests that 1R effectively acts as a “sergeant” and the observed supramolecular chirality arises from a “sergeants and soldiers” effect. Such inversion of chirality has been reported for self-assembled mixed (chiral/achiral) OPV gels.9 At “doping” concentrations of 19 mM (60 mol %) and beyond, strong positive CD signals were obtained (Figure 3b), indicating a switchover of the supramolecular aggregates to a right-handed helical arrangement. Under these conditions, the chiral gelator is now the major component in the mixed assembly with 3 and there should be a substantial presence of the homoassembly of 1R, resulting in a right-handed helical supramolecular chirality. However, the mixed assembly of 3/2R did not show any amplification of chirality. Both the control (with only the chiral components) as well as the mixed systems (chiral/achiral) exhibited similar variation of the ellipticity with increasing concentration of the chiral component (Figure 3c). This result suggests that the molecular structure of the chiral dopant plays an important role in determining the overall chirality of the supramolecular assembly. Langmuir 2010, 26(20), 16141–16149

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Figure 4. Tapping mode AFM height images of (a) isooctane gel (∼30 mM) of 1R and (b) isooctane gel (∼30 mM) of 2R.

Structural Characterization. The doped/undoped gels were investigated by SEM and AFM. For AFM analysis, the xerogels of 1R and 2R were prepared by drop-casting the hot isooctane sols on freshly cleaved mica followed by air drying. Tapping mode AFM images showed a highly entangled fibrous network, with similar fiber diameters ranging from 70 to 150 nm (Figure 4). The thinnest fiber observed was 40 nm in diameter. The morphological similarity is in agreement with the conclusion reached from spectroscopic studies that both molecules have similar aggregation modes. No helicity in the fibers could be observed from these images, suggesting either that the chirality of the supramolecular aggregates does not translate to the macroscopic aggregates or that the helix pitch is too small to be resolved by microscopy.23b However, the SEM images of the dried isooctane/dodecane gels of the achiral urethane 3 doped with 10 mol % of the chiral analogues (1R and 2R) revealed distinct morphological differences between the doped and undoped gels (Figure 5), suggesting profound influence of the chiral dopant on the SAFIN structure. The xerogel obtained from the isooctane gels of 3 showed long, flat, tapelike fibers (Figure 5a). On doping with 10 mol % 1R, broken fibers were observed (Figure 5b). In contrast, for the 3/2R (10 mol %) isooctane gel, a morphology similar to that of pure 3 gel was obtained, except for the fact that the width of the fibers was more uniform (Figure 5c). Similarly, the SEM of the xerogel of 3/2R (10 mol %) from dodecane revealed the presence of tapelike fibers which clearly had larger width compared to the dodecane gel of pure 3 (Figure 5d and f), whereas the fibers from the xerogel of 3/1R from dodecane were clearly thinner, more aggregated, and cylindrical (Figure 5e). These results clearly suggest33 that the packing of 3/2R in the gel fibers is more favorable than that of 3/1R at the level of 10 mol % of doping. Since an inversion of chirality was observed at intermediate doping concentrations for the dried mixed assembly of 3/1R (vide supra), the morphology of this mixed assembly was investigated by SEM as a function of increasing concentration of the chiral dopant. Large aggregated structures, and not well-defined fibers, were observed from 20 to 60 mol % doping (Supporting Information Figure S4a-c). Spherical aggregates appeared at still higher doping concentration of 80 mol %, that is, ∼25 mM (Supporting Information Figure S4d). The formation of spherical aggregates is not likely due to the rigorous drying process employed for the SEM sample preparation, since an air-dried xerogel also revealed the presence of (33) In these self-assembled systems, the thinner fibers and more meshlike aggregates as observed for dodecane gel of 3/1R indicate a diminished efficiency of the self-assembly and eventually the gelation process as evidenced by gel melting and rheology results. However, we believe that such correlation of gel morphology with the efficiency of aggregation may not hold for other self-assembled systems and hence cannot be generalized. For example, racemic inositol gelators have been shown to have improved gelation capacity over the corresponding chiral gels due to the formation of thinner and more connected fibers. See: Watanabe, Y.; Miyasou, T.; Hayashi, M. Org. Lett. 2004, 6, 1547.

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Figure 5. SEM images of the xerogels obtained from isooctane sols of (a) 3, (b) 3/1R, and (c) 3/2R and the xerogels obtained from dodecane sols of (d) 3, (e) 3/1R, and (f) 3/2R. The white bars on the top right corner of the images (a, b, d-f) approximately represent a length of 500 nm.

such aggregates in contact mode AFM (Supporting Information Figure S4e). Interestingly, the SEM investigation of the xerogel of 3/2R, at similar doping concentrations of 2R (80 mol %), showed a fibrous network similar to the morphology of an isooctane gel of pure 2R (Supporting Information Figure S4f). This result is consistent with the morphological data obtained at 10 mol % doping and demonstrates that 3 and 1R do not pack well in the gel fibers even at a higher level of doping with 1R, as opposed to the 3/2R pair. As we would see later, these conclusions also provide consistent explanations for the rheological data. Thermal Stability of the Isooctane Gels. The gel to sol transition temperature gives an idea about the thermal stability of the gels. These experiments were conducted by the inverted test tube method.34 The isooctane gels of 3 doped with 10 mol % 1R showed lower thermal stability compared to the undoped gel (Table 1). Importantly, the mixed isooctane gel of 3/2R showed similar thermal stability as the pure gel of 3 (∼60 °C), which is ∼4 °C higher than that of 3/1R (56 °C). This indicates that structural recognition may be playing an important role so that the thermal stability is unaltered due to a favorable packing between 3 and 2R. This result is also consistent with the results of the morphological investigations of the mixed gels (vide supra) and data from the rheological experiments on the mixed dodecane gels (vide infra). (34) (a) Raghavan, S. R.; Cipriano, B. H. Gel Formation: Phase Diagrams using Tabletop Rheology and Calorimetry. In Molecular Gels: Materials with SelfAssembled Fibrillar Networks; Weiss, R. G., Terech, P., Eds.; Springer: Dordrecht, The Netherlands, 2006; Chapter 8.(b) Eldridge, J. E.; Ferry, J. D. J. Phys. Chem. 1954, 58, 992.

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Figure 6. (a) Dynamic frequency sweep experiments (stress kept constant at 1 Pa) on dodecane gels of 1R (G0 , black squares; G00 , red circles)

and 2R (G0 , blue triangles; G00 , green triangles). (b) Stress sweep experiments (frequency kept constant at 0.5 Hz) on dodecane gels of 1R (G0 , black squares) and 2R (G0 , red circles). (c) Comparison of the dynamic frequency sweep experiments (stress kept constant at 1 Pa) on dodecane gels of 1R (G0 , black squares) and 1R at 80% ee (G0 , red circles). (d) Variation of the shear storage modulus (G0 ) as a function of temperature (at a constant stress of 1 Pa and a constant frequency of 1 Hz) for n-dodecane gels of 1R (black squares) and 1R at 80% ee (red circles).

Another important observation in the gel melting experiments is that as the isooctane gel of 1R was progressively doped with the enantiomer 1S, the thermal stability of the gel decreased. In particular, the melting of the racemate-derived gel took place about 10 °C lower than the gel from the pure enantiomer (Table 1), demonstrating that the presence of the other enantiomer considerably affects the bulk properties of the gel. Rheological Properties. Rheological studies provide information about the mechanical strength35 and fragility of the gels derived from low molecular mass gelators, since they behave like soft solids under the conditions of the experiment.36 It is useful here to recall that the chirally doped gels of achiral 3 in dodecane failed to show amplification of chirality. A comparative study of the isooctane and dodecane gels would be useful in that context. The dodecane gels were well suited for rheological experiments because of the higher boiling point of this solvent, minimizing solvent evaporation during the experiment. Dynamic frequency sweep experiments on the dodecane gels of 1R and 2R showed that these gels exhibited high and comparable shear storage moduli (G0 ∼ 40 000 Pa) (Figure 6a). However, the 2R gel was more fragile, indicated by its lower yield stress value (σ* ≈ 350 Pa) compared to the 1R gel (σ* ≈ 800 Pa) (Figure 6b). Interestingly, a dodecane gel of 1R at 80% ee (i.e., 27 mM in 1R and 3 mM in 1S) shows almost 2.5 times reduced shear storage modulus (∼16 000 Pa) compared to the gel with 100% enantiomeric purity (Figure 6c). Also, this gel began to flow at a considerably lower applied stress (σ* ≈ 350 Pa; Supporting Information Figure S5). Variable temperature rheology experiments on these gels indicated that the gel with 80% ee of 1R has lower thermal stability. These experiments are in agreement with the results from the thermal stability studies obtained for the corresponding isooctane gels (wherein a decrease in thermal stability was observed as a function (35) Goodwin, J. W.; Hughes, R. W. Rheology for Chemists: an Introduction; The Royal Society of Chemistry: Cambridge, 2000. (36) (a) Almdal, K.; Dyre, J.; Hvidt, S.; Kramer, O. Polym. Gels Networks 1993, 1, 5. (b) Lortie, F.; Boileau, S.; Bouteiller, L.; Chassenieux, C.; Deme, B.; Ducouret, G.; Jalabert, M.; Laupr^etre, F.; Terech, P. Langmuir 2002, 18, 7218.

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Table 2. Dynamic Rheology of Gels a entry

dodecane gels

G0 (Pa)

G00 (Pa)

σ* (Pa)

1 2 3 4 5 6

1R (30 mM) 40 200 2100 780 1R (80% ee) 15 900 870 350 2R (30 mM) 43 100 2200 350 3 (30 mM) 4600 900 100 3 (27 mM)/1R(3 mM) 290 20 25 3 (27 mM)/2R(3 mM) 17 400 870 560 a The G0 and G00 values (at f = 1.0 Hz) are obtained from the frequency sweep experiment at a fixed stress of 1.0 Pa, and the yield stress (σ*) values are obtained from the stress sweep experiment at a fixed frequency of 0.5 Hz.

of decreasing ee; see Table 1). The data from the rheology experiments are summarized in Table 2. When the dodecane gel of 3 was doped with 10 mol % 1R, the mechanical strength of the gel was severely affected (Figure 7a). The shear storage modulus for the doped gel at 1.0 Hz was ∼300 Pa (entry 5, Table 2), about 15 times less than the undoped gel (∼4600 Pa). The yield stress of the gel was also small (∼25 Pa). On the contrary, doping with 2R showed strikingly different data (entry 6, Table 2). The doped gel (3/2R) exhibited a markedly improved yield stress value (∼560 Pa, almost 6 times the value of the undoped gel). Variable temperature rheology experiments showed that the gel doped with 1R had considerably lower Tgel compared to both 3 and 3/2R gels (Figure 7c). These results are consistent with the data obtained from gel melting experiments on the corresponding isooctane gels discussed earlier. They also match well with the morphological studies on these dodecane gels and further corroborate our earlier conclusion that the packing of 3/2R in the gel fibers is better compared to 3/1R (possibly because of the similar length of the alkyl side chain). Molecular Modeling. Molecular modeling was done to analyze the energetically most favorable conformations of the chiral/ achiral molecules and also to model the structure of the primary gel aggregates. Semiempirical PM3 and ab initio Hartree-Fock/ 6-311G** geometry optimization suggested that the orientation of Langmuir 2010, 26(20), 16141–16149

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Figure 7. Comparison of (a) frequency sweep, (b) stress sweep, and (c) variable temperature rheology experiments of the dodecane gels of 3 (squares), 3 doped with 10 mol % 1R (circles), and 3 doped with 10 mol % of 2R (triangles).

Figure 8. (a) Orientation of hydrocarbon side chain (3).The most electron rich areas (localized lone pairs of electrons) are shown. On the right is shown one of the minimum energy conformers of 3, with no repulsive overlap between the electron clouds of the lone pairs on the carbonyl and ester oxygens. (b) Minimum energy conformers of the chiral urethanes 1R and 1S. 1S adopts the conformation shown on the left, which leads to left-handed helix formation. 1R adopts the conformation on the right, leading to right-handed helix formation. (c) Helix formation from two different conformers of chiral 1R, shown with two molecules. Molecules on the top and bottom are shown in tube and ball-stick representation, respectively. The structure on the right is energetically favored based on semiempirical and molecular mechanics calculations.

hydrocarbon side chain has a strong effect on the total energy of the molecule, coming from the repulsive overlap of the electron clouds of the lone pair electrons surrounding the ester and carbonyl oxygens (Figure 8a, left). For this reason, the minimum energy structures of the studied molecules have an orientation of the Langmuir 2010, 26(20), 16141–16149

hydrocarbon side chain as shown in Figure 8a, right. The structures analyzed in Figure 8a are that of the achiral urethane 3. The conformation, as depicted on the right of Figure 8a (the favored conformation), leads to the formation of right-handed helical aggregates. For achiral urethane 3, another conformation of same energy is DOI: 10.1021/la1029905

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Figure 9. Energy-optimized assembly of 60 molecules of 3, showing the formation of both right- (30 molecules) and left-handed helical structures (30 molecules). The enlarged image clearly demonstrates the π-π stacking, H-bonding, and van der Waals interactions in the aggregates.

possible which will lead to left-handed helices (Figure 8b, left, 1S). The helices (right and left) formed from the two conformers of 3 will be equal in energy. Indeed, CHARMm24 force field calculations with ab initio Hartree-Fock/6-311G** calculated atomic charges (Mulliken population analysis) for large oligomers (up to 100 molecules) indicated the formation of both right and left-handed local helical structures for the achiral urethanes (Figure 9), consistent with the expectation. In these assemblies, the intermolecular hydrogen bonding network (between the carbonyl oxygen and the amide hydrogen of an adjacent molecule) is locally broken, and, at that point, the adjacent stacked molecules have the mirror image conformation, leading to the subsequent formation of the opposite helix structure. When we analyze the favored conformations of the chiral urethanes, the enantiomers 1R and 1S cannot adopt both the conformations (as discussed in the preceding paragraph for achiral 3). Although both the conformers have equal energy, the helical assemblies formed from these conformers are different in energy for a particular enantiomer. The presence of the methyl group in the side chain plays an important role in the choice of the preferred conformation. As the helix is progressively built from an enantiomer, an “out of aromatic plane” orientation of the methyl group is required for the formation of energetically stable helical assemblies. In Figure 8c, the formation of a certain type of helix is schematically demonstrated with two molecules of 1R. The assembly on the left leads to left-handed helical aggregates and that on the right forms a right-handed helix. For the structure on the left (Figure 8c), optimization of the stacking distance of the aromatic skeletons, keeping the distance between the carbonyl and ester groups as large as possible, brings the methyl group of the lower aromatic skeleton (ball and stick representation) close to the aromatic skeleton of the adjacent molecule. This situation leads to an additional repulsive interaction. However, such a situation does not arise for the structure on the right in Figure 8c, because of the “out of aromatic plane” orientation of the methyl group. Hence, the helix formation is energetically much more favorable for the conformation on the right, leading to only a right-handed helix for 1R. On the basis of the results of these calculations, the positive Cotton effect observed for the isooctane and dodecane gels of 1R (or 2R) can be assigned to the right-handed helical aggregates in these gels. X-ray Crystallography. The X-ray crystal structure of a single crystal of 3 grown by slow evaporation from a solution of 3 in EtOH shows that 3 crystallizes in the centrosymmetric monoclinic space group P21/c with four molecules in the unit cell 16148 DOI: 10.1021/la1029905

Figure 10. Packing of 3 along the y-axis (top) and packing of three molecules of 3 along the x-axis (space-filled model).

(Figure 10). The amide group attached to the alkyl chain is tilted 42.7° from the pyrene plane. The molecules pack along the crystallographic b-axis. Strong hydrogen bonding exists between the amide hydrogen of one molecule and the carbonyl oxygen of an adjacent molecule. The hydrogen bond parameters are 0.860 A˚ (N1-H1), 2.027 A˚ (H1-O1#), 2.875 A˚ (N1-O1#), and 168.11° (N1-H1-O1#). No helical organization is present in the solid state, probably because of the tight packing in the crystal structure. This result is similar to what was previously observed for the crystals of 1R.19 The crystal structure clearly shows the presence of π-π stacking, H-bonding, and van der Waals interactions in the solid state. These are likely to be the most important interactions in the gel aggregates as well. Summary. Alkyl-1-pyrenyl urethanes have been shown to form helical supramolecular aggregates in isooctane and dodecane. The chiral urethanes derived from (R)-2-octanol and (R)-2-decanol (1R and 2R, respectively) form right-handed helical organization, as evidenced by CD spectroscopy and supported by modeling studies. The isooctane gels of the mixed assembly of achiral/chiral urethanes showed amplification of chirality through the “sergeants and soldiers” principle. However, no chirality amplification was observed in the corresponding dodecane gels. The xerogel films of 3/1R obtained from isooctane showed inversion of chirality of the supramolecular assembly at intermediate doping concentrations of 1R. However, the xerogel films of 3/2R did not show any amplification of chirality at different doping levels of 2R. None of the gels discussed showed macroscopic chirality in the SAFIN structure (SEM/AFM investigation). The gels of 3/1R were thermally and mechanically weaker and more fragile compared to the gels obtained from pure 3. In contrast, the 3/2R gels showed comparable thermal stability (to the native gel of 3), significantly improved yield stress, and fibers of larger width. Langmuir 2010, 26(20), 16141–16149

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Conclusions The work described in this Article reports the first thorough and multitechnique studies on the effect of two structurally related but chirally different components on the chiral supramolecular structures of the self-assembled organogel network. The presence of strong CD in xerogel films and the absence of macroscopic chirality (AFM/SEM) suggest that, for the selfassembly of chiral low molecular mass gelators, the transcription of supramolecular chirality to the macroscopic domain is not guaranteed. The differences observed in the chiral properties of the xerogels of 3/1R and 3/2R indicate the operation of a different mechanism of chirality transcription in the mixed assembly in the absence of the solvent. This mechanism is also different from the one operating in the formation of the homochiral assembly of the pure chiral urethanes, since the supramolecular chirality of the homoaggregates was retained even as the solvent evaporates (vide supra), whereas, for example, in the case of 3/2R, the “sergeants and soldiers” effect is lost as the aggregates are dried. The “sergeants and soldiers” principle, responsible for the amplification of chirality in the mixed organogels of urethanes, has previously been shown to operate in other self-assembled organogels.9,11 The absence of chirality amplification in the dodecane gels suggests an important role of the solvent in the transcription of molecular chirality to the supramolecular domain in a mixed assembly. Even the interactions in the coassembly of the individual enantiomers are significantly influenced by the solvent as evidenced from the failure of the racemate to form gel in dodecane, whereas gelation occurred in isooctane. In this context, it is significant that the chiral gelators themselves form chiral supramolecular aggregates in dodecane, as demonstrated by the strong Cotton effect in the gels obtained from the pure chiral urethanes. In contrast to the 3/2R gels, the 3/1R gels (both in isooctane and dodecane) were characterized by lower thermal stability, severely affected yield stress and fibers of smaller width. These results clearly demonstrate the important role played by the molecular recognition process, leading to a more favorable packing of 3 and 2R in the gel fibers, probably because of the similar length of the side-chain alkyl groups. Moreover, such similarity in the macroscopic properties such as thermal and mechanical strength of the gels from the two solvents (dodecane and isooctane) despite significant differences in the chiroptical properties of the supramolecular aggregates indicates the hierarchical nature of the aggregation process in the mixed chiral/achiral urethanes. This may be correlated with the failure of the chirality of the initially formed

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fibrils to get transferred to the assemblies of higher order. The absence of helical packing in the crystal structure of 3 indicates that the organization of molecules in the gel state is different from that in the crystal. In this case, the crystal structure does not represent a good model for the packing of molecules in the helical gel fibrils. This is probably expected, since the crystals were grown from a nongelling solvent (EtOH). However, the presence of π-π stacking and H-bonding interactions as the main interactions in the solid state provides a basis for modeling the primary gel aggregates. Besides, the modeling studies show the preference of right-handed helical organization for 1R and 2R, and hence, the observed positive CD signal from the gels of these molecules may be correlated with right-handed helical aggregates. The propensity of the formation of both right and left-handed local helical structures for the achiral gelator 3, as revealed from energy-optimization studies, suggests why we might expect the amplification of chirality in the mixed gels. In conclusion, the present study on the self-assembled alkyl 1-pyrenyl urethanes demonstrates that the mechanism of the expression of molecular chirality into the supramolecular and macroscopic domain is quite complex in this system. We believe that the results from this study will enhance the scope of further research toward the effect of the structure-property relationship of small self-assembling organic molecules on the manifestation of chirality of the aggregates. This should culminate into designing synthetic self-assembled structures with more control and better prediction on the tuning of supramolecular and macroscopic chirality. Acknowledgment. We thank the Department of Science and Technology, New Delhi (Grant No. SP/S1/OC-11/2004) and the Jawaharlal Nehru Centre for Advance Scientific Research, Bangalore, for financial support. R.K.D. and R.K. thank the CSIR for Senior Research Fellowship. Mr. Saikat Sen is acknowledged for his help in obtaining the crystal structure. We also acknowledge the Institute Nanoscience Initiative (INI) for the SEM images. J.L. acknowledges the support from Academy of Finland under research Contract No. 123801. Supporting Information Available: Physical characterization data for the compounds, absorption spectra, LD spectra of the gel samples and xerogels, additional CD spectra, SEM images, rheology data, and details of molecular modeling studies. This material is available free of charge via the Internet at http://pubs.acs.org.

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