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Striking Correlation between the Unusual Trigonal Crystal Packing and the Ability to Self-Assemble into Nanofibers of 2,3-Di-n-alkyloxyanthracenes† Alexandre G. L. Olive,‡ Guillaume Raffy,‡ Hassan Allouchi,§ Jean-Michel Leger, Andre Del Guerzo,*,‡ and Jean-Pierre Desvergne*,‡
Universit e Bordeaux 1, CNRS, Institut des Sciences Mol eculaires UMR 5255, Nanostructures Organiques e de Tours, Laboratoire de Chimie Physique, PCMB EA 4244, NEO, 33405 Talence, France, §Universit 37200 Tours, France and Universit e Victor Segalen Bordeaux II, Laboratoire de Pharmacochimie, 33076 Bordeaux, France )
‡
Received December 21, 2008. Revised Manuscript Received February 4, 2009 The space group of the crystals of derivatives of 2,3-dialkoxyanthracenes is monoclinic P21/a (herringbone structure) with the linear ethyl or propyl chains but abruptly changes to the trigonal P3 or R3 space group for butyl to heptyl chains. Strikingly, this switch is correlated with the capacity of these compounds to self-assemble into nanofibers and organogels. Besides, compounds with a chain length exceeding seven carbon atoms could not be crystallized in accordance with the analysis of the projected crystal structure but are nevertheless excellent organogelators. The study of this series of compounds suggests a tight link between the molecular structure of the crystals and that of the organogels.
Introduction The formation of well-defined self-assembled structures with nanoscopic dimensions is of importance in a variety of research fields ranging from nanoscience to biomedical engineering.1 In this perspective, physical gels are of interest because they can be simply and spontaneously formed when generating fibrils of nanosize diameters. More specifically, gelling agents based on small molecules are being extensively investigated because of their potential to produce thermoreversible nanostructured materials with desired and stimuli-tuned properties.2-9 Although a knowledge of the supramolecular organization within the fibrils is of crucial importance from both an academic standpoint and the development of novel soft materials, no straightforward technique † Part of the Gels and Fibrillar Networks: Molecular and Polymer Gels and Materials with Self-Assembled Fibrillar Networks special issue. *Corresponding authors. E-mail:
[email protected];
[email protected].
(1) Whitesides, G. M.; Matthias, J. P.; Seto, C. T. Science 1991, 254, 1312–1319. (2) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133–3160. (b) Molecular Gels: Materials with Self-Assembled Fibrillar Networks; Weiss R. G., Terech P., Eds.; Springer: Dordrecht, the Netherlands, 2006. (c) Sangeetha, N. M.; Maitra, U. Chem. Soc. Rev. 2005, 34, 821–836. (3) Desvergne, J.-P.; Olive, A. G. L.; Sangeetha, N. M.; Reichwagen, J.; Hopf, H; Del Guerzo, A. Pure Appl. Chem. 2006, 78, 2333–2339. (4) Del Guerzo, A.; Olive, A. G. L.; Reichwagen, J.; Hopf, H.; Desvergne, J.-P. J. Am. Chem. Soc. 2005, 127, 17984–17985. (5) Shklyarevskiy, I. O.; Jonkheijm, P.; Christianen, P. C. M.; Schenning, A. P. H. J.; Del Guerzo, A.; Desvergne, J.-P.; Meijer, E. W.; Maan, J. C. Langmuir 2005, 21, 2108–2112. (6) (a) Srinivasan, S.; Babu, S. S.; Praveen, V. K.; Ajayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 5746–5749. (b) Brizard, A.; Stuart, M.; Van Bommel, K.; Friggeri, A.; De Jong, M.; Van Esch, J. Angew. Chem., Int. Ed. 2008, 47, 2063–2066. (7) Mizrahi, S.; Rizkov, D.; Hayat, N.; Lev, O. Chem. Commun. 2008, 2914–2916. (8) Yang, Y.; Chen, T.; Xiang, J.-F.; Yan, H.-J.; Chen, C.-F.; Wan, L.-J. Chem.;Eur. J. 2008, 14, 5742–5746. (9) Basit, H.; Pal, A.; Sen, S.; Bhattacharya, S. Chem.;Eur. J. 2008, 14, 6534–6545.
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is available or fully satisfactory to reveal this organization; consequently, much effort is being devoted to unraveling the relationships between the molecular packing and physical properties of gels.2 One of the best methods is to extrapolate the structure of the gel phase from X-ray crystallography on single crystals obtained from the same molecule or from a close analogue.10-12 2,3-Dialkoxyanthracene derivatives DAOA (n = 6-16) were shown to be very efficient organogelators for a large variety of solvents (alcohols, amines, alkanes, and nitriles). The gelling properties were empirically found to be dependent on a subtle balance between the chain length and the shape of the aromatic moiety,13 and the optimal chain dimension was found for 10-12 carbons for the 2,3-disubstituted derivative. With decreasing chain length, the gelation process is less effective and is found to be strongly sensitive to the experimental conditions (solvent, temperature gradient, etc.); it is in competition with crystallization that predominates for the shortest chains (n < 6 carbons). In a recent paper, it was reported that DHxOA (n = 6) forms both a gel and crystals. DHxOA crystallizes in space group R3 with 18 molecules in the unit cell.14 The molecules are organized in triads themselves packed head-to-tail in layers. This unanticipated molecular arrangement, contrasting with the usual herringbone structure of simple anthracenes, was also suggested from molecular modeling and spectroscopic investigations to be operative in the gel phase.15 A similar molecular model was (10) Ostuni, E.; Kamaras, P.; Weiss, R. G. Angew. Chem., Int. Ed. 1996, 35, 1324–1326. (11) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237–1247. (12) Ballabh, A.; Trivedi, D. R.; Dastidar, P. Chem. Mater. 2003, 15, 2136–2140. (13) Desvergne, J.-P.; Brotin, T.; Meerschaut, D.; Clavier, G.; Placin, F.; Pozzo, J.-L.; Bouas-Laurent, H. New J. Chem. 2004, 28, 234–243. (14) Pozzo, J.-L.; Desvergne, J.-P.; Clavier, G. M.; Bouas-Laurent, H.; Jones, P. G.; Perlstein, J. J. Chem. Soc., Perkin Trans. 2 2001, 824-826. (15) Placin, F.; Desvergne, J.-P.; Belin, C.; Buffeteau, T.; Desbat, B.; Ducasse, L.; Lassegues, J.-C. Langmuir 2003, 19, 4563–4572.
Published on Web 03/20/2009
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Scheme 1. General Formulas of 2,3-Di-n-alkyloxyanthracene DAOA
also projected for DDOA, a member of the DAOA family bearing longer chains (n = 10) with marked structural variations due to the larger conformational flexibility of the chains.3,15 These results were intriguing because they contrast with many structures described in the literature that revealed face-to-face stacking of aromatic cores. This prompted us to investigate the molecular packing mode of other members of the series that crystallized, in relation to the chain length and their gelling or aggregative properties. Indeed, it was crucial to know whether the trigonal space group disclosed for DHxOA is an exception in the series or rather the favorite packing mode for these rodlike self-assembling molecules. Knowing whether and why this organization is always favored would in turn be exploited to describe the gels, often considered to result from aborted crystallization,2 and influence the design of novel compounds. Single crystals suitable for an X-ray study could be grown for anthracenes with chain lengths ranging from two to seven carbon atoms (n = 2-7), whereas the derivatives with longer chains (n > 7) gave, under our experimental conditions, only gels or a nonexploitable solid material (xerogel).
Materials and Methods Confocal Microscopy. Fluorescence images have been obtained with a Picoquant Microtime 200 microscope, using Symphotime software, coupled to a pulsed picosecond laser (6 ps pulses) operating at about 4.8 MHz frequency, tuned to 770 nm and frequency doubled in an SHG unit (thus generating 385 nm pulses). The fluorescence was filtered with interference band-pass filters. The Raman spectra were recorded in backscattering geometry on a Labram I (Dilor-France) microspectrometer in conjunction with a confocal microscope. A 632.8 nm incident beam from a He-Ne laser was used and filtered out by a Notch filter.
Results and Discussion Synthesis and Crystallization. The preparation and characterization of the compounds were described elsewhere.13,16 Single crystals were obtained from a 10 mM chloroform solution of DAOA subjected to slow heptane vapor diffusion. The noncolored solids display a similar crystal shape for all of the compounds (plate) except for DHOA (n = 6), which produces hexagonal tablets (Table 1). Other experimental conditions for growing single crystals met with failure (i.e., cooling a hot solution of DAOA (n > 3) led to gels or a noncrystalline solid). X-ray Crystal Structures. Table 1 shows the details of the X-ray crystal structures, with cell parameters and R values. An inspection of Table 1 shows that the molecular packing (16) Pozzo, J.-L.; Clavier, G. M.; Colomes, M.; Bouas-Laurent, H. Tetrahedron 1997, 53, 6377–6390.
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and the space group of DAOA compounds are dependent on the length of the alkyl substituents. Thus, the compounds have been classified into two main categories according to the substituent size and the space group: the short chains (n = 2 and 3) promote crystallization in a herringbone mode whereas the long chains (n = 4 to 7) induce packing into triads (vide infra). DEOA (n = 2) and DPrOA (n = 3). With the short chains (n = 2 and 3), the molecules crystallize in monoclinic space group P21/a, with a packing mode similar to that of anthracene, the nonsubstituted parent compound. As already pointed out by Robertson,17 this crystallization is not unexpected because of the ellipsoidal shape of the molecules, similar to that of anthracene that is recognized to induce a herringbone structure of so-called type A (molecules experience a face-to-edge arrangement with an angle of ca. 63° for DEOA and ca. 67° for DPrOA, respectively; Table 2 and Figure 1). This packing mode severely limits the π-orbital overlap of the closest molecules. Interestingly, the unit cell parameters are very similar, with the c value varying with the chain dimension in agreement with the orientation of the molecules in the crystal. For anthracene,18,19 the c parameter is about half the value found in the unit cell of the two disubstituted derivatives (a = 8.56, b = 6.03, c = 11.16 A˚, and β = 124.7°), in correlation with the highest symmetry of the parent compound that crystallizes with only two molecules per unit cell instead of four for DEOA and DPrOA crystals. In the crystal, the aromatic core in DEOA and DPrOA is quasi-planar, with bonds and angles being close to those observed for anthracene itself, indicating a minor effect of the substituents. The latter are not symmetrical and present a slight distortion with respect to the aromatic plane (Figure 1). Finally, no specific interaction between the chains and the aromatics appears in the packing, with the latter being essentially controlled by the anthracene unit that imposes the herringbone organization. The chains entrapped in the crystalline lattice produce only an extension of the c axis but have no influence on the packing mode. One can notice, however, that sheet-sheet boundaries display anthraceneanthracene or chain-chain interactions, limiting the occurrence of chain-aromatic interactions within a sheet. DBOA (n = 4), DPtOA (n = 5), DHxOA (n = 6), and DHpOA (n = 7). These four flag-shaped compounds display the most striking crystalline structures, all belonging to a trigonal space group. A slight dependence on the parity of the chain is observed because with an odd n they pack in a P3 group with 6 molecules per unit cell, whereas with an even n, the group is R3 with 18 molecules per unit cell. As reported in this article (vide infra), DPtOA is dimorphic and also crystallizes in the monoclinic space group Cc. In these centrosymmetric trigonal space groups, three molecules are packed with the same orientation in a tight triad. These pack with an alternating head-to-tail orientation into layers (Figures 2, 3, S1, and S2). The triads look like a truncated regular tetrahedron in which the rodlike molecules form an angle of ca. 60° (equilateral section) (Figures 2 and S1 and Table 2). Interaromatic (CH-π) and interchain interactions appear as the major van der Waals binding forces in the triads. The mutual head-to-tail arrangement (17) Robertson, J. M. Proc. R. Soc. London 1951, A207, 101–110. (18) Cruickshank, D. W. J. Acta Crystallogr. 1956, 9, 915–923. (19) Mathieson, A. M.; Robertson, J. M.; Sinclair, V. C. Acta Crystallogr. 1950, 3, 245–250.
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Olive et al. Table 1. Crystallographic Parameters of DAOA Compounds (n = 2-7) DEOA
C18H18O2 266.32 plate colorless 0.10 0.10 0.25, monoclinic P21/a 8.6007(14) 7.8074(11) 21.612 (4) 90.00 100.929 (8) 90.00 1424.9(4) 4 1.241 150 0.71073 0.080 54.78 -11 e h e 10 -10 e k e 8 -27 e l e 27 F(000) 568 reflns measd 5395 unique reflns 2985 reflns used I > 2σ(I) 1350 no. of parameters 182 restraints 0 1.036 GOF on F2 0.0826 R1 [I > 2σ(I)] 0.1572 wR2 max 0.220 ΔF (eA˚-3)
formula formula wt crystal habit crystal color crystal size (mm3), crystal system space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dcalc (g 3 cm-1) T (K) λ (KR) μ (mm-1) 2θ max (deg) limiting indices
DPrOA
DBOA
DPtOA
DPtOA
DHxOA
DHpOA
C20H22O2 294.38 plate colorless 0.03 0.15 0.28, monoclinic P21/a 8.5062(3) 7.9640(3) 24.0314(10) 90.00 91.547(2) 90.00 1627.37(11) 4 1.202 160(2) 0.71073 0.076 52.02 -10 e h e 10 -9 e k e 9 -26 e l e 29 632 10 340 3200 2110 199 0 0.999 0.0445 0.0952 0.152
C22H26O2 322.5 plate colorless 0.03 0.15 0.27, trigonal R3 14.6672(4) 14.6672(4) 46.706(2) 90.00 90.00 120.00 8701.5(8) 18 1.108 295 0.71073 0.075 44.00 -15 e h e 15 -12 e k e 12 -45 e l e 47 3132 3242 2048 1023 322 0 0.945 0.044 0.097 0.068
C24H30O2 350.48 plate colorless 0.10 0.15 0.15, trigonal P3 14.5525(8) 14.5525(8) 17.0822(17) 90.00 90.00 120.00 3132.9(4) 6 1.115 213(2) 1.54180 0.633 124.76 -16 e h e 16 -16 e k e 16 -13 e l e 14 1266 13 345 2620 2560 237 0 1.065 0.0794 0.2672 0.243
C24H30O2 350.48 plate colorless 0.15 0.15 0.20, monoclinic Cc 32.990(3) 14.4951(11) 8.8108(12) 90.00 99.065(5) 90.00 4160.6(7) 8 1.119 213(2) 1.54180 0.536 144.42 -40 e h e 40 -17 e k e 17 -8 e l e 10 1520 20 542 7030 6972 500 20 1.000 0.0517 0.1435 0.243
C26H34O2 378.53 hexagonal tablet colorless 0.20 0.50 0.55, trigonal R3 14.639(2) 14.639(2) 54.435(7) 90.00 90.00 120.00 10102.6(23) 18 1.120 173 0.71073 0.069 50.00 -15 e h e 17 -17 e k e 0 -64 e l e0 3708 6265 3968 2093 255 248 1.024 0.0413 0.0821 0.154
C28H38O2 406.58 plate colorless 0.05 0.15 0.17, trigonal P3 14.660(2) 14.660(2) 19.643(4) 90.00 90.00 120.00 3656.0(10) 6 1.108 150 0.71073 0.067 49.38 -16 e h e17 -16 e k e 16 -22 e l e 22 1332 16 847 3925 1073 272 0 0.939 0.1061 0.1286 0.186
Table 2. Packing Mode, Angles between Aromatic Rings, and Shortest Intermolecular Distances
compound
packing mode
angle between aromatic planes (deg)
anthracene18,19 DEOA DPrOA DBOA DPtOA DHxOA DHpOA
herringbone herringbone herringbone triad triad triad triad
46.7 63.6 66.7 60.7 60.8 60.2 60.5
a
shortest CH-π (chainaromatic plane) distance (A˚)
2.73 2.62 2.51 2.53
shortest π-CH (aromatic plane) distance (A˚) 2.457 2.659 2.695 2.775 2.783 2.865 2.802
shortest CH aromO distance (A˚)
2.787/2.844a 2.738/2.773a 2.882/3.001b 2.880/3.032b
CH in the 9 position of the central ring. b CH in the 1 position of the lateral anthracenic ring. See Figures 4 and S3.
of adjacent triads rests on oriented π-CH interactions (the chain belonging to a triad is close to the aromatic ring of the neighboring triad) and on CH (aromatic) 3 3 3 O (vide infra) and dipole-dipole interactions (each molecule in a triad has a dipole moment of 1.9 D). It is also striking that the molecules are all oriented perpendicular to the sheets, whereas in the more common herringbone structure the molecules display an angle of about 40° within an equivalent sheet plane. From the packing energy computation, it has already been demonstrated for n = 614 that most of the crystal energy (88%) is found in the layer formation (only 12% of the total energy of ca. 6 kcal/mol is used for packing the layers), with the triad formation requiring 33% of the total energy. Although the parity effect on the packing mode is not rare in the n-alkane series,20 it was a priori not expected in the present situation because of the presence of a large aromatic (20) Robles, L.; Mondieig, D.; Haget, Y.; Cuevas-Diarte, M. A. J. Chim. Phys. 1998, 95, 92–111 and references therein.
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substrate, the anthracene core, which should have masked the chain effect. The parity effect in the DAOA series appears clearly on the structure of the triads and their organization. Slight but significant differences occur in the triads: for n = 4 and 6, the terminal CH3 groups point outward whereas for n = 5 and 7 they are oriented inward with a clear shift (ca. one C-C σ bond) in the mutual position of the adjacent triads. It appears that in all four structures a similar orientation and distance of the C-H(arom) 3 3 3 O interactions are preserved between the neighbor triads, indicating the decisive contribution of these weak H-bonds. Nonetheless, all of the interactions participating in the crystal packing are materialized by equivalent interatomic distances in the four structures. One notes that depending on the chain length either the CH of the central ring or that of the lateral ring is involved in the interaction with oxygen (Table 2 and Figures 4 and S3). The molecular packing is also ensured for the four compounds by similar intermolecular C-H (chain) 3 3 3 π-cloud, C-H(chain) 3 3 3 C-H(chain), and C-H (arom) 3 3 3 π-cloud interactions (Figure 4). Langmuir 2009, 25(15), 8606–8614
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Figure 1. Molecular packing and conformations of DEOA (left) and DPrOA (right) in the crystal. (Top) Viewed along the long axis of the anthracene core. Note that this view is not along any viable crystallographic main axis. (Bottom) Note the dissymmetrical arrangement of the chains (for DEOA, the chains are on both sides of the plane). Oxygen atoms are red, and carbon atoms are gray. For clarity, hydrogen atoms are not represented.
In the crystals of DAOA (n g 4), whereas the bonds and angles of the anthracene subunit are in the range of those observed for DEOA and DPrOA, the aromatic core is in fact not coplanar. The torsion of the aromatic ring increases with the chain length: quasi-planar with the short sequence (n = 2 and 3), it presents a maximum out-of-plane deformation with the longest chain (n = 7) (ca. 0.5 A˚ vs the plane delineated by the central ring and the nonsubstituted ring of the aromatic; see Figures 5 and S4). The substituents are not symmetrical and present a slight distortion with respect to the aromatic plane. However, this distortion decreases with increasing chain length, thus for n = 7, the chains display a more regular and symmetrical zigzag reminiscent of the regular conformation of n-alkanes in the crystal.21 Minimized structures in the gas phase display chains that are coplanar with the aromatic core and point outward,15 but if the chains are forced to extend in the prolongation of the anthracene core axis (one gauche C-C bond introduced), then the plane of the chains forms an angle with that of the anthracene. Coplanarity can thus be achieved only by putting a strain on the conformation of the aromatic core or the molecule in general. Besides, the alkoxy function, in contrast to a C-C link, also imposes an intramolecular separation between the chains that favors a partial intermolecular interdigitation and tight packing of the six chains within a triad. From these data, it emerges that the crystallization for n g 4 is controlled neither by the anthracene core as encountered for n = 2 and 3 nor by the chain as for n-alkanes20 but by a combination of both and the rodlike shape of the molecules. When the aliphatic/aromatic ratio increases, the chain approaches a regular anti conformation at the (21) Nyburg, S. C.; Gerson, A. R. Acta Crystallogr. 1992, B48, 103–106 and references therein.
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expense of the anthracene nucleus, which is subject to deformation. DPtOA. A second crystalline form has been isolated for DPtOA (Figures 6, S5, and S6). It belongs to the noncentrosymmetric monoclinic space group Cc, with eight molecules in the unit cell. The chain of one of the two molecules in the asymmetric unit presents disorder and occupies two equivalent positions. The substituents present an important dissymmetry and largely stand out of the plane delineated by the aromatic nucleus. In the crystal, the molecules are organized in layers in which no significant π-π stacking occurs as the angle between the closest aromatics varies from 78 to 85°. However, the neighboring molecules display similar C-H(arom) 3 3 3 O and C-H(chain)-C-H(chain) interactions (Figure S6) to those observed for compounds crystallizing in the trigonal space group (n = 4f7). Thus, to first approximation this structure can be viewed as intermediate between the herringbone and the triad packing mode. DDOA and Spectroscopy. No single crystal of DDOA could be obtained despite various attempts. The WAXS reflections of powder and of bundles in xerogels have both been shown to display a reticular distance of 66.1 A˚ in hexagonal symmetry,22 but no X-ray diffraction measurements sufficed to assign a definitive molecular packing of the DDOA gels. Spectroscopic data13,15 also underline the similarities between gels and single crystals. UV-visible absorption spectra of the gelators, as shown previously for DDOA, undergo a red shift and an increase in absorption of the 0-0 transition when going from solution to the gel phase (22) (a) Terech, P.; Clavier, G.; Bouas-Laurent, H.; Desvergne, J.-P.; Deme, B.; Pozzo, J.-L. J. Colloid Interface Sci. 2006, 302, 633–642. (b) Terech, P.; Bouas-Laurent, H.; Desvergne, J.-P. J. Colloid Interface Sci. 1995, 174, 258–263.
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Figure 2. Molecular packing of DBOA and DHpOA in the crystal. Viewed along the b and c axes, respectively. as a result of the rearrangement of the alkoxy chain leading to a larger delocalization of the oxygen lone pair on the aromatic ring.13 In the gel and the powder, the spectra are the same (Figure S7). The well-structured absorption and emission spectra4 also support the absence of face-to-face packing of the aromatic cores because it has been shown in the literature that such structures would induce blue or red shifts in the spectra as well as a loss of vibronic structure. Similarly, the IR absorption spectra of DDOA in the condensed phases (Figure S7) are almost identical and differ from the solution spectrum. The vibrations of the aromatic core (signals a, Figure S7) are mostly split into two signals, suggesting at least two different environments for the aromatic core. The vibronic progression series in the 7201350 cm-1 region (signals b, Figure S7) typically arises from alkane chains. In particular, the transition at 721 cm-1 confirms the presence of the CH2 rocking mode and thus the all-trans conformation of a large part of the chains (at least four CH2 groups in a row). The presence of gauche conformations is also confirmed in the xerogel by the 739 cm-1 transition but is not attributed in the organogel due to the overlapping CCl4 absorption. The progression bands usually observed between 1150 and 1350 cm-1 are unfortunately also unavailable because of the intense absorption of ether in that region. Further comparison between crystals and desolvated gels is obtained by Raman spectroscopy performed under 8610 DOI: 10.1021/la804206n
confocal microscopy. Figure 7 shows the spectra obtained for a DHxOA crystal and a bundle of DDOA aerogel fibers (obtained23 in supercritical CO2). Although the spectrum of the aerogel is less well resolved because of lower signal intensity, most transitions around 2900 cm-1 are observed both in the crystal and in the gel. It has been shown24 that in the range of 2830-2950 cm-1 a series of peaks, including the most intense transition around 2880 cm-1, are representative of all-trans conformations in alkane chains. Although gauche conformations, expected in our case, weaken the central feature and decrease the structure of the band, this effect appears to be limited.
General Comments and Conclusions From the above data, it emerges that the trigonal space group (P3 or R3) is the favorite packing mode in crystals of DAOA compounds holding alkoxy chains with a number of carbon atoms g4. The molecules are arranged in triads, packing in head-to-tail orientation in layers. This packing mode is controlled by the rodlike shape of the molecules, in contrast with compounds holding the short chains (n = 2 and 3) where the crystallization is directed by the ellipsoid shape of the aromatic moiety imposing a herringbone (23) Placin, F.; Desvergne, J.-P.; Cansell, F. J. Mater. Chem. 2000, 10, 2147–2149. (24) Ricard, L.; Abbate, S.; Zerbi, G. J. Phys. Chem. 1985, 89, 4793–4799.
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Figure 3. Head-to-tail organization of the triads in the crystal of DAOA (n = 4-7). Note the shift of the triads for n = 5 and 7 and the orientation of terminal CH3 vs the chain parity. packing. The driving force for the crystallization had been shown for DHxOA to result mainly from the intratriad aromatic core interactions (VdW) and to a lesser extent from the intertriad dipolar attraction that induces the head-to-tail configuration. Unfortunately, no single crystals could be grown in other solvents; consequently, the solvent effect on the crystal and on polymorphism was not studied. The analysis of the crystal structures reveals some reasons that could explain the failing crystallization of longer-chain derivatives (8 e n e 16). For these derivatives, preserving the same trigonal structure would suggest two possibilities as illustrated for DDOA (n = 10). In the first case, as happens in the crystals for n = 4 to 7, the aromatic cores of two neighboring triads slide apart until the end borders are aligned (Figure 8a). This would lead to the impossible overlap of two chains (Figure 8b,c), and an adaptation of the structure with a variation of the a and b unit cell dimensions would be necessary. In the second alternative, the relative position of the aromatics and the C-H(arom) 3 3 3 O interactions would be preserved, but because the chain lengths would no longer match the aromatic core lengths, large void gaps would appear in the structure (Figure 8d). These less-compact structures could thus be unadapted for single-crystal formation. Besides, the chain packing implies a conformational preorganization of the chains, which is likely to be slower in the case of the more-flexible long chains. Because the gel could be considered to be an aborted crystallization, it is tempting to correlate the crystalline packing mode of DAOA derivatives directly with their gelling properties as often speculated in the literature2,25 and already proposed3,15 for n = 10. In contrast to DEOA and DPrOA,
which crystallize in the monoclinic P21/c space group, all derivatives that can form crystals in a trigonal space group can also form either a gel (n g 6) or self-assembled nanofibers, as shown in Figure 9 for DBOA (n = 4). This Figure shows fluorescence confocal microscopy images of fibers formed by the evaporation of a DBOA solution in chloroform (i.e., using the same solvent as that used for singlecrystal growth but largely accelerating the process by selfassembly in seconds vs crystallization in days). The dimension and shape of these fibers nicely compares with those observed in gels obtained with the other members of the series. Figure 9 also shows fibers of DHpOA grown on a microscopy slide next to a small crystal. The first steps of the self-assembly process should be driven by the previously described interaromatic interactions, and it is tempting to consider that both in the crystallization and in the gel formation the packing process is initiated by the formation of triads, independently of the chain length of the substituent. Thereafter, the constitution of small sheets with regular head-to-tail orientation of these triads is expected to happen more easily with chains of n e 7. Further growth could be solvent-assisted26 and should be combined with the stacking of sheets through quite weak interactions. Although a chain length of n g 8 implies the failure of crystallization for the above-mentioned reasons, this is not directly correlated with the self-assembly ability of all compounds with n g 4. Gel formation is thus not hampered by the lack of perfect packing, as is the case for the strongest gelator in the series, DDOA. Although we speculate possible voids in these structures (for n > 7) and thus an alteration of the intersheet interactions, the kinetically fast gelation process is not inhibited. Previous SANS measurements corroborate
(25) Low Molecular Mass Gelators: Design, Self-Assembly, Function; Topics in Current Chemistry; Fages, F., Ed.; Springer: Berlin, 2005; Vol. 256.
(26) Jonkheijm, P.; van der Schoot, P.; Schenning, A.P.H.J.; Meijer, E. W. Science 2006, 313, 80–83.
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Figure 4. Shortest intermolecular distances separating two molecules in neighboring triads (top) or within a triad (bottom).
Figure 5. Molecular conformations of DBOA and DHpOA in the crystal. Note that the dissymmetry of the chains decreases with the length. (See also Figure S4.)
the presence of a sheet structure within the gel fibers but also suggest that they stack together with different orientations.22 This lack of macroscopic crystallographic order could be at the origin of the absence of intense X-ray diffraction of the gels. Additionally, the spectroscopic data in the UV-visible and vibrational regions confirm the conformational similarity of the molecules in crystals and gels. The UV-vis spectra 8612 DOI: 10.1021/la804206n
confirm the absence of aromatic face-to-face interactions, as expected by the present crystal structures that display only edge-to-face interactions, and the strain on the alkoxy bond that leads to the red shift in absorption. The IR and Raman spectra, however, also support a mostly trans conformation of the alkyl chains, as seen in the crystal structures. These data do not allow us to determine what draws a line between gel formation and crystallization when 4 e n e 7, Langmuir 2009, 25(15), 8606–8614
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Figure 6. Cc crystals of DPtOA. (Left) Asymmetric unit. The aromatic planes of the two independent molecules, which do not superimpose, form an angle of 25.68°. In green are indicated the shortest distances between the two molecules. Note the disorder experienced by one of the chains of one molecule. (Right) Packing viewed along the b axis (top) and the a axis (bottom), respectively.
Figure 7. Raman spectra of (left) a crystal of DHxOA. (Right) Bundle of fibers of the DDOA aerogel.
Figure 8. Graphic representation of possible packing of two DDOA molecules of neighboring triads based on the crystal structure of DHpOA. (a, b) Sliding of the molecules within the original plane of the aromatic core. (c) Same geometry as in panel a showing the overlap of hydrogens evidenced by the black ring. (d) Geometry obtained by extension of the chain length (7 to 10) only; void gaps evidenced by the black rings.
as well as the role of dimorphism. Nevertheless, within this series (2 e n e 7) the direct correlation between trigonal space group crystallization and self-assembly behavior supports the interpretation that these processes are tightly linked, presumably through a common molecular structure. Therefore, although by X-ray diffraction we could extract only reticular distances on xerogels but no structure from the fibers in the organogels, we believe that the molecular Langmuir 2009, 25(15), 8606–8614
arrangement in all of the gels of the series resembles that of the crystals (n g 4). The impact of this original packing mode on the dynamics of excited states in the fibers is currently under investigation by confocal fluorescence microscopy measurements.27 It will also be of interest to
(27) Olive, A. G. L. et al. Manuscript in preparation.
DOI: 10.1021/la804206n
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Figure 9. Fluorescence confocal microscope image of fibers formed by the evaporation of chloroform solutions of gelator on a glass slide: λex, 385
nm; 400 nm < λem < 450 nm; (left) DBOA 0.3 mM (18 μm 18 μm, 80 nm/pixel, fluence 400 W/cm2). The section of the fluorescent fibers is in the range of those observed in the gels formed by the upper members of the series (n = 6-10).27 (Right) Fibers and crystal of DHpOA (36.8 μm 36.8 μm, 100 nm/pixel, fluence 3 W/cm2).
explore the impact of this packing mode on the electronic properties of 2,3-disubstituted tetracene28 and pentacene29 derivatives. (28) Reichwagen, J.; Hopf, H.; Del Guerzo, A.; Belin, C.; Desvergne, J.-P.; Bouas-Laurent, H. Org. Lett. 2005, 7, 971–974. (29) Reichwagen, J.; Hopf, H.; Desvergne, J.-P.; Del Guerzo, A.; BouasLaurent, H. Synthesis 2005, 20, 3505–3507.
8614 DOI: 10.1021/la804206n
Acknowledgment. A.G.L.O. received a Ph.D. fellowship from the Region Aquitaine. A.D.G. and J.-P.D. thank the Region Aquitaine, the Ministry of Education and Research, and the CNRS for financial support. Supporting Information Available: Crystal structures and absorption spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2009, 25(15), 8606–8614
