Molecular Arrangement in the Gel Fibers of 2,3-Didecyloxyanthracene

A model of the molecular arrangement of the DDOA molecules within the fibers is proposed from ab initio calculations combined with IR and fluorescence...
1 downloads 0 Views 316KB Size
Download PDF
Langmuir 2003, 19, 4563-4572

4563

Molecular Arrangement in the Gel Fibers of 2,3-Didecyloxyanthracene (DDOA): A Spectroscopic and Theoretical Approach Fre´de´ric Placin,†,‡ Jean-Pierre Desvergne,† Colette Belin,‡ Thierry Buffeteau,‡ Bernard Desbat,‡ Laurent Ducasse,‡ and Jean-Claude Lasse`gues*,‡ Laboratoire de Chimie Organique et Organome´ tallique, UMR 5802 CNRS, Universite´ Bordeaux I, and Laboratoire de Physico-Chimie Mole´ culaire, UMR 5803 CNRS, Universite´ Bordeaux I, 351 Cours de la Libe´ ration, 33405 Talence, France Received December 20, 2002. In Final Form: March 14, 2003 2,3-Didecyloxyanthracene (DDOA) is able to gel a large variety of organic solvents at very low concentrations by forming a three-dimensional network of fibers. A vibrational assignment is proposed for the midinfrared absorptions of DDOA in the gel and solution states, showing that some out-of-plane vibrations of the anthracene group constitute pertinent local probes of the molecular organization in the three-dimensional supramolecular architecture. A model of the molecular arrangement of the DDOA molecules within the fibers is proposed from ab initio calculations combined with IR and fluorescence dichroism experiments performed on oriented bundles of aerogel fibers.

1. Introduction Although gels are best known to arise from polymers, proteins, and inorganics, low-molecular-weight organic compounds can also display gelating properties, which are of interest in various fields.1 Thus, supramolecular thermoreversible gels could be generated via the spontaneous self-assembly of small-size molecules in various organic or inorganic fluids under nonequilibrium conditions. If hydrogen bonding is a well-known driving force for promoting aggregation, other noncovalent intermolecular forces, such as hydrophobic effects, van der Waals and dipole-dipole interactions, or π-π stacking, are recognized to play a basic role in the fiber formation, which in turn immobilize the solvent within delimited microcavities inside a three-dimensional architecture.2 The control of the gelation phenomena and design of novel types of organogelators are challenging, particularly for the non-hydrogen bonding candidates, in which only weak nondirectional intermolecular interactions could be available or emulated. In these systems, the structural requirements for a molecule to be a successful gelator are still based on empirical considerations.2,3 Indeed, if the classical techniques used for the structure investigation (combined with rheological experiments) give information on the shape, size of the three-dimensional architectures, and mechanical properties of the soft material, they unfortunately do not provide any clear evidence on the molecular organization inside the fibers.4 * Corresponding author. Phone: +33 5 56 84 63 55. Fax: +33 5 56 84 84 02. E-mail: [email protected]. † Laboratoire de Chimie Organique et Organome ´ tallique, Universite´ Bordeaux I. ‡ Laboratoire de Physico-Chimie Mole ´ culaire, Universite´ Bordeaux I. (1) (a) Derossi, D.; Kajiwara, K.; Osada, Y.; Yamauchi, A. Polymer gels. Fundamentals and Biomedical Applications; Plenum Press: New York, 1991. (b) Jung, J. H.; Ono, Y.; Shinkai, S. Chem.sEur. J. 2000, 6, 4552. (c) Terech, P. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 1630. (d) George, M.; Weiss, R. G. Langmuir 2002, 18, 7124. (2) (a) Terech, P.; Weiss, R. G. Chem. Rev. 1997, 97, 3133. (b) Terech, P.; Furman, I.; Weiss, R. G. J. Phys. Chem. 1995, 99, 9558. (3) Aggeli, A.; Nyrkova, I. A.; Bell, M.; Harding, R.; Carrick, L.; McLeish, T. C. B.; Semenov, A. N.; Boden, N. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 11857.

Herein, we report a structural investigation of the 2,3didecyloxyanthracene (DDOA) aerogel fibers. Because X-ray diffraction experiments on xerogels or aerogels do not provide any Bragg diffraction patterns, we have used polarized IR and fluorescence spectroscopies. The dichroic effects observed with both spectroscopies are consistent with a preferential orientation of the DDOA molecules inside the fibers, which could be related to, as is supported by theoretical considerations, a special molecular organization different from the crystalline packing mode. 2. Experimental Section 2.1. Synthesis and Spectroscopy. The synthesis of the dialkoxyanthracene derivatives5,6 and preparation of the DDOA aerogel fibers via the supercritical CO2 route7 have previously been described. Let us recall that the 2,3-dimethyloxyanthracene (DMOA), 2,3-dipropyloxyanthracene (DPOA), and 2,3-dihexyloxyanthracene (DHOA) molecules and DDOA derivatives involve n ) 1, 3, 6, and 10 carbon atoms per aliphatic chain, respectively. Electronic absorption spectra were recorded with a Hitachi spectrometer using quartz cells of 1- and 10-mm thickness. Polarized fluorescence spectra were recorded using a Fluorolog 212 (SPEX) spectrometer with an excitation wavelength of 345 nm on aligned DDOA aerogel fibers. Let us recall that the DDOA aerogel is an extremely light (∼0.002 g/cm3) cottonlike material made of very long (∼1 mm) and thin (70- to 150-nm diameter) fibers.7 The careful and repeated mechanical stretching of these interlaced fibers under a microscope allows some oriented parts to be isolated and aligned on a piece of black paper. For the IR measurements, the solid samples are prepared in KBr pellets, and the gel or solutions are contained in a cell equipped with CsI windows. The IR spectra are recorded with a spectral resolution of 4 cm-1 using a Nicolet 740 spectrometer in the 4000-400(4) Abdallah, D. J.; Sirchio, S. A.; Weiss, R. G. Langmuir 2000, 16, 7558. This paper reports the first determination of the molecular packing in a low-molecular-mass organogelator from X-ray diffraction experiments. (5) Brotin, T.; Utermo¨lhen, R.; Fages, F.; Bouas-Laurent, H.; Desvergne, J. P. Chem. Commun. 1991, 416. (6) (a) Brotin, T.; Desvergne, J.-P.; Fages, F.; Utermo¨hlen, R.; Bonneau, R.; Bouas-Laurent, H. Photochem. Photobiol. 1992, 55, 349. (b) Pozzo, J.-L.; Clavier, G. M.; Colome`s, M.; Bouas-Laurent, H. Tetrahedron 1997, 53, 6377. (7) Placin, F.; Desvergne, J.-P.; Cansell, F. J. Mater. Chem. 2000, 10, 2147.

10.1021/la0270439 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/30/2003

4564

Langmuir, Vol. 19, No. 11, 2003

Placin et al.

Figure 1. Conformation adopted by the DHOA molecule in the crystal10 (a, b) and, according to ab initio calculations, by the isolated DPOA molecule in its ground state (c, d) and higher-energy conformation (e, f). cm-1 range. This spectrometer is equipped with a Nic-Plan microscope, allowing the dichroism measurements to be performed on oriented fibers in the 4000-650-cm-1 transmission range. The gels have been prepared from DDOA freshly purified by chromatography. Indeed, aged DDOA gives yellowish solutions and optically inhomogeneous gels characterized by crystalline nuclei growing around the impurities. The reproducibility of the gels is ascertained by repeatable spectroscopic responses and sol-gel transition temperatures. 2.2. Calculations. The geometries and energies of free molecules and clusters for various dialkoxyanthracene derivatives (n ) 3-11) have been calculated by means of ab initio quantum chemical approaches and molecular mechanics (MM) using the Assisted Model Building with Energy Refinement (AMBER) force field. Furthermore, the vibrational frequencies of model molecules with relatively short aliphatic chains (n ) 3, 4) were also calculated. The geometries were fully optimized at the Hartree-Fock (HF) level with a 6-31g* basis set or by using the density functional theory (DFT) with the nonlocal functional B3LYP and the same basis set. The vibrational spectra were calculated at the HF level for both the IR intensities and the Raman activities. Only the vibrational frequencies were evaluated at the DFT level because the calculation of the Raman activities is too time-consuming. All the ab initio calculations were performed using the Gaussian 98 package,8 and the MM studies were performed with the MacroModel package, in which the AMBER program is implemented.9

3. Results and Discussion 3.1. Conformation of the DDOA Molecule. Before any spectroscopic studies were performed, the conformation or point group of the investigated molecule and eventually the conformational changes occurring between different physical states have to be determined. This

information is not available for DDOA because single crystals could not be grown, and we have supposed that the DDOA molecular geometry in the condensed state is similar to the known DHOA molecular geometry in the crystal.10 Projections of the latter are reported in Figure 1a,b. The optimized geometry of the isolated DPOA molecule calculated at the B3LYP/6-31g* level is depicted in Figure 1c,d. The anthracene ring and COC groups are coplanar in the isolated DPOA molecule, in the same way as in the DHOA crystal. This is ascribable to strong electronic effects occurring between the ring and substituted O-C bond. As a result, the O-C bond with the aromatic group [O-C(Ar) ) 1.362 Å] is shorter (∼0.07 Å) than the O-C bond with the aliphatic chain [C-O(R) ) 1.434 Å]. This theoretical result is very sim(8) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.7; Gaussian, Inc.: Pittsburgh, PA, 1998. (9) MacroModel, version 6.5; Shro¨dinger: New York. Mohamadi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.; Lipton, M.; Caulfield, C.; Chang, G.; Hendrickson, T.; Still, W. C. J. Comput. Chem. 1990, 11, 440. (10) 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.

Molecular Arrangement in DDOA Gel Fibers

ilar to the crystal data (1.358 and 1.427 Å, respectively).10 A marked difference between the calculated geometry of DPOA and the molecular geometry of DHOA in the crystal structure concerns the orientation of the aliphatic chains with respect to the aromatic plane. In the optimized DPOA geometry (Figure 1c), the all-trans chains are oriented according to a dihedral angle O1-C1-C2-C3 of 180°, that is, roughly along the bisector of the yA and zA axes. This situation does not favor any π-π packing. In the DHOA crystal, the chains are also contained in the aromatic plane, but their axis yC is almost collinear to yA. Another stationary point may be calculated for DPOA with an energy higher than the global minimum by less than 4 kJ mol-1. Its geometry is deduced from the previous one by rotating the terminal methyl groups by 120° (i.e., the dihedral angle O1-C1-C2-C3 is now equal to 60°), and two projections of this geometry are reported in Figure 1e,f. The anthracene ring and COC groups are still coplanar, but the position of the aliphatic chains with respect to the anthracene is very similar to that in the case of the crystal structure. A subtle difference between DPOA and DHOA concerns the respective orientation of the two aliphatic chains: in the isolated DPOA molecule, the CCC planes of the two chains are calculated to be symmetrical with respect to the (xA, yA) plane (Figure 1f), whereas in the crystal (Figure 1b), they are not because of the triad arrangement. In conclusion, it can be inferred that the anthracene ring and COC groups are practically coplanar in the DDOA molecule as well as for the other derivatives, this feature remaining rather insensitive to the physical state. On the other hand, the orientation of the aliphatic chains with respect to the aromatic plane is certainly strongly dependent on molecular interactions, and large differences are expected between the solution, gel, and solid states. 3.2. IR Characterization of DDOA in Various States. The IR spectra of some dialkoxyanthracene derivatives and anthracene are presented in Figure 2. The D2h point-group symmetry of anthracene becomes, at best, C2v after 2,3-disubstitution. If the C2 axis of the 2,3disubstituted derivatives is kept along the in-plane long axis yA, the symmetry of the 66 anthracene vibrations are transformed from D2h to C2v according to the following:

Langmuir, Vol. 19, No. 11, 2003 4565

Figure 2. IR spectra of anthracene (a), DMOA (b), DHOA (c), DDOA xerogel (d), DDOA gel (e) and DDOA solution (d). Spectra a-d are recorded from KBr pellets and spectra e and f from a 0.1 M carbon tetrachloride solution at 0 °C (e) or 25 °C (f). The dashed surface indicates the spectral range where intense CCl4 absorptions cannot be subtracted.

12Ag + 11B2u(yA) f 23A1(y) 6B2g + 5Au f 11A2 11B3g + 11B1u(zA) f 22B1(z) 4B1g + 6B3u(xA) f 10B2(x) where the IR-active vibrations are indicated by their transition moments along x, y, or z. Despite this symmetry lowering, one can see in Figure 2 that most of the intense and well-separated bands of anthracene in the 1000400-cm-1 range, due to characteristic out-of-plane vibrations, undergo only moderate frequency and intensity changes in the disubstituted derivatives. In the 10001350-cm-1 region, vibrations of the COC groups induce strong absorptions. The rather large bond-length difference between the two C-O bonds has led some authors to assign qualitatively the more intense absorption bands in the 1150-1350-cm-1 range to stretching vibrations of the short O-C(Ar) bonds and the less intense features near 1000 cm-1 to stretching vibrations of the longer

Figure 3. Comparison of the calculated IR frequencies of the free DPOA molecule with those from the experimental spectrum of the DDOA xerogel in the region of the stretching vibrations of the O-C(Ar) bonds; a schematic representation of the DPOA atomic motions in the four vibrations a-d is given below.

O-C(R) bonds.11,12 Actually, these two oscillators that are coupled to other motions of the anthracene and aliphatic groups do not vibrate quite independently, and the presence of two ortho-substituted COC groups provokes (11) Varsanyi, G. Assignments for Vibrational Spectra of Seven Hundred Benzene Derivatives; Adam Hilger: London, 1974; Vol 1. (12) Reddy, B. V.; Rao, G. R. Vib. Spectrosc. 1994, 6, 251.

4566

Langmuir, Vol. 19, No. 11, 2003

Placin et al.

Table 1. Symmetry Assignment of the IR-Active Vibrations of Anthracene below 1650 cm-1 According to Ref 16 (First Column) and Observed IR Wavenumbers for DDOA in Its Xerogel and Aerogel Phasesa anthracene (cm-1)

1624 (zA) B1u 1536 (yA) B2u 1454 (zA) B1u 1447 (yA) B2u 1398 (yA) B2u

DDOA aerogel (cm-1)

DDOA aerogelb (R ) A|/A⊥)

2960 2956 2943 2927 2917 2897 2872 2850 1630 1568 1491 1481

2960 2955 2943 2927 2915 (xC) 2897 (zC) 2875 2851.5 (zC) 1629 (zA) 1569 (yA) 1491 (yA) 1481.5

1.18 0.80 n.m. 1.03 0.45 0.60 0.55 1.13 1.13 n.m. 1.59 3.60

1466 1397 1385 1377

1467 (yA) 1398 (yA) 1387 (yC)? 1377 (zC)?

1.90 2.6 1.93 0.90

1304 1287 1262 1223 1193 1165

1304 1288 (yA) 1262 (zA) 1224 (yA) 1193 (zA) 1165 (zA)

0.22 1.87 1.12 2.0 1.02 0.95

1123 1065 1044 1014 983 957 933 914 890 835 748.5 739 721

1152 (zA) 1123 (yA) 1066 1046 1015 (zA) 982 (yA) 958 (xA) 934 915 (zA) 891 (xA) 835 (xA) 748.5 (yA) 739 (xA) 721 (xC)

n.m. n.m. 1.04 0.94 0.91 0.77 0.21 1.3 0.94 0.33 0.19 2.16 0.215 0.21 n.m. n.m. n.m.

DDOA xerogel (cm-1)

1346 (yA) B2u 1315 (zA) B1u 1271 (zA) B1u

1165 (yA) B2u 1148 (zA) B1u 1124 (yA) B2u

1000 (yA) B2u 953 (xA) B3u 907 (zA) B1u 876 (xA) B3u 725 (xA) B3u 651 (zA) B1u 601 (yA) B2u 468 (xA) B3u

594 478

assignmentc νa CH3 i.p. νa CH3 o.p. νa (O)CH2 ? νs + 2δs CH3 F νa CH2 νs + 2δs CH2 F νs CH3 νs CH2 R(CC) B1 R(CC) A1 R(CC) A1 ? R(CC) B1 R(CC) A1 β(CH) A1 w CH2 δs CH3 ? w CH2 δs CH3 ? β(CH) A1 R(CC) B1 ? νs[O-C(A)] + β(CH) A1d β(CH) B1 νs[O-C(A)] + β(CH) A1d νa[O-C(A)] + β(CH) B1d νa]O-C(A)] + β(CH) B1d β(CH) A1 β(CH) B1 R(CC) A1 ? CH2 ? CH2 ν[O-C(R)] R(CC) A1 (CH) B2 ? CH2 R(CCC) B1 (CH) B2 (CH) B2 ? R(CCC) A1 ? (CH) B2 rCH2 R(CCC) B1 R (CCC) A1 τ(CCCC) B2

a The dichroic ratios for the oriented aerogel fibers are reported together with a tentative vibrational assignment of DDOA in the last two columns. b n.m. ) non measurable. c The nomenclature for the anthracene vibrations, taken from ref 25, is τ(CCCC) ) out-of-plane cycle deformation, R(CCC) ) in-plane cycle deformation, (CH) ) out-of-plane CH wagging, β(CH) ) in-plane CH rocking and scissoring, and R(CC) ) CC stretching. For the ether and aliphatic groups, νs (δs) and νa (δa) stand for symmetric and antisymmetric stretchings (or bendings), respectively, w stands for wagging, and r stands for rocking; i. p. ) in the CCC plane, o.p. ) out of the CCC plane, F ) Fermi resonance. d The vibrations are the same as those described in Figure 3.

additional splitting between the in-phase and the outof-phase motions. For a more accurate description of these vibrations, we have performed DFT calculations of the IR frequencies and intensities on the optimized DPOA structure displayed in Figure 1e,f. The results are illustrated in Figure 3 for the intense pattern of absorptions observed in the 1350-1100-cm-1 region. One can see that the four main bands denoted as a, b, c, and d are satisfactorily reproduced. They effectively involve in-phase and out-of-phase stretching vibrations of the short O-C(Ar) bond, but these motions are strongly coupled to specific in-plane motions of the anthracene group, as is shown in the lower panel of Figure 3. Another great advantage of this calculation is the prediction of the dipolemoment components corresponding to each of these absorptions. They are clearly directed along yA for bands a and b and along zA for c and d. We checked that these absorptions are nearly independent of the size of the aliphatic chain by performing a similar calculation on the n ) 4 derivative.

Finally, the variation of the substituted alkoxy-chain length from DMOA to DHOA and DDOA allows some vibrations of the aliphatic chain to be located (Figure 2). This is clearly the case for the in-phase rCH2 rocking mode at 721 cm-1 and the stretching vibrations of the methylene and methyl groups in the 3000-2800-cm-1 range. In the DDOA gel or xerogel, the aliphatic chains are supposed to adopt an all-trans conformation, in the same manner as in the DHOA crystal. Unfortunately, the progression bands usually observed in the 1150-1350-cm-1 range (wagging and twisting of the methylene groups) are hidden by the intense absorptions of the ether group. One can simply remark that a series of weak bands observed between 1100 and 900 cm-1 for the solid or gel DDOA phases (Figure 2d,e) transform into a broad envelop in the carbon tetrachloride solution (Figure 2f) and that the frequencies of the methylene stretching vibrations are close to the expected values for the all-trans chains.13 The assignment of the main IR absorptions of the DDOA

Molecular Arrangement in DDOA Gel Fibers

Figure 4. Comparison of the IR spectra in the region of the R(CCC) and τ(CCCC) vibrations for DDOA in the xerogel form (KBr pellet) and heptane solution (1.2 10-2 M) at various temperatures. The sol-gel transition occurs at 22 °C.

Figure 5. Comparison of the IR spectra in the region of the R(CCC) and τ(CCCC) vibrations for DHOA in the crystal, gel, and solution phases.

xerogel phase coming from the anthracene, ether, and aliphatic groups are reported in Table 1. Once this preliminary vibrational assignment was established, a comparison of the DDOA solid, liquid, and gel phases reveals that some vibrational modes are split in the gel state, one component being characteristic of the solid state and the other of the disordered liquid state. An example is given in Figure 4 for the τ(CCCC) out-of-plane anthracene mode; a rather symmetric absorption band is observed at 469 cm-1 in a heptane solution, but when gelification occurs at about 22 °C, a shoulder appears at 478 cm-1. The relative intensity of this second absorption increases by lowering the temperature, and its position is close to that observed for the xerogel. It will be shown later on that the (CH) out-of-plane mode near 890 cm-1 undergoes a similar behavior. Note that the in-plane R(CCC) mode at 594 cm-1 in Figure 4 is relatively insensitive to the sol-gel transformation. From the spectral changes observed for the out-of-plane vibrations, it is possible to determine the gel transition temperature as a function of the DDOA concentration. The phase diagram thus established is in very good agreement with that previously found by other techniques.6b,14 So, the sensitivity of some out-of-plane vibrations of the anthracene part to molecular interactions and the local structure is sufficient to distinguish in the gel a quasicrystalline state and a liquidlike one. This is a rather general property because DHOA exhibits the same behavior (Figure 5). The similarity of the IR spectra of DDOA and DHOA suggests that the molecular arrangement in the DDOA gel fibers might be of the same kind (13) Ricard, L.; Abbate, S.; Zerbi, G. J. Phys. Chem. 1985, 89, 4793. (14) Placin, F.; Desvergne, J.-P.; Lasse`gues, J-C. Chem. Mater. 2001, 13, 117.

Langmuir, Vol. 19, No. 11, 2003 4567

Figure 6. Comparison of the IR spectra of DDOA in the CCl4 solution (a), CCl4 gel (b), aerogel (c), and xerogel (d) phases for the (CH) band near 890 cm-1.

Figure 7. IR spectra of DDOA aerogel fibers recorded with the incident electric vector parallel (bottom) or perpendicular (top) to the fiber axis. A video image of the investigated bundles of aerogel fibers is given in the inset. The size of the inner rectangle is 50 × 150 µm.

as that in crystalline DHOA. However, we have tried to get some further information from IR and fluorescence dichroism experiments on oriented DDOA fibers.15 3.3. IR Dichroism of DDOA Fibers. The DDOA fibers produced from supercritical CO2 are ideal for spectroscopic measurements because they are free from solvent.7 In addition, one can infer that the organization of the DDOA molecules in the gel fibers is maintained in the aerogel fibers after the removal of CO2. This can be illustrated from the (CH) out-of-plane mode near 890 cm-1 (Figure 6). It undergoes a similar evolution as the τ(CCCC) outof-plane vibration near 470 cm-1 (Figure 4) because the gel phase is characterized by the 882-cm-1 band of the isotropic solution and appearance of a new band at 891 cm-1. The former corresponds to the (CH) vibration of the DDOA monomers surrounded by several CCl4 solvation shells and the latter to the same vibration in a fiber of interacting DDOA molecules. Interestingly, the aerogel spectrum exhibits the only 891-cm-1 component, as was expected for all the DDOA molecules interacting with each another in a relatively well-preserved fiber arrangement. By comparison, the molecular arrangement in the xerogel appears to be rather strongly perturbed because the 891 cm-1 component is much broader and is accompanied by a shoulder at ∼878 cm-1. The solvent evaporation to obtain the xerogel produces a partial collapse of the molecular organization, whereas the supercritical route is a relatively soft treatment. Note that the blue shifts of the τ(CCCC) (15) The molecular arrangement of DDOA in a liquid crystal has recently been investigated using polarized IR spectroscopy: Kato, T.; Kutsuna, T.; Yabuuchi, K.; Mizoshita, N. Langmuir 2002, 18, 7086. From their data, the authors could only propose a general orientation of the gelator versus the director of the liquid crystal phase.

4568

Langmuir, Vol. 19, No. 11, 2003

Placin et al.

Table 2. Orientation Factors of Anthracene and DDOA in Stretched Polyethylene (PE) and DDOA in the Aerogel Fibers orientation factor KxA

K yA

KzA

anthracene/PE 0.11 0.63 0.26 (ref 16) DDOA/PE 0.12 0.73 0.15 (ref 18) DDOA/aerogel 0.095 0.51 0.33 (this work) ( 0.012 ( 0.05 ( 0.02

KxA + KyA + KzA 1.0 1.0 0.94 ( 0.08

and (CH) bands from the isotropic solution to the gel phase are identical (9 cm-1). These out-of-plane vibrations are similarly affected by molecular interactions that might be similar to the CH-π interactions involved in the DHOA crystal between one CH bond of a methylene group and the anthracene π electrons. Bundles of aerogel fibers have been aligned manually, as was explained in the Experimental section, and the result is shown in the inset of Figure 7. The zone delimited by a rectangle in this image was selected through the microscope to measure the parallel (A|) and perpendicular (A⊥) polarized spectra relative to the mean fiber axis Z.16,17 Let us recall that the integrated intensity A of a given IR absorption band is proportional to the square of the product of the transition moment of the molecular vibration causing the band by the electric-field vector. From the integrated intensities Ai,| and Ai,⊥ of each vibration i, one can measure the dichroic ratio:

Ri ) Ai,|/Ai,⊥

(1)

which is sometimes converted into the orientation factor

Ki ) 〈cos2 R〉 ) Ri/(Ri + 2)

(2)

where R is the angle made by the transition moment of vibration i with the fiber axis Z. If the dichroic ratios related to the three main molecular axes are measurable, it can be checked that the sum of the orientation factors for the three mutually orthogonal directions add up to unity:

K x + Ky + Kz ) 1

Figure 8. UV-vis absorption spectra of DHOA (solid line) and DDOA (broken line) in methanol solution (a) and either the solid state (DHOA, KBr pellet) or the gel phase (DDOA, methanol; b).

(3)

The results reported in Figure 7 reveal rather strong intensity variations for some absorptions; hence the DDOA molecules do adopt specific orientations within the fibers. For example, it can be immediately seen that Ai,| is much smaller than Ai,⊥ for the out-of-plane vibrations of the anthracene group situated in the 1000-700-cm-1 spectral range. Because the transition moment of these vibrations lies along xA, this molecular axis should be rather perpendicular to the fiber axis Z. The data of Figure 7 have been quantitatively exploited by fitting step-by-step mixed Lorentzian/Gaussian profiles (about 5-10% Gaussian) through a series of 3-4 bands. The ratios of the areas of these components in the parallel and perpendicular polarizations are given in Table 1. A similar procedure has been used for the spectral range of the methyl and methylene stretching vibrations, and a minimum of eight components are needed to reproduce the whole profile between 3000 and 2800 cm-1. However, most of these components are rather strongly overlapped, and the (16) Radziszewski, J. G.; Michl, J. J. Chem. Phys. 1985, 82, 3527. (17) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light; VCH Publishers: New York, 1986.

Figure 9. Fluorescence emission spectra of the manually oriented aerogel fibers of DDOA as a function of the polarization angle. The excitation wavelength is 345 nm. The polarization of the electronic transitions of the anthracene monomer (around 420 nm) and excimer (around 600 nm) are given on top of the corresponding bands.

corresponding dichroic ratios are not very reliable. Their positions and assignments are given in Table 1. It is interesting to note that the orientation of anthracene16 or DDOA18 molecules contained in stretched polyethylene has already been studied by IR and UV dichroism. Table 2 indicates that the measured orientation factors are in the same range as those of DDOA in the aerogel fibers. 3.4. UV Absorption and Fluorescence Emission of DDOA in Various States. The electronic absorption and fluorescence emission spectra of DDOA in the isotropic and gel phases have previously been reported.18 A comparison of the electronic spectra of DHOA and DDOA shows that these two molecules exhibit, as was expected, very similar features: the structured band observed in the methanol solution and originating from 1La and 1Lb transitions18 (Figure 8a) is red-shifted toward a new structured band in both solid DHOA and gelled DDOA (Figure 8b). One can thus infer that the DDOA molecules in the gel phase undergo local perturbations similar to those in the crystal with prominent CH-π interactions between the head-to-tail anthracene moieties. It has also been previously shown that the maximum of the fluorescence emission is red-shifted upon the gelification of a DDOA solution. Fluorescence emission spectra reveal that significant changes occur in the aerogel phase: the first structured emission band ranging from 370-480 nm (18) Brotin, T.; Waluk, J.; Desvergne, J.-P.; Bouas-Laurent, H.; Michl, J. Photochem. Photobiol. 1992, 55, 335.

Molecular Arrangement in DDOA Gel Fibers

Langmuir, Vol. 19, No. 11, 2003 4569

For the uniaxial orientation of yA around Z, that is, random distribution of the anthracene long axis in the (X, Y) plane (Figure 10), integration must be performed over the angle φ, and the absorption intensities can then be developed from eq 4 according to

A⊥ ) AX ) AY ) 1/2(〈cos2 ψ〉 + 〈cos2 θ〉〈sin2 ψ〉)AxA + 1/2〈sin2 θ〉AyA + 1/2(〈sin2 ψ〉 + 〈cos2 θ〉〈cos2 ψ〉)AzA (5) Figure 10. Model of the uniaxial orientation of the yA long axis and noncylindrical orientation of the xA and zA axes around the fiber axis Z. The anthracene plane is schematized by a rectangle.

(1La transition, polarized along the anthracene ring short axis),18 also observed for the gel phase, is accompanied by a nonstructured broad band localized between 500 and 650 nm. This new emission band might be due to the presence of excimers19 with the two anthracene moieties having a greater overlap in the aerogel phase than in the gel phase. 3.5. Linear Dichroism in the Fluorescence Emissions. Manually oriented aerogel fibers have been studied using a polarizer between the sample and detector. The polarization plane has been rotated from the parallel to the perpendicular positions by steps of 10°. The results reported in Figure 9 indicate that the intensity of the structured band (370-480 nm) is weakly increased with the parallel polarization. Thus, the angle of the zA axis with respect to the fiber axis Z is expected to be slightly larger than the magic angle (54.7°). On the other hand, the second component (500-650 nm) exhibits a stronger variation and reaches its maximum intensity with the perpendicular polarization. This is an indication that the xA axis adopts an orientation nearly perpendicular to Z, in agreement with the IR conclusions. 3.6. Molecular Arrangement in the DDOA Aerogel Fibers. Although the IR and fluorescence experiments already described give consistent results, they do not provide sufficient information to establish the threedimensional arrangement of the gelator. Important features have previously been mentioned of the morphology of the gel fibers: they display very long filaments (>10 µm) with a limited diameter (ca. 10-50 nm)20 and they appear to be hollow, as was revealed by transmission electron microscopy images.21 Thus, the uniaxial orientation of the long axis yA and noncylindrical symmetry for the other two axes could reasonably be envisioned for the molecular organization (Figure 10). The following Euler transformation matrix relates the transition moments in the molecular frame (xA, yA, zA) to their components in the OXYZ laboratory frame, where Z is taken along the fiber axis:

|||

MX cos ψ cos φ - cos θ sin φ sin ψ MY ) cos ψ sin φ + cos θ cos φ sin ψ MZ sin θ sin ψ

sin θ sin φ -sin θ cos φ cos θ

-sin ψ cos φ - cos θ sin φ cos ψ -sin ψ sin φ - cos θ cos φ cos ψ sin θ cos ψ

|| | MxA MyA MzA

and

A| ) AZ ) 〈sin2 θ〉〈sin2 ψ〉AxA + 〈cos2 θ〉AyA + 〈sin2 θ〉〈cos2 ψ〉AzA (6) The theoretical dichroic ratios for the three kinds of vibrations of the anthracene group are then given by

RxA ) 2〈sin2 θ〉〈sin2 ψ〉/(〈cos2 ψ〉 + 〈cos2 θ〉〈sin2 ψ〉) (7) RyA ) 2〈cos2 θ〉/〈sin2 θ〉

(8)

RzA ) 2〈sin2 θ〉〈cos2 ψ〉/(〈sin2 ψ〉 + 〈cos2 θ〉〈cos2 ψ〉) (9) The experimental values are affected by rather large uncertainties: RyA ) 2.1 ( 0.5, RxA ) 0.21 ( 0.03, and RzA ) 0.99 ( 0.08 (Table 1). However, the introduction of these values with their error bars in the three above equations leads to the unique solution θ ) 41° and ψ ) 28°. Then, simple trigonometric transformations show that the xA, yA, and zA molecular axes make mean angles of 72, 41, and 54.6° with the fiber axis Z, respectively. Within this model, the long axis yC of the aliphatic chains should also be uniaxially oriented. Unfortunately, only a few well-isolated vibrations of the CH2 groups are available, and none of them have a transition moment along yC (Table 1). The more reliable results concern the vibrations displaying their transition moments along xC (rCH2) or zC (νsCH2). It emerges that the dichroic ratios for these vibrations are around 0.2 for rCH2 and 1.1 for νsCH2 (Table 1), that is, very similar to those previously found for the anthracene core vibrations along xA and zA, respectively. 3.7. Modeling of the Molecular Arrangement. A tentative modeling of the gelator structure should take into account the available experimental information: (i) the IR and UV-visible spectra of solid DHOA and gelled DDOA are very similar, (ii) the mean orientation of a given molecule with respect to the fiber axis has been evaluated from the IR dichroism, and (iii) the fluorescence emission implies the occurrence of excimers. In addition, it has been reported in the literature that the crystal packing mode could be helpful for designing new gelators,22 and it is tempting to extract some pertinent molecular information from the DHOA crystal structure to disclose the gel structure.23 The DHOA crystal unit cell exhibits two remarkable features: (i) the c axis is very large (54.43 Å) and the molecules pack together with their long molecular

(4)

(19) (a) Chandross, E. A.; Ferguson, J. J. Chem. Phys. 1966, 45, 3554. (b) Morita, M.; Kishi, T.; Tanaka, M.; Tanaka, J.; Ferguson, J.; Sakata, Y.; Misumi, S.; Hayashi, T.; Mataga, N. Bull. Chem. Soc. Jpn. 1978, 51, 3449.

(20) Terech, P.; Bouas-Laurent, H.; Desvergne, J. P. J. Colloid Sci. 1995, 174, 258. (21) Placin, F. Thesis n° 2125, University Bordeaux 1, Talence, France, 1999. (22) Menger, F. M.; Caran, K. L. J. Am. Chem. Soc. 2000, 122, 11679. (23) As quoted from the following, it must be used with caution because of the polymorphism observed in many organogels: Osumi, E.; Kamuras, P.; Weiss, R. G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1324.

4570

Langmuir, Vol. 19, No. 11, 2003

Placin et al.

Figure 11. (a) Projection of the DHOA unit cell along the a axis. The cell includes three blocks (numbered 1-3) that contain two triads each. (b) AMBER-optimized cluster of 18 DHOA molecules, with the same projection as in part a. (c and d) AMBER-optimized geometry of the head-to-tail central DHOA dimer of block 1 (a1 belongs to one triad and a2 belongs to the neighboring triad). The projection is along (c) or perpendicular to (d) the anthracene part of the a1 DHOA molecule.

Figure 12. AMBER-optimized cluster of 18 DDOA molecules with (a) the same projection as in Figure 11b and (b) a view along a direction perpendicular to the one used in part a. (c and d) AMBER-optimized geometry of the a1-a2 DDOA dimer. The projection is along (c) or perpendicular to (d) the anthracene part of the a1 DDOA molecule.

axis parallel to c and (ii) an angle of 60° is experienced between the aromatic planes of neighboring molecules inside the triads. The first feature is in good agreement with the quite generally accepted guideline for designing low-molecular-weight gelators, that is, the existence of unidirectional interactions forces the one-dimensional

assemblies into gels.24 Furthermore, a mean angle of 72° is found between the xA molecular axis and the fiber axis (24) (a) Gronwald, O.; Shinkai, S. Chem.sEur. J. 2001, 7, 4329. (b) Lu, L.; Cocker, T. M.; Bachman, R. E.; Weiss, R. G. Langmuir 2000, 16, 20. (25) Pauzat, F.; Ellinger, Y. Chem. Phys. 2002, 280, 267.

Molecular Arrangement in DDOA Gel Fibers

Figure 13. Schematic representation of the proposed arrangement of DDOA in the gel fibers.

Z, but the sign of this angle is undetermined. This geometry is not very different from that encountered in the triad packing, where an angle of 60° is formed between each aromatic molecular plane. These remarks prompted us to investigate the gel structure by means of MM. The optimized geometries of two clusters containing, on one hand, 18 molecules of DHOA (i.e., the molecular aggregation within the DHOA crystal unit cell, see Figure 11) and, on the other hand, 18 molecules of DDOA have been compared. The clusters involve three blocks (numbered 1-3 in Figures 11 and 12), each of them made of two interacting triads. The starting geometry of DDOA was built from the DHOA cluster by displacing the external blocks 2 and 3 from the central block 1. The optimized structures of the clusters are shown in parts b and c of Figure 11 for DHOA and Figure 12 for DDOA. In both cases, the central block is hardly changed with respect to the crystal structure, as is shown by the projection along the pseudo-c axis. The reduction of the transversal interactions induced by the severe truncation along the a and b directions of the 18-mer cluster is not sufficient to decrease noticeably the stability of this central part. This result reflects the leading role of the unidirectional interactions parallel the pseudo-c axis. The sole noticeable geometrical change of the central block mainly concerns the peripheral molecules. The aliphatic chains of these molecules lack some CH-π interactions along a or b and, consequently, adopt an angular orientation with respect to the aromatic plane, as was found for the optimized structure of the isolated molecule (see Figure 1d). The main difference between DHOA and DDOA is related to the respective orientation of the neighboring blocks. For DDOA, there is a strong tendency for an angle to emerge between blocks 1 and 2 and blocks 1 and 3. This misalignment should induce a curved one-dimensional stacking. This difference might be related to subtle differences in the intermolecular interactions, as is illustrated in Figures 11c,d and 12c,d for the central block 1 of DHOA and DDOA, respectively. In the DHOA unit cell, the a1 and a2 molecules belong to adjacent triads and stack into a headto-tail dimer. One notes that the intermolecular interactions in the DHOA crystal are optimum as a result of the perfect fit between the length of the aromatic part and of

Langmuir, Vol. 19, No. 11, 2003 4571

the aliphatic chains. The a1-a2 optimized DHOA dimer (Figure 11c,d) is very similar to the crystal dimer. In the case of DDOA, where the length of the aliphatic chain is longer than the length of the anthracene, the a1-a2 optimized dimer corresponds to a slide of one molecule with respect to the other one along the long molecular axis. Some interactions now involve O(a1)-CH2(a2) contacts that provoke a tilt of the chains relative to the aromatic plane. The geometries of a1 or a2 DDOA molecules correspond to the geometry of the isolated molecule (see Figure 1c,d). The fitting of the chain and anthracene lengths favor a “parallel” packing involving roughly coplanar molecules, which in turn induces a route to crystal-type packing (DHOA case). It has been observed that DHOA is also able to form some gels if the kinetics of the preparation are well-tuned. This might result from the tendency of the isolated molecules (appearing, for example, as crystallization defects) to present a curved geometry. Consequently, the crystal packing is prevented from developing at the expense of the gel formation. Besides, shorter chains (n < 6) are not efficient enough to build large π-CH2-type interactions in such a way that the gels are not formed for these derivatives. The mis-fit of the chain and anthracene lengths favors an “angular” packing involving curved molecules, which, consequently, promotes a curved-type one-directional packing (such as for DDOA and the parent derivatives). From the previously mentioned considerations, we propose the three-dimensional arrangement schematized in Figure 13. The DDOA molecules (or other derivatives) would be situated on concentric cylinders, forming helicoidal coils on each cylinder with the long axis yA making an angle of about 41° with the fiber axis Z. This arrangement is consistent with an uniaxial orientation of yA around Z, with head-to-tail interactions between the molecules in a direction perpendicular to Z, that is, between the molecules belonging to successive cylinders. One can also infer that the fibers are hollow because repulsive forces impose a minimum diameter to the cylinders, whereas cohesive forces are unable to ensure larger external diameters. 4. Conclusion DDOA is an efficient low-molecular-mass organic gelator, although the cohesive forces between DDOA molecules to form a three-dimensional network are of the van der Waals type. The directions of these intermolecular forces are more difficult to predict than, for example, hydrogen bonds, and the gelation mechanism of DDOA is puzzling. To determine the molecular arrangement of DDOA inside the gel, IR and fluorescence linear dichroism experiments on aerogel fibers have been combined with theoretical calculations. The calculations are very helpful in predicting the transition moments of the anthracenegroup vibrations and conformation of the two interacting DDOA molecules. Besides, the IR dichroism results are surprisingly similar to those obtained on anthracene or DDOA molecules oriented in stretched polyethylene films. However, it remains necessary to propose models for the molecular orientation and arrangement inside the fibers. From the experimental data, we can infer that the DDOA long axis is uniaxially oriented around the fiber axis Z and that the DDOA molecules are arranged along helicoidal coils in concentric cylinders. The only constraints on this model are to satisfy the experimental dichroic ratios, theoretical predictions, and hollow characters of the fibers. Other techniques, such as small-angle scattering (X-ray and neutrons), should be applied on the aerogel fibers to check the validity of this description.

4572

Langmuir, Vol. 19, No. 11, 2003

Acknowledgment. The authors are grateful to M. Besnard and G. Clavier for their help with the experiments and for stimulating discussions. They also dedicate the present paper to Professor H. Bouas-Laurent for his 70th

Placin et al.

birthday because he was greatly involved in the development of these gels. LA0270439