Langmuir 1996, 12, 4321-4323
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Molecular Rods in a Zinc(II) Porphyrin/Cyclohexane Physical Gel: Neutron and X-ray Scattering Characterizations P. Terech,*,†,‡ G. Gebel,† and R. Ramasseul‡,§ De´ partement de Recherche Fondamentale sur la Matie` re Condense´ e, SI3M-PCM, and SCIB-CC, CNRS URA1194, C.E.A-Grenoble, 17, rue des Martyrs, 38054 Grenoble Ce´ dex 09, France Received April 19, 1996. In Final Form: June 21, 1996X Aggregation of some low-molecular weight compounds in organic liquids is known to result in the formation of rod-shaped colloidal particles.1 Beyond the overlap concentration of the aggregates, the systems usually exhibit remarkable viscoelastic features which account for their description as gel-like materials. In particular, their typical consistency results from the development of a three-dimensional network made of interconnected colloidal aggregates. The sensitivity of the aggregation process to parameters such as temperature, concentration, and impurities, the fragility of the resulting network, and its thermal reversibility characterize such systems whose structural study raises specific problems.1 By contrast to the commonly used electron microscopy technique, the small-angle scattering techniques enable a nondestructive investigation of these materials swollen by their solvents. Furthermore, comparative X-ray and neutron studies can provide complementary information concerning the internal structure of the aggregates, such as their cross-sectional heterogeneity or contrast profile.2
Aggregation of porphyrin derivatives in fiber-like structures is a phenomenon which has been well studied as it possesses a potential interest in photoelectronic conversion, molecular recognition, and biomimetic fields. Among others, various examples are recalled below. In aqueous solutions, amphiphilic porphyrins are known to form micellar fibers3,4 in which the fundamental unit results from a dimeric molecular association. Some metalloporphyrins exhibit a formation of discotic mesophases in which the disk-like porphyrin units behave as rods.5 Fibers of zinc(II) metalloporphyrins in aqueous solutions have been observed6 and their diameter (70 Å) corresponds to approximately twice the molecular length of the lipid-porphyrinatozinc(II) molecules used. Other amphiphilic tin(IV) porphyrinate dichlorides have been reported to form fibers with a 50 Å diameter,7 consistent with a lateral assembly of the molecules. So far, examples of fibrillar aggregates of metalloporphyrinates in organic solvents have not been reported. However, aggregation of a cationic meso-tetrakis(4-N-stearylpyridyl)porphyrin tetraiodide metallic complex in organic solvents of variable polarity has been demonstrated and studied8 using the ESR and NMR techniques, but the corresponding aggregate morphology remains uncharacterized. Here, we report on the first characterized system of rodlike aggregates of a trisubstituted metallo (ZnII) porphyrin (abbreviated below as ZnP3) in an apolar organic * Author to whom correspondence should be addressed. † SI3M-PCM. ‡ Member of CNRS. § SCIB-CC. X Abstract published in Advance ACS Abstracts, August 15, 1996. (1) Terech, P. In Low-molecular weight organogelators, Specialist surfactants, Blackie Academic & Professional, Chapman & Hall, in press. (2) See for instance: Ibel, K.; Stuhrmann, H. B. J. Mol. Biol. 1975, 93, 255. (3) Fuhrhop., J.-H.; Demoulin, C.; Boettcher, C.; Ko¨ning, J.; Siggel, U. J. Am. Chem. Soc. 1992, 114, 4159. (4) Arimori, S.; Takeuchi, M.; Shinkai, S. J. Am. Chem. Soc. 1996, 118, 245. (5) Bruce, D. W.; Wali, M. A.; Min Wang Q. J. Chem. Soc., Chem. Commun. 1994, 2089. (6) Tsuchida, E.; Komatsu, T.; Toyano, N.; Kumamoto, S.; Nishide, H. J. Chem. Soc., Chem. Commun. 1993, 1731. (7) Fuhrhop, J.-H.; Bindig, U.; Siggel, U. J. Chem. Soc., Chem. Commun. 1994, 1583. (8) Yamamura, T. Chem. Lett. 1977, 773.
S0743-7463(96)00381-2 CCC: $12.00
liquid (cyclohexane) using small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) experiments.
Zinc(II) 5-(p-carboxyphenyl)-10,15,20-tris[((p-hexadecyloxy)carbonyl)phenyl]porphyrinate (ZnP3) was obtained as a side product during the purification of the corresponding tetraester, zinc(II) 5,10,15,20-tetrakis[((phexadecyloxy)carbonyl)phenyl]porphyrinate9 by column chromatography on basic alumina (yield 40-60%).10 ZnP3 was dissolved with difficulty in cyclohexane by heating, and a gelatinous dark-red material was subsequently formed on cooling to room temperature without agitation. On shaking, the jelly was broken down to a liquid which reversibly recovered the typical viscoelastic consistency when allowed to stand at room temperature. The metallic element and the presence of one acid group attached to the porphyrin molecule were observed to be compulsory requisites for the observation of the gelation phenomenon of cyclohexane by ZnP3. The SAXS experiments were performed at the synchrotron source LURE (Orsay, France) using the D22 spectrometer. The explored Q-values varied from Q ) 0.006 to 0.3 Å-1, where Q was the momentum transfer (defined as Q ) (4π sin θ)/λ for elastic scattering, with λ being the radiation wavelength and 2θ the scattering angle). Cells (1 mm thickness) with Kapton windows were (9) Ramasseul, R.; Maldivi, P.; Marchon, J.-C.; Taylor, M.; Guillon, D. Liq. Cryst. 1993, 13, 729. (10) In a same manner the tetraesterporphyrin free base gives the carboxy triester porphyrin free base. These porphyrins were characterized by mass spectroscopy, 1H NMR spectroscopy, and UV-vis.
© 1996 American Chemical Society
4322 Langmuir, Vol. 12, No. 18, 1996
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used as sample holders. The SANS experiments were performed on the PAXE spectrometer (Laboratoire Le´on Brillouin, Orphe´e reactor, Saclay, France). Quartz cells (1 mm thickness) and deuterated cyclohexane were used. The Q-range available was 0.006 Å-1 < Q < 0.35 Å-1. Usual corrections for background subtraction, transmission, and normalization were applied.11 The consistency of the samples indicates that a very fragile network of ZnP3 aggregates grows in the organic apolar medium. The thixotropic material can be considered as a solid-like system with a weak yield stress value and as a liquid-like system beyond this value. The samples investigated at rest are gel-like. Considering the high dispersion state of the samples (concentration of the order of 1%), it is clear that the partitioning of the organic medium is most probably achieved by interacting colloidal aggregates. The related structural investigation can be conveniently performed by the small-angle scattering technique. SANS12 or SAXS13 can provide information on long correlation distances in a range typical of colloidal systems (20 Å < d < 1000 Å) and can furthermore take advantage of some contrast variation opportunities. The angular distribution of the intensity scattered at small angles can be decomposed into a term related to the formfactor of the scatterers and a term reflecting their spatial distribution. Within low-concentration conditions, the contribution of the interference term can be neglected. The fact that the ZnP3-cyclohexane solutions exhibit superimposable scattering curves in the concentration range 1-4% has been checked experimentally. For neutron scattering, the contrast of the aggregates in the medium (deuterated cyclohexane) has a direct dependence on the isotopic composition of the two-phase system. For X-ray scattering, the contrast is a function of the atomic number Z of the constitutive elements in the scatterer with respect to the embedding liquid. As a consequence, SAXS will probe mainly the metalloporphyrinic core of a ZnP3 molecule while SANS will be more sensitive to the aliphatic chains. Scattering experiments using the two radiations can thus be very useful to probe the structural heterogeneity of aggregates as demonstrated below for the ZnP3 gel-like systems. The main objective of the present study is to search simple and outstanding scattering features describing the overall shape of the colloidal aggregates. For instance, we can calculate the theoretical intensity scattered by rodlike structures which are uncorrelated in position and orientation. The intensity is a sum of the spherically averaged intensity for each long rod of length L and radius r (L . 2r) and reduces to the simplified form-factor expression.13
I(Q) )
( )[ πL Q
A∆F
]
2J1(Qr) Qr
2
(1)
In expression 1, A is the cross-sectional area of the rod, J1 is the Bessel function of the first order and ∆F represents the amplitude of the fluctuations of the volumic contrast F(r b). Expression 1 appears as the product of an axial term proportional to Q-1 and a cross-sectional term proportional to J1. At low-angles, Q-expansions of expression 1 indicate that an asymptotic Q-1 behavior is typical of the rodlike scatterers. Consequently, a QI versus Q log-log plot is a convenient graphic representation in order to demon(11) Lindner, P.; Zemb, T. Neutron, X-rays and light scattering: Introduction to an investigate tool for colloidal and polymeric systems; North Holland-Elsevier: Amsterdam, 1991. (12) Cabane, B. Surfactant solutions; Surfactant Science Series 22: 57; Zana, R., Ed.; Marcel Dekker, Inc.: New York, 1987. (13) Glatter, O.; Kratky, O. Small angle X-ray scattering; Academic Press: London, 1982.
Figure 1. SAS data of ZnP3 aggregates in cyclohexane. Points are experimental and full lines are calculations of expression 3. A: neutron scattering, (b) φ ) 0.019, (-) r ) 25 Å. B: X-ray scattering, (b) φ ) 0.0062, (-) r ) 14.5 Å.
strate the unidirectionality of scatterers, as shown by the low-angle plateau of Figure 1 observed for the ZnP3 aggregates in cyclohexane. The use of the scattering invariant (INV) enables the extraction of the geometrical radius of the rods without making any further assumptions concerning the contrast of the aggregates. It can be conveniently used to compare SAXS and SANS data directly. INV is calculated by a Q-integration of the scattering curve following expression 213
INV )
∫0∞Q2I(Q) dQ ) (∆F)2 φ (1 - φ) 2π2
(2)
where φ is the volume fraction of the rods. Expression 3 is obtained by combining expressions 1 and 2.
(
)
2J1(Qr) QI(Q) r2 ) INV Qr 2(1 - φ)
2
(3)
SANS and SAXS experiments are represented in Figure 1 as QI/INV versus Q plots. The full lines are calculated scattering curves at low-angles obtained using expression 3. In such representations, the level of the plateau is fixed by the radius and volume fraction values while the curvature of the intensity decay (Gaussian) in the intermediate Q-range, due to the finite size of the rod cross sections, is fixed by the radius value. The oscillating large-angle part of the theoretical curve (dotted line) reveals the shape, the polydispersity, and the contrast homogeneity of the cross sections.13 For the SANS experiments, in the intermediate Q-range, described by expression 3, a satisfactory agreement with the experimental data is found if a value of r ) 25 Å is used. This is evident considering both the shape and level of the scattered intensity. The analysis demonstrates the unidirectionality of the ZnP3 aggregates and provides the
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related cross-sectional radius value. Comparable conclusions can be obtained using the “classical” so-called “Guinier analysis”13 of the cross-sectional intensity. Figure 1B compares the SAXS data and a calculated scattering curve following expression 3 for r ) 14.5 Å. The agreement is also satisfactory considering the level of the QI plateau, the shape of the cross-sectional intensity decrease, and the position of the intensity minimum at large Q. As for the SANS experiments, the important point of the demonstration is the agreement in the Q-range around the Gaussian decay. Both the low-angle departure from the expected plateau observed by SAXS and the large-angle damped oscillations observed by SANS are indications that some refinements to the perfect cylinder model have to be considered but are currently out of the scope of the present paper. The geometrical radius measured by SAXS is rSAXS ) 14.5 ( 0.5 Å while that measured by SANS is rSANS ) 25 ( 1 Å. When comparing the rSAXS value to the molecular size of the ZnP3 metalloporphyrinic rigid core (ca.17 Å), it appears that most probably only one molecule lies within the ZnP3 rodlike aggregate. The important difference between the rSANS and rSAXS values can be explained by considering the influence of the cross-sectional contrast profile of a special rodlike morphology upon the scattering. For instance, if a molecular thread structure is considered where the ZnP3 molecules are stacked in a manner that an internal organometallic core is formed (without prejudging of the detailed molecular mechanism and structural arrangement), the above scattering features become consistent with the experimental data. In such a schematic model of a circular heterogeneous cross section, the neutron contrast arises mostly from the aliphatic chains with respect to deuterated cyclohexane while the X-ray contrast is mainly due to the metalloporphyrinic core. The
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present data strongly support the model in which the ZnP3 molecules autoassociate in the simplest structure possible for an organometallic molecular thread. Interestingly, a comparable striking structural feature was also observed for another class of organometallic compounds in cyclohexane (bicopper(II) tetracarboxylate complexes).14 These two systems constitute the first well-characterized examples of molecular threads in organic liquids. Despite their structural similarity, these systems exhibit very different rheological properties. For example, the bicopper(II) tetracarboxylate complexes can be described as “living polymers” while the ZnP3-cyclohexane systems are thixotropic materials. To conclude, we have proved the existence of rodlike aggregates in ZnP3-cyclohexane solutions. Comparisons of the diameter of the aggregates and molecular dimensions, as well as contrast arguments, indicate that the aggregates are molecular threads. Numerous autoassociative systems forming unidirectional aggregates are known but the ZnP3-cyclohexane solutions constitute a very rare example of molecular threads with only one molecule per cross section. Further experiments are needed to characterize both the axial and cross-sectional structures as well as the mechanical properties of the gel-like materials. Acknowledgment. Drs C. William (LURE) and J. Teixera (LLB) are thanked for their help during the scattering experiments. Mrs. C. Dumon and C. Balbine are acknowledged for their contribution to the synthesis work. LA960381N (14) Terech, P.; Schaffhauser, V.; Maldivi, P.; Guenet, J. M. Europhys. Lett. 1992, 17, 515.
