Encyclopedia of Smart Materials (Vols 1 and 2) - M. Schwartz (2002) WW Part 7 pps

P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-G-DRV January 12, 2002 4:11 472 GELATORS, ORGANIC (a)(b) OH OH HO HO N N N N N N N N O O 1 23 Figure 3. Crystal structures of (a) 1·2 complex and (b) 1·3 complex (solvent molecules have been omitted for clarity). concentration-dependent. These assemblies range from a rigid-rod character at very dilute concentrations to a lyotropic liquid–crystalline gel at higher concentrations (5). Most interestingly, Meijer developed supramolecular polymers of type 5, that are held together by quadruple hydrogen bonding between the ureidopyrimidone units. This material displays most, if not all, properties of macro- scopic polymers based only on non-covalent connections (Fig. 4) (6). Materials based principally on hydrogen bonding and other intermolecular interactions have been generated using low molecular weight organogelators (7). Recently, mi- crocellular organic materials have been prepared by drying of organogels in supercritical CO 2 (8). The field of organogelation has evolved from molecules that have different structural and recognition properties to the ratio- nal design and fine-tuning of materials. In the following section, we describe advances in organogelation and the use of intermolecular interactions in developing these new supramolecular structures. INTERMOLECULAR INTERACTIONS Hydrogen Bonding Hydrogen bonds are usually formed when a donor (D) that has an available acidic hydrogen is brought into close contact with an acceptor (A) that possesses a lone pair of elec- trons (Fig. 5). Hydrogen bonding has been the subject of statistical investigations (9,10), X-ray diffraction analysis (11–13), and theoretical studies. The hydrogen bond can vary in strength from 1 kcal/mol for C–H ··· O hydrogen bonding (14,15) to 40 kcal/mol for the HF − 2 ion in the gas phase (16,17). Hydrogen bonding has played a critical role in the development of areas such as self-assembly (18) and crystal engineering (19,20). Solid-state and solution studies of the hydrogen bond have provided evidence that this interaction is a highly ordered phenomenon, not a random event. In the solid state, Zaworotko demonstrated that the inorganic complex 6 that has four hydrogen bond donors oriented in a tetrahedral geometry can form hydrogen bonds to nitrogen and to π-systems (21). The crystal structures from this study show supramolecular diamond-like arrangements, where the size of the network cavities de- pend on the size of the hydrogen bond acceptor (see Fig. 6). Hydrogen-bond strength is influenced by secondary electrostatic interactions. A particularly strong hydrogen- bonded complex is formed when a molecule which is comprised of all hydrogen-bond donors binds to an all hydrogen-bond acceptor (Fig. 7) (22). The calculated indi- vidual secondary electrostatic interactions in these complexes are ±2.5 kcal/mol in chloroform (23). Schneider es- timated the contribution of a related series of secondary interactions at ±0.7 kcal/mol from a large number of examples in the literature (24). The presence of a competitive hydrogen-bonding solvent can also influence the strength of hydrogen bonding. Lorenzi found that the dimerization constant of a small cyclic peptide is 80 M −1 in tetrachloromethane, whereas the same molecule does not dimerize in chloroform (25). Wilcox also studied the effects of the concentration of water on the binding free energy of hydrogen-bonding molecules in chloroform (26). In competitive solvents, such as water/methanol mixtures, guanidinium receptors and carboxylate substrates are highly solvated. Schmidtchen has observed that as the polarity of the solvent (higher percentage of water in methanol) increases, the enthalphic P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-G-DRV January 12, 2002 4:11 GELATORS, ORGANIC 473 O N H N N N H O OR OR OR O N H N N N H O OR RO RO ON H N N N H O RO OR OR N N C 13 H 27 O H N H N O H N H N O H N NO H C 13 H 27 4 5 Figure 4. Examples of supramolecular materials: Molecule 4 forms liquid-crystal gels and 5 forms polymers. A = F, Cl, O, S, N, π-system D = F, Cl, O, S, N, C ADH Figure 5. Hydrogen bond formed between an acidic hydrogen (D–H) and an acceptor (A). component of the binding free energy and the binding affinity decreases (27). π–π Interactions Planar aromatic molecules, it is known, interact with one another in three possible geometrical arrangements: stacking (e.g., face-to-face overlap of duroquinone, Fig. 8); offset stacking (e.g., laterally shifted overlap in [18]annulene); and herringbone (e.g., T-shaped edge-to-face interactions in benzene, Fig. 8) (20). The greatest van der Waals inter- active energy is found in the face-to-face overlap arrangement in which there is the highest number of C···C intermolecular contacts. If van der Waals forces were solely to determine the packing of flat aromatic molecules, the offset stack and herringbone arrangements would not be commonly observed. In effect, there is a barrier to face-to-face stacking due to π ···π repulsions. As a result, the offset stack arrangement is the most commonly observed (28). Similarly, the herringbone interaction in many aromatic hydrocarbons offers evidence for the C(δ−)H(δ+) nature of this interaction (29). The slightly electrostatic character of the herringbone interaction may predispose molecules during crystallization toward inclined geometries and re- flects its character as a weak C–H ···π hydrogen bond (30). van der Waals Interactions Van der Waals interactions are dispersive forces caused by fluctuating multipoles in adjacent molecules that lead to attraction between them. In the solid state, the packing of aliphatic side chains is governed by van der Waals interactions. When the side chains are longer than five car- bon atoms, H ···H interactions predominate. Similar effects have been observed in molecular recognition solution studies. Kataigorodskii’s close-packing principle assumes that the potential energy of the system is minimized by P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-G-DRV January 12, 2002 4:11 474 GELATORS, ORGANIC H O Mn Mn H O H O O Mn Mn H 6 = 6 = , a N N Figure 6. The hydroxyl groups of inorganic complex 6 can hydrogen bond to 4,4-dipyridine or benzene to form diamondoid networks (a). molecules making a maximum number of intermolecular interactions (31). Therefore, because van der Waals interactions are nondirectional, the energy differences between alternative molecular arrangements are small. However, computational studies of rigid molecules that assemble into one-dimensional aggregates give insight into the importance of van der Waals and coulombic terms (32,33). The importance of van der Waals interactions in organogelation should not be underestimated because most organogelators have long alkyl chain groups. ORGANOGELATION Many attempts have been made to define the phenomenon of gelation. In 1993, Kramer and colleagues proposed that use of the term “gel” be limited to systems that fulfill the following phenomenological characteristics: (1) They con- sist of two or more components, one of which is a liquid, and (2) they are soft, solid, or solid-like materials (34). The authors further described their definition of “solid- like” and also reviewed other existing definitions of gels. However, there is no precise definition for gelation, and in recent literature it is a phenomenon that is described rather than defined. Organogels are usually formed when an organogelator is heated and dissolved in an appropriate solvent. The mixture is allowed to cool to its gel transition temperature resulting in the formation of a matrix that traps the solvent due to surface tension. In general, the Figure 7. Repulsive and atractive secondary interactions in triple hydrogen-bonded donor–acceptor complexes. DDD DAD ADA NN O N O HH N NOO C 6 H 11 H AAA NN H HH N H H OO O Ar O NNN Repulsive secondary interactions Atractive secondary interactions O O OO Stack Offset stack Edge-to-fac e Figure 8. The most common π–π interactions are stacked, offset stacked, and edge-to-face. amount of organogelator needed to gel a certain solvent is small with respect to solvent, and concentrations can be as low as 2% by weight. Organogelators are usually divided into two distinct classes based on the nature of the chemical forces that stabilize them. Chemical organogelators are formed through covalent networks; examples include cross-linked polymer gels and silica gels. One common feature of these gels is that their formation is irreversible. In contrast, physical gels are stabilized by noncovalent forces that range from hydrogen bonding to π–π stacking interactions. These types of gels are thermoreversible from the gel phase to solution. The molecules that encompass this class of P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-G-DRV January 12, 2002 4:11 GELATORS, ORGANIC 475 compounds range from peptides to carbohydrates to very simple organic molecules. We focus most of our discussion on these types of organogelators. Several papers discuss the structures of organogelators and their properties. Terech and Weiss presented extensive research on anthryl and anthraquinone appended chloles- terol derivatives (35). Extensive work on amide and urea organogelators is presented in van Esch and Feringa’s 1999 book chapter (36). Hamilton and co-workers reported examples of organogelator design derived in many cases from molecular recognition and self-assembly (37). Although the mechanism of gelation is not fully un- derstood, there have been many attempts to understand the structure of gels. In a recent paper, Terech discussed and compared three methods for measuring phase transition temperatures in physical organogels (38). The three methods analyzed were the “falling ball” technique, nu- clear magnetic resonance (NMR) spectroscopy, and rheology. The authors concluded that the rheology method is the most reliable. Examples of Organogelators Low molecular weight organogelators encompass a variety of compounds that can self-assemble into a fibrous matrix that can trap solvent molecules within its cavities. The noncovalent interactions that hold these structures together are various in nature. As a result, organogelators are usually classified by their chemical consititution. We will use this same type of classification previously em- ployed by Terech, Weiss, van Esch, and Feringa. Fatty Acid and Surfactant Gelators. Some of the first organogelators were based on substituted fatty acids. 12- Hydroxyoctadecanoic acid 7 and its monovalent salts form organogels in a variety of solvents (Fig. 9) (39,40). Obser- vation of circular dichroism was used as evidence for the formation of supramolecular helical strands, although the N + I − N + N + C 16 H 33 C 16 H 33 HO CO 2 −− O 2 C OH HO CO 2 −− O 2 C OH COOH OH L-tartarate D-tartarate 2 X − 2X − = L-tartarate, 9 = D-tartarate, 10 7 8 Figure 9. Examples of gemini surfactants and fatty acid organogelators. maximum of the signal depended on the solvent. For D-7, most of the helices were left-handed, whereas for L-7 they were right-handed. Tetraalkylammonium derivatives such as compound 8 behave as surfactants and organogelators (41). Gemini (dimeric) surfactants formed by cetyltrimethylammonium ions (CTA) with various coun- terions gel organic solvents (see Fig. 9) (42). The most probable structure for the gels of 9 and 10 is an entan- gled network of long fibers that have polar groups at the core of the aggregate and long alkyl chains in contact with the solvent. These compounds gel chlorinated solvents most effectively at concentrations as low as 10 mM. However, they gel other solvents such as toluene, xylenes, chlorobenzene, and pyridine at concentrations from 20– 30 mM. Anthracene and Anthraquinone Derivatives. Anthracene and anthraquinone derivatives gel various alkanes, al- cohols, aliphatic amines, and nitriles. These aromatic structures to form gels through π–π interactions. However, when the anthryl ring of 11 is partially hydrogenated, compounds 12 and 13 still form organogels (Fig. 10) (43). The interesting photochromatic properties of anthraquinones, such as 14, has led to the study of substitution patterns on the ring. Dialkoxy-2,3-anthraquinone derivatives are the most effective agents (44). Both anthracene and anthraquinone substituents have been coupled to cholesterol groups for organogelation purposes. These are discussed later. Amides and Ureas. The hydrogen bonding properties of amides and ureas have been the subject of solid-state and solution studies. Many low molecular weight organogelators have been designed by using this recognition element. The amide functional group can form eight-membered, hydrogen-bonded dimers and one-dimensional infinite arrays (Fig. 11). Primary and secondary amides (45) and pyrimidones (46) form cyclic dimers. P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-G-DRV January 12, 2002 4:11 476 GELATORS, ORGANIC Figure 10. Examples of organogelators based on anthracene and anthraquinone derivatives. OC 10 H 21 OC 10 H 21 OC 10 H 21 OC 10 H 21 OC 10 H 21 OC 10 H 21 11 13 12 H 2 (1 atm) Pd/C EtOH O O OC 10 H 21 OC 10 H 21 14 Hanabusa reported that trans-cyclohexane-1,2-diamide 15 gels organic solvents, silicon oil, and liquid paraffinat concentrations as low as 2 g/L (47). Enantiomerically pure 15 produces stable gels, and circular dichroism indicates a chiral helical arrangement of the diamides. Electron mi- crographs of a gel produced from 15 in acetonitrile showed the presence of the helical superstructures. However, the racemic mixture of 15 and 16 produces only unstable gels. The authors believe that the helical superstructures could arise from stacked, hydrogen–bonded, infinite aggregates. Therefore, the orientation of the amide groups in a NN R O R O NN R O R O NN R O R O HH H H H H (a) N H O N H O (b) Figure 11. Amides can form linear array (a) ordimers (b) through hydrogen bonding. antiparallel trans configuration (both equatorial) is critical for the complementary interaction. Interestingly, 17 which has cis-amide groups (one equatorial and one axial) cannot form this interaction favorably and is not observed to gel any solvents (Fig. 12). This type of stacked amide hydrogen bonding has been observed in the crystal structure of a cyclohexane-1,3,5-triamide reported by Hamilton (48). Shirota reported similar amide-containing molecules in which the hydrogen-bonding groups are arranged around a rigid core. Compound 18 gels various solvents. To prove that hydrogen bonding is essential in the gelling ability of these molecules, the N-methyl analog 19 was synthesized, and no gelation was observed (49). Increasing the distance between the hydrogen-bonding groups does not have ad- verse effects on the gelling capacities of these compounds (Fig. 13), as was observed for 20 (50). Hanabusa also reported long alkyl chain trisubsti- tuted organogelators based on a flexible core (51). cis- 1,3,5-Cyclohexanetricarboxamide derivatives 21–24 show a trend in improved gelation when the alkyl chains are longer (Fig. 14). These results suggest that intermolecular hydrophobic interactions among the alkyl chains are critical in stabilizing the gel network. These molecules increase the viscosity of solvents at very low concentrations. Compound 22 causes an increase in the viscosity of CCl 4 to 250.0 cP at 25 ◦ C from 0.908 cP in the absence of the gelling agent. NHCOC 11 H 23 NHCOC 11 H 23 NHCOC 11 H 23 NHCOC 11 H 23 15 trans ( 1R, 2R ) 16 trans ( 1S, 2S ) 17 cis derivative Figure 12. Organogelators based on 1,2-diaminocyclohexane derivatives. P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-G-DRV January 12, 2002 4:11 GELATORS, ORGANIC 477 ON O N N O H C 18 H 37 HC 18 H 37 C 18 H 37 H ON O N N O CH 3 C 18 H 37 H 3 CC 18 H 37 C 18 H 37 CH 3 18 19 N ON H C 17 H 35 O N HC 17 H 35 O N H C 17 H 35 20 Figure 13. Low molecular weight organogelators based on amide hydrogen bonds. Recent applications of organogelators have included trapping liquid crystals within the gel matrix. Kato used 15 to gel liquid crystals such as 25 and 26 (Fig. 15) in concentrations as low as 1 mol% (52). These gels were stable at room temperature for several months. Measurements of the response to an electric field were made on gels of 15 and 25, and interestingly, the threshold voltage of the gel (5.0 V) is larger than that of the liquid crystal alone (1.1 V). The authors propose that the solvent (liquid crystal) is oriented within the gel and that the structure resembles the cartoon shown in Fig. 15. As a result of this property, these materials may have applications in electro-optical devices. Organogels formed by hydrogen bonding of small molecules have also been stabilized by polymerization. For example, Masuda used amide hydrogen bonds as in 1-aldosamide 27 to template the position of diacetylene groups close to each other for polymerization (53a). IR stretching frequencies were consistent with additional hydrogen bonding between the aminosaccharides. Robust nanofibers are observed in 27 using electron microscopy. However, 1-galactosamide containing 28 forms amorphous solids presumably due to steric hindrance by the axial OAc group that leads to the formation of an infinite amide CONHR CONHRRHNOC 21; R = CH 2 (CH 2 ) 4 CH 3 22; R = CH 2 (CH 2 ) 10 CH 3 23; R = CH 2 (CH 2 ) 16 CH 3 24; R = CH 2 CH 2 CH(CH 3 )(CH 2 ) 3 CH(CH 3 ) 2 Figure 14. Long alkyl chain organogelators derivatives with hydrogen bonding groups around a cyclic core. hydrogen-bonded network. Polymerization of 27 was confirmed by UV absorption and gel permeation chromatogra- phy. In a similar example, Shinkai polymerized 29 in situ (53b). The absorption spectra of the gels before and after photoirradiation show a distinct change that is consistent with polymerization (Fig. 16). Recently, Tamaoki published on the gelation and polymerization of 30 (see Fig. 17) (54). Compound 30 contains two cholestryl ester units at the ends of a diyne spacer. 30 shows liquid crystal behavior when heated between 101 and 133 ◦ C and gels nonpolar solvents at low concentrations. Gels formed in cyclohexane were irradiated by UV light (500-W high-pressure Hg lamp), and their color changed from colorless to dark blue. The absorption spectra provided evidence for the presence of the exciton band of polydiacetylene. The T gel for a gel at 5.3 mM was 45 ◦ C before polymerization. However, after UV irradiation, the gel maintained its shape even above the boiling point of cyclohexane (80.7 ◦ C). Ureas have been studied in detail due to their hydrogen- bonding complementarity to carboxylates and other an- ions. Moran synthesized 31 (Fig. 18), which has additional hydrogen-bonding groups to promote stronger association (55). The bis-urea 32 developed by Rebek P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-G-DRV January 12, 2002 4:11 478 GELATORS, ORGANIC Figure 15. Compound 15 can gel liquid crystals 25 and 26. It is believed that the structure of the gels is as depicted in the illustration. CN OCN 25 26 CN CN CN CN CN NC NC CN NC NC CN O H N N O H O H N N O H O H N N O H O H N N O H H O AcO AcO N OAc H O N O O OAc AcO H AcO OAc OAc O AcO N OAc H O N O O AcO H AcO OAc OAc OAc OAc NH NH O O 27 28 29 Figure 16. Aldosamide 27 is stabilized by one-dimensional hydrogen bonds, whereas 28 has axial acetyl groups that hinder the formation of such structures. Diamide 29 also forms one-dimensional strands and polymerizes in situ. P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-G-DRV January 12, 2002 4:11 GELATORS, ORGANIC 479 O O N H O O O O N H O O 30 Figure 17. Polymerization of the gel formed by 30 in cyclohexane causes it to turn deep blue after 1 minute of photoirradiation. N H N O HOO ON H Bu NO H Bu OO Br Br O N H O N RH N H O N HR 31 32 Figure 18. Urea molecules used for binding carboxylates. Molecule 31 has additional hydrogen-bonding sites, and 32 has chiral R groups for enantioselective recognition. complexes carboxylates enantioselectively when R is chiral (56). Etter and co-workers observed that bidentate hydrogen bonding predominates in bis-ureas in the solid state. Graph sets and hydrogen-bonding rules were derived from this data and used to predict hydrogen-bonding patterns in related structures (57). Lauher has published crystal structures of a family of ureylenedicarboxylic acids that form hydrogen-bonded sheets through the carboxylic acid dimer, as well as urea hydrogen-bonded one-dimensional strands (58,59). Examples are shown in Fig. 19. Hanabusa synthesized and studied gelators based on the urea hydrogen-bonding group. Molecules that have rigid spacers between the urea functional groups 33 and 34 gel only toluene and tetrachloromethane, respectively (60). However, cyclic bis-ureas 35–37 show remarkable gelling properties in different organic solvents. Similar to the results with 1,2-bisamidocyclohexanes, these molecules show gelation that depends on the length of the alkyl chains. The antiparallel orientation of the bis-urea groups is also important because the cis analog does not gel any solvent. See Fig. 20. Bis-urea molecules have been reported by Kellogg and Feringa (see Fig. 21) (61). The morphology of the dried gels observed is thin rectangular sheets. These compounds gel only a selective number of solvents at concentrations around 10 mg/mL. These gels are stable up to temperatures of 100 ◦ C and for months at room temperature. Hamilton reported a family of bis-urea molecules, shown in Fig. 22, that gels mixtures of solvents at 5 ◦ C (62). The crystal structure of 38 confirmed the formation of extensive hydrogen-bonded arrays by both urea groups. As seen in Fig. 22, all of the urea groups point in the same direction, which makes the aggregates chiral. In this particu- lar case, chirality is translated to the entire crystal because all strands point in the same direction. The urea hydrogen bonding distances N–H ···O are 2.18 and 2.23 ˚ A, which are within the expected range. Cyclic bis-ureas derived from trans-1,2-diaminocyclohexane and 1,2-diaminobenzene derivatives have been ex- tensively studied by Kellogg and Feringa (63). They prepared polymerizable derivatives 39 and 40 (Fig. 23). It is interesting to note that 39 forms gels only in tetralin, but 40 gels a variety of solvents. After photoirradiation and polymerization of the methacrylate groups, there is a slight turbidity and stability increase in the gels. A highly porous material was obtainedafterremovingthesolvent by freeze- drying. Functional organogels can also be generated by in- troducing reactive groups in the spacer between the ureas. Thiophene 41 and bis-thiophene 42 show efficient charge transport within the organogels that they form (64). These gels have potential application as organic semiconductors and have been further investigated. Molecular modeling of 1,2-bisaminocyclohexane and 1,2-bisamidobenzene derivatives indicates that trans-antiparallel orientation of the bis-urea groups is the most favorable for forming an extensive hydrogen-bonding network. However, in crystal structure 43, both urea groups are oriented in the same direction (65). The authors used a variety of techniques, such as infrared spectroscopy, differential scanning calorimetry P1: FCH/FYX P2: FCH/FYX QC: FCH/UKS T1: FCH PB091-G-DRV January 12, 2002 4:11 480 GELATORS, ORGANIC (CH 2 ) n NN (CH 2 ) n O HH O O H O O H (CH 2 ) n NN (CH 2 ) n O HH O O H O O H (CH 2 ) n NN (CH 2 ) n O HH O O H O O H (CH 2 ) n NN (CH 2 ) n O HH O O H O O H (CH 2 ) n NN O HH O O H O (CH 2 ) n O H (CH 2 ) n NN O HH O O H O (CH 2 ) n O H n = 1, 2, 3 Figure 19. Formation of urea bidentate hydrogen-bonded chains prevail in the presence of other hydrogen-bonding groups such as carboxylic acids. and electron microscopy, to elucidate the supramolecular structure of the gelators. Barbiturate-Melamine and 2,6-Diaminopyridine- Barbiturate Organogelators. Hydrogen-bond complementarity between two different functional groups has been studied in many systems for recognition in host–guest chemistry and for the formation of infinite aggregates in solution and in the solid state. Two of these patterns that have been incorporated into organogelators are the barbiturate–melamine pair and 2,6-diaminopyridine- barbiturate. Whitesides and Lehn exploited the hydrogen-bonding complementarity of melamine-barbituric/cyanuric acid (13,66). The focus of their research was to study the NN C n H 2n+1 O NN C n H 2n+1 O H H H H N H N H N H X C 12 H 25 N H X C 12 H 25 33; X = O 34; X = S trans (1R, 2R) 35; n = 4 36; n = 12 37; n = 18 Figure 20. Organogelators based on bis-urea derivatives. preferential formation of cyclic or linear aggregates. Their approach was to use steric hindrance to enhance formation of cyclic aggregates over linear ones. When the melamine derivative has methyl substitiuents on the phenyl rings (44)orann-butyl substituent at the 3-position on the melamine derivative (45), the aggregates cocrystalize with diethylbarbiturate 46 in a linear arrangement. However, when the substituents were bulky, as in t-butyl-substituted phenyl groups (47), the cocrystallized aggregates were cyclic (see Fig. 24). These types of molecular recognition motifs have been coupled to long alkyl chains to generate two-component organogelators. Hanabusa used a 1:1 mixture of 48 and 49 to gel N,N-dimethylformamide, chloroform, tetrachloromethane and cyclohexane at concentrations as low as 0.04 mol/mL (Fig. 25) (67). N H N H O (CH 2 ) n N H N H O R N H N H O R = -(CH 2 ) 7 CH 3 R = R = n = 3 n = 6 n = 9 n = 12 Figure 21. 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