Supramolecular Assemblies of Amide-Derived ... - ACS Publications

Invited Feature Article pubs.acs.org/Langmuir

Supramolecular Assemblies of Amide-Derived Organogels Featuring Rigid π‑Conjugated Phenylethynyl Frameworks M. Rajeswara Rao and Shih-Sheng Sun* Institute of Chemistry, Academia Sinica, Taipei, 115, Taiwan, Republic of China ABSTRACT: Organogels, being an important class of soft materials, have evolved to be one of the most attractive subjects bridging supramolecular chemistry and material sciences due to their structural diversity and associated physical properties. Myriad applications in fields such as optoelectronics, light harvesting, environmental science, and regenerative medicine are being envisaged. Supramolecular gels usually are formed through self-aggregation of small-molecule gelators to form entangled self-assembled fibrillar networks through a combination of non covalent interactions such as hydrogen bonding, π−π stacking, electrostatic forces, donor−acceptor interactions, metal coordination, solvophobic forces, and van der Waals interactions. This feature article discusses recent and current state of research on amide derived organogelators bearing rigid conjugated phenylethynyl building blocks. Selective examples from our works along with some closely related examples from literature have been highlighted to showcase the structural diversity and their potential applications in supramolecular chemistry and materials science.

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INTRODUCTION Gels are semisolid materials, which can serve a variety of purposes and are permeated ubiquitously in our daily lives in a variety of forms including toothpaste, soap, shampoo, hair gel, contact lenses, gel pens, and so forth. As a matter of fact, gels are prevalent in nature within cells and tissues of bodies. They are also present in a variety of artificial materials. These soft materials attracted great attentions in the past decade in fundamental scientific research, and the applications have been envisaged in diverse fields such as drug delivery, materials science, supramolecular chemistry, catalysis, regenerative medicine, environmental science, and so on.1−3 In general, gels can be classified into two classes based on the manner in which the gel matrix is formed. The gel is referred to a polymer gel if this matrix is formed via covalent bond. Polymer gels are usually thermally irreversible and cannot be redissolved because of the nature of covalent cross-linking networks. However, the covalent nature of polymer gels renders these gels being robust and possessing a high degree of elasticity.4 The other class of gel is supramolecular gels or physical gels, in which the gels are formed by noncovalent interactions. The term “supramolecular gel” is employed to comprehend any gelation species utilizing noncovalent interactions to form gels. The mechanism for the formation of supramolecular gels is generally accepted to have the gelator molecules self-assemble through highly specific interactions that allow preferential one-dimensional growth and usually to form microscale fibers, strands, or tapes.5 The prolonged objects join in three-dimensional networks that subsequently encapsulate the liquid components and suppress their flow. The conjunction points between fibers, strands, or tapes provide rigidity to the resulting microstructures.5 These self-assembled fibrillar networks form through © XXXX American Chemical Society

a combination of noncovalent interactions such as hydrogen bonding, electrostatic, π−π stacking, solvophobic, metal coordination, van der Waals, and charge-transfer interactions. Unlike polymeric gels, the gelation process in supramolecular gels is typically sensitive to external factors such as temperature, chemical additives, and solvent polarity due to the weak noncovalent interactions.1,5 The presence of functional groups in gelator molecules with specific physical or chemical properties can respond to various stimuli such as the change in pH,6 presence of enzymes,7,8 mechanical stress,9 magnetic field,10 electric field,11 metal cations,12 anion13−15 or exposure to heat16 and light.17 Supramolecular gels have recently received great attention due to their reversible gelation and easily tunable molecular structures and properties, which render them promising candidates for chemical sensors,12 organocatalysts,18 gel electrolytes,19 liquid crystals,20 photoresponsive materials,21 pharmaceuticals,22 optoelectronic and nanoelectronic devices,23−25 templates for growing nanomaterials,26,27 environmental cleaning agents,28 and also cytotoxic agents.29Supramolecular gels can be referred as organogels when the immobilizing medium of the gel is an organic solvent. There is a large structural diversity that can be utilized for providing noncovalent interactions within organogels. Common examples include hydrocarbons,30 fatty acids,31 saccharides,32 steroids,33 amides,34−37 aminoacids,38−40 ureas,41 aromatic molecules,42 metal complexes43 and multicomponent systems44−46 etc. In this feature article, we focus our attention on recent developments of selective organogelators Received: July 4, 2013 Revised: September 13, 2013

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dx.doi.org/10.1021/la402449e | Langmuir XXXX, XXX, XXX−XXX

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Invited Feature Article

Chart 1. Long-Chain Pyridine-2,6-dicarboxamide Phenylethnyl Functionalized Compounds 1−11 and PM-chiral

Figure 1. (a) Variable-temperature fluorescence spectra of compound 1 in toluene (1 × 10−5 M). (b) Photograph of fluorescence of compound 1 in toluene upon cooling and heating. Reproduced with permission from reference 47.

with the amide functional group, in particular, those comprised of rigid phenylethynyl linkers and their potential applications in various fields.

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ORGANOGELATORS WITH AMIDE FUNCTIONAL GROUPS Hydrogen bonding interaction is one of the most important noncovalent interactions in biological systems and has been commonly employed in constructing supramolecular species via the self-assembly strategy. The N−H···OC hydrogen bond in an amide motif has an N−H donor and CO acceptor, which provides a strong and directional self-complementary hydrogen bonding interaction. Amide derivatives are one of the widely used building blocks in the process of gelation to self-assemble into fiber-like nanostructures with three-dimensional networks. Despite the common usages of amide derivatives in designing organogelators, those incorporating rigid phenylethynyl framework are relatively less known. In this regard, we reported the first examples of structural rigid π-conjugated organogelators with phenylene ethynylene backbone incorporating long-chain

Figure 2. Micrographs of (a) 5 at 172 °C; (b) 6 at 158 °C; (c) 7 at 213 °C; and (d) 10 at 160 °C under POM. Reproduced with permission from reference 48.

pyridine-2,6-dicaboxamides.47−49 Indeed, compounds 1 and 2 were originally designed and synthesized for the purpose of dihydrogen phosphate recognition. The large steric requirement by the two pyridine 2,6-dicaboxamide moieties is expected to generate a cleft to accommodate a tetrahedral shape dihydrogen phosphate anion by multiple hydrogen bonding interactions with the four amide groups. Despite very good selectivity for dihydrogen phosphate, the binding constants for both B


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Chart 2. Structures of Amide and Ethynyl Functionalized Mesomorphic BODIPY Fluorophores

Chart 3. Amide-Functionalized Ethynylphenyl-Pyrene Organogelators 14 and 15

1, 5, 7, 9, and 11 with three dodecyloxy chains in the end of each phenyl ring are thoroughly soluble in all organic solvents thereby fail to exhibit gelation. The gelation ability of compounds 1, 4, 6, 8, 10, and PM-chiral was found to be eminent in some organic solvents with minimum gel concentration of 0.1%, hence they can be referred to as “supergelators”.50 It is interesting to note that the formation of organogels is faster with gelators bearing shorter alkoxyl chains than those with longer alkoxyl chains. On the other hand, the organogels with longer alkoxyl chains show better stability in gel forms than their congeners with shorter alkoxyl chains. It seems the interactions between the shorter alkoxyl chains and solvent molecules allow the gelating system to reach the metastable state kinetically faster but sustain less thermodynamic stability. Variable-temperature 1H NMR and electronic absorption spectra indicated that the driving forces for the formation of gel were intermolecular hydrogen bonds between amides and π−π stacking interactions among aromatic moieties. The prominence of intermolecular hydrogen bonding interaction was observed by the phase transition from gel to liquid upon addition of MeOH or DMSO where the collapse of hydrogen bonding interactions was induced by these hydrogen bond competing solvents. Interestingly, these compounds were found to exhibit very unique optical

Figure 3. AFM images of helical fibers of gelator 3 from a solution of toluene deposited on a silicon wafer. Left: 5.0 × 10−5 M. Right: 6.0 × 10−4 M (image size 2.0 μm × 2.0 μm). Reproduced with permission from reference 49.

compounds 1 and 2 are less than 2000 M−1. During the process of growing single crystal for compound 1, we surprisingly discovered that compound 1 possessed superior ability to gel a variety of common organic solvents. It was latter found that compounds 1, 4, 6, 8, 10, and PM-chiral with single alkoxyl chains (Chart 1) at the end of each phenyl ring exhibited remarkable ability of gelation in a variety of organic solvents of different polarities such as toluene, p-xylene, chloroform, dichloromethane, 1-heptanol, and so forth, while compounds C


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Figure 4. A schematic representation of the self-assembled organogelators into helical nanostrucutures and the effect of doping chiral gelator PM-chiral on the energy disparity of the enantiomeric helical formation. Reproduced with permission from reference 49.

Figure 5. Temperature dependence of the UV−vis (top) and CD (bottom) spectra of a toulene solution of achiral gelator 4 (1 × 10−3 M) and chiral gelator PM-chiral (5 mol %) coassembly at different temperature. Reproduced with permission from reference 49.

properties upon sol-to-gel transition. The fluorescence intensity of compound 1 increased from solution to gel state, which was

attributed to be the consequence of gelation induced enhanced emission (Figure 1). Similar gelation-induced enhanced emission D


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Chart 4. Platinum Acetylide Metallogelators 16−20

Figure 6. (a) Emission spectra of 16 in toluene at different temperatures; (b) Photograph of toluene solution of 16; right: viscous form (328 K), middle: gel form (278 K) under UV lamp, and left: the gel form (298 K). Reproduced with permission from reference 63. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA.

fluidic birefringent columnar mesophase with focalconic and filament textures by cooling from isotropic melt and a second columnar mesophase with broken-fan textures upon further cooling. In the case of compound 10, a viscous but fluidic birefringent smectic phase with filament textures was observed. For compounds showing liquid crystallinity, the multiple intermolecular hydrogen bonding interactions of the four amide groups may have been counterbalanced by the rotational freedom of the single bonds, which can afford flexibility of the core geometry, to result in the formation of mesophases.48 Transmission electron microscopy (TEM) images of xerogels 1, 4, 6, 8, and 10 from various solvents showed interconnected networks of fibers. The density of entangled fibers and the fiber dimensions are generally correlated to the gelation efficiency. The difference in the gelation behaviors and morphological features is likely originated from highly directional intermolecular interactions due to the effect of the hydrogen-bonding group in the hierarchical positions to induce directional growth of the self-assembled nanostructures. Due to the strong propensity for π-stacking, the self-assembly of organic π-conjugated molecules functionalized with H-bonding moieties into luminescent organogels have great potential applications in field effect transistors (FETs), organic light-emitting diodes (OLEDs), organic light-emitting transistors (OLETs), and sensors. Moreover, some organogels displayed unique liquid crystalline properties, which are potential candidates for developing optical electronics.23−25 An elegant organogelating system based on a strucutrally similar ethynyl phenyl dicarboxamide decorating with 4,4difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) fluorophores

was also observed for compounds 4, 6, 8, and 10. Unlike in solution, the conformational flexibility greatly reduces in gel state due to the strong intermolecular hydrogen bonding and π−π stacking interactions in the gels and, therefore, the nonradiative decay process was slowed down with enhanced fluorescence quantum yields. The mechanism for observed gelation induced enhanced emission was further supported by excited-state lifetime measurements. When the lifetime measurements were performed on the gel samples, the fluorescence decays became a biexponential function with one fitted lifetime similar to the lifetime of molecularly dissolved species measured in dilute solution. The additional lifetime component in the gel samples typically exhibited a larger value compared to the ones in solution, which indicates the simultaneous presence of two forms of gelator in the gel sample, i.e., in its molecularly dissolved form and in an aggregated form. The increase of the lifetime values for the aggregated species is apparently due to the decreased motion of the molecules within the gel aggregate. Therefore, a decreased nonradiative deactivation of the excited state and an enhanced emission was observed in the gel state. With rigid π-frameworks attaching with flexible alkoxyl chains in the molecular structures for these compounds shown in Chart 1, it is not surprising to realize that some of them show liquid crystallinity. Indeed, as revealed by differential scanning calorimetry (DSC) and polarized optical microscopy (POM) studies, compounds 5, 6, 7, and 10 display unique liquid crystalline properties over a large range of temperatures (Figure 2). Under POM, compounds 5 and 6 exhibited fanlike and dendritic textures indicative of a columnar phase upon cooling from isotropic liquid, whereas compound 7 initially showed a E


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Figure 7. A schematic representation of excimer formation of platinum acetylide metallogelator 16 in the excited state at high temperatures. Reproduced with permission from reference 63. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA.

Chart 5. Platinum Acetylide Metallogelators 21−30

was reported by Ziessel (Chart 2).51 It was found that 13B8ethynyl was deprived of the liquid cystalline behavior whereas the 13-B12ethynyl and 13-B16ethynyl exhibited mesomorphism in the temperature range 184−158 °C and 167−114 °C, respectively, assigned to columnar hexagonal mesophases. It was noticed that upon increasing the alkyl chain length from eight to dodecyl and to hexadecyl, there found a considerable changes in the mesomorphic effects such as, first, induction of the

mesomorphism, second, increase of the mesomorphic temperature domain from 25 °C for the dodecyl to 51 °C for the octadecyl derivative, and third, decrease of the liquid crystal transition temperature down to 114 °C for the hexadecyl derivative. In contrast to the ethnyl derivatives, the analogous 12Bnamide exhibited mesomorphism over a large temperature range with the exsistence of the hexagonal columnar mesophases due to the presence of third amide functional group. Based on the F


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Figure 8. A schematic illustration of the formation of supramolecular honeycomb patterns. Reproduced with permission from reference 67. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA.

crystal structure of the model compound 12-B1amide derivative, the columnar mesophase was likely driven by the formation of a dimer stabilized by hydrogen bonding. Interestingly, the trisamide derivative 12-B16amide displayed excellent gelation properties in linear alkanes from C9 to C12 carbon chains and in dimethylformamide (DMF). Various spectropscopic techniques including temperature-dependent 1H NMR, UV−vis, and steady-state fluorescence were employed to study the gelation properties and the results suggested that the formation of the gels originates from a combination of intermolecular hydrogen bonds between the amides functionalities and fluorophore interactions, but the fluorophore aggegation is the main driving force for the formation of the gel. Ziessel et al. also reported an efficient organogelator based on the phenylethynyl pyrene skeleton comprising of amide functional groups (Chart 3).52 The double substitution of 4ethynylphenylaminoacyl on pyrene (14) renders gel formation in organic solvents such as toluene, cyclohexane, and DMF, whereas the tetra-substituted pyrene core (15) does not form gel but rather a stable liquid crystalline material due to the presence of four donor−acceptor groups that causes stronger intermolecular interaction and aggregation capabilities leading to the formation of precipitates. The bisamide-pyrene compound 14 was able to form thermally reversible transparent gels with cylcohexane and turbid gels with toluene and DMF. The spectroscopic and microscopic studies showed that the emission and morphology properties of the gel were strongly dependent on the nature of the gelating solvents. SEM images of cyclohexane xerogels of compound 14 appeared as thin fibers of three-dimensional networks with diameter in the range of 40− 70 nm extending over micrometers. In toluene, similar thin fibers with increased diameter in the range of 50−110 nm were observed indicative of the enhanced robustness of the gels

compared to cyclohexane gels. Interestingly, morphology of the gel in DMF appeared completely different compared to earlier two solvents. A dense network of entangled fibers with diameter in the range of 500−1000 nm was identified which is the combination of several elementary fibers. The hydrogen bonding between amide protons, π−π interactions among pyrene cores and van der Waals interactions of aliphatic alkyl chains were found to be the driving force to form robust gels in DMF. The role of hydrogen bonding in the formation of the observed fibers was established by IR spectroscopy in cyclohexane, toluene, and DMF gels. A single CO stretching band was observed at 1644, 1643, and 1659 cm−1 in cyclohexane, toluene, and DMF, respectively. In addition, a large NH stretching vibration at 3261, 3265, and 3332 cm−1 in cyclohexane, toluene, and DMF, respectively, was observed in the gelled solvents. These spectroscopic data are an unambiguous signature of hydrogenbonded amides inducing a hydrogen-bonded network. Tetrasubstituted pyrene derivative 15 exhibited mesotropic behavior with a temperature domain from room temperature to 200 °C. Optical microscope showed typical texture of the hexagonal columner phase with birefringent chromosome-like domains and large homotropic regions. However, under a luminiscent microscope, interestingly, chromosome-like motifs were nonluminiscent, whereas the homotropic flower like domains appeared highly luminiscent. Self-assembly of small molecules into one-dimensional nanostructures offers a number of potential applications in electronically and biologically active materials. In this context, organogels represent promising candidates for creating such unique nanoscopic structures due to their ability of selforganization into highly ordered hierarchical nanomorphology. An interesting example of gelation assisted supramolecular chiral amplification of one-dimensional helical nanostructures has been G


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Chart 6. Alkynylplatinum(II) Terpyridyl Metallogels 31−39

discovered based on a π-conjugated phenylethynyl thiophene consisting of long-chain dicarboxamides (Chart 1).49 Both chiral gelator PM-chiral and achiralgelator 3 possessed excellent gelation capabilities in various organic solvents such as aromatic and chlorinated solvents with minimum gel concentration as low as 0.5 mg/mL. Atomic force microscopy (AFM) images of toluene xerogels of the achiral gelator 3 showed entangled leftand right-handed helical fibers (Figure 3). The pitch distribution range varies from ∼11−200 nm dependent on the gel concentration. The AFM images of the chiral gelator PM-chiral displayed scattered helical fibers out of normal linear fibers, which was attributed to thermodynamic instability of chiral aggregates during the solvent evaporation. Molecular mechanics and dynamics simulations were further carried out to unveil the nature of noncovalent interactions and supramolecular aggregation at these gelator systems. By taking advantage of the intramolecular hydrogen bonding interactions

between the amide N-Hs and the pyridine N atom, the pyridyl dicarboxamide moiety adopts a rigid planar conformation and affords the corresponding twisted structures of an enantiomeric pair in gelator 3. This stereo requirement develops extraordinary features, leading to the enantiomeric stacking arrangements between the neighboring gelators with slightly clockwise and counter-clockwise oriented stackings. The optimized stackings are stabilized by the strong intermolecular hydrogen bonding interactions of the amide N-Hs with the next adjacent amide carbonyl O atoms and the π−π interactions. The optimized spacing and the offset angle between the two adjacent gelator molecules are found to be 3.7 Å and 11°, respectively. Figure 4 illustrates the nanocoil aggregation structures of gelator 3 in which the relevant structural parameters are closely matched with the TEM and AFM images with a diameter around 10 nm and a pitch length of 11 nm consisting of 33 aggregated gelator molecules. This gelating system provides a remarkable example H


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Figure 9. Pictorial representations of the different self-assembly processes of 40 in the absence and presence of cyanurate (dCA). Reproduced with permission from reference 70. Copyright 2009 Royal Society of Chemistry.

responses until the concentration was higher than 7.5 × 10−5 M with three negative CD bands appeared at 360, 412, and 438 nm. Upon doping of chiral gelator PM-chiral (sergeant) over 5%, a CD signal of achiral 3 (soldier) with negative Cotton effect was observed and the CD signal was further amplified with doping amount up to 30%. This observation supports the transfer of chiral memory of the chiral “sergeant” at the supramolecular level to the achiral “soldiers” in the same helical aggregate and strongly biases its handedness. It was also observed that the degree of chiral amplification strongly depends on the temperature because the intermolecular interactions between the gelator molecules were enhanced while decreasing the temperature, which further leads to the favorable chiral amplification (Figure 5). These chiral supramolecular assemblies may find a considerable attention as

where the supramolecular self-organization of nanohelices observed by microscopic images can be realized into a molecular level. As expected, no macromolecular chirality was observed for the achiral derivative 3, which was circular dichroism (CD) silent. Nevertheless, the helicity of achiral units can be biased in the presence of chiral handles and strong noncovalent interactions between the monomeric units with an intrinsic conformational chirality of the assembly and careful matching of secondary interactions to obtain a well-defined chiral conformation in the aggregation process.53−55 It was demonstrated that the supramolecular chirality can be induced by coassemby of a chiral conductor PM-chiral and nonchiral 3 via the “sergeant and soldier” principle.56 Chiral gelator PM-chiral showed no CD I


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For instance, the thick and dense fibrous networks of 21b were varied to rope-like fibers upon changing solvents from n-hexane to n-heptane whereas sheet-like morphology was identified in npropanol. It was proposed that primary aggregates initially formed driven by the intermolecular hydrogen-bonding interactions by amide functionalities as well as hydrophobic and hydrophilic interactions from the simple building blocks. Subsequently, these primary aggregates developed to secondary fiber-like structures and ultimately intertwined to form large intense three-dimensional networks. Furthermore, complex 24a with four platinum−acetylide fragments featured intense emission in the longer wavelength region with a quantum yield of 35% relative to other complexes reported. Another class of platinum acetylides 25−30 was prepared by incorporating the azobenzene groups (Chart 5).67 Interestingly, complexes 25−27 lack any gelation properties, but amidefunctionalized compounds 29 and 30 showed efficient gelation of various nonpolar alkanes such as cyclohexane, n-hexane, nheptane, n-octane, n-decane, and dodecane that corroborate the crucial role of hydrogen bonding interactions between the amide groups. Interestingly, SEM investigations of morphology of these metallogels revealed an unusal formation of honeycomb patterns on a large scale with highly ordered microstructures in a variety of nonpolar solvents. Furthermore, the presence of longer alkyl chain in compound 30 (1.2−1.6 μM) improved the average pore-size of the honeycomb structures over 29 (0.8−1.2 μM). Concentration-dependent 1H NMR spectroscopy and competitive experiment between hydrogen bonds corroborated the importance of hydrogen bonding interactions during the formation of these ordered honeycomb supramolecular aggregates. A possible mechanism for the formation of these aggregates was explained by presuming the formation of primarly ordered aggregates initially driven by intermolecular hydrogen bonding interactions between the simple building blocks, which further developed to secondary shell-like structures in nonpolar solvents through the hydrophobic/hydrophilic interactions. Finally, micropores will evolve by the bursting of the unstable spherical structures (Figure 8). Gallate-substituted derivatives of σ-alkynyl platinum(II) terpyridine luminophoric complexes 31−33 were synthesized with a deliberate design by incorporating Pt(II) to favor spin− orbit coupling for phosphorescence and to facilitate metal−metal interactions (Chart 6).68 Compound 32 was found to be a good gelator of dodecane, while 31 is insoluble in alkane solvents; the deprived gelation could be due to the absence of long alkyl chains. On the other hand, compound 33 is highly soluble and exhibited liquid-crystalline property over a large temperature range. Changing from linear chains in 32 to the branched chains in 33, the formation of nanofibril structures and extended columnar mesophases were observed, respectively. In the case of gelator 32, TEM and AFM studies confirmed the presence of linear fibrils with the length of several hundred nanometers and the width of 2 nm. The formation of elongated fibers suggests that the strong directional intermolecular interactions such as Pt···Pt and hydrogen bonding of amide functions plays a crucial role in the self-assembly of 32. A series of alkylplatinum(II) terpyridyl complexes 34−39 has been reported by Yam et al. (Chart 6) and studied their gelation properties.69 It has been observed that all the complexes except 37-OTf and 39-OTf showed thermotropic gelation properties tested for various organic solvents ranging from polar aprotic DMSO or polar protic methanol to nonpolar dodecane. The reason for complexes 37-OTf and 39-OTf not forming gels was

candidate materials for use in biological and electro-optical applications as the recognition and transcription of chirality in the self-assembly process has a key role in mimicking the development of biological helical structures with specific function as well as in controlling unique optoelectronic device applications.57 Recently, there has been growing interest in the study of metallogels, such as Au(I),58 Pt(II),43,59 Cu(I),60 and Re(I).61 The availability and the diversity of metal−ligand chromophores and their associated rich spectroscopic and luminescence properties that might allow potential applications as electroluminescent materials or as dopants in organic light-emitting devices (OLEDs).62 In this regard, a series of new metallogelators composed of Pt(II) acetylides conjugated to longchain pyridine-2,6-dicarboxamides 16−19 were reported (Chart 4).63 Both complexes 16 and 19 formed stable yellowish gels in 1,2-dicholoethane, toluene, and 1-butanol, and p-dioxane. On the other hand, complexes 17 and 18 failed to form gels in any of the solvents tested, which was attributed to lower tendency to form strong aggregates. Temperature-dependent 1H NMR studies of platinum(II) acetylide complexes 16 and 19 revealed the process of molecular aggregation, and the TEM images of xerogels indicated formation of three-dimensional elongated nanostructures. It has been discovered that 16 in toluene showed emission enhancement at the elevated temperatures upon gel to sol transition (Figure 6). This unusal behavior was attributed to the formation of excimer in the excited state due to higher molecular degree of freedom of individual metallogelators at the elevated temperatures, which allows rearrangement of molecular aggregates in the excited states to lower energy assemblies and the enhanced emission at longer wavelength was observed due to increased aromatic π−π interactions (Figure 7). A closely related luminescence enhancement of an excimer emission at elevated temperature upon gel-to-sol phase transition was observed in the metallogel of alkynylplatinum(II) complex 20 reported by Yam and co-workers (Chart 4).64 Previously, Aida and co-workers first reported luminescence diminishment upon sol-to-gel phase transition for a series of Au(I) pyrazolate gelators.58 Kamikawa and Kato also reported a series of pyrene-containing oligoglutamic acids that showed enhanced excimer emission at elevated temperature.65 On the contrary, 17 was not found to show emission enhancement at the elevated temperatures due to low tendency to form strong aggregates in toluene that leads to the formation of excimer in the excited state. A series of luminescent platinum−acetylide complexes containing ethynyl pyrene moieties 21−24 were prepared and investigated their gelation properties (Chart 5).66 According to stable-to-inversion-of-a-test tube method, most of the compounds formed stable gels in nonpolar alkanes or polar alcohols such as cyclohexane, n-hexane, n-propanol, etc. The structural effects of the compounds such as number of platinum acetylide units and the presence of amide functionalities have a considerable effect on the gelation. For instance, amide group containing compounds 21b and 22b formed stable gels with lower CGCs compared to their counter parts 21a and 22a, supporting that the presence of amide functionalities induced stronger intermolecular hydrogen bonding interactions to result in stable gel formation. While compounds 23a and 24a exhibited gelation, 23b and 24b were insoluble in most of the solvents tested attributed to the enhanced intermolecular hydrogen bonds due to the presence of more amide groups. The morphologies of the compounds under SEM were greatly influenced by the molecular shapes and the solvent of gelation. J


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crystalline properties, which are potential candidates for developing optical electronics. The performance of these optoelectronic materials strongly depends on the nature of the interactions, and the three-dimensional network from the supramolecular assemblies of organogels allows efficient charge transport, which is ultimately important to the functional properties of the active optoelectronic devices. The excellent ability of organogelator to form three-dimensional network fibrils with nanometer to micrometer in length would create an interpenetrating interface. In this way, a large interface suitable for charge separation would appear in the p-n type solar cell devices. It also provides charge percolation pathways on the nanometer scale. The ability to manipulate the intermolecular organization between the building blocks and produce a large variety of molecular architectures with judicious design of the functionalities has rendered these novel supramolecular gels many possible applications in the near future.

attributed to high solubility in organic solvents for the former, while the latter complex lacks adequate hydrophobic−hydrophobic interactions. In addition to the cooperative effect of hydrogen bonding, π−π, and hydrophobic interactions, Pt···Pt interactions also observed to play a significant role in the gel formation in these metallogelators, which was confirmed by variable UV−vis absorption and emission spectra. Morphology studies revealed the formation of typical fibrous networks with the length longer than 5 μM and the diameter in the range of 400−900 nm. Interestingly, depending on the nature of the counteranion, the gel complexes 35-OTf, 36-OTf, and 38-PF6 showed different colors that are influenced by the degree of aggregation, Pt···Pt, and π−π interactions. A dual hydrogen-bonding system 40 based on melaminelinked tri(π-phenyleneethynylene) was prepared (Figure 9)70 and used to demonsrate the role of the cyanurate (dCA) in the self-assembly process. Interestingly, in the absence of cyanurate moitiey, 40 formed opeque and weak gels in the aliphatic solvents, whereas strong and transparent gels were observed in the presence of aliquots of dCA. The CGC of 40 in methylcyclohexane is 3 mM with the gel melting temperature (Tgel) found to be 38 °C; on the other hand, for a 1:1 complex of 40-dCA, the CGC decreased to 2 mM with enhanced Tgel of about 4 °C, indicating that 40-dCA gel is stabilized by the complementary H-bonding interaction with cyanurates. In addition, the diversity in the fluorescence emission, physical properties of the gels, and morphological differences of 40 in the presence and absence of dCA was attributed to the formation of a 2:2 dimer during the initial stage of the self-assembly in 40-dCA (Figure 9). In this dimer, the molecule 40 is strongly bonded in an H-type fashion through 12 H-bonds. These dimers may further self-assemble to form hierarchical structures in a J-type fashion through amide H-bonding. In the case of 40 alone, initially, a linear supramolecular polymeric assembly could be favored through amide−amide H-bonding. In such a situation, interchromophoric interaction is minimized. Under higher concentration, these noncovalent polymer chains may undergo further aggregation leading to the gelation of the solvents.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

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CONCLUSIONS In summary, the field of gelation by low molecular mass gelators is an area of intense current activity with gelation occurring reproducibly in a wide and increasing range of organic substructure types. Supramolecular gels formed from the assembly of small-molecule gelators are a highly tailorable class of materials that have found applications ranging from biomaterials to sensors. In this feature article, we have summarized some selective supramolecular structures that have been built based on amide functionalities with rigid π-conjugated framework, especially those with phenylethynyl linker that exhibit gelation with various nanostructures. Judging from the selective examples discussed, the construction of the gel structure in the amide functionalized organogelators is mainly based on the intermolecular hydrogen bonding between amide groups with bestowed support of aromatic π−π stacking, metal−metal, and van der Waals interactions. Such self-organization process will enable the transcription of molecular functions onto huge amplification of the functional output in the nano- or microscale. The strong propensity for π-stacking in the linear π-conjugated systems functionalized with H-bonding moieties render the organogels self-organized from these organic π-conjugated molecules great potential applications in optoelectronic devices and sensors. Moreover, some organogels displayed unique liquid

Rajeswara Rao received his Ph.D. degree in 2011 from the Indian Institute of Technology Bombay (IITB, India) working on the design, study and applications of fluorescent systems based on porphyrins and BODIPYs under the supervision of Prof. M. Ravikanth. In 2011, he joined the group of Dr. Shih-Sheng Sun at Academia Sinica (Taiwan) as a postdoctoral research fellow to work on the development of new solidstate stimuli responsive materials.

Shih-Sheng Sun completed his Ph.D. (2001) in Supramolecular Chemistry at State University of New York at Binghamton with Prof. K


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(18) Rodriguez-Llansola, F.; Escuder, B.; Miravet, J. F. Switchable Perfomance of an L-Proline-Derived Basic Catalyst Controlled by Supramolecular Gelation. J. Am. Chem. Soc. 2009, 131, 11478−11484. (19) Wicklein, A.; Ghosh, A.; Sommer, M.; Würthner, F.; Thelakkat, M. Self-Assembly of Semiconductor Organogelator Nanowires for Photoinduced Charge Separation. ACS Nano 2009, 3, 1107−1114. (20) Kuang, G.-C.; Jia, X.-R.; Teng, M.-J.; Chen, E.-Q.; Li, W.-S.; Ji, G. Organogels and Liquid Crystalline Properties of Amino Acid-Based Dendrons: A Systematic Study on Structure−Property Relationship. Chem. Mater. 2012, 24, 71−80. (21) Iwaura, R.; Shimizu, T. Polyhedral Silver Nanocrystals with Distinct Scattering Signatures. Angew. Chem., Int. Ed. 2006, 45, 4601− 4604. (22) Bastiat, G.; Leroux, J. C. Pharmaceutical Organogels Prepared from Aromatic Amino Acid Derivatives. J. Mater. Chem. 2009, 19, 3867− 3877. (23) Babu, S. S.; Prasanthkumar, S.; Ajayaghosh, A. Self-Assembled Gelators for Organic Electronics. Angew. Chem., Int. Ed. 2012, 5, 1766− 1776. (24) Kartha, K. K.; Mukhopadhyay, R. D.; Ajayaghosh, A. Supramolecular Gels and Functional Materials Research in India. Chimia 2013, 67, 51−63. (25) Kitahara, T.; Shirakawa, M.; Kawano, S.-I.; Beginn, U.; Fujita, N.; Shinkai, S. Creation of a Mixed-Valence State from One-Dimensionally Aligned TTF Utilizing the Self-Assembling Nature of a Low MolecularWeight Gel. J. Am. Chem. Soc. 2005, 127, 14980−14981. (26) Ajayaghosh, A.; Varghese, R.; Praveen, V. K.; Mahesh, S. Evolution of Nano- to Microsized Spherical Assemblies of a Short Oligo(p-phenyleneethynylene) into Superstructured Organogels. Angew. Chem., Int. Ed. 2006, 45, 3261−3264. (27) Ajayaghosh, A.; Varghese, R.; Mahesh, S.; Praveen, V. K. From Vesicles to Helical Nanotubes: A Sergeant-and-Soldiers Effect in The Self-Assembly of Oligo(p-phenyleneethynylene)s. Angew. Chem., Int. Ed. 2006, 45, 7729−7732. (28) Dutta, S.; Das, D.; Dasgupta, A.; Das, P. K. Amino Acid Based Low-Molecular-Weight Ionogels as Efficient Dye-Adsorbing Agents and Templates for the Synthesis of TiO2 Nanoparticles. Chem.−Eur. J. 2010, 16, 1493−1505. (29) Yang, Z.; Xu, K.; Guo, Z.; Guo, Z.; Xu, B. Intracellular Enzymatic Formation of Nanofibers Results in Hydrogelation and Regulated Cell Death. Adv. Mater. 2007, 19, 3152−3156. (30) Abdallah, D. J.; Weiss, R. G. n-Alkanes Gel n-Alkanes (and Many Other Organic Liquids). Langmuir 2000, 16, 352−355. (31) Mieden-Gundert, G.; Klein, L.; Fischer, M.; Vögtle, F.; Heuzé, K.; Pozzo, J.-L.; Vallier, M.; Fages, F. Combinatorial Discovery of New Photocatalysts for Water Purification with Visible Light. Angew. Chem., Int. Ed. 2001, 40, 3164−3166. (32) Chen, Q.; Lv, Y.; Zhang, D.; Zhang, G.; Liu, C.; Zhu, D. Cysteine and pH-Responsive Hydrogel Based on a Saccharide Derivative with an Aldehyde Group. Langmuir 2010, 26, 3165−3168. (33) Svobodová, H.; Noponen, V.; Kolehmainen, E.; Sievänen, E. Recent Advances in Steroidal Supramolecular Gels. RSC Advances 2012, 2, 4985−5007. (34) Shirakawa, M.; Fujita, N.; Shinkai, S. [60]Fullerene-Motivated Organogel Formation in a Porphyrin Derivative Bearing Programmed Hydrogen-Bonding Sites. J. Am. Chem. Soc. 2003, 125, 9902−9903. (35) Bhattacharya, S.; Samanta, S. K. Soft Functional Materials Induced by Fibrillar Networks of Small Molecular Photochromic Gelators. Langmuir 2009, 25, 8378−8381. (36) Samanta, S. K.; Bhattacharya, S. Wide-Range Light-Harvesting Donor−Acceptor Assemblies through Specific Intergelator Interactions via Self-Assembly. Chem.Eur. J. 2012, 18, 15875−15885. (37) Tseng, K.-P.; Kao, M.-T.; Tsai, T. W. T.; Hsu, C.-H.; Chan, J. C. C.; Shyue, J.-J.; Sun, S.-S.; Wong, K.-T. Manipulating the Nanostructure of Organogels Generated from Molecules with a 3-Dimensional Truxene Core. Chem. Commun. 2012, 48, 3515−3517. (38) Tomasini, C.; Castellucci, N. Peptides and Peptidomimetics That Behave as Low Molecular Weight Gelators. Chem. Soc. Rev. 2013, 42, 156−172.

Alistair J. Lees. After two-year postdoctoral studies at Northwestern University with Prof. Joseph T. Hupp and Prof. Son Bin T. Nguyen, he joined the Institute of Chemistry, Academia Sinica as an Assistant Research Fellow in 2003 and was promoted to an Associated Research Fellow in 2009. His research interests focus on supramolecular materials chemistry and their applications in optoelectronic materials.

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ACKNOWLEDGMENTS We are grateful to the National Science Council of Taiwan and Academia Sinica for support of the research. The authors also thank all their co-workers who appear in the original publications.

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REFERENCES

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