Supramolecular Gels

3 SUPRAMOLECULAR GELS Eduardo Rezende Triboni1, Thaisa Branda˜o Ferreira Moraes1 and Mario Jose Politi2 1

Laboratory of Nanotechnology and Processing Engineering, Lorena Engineering School of the University of Sa˜o Paulo, Sa˜o Paulo, Brazil 2 Laboratory of Photochemistry and Fast Kinetics, Institute of Chemistry of the University of Sa˜o Paulo, Sa˜o Paulo, Brazil

Abstract Nanotechnology based on intermolecular arrangements of molecules or atoms develops properly by the capacity of the formation of different structures and geometries directed by the functional groups and forces of interaction. Efforts to achieve control of such arrangements are a major challenge in softmatter research. Supramolecular gels emerge as odd systems in their occurrence is due to the ordered one dimensional nanostructured building block forming fibers that immobilize an enormous amount of solvent. The structural and electronic complexity of these systems allows the obtaining of materials with reversible characteristics for use in several technological areas. Herein are outlined some key concepts and some studies and applications of this class of fascinating and very promising material for the improvement and evolution of functional materials. Keywords: Supramolecular gels; organogels; hydrogels; functional materials; self-sorting; soft matter CHAPTER OUTLINE 3.1 Introduction 36 3.2 Gel Structuring Process 38 3.3 Host
Guest Systems and Gelators 43 3.4 Nanostructure Synthesis With Supramolecular Gels 46 3.5 Nanoscale Ordering as a Background for Advanced Gels 3.5.1 Charge Transfer 57 References 60 Further Reading 69

Nano Design for Smart Gels. DOI: https://doi.org/10.1016/B978-0-12-814825-9.00003-5 © 2019 Elsevier Inc. All rights reserved.

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confira tudo que respira conspira Paulo Leminsky (Brazilian poet)

conferes all that breathes conspires (free translation)

3.1

Introduction

The epigraph alludes to the “intelligent” character attributed to the so-called supramolecular gels. Such emphasis is given by the structural hierarchy characteristics by which they are formed, involving processes of self-organization directed by noncovalent interactions that at first can be shaped in a rational way. Thus it is natural that a major source of inspiration for the formation and application of supramolecular systems is the functioning of living systems, including their main components commonly found as amino acids, steroids, peptides, alkyl long chains, saccharides, fatty acids, aromatic fragments, organic salts, and host
guest systems (Atwood, 2017; Davis, 2004; Lehn, 1985, 2002; Uhlenheuer, Petkau, & Brunsveld, 2010). Supramolecular or physical gels are formed strictly by intermolecular interactions that are established between molecules or structural motifs being so called low weight molecular gelators (LWMG). The structuring of the gel is directed through functional groups acting as noncovalent bonding algorithms, such as hydrogen bonding, metal-bonding coordination, cation
anion electrostatic bonding, ion-dipole, dipoles orientation, van der Waals, and π-interaction (Adalder et al., 2014; Seiffert, 2016; Terech & Weiss, 1997) (see Chapter 2: Sol
Gel Chemistry—Deals With Sol
Gel Processes). Because of the oriented structuring, this soft-material category is referred to as a “smart” system based on the possibility of premolded designs or, in other words, on the combination of supramolecular building blocks that enable the achievement of specific properties. Mostly they are formed by the dissolution of a solid in an enormous quantity of liquid that effectively confers to the dispersant a fundamental role for gelation by mutual interactions established between the building blocks and the solvent. Thus the primordium of any supramolecular gelator is the analysis of its gellability which is considered by weight-by-volume assays in a

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series of solvents. This is due to the fact that it is not only a supramolecular system dispersed in a solvent medium, made of supermolecules or a combination thereof, but a system which can induce solvent structuring through mutual molecule
solvent interactions by the most varied triggers. Two very important mechanical dynamics of this class of soft material, addressed by noncovalent interactions, are the ability to respond to external stimuli and the inherent thermal reversibility. Thus gelled systems were found that are efficiently responsive to different physical and chemical promoters, such as temperature, photons, electricity, magnetic field, sound, enzymes, concentration gradients of molecules or ions, redox changes, etc. ˇ cˇ manec, (Aggeli et al., 1997; Babincova´, Leszczynska, Sourivong, Ci & Babinec, 2001; Bardelang et al., 2008; Chung, An, & Park, 2008; Leeb, Lupton, Yu, & Hovorka, 1997; Lloyd & Steed, 2009; SegarraMaset, Nebot, Miravet, & Escuder, 2013; Steed, 2011; Weng, Beck, Jamieson, & Rowan, 2006; Yagai, 2011; Yang, Zhang, & Zhang, 2012; Zhang et al., 2015). Additionally, the gels are able to maintain several cycles of formation and disintegration promoted by different structural triggers, shown in Fig. 3.1 (Wang, Shen, Feng, & Tian, 2006). Notice that in some cases the supramolecular gels are irreversible to their polymeric gel counterpart (Ghosh, Mahapatra, Das, & Dey, 2014). The routine preparation of a gel involves the heating of a saturated solution and then cooling, causing the trapping of the solvent within a microscopic solid network. Further, the gel can be broken by heating it above the gel-to-sol (Tgel) transition temperature, which corresponds to the collapse of the solid microstructure leading to resolubilization or precipitation. Mostly the rheological behavior obeys the viscoelastic regime and it can be subdivided into hard or weak gels (Piepenbrock et al., 2010). The former being stiffer with more brittleness, the latter having greater shear against an applied stress. In this example, a very small ratio of LWMG/solvent was used, in a mass per volume ratio (w/v), with the proportion ranging from 0.1% to 5% is sufficient to form the gel. In some cases a ratio of less than 0.1% is still sufficient to obtain gels, that is, in the case of LWMG supergelators (Krishnan & Sureshan, 2018; Luboradzki et al., 2000; Manchineella, Murugan, & Govindaraju, 2017). The metastable nature of this category of gel gives these soft materials a very exciting property which is the ability to obtain nanofibers with different morphologies and polymorphisms, depending on the type of external stimuli or with changes in the preparation condition, yet before gel formation or even after the formation of the three dimensional (3D)-entangled network.

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Figure 3.1 Sol
gel reversible transformation of the open and closed forms shown in the molecular scheme, and the gel
sol transitions under the cooperative effects of light, thermal, fluoride anions, and protons: (A) Gel(open); (B) Gel(closed); (C) Sol(open); (D) Sol(closed); (E) Sol(open) 1 F2; (F) Sol(closed) 1 F2 (Wang et al., 2006). Republished with permission from Chemical Communications by Royal Society of Chemistry (Great Britain). Reproduced with permission from Royal Society of Chemistry in the format Book via Copyright Clearance Center.

3.2

Gel Structuring Process

Fig. 3.2 outlines some triggers and stimuli that have led to the formation of supramolecular fibers within gels. LWMG gels closely follow the concepts of supramolecular chemistry and it may be recognized in the broad approach for the preparation of gels the use of supramolecular structural elements. In a general sense, the fundamental concept behind a

Chapter 3 SUPRAMOLECULAR GELS

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Figure 3.2 Physical stimuli and chemical triggers that lead to gel structuring (Escuder, Rodrı´guez-Llansola, & Miravet, 2010). Chemical Society reviews by Royal Society of Chemistry (Great Britain). Reproduced with permission from Royal society OF Chemistry in the format Book via Copyright Clearance Center.

molecular gelator is a priori simple: a structure loaded with functional groups and with topological aspects that enable a hierarchical assembly process leading to the formation of self-sorted solid network within the liquid. This solid matrix is originated in a nucleation stage and through its expected 1D structure, guided by intermolecular forces and mutual interaction with the solvent, forms unidirectional fibers that emerge and may entangle themselves, with the whole liquid forming 3D self-assembled fibrillar networks—SAFINs (Fig. 3.3). This is the microstructural picture of gels derived from small molecules, be they hydrogels, organogels, metallogels, twocomponent gels, host
guest gels, stimuli-responsive gels, etc. It has even been compared with structuring steps of the proteins within a perspective of hierarchical association (Piepenbrock et al., 2010): 1. dimerization of two individual molecules; 2. oligomer formation by interaction of dimers with further molecules; 3. formation of polymer fibrils of approximately the same width as the molecular building blocks (c.1
2 nm) by extension of the oligomers; 4. fiber formation by bundling of fibrils (c.20
50 nm width); 5. interactions of fibers to give an effectively infinite, interconnected network spanning the entire sample (the least well understood aspect of the gelation process); and 6. immobilization of the solvent by the fiber network generally by surface tension effects.

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Figure 3.3 From molecules to SAFINs: steps of molecular assembly until reaching a supramolecular 3D network.

According to the definition of Flory (Chapter 1: Introduction) on the chemical systems that potentially lead to gelation, it is quite reasonable to distinguish between gelation systems that gel through supramolecular small molecules and those that are propelled by colloidal particles previously formed in the sol phase. These colloidal particles can be formed by chemical covalent bonds, differently from usual supramolecular ordering. This case is quite characteristic of silicon-based gels and other inorganic systems via the sol
gel route (Livage, 1998). Another issue is the extension of the concept and the subdescriptions of the types of agents that act in the gelation. Currently, both polymers and lyotropic systems can be classified as supramolecular gelators depending only upon whether the driving force for gelation is based predominately on the intermolecular interaction (Hu, Han, Ge, & Guo, 2015; Mariani et al., 2017). The formation of the molecular gels competes directly with the precipitation and crystallization of the small molecule in a solvent and the balance between solubility/insolubility exerts a great influence. In fact, there is still a bit of trial and error—a case of serendipity—whether a molecule or the combination of two or more will aggregate to form 1D-columnar fibrils. However, there are empirical and theoretical studies aiming at the better understanding of the formation and stability of these 1D objects and their strict influence on the formation of the gel state (Abdallah &Weiss, 2000; Corezzi, Fioretto, De Michele, Zaccarelli, & Sciortino, 2010; George & Weiss, 2006; Gra´na´sy, Pusztai, Bo¨rzso¨nyi, Warren, & Douglas, 2004; Li, Liu, Strom, & Xiong, 2006; Toledano, Sciortino, & Zaccarelli, 2009). One can mention two empirical attempts that are very attractive and have established a kind of paradigm about the predictability of gel formation through microstructural interactions: (1) the use of

Chapter 3 SUPRAMOLECULAR GELS

solvent parameters based on solvation properties, mainly the Harnessing Hansen solubility scale and (2) analysis of the SAFINs’ features formed by small modification in recognized gelators (Diehn et al., 2014; Gao, Wu, & Rogers, 2012; Hashemnejad et al., 2017; Lo¨fman et al., 2015; Rogers & Marangoni, 2016; Singh et al., 2017). The latter trend is based on the comparative method proposed by Weiss—between the crystalline structure of the pure molecule, the fibers formed in the gel, and those formed in xerogel (Terech & Weiss, 1997). Most gelators have more than one functionality in their molecular moieties and what really matters it is the nature of the interaction promoted by these structural elements providing hydrogen bonding, van der Waals interaction, metal ion coordination, electrostatic interaction, guest
host pairing, etc. Notice that the handling of these functionalities allows the acquisition of materials that do not solely have structural planning but also ensure the generation of mechanically dynamic structures. Among the intermolecular interactions the hydrogen bonding is one notorious directing force for the growth of long 1D structures (Prins, Reinhoudt, & Timmerman, 2001; Yagai, 2006). Its excellent directional alignment, selectivity, and binding strength maximize the approximation of the molecules and bring stability to the fibers. This fact is not so surprising, if you think about the importance they have in the organization of ribonucleic and deoxyribonucleic acids, proteins, and in the systems of recognition of activated sites and crystallization (Desiraju & Steiner, 2001; Jeffrey & Jeffrey, 1997; McDonald & Thornton, 1994; Parthasarathi, Subramanian, & Sathyamurthy, 2006). The interaction between aromatic molecular chains or π-interactions are also very interesting and well exploited for the preparation of gels, being good auxiliary modulators, and sometimes the main feature to gel through different modes of packaging (Babu, Praveen, & Ajayaghosh, 2014). These π-gelators or π-modulators are found among the moieties that give rise to photophysical and photochemical events that are commonly studied in the different condensed states, such as luminescence shifts and intensity changes, charge and energy transfer, J- and H-aggregates, etc. Currently there is good literature on the different subclasses of supramolecular gels (Abdallah & Weiss, 2000; Astruc, Boisselier, & Ornelas, 2010; Buerkle & Rowan, 2012; Das et al., 2018; Delbecq, 2014; Lloyd & Steed, 2009; Mandal, Kar, & Das, 2014; Praveen, Ranjith, & Armaroli, 2014; Ressouche et al., 2016; Steed, 2010; Suzuki & Hanabusa, 2009; Weng et al., 2006; Zhang, Hu, & Li, 2018). Fig. 3.4 illustrates some structural motifs and functional groups recurrently used for making supramolecular gels.

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R OH

NH2

Carboxylic acid

Amide

O

O

O

O

N N H H Urea

R

O

O

R

R N

N

nN

N

H

H

H

H

R OH Fat acid

Bis-urea

OH N

R

O

R

R

NH2

O

N

OH

O

OH HN

NH

O

O

O

Azo derivatives

Aminoacid

OH

O

OH

OH

O

OH

HO

O R

OH

Barbituric acid

Gluconic acid DBS R NH

OH

NH2

O

O

OH

OH O

OH

NH R

Benzo di-, tri- carboxylic

1,2 diamine cyclohexane

NH2

R

R

OH

R

R

-

O

Phenols

Benzo di-, tri- amide

R N

O

R

Cathecols

O R

n Alkyl chains

O R Cholestheryl derivatives

O

N R

O R

Aromatic diimide

R R

O

R

R

R

R

R R R

O p-PhenyleneVinylenes

Anthraquinone

L L

M L

L +

L A

L L

-

L

Electrostatic: Metal-ligants and anion tuning

Figure 3.4 Structural motifs and functional groups to build supramolecular gels.

Fused aromatics

Chapter 3 SUPRAMOLECULAR GELS

3.3

Host
Guest Systems and Gelators

Another very interesting trend for obtaining and maintaining dynamical and mechanical properties of a gel is the use of host
guest systems (Foster & Steed, 2010; Qi & Schalley, 2014; Zhang et al., 2012). In these the outgoing and incoming events in the host cavities may lead to gels as well disrupting them over repetitive cycles operating as an off
on switch. These materials open opportunities to incorporate substrates within the gel cavities and to release them on demand. Supramolecular-based host
guest gels are one of the more soft materials where the dynamical properties are enhanced through complementary synergic units, producing gels with responsiveness and function (Fig. 3.5). It is worth mentioning the case of cholesterol as a gelator. Cholesterol is a very important structural element for the organization and stability of the cell membrane. It acts as a bidirectional regulator of membrane fluidity; at high temperatures it stabilizes the membrane and increases its melting point, while at low temperatures it intercalates between the phospholipids and prevents them from clumping and stiffening (Cooper, 1978). Likely the high gelation ability, per van der Waals interaction, of cholesteryl derivatives may be closely correlated with their physiological function. A very illustrative example of this function can be seen in the obtaining of a self-healing gel made from liposomes and bridged cholesteryl linkers at the edge (Fig. 3.6) (Strandman & Zhu, 2016). Cholesterol-based gels are considered to be the first rational synthesis proposed by Weiss and coworkers (Svobodova´ et al., 2012). The structure comprising an aromatic fragment attached to a steroidal group via a functionalized link could exhibit a certain predictability to gel, this system was labeled as ASL ˇ c, Vo¨gtle, & Fages, 2005). As a conse(Terech & Weiss, 1997; Zini´ quence of these studies, structures have emerged with the same ASL concept with a diversity of combinations between structural units and thus qualify the molecular gelators as powerful tools for nanotechnological enhancement. Unlike their polymer counterparts, we do not easily find supramolecular gel-based products on the market shelves, such as hair products, cosmetics, pharmaceuticals, food ingredients, and products for use in biomedicine. This is most probably because of the relatively short time of technological research, the large scale of polymer production and its atavistic ability to gel. However, some gels from low molecular mass gelators have been used for very specific purposes. The use of inorganic vanadium

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Figure 3.5 Macrocycles for functional supramolecular gels. Reprinted (adapted) with permission from Qi, Z., & Schalley, C. A. (2014). Exploring macrocycles in functional supramolecular gels: From stimuli responsiveness to systems chemistry, Accounts of Chemical Research, 47(7), 2222
2233. Copyright r 2014, American Chemical Society.

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Figure 3.6 Lithotropic system modulated by cholesterol-based linkers and their interconnections (Taguchi et al., 2011). Reprinted from Rao, Z., Inoue, M., Matsuda, M., & Taguchi, T. (2011). Quick self-healing and thermo-reversible liposome gel, Colloids and Surfaces B: Biointerfaces, 82(1), 196
202, Copyright 2011, with permission from Elsevier.

oxide gels in electrochemical studies can be mentioned (Livage, 1996); these are amphiphilic molecules whose organization in lyotropic phases serves as synthetic molds for mesostructures (Shimizu, Masuda, & Minamikawa, 2005) and in the formulation of lubricants (Donahue, 2006). Sorbitol is a small molecule widely used in the food and pharmaceutical industry (Grassi et al., 2011). An important example of the application of a colloidal gel is the mixture of aluminum salts, naphthenic and palmitic acids (NAPALM - NA-phthene 1 PALM-itate), which was widely used as an incendiary weapon during World War II. However, it is from the 1990s that increased attention was given to the myriad applications to which these materials could be applied. Since then, there has been an enormous growth of scientific reports and investigations regarding the use of supramolecular gels of the most varied types and in different technological areas. Nowadays, molecular gels have gained the attention of the food, cosmetic, and pharmaceutical industries (Aditya, Espinosa, & Norton, 2017; McClements, 2017a, 2017b). Other prognostics of technological support that are addressed to molecular gelators are as follows: organic reactions, templates or directing agents for the formation of inorganic structures, (opto) electrochemical and photonic systems, electrochemical electrolytes, photo harvesting materials, drug delivery, environmental remediation, liquid crystalline systems, tissue regeneration, and sensors (Christoff-tempesta & Lew, 2018; Escuder et al., 2010;

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Microstructural architecture and potentialites

Junctions CT events Energy transfer Exciton splitting Electrical conduction Magnetism

Templates and scaffolds Chiral induction Chemical reactions Nanoparticle synthesis Self-healing

Dynamical flow Drug delivery Sensing Selective membranes Electrolytes Environmental remediation

Figure 3.7 Different loci within a gel. The whole structure could be useful for applications.

Hirst et al., 2008; Jadhav et al., 2010; Okesola & Smith, 2016; Sangeetha & Maitra, 2005; Steed, 2011; Xue et al., 2017; Yagai, 2011; Zhang et al., 2014). From this general prospect and comprising of several research areas and a wide variety of experimental attempts, it is known that some functional groups or structural motifs play key roles in the gelation, with solvophobic and directional forces acting on the nucleation and growth of aggregates that will compose 3D dimensionality, solvents have an active influence for gelation, and mostly molecular gelators have more than one component to intermolecular interaction (although there are exceptions). Hence, there are different ways to choose the building blocks or components of gel-forming that could allow the tailoring of microstructure and final properties in order to obtain many technological and innovative functionalities. Nevertheless, the whole system should be taken into account because the 3D entangled network allows the formation of interstices and solvent/solid interfaces which can be very active. Fig. 3.7 depicts the conceptual design for the programmed building block gelator and the locations within the gel matrix that can offer suitable functional sites.

3.4

Nanostructure Synthesis With Supramolecular Gels

Supramolecular organization at the nanoscale provides a great perspective for molecule-based nanodevices, since important requirements for them are the geometries and positioning that are established between interacting moieties in this dimensionality (Altay, Tezcan, & Otto, 2017; Avinash & Govindaraju, 2018; Carne´ et al., 2011; Desmarchelier et al., 2016;

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47

Lakshmi, Dorhout, & Martin, 1997; Li et al., 2010; Yagai, 2011; Zhang et al., 2014). Thus using gels as a bottom-up platform to create nanostructures is a powerful tool for the improvement of nanomolecular-based materials. An example of how a nanofiber is employed in the nanotechnological approach is its use as a template or as tailoring mold for growing nanoparticles (NPs). For instance, consider amphiphilic fibers based on pillar[n]arenes, which are a new class of ball-surfactants that give rise to supramolecular nanoarchitectures in aqueous and organic media (Chen et al., 2015; Yao et al., 2018). In certain conditions these supermolecules may be associated in microtubes in which gold nanoparticles (Au-NPs) may be deposited by the photoreduction of their precursor salt (according to Fig. 3.8). This hybrid material

Figure 3.8 Synthetic routes and chemical structure of bola-amphiphilic pillar[5]arene bola-AP5 (Chen et al., 2015). Reprinted (adapted) with permission from Chen, R., Jiang, H., Gu, H., Zhou, Q., Zhang, Z., Wu, J., & Jin, Z. (2015). Tubular structures self-assembled from a bola-amphiphilic pillar [5] arene in water and applied as a microreactor. Organic Letters, 17 (17), 4160
4416. Copyright r 2015, American Chemical Society.

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comprising microtubes and Au-NPs seems to be a microreactor that maintains its catalytic capacity for several cycles without losing its reactivity and conversion rate. With this example one can glimpse the application of artificially nano-assembled systems that could be developed for use in catalytic industrial processes. Molecular-based hydrogels and organogels are suitable for inoculation of metallic hosts or nanoparticles. The formation of NPs and nanostructures within gels demonstrate the feasibility of these materials for improved catalytical applications showed above (Sangeetha & Maitra, 2005; Sone, Zubarev, & Stupp, 2002; Zhang & Xie, 2016). Compared with traditional molecular amphiphilic surfactants, they have an innovative practical possibility, which is the possibility to be used in a wide range of organic solvents and be easily removed by gel disruption. Love et al. (2005) described the synthesis of Au-NPs from a matrix composed of an amino acid dendritic gelator in toluene. After gelation the gel was filled with HAuCl4 by simple diffusion aided by tetraoctylammonium bromide. In this study, they observed that the NPs, obtained by photoirradiation with average diameter B13 nm, were not formed on the fibrillar surface. Another point reported was the lack of participation of the amino acid dithiol groups in the production of the Au-NPs. Further, octanotiol was infiltrated to stabilize the surface of the Au-NPs within the gel, and after heating above the Tgel the NPs were released and stayed dispersed. The eco-friendly synthesis of Ag0 silver clusters from a matrix of a dipeptidehydrogel had the active participation of aspartic acid residues through the complexation of the Ag1 silver ions, that with photoirradiation formed nanoclusters of Ag0 (Adhikari & Banerjee, 2010). In both reports the nucleation and the growth of NPs did not occur along the SAFINs, but within interstices or in specific loci of contact between the solvent and the fibers. Nevertheless, the growth of NPs along SAFINs can be successfully achieved (Jung et al., 2014). Jung and coworkers have built a chiral arrangement of Au0 NPs through modulated synthesis in which nucleation and growth occur at spatially established sites along the chiral helical nanofiber. Beyond the tuning of NP diameters the method allowed one to obtain left- and right-hand helicates with customizable chiroptical properties (Fig. 3.9). In general, gels with chiral fibers can work as improved templates to translate the chirality into inorganic nanomaterials giving chiroptical materials, and chiral recognition and asymmetric catalysis could be enhanced (Zhang et al., 2014).

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Figure 3.9 TEM images of nanoparticle superstructures from chiral hydrogel. Exposure to UV irradiation controls the size of the gold nanoparticles on the helical nanofiber template (Jung et al., 2014). Reprinted (adapted) with permission from Jung, S. H., Jeon, J., Kim, H., Jaworski, J., & Jung, J.H., et al. (2014). Chiral arrangement of achiral Au nanoparticles by supramolecular assembly of helical nanofiber templates, Journal of the American Chemical Society, 136(17), 6446
6452. Copyright r 2014, American Chemical Society.

Very interesting charge transfer (CT) systems can be made mainly within organogels due to the hydrophobic character of the majority of electron donor and receptor components (Ajayaghosh, Praveen, & Vijayakumar, 2008; Friggeri, Gronwald, Van Bommel, Shinkai, & Reinhoudt, 2002; Wang, Zhang, & Zhu, 2005). A counterpoint is the study of Reddy et al. (2017) in which they designed a supramolecular structure enabled to gelling in both organic solvent and water—an ambidextrous gelator. It is worth mentioning the structural programming of the groups linked to the gelator (FmKPy) (Fig. 3.10). Purposely the gelator was attached to different solvophobic ambiguities which contributed to the stabilization of the SAFINs and at the same time acted as host units to the acceptor moiety TNF. The possibility of producing order along the supramolecular nanofibers has attracted the use of gels for the formation of well-ordered crystalline solids or xerogels, from reagents that can be transformed by external stimuli within the matrix. Since

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Figure 3.10 (A) Chemical structure of the FmKPy gelator and TNF. The highlighted part in FmKPy indicates the possible intermolecular interactions during the supramolecular self-assembly. (B) Proposed molecular packing in the FmKPy ambidextrous gel (left) and in the CT ambidextrous gel (right) (Reddy et al., 2017). Reprinted (adapted) with permission from Reddy, S. M. M., Dorishetty, P., Augustine, G., Deshpande, A. P., Ayyadurai, N., & Shanmugam, G. (2017). A low-molecular-weight gelator composed of pyrene and fluorene moieties for effective charge transfer in supramolecular ambidextrous gel. Langmuir, 33(47), 13504
13514. Copyright r 2017, American Chemical Society.

topochemical reactions are made in crystalline form and are compromised by proximity-driven and regio/stereospecific features, their promotion within gelled soft materials can lead to better results. The use of gel for this purpose has been described in polymerizations, cycloadditions, and pericyclic dimerizations (Dawn et al., 2010; Krishnan, Mukherjee, Aneesh, Namboothiry, & Sureshan, 2016; Zhang et al., 2017). Krishnan and Sureshan (2017) described a topochemical reaction within a xerogel based on a dipeptide functionalized with azide and alkyne groups which was obtained from both organogel and hydrogel (Fig. 3.11). The alignment achieved in the gel matrix between electron donor and acceptor molecules is a very important bias for molecular-based systems with optical, electronic, and optoelectronic properties. Most of the studies employing gels deposited them as thin films or xerogels for performance tests, although there were exceptions where the gel itself was used for assembling the devices (Kubo et al., 2001, 2002; Mohmeyer et al., 2004; Zhang et al., 2017). The investment in the assembly of solar cells based on organogels has led to promising impacts on performance other than simple deposition of organic molecules by the traditional coating methods. The recurrent strategy is to use molecular p- and n-type semiconductors or conductors

Chapter 3 SUPRAMOLECULAR GELS

Figure 3.11 Thermal topochemical reaction inside the xerogel obtained from organogels and hydrogels (Krishnan & Sureshan, 2017). Reprinted (adapted) with permission from Krishnan, B. P., & Sureshan, K. M. (2017). Topochemical azide—alkyne cycloaddition reaction in gels: Size-tunable synthesis of triazole-linked polypeptides, pp. 1
8. Copyright r 2017, American Chemical Society.

from this field of research, such as aromatic diimides, oligothiophenes, tetrathiafulvalenes, graphenes, and extended aromatics (Ajayaghosh & Praveen, 2007; Babu et al., 2014; Ghosh, Praveen, & Ajayaghosh, 2016; Hasegawa et al., 2017; Jiao et al., 2014; Yuan et al., 2017). Organogels are preferable because they are easily volatilized, which aids the favored methods of casting. Some strategies are used so that the microstructural configuration favors the electronic processes. Usually two main modes of arrangement are explored, one of them being the cross-linking of the SAFINs network, putting the donor and receptor molecules into contact, thus favoring the formation of carriers of charge. The other is the tailored complementarity or intercalation of the electron donor and acceptor moieties which allows charge and energy transfer events. Pandeeswar et al. (2016) achieved interesting results involving a multi-responsive hydrogel matrix. They produced a material with stable ferroelectric behavior at room temperature being the driving force of targeting and stability the π-stacked nanofiber between an electron donor (pyrene) and a receptor

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molecule (diimides). Additionally, the hydrogel was shown to be optically and electronically responsive and had the ability to recover from its initial state after several cycles, which opens an opportunity for the design of multistate thin-film memory devices. Further, the diimide molecule was synthesized with R and S configuration amino acids allowing the formation of different chirality helical 1D fibers (Fig. 3.12).

Figure 3.12 Scheme of molecular structures and schematic representation of a multistimuli-responsive TIS
CT coassembly of DPT and A1/A2. (A) Molecular structures and schematic representation of acceptors (A1, L-alanine methyl ester conjugated NDI; A2, D-alanine methyl ester conjugated NDI), donor (DPT), and A• 2 radicals. (B) Images of aqueous solution/solids of A1, DPT, and mechanical-stimuli-induced ultrasonication for solid powders. A1 1 DPT TIS
CT coassembled (A1:DPT 2:1) hydrogel (inverted vial) and solid powders under UV (i) and visible light (ii); schematic representation of corresponding P-type (right handed) self-assembly of A1 and TIS
CT coassembly. (C) Schematic illustration of an electric field (E)-induced net polarization (P) switching, temperature (Δ) driven disassembly, and UV irradiation (hν 365 nm) responsive photo-induced single-electron transformation reaction within the TIS
CT coassembly, respectively (Pandeeswar et al., 2016). Reprinted (adapted) with permission from Pandeeswar, M., Senanayak, S. P., Narayan, K. S., & Govindaraju, T. (2016). Multi-stimuli-responsive charge-transfer hydrogel for room-temperature organic ferroelectric thin-film devices. Journal of the American Chemical Society, 138(26), 8259
8268. Copyright r 2016, American Chemical Society.

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53

Figure 3.13 Liquid crystals and supramolecular gelators (Moriyama et al., 2003). Reproduced with permission from John Wiley & Sons and Copyright Clearance Center.

The combination of liquid crystals (LCs) and gelling agents or even the formation of systems with reversible gel-LCs transition with the use of stimuli is another great strand for nanotechnology (Ajayaghosh, 2018; Amabilino, Smith, & Steed, 2017; Moore, 2000; Moriyama, Mizoshita, Yokota, Kishimoto, & Kato, 2003; Zhang et al., 2010). These systems allow different orientations of the supramolecular fibers along the directional organization of the mesomorphic phase (Fig. 3.13).

3.5

Nanoscale Ordering as a Background for Advanced Gels

For now it is important to consider the principles and classifications currently established for structured molecular systems that are oriented by intermolecular interactions and that will lead to architectures and supramolecular arrangements having characteristics such as self-discrimination and self-assembly, thus composing an auspicious field of investigation of selfsorting (Elemans, Lei, & De Feyter, 2009; Kramer, Lehn, & Marquis-Rigault, 1993). Isaacs and coworkers (Mukhopadhyay,

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Wu, & Isaacs, 2004; Wu & Isaacs, 2003) defined a self-sorted event as: “a recognition phenomenon for an object in a complex mixture of components,” based on recognition codes markedly directed by intermolecular alignment. There is an important differentiation within the concept of self-sorting concerning the form of the molecular components that are disposed that define their orientation toward each other. Then if identical objects recognize themselves and associate there is a self-recognition— narcissistic self-sorting (Taylor & Anderson, 1999)—whereas if there is affinity for another structure there is a process of selfdiscrimination—social self-sorting (Shivanyuk & Rebek, 2002). The discrimination and/or molecular selection for one process or another is closely correlated with general structural aspects such as shape, size, coordinate number, electronic distribution, and charge, and are greatly influenced by external or empirical conditions. Asymmetric centers are also important for leading chiral architectures such as homochiral, heterochiral ordinations as well chirality transfer and amplification (Duan, Cao, Zhang, & Liu, 2014; Ję drzejewska & Szumna, 2017; SafontSempere, Ferna´ndez, & Wü rthner, 2011; Singh & Sun, 2012). In addition, self-sorting processes can be governed by kinetic or thermodynamic characteristics: the first relates to the trapped metastable structures generated, while the latter refers to thermodynamic equilibrium systems (Masson, Lu, Ling, & Patchell, 2009; Mukhopadhyay, Zavalij, & Isaacs, 2006; Osowska & Miljani´c 2011; Shi, Sun, Lin, Liu, & Liu, 2017; Tena-Solsona et al., 2015). Self-sorting materials are further subdivided into integrative and nonintegrative depending on the exclusive or inclusive combination of the components. An integrative system refers to the assembly for a superstructure combined with all constituents of a mixture, on the other hand, a nonintegrative indicates the formation of different combinations between the constituents of the mixture, thus involving social or narcissistic ensembles (Holloway, Bogie, & Hooley, 2017; Jiang, Winkler, & Schalley, 2008a, 2008b) (Fig. 3.14). Nevertheless, these concepts can be only covered and realized on the basis of the empirical outcomes obtained, in spite of a programmed design for a complex system, since most of them behave in an imprecise dynamic. The balance of intermolecular strengths and geometric aspects ensures a fundamental role for the nano and/or micro self-assembled structures or even for specific insights of molecular arrangements. These systems are extremely sensitive to external conditions, and they may be tuned in different fashions through several experimental

Chapter 3 SUPRAMOLECULAR GELS

55

Figure 3.14 Types of self-sorting complex structures that can be obtained in a mixture of associative building blocks.

conditions such as temperature, solvent, concentration, sensitivity to light, ions, metal coordination, pH, and so on (Fukui, Takeuchi, & Sugiyasu, 2017; Grommet et al., 2018; Lehn, 2013; Rest, Mayoral, & Ferna´ndez, 2013). Given that, several types of molecules that can be assembled in different levels of organization can generate many functional properties, taking into account the features that are sensitive to external factors that may be a concern to applications. This reasoning certainly serves as a guideline to achieve the complexity that approaches extremely complex systems. In the light of some empirical data involving the concentration effect of the components of a mixture, one can delineate a rule-of-thumb: self-recognition is most efficient when all components are present at the same concentration. On the other hand, temperature variation may lead to an increased amount of crossover moieties, that is, mostly to social self-sorting products (Wu & Isaacs, 2003). Considerations about the experimental setup on the supramolecular outcomes have already been established for supramolecular systems governed by all types of noncovalent forces, which are the prime driving forces that would alter the mass balance and even modify preformed microstructures in the solutions (Rest, Kandanelli, & Ferna´ndez, 2015). Studies with solvents also indicate the sensitivity that selfassembled systems have with polarity changes and other specific properties of the solvents (Safont-Sempere et al., 2011;

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Singh & Sun, 2012). Most studies that attempt to understand solvent interference in molecular choices to self-sort are performed by spectroscopic techniques such as NMR, UV
vis, circular dichroism, dynamic light scattering, and fluorescence, as well as other identification techniques such as mass spectrometry and chromatography. Promptly some structural characteristics are called molecular codes for the self-sorting of complex mixtures. Safont-Sempere et al. (2011) refer to molecular codes as intrinsic information encoded in the molecular structuring of a given moiety interacting with itself and with other components of the mixture. This concept seems to follow the idea of maximum interaction that is known for the enzymatic active cavities in which the guest molecules pursue better interactions with the host. To self-sort in a mixture the structures tend to be stabilized by factors such as: 1. geometrical complementary by size and shape; 2. complementarity in hydrogen-bonded systems; 3. steric effects; 4. coordination sphere in metal
ligand interactions; and 5. charge transfer. Size and shape are primarily the main protagonists of this process of interaction for molecular recognition. Certainly a good approximation of the structures is directly related to the extent and the strength of intermolecular interactions that are established during the pairing. The surface area of these objects provides spatial geometric arrangements with precise positions to obtain a self-sorted outcome. Hydrogen bonding, π
π stacking, and metal ion coordination are very useful for these molecular recognition events directed through size and shape. As for complementarity in hydrogen-bonded systems, a striking example of this phenomenon is found in the biological pairing of the nitrogen bases (A
T and C
G) in the DNA double helix (Yakovchuk, Protozanova, & Frank-Kamenetskii, 2006). Despite the extreme exigency imposed by geometrical elements, the match and mismatch between donors and acceptors will decide the outcome of a pairing recognition event. A significant number of experiments on hydrogen bonding for social and narcissistic self-sorting studies have been developed (Jiang et al., 2008a, 2008b; Mukhopadhyay et al., 2004). Steric effects originate from spatial arrangements and the bulkiness of atoms or molecules. They are important for selfsorted architectures, although they are not the main alignment force, but are useful as a fine adjustment of the final products. Steric codes can lead to stable systems in structural equilibrium

Chapter 3 SUPRAMOLECULAR GELS

furnishing integrative and/or nonintegrative domains, depending on the initial combinations of components and groups. Metallosupramolecular systems, pseudorotaxanes, and calix[4] arenes have provided excellent results for the effects of steric codes (Johnson & Hooley, 2011; Safont-Sempere et al., 2011). Coordination spheres in metal
ligand interactions are very suitable for the design of self-sorted materials. It is important to take into account the choice of the ligand and metal due to the shape and size of the ligand and the coordination number of the metal center that may have a key action (Safont-Sempere et al., 2011). In multimetallic systems the coordination sphere becomes very influential to the metal
ligand interaction and to the recognition events. The ligand and the metal center form a coordinated bond, being a well-established field with all the electronic peculiarities in which p orbitals of the ligand interact with the d orbital of the metal ion (Langner et al., 2007). Transition metals with vacant d-orbitals would be more available for supramolecular recognition codes due to their quite specific coordination modes which are more appealing for obtaining supramolecular self-sorted metal
ligand structures (Albrecht, 2000; Enemark & Stack, 1998; Machado, Baxter, & Lehn, 2001).Three factors are considered very important to self-sort metallosupramolecular materials: 1. the size, charge, hardness, and polarizability of the metal ions; 2. the identity (and sterics) of the ligands; and 3. the electronic interaction between the ligands and the central metal ion.

3.5.1

Charge Transfer

Processes that are established by charge transfer (CT) between donor and electron acceptor are very important for the development of materials of molecular electronics and optoelectronics. The ordering of the nanometric molecular domains and other domains throughout the arrangement and assembly of the devices is extremely important so that good bulk heterojunctions are formed preferably in order of the sizes of the exciton diffusion length (10
20 nm), ensuring efficient CT at the interfaces (Chen, Zhang, Huang, Chen, & Wee, 2008; Otero et al., 2007). CT events are not decisive for the self-association processes, being considered to be lightweight molecular codes in the arrangements. However, when size and shape conditions and other spatial characteristics are similar, they become important elements in the ordering and choice of the ordered molecular cluster or architecture.

57

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These considerations on some of the molecular codes that play a decisive role in the processes of self-ordering and selfdiction are closely correlated with all soft-matter research that pursues fine-tuning materials and specific arrangements leading to high-performance properties. Regardless of the experimental difficulty in controlling them, the knowledge of molecular codes certainly becomes an optimal tool for the development of supramolecular organogels and hydrogels (Colquhoun et al., 2014; Das & Ghosh, 2011; Hirst, Huang, Castelletto, Hamley, & Smith, 2007; Morris et al., 2013; Smith & Smith, 2011). It is worth mentioning some systems of supramolecular gels in which some principles of self-sorting were explored. Perylene diimides are good aromatic building blocks for selfsorted events due to their extended π-system surface areas and the possibility of the inclusion of side chains of different sizes favoring Van der Waals interactions and different arrangements of molecular planes by π-stacking, leading to different interactions of their Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) orbitals in social or narcissistic recognition processes that provide aggregates such as excimer and excimer aggregates of type J and H. The effects of steric code can be observed on derivatives with linear alkyl chains and with branched alkyl chains. The former carry H-type aggregates which form red gels, whereas in the second case there is the formation of J-type aggregates which form yellow gels (Li ¨rthner, Bauer, Stepanenko, & Yagai, 2008). When et al., 2006; Wu mixtures of the alkyls, linear and branched derivatives, are responsible for the influence of steric bulkiness of the stacking of molecules it shows that this process can be also self-assemblylike. These studies also take into account the effect of different solvent mixtures on these aggregation phenomena. The steric effect was also observed in supramolecular gels having hydrogen bonds as the main interaction force. Moffat and (Smith, 2009) make use of similar H-bonding molecular structures of different amides and carbamides to further study the recognition events and the influence of steric effects on self-sort. The logic of the study uses the fact that the two H-building blocks used can be organized within the gel through different nanoscale assembly preferences. Thus variations in the initial and solvent compositions lead to elucidations of the predilections of the aggregation system and then to the gel property and other properties of the gel. The authors emphasize the possibility of programmed synthesis of building blocks that favor the pursuit and development of functional materials based on hierarchical arrangements that take place in the nanodomain, which is the stage of the balance

Chapter 3 SUPRAMOLECULAR GELS

of intermolecular forces that will establish the balances of selfrecognition and self-discrimination. CT systems are capable of providing an equilibrium, or modulations for solutions involving self-assembly molecular blocks or other processes (Molla, Das, & Ghosh, 2010; Sugiyasu, Kawano, Fujita, & Shinkai, 2008). The assumptions of a good approximation and definition of surface geometric configurations that allow greater electron transfer efficiency between donor and acceptor are certainly the dominant ones for any organizational event. However, mixtures of oligophenylenevinylenes and perylene diimides conducted in organic solvents illustrate peculiar cases in which CT processes appear to decisively influence aggregation leading to self-discrimination rather than self-recognition (Beckers et al., 2006; van Herrikhuyzen, Syamakumari, Schenning, & Meijer, 2004); apparently, even though it was considered a weak attraction force for such events, the interaction of the donor with the electron acceptor induced the formation of intercalated aggregates. All these elements of supramolecular qualification are very interesting and can be thought of in the planning of a gelator so that it can form a gel in water, as well as organic solvents and mixtures of solvents. Nevertheless, the initial and external conditions to which the experiments are exposed play key roles in structuring the nanometric fibers within the 3D matrix, as well as the chemical, physical, and mechanical impulses that can modify the molecular interrelations leading to new formations and aggregations. As a result of a well-established synthetic apparatus for the various organic building blocks, a knowledge of physicochemical properties of the constituent units of the system, analytical techniques that allow data collection, and the interpretative quality and supramolecular principles of complementarity, recognition, and transformation has a vast field of research in which considerations about the nanometric arrangements will help in the development of functional gels. Supramolecular gels open up a huge field of research for materials science and soft materials. Gelling systems of various types can be designed for a given behavior, starting from supramolecular building blocks and from physical and chemical properties of their components, which requires a joint effort of specialists from different areas of chemistry, physics, engineering, and others. Nanotechnology is an auspicious field that allows the manipulation, hierarchization, and control of nanostructures that will eventually bring innovations and improvements to the different fields that encompass this scientific area.

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Further Reading Li, J. L., & Liu, X. Y. (Eds.), (2013). Soft fibrillar materials: Fabrication and applications. John Wiley & Sons. Rao, Z., Inoue, M., Matsuda, M., & Taguchi, T. (2011). Quick self-healing and thermo-reversible liposome gel. Colloids and Surfaces B: Biointerfaces, 82(1), 196
202. Available from https://doi.org/10.1016/j.colsurfb.2010.08.038. Smith, D. K. (2009). Lost in translation? Chirality effects in the self-assembly of nanostructured gel-phase materials. Chemical Society Reviews, 38(3), 684
694. Zhang, X., Deng, C., Wang, M., Liu, X., Lin, C., Peng, L., & Wang, L. (2017). Topochemical polymerisation of assembled diacetylene macrocycle bearing dibenzylphosphine oxide in solid state. Supramolecular Chemistry, 29(2), 94
101.

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