157
Functional πGelators and Their Applications Sukumaran Santhosh Babu, Vakayil K. Praveen, and Ayyappanpillai Ajayaghosh* Photosciences and Photonics Group, Chemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Trivandrum 695019, India CONTENTS 1. Introduction 1973 1.1. Gelator and the Solvent 1974 1.2. Mechanistic Considerations 1974 1.3. Structural Requirements 1975 1.4. π-Gelators vs Liquid Crystals 1975 2. Photoresponsive Gelators 1976 2.1. Azobenzenes 1976 2.2. Stilbenes 1988 2.3. Butadienes 1991 2.4. Dithienylethenes 1992 2.5. Spiropyrans and Spirooxazines 1994 2.6. 2H-Chromenes 1994 2.7. Diacetylenes 1994 2.8. Cinnamic Acid Derivatives 2001 2.9. Miscellaneous Photoresponsive Gelators 2001 3. Functional Dye Derived Gelators 2002 3.1. Merocyanines 2002 3.2. Naphthalimides 2004 3.3. Perylene Bisimides 2008 3.4. BODIPY Dyes 2013 3.5. Phthalocyanines 2013 3.6. Porphyrins 2014 3.7. Squaraines 2020 3.8. Azo Dyes 2020 3.9. Coumarins 2021 3.10. Dye Doped Gels 2022 3.11. Miscellaneous Dye Based Gelators 2024 4. Gelators Based on Fused Aromatics 2025 4.1. Pyrenes 2025 4.2. Triphenylenes 2029 4.3. n-Acenes 2031 4.3.1. Naphthalenes 2031 4.3.2. Anthracenes and Anthraquinones 2040 4.3.3. Tetracenes and Pentacenes 2046 4.4. Fluorenes 2046 5. Heterocycles as Gelators 2055 5.1. Pyrrole 2055 5.2. Indole 2056 5.3. Adenines, Guanines, and Guanosines 2057 5.4. Pyridine Derivatives 2059 5.4.1. Bipyridines 2059 5.4.2. Terpyridines 2060 5.4.3. Phenanthrolines 2060 5.4.4. Miscellaneous Pyridine Based Gelators 2061 5.5. Quinolines 2062 5.6. Phenazines 2063 5.7. Riboavins 2064 5.8. Carbazoles 2064 5.9. Oxadiazoles, Benzoxazoles, and Phenylisox- azoles 2064 5.10. Benzofurans, Benzimidazoles, and Benzo- thiadiazoles 2066 5.11. Tetrathiafulvalenes 2067 5.12. Miscellaneous Gelators 2070 6. Oligomeric π-Systems Based Gels 2073 6.1. Thiophenes 2074 6.2. Phenylenevinylenes 2079 6.3. Phenyleneethynylenes 2087 6.4. Phenylenes 2090 7. Carbon Allotrops and Related Gelators 2095 7.1. Fullerenes (C 60 s) 2095 7.2. Carbon Nanotubes 2095 7.3. Graphenes 2098 7.4. Coronenes 2099 8. Gels Formed by Miscellaneous Chromophores 2100 9. Applications of π-Gels 2101 9.1. Organic Electronics 2102 9.1.1. Field Eect Transistors (FETs) 2103 9.1.2. Photovoltaic Devices (PVDs) 2103 9.2. Imaging and Sensing Applications 2104 10. Conclusion and Outlook 2106 Appendix 2106 Author Information 2106 Corresponding Author 2106 Present Address 2106 Notes 2106 Biographies 2107 Acknowledgments 2107 Abbreviations 2107 References 2108 Received: April 10, 2013 Published: January 8, 2014 Review pubs.acs.org/CR © 2014 American Chemical Society 1973 dx.doi.org/10.1021/cr400195e | Chem. Rev. 2014, 114, 19732129

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  • Functional Gelators and Their ApplicationsSukumaran Santhosh Babu, Vakayil K. Praveen, and Ayyappanpillai Ajayaghosh*

    Photosciences and Photonics Group, Chemical Sciences and Technology Division, CSIR-National Institute for InterdisciplinaryScience and Technology (CSIR-NIIST), Trivandrum 695019, India

    CONTENTS

    1. Introduction 19731.1. Gelator and the Solvent 19741.2. Mechanistic Considerations 19741.3. Structural Requirements 19751.4. -Gelators vs Liquid Crystals 1975

    2. Photoresponsive Gelators 19762.1. Azobenzenes 19762.2. Stilbenes 19882.3. Butadienes 19912.4. Dithienylethenes 19922.5. Spiropyrans and Spirooxazines 19942.6. 2H-Chromenes 19942.7. Diacetylenes 19942.8. Cinnamic Acid Derivatives 20012.9. Miscellaneous Photoresponsive Gelators 2001

    3. Functional Dye Derived Gelators 20023.1. Merocyanines 20023.2. Naphthalimides 20043.3. Perylene Bisimides 20083.4. BODIPY Dyes 20133.5. Phthalocyanines 20133.6. Porphyrins 20143.7. Squaraines 20203.8. Azo Dyes 20203.9. Coumarins 20213.10. Dye Doped Gels 20223.11. Miscellaneous Dye Based Gelators 2024

    4. Gelators Based on Fused Aromatics 20254.1. Pyrenes 20254.2. Triphenylenes 20294.3. n-Acenes 2031

    4.3.1. Naphthalenes 20314.3.2. Anthracenes and Anthraquinones 20404.3.3. Tetracenes and Pentacenes 2046

    4.4. Fluorenes 20465. Heterocycles as Gelators 2055

    5.1. Pyrrole 20555.2. Indole 20565.3. Adenines, Guanines, and Guanosines 20575.4. Pyridine Derivatives 2059

    5.4.1. Bipyridines 20595.4.2. Terpyridines 20605.4.3. Phenanthrolines 20605.4.4. Miscellaneous Pyridine Based Gelators 2061

    5.5. Quinolines 20625.6. Phenazines 20635.7. Riboavins 20645.8. Carbazoles 20645.9. Oxadiazoles, Benzoxazoles, and Phenylisox-

    azoles 20645.10. Benzofurans, Benzimidazoles, and Benzo-

    thiadiazoles 20665.11. Tetrathiafulvalenes 20675.12. Miscellaneous Gelators 2070

    6. Oligomeric -Systems Based Gels 20736.1. Thiophenes 20746.2. Phenylenevinylenes 20796.3. Phenyleneethynylenes 20876.4. Phenylenes 2090

    7. Carbon Allotrops and Related Gelators 20957.1. Fullerenes (C60s) 20957.2. Carbon Nanotubes 20957.3. Graphenes 20987.4. Coronenes 2099

    8. Gels Formed by Miscellaneous Chromophores 21009. Applications of -Gels 2101

    9.1. Organic Electronics 21029.1.1. Field Eect Transistors (FETs) 21039.1.2. Photovoltaic Devices (PVDs) 2103

    9.2. Imaging and Sensing Applications 210410. Conclusion and Outlook 2106Appendix 2106Author Information 2106

    Corresponding Author 2106Present Address 2106Notes 2106Biographies 2107

    Acknowledgments 2107Abbreviations 2107References 2108

    Received: April 10, 2013Published: January 8, 2014

    Review

    pubs.acs.org/CR

    2014 American Chemical Society 1973 dx.doi.org/10.1021/cr400195e | Chem. Rev. 2014, 114, 19732129

  • 1. INTRODUCTION

    Supramolecular chemistry originally dened by Lehn aschemistry beyond molecules exploits noncovalent interac-tions between molecules to form 1D, 2D, and 3Darchitectures.13 Molecular self-assembly is an oshoot ofsupramolecular chemistry, which originated as a laboratorycuriosity among chemists.4,5 Over the years, the study ofmolecular self-assembly has grown to a matured eld ofinterdisciplinary research by which both biology and materialsciences have beneted.68 When molecules with certainstructural features self-assemble, the resulting supramoleculararchitectures can immobilize a large volume of solvents leadingto gelation of the solvent mass.924 Such molecules can gelateboth organic as well as aqueous solvents. In recent years, thisarea of research has been aggressively pursued by chemists andmaterials scientists resulting in a wide variety of softmaterials.2531 A large number of review articles have appearedin the literature pertaining to general aspects and mechanismsof gelation,924,3249 dierent types of gelators,5078 and theirapplications.2531,79111 The basic principles, the structuralfeatures of gelators, inuence of solvents and role ofnoncovalent interactions, properties of gels, and their character-ization have been discussed in many of those reviews. Readersare advised to refer to previous reviews for a generalunderstanding of the gel chemistry. Even though in many ofthose reviews chromophore based gels and their propertieshave been discussed, a comprehensive analysis of the progresswith dierent types of -systems is not available. Therefore, themain purpose of this review is to bring together a detaileddiscussion of organogelators based on -systems, which arenamed as -gelators.28,44,57 Organogels derived from -gelators are called -gels which are soft, nonowing materialsderived from gelators with more than one aromatic -unitwhich are either fused as in the case of naphthalene, anthracene,pyrene, etc. or conjugated as in the case of p-phenyl-enevinylenes, p-phenyleneethynylenes, thienylenevinylenes, etc.-systems, by virtue of their delocalized -electrons,112 have

    inherent electronic properties such as luminescence,113 chargecarrier mobility, and electronic conductivity.114 Therefore, -conjugated molecules are extensively used in organic electronicdevices such as, LEDs,115 FETs,116,117 and PVDs.118,119 When-systems are used in devices by deposition or coating, theyexist as aggregates of dierent size and shape which haveinuence on electronic properties.120 Therefore, controlling thesize and shape of molecular aggregates are important foroptimum electronic properties.121 One way to control the sizeand shape of aggregates is by the bottom-up self-assemblyapproach, utilizing weak forces such as hydrogen bonding and-stacking interactions.122 In this context, gel chemistry hasbeen recognized as a convenient strategy to exploit molecularself-assembly for the creation of supramolecular architectures ofnano to microscale dimensions. However, the judicious choiceof the molecular components plays a signicant role in thedesign of functional -gels with controlled size and shape.

    1.1. Gelator and the Solvent

    In an organogels, solvent is the major component which getsgelled by the gelator which is the minor component. Therefore,nature of the solvent is a crucial factor in the gelation process.For example, polarity, functional groups, hydrophobichydro-philic character, viscosity etc. are inuential in deciding the netproperty of the resultant gel. The balance of gelatorsolventand gelatorgelator interactions is determined by the solvent

    polarity and temperature.41,123,124 Ability of the solvent todisrupt the hydrogen bonding between the gelator moleculescan severely retard the gelation of solvents. Solvents have acritical role in assisting the nucleation and growth processes ofthe self-assembly125 and the consequent gelation.41,126 For ahomogeneous solution of the gelator, the solvent should havethe ability to compete with the intergelator interaction whereasin the case of gelation, solvents favor the intergelatorinteractions.41 Hence the dielectric constants127 and theHansen solubility parameters128130 for the gelator as well asfor the solvent are important.131,132 Recent studies revealed thatonce the gelation is complete, the solvent trapped inside the gelbrous structures becomes mechanically passive.133136 Themechanical strength of gels and the ability to reset could bevaried with the volume fraction of the solvent used.133136 Theimportance of the chemical structure of the gelator and the typeof solvent molecules used in gelation have been established in arecent study.137 In the case of most of the -gelators, nonpolaror lowpolar hydrocarbon solvents have been found to besuitable for gelation. Neverthless, the nature of the gelator isalso a deciding factor on the choice of the appropriate solventfor the preparation of the gels.1.2. Mechanistic Considerations

    Even though, there are numerous approaches toward the designof supramolecular gelators, in the earlier period most gelatorsare found rather by serendipity than by design. However, thefollowing features of gelation are considered to be important inthe design of new organogelators: (1) formation of 1Daggregates via anisotropic growth process, (2) intertwining ofthese 1D aggregates to form a 3D network, and (3) theprevention of crystallization or precipitation of the self-assembled aggregate through a delicate balance betweenorder and disorder (Figure 1). Hence, the design of new

    gelling agents continues to be a challenging task. Theknowledge gained from the aggregation of gelator moleculesduring the past decade, has enabled gelators to be designedthrough the incorporation of structural features that are knownto promote aggregation. This novel class of supramolecularmaterials exhibits striking properties with respect to self-assembly phenomena leading to diverse architectures.Gelation is normally observed when a homogeneous solution

    of the gelator, obtained by heating a required amount in asuitable solvent, is cooled to room temperature or below.Supersaturation-mediated nucleation and growth is found to bethe driving force for the gel nanostructure formation.41 Asevidenced in the recent studies of chiral and achiral perylenebisiimide gelators, a marked dierence is observed in theaggregation process at low and high concentration of the

    Figure 1. Schematic representation of the self-assembly of lowmolecular weight gelators into one-dimensional aggregates and thesubsequent formation of an entangled network.

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  • gelator upon slow cooling.138 At a lower concentrationthermodynamically stable aggregates are formed, whereas thekinetically favored products are preferably formed at higherconcentrations, especially for organogels upon cooling. The fastnucleation and growth process at the initial stages ofaggregation rapidly converts to kinetically stable brousaggregates, which hold a large excess of solvent molecules toform organogels.

    1.3. Structural Requirements

    Even though the chemical structure of many of the gelatorslooks rather simple, it is dicult to precisely predict thestructural requirements for molecules to show gelationproperty. However, the information gained from a variety ofdierent types of gelators helped in rationalizing the generalstructural requirements of molecular gelators. In majority of thecases, presence of functional groups such as hydroxyl, amide,urea and carboxylic acid that are capable of forming hydrogenbonded assemblies has been proved to be essential for gelation.The directionality, specicity and rigidity of multiple hydrogenbonding interactions help spatial arrangement of functionalchromophores in achieving ecient gelation. For example, thecomplexation between ditopic N,N-disubstituted melaminetype DAD module and barbiturate or cyanurate type ADAmodules as well as the complementary interactions of acid-pyridine derivatives have also been widely used for gelation(Chart 1a). Molecules with structural motifs such as aminoacids, peptides, cholesterol, sugar, cyclohexyl amine, chiral/achiral aliphatic/oligoethylene chains (Chart 1b), etc. havebeen found to facilitate gelation of a variety of solvents.Presence of an aromatic core that helps stacking is

    another requirement for the design of organogelators.Molecules that facilitate dipoledipole and donoracceptor

    interactions can also lead to the gelation of solvents. Anotherimportant structural requirement for molecules to assistgelation is the presence of long hydrocarbon chains with acertain optimum chain length. For example, functionalization ofmolecules with 3,4,5- or 3,5-alkoxy substituted benzene (Chart1c) moieties helps to induce gelation. The presence of alkoxychains maintains the subtle balance between solubility andprecipitation of the gelator molecules in a given solvent. Thepresence of hydrocarbon chains also facilitates van del Waalsinteraction of gelator molecules. In a majority of cases, morethan one such structural requirement is essential for the designof gelator molecules, even though there are exceptions.

    1.4. -Gelators vs Liquid Crystals

    -gelators and liquid crystals (LCs) are a signicantly dierentclass of soft materials with distinct properties. They haveimportant applications in certain domains of advancedmaterials. In this respect LCs have an advantage over -gelators due to the proven application history of the formerwhereas -gelators are still waiting for a major breakthrough inapplication. However, -gelators have advantages over LCs interms of the diversity of the architectural shape and size withtunable properties. Apart from the aesthetic architecturalfeatures, chromophore based gelators are interesting from theviewpoint of their electronic properties which can besignicantly modulated by self-assembly and gelation. Afterdiscussing the gelation, morphology, and associated propertiesof -gelators derived from a dierent class of molecules, webriey review their application, mainly in organic electronicdevices, sensing, and imaging. Even though a large number ofgelators have already been investigated for their functionalproperties, these soft materials are yet to be exploited foradvanced applications. It is an emerging area, and therefore

    Chart 1. Structural Requirements for the Design of Gelators

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  • viable strategies with innovative ideas and immense care needto be implemented to improve the quality and eciency of thedevices based on -gelators.The progress achieved in gel chemistry has been tremendous

    and almost every class of molecules has been touched so far insearch of new gelators. Atleast one new molecule may begetting added everyday as judged from the frequency of reportsappearing in various scientic journals. Therefore, it is dicultto discuss all of them at one place. Due to this reason, whilediscussing dierent types of -gelators, we have excludedgelators based on metallo supramolecular assemblies, hydrogels,and ionic gels derived from simple amino acids, peptides, andcarbohydrates since several recent reviews are available on thesetopics. We rather focus on -gelators based on photoresponsivechromophores, functional dyes, fused polyaromatics, hetero-cycles, functional dyes, oligomeric conjugated -systems, andcarbon allotrops. We have minimized discussions related togelators obtained from simple functional derivatives of aromaticmolecules such as benzene, pyrrole, or thiophene. However, wehave included peptide and sugar based gelators having fusedaromatics and functional dyes attached to them as borderlinecases of -gelators for the benet of readers. The nonaromaticdiacetylenes and several miscellaneous systems such as DNAbases and riboavines are also included in this review to give awider perspective of the topic to readers. For a betterunderstanding, we have classied gelators discussed in thisreview into dierent categories according to the generalnomenclature of molecules.

    2. PHOTORESPONSIVE GELATORSOrganogels based on photoresponsive chromophores are ofgreat interest since their properties can be modulated usinglight as an external trigger.139149 For example, uponphotoirradiation, gel can be transformed into a viscous liquid

    or a solution. In many cases, the size and shape of the xerogelmorphology can be manipulated with light, leading to a changein their electronic properties. Such change in photoinducedphysical processes can be achieved by transcis isomerization,2 + 2 dimerization, photoscission, or photopolymerizations ofthe chromophore derived gelators. The most commonly usedphotoresponsive chromophores for the design of photo-responsive gels are azobenzene, stilbene, diarylethene, andspiropyran.139151

    2.1. Azobenzenes

    Azobenzene derivatives have been studied for the design ofphotoresposive gels. The reversible photoinduced transcisisomerization of azobenzene unit140,150152 signicantlyinuence the gelation ability of the associated gelators. Amajor limitation of the azobenzene systems is the thermal backisomerization under dark conditions. Moreover, a completeconversion of the trans to the cis isomer cannot generally beaccomplished. However, the large volume and polarity changesassociated with the trans to the cis isomerization can induceconsiderable variation in the properties even with partialisomerization yield. Due to the large steric repulsion of the cisisomers, they have relatively weak association constantsresulting in the destruction of the self-assembly.Shinkai and co-workers have made signicant contributions

    to azobenzene derived gelators. In their rst report, gelation oftwo types of gelators 16 (Chart 2 and Figure 2) with steroidskeletons having either the natural (S) conguration or theinverted (R) conguration have been studied.153,154 Amongthem, the gelator with a p-alkoxy azobenzene moiety is themost ecient and can gelate either nonpolar solvents (Sderivatives, 2ae) or polar solvents (R derivatives, 3af). It ispossible to read-out the solgel phase transition as well as thechirality of the supramolecular stacks using CD spectrosco-

    Chart 2

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  • py.153,154 For example, the 1-butanol gel of 2a showed a

    positive exciton coupling whereas methanol gel of 3b showed a

    negative exciton coupled band characteristic of the clockwise

    and anticlockwise orientation of the azobenzene dipoles in the

    gel state, respectively. The SEM pictures of the xerogels showed

    helical bers that complement the observed CD spectra.Detailed morphological studies of a variety of azacrown

    appended cholesterol gelators, have revealed the formation of

    supramolecular architectures of dierent shape and size.153162

    Figure 2. SEM and TEM images of gel nanostructures of azacrown appended cholesterol gelators. (Panels ac, reprinted with permission from ref157. Copyright 2000 American Chemical Society. Panels d and e reprinted with permission from ref 159. Copyright 2000 American ChemicalSociety. Panel f, reprinted with permission from ref 161. Copyright 2001 American Chemical Society.)

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  • The xerogel of 6 in cyclohexane showed the brous networkstructure with 2562 nm diameter, which were partially twistedin a helical fashion (Figure 2a).157 The SEM pictures of thedried samples of 7a (Figure 2b) showed curved lm likeaggregates with 3040 nm thickness.157,158 As shown in Figure2c, the xerogel of 7b exhibited a tubular structure with 4575nm wall thickness and 170390 nm inside tube diameter.157The growth of a few layered and curved lamellae is observed inthe case of 7a (Figure 2b) whereas continous growth ofmultilayered paper like roll structure is observed for 7b(Figure 2c). In contrast to the common brous or plate likeaggregates of organogels, a multilayered spherical morphologywas observed for the xerogel of 7c (Figure 2d) whereas a

    cyclohexane gel of 7d showed the presence of rolled lm-likestructure (Figure 2e).159,160 The xerogel of 7e exhibited atubular structure with 520 nm outer diameter and also showedthe presence of linear and the helical ribbons with 17001800nm pitch (Figure 2f).161,162 Observation of such a wide varietyof self-assembled structures indicate the feasibility of preparingdiverse architectures with controlled morphology which areotherwise dicult to obtain.An interesting aspect of the above studies is the use of the

    self-assembled architectures as templates for the creation ofexotic inorganic silica structures which are dicult to obtain byconventional methods.164 For example the solgel polymer-ization of TEOS in the presence of various supramolecular

    Chart 3

    Figure 3. (a) Schematic representation of 1D molecular stacking of 11a. TEM images of (b) helical ribbon and (c) tubular structure of self-assembled 11b in the absence and presence of Pd metal ion. (Panel a reprinted with permission from ref 170. Copyright 2003 American ChemicalSociety. Panels b and c reprinted with permission from ref 171. Copyright 2005 The Royal Society of Chemistry.)

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  • structures of the gelators resulted in spherical, tubular androlled-paper-like silica structures.163166 SAXS studies revealedthat the gel bers of the ionic gelator 8 (Chart 3) is amorphouswhereas that of the nonionic gelator 3c (Chart 2) iscrystalline.164 An important nding of these studies is thationic as well as amorphous nature of the ber is essential forsilica transcription by template reaction.163166

    The Gemini type azobenzene functionalized cholesterolbased gelators 9ae (Chart 3) exhibit an oddeven eect ingelation.167,168 Compounds 9a and 9c having an even numberof methylene units between NH and NH2 formed translucentorganogels. The SEM images showed lamellar structured lm-like aggregates with 60330 nm thickness and a fewmicrometer lengths. The gelators 9b and 9d having an oddnumber of methylene units formed transparent organogelshaving brous structures of diameters 2050 and 1070 nm,respectively. In another study, it has been demonstrated thatthe morphologies of the self-assembled azobenzene-cholesterolgelators could be controlled by the balance of the solvophilicand solvophobic groups in organic solvents.169 The self-assembled gelator 9e (Chart 2) showed a brous structure with200300 nm diameters, whereas the gelator 10 (Chart 3)exhibited a tubular structure with a uniform external diameterof ca. 560 nm, with a wall thicknesses of 50 nm.169

    The dimeric azobenzene appended cholesterol basedorganogelator 11a (Figure 3) adopts a folded conformationin the self-assembled state to have an ecient intramolecularand intermolecular cholesterol-cholesterol and azobenzene-azobenzene interactions (Figure 3a).170 Compound 11b(Figure 3) is found to form helical ribbon and tubularstructures in the absence and presence of Pd metal ion,

    respectively (Figure 3b,c).171 This is attributed to the change inthe balance between hydrophobicity and hydrophilicity in 11bupon complexation with metal ions. Using this gelator, the solgel transcription method has been shown to be eective to theformation of palladium-doped double-walled silica nano-tubes.172

    Azobenzenes 12ac (Chart 4), functionalized with twourethane moieties linked to two cholesteryl ester units formgels that exhibit solgel phase transitions upon photo-irradiation as a result of transcis isomerization of theazobenzene units. During the solgel phase transitions,hydrogen bonds, which are partly responsible for stabilizingthe gels are broken or reformed.173 In a bischolestrolfunctionalized gelator 13 (Chart 4), in addition to thephotoresponsive property of the azobenzene moiety, a redoxactivity has been introduced by adding a TTF moiety.Therefore, this gelator allows both photo and redox controlof the electronic properties.174 Recently, a series of photo-responsive dicholesterol linked azobenzene gelators 14ac(Chart 4) with dierent spacer lengths have been reported.175

    Detailed studies revealed that spacer groups dramatically aectthe gelation properties. Accordingly, 14b with a spacer of twomethylene units is found to be the best gelator. In addition tothe eect of spacer groups, the eect of methanol as a cosolventon the gelation of cyclopentanone by 14b has been studied.176

    The addition of methanol is found to modulate the speed ofgelation, photoresponsive property and the morphology of thegel nanostructures. This study highlights the importance of thesolvent-gelator interaction in tuning the properties of theresultant gel. A cholesterol imide appended azobenzene gelator15 (Chart 4) exhibited reversible solgel transition via

    Chart 4

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  • photoisomerization of the azobenzene moiety upon irradiationwith UV and visible lights.177

    A family of hydrogelators based on azobenzene appendedsugar bolaamphiphiles, 16ad (Chart 5), has been reportedjointly by Shinkai and Reinhoudt.178 These bolaamphiphilesconsist of two solvophilic aminophenyl sugar skeletons as thechiral aggregate forming site and a solvophobic azobenzenemoiety as the stacking site. Compound 16a is found to bea super hydrogelator, as it can form gels even atconcentrations as low as 0.05 wt %, whereas compound 16bcould gelate only a 1:1 (v/v) DMSO-water solvent mixture. Onthe otherhand, 16c and 16d could not gelate any of the solventsinvestigated. These observations reveal that the conformationof the sugar moieties has signicant inuence on the gelationproperty.The sugar derivatives 16a and 16b were reported to form

    right handed helical aggregates in 1:1 (v/v) DMSO-water,whereas 16c and 16d formed left handed helical aggregates as isevident from the corresponding exciton coupled CD spectra.TEM studies revealed right handed helical brillar structures(Figure 4b) for the aggregates of 16a and 16b, reecting themicroscopic 1D columnar orientation of azobenzene chromo-phores. However 16c and 16d tend to aggregate into vesicularstructures as a result of their 2D aggregation mode and failed tofrom gels. Addition of boronic acid appended poly(L-lysine)(Figure 4a) to the gel phase of 16b resulted in the formation ofsol phase with a morphological transition from brousaggregate (Figure 4b) to vesicular aggregate (Figure 4c,d).179

    These macroscopic and microscopic level changes are due to

    the specic boronic acid-sugar interaction and can be revertedusing D-fructose, which has high anity toward the boronicacid group.The sugar coated nanobers formed by a combination of

    azobenzene and disaccharide lactones 17ac (Chart 5)provided a bioactive interface for cell attachment and exhibitedlectin binding.180 In addition, the hydrogels exhibited areversible solgel transition in response to temperature andUV irradiation. The latest report on a photoresponsive gelatorpertains to a sugar based amphiphilic system containing anazobenzene moiety 18 and 19 (Chart 5).181 The partial transcis isomerization of the azobenzene moiety allows photo-induced chopping of the entangled long bers to short bers,resulting in controlled ber length and gelsol transition.These gelators facilitate phase selective gelation of aromaticsolvents from aqueous emulsion. Such a phase selective gelationis useful for the removal of small amounts of toxic solvents fromwater.Feringa and co-workers have reported chiral recognition in

    hybrid gel assemblies of alkyl substituted 1,2-bis-(uriedocyclohexane) derivatives (S)-20, (R)-20, and theazobenzene incorporated derivatives (S)-21 and (R)-21 (Figure5).182 The CD spectra of (R)-21 in 1-butanol gel of (S)-20showed a slightly more intense bisignate CD signal than that ofa solution of (R)-21 in 1-butanol, indicating that theenvironment of (R)-21 is less polar when it is incorporatedinto aggregates of (S)-20 than in 1-butanol (Figure 5a).However, incorporation of (R)-21 in a gel of (R)-20 results in astrong positive Cotton eect, which is not exciton coupled,

    Chart 5

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  • indicating that the azobenzene groups are no longer stacked butincorporated between the closely packed alkyl chains of (R)-20aggregate as shown in Figure 5b.The simple bisurea appended azobenzene derived organo-

    gelators 22a and 22b (Figure 5) are reported to show distinctgelation behavior.183 For example, the 4,4-disubstitutedderivative (22a) is found to be a poor gelator, whereas the2,2-disubstituted derivative (22b) is an excellent gelator of avariety of organic solvents at concentrations as low as 0.2 mM.Interestingly, 22b showed remarkable polymorphism in theirgel state leading to two dierent types of supramolecularaggregates, which dier in the stacking of chromophore moietyas well as in their hydrogen bonding patterns, as evident fromthe dierences in the UV/vis absorption and IR spectra. Thisnovel observation of polymorphism in organogels was found todepend on the solvent polarity and kinetic factors.Use of a liquid crystal (LC) as a medium to self-assemble

    azobenzene derivatives 23 and 24 (Chart 6) is a facile way ofpreparing an LC gel with aligned gelator molecules.184188

    Thus, the anisotropic LC physical gels can be prepared in theabsence of any external eect, by slowly cooling a nematic LC/23 mixture, cast as a thin lm on a glass surface or calciumuoride crystal windows from the isotropic phase.184 It hasbeen found that the LC environment has strong inuence onthe gelation properties of 24.185 By utilizing the light inducedstructural reorganization of 23 in the LC gel state, electricallyswitchable volume gratings can be prepared without usingpatterned electrodes.186 The photoluminescence intensity ofCdSe/ZnS QDs dispersed in a cholesteric LC can be switchedusing the gel network of 23 in presence of a covalently cross-linked polymer to congure the changes in the LC orientationalstate in response to an electric eld.187 In addition, using the

    gelator 24 and a ferroelectric LC, gels showing interestingelectrooptical properties could be designed.188

    Ikeda, Kato and co-workers have reported a chiralazobenzene 25 (Chart 6) containing a cyclic syn-carbonatemoiety that self-assembles via dipoledipole interactions toform a photoswitchable organogel.189 They have alsodemonstrated photoresponsive properties of anisotropicphysical gels (Figure 6) composed of 26 (Chart 6) and aroom temperature nematic LC 4-cyano-4-pentyl-biphenyl.190The photoinduced gelsol transition of the physical gel uponUV irradiation at room temperature leads to the transcisphotoisomerization which induces the transition from nematicLC gel (Figure 6b) to a cholesteric sol phase (Figure 6c).Subsequent visible light irradiation or keeping the sol at roomtemperature causes cistrans photoisomerization of theazobenzene moiety leading to regelation. During this process,the cholesteric molecular alignment in the sol phase (Figure 6c)behaves as a template for the aggregation of gelators intocholesteric LC gel (Figure 6d). The polarity change of theazobenzenes in 26 should be a key factor to the induction ofphoto stimulated ono switching of the hydrogen bondingwhich eventually led to the gelsol transition.191,192 Lightinduced complex structural changes were also reported for LCgels consisting of 26 and a discotic LC 2,3,6,7,10,11-hexahexyloxytriphenylene.191

    Another series of azobenzene based photoresponsiveorganogelators functionalized with semicarbazide groups27ae (Chart 6) as hydrogen bonding motifs have beenreported by Zentel and co-workers.193195 Compounds 27cand 27e were found to gelate smectic phase of a chiral smecticC* LC material and stabilize the LC director pattern presentduring the gel formation upon application of a DC electriceld.194 The photoinduced gel to sol transition of the LCphysical gel of 27c and 27e could be monitored by usingmagnetic polymer colloids entrapped in the gel bers. Uponirradiation, the magnetic polymer colloids start to move andalign in the direction of the external magnet, indicating collapseof the gel nanostructures.195

    The azobenzene appended melamine (28) and thecomplementary H-bonding barbiturate (B1) or cyanurate(C1) derivatives (Figure 7) have been exploited for thepreparation of photoresponsive rosette assemblies.196 Inaliphatic solvents, a rosette possessing the sterically bulkytridodecyloxyphenyl substituent in the barbiturate componentB1 does not hierarchically organize into higher order columnaraggregates. However, the sterically nondemanding N-dodecyl-cyanurate C1 results hierarchically organized elongatedcolumnar brous aggregates in cyclohexane, which eventuallyleads to the formation of an organogel (Figure 7). Dynamiclight scattering and UV/vis studies revealed that thedissociation and the reformation of columnar aggregatescould be controlled by the transcis isomerization of theazobenzene moiety. The molecular modeling study indicatesthat the rosette possessing azobenzene side chains loses itsplanarity upon irradiation resulting in the disruption of theaggregates and hence the dissociation of the organogel.Due to tight molecular packing in the supramolecular

    polymeric gel bers of the coassembled azobenzene tetheredmelamine dimer 29 (Chart 7) and cyanurate/barbiturates (C1/B1 and B2; Figure 7 and Chart 7), the azobenzene moiety inthe gel state showed resistance to photoisomerization.197

    Therefore, this system loses the photoresponsive character ofthe assembly. On the other hand, the azobenzene-function-

    Figure 4. (a) Structure of a boronic acid appended poly(L-lysine) usedas an additive to the gelator 16b. TEM images of 16b (b) alone and (cand d) with increasing concentration of boronic acid appended poly(L-lysine). (Reprinted with permission from ref 179. Copyright 2002 TheRoyal Society of Chemistry.)

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  • alized diaminopyrimidinone derivatives 30a and 30b (Chart 7)hierarchically organize into lamellar superstructures to formorganogels in nonpolar media, which undergo photoinduceddisruption and reformation as demonstrated by the photo-chemically reversible solgel transition.198 Irradiation with UV

    light (350 nm) slowly dissolves the macroscopic aggregates,

    resulting in the collapse of the gel into soluble supramolecular

    tapes. A zinc complex prepared from the cyclen (1,4,7,10-

    tetraazacyclododecane) based ligand 31 functionalized with

    Figure 5. Model for the incorporation of (R)-21 in (a) an aggregate of (S)-20 with opposite conguration and (b) an aggregate of (R)-20 with thesame conguration, showing the distinct environments for the azobenzene groups. (Reprinted with permission from ref 182. Copyright 2001 Wiley-VCH.)

    Chart 6

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  • 1,3,5-triazine featuring azobenzene with long alkyl chains(Chart 7) is reported to form organogels.199

    Amino acids and peptides are versatile building blocks tocreate functional helical assemblies and supramolecular gelswhen integrated with an appropriate chromophore.81,82,84,90,93

    For example, the gelators based on p-nitro-azobenzene-coupledbis-alanines 32ac (Chart 8)200 and the glycine (33e35e), L-alanine (33d35d), L-leucine (33c35c), L-valine (33b35b),and L-isoleucine (33a35a) functionalized azobenzenes (Chart8) form gels in many solvents including oils.201 The 4-substituted benzoic acid derivatives 34a and 34b and thedisubstituted azobenzene derivatives 35a and 35b showedbetter gelation abilities than the monosubstituted azobenzenederivative 33a and 33b.The photoresponsive gel formed by 36 (Chart 9) is the rst

    example of an enzymatic reaction catalyzed generation ofphotoresponsive hydrogelator.202 A large number of aminoacids based photoresponsive gelators 3739 (Chart 9) havebeen reported which upon transcis photoisomerization inducebreaking and forming of hydrogen bonds resulting in gelsol

    transition.203206 Zhang and co-workers have conductedcomprehensive studies on the gelation properties of a largevariety of short peptide functionalized azobenzene derivatives38ad (Chart 9).205 These studies revealed that gelation abilitycan be modulated by the amino acid residues, pH, and by salteect. The presence of aromatic amino acids such asphenylalanine and tyrosine was found to favor gelation ofwater at an appropriate pH range whereas cationic amino acidresidues such as arginine and lysine showed adverse eect. Thedierent strength of intermolecular interactions in the gelmatrix was found to inuence the sensitivity of the hydrogeltoward light. The potential of the hydrogel matrix for biologicalapplication is demonstrated by the controlled photoresponsiverelease of vitamin B12. Huang and co-workers have reportedthe salt promoted hydrogelation of the dipeptide functionalizedazobenzene amphiphiles 39a,b (Chart 9).206 Upon UVirradiation, the laminated ribbon-like morphology of the gelnanostructures was found to undergo a morphologicaltransition to short bers, which revert to nanoribbons byvisible light exposure.

    Figure 6. POM pictures and the respective schematic illustration of photoinduced structural changes in LC physical gels consisting of 4-cyano-4-pentyl-biphenyl containing 3 wt % of 26: (a) isotropic liquid state at 120 C; (b) nematic gel state at room temperature; (c) cholesteric LC phase(LC sol state) at room temperature after UV irradiation of the nematic gel for 15 min; (d) cholesteric gel state at room temperature after keeping thecholesteric LC phase. (Reprinted with permission from ref 190. Copyright 2003 Wiley-VCH.)

    Figure 7. Schematic representation showing aggregation of the melamineazobenzene conjugate (28) with barbiturate (B1) and cyanurate (C1)derivatives. Rosette 283C13 can hierarchically organize into intertwined bers (red arrows). The ber formation can be regulated by light (bluearrows). (Reprinted with permission from ref 196. Copyright 2005 American Chemical Society.)

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  • The chiroptical properties of a coassembled gel formed froma mixture of an azobenzene containing lipid molecule 40a and asimple L-glutamate derivative 40b (Figure 8) is reported byIhara and co-workers.207 The gelator 41 showed a positive CDspectrum in polar solvents while it is inverted to a negative onein nonpolar solvents (Figure 8).208 This is rationalized in termsof the dierence in interchromophoric interaction under therespective conditions. Detailed chiroptical studies and semi-empirical quantum mechanical calculations revealed that thegelator self-assembled into H-type aggregates with stronginterchromophoric interaction in polar solvents. In nonpolarsolvents also, the gelator forms H-type aggregates but with

    weak interchromophoric interactions and strong hydrogenbonding interactions by an interdigitated stacking mode. Thegelation is found to be responsive to temperature, photo-irradiation and polarity of the solvents (Figure 8). Thechiroptical switching properties of a gel with nanotubularmoropology formed by the coassembly of a lipid gelator and anazobenzene derivative is also reported in the literature.209

    Lee et al. reported the gelation properties of small peptideswith laterally grafted azobenzene. The photoresponsive hydro-gels formed by 42a and 42c (Chart 10) showed morphologicalswitching from long nanobers to discrete spherical aggregateswhereas the gel of 42b remained unchanged upon photo-

    Chart 7

    Chart 8

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  • irradiation.210 The gel matrix of 42a has been tested for thecontrolled encapsulation and release of dye molecules such asrhodamine B. Several photoreversible dendritic organogelators

    such as 4347 (Chart 10 and 11) are known in theliterature.211216 A poly(benzyl ether) dendritic supergelator43 (Chart 10) containing azobenzene moiety exhibited multi

    Chart 9

    Figure 8. Illustration of the multichannel supramolecular chiroptical switches composed by 41. (Reprinted with permission from ref 208. Copyright2011 American Chemical Society.)

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  • stimuli responsive gelation.211 Microsphere morphology in thesolution was changed to nanobers, by sonication whichtriggered the gelation processes.Pauli and Banerjee have reported uorescent organogels

    from self-assembly of an azobenzene molecule appended withfour amino acid (isoleucine) residues 45a (Chart 11). Thestructurally analogous 45b,c (Chart 11) failed to gelate any ofthe solvents tried suggesting the crucial role of amino acid sidechain in the self-assembling process.213 Kim and co-workershave reported self-assembly and gelation of an asymmetric bis-dendritic gelator consisting of an azobenzene dendron and analiphatic amide dendron 46 (Chart 11).214 Sterically bulkyazobenzene groups at the periphery of the dendron allowed arapid and reversible gelsol transition by light throughdisruption of hydrogen bonds between the aromatic andaliphatic secondary amides in the molecule. Hydrogel formedfrom the tetrameric sugar derivative 47 (Chart 11) showedresistance toward photoinduced isomerization due to the strongpacking of the molecules assisted by stacking andhydrogen bonding.215

    Yi, Huang and co-workers have reported the gelationproperty of the tripodal gelators 48 and 49, functionalizedwith azobenzene bearing long alkyl chains (Chart 12).216,217

    The gelator 48 is capable of forming anisotropic bilayers whichare hydrophilic along the plane but hydrophobic near therim.216 The xerogels formed by 48 displayed morphologies andsurface properties that are strongly depend on the nature of thegelling solvent. A cabbage-like topography and super hydro-phobicity were observed in the xerogel formed from a low polararomatic solvent such as xylene. The wettability of the xerogelof 48 could be turned from hydrophobicity to hydrophilicity byapplying a solgel process with dierent solvents. The C3-symmetrical photoresponsive trisurea compounds connectedwith three azobenzene moieties through exible alkyl spacers(50a or 50b) when mixed with another trisamide gelator (TG)formed a stable gel in 1,4-dioxane (Chart 12).218 Since theazobenzene moiety is present at the rim of the packing, thetwo-component gel exhibits reversible photoisomerization fromtranscis without the breakage of the gel state.

    Chart 10

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  • Some of the interesting photoresponsive gelators are shownin Chart 13. The gelator 51 (Chart 13) is based on a twistedintramolecular charge transfer probe 9-(9-acridyl)carbazoleand an azobenzene unit that shows photoinduced reversibleuorescence wavelength shift between the gel and the solstate.219 Rebek and co-workers have demonstrated thephotoresponsive properties of organogels prepared from anazobenzene appended glycoluril derivate 52 (Chart 13).220 Thegelsol transition of 52 occurred in the presence of UV light,uoride ions or a suitable guest such as n-tetradecane. Thephotoresponsive supramolecular gel prepared from a 1:1hydrogen bonded complex of an azobenzene appended 2,6-

    bis(benzoxazol-2-yl)pyridine ligand 53 (Chart 13) and apolymeric secondary dialkyl ammonium salt exhibited increasein the uidity upon illumination due to transcis isomerizationdriven gelsol phase transition.221 A rotaxane composed ofazobenzene chromophore and -CD dispersed in anamphoteric thermoreversible hydrosol gels showed uorescentbinary signals when compared to that in solution state.222

    Tuning of amphiphilic properties of 54 (Chart 13) bycomplexation with -CD was found to enhance its gelationability which can be reversibly modulated by photoirradia-tion.223 Photoinduced ber to vesicle morphological transitionwas exhibited by an organogelator bearing azobenzene and

    Chart 11

    Chart 12

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  • hydrazide groups 55 (Chart 13).224 A lipophilic azobenzenefunctionalized 2,2,4-triazole Fe(II) complex reported byKimizuka and co-workers formed molecular wires and gels inorganic solvents.225 Organogels formed by primary ammoniumdicarboxylate salts of azobenzene-4,4-dicarboxylic acid andprimary alkyl amines exhibited reverse-thermal gelation.226 A1:3 molar ratio mixture of 12-hydroxystearic acid and anazobenzene amine derivative was reported to show lighttriggred gel to sol transition in toluene and sol to gel transitionin chloroform, which is a very rare observation for a givenphotoresponsive system.227

    Apart from the above-mentioned systems, a large variety ofazobenzene containing polymers have been reported to showgelation behavior under suitable experimental condi-tions.228243 Most of these systems show interesting reversibleproperty change upon UVvis light irradiation. An interestingcase is the photoinduced anisotropic bending and unbendingbehavior of a LC gel prepared by the polymerization ofmixtures containing azobenzene monomers and cross-linkerswith azobenzene moieties, reported by Ikeda et al.230232 Thedierence in the association constants of trans and cisazobenzene inclusion complex with cyclodextrin has beenexploited to the design of a variety of smart gels havingreversible gelsol property.234241 The viscosity of a gelprepared from a mixture of hydrophobically modied PAA anda cationic photosensitive surfactant, azobenzene trimethylam-monium bromide, can be controlled reversibly by UV andvisible light irradiation.242 In response to the photoswitching ofazobenzene moiety, an azobenzene containing heteropolymer

    gel in an ionic liquid exhibited thermally reversible volumephase transition.243

    2.2. Stilbenes

    Stilbenes are recognized as one of the thoroughly studiedphotochemical systems.146,244 Upon direct excitation ofstilbenes, photoisomerization is conned entirely to the singletexcited state and competes with uorescence on the trans sideand photocyclization on the cis side. Stilbenes in the cis-formare not thermally stable and return to the trans-form in thedark. For most of the stilbene derivatives, the thermal activationbarrier is around 154 kJ mol1. Owing to their photoresponsiveconformational changes, stilbenes are suitable moieties for thedesign of smart gelators. However, the thermal backisomerization is the limitation of stilbene based systemswhich does not allow a photochemical control on theproperties. Despite this limitation, the photochemical proper-ties and self-assembly of stilbenes have been the subjects ofnumerous studies.The stilbene appended cholesterol derivative 56 (Figure 9),

    reported by Whitten and co-workers, self-assembles to formorganogels in appropriate solvents.245 Time dependentevolution of the gel nanostructures of 56 has been studiedusing AFM technique, in order to get a direct observation onthe role of solvent during the gel formation (Figure 9).246 Thegel formation was initiated by a change in the gelatorsolventinteraction, resulting in dewetting of the sol phase from a solidsupport. Time transient AFM images of 56 in 1-octanol onHOPG revealed that at rst the solution is dewetted from thesurface, forming circular and elongated droplets (Figure 9b),

    Chart 13

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  • followed by the formation of ne bers (Figure 9c), withaverage length of 250 5 nm and width as thin as 6.5 0.5nm. Further, the growth process was continued through thecombination of the neighboring ne bers into thicker ones(Figure 9d). Detailed 1H NMR investigation on thecomposition, structure and dynamics of the stilbene-cholesterolgels were also reported by the same research group.133

    Eastoe et al. have made a photoresponsive organogel systemby reuxing stilbene-containing photosurfactant 57 (Figure 10),and N,N-dimethyldodecylamine in toluene-d8 for several hoursfollowed by cooling to room temperature.247 Upon exposure toUV light, this gel transformed to sol phase with a spatial controland has been demonstrated within the sample as showed inFigure 10.

    As in the case of azobenzene derivatives, gelation ability ofstilbene derivatives becomes weak upon photoisomerization.The cis stilbene derivatives, 58a-d (Chart 14) showed poor

    gelation ability when compared to the corresponding trans-stilbenes 59a-f (Chart 14).248,249 However, a concentratedsolution of the cis isomer turns into a gel upon exposure toirradiation. Detailed studies revealed that, apart from thehydrogen bonding between the oxalyl amide units, thelipophilic interactions between long alkyl chains and the -stacking between the trans-stilbene units are responsible for thegelation.The polycondensation of silsesquioxanes in the organogel

    phase of the diureido stilbene derivative bearing triethoxysilaneend functional group 60 (Chart 15) allowed the transcriptionand xation of the gel nanostructures stabilized by weakintermolecular interactions to a rigid hybrid network frozen bycovalent siloxane linkages.250 Hydrogen bonded complexes ofL-tartaric acid and alkoxyl substituted stilbazoles 61ac (Chart15) form organogels with strongly enhanced uorescenceproperty.251 Similarly, a H-bonded complex of 3-cholesteryl 4-(trans-2-(4-pyridinyl)vinyl)phenyl succinate and 3-cholestery-loxycarbonylpropanoic acid 62 (Chart 15) showed veryecient gelation as well as thermotropic LC properties.252

    AIEE is an interesting property of several modied stilbenederivatives.253 A series of such organogelators 63-65 (Chart 16)has been reported by Park and co-workers.254260 The gelationof 63a and 63b is attributed to the cooperative eect of the -stacking interactions of the rigid rod-like aromatic segmentsand the intermolecular interactions induced by the four CF3units.254257 Organogels formed from 63b was found toundergo proton induced gelsol phase transition with theconcomitant uorescence intensity modulations.256 The brous

    Figure 9. Time transient AFM images (in amplitude mode) of solgelphase transition of the gelators 56. Images were acquired after theheated sol phase was cooled to room temperature for (a) 0, (b) 10, (c)21, and (d) 31 min. The scale of all images is 12 12 m. (Reprintedwith permission from ref 246. Copyright 2000 American ChemicalSociety.)

    Figure 10. Photoinduced gel-to-sol transition of 57N,N-dimethyldodecylamine organogel in toluene-d8: (a) initial gel state, (b) after irradiation,and (c) after irradiation through a mask. (Reprinted with permission from ref 247. Copyright 2004 The Royal Society of Chemistry.)

    Chart 14

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  • self-assembly of 63a and 63b is highly emissive when comparedto that of the isotropic solution (Figure 11).254256 AIEE in thegel state is attributed to the synergetic eect of intramolecularplanarization and J-type aggregate formation (restricted excimerformation). The photoluminescence, arising from the brousaggregates of 63a can be repeatedly switched by an electric

    eld.257 Modulation of the excitation wavelength throughelectric eld induced LC orientation oers new ways to achieveand explore electrically controllable photoluminescence.Aggregation of 63a conned in the LC droplets resulted incircular and highly folded nanoscale bers, in contrast to theextended ones formed in the bulk LC gel.258 This connementwas found to inuence the switching times and contrast ofsingle LC gel droplets which is largely determined by thedroplet size. The organogel of the hydrogen bonded complex64 (Chart 16), formed from the nonuorescent 2-(3,5-bis-triuoromethyl-biphenyl-4-yl)-3-(4-pyridin-4-yl-phenyl)-acrylo-nitrile monomer and 3,5-bistriuoromethyl benzoic acid, wasfound highly transparent and uorescent.260 The complexationof the monomer 65 with a chiral dicarboxylic acid L- or D-tartaric acid (Chart 16) also resulted in a stable gel,accompanied by a drastic uorescence enhancement as wellas chirality induction.260

    The D--A type organogelators 66a and 66b (Chart 17)exhibited AIEE during the solgel phase transformation.261Interestingly, 66b showed solvent dependent gelation andemission properties, such as yellow uorescence from THFwater gel, orange emission from acetone gel and reduorescence from DMSO gel. A gel like material obtained

    Chart 15

    Chart 16

    Figure 11. (a) Photographs of gel and sol states of 63b; inset showsthe uorescence emission of the sol and gel states of 63b. (b)Fluorescence spectra of 63b in the sol and gel states. (Reprinted withpermission from ref 256. Copyright 2008 American Chemical Society.)

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  • from the self-assembly of the stilbene functionalized binaphthylderivative 67 (Chart 17) has shown enhanced sensitivitytoward the detection of DNT and TNT vapors in comparisonto the solution state.262 Compound 68 (Chart 17) is anexample of multistimuli responsive organogelator whichshowed ultrasound induced gelationas well as uoride andproton controlled reversible gel to sol transition.263 Thestilbene-cored gelator 69 (Chart 17) having benzophenoneperipheries with alkyl chains self-assembles to form a gel inmethylcyclohexane.264 A photoresponsive hydrogel has beenreported from a cucurbit[7]uril derivative in the presence of a

    small amount of 4,4-diaminostilbene dihydrochloride as aguest.265 The cyanostilbene derivative switches the uorescenceintensity in the gel matrix through AIEE, enabled thequantitative determination of enantiomer composition.266

    2.3. Butadienes

    The DA substituted amphiphilic butadienes 70ad undergoconcentration dependent hierarchical self-assembly fromvesicles to gels due to the formation of globular aggregateswith entrapped solvent within them (Figure 12).146,267 Theself-assembly process was associated with unique changes in the

    Chart 17

    Figure 12. Chemical structure of 70 and schematic representation of the hierarchical organization of 70d from small to large vesicles and nally togels. (Reprinted with permission from ref 267. Copyright 2006 Wiley-VCH.)

    Chart 18

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  • uorescence of the system.267 At room temperature, thesolution of 70d exhibited a weak uorescence at 540 nm with avery low quantum yield whereas upon aggregation at a lowertemperature of 7 C, nearly 40-fold enhancement inuorescence accompanied by a shift in the maximum to 602nm was observed. This AIEE eect can be attributed to theformation of H-type aggregates, which involves a parallelinteraction mode of the chromophores. Apart from thisobservation, the presence of a photoisomerizable butadienechromophore makes these materials photoresponsive.Functionalization of the butadiene core with a cholesterol

    moiety (Chart 18) resulted in the gelation with interestingmorphological features.146,268 The correlation of the morphol-ogy with absorption and emission spectral studies as a functionof concentration and temperature revealed that the mono-cholesterol derivatives 71a,b form J-type aggregates and thebischolesterol derivatives 72a,b form H-type aggregates.268 Theslipped stacking of the monocholesterol J-type aggregateresulted in the formation of helically twisted bers, whereasthe cofacial arrangement in the bischolesterol H-type aggregateresulted in agglomerated spheres.

    2.4. Dithienylethenes

    Dithienylethenes are a distinct class of photochromic systemthat can undergo a reversible ring-closure reaction upon irradi-ation with UV and visible light, respectively.142,150,151,269273Apronounced change of the electronic properties and conforma-tional exibility occurs by this transformation. In the openform, the two thienyl moieties are not conjugated and canrotate around the bond connecting them with the cyclopentenering. However, in the ring-closed form, the conjugation extendsthroughout the molecule and the rotational freedom is lost.The research groups of van Esch and Feringa have conducted

    extensive studies on photoresponsive as well as chiropticalproperties of organogels derived from dithienylethene basedmolecules 73 and 74 (Chart 19).274279 For instance, they have

    demonstrated the reversible optical transcription of supra-molecular chirality into molecular chirality using an organo-gelator 73a.275,279 The dithienylethene molecule 73a exists intwo antiparallel interconvertible open forms with P- and M-helicity, which cyclize in a fully reversible manner uponirradiation with UV light to form two diastereomers of the ringclosed product of 73a. The open form 73a is found to be anecient organogelator of nonpolar solvents. CD and TEMstudies of the toluene gels revealed that the molecule 73a self-assemble to helical bers, owing to the expression of the M- orP-chirality of the open form at the supramolecular level.The photocyclization of 73a in the gel state showed nearly

    absolute stereocontrol though there is no stereoselectivity inthe nongelled solution. Both the open and closed forms of 73aexist in two dierent chiral gel states, denoted as and ,leading to a four-state chiroptical supramolecular switch, sincethese four states can be cycled by a sequence of photochemicalreactions as shown in Figure 13. A stable gel of ()-73a (P-helicity) is obtained on cooling the isotropic solution of theopen form 73a. Photocyclization results in the formation of themetastable gel ()-73a, which convert into the thermodynami-cally stable gel ()-73a (M-helicity) upon the heatingcoolingcycles. Irradiation of the gel ()-73a closed form with visible

    Chart 19

    Figure 13. Four dierent chiral aggregated states and the switchingprocesses of the chiroptical supramolecular switch consisting of theopen form 73a and the closed form of 73a. UV, = 313 nm; visible(Vis), > 460 nm; * = cooling; = heating.

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  • light results in the metastable gel ()-73a open form, whichthen change into the original stable gel ()-73a open form viaan isotropic solution of 73a, after a heatingcooling cycle. Theoptical switching between dierent supramolecular chiralaggregates and the interplay of molecular and supramolecularchirality in these systems is attractive for designing molecularmemory systems and smart functional materials. Later, bymanipulating the delicate balance between the solution and self-

    assembling properties of these molecules, dynamic andreversible pattern formation was realized.276

    The coassembly of chiral and achiral gel formingdithienylethene molecules results in induction and amplica-tion of chirality via sergeant and soldier principle.278281

    During the aggregation, the chiral molecules (73ac,sergeants) select only one of the photoactive conformers andcontrolled the stereochemistry of the achiral molecules (73dj,

    Chart 20

    Chart 21

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  • soldiers).278,279 The supramolecular chirality of the assemblywas found to be essential in transferring the chirality of thechiral sergeant molecules to the achiral soldiers. Apart fromthis, the solvents used for the coassembly and gelation werealso found to play a crucial role in transferring thesupramolecular chirality and decide whether four- or two-gelstates are accessible.Dithienylethenes are found in a variety of -gelators 7577

    (Chart 20) which show reversible photoinduced changes in thegelation and electronic properties.282285 For example, Tianand co-workers have reported a photochromic uorescentorganogel based on bisthienylethene-bridged naphthalimides75.282 The presence of naphthalimides on the photochromicunit enables the photoswitching of uorescence with highcontrast whereas the cholesterol group leads to a chiralassembly of the chromophores. The dithienylethene having the2,1,3-benzothiadiazole unit as the center ethene bridge showsgood photochromic performance, with a high cyclizationquantum yield and moderate fatigue resistance in an organogelmedia.283

    2.5. Spiropyrans and Spirooxazines

    The photochromic reaction of spiropyrans and closely relatedspirooxazines involves reversible photochemical cleavage of theCO bond of the spiro unit.139,150,151,286288 Because of theirinteresting photochromic properties, spiroheterocyclic com-pounds have been investigated for the design of opticalmemories, switches, and displays.286288The rst report aboutthe spiropyran based organogel system is reported byHachisako et al.289 They have determined the criticalaggregation concentrations of organogelators by following thekinetics of the thermal merocyanine-spiropyran isomerizationof L-glutamic acid-derived lipids with spiropyran head groups,78ac (Chart 21). Similar observations are also made whenspiropyran doped into organogels formed from L-glutamic acid-derived lipids.290

    The spiropyran linked dipeptide molecule 79 (Chart 21) isreported to form a hydrogel upon photoisomerization into amerocyanine form.291 In addition, the hydrogel is found toundergo gel to sol transition upon interaction with vancomycindue to the strong interaction of the later with peptide unit ofthe gelator thereby weakening the supramolecular interactionsresponsible for the gelation. The spiropyran derived dendronmolecule 80 (Chart 21) shows dierent self-assemblingproperties depending upon the processing conditions.292 Forinstance, cooling of a hot solution of 80 to 30 C followed byUV irradiation to the open merocyanine leads to the formationof nano/microsized particles, while cooling into 0 C results inbrous morphology and gel. Visible light irradiation of the gelcaused merocyanine-spiropyran isomerization and gel-to-soltransition. Apart from this, aggregation of 80 was found toenhance the uorescence property of the photomerocyanine byminimizing the nonradiative decay pathways of the excitedstate.Doping of spiropyran into an organogel system based on 4-

    tert-butyl-1-phenylcyclohexanol was found to increase thelifetime of the photomerocyanine and also stabilize the organicphotochromic material.293 Recently, Raghavan and co-workershave reported photorheological properties of a gel preparedfrom a lecithin/sodium deoxycholate reverse micelles dopedwith spiropyran.294 The gel consisting of long worm like reversemicelles changed its dimension upon UV induced isomerizationof spiropyran to merocyanine and caused about 10-fold

    decrease in the viscosity. When the UV irradiation wasswitched o, merocyanine was reverted back to spiropyranform, and the viscosity recovered its initial value. This cycle canbe repeated several times without loss of response.Spironaphthoxazines appended to gallic acid 81a,b and

    cholesterol 82 form photoresponsive gels (Chart 22).295 In the

    presence of p-toluenesulfonic acid, the gelation ability of thesemolecules is further enhanced owing to an acid mediated ring-opening of the photochromic moiety. Kinetic studies revealedthat the rate of bleaching of the open to closed form is muchslower in the gel state when compared to that in the solution.2.6. 2H-Chromenes

    2H-Chromenes are known to display interesting photochromicproperties based on their photoirradiation induced fatigueresistant reversible color change.151,296 The closed form iscolorless, whereas its photoisomerized open form is colored.The latter is thermally unstable and reverts back to its originalform by an electrocyclization process. Pozzo and co-workersreported light and pH sensitive properties of organogels of 2H-chromene derivative 83 (Chart 23).297 The neutral carboxylicacid form was found to be readily soluble in polar organicsolvents which upon addition of NaOH turned to gel.Furthermore, on irradiation at 366 nm, a yellow color wasdeveloped and the gel started to ow upon inversion. In thedark, a colorless viscous solution was formed, which on heatingfollowed by cooling regenerated the original gel. Thesetransitions were caused by the photoinduced ring-opening ofthe colorless cyclic form to the colored acyclic form, whichpartially disrupts the gel structure due to its incompatibilitywith the network. The acyclic form is, however, thermallyunstable and returns to the cyclic form upon heating andcooling, resulting in the formation of a gel. A series of 2H-chromene derivatives functionalized with N-acyl-1,-aminoacids 84ai (Chart 23) and dipeptide 84j (Chart 23) alsoreported to form gels in presence of sodium salts.298

    2.7. Diacetylenes

    In the recent past, there has been considerable interest in theself-assembly of diacetylene derivatives as they allow thecovalent xation of supramolecular assemblies through photo-polymerization reactions to give polydiacetylenes, which areattractive candidates as conducting nanowires.143,299302 Gelforming diacetylenes are particularly interesting because the gelnetwork which is stabilized by the noncovalent interactions canbe permanently supported by strong covalent bonds under

    Chart 22

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  • photolytic conditions, thereby retaining the morphologicalcharacteristics with increased thermal stability.143,299302

    The diacetylene-1-glucosamide derivatives 85 and 86 (Chart24) have been reported to form nanobres upon UV irradiationleading to the gelation of organic solvents.303305 UV or -raypolymerization of the nanobers of 86 by utilizing gel state as atemplate resulted in the formation of insoluble bers. UVirradiation of the hydrogel derived helical ribbons of 87 (Chart24) at 254 nm resulted in the polymerization of self-assembledgelators leading to a red coloration.306 Stimuli responsivepolydiacetylenes were prepared by the cross-linking of thediacetylene-containing glycolipids 8890 (Chart 24).307,308The diacetylene amide derivatives 9193 (Figure 14) form gelswhich upon UV irradiation undergo polymerization.309311

    Interesingly, the gelators 93af, exhibited oddeven eect on

    the color of the polymer obtained.310 The oddeven eect inthe carbon atoms of the aliphatic chains controls the planarityof the polymers and hence the color of the odd chain gels is redand that of the even chain is blue (Figure 14). In addition, theresulted gel derived polymers showed electric eld emissioncharacteristics.The diacetylene dicholesteryl ester derivative 94j (Chart 25)

    undergo photopolymerization in the organogel state resultingin nanowires.312 TEM images of the unstained specimensrevealed that the bril structure formed in the gel did not showany change even after polymerization. A series of diacetylenecholesteryl esters 94am, and 95a,b (Chart 25) having twourethane linkages have been reported and the relationshipbetween their gelation properties and chemical structures hasbeen established.313 Most of these gelators could be

    Chart 23

    Chart 24

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  • polymerized to give polydiacetylenes upon UV irradiation, withconcomitant change in the absorption properties. Thecompounds with excellent gelation ability exhibited highpolymerization rate and yield, as in the case of the cyclohexanegel of 94f, with a polymerization yield of 52%.The diacetylene based gelators 96af (Figure 15) exhibit

    interesting oddeven eect in the color of the resultedphotopolymers.314 The cyclohexane gels of 96a, 96c, and 96e,with odd numbers of carbon atom spacers, showed remarkable

    color changes to either orange (96a) or blue-violet (96c and96e) upon photoirradiation, whereas in the case of gels witheven number of methylene units (96b, 96d, and 96f),photopolymerization was not observed. Gelation properties ofmacrocyclic dimers and linear derivatives of diacetylenecontaining L-glutamic acid units 97 and 98 (Figure 15) havealso been investigated.315 Macrocyclic compounds with a longspacer length 97be and 98be formed stable gels and henceundergone photoirradiation with a signicant color change

    Figure 14. (a) Time dependence (0180 min) of UVvis spectral change on photoirradiation of the n-hexane gels prepared from 93c (left) and93d (right). Insets show the photographs of the irradiated gels. (b) Photographs of n-hexane gels of 93c93f after photoirradiation for 3 h.(Reprinted with permission from ref 310. Copyright 2007 American Chemical Society.)

    Chart 25

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  • from colorless to red or orange. On the other hand, forcompounds with a relatively short spacer length 97a, 98a, and98f, no change in the visible color upon irradiation wasobserved. It indicates the inability to undergo photopolymeri-zation due to the slight dierence in the optimum arrangementof the diacetylene moieties for polymerization. Weiss et al. have

    reported the gelation and polymerization of 10,12-pentacosa-diynoic acid derived amides 99ac, ester 100 and the chiralamidoester 101 (Figure 15).316,317 Photopolymerization andsubsequent heating led to signicant changes in the absorptionspectrum and also change of the visible color from blue tored.316

    Figure 15. (a) Structures of dierent diacetylene gelators; (b) Photographs of the photoirradiated cyclohexane gels of 96af. (Reprinted withpermission from ref 314. Copyright 2004 American Chemical Society.)

    Figure 16. SEM images of self-assemblies of 104b (a) in the absence and (b) in the presence of an applied magnetic eld of 20 T. The arrowindicates the direction of the magnetic eld. The scale bar is 2 m. (Reprinted with permission from ref 320. Copyright 2007 Wiley-VCH.)

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  • There are several reports of gelators based on peptideamphiphiles containing photopolymerizable diacetylenes 102-114 (Figure 16 and Charts 26 and 27).318330 The presence ofamino acid units direct the self-assembly and gelation, enablingthe diacetylene moieties to ideally position for photo-polymerization. UV irradiation of gels of 103a and 103b(Figure 16) at 254 nm produced red colored gels with straightand tightly packed bers due to cross-linking.319 Studies on theber forming properties and polymerization characteristics oftwo peptide amphiphile based gelators 104a,b revealed that thepeptide sequence GANPNAAG allowed remarkably stable and

    twisted ribbons to be prepared which can be aligned with thehelp of an applied magnetic eld (Figure 16).320,321 The stable,aligned -sheet bers formed by the diyne containing peptideamphiphiles did not change the ber structure, even after thepolymerization.Stupp et al. have reported the preparation of supramolecular

    nanobers of 105a and 105b (Chart 26) having -sheetstructures.322 As a clear indication of the eect of molecularorder on the eciency of polymerization, 105b with branchedarchitectures exhibited less ecient polymerization. Thetopographical patterns obtained from peptide amphiphiles

    Chart 26

    Chart 27

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  • Chart 28

    Chart 29

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  • 105a and 105c (Chart 26) have been used to create cellenvironments with microtextures and well-dened nanoscalebioactivity.323 The photopolymerization of diacetylene macro-monomers 106 and 107 (Chart 27) consisting of hydrogenatedpoly(isoprene) and -sheet forming oligopeptide upon UVirradiation gave supramolecular polymers with a double-helicaltopology.302,324329 In these structures, the degree of order inthe parallel -sheets is governed by the number of hydrogenbonds at the end group that controls degree of diacetylenespolymerization. This is clear from the successful photo-polymerization of supramolecular polymers of macromonomers106 and 107 and organogels of amphiphilic diacetylene modelcompounds 110112 (Chart 27). However, gels obtained from108 and 109 failed to do so.328,329 The peptide-diacetylene-peptide molecules 113 and 114 (Chart 28) are shown to formaligned macroscopic hydrogels consisting of 1D nanostructuresin acidic water.330 The internal diacetylene unit of 113b can bephotopolymerized into polydiacetylenes that run parallel to thenanostructure long axis and the resulting polydiacetylenesexhibited ambipolar charge transport.Photopolymerization of nanotubules and ribbons of

    phospholipid diacetylene conjugate 115 (Chart 29) revealedthat nanotubules are spontaneously transformed to helicalribbons.331 In the case of the gelator 116 (Chart 29), helicalribbons were formed which are responsible for the 3D networkof the photopolymerized gel.332 Polymerization and stabiliza-tion of the supramolecular nanostructures formed by theurethane-amide dendrons 117 and 118 (Chart 29) withdiacetylenes moieties at the alkyl periphery have beenreported.333 Tris[4-(4-alkyloxyphenyl) butadiynylphenyl]-1,3,5-benzenetricarboxamides 119a,b (Chart 29) form stablegels in THF/cyclohexane mixture.334 However, the diacetylenicgroups in 119a exhibits poor photoreactivivity in the gel statedue to the inappropriate alignment for polymerization.The amphiphilic diacetylene 120 self-assembles into helical

    tapes or nanotubes (Figure 17a) with a bilayer structure.335

    Photopolymerization enabled the conversion of the nanotubesto helical tapes and at ribbons as indicated by the TEM images(Figure 17b).The heterobifunctional molecules 121ac (Figure 18) form

    gels in hexyl methacrylate as the gelling solvent.336,337 The

    photopolymerization of the acryloyl and diacetylene groups ofthe gelator resulted in nanobers with a cross-linked structurewhereas the simultaneous polymerization of hexyl methacrylateformed a polymer matrix. Interestingly, polymerizationoccurred in the bulk of the monomeric solvent, inside theself-assembled nanobers and at the interface between thenanobers and the solvent. Thus, free-standing lms of thepolymerized gels could be obtained as shown in Figure 18a.The nonuorescent sol state of 121a showed enhancedemission properties in the gel state due to AIEE pheneomenon(Figure 18b).High molecular weight polydiacetylenes (Mn = 27.9 kDa)

    with low polydispersity index (PI = 1.8) has been achieved ingood yield from a butadiyne derivative 122 (Chart 30) havingsterically demanding phenyl groups, using a light-promotedtopochemical reaction.338 A cholic acid derivative with adiacetylene functionality 123 (Chart 30) is found to be a goodgelator but failed to undergo photopolymerization in the gelstate.339 Photopolymerization of the diacetylene-urea-trialkox-ysilyl gelators 124a,b (Chart 30) gave thermochromic ureido-polydiacetylene gels.340 The multichromic assemblies could be

    Figure 17. (a) TEM (0.1% aqueous solution) (top) and cryo-TEM of the self-assembled nanotubes formed by 120 (0.05% aqueous solution)(bottom). (b) Corresponding cryo-TEM of polymerized nanotubes (0.05% aqueous solution). (Reprinted with permission from ref 335. Copyright2011 American Chemical Society.)

    Figure 18. (a) Photographs of the free-standing lm of thepolymerized gel of 121a and (b) photographs of vials containing asol in n-decane at 50 C (left), a gel in n-decane at room temperature(middle), and a polymerized gel (right) under 365 nm UV light.(Reprinted with permission from ref 337. Copyright 2008 AmericanChemical Society.)

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  • further stabilized by the covalent siloxane network throughpolycondensation of the trialkoxysilyl groups. This studyprovided a clear insight about the molecular mechanism ofthe blue to red color transition and of the reversibility of thepurple to red color transition as generally observed inpolydiacetylene thermochromism. The dehydrobenzoannulenes125 (Chart 30) with methyl ester groups was designed in sucha way to have boomerang shape with highly polarizedsubstituents in the periphery that could help to align themolecules anisotropically to form stable organogel.341 Theparallely aligned bers of 126 (Chart 30) bridging two goldelectrodes were obtained under an electric eld.342 Thenegative dielectric anisotropy of uorinated rod-shaped LCside groups of 126 is considered to assist the alignement ofbers. In situ photopolymerization enabled to get themacroscopic orientation of the aligned bers. Optically activepolydiacetylenes were prepared by UV-irradiation of thexerogel of the achiral diacetylene 10,12-pentacosadiynoic acidand a chiral LMOG N,N-9-bis(octadecyl)-Boc-glutamic dia-mide.343 Mechanistic pathway of the formation of helicalmicrotubules from an initial solution of nanoscale vesicles inreal time was investigated.344 An aqueous mixture of achiralcomponents such as single tailed diacetylenic surfactant, 10,12-pentacosadiynoic acid and a short chain alcohol, geraniolformed vesicles and gels which changed to microtubules withtime.

    2.8. Cinnamic Acid Derivatives

    A photoresponsive reversible gelsol transition has beenobserved in p-nitrocinnamate containing amino acid-baseddendron gelator 127 (Chart 31).345 A similar observation wasreported by Xu and co-workers using cinnamic acid function-alized amino acid derivative 128 (Chart 31).346 Raghavan andco-workers have investigated combinations of surfactants andcinnamic acid derivatives to tune the gelation properties.347349

    They have used combinations of cationic surfactant cetyltrimethylammonium bromide and o-methoxy-cinnamic acid,347

    zwitterionic surfactant erucyl dimethyl amidopropyl betaineand o-methoxy-cinnamic acid,348 and phospholipid lecithin andp-coumaric acid.349 Upon irradiation by UV light (

  • subunit disrupted supramolecular gel network of 133 and 134(Chart 32) and can be reverted back by using temperature,light, or pH as external stimuli.357

    Compounds 135 and 136 (Chart 33) are examples ofphotoresponsive gelators that exhibit photochromism uponirradiating with UV light.358,359 The cholesterol appendedsalicylideneaniline derivatives 137 and 138a (Chart 33)undergo isomerization of salicylideneaniline and ketoenoltautomerism under UV-light, accompanied by gelsol tran-sition.360,361 Interstingly, replacement of the cholesterol groupand ester linkage with 3,4,5-tridodecyloxybenzoic acid unit andamide linkage resulted in a gelator 138b (Chart 33) thatshowed thermochromic/nonphotochromic behavior in the self-assembled state.362 In adittion, these molecules showed AIEE inthe gel state due to J-aggregation and inhibition of nonradiative

    decay via intramolecular rotation.360362 In addition, photo-responsive gelators based on maleic and fumaric acids are alsoreported.363368

    3. FUNCTIONAL DYE DERIVED GELATORS

    Organic dyes are widely exploited in materials as well asbiological applications.369371 Since most of the dyes have astrong tendency to aggregate and to exhibit solvatochromicbehavior, self-assembly and gelation of dyes have generatedinterests among chemists. Synthetic modication of organicdyes with suitable functional groups has resulted in a largenumber of gelators exhibiting interesting optoelectronicproperties. In addition, there has been interest in dye dopedgels in which the encapsulated dyes undergo absorption andemission changes depending upon the nature of the gel. A largenumber of organic functional dyes have been exploited for thedesign of organogel that are discussed in this section.

    3.1. Merocyanines

    Merocyanines are intensely colored uorescent dyes with largeextinction coecients, dipole moments and polarizabilities.Due to the tunable optoelectronic properties, merocyanine dyeshave attracted much attention in photovoltaic devices(PVDs).372 In addition, merocyanines are used in nonlinearoptics and as photorefractive materials. Due to the strongtendency to form higher order aggregates, merocyanines areknown for their gelation properties when suitably function-alized.Wurthner and co-workers have reported the hierarchical self-

    organization of the merocyanine dye 139 (Figure 19) tosupramolecular polymers and organogels, through antiparallelH-type aggregate formation, thus minimizing the electrostaticenergy.373375 Based on the detailed optical as well asmorphological studies, a structural model for the gelation of139 has been suggested as depicted in Figure 19. This moleculeexists as monomeric units in polar solvents at lowconcentrations. Increasing concentration or changing thesolvent polarity leads to the formation of a well-dened helicalpolymer with a uniform pitch of about 5 nm. Six of thesepreorganized strands of the initially formed supramolecularhelical polymer further intertwined to form rods laminated with

    Chart 32

    Chart 33

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  • alkyl chains. In aliphatic solvents, increase in concentrationallowed alkyl chains at the periphery to entangle, resulting in anincrease in the viscoelastic and gelation properties and alyotropic mesophase formation. A self-complementary Hamil-ton receptor functionalized merocyanine dye 140 (Figure 19)self-assembles in a h-t fashion through six hydrogen bonds,exhibiting enhanced emission in the gel state.376

    Complementary hydrogen bonding interaction betweenexible bismelamine receptors BMn with dierent alkyl linkerlengths (n = 512) and the barbituric acid type merocyaninedyes 141143 (Chart 34) is a good way of creatingsupramolecular assemblies and gels.377379 The merocyaninedyes 141143 exhibit ecient gelation in the presence of anequimolar amount of bismelamine receptor BM12 in nonpolar

    organic solvents.377379 141.BM12 exhibited an entangledbrous network whereas 143.BM12 showed a 2D sheet-likestructure.378 By varying the alkyl linker lengths, columnarstructures with and without 2D ordering have been obtained.379

    In addition, hierarchical organization aorded phase separatedcrystalline nanobers for n = 5, 7 and soft nanobrils,agglomerating into worm-like objects for n = 8, gel-formingcontinuous globular networks for n = 10, and nanobers for n =11, 12 (Figure 20). Barbituric acid functionalized merocyaninedye 144 (Chart 34) forms vesicular aggregates (deformed tocoee-bean-like ellipsoidal morphology upon solvent evapo-ration) through J-type aggregates.380 The aggregation mode ischanged to H-type through dipolar interactions in presence ofthe barbituric acid receptor B1 (Chart 34) through the

    Figure 19. Chemical structures of 139 and 140. Structural models for the dierent levels of organization observed for bis(merocyanine) dye 139 independence on the solvent polarity and the concentration. (a) Monomer, (b) polymeric chain constituted by antiparallel pairing of merocyaninedyes, and (c) tubular ber consisting of H-type aggregate dyes. The arrow around the ber indicates the direction of H-aggregation with a closer anda more distant antiparallel dye neighbor and (d) hexagonal packing of the rods. In methylcyclohexane/THF mixtures, all processes can be controlledin a reversible manner at ambient temperature. (Reprinted with permission from ref 375. Copyright 2004 American Chemical Society.)

    Chart 34

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  • formation of hydrogen bonded complex, leading to gelationand ribbon-like aggregates.Aggregation and gelation properties of cholesterol appended

    merocyanines 145a,b (Chart 34) in various organic solventswith dramatically dierent chromophore packing and distinctoptical properties were reported by Yagai and co-workers.381

    Interestingly, morphological transition from kinetically formednanobers to thermodynamically stable globules with thedisappearance of excitonic interactions between the chromo-phoric units is also observed. -Aminoalkyloyl L-glutamic acidamphiphiles form chiral assemblies and induce chirality toachiral cyanine dye which is bonded through specic bindingsites.382

    3.2. Naphthalimides

    Naphthalene mono- and diimides have attracted muchattention due to their tendency to form n-type semiconductormaterials.383385 The naphthalimides belong to electrondecient class of aromatic compound and found to be versatiledue to easy functionalization through the imide nitrogen or viacore substitution.383385 Naphthalimides exhibit good electrontransport properties, and hence their self-assembly and gelation

    have signicance with respect to the creation of supramolecularstructures of dierent size and shape.A systematic investigation of the organogelation behavior of

    the amino acid functionalized naphthalene monoimides 146af(Chart 35) in a wide range of solvents to establish a

    relationship between the nature of the solvent and the gelformation has been reported.134,135,386389 The side chains of146a and 146b inuence the organization of the aggregates, insuch a way to form a J-type aggregate for 146b and an H-typefor 146a in toluene.387 IR and 1H NMR experiments enabled tounderstand the aggregation behavior of 146a,b and found thatthe preformed hydrogen bonded h-t stacks, resulting in self-assembled columns. Further, small aggregates and gels areformed through inter columnar stacking.388 The

    Figure 20. Schematic representation of the hierarchical organization of 141.BMn. (Reprinted with permission from ref 379. Copyright 2007American Chemical Society.)

    Chart 35

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  • importance of hydrogen bonding, Hansen parameter and

    Hildebrand parameter pertaining to the gelation was also

    investigated.389 The dynamical behavior of solvent in a gel

    phase of 146a and intermolecular structural features of gelator

    in the gel nanober have been investigated using NMR spin

    relaxation and diusion experiments.134,135

    Yi and co-workers have made signicant contribution in thearea of naphthalimide based organogelators.390399 Externalstimuli such as heat and ultrasound have been used to achieve athermodynamic balance between hydrogen bonding, hydro-phobic interactions, and interactions, to control thesolubility, gelation, and morphological features of a family ofasymmetric cholesterol containing naphthalimide based orga-

    Chart 36

    Figure 21. Schematic representation of the mechanism of formation of a regular pattern; (a) vesicle structure; (b) arrangement of the vesicles insolution, the red arrow indicates the direction of molecular movement with evaporation; formation of the porous pattern on (c) the rst layer and(d) multilayer. (Reprinted with permission from ref 394. Copyright 2010 The Royal Society of Chemistry.)

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  • nogelators 147150 (Chart 36) having dierent alkyl chainspacers.390392 Comparison of the gelation studies of 148 and149 indicate that the presence of a chiral center at the middleposition of the linker facilitate the gelation of the latter.392

    Moreover, upon sonication, an instantaneous gel-to-geltransition occurred with morphological change from micellestructure of 148 and the coreshell microsphere of 149 toentangled bers. The gelator 151b formed a vesicular structurewhereas the gelator 151a having a heteroatom showed aspherical morphology (Chart 36).393

    A self-assembly approach has been utilized to generate largearea honeycomb patterned lms of the gelator 152 (Figure 21),by casting the solution onto solid substrates followed byevaporation.394 As shown in Figure 21, a model was proposedto explain these results. At the beginning, quasi amphiphiliccharacter of the folded structure of 152 allowed the formationof vesicles in CH2Cl2, which was then deposited on the surfaceof a substrate (Figure 21a). As the solvent began to vaporize,these structures started to break around the surface of thesolvent due to the pressure dierence between the inside andoutside of the vesicle. With further evaporation of solvent, themolecules moved to the more diluted area and reaggregated(Figure 21b). On reaching the surface of the liquid to thebottom of the vesicles, the molecules moved from the upperpart of the vesicles and aggregated in the gap between adjacentvesicles to form a hexagonal grid-like pattern (Figure 21c). Inaddition, in the case of multilayer structure, cross bed patternwas also found (Figure 21d). These studies indicate that thepreorganization of deposited vesicles in a packed and orderedmanner is essential to obtain such a regular hexagonal pattern.A cross-linked ring structure with nanoscale to microscale

    pores has been obtained from thixotropic, self-healing organo-gel of 153 (Chart 37).395 The reversible rheological switching

    was observed with respect to alternate shaking and resting dueto ring disintegration and reconstitution of the gel network. Avariety of up conversion nanoparticle (UCNP)-organogelnanocomposites of cholic acid-based tripeptide containingnaphthalimide uorophore 154a,b (Chart 37) with dierentUCNPs, such as NaYF4:20 mol % Yb, 2% Er (UCNP-Red),NaYF4:25 mol % Yb, 2% Er (UCNP-Green), and NaYF4:20

    mol % Yb, 1% Tm (UCNP-Blue) have been reported.396 Thesehybrid gels show dierent colors under normal light and onexcitation with a near IR light.White