Composites of N,N′-bis-(pyridyl) urea-dicarboxylic acid as new hydrogelators—a crystal engineering approach

Adarsh, N. N.; Kumar, Deepak; Dastidar, Parthasarathi

doi:10.1016/j.tet.2007.02.005

Cited by 55 publications

(44 citation statements)

References 40 publications

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“…Related work by the Dastidar group has shown that co-gels 55 comprising the simple N,N'-di-(n-pyridyl) urea (n = 3 or 4) in conjunction with carboxylic acids also produces a range of composite materials, some of which form gels and others of which are crystalline. 48 This builds on earlier reports of the effective hydrogelation ability of single component gelators 60 containing both urea and carboxylic acid functionality. 49 The Dastidar group characterised a range of materials by single crystal X-ray diffraction which revealed a variety of supramolecular synthons of the types shown in Fig.…”

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confidence: 57%

“…48 We reasoned that hydrogen bonding to the pyridyl nitrogen atom or its protonation could facilitate urea tape formation and hence result in gel formation. While no urea -tape motifs were observed in the interesting range of X-ray crystal 5 structures reported by the Dastidar group, 48 it is possible that the crystalline structures are not fully representative of the gel phase material.50 Alternatively gelation by anion-mediated hydrogen bonded tape formation may be involved. 51 In this report we examine multi-component gel formation with extended bis(3-10 pyridyl urea)s. As single components these pyridyl ureas are poor gelators or non-gelators and hence co-gel formation with carboxylic acids offers the possibility of 'turn-on' gelation and other complex, emergent properties.…”

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Triggered formation of thixotropic hydrogels by balancing competitive supramolecular synthons

Liu

Steed

2013

Soft Matter

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Publisher's copyright statement: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Two component hydrogels have been obtained by formation of 1:1 complexes of bis(pyridyl urea)s with a range of dicarboxylic acids. The gels are thixotropic and undergo an assembly process in which short segments can reversibly assemble into an interconnected fibrous network. NMR and IR spectroscopic data suggest that the gelators form neutral gelator-acid complexes rather than salts. The use of dicarboxylic acids to trigger gelation in bis(pyridyl urea)s parallels analogous triggered gelation of metal 10 ions and halogen bond donors in related systems. IntroductionGels derived from low molecular weight gelators (LMWG) have experienced an explosion of interest in recent years. 14 The growing interest in their properties stems from their generally facile synthesis, their synthetic and structural versatility, and the possibility of adaptive or reversible gelation offered by the weak, dynamic supramolecular interactions holding the fibres together. 7 The 25 mechanism of the non-equilibrium self-assembly process involved in gel formation by LMWG is also fascinating from a fundamental viewpoint and is perhaps a more tractable problem in well-defined small molecules than in more conventional silica or biopolymer based hydrogels. Of particular current interest are 30 multicomponent gels. 25These systems may comprise stoichiometric co-gels in which two non-gelator components combine in a well-defined way to produce a gel-forming supermolecule, or they may be blends of LMWG (sometimes termed 'multi-gelator gels') that are individually gelators. 26-32 35 Some metallogels arising from metal cross-linking of gelating ligands, 4,6, 33 or anion influenced gels also fall into the broad category of multicomponent gels. [34][35][36] In previous work we have looked at triggered gelation in 'inhibited gelators', particularly pyridyl ureas. 37,38 Intramolecular CH O interactions coupled 40 with the good hydrogen bond acceptor ability of the pyridyl nitrogen atom make pyridyl ureas particularly poor gelators (Fig. 1b) 37, 39-41 because they cannot effectively form the typical urea -tape motif generally thought to be responsible for onedimensional fibre growth and hence gel formation (Fig. 1a). 42, 43 45 Addition of a co-gelator such as a metal ion 38,[44][45][46] or halogen bond donor 47 results in coordination to the pyridyl nitrogen atom hence freeing the urea functionality and switching the system from the urea-pyridyl hydrogen bonding motif to the ...

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Triggered formation of thixotropic hydrogels by balancing competitive supramolecular synthons

Liu

Steed

2013

Soft Matter

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“…More recently, halogenbonding has been demonstrated to be also useful in solution, for example in anion binding by preorganised tripodal anion hosts 43 . In our hypothesis, halogen-bonding should be of sufficient strength to 'tip the balance' and engender a switch from pyridyl urea hydrogenbonding to urea tape interactions even in competitive media 44,45 and result in multicomponent gels [46][47][48] . We now report the use of halogen-bonding to 'turn-on' gelation in bis(pyridyl urea) gelators 1 and 2 in the presence of 1,4-diiodotetrafluorobenzene 3, and halogen-bonding induced gelation in the halogen-bond donor gelator 4 when treated with either 4,4'-bipyridine 5, tetrabutylammonium iodide or pyridyl derivative 1 (Figure 2).…”

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Halogen-bonding-triggered supramolecular gel formation

et al. 2012

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The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Tunable gel phase materials with novel properties such as switchable rheology and controlled flow characteristics are an emerging topic of interest in potential applications as diverse as pharmaceutical crystallization, catalysis, drug delivery and controlled release, wound healing, and stabilization of drilling mud [1][2][3][4][5][6][7][8][9] . Within this context supramolecular low molecular weight gelators (LMWG), with their reversible and dynamic intermolecular interactions are achieving increasing prominence [10][11][12][13][14][15] . Work on switchable gels includes 2 systems involving photo-, pH, and redox based switching, ultrasound induced gelation, and switchable catalysis [16][17][18][19][20][21][22][23] . In order to systematically develop smart materials with controllable and well understood changes in bulk properties, an understanding of the intermolecular interactions in the system and the way in which they engage in hierarchical self-assembly in order to produce emergent morphologies with complex behavior is required. The link between even primary supramolecular interactions and bulk material properties is often unclear, however. Recent work has highlighted some simple approaches to controllable or smart materials that have met with considerable success. A key theme in particular is setting up a 'tipping point' in a system comprising competing intermolecular interactions which contribute to either gel assembly or gel dissolution. In this way a number of research groups have shown that anion binding in competition with urea self-assembly can be used to 'turn-down' and ultimately 'turn-off' gelation behavior 24,25 . Interestingly, in alternative systems, anion binding has also been used to induce gelation 26,27 . In a similar way metal coordination has also been used to tune gel properties 25 . In a series of bis(urea) gels we have shown how a combination of metal-and anion-binding can allow comprehensive control over the system. Competitive anion binding reduces gel strength by inhibiting urea -tape hydrogen-bond formation [28][29][30][31] .Conversely, metal coordination in pyridyl-urea compounds results in metal binding to the pyridyl group which suppresses the alternative, gel-inhibiting urea-pyridyl hydrogenbonding interaction, freeing the urea groups to form fibrils, and hence gels, as a result of the urea tape hydrogen-bonding motif [32][33][34] . These competitive interaction modes are summarised in Figure 1. 3

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“…Coupled orthogonal self-assembly can give rise to single continuous gel networks comprising two or more intimately mixed gelators either in a stoichiometric ratio to give a co-gel arising from a well-defined gelating supermolecule, or in continuous, nonstoichiometric blends (analogous to polymer blends). 5 Stoichiometric supramolecular co-gels have recently become topical, as in the work of Dastidar on dipyridylurea-carboxylic acid derivatives 22 and the work of Smith on diamine-linked dendrons. 23 There is also a significant field based on metallogelators where the components are metals and gelating 10 ligands.…”

Section: Non-equilibrium Self-assemblymentioning

confidence: 99%

Supramolecular gel phase crystallization: orthogonal self-assembly under non-equilibrium conditions

2014

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Composites of N,N′-bis-(pyridyl) urea-dicarboxylic acid as new hydrogelators—a crystal engineering approach

Cited by 55 publications

References 40 publications

Triggered formation of thixotropic hydrogels by balancing competitive supramolecular synthons

Triggered formation of thixotropic hydrogels by balancing competitive supramolecular synthons

Halogen-bonding-triggered supramolecular gel formation

Supramolecular gel phase crystallization: orthogonal self-assembly under non-equilibrium conditions

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