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Cite this: Chem. Commun., 2014, 50, 11690

Pyridylenevinylene based Cu2+-specific, injectable metallo(hydro)gel: thixotropy and nanoscale metal–organic particles†

Received 20th June 2014, Accepted 25th July 2014

Subham Bhattacharjeea and Santanu Bhattacharya*ab

DOI: 10.1039/c4cc04712e www.rsc.org/chemcomm

A Cu2+-selective metallo(hydro)gelation of a p-pyridyl ended oligophenylenevinylene system is reported over its respective meta- and ortho-regioisomers. The metallogel formed via the self-assembly of the nanoscale-metal–organic particles is injectable and also shows multi-stimuli responsiveness, including thixotropy.

The coordination of a metal ion to a suitably crafted molecule containing appropriate binding sites could trigger supramolecular self-assembly and might even lead to physical gelation in certain cases.1 This opens up the fascinating area of research on metallogels at the frontiers of supramolecular chemistry, coordination chemistry, and material science. This is highly promising since metallogels are potentially useful in molecular recognition, catalysis, and in materials chemistry.1 Indeed elegant design strategies and intriguing architectural plans have been implemented to construct a wide variety of molecular building blocks that constitute the so-called metallogels through binding with specific metal ions.2 However, careful design on oligo-p-phenylenevinylene (OPV)-based metallogels is still unexplored, although a great deal of work on the self-assembly of OPV-based systems has been reported.3 Recently, we reported the first example of one-component, as well as two-component, salt-type OPV-based hydrogels.4 While the majority of metallogels are constructed through the entanglement of the fibrous nanostructures, the entrapment of solvents through the self-organization of the nanoscale metal–organic particles (NMOPs) leading to the fabrication of macroscopic metallogel is rare.5 Recently discovered NMOPs have attracted considerable attention because of their potential applications in drug delivery, sensing, and in molecular electronics, etc.6 Herein, we describe the Cu2+-triggered, regioisomer-specific hydrogelation of a p-pyridyl appended OPV derivative for the first time. The metallogelation was found to be a three-component

process and the gel also showed multi-stimuli responsive properties, including thixotropy. The 3-D gel is constructed through the selforganization of the individual NMOPs. The latter renders this gel as an aesthetically appealing candidate with significant potential in the fields of medicinal and materials chemistry. Compound 1, a waxy solid with a low melting point (m.p. = 45–46 1C), is soluble in water and produces a yellow colored solution (Fig. 1a). The solubility of 1 in water may be attributed to the presence of oligo-oxyethylene chains in the middle of the backbone of 1, which has the ability to associate with water molecules via extensive H-bonds.2a,7 The end pyridyl N-atoms in the p-position of 1 also play a significant role in solubilizing it in water. Thus, although 1 itself is not a gelator, it has two p-pyridyl segments, which are known to be classic metal coordination sites.2 Hence, the structural feature of 1 is ideal for achieving metal-ion-induced selfassembly in water. Accordingly, we checked the metallogelation ability of 1 with a variety of transition metal cations, such as Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Hg2+ as their chloride salts and Ag+ as silver trifluoromethanesulfonate (AgOTf). The addition of an equal amount of Co2+ or Ni2+ to the solution of 1 in water resulted in the formation of water-soluble complexes or sol under ambient conditions.

a

Department of Organic Chemistry, Indian Institute of Science, Bangalore, India. E-mail: [email protected]; Fax: +91-80-23600529; Tel: +91-80-22932664 b Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India † Electronic supplementary information (ESI) available: Synthesis, characterization, Fig. S1–S9 and experimental section. See DOI: 10.1039/c4cc04712e

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Fig. 1 Chemical structures of the OPV-based ligands 1, 2, and 3 (top). Photographs of the solutions of ligand 1 in water without (a) and with (b) Cu2+ ions. (c) Digital images of the gel–sol transition of the metallogel of 1/CuCl22H2O = 1 : 1 in water induced by various stimuli ([1] = 8 mM).

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In the presence of AgOTf or ZnCl2, 1 however, precipitated immediately. The complexes of 1–CdCl2 or 1–HgCl2 resulted in the formation of viscous precipitates in water. The gelation propensity of 1 in water was also checked with an equimolar amount of Pd(NO3)2 and K2PtCl4. In each instance, we observed the instantaneous formation of a viscous solution, from which the Pt-complex eventually precipitated upon aging and no gelation was observed. Interestingly, the addition of CuCl22H2O to the aqueous solution of 1 in a 1 : 1 molar ratio resulted in instantaneous and robust gel formation, which remained highly stable over several months without any deterioration (Fig. 1c). More specifically, 3.7 mg (6.08 mM) of ligand 1, along with 1 mg of CuCl22H2O (6.08 mM), could immobilize 1 mL of water. Thus the metallo-gelation of 1 manifested excellent selectivity to Cu2+ only. Additional evidence of metal ion coordination was obtained from the yellow to red color change of the solution of 1 (Fig. 1a and b) and also from the FT-IR and ESI-MS of the 1–Cu2+ complexes. The CQN band of the pyridyl ring of the ligand 1 at 1596 cm1 shifts to higher frequencies (1607 cm1) upon complexation with Cu2+ ion, thus indicating metal ion coordination (Fig. S1a, ESI†).2k The mass spectra also indicate the formation of 1 : 1 and also 1 : 2 1/Cu2+ complexes (Fig. S1c, ESI†). A 1 : 1 stoichiometry of interaction is indicated on the basis of Job’s plot (Fig. S2c and d, ESI†) along with the significantly high binding constant (log K) 5.41  0.04 (Fig. S2a and b, ESI†). Furthermore, the strong fluorescence of 1 in water was quenched immediately after the addition of Cu2+ (Fig. S2e and f, ESI†), indicating either an electron transfer or energy transfer to the metal ion from 1, causing the rapid non-radiative decay. The concentrationdependent emission spectra of the metallogel showed quenching of the photoluminescence (PL) of the solution at a relatively high concentration. However, the PL of the solution gradually increased with a decreasing concentration of the solution (Fig. S3, ESI†). Electroluminescence (EL) studies of the spin-coated and dropcasted dilute metallogel were also performed. However, we could not detect any EL emission from the sample under this condition. The gelation ability of 2 and 3, which are isomeric to 1, was also checked in water. 2 is fairly soluble in water. Unlike the hydrogelation of 1, 2 induced the formation of a viscous solution when interacted with 1 equiv. of CuCl22H2O in water. 3 also failed to form gel with Cu2+ in water. Thus the position of the pyridyl N-atom in the molecular backbone of the OPV appears to be crucial for the formation of a supramolecular metallogel. The gelation ability of 1 was also tested in the presence of other Cu2+ salts. The gelation of 1 was observed in the presence of CuBr2 or Cu(NO3)2. The addition of CuSO4 or Cu(OAc)2 to the aqueous solution of 1 in a 1 : 1 molar ratio led to either precipitation or a viscous solution, respectively. Thus the presence of Cl, Br, or NO3, along with the Cu2+ cation, plays a pivotal role in metallogel formation. Therefore, it appears that the metallo-gelation of 1 in water is a three-component process. To demonstrate this fact, an equimolar amount of NiCl2 was added to an aqueous solution of 1 to act as a source of the Cl ion. Next, Cu(OAc)2 was mixed to the sol of 1–NiCl2, and the mixture resulted in immediate gelation. That experiment was also performed in a reverse manner, that is, NiCl2 or tetrabutylammonium chloride (TBACl) was added as the source of Cl ion to a sol of a 1–Cu(OAc)2 mixture. This produced

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a stable gel, thus confirming the involvement of the threecomponents. The metallogel of 1 also responded to a variety of external stimuli. The metallogel, on shaking, transformed into a viscous sol, which on resting again transformed into a robust gel, indicating a thixotropic behavior (Fig. 1c). The addition of a little excess of sodium L-ascorbate under Ar atmosphere, led to precipitation from the gel state, presumably because of the reduction of Cu2+ to Cu+ under this condition (Fig. 1c).2n The reversible gel formation was realized when the precipitate was kept under an oxygen environment for B1 day, which again oxidized Cu+ to Cu2+. The addition of 1.5 equiv. of ethylenediaminetetraacetic acid disodium salt (EDTA) to the preformed metallogel of 1, which binds preferentially to the Cu2+ ion, resulted in the instantaneous dissolution of the gel (Fig. 1c).2k The hydrogel could be disrupted progressively when a few drops of conc. ammonia solution were added on the top of the preformed gel of 1 (Fig. 1c and Fig. S4, ESI†). The addition of a few drops of pyridine or ethylenediamine also instantaneously abolished the hydrogel (Fig. 1c). The morphological features of the hydrogel in the nanoscale dimension were primarily investigated using scanning electron microscopy (SEM) of a diluted sample of the gel ([1] = 0.3 mM), which showed the presence of spherical aggregates (NMOPs) with a diameter in the range of 100–300 nm (Fig. 2a and Fig. S5a and b, ESI†). However, cluster-type aggregates were observed when the SEM was performed from a freeze-dried gel sample above the CGC (Fig. S5c, ESI†). A closer look of the aggregates indeed reveals the formation of the former by fusion of the individual NMOPs. Atomic force microscopy (AFM) of a relatively diluted sample ([1] = 0.1 mM) also revealed the formation of NMOPs with an average diameter of B100 nm and a height in the range of 10–12 nm (Fig. 2b–d and Fig. S5d and e, ESI†). Dynamic light scattering experiments of the solution of the metallogel showed the presence of NMOPs in solution ([1] = Cu2+ = 2 mM) with an average hydrodynamic diameter (Dh) of 953  17 nm (Fig. S6a, ESI†). The addition of 1 equiv. of pyridine to the

Fig. 2 (a) SEM ([1] = 0.3 mM) and (b,c) AFM ([1] = 0.1 mM) images of the aggregates of the metallogel and (d) the height image of a single NMOP. (e) Energy minimized structure of 1–Cu2+ complex. (f) X-ray diffraction pattern of the xerogel.

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above solution resulted in a decrease in the aggregate size of the NMOPs drastically to Dh 403  9 nm (Fig. S6b, ESI†). Interestingly, the size of the aggregates reduced to Dh 278  4 nm on the further addition of pyridine (1 : pyridine = 1 : 3) to the resultant solution (Fig. S6c, ESI†). It is important to note that, OPV 1 itself form aggregates in water and gave Dh of 295  12 nm under DLS (Fig. S6d, ESI†). Therefore, it may be concluded that the addition of pyridine to the solution of the metallogel breaks the NMOPs in the solution by making the OPV 1 free in the solution, the aggregation of which resulted in the formation of aggregates of a relatively smaller size in solution. Furthermore, X-ray diffraction (XRD) studies of the xerogel were carried out in an attempt to investigate the probable molecular arrangements in the metallogel. Three well-resolved Bragg reflection peaks at 13.4, 6.8, and 4.5 Å were observed with reciprocal spacing ratios of 1 : 1/2 : 1/3, indicating the presence of 100, 200, and 300 reflections in the layered structure with a periodicity of 13.4 Å (Fig. 2f). Actually, the layered spacing of 13.4 Å is slightly shorter than the distance between the central aromatic ring of 1 to the end methyl group of the oxyethylene chain B14.4 Å (Fig. 2e), suggesting interdigitation among the oxyethylene chains in the metal-ion-induced self-assembly of 1.4b The concentration-dependent UV-Vis absorption spectra of the metallogel were recorded. However, the characteristic d–d transition was not observed. This is presumably because of the tailing of the intense absorption of the metal–ligand complex, which eventually masks the d–d transition (Fig. S7a, ESI†). The EPR spectral profile of the metallogel showed two g-values i.e., gJ and g> at 2.22 and 2.08, respectively, both of which are higher than the free electron g-value and which also indicates that the ground state is a dx2y2 orbital (Fig. S7b, ESI†). These observations confirm the presence of a typical distorted squarebased geometry of the Cu2+ ion in the metallogel which is substantiated by the elemental analysis. The elemental analysis of the metallogel revealed the molecular formula or repetitive unit of the metallogel as Cu(Py)2Cl2(H2O)2 (Py denotes the end pyridyl units of compound 1) (see ESI†). Based on these observations, a probable coordination environment around the Cu2+ ion in the metal–ligand complex was proposed, and which shows that the Cu2+ ion is coordinated to the two pyridyl N-atoms of the two different molecules of 1 and two Cl ions in a distorted square based geometry. The two water molecules probably remain in the hydrate form associated with the hydrophilic oxyethylene chains of 1 (Fig. 3a). Thus a distorted square-based geometry of the Cu2+ ion in the complex is indicated (Fig. 3a). On this basis, it is quite possible that the bending in the metal–ligand supramolecular polymers8 due to the distorted square-based geometry of the Cu2+ ion may lead to the formation of both linear and cyclic supramolecular oligomers. However a longer supramolecular polymer, with a high aspect ratio, may behave like a flexible macromolecule and could fold due to the van der Waals interactions among the different parts of the oligomer. Based on these observations, a model for the formation of the supramolecular NMOPs is proposed. This comprises the metal ion coordination triggered bending of the metal–ligand supramolecular polymers (curvature driving forces), p–p interactions among the

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Fig. 3 (a) Coordination geometry of the Cu2+ ion in the metallogel and (b) schematic illustration of supramolecular metallogel formation through the aggregation of individual NMOPs (the Cl ion coordinated to the Cu2+ ion was not shown in the model of NMOPs for clarity).

aromatic planes, and interdigitation of the oxyethylene chains through the van der Waals interactions (Fig. 3b). Eventually, the metallogel could be regarded as a supramolecular polymer, whose monomer components are individual NMOPs. Individual NMOPs further self-assemble to a 3-D gel through the strong inter-NMOP cohesive forces that can arise due to the protrusion of the oxyethylene chains from the surface of the sole NMOPs. The mechanical strength of the metallogel was further probed using oscillatory amplitude sweep experiments under variable concentrations.4b This experiment revealed that the viscoelasticity (G 0 value) of the gel increases with the increasing concentration of the gelators (Fig. 4a). Frequency sweep experiments at variable concentrations reveal that the storage modulus G 0 is always greater than the respective loss modulus G00 under an applied strain of 0.01% in the angular frequency range of 1–100 rad s1, and is also invariant to the applied angular frequency, which confirm the viscoelastic nature of the gel over the entire angular frequency range (Fig. S8, ESI†). The thixotropic nature of the metallogel was further examined using a hysteresis loop test. A loop test was carried out by applying successive low/high strains, separated by enough time to assure the complete gel-to-sol (G00 4 G0 ) and sol-to-gel (G0 4 G00 ) conversion. Remarkably, the original mechanical strength of the metallogel was fully regained within B1 min after the cessation of the large strain, indicating the thixotropic behavior of the gel (Fig. 4b). This unique physical behavior makes the hydrogel injectable through a narrow needle, and thus it may be moulded into different alphabetical letters (Inset in Fig. 4a and Fig. S9, ESI†). In this communication, a novel Cu2+-specific metallogelation of 1 in water is reported. The gelation-triggered selective-detection of Cu2+ ions is promising in terms of the analytical applications. A remarkable regioisomer control in hydrogelation was also realized.

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Fig. 4 (a) Oscillatory amplitude sweep experiments of the metallogel at three different concentrations. (b) Hysteresis loop test of the metallogel (1 = 30 mM). The inset in (a) demonstrates the injectability of the metallogel.

In addition to thixotropy, the hydrogel responds to several other external stimuli. The reversible nature of the gel–sol transition is also demonstrated using Cu2+/Cu+ redox chemistry. Morphological studies suggest the formation of NMOPs instead of typical fibrous structures. The lamellar arrangement of the metal–ligand complex in the NMOPs was evidenced from the XRD studies. The strong cohesive forces among the individual NMOPs, which probably arise due to the projection of the oxyethylene chains from the surface of the NMOPs, result in further aggregation, presumably through the interdigitation of the oxyethylene chains that eventually give rise to the hydrogel network. To the best of our knowledge, this is the first report of an OPV-based metallo(hydro)gel with excellent regioisomer and metal ion selectivity in gelation, NMOPs morphologies, injectability, and multi-stimuli responsiveness. Accordingly, numerous applications are envisaged and work is currently underway to exploit such opportunities. We thank Prof. R. N. Mukherjee, Director, IISER, Kolkata for providing the EPR spectra and Prof. S. K. Ray of IIT, Kharagpur for the EL measurement. Prof. S.B. thanks DST (J.C. Bose Fellowship) for the financial support of this work. Mr S.B. thanks CSIR for the SRF.

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Chem. Commun., 2014, 50, 11690--11693 | 11693

Pyridylenevinylene based Cu(2+)-specific, injectable metallo(hydro)gel: thixotropy and nanoscale metal-organic particles.

A Cu(2+)-selective metallo(hydro)gelation of a p-pyridyl ended oligo-phenylenevinylene system is reported over its respective meta- and ortho-regioiso...
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