A novel low molecular weight supergelator showing an excellent gas adsorption, dye adsorption, self-sustaining and chemosensing properties in the gel state

Abstract

In this manuscript we have described the role of a novel pyridine–pyrazole based bis amide molecule namely N¹,N³-bis[5-(pyridin-2-yl)-1H-pyrazole-3-yl]-terephthalamide (BPPTP) which acts as a supergelator in forming very strong gels with copper triflate at a concentration as low as 0.3 wt%. The gel was found to be highly stable towards heat, mineral acids and mechanical stress thus showing self-sustainability. The xerogel showed not only high N₂ gas sorption properties seldom reported for metallogels, but also good toxic dye sorption and rare amine sensing properties in gel state.

---

Introduction

Supramolecular gels, constructed from low molecular mass gelators (LMMGs), have garnered monumental importance in recent years as precursors for the development of smart materials.^1–5 They could change their physical or chemical behavior, often reversibly, when acted upon by some external stimulus such as sound, light, chemicals, pH, temperature, redox and magnetic field.^6–11 As a result they find important applications in the field of drug delivery,¹² light harvesting,^13–15 tissue engineering,^16,17 for templated growth of nanoparticles,^18,19 sensing²⁰ and catalysis.²¹ Metallogels are one such class of supramolecular gels which utilizes metal complexes or clusters in gel formation.^22–26 Generally formation of metallogels involves two different strategies, either discrete metal complexes act as gelators by self assembling via hydrogen bonding, π–π interactions, metal–metal interactions etc. or the metal–ligand interact to form highly cross linked, intertwined coordination polymers which acts as gelators by entrapping the substantial amount of solvent molecules into the network. In other words, metallogels can be thought of a gel induced by a type of aggregation that lies between that of highly long-range ordered aggregates (crystals) and that of random aggregates (amorphous solids) into which coordination bonding interactions are introduced. The use of metals in the self-assembling process not only provide extra strength stability in metallogels but also induce properties solely dependent on metals and thus can be tuned to impart redox, magnetic, catalytic and spectroscopic properties.^27–30

Recently, a lot of studies have been carried out using urea and amide derivatives of pyridine for the generation of organo and metallogels using the urea α-tape and amide (–C=O) N–H hydrogen bonding interactions.^31–33 However most of the examples suffer from a serious shortcoming in which the pyridine moiety being divergent with well-directional, single coordinating site may lead to the formation of solid coordination polymeric networks. Not to mention this goes against the basic principle of gel formation, which basically aims to arrest a phase in between the solid aggregate and dynamic solution phase. Naturally any judicious design of gelator molecule should aim to restrict coordination polymer formation. In our quest to restrict such coordination polymer formation, we have successfully exploited a strategy of designing bis-amide based gelator molecule fitted with a converging or chelating functional group, pyridine–pyrazole moiety. The presence of pyrazole moiety juxtaposed with the pyridine ring can be considered as 2,2′-bipyridine analogue and indeed results in chelation in which the metal ion simultaneously binds with both pyrazole and pyridine nitrogens thus hindering coordination polymeric precipitate formation. Furthermore, the 1-H pyrazole group can play a dual role in metal coordination as well as hydrogen bond formation with an inbuilt hydrogen bonding site (N–H), which along with amide groups turn out to be highly beneficial in immobilizing solvent molecules. The presence of metal ions, on the other hand, helped in designing various functional gel materials with various potential applications ranging from waste water treatment, catalysis to adsorption.^34–36

Inspired by the aforesaid facts, we now extend our work along these lines by employing another novel convergent chelating pyridine–pyrazole based bis-amide ligand namely N¹,N³-bis[5-(pyridin-2-yl)-1H-pyrazole-3-yl] terephthalamide (BPPTP). The BPPTP ligand acts as a supergelator in forming very strong gels with copper salts. The resultant gel material exhibits remarkable self-sustainability by virtue of high stability towards heat, mineral acids and mechanical stress. The xerogel was also found to be an excellent adsorbent material. Furthermore, responses of the gel to various external chemical stimuli, including an unprecedented metal stimuli coordination mediated gel-to-gel transformation with potential application in colorimetric sensing of amines, have also been described.

Results and discussion

The BPPTP ligand was prepared by the reaction of 3{5}-amino-5{3}-(pyrid-2-yl)-1H-pyrazole with terephthaloyl chloride in presence of a base. The ligand was practically insoluble in all common organic solvents, partially soluble in hot DMF but completely soluble in DMSO. Water was found to be imperative towards gelation as no gels were obtained in DMSO alone. Interestingly, BPPTP is not a self-gelator but in presence of copper salts gives rise to green transparent gels. The whole gelation study was conducted using copper triflate although the gelation process was not anion dependent as gels with other copper salts such as halides, nitrate and tetrafluoroborate were also obtained. In a typical gelation experiment, a DMSO solution of the ligand was taken and to it aqueous solution of the metal salts was added at concentration ranging from 1 eq. to 3 eq. with respect to the ligand. Addition of 1 eq. of copper triflate gave cloudy solution whereas partial gel was obtained when 2 eq. copper triflate was added. Addition of 3 eq. of copper triflate instantly rendered the total mixture immobile thereby forming a near transparent green gel (Fig. 1). Interestingly the ligand was found to act as a supergelator in forming metallogels at a concentration of as low as 0.3 wt%. Sonication also plays a key role in accelerating the gelation rate. Interestingly, the resultant metallogels show some remarkable thermal, mechanical as well as chemical stability, suggesting a robust metal coordinated polymeric network formation. The gels were found to be thermo-irreversible and quite resistant to heat as heating even at the boiling point of the solvent did not result in sol formation or change in appearance of the gel. In addition to being heat resistant, the gel was also unaffected by strong mineral acids such as nitric acid, sulphuric acid, hydrochloric acid and perchloric acid. Few drops of each of the mineral acid added on top of the gel did not change the latter's appearance nor resulted in its breakdown even after several weeks. The gels were also stable under ambient conditions on a bench top over several weeks without any noticeable change in appearance.

Fig. 1 Pictures of inverted vials of copper triflate gels when (a) 1 equivalent, (b) 2 equivalents and (c) 3 equivalents of metal salt added with respect to BPPTP. Note the formation of bubbles due to instant gelation.

Microscopy studies

In order to gain some insight into the morphology of the gels, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed (Fig. 2). In both the studies the gel micrographs showed the presence of highly entangled intertwined network made by the self-assembling of nanofibres measuring several micrometers in length and ca. 50 nanometers in diameter. Individual fibres were found to assemble to generate bundled aggregates. The fibrous nature of the sample is in agreement with the nature generally observed for gels.

Fig. 2 (a) SEM and (b) TEM images of copper metallogel.

Spectral studies

In order to gain further insight into the metal–ligand coordination and participation of different functional groups in the self-assembly process through hydrogen-bonding interactions, detail spectroscopic experiments were performed with the free ligand and its corresponding xerogel. The infrared (IR) spectrum (Fig. S1 in the ESI†) of the free ligand shows a very broad peak centered around 3409 cm⁻¹ which can be assigned to pyrazole N–H stretching which gets shifted to 3431 cm⁻¹ in the metallogel. The peaks at 3238, 1672 and 1545 cm⁻¹ were assigned to N–H amide stretching, C=O stretching and N–H amide bending vibrations respectively in the free ligand. The corresponding stretching peaks were shifted to 3211, 1662 cm⁻¹ respectively in the metallogel. The N–H bending peak intensity was broadened and shortened with peaks shifting to 1547 cm⁻¹ in the metallogel. The decrease in the N–H and C=O stretching frequency coupled with an increase in N–H bending frequency in the xerogel is consistent with increased hydrogen bonding via amide groups in the metallogel.

In the UV-visible spectrum (Fig. 3), a 1 × 10⁻⁵ M solution of the ligand prepared in DMF/water (1 : 1) medium showed two distinct absorption peaks at 260 nm (peak 1) and 294 (peak 2) nm corresponding to π–π* transitions. Spectrum recorded with the metallogel did not show any shift in position of peak 1 while peak 2 showed a red shift to 310 nm with a decrease in intensity. A very weak broad band also appeared centered around 370 nm which can be assigned to intraligand charge transfer (ILCT). On increasing the concentration, a broad peak centered on 620 nm was observed corresponding to d–d (²B_1g → ²A_1g) transition, which complies with the energy range observed for copper ions with four-fold nitrogen coordination in the equatorial plane thus confirming square planar geometry around the metal ion.

Fig. 3 UV-vis spectra of BPPTP ligand (black), after complexation with copper (red) and after addition of ammonia (blue). Inset image shows d–d transitions for copper complex (black) and after addition of ammonia (red).

Further corroboration on the square planar geometry around the metal ion comes from the EPR spectrum. The X-band EPR spectrum (Fig. 4) recorded at 77 K revealed an anisotropic character that contained four super hyperfine well resolved absorption-like peaks in the low field region that correspond to g_‖, thus indicating the interaction of the copper(II) odd electron with four similar atoms, which in the present case is nitrogen. Moreover, the g-tensor values, g_‖ (2.38) > g_⊥ (2.06) > 2.0023, indicate that the unpaired electron of copper lies in the d_x²−y² orbital giving ²B_1g as the ground state and has a square planar geometry. The exchange interaction parameter G value (calculated as G = (g_‖ − 2)/(g_⊥ − 2)) was found to be 6.33 (that is, G > 4), which indicates that the exchange interaction between copper centers in the gel state is negligible. Moreover, the absence of any half-field signals around 1600G due to a ΔM_s = ±2 transition also rules out any Cu–Cu interaction, which suggests that the complex is mononuclear.

Fig. 4 EPR spectra of BPPTP–copper triflate gel.

Based on the above mentioned spectral studies we can propose that the copper forms square planar infinite network by coordinating with four nitrogen atoms of two chelating BPPTP molecules. These 1D networks form the basic building unit of gel which then self-assemble via hydrogen bonding with the amide groups and π–π stacking giving rise to three dimensional networks, solvent molecules being immobilized through hydrogen bonding with the amide groups (Fig. 5).

Fig. 5 Schematic representation of the probable local structure of the gel showing the coordination sphere around copper having square planar geometry. Please note the functional groups responsible for gel formation via hydrogen bond and π–π interactions are highlighted in blue.

Rheological studies

The strength of the copper triflate gels of BPPTP (1 wt%) was further estimated by using stress sweep rheometry (Fig. 6) in which the viscous modulus and the elastic modulus was measured as a function of increasing strain amplitude from 0.01 to 200% keeping the frequency constant at 1 Hz and temperature at 298 K. For metallogels, the criteria of independence of frequency of the dynamic storage modulus and also the elastic modulus should be larger than the viscous modulus by atleast an order of magnitude must be met. The viscous modulus (G′′) of the metallogel was found to be 5–7 times smaller than the elastic modulus (G′) over the entire range which is suggestive of gel-like materials moderately tolerant to external forces. The yield stress was found to be of ca. 600 Pa which indicates strong gel.

Fig. 6 Plot of elastic modulus (G′, black squares) and viscous modulus (G′′, red circles) for BPPTP–copper triflate gel, as a function of applied stress [Pa] for 1 wt% gel.

Gas and dye adsorption

Metallogels possess two intrinsic characteristic features that can be envisaged as the two most important prerequisites for an adsorbent material, viz., porous network and immobilized functional groups on the porous surface that can facilitate direct interaction with foreign guest molecules. This is particularly true for co-ordination polymer based metallogels which by default contain porous networks, filled with considerably large amount of solvent molecules, primarily facilitated by the weak supramolecular interactions between gelator and solvent molecules. Of course, retaining network integrity from gel state to solid xerogel is a major obstacle preventing gel materials from being natural adsorbents. In the present context, in order to probe the adsorbent nature of the gel network, gas sorption and dye adsorption studies were performed with xerogel prepared using 3 eq. of copper triflate. The xerogel was activated following a standard method prior to the sorption experiment. 100 mg of xerogel was soaked in 1 : 1 DCM–methanol mixture and was heated in vacuum at 60 °C for 12 h. Afterwards the xerogel was outgassed for 16 h at 80 °C and the N₂ adsorption measurement was performed at liquid nitrogen temperature (77 K) within a pressure range of 0 to 1 atm. As supported by the isotherm (Fig. 7), the xerogel exhibits a reasonably high N₂ uptake of ∼62 cm³ g⁻¹ for a xerogel with a considerable BET surface area of ∼106.9 m² g⁻¹. The Barrett–Joyner–Halenda (BJH) pore size distribution shows most of the pores lying between ∼5 and ∼10.0 nm suggesting a characteristic mesoporous nature of the xerogel (Fig. S2†).

Fig. 7 N₂ adsorption–desorption isotherm of copper triflate xerogel.

Keeping in mind the presence of immobilized functional groups like amide and pyrazole moiety, we further extends adsorption studies to get some insight on functional groups –CO₂ gas interactions, which are of great current interest. After activating the sample for 16 h at 80 °C, CO₂ adsorption measurement was performed at room temperature within a pressure range of 0 to 1 atm. The xerogel was found to adsorb CO₂ in a stepwise manner showing a sharp increase upto pressure 300 mm Hg, after that another sharp increase was observed finally attaining saturation at 800 mm Hg with a CO₂ uptake capacity of ∼8 cm³ g⁻¹ (Fig. S3†). However the adsorption–desorption does not follow the same path showing hysteresis thus confirming that the adsorbed CO₂ is not immediately released on decreasing the external pressure and some of it remains trapped within the network even at zero pressure.

The potential of the gel as adsorbent material was further explored using water soluble toxic organic dyes. Azo-based dyes, although used for industrial purposes are a potent water pollutant.^37–39 The physico-chemical processes employed in purification of such dye-laden wastewater are not only expensive but also require a pretreatment step. Adsorption techniques, on the other hand, are a much cheaper alternative for removal of dye if the adsorbent is inexpensive. In view of the above mentioned facts, we tested our xerogel for the adsorption of toxic dye, methyl orange. The presence of hydrogen bonding groups in methyl orange mean that it can be adsorbed via hydrogen bonding interactions to the xerogel surface. For the xerogels to participate in hydrogen bonding interactions with the dye, the amide groups in the xerogel must be free to interact with the hydrogen bonding groups of the dye. Time dependent UV-visible spectra (Fig. 8) of the dye solution (3 mg of dye in 100 ml water) containing the xerogel (5 mg) showed a bathochromic shift in the absorption maxima of the dye from 465 nm to 505 nm which indicates interaction of the dye molecules with the metallogels. Over time the intensity of the peak decreased with a concomitant bleaching of the orange colour of the dye was observed after about 24 hours. The dye adsorption capacity of the xerogel, calculated over a period of 8 hours from the start of adsorption, was found to be about 5 mg g⁻¹. The reusability of dye adsorbed xerogel was examined by washing it with 0.2 M HCl and 0.2 M NaOH solutions, and then at neutral pH. The resultant dye-free xerogel was found to be equally potent in adsorbing a fresh batch of dye molecules, suggesting that the adsorption was mainly through electrostatic hydrogen bonding interactions with the amide groups of the ligand. The adsorption/desorption process can be repeated several times without any loss of activity of the xerogel.

Fig. 8 UV-visible spectra of methyl orange solution after addition of copper triflate xerogel taken at 30 minutes time interval.

Amine sensing

Over the past few decades, development of sensory materials for the selective detection of volatile amine vapours had attracted immense interest due to their use in chemical, pharmaceutical and food industries.^40,41 However these amines being toxic, their detection at a trace vapour level is necessary for environmental and industrial pollution monitoring, in medical diagnosis and for quality control of food. A great deal of research has been done in the field of fluorescent and colorimetric sensing of such primary, secondary and tertiary amines in solution.^42–45 However colorimetric detection of such amines in solid or gel state has been left quite unexplored.^46–48 Here we report the first metallogel, to the best of our knowledge, which could detect amine vapours colorimetrically in the gel state which is a rare phenomenon. Furthermore, we would like to emphasize the unique ability of the gel to detect both primary, secondary, tertiary amines as well as aromatic amine. Evidently, the metallogel acts as an excellent chemosensor in detecting ammonia, diethylamine, triethyl amine and pyridine vapours in gel state by change of the green gel to deep-red gel in presence of aforementioned amine vapours.⁴⁹ However, the gel shrunk in size due to exudation of water molecules without any loss of integrity of the gel network (Fig. 9). The change in colour of the gel may be due to the change in coordination environment around the copper ion as ammonia is coordinated to the metal's vacant site. UV-vis studies of the gel show a complete change of spectra on addition of ammonia which suggests that the geometry around the metal ion changes with the exclusion of solvent molecules and coordination of ammonia molecules to copper. The d–d transition peak of ligand–copper complex at 620 nm shows a hypsochromic shift to 550 nm on addition of ammonia which suggests a change in geometry from square planar to tetragonally distorted octahedral geometry (Fig. 3, inset image). Moreover, in presence of HCl vapours, the ammonia-coordinated gel reverted back to the original state.

Fig. 9 Reversible colorimetric change of copper triflate gel in presence of ammonia and HCl vapours. Note a simultaneous exudation of solvent molecules and shrinkage of the gel was observed on addition of ammonia.

Self-sustainability

Self-sustainability of a gel is the ability of the gel to stand by itself and withstand change or deformation when acted upon by external forces.⁵⁰ In order to study the self-sustaining and load-bearing property of the metallogel, a cylindrically cut gel having a diameter of 2 cm and a height of 1 cm was taken and coins each weighing 9 grams were placed on it in succession (Fig. 10a). It was observed that the gel could withstand the load exerted by more than 15 such coins without deformation or leaching of solvent. Moreover, the gel being self-sustaining can be cut into small pieces, each piece being able to retain their shape under considerable stress (Fig. 10b).

Fig. 10 Self-sustainability nature of the gel when (a) 15 coins were loaded with no significant solvent leaching and (b) cut to small pieces.

Conclusions

In summary, we have synthesized a novel pyridine–pyrazole based terephthaloyl amide based gel, BPPTP, capable of not only gelating copper salts but also acts as a supergelator in forming gels at a concentration as low as 0.3 wt% in an eco-friendly solvent medium. The gels obtained were found to be highly stable towards heat and mineral acids. Being self-sustaining, the gel was found to resist deformation or breakdown under appreciable mechanical stress. Due to highly mesoporous nature of the fibres which constitute the gel, it shows high nitrogen adsorption rarely observed for a metallogel and good toxic dye adsorption properties. Moreover the metallogel was found to display wide range of amine sensing properties in detecting vapours containing nitrogen colorimetrically by changing colour from green to deep-red in the gel state which is a rare phenomenon with a potential to be practically useful in near future for industrial and medical purposes.

Experimental part

Terephthaloyl chloride was commercially available (Aldrich) and used without further purification. FT-IR spectra were performed on a Nicolet MAGNA-IR 750 spectrometer with the KBr pellets containing the samples. ¹H spectrum was recorded on Bruker spectrometers operating at 400 MHz in DMSO-d₆ solvent. Mass spectra were collected from Micromass Q-Tof Micro instrument and UV visible studies were performed in Perkin Elmer Lambda 950 UV/VIS instrument. The elemental analyses were carried out using a Perkin-Elmer 2400 Series-II CHN analyser. EPR spectra were recorded on a JEOL instrument. Electron microscopic studies were made using a JEOL, JMS-6700F field emission scanning electron microscope (FESEM) and JEOL JEM 2010 high-resolution microscope instrument for TEM images. Rheology experiments were performed in SDT Q Series Advanced Rheometer AR 2000.

N¹,N³-Bis[5-(pyridin-2-yl)-1H-pyrazole-3-yl]-terephthalamide (BPPTP)

Terephthaloyl chloride (2.03 g, 10 mmol) was dissolved in dry acetonitrile and to it was added an acetonitrile solution of 3{5}-amino-5{3}-(pyrid-2-yl)-1H-pyrazole⁵¹ (3.2 g, 20 mmol), giving an immediate precipitate. The reaction mixture was refluxed overnight, cooled and filtered. The precipitate was washed with acetonitrile and dried in vacuum. To the suspension of the hydrochloride salt in water, was added saturated sodium bicarbonate solution till slightly basic and stirred at room temperature overnight. The precipitated solid was filtered, washed with water, dried and purified from DMSO/methanol. Yield 50%; mp > 200 °C. Elemental analysis calcd (%) for C₂₄H₁₈N₈O₂: C 63.99, H 4.03, N 24.88; found: C 63.54, H 4.05, N 24.61. ¹H NMR spectra/δ (ppm) (400 MHz, DMSO-d₆): 13.21 (s, 2H, pz NH), 11.08 (s, 2H, amide NH), 8.62 (d, 2H, py H⁶), 8.13 (s, 4H, Ph H), 7.87 (m, 4H, py H³ + H⁴), 7.35 (d, 2H, py H⁵), 7.26 (s, 2H, pz C–H). ¹³C NMR spectra/ppm 96.11 (pz C⁴), 119.76 (py C⁵), 122.96 (py C³), 127.9 (Ph C^2/3/5/6), 136.6 (Ph C^1/4), 137.27 (py C⁴), 142 (pz C⁵), 148.2 (pz C³), 149.39 (py C^6/2), 163.93 (C=O). IR (KBr, cm⁻¹): 3409(br), 1672(s), 1585(s), 1545(s), 1489(s), 1427(s), 1321(s), 1280(s).

Gelation test. To 10 mg of the ligand in 0.5 ml of DMSO kept in a screw-capped sample vial, was added the corresponding amount of the metal salt dissolved in 0.5 ml water and briefly sonicated. The gel obtained was tested by the “stable-to-inversion of the vial” method.

Acknowledgements

RM gratefully acknowledges SERB (Project No. SR/S1/IC-65/2012) India, for financial assistance. SSG is thankful to CSIR, India for Research Fellowships.

Notes and references

1. Z. Qi and C. A. Schalley, Acc. Chem. Res., 2014, 47, 2222 and references therein.
2. D. Krisna Kumar and J. W. Steed, Chem. Soc. Rev., 2014, 43, 2080 and references therein.
3. S. S. Babu, V. K. Praveen and A. Ajayaghosh, Chem. Rev., 2014, 114, 1973 and references therein.
4. J. Boekhoven, J. M. Poolman, C. Maity, F. Li, L. van der Mee, C. B. Minkenberg, E. Mendes, J. H. van Esch and R. Eelkema, Nat. Chem., 2013, 5, 433.
5. L. E. Buerkle and S. J. Rowan, Chem. Soc. Rev., 2012, 41, 6089 and references therein.
6. N. Kotobuki, K. Murata and K. Haraguchi, J. Biomed. Mater. Res., Part A, 2013, 101, 537.
7. G. Cravotto and P. Cintas, Chem. Soc. Rev., 2009, 38, 2684.
8. J. T. van Herpt, M. C. A. Stuart, W. R. Browne and B. L. Feringa, Langmuir, 2013, 29, 8763.
9. Y. Li, G. Huang, X. Zhang, B. Li, Y. Chen, T. Lu, T. J. Lu and F. Xu, Adv. Funct. Mater., 2013, 23, 660.
10. X. Du, J. Zhou, J. Shi and B. Xu, Chem. Rev., 2015, 115, 13165 and references therein.
11. Z. Sun, Q. Huang, T. He, Z. Li, Y. Zhang and L. Yi, ChemPhysChem, 2014, 15, 2421.
12. K. J. Skilling, F. Citossi, T. D. Bradshaw, M. Ashford, B. Kellam and M. Marlow, Soft Matter, 2014, 10, 237 and references therein.
13. P. Duan, N. Yanai, H. Nagatomi and N. Kimizuka, J. Am. Chem. Soc., 2015, 137, 1887.
14. V. K. Praveen, C. Ranjith and N. Armarol, Angew. Chem., Int. Ed., 2014, 53, 365.
15. C. Vijayakumar, V. K. Praveen and A. Ajayaghosh, Adv. Mater., 2009, 21, 2059.
16. K. Y. Lee and D. J. Mooney, Chem. Rev., 2001, 101, 1869.
17. L. E. Beurkle, H. A. von Recum and S. J. Rowan, Chem. Sci., 2012, 3, 564.
18. J. H. Lee, S. Kang, J. Y. Lee and J. H. Jung, Soft Matter, 2012, 8, 6557.
19. M.-O. M. Piepenbrock, N. Clarke and J. W. Steed, Soft Matter, 2011, 7, 2412.
20. J. H. Jung, M. Park and S. Shinkai, Chem. Soc. Rev., 2010, 39, 4286 and references therein.
21. A. Döring, W. Birnbaum and D. Kuckling, Chem. Soc. Rev., 2013, 42, 7391 and references therein.
22. X. Yan, T. R. Cook, J. B. Pollock, P. Wei, Y. Zhang, Y. Yu, F. Huang and P. J. Stang, J. Am. Chem. Soc., 2014, 136, 4460.
23. A. Y. Y. Tam and V. W. W. Yam, Chem. Soc. Rev., 2013, 42, 1540 and references therein.
24. O. Kotova, R. Daly, C. M. G. dos Santos, M. Boese, P. E. Kruger, J. J. Boland and T. Gunnlaugsson, Angew. Chem., Int. Ed., 2012, 51, 7208.
25. W. J. Gee and S. R. Batten, Chem. Commun., 2012, 48, 4830.
26. R. E. Bachman, A. J. Zucchero and J. L. Robinson, Langmuir, 2012, 28, 27.
27. J. Zhang, X. Wang, L. He, L. Chen, C. Y. Su and S. L. James, New J. Chem., 2009, 33, 1070.
28. T. W. Assenmacher, H. Peterlik, R. Weisbarth, M. Nieger and K. H. Dötz, Angew. Chem., Int. Ed., 2007, 46, 6368.
29. W. Weng, J. B. Beck, A. M. Jamieson and S. J. Rowan, J. Am. Chem. Soc., 2006, 128, 11663.
30. S. Kawano, N. Fujita and S. Shinkai, J. Am. Chem. Soc., 2004, 126, 8592.
31. M. O. M. Piepenbrock, N. Clarke and J. W. Steed, Soft Matter, 2010, 6, 3541.
32. J. W. Steed, Chem. Soc. Rev., 2010, 39, 3686 and references therein.
33. M. O. M. Piepenbrock, N. Clarke and J. W. Steed, Soft Matter, 2011, 7, 2412.
34. S. Sengupta and R. Mondal, J. Mater. Chem. A, 2014, 2, 16373.
35. S. Sengupta, A. Goswami and R. Mondal, New J. Chem., 2014, 38, 2470.
36. S. Sengupta and R. Mondal, Chem.–Eur. J., 2013, 19, 5537.
37. M. Arkas, D. Tsiourvas and C. M. Paleos, Chem. Mater., 2005, 17, 3439.
38. A. Gürses, Ç. Doğar, M. Yalçın, M. Açıkyıldız, R. Bayrak and S. Karaca, J. Hazard. Mater., 2006, 131, 217.
39. A. Sayari, S. Hamoudi and Y. Yang, Chem. Mater., 2005, 17, 212.
40. J. M. Landete, B. Rivas, A. Marcobal and R. Muñoz, Int. J. Food Microbiol., 2007, 117, 258.
41. V. P. Aneja, P. A. Roelle, G. C. Murray, J. Scutherland, J. W. Erisman, D. Faler, W. A. H. Asman and N. Patni, Atmos. Environ., 2001, 35, 1903.
42. S. W. Thomas III, G. D. Joly and T. M. Swager, Chem. Rev., 2007, 107, 1339.
43. P. Montes-Navajas, L. A. Baumes, A. Corma and H. Garcia, Tetrahedron Lett., 2009, 50, 2301 and references therein.
44. S. Körsten and G. J. Mohr, Chem.–Eur. J., 2011, 17, 969.
45. C. Patze, K. Broedner, F. Rominger, O. Trapp and U. H. F. Bunz, Chem.–Eur. J., 2011, 17, 13720 and references therein.
46. H. Y. Lee, X. Song, H. Park, M. H. Baik and D. Lee, J. Am. Chem. Soc., 2010, 132, 12133.
47. H. Iida, M. Miki, S. Iwahana and E. Yashima, Chem.–Eur. J., 2014, 20, 4257 and references therein.
48. S. Wang, P. Xue, P. Wang and B. Yao, New J. Chem., 2015, 39, 6874 and references therein.
49. S. Bhattacharya, S. Sengupta, S. Bala, A. Goswami, S. Ganguly and R. Mondal, Cryst. Growth Des., 2014, 14, 2366.
50. S. Samai and K. Biradha, Chem. Mater., 2012, 24, 1165.
51. C. M. Pask, K. D. Camm, C. A. Kilner and M. A. Halcrow, Tetrahedron Lett., 2006, 47, 2531.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26713g

---