Unusual C–I···O Halogen Bonding in Triazole Derivatives: Gelation

It was found in our previous studies that the iodination of the 5-position of 1,2,3-triazole could change the gelling ability of triazole derivatives ...
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Unusual C−I···O Halogen Bonding in Triazole Derivatives: Gelation Solvents at Two Extremes of Polarity and Formation of Superorganogels Yaodong Huang,* Huimin Li, Ziyan Li, Yan Zhang, Wenwen Cao, Luyuan Wang, and Shuxue Liu Key Laboratory of Systems Bioengineering (Ministry of Education), School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, PR China S Supporting Information *

ABSTRACT: To investigate the influence of halogen bond (XB) on the gelation of a one-component organogel system, a new family of 5-iodo-1H-1,2,3-triazole and 1H-1,2,3-triazole gelators was designed and synthesized. The iodo gelators (1I, 3I) gelled various solvents at low concentrations and formed many superorganogels, whereas the hydrogenous gelators (1H, 3H) showed much poorer gelling performance. An X-ray analysis of the single crystals of two reference compounds (16I, 16H) reveals that the unusual C−I···O XB interaction is responsible for this difference. The results of spectroscopic examinations (XRD, SEM, 1H NMR, and UV) are well consistent with those of single-crystal analyses. Under the guidance of the XB interaction and the weak π−π interaction, 1I and 3I self-assemble to hexagonal columnar aggregations in the gel state, whereas 1H and 3H, driven by CH−π interactions, feature the formation of gels with a lamellar structure. The mechanical property of iodo gels is much better than that of hydrogenous gels under the same concentration. Gels from 1I respond to the stimuli of Hg2+, Cu2+, Zn2+, and Mg2+ as perchlorate salts, and gels from 1H are selectively responsive to Hg2+ solely.

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INTRODUCTION Supramolecular organogels have been studied with great interest in the past two decades because they are highly desirable for various applications in functional materials, such as drug delivery,1−4 biofunctional materials,5,6 chemosensors,7−9 and optical/electronic materials,10−13 and so forth. Stemmed from the spontaneous self-assembly of simple molecules (gelators), the formation of supramolecular organogels is driven by weak physical interactions such as hydrogen bonding (HB), van der Waals forces, π−π stacking, electrostatic interaction, dipole−dipole interaction, solvophobic interaction, metal-coordination, and host−guest interaction. Among these noncovalent interactions, HB is a common, yet very important interaction because of its strength, directionality, reversibility, and selectivity. As the workhorse of the supramolecular toolhouse, HB is widely used in the gelation of peptide-,14,15 carbohydrate-,16−18 amino acid-,19−21 nucleic acid-,22−24 and urea-based25−27 compounds. Halogen bonding (XB), known in general as D···X−Y (D = N, O, S, Se, Cl, Br, I, I−, Br−, Cl−, F−, ···; X = I, Br, and Cl; Y = C, N, halogen; so forth), is also a very important noncovalent interaction. In D···X−Y, X is the electrophilic halogen atom and it acts as a XB donor, whereas D is a donor of electron density (XB acceptor). As a result, D can be both neutral species and anions and the latter are particularly efficient XB acceptors. XB can compete with HB in many aspects such as energetic and spectroscopic features. Notably, in regard to geometric trends, XB is more directional than HB. Notwithstanding this, XB is © XXXX American Chemical Society

indeed less exploited than HB. Although XB was heard of as early as 1814 and the subsequent studies about it were fruitful, the nature and applications of XB did not begin to be intensively investigated until the mid-1990s. Until now, XB has been extensively found in crystal engineering,28,29 materials chemistry,30−34 and biological systems.35−37 In two recent reviews, the achievements of XB study and XB application were reviewed in detail.38,39 In fact, XB has an influence on all research fields in which the control of intermolecular recognition and self-assembly processes play a key role. Although the reports on XB have been well documented, its application in gels is rare. A careful literature survey indicates that there are only three reports involving the formation of gels. Among them, two reports are about the halogen-bond-induced gelation, in which diiodo compounds and bipyridine compounds were used simultaneously to form two-component organogels;40,41 the third report is about the hydrogelation enhancement owing to iodination, in which HB was thought to participate in the self-assembly of the halogenated peptide DFNKF.42 Very recently, Philp and co-workers reported that some iodotriazoles could assemble in solutions through the support of XB.43 It was found in our previous studies that the iodination of the 5-position of 1,2,3-triazole could change the gelling ability of Received: October 10, 2016 Revised: December 3, 2016

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DOI: 10.1021/acs.langmuir.6b03691 Langmuir XXXX, XXX, XXX−XXX

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Langmuir Scheme 1. Synthetic Routes for Compounds 1I, 1H, 3I, 3H, 16I, and 16Ha

a

(a) Trimethylsilylacetylene, CuI, Pd(Ph3P)2Cl2, THF, TEA; (b) K2CO3, THF, MeOH, rt, 24 h; (c) N,N-diisopropylethylamine (DIPEA), NBS, CuI, THF, rt, 48 h; (d) CuSO4, Vc, THF, EtOH, rt, 48 h; (e) C12H25Br, K2CO3, dimethylformamide (DMF), 70−80 °C, 6 h; (f) LiCl, KBH4, THF, reflux, 8 h; (g) SOCl2, DMF, rt, 4 h; (h) NaN3, DMF, rt, 4 h; and (i) Me2SO4, NaOH, 1,4-dioxane, 75 °C, 6 h.

triazole derivatives dramatically.44,45 Although we thought that it should be the result of XB interaction, we could not provide evidence for the existence of XB. To reveal the real interactions existing in such a kind of iodinated compounds, we designed and synthesized new compounds and tried to obtain convincing information. In this paper, we report the synthesis and properties of 1H-1,2,3-triazole-based gelators, along with the proof of C−I···O XB as the driving force of gelation using single-crystal X-ray analysis.

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distilled from Na/benzophenone under a slight positive atmosphere of N2. Other chemicals and solvents were used as received without further purification. Measurements. 1H nuclear magnetic resonance (1H NMR) spectra were recorded using a Bruker 400 spectrometer, and 13C nuclear magnetic resonance (13C NMR) spectra were recorded using a Bruker 500 spectrometer. High-resolution mass spectroscopy (HRMS) was conducted using an Agilent Technologies 6520 Accurate-Mass Quadrupole Time-of-Flight liquid chromatography (LC)/MS instrument under the electrospray ionization model. Ultraviolet−visible (UV−vis) absorption spectra were recorded using a Lambda 35 spectrometer. Scanning electron microscopy (SEM) images were observed using a Hitachi S-4800 microscope. The samples for SEM measurements were prepared by casting the gels on copper slides and were dried at room temperature for 48 h. X-ray diffraction (XRD) was checked using a Bruker diffractometer (Cu Kα radiation k

MATERIALS AND METHODS

Materials. All chemicals and solvents were purchased from commercial chemical suppliers and were of reagent grade. Triethylamine (TEA) was dried by distillation from potassium carbonate and stored on activated 4 Å molecular sieves. Tetrahydrofuran (THF) was B

DOI: 10.1021/acs.langmuir.6b03691 Langmuir XXXX, XXX, XXX−XXX

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for C18H16IN3O3 [M + Na]+ 472.0144, found 472.0129. Anal. Calcd for C18H16IN3O3: C 48.12, H 3.59, N 9.35; found: C 48.09, H 3.42, N 9.27. Synthesis of 4-[1-(4-Dodecyloxy-benzyl)-1H-[1,2,3]triazol-4-yl]benzoic Acid Methyl Ester (1H). A solution of Vc (2.04 g, 11.582 mmol) and copper(II) sulfate pentahydrate (1.65 g, 6.608 mmol) in H2O (30 mL) was added to a mixture of 6 (1.26 g, 3.972 mmol) and 5 (0.62 g, 3.874 mmol) in THF (30 mL) under a nitrogen atmosphere. After stirring at room temperature for 48 h, the reaction mixture was transferred with 50 mL of dichloromethane to a separatory funnel. The organic solution was washed with water and then dried over anhydrous Na2SO4. The solvents were evaporated in vacuum, and the residue was purified using silica gel column chromatography with MeOH/CH2Cl2 (1:45, v/v) as the eluant to give 1H as a white solid (1.17 g) in 63% yield. Mp, 146−148 °C. 1H NMR (400 MHz, CDCl3, δ ppm): 8.07 (d, J = 8.4 Hz, 2H), 7.89 (d, J = 8.4 Hz, 2H), 7.72 (s, 1H), 7.27 (d, J = 8.8 Hz, 2H), 6.91 (d, J = 8.8 Hz, 2H), 5.53 (s, 2H), 3.94 (m, 5H), 1.78 (m, 2H), 1.44 (m, 2H), 1.26−1.30 (m, 16H), 0.87 (t, 3H). 13C NMR (500 MHz, CDCl3, δ ppm): 166.77, 159.71, 147.06, 134.94, 130.18, 129.75, 129.57, 126.07, 125.46, 120.13, 115.15, 68.19, 53.98, 52.12, 31.92, 29.20−29.70, 26.03, 22.69, 14.11. HRMS: calculated for C29H39N3O3 [M + Na]+ 500.2889, found 500.2887. Anal. Calcd for C29H39N3O3: C 72.77, H 8.42, N 8.78; found: C 72.64, H 8.37, N 8.62. Synthesis of 4-[1-(3,4,5-Tridodecyloxy-benzyl)-1H-[1,2,3]triazol-4yl]-benzoic Acid Methyl Ester (3H). A solution of Vc (1.45 g, 8.233 mmol) and copper(II) sulfate pentahydrate (1.13 g, 4.526 mmol) in H2O (30 mL) was added to a mixture of 11 (1.92 g, 2.800 mmol) and 5 (0.44 g, 2.749 mmol) in THF (30 mL) under a nitrogen atmosphere. After stirring at room temperature for 48 h, the reaction mixture was transferred with 50 mL of dichloromethane to a separatory funnel. The organic solution was washed with water and then dried over anhydrous Na2SO4. The solvents were evaporated in vacuum, and the residue was purified using silica gel column chromatography with MeOH/CH2Cl2 (1:60, v/v) as the eluant to give 3H as a white solid (1.17 g) in 51% yield. Mp, 79−81 °C. 1H NMR (400 MHz, CDCl3, δ ppm): 8.09 (d, J = 8.4 Hz, 2H), 7.90 (d, J = 8.4 Hz, 2H), 7.75 (s, 1H), 6.51 (s, 2H), 5.47 (s, 2H), 3.93 (m, 9H), 1.78 (m, 6H), 1.45 (m, 6H), 1.26−1.30 (m, 48H), 0.87 (m, 9H). 13C NMR (500 MHz, CDCl3, δ ppm): 166.74, 153.73, 147.12, 138.76, 134.87, 130.22, 129.64, 129.16, 125.48, 120.29, 106.86, 73.51, 69.36, 54.73, 52.13, 31.94, 29.37−29.76, 26.09, 22.70, 14.11. HRMS: calculated for C53H87N3O5 [M + Na]+ 868.6544, found 868.6457. Anal. Calcd for C53H87N3O5: C 75.22, H 10.36, N 4.97; found: C 75.14, H 10.28, N 4.85. Synthesis of 4-[1-(4-Methoxy-benzyl)-1H-[1,2,3]triazol-4-yl]-benzoic Acid Methyl Ester (16H). A solution of Vc (1.89 g, 10.731 mmol) and copper(II) sulfate pentahydrate (1.60 g, 6.408 mmol) in H2O (30 mL) was added to a mixture of 15 (0.82 g, 5.029 mmol) and 5 (0.78 g, 4.873 mmol) in THF (30 mL) under a nitrogen atmosphere. After stirring at room temperature for 48 h, the reaction mixture was transferred with 50 mL of dichloromethane to a separatory funnel. The organic solution was washed with water and then dried over anhydrous Na2SO4. The solvents were evaporated in vacuum, and the residue was purified using silica gel column chromatography with MeOH/CH2Cl2 (1:60, v/v) as the eluant to give 16H as a white solid (1.10 g) in 70% yield. Mp, 172−174 °C. 1H NMR (400 MHz, CDCl3, δ ppm): 8.07 (d, J = 8.4 Hz, 2H), 7.88 (d, J = 8.4 Hz, 2H), 7.71 (s, 1H), 7.29 (d, J = 8.8 Hz, 2H), 6.92 (d, J = 8.8 Hz, 2H), 5.53 (s, 2H), 3.92 (s, 3H), 3.82 (s, 3H). 13C NMR (500 MHz, CDCl3, δ ppm): 166.77, 160.10, 147.10, 134.94, 130.18, 129.76, 129.57, 120.16, 114.62, 55.37, 53.94, 52.12. HRMS: calculated for C18H17N3O3 [M + Na]+ 346.1168, found 346.1162. Anal. Calcd for C18H17N3O3: C 66.86, H 5.30, N 13.00; found: C 66.71, H 5.19, N 12.87.

= 1.54056 Å). The gels were scooped onto the glass plates and dried in the atmosphere for the XRD measurement. Single-crystal XRD data for the crystal of 16I were collected on a Rigaku D/Max 2500 diffractometer using Cu Kα radiation (λ = 0.71073 Å) at 293 K, whereas that of 16H were collected on a Rigaku 007HF XtaLAB P200 diffractometer using Mo Kα radiation (λ = 0.71073 Å) at 113 K. The structures were solved using SHELXS-97 (direct methods) and refined using SHELXL-97 (full-matrix least-squares on F2) contained in WinGX software packages. Synthesis. The synthetic route for 1I, 1H, 3I, 3H, 16I, and 16H is shown in Scheme 1. Among the six compounds, 16I and 16H were used as reference compounds for the single-crystal X-ray analysis. 4Ethynyl-benzoic methyl ester (5), 1-azidomethyl-4-dodecyloxy-benzene (6), 5-azidomethyl-1,2,3-trisdodecyloxy-benzene (11), and 1azidomethyl-4-methoxy-benzene (15) were prepared according to the literature.44 5-Iodo-1H-1,2,3-triazole was achieved by a one-pot click reaction of azide with alkyne in the presence of CuI and Nbromosuccinimide (NBS) catalytic system, whereas 1H-1,2,3-triazole was synthesized with copper(II) sulfate pentahydrate and vitamin C (Vc) as a catalyst. The synthesized 1I, 3I, 16I and 1H, 3H, 16H were characterized using 1H NMR, 13C NMR, and HRMS. Elemental analyses were performed by the Elemental Analysis Center of Tianjin University. Synthesis of 4-[1-(4-Dodecyloxy-benzyl)-5-iodo-1H-[1,2,3]triazol4-yl]-benzoic Acid Methyl Ester (1I). A mixture of 6 (1.09 g, 3.436 mmol), 5 (0.50 g, 3.124 mmol), CuI (0.65 g, 3.413 mmol), DIPEA (0.60 mL, 3.435 mmol), and NBS (0.62 g, 3.484 mmol) in 40 mL of THF was stirred at room temperature for 48 h. The solvent was removed in vacuum, and the residue was purified using silica gel column chromatography with MeOH/CH2Cl2 (1:50, v/v) as the eluant to get the titled compound as a pale yellow solid (1.13 g) in 60% yield. Mp, 145−147 °C. 1H NMR (400 MHz, CDCl3, δ ppm): 8.11 (d, J = 8.4 Hz, 2H), 8.05 (d, J = 8.4 Hz, 2H), 7.28 (d, J = 8.8 Hz, 2H), 6.87 (d, J = 8.8 Hz, 2H), 5.62 (s, 2H), 3.94 (m, 5H), 1.75 (m, 2H), 1.43 (m, 2H), 1.26−1.30 (m, 16H), 0.87 (t, 3H). 13C NMR (500 MHz, CDCl3, δ ppm): 166.74, 159.42, 149.05, 134.65, 129.97, 129.83, 129.44, 127.16, 125.89, 114.84, 68.11, 54.15, 52.19, 31.92, 29.21− 29.65, 26.03, 22.69, 14.11. HRMS: calculated for C29H38IN3O3 [M + Na]+ 626.1856, found 626.1850. Anal. Calcd for C29H38IN3O3: C 57.71, H 6.35, N 6.96; found: C 57.53, H 6.27, N 6.68. Synthesis of 4-[1-(3,4,5-Tridodecyloxy-benzyl)-5-iodo-1H-[1,2,3]triazol-4-yl]-benzoic Acid Methyl Ester (3I). A mixture of 11 (2.64 g, 3.851 mmol), 5 (0.57 g, 3.561 mmol), CuI (0.74 g, 3.886 mmol), DIPEA (0.60 mL, 3.435 mmol), and NBS (0.61 g, 3.427 mmol) in 60 mL of THF was stirred at room temperature for 48 h. The solvent was removed in vacuum, and the residue was purified using silica gel column chromatography with MeOH/CH2Cl2 (1:60, v/v) as the eluant to get the titled compound as a pale yellow solid (1.57 g) in 45% yield. Mp, 67−69 °C. 1H NMR (400 MHz, CDCl3, δ ppm): 8.13 (d, J = 8.4 Hz, 2H), 8.06 (d, J = 8.4 Hz, 2H), 6.55 (s, 2H), 5.56 (s, 2H), 3.94 (m, 9H), 1.77 (m, 6H), 1.44 (m, 6H), 1.26 (m, 48H), 0.87 (m, 9H). 13C NMR (500 MHz, CDCl3, δ ppm): 166.71, 153.46, 149.05, 138.57, 134.61, 130.02, 129.85, 128.89, 127.15, 106.82, 73.46, 69.31, 54.77, 52.19, 31.94, 29.38−29.76, 26.10, 22.70, 14.12. HRMS: calculated for C53H86IN3O5 [M + Na]+ 994.5510, found 994.5423. Anal. Calcd for C53H86IN3O5: C 65.48, H 8.92, N 4.32; found: C 65.39, H 8.87, N 4.21. Synthesis of 4-[1-(4-Methoxy-benzyl)-5-iodo-1H-[1,2,3]triazol-4yl]-benzoic Acid Methyl Ester (16I). A mixture of 15 (0.82 g, 5.029 mmol), 5 (0.77 g, 4.811 mmol), CuI (0.92 g, 4.831 mmol), DIPEA (0.85 mL, 4.867 mmol), and NBS (0.87 g, 4.888 mmol) in 30 mL of THF was stirred at room temperature for 48 h. The solvent was removed in vacuum, and the residue was purified using silica gel column chromatography with MeOH/CH2Cl2 (1:70, v/v) as the eluant to get the titled compound as a pale yellow solid (1.45 g) in 67% yield. Mp, 169−171 °C. 1H NMR (400 MHz, CDCl3, δ ppm): 8.12 (d, J = 8.4 Hz, 2H), 8.04 (d, J = 8.4 Hz, 2H), 7.30 (d, J = 8.8 Hz, 2H), 6.89 (d, J = 8.8 Hz, 2H), 5.62 (s, 2H), 3.94 (s, 3H), 3.80 (s, 3H). 13 C NMR (500 MHz, CDCl3, δ ppm): 166.74, 159.82, 149.07, 134.64, 129.97, 129.83, 129.47, 114.31, 55.32, 54.10, 52.19. HRMS: calculated

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RESULTS AND DISCUSSION

To assess the noncovalent interactions existing in the synthesized compounds, an X-ray crystallographic analysis of their single crystals is the best choice. Initially, 1I, 1H, 3I, and 3H were all selected for the growth of single crystals. However, all endeavors resulted in failure. Fortunately, we succeeded in C

DOI: 10.1021/acs.langmuir.6b03691 Langmuir XXXX, XXX, XXX−XXX

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Langmuir the growth of single crystals from 16I and 16H, two control compounds bearing methoxy chains. The single crystal of 16I was obtained in the mixture of petroleum ether/CH2Cl2 using a slow solvent evaporation method, whereas that of 16H was crystallized from acetonitrile at room temperature. Their crystal data are listed in Table S1. The perspective view of 16I and 16H molecules is shown in Figures S1 and S2, respectively. The crystal structure of 16I, with the display of its C−I···O XB interaction, is shown in Figure 1. Figure 2 reveals the π−π

Figure 3. Crystal-packing modes of 16H. Color of the atoms: carbon (light gray), nitrogen (blue), and oxygen (red). Hydrogen atoms are omitted for clarity.

and the infinite chains will make a lamellar network. The selfassembled chains adopt a herringbone arrangement rather than a linear one, suggesting that the axes of the weak interactions in 16H are nonparallel. Figure 4 shows the noncovalent Figure 1. Crystal-packing modes of 16I exhibiting C−I···O XB interaction with the distance of 3.0278 Å (fragmented lines). Color of the atoms: carbon (light gray), nitrogen (blue), oxygen (red), and iodine (pink). Hydrogen atoms are omitted for clarity.

Figure 4. CH−π interactions (fragmented lines) in 16H. Color of the atoms: carbon (light gray), nitrogen (blue), oxygen (red), and hydrogen (light gray, small balls). Figure 2. π−π interaction with the distance of 3.7507 Å (fragmented lines) that existed in 16I. Color of the atoms: carbon (light gray), nitrogen (blue), oxygen (red), iodine (pink), and hydrogen (light gray, small balls).

interactions in the crystal of 16H. Five kinds of CH−π interactions are found, and the distances of hydrogen to the centroid of rings are 2.70, 2.78, 2.88, 2.91, and 2.95 Å, respectively (cf. Table S2). Also from Figure 4, it is clear that the molecules are packed with a longitudinal displacement. The two adjacent molecules are slightly oblique, whereas the two alternate ones are stacked on top of each other without any deviation. The difference in the arranged models of 16I and 16H reveals that the XB interaction does influence the structure of aggregates greatly. It is interesting, yet challenging to correlate the supramolecular self-assembly patterns of a compound with its gelling/nongelling behavior from its single-crystal data. Recent advances in this topic have been described by Dastidar.46 Efforts on the studies are still limited because it is difficult to get X-ray quality single crystals of gelators. To try to show the structure−property correlation in the synthesized compounds, the gelation behaviors of 1I, 1H, 3I, and 3H were tested in various solvents using the routine “inverted test tube” method, and the results are listed in Table 1. As summarized in Table 1, 1I is a very powerful gelator. It can gel almost all 24 tested solvents except for chloroform, pyridine, and dioxane. Moreover, most of the minimum gel concentrations (MGCs) are very low. Besides twelve of them are smaller than 1%, it is worth noting that seven MGCs are less than 0.5%, indicating the massive formation of superorganogels. By contrast, 1H can gel twelve tested solvents and none of the MGC is less than 1%, suggesting that the gelling ability of 1H is much inferior to that

interaction found in 16I. Obviously, the molecules are packed in the head−tail-alternation mode to form a one-dimensional packing column, and the neighboring columns are connected by the C−I···O XB interaction. In the column, the adjacent two molecules are rotationally arranged along the direction of the methylene carbons that are connected with N 1s of 1,2,3triazole rings, and the π−π interaction is found between the stacked benzene rings. The classical XB between the basic carbonyl O-atom (Lewis base, donor of electron density) and the I-atom (Lewis acid, acceptor of electron density) of another molecule has a C−I···O bond angle of 175.70° and an I···O distance of 3.0278 Å (cf. Table S2). The angle of XB in 16I is very close to the optimum 180°, and the I···O distance is shorter than the sum of mean van der Waals radii of the two atoms (3.4978 Å, cf. Table S2), indicating that the XB interaction is sufficiently strong in 16I. The centroid−centroid distance between the stacked benzene rings is 3.7507 Å (cf. Figure 2 and Table S2), a value approximately being the maximum contact for which the π−π interaction is accepted, suggesting that the π−π interaction in 16I is weak. The crystal structure obtained from 16H is completely different from that obtained from 16I. As shown in Figure 3, the molecules of 16H arrange in a b−c plane to form chains, D

DOI: 10.1021/acs.langmuir.6b03691 Langmuir XXXX, XXX, XXX−XXX

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important role in the formation of gels, reflecting by the fact that a certain gelator can gel certain solvents, yet cannot gel others. Moreover, it has been reported that the solvent can also affect the self-assembled nanostructures greatly.47 To evaluate the stability of the C−I···O XB interaction, 1I was further used to gel aqueous ethanol and aqueous THF. The results showed that it appeared to be an effective gelator for the organic solvents in water. For example, heating the mixtures of 1I (10 mg)−ethanol (1.2 mL)−H2O (0.4 mL) and 1I (30 mg)−THF (0.5 mL)−H2O (1 mL), respectively, to homogenous liquids followed by cooling to room temperature resulted in the formation of organogels, which were located at the top of the ungelled water. Somewhat surprised by the results, we sought to determine the possibility of gelling aqueous organic solutions. It was found that 1I could gel aqueous THF completely when the mixture had a proper ratio of water to THF. Moreover, even below the MGC for THF, 1I may gel the corresponding aqueous solution totally. For example, 10 mg of 1I cannot gel 0.5 mL of THF at 5 °C; however, it can completely gel the mixtures of 0.5 mL of THF containing 0.1− 0.3 mL of water. Figure 5 shows the results of the gelling

Table 1. Gelation Performances of 1I, 1H, 3I, and 3H in Organic Solventsa solvent acetonitrile acetone benzene chlorobenzene p-dimethyl benzene toluene chloroform dichloromethane 1,2-dichloroethane tetrachloromethane methanol ethanol isopropanol n-butylalcohol petroleum ether n-hexane cyclohexane diethyl ether dioxane ethyl acetate pyridine N,N-dimethylformamide triethylamine tetrahydrofuran

1I 0.3 0.5 2.8 4.3 1.6 2.8 S 5.7 2.6 0.6 0.1 0.5 0.4 1.9 0.2 0.4 0.3 0.6 P 0.5 S 2.6 0.8 4.3

(T) (T) (T) (T) (T) (T) (T) (T) (T) (T) (T) (T) (T) (T) (T) (T) (T) (T) (T) (T) (T)

1H 1.4 P 4.4 5.7 7.2 5.5 3.3 P P P Ins 1.7 2.3 3.4 Ins Ins Ins Ins 3.1 P 4.9 2.0 P P

(O) (O) (O) (O) (O) (O)

(O) (O) (O)

(O) (O) (O)

3I 0.1 0.9 S S S S S S S S 0.4 0.6 1.2 2.0 2.0 3.5 0.5 S S 0.7 S 1.5 S S

(T) (T)

(T) (T) (T) (T) (T) (T) (T)

(T) (T)

3H 1.4 1.5 P S P S S S 7.4 S 1.1 1.3 3.0 3.4 P P 0.6 P 7.2 P 6.4 2.6 P S

(O) (O)

(O) (O) (O) (O) (O)

(O) (O) (O) (O)

a

Values denote the MGC (in wt %) to achieve gelation at room temperature. S: solution; Ins: insoluble; P: precipitation; T: transparent; O: opaque.

of 1I. In the column of 3I, lower MGCs and three superorganogels are observed. Apart from that, an impressive observation is that it contains no precipitation item. Eleven gels and thirteen solutions constitute the whole result. It is presumable that, compared with 1I, the increase in the amount of alkoxy chains can improve solubility and thus make 3I more soluble. For the same reason, it is obvious that 3H is more soluble than 1H, which is reflected by the six solutions and seven precipitations of the former and the twelve insoluble/ precipitation items of the latter (Table 1). The great difference in the gellation performances of iodo gelators (1I, 3I) and of hydrogenous gelators (1H, 3H) clearly shows that the iodine atom at the 5-position of the triazole ring plays an important role in the formation of gels. The above-mentioned X-ray crystallographic analyses manifest that the introduction of the iodine atom results in the formation of a strong C−I···O XB interaction and a weak π−π interaction, although there is a loss in weak CH−π interactions at the same time. This is the reason why the gelling ability of iodo gelators is much better than that of hydrogenous gelators. Although the gelling ability of iodo gelators is much more powerful than that of hydrogenous gelators as a whole, it is worth noting that both 1I and 3I cannot form gel in dioxane or pyridine whereas 1H and 3H can gel them efficiently (Table 1). The observations should result from the gelator−solvent interactions, that is, the formation of XB between the iodo gelators and the solvents. In other words, dioxane and pyridine can destroy the XB between the gelator molecules and they will form a XB with the gelators. The collapse of XB between the gelator molecules results in the loss of gelling ability. Undoubtedly, the gelator−solvent interactions play an

Figure 5. Gelling aqueous THF with 1I. (A) 30 mg 1I + 0.5 mL THF; (B) 30 mg 1I + 0.5 mL THF + 0.3 mL H2O; (C) 30 mg 1I + 0.5 mL THF + 0.4 mL H2O; (D) 30 mg 1I + 0.5 mL THF + 0.6 mL H2O; (E) 10 mg 1I + 0.5 mL THF (5 °C); (F) 10 mg 1I + 0.5 mL THF + 0.2 mL H2O (5 °C); (G) 10 mg 1I + 0.5 mL THF + 0.3 mL H2O (5 °C); and (H) 10 mg 1I + 0.5 mL THF + 0.4 mL H2O (5 °C).

aqueous THF test. Obviously, the water in aqueous THF changes the nature of the solvent and thus “promotes” the gelling ability of 1I. It has been reported that some aliphatic 2,3,ω-1,ω-diesters are capable of gelling solvents from low polarity such as cyclohexane to extreme high polarity such as aqueous ethanol and water.48 Although 1I cannot gel pure E

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Langmuir water, it is very similar to those diester omnigelators in view of gelling solvents at both extremes of polarity. These observations imply that the C−I···O XB interaction is very stable and it can resist the impact from high polar water. Indeed, 1I is so powerful that it can gel not only nonpolar and intermediate polar solvents but also high polar solvents at very low MGCs, suggesting that the C−I···O XB interaction, the van der Waals interaction, and the weak π−π interaction act cooperatively and efficiently in the course of gelation. The gels formed by the as-synthesized gelators were all thermoreversible even after twenty heating−cooling cycles, and they were found to be stable for 6 months at room temperature. To compare the thermal stability of iodo gels, the gel−sol phase transition temperatures (Tgel) of 1I at different concentrations in ethanol, acetonitrile, and acetone were determined, respectively. Figure 6 shows the checked results. Predictably,

Figure 7. Objects for the shape-persistent experiment taken from the acetonitrile gels of (A) 1I, (B) 1H, (C) 3I, and (D) 3H. The concentrations of the gels are 0.07 mmol/mL. Figure 6. Plots of Tgel against the concentration of 1I in ethanol (black), acetonitrile (red), and acetone (blue) with the heating rate of 2 °C/min in a water bath.

respectively. As is shown in Figure 8a, only a weak sharp peak appears at 2θ = 7.42° (d = 11.9 Å) in the small-angle region; therefore, it is difficult to identify its structure from the XRD result. In the case of 16H, three peaks are found at 2θ = 2.02°, 5.49°, and 6.00°, and the d values are 43.7, 16.1, and 14.7 Å, respectively (Figure 8b). Among the three peaks, the first and the third are the reflections of Miller indices 100 and 200, respectively. In the small-angle region, two theoretical spectra are well in accordance with their experimental ones. The peak at 2θ = 7.38° (d = 12.0 Å) in Figure 8c and the peak 2θ = 5.40° (d = 16.4 Å) in Figure 8d are the counterparts of the peak at 2θ = 7.42° (d = 11.9 Å) in Figure 8a and the one at 2θ = 5.49° (d = 16.1 Å) in Figure 8b, respectively. The XRD spectra of xerogels obtained from different gels are shown in Figure 9. The curve for 1I/acetonitrile xerogels gives two sets of d values: 76.2, 43.7, 38.7, 19.1 Å; 28.8, 17.6, and 14.4 Å (Figure 9a). They match the periodicity of 1:1/√3:1/ 2:1/4 and 1:1/√3:1/2, respectively, indicating that the gelator molecules aggregate to two different hexagonal column structures. Figure 9b shows that the two peaks at 2θ = 2.48° (d = 35.6 Å), 5.86° (d = 15.1 Å), and the former correspond to Miller indice 200 reflection. Interestingly, the spectrum shows the absence of 100 and 110 reflections, whereas the one of 1I/ acetonitrile gives two sets of complete reflections. At this point, 3I/acetonitrile xerogels are much more similar to 16I/ acetonitrile xerogels than 1I/acetonitrile xerogels, although 1I and 16I are much more structurally similar. The spectra of Figures 8d and 9c indicate the lamellar structure of the 1H/ acetonitrile gel and 3H/acetonitrile gel distinctly. To gain a visual insight into the gel microstructures, SEM was used to study the xerogels obtained from the four gelators.

the Tgel value increases with the increase in the concentration of gelators, indicating that the concentration of gelators has a positive influence on the stability of gel networks. The Tgel values appear in the order of ethanol > acetonitrile > acetone, the same order of their polarity. It is worth noting that, under the concentration of 30 mg/mL, the Tgel value of 1I/ethanol gel reaches 85 °C and it is higher than the boiling point of ethanol. To provide a more conclusive claim on the superior gelation property of the iodo compounds, the thermal stability of different iodo gels and hydrogenous gels was checked, and the results are shown in Figure S3. It is very apparent that under the same molar concentration, the Tgel value of 1I is obviously higher than that of 1H and the maximum is 42 °C (0.03 mmol/ L). As to the hydrogenous gels, the Tgel value of 3H is higher than that of 1I and the maximum is 10 °C (0.05 mmol/L). As shown in Figure 7, the iodo gels have a much better mechanical property than the hydrogenous gels under the same concentration. Both blocks of 1I and 3I gels can be clipped integrally with ease. By contrast, the block of 1H is very fragile and it can hardly be clipped. The mechanical property of 3H is better than that of 1H, but it is still weak and easy to break. The good performance of the iodo gels in shape-persistence slightly contradicts the preconception that the physical gels are not stable and have a poor mechanical property. The XRD spectra of the thin solid layers obtained from 16I in acetonitrile and 16H in acetonitrile are shown in Figure 8a,b, respectively. As a contrast, their theoretical XRD spectra drawn from their X-ray crystallographic data are shown in Figure 8c,d, F

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Figure 8. XRD spectra of (a) the thin solid layer obtained from 16I/acetonitrile, (b) the thin solid layer obtained from 16H/acetonitrile, (c) theoretical outline illustrated using the X-ray crystallographic data of 16I, and (d) theoretical outline illustrated using the X-ray crystallographic data of 16H.

Figure 9. XRD spectra of the xerogels obtained from the gels of (a) 1I/acetonitrile, (b) 3I/acetonitrile, (c) 1H/acetonitrile, and (d) 3H/acetonitrile.

G

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to some two-dimensional assembly. It is known that the onedimensional aggregation is appropriate to form a needlelike assembly and the two-dimensional aggregation tends to form a sheetlike aggregate. The macroscopic structures identified using SEM are exactly predicted by the X-ray crystallographic study. The variable temperature 1H NMR of 1I in CD3CN and 1H in CD3CN was performed to intend to determine the driving forces of the gelation spectroscopically, and their spectra are partially shown in Figure 11. Before the examination of 1H NMR, the gel−sol phase transition temperatures of the two samples were tested, and they were under the gel state at 25 °C, the gel−sol transition state at 50 °C (1I−CD3CN) and 45 °C (1H−CD3CN), and the sol state both at 60 and 70 °C, respectively. As shown in Figure 11, the spectra obtained at 25 °C are featured by their weak and broad proton signals, which result from the extensive aggregation of 1I and 1H molecules in the gel state. Upon warming from 25 to 70 °C by three steps, the weak and broad peaks steadily sharpen and shift to upfield, reflecting the reduced π−π interaction or the CH−π interaction between the molecules of gelators upon warming. The small upshift of the peaks indicates that the π−π interaction and the CH−π interaction are weak ones, which are in accordance with the observations in the crystals of 16I and 16H. UV absorption spectral measurements were taken to investigate the stacking model of gelator molecules. A quartz cuvette with 1.0 mm width was used for the samples to express the ordinate with an arbitrary unit. As shown in Figure 12, the acetonitrile sol of 1I gave two characteristic absorptions at 225 and 274 nm. Upon gelation, the peak at 274 nm shifted to 268 nm whereas the other one could not be recorded. The blue shift of the absorption from sol to gel indicates the formation of H-aggregates. The gels from 1H are all opaque and they cannot be used for the UV measurement. As an alternate, the translucent gel of 3H/cyclohexane was measured with the gel sample sandwiched between two quartz glass plates. The results show that 3H also has a hypsochromic shift upon gelation (cf. Figure S4), indicating that the molecules adopt a H-aggregation mode in the gel phase. Gels of 1I/acetone and 1H/dioxane were chosen as samples to investigate their response to the metal-cation-stimuli test. To four vials of 1I/acetone gels and four vials of 1H/dioxane gels,

As shown in Figure 10, all gels formed by 1I or 3I in different solvents are fibrous, whereas those formed by 1H or 3H in

Figure 10. SEM images of the xerogels from the gels of (a) 3I/ methanol, (b) 3I/acetonitrile, (c) 1I/acetone, (d) 1I/acetonitrile, (e) 1H/ethanol, (f) 1H/acetonitrile, (g) 3H/acetone, and (h) 3H/ methanol.

different solvents are sheetlike. Videlicet, the morphologies of these gels are totally dependent on the gelator and independent of the gelating solvent. The foregoing X-ray crystallographic study suggests that the iodo gelator grows to some onedimensional assembly, whereas the hydrogenous gelator grows

Figure 11. Partial variable temperature 1H NMR spectra of (a) gel 1I/CD3CN (1.8 wt %) and (b) gel 1H/CD3CN (2.2 wt %). H

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Figure 12. UV absorption spectra of 1I in the sol phase (acetonitrile, 0.2 mg/mL, red line) and in the gel state (acetonitrile, 2.8 mg/mL, black line).

2 equiv of Hg2+, Zn2+, Cu2+, and Mg2+ (as perchlorate hydrate) was added carefully. Within 10 min, both the 1I gel and the 1H gel were destroyed by Hg2+ and they disintegrated to solutions completely. Other 1I gels with respective Zn2+, Cu2+, and Mg2+ were gradually collapsed, and most of the parts became solutions around 3 h after the addition of cations. At the same time, the gels of 1H with respective Zn2+, Cu2+, and Mg2+ were remained unchanged. The results of the test are graphically shown in Figure 13. It is worth noting that, on the one hand, the remanent gels of 1I gels with respective Zn2+, Cu2+, and Mg2+ are still insoluble even after the addition of great excessive cations and lasts for several months; on the other hand, the gels of 1H with respective Zn2+, Cu2+, and Mg2+ do not show any evidence of collapse even 6 months later. The derivatives of 1,2,3-triazole are excellent ligands in coordination chemistry, and myriads of applications of those ligand architectures have been reported.49−51 It is presumable that the collapse of gels results from the formation of metal complexes, which will disturb or destroy the weak interactions among the gelator molecules. Moreover, the observation of the response of 1I/ acetone gels to Zn2+, Cu2+, and Mg2+ suggests that the formation of a metal-coordination bond is competitive with the formation of C−I···O XB in these gels. They reach a balance finally and the gels disintegrate partly. To show their complexation motifs, we tried to culture the single crystals of the metal complexes from Hg2+, Zn2+, Cu2+, Mg2+ and 1I, 1H, 16I, 16H. However, all attempts failed. To confirm the breaking of the XB interaction upon metal complexation, the sols of the above-mentioned collapsed gels were spilt on the copper slides and dried at room temperature for 48 h and then were checked using SEM. Figure 14 shows their morphologies.

Figure 14. SEM images of the “xerosols” from the collapsed gels of (A) 3I/acetonitrile−Hg2+, (B) 1I/acetonitrile−Hg2+, (C) 3H/ acetonitrile−Hg2+, (D) 1H/acetonitrile−Hg2+, (E) 3I/acetonitrile− Cu2+, and (F) 1I/acetonitrile−Cu2+.

It can be seen that none of them is exact fibrous or sheetlike the morphologies of the original iodo gels and hydrogenous gels, which contain no metal ions. The “xerosols” with Hg2+ are porous (images A−D), whereas the two with Cu2+ are more compact, and the fibrous motifs still remained in a certain degree (images E and F). The observations are consistent with the facts that Hg2+ destroys the gels completely, whereas Cu2+ just integrates them partly. The interactions between the gelators and Hg2+ were further investigated using UV spectroscopy. Figure 15 shows their spectral changes after the addition of Hg2+. As shown in Figure 15a, with the progressive addition of Hg2+ to the acetonitrile solution of 1I, the intensity of the absorption peak at 274 nm decreases gradually, and the peak undergoes blue-shifting at the same time. However, the situation of 1H is not the same as 1I. For the acetonitrile solution of 1H, the addition of Hg2+ increases the intensity of the absorption peak at 275 nm and causes the peak shifts to shorter wavelengths within a certain Hg2+ amounts (0−2.2 equiv, Figure 15b). Then, the continuous addition decreases the intensity and finally both the intensity and the wavelength change very less when the cation amount is larger than 3 equiv. Although it is difficult to draw the exactly

Figure 13. Metal-cation stimuli test of 1I gel and 1H gel. I

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Figure 15. UV absorption spectra of (a) 1I acetonitrile solution (0.2 mg/mL) and (b) 1H acetonitrile solution (0.5 mg/mL) under different amounts of Hg2+ at room temperature.

coordinating motifs based on the UV results, it is obvious that the coordinating motifs of 1I and 1H with cation are different. It is reasonable to deduce that the binding behavior of triazole rings with the cation destroys the noncovalent interactions (XB, π−π or CH−π interactions) existing in the systems and causes the collapse of the gel completely.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

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ORCID

CONCLUSIONS In summary, we have reported the examples of organogelators with XB interaction and showed the enhancement in their gelling ability induced by the XB interaction through the X-ray crystallographic analysis for the first time. The XB interaction changes not only the noncovalent force motif of gelators but also the properties of their organogels. Five different CH−π interactions in hydrogenous gelators make themselves assemble to lamellar aggregations, whereas the strong XB interaction and the weak π−π interaction in iodo gelators guide the gelator molecules to form hexagonal columnar networks. The morphologies of gels from iodo gelators are all fibrous and the ones from hydrogenous gelators are all sheetlike. They are determined by the nature of gelators and independent of the gelling solvents. Finally, the organogels from the iodo gelators have better mechanical property than that from the hydrogenous gelators, and they respond to the stimuli of Hg2+, Zn2+, Mg2+, and Cu2+, but the gels from hydrogenous gelators respond to Hg2+ solely.

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mass spectra of compounds 16I, 16H, 1I, 1H, 3I, and 3H (PDF)

Yaodong Huang: 0000-0002-8294-5395 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Tianjin (no. 15JCYBJC20100). REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b03691. Crystallographic information of 16H (CIF) Crystallographic information of 16H (PDF) Crystallographic information of 16I (CIF) Crystallographic information of 16I (PDF) Crystallographic data of 16H and 16I, noncovalent interactions found in 16I and 16H, selected bond lengths (Å) and angles (°) of 16I and 16H, ORTEP representations for 16I and 16H, plots of Tgel against the concentration of 1I versus 1H and 3I versus 3H, UV absorption spectra of 3H in the sol phase and gel phase, 1 H NMR spectra, 13C NMR spectra, and high-resolution J

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