Chiral Supramolecular Gels with Lanthanide Ions - ACS Publications

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Chiral Supramolecular Gels with Lanthanide Ions: Correlation between Luminescence and Helical Pitch Chaelin Kim, Ka Young Kim, Ji Ha Lee, Junho Ahn, Kazuo Sakurai, Shim Sung Lee, and Jong Hwa Jung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13916 • Publication Date (Web): 06 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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ACS Applied Materials & Interfaces

Chiral Supramolecular Gels with Lanthanide Ions: Correlation between Luminescence and Helical Pitch Chaelin Kim,†,§ Ka Young Kim,†,§ Ji Ha Lee, †,‡,§, Junho Ahn,† Kazuo Sakurai,‡ Shim Sung Lee,† and Jong Hwa Jung*,† †

Department of Chemistry and Research Institute of Natural Sciences Gyeongsang National

University, Jinju, 660-701, Korea. ‡

Department of Chemistry, Kitakyushu University, Kitakyushu, 819-0395, Japan.

KEYWORDS Helical Pitch, Supramolecular Gel, Lanthanide Ion, Luminescence, Inkjet Printing

ABSTRACT: We report the correlation between the fluorescence intensity and the helical pitch of supramolecular hydrogels with Tb(III) and Eu(III) as well as their inkjet printing patterning as an application. The luminescent gels, which exhibited three different emissions of red, green, and blue, could be prepared without and with Eu(III) and Tb(III). The luminescence intensity of supramolecular gels (gel-Tb and gel-Eu) composed of Tb(III) and Eu(III) was ca. 3 fold larger than that of the sol (1+Tb(III) or 1+Eu(III)), which was attributed to large tilting angles between molecules. By AFM observations, these gels showed well-defined right-handed helical nanofibers formed by coordination bonds in which the helical pitch lengths were strongly

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dependent on the concentrations of lanthanide ions. In particular, the large luminescence intensity of gel-Tb exhibited a smaller helical pitch length than that of gel-1 due to relatively weak π-π stacking with large tilting angles between molecules. The luminescence intensities were enhanced linearly with increasing concentrations of lanthanide ions. This is the first example of the correlation between the helical pitch length and the luminescence intensity of supramolecular materials. The coordination bonding in supramolecular hydrogels had a strong influence on rheological properties. We also developed a water-compatible inkjet printing system to generate luminescent supramolecular gels on A4-sized paper. The images of a logo and the text were composed of three different emissions and were well-printed on A4 sized paper coated with gel-1.

INTRODUCTION Self-assembled gels composed of low molecular weight, oligomeric, or polymeric species are able to be formed by noncovalent interactions such as hydrophobic interactions, π–π stacking, H–bonding, and charge transfer interactions.1-10 These supramolecular gels have attracted great interest in the materials science field and applied as smart or adaptive materials,11-13 informationrecording materials,14 and drug delivery system in a clinical field.15-16 Of particular interest are supramolecular metallogels composed of lanthanide ions because of the narrow fluorescence band and high quantum yields shown by them when bound to a suitable π-chromophores.17-19 The foregoing was explicitly shown by Steed,20 Maitra,21 Rowan,22 De Cola,23 Yam,14 Gunnlaugsson,18,19 and co-workers. For instance, Gunnlaugsson et al. 18,19 have reported a novel methodolgy for the preparation of Tb(III) and Eu(III) complexed luminescent gels prepared with a picolinic acid derivatives. Gunnlaugsson’s group19 has controlled the


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luminescence properties of lanthanide ions-based supramolecular gels by mixing the Tb(III) : Eu(III) stoichiometric ratios. In addition, Maji et al.24 demonstrated that bimetallic metallogels showing controllable yellow and near white emission could be prepared through a self-assembly of terpyridine-based ligands with a mixture of Tb(III) and Eu(III) by coordination bond. Such unique luminescence property of lanthanide cations, originating from f-f electronic transition, has been studied in crystallized coordination polymers and applied for solid state lighting and sensing area.25-29 Recently, our group has also reported fluorescent supramolecular gels generated by combination of typical self-assembly concepts (such as H-bonding and π-π stacking) and coordination with metal ions, for which those ligands that can bind to transition metal cations were employed.30-35 Even though supramolecular metallogels have been applied in various fields such as device fabrication, biological science, and sensors, lanthanide ions-based supramolecular gels have been less well studied, and these studies basically focused on the luminescence and the rheological properties.36-38 Scientists have taken great interest in supramolecular chirality for its structural beauty, vital role in biological fields and potential for practical applications such as chiral separations and asymmetric catalysis.39-40 Control of the helical properties such as handedness, helical pitch, and helical pitch angles is very important for full exploitation of the potential of helices.41-47 The secondary helical structures of polymers consisting of covalent bonds have recently been fully established in several cases so that their helical pitch, helical sense, and packing angles could be seen.48-50 In contrast, the control of helical pitch, and helical sense in supramolecular gels induced by non-covalent bonds has been less well studied.51-53 Furthermore, the correlation between the luminescence intensity and the helical pitch length of supramolecular gels has not yet been reported, because the control of helical pitch length is difficult. Thus, if the relationship


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between the luminescence and the helical pitch can be clarified, in particular, the multifunctionalized materials that have fluorescence and helical properties could be significantly utilized in the chiral fluorescence sensing and chiral fluorescence imaging fields. In these regards, we have used the alanine moiety as a linker for self-assembly through intermolecular hydrogen-bonding interactions as well as control of helicity. The helically molecular arrangement of terpyridine moieties would prevent the quenching effect with π-π stacking, and was controlled by the concentrations of lanthanide ions. Herein, we describe the correlation between luminescence and helical pitch of a terpyridine-appended supramolecular gel with or without Tb(III) and Eu(III) incorporated in the structure. The luminescence properties of the supramolecular gels and the solution states of the gels with or without Tb(III) and Eu(III) were investigated by UV-Vis spectroscopy, luminescence spectroscopy, circular dichroism (CD) spectroscopy, and atomic force microscope (AFM). We also devised a new strategy for largearea patterns of lanthanide ions-based supramolecular gels on a solid substrate employing a common office inkjet printer. EXPERIMENTAL SECTION Synthesis of Compound 2. The compound 2 was prepared according to a literature procedure. 53

(R)-(-)-2-amino-1-propanol (0.28 g, 3.7 mmol) was put into a stirred suspension of powdered

KOH (1.05 g, 18.7 mmol) in dry DMSO (20 mL) at 60 °C. After 30 min, 4’-chloro-2,2’:6’,2”terpyridine 3 (1.00 g, 3.7 mmol) was put into the mixture. The mixture was then stirred for 4 h at 70 °C and poured into 600 mL of distilled water thereafter. CH2Cl2 (3×200 mL) was used to extract the aqueous phase. Residual water in dichloromethane was dried over Na2SO4 and CH2Cl2 was removed by vacuo, and the desired product was purified by recrystallization with


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ethyl acetate to give 0.72 g (72 %) of 1. Mp = 118.3 °C, 1H-NMR (300 MHz, CDCl3) 8.7(d, 2H, ArH, J =4.7 Hz), 8.6(d, 2H, ArH, J =7.4 Hz), 8.0(m, 4H, ArH), 7.5(m, 2H, ArH), 4.0(m, 2H, -OCH2-CH-), 3.2(dd, 1H, -CH-, J =6.3 Hz and J =12.5 Hz), 1.7(s, 2H, -NH2), 1.1(d, 3H, -CH3, J =6.4 Hz).

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C NMR (125 MHz, CDCl3) 160.2, 156.5, 154.2, 148.9, 123.7, 120.8, 105.2, 70.1,

48.1, 21.4. ESI-MS (m/z) calculated for C18H18N4O: 305.35, found: 306.2; 307.4 [M+H]+. Element analysis: calculated for C18H18N4O: C 70.6, H 5.9, N 18.3; Found: C 70.1, H 5.5, N 17.8. Synthesis of Compound 1. In a two neck flask, 2 (0.50 g, 1.64 mmol) and TEA (0.1 mL, 0.72 mmol) were poured into dry CH2Cl2 (10 mL). After cooling the solution in an ice bath and magnetically stirring the solution, sebacoyl chloride 4 (0.16 mL, 0.75 mmol) was added dropwise for reactions. The reactant was stirred for 3 h at room temperature. The crude product was recrystallized from CH2Cl2 to give a white crystalline solid 1 in 49.68 % yield (0.632 g). Mp = 198 °C, 1H NMR (300 MHz, DMSO-d6) 8.76 (dd, J = 12.39, 6.12 Hz, 4H), 8.16 (dd, J = 11.12, 4.21 Hz, 4H), 8.06 (s, 4H), 7.93 (d, J = 6.88 Hz, 2H), 7.63 (dd, J = 6.69, 5.42 Hz, 4H), 4.37-4.02 (m, 6H), 2.04 (t, J = 7.25, 7.25 Hz, 4H), 1.42 (dd, J = 8.33, 3.71 Hz, 4H), 1.18 (dd, J = 16.73, 8.85 Hz, 8H), 13C NMR (125 MHz, DMSO-d6) 173.8, 160.5, 156.4, 155.3, 149.2, 137.2, 123.6, 121.4, 106.1, 68.1, 47.0, 36.8, 28.9, 28.6, 25.6, 17.3 ESI-MS (m/z) calculated for C46H48N8O4: 776.38, found: 777.32 [M+H]+. Element analysis: calculated for C46H48N8O4: C 71.11, H 6.23, N 14.42; Found: C 71.08, H 6.15, N 14.32. RESULTS AND DISCUSSION Synthesis and Characterization of Ligand 1 and its Complexes with Tb(III) and Eu(III). Terpyridine-based ligand (1) was synthesized in three steps with 4´-chloro-2,2´:6´,2´´-terpyridine


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3 (Scheme 1). Compound 3 was treated with (R)-(2)-amino-1-propanol in KOH in DMSO to introduce chirality. Compound 5 was chlorinated. Thereafter, compound 2 and compound 4 were reacted in dichloromethane in the presence of triethyl amine to afford anticipated ligand 1 in 85% yield. Ligand 1 was completely characterized by 1H,

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C NMR, ESI-MS spectroscopy and

elementary analysis.

Scheme 1. Synthesis of terpyridine-based ligand 1. The self-assembly of 1 with Tb(III) and Eu(III) in the solution of H2O/DMSO (3:7 v/v) at low concentration was investigated. The absorption spectrum of 1 centered at 279 nm (ε279 = 2.56 x 104 M-1 cm-1), which was consisted of two main bands and a shoulder band at 311 nm (ε311 = 3.4 x 103 M-1 cm-1). When Tb(III) or Eu(III) was added to the solution of 1 in H2O/DMSO (3:7 v/v), no significant changes occurred in the absorption spectra of the solution (Figure S1 in Supporting Information), whereas bands at 311 nm and 325 nm increased. These bands originated from metal-ligand charge transfer (MLCT) between Tb(III) or Eu(III) and the terpyridine moieties of 1.7,19 Furthermore, we also observed the luminescence spectra of the solution of 1 and Tb(III) or Eu(III) with excitation at 325 nm (Figure 1). When lanthanide ions were added, the luminescence


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intensity increased for both systems indicating that the coordination between lanthanide ions and 1 led to successful transition to the excited electronic states of these cations; the characteristic emission bands appeared at 489, 545, 585, and 622 nm (assigned to 5D4→7FJ (J=6-3)), while for Eu(III) these bands were shown at 579, 593, 616, 650, and 696 nm (assigned to 5D0→ 7FJ (J=04)). These luminescence spectra were gradually increased until 1 equivalent of Tb(III) or Eu(III) was added. These emissions almost reached equilibrium after the addition of 1 equivalent of metal ions, indicating that one Tb(III) or Eu(III) ions was formed from one molecule of ligand 1 with a 1:1 stoichiometric ratio. The nonlinear regression analysis program SPECFIT was used to fit the data for analysis of the variations in the ground states and excited states for both titrations.54-55 It was suggested that the formation of one species and changes of the spectroscopic data were clearly fitted to 1:1 stoichiometry by factor analysis of the overall variations observed with these titrations. The binding constants of 1 for Tb(III) and Eu(III) were calculated to be log K = 1.30 x 105 and 3.00 x 104, respectively.


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Figure 1. The luminescence spectra of solution 1 (5×10-4 M) upon titrating with (A) Tb(NO3)3 and (C) Eu(NO3)3 (0-6 equivalents) in H2O/DMSO (3:7 v/v). Plot of fluorescence intensity of solution 1 (5×10-4 M) upon titrating with (B) Tb(NO3)3 (at (a) 489, (b) 545, (c) 585, (d) 622 nm) and (D) Eu(NO3)3 (at (a) 579, (b) 593, (c) 616, (d) 650, (e)696 nm) (0-6 equivalents) in H2O/DMSO (3:7 v/v) (excitation : 325 nm, cell width : 10 mm). Formation of Lanthanide Ion-Based Helical Supramolecular Gels. After demonstrating the formation of a higher order self-assembly in solution, we paid our attention to the formation of self-assemblies inducing large supramolecular polymer at higher concentration because it was expected to lead to the preparation of supramolecular gels. Thus, we evaluated the gelation ability of 1 upon addition of Tb(III) and Eu(III), respectively, in various solvents. For instance, a


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coordinated supramolecular gel 1 could be prepared when 1wt% of 1 was dissolved in a mixture of DMSO and various solvents such as CH2Cl2, CHCl3, toluene, ethyl acetate, MeOH, EtOH, acetonitrile, acetone, tetrahydrofuran, and water (Figure S2). A small volume of Tb(III) or Eu(III) aqueous solution in concentrations of 0-1 equivalent to the ligand concentration was added to the solution of 1, because ligand 1 was formed 1:1 complexes with Tb(III) or Eu(III) in the solution. Then, the mixed solution samples were maintained for a week at room temperature. Table S1 summarizes the results of the gelation behavior of 1 with Tb(III) and Eu(III) in various solvents. Opaque gels were obtained without and with Tb(III) or Eu(III) in a solution of H2O and DMSO (3:7 v/v) within 10 min (Figure 2), which led to instant formation of mechanically strong gels possessing high resistance to inversion tests as revealed for both the Tb(III) and Eu(III) gels (Figure 2) (abbreviated as gel-Tb and gel-Eu). These gels were also maintained stably for more than 6 months, indicating that ligand 1 formed a network structure with Tb(III) or Eu(III) through coordination bonding. In addition, ligand 1 formed gel in the absence of metal ion (abbreviated as gel-1). When exposed to UV-light irradiation, these gels were found to be strongly luminescent, which displayed green-, red-, and blue emission colors.


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Figure 2. Photographs of supramolecular gels (1 wt%) without and with various concentration of (A) Tb(III) and (B) Eu(III) (0~1 equivalent). We observed the changes of absorption and luminescence spectra upon addition of Tb(NO3)3 and Eu(NO3)3 to gel-1 in H2O/DMSO (3:7 v/v) (Figure 3). The absorption band of gel-1 without metal ions appeared at one main band centered at 279 nm (ε279 = 2.29x 104 M-1 cm-1) with a shoulder band at 311 nm (ε311 = 3.5 x 103 M-1 cm-1), which is quite similar to the solution. These absorption bands were assigned as the π(tpy) → π*(tpy) transition typical of terpyridine


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derivatives.7 The absorbance at 325 nm slightly increased with increasing concentrations of Tb(III) and Eu(III), which was caused complex formation between 1 and the lanthanide ions. The emission spectrum of gel-1 was also obtained at 425 nm (λex=325 nm), which is owing to the π→ π* fluorescence of terpyridine. The luminescence intensity of gel-1 at 425 nm was ca. 9 fold enhanced in compared to sol 1 at the same concentration (Figure S3), suggesting that the blue emission of gel-1 is due to helical aggregation between terpyridine and terpyridine moieties with weak π-π stacking, which was attributed to the aggregation-induced emission (AIE) effect.56-57,25 More interestingly, the gels obtained with Tb(III) or Eu(III) exhibited narrow emission bands at 579, 593, 616, 650, and 696 nm for Eu(III) (5D0→ 7FJ (J=0-4)) and 489, 545, 585, 622 nm for Tb(III) (5D4→7FJ (J=6-3)) (Figure 3E, 3G). These emission bands were structurally analogous to the emission bands appeared in the spectrum obtained from the self-assembly formation that occurred in the solution state discussed above. More interestingly, the emission intensities at 426 nm in gel-Tb and gel-Eu were ca. 2 fold enhanced as compared to gel-1, which originated from the terpyridine moiety. The enhanced emission of both gel-Tb and gel-Eu was due to the effective helical molecular arrangement with weak π-π stacking between the ligand molecules 1. In contrast, the emission peaks of gel-Tb and gel-Eu between 489 nm to 622 nm were ca. 3 fold increased as compared to the solution (1+Tb(III) or 1+Eu(III)). The dramatic luminescence enhancement should be attributed to facile energy transfer from terpyridine moiety to Tb(III) or Eu(III) ions. We analyzed the color perception of gels without and with metal ions by using the CIE chromaticity diagram. The color of gel without lanthanide ions is consistent with coordinates of (0.229, 0.240) on the CIE diagram (Figure S4), which is indicative of a typical blue color. In contrast, the colors of gels with Tb(III) and Eu(III) correspond to coordinates of


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(0.302, 0.546) and (0.544, 0.264), respectively. These CIE values are slightly different from a typical gel prepared with Tb(III) or Eu(III) as reported previously in the literature,19,25,29 which were correlated with the blue emission of the terpyridine moiety of 1. The quantum yields of gelTb (1.0 equivalent) and gel-Eu (1.0 equivalent) have been measured by previous reported method.58 The values obtained were Φ=0.25 for gel-Tb and Φ=0.20 for gel-Eu at 25 oC.


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Figure 3. The absorption spectra of gel-1 (1×10-4 M) upon titrating with (A) Tb(NO3)3 and (C) Eu(NO3)3 (0-1 equivalents) in H2O/DMSO (3:7 v/v). Plot of absorption intensity of gel-1 (1×10-4 M) upon titrating with (B) Tb(NO3)3 and (D) Eu(NO3)3 (0-1 equivalents) (at (a) 279, (b) 311, (C) 325 nm) in H2O/DMSO (3:7 v/v) (cell width : 2 mm). (E) The luminescence spectra of gel-1 (1.30X10-3 M) upon titrating with Tb(NO3)3 and (G) Eu(NO3)3 (0-1 equivalents) in H2O/DMSO (3:7 v/v). Plot of fluorescence intensity of gel-1 (1.30X10-3 M) upon titrating with (F) Tb(NO3)3 (at (a) 423, (b) 489, (c) 545, (d) 585, (e) 622 nm) and (H) Eu(NO3)3 (0-1 equivalents) (at (a) 421, (b) 579, (c) 593, (d) 616, (e) 650, (f) 696 nm) in H2O/DMSO (3:7 v/v) (excitation : 325 nm, cell width : 2 mm). To obtain evidence for the helical molecular arrangement, we observed the CD spectra of 1 in the sol state and gel state (Figure S5). The ICD (induced circular dichroism) spectrum of gel-1 exhibited the positive sign at 312 nm (Figure S5A), which originated from the terpyridine moiety of 1, but not the alanine moiety. In contrast, sol 1 did not showed any CD signal at the same wavelength (Figure S5A). These findings suggest the view that molecules of 1 were helically arranged through the intermolecular H-bonding and π-π interactions with a tilted molecular arrangement. Interestingly, the ICD intensities of both gel-Tb and gel-Eu were higher than that of gel-1 under the same conditions (Figure S5B, S5C), indicating that lanthanide ions in gel-Tb and gel-Eu could induce a relatively well-defined helical molecular arrangement of 1 in complex with lanthanide ions, as compared to gel-1 alone. The circularly polarized luminescence (CPL) spectra of gel-Tb (1.0 equivalent) and gel-Eu (1.0 equivalent) were observed in Figure S6. The CPL spectra of both gel-Tb and gel-Eu inhibited the positive sign at 540 nm and 619 nm, respectively, indicating that the lanthanide ions in gel-Tb and gel-Eu would arranged into the right-handed helicity. Also, there were significant differences on the line shapes of the observed


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spectra for two lanthanide complexes, which was attributed to crystal-field splitting of the electronic level.59-60 The gel-Tb showed a higher circularly polarized luminescence intensity than that of the gel-Eu. The effects of variations in temperature on the absorption and luminescence spectra of gels prepared without and with lanthanide ions were also observed (Figures S7 and S8). When the temperature of the dilute solution of gel-1 was increased from 293 to 393 K with and without Tb(III) or Eu(III) (1.0 equivalent), the emission band at 425 nm decreased in intensity. This luminescence intensity at 425 nm in gel-1, gel-Tb, and gel-Eu disappeared at 60 °C, 75 °C and 70 °C, respectively. These temperatures define the sol-gel transition temperatures for each preparation. In addition, the weak luminescent spectra of gels at high temperature were due to the aggregation species of 1 transforming to the monomer species. In contrast, the emission bands between 489 nm to 696 nm became relatively decreased in intensity (Figure S8) in comparison to the terpyridine moiety at 425 nm. Since the emission band at 425 nm is distinctively temperature-dependent, we propose that the emission originates from the formation of the aggregated species in H2O/DMSO (3:7 v/v), as it is well known that a decrease of temperature would favor aggregate formation.59,35 This temperature-dependent emission band has been tentatively suggested to derive from the aggregation species. We also measured the solgel transition temperatures of gels by the vial inversion test method (Figure S9-S11). The gel-1 began to become a partial sol at around 60 °C and was complete changed into the sol at 80 °C, while the gel-Tb and gel-Eu became complete sol at around 75 °C and 70 °C, respectively, upon heating without stirring.


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Morphological Properties of Gels Prepared with Tb(III) or Eu(III). The morphologies of the prepared gel-1, gel-Tb, and gel-Eu prepared were investigated by atomic force microscopy (AFM). The AFM image of gel-1 without lanthanide ions exhibited a clear right-handed helical structure with 44.5 nm of diameter, 48.9 nm of pitch length, and 35 ± 1 o of pitch angle (Figure 4A), which were in agreement with CD observation. These findings also support the view that the helicity of 1 at the molecular level was reflected into the macro- or supramolecular-level helicity. In contrast, the helical pitch lengths of gel-Tb with increasing concentrations of Tb(III) decreased from 48.9 nm to 30.7 nm, whereas the helical pitch angels of gel-Tb were no large changed by the concentration of Tb(III) (Figure 4A, 4C). The gel-Eu was also changed from 48.9 nm to 39.1 nm of pitch length by increasing concentrations of Eu(III) (Figure 4B, 4D). These findings indicate that the molecules 1 of gel-Tb were helically well-arranged, and the π-π stacking between molecules of 1 was weaker than in gel-1. Conclusively, the gel-Tb, which has a smaller pitch length, showed higher luminescence enhancement than did gel-1 and gel-Eu, which was ascribed to energy transfer (ET) from the terpyridine moiety to Tb(III) by the helical molecular arrangement as well as the relative weaker π–π stacking with high tilting angle


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between

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molecules.

Figure 4. (A) AFM images of gel-Tb and prepared with different Tb(III) concentrations; (a) 0, (b) 0.25, (c) 0.5, (d) 0.75, and (e) 1.0 equiv. (B) AFM images of gel-Eu prepared with different Eu(III) concentrations; (a) 0.25, (b) 0.5, (c) 0.75, and (d) 1.0 equiv. 3D models of right-handed helical sturtures of (C) gel-Tb and (D) gel-Eu of the internal and external helical structures (upper: helical pitch, below: the titling angle between lanthanide ion coordinated two molecules 1). The fluorescence image of gel-1 emitted a strong blue color by fluorescence microscopy (Figure S12). In contrast, the green- and the red-like fluorescence images of gel-Tb and gel-Eu, respectively, were also obtained (Figure S13, S14), which originated from the formation of a complex of 1 and lanthanide ions by coordination bonds between them. The powder-XRD patterns of gel-1, gel-Tb, and gel-Eu were also observed (Figure S15, S16). For the gel-1, a broad peak was observed at a 2θ value of 25.22, indicating an interlayer distance


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of 3.5 Å between the aromatic groups of the gelator. Furthermore, the distance between the aromatic groups of the gel-Tb and gel-Eu prepared with different concentrations of Tb(III) and Eu(III) showed the same component at 3.5 Å. These results indicate that the interlayer distance of the aromatic groups of the gelator in gel-1 was the same as those of gel-Tb and gel-Eu. Correlation between Luminescence Intensity and Helical Pitch. To investigate the correlation between luminescence intensity and the helical pitch length of supramolecular gels, we observed AFM images of supramolecular gels prepared with different concentrations (0, 0.25, 0.5, 0.75, and 1.0 equiv.) of Tb(III) or Eu(III). In the cases of both Tb(III) and Eu(III), the helical pitch lengths (from 48.9 nm to 39.1 nm for Eu(III), from 48.9 nm to 30.7 nm for Tb(III)) decreased linearly with increasing concentrations of lanthanide ions (Figure 4, 5). In contrast, increases of concentration of Tb(III) showed large helical pitch change in comparison to Eu(III). The helical fiber width and pitch angles were not greatly changed by increasing concentrations of lanthanide ions (Table 1). The correlation between the luminescence intensities and the helical pitch lengths is shown in Figure 5. Very interestingly, Figure 5 showed a good linearity. As mentioned above, the distance between two gelator molecules with π-π stacking was constant (~3.5 Å) by powder XRD observations. The tilting angle between two molecules in a short helical pitch length was relatively large as compared to the large helical pitch length (Table 1, Figure 4C, 4D). The tilting angle between molecules in gel-Tb was larger than those of gel-Eu and gel-1. The large tilting angle induced weak π-π stacking, and resulted in facile energy transfer from the terpyridine moiety to the lanthanide ions, which caused the large luminescence intensity. This is the first example of a correlation between the luminescence intensity and the helical pitch length.


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Table 1. Parameters for helical fiber in supramolecular gels.

Tb (III) 1+0.25 equiv.

1+0.5 equiv.

1+0.75 equiv.

1+1.0 equiv.

48.9

45.8

38.0

35.2

30.7

35.4

34.9

35.1

34.9

2.5

2.8

3.3

3.6

1 Pitch (nm) Pitch angel (°) Tilting angel (°)

Eu (III) 1+0.25 equiv.

1+0.5 equiv.

1+0.75 equiv.

1+1.0 equiv.

48.9

47.7

46.9

43.2

39.1

34.7

35.4

34.9

34.7

35.1

34.8

4.1

2.5

2.6

2.7

2.9

3.1

1


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Figure 5. The correlation between luminescence intensity vs. the helical pitch length of (A) gelTb (1.30×10-3 M, at 489 nm) and (B) gel-Eu (1.30×10-3 M, at 616 nm). Luminescence Lifetimes of Supramolecular Gels. A time-resolved fluorescence emission study of the gels was carried out with different concentrations of lanthanide cations (0, 0.2, 0.4, 0.6, 0.8 and 1 equiv.; Figure S17, S18) with a view to understanding the excited-state behavior of the gels without and with lanthanide ions. The luminescence lifetime of 1 was calculated to be on the order of less than a nanosecond in the sol state. On the other hand, the luminescence lifetime of gel-1 was found to be 1.75 ns (Table S2), which was similar to soft nanomaterials reported in the literature.61-62 Thus, the blue emission of gel-1 was originated from the aggregation species of 1 as observed by fluorescence spectroscopy. More interestingly, in gel-Tb and gel-Eu, the lifetimes linearly increased as the concentration of Tb(III) or Eu(III) increased (Table S2). The luminescence lifetimes of gel-Tb and gel-Eu dramatically increased with upon addition of one equivalent of lanthanide ions and were calculated to be 1008 µs and 568 µs, respectively, which is 6x105~3x105 fold longer than that of gel-1. These lifetimes on the order of microseconds of gel-Tb and gel-Eu originated from formation of 1-Tb(III) and 1-Eu(III) complexes formed by coordination bonds. These lengthy lifetimes of both gel-Tb and gel-Eu will be useful in optical electronics and sensing applications. Rheological Properties of Gels. To gain an understanding of the mechanical properties of these unusual gels, the mechanical properties of the gels including strain, and frequency sweeps were measured by using a rheometer. The viscoelastic properties of the three gels obtained without and with lanthanide ions were typical of those seen in a cross-liked polymer network


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(Figure 6). Notably, for averaging the data represented in Figure 6, five successive trials on the same gel were carried out. The three gels showed a typical elastic property by the strain amplitude sweeps. (Figure 6). A rapid decrease in the loss modulus of the three gels above critical strain region (γ = 18%) was observed, suggesting a collapse of the gel into a quasi-liquid state. The rheological properties were recovered immediately after a large amplitude oscillatory break-down. When the largeamplitude oscillatory force (γ = 100%; frequency, ω = 6.0 rad s-1 (1.0 Hz)) was applied, the G´ value of gel-Tb decreased from 1.07 MPa to 0.5 Pa, leading to a quasi-liquid state (tan δ = G”/ G’ ≈ 10). The G´ recovered rapidly to the initial value and the system returned to a quasi-solid state (tan δ = G”/ G’≈ 0.15) coincidentally as the amplitude was decreased (γ = 0.1%) at the same frequency (1.0 Hz). When lanthanide ions were added, the G’ and G” values of gel-Tb and gel-Eu increased ca. 2.2- and 1.6-fold, respectively, compared to that of gel-1, suggesting that the gels formed by lanthanide ions were much stronger than that gel-1 formed without metal ion because of the formation of the network structure. The high G’ and G” values of gel-Tb compared to those of gel-Eu were attributed to strong bonding between Tb(III) and 1 by intermolecular hydrogen-bond interactions and coordination bonds. These findings indicate that increases of rheological properties of gel-Tb are consistent with the luminescence enhancement.


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Figure 6. Strain sweep (frequency: 0.01 rad s-1) of supramolecular gel-1 (2 wt%) (A) without and (B) with Eu(III) and (C) Tb(III) (1.0 equivalent). Creating Patterned Luminescence Imaging by using an Inkjet Printing Method. The inkjet printing technology has attracted great attention due to its utilization in the easy application for large-scale imaging with high throughput, low costs for manufacture, and low occurrence of waste products associated with its employment.63-66 The inkjet printing method has been applied on hard surfaces, including silicon films, polymer layers, and glass substrates. In contrast, supramolecular gels-applied inkjet printing on paper substrates has never been employed. If an adaptation of supramolecular gels to inkjet printing system by using an ordinary office inkjet printer could be succeeded, it should be possible to utilize paper-based devices to the fullest extent in both scientific researches and biological application fields. Thus, we explored the possibility of generating gel-1 with Tb(III) and Eu(III) on the paper using the inkjet printing method. The various concentrations of diluted gel-1 were printed on paper by inkjet printer. The luminescence intensities of gel-1 coated papers (gel-1-P) were observed by fluorimeter (Figure S19). The luminescence intensity of papers (gel-1-P) increased linearly as the concentration of gel-1 increased. The gel-1-P (ca. 1 x 10-7 M, 0.1 wt%) exhibited blue emission under irradiation with a UV lamp. These results indicate that gelator 1 formed an aggregate structure in the solution state. Then, we carried out inkjet printing with various concentrations of Tb(III) or


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Eu(III) solution on the gel-1-P (Figure S20). Both green and red luminescence was emitted by using the inkjet printing method. These colors were revealed clearly with a highly diluted solution (ca. 1 x 10-6 M) of Tb(III) and Eu(III) on the blue colored paper (gel-1-P), suggesting that the special color patterned imaging can be efficiently achieved on the gel-1-P even with highly diluted concentrations of lanthanide ions. Furthermore, examined the fluorescence emission of a logo and the text prepared on the gel-1-P after dipping it in various concentrations of Tb(III) and Eu(III) solutions. Strong green, red, and blue colors were observed on the gel-1-P when irradiated with a UV lamp (Figure 7). Thus, an emissive gel formed by Tb(III) or Eu(III) can be useful to make a specific secret code on paper. The logo of our University and the text were also printed on the gel-1-P (Figure 7). As expected, the logo with the green and the red colors was clearly displayed on the gel-1-P background with blue color when irradiated UVlamp.

Figure 7. Photographs of the fluorescent pattern by (A) logo and (B) text printing under the UV lamp. CONCLUSIONS


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In conclusion, we have demonstrated that a terpyridine-appended ligand formed luminescence supramolecular gels without and with Tb(III) or Eu(III) in a mixed H2O/DMSO (3:7 v/v) solution. The supramolecular gels (gel-Tb(III) and gel-Eu(III)) showed a remarkably enhanced luminescence along with a long lifetime, in comparison with the gel-1, which was dependent on the concentration of lanthanide cations. Furthermore, the gel-Tb and gel-Eu showed well-defined right-handed helicity with strong coordination bonding, and intermolecular hydrogen-bonding interaction as compared to the gel-1. Interestingly, the well-defined helicity of gel-Tb and gel-Eu induced stronger luminescent properties than that of gel-1 due to energy transfer from the terpyridine moiety to the lanthanide ions. The helical pitch lengths of supramolecular gels were well-controlled by the concentrations of lanthanide ions. Interestingly, the short helical pitch lengths showed the enhancement of the luminescence intensities. The rheological property was confirmed by rheometer, which demonstrated that a difference existed between the three gels (gel-Tb, gel-Eu and gel-1), an outcome of the self-assembled supramolecular interaction including coordination bond formed between the lanthanide ions and the gelators in the gel formation. These results revealed that gelator-metal ion interaction such as coordination bond is a significant role on gelation process. In addition, we developed a water-compatible inkjet printing system that can be employed to generate luminescent supramolecular gel on the paper. The ink solutions were facilely shifted to a paper substrate by employing an ordinary office inkjet printer, which resulted in green and red luminescence colors. We truly believe that our strategy for these supramolecular gels could support innovative designs and new prospects for development of robust and facilely processed emissive materials for potential applications in optoelectronics devices, sensors, paper-based devices, and specific secret barcode systems. ASSOCIATED CONTENT


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Supporting Information. UV–Vis data (Figures S1 and S7), luminescence data (Figures S3, S8, S19, and S20), CIE image (Figure S4), CD data (Figure S5), fluorescent microscopy image (Figures S12, S13, and S14) and XRD data (Figure S15 and S16). AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions § These

authors contributed equally to this work.

ACKNOWLEDGMENT This work was supported by the NRF (2015R1A2A2A05001400 and 2012R1A4A1027750) from the Ministry of Education, Science and Technology, Korea. In addition, this work was partially supported by a grant from the Next-Generation Bio Green 21 Program (SSAC, grant#: PJ011177022016), Rural development Administration, Korea. J. H. L. thanks the Japan Society for the Promotion of Science (JSPS) for research fellowships 15F15342. REFERENCES (1) Fages, F. Metal Coordination to Assist Molecular Gelation. Angew. Chem., Int. Ed. 2006, 45, 1680-1682. (2) Abdallah, D. J.; Weiss, R.G. Organogels and Low Molecular Mass Organic Gelators. Adv. Mater. 2000, 12, 1237-1247. (3) Goh, C. Y.; Becker, T.; Brown, D. H.; Skelton, B. W.; Jones, F.; Mocerino, M.; Ogden, M. I. Self-Inclusion of Proline-Functionalised Calix[4]arene leads to Hydrogelation. Chem. Commun. 2011, 47, 6057-6059. (4) Xing, B.; Choi, M.-F.; Xu, B. A Stable Metal Coordination Polymer Gel Based on a Calix[4]arene and Its Uptake of Non-ionic Organic Molecules from the Aqueous Phase. Chem. Commun. 2002, 4, 362-363.


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Table of Contents (TOC)


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Chiral Supramolecular Gels with Lanthanide Ions - ACS Publications

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