Fei
Chen
a,
Yong-Mei
Wang
a,
Weiwei
Guo
a and
Xue-Bo
Yin
*ab
aState Key Laboratory of Medicinal Chemical Biology, Tianjin Key Laboratory of Biosensing and Molecular Recognition, College of Chemistry, Nankai University, Tianjin, 300071, China. E-mail: xbyin@nankai.edu.cn; Fax: +86-22-23503034
bCollaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin, 300071, China
First published on 28th November 2018
Fluorescence significantly improves the performance of gels. Various strategies, such as embedment and crosslinking, have been used to integrate extrinsic luminophores into gel systems, but the procedures are usually complex. Herein, for the first time, we report gels with intrinsic and tunable emission color prepared with 5-boronoisophthalic acid (5-bop) and Eu3+, Tb3+, and/or Dy3+ similar to the procedure for the preparation of metal–organic frameworks (MOFs). The single-metal gels exhibit intrinsic trichromatic fluorescence, due to which full-color emissions are readily obtained by tuning the type and/or ratio of Ln3+ ions to prepare mixed-metal gels. The emission is governed by an antenna effect and is thus excited with single-wavelength at 275 nm. The nucleation-growth mechanism reveals that the Ln3+ ions and 5-bop produce separated layers, which then grow anisotropically to form nanoribbons by high coordinated valence of Ln3+ ions and biased carboxyl distribution as well as steric hindrance and hydrogen bonds of the boric acid group in 5-bop. The nanoribbons entangle together to generate chemical-physical hybrid gels. To the best of our knowledge, this is the first example of gels with inherent and tunable emission color. Due to their optical and viscoelastic properties, the gels have numerous potential applications such as tunable emission and multi-target detection.
Although a complex procedure is required to introduce extrinsic luminophores,4,16,17 fluorescent gels are still less reported. An alternative assembly strategy, in combination with unconventional precursors, is thus essential to establish gels with intrinsic emission. Metal–organic frameworks (MOFs) are the assembly between metal ions (or clusters) and organic ligands.18 Some MOF gels were reported,19–24 but the formed MOF nanoparticles coordinated with mismatched growth and gelation.19 Therefore, the so-called “gels” were still MOFs but in amorphous state.19–24 The other strategy is tantamount to embed nanoscale MOFs into the polymer matrix.25,26 Can the assembly procedure used for the formation of MOFs be employed in the preparation of gels? Similar to low-molecular-weight gels,1,2 if metal ions and organic ligands grow anisotropically to constitute flexible fibers or ribbons, fibers or ribbons may entangle together to form valid MOF gels.
Herein, we report that the ligand with biased carboxyl distribution, such as isophthalic acid (ISP) derivative, is selected to form a ribbon-like structure, followed by gelation upon the entanglement of nanoribbons. The introduction of a boric acid group in ISP improves the anisotropic growth through steric hindrance and hydrogen bonds (Fig. 1A). Metal ions with high coordination valences also enhance the anisotropic growth.27,28 Therefore, a series of emissive MOF gels are prepared successfully with 5-boronoisophthalic acid (5-bop) as the ligand and single- or mixed-Ln3+ ions, including Eu3+, Tb3+, and Dy3+, as metal nodes for the first time. Single-metal MOFs exhibit trichromatic fluorescence; thus, full-color emissions are readily obtained by tuning the types and/or ratios of Ln3+ ions to prepare mixed-metal MOF gels (Fig. 1A). Tb3+ and Dy3+ ions form MOF gels easily with 5-bop as the ligand because their coordination environments are different from that of Eu3+ ions (Fig. 1B). However, mixed-metal MOF gels are easily prepared to obtain gels with different colors. Formation mechanism study indicates that the precursors form separated layers (Fig. 1C), which grow anisotropically to form nanoribbons by the high coordination valence of Ln3+ ions and biased carboxyl distribution as well as steric hindrance and hydrogen bonds of the boric acid group. Nanoribbons entangle together to form MOF gels. Nanoribbons are formed through chemical bonding between 5-bop and metal ions similar to chemical gels; the nanoribbons entangle through non-covalent interaction, which is the essential property of physical gels. Chemical/physical hybrid gels with intrinsic fluorescence emission were thus reported.
Rheological measurements confirmed the viscoelastic properties of Tb- and Dy-MOF gels. The linear viscoelastic region of the MOF gel systems was delineated by a strain amplitude sweep (Fig. 3A and C). The results in Fig. 3B and D depict the frequency sweep results of the gel systems, where the value of storage modulus G′ is markedly higher than that of the loss modulus G′′ throughout the test region (ω = 0.1–100 rad s−1). Strain amplitude sweep measurements show that both the G′ and G′′ moduli remain constant in a certain range of strain. The G′′ modulus plateau is at a higher value than that of G′, illustrating the degradation of the gel structure. High-yield strain (γ value) suggests that the gels are strongly damage resistant. The gel properties of Tb- and Dy-MOFs were thus confirmed.
The characteristic emission peaks of Ln3+ ions were clearly observed from the fluorescence profiles of Eu-, Tb-, and Dy-MOFs (Fig. 3E–G);30 interestingly, their emissions show the trichromatic colors red, green, and blue, respectively (Fig. 3H). Therefore, full-color emissions are easily obtained according to the trichromatic theory. The emission is governed by the antenna effect, as schematized in Fig. 3I.31 The ligand 5-bop is excited to its singlet state (S1, Table S1†) and then transfers to its triplet state (T1) through intersystem crossing.31 Ln3+ ions are then sensitized by the triplet state to excite their emissions. The emissions are observed from Eu- and Tb-MOFs as procedures “a” and “b” in Fig. 3H. Competing energy transfer occurs in Dy-MOF gels because the T1 energy of 5-bop is close to that of 4F9/2 of Dy3+ for low radiative energy transition. Instead, nonradiative transition from 4F9/2 of Dy3+ to T1 of 5-bop occurs easily. Thus, the blue emission of Dy-MOF gels was partly due to 5-bop with the procedure “c” shown in Fig. 3H. The antenna effect governs the emission of MOFs; thus, single-wavelength excitation achieves all the emissions to facilitate their practical application.32
Tb-MOFs with different reaction times were characterized to reveal the formation of the gels (Fig. S3†). The precursors were still transparent in the solution state at 1 h. After 2 h, the fluidity of the solution decreased considerably, and “non-flowing” gel property was observed at 3 h. The transparency of the gels decreased gradually as the reaction time increased from 3 to 12 h. TEM images revealed the structural change. After 1 h, the precursors formed irregular clusters (Fig. S4A†). Filiform products were observed at 1.5 h (Fig. S4B†). With increasing reaction time from 1.5 to 3 h, the content of filiform products increased, whereas the clusters decreased and disappeared (Fig. S4B–E†). After 6 h, nanoribbons were formed and they entangled to form MOF gels (Fig. S4F†). Moreover, high yield was achieved as almost all the precursors formed MOF gels, as revealed by the TEM images (Fig. S4F†).
The precursor concentration was tested. When the content of Tb3+ was lower than 0.2 mmol in 10 mL of DMF/H2O, the MOF gel was formed in a normal manner (Fig. S5A†). Tb-MOFs became precipitates when the content was higher than 0.4 mmol. The morphology was clearly evident from their TEM images (Fig. S5B–E†). The nanoribbons of Tb-MOFs gradually became shorter and thicker with increasing Tb content. Moreover, the nanoribbons became rigid to prevent their entangling to form gels. Therefore, the formation of MOF gels was governed by the nucleation-growth mechanism. A rapid nucleation occurred under the initial period, whereas diluted solution yielded modest amounts of crystal nuclei, which then grew slowly to form long nanoribbons. The nanoribbons were flexible and entangled to constitute the gel. Aligned hydrogen bonds between the nanoribbons were present, resulting in the stability of the assembled MOF gels.
Single-crystal data can provide evidence of gel formation at the atomic level. Single-crystal MOFs were thus prepared with ISP as the ligand and Eu3+, Tb3+, or Dy3+ as the metal nodes. ISP-MOFs exhibit a monoclinic system with space group P21/n or P21/c (Tables S2–S4†). The fundamental building unit of Eu-ISP MOFs contains two non-equivalent Eu3+ ions, three deprotonated ISP molecules, and two H2O molecules, with the formula Eu2(ISP)3(H2O)2. The H2O molecules are coordinated to the two Eu3+ ions (Fig. 4A). Tb- and Dy-ISP MOFs show the same configuration with the formulae of Tb2(ISP)3(H2O)2·H2O and Dy4(ISP)6(H2O)4·H2O2, respectively. While two H2O molecules are coordinated to single Tb3+ or Dy3+ ion, the third H2O molecule exists in the channel (Fig. 4B and C). Tb1 and Dy1 are eight-coordinated, but Tb2, Dy2, Eu1, and Eu2 ions show seven-coordinated pentagonal bipyramidal geometry (Fig. 4A–C). Due to slight differences in the coordination environments between Eu3+ and Tb3+ or Dy3+, Eu3+ ions cannot form MOF gels easily.
Eu1/Tb1/Dy1 and Eu2/Tb2/Dy2 atoms were arranged alternately and linked by discrete carboxylate groups of ISP to form 1D ribbon-like chains (Fig. 4D–F). Adjacent 1D chains connect with each other through the ligand to form a 2D layer structure. The high coordination valence of Ln3+ ions and biased carboxyl distribution in the ligands result in anisotropic growth to form a ribbon-like structure, as shown with arrows in Fig. 4D–F. If 5-bop is chosen as the ligand, the steric hindrance of the boric acid group enhances the anisotropic growth for the formation of nanoribbons. Interestingly, different layers are clearly separated without any chemical cross-linking, as illustrated with dotted lines in Fig. 4D–F. Thus, a layer structure was formed and also confirmed with high-resolution TEM images of Eu-ISP and Tb-ISP MOFs (Fig. S6†). The introduction of the boric acid group further led to the separation of the layers to form nanoribbons.
To prove the role of the boric acid unit in isophthalic acid for anisotropic growth, Ln-MOFs with [1,1′-biphenyl]-3,5-dicarboxylic acid as ligand were synthesized to confirm the effect of steric hindrance. 5-Hydroxyisophthalic acid and 5-aminoisophthalic acid were used as the ligands to verify the role of hydrogen bonds. All Ln-MOFs existed as precipitates (Fig. S7†). Their different morphologies were revealed from the TEM images (Fig. S8†). The morphology of Tb-MOFs was similar to that of Dy-MOFs but different to that of Eu-MOFs because of the different coordination environments of Ln3+ ions, as validated with the single-crystal results (Fig. 4). The gels could not form under the condition of single steric resistance taking the place of boric acid with the phenyl ring (Fig. S8A–C†); also, a ribbon-like structure was observed from Eu-[1,1′-biphenyl]-3,5-dicarboxylic acid-MOFs. Moreover, we did not obtain gels with single hydrogen bonds using amino and hydroxyl groups instead of boric acid (Fig. S8D–I†). However, an irregular slice layered structure was observed from Ln-MOFs prepared with 5-hydroxyisophthalic acid and 5-aminoisophthalic acid as the ligands. Therefore, the gel could be formed merely with the combination of steric hindrance and hydrogen bonds by the introduction of the boric acid group.
Powder XRD patterns of Eu-MOF, Tb-MOF gels, and Dy-MOF gels were recorded and compared with those of the single crystals of Eu-, Tb-, and Dy-ISP MOFs, respectively. Similar XRD patterns were observed for MOFs with 5-bop and ISP as the ligands (Fig. S9†). Thus, single-crystal data provided evidence for the formation of nanoribbons and the differences among Ln3+ ions as metal nodes. High-resolution TEM images reveal ribbons with thickness less than 20 nm (Fig. S8†). The length was greater than 10 μm and thus, the aspect ratio was higher than 500. The nanoribbons were formed through covalent bonds, which is a property of chemical gels;7–10 no coordinative cross-linking was found between the nanoribbons (Fig. S6 and S10†) similar to that observed for supramolecular physical gels.1,2 Therefore, our MOF gels were chemical/physical hybrid gels.
Element distribution of the mixed-metal MOF gels was first tested with scanning transmission electron microscopy (STEM) and energy-dispersive X-ray (EDX) elemental mapping. Each element was clearly observed in the images and distributed in the entire area of the MOF gels uniformly (Fig. 5C). Therefore, Eu, Tb, and Dy were successfully integrated into the mixed-metal MOF gels. The expected metals were observed from X-ray photoelectron spectroscopy (XPS) patterns, which also confirmed the formation of mixed-metal MOF gels (Fig. S13†). The mole ratios of Ln3+ ions were 1:0.80, 1:1.09, 1:1.21, and 1:0.94:0.91 in Eu–Tb, Eu–Dy, Tb–Dy, and Eu–Tb–Dy MOF gels, respectively, as revealed from the inductively coupled plasma-atomic emission spectrometry results (Table S5†). Rheological measurements confirmed that the moduli of the gels were a function of the applied angular frequency to disclose their gel properties (Fig. S14†). Therefore, mixed-metal MOF gels were formed by integrating different metal ions together.
The characteristic emissions of Ln3+ ions were well retained in the bimetallic MOF gels, but the intensity was markedly different from that shown in Fig. 3. The emission of Eu3+ ions in Eu–Tb MOF gels was retained, but the emission of Tb3+ ions was heavily suppressed (Fig. S15A†). We considered that part of the energy of 5D4 of Tb3+ was transferred to 5D0 of Eu3+ before radiative transition occurred with the “d” procedure shown in Fig. 3H.32,34 The emission of Eu3+ ion was suppressed in Eu–Dy MOF gels (Fig. S15B†); 4F9/2 of Dy3+ has much closer energy to T1 of 5-bop than the 5D0 state of Eu3+, suggesting that energy transfers easily from 5-bop to Dy3+ ions as procedure “c” in Fig. 3H. Thus, blue emission was clearly observed, whereas that of the Eu3+ ions was heavily suppressed in Eu–Dy MOF gels.
The emission intensity of Tb3+ was retained in the Tb–Dy MOF gels, but the blue emission decreased clearly (Fig. S15C†). Energy was transferred from 4F9/2 of Dy3+ to 5D4 of Tb3+ with the “e” procedure shown in Fig. 3H. Fig. S15D† shows that the emissions of the mixed-metal MOF gels were not the simple summation from those of single-metal MOFs. Therefore, metal ions were integrated at the atomic level, enabling energy transfer between different Ln3+ ions. Isostructural MOF gels were easily prepared by tuning the types and ratios of Ln3+ ions for combined emissions, which show promise for colorful applications.
Light-emitting diodes (LEDs) are the next generation of lighting systems by virtue of their environmental friendliness, high efficiency, and long lifetime.38,39 White-emitting LED devices have been prepared by coating MOFs onto a UV lamp.38,39 Our MOF gels were also coated onto a UV torch to reveal their UV-white emission transfer (Fig. 7). A white-emitting MOF gel was obtained with the ratio of Eu/Tb/Dy as 1:1:2, whereas colorful emissions were easily obtained from mixed-metal MOF gels (Fig. 7).
Footnote |
† Electronic supplementary information (ESI) available: All experimental details, crystallographic data collection and refinement statistics, details of chemical synthesis, additional figures and tables. See DOI: 10.1039/c8sc04732d |
This journal is © The Royal Society of Chemistry 2019 |