Coordination Polymer Gels with Modular Nanomorphologies, Tunable

Aug 9, 2017 - Synopsis. Coordination-driven self-assembly of an amphiphilic, tripodal LMWG with different metal ions generates CPGs that exhibit modul...
23 downloads 20 Views 4MB Size
Download PDF
Article pubs.acs.org/IC

Coordination Polymer Gels with Modular Nanomorphologies, Tunable Emissions, and Stimuli-Responsive Behavior Based on an Amphiphilic Tripodal Gelator Papri Sutar and Tapas Kumar Maji* Molecular Materials Laboratory, Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Banglore 560064, India S Supporting Information *

ABSTRACT: The recent upsurge in research on coordination polymer gels (CPGs) stems from their synthetic modularity, nanoscale processability, and versatile functionalities. Here we report self-assembly of an amphiphilic, tripodal low-molecular weight gelator (L) that consists of 4,4′,4-[1,3,5-phenyl-tri(methoxy)]-tris-benzene core and 2,2′:6′,2″-terpyridyl termini, with different metal ions toward the formation of CPGs that show controllable nanomorphologies, tunable emission, and stimuli− responsive behaviors. L can also act as a selective chemosensor for ZnII with very low limit of detection (0.18 ppm) in aqueous medium. Coordination-driven self-assembly of L with ZnII in H2O/MeOH solvent mixture results in a coordination polymer hydrogel (ZnL) that exhibits sheet like morphology and charge-transfer emission. On the other hand, coordination of L with TbIII and EuIII in CHCl3/tetrahydrofuran solvent mixture results in green- and red-emissive CPGs, respectively, with nanotubular morphology. Moreover, precise stoichiometric control of L/EuIII/TbIII ratio leads to the formation of bimetallic CPGs that show emissions over a broad spectral range, including white-lightemission. We also explore the multistimuli responsive properties of the white-light-emitting CPG by exploiting the dynamics of LnIII-tpy coordination.

■

INTRODUCTION Recently, enormous attention has been focused on the development of multifunctional coordination polymer gels (CPGs), achieved by the self-assembly of metal ions and low molecular-weight gelators (LMWGs).1−5 CPGs are highly promising soft-hybrid materials, as the presence of metal ions endows them with unique redox, optical, electronic, and catalytic properties.6−10 Also, the metal−LMWG interactions allow the controlled growth of different nanostructures, such as fibers, tubes, rings, ribbons, and vesicles in CPGs.11,12 Many novel functionalities such as tunable emission, catalysis, proton conductivity, sensing, and gas storage have been explored in CPGs, which arise due to the synergistic combination of inorganic and organic components. 13−24 Among many divergent classes of LMWGs reported, the amphiphilic gelator containing donor and acceptor π-chromophores connected by a flexible alkylamide chain is intriguing.25 This is because dynamic self-assembly of such LMWGs in aqueous or organic solvents result in hydrogel or organogel with different nanostructures and properties.26−29 Recently, we have reported a flexible, amphiphilic LMWG (L) that consists of 4,4′,4″[1,3,5-phenyl-tri(methoxy)]-tris-benzene (tmb) core and 2,2′:6′,2″-terpyridyl (tpy) termini that self-assembled to hydrogel (HG) and organogel (OG) with nanosphere and nanofiber morphologies, respectively.26 Different spatial organization of tmb (donor) and tpy (acceptor) moieties of L resulted in charge-transfer (CT) emission in HG and LMWG© 2017 American Chemical Society

based emission in OG. Herein, we envisage that coordination of specific metal ions to the tpy units of L could drive the selfassembly in different way and result in CPGs with added complexity and functionality. Moreover, metal binding could provide fluorescence readout signal, which can be useful for specific recognition process. Example of LMWG that can act as a chemosensor for a metal ion and form CPG with the same metal ion is very rare.4 In this regard, designing of LMWG for selective detection ZnII from water is important, as it is the second most abundant metal ion in the human body. ZnII is an important structural cofactor in many biological processes.30 However, it is also a significant metal pollutant present in drinking water, which causes several diseases including Alzheimer, epilepsy, and ischemic stroke.31−37 Long-term exposure to high levels of ZnII could also induce liver and kidney damage. Hence, detection of ZnII from drinking water is essential. Among various detection procedures, chelation enhanced fluorescence (CHEF) is highly effective, as it provides fluorescence enhancement with or without any spectral shifts after chelation with specific metal ions.38−42 Hence the presence of metal ion could be easily detected by naked eye. One important feature of the gel materials is stimuli− responsive behavior. In this regard, significant amount of work Received: April 20, 2017 Published: August 9, 2017 9417

DOI: 10.1021/acs.inorgchem.7b01002 Inorg. Chem. 2017, 56, 9417−9425

Article

Inorganic Chemistry Scheme 1. Schematic Showing the Structure of L and Its Functionalitya

a

(a) L acts as a chemosensor for ZnII ion. (b) Coordination of L with TbIII/EuIII (in different ratios) results in CPGs with tunable emission property, including white-light emission. (c) The white-light-emitting CPG shows multistimuli−responsive behaviours.

■

has been done on pH-, mechano-, photoresponsive behavior of organogels or hydrogels, synthesized from purely organic systems.43−48 In spite of such overwhelming progress, there are still very few reports on stimuli−responsive CPGs. In this respect, lanthanide (LnIII)-based coordination polymer gels (LnIII-CPG) could be a potential platform for studying stimuli− responsive sol−gel transition and corresponding emission change.15 This is because LnIII-based materials exhibit narrow band emissions (red: EuIII, PrIII, SmIII; green: TbIII, ErIII; blue: TmIII, CeIII) with high quantum yields when coordinated to suitable πchromophoric chelator, such as terpyridine (NΛNΛN). Recently, many LnIII-based materials, including LnIII-CPGs with unique emission properties, stemming from sensitized f−f electronic transition have been exploited for sensing and solidstate lighting applications.49−66 However, little attention has been given on fabricating processable white-light-emitting CPG and exploiting their stimuli−responsive behaviors. In this respect, LnIII−CPGs having weak LnIII−NΛNΛN interactions (hard acid−soft base) is interesting, as they could be explored for the mechanistic understanding of stimuli−responsive reversible sol−gel transition and corresponding emission dynamics.15 Herein, we report the detection of ZnII in parts per million level from aqueous solution by utilizing gelator L based on fluorescence turn-on response. Furthermore, the coordination-driven self-assembly of L with ZnII in H2O/ MeOH solvent mixture leads to the formation of a coordination polymer hydrogel (ZnL) that exhibits sheetlike morphology. The ZnL shows CT emission similar to the previously reported HG. Moreover, coordination of L with TbIII and EuIII in CHCl3/tetrahydrofuran (THF) solvent mixture leads to the formation of CPGs TbL and EuL, which exhibit green and red emission, respectively. By controlling the stoichiometry of L/ EuIII/TbIII we are able to prepare bimetallic CPGs that show tunable emission over a broad spectral range, including whitelight emission. Moreover, the multi-stimuli−responsive behaviors of the bimetallic CPGs are explored based on the LnIII−tpy coordination dynamics (Scheme 1).

EXPERIMENTAL SECTION

Materials and Methods. 1,3,5-Tris(bromomethyl)benzene, methyl 4-hydroxybenzoate, 4′-chloro-2,2′:6′,2″-terpyridine, 1,3-diaminopropane, triphenylphosphine (PPh3), and trichloroisocyanuric acid (TCIC) were purchased from Sigma-Aldrich chemical Co. Ltd. All solvents were purchased from Spectrochem Pvt. Ltd. All the metal salts Zn(NO3)2·6H2O, Tb(NO3)3·6H2O, and Eu(NO3)3·6H2O were purchased from Alfa Aesar. All the other chemicals were purchased from Spectrochem Pvt. Ltd. All solvents were predried using standard procedures before using. For UV−vis experiments spectroscopic grade solvents were purchased from Spectrochem Pvt. Ltd. 1H NMR is recorded on a Bruker AV-400 spectrometer with chemical shifts recorded as parts per million, and all spectra were calibrated against tetramethylsilane (TMS). High-resolution mass spectra (HRMS) were recorded on Agilent 6538 Ultra High Definition (UHD) AccurateMass Q-TOF-LC/MS system using electrospray ionization (ESI) modes. Elemental analysis was performed on a Thermo Fisher Flash 2000 Elemental Analyzer. UV−vis spectra were recorded on a PerkinElmer lamda 900 spectrometer. Fluorescence studies were accomplished using PerkinElmer Ls55 Luminescence spectrometer. Fourier transform infrared spectroscopy (FTIR) studies were performed by making samples with KBr pellets using Bruker FTIR spectrometer. Morphology studies were performed using Lica-S440I field emission scanning electron microscopy (FESEM). The xerogel samples were dispersed in ethanol and drop-casted on silicon wafer. The FESEM images were taken under high vacuum using accelerating voltage of 10 kV. Elemental mapping was also performed in the same instrument. Transmission electron microscopy (TEM) analysis was performed using JEOL JEM-3010 with accelerating voltage of 300 kV. For this analysis the xerogel was dispersed in ethanol and then dropcasted on a carbon-coated copper grid. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) measurements were performed on PerkinElmer Optima 7000dv ICP-AES. Synthesis of L. The synthesis of L is performed in multisteps from commercially available 1,3,5-tris(bromomethyl)benzene, methyl 4hydroxybenzoate, and 4-chloro-2,2′:6′,2″-terpyridine, as reported previously.26 Reaction of 1,3,5-tris(bromomethyl)benzene with methyl 4-hydroxybenzoate first results in methyl 4,4′,4″-[1,3,5-phenyl-tri(methoxy)]-tris-benzoate, which is subsequently hydrolyzed to corresponding acid. The resulting acid is further coupled with 2,2′:6′,2″-terpyridin-4-yl-propane-1,3-diamine in 1:3 stoichiometry to 9418

DOI: 10.1021/acs.inorgchem.7b01002 Inorg. Chem. 2017, 56, 9417−9425

Article

Inorganic Chemistry get the targeted L in ∼88% yield. The formation of L is confirmed by 1 H NMR, HRMS, and FTIR spectra and melting point. mp of L ≈ 300 °C. 1H NMR (400 MHz, deuterated dimethyl sulfoxide (DMSO-d6)) δ: 8.65 (d, 2H, ArH), 8.89 (d, 2H, ArH), 8.23 (m, 2H, ArH), 7.90 (s, 2H, ArH), 7.78 (m, 2H, ArH), 7.73 (m, 2H, ArH), 7.53 (s, 1H, ArH), 7.05 (d, 2H, ArH), 5.21 (s, 2H, ArCH2OAr), 3.72 (m, 2H, CONHCH2), 3.02 (m, 2H, CH2), 2.04 (m, 2H, CH2NH)). HRMS (ESI, m/z): [M + H]+ calculated for C84H75N16O6, 1391.4252; found: 1391.4254. Selected FTIR data (KBr, cm−1): 3436 (b), 3039 (b), 2779 (s), 2676 (s), 2493 (s), 1721 (sh), 1695(sh), 1600 (m), 1470 (m), 1399 (m), 1249 (s), 1167 (s), 850 (s), 790 (m), 534 (m). Anal. Calcd for C84H75N15O6 Calc: C, 72.53; H, 5.43; N, 15.11%; Expt: C, 72.61; H, 5.53; N, 15.05%. Synthesis of ZnL Hydrogel. L (9 mg) was dissolved in 600 μL of MeOH, while 1.9 mg of Zn(NO3)3·6H2O was dissolved in 400 μL of water. The solution of ZnII was slowly added into the methanolic solution of L. The mixture was sonicated for 2−3 min. The mixture immediately converted to opaque ZnL gel. The formation of gel was confirmed by inversion-test method. Selected FTIR data (KBr, cm−1): For ZnL xerogel: 3215 (m), 3063 (b), 2978−2749 (m), 2673 (sh), 2493 (s), 1705 (sh), 1601 (m), 1470 (sh), 1397 (sh), 1245 (m), 1168 (s), 1055 (m), 848 (s), 788 (sh), 531 (sh). Synthesis of TbL and EuL Gels. Solution of L (500 μL, 1 × 10−3 M) in CHCl3/THF (2:1) was mixed with 500 μL solution of Tb(NO3)3·6H2O (1 × 10−3 M, in THF). The mixture was heated at 90 °C for 2−3 min to prepare a viscous solution, which eventually resulted in stable opaque TbL gels upon cooling. Similarly EuL gel was also prepared. Solution of L (500 μL, 1 × 10−3 M) in CHCl3/THF (2:1) was mixed with 500 μL solution of Eu(NO3)3·6H2O (1 × 10−3 M, in THF). The mixtures were heated at 90 °C for 2−3 min to prepare a viscous solution that converts to opaque EuL gels upon cooling. In both cases, the formation of gels was confirmed by inversion-test method. Selected FTIR data (KBr, cm−1) for TbL xerogel: 3426 (b), 2973−2829 (m), 1640 (m), 1457 (s), 1384 (sh), 1168 (s), 997 (s), 973 (s), 840 (s), 808 (s). Selected IR data (KBr, cm−1) for EuL xerogel: 3419 (b), 2426 (s), 1637 (sh), 1469 (s), 1382 (sh), 1041 (s), 975 (s), 746 (s). Synthesis of Bimetallic CPGs. First 1 × 10−3 M solutions of Tb(NO3)3·6H2O and Eu(NO3)3·6H2O are prepared by dissolving the metal salts in THF. After that, 250 μL solution of Tb(NO3)3·6H2O and 250 μL solution of Eu(NO3)3·6H2O are added into 500 μL solution of L (1 × 10−3 M). The mixture is heated at 90 °C for few minutes to form viscous solution, which is eventually converted to stable gel (CPG1) after cooling. Similarly CPG2 was prepared by using (1 × 10−3 M) and (2 × 10−3 M) of Tb(NO3)3·6H2O and Eu(NO3)3·6H2O in THF, respectively. The CPG3 was prepared by using (1 × 10−3 M) and (2.2 × 10−3 M) solutions of Tb(NO3)3·6H2O and Eu(NO3)3·6H2O in THF, respectively. Selected FTIR data (KBr, cm−1): For CPG1 xerogel: 3420 (b), 2940−2824 (m), 1635 (sh), 1457 (s), 1382 (sh), 1155 (s), 997 (s), 746 (s). For CPG2 xerogel: 3421 (b), 2938−2827 (m), 1632 (sh), 1455 (s), 1384 (sh), 1157 (s), 980 (s), 743 (s). For CPG3 xerogel: 3417 (b), 2941−2824 (m), 1630 (sh), 1457 (s), 1390 (sh), 1163 (s), 987 (s), 739 (s).

Figure 1. (a) Change in absorption spectra of methanolic solution of L after addition of different equivalents of ZnII. (b) Change in emission spectra of methanolic solution of L after addition of different metal ions. Turn-on emission is observed after addition of ZnII. (c) Relative fluorescence intensity of L in MeOH after addition of different metal ion solution. (d) Photograph of methanolic solution of L and after addition of ZnII under UV light.

addition of ZnII, absorption maxima of L at 280 nm shifts to 297 nm, and a broad shoulder appears at 318 nm, which suggests complexation of ZnII to tpy groups of L (Figure 1a). The titration curve saturates after reaching L/ZnII = 2:3, which suggests the formation of coordination polymer with octahedral geometry of ZnII. The fluorescence response of L in titration with different metal ions is shown in Figure 1b. For this experiment, 1 × 10−5 M solution of L in methanol was mixed with different metal ions, and the final concentration of the metal ions was fixed at 1.5 × 10−5 M. A remarkable enhancement (10-fold) in fluorescence intensity at 420 nm is observed only after addition of ZnII (Figure 1c). The change in fluorescence could also be observed by bare eye under UV light (Figure 1d). Such enhancement in fluorescence is probably attributed to CHEF effect.67−69 As a control experiment, we studied the fluorescence response of a model compound, 2,2′:6′,2″-terpyridin-4-yl-propane-1,3-diamine in the presence of different metal ions under similar condition. We observe enhancement of fluorescence after addition of ZnII, CdII, AlIII, and FeIII, among which ZnII shows highest enhancement. Also we observed slight diminution of fluorescence in some cases (CuII, CoII, NiII). This suggests that the model compound does not show selectivity. (Supporting Information, Figure S6). The enhancement in fluorescence occurs due to the spatial reorganization of the terpyridine moieties in L after complexation with ZnII, which is evident from the UV−vis titration study. No significant fluorescent enhancement is observed when metal ions that prefer N donors, such as AgI and CuI, are added to the solution of L (Supporting Information, Figure S7). Above observations indicate that L could be used as chemosensor for ZnII. Since methanol is a biohazardous solvent and detection of ZnII from water is more important, we performed the sensing experiment using solution of L (1 × 10−5 M) in H2O/DMSO (99.9:0.1) solvent mixture and aqueous solution of ZnII (1 × 10−5 M). Similar turn-on fluorescence response is observed after gradual addition of ZnII to L (Figure 2a). To prove the selectivity of L toward ZnII, the calculation of limit of detection (LOD) is performed through

■

RESULTS AND DISCUSSION The solution of L (1 × 10−4 M) in methanol shows a broad absorption band at 250−320 nm, attributed to the π−π* transition of the tmb and tpy moieties (Supporting Information, Figure S1).26 Since terpyridine is a well-known metal chelating group, we study the interaction of L with different metal ions (Mn+ = CuII, AllII, CaII, CdII, CoII, FeIII, MgII, MnII, NiII, PbII, HgII, ZnII, CuI, AgI) by gradually adding Mn+ solution (1 × 10−4 M) to the solution of L (1 × 10−5 M; Supporting Information, Figures S2−S5). Absorption spectrum of L does not show any spectral shift after addition of 3 equiv of CuII, AllII, CaII, CdII, CoII, FeIII, MgII, MnII, NiII, PbII, HgII, AgI, CuI. However, a significant change in absorption spectra is observed after addition of ZnII (Figure 1a). With incremental 9419

DOI: 10.1021/acs.inorgchem.7b01002 Inorg. Chem. 2017, 56, 9417−9425

Article

Inorganic Chemistry

FESEM and TEM images of ZnL xerogel reveal the formation of micron-sized sheets (Supporting Information, Figure S9). High-resolution TEM images show that sheets are stacked upon one another (Figure 3d). Formation of sheetlike morphology can be explained based on the coordination driven self-assembly of L as shown in Scheme 2. First, in polar solvent Scheme 2. Schematic Showing the Self-Assembly of L in H2O/MeOH Leads to the Formation of Hydrogel (HG) with Nanosphere Morphologya Figure 2. (a) Fluorescence spectra of L in H2O/DMSO (99.9:0.1) upon addition of aqueous solution of ZnII in ppm scale. (b) Plot of fluorescence intensity of L at 420 nm vs concentration of ZnII.

standard deviation and linear fitting. By plotting the relative fluorescence intensity (I/I 0 ) change as a function of concentration of ZnII the LOD is calculated as 6.25 × 10−7 M (0.18 ppm; Figure 2b). According to world health organization (WHO) the acceptable limit for ZnII in drinking water is 5 ppm. Hence L could act as good chemosensor for the detection of ZnII from water. In our previous work, we discussed how self-assembly of L in water/MeOH and CHCl3/THF solvent mixtures resulted in hydrogel (HG) and organogel (OG) with nanosphere and nanofiber morphology, respectively.26 To know how coordination of metal ions influences the gel nanostructure and corresponding emission properties, we check the gelation propensity of L in the presence of ZnII and TbIII/EuIII. A 400 μL aqueous solution of Zn(NO3)2·6H2O (6.4 × 10−3 M) is added to the 600 μL methanolic solution of L (6.5 × 10−3 M), and the mixture is kept undisturbed at room temperature. The mixture converts to gel (ZnL) within 2−3 min. The formation of gel is confirmed by inversion test method (Figure 3a). However, when more than 1 equiv of ZnII is added, instead of gelation, precipitation is observed. The precipitate is crystalline in nature (Supporting Information, Figure S8). Insight into the morphology of ZnL is obtained by recording FESEM and TEM images of the corresponding xerogel (Figure 3b−d). The

a

In presence of 1 equiv of ZnII the coordination polymer gel (ZnL) with sheetlike morphology is formed. Addition of more than 1 equiv of ZnII leads to the formation of precipitate.

L is assembled through intermolecular π−π stacking between the tmb and tpy moieties.26 In absence of metal ions like ZnII, the π-stacked tmb and tpy groups tend to minimize their contact with H2O/MeOH due to the hydrophobic effect. However, the hydrophilic amide groups tend to be exposed toward the polar solvent medium due to H-bonding interaction with H2O/MeOH. As a result, the initial π-stacked nanostructures are folded into nanospheres whose exteriors are decorated with amide groups.26 When ZnII ions are added into the solution of L (L/ZnII = 1:1), tpy groups of L coordinate with ZnII and reduce the flexibility of the initially formed π-stacked nanostructures. Hence, after coordination with ZnII further folding of the two-dimensional (2D) assembly to nanospheres could not happen. Rather, the coordination of ZnII propagates the self-assembly of L in 2D plane and result in 2D sheet morphology of ZnL (Scheme 2). The presence of ZnII in ZnL xerogel is confirmed by energy-dispersive X-ray spectroscopy (EDXS) analysis (Figure 4a). The elemental mapping of ZnL xerogel shows uniform distribution of C, N, O, and ZnII throughout the 2D sheet of ZnL (Figure 4b−e). The loading of ZnII in ZnL xerogel is measured by ICP-AES analysis, which shows 39 mg of ZnII per gram of ZnL. This indicates L/ZnII = 1:0.85 in the ZnL xerogel. The X-ray photoelectron spectroscopy (XPS) of ZnL shows the binding energy of Zn 2p at 1029 eV (Supporting Information, Figure S10). These results clearly suggest that coordination of ZnII to L has propagated the selfassembly of L to ZnL gel. However, it can be noted that some of the tpy groups remain uncoordinated in the ZnL hydrogel, as two tpy groups of L participate in coordination at L/ZnII = 1:1 ratio (Scheme 2). The UV−vis spectrum of ZnL xerogel exhibits a broad absorption with maxima at 410 nm that is attributed to the interligand CT transition between π-stacked

Figure 3. (a) Photograph of ZnL CPG under daylight and UV light. (b) FESEM image of ZnL xerogel showing the presence of sheet morphology. (c) TEM image of ZnL xerogel showing sheet morphology. (d) High-resolution TEM image of ZnL xerogel showing layering of sheets. 9420

DOI: 10.1021/acs.inorgchem.7b01002 Inorg. Chem. 2017, 56, 9417−9425

Article

Inorganic Chemistry

in micrometer range (Figure 6a and Supporting Information, Figure S13).

Figure 6. (a) FESEM image of TbL xerogel. (b) TEM image of TbL xerogel. (c) High-resolution TEM image of TbL xerogel showing the formation of nanotubes. (d) Schematic showing the self-assembly for the formation of TbL nanotubes. (e) Probable coordination environment around TbIII in TbL that leads to the formation of nanotube morphology.

Figure 4. (a) EDXS analysis of ZnL xerogel showing the presence of ZnII ions in xerogel. Elemental mapping of ZnL xerogel for (b) C, (c) N, (d) O, (e) ZnII showing uniform distribution of all the elements throughout the xerogel matrix.

tmb (π-donor) and tpy groups (π-acceptor; Figure 5a). Similar CT transition was also observed in HG xerogel.26 When excited

However, high-resolution TEM images of TbL xerogel reveal that the 1D nanostructures are actually nanotubes with uniformly spaced dark lines separated by bright core (Figure 6b,c). Nanotubes are several micrometers long with an approximate diameter in the 90−110 nm range. The presence of TbIII on the nanotubes of TbL xerogel is confirmed by EDXS analysis (Supporting Information, Figure S14). The elemental mapping of TbL xerogel reveals uniform distribution of TbIII ions throughout the xerogel matrix (Supporting Information, Figure S15). The XPS data reveal coordination of TbIII to tpy group of L (Supporting Information, Figure S16). These results confirm that self-assembly of L in TbL has been propagated by coordination of TbIII to L. The formation of nanotubes in TbL xerogel can be explained based on coordination-driven selfassembly of L as shown in Figure 6d. Initially, L self-assembled in CHCl3/THF solvent mixture by complementary H-bonding between amide groups and π−π stacking between tmb cores of neighboring L and formed the nanofiber. When TbIII is added to the solution of L in CHCl3/THF solvent mixture, the pendent tpy groups present on the outer surface of the 1D assembly coordinate with the TbIII and further extend the assembly. The coordinated tpy units present around TbIII ions orient themselves orthogonally to reduce the steric repulsion and thus facilitate bending of primary 1D nanostructures into nanotubes (Figure 6d,e).12 Similarly, the presence of EuIII in the EuL xerogels is confirmed by EDXS analysis (Supporting Information, Figure S17). The elemental mapping of EuL xerogel shows even distribution of EuIII ions throughout the xerogel network of EuL (Supporting Information, Figure S18). The ICP-AES analysis of TbL exhibits 130.75 mg loading of TbIII per gram of TbL xerogel. This indicates L/TbIII = 1:1.02 in TbL. Similarly, ICP-AES analysis of EuL shows 66.8 mg loading of EuIII per gram of EuL. This indicates L/EuIII= 1:0.7 in EuL. The XPS shows coordination of EuIII to tpy group of L

Figure 5. (a) Absorption spectra of ZnL xerogel. (b) Emission spectra of ZnL xerogel after excitation at 400 nm.

at 400 nm ZnL xerogel shows cyan emission with maxima at λ = 490 nm (Figure 5b). Such red-shifted emission in ZnL xerogel compared to L solution arises due to CT interaction.70,71 Moreover, excitation spectrum of ZnL xerogel shows the presence of ground-state interaction, thus ruling out the possibility of excited-state exciplex formation (Supporting Information, Figure S11). Next, we investigate the gelation propensity of L in the presence of different lanthanide ions in CHCl3/THF mixture. Heating the solution of L and Tb(NO3)3·6H2O (molar ratio = 1:1) in CHCl3/THF (2:1) solvent mixture, followed by cooling, results in stable CPG (TbL) (Supporting Information, Figure S11). Likewise, the EuIII-based CPG (EuL) is also prepared by heating and cooling the mixture of L and Eu(NO3)3·6H2O (1:1) in the aforementioned solvent mixture (Supporting Information, Figure S12). To gain insight into the mode of packing, TbL xerogel is analyzed by FESEM and TEM images. FESEM images of TbL show the presence of uniformly distributed one-dimensional (1D) nanostructures with length 9421

DOI: 10.1021/acs.inorgchem.7b01002 Inorg. Chem. 2017, 56, 9417−9425

Article

Inorganic Chemistry

corresponding to L, TbIII, and EuIII in which the L-centered emission at 440 nm (blue), TbIII-centered emission at 545 nm (green), and EuIII-centered emission at 615 nm (red) have comparable intensities (Figure 7e). The Commission International de L’Eclairage (CIE) coordinates of CPG2 are calculated to be (0.29, 0.29), which is quite near to the CIE of pure white light (0.33, 0.33; Figure 7f). The emission color of CPGs are tuned over a broad spectrum by using a simple strategy that leads to the formation of red, pink, white, cyan, and green lightemitting CPGs (Supporting Information, Figure S22). The presence of both TbIII and EuIII in bimetallic CPGs was confirmed by EDXS analysis (Supporting Information, Figures S23−S25). The elemental mapping of CPG2 shows even distribution of both TbIII and EuIII ions throughout the gel matrix (Supporting Information, Figure S26). ICP-AES analysis is performed to confirm the ratio of TbIII and EuIII loading in CPG1, CPG2, and CPG3. This shows TbIII/EuIII = 1:0.88, 1:1.53, 1:2.2 in CPG1, CPG2, and CPG3, respectively. The XPS of CPG2 shows coordination of both TbIII and EuIII to tpy groups of L (Supporting Information, Figures S27 and S28). The emission lifetime of CPG1, CPG2, and CPG3 at 543 nm are found to be 205, 165, and 150 μs, respectively (Supporting Information, Table S1). The absolute quantum yield (QY) values upon excitation at 365 nm is 45.3%, 18.4%, 7.6%, 21.7%, and 7.6% for TbL, EuL, CPG1, CPG2, and CPG3, respectively (Supporting Information, Table S1). These values are comparable with reported values of the TbIII/EuIII-based matallogel and coordination polymers.58,60,64 Furthermore, mechano-, thermo-, and chemo-responsive behaviors of green (TbL), red (EuL), and white light-emitting (CPG2) gels are investigated. When CPG2 is exposed to sonication for 5 min, a gradual gel-to-sol transition is observed (Figure 8a). In contrast to the intense white luminescence of

(Supporting Information, Figure S19). The UV−vis spectra of TbL and EuL xerogels show broad absorption with maxima at 340 nm (Supporting Information, Figures S20 and S21). When excited at 330 nm, the emission spectrum of TbL xerogel displays peaks at 488, 543, 584, and 620 nm, which can be attributed to the 5D4−7FJ (J = 6−3) transitions of TbIII (Figure 7a). Moreover, L-centered peak at 440 nm is also observed in

Figure 7. Emission spectra of (a) TbL, (b) EuL, (c) CPG1, (d) CPG3, and (e) CPG2 (λex = 280 nm). (insets) Photographs of gels showing green, red, cyan, pink, and white emissions under UV light. (f) CIE chromatography of TbL (●), EuL (■), and CPG2 (▲).

the emission spectrum of TbL. When excited at 330 nm, the emission spectrum of EuL shows characteristics peaks for 5 D0−7FJ (J = 1−3) transition of EuIII at 592, 615, and 687 nm, respectively, along with L-centered peak at 440 nm (Figure 7b). Under UV light TbL and EuL CPG show intense green and red emission, respectively (insets of Figure 7a,b). The TbL and EuL exhibit long emission lifetime of 303 μs and 1 ms when monitored at 543 and 616 nm, respectively. We also investigate how the emission property of CPGs can be tuned by modulating the stoichiometry of two lanthanide ions (TbIII and EuIII) and L. A series of bimetallic CPGs (CPG1, CPG2, CPG3) are prepared by simultaneously adding different ratios of TbIII and EuIII metal ions into the solution of L during gelation (Supporting Information, Figure S22). The emission spectra of the bimetallic CPGs reveal that the intensity of the green band at 543 nm decreases and that the intensity of the red band at 615 nm increases gradually with increasing EuIII concentration (Figure 7c−e). At 1:1 and 1:2.2 TbIII/EuIII ratios the cyan- and pink-emitting CPGs (CPG1 and CPG3) are formed. The emission spectra of CPG1 and CPG3 show the characteristic peaks of both TbIII and EuIII when excited at 330 nm (Figure 7c,d). Interestingly, a white-light-emitting gel (CPG2) is formed by using TbIII/EuIII = 1:2 during the gelation. The emission spectrum of CPG2 displays bands

Figure 8. (a) Multistimuli−responsive properties of white-lightemitting CPG2, (b) comparison of emission spectra of sols obtained after sonication (red), addition of TFA (blue) and heating (green) with the xerogel of CPG2 (black).

CPG2 gel, the sol displays low-intensity bluish-white emission under UV light (Figure 8a). The emission spectrum of mechanically induced sol shows a broad band around 420 nm, similar to the emission maxima of free L (Figure 8b). This indicates that sonication induces the cleavage of Ln−Ntpy coordination bonds, which hampers not only the emission color but also the stability of gel network. However, because of incomplete cleavage of all Ln−Ntpy bonds, the sol shows partial retention of TbIII- and EuIII-centered peaks. The sol converts back to gel upon resting at room temperature and recovers the white light emission. Such sonication induced reversible gel−sol transition and corresponding change in emission color happen due to the dynamic nature of the Ln−Ntpy coordination 9422

DOI: 10.1021/acs.inorgchem.7b01002 Inorg. Chem. 2017, 56, 9417−9425

Inorganic Chemistry

■

bonds.72,73 Comparative studies with TbL and EuL show similar gel-to-sol transformation (Supporting Information, Figures S29 and S30). The TbL and EuL sols exhibit lowintensity blue and bluish-pink emissions, respectively, under UV light. The emission spectra of TbL and EuL sols show broad L-centered emission at 400 nm and very low-intensity TbIII- and EuIII-centered peaks, respectively (Supporting Information, Figures S31 and S32). To prove that the gel-tosol transition is not happening due to the presence of water, we layered 1 mL of water on the top of TbL gel. We did not observe gel-to-sol transition even after 6 h (Supporting Information, Figure S33). The CPG2 also exhibits reversible thermoresponsive gel−sol transition (Figure 8a). The emission spectrum of the sol shows a broad L-centered emission at 420 nm and weak TbIII and EuIII-centered peaks (Figure 8b). Furthermore, when CPG2 is exposed to the vapor of trifluoroacetic acid (TFA) the gel-to-sol conversion is observed, along with the change in emission color (Figure 8a). Emission spectrum of the sol shows a broad emission at 440 nm, attributed to the free L (Figure 8b). Diffusion of TFA vapor in the CPG2 gel matrix results in the protonation of the Ntpy atoms.74,75 The protonated terpyridine (tpyH+) shows a lower affinity for the LnIII ions due to increased electrostatic repulsion. Hence Ln−Ntpy coordination bonds break, and gel converts to sol. However, after the sol is exposed to the vapor of triethylamine (NEt3) it converts back to gel. This is because NEt3 deprotonates the tpyH+ and reforms the Ln−Ntpy coordination bonds.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tapas Kumar Maji: 0000-0002-7700-1146 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS T.K.M. is grateful to the SERB, Department of Science and Technology (DST, Project No. MR-2015/001019), Government of India (GOI), and JNCASR for financial support. P.S. is thankful to the Council of Scientific and Industrial Research (CSIR), GOI, for a fellowship. The authors acknowledge Prof. S. J. George and S. Kuila for lifetime measurements. The authors acknowledge Prof. K.S. Narayan and A. Ghorai for quantum yield measurements.

■

REFERENCES

(1) Sutar, P.; Maji, T. K. Coordination polymer gels: soft metal− organic supramolecular materials and versatile applications. Chem. Commun. 2016, 52, 8055−8074. (2) Jung, J. H.; Lee, J. H.; Silverman, J. R.; John, G. Coordination polymer gels with important environmental and biological applications. Chem. Soc. Rev. 2013, 42, 924−936. (3) Tam, A. Y.-Y.; Yam, V. W.-W. Recent advances in metallogels. Chem. Soc. Rev. 2013, 42, 1540−1567. (4) Piepenbrock, M.-O. M.; Lloyd, G. O.; Clarke, N.; Steed, J. W. Metal- and Anion-Binding Supramolecular Gels. Chem. Rev. 2010, 110, 1960−2004. (5) Nath, K.; Husain, A.; Dastidar, P. Metallogels and Silver Nanoparticles Generated from a Series of Transition Metal-Based Coordination Polymers Derived from a New Bis-pyridyl-bis-amide Ligand and Various Carboxylates. Cryst. Growth Des. 2015, 15, 4635− 4645. (6) Tu, T.; Fang, W.; Bao, X.; Li, X.; Dötz, K. H. Visual Chiral Recognition through Enantioselective Metallogel Collapsing: Synthesis, Characterization, and Application of Platinum−Steroid LowMolecular-Mass Gelators. Angew. Chem., Int. Ed. 2011, 50, 6601−6605. (7) Maity, M.; Maitra, U. An easily prepared palladium-hydrogel nanocomposite catalyst for C−C coupling reactions. J. Mater. Chem. A 2014, 2, 18952−18958. (8) Mehdi, H.; Pang, H.; Gong, W.; Dhinakaran, M. K.; Wajahat, A.; Kuang, X.; Ning, G. A novel smart supramolecular organic gelator exhibiting dual-channel responsive sensing behaviours towards fluoride ion via gel−gel states. Org. Biomol. Chem. 2016, 14, 5956−5964. (9) Peveler, W. J.; Bear, J. C.; Southern, P.; Parkin, I. P. Organic− inorganic hybrid materials: nanoparticle containing organogels with myriad applications. Chem. Commun. 2014, 50, 14418−14420. (10) Xie, Z.; Wang, F.; Liu, C.-y. Organic−Inorganic Hybrid Functional Carbon Dot Gel Glasses. Adv. Mater. 2012, 24, 1716− 1721. (11) Jung, J. H.; Ono, Y.; Shinkai, S. Sol-Gel Polycondensation of Tetraethoxysilane in a Cholesterol-Based Organogel System Results in Chiral Spiral Silica. Angew. Chem., Int. Ed. 2000, 39, 1862−1865. (12) Suresh, V. M.; De, A.; Maji, T. K. High aspect ratio, processable coordination polymer gel nanotubes based on an AIE-active LMWG with tunable emission. Chem. Commun. 2015, 51, 14678−14681. (13) Sutar, P.; Suresh, V. M.; Maji, T. K. Tunable emission in lanthanide coordination polymer gels based on a rationally designed blue emissive gelator. Chem. Commun. 2015, 51, 9876−9879. (14) Aiyappa, H. B.; Saha, S.; Wadge, P.; Banerjee, R.; Kurungot, S. Fe(III) phytate metallogel as a prototype anhydrous, intermediate temperature proton conductor. Chem. Sci. 2015, 6, 603−607. (15) Chen, P.; Li, Q.; Grindy, S.; Holten-Andersen, N. White-LightEmitting Lanthanide Metallogels with Tunable Luminescence and

■

CONCLUSION In conclusion, the preceding results demonstrate that designing an amphiphilic, tripodal LMWG could be apposite for synthesizing CPGs with modular nanomorphologies and versatile functionalities. The LMWG not only acts as a chemosensor for ZnII but also forms ZnL coordination polymer hydrogel that exhibits layered sheet morphology and CT emission. Moreover, the coordination-driven self-assembly of L with TbIII/ EuIII results in bright green/red luminescent CPGs. Furthermore, coordination of both TbIII and EuIII to L results in bimetallic CPGs whose emissions are tuned over a broad spectral range by simply tuning L/TbIII/EuIII ratios. By taking advantage of Ln−Ntpy bond dynamics, we are able to demonstrate the reversible mechano-, thermo-, and chemoresponsive behaviors of luminescent CPGs. Our strategy of exploiting a flexible, amphiphilic LMWG for synthesizing a series of monometallic and bimetallic CPGs can open a new platform for developing multifunctional CPGs from a single LMWG.

■

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01002. Absorption and emission spectra of L, titration results with all the metal ions, FESEM images and TEM images of ZnL and TbL xerogels, excitation spectra of ZnL and L, absorption spectra of TbL and EuL xerogel, EDAX and elemental mapping of TbL, EuL, and CPG2, picture of all CPGs, stimuli−responsive gel−sol transition of TbL and EuL, and corresponding change in emission. (PDF) 9423

DOI: 10.1021/acs.inorgchem.7b01002 Inorg. Chem. 2017, 56, 9417−9425

Article

Inorganic Chemistry Reversible Stimuli-Responsive Properties. J. Am. Chem. Soc. 2015, 137, 11590−11593. (16) Martinez-Calvo, M.; Kotova, O.; Mobius, M. E.; Bell, A. P.; McCabe, T.; Boland, J. J.; Gunnlaugsson, T. Healable Luminescent Self-Assembly Supramolecular Metallogels Possessing Lanthanide (Eu/Tb) Dependent Rheological and Morphological Properties. J. Am. Chem. Soc. 2015, 137, 1983−1992. (17) Lee, H.; Jung, S. H.; Han, W. S.; Moon, J. H.; Kang, S.; Lee, J. Y.; Jung, J. H.; Shinkai, S. A Chromo-Fluorogenic Tetrazole-Based CoBr2 Coordination Polymer Gel as a Highly Sensitive and Selective Chemosensor for Volatile Gases Containing Chloride. Chem. - Eur. J. 2011, 17, 2823. (18) Barman, S.; Anand Garg, J.; Blacque, O.; Venkatesan, K.; Berke, H. Triptycene based luminescent metal−organic gels for chemosensing. Chem. Commun. 2012, 48, 11127−11129. (19) Xing, B.; Choi, M.-F.; Xu, B. Design of Coordination Polymer Gels as Stable Catalytic Systems. Chem. - Eur. J. 2002, 8, 5028−5032. (20) Ye, Y.-X.; Liu, W.-L.; Ye, B.-H. A highly efficient and recyclable Pd(II) metallogel catalyst: A new scaffold for Suzuki-Miyaura coupling. Catal. Commun. 2017, 89, 100−105. (21) Lee, J. H.; Kang, S.; Lee, J. Y.; Jung, J. H. A tetrazole-based metallogel induced with Ag+ ion and its silver nanoparticle in catalysis. Soft Matter 2012, 8, 6557−6563. (22) Feldner, T.; Häring, M.; Saha, S.; Esquena, J.; Banerjee, R.; Díaz, D. D. Supramolecular Metallogel That Imparts Self-Healing Properties to Other Gel Networks. Chem. Mater. 2016, 28, 3210−3217. (23) Sarkar, S.; Dutta, S.; Bairi, P.; Pal, T. Redox-Responsive Copper(I) Metallogel: A Metal−Organic Hybrid Sorbent for Reductive Removal of Chromium(VI) from Aqueous Solution. Langmuir 2014, 30, 7833−7841. (24) Lohe, M. R.; Rose, M.; Kaskel, S. Metal−organic framework (MOF) aerogels with high micro- and macroporosity. Chem. Commun. 2009, 6056−6058. (25) Das, U. K.; Dastidar, P. Supramolecular Chirality in Organo-, Hydro-, and Metallogels Derived from Bis-amides of L-(+)-Tartaric Acid: Formation of Highly Aligned 1D Silica Fibers and Evidence of 5c Net SnS Topology in a Metallogel Network. Chem. - Eur. J. 2012, 18, 13079−13090. (26) Sutar, P.; Maji, T. K. Bimodal self-assembly of an amphiphilic gelator into a hydrogel-nanocatalyst and an organogel with different morphologies and photophysical properties. Chem. Commun. 2016, 52, 13136−13139. (27) Zhang, L.; Wang, X.; Wang, T.; Liu, M. Tuning Soft Nanostructures in Self-assembled Supramolecular Gels: From Morphology Control to Morphology-Dependent Functions. Small 2015, 11, 1025−1038. (28) Busseron, E.; Ruff, Y.; Moulin, E.; Giuseppone, N. Supramolecular self-assemblies as functional nanomaterials. Nanoscale 2013, 5, 7098−7140. (29) Liu, C.; Jin, Q.; Lv, K.; Zhang, L.; Liu, M. Water tuned the helical nanostructures and supramolecular chirality in organogels. Chem. Commun. 2014, 50, 3702−3705. (30) Frederickson, C. J.; Koh, J. Y.; Bush, A. I. The neurobiology of zinc in health and disease. Nat. Rev. Neurosci. 2005, 6, 449−462. (31) Chang, M.; McBroom, M. W.; ScottBeasley, R. Roofing as a source of nonpoint water pollution. J. Environ. Manage. 2004, 73, 307− 315. (32) Reddad, Z.; Gerente, C.; Andres, Y.; Le Cloirec, P. Adsorption of Several Metal Ions onto a Low-Cost Biosorbent: Kinetic and Equilibrium Studies. Environ. Sci. Technol. 2002, 36, 2067−2073. (33) Zhang, Y.; Wang, Y.; Yu, R.; Zhang, S.; Wu, Z. Effects of heavy metals Cd2+, Pb2+ and Zn2+ on DNA damage of loach Misgurnus anguillicaudatus. Front. Biol. China 2008, 3, 50. (34) Bush, A. I.; Pettingell, W. H.; Multhaup, G.; d Paradis, M.; Vonsattel, J.-P.; Gusella, J. F.; Beyreuther, K.; Masters, C. L.; Tanzi, R. E. Rapid induction of Alzheimer A beta amyloid formation by zinc. Science 1994, 265, 1464−1467. (35) Lee, J.-Y.; Cole, T. B.; Palmiter, R. D.; Suh, S. W.; Koh, J.-Y. Contribution by synaptic zinc to the gender-disparate plaque

formation in human Swedish mutant APP transgenic mice. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 7705−7710. (36) Suh, S. W.; Jensen, K. B.; Jensen, M. S.; Silva, D. S.; Kesslak, P. J.; Danscher, G.; Frederickson, C. J. Histochemically-reactive zinc in amyloid plaques, angiopathy, and degenerating neurons of Alzheimer’s diseased brains. Brain Res. 2000, 852, 274−278. (37) Koh, J.-Y.; Suh, S. W.; Gwag, B. J.; He, Y. Y.; Hsu, C. Y.; Choi, D. W. The role of zinc in selective neuronal death after transient global cerebral ischemia. Science 1996, 272, 1013−1016. (38) Nugent, J. W.; Lee, H.; Lee, H.-S.; Reibenspies, J. H.; Hancock, R. D. Mechanism of chelation enhanced fluorescence in complexes of cadmium(II), and a possible new type of anion sensor. Chem. Commun. 2013, 49, 9749−9751. (39) Chae, M.-Y.; Yoon, J.; Czarnik, A. W. Chelation-enhanced fluorescence chemosensing of Pb(II), an inherently quenching metal ion. J. Mol. Recognit. 1996, 9, 297−303. (40) Guo, L. E.; Liu, X. Y.; Wang, H.; Chen, Q. L.; Wang, G. K.; Luo, K.; Mi, Q. L.; Zhou, Y.; Zhang, J. F. Iron(III)-Selective ChelationEnhanced Fluorescence Sensing for In Vivo Imaging Applications. Chem. - Asian J. 2015, 10, 1898−1902. (41) Akkaya, E. U.; Huston, M. E.; Czarnik, A. W. Chelationenhanced fluorescence of anthrylazamacrocycle conjugate probes in aqueous solution. J. Am. Chem. Soc. 1990, 112, 3590−3593. (42) Zhang, X.; Li, H.; Liu, G.; Pu, S. A novel diarylethene-based fluorescent switch with a carboxamidoquinoline unit for sensing of Zn(II) ion. J. Photochem. Photobiol., A 2016, 330, 22−29. (43) Fages, F. Low Molecular Mass Gelators. Top. Curr. Chem. 2005, 256, 283. (44) van Esch, J. H.; Feringa, B. L. New Functional Materials Based on Self-Assembling Organogels: From Serendipity towards Design. Angew. Chem., Int. Ed. 2000, 39, 2263−2266. (45) Yagai, S.; Nakajima, T.; Kishikawa, K.; Kohmoto, S.; Karatsu, T.; Kitamura, A. Hierarchical Organization of Photoresponsive HydrogenBonded Rosettes. J. Am. Chem. Soc. 2005, 127, 11134. (46) Ahn, S.-K.; Kasi, R. M.; Kim, S.-C.; Sharma, N.; Zhou, Y. Stimuli-responsive polymer gels. Soft Matter 2008, 4, 1151−1157. (47) Grove, T. Z.; Osuji, C. O.; Forster, J. D.; Dufresne, E. R.; Regan, L. Stimuli-Responsive Smart Gels Realized via Modular Protein Design. J. Am. Chem. Soc. 2010, 132, 14024−14026. (48) Ninawe, P. R.; Parulekar, S. Drug Delivery Using StimuliResponsive Polymer Gel Spheres. Ind. Eng. Chem. Res. 2012, 51, 1741−1755. (49) Kotova, O.; Daly, R.; dos Santos, C. M. G.; Boese, M.; Kruger, P. E.; Boland, J. J.; Gunnlaugsson, T. Europium-Directed SelfAssembly of a Luminescent Supramolecular Gel from a Tripodal Terpyridine-Based Ligand. Angew. Chem., Int. Ed. 2012, 51, 7208− 7212. (50) Bhowmik, S.; Banerjee, S.; Maitra, U. A self-assembled, luminescent europium cholate hydrogel: a novel approach towards lanthanide sensitization. Chem. Commun. 2010, 46, 8642−8644. (51) Roy, S.; Chakraborty, A.; Maji, T. K. Lanthanide−organic frameworks for gas storage and as magneto-luminescent materials. Coord. Chem. Rev. 2014, 273−274, 139−164. (52) Liu, Y.; Tu, D.; Zhu, H.; Chen, X. Lanthanide-doped luminescent nanoprobes: controlled synthesis, optical spectroscopy, and bioapplications. Chem. Soc. Rev. 2013, 42, 6924−6958. (53) Meyer, L. V.; Schönfeld, F.; Müller-Buschbaum, K. Lanthanide based tuning of luminescence in MOFs and dense frameworks−from mono- and multimetal systems to sensors and films. Chem. Commun. 2014, 50, 8093−8108. (54) Dannenbauer, N.; Matthes, P. R.; Scheller, T. P.; Nitsch, J.; Zottnick, S. H.; Gernert, M. S.; Steffen, A.; Lambert, C.; MüllerBuschbaum, K. Near-Infrared Luminescence and Inner Filter Effects of Lanthanide Coordination Polymers with 1,2-Di(4-pyridyl)ethylene. Inorg. Chem. 2016, 55, 7396−7406. (55) Dechnik, J.; Mühlbach, F.; Dietrich, D.; Wehner, T.; Gutmann, M.; Lühmann, T.; Meinel, L.; Janiak, C.; Müller-Buschbaum, K. Luminescent Metal−Organic Framework Mixed-Matrix Membranes 9424

DOI: 10.1021/acs.inorgchem.7b01002 Inorg. Chem. 2017, 56, 9417−9425

Article

Inorganic Chemistry from Lanthanide Metal−Organic Frameworks in Polysulfone and Matrimid. Eur. J. Inorg. Chem. 2016, 2016, 4408−4415. (56) Surender, E. M.; Comby, S.; Martyn, S.; Cavanagh, B.; Lee, T. C.; Brougham, D. F.; Gunnlaugsson, T. Cyclen lanthanide-based micellar structures for application as luminescent [Eu(III)] and magnetic [Gd(III)] resonance imaging (MRI) contrast agents. Chem. Commun. 2016, 52, 10858−10861. (57) Barry, D. E.; Caffrey, D. F.; Gunnlaugsson, T. Lanthanidedirected synthesis of luminescent self-assembly supramolecular structures and mechanically bonded systems from acyclic coordinating organic ligands. Chem. Soc. Rev. 2016, 45, 3244−3274. (58) Mohapatra, S.; Adhikari, S.; Riju, H.; Maji, T. K. Terbium(III), Europium(III), and Mixed Terbium(III)−Europium(III) Mucicate Frameworks: Hydrophilicity and Stoichiometry-Dependent Color Tunability. Inorg. Chem. 2012, 51, 4891−4893. (59) Bhattacharyya, S.; Chakraborty, A.; Jayaramulu, K.; Hazra, A.; Maji, T. K. A bimodal anionic MOF: turn-off sensing of CuII and specific sensitization of EuIII. Chem. Commun. 2014, 50, 13567−13570. (60) McCarney, E. P.; Byrne, J. P.; Twamley, B.; Martínez-Calvo, M.; Ryan, G.; Möbius, M. E.; Gunnlaugsson, T. Self-assembly formation of a healable lanthanide luminescent supramolecular metallogel from 2,6bis(1,2,3-triazol-4-yl)pyridine (btp) ligands. Chem. Commun. 2015, 51, 14123−14126. (61) Bradberry, S. J.; Savyasachi, A. J.; Peacock, R. D.; Gunnlaugsson, T. Quantifying the formation of chiral luminescent lanthanide assemblies in an aqueous medium through chiroptical spectroscopy and generation of luminescent hydrogels. Faraday Discuss. 2015, 185, 413−431. (62) Kotova, O.; Daly, R.; dos Santos, C. M. G.; Kruger, P. E.; Boland, J. J.; Gunnlaugsson, T. Cross-Linking the Fibers of Supramolecular Gels Formed from a Tripodal Terpyridine Derived Ligand with d-Block Metal Ions. Inorg. Chem. 2015, 54, 7735−7741. (63) Daly, R.; Kotova, O.; Boese, M.; Gunnlaugsson, T.; Boland, J. J. Chemical Nano-Gardens: Growth of Salt Nanowires from Supramolecular Self-Assembly Gels. ACS Nano 2013, 7, 4838−4845. (64) Feng, W.-X.; Yin, S.-Y.; Pan, M.; Wang, H.-P.; Fan, Y.-N.; Lü, X.-Q.; Su, C.-Y. PMMA-copolymerized color tunable and pure whitelight emitting Eu3+−Tb3+ containing Ln-metallopolymers. J. Mater. Chem. C 2017, 5, 1742−1750. (65) Pan, M.; Du, B.-B.; Zhu, Y.-X.; Yue, M.-Q.; Wei, Z.-W.; Su, C.-Y. Highly Efficient Visible-to-NIR Luminescence of Lanthanide(III) Complexes with Zwitterionic Ligands Bearing Charge-Transfer Character: Beyond Triplet Sensitization. Chem. - Eur. J. 2016, 22, 2440−2451. (66) Yang, Q.-Y.; Pan, M.; Wei, S.-C.; Li, K.; Du, B.-B.; Su, C.-Y. Linear Dependence of Photoluminescence in Mixed Ln-MOFs for Color Tunability and Barcode Application. Inorg. Chem. 2015, 54, 5707−5716. (67) Singhal, D.; Gupta, N.; Singh, A. K. Chromogenic ‘naked eye’ and fluorogenic ‘turn on’ sensor for mercury metal ion using thiophene-based Schiff base. RSC Adv. 2015, 5, 65731−65735. (68) Shyamal, M.; Mazumdar, P.; Maity, S.; Samanta, S.; Sahoo, G. P.; Misra, A. Highly Selective Turn-On Fluorogenic Chemosensor for Robust Quantification of Zn(II) Based on Aggregation Induced Emission Enhancement Feature. ACS. Sens. 2016, 1, 739−747. (69) Hancock, R. D. The pyridyl group in ligand design for selective metal ion complexation and sensing. Chem. Soc. Rev. 2013, 42, 1500− 1524. (70) Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.; Summerton, G. C.; Gertenbach, J. A.; Field, J. S.; Haines, R. J.; McMillin, D. R. Long-Lived Emissions from 4′-Substituted Pt(trpy)Cl+ Complexes Bearing Aryl Groups. Influence of Orbital Parentage. Inorg. Chem. 2001, 40, 2193− 2200. (71) Lee, Y. H.; Nghia, N. V.; Go, M. J.; Lee, J.; Lee, S. U.; Lee, M. H. Terpyridine−Triarylborane Conjugates for the Dual Complexation of Zinc(II) Cation and Fluoride Anion. Organometallics 2014, 33, 753− 762.

(72) Balkenende, D. W. R.; Coulibaly, S.; Balog, S.; Simon, Y. C.; Fiore, G. L.; Weder, C. Mechanochemistry with Metallosupramolecular Polymers. J. Am. Chem. Soc. 2014, 136, 10493−10498. (73) Burnworth, M.; Tang, L.; Kumpfer, J. R.; Duncan, A. J.; Beyer, F. L.; Fiore, G. L.; Rowan, S. J.; Weder, C. Optically healable supramolecular polymers. Nature 2011, 472, 334−337. (74) Cao, D.-K.; Hu, J.-S.; Li, M.-Q.; Gong, D.-P.; Li, X.-X.; Ward, M. D. Two bisthienylethene−Ir(III) complexes showing acid/baseinduced structural transformation and on−off luminescence switching in solution. Dalton. Trans. 2015, 44, 21008−21015. (75) Alam, P.; Kaur, G.; Chakraborty, S.; Roy Choudhury, A.; Laskar, I. R. Aggregation induced phosphorescence” active “rollover” iridium(III) complex as a multi-stimuli-responsive luminescence material. Dalton. Trans. 2015, 44, 6581−6592.

9425

DOI: 10.1021/acs.inorgchem.7b01002 Inorg. Chem. 2017, 56, 9417−9425