Tunable Luminescent Heterometallic Zn2Ln2 Edge-Defective

Jun 1, 2017 - ... including ligand-based yellow-green light emission for 1 and 4 and lanthanide-center emission for 2 and 3. The multiple stimuli-resp...
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Tunable Luminescent Heterometallic Zn2Ln2 Edge-Defective Molecular Cubane with Stimuli-Responsive Properties Wei Huang, Ming Zhang, Shuaidan Huang, and Dayu Wu* Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Collaborative Innovation Center of Advanced Catalysis & Green Manufacturing, School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China S Supporting Information *

to external stimuli, such as heat, solvent wetting, or recrystallization.9 Recently, studies on luminescent metal− organic frameworks (MOFs) have revealed that the fluorescent intensity and emissive color can be altered by temperature,10 vapor,11 pressure,12 and light.13 However, the tunable fluorescence system concerning the metal−organic discrete molecules remains virtually challenging.14 The Schiff base ligand HL requires a sophisticated synthesis (five steps) to combine d/f metal binding sites with the large πconjugate component in one molecule for the assembly of multifunctional luminescent metal−organic discrete molecules (Scheme S1) We herein describe the interesting Zn2Ln2 edgedefective cubane-like clusters [Ln = Gd (1), Eu (2), Tb (3), Dy (4)] obtained via a compartmentalized ligand approach (Scheme S2). The photophysical properties demonstrate that Zn2Ln2 cubanes show the Ln(III)-tunable luminescence and the emissive property can respond to external stimuli, i.e., temperature, pressure, and solvent. The free ligand HL exhibits absorption bands at 229 and 345 nm, which are weakly perturbed upon coordination to metal ions in complexes 1−4 (Figure S1). Complexes 1−4 exhibit similar absorption spectra in the solid state, and the additional 420 nm may be ascribed to metal-to-ligand charge-transfer bands due to Zn(II) and Ln(III) coordination. Single-crystal X-ray diffraction (XRD) analysis of complexes 1 and 4 revealed that they are isomorphous. The powder XRD patterns for complexes 2 and 3 are consistent with the pattern simulated from the single-crystal data of complex 1, indicating that Tb(III) and Eu(III) complexes are also isostructural to the Gd(III) complex (Figure S2). As a representative, the crystal structure of complex 1 at 298 K is shown in Figure 1. The crystal structure consists of cation entities [[Gd2Zn2(L)2(Ac)5(OH)2]+, one [ZnCl3(H2O)]− anion for charge balance, and solvent molecules of crystallization. The asymmetric unit in 1 consists of a distorted {Zn2Gd2O4} cubane core with two μ3-OH− and two μ2-O phenolate groups as vertices. Two Gd(III) ions show the slightly different eightcoordinate environment. Peripheral ligation is provided by two monodeprotonated L ligands. Significant intramolecular π···π stacking interactions are found between two five-membered Zn(II)-chelated metallocycles with the shortest interatomic contact of 3.348(2) Å, in addition to the terminal phenyl/pyridyl rings with an interatomic contact of ca. 3.410(2)−3.652(2) Å between two parallel L ligands.15 Each Zn(II) ion is coordinated in a [N2O3] square-pyramidal environment. Gd1 and Gd2 are

ABSTRACT: Heterometallic Zn2Ln2 [Ln = Gd (1), Eu (2), Tb (3), Dy (4)] discrete molecules with edgedefective cubane structure are assembled from a multifunctional fluorescent conjugate ligand and LnIII/ZnII mixed-metal ions; they exhibit the tunable luminescence, including ligand-based yellow-green light emission for 1 and 4 and lanthanide-center emission for 2 and 3. The multiple stimuli-responsive photoluminescences were investigated to show a two-step thermal-responsive luminescence increase in the intensity upon cooling and piezochromic luminescence.

M

olecular systems whose physical properties could be changed in response to a single stimulus or multiple stimuli are called “smart” or stimuli-responsive materials. Stimuliresponsive luminescent materials have attracted great attention because of their potential applications in fluorescent sensors and optical recording devices.1 A large variety of organic solids have been reported to be “smart” or “intelligent” materials because their luminescence properties can be altered in response to external stimuli or environmental variations, such as pressure, temperature, pH, vapor, light irradiation, and so on.2 However, the responsive properties have rarely been extended to the study of metal−organic polynuclear discrete molecules because of structural uncontrollability, low quantum efficiency, etc.3 The highly ordered nature of individual aggregates or clusters, which combine the properties of organic ligands and metal centers, offers a powerful platform for the development and application of fluorescent materials.4 Usually, the use of Ln ions often provides efficient luminescence, which is typically accomplished by an antenna effect.5 On the other hand, the polynuclear aggregates through the combination of d10 transition metals and highly conjugated organic linking groups can emit intense ligand-based luminescence.6 A possible advantage for the utilization of ligand-based emission is that it could be easily tunable by solvatochromism, piezochromism, and variation of substitution or the pH.7 Piezochromic luminescent (PCL) molecules are a class of “smart” materials whose fluorescence properties can respond to external pressure or mechanical grinding.8 Generally speaking, the conformational planarization of a π system induced by high pressure accounts for the red shift in the photoluminescence (PL) spectra of luminogens. This process can be reversible and the original emission color can be revised by altering the aromatic π···π and/or C−H···π packing mode in the solid state in response © 2017 American Chemical Society

Received: March 3, 2017 Published: June 1, 2017 6768

DOI: 10.1021/acs.inorgchem.7b00567 Inorg. Chem. 2017, 56, 6768−6771

Communication

Inorganic Chemistry

us to consider the potential luminescence properties of its mixed d/f metal complexes. Complex 1 shows the strong ligand-based luminescence centered at 530 nm in air, and the emission peaks are red-shifted by 100 nm from that of free HL. The type of red shift is likely to be ascribed to metal-chelation-enhanced conjugation in the complexes. Under the same conditions, complex 3 simultaneously showed a typical emission of the Tb(III) ion, with the strongest peak at 546 nm corresponding to a 5D4 → 7F5 transition. However, complex 2 showed a weak ligand-based emission and exhibited typical emissions at 590, 616, 650, and 698 nm, corresponding to 5D0 → 7FJ (J = 1, 2, 3, and 4) transitions of the Eu(III) ion, indicative of an efficient energy transfer from the ligand to the Eu(III) center. When the as-synthesized powder of 1 (noted as A) was thoroughly ground in an agate mortar for 5 min, the emission shows a slight weakening in the intensity for the ground sample (G), and a significant bathochromic shift (Δλ = 24 nm) was observed (Figures 2a and S6). To investigate the solvent-revised PCL behavior, G was wetted with the dropwise addition of a methanol (MeOH) solvent (noted as M), and the emission peak shows a hypochromatic shift of ca. 12 nm within a few seconds. Notably, the emission could perfectly revert to G when M was further ground. For 3, the grinding gave rise to a similar ligand-based bathochromic shift (Δλ = 22 nm) and fluorescence decrease. The subsequent wetting with MeOH can perfectly revert the ligand-based emission to that of as-synthesized sample A. In order to establish the origin and mechanism of this reversible piezochromic behavior, the UV−vis spectra in the solid state of the A, G, and M forms of complex 1 were obtained. The results show complete consistency of the absorption bands for all of the complexes (Figure S8). The changed emission spectra in the grinding process are, therefore, attributed to physical processes. Powder XRD was carried out to investigate the possible phase changes in 1. The sharp peaks in the XRD pattern (Figure 2d) unambiguously indicate that the A sample has a wellordered crystalline structure. The ground sample G exhibits identical diffraction signals, which indicates that the grinding cannot form the amorphous phase. Because the crystalline phase is unresponsive to mechanical grinding, the distinct luminescence response to stress implies that the aromatic π···π and C···C contacts are strong enough for stabilization of the molecular packing patterns under mechanical grinding. Such a red shift frequently arises from the conformational planarization induced by the high pressure. However, the lanthanide center emitted faintly upon grinding 2 and 3. It is normal because the luminophores often emit weakly when molecules meet with varying degrees of an aggregation-caused quenching effect. Next, we examined the quantitative response of the assynthesized 1 to pressure. When deposited samples in a vessel were subjected to different pressures, complex 1 uniformly exhibited positive piezofluorochromism, that is, a bathochromic shift of the emission band with increasing pressure (Figure 3a). On the basis of the Commission Internacionale d’Eclairage (CIE) chromaticity diagram, the corresponding CIE coordinates move from (0.375, 0.551) for the as-synthesized sample to (0.385, 0.536), (0.393, 0.531), (0.407, 0.524), and (0.417, 0.511) subject to 5, 10, 20, and 28 MPa, respectively (Figure S9). The maximum emission shift exhibited an excellent linear correlation with the pressure values (Figure 3b), demonstrating that 1 can function as an indicator of the pressure. Another remarkable feature is the temperature-dependent emission of the molecular cubane under UV excitation. As shown

Figure 1. (a) Cation core structure of 1 with the selected atom···atom contacts. All H atoms, anions, and solvent molecules are omitted for clarity. (b) Perspective view of the coordination polyhedron for Zn(II) ions in 1. (c) Perspective view of the coordination polyhedron for Gd(III) ions in 1. (d) View of the edge-defective Zn2Gd2O4 cubane core.

triply bridged by two μ3-OH− O atoms and one acetate group. However, the Zn(II) linkage is broken by the long Zn···O contact with 2.962(3) and 3.032(5) Å for Zn1···O1 and Zn2···O4, respectively, so that a heterometallic cubane forms with two defective edges. The strong intermolecular offset π···π stacking was also evidenced by the shortest interatomic distance of 3.253(4) Å between uncoordinated pyrazolyl and pyridyl rings (Figure S3) The solid-state PL spectra were recorded at room temperature (Figures 2 and S4). Free ligand HL exhibits strong blue

Figure 2. (a) Emission spectra of the as-synthesized sample 1 (A), ground sample (G), and ground sample wetted with MeOH (M). (b and c) Spectra of the A, G, and M samples of 2 and 3, respectively. λex = 350 nm; emission spectra were normalized at 530 nm. (d) Powder XRD patterns of A, G, and M for 1 together with the pattern simulated from the single-crystal data.

luminescence centered at 430 nm in the solid state under excitation at 350 nm, and the luminescence quantum yield (QY) is 1.26% at 298 K. The luminescence spectra under different pressures revealed that ligand HL is unresponsive to mechanical compression (Figure S5). The strong luminescence of HL is mostly due to the fact that ligand HL has highly conjugated π systems, which could act as a source of luminescence and prompt 6769

DOI: 10.1021/acs.inorgchem.7b00567 Inorg. Chem. 2017, 56, 6768−6771

Communication

Inorganic Chemistry

room temperature, complex 2 shows a weak lanthanide-center emission (QY ∼ 0.1%). The luminescent intensity increases gently as the temperature decreases to 150 K, and a steep fluorescent increase is observed upon cooling to 90 K (QY ∼ 7.8%), with a relative sensitivity of 15.3% per K, followed by a sudden decrease upon further cooling. Such a temperaturedependent emission color and intensity change could be readily observed by the naked eye as well (inset of Figure 4d). When the temperature was gradually released to ambient conditions, the original PL spectrum was restored. It is worth noting that the temperaure-dependent multistep fluorescent modulation has been encountered in a Ag+-containing MOF,16 while investigations on discrete molecules exhibiting multistep changes with temperature have rarely been done.17 In conclusion, the smart tetranuclear Zn2Ln2 cubane-like complexes have been synthesized from a fluorophore-based multifunctional ligand. They exhibit Ln(III)-tunable luminescence, including ligand-center emission from Gd(III) and Dy(III) complexes, lanthanide-center emission from the Eu(III) complex, and mixing character for the Tb(III) complex. The complexes showed unique temperature-responsive emissive properties. An obvious bathochromic shift occurs in the solid state upon mechanical pressure, and the ligand-based emission of the samples subjected to wetting with a MeOH solvent were restored to the original one. Therefore, these findings indicate that Zn2Ln2 molecular cubanes exhibit ligand-based and/or lanthanide-center luminescence, which could expand applications in pressure and temperature sensing.

Figure 3. Piezochromic response of complex 2. (a) Normalized luminescence intensity of 2 under different pressures with excitation at 350 nm. (b) Maximum emission shift versus different pressures.

in Figure 4a, the emission spectra of 1 were recorded from 78 K to room temperature. The emission position centered at 530 nm

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ASSOCIATED CONTENT

* Supporting Information S

Figure 4. Temperature-dependent fluorescence spectra of complexes 1 (a) and 2 (c), excited at 350 nm. The three sharp peaks marked with a gray rectangle in part a are the emission spectra of a background light source. (b and d) Relationship between the emission intensity and temperature. Inset: Luminescence photograph of 1 at 78 K and 2 at 300, 130, and 90 K.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00567. Additional structural and spectroscopic characterization data (PDF) Accession Codes

basically remains unchanged with changing temperature, while the band intensity linearly increases when the temperature is progressively lowered between room temperature and 208 K. The temperature can be linearly related to the relative intensity (I) by the equation I = 11.21−0.031T with R2 = 0.9982. A sudden decrease of the intensity is seen from 208 to 188 K, and then another linear increase of the intensity is observed upon continuous cooling to lower temperatures (Figure 4b). The linear relationship can be expressed between 128 and 188 K as I = 12.34−0.044T with R2 = 0.9931. The temperature sensitivities for the ranges of 1 and 2 are 1.42% and 1.02% per K, respectively. Generally, cold conditions should be favorable for the rigidity of ligands; thereby, the radiationless decay of the intraligand (π···π*) excited state could be reduced to some extent, and the increase of the QYs should be observed eventually by lowering the temperature. The multistep change is unexpected and suggests that the new mechanisms are operating in Zn2Gd2 (1), which causes the temperature-dependent emission spectra to be quite different. Note that the PL of complex 1 responding to the temperature change is very fast and reversible, underlying their potential as optical sensors for temperature detection. The temperature dependence of the emission spectra of 2 from 77 to 300 K is illustrated in Figure 4c, and the integrated intensities of the 5D0 → 7F2 transitions are shown in Figure 4d. At

CCDC 1519475−1519476 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dayu Wu: 0000-0002-4132-4795 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for financial support by the PAPD of Jiangsu Higher Education Institutions. This work is supported by the NSFC program (Grants 21671027 and 21471023) and sponsored by the Jiangsu Provincial QingLan Project. 6770

DOI: 10.1021/acs.inorgchem.7b00567 Inorg. Chem. 2017, 56, 6768−6771

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b00567 Inorg. Chem. 2017, 56, 6768−6771