A Piezochromic Dysprosium(III) Single-Molecule Magnet Based on an

Jul 10, 2017 - (1) An efficient approach toward achieving multifunctional materials is to assemble organic/inorganic hybrids via coordination chemistr...
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A Piezochromic Dysprosium(III) Single-Molecule Magnet Based on an Aggregation-Induced-Emission-Active Tetraphenylethene Derivative Ligand Wen-Bin Chen, Yan-Cong Chen, Jun-Liang Liu,* Jian-Hua Jia, Long-Fei Wang, Quan-Wen Li, and Ming-Liang Tong* Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, P. R. China S Supporting Information *

complexes can emit photoluminescence (PL) with different colors upon mechanical stimuli.10 It is conceivable that combining piezochromism with singlemolecule magnetism would result in piezochromic SMMs. Such multifunctional materials provide a potential way for modulating the crystal-field environment of the SMMs via mechanical stimuli, consequently tuning the magnetic relaxation. If it is switchable, the bifunctional compounds could bear boundless possibilities, unlimited to information storage, sensors, switches, etc. However, to the best of our knowledge, a study focusing on bifunctional complexes with both piezochromism and SMM behavior has not yet been explored. With the above concerns, we manage to combine a new AIEactive ligand, HTPEIPOMe, together with a DyIII ion, resulting a dinuclear molecule, [Dy2(HTPEIPOMe)2(OAc)4(NO3)2] (1). The HTPEIPOMe ligand and complex 1 both display switchable piezochromism by external stimuli including a pressing−fuming cycle. Magnetic studies revealed that complex 1 is an SMM with an energy barrier of 168(15) K at zero field. Pressure as a stimuli to complex 1 actually affects the magnetic anisotropy of the DyIII center, leading to different magnetic behaviors. The HTPEIPOMe ligand is prepared by a Schiff-base reaction of 4-(1,2,2-triphenylvinyl)aniline and 3-methoxysalicyl aldehyde in CH3CN (Scheme S1).11 The reaction of Dy(OAc)3·4H2O and Dy(NO3)3·6H2O, together with HTPEIPOMe, in CH3CN produces yellow block crystals of 1 (see the Supporting Information for details). As shown in the thermogravimetric analysis (TGA) and temperature-dependent powder X-ray diffraction (PXRD) patterns in Figures S1 and S2, complex 1 does not decompose until 250 °C, indicating high thermal stability. A single-crystal X-ray diffraction study (Tables S1 and S2) was performed for the HTPEIPOMe ligand and complex 1 to understand the molecular structures and packing in the solid state. For the HTPEIPOMe ligand, each asymmetric unit contains a neutral HTPEIPOMe molecule, as shown in Figure S3, and the TPE group adopts a twisted conformation. The bond lengths of C1−O1(phenol) = 1.358(3) Å and N1−C8 = 1.278(3) Å indicate that the ligand possesses a phenolimine form. Complex 1 crystallizes in the triclinic space group P1̅. Each unit cell contains two DyIII ions, with two coordinated

ABSTRACT: A bifunctional dysprosium(III) dimer, [Dy2(HTPEIPOMe)2(OAc)4(NO3)2] (1), comprising an AIE-active (AIE = aggregation-induced emission) ligand of 2-methoxy-6-[[[4-(1,2,2-triphenylvinyl)phenyl]imino]methyl]phenol (HTPEIPOMe), was successfully synthesized. It not only behaves as a single-molecule magnet (SMM) with an energy barrier of 168(15) K at zero field but also exhibits piezochromism during the pressing− fuming cycle with switchable color, photoluminescence, and magnetic response.

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he investigation of multifunctional molecular materials has been a highlighted research topic in recent years because of their promising prospects in information storage, electronics, sensors, and biomedicine.1 An efficient approach toward achieving multifunctional materials is to assemble organic/ inorganic hybrids via coordination chemistry and crystal engineering to take advantage of their versatile and tunable physical properties.2 Ln-based complexes are among the attractive organic/inorganic hybrids in the development of advanced magnetic and luminescent materials thanks to the intrinsic and characteristic magnetic/optical properties of LnIII ions.3 In terms of molecular magnetism, single-molecule magnets (SMMs) have attracted increasing attention for their interesting magnetic dynamics and potential applications.4 Since the discovery of {LnPc2} SMMs in 2003,5 lanthanide SMMs have become more and more popular because of their intrinsic large spin states and high magnetic anisotropy.6 It is worth noting that the DyIII ions with oblate-shaped charge distribution, when accommodated in an axial strong ligand field, can bear a |±15/2⟩ Kramers doublet, making it promising for high-performance SMMs.7 Meanwhile, organic moieties have crucial roles in functional hybrid materials because of their organic functionality of ligands as well as the synergy of their coordination environments and properties.8 Among them, tetraphenylethene (TPE) derivatives are a family of typical aggregation-induced-emission (AIE)-active organic compounds.9 This series is nonemissive in the molecularly dissolved state but becomes highly emissive in the aggregated state. Moreover, some TPE-based molecules also display switchable solid-state mechanochromism, in which the © 2017 American Chemical Society

Received: April 26, 2017 Published: July 10, 2017 8730

DOI: 10.1021/acs.inorgchem.7b01059 Inorg. Chem. 2017, 56, 8730−8734

Communication

Inorganic Chemistry HTPEIPOMe molecules in the opposite direction, four μ-OAc− anions, and two chelating NO3− anions (Figure 1). The DyIII ion

Figure 1. (a) Crystal structure of 1 (symmetry code: A, −x + 1, −y + 1, −z). (b) Coordination geometry of lanthanide ions. H atoms are omitted for clarity. Dy−O bond lengths: 2.2735(16)−2.6529(16) Å.

is nine-coordinate, surrounded by nine O atoms, two from HTPEIPOMe, five from four μ-OAc−, and two from one NO3−, which leads to a distorted monocapped square-antiprismatic geometry (Figure 1b). The Dy−O bond distances range from 2.2735(16) to 2.6529(16) Å. μ-OAc− in the dinuclear complex exhibits two different bridging modes (Figure 1a), μ-η1:η1 and μη2:η1, forming a centrosymmetric {Dy2(μ-η2:η1-OAc)2(μ-η1:η1OAc)2} unit with an intramolecular Dy1···Dy1A distance of 3.8117(3) Å and a Dy1−O3−Dy1A bond angle of 104.04(6)° (Figure 1a). The packing of the crystal structure indicates that the shortest intermolecular Dy···Dy separations were found to be 8.9457(4) Å, suggesting well-isolated dinuclear molecules. As shown in Table S2, the C1−O1(phenol) bond length in 1 is shorter than that in the HTPEIPOMe ligand [1.304(3) vs 1.358(3) Å], while the N1−C8 one is longer [1.305(3) vs 1.278(3) Å]. This indicates that HTPEIPOMe is a ketoamine form in complex 1. The dihedral angle between the phenol ring and its neighboring benzene ring decreases to 9.26° compared to the ligand structure (62.46°). PL spectroscopy measurements were performed to elucidate the AIE property of the ligand. The emission spectra of 20 μM HTPEIPOMe in water/acetonitrile (CH3CN) mixtures with different water contents are shown in Figure S4. When the volume fraction (f w) is lower than 50 vol %, the HTPEIPOMe solutions have virtually no emission. However, further addition of water into the mixture results in a significant enhancement of the PL intensity, which is due to the restriction of intramolecular rotation of C−C bonds upon aggregation.9b At 90 vol % water content, the red fluorescence (612 nm) is more than 24-fold stronger than that in the pure CH3CN solution (Figure S4b). These results clearly indicate that HTPEIPOMe is AIE-active. However, because complex 1 decomposes in CH3OH, CH3CN, and THF as observed in ESI-MS, PL spectroscopy measurements of complex 1 in solution were not performed. Complex 1 grows in yellow crystals, and no obvious color change was observed when the crystals were ground. As shown in Figure 2a, when the crystalline sample of 1 was pressed under a pressure of 0.5 GPa, its color changed from yellow to orange. After solvent fuming with CH3CN vapor, the pressed powder recovered its initial color. The piezochromic phenomenon was confirmed by UV−vis diffuse-reflectance and PL spectra (Figure 2b). The initial powder displays two distinguishable absorption peaks centered at 380 and 475 nm. After pressing, the intensity of the absorption spectrum weakens at 475 nm and enhances at 630 nm (Figure 2b). PL spectroscopy measurements show that the

Figure 2. (a) Photograph of 1. (b) Diffuse-reflectance (dotted lines) and emission (solid lines; excitation wavelength = 340 nm) spectra of 1. (c) PXRD patterns of 1.

emission peak of the initial powder is shifted to longer wavelength from 593 to 603 nm after pressing. Upon fuming with CH3CN, the red-shifted PL recovers to the initial peak position (593 nm). To check the crystallinity of complex 1, PXRD measurements were taken for the samples (Figure 2c). The PXRD pattern of the initial powder is sharper and stronger, indicating the ordered crystalline structure. The peak positions of the simulated and experimental PXRD patterns are in agreement with each other, confirming its good phase purity. The profile of the pressed powder lacks any clearly discernible peaks, which suggests that the pressed powder to some extent loses its crystallinity. After fuming with CH3CN vapor, the diffraction pattern of the fumed powder is almost identical with the initial one, demonstrating that the amorphous powder has recovered to the crystalline phase. These results clearly show that the color change from yellow to orange is caused by the morphology transition from the crystalline state to amorphous state, which is in good agreement with those found in other TPE derivatives.10 Likewise, HTPEIPOMe can also display similar piezochromic properties from ivory to brown (Figure S5). The static magnetic susceptibilities were measured on the polycrystalline sample under a 1 kOe direct-current (dc) field (Figure S6). The χMT value at room temperature of 27.39 cm3 K mol−1 is in agreement with the expected values for two isolated DyIII ions (S = 5/2, L = 5, 6H15/2, J = 15/2, and gJ = 4/3).3a Upon cooling, the χMT value decreases slightly to 26.55 cm3 K mol−1 at 16 K, which is attributed to thermal depopulation of the excited crystal-field sublevels.7 On further cooling, the intramolecular ferromagnetic interaction between DyIII ions leads to a significant increase of χMT, reaching 31.12 cm3 K mol−1 at 2 K.6g The fielddependent magnetization rises rapidly at low fields and then increases gently to 10.57 Nβ at 7 T (Figure S6, inset). 8731

DOI: 10.1021/acs.inorgchem.7b01059 Inorg. Chem. 2017, 56, 8730−8734

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Inorganic Chemistry

relaxation time, the relaxation times were extracted using the generalized Debye model (Table 1). At 2 K, the relaxation times

Variable-temperature and variable-frequency alternatingcurrent (ac) magnetic susceptibilities were carried out (Figures 3a and S7). A set of temperature- and frequency-dependent

Table 1. Relaxation Times (τ) from the Best Fits of the ac Susceptibilities for the Initial, Pressed, And Fumed Powders of 1 at 0 and 1 kOe Fields at 2 K τ0 Oe/ms

τ1 kOe/s

13.5(2) 8.9(2) 14.3(2)

0.35(2) 0.19(2) 0.28(2)

initial powder pressed powder fumed powder

for the initial powder are approximately 2 times longer than those of the pressed powder. After fuming with CH3CN vapor, the relaxation times are similar to those of the initial sample, suggesting a reversible switching. In summary, a novel AIE-active ligand and its thermally stable dinuclear dysprosium(III) complex (1) were synthesized. Upon pressing and solvent fuming for the HTPEIPOMe ligand and complex 1, their color and emission spectra can be reversibly switched. Magnetic studies revealed that complex 1 is an SMM with an energy barrier of 168 K at zero field. Mechanical stimulation of pressure on complex 1 actually affects the magnetic dynamics of the DyIII center, leading to switchable relaxation time in the low temperature. This work reports an unprecedented case of a coordination complex that exhibits both piezochromism and slow relaxation of magnetization as a bifunctional material, providing a new route to design and modify the multifunctional SMMs.

Figure 3. (a) Frequency dependence of the in-phase (χ′MT) and out-ofphase (χ″M) products at zero field for 1. The solid lines are guides to the eyes. (b) Plots of ln(τ) versus T−1 for 1 under zero field. The dashed lines represent the best fits using eq 1. The solid lines represent the term of an Orbach process. Inset: Cole−Cole plots for the ac susceptibilities for 1 at zero field. The solid lines represent the best fit to the generalized Debye model.

peaks of χM″ are observed under zero field, which is the typical SMM behavior. The relaxation time (τ) can thus be extracted using the generalized Debye model in the range of 2−13 K (Figure 3b, inset), giving a narrow-to-moderate distribution coefficient (α) of 0.01−0.22. Taking quantum tunneling of magnetization (QTM), Raman, and Orbach relaxation processes into account, the data were fitted using eq 1 (Figure 3b),12 resulting in an effective energy barrier Ueff/kB = 168(15) K, preexponential factor τ0 = 9.2 × 10−11 s, C = 0.025(9) s−1 K−n, n = 5.1(2), and τQTM = 0.0125(6) s. τ −1 = τQTM −1 + CT n + τ0−1 exp( −Ueff /kBT )

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ASSOCIATED CONTENT

S Supporting Information *

When a field of 1 kOe is applied, the relaxation time remains virtually the same at high temperatures and becomes longer at low temperatures compared to those at zero field (Figure S9). This is commonly observed as a result of suppression of QTM upon application of a dc field.13 The best fit of the relaxation time using eq 1 under a magnetic field gives a similar result except for the quantum tunneling rate: Ueff/kB = 163(18) K, preexponential factor τ0 = 1.0 × 10−10 s, C = 0.008(2) s−1 K−n, n = 5.6(1), and τQTM = 0.28(2) s. Ac susceptibilities were also carried out for the pressed powder of 1. The temperature-dependent ac data show that the ν = 1488 Hz peak is located at 11 K (Figure S11), which is similar to the initial sample. The differences between them are the relaxation times at low temperatures. It is clear that the spin−lattice relaxation rates for the pressed powder are faster than those for the initial powder because the peaks of χ″M signals are located at higher frequencies for the pressed powder whether under an applied field or not (Figure 4). To quantitatively evaluate the

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01059. Experimental section, TGA, temperature-dependent PXRD patterns, structural figures, crystallographic data spectra, and magnetic characterization (PDF) Accession Codes

CCDC 1531710−1531711 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 Authors

*E-mail: [email protected] (J.-L.L.). *E-mail: [email protected] (M.-L.T.). ORCID

Ming-Liang Tong: 0000-0003-4725-0798 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the 973 Project (Grant 2014CB845602), the NSFC (Grants 21371183, 91422302, and 21620102002), the NSF of Guangdong (Grant S2013020013002), and the Fundamental Research Funds for the Central Universities (Grants 17lgjc13 and 17lgpy81).

Figure 4. Frequency-dependent ac susceptibilities for the initial, pressed, and fumed powders of 1 at 0 (a) and 1 kOe (b) dc fields at 2 K. The solid lines are guides to the eyes. 8732

DOI: 10.1021/acs.inorgchem.7b01059 Inorg. Chem. 2017, 56, 8730−8734

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DOI: 10.1021/acs.inorgchem.7b01059 Inorg. Chem. 2017, 56, 8730−8734

Communication

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DOI: 10.1021/acs.inorgchem.7b01059 Inorg. Chem. 2017, 56, 8730−8734